A Crystalline Nitrogen Chain Radical Anion

A Crystalline Nitrogen Chain Radical Anion | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article A Crystalline Nitrogen Chain Radical Anion Meera Mehta, Reece Lister-Roberts, Daniel Galano, Bono van IJzendoorn, and 4 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6055289/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 10 Feb, 2026 Read the published version in Nature Chemistry → Version 1 posted You are reading this latest preprint version Abstract Long chain nitrogen ions and radicals ([N n ] x+ / [N n ] x– , n > 3) are naturally occurring under the intense radiative conditions of the Earth’s ionosphere, and those of other planetary bodies. However, the strong thermodynamic driving force to lose N 2 renders these types of molecules extremely reactive under ambient conditions such that they can typically be studied only under extreme conditions, for example at ultrahigh pressures (10 to >200 GPa). We now report the isolation of a molecule featuring a metal-unsupported {N 4 } •– unit under ambient conditions which demonstrates remarkable multi-week long persistence in the solid-state. Spectroscopic, crystallographic and computational studies provide insight into the bonding across the {N 4 } •– chain. Reactivity studies reveal that the chain can cleave into N1 and N3 fragments, and can act as a source of nitrene radical anion, an observation that such molecules could act as storable nitrogen group transfer reagents. Physical sciences/Chemistry/Coordination chemistry Physical sciences/Chemistry/Inorganic chemistry/Chemical bonding Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 4 Figure 5 Figure 5 Figure 6 Figure 7 Figure 8 Introduction Carbon’s ability to form linear molecular chains is unmatched, and central to how our biology and the materials in our world operate. In stark contrast, its neighbour nitrogen heavily disfavours chain formations. In part this is due to the disproportionally strong N ≡ N triple bond when compared to N–N single and double bonds, making loss of dinitrogen (N 2 ) gas an enormous enthalpic and entropic driving force from catenated nitrogen. 1 This ability for nitrogen chains to rapidly and readily release N 2 renders them potent high-energy-density materials with applications as propellants and explosives, and as gas generators in airbag technology. 2 , 3 Linear chains of N n where n > 3 tend to be especially reactive and difficult to handle. Nonetheless, nitrogen chains and their corresponding ions are of enormous fundamental interest. For example, sandwiched between the lower atmosphere and the magnetosphere, the ionosphere makes life on earth possible by absorbing harmful radiation from the Sun. 4 By absorbing this radiation, the ionosphere also increases the fidelity of radio communication and navigation. In this region of the upper atmosphere, where N 2 is bombarded by solar radiation and galactic cosmic rays, and under artificial plasma conditions, various nitrogen chain ions and radicals have been detected, including [N 4 ] + , [N 5 ] + , and [N 5 ] – . 5 Similar ions are also thought to exist in the turbulent atmosphere of Titan (Saturn’s largest moon). 6 , 7 These fleetingly stable molecules have also been detected in mass spectrometers and trapped at very low temperatures in inert gas matrices. 8 , 9 Furthermore, at ultrahigh pressures the existence of different nitrogen phases has been demonstrated, and crystalline N n (n > 3) chains identified in technologically-relevant metal nitrides. 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 Yet, studying nitrogen chain ions under ambient conditions presents a formidable challenge, and better understanding of their electronic structures should reveal a wealth of hitherto untapped chemical space. Nitrogen chain anions substituted with organic groups are similarly under-investigated. An {N 4 } dianion flanked with organic groups was first reported in the form [Li] 2 [(Ph) 2 N 4 ], 21 although this compound was characterized only on the basis of its subsequent reactivity with electrophiles. The corresponding radical monoanion ([(Ph) 2 N 4 ] •– ) remains particularly elusive, 22 presumably due to its radical nature in addition to the N chain structure. In 1980, McDonald detected [(Ph) 2 N 4 ] •– in a mass spectrometer, and believed it to be generated from N 2 loss from the phenyl azide (PhN 3 ) to give the corresponding nitrene radical anion which then coordinates a second equivalent of PhN 3 . 23 As with the unsubstituted ions, this extant body of literature suggests that organic compounds featuring {N 4 } •– chains are fleetingly stable and only accessible under extreme conditions. One common strategy to isolate highly reactive molecular fragments is to trap them in the coordination sphere of metals. For example, Cummins has shown this with phosphorus monoxide on a molybdenum complex, 24 and Braunschweig stabilized a boron-oxygen triple bond within the coordination sphere of a platinum atom. 25 Also using Pt, Holthausen and Schneider isolated a triplet metalonitrene. 26 In the context of {N 4 } chemistry, numerous research groups have coordinated tetrazene (R−N = N-N = N−R) ligands to metals from across the d-block, and Mg and K. 21 , 27 , 28 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 When coordinated to d-block metals, the [R 2 N 4 ] ligand can exhibit redox non-innocence. 29 , 30 These groups often report that the naked [R 2 N 4 ] ligand could not be isolated and thus the {N 4 } unit is generated within the metal coordination sphere by coupling organoazides. Most relevant to this work, Braunschweig and co-workers reported the compound [({cAAC}BTip) 2 (µ 2 -K) 2 N 4 ] (cAAC = 1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene;Tip = 2,4,6-triisopropylphenyl), featuring a potassium supported {N 4 } 2 – unit constructed, remarkably, directly from N 2 gas! 31 However, in this approach interactions between the metal and fragment, e.g., σ-donation and π-backdonation, can significantly perturb the electronic structure of said fragment and alter its geometry, raising questions as to whether the coordinated fragment accurately represents the unsupported species. Another common approach to enable isolation of reactive fragments is to invoke kinetic stabilization and this has very recently been beautifully demonstrated by Hupf and Beckmann, and the Ye and Tan groups, with the isolation of a triplet nitrene. 41 , 42 However, this tactic may not be the ideal solution to stabilizing ‘longer’ N n chains, as the sterically encumbered substituents required are often labour intensive to prepare and need to be big enough to span multiple atomic units. Thus, in this context delocalization of charge is considered to stabilize the otherwise difficult-to-catch {N 4 } •– unit. In this article we report the synthesis of a robust metal-unsupported nitrogen chain radical anion, [(4-BrC 6 H 4 ) 2 N 4 ] •– ([ 1 ] •– ). The electronic structure of compound [ 1 ] •– is computationally and experimentally studied, and the radical character is confirmed to be distributed across the {N 4 } chain. The subsequent chemical reactivity of [ 1 ] •– is also surveyed, and indicates the possibility that such a molecule can act as an N1 source, highly relevant in chemical synthesis. Results and Discussion Synthesis and Characterization First, 2 equivalents (equiv.) of 1-azido-4-bromobenzene (4-BrC 6 H 4 N 3 ) were reacted with 1 equiv. of potassium graphite (KC 8 ) in the presence of 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (crypt) as shown in Fig. 1 a. The reaction mixture was analyzed by nuclear magnetic resonance (NMR) spectroscopy, and only the cation sequestering agent could be observed by 1 H NMR spectroscopy, consistent with it being diamagnetic in the reaction mixture (see Supplementary Information Section 2.2, Figure S1 ). No other diamagnetic species were observed by the NMR studies. Crystals suitable for single crystal X-ray diffraction (XRD) studies were obtained by slow vapour diffusion of hexane into a tetrahydrofuran (THF) solution, and authenticated the nitrogen-containing product in the reaction mixture as the {N 4 } radical anion [ 1 ] •– (Fig. 1 b). [K(crypt)][ 1 ] is black in appearance, and the anion was confirmed to be paramagnetic by electron paramagnetic resonance (EPR) spectroscopy, vide infra . Compound [K(crypt)][ 1 ] was isolated in 66% yield at this scale, but the reaction could be scaled 30× (4.44 mmol of 4-BrPhN 3 ) with only a small decrease in yield, to 44%. Bulk purity of [K(crypt)][ 1 ] was assessed by powder X-ray diffraction and spin counting using continuous wave EPR spectroscopy, and both methods confirmed high purity. The structural data of [ 1 ] •– reveal the terminal N1A–N2A (which is the same as N1B–N2B) bond length to be 1.324(8) Å and the internal N2A–N2B bond length to be 1.37(2) Å, with the anion sitting on a crystallographic special position (Fig. 1 b). These bond lengths are shorter than that of a hydrazine N–N single bond (1.45 Å) and longer than a trans -azobenzene N = N double bond (1.19 Å), 43 , 44 consistent with partial multiple bond character between the N–N bonds of [ 1 ] •– . Density functional theory (DFT) calculations were conducted to further understand the structure and bonding of [ 1 ] •– and, consistent with the slightly shorter N1A–N2A crystallographic bond length, the Wiberg bond indices indicate that the N1– N2 bond has slightly higher bond order (N1–N2: 1.430; N2–N2: 1.389). These data are consistent with the bonding across the {N 4 } unit to be both delocalized and to have partial multiple bond character between a single and double bond. This description is also supported by infrared (IR) spectroscopy, in which the stretches related to the {N 4 } unit are observed at 1236 cm – 1 (in good agreement with the DFT calculated stretches, for which the largest intensity stretch is at 1272 cm – 1 ) and appear between literature reported bond stretches for hydrazine (1077 cm – 1 , N–N) and azobenzene (1440 cm – 1 , N = N). 45, 46 A range of partial atomic charge types were also computed, and Natural Bond Orbital, Hirshfeld, Löwdin, and Mulliken data all agree that the charge of [ 1 ] •– is significantly delocalized across the whole structure (Supplementary Information Section 2.5, Table S4). Natural resonance theory (NRT) calculations of [ 1 ] •– also support a highly delocalized electronic structure with 248 α spin resonances and 134 β spin resonances found, and with no structure contributing more than 2.6%. To simplify the picture and focus on the N 4 core, the model system [HN 4 H] •– was investigated. NRT studies of [HN 4 H] •– found six key α spin resonance forms with a total weight of 87% and four key β spin resonance forms with a total weight of 90%, with Fig. 1 c showing the unique resonance forms. The NRT data show that there is significant radical and anion character at the terminal (N1) and internal (N2) nitrogens, but which sites - terminal or internal nitrogens - are prone to subsequent reactivity is unclear. The Kohn-Sham molecular orbitals of [ 1 ] •– were also analyzed, and as expected showed significant delocalization over the whole molecule (see Supplementary Information Section 2.6, Figure S4 for a pictorial representation of orbitals HOMO-17 to LUMO + 6 with their corresponding energies). Consistent with the isolobal relationship between 1,3-butadiene and the neutral [RN 4 R], 47 and that addition of one electron to [RN 4 R] gives [RN 4 R] •– , we observed the expected molecular orbitals corresponding to the Hückel theory description of π bonding shown in Fig. 2 . 48 Further, the HOMO was found to be the π 3 combination with anti-bonding character between the terminal and internal nitrogens and bonding character between the two internal nitrogens. EPR spectroscopy of [K(crypt)][ 1 ] confirmed the presence of an unpaired electron centred at g = 2.00575 (Fig. 3 a). The complexity of the spectrum is in line with the delocalization of the radical with multiple coupling partners. A single point DFT calculation gave an estimate of the isotropic hyperfine values and suggested that the electron has the largest coupling to the N1 atoms with A iso = 13.1 MHz (see Supplementary Information Section 2.7 for the full list of values and subsequent simulated spectra). In order to further understand the key hyperfine couplings involved in the spectra, a simplified model involving only the three environments (2 equiv. nitrogens, 4 equiv. hydrogens, 4 equiv. hydrogens) with the largest calculated hyperfine coupling constants were refined by iterative simulation (Fig. 3 a, see Supplementary Information Figure S7 for the overlayed spectra). The simplified simulation finds that the electron is indeed most strongly coupled to one of the nitrogen environments (×2 A N = 14.50 MHz) with significantly smaller couplings from the rest of the molecule, modelled as two proton environments with four protons in each environment (×4 A H = 4.20 MHz, ×4 A H = 1.92 MHz). Addition of further nuclei to the simulation did not improve the fit to the data. The spin densities were also calculated using a variety of established methods, Mulliken, Löwdin, and Hirshfeld (see Supplementary Information Section 2.8, Table S6). All of these methods support delocalization of the radical across the structure with the {N 4 } unit having the largest portion of the spin density (62% when using Mulliken, Fig. 3 b). More specifically, these calculations revealed that the largest individual spin densities are on the terminal nitrogens (N1A and N1B), in agreement with the large hyperfine value from the simulation and conclusions from the NRT calculation on the [HN 4 H] •– model system. These findings suggest the terminal nitrogens as potential sites for subsequent reactivity. The stability of [K(crypt)][ 1 ] was probed with half-life experiments using EPR spectroscopy recorded on samples prepared anaerobically and sealed in a J Young EPR tube (Fig. 3 c). An EPR spectrum was recorded every 90 seconds (600 seconds for powder experiment) and the peak intensities were used to generate decay curves which revealed that [K(crypt)][ 1 ] is more stable in 1,2-difluorobenzene ( o DFB) than in THF ( o DFB: t 1/2 = ~ 4 h; THF: t 1/2 = ~ 30 mins; see Supplementary Information Section 2.10). In the solid state [K(crypt)][ 1 ] was found to be remarkably stable, with only a negligible decrease in EPR resonance intensity observed after 2 days and the radical still present after 6 weeks when stored under anaerobic conditions. The magnetism of [K(crypt)][ 1 ] was measured using superconducting quantum interference device (SQUID) magnetometry and, as expected, the molar magnetic susceptibility showed the typical behaviour for a single unpaired electron (Supplementary Information Section 2.11). Next, [K(crypt)][ 1 ] was investigated by cyclic voltammetry (CV) to identify redox events (Supplementary Information Section 2.12). This cyclic voltammogram revealed a reversible single-electron reduction event at − 1.21 V (vs Ag/AgCl). The one electron reduction of [ 1 ] •– would result in formation of the diamagnetic [ 2 ] 2 – (Fig. 4 a), the phenyl-substituted analogue of which has been reported as the lithium salt. 21 Note, however, that in this literature report the dianionic lithium salt is not spectroscopically characterized. Although a related Mg salt with bulkier hydrocarbon groups on the nitrogens has been isolated, and the Mg cations were found to coordinate the {N 4 } 2 – chain presumably increasing its stability. 28 Similar reversible reduction of {N 4 } •– to {N 4 } 2 – has also been reported in the context of a tetrazene ligand coordinated to an iron centre. 49 Thus, efforts were made to chemically reduce [ 1 ] •– using KC 8 and cobaltocene. No reaction was observed in the case of cobaltocene, and with KC 8 rapid gas evolution (believed to be N 2 loss) was immediately observed and only decomposition detected by NMR spectroscopy. However, when direct reduction of 1 equiv. of 4-BrC 6 H 4 N 3 was tested with 1 equiv. of KC 8 in the absence of crypt, single crystals of potassium-coordinated [ 2 ] 2 – could be obtained from the reaction mixture (Fig. 4 a). This dianion was not characterized further, as it was found to detonate. Considering the molecular orbital diagram in Fig. 2 , the observed lower stability of [ 2 ] 2 – is not entirely surprising, as addition of a second electron would increase electron density in the HOMO resulting in an increase of anti-bonding character between the N1 and N2 atoms and bonding character between the two internal nitrogens which in turn promotes N 2 elimination. Two irreversible oxidation events were also observed by CV. Sweeping in both positive and negative directions from − 0.8 V resulted in the same series of events, with a small decrease in the reduction event when sweeping to oxidative potentials first, likely due to a small amount of [K(crypt)][ 1 ] decomposing during the irreversible oxidation events. Scanning the reversible reduction event at different scan rates (see Supplementary Information Figure S17) revealed that the redox event appears to be more reversible at lower scan rates, consistent with this event being chemically reversible on the CV timescale but demonstrating slow electron transfer which could be the result of a structural change happening between the two states. 50 UV-Vis studies revealed a strong absorption of blue / green light as well as a weaker broad absorption tailing the whole of the visible region, in line with the black appearance of [K(crypt)][ 1 ]. Time-dependent density functional theory (TD-DFT, see Supplementary Information Section 2.14.2) calculations were undertaken to better understand the nature of the electronic transitions resulting in this spectrum. Four key excitations were found and investigated further using natural transition orbitals (NTOs). The NTO data revealed that all four of the excitations are primarily underpinned by the same two orbital pairs with differing occupations; these are pictured in Fig. 4 b. The transition identified with the grey dot can be described as primarily a movement of electron density from the {N 4 } chain onto the aromatic rings, and the one with a blue dot with the opposite character. Reactivity Studies Radical-radical recombination reactions have become useful tools to pinpoint the reactive site for paramagnetic materials. 51 For this reason, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and 1,1′,1′′-{[4-(diphenylmethylidene)cyclohexa-2,5-dien-1-yl]methanetriyl}tribenzene (Gomberg’s dimer) were independently reacted with a solution of [K(crypt)][ 1 ] in THF (Fig. 5 ). In both reactions, these persistent radicals showed no reactivity towards [K(crypt)][ 1 ], further highlighting the high stability of [ 1 ] •– . Next, triphenyltin hydride was investigated as a source of H • , 52 a far less stable and less bulky radical, which did lead to radical quenching. Single crystals of hexaphenyldistannane ( 3 ) were obtained from the reaction mixture, 53 confirmed by XRD studies, and is a known by-product from H • elimination from tin hydrides. 52 NMR spectroscopy studies of the crude reaction mixture confirmed the presence of the amide [ 4 ] – . After acidic workup, the product [ 4 ] – was converted to 4-bromoanilinium chloride ( 5 ), confirmed by 1 H NMR spectroscopy. Whereas, when 4 equiv. of 4-methylthiophenol (TolSH, a known source of H • and H + ) 54, 55 was investigated in place of the triphenyltin hydride, [K(crypt)][ 4 ] + 6 was formed with an NMR conversion of 68% (confirmed by XRD analysis of single crystals obtained from the reaction mixture). It is worth noting that the diagnostic 1 H NMR resonances corresponding to a mixture of [K(crypt)][ 4 ] + 6 shift based on the relative concentrations of the two species, consistent with a proton from 6 shuttling to [ 4 ] – and back again. [ 4 ] – + 6 co-crystallize with the sequestered potassium cation sitting between them and the sulfur and nitrogen-groups pointing towards one another. These data are consistent with the mixture being described as [K(crypt)][ 4 ] + 6 with the nitrogen deprotonated or [K(crypt)][ 6 ] + 4 with the sulfur deprotonated. To better understand the thiol reactivity, 1 equiv. of TolSH was reacted with [K(crypt)][ 1 ] and immediately inspected by 1 H NMR spectroscopy. To our delight, the formation of 4-BrPhN 3 was observed (Supplementary Information Section 3.3.1, Figure S34). Almost half a century ago, in McDonald’s work where [(Ph) 2 N 4 ] •– was detected in a mass spectrometer, it was postulated that the {N 4 } •– was generated by first ionizing 1 equiv. of azide to give the nitrene radical anion [PhN] •– , which subsequently forms an adduct with another equiv. of azide (pictorial description in Fig. 5 ). 23 We computed the Gibbs reaction energy of the dissociation of this ‘adduct’ starting from [ 1 ] •– to the corresponding azide and nitrene radical anion to be ~ + 19 kJ/mol. To our knowledge this description of the bonding of such molecules has not previously been reported, in part because until now molecules featuring metal-unsupported {N 4 } •– units were not isolated. However, detection of 4-BrC 6 H 4 N 3 in the thiol reaction demonstrates that [ 1 ] •– can fall apart in a manner consistent with this bonding description. Nitrene species are known to react with aldehydes to undergo carbonyl C–H activation. 56 And as [K(crypt)][ 1 ] may act as a source of nitrene radical anion, it was investigated with 4-iodobenzaldehyde (4-IC 6 H 4 CHO) in o DFB, and indeed the corresponding amide 7 could be isolated. Furthermore, the crude reaction mixture from this aldehyde reaction again revealed the presence of the expected 4-BrPhN 3 molecule (Supplementary Information Section 3.4.1, Figure S38). It is also worth noting that 4-BrC 6 H 4 N 3 does not react with 4-IC 6 H 4 CHO under the same reaction conditions, and independently prepared [K(crypt)][ 4 ] reacts with 4-IC 6 H 4 CHO to give the corresponding imine, not amide. Conclusion Almost half a century after the first detection of a compound featuring a {N 4 } •– chain in a mass spectrometer, we have shown that a molecule featuring this moiety can be isolated under ambient conditions. Electronically stabilized by aromatic units with bromines, crystalline [(4-BrC 6 H 4 ) 2 N 4 ] •– is isolated as a storable solid with multi-day stability under anaerobic conditions. The propensity of nitrogen catenates to rapidly degrade and release N 2 has thus far been an impediment to fully understanding the electronic structure of such chains, but the high stability of the metal-unsupported [(4-BrC 6 H 4 ) 2 N 4 ] •– has allowed this barrier to be overcome. Computational and experimental electronic structure studies into [(4-BrC 6 H 4 ) 2 N 4 ] •– are consistent with the {N 4 } chain having partial multiple bond character and there being significant radical character on the terminal nitrogen atoms bonded to the aromatic units. These conclusions are consistent with the subsequent reactivity studies which reveal that this {N 4 } chain can decompose into N1 and N3 units, with reactivity consistent with the generation of a nitrene radical anion, exemplified by its reaction with an aldehyde where the carbonyl C–H bond is activated to give an amide. Efforts are now focused on exploring the additional reactivity patterns of {N 4 } •– -containing molecules, to enhance our understanding of their chemical properties, and fully unlock their potential as gram-scale storable nitrene synthons. Methods General considerations NMR spectra were recorded on a Bruker AVIII 400 spectrometer at ambient temperatures. Cyclic voltammetry was carried out in the glovebox under inert conditions with EMStat4s. Ultraviolet-visible electronic absorption spectra were recorded on a Mettler Toledo UV5Bio spectrophotometer using 10 mm path length quartz J Young cuvettes. Electron paramagnetic resonance spectra were recorded at X band (9.4 GHz) with a Bruker EMXmicro spectrometer at 298 K. Spin counting was conducted on a Bruker Magnettech ESR5000 at X band (9.8 GHz), 298 K using the ESR Studio software’s incorporated spin counting function. Mass spectrometry samples were analyzed using an electrospray ionization (ESI) equipped Waters RDa bench-top time of flight mass spectrometer. ATR-IR spectra were recorded using a Bruker Alpha II under an inert atmosphere. X-ray diffraction data was collected for compound [K(crypt)][ 1 ] on a dual source Rigaku FR-X rotating anode at 100K with Cu Kα (1.54184 Å) radiation. X-ray diffraction data was collected for all other compounds on an Oxford Diffraction Supernova dual-source diffractometer at 150K using Cu Kα (1.54184 Å). Preparation of [K(crypt)][ 1 ] In the glovebox, KC 8 (10 mg, 0.074 mmol, 1 equiv.) and 2.2.2-cryptand (crypt; 28 mg, 0.074 mmol, 1 equiv.) were suspended in THF in a vial. 4-BrC 6 H 4 N 3 (19 uL, 0.148 mmol, 2 equiv.) was added and the vial shaken for 30 seconds. The solution was filtered and ether added to precipitate a black solid. The solid was filtered and washed with ether before drying under vacuum yielding [K(crypt)][ 1 ] as a black crystalline solid. Single crystals were obtained by slow vapour diffusion of hexane into THF in the freezer. A mortar and pestle was used to grind the crystalline powder to a fine powder in the glovebox for use in powder X-ray diffraction studies. Isolated yield: 38.3mg, 66%. Computational Methodology DFT calculations were carried out using the Gaussian 16 package, revision C.01. 60 Following extensive benchmarking (Supplementary Information Section 2.3) the TPSS functional was used, 61 together with the Def2-TZVP basis set. 62 , 63 Geometry optimizations were performed, and all minima verified as true by harmonic vibrational frequency calculations. These frequencies were employed in conjunction with Grimme’s quasi-harmonic approach for computation of the Gibbs free energies. 62 , 64 The solvent environment was modelled using the smd method, 65 with parameters appropriate to THF and cyclopentanone (a suitable model for o DFB). Natural bond orbital (NBO) and natural resonance theory (NRT) calculations were carried out using NBO 7.0. 66 TDDFT calculations were performed using the TPSSh functional 67 and Def2-TZVP basis set. EPR related calculations were carried out using ORCA 5.0.4 with the B3LYP functional, 68 , 69 , 70 with the aug-cc-pVTZ basis set used for Br, 71 and the EPR-III basis set for all other atoms. 72 Declarations Data Availability All data are available in the main text or Supplementary Information. Crystallographic data for structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers 2423978-2423981. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Cartesian coordinates of optimized structures and all other source data are provided in the Source Data Folder. All data are also available from the corresponding authors upon request. Acknowledgments This research was funded in part by UKRI [EP/Y037391/1]. For the purpose of Open Access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript (AAM) version arising from this submission. We thank UKRI for their support, and EPSRC for funding PhD students R.L-R and D.G. We also thank the EPSRC for supporting the National Research Facility for EPR (EP/W014521/1, EP/Z530670/1, EP/X034623/1, EP/V035231/1) and SQUID magnetometer (EP/S033181/1) used in this work. A.M.B. is grateful to The Royal Society and the EPSRC for a Dorothy Hodgkin Fellowship (DH160004 and DHF\R\221018), and the University of Manchester for a Dame Kathleen Ollerenshaw Fellowship. A.M.B. also thanks the Royal Society of Chemistry for a Community for Analytical and Measurement Science fellowship (CAMS Fellowship 2020 ACTF ref 600310/09). We thank Prof. Eric McInnes for useful discussions regarding SQUID measurements, Agamemnon Crumpton for useful discussions regarding modelling disorder across special positions, and Anne Davies and Martin Jennings for elemental analyses. We are grateful to The University of Manchester for access to its Computational Shared Facility and associated support services. Author information Reece Lister-Roberts, 1,2 Daniel Galano, 1 Bono van IJzendoorn, 2 George F. S. Whitehead, 1 Adam Brookfield, Alice M. Bowen, 1,3 * Nikolas Kaltsoyannis, 1 * Meera Mehta 2 * [email protected] , [email protected] , [email protected] . 1. Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, U.K. 2. Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K. 3. The EPSRC National Research Facility for Electron Paramagnetic Resonance, Photon Science Institute, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K. Author Contributions R.L-R. performed all synthesis, and subsequent analysis and interpretation, with experiments designed in collaboration with M.M. and EPR studies performed in collaboration with A.M.B. and A.B. R.L-R and A.B recorded the SQUID data. D.G. and N.K. performed all computational investigations. B.v.I. performed preliminary investigations, and single crystal XRD studies along with R.L-R. G.F.S.W. performed powder XRD studies and subsequent data interpretation. R.L-R., D.G. and M.M. wrote the initial drafts of the manuscript with editing from R.L-R., D.G. B.v.I., N.K., M.M. and A.M.B. Ethics Declaration Competing interests The authors declare no competing interests. References Greenwood NN, Earnshaw A. Chemistry of the Elements . Elsevier, 2012. Klapötke TM. Chemistry of High-Energy Materials . 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Kendall RA, Dunning TH, Jr., Harrison RJ. Electron Affinities of the First‐row Atoms Revisited. Systematic Basis Sets and Wave Functions. J. Chem. Phys. 1992, 96 (9) : 6796–6806. Barone V. Structure, Magnetic Properties and Reactivities of Open-Shell Species From Density Functional and Self-Consistent Hybrid Methods. Recent Advances in Density Functional Methods , pp 287–334. Additional Declarations There is NO Competing Interest. Supplementary Files 20250217SupportingInformation.pdf Supplementary Information NCHEM25020567Mehtacifandcheckcif.zip Cif and Checkcifs for XRD Structures Cite Share Download PDF Status: Published Journal Publication published 10 Feb, 2026 Read the published version in Nature Chemistry → Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6055289","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":420813912,"identity":"c36df22e-e464-447e-8a92-531da536f5de","order_by":0,"name":"Meera Mehta","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABB0lEQVRIiWNgGAWjYDADAwYGxgeMDUgiEgS1sDEwG5CshU2CKC267c3HPvxg2CZnLt/8rOLnDptoBvYeA4YfNQyJMxuwazE7cyx5Zg/DbWPLNjazm71n0nIbeI4lMPYcY0icjcMWsxs5xgw8DLcTNxxjMLvN2HY4t0Ei+QADbwND4jxcWu6//8z4h+F2/YZj7N+KGdv+A7UkNjD+xaflBg8zM9CWBINjPGbMjG0HwLYwg2zB6bAzacbMMga3DTccyymW7G1Lzm0D+uWwzDEJY5zeP374MeObitvyBoePb/zws80ut5+9x/Dhmxob2RkHcFgDBgZIbDYgPkBERI6CUTAKRsEowAMASgFbACwZxs0AAAAASUVORK5CYII=","orcid":"https://orcid.org/0000-0002-6833-5574","institution":"University of Oxford","correspondingAuthor":true,"prefix":"","firstName":"Meera","middleName":"","lastName":"Mehta","suffix":""},{"id":420813913,"identity":"17610fda-46cc-40f4-b55c-9e8cf20774ac","order_by":1,"name":"Reece Lister-Roberts","email":"","orcid":"https://orcid.org/0000-0001-9277-7586","institution":"University of Manchester, University of Oxford","correspondingAuthor":false,"prefix":"","firstName":"Reece","middleName":"","lastName":"Lister-Roberts","suffix":""},{"id":420813914,"identity":"e25d2aa1-67a2-4580-b489-812b2869b741","order_by":2,"name":"Daniel Galano","email":"","orcid":"https://orcid.org/0009-0006-4370-5336","institution":"University of Manchester","correspondingAuthor":false,"prefix":"","firstName":"Daniel","middleName":"","lastName":"Galano","suffix":""},{"id":420813915,"identity":"279da5e9-f284-4c3b-9f68-95d4e890de0f","order_by":3,"name":"Bono van IJzendoorn","email":"","orcid":"https://orcid.org/0000-0003-1010-4834","institution":"University of Oxford","correspondingAuthor":false,"prefix":"","firstName":"Bono","middleName":"van","lastName":"IJzendoorn","suffix":""},{"id":420813916,"identity":"2f0ca6e6-6a4b-43b1-8cd7-d61d6f698f12","order_by":4,"name":"George Whitehead","email":"","orcid":"https://orcid.org/0000-0003-1949-4250","institution":"University of Manchester","correspondingAuthor":false,"prefix":"","firstName":"George","middleName":"","lastName":"Whitehead","suffix":""},{"id":420813917,"identity":"912db296-40fd-47b2-80ea-aee818cb8851","order_by":5,"name":"Adam Brookfield","email":"","orcid":"","institution":"University of Manchester","correspondingAuthor":false,"prefix":"","firstName":"Adam","middleName":"","lastName":"Brookfield","suffix":""},{"id":420813918,"identity":"7f753e9c-96d5-48e8-8053-041e807bc26c","order_by":6,"name":"Alice M. Bowen","email":"","orcid":"","institution":"University of Manchester","correspondingAuthor":false,"prefix":"","firstName":"Alice","middleName":"M.","lastName":"Bowen","suffix":""},{"id":420813919,"identity":"21b23849-111d-4153-834e-c9c0aa74c5a4","order_by":7,"name":"Nikolas Kaltsoyannis","email":"","orcid":"https://orcid.org/0000-0003-0293-5742","institution":"University of Manchester","correspondingAuthor":false,"prefix":"","firstName":"Nikolas","middleName":"","lastName":"Kaltsoyannis","suffix":""}],"badges":[],"createdAt":"2025-02-18 10:36:17","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6055289/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6055289/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41557-025-02040-2","type":"published","date":"2026-02-10T05:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":77308437,"identity":"2981a387-f186-4c03-a0ed-c95b44ba3527","added_by":"auto","created_at":"2025-02-27 09:26:09","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":144270,"visible":true,"origin":"","legend":"\u003cp\u003ea) Synthesis of [K(crypt)][\u003cstrong\u003e1\u003c/strong\u003e]. b) Molecular structure of [\u003cstrong\u003e1\u003c/strong\u003e]\u003csup\u003e•–\u003c/sup\u003e in [K(crypt)][\u003cstrong\u003e1\u003c/strong\u003e] showing anisotropic displacement ellipsoids at 50% probability. Counter cation and hydrogens omitted for clarity. Nitrogen: blue; carbon: white; bromine: brown. c) Natural Resonance Theory (NRT) calculated resonance forms of [HN\u003csub\u003e4\u003c/sub\u003eH]\u003csup\u003e•–\u003c/sup\u003e.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/84254ff4ab2246c53ff31976.png"},{"id":77307301,"identity":"c6d6767f-d6e5-4375-a0a7-39646daea701","added_by":"auto","created_at":"2025-02-27 09:18:08","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":141041,"visible":true,"origin":"","legend":"\u003cp\u003eIsolobal relationship between 1,3-butadiene and [RN\u003csub\u003e4\u003c/sub\u003eR], and selected Kohn-Sham molecular orbitals of [\u003cstrong\u003e1\u003c/strong\u003e]\u003csup\u003e•–\u003c/sup\u003e with simplified molecular orbital diagram focusing on the π-bonding of the {N\u003csub\u003e4\u003c/sub\u003e} unit.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/78e38337d30d1cc6a6b259e8.png"},{"id":77307304,"identity":"0d023a28-ef11-40bc-bee0-a45e15a6c36d","added_by":"auto","created_at":"2025-02-27 09:18:09","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":163206,"visible":true,"origin":"","legend":"\u003cp\u003ea) Experimental (blue) and simulated (grey, ×2 A\u003csub\u003eN\u003c/sub\u003e = 14.50 MHz, ×4 A\u003csub\u003eH\u003c/sub\u003e = 4.20 MHz, ×4 A\u003csub\u003eH\u003c/sub\u003e = 1.92 MHz) continuous wave EPR spectroscopy (298 K, 0.1 G, 2 scans, 9.389934 GHz, sample sealed in J Young tube under N\u003csub\u003e2\u003c/sub\u003e) spectrum of [K(crypt)][\u003cstrong\u003e1\u003c/strong\u003e]. b) Calculated Mulliken spin densities. c) Decay curves of [K(crypt)][\u003cstrong\u003e1\u003c/strong\u003e] determined by continuous wave EPR spectroscopy, where the integrated resonance intensity from a continuous wave EPR spectrum recorded every 90 seconds is reported (600 seconds for powder).\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/5a95480ff34e59e8be834e0b.png"},{"id":77308438,"identity":"627bcbf5-b281-41d9-acaa-c238757ec876","added_by":"auto","created_at":"2025-02-27 09:26:09","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":133005,"visible":true,"origin":"","legend":"a) Synthesis of [K(crypt)][]. b) Molecular structure of [] in [K(crypt)][] showing anisotropic displacement ellipsoids at 50% probability. Counter cation and hydrogens omitted for clarity. Nitrogen: blue; carbon: white; bromine: brown. c) Natural Resonance Theory (NRT) calculated resonance forms of [HNH].","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/074cb2045aa1d3a7e7118717.png"},{"id":77307309,"identity":"4c978847-610f-4d40-a816-a21fd279bb55","added_by":"auto","created_at":"2025-02-27 09:18:09","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":164968,"visible":true,"origin":"","legend":"\u003cp\u003ea) Cyclic Voltammogram of [K(crypt)][\u003cstrong\u003e1\u003c/strong\u003e] ([nBu\u003csub\u003e4\u003c/sub\u003eN][PF\u003csub\u003e6\u003c/sub\u003e], 100 equiv.) 3 mM in THF at 0.1 V/s starting at -0.8 V and scanning independently in the positive direction first (grey trace) and the negative direction first (blue trace). Glassy carbon working electrode, platinum wire counter electrode and leak-proof Ag/AgCl reference electrode were used. Molecular structure of [K(THF)\u003csub\u003e2\u003c/sub\u003e]\u003csub\u003e2\u003c/sub\u003e[\u003cstrong\u003e2\u003c/strong\u003e] showing anisotropic displacement ellipsoids at 50% probability with hydrogen atoms omitted for clarity. Disorder and coordinated THF molecules omitted for clarity and major component (\u003cem\u003eca.\u003c/em\u003e 60% occupancy) shown.\u0026nbsp; Nitrogen: blue; carbon: white; bromine: brown; potassium: violet; oxygen: red. b) Experimental (in \u003cem\u003eo\u003c/em\u003eDFB solvent) and calculated UV-Vis spectra of [\u003cstrong\u003e1\u003c/strong\u003e]\u003csup\u003e•–\u003c/sup\u003e, showing the key natural transition orbitals (NTOs) and their occupations with blue and grey dots in computed transitions 1–4.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/5bee5d72711cc7065028ec36.png"},{"id":77308439,"identity":"f0cee456-581f-4a1f-b26d-1abdb1727d8f","added_by":"auto","created_at":"2025-02-27 09:26:09","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":98704,"visible":true,"origin":"","legend":"\u003cp\u003eReactivity studies of [K(crypt)][\u003cstrong\u003e1\u003c/strong\u003e]. No thiyl radical species or disulfide species is detected in the thiol chemistry, and the radical is believed to be quenched by H\u003csup\u003e• \u003c/sup\u003eabstraction from either solvent or crypt.\u003csup\u003e57\u003c/sup\u003e Thiyl radicals are known to abstract H\u003csup\u003e• \u003c/sup\u003efrom ethers.\u003csup\u003e58, 59\u003c/sup\u003e The crude reaction mixture from the reaction with 4-iodobenzaldehyde revealed unidentified decomposition products by \u003csup\u003e1\u003c/sup\u003eH NMR spectroscopy, as is typical in nitrene C–H activation chemistry.\u003csup\u003e56\u003c/sup\u003e\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/96c84b94891cee3525e88849.png"},{"id":77308440,"identity":"16bd7b27-c115-4a89-b510-e03b997b1f9d","added_by":"auto","created_at":"2025-02-27 09:26:09","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":203827,"visible":true,"origin":"","legend":"Isolobal relationship between 1,3-butadiene and [RNR], and selected Kohn-Sham molecular orbitals of [] with simplified molecular orbital diagram focusing on the π-bonding of the {N} unit.","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/bd96b4f26bfc0b9bb93fb6ba.png"},{"id":77310041,"identity":"d40d8998-151b-48d5-bdad-6ee3b19e9365","added_by":"auto","created_at":"2025-02-27 09:42:09","extension":"png","order_by":6,"title":"Figure 6","display":"","copyAsset":false,"role":"figure","size":138138,"visible":true,"origin":"","legend":"a) Experimental (blue) and simulated (grey, \u0026times;2 A = 14.50 MHz, \u0026times;4 A = 4.20 MHz, \u0026times;4 A = 1.92 MHz) continuous wave EPR spectroscopy (298 K, 0.1 G, 2 scans, 9.389934 GHz, sample sealed in J Young tube under N) spectrum of [K(crypt)][]. b) Calculated Mulliken spin densities. c) Decay curves of [K(crypt)][] determined by continuous wave EPR spectroscopy, where the integrated resonance intensity from a continuous wave EPR spectrum recorded every 90 seconds is reported (600 seconds for powder).","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/7978e6cad57e58b95ea6593c.png"},{"id":77308442,"identity":"0a2e132a-f885-4e69-9500-d6336099f15e","added_by":"auto","created_at":"2025-02-27 09:26:09","extension":"png","order_by":7,"title":"Figure 7","display":"","copyAsset":false,"role":"figure","size":131278,"visible":true,"origin":"","legend":"a) Cyclic Voltammogram of [K(crypt)][] ([nBuN][PF], 100 equiv.) 3 mM in THF at 0.1 V/s starting at -0.8 V and scanning independently in the positive direction first (grey trace) and the negative direction first (blue trace). Glassy carbon working electrode, platinum wire counter electrode and leak-proof Ag/AgCl reference electrode were used. Molecular structure of [K(THF)][] showing anisotropic displacement ellipsoids at 50% probability with hydrogen atoms omitted for clarity. Disorder and coordinated THF molecules omitted for clarity and major component ( 60% occupancy) shown. Nitrogen: blue; carbon: white; bromine: brown; potassium: violet; oxygen: red. b) Experimental (in DFB solvent) and calculated UV-Vis spectra of [], showing the key natural transition orbitals (NTOs) and their occupations with blue and grey dots in computed transitions 1\u0026ndash;4.","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/1279a8aa2802eab12d1af6ee.png"},{"id":77308727,"identity":"b9ce817a-91e2-4f35-a8e6-3ee1d4af7835","added_by":"auto","created_at":"2025-02-27 09:34:09","extension":"png","order_by":8,"title":"Figure 8","display":"","copyAsset":false,"role":"figure","size":67186,"visible":true,"origin":"","legend":"Reactivity studies of [K(crypt)][]. No thiyl radical species or disulfide species is detected in the thiol chemistry, and the radical is believed to be quenched by H abstraction from either solvent or crypt. Thiyl radicals are known to abstract H from ethers. The crude reaction mixture from the reaction with 4-iodobenzaldehyde revealed unidentified decomposition products by H NMR spectroscopy, as is typical in nitrene C\u0026ndash;H activation chemistry.","description":"","filename":"floatimage5.png","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/a1152ef165345eb17e417fb0.png"},{"id":102387571,"identity":"edb84260-fc24-403c-973b-c650d9d57689","added_by":"auto","created_at":"2026-02-11 08:11:37","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2215726,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/da06766d-26da-4356-a99a-adc03b650dec.pdf"},{"id":77308444,"identity":"b5575f0c-ddc0-4739-a1a0-ec350fe9007f","added_by":"auto","created_at":"2025-02-27 09:26:09","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3272496,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"20250217SupportingInformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/a259a5da3ce56d2d55fad3cf.pdf"},{"id":77307311,"identity":"94080669-119f-4c36-8fe2-eb5b9592cdc2","added_by":"auto","created_at":"2025-02-27 09:18:09","extension":"zip","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":1883651,"visible":true,"origin":"","legend":"Cif and Checkcifs for XRD Structures","description":"","filename":"NCHEM25020567Mehtacifandcheckcif.zip","url":"https://assets-eu.researchsquare.com/files/rs-6055289/v1/1879393e2e43df6ca18f05f3.zip"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"A Crystalline Nitrogen Chain Radical Anion","fulltext":[{"header":"Introduction","content":"\u003cp\u003eCarbon\u0026rsquo;s ability to form linear molecular chains is unmatched, and central to how our biology and the materials in our world operate. In stark contrast, its neighbour nitrogen heavily disfavours chain formations. In part this is due to the disproportionally strong N\u0026thinsp;\u0026equiv;\u0026thinsp;N triple bond when compared to N\u0026ndash;N single and double bonds, making loss of dinitrogen (N\u003csub\u003e2\u003c/sub\u003e) gas an enormous enthalpic and entropic driving force from catenated nitrogen.\u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e This ability for nitrogen chains to rapidly and readily release N\u003csub\u003e2\u003c/sub\u003e renders them potent high-energy-density materials with applications as propellants and explosives, and as gas generators in airbag technology.\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e, \u003cspan citationid=\"CR3\" class=\"CitationRef\"\u003e3\u003c/span\u003e\u003c/sup\u003e Linear chains of N\u003csub\u003en\u003c/sub\u003e where n\u0026thinsp;\u0026gt;\u0026thinsp;3 tend to be especially reactive and difficult to handle.\u003c/p\u003e \u003cp\u003eNonetheless, nitrogen chains and their corresponding ions are of enormous fundamental interest. For example, sandwiched between the lower atmosphere and the magnetosphere, the ionosphere makes life on earth possible by absorbing harmful radiation from the Sun.\u003csup\u003e\u003cspan citationid=\"CR4\" class=\"CitationRef\"\u003e4\u003c/span\u003e\u003c/sup\u003e By absorbing this radiation, the ionosphere also increases the fidelity of radio communication and navigation. In this region of the upper atmosphere, where N\u003csub\u003e2\u003c/sub\u003e is bombarded by solar radiation and galactic cosmic rays, and under artificial plasma conditions, various nitrogen chain ions and radicals have been detected, including [N\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e+\u003c/sup\u003e, [N\u003csub\u003e5\u003c/sub\u003e]\u003csup\u003e+\u003c/sup\u003e, and [N\u003csub\u003e5\u003c/sub\u003e]\u003csup\u003e\u0026ndash;\u003c/sup\u003e.\u003csup\u003e\u003cspan citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u003c/sup\u003e Similar ions are also thought to exist in the turbulent atmosphere of Titan (Saturn\u0026rsquo;s largest moon).\u003csup\u003e\u003cspan citationid=\"CR6\" class=\"CitationRef\"\u003e6\u003c/span\u003e, \u003cspan citationid=\"CR7\" class=\"CitationRef\"\u003e7\u003c/span\u003e\u003c/sup\u003e These fleetingly stable molecules have also been detected in mass spectrometers and trapped at very low temperatures in inert gas matrices.\u003csup\u003e\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e, \u003cspan citationid=\"CR9\" class=\"CitationRef\"\u003e9\u003c/span\u003e\u003c/sup\u003e Furthermore, at ultrahigh pressures the existence of different nitrogen phases has been demonstrated, and crystalline N\u003csub\u003en\u003c/sub\u003e (n\u0026thinsp;\u0026gt;\u0026thinsp;3) chains identified in technologically-relevant metal nitrides.\u003csup\u003e\u003cspan citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e, \u003cspan citationid=\"CR11\" class=\"CitationRef\"\u003e11\u003c/span\u003e, \u003cspan citationid=\"CR12\" class=\"CitationRef\"\u003e12\u003c/span\u003e, \u003cspan citationid=\"CR13\" class=\"CitationRef\"\u003e13\u003c/span\u003e, \u003cspan citationid=\"CR14\" class=\"CitationRef\"\u003e14\u003c/span\u003e, \u003cspan citationid=\"CR15\" class=\"CitationRef\"\u003e15\u003c/span\u003e, \u003cspan citationid=\"CR16\" class=\"CitationRef\"\u003e16\u003c/span\u003e, \u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e, \u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e\u003c/sup\u003e Yet, studying nitrogen chain ions under ambient conditions presents a formidable challenge, and better understanding of their electronic structures should reveal a wealth of hitherto untapped chemical space.\u003c/p\u003e \u003cp\u003eNitrogen chain anions substituted with organic groups are similarly under-investigated. An {N\u003csub\u003e4\u003c/sub\u003e} dianion flanked with organic groups was first reported in the form [Li]\u003csub\u003e2\u003c/sub\u003e[(Ph)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e],\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e although this compound was characterized only on the basis of its subsequent reactivity with electrophiles. The corresponding radical monoanion ([(Ph)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e) remains particularly elusive,\u003csup\u003e\u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e\u003c/sup\u003e presumably due to its radical nature in addition to the N chain structure. In 1980, McDonald detected [(Ph)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e in a mass spectrometer, and believed it to be generated from N\u003csub\u003e2\u003c/sub\u003e loss from the phenyl azide (PhN\u003csub\u003e3\u003c/sub\u003e) to give the corresponding nitrene radical anion which then coordinates a second equivalent of PhN\u003csub\u003e3\u003c/sub\u003e.\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e As with the unsubstituted ions, this extant body of literature suggests that organic compounds featuring {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e chains are fleetingly stable and only accessible under extreme conditions.\u003c/p\u003e \u003cp\u003eOne common strategy to isolate highly reactive molecular fragments is to trap them in the coordination sphere of metals. For example, Cummins has shown this with phosphorus monoxide on a molybdenum complex,\u003csup\u003e\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e\u003c/sup\u003e and Braunschweig stabilized a boron-oxygen triple bond within the coordination sphere of a platinum atom.\u003csup\u003e\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e\u003c/sup\u003e Also using Pt, Holthausen and Schneider isolated a triplet metalonitrene.\u003csup\u003e\u003cspan citationid=\"CR26\" class=\"CitationRef\"\u003e26\u003c/span\u003e\u003c/sup\u003e In the context of {N\u003csub\u003e4\u003c/sub\u003e} chemistry, numerous research groups have coordinated tetrazene (R\u0026minus;N\u0026thinsp;=\u0026thinsp;N-N\u0026thinsp;=\u0026thinsp;N\u0026minus;R) ligands to metals from across the d-block, and Mg and K.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR27\" class=\"CitationRef\"\u003e27\u003c/span\u003e, \u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e, \u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e, \u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e, \u003cspan citationid=\"CR32\" class=\"CitationRef\"\u003e32\u003c/span\u003e, \u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e, \u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e, \u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e, \u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e, \u003cspan citationid=\"CR38\" class=\"CitationRef\"\u003e38\u003c/span\u003e, \u003cspan citationid=\"CR39\" class=\"CitationRef\"\u003e39\u003c/span\u003e, \u003cspan citationid=\"CR40\" class=\"CitationRef\"\u003e40\u003c/span\u003e\u003c/sup\u003e When coordinated to d-block metals, the [R\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e] ligand can exhibit redox non-innocence.\u003csup\u003e\u003cspan citationid=\"CR29\" class=\"CitationRef\"\u003e29\u003c/span\u003e, \u003cspan citationid=\"CR30\" class=\"CitationRef\"\u003e30\u003c/span\u003e\u003c/sup\u003e These groups often report that the naked [R\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e] ligand could not be isolated and thus the {N\u003csub\u003e4\u003c/sub\u003e} unit is generated within the metal coordination sphere by coupling organoazides. Most relevant to this work, Braunschweig and co-workers reported the compound [({cAAC}BTip)\u003csub\u003e2\u003c/sub\u003e(\u0026micro;\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u003c/sup\u003e-K)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e] (cAAC\u0026thinsp;=\u0026thinsp;1-(2,6-diisopropylphenyl)-3,3,5,5-tetramethylpyrrolidin-2-ylidene;Tip\u0026thinsp;=\u0026thinsp;2,4,6-triisopropylphenyl), featuring a potassium supported {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003c/sup\u003e unit constructed, remarkably, directly from N\u003csub\u003e2\u003c/sub\u003e gas!\u003csup\u003e\u003cspan citationid=\"CR31\" class=\"CitationRef\"\u003e31\u003c/span\u003e\u003c/sup\u003e However, in this approach interactions between the metal and fragment, e.g., σ-donation and π-backdonation, can significantly perturb the electronic structure of said fragment and alter its geometry, raising questions as to whether the coordinated fragment accurately represents the unsupported species. Another common approach to enable isolation of reactive fragments is to invoke kinetic stabilization and this has very recently been beautifully demonstrated by Hupf and Beckmann, and the Ye and Tan groups, with the isolation of a triplet nitrene.\u003csup\u003e\u003cspan citationid=\"CR41\" class=\"CitationRef\"\u003e41\u003c/span\u003e, \u003cspan citationid=\"CR42\" class=\"CitationRef\"\u003e42\u003c/span\u003e\u003c/sup\u003e However, this tactic may not be the ideal solution to stabilizing \u0026lsquo;longer\u0026rsquo; N\u003csub\u003en\u003c/sub\u003e chains, as the sterically encumbered substituents required are often labour intensive to prepare and need to be big enough to span multiple atomic units. Thus, in this context delocalization of charge is considered to stabilize the otherwise difficult-to-catch {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e unit.\u003c/p\u003e \u003cp\u003eIn this article we report the synthesis of a robust metal-unsupported nitrogen chain radical anion, [(4-BrC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e ([\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e). The electronic structure of compound [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e is computationally and experimentally studied, and the radical character is confirmed to be distributed across the {N\u003csub\u003e4\u003c/sub\u003e} chain. The subsequent chemical reactivity of [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e is also surveyed, and indicates the possibility that such a molecule can act as an N1 source, highly relevant in chemical synthesis.\u003c/p\u003e"},{"header":"Results and Discussion","content":"\u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003eSynthesis and Characterization\u003c/h2\u003e \u003cp\u003eFirst, 2 equivalents (equiv.) of 1-azido-4-bromobenzene (4-BrC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e) were reacted with 1 equiv. of potassium graphite (KC\u003csub\u003e8\u003c/sub\u003e) in the presence of 4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane (crypt) as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea. The reaction mixture was analyzed by nuclear magnetic resonance (NMR) spectroscopy, and only the cation sequestering agent could be observed by \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH NMR spectroscopy, consistent with it being diamagnetic in the reaction mixture (see Supplementary Information Section 2.2, Figure \u003cspan refid=\"MOESM1\" class=\"InternalRef\"\u003eS1\u003c/span\u003e). No other diamagnetic species were observed by the NMR studies. Crystals suitable for single crystal X-ray diffraction (XRD) studies were obtained by slow vapour diffusion of hexane into a tetrahydrofuran (THF) solution, and authenticated the nitrogen-containing product in the reaction mixture as the {N\u003csub\u003e4\u003c/sub\u003e} radical anion [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). [K(crypt)][\u003cb\u003e1\u003c/b\u003e] is black in appearance, and the anion was confirmed to be paramagnetic by electron paramagnetic resonance (EPR) spectroscopy, \u003cem\u003evide infra\u003c/em\u003e. Compound [K(crypt)][\u003cb\u003e1\u003c/b\u003e] was isolated in 66% yield at this scale, but the reaction could be scaled 30\u0026times; (4.44 mmol of 4-BrPhN\u003csub\u003e3\u003c/sub\u003e) with only a small decrease in yield, to 44%. Bulk purity of [K(crypt)][\u003cb\u003e1\u003c/b\u003e] was assessed by powder X-ray diffraction and spin counting using continuous wave EPR spectroscopy, and both methods confirmed high purity.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe structural data of [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e reveal the terminal N1A\u0026ndash;N2A (which is the same as N1B\u0026ndash;N2B) bond length to be 1.324(8) \u0026Aring; and the internal N2A\u0026ndash;N2B bond length to be 1.37(2) \u0026Aring;, with the anion sitting on a crystallographic special position (Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb). These bond lengths are shorter than that of a hydrazine N\u0026ndash;N single bond (1.45 \u0026Aring;) and longer than a \u003cem\u003etrans\u003c/em\u003e-azobenzene N\u0026thinsp;=\u0026thinsp;N double bond (1.19 \u0026Aring;),\u003csup\u003e\u003cspan citationid=\"CR43\" class=\"CitationRef\"\u003e43\u003c/span\u003e, \u003cspan citationid=\"CR44\" class=\"CitationRef\"\u003e44\u003c/span\u003e\u003c/sup\u003e consistent with partial multiple bond character between the N\u0026ndash;N bonds of [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e. Density functional theory (DFT) calculations were conducted to further understand the structure and bonding of [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e and, consistent with the slightly shorter N1A\u0026ndash;N2A crystallographic bond length, the Wiberg bond indices indicate that the N1\u0026ndash; N2 bond has slightly higher bond order (N1\u0026ndash;N2: 1.430; N2\u0026ndash;N2: 1.389). These data are consistent with the bonding across the {N\u003csub\u003e4\u003c/sub\u003e} unit to be both delocalized and to have partial multiple bond character between a single and double bond. This description is also supported by infrared (IR) spectroscopy, in which the stretches related to the {N\u003csub\u003e4\u003c/sub\u003e} unit are observed at 1236 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e (in good agreement with the DFT calculated stretches, for which the largest intensity stretch is at 1272 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e) and appear between literature reported bond stretches for hydrazine (1077 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, N\u0026ndash;N) and azobenzene (1440 cm\u003csup\u003e\u0026ndash;\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003e, N\u0026thinsp;=\u0026thinsp;N).\u003csup\u003e45, \u003cspan citationid=\"CR46\" class=\"CitationRef\"\u003e46\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003eA range of partial atomic charge types were also computed, and Natural Bond Orbital, Hirshfeld, L\u0026ouml;wdin, and Mulliken data all agree that the charge of [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e is significantly delocalized across the whole structure (Supplementary Information Section 2.5, Table S4). Natural resonance theory (NRT) calculations of [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e also support a highly delocalized electronic structure with 248 α spin resonances and 134 β spin resonances found, and with no structure contributing more than 2.6%. To simplify the picture and focus on the N\u003csub\u003e4\u003c/sub\u003e core, the model system [HN\u003csub\u003e4\u003c/sub\u003eH]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e was investigated. NRT studies of [HN\u003csub\u003e4\u003c/sub\u003eH]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e found six key α spin resonance forms with a total weight of 87% and four key β spin resonance forms with a total weight of 90%, with Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec showing the unique resonance forms. The NRT data show that there is significant radical and anion character at the terminal (N1) and internal (N2) nitrogens, but which sites - terminal or internal nitrogens - are prone to subsequent reactivity is unclear.\u003c/p\u003e \u003cp\u003eThe Kohn-Sham molecular orbitals of [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e were also analyzed, and as expected showed significant delocalization over the whole molecule (see Supplementary Information Section 2.6, Figure S4 for a pictorial representation of orbitals HOMO-17 to LUMO\u0026thinsp;+\u0026thinsp;6 with their corresponding energies). Consistent with the isolobal relationship between 1,3-butadiene and the neutral [RN\u003csub\u003e4\u003c/sub\u003eR],\u003csup\u003e\u003cspan citationid=\"CR47\" class=\"CitationRef\"\u003e47\u003c/span\u003e\u003c/sup\u003e and that addition of one electron to [RN\u003csub\u003e4\u003c/sub\u003eR] gives [RN\u003csub\u003e4\u003c/sub\u003eR]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e, we observed the expected molecular orbitals corresponding to the H\u0026uuml;ckel theory description of π bonding shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e.\u003csup\u003e\u003cspan citationid=\"CR48\" class=\"CitationRef\"\u003e48\u003c/span\u003e\u003c/sup\u003e Further, the HOMO was found to be the π\u003csub\u003e3\u003c/sub\u003e combination with anti-bonding character between the terminal and internal nitrogens and bonding character between the two internal nitrogens.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eEPR spectroscopy of [K(crypt)][\u003cb\u003e1\u003c/b\u003e] confirmed the presence of an unpaired electron centred at g\u0026thinsp;=\u0026thinsp;2.00575 (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea). The complexity of the spectrum is in line with the delocalization of the radical with multiple coupling partners. A single point DFT calculation gave an estimate of the isotropic hyperfine values and suggested that the electron has the largest coupling to the N1 atoms with \u003cem\u003eA\u003c/em\u003e\u003csub\u003eiso\u003c/sub\u003e = 13.1 MHz (see Supplementary Information Section 2.7 for the full list of values and subsequent simulated spectra). In order to further understand the key hyperfine couplings involved in the spectra, a simplified model involving only the three environments (2 equiv. nitrogens, 4 equiv. hydrogens, 4 equiv. hydrogens) with the largest calculated hyperfine coupling constants were refined by iterative simulation (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, see Supplementary Information Figure S7 for the overlayed spectra). The simplified simulation finds that the electron is indeed most strongly coupled to one of the nitrogen environments (\u0026times;2 A\u003csub\u003eN\u003c/sub\u003e = 14.50 MHz) with significantly smaller couplings from the rest of the molecule, modelled as two proton environments with four protons in each environment (\u0026times;4 A\u003csub\u003eH\u003c/sub\u003e = 4.20 MHz, \u0026times;4 A\u003csub\u003eH\u003c/sub\u003e = 1.92 MHz). Addition of further nuclei to the simulation did not improve the fit to the data. The spin densities were also calculated using a variety of established methods, Mulliken, L\u0026ouml;wdin, and Hirshfeld (see Supplementary Information Section 2.8, Table S6). All of these methods support delocalization of the radical across the structure with the {N\u003csub\u003e4\u003c/sub\u003e} unit having the largest portion of the spin density (62% when using Mulliken, Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb). More specifically, these calculations revealed that the largest individual spin densities are on the terminal nitrogens (N1A and N1B), in agreement with the large hyperfine value from the simulation and conclusions from the NRT calculation on the [HN\u003csub\u003e4\u003c/sub\u003eH]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e model system. These findings suggest the terminal nitrogens as potential sites for subsequent reactivity.\u003c/p\u003e \u003cp\u003eThe stability of [K(crypt)][\u003cb\u003e1\u003c/b\u003e] was probed with half-life experiments using EPR spectroscopy recorded on samples prepared anaerobically and sealed in a J Young EPR tube (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec). An EPR spectrum was recorded every 90 seconds (600 seconds for powder experiment) and the peak intensities were used to generate decay curves which revealed that [K(crypt)][\u003cb\u003e1\u003c/b\u003e] is more stable in 1,2-difluorobenzene (\u003cem\u003eo\u003c/em\u003eDFB) than in THF (\u003cem\u003eo\u003c/em\u003eDFB: t\u003csub\u003e1/2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;~\u0026thinsp;4 h; THF: t\u003csub\u003e1/2\u003c/sub\u003e\u0026thinsp;=\u0026thinsp;~\u0026thinsp;30 mins; see Supplementary Information Section 2.10). In the solid state [K(crypt)][\u003cb\u003e1\u003c/b\u003e] was found to be remarkably stable, with only a negligible decrease in EPR resonance intensity observed after 2 days and the radical still present after 6 weeks when stored under anaerobic conditions. The magnetism of [K(crypt)][\u003cb\u003e1\u003c/b\u003e] was measured using superconducting quantum interference device (SQUID) magnetometry and, as expected, the molar magnetic susceptibility showed the typical behaviour for a single unpaired electron (Supplementary Information Section 2.11).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eNext, [K(crypt)][\u003cb\u003e1\u003c/b\u003e] was investigated by cyclic voltammetry (CV) to identify redox events (Supplementary Information Section 2.12). This cyclic voltammogram revealed a reversible single-electron reduction event at \u0026minus;\u0026thinsp;1.21 V (vs Ag/AgCl). The one electron reduction of [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e would result in formation of the diamagnetic [\u003cb\u003e2\u003c/b\u003e]\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003c/sup\u003e (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea), the phenyl-substituted analogue of which has been reported as the lithium salt.\u003csup\u003e\u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e\u003c/sup\u003e Note, however, that in this literature report the dianionic lithium salt is not spectroscopically characterized. Although a related Mg salt with bulkier hydrocarbon groups on the nitrogens has been isolated, and the Mg cations were found to coordinate the {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003c/sup\u003e chain presumably increasing its stability.\u003csup\u003e\u003cspan citationid=\"CR28\" class=\"CitationRef\"\u003e28\u003c/span\u003e\u003c/sup\u003e Similar reversible reduction of {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e to {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003c/sup\u003e has also been reported in the context of a tetrazene ligand coordinated to an iron centre.\u003csup\u003e\u003cspan citationid=\"CR49\" class=\"CitationRef\"\u003e49\u003c/span\u003e\u003c/sup\u003e Thus, efforts were made to chemically reduce [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e using KC\u003csub\u003e8\u003c/sub\u003e and cobaltocene. No reaction was observed in the case of cobaltocene, and with KC\u003csub\u003e8\u003c/sub\u003e rapid gas evolution (believed to be N\u003csub\u003e2\u003c/sub\u003e loss) was immediately observed and only decomposition detected by NMR spectroscopy. However, when direct reduction of 1 equiv. of 4-BrC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e was tested with 1 equiv. of KC\u003csub\u003e8\u003c/sub\u003e in the absence of crypt, single crystals of potassium-coordinated [\u003cb\u003e2\u003c/b\u003e]\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003c/sup\u003e could be obtained from the reaction mixture (Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea). This dianion was not characterized further, as it was found to detonate. Considering the molecular orbital diagram in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003e, the observed lower stability of [\u003cb\u003e2\u003c/b\u003e]\u003csup\u003e\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e\u0026ndash;\u003c/sup\u003e is not entirely surprising, as addition of a second electron would increase electron density in the HOMO resulting in an increase of anti-bonding character between the N1 and N2 atoms and bonding character between the two internal nitrogens which in turn promotes N\u003csub\u003e2\u003c/sub\u003e elimination. Two irreversible oxidation events were also observed by CV. Sweeping in both positive and negative directions from \u0026minus;\u0026thinsp;0.8 V resulted in the same series of events, with a small decrease in the reduction event when sweeping to oxidative potentials first, likely due to a small amount of [K(crypt)][\u003cb\u003e1\u003c/b\u003e] decomposing during the irreversible oxidation events. Scanning the reversible reduction event at different scan rates (see Supplementary Information Figure S17) revealed that the redox event appears to be more reversible at lower scan rates, consistent with this event being chemically reversible on the CV timescale but demonstrating slow electron transfer which could be the result of a structural change happening between the two states.\u003csup\u003e\u003cspan citationid=\"CR50\" class=\"CitationRef\"\u003e50\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eUV-Vis studies revealed a strong absorption of blue / green light as well as a weaker broad absorption tailing the whole of the visible region, in line with the black appearance of [K(crypt)][\u003cb\u003e1\u003c/b\u003e]. Time-dependent density functional theory (TD-DFT, see Supplementary Information Section 2.14.2) calculations were undertaken to better understand the nature of the electronic transitions resulting in this spectrum. Four key excitations were found and investigated further using natural transition orbitals (NTOs). The NTO data revealed that all four of the excitations are primarily underpinned by the same two orbital pairs with differing occupations; these are pictured in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. The transition identified with the grey dot can be described as primarily a movement of electron density from the {N\u003csub\u003e4\u003c/sub\u003e} chain onto the aromatic rings, and the one with a blue dot with the opposite character.\u003c/p\u003e \u003c/div\u003e\n\u003ch3\u003eReactivity Studies\u003c/h3\u003e\n\u003cp\u003eRadical-radical recombination reactions have become useful tools to pinpoint the reactive site for paramagnetic materials.\u003csup\u003e\u003cspan citationid=\"CR51\" class=\"CitationRef\"\u003e51\u003c/span\u003e\u003c/sup\u003e For this reason, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) and 1,1\u0026prime;,1\u0026prime;\u0026prime;-{[4-(diphenylmethylidene)cyclohexa-2,5-dien-1-yl]methanetriyl}tribenzene (Gomberg\u0026rsquo;s dimer) were independently reacted with a solution of [K(crypt)][\u003cb\u003e1\u003c/b\u003e] in THF (Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e). In both reactions, these persistent radicals showed no reactivity towards [K(crypt)][\u003cb\u003e1\u003c/b\u003e], further highlighting the high stability of [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e. Next, triphenyltin hydride was investigated as a source of H\u003csup\u003e\u0026bull;\u003c/sup\u003e,\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e a far less stable and less bulky radical, which did lead to radical quenching. Single crystals of hexaphenyldistannane (\u003cb\u003e3\u003c/b\u003e) were obtained from the reaction mixture,\u003csup\u003e\u003cspan citationid=\"CR53\" class=\"CitationRef\"\u003e53\u003c/span\u003e\u003c/sup\u003e confirmed by XRD studies, and is a known by-product from H\u003csup\u003e\u0026bull;\u003c/sup\u003e elimination from tin hydrides.\u003csup\u003e\u003cspan citationid=\"CR52\" class=\"CitationRef\"\u003e52\u003c/span\u003e\u003c/sup\u003e NMR spectroscopy studies of the crude reaction mixture confirmed the presence of the amide [\u003cb\u003e4\u003c/b\u003e]\u003csup\u003e\u0026ndash;\u003c/sup\u003e. After acidic workup, the product [\u003cb\u003e4\u003c/b\u003e]\u003csup\u003e\u0026ndash;\u003c/sup\u003e was converted to 4-bromoanilinium chloride (\u003cb\u003e5\u003c/b\u003e), confirmed by \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH NMR spectroscopy. Whereas, when 4 equiv. of 4-methylthiophenol (TolSH, a known source of H\u003csup\u003e\u0026bull;\u003c/sup\u003e and H\u003csup\u003e+\u003c/sup\u003e)\u003csup\u003e54, \u003cspan citationid=\"CR55\" class=\"CitationRef\"\u003e55\u003c/span\u003e\u003c/sup\u003e was investigated in place of the triphenyltin hydride, [K(crypt)][\u003cb\u003e4\u003c/b\u003e]\u0026thinsp;+\u0026thinsp;\u003cb\u003e6\u003c/b\u003e was formed with an NMR conversion of 68% (confirmed by XRD analysis of single crystals obtained from the reaction mixture). It is worth noting that the diagnostic \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH NMR resonances corresponding to a mixture of [K(crypt)][\u003cb\u003e4\u003c/b\u003e]\u0026thinsp;+\u0026thinsp;\u003cb\u003e6\u003c/b\u003e shift based on the relative concentrations of the two species, consistent with a proton from \u003cb\u003e6\u003c/b\u003e shuttling to [\u003cb\u003e4\u003c/b\u003e]\u003csup\u003e\u0026ndash;\u003c/sup\u003e and back again. [\u003cb\u003e4\u003c/b\u003e]\u003csup\u003e\u0026ndash;\u003c/sup\u003e + \u003cb\u003e6\u003c/b\u003e co-crystallize with the sequestered potassium cation sitting between them and the sulfur and nitrogen-groups pointing towards one another. These data are consistent with the mixture being described as [K(crypt)][\u003cb\u003e4\u003c/b\u003e]\u0026thinsp;+\u0026thinsp;\u003cb\u003e6\u003c/b\u003e with the nitrogen deprotonated or [K(crypt)][\u003cb\u003e6\u003c/b\u003e]\u0026thinsp;+\u0026thinsp;\u003cb\u003e4\u003c/b\u003e with the sulfur deprotonated. To better understand the thiol reactivity, 1 equiv. of TolSH was reacted with [K(crypt)][\u003cb\u003e1\u003c/b\u003e] and immediately inspected by \u003csup\u003e\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e\u003c/sup\u003eH NMR spectroscopy. To our delight, the formation of 4-BrPhN\u003csub\u003e3\u003c/sub\u003e was observed (Supplementary Information Section 3.3.1, Figure S34). Almost half a century ago, in McDonald\u0026rsquo;s work where [(Ph)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e was detected in a mass spectrometer, it was postulated that the {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e was generated by first ionizing 1 equiv. of azide to give the nitrene radical anion [PhN]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e, which subsequently forms an adduct with another equiv. of azide (pictorial description in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003e).\u003csup\u003e\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e\u003c/sup\u003e We computed the Gibbs reaction energy of the dissociation of this \u0026lsquo;adduct\u0026rsquo; starting from [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e to the corresponding azide and nitrene radical anion to be ~\u0026thinsp;+\u0026thinsp;19 kJ/mol. To our knowledge this description of the bonding of such molecules has not previously been reported, in part because until now molecules featuring metal-unsupported {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e units were not isolated. However, detection of 4-BrC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e in the thiol reaction demonstrates that [\u003cb\u003e1\u003c/b\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e can fall apart in a manner consistent with this bonding description. Nitrene species are known to react with aldehydes to undergo carbonyl C\u0026ndash;H activation.\u003csup\u003e\u003cspan citationid=\"CR56\" class=\"CitationRef\"\u003e56\u003c/span\u003e\u003c/sup\u003e And as [K(crypt)][\u003cb\u003e1\u003c/b\u003e] may act as a source of nitrene radical anion, it was investigated with 4-iodobenzaldehyde (4-IC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eCHO) in \u003cem\u003eo\u003c/em\u003eDFB, and indeed the corresponding amide \u003cb\u003e7\u003c/b\u003e could be isolated. Furthermore, the crude reaction mixture from this aldehyde reaction again revealed the presence of the expected 4-BrPhN\u003csub\u003e3\u003c/sub\u003e molecule (Supplementary Information Section 3.4.1, Figure S38). It is also worth noting that 4-BrC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e does not react with 4-IC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eCHO under the same reaction conditions, and independently prepared [K(crypt)][\u003cb\u003e4\u003c/b\u003e] reacts with 4-IC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eCHO to give the corresponding imine, not amide.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eAlmost half a century after the first detection of a compound featuring a {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e chain in a mass spectrometer, we have shown that a molecule featuring this moiety can be isolated under ambient conditions. Electronically stabilized by aromatic units with bromines, crystalline [(4-BrC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e is isolated as a storable solid with multi-day stability under anaerobic conditions. The propensity of nitrogen catenates to rapidly degrade and release N\u003csub\u003e2\u003c/sub\u003e has thus far been an impediment to fully understanding the electronic structure of such chains, but the high stability of the metal-unsupported [(4-BrC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e has allowed this barrier to be overcome. Computational and experimental electronic structure studies into [(4-BrC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003e)\u003csub\u003e2\u003c/sub\u003eN\u003csub\u003e4\u003c/sub\u003e]\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e are consistent with the {N\u003csub\u003e4\u003c/sub\u003e} chain having partial multiple bond character and there being significant radical character on the terminal nitrogen atoms bonded to the aromatic units. These conclusions are consistent with the subsequent reactivity studies which reveal that this {N\u003csub\u003e4\u003c/sub\u003e} chain can decompose into N1 and N3 units, with reactivity consistent with the generation of a nitrene radical anion, exemplified by its reaction with an aldehyde where the carbonyl C\u0026ndash;H bond is activated to give an amide. Efforts are now focused on exploring the additional reactivity patterns of {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e\u0026bull;\u0026ndash;\u003c/sup\u003e-containing molecules, to enhance our understanding of their chemical properties, and fully unlock their potential as gram-scale storable nitrene synthons.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003eGeneral considerations\u003c/p\u003e \u003cp\u003eNMR spectra were recorded on a Bruker AVIII 400 spectrometer at ambient temperatures. Cyclic voltammetry was carried out in the glovebox under inert conditions with EMStat4s. Ultraviolet-visible electronic absorption spectra were recorded on a Mettler Toledo UV5Bio spectrophotometer using 10 mm path length quartz J Young cuvettes. Electron paramagnetic resonance spectra were recorded at X band (9.4 GHz) with a Bruker EMXmicro spectrometer at 298 K. Spin counting was conducted on a Bruker Magnettech ESR5000 at X band (9.8 GHz), 298 K using the ESR Studio software\u0026rsquo;s incorporated spin counting function. Mass spectrometry samples were analyzed using an electrospray ionization (ESI) equipped Waters RDa bench-top time of flight mass spectrometer. ATR-IR spectra were recorded using a Bruker Alpha II under an inert atmosphere. X-ray diffraction data was collected for compound [K(crypt)][\u003cb\u003e1\u003c/b\u003e] on a dual source Rigaku FR-X rotating anode at 100K with Cu Kα (1.54184 \u0026Aring;) radiation. X-ray diffraction data was collected for all other compounds on an Oxford Diffraction Supernova dual-source diffractometer at 150K using Cu Kα (1.54184 \u0026Aring;).\u003c/p\u003e \u003cp\u003ePreparation of [K(crypt)][\u003cb\u003e1\u003c/b\u003e]\u003c/p\u003e \u003cp\u003eIn the glovebox, KC\u003csub\u003e8\u003c/sub\u003e (10 mg, 0.074 mmol, 1 equiv.) and 2.2.2-cryptand (crypt; 28 mg, 0.074 mmol, 1 equiv.) were suspended in THF in a vial. 4-BrC\u003csub\u003e6\u003c/sub\u003eH\u003csub\u003e4\u003c/sub\u003eN\u003csub\u003e3\u003c/sub\u003e (19 uL, 0.148 mmol, 2 equiv.) was added and the vial shaken for 30 seconds. The solution was filtered and ether added to precipitate a black solid. The solid was filtered and washed with ether before drying under vacuum yielding [K(crypt)][\u003cb\u003e1\u003c/b\u003e] as a black crystalline solid. Single crystals were obtained by slow vapour diffusion of hexane into THF in the freezer. A mortar and pestle was used to grind the crystalline powder to a fine powder in the glovebox for use in powder X-ray diffraction studies. Isolated yield: 38.3mg, 66%.\u003c/p\u003e \u003cp\u003eComputational Methodology\u003c/p\u003e \u003cp\u003eDFT calculations were carried out using the Gaussian 16 package, revision C.01.\u003csup\u003e\u003cspan citationid=\"CR60\" class=\"CitationRef\"\u003e60\u003c/span\u003e\u003c/sup\u003e Following extensive benchmarking (Supplementary Information Section 2.3) the TPSS functional was used,\u003csup\u003e\u003cspan citationid=\"CR61\" class=\"CitationRef\"\u003e61\u003c/span\u003e\u003c/sup\u003e together with the Def2-TZVP basis set.\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e, \u003cspan citationid=\"CR63\" class=\"CitationRef\"\u003e63\u003c/span\u003e\u003c/sup\u003e Geometry optimizations were performed, and all minima verified as true by harmonic vibrational frequency calculations. These frequencies were employed in conjunction with Grimme\u0026rsquo;s quasi-harmonic approach for computation of the Gibbs free energies.\u003csup\u003e\u003cspan citationid=\"CR62\" class=\"CitationRef\"\u003e62\u003c/span\u003e, \u003cspan citationid=\"CR64\" class=\"CitationRef\"\u003e64\u003c/span\u003e\u003c/sup\u003e The solvent environment was modelled using the smd method,\u003csup\u003e\u003cspan citationid=\"CR65\" class=\"CitationRef\"\u003e65\u003c/span\u003e\u003c/sup\u003e with parameters appropriate to THF and cyclopentanone (a suitable model for \u003cem\u003eo\u003c/em\u003eDFB). Natural bond orbital (NBO) and natural resonance theory (NRT) calculations were carried out using NBO 7.0.\u003csup\u003e\u003cspan citationid=\"CR66\" class=\"CitationRef\"\u003e66\u003c/span\u003e\u003c/sup\u003e TDDFT calculations were performed using the TPSSh functional\u003csup\u003e\u003cspan citationid=\"CR67\" class=\"CitationRef\"\u003e67\u003c/span\u003e\u003c/sup\u003e and Def2-TZVP basis set. EPR related calculations were carried out using ORCA 5.0.4 with the B3LYP functional,\u003csup\u003e\u003cspan citationid=\"CR68\" class=\"CitationRef\"\u003e68\u003c/span\u003e, \u003cspan citationid=\"CR69\" class=\"CitationRef\"\u003e69\u003c/span\u003e, \u003cspan citationid=\"CR70\" class=\"CitationRef\"\u003e70\u003c/span\u003e\u003c/sup\u003e with the aug-cc-pVTZ basis set used for Br,\u003csup\u003e\u003cspan citationid=\"CR71\" class=\"CitationRef\"\u003e71\u003c/span\u003e\u003c/sup\u003e and the EPR-III basis set for all other atoms.\u003csup\u003e\u003cspan citationid=\"CR72\" class=\"CitationRef\"\u003e72\u003c/span\u003e\u003c/sup\u003e\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eData Availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll data are available in the main text or Supplementary Information. Crystallographic data for structures reported in this Article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers 2423978-2423981. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Cartesian coordinates of optimized structures and all other source data are provided in the Source Data Folder. All data are also available from the corresponding authors upon request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgments\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis research was funded in part by UKRI [EP/Y037391/1]. For the purpose of Open Access, the author has applied a CC BY public copyright licence to any Author Accepted Manuscript (AAM) version arising from this submission. We thank UKRI for their support, and EPSRC for funding PhD students R.L-R and D.G. We also thank the EPSRC for supporting the National Research Facility for EPR (EP/W014521/1, EP/Z530670/1, EP/X034623/1, EP/V035231/1) and SQUID magnetometer (EP/S033181/1) used in this work. A.M.B. is grateful to The Royal Society and the EPSRC for a Dorothy Hodgkin Fellowship (DH160004 and DHF\\R\\221018), and the University of Manchester for a Dame Kathleen Ollerenshaw Fellowship. A.M.B. also thanks the Royal Society of Chemistry for a Community for Analytical and Measurement Science fellowship (CAMS Fellowship 2020 ACTF ref 600310/09). We thank Prof. Eric McInnes for useful discussions regarding SQUID measurements, Agamemnon Crumpton for useful discussions regarding modelling disorder across special positions, and Anne Davies and Martin Jennings for elemental analyses. We are grateful to The University of Manchester for access to its Computational Shared Facility and associated support services.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor information\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eReece Lister-Roberts,\u003csup\u003e1,2\u003c/sup\u003e Daniel Galano,\u003csup\u003e1\u003c/sup\u003e Bono van IJzendoorn,\u003csup\u003e2\u003c/sup\u003e George F. S. Whitehead,\u003csup\u003e1\u003c/sup\u003e Adam Brookfield, Alice M. Bowen,\u003csup\u003e1,3\u003c/sup\u003e*\u0026nbsp;Nikolas Kaltsoyannis,\u003csup\u003e1\u003c/sup\u003e* Meera Mehta\u003csup\u003e2\u003c/sup\u003e*\u003c/p\u003e\n\u003cp\[email protected], [email protected], [email protected].\u003c/p\u003e\n\u003cp\u003e1. Department of Chemistry, University of Manchester, Oxford Road, Manchester M13 9PL, U.K.\u003c/p\u003e\n\u003cp\u003e2. Department of Chemistry, University of Oxford, 12 Mansfield Road, Oxford OX1 3TA, U.K.\u003c/p\u003e\n\u003cp\u003e3. The EPSRC National Research Facility for Electron Paramagnetic Resonance, Photon Science Institute, University of Manchester, Oxford Road, Manchester, M13 9PL, U.K.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eR.L-R. performed all synthesis, and subsequent analysis and interpretation, with experiments designed in collaboration with M.M. and EPR studies performed in collaboration with A.M.B. and A.B. R.L-R and A.B recorded the SQUID data. D.G. and N.K. performed all computational investigations. B.v.I. performed preliminary investigations, and single crystal XRD studies along with R.L-R. G.F.S.W. performed powder XRD studies and subsequent data interpretation. R.L-R., D.G. and M.M. wrote the initial drafts of the manuscript with editing from R.L-R., D.G. B.v.I., N.K., M.M. and A.M.B.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics Declaration\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors declare no competing interests.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eGreenwood NN, Earnshaw A. \u003cem\u003eChemistry of the Elements\u003c/em\u003e. Elsevier, 2012.\u003c/li\u003e\n\u003cli\u003eKlap\u0026ouml;tke TM. \u003cem\u003eChemistry of High-Energy Materials\u003c/em\u003e. De Gruyter: Berlin, Boston, 2022.\u003c/li\u003e\n\u003cli\u003eO\u0026rsquo;Sullivan OT, Zdilla MJ. Properties and Promise of Catenated Nitrogen Systems As High-Energy-Density Materials. \u003cem\u003eChem. Rev.\u003c/em\u003e 2020, \u003cstrong\u003e120\u003c/strong\u003e(12)\u003cstrong\u003e:\u003c/strong\u003e 5682\u0026ndash;5744.\u003c/li\u003e\n\u003cli\u003eHeelis RA, Maute A. 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Structure, Magnetic Properties and Reactivities of Open-Shell Species From Density Functional and Self-Consistent Hybrid Methods. \u003cem\u003eRecent Advances in Density Functional Methods\u003c/em\u003e, pp 287\u0026ndash;334.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6055289/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6055289/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eLong chain nitrogen ions and radicals ([N\u003csub\u003en\u003c/sub\u003e]\u003csup\u003ex+\u003c/sup\u003e / [N\u003csub\u003en\u003c/sub\u003e]\u003csup\u003ex–\u003c/sup\u003e, n \u0026gt; 3) are naturally occurring under the intense radiative conditions of the Earth’s ionosphere, and those of other planetary bodies. However, the strong thermodynamic driving force to lose N\u003csub\u003e2\u003c/sub\u003e renders these types of molecules extremely reactive under ambient conditions such that they can typically be studied only under extreme conditions, for example at ultrahigh pressures (10 to \u0026gt;200 GPa). We now report the isolation of a molecule featuring a metal-unsupported {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e•–\u003c/sup\u003e unit under ambient conditions which demonstrates remarkable multi-week long persistence in the solid-state. Spectroscopic, crystallographic and computational studies provide insight into the bonding across the {N\u003csub\u003e4\u003c/sub\u003e}\u003csup\u003e•–\u003c/sup\u003e chain. Reactivity studies reveal that the chain can cleave into N1 and N3 fragments, and can act as a source of nitrene radical anion, an observation that such molecules could act as storable nitrogen group transfer reagents.\u003c/p\u003e","manuscriptTitle":"A Crystalline Nitrogen Chain Radical Anion","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-02-27 09:18:04","doi":"10.21203/rs.3.rs-6055289/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-chemistry","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"nchem","sideBox":"Learn more about [Nature Chemistry](http://www.nature.com/nchem/)","snPcode":"","submissionUrl":"","title":"Nature Chemistry","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Research","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"4560cc44-3f35-4b05-afd5-bff40c39cee2","owner":[],"postedDate":"February 27th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":44857317,"name":"Physical sciences/Chemistry/Coordination chemistry"},{"id":44857318,"name":"Physical sciences/Chemistry/Inorganic chemistry/Chemical bonding"}],"tags":[],"updatedAt":"2026-02-11T08:10:42+00:00","versionOfRecord":{"articleIdentity":"rs-6055289","link":"https://doi.org/10.1038/s41557-025-02040-2","journal":{"identity":"nature-chemistry","isVorOnly":false,"title":"Nature Chemistry"},"publishedOn":"2026-02-10 05:00:00","publishedOnDateReadable":"February 10th, 2026"},"versionCreatedAt":"2025-02-27 09:18:04","video":"","vorDoi":"10.1038/s41557-025-02040-2","vorDoiUrl":"https://doi.org/10.1038/s41557-025-02040-2","workflowStages":[]},"version":"v1","identity":"rs-6055289","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6055289","identity":"rs-6055289","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}