Two-Coordinated Au(I) Complex Photoredoxcatalyst: Highly Efficient Catalysis in C-C Cross-coupling Reactions and the Underlying Mechanism

Two-Coordinated Au(I) Complex Photoredoxcatalyst: Highly Efficient Catalysis in C-C Cross-coupling Reactions and the Underlying Mechanism | 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 Two-Coordinated Au(I) Complex Photoredoxcatalyst: Highly Efficient Catalysis in C-C Cross-coupling Reactions and the Underlying Mechanism Youngmin You, Byung Hak Jhun, Jihoon Jang, Shinae Lee, Eun Jin Cho This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-3938892/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 03 Aug, 2024 Read the published version in Nature Communications → Version 1 posted You are reading this latest preprint version Abstract Photocatalysis provides a versatile approach to redox activation of various organic substrates for synthetic applications. To broaden the scope of photoredox catalysis, developing catalysts with strong photoredox power is imperative. Photoredox catalysts with excited-state properties that include cathodic oxidation potentials and long lifetimes are particularly demanded. In this research, we demonstrate the high-efficiency catalytic utility of two-coordinated Au(I) complex photocatalysts that exhibit an exclusive ligand-to-ligand charge-transfer (LLCT) transition in C-C cross-coupling reactions between N -heterocycles and (hetero)aryl halides, including redox-resistant (hetero)aryl chlorides. Our photocatalysis system can steer reactions under visible-light irradiation at a catalyst loading as low as 0.1 mol% and exhibits a broad substrate scope with high chemo- and regioselectivity. Our mechanistic investigations provide direct spectroscopic evidence for each step in the catalysis cycle and demonstrate that the LLCT-active Au(I) complex catalysts offer several benefits, including strong visible-light absorption, a 207 ns-long excited-state lifetime without short-lived components, and a 91% yield in the production of free-radical intermediates. Given the wide structural versatility of the proposed catalysts, we envision that our research will provide useful insights into the future utilization of the LLCT-active Au(I) complex for organic transformations. Physical sciences/Chemistry/Catalysis/Photocatalysis Physical sciences/Chemistry/Photochemistry/Photocatalysis Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis Figures Figure 1 Figure 2 Figure 3 Introduction Visible-light-activatable, homogeneous photoredox catalysis has emerged as a useful tool for a range of organic transformations. 1,2 The synthetic utility benefits from the ability of catalysts to generate free-radical intermediates through heterobimolecular photoinduced electron transfer. 3 To expand the synthetic utility, researchers have devoted enormous efforts to identifying catalyst molecules capable of mediating photoredox reactions. 4,5 Coordinatively saturated complexes with d 6 metals, such as fac -[Ir(ppy) 3 ] (ppy = 2-phenylpyridinato) and [Ru(bpy) 3 ] 2+ (bpy = 2,2¢-bipyridyl), constitute a family of successful molecular photoredox catalysts. 6 Their catalytic efficiencies benefit from the tunable, redox-active triplet metal-to-ligand charge-transfer ( 3 MLCT) transition state. 7 The 3 MLCT-active metal complexes, however, sometimes exhibit insufficient catalytic performance, especially for activating substrates with redox-resistant bonds, such as the C-Cl bond. 8 This limitation stems from detrimental effects of the central metal, including 1) an unavoidable energy loss in intersystem crossing from the initially photoexcited singlet state to the 3 MLCT transition state, 2) the occurrence of a metal-centered non-radiative process, 9 and 3) the presence of electron-deficient metal centers, such as Ir(III), which are disadvantageous for achieving strong excited-state reducing power. We envisioned that low-valent metal complexes that avoid metal-involved electronic transitions could be promising candidates for overcoming the limitations of the 3 MLCT-active photoredox catalysts. To investigate this idea, we selected two-coordinated d 10 Au(I) complexes with charge-neutral carbene and monoanionic amido ligands (Fig. 1a). This heteroleptic structure exhibits strong visible-light absorption because of the amido ligand-to-carbene ligand charge-transfer (LLCT) transition through redox-innocent mediation of the Au 5 d orbitals. 10,11 The singlet and triplet LLCT transition states are near-degenerate and in a rapid equilibrium, effectively alleviating energy loss in the photocatalysis, which is substantial for 3 MLCT-active Ir(III) and Ru(II) complex photocatalysts. 12,13 Note that the Au center is hardly involved in redox processes or in the electronic transition in LLCT-active Au(I) complexes, which enables facile control of the excited-state oxidation ( E * ox ) and reduction ( E * red ) potentials. In addition, the LLCT-active Au(I) complexes exhibit an excited-state lifetime ( t obs ) as long as several microseconds, 14-16 without substantial contaminations by short-lived components; such contaminations are routinely observed for organic thermally activated delayed fluorescence (TADF) molecules, such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) shown in Fig. 1b. 17 We thus speculate that 1) the negligible electronic energy loss, 2) the absence of metal-centered valence change, 3) the less-electron-deficient Au(I) center, and 4) the efficient utilization of the long-lived excited state will make LLCT-active Au(I) complexes potent photoreducing catalysts. Previous research on photocatalytic Au complexes have mainly focused on identification of metal-centered redox behaviors. For example, a dinuclear Au(I) complex with bisphosphino bridging ligands exhibited an E * ox value as negative as −1.6 V vs a NaCl-saturated calomel electrode because of the Au(I/II) redox cycle (Fig. 1b). 18,19 The two-electron cycle between Au(I) and Au(III) species has also garnered research interest because it complies with the well-established oxidative addition-reductive elimination catalytic steps. 20-22 These ground-breaking advances promise the synthetic potential of Au(I) complexes, although their catalytic utility is still based on the metal-centered redox processes involving Au(II) or Au(III), analogous to the case of 3 MLCT-active complexes. The photocatalysis ability of LLCT-active Au(I) complexes was recently validated for [2+2] cycloaddition 23 and water reduction reactions. 24 The catalysis involved energy transfer and the reductive formation of metal nanoparticles, respectively, as the key processes. To the best of our knowledge, organic transformations driven by LLCT-active Au(I) complex photoredox catalysts remain unexplored. Photocatalytic C-C bond formation plays a critical role in chemistry, driving molecular diversity and facilitating the synthesis of complex functional molecules, as briefly outlined in Fig. 1a-i. 25-28 The pioneering work conducted independently by the Lee 29 and Stephenson 30 groups on the intramolecular C-C bond formation of aryliodides and bromides with π-substrates marked important advances. Despite under UVA irradiation, the MLCT-active dinulear Au complex was also employed by the Barriault group for an intramolecular C-C coupling process. 31 Following these contributions, the scope of research has been significantly expanded to include intermolecular processes with a broader spectrum of substrates, particularly the more challenging aryl chloride substrates. This expansion has been based on strongly photoreducing catalysts such as the photoexcited doublet species utilized by König and coworkers. 32 Despite recent advances, 33-37 a persistent need exists to develop more efficient and selective photocatalytic systems capable of activating redox-resistant substrates, such as aryl chlorides, for C-C cross-coupling reactions. Herein, we report unprecedented photoredox catalytic reactivity and the corresponding mechanism of LLCT-active Au(I) complexes for C-C cross-coupling reactions (Fig. 1a-ii). Our Au(I) complex exhibited an E * ox value as negative as -2.16 V vs standard calomel electrode (SCE), demonstrating the high reducing power as close as sodium mercury amalgam (-2.21 V vs SCE) and metallic potassium (-2.23 V vs SCE) (see Fig. 1b and Supplementary Tables 1 and 2). Our catalysis system features notable advantages, including a catalyst loading as low as 0.1 mol%, reaction yields greater than 80%, and a broad substrate scope including heteroaryl chlorides that are difficult to be activated. Our mechanistic investigations based on steady-state and transient electronic spectroscopy, spectroelectrochemistry, and quantum chemical calculations revealed that the Au(I) complex photocatalyst exhibits strong photoreducing power, with the ability to efficiently retard charge recombination, characterized by a 91% efficiency for the reductive generation of a key radical intermediate from an aryl chloride. In addition, our spectroscopic studies identified neutralization of the one-electron-oxidized catalyst as the rate-determining step in the overall photoredox catalysis cycle. Results Photocatalytic C - C cross-coupling reactions A series of two-coordinated Au(I) complexes having N -heterocyclic carbene and carbazolide ligands with various alkyl substituents were tested to evaluate their photoredox catalysis performance (Table 1). These complexes exhibit visible absorption due to the LLCT transition (Supplementary Fig. 1). For example, the ultraviolet-visible (UV-Vis) absorption spectrum of [Au(BZI)(TMCz)] (BZI = 1,3-diphenylbenzo[ d ]imidazolylidene; TMCz = 1,3,6,8-tetramethylcarbazolide) has an onset wavelength of 470 nm (Supplementary Fig. 1 and Table 3). The excited-state [Au(BZI)(TMCz)] is long-lived, with a t obs as long as 207 ns in deaerated toluene, without any short-lived components (Supplementary Fig. 2). [Au(BZI)(TMCz)] exhibits ground-state oxidation ( E ox ) and reduction ( E red ) potentials of [Au(BZI)(TMCz)] of 0.56 and −2.26 V vs SCE, respectively (Supplementary Fig. 3). The corresponding E * ox value is calculated to be −2.16 V vs SCE, being more cathodic than those of widely used photoredox catalysts such as fac -[Ir(ppy) 3 ] (−1.73 V vs SCE), 38 4CzIPN (−1.18 V vs SCE), 5 and Mes-Acr + (−0.57 V vs SCE). 39 It should be emphasized that, as compared in Fig. 1b, [Au(BZI)(TMCz)] exhibits a balance between t obs and E * ox , which is a prerequisite for a potent photoreducing catalyst. The catalytic efficacy of Au complexes was evaluated through comparisons with several established photoredox catalysts for an intermolecular C-C coupling reaction between model (hetero)aryl chlorides (specifically, methyl-4-chlorobenzoate ( 1a ) and chloroquinoline ( 1b )) and N -methylpyrrole ( 2a ) (Table 1). The reactions were carried out with a 0.25 mol% or a 0.1 mol% photocatalyst for 1a or 1b , respectively, and 2 equivalents of N , N -di(isopropyl)ethylamine (DIPEA) in Ar-saturated dimethyl sulfoxide (DMSO, 0.50 M) under blue LEDs irradiation. Notably, Au complexes without alkyl substituents in the carbene ligands, [Au(BZI)(Cz)] and [Au(BZI)(TMCz)] (Cz = carbazolide), exhibited superior reactivity, successfully yielding the desired C-C coupled product 3 in excellent yields. The distinctly superior catalytic performance of [Au(BZI)(Cz)] and [Au(BZI)(TMCz)] compared with that of the other Au(I) complexes is attributable to their interactions with 2a being less sterically encumbered. The added 2a results in an increase in t obs , suggesting that 2a is not engaged in photoinduced electron transfer but suppresses nonradiative relaxation of the Au(I) complexes (Supplementary Fig. 2). By contrast, the commonly used fac -[Ir(ppy) 3 ] demonstrated lower reactivity, and 4CzIPN, fluorescein, and the Pt(II) complex photocatalyst 40 were inactive in this transformation. Optimization of the reaction parameters enabled the synthesis of the heteroaryl–heteroaryl coupled product ( 3ba ) in 84% yield in the presence of only 0.1 mol% [Au(BZI)(TMCz)]. The optimization results are compiled in Supplementary Table 4. Notably, this catalytic loading represents an advance because such a low loading has not been achieved in any previously reported photocatalytic processes. The quantum yield for the reaction of 1a with 2a , which was determined using the standard ferrioxalate actinometry, is as large as 34%. Finally, control experiments revealed that the reaction requires both the photocatalyst and irradiation with visible light (Supplementary Table 4). With the optimized parameters in hand, we explored the versatility of the Au(I) complex-catalyzed C–C coupling protocol in synthesizing various (hetero)aryl– (hetero)aryl products ( 3 ). [Au(BZI)(TMCz)], which has an E * ox as negative as –2.16 V vs SCE, enables the incorporation of a range of redox-resistant (hetero)aryl chlorides ( 1a , 1b , 1c , 1k , 1m , and 1p ) with reduction potentials ( E red s) in the range –1.61 to –2.12 V vs SCE (see Supplementary Fig. 4 for the corresponding voltammograms), as well as bromides and iodides, in the C-C coupled products (Table 2). We observed a distinct variation in the reactivity among different (hetero)aryl halides, leading to a chemoselective process. Specifically, in the case of dihalogenated substrates, 1q and 1r , the iodo group demonstrated selective reactivity over the bromo substituent, resulting in 3qa and 3ra as the major products, respectively. For 1m , which contains two chloro substituents, regioselective reactivity was noted at the chloride positioned para to the CF 3 group, yielding 3ma . In general, N -heteroaryl halides exhibited higher reactivities than aryl halides, even with a reduced catalyst loading (0.1 mol% vs 0.25 mol%). Furthermore, the substitution patterns did not affect reactivity: ortho -, meta -, and para -substituted aryl halides were all suitable. Notably, our photoredox catalysis protocol tolerated the presence of various functional groups, including aldehyde ( 3ja ), ketone ( 3ia ), ester ( 3aa , 3ra , 3ab , and 3ac ), and nitrile ( 3ka ) groups, as well as medicinally significant fluoride ( 3oa ) and CF 3 ( 3da , 3la , 3ma , 3na ) groups. In general, dehalogenation side products were also observed, albeit in negligible amounts. In addition, the high reactivity of our photoredox catalysis protocols was further demonstrated by application to the C–B bond-forming reaction, where bis(pinacolato)diboron was used as the coupling partner. Detailed results of the reactions are provided in Supplementary Table 5. The resultant arylboronates are versatile synthons, applicable in a broad range of transition-metal-catalyzed transformations. Mechanistic investigations Having demonstrated the photoredoxcatalytic ability of the Au(I) complex, we sought to elucidate the role of the catalyst in the C−C cross-coupling reaction. The E * ox value of [Au(BZI)(TMCz)] (−2.16 V vs SCE) is more negative than the E red value of 1a (−1.81 V vs SCE), implying that the excited-state of [Au(BZI)(TMCz)] (denoted as [Au(BZI)(TMCz)]* hereafter) can be oxidatively quenched by 1a with the driving force for heterobimolecular one-electron transfer (−D G eT , −D G eT = e ×[ E * ox ([Au(BZI)(TMCz)]) − E red ( 1a )], where e is the elementary charge and we ignore the Coulomb term because of our use of the polar solvent DMSO) of 0.35 eV. By contrast, reductive quenching of [Au(BZI)(TMCz)]* ( E * red = 0.46 V vs SCE) by DIPEA ( E ox = 0.64 V vs SCE) is predicted to be disfavored due to the negative −D G eT of −0.18 eV. We validated the aforementioned thermodynamic considerations for the initial electron transfer by using photoluminescence titration experiments. As shown in Fig. 2a and 2b, the increased concentrations of 1a (0−100 mM) elicit a concentration-dependent decrease in the photoluminescence intensity of 50 mM [Au(BZI)(TMCz)]. An analogous decrease is also observed for t obs (Fig. 2c). The apparent quenching rate computed through the relationship, quenching rate = 1/ t obs ( 1a ) − 1/ t obs (0), where t obs ( 1a ) and t obs (0) are the t obs of [Au(BZI)(TMCz)] in the presence and absence of 1a , respectively, increases with increasing molarity of 1a . Pseudo-first-order kinetics analysis for the quenching rate reveals strict adherence to the straight line and returns an apparent heterobimolecular quenching rate constant ( k q ) as large as 7.2 × 10 8 M −1 s −1 . Deconvolution of the diffusion rate constant ( k diff , 3.3 × 10 9 M −1 s −1 for DMSO at 298 K) from k q yields a second-order quenching rate constant ( k Q ) of 9.1 × 10 8 M - 1 s - 1 . The quantum yield for quenching ( F Q ) is as large as 95% in the presence of 0.50 M 1a , which is estimated according to the relationship F Q = k Q ×[ 1a ] / ( k Q ×[ 1a ] + k d ), where [ 1a ] is expressed in molarity (0.50 M) and k d is the intrinsic decay rate of [Au(BZI)(TMCz)]* (2.2 × 10 7 s - 1 ). In stark contrast, DIPEA does quench the photoluminescence of [Au(BZI)(TMCz)]* even at a concentration as high as 500 mM (Fig. 2e and 2f). We exclude the energy-transfer pathway because [Au(BZI)(TMCz)] not only exhibits negligible spectral overlap with 1a , 2a , and DIPEA but also exhibits a T 1 energy (2.79 eV) lower than that of 1a (3.16 eV) (refer to Supplementary Fig. 5 for details). Collectively, our electrochemical and photoluminescence results strongly indicate rapid and exclusive electron transfer from [Au(BZI)(TMCz)]* to 1a . The photoinduced electron transfer produces a radical-ion pair (RIP) consisting of [Au(BZI)(TMCz)] · + and 1a ·- . We used nanosecond visible–near-infrared (Vis–NIR) transient absorption spectroscopy to directly monitor the genesis of the radical ion species. As shown in Fig. 3a, the heat map of the photoinduced Vis–NIR absorption difference spectra of 100 mM [Au(BZI)(TMCz)] after nanosecond pulsed laser photoexcitation at a wavelength of 355 nm contains negative signals in the visible region because of the stimulated emission (see also the top- and bottom-most panels in Fig. 3c). In sharp contrast, positive signals emerge at a peak wavelength of 870 nm in the presence of 200 mM 1a (Fig. 3b and the second panel in Fig. 3c). The NIR signals are attributable to [Au(BZI)(TMCz)] · + because [Au(BZI)(TMCz)] · + electrochemically generated at an anodic potential of 0.45 V vs Ag +/0 exhibits a broad absorption band in this region (third panel in Fig. 3c). This spectral assignment is further corroborated by the close match with the [Au(BZI)(TMCz)] · + electronic transition spectrum quantum chemically simulated at the CAM-B3LYP level of theory with a conductor-like polarizable continuum model parameterized to DMSO (fourth panel in Fig. 3c). The transient spectroscopy and calculation investigations provide direct evidence for photoinduced electron transfer from [Au(BZI)(TMCz)]* to 1a . Once formed, the RIP is rapidly annihilated by charge recombination through back electron transfer from 1a ·- to [Au(BZI)(TMCz)] · + because the driving force for charge recombination (−D G CR , −D G CR = e ×[ E red ( 1a ) − E ox ([Au(BZI)(TMCz)])]) is as large as 2.37 eV.Notably, in many photoredox catalysis processes, the charge recombination is ultrafast and detrimental to the catalysis cycle, limiting the overall photocatalytic performance. 40,41 The charge recombination process in [Au(BZI)(TMCz)] · + can be monitored at a wavelength of 870 nm. Surprisingly, [Au(BZI)(TMCz)] · + is long-lived, with an apparent lifetime of 18 ms (Fig. 3d). Second-order kinetics analyses based on the molar absorbance of [Au(BZI)(TMCz)] · + at 870 nm (96 M - 1 cm - 1 ) indicate that the rate constant for charge recombination ( k CR ) with 1a ·- is 3.3 × 10 8 M - 1 s - 1 (Fig. 3e). Notably, the yield for the liberation of free-radical ion species from the geminate RIP (i.e., [[Au(BZI)(TMCz)] · + ××× 1a ·- ] ® [Au(BZI)(TMCz)] · + and 1a ·- ) can be computed according to the relationship k - diff / ( k - diff + k CR ) to be as large as 91%. This value is greater than the yield for charge recombination of cyclometalated Pt(II) complexes with CF 3 I ·- (47–77%) which are highly reducing photocatalysts established by us. 40 The k CR values with other aryl halide substrates were also determined to be in the range (0.7–2.9) × 10 8 M - 1 s - 1 (Supplementary Fig. 6). Finally, we found that the k CR values adhere to the Jortner curves for electron transfer with large reorganization energies, which suggested strong interactions with solvents (Supplementary Fig. 7). The liberated 1a ·- is cleaved into an aryl radical species and Cl - . The aryl radical species subsequently reacts with 2a to form a C–C cross-coupled adduct ( 3aaH • in Scheme 1). We hypothesized that the 3aaH • species plays a key role in completing the photoredox catalysis cycle. Specifically, we hypothesized that the [Au(BZI)(TMCz)] · + is neutralized to the original [Au(BZI)(TMCz)] state through one-electron withdrawal from 3aaH • and that DIPEA subsequently deprotonates the resultant one-electron-oxidized 3aaH + intermediate, producing the final product ( 3aa ). To validate this hypothesis, we performed nanosecond laser flash photolysis for the mixture of 100 mM [Au(BZI)(TMCz)] and 50 mM 1a with the addition of 2a in various concentrations (0–300 mM). We expected that the effective concentration of 3aaH • would increase with the increased 2a concentration. Gratifyingly, we found that the increase in concentration of 2a shortens the lifetime of the 870 nm signal of [Au(BZI)(TMCz)] · + (Fig. 3f). The concentration-dependent decays can be best interpreted as the recovery of [Au(BZI)(TMCz)] through electron transfer from 3aaH •to [Au(BZI)(TMCz)] · + . Our pseudo-first order kinetics analysis of the 870 nm signals as a function of [ 2a ] yields a rate constant of 3.8 × 10 5 M - 1 s - 1 for bimolecular electron transfer ( k eT ) from 3aaH •to [Au(BZI)(TMCz)] · + (Fig. 3g). Results obtained with the other substrates are summarized in Supplementary Fig. 8. Although the determined value should serve as a lower limit due to the pre-steps for the generation of 3aaH •, this k eT value is three orders of magnitude smaller than the rate constants for the other electron-transfer steps, k Q (9.1 × 10 8 M - 1 s - 1 ) and k CR (3.3 × 10 8 M - 1 s - 1 ). This comparison indicates that the catalyst recovery is the rate-determining step in the overall photoredox catalysis cycle. The relatively slow electron transfer is presumably attributable to the multiple steps, including C-Cl bond cleavage and the radical addition to 2a , required to form 3aaH •. Discussion Given its enormous potential, photoredox catalysis will continue to expand our horizon of synthetic organic methodologies. This exploration inevitably requires the development of catalysts with strong redox power; it also requires kinetic compatibility of electronic processes involved in the catalytic cycle. In this research, we investigated the homogeneous photoredox catalysis of two-coordinated Au(I) complexes showing exclusive LLCT transition. The advantages of Au(I) complex photoredox catalysts include 1) strong visible-light absorption, 2) cathodic E * ox s capable of reductively activating redox-resistant (hetero)aryl chloride substrates, 3) submicrosecond τ obs , enabling efficient excited-state heterobimolecular processes, 4) effective suppression of charge recombination, and 5) versatility for controlling the E * ox and τ obs values through ligand structures. We exploited these features to catalyze C−C cross-coupling reactions between (hetero)aryl halides, including (hetero)aryl chlorides, and N -heterocycles. Notably, our photoredox catalysis protocols require a minimal catalyst loading as low as 0.1 mol%. In addition, the catalysis exhibits a broad substrate scope and is applicable to C–B bond formation reactions. Our mechanistic investigations revealed that the catalytic cycle of the C–C cross-coupling reaction is initiated by one-electron transfer from the excited-state Au complex to the aryl chloride substrate. Of particular interest is the long-lived (18 µs) radical-ion species because most photoredox catalysts suffer from rapid charge recombination before the subsequent catalysis step. Indeed, the yield for the radical intermediate is as large as 91%, validating the highly efficient production pathway. We also discovered that the final step (i.e., the recovery of the photocatalyst) is completed by electron transfer from the radical species of the C−C cross-coupled adduct and that the overall catalysis is governed by this step. Our research provides the first photoredoxcatalytic organic transformation by LLCT-active Au(I) complexes. Mechanistic investigations also indicated electron-transfer behaviors beneficial for a range of photoredox catalytic applications, which will inspire future research toward developing organic synthesis methodologies that require strong photoreduction capability. Methods Photoredoxcatalytic C(Ar) - C(Ar) cross-coupling reactions . An oven-dried reaction vial equipped with a magnetic stir bar was charged with (hetero)aryl halide 1 (0.50 mmol, 1.0 equiv), [Au(BZI)(TMCz)] (0.3 mg , 0.1 mol%), N -heterocycle 2 (5.0 mmol, 10 equiv), DIPEA (1.0 mmol, 2 equiv), and anhydrous DMSO (0.50 M, 1.0 mL). The vial was sealed with a silicon septum screw cap and purged with Ar via a balloon for 10 min. The setup was then exposed to blue LEDs (18 W) in a photoreactor with continuous stirring. The reaction progress was monitored using thin-layer chromatography (TLC) or gas chromatography (GC). Upon completion, the reaction was quenched by adding water. The reaction mixture was extracted three times using dichloromethane. The combined organic phases were dried over MgSO 4 , concentrated under reduced pressure using a rotary evaporator, and purified by silica gel flash column chromatography using a hexane–ethyl acetate solution (10/1) as the eluent to give the corresponding C-C cross-coupled product 3 . Nanosecond laser flash photolysis . Ar-saturated DMSO containing 100 μM [Au(BZI)(TMCz)] in a 1 cm × 1 cm quartz cell was excited by a Nd:YAG laser (Continuum, SLII-10; 355 nm and 5 mJ pulse −1 ) in the absence and presence of 200 mM 1a . Time courses of the transient absorption were measured using a photomultiplier tube (visible region) and an InGaAs-PIN photodiode (Hamamatsu 2949) (NIR region). The output from the photomultiplier tube and photodiode was recorded with a digitized oscilloscope (Tektronix, TDS3032; 300 MHz). All experiments were performed at 298 K. For titration experiments, stock solutions of 100 mM [Au(BZI)(TMCz)] and 1.11 M 2a were prepared in DMSO. [Au(BZI)(TMCz)] solution (3.0 mL) was added to a 1 cm × 1 cm quartz cell. Decay traces of [Au(BZI)(TMCz)] · + were recorded with the continuous addition of 2a . The final concentration of 2a was 300 mM. Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. Declarations Acknowledgements This work was supported by Midcareer Research Programs (RS-2023-00208856 and NRF-2020R1A2C2009636) through National Research Foundation grants funded by the Ministry of Science, Information, and Communication Technology (ICT) and Future Planning (MSIP), and by the Yonsei University Research Fund (2023-22-0132). Author Contributions B.H.J. performed spectroscopic and quantum chemical investigations, analyzed the data, and wrote the manuscript. J.J. designed and synthesized the substrates, evaluated the catalysis performance, and co-wrote the manuscript. S.L. designed and performed spectroscopic and electrochemical experiments, analyzed the data, and co-wrote the manuscript. E.J.C. supervised the work at Chung-Ang University and co-wrote the manuscript. Y.Y. coordinated all of the experiments and analyses, and co-wrote the manuscript. All authors contributed to discussion on the study, and edited the manuscript. Additional information Supplementary information is available in the online version of the paper. Reprints and permissions information are available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to Y.Y. ( [email protected] ) or E.J.C. ( [email protected] ). Competing interests The authors declare no competing interests. References Romero, N. A. & Nicewicz, D. A. Organic photoredox catalysis. Chem. Rev. 116 , 10075-10166 (2016). Barham, J. P. & König, B. Synthetic photoelectrochemistry. Angew. Chem., Int. Ed. 59 , 11732-11747 (2020). Staveness, D., Bosque, I. & Stephenson, C. R. J. Free radical chemistry enabled by visible light-induced electron transfer. Acc. Chem. Res. 49 , 2295-2306 (2016). Arias-Rotondo, D. M. & McCusker, J. K. The photophysics of photoredox catalysis: a roadmap for catalyst design. Chem. Soc. Rev. 45 , 5803-5820 (2016). Speckmeier, E., Fischer, T. G. & Zeitler, K. 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G. W., Chernowsky, C. P., Williams, O. P. & Wickens, Z. K. Potent reductants via electron-primed photoredox catalysis: unlocking aryl chlorides for radical coupling. J. Am. Chem. Soc. 142 , 2093–2099 (2020). Kirgan, R. A., Sullivan, B. P. & Rillema, D. P. Photochemistry and Photophysics of Coordination Compounds: Rhenium. Top. Curr. Chem. 281 , 45–100 (2007). Xiang, M., Xin, Z.-K., Chen, B., Tung, C.-H. & Wu, L.-Z. Exploring the reducing ability of organic dye (Acr + -Mes) for fluorination and oxidation of benzylic C(sp 3 )–H bonds under visible light irradiation. Org. Lett. 19 , 3009–3012 (2017). Choi, W. J. et al. Mechanisms and applications of cyclometalated Pt(II) complexes in photoredox catalytic trifluoromethylation. Chem. Sci. 6 , 1454-1464 (2015). Zou, C., Miers, J. B., Ballew, R. M., Dlott, D. D. & Schuster, G. B. Electron transfer and back electron transfer in photoexcited ion pairs: forward and back directions have different maximum rates. J. Am. Chem. Soc. 113 , 7823–7825 (1991). Tables Tables 1 to 2 are available in the Supplementary Files section Scheme Scheme 1 is available in the Supplementary Files section. Additional Declarations There is NO Competing Interest. Supplementary Files Supplementaryinformation.pdf Scheme1.png Scheme 1. Catalysis cycle. Plausible mechanism of the photoredox catalytic C–C cross-coupling reaction. Tables.docx Cite Share Download PDF Status: Published Journal Publication published 03 Aug, 2024 Read the published version in Nature Communications → 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. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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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-3938892","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":273057364,"identity":"72f133c2-4ce8-42c5-9e4a-f3d01dfea7fa","order_by":0,"name":"Youngmin You","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA1klEQVRIiWNgGAWjYFCCBAOGDxUQ5gEQIUGMFsYZZ0jVwszbhsQnqIWfPXnzB955h+XM+Q8fPMBQY8cgOfsAfi2SPc/KJCS3HTa2nJGWcIDhWDKDNF8Cfi0GN3LMGAy3HU7ccIPH4AAD2wEGOR4CDgNqMf6QOAeo5fz5DwcY/hGnxUDiYANQy4EchgOMbQcYpAlpAflFsuFYurHBjTSDA4l9yTySPQS0gELs858aazmD84cff/jwzU5O4gwBLVDQDKESGBgIOQsO6ohVOApGwSgYBSMRAAC1rEWIB2J9WgAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-5633-6599","institution":"Yonsei University","correspondingAuthor":true,"prefix":"","firstName":"Youngmin","middleName":"","lastName":"You","suffix":""},{"id":273057365,"identity":"52afdab7-213f-4635-9e5d-4928f566694f","order_by":1,"name":"Byung Hak Jhun","email":"","orcid":"","institution":"Yonsei University","correspondingAuthor":false,"prefix":"","firstName":"Byung","middleName":"Hak","lastName":"Jhun","suffix":""},{"id":273057366,"identity":"b05b4ea8-6de6-4444-8098-3443b555632a","order_by":2,"name":"Jihoon Jang","email":"","orcid":"","institution":"Chung-Ang University","correspondingAuthor":false,"prefix":"","firstName":"Jihoon","middleName":"","lastName":"Jang","suffix":""},{"id":273057367,"identity":"ebfddd80-516e-490f-870d-c4dac692147e","order_by":3,"name":"Shinae Lee","email":"","orcid":"","institution":"Yonsei University","correspondingAuthor":false,"prefix":"","firstName":"Shinae","middleName":"","lastName":"Lee","suffix":""},{"id":273057368,"identity":"43365851-f599-490b-b2a3-3702d72b4b7d","order_by":4,"name":"Eun Jin Cho","email":"","orcid":"https://orcid.org/0000-0002-6607-1851","institution":"Chung-Ang University","correspondingAuthor":false,"prefix":"","firstName":"Eun","middleName":"Jin","lastName":"Cho","suffix":""}],"badges":[],"createdAt":"2024-02-08 05:31:00","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-3938892/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-3938892/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1038/s41467-024-50979-6","type":"published","date":"2024-08-03T04:00:00+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":51745452,"identity":"9f90c86f-8c72-4372-8d79-2d31757cc346","added_by":"auto","created_at":"2024-02-28 10:20:28","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":415583,"visible":true,"origin":"","legend":"\u003cp\u003ePhotoredoxcatalytic C(Ar)-C(Ar) cross-coupling reactions. a, Previous and our methods for photocatalytic C(Ar)-C(Ar) cross-coupling reactions. b, Comparisons of excited-state lifetime and excited-state oxidation potentials of representative photocatalysts and [Au(BZI)(TMCz)]. Shown at the bottom are chemical reducing agents. Refer to Supplementary Tables 1 and 2 for the values.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-3938892/v1/e6ae1748a8c12e591f04b70b.png"},{"id":51745119,"identity":"9281e612-7ed8-473e-891d-eb6f36923c31","added_by":"auto","created_at":"2024-02-28 10:12:28","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":241388,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOxidative quenching.\u003c/strong\u003e \u003cstrong\u003ea,\u003c/strong\u003e Photoluminescence (\u003cem\u003el\u003c/em\u003e\u003csub\u003eex\u003c/sub\u003e = 380 nm) spectra of Ar-saturated DMSO containing 50 mM [Au(BZI)(TMCz)], recorded with increasing concentration of \u003cstrong\u003e1a\u003c/strong\u003e (0–100 mM). The peak marked with an asterisk (*) is the Raman signal of the solvent. \u003cstrong\u003eb\u003c/strong\u003e, Photographs showing photoluminescence emissions of Ar-saturated DMSO of 50 mM [Au(BZI)(TMCz)] in the presence of various concentrations of \u003cstrong\u003e1a\u003c/strong\u003e (from left: 0, 5, 50, and 500 mM \u003cstrong\u003e1a\u003c/strong\u003e) illuminated under 365 nm. \u003cstrong\u003ec\u003c/strong\u003e, Photoluminescence decay traces of Ar-saturated DMSO containing 50 mM [Au(BZI)(TMCz)], recorded with increasing concentration of \u003cstrong\u003e1a\u003c/strong\u003e (0–100 mM) at a wavelength of 540 nm after picosecond pulsed laser photoexcitation at 377 nm (pulse duration = 25 ps). \u003cstrong\u003ed\u003c/strong\u003e, Corresponding pseudo-first-order kinetics analysis of the quenching rate as a function of added \u003cstrong\u003e1a\u003c/strong\u003e. The quenching rate was calculated according to the relationship rate = 1/\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e(\u003cstrong\u003e1a\u003c/strong\u003e) − 1/\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e(0), where \u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e(\u003cstrong\u003e1a\u003c/strong\u003e) and \u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e(0) are the observed photoluminescence lifetime of 50 mM [Au(BZI)(TMCz)] in the presence and absence, respectively, of \u003cstrong\u003e1a\u003c/strong\u003e. \u003cstrong\u003ee\u003c/strong\u003e, Photoluminescence (\u003cem\u003el\u003c/em\u003e\u003csub\u003eex\u003c/sub\u003e = 380 nm) spectra of Ar-saturated DMSO containing 50 mM [Au(BZI)(TMCz)] recorded with increasing concentration of DIPEA (0-500 mM). \u003cstrong\u003ef\u003c/strong\u003e, Photographs showing photoluminescence emissions of Ar-saturated DMSO of 50 mM [Au(BZI)(TMCz)] in the presence of various concentrations of DIPEA (from left: 0, 5, 50, and 500 mM DIPEA) illuminated under 365 nm.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-3938892/v1/46ddee423eb91d8df67053c8.png"},{"id":51745122,"identity":"5d53f716-3cc1-439f-ae38-b0bdcd52e2cc","added_by":"auto","created_at":"2024-02-28 10:12:28","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":558559,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eElectron transfer. a,b,\u003c/strong\u003e Heat maps showing nanosecond photoinduced transient Vis–NIR absorption difference signals of Ar-saturated DMSO containing 100 mM [Au(BZI)(TMCz)] recorded in the absence (\u003cstrong\u003ea\u003c/strong\u003e) and presence (\u003cstrong\u003eb\u003c/strong\u003e) of 200 mM \u003cstrong\u003e1a\u003c/strong\u003e, recorded after 355 nm pulsed laser photoexcitation. \u003cstrong\u003ec\u003c/strong\u003e, Top-most panel, selected photoinduced transient Vis–NIR absorption difference spectra of Ar-saturated DMSO containing 100 mM [Au(BZI)(TMCz)]; second panel, selected photoinduced transient Vis–NIR absorption difference spectra of Ar-saturated DMSO containing 100 mM [Au(BZI)(TMCz)] recorded in the presence of 200 mM \u003cstrong\u003e1a\u003c/strong\u003e; third panel, Vis–NIR absorption difference spectra of 2.0 mM [Au(BZI)(TMCz)] recorded under an anodic potential of 0.45 V vs Ag\u003csup\u003e+/0\u003c/sup\u003e (conditions: Pt mesh working electrode, Pt coil counter electrode, Ag/AgNO\u003csub\u003e3\u003c/sub\u003e pseudo-reference electrode, and Ar-saturated DMSO containing 0.10 M Bu\u003csub\u003e4\u003c/sub\u003eNPF\u003csub\u003e6\u003c/sub\u003e and the Au(I) complex); fourth panel, the absorption spectrum simulated for [Au(BZI)(TMCz)]\u003csup\u003e·+\u003c/sup\u003e (CAM-B3LYP and LANL3DZ basis sets for Au and 6-311+g(d,p) basis set for the other atoms), where the vertical bars indicate oscillator strengths; bottom-most panel, photoluminescence spectrum of Ar-saturated DMSO containing 10 mM [Au(BZI)(TMCz)]. \u003cstrong\u003ed\u003c/strong\u003e, Temporal changes of the 870 nm [Au(BZI)(TMCz)]\u003csup\u003e·+\u003c/sup\u003e traces. \u003cstrong\u003ee\u003c/strong\u003e, Second-order kinetics analysis for charge recombination between [Au(BZI)(TMCz)]\u003csup\u003e·+\u003c/sup\u003e and \u003cstrong\u003e1a\u003c/strong\u003e\u003csup\u003e·-\u003c/sup\u003e. See Supplementary Fig. 6 for the results for the other substrates. \u003cstrong\u003ef\u003c/strong\u003e, Decay traces recorded at 870 nm in the presence of 50 mM \u003cstrong\u003e1a\u003c/strong\u003e and increased concentrations of \u003cstrong\u003e2a\u003c/strong\u003e (0-300 mM). \u003cstrong\u003eg\u003c/strong\u003e, Pseudo-first-order kinetics analysis for the catalyst recovery through electron transfer to [Au(BZI)(TMCz)]\u003csup\u003e·+\u003c/sup\u003e. See Supplementary Fig. 8 for the results for the other substrates.\u003c/p\u003e\n\u003cp\u003e\u0026nbsp;\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-3938892/v1/74ffb6b8b15e4574d7957ab7.png"},{"id":61711851,"identity":"2488c618-9810-410d-906b-6a6d6408b53e","added_by":"auto","created_at":"2024-08-04 07:06:46","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":2244192,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3938892/v1/d263c427-051b-4c1f-97a1-5c2a2cafcf4a.pdf"},{"id":51745453,"identity":"d07c5a61-013d-4768-9b82-132579bdeba2","added_by":"auto","created_at":"2024-02-28 10:20:29","extension":"pdf","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":3926407,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cbr\u003e\u003c/p\u003e","description":"","filename":"Supplementaryinformation.pdf","url":"https://assets-eu.researchsquare.com/files/rs-3938892/v1/21431bbf758e2040398fbfa8.pdf"},{"id":51745118,"identity":"7cc2ddd4-e97b-4e2a-b259-d9751eaa539c","added_by":"auto","created_at":"2024-02-28 10:12:28","extension":"png","order_by":2,"title":"","display":"","copyAsset":false,"role":"supplement","size":75112,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eScheme 1. Catalysis cycle. \u003c/strong\u003ePlausible mechanism of the photoredox catalytic C–C cross-coupling reaction.\u003c/p\u003e","description":"","filename":"Scheme1.png","url":"https://assets-eu.researchsquare.com/files/rs-3938892/v1/b783cee298e38751f810cb65.png"},{"id":51745120,"identity":"7ab2f533-7aad-4a24-b510-9b76c0ad05bb","added_by":"auto","created_at":"2024-02-28 10:12:28","extension":"docx","order_by":3,"title":"","display":"","copyAsset":false,"role":"supplement","size":209263,"visible":true,"origin":"","legend":"","description":"","filename":"Tables.docx","url":"https://assets-eu.researchsquare.com/files/rs-3938892/v1/2dc718737130e641d28b3c4c.docx"}],"financialInterests":"There is \u003cb\u003eNO\u003c/b\u003e Competing Interest.","formattedTitle":"Two-Coordinated Au(I) Complex Photoredoxcatalyst: Highly Efficient Catalysis in C-C Cross-coupling Reactions and the Underlying Mechanism","fulltext":[{"header":"Introduction","content":"\u003cp\u003eVisible-light-activatable, homogeneous photoredox catalysis has emerged as a useful tool for a range of organic transformations.\u003csup\u003e1,2\u003c/sup\u003e The synthetic utility benefits from the ability of catalysts to generate free-radical intermediates through heterobimolecular photoinduced electron transfer.\u003csup\u003e3\u003c/sup\u003e To expand the synthetic utility, researchers have devoted enormous efforts to identifying catalyst molecules capable of mediating photoredox reactions.\u003csup\u003e4,5\u003c/sup\u003e Coordinatively saturated complexes with \u003cem\u003ed\u003c/em\u003e\u003csup\u003e6\u003c/sup\u003e metals, such as \u003cem\u003efac\u003c/em\u003e-[Ir(ppy)\u003csub\u003e3\u003c/sub\u003e] (ppy = 2-phenylpyridinato) and [Ru(bpy)\u003csub\u003e3\u003c/sub\u003e]\u003csup\u003e2+\u003c/sup\u003e (bpy = 2,2¢-bipyridyl), constitute a family of successful molecular photoredox catalysts.\u003csup\u003e6\u003c/sup\u003e Their catalytic efficiencies benefit from the tunable, redox-active triplet metal-to-ligand charge-transfer (\u003csup\u003e3\u003c/sup\u003eMLCT) transition state.\u003csup\u003e7\u003c/sup\u003e The \u003csup\u003e3\u003c/sup\u003eMLCT-active metal complexes, however, sometimes exhibit insufficient catalytic performance, especially for activating substrates with redox-resistant bonds, such as the C-Cl bond.\u003csup\u003e8\u003c/sup\u003e This limitation stems from detrimental effects of the central metal, including 1) an unavoidable energy loss in intersystem crossing from the initially photoexcited singlet state to the \u003csup\u003e3\u003c/sup\u003eMLCT transition state, 2) the occurrence of a metal-centered non-radiative process,\u003csup\u003e9\u003c/sup\u003e and 3) the presence of electron-deficient metal centers, such as Ir(III), which are disadvantageous for achieving strong excited-state reducing power.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe envisioned that low-valent metal complexes that avoid metal-involved electronic transitions could be promising candidates for overcoming the limitations of the \u003csup\u003e3\u003c/sup\u003eMLCT-active photoredox catalysts. To investigate this idea, we selected two-coordinated \u003cem\u003ed\u003c/em\u003e\u003csup\u003e10\u003c/sup\u003e Au(I) complexes with charge-neutral carbene and monoanionic amido ligands (Fig. 1a). This heteroleptic structure exhibits strong visible-light absorption because of the amido ligand-to-carbene ligand charge-transfer (LLCT) transition through redox-innocent mediation of the Au 5\u003cem\u003ed\u003c/em\u003e orbitals.\u003csup\u003e10,11\u003c/sup\u003e The singlet and triplet LLCT transition states are near-degenerate and in a rapid equilibrium, effectively alleviating energy loss in the photocatalysis, which is substantial for \u003csup\u003e3\u003c/sup\u003eMLCT-active Ir(III) and Ru(II) complex photocatalysts.\u003csup\u003e12,13\u003c/sup\u003e Note that the Au center is hardly involved in redox processes or in the electronic transition in LLCT-active Au(I) complexes, which enables facile control of the excited-state oxidation (\u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003e) and reduction (\u003cem\u003eE\u003c/em\u003e*\u003csub\u003ered\u003c/sub\u003e) potentials. In addition, the LLCT-active Au(I) complexes exhibit an excited-state lifetime (\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e) as long as several microseconds,\u003csup\u003e14-16\u003c/sup\u003e without substantial contaminations by short-lived components; such contaminations are routinely observed for organic thermally activated delayed fluorescence (TADF) molecules, such as 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) shown in Fig. 1b.\u003csup\u003e17\u003c/sup\u003e We thus speculate that 1) the negligible electronic energy loss, 2) the absence of metal-centered valence change, 3) the less-electron-deficient Au(I) center, and 4) the efficient utilization of the long-lived excited state will make LLCT-active Au(I) complexes potent photoreducing catalysts.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003ePrevious research on photocatalytic Au complexes have mainly focused on identification of metal-centered redox behaviors. For example, a dinuclear Au(I) complex with bisphosphino bridging ligands exhibited an \u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003e value as negative as −1.6 V vs a NaCl-saturated calomel electrode because of the Au(I/II) redox cycle (Fig. 1b).\u003csup\u003e18,19\u003c/sup\u003e The two-electron cycle between Au(I) and Au(III) species has also garnered research interest because it complies with the well-established oxidative addition-reductive elimination catalytic steps.\u003csup\u003e20-22\u003c/sup\u003e These ground-breaking advances promise the synthetic potential of Au(I) complexes, although their catalytic utility is still based on the metal-centered redox processes involving Au(II) or Au(III), analogous to the case of \u003csup\u003e3\u003c/sup\u003eMLCT-active complexes. The photocatalysis ability of LLCT-active Au(I) complexes was recently validated for [2+2] cycloaddition\u003csup\u003e23\u003c/sup\u003e and water reduction reactions.\u003csup\u003e24\u003c/sup\u003e The catalysis involved energy transfer and the reductive formation of metal nanoparticles, respectively, as the key processes. To the best of our knowledge, organic transformations driven by LLCT-active Au(I) complex photoredox catalysts remain unexplored.\u003c/p\u003e\n\u003cp\u003ePhotocatalytic C-C bond formation plays a critical role in chemistry, driving molecular diversity and facilitating the synthesis of complex functional molecules, as briefly outlined in Fig. 1a-i.\u003csup\u003e25-28\u003c/sup\u003e The pioneering work conducted independently by the Lee\u003csup\u003e29\u003c/sup\u003e and Stephenson\u003csup\u003e30\u003c/sup\u003e groups on the intramolecular C-C bond formation of aryliodides and bromides with π-substrates marked important advances. Despite under UVA irradiation, the MLCT-active dinulear Au complex was also employed by the Barriault group for an intramolecular C-C coupling process.\u003csup\u003e31\u003c/sup\u003e Following these contributions, the scope of research has been significantly expanded to include intermolecular processes with a broader spectrum of substrates, particularly the more challenging aryl chloride substrates. This expansion has been based on strongly photoreducing catalysts such as the photoexcited doublet species utilized by König and coworkers.\u003csup\u003e32\u003c/sup\u003e Despite recent advances,\u003csup\u003e33-37\u003c/sup\u003e a persistent need exists to develop more efficient and selective photocatalytic systems capable of activating redox-resistant substrates, such as aryl chlorides, for C-C cross-coupling reactions.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eHerein, we report unprecedented photoredox catalytic reactivity and the corresponding mechanism of LLCT-active Au(I) complexes for C-C cross-coupling reactions (Fig. 1a-ii). Our Au(I) complex exhibited an \u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003e value as negative as -2.16 V vs standard calomel electrode (SCE), demonstrating the high reducing power as close as sodium mercury amalgam (-2.21 V vs SCE) and metallic potassium (-2.23 V vs SCE) (see Fig. 1b and Supplementary Tables 1 and 2). Our catalysis system features notable advantages, including a catalyst loading as low as 0.1 mol%, reaction yields greater than 80%, and a broad substrate scope including heteroaryl chlorides that are difficult to be activated. Our mechanistic investigations based on steady-state and transient electronic spectroscopy, spectroelectrochemistry, and quantum chemical calculations revealed that the Au(I) complex photocatalyst exhibits strong photoreducing power, with the ability to efficiently retard charge recombination, characterized by a 91% efficiency for the reductive generation of a key radical intermediate from an aryl chloride. In addition, our spectroscopic studies identified neutralization of the one-electron-oxidized catalyst as the rate-determining step in the overall photoredox catalysis cycle.\u0026nbsp;\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003ePhotocatalytic C\u003c/strong\u003e\u003cstrong\u003e-\u003c/strong\u003e\u003cstrong\u003eC cross-coupling reactions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eA series of two-coordinated Au(I) complexes having \u003cem\u003eN\u003c/em\u003e-heterocyclic carbene and carbazolide ligands with various alkyl substituents were tested to evaluate their photoredox catalysis performance (Table 1). These complexes exhibit visible absorption due to the LLCT transition (Supplementary Fig. 1). For example, the ultraviolet-visible (UV-Vis) absorption spectrum of [Au(BZI)(TMCz)] (BZI = 1,3-diphenylbenzo[\u003cem\u003ed\u003c/em\u003e]imidazolylidene; TMCz = 1,3,6,8-tetramethylcarbazolide) has an onset wavelength of 470\u0026nbsp;nm (Supplementary Fig. 1 and Table 3). The excited-state [Au(BZI)(TMCz)] is long-lived, with a\u0026nbsp;\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e as long as 207 ns in deaerated toluene, without any short-lived\u0026nbsp;components (Supplementary Fig. 2). [Au(BZI)(TMCz)] exhibits ground-state oxidation (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eox\u003c/sub\u003e) and reduction (\u003cem\u003eE\u003c/em\u003e\u003csub\u003ered\u003c/sub\u003e) potentials of [Au(BZI)(TMCz)] of 0.56 and −2.26 V vs SCE, respectively\u0026nbsp;(Supplementary Fig. 3). The corresponding \u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003e value is calculated to be −2.16 V vs SCE, being more cathodic than those of widely used photoredox catalysts such as \u003cem\u003efac\u003c/em\u003e-[Ir(ppy)\u003csub\u003e3\u003c/sub\u003e] (−1.73 V vs SCE),\u003csup\u003e38\u003c/sup\u003e 4CzIPN (−1.18 V vs SCE),\u003csup\u003e5\u003c/sup\u003e and Mes-Acr\u003csup\u003e+\u003c/sup\u003e (−0.57 V vs SCE).\u003csup\u003e39\u003c/sup\u003e It should be emphasized that, as\u0026nbsp;compared in Fig. 1b, [Au(BZI)(TMCz)] exhibits a balance between\u0026nbsp;\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e and \u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003e, which is a prerequisite for a potent photoreducing catalyst.\u003c/p\u003e\n\u003cp\u003eThe catalytic efficacy of Au complexes was evaluated through comparisons with several established photoredox catalysts for an intermolecular C-C coupling reaction between model (hetero)aryl chlorides (specifically, methyl-4-chlorobenzoate (\u003cstrong\u003e1a\u003c/strong\u003e) and chloroquinoline (\u003cstrong\u003e1b\u003c/strong\u003e)) and \u003cem\u003eN\u003c/em\u003e-methylpyrrole (\u003cstrong\u003e2a\u003c/strong\u003e) (Table 1). The reactions were carried out with a 0.25 mol% or a 0.1 mol% photocatalyst for \u003cstrong\u003e1a\u003c/strong\u003e or \u003cstrong\u003e1b\u003c/strong\u003e, respectively, and 2 equivalents of \u003cem\u003eN\u003c/em\u003e,\u003cem\u003eN\u003c/em\u003e-di(isopropyl)ethylamine (DIPEA) in Ar-saturated dimethyl sulfoxide (DMSO, 0.50 M) under blue LEDs irradiation. Notably, Au complexes without alkyl substituents in the carbene ligands, [Au(BZI)(Cz)] and [Au(BZI)(TMCz)] (Cz = carbazolide), exhibited superior reactivity, successfully yielding the desired C-C coupled product \u003cstrong\u003e3\u003c/strong\u003e in excellent yields. The distinctly superior catalytic performance of [Au(BZI)(Cz)] and [Au(BZI)(TMCz)] compared with that of the other Au(I) complexes is attributable to their interactions with \u003cstrong\u003e2a\u0026nbsp;\u003c/strong\u003ebeing less sterically encumbered. The added \u003cstrong\u003e2a\u003c/strong\u003e results in an increase in\u0026nbsp;\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e, suggesting that \u003cstrong\u003e2a\u003c/strong\u003e is not engaged in photoinduced electron transfer but suppresses nonradiative relaxation of the Au(I) complexes (Supplementary Fig. 2).\u003c/p\u003e\n\u003cp\u003eBy contrast, the commonly used \u003cem\u003efac\u003c/em\u003e-[Ir(ppy)\u003csub\u003e3\u003c/sub\u003e] demonstrated lower reactivity, and 4CzIPN, fluorescein, and the Pt(II) complex photocatalyst\u003csup\u003e40\u003c/sup\u003e were inactive in this transformation. Optimization of the reaction parameters enabled the synthesis of the heteroaryl–heteroaryl coupled product (\u003cstrong\u003e3ba\u003c/strong\u003e) in 84% yield in the presence of only 0.1 mol% [Au(BZI)(TMCz)]. The optimization results are compiled in Supplementary Table 4. Notably, this catalytic loading represents an advance because such a low loading has not been achieved in any previously reported photocatalytic processes. The quantum yield for the reaction of \u003cstrong\u003e1a\u003c/strong\u003e with \u003cstrong\u003e2a\u003c/strong\u003e, which was determined using the standard ferrioxalate actinometry, is as large as 34%. Finally, control experiments revealed that the reaction requires both the photocatalyst and irradiation with visible light (Supplementary Table 4).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWith the optimized parameters in hand, we explored the versatility of the Au(I) complex-catalyzed C–C coupling protocol in synthesizing various (hetero)aryl– (hetero)aryl products (\u003cstrong\u003e3\u003c/strong\u003e). [Au(BZI)(TMCz)], which has an \u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003e as negative as –2.16 V vs SCE, enables the incorporation of a range of redox-resistant (hetero)aryl chlorides (\u003cstrong\u003e1a\u003c/strong\u003e, \u003cstrong\u003e1b\u003c/strong\u003e, \u003cstrong\u003e1c\u003c/strong\u003e,\u003cstrong\u003e\u0026nbsp;1k\u003c/strong\u003e, \u003cstrong\u003e1m\u003c/strong\u003e, and \u003cstrong\u003e1p\u003c/strong\u003e) with reduction potentials (\u003cem\u003eE\u003c/em\u003e\u003csub\u003ered\u003c/sub\u003es) in the range –1.61 to –2.12 V vs SCE (see Supplementary Fig. 4 for the corresponding voltammograms), as well as bromides and iodides, in the C-C coupled products (Table 2). We observed a distinct variation in the reactivity among different (hetero)aryl halides, leading to a chemoselective process. Specifically, in the case of dihalogenated substrates, \u003cstrong\u003e1q\u003c/strong\u003e and \u003cstrong\u003e1r\u003c/strong\u003e, the iodo group demonstrated selective reactivity over the bromo substituent, resulting in \u003cstrong\u003e3qa\u003c/strong\u003e and \u003cstrong\u003e3ra\u003c/strong\u003e as the major products, respectively. For \u003cstrong\u003e1m\u003c/strong\u003e, which contains two chloro substituents, regioselective reactivity was noted at the chloride positioned para to the CF\u003csub\u003e3\u003c/sub\u003e group, yielding \u003cstrong\u003e3ma\u003c/strong\u003e. In general, \u003cem\u003eN\u003c/em\u003e-heteroaryl halides exhibited higher reactivities than aryl halides, even with a reduced catalyst loading (0.1 mol% vs 0.25 mol%). Furthermore, the substitution patterns did not affect reactivity: \u003cem\u003eortho\u003c/em\u003e-, \u003cem\u003emeta\u003c/em\u003e-, and \u003cem\u003epara\u003c/em\u003e-substituted aryl halides were all suitable. Notably, our photoredox catalysis protocol tolerated the presence of various functional groups, including aldehyde (\u003cstrong\u003e3ja\u003c/strong\u003e), ketone (\u003cstrong\u003e3ia\u003c/strong\u003e), ester (\u003cstrong\u003e3aa\u003c/strong\u003e, \u003cstrong\u003e3ra\u003c/strong\u003e, \u003cstrong\u003e3ab\u003c/strong\u003e, and \u003cstrong\u003e3ac\u003c/strong\u003e), and nitrile (\u003cstrong\u003e3ka\u003c/strong\u003e) groups, as well as medicinally significant fluoride (\u003cstrong\u003e3oa\u003c/strong\u003e) and CF\u003csub\u003e3\u003c/sub\u003e (\u003cstrong\u003e3da\u003c/strong\u003e, \u003cstrong\u003e3la\u003c/strong\u003e, \u003cstrong\u003e3ma\u003c/strong\u003e, \u003cstrong\u003e3na\u003c/strong\u003e) groups. In general, dehalogenation side products were also observed, albeit in negligible amounts. In addition, the high reactivity of our photoredox catalysis protocols was further demonstrated by application to the C–B bond-forming reaction, where bis(pinacolato)diboron was used as the coupling partner. Detailed results of the reactions are provided in Supplementary Table 5. The resultant arylboronates are versatile synthons, applicable in a broad range of transition-metal-catalyzed transformations.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eMechanistic investigations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eHaving demonstrated the photoredoxcatalytic ability of the Au(I) complex, we sought to elucidate the role of the catalyst in the C−C cross-coupling reaction. The \u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003e value of [Au(BZI)(TMCz)] (−2.16 V vs SCE) is more negative than the \u003cem\u003eE\u003c/em\u003e\u003csub\u003ered\u003c/sub\u003e value of \u003cstrong\u003e1a\u003c/strong\u003e (−1.81 V vs SCE), implying that the excited-state of [Au(BZI)(TMCz)] (denoted as [Au(BZI)(TMCz)]* hereafter) can be oxidatively quenched by \u003cstrong\u003e1a\u003c/strong\u003e with the driving force for heterobimolecular one-electron transfer (−D\u003cem\u003eG\u003c/em\u003e\u003csub\u003eeT\u003c/sub\u003e, −D\u003cem\u003eG\u003c/em\u003e\u003csub\u003eeT\u003c/sub\u003e = \u003cem\u003ee\u003c/em\u003e×[\u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003e([Au(BZI)(TMCz)]) − \u003cem\u003eE\u003c/em\u003e\u003csub\u003ered\u003c/sub\u003e(\u003cstrong\u003e1a\u003c/strong\u003e)], where \u003cem\u003ee\u003c/em\u003e is the elementary charge and we ignore the Coulomb term because of our use of the polar solvent DMSO) of 0.35 eV. By contrast, reductive quenching of [Au(BZI)(TMCz)]* (\u003cem\u003eE\u003c/em\u003e*\u003csub\u003ered\u003c/sub\u003e = 0.46 V vs SCE) by DIPEA (\u003cem\u003eE\u003c/em\u003e\u003csub\u003eox\u003c/sub\u003e = 0.64 V vs SCE) is predicted to be disfavored due to the negative −D\u003cem\u003eG\u003c/em\u003e\u003csub\u003eeT\u003c/sub\u003e of −0.18 eV.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe validated the aforementioned thermodynamic considerations for the initial electron transfer by using photoluminescence titration experiments. As shown in Fig. 2a and 2b, the increased concentrations of \u003cstrong\u003e1a\u003c/strong\u003e (0−100 mM) elicit a concentration-dependent decrease in the photoluminescence intensity of 50\u0026nbsp;mM [Au(BZI)(TMCz)]. An analogous decrease is also observed for\u0026nbsp;\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e (Fig. 2c). The apparent quenching rate computed through the relationship, quenching rate = 1/\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e(\u003cstrong\u003e1a\u003c/strong\u003e) − 1/\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e(0), where\u0026nbsp;\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e(\u003cstrong\u003e1a\u003c/strong\u003e) and\u0026nbsp;\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e(0) are the\u0026nbsp;\u003cem\u003et\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e of [Au(BZI)(TMCz)] in the presence and absence of \u003cstrong\u003e1a\u003c/strong\u003e, respectively, increases with increasing molarity of \u003cstrong\u003e1a\u003c/strong\u003e. Pseudo-first-order kinetics analysis for the quenching rate reveals strict adherence to the straight line and returns an apparent heterobimolecular quenching rate constant (\u003cem\u003ek\u003c/em\u003e\u003csub\u003eq\u003c/sub\u003e) as large as 7.2 × 10\u003csup\u003e8\u003c/sup\u003e M\u003csup\u003e−1\u003c/sup\u003e s\u003csup\u003e−1\u003c/sup\u003e. Deconvolution of the diffusion rate constant (\u003cem\u003ek\u003c/em\u003e\u003csub\u003ediff\u003c/sub\u003e, 3.3 × 10\u003csup\u003e9\u003c/sup\u003e M\u003csup\u003e−1\u003c/sup\u003e s\u003csup\u003e−1\u003c/sup\u003e for DMSO at 298 K) from \u003cem\u003ek\u003c/em\u003e\u003csub\u003eq\u003c/sub\u003e yields a second-order quenching rate constant (\u003cem\u003ek\u003c/em\u003e\u003csub\u003eQ\u003c/sub\u003e) of 9.1 × 10\u003csup\u003e8\u003c/sup\u003e M\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e s\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e. The quantum yield for quenching (\u003cem\u003eF\u003c/em\u003e\u003csub\u003eQ\u003c/sub\u003e) is as large as 95% in the presence of 0.50 M \u003cstrong\u003e1a\u003c/strong\u003e, which is estimated according to the relationship\u0026nbsp;\u003cem\u003eF\u003c/em\u003e\u003csub\u003eQ\u003c/sub\u003e = \u003cem\u003ek\u003c/em\u003e\u003csub\u003eQ\u003c/sub\u003e×[\u003cstrong\u003e1a\u003c/strong\u003e] / (\u003cem\u003ek\u003c/em\u003e\u003csub\u003eQ\u003c/sub\u003e×[\u003cstrong\u003e1a\u003c/strong\u003e] + \u003cem\u003ek\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e), where [\u003cstrong\u003e1a\u003c/strong\u003e] is expressed in molarity (0.50 M) and \u003cem\u003ek\u003c/em\u003e\u003csub\u003ed\u003c/sub\u003e is the intrinsic decay rate of [Au(BZI)(TMCz)]* (2.2 × 10\u003csup\u003e7\u003c/sup\u003e s\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e). In stark contrast, DIPEA does quench the photoluminescence of [Au(BZI)(TMCz)]* even at a concentration as high as 500 mM (Fig. 2e and 2f). We exclude the energy-transfer pathway because [Au(BZI)(TMCz)] not only exhibits negligible spectral overlap with \u003cstrong\u003e1a\u003c/strong\u003e, \u003cstrong\u003e2a\u003c/strong\u003e, and DIPEA but also exhibits a \u003cem\u003eT\u003c/em\u003e\u003csub\u003e1\u003c/sub\u003e energy (2.79 eV) lower than that of \u003cstrong\u003e1a\u003c/strong\u003e (3.16 eV) (refer to Supplementary\u0026nbsp;Fig. 5 for details). Collectively, our electrochemical and photoluminescence results strongly indicate rapid and exclusive electron transfer from [Au(BZI)(TMCz)]* to \u003cstrong\u003e1a\u003c/strong\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe photoinduced electron transfer produces a radical-ion pair (RIP) consisting of [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e and \u003cstrong\u003e1a\u003c/strong\u003e\u003csup\u003e·-\u003c/sup\u003e. We used nanosecond visible–near-infrared (Vis–NIR) transient absorption spectroscopy to directly monitor the genesis of the radical ion species. As shown in Fig. 3a, the heat map of the photoinduced Vis–NIR absorption difference spectra of 100\u0026nbsp;mM [Au(BZI)(TMCz)] after nanosecond pulsed laser photoexcitation at a wavelength\u0026nbsp;of 355 nm contains negative signals in the visible region because of the stimulated emission (see also the top- and bottom-most panels in Fig. 3c). In sharp contrast, positive signals emerge at a peak wavelength of 870 nm in the presence of 200 mM \u003cstrong\u003e1a\u003c/strong\u003e (Fig. 3b and the second panel in Fig. 3c). The NIR signals are attributable to [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e because [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e electrochemically generated at an anodic potential of 0.45 V vs Ag\u003csup\u003e+/0\u003c/sup\u003e exhibits a broad absorption band in this region (third panel in Fig. 3c). This spectral assignment is further corroborated by the close match with the [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e electronic transition spectrum quantum chemically simulated at the CAM-B3LYP\u0026nbsp;level of theory with a conductor-like polarizable continuum model parameterized to DMSO (fourth panel in Fig. 3c). The transient spectroscopy and calculation investigations provide direct evidence for photoinduced electron transfer from [Au(BZI)(TMCz)]* to \u003cstrong\u003e1a\u003c/strong\u003e.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eOnce formed, the RIP is rapidly annihilated by charge recombination through back electron transfer from \u003cstrong\u003e1a\u003c/strong\u003e\u003csup\u003e·-\u003c/sup\u003e to [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e because the driving force for charge recombination (−D\u003cem\u003eG\u003c/em\u003e\u003csub\u003eCR\u003c/sub\u003e, −D\u003cem\u003eG\u003c/em\u003e\u003csub\u003eCR\u003c/sub\u003e = \u003cem\u003ee\u003c/em\u003e×[\u003cem\u003eE\u003c/em\u003e\u003csub\u003ered\u003c/sub\u003e(\u003cstrong\u003e1a\u003c/strong\u003e) − \u003cem\u003eE\u003c/em\u003e\u003csub\u003eox\u003c/sub\u003e([Au(BZI)(TMCz)])]) is as large as 2.37 eV.Notably, in many photoredox catalysis processes, the charge recombination is ultrafast and detrimental to the catalysis cycle, limiting the overall photocatalytic performance.\u003csup\u003e40,41\u003c/sup\u003e The charge recombination process in [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e can be monitored at a wavelength of 870 nm. Surprisingly, [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e is long-lived, with an apparent lifetime of 18\u0026nbsp;ms (Fig. 3d). Second-order kinetics analyses based on the molar absorbance of [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e at 870 nm (96\u0026nbsp;M\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e cm\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e) indicate that the rate constant for charge recombination (\u003cem\u003ek\u003c/em\u003e\u003csub\u003eCR\u003c/sub\u003e) with \u003cstrong\u003e1a\u003c/strong\u003e\u003csup\u003e·-\u003c/sup\u003e is 3.3 × 10\u003csup\u003e8\u003c/sup\u003e M\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e s\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e (Fig. 3e). Notably, the yield for the liberation of free-radical ion species from the geminate RIP (i.e., [[Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e×××\u003cstrong\u003e1a\u003c/strong\u003e\u003csup\u003e·-\u003c/sup\u003e]\u0026nbsp;®\u0026nbsp;[Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e and \u003cstrong\u003e1a\u003c/strong\u003e\u003csup\u003e·-\u003c/sup\u003e) can be computed according to the relationship \u003cem\u003ek\u003c/em\u003e\u003cem\u003e\u003csub\u003e-\u003c/sub\u003e\u003c/em\u003e\u003csub\u003ediff\u003c/sub\u003e / (\u003cem\u003ek\u003c/em\u003e\u003cem\u003e\u003csub\u003e-\u003c/sub\u003e\u003c/em\u003e\u003csub\u003ediff\u003c/sub\u003e + \u003cem\u003ek\u003c/em\u003e\u003csub\u003eCR\u003c/sub\u003e) to be as large as 91%. This value is greater than the yield for charge recombination of cyclometalated Pt(II) complexes with CF\u003csub\u003e3\u003c/sub\u003eI\u003csup\u003e·-\u003c/sup\u003e (47–77%) which are highly reducing photocatalysts established by us.\u003csup\u003e40\u003c/sup\u003e The \u003cem\u003ek\u003c/em\u003e\u003csub\u003eCR\u003c/sub\u003e values with other aryl halide substrates were also determined to be in the range (0.7–2.9) × 10\u003csup\u003e8\u003c/sup\u003e M\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e s\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e (Supplementary Fig. 6). Finally, we found that the \u003cem\u003ek\u003c/em\u003e\u003csub\u003eCR\u003c/sub\u003e values adhere to the\u0026nbsp;Jortner curves for electron transfer with large reorganization energies, which suggested strong interactions with solvents (Supplementary Fig. 7).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe liberated \u003cstrong\u003e1a\u003c/strong\u003e\u003csup\u003e·-\u003c/sup\u003e is cleaved into an aryl radical species and Cl\u003csup\u003e-\u003c/sup\u003e. The aryl radical species subsequently reacts with \u003cstrong\u003e2a\u003c/strong\u003e to form a C–C cross-coupled adduct (\u003cstrong\u003e3aaH\u003c/strong\u003e• in Scheme 1). We hypothesized that the \u003cstrong\u003e3aaH\u003c/strong\u003e• species plays a key role in completing the photoredox catalysis cycle. Specifically, we hypothesized that the [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e is neutralized to the original [Au(BZI)(TMCz)] state through one-electron withdrawal from \u003cstrong\u003e3aaH\u003c/strong\u003e• and that DIPEA subsequently deprotonates the resultant one-electron-oxidized \u003cstrong\u003e3aaH\u003csup\u003e+\u003c/sup\u003e\u003c/strong\u003e intermediate, producing the final product (\u003cstrong\u003e3aa\u003c/strong\u003e).\u003c/p\u003e\n\u003cp\u003eTo validate this hypothesis, we performed nanosecond laser flash photolysis for the mixture of 100\u0026nbsp;mM [Au(BZI)(TMCz)] and 50 mM \u003cstrong\u003e1a\u003c/strong\u003e with the addition of \u003cstrong\u003e2a\u003c/strong\u003e in various concentrations (0–300 mM). We expected that the effective concentration of \u003cstrong\u003e3aaH\u003c/strong\u003e• would increase with the increased \u003cstrong\u003e2a\u003c/strong\u003e concentration. Gratifyingly, we found that the increase in concentration of \u003cstrong\u003e2a\u003c/strong\u003e shortens the lifetime of the 870 nm signal of [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e (Fig. 3f). The concentration-dependent decays can be best interpreted as the recovery of [Au(BZI)(TMCz)] through electron transfer from \u003cstrong\u003e3aaH\u003c/strong\u003e•to [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e. Our pseudo-first order kinetics analysis of the 870 nm signals as a function of [\u003cstrong\u003e2a\u003c/strong\u003e] yields a rate constant of 3.8 × 10\u003csup\u003e5\u003c/sup\u003e M\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e s\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u0026nbsp;\u003c/sup\u003efor bimolecular electron transfer (\u003cem\u003ek\u003c/em\u003e\u003csub\u003eeT\u003c/sub\u003e) from \u003cstrong\u003e3aaH\u003c/strong\u003e•to [Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e (Fig. 3g). Results obtained with the other substrates are summarized in\u0026nbsp;Supplementary Fig. 8. Although the determined value should serve as a lower limit due to the pre-steps for the generation of \u003cstrong\u003e3aaH\u003c/strong\u003e•, this \u003cem\u003ek\u003c/em\u003e\u003csub\u003eeT\u003c/sub\u003e value is three orders of magnitude smaller than the rate constants for the other electron-transfer steps, \u003cem\u003ek\u003c/em\u003e\u003csub\u003eQ\u003c/sub\u003e (9.1 × 10\u003csup\u003e8\u003c/sup\u003e M\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e s\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e) and \u003cem\u003ek\u003c/em\u003e\u003csub\u003eCR\u003c/sub\u003e (3.3 × 10\u003csup\u003e8\u003c/sup\u003e M\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e s\u003csup\u003e-\u003c/sup\u003e\u003csup\u003e1\u003c/sup\u003e). This comparison indicates that the catalyst recovery is the rate-determining step in the overall photoredox catalysis cycle. The relatively slow electron transfer is presumably attributable to the multiple steps, including C-Cl bond cleavage and the radical addition to \u003cstrong\u003e2a\u003c/strong\u003e, required to form \u003cstrong\u003e3aaH\u003c/strong\u003e•.\u0026nbsp;\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eGiven its enormous potential, photoredox catalysis will continue to expand our horizon of synthetic organic methodologies. This exploration inevitably requires the development of catalysts with strong redox power; it also requires kinetic compatibility of electronic processes involved in the catalytic cycle. In this research, we investigated the homogeneous photoredox catalysis of two-coordinated Au(I) complexes showing exclusive LLCT transition. The advantages of Au(I) complex photoredox catalysts include 1) strong visible-light absorption, 2) cathodic \u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003es capable of reductively activating redox-resistant (hetero)aryl chloride substrates, 3) submicrosecond \u003cem\u003eτ\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e, enabling efficient excited-state heterobimolecular processes, 4) effective suppression of charge recombination, and 5) versatility for controlling the \u003cem\u003eE\u003c/em\u003e*\u003csub\u003eox\u003c/sub\u003e and \u003cem\u003eτ\u003c/em\u003e\u003csub\u003eobs\u003c/sub\u003e values through ligand structures. We exploited these features to catalyze C\u0026minus;C cross-coupling reactions between (hetero)aryl halides, including (hetero)aryl chlorides, and \u003cem\u003eN\u003c/em\u003e-heterocycles. Notably, our photoredox catalysis protocols require a minimal catalyst loading as low as 0.1 mol%. In addition, the catalysis exhibits a broad substrate scope and is applicable to C\u0026ndash;B bond formation reactions. Our mechanistic investigations revealed that the catalytic cycle of the C\u0026ndash;C cross-coupling reaction is initiated by one-electron transfer from the excited-state Au complex to the aryl chloride substrate. Of particular interest is the long-lived (18 \u0026micro;s) radical-ion species because most photoredox catalysts suffer from rapid charge recombination before the subsequent catalysis step. Indeed, the yield for the radical intermediate is as large as 91%, validating the highly efficient production pathway. We also discovered that the final step (i.e., the recovery of the photocatalyst) is completed by electron transfer from the radical species of the C\u0026minus;C cross-coupled adduct and that the overall catalysis is governed by this step. Our research provides the first photoredoxcatalytic organic transformation by LLCT-active Au(I) complexes. Mechanistic investigations also indicated electron-transfer behaviors beneficial for a range of photoredox catalytic applications, which will inspire future research toward developing organic synthesis methodologies that require strong photoreduction capability.\u003c/p\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003ePhotoredoxcatalytic C(Ar)\u003c/strong\u003e\u003cstrong\u003e-\u003c/strong\u003e\u003cstrong\u003eC(Ar) cross-coupling reactions\u003c/strong\u003e. An oven-dried reaction vial equipped with a magnetic stir bar was charged with (hetero)aryl halide \u003cstrong\u003e1\u003c/strong\u003e (0.50 mmol, 1.0 equiv), [Au(BZI)(TMCz)] (0.3 mg , 0.1 mol%), \u003cem\u003eN\u003c/em\u003e-heterocycle \u003cstrong\u003e2\u003c/strong\u003e (5.0 mmol, 10 equiv), DIPEA (1.0 mmol, 2 equiv), and anhydrous DMSO (0.50 M, 1.0 mL). The vial was sealed with a silicon septum screw cap and purged with Ar via a balloon for 10 min. The setup was then exposed to blue LEDs (18 W) in a photoreactor with continuous stirring. The reaction progress was monitored using thin-layer chromatography (TLC) or gas chromatography (GC). Upon completion, the reaction was quenched by adding water. The reaction mixture was extracted three times using dichloromethane. The combined organic phases were dried over MgSO\u003csub\u003e4\u003c/sub\u003e, concentrated under reduced pressure using a rotary evaporator, and purified by silica gel flash column chromatography using a hexane–ethyl acetate solution (10/1) as the eluent to give the corresponding C-C cross-coupled product \u003cstrong\u003e3\u003c/strong\u003e.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eNanosecond laser flash photolysis\u003c/strong\u003e. Ar-saturated DMSO containing 100 μM [Au(BZI)(TMCz)] in a 1 cm × 1 cm quartz cell was excited by a Nd:YAG laser (Continuum, SLII-10; 355 nm and 5 mJ pulse\u003csup\u003e−1\u003c/sup\u003e) in the absence and presence of 200 mM \u003cstrong\u003e1a\u003c/strong\u003e. Time courses of the transient absorption were measured using a photomultiplier tube (visible region) and an InGaAs-PIN photodiode (Hamamatsu 2949) (NIR region). The output from the photomultiplier tube and photodiode was recorded with a digitized oscilloscope (Tektronix, TDS3032; 300 MHz). All experiments were performed at 298 K. For titration experiments, stock solutions of 100\u0026nbsp;mM [Au(BZI)(TMCz)] and 1.11 M \u003cstrong\u003e2a\u003c/strong\u003e were prepared in DMSO. [Au(BZI)(TMCz)] solution (3.0 mL) was added to a 1 cm × 1 cm quartz cell. Decay traces of\u0026nbsp;[Au(BZI)(TMCz)]\u003csup\u003e·\u003c/sup\u003e\u003csup\u003e+\u003c/sup\u003e were recorded with the continuous addition of\u0026nbsp;\u003cstrong\u003e2a\u003c/strong\u003e. The final concentration of \u003cstrong\u003e2a\u003c/strong\u003e was 300 mM.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eData availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eAcknowledgements\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThis work was supported by Midcareer Research Programs (RS-2023-00208856 and NRF-2020R1A2C2009636) through National Research Foundation grants funded by the Ministry of Science, Information, and Communication Technology (ICT) and Future Planning (MSIP), and by the Yonsei University Research Fund (2023-22-0132).\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor Contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eB.H.J. performed spectroscopic and quantum chemical investigations, analyzed the data, and wrote the manuscript. J.J. designed and synthesized the substrates, evaluated the catalysis performance,\u0026nbsp;and co-wrote the manuscript.\u0026nbsp;S.L. designed and performed spectroscopic and electrochemical experiments, analyzed the data, and\u0026nbsp;co-wrote the manuscript.\u0026nbsp;E.J.C. supervised the work at Chung-Ang University and co-wrote the manuscript.\u0026nbsp;Y.Y. coordinated all of the experiments and analyses, and co-wrote the manuscript.\u0026nbsp;All authors contributed to discussion on the study, and edited the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAdditional information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eSupplementary information is available in the online version of the paper. Reprints and permissions information are available online at www.nature.com/reprints. Correspondence and requests for materials should be addressed to Y.Y. ([email protected]) or E.J.C. ([email protected]).\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\u003eRomero, N. A. \u0026amp; Nicewicz, D. A. Organic photoredox catalysis. \u003cem\u003eChem. Rev.\u003c/em\u003e \u003cstrong\u003e116\u003c/strong\u003e, 10075-10166 (2016). \u003c/li\u003e\n\u003cli\u003eBarham, J. P. \u0026amp; K\u0026ouml;nig, B. Synthetic photoelectrochemistry. \u003cem\u003eAngew. Chem., Int. Ed.\u003c/em\u003e \u003cstrong\u003e59\u003c/strong\u003e, 11732-11747 (2020). \u003c/li\u003e\n\u003cli\u003eStaveness, D., Bosque, I. \u0026amp; Stephenson, C. R. J. Free radical chemistry enabled by visible light-induced electron transfer. \u003cem\u003eAcc. Chem. Res.\u003c/em\u003e \u003cstrong\u003e49\u003c/strong\u003e, 2295-2306 (2016). \u003c/li\u003e\n\u003cli\u003eArias-Rotondo, D. M. \u0026amp; McCusker, J. K. 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Soc.\u003c/em\u003e \u003cstrong\u003e113\u003c/strong\u003e, 7823\u0026ndash;7825 (1991).\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Tables","content":"\u003cp\u003eTables 1 to 2 are available in the Supplementary Files section\u003c/p\u003e"},{"header":"Scheme","content":"\u003cp\u003eScheme 1 is available in the Supplementary Files section.\u0026nbsp;\u003c/p\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-3938892/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-3938892/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003e\u003cstrong\u003ePhotocatalysis provides a versatile approach to redox activation of various organic substrates for synthetic applications. To broaden the scope of photoredox catalysis, developing catalysts with strong photoredox power is imperative. Photoredox catalysts with excited-state properties that include cathodic oxidation potentials and long lifetimes are particularly demanded. In this research, we demonstrate the high-efficiency catalytic utility of two-coordinated Au(I) complex photocatalysts that exhibit an exclusive ligand-to-ligand charge-transfer (LLCT) transition in C-C cross-coupling reactions between \u003c/strong\u003e\u003cem\u003e\u003cstrong\u003eN\u003c/strong\u003e\u003c/em\u003e\u003cstrong\u003e-heterocycles and (hetero)aryl halides, including redox-resistant (hetero)aryl chlorides. Our photocatalysis system can steer reactions under visible-light irradiation at a catalyst loading as low as 0.1 mol% and exhibits a broad substrate scope with high chemo- and regioselectivity. Our mechanistic investigations provide direct spectroscopic evidence for each step in the catalysis cycle and demonstrate that the LLCT-active Au(I) complex catalysts offer several benefits, including strong visible-light absorption, a 207 ns-long excited-state lifetime without short-lived components, and a 91% yield in the production of free-radical intermediates. Given the wide structural versatility of the proposed catalysts, we envision that our research will provide useful insights into the future utilization of the LLCT-active Au(I) complex for organic transformations.\u003c/strong\u003e\u003c/p\u003e","manuscriptTitle":"Two-Coordinated Au(I) Complex Photoredoxcatalyst: Highly Efficient Catalysis in C-C Cross-coupling Reactions and the Underlying Mechanism","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-02-28 10:12:24","doi":"10.21203/rs.3.rs-3938892/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"79724b0c-cd14-4bd0-b9e1-bfc9f722e377","owner":[],"postedDate":"February 28th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[{"id":28785450,"name":"Physical sciences/Chemistry/Catalysis/Photocatalysis"},{"id":28785451,"name":"Physical sciences/Chemistry/Photochemistry/Photocatalysis"},{"id":28785452,"name":"Physical sciences/Materials science/Materials for energy and catalysis/Photocatalysis"}],"tags":[],"updatedAt":"2024-08-04T07:06:40+00:00","versionOfRecord":{"articleIdentity":"rs-3938892","link":"https://doi.org/10.1038/s41467-024-50979-6","journal":{"identity":"nature-communications","isVorOnly":false,"title":"Nature Communications"},"publishedOn":"2024-08-03 04:00:00","publishedOnDateReadable":"August 3rd, 2024"},"versionCreatedAt":"2024-02-28 10:12:24","video":"","vorDoi":"10.1038/s41467-024-50979-6","vorDoiUrl":"https://doi.org/10.1038/s41467-024-50979-6","workflowStages":[]},"version":"v1","identity":"rs-3938892","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-3938892","identity":"rs-3938892","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}