Uncovering Redox Non-Innocent H-Bonding in Cu(I)-Diazene Complexes

The life-sustaining reduction of N 2 to NH 3 is thermoneutral yet kinetically challenged by high energy intermediates such as N 2 H 2 . Exploring intramolecular H-bonding as a potential strategy to stabilize diazene intermediates, we employ a series of [ xHet TpCu] 2 (𝜇-N 2 H 2 ) complexes that exhibit H-bonding between pendant aromatic N-heterocycles ( X Het) such as pyridine and a bridging trans -N 2 H 2 ligand at copper(I) centers. X-ray crystallography and IR spectroscopy clearly reveal H-bonding in [ pyMe TpCu] 2 (𝜇-N 2 H 2 ) while low temperature 1 H NMR studies coupled with DFT analysis reveals a dynamic equilibrium between two closely related, symmetric H-bonded structural motifs. Importantly, the x Het pendant negligibly influences the electronic structure of xHet TpCu I centers in xHet TpCu(CNAr 2,6-Me2 ) complexes that lack H-bonding as judged by nearly indistinguishable n(CN) frequencies (2113 - 2117 cm -1 ). Nonetheless, H-bonding in the corresponding [ xHet TpCu] 2 (𝜇-N 2 H 2 ) complexes results in marked changes in n(NN) (1398 - 1419 cm -1 ) revealed through rRaman studies. Due to the closely matched N-H BDE’s of N 2 H 2 and the neutral pyH 0 cation radical, the aromatic N-heterocylic pendants may encourage partial H-atom transfer (HAT) from N 2 H 2 to x Het through redox non-innocent H-bonding in [ xHet TpCu] 2 (𝜇-N 2 H 2 ). DFT studies reveal modest thermodynamic barriers for concerted transfer of both H-atoms of coordinated N 2 H 2 to the x Het pendants to generate tautomeric [ xHetH TpCu] 2 (𝜇-N 2 ) complexes, identifying concerted dual HAT as a thermodynamically favorable pathway for N 2 / N 2 H 2 interconversion.