Uncovering Redox Non-innocent Hydrogen-Bonding in Cu(I)-Diazene Complexes

The life-sustaining reduction of N2 to NH3 is thermoneutral yet kinetically challenged by high-energy intermediates such as N2H2. Exploring intramolecular H-bonding as a potential strategy to stabilize diazene intermediates, we employ a series of [ xHetTpCu]2(μ-N2H2) complexes that exhibit H-bonding between pendant aromatic N-heterocycles (xHet) such as pyridine and a bridging trans-N2H2 ligand at copper­(I) centers. X-ray crystallography and IR spectroscopy clearly reveal H-bonding in [pyMeTpCu]2(μ-N2H2) while low-temperature 1H NMR studies coupled with DFT analysis reveals a dynamic equilibrium between two closely related, symmetric H-bonded structural motifs. Importantly, the xHet pendant negligibly influences the electronic structure of xHetTpCuI centers in xHetTpCu­(CNAr2,6‑Me2 ) complexes that lack H-bonding as judged by nearly indistinguishable ν­(CN) frequencies (2113–2117 cm–1). Nonetheless, H-bonding in the corresponding [ xHetTpCu]2(μ-N2H2) complexes results in marked changes in ν­(NN) (1398–1419 cm–1) revealed through resonance Raman studies. Due to the closely matched N–H BDEs of N2H2 and the pyH0 cation radical, the aromatic N-heterocyclic pendants may encourage partial H-atom transfer (HAT) from N2H2 to xHet through redox-non-innocent H-bonding in [ xHetTpCu]2(μ-N2H2). DFT studies reveal modest thermodynamic barriers for concerted transfer of both H-atoms of coordinated N2H2 to the xHet pendants to generate tautomeric [ xHetHTpCu]2(μ-N2) complexes, identifying metal-assisted concerted dual HAT as a thermodynamically favorable pathway for N2/N2H2 interconversion.