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

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