Uncovering Redox Non-Innocent H-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 ^xHetTpCu^I centers in ^xHetTpCu(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 [^xHetTpCu]₂(𝜇-N₂H₂) 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 N₂H₂ and the neutral pyH⁰ cation radical, the aromatic N-heterocylic 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 concerted dual HAT as a thermodynamically favorable pathway for N₂ / N₂H₂ interconversion.