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

19 May 2021, Version 1
This content is a preprint and has not undergone peer review at the time of posting.


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(πœ‡-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(πœ‡-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 n(CN) frequencies (2113 - 2117 cm-1). Nonetheless, H-bonding in the corresponding [xHetTpCu]2(πœ‡-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(πœ‡-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(πœ‡-N2) complexes, identifying concerted dual HAT as a thermodynamically favorable pathway for N2 / N2H2 interconversion.


Redox Non-Innocence

Supplementary materials



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