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

• DOI: 10.1021/jacs.1c04108
• Publication Date: 2021-09-21T17:27:11Z
• Authors: Evan J. Gardner; Sean C. Marguet; Caitlyn R. Cobb; Dominic M. Pham; Josalyne A. M. Be...; Jeffery A. Bertke; Hannah S. Shafaat; Timothy H. Warren
• Keywords: [< sup; lack h; ray crystallography; innocent hydrogen; thermodynamically favorable pathway; Biochemistry; 01 natural sciences; two closely related; identifying metal; Environmental Sciences not elsewhere classified; dft analysis reveals; potential strategy; x </ sup; 0 </ sup; resonance raman studies; )- diazene complexes; sustaining reduction; heterocycles (< sup; copper ­(; Infectious Diseases; 2 </ sub; energy intermediates; het </ sup; symmetric h; Biotechnology; Biological Sciences not elsewhere classified; Chemical Sciences not elsewhere classified; het pendants; pendant aromatic n; Biophysics; Marine Biology; </ sup; bonded structural motifs; pyme </ sup; innocent h; tpcu ]< sub; heth </ sup; 1 </ sup; tpcu ­( cnar; stabilize diazene intermediates; marked changes; ν ­( nn; Cell Biology; electronic structure; exploring intramolecular h; 0104 chemical sciences; uncovering redox non; sup >< sup; exhibit h; 3 </ sub; trans </; aromatic n

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.