N-N Bond Cleavage of 1,2-Diarylhydrazines and N-H Bond Formation via H-Atom Transfer in Vanadium Complexes Supported by a Redox-Active Ligand

Addition of stoichiometric quantites of 1,2-diarylhydrazines to the bis(imino)pyridine vanadium dinitrogen complex, [{((BPDI)-B-iPr)V(THF)}(2)(mu(2)-N-2)] ((BPDI)-B-iPr = 2,6-(2,6-iPr(2)-C6H3N=CPh)(2)C5H3N), resulted in N-N bond cleavage to yield the corresponding vanadium bis(amido) derivatives, ((BPDI)-B-iPr)V(NHAr)(2) (Ar = Ph, Tol). Spectroscopic, structural, and computational studies support an assignment as vanadium(III) complexes with chelate radical anions, [BPDI](center dot-) With excess 1,2-diarylhydrazine, formation of the bis(imino)pyridine vanadium imide amide compounds, ((BPDI)-B-iPr)V(NHAr)NAr, were observed along with the corresponding aryldiazene and aniline. A DFT-computed N-H bond dissociation free energy of 69.2 kcal/mol was obtained for ((BPDI)-B-iPr)V(NHPh)NPh, and interconversion between this compound and ((BPDI)-B-iPr)V(NHPh)(2) with (2,2,6,6-tetrarnethylpiperidin-1-yl)oxidanyl (TEMPO), 1,2-diphenylhydrazine, and xanthene experimentally bracketed this value between 67.1 and 73.3 kcal/mol. For ((BPDI)-B-iPr)V(NHPh)(2), the N-H BDFE was DFT-calculated to be 64.1 kcal/mol, consistent with experimental observations. Catalytic disproportionation of 1,2-diarylhydrazines promoted by ((BPDI)-B-iPr)V(NHAr)NAr was observed, and crossover experiments established exchange of anilide (but not imido) ligands in the presence of free hydrazine. These studies demonstrate the promising role of redox-active active ligands in promoting N-N bond cleavage with concomitant N-H bond formation and how the electronic properties of the metal-ligand combination influence N-H bond dissocation free energies and related hydrogen atom transfer processes.