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.

ABSTRACT

Addition of stoichiometric quantites of 1,2-diarylhydrazines to the bis(imino)pyridine vanadium dinitrogen complex, [{((iPr)BPDI)V(THF)}2(µ2-N2)] ((iPr)BPDI = 2,6-(2,6-iPr2-C6H3N═CPh)2C5H3N), resulted in N-N bond cleavage to yield the corresponding vanadium bis(amido) derivatives, ((iPr)BPDI)V(NHAr)2 (Ar = Ph, Tol). Spectroscopic, structural, and computational studies support an assignment as vanadium(III) complexes with chelate radical anions, [BPDI](•-). With excess 1,2-diarylhydrazine, formation of the bis(imino)pyridine vanadium imide amide compounds, ((iPr)BPDI)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 ((iPr)BPDI)V(NHPh)NPh, and interconversion between this compound and ((iPr)BPDI)V(NHPh)2 with (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO), 1,2-diphenylhydrazine, and xanthene experimentally bracketed this value between 67.1 and 73.3 kcal/mol. For ((iPr)BPDI)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 ((iPr)BPDI)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.