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, [{(iPrBPDI)­V­(THF)}2(μ2-N2)] (iPrBPDI = 2,6-(2,6-iPr2-C6H3NCPh)2C5H3N), resulted in N–N bond
cleavage to yield the corresponding vanadium bis­(amido) derivatives,
(iPrBPDI)­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,
(iPrBPDI)­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 (iPrBPDI)­V­(NHPh)­NPh, and interconversion between this compound and (iPrBPDI)­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 (iPrBPDI)­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 (iPrBPDI)­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.