Catalytic N-H Bond Formation Promoted by a Ruthenium Hydride Complex Bearing a Redox-Active Pyrimidine-Imine Ligand

The synthesis of a piano-stool ruthenium hydride, [(Ε⁵-C₅Me₅)Ru(PmIm)H] (PmIm = (N-(1,3,5-trimethylphenyl)-1-(pyrimidin-2-yl)ethan-1-imine), for the dual purpose of catalytic dihydrogen activation and subsequent hydrogen atom transfer for the formation of weak chemical bonds is described. The introduction of a neutral, potentially redox-active PmIm supporting ligand was designed to eliminate the possibility of deleterious C(sp²)-H reductive coupling and elimination that has been identified as a deactivation pathway with related rhodium and iridium catalysts. Treatment of [(Ε⁵-C₅Me₅)RuCl₂]_nwith one equivalent PmIm ligand in the presence of zinc and sodium methoxide resulted in the isolation of the diruthenium complex, [(Ε⁵-C₅Me₅)Ru(PmIm)]₂, arising from the C-C bond formation between two PmIm chelates. Addition of H₂to the ruthenium dimer under both thermal and blue light irradiation conditions furnished the targeted hydride, [(Ε⁵-C₅Me₅)Ru(PmIm)H], which has a relatively weak DFT-calculated Ru-H bond dissociation free energy (BDFE) of 47.9 kcal/mol. Addition of TEMPO to [(Ε⁵-C₅Me₅)Ru(PmIm)H] generated the 17-electron metalloradical, [(Ε⁵-C₅Me₅)Ru(PmIm)], which was characterized by EPR spectroscopy. The C-C bond forming process was reversible as the irradiation of [(Ε⁵-C₅Me₅)Ru(PmIm)]₂generated [(Ε⁵-C₅Me₅)Ru(PmIm)H] and a piano-stool ruthenium complex containing an enamide ligand derived from H-atom abstraction from the PmIm chelate. Equilibration studies were used to establish an experimental estimate of the effective Ru-H BDFE, and a value of 50.8 kcal/mol was obtained, in agreement with the observed loss of H₂and the DFT-computed value. The ruthenium hydride was an effective catalyst for the thermal catalytic hydrogenation of TEMPO, acridine, and a cobalt-imido complex and for the selective reduction of azobenzene to diphenylhydrazine, highlighting the role of this complex in catalytic weak bond formation using H₂as the stoichiometric reductant.