Protonation of a Cobalt Phenylazopyridine Complex at the Ligand Yields a Proton, Hydride, and Hydrogen Atom Transfer Reagent

Protonation of the Co(I) phenylazopyridine (azpy) complex [CpCo(azpy)] 2 occurs at the azo nitrogen of the 2-phenylazopyridine ligand to generate the cationic Co(I) complex [CpCo(azpyH)](+) 3 with no change in oxidation state at Co. The N-H bond of 3 exhibits diverse hydrogen transfer reactivity, as studies with a variety of organic acceptors demonstrate that 3 can act as a proton, hydrogen atom, and hydride donor. The thermodynamics of all three cleavage modes for the N-H bond (i.e., proton, hydride, and hydrogen atom) were examined both experimentally and computationally. The N-H bond of 3 exhibits a pK(a) of 12.1, a hydricity of Delta G degrees(H-) = 89 kcal/mol, and a bond dissociation free energy (BDFE) of Delta G degrees(H center dot) = 68 kcal/mol in CD3CN. Hydride transfer from 3 to the trityl cation (Delta G degrees(H-) = 99 kcal/mol) is exergonic but takes several hours to reach completion, indicating that 3 is a relatively poor hydride donor, both kinetically and thermodynamically. Hydrogen atom transfer from 3 to 2,6-di-tert-butyl-4-(4'-nitrophenyl)phenoxyl radical ((Bu2NPArO center dot)-Bu-t, Delta G(H center dot) = 77.8 kca/mol) occurs rapidly, illustrating the competence of 3 as a hydrogen atom donor.