Well-Defined Cobalt(I) Dihydrogen Catalyst: Experimental Evidence for a Co(I)/Co(III) Redox Process in Olefin Hydrogenation

The synthesis of a cobalt dihydrogen Co-I-(H-2) complex prepared from a Co-I-(N-2) precursor supported by a monoanionic pincer bis(carbene) (CCC)-C-Mes ((CCC)-C-Mes = bis(mesityl-benzimidazol-2-ylidene)phenyl), is described. This species is capable of H-2/D-2 scrambling and hydrogenating alkenes at room temperature. Stoichiometric addition of HCl to the Co-I-(N-2) cleanly affords the Co-III hydridochloride complex, which, upon the addition of Cp2ZrHCl, evolves hydrogen gas and regenerates the Co-I-(N-2) complex. Furthermore, the catalytic olefin hydrogenation activity of the Co-I species was studied by using multinuclear and parahydrogen (p-H-2) induced polarization (PHIP) transfer NMR studies to elucidate catalytically relevant intermediates, as well as to establish the role of the Co-I-(N-2) in the Co-I/Co-III redox cycle.