Abstract
Exposure of cyclohexane solutions of ((iPr)DI)-Co-(η(3)-C(3)H(5)) ((iPr)DI = [2,6-(i)Pr(2)-C(6)H(3)N = C-(CH(3))](2)) to a H(2) atmosphere in the presence of excess arene resulted in the formation of a transient cobalt hydride [((iPr)DI)-Co-H], which formed the corresponding cobalt cyclohexadienyl compounds arising from arene insertion. Competing formation of a dinuclear bridging dicobalt dihydride, [((iPr)DI)-Co-(H)](2), was identified and contributed to the observed modest yields. The dimeric structure of [((iPr)DI)-Co-(H)](2) was confirmed in the solid state by X-ray diffraction, and the presence of two bridging hydride ligands was quantified by a Toepler pump experiment. The putative monomeric [((iPr)DI)-Co-H] favored insertion at the ortho and meta positions of representative monosubstituted arenes. Reducing the size of the 2,6-aryl substituents on the α-diimine ligand from (i)Pr to Me enabled the synthesis of [((Mes)DI)-Co-(η(3)-C(3)H(5))-(μ-N(2))](2) ((Mes)DI = 2,4,6-Me(3)-C(6)H(3)N = C-(CH(3))](2)), which was characterized by X-ray diffraction. Exposure to H(2) generated the putative [((Mes)DI)-Co-H] whose reduced steric profile supported insertion of more sterically demanding arenes such as tert-butylbenzene and xylenes, with ortho- and meta-substituent site selectivity. Overall, the ligand donacity conferred by the α-diimine ligand promoted unique reactivity and selectivity on the process of arene insertion into a cobalt-hydride bond, as compared to reported bis-(phosphine) cobalt complexes.