Arene Insertion Reactivity of α-Diimine-Supported Cobalt(I) Hydrides

Exposure of cyclohexane solutions of (iPrDI)Co(η3-C3H5) (iPrDI = [2,6-iPr2-C6H3N = C(CH3)]2) to a H2 atmosphere in the presence of excess arene resulted in the formation of a transient cobalt hydride [(iPrDI)Co–H], which formed the corresponding cobalt cyclohexadienyl compounds arising from arene insertion. Competing formation of a dinuclear bridging dicobalt dihydride, [(iPrDI)Co(H)]2, was identified and contributed to the observed modest yields. The dimeric structure of [(iPrDI)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 [(iPrDI)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 iPr to Me enabled the synthesis of [(MesDI)Co(η3-C3H5)(μ-N2)]2 (MesDI = 2,4,6-Me3-C6H3N = C(CH3)]2), which was characterized by X-ray diffraction. Exposure to H2 generated the putative [(MesDI)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.