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.