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.