Olefin Coupling Catalyzed by (Pybox)Os Complexes via Osmacyclopentane Intermediates: Comparison with Isoelectronic (Phebox)Ir

(Pybox)Os is found to catalyze alkene hydrovinylation, effecting the dimerization of ethylene, tail-to-tail coupling of propene and 1-butene, and cross-coupling of ethylene with higher α-olefins. This reactivity contrasts with the previously reported dehydrogenative coupling of ethylene to give butadiene catalyzed by the isoelectronic fragment (Phebox)-Ir. The reaction mechanism was investigated through computational and experimental means. Both the Os- and Ir-catalyzed reactions proceed through a [2 + 2 + 1] cyclization of the corresponding bis-olefin complex to yield an experimentally observed metallacyclopentane intermediate. In both cases, the metallacyclopentane undergoes β-H elimination, via a dechelated κ²-pincer-ligated intermediate, to yield a σ-π-but-3-enyl hydride complex or derivative. Both the greater reactivity and the distinct chemoselectivity of the Os system relative to the Ir system are attributable to C-H reductive elimination by the σ-π-but-3-enyl hydride having a barrier for Os much lower than that for Ir. This lower barrier to C-H elimination for Os is unexpected given that the thermodynamic driving force for elimination is much less for Os than for Ir. Computational studies of model complexes were conducted, comparing (Pybox)-Os-(L)-(CH₃)-(H) with the isoelectronic (Phebox)-Ir-(L)-(CH₃)-(H). The results indicate that the more facile kinetics with Os relative to Ir may be general for C-H elimination from six-coordinate d⁶ complexes of the two metals, as well as for the microscopic reverse, i.e., C-H addition to the corresponding four-coordinate d⁸ species.