Why Two Metals Are Better Than One for Heterodinuclear Cobalt–Zirconium-Catalyzed Kumada Coupling

Heterodinuclear transition metal
complexes with a direct metal–metal
interaction offer the potential of unique reactivity compared with
mononuclear catalysts. Heterodinuclear Co–Zr complexes with
phosphinoamide ligands bridging Co and Zr metal centers are effective
precatalysts for Kumada C–C bond coupling reactions between
alkyl halides and alkyl Grignards. In contrast, the analogous mononuclear
Co tris­(phosphinoamine) complex without Zr provides very inefficient
catalysis. Here we describe density functional theory calculations
that reveal the mechanistic and reactivity impact of the Co–Zr
metal–metal interaction and phosphinoamide ligands on alkyl
halide–alkyl Grignard Kumada coupling catalysis. The Co–Zr
interaction enables two-electron reduction of the precatalyst to form
an active catalyst, which then promotes a low-energy electron-transfer
alkyl halide oxidative addition mechanism. The Co–Zr interaction
and the phosphinoamide ligands bridging the metal centers provide
a dialkyl intermediate with a low-energy C–C bond forming reductive
elimination route through a phosphine dissociation pathway, which
is not viable for an analogous mononuclear Co complex.