A unique coplanar multi-center bonding network in doubly acetylide-bridged binuclear zirconocene complexes: A density functional theory study

A unique π-conjugative interaction pattern was experimentally revealed in the doubly acetylide-bridged binuclear group 4 metallocene complexes, which was involved in C–C coupling/cleavage reactions of acetylides and σ-alkynyl migrations. To elucidate how this multi-center bonding network affects the structural and reaction properties of these complexes, density functional theory (DFT) calculations and molecular orbital (MO) analysis were carried out on the electronic structure and σ-alkynyl migration mechanisms of the doubly acetylide-bridged binuclear Zr complexes, (L2Zr)2(μ-Ctriple bond; length of mdashCH)2 (L = Cp, Cl). The B3LYP calculations suggested that the doubly [σ,π] acetylide-bridged complex C2h-(L2Zr)2(μ-Ctriple bond; length of mdashCH)2 was produced by the reaction of L2Zr(Ctriple bond; length of mdashCH)2 with L2Zr through a C2v-(L2Zr)2(μ-Ctriple bond; length of mdashCH)2 intermediate followed by an isomerization process. In particular, the isomerization of C2h- or C2v-(L2Zr)2(μ-Ctriple bond; length of mdashCH)2 is almost thermoneutral through a low barrier of 15.3–17.0 kcal/mol. The MO Walsh diagram revealed that the two isomers have a very similar six-center-six-electron bonding network. The coplanar π-conjunctive interaction by the electron donating and back-donating interactions between the metal centers and acetylide ligands significantly stabilizes the doubly acetylide-bridged binuclear group 4 metallocene complexes and the isomerization transition state.