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

Abstract

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, (L 2Zr) 2(μ-C CH) 2 (L = Cp, Cl). The B3LYP calculations suggested that the doubly [σ,π] acetylide-bridged complex C 2 h -(L 2Zr) 2(μ-C CH) 2 was produced by the reaction of L 2Zr(C CH) 2 with L 2Zr through a C 2 v -(L 2Zr) 2(μ-C CH) 2 intermediate followed by an isomerization process. In particular, the isomerization of C 2 h - or C 2 v -(L 2Zr) 2(μ-C CH) 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.