PNNL

Abstract

The research described in this product was performed in part in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. A unique p-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 r-alkynyl migrations. To elucidate how this multicenter 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 r-alkynyl migration mechanisms of the doubly acetylide-bridged binuclear Zr complexes, (L2Zr)2(l-C„CH)2 (L = Cp,Cl). The B3LYP calculations suggested that the doubly [r,p] acetylide-bridged complex C2h-(L2Zr)2(l-C„CH)2 was produced by the reaction of L2Zr(C„CH)2 with L2Zr through a C2v- (L2Zr)2(l-C„CH)2 intermediate followed by an isomerization process. In particular, the isomerization of C2h- or C2v-(L2Zr)2(l- 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 p-conjunctive interaction by the electron donating and backdonating interactions between the metal centers and acetylide ligands significantly stabilizes the doubly acetylide-bridged binuclear group 4 metallocene complexes and the isomerization transition state.