d0 Re-based olefin metathesis catalysts, Re([triple bond]CR)(=CHR)(X)(Y): the key role of X and Y ligands for efficient active sites

DFT(B3PW91) calculations show that the reaction pathways for ethylene metathesis with Re(triple bond]CMe)(=CHMe)(X)(Y) (X/Y = CH2CH3/CH2CH3; CH2CH3/OSiH3; OSiH3/CH2CH3; OCH3/OCH3, CH2CH3/OCH3, and OCF3/OCF3) occur in two steps: first, the pseudo-tetrahedral d0 Re complexes distort to a trigonal pyramid to open a coordination site for ethylene, which remains far from Re (early transition state for C-C bond formation). The energy barrier, determined by the energy required to distort the catalyst, is the lowest for unsymmetrical ligands (X not equal Y) when the apical site of the TBP is occupied by a good sigma-donor ligand (X) and the basal site by a poor sigma-donor (Y). Second, the formation of metallacyclobutanes (late transition state for C-C bond formation) has a low energy barrier for any type of ligands, decreasing for poor sigma-donor X and Y ligands, because they polarize the Re-C alkylidene bond as Re(+delta)=C(-delta), which favors the reaction with ethylene, itself polarized by the metal center in the reverse way. The metallacyclobutane is also a TBP, with apical alkylidyne and Y ligands, and it is stabilized by poor sigma-donor X and Y. The best catalyst will have the more shallow potential energy surface, and will thus be obtained for the unsymmetrical set of ligands with X = a good sigma-donor (alkyl) and Y = a poor sigma-donor (O-based ligand). This rationalizes the high efficiency of well-defined Re alkylidene supported on silica, compared to its homogeneous equivalent, Re(triple bond]CMe)(=CHMe)(OR)2.