d0 Re-Based Olefin Metathesis Catalysts, Re(⋮CR)(CHR)(X)(Y): The Key Role of X and Y Ligands for Efficient Active Sites

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Abstract
DFT(B3PW91) calculations show that the reaction pathways for ethylene metathesis with Re(⋮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 ≠ Y) when the apical site of the TBP is occupied by a good σ-donor ligand (X) and the basal site by a poor σ-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 σ-donor X and Y ligands, because they polarize the Re−C alkylidene bond as Re+δC-δ, 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 σ-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 σ-donor (alkyl) and Y = a poor σ-donor (O-based ligand). This rationalizes the high efficiency of well-defined Re alkylidene supported on silica, compared to its homogeneous equivalent, Re(⋮CMe)(CHMe)(OR)2.