Understanding Structural and Dynamic Properties of Well-Defined Rhenium-Based Olefin Metathesis Catalysts, Re(⋮CR)(CHR)(X)(Y), from DFT and QM/MM Calculations

DFT(B3PW91) and QM/MM (B3PW91:UFF) calculations have been carried out to rationalize the structural and dynamical properties of quasi-tetrahedral alkylidyne-alkylidene rhenium complexes Re(CR)(CHR)(X)(Y) (R = alkyl, X = Y = alkyl; X = alkyl, Y = siloxy; X = Y = alkoxy), which are catalysts for olefin metathesis with efficiency depending on X and Y. The optimized geometries of all complexes are pseudo-tetrahedral and yield syn and anti isomers. The presence of the C−H agostic interaction in the syn isomers is evidenced by geometrical features as well as the ν C - H stretching frequencies and J C - H NMR coupling constants calculated to be lower than in the anti isomers, as observed experimentally in the case of the J C - H . Ancillary X and Y ligands that are weak σ-donors and π-donors (OR) compete with the agostic interaction. The calculated syn/anti ratios are in good agreement with experimental data. The syn/anti interconversion occurs preferentially via the alkylidene rotation, while H transfers between perhydrocarbyl ligands have much higher energy barriers, showing that this process does not compete with alkylidene rotation. Molecular orbital, natural bonding orbital (NBO), and Bader atoms-in-molecules (AIM) analyses have been carried out to rationalize the results.