Predicting Catalytic Activity from C-13(CH) Alkylidene Chemical Shift in Cationic Tungsten Oxo Alkylidene N-Heterocyclic Carbene Complexes

A series of cationic tungsten oxo alkylidene N-heterocyclic carbene (NHC) complexes was synthesized and structurally characterized by single crystal X-ray diffraction. The C-13 NMR chemical shifts of the alkylidene C atoms of these complexes were correlated with the diamagnetic, paramagnetic and spin-orbit chemical shifts calculated by DFT. A good correlation (R-2=0.90) between the DFT isotropic chemical shifts and the experimental chemical shift as well as a strong correlation between the DFT isotropic chemical shifts and the TOF1min for the RCM of 1,7-octadiene was found. Further, a comparison of the catalyst geometries allowed for assigning tetracoordinate pseudotetrahedral catalysts to the most deshielded alkylidenes and to the highest TOF1min, pentacoordinate square-planar catalysts to the intermediate deshielded alkylidenes and intermediate TOF1min, and hexacoordinate and octahedral catalyst to the most shielded alkylidene and lowest TOF1min. Analysis of the magnetic shielding tensors allowed for ascribing variations in the chemical shifts to electronic transitions between occupied molecular orbitals corresponding to the alkylidene-C and alkylidene-H sigma-bonds and the empty molecular orbital corresponding to the W-alkylidene sigma*-bond.