Predicting Catalytic Activity from 13CCH 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.

Automated summary

A series of cationic tungsten oxo alkylidene N-heterocyclic carbene (NHC) complexes were synthesized and characterized by single crystal X-ray diffraction. The correlation between the C-13 NMR chemical shifts of alkylidene C atoms and the DFT calculated chemical shifts was studied, revealing a good correlation and strong correlation with the rate of ring-closing metathesis. Analysis of catalyst geometries and magnetic shielding tensors allowed for assigning different structures to different levels of shielded alkylidene C atoms and corresponding reaction rates.