The influence of the cationic carbenes on the initiation kinetics of ruthenium-based metathesis catalysts; a DFT study

Abstract

Cationic carbenes are a relatively new and rare group of ancillary ligands, which have shown their superior activity in a number of challenging catalytic reactions. In ruthenium-based metathesis catalysis they are often used as ammonium tags, to provide water-soluble, environment-friendly catalysts. In this work we performed computational studies on three cationic carbenes with the formal positive charge located at different distances from the carbene carbon. We show that the predicted initiation rates of Grubbs, indenylidene, and Hoveyda–Grubbs-like complexes incorporating these carbenes show little variance and are similar to initiation rates of standard Grubbs, indenylidene, and Hoveyda–Grubbs catalysts. In all investigated cases the partial charge of the carbene carbon atom is similar, resulting in comparable Ccarbene–Ru bond strengths and Ru–P/O dissociation Gibbs free energies.

Impact of the Carbene Derivative Charge on the Decomposition Rates of Hoveyda–Grubbs-like Metathesis Catalysts

“…These results are consistent with our former calculations and experimental results, indicating that the introduction of ammonium tags at different distances from the carbene core has limited influence on the catalytic properties of ruthenium metathesis catalysts bearing such carbenes. 31 , 32 , 51 …”

“…These results reinforce the idea that the influence of the additional +1 charge on the catalytic properties of Hoveyda–Grubbs catalysts is negligible, while the impact of the additional −1 charge is small, but may lead to substantial differences in catalytic activities. 32 In the case of 4 / 5 , the differences, however, can also be attributed to the lack of the mesityl moieties, which were shown to incorporate the excess negative charge. 52 …”

“…This is in line with our previous investigation and a number of experimental studies showing that the additional +1 charge on carbene has also little impact on the overall reactivity of ruthenium metathesis catalysts. 32 , 51 Considering the bimolecular coupling decomposition pathway, however, the formal positive charge close to the imidazole moiety destabilizes the diruthenium intermediate, making this pathway highly unlikely for positively charged carbenes.…”

“… 26 , 28 − 30 Interestingly, it has been shown both experimentally and theoretically that the impact of the additional positive charge on the reactivity of such systems is very low. 31 , 32 …”

Molecular Modeling of Mechanisms of Decomposition of Ruthenium Metathesis Catalysts by Acrylonitrile

“…Others have pointed out that M06-D3 may overestimate the effect of dispersion due to double-counting of these effects . Our recent benchmark study for ruthenium metathesis catalysts shows that the use of this correction is crucial for obtaining accurate values for M06 functional for initiation of the Hoveyda–Grubbs-type catalysts . Since, many different reactions are studied in this work, we decided to include and discuss both the D3-corrected and uncorrected (in parentheses in all figures) Gibbs free energies.…”