Getty, K.; Delgado-Jaime, M. U.; Kennepohl, P. An electronic rationale for observed initiation rates in ruthenium-mediated olefin metathesis: charge donation in phosphine and N-heterocyclic carbene ligands. J. Am. Chem. Soc. 2007, 129, 15774-15776.
The simple substitution of N-heterocyclic carbene (NHC) ligands for phosphines in ruthenium olefin metathesis catalysts resulted in a significant increase in catalyst performance. The greater basicity and stronger σ-donating properties of the NHCs compared to phosphines have been generally accepted to account for the greater catalyst activity. However, recent studies are starting to paint a different picture about the electronic differences in metathesis catalysts with phosphines vs. NHCs.
Kennepohl and coworkers have utilized X-ray absorption spectroscopy to directly measure the electronic environment at the metal center on 4, 5, and 6 coordinate 1st and 2nd generation-type catalysts. From comparing ionization energies, the data show that the charge on the metal ion is larger in NHC bearing catalysts than their phosphine analogs. In addition, theoretical calculations show stronger pi-acceptor character associated with NHC ligation than with the bis-phosphine catalysts. These results support the slower initiation kinetics and higher reactivity with olefins (once initiated) observed experimentally with the NHC catalysts.
The results from this study are an attempt to discern the activity increase when using 1stversus 2nd generation catalysts. More examples of catalysts bearing strong σ-donating and pi-accepting ligands could prove as useful support for the presented idea.