Imidazo[1,5-a]pyridin-3-ylidenes as π-accepting carbene ligands: substituent effects on properties of N-heterocyclic carbenes.

Various 1-substituted-imidazo[1,5-a]pyridin-3-ylidenes were prepared and characterized. The fundamental character and the effects of substituents on the imidazo[1,5-a]pyridine backbone on the electronic character of carbenes were evaluated using rhodium complexes and selenium adducts. C NMR chemical shifts of carbene carbons in imidazo[1,5-a]pyridin-3-ylidene and H and C NMR spectra of olefin moieties trans to the carbene in Rh(nhc)(cod)Cl complexes shifted relatively downfield compared to those for the corresponding complexes bearing conventional N-heterocyclic carbenes (NHCs). The IR peak of the trans-carbonyl moiety of Rh(nhc)(CO)Cl complexes appeared at a higher wavenumber than for conventional NHCs. These observations suggest that imidazo[1,5-a]pyridin-3-ylidene ligands have strong π-accepting character. Rh(nhc)(CO)Cl complexes were further characterized by X-ray diffraction analyses. To obtain further insight into the electronic character of imidazo[1,5-a]pyridin-3-ylidenes, selenoureas were prepared. We found that a combination of both the Se NMR chemical shift and J coupling constant of C-Se bonds are appropriate for a rigorous evaluation of the electronic characters of imidazo[1,5-a]pyridin-3-ylidenes, but did not correspond to those for conventional NHCs. DFT calculations for imidazo[1,5-a]pyridin-3-ylidenes revealed that a hybrid accepting orbital comprised of a vacant p-orbital of carbene and a π* orbital of the pyridine ring newly occurred to result in significantly increased π-accepting character. Finally, the strong π-accepting character of the ligand was demonstrated by the Rh-catalyzed polymerization of phenylacetylene.