Reversible Insertion of Carbenes into Ruthenium-Silicon Bonds

The five-coordinate carbene complexes [Ru{kappa P,P,Si-Si(Me)(C6H4-2-PiPr(2))(2)}Cl(=CHR)] (2, R = Ph; 3, R = SiMe3), analogues of the Grubbs catalyst, were prepared from the dimer [Ru(mu-Cl){kappa P,Si-Si(Me)(C6H4-2-PiPr(2))(2)}](2) (1) and the corresponding diazoalkane N2CHR. The particular structural features that result from the presence of a strongly trans directing silyl group at the pincer ligand of these complexes are discussed on the basis of NMR information and the crystal structure of the vinylidene analogue [Ru{kappa P,P,Si-Si(Me)(C6H4-2-PiPr(2))(2)}Cl(=C=CHPh)] (4), which was also obtained from 1 and phenylacetylene. The reactions of 3 with reagents such as P(OMe)(3), CO, NCMe, and K(acac) illustrate that the first response of these carbene complexes to an increase of the coordination number around ruthenium is the insertion of the carbene ligand into the Ru-Si bond. These reactions also indicate that the insertion process is reversible and allows typical transformations of carbene ligands such as C-H functionalizations via carbene insertion (in the acac ligand) or the formation of ketene from CO. In addition, the reactions of 3 with terminal alkynes such as phenylacetylene or 3,3-dimethyl-1-butyne show that the inserted carbenes can also undergo reactions typical of metal-bound alkyls such as alkyne insertion and C-H reductive elimination.