Reversible Insertion of Carbenes into Ruthenium–Silicon Bonds

The five-coordinate carbene complexes
[Ru­{κP,P,Si-Si­(Me)­(C6H4-2-PiPr2)2}­Cl­(CHR)]
(2, R = Ph; 3, R = SiMe3), analogues
of the Grubbs catalyst, were prepared from the dimer [Ru­(μ-Cl)­{κP,P,Si-Si­(Me)­(C6H4-2-PiPr2)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{κP,P,Si-Si­(Me)­(C6H4-2-PiPr2)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.