Double C(sp3) dehydrogenation as a route to coordinated Arduengo carbenes:: experiment and computation on comparative π-acidity

Reaction of [RuHClL2](2) (L = (PPr3)-Pr-i) with the C/C unsaturated cyclic carbene:C(NMeCH)(2) produces the 16-electron square-pyramidal RuHCl[CNMeCH=CHNMe]L-2 by a chloride bridge-splitting reaction. Double H2C(sp(3)) dehydrogenation of cyclic H2C(NMeCH2)(2) is successful for producing the C/C saturated carbene (boundto Ru); the two hydrogens removed are found as RuHCl(H-2)L-2. In this case, the free carbene is unstable with respect to dimerization to the olefin. The C-13 chemical shifts of the carbene carbons of these two complexes, the Ru/C distance, the N-C(carbene) distance, and a variety of reaction energies (from DFT calculations) and calculated atomic charges are generally consistent with these two carbenes, aromatic and non-aromatic, both binding similarly, and with little back donation from this electron-rich center. The C-13 chemical shifts are perhaps the most sensitive parameter. Collectively, these results suggest that, if the C/C unsaturated and the C/C saturated Arduengo carbenes differ in their binding to this electron-rich metal center, the difference is at or below detection limits.