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

Abstract

Reaction of [RuHClL2]2 (L=PiPr3) with the C/C unsaturated cyclic carbene:C(NMeCH)2 produces the 16-electron square-pyramidal RuHCl[CNMeCHCHNMe]L2 by a chloride bridge-splitting reaction. Double H2C(sp3) dehydrogenation of cyclic H2C(NMeCH2)2 is successful for producing the C/C saturated carbene (bound to Ru); the two hydrogens removed are found as RuHCl(H2)L2. In this case, the free carbene is unstable with respect to dimerization to the olefin. The 13C 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 13C 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.