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.