Experimental and Theoretical Insights into the Electronic Properties of Anionic N-Heterocyclic Dicarbenes through the Rational Synthesis of Their Transition Metal Complexes.

The lithiation of the NHC ligand backbone in Cp(CO)Mn(IMes) followed by transmetalation on the C4 carbenic position with Cp(CO)FeI led to the heterobimetallic complex Cp(CO)Mn(IMes)Fe(CO)Cp bearing the anionic ditopic imidazol-2,4-diylidene IMes ligand. Subsequent treatment of the later with TfOH induced a selective decoordination of the [Cp(CO)Mn] fragment to form the cationic abnormal NHC complex Cp(CO)Fe(IMes), which was further derivatized to the bis(iron) IMes complex Cp(CO)Fe(IMes)Fe(CO)Cp by reaction with AmOK and Cp(CO)FeI. The effect of the metalation at the C4 or C2 position on the imidazole ring on the electronic donation properties of the associated C2 and C4 carbenic centers in the IMes ligand was quantified through systematic experimental and theoretical studies of IMes, IMes, and IMes complexes. The evaluation of the catalytic activity of the series of cationic Fe(II) complexes based on IMes, IMes, and IMes ligands in a benchmark ketone hydrosilylation showed the superiority of the bimetallic derivative.