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)2Mn­(IMes)
followed by transmetalation on the C4 carbenic position
with Cp­(CO)2FeI led to the heterobimetallic complex Cp­(CO)2Mn­(μ-dIMes)­Fe­(CO)2Cp bearing
the anionic ditopic imidazol-2,4-diylidene dIMes
ligand. Subsequent treatment of the later with TfOH induced a selective
decoordination of the [Cp­(CO)2Mn] fragment to form the
cationic abnormal NHC complex [Cp­(CO)2Fe­(aIMes)]­(OTf), which was further derivatized to the bis­(iron) dIMes complex [Cp­(CO)2Fe­(μ-dIMes)­Fe­(CO)2Cp]­(OTf) by reaction with tAmOK and Cp­(CO)2FeI. 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 dIMes ligand was quantified through systematic experimental
and theoretical studies of IMes, aIMes, and dIMes complexes. The evaluation of the catalytic activity
of the series of cationic Fe­(II) complexes based on IMes, aIMes, and dIMes ligands in a benchmark
ketone hydrosilylation showed the superiority of the bimetallic derivative.