(Cyclopentadienyl)iron(II) complexes of N-heterocyclic carbenes bearing a malonate or imidate backbone: Synthesis, structure, and catalytic potential in hydrosilylation
Résumé
The backbone-functionalized anionic carbenes maloNHC (1R; malonate backbone) and imidNHC (2; imidate backbone) were generated in situ from their respective zwitterionic precursors and treated with FeCp(CO)2I to afford the zwitterionic complexes {FeCp(CO)2(1R)} (3R; 59-84% yield), and {FeCp(CO)2(2)} (4; 77% yield), respectively. Methylation of the malonate complex 3Me takes place at one of the backbone oxygen atoms to give the cationic adduct [FeCp(CO)2(1MeMe)](OTf) ([5Me](OTf); 96% yield), whereas methylation of 4 takes place at the imidate nitrogen atom to produce the cationic adduct [FeCp(CO)2(2Me)](OTf) ([6Me](OTf); 84% yield). All of the complexes were characterized by NMR and IR in solution, while X-ray structure analyses were carried out for 3Me, 4, and [6Me](OTf). In addition, a detailed experimental and theoretical investigation of the electron density within the archetypal zwitterionic complex 3Me was carried out. The observation of short intramolecular contacts between Cipso or Cortho of the mesityl groups of the carbene and the proximal carbonyl groups is rationalized in terms of a noncovalent "through space" π-π* interaction involving a two-electron delocalization of the occupied π(Cipso=Cortho) molecular orbital (MO) of the aryl ring into one vacant π*(C=O) MO of the carbonyl ligand. A theoretical analysis carried out on dissymmetrical model complexes reveals that the magnitude of such an interaction is correlated with the donor properties of aryl group substituents. A catalyst screening of the above complexes in the hydrosilylation of benzaldehyde under visible light irradiation revealed a dramatic effect of the electronic donor properties of these carbenes on the performances of their complexes, with the more nucleophilic carbene 1tBu- in the zwitterionic species 3tBu appearing as the most efficient. This complex shows good efficiency and excellent chemoselectivity in the hydrosilylation of various aldehydes bearing reactive functional groups. It is also moderately active in the hydrosilylation of a few ketone substrates and exhibits very good performance in the hydrosilylation of representative aldimines and ketimines.