Cationic iron(II) complexes of the mixed cyclopentadienyl (Cp) and the N-heterocyclic carbene (NHC) ligands as effective precatalysts for the hydrosilylation of carbonyl compounds

A series of iron(II) complexes of N-heterocyclic carbene ligands was synthesized and fully structurally characterized. Specifically, the benzimidazole based {Cp[1,3-di-R-benzimidazol-2-ylidene]-Fe(CO)2}I [R = Et (1b), i-Pr (2b) and n-Bu (3b)] and the imidazole based {Cp[1-benzyl-3-R-imidazol-2-ylidene]Fe(CO)2}PF6 [R = Me (4b) and Et (5b)] type of complexes were synthesized from their respective benzimidazolium iodide (1−3)a and their imidazolium hexafluorophosphate (4−5)a salts by the reaction with CpFe(CO)2I in the presence of MN(SiMe3)2 (M = Li or K) as a base. The molecular structures of the (1−5)b complexes reveal that the metal center display a conventional piano stool structure. More importantly, the (1−5)b complexes, when irradiated with visible light, effectively catalyzed the hydrosilylation reaction of carbonyl compounds namely, of the aldehyde and ketone substrates, using organosilane reagents. Specifically, the (1−5)b complexes performed the hydrosilylation of a representative benzaldehyde substrate using phenylsilane in ambient conditions at 30 °C while that of the representative acetophenone substrate at a more elevated temperature of 70 °C. The benzimidazole derived complexes (1−3)b displayed superior activity than the imidazole derived (4−5)b complexes.