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)(2)I 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 degrees C while that of the representative acetophenone substrate at a more elevated temperature of 70 degrees C. The benzimidazole derived complexes (1-3)b displayed superior activity than the imidazole derived (4-5)b complexes. (C) 2014 Elsevier B. V. All rights reserved.