A 1,2,3-triazole-derived pincer-type mesoionic carbene complex of iron(ii): carbonyl elimination and hydrosilylation of aromatic aldehydes via the concerted reaction with hydrosilane and a base

Abstract

Iron complexes bearing 1,2,3-triazol-5-ylidene were synthesized and applied to the reaction with hydrosilane and homogeneous catalytic hydrosilylation of aromatic ketones and aldehydes. Addition of a free carbene to a solution of Fe(CO)₄Br₂ yielded an octahedral, diamagnetic and cationic iron(II) complex [Fe(1,2,3-triazolylidene)(CO)₂Br]⁺. Pyrolysis of the dicarbonyl complex eliminated the two CO ligands to form a paramagnetic four-coordinate complex. A theoretical study using DFT calculations indicated that the spin state changed from singlet to quintet during ligand elimination. Investigations of the successful hydrosilylation of acetophenone and benzaldehyde derivatives using MIC–iron(II) bromide suggested the importance of the base for efficient conversion in the catalytic process. The bromide-to-hydride exchange reaction, transmetallation, of MIC–iron(II) bromide in the presence of KO^tBu and HSi(OEt)₃ which could occur in the initial process of hydrosilylation was proposed, and supported by a theoretical study.