Deprotonation/protonation-driven change of the σ-donor ability of a sulfur atom in iron(ii) complexes with a thioamide SNS pincer type ligand

Abstract

A new iron complex with a thioamide SNS pincer type ligand, [FeBr₂(κ³-H₂L^DPM)] (κ³-H₂L^DPM = 2,6-bis(N-2,6-bis(diphenylmethyl)-4-isopropylphenylthioamide)pyridine), was synthesized. This complex reacts with NaH in THF to yield a unique Fe(II) complex with two THF molecules, [Fe(THF)₂(κ³-L^DPM)] (κ³-L^DPM = 2,6-bis(N-2,6-bis(diphenylmethyl)-4-isopropylphenyliminothiolate)pyridine). The THF molecules of [Fe(THF)₂(κ³-L^DPM)] can be substituted with CO and CN-xylyl to give [Fe(CO)₃(κ³-L^DPM)] and [Fe(CN-xylyl)₃(κ³-L^DPM)], respectively. The complex [Fe(CN-xylyl)₃(κ³-L^DPM)] reacts with HBF₄ to produce [Fe(CN-xylyl)₃(κ³-H₂L^DPM)]²⁺ with protonated thioamide units. The differences of the IR spectra before and after protonation indicate that the major binding mode of CN-xylyl to iron(II) changes from π-back donation from metal to isocyanide to σ-donation from isocyanide to iron(II). This indicates that the σ-donor ability of the thioamide sulfur atom is tuned by deprotonation/protonation of thioamide.