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

A new iron complex with a thioamide SNS pincer type ligand, [FeBr2(κ(3)-H2L(DPM))] (κ(3)-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(κ(3)-L(DPM))] (κ(3)-L(DPM) = 2,6-bis(N-2,6-bis(diphenylmethyl)-4-isopropylphenyliminothiolate)pyridine). The THF molecules of [Fe(THF)2(κ(3)-L(DPM))] can be substituted with CO and CN-xylyl to give [Fe(CO)3(κ(3)-L(DPM))] and [Fe(CN-xylyl)3(κ(3)-L(DPM))], respectively. The complex [Fe(CN-xylyl)3(κ(3)-L(DPM))] reacts with HBF4 to produce [Fe(CN-xylyl)3(κ(3)-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.