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.

Nickel(II) complexes containing a pyrrole-diphosphine pincer ligand

A new pincer ligand, (P(2)(Ph)Pyr)(-), based on the anion of 2,5-bis[(diphenylphosphino)methyl]pyrrole has been prepared in four steps from pyrrole. The ligand undergoes oxidation to diphosphine oxide under ambient conditions and was therefore isolated as its borane adduct, H(P(2)(Ph)Pyr)·2BH(3) (2). Delivery of the ligand to nickel(II) was accomplished by the direct reaction of NiCl(2) with 2 in the presence of Et(2)NH to afford [NiCl(P(2)(Ph)Pyr)]. Salt metathesis reactions of the chloro complex afford new compounds including [Ni(CH(3))(P(2)(Ph)Pyr)] and [Ni(NCCH(3))(P(2)(Ph)Pyr)](OTf). In all cases, the ligand gives rise to diamagnetic square-planar complexes, which have been fully characterized in solution and the solid state. All complexes examined display an irreversible oxidation to nickel(III) according to cyclic voltammetry. Reduction of the chloro complex in dichloromethane results in an electrocatalytic process, whereas reduction in tetrahydrofuran leads to the irreversible formation of a nickel(I) species.