Glyoxalbis(2-methylmercaptoanil) complexes of nickel and ruthenium: radical versus non-radical states

Abstract

The coordination chemistry of a N₂S₂ donor α-diimine ligand, glyoxalbis(2-methylmercaptoanil) (gmma), with transition metal ions was explored. The study authenticated that gmma is a redox non-innocent flexidentate ligand. The nickel(II) and ruthenium(II) complexes of types trans-[Ni^II(gmma)Cl₂] (1), trans-[Ru^II(gmma)Cl₂] (2) and [Ru^II(gmma)(PPh₃)Cl]Cl (3⁺Cl⁻) were isolated. Single crystal X-ray crystallography established that the diimine fragment particularly in 2 and 3⁺Cl⁻·H₂O is severely distorted. EPR spectroscopy and density functional theory (DFT) calculations authenticated that the octahedral 1⁻ and 1²⁻ ions are the gmma anion radical (gmma˙⁻) and gmma diimide (gmma²⁻) complexes of types trans-[Ni^II(gmma˙⁻)Cl₂]⁻ (1⁻) and trans-[Ni^II(gmma²⁻)Cl₂]²⁻ (1²⁻) which are unstable in solution. The chemically and electrochemically reduced ions undergo a chemical change to a dinuclear square planar nickel(II) complex of type [(μ-NSNS-gmma)Ni₂Cl₃(PF₆)] (4^PF6) in the presence of PF₆⁻ salt, predicted by ESI mass spectroscopy, where μ-NSNS-gmma is a bis(methylthio-imine) donor bridging ligand. The CSS solution of 2 is stable. In contrast to 1⁻ and 1²⁻ ions, 2⁻ in solution does not undergo any chemical change and it is defined as a resonance hybrid of trans-[Ru^II(gmma˙⁻)Cl₂]⁻ and trans-[Ru^III(gmma²⁻)Cl₂]⁻ states. Similarly, 3 is defined as a resonance hybrid of [Ru^II(gmma˙⁻)(PPh₃)Cl] and [Ru^III(gmma²⁻)(PPh₃)Cl] states. In 2⁻ and 3 the atomic spin is delocalised over both ruthenium and the α-diimine fragment, authenticated by the unrestricted DFT calculations. The UV-vis absorption spectra of the isolated complexes and their reduced analogues were analyzed by spectroelectrochemical measurements and TD DFT calculations.