Helical dinuclear transition metal complexes with bis(bidentate) [S,N] ligands: synthesis, structural and computational studies

A diprotic bis(β-thioketoimine) ligand featuring a flexible 4,4’-methylbis(aniline) linker, H22, was synthesised via treatment of the corresponding bis(β-ketoimine) with Lawesson’s reagent. Proligand H22 underwent lithiation and coordination with one equivalent of d-block metal(II) chlorides MCl2(THF)x (M = Fe, Co and Zn) to yield a corresponding series of homoleptic dinuclear complexes, [M2(μ-2)2]. X-ray diffraction analysis reveals a tetrahedral geometry for the two metals and a double-stranded helicate structure arising from inter-strand face-face π-stacking. These interactions create a helical ‘twist’ of ca. 70°. Utilising a bulky mononucleating β-thioketoiminate ligand, [3]–, the analogous series of heteroleptic monometallic complexes, [M(3)2] (M = Fe, Co and Zn), were prepared and characterised by spectroscopic, analytical and X-ray diffraction techniques. A comprehensive DFT study of all complexes reveals a stronger M–S bonding compared to M–N due to a higher degree of covalency. Solution magnetic studies and Natural Bonding Orbital calculations are consistent with high-spin tetrahedral Fe(II) and Co(II) centres, and cyclic voltammetry reveals both oxidation and reduction processes are accessible.