Helical dinuclear 3d metal complexes with bis(bidentate) [S,N] ligands: synthesis, structural and computational studies

Abstract

A diprotic bis(β-thioketoimine) ligand precursor featuring a flexible 4,4′-methylbis(aniline) linker, H₂2, was synthesised via treatment of the corresponding bis(β-ketoimine) with Lawesson's reagent. Lithiation of H₂2 and coordination with one equivalent of d-block metal(II) chlorides MCl₂(THF)_x (M = Fe, Co and Zn) yielded 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 homoleptic monometallic complexes, [M(3)₂] (M = Fe, Co and Zn), were prepared and characterised by spectroscopic and analytical 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 on the mono- and dinuclear iron and cobalt complexes are consistent with high-spin tetrahedral Fe(II) and Co(II) centres, and cyclic voltammetry reveals both oxidation and reduction processes are accessible.