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

A diprotic bis(beta-thioketoimine) ligand precursor featuring a flexible 4,4 '-methylbis(aniline) linker, H22, was synthesised via treatment of the corresponding bis(beta-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(mu-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 pi-stacking. These interactions create a helical 'twist' of ca. 70 degrees. Utilising a bulky mononucleating beta-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. A dinucleating [S,N] ligand with a flexible linker forms homobimetallic complexes with Fe, Co and Zn. These show a double-stranded helicate structure, stabilised by intramolecular pi-pi interactions, with homochirality at the two tetrahedral centres.