Homoleptic Mononuclear Tris-Chelate Complexes of FeII, CoII, NiII, and ZnII Based on a Redox-Active Imidazolyl-2-thione Ligand: Structural and Electrochemical Correlation

Five mononuclear tris-chelate complexes of Fe-II, Co-II, Ni-II and Zn-II, containing the bidentate ligand 3-methyl-1-(2-pyridyl)imidazolyl-2-thione (L), have been synthesised and characterised, namely [M-II(L)(3)](BF4)(2), where M = Fe (1), Co (2), Ni (3) and Zn (4), and [Fe-II(L)(3)](PF6)(2) {1-(PF6)(2)}. The complexes have been characterised by standard methods and single-crystal X-ray diffraction, showing that all the complexes are isostructural and isomorphs, crystallising in the P1 space group, except for 1-(PF6)(2), which crystallises in the P2(1)/c, as expected due to the different size and symmetry of the anion. The metal centre is found in a distorted meridional-N3S3 octahedral geometry. In all the complexes, two ligand strands within the same complex cation unit interact via an almost perfect face-to-face pi-pi stacking. The double intramolecular interaction occurs between a pyridyl ring of one ligand strand and an imidazolyl ring of a second ligand strand and vice versa. The Fe-II and Co-II complexes, 1 and 2, are stabilised in the high spin state, based on the bond lengths and angles obtained by SC-XRD and by VT-magnetic studies in the solid state and NMR Evans method in CD3CN solution. The cyclic voltammetry behaviour for compounds 1-4 showed an EC mechanism for the oxidation process, in which two ligand strands are electrochemically oxidised, instead of the metal centre, followed by a chemical process (de-coordination and dimerisation). Whereas, the reduction process involved metal electrodeposition for the Fe-II, Ni-II and Zn-II complexes. In the case of the Co-II complex, the voltammetric response suggested a multistep decomposition process during the electrochemical reduction. Finally, a linear correlation between the oxidation potential of the ligand and the average M-N bond length has been found, permitting the modification of the oxidation potential by selection of the metal centre.