Modification of a Hexadentate Amine based Ligand System by N-Methylation and Effects on Spin State and Redox Behavior of the Corresponding Transition Metal Complexes

Recently, the synthesis of hexadentate ligands based on N, N'-bis-(2,2'-bipyridine-6-ylmethyl)-2,2'-biphenylenediamine (1a-d) and the corresponding iron(II) complexes (3a-d) was reported by our group. In this contribution we present the synthesis of the analogous cobalt(II) complexes 4a-d. Together with the iron(II) complex the electrochemical behavior of the complexes 3 and 4 was investigated by cyclic voltammetry (CV). The aminomethyl substructure was identified as the main source of ligand degradation caused by chemical oxidation with air. Upon exposure to air the amine group in complexes 3 and 4 is oxidized to imine and even amide groups. Some examples (8, 9) of the oxidation products were characterized by X-ray structure analysis. In order to increase the robustness of the Fe-II and Co-II complexes towards oxidation, the ligand scaffold of 1a was modified by N-methylation of the amino group yielding the tertiary amine 2. The corresponding iron(II) and cobalt(II) complexes employing 2 as ligand were synthesized {[Fe(2)][PF6](2) (5), [Co(2)][PF6](2) (6)} and fully characterized, their redox behavior and spin-state was investigated by CV and Evans' method. It was found, that the introduced N-methyl group leads to a substantial anodic shift of the M-II-M-III redox potential and the stabilization of Fe-II and Co-II high-spin state. By means of X-ray structure analysis these effects could be explained by repulsive steric effects of the methyl group.