Cobalt Complexes of Tripodal Hexadentate Ligands: Electrochemically Driven Rearrangements

Abstract

The CoIII complexes of the hexadentate tripodal ligands HOsen (3-(2´-aminoethylamino)-2,2-bis((2´´-aminoethylamino)methyl)propan-1-ol) and HOten (3-(2´-aminoethylthia)-2,2-bis((2´´-aminoethylthia)methyl)propan-1-ol) have been synthesized and fully characterized. The crystal structures of [Co(HOsen)]Cl3∙H2O and [Co(HOten)](ClO4)Cl2 are reported and in both cases the ligands coordinate as tripodal hexadentate N6 and N3S3 donors, respectively. Cyclic voltammetry of the N3S3 coordinated complex [Co(HOten)]3+ is complicated and electrode dependent. On a Pt working electrode an irreversible CoIII/II couple (formal potential –157 mV versus Ag|AgCl) is seen, which is indicative of dissociation of the divalent complex formed at the electrode. The free HOten released by the dissociation of [Co(HOten)]2+ can be recaptured by Hg as shown by cyclic voltammetry experiments on a static Hg drop electrode (or in the presence of Hg2+ ions), which leads to the formation of an electroactive HgII complex of the N3S3 ligand (formal potential +60 mV versus Ag|AgCl). This behaviour is in contrast to the facile and totally reversible voltammetry of the hexaamine complex [Co(HOsen)]3+ (formal potential (CoIII/II) –519 mV versus Ag|AgCl), which is uncomplicated by any coupled chemical reactions. A kinetic and thermodynamic analysis of the [Co(HOten)]2+/[Hg(HOten)]2+ system is presented on the basis of digital simulation of the experimental voltammetric data.