Dinuclear Zinc and Cobalt Complexes with Imidazolyl and N ‐Methylmidazolyl Units and Their Solution Speciation and Redox Properties

Abstract

Herein, we report the synthesis of a new ligand, which is preorganised for the complexation of two metal ions and has two 1H-imidazolyl units in the outer ligand sphere. The zinc and cobalt complexes with the ligand were synthesised and fully characterised, and their solid-state structures were determined. The complexes show a highly symmetric [M2L2]2+ structural motif. The thermodynamic data, that is, the overall stability constants and redox potential of the Co2+/Co3+ couple, were determined. Potentiometric studies revealed the presence of mononuclear (below pH 7) and differently protonated dimeric species (above pH 7). The formation of distinct species was corroborated by ESI-MS measurements. Similar species were obtained for the corresponding zinc complexes. Additionally, we determined the dependence of the Co2+/Co3+ redox couple on the protonation state of the complex. The studies showed that there is a considerably strong thermodynamic coupling between the NH protons of the imidazolyl units and the oxidation state of the metal ion. Upon deprotonation of all four NH units, the potential of the redox couple shifted by ca. 1.1 V. We also synthesised the analogous ligand with N-methylimidazolyl units to investigate the influence of the NH protons on the properties of the complexes. The methylation of the NH unit has virtually no influence on the redox, magnetic and structural properties of the zinc and cobalt complexes.