New dinuclear cobalt(II) octaaza macrocyclic complexes with high oxidation redox potentials: Their crystal structure and unusual magnetic properties

Dinuclear cobalt(II) complexes were synthesised from 1,4,8,11-tetrakis-(2-pyridylmethyl)-1,4,8,11-tetraazacyclotetradecane (tmpc), [Co2(tmpc)Cl2][CoCl4] (1), [Co2(tmpc)Cl2][PF6]2 (1a) and [Co2(tmpc)(NO3)2][NO3]2 · MeOH (2) and characterised by magnetic, spectroscopic and electrochemical techniques and by single-crystal X-ray diffraction. The X-ray structures of 1 and 2 demonstrate that in both complexes the metal ion is exo-coordinated with respect to the macrocyclic ligand. In 1, the Co(II) ions are fivefold coordinated with trigonal bipyramidal geometry, while in 2 they are sixfold coordinated with a distorted octahedral geometry. The high magnetic moments obtained for these complexes are explained in terms of a spin–orbit coupling. Magnetic measurements show a deviation from the Curie–Weiss law at low temperatures. Two magnetic orderings were observed, at high temperatures an antiferromagnetic coupling was found, below 20 K a change to a weak ferromagnetic coupling was observed. Isothermal magnetic measurements at low temperature show a weak hysteresis, which was confirmed by the small coercive field found at low temperature. In addition, for the first time an explanation for the observation that these cobalt(II) compounds are very stable towards oxidation is offered in terms of the high redox potential values obtained for 1, 1a, and 2.