Tuning the physical properties of a metal complex by molecular techniques: the design and the synthesis of the simplest cobalt-o-dioxolene complex undergoing valence tautomerism

A family of complexes of general formula [Co(CTH)(Phendiox)]Y·solv (CTH ¼ tetraazamacrocycle; Phendiox ¼ catecholato or semiquinonato form of 9,10-dioxophenanthrene; Y ¼ PF6, I, BPh4; solv ¼ H2O, CH2Cl2, C6H5CH3) were found to undergo valence tautomeric equilibria in the solid state. The interconversion involves an intramolecular electron transfer between a six-coordinated diamagnetic cobalt(III) metal ion and the coordinated polyoxolene ligand yielding a cobalt(II)–semiquinonato complex, the metal ion being in high spin configuration. All the synthesised complexes are diamagnetic at low temperature and high pressure and paramagnetic at high temperature and low pressure. These complexes were designed following electrochemical data. The critical interconversion temperature was found to depend on the nature of the counterion Y and on the nature of the solvent trapped in the lattice. No hysteresis was detected in the interconversion processes. However significant hysteresis effects were detected by substituting solvent from the lattice with its deuterated analogue and by using a partially deuterated ancillary macrocyclic ligand. The observed results are totally unpredictable and show how the emerging properties of a given set of molecules can be deeply influenced by very subtle and not controlled factors. Studies on closely related complexes were carried out for elucidating the electronic properties of these systems. In particular it was found that the [CoII(Me4cyclam)(SQ)]Y complexes (Me4cyclam ¼ tetraazamacrocycle; SQ ¼ semiquinonato radical ligand) are characterised by a weak anti-ferromagnetic coupling between the high-spin cobalt(II) ion and the paramagnetic ligand. In addition it was shown that the valence tautomeric interconversion could be photoinduced, the process involving two separate steps. DFT calculations show that in addition to the cobalt(III)–catecholato and hs-cobalt(II)– semiquinonato species which are involved in the thermal equilibrium, the third valence tautomer ls-cobalt(II)–semiquinonato species could be important from a photomagnetic point of view.