A series of cobalt complexes [Co(Me(n)tpa)(diox)]PF(6)sol (diox=3,5-di-tert-butyl-1,2-dioxolene; sol=ethanol, toluene; tpa=tris(2-pyridylmethyl)amine) were prepared by using tripod-like Me(n)tpa (n=0, 1, 2, 3), derived from tpa by successive introduction of methyl groups into the 6-position of the pyridine moieties, as an ancillary ligand. The steric hindrance induced by this substitution modulates the redox properties of the metal acceptor, thus determining the charge distribution of the metal-dioxolene moiety at room temperature. All of these complexes were characterised by using diffractometric studies, electronic spectroscopic analysis, and magnetic susceptibility measurements. In the solid state, the [Co(Me(n)tpa)(diox)](+) ions (n=0, 1) can be described as diamagnetic cobalt(III)-catecholato derivatives, whereas a cobalt(II)-semiquinonato description seems appropriate for the paramagnetic [Co(Me(3)tpa)(diox)](+) complex. The complex [Co(Me(2)tpa)(diox)]PF(6)C(2)H(5)OH undergoes entropy-driven valence tautomeric interconversion at room temperature. Optically induced valence tautomerism was observed by irradiation of [Co(Me(n)tpa)(diox)]PF(6) complexes (n=0, 1, 2) at cryogenic temperatures. The different relaxation kinetics of the photoinduced metastable phases are related to the respective free-energy changes of the interconversion, as estimated by cyclic voltammetric experiments at room temperature, and to the different lattice interactions, as supported by structural data. These results show the importance of molecular techniques for controlling the relaxation properties of photoinduced metastable species. At the same time, this behaviour strongly suggests that this paradigm exhibits intrinsic limits because of the less controllable factors that affect the process.

The observation of very long lifetimes of the metastable states of two cobalt-dioxolene complexes undergoing photoinduced and high T(c) thermally-induced valence tautomer interconversion opens new research perspectives.