Cobalt lawsone complexes: searching for new valence tautomers.

Bi-stable molecular systems presenting valence tautomerism are associated with the development of new functional materials, which can be used for applications in organic electric conductors, optoelectronic and molecular magnetic devices. The properties of these materials can be adjusted with slight chemical changes and can be induced by external stimuli. Typical examples of valence tautomer compounds are coordination complexes of Co and o-dioxolene ligands, notably quinone like ones. In the search for a new class of cobalt complexes presenting valence tautomerism we report herein the synthesis and characterization of five new coordination compounds of cobalt and 2-hydroxy-1,4-naphthoquinone (lawsone or shortly Law). Complexes [Co(Law)2(im)2]·C6H5CH3 (1), [Co(Law)2(py)2]·CH3OH (2), [Co(Law)2(phen)]·(C4H8O)2 (3), [Co(Law)2(2,2-bpy)]·C6H5CH3 (4) and [Co(Law)2(2,2-bpy)] (5) were synthesized and fully characterized by X-ray diffraction and EPR techniques in a wide range of temperatures and under illumination. Despite presenting similar molecular and geometry packing of the valence tautomer complexes of cobalt and o-dioxolenes, neither structural nor electronic evidence of valence tautomerism could be found in the Co and lawsone complexes.