Spectroscopic and thermodynamic characterization of a cobalt-verdazyl valence tautomeric system. influence of crystal structure, solvent and counterion.

Crystallization of the verdazyl-based valence tautomeric ion [Co(dipyvd) 2 ] 2+ (where dipyvd is the radical ligand 1-isopropyl-3,5-di(2'-pyridyl)-6-oxoverdazyl) with a variety of different counterions results in materials that show varying degrees of valence tautomeric (VT) transition in the solid state. The X-ray structure of the SbF 6 salt at 150 K reveals a localized structure for the S = 1/2 tautomer, with a Co 3+ cation and distinct anionic and radical ligands. Comparison with the structure of the same material at 300 K reveals large structural changes in the ligand as a result of the valence tautomeric equilibrium. Data for the S = 3/2 form is less conclusive; X-ray spectroscopy on the PF 6 salt suggests a degree of low spin Co 2+ character for the S = 3/2 tautomer at very low temperature though this is inconsistent with EPR data at similar temperatures and structural information at 150 K. Magnetic measurements on the [BAr F 4 ] - and triflate salts in organic solvents show that the VT equilibrium is dependent on solvent and ion pairing effects.