A new class of ferromagnetically-coupled mixed valence vanadium(IV/V) polyoxometalates

Abstract

Reaction of [VVIOCl2(thf)2] with a bidentate nitrogen-donor ligand (L: phen = 1,10-phenanthroline, 5-mephen = 5-methyl-1,10-phenanthroline, bipy = 2,2′-bipyridine, 5,5′-me2bipy = 5,5′-dimethyl-2,2′-bipy) in methyl alcohol, in the presence of triethylamine, leads to the formation of hexameric [V2IVV4V] oxo-alkoxo-vanadates of the general formula [V6O12-μ2- OCH3)4(L)4]·xH2O [L = phen (1· 4H2O), 5-mephen (2·6H2O), bipy (3· 4H2O), 5,5′-me2bipy (4·H2O)]. X-ray structure analysis of 1·2H2O and 4· 8CH3OH revealed a pair of V3O13N4 trimeric units sharing two corners, with a centrosymmetric planar V6-core. In addition, a fully oxidized VV species [VV4O8(OCH3)2 (μ3-OCH3)2(5,5′- me2bipy)2]·3CH3OH (5·3CH3OH) was isolated from the reaction mixture used for the synthesis of 4.H2O. The crystal structure of 5·3CH3OH revealed a dicubane-like framework with two missing vertices. Electron paramagnetic resonance (EPR) and variable temperature magnetic susceptibility studies for the hexamers 1·4H2O and 3·4H2O showed the complete localization of the single 3d electrons on the VIV ions and unusual ferromagnetic interaction between the two paramagnetic vanadium(IV) ions separated by a distance of about 5.1 Å. Furthermore, intermolecular antiferromagnetic interactions through π-contacts of phenyl rings were observed for these species below 8 K. The ferromagnetic exchange coupling observed in the hexanuclear compounds 1·4H2O and 3·4H2O is also discussed using ab initio UHF calculations on a model compound. The value of the exchange coupling constant (3.7 cm-1) for this model compound, calculated using the broken symmetry approach, is in good agreement, both in sign and magnitude, with the experimental J values (6.00 cm-1 for 1·4H2O and 8.54 cm-1 for 3·4H2O).