A New Class of Ferromagnetically-Coupled Mixed Valence Vanadium(IV/V) Polyoxometalates

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]⋅x H2O [L=phen (1⋅4 H2O), 5‐mephen (2⋅6 H2O), bipy (3⋅4 H2O), 5,5′‐me2bipy (4⋅H2O)]. X‐ray structure analysis of 1⋅2 H2O and 4⋅8 CH3OH 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]⋅3 CH3OH (5⋅3 CH3OH) was isolated from the reaction mixture used for the synthesis of 4⋅H2O. The crystal structure of 5⋅3 CH3OH revealed a dicubane‐like framework with two missing vertices. Electron paramagnetic resonance (EPR) and variable temperature magnetic susceptibility studies for the hexamers 1⋅4 H2O and 3⋅4 H2O 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⋅4 H2O and 3⋅4 H2O 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⋅4 H2O and 8.54 cm−1 for 3⋅4 H2O). A new structural motif is reported, which was found after isolation of the hexameric [V2IVV4V] oxo‐alkoxo‐vanadates of the general formula [V6O12(μ2‐OCH3)4(L)4]⋅x H2O (see figure, L = nitrogen‐donor chelate ligand). In these compounds the single 3d electrons are completely localized on the two VIV ions and interact ferromagnetically through a long diamagnetic path of about 5.1 Å, something which is quite surprising.