SWITCHING THE MECHANISM OF SPIN-EXCHANGE COUPLING IN (MU-OXO)BIS(MU-CARBOXYLATO)DIVANADIUM(III) COMPLEXES BY PROTONATION OF THE OXO BRIDGE

Hydrolysis of LVCl3 in methanol/water (4:1) mixtures with the sodium salts of a variety of carboxylic acids (CF3COOH, FCH2CO2H, ClCH2CO2H, BrCH2CO2H, HCO2H, CH3COOH, C6H5COOH) affords green dinuclear complexes [L2V(III)2(mu-O)(mu-carboxylato)2]2+, which have been isolated as solid hexafluorophosphate or iodide salts (L = 1,4,7-trimethyl-1,4,7-triazacyclononane, C9H21N3). From temperature-dependent magnetic susceptibility measurements it was found that the spins of both vanadium(III) ions (d2) are fully aligned between 10 and 298 K, indicating very strong intramolecular ferromagnetic coupling. From CH3CN solutions of these green complexes the pink protonated forms [L2V(III)2(mu-OH)(mu-carboxylato)2]3+ are generated by addition of concentrated HBr or HClO4. [L2V2(mu-OH)(mu-C6H5CO2)2]Br3 and [L2V2(mu-OH)(mu-CH3CO2)2](ClO4)3.H2O have been isolated as red crystals. The former exhibits intramolecular antiferromagnetic spin-exchange coupling (H = -2JS1.S2, S1 = S2 = 1; J = -36 cm-1, g = 1.87). A model is presented to rationalize the dramatic change of mechanism of the spin-exchange coupling upon protonation of the mu-oxo group.