Crystal structure of the VV dimer [V2O2(μ-O)(dmpp)2(OCH3)2] and its equilibrium with the VV trimer [V3O3(μ-O)3(dmpp)3(H2O](H2O)2 in methanol/water solutions

The behaviour of the cyclic trimeric V-V complex [V3O3(mu-O)3(dmpp)(3)(H2O)](H2O)(2), V3L3, (L = Hdmpp = 3-hydroxy-1,2-dimethyl-4-pyridinone) was studied in methanol and methanol/water solutions by using V-51 and 1D- and 2D H-1 NMR spectroscopy. Red crystals, isolated from a highly concentrated methanol solution of the trimeric complex, were analysed by X-ray crystallography. The solid-state structure of the compound showed the presence of a new dinuclear V-V cluster and allowed for its formulation as a [V2O2(mu-O)- (dmpp)(2)(OCH3)(2)] complex, V2Y2L2 (Y = OCH3). This complex crystallises in the monoclinic system: P2(1)/c, a = 8.4573(11) b = 15.034(2) angstrom and c = 15.849(2) angstrom, beta = 105.300(2)degrees, V 1943,7(4) angstrom(3), Z = 2, and R-1(wR(2)) = 0.0492(0.1706). The trimer V3L3 complex dissolved in a dry methanol solution fully decomposes, as shown by the 51V NMR signals at -388, -450 and -551 ppm, which are assigned to a monomer complex [VO(OMe)(dmpp)(2)] (VYL2), the dimer V2Y2L2 and the monomethyl ester of monovanadate, V1Y (V-1 = monovanadate; Y = OCH3) respectively. In methanol/water solutions, a new V-51 NMR signal appears at 6 = -492 ppm, which is assigned to the [VO2(dmpp)(H2O)(2)] (VL) complex. When the percentage of water in the mixture increases, the relative intensities of the V2Y2L2 and V1Y signals decrease sharply, and a broad signal at -488 ppm appears, corresponding to the original V-V trimer complex, which is the only species present in 94% water. A temperature-dependent H-1 NMR study of a CD3OD solution Of V3L3 confirmed the presence, at room temperature, of the dinuclear V2L,2 complex and the VL2 species. At temperatures below 0 degrees C down to -50 degrees C, the appearance of new signals reflects the presence of isomers for the V2Y2L2 and VYL2 species with different stabilities and symmetries. 2D H-1 homonuclear NMR exchange experiments (EXSY) allowed us to establish the isomeric equilibria that take place in solution, and indicates intramolecular exchange between the two ligands of the major isomer of VYL2 and intermolecular exchange between the major and minor isomers of species of different nuclearity, V2Y2L2 and VYL2. However, no evidence was found for intermolecular exchange between the major isomers and between the minor isomers of species of different nuclearity or between isomers of species of the same nuclearity.