Chemical routes to oxides: alkoxide vs. alkoxide-acetate routes: synthesis, characterization, reactivity and polycondensation of MNb2(OAc)(2)(OPri)(10) (M = Mg, Cd, Pb) species

The molecular constitution of solutions containing niobium alkoxides and divalent metal acetates M(OAc)(2) (M = Mg, Ba, Pb, Zn, Cd) has been examined. The heterometallic aggregates MNb2(mu-OAc)(2)(mu-OPri)(4)(OPri)(6) (M = Mg 1, Cd 2, Pb 3a) have been isolated and characterized by elemental analysis, FTIR, multinuclear NMR (H-1,C-13,Pb-207,Cd-113) and by single-crystal X-ray diffraction for M = Mg and Cd. The magnesium derivative crystallizes in the monoclinic system, space group P2(1)/c with the unit-cell parameters a=21.238(5), b=10.127(10), c=24.861(3) Angstrom,beta=107.77(2)degrees and Z = 4. The thermal stability of the various species has been investigated. Condensation is induced thermally for PbNb2(OAc)(2)(OPri)(10). The reactivity between MNb2(mu-OAc)(2)-(mu-OPri)(4)(OPri)(6) (M = Pb, Mg) and other metallic species has been evaluated. A terheterometallic compound PbMgNb2O(OAc)(2)(OPri)(10) has been isolated. Whereas no reaction is observed between Ba(OAc)(2) and Nb(OPri)(5), the reaction between metal alkoxides affords BaNb2(OPri)(12)((PrOH)-O-i)(2). Its reactivity shows that the absence of an assembling acetate ligand induces facile separation between the metals. Results of the hydrolysis experiments of 1-3a are given. The powders have been analyzed by TG, SEM-EDX, light scattering and XRD. The merit of the assembling acetate ligand for avoiding the segregation of the metals is emphasized.