Hydrothermal synthesis and structure of a mixed valent heteropoly-oxometallate Keggin salt: [PMo4.27W7.73O406-][H3N(CH2)(6)NH32+](3)

The polyoxometallate Keggin (alpha type) [PMo4.27W7.73O406-] [H3N(CH2)(6)NH32+](3), involving molybdenum and tungsten present in both V and VI oxidation states, has been synthesized under hydrothermal conditions and its structure determined by single crystal X-ray analysis. Black platelet crystals are formed in the monoclinic system, space group C2/m (No. 12), a = 10.336(9), b = 20.097(5), c = 13.276(6) Angstrom, beta = 106.41(7)degrees, V-cell = 2645(3) Angstrom(3), Z = 8, d(calc) = 3.555 Mg m(-3) (CuK alpha radiation, 2 theta(max) = 144 degrees, R(F) = 6.56 for 1729-reflections). Due to its location at the origin (2/m site), the anion is found to be disordered, its four independent metal sites occupied with molybdenum and tungsten atoms; a significant preferred localization of the molybdenum on two sites is also reported. The structure has a lamellar organization formed from alternating organic-inorganic planes with their normal parallel to c. A study by ESR has proven the presence of paramagnetic Mo/W(V) in the compound with the observation of a strong signal (g = 1.983). (C) 1997 Academic Press.