Polymorphism in tetra-butylammonium salts of Keggin-type polyoxotungstates

Abstract

Some tetra- n-butylammonium salts of Keggin-type polyoxotungstates, namely α-[PW 11O 39] 7−, α-[PW 11M(H 2O)O 39] 5−, M II=Co, Ni, α-[PW 11CuO 39] 5−, α-[PW 11Fe(H 2O)O 39] 4− and α-[PW 11Fe(OH)O 39] 5−, with the general formula [(C 4H 9) 4N] 4H x [anion]· nH 2O, were studied by powder X-ray diffraction. The salts α-[(C 4H 9) 4N] 4H 3[PW 11O 39] and α-[(C 4H 9) 4N] 4H[PW 11CuO 39] were obtained in two crystalline phases, a behavior that has been scarcely documented in compounds with Keggin polyoxometalates. One of the phases corresponds to a body-centered cubic unit cell ( Im 3 ̄ symmetry) and the other has a body-centered tetragonal unit cell ( I4 1 or I4 122 symmetry). For the compounds with Cu, the cell parameters of the cubic and tetragonal structures were a=17.675(2) and a=18.775(2), c=14.646(2) Å, respectively. The cubic phase was obtained by precipitation from aqueous solution. The tetragonal phase was isolated by crystallization from acetonitrile. The remaining compounds were found to crystallize with one of these two crystal structures. α-[(C 4H 9) 4N] 4H x [PW 11M(H 2O)O 39]· nH 2O, with M=Co, Ni, x=1, and M=Fe, x=0, n=1–2, were found to crystallize with the body-centered cubic structure. The method of synthesis used to prepare α-[(C 4H 9) 4N] 4H[PW 11Fe(OH)O 39] afforded the tetragonal phase. The conversion of the cubic into the tetragonal phase and the stability of each one are discussed.