Synthesis, X-ray crystal structure, and EPR study of [Na(H2O)2]2[VO(H2O)5][SiW12O40]•4H2O

The vanadyl salt [Na(H2O)(2)](2)[VO(H2O)(5)][SiW12O40]. 4H(2)O has been synthesized in mild conditio ns by cationic exchanges from dodecasilicotungstic acid. Structural determination and EPR study have been achieved on single crystals. They are tetragonal, space group P4/n with a = 14.7759(1), c = 10.4709(2) Angstrom, V = 2286(1) Angstrom (3) and Z = 2. A three-dimensional framework built from Keggin anions [SiW12O40](4-) linked by sodium cations in (110) and (1 (1) over bar0) planes generates channels along the c axis in which are localized aquo vanadyl complexes [VO(H2O)(5)](2+) and water molecules. Single crystal EPR spectra show eight hyperfine lines of the vanadium atom (I = 7/2) which are split into 1:2:1 pattern when the magnetic field is parallel to the c axis. The triplet pattern may be attributed to weak dipolar interactions between the nearest-neighbor vanadium atoms which are 10.47 Angstrom apart in the infinite chain. A ring model was used to simulate the spectrum, and a very small antiferromagnetic exchange interaction was determined accurately. The EPR parameters determined are g(x) = g(y) = 1.980, g(z) = 1.9336, A(x) = A(y) = 0.0072 cm(-1), and A(z) = 0.01805 cm(-1), J = -0.00025 cm(-1). (C) 2001 Editions scientifiques et medicales Elsevier SAS. All rights reserved.