Synthesis, X-ray crystal structure, and EPR study of (Na(H_2O)_2)_2(VO(H_2O)_5)(SiW_(12)O_(40)) centre dot 4H_2O

摘要

The vanadyl salt (Na(H_2O)_2)_2(VO(H_2O)_5)(SiW_(12)O_(40)) centre dot 4H_2O has been synthesized in mild conditions by cationic exchanges from dodecasilicotungstic acid. Structural determination and EPR study have been achieved on single crystals. They are tetragonal, space group P4 with a = 14.7759(1), c = 10.4709(2) A, V = 2286(1) A~3 and Z = 2. A three-dimensional framework built from Keggin anions (SiW_(12)O_(40))~(4-) linked by sodium cations in (110) and (11-bar0) planes generates channels along the c axis in which are localized aquo vanadyl complexes (VO(H_2O)_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 A 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 =0.0072cm~(-1), and A_z=0.01805 cm~(-1), J= -0.00025cm~(-1).