Photochemistry of polyoxovanadates. Part 4. Crystal and electronic structures and magnetic susceptibility of the photochemically prepared layered vanadyl phosphate Na(VO)(2)(PO4)(2)center dot 4H(2)O

摘要

Prolonged photolysis of aqueous solutions containing Na3VO4, NaH2PO4, and MeOH at pH 1.8 adjusted by use of H3PO4 led to the formation of a layered vanadyl phosphate, Na(VO)(2)(PO4)(2). 4H(2)O 1. A single-crystal X-ray structural analysis of a dark green crystal of 1 showed that the structure contains layers of vanadium (randomly with 1 : 1 ratio of V4+ and V5+) and phosphorus oxide with the water molecules and sodium cations between the layers. The layer is a 4-connected net in which corner-sharing vanadium oxygen octahedra and phosphate tetrahedra alternate. The molecular structure is identical with that of a hydrothermally prepared Na+ compound but reveals a non-zero dihedral angle (2.61 degrees) between the least-squares planes (containing four equatorial oxygen atoms) of the VO6 octahedra in contrast to the parallel arrangement of the basal least-squares planes for the latter. Magnetic susceptibility measurements for 1 show weak ferromagnetic coupling (J/k = 0.87 K) at T less than or equal to 100 K and weak antiferromagnetic behavior at T > 100 K. The magneto/structural relationship for the retracted-chair-like V(OPO)(2)V rings is compared among other layered vanadyl phosphate compounds. The ferromagnetic property of the d(xy) orbital at one V-IV in the retracted-chair V-IV(OPO)(2)V-IV ring can be explained in terms of interaction with the V=O pi* orbital at the opposite V-IV, which is possible when both distances of (O=)V ... V(=O) and (O=)V ... O(=V) are short (approximate to 4.55 and approximate to 4.36 Angstrom, respectively). Otherwise, an antiferromagnetic property due to the superexchange interaction between magnetic d(xy) orbitals at both V-IV is operative. The results of extended Huckel (EH) calculations for a fragment model [V8P8O40](4-) (with V-IV/V-V = 1/1) support a variety of magnetic interactions of the unpaired electron at V-IV in the V-P-O layer consisting of the retracted-chair V(OPO)(2)V rings. [References: 26]