Crystal and electronic structures and magnetic susceptibility of the photochemically prepared layered vanadyl phosphate Na(VO)2(PO4)2·4H2O

Abstract

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)₂·4H₂O 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°) 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 ≤ 100 K and weak antiferromagnetic behavior at T > 100 K. The magneto/structural relationship for the retracted-chair-like V(OPO)₂V 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)₂V^IV ring can be explained in terms of interaction with the VO π* orbital at the opposite V^IV, which is possible when both distances of (O)V⋯V(O) and (O)V⋯O(V) are short (≈4.55 and ≈4.36 Å, 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 Hückel (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)₂V rings.