Structures and magnetism of rubidium and cesium salts of dimeric oxovanadium(IV) tartrate(4−) complexes

Complexes of type A₄VO(tart)]₂·nH₂O, where A = Rb or Cs and tart =d,l-tartrate(4?) (n = 2) or d,d-tartrate(4?) (n = 2 for Rb and n = 3 for Cs), were prepared from an aqueous mixture of V₂O₅, AOH and H₄tart. These complexes were studied by single-crystal X-ray diffraction methods: Rb₄VO(d,l-tart)]₂·2H₂O, space group P1 with a = 8.156(1),b = 8.246(1),c = 8.719(1)Å, = 66.09(1)°, β = 65.07(1)°, γ = 82.40(1)°,Z = 2, 1917 observed reflections, and final R_w = 0.035; Cs₄VO(d,l-tart)]₂·2H₂O, space group P₂₁/c with a = 9.350(1),b = 13.728(2),c = 8.479(1)Å, β = 106.77(1)°,Z = 4, 2235 observed reflections, and final R_w = 0.054; Rb₄VO(d,d-tart)]₂·2H₂O, space group P₄₁₂₂ with a = 8.072(1),c = 32.006(3)Å,Z = 8, 1014 observed reflections and final R_w = 0.038; Cs₄VO(d,d-tart)]₂·3H₂O, space group P₁₂₂ with a = 8.184(1),c = 33.680(5)Å,Z = 8, 1310 observed reflections, and final R_w = 0.063. Bulk magnetic susceptibility data (1.5–300 K) for these compounds and A₄VOl,l-tart)]₂·nH₂O (A = Rb, Cs) were obtained on polycrystalline samples. These data were analyzed in terms of a Van Vleck exchange coupled S = 1/2 model which was modified to include an interdimer exchange parameters Θ. Analysis of the low-temperature (1.5–20 K) susceptibility data gave 2J = +1.30 cm^?1 and Θ = ?1.86 K for Rb₄VO(d,l-tart)]₂·2H₂O, 2J = +1.16 cm^?1 and Θ = ?1.69 K for Cs₄VO(d,l-tart)]₂·2H₂O, 2J = +1.90 cm^?1 and Θ = ?0.82 K for Rb₄VO(d,d-tart)]₂·2H₂O, 2J = +2.04 cm^?1 and Θ = ?0.80 K for Rb₄VO(l,l-tart)]₂·2H₂O, 2J = +1.52 cm^?1 and Θ = ?0.25 K for Cs₄VO(d,d-tart)]₂·3H₂O, and 2J = +1.64 cm^?1 and Θ = ?0.31 K for Cs₄VO(l,l-tart)]₂·3H₂O. These results suggest the magnitudes of intradimer (ferromagnetic and interdimer (antiferromagnetic) exchange interactions are similar in these complexes, as observed for the analogous Na salts.