Structural characterization and magnetic properties of sandwich-type tungstoarsenate complexes. Study of a mixed-valent VIV2/VV heteropolyanion

Complexes K11Na1As2W18(Mn(H2O))3O66]x27H2O (1) and Na12As2W18(Co(H2O))3O66]x34H2O (2) have been characterized. 1 crystallizes in the orthorhombic space group Pnma, with a = 30.6484(4) A, b = 14.9946(2) A, and c = 19.17080(10) A (Z = 4), while 2 crystallizes in the monoclinic space group C2/c, with a = 14.124(2) A, b = 23.294(3) A, c = 32.247(3) A, and beta = 98.935(10) degrees (Z = 4). Structures of the anions of 1 and 2 are similar, the divalent metals adopting a square pyramidal environment. K11As2W18(VO)3O66]x23H2O (3) crystallizes in the orthorhombic space group Pnma, with a = 30.6240(5) A, b = 14.9861(2) A, and c = 19.2651(3) A (Z = 4). The structure has revealed a disorder on two of the three metals linking the alpha-AsW9O33]9- parts. For these two vanadium atoms, the V=O bonds are directed alternatively toward the inside or the outside of the alpha-AsW9O33]9- cavity. The remaining vanadium shows a V=O bond always directed toward the outside of the cavity. Titration of VIV by CeIV revealed that 3 is the mixed-valent VIV2VV species. Magnetic measurements are in agreement with this formulation. The high-temperature molar magnetic susceptibility of a powdered sample of 3 clearly confirms the presence of two d1 centers. The two VIV are antiferromagnetically coupled, with J = -2.9 cm-1 and g = 1.93. Crystallographic data do not permit the location of the two VIV to be distinguished from the location of the VV. As expected, the Mn(II) are very weakly antiferromagnetically coupled in compound 1. The complex Na8Ni(H2O)6]2As2W18(Ni(H2O))3O66]x20H2O (4) has been synthesized. The anion crystallized with two octahedral Ni(H2O)6]2+ as counterions. Magnetic data have been fitted assuming that the only exchange-coupled centers are the nickels of the polyanion. 4 exhibits an antiferromagnetic coupling with J = -1.7 cm-1, g = 2.27, and theta = -1.5 K.