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

Complexes K11Na1[As2W18{Mn(H2O)}(3)O-66]. 27H(2)O (1) and Na-12[As2W18{Co(H2O)}(3)O-66]. 34H(2)O (2) have been characterized. 1 crystallizes in the orthorhombic space group Pnma, with a 30.6484(4) Angstrom, b = 14.9946(2) Angstrom, and c = 19.17080(10) Angstrom (Z = 4). while 2 crystallizes in the monoclinic space group C2/c, with a = 14.124(2) Angstrom, b = 23.294(3) Angstrom, c 32.247(3) Angstrom. 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. K-11[As2W18(VO)(3)O-66]. 23H(2)O (3) crystallizes in the orthorhombic space group Pnma, with a = 30.6240(5) Angstrom, b = 14.9861(2) Angstrom, and c = 19.2651(3) Angstrom (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 V-IV by Ce-IV revealed that 3 is the mixed-valent V(IV)2V(V) 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 d(1) centers. The two V-IV are antiferromagnetically coupled, with J = -2.9 cm(-1) and g = 1.93. Crystallographic data do not permit the location of the two V-IV to be distinguished from the cm location of the VV. As expected, the Mn-II are very weakly antiferromagnetically coupled in compound 1. The complex Na-8[Ni(H2O)(6)](2)[As2W18{Ni(H2O)}(3)O-66]. 20H(2)O (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.