Synthesis, magnetism, and x-ray molecular structure of the mixed-valence vanadium(IV/V)-oxygen cluster [VO4 subset of(V18O45)](9-)

Within the transition metal oxide systems, vanadium presents a unique chemistry due to its capacity to form a great number of mixed-valence oxo clusters which often have the peculiarity to incorporate species that function, for size, shape, and charge, as templates. Prismatic, lustrous dark brown crystals of [(n-C4H9)NH3]9[V19O49],7H2O are obtained by reacting (n-C4H9NH3)VO3, VOSO4, and (n-C4H9)NH2 in H2O. The X-ray crystal structure shows an ellipsoidal metal-oxo cluster formed by 15 VO5 and 3 VO4 polyhedra surrounding an almost regular VO4 tetrahedron located on the 3-fold axis of a trigonal cell of dimensions a ) 19.113(5) Å and c ) 13.743(5) Å with space group P3 and Z ) 2. Exponentially weighted bond valence sum calculations, manganometric titration of h the VIV centers, and magnetic measurements are consistent with the presence of three localized and three delocalized electrons. Variable-temperature solid-state susceptibility studies indicate antiferromagnetic coupling between VIV centers. Cyclic voltammetry in acetonitrile shows a irreversible reduction at -1.24 V and a reversible oxidation at +0.17 V (vs Ag/AgCl). The title compound converts quantitatively to the metal oxide K2V3O8 with an extended layered structure as soon as a potassium salt is added to a neutral aqueous solution of the polyoxoanion.