Surface complexation of [Nb6O19]8− with NiII: solvothermal synthesis and X-ray structural characterization of two novel heterometallic NiNb-polyoxometalates

Solvothermal reaction of NiCl2, Nb(OEt)5 and 1,3,5-triamino-1,3,5-trideoxy-cis-inositol (taci) in a 1:1:1 ratio under alkaline conditions (NaOH) resulted in the formation of two new NbNi heteropolyoxometalate complexes. [Ni(taci)2]2{trans-[Nb6O19][Ni(taci)]2}·26H2O (1) and [Na(H2O)6]2{cis-[H2Nb6O19][Ni(taci)]2}·18H2O (2), were characterized by single crystal X-ray analysis. In both compounds, two [Ni(taci)]2+ entities are each bonded to three NbONb bridges of a central Nb6O19 unit, resulting in the formation of an approximately C3v symmetric NiN3O3 coordination geometry. The mononuclear analogue [Ni(taci)(H2O)3]2+ was crystallized as the sulfate and structurally characterized for comparison. The two isolated anions {trans-[Nb6O19][Ni(taci)]2}4− and {cis-[H2Nb6O19][Ni(taci)]2}2− have approximate D3d and C2v symmetry, respectively. Compound 2 crystallized, however, in the cubic space group Pnm (No. 224) with the two Ni centers equally distributed over four symmetry equivalent sites. In this structure, the [Na(H2O)6]+ cations and the {cis-[H2Nb6O19][Ni(taci)]2}2− anions represent linearly and tetrahedrally connecting building blocks in a hydrogen bonded supramolecular architecture which can be described in terms of two independent but interpenetrating diamond-type nets.