Cupric siliconiobate. Synthesis and solid-state studies of a pseudosandwich-type heteropolyanion

The Na+ and [Cu(en)(2)(H2O)(2)](2+) (en = ethylenediamine) salt of a pseudosandwich-type heteropolyniobate forms upon prolonged heating of Cu(NO3)(2) and hydrated Na-14[(SiOH)(2)Si2Nb16O54] in a mixed water-en solution. The structure [a = 14.992(2) angstrom, b = 25.426(4) angstrom, c = 30.046(4) angstrom, orthorhombic, Pnn2, R1 = 6.04%, based on 25869 unique reflections] consists of two [Na(SiOH)(2)Si2Nb16O54](13)- units linked by six sodium cations, and this sandwich is charge-balanced by five [Cu(en)(2)(H2O)(2)](2+) complexes, seven protons, and three additional sodium atoms (all per a sandwich-type cluster). Diffuse-reflectance UV-vis indicates that there is a lambda(max) at 383 nm for the Cu-II d-d transition and the Si-29 MAS NMR spectrum has two peaks at -78.2 ppm (151 Hz) and -75.5 ppm (257 Hz) for the two pairs of symmetry-equivalent internal [SiO4](4-) and external [SiO3(OH)](3-) tetrahedra, respectively. Unlike tungsten-based sandwich-type complexes, the [Na(SiOH)(2)Si2Nb16O54](13-) units are linked exclusively by Na+ instead of one or more d-electron metals.