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 Na14[(SiOH)2Si2Nb16O54] in a mixed water−en solution. The structure [a = 14.992(2) A, b = 25.426(4) A, c = 30.046(4) A, orthorhombic, Pnn2, R1 = 6.04%, based on 25869 unique reflections] consists of two [Na(SiOH)2Si2Nb16O54]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 λmax at 383 nm for the CuII d−d transition and the 29Si 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)2Si2Nb16O54]13− units are linked exclusively by Na+ instead of one or more d-electron m...