Comparative Study of Inorganic Cluster−Surfactant Arrays

Abstract

We have investigated inorganic cluster−surfactant materials to better understand the structural evolution of these phases as the surfactant:cluster ratio increases above 4:1 and the cluster charge increases beyond −4. Our studies suggest that both ordering of the surfactant molecules into bilayers as well as the cluster charge are the primary influences on the hybrid cluster−surfactant phase structure. However, cluster geometry, inclusion of solvent molecules, surfactant tail length, cation head size, etc. also influence the self-assembly of these materials. We present the synthesis, characterization, and single-crystal X-ray structure of [SiMo12O40][C16H33N(CH3)3]4 (monoclinic P21/c, a = 13.136(1) Å, b = 20.139(2) Å, c = 41.030(3) Å, β = 93.443(1)°, and V = 10834.7(17) Å3) and the synthesis and characterization of a related phase that forms upon chemical reduction of the silicomolybdate anion. Structural and chemical comparisons are made between these two compounds, as well as other phases formed from polyoxometalates (with charges ranging from −3 to −16) and surfactants. The structure of [SiMo12O40][C16H33N(CH3)3]4 is compared to the structures of other reported cluster−surfactant phases that also have a 1:4 cluster:surfactant ratio. Analyses of these phases provide some insight as to why it is thus far only this 1:4 ratio that provides crystals suitable for single-crystal diffraction studies and how the structure of the cluster−surfactant phases evolves as the surfactant/cluster ratio is increased.