Structure and properties of the dendron-encapsulated polyoxometalate (C52H60NO12)(12)(Mn(H2O))(3)(SbW9O33)(2), a first generation dendrizyme
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 124 (35):
10489-10496 (September 2002)DOI: 10.1021/ja017613b
Abstract
Combining analytical and theoretical methods, we present a detailed study of a heteropolytungstate cluster encapsulated in a shell of dendritically branching surfactants, namely (C52H60NO12)(12)(Mn(H2O))(3)(SbW9O33)(2), 3. This novel surfactant-encapsulated cluster (SEC) self-assembles spontaneously from polyoxometalate-containing solutions treated with a stoichiometric amount of dendrons. Compound 3 exhibits a discrete supramolecular architecture in which a single polyoxometalate anion resides in a compact shell of dendrons. Our approach attempts to combine the catalytic activity of polyoxometalates with the steric properties of tailored dendritic surfactants into size-selective catalytic systems. The structural characterization of the SEC is based on analytical ultracentrifugation (AUC) and small-angle neutron scattering (SANS). The packing arrangement of dendrons at the cluster surface is gleaned from molecular dynamics (MD) simulations, which suggests a highly porous shell structure due to the dynamic formation of internal clefts and cavities. From analysis of the MD trajectory of 3, a theoretical neutron-scattering function is derived that is in good agreement with experimental SANS data. Force field parameters used in MD simulations are partially derived from a quantum mechanical geometry optimization of (Zn(H2O))(3)(SbW9O33)(2)(12-), 2b, at the density functional theory (DFT) level. DFT calculations are corroborated by X-ray structure analysis of Na6K6(Zn(H2O))(3)(SbW9O33)(2).23H(2)O, which is isostructural with the catalytically active Mn derivative 2a. The combined use of theoretical and analytical methods aims at rapidly prototyping smart catalysts (''dendrizymes''), which are structurally related to naturally occurring metalloproteins.