Preparation of Monodispersed Nanoparticles by Electrostatic Assembly of Keggin-Type Polyoxometalates and 1,4,7-Triazacyclononane-Based Transition-Metal Complexes

Complexation of Keggin-type polyoxometalates (POMs), [alpha-PW12O40](3-) (PW), [alpha-SiW12O40] (SiW), and [gamma-SiV2W10O38(OH)(2)](4-) (SiVWH) with cationic transition-metal complexes having a triazacyclononane ligand, [M(tacn)2](n+) (M-tacn; M = Co-III and Ni-II tacn = 1,4,7-triazacyclononane), yields fine particles of inorganic-organic composites. The strong electrostatic interaction between the highly negatively charged POMs and the +2- or +3-charged [M(tacn)(2)](n+) as well as the hydrophobicity of [M(tacn)(2)](n+) results in the formation of the water-insoluble binary composites. The crystal structure of the 1: 1 (=the molar ratio of POM to M-tacn) composite [Co(tacn)(2)][alpha-PW12O40]center dot 2H(2)O (1 center dot H2O) shows the close packing of PW and Co-tacn in the crystal lattice. The guest sorption properties of the corresponding anhydrous compound I show that the amounts of hydrophobic molecules sorbed are comparable to that of water. The reaction of SiVWH with Co-tacn yields fine particles of [Co(tacn)(2)](2)[gamma-SiV2W10O40]center dot 6H(2)O (2 center dot H2O), and the molar ratio of POM to M-tacn is 1:2. The reaction of SiW with +2-charged Ni-tacn yields fine particles of [Ni(tacn)(2)](2)[(alpha-SiW12O40]center dot 4H(2)O (3 center dot H2O). The crystal structure of 3 center dot H2O shows the honeycomb packing of SiW and Ni-tacn, and the ionic components are closely packed in the crystal lattice. The reaction of SiVWH with Ni-tacn yields fine particles of [Ni(tacn)(2)](2)[gamma-SiV2W10O38(OH)(2)]center dot 3H(2)O (4 center dot H2O). The crystal structure of 4 center dot H2O is analogous to that of 3 center dot H2O. Compound 4 center dot H2O can heterogeneously catalyze the epoxidation of olefins with H2O2; maintaining the stereoselectivity of the tetra-n-butylammonium salt of SiVWH in the homogeneous reaction system. On the other hand, 2 center dot H2O is inactive probably because the POM in 2 center dot H2O is deprotonated.