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

Abstract

Complexation of Keggin-type polyoxometalates (POMs), [α-PW 12 O 40 ] 3- (PW), [α-SiW 12 O 40 ] 4- (SiW), and [γ-SiV 2 W 10 O 38 (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 ][α-PW 12 O 40 ]·2H 2 O ( 1 ·H 2 O) shows the close packing of PW and Co−tacn in the crystal lattice. The guest sorption properties of the corresponding anhydrous compound 1 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 [γ-SiV 2 W 10 O 40 ]·6H 2 O ( 2 ·H 2 O), 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 [α-SiW 12 O 40 ]·4H 2 O ( 3 ·H 2 O). The crystal structure of 3 ·H 2 O 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 [γ-SiV 2 W 10 O 38 (OH) 2 ]·3H 2 O ( 4 ·H 2 O). The crystal structure of 4 ·H 2 O is analogous to that of 3 ·H 2 O. Compound 4 ·H 2 O can heterogeneously catalyze the epoxidation of olefins with H 2 O 2, maintaining the stereoselectivity of the tetra- n -butylammonium salt of SiVWH in the homogeneous reaction system. On the other hand, 2 ·H 2 O is inactive probably because the POM in 2 ·H 2 O is deprotonated.