Tetrameric,Tri-Titanium(IV)-Substituted Polyoxometalates with an a-Dawson Substructure as Soluble Metal Oxide Analogues.Synthesis and Molecular Structure of Three Giant 'Tetrapods' Encapsulating Different Anions (Br~-,I~-,and NO_3~-)

摘要

Preparation and structural characterization of giant "tetrapod"-shaped polyoxometalates (POMs) with approximately T_d symmetry,[{alpha-W15O59(OH)3}_4{mu_3-Ti(H2O)_3}_4X]~(21-) (X=Br~-(la),I~-(2a),and NO_3~-(3a)),consisting of four tri-titanium(IV)-substituted a-Dawson subunits,four octahedral Ti(H2O)3 bridging groups and one encapsulated anion X,are described.Sodium salts of three giant "tetrapod"-shaped POMs encapsulating different anions 1-3 were prepared as analytically pure crystals in 1:5 molar-ratio reactions in water of the tri-lacunary Dawson POM with the in situ-generated "TiX4" (X=Br~-,I~-,and NO_~-),which were prepared by the reactions of Ti(SO4)2 with BaX2 in aqueous solution [Note:The polyoxoanion is represented by la,2a,3a,...,and the whole formula,which contains counter ions,polyoxoanion,hydrated waters,and so on,is represented by 1,2,3,...].The compounds were characterized by using elemental analysis,thermogravimetric and differential thermal analyses (TG/DTA),solid-state and solution IR,solid-state and solution ~(31)PNMR spectroscopy,and X-ray crystallography.Molecular structures of la-3a are analogous to the previously prepared POM with X=Cl~ (4a).The fact that encapsulation of the different anions (Cl~-Br~-,I~-,and NO_3~-) was achieved in the central cavity of the giant "tetrapod"-shaped POM shows the cationic character of the central framework constituted by the protonated Ti-O-Ti bonds,i.e.,the Ti-OH-Ti bonds.The solid-state and solution ~(31)PNMR measurements showed that la-3a were stable in the solid-state and in DMSO,but in water,they partially decomposed.These properties of la-3a in water are in contrast to that of 4a.