Reaction products of titanium(IV) sulfate with the two, dimeric precursors, 1,2,3-tri-titanium(IV)- and 1,2-di-titanium(IV)-substituted α-Keggin polyoxometalates (POMs), under acidic conditions. A tetra-titanium(IV) oxide cluster and one coordinated sulfa

A novel, monomeric species containing a tetra-titanium(IV) oxide cluster and one coordinated sulfate ion grafted on a di-lacunary Keggin polyoxometalate (POM), [[{Ti(H2O)3}2{Ti(H2O)2}2(μ-O)3(SO4)](PW10O37)]−1, was derived from reactions of Ti(SO4)2 with two precursors, i.e., [(α-1,2-PW10Ti2O38)2O2]10 −2 and [(α-1,2,3-PW9Ti3O37)2O3]12 −3, under strongly acidic conditions. [Note: the potassium salts of POMs 1–3 are represented as K1 to K3, respectively.] The potassium salt of 1, which was stable in the solid state, but unstable in water, was unequivocally characterized by complete elemental analysis, TG/DTA, FTIR, X-ray crystallography, and solid-state 31P CPMAS and solution 31P NMR spectroscopy. Both precursors have been considered as relatively stable forms of titanium(IV)-substituted Keggin POMs. A deposit of crystals from the reaction system consisting of precursor 3, Ti(SO4)2, and a strong acid (HCl, HNO3, or H2SO4) was significantly affected by the conjugate base of the acid, while crystallization from the reaction using precursor 2 was not affected by the base. The Ti4 center in 1 belongs to a class of a 2-host (di-lacunary site)/4-guest (four Ti atoms) coordination relationship, or a combination of two sets of a 1-host/2-guest coordination.

Graphical abstractThe title polyoxometalate 1 as potassium salt was synthesized by reactions of Ti(SO4)2 with the two, relatively stable forms of Ti(IV)-substituted Keggin POMs under strongly acidic conditions, and unequivocally characterized in the solid state and in solution. The Ti4 center in 1 belongs to a class of a 2-host (di-lacunary site)/4-guest (4 Ti atoms) coordination relationship, or a combination of two sets of a 1-host/2-guest coordination.Figure optionsDownload full-size imageDownload as PowerPoint slideHighlights► A Ti4 oxide cluster coordinated by SO42 − ion grafted on a Keggin POM was prepared. ► The POM was derived from reactions of Ti(SO4)2 with two different precursors. ► A deposit of crystals was affected by the conjugate bases of the acids used. ► The POM has been unequivocally characterized by various measurements. ► The POM is in a class of a 2-host/4-guest coordination relationship.