Synthesis and Molecular Structure of a Novel Compound Containing a Carbonate-Bridged Hexacalcium Cluster Cation Assembled on a Trimeric Trititanium(IV)-Substituted Wells-Dawson Polyoxometalate

Abstract

A novel compound containing a hexacalcium cluster cation, one carbonate anion, and one calcium cation assembled on a trimeric trititanium(IV)-substituted Wells-Dawson polyoxometalate (POM), [{Ca₆(CO₃)(μ₃-OH)(OH₂)₁₈}(P₂W₁₅Ti₃O₆₁)₃Ca(OH₂)₃]^19- (Ca₇Ti₉Trimer), was obtained as the Na₇Ca₆ salt (NaCa-Ca₇Ti₉Trimer) by the reaction of calcium chloride with the monomeric trititanium(IV)-substituted Wells-Dawson POM species "[P₂W₁₅Ti₃O₅₉(OH)₃]^9-" (Ti₃Monomer). Ti₃Monomer was generated in situ under basic conditions from the separately prepared tetrameric species with bridging Ti(OH₂)₃ groups and an encapsulated Cl^- ion, [{P₂W₁₅Ti₃O₅₉(OH)₃}₄{μ₃-Ti(H₂O)₃}₄Cl]^21- (Ti₁₆Tetramer). The Na₇Ca₆ salt of Ca₇Ti₉Trimer was characterized by complete elemental analysis, thermogravimetric (TG) and differential thermal analyses (DTA), FTIR, single-crystal X-ray structure analysis, and solution ¹⁸³W and ³¹P NMR spectroscopy. X-ray crystallography revealed that the [Ca₆(CO₃)(μ₃-OH)(OH₂)₁₈]⁹⁺ cluster cation was composed of six calcium cations linked by one μ₆-carbonato anion and one μ₃-OH^- anion. The cluster cation was assembled, together with one calcium ion, on a trimeric species composed of three tri-Ti(IV)-substituted Wells-Dawson subunits linked by Ti-O-Ti bonds. Ca₇Ti₉Trimer is an unprecedented POM species containing an alkaline-earth-metal cluster cation and is the first example of alkaline-earth-metal ions clustered around a titanium(IV)-substituted POM.