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.