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.

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), [{Ca(CO)(μ-OH)(OH)}(PWTiO)Ca(OH)] (CaTiTrimer), was obtained as the NaCa salt (NaCa-CaTiTrimer) by the reaction of calcium chloride with the monomeric trititanium(IV)-substituted Wells-Dawson POM species "[PWTiO(OH)]" (TiMonomer). TiMonomer was generated in situ under basic conditions from the separately prepared tetrameric species with bridging Ti(OH) groups and an encapsulated Cl ion, [{PWTiO(OH)}{μ-Ti(HO)}Cl] (TiTetramer). The NaCa salt of CaTiTrimer was characterized by complete elemental analysis, thermogravimetric (TG) and differential thermal analyses (DTA), FTIR, single-crystal X-ray structure analysis, and solution W and P NMR spectroscopy. X-ray crystallography revealed that the [Ca(CO)(μ-OH)(OH)] cluster cation was composed of six calcium cations linked by one μ-carbonato anion and one μ-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. CaTiTrimer 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.