Syntheses,Structures And Properties Of Novel Polyoxotantalates

Compared to polyoxometalates of W(Mo,V,Nb),the development and related reports of polyoxometalates of Ta(polyoxotantalates)are still restricted by its harsh synthesis conditions and difficulty,which is not only owing to the inertness of tantalum,but also due to the stable existence of the soluble precursor K8[Ta6O19]·17H2O requires the strong basicity and narrow range of pH values in solution,otherwise it is easy to hydrolyze.In this paper,K8[Ta6O19]·17H2O、transition metal salts and other compounds were used as raw materials,and four different structural types of novel polyoxotantalates were obtained by optimizing the reaction time,temperature,pH and other conditions.The synthesized compounds were characterized by X-ray powder diffraction,elemental analysis,infrared spectroscopy,thermogravimetric analysis,and the properties were studied by solid UV-Vis spectra,electrospray mass spectrometry and magnetic properties1.Two polyoxotantalates based on high-nuclearity transition metal clusters were prepared by combining conventional aqueous solution method and diffusion method:Na21K12[Fe10.5(OH)8(H2O)2Ta37.5O122]·62H2O(1)Cs2K12[Ni13(OH)8(H2O)4Ta36098(OH)16]·52H2O(2)Both compounds 1 and 2 are polyoxotantalates with high degree of polymerization by connecting six{Ta6O19} building blocks with the central transition metal cluster through bridging oxygen,presenting an overall trigonal-antiprismatic configuration.Compound 1 is the first hexamer polyoxotantalates with a high-nuclearity ferrum-cluster as the center,and the centrosymmetric high-nuclearity ferrum-cluster{Fe10.5(OH)8(H2O)2O8} has a three-layer structure,which is formed by connecting the upper and lower{Fe3.25(H2O)O2} units with {Fe4(OH)4} units through bridging oxygen;Compound 2 is the first hexamer polyoxotantalates with a high-nuclearity nickel-cluster as the center,as well as the hexamer polyoxotantalates with highest-nuclearity transition metal cluster until now,appearing an overall trigonal-antiprismatic assembly,and the central nickel-cluster {Ni13(OH)8(H2O)4O6} also exhibits an trigonal-antiprismatic configuration,which is formed by the connection of six Ni through the bridging oxygen with a double-cubane core {Ni7(OH)8}.This double trigonal-antiprismatic configuration is rare in the field of polyoxometalates.The electrospray mass spectrometry showed that compound 1 has a certain stability in aqueous solution as well as compound 2,and the magnetic measurement results showed that compounds 1 and 2 both exhibits antiferromagnetic coupling.2.A binuclear transition metal bridging polyoxotantalate was synthesized by conventional aqueous solution method and diffusion method K11H9[Cu2O(Ta6O19)2)·24H2O(3)Compound 3 is the first pure inorganic dimer of polyoxotantalate with transition metal Cu as the connecting center.Its anions can be regarded as two Lindqvist type {Ta6O19} building units connected by two Cu atoms.Each adjacent dimer is connected through two μ2-O bridges,and forming a one-dimensional chain structure.The magnetic test showed that there is antiferromagnetic interaction in compound 3.3.Two transition metal derivatives based on phosphate/tantalum heteropolytantalate were synthesized by conventional aqueous solution method:Cs3NaH4[Zn(H2O)4{P4Ta6(O2)6O24}]·13H2O(4)Cs3KH4[Cd(H2O)4{P4Ta6(O2)6O24}]·8H2O(5)Compounds 4 and 5 are both transition metal derivatives based on the anion[P4Ta6(O2)6O24]10-(P4Ta6),and their anions are isostructural,which is formed by a {M(H2O)4} unit(M=Cd,Zn)connected with the P4Ta6 unit through the double bridging oxygen μ2-O.Moreover,the transition metal and the counterbalance cations are used as the connecting nodes to expand into a two-dimensional planar configuration.The electrospray mass spectrometry showed that compounds 4 and 5 are both stable to a certain extent in aqueous solution.