Synthesis and structure of sandwich-type polytungstoarsenates of [As_2W_(18)Cd_4(H_2O)_2O_(68)]~(10-) and [As_2W_(18)Zn_2(CdOH_2)_20_(68)]~(10-)

摘要

Isomerically pure complexes of [As_2W_(18)Cd_4(H_2O)_2O_(68)]~(10-) and [As_2W_(18)Zn_2(CdOH_2)_2O_(68)]~(10-) were synthesized in high yield by self-assembly of WO_4~(2-) and HAsO_4~(2-) (18:2) solution with the stoichiometric ratio of Cd~(2+) and Zn~(2+) ions. The complexes were characterized by elemental analysis, IR, Raman, ~(113)Cd NMR spectroscopy, and single crystal structure analysis. Single crystal structure analysis was carried out on (NH_4)_(10)[As_2W_(18)Cd_4(H_2O)_2O_(68)] centre dot 23H_2O which crystallized in the monoclinic system, space group C2/c with a = 23.512(3)A, b- 12.480(2)A, c = 30.583(4) A, beta = 95.597(3) deg, 2=4, and V= 8931(2) A~3. The structure consists of two B-AsW_9O_(34)~(9-) Keggin moieties linked via a rhomb-like Cd_4O_(14) group leading to a C2h symmetry sandwich-type complex. Double- and single-line ~(113)Cd NMR spectra of [As_2W_(18)Cd_4(H_2O)_2O_(68)]~(10-) and [As2_2W_(18)Zn_2(CdOH_2)_2O_(68)]~(10-), respectively, confirm C_(2h) symmetry for both anions in solution. Replacement of Cd~(2+) with Zn~(2+) in [As_2W_(18)Zn_4(H_2O)_2O_(68)]~(10-) takes place only under vigorous conditions.