Synthesis and characterization of iron(III)-substituted, dimeric polyoxotungstates, [Fe4(H2O)10(β-XW9O33)2]n- (n=6, X = AsIIISbIII; n=4, X = SeIV, TeIV)

Interaction of the lacunary [alpha-XW9O33](9-) (X = As-III, Sb-III) with Fe3+ ions in acidic, aqueous medium leads to the formation of dimeric polyoxoanions, [Fe-4(H2O)(10)(beta-XW9O33)(2)](6-) (X = As-III, Sib(III)) in high yield. X-ray single-crystal analyses were carried out on Na-6[Fe-4(H2O)(10)(beta-AsW9O33)(2)].32H(2)O, which crystallizes in the monoclinic system, space group C2/m, with a = 20.2493(18) Angstrom, b = 15.2678(13) Angstrom, c = 16.0689(14) Angstrom, beta = 95.766(2)degrees, and Z = 2; Na-6[Fe-4(H2O)(10)(beta-SbW9O33)(2)].32H(2)O is isomorphous with a = 20.1542(18) Angstrom, b = 15.2204(13) Angstrom, 0 = 16.1469(14) Angstrom, and beta = 95.795(2)degrees. The selenium and tellurium analogues are also reported, [Fe-4(H2O)(10)(beta-XW9O33)(2)](4-)(X = Se-IV, Te-IV). They are synthesized from sodium tungstate and a source of the heteroatom as precursors. X-ray single-crystal analysis was carried out on Cs-4[Fe-4(H2O)(10)(beta-SeW9O33)(2)].21H(2)O, which crystallizes in the triclinic system, space group P (1) over bar, with a = 12.6648(10) Angstrom, b = 12.8247(10) Angstrom, c = 16.1588(13) Angstrom, alpha = 75,6540(10)degrees, beta = 87,9550(10)degrees, gamma = 64.3610(10)degrees, and Z = 1. All title polyanions consist of two (beta-XW9O33) units joined by a central pair and a peripheral pair of Fe3+ ions leading to a structure with idealized C-2h symmetry. It was also possible to synthesize the Cr(III) derivatives [Cr-4(H2O)(10)(beta-XW9O33)(2)](6-) (X = As-\\\, Sb-\\\), the tungstoselenates(IV) [M-4(H2O)(10)(beta-SeW9O33)(2)](16-4n)(-) (Mn+ = Cr3+, Mn2+, Co2+, Ni2+, Zn2+, Cd2+, and Hg2+), and the tungstotellurates(IV) [M-4(H2O)(10)(beta-TeW9O33)(2)](16-4n)(-) (Mn+ = Cr3+, Mn2+, Co2+, Ni2+, Cu2+, Zn2+, Cd2+, and Hg2+), as determined by FTIR. The electrochemical properties of the iron-containing species were also studied. Cyclic voltammetry and controlled potential coulometry aided in distinguishing between Fe3+ and W6+ waves. By variation of pH and scan rate, it was possible to observe the stepwise reduction of the Fe3+ centers.