Hydrothermal syntheses:: A route to the stepwise condensation of reduced Keggin polyanions.: From reduced β-[HmSiMo12O40]n- monomers to bicapped dimerized [Si2Mo28O84(H2O)2]6- anions

The five reduced polyoxomolybdate anions [HSiMo12O40](4-) (HIbeta), [H2SiMo12O40](4-) (H2IIbeta), [H5SiMo12O40](3-)(H5IVbeta), [SiMo14O44](4-) (SiMo14), and [Si2Mo28O84(H2O)(2)](6-) (Si2Mo28) have been synthesized as tetramethylammonium salts, by hydrothermal reactions, and characterized by IR spectroscopy, redox titrations (HIbeta, H2IIbeta), and X-ray crystal structure analyses. Crystal data: (TMA)(4)HIbeta. 1.5H(2)O, monoclinic space group P2(1)/n, a = 12.2487(3) Angstrom, b = 39.9307(11) Angstrom, c = 12.4761(3) Angstrom, beta = 116.542(1)degrees; (TMA)(4)H2IIbeta. 2H(2)O, monoclinic space group P2(1)/n, a = 12.7930(2) Angstrom, b = 21.6171(2) Angstrom, c 19.7222(2) Angstrom, beta = 96.017(1)degrees; (TMA)(3)H5IV beta . 2H(2)O, monoclinic space group P2(1)/n, a = 12.7763(5) Angstrom, b = 20.2277(8) Angstrom, c = 20.9784(9) Angstrom, beta = 98.037(1)degrees; (TMA)(4)SiMo14, monoclinic space group P2(1)/n, a = 12.0502(4) Angstrom, b = 11.7500(4) Angstrom, c = 19.8001(5) Angstrom, beta = 100.525(1)degrees; (TMA)(6)Si2Mo28. 2H(2)O, monoclinic space group P2(1)/n, a = 19.3591(2) Angstrom, b = 13.1119(2) Angstrom, c = 21.1265(2) Angstrom, beta = 101.124(1)degrees. HIbeta, H2IIbeta, and H5IVbeta are respectively one-, two-, and four-electron-reduced {SiMo12O40} anions with the beta-Keggin type structure. SiMo14 and Si2Mo28 are two novel anions containing alpha-Keggin cores; in SiMo14 the Keggin core is capped by two {MoO2} units while in Si2Mo28 two bicapped anions are Linked through terminal oxo groups. A comparison of the molecular structures of the five anions shows the role of the protonation, the reduction, and the aggregation of capping {MoO2} units on the structural features of the Keggin core. The condensation process is discussed in terms of Lewis-type interactions between reduced, highly charged basic polyanions and reactive {MoO2}(2+) units.