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 [HSiMo 12 O 40 ] 4 - (HI β ), [H 2 SiMo 12 O 40 ] 4 - (H 2 II β ), [H 5 SiMo 12 O 40 ] 3 - (H 5 IV β ), [SiMo 14 O 44 ] 4 - (SiMo 14 ), and [Si 2 Mo 28 O 84 (H 2 O) 2 ] 6 - (Si 2 Mo 28 ) have been synthesized as tetramethylammonium salts, by hydrothermal reactions, and characterized by IR spectroscopy, redox titrations (HI β, H 2 II β ), and X-ray crystal structure analyses. Crystal data: (TMA) 4 HI β ·1.5H 2 O, monoclinic space group P 2 1 / n, a = 12.2487(3) Å, b = 39.9307(11) Å, c = 12.4761(3) Å, β = 116.542(1)°; (TMA) 4 H 2 II β ·2H 2 O, monoclinic space group P 2 1 / n, a = 12.7930(2) Å, b = 21.6171(2) Å, c = 19.7222(2) Å, β = 96.017(1)°; (TMA) 3 H 5 IV β ·2H 2 O, monoclinic space group P 2 1 / n, a = 12.7763(5) Å, b = 20.2277(8) Å, c = 20.9784(9) Å, β = 98.037(1)°; (TMA) 4 SiMo 14, monoclinic space group P 2 1 / n, a = 12.0502(4) Å, b = 11.7500(4) Å, c = 19.8001(5) Å, β = 100.525(1)°; (TMA) 6 Si 2 Mo 28 ·2H 2 O, monoclinic space group P 2 1 / n, a = 19.3591(2) Å, b = 13.1119(2) Å, c = 21.1265(2) Å, β = 101.124(1)°. HI β, H 2 II β, and H 5 IV β are respectively one-, two-, and four-electron-reduced {SiMo 12 O 40 } anions with the β-Keggin type structure. SiMo 14 and Si 2 Mo 28 are two novel anions containing α-Keggin cores; in SiMo 14 the Keggin core is capped by two {MoO 2 } units while in Si 2 Mo 28 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 {MoO 2 } 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 {MoO 2 } 2+ units.