Giant polyoxometalates with catalytic, magnetic, and antiviral properties, which are in part attributable to their structures, are currently of great interest. Herein is described the synthesis and characterization of 1, a structurally novel tetrameric complex from Keggin ions (see picture). This complex is stable under the physiological conditions of the stomach (pH 1-2), which is interesting since related keggin anions are among the least toxic and yet most potent antiviral agents of the more than 300 polyoxometalates investigated biologically and pharmacologically.

The mono- and trisubstituted peroxyniobium polyoxotungstates of formulas [(CH3)3NH]7[Si-(NbO2)3W9O37], Cs7[Si(NbO2)3W9O37], alpha-K5[Si(NbO2)W11O39] and alpha-[(CH3)3NH]5[Si(NbO2)-W11O39], have been prepared, purified, and characterized spectroscopically by 29Si NMR, 183W NMR, and IR. The presence of peroxo groups was verified by the yellow color of the product and quantified by iodometric titration. The potency of both the complexes and the precursor complexes was evaluated in human peripheral blood mononuclear cells (PBMC) acutely infected with human immunodeficiency virus type 1 (HIV-1). Hexaniobate (K7H[Nb6O19]) was the least effective with a median effective concentration (EC50) of > 100 microM, while Cs7[Si(NbO2)3W9O37] was one of the most effective compounds with an EC50 of 1.0 microM. None of the compounds were toxic to uninfected PBMC with the exception of alpha-K8[SiW11O39], which had a median inhibitory concentration (IC50) of 79 microM. The potency and selectivity of the complexes against HIV-1 reverse transcriptase was also evaluated and shown to be quite high (IC50 values from 0.03 to 0.06 microM). The trimethylammonium salts of the complexes were tested for their ability to inhibit the interaction between gp120 and CD4 using a cell-free system. The complex [(CH3)3NH]7[Si(NbO2)3W9O37] inhibited this interaction by 70% at 25 microM.

The reaction of K(7)[HNb(6)O(19)], H(2)O(2) and A-Na(9)[PW(9)O(34)] in water followed by treatment with Cs(+) or (n-Bu(4)N)(+) (TBA) affords the corresponding salts of the tris(peroxoniobium) heteropolyanion A,beta-[(NbO(2))(3)PW(9)O(37)](6)(-) (1) in approximately 60% isolated yields. An X-ray structure of the Cs salt, Cs1 (monoclinic P2/c; a = 16.92360(10) Å, b = 13.5721(2) Å, c = 22.31890(10) Å, beta = 92.0460(10) degrees, and Z = 4) confirms the A-type substitution pattern of the three Nb atoms and clarifies the M(3) rotational (Baker-Figgis) isomerism in the Keggin unit as beta. The three terminal eta(2)-O(2)(2)(-) groups on the Nb atoms give 1 an overall symmetry approximating the chiral C(3). These terminal peroxo ligands, and these groups only, thermally decompose when either Cs1 or TBA1 is in solution unless additional H(2)O(2) is present. The peroxo groups can be titrated with triphenylphosphine (2.8 +/- 0.3 peroxide groups found per molecule). Refluxing TBA1 in acetonitrile for 24 h in the presence of base generates the parent heteropolyanion, [Nb(3)PW(9)O(40)](6)(-) (2) in 80% yield after isolation. Treatment of 2 with glacial acetic acid in acetonitrile converts it to [Nb(6)P(2)W(18)O(77)](6)(-) (3) in approximately 100% yield, while treatment of TBA3 with hydroxide converts it back to 2 in high yield. Spectroscopic (FTIR, Raman, (183)W NMR, and (31)P NMR), titrimetric, mass spectrometric (FABMS), and elemental analysis data are all consistent with these formulas. The addition of TBA1 to solutions of alkenes and 33% aqueous peroxide in acetonitrile at reflux results in the generation of the corresponding vicinal diols in high selectivity and yield at high conversion of substrate. Several spectroscopic and kinetics experiments, including a novel one correlating the incubation time of TBA1 under the reaction conditions with the rates of alkene oxidation, establish that TBA1 functions primarily as a catalyst precursor and that much of the catalytic activity is derived from generation of tungstate under the reaction conditions.

The reversible, stepwise formation of individual Nb-mu-O-Nb linkages during acid condensation of 2 equiv of A-alpha-[SiNb(3)W(9)O(40)](7-) (1) to the tri-mu-oxo-bridged structure A-alpha-[Si(2)Nb(6)W(18)O(77)](8-) (4) is demonstrated by a combination of X-ray crystallography and variable-pD solution (183)W and (29)Si NMR spectroscopy. Addition of DCl to a pD 8.4 solution of 1 (Li(+) salt in D(2)O) results in formation of a mono-Nb-mu-O-Nb-linked dimer, A-alpha-[Si(2)Nb(6)W(18)O(79)](12-) (2; pD = 3.0-1.3). At pD values between 1.6 and 0.3, two isomers (syn and anti) of the di-mu-oxo-bridged dimer, A-alpha-[Si(2)Nb(6)W(18)O(78)](10-) (3), are observed by (183)W NMR (C(2v) and C(2h) symmetry for the syn and anti isomers, respectively; 5 (183)W NMR signals for each isomer in the ratio 2:2:2:2:1). X-ray-quality crystals of syn-3 were isolated in 53% yield (syn-A-alpha-Cs(8)H(2)[Si(2)Nb(6)W(18)O(78)].18H(2)O, orthorhombic, Cmcm, a = 40.847(2), b = 13.2130(7), and c = 16.8179(9) A at 173K, Z = 4, final R(1) = 0.0685). At the low-pD limit of -0.08 (1.2 M DCl), 4 alone is observed. Additional supporting data are provided by variable-pD (29)Si NMR spectroscopy. Reversibility of the above processes was subsequently demonstrated by acquisition of (183)W NMR spectra after incremental additions of LiOH to D(2)O solutions of 4 to effect its stepwise hydrolysis to 2 equiv of 1.

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