Polyoxometalate-based gelating networks for entrapment and catalytic decontamination.

We report the synthesis and characterization of a new class of organic/inorganic hybrid polymers composed of covalently-bound 1,3,5-benzenetricarboxamide linkers and anionic polyoxovanadate clusters with varying counter-cations. These materials form gels within seconds upon contact with polar aprotic organic liquids and catalyze the degradation of odorants and toxic molecules under mild conditions including aerobic oxidation of thiols, hydrogen peroxide-catalyzed oxidation of sulfides, and hydrolysis of organophosphate chemical warfare agent analogues.

Revisiting the polyoxometalate-based late-transition-metal-oxo complexes: the "oxo wall" stands.

Terminal oxo complexes of the late transition metals Pt, Pd, and Au have been reported by us in Science and Journal of the American Chemical Society. Despite thoroughness in characterizing these complexes (multiple independent structural methods and up to 17 analytical methods in one case), we have continued to study these structures. Initial work on these systems was motivated by structural data from X-ray crystallography and neutron diffraction and (17)O and (31)P NMR signatures which all indicated differences from all previously published compounds. With significant new data, we now revisit these studies. New X-ray crystal structures of previously reported complexes K(14)[P(2)W(19)O(69)(OH(2))] and "K(10)Na(3)[Pd(IV)(O)(OH)WO(OH(2))(PW(9)O(34))(2)]" and a closer examination of these structures are provided. Also presented are the (17)O NMR spectrum of an (17)O-enriched sample of [PW(11)O(39)](7-) and a careful combined (31)P NMR-titration study of the previously reported "K(7)H(2)[Au(O)(OH(2))P(2)W(20)O(70)(OH(2))(2)]." These and considerable other data collectively indicate that previously assigned terminal Pt-oxo and Au-oxo complexes are in fact cocrystals of the all-tungsten structural analogues with noble metal cations, while the Pd-oxo complex is a disordered Pd(II)-substituted polyoxometalate. The neutron diffraction data have been re-analyzed, and new refinements are fully consistent with the all-tungsten formulations of the Pt-oxo and Au-oxo polyoxometalate species.

Relative energies of alpha and beta isomers of Keggin dodecatungstogallate.

The relative energies of beta Keggin heteropolytungstates, X(n+) W12O40(8-n)-, decrease as X(n+) is varied within period 3, from P5+ to Si4+ to Al3+. With heating of alpha-H5Ga3+ W12O40 at 200 degrees C in water, an equilibrated mixture of alpha (T(d); one 183W NMR signal) and beta (C(3v); three signals; 1:2:1 ratio) isomers is obtained. From deltaG(exp) = -RT ln K(beta-->alpha), in which (from 71Ga NMR spectra) K(beta-->alpha) (= [alpha]/[beta]) = 5.0, beta-GaW12O40(5-) is 0.65 kcal mol(-1) higher in energy than alpha-GaW12O40(5-). This finding is evaluated by analysis of the X-ray crystal structure alpha-K2Na3[GaW12O40] x 9.3 H2O [trigonal, space group P3(2)21, a = 18.9201(13) A, b = 18.9201(13) A, c = 12.5108(12) A, Z = 3, T = 100(2)K], comparison of the Shannon and Prewitt radii and Pauling electronegativities of Al3+ and Ga3+, and insight from density functional theory calculations, which predicted Ebeta - Ealpha = 0.32 kcal mol(-1).

A palladium-oxo complex. Stabilization of this proposed catalytic intermediate by an encapsulating polytungstate ligand.

A terminal Pd-oxo unit is reported. The unit is encapsulated in a cavity defined by two [A-alpha-PW9O34]9- units fused together by one [WO(OH2)]4+ center and forms from Pd(II) in buffered media in the presence of O2. Both X-ray diffraction and EXAFS data are consistent with a Pd-oxo bond distance of ca. 1.65 A. 17O NMR studies confirm that the solid-state structure is maintained in solution.

Biomimetic synthesis of trans,syn,trans-fused polyoxepanes: remarkable substituent effects on the endo-regioselective oxacyclization of polyepoxides.

The biogenesis of trans,syn,trans-fused polycyclic ether substructures of neurotoxic natural products of the brevetoxin-ciguatoxin family may involve regio- and stereoselective oxacyclizations of polyepoxides. We report that a biomimetic Lewis acid-promoted cyclization of 1 to 2 proceeds with endo-regioselectivity and anti-stereospecificity at each position of nucleophilic addition (C3, C7, C11, and C15 in 1) when the tandem cyclization cascade is terminated by a carbonyl oxygen nucleophile. In contrast to expectations from earlier reports with simpler epoxide substrates, alkyl substituents are not required at internal epoxide positions corresponding to C7 and C11 in polyepoxide 1. Furthermore, we report that trimethylsilyl serves as a removable regioselectivity-directing substituent at positions corresponding to C3 of 1. These results greatly expand the potential applications of this biomimetic oxacyclization cascade to the synthesis of fused polycyclic ether natural products.

Aerobic oxidation of formaldehyde mediated by a Ce-containing polyoxometalate under mild conditions.

An evaluation of over 50 polyoxometalates (POMs) identified the complex NaH3[SiW11Ce(IV)O39] (NaH3(1)) as a selective and effective catalyst for the aerobic oxidation of formaldehyde to formic acid under very mild (including ambient) conditions. 183W NMR, UV-vis, cyclic voltammetry, and potentiometric titration establish that the catalyst is a monomer (Cs symmetry), 1, in solution, while X-ray crystallography (a = 12.9455(15) A, b = 13.2257(16) A, c = 14.5288(17) A, alpha = 81.408(2) degrees , beta = 85.618(2) degrees , gamma = 80.726(2) degrees , P, Z = 1, R1 = 5.79% based on 17244 independent reflections) and IR establish it to be a dimer (Ci symmetry), 1(2), in the solid state. Several lines of evidence, including the parabolic kinetic order in 1, nonlinear Arrhenius plot, independence of the rate on O2 pressure, presence of titratable H2O2 and HCO3H intermediates, and inhibition by conventional radical scavengers, all indicate the O2-based oxidations proceed by complex homolytic chemistry (autoxidation and Haber-Weiss radical-chain processes) likely initiated by protonated 1.

Multi-iron silicotungstates: synthesis, characterization, and stability studies of polyoxometalate dimers.

The reaction of Fe(III) with Na(+) and K(+) salts of the trivacant [alpha-SiW(9)O(34)](10)(-) ligand have been investigated at pH 6 and pH 1. A new dimer, [(alpha-SiFe(3)W(9)(OH)(3)O(34))(2)(OH)(3)](11-) (1), is synthesized by reacting Na(7)H(3)[alpha-SiW(9)O(34)] or K(10)[alpha-SiW(9)O(34)] with exactly 3 equiv of Fe(III) in a 0.5 M sodium acetate solution (pH 6). The structure of 1, determined by single-crystal X-ray diffraction (a = 22.454(2) A, b = 12.387(2) A, c = 37.421(2), beta = 100.107(8) degrees , monoclinic, C2/c, Z = 4, R(1) = 5.11% based on 12739 independent reflections), consists of two [alpha-SiFe(3)W(9)(OH)(3)O(34)](4-) units linked by three Fe-mu-OH-Fe bonds. Reaction of K(10)[alpha-SiW(9)O(34)] with 3 equiv of Fe(III) in water (pH 1) yields [(alpha-Si(FeOH(2))(2)FeW(9)(OH)(3)O(34))(2)](8)(-2). The structure of 2 was also determined by single-crystal X-ray diffraction (a = 36.903(2) A, b = 13.9868(9) A, c = 21.7839(13) A, beta = 122.709(1) degrees , monoclinic, C2/c, Z = 4, R(1) = 4.57% based on 11787 independent reflections). It consists of two [alpha-Si(FeOH(2))(2)FeW(9)(OH)(3)O(34)](4-) Keggin units linked by a single edge. The terminal ligand on Fe1 in each trisubstituted Keggin unit becomes a mu(2) oxo ligand bridging to a [WO(6)](2-) moiety. The UV-vis spectra of both complexes show the characteristic oxygen-to-metal-charge-transfer bands of polyoxometalates as well as an Fe(III)-centered band at 436 nm (epsilon = 146 M(-1) cm(-1)) and 456 nm (epsilon = 104 M(-1) cm(-1)) for complexes 1 and 2, respectively. Differential scanning calorimetry data show that complex 1 decomposes between 575 and 600 degrees C whereas no decomposition is observed for complex 2 up to temperatures of 600 degrees C.

Alkene substituents for selective activation of endo-regioselective polyepoxide oxacyclizations.

The presence of an alkenyl substituent on the terminal epoxide of a polyepoxide substrate enhances the yield of all-endo-regioselective tandem oxacyclization to trans-syn-trans-fused polycyclic ethers. For a substrate in which the epoxide and alkene functional groups are separated by two methylene substituents, a novel bromonium ion-induced endo-regioselective cyclization to bromooxepane is also described. [reaction: see text]

A late-transition metal oxo complex: K7Na9[O=PtIV(H2O)L2], L = [PW9O34]9-.

Terminal mono-oxo complexes of the late transition metal elements have long been considered too unstable to synthesize because of repulsion between the oxygen electrons and the mostly filled metal d orbitals. A platinum(IV)-oxo compound flanked by two polytungstate ligands, K7Na9[O=Pt(H2O)L2], L = [PW9O34(9-)], has now been prepared and isolated at room temperature as air-stable brown crystals. X-ray and neutron diffraction at 30 kelvin revealed a very short [1.720(18) angstrom] Pt-O bond and no evidence of a hydrogen atom at the terminal oxygen, ruling out a better precedented Pt-OH complex. Density functional theory and spectroscopic data account for the stability of the Pt(IV)-oxo unit by electron withdrawal into delocalized orbitals of the polytungstates.

First isolated active titanium peroxo complex: characterization and theoretical study.

The protonated titanium peroxo complex [Bu(4)N](4)[HPTi(O(2))W(11)O(39)] (1) has been first prepared via interaction of the micro-oxo dimeric heteropolytungstate [Bu(4)N](8)[(PTiW(11)O(39))(2)O] (3) with an excess of 30% aqueous H(2)O(2) in MeCN. Peroxo complex 1 has been characterized by using elemental analysis, UV-vis, IR, resonance Raman (RR), (31)P and (183)W NMR spectroscopy, cyclic voltammetry, and potentiometric titration. The electronic and vibrational spectra of 1 are very similar to those of the well-known unprotonated titanium peroxo complex [Bu(4)N](5)[PTi(O(2))W(11)O(39)] (2), while (31)P and (183)W NMR spectra differ significantly. A compilation of the physicochemical techniques supports a monomeric Keggin type structure of 1 bearing one peroxo ligand attached to Ti(IV) in a eta(2)-coordination mode. The protonation of the titanium peroxo complex results in an increase of the redox potential of the peroxo group, E(1/2) = 1.25 and 0.88 V relative to Ag/AgCl reference electrode for 1 and 2, respectively. In contrast to 2, 1 readily reacts with 2,3,6-trimethylphenol (TMP) at 40 degrees C in MeCN to give 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol (BP) and 2,3,5-trimethyl-p-benzoquinone (TMBQ). The proportion between BP and TMBQ in the reaction products depends on the TMP/1 ratio. When a 2-fold excess of TMP is used, the main reaction product is BP (90%), while using a 2-fold excess of 1 leads to TMBQ (95%). On the basis of the product study, a homolytic oxidation mechanism that implicates the formation of phenoxyl radicals is suggested. The RR deuterium labeling experiments show that the activating proton is most likely localized at a Ti-O-W bridging oxygen rather than at the peroxo group. Theoretical calculations carried out at the DFT level on the protonated and unprotonated titanium peroxo derivatives also propose that the most stable complex is formed preferentially after protonation of the Ti-O-W site; however, both Ti-OH-W and TiOO-H protonated anions could coexist in solution.

Decomposition of a-type sandwiches. Synthesis and characterization of new polyoxometalates incorporating multiple d-electron-centered units.

The controlled decomposition of the sandwich-type polyoxometalates K(12)[(M(OH(2))(2))(3)(A-alpha-PW(9)O(34))(2)] (where M = Mn(II) or Co(II)) in 0.5 M NaCl yields a new family of transition metal substituted POMs of the general formula [((MOH(2))M(2)PW(9)O(34))(2)(PW(6)O(26))](17)(-) (where M = Mn(II) (1Mn) or Co(II) (1Co)). The structure of 1Mn, determined by single-crystal X-ray diffraction (a = 17.4682(10) A, b = 22.3071(12) A, c = 35.1195(18) A, beta = 95.898(1) degrees, monoclinic, P2(1)/c, Z = 4, R(1) = 6.19%, based on 50264 independent reflections), consists of two B-alpha-(Mn(II)OH(2))Mn(II)(2)PW(9)O(34)(3)(-) units joined by a B-type hexavacant PW(6)O(26)(11)(-) fragment to form a C-shaped polyoxometalate. A low resolution X-ray structure of the Co(II) analogue, 1Co, was also obtained. The UV-visible spectrum of 1Co shows the characteristic charge-transfer bands of polyoxometalates as well as a new Co-centered peak (560 nm, epsilon = 416 M(-)(1) cm(-)(1)) which appears at a higher wavelength relative to that exhibited by the parent A-type sandwich, K(12)[(Co(OH(2))(2))(3)(A-alpha-PW(9)O(34))(2)]. The methyltricaprylammonium salt of 1Mn is an effective catalyst for the H(2)O(2)-based epoxidation of cis-cyclooctene, cyclohexene, and 1-hexene.

Yttrium polyoxometalates. Synthesis and characterization of a carbonate-encapsulated sandwich-type complex.

Reaction of A-alpha-PW(9)O(34)(9)(-) with YCl(3) in an aqueous Na(2)CO(3) solution produces a dianion-encapsulated A-type sandwich polyoxometalate, (YOH(2))(3)(CO(3))(A-alpha-PW(9)O(34))(2)(11)(-). The X-ray structure of this complex reveals that three Y(III) ions are sandwiched between two A-alpha-PW(9)O(34)(9)(-) moieties and that a carbonate dianion is encapsulated in the same plane as the three Y(III) atoms. The oxygen atoms of the CO(3)(2)(-) are sitting at the midpoints of the sides of the triangle formed by the three Y(III) ions. (31)P and (13)C NMR studies confirm that this complex is significantly more stable than the analogous A-type sandwich polyoxometalates containing divalent metals.

Dimerization of A-alpha-[SiNb(3)W(9)O(40)](7-) by pH-controlled formation of individual Nb-mu-O-Nb linkages.

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.

Biomimetic synthesis of fused polypyrans: oxacyclization stereo- and regioselectivity is a function of the nucleophile.

[reaction: see text] The stereoselectivity of Lewis acid-induced endo-regioselective oxacyclizations of 1,4-diepoxides is dependent upon the nature of the terminating nucleophile. For instance, the tert-butyl carbonate-substituted diepoxide of 3,6-dimethylhepta-2,5-dien-1-ol provides a cis-fused bicyclic product, whereas the N,N-dimethylcarbamate derivative affords the trans-fused diastereomer. Stereospecific and regioselective conversion of the tertiary carbamate-terminated 1,4,7-triepoxide (I) to tricyclic all-trans-fused polypyran (II) is also demonstrated.

Stability and structure in alpha- and beta-Keggin Heteropolytungstates, [X(n)(+)W12O40)](8-n)(-), X = p-block cation.

beta-[SiW(12)O(40)](4)(-) (C(3)(v) symmetry) is sufficiently higher in energy than its alpha-isomer analogue that effectively complete conversion to alpha-[SiW(12)O(40)](4)(-) (T(d)) is observed. By contrast, beta- and alpha-[AlW(12)O(40)](5)(-) (beta- and alpha-1; C(3)(v) and T(d), respectively) are sufficiently close in energy that both isomers are readily seen in (27)Al NMR spectra of equilibrated (alpha-beta) mixtures. Recently published DFT calculations ascribe the stability of beta-1 to an electronic effect of the large, electron-donating [AlO(4)](5)(-) (T(d)) moiety encapsulated within the polarizable, fixed-diameter beta-W(12)O(36) (C(3)(v)) shell. Hence, no unique structural distortion of beta-1 is needed or invoked to explain its unprecedented stability. The results of these DFT calculations are confirmed by detailed comparison of the X-ray crystal structure of beta-1 (beta-Cs(4.5)K(0.5)[Al(III)W(12)O(40)].7.5H(2)O; orthorhombic, space group Pmc2(1), a = 16.0441(10) A, b = 13.2270(8) A, c = 20.5919(13) A, Z = 4 (T = 100(2) K)) with previously reported structures of alpha-1, alpha- and beta-[SiW(12)O(40)](4)(-), and beta(1)-[SiMoW(11)O(40)](4)(-).

Endo-oxacyclizations of polyepoxides: biomimetic synthesis of fused polycyclic ethers.

Boron trifluoride-etherate promotes the endo-selective oxacyclization of polyepoxides derived from various acyclic terpenoid polyalkenes, including geraniol, farnesol, and geranylgeraniol, providing an efficient and stereoselective synthesis of substituted oxepanes and fused polyoxepanes. The mechanism of the oxacyclization reaction probably involves intramolecular nucleophilic addition of epoxide oxygen to open another epoxide that is activated as an electrophile by the Lewis acid. These oxacyclizations proceed stereospecifically with inversion of configuration upon opening of each epoxide to provide trans-fused polycyclic products. The oxacyclization cascade is terminated by a tethered nucleophile, which may be the carbonyl oxygen of a ketone, ester, or carbonate, or a trisubstituted alkene. The best oxacyclization yields are generally observed with tert-butyl carbonate as the terminating nucleophile, although in some cases the oxacyclization products include formation of tert-butyl ethers as a minor product. The oxacyclization transformations described herein may mimic ring-forming steps in the biosynthesis of trans-syn-trans-fused polycyclic ether marine natural products.