A dimeric titanium-containing polyoxometalate. Synthesis, characterization, and catalysis of H2O2-based thioether oxidation

The previously unknown titanium(IV)-containing mu-hydroxo dimeric heteropolytungstate (Bu4N)7[(PTiW11O39)2-OH] (TBA salt of H1) has been synthesized, starting from H5PTiW11O40, and characterized by elemental analysis, multinuclear (31P, 17O, 183W) NMR, IR, FAB-MS, cyclic voltammetry, and potentiometric titration. 31P NMR reveals that H1 (delta -12.76) readily forms in MeCN from the Keggin monomer (POM), PTiW11O40(5-) (2, delta -13.34), upon the addition of 1.5 equiv of H+, via the protonated species, P(TiOH)W11O39(4-) (H2, delta -13.44). The ratio of H1, 2, and H2, which are present in equilibrium in MeCN solution at 25 degrees C, depends on the concentration of both H+ and H2O. The Ti-O-Ti linkage readily reacts with nucleophilic reagents, such as H2O and ROH, to yield monomeric Keggin derivatives. mu-Hydroxo dimer H1 shows higher catalytic activity than 2 for thioether oxidation by hydrogen peroxide in acetonitrile. The reaction proceeds readily at room temperature and affords the corresponding sulfoxide and sulfone in ca. quantitative yield. The addition of H2O2 to H1 or H2 results in the formation of a peroxo complex, most likely the hydroperoxo complex P(TiOOH)W11O39(4-) (I), which has 31P NMR resonance at -12.43 ppm. The rate of the formation of I is higher from H2 than from H1. When H1 is used as a catalyst precursor, the rates of the thioether oxidation and peroxo complex formation increase with increasing H2O concentration, which favors the cleavage of H1 to H2. H2O2 in MeCN slowly converts 2 to another peroxotitanium complex, P(TiO2)W11O39(5-) (II), which has 31P NMR resonance at -12.98 ppm. Peroxo complexes I and II differ in their protonation state and interconvert fast on the 31P NMR time scale. Addition of 1 equiv of H+ completely converts II to I, while 1 equiv of OH- completely converts I to II. 31P NMR confirms that I is stable under turnover conditions (thioether, H2O2, MeCN). Contrary to two-phase systems such as dichloroethane/aqueous H2O2, no products resulting from the destruction of the Keggin POM were detected in MeCN in the presence of H2O2 (a 500-fold molar excess). The reactivity of I, generated in situ from II by adding 1 equiv of H+, toward organic sulfides under stoichiometric conditions was confirmed using both 31P NMR and UV-vis spectroscopy. This is a rare demonstration of the direct stoichiometric oxidation of an organic substrate by a titanium peroxo complex.     

Synthesis, characterization, and reactivity of Ti(IV)-monosubstituted Keggin polyoxometalates

Ti(IV)-monosubstituted Keggin-type polyoxometalates (Ti-POMs), mu-oxo dimer [Bu4N]8[(PTiW11O39)2O] (1), and three monomers [Bu4N]4[PTi(L)W11O39], where L = OH (2), OMe (3), and OAr (4, ArOH = 2,3,6-trimethylphenol (TMP)), have been prepared starting from mu-hydroxo dimer [Bu4N]7[(PTiW11O39)2OH] (5) or heteropolyacid H5PW11TiO40 or both. The compounds have been characterized by elemental analysis, IR, UV-vis, and multinuclear (31P, 1H, 183W) NMR. The interaction of 1 and 3-5 with H2O in MeCN produces 2. The hydrolysis constants, estimated from 31P and 1H NMR data, are 0.006 and 0.04 for 1 and 3, respectively. Studies by 31P NMR, IR, potentiometric titration, and cyclic voltammetry revealed that 1-3 and 5 afford the same protonated titanium peroxo complex [Bu4N]4[HPTi(O2)W11O39] (I) upon interaction with aqueous H2O2 in MeCN. The rates of formation of I correlate with the rates of hydrolysis of the Ti-POMs and follow the order of 5 > 1 > 3. A two-step mechanism of the reaction of Ti-POMs with H2O2, which involves hydrolysis of the Ti-L bonds to yield 2 followed by fast interaction of 2 with hydrogen peroxide producing I, is suggested. The equilibrium constant for the reaction of 2 with H2O2 to yield I and H2O, estimated using 31P NMR, is 10. The interaction of the Ti-POMs with TMP follows the trends similar to their interaction with H2O) and requires preliminary hydrolysis of the Ti-L bonds. All of the Ti-POMs catalyze the oxidation of TMP with H2O2 in MeCN to give 2,3,5-trimethyl-p-benzoquinone and 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol. The product distribution is similar for all of the Ti-POMs. The catalytic activities of the Ti-POMs correlate with the rates of formation of I and follow the order of 2 > 5 > 1 > 3. The findings lay a basis for a better understanding of the nature of the reactivity of titanium in Ti-catalyzed oxidations.     

Synthesis, characterization, and photochemical behavior of {Ru(arene)}2+ derivatives of alpha-[PW11O39]7-: an organometallic way to ruthenium-substituted heteropolytungstates

Reaction of [Ru(arene)Cl(2)](2) (arene = benzene, toluene, p-cymene, hexamethylbenzene) with K(7)[PW(11)O(39)].14H(2)O provided two series of organometallic derivatives of heteropolytungstates: type-1 and type-2 complexes of general formulas [PW(11)O(39){Ru(arene)(H(2)O)}](5-) and [{PW(11)O(39){Ru(arene)}}(2){WO(2)}](8-), respectively. All compounds were characterized by infrared and multinuclear NMR ((1)H, (31)P, (183)W) spectroscopies. The crystal structures of Na(4)K(4)[{PW(11)O(39){Ru(benzene)}}(2){WO(2)}].6H(2)O (NaK-2a.6H(2)O), K(7)H[{PW(11)O(39){Ru(toluene)}}(2){WO(2)}].4H(2)O (K-2b.4H(2)O), and Cs(3)K(2)[PW(11)O(39){Ru(p-cymene)(H(2)O)}].4H(2)O (CsK-1c.4H(2)O) were obtained and revealed that the {Ru(arene)} fragment is supported on the oxometallic framework. Photochemical reactivity of [PW(11)O(39){Ru(arene)(H(2)O)}](5-) (arene = toluene, p-cymene) in the presence of various ligands L (L = H(2)O, dimethyl sulfoxide, tetramethylene sulfoxide, and diphenyl sulfoxide) was investigated, and led to the formation of [PW(11)O(39){Ru(L)}](5-), in which the ruthenium is incorporated into the lacunary [PW(11)O(39)](7-) anion.     

Phenylimido Functionalization of alpha-[PW12O40]3-

A synthetic route of potentially wide scope is reported herein for the organoimido functionalization of polyoxotungstates. This report focuses on the reaction between the monovacant lacunary polyoxotungstate, alpha-((n-C4H9)4N)4H3[PW11O39], and W(NC6H5)Cl4 in anhydrous acetonitrile. Evidence from 1H, 31P, 183W, and 1H-183W HMQC NMR spectroscopy, as well as cyclic voltammetry, electronic absorption, and elemental analysis, is presented for the formation of alpha-[PW12O39(NC6H5)]3- (2) of Cs symmetry, which is structurally related to Td alpha-[PW12O40]3- (3) by formal oxide substitution. The electronic structure of 2 is significantly perturbed from 3 with significant arylimido-->tungsten charge transfer, primarily localized to the W(NC6H5) fragment with secondary charge delocalization onto the remaining W and corner-shared bridging O atoms. This is consistent with the approximately 800 ppm downfield 183W NMR shift for the phenylimido-tungsten, modest cathodic shifts in reversible redox potentials, electronic and IR spectra, and density functional theory calculations.     

Functionalized heteropolyanions: high-valent metal nitrido fragments incorporated into a Keggin polyoxometalate structure

Three examples of nitrido-functionalized polyoxometalate species are reported, namely (n-Bu4N)4[PW11O39(OsN)] (1), (n-Bu4N)4[PW11O39(ReN)] (2), and (n-Bu4N)3[PW11O39(ReN)] (3), which feature the incorporation of [OsVI identical to N]3+, [ReVI identical to N]3+ and [ReVII identical to N]4+ fragments, respectively, into the framework of a Keggin-type heteropolyanion.     

Synthesis and structural characterization of a gamma-Keggin-type dimeric silicotungstate with a bis(mu-hydroxo) dizirconium core [(gamma-SiW10O36)2Zr2(mu-OH)2]10-

A novel dinuclear zirconium sandwich-type silicotungstate cluster of [(gamma-SiW(10)O(36))(2)Zr(2)(mu-OH)(2)](10-) (1) was synthesized by the reaction of a divacant lacunary gamma-Keggin silicotungstate [gamma-SiW(10)O(36)](8-) with ZrOCl(2).8H(2)O. The anion consisted of two [gamma-SiW(10)O(36)](8-) units sandwiching a diamond Zr(2)(mu-OH)(2) core, and each zirconium atom in 1 was six-coordinated to two mu-OH ligands and four oxygen atoms of two [gamma-SiW(10)O(36)](8-) units. The Zr(2)(mu-OH)(2) core in 1 reacted with methanol to give the corresponding monomethoxo derivative [(gamma-SiW(10)O(36))(2)Zr(2)(mu-OH)(mu-OCH(3))](10-) (2).     

Aqueous speciation studies of europium(III) phosphotungstate

The incorporation of lanthanide ions into polyoxometalates may be a unique approach to generate new luminescent, magnetic, and catalytic functional materials. To realize these new applications of lanthanide polyoxometalates, it is imperative to understand the solution speciation chemistry and its impact on solid-state materials. In this study we find that the aqueous speciation of europium(III) and the trivacant polyoxometalate, PW9O34 9-, is a function of pH, countercation, and stoichiometry. For example, at low pH, the lacunary (PW11O39)7- predominates and the 1:1 Eu(PW11O39)4-, 2, forms. As the pH is increased, the 1:2 complex, Eu(PW11O39)2 11- species, 3, and (NH4)22[(Eu2PW10O38)4(W3O8(H2O)2(OH)4].44H2O, a Eu8 hydroxo/oxo cluster, 1, form. Countercations modulate this effect; large countercations, such as K+ and Cs+, promote the formation of species 3 and 1. Addition of Al(III) as a counterion results in low pH and formation of [Eu(H2O)3(alpha-2-P2W17O61)]2, 4, with Al(III) counterions bound to terminal W-O bonds. The four species observed in these speciation studies have been isolated, crystallized, and characterized by X-ray crystallography, solution multinuclear NMR spectroscopy, and other appropriate tech-niques. These species are 1, (NH4)22[(Eu2PW10O38)4(W3O8(H2O)2(OH)4].44H2O (P; a=20.2000(0), b=22.6951(6), c=25.3200(7) A; alpha=65.6760(10), beta=88.5240(10), gamma=86.0369(10) degrees; V=10550.0(5) A3; Z=2), 2, Al(H3O)[Eu(H2O)2PW11O34].20H2O (P, a=11.4280(23), b=11.5930(23), c=19.754(4) A; alpha=103.66(3), beta=95.29(3), gamma=102.31(3) degrees; V =2456.4(9) A3; Z=2), 3, Cs11Eu(PW11O34)2.28H2O (P; a=12.8663(14), b=19.8235(22), c=21.7060(23) A; alpha=114.57(0), beta=91.86(0), gamma=102.91(0) degrees ; V=4858.3(9) A3; Z=2), 4, Al2(H3O)8[Eu(H2O)3(alpha-2-P2W17O61)]2.29H2O (P; a=12.649(6), b=16.230(8), c=21.518(9) A; alpha=111.223(16), beta=94.182(18), gamma=107.581(17) degrees ; V=3842(3) A3; Z=1).     

Zirconium-substituted isopolytungstates: structural models for zirconia-supported tungsten catalysts

The synthesis and characterization of a series of mixed W-Zr polynuclear Lindqvist-type complexes, deriving from hexatungstate [W6O19]2-, are described in this work. This family of compounds is built from {W5O18Zr}2- moieties as shown by the X-ray structures of the monomeric [W5O18Zr(H2O)(3-n)(DMSO)n]2- (n = 1 and 2) and dimeric [{W5O18Zr(mu-OH)}2]6- anions. A comprehensive spectroscopic study (183W NMR, FTIR, Raman, EXAFS, and EPR) of these compounds is presented. The goal of incorporating Zr(IV) cations into an oxotungstic core is to obtain spectroscopic models that could mimic the interactions that develop in supported catalysts between the active phase and the supporting oxide. This work tends to show that these molecular compounds can be regarded as soluble structural analogues of WOx/ZrO2 catalysts, which are interesting candidates for the skeletal isomerization of light n-alkanes.     

Dimerization of mono-ruthenium substituted alpha-Keggin-type tungstosilicate [alpha-SiW11O39RuIII(H2O)]5- to micro-oxo-bridged dimer in aqueous solution: synthesis, structure, and redox studies

We report the dimerization of a mono-ruthenium(III) substituted alpha-Keggin-type tungstosilicate [alpha-SiW(11)O(39)Ru(III)(H2O)](5-) to a micro-oxo-bridged dimer [{alpha-SiW(11)O(39)Ru(m)}2O](n-) (m = III, n = 12; m = IV/III, n = 11; m = IV, n = 10). Single crystal X-ray structure analysis of Rb(10)[{alpha-SiW(11)O(39)Ru(IV)}2O].9.5H2O (triclinic, P1, with a = 12.7650(6) A, b = 18.9399(10) A, c = 20.2290(10) A, alpha = 72.876(3) degrees, beta = 88.447(3) degrees, gamma = 80.926(3) degrees, V = 4614.5(4) A(3), Z = 2) reveals that two mono-ruthenium substituted tungstosilicate alpha-Keggin units are connected through micro-oxo-bridging Ru-O-Ru bonds. Solution (183)W-NMR of [{SiW(11)O(39)Ru(IV)}2O](10-) resulted in six peaks (-63, -92, -110, -128, -132, and -143 ppm, intensities 2 : 2 : 1 : 2 : 2 : 2) confirming that the micro-oxo bridged dimer structure is maintained in aqueous solution. The dimerization mechanism is presumably initiated by deprotonation of the aqua-ruthenium complex [alpha-SiW(11)O(39)Ru(III)(H2O)](5-) leading to a hydroxy-ruthenium complex [alpha-SiW(11)O(39)Ru(III)(OH)](6-). Dimerization of two hydroxy-ruthenium complexes produces the micro-oxo bridged dimer [{alpha-SiW(11)O(39)Ru(III)}2O](12-) and a water molecule. The Ru(III) containing dimer is oxidized by molecular oxygen to produce a mixed valence species [{alpha-SiW(11)O(39)Ru(IV-III)}2O](11-), and further oxidation results in the Ru(IV) containing [{alpha-SiW(11)O(39)Ru(IV)}2O](10-).     

A monovacant heteropolytungstate thioderivative: Synthesis and characterization of [(PW11O39)2(H4Mo4S4O6)](10-) and related isomers

[(PW(11)O(39))(2)(Mo(4)S(4)O(4)(OH(2))(2))](10-) anions were obtained through the stereospecific addition of the [Mo(2)S(2)O(2)](2+) oxothiocation to the monovacant alpha-[PW(11)O(39)](7-) anion. K(10)[(PW(11)O(39))(2)(Mo(4)S(4)O(4)(OH(2))(2))].25H(2)O has been isolated as crystals and characterized by X-ray diffraction. The structure revealed a "sandwich-like" dimer of two alpha-[PW(11)O(39)](7-) subunits assembled by the noteworthy central cluster [H(4)Mo(4)S(4)O(6)]. The crystallization of the crude product produces an isomerically pure compound, which was characterized by (31)P and (183)W NMR. IR data were also supplied. In solution, the compound isomerizes, giving a second diastereoisomer. A kinetic experiment, carried out by (31)P NMR, allowed the conditions of the thermodynamic equilibrium to be determined. A structural relationship between the two isomers is proposed, fully consistent with NMR data. Cisoid and transoid isomers result in the relative disposition of each [PW(11)O(39)](7-) subunit, either staggered or eclipsed. An investigation of the formation of the [Mo(2)O(2)S(2)](2+) unit from the polycondensed cyclic precursor [Mo(10)S(10)O(10)(OH)(10)(H(2)O)(5)] and the aggregation process resulting in the oxothio [(PW(11)O(39))(2)(Mo(4)S(4)O(4)(OH(2))(2))](10-) compound has been undertaken. The studies were monitored by (31)P NMR and UV-vis spectroscopies. The reaction is quantitative in nearly stoichiometric conditions.     

Synthesis and catalysis of di- and tetranuclear metal sandwich-type silicotungstates [(gamma-SiW10O36)2M2(mu-OH)2]10- and [(gamma-SiW10O36)2M4(mu4-O)(mu-OH)6]8- (M = Zr or Hf)

The di- and tetranuclear metal sandwich-type silicotungstates of Cs10[(gamma-SiW10O36)2{Zr(H2O)}2(mu-OH)2] x 18 H2O (Zr2, monoclinic, C2/c (No. 15), a = 25.3315(8) A, b = 22.6699(7) A, c = 18.5533(6) A, beta = 123.9000(12) degrees, V = 8843.3(5) A(3), Z = 4), Cs10[(gamma-SiW10O36)2{Hf(H2O)}2(mu-OH)2] x 17 H2O (Hf2, monoclinic, space group C2/c (No. 15), a = 25.3847(16) A, b = 22.6121(14) A, c = 18.8703(11) A, beta = 124.046(3) degrees, V = 8974.9(9) A(3), Z = 4), Cs8[(gamma-SiW10O36)2{Zr(H2O)}4(mu4-O)(mu-OH)6] x 26 H2O (Zr4, tetragonal, P4(1)2(1)2 (No. 92), a = 12.67370(10) A, c = 61.6213(8) A, V = 9897.78(17) A(3), Z = 4), and Cs8[(gamma-SiW10O36)2{Hf(H2O)}4(mu4-O)(mu-OH)6] x 23 H2O (Hf4, tetragonal, P4(1)2(1)2 (No. 92), a = 12.68130(10) A, c = 61.5483(9) A, V = 9897.91(18) A(3), Z = 4) were obtained as single crystals suitable for X-ray crystallographic analyses by the reaction of a dilacunary gamma-Keggin silicotungstate K8[gamma-SiW10O36] with ZrOCl2 x 8 H2O or HfOCl2 x 8 H2O. These dimeric polyoxometalates consisted of two [gamma-SiW10O36](8-) units sandwiching metal-oxygen clusters such as [M2(mu-OH)2](6+) and [M4(mu4-O)(mu-OH)6](8+) (M = Zr or Hf). The dinuclear zirconium and hafnium complexes Zr2 and Hf2 were isostructural. The equatorially placed two metal atoms in Zr2 and Hf2 were linked by two mu-OH ligands and each metal was bound to four oxygen atoms of two [gamma-SiW10O36](8-) units. The tertanuclear zirconium and hafnium complexes Zr4 and Hf4 were isostructural and consisted of the adamantanoid cages with a tetracoordinated oxygen atom in the middle, [M4(mu4-O)(mu-OH)6](8+) (M = Zr or Hf). Each metal atom in Zr4 and Hf4 was linked by three mu-OH ligands and bound to two oxygen atoms of the [gamma-SiW10O36](8-) unit. The tetra-nuclear zirconium and hafnium complexes showed catalytic activity for the intramolecular cyclization of (+)-citronellal to isopulegols without formation of byproducts resulting from etherification and dehydration. A lacunary silicotungstate [gamma-SiW10O34(H2O)2](4-) was inactive, and the isomer ratio of isopulegols in the presence of MOCl2 x 8 H2O (M = Zr or Hf) were much different from that in the presence of tetranuclear complexes, suggesting that the [M4(mu4-O)(mu-OH)6](8+) core incorporated into the POM frameworks acts as an active site for the present cyclization. On the other hand, the reaction hardly proceeded in the presence of dinuclear zirconium and hafnium complexes under the same conditions. The much less activity is possibly explained by the steric repulsion from the POM frameworks in the dinuclear complexes.     

Reactions of in situ generated hydrated organotin cations with chelating O,O- or O,N-ligands: a possible structure-directing influence of the organic substituent on tin

The reaction of [n-Bu(2)SnO](n) with 1,5-naphthalenedisulfonic acid tetrahydrate in a 1:1 stoichiometry followed by reaction with 2,2'-bipyridine-N,N'-dioxide (BPDO-I) afforded a 1D-coordination polymer [n-Bu(2)Sn(BPDO-I)(1,5-C(10)H(6)(SO(3))(2))](n) (1) where the disulfonate ligand acts as a bridging ligand between two tin centers. An analogous reaction involving [Ph(2)SnO](n) afforded a trihydrated O,O'-chelated diorganotin cation [{Ph(2)Sn(BPDO-I)(H(2)O)(3)}(2+)][C(10)H(6)(SO(3)(-))(2)]·2CH(3)OH (2·2CH(3)OH). Utilizing two equivalents of BPDO-I in this reaction resulted in the ionic complex [{Ph(2)Sn(BPDO-I)(2)(H(2)O)}(2+)][C(10)H(6)(SO(3)(-))(2)]·3H(2)O (3·3H(2)O). In 2 and 3 the sulfonate ligands are not present in the coordination sphere of tin. Reaction of [n-Bu(2)SnO](n) and 1,5-naphthalenedisulfonic acid tetrahydrate, followed by reaction with [bis(diphenylphosphoryl)methane (DPPOM)] resulted in the formation of, [{n-Bu(2)Sn(DPPOM)(2)(H(2)O)(1,5-C(10)H(6)(SO(3))(SO(3)(-))}]·H(2)O (4·H(2)O). Of the two coordinating groups present in DPPOM, only one P=O group is coordinated to the tin atom. The remaining P=O motif is free and is involved in intramolecular H-bonding with the tin-bound water molecule. Using [Ph(2)SnO](n) instead of [n-Bu(2)SnO](n) afforded the ionic complex [{Ph(2)Sn(DPPOM)(2)}(2+){1,5-C(10)H(6)(SO(3)(-))(2)}] (5) where the DPPOM functions as a chelating ligand. The reaction of [n-Bu(2)SnO](n) with 1,5-naphthalenedisulfonic acid tetrahydrate followed by addition of one equivalent of 8-hydroxyquinoline (8-HQ) in presence of triethylamine afforded the neutral dinuclear complex, [(H(2)O)(8-Q)n-Bu(2)Sn(μ-1,5-C(10)H(6)(SO(3))(2))n-Bu(2)Sn(8-Q)(H(2)O)] (6) where the two tin atoms are bridged by the disulfonate ligand. Compounds 1-6 are thermally stable as shown by their thermogravimetric analyses.     

Synthesis, structural characterization, and catalytic performance of dititanium-substituted gamma-Keggin silicotungstate

A novel titanium-substituted silicotungstate cluster of [{gamma-SiTi2W10O36(OH)2}2(mu-O)2]8- (1) is synthesized by the introduction of titanium(IV) ions into a divacant lacunary gamma-Keggin-type silicotungstate of [gamma-SiW10O36]8-. This titanium-substituted polyoxometalate, 1, exhibits a dimeric structure. One half of the gamma-Keggin fragment of 1 contains a dinuclear titanium center bridged by two hydroxo groups, and the resulting Ti2(mu-OH)2 core connects to the other Ti2(mu-OH)2 core of the paired gamma-Keggin subunit through Ti-O-Ti linkages. The Ti2(mu-OH)2 core of 1 reacts with MeOH to form the corresponding alkoxo derivative, [{gamma-SiTi2W10O36(OH)(OMe)}2(mu-O)2]8- (2). Two of four hydroxo groups of the Ti2(mu-OH)2 cores in 1 are replaced by methoxo groups to give the Ti2(mu-OH)(mu-OMe) core, and the Ti-O-Ti linkages connecting two gamma-Keggin subunits are maintained in 2. The gamma-Keggin dititanium-substituted silicotungstate 1 catalyzes mono-oxygenation reactions, such as the epoxidation of olefins and sulfoxidation of sulfides with hydrogen peroxide under mild conditions, while the monotitanium-substituted silicotungstate, [alpha-SiTiW11O39]4- (3), and the fully occupied silicododecatungstate, [gamma-SiW12O40]4-, are inactive. The epoxidation with 1 is stereospecific; the configurations around the C=C double bonds of the cis- and trans-olefins are completely retained in the corresponding epoxides. For the competitive epoxidation of cis- and trans-2-octenes, the ratio of the formation rate of cis-2,3-epoxyoctane to that of the trans isomer (R(cis)/R(trans)) is relatively high (21.3) in comparison with those observed for the tungstate catalysts, including [gamma-SiW10O34(H2O)2]4-. The epoxidation of 3-methyl-1-cyclohexene is highly diastereoselective and gives the corresponding epoxide with an anti configuration. The molecular structure of 1 is preserved during the catalysis because the 29Si and 183W NMR spectra of the catalyst recovered after completion of the oxidation are consistent with those of as-prepared compound 1. All these facts suggest the contribution of rigid nonradical oxidants generated on the multinuclear titanium center of 1.     

Aerosol assisted chemical vapour deposition of tungsten oxide films from polyoxotungstate precursors: active photocatalysts

Aerosol assisted chemical vapour deposition of polyoxotungstate precursors [n-Bu4N]2[W6O19] and [n-Bu4N]4H3[PW11O39] produces films of WO(3 - x) and WO3 on glass substrates; the WO3 films show significant photocatalytic decomposition of a test organic pollutant--stearic acid--when irradiated with either 254 or 365 nm radiation.     

Synthesis and reactivity of the methoxozirconium pentatungstate (nBu4N)6[{(mu-MeO)ZrW5O18}2]: insights into proton-transfer reactions, solution dynamics, and assembly of {ZrW5O18}2- building blocks

The methoxo-bridged, dimeric, ZrIV-substituted Lindqvist-type polyoxometalate (POM) (nBu4N)6[{(mu-MeO)ZrW5O18}2], (TBA)61, has been synthesized by stoichiometric hydrolysis of Zr(OnPr)4, [{Zr(OiPr)3(mu-OnPr)(iPrOH)}2], or [{Zr(OiPr)4(iPrOH)}2] and [{WO(OMe)4}2] in the presence of (nBu4N)2WO4, providing access to the systematic nonaqueous chemistry of ZrW5 POMs for the first time and an efficient route to 17O-enriched samples for 17O NMR studies. 1H NMR provided no evidence for dissociation of 1 in solution, although exchange with MeOH was shown to be slow by an EXSY study. Reactions with HX at elevated temperatures gave a range of anions [{XZrW5O18}n]3n- (X = OH, 3; OPh, 4; OC6H4Me-4, 5; OC6H4(CHO)-2, 6; acac, 7; OAc, 8), where n = 2 for 3 and n = 1 for 4-8, while 1H and 17O NMR studies of hydrolysis of 1 revealed the formation of an intermediate [(mu-MeO)(mu-HO)(ZrW5O18)2]6-. Electrospray ionization mass spectrometry of 1 and 3 illustrated the robust nature of the ZrW5O18 framework, and X-ray crystal structure determinations showed that steric interactions between ligands X and the ZrW5O18 surface are important. The coordination number of Zr is restricted to six in aryloxides 4 and 5, while seven-coordination is achieved in the chelate complexes 6-8. Given the inert nature of the methoxo bridges in 1, protonation of ZrOW sites is proposed as a possible step in reactions with HX. The diphenylphosphinate ligand in [(Ph2PO2)ZrW5O18]3- was found to be labile and upon attempted recrystallization the aggregate [(mu3-HO)2(ZrW5O18)3H]7- 9 was formed, which was found to be protonated at ZrOZr and ZrOW sites. This work demonstrates the flexibility of the {ZrW5O18}2- core as a molecular platform for modeling catalysis by tungstated zirconia surfaces.     

Synthesis and characterization of [PW11O39Ir(H2O)]4-: successful incorporation of Ir into polyoxometalate framework and study of the substitutional lability at the Ir(III) site

The first Keggin-type heteropolyanion, [PW(11)O(39)Ir(H(2)O)](4-) (1), was synthesized by hydrothermal reaction from two different polytungstate precursors and [IrF(6)](2-). It was isolated as (Bu(4)N)(4)[PW(11)O(39)Ir(H(2)O)] (1a), which was completely characterized by multinuclear (31)P and (183)W NMR, ESI-mass spectrometry and cyclic voltammetry. A rapid screening methodology to ascertain the intrinsic substitutional lability at the Ir site is also presented, based on ESI-MS.     

SHOP-type nickel complexes with alkyl substituents on phosphorus, synthesis and catalytic ethylene oligomerization

The beta-keto phosphorus ylides (n-Bu)3P=CHC(O)Ph 6, (t-Bu)2PhP=CHC(O)Ph 7, (t-Bu)Ph2P=CHC(O)Ph 8, (n-Bu)2PhP=CHC(O)Ph 9, (n-Bu)Ph2P=CHC(O)Ph 10, Me2PhP=CHC(O)Ph 11 and Ph3P=CHC(O)(o-OMe-C6H4) 12 have been synthesized in 80-96% yields. The Ni(II) complexes [NiPh{Ph2PCH...C(...O)(o-OMeC6H4)}(PPh3)] 13, [NiPh{Ph(t-Bu)PCHC(O)Ph}(PPh3)] 15, [NiPh{(n-Bu)2PCH...C(...O)Ph}(PPh3)] 16 and [NiPh{Ph(n-Bu)PCH...C(...O)Ph}(PPh3)] 17 have been prepared by reaction of equimolar amounts of [Ni(COD)2] and PPh3 with the beta-keto phosphorus ylides 12 or 8-10, respectively, and characterized by 1H and 31P{1H} NMR spectroscopy. NMR studies and the crystal structure determination of 13 indicated an interaction between the hydrogen atom of the C-H group alpha to phosphorus and the ether function. The complexes [NiPh{Ph2PCHC(O)Ph}(Py)] 18, [NiPh{Ph(t-Bu)PCHC(O)Ph}(Py)] 19, [NiPh{(n-Bu)2PCH...C(...O)Ph}(Py)] 20, [NiPh{Ph(n-Bu)PCH...C(...O)Ph}(Py)] 21 and [NiPh{Me2PCH...C(...O)Ph}(Py)] 22 have been isolated from the reactions of [Ni(COD)2] and an excess of pyridine with the -keto phosphorus ylides Ph3PCH=C(O)Ph 3 or 8-11, respectively, and characterized by 1H and 31P{1H} NMR spectroscopy. Ligands 3, 8, 10 and 12 have been used to prepare in situ oligomerization catalysts by reaction with one equiv. of [Ni(COD)2] and PPh3 under an ethylene pressure of 30 or 60 bar. The catalyst prepared in situ from 12, [Ni(COD)2] and PPh3 was the most active of the series with a TON of 12700 mol C2H4 (mol Ni)-1 under 30 bar ethylene. When the beta-keto phosphorus ylide 8 was reacted in situ with three equiv. of [Ni(COD)2] and one equiv. of PPh3 under 30 bar of ethylene, ethylene polymerization was observed with a TON of 5500 mol C2H4 (mol Ni)-1.     

Cyano-bridged hexanuclear Fe4M2 (M = Ni, Co, Mn) clusters: spin-canted antiferromagnetic ordering of Fe4Ni2 cluster

Heterobimetallic hexanuclear cyano-bridged complexes, [{Fe(Tp)(CN)3}4{M(MeCN)(H2O)2}(2)].10H2O.2MeCN [M = Ni (1), Co (2), Mn (3); Tp = hydrotris(1-pyrazolyl)borate], have been synthesized in H2O-MeCN solution. Complexes 1-3 are isostructural and hexanuclear with [{Fe(Tp)(CN)3}4{M(MeCN)(H2O)2}2] units linked by hydrogen bonds to form a 2D-structure in the solid state. Complex 1 is a canted antiferromagnet that undergoes a field-induced spin-flop-like transition at approximately 1 T and 2 K. At 4.45 K 1 has a transition to paramagnetic state of noninteracting S = 4 magnetic clusters. However, 2 and 3 show antiferromagnetic intracluster coupling. Facile loss of solvent from 2 alters the local symmetry resulting in changing the intracluster interaction from antiferro- to ferromagnetic.     

Organic-inorganic hybrids assembled by bis(undecatungstophosphate) lanthanates and dinuclear copper(II)-oxalate complexes

A series of organic-inorganic hybrid compounds, K2H7[{Ln(PW11O39)2}{Cu2(bpy)2(mu-ox)}].xH2O (Ln = La, x approximately = 18 (1); Ln = Pr, x approximately = 18(2); Ln = Eu, x approximately = 16(3); Ln = Gd, x approximately 22(4); Ln = Yb, x approximately = 19 (5); bpy = 2,2'-bipyridine and ox = oxalate), have been isolated by the conventional solution method. Single-crystal X-ray diffraction studies reveal that compounds 1-5 are isomorphic and consist of one-dimensional chains, which are constructed by alternating bis(undecatungstophosphate) lanthanates [Ln(PW11O39)2](11-) and dinuclear copper(II)-oxalate complexes [Cu2(bpy)2(mu-ox)]2+.pi-pi interactions of the bpy ligands from adjacent chains lead to their three-dimensional structures. An analogue of potassium K2H9[{K(PW11O39)2}{Cu2(bpy)2(mu-ox)}1].approximately 20.5H2O(6) has also been obtained. The syntheses and structures of these compounds are reported here. Magnetic properties of 1, 2 and 3 are discussed as well. Attempts to crystallize similar compounds containing Co(II) and Ni(II) were unsuccessful.     

Fe2 and Fe4 clusters encapsulated in vacant polyoxotungstates: hydrothermal synthesis, magnetic and electrochemical properties, and DFT calculations

While the reaction of [PW(11)O(39)](7-) with first row transition-metal ions M(n+) under usual bench conditions only leads to monosubstituted {PW(11)O(39)M(H(2)O)} anions, we have shown that the use of this precursor under hydrothermal conditions allows the isolation of a family of novel polynuclear discrete magnetic polyoxometalates (POMs). The hybrid asymmetric [Fe(II)(bpy)(3)][PW(11)O(39)Fe(2) (III)(OH)(bpy)(2)]12 H(2)O (bpy=bipyridine) complex (1) contains the dinuclear {Fe(micro-O(W))(micro-OH)Fe} core in which one iron atom is coordinated to a monovacant POM, while the other is coordinated to two bipyridine ligands. Magnetic measurements indicate that the Fe(III) centers in complex 1 are weakly antiferromagnetically coupled (J=-11.2 cm(-1), H=-JS(1)S(2)) compared to other {Fe(micro-O)(micro-OH)Fe} systems. This is due to the long distances between the iron center embedded in the POM and the oxygen atom of the POM bridging the two magnetic centers, but also, as shown by DFT calculations, to the important mixing of bridging oxygen orbitals with orbitals of the POM tungsten atoms. The complexes [Hdmbpy](2)[Fe(II)(dmbpy)(3)](2)[(PW(11)O(39))(2)Fe(4) (III)O(2)(dmbpy)(4)]14 H(2)O (2) (dmbpy=5,5'-dimethyl-2,2'-bipyridine) and H(2)[Fe(II)(dmbpy)(3)](2)[(PW(11)O(39))(2)Fe(4) (III)O(2)(dmbpy)(4)]10 H(2)O (3) represent the first butterfly-like POM complexes. In these species, a tetranuclear Fe(III) complex is sandwiched between two lacunary polyoxotungstates that are pentacoordinated to two Fe(III) cations, the remaining paramagnetic centers each being coordinated to two dmbpy ligands. The best fit of the chi(M)T=f(T) curve leads to J(wb)=-59.6 cm(-1) and J(bb)=-10.2 cm(-1) (H=-J(wb)(S(1)S(2)+S(1)S(2*)+S(1*)S(2)+S(1*)S(2*))-J(bb)(S(2)S(2*))). While the J(bb) value is within the range of related exchange parameters previously reported for non-POM butterfly systems, the J(wb) constant is significantly lower. As for complex 1, this can be justified considering Fe(w)--O distances. Finally, in the absence of a coordinating ligand, the dimeric complex [N(CH(3))(4)](10)[(PW(11)O(39)Fe(III))(2)O]12 H(2)O (4) has been isolated. In this complex, the two single oxo-bridged Fe(III) centers are very strongly antiferromagnetically coupled (J=-211.7 cm(-1), H=-JS(1)S(2)). The electrochemical behavior of compound 1 both in dimethyl sulfoxide (DMSO) and in the solid state is also presented, while the electrochemical properties of complex 2, which is insoluble in common solvents, have been studied in the solid state.