Theoretical study of the relative stabilities of the alpha/beta3-[XW11O39](m-) lacunary polyoxometalates (X = P, Si)

A computational study of the relative stability of the monolacunary Keggin polyoxotungstates alpha and beta 3-[XW 11O 39] ( m- ) (X = P, m = 7; X = Si, m = 8) was performed. The influence of the nature of different grafted cations and of the central anion XO 4 ( n- ) on the relative stabilities of the lacunary isomers was analyzed. From these results, an interpretation of the structural difference in the metallic frameworks of alpha-[PW 11O 39{Ru(DMSO) 3(H 2O)}] (5-), alpha-[PW 11O 39{Ru(C 6H 6)(H 2O)}] (5-), and beta 3-[SiW 11O 39{Ru(DMSO) 3(H 2O)}] (6-) is proposed, and conclusions are drawn as to how to favor the formation of beta 3 derivatives in future syntheses.     

Emission Spectra of Some Heteropolytungstates Containing Europium

The emission spectra of some heteropolytungstates containing europium, K.[Eu(XW11 O39)2](X = P, Ge, As, or Ga), K17[EuCAs2W17O61)2] and K16H9EuAs4W40O140 have been investigated and the structures of these compounds are discussed. The emission spectra at room temperature are very similar to each other in terms of both the number of bands and transition frequencies, showing that they have the similar structures to those of K16[Ce(P2W17O61)2] and K12[U(GeW11O39)2].     

Synthesis and Spectroscopic Characterization of Organothiophosphoryl Polyoxotungstates α- [C6H11P (S) ]2Xn+W11O39(8-n)- (Xn+ = P5+, Si4+, Ge4+, Ga3+)

Organothiophosphoryl polyoxotungstate derivatives α-C6H11P(S)]2Xn W11O39(8-n)- (X=P, Si, Ge, Ga)were obtained by the reactions of the monovacant α-[Xn W11O39](12-n)- (X=P, Si, Ge, Ga) anions with electrophilic C6H11P(S)Cl2 in acetonitrile. These new organic-inorganic hybrid anions were characterized by elemental analyses, IR, 31P and 183W NMR spectrometries. The six-line 183W NMR spectrum indicates that [C6H11P (S) ]2Xn W11O39(8-n)- (X= P, Si, Ge, Ga) anions possess true Cs symmetry in acetonitrile. According to the spectroscopic observation and the chemical analyses, it is known that each of the hybrid anions consists of an α-[XW11O3)] framework on which two equivalent C6H11P(S) groups are grafted through P-O-W bridges.     

Borotungstate polyoxometalates: multinuclear NMR structural characterization and conversions in solutions

The unique heteropolyanion [H(3)BW(13)O(46)](8-) (BW(13)), previously suggested on the basis of indirect evidence, and protonated lacunary heteropolyanion [HBW(11)O(39)](8-) (BW(11)) have been identified in aqueous solutions at pH 5-7.5 from NMR spectra. The pattern of tungsten-tungsten connectivities based on the analysis of the (2)J(W-O-W) coupling satellites in the (183)W NMR spectrum of BW(11), containing six peaks of relative intensities ～2:2:2:1:2:2, indicates that the latter is the α isomer. The (17)O NMR spectrum confirms the protonated state of the polyanion with the proton delocalized on two out of four terminal O atoms surrounding the tungsten vacancy. The (183)W NMR spectrum of BW(13) contains seven peaks of relative intensities ～2:1:2:2:2:2:2 with additional large couplings due to the connectivity between BW(11) and [W(2)O(7)](2-) fragments. According to the (17)O NMR spectrum, two protons of [BW(13)O(46)H(3)](8-) are delocalized on the two terminal trans O atoms of the dimeric fragment while the third one is linked to its bridging O atom. The conversions of BW(11) and BW(13) in solution were followed by using (183)W NMR spectra at a "fingerprint" level. In the pH range from ～7.5 to 6, BW(11) transforms to BW(13), transforming further to [BW(12)O(40)](5-) (BW(12)) and [B(3)W(39)O(132)H(n)](n-21) (B(3)W(39)) in different ratios. Conversion of BW(13) to BW(12) proceeds through an intermediate complex of suggested composition [BW(11)O(39)·WO(2)](7-). At high acidity (pH ～ 0), B(3)W(39) gradually decomposes into tungstic acid, BW(12) and H(3)BO(3). Polyanion BW(12) persists in the pH range ～0-7.5.     

Kinetics of Photocatalytic Degradation of Formic Acid over Silica Composite Films Based on Polyoxometalates

The composite films, XW11O39^n-/SiO2,(X refers to Si,Ge or P,respectively) were prepared by tetraethoxysilane (TEOS) hydrolysis sol-gel method via spin-coating technique. Formation of the composite films is due to strong chemical reaction of organic silanol group with the surface oxygen atoms of XW11O39^n-, resulted in the saturation of the surface of the lacunary polyoxometalates (POMs). Therefore,the coordination structural model of the films was proposed. As for the films, retention of the primary Keggin structure was confirmed by UV-vis, FT-IR spectra and MAS NMR. The surface morphology of the films was characterized by SEM, indicating that the film surface is relatively uniform, and the layer thickness is in the range of 250-350nm. Aqueous formic acid (FA) (0-20mmol/L) was degraded into CO2 and H2O by irradiating the films in the near-UV area. The results show that all the films have photocatalytic activities and the degradation reaction follows Langmuir-Hinshelwood first order kinetics.     

Synthesis and characterization of the Keggin-type ruthenium-nitrido derivative [PW11O39{RuN}]4- and evidence of its electrophilic reactivity

The ruthenium-nitrido POM derivative [PW11O39{RuVIN}]4- has been synthesized by reaction between [PW11O39]7- and [RuVINCl5]2- or [RuVINCl4]-. Its molecular structure has been confirmed from multinuclear 31P and 183W NMR spectroscopy together with an EXAFS study, while the oxidation state of the ruthenium bearing the nitrido ligand has been inferred both from 183W NMR and XANES analysis at the Ru-K edge. The potential of [PW11O39{RuVIN}]4- in N-atom transfer reactions has been demonstrated through reaction with triphenylphosphine, which ultimately leads to the release of the bis(triphenylphosphane)iminium cation [PPh3=N=PPh3]+ through several intermediates, among which the phosphoraniminato derivative [PW11O39{RuVNPh3}]3- has been structurally characterized. Its unusual oxidation state is in accordance with its EPR spectrum.     

The ruthenium(II)-supported heteropolytungstates [Ru(dmso)3(H2O)-XW11O39]6- (X = Ge, Si)

The novel Ru(II)-supported heteropolytungstates [Ru(dmso)(3)(H(2)O)XW(11)O(39)](6-)(X = Ge, Si) have been synthesized and characterized by single-crystal X-ray diffraction, multinuclear NMR ((183)W, (13)C, (1)H, (29)Si) and IR spectroscopy, elemental analysis and electrochemistry. The novel polyanion structure consists of a Ru(dmso)(3)(H(2)O) unit linked to a monolacunary [XW(11)O(39)](8-) Keggin fragment via two Ru-O-W bonds resulting in an assembly with C(1) symmetry. Polyanions 1 and 2 were synthesized by reaction of cis-Ru(dmso)(4)Cl(2) with [A-alpha-XW(9)O(34)](10-) in aqueous, acidic medium (pH 4.8). Tungsten-183 NMR of 1 leads to a spectrum with 11 peaks of equal intensity, indicating that the solid-state structure is preserved in solution. Electrochemistry studies revealed that 1 and 2 are stable in solution at least from pH 0 to 7, even in the presence of dioxygen. Their cyclic voltammetry patterns show mainly two two-electron reversible W-waves, those of the Si derivative 2 being located at slightly more negative potentials than those of the Ge derivative 1. The observed stability of 1 and 2 might be attributed to a stabilization of the Ru-center both by the strongly bound dmso ligands and the Keggin moiety. This stabilization drives the redox waves of Ru outside the accessible potential range. However, conditions were found to reveal, at least partially, the redox behavior of Ru in 1 and 2.     

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.     

Noble metals in polyoxometalate chemistry: palladium-containing derivatives of the monolacunary Keggin and Wells-Dawson tungstophosphates

We have prepared the three novel Pd(II)-containing tungstophosphates [Pd(2)(α-PW(11)O(39)H(0.5))(2)](9-) and two structural isomers of [Pd(2)(α(2)-P(2)W(17)O(61)H(n))(2)]((16-2n)-) via simple synthetic procedures and characterized their potassium salts by single-crystal X-ray diffraction, elemental analysis, and IR and multinuclear ((31)P and (183)W) NMR spectroscopy. This study sheds light on the long-standing question about the nature and structure of the actual products formed in the reaction of Pd(II) ions with monolacunary Keggin-type [α-XW(11)O(39)](n-) and Wells-Dawson-type [α(2)-P(2)W(17)O(61)](10-) heteropolytungstates.     

Structural and electronic properties of hetero-transition-metal Keggin anions: a DFT Study of alpha/beta-[XW12O40]n- (X = CrVI, VV, TiIV, FeIII, CoIII, NiIII, CoII, and ZnII) relative stability

Density functional theory calculations have been carried out to investigate the electronic structures and the alpha/beta relative stability of Keggin-typed [XW(12)O(40)]n- anions with transition metal as heteroatom X (X = Cr(VI), V(V), Ti(IV), Fe(III), Co(III), Ni(III), Co(II) and Zn(II)). Nice agreement in geometries between computation and experiment has been obtained, and the higher stability of the alpha isomer over the beta one has been confirmed. Structural parameter analysis reveals that the {M(3)O(13)} triads in both alpha and beta isomers contract considerably with the increase of the negative anionic charge, while the overall size of both isomers shrinks only slightly. Fragment molecular orbital analysis shows that except alpha/beta-[TiW(12)O(40)]4-, the electronic structures of Keggin anions can be described by the insertion of the e and/or t2 orbital of XO4n- into the frontier orbitals of W(12)O(36) cage, and this leads to the specific redox property, which is different from that of the Keggin anions with main-group elements as heteroatoms. Energy decomposition analysis shows that the enhanced intrinsic stability of the alpha isomer in Td arrangement of W(12)O(36) shell and the larger deformation of the alpha over the beta isomer are two dominating factors and contribute oppositely to the alpha/beta relative stability.     

The monolanthanide-containing silicotungstates [Ln(beta2-SiW11O39)2]13- (Ln = La, Ce, Sm, Eu, Gd, Tb, Yb, Lu): a synthetic and structural investigation

We have synthesized and structurally characterized the monolanthanide-containing polyanions [Ln(beta2-SiW11O39)2]13- (Ln = La (1), Ce (2), Sm (3), Eu (4), Gd (5), Tb (6), Yb (7), Lu (8)). Synthesis was accomplished by reaction of the respective lanthanide ion with the monolacunary Keggin-type precursor [beta2-SiW11O39]8- in a 1:2 molar ratio in 1 M KCl medium at pH 5. Polyanions 1-8 were isolated as potassium salts and then characterized by IR, single-crystal X-ray diffraction, and elemental as well as thermogravimetric analysis. The structures of 1-8 are composed of an eight-coordinated Ln3+ center sandwiched by two chiral (beta2-SiW11O39) units. Large Ln3+ ions appear to favor an (R,R) (or (S,S)) configuration (point group C2) of the Keggin units, with an increasing amount of (R,S) (or (S,R)) configuration (point group C1) found in the solid state as the Ln3+ ion decreases in size. This trend is also supported by solution 183W NMR results for the diamagnetic La3+ and Lu3+ derivatives.     

Synthesis and structure of dilacunary decatungstogermanate, [gamma-GeW10O36]8-

The dilacunary decatungstogermanate [gamma-GeW10O36]8- (1) has been synthesized and structurally characterized in solution and in the solid state. Reaction of germanium dioxide with sodium tungstate in aqueous acidic medium results in the formation of [beta2-GeW11O39]8- (2), which is then used as a precursor for the synthesis of 1. The (183)W spectrum of 2 shows the expected 11 peaks of equal intensity, whereas that of 1 exhibits the expected three peaks with relative intensities 2:2:1. Polyanion 1 represents a novel lacunary polyoxometalate, giving rise to a multitude of derivatives by reaction with transition metals, lanthanides, and other electrophiles.     

About the Keggin isomers: crystal structure of [N(C4H9)4]-gamma-[SiMo2W10O40]n- (n= 4 or 6)

The tetrabutylammonium gamma-dodecatungstosilicate has been crystallized in a 6/1 acetonitrile/water solvent. An X-ray single-crystal analysis was carried out on [N(C4H9)4]4-gamma-[SiW12O40] which crystallizes in the orthorhombic system, space group P2(1)2(1)2(1), with a = 19.0881(3) A, b = 21.4435(3) A, c = 26.0799(1) A, V = 10674.9(2) A3, Z = 4, and rho(calcd) = 2.392 g/cm3. The idealized C2v arrangement of the anion results from the rotation of 60 degrees of two trigonal [W3O13] groups in the Keggin anion. Taking as reference the geometrical characteristics of the Keggin anion, it appears that the bond lengths and bonds angles within the four [W3O13] groups are not significantly modified while the mu-oxo junctions between the two rotated groups and those between the two unrotated groups involve more acute and opened W-O-W angles, respectively. The syntheses and 183W NMR characterizations of the mixed gamma-[SiW10Mo2O40]n- compounds corresponding to the oxidized (Mo(VI); n = 4) and to the two electron-reduced (Mo(V); n = 6) anions are reported. Structural analysis by 183W NMR has proved unambiguously that the C2v structure of the gamma-[SiW10O36]8- subunit is retained in both the compounds. The electronic behavior of the series gamma-[SiW10M2E2O36]6- (M = Mo or W; E = O or S) is examined, compared and related to 183W NMR data.     

A strategy for the analysis of chiral polyoxotungstates by multinuclear (31P, 183W) NMR spectroscopy applied to the assignment of the 183W NMR spectra of alpha1-[P2W17O61]10- and alpha1-[YbP2W17O61]7-

This paper describes the complete assignment of the 183W NMR spectra of the chiral polyoxometalates alpha1-[P2W17O61]10- and alpha1-[YbP2W17O61]7- in aqueous solution. These spectra display each 17 lines of equal intensity with a relatively narrow chemical shift distribution. The identification of signals is based on selective 31P-183W decoupling and recognition of particular sets of coupling constants for tungsten atoms around the lacunary site. Further assignment is obtained by 183W 2D-COSY NMR experiments. We demonstrate herewith a new way for the unambiguous assignment of 183W NMR spectra of polyoxotungstates without any symmetry elements or tungsten atoms in special positions. This way relies on the correlation of the magnitude of 2J(W-W) coupling constants with the geometry of oxo-bridges in polyoxotungstates. These results open the way to monitor interaction sites of chiral polyoxotungstates with organic ligands.     

A new method for the synthesis of organopolyoxometalate hybrid compounds

The reaction of a quaternary ammonium salt of the tin chloride-substituted polyoxometalate, [PSn(Cl)W11O39]4-, with a variety of n-nucleophiles including primary, secondary, and tertiary amines and a tertiary phosphine, yielded tin-centered Lewis acid-base adducts, [PSn(Cl)W11O39]4--n-nucleophile; with more nucleophilic secondary amines such as diisopropylamine, apparently some [PSnN[CH(CH3)2]2W11O39]4- was formed as a minor product. The compounds were identified by 1H, 119Sn, 15N, 31P, and 183W NMR, ESI-MS, and elemental analyses. The key connectivity of the Sn-Cl center with the amine was clarified by the observation of 3J Sn-H couplings (Sn from the polyoxometalate cluster and H from the amine moiety) in a 2D 119Sn-1H heteronuclear multiple-bond correlation NMR experiment. This new, rather simple synthetic method was also utilized for preparing amino acid-polyoxometalate hybrid compounds.     

Functionalization of heteropolyanions-osmium and rhenium nitrido derivatives of Keggin- and Dawson-type polyoxotungstates: synthesis, characterization and multinuclear ((183)W, (15)N) NMR, EPR, IR, and UV/Vis fingerprints

Reaction of K(10)[alpha(2)-P(2)W(17)O(61)] or K(10)[alpha(1)-P(2)W(17)O(61)] or [Bu(4)N][OsCl(4)N] in a water/methanol mixture, and subsequent precipitation with (Bu(4)N)Br provided [alpha(2)-P(2)W(17)O(61){Os(VI)N}](7-) and [alpha(1)-P(2)W(17)O(61){Os(VI)N}](7-) Dawson structures as tetrabutylammonium salts. Reactions of [(Bu(4)N)(4)][alpha-H(3)PW(11)O(39)] with either [ReCl(3)(N(2)Ph(2))(PPh(3))(2)] or [Bu(4)N][ReCl(4)N] are alternatives to the synthesis of [(Bu(4)N)(4)][alpha-PW(11)O(39){Re(VI)N}]. (183)W and (15)N NMR, EPR, IR, and UV-visible spectroscopies and cyclic voltammetry have been used to characterize these compounds and the corresponding [(Bu(4)N)(4)][alpha-PW(11)O(39){Os(VI)N}] Keggin derivative.     

Heteropolytungstates containing organotin and cobalt or zinc atoms

Heteropolytungstates containing an organotin moiety plus Co or Zn as central atom, M6H[XW11O39SnPh] · nH2O (M=Me4N or Bu4N; X=Zn or Co) were prepared and characterized by elemental analysis, i.r., u.v.–vis. spectroscopy, e.s.r. and n.m.r. spectroscopy and electrochemistry. The results show that these two anions possess Keggin-like structures. This revised version was published online in June 2006 with corrections to the Cover Date.     

Prediction of second-order optical nonlinearity of trisorganotin-substituted beta-Keggin polyoxotungstate

The dipole polarizabilities, second-order polarizabilities, and origin of second-order nonlinear optical (NLO) properties of trisorganotin-substituted beta-Keggin polyoxotungstate [XW9O37(SnR)3](11-n)- (X = P, Si, Ge, R = Ph; X = Si, R = PhNO2, PhCCPh) have been investigated by using time-dependent density functional response theory. This class of organic-inorganic hybrid complexes possesses a remarkably large molecular second-order NLO response, especially for [SiW9O37(SnPhCCPh)3]7- (system 5) with the static second-order polarizability (beta(vec)) computed to be 1569.66 x 10(-30) esu. Thus, these complexes have the possibility to be excellent second-order nonlinear optical materials. Analysis of the major contributions to the beta(vec) value suggests that the charge transfer from the heteropolyanion to the organic segment along the z-axis plays the key role in the NLO response of [XW9O37(SnR)3](11-n)-. The computed beta(vec) values increase as a heavy central heteroatom changes in the order Ge > Si > P. Furthermore, nitro substitution on the aryl segment and the lengthening of organostannic pi-conjugation are more important in enhancing the optical nonlinearity, especially for the latter factor. The present investigation provides important insight into the origin of the NLO properties of trisorganotin-substituted heteropolyoxotungstate.     

Organotin and organogermanium linkers for simple, direct functionalization of polyoxotungstates

Fifteen Keggin-anion-derived polytungstates [TW11O39[MCH2CH2X]]n- (T = Si, Ge, Ga; M = Sn, Ge; X = COOH, COOCH3, CONH2, CN; n = 5, 6) were prepared in aqueous or aqueous-organic solution from the corresponding lacunary polytungstates and trichlorotin and -germanium precursors, and were isolated as caesium salts. The derivatized polytungstates were characterized by elemental analysis, multinuclear NMR spectroscopy, and cyclic voltammetry; they are stable in aqueous solution to pH 6-7. NMR spectroscopy revealed the presence of a second (beta1 or beta3) isomer in the tungstogallate derivatives. Acid hydrolysis of the ester and nitrile derivatives could be achieved without decomposition of the polytungstate moieties, and esterification and amidation of the carboxylate functions was straightforward using standard coupling techniques, e.g. the formation, isolation and characterization of [SiW11O39[Ge(CH2)2CONHCH2COOCH3]]5- from glycine methyl ester. Since the Cl3MCH2CH2X precursors are readily accessible by hydrostannation/germanation reactions with the corresponding alkenes, novel coupled polytungstates, such as [(SiW11O39GeCH2CH2COOCH2)4C]20- from pentaerythritol tetraacrylate, can also be prepared.