New polyoxometalate compounds built up of lacunary Wells-Dawson anions and trivalent lanthanide cations

Five new polyoxometalate compounds built on lacunary Wells-Dawson anions and trivalent lanthanide cations, KNa3[Nd2(H2O)10(alpha2-P2W17O61)].11H2O (1), (H3O)[Nd3(H2O)17(alpha2-P2W17O61)].6.75H2O (2), (H2bpy)2[Nd2(H2O)9 (alpha2-P2W17O61)].4.5H2O (3), (H2bpy)2[La2(H2O)9(alpha2-P2W17O61)].4.5H2O (4), and (H2bpy)2[Eu2(H2O)9(alpha2-P2W17O61)].5H2O (5), have been synthesized and characterized by elemental analysis, IR, TG, and single-crystal X-ray diffraction. Compound 1 shows a bisupporting polyoxometalate cluster structure where two {Nd(H2O)7}3+ fragments are supported on the polyoxometalate dimer [{Nd(H2O)3(alpha2-P2W17O61)}2]14-; this represents the first bisupporting polyoxometalate compound based on a polyoxometalate dimer. Compound 2 displays a 1D chain structure built up of bisupporting polyoxoanions [{Nd(H2O)7}2{Nd(H2O)3(alpha2-P2W17O61)}2]8- and Nd3+ ions. Compounds 3-5 are isostructural and show a 2D structure constructed of 1D polyoxometalate chains of [Ln(H2O)2(alpha2-P2W17O61)]n(7n-) linked by Ln3+ ions. Compounds 2-5 represent the first extended structures formed by lacunary Wells-Dawson anions and trivalent lanthanide ions. The influence of the Ln3+/[alpha2-P2W17O61]10- ratio on the syntheses of these five compounds has been studied. Furthermore, the fluorescent activity of compound 5 is reported.     

Lanthanide complexes of [alpha-2-P2W17O61]10-: solid state and solution studies

We have isolated the 1:1 Ln:[alpha-2-P2W17O61]10- complexes for a series of lanthanides. The single-crystal X-ray structure of the Eu3+ analogue reveals two identical [Eu(H2O)3(alpha-2-P2W17O61)]7- moieties connected through two Eu-O-W bonds, one from each polyoxometalate unit. An inversion center relates the two polyoxometalate units. The Eu(III) ion is substituted for a [WO]4+ unit in the "cap" region of the tungsten-oxygen framework of the parent Wells-Dawson ion. The point group of the dimeric molecule is Ci. The extended structure is composed of the [Eu(H2O)3(alpha-2-P2W17O61)]214- anions linked together by surface-bound potassium cations. The space group is P, a = 12.7214(5) A, b = 14.7402(7) A, c = 22.6724(9) A, alpha = 71.550(3), beta = 84.019(3)degrees, gamma = 74.383(3), V = 3883.2(3) A3, Z = 1. The solution studies, including 183W NMR spectroscopy and luminescence lifetime measurements, show that the molecules dissociate in solution to form monomeric [Ln(H2O)4(alpha-2-P2W17O61)]7- species.     

Sensing the chirality of Dawson lanthanide polyoxometalates [alpha1-LnP2W17O61]7- by multinuclear NMR spectroscopy

Lanthanide complexes of the chiral Dawson phosphotungstate [alpha(1)-P(2)W(17)O(61)](10-) were used to study the formation of diastereomers with optically pure organic ligands. The present work started with the full assignment of the (183)W NMR spectra of [alpha(1)-Yb(H(2)O)(4)P(2)W(17)O(61)](7-) at different temperatures and concentrations, which allowed the structure of the dimerized form in aqueous solution to be established. Different enantiopure amino acids and phosphonic acids were screened as ligands. Both types allowed chiral differentiation by multinuclear NMR spectroscopy under fast-exchange conditions. Functional groups with a good affinity for the oxo framework of the polyoxometalate were identified, and maps of the interactions between L-serine and N-phosphonomethyl-L-proline with [alpha(1)-Yb(H(2)O)(4)P(2)W(17)O(61)](7-) were established. This demonstrates the power of (183)W NMR spectroscopy to elucidate the molecular recognition of inorganic molecules by organic compounds. N-Phosphonomethyl-L-proline appears to be a convenient ligand to promote separation of the diastereomers and ultimately resolution of the enantiomers of [alpha(1)-Yb(H(2)O)(4)P(2)W(17)O(61)](7-).     

Synthesis and structure of dawson polyoxometalate-based, multifunctional, inorganic-organic hybrid compounds: organogermyl complexes with one terminal functional group and organosilyl analogues with two terminal functional groups

Four novel multifunctional polyoxometalate (POM)-based inorganic-organic hybrid compounds, [α(2)-P(2)W(17)O(61){(RGe)}](7-) (Ge-1, R(1) = HOOC(CH(2))(2(-)) and Ge-2, R(2) = H(2)C═CHCH(2(-))) and [α(2)-P(2)W(17)O(61){(RSi)(2)O}](6-) (Si-1, R(1) and Si-2, R(2)), were prepared by incorporating organic chains having terminal functional groups (carboxylic acid and allyl groups) into monolacunary site of Dawson polyoxoanion [α(2)-P(2)W(17)O(61)](10-). In these POMs, new modification of the terminal functional groups was attained by introducing organogermyl and organosilyl groups. Dimethylammonium salts of the organogermyl complexes, (Me(2)NH(2))(7)[α(2)-P(2)W(17)O(61)(R(1)Ge)]·H(2)O MeN-Ge-1 and (Me(2)NH(2))(7)[α(2)-P(2)W(17)O(61)(R(2)Ge)]·4H(2)O MeN-Ge-2, were obtained as analytically pure crystals, in 22.8% and 55.3% yields, respectively, by stoichiometric reactions of [α(2)-P(2)W(17)O(61)](10-) with separately prepared Cl(3)GeC(2)H(4)COOH in water, and H(2)C═CHCH(2)GeCl(3) in a solvent mixture of water/acetonitrile. Synthesis and X-ray structure analysis of the Dawson POM-based organogermyl complexes were first successful. Dimethylammonium salts of the corresponding organosilyl complexes, (Me(2)NH(2))(6)[α(2)-P(2)W(17)O(61){(R(1)Si)(2)O}]·4H(2)O MeN-Si-1 and (Me(2)NH(2))(6)[α(2)-P(2)W(17)O(61){(R(2)Si)(2)O}]·6H(2)O MeN-Si-2, were also obtained as analytically pure crystalline crystals, in 17.1% and 63.5% yields, respectively, by stoichiometric reactions of [α(2)-P(2)W(17)O(61)](10-) with NaOOC(CH(2))(2)Si(OH)(2)(ONa) and H(2)C═CHCH(2)Si(OEt)(3). These complexes were characterized by elemental analysis, thermogravimetric and differential thermal analyses (TG/DTA), FTIR, solid-state ((31)P) and solution ((31)P, (1)H, and (13)C) NMR, and X-ray crystallography.     

Production and reactions of organic-soluble lanthanide complexes of the monolacunary Dawson [alpha1-P2W17O61]10- polyoxotungstate

The incorporation of lanthanides into polyoxometalates provides entry to new classes of potentially useful materials that combine the intrinsic properties of both constituents. To utilize the [alpha1-Ln(H2O)4P2W17O61]7- species in applications of catalysis and development of luminescent materials, the chemistry of this family of lanthanide polyoxometalates in organic solvents has been developed. Organic-soluble polyoxometalate-lanthanide complexes TBA5H2[alpha1-Ln(H2O)4P2W17O61] (Ln = La(III), Sm(III), Eu(III), Yb(III)) were prepared and characterized by elemental analysis, acid-base titration, IR, 31P NMR, and mass spectrometry. The synthetic procedure involves a cation metathesis reaction in aqueous solution under strict pH control. A solid-liquid-phase transfer protocol yielded a unique species (TBA)8K3[Yb(alpha1-YbP2W17O61)2] with three ytterbium ions and two [alpha1-P2W17O61]10- polyoxotungstates. A centrosymmetric dimeric complex [{alpha1-La(H2O)4P2W17O61}2]14- was crystallized from aqueous solution and characterized by X-ray diffraction. ESI mass spectral analysis of the complexes TBA5H2[alpha1-Ln(H2O)4P2W17O61] shows that similar dimers exist in organic solution, in particular for the early lanthanides. Fragmentation in the mass spectrometer of the complexes from dry acetonitrile solution involves double protonation of an oxo ligand and loss of one water molecule. Low mass tungstate fragments combine into [(WO3)n]2- (n = 1-5) ions and their condensation products with phosphate. Reaction of TBA5H2[alpha1-Eu(H2O)4P2W17O61] with 1,10-phenanthroline or 2,2'-bipyridine showed an increase of the europium luminescence. This result is explained by the formation of a ternary complex of [alpha1-Eu(H2O)4P2W17O61]7- and two sensitizing ligands.     

Increased Lewis acidity in hafnium-substituted polyoxotungstates

Monolacunary polyoxotungstates [alpha(1)-P(2)W(17)O(61)](10-) and [alpha-PW(11)O(39)](7-) react with HfCl(4) to yield [alpha(1)-HfP(2)W(17)O(61)](6-) and [alpha-Hf(OH)PW(11)O(39)](4-), isolated as organo-soluble tetrabutylammonium (TBA) salts. Subsequent analyses, including mass spectrometry, show that they are stronger Lewis acids than (TBA)(5)H(2)[alpha(1)-YbP(2)W(17)O(61)]. The new polyoxotungstates catalyze Lewis acid mediated organic reactions, such as Mukaiyama aldol and Mannich-type additions. In particular, reactions with aldehydes, which were impossible with lanthanide polyoxotungstates, are made possible. Thus these modifications of the polyoxometalate composition allowed fine tuning of the Lewis acidity. The catalysts could be easily recovered and reused.     

Preparation and Tungsten-183 NMR Characterization of [alpha-1-P(2)W(17)O(61)](10)(-), [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-), and [alpha-2-Zn(H(2)O)P(2)W(17)O(61)](8)(-)

The preparation of the alpha-1 and alpha-2 isomers of the Wells-Dawson 17 tungsto derivatives by standard methods is accompanied by a significant proportion of the other isomer present as an impurity. In this study, the alpha-1 and alpha-2 isomers of [Zn(H(2)O)P(2)W(17)O(61)](8)(-) have been prepared in >98% purity by reacting isomerically pure K(9)Li[alpha-1-P(2)W(17)O(61)] and K(10)[alpha-2-P(2)W(17)O(61)], respectively, with ZnCl(2), while rigorously controlling the pH at 4.7. The molecules were isolated as potassium salts. For (183)W NMR and (31)P NMR characterization, both molecules were ion exchanged by cation-exchange chromatography, maintaining the pH at 4.7, to obtain the lithium salts. Removal of water and isolation of a solid sample of [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-) was achieved by lyophilization at -40 degrees C. The chemical shift data from (31)P and (183)W NMR spectroscopy of the isolated [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-) and [alpha-2-Zn(H(2)O)P(2)W(17)O(61)](8)(-) isomers are consistent with a mixture of the alpha-1 and alpha-2 isomers reported previously;(1) the molecules have the expected C(1) and C(s)() symmetry, respectively. The [alpha-1-Zn(H(2)O)P(2)W(17)O(61)](8)(-) isomer is stable in the pH range of 4.6-6 at temperatures <35 degrees C. Using the same ion exchange and lyophilization techniques, the lacunary [alpha-1-P(2)W(17)O(61)](10)(-) isomer was isolated as the lithium salt; characterization by (183)W NMR spectroscopy confirms the C(1) symmetry.     

缺位的Wells-Dawson型磷钨酸盐和Eu(Ⅲ)构筑的一维链状多金属氧酸盐化合物

在水溶液条件下以六缺位的[H2P2W12O48)]12-和Eu髥为反应前躯体得到了1个新的多金属氧酸盐化合物[Eu3(H2O)17(α2-HP2W17O61)].5H2O(1),对其进行了元素分析、红外光谱、热重、荧光等表征,并用X-射线单晶衍射测定了它的晶体结构。化合物1含有双支撑的多阴离子簇[{Eu(H2O)7}2{Eu(H2O)3(α2-P2W17)}2]8-,并进一步通过Eu髥连接形成了延c轴方向的一维链。室温条件下的荧光光谱研究表明:化合物1显示了强的红光发射。     

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.     

Peptide Bond Hydrolysis Catalyzed by the Wells-Dawson Zr(α(2)-P(2)W(17)O(61))(2) Polyoxometalate

In this paper we report the first example of peptide hydrolysis catalyzed by a polyoxometalate complex. A series of metal-substituted Wells-Dawson polyoxometalates were synthesized, and their hydrolytic activity toward the peptide bond in glycylglycine (GG) was examined. Among these, the Zr(IV)- and Hf(IV)-substituted ones were the most reactive. Detailed kinetic studies were performed with the Zr(IV)-substituted Wells-Dawson type polyoxometalate K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O which was shown to act as a catalyst for the hydrolysis of the peptide bond in GG. The speciation of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O which is highly dependent on the pD, concentration, and temperature of the solution, was fully determined with the help of (31)P NMR spectroscopy and its influence on the GG hydrolysis rate was examined. The highest reaction rate (k(obs) = 9.2 (±0.2) × 10(-5) min(-1)) was observed at pD 5.0 and 60 °C. A 10-fold excess of GG was hydrolyzed in the presence of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O proving the principles of catalysis. (13)C NMR data suggested the coordination of GG to the Zr(IV) center in K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O via its N-terminal amine group and amide carbonyl oxygen. These findings were confirmed by the inactivity of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O toward the N-blocked analogue acetamidoglycylglycinate and the inhibitory effect of oxalic, malic, and citric acid. Triglycine, tetraglycine, and pentaglycine were also fully hydrolyzed in the presence of K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O yielding glycine as the final product of hydrolysis. K(15)H[Zr(α(2)-P(2)W(17)O(61))(2)]·25H(2)O also exhibited hydrolytic activity toward a series of other dipeptides.     

Coordination of rare-earth elements in complexes with monovacant Wells-Dawson polyoxoanions

The alpha-1 and alpha-2 isomers of the monovacant Wells-Dawson heteropolyoxoanion [P(2)W(17)O(61)](10-) are complexants of trivalent rare-earth (RE) ions and serve to stabilize otherwise reactive tetravalent lanthanide (Ln) and actinide (An) ions in aqueous solution. Aspects of the bonding of Ln ions with alpha-1-[P(2)W(17)O(61)](10-) and alpha-2-[P(2)W(17)O(61)](10-) were investigated to address issues of complex formation and stability. We present structural insights about the Ln(III) coordination environment and hydration in two types of stoichiometric complexes, [Ln(alpha-1-P(2)W(17)O(61))](7-) and [Ln(alpha-2-X(2)W(17)O(61))(2)](17-) (for Ln identical with Sm, Eu, Lu; X identical with P, As). The crystal and molecular structures of [(H(2)O)(4)Lu(alpha-1-P(2)W(17)O(61))](7-) (1) and [Lu(alpha-2-P(2)W(17)O(61))(2)](17-) (2) were solved and refined through use of single-crystal X-ray diffraction. The crystallographic results are supported with corresponding insights from XAFS (X-ray absorption fine structure) for a series of nine solid-state complexes as well as from optical luminescence spectroscopy of the Eu(III) analogues in aqueous solution. All the Ln ions are eight-coordinate with oxygen atoms in a square antiprism arrangement. For the 1:1 stoichiometric Ln/alpha-1-[P(2)W(17)O(61)](10-) complexes, the Ln ions are bound to four O atoms of the lacunary polyoxometalate framework in addition to four O atoms from solvent (water) molecules as [(H(2)O)(4)Ln(alpha-1-P(2)W(17)O(61))](7-). This structure (1) is the first of its kind for any metal complex of alpha-1-[P(2)W(17)O(61)](10-), and the data indicate that the general stoichiometry [(H(2)O)(4)Ln(alpha-1-P(2)W(17)O(61))](7-) is maintained throughout the lanthanide series. For the 1:2 stoichiometric Ln/alpha-2-[X(2)W(17)O(61)](10-) complexes, no water molecules are in the Ln-O(8) coordination sphere. The Ln ions are bound to eight O atoms-four from each of two heteropolyanions-as [Ln(alpha-2-X(2)W(17)O(61))(2)](17-). The average Ln-O interatomic distances decrease across the lanthanide series, consistent with the decreasing Ln ionic radius.     

Characterization of manganese(V)-oxo polyoxometalate intermediates and their properties in oxygen-transfer reactions

A manganese(III)-substituted polyoxometalate, [alpha2-P2MnIII(L)W17O61]7- (P2W17MnIII), was studied as an oxidation catalyst using iodopentafluorobenzene bis(tifluoroacetate) (F5PhI(TFAc)2) as a monooxygen donor. Pink P2W17MnIII turns green upon addition of F5PhI(TFAc)2. The 19F NMR spectrum of F5PhI(TFAc)2 with excess P2W17MnIII at -50 degrees C showed the formation of an intermediate attributed to P2W17MnIII-F5PhI(TFAc)2 that disappeared upon warming. The 31P NMR spectra of P2W17MnIII with excess F5PhI(TFAc)2 at -50 and -20 degrees C showed a pair of narrow peaks attributed to a diamagnetic, singlet manganese(V)-oxo species, P2W17MnV=O. An additional broad peak at -10.6 ppm was attributed to both the P2W17MnIII-F5PhI(TFAc)2 complex and a paramagnetic, triplet manganese(V)-oxo species. The electronic structure and reactivity of P2W17MnV=O were modeled by DFT calculations using the analogous Keggin compound, [PMnV=OW11O39]4-. Calculations with a pure functional, UBLYP, showed singlet and triplet ground states of similar energy. Further calculations using both the UBLYP and UB3LYP functionals for epoxidation and hydroxylation of propene showed lowest lying triplet transition states for both transformations, while singlet and quintet transition states were of higher energy. The calculations especially after corrections for the solvent effect indicate that [PMnV=OW11O39]4- should be highly reactive, even more reactive than analogous MnV=O porphyrin species. Kinetic measurements of the reaction of P2W17MnV=O with 1-octene indicated, however, that P2W17MnV=O was less reactive than a MnV=O porphyrin. The experimental enthalpy of activation confirmed that the energy barrier for epoxidation is low, but the highly negative entropy of activation leads to a high free energy of activation. This result originates in our view from the strong solvation of the highly charged polyoxometalate by the polar solvent used and adventitious water. The higher negative charge of the polyoxometalate in the transition versus ground state leads to electrostriction of the solvent molecules and to a loss of degrees of freedom, resulting in a highly negative entropy of activation and slower reactions.     

Europium(III) reduction and speciation within a Wells-Dawson heteropolytungstate

The redox speciation of Eu(III) in the 1:1 stoichiometric complex with the alpha-1 isomer of the Wells-Dawson anion, [alpha-1-P 2W 17O 61] (10-), was studied by electrochemical techniques (cyclic voltammetry and bulk electrolysis), in situ XAFS (X-ray absorption fine structure) spectroelectrochemistry, NMR spectroscopy ( (31)P), and optical luminescence. Solutions of K 7[(H 2O) 4Eu(alpha-1-P 2W 17O 61)] in a 0.2 M Li 2SO 4 aqueous electrolyte (pH 3.0) show a pronounced concentration dependence to the voltammetric response. The fully oxidized anion and its reduced forms were probed by Eu L 3-edge XANES (X-ray absorption near edge structure) measurements in simultaneous combination with controlled potential electrolysis, demonstrating that Eu(III) in the original complex is reduced to Eu(II) in conjunction with the reduction of polyoxometalate (POM) ligand. After exhaustive reduction, the heteropoly blue species with Eu(II) is unstable with respect to cluster isomerization, fragmentation, and recombination to form three other Eu-POMs as well as the parent Wells-Dawson anion, alpha-[P 2W 18O 62] (6-). EXAFS data obtained for the reduced, metastable Eu(II)-POM before the onset of Eu(II) autoxidation provides an average Eu-O bond length of 2.55(4) A, which is 0.17 A longer than that for the oxidized anion, and consistent with the 0.184 A difference between the Eu(II) and Eu(III) ionic radii. The reduction of Eu(III) is unusual among POM complexes with Lindqvist and alpha-2 isomers of Wells-Dawson anions, that is, [Eu(W 5O 18) 2] (9-) and [Eu(alpha-2-As 2W 17O 61) 2] (17-), but not to the Preyssler complex anion, [EuP 5W 30O 110] (12-), and fundamental studies of materials based on coupling Eu and POM redox properties are still needed to address new avenues of research in europium hydrometallurgy, separations, and catalysis sciences.     

Modeling reactive metal oxides. Kinetics,thermodynamics, and mechanism of M(3) cap isomerization in polyoxometalates

An investigation of M(3)O(13) unit ("M(3) cap") isomerization in the classical polytungstodiphosphates alpha- and beta-P(2)W(18)O(62)(6)(-) has been undertaken because cap isomerism is an important and structurally well-studied phenomenon in many polyoxometalate families. The relative thermodynamic stabilities of the alpha (more stable) versus beta isomers were established both in the solid state by differential scanning calorimetry (4.36 +/- 0.64 kcal/mol) and in solution by (31)P NMR (3.80 +/- 0.57 kcal/mol). The isomerization of beta-P(2)W(18)O(62)(6)(-) to alpha-P(2)W(18)O(62)(6)(-), followed by (31)P NMR, has a bimolecular rate constant k(2) of 9.3 x 10(-)(1) M(-)(1) s(-)(1) at 343 K in pH 4.24 acetate buffer. Several lines of evidence establish the validity of suggestions in the literature that isomerization goes through a lacunary (defect) intermediate. First, the rate is proportional to [OH(-)]. Second, isomerization increases at higher ionic strengths, and a Debye-Hückel plot is consistent with a rate-limiting reaction between beta-P(2)W(18)O(62)(6)(-) and OH(-) (two species with a charge product of 6). Third, alkali-metal cations stabilize the bimolecular transition state (K(+) > Na(+) > Li(+)), consistent with recent ion-pairing studies in polyoxometalate systems. Fourth, the monovanadium-substituted products alpha(1)- and alpha(2)-P(2)VW(17)O(62)(7)(-) ((51)V NMR delta -554 ppm) form during isomerization in the presence of VO(2+). The known lacunary compounds (alpha(1)- and alpha(2)-P(2)W(17)O(61)(10)(-)) also react rapidly with the same vanadium precursor. Fifth, solvent studies establish that isomerization does not occur when OH(-) is absent. A mechanism is proposed involving attack of OH(-) on beta-P(2)W(18)O(62)(6)(-), loss of monomeric W(VI) from the M(3) (M(3)O(13)) terminal cap, isomerization of the resulting lacunary compound to alpha-P(2)W(17)O(61)(10)(-), and finally reaction of this species with monomeric W(VI) to form the thermodynamic and observed product, alpha-P(2)W(18)O(62)(6)(-).     

Influence of steric and electronic properties of the defect site, lanthanide ionic radii, and solution conditions on the composition of lanthanide(III) alpha1-P2W17O6110- polyoxometalates

This study identifies the principles that govern the formation and stability of Ln complexes of the (alpha(1)-P(2)W(17)O(61))(10-) isomer. The conditional stability constants for the stepwise formation equilibria, K(1cond) and K(2cond), determined by (31)P NMR spectroscopy, show that the high log K(1cond)/log K(2cond) ratio predicts the stabilization of the 1:1 Ln/ (alpha(1)-P(2)W(17)O(61))(10-) species. The value of log K(1cond) increases as the Ln series is traversed, consistent with the high charge/size requirement of the basic alpha(1) defect site. The conditional stability constants, K(2), are very low and are highly dependent on the countercations in the buffer. The source of the instability is understood from the crystal structures of the early-mid lanthanide analogues, where the close contact of the (alpha(1)-P(2)W(17)O(61))(10-) units result in severe steric encumbrance. The electronic properties of the alpha(1) defect along with the lanthanide ionic radii and countercation composition are important parameters that need to be considered for a rational synthesis of lanthanide polyoxometalates.     

Syntheses and X-ray crystal structures of zirconium(IV) and hafnium(IV) complexes containing monovacant wells-Dawson and Keggin polyoxotungstates

The syntheses and crystal structures of a series of zirconium(IV) and hafnium(IV) complexes with Dawson monovacant phosphotungstate [alpha2-P2W17O61](10-) and in situ-generated Keggin monovacant phosphotungstate [alpha-PW11O39](7-), which was obtained by a reaction of [alpha-PW12O40](3-) with Na2CO3, are described. K15H[Zr(alpha2-P2W17O61)2].25H2O (K-1), K16[Hf(alpha2-P2W17O61)2].19H2O (K-2), (Et2NH2)10[Zr(alpha-PW11O39)2].7H2O (Et2NH2-3), and (Et2NH2)10[Hf(alpha-PW11O39)2].2H2O (Et2NH2-4), being afforded by reactions in aqueous solutions of monolacunary Dawson and Keggin polyoxotungstates with ZrCl2O.8H2O and HfCl2O.8H2O followed by exchanging countercations, were obtained as analytically pure, homogeneous colorless crystals. Single-crystal X-ray structure analyses revealed that the Zr(IV) and Hf(IV) ions are in a square antiprismatic coordination environment with eight oxygen atoms, four of them being provided from each of the two monovacant polyanion ligands. Although the total molecular shapes and the 8-coordinate zirconium and hafnium centers of complexes 1-4 are identical, the bonding modes (bond lengths and bond angles) around the zirconium(IV) and hafnium(IV) centers were dependent on the monovacant structures of the polyanion ligands. Additionally, the characterization of complexes 1-4 was accomplished by elemental analysis, TG/DTA, FTIR, and solution (31P and 183W) NMR spectroscopy.     

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.     

The classic Wells-Dawson polyoxometalate, K6[alpha-P2W18O62].14H2O. Answering an 88 year-old question: what is its preferred, optimum synthesis?

The 88-year-old problem of developing a preferred, optimized synthesis of the prototype Wells-Dawson polyoxometalate, K6[alpha-P2W18O62].14H2O, is addressed herein. Specifically, six published syntheses of K6[alpha-P2W18O62].14H2O are listed and discussed, with emphasis given to the two most recent syntheses, Nadjo and co-workers' 2004 synthesis and a 1997 Inorganic Syntheses procedure by Droege, Randall, Finke et al. (hereafter D-R-F). For the starting experiment, the synthesis by Nadjo and co-workers was repeated. Next, the D-R-F synthesis and then the earlier (1984) synthesis in Droege's Ph.D. thesis were repeated and reinvestigated. The results demonstrate that the Nadjo synthesis produces over 200 g of high alpha-isomer purity (> or =97% by (31)P NMR) K6[alpha-P 2W18O62].14H2O in four steps over 8 days in 93% yield in our hands. A recrystallization step added as part of this work (for a total of five steps over 12 days) produces an increase in purity (>99%) with a concomitant loss of 8% yield (i.e., 85% overall yield) for the Nadjo-plus-recrystallization synthesis. Next, the D-R-F Inorganic Syntheses procedure was reinvestigated to determine the cause of "failed syntheses" occasionally encountered in our laboratories, the most recent and worst example to date being when one of us (C.R.G.) found 150 g of K10[alpha2-P 2W17O61] as an undesired side product when, as it turns out, the D-R-F Inorganic Syntheses procedure is followed rather than the earlier Droege synthesis. Specifically, it is shown that the problem in the Inorganic Syntheses procedure is that it ambiguously says to add 210 mL of HCl until a pH of 3-4 is reached when, in fact, it takes only 130-150 mL of HCL to reach a pH 3-4. Adding the full 210 mL of HCl ensures that a pH <2 is reached, as is required to produce isomerically pure K6[alpha-P 2W18O62].14H2O from the K 10[alpha 2-P 2W 17O 61] intermediate. The result is K6[alpha-P2W18O62].14H2O in five steps over 10 days in 82% yield and > or =97% purity. A table is provided comparing the details of the two best syntheses as reported herein: the Nadjo-plus-recrystallization synthesis and the D-R-F synthesis (with sufficient added HCl/proper pH control). That table makes apparent that the Nadjo-plus-recrystallization synthesis is improved on the basis of its better atom economy, its slightly higher product yields (85% vs 82%), slightly better purity (>99% vs >97%), and its comparable time (2 days shorter without recrystallization but 2 days longer with recrystallization) in comparison to the D-R-F synthesis with proper pH <2 control. Perhaps most importantly, some take-home messages concerning polyoxometalate synthesis illustrated by the iterative, 88 year-old quest to the best K6[alpha-P 2W18O62].14H2O synthesis are summarized and briefly discussed.     

Photoreduction of 99Tc pertechnetate by nanometer-sized metal oxides: new strategies for formation and sequestration of low-valent technetium

Technetium-99 ((99)Tc) (β(-)(max): 293.7 keV; t(1/2): 2.1 × 10(5) years) is a byproduct of uranium-235 fission and comprises a large component of radioactive waste. Under aerobic conditions and in a neutral-basic environment, the pertechnetate anion ((99)TcO(4)(-)) is stable. (99)TcO(4)(-) is very soluble, migrates easily through the environment and does not sorb well onto mineral surfaces, soils, or sediments. This study moves forward a new strategy for the reduction of (99)TcO(4)(-) and the chemical incorporation of the reduced (99)Tc into a metal oxide material. This strategy employs a single material, a polyoxometalate (POM), α(2)-[P(2)W(17)O(61)](10-), that can be photoactivated in the presence of 2-propanol to transfer electrons to (99)TcO(4)(-) and incorporate the reduced (99)Tc covalently into the α(2)-framework to form the (99)Tc(V)O species, (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-). This occurs via the formation of an intermediate species that slowly converts to (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-). Extended X-ray absorption fine structure and X-ray absorption near-edge spectroscopy analysis suggests that the intermediate consists of a (99)Tc(IV) α(2)- species where the (99)Tc is likely bound to two of the four W-O oxygen atoms in the α(2)-[P(2)W(17)O(61)](10-) defect. This intermediate then oxidizes and converts to the (99)Tc(V)O(α(2)-P(2)W(17)O(61))(7-) product. The reduction and incorporation of (99)TcO(4)(-) was accomplished in a "one pot" reaction using both sunlight and UV irradiation and monitored as a function of time using multinuclear nuclear magnetic resonance and radio thin-layer chromatography. The process was further probed by the "step-wise" generation of reduced α(2)-P(2)W(17)O(61)(12-) through bulk electrolysis followed by the addition of (99)TcO(4)(-). The reduction and incorporation of ReO(4)(-), as a nonradioactive surrogate for (99)Tc, does not proceed through the intermediate species, and Re(V)O is incorporated quickly into the α(2)-[P(2)W(17)O(61)](10-) defect. These observations are consistent with the periodic trends of (99)Tc and Re. Specifically, (99)Tc is more easily reduced compared to Re. In addition to serving as models for metal oxides, POMs may also provide a suitable platform to study the molecular level dynamics and the mechanisms of the reduction and incorporation of (99)Tc into a material.     

Synthesis and Structure of One Novel Polyoxometalate Compound Connected via Cerium(III) Cation with Three-dimensional Framework H_2{[K(H_2O)_2]_2[Ce(H_2O)_5]_2(H_2Mo_(1.16)W_(10.84)O_(42))}·8H_2O

One novel polyoxometalate compound connected via trivalent cerium cation as bridge H2{[K(H2O)2]2[Ce(H2O)5]2(H2Mo1.16W10.84O42)}·8H2O 1 was designed and synthesized in aqueous solution. X-ray diffraction analysis reveals that the structure of 1 is a three-dimensional framework assembled from the arrangement of H2Mo1.16W10.84O42 (named paradodecmetalate-B) and Ce(H2O)53+ containing two planes, which are constructed through the unification of H2Mo1.16W10.84O4210- and Ce(H2O)53+ along the [100] and [001] directions. Crystal data: H96Ce4K4Mo2.32O128W21.68, Mr = 7074.89, monoclinic, P21/n, a = 12.5037(17), b = 17.002(2), c = 12.7473(17) A, β = 105.966(2)°, V = 2605.4(6) 3, Z = 1, Dc = 4.509 g/cm3, F(000) = 3132, μ = 26.098 mm-1, R = 0.0377 and wR = 0.0789 (I > 2σ(I)).