Polyoxomolybdate promoted hydrolysis of a DNA-model phosphoester studied by NMR and EXAFS spectroscopy

Hydrolysis of (p-nitrophenyl)phosphate (NPP), a commonly used phosphatase model substrate, was examined in molybdate solutions by means of (1)H, (31)P, and (95)Mo NMR spectroscopy and Mo K-edge Extended X-ray Absorption Fine Structure (EXAFS) spectroscopy. At 50 °C and pD 5.1 the cleavage of the phosphoester bond in NPP proceeds with a rate constant of 2.73 × 10(-5) s(-1) representing an acceleration of nearly 3 orders of magnitude as compared to the hydrolysis measured in the absence of molybdate. The pD dependence of k(obs) exhibits a bell-shaped profile, with the fastest cleavage observed in solutions where [Mo(7)O(24)](6-) is the major species in solution. Mixing of NPP and [Mo(7)O(24)](6-) resulted in formation of these two intermediate complexes that were detected by (31)P NMR spectroscopy. Complex A was characterized by a (31)P NMR resonance at -4.27 ppm and complex B was characterized by a (31)P NMR resonance at -7.42 ppm. On the basis of the previous results from diffusion ordered NMR spectroscopy, performed with the hydrolytically inactive substrate phenylphosphonate (PhP), the structure of these two complexes was deduced to be (NPP)(2)Mo(5)O(21)(4-) (complex A) and (NPP)(2)Mo(12)O(36)(H(2)O)(6)(4-) (complex B). The pH studies point out that both complexes are hydrolytically active and lead to the hydrolysis of phosphoester bond in NPP. The NMR spectra did not show evidence of any paramagnetic species, excluding the possibility of Mo(VI) reduction to Mo(V), and indicating that the cleavage of the phosphomonoester bond is purely hydrolytic. The Mo K-edge XANES region also did not show any sign of Mo(VI) to Mo(V) reduction during the hydrolytic reaction. (95)Mo NMR and Mo K-edge EXAFS spectra measured during different stages of the hydrolytic reaction showed a gradual disappearance of [Mo(7)O(24)](6-) during the hydrolytic reaction and appearance of [P(2)Mo(5)O(23)](6-), which was the final complex observed at the end of hydrolytic reaction.     

Characterization of active phosphorus surface sites at synthetic carbonate-free fluorapatite using single-pulse 1H, 31P, and 31P CP MAS NMR

The chemically active phosphorus surface sites defined as PO(x), PO(x)H, and PO(x)H2, where x = 1, 2, or 3, and the bulk phosphorus groups of PO4(3-) at synthetic carbonate-free fluorapatite (Ca5(PO4)3F) have been studied by means of single-pulse 1H,31P, and 31P CP MAS NMR. The changes in composition and relative amounts of each surface species are evaluated as a function of pH. By combining spectra from single-pulse 1H and 31P MAS NMR and data from 31P CP MAS NMR experiments at varying contact times in the range 0.2-3.0 ms, it has been possible to distinguish between resonance lines in the NMR spectra originating from active surface sites and bulk phosphorus groups and also to assign the peaks in the NMR spectra to the specific phosphorus species. In the 31P CP MAS NMR experiments, the spinning frequency was set to 4.2 kHz; in the single-pulse 1H MAS NMR experiments, the spinning frequency was 10 kHz. The 31P CP MAS NMR spectrum of fluorapatite at pH 5.9 showed one dominating resonance line at 2.9 ppm assigned to originate from PO4(3-) groups and two weaker shoulder peaks at 5.4 and 0.8 ppm which were assigned to the unprotonated PO(x) (PO, PO2-, and PO3(2-)) and protonated PO(x)H (PO2H and PO3H-) surface sites. At pH 12.7, the intensity of the peak representing unprotonated PO(x) surface sites has increased 1.7% relative to the bulk peak, while the intensity of the peaks of the protonated species PO(x)H have decreased 1.4% relative to the bulk peak. At pH 3.5, a resonance peak at -4.5 ppm has appeared in the 31P CP MAS NMR spectrum assigned to the surface species PO(x)H2 (PO3H2). The results from the 1H MAS and 31P CP MAS NMR measurements indicated that H+, OH-, and physisorbed H2O at the surface were released during the drying process at 200 degrees C.     

Cadmium(II) N-acetylcysteine complex formation in aqueous solution

The complex formation between Cd(II) ions and N-acetylcysteine (H(2)NAC) in aqueous solution was investigated using Cd K- and L(3)-edge X-ray absorption and (113)Cd NMR spectroscopic techniques. Two series of 0.1 M Cd(II) solutions with the total N-acetylcysteine concentration c(H2NAC) varied between 0.2-2 M were studied at pH 7.5 and 11.0, respectively. At pH = 11 a novel mononuclear [Cd(NAC)(4)](6-) complex with the average Cd-S distance 2.53(2) ? and the chemical shift δ((113)Cd) = 677 ppm was found to dominate at a concentration of the free deprotonated ligand [NAC(2-)] > 0.1 M, consistent with our previous reports on cadmium tetrathiolate complex formation with cysteine and glutathione. At pH 7.5 much higher ligand excess ([HNAC(-)] > 0.6 M) is required to make this tetrathiolate complex the major species. The (113)Cd NMR spectrum of a solution containing c(Cd(II)) = 0.5 M and c(H2NAC) = 1.0 M measured at 288 K showed three broad signals at 421, 583 and 642 ppm, which can be attributed to CdS(3)O(3), CdS(3)O and CdS(4) coordination sites, respectively, in oligomeric Cd(II)-NAC species with single thiolate bridges between the cadmium ions.     

Probing crystal structures and transformation reactions of ammonium molybdates by 14N MAS NMR spectroscopy

The unique high-resolution feature offered by 14N magic-angle spinning (MAS) NMR spectroscopy of ammonium ions has been used to characterize the crystal structures of various ammonium molybdates by their 14N quadrupole coupling parameters, i.e., CQ, the quadrupole coupling constant, and etaQ, the asymmetry parameter. Two polymorphs of diammonium monomolybdate, (NH4)2MoO4, recently structurally characterized by single-crystal X-ray diffraction (XRD) and named mS60 and mP60, show distinct but different 14N MAS NMR spectra from each of which two sets of characteristic 14N CQ and etaQ values have been obtained. Similarly, the well-characterized ammonium polymolybdates (NH4)2Mo2O7, (NH4)6Mo7O24.4H2O, and (NH4)6Mo8O27.4H2O also give rise to distinct and characteristic 14N MAS NMR spectra. In particular, it is noted that simulation of the experimental (NH4)6Mo7O24.4H2O spectrum requires an iterative fit with six independent NH4+ sites. For the slow spinning frequencies employed (nu(r) = 1500-3000 Hz), all 14N MAS NMR spectra of the ammonium molybdates in this study are fingerprints of their identity. These different 14N MAS NMR fingerprints are shown to be an efficient tool in qualitative and quantitative assessment of the decomposition of (NH4)2MoO4 in humid air. Finally, by a combination of the 14N and 95Mo MAS NMR experiments performed here, it has become clear that a recent report of the 95Mo MAS spectra and data for the mS60 and mP60 polymorphs of (NH4)2MoO4 are erroneous because the sample examined had decomposed to (NH4)2Mo2O7.     

4-amino- and 4-nitrodipicolinatovanadium(V) complexes and their hydroxylamido derivatives: synthesis, aqueous, and solid-state properties

A number of 4-substituted, dipicolinatodioxovanadium(V) complexes and their hydroxylamido derivatives were synthesized to characterize the solid state and solution properties of five- and seven-coordinate vanadium(V) complexes. The X-ray crystal structures of Na[VO2dipic-NH2].2H2O (2) and K[VO2dipic-NO2] (3) show the vanadium adopting a distorted, trigonal-bipyramidal coordination environment similar to the parent coordination complex, [VO2dipic]- (1), reported previously as the Cs+ salt. The observed differences in the chemical shifts of the complexes both in the 1H (ca. 0.7-1.4 ppm) and 51V (ca. 1-11 ppm) NMR spectra were consistent with the electron-donating or electron-withdrawing properties of the substituent groups, respectively. Stoichiometric addition of a series of hydroxylamine ligands (H2NOH, MeHNOH, Me2NOH, and Et2NOH) to complexes 1-3 led to the formation of seven-coordinate vanadium(V) complexes. The X-ray crystal structure of [VO(dipic)(Me2NO)(H2O)].0.5H2O (1c) was found to be similar to the previously characterized complexes [VO(dipic)(H2NO)(H2O)] (1a) and [VO(dipic)(OO-tBu)(H2O)]. While only slight differences in the 1H NMR spectra were observed upon addition of the hydroxylamido ligand, the signals in the 51V NMR spectra change by up to 100 ppm. The addition of the hydroxylamido ligand increased the complex stability of complexes 2 and 3. Evidence for a nonstoichiometric redox reaction was found for the monoalkyl hydroxylamine ligand. The reaction of an unsaturated five-coordinate species with a hydroxylamine to form a seven-coordinate vanadium complex will, in general, dramatically increase the amounts of the vanadium compound that remain intact at pH values near neutral.     

Stable enols of carboxylic esters. The poly(methoxycarbonyl)cyclopentadiene systems

The structures of the poly(methoxycarbonyl)cyclopentadienes C(5)H(6)(-)(n)()(CO(2)Me)(n)(), n = 5 (Cp-5), n = 4 (Cp-4), n = 3 (1, 2,4-; Cp-3) and n = 2 (1,2-; 1,2-Cp-2-I) were investigated. The X-ray diffractions of Cp-5 (known), Cp-4, and Cp-3 showed an enol of ester structure in the solid state. The enolic hydrogen forms a symmetrical hydrogen bond to a neighboring ester carbonyl, so that the vicinal "enolic" CO(2)Me groups in the 1, 2-C(=CO(2)Me)-C(CO(2)Me)(4) moiety are identical. The relevant X-ray parameters for the three enols are similar. The CP-MAS spectra of Cp-3-Cp-5 generally resemble their (13)C NMR spectra in CDCl(3) except for some differences of mostly <1 ppm. The (1)H, (13)C, and (17)O NMR spectra of Cp-3-Cp-5 in CDCl(3) are consistent with those of the hydrogen bonded enols. Most characteristic are the (1)H and (17)O signals of the OH groups at 19.7-20.1 and 221-225 ppm, respectively. Proton addition to sodium 1, 2-bis(methoxycarbonyl)cyclopentadienide gave a mixture of four 1, 2-bis(methoxycarbonyl)cyclopentadienes. The isomer (1,2-Cp-2-I) formed in 10-20% displays delta(O(1)H) at 19.3 ppm and is the enol analogue of Cp-5 whereas its main isomer (30-55%) (1,2-Cp-2-IV) has the ester structure. In CD(3)CN and DMSO-d(6) only one signal was observed at room temperature for each type of H, C, or O of Cp-5, suggesting a complete ionization to the symmetrical anion of Cp-5. In contrast, Cp-4 and Cp-3 in CD(3)CN at room temperature display OH signals in both (1)H and (17)O NMR spectra, and Cp-5 shows a broad OH signal in the (1)H spectrum at 240 K. The enol of ester structure is the main species, although exchange with the corresponding anion is possible. On standing in DMF-d(7) at room temperature, new signals are observed for Cp-3 and Cp-4. On raising the temperature in Cl(2)CDCDCl(2), Cp-3-Cp-5 show line broadening and appearance of new signals. These were ascribed to rearrangment and decomposition processes.     

Cluster oxalate complexes [M3(mu3-Q)(mu2-Q2)3(C2O4)3]2- and [Mo3(mu3-Q)(mu2-Q)3(C2O4)3(H2O)3]2- (M = Mo, W; Q = S, Se): mechanochemical synthesis and crystal structure

Mechanochemical reaction of cluster coordination polymers 1infinity[M3Q7Br4] (M = Mo, W; Q = S, Se) with solid K2C2O4 leads to cluster core excision with the formation of anionic complexes [M3Q7(C2O4)3]2-. Extraction of the reaction mixture with water followed by crystallization gives crystalline K2[M3Q7(C2O4)3].0.5KBr.nH2O (M = Mo, Q = S, n = 3 (1); M = Mo, Q = Se, n = 4 (2); M = W, Q = S, n = 5 (3)). Cs2[Mo3S7(C2O4)3].0.5CsCl.3.5H2O (4) and (Et4N)1.5H0.5K{[Mo3S7(C2O4)3]Br}.2H2O (5) were also prepared. Close Q...Br contacts result in the formation of ionic triples {[M3Q7(C2O4)3](2)Br}5- in 1-4 and the 1:1 adduct {[Mo3S7(C2O4)3]Br}3- in 5. Treatment of 1 or 2 with PPh(3) leads to chalcogen abstraction with the formation of [Mo3(mu3-Q)(mu2-Q)3(C2O4)3(H2O)3]2-, isolated as (Ph4P)2[Mo3(mu3-S)(mu2-S)3(C2O4)3(H2O)3].11H2O (6) and (Ph4P2[Mo3(mu3-Se)(mu2-Se)3(C2O4)3(H2O)3].8.5H2O.0.5C2H5OH (7). All compounds were characterized by X-ray structure analysis. IR, Raman, electronic, and 77Se NMR spectra are also reported. Thermal decomposition of 1-3 was studied by thermogravimetry.     

Syntheses and Characterization of gamma-[SiW(10)M(2)S(2)O(38)](6)(-) (M = Mo(V), W(V)). Two Keggin Oxothio Heteropolyanions with a Metal-Metal Bond

The oxothio polyanions gamma-[SiW(10)M(2)S(2)O(38)](6)(-) (M = Mo(V), W(V)) were obtained through stereospecific addition of the dication [M(2)S(2)O(2)](2+) (M = Mo, W) to the divacant gamma-[SiW(10)O(36)](8)(-) anion in dimethylformamide. These compounds were isolated as crystals and are stable in usual organic solvents and in aqueous medium from pH = 1 to pH = 7. NEt(4)Cs(3)H(2)[SiW(10)Mo(2)S(2)O(38)].6H(2)O (a gamma-isomer derived from the alpha Keggin structure capped by the [Mo(2)S(2)O(2)](2+) fragment containing a metal-metal bond) crystallizes in the triclinic space group P&onemacr; with a = 12.050(3) ?, b = 12.695(2) ?, c = 20.111(4) ?, alpha = 74.35(2) degrees, beta = 86.83(2) degrees, gamma = 63.50(2) degrees, Z = 2. NEt(4)Cs(5)[SiW(12)S(2)O(38)].7H(2)O is isostructural and crystallizes in the triclinic space group P&onemacr; with a = 12.197(4) ?, b = 12.714(3) ?, c = 20.298(3) ?, alpha = 74.75(1) ?, beta = 86.48(2) degrees, gamma = 61.80(2) degrees, Z = 2. (183)W NMR spectra of Li(+) salts in aqueous solution agree with the solid state structures and reveal 100% purity for both anions. Polarographic, infrared and UV-vis data are also given.     

钼(钨)-钼-硫簇合物的~(95)Mo NMR研究

对含有[MS4]2-（M＝Mo，W）单元的一系列钼（钨）-钼-硫簇合物进行了95MoNMR研究，定性分析了95Mo化学位移随金属原子配位数、配位金属种类和配位金属配体改变而变化的规律。结果表明，随着[MoS4]2-配位金属原子数的增加，[MoS4]2-上Mo的化学位移逐渐向高场移动，这可归因于低氧化态MO0上的电子通过硫桥离域到高氧化态的Mo上。为了解析Mo0上化学位移的实验结果，采用MM＋力场对[（OC）4MOS2MoS2]2-和[｛（OC）4Mo｝MoS4]2-的晶体结构进行几何优化，使之更接近于溶液中的结构，然后利用Fenske－Hall方法计算Mo原子上的净电荷分布，计算结果较好地好择了Mo和Mo0化学位移的变化趋势。     

13C-labeled N-acetyl-neuraminic acid in aqueous solution: detection and quantification of acyclic keto, keto hydrate, and enol forms by 13C NMR spectroscopy

Aqueous solutions of N-acetyl-neuraminic acid (Neu5Ac, 1) labeled with (13)C at C1, C2, and/or C3 were analyzed by (13)C NMR spectroscopy to detect and quantify the acyclic forms (keto, keto hydrate, enol) present at varying pHs. In addition to pyranoses, solutions contained the keto form, based on the detection of C2 signals at approximately 198 ppm (approximately 0.7% at pH 2). Spectra of [2-(13)C] and [3-(13)C] isotopomers contained signals arising from labeled carbons at approximately 143 and approximately 120 ppm, respectively, which were attributed to enol forms. Solution studies of [1,2,3-(13)C3] 1 substantiated the presence of enol (approximately 0.5% at pH 2). Enol was not detected at pH > 6.0. A C2 signal observed at approximately 94 ppm was identified as C2 of the keto hydrate (approximately 1.9% at pH 2), based partly on its abundance as a function of solution pH. Density functional theory (DFT) calculations were used to study the effect of enol and hydrate structure on J(CH) and J(CC) values involving C2 and C3 of these forms. Solvated DFT calculations showed that (2)J(C2,H3) in cis and trans enols have similar magnitudes but opposite signs, making this J-coupling potentially useful to distinguish enol configurations. Solvent deuterium exchange studies of 1 showed rapid incorporation of (2)H from (2)H2O at H3 axial in the pyranoses at p(2)H 8.0, followed by slower exchange at H3 equatorial. The acyclic keto form, which presumably participates in this reaction, must assume a pseudo-cyclic conformation in solution in order to account for the exchange selectivity. Weak (13)C signals arising from labeled species were also observed consistently and reproducibly in aqueous solutions of (13)C-labeled 1, possibly arising from products of lactonization or intermolecular esterification.     

Ligand Substitution Reactions at the Nickel of [Mo(3)NiS(4)(H(2)O)(10)](4+) with Two Water Soluble Phosphines, CO, Br(-), I(-), and NCS(-) and the Inertness of the 1,4,7-Triazacyclononane (L) Complex [Mo(3)(NiL)S(4)(H(2)O)(9)](4+)

Comparisons (25 degrees C) are made of substitution reactions, X replacing H(2)O, at the tetrahedral Ni of the heterometallic sulfido cuboidal cluster [Mo(3)NiS(4)(H(2)O)(10)](4+), I = 2.00 M (LiClO(4)). Stopped-flow formation rate constants (k(f)/M(-)(1) s(-)(1)) for six X reagents, including two water soluble air-stable phosphines, 1,3,5-triaza-7-phosphaadamantane PTA (119) and tris(3-sulfonatophenyl)phosphine TPPTS(3)(-) (58), and CO (0.66), Br(-) (14.6), I(-) (32.3), and NCS(-) (44) are reported alongside the previous value for Cl(-) (9.4). A dependence on [H(+)] is observed with PTA, which gives an unreactive form confirmed by NMR as N-protonated PTA (acid dissociation constant K(a) = 0.61 M), but in no other cases with [H(+)] in the range 0.30-2.00 M. The narrow spread of rate constants for all but the CO reaction is consistent with an I(d) dissociative interchange mechanism. In addition NMR studies with H(2)(17)O enriched solvent are too slow for direct determination of the water-exchange rate constant indicating a value <10(3) s(-)(1). Equilibrium constants/M(-)(1) for 1:1 complexing with the different X groups at the Ni are obtained for PTA (2040) and TPPTS(3)(-) (8900) by direct spectrophotometry and from kinetic studies (k(f)/k(b)) for Cl(-) (97), Br(-) (150), NCS(-) (690), and CO (5150). No NCS(-) substitution at the Ni is observed in the case of the heterometallic cube [Mo(3)Ni(L)S(4)(H(2)O)(9)](4+), with tridentate 1,4,7-triazacyclononane(L) coordinated to the Ni. Substitution of NCS(-) for H(2)O, at the Mo's of [Mo(3)NiS(4)(H(2)O)(10)](4+) and [Mo(3)(NiL)S(4)(H(2)O)(9)](4+) are much slower secondary processes, with k(f) = 2.7 x 10(-)(4) M(-)(1) s(-)(1) and 0.94 x 10(-)(4) M(-)(1) s(-)(1) respectively. No substitution of H(2)O by TPPTS(3)(-) or CO is observed over approximately 1h at either metal on [Mo(3)FeS(4)(H(2)O)(10)](4+), on [Mo(4)S(4)(H(2)O)(12)](5+) or [Mo(3)S(4)(H(2)O)(9)](4+).     

Structural and kinetic studies on uranyl(V) carbonate complex using 13C NMR spectroscopy

We have measured 13C NMR spectra of uranyl(V) carbonate complex in D2O solution containing 1.003 M Na2(13)CO3 at various temperatures. Two singlet signals corresponding to free and coordinated CO3(2-) were observed at 169.13 and 106.70 ppm, respectively. From the peak area ratio, the structure of the uranyl(V) carbonate complex was determined as [U(V)O2(CO3)3]5-. Furthermore, kinetic analyses of the exchange reaction of free and coordinated CO3(2-) in [U(V)O2(CO3)3]5- were carried out using 13C NMR line-broadening. As a result, the first-order rate constant at 298 K and the activation parameters for CO3(2-) exchange reaction in [U(V)O2(CO3)3]5- were evaluated as 1.13 x 10(3) s(-1) and deltaH(double dagger) = 62.0 +/- 0.7 kJ x mol(-1), deltaS(double dagger) = 22 +/- 3 J x mol(-1) x K(-1), respectively. We suggest that the exchange follows a dissociative mechanism as in the corresponding [U(VI)O2(CO3)3]4- complex.     

Structural characterization of molybdenum(V) species in aqueous HCl solutions

Mo(V) aqua-chloro complexes in hydrochloric acid solutions have been studied by means of Mo K- and L2,3-edge X-ray absorption and Raman spectroscopic methods. The solid compounds (HPPh3)2[MoOCl5] (1), 6[MoOCl4(H2O)]-.10(pyH)+.4Cl- (2), and (pyH)2[Mo2O4Cl4(trans-OH2)2] (3) were used for structural comparisons. The compound 2 crystallizes in the orthorhombic space group Pmma (no. 51) with a=21.398(3), b=8.057(4), c=13.330(4) A, and Z=4. In 0.2 M solutions of MoCl5 in 7.4-9.4 M HCl the mononuclear [MoOCl4(OH2)]- complex dominates with the bond distances Mo=O 1.66(2) A, Mo-Cl 2.38(2) A, and Mo-OH2 2.30(2) A. Its Raman band at 994 cm-1 for the Mo=O symmetric stretching vibration is closer to that of 2 (988 cm-1) than of 1 (969 cm-1). The Mo K-edge EXAFS spectrum for 0.2 M MoCl5 in 1.7 M HCl solution reveals a dinuclear [Mo2O4Cl6-n(OH2)n]n-4 (n=2, 3) complex with a double oxygen bridge and the average distances Mo=O 1.67(2) A, Mo-(mu-O) 1.93(2) A, Mo-Cl 2.47(3) A, Mo-Mo 2.56(2) A, and a short Mo-OH2 distance of 2.15(2) A, which implies that at least one of the aqua ligands is in equatorial position relative to the two axial Mo=O bonds. This position differs from the Mo-OH2 configuration exclusively trans to the M=O groups of the isomeric (with n=2) dinuclear complex in 3. The difference in the ligand field is also reflected in their L2,3-edge XANES spectra. For 0.2 M MoCl5 solutions in intermediate HCl concentrations (3.7-6.3 M) the Raman bands at 802 cm-1 (Mo-O-Mo) and 738 cm-1 (Mo-(mu-O)2-Mo) verify three coexisting classes of Mo(V) complexes: mononuclear complexes together with dinuclear mono-oxo (e.g., [Mo2O3Cl6(H2O)2]2-) and dioxo bridged species, even though principal component analysis (PCA) of the corresponding series of EXAFS spectra only could distinguish two major components. By fitting linear combinations of the appropriate EXAFS oscillation components, dioxo-bridged dinuclear complexes were found to dominate at HCl concentrations相似文献   

Polyoxomolybdenum(V/VI)-sulfite compounds: synthesis, structural, and physical studies

Reaction of Na(2)Mo(VI)O(4) x 2H(2)O with (NH(4))(2)SO(3) in the mixed-solvent system H(2)O/CH(3)CN (pH = 5) resulted in the formation of the tetranuclear cluster (NH(4))(4)[Mo(4)(VI)SO(16)] x H(2)O (1), while the same reaction in acidic aqueous solution (pH = 5) yielded (NH(4))(4)[Mo(5)(VI)S(2)O(21)] x 3H(2)O (2). Compound {(H(2)bipy)(2)[Mo(5)(VI)S(2)O(21)] x H(2)O}(x) (3) was obtained from the reaction of aqueous acidic solution of Na(2)Mo(VI)O(4) x 2H(2)O with (NH(4))(2)SO(3) (pH = 2.5) and 4,4'-bipyridine (4,4'-bipy). The mixed metal/sulfite species (NH(4))(7)[Co(III)(Mo(2)(V)O(4))(NH(3))(SO(3))(6)] x 4H(2)O (4) was synthesized by reacting Na(2)Mo(VI)O(4) x 2H(2)O with CoCl(2) x 6H(2)O and (NH(4))(2)SO(3) with precise control of pH (5.3) through a redox reaction. The X-ray crystal structures of compounds 1, 2, and 4 were determined. The structure of compound 1 consists of a ring of four alternately face- and edge-sharing Mo(VI)O(6) octahedra capped by the trigonal pyramidal sulfite anion, while at the base of the Mo(4) ring is an oxo group which is asymmetrically shared by all four molybdenum atoms. Compound 3 is based on the Strandberg-type heteropolyion [Mo(5)(VI)S(2)O(21)](4-), and these coordinatively saturated clusters are joined by diprotonated 4,4'-H(2)bipy(2+) through strong hydrogen bonds. Compound 3 crystallizes in the chiral space group C2. The structure of compound 4 consists of a novel trinuclear [Co(III)Mo(2)(V)SO(3)(2-)] cluster. The chiral compound 3 exhibits nonlinear optical (NLO) and photoluminescence properties. The assignment of the sulfite bands in the IR spectrum of 4 has been carried out by density functional calculations. The cobalt in 4 is a d(6) octahedral low-spin metal atom as it was evidenced by magnetic susceptibility measurements, cw EPR, BVS, and DFT calculations. The IR and solid-state UV-vis spectra as well as the thermogravimetric analyses of compounds 1-4 are also reported.     

Chiral polyoxotungstates. 1. Stereoselective interaction of amino acids with enantiomers of [Ce(III)(alpha1-P2W17O61)(H2O)x]7-. The structure of DL-[Ce2(H2O)8(P2W17O61)2]14-

The ammonium salt of the 1:1complex (1) of Ce(III) with alpha(1)-[P(2)W(17)O(61)](10)(-) was prepared and characterized by elemental analysis, vibrational and NMR spectroscopy ((31)P, (183)W), cyclic voltammetry, and single-crystal X-ray analysis (P1; a = 15.8523(9) A, b = 17.4382(10) A, c = 29.3322(16) A, alpha = 99.617(1) degrees, beta = 105.450 (1) degrees, gamma = 101.132(1) degrees, V = 7460.9(7) A(3), Z = 2). The anion consists of a centrosymmetric head-to-head dimer, [[Ce(H(2)O)(4)(P(2)W(17)O(61))](2)],(14-) with each 9-coordinate Ce cation linked to four oxygens of one tungstophosphate anion and to one oxygen of the other anion. On the basis of P NMR spectroscopy, a monomer-dimer equilibrium exists in solution with K = 20 +/- 4 M(-1) at 22 degrees C. Addition of chiral amino acids to aqueous solutions of 1 results in splitting of the (31)P NMR signals as a result of diastereomer formation. No such splitting is observed with glycine or DL-proline, or when chiral amino acids are added to the corresponding complex of the achiral alpha(2)-isomer of [P(2)W(17)O(61)](10)(-). From analysis of the (31)P NMR spectra, formation constants of the two diastereomeric adducts of 1 with L-proline are 7.3 +/- 1.3 and 9.8 +/- 1.4 M(-1).     

A vanadium-51 NMR study of the binding of vanadate and peroxovanadate to proteins

51V quadrupolar central transition NMR spectra of buffered (pH 7.6-8.0) solutions of bovine apo-transferrin (Tf) and bovine prostatic acid phosphatase (Pp) treated with vanadate show normal features (chemical shifts between -515 and -542 ppm) corresponding to the complexation of VO2+ to the Tf binding site and the Pp active centre, respectively. Addition of H2O2 leads to the temporary formation of complexed VO(O2)+ (delta approximately -595). Vanadate-dependent bromoperoxidase from the alga Ascophyllum nodosum exhibits an unusually high shielding both for the native (delta = -931) and the peroxo form (delta = -1135) of the enzyme. A resonance at -471 ppm is traced back to an inactive form with oxovanadium(V) in a trigonal-bipyramidal array.     

Synthesis,Crystal Structure of Cis—dioxo—catecholatotungsten（VI） Complex and Its NMR Studies on the Interaction with ATP

Cis-dioxo-catecholatotungsten(VI) complex anion[W^(VI)O2-(OC6H4O)2]^2- was obtained with discrete protonated ethylenediamine (NH2CH2CH2NH3)^ cations by the reaction of tetrabutyl ammonium decatungstate with catechol in the mixed solvent of CH3OH,CH3CN and ethylenediamine,and compared with its molybdenum anaogue [Mo^(V) O2(OC6H4O)2]^3- by crystal structure,UV,EPR,The results of the UV and EPR spectra show that tungsten is less redox active than molybdenum since the molybdenum is reduced from Mo(VI) to Mo(V) but tungsten stays in the original highest oxidized state Mo(VI) when they are crystallized from the solution above.It is worthy to note that [W^(VI)O2(OC6H4O)]^2- shows the same coordination structure as its molybdenum analogue in which the metal center exhibits distorted octahedral coordination geometry with two cis-dioxocatecholate ligands and might have the related coordination structure feature with the cofactor of flavoenzyme because [Mo^(V)O2(OC6H4O)2]^3- presented essentially the same EPR spectra as flavoenzyme.The NMR studies on the interaction of the title complex with ATP reveal that the reduction of W(VI) to W(V) occurs when the title complex is dissolved in D2O and the W(V) is oxidized again when ATP solution is mixed with original solution and the hydrolysis of the catecholato ligand take places at mean time being monitored by ^1H NMR and ^13C NMR spectra.     

Zr(IV)-monosubstituted Keggin-type dimeric polyoxometalates: synthesis, characterization, catalysis of H2O2-based oxidations, and theoretical study

The previously unknown Zr(IV)-monosubstituted Keggin-type polyoxometalates (Zr-POMs), (n-Bu4N)7H[{PW11O39Zr(mu-OH)}2] (1), (n-Bu4N)8[{PW11O39Zr(mu-OH)}2] (2), and (n-Bu4N)9[{PW11O39Zr}2(mu-OH)(mu-O)] (3) differing in their protonation state, have been prepared starting from heteropolyacid H5PW11ZrO40.14H2O. The compounds were characterized by elemental analysis, potentiometric titration, X-ray single-crystal structure, and IR, Raman, and 31P and 183W NMR spectroscopy. The single-crystal X-ray analysis of 2 reveals that two Keggin structural units [PW11O39Zr]3- are linked through two hydroxo bridges Zr-(OH)-Zr with Zr(IV) in 7-fold coordination. The IR spectra of 1 and 2 show a characteristic band at 772 cm(-1), which moves to 767 cm(-1) for 3, reflecting deprotonation of the Zr-(OH)-Zr bond. Potentiometric titration with methanolic Bu4NOH indicates that 1-3 contain 2, 1, and 0 acid protons, respectively. (83W NMR reveals Cs symmetry of 2 and 3 in dry MeCN, while for 1, it discovers nonequivalence of its two subunits and their distortion resulting from localization of the acidic proton on one of the Zr-O-W bridging O atoms. The (31)P NMR spectra of 2 and 3 differ insignificantly in dry MeCN, showing only signals at delta -12.46 and -12.44 ppm, respectively, while the spectrum of 1 displays two resonances at delta -12.3 (narrow) and -13.2 (broad) ppm, indicating slow proton exchange on the (31)P NMR time scale. The theoretical calculations carried out at the density functional theory level on the dimeric species 1-3 propose that protonation at the Zr-O-Zr bridging site is more favorable than protonation at Zr-O-W sites. Calculations also revealed that the doubly bridged hydroxo structure is thermodynamically more stable than the singly bridged oxo structure, in marked contrast with analogous Ti- and Nb-monosubstituted polyoxometalates. The interaction of 1-3 with H(2)O and H(2)O(2) in MeCN has been studied by both (31)P and (183)W NMR. The stability of the [PW(11)O(39)ZrOH](4-) structural unit toward at least 100-fold excess of H2O2 in MeCN was confirmed by both NMR and Raman spectroscopy. The interaction of 1 and 2 with H2O in MeCN produces most likely monomeric species (n-Bu4N)3+n[PW11O39Zr(OH)(n(H2O)(3-n)] (n = 0 and 1) showing a broad 31P NMR signal at delta -13.2 ppm, while interaction with H2O2 leads to the formation of an unstable peroxo species (delta -12.3 ppm), which reacts rapidly with cyclohexene, producing 2-cyclohexen-1-one and trans-cyclohexane-1,2-diol. Both 1 and 2 show a pronounced catalytic activity in H2O2 decomposition and H2O2-based oxidation of organic substrates, including cyclohexene, alpha-pinene, and 2,3,6-trimethylphenol. The oxidation products are consistent with those of a homolytic oxidation mechanism. On the contrary, 3 containing no acid protons reacts with neither H2O nor H2O2 and shows negligible catalytic activity. The Zr-monosubstituted polyoxometalates can be used as tractable homogeneous probes of Zr single-site heterogeneous catalysts in studying mechanisms of H2O2-based oxidations.     

Synthesis,crystal structure,and solid state NMR spectroscopy of NH(4)[(V(2)O(3))(2)(4,4'-bpy)(2)(H(2)PO(4))(PO(4))(2)].0.5H(2)O,a mixed-valence vanadium(IV,V) phosphate with a pillared layer structure

A mixed-valence vanadium phosphate, NH(4)[(V(2)O(3))(2)(4,4'-bpy)(2)(H(2)PO(4))(PO(4))(2)].0.5H(2)O, has been synthesized under hydrothermal conditions and structurally characterized by single-crystal X-ray diffraction. It crystallizes in the monoclinic space group C2/c (No. 15) with a = 12.6354(8) A, b = 9.9786(6) A, c = 23.369(1) A, beta = 92.713(1) degrees, and Z = 4 with R(1) = 0.0389. The structure consists of dimers of edge-sharing vanadium(IV,V) octahedra that are connected by corner-sharing phosphate tetrahedra to form layers in the ab-plane, which are further linked through 4,4'-bipyridine pillars to generate a 3-D framework. Magnetic susceptibility confirms the valence of the vanadium atoms. The (31)P MAS NMR spectrum shows a resonance centered at 80 ppm with a shoulder at ca. 83 ppm in an intensity ratio close to 1:2, which correspond to two distinct P sites. The observed large downfield (31)P NMR shifts can be ascribed to magnetic exchange coupling involving phosphorus atoms. The unpaired electron spin density at the phosphorus nucleus was determined from variable-temperature (31)P NMR spectra. The (1)H MAS NMR spectrum was fitted to six components in accordance with the structure as determined from X-ray diffraction.