Sib Sankar Mal

6-Peroxo-6-Zirconium Crown and Its Hafnium Analogue Embedded in a Triangular Polyanion: [M6(O2)6(OH)6(γ-SiW10O36)3]18− (M = Zr, Hf)

We have synthesized and structurally characterized the unprecedented peroxo-zirconium(IV) containing [Zr6(O2)6(OH)6(gamma-SiW10O36)3]18- (1). Polyanion 1 comprises a cyclic 6-peroxo-6-zirconium core stabilized by three decatungstosilicate units. We have also prepared the isostructural hafnium(IV) analogue [Hf6(O2)6(OH)6(gamma-SiW10O36)3]18- (2). We investigated the acid/base and redox properties of 1 by UV-vis spectroscopy and electrochemistry studies. Polyanion 1 represents the first structurally characterized Zr-peroxo POM with side-on, bridging peroxo units. The simple, one-pot synthesis of 1 and 2 involving dropwise addition of aqueous hydrogen peroxide could represent a general procedure for incorporating peroxo groups into a large variety of transition metal and lanthanide containing POMs.

Nucleation Process in the Cavity of a 48‐Tungstophosphate Wheel Resulting in a 16‐Metal‐Centre Iron Oxide Nanocluster

The 16-Fe(III)-containing 48-tungsto-8-phosphate [P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)](20-) (1) has been synthesised and characterised by IR and ESR spectroscopy, TGA, elemental analyses, electrochemistry and susceptibility measurements. Single-crystal X-ray analyses were carried out on Li(4)K(16)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)]66 H(2)O2 KCl (LiK-1, orthorhombic space group Pnnm, a=36.3777(9) A, b=13.9708(3) A, c=26.9140(7) A, and Z=2) and on the corresponding mixed sodium-potassium salt Na(9)K(11)[P(8)W(48)O(184)Fe(16)(OH)(28)(H(2)O)(4)].100 H(2)O (NaK-1, monoclinic space group C2/c, a=46.552(4) A, b=20.8239(18) A, c=27.826(2) A, beta=97.141(2) degrees and Z=4). Polyanion 1 contains--in the form of a cyclic arrangement--the unprecedented {Fe(16)(OH)(28)(H(2)O)(4)}(20+) nanocluster, with 16 edge- and corner-sharing FeO(6) octahedra, grafted on the inner surface of the crown-shaped [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) precursor. The synthesis of 1 was accomplished by reaction of different iron species containing Fe(II) (in presence of O(2)) or Fe(III) ions with the P(8)W(48) anion in aqueous, acidic medium (pH approximately 4), which can be regarded as an assembly process under confined geometries. One fascinating aspect is the possibility to model the uptake and release of iron in ferritin. The electrochemical study of 1, which is stable from pH 1 through 7, offers an interesting example of a highly iron-rich cluster. The reduction wave associated with the Fe(III) centres could not be split in distinct steps independent of the potential scan rate from 2 to 1000 mV s(-1); this is in full agreement with the structure showing that all 16 iron centres are equivalent. Polyanion 1 proved to be efficient for the electrocatalytic reduction of NO(x), including nitrate. Magnetic and variable frequency EPR measurements on 1 suggest that the Fe(III) ions are strongly antiferromagnetically coupled and that the ground state is tentatively spin S=2.

Peroxo-Zr/Hf-Containing Undecatungstosilicates and -Germanates

A family of dimeric, peroxo-containing heteropolytungstates, [M(2)(O(2))(2)(XW(11)O(39))(2)](12-) [M = Zr(4+), X = Si (1), Ge (2); M = Hf(4+), X = Si (3)], have been synthesized by reacting ZrCl(4)/HfCl(4) with the respective monolacunary Keggin precursor [XW(11)O(39)](8-) (X = Si, Ge) in an aqueous acidic medium (pH 4.8). The isostructural polyanions 1-3 are composed of two (XW(11)O(39)) Keggin units encapsulating a central diperoxo-dimetal fragment {M(2)(O(2))(2)}(4+) (M = Zr(4+), Hf(4+)). Cyclic voltammetry and exhaustive electrolysis studies indicate fast reductive release of the peroxo ligands upon reduction of 1-3. Stoichiometric oxo-transfer studies from 1-3 to the substrate l-methionine were performed, and the reactions were monitored by (1)H NMR.

Reactive ZrIV and HfIV Butterfly Peroxides on Polyoxometalate Surfaces: Bridging the Gap between Homogeneous and Heterogeneous Catalysis

At variance with previously known coordination compounds, the polyoxometalate (POM)-embedded Zr(IV) and Hf(IV) peroxides with formula: [M(2)(O(2))(2)(α-XW(11)O(39))(2)](12-) (M=Zr(IV), X=Si (1), Ge (2); M=Hf(IV), X=Si (3)) and [M(6)(O(2))(6)(OH)(6)(γ-SiW(10)O(36))(3)](18-) (M=Zr(IV) (4) or Hf(IV) (5)) are capable of oxygen transfer to suitable acceptors including sulfides and sulfoxides in water. Combined (1)H NMR and electrochemical studies allow monitoring of the reaction under both stoichiometric and catalytic conditions. The reactivity of peroxo-POMs 1-5 is compared on the basis of substrate conversion and kinetic. The results show that the reactivity of POMs 1-3 outperforms that of the trimeric derivatives 4 and 5 by two orders of magnitude. Reversible peroxidation of 1-3 occurs by H(2)O(2) addition to the spent catalysts, restoring oxidation rates and performance of the pristine system. The stability of 1-3 under catalytic regime has been confirmed by FT-IR, UV/Vis, and resonance Raman spectroscopy. The reaction scope has been extended to alcohols, leading to the corresponding carbonyl compounds with yields up to 99% under microwave (MW) irradiation. DFT calculations revealed that polyanions 1-3 have high-energy peroxo HOMOs, and a remarkable electron density localized on the peroxo sites as indicated by the calculated map of the electrostatic potential (MEP). This evidence suggests that the overall description of the oxygen-transfer mechanism should include possible protonation equilibria in water, favored for peroxo-POMs 1-3.

Cobalt, Manganese, Nickel, and Vanadium Derivatives of the Cyclic 48-Tungsto-8-Phosphate [H7P8W48O184]33−

The cobalt(II) containing tungstophosphate [Co(4)(H(2)O)(16)P(8)W(48)O(184)](32-) (1) has been synthesized by addition of Co(2+) ions to an aqueous solution of [H(7)P(8)W(48)O(184)](33-) (P(8)W(48)) and characterized by single-crystal XRD, IR, and UV-vis spectroscopy, elemental analysis, electrochemistry, and magnetochemistry. The novel polyanion 1 is a derivative of the superlacunary P(8)W(48) with four cobalt(II) ions coordinated to the rim of the central cavity and two additional cobalt(II) ions linked on the outside bridging neighboring polyanions. Using similar synthetic procedures, but adding a few drops of H(2)O(2), we isolated the manganese(II) derivative [Mn(4)(H(2)O)(16)(P(8)W(48)O(184))(WO(2)(H(2)O)(2))(2)](28-) (2) and its nickel(II) analogue [Ni(4)(H(2)O)(16)(P(8)W(48)O(184))(WO(2)(H(2)O)(2))(2)](28-) (3). Both polyanions have picked up two equivalents of tungsten resulting in the unprecedented {P(8)W(50)} host framework. We also made the vanadium(V) derivative [(VO(2))(4)(P(8)W(48)O(184))](36-) (4), with four tetrahedral vanadate groups grafted to the P(8)W(48) host. The voltammetric patterns associated with the W-centers in polyanions 1, 2, and 4 display enough distinct features allowing for a qualitative classification according to relative basicity of the reduced polyanions: 2 > P(8)W(48) > 1 > 4. The electrochemistry of 1 offers a new example for detection of the Co(2+) centers in a multicobalt containing polyanion. During a study of the Mn(2+) centers of 2 at pH 5, a film deposition is observed. The vanadium(V) centers of 4 are well-behaved in a pH 0.33 medium. Temperature and magnetic field dependence of the magnetic moment of 1-3 were performed on a SQUID magnetometer over the temperature range 1.8-250 K and field range 0-7 T. The results are consistent with the model of noninteracting 3d metal ions. Variable temperature (4-295 K) and variable frequency (34-413 GHz) EPR measurements support the magnetic susceptibility results. The zero-field splitting D and g values obtained for 1-3 are in agreement with those reported for high-spin Co(2+), Mn(2+), and Ni(2+) ions in axially distorted octahedral environments.

Wheel-Shaped Cu20-Tungstophosphate [Cu20X(OH)24(H2O)12(P8W48O184)]25- Ion (X = Cl, Br, I) and the Role of the Halide Guest

We have synthesized the known [Cu(20)Cl(OH)(24)(H(2)O)(12)(P(8)W(48)O(184))](25-) (1) and report here its bromide and iodide analogues, [Cu(20)Br(OH)(24)(H(2)O)(12)(P(8)W(48)O(184))](25-) (2) and [Cu(20)I(OH)(24)(H(2)O)(12)(P(8)W(48)O(184))](25-) (3). These polyanions were characterized in the solid state by IR spectroscopy and single-crystal X-ray diffraction. Magnetic susceptibility and magnetization data over 1.8-300 K show that the Cu(2+) ions in 1-2 are antiferromagnetically coupled, leading to a diamagnetic ground state. The effective exchange coupling constant J(eff) was estimated as approximately -3 K for both 1 and 2. Electron paramagnetic resonance measurements were made on 1 and 2 over 5-295 K at microwave frequencies of 9.5, 34, and 220 GHz. The observed (weak) signals were characteristic of randomly distributed Cu(2+) ions only, with g values and hyperfine constants typical of the unpaired electron in a 3d(x(2)-y(2)) orbital of Cu(2+). No signals attributable to the copper-hydroxo cluster were detected, supporting the conclusions from the magnetization measurements. DFT calculations were performed as well to obtain additional information on the anionic guest inside the cavity created by the copper-hydroxo cage related to electronic structure and energies of encapsulation. The polyanions 2 and 3 were also characterized by cyclic voltammetry (CV) in a pH 5 medium. Their CVs are composed by an initial two-step reduction of the Cu(2+) centers to Cu(0) through Cu(+), followed at more negative potential by the redox processes of the W centers. A comparison with the CV characteristics of the previously studied compound 1 indicates that the potential locations of the Cu or W waves of the three analogues do not depend significantly upon the identity of the central halide X. This observation is in accordance with conclusions of DFT calculations. The modified electrodes based on 2 and the room-temperature ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate triggers an efficient reduction of nitrate. To our knowledge, this is the first example of electrocatalytic nitrate reduction by a polyanion entrapped in room-temperature ionic liquid films.

Yttrium(III)-containing tungstoantimonate(III) stabilized by tetrahedral WO4(2-) capping unit, [{Y(α-SbW9O31(OH)2)(CH3COO)(H2O)}3(WO4)]17-.

The yttrium(III)-containing tungstoantimonate(III) [{Y(α-SbW(9)O(31)(OH)(2))(CH(3)COO)(H(2)O)}(3)(WO(4))](17-) (1) has been synthesized in a simple one-pot reaction of Y(3+) ions with [α-SbW(9)O(33)](9-) and WO(4)(2-) in a 3:3:1 molar ratio in 1 M LiOAc/AcOH buffer at pH 5.3. Polyanion 1 is composed of three (α-SbW(9)O(33)) units linked by three Y(3+) ions and a capping, tetrahedral WO(4)(2-) capping unit, resulting in an assembly with C(3v) symmetry. The hydrated ammonium-sodium salt of 1 was investigated in the solid state by single-crystal XRD, FT-IR spectroscopy, thermogravimetric and elemental analyses, and in solution by multinuclear NMR spectroscopy.

Hexazirconium‐ and Hexahafnium‐Containing Tungstoarsenates(III) and Their Oxidation Catalysis Properties

Exploring the interaction of lacunary polyoxometalates (POMs) with Group 4 (Ti, Zr, and Hf) transition-metal ions has been mainly driven by the fact that potential products may serve as oxidation catalysts or even as soluble molecular analogues of known Tiand Zr-containing heterogeneous catalysts. Bearing in mind that zirconium(IV) and hafniACHTUNGTRENNUNGum(IV) can have larger coordination numbers than titaniACHTUNGTRENNUNGum(IV), the chemistry of the former pair compared to the lighter congener is expected to be different, leading to different structural assemblies. There are significantly fewer reports on Zr/Hf-POMs than Ti-POMs in the literature, and the former can be classified according to the composing lacunary POM fragments as follows: Keggin type ([PW11O39] 7 , [SiW11O39] 8 , [GeW11O39] 8 , [b-SiW10O37] 10 , [g-SiW10O36] 8 , [PW9O34] 9 , and [SiW9O34] 10 ), Wells– Dawson type ([a2-P2W17O61] 10 , [a-P2W16O59] 12 , and [aP2W15O56] 12 ), and Lindqvist type ([W5O18] 6 ). Our group has reported the first two examples of peroxo-Zr/Hf polyACHTUNGTRENNUNGanions.[2h,k] In addition, the unique, asymmetric structure [Zr2ACHTUNGTRENNUNG(m-OH)ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(AsOH)2ACHTUNGTRENNUNG(AsW7O28) ACHTUNGTRENNUNG(AsW10O36)]9 has been obtained from the reaction of [NaAs4W40O140] 27 with Zr ions, and does not belong to any of the above-mentioned classes. Whereas the reactivity of the classical trilacunary Keggin ions [PW9O34] 9 and [SiW9O34] 10 with Ti, Zr, and Hf ions is fairly well understood (vide supra), this is not the case for the lone pair containing Keggin family [XW9O33] 9 (X=As, Sb, Bi), which displays different chemical reactivity and consequently results in structurally distinct products. For all the above-mentioned reasons we decided to study in detail the reactivity of [AsW9O33] 9 with zirconium(IV) and hafnium(IV). Here we report on the two Zr6and Hf6-containing tungstoarsenates ACHTUNGTRENNUNG(III) [M6O4(OH)4 ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(CH3COO)5ACHTUNGTRENNUNG(AsW9O33)2]11 (M=Zr, 1; Hf, 2, see Figure 1), which have been isolated as the hydrated mixed cesium–sodium salts Cs6Na5ACHTUNGTRENNUNG[Zr6O4(OH)4 ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(CH3COO)5ACHTUNGTRENNUNG(AsW9O33)2]·80 H2O (CsNa-1) and Cs6Na5ACHTUNGTRENNUNG[Hf6O4(OH)4ACHTUNGTRENNUNG(H2O)2ACHTUNGTRENNUNG(CH3COO)5ACHTUNGTRENNUNG(AsW9O33)2]·80H2O (CsNa-2). These compounds were characterized in the solid state by single-crystal X-ray dif-

Preparation and characterization of Langmuir-Blodgett films of wheel-shaped Cu-20 tungstophosphate and DODA by two different strategies.

A novel surfactant-encapsulated organic-inorganic hybrid compound (DODA)(24)Li[Cu(20)Cl(OH)(24)(P(8)W(48)O(184))] x 18 H(2)O (DODA-Cu20) has been prepared from the wheel-shaped tungstophosphate salt K(12)Li(13)[Cu(20)Cl(OH)(24)(H(2)O)(12)(P(8)W(48)O(184))] x 22 H(2)O (Cu20) and dimethyldioctadecylammonium bromide (DODA), and it has been characterized by elemental analysis (EA), thermogravimetric analysis (TGA), (1)H NMR, Fourier transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) measurements. Monolayer and multilayer films of DODA-Cu20 were fabricated on different substrates by the Langmuir-Blodgett (LB) technique using H(2)O as the subphase. Another type of organic-inorganic hybrid film, DODA/Cu20, was also deposited on the same substrates as used for the film containing DODA-Cu20 under the same conditions by the LB technique using a Cu20 aqueous solution as the subphase and DODA as the cationic amphiphile for comparison. Both thus-prepared organic-inorganic hybrid films were characterized by UV-vis spectroscopy, XRD, transmission and polarized FT-IR spectroscopy, X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The results indicate that stable monolayers at the air-water interface for DODA-Cu20 and at the air-Cu20 solution interface for DODA can be formed and that two LB films containing DODA-Cu20 and DODA/Cu20 constructed by two different methods both exhibit well-ordered lamellar structures. It is proposed that Cu20 exhibits different packing modes in the two LB films depending on the deposition strategy used.

Heterogeneous Wheel-Shaped Cu20-Polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25− Catalyst for Solvent-Free Aerobic Oxidation of n-Hexadecane

The selective oxidation of alkanes as a green process remains a challenging task because partial oxidation is easier to achieve with sacrificial oxidants, such as hydrogen peroxide, alkyl hydroperoxides or iodosylbenzene, than with molecular oxygen or air. Here, we report on a heterogeneous catalyst for n-hexadecane oxidation comprised of the wheel shaped Cu20-polyoxotungstate [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- anchored on 3-aminopropyltriethoxysilane (apts)-modified SBA-15. The catalysts were characterized by powder X-ray diffraction (XRD), N2-adsorption measurements and Fourier transform infrared reflectance (FT-IR) spectroscopy. The heterogeneous Cu20-polyanion system catalyzed the solvent-free aerobic oxidation of n-hexadecane to alcohols and ketones by using air as the oxidant under ambient conditions. The catalyst exhibits an exceptionally high turn over frequency (TOF) of 20,000 h(-1) at 150 degrees C and is resistant to poisoning by CS2. Moreover, it can be easily recovered and reused by filtration without loss of its catalytic activity. Possible homogeneous contributions also have been examined and eliminated. Thus, this system can use air as oxidant, which, in combination with its good overall performance and poison tolerance, raises the prospect of this type of heterogeneous catalyst for practical applications.