Central Heteroatom Research Articles

Influence of the Heteroatom Size on the Redox Potentials of Selected Polyoxoanions

The apparent formal potentials for the one-electron redox process of most Keggin-type heteropolytungstates, XW(12)O(40)(q-), have long been shown to linearly depend on their overall negative charges, in the absence of proton interference in the process. However, for a given overall negative charge, these formal potentials are also shown here to depend on the specific central heteroatom X. In the present work, cyclic voltammetry was used to study a large variety of Keggin-type anions, under conditions where their comparisons are straightforward. In short, apparent potential values get more negative (the clusters are more difficult to reduce) for smaller central heteroatoms within a given family of Keggin-type heteropolyanions carrying the same overall negative charge. Density functional theory calculations were performed on the same family of Keggin compounds and satisfactorily reproduce these trends. They show that internal XO(4) units affect differently the tungstate oxide cage. The electrostatic potential created by each internal anionic unit in a fragment-like approach (XO(4)(q-)@W(12)O(36)) was analyzed, and it is observed that X atoms of the same group show slight differences. Within each group of the periodic table, X atoms with lower atomic numbers are also smaller in size. The net effect of such a tendency is to produce a more negative potential in the surroundings and thus a smaller capacity to accept electrons. The case of [BW(12)O(40)](5-) illustrates well this conclusion, with the smallest heteroatom of the Keggin series with group III central elements and a very negative reduction potential with respect to the other elements of the same group. Particularly in this case, the electronic structure of the Keggin anion shows the effects of the small size of boron: the highest occupied molecular orbitals of [BW(12)O(40)](5-) appear to be approximately 0.35 eV higher than those in the other clusters of the same charge, explaining that the BO(4) unit is more unstable than AlO(4) or GaO(4) despite carrying the same formal charge.

Prediction of second-order optical nonlinearity of porphyrin–metal–polyoxometalate sandwich compounds

The second-order nonlinear optical (NLO) properties of porphyrin-metal-polyoxometalate (por-metal-POM) sandwich structures [(por)M(PW(11)O(39))](5-) (por = TPP, TPyP, TPPF(20), M = Hf; por = TPP, M = Zr) and [(TPP)Hf(XW(11)O(36))](6-) (X = Si, Ge) are investigated by time-dependent density functional theory (TDDFT). The character of charge-transfer transition indicates that the porphyrin ligand acts as an electron acceptor and the lacunary Keggin-type POM acts as an electron donor. Our results show that this kind of organic-inorganic hybrid compound possesses remarkably large molecular second-order NLO polarizability, approximately 100 x 10(-30) esu, and might be an excellent second-order NLO material. Furthermore, the NLO response can be tuned by the element substituents. The computed beta(0) values increase with the auxiliary electron-accepting group on the porphyrin ring (TPPF(20) > TPyP > TPP) and a heavy central heteroatom (Ge > Si > P). The present investigation provides important insight into the NLO properties of this class of por-metal-POM sandwich compound.

Effects of vanadium(V)-substitution on the oxidative properties of α-Keggin-type heteropolyanion clusters—progress in DFT theoretical studies

The redox properties of α-Keggin-type heteropolyanion clusters [XM12O40] n− (X = Si, P; M = Mo, W) mainly depend on their constituent outer metal-oxygen cages {M12O36}. They act as “reservoirs”, through which the transfer and transition of electrons and protons may occur. At the atomic and molecular level, the redox properties of these clusters can be controlled and also tuned by modifying the metal M in the cages and the central heteroatom X of the clusters. Combined with relevant experimental results, this review summarizes our recent theoretical investigations of the effect of vanadium substitution on the redox properties of Keggin anion clusters. Theoretical modeling and calculation results showed that the oxidative ability of the modified species was increased by partial substitution of the cage M atoms of the Keggin clusters by vanadium atoms which have lower electronegativity. A linear correlation between the catalytic efficiency per vanadium atom and the microstructures of the vanadium(V)-substituted heteropolyanions [PV n Mo12−n O40](3+n)− (n = 1−3) was established for the first time. This relationship may be suitable to interpret the catalytic behavior of the title compounds in the hydroxylation of benzene to phenol, and may also be used in understanding other reactions such as the oxidative dehydrogenation of isobutyric acid and the nitration of adamantine. The establishment of this nearly linear structure-property relationship may lay the foundations of understanding the behavior of the title compounds in homogeneous catalytic oxidation reactions, and may direct the design of future catalysts and the choice of other catalytic reactions.

Use of a 12-molybdovanadate(V) modified ionic liquid carbon paste electrode as a bifunctional electrochemical sensor

Vanadium (V) was used as the central heteroatom in the synthesis of the Keggin-type polyoxometalate {[CH 3(CH 2) 7] 4N} 3[VMo 12O 40] ( 1). This compound was characterized by elemental analysis and single-crystal X-ray diffraction. Compound 1 was used as a bulk-modifier to fabricate the chemically modified ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF 6]) carbon paste electrode ( 1-ILCPE) by direct mixing. The bulk-modified 1-ILCPE showed well-defined cyclic voltammograms with four reversible redox couples in H 2SO 4 + Na 2SO 4. The first wave could be ascribed to the reduction of the central V and the last three waves corresponded to the redox processes of the peripheral Mo atoms. The 1-ILCPE presents good electro-catalytic activity not only toward the oxidation of l-cysteine, which is attributed to the function of the central V, but also toward the reduction of hydrogen peroxide, which is attributed to the function of the Mo atoms. Compared with the conventional carbon paste electrode (CPE), the excellent experimental results suggest that the newly developed paste electrode holds great promise in the design of electrochemical devices, such as sensors and biosensors. The promising results are ascribed to the use of an ionic liquid, which forms an excellent charge-transfer bridge and wide electrochemical windows in the bulk of carbon paste electrode.

A Remarkable Member of the Polyoxometalates: The Eight-Nickel-Capped α-Keggin Polyoxoazonickelate

The eight-nickel-capped polyoxoazonickelate, [Ni(20)(OH)(24)(MMT)(12)(SO(4))](NO(3))(2).6H(2)O (1; MMT = 2-mercapto-5-methyl-1,3,4-thiadiazole), has been synthesized, which has an alpha-Keggin structure with eight nickel caps. In this structure, the polyatom is the late transition metal Ni(II); the central heteroatom is S, and the organic terminal ligand becomes the primary part of the Keggin structure. This is a Keplerate-type cluster, which shows a central Ni(II)(12) cuboctahedron formed by the 12 Ni(II) centers of the classical alpha-Keggin core and a Ni(II)(8) hexahedron formed by the eight nickel caps.

Surface charge of polyoxometalates modulates polymerization of the scrapie prion protein

Prions are composed solely of an alternatively folded isoform of the prion protein (PrP), designated PrP(Sc). N-terminally truncated PrP(Sc), denoted PrP 27-30, retains infectivity and polymerizes into rods with the ultrastructural and tinctorial properties of amyloid. We report here that some polyoxometalates (POMs) favor polymerization of PrP 27-30 into prion rods, whereas other POMs promote assembly of the protein into 2D crystals. Antibodies reacting with epitopes in denatured PrP 27-30 also bound to 2D crystals treated with 3 M urea. These same antibodies did not bind to either native PrP(Sc) or untreated 2D crystals. By using small, spherical POMs with Keggin-type structures, the central heteroatom was found to determine whether prion rods or 2D crystals were preferentially formed. An example of a Keggin-type POM with a phosphorous heteroatom is the phosphotungstate anion (PTA). Both PTA and a Keggin-type POM with a silicon heteratom have low-charge densities and favor formation of prion rods. In contrast, POMs with boron or hydrogen heteroatoms exhibiting higher negative charges encouraged 2D crystal formation. The 2D crystals of PrP 27-30 produced by selective precipitation with POMs were larger and more well ordered than those obtained by sucrose gradient centrifugation. Our findings argue that the negative charge of Keggin-type POMs determines the quaternary structure adopted by PrP 27-30. The mechanism by which POMs function in competing prion polymerization pathways--one favoring 2D crystals and the other, amyloid fibrils--remains to be established.

Heterocyclic columnar hexacatenar bisthiazoles

Three new series of hexacatenar liquid crystals derived from heterocyclic bisthiazoles and bisoxazoles exhibiting columnar phases were reported, and the formation of columnar phases was strongly sensitive to central heteroatoms and/or their positions incorporated.

Density functional study of magnetic exchange of dinuclear manganese complexes with the heteropolymolyanion: [MnII 2(X n+Mo9O33)2]2(n−10)− (X = PV, AsV, SeVI)

The magnetic exchange interactions between the dimanganese(II)-substituted complexes and the heteropolymolyanion, [MnII2(Xn+Mo9O33)2]2(n−10)− (X = PV(I), AsV(II) and SeVI(III)), are investigated by using density functional theory combined with broken-symmetry approach (DFT-BS) method. The calculated magnetic exchange coupling constant (J) of complex II is in reasonable agreement with the responding experimental value and the negative J values indicate that antiferromagnetic exchange interactions exist in these complexes. Furthermore, the influence of the central heteroatom on the exchange coupling within the dimanganese core unit is studied from standpoints of geometry, spin density and frontier orbitals. It demonstrates that the change of the heteroatom X via PV-AsV-SeVI elongates the distances of Mn1...Mn2 and shortens the distances of Ob...Ob, and reduces the effectiveness of the superexchange pathways, consequently, decreasing the magnitude of the antiferromagnetic coupling constant, J, of these species.

Argon cluster-mediated isolation and vibrational spectra of peroxy and nominally D3h isomers of CO3− and NO3−

Vibrational predissociation spectra are reported for two isomeric forms of the gas-phase ions, CO(3)(-) and NO(3)(-). The peroxy forms, (OOCO(-) and OONO(-)) were isolated using an Ar-mediated synthetic scheme involving exchange of CO and NO for the more weakly bound Ar ligands in O(2)(-)Ar(m) clusters, while the forms based on a central heteroatom (CO(3)(-) and NO(3)(-)) were generated by electron impact on CO(2) and HNO(3) vapor. The simple two-band spectrum of OOCO(-) indicates that it is best described as the O(2)(-) x CO ion-molecule complex, whereas the covalently bound CO(3)(-) form yields a much more complicated vibrational spectrum with bands extending out to 4000 cm(-1). In contrast, the NO(3)(-) ion yields a simple spectrum with only one transition as expected for the antisymmetric NO stretching fundamental of a species with D(3h) structure. The spectrum of the peroxynitrite isomer, OONO(-), displays intermediate complexity that can be largely understood in the context of fundamentals associated with its cis and trans structures previously characterized in an Ar matrix.

Electrochemical Behavior and Multilayer Assembly Films with Fine Functional Activities of the Sandwich‐Type Polyoxometallate [Sb2W20Fe2O70(H2O)6]8−

AbstractThe iron‐substituted sandwich‐type polyoxometalate (POM), comprised of the main group element Sb(III) as the central heteroatom, [Sb2W20Fe2O70(H2O)6]8− (Sb2W20Fe2), is one of the Krebs‐type derivatives. For the first time, the POMs' redox electrochemistry has been elucidated under acidic conditions employing cyclic voltammetry. It exhibited what is believed to be a bielectronic redox couple associated with the two Fe(III) centers followed by four‐electron and two‐electron redox processes, respectively, with these being attributed to redox processes of the tungsten‐oxo framework. The oxidized form of this POM was found to be stable from pH 1.5 to pH 6. Release of the iron centers from the complex, namely demetallization, was observed upon reduction of the Fe(III) sites at room temperature, with an influence of the solution pH being observed. Through the technique of layer‐by‐layer (LBL) assembly, the POM was successfully immobilized on both quartz and glassy carbon electrode (GCE) surfaces by alternate deposition with the polyelectrolyte poly(diallydimethylammonium chloride) (PDDA). Thus‐prepared multilayer films have been characterized by cyclic voltammetry (CV), UV‐vis spectroscopy (UV‐vis) and XPS. The electrocatalytic activities of the multilayer films containing Sb2W20Fe2 have been investigated towards the reduction of NO3− and IO3−. With an increase in the number of Sb2W20Fe2 monolayers within the assembly, the catalytic current towards the reduction of IO3− was enhanced.

Solvent Effect on the Electronic Spectra of Azine Dyes under Alkaline Condition

Thiazine dye, methylene blue (MB), oxazine dye, nile blue (NB), and phenazine-based dye, neutral red (NR), bear a similar basic dye skeleton with a distinctively different central heteroatom. All of them are extracted into nonpolar organic solvent from alkaline solution. The role of the heteroatom on the respective dye skeletons and redox potentials of the dyes has been examined to signature the stability of the species in organic solvent and the results have been substantiated through geometry optimization and wave function analysis at the density functional theory level. The effect of solvent polarity on the electronic absorption spectra of the three nonionic benzenoid species has been investigated with an intention to investigate the solvatochromic behavior of these compounds.

The novel sandwich-type heteropolyoxotungstates [M2Bi2(β-B-MW9O34)2]14– (M = CoII, ZnII): β-type dimeric heteropolyanions with a transition metal as the central heteroatom and BiIII and M as linking atoms

Two new sandwich-type heteropolyoxometalates, Na(14)[Co(2)Bi(2)(beta-B-CoW(9)O(34))(2)].48H(2)O (1) and Na(14)[beta-B-Zn(2)Bi(2)(ZnW(9)O(34))(2)].51H(2)O (2), have been synthesized at pH = 7.5-8 and characterized by elemental analysis, IR, UV-vis, TG-DSC and electrochemistry. Structural analysis indicates that both polyanions, M(2)Bi(2)(beta-B-MW(9)O(34))(2)](14-) (M = Co(II) and Zn(II)), are isomorphic and consist of two unprecedented [beta-B-MW(9)O(34)](12-) subunits linked by two M(II) and two Bi(III) which are coplanar. The polyanions are also the first examples of dimeric heteropolyanions with Bi(III) only as the second (linking) heteroatom and the transition metals (Co(II) or Zn(II)) as the first and second heteroatoms as well. The lower absorption of nu(W-O(d)) (915 cm(-1)) in the IR is a simple and feasible judgment of sandwich-type polyanions with a transition metal ion as a central heteroatom.

How Do the Different Defect Structures and Element Substitutions Affect the Nonlinear Optical Properties of Lacunary Keggin Polyoxometalates? A DFT Study.

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How Do the Different Defect Structures and Element Substitutions Affect the Nonlinear Optical Properties of Lacunary Keggin Polyoxometalates? A DFT Study

AbstractSystematic DFT calculations have been carried out on the lacunary α‐Keggin polyoxometalate derivatives [PW11O39]7–, [XW9O34]n– (X = AlIII, SiIV, GeIV, PV, AsV, and SbV), [XW9M2O39]n–, and [XW9M3O40]n– (X = PV and SiIV, M = MoVI, VV, NbV, and TaV) to investigate the geometric structure and element substitution effects on the molecular nonlinear optical response. Analysis of the computed static second‐order polarizability (β0) predicts that the molecular nonlinear optical activity of lacunary Keggin polyoxometalate derivatives can be modified by replacing the central heteroatom and the addenda metal atom. Substitution of the central Al atom or the addenda V atom causes significant enhancement in the molecular nonlinearity. Moreover, the β0 values are substantially dependent on the defect structures. This class of inorganic complexes possesses remarkably large molecular optical nonlinearity, especially for the partial substitution complex [SiW9Nb2O39]10– (IIIc), which has a computed β0 value of 2071.0 a.u. Thus, lacunary Keggin polyoxometalates could become excellent candidates in the field of second‐order NLO. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Prediction of Second-Order Optical Nonlinearity of Trisorganotin-Substituted β-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.

Computational Study of Solvent Effects and the Vibrational Spectra of Anderson Polyoxometalates

The structures and vibrational frequencies of the type II Anderson heteropolyanions [TeMo6O24]6- and [IMo6O24]5- have been calculated by using density functional theory using a number of common functionals and basis sets. For the first time, Raman intensities have been calculated and the effect of solvent on the modeling has been investigated. The calculated IR and Raman spectral traces are in good agreement with experiment allowing the characteristic group frequencies for this class of polyoxometalate to be identified. The stretching vibrations of the molybdenum-oxygen bonds are predicted to occur at somewhat lower frequencies than in the type I polyoxometalates. Stretching of the heteroatom-oxygen bonds occurs at significantly lower frequencies than in the Keggin anions as a simple consequence of the higher coordination number of the central heteroatom in the Anderson systems. For the [Mo2O7]2- and [Mo6O19]2- ions, the relatively low negative charge leads to small structural changes when solvent is included. In these systems, solvent leads to an increase in the bond polarity and a decrease in the covalent bond orders, resulting in decreases in the calculated frequencies. For the Anderson anions, the higher negative charges leads to greater solvent effects with contraction of the clusters and increases in the frequencies of bands due to stretching of the two, cis-related molybdenum-oxygen bonds.

Vanadium-substituted Dawson-type polyoxometalates as versatile electrocatalysts

Abstract A selected series of mono- and multi- V-substituted derivatives of Dawson type structure were synthesized and characterized with the aim of exploring their electrochemistry and their electrocatalytic abilities. The focus was placed on the electrochemistry of [P 2 V 2 W 16 O 62 ] 8- as a representative example. The redox processes of its two V-centers were observed in a potential domain well positive of those of the W-centers. They are pH-dependent, the first redox process exhibiting only modest and progressively smaller potential shifts from pH 0 to 4, while the second wave was far more sensitive to acidity changes from pH 0 to 8. In contrast, mono-substituted derivatives display very small or no pH-dependence of the V-wave. Finally, combination of this diversity in the number of V atoms with the presence of As or P as the central heteroatom in these tungstic and molybdo-tungstic structures modulates substantially the apparent formal potentials that span the range from + 569 mV to + 122 mV vs SCE at pH 7. This leaves considerable flexibility in the choice of POMs for electrocatalytic purposes. The homogeneous oxidation and the electrocatalytic reduction of nitrite and the electrocatalytic oxidation of NAD(P)H by an appropriate selection of these V-substituted anions were studied. To cite this article: B. Keita et al., C. R. Chimie 8 (2005)

Icosahedral gold cage clusters: M@Au12− (M=V, Nb, and Ta)

We report the observation and characterization of a series of stable bimetallic 18-valence-electron clusters containing a highly symmetric 12-atom icosahedral Au cage with an encapsulated central heteroatom of Group VB transition metals, M@Au12− (M=V,Nb,Ta). Electronic and structural properties of these clusters were probed by anion photoelectron spectroscopy and theoretical calculations. Characteristics of the M@Au12− species include their remarkably high binding energies and relatively simple spectral features, which reflect their high symmetry and stability. The adiabatic electronic binding energies of M@Au12− were measured to be 3.70±0.03, 3.77±0.03, and 3.76±0.03 eV for M=V, Nb, and Ta, respectively. Comparison of density-functional calculations with experimental data established the highly symmetric icosahedral structures for the 18-electron cluster anions, which may be promising building blocks for cluster-assembled nanomaterials in the form of stoichiometric [M@Au12−]X+ salts.

Synthesis, characterization and electrocatalytic behaviors of α-K6[As2W12Mo6O62] · 12H2O

Pure α-K6[As2W12Mo6O62]·12H2O was synthesized for the first time. It was obtained in high yield by careful consideration of the main experimental parameters. Keys to the successful achievement of this synthesis include the high acidity of the medium, the order of addition of reactants, the necessity of keeping a larger than stoichiometric molybdate to lacunary polyoxometalates ratio during the synthesis and the recrystallization in fairly acidic medium. Purity of the isolated product was checked by elemental analysis, electrochemistry, IR spectroscopy, UV–Vis spectroscopy and 183W NMR spectroscopy. Comparison of the physical characteristics of α-K6[As2W12Mo6O62]·12H2O with the corresponding ones of its known phosphorus analogue, α-K6[P2W12Mo6O62]·14H2O, confirms that the two complexes have the same structure. The expected positive potential shift of the first several voltammetric waves linked with the presence of As instead of P as the central heteroatom in these molecules was observed. The behaviours of α-K6[As2W12Mo6O62]·12H2O and α-K6[P2W12Mo6O62]·14H2O in the electrocatalytic reduction of nitrous acid and nitrogen monoxide were studied and compared with each other and with the activity of α-K6[As2W18O62]·14H2O.

Correlation of Negative Differential Resistance (NDR) Peak Voltages of Nanostructured Heteropolyacid (HPA) Monolayers with One Electron Reduction Potentials of HPA Catalysts

An extensive and quantitative study on the surface electronic properties of nanostructured Keggin-type heteropolyacid (HPA) monolayers was carried out using scanning tunneling microscopy (STM) to relate surface electronic properties to bulk redox properties of HPAs. Cation-exchanged RPMo12O40 (R = H3, Zn3/2, Co3/2, Cu3/2, Bi1); heteroatom-substituted HnXW12O40 (X = P, Si, B, Co) and HnXMo12O40 (X = P, As, Si); and polyatom-substituted HnPW11M1O40 (M = W, Mo, V), H3PMoxW12-xO40 (x = 0, 3, 6, 9, 12), H3+xPMo12-xVxO40 ( x= 0−3), and H3+xPW12-xVxO40 (x = 0−3) HPAs were examined to elucidate the effect of different substitutions. All HPA samples formed two-dimensional well-ordered monolayer arrays on graphite surfaces and exhibited negative differential resistance (NDR) behavior in their tunneling spectra. Substitution of more electronegative atoms for countercations or for the central heteroatom shifted the NDR peaks to less negative voltages, corresponding to increased reduction potentials of the HPAs. On th...