Ana M.V. Cavaleiro

Keggin-type polyoxotungstates as catalysts in the oxidation of cyclohexane by dilute aqueous hydrogen peroxide

Abstract Oxidation of cyclohexane by hydrogen peroxide in the presence of catalytic amounts of the Keggin-type heteropolytungstates [PW 11 O 39 ] 7− and [PW 11 M(L)O 39 ] (7− m )− , M m + =first row transition metal cation, L=H 2 O or CH 3 CN, was found to produce cyclohexanol, cyclohexanone and, in certain cases, cyclohexyl hydroperoxide. The presence of the latter was demonstrated by negative chemical ionization GC-MS. The reactions were carried out in acetonitrile, using tetra n -butylammonium salts of the catalysts and aqueous 30% hydrogen peroxide as oxidant. The polyanions [PW 11 O 39 ] 7− and [PW 11 Fe(H 2 O)O 39 ] 4− showed higher catalytic activity and different selectivity for the oxidation of cyclohexane than did the corresponding Cu-, Co-, Mn- and Ni-substituted complexes.

Synthesis, properties and photochromism of novel charge transfer compounds with Keggin anions and protonated 2,2′-biquinoline

Abstract Several novel compounds with protonated 2,2′-biquinoline (biqui) and Keggin polyoxoanions (α-isomers), having the general formula (Hbiqui) m [XM 12 O 40 ]· n solv, X=P ( m =3), Si ( m =4); M=Mo, W; n =0, 3; solv=H 2 O, N , N -dimethylformamide (dmf), were synthesized and characterized by analytical, spectroscopic and X-ray diffraction techniques. Electronic spectroscopy (visible/UV) indicated the presence of intermolecular charge transfer between the organic and inorganic moieties in the solid state. Evidence for the existence of intermolecular electronic interaction in solution was found for compounds with the [SiW 12 O 40 ] 4− anion, a fact quite uncommon for charge transfer compounds based on Keggin anions. A single crystal X-ray diffraction study could be performed on (Hbiqui) 4 [SiW 12 O 40 ]·3dmf crystals, but the refinements revealed the presence of highly disordered 2,2′-biquinoline molecules and α-[SiW 12 O 40 ] 4− anions. Photosensitivity to sunlight and to W-lamp visible light was assessed for all compounds. Photochromic properties were found for solids with [XMo 12 O 40 ] m − Keggin anions. Reduction of these anions upon irradiation was observed by diffuse reflectance and EPR spectroscopy. After a minimum of 5 h under sunlight, the extent of anion reduction followed the order (Hbiqui) 3 [PMo 12 O 40 ]·3dmf>(Hbiqui) 4 [SiMo 12 O 40 ]·3dmf>(Hbiqui) 3 [PMo 12 O 40 ]>(Hbiqui) 4 [SiMo 12 O 40 ]·4H 2 O. The Kurtz powder test was used to evaluate the second-order non-linear optical properties of the prepared compounds. (Hbiqui) 4 [SiW 12 O 40 ]·3H 2 O originated a second harmonic generation signal with intensity about 15% that of urea for 1064 nm radiation.

Oxidation of monoterpenes with hydrogen peroxide catalysed by Keggin-type tungstoborates

The Keggin-type anions [MnIII(H2O)BW11O39]6− and [BW12O40]5− were used for the first time as catalysts in studies of oxidation of monoterpenes by hydrogen peroxide. The oxidation of geraniol 1, nerol 2, (+)-3-carene 3, thymol 4 and carvacrol 5 was examined. The reactions were carried out in acetonitrile, at room temperature (for 1 and 2) or at reflux (for 3, 4 and 5). Compounds 1 and 2 were preferentially epoxidised at the C2–C3 double bond, whereas 3, under controlled reaction conditions, could afford only the α-epoxide. The oxidation of 4 and 5 yielded a mixture of benzoquinones. For tetrabutylammonium salts, the MnIII substituted anion was found to be a more efficient catalyst than [BW12O40]5−. The infrared spectra of the recovered solids at the end of reaction showed that the Keggin anions were still present, but the electronic spectra of the manganese residues indicate that oxidation of MnIII had taken place.

Homogeneous catalytic oxidation of styrene and styrene derivatives with hydrogen peroxide in the presence of transition metal-substituted polyoxotungstates

The tetrabutylammonium (TBA) salts of the Keggin-type polyoxotungstates with general formula [XW11M(H2O)O39](n−m)−, where X = P, B or Si and M = Mn, Fe or Co, were evaluated as catalysts in the oxidation of styrene, α-methylstyrene, p-methylstyrene, α,p-dimethylstyrene, p-chlorostyrene, p-nitrostyrene, and p-methoxystyrene under mild conditions, using aqueous H2O2 as an eco-sustainable oxidant. In this study, the influence of the catalysts and of the different styrene substituents on the oxidation reaction profile was evaluated in terms of conversion and selectivity. For all the performed catalytic studies, the main product results from the oxidative cleavage of the vinyl double bond, except in the case of the oxidation of p-methoxystyrene catalysed by BW11Mn, for which p-methoxyphenol is the main product. The catalysts BW11Mn and SiW11Co give rise to 100% conversion for almost all of the substrates, excluding p-methoxystyrene and p-nitrostyrene for both catalysts and α,p-dimethylstyrene only in the case of BW11Mn. The selectivity for CC cleavage products resulting from the oxidative cleavage of the vinyl double bond can be as high as 98%, reaching 98% conversion for p-nitrostyrene when SiW11Co was used as a catalyst. Possible pathways are discussed and the oxidation of a few presumed intermediates was carried out. The systematic study of several substituted styrene derivatives suggests a possible reactivity order for these compounds in the catalytic system considered.

N.m.r. studies of complexes of molybdenum(VI) with tartaric acid in aqueous solution

SummaryComplex formation between molybdate ions and R,R-(+)-tartaric acid has been studied in aqueous solution, and to a limited extent in DMSO, over a range of concentrations and pH at 298K, using1H and13C n.m.r. techniques. Only four main species are evident; two, which have Mo: L ratios of 1∶1 and 1∶2, are bondedvia a carboxylic oxygen atom and an adjacent hydroxyl oxygen atom. The 1∶2 species forms stereospecifically. The other two main species contain bridging tartaric acid molecules, and a 2∶2 species, also formed stereo-specifically in solution, have been isolated and characterised in the solid state. A limited number of experiments with racemic acid and meso-tartaric acid indicate that only a few species are formed in these systems and with a good deal of stereospecificity also.

Oxidation of Polycyclic Aromatic Hydrocarbons with Hydrogen Peroxide in the Presence of Transition Metal Mono-Substituted Keggin-Type Polyoxometalates

The oxidation of anthracene, naphthalene, 1‐ethylnaphthalene, and 2‐ethylnaphthalene with environmentally benign hydrogen peroxide has been studied in the presence of the tetrabutylammonium salts of iron and manganese mono‐substituted Keggin‐type polyoxometalates [XW11M(H2O)O39]n− (X=P, Si, or B, M=MnIII or FeIII). The reactions were performed under homogeneous conditions at 80 °C, using acetonitrile as the solvent. For all the catalysts studied, anthracene was selectively oxidized to 9,10‐anthraquinone at 100 % conversion in the presence of [BW11Mn(H2O)O39]6−. The oxidation of 1‐ethylnaphthalene and 2‐ethylnaphthalene occurs mainly at the alkyl substituent, along with the formation of phthalic anhydrides, which resulted from aromatic ring oxidation, with iron substituted anions. Naphthalene could not be oxidized with the conditions used. To understand the reaction pathways, the oxidation of several products was performed in the presence of [PW11Fe(H2O)O39]4−. Under the conditions used, the oxidation of vinylnaphthalene derivatives gave rise to mainly carbon–carbon double‐bond cleavage, affording the corresponding naphthaldehydes in high yields (>67 %). Possible reaction pathways are presented and the role of different catalysts is discussed.

Novel charge transfer supramolecular assemblies with Keggin anions and 2-amino-5-nitropyridine

Several novel compounds with the non-linear optical chromophore 2-amino-5-nitropyridine (2A5NP) and Keggin polyoxoanions (alpha-isomers), having the general formula (2A5NP)(m)H(n)[XM12O40].xH2O, M = Mo, W, were synthesised. Compounds were obtained with X = P, n = 3, m = 3 and 4 and X = Si, n = m = 4 (x = 2-6). Thus, for each of the anions [PMo12O40]3- and [PW12O40]3- two different compounds were obtained, with the same anion and organic counterpart but with a different stoichiometric ratio. These presented different charge transfer properties and thermal stability. All compounds were characterised by spectroscopic and analytical techniques. The single crystal X-ray diffraction structure of (2A5NP)4H3[PMo12O40].2.5H2O.0.5C2H5OH showed that the water solvent molecules and the organic chromophores are assembled via infinite one-dimensional chains of hydrogen bonds with formation of open channels, which accommodate [PMo12O40]3- and ethanol solvent molecules.

Study of polyoxotungstates with the Keggin structure by cyclic voltammetry

Abstract Cyclic voltammetry was used to study, in aqueous solution (pH 2–6), polyoxotungstates with the Keggin structure with the general formula α-[XW11M(H2O)O39]n−, X=P, Si, B and M=Cr(III), Fe(III), Co(II,III), Ni(II). The results provide valuable information on the comparative ease of reduction of the several species studied and on the stability of the reduced species. The transition metal substituted heteropolytungstates proved to be, in general, more difficult to reduce than the corresponding lacunary species, under the same conditions. For all species, the peak potentials shift to more negative values as the pH increases. In the range of potentials studied (−1100 mV to 1100 mV), only the boron compounds could be reduced irreversibly. The dependence of the reduction potentials on the pH was studied for the Ni(II) complexes. For some of the complexes, in acidic solutions (pH 2.2), namely [PW11Ni(H2O)O39]5−, a new species, resulting from the two-electron reduction, was observed. This was attributed to an isomerization α → β of the reduced anions.

Polymorphism in tetra-butylammonium salts of Keggin-type polyoxotungstates

Abstract Some tetra-n-butylammonium salts of Keggin-type polyoxotungstates, namely α-[PW11O39]7−, α-[PW11M(H2O)O39]5−, MII=Co, Ni, α-[PW11CuO39]5−, α-[PW11Fe(H2O)O39]4− and α-[PW11Fe(OH)O39]5−, with the general formula [(C4H9)4N]4Hx[anion]·nH2O, were studied by powder X-ray diffraction. The salts α-[(C4H9)4N]4H3[PW11O39] and α-[(C4H9)4N]4H[PW11CuO39] were obtained in two crystalline phases, a behavior that has been scarcely documented in compounds with Keggin polyoxometalates. One of the phases corresponds to a body-centered cubic unit cell (Im 3 symmetry) and the other has a body-centered tetragonal unit cell (I41 or I4122 symmetry). For the compounds with Cu, the cell parameters of the cubic and tetragonal structures were a=17.675(2) and a=18.775(2), c=14.646(2) A, respectively. The cubic phase was obtained by precipitation from aqueous solution. The tetragonal phase was isolated by crystallization from acetonitrile. The remaining compounds were found to crystallize with one of these two crystal structures. α-[(C4H9)4N]4Hx[PW11M(H2O)O39]·nH2O, with M=Co, Ni, x=1, and M=Fe, x=0, n=1–2, were found to crystallize with the body-centered cubic structure. The method of synthesis used to prepare α-[(C4H9)4N]4H[PW11Fe(OH)O39] afforded the tetragonal phase. The conversion of the cubic into the tetragonal phase and the stability of each one are discussed.

Mono-substituted silicotungstates as active catalysts for sustainable oxidations: homo- and heterogeneous performance

A series of tetrabutylammonium (TBA) salts of the transition metal mono-substituted silicotungstates [SiW11M(H2O)O39]n−, M = CoII, FeIII, MnIII, (SiW11M) were explored as homogeneous catalysts for the oxidation of geraniol and styrene with H2O2. The most active homogeneous catalysts (SiW11Co and SiW11Fe) were immobilized onto an amine-functionalized SBA-15 (aptesSBA-15) and the resulting composites were characterized using several techniques (FT-IR, FT-Raman, UV-Vis/DRS, elemental analysis, powder XRD, SEM and N2 adsorption–desorption isotherms). The catalytic performance of the new composites SiW11Co@aptesSBA-15 and SiW11Fe@aptesSBA-15 was investigated under similar experimental conditions to those used for homogeneous counterparts. 2,3-Epoxygeraniol and benzaldehyde were the main products obtained from geraniol and styrene oxidation, respectively, for all the catalysts. SiW11Co and SiW11Co@aptesSBA-15 showed to be the most active catalysts for the oxidation of geraniol and styrene. The recyclability of the composite SiW11Co@aptesSBA-15 was investigated for three reaction cycles. The stability of the composites was confirmed using several techniques after catalytic cycles.