Synthesis of coenzyme Q0 through divanadium-catalyzed oxidation of 3,4,5-trimethoxytoluene with hydrogen peroxide.

The selective oxidation of methoxy/methyl-substituted arenes to the corresponding benzoquinones has been first realized using aqueous hydrogen peroxide as a green oxidant, acid tetrabutylammonium salts of the γ-Keggin divanadium-substituted phosphotungstate [γ-PW10O38V2(µ-O)2]5- (I) as a catalyst, and MeCN as a solvent. The presence of the dioxovanadium core in the catalyst is crucial for the catalytic performance. The reaction requires an acid co-catalyst or, alternatively, a highly protonated form of I can be prepared and employed. The industrially relevant oxidation of 3,4,5-trimethoxytoluene gives 2,3-dimethoxy-5-methyl-1,4-benzoquinone (ubiquinone 0 or coenzyme Q0, the key intermediate for coenzyme Q10 and other essential biologically active compounds) with 73% selectivity at 76% arene conversion. The catalyst retains its structure under turnover conditions and can be easily recycled and reused without significant loss of activity and selectivity.

Tetrapalladium-containing polyoxotungstate [Pd(II)4(α-P2W15O56)2]16-: a comparative study.

The novel tetrapalladium(II)-containing polyoxometalate [Pd(II)4(α-P2W15O56)2](16-) has been prepared in aqueous medium and characterized as its hydrated sodium salt Na16[Pd4(α-P2W15O56)2]·71H2O by single-crystal XRD, elemental analysis, IR, Raman, multinuclear NMR, and UV-vis spectroscopy. The complex exists in anti and syn conformations, which form in a 2:1 ratio, and possesses unique structural characteristics in comparison with known {M4(P2W15)2} species. (31)P and (183)W NMR spectroscopy are consistent with the long-term stability of the both isomers in aqueous solutions.

Borotungstate polyoxometalates: multinuclear NMR structural characterization and conversions in solutions.

The unique heteropolyanion [H(3)BW(13)O(46)](8-) (BW(13)), previously suggested on the basis of indirect evidence, and protonated lacunary heteropolyanion [HBW(11)O(39)](8-) (BW(11)) have been identified in aqueous solutions at pH 5-7.5 from NMR spectra. The pattern of tungsten-tungsten connectivities based on the analysis of the (2)J(W-O-W) coupling satellites in the (183)W NMR spectrum of BW(11), containing six peaks of relative intensities ∼2:2:2:1:2:2, indicates that the latter is the α isomer. The (17)O NMR spectrum confirms the protonated state of the polyanion with the proton delocalized on two out of four terminal O atoms surrounding the tungsten vacancy. The (183)W NMR spectrum of BW(13) contains seven peaks of relative intensities ∼2:1:2:2:2:2:2 with additional large couplings due to the connectivity between BW(11) and [W(2)O(7)](2-) fragments. According to the (17)O NMR spectrum, two protons of [BW(13)O(46)H(3)](8-) are delocalized on the two terminal trans O atoms of the dimeric fragment while the third one is linked to its bridging O atom. The conversions of BW(11) and BW(13) in solution were followed by using (183)W NMR spectra at a "fingerprint" level. In the pH range from ∼7.5 to 6, BW(11) transforms to BW(13), transforming further to [BW(12)O(40)](5-) (BW(12)) and [B(3)W(39)O(132)H(n)](n-21) (B(3)W(39)) in different ratios. Conversion of BW(13) to BW(12) proceeds through an intermediate complex of suggested composition [BW(11)O(39)·WO(2)](7-). At high acidity (pH ∼ 0), B(3)W(39) gradually decomposes into tungstic acid, BW(12) and H(3)BO(3). Polyanion BW(12) persists in the pH range ∼0-7.5.

95Mo and 17O NMR studies of aqueous molybdate solutions.

Aqueous molybdate solutions with molybdenum concentrations of [Mo]=1, 0.4, and 0.2 M have been studied by NMR at pH 7-1 and in 0.3-6 M HClO4. The 95Mo NMR spectrum of isopolyanion (IPA) Mo7O24(6-) (I) at pH=5 consists of a signal at 210 ppm and two overlapping peaks at 32 and approximately 15 ppm with the intensity ratio approximately 1:4:2, and that of beta-Mo8O26(4-) (II) consists of two signals at approximately 100 and 10 ppm with the intensity ratio approximately 1:3. A broad 95Mo NMR line at around 0 ppm was observed in the pH range of IPA Mo36O112(8-) (III), and a signal of cationic oxospecies including MoO2(2+) (IV) was observed from -62 to -69 ppm. Two protonation sites of IPA I have been identified from 17O NMR spectra, which suggests binding of up to two protons. The distribution diagram, derived from the 95Mo NMR spectra, is given for [Mo]=0.4 M. The 95Mo NMR signals shift to lower frequencies with increasing number and strength of the Mo-O terminal bonds.

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, characterization, and reactivity of Ti(IV)-monosubstituted Keggin polyoxometalates.

Ti(IV)-monosubstituted Keggin-type polyoxometalates (Ti-POMs), mu-oxo dimer [Bu4N]8[(PTiW11O39)2O] (1), and three monomers [Bu4N]4[PTi(L)W11O39], where L = OH (2), OMe (3), and OAr (4, ArOH = 2,3,6-trimethylphenol (TMP)), have been prepared starting from mu-hydroxo dimer [Bu4N]7[(PTiW11O39)2OH] (5) or heteropolyacid H5PW11TiO40 or both. The compounds have been characterized by elemental analysis, IR, UV-vis, and multinuclear (31P, 1H, 183W) NMR. The interaction of 1 and 3-5 with H2O in MeCN produces 2. The hydrolysis constants, estimated from 31P and 1H NMR data, are 0.006 and 0.04 for 1 and 3, respectively. Studies by 31P NMR, IR, potentiometric titration, and cyclic voltammetry revealed that 1-3 and 5 afford the same protonated titanium peroxo complex [Bu4N]4[HPTi(O2)W11O39] (I) upon interaction with aqueous H2O2 in MeCN. The rates of formation of I correlate with the rates of hydrolysis of the Ti-POMs and follow the order of 5 > 1 > 3. A two-step mechanism of the reaction of Ti-POMs with H2O2, which involves hydrolysis of the Ti-L bonds to yield 2 followed by fast interaction of 2 with hydrogen peroxide producing I, is suggested. The equilibrium constant for the reaction of 2 with H2O2 to yield I and H2O, estimated using 31P NMR, is 10. The interaction of the Ti-POMs with TMP follows the trends similar to their interaction with H2O) and requires preliminary hydrolysis of the Ti-L bonds. All of the Ti-POMs catalyze the oxidation of TMP with H2O2 in MeCN to give 2,3,5-trimethyl-p-benzoquinone and 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol. The product distribution is similar for all of the Ti-POMs. The catalytic activities of the Ti-POMs correlate with the rates of formation of I and follow the order of 2 > 5 > 1 > 3. The findings lay a basis for a better understanding of the nature of the reactivity of titanium in Ti-catalyzed oxidations.

Aerobic oxidation of formaldehyde mediated by a Ce-containing polyoxometalate under mild conditions.

An evaluation of over 50 polyoxometalates (POMs) identified the complex NaH3[SiW11Ce(IV)O39] (NaH3(1)) as a selective and effective catalyst for the aerobic oxidation of formaldehyde to formic acid under very mild (including ambient) conditions. 183W NMR, UV-vis, cyclic voltammetry, and potentiometric titration establish that the catalyst is a monomer (Cs symmetry), 1, in solution, while X-ray crystallography (a = 12.9455(15) A, b = 13.2257(16) A, c = 14.5288(17) A, alpha = 81.408(2) degrees , beta = 85.618(2) degrees , gamma = 80.726(2) degrees , P, Z = 1, R1 = 5.79% based on 17244 independent reflections) and IR establish it to be a dimer (Ci symmetry), 1(2), in the solid state. Several lines of evidence, including the parabolic kinetic order in 1, nonlinear Arrhenius plot, independence of the rate on O2 pressure, presence of titratable H2O2 and HCO3H intermediates, and inhibition by conventional radical scavengers, all indicate the O2-based oxidations proceed by complex homolytic chemistry (autoxidation and Haber-Weiss radical-chain processes) likely initiated by protonated 1.

First isolated active titanium peroxo complex: characterization and theoretical study.

The protonated titanium peroxo complex [Bu(4)N](4)[HPTi(O(2))W(11)O(39)] (1) has been first prepared via interaction of the micro-oxo dimeric heteropolytungstate [Bu(4)N](8)[(PTiW(11)O(39))(2)O] (3) with an excess of 30% aqueous H(2)O(2) in MeCN. Peroxo complex 1 has been characterized by using elemental analysis, UV-vis, IR, resonance Raman (RR), (31)P and (183)W NMR spectroscopy, cyclic voltammetry, and potentiometric titration. The electronic and vibrational spectra of 1 are very similar to those of the well-known unprotonated titanium peroxo complex [Bu(4)N](5)[PTi(O(2))W(11)O(39)] (2), while (31)P and (183)W NMR spectra differ significantly. A compilation of the physicochemical techniques supports a monomeric Keggin type structure of 1 bearing one peroxo ligand attached to Ti(IV) in a eta(2)-coordination mode. The protonation of the titanium peroxo complex results in an increase of the redox potential of the peroxo group, E(1/2) = 1.25 and 0.88 V relative to Ag/AgCl reference electrode for 1 and 2, respectively. In contrast to 2, 1 readily reacts with 2,3,6-trimethylphenol (TMP) at 40 degrees C in MeCN to give 2,2',3,3',5,5'-hexamethyl-4,4'-biphenol (BP) and 2,3,5-trimethyl-p-benzoquinone (TMBQ). The proportion between BP and TMBQ in the reaction products depends on the TMP/1 ratio. When a 2-fold excess of TMP is used, the main reaction product is BP (90%), while using a 2-fold excess of 1 leads to TMBQ (95%). On the basis of the product study, a homolytic oxidation mechanism that implicates the formation of phenoxyl radicals is suggested. The RR deuterium labeling experiments show that the activating proton is most likely localized at a Ti-O-W bridging oxygen rather than at the peroxo group. Theoretical calculations carried out at the DFT level on the protonated and unprotonated titanium peroxo derivatives also propose that the most stable complex is formed preferentially after protonation of the Ti-O-W site; however, both Ti-OH-W and TiOO-H protonated anions could coexist in solution.