Discovery of a Neutral 40-PdII-Oxo Molecular Disk, [Pd40O24(OH)16{(CH3)2AsO2}16]: Synthesis, Structural Characterization, and Catalytic Studies.

We report on the synthesis and structural characterization of a giant, discrete, and neutral molecular disk, [Pd40O24(OH)16{(CH3)2AsO2}16] (Pd40), comprising a 40-palladium-oxo core that is capped by 16 dimethylarsinate moieties, resulting in a palladium-oxo cluster (POC) with a diameter of ∼2 nm. Pd40, which is the largest known neutral Pd-based oxo cluster, can be isolated either as a discrete species or constituting a 3D H-bonded organic-inorganic framework (HOIF) with a 12-tungstate Keggin ion, [SiW12O40]4- or [GeW12O40]4-. 1H and 13C NMR as well as 1H-DOSY NMR studies indicate that Pd40 is stable in aqueous solution, which is also confirmed by ESI-MS studies. Pd40 was also immobilized on a mesoporous support (SBA15) followed by the generation of size-controlled Pd nanoparticles (diameter ∼2-6 nm, as based on HR-TEM), leading to an effective heterogeneous hydrogenation catalyst for the transformation of various arenes to saturated carbocycles.

Discrete Polyoxopalladates as Molecular Precursors for Supported Palladium Metal Nanoparticles as Hydrogenation Catalysts.

We have used discrete polyoxopalladates(II) (POPs) of the MPd12X8 nanocube- and Pd15X10 nanostar-types (M = central metal ion, X = capping group) as molecular precursors (diameter ca. 1 nm) for the formation of supported (SBA-15) metallic nanoparticles. These materials proved to be highly active in the hydrogenation of o-xylene. The characterization of such hydrogenation catalysts revealed that the average size of the resulting alloy particles is quite uniform with diameters ranging from 1 to 3 nm (indicating little to no agglomeration). The central transition-metal ion M n+ (MnII, FeIII, CoII, NiII, CuII, ZnII, PdII) in the POP structure and also the nature of the capping group (AsO43-, SeO32-, PO43-, phenyl-AsO32-) influence the resulting catalytic performance.

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.

Organo-ruthenium supported heteropolytungstates: synthesis, structure, electrochemistry, and oxidation catalysis.

The reaction of [Ru(arene)Cl(2)](2) (arene = benzene, p-cymene) with [X(2)W(22)O(74)(OH)(2)](12-) (X = Sb(III), Bi(III)) in buffer medium resulted in four organo-ruthenium supported heteropolytungstates, [Sb(2)W(20)O(70)(RuC(6)H(6))(2)](10-) (1), [Bi(2)W(20)O(70)(RuC(6)H(6))(2)](10-) (2), [Sb(2)W(20)O(70)(RuC(10)H(14))(2)](10-) (3), and [Bi(2)W(20)O(70)(RuC(10)H(14))(2)](10-) (4), which have been characterized in solution by multinuclear ((183)W, (13)C, (1)H) NMR, UV-vis spectroscopy, electrochemistry, and in the solid state by single-crystal X-ray diffraction, IR spectroscopy, thermogravimetric analysis, and elemental analysis. Polyanions 1, 2, and 4 crystallize in the triclinic system, space group P1 with the following unit cell parameters: K(5)Na(5)[Sb(2)W(20)O(70)(RuC(6)H(6))(2)] x 22 H(2)O (KNa-1), a = 12.1625(2) A, b = 13.1677(2) A, c = 16.0141(3) A, alpha = 78.9201(7) degrees, beta = 74.4442(8) degrees, gamma = 78.9019(8) degrees, and Z = 1; Cs(2)Na(8)[Bi(2)W(20)O(70)(RuC(6)H(6))(2)] x 30 H(2)O (CsNa-2), a = 11.6353(7) A, b = 13.3638(7) A, c = 16.7067(8) A, alpha = 79.568(2) degrees, beta = 71.103(2) degrees, gamma = 80.331(2) degrees, and Z = 1; Na(10)[Bi(2)W(20)O(70)(RuC(10)H(14))(2)].35H(2)O (Na-4), a = 15.7376(12) A, b = 15.9806(13) A, c = 24.2909(19) A, alpha = 92.109(4) degrees, beta = 101.354(4) degrees, gamma = 97.365(3) degrees, and Z = 2. Polyanions 1-4 consist of two (L)Ru(2+) (L = benzene or p-cymene) units linked to a [X(2)W(20)O(70)](14-) (X = Sb(III), Bi(III)) fragment via Ru-O(W) bonds resulting in an assembly with idealized C(2h) symmetry. Polyanions 1-4 are stable in solution as indicated by the expected (183)W, (13)C, and (1)H NMR spectra. The electrochemistry of 1-4 is described by considering the reduction and the oxidation processes. The nature of the arene in Ru(arene) has practically no influence on the formal potentials of the W-centers, which are more sensitive to the Sb or Bi hetero atoms. The results suggest that the respective Sb- and Bi derivatives have very different pK(a) values, with the reduced form of 1 being the most basic, thus permitting the observation of two well-developed voltammetric waves at pH 6. In contrast, the identity of the arene influences the oxidation processes, thus permitting to distinguish them. A strong electrocatalytic water oxidation peak is observed that is more positive than the one corresponding to the Ru(arene) oxidation process. Also a stepwise oxidation of the Ru(benzene) group could be observed at pH 3. The catalytic efficiency, on the other hand, of 1-4 toward the oxidation of n-hexadecane and p-xylene illustrated the effect of ruthenium substitution on the polyanion catalytic performance.

Solvent-free aerobic oxidation of n-alkane by iron(III)-substituted polyoxotungstates immobilized on SBA-15.

The tetrairon(III)-substituted polytungstates [Fe(4)(H(2)O)(10)(beta-XW(9)O(33))(2)](n-) (n = 6, X = As(III), Sb(III); n = 4, X = Se(IV), Te(IV)) were immobilized on (3-aminopropyl)triethoxysilane-modified SBA-15 and showed an excellent catalytic performance for solvent-free aerobic oxidation of long-chain n-alkanes using air as the oxidant under ambient conditions through a classical free-radical chain autoxidation mechanism.