Mild, aqueous, aerobic, catalytic oxidation of methane to methanol and acetaldehyde catalyzed by a supported bipyrimidinylplatinum-polyoxometalate hybrid compound

We have demonstrated that a bipyrimidinylplatinum-polyoxometalate, [Pt(Mebipym)Cl2]+[H4PV2Mo10O40]-, supported on silica is an active catalyst for the aerobic oxidation of methane to methanol in water under mild reaction conditions. Further oxidation of methanol yields acetaldehyde. The presence of the polyoxometalate is presumed to allow the facile oxidation of a Pt(II) intermediate to a Pt(IV) intermediate and to aid in the addition of methane to the Pt catalytic center.     

Phenanthroline-polyoxometalate hybrid compounds and the observation of intramolecular charge transfer

A phenanthroline ligand has been covalently modified at the 2 and 9 positions by an aminophenylhexamolybdate substituent. The 1H NMR spectrum indicated a strong electron-withdrawing effect of the hexamolybdate (Mo6O19(2-)) moiety on the phenanthroline ligand. UV-vis and cyclic voltammetry showed extended conjugation of the hybrid phenanthroline-polyoxometalate compound and the possibility of easy oxidation of the extended phenanathroline ligand. Further EPR experiments provided strong evidence for an intramolecular charge-transfer process with the formation of a phenanthroline cation radical and a reduced hexamolybdate.     

Ligand structural effects on Cu2S2 bonding and reactivity in side-on disulfido-bridged dicopper complexes

To assess supporting ligand effects on S-S bond activation, a series of [Cu2(mu-eta2:eta2-S2)]2+ complexes supported by various beta-diketiminate or anilido-imine ligands (L) were synthesized via the reaction of Cu(I) precursors LCu(CH(3)CN) with S8. For the cases where L = beta-diketiminate, the syntheses were complicated by formation of clusters [Cu(SR)]4, where SR represents the ligand functionalized by sulfur at the central methine position. The [Cu2(mu-eta2:eta2-S2)]2+ products were characterized by X-ray crystallography and electronic absorption and resonance Raman spectroscopy. Correlations among the Cu-S, Cu-Cu, and S-S distances and the nu(S-S) values were observed and interpreted within the framework of a previously described bonding picture (Chen, P.; Fujisawa, K.; Helton, M. E.; Karlin, K. D.; Solomon, E. I. J. Am. Chem. Soc. 2003, 125, 6394). Comparison of these data to those for other relevant species revealed a remarkable degree of S-S bond activation in the compounds supported by the beta-diketiminate and anilido-imine ligands, which through strong electron donation increase backbonding from the copper ions into the S-S sigma* orbital and cause S-S bond weakening. Reactions of one of the complexes supported by an anilido-imine ligand with PPh(3) and xylyl isocyanide were explored, revealing facile transfer of sulfur to PPh(3) but only displacement of sulfur to yield a LCu(I)-CNAr (Ar = xylyl) complex with the isocyanide.     

Structural diversity in copper-sulfur chemistry: synthesis of novel Cu/S clusters through metathesis reactions

With the ultimate goal of understanding the Cu(4)S cluster in nitrous oxide reductase, studies of the fundamental chemistry of nitrogen-donor ligand-supported copper-sulfur species have been pursued. Reactions of Cu(II)X(2) (X = Cl(-) or CF(3)SO(3)(-)), N,N,N',N'-tetramethyl-trans-(1R,2R)-diaminocyclohexane, and Li(2)S or Na(2)S(2) yielded clusters that contain [Cu(2)(micro-S(2))(2)](2+), [Cu(3)(micro-S)(2)](3+), [Cu(4)(micro-S(2))(2)](4+), and/or [Cu(6)(micro-S(2))(4)](4+) cores, depending on the specific reaction conditions, notably the nature of X and the sulfur source used. Copper(II) and/or Copper(III) and variable sulfur oxidation levels, including S(2-), S(2)(2-), and S(2)(-*), were identified by X-ray crystallography and spectroscopy.     

A new method for the synthesis of organopolyoxometalate hybrid compounds

The reaction of a quaternary ammonium salt of the tin chloride-substituted polyoxometalate, [PSn(Cl)W11O39]4-, with a variety of n-nucleophiles including primary, secondary, and tertiary amines and a tertiary phosphine, yielded tin-centered Lewis acid-base adducts, [PSn(Cl)W11O39]4--n-nucleophile; with more nucleophilic secondary amines such as diisopropylamine, apparently some [PSnN[CH(CH3)2]2W11O39]4- was formed as a minor product. The compounds were identified by 1H, 119Sn, 15N, 31P, and 183W NMR, ESI-MS, and elemental analyses. The key connectivity of the Sn-Cl center with the amine was clarified by the observation of 3J Sn-H couplings (Sn from the polyoxometalate cluster and H from the amine moiety) in a 2D 119Sn-1H heteronuclear multiple-bond correlation NMR experiment. This new, rather simple synthetic method was also utilized for preparing amino acid-polyoxometalate hybrid compounds.     

Synthesis, characterization and catalytic activity of a Wilkinson's type metal-organic-polyoxometalate hybrid compound

A metal-organic-polyoxometalate hybrid compound with two functional centers consisting of a rhodium(I)bis(diphenylphospine) unit connected through two alkylene bridging groups to a lacunary Keggin type polyoxometalate was synthesized and used as an effective, recyclable hydrogenation catalyst in monophasic and aqueous biphasic reaction modes.     

Synthesis and Characterization of Polyoxometalate–Polyamino Dendritic Hybrid Compounds

Organic–inorganic hybrid compounds were prepared by the reaction of a tin chloride-substituted polyoxometalate, [PSn(Cl)W₁₁O₃₉]⁴⁻ with tris(2-aminoethyl)amine, and poly(propylene)imine (DAB-Am) tetraamine and octaamine dendrimers. Translational diffusion coefficients of the hybrid compounds were measured in DMSO-d ₆ by the stimulated echo diffusion (STE) NMR technique. Molecular radii were derived from the diffusion coefficients by the Stokes–Einstein equation and appeared to be incorrect because of fast exchange on the NMR time scale of the counter cation in the solution, which led to an averaging of the NMR signal and high diffusion coefficients. An effective hydrodynamic diameter of the [PSn(Cl)W₁₁O₃₉]⁴⁻–polypropylenimine octaamine hybrid adduct was measured in a light scattering experiment.     

Organometallic[bond]polyoxometalate hybrid compounds: metallosalen compounds modified by Keggin type polyoxometalates

Hybrid compounds with two functional centers consisting of a metallosalen moiety (M[bond]salen; M = Mn, Co, Ni, and Pd) connected by an alkylene bridging group to a lacunary Keggin type polyoxometalate were synthesized and characterized. In these metallosalen-polyoxometalate compounds (M[bond]salen[bond]POM) it was shown by the use of a combination of UV[bond]vis, (1)H NMR, EPR, XPS, and cyclic voltammetry measurements that the polyoxometalate exerts a significant intramolecular electronic effect on the metallosalen moiety leading to formation of an oxidized metallosalen moiety. For the Mn[bond]salen[bond]POM, the metallosalen center is best described as a metal[bond]salen cation radical species; that is, a localized "hole" is formed on the salen ligand. For the other M[bond]salen[bond]POM compounds, the metallosalen moiety can be described as a hybrid of a metal[bond]salen cation radical species and an oxidized metal[bond]salen species, that is, a delocalized "hole" is formed at the metallosalen center. It is proposed that these oxidized metallosalen centers are best characterized as stabilized charge transfer (metallosalen donor[bond]polyoxometalate acceptor) complexes despite the relatively large distance between the two functional centers.