Isostructural Pd(II) and Pt(II) pyrophosphato complexes: polymorphism and unusual bond character in d8-d8 systems

Isostructural, "clamshell"-like, neutral dimeric pyrophosphato complexes of general formula {[M(bipy)](2)(μ-P(2)O(7))} [M = Pd(II) (1) or Pt(II) (2)] were synthesized and studied through single-crystal X-ray diffraction, IR, (31)P NMR spectroscopy, and MALDI-TOF mass spectrometry. Compound 1 was synthesized through the reaction of palladium(II) acetate, 2,2'-bipyridine (bipy), and sodium pyrophosphate (Na(4)P(2)O(7)) in water. Compound 2 was prepared through two different routes. The first involved the reaction of the Pt(IV) precursor Na(2)PtCl(6), bipy, and Na(4)P(2)O(7) in water, followed by reduction in DMF. The second involved the reaction of the Pt(II) precursor K(2)PtCl(4), bipy, and Na(4)P(2)O(7) in water. Both complexes crystallize in the monoclinic chiral space group Cc as hexahydrates, 1·6H(2)O (1a, yellow crystals) and 2·6H(2)O (2a, orange crystals), and exhibit a zigzag chain-like supramolecular packing arrangement with short and long intra/intermolecular metal-metal distances [3.0366(3)/4.5401(3) ? in 1a; 3.0522(3)/4.5609(3) ? in 2a]. A second crystalline phase of the Pt species was also isolated, with formula 2·3.5H(2)O (2b, deep green crystals), characterized by a dimer-of-dimers (pseudo-tetramer) structural submotif. Green crystals of 2b could be irreversibly converted to the orange form 2a by exposure to air or water, without retention of crystallinity, while a partial, reversible crystal-to-crystal transformation occurred when 2a was dried in vacuo. (31)P NMR spectra recorded for both 1 and 2 at various pHs revealed the occurrence of a fluxional protonated/deprotonated system in solution, which was interpreted as being composed, in the protonated form, of [HO=PO(3)](+) (P(α)) and O=PO(3) (P(β)) pyrophosphate subunits. Compounds 1 and 2 exhibited two successive one-electron oxidations, mostly irreversible in nature; however, a dependence upon pH was observed for 1, with oxidation only occurring in strongly basic conditions. Density functional theory and atoms in molecules analyses showed that a d(8)-d(8) interaction was present in 1 and 2.     

Novel NdCo<Subscript>5</Subscript> nanoflakes and nanoparticles produced by surfactant-assisted high-energy ball milling

High-energy ball milling has been shown to be a promising method for the fabrication of rare earth—transition metal nanopowders. In this work, NdCo₅ nanoflakes and nanoparticles have been produced by a two-stage high-energy ball milling (HEBM), by first using wet HEBM to prepare precursor nanocrystalline powders followed by surfactant-assisted HEBM. NdCo₅ flakes have a thickness below 150 nm and an aspect ratio as high as 10²–10³; the nanoparticles have an average size of 7 nm. Both the nanoparticles and nano-flakes exhibited high coercivities at low temperatures, with values at 50 K of 3 and 3.7 kOe, respectively. The high values of coercivity can be attributed to the large surface anisotropy of nanoparticles that leads to an effective uniaxial-type of behavior in contrast to the planar anisotropy of the bulk samples. Angle-dependent magnetization measurements at different temperatures were used to determine the spin reorientation transitions in the nanopowders and nanoparticles. The nanoparticles showed spin reorientation temperatures, T _SR1 = 276 and T _SR2 = 237 K which are lower when compared with the values of 290 and 245 K, respectively for bulk.     

Mild, Reversible Reaction of Iridium(III) Amido Complexes with Carbon Dioxide

Unlike some other Ir(III) hydrides, the aminopyridine complex [(2-NH(2)-C(5)NH(4))IrH(3)(PPh(3))(2)] (1-PPh(3)) does not insert CO(2) into the Ir-H bond. Instead 1-PPh(3) loses H(2) to form the cyclometalated species [(κ(2)-N,N-2-NH-C(5)NH(4))IrH(2)(PPh(3))(2)] (2-PPh(3)), which subsequently reacts with CO(2) to form the carbamato species [(κ(2)-O,N-2-OC(O)NH-C(5)NH(4))IrH(2)(PPh(3))(2)] (10-PPh(3)). To study the insertion of CO(2) into the Ir-N bond of the cyclometalated species, a family of compounds of the type [(κ(2)-N,N-2-NR-C(5)NH(4))IrH(2)(PR'(3))(2)] (R = H, R' = Ph (2-PPh(3)); R = H, R' = Cy (2-PCy(3)); R = Me, R' = Ph (4-PPh(3)); R = Ph, R' = Ph (5-PPh(3)); R = Ph, R' = Cy (5-PCy(3))) and the pyrimidine complex [(κ(2)-N,N-2-NH-C(4)N(2)H(3))IrH(2)(PPh(3))(2)] (6-PPh(3)) were prepared. The rate of CO(2) insertion is faster for the more nucleophilic amides. DFT studies suggest that the mechanism of insertion involves initial nucleophilic attack of the nitrogen lone pair of the amide on CO(2) to form an N-bound carbamato complex, followed by rearrangement to the O-bound species. CO(2) insertion into 1-PPh(3) is reversible in the presence of H(2) and treatment of 10-PPh(3) with H(2) regenerates 1-PPh(3), along with Ir(PPh(3))(2)H(5).     

Reductive coupling of carbon monoxide by U(III) complexes--a computational study

The role of U((η-C(8)H(6){Si(i)Pr(3)-1,4}(2))(η-C(5)Me(5)) and U((η-C(8)H(6){Si(i)Pr(3)-1,4}(2))(η-C(5)Me(4)H) in the reductive di- tri- and tetramerization of CO has been modelled using density functional methods and U(C(8)H(8))(C(5)H(5)) as the metal fragment. The orbital structure of U(C(8)H(8))(C(5)H(5)) is described. CO binding to form a monocarbonyl U(C(8)H(8))(C(5)H(5))(CO) is found, by a variety of methods, to place spin density on the CO ligand via back-bonding from the U5f orbitals. A possible pathway for formation of the yne diolate complex [U(C(8)H(8))(C(5)H(5))](2)C(2)O(2) is proposed which involves dimerization of U(C(8)H(8))(C(5)H(5))CO via coordination of the CO O atoms to the opposing U atoms followed by C-C bond formation to form a zig-zag intermediate, stable at low temperatures. The intermediate then unfolds to form the yne diolate. The structures of [U(C(8)H(8))(C(5)H(5))]C(2)O(2), the deltate complex [U(C(8)H(8))(C(5)H(5))]C(3)O(3) and the squarate complex [U(C(8)H(8))(C(5)H(5))]C(4)O(4) are optimized and provide good models for the experimental compounds. The reaction of further CO with a zig-zag intermediate to form deltate and squarate complexes was explored using Th(C(8)H(8))(C(5)H(5)) as a model and low energy pathways are proposed.     

Palladium catalyzed carboxylation of allylstannanes and boranes using CO2

A family of well-defined (η(3)-allyl)Pd(L)(carboxylate) (L = PR(3) or NHC) complexes are by far the most efficient catalysts reported to date for the catalytic carboxylation of allylstannanes into allylcarboxylates using CO(2). The substrate scope of this reaction is extended to both substituted allylstannanes and allylboranes.     

Cauchy problem for a higher-order Boussinesq equation

In this study we establish global well-posedness of the Cauchy problem for a higher-order Boussinesq equation. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Solid State Regeneration of Ketones From Semicarbazones Using Antimony Trichloride Under Microwave Irradiation

Microwave irradiation of ketosemicarbazones with wet antimony trichloride under solvent-free condition provides a fast, efficient and simple method for the regeneration of ketones in good yields.     

Synthesis of PCP-supported nickel complexes and their reactivity with carbon dioxide

The Ni amide and hydroxide complexes [(PCP)Ni(NH(2))] (2; PCP=bis-2,6-di-tert-butylphosphinomethylbenzene) and [(PCP)Ni(OH)] (3) were prepared by treatment of [(PCP)NiCl] (1) with NaNH(2) or NaOH, respectively. The conditions for the formation of 3 from 1 and NaOH were harsh (2 weeks in THF at reflux) and a more facile synthetic route involved protonation of 2 with H(2)O, to generate 3 and ammonia. Similarly the basic amide in 2 was protonated with a variety of other weak acids to form the complexes [(PCP)Ni(2-Me-imidazole)] (4), [(PCP)Ni(dimethylmalonate)] (5), [(PCP)Ni(oxazole)] (6), and [(PCP)Ni(CCPh)] (7), respectively. The hydroxide compound 3, could also be used as a Ni precursor and treatment of 3 with TMSCN (TMS=trimethylsilyl) or TMSN(3) generated [(PCP)Ni(CN)] (8) or [(PCP)Ni(N(3))] (9), respectively. Compounds 3-7, and 9 were characterized by X-ray crystallography. Although 3, 4, 6, 7, and 9 are all four-coordinate complexes with a square-planar geometry around Ni, 5 is a pseudo-five-coordinate complex, with the dimethylmalonate ligand coordinated in an X-type fashion through one oxygen atom, and weakly as an L-type ligand through another oxygen atom. Complexes 2-9 were all reacted with carbon dioxide. Compounds 2-4 underwent facile reaction at low temperature to form the κ(1)-O carboxylate products [(PCP)Ni{OC(O)NH(2)}] (10), [(PCP)Ni{OC(O)OH}] (11), and [(PCP)Ni{OC(O)-2-Me-imidazole}] (12), respectively. Compounds 10 and 11 were characterized by X-ray crystallography. No reaction was observed between 5-9 and carbon dioxide, even at elevated temperatures. DFT calculations were performed to model the thermodynamics for the insertion of carbon dioxide into 2-9 to form a κ(1)-O carboxylate product and understand the pathways for carbon dioxide insertion into 2, 3, 6, and 7. The computed free energies indicate that carbon dioxide insertion into 2 and 3 is thermodynamically favorable, insertion into 8 and 9 is significantly uphill, insertion into 5 and 7 is slightly uphill, and insertion into 4 and 6 is close to thermoneutral. The pathway for insertion into 2 and 3 has a low barrier and involves nucleophilic attack of the nitrogen or oxygen lone pair on electrophilic carbon dioxide. A related stepwise pathway is calculated for 7, but in this case the carbon of the alkyne is significantly less nucleophilic and as a result, the barrier for carbon dioxide insertion is high. In contrast, carbon dioxide insertion into 6 involves a single concerted step that has a high barrier.     

Vibronic spectroscopy of single C60 molecules and monolayers with the STM

A scanning tunneling microscope is used to study the differential conductance (dI/dV) of single C(60) molecules in isolation and in monolayers adsorbed on NiAl(110) and on an ultrathin alumina film grown on the NiAl(110) surface. On the oxide layer, the electronic bands in the dI/dV spectra display a series of equally spaced features, attributed to the vibronic states of the molecules, which are absent when the molecules are adsorbed on the metal. A comparison between the molecular spectra measured on the oxide film reveals the effect of adsorption temperature and geometry, as well as intermolecular interactions on the vibronic features.     

Improvement in the photocatalytic activity of TiO2 by the partial oxidation of the C impurities

TiO₂ was treated by water in an ultrasonic bath, resulting in the enhancement of the photocatalytic activity for the decomposition of methylene blue under UV and visible light irradiation. No change in the crystallinity and optical properties of TiO₂ by the H₂O-treatment was observed. The X-ray photoelectron spectroscopy (XPS) and FT-IR data revealed that the C impurities were oxidized by this treatment, indicating that the change in the structure of the C impurities plays a pivotal role in the photocatalytic activity of TiO₂.