Theoretical study on second hyperpolarizabilities of singlet diradical square planar nickel complexes involving o-semiquinonato type ligands

Hybrid density functional theory method is applied for investigating the diradical character dependence of the second hyperpolarizability (gamma) of square planar nickel complexes involving several types of bidentate ligands [o-C6H4XY, where X = Y = O, NH, S, Se, and PH as well as (X, Y) = (NH, NH2) and (S, NH2)]. It is found that, as a function of the donor atoms, the diradical character of these complexes varies from 0.0 to 0.884 and is associated with substantial variations of gamma ranging from 14 x 10(3) to 819 x 10(3) au. In particular, the largest gamma values are associated with intermediate diradical characters in good agreement with the structure-property relationship obtained for pure hydrocarbon systems. Increasing the electronegativity of the X and Y donor groups of the ligands leads to larger diradical characters as a result of the enhancement of the double bond nature of the C=X(Y) bonds, which further stabilizes the diradicals on both-end benzene rings. This demonstrates that the electronegativities of the donor atoms of the ligands become a tuning parameter of the diradical character and then of the gamma values of these complexes.     

Preparation of apatite-type-silicate-supported precious metal catalysts for selective catalytic reduction of NOx

Apatite-type silicate supported precious metal catalysts were prepared and investigated for their catalytic activity in selective catalytic NO reduction. Single-phase La_9.33Si₆O₂₆ and La_8.33ASi₆O_25.5 (A=Ca, Sr, Ba) were obtained by a sol-gel method. Pd/La_9.33Si₆O₂₆ catalyst exhibited high activity for oxidation of C₃H₆, comparable to Pd/Al₂O₃ catalyst, although the specific surface area of La_9.33Si₆O₂₆ was lower than that of Al₂O₃. In addition, Pt/La_9.33Si₆O₂₆ catalyst exhibited higher activity for selective catalytic reduction of NO than Pt/Al₂O₃ catalyst. Substitution of Ba²⁺ for La³⁺ of La_9.33Si₆O₂₆ led to increased catalytic activity at low temperature.     

Synthesis of nanometer-scale porphyrin wheels of variable size

Starting from 1,3-phenylene linked diporphyrin zinc(II) complex 2ZA, repeated stepwise Ag I-promoted coupling reactions provided linear oligomers from 2nZA up to 128ZA. Of these zigzag shaped porphyrin arrays, the Ag I-promoted intramolecular cyclization reaction of 2 nZA (n=5, 6, 8, 9, 12, and 16) under dilute conditions gave the corresponding cyclic porphyrin wheels C2nZA (n=5, 6, 8, 9, 12, and 16), whereas large arrays 2nZA (n=24, 32, and 48) did not provide cyclic porphyrin products. These large discrete porphyrin arrays and wheels were fully characterized by means of 1H NMR spectroscopy, MALDI-TOF mass spectrometry, UV/Vis absorption spectroscopy, GPC-HPLC analysis, and the scanning tunneling microscopy (STM) technique. The STM images of C12ZA and C18ZA reveal their large circular structures. In the cyclic structures of C2nZA in solution, however, the gradual decrease in fluorescence quantum yields and fluorescence lifetimes are observed, reflecting some conformational heterogeneities. Collectively, the present work provides an important contribution to the construction of fully covalently linked large cyclic arranged porphyrin arrays with ample electronic interactions as a model of light-harvesting antenna.     

Coordination-driven face-directed self-assembly of trigonal prisms. Face-based conformational chirality

The coordination-driven self-assembly of four different trigonal prisms from 3 equiv of one of four different tetrapyridyl star connectors and 6 equiv of a platinum linker dication in nitromethane is presented. This face-directed approach affords high yields without template assistance. The prisms have been characterized by multinuclear and DOSY NMR and dual ESI-FT-ICR mass spectrometry. The use of a conformationally chiral star connector leads to a conformationally chiral prism when connector arm ends attached to a vertex have a strongly correlated twist sense and chirality is communicated across polyhedral faces, edges, and vertices. Molecular mechanics results suggest that in the smallest prism 3d collective effects dominate and the all-P and all-M conformers are strongly favored. NMR data prove that the two edges of the pyridine rings in the triflate salts of 3a-3d are distinct. An Eyring plot of rates obtained from line-shape analysis and 1-D EXCHSY NMR yields an activation enthalpy DeltaH(double dagger) of approximately 12 kcal/mol and activation entropy DeltaS(double dagger) of approximately -15 cal/mol x K for the edge interconversion process, compatible with pyridine rotation around the Pt-N bond. For 3c, this behavior is observed only up to approximately 318 K. At higher temperatures, the Eyring plot is again linear but follows a very different straight line, with a DeltaH(double dagger) of approximately 35 kcal/mol and DeltaS(double dagger) of approximately 60 cal/mol x K. This highly unusual result is further investigated and discussed in the following companion paper.     

Trans-selective asymmetric aziridination of diazoacetamides and N-Boc imines catalyzed by axially chiral dicarboxylic acid

Axially chiral dicarboxylic acid (R)-1d catalyzed reaction of diazoacetamides and N-Boc imines provided a novel organocatalytic means for the formation of enantiomerically enriched N-Boc protected trans aziridines.     

Nickel-catalyzed decarbonylative addition of phthalimides to alkynes

An intermolecular nickel-catalyzed addition reaction has been developed where N-arylphthalimides react with alkynes to afford substituted isoquinolones. A mechanistic rationale is proposed, implying nucleophilic attack of Ni(0) to an amide as the primary step of the catalytic cycle.     

Poly(arylene ether) ionomers containing sulfofluorenyl groups: Effect of electron-withdrawing groups on the properties

For polymer electrolyte membrane fuel membrane cell (PEMFC) applications, the effect of electron-withdrawing groups on the properties of sulfonated poly(arylene ether) (SPE) ionomer membranes was investigated. A series of poly(arylene ether)s containing fluorenyl groups and electron-withdrawing groups (sulfone, nitrile, or fluorine) was synthesized, which were sulfonated with chlorosulfonic acid using a flow reactor to obtain the title ionomers. The ionomers had high molecular weight (M_n > 77 kDa, M_w > 238 kDa) and gave tough, ductile membranes by solution casting. The ion exchange capacity (IEC) of the membranes ranged from 1.6 to 3.5 mequiv/g as determined by titration. The electron-withdrawing groups did not appear to affect the thermal properties (decomposition temperature higher than 200 °C). The presence of nitrile groups, especially at positions meta to the ether linkages, improved the oxidative stability of the SPE membranes, while it led to a deterioration of the hydrolytic stability. The perfluorinated biphenylene groups were effective in providing high mechanical strength with reasonable dimensional change, probably due to a somewhat decreased water absorbability. The SPE membrane containing sulfone groups showed the highest proton conductivity (10⁻³-10⁻¹ S/cm) at 20-93% RH (relative humidity) and 80 °C. The nitrile-containing SPE membrane showed smaller apparent activation energies for oxygen and hydrogen permeability and is thus considered to be a possible candidate for applications in PEMFCs.     

Chiral distortion in a Mn(IV)(salen)(N3)2 derived from Jacobsen's catalyst as a possible conformation model for its enantioselective reactions

The Mn (IV)(salen)(N 3) 2 complex ( 3) from Jacobsen's catalyst is synthesized, and the X-ray crystal structures of 3 as well as the starting Mn (III)(salen)(N 3)(CH 3OH) complex ( 2) are determined in order to investigate the conformation of the high-valent Mn (IV)(salen) molecule in comparison with that of Mn (III)(salen). The asymmetric unit of the crystal of 3 contains four complexes, all of which adopt a nonplanar stepped conformation effectively distorted by the chirality of the diimine bridge. The asymmetric unit of 2 also contains four complexes. Two of them show a stepped conformation of a lesser degree, but the other two adopt a bowl-shaped conformation. Comparison of the structural parameters shows that the Mn center in 3 is coordinated from both sides by two external axial N 3 ligands with significantly shorter bond lengths, which could induce greater preference for the stepped conformation in 3. The CH 3CN solution of 3 shows circular dichroism with a significantly strong band at 275 nm as compared to 2, suggesting that 3 may adopt a more chirally distorted conformation also in solution. The circular dichroism spectrum of 3 is slightly altered with isodichroic points from 298 to 253 K and shows no further change at temperatures lower than 253 K, suggesting that the solution of 3 contains an equilibrium between two conformers, where a low-energy conformer with more chiral distortion is predominantly favored even at room temperature. Complexes 2 and 3 are thoroughly characterized using various techniques including cyclic voltammetry, magnetic susceptibility, UV-vis, electron paramagnetic resonance, (1)H NMR, infrared spectroscopy, and electrospray ionization mass spectrometry.     

Exo- and endo-Isomers of Plumbylenes Supported by 1,3-Diethers of Calix[4]arene

Calix[4]arene-plumbylene complexes Pb[Calix^t-Bu(O)₂(OR)₂], where R=benzyl, 9-fluorenyl, Si^tBuMe₂, and SiⁱPr₃, have been synthesized by the reaction of Lappert's plumbylene Pb[N(SiMe₃)₂]₂ with the corresponding 1,3-diethers of calix[4]arene. The products have been fully characterized by ¹H, ¹³C{¹H}, and ²⁰⁷Pb NMR spectroscopy, elemental analysis, and X-ray diffraction analysis. The products adopt two different forms depending on the R groups: the alkyl derivatives were obtained as exo-isomers in which the lead atom resides outside the cavity of the calix[4]arene, while the silyl derivatives were isolated as endo-isomers where the lead atom is situated inside the cavity. X-ray diffraction studies revealed that the lead(II) atoms in the exo- and endo-isomers are coordinated by the ethereal oxygen atoms (OR) and the aromatic rings (ArOR), respectively, which are maintained in solution as evidenced by ²⁰⁷Pb NMR studies.