Preparation and characterization of carboximidate iron(II) complexes

The reactions of C₅H₄NCHNNHC(O)Ph (1) with Fe(II) chloride gave [Fe₂(C₅H₄NC(OEt)NNHC(O)Ph)₂(μ-OEt)₂Cl₂]) (2) in ethanol and [Fe₂(C₅H₄NC(OMe)NNHC(O)Ph)₂(μ-OMe)₂Cl₂] (3) in methanol as well as [Fe(C₅H₄NCHNNHC(O)Ph)Cl₂] (4) in tetrahydrofuran, respectively. The X-ray diffraction analysis reveals their structures and complex 4 is proposed as an intermediate of formation of complexes 2 and 3.     

Unsymmetrical cyrhetrenyl and ferrocenyl azines derived from 5-nitrofurane: Synthesis,structural characterization and electrochemistry

A new series of unsymmetrical cyrhetrenyl and ferrocenyl azines that were monosubstituted [(η⁵-C₅H₄)–C(R)N–NCH(5-NO₂–2-C₄H₂O)]M {with MRe(CO)₃ and RH (1a) or RMe (1b); MFe(η⁵-C₅H₅) and RH (2a) or RMe (2b)} and disubstituted [Fe{(η⁵-C₅H₄)–C(Me)N–NCH(5-NO₂–2-C₄H₂O)}₂] (3a) were prepared by condensation reactions of the corresponding organometallic hydrazone [(η⁵-C₅H₄)–C(R)N–NH₂)]M with 5-nitro-2-furaldehyde. The ¹H and ¹³C{¹H} NMR spectra indicated that these compounds adopted an (E,E)-configuration about the ˃CN − bond and an s-trans conformation about the N1–N2 bond, and this result was confirmed by X-ray crystallographic analyses of 1a and 2b. The opposite electronic effects of the organometallic fragments correlate with the co-planarity of the [(η⁵-C₅H₄)–C(R)N–NCH(5-NO₂–2-C₄H₂O)] system, the reduction potential of the nitro group (E_1/2) and the chemical shifts of the iminic carbons.     

Nonlinear optical properties in tetrametallic Fe3Mn complexes with pseudo-C3 symmetry

New type of 3D chromophores [{CpFeC₅H₄CRCH4-py}₃Mn(CO)₃]BF₄ (1) with weakly interaction subchromophore were synthesized and found to display improved nonlinearity compared with their 1D reference systems [(CpFeC₅H₄CRCH4-py)Mn(CO)₅]BF₄ (2).     

Synthesis and reactivity of an 2-azabutadiene-based π-conjugated dithioether: Formation of a N,S-ligated molybdenum chelate complex and C,N,S-pincer complexes of palladium and platinum

Nucleophilic attack of sodium isopropylthiolate on 4,4-dichloro-1,1-diphenyl-2-azabuta-1,3-diene [Cl₂CC(H)–NCPh₂}] (1) affords the 2-azabutadiene derivative [(i-PrS)₂CC(H)–NCPh₂] (2). Upon irradiation of Mo(CO)₆ in THF in the presence of 2, the chelate complex cis-[(OC)₄Mo{(i-PrS)₂CC(H)–NCPh₂}] (3) is obtained. Coordination on Mo occurs through the imine nitrogen and a thioether group. Polydentate dithioether 2 acts as N,C,S-pincer ligand after orthometallation reaction with Pd(II) or Pt(II). The molecular structures of 2 and (C,N,S)-[(i-PrS)₂CC(H)–NC(Ph)C₆H₄)PtCl] (4b) have been determined by X-ray diffraction studies.     

Synthesis and structural characterizations of organolanthanide complexes with amino-tethered guanidinate ligand (C5H5)2Ln[H2N(CH2)3NC(NHiPr) NiPr)] (Ln = Yb,Y, Er,Dy)

Reaction of N,N′-diisopropylcarbodiimide (ⁱPrNCNⁱPr) with H₂N(CH₂)₃NH₂ and (C₅H₅)₃Ln, give (C₅H₅)₂Ln[H₂N(CH₂)₃NC(NHⁱPr) NⁱPr)] in high yields, indicating that the N–H bonds of one NH₂ group readily add to the CN double bonds of carbodiimide and one cyclopentadienyl group is eliminated to construct a novel amino-tethered guanidinate anionic ligand [H₂N(CH₂)₃N C(NHⁱPr)NⁱPr)]⁻.     

The effect of Mg(OH)2 on furfural oxidation with H2O2

The oxidation of furfural in H₂O₂ and H₂O₂–Mg(OH)₂ system were systematically investigated and a rational explanation for the reaction mechanism was proposed. 2-formyloxyfuran, from selective oxidation of HCO group in furfural, was a crucial intermediate. The addition of Mg(OH)₂ suppressed the oxidation of furan ring of furfural and enhanced selectivities of 2(5H)-furanone (44.8%) and succinic acid (38.0%). FT-IR, Gaussian calculation and experimental results indicated that the process of furfural oxidation with H₂O₂ is homogeneous, and the synergy between dissolved Mg^2 + cations and OH⁻ ions facilitates the HOO⁻ attacking the carbon atom of HCO other than the CC bound of furan ring.     

Origin of the cluster-size effect in the hydrogenation of cinnamaldehyde over supported Au catalysts

Gold nanoclusters supported on γ-Al₂O₃ were more active and selective than platinum nanoclusters in the high-pressure liquid-phase hydrogenation of cinnamaldehyde to cinnamylalcohol, while in the hydrogenation of cyclohexene gold was less active than platinum. The differences in catalytic performance are ascribed to the weaker interaction of gold with the reactants and products compared to platinum. Gold clusters with a diameter below 2 nm are essential to obtain a high activity and selectivity in the hydrogenation of cinnamaldehyde. The size dependence of the selectivity originates from the stronger dependence of the CO hydrogenation rate on cluster size compared to the CC hydrogenation rate. Small clusters exhibit an enhanced π backbonding, which favors CO adsorption over CC adsorption.     

Thermally stable pseudo-third-generation Grubbs ruthenium catalysts with pyridine–phosphinimine ligand

The ligand precursors 2-(R₃PN)CH₂Py (R = Ph(1a), Cy(2a)) were prepared from reaction of pyridine azide with various phosphine ligands. Reaction of 1a or 2a with RuCl₂(CHPh)(Py)₂(H₂IMes) (Py = pyridine) afforded the ruthenium alkylidene complex RuCl₂(CHPh)(PyCH₂(NPR₃))(H₂IMes) (R = Ph(1), Cy(2)). Both catalysts showed good thermal stability and latent behavior toward RCM and ROMP reactions.     

Pt nanoparticles deposited over carbon nanotubes for selective hydrogenation of cinnamaldehyde

Two kinds of carbon nanotubes (CNTs) with different inner diameter (less than 10 nm: CNTs-1 and between 60 and 100 nm: CNTs-2) were used as catalyst supports. The platinum particles were simply deposited on the outside surface (CNTs-1) and inside (CNTs-2) and were easily reduced to Pt⁰ by refluxing. The catalysts exhibit high activities in the selective hydrogenation of cinnamaldehyde which contains both CC and CO bonds. But the selectivity of these two catalysts was quite different under same reaction conditions. The high selective hydrogenation of CO bond was observed over catalyst 3%Pt/CNTs-2, while the completely hydrogenation of both CC and CO bonds was found over catalyst 3%Pt/CNTs-1.     

Preparation of high-temperature stable SiBCN fibers from tailored single source polyborosilazanes

Bulk SiBCN ceramics derived from polyborosilazanes of the type [B(C₂H₄SiRNH)₃]_n (1a, R = CH₃; 2a, R = H; C₂H₄ = CHCH₃, CH₂CH₂) exhibit an exceptional structural stability at high temperature. Therefore, such quaternary systems are of great scientific and technical interest as fibrous reinforcements intended for high-temperature applications. In this context, the design of novel polyborosilazanes, which display properties tailored for the preparation of SiBCN fibers, is studied. Boron-modified polysilazanes of the type [B(C₂H₄SiRNCH₃)₃]_n (1b, R = CH₃; 2b, R = H) are prepared via aminolysis of the tris(dichlorosilylethyl)boranes B(C₂H₄SiRCl₂)₃ (1, R = CH₃; 2, R = H). It is shown that the functionalisation of the precursors with NCH₃ units improves their processability (i.e. solubility) compared to that of their ammonolysed analogs [B(C₂H₄SiRNH)₃]_n (1a, R = CH₃; 2a, R = H). In addition to the influence of the NCH₃ units, the presence of the SiCH₃ functions in such polymers offers the best potential for the preparation of fibers by melt-spinning. As-spun fibers are then converted under controlled atmosphere into high-temperature stable SiBCN fibers according to the polymer-derived ceramic route.     

Thiourea-catalyzed dearomatizing [4+2] cycloadditions of 3-nitroindole

We have developed two efficient thiourea promoted dearomatizing processes involving the cycloadditions of 3-nitroindoles. The C2C3 double bond of the heteroarene can be involved as electron-poor 2π dienophile in [4+2] cycloadditions. While the uncatalyzed process requires harsh conditions, the organocatalyzed reaction takes place at room temperature and atmospheric pressure. The C2C3NO motif of the heteroarene can also react as an electron-poor 4π heterodiene in [4+2] / [3+2] cycloadditions cascades, under high pressure. In contrast to Lewis acid activation, thiourea promotion thus proves efficient even under unconventional activation conditions and in the presence of acid sensitive reactants such as enol ethers.     

Synthesis,X-ray structure,spectroscopic properties,and theoretical investigations of dinuclear imidazole molybdenum(VI) complex with long MoO bands

One new dinuclear multioxomolybdenum(VI) complex containing long MoO bands with imidazole, [Mo₂O₆(imi)₄] (1) (imi = imidazole), has been prepared and characterized, and the geometric structure, electronic structure and spectroscopic properties, investigated experimentally and theoretically. The rarely observed long MoO bonds in 1 (ca. 0.15 Å longer than documents previously) are analysed by orbital structure analysis. The electronic origins of the optical transitions are also mainly related to the MoO groups with ligand-to-metal charge transfer (LMCT) nature (P_terminal-oxo → d_Mo).     

Investigation of bifurcated hydrogen bonds within the thermotropic liquid crystalline polyurethanes

The polyurethanes are synthesized from the biphenyl-4,4′-diol (mesogenic biphenol) and 1,3-Bis(isocyanatomethyl) cyclohexane, using (CH₂) of 2, 6 and 11 units as flexible alkylene spacer, respectively. FTIR detects the hydrogen bond in the thermotropic liquid crystalline polyurethane. FTIR spectra show a new CO absorption with lower wavenumber at around 1658 cm^?1 is assigned to “bifurcated” hydrogen bonded CO group—a CO with higher strength hydrogen bonds. The distributions of “bifurcated” hydrogen bonded CO are increased substantially along with increasing the flexible spacer length in polymer backbone. The “bifurcated” hydrogen bond existed not only at the temperature below T_g, but also existed at the temperature far higher than T_m and T_i. It almost is independent of temperature and exhibits a stable interaction (or strucuture) throughout a wide temperature range, differences from the normal liquid crystalline polyurethanes. It is worthy of predicting the thermotropic liquid crystalline polyurethane with “bifurcated” hydrogen bond would enhance its performances.     

Catalytic reactions of dioxygen with ethane and methane on platinum clusters: Mechanistic connections,site requirements,and consequences of chemisorbed oxygen

C₂H₆ reactions with O₂ only form CO₂ and H₂O on dispersed Pt clusters at 0.2–28 O₂/C₂H₆ reactant ratios and 723–913 K without detectable formation of partial oxidation products. Kinetic and isotopic data, measured under conditions of strict kinetic control, show that CH₄ and C₂H₆ reactions involve similar elementary steps and kinetic regimes. These kinetic regimes exhibit different rate equations, kinetic isotope effects and structure sensitivity, and transitions among regimes are dictated by the prevalent coverages of chemisorbed oxygen (O^*). At O₂/C₂H₆ ratios that lead to O^*-saturated surfaces, kinetically-relevant CH bond activation steps involve O^*O^* pairs and transition states with radical-like alkyls. As oxygen vacancies (¹) emerge with decreasing O₂/alkane ratios, alkyl groups at transition states are effectively stabilized by vacancy sites and CH bond activation occurs preferentially at O^** site pairs. Measured kinetic isotope effects and the catalytic consequences of Pt cluster size are consistent with a monotonic transition in the kinetically-relevant step from CH bond activation on O^*O^* site pairs, to CH bond activation on O^** site pairs, to O₂ dissociation on ^** site pairs as O^* coverage decrease for both C₂H₆ and CH₄ reactants. When CH bond activation limits rates, turnover rates increase with increasing Pt cluster size for both alkanes because coordinatively unsaturated corner and edge atoms prevalent in small clusters lead to more strongly-bound and less-reactive O^* species and lower densities of vacancy sites at nearly saturated cluster surfaces. In contrast, the highly exothermic and barrierless nature of O₂ activation steps on uncovered clusters leads to similar turnover rates on Pt clusters with 1.8–8.5 nm diameter when this step becomes kinetically-relevant at low O₂/alkane ratios. Turnover rates and the O₂/alkane ratios required for transitions among kinetic regimes differ significantly between CH₄ and C₂H₆ reactants, because of the different CH bond energies, strength of alkylO^* interactions, and O₂ consumption stoichiometries for these two molecules. Vacancies emerge at higher O₂/alkane ratios for C₂H₆ than for CH₄ reactants, because their weaker CH bonds lead to faster scavenging of O^* and to lower O^* coverages, which are set by the kinetic coupling between CH and OO activation steps. The elementary steps, kinetic regimes, and mechanistic analogies reported here for C₂H₆ and CH₄ reactions with O₂ are consistent with all rate and isotopic data, with their differences in CH bond energies and in alkyl binding, and with the catalytic consequences of surface coordination and cluster size. The rigorous mechanistic interpretation of these seemingly complex kinetic data and cluster size effects provides useful kinetic guidance for larger alkanes and other catalytic surfaces based on the thermodynamic properties of these molecules and on the effects of metal identity and surface coordination on oxygen binding and reactivity.     

Property influence and poisoning mechanism of HgCl2 on V2O5-WO3/TiO2 SCR-DeNOx catalysts

In this paper, HgCl₂ was loaded by impregnation method to study its property influence and poisoning mechanism on V₂O₅-WO₃/TiO₂ SCR-DeNO_x catalyst. For HgCl₂-containing catalysts, deactivation was observed in simulated gas stream, and its catalytic activity declined with the increase of HgCl₂ loadings. Brønsted acid sites (VOH) and VO bond were affected evidently by HgCl₂, new NH₃ adsorption site ClVOH was generated after HgCl₂ addition. Interactions between HgCl₂ and V₂O₅ resulted in the loss of SCR active sites, and –HgCl existed as the stable form on the bridge site. Finally, the probable schematic diagram of HgCl₂ poisoning mechanism was also proposed.     

The structure of MB2MCC (MZr,Hf, Ta) multi-phase ceramic coatings on graphite

A new class of MB₂MCC multi-phase ceramic coatings were fabricated via pilling up MB₂ (MZr, Hf, Ta) powders on the surface of graphite plates followed by heating at 2823 K. Results showed that with the increase of atomic number of Zr, Hf, and Ta, the wettability of the ceramics on graphite plates increased. The surface and inside morphology of the ceramic coatings were different from each other. The graphite inside these ceramic coatings was highly ordered with graphite interlayer distance closing to that of single crystal graphite (0.3354 nm) and some B of MB₂ diffused into graphite causing the decrease of local symmetry without disrupting the graphite structure. This approach is envisaged as a promising path to develop new kinds of ultra-high temperature material coatings for carbon materials.     

Ultraviolet,water, and thermal aging studies of a waterborne polyurethane elastomer-based high reflectivity coating

A waterborne aliphatic polyurethane-based coating was studied for accelerated ultra-violet (UV), water (WT), and thermal (TH) aging for a period of 1000 h. To monitor the coating durability, samples were tested every 200 h. ATR-FTIR spectroscopy was used to monitor the chemical changes occurring during the aging process. UV–vis with integrating sphere was used to track the change in diffused reflectance, while the optical microscope and the scanning white light interferometry (SWLI) were used for surface characterization. FTIR studies of coatings subjected to UV exposure indicated a decrease in functional groups such as CONH, CH, CO, and COC. The appearance of functional groups such as NH is attributed to chain scission of the polyurethane binder in the coating. Investigation of the degradation mechanism in water and thermal aging showed physical effects through water penetration and the mismatch in the coefficient of thermal expansion as the primary causes of degradation. In all aging scenarios, the reduction of reflectivity was largely due to physical defects caused by the different aging mechanisms.     

Crystalline MoVO based complex oxides as selective oxidation catalysts of propane

Three single crystalline MoVO based oxides, MoVO, MoVTeO and MoVTeNbO, all of which have the same orthorhombic layer-type structure with the particular arrangement of MO₆ (M = Mo, V, Nb) octahedra forming slabs with pentagonal, hexagonal, and heptagonal rings in (1 0 0) plane, were synthesized by hydrothermal method and their catalytic performance in the selective oxidation of propane to acrylic acid were compared in order to elucidate the roles of constituent elements and crystal structure in the course of the propane oxidation. It was observed that the rate of propane oxidation was almost the same over all three catalysts, revealing that Mo and V, which were indispensable elements for the structure formation, were responsible for the catalytic activity for propane oxidation. The Te-containing catalysts showed much higher selectivity to acrylic acid than the MoVO catalyst. Since propene was formed as a main product at low conversion levels over every catalyst, it can be concluded that Te located in the central position of the hexagonal ring promoted the conversion of intermediate propene effectively to acrylic acid. The catalyst with Nb occupying the same structural position of V clearly showed the improved selectively to acrylic acid particularly at high conversion region, because the further oxidation of acrylic acid to CO_x was greatly suppressed. These conclusions were further supported by the additional studies of the determination of activation energy and catalytic oxidations of intermediate products of the propane oxidation.     

Studies on the electrochemical degradation of Acid Orange II wastewater with cathodes modified by quinones

The electrochemical oxidation of dye wastewater with the cathode modified by quinones has been investigated. Under the optimal conditions, the decolorization efficiency can reach 96.0% after 120 min. The addition of tert-butanol significantly decreased the decolorization efficiency, suggesting the involvement of •OH radicals in the decomposition of H₂O₂. A potential difference may result in a quinone radical on the cathode which can catalyze the reduction of O₂ to H₂O₂. UV–vis spectrum and GC/MS analysis revealed that the dye molecule was firstly initiated by the cleavage of NN, and decomposed to aromatic intermediates, further degraded to ring opening products and finally mineralized.