A Molecular Iron Catalyst for the Acceptorless Dehydrogenation and Hydrogenation of N‑Heterocycles

A well-defined iron complex (<b>3</b>) supported by a bis­(phosphino)­amine pincer ligand efficiently catalyzes both acceptorless dehydrogenation and hydrogenation of N-heterocycles. The products from these reactions are isolated in good yields. Complex <b>3</b>, the active catalytic species in the dehydrogenation reaction, is independently synthesized and characterized, and its structure is confirmed by X-ray crystallography. A <i>trans</i>-dihydride intermediate (<b>4</b>) is proposed to be involved in the hydrogenation reaction, and its existence is verified by NMR and trapping experiments

Acceptorless, Reversible Dehydrogenation and Hydrogenation of <i>N</i>‑Heterocycles with a Cobalt Pincer Catalyst

Acceptorless, reversible dehydrogenation and hydrogenation reactions involving <i>N</i>-heterocycles are reported with a well-defined cobalt complex supported by an aminobis­(phosphine) [PN­(H)­P] pincer ligand. Several <i>N</i>-heterocycle substrates have been evaluated under dehydrogenation and hydrogenation conditions. The cobalt-catalyzed amine dehydrogenation step, a key step in the dehydrogenation process, has been independently verified. Control studies with related cycloalkanes suggest that a direct acceptorless alkane dehydrogenation pathway is unlikely. The metal–ligand cooperativity is probed with the related [PN­(Me)­P] derivative of the cobalt catalyst. These results suggest a bifunctional dehydrogenation pathway and a nonbifunctional hydrogenation mechanism

A Single Nickel Catalyst for the Acceptorless Dehydrogenation of Alcohols and Hydrogenation of Carbonyl Compounds

A single homogeneous nickel­(II) complex, supported by the tris­(3,5-dimethylpyrazolyl)­borate ligand and 2-hydroxyquinoline ancillary ligand, is shown to catalyze both acceptorless dehydrogenation of alcohols and hydrogenation of carbonyl compounds under mild conditions. Products from the catalytic reactions were isolated with good yields. A mechanistic investigation highlights the critical role of the 2-hydroxyquinoline ligand in the catalysis and argues against a stepwise dehydrogenation pathway

A Single Nickel Catalyst for the Acceptorless Dehydrogenation of Alcohols and Hydrogenation of Carbonyl Compounds

A single homogeneous nickel­(II) complex, supported by the tris­(3,5-dimethylpyrazolyl)­borate ligand and 2-hydroxyquinoline ancillary ligand, is shown to catalyze both acceptorless dehydrogenation of alcohols and hydrogenation of carbonyl compounds under mild conditions. Products from the catalytic reactions were isolated with good yields. A mechanistic investigation highlights the critical role of the 2-hydroxyquinoline ligand in the catalysis and argues against a stepwise dehydrogenation pathway

Determination of Pre-Steady-State Rate Constants on the Escherichia coli Pyruvate Dehydrogenase Complex Reveals That Loop Movement Controls the Rate-Limiting Step

Spectroscopic identification and characterization of covalent and noncovalent intermediates on large enzyme complexes is an exciting and challenging area of modern enzymology. The Escherichia coli pyruvate dehydrogenase multienzyme complex (PDHc), consisting of multiple copies of enzymic components and coenzymes, performs the oxidative decarboxylation of pyruvate to acetyl-CoA and is central to carbon metabolism linking glycolysis to the Krebs cycle. On the basis of earlier studies, we hypothesized that the dynamic regions of the E1p component, which undergo a disorder–order transition upon substrate binding to thiamin diphosphate (ThDP), play a critical role in modulation of the catalytic cycle of PDHc. To test our hypothesis, we kinetically characterized ThDP-bound covalent intermediates on the E1p component, and the lipoamide-bound covalent intermediate on the E2p component in PDHc and in its variants with disrupted active-site loops. Our results suggest that formation of the first covalent predecarboxylation intermediate, C2α-lactylthiamin diphosphate (LThDP), is rate limiting for the series of steps culminating in acetyl-CoA formation. Substitutions in the active center loops produced variants with up to 900-fold lower rates of formation of the LThDP, demonstrating that these perturbations directly affected covalent catalysis. This rate was rescued by up to 5-fold upon assembly to PDHc of the E401K variant. The E1p loop dynamics control covalent catalysis with ThDP and are modulated by PDHc assembly, presumably by selection of catalytically competent loop conformations. This mechanism could be a general feature of 2-oxoacid dehydrogenase complexes because such interfacial dynamic regions are highly conserved

Fig3B_Glycine

Microsoft excel file containing raw data for Fig3B Glycine

Effect of Jute as Fiber Reinforcement Controlling the Hydration Characteristics of Cement Matrix

The present investigation deals with the effect of jute as a natural fiber reinforcement on the setting and hydration behavior of cement. The addition of jute fiber in cement matrix increases the setting time and standard water consistency value. The hydration characteristics of fiber reinforced cement were investigated using a variety of analytical techniques including thermal, infrared spectroscopy, X-ray diffraction, and free lime estimation by titration. Through these analyses it was demonstrated that the hydration kinetics of cement is retarded with the increase in jute contents in cement matrix. A model has been proposed to explain the retarded hydration kinetics of jute fiber reinforced cement composites. The prolonged setting of these fiber reinforced cement composites would be beneficial for applications where the premixed cement aggregates are required to be transported from a distant place to the construction site

Adsorption of Anionic-Azo Dye from Aqueous Solution by Lignocellulose-Biomass Jute Fiber: Equilibrium, Kinetics, and Thermodynamics Study

The present investigation describes the evaluation of feasibility of lignocellulosic-biomass jute fiber (JF) toward adsorptive removal of anionic-azo dye from aqueous solution. Batch studies illustrated that dye uptake was highly dependent on different process variables, pH, initial dye concentration of solution, adsorbent dosage, and temperature. Further, an attempt has been taken to correlate these process variables with dye absorption and was optimized through a full-factorial central composite design (CCD) in response surface methodology (RSM). Maximum adsorption capacity (29.697 mg/g) under optimum conditions of variables (pH 3.91, adsorbent dose 2.04 g/L, adsorbate concentration 244.05 mg/L, and temperature 30 °C), as predicted by RSM, was found to be very close to the experimentally determined value (28.940 mg/g). Exothermic and spontaneous nature of adsorption was revealed from thermodynamic study. Equilibrium adsorption data were highly consistent with Langmuir isotherm yielding <i>R</i>² = 0.999. Kinetic studies revealed that adsorption followed pseudo second-order model regarding the intraparticle diffusion. Activation parameters for the adsorption process were computed using Arrhenius and Eyring equations. Maximum desorption efficiency of spent adsorbent was achieved using sodium hydroxide solution (0.1 M)

Pincer-Ligated Nickel Hydridoborate Complexes: the Dormant Species in Catalytic Reduction of Carbon Dioxide with Boranes

Nickel pincer complexes of the type [2,6-(R₂PO)₂C₆H₃]­NiH (R = ^tBu, <b>1a</b>; R = ⁱPr, <b>1b</b>; R = ^cPe, <b>1c</b>) react with BH₃·THF to produce borohydride complexes [2,6-(R₂PO)₂C₆H₃]­Ni­(η²-BH₄) (<b>2a</b>–<b>c</b>), as confirmed by NMR and IR spectroscopy, X-ray crystallography, and elemental analysis. The reactions are irreversible at room temperature but reversible at 60 °C. Compound <b>1a</b> exchanges its hydrogen on the nickel with the borane hydrogen of 9-BBN or HBcat, but does not form any observable adduct. The less bulky hydride complexes <b>1b</b> and <b>1c</b>, however, yield nickel dihydridoborate complexes reversibly at room temperature when mixed with 9-BBN and HBcat. The dihydridoborate ligand in these complexes adopts an η²-coordination mode, as suggested by IR spectroscopy and X-ray crystallography. Under the catalytic influence of <b>1a</b>–<b>c</b>, reduction of CO₂ leads to the methoxide level when 9-BBN or HBcat is employed as the reducing agent. The best catalyst, <b>1a</b>, involves bulky substituents on the phosphorus donor atoms. Catalytic reactions involving <b>1b</b> and <b>1c</b> are less efficient because of the formation of dihydridoborate complexes as the dormant species as well as partial decomposition of the catalysts by the boranes

Fig1A

Microsoft excel file containing raw data for Fig1a