Stepwise Hydrogen Transfer Research Articles

Highly Poison-Resistant Single-Atom Co-N4 Active Sites with Superior Operational Stability over 460h for H2 S Catalytic Oxidation.

Efficient catalytic elimination of hydrogen sulfide (H2 S) with high activity and durability in nature gas and blast-furnace gas is very critical for both fundamental catalytic research and applied environmental chemistry. Herein, atomically dispersed Co atom catalysts with Co-N4 sites that can transform H2 S into S with conversion rate of ≈100% are designed and prepared. The representative 4Co-N/NC achieves a sulfur yield of nearly 100% and TOF(Co) of 869h-1 at 180°C. Importantly, remarkable long-term durability is achieved as well, with no obvious loss of catalytic activity in the run of 460h, outperforming most of the reported catalysts. The short bond length and strong cooperation of Co-N are beneficial to improve the structural stability of the Co-N4 centers, and significantly enhanced resistance of water and sulfation over single-atom Co-catalyst. The present mechanism involves the stepwise hydrogen transfer process via the adsorbed *HOO and *HS intermediates.

Synthesis, Electrocatalytic and Gas Transport Characteristics of Pentagonally Structured Star-Shaped Nanocrystallites of Pd-Ag.

The method of synthesis of bimetallic Pd–Ag pentagonally structured catalyst “nanostar” on the surface of Pd-23%Ag alloy films has been developed. The resulting catalyst was studied as a highly active functional layer for methanol oxidation reaction (MOR) in alkaline media and the intensification of hydrogen transport through the Pd-23%Ag membrane in the processes of hydrogen diffusion purification. A modifying layer with a controlled size, composition and excellent electrocatalytic activity was synthesized by electrochemical deposition at a reduced current density compared to classical methods. The low deposition rate affects the formation of pentagonally structured nanocrystallites, allowing Pd and Ag particles to form well-defined structures due to the properties of the surfactant used. Electrochemical studies have demonstrated that the catalyst synthesized by the “nanostar” method shows better electrocatalytic activity in relation to MOR and demonstrates a higher peak current (up to 17.82 µA cm−2) in comparison with one for the catalyst synthesized by the “nanoparticle” method (up to 10.66 µA cm−2) in a cyclic voltammetric study. The nanostar catalyst electrode releases the highest current density (0.25 µA cm−2) for MOR and demonstrates higher catalytic activity for the oxidation of possible intermediates such as sodium formate in MOR. In the processes of diffusion membrane purification of hydrogen, a multiple increase in the density of the penetrating flux of hydrogen through the membranes modified by the “nanostar” catalyst (up to 10.6 mmol s−1 m−2) was demonstrated in comparison with the membranes modified by the “nanoparticles” method (up to 4.49 mmol s−1 m−2). Research data may indicate that the properties of the developed pentagonally structured catalyst “nanostar” and its enhanced activity with respect to reactions involving hydrogen increase the desorption activity of the membrane, which ultimately accelerates the overall stepwise transfer of hydrogen across the membrane.

Ammonia Borane Dehydrogenation Promoted by a Pincer-Square-Planar Rhodium(I) Monohydride: A Stepwise Hydrogen Transfer from the Substrate to the Catalyst.

The pincer d(8)-monohydride complex RhH{xant(P(i)Pr2)2} (xant(P(i)Pr2)2 = 9,9-dimethyl-4,5-bis(diisopropylphosphino)xanthene) promotes the release of 1 equiv of hydrogen from H3BNH3 and H3BNHMe2 with TOF50% values of 3150 and 1725 h(-1), to afford [BH2NH2]n and [BH2NMe2]2 and the tandem ammonia borane dehydrogenation-cyclohexene hydrogenation. DFT calculations on the ammonia borane dehydrogenation suggest that the process takes place by means of cis-κ(2)-PP-species, through four stages including: (i) Shimoi-type coordination of ammonia borane, (ii) homolytic addition of the coordinated H-B bond to afford a five-coordinate dihydride-boryl-rhodium(III) intermediate, (iii) reductive intramolecular proton transfer from the NH3 group to one of the hydride ligands, and (iv) release of H2 from the resulting square-planar hydride dihydrogen rhodium(I) intermediate.

Ruthenium(II) Complexes Containing Lutidine-Derived Pincer CNC Ligands: Synthesis, Structure, and Catalytic Hydrogenation of C-N bonds.

A series of Ru complexes containing lutidine-derived pincer CNC ligands have been prepared by transmetalation with the corresponding silver-carbene derivatives. Characterization of these derivatives shows both mer and fac coordination of the CNC ligands depending on the wingtips of the N-heterocyclic carbene fragments. In the presence of tBuOK, the Ru-CNC complexes are active in the hydrogenation of a series of imines. In addition, these complexes catalyze the reversible hydrogenation of phenantridine. Detailed NMR spectroscopic studies have shown the capability of the CNC ligand to be deprotonated and get involved in ligand-assisted activation of dihydrogen. More interestingly, upon deprotonation, the Ru-CNC complex 5 e(BF4 ) is able to add aldimines to the metal-ligand framework to yield an amido complex. Finally, investigation of the mechanism of the hydrogenation of imines has been carried out by means of DFT calculations. The calculated mechanism involves outer-sphere stepwise hydrogen transfer to the C-N bond assisted either by the pincer ligand or a second coordinated H2 molecule.

Computational Mechanistic Study of the Hydrogenation of Carbonate to Methanol Catalyzed by the RuIIPNN Complex

Density functional theory computations have been carried out to study the mechanism of hydrogenation-based transformation of dimethyl carbonate to methanol, catalyzed by Ru(II)PNN catalyst. The energetic results show that the catalytic transformation includes three sequential stages consistently involving the catalyst: (stage I) transformation of dimethyl carbonate (3) to methyl formate (5) and methanol; (stage II) transformation of methyl formate 5 to formaldehyde and methanol; (stage III) hydrogenation of formaldehyde to methanol. Stages I and II proceed similarly and follow three steps: hydrogen activation, formation of a hemiacetal intermediate via stepwise hydrogen transfer to dimethyl carbonate in stage I or methyl fomate in stage II, and subsequent decomposition of the hemiacetal intermediate to afford methanol. Hydrogenation via carbonyl insertion into the Ru-H bond is less favorable than the stepwise hydrogen-transfer mechanism. Decomposition of hemiacetal takes places by first breaking the hemiacetal O-H bond to give an alkoxide complex, followed by deprotonation of the benzylic arm ligand to the adjacent methoxy group. Comparing the hydrogenation steps in the three stages, hydrogenation in stage I is most difficult, that in stage II is less difficult, and that in stage III is easiest in terms of both kinetics and thermodynamics. This can be ascribed to the stronger electrophilicity of the carbonyl group in methyl formate or formaldehyde than that in dimethyl carbonate and fewer steric effects between the catalyst and methyl formate or formaldehyde than that between the catalyst and dimethyl carbonate. Thermodynamically, both stages I and II are uphill, but stage III is downhill significantly, which is the driving force for the catalytic transformation. The study indicates that the methanol product could facilitate the hydrogen activation involved in the transformation, implying that transformation could be accelerated by initially adding methanol.

Concerted or Stepwise Hydrogen Transfer in the Transfer Hydrogenation of Acetophenone Catalyzed by Ruthenium–Acetamido Complex: A Theoretical Mechanistic Investigation

In this paper, the mechanism of transfer hydrogenation of acetophenone catalyzed by ruthenium-acetamido complex was studied using density function theory (DFT) method. The catalytic cycle of transfer hydrogenation consists of hydrogen transfer (HT) step and dehydrogenation (DH) step of isopropanol (IPA). Inner sphere mechanism (paths 1 and 7) and outer sphere mechanism (paths 2-6) in HT step are fully investigated. Calculated results indicate that DH step of IPA (from (i)1 to (i)2) is the rate-determining step in the whole catalytic cycle, which has a potential energy barrier of 16.2 kcal/mol. On the other hand, the maximum potential energy barriers of paths 1-7 in the HT step are 5.9, 12.7, 24.4, 16.8, 23.7, 7.2, and 6.1 kcal/mol, respectively. The inner sphere pathways (paths 1 and 7) are favorable hydrogen transfer modes compared with outer sphere pathways, and the proton transferred to the oxygen atom of acetophenone comes from the hydroxyl group but not from amino group of acetamido ligand. Those theoretical results are in agreement with experimental report. However, in view of this DFT study in the inner sphere mechanism of HT step, hydride transfer and proton transfer are concerted and asynchronous hydrogen transfer but not a stepwise one, and hydride transfer precedes proton transfer in this case.

Nature of Asynchronous Hydrogen Transfer in Ketone Hydrogenation Catalyzed by Ru Complex

Three different reaction pathways—concerted hydride transfer, concerted proton transfer, and stepwise hydrogen transfer—in the carbonyl hydrogenation step of the catalytic cycle for ketone hydrogenation catalyzed by Ru complexes are investigated in this work. Our results from this study suggest that hydride and proton asynchronous transfers in the ketone hydrogenation process are stepwise in general. It is the nature of the substrate, electron withdrawing effect, and characteristics of catalytic ligands that make one of two transition states disappear in the concerted mechanism.

Mechanistic Study of Hydrogen Transfer to Imines from a Hydroxycyclopentadienyl Ruthenium Hydride. Experimental Support for a Mechanism Involving Coordination of Imine to Ruthenium Prior to Hydrogen Transfer

Reaction of [2,3,4,5-Ph(4)(eta(5)-C(4)COH)Ru(CO)(2)H] (2) with different imines afforded ruthenium amine complexes at low temperatures. At higher temperatures in the presence of 2, the complexes decomposed to give [Ru(2)(CO)(4)(mu-H)(C(4)Ph(4)COHOCC(4)Ph(4))] (1) and free amine. Electron-rich imines gave ruthenium amine complexes with 2 at a lower temperature than did electron-deficient imines. The negligible deuterium isotope effect (k(RuHOH)/k(RuDOD) = 1.05) observed in the reaction of 2 with N-phenyl[1-(4-methoxyphenyl)ethylidene]amine (12) shows that neither hydride (RuH) nor proton (OH) is transferred to the imine in the rate-determining step. In the dehydrogenation of N-phenyl-1-phenylethylamine (4) to the corresponding imine 8 by [2,3,4,5-Ph(4)(eta(4)-C(4)CO)Ru(CO)(2)] (A), the kinetic isotope effects observed support a stepwise hydrogen transfer where the isotope effect for C-H cleavage (k(CHNH)/k(CDNH) = 3.24) is equal to the combined (C-H, N-H) isotope effect (k(CHNH)/k(CDND) = 3.26). Hydrogenation of N-methyl(1-phenylethylidene)amine (14) by 2 in the presence of the external amine trap N-methyl-1-(4-methoxyphenyl)ethylamine (16) afforded 90-100% of complex [2,3,4,5-Ph(4)(eta(4)-C(4)CO)]Ru(CO)(2)NH(CH(3))(CHPhCH(3)) (15), which is the complex between ruthenium and the amine newly generated from the imine. At -80 degrees C the reaction of hydride 2 with 4-BnNH-C(6)H(9)=NPh (18), with an internal amine trap, only afforded [2,3,4,5-Ph(4)(eta(4)-C(4)CO)](CO)(2)RuNH(Ph)(C(6)H(10)-4-NHBn) (19), where the ruthenium binds to the amine originating from the imine, showing that neither complex A nor the diamine is formed. Above -8 degrees C complex 19 rearranged to the thermodynamically more stable [Ph(4)(eta(4)-C(4)CO)](CO)(2)RuNH(Bn)(C(6)H(10)-4-NHPh) (20). These results are consistent with an inner sphere mechanism in which the substrate coordinates to ruthenium prior to hydrogen transfer and are difficult to explain with the outer sphere pathway previously proposed.

Transfer Hydrogenation of Carbonyl Compounds Catalyzed by a Ruthenium−Acetamido Complex: Evidence for a Stepwise Hydrogen Transfer Mechanism

The ruthenium−acetamido complex, prepared from the reaction of [(PCy3)2(CO)(CH3CN)2RuH]+BF4- with KOH in 2-propanol, was found to be an effective catalyst for the transfer hydrogenation of carbonyl compounds and imines. Observation of both the inverse deuterium isotope effect and the competitive inhibition by added phosphine provided strong evidence for a stepwise mechanism of proton and hydride transfer via a coordinatively unsaturated ruthenium−amido species.

In situ polymerization of functional monomers in rubbers: 1. Modification of silicone rubbers by a poly(ester thioether amine) based on piperazine

Modified silicone rubber intended mainly for biomedical applications was obtained by in situ polymerization. The process involved swelling of the rubber with a mixture of solvent and monomers, polymerization, and finally evaporating the solvent under reduced pressure. The polymerization system selected was the stepwise hydrogen-transfer polyaddition of 1,4-piperazinediyl diethanethiol to ethylene glycol dimethacrylate, leading to a poly(ester thioether amine). The modified silicone was physically and mechanically characterized by several analytical techniques (Fourier-transform infra-red spectroscopy, differential scanning calorimetry, dynamic mechanical thermal analysis, dynamic contact angle and scanning electron microscopy), and the results obtained were compared with those relative to both the linear guest polymer and the unmodified rubber.