Hydrogenation Of N-heterocycles Research Articles

Thermodynamic evaluations of the acceptorless dehydrogenation and hydrogenation of pre-aromatic and aromatic N-heterocycles in acetonitrile.

N-heterocycles are important chemical hydrogen-storage materials, and the acceptorless dehydrogenation and hydrogenation of N-heterocycles as organic hydrogen carriers have been widely studied, with the main focus on the catalyst synthesis and design, investigation of the redox mechanisms, and extension of substrate scope. In this work, the Gibbs free energies of the dehydrogenation of pre-aromatic N-heterocycles (YH2) and the hydrogenation of aromatic N-heterocycles (Y), i.e., ΔGH2R(YH2) and ΔGH2A(Y), were derived by constructing thermodynamic cycles using Hess' law. The thermodynamic abilities for the acceptorless dehydrogenation and hydrogenation of 78 pre-aromatic N-heterocycles (YH2) and related 78 aromatic N-heterocycles (Y) were well evaluated and discussed in acetonitrile. Moreover, the applications of the two thermodynamic parameters in identifying pre-aromatic N-heterocycles possessing reversible dehydrogenation and hydrogenation properties and the selection of the pre-aromatic N-heterocyclic hydrogen reductants in catalytic hydrogenation were considered and are discussed in detail. Undoubtedly, this work focuses on two new thermodynamic parameters of pre-aromatic and aromatic N-heterocycles, namely ΔGH2R(YH2) and ΔGH2A(Y), which are important supplements to our previous work to offer precise insights into the chemical hydrogen storage and hydrogenation reactions of pre-aromatic and aromatic N-heterocycles.

Hydrogenation of N-Heterocycles Using Frustrated Lewis Pairs Incorporated in NU-1000 MOF

Hydrogenation of N-Heterocycles Using Frustrated Lewis Pairs Incorporated in NU-1000 MOF

Poisoning and Reuse of Supported Precious Metal Catalysts in the Hydrogenation of N-Heterocycles, Part II: Hydrogenation of 1-Methylpyrrole over Rhodium

Poisoning effect of nitrogen on heterogeneous, supported precious metal catalysts, along with their recycling, was further examined in the liquid-phase hydrogenation of 1-methylpyrrole (MP) to 1-methylpyrrolidine (MPD) over rhodium on carbon or γ-alumina, in methanol, under non-acidic conditions, at 25–50 °C and 10 bar. Reusing a spent, unregenerated 5% Rh/C or 5% Rh/γ-Al2O3 catalyst, it was found that the conversion of this model substrate and the activity of the catalyst were strongly dependent on the amount of catalyst, the type of support, the catalyst pre- or after-treatment, the temperature, and the number of recycling, respectively. An unexpected catalytic behaviour of rhodium was observed when it was used in a prehydrogenated form, because no complete conversion of MP was achieved over even the fresh Rh/C or Rh/γ-Al2O3, contrary to the untreated one. In addition, there was a significant difference in the reusability and activity of these rhodium catalysts, depending on their supports (activated carbon, γ-alumina). These diversions were elucidated by applying dispersion (O2- and H2-titration), temperature-programmed desorption of ammonia (NH3-TPD), and transmission electron microscopy (TEM) measurements.

Dual Role of Graphene as Support of Ligand-Stabilized Palladium Nanoparticles and Carbocatalyst for (De)Hydrogenation of N-Heterocycles

Dual Role of Graphene as Support of Ligand-Stabilized Palladium Nanoparticles and Carbocatalyst for (De)Hydrogenation of N-Heterocycles

Nitrogen-coordinated cobalt nanocrystals for oxidative dehydrogenation and hydrogenation of N-heterocycles.

To endow non-noble metals with the high catalytic activity that is typically exhibited by noble metals is the central yet challenging aim for substituting noble metals. In this regard, by exploiting the coordination effect of nitrogen, we prepared cobalt nanocrystals stabilized by nitrogen-doped graphitized carbon (Co NCs/N-C). The obtained Co NC/N-C catalyst showed extraordinary performances toward both oxidative dehydrogenation of N-heterocycles and its reverse hydrogenation process under extremely mild conditions. A nearly quantitative conversion could be achieved for oxidative dehydrogenation even at room temperature (25 °C), for which the coordination effect of nitrogen is responsible: the interaction of Co-N induces a partial positive charge on the Co surface, thereby promoting the reaction. In contrast, cobalt nanocrystals supported by pristine carbon (Co NCs/C) proved to be inactive for oxidative dehydrogenation, owing to the lack of nitrogen. Moreover, in Co NCs/N-C, the N-doped graphitized carbon formed a protective layer for Co NCs, which preserved the active valence of Co species and prevented the catalyst from leaching. It was found that the catalyst still retained its excellent catalytic activity after five regeneration cycles; in comparison, its cobaltous oxide counterpart (CoO x /N-C) was barely active. As for the mechanism, electron paramagnetic resonance (EPR) analysis revealed the formation of superoxide anion radicals during the dehydrogenation process. Interestingly, the pressure of feed hydrogen had little effect on the hydrogenation process. Our Co NC/N-C catalyst is capable of activating molecular oxygen and hydrogen as effectively as noble metals; the coordination effect of nitrogen and the protection by the carbon layer in combination confer tremendous potential on the Co NCs/N-C for substituting noble-metal-based catalysts and soluble catalysts for homogeneous reactions.

Mechanisms for dehydrogenation and hydrogenation of N-heterocycles using PNP-pincer-supported iron catalysts: a density functional study.

The catalytic dehydrogenation and hydrogenation of N-heterocycles have potential applications in organic hydrogen storage. Recently, Fe(HPNP)(CO)(H)(HBH3) (cp1) and Fe(HPNP)(CO)(H)(Br) (cp2), the iron(ii) complexes supported by bis(phosphino)amine pincer (Fe-PNP) (PNP = N(CH2CH2P(i)Pr2)2), have been reported to be the starting complexes which can catalyze the dehydrogenation and hydrogenation of N-heterocycles. The active species were proposed to be the trans-dihydride complexes, Fe(HPNP)(CO)(H)2 (cp4) and Fe(PNP)(CO)(H) (cp3), which can be interconverted. Here, our density functional study revealed that the N-heterocyclic substrate plays a role in the formation of cp4 from cp1, while the tert-butoxide base assists with the formation of cp3 from cp2. The mechanism for cp3 catalyzed dehydrogenation of a 1,2,3,4-tetrahydroquinoline (THQ) substrate to quinoline (Q) involves two main steps: (i) dehydrogenation of THQ to 3,4-dihydroquinoline (34DHQ) and (ii) dehydrogenation of 34DHQ to Q. In each dehydrogenation step, the proton is transferred from the substrate to the N of the PNP ligand of cp3. An ion-pair complex between Fe-PNP and the deprotonated substrate is then formed before the hydride at the adjacent C is transferred to Fe. Notably, the isomerization of 34DHQ to 14DHQ or 12DHQ is not necessary, as the bifunctionality of Fe-PNP in cp3 can stabilize the ion-pair complex and facilitate direct dehydrogenation of the C3-C4 bond in 34DHQ. On the other hand, the mechanism for hydrogenation of Q involves the initial formation of 14DHQ, which can easily isomerize to 34DHQ with the assistance of a tert-butoxide base. Finally, 34DHQ is dehydrogenated to THQ. As the overall energy barriers for cp3 catalyzed dehydrogenation of THQ (+27.6 kcal mol(-1)) and cp4 catalyzed hydrogenation of Q (+23.8 kcal mol(-1)) are only slightly different, reaction conditions can be conveniently adjusted to favor either the dehydrogenation or hydrogenation process. Insights into the role of metal-ligand cooperativity in Fe-PNP complexes in promoting the dehydrogenation and the hydrogenation of N-heterocycles should benefit the development of efficient catalysts for organic hydrogen storage.

A molecular iron catalyst for the acceptorless dehydrogenation and hydrogenation of N-heterocycles.

A well-defined iron complex (3) 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 3, the active catalytic species in the dehydrogenation reaction, is independently synthesized and characterized, and its structure is confirmed by X-ray crystallography. A trans-dihydride intermediate (4) is proposed to be involved in the hydrogenation reaction, and its existence is verified by NMR and trapping experiments.

Rate and Stereoselectivity Changes During Hydrogenation of n-Heterocycles

High diastereoselectivities were obtained in the heterogeneous catalytic hydrogenation of chiral pyrrole and pyridine derivatives with complete conversion, in non-acidic medium. The products of the hydrogenations, secondary and tertiary amines act as catalyst modifiers, increasing the d.e. values in the last period of the reactions.