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.

On the mechanism of acceptorless dehydrogenation of N-heterocycles catalyzed by tBuOK: a computational study

The catalytic acceptorless dehydrogenation (ADH) of saturated N-heterocycles has recently gained considerable attention as a promising strategy for hydrogen release from liquid organic hydrogen carriers (LOHCs). Recently, a simple ^tBuOK base-promoted ADH of N-heterocycles was developed by Yu et al. (Adv. Synth. Catal. 2019, 361, 3958). However, it is still open as to how the ^tBuOK plays a catalytic role in the ADH process. Herein, our density functional study reveals that the ^tBuOK catalyzes the ADH of 1,2,3,4-tetrahydroquinoline (THQ) through a quasi-metal-ligand bifunctional catalytic channel or a base-catalyzed pathway with close energy barriers. The hydride transfer in the first dehydrogenation process is determined to be the rate determining step, and the second dehydrogenation can proceed directly from 34DHQ regulated by the ^tBuOK. In addition, the computational results show that the cooperation of a suitable alkali metal ion with the ^tBuO^- group is so critical that the ^tBuOLi and the isolated ^tBuO^- are both inferior to ^tBuOK as a dehydrogenation catalyst.

Iron Dihydride Complex Stabilized by an All-Phosphorus-Based Pincer Ligand and Carbon Monoxide

PNP-pincer-stabilized iron carbonyl dihydride complexes are key intermediates in catalytic hydrogenation and dehydrogenation reactions; however, decomposition through these intermediates has been observed. This inspires the development of a PPP-pincer system that may show improved catalyst stability. In this work, bis[2-(diisopropylphosphino)phenyl]phosphine (or ^iPrPP^HP) is used to react with FeCl₂ under a carbon monoxide (CO) atmosphere to yield trans-(^iPrPP^HP)Fe(CO)Cl₂. A subsequent reaction with NaBH₄ produces syn/anti-(^iPrPP^HP)FeH(CO)Cl or cis,anti-(^iPrPP^HP)Fe(CO)H₂, depending on the amount of NaBH₄ employed. The cis-dihydride complex shows catalytic activity for the conversion of PhCHO to PhCH₂OH (under H₂) or PhCO₂CH₂Ph (under Ar). It also catalyzes the dehydrogenation of PhCH₂OH to PhCHO and PhCO₂CH₂Ph, albeit with limited turnover numbers. A more efficient catalytic process is the dehydrogenation of formic acid to carbon dioxide (CO₂), which can operate under additive-free conditions. Mechanistic investigation suggests that the cis-dihydride complex undergoes protonation with formic acid to release H₂ while forming anti-(^iPrPP^HP)FeH(CO)(OCHO)·HCO₂H, in which the CO ligand has shifted and the formate is hydrogen-bonded to formic acid. The hydrido formate complex loses CO₂ under ambient conditions, completing the catalytic cycle by reforming the cis-dihydride complex.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

Mechanistic insights and computational design of half-sandwich iridium and rhodium complexes for hydrogenation of quinoline

DFT calculations reveal a stepwise proton and hydride transfer mechanism for the hydrogenation of quinoline to 1,2,3,4-tetrahydroquinoline catalysed by half-sandwich Cp*Ir(NHC) complexes.

Transition metal-catalyzed dehydrogenation of amines

This review focuses on the use of homogeneous transition metal complexes for the catalytic dehydrogenation of amines for synthetic purposes, and for hydrogen storage applications. The catalytic dehydrogenation of primary, secondary and cyclic amines is reviewed looking at reaction conditions, different catalysts and common side reactions. Recent developments in this active field of research showcase how cooperative ligands and photocatalysts can overcome the need for noble metals or harsh reaction conditions.