Synergistic Effect of Pendant N Moieties for Proton Shuttling in the Dehydrogenation of Formic Acid Catalyzed by Biomimetic IrIII Complexes

Hydrogenation of CO2 to formate catalyzed by a Ru catalyst supported on a copolymerized porous organic polymer

The catalytic hydrogenation of carbon dioxide to formate is of great interest due to its significant role in CO2 utilization. In this study, a novel heterogeneous Ru(III) catalyst was prepared by immobilizing RuCl3 on a porous organic polymer (POP) obtained from 1,4-phthalaldehyde (PTA) and 4,4'-biphenyldicarboxaldehyde (BPDA) with melamine. A copolymerization strategy utilizing monomers of varying lengths was employed to prepare the POP-supported Ru catalyst with adjustable porosity. The optimization of the framework porosity resulted in enhanced CO2 affinity, accelerated mass transfer, and a remarkable enhancement in catalytic activity. A high turnover number (TON) of 2458 was achieved for the CO2 hydrogenation to formate in 2 h with catalyst Cat-3 under 3 MPa (CO2/H2 = 1 : 1) at 120 °C in 1 M Et3N aqueous solution. Moreover, the Cat-3 demonstrated good recyclability and was able to be reused for five consecutive runs, resulting in a high total TON of 9971.

Formic Acid Dehydrogenation through Ligand Design Strategy of Amidation in Half-Sandwich Ir Complexes

A series of Cp*Ir (Cp* = pentamethylcyclopentadienyl) complexes with amidated 8-aminoquinoline ligands were synthesized and tested for formic acid (FA) dehydrogenation. These complexes showed improved activities compared to pristine 8-anminquinoline (L1). Specially, amidation changed the outer coordination sphere of the complex (3) bearing N-8-quinolinylformamide (L3), and 3 was proved to be a proton-responsive catalyst. Our experimental results and DFT calculations demonstrated that the deprotonated carbanion in L3 could interact with a water molecule to stabilize the transition states and lower the reaction energy barrier, which improved the reaction activity. A turnover frequency of 206250 h-1 was achieved by 3 under optimized conditions. This study presents a method to develop new ligands and modify the existing ligands for efficient FA dehydrogenation.

N-Site Regulation of Pyridyltriazole in Cp*Ir(N̂N)(H2O) Complexes Achieving Catalytic FA Dehydrogenation

A series of novel Cp*Ir complexes with nitrogen-rich N̂N bidentate ligands were developed for the catalytic dehydrogenation of formic acid in water under base-free conditions. These complexes were synthesized by using pyridyl 1,2,4-triazole, methylated species, or pyridyl 1,2,3-triazole as a N-site regulation ligand and were fully characterized. Complex 1-H2O bearing 1,2,4-triazole achieved a high turnover frequency of 14192 h-1 at 90 °C in 4 M FA aqueous solution. The terminal and bridged Ir-H intermediates of 1-H2O were successfully detected by 1H NMR and mass spectrometry measurements. Kinetic isotope effect experiments and density functional theory (DFT) calculations were performed; then a plausible mechanism was proposed involving the β-hydride elimination and formation of H2. Water-assisted H2 release was proven to be the rate-determining step of the reaction. The distribution of Mulliken charges on N atoms of triazole ligand internally revealed that the ortho site N2 of 1-H2O with a higher electron density was conducive to efficient proton transfer. Additionally, the advantage of water-assisted short-range bridge of 1,2,4-triazole moieties led to a higher catalytic activity of 1-H2O. This study demonstrated the effectiveness of nitrogen-rich ligands on FA dehydrogenation and revealed a good strategy for N site regulation in the development of new homogeneous catalysts.

Iridium-(κ2-NSi) catalyzed dehydrogenation of formic acid: effect of auxiliary ligands on the catalytic performance

The iridium(III) complexes [Ir(H)(Cl)(κ²-NSi^tBu₂)(κ²-bipy^Me₂)] (2) and [Ir(H)(OTf)(κ²-NSi^tBu₂)(κ²-bipy^Me₂)] (3) (NSi^tBu₂ = {4-methylpyridine-2-yloxy}ditertbutylsilyl) have been synthesized and characterized including X-ray studies of 3. A comparative study of the catalytic activity of complexes 2, 3, [Ir(H)(OTf)(κ²-NSi^tBu₂)(coe)] (4), and [Ir(H)(OTf)(κ²-NSi^tBu₂)(PCy₃)] (5) (0.1 mol%) as catalysts precursors for the solventless formic acid dehydrogenation (FADH) in the presence of Et₃N (40 mol%) at 353 K has been performed. The highest activity (TOF_{5 min} ≈ 3260 h^-1) has been obtained with 3 at 373 K. However, at that temperature the FTIR spectra show traces of CO together with the desired products (H₂ and CO₂). Thus, the best performance was achieved at 353 K (TOF_{5 min} ≈ 1210 h^-1 and no observable CO). Kinetic studies at variable temperature show that the activation energy of the 3-catalyzed FADH process is 16.76 kcal mol^-1. Kinetic isotopic effect (5 min) values of 1.6, 4.5, and 4.2 were obtained for the 3-catalyzed dehydrogenation of HCOOD, DCOOH, and DCOOD, respectively, at 353 K. The strong KIE found for DCOOH and DCOOD evidenced that the hydride transfer from the C-H bond of formic acid to the metal is the rate-determining step of the process.

Recent Developments in Reactions and Catalysis of Protic Pyrazole Complexes

Protic pyrazoles (N-unsubstituted pyrazoles) have been versatile ligands in various fields, such as materials chemistry and homogeneous catalysis, owing to their proton-responsive nature. This review provides an overview of the reactivities of protic pyrazole complexes. The coordination chemistry of pincer-type 2,6-bis(1H-pyrazol-3-yl)pyridines is first surveyed as a class of compounds for which significant advances have made in the last decade. The stoichiometric reactivities of protic pyrazole complexes with inorganic nitrogenous compounds are then described, which possibly relates to the inorganic nitrogen cycle in nature. The last part of this article is devoted to outlining the catalytic application of protic pyrazole complexes, emphasizing the mechanistic aspect. The role of the NH group in the protic pyrazole ligand and resulting metal-ligand cooperation in these transformations are discussed.

A ligand design strategy to enhance catalyst stability for efficient formic acid dehydrogenation

New Ir complexes bearing N-(methylsulfonyl)-2-pyridinecarboxamide (C1) and N-(phenylsulfonyl)-2-pyridinecarboxamide (C2) were employed as catalysts for aqueous formic acid dehydrogenation (FADH). The ligands were designed to maintain the picolinamide skeleton and introduce strong sigma sulfonamide moieties. C1 and C2 exhibited good stability towards air and concentrated formic acid (FA). During 20 continuous cycles, C1 and C2 could achieve the complete conversion of FA with TONs of 172 916 and 172 187, respectively. C1 achieved a high TOF of 19 500 h^-1 at 90 °C and an air-stable Ir-H species was observed by ¹H NMR spectroscopy.

Dehydrogenation of Formic Acid Using Iridium-NSi Species as Catalyst Precursors

Using a low loading of the iridium(III) complexes [Ir(CF3SO3)(κ2-NSiiPr)2] (1) (NSiiPr = (4-methylpyridin-2-iloxy)diisopropylsilyl and [{Ir(κ2-NSiMe)2}2(µ-CF3SO3)2] (2) (NSiMe = (4-methylpyridin-2-iloxy)dimethylsilyl) in presence of Et3N, it has been possible to achieve the...

Recent Advances in Homogeneous/Heterogeneous Catalytic Hydrogenation and Dehydrogenation for Potential Liquid Organic Hydrogen Carrier (LOHC) Systems

Here, we review liquid organic hydrogen carriers (LOHCs) as a potential solution to the global warming problem due to the increased use of fossil fuels. Recently, hydrogen molecules have attracted attention as a sustainable energy carrier from renewable energy-rich regions to energy-deficient regions. The LOHC system is one a particularly promising hydrogen storage system in the “hydrogen economy”, and efficient hydrogen mass production that generates only benign byproducts can be applied in the industry. Therefore, this article presents hydrogenation and dehydrogenation, using homogeneous or heterogeneous catalysts, for several types of LOHCs, including formic acid/formaldehyde/ammonia, homocyclic compounds, nitrogen- and oxygen-containing compounds. In addition, it introduces LOHC system reactor types.

DFT Probe into the Mechanism of Formic Acid Dehydrogenation Catalyzed by Cp*Co, Cp*Rh, and Cp*Ir Catalysts with 4,4'-Amino-/Alkylamino-Functionalized 2,2'-Bipyridine Ligands

The mechanistic landscape of H₂ generation from formic acid catalyzed by Cp*M(III) complexes (M = Co or Rh or Ir) with diamino-/dialkylamino-substituted 2,2'-bipyridine ligand architectures have been unveiled computationally. The calculations indicate that the β-hydride elimination process is the rate-determining step for all the investigated catalysts. The dialkylamino moieties on the 2,2'-bipyridine ligand were found to reduce the activation free energy required for the rate-limiting β-hydride elimination step and increase the hydridic nature of the Ir-hydride bond, which accounts for the experimentally observed enhanced catalytic activity. Furthermore, the protonation by H₃O⁺ ion was found to be the kinetically most favorable route than the conventional protonation by formic acid. The origin for this preference lies in the increased electrophilicity of the proton from hydronium ion which facilitates easy protonation of the metal-hydride with low activation energy barrier. The Co and Rh analogues of the chosen iridium catalyst were computationally designed and were estimated to possess a rate-determining activation barrier of 16.9 and 14.5 kcal/mol, respectively. This illustrates that these catalysts are potential candidates for FAD. The insights derived in this work might serve as a vital knowledge that could be capitalized upon for designing cost-effective catalyst for FAD in future.

Deciphering the Mechanistic Details of Manganese-Catalyzed Formic Acid Dehydrogenation: Insights from DFT Calculations

A comprehensive density functional theory investigation has been carried out to unravel the complete mechanistic landscape of aqueous-phase formic acid dehydrogenation (FAD) catalyzed by a pyridyl-imidazoline-based Mn(I) catalyst [Mn(PY-^NHIM)(CO)₃Br], which was recently reported by Beller and co-workers. The computed free energy profiles show that for the production of a Mn-formate intermediate [Mn(HCO₂^-)], a stepwise mechanism is both kinetically and thermodynamically favorable compared to the concerted mechanism. This stepwise mechanism involves the dissociation of a Br^- ion from a Mn-bromide complex [Mn(Br)] to create a vacant site and coordination of water solvent to this vacant site, followed by the dissociative exchange of the aqua ligand with the formate ion to form Mn(HCO₂^-). Non-covalent interaction analysis revealed that the steric hindrance at the transition state is the cardinal reason for the preference to a stepwise mechanism. The β-hydride elimination process was estimated to be the rate-determining step with a barrier of 19.0 kcal/mol. This confirms the experimental observation. The generation of a dihydrogen-bound complex was found to occur through the protonation of Mn-hydride by a hydronium ion instead of formic acid. The mechanistic details and insights presented in this work would promote future catalytic designing and exploration of earth-abundant Mn-based catalytic systems for potential applications toward FAD.

Formic Acid as a Potential On-Board Hydrogen Storage Method: Development of Homogeneous Noble Metal Catalysts for Dehydrogenation Reactions

Hydrogen can be used as an energy carrier for renewable energy to overcome the deficiency of its intrinsically intermittent supply. One of the most promising application of hydrogen energy is on-board hydrogen fuel cells. However, the lack of a safe, efficient, convenient, and low-cost storage and transportation method for hydrogen limits their application. The feasibility of mainstream hydrogen storage techniques for application in vehicles is briefly discussed in this Review. Formic acid (FA), which can reversibly be converted into hydrogen and carbon dioxide through catalysis, has significant potential for practical application. Historic developments and recent examples of homogeneous noble metal catalysts for FA dehydrogenation are covered, and the catalysts are classified based on their ligand types. The Review primarily focuses on the structure-function relationship between the ligands and their reactivity and aims to provide suggestions for designing new and efficient catalysts for H₂ generation from FA.