Additive-Free Formic Acid Dehydrogenation Catalyzed by a Cobalt Complex

Formoxyboranes as hydroborane surrogates for the catalytic reduction of carbonyls through transfer hydroboration

A new class of Lewis base stabilized formoxyboranes demonstrates the feasibility of catalytic transfer hydroboration. In the presence of a ruthenium catalyst, they have shown broad applicability for reducing carbonyl compounds. Various borylated alcohols are obtained in high selectivity and yields up to 99%, tolerating several functional groups. Computational studies enabled to propose a mechanism for this transformation, revealing the role of the ruthenium catalyst and the absence of hydroborane intermediates.

Catalytic Dehydrogenation of Formic Acid Promoted by Triphos-Co Complexes: Two Competing Pathways for H2 Production

In this study, we reported the synthesis and structural characterization of a triphos-CoII complex [(κ3-triphos)CoII(CH3CN)2]2+ (1) and a triphos-CoI-H complex [(κ2-triphos)HCoI(CO)2] (4). The facile synthetic pathways from 1 to [(κ3-triphos)CoII(κ2-O2CH)]+ (1') and [(κ3-triphos)CoI(CH3CN)]+ (2), respectively, as well as the interconversion between [(κ3-triphos)CoI(CO)2]+ (3) and 4 have been established. The activation energy barrier, associated with the dehydrogenation of a coordinated formate fragment in 1' yielding the corresponding 2 accompanied by the formation of H2 and CO2, was experimentally determined as 23.9 kcal/mol. With 0.01 mol % loading of 1, a maximum TON ∼ 1735 within 18 h and TOF ∼ 483 h-1 for the first 3 h could be achieved. Kinetic isotope effect (KIE) values of 2.25 (kHCOOH/kDCOOH) and 1.36 (kHCOOH/kHCOOD) for the dehydrogenation of formic acid and its deuterated derivatives, respectively, implicate that the H-COOH bond cleavage is likely the rate-determining step. The catalytic mechanism proposed by density functional theory (DFT) calculations coupled with experimental 1H NMR and gas chromatography-mass spectrometry (GC-MS) analysis unveils two competing pathways for H2 production; specifically, deprotonating a HCOO-H bond by a proposed Co-H intermediate C and homolytic cleavage of the CoII-H moiety of C, presumably via a dimeric Co intermediate D containing a [Co2(μ-H)2]2+ core, to yield the corresponding 2 and H2.

Formate dehydrogenase activity by a Cu(II)-based molecular catalyst and deciphering the mechanism using DFT studies

Due to the requirement to establish renewable energy sources, formic acid (FA), one of the most probable liquid organic hydrogen carriers (LOHCs), has received great attention. Catalytic formic acid dehydrogenation in an effective and environmentally friendly manner is still a challenge. The N3Q3 ligand (N3Q3 = N,N-bis(quinolin-8-ylmethyl)quinolin-8-amine) and the square pyramidal [Cu(N3Q3)Cl]Cl complex have been synthesised in this work and characterised using several techniques, such as NMR spectroscopy, mass spectrometry, EPR spectroscopy, cyclic voltammetry, X-ray diffraction and DFT calculations. This work investigates the dehydrogenation of formic acid using a molecular and homogeneous catalyst [Cu(N3Q3)Cl]Cl in the presence of HCOONa. The mononuclear copper complex exhibits catalytic activity towards the dehydrogenation of formic acid in H2O with the evolution of a 1 : 1 CO2 and H2 mixture. The activation energy of formic acid dehydrogenation was calculated to be Ea = 86 kJ mol-1, based on experiments carried out at various temperatures. The Gibbs free energy was found to be 82 kJ at 298 K for the decomposition of HCOOH. The DFT studies reveal that [Cu(N3Q3)(HCOO-)]+ undergoes an uphill process of rearrangement followed by decarboxylation to generate [Cu(N3Q3)(H-)]+. The initial uphill step for forming a transition state is the rate-determining step. The [Cu(N3Q3)(H-)]+ follows an activated state in the presence of HCOOH to liberate H2 and generate the [Cu(N3Q3)(OH2)]2+.

On the Demise of PPP-Ligated Iron Catalysts in the Formic Acid Dehydrogenation Reaction

The PPP-ligated iron complexes, cis-(iPrPPRP)FeH2(CO) [iPrPPRP = (o-iPr2PC6H4)2PR (R = H or Me)], catalyze the dehydrogenation of formic acid to carbon dioxide but lose their catalytic activity over time. This study focuses on the analysis of the species formed from the degradation of cis-(iPrPPMeP)FeH2(CO) over its course of catalyzing the dehydrogenation reaction. These degradation products include species both soluble and insoluble in the reaction medium. The soluble component of the decomposed catalyst is a mixture of cis-[(iPrPPMeP)FeH(CO)2][(HCO2)(HCO2H)x], protonated iPrPPMeP, and oxidation products resulting from adventitious O2. The precipitate is solvated Fe(OCHO)2. Further mechanistic investigation suggests that cis-[(iPrPPMeP)FeH(CO)2][(HCO2)(HCO2H)x] displays diminished but measurable catalytic activity, likely through the displacement of a CO ligand by the formate ion. The formation of Fe(OCHO)2 along with the dissociation of iPrPPMeP is responsible for the eventual loss of catalytic activity.

Efficient additive-free formic acid dehydrogenation with a NNN-ruthenium complex

A new ruthenium complex containing a pyridylidene amine-based NNN ligand was developed as a catalyst precursor for formic acid dehydrogenation, which, as a rare example, does not require basic additives to display high activity (TOF ∼10 000 h-1). Conveniently, the complex is air-stable, but sensitive to light. Mechanistic investigations using UV-vis and NMR spectroscopic monitoring correlated with gas evolution profiles indicate rapid and reversible protonation of the central nitrogen of the NNN ligand as key step of catalyst activation, followed by an associative step for formic acid dehydrogenation.

Methyl Effects on the Stereochemistry and Reactivity of PPP-Ligated Iron Hydride Complexes

Iron dihydride complexes are key intermediates in many iron-catalyzed reactions. Previous efforts to study molecules of this type have led to the discovery of a remarkably stable cis-FeH₂ complex, which is supported by bis[2-(diisopropylphosphino)phenyl]phosphine (^iPrPP^HP) along with CO. In this work, the hydrogen on the central phosphorus has been replaced with a methyl group, and the corresponding iron carbonyl dichloride, hydrido chloride, and dihydride complexes have been synthesized. The addition of the methyl group favors the anti configuration for the Me-P-Fe-H moiety and the trans geometry for the H-Fe-CO motif, which is distinctively different from the ^iPrPP^HP system. Furthermore, it increases the thermal stability of the dihydride complex, cis-(^iPrPP^MeP)Fe(CO)H₂ (^iPrPP^MeP = bis[2-(diisopropylphosphino)phenyl]methylphosphine). The variations in stereochemistry and compound stability contribute greatly to the differences between the two PPP systems in reactions with PhCHO, CS₂, and HCO₂H.

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.

Recent Progress in Homogeneous Catalytic Dehydrogenation of Formic Acid

Recently, there has been a strong demand for technologies that use hydrogen as an energy carrier, instead of fossil fuels. Hence, new and effective hydrogen storage technologies are attracting increasing attention. Formic acid (FA) is considered an effective liquid chemical for hydrogen storage because it is easier to handle than solid or gaseous materials. This review presents recent advances in research into the development of homogeneous catalysts, primarily focusing on hydrogen generation by FA dehydrogenation. Notably, this review will aid in the development of useful catalysts, thereby accelerating the transition to a hydrogen-based society.

Metal-Ligand Cooperation in Cp*Ir-Pyridylpyrrole Complexes: Rational Design and Catalytic Activity in Formic Acid Dehydrogenation and CO2 Hydrogenation under Ambient Conditions

Interconversion between CO₂ + H₂ and FA/formate is the most promising strategy for the fixation of carbon dioxide and reversible hydrogen storage; however, FA dehydrogenation and CO₂ hydrogenation are usually studied separately using different catalysts for each reaction. This report describes of the catalysis of [Cp*Ir(N∧N)(X)]ⁿ⁺ (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; X = Cl, n = 0; X = H₂O, n = 1) bearing a proton-responsive N∧N pyridylpyrrole ligand for both reactions. Complex 2-H₂O catalyzes FA dehydrogenation at 90 °C with a TOF_max of 45 900 h^-1. Its catalysis is more active in aqueous solution than in neat solution under base-free conditions. These complexes also catalyze CO₂ hydrogenation in the presence of base to formate under atmospheric pressure (CO₂/H₂ = 0.05 MPa/0.05 MPa) at 25 °C with a TOF value of 4.5 h^-1 in aqueous solution and with a TOF value of 29 h^-1 in a methanol/H₂O mixture solvent. The possible mechanism is proposed by intermediate characterization and KIE experiments. The extraordinary activity of these complexes are mainly attributed to the metal-ligand cooperative effect of the the pyrrole group to accept a proton in the dehydrogenation of formic acid and assist cooperative heterolytic H-H bond cleavage in CO₂ hydrogenation.

Advances in Nonprecious Metal Homogeneously Catalyzed Formic Acid Dehydrogenation

Formic acid (FA) possesses a high volumetric concentration of H2 (53 g L−1). Moreover, it can be easily prepared, stored, and transported. Therefore, FA stands out as a potential liquid organic hydrogen carrier (LOHC), which allows storage and transportation of hydrogen in a safe way. The dehydrogenation to produce H2 and CO2 competes with its dehydration to give CO and H2O. For this reason, research on selective catalytic FA dehydrogenation has gained attention in recent years. Several examples of highly active homogenous catalysts based on precious metals effective for the selective dehydrogenation of FA have been reported. Among them are the binuclear iridium-bipyridine catalysts described by Fujita and Himeda et al. (TOF = 228,000 h−1) and the cationic species [IrClCp*(2,2′-bi-2-imidazoline)]Cl (TOF = 487,500 h−1). However, examples of catalytic systems effective for the solventless dehydrogenation of FA, which is of great interest since it allows to reduce the reaction volume and avoids the use of organic solvents that could damage the fuel cell, are scarce. In this context, the development of transition metal catalysts based on cheap and easily available nonprecious metals is a subject of great interest. This work contains a summary on the state of the art of catalytic dehydrogenation of FA in homogeneous phase, together with an account of the catalytic systems based on non-precious metals so far reported.

Insights into Formate Oxidation by a Series of Cobalt Piano-Stool Complexes Supported by Bis(phosphino)amine Ligands

A series of (cyclopentadienyl)cobalt(III) half-sandwich complexes (1-4) supported by bidentate bis(phosphino)amine ligands was synthesized and characterized by NMR spectroscopy, X-ray crystallography, and cyclic voltammetry. The Co^III-hydride complex 4-H bearing the bis(cyclohexylphosphine) ligand derivative was successfully isolated via protonation of the neutral reduced Co^I complex 5 with a weak acid. Experimental and computational methods were used to determine the thermodynamic hydride accepting ability of these Co^III centers and to evaluate their reactivity toward the oxidation of formate. We find that the hydride accepting ability of 1-4 ranges from 71 to 74 kcal/mol in acetonitrile, which should favor a highly exergonic reaction with formate through direct hydride transfer. Formate oxidation was demonstrated at elevated temperatures in the presence of stoichiometric quantities of 4, generating carbon dioxide and the Co^III-hydride complex 4-H in 72% yield.