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Efficient hydrogen storage and release are essential for effective use of hydrogen as an energy carrier. In principle, formic acid could be used as a convenient hydrogen storage medium via reversible CO2 hydrogenation. However, noble metal-based catalysts are currently needed to facilitate the (de)hydrogenation, and the CO2 produced during hydrogen release is generally released, resulting in undesirable emissions. Here we report an α-amino acid-promoted system for reversible CO2 hydrogenation to formic acid using a Mn-pincer complex as a homogeneous catalyst. We observe good stability and reusability of the catalyst and lysine as the amino acid at high productivities (CO2 hydrogenation: total turnover number of 2,000,000; formic acid dehydrogenation: total turnover number of 600,000). Employing potassium lysinate, we achieve >80% H2 evolution efficiency and >99.9% CO2 retention in ten charge–discharge cycles, avoiding CO2 re-loading steps between each cycle. This process was scaled up by a factor of 18 without obvious drop of the productivity.

Recent developments of CO

Diesters are of fundamental importance in the chemical industry and are used for many applications, e.g. as plasticizers, surfactants, emulsifiers, and lubricants. Herein, we present a straightforward and efficient method for the selective synthesis of diesters via palladium-catalyzed direct carbonylation of di- or polyols with readily available alkenes. Key-to-success is the use of a specific palladium catalyst with the “built-in-base” ligand L16 providing esterification of all alcohols and a high n/iso ratio. The synthesized diesters were evaluated as potential plasticizers in PVC films by measuring the glass transition temperature (Tg) via differential scanning calorimetry (DSC).

A series of alkyl ammonium (or imidazolium) based ionic liquids was tested as novel and potentially green absorbent for CO

Metal–organic framework (MOF)‐derived Co‐N‐C catalysts with isolated single cobalt atoms have been synthesized and compared with cobalt nanoparticles for formic acid dehydrogenation. The atomically dispersed Co‐N‐C catalyst achieves superior activity, better acid resistance, and improved long‐term stability compared with nanoparticles synthesized by a similar route. High‐angle annular dark‐field–scanning transmission electron microscopy, X‐ray photoelectron spectroscopy, electron paramagnetic resonance, and X‐ray absorption fine structure characterizations reveal the formation of CoIINx centers as active sites. The optimal low‐cost catalyst is a promising candidate for liquid H2 generation., The small print: Co‐N‐C single‐atom catalysts were derived from metal– organic frameworks and compared with related nanoparticles in the selective dehydrogenation of formic acid. The highly dispersed single CoIINx sites demonstrated improved reactivity and resistance to acid, constituting the current state‐of‐the‐art in low‐cost earth‐abundant catalysts for this transformation.

Ruthenium PNP pincer complexes bearing supplementary cyclometalated C,N‐bound ligands have been prepared and fully characterized for the first time. By replacing CO and H− as ancillary ligands in such complexes, additional electronic and steric modifications of this topical class of catalysts are possible. The advantages of the new catalysts are demonstrated in the general α‐alkylation of ketones with alcohols following a hydrogen autotransfer protocol. Herein, various aliphatic and benzylic alcohols were applied as green alkylating agents for ketones bearing aromatic, heteroaromatic or aliphatic substituents as well as cyclic ones. Mechanistic investigations revealed that during catalysis, Ru carboxylate complexes are predominantly formed whereas neither the PNP nor the CN ligand are released from the catalyst in significant amounts., Alkylation with ruthenium: Eight new Ru pincer complexes were prepared by introduction of C,N‐bound phenyl heterocycle ligands. These complexes were applied as catalysts in the α‐alkylation of ketones with alcohols through hydrogen autotransfer. Here, superior yields in comparison to [RuHCl(CO)(HN(CH2CH2PPh2)2)] (Ru‐MACHO) were achieved and 21 different alkylated ketones were synthesized in up to 97 % yield. Moreover, investigations regarding the catalyst species present during the reaction were conducted.

A homogeneous Ru-pincer complex was immobilized by the supported liquid phase (SLP) technique and tested in continuous gas-phase steam reforming of methanol. Variations of catalyst composition (liquid phase, base and catalyst loading) as well as process parameters (temperature, flow rate) were carried out. High activity was obtained when ionic liquids were used for catalyst immobilization. However, better stability was obtained in the absence of ionic liquids using pure KOH.

The rise of CO2 in atmosphere is considered as the major reason for global warming. Therefore, CO2 utilization has attracted more and more attention. Among those, using CO2 as C1-feedstock for the chemical industry provides a solution. Here we show a two-step cascade process to perform catalytic carbonylations of olefins, alkynes, and aryl halides utilizing CO2 and H2. For the first step, a novel heterogeneous copper 10Cu@SiO2-PHM catalyst exhibits high selectivity (≥98%) and decent conversion (27%) in generating CO from reducing CO2 with H2. The generated CO is directly utilized without further purification in industrially important carbonylation reactions: hydroformylation, alkoxycarbonylation, and aminocarbonylation. Notably, various aldehydes, (unsaturated) esters and amides are obtained in high yields and chemo-/regio-selectivities at low temperature under ambient pressure. Our approach is of interest for continuous syntheses in drug discovery and organic synthesis to produce building blocks on reasonable scale utilizing CO2.

Photocatalytic carbon dioxide reduction utilizing metal complexes based on the earth-abundant transition metals iron and copper was transferred from organic solvents into ionic liquids with high selectivity and moderate turn-over numbers. Different classes of ionic liquids were evaluated to determinate the CO/H2 selectivity of the process. High-purity conditions were utilized to validate that CO was a result of CO2 reduction and not resulting from other carbon sources. In addition, a novel copper-based photosensitizer was synthesized and its photo- and electrochemical properties were investigated.

Hydrogen use as a sustainable energy vector is of growing interest and is the subject of current intensive research. We report herein an upgraded follow-up system for formic acid (FA) dehydrogenation (DH) catalyzed by [Mn(BisIm)(CO)3Br] (BisIm = 4,4′,5,5′-tetrahydro-1H,1′H-2,2′-bisimidazole). The latter belongs to a family of novel catalysts we developed, which are interesting since they bear easily accessible ligands from cheap, commercially available starting materials. The novel catalyst system allows for improved hydrogen generation and better catalyst turnover numbers (7883 after 12 cycles of FA dosage). In addition, a significantly lower CO content in the gas phase was also achieved (47 ppm). Extended investigation of the influence of the ligand structure on the catalytic performances confirmed that the non-bonding electron pair on the N–H moiety, on the 3rd position of each imidazoline ring, is crucial for the transformation.

Heterogeneous copper particles have been widely investigated in photocatalytic reduction processes to produce hydrogen. In this contribution, copper nanoparticles were in situ generated and supported on different C- and SiO2-based additives in the presence of an iridium photosensitizer (PS), which itself was activated by visible light absorption. Thereby, the influence of the supports on the formation of the active species and their performance as water reduction catalysts were studied. The additives ensure heterogeneous nucleation leading to lower sizes of the formed copper particles and prevent their further growth during the reaction which was verified by STEM analysis. Consequently, the hydrogen productivity in the presence of the supports was enhanced and the best carbon material rGO(H2) revealed a 3 times higher activity compared to the non-supported system after 20 h. Photoluminescence measurements confirmed the proposed reductive quenching mechanism in this system and suggested that rGO(H2) mediates reoxidation of the PS. However, in the presence of the ligand 2,2′-bipyridine (bpy) the application of the silica MCM-41 showed a fourfold increase of hydrogen evolution compared to the non-supported system, which was still active after 160 h and could be successfully reused in recycling experiments.

Heterogeneous Cu catalysts are widely used in photocatalytic hydrogen generation. The typical working mode includes the transfer of photo-induced charges from a semiconductor to CuO which itself is reduced to Cu2O to initiate the catalytic cycle. In this contribution a photosensitizer (dye), excited by visible light absorption, was used to transfer an electron to the CuO which after reduction catalyzes the water reduction to hydrogen. Several copper(II)-catalysts on high surface silica were prepared by impregnation or precipitation and applied in photocatalytic water reduction. The best catalyst CuO(Cl0.1)/SiO2, synthesized via incipient wetness impregnation of CuCl2 in MCM-41 and a following precipitation with NaOH, achieved a 6 times higher activity (1702 mmol h−1 g−1) compared to a previously investigated copper system (280 mmol h−1 g−1 for CuI) under the same reaction conditions. All materials were fully characterized by XRD, TEM and N2 sorption and further by magnetic resonance and X-ray methods EPR, ASAXS and XAS. In situ measurements evidenced a reduction of the initial Cu(II)-species, which confirms the (proposed) photocatalytic mechanism.

Summary A highly porous photocatalyst (copper on TiO2 aerogel) was synthesized and applied in aqueous CO2 reduction without using external sacrificial electron donors. For the first time, complete selectivity toward CO and improved catalyst productivity are observed in the presence of oxygen. The optimal activity is achieved in a feed containing 0.5 vol% O2 in CO2. In situ XAS, EPR, and UV-vis measurements suggest, among different Cu oxidation states, Cu(I) to be the most active species in photocatalytic CO2 reduction. Oxygen sensing of the catalyst in the presence of O2/CO2 mixtures indicated an unexpected photoadsorption of oxygen on the titania surface. We propose photooxidation of surface hydroxyl groups to be the electron source for CO2 reduction, which is supported by hydroxyl group consumption, detection of hydroxyl radicals using in situ EPR, and detection of surface peroxide species after the reaction.

Among the known liquid organic hydrogen carriers, formic acid attracts increasing interest in the context of safe and reversible storage of hydrogen. Here, the first molecularly defined cobalt pincer complex is disclosed for the dehydrogenation of formic acid in aqueous medium under mild conditions. Crucial for catalytic activity is the use of the specific complex 3. Compared to related ruthenium and manganese complexes 7 and 8, this optimal cobalt complex showed improved performance. DFT computations support an innocent non‐classical bifunctional outer‐sphere mechanism on the triplet state potential energy surface.

Complexes Ru(H)(Cl)(CO)(HN{CH2CH2P(iPr)(2)}(2)) 7 and Ru(H)(Cl)(CO)(CH3N{CH2CH2P(iPr)(2)}(2)) 8 were compared for the selective dehydrogenation (DH) of formic acid (FA) at different pH values. Remarkably, highest activities were observed in acidic environment (pH 4.5). Under all investigated conditions, the N-methylated complex 8 showed improved performance compared to 7. These observations can be mechanistically explained with protonation of 4H/3Me being the key step in formic acid DH for both complexes.

Electrochemical CO2 reduction to valuable chemicals represents a greenand sustainable approach to close the anthropogenic carbon cycle, but hasbeen impeded by low efficiency and high cost of electrocatalysts. Here, acost-effective hybrid catalyst consisting of hemin (chloroprotoporphyrin IXiron(III)), a product recovered from bovine blood, adsorbed onto commercialVulcan carbon is reported. Upon heat treatment, this material shows significantlyimproved activity and selectivity for CO2 reduction in water whileexhibiting good stability for more than 10 h. The heat treatment leads toconsecutive removal of the axial chlorine atom and decomposition of the ironporphyrin ring, restructuring to form atomic Fe sites. The optimized hybridcatalyst obtained at 900 °C shows near-unity selectivity for reduction of CO2to CO at a small overpotential of 310 mV. The insight into transformation ofadsorbed Fe complexes into single Fe atoms upon heat treatment providesguidance for development of single atom catalysts.

Herein, we report a novel amino acid based reaction system for CO2 capture and utilization (CCU) to produce formates in the presence of the naturally occurring amino acid l-lysine. Utilizing a specific ruthenium-based catalyst system, hydrogenation of absorbed carbon dioxide occurs with high activity and excellent productivity. Noteworthy, following the CCU concept, CO2 can be captured from ambient air in the form of carbamates and converted directly to formates in one-pot (TON > 50 000). This protocol opens new potential for transforming captured CO2 from ambient air to C1-related products., A novel amino acid based reaction system for CO2 capture and utilization (CCU) to produce formates is presented applying a ruthenium-based catalyst. Noteworthy, CO2 can be captured from ambient air and converted to formates in one-pot (TON > 50 000).

Time‐resolved X‐ray absorption spectroscopy has been utilized to monitor the bimolecular electron transfer in a photocatalytic water splitting system. This has been possible by uniting the local probe and element specific character of X‐ray transitions with insights from high‐level ab initio calculations. The specific target has been a heteroleptic [IrIII(ppy)2(bpy)]+ photosensitizer, in combination with triethylamine as a sacrificial reductant and Fe3(CO)12 as a water reduction catalyst. The relevant molecular transitions have been characterized via high‐resolution Ir L‐edge X‐ray absorption spectroscopy on the picosecond time scale and restricted active space self‐consistent field calculations. The presented methods and results will enhance our understanding of functionally relevant bimolecular electron transfer reactions and thus will pave the road to rational optimization of photocatalytic performance., The light induced dynamics of an Iridium‐based photosensitizer for hydrogen generation were studied by picosecond‐resolved X‐ray absorption spectroscopy (XAS) during the primary steps of the photocatalytic cycle. We used total fluorescence yield and high‐resolution energy detection (HERFD) XAS spectra to follow the oxidation state of the central metal of an iridium‐based photosensitizer subject to photoinduced electron transfer. The detailed analysis of the main X‐ray transitions from the ground and excited states are guided by high‐level ab initio calculations.

Ru‐pincer complexes were immobilized as supported liquid phase (SLP) materials to allow the methanol reforming reaction as continuous gas phase process. Under reaction conditions, the liquid phase forms from the hydroxide coating. Several hydroxides were screened and CsOH showed highest activity compared to the standard KOH coating. The well‐known Ru‐pincer complex carbonylchlorohydrido [bis(2‐di‐i‐propylphosphinoethyl)amine]ruthenium(II) is limited in catalyzing the final step of the methanol reforming. Addition of a second complex, having a methylated backbone in the pincer‐ligand, could overcome these limitations. Significant enhancement of the overall catalytic activity was observed.

Molybdenum(0) complexes with aliphatic aminophosphine pincer ligands have been prepared which are competent for the disproportionation of formic acid, thus representing the first example so far reported of non-noble metal species to catalytically promote such transformation. In general, formic acid disproportionation allows for an alternative access to methyl formate and methanol from renewable resources. MeOH selectivity up to 30% with a TON of 57 could be achieved while operating at atmospheric pressure. Selectivity (37%) and catalyst performance (TON = 69) could be further enhanced when the reaction was performed under hydrogen pressure (60 bars). A plausible mechanism based on experimental evidence is proposed., Mo(0) complexes with aliphatic PNP-pincer ligands enable the first example of non-noble metal catalyzed formic acid disproportionation leading to methanol with a selectivity of up to 37% and a turnover number up to 69.

Energy & environmental science 14(8), 4427-4436 (2021). doi:10.1039/D1EE01053K, Published by RSC Publ., Cambridge

Correction for ‘HCOOH disproportionation to MeOH promoted by molybdenum PNP complexes’ by Elisabetta Alberico et al., Chem. Sci., 2021, 12, 13101–13119, DOI: 10.1039/D1SC04181A.

International audience; Facet-engineered monoclinic scheelite BiVO4 particles decorated with various cocatalysts were successfully synthesized by selective sunlight photodeposition of metal or metal oxy(hydroxide) nanoparticles onto the facets of truncated bipyramidal BiVO4 monoclinic crystals coexposing {010} and {110} facets. X-ray photoelectron spectroscopy, scanning electron microscopy, and scanning Auger microscopy revealed that metallic silver (Ag) and cobalt (oxy)hydroxide (CoOx(OH)y) particles were selectively deposited onto the {010} and {110} facets, respectively, regardless of the cocatalyst amount. By contrast, the nickel (oxy)hydroxide (NiOx(OH)y) photodeposition depends on the nickel precursor amount with an unprecedented selectivity for 0.1 wt % NiOx(OH)y/BiVO4 with a preferential deposition onto the {010} facets and the edges between the {110} facets. Moreover, these noble metal-free heterostructures led to remarkable photocatalytic properties for rhodamine B photodecomposition and sacrificial water oxidation reactions. For instance, 0.2 wt % CoOx(OH)y/BiVO4 led to one of the highest oxygen evolution rates, i.e., 1538 μmol h–1 g–1, ever described which is ten times higher than that found for bare BiVO4. The selective deposition of cobalt (oxy)hydroxide species onto the more electron-deficient facet of truncated bipyramidal monoclinic BiVO4 particles favors photogenerated charge carrier separation and therefore plays a key role for efficient photochemical oxygen evolution.

Facet-engineered monoclinic scheelite BiVO

An efficient water reduction system was developed using thermally activated delayed fluorescence (TADF) dyes as organic photosensitizers and a palladium complex as water reduction catalyst. 2,4,5,6-tetrakis(carbazol-9-yl)-1,3-dicyanobenzene (4CzIPN) that has the best fluorescence efficiency displayed the highest photoinduced productivity (TON up to 2567) and was active for 5 days. Based on time-resolved photoluminescence experiments on the photocatalytic system, reductive quenching of the TADF dye was found as main pathway occurring before intersystem crossing takes place.

Nickel-based nanocatalysts were used in acceptorless, reversible dehydrogenation and hydrogenation reactions of N-heterocycles. Both processes were realized in the same solvent using a single catalyst, without isolation of products and workup, which makes it attractive for hydrogen storage purposes. This concept has been demonstrated in a continuous hydrogenation/dehydrogenation sequence of quinaldine with negligible loss in activity of the nickel catalyst after three hydrogen storage cycles. The scope of acceptorless dehydrogenation has been explored and control experiments suggest that hydrogen liberation is initiated via amine dehydrogenation and supports the direct alkane dehydrogenation from the partially oxidized N-heterocycles.

An ideal photosensitizer should strongly absorb visible light over a broad range and exhibit long-lived excited states. Therefore, an anthracene moiety attached to a bipyridine sphere was chosen to study the impact of a carbon extended π-system in the back on the structural, electrochemical and photophysical properties of the resulting copper photosensitizers. Consequently, four novel homo- and heteroleptic Cu(I) complexes of the type [Cu(N^N)2]+ and [(P^P)Cu(N^N)]+ were prepared and fully characterized. The solid state structures possess a strong twist of the anthracene system due to sterical constraints. At the same time the absorption maxima are red-shifted and the excited states display unprecedented lifetimes up to 4 μs in acetonitrile stemming from different excitation processes unique for the anthracene moiety. The excited state properties were then further analyzed by transient absorption spectroscopy and DFT calculations. Moreover, it was found that the anthracene extension has a slight positive effect on the photocatalytic reduction of protons, which was selected as a model reaction to test the capability of these photosensitizers.

Homogenous photocatalytic systems based on copper photosensitizers are promising candidates for noble metal free approaches in solar hydrogen generation. To improve their performance, a detailed understanding of the individual steps is needed. Here, we study the interaction of a heteroleptic copper (I) photosensitizer with an iron catalyst by time-resolved spectroscopy and ab initio calculations. The catalyst leads to rather efficient quenching of the 3MLCT state of the copper complex, with a bimolecular rate being about three times smaller than the collision rate. Using control experiments with methyl viologen, an appearing absorption band is assigned to the oxidized copper complex demonstrating that an electron transfer from the sensitizer to the iron catalyst occurs and the system reacts along an oxidative pathway. However, only about 30% of the quenching events result in an electron transfer while the other 70% experience deactivation indicating that the photocatalytic performance might suffer from geminate recombination.

This work is focused on the identification and investigation of the catalytically relevant key iron species in a photocatalytic proton reduction system described by Beller and co-workers. The system is driven by visible light and consists of the low-cost [Fe3 (CO)12 ] as catalyst precursor, electron-poor phosphines P(R)3 as co-catalysts, and a standard iridium-based photosensitizer dissolved in a mixture of THF, water, and the sacrificial reagent triethylamine. The catalytic reaction system was investigated by operando continuous-flow FTIR spectroscopy coupled with H2 gas volumetry, as well as by X-ray absorption spectroscopy, NMR spectroscopy, DFT calculations, and cyclic voltammetry. Several iron carbonyl species were identified, all of which emerge throughout the catalytic process. Depending on the applied P(R)3 , the iron carbonyl species were finally converted into [Fe2 (CO)6 (μ-CO){μ-P(R)2 }]- . This involves a P-C cleavage reaction. The requirements of P(R)3 and the necessary reaction conditions are specified. [Fe2 (CO)6 (μ-CO){μ-P(R)2 }]- represents a self-assembling, sulfur-free [FeFe]-hydrogenase active-site mimic and shows good catalytic activity if the substituent R is electron poor. Deactivation mechanisms have also been investigated, for example, the decomposition of the photosensitizer or processes observed in the case of excessive amounts of P(R)3 . [Fe2 (CO)6 (μ-CO){μ-P(R)2 }]- has potential for future applications.

In a previous paper we have demonstrated that the easily-synthesized class of iron(0) cyclopentadienone complexes constitutes a promising catalyst platform for the electrochemical conversion of CO2 to CO and H2O. One of the unusual features of these catalysts is that catalysis proceeds efficiently in aprotic electrolytes in the absence of acidic additives. Herein we present a detailed study of the underlying catalytic mechanisms. Using a combination of FTIR spectroelectrochemistry, DFT calculations, and nonelectrochemical control experiments, we have identified a number of catalytic intermediates including the active species and the product of catalyst deactivation. On the basis of these insights, we have carried out digital simulations in order to decipher the voltammetric profiles of the iron(0) cyclopentadienones. Further control experiments revealed that the anodic oxidation of the electrolyte constitutes the terminal proton source for the formation of CO and H2O. Taken together, our results suggest a...

Hydrogen is playing an increasingly larger role in clean energy technologies and the emerging hydrogen economy. However, efficient and selective H2 production from renewable resources is rare so far. Herein, we describe a dehydrogenation route that is applicable to various kinds of non-food-related biomass and daily waste, such as wheat straw, corn straw, rice straw, reed, bagasse, bamboo sawdust, cardboard and newspaper. H2 yields up to 95% were achieved by a one-pot, two-step reaction with a 69 ppm molecularly defined iridium catalyst bearing an imidazoline moiety from formic acid, which was in turn obtained via a 1 v% dimethyl sulfoxide-promoted hydrolysis–oxidation of biomass. Formation of the unwanted side products CO and CH4 was no more than 22 and 2 ppm, respectively, while CO2 was captured as carbonate. The resulting hydrogen gas can be directly applied in proton exchange membrane fuel cells. Access to renewable hydrogen represents an important target for the success of the hydrogen economy. Now, a one-pot method is presented for the conversion of cellulosic biomass into hydrogen via formic acid as the intermediate, followed by its application to a fuel cell.

Novel nanostructured catalysts with highly dispersed cobalt have been synthesized by the pyrolysis of metal phenanthroline complexes. Materials with significantly different properties were obtained by simply tuning the metal/ligand ratio. The catalytic potential of this class of compounds is shown by the first example of the dehydrogenation of formic acid under the catalysis of atomically dispersed cobalt. From TEM, XPS, and XRD characterization, KSCN poisoning, and acid leaching, the formation of CoNx species as the active site seems key to the success of this reaction. Excellent stability and recyclability make this new catalyst also attractive for other applications.

Driven by the intention to improve classic heteroleptic copper photosensitizers two novel Cu(I) complexes applying a hetero-bidentate P^N ligand were prepared. A combined photophysical, electrochemical, and theoretical study gives insights into structure-activity relationships and revealed an increased absorptivity. Both complexes were tested for the light-driven production of H2 .

A catalytic system based on an iridium-PNP complex was developed for the aqueous-phase reforming of methanol. Investigations revealed higher activity at low base concentrations and higher stability at high base concentrations as an opposing trend relative to that of known Ru and Fe PNP-pincer-containing systems for methanol dehydrogenation. During mechanistic investigations it was possible to identify a CO containing derivate of the deactivation species.

A series of phenanthroline-based ligands have been synthesised and their influence as bidentate nitrogen ligands in heteroleptic [Cu(P^P)(N^N)]+ photosensitisers in light-driven water reduction has been studied. In this noblemetal- free Cu–Fe-based photocatalytic water reduction system, the structural effects of the nitrogen ligands have been explored, including the steric and electronic effects of substituents at the 2,9- and 4,7-positions of phenanthroline. Ligands were prepared that led to increased hydrogen generation, with turnover numbers (TONCu) of up to 1388 being observed. All the new complexes were electrochemically and photophysically characterised. We demonstrate for the first time that the presence of fluorine in nitrogen ligands increases the efficacy of copper complexes in photocatalytic hydrogen production.

Formic acid dehydrogenation (FAD) is considered as a promising process in the context of hydrogen storage. Its low toxicity, availability and convenient handling make FA attractive as a potential hydrogen carrier. To date, most promising catalysts have been based on noble metals, such as ruthenium and iridium. Efficient non-noble metal systems like iron were designed but manganese remains relatively unexplored for this transformation. In this work, we present a panel of phosphine free manganese catalysts which showed activity and stability in formic acid dehydrogenation. The most promising results were obtained with Mn(pyridine-imidazoline)(CO)(3)Br yielding >14 l of the H-2/CO2 mixture and proved to be stable for more than 3 days. Additionally, this study provides insights into the mechanism of formic acid dehydrogenation. Kinetic experiments, Kinetic Isotopic Effect (KIE), in situ observations, NMR labeling experiments and pH monitoring allow us to propose a catalytic cycle for this transformation.