Hydrogen generation from formic acid (FA), one of the most promising hydrogen storage materials, has attracted much attention due to the demand for the development of renewable energy carriers. Catalytic dehydrogenation of FA in an efficient and green manner remains challenging. Here, we report a series of bio-inspired Ir complexes for highly robust and selective hydrogen production from FA in aqueous solutions without organic solvents or additives. One of these complexes bearing an imidazoline moiety (complex 6) achieved a turnover frequency (TOF) of 322,000 h −1 at 100 ºC, which is higher than ever reported. The novel catalysts are very stable and applicable in highly concentrated FA. For instance, complex 3(1 µmol) affords an unprecedented turnover number (TON) of 2,050,000 at 60 ºC. Deuterium kinetic isotope effect experiments and density functional theory (DFT) calculations employing a "speciation" approach demonstrated a change in the rate determining step with increasing solution pH. This study provides not only more insight into the mechanism of dehydrogenation of FA, but also offers a new principle for the design of effective homogeneous organometallic catalysts for H 2 generation from FA.

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A series of Ir catalysts bearing amide-based ligands generated by a deprotonated amide moiety was prepared with the hypotheses that the strong electron-donating ability of the coordinated anionic nitrogen atom and the proton-responsive OH group near the metal center will improve the catalytic activity for CO2 hydrogenation and formic acid (FA) dehydrogenation. The effects of the modifications of the ligand architecture on the catalytic activity were investigated for CO2 hydrogenation at ambient conditions (25 °C with 0.1 MPa H2/CO2 (v/v = 1/1)) and under slightly harsher conditions (50 °C with 1.0 MPa H2/CO2) in basic aqueous solutions together with deuterium kinetic isotope effects (KIEs) with selected catalysts. Cp*Ir­(L12)­(H2O)­HSO4 (L12 = 6-hydroxy-N-phenylpicolinamidate) that has an anionic coordinating N atom and an OH group in the second coordination sphere, exhibits a turnover frequency (TOF) of 198 h–1 based on the initial 1 h of reaction. This TOF which, to the best of our knowledge, is the highest value ever reported under ambient conditions in basic aqueous solutions. However, Cp*Ir­(L10)­(H2O)­HSO4 (L10 = (4-hydroxy-N-methyl­picolinamidate) performs better in long-term CO2 hydrogenation (up to a TON of 14 700 with [Ir] = 10 μM after 348 h and the final formate concentration of 0.643 M with [Ir] = 250 μM) at ambient conditions. Further, the catalytic activity for FA dehydrogenation was examined under three different conditions (pH 1.6, 2.3, and 3.5). The Cp*Ir­(L12)­(H2O)­HSO4 complex in any of these conditions is less active compared to the picolinamidate catalysts without ortho-OH, owing to its instability. The complex without OH group, Cp*Ir­(L8)­(H2O)­HSO4 (L8 = N-phenyl-picolinamidate), exhibits a high TOF (up to 118 000 h−1) at 60 °C. Theoretical calculations were performed to examine the catalytic mechanism, and a step-by-step mechanism has been proposed for both CO2 hydrogenation and FA dehydrogenation reactions. Density functional theory calculations of [Cp*Ir­(L3)­(H2O)]­HSO4 (L3 = picolinamidate) and the X-ray structure of the [Cp*Ir­(L7)­(H)]·H2O (L7 = N-methylpicolinamidate) complex imply a pH-dependent conformational change from N,N coordination to N,O coordination upon lowering the pH of the aqueous solution.

We are engaged in research and development to reduce CO 2 emissions. Longterm increase of CO 2 concentration in the atmosphere has a great influence on climate change. [1] Therefore, developing technologies aiming to reduce CO 2 emissions and to overcome the present society depending on fossil fuels as primary energy. Additionally, transition of fossil fuels into renewable energies is also important for realizing future sustainable society. [2] The most suitable material for this goal is considered to be hydrogen, because its combustion emits only water as a byproduct. In addition, hydrogen possesses almost three times as much energy as natural gases, and can supply electric power very efficiently for fuel cells without releasing greenhouse gases and air pollutants. [3] However, hydrogen also has serious drawbacks for practical applications. Most serious problem is that, hydrogen forms as a gas at ambient conditions with very low density (0.0899 kg m −3 at 0 °C, 0.10 MPa) over ten times lower than air (1.293 kg m −3 at 0 °C, 0.10 MPa). As a result, it becomes difficult to store and transport hydrogen safely. Developing safe and efficient hydrogen storage materials is one of the most difficult challenges for the transformation from the fossil fuel-based economy to hydrogen-based one as a long-term solution for a safe energy future.Hydrogen has attracted considerable attention as an energy source, and various attempts to develop suitable methods for hydrogen generation are made at the National Institute of Advanced Industrial Science and Technology. In this paper, the authors introduce their recent strategies to store hydrogen using formic acid (FA) as a hydrogen carrier. FA, which is believed to be one of the most promising liquid organic hydrogen carriers, can provide a viable method for safe hydrogen transportation. In order to optimize the performance of hydrogen storage with FA, the authors have investigated both homogeneous and heterogeneous catalysts. For example, Ir catalysts anchoring N^N-bidentate ligands show high catalytic activity for both the reactions of FA synthesis and hydrogen generation from FA. Ultrafine Pd-based nanoparticles are also immobilized on various supports, which show excellent catalytic performance for FA dehydrogenation under mild conditions. The authors also develop both homogeneous and heterogeneous catalysts to generate high-pressure gases (H 2 and CO 2 ) over 120 and 35 MPa, respectively,

In an effort to design concepts for highly active catalysts for the hydrogenation of CO2 to formate in basic water, we have prepared several catalysts with picolinic acid, picolinamide, and its derivatives, and we investigated their catalytic activity. The CO2 hydrogenation catalyst having a 4-hydroxy-N-methylpicolinamidate ligand exhibited excellent activity even under ambient conditions (0.1 MPa, 25 °C) in basic water, exhibiting a TON of 14700, a TOF of 167 h–1, and producing a 0.64 M formate concentration. Its high catalytic activity originates from strong electron donation by the anionic amide moiety in addition to the phenolic O– functionality.

The catalytic cycle for the production of formic acid by CO2 hydrogenation and the reverse reaction have received renewed attention because they are viewed as offering a viable scheme for hydrogen storage and release. In this Forum Article, CO2 hydrogenation catalyzed by iridium complexes bearing sophisticated N^N-bidentate ligands is reported. We describe how a ligand containing hydroxy groups as proton-responsive substituents enhances the catalytic performance by an electronic effect of the oxyanions and a pendent-base effect through secondary coordination sphere interactions. In particular, [(Cp*IrCl)2(TH2BPM)]Cl2 (Cp* = pentamethylcyclopentadienyl; TH2BPM = 4,4',6,6'-tetrahydroxy-2,2'-bipyrimidine) enormously promotes the catalytic hydrogenation of CO2 in basic water by these synergistic effects under atmospheric pressure and at room temperature. Additionally, newly designed complexes with azole-type ligands were applied to CO2 hydrogenation. The catalytic efficiencies of the azole-type complexes were much higher than that of the unsubstituted bipyridine complex [Cp*Ir(bpy)(OH2)]SO4. Furthermore, the introduction of one or more hydroxy groups into ligands such as 2-pyrazolyl-6-hydroxypyridine, 2-pyrazolyl-4,6-dihydroxypyrimidine, and 4-pyrazolyl-2,6-dihydroxypyrimidine enhanced the catalytic activity. It is clear that the incorporation of additional electron-donating functionalities into proton-responsive azole-type ligands is effective for promoting further enhanced hydrogenation of CO2.

A Cp*Ir complex with pyridyl-imidazoline achieved the quantity production of 1 m3 of H2/CO2 gases from only HCO2H in water without any additives.

Production of methanol (MeOH) from CO 2 is strongly desired as a key chemical feedstock and a fuel. However, the conventional process requires elevated temperature and pressure, and high temperature restricts the productivity of MeOH due to equilibrium limitations between CO 2 and MeOH. This paper describes the efficient hydrogenation/disproportionation of formic acid (FA) to MeOH by using iridium catalysts with electronically tuned ligands and by optimizing reaction conditions. An iridium complex bearing 5,5′-dimethyl-2,2′bipyridine in FA hydrogenation achieved MeOH selectivity with H 2 of up to 47.1% for FA hydrogenation under 4.5 MPa of H 2 in the presence of H 2 SO 4 . The final concentration of MeOH of 3.9 M and a TON of 1314 were obtained in 12 M FA aqueous solution including 10 mol % of H 2 SO 4 at 60 °C under 5.2 MPa of H 2 . Even under atmospheric pressure without introduction of external hydrogen gas, the FA disproportionation under deuterated conditions produced MeOH with 15.4% selectivity. Furthermore, the isotope effect and NMR studies revealed mechanistic insight into the catalytic hydrogenation of FA to MeOH.