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.

To develop highly efficient catalysts for dehydrogenation of formic acid in water, we investigated several Cp*Ir catalysts with various amide ligands. The catalyst with an N-phenylpicolinamide ligand exhibited a TOF of 118 000 h at 60 °C. A constant rate (TOF>35 000 h ) was maintained for six hours, and a TON of 1 000 000 was achieved at 50 °C.

Cp*Ir (Cp* = pentamethylcyclopentadienyl) complexes with an N,N-bidentate ligand such as 2,2'-bipyridine serve as catalysts for both carbon dioxide (CO 2 ) hydrogenation to formate and formic acid dehydrogenation in water. Previously, it was shown that the introduction of an electron-donating substituent on 2,2'-bipyridine is an effective method to improve the catalytic activity. Especially, the highly electron-donating hydroxyl (OH) substituent performs much better than other substituents such as methyl or methoxy under basic conditions. However, the introduction of an OH substituent on the ligand has been limited to six-membered rings such as pyridine or pyrimidine. These results prompted us to develop a new ligand comprising a pyridyl-pyrazole with an OH group on the pyrazole moiety for Cp*Ir-catalyzed CO 2 hydrogenation and formic acid dehydrogenation. The resultant catalyst showed high catalytic activity in CO 2 hydrogenation and excellent robustness in formic acid dehydrogenation with a turnover number of 10 million.New fuels without emission of air pollutants or greenhouse gases are being sought, and among them the use of hydrogen gas has drawn much attention. Safe and efficient utilization of hydrogen provides a convenient way to help combat the pressing challenge of global greenhouse gas emissions. However, significant technical and safety concerns regarding cryogenic liquid and compressed gaseous hydrogen together with its high flammability result in considerable barriers to widespread use. Therefore, rapid and controlled storage/ production of hydrogen in a safe and efficient manner is being extensively studied. Because of its relatively high gravimetric hydrogen content (4.3 wt%), its low toxicity toward the human body and the environment, and its easy accessibility by biomass processing, [1] formic acid (FA) has been widely considered as a liquid storage medium capable of releasing H 2 via its catalytic dehydrogenation. In addition, when FA is stored as an aqueous solution (< 85 wt%), it is not combustible. Toward the efficient production and storage of hydrogen, it is of considerable interest to develop highly active and durable catalysts that exhibit selective FA dehydrogenation to H 2 and CO 2 , and convert thermodynamically stable CO 2 and H 2 to formate.To date, much effort has been devoted to discover new and effective homogeneous catalysts [2] for both CO 2 hydrogenation to formate [3] and FA dehydrogenation. [3j,ae,4] Remarkable performance of catalysts for CO 2 hydrogenation has been achieved by many researchers. Nozaki and coworkers showed that an Ir complex coordinated by a PNP pincer ligand exhibited the high TOF (turnover frequency) of 150,000 h À1 and high TON (turnover number) of 3,500,000 at 200 8C