CO₂ Hydrogenation Catalyzed by Iridium Complexes with a Proton-Responsive Ligand

Abstract

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.

An Iridium(III) Complex as a Photoactivatable Tool for Oxidation of Amyloidogenic Peptides with Subsequent Modulation of Peptide Aggregation

“…For the oxidation of amyloidogenic peptides simply upon photoactivation under mild conditions, Ir-1 (Figure 1b) was rationally designed as a chemical tool by taking into account the characteristics found in previously reported photoactivatable Ir III complexes, as well as specificity for amyloidogenic peptides: 1) photoproperties with relatively low-energy radiation (i.e., visible light); [8d,e] 2) the ability to produce ROS [e.g., singlet oxygen ( 1 O 2 )] from redundant O 2 , which is responsible for peptide oxidation, upon photoactivation; [5a,8c] 3) robust octahedral coordination, which can provide relative structural stability without any structural modifications when light is introduced; [8c] 4) strong spin-orbit coupling of the Ir III center, which could facilitate electronic transitions without the assistance of additional electron acceptors; [8d,e] and 5) incorporation of a ligand ( 1 ) containing a dimethylamino group, which is suggested to be crucial for interactions with amyloidogenic peptides, [8a,b] onto the Ir III center (Figure 1b). …”

“…Ir-1 was rationally designed as such a tool through incorporating the general properties of Ir III complexes previously reported for various applications, including photoactivation, formation of reactive oxygen species (ROS) upon light exposure, and relatively stable octahedral geometry, as well as relatively easy introduction of a ligand containing a structural moiety, suggested to be important for interactions with amyloidogenic peptides, on the Ir III center. [5a,8] Representative amyloidogenic peptides (i.e., Aβ, α-Syn, and hIAPP) were indicated to be noticeably oxidized upon treatment of Ir-1 with photoactivation under aerobic conditions, as monitored by electrospray ionization mass spectrometry (ESI-MS), and their oxidation sites (as potential sites, methionine, histidine, and tyrosine residues) were identified by tandem MS (ESI-MS 2 ). Even with oxidative modifications of a few residues in these amyloidogenic peptides, the aggregation pathways were noticeably modulated, resulting in distinct morphological features (e.g., smaller-sized or amorphous peptide species, instead of fibrils).…”

Development of Effective Catalysts for Hydrogen Storage Technology Using Formic Acid

“…In the case of the single crystals grown under basic conditions, the distance of CO bond on pyridine ligand was shorter than that of the crystal grown under acidic conditions due to the resonance. [32,33] Furthermore, the imidazoline, which is a nonaromatic five-membered ring and is derived from imidazole by the reduction of the CC double bond, was also evaluated as a ligand moiety in CO 2 hydrogenation. The improvement of catalytic activity by oxyanion substituent was also observed in system using phenanthroline catalysts 4 and 5 (Entry 4, TOF: 3 h −1 and Entry 5, TOF: 2600 h −1 ) at 80 °C under 4.0 MPa of P H2/CO2 .…”

Molecular Catalysis of the Electrochemical and Photochemical Reduction of CO2 with Earth-Abundant Metal Complexes. Selective Production of CO vs HCOOH by Switching of the Metal Center

“…Following the pioneering work on Ni and Co macrocyclic complexes as potential catalysts for electrochemical CO 2 reduction, 5,6 various metal complexes have been used in this purpose, including mainly Re, Ru, Ir, Rh, Os, Pd, Mo, Cu, Co, Ni, Mn and Fe. 1,[7][8][9][10][11][12][13][14] The reduction products may be oxalate, CO or more rarely formic acid. Only rare example has been shown to catalyze CO 2 reduction with more than two electrons in electrochemical conditions (formaldehyde production).…”

“…9,23,24 Mechanistic insights have been gained recently too with Re, [28][29][30] Mn 31,32 and Ni 33-36 based catalysts through electrochemical and spectroscopic analysis, and quantum chemistry calculations. The reduction of carbon dioxide into formate has been more rarely observed using molecular catalysts, in electrochemical conditions with Fe, Ni, Co, Ru, Rh or Ir based catalysts [37][38][39][40][41][42][43][44][45][46][47][48][49] (see also tables 2 and 3 in reference 7) and in photochemical conditions 13,14,[50][51][52][53] with Re, Ru, Ir main-2 ly and very recently Mn complexes. 53 Mechanistic studies have accordingly been scarce.…”