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

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.

Conductive poly(2,5-substituted aniline)s highly soluble both in water and organic solvents.

Highly soluble conductive polyanilines were synthesized from newly designed aniline derivatives: 2,5-bis(2-methoxyethoxy)aniline (2) and 2,5-bis[2-(2-methoxyethoxy)ethoxy]aniline (3). The corresponding polyanilines, P2 and P3, were characterized by means of fourier transform infrared (FT-IR), thermogravimetric analysis (TGA), and UV-VIS-NIR spectroscopies. The electrical conductivities at room temperature of emeraldine salt forms of P2 (P2-ES) and P3 (P3-ES) were evaluated to be sigma(rt) = 2.4 x 10(-3) and 1.7 x 10(-3) S/cm, respectively. The length of ethylene-1,2-dioxy chains on the polyaniline scarcely affected the electronic conductivity. A simple modification at 2,5-positions of aniline by introducing 1,2-ethlenedioxy groups dramatically altered the solubility of polyanilines in common organic solvents and water (400 g/L for P3-ES).

Synthesis of polyaniline with low polydispersity by using a supramolecular ionic assembly as the reaction medium.

A supramolecular ionic assembly comprised of an anionic oligo(phenylene ethynylene) and anilinium cations provides a unique reaction medium in which anilinum cations are concentrated and aligned. The oxidative polymerization (see figure) of aniline using the supramolecular ionic assembly (gray) yielded polyaniline (green/blue) with a number-average molar mass of 20,500 and polydispersity of 1.3.

Potassium complexes supported by monoanionic tetradentate amino-phenolate ligands: synthesis, structure and catalysis in the ring-opening polymerization of rac-lactide.

A series of potassium complexes bearing monoanionic tetradentate amino-phenolate ligands, [LK]2 (L = {(2-R1)C6H4CH2N[(CH2)2R2]CH2(4-R4-6-R3)C6H2O-}, R1 = NMe2, R2 = NEt2, R3 = CPh3, R4 = Me (1); R1 = R2 = NEt2, R3 = CPh3, R4 = Me (2); R1 = NMe2, R2 = NEt2, R3 = R4 = cumyl (4); R1 = R2 = OMe, R3 = tBu, R4 = Me (6); L = (2-NMe2)C6H4CH2N[[CH2-(S)-1-butylpyrrolidinyl]CH2(4-Me-6-CPh3)C6H2O-] (3)), have been synthesized via reactions of KN(SiMe3)2 and 1 equiv. of the corresponding aminophenols. The solid-state structures of typical complexes 4 and 6 are determined via X-ray diffraction studies, which reveal the dinuclear nature of these complexes. By contrast, DOSY measurements of 1, 4 and 6 suggest that these complexes are monomeric in solution. It is noteworthy that the coordination chemistry of these potassium complexes is versatile, which is closely related to the nature of the ortho-substituent of the phenolate ring, as indicated by the results of the corresponding spectroscopic studies. In the presence of iPrOH, 1-4 and 6 could initiate the polymerization of 500 equiv. of rac-lactide to achieve high monomer conversions within several minutes but afford atactic PLAs with slightly isotactic-enriched microstructures (Pm = 0.58-0.60). Experimental results also demonstrated that a bulky trityl substituent at the ortho-position of the phenolate ring of the ligand framework is beneficial for the enhancement of the activities of these potassium complexes.

Formic acid dehydrogenation with bioinspired iridium complexes: a kinetic isotope effect study and mechanistic insight.

Highly efficient hydrogen generation from dehydrogenation of formic acid is achieved by using bioinspired iridium complexes that have hydroxyl groups at the ortho positions of the bipyridine or bipyrimidine ligand (i.e., OH in the second coordination sphere of the metal center). In particular, [Ir(Cp*)(TH4BPM)(H2 O)]SO4 (TH4BPM: 2,2',6,6'-tetrahydroxyl-4,4'-bipyrimidine; Cp*: pentamethylcyclopentadienyl) has a high turnover frequency of 39 500 h(-1) at 80 °C in a 1 M aqueous solution of HCO2 H/HCO2 Na and produces hydrogen and carbon dioxide without carbon monoxide contamination. The deuterium kinetic isotope effect study clearly indicates a different rate-determining step for complexes with hydroxyl groups at different positions of the ligands. The rate-limiting step is ß-hydrogen elimination from the iridium-formate intermediate for complexes with hydroxyl groups at ortho positions, owing to a proton relay (i.e., pendent-base effect), which lowers the energy barrier of hydrogen generation. In contrast, the reaction of iridium hydride with a proton to liberate hydrogen is demonstrated to be the rate-determining step for complexes that do not have hydroxyl groups at the ortho positions.