Precisely Controlling Ancillary Ligands to Improve Catalysis of Cp*Ir Complexes for CO2 Hydrogenation.

The regulation of ancillary ligands is critical to improve catalysis of Cp*Ir complexes for CO2 hydrogenation. Herein, a series of Cp*Ir complexes with N^N or N^O ancillary ligands were designed and synthesized. These N^N and N^O donors were derived from the pyridylpyrrole ligand. The solid-state structures of Cp*Ir complexes featured a pendant pyridyl group in 1-Cl and 1-SO4 and a pyridyloxy group in 2-Cl, 3-Cl, 2-SO4, and 3-SO4. These complexes were employed as catalysts for CO2 hydrogenation to formate in the presence of alkali under a pressure range of 0.1-8 MPa and temperature range of 25-120 °C. The catalytic activity of 2-SO4 with a pyridyloxy pendant group dramatically outperformed that of 1-SO4 and 3-SO4. The TOF of conversion of CO2 into formate reached 263 h-1 at 25 °C under a total pressure of 8 MPa (CO2/H2 = 1:1). The experiments and density functional theory calculations revealed that a pendant base in metal complexes plays a key role in the rate-determining heterolytic H2 splitting and enhancing the proton transfer by forming a hydrogen bonding bridge thereby improving the catalytic activity.

Metal-Ligand Cooperation in Cp*Ir-Pyridylpyrrole Complexes: Rational Design and Catalytic Activity in Formic Acid Dehydrogenation and CO2 Hydrogenation under Ambient Conditions.

Interconversion between CO2 + H2 and FA/formate is the most promising strategy for the fixation of carbon dioxide and reversible hydrogen storage; however, FA dehydrogenation and CO2 hydrogenation are usually studied separately using different catalysts for each reaction. This report describes of the catalysis of [Cp*Ir(N∧N)(X)]n+ (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; X = Cl, n = 0; X = H2O, n = 1) bearing a proton-responsive N∧N pyridylpyrrole ligand for both reactions. Complex 2-H2O catalyzes FA dehydrogenation at 90 °C with a TOFmax of 45 900 h-1. Its catalysis is more active in aqueous solution than in neat solution under base-free conditions. These complexes also catalyze CO2 hydrogenation in the presence of base to formate under atmospheric pressure (CO2/H2 = 0.05 MPa/0.05 MPa) at 25 °C with a TOF value of 4.5 h-1 in aqueous solution and with a TOF value of 29 h-1 in a methanol/H2O mixture solvent. The possible mechanism is proposed by intermediate characterization and KIE experiments. The extraordinary activity of these complexes are mainly attributed to the metal-ligand cooperative effect of the the pyrrole group to accept a proton in the dehydrogenation of formic acid and assist cooperative heterolytic H-H bond cleavage in CO2 hydrogenation.

Neutral Cyclometalated Ir(III) Complexes with Pyridylpyrrole Ligand for Photocatalytic Hydrogen Generation from Water.

To explore structure-activity relationships with respect to light-harvesting behavior, a family of neutral iridium complexes [Ir(ppy)2(LR)] 1-4 (where ppy = 2-phenylpyridine, and N̂N = 2-(1H-pyrrol-2-yl)pyridine and its functionalized derivatives) were designed and synthesized. The structural modifications in metal complexes are accomplished through the attributions of electron-donating CH3 in 2, OCH3 in 3, and electron-withdrawing CF3 in 4. The structural analysis displays that the pyridylpyrrole acts as one-negative charged bidentated ligand to chelate the iridium center. The electrochemical and photophysical properties of these complexes were systematically studied. The neutral 1-4 as well as the ionic structurally analogous [Ir(ppy)2(bpy)](PF6) (5) were utilized as PSs in photocatalytic hydrogen generation from water with [Co(bpy)3](PF6)2 as catalyst and triethanolamine (TEOA) as electron sacrificial agent in the presence of salt LiCl. Complex 1 maintains activity for more than 144 h under irradiation, and the total turnover number is up to 1768. The electrochemical properties and the quenching reaction indicate the H2 generation by neutral complexes 1-4 is involved exclusively in the oxidative quenching process.

3D Uranyl Organic Frameworks Supported by Rigid Octadentate Carboxylate Ligand: Synthesis, Structure Diversity, and Luminescence Properties.

Seven three dimensional (3D) uranyl organic frameworks (UOFs), formulated as [NH4 ][(UO2 )3 (HTTDS)(H2 O)] (1), [(UO2 )4 (HTTDS)2 ](HIM)6 (2, IM=imidazole), [(UO2 )4 (TTDS)(H2 O)2 (Phen)2 ] (3, Phen=1,10-phenanthroline), [Zn(H2 O)4 ]0.5 [(UO2 )3 (HTTDS)(H2 O)4 ] (4), and {(UO2 )2 [Zn(H2 O)3 ]2 (TTDS)} (5), {Zn(UO2 )2 (H2 O)(Dib)0.5 (HDib)(HTTDS)} (6, Dib=1,4-di(1H-imidazol-1-yl)benzene) and [Na]{(UO2 )4 [Cu3 (u3 -OH)(H2 O)7 ](TTDS)2 } (7) have been hydrothermally prepared using a rigid octadentate carboxylate ligand, tetrakis(3,5-dicarboxyphenyl)silicon(H8 TTDS). These UOFs have different 3D self-assembled structures as a function of co-ligands, structure-directing agents and transition metals. The structure of 1 has an infinite ribbon formed by the UO7 pentagonal bipyramid bridged by carboxylate groups. With further introduction of auxiliary N-donor ligands, different structure of 2 and 3 are formed, in 2 the imidazole serves as space filler, while in 3 the Phen are bound to [UO2 ]2+ units as co-ligands. The second metal centers were introduced in the syntheses of 4-7, and in all cases, they are part of the final structures, either as a counterion (4) or as a component of framework (5-7). Interesting, in 7, a rare polyoxometalate [Cu3 (µ3 -OH)O7 (O2 CR)4 ] cluster was found in the structure. It acts as an inorganic building unit together with the dimer [(UO2 )2 (O2 CR)4 ] unit. Those uranyl carboxylates were sufficiently determined by single crystal X-ray diffraction, and their topological structures and luminescence properties were analyzed in detail.

Zinc complexes supported by pyridine-N-oxide ligands: synthesis, structures and catalytic Michael addition reactions.

New dipyridylpyrrole N-oxide ligands HL1 and HL2 are designed and synthesized via oxidation of 2-(5-(pyridin-2-yl)-1H-pyrrol-2-yl)pyridine (Hdpp) by using 3-chloroperbenzoic acid (m-CPBA) in CH2Cl2. The treatment of ZnEt2 with two equiv. of HL1 and HL2 affords [Zn(L1)2] and [Zn(L2)2] in medium yield, respectively. These ligands and zinc complexes are fully characterized by NMR, IR, UV-vis and ESI-MS spectroscopy and X-ray diffraction analysis. The structure of HL1 and HL2 shows a planar geometry. The intramolecular hydrogen-bond interactions between the imino hydrogen and N-oxide oxygen atom are observed. In [Zn(L1)2] and [Zn(L2)2], two ligands chelate to the zinc metal with a cross perpendicular geometry. The zinc complexes were employed as a highly efficient catalyst for the thiol-Michael addition of thiols to α,ß-unsaturated ketones in EtOH at room temperature. The loading of the catalyst is lowered to 0.01 mol%. The catalytic mechanism was proposed based on NMR and ESI-MS experiments.