AbstractsBiology & Animal Science

Electrocatalytic Carbon Dioxide Reduction Using Cationic Cp*-Iridium Complexes Bearing Unsymmetrically Substituted Bipyridine Ligands

by Fanni Daruny Sypaseuth

Institution: Freie Universität Berlin
Department: FB Biologie, Chemie, Pharmazie
Degree: PhD
Year: 2015
Record ID: 1109251
Full text PDF: http://edocs.fu-berlin.de/diss/receive/FUDISS_thesis_000000098708


Due to environmental considerations and the limitation of fossil based energy resources, carbon diox- ide reduction into liquid fuels and energy storage materials, such as formic acid, is one of the most important topics of recent research. Ir complexes play a significant role in both the hydrogenation and electrocatalytic conversion of CO2 . This thesis describes the synthesis of new Ir complexes and their elecrocatalytic activity in CO2 reduction. Unsymmetrically substituted bipyridine ligands were prepared by Pd catalyzed C−H bond arylation of pyridine N -oxides using halopyridines. This method was extended towards the formation of asymmetrically substituted terpyridines. The substituents on both the pyridine N -oxide and halopyridine rings played a crucial role in the yields of the formed products. The presence of the electron withdrawing ethoxycarbonyl group in the 4 position on the pyridine N -oxide ring was essential to drive the reaction to moderate to good yields. As for the arylating reagents, using 2-bromopyridine and 2-bromo-4-tert-butylpyridine resulted in the best yields. The bi- and terpyridine N -oxides were reduced to the corresponding bi- and terpyridines in excellent yields. [Cp*Ir(bpy)Cl] + -type complexes were subsequently prepared, bearing symmetrically and unsymmet- rically substituted, as well as unsubstituted bpy ligands. The complexes were characterized by standard methods and their electronic properties were investigated by cyclic voltammetry. The expected trend was observed in the potentials of the Ir centered reduction event of the complexes: electron donating substituents shifted the reduction potentials to more negative values, whereas complexes bearing electron withdrawing CO2 Et group were reduced at more positive potentials. In CO2 saturated MeCN/H2O 9:1, v/v mixture all complexes displayed a catalytic current wave between −1.80 and −2.05 V vs Fc/Fc + . To further test the catalytic behavior of the complexes, controlled potential electrolysis experiments were carried out in a self-made two compartment cell. First, the electrolysis conditions were examined using [Cp*Ir(bpy)Cl]Cl complex bearing unsubstituted 2,2' -bipyridine. At −1.80 V vs Ag wire, formation of formate and formic acid were observed after electrolysis for 3 h in MeCN/H 2 O 95:5, v/v and MeCN/MeOH 1:1, v/v solvent mixtures. Occasionally, another product, possibly formaldehyde was also obeserved, however its formation was not reproducible. After optimization of the conditions, the catalysts were tested to determine turnover frequencies with respect to HCO2H formation. Although a straightforward trend between electronic properties of the catalyst and their activity in the CO2 reduction could not be observed, an influence of the ligand substituents on the activities was clear. All new complexes showed higher activities than the [Cp*Ir(bpy)Cl]Cl complex bearing unsubstituted 2,2' -bipyridine ligand. Durch zunehmende Umweltbedenken und die Begrenzung fossiler Ressourcen hat die Reduktion von…