Oxidation reactions of both inorganic and organic substrates are among the most important chemical transformations, with application in sustainable chemistry and chemical synthesis. A library of half sandwich IrIII oxidation complexes with varying ligands were synthesised (54-78% yield) and fully characterised (NMR, UV-vis, crystal data), in order to investigate the effect on catalyst activity for water and C-H oxidation reactions. The electrochemical transition of IrIII to IrIV was investigated (cyclic voltammetry (CV)) and found to vary between ligand sets, such that alkyl substituted compounds had a lower midpoint potential than aryl substituted. Solution speciation under aqueous conditions was also investigated for all complexes (UV-vis spectroscopy), as well as investigation into catalyst activation by oxidative loss of the pentamethylcyclopentadienyl ligand (1H NMR, UV-vis spectroscopy).The catalytic activity for complexes Ir1-Ir7 was investigated for water oxidation with chemical oxidants, by oxygen evolution assays with a Clark electrode. All complexes evolved oxygen to some extent, with ligand effects causing significant variation in the rate of water oxidation (4.60 mM min-1 to 0.02 mM min-1 with sodium periodate in pure H2O). Mechanistic studies including H/D kinetic isotope effects and reaction progress kinetic analysis showed primary KIEs of 1.3-2.5, indicating O-H cleavage to be in the rate determining step. Determination of the catalyst order revealed an order in iridium of 0.5-0.6 for Ir1-Ir6 and 0.9 for Ir7, which was proposed to be due differences in the activecatalyst species for Ir7.The complexes were also tested for C-H oxidation performance and followed by 1H NMR. The reaction profiles for precatalysts Ir1-Ir7 all showed a plateau conversion with C-H oxidation, varying between 52% - 88%, which was attributed to a competition reaction with water oxidation. The varying ligands impart C-H/water oxidation selectivity onto the catalysts. The C-H oxidation scope was extended by investigations into the oxidation of several terpene-based compounds were also conducted (gas chromatography mass spectrometry, 1H NMR).Comparison of the water oxidation activity of the catalysts as driven by electrochemical potential (as followed by Clark electrode and chronoamperometry) exposed surprising trends that did not correlate with the chemical oxidant data and highlights the importance of reaction conditions when comparing water oxidation activity. Catalyst immobilisation was also attempted with a range of metal oxide supports (indium tin oxide on fluorine doped tin oxide glass, BiVO4, Fe2O3) and analysis of the resulting electrode assessed by CV. Fourier transform alternating current voltammetry (FTACV) was employed in order to investigate the oxidation state of the iridium during electrochemical water oxidation, revealing a clear [Ir] redox transition at the foot of the catalytic wave, proposed to be a key transition to the active catalytic species.
|Date of Award||22 Nov 2018|
|Supervisor||Ulrich Hintermair (Supervisor) & Frank Marken (Supervisor)|