Ligand Exchange and Redox Processes in Iridium Triazolylidene Complexes Relevant to Catalytic Water Oxidation

Abstract

Iridium(III) complexes containing a bidentate spectator ligand have emerged as powerful catalyst precursors for water oxidation. Here we investigate the initial steps of transformation at the iridium center when using complex [IrCp*(pyr-trz)Cl] 1 (Cp* = pentamethylcyclopentadienyl, pyr-trz = 4-(2-pyridyl-)1,2,3-triazol-5-ylidene), a potent water oxidation catalyst precursor. Ligand exchange with water is facile and is reversed in the presence of chloride solutions, while MeCN substitution is only effective from the corresponding aqua complex. A pK a = 8.3 of the aqua complex was determined, which is in agreement with strong electron donation from the triazolylidene ligand that is comparable to aryl anions. Evaluation of the pH-dependent oxidation process in aqueous media reveals two regimes, between pH 4-8.5 and above 10.5, where proton-coupled electron transfer processes are occurring. These investigations will help to further optimize water oxidation catalysts and indicate that MeCN as a co-solvent has adverse effects for initiating water coordination in the oxidation process.3

Selective Conversion of Various Monosaccharaides into Sugar Acids by Additive‐Free Dehydrogenation in Water

“…The solubility of 2 in water is promoted by the polar nature of the triazolylidene ligand and a facile chloride-water ligand exchange at iridium (Scheme 1) as identify by the chemical speciation determined from ESI-MS and NMR spectroscopy. [33] Aqueous solutions of 2 were stirred and aliquots were extracted at different time intervals, diluted with water to a final concentration of 5 × 10 -4 M (based on the initial Ir concentration), and directly introduced to the mass spectrometer. The ESI-MS of 2 in water ( Figure 5) Figures S55, S56).…”

Unveiling the role of ancillary ligands in acceptorless benzyl alcohol dehydrogenation and etherification mediated by mesoionic carbene iridium complexes

“…As discussed above, this ratio erodes with increasing reaction time and converges to about 2 at higher conversions. The aniline complex 4 does not show any change of ratio throughout the reaction and forms the ether and the aldehyde in equal amounts (entries [10][11][12].…”

“…of lithium bis(trimethylsilyl)amide (LiHMDS), exploiting the relatively high aciditiy of the iridium-coordinated water ligand (pKa = 8.3). 10 The hydroxyl complex 6 was isolated as an orange solid in moderate yields (51%). Compound 6 is highly hygroscopic, and probably for this reason attempts to obtain microanalysis were unsuccessful.…”

“…We also noted that the iridiumbound water molecule can be easily replaced by acetonitrile, while the chloride derivative does not undergo exchange with acetonitrile but exchanges partly with water when solubilized in aqueous solvents. 10 Based on these observations, we were hypothesizing that the ancillary ligand in these complexes plays a distinct role in controlling the reactivity of the iridium center. We therefore targeted a set of triazolylidene pyridyl iridium complexes containing different neutral or anionic ancillary ligands to evaluate their impact on the catalytic activity.…”

Influence of the Protonatable Site in the Photo-Induced Proton-Coupled Electron Transfer between Rhenium(I) Polypyridyl Complexes and Hydroquinone

“…Different reports can be found in the literature involving Ru II polypyridyl complexes [12][13][14][15][16] and, more recently, Re I tricarbonyl diimines [17][18][19][20][21] and Ir III complexes. 22,23 Phenols have been typically used as quenchers in these studies, partly by their relevant function in biological systems, but also by its simplicity which make mechanistic investigations easier.…”