Highly Effective Water Oxidation Catalysis with Iridium Complexes through the Use of NaIO₄

Abstract

Exceptional water oxidation (WO) turnover frequencies (TOF=17,000 h(-1)), and turnover numbers (TONs) close to 400,000, the largest ever reported for a metal-catalyzed WO reaction, have been found by using [Cp*Ir(III)(NHC)Cl2] (in which NHC=3-methyl-1-(1-phenylethyl)-imidazoline-2-ylidene) as the pre-catalyst and NaIO4 as oxidant in water at 40 °C. The apparent TOF for [Cp*Ir(III)(NHC)X2] (1X, in which X stands for I (1I), Cl (1Cl), or triflate anion (1OTf)) and [(Cp*-NHCMe)Ir(III)I2] (2) complexes, is kept constant during almost all of the O2 evolution reaction when using NaIO4 as oxidant. The TOF was found to be dependent on the ligand and on the anion (TOF ranging from ≈600 to ≈1100 h(-1) at 25 °C). Degradation of the complexes by oxidation of the organic ligands upon reaction with NaIO4 has been investigated. (1)H NMR, ESI-MS, and dynamic light-scattering measurements (DLS) of the reaction medium indicated that the complex undergoes rapid degradation, even at low equivalents of oxidant, but this process takes place without formation of nanoparticles. Remarkably, three-month-old solution samples of oxidized pre-catalysts remain equally as active as freshly prepared solutions. A UV/Vis feature band at λmax =405 nm is observed in catalytic reaction solutions only when O2 evolves, which may be attributed to a resting state iridium speciation, most probably Ir-oxo species with an oxidation state higher than IV.

Carbene iridium complexes for efficient water oxidation: scope and mechanistic insights

“…Scattering indicating sizes as large as 1000 nm were observed by the fourth addition of 20 equivalents; however application of a slight vacuum to the solution causes a decrease in the observed size. Additionally, after 40 hours at atmospheric pressure, particle sizes returned to the baseline measurements of typical cerium(IV)/cerium(III) solutions 71 (cf. ibid.…”

Extending the range of pentasubstituted cyclopentadienyl compounds: The synthesis of a series of tetramethyl(alkyl or aryl)cyclopentadienes (Cp ∗ R ), their iridium complexes and their catalytic activity for asymmetric transfer hydrogenation

“…One of the first Cp ⁄ metal complexes was made by reaction between IrCl 3-Á3H 2 O and hexamethyldewarbenzene [7]. A complete review of Cp ⁄ complexes would be massive, but recent publications show that Cp ⁄ may be used in complexes for transfer hydrogenation [8][9][10][11], water oxidation [12][13][14], anti-cancer activity [15,16], and antimicrobial activity [17]. The development of much early metal chemistry was made possible in large part to the use of pentamethylcyclopentadiene.…”

Rate and Stability of Photocatalytic Water Oxidation using [Ru(bpy)3]2+ as Photosensitizer

“…1,2 While in attempts to mimic nature the first catalysts were based on manganese, more recently catalytically active compounds for water oxidation have been reported based on cobalt, 3-7 nickel, [8][9][10] iron, [11][12][13] copper, 14-20 ruthenium [21][22][23][24][25][26][27][28][29][30][31][32][33][34] and iridium. [35][36][37][38][39] Generally, the catalytic activity of these compounds is investigated using the oxidant (NH4)2[Ce(NO3)6] (CAN). [21][22][23] In three decades the turnover frequencies (TOF) of water-oxidation catalysts have increased from 2 × 10 -3 s -1 to more than 300 s -1 under optimal conditions.…”