Reaction Progress Kinetic Analysis as a Tool To Reveal Ligand Effects in Ce(IV)-Driven IrCp*-Catalyzed Water Oxidation

Abstract

A series of iridium-based complexes have been evaluated in Ce(IV)-driven water oxidation catalysis. Detailed kinetic data have been obtained from UV−vis stopped-flow experiments, and these data have been analyzed using reaction progress kinetic analysis. The graphical plots show that there are three clear phases in the reaction: catalyst activation, water oxidation catalysis, and cerium concentration controlled catalysis at the end of the reaction. The ligand attached to the IrCp* complex has a clear influence on both the activation as well as the catalysis. Some bidentate ligands result in relatively slow catalysis, and the first-order in iridium supports the presence of mononuclear active species; however, other bidentates form the more active dinuclear species. Monodentate ligands allow the formation of bis-μ-oxo bridged dimeric species, supported by kinetics displaying 1.6-order in [Ir], leading to high reaction rates.

Rational Design Rules for Molecular Water Oxidation Catalysts based on Scaling Relationships

“…[20] The dimeric nature of this type of iridium catalysts is also supported by kinetic analysiso f the reaction progress. [22] When mononucleari ridiumc atalyst 4 was anchored to an ITOs urface, with no freedom to come in close proximity to as econd iridium center,p reventing ROC, indeed am uch higher overpotential of 462 mV was found at the onset of water oxidation ( Figure 5, 4@MO x ). [23] These examples show that the ROC mechanism can indeed lead to powerful catalysts for water oxidation that are active at low overpotential.…”

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“…[25] This was attributed to the possibility of WOCs to form multimetallic species exhibiting ah ighera ctivity than the single-site precursors. It is interesting to notice that m = 1.6/1.7 was found for catalysts 2, 4,a nd 5 when the WO reaction is driven by CAN.…”

“…It was decided to test them using NaIO 4 as sacrificial oxidantbecause, being less aggressive than cerium ammonium nitrate (CAN) [22][23][24] and not showingb yi tself any tendency to form nanoparticles (NPs), as CAN does, it avoids complications in the kinetic studies. [25] The following catalysts were selected for this study (Scheme 1): cis-[Ir(ppy) 2 (H 2 O) 2 ]OTf (ppy = 2-phenylpyridine) (1), the first Ir molecular WOC, which is also the first organometallic WOC in general,r eported by Bernhard and co-workersi n 2008; [26] [Cp*Ir(H 2 O) 3 ]NO 3 (Cp* = 1,2,3,4,5-pentamethyl-cyclopentadienyl anion) (2), the tris-acquoC p* complex independently reportedb yC rabtree and co-workers [27] and our group in 2010; [28] [Cp*Ir(bzpy)Cl] (bzpy = 2-benzoylpyridine) (3), [28][29][30] analogoust ot he first IrCp* WOC reported by Crabtree and coworkers [Cp*Ir(ppy)Cl] in 2009, [31] but slightly more active and more soluble in water;[Cp*IrCl 2 (Me 2 -NHC)] (Me 2 -NHC = N-dimethylimidazolin-2-ylidene) (4), [32][33][34] as ar epresentative of the Ir carbene WOCs; [35,36] [Cp*Ir(pyalk)Cl] (pyalk = 2-pyridine-isopropanoate) (5), [37,38] perhaps the most investigated Ir WOC; [39] [Cp*Ir(pic)NO 3 ]( pic = 2-pyridine-carboxylate) (6), apparently the Water oxidation (WO) is ac entral reactioni nt he photo-and electro-synthesis of fuels. [25] The following catalysts were selected for this study (Scheme 1): cis-[Ir(ppy) 2 (H 2 O) 2 ]OTf (ppy = 2-phenylpyridine) (1), the first Ir molecular WOC, which is also the first organometallic WOC in general,r eported by Bernhard and co-workersi n 2008; [26] [Cp*Ir(H 2 O) 3 ]NO 3 (Cp* = 1,2,3,4,5-pentamethyl-cyclopentadienyl anion) (2), the tris-acquoC p* complex independently reportedb yC rabtree and co-workers [27] and our group in 2010; [28] [Cp*Ir(bzpy)Cl] (bzpy = 2-benzoylpyridine) (3), [28][29][30] analogoust ot he first IrCp* WOC reported by Crabtree and coworkers [Cp*Ir(ppy)Cl] in 2009, [31] but slightly more active and more soluble in water;[Cp*IrCl 2 (Me 2 -NHC)] (Me 2 -NHC = N-dimethylimidazolin-2-ylidene) (4), [32]…”

The Middle‐Earth between Homogeneous and Heterogeneous Catalysis in Water Oxidation with Iridium

“…The activity of Ir WOCs was initially evaluated using CAN as sacrificial oxidant [Equation (2)] . This means that the monitored reaction is:…”