Evolution of Iridium-Based Molecular Catalysts during Water Oxidation with Ceric Ammonium Nitrate

Abstract

Organometallic iridium complexes have been reported as water oxidation catalysts (WOCs) in the presence of ceric ammonium nitrate (CAN). One challenge for all WOCs regardless of the metal used is stability. Here we provide evidence for extensive modification of many Ir-based WOCs even after exposure to only 5 or 15 equiv of Ce(IV) (whereas typically 100-10000 equiv are employed during WOC testing). We also show formation of Ir-rich nanoparticles (likely IrO(x)) even in the first 20 min of reaction, associated with a Ce matrix. A combination of UV-vis and NMR spectroscopy, scanning transmission electron microscopy, and powder X-ray diffraction is used. Even simple IrCl(3) is an excellent catalyst. Our results point to the pitfalls of studying Ir WOCs using CAN.

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

“…As observed previously with related Ir(Cp*) complexes, 67 addition of CAN induced a color change of the initial yellow solution to blue within seconds. While the formation of a blue mixture or a blue layer 67 has periodically been attributed to IrO x formation due to the oxide's general tendency to absorb broadly around 580 nm 30,33 similar color changes have previously been noted in high-valent iridium aquo complexes. 68 To further investigate this issue with complex 4a, an experiment was performed using the previously described digital pressure transducers to manometrically follow the evolution of oxygen from several equal additions of oxidant with concurrent UV/Vis measurements When a single addition of 20 equivalents of CAN was added to a 0.5 mM solution of 4a no measurable oxygen production was observed ( Figure 6) though there is a rapid color change from yellow to blue and back over the course of 15 minutes (Figure 7, Top).…”

“…More recently, a variety of Cp*Ir complexes (Cp* = C 5 Me 5 -) have been shown to act as efficient water oxidation precatalysts. [27][28][29][30][31][32][33] These complexes are remarkable for their high activity compared to original iridium catalysts with 2-phenylpyridine ligand spheres. Mechanistic work under different reaction conditions led to controversial conclusions: while a heterogeneous mode of action has been put forward in some studies, [33][34][35] support for homogeneous catalysis has been obtained e.g.…”

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

“…[6][7][8][9][10][11][12][13][14][15][16][17][18] Depending on the ligand design, very high turnover numbers have been achieved. 19 Moreover, kinetic and mechanistic studies have provide increasingly compelling evidence that some complexes are precursors for homogeneous rather than heterogeneous [20][21][22] water oxidation catalysts and that the oxidation therefore occurs at an iridium center that is in a well-defined environment. 19,[23][24][25][26][27][28] This environment has remained elusive up to now despite various efforts to trap and isolate catalytically competent species.…”

Quantum Chemical Modeling of Homogeneous Water Oxidation Catalysis

“…[23,168,169] Heterogeneity problems have also been well documented by Crabtreei nareview on the study of homogeneoust ransition-metal catalysis. [170] Various experimental controlm ethods have been suggested to solve this problem.…”