Relevance of Chemical vs. Electrochemical Oxidation of Tunable Carbene Iridium Complexes for Catalytic Water Oxidation

Abstract

Based on previous work that identified iridium(III) Cp* complexes containing a C,N‐bidentate chelating triazolylidene‐pyridyl ligand (Cp* = pentamethylcyclopentadienyl, C5Me5–) as efficient molecular water oxidation catalysts, a series of new complexes based on this motif has been designed and synthesized in order to improve catalytic activity. Modifications include specifically the introduction of electron‐donating substituents into the pyridyl unit of the chelating ligand (H, a; 5‐OMe, b; 4‐OMe, c; 4‐tBu, d; 4‐NMe2, e), as well as electronically active substituents on the triazolylidene C4 position (H, 8; COOEt, 9; OEt, 10; OH, 11; COOH, 12). Chemical oxidation using cerium ammonium nitrate (CAN) indicates a clear structure‐activity relationship with electron‐donating groups enhancing catalytic turnover frequency, especially when the donor substituent is positioned on the triazolylidene ligand fragment (TOFmax = 2500 h–1 for complex 10 with a MeO group on pyr and a OEt‐substituted triazolylidene, compared to 700 h–1 for the parent benchmark complex without substituents). Electrochemical water oxidation does not follow the same trend, and reveals that complex 8b without a substituent on the triazolylidene fragment outperforms complex 10 by a factor of 5, while in CAN‐mediated chemical water oxidation, complex 10 is twice more active than 8b. This discrepancy in catalytic activity is remarkable and indicates that caution is needed when benchmarking iridium water oxidation catalysts with chemical oxidants, especially when considering that application in a potential device will most likely involve electrocatalytic water oxidation.

Chemical and electrochemical water oxidation mediated by bis(pyrazol-1-ylmethyl)pyridine-ligated Cu(i) complexes

“…À1 for electrocatalytic water oxidation. The difference in activity trends between chemical water oxidation and electrocatalytic water oxidation was also observed by Olivares et al31 with iridium complexes containing a C,N-bidentate chelating triazolylidene-pyridyl ligands as catalysts. They observed enhanced activity for CAN-driven water oxidation in the presence of electron-donating groups on the ligand scaffold whereas in electrocatalytic water oxidation the activity is highest when the triazolylidene ligand is unsubstituted.…”

Synthesis, stability, and reactivity of mesoionic carbene iridium dihydride complexes

“…Synthesis and characterization of a series of dihydride iridium complexes with mesoionic carbene ligands. Metalation of the known 16,17 triazolium salts 1-3 was accomplished upon reaction with IrH5(PPh3)2 in toluene at elevated temperature (Scheme 1), similar to related work with imidazolium salts. 18 These conditions induced triazolium C-H bond activation via cyclometalation and afforded complexes 4-6 as pale yellow solids.…”

“…General. Ligands 1-3 and IrH5(PPh3)2 were prepared according to literature methods; 16,17,29 all other reagents and solvents were used as obtained from commercial suppliers. Unless specified, NMR spectra were recorded at 25 °C on Bruker spectrometers operating at 300 or 400 MHz General procedure for the synthesis of complexes 4-6: A mixture of the pyridyl-triazolium salt (1 eq) and IrH5(PPh3)2 (1 eq) in toluene (15 mL) was refluxed for 24 h. The solvent was removed under reduced pressure and the residue was dissolved in CH2Cl2 and layered with Et2O to induce precipitation of a light yellow solid, which was collected by decantation and dried in vacuo to afford the title complex.…”

Noncovalent Immobilization of Pentamethylcyclopentadienyl Iridium Complexes on Ordered Mesoporous Carbon for Electrocatalytic Water Oxidation

“…Many leading efforts to study these molecular Ir catalysts have focused on the use of chemical oxidants (e.g., NaIO 4 and ceric ammonium nitrate), [9,[11][12][13][14][15][16][17][18][19][20][21][22][23][24] with perhaps fewer studies on electrochemically driven water oxidation. [25][26][27][28][29] For example, Crabtree and coworkers identified the tris-aqua complex [Cp*Ir(H 2 O) 3 ] 2 (A) (Cp* ¼ pentamethylcyclopentadienyl) and the complex bearing the 2-(2-pyridyl)-2-propanolate ligand, B, as molecular water oxidation catalyst precursors at pH 7 and 1.7 V versus normal hydrogen electrode…”

“…Electron-donating groups on the triazolylidene ligand increase chemical water oxidation activity, in contrast to electrochemical oxidation where the best activity was found for the unsubstituted version. [25] Homogeneous electrocatalysts often suffer from limitations including a) catalyst crossover between anode and cathode, which can be kinetically inhibiting, b) catalytic activity limited by diffusion to the electrode, and c) a lack of stability of the catalyst. [30] Although, in some cases heterogeneous electrocatalysts can overcome these limitations that are often present for homogeneous catalytic systems, it is challenging to systemically tune and optimize the catalyst active sites of heterogeneous materials.…”