Catalytic Water Splitting with an Iridium Carbene Complex: A Theoretical Study

Abstract

Catalytic water oxidation at Ir(OH)(+) (Ir = IrCp*(Me2NHC), where Cp* = pentamethylcyclopentadienyl and Me2NHC = N,N'-dimethylimidazolin-2-ylidene) can occur through various competing channels. A potential-energy surface showing these various multichannel reaction pathways provides a picture of how their importance can be influenced by changes in the oxidant potential. In the most favourable calculated mechanism, water oxidation occurs via a pathway that includes four sequential oxidation steps, prior to formation of the O-O bond. The first three oxidation steps are exothermic upon treatment with cerium ammonium nitrate and lead to formation of Ir(V) (=O)(O(·))(+), which is calculated to be the most stabile species under these conditions, whereas the fourth oxidation step is the potential-energy-determining step. O-O bond formation takes place by coupling of the two oxo ligands along a direct pathway in the rate-limiting step. Dissociation of dioxygen occurs in two sequential steps, regenerating the starting material Ir(OH)(+). The calculated mechanism fits well with the experimentally observed rate law: v = kobs[Ir][oxidant]. The calculated effective barrier of 24.6 kcal mol(-1) fits well with the observed turnover frequency of 0.88 s(-1). Under strongly oxidative conditions, O-O bond formation after four sequential oxidation steps is the preferred pathway, whereas under milder conditions O-O bond formation after three sequential oxidation steps becomes competitive.

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

“…Successively, many other molecular Ir WOCs were reported, and many excellent and exhaustive reviews recently appeared in the literature , , . Herein, the focus is on the stability and activity of Ir WOCs, two strictly interlocked and crucial aspects.…”

Anode Preparation Strategies for the Electrocatalytic Oxidation of Water Based on Strong Interactions between Multiwalled Carbon Nanotubes and Cationic Acetylammonium Pyrene Moieties in Aqueous Solutions

“…There are, however, no examples of molecular‐based iridium water oxidation catalysts that, combined with MWCNTs, provide stable and efficient anodes that are active in the electrocatalytic oxidation of H 2 O. In this report the properties of compound 1 as a dispersing agent for MWCNTs in aqueous solutions combined with the excellent catalytic properties of NHC IrCp*Cl 2 are discussed . This is realized by the direct covalent attachment of the catalyst to the pyrene anchor to obtain the novel catalytic precursor 3 .…”

Energetic Effects of a Closed System Approach Including Explicit Proton and Electron Acceptors as Demonstrated by a Mononuclear Ruthenium Water Oxidation Catalyst

“…In the optimisation of catalysts, computational techniques are increasingly employed to test proposed mechanisms and to infer catalyst design principles . When evaluating proposed catalytic cycles, the free energies of the various catalytic intermediates are compared .…”