Revisiting the O-O Bond Formation through Outer Sphere Wa- ter Molecules vs Bimolecular Mechanism in Water Oxidation Catalysis (WOC) by Cp*Ir-based Complexes

Abstract : Herein we present a detailed computational investigation of the mechanistic aspects of the water oxidation catalysis (WOC) for iridium based catalysts, Cp*Ir-L x=1-4 , (where Cp* = pentamethylcyclopentadiene; L 1 = bph = bi-phenyl; L 2 = phpy = 2-phenylpyridine; L 3 = bpy = 2,2′-bipyridyl; and L4 = bnql = benzo[n]quinoline or 1-naphthoquinoline). Our density functional theory (DFT) calculations not only confirmed that the O-O coupling step is the rate-limiting step as expected, but also provided useful insights about the number of water molecules involved in the catalytic cycle, which is under immense debate from a kinetic stand point. To test the effect of the metal environment, we tuned the ligands choosing four ligands (L 1-L 4) holding four kinds of chelation: CC , N-C, N-N and C-N' (L 1 = bph = bi-phenyl; L 2 = phpy = 2-phenylpyridine; L 3 = bpy = 2,2-bipyridyl; and L 4 = bnql = benzo[n]quinoline or 1-naphthoquinoline) ligands, respectively. A screening analysis of the potential energy surface revealed the water oxidation mechanism together with the optimum number of water molecules, concluding that three water molecules are mandatory for the right recipe, and that a highly positive iridium oxo center with high oxidation state (Ir(V)) pulls the electron density from lone pair of oxo oxygen and O center shows positive density. The most external water molecule or second OH 2 (W2) becomes nucleophilic while approaching the iridium center because of the proton abstraction by another water molecule (W3), and the nucleophilic water (W1) attacks the electrophilic oxygen facilitating the O-O bond formation. Moreover, the bimolecular mechanism for the O-O bond step was also calculated for terms of comparison. This study reveals that high cationic character of the metal is helpful for the O-O coupling. Water oxidation catalysis (WOC) is a subject of great scientific interest during last couple of decades owing to its potential application to make renewable energy carriers. 1 Our limited understanding of WOC, which is considered to be a difficult process in terms of chemical or electrochemical driving force, is the main hurdle in realizing this alternative energy scheme. 2 The one electron process (HO* + H + + e-) occurs at E=2.848 V vs NHE, while the two electron process and formation of H 2 O 2 has a barrier about E=1.776 V vs NHE. 3 The most widely studied four electron oxidation 2H 2 O  O 2 + 4e
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Farhan Pasha, Sai Vummaleti, Theodorus de Bruin, Albert Poater, Jean Basset. Revisiting the O-O Bond Formation through Outer Sphere Wa- ter Molecules vs Bimolecular Mechanism in Water Oxidation Catalysis (WOC) by Cp*Ir-based Complexes. European Journal of Inorganic Chemistry, Wiley-VCH Verlag, 2019, 2019 (15), pp.2093-2100. ⟨10.1002/ejic.201800500⟩. ⟨hal-02158037⟩

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