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Issue 9, 2014
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Density functional theory calculations of catalytic mechanistic pathways for the formation of O2 involving triazolylidene iridium complexes

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Abstract

We have studied Ir(III)-catalysed water oxidation pathways at the CAM-B3LYP level of Density Functional Theory (DFT), involving a novel oxygen-evolving catalyst (OEC), for a low-concentration mechanism. Starting from the dichloride complex IrIII–Cl (Ir = [IrCp*Cl(triazolylidene)], Cp* = C5Me5), DFT predicts reaction with one water molecule and formation of a cationic solvento complex [IrIII–OH2]+, followed by two proton-coupled electron transfer (PCET) steps; one involves an [IrIII–OH2]+/[IrIV–OH]+ transition, whilst the other is an [IrIV–OH]+ to [IrV[double bond, length as m-dash]O]+ transformation. The oxidation potentials and pKa values were calculated for the transformation of [IrIII–OH2]+ to [IrV[double bond, length as m-dash]O]+ for both sequential orders of H+/e and e/H+ transfer. Deprotonation of [IrIII–OH2]+ was found to be feasible even in acidic conditions (with a pKa of 1.08), while the deprotonation of [IrIV–OH]+ is strongly disfavoured (pKa = 10.33). The computed oxidation potentials are rather high (1.81 V and 2.01 V for [IrIII–OH2]+ and [IrVI–OH]+, respectively); however, subsequent proton dissociation is more favoured (with pKa values for the oxidised intermediates of −16.1 and −8.1, respectively). This indicates the possibility of concerted, as opposed to stepwise, H+/e transfer (PCET). Further, the [IrV[double bond, length as m-dash]O]+ intermediate was found to react with two water molecules to release O2. This sequence involves O–O bond formation followed by one PCET step, while a further single electron transfer (oxidation) step generates an [IrV–O–O]+ species, which liberates O2 and regenerates the [IrIII–OH2]+ complex.

Graphical abstract: Density functional theory calculations of catalytic mechanistic pathways for the formation of O2 involving triazolylidene iridium complexes

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Supplementary files

Article information


Submitted
03 Mar 2014
Accepted
12 May 2014
First published
28 May 2014

New J. Chem., 2014,38, 4060-4070
Article type
Paper

Density functional theory calculations of catalytic mechanistic pathways for the formation of O2 involving triazolylidene iridium complexes

S. Bandaru, N. J. English and J. M. D. MacElroy, New J. Chem., 2014, 38, 4060
DOI: 10.1039/C4NJ00321G

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