Computational Investigation and Design of Cobalt Aqua Complexes for Homogeneous Water Oxidation

Abstract

We study the water oxidation mechanism of the cobalt aqua complex [Co(H 2 O) 6 ] 2+ in a photocatalytic setup by means of density functional theory. Assuming a waternucleophilic-attack or radical coupling mechanism, we investigate how the oxidation state and spin configuration change during the catalytic cycle. In addition, different ligand environments are employed by substituting a water ligand with a halide, pyridine, or derivative thereof. This allows exploration of the effect of such ligands on the frontier orbitals and the thermodynamics of the water oxidation process. Moreover, the thermodynamically most promising water oxidation catalyst can be identified by comparing the computed free energy profiles to the one of an "ideal catalyst". Examination of such simple (hypothetical) water oxidation catalysts provides a basis for the derivation of design guidelines, which are highly sought for the development of efficient homogeneous water oxidation catalysts.

Water Oxidation Electrocatalysis with a Cobalt‐Borate‐Based Hybrid System under Neutral Conditions

“…[56] As schematized in Figure 8, the LUMO is the acceptor orbitalt argeted by the lone pair of water.F ollowing simple MO interaction arguments, it is expected to bear an antibonding character of Co d z 2 and oxygen p z orbitals. [52] Our calculations show that the ground states of both L and L' systems were overall quartet spin states, and the low spin (LS) doublet and high spin (HS) sextet werew ell separated (Table S3,S upporting Information). For the above purposes,t he model outlined in Figure 8w as modified, and an all-oxygenc oordinating environmentw as generated as well (denoted L').…”

“…Previously the second model was tested computationally. [52] Our calculations show that the ground states of both L and L' systems were overall quartet spin states, and the low spin (LS) doublet and high spin (HS) sextet werew ell separated (Table S3,S upporting Information). Therefore, the reactivity is attributed to the intermediate spin state.…”

“…[50,51] Despite previous reports on modeling the mechanism of water oxidation for similars ystems, [52][53][54] there had been no attempts to utilize multi-configurationalm ethods for any relatedC os ystem used as aw ater oxidation catalyst (WOC). [50,51] Despite previous reports on modeling the mechanism of water oxidation for similars ystems, [52][53][54] there had been no attempts to utilize multi-configurationalm ethods for any relatedC os ystem used as aw ater oxidation catalyst (WOC).…”

“…To gain insight into the catalytic performance of the cobalt borate catalyst of interest, we performed electronic structure calculations utilizingt he multi-configurational Complete Active Space Self ConsistentF ield (CASSCF) method. [50,51] Despite previous reports on modeling the mechanism of water oxidation for similars ystems, [52][53][54] there had been no attempts to utilize multi-configurationalm ethods for any relatedC os ystem used as aw ater oxidation catalyst (WOC). Considering the existing mechanisticd ata in the literature [53,55,56] and the studies pursuedi no ur research group, [38,40,56] as eries of PCET steps to oxidize water can be envisioned as shown in Figure 8.…”

Dehydrogenation Free Energy of Co2+(aq) from Density Functional Theory-Based Molecular Dynamics

“…15 Inspired by these observations, Schilling et al computationally investigated two potential water oxidation mechanisms of an octahedral model complex [Co II (H 2 O) 6 ] 2+ . 16 Apart from a water-nucleophilic-attack and the release of O 2 at the end of the cycles, all catalytic states were assumed to be connected by proton coupled electron transfer (PCET) steps. Therefore, free energy differences between those states were calculated employing an approach by Nørskov et al, [17][18][19][20] in which only the free energy of the electron-proton pair, not the individual chemical potential needs to be calculated.…”