What Determines Catalyst Functionality in Molecular Water Oxidation? Dependence on Ligands and Metal Nuclearity in Cobalt Clusters

Abstract

The metal-oxo M4O4 "cubane" topology is of special significance to the field of water oxidation as it represents the merging of bioinspired structural principles derived from natural photosynthesis with successful artificial catalysts known to date. Herein, we directly compare the rates of water oxidation/O2 evolution catalyzed by six cobalt-oxo clusters including the Co4O4 cubanes, Co4O4(OAc)4(py)4 and [Co4O4(OAc)2(bpy)4](2+), using the common Ru(bpy)3(2+)/S2O8(2-) photo-oxidant assay. At pH 8, the first-order rate constants for these cubanes differ by 2-fold, 0.030 and 0.015 s(-1), respectively, reflecting the number of labile carboxylate sites that allow substrate water binding in a pre-equilibrium step before O2 release. Kinetic results reveal a deprotonation step occurs on this pathway and that two electrons are removed before O2 evolution occurs. The Co4O4 cubane core is shown to be the smallest catalytic unit for the intramolecular water oxidation pathway, as neither "incomplete cubane" trimers [Co3O(OH)3(OAc)2(bpy)3](2+) and [Co3O(OH)2(OAc)3(py)5](2+) nor "half cubane" dimers [Co2(OH)2(OAc)3(bpy)2](+) and [Co2(OH)2(OAc)3(py)4](+) were found capable of evolving O2, despite having the same ligand sets as their cubane counterparts. Electrochemical studies reveal that oxidation of both cubanes to formally Co4(3III,IV) (0.7 V vs Ag/AgCl) occurs readily, while neither dimers nor trimers are oxidized below 1.5 V, pointing to appreciably greater charge delocalization in the [Co4O4](5+) core. The origin of catalytic activity by Co4O4 cubanes illustrates three key features for water oxidation: (1) four one-electron redox metals, (2) efficient charge delocalization of the first oxidation step across the Co4O4 cluster, allowing for stabilization of higher oxidizing equivalents, and (3) terminal coordination site for substrate aquo/oxo formation.

What Influences the Water Oxidation Activity of a Bioinspired Molecular CoII4O4 Cubane? An In-Depth Exploration of Catalytic Pathways

“…Nuclearity of clusters, ligand lability, interactions with buffer molecules, as well as the nature of the oxidant can strongly influence catalysis. [40][41][42] An example for the possibility of tuning WOC activity is a study by Duan et al 40 where they compared two mononuclear Ru-WOCs differing only in the nature of their axial ligands. They found the formation of a Ru-O-O-Ru dimer to be favored by ligands capable of π-π-stacking interactions, which led to that catalyst exhibiting a ten times higher turnover frequency than its analog.…”

Oxygen‐Atom Transfer Chemistry and Thermolytic Properties of a Di‐tert‐Butylphosphate‐Ligated Mn4O4 Cubane

“…Reactivity of its core oxo ligands to generate dioxygen (on irradiation)20 and water (on treatment with a H ⋅ donor)21 comprises a partially realized water oxidation cycle. In light of previous work on the improvements made possible by immobilization of Mn–oxo clusters,22–24 complexes containing the Mn 4 (μ 3 ‐O) 4 cubane unit (hereafter Mn 4 O 4 ) are promising candidates25, 26 for molecular precursor chemistry designed to incorporate the Mn 4 O 4 structure into a stable, inorganic network.…”

Molecular complexes in water oxidation: Pre-catalysts or real catalysts

“…First, neither trimmers (Co 3 O 3 and Co 3 O 4 ) nor dimer (Co 2 O 2 ) was found to be capable of releasing O 2 , suggesting that the metal nuclearity in cobalt clusters is a crucial factor in water oxidation. Second, ligand environment affected the stabilization of higher oxidizing intermediates [110]. …”