Concerted Proton–Electron Transfer Reactivity at a Multimetallic Co₄O₄ Cubane Cluster

Abstract

High-valent oxocobalt(IV) species have been invoked as key intermediates in oxidative catalysis, but investigations into the chemistry of proton-coupled redox reactions of such species have been limited. Herein, the reactivity of an established water oxidation catalyst, [Co 4 O 4 (OAc) 4 (py) 4 ][PF 6 ], toward H-atom abstraction reactions is described. Mechanistic analyses and density functional theory (DFT) calculations support a concerted proton−electron transfer (CPET) pathway in which the high energy intermediates formed in stepwise pathways are bypassed. Natural bond orbital (NBO) calculations point to cooperative donor−acceptor σ interactions at the transition state, whereby the H-atom of the substrate is transferred to an orbital delocalized over a Co 3 (μ 3 -O) fragment. The mechanistic insights provide design principles for the development of catalytic C−H activation processes mediated by a multimetallic oxo metal cluster.

C–H Activation by RuCo3O4 Oxo Cubanes: Effects of Oxyl Radical Character and Metal–Metal Cooperativity

“…Significantly, electronic changes at the ligand set are transmitted through all three Co centers to the Ru–oxo moiety, indicating that the metal centers are working cooperatively as an assembly in oxidizing C–H substrates. Similar multimetallic cooperative effects have been predicted by DFT in the C–H activation study of the related Co 4 O 4 cubane . These studies indicate a direct influence of the electronic communication within the cubane clusters on C–H activation reactivity.…”

“…Similar multimetallic cooperative effects have been predicted by DFT in the C−H activation study of the related Co 4 O 4 cubane. 18 These studies indicate a direct influence of the electronic communication within the cubane clusters on C−H activation reactivity.…”

“…While significant efforts have elucidated the mechanisms of metal–oxo mediated PCET reactions, − a limited subset of these studies employ high-valent multimetallic-oxo complexes. − Indeed, reactive multimetallic-oxo species bearing well-characterized terminal metal–oxo/oxyl motifs are exceedingly rare despite their importance as key intermediates in oxidative catalysis. ,,− To date, few spectroscopically and computationally characterized high-valent multimetallic-oxo/oxyl species exist . Meyer and co-workers have provided evidence for a dioxo–Ru V intermediate, [(bpy) 2 (O)­Ru V O­Ru V (O)­(bpy) 2 ] 4+ (bpy = 2,2′-bipyridine), derived from PCET reactions of the binuclear oxo “blue dimer” [(bpy) 2 (H 2 O)­Ru III O­Ru III (H 2 O)­(bpy) 2 ] 4+ (Scheme c). − This dioxo–Ru V intermediate appears to possess considerable spin density at the terminal oxo ligands, and a radical coupling between the two terminal Ru V –oxo moieties has been proposed to mediate O–O bond formation. , Indeed, transient intermediate Ru V –oxo species are commonly invoked in water and organic oxidation catalysis. − Although metal–oxyl species are known to exhibit unique reactivity, it is unclear how the spin density localized on oxo ligands contributes to the PCET reactivity of multimetallic-oxo clusters. , Studies of high-valent multimetallic-oxo clusters bearing terminal oxo/oxyl moieties present an opportunity to probe the influence of metal–metal cooperativity and provide insight into the role of oxyl radical character in PCET reactions.…”

Concerted Multiproton–Multielectron Transfer for the Reduction of O2 to H2O with a Polyoxovanadate Cluster

“…In both examples, the BDFE­(O–H) of the bridging hydroxide ligands of the resultant 2e – /2H + reduced assembly are greater than 71 kcal/mol, on the basis of the reactivity of these complexes with sacrificial hydrogen atom donors with well-established BDFEs. Seminal work from Tilley and co-workers describes the BDFE­(O–H) of a μ 2 –OH moiety embedded in a cobalt oxide cubane (74 kcal/mol) . In all cases, the experimentally established BDFE­(O–H) values of the polynuclear metal oxide clusters are significantly larger (>10 kcal/mol) than that of complex V 6 O 7 (OH) 6 .…”

Multi-active Site Dynamics on a Molecular Cr/Co/Se Cluster Catalyst

“…21,22 There are only a few examples of metal-oxide and -chalcogenide clusters that are catalytically active, including [Mo3S13] 2and [MoFe3(µ3-S)4], for hydrogen evolution and hydrazine reduction, 23,24 respectively, and the cubane-type cluster [Co4(µ3-O)4], for catalytic C-H activation and water oxidation. 25,26 In contrast, metal clusters, particularly of gold, can catalyze a wide range of reactions. 27,28 However, in part due to scalability challenges, the elucidation of catalytic intermediates remains challenging and relies heavily on computational modeling.…”