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

Abstract

High-valent multimetallic-oxo/oxyl species have been implicated as intermediates in oxidative catalysis involving proton-coupled electron transfer (PCET) reactions, but the reactive nature of these oxo species has hindered the development of an in-depth understanding of their mechanisms and multimetallic character. The mechanism of C–H oxidation by previously reported RuCo3O4 cubane complexes bearing a terminal RuV–oxo ligand, with significant oxyl radical character, was investigated. The rate-determining step involves H atom abstraction (HAA) from an organic substrate to generate a Ru–OH species and a carbon-centered radical. Radical intermediates are subsequently trapped by another equivalent of the terminal oxo to afford isolable radical-trapped cubane complexes. Density functional theory (DFT) reveals a barrierless radical combination step that is more favorable than an oxygen-rebound mechanism by 12.3 kcal mol–1. This HAA reactivity to generate organic products is influenced by steric congestion and the C–H bond dissociation energy of the substrate. Tuning the electronic properties of the cubane (i.e., spin density localized on terminal oxo, basicity, and redox potential) by varying the donor ability of ligands at the Co sites modulates C–H activations by the RuV–oxo fragment and enables construction of structure–activity relationships. These results reveal a mechanistic pathway for C–H activation by high-valent metal–oxo species with oxyl radical character and provide insights into cooperative effects of multimetallic centers in tuning PCET reactivity.

Oxyl Character and Methane Hydroxylation Mechanism in Heterometallic M(O)Co3O4 Cubanes (M = Cr, Mn, Fe, Mo, Tc, Ru, Rh)

“…As we have shown in previous work, the reactivity of the terminal M(O) moiety is linearly correlated with O for [Ru(O)Co3O4(OAc)4(4-R-py)3] cubanes (R = OMe, H, CF3 and Me), 18 implying that this spin density (i.e., the oxyl character) can provide a useful measure in the design of C-H activation catalysts based on heterometallic cubanes. Natural bond orbital (NBO) analyses revealed a spin density of O = 0.36 in Ru(O)Co3O4 (Figure 3) and a significantly larger value of O = 0.93 in Fe(O)Co3O4 (Figure 3).…”

“…Nonetheless, the HAA TS remains 25.7 kcal mol -1 higher in energy than the Ru(O)Co3O4⋯C6H12 reactant complex. The significant height of this barrier is consistent with the ease of isolating and characterizing the Ru cubane 15,18 despite its oxyl character. Once the HAA step has been completed, however, the oxygen rebound step proceeds readily over a low barrier of only 5.2 kcal/mol, yielding the RuCo3O4⋯C6H11OH product, which lies 5.0 kcal/mol below the initial Ru(O)Co3O4⋯C6H12 reactant complex.…”

“…39 The hybrid meta-GGA exchange-correlation functional TPSSh 40 was used based on the good agreement between computational geometry optimizations and experimentally observed X-ray crystal structures, and between computed and experimental oxidation potentials. [15][16][17][18] Geometry optimizations and single-point energy refinement calculations were carried out with the double- def2-SVP and triple- def2-TZVP basis sets, 41 respectively. For numerical integration, we used Gaussian's ultrafine integration gridthat is, a pruned (99,590) grid with 99 radial shells and 590 angular points per shell.…”

A Single Bioinspired Hexameric Nickel Catechol–Alloxazine Catalyst Combines Metal and Radical Mechanisms for Alkene Hydrosilylation

“…To complete this arsenal, additional enzymatic strategies for electron transfer use bimetallic or polymetallic assemblies such as iron‐nickel [Fe−Ni] or iron‐iron [Fe−Fe] cores involved in hydrogenases, and iron‐sulfur [3Fe‐4S] and [4Fe‐4S] clusters found in ferredoxins. A recent example has shown that a H atom abstraction step performed by RuCo 3 O 4 oxo cubanes with oxyl radical ligands in the context of proton‐coupled electron transfer (PCET) reactivity is facilitated by metal‐metal cooperation in the multimetallic cluster [11] . Cooperative mechanisms are however not limited to metals, and the crystal structure of dihydromethanopterin reductase is a textbook example of this.…”

“…A recent example has shown that a H atom abstraction step performed by RuCo 3 O 4 oxo cubanes with oxyl radical ligands in the context of proton-coupled electron transfer (PCET) reactivity is facilitated by metal-metal cooperation in the multimetallic cluster. [11] Cooperative mechanisms are however not limited to metals, and the crystal structure of dihydromethanopterin reductase is a textbook example of this. This enzyme catalyzes the final step of methanopterin biosynthesis and is a 24-subunit cubic protein cage containing 48 molecules of FMN, which act as either cofactor or electron donor.…”