The functionalization of methane, ethane, and other alkanes derived from fossil fuels is a central goal in the chemical enterprise. Recently, a photocatalytic system comprising [CeIVCl5(OR)]2− [CeIV, cerium(IV); OR, –OCH3 or –OCCl2CH3] was disclosed. The system was reportedly capable of alkane activation by alkoxy radicals (RO•) formed by CeIV–OR bond photolysis. In this work, we present evidence that the reported carbon-hydrogen (C–H) activation of alkanes is instead mediated by the photocatalyst [NEt4]2[CeCl6] (NEt4+, tetraethylammonium), and RO• are not intermediates. Spectroscopic analyses and kinetics were investigated for C–H activation to identify chlorine radical (Cl•) generation as the rate-limiting step. Density functional theory calculations support the formation of [Cl•][alcohol] adducts when alcohols are present, which can manifest a masked RO• character. This result serves as an important cautionary note for interpretation of radical trapping experiments.

A series of mononuclear pseudomacrocyclic cobalt complexes have been investigated as catalysts for O reduction. Each of these complexes, with Co reduction potentials that span nearly 400 mV, mediate highly selective two-electron reduction of O to HO (93-99%) using decamethylferrocene (Fc*) as the reductant and acetic acid as the proton source. Kinetic studies reveal that the rate exhibits a first-order dependence on [Co] and [AcOH], but no dependence on [O] or [Fc*]. A linear correlation is observed between log(TOF) vs E(Co) for the different cobalt complexes (TOF = turnover frequency). The thermodynamic potential for O reduction to HO was estimated by measuring the H/H open-circuit potential under the reaction conditions. This value provides the basis for direct assessment of the thermodynamic efficiency of the different catalysts and shows that HO is formed with overpotentials as low as 90 mV. These results are compared with a recently reported series of Fe-porphyrin complexes, which catalyze four-electron reduction of O to HO. The data show that the TOFs of the Co complexes exhibit a shallower dependence on E(M) than the Fe complexes. This behavior, which underlies the low overpotential, is rationalized on the basis of the catalytic rate law.

The selective reduction of O 2 , typically with the goal of forming H 2 O, represents a long-standing challenge in the field of catalysis. Macrocyclic transition-metal complexes, and cobalt porphyrins in particular, have been the focus of extensive study as catalysts for this reaction. Here, we show that the mononuclear Co-tetraarylporphyrin complex, Co(por OMe ) (por OMe = meso-tetra(4-methoxyphenyl)porphyrin), catalyzes either 2e – /2H + or 4e – /4H + reduction of O 2 with high selectivity simply by changing the identity of the Brønsted acid in dimethylformamide (DMF). The thermodynamic potentials for O 2 reduction to H 2 O 2 or H 2 O in DMF are determined and exhibit a Nernstian dependence on the acid p K a , while the Co III/II redox potential is independent of the acid p K a . The reaction product, H 2 O or H 2 O 2 , is defined by the relationship between the thermodynamic potential for O 2 reduction to H 2 O 2 and the Co III/II redox potential: selective H 2 O 2 formation is observed when the Co III/II potential is below the O 2 /H 2 O 2 potential, while H 2 O formation is observed when the Co III/II potential is above the O 2 /H 2 O 2 potential. Mechanistic studies reveal that the reactions generating H 2 O 2 and H 2 O exhibit different rate laws and catalyst resting states, and these differences are manifested as different slopes in linear free energy correlations between the log(rate) versus p K a and log(rate) versus effective overpotential for the reactions. This work shows how scaling relationships may be used to control product selectivity, and it provides a mechanistic basis for the pursuit of molecular catalysts that achieve low overpotential reduction of O 2 to H 2 O.

The oxygen reduction reaction catalyzed by homogeneous cobalt macrocycles typically leads to selective 2e − /2H + reduction of O 2 to H 2 O 2 . Variations in the reaction conditions make it difficult to compare the performance characteristics of different catalysts, however, and limits the ability to leverage insights to design improved catalysts. Here, we show that free energy relationships between the logarithm of the turnover frequency [log(TOF)] and the effective overpotential (η eff ) for the ORR enable systematic comparison of the catalytic performance of diverse Co−macrocycles under a variety of reaction conditions. The study is initiated by evaluating the ORR log(TOF)/η eff correlation for a series of Co(porphyrin) catalysts. The data show that these catalysts exhibit a different linear free energy relationship relative to previously reported Co(N 2 O 2 ) complexes and that this difference correlates with different rate laws associated with the two different classes of catalysts. These linear relationships are then compared to log(TOF)/η eff data for a diverse collection of other homogeneous cobalt ORR catalysts reported previously in the literature, and the collective analysis shows how different catalyst systems and their performance may be compared, even when the reactions are conducted under different conditions. This benchmarking method is recommended as a general strategy for systematic comparison of other (electro)catalysts and catalytic reactions.

A soluble, bis-ketiminate-ligated Co complex [Co(NO)] was recently shown to catalyze selective reduction of O to HO with an overpotential as low as 90 mV. Here we report experimental and computational mechanistic studies of the Co(NO)-catalyzed O reduction reaction (ORR) with decamethylferrocene (Fc*) as the reductant in the presence of AcOH in MeOH. Analysis of the Co/O binding stoichiometry and kinetic studies support an O reduction pathway involving a mononuclear cobalt species. The catalytic rate exhibits a first-order kinetic dependence on [Co(NO)] and [AcOH], but no dependence on [Fc*] or [O]. Differential pulse voltammetry and computational studies support Co-hydroperoxide as the catalyst resting state and protonation of this species as the rate-limiting step of the catalytic reaction. These results contrast previous mechanisms proposed for other Co-catalyzed ORR systems, which commonly feature rate-limiting protonation of a Co-superoxide adduct earlier in the catalytic cycle. Computational studies show that protonation is strongly favored at the proximal oxygen of the Co(OOH) species, accounting for the high selectivity for formation of hydrogen peroxide. Further analysis shows that a weak dependence of the ORR rate on the p K values of the protonated Co(OOH) species across a series of Co(NO) catalysts provides a rationale for the unusually low overpotential observed for O reduction to HO.

The utilization of earth‐abundant low‐toxicity metal ions in the construction of highly active and efficient molecular catalysts promoting the water oxidation reaction is important for developing a sustainable artificial energy cycle. However, the kinetic and thermodynamic properties of the currently available molecular water oxidation catalysts (MWOCs) have not been comprehensively investigated. This Review summarizes the current status of MWOCs based on first‐row transition metals in terms of their turnover frequency (TOF, a kinetic property) and overpotential (η, a thermodynamic property) and uses the relationship between log(TOF) and η to assess catalytic performance. Furthermore, the effects of the same ligand classes on these MWOCs are discussed in terms of TOF and η, and vice versa. The collective analysis of these relationships provides a metric for the direct comparison of catalyst systems and identifying factors crucial for catalyst design.

A photochemical C(sp3)–H oxygenation of arene and alkane substrates (including methane) catalyzed by [NEt4]2[CeIVCl6] under mild conditions (1 atm, 25 °C) is described.

Molecular ruthenium (Ru) complexes derived from the Ru blue dimer complex have been extensively studied for water oxidation. For example, monomeric Ru catalysts of polypyridyl-type ligands, such as 2,2-bipyridine-6,6-dicarboxylate (bda),...