Improved electrocatalysts for the oxygen reduction reaction (ORR) are critical for the advancement of fuel cell technologies. Herein, we report a series of 11 soluble iron porphyrin ORR electrocatalysts that possess turnover frequencies (TOFs) from 3 s–1 to an unprecedented value of 2.2 × 106 s–1. These TOFs correlate with the ORR overpotential, which can be modulated by changing the E1/2 of the catalyst using different ancillary ligands, by changing the solvent and solution acidity, and by changing the catalyst’s protonation state. The overpotential is well-defined for these homogeneous electrocatalysts by the E1/2 of the catalyst and the proton activity of the solution. This is the first such correlation for homogeneous ORR electrocatalysis, and it demonstrates that the remarkably fast TOFs are a consequence of high overpotential. The correlation with overpotential is surprising since the turnover limiting steps involve oxygen binding and protonation, as opposed to turnover limiting electron transfer commonly found in Tafel analysis of heterogeneous ORR materials. Computational studies show that the free energies for oxygen binding to the catalyst and for protonation of the superoxide complex are in general linearly related to the catalyst E1/2, and that this is the origin of the overpotential correlations. This analysis thus provides detailed understanding of the ORR barriers. The best catalysts involve partial decoupling of the influence of the second coordination sphere from the properties of the metal center, which is suggested as new molecular design strategy to avoid the limitations of the traditional scaling relationships for these catalysts.

Plot of turnovers versus time for catalytic oxidative hydrophenylation of propylene with Cu(II) oxidant that is regenerated in situ (using (5-FP)Rh(TFA)( 2-C2H4) (1) as catalyst

Cationic platinum(II) complexes [( t bpy)Pt(Ph)(L)]+ [ t bpy =4,4′-di-tert-butyl-2,2′-bipyridyl; L = THF, NC5F5, or NCMe] catalyze the hydrophenylation of ethylene to generate ethylbenzene and isomers of diethylbenzene. Using ethylene as the limiting reagent, an 89% yield of alkyl arene products is achieved after 4 h at 120 °C. Catalyst efficiency for ethylene hydrophenylation is diminished only slightly under aerobic conditions. Mechanistic studies support a reaction pathway that involves ethylene coordination to Pt(II), insertion of ethylene into the Pt–phenyl bond, and subsequent metal-mediated benzene C–H activation. Studies of stoichiometric benzene (C6H6 or C6D6) C–H/C–D activation by [( t bpy)Pt(Ph-d n )(THF)]+ (n = 0 or 5) indicate a k H/k D = 1.4(1), while comparative rates of ethylene hydrophenylation using C6H6 and C6D6 reveal k H/k D = 1.8(4) for the overall catalytic reaction. DFT calculations suggest that the transition state for benzene C–H activation is the highest energy species along the catalytic cycle. In CD2Cl2, [( t bpy)Pt(Ph)(THF)][BAr′4] [Ar′ = 3,5-bis(trifluoromethyl)phenyl] reacts with ethylene to generate [( t bpy)Pt(CH2CH2Ph)(η2-C2H4)][BAr′4] with k obs = 1.05(4) × 10–3 s–1 (23 °C, [C2H4] = 0.10(1) M). In the catalytic hydrophenylation of ethylene, substantial amounts of diethylbenzenes are produced, and experimental studies suggest that the selectivity for the monoalkylated arene is diminished due to a second aromatic C–H activation competing with ethylbenzene dissociation.

Linear alkyl benzenes (LAB) are global chemicals that are produced by acid-catalyzed reactions that involve the formation of carbocationic intermediates. One outcome of the acid-based catalysis is that 1-phenylalkanes cannot be produced. Herein, it is reported that [Rh(μ-OAc)(η-CH)] catalyzes production of 1-phenyl substituted alkene products via oxidative arene vinylation. Since C═C bonds can be used for many chemical transformations, the formation of unsaturated products provides a potential advantage over current processes that produce saturated alkyl arenes. Conditions that provide up to a 10:1 linear:branched ratio have been achieved, and catalytic turnovers >1470 have been demonstrated. In addition, electron-deficient and electron-rich substituted benzenes are successfully alkylated. The Rh catalysis provides ortho:meta:para selectivity that is opposite to traditional acid-based catalysis.

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