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.

Cooperative reactivity between multiple metal centers in close proximity is a common phenomenon for enzymatic systems, 1 whereas most synthetic homogeneous transition metal catalysts consist of one metal center. Recently, there have been growing efforts to develop bimetallic catalysts enabling cooperative, simultaneous activation of both an electrophile and a nucleophile in asymmetric catalysis. 2 For example, chiral metal salen complexes developed by Jacobsen and co-workers are highly efficient catalysts for the asymmetric epoxide opening reactions, wherein two metal centers are involved in the transition state. 3 This discovery led to the development of a number of covalently linked macromolecular salen structures 4 showing improved reactivity as well as enantioselectivity.The self-assembly or supramolecular approach using reversible noncovalent bonding interactions 5 such as hydrogen bonding 6 is a highly attractive strategy which enables rapid construction of multinuclear systems in solution via self-assembly of monomeric units. Herein we wish to report a novel dinuclear Co(II)-salen catalyst self-assembled through complementary hydrogen bonding interactions which results in significant enhancement of reaction rates and enantioselectivity of Henry reactions. Our catalyst design features two 2-pyridone/aminopyridine hydrogen bonding pairs 6e,f to create self-assembled dimers in solution (Figure 1). The unsymmetrical (2a) and symmetrical (2b-c) 7 salen ligands were prepared by condensation of salicylaldehydes and (R,R)-1,2cyclohexane diamine (Figure 1). 8 Cobalt (II) complexes 1a-d were prepared by reacting the corresponding salens with Co(OAc) 2 • 4H 2 O under argon.

Control of enantioselectivity remains a major challenge in radical chemistry. The emergence of metalloradical catalysis (MRC) offers a conceptually new strategy for addressing this and other outstanding issues. Through the employment of D 2 -symmetric chiral amidoporphyrins as the supporting ligands, Co(II)-based MRC has enabled the development of new catalytic systems for asymmetric radical transformations with a unique profile of reactivity and selectivity. With the support of new-generation HuPhyrin chiral ligands whose cavity environment can be fine-tuned, the Co-centered d-radicals enable to address challenging issues that require exquisite control of fundamental radical processes. As showcased with asymmetric 1,5-C−H amination of sulfamoyl azides, the enantiocontrol of which has proven difficult, the judicious use of HuPhyrin ligand by tuning the bridge length and other remote nonchiral elements allows for controlling both the degree and sense of asymmetric induction in a systematic manner. This effort leads to successful development of new Co(II)-based catalytic systems that are highly effective for enantiodivergent radical 1,5-C−H amination, producing both enantiomers of the strained five-membered cyclic sulfamides with excellent enantioselectivities. Detailed deuterium-labeling studies, together with DFT computation, have revealed an unprecedented mode of asymmetric induction that consists of enantiodifferentiative H-atom abstraction and stereoretentive radical substitution.

Both arylsulfonyl and alkylsulfonyl azides can be effectively activated by the cobalt(II) complexes of D 2symmetric chiral amidoporphyrins for enantioselective radical 1,5-C−H amination to stereoselectively construct 5-membered cyclic sulfonamides. In addition to C−H bonds with varied electronic properties, the Co(II)-based metalloradical system features chemoselective amination of allylic C−H bonds and is compatible with heteroaryl groups, producing functionalized 5membered chiral cyclic sulfonamides in high yields with high enantioselectivities. The unique profile of reactivity and selectivity of the Co(II)-catalyzed C−H amination is attributed to its underlying stepwise radical mechanism, which is supported by several lines of experimental evidence.

A new catalytic radical system involving CoII-based metalloradical catalysis is effective in activating sulfamoyl azides for enantioselective radical 1,6-amination of C(sp3)-H bonds, affording six-membered chiral heterocyclic sulfamides in high yields with excellent enantioselectivities. The CoII-catalyzed C–H amination features an unusual degree of functional-group tolerance and chemoselectivity. The unique reactivity and stereoselectivity is attributed to the underlying stepwise radical pathway. The resulting optically active cyclic sulfamides can be readily converted into synthetically useful chiral 1,3-diamine derivatives without loss in enantiopurity.