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The ability of earth-abundant metals to serve as catalysts for the oxygen reduction reaction is of increasing importance given the prominence of this reaction in several emerging technologies. It is now recognized that both the primary and secondary coordination environments of these catalysts can be modulated to optimize their performance. In this present work, we describe two CoII complexes [CoII(PaPy2Q)](OTf) (1) and [CoII(PaPy2N)](OTf) (2) that catalyze chemical and electrochemical dioxygen reduction. Both 1 and 2 contain CoII centers in a N5− coordination environment, but 2 has a naphthyridine group that places a nitrogen atom in the secondary coordination sphere. Solid-state X-ray crystallography and solution-state spectroscopic measurements reveal that, apart from this second-sphere nitrogen in 2, complexes 1 and 2 have essentially identical properties. Despite these similarities, 2 performs the chemical reduction of dioxygen ~10-fold more rapidly than 1. In addition, 2 has an enhanced performance in the electrochemical reduction of dioxygen compared to 1. Both complexes yield a significant amount of H2O2 in the chemical reduction of dioxygen (>25%). The enhanced catalytic performance of 2 is attributed to the presence of the second-sphere nitrogen atom, which might enable the efficient protonation of cobalt–oxygen intermediates formed during turnover. 

Abstract Manganese catalysts that activate hydrogen peroxide have seen increased use in organic transformations, such as olefin epoxidation and alkane C−H bond oxidation. Proposed mechanisms for these catalysts involve the formation and activation of Mn^III‐hydroperoxo intermediates. Examples of well‐defined Mn^III‐hydroperoxo complexes are rare, and the properties of these species are often inferred from Mn^III‐alkylperoxo analogues. In this study, we show that the reaction of the Mn^III‐hydroxo complex [Mn^III(OH)(^6Medpaq)]⁺(1) with hydrogen peroxide and acid results in the formation of a dark‐green Mn^III‐hydroperoxo species [Mn^III(OOH)(^6Medpaq)]⁺(2). The formulation of2is based on electronic absorption,¹H NMR, IR, and ESI‐MS data. The thermal decay of2follows a first order process, and variable‐temperature kinetic studies of the decay of2yielded activation parameters that could be compared with those of a Mn^III‐alkylperoxo analogue. Complex2reacts with the hydrogen‐atom donor TEMPOH two‐fold faster than the Mn^III‐hydroxo complex1. Complex2also oxidizes PPh₃, and this Mn^III‐hydroperoxo species is 600‐fold more reactive with this substrate than its Mn^III‐alkylperoxo analogue [Mn^III(OO^tBu)(^6Medpaq)]⁺. DFT and time‐dependent (TD) DFT computations are used to compare the electronic structure of2with similar Mn^III‐hydroperoxo and Mn^III‐alkylperoxo complexes.