Primary and Secondary Coordination Sphere Effects in the Cobalt Complex-Catalyzed Electrocatalytic O ₂ Reduction to Water

Molecular cobalt-based complexes generally favor O₂ reduction to H₂O₂. However, a full reduction of O₂ to H₂O is desired for fuel cell applications, which drives the current research in oxygen reduction electrocatalysis. We report a new Co(III) complex based on an N₄O-coordinating ligand (UMAPA) that electrochemically reduces oxygen to water in the presence of acetic acid in acetonitrile. By combining electrochemistry-coupled electrospray ionization mass spectrometry (EC-ESI-MS), cyclic voltammetry, ultraviolet-visible (UV-vis) spectrophotometry, and rotating ring disk electrode voltammetry analyses, we unraveled the rarely reported integral role of both primary and secondary coordination in a single catalyst. The chronoamperometry EC-ESI-MS experiments allowed us to effectively monitor the catalyst's activation steps and the formation of the cobalt(III) hydroperoxo intermediate. Consequently, we deciphered how acetic acid and the secondary coordination sphere of the Co(III) complex facilitated the subsequent water-forming steps. Furthermore, we characterized the key Co(III)-hydroperoxo and Co(III)-hydroxo intermediates by using cryogenic ion spectroscopy. Our experiments also showed that the cobalt complex undergoes ligand hydroxylation under oxygen-rich conditions. Interestingly, chronoamperometry EC-ESI-MS showed that the hydroxylated cobalt complex remains active for electrocatalytic oxygen reduction (ORR). We then explored the significance of different coordination configurations of the same ligand around the Co center in ORR. The Co complex with the protonated UMAPA ligand exhibited N₄ coordination and slower preactivation steps than the Co complex with the deprotonated N₄O-coordinating UMAPA. Surprisingly, the CV experiments revealed that these small changes in the ligand configuration around the Co center result in a drastic change in the ORR activity. EC-ESI-MS was instrumental in determining the molecular-level details of the electrocatalytic ORR catalyzed by the Co complex, which conventional electroanalytical techniques alone cannot provide.