Ligand architecture and secondary‐sphere effects in a well‐defined cobalt( IV )–oxo complex

Abstract High‐valent terminal metal–oxo species are key intermediates in biological and synthetic oxidation chemistry, yet late transition metal–oxo complexes remain challenging to access due to the ‘Oxo Wall’. Herein, we report the generation, detailed spectroscopic characterization, and reactivity of a well‐defined cobalt(IV)–oxo complex, (13‐TBC)Co IV (O) 2+ , supported by a neutral tetrabenzyl‐substituted tetraazamacrocyclic ligand (13‐TBC). The Co‐oxo intermediate is formed cleanly in the oxidation of the corresponding Co(II) precursor by iodosylbenzene (PhIO) and is stabilized at low temperature, enabling comprehensive analysis by UV–vis, EPR, rRaman and cold‐spray mass spectroscopies, and 18 O‐labeling experiments. These data unambiguously establish the presence of a terminal cobalt–oxo unit and indicate a ligand‐protonated structure with intramolecular hydrogen bonding that enforces a distorted coordination geometry. Reactivity studies reveal that the cobalt(IV)–oxo species functions as an electrophilic oxidant capable of both oxygen‐atom transfer (OAT) to thioanisoles and hydrogen‐atom transfer (HAT) from weak C–H bonds, with distinct kinetic signatures and product selectivity. Notably, protonation exerts opposing effects on these pathways, markedly enhancing OAT while suppressing HAT, underscoring the critical role of secondary coordination sphere interactions. Comparative studies highlight the critical role of ligand ring size and benzyl substitution in tuning the structure and oxidative reactivity of cobalt–oxo intermediates. These findings provide new insight into structure–reactivity relationships of cobalt–oxo species and offer guiding principles for the rational design of late transition metal catalysts.