Formation and Reactivity of an Elusive Monomeric Mn(IV)-Oxo Species Inside a Cavitand Pore

Metal-functionalized cavitands are promising platforms for mimicking the chemical environments of hydrophobic pockets in natural metalloenzymes. However, successfully combining the unique supramolecular capabilities of cavitand scaffolds with the high reactivities of transition metal complexes still remains a major challenge. In this study, we present an original cavitand architecture featuring a coordinatively unsaturated Mn(II) center embedded deep within its pore. This metallocavitand was employed to generate a Mn(IV)-oxo species inside a molecular cavity. This elusive intermediate was fully characterized spectroscopically (UV-vis, EPR, X-ray photoelectric spectroscopy (XPS), and HRMS) and, for the first time for a pseudo-octahedral Mn(IV)-oxo species, also by XRD. The experimental data was corroborated by detailed ab initio/time-dependent density functional theory (TDDFT) and natural bond orbital (NBO) calculations, confirming the Mn(IV)-oxo (rather than Mn(III)-oxyl) electronic character of this species. Reactivity and mechanistic studies, including monitoring the decay of this complex in various chlorinated solvents and its reactions with representative substrates, revealed that, despite the steric protection provided by the cavitand scaffold, its Mn(IV)-oxo core remains highly reactive in both H atom abstraction (HAA) and O atom transfer (OAT) reactions. Moreover, this reactivity is subject to a high degree of steric control imposed by the cavitand framework capable of discriminating between potential substrate molecules based on their size and shape. This was further demonstrated by the regioselective oxidation of a bisphosphine substrate, emulating the regioselectivity of natural metalloenzymes.