With the fully characterized Mn(II)-metallocavitand 8 in hand, we set out to explore the potential of this novel ligand platform to stabilize a Mn(IV)-oxo species. Adding a DCM solution of excess 1-(tert-butylsulfonyl)-2-iodosylbenzene (sPhIO) to a solution of 8 in the same solvent resulted in an immediate color change from pinkish orange to wine red (Figure ). A similar, albeit slower, color change occurred also upon stirring the DCM solution of 8 with solid iodosyl benzene (PhIO). In both cases, ESI-HRMS detected a molecular ion mass of [M] + = 1423.3147 amu, matching the empirical formula of 8 plus an O atom, suggesting the likely formation of a monomeric Mn-oxo species.