Remarkably, the obtained crystal structure also features an entrapped DCM molecule hydrogen-bonded to the metal-oxo moiety, positioned and orientated in a manner reminiscent of a putative transition state of an HAA reaction taking place via the σ-approach (that has so far been largely considered inaccessible to Mn(IV)-oxo systems). Regardless of the exact mechanism, the HAA reaction was found to be the primary pathway for the decomposition of this Mn(IV)-oxo intermediate in chlorinated solvents, which exhibited a notable rate dependence on the molecular dimensions of the solvent used. Accordingly, the cavitand framework was found to impose a high degree of size and shape selectivity on the Mn(IV)-oxo center in its HAA and OAT reactions with weak C-H bond hydrocarbons and tertiary phosphines, respectively. This steric control could also be harnessed for the regioselective oxidation of an asymmetric bisphosphine substrate -a transformation that is considered highly challenging to accomplish in non-enzymatic systems.