In addition to providing a theoretical justification for the simulated EPR spectrum of 9, the above CASSCF calculations also validated the formal description of its core as a Mn(IV)oxo species -indeed, predominating the ground state electronic configuration (72%) -rather than a Mn(III)-oxyl radical, the relative contribution of which is negligibly small (4%) (Fig. ). This is quite similar to the pseudo-octahedral Mn(IV)-oxo complexes of the poly-pyridyl ligands (NPY4), extensively studied by Jackson. Finally, an XRD-quality single crystal of complex 9 was grown at -30°C from its anhydrous DCM solution under rigorously air-free conditions (nitrogen glovebox). The obtained structure (Figure ) clearly shows the presence of a metal-coordinated oxo ligand located within the interior of the cavity and facing an entrapped DCM molecule. However, unlike in complex 8•H2O, here the DCM molecule in the cavity acquires a single well-defined spatial orientation, due to a strong H-bond between the oxo-moiety and one of its methylene protons (CH--O: 1.981Å, Figure ). Accordingly, the Mn-O bond of 1.715(9)Å found was slightly longer than that of the pseudo-tetrahedral Mn(IV)-oxo species reported by Nocera (1.673Å), but in an excellent agreement with the corresponding bond lengths of the pseudo-octahedral Mn(IV)-oxos stabilized by H-bonding or Brønsted/Lewis acids, as inferred from EXAFS measurements and predicted computationally. The fact that crystals of 9 could be grown from its DCM solution was quite surprising, given that this highly reactive species decays rapidly in that solvent at r.t, even when kept under strictly anhydrous and air-free conditions (Figure ). This observation is in line with other Mn(IV)-oxo systems found in the literature, as most reported pseudo-octahedral Mn(IV)-oxo intermediates have short lifetimes in solutionranging from several minutes to several hours. However, while in other metal-oxo systems this decay occurs via the formation of µ-oxo-bridged dimers, the bulky cavitand framework of complex 9 renders such dimerization impossible, suggesting that a different process takes place here.