Since their first preparation by Cram, resorcinarene-based cavitands have been extensively studied as possible platforms for constructing molecular systems with distinct enzymemimicking properties, i.e. substrate recognition, stabilization of reactive intermediates, and reaction rate acceleration. The enzyme-like reactivity in these cavitands was often achieved by means of introducing inwardoriented reactive functional groups (such as carboxylic acids, 39 aldehydes 44 or esters ) within their cavities. Building on the same concept, the idea of mimicking natural metalloenzymes by functionalizing the cavitands with catalytically-active transition metals was first raised by Rebek, and further explored by him and others in a series of subsequent works. In almost all such systems, the metal center was introduced via the 'dangling arm' approach, i.e. by appending the metal coordination site to the aromatic walls at the upper rim of the cavitand (complexes VI and VII, Fig. ). While this architecture is easier to achieve synthetically, it inevitably exposes the metal center not only to the interior of the cavitand, but also to the bulk solution. As a result, such cavitand systems were often found to have little to no effect on the reactivity of the appended catalyst, compared to a native one. Conversely, cavitand-based systems where the metal center is buried deep within their molecular cavity (i.e., at the lower rim), are expected to better mimic the active sites of metalloenzymes, often located inside hydrophobic pockets, more closely. A noteworthy example of such biomimetic cavitand design, featuring a tetradentate tripodal ligand capped with a calix [6]-arene macrocycle, was demonstrated by Reinaud (complex VIII, Fig. ). Yet, the skeletons of traditional resorcinarene-based cavitands are poorly suited for this concept, as they lack modifiable functional groups at their lower rim. Indeed, the few reported catalytic systems, where the active metal center was positioned at the bottom of a molecular cavity, were limited to cyclodextrin or podant-type ligands, whose inner cavities suffer from poor structural definition and have limited guest binding capabilities. Herein, we report the design and synthesis of a conceptually novel metallocavitand system (IX, Fig. ) featuring a rigid pentacoordinate metal-binding cage located at the lower-rim. Such an architecture effectively shields the coordinated metal center from the bulk of the solution and ensures that the former is accessible only from within the molecular cavity. This design enabled the isolation and full characterization (including by XRD) of a highly reactive Mn(IV)-oxo species in a pseudo-octahedral ligand field. Moreover, in this study we explore the reactivity of this unique system and demonstrate that the cavitand framework imposes size and shape selectivity on the HAA and OAT reactions it undergoes.