In consistency with the trend observed in chlorinated solvents, the smallest substrate (CHD) exhibited the highest reaction rate -two orders of magnitude faster than that of DHA (Figure , green squares and blue triangles, respectively), despite having the same number of reactive C-H bonds and nearly identical CH BDE values (Table , entries 1 and 2). On the other hand, with TPM as a substrate, the decay rate of 9 was not perceptibly different than that in plain DCE (Figure , red circles), despite the much weaker central C-H bond of TPM compared to those of the solvent molecules (Table , entry 3) -once again showcasing the size and shape selectivity imposed by the cavitand scaffold of 9 on its Mn(IV)oxo core. A similar steric substrate control was also found for the OAT reactivity of intermediate 9 with different tertiary phosphines. Specifically, when reacted with a 1:1 mixture of a small trimethylphosphine (PMe3) and a bulky tricyclohexylphosphine (PCy3), the oxo-moiety of 9 was fully transferred to the former within minutes, while leaving the latter unaffected, as confirmed by a quantitative P NMR measurement (a). Interestingly, while the 31 P NMR signal of PCy3 remained sharp, the signals of both PMe3 and the oxide Me3P(O) showed a significant broadening, presumably due to the labile interaction of these small molecules with the coordinatively unsaturated Mn(II) center in the resulting complex 8 -the formation of which was evident by UV-vis spectroscopy (Fig. ). Unlike tricyclohexylphosphine, a long chain dimethyloctylphosphine (12) 91 did undergo a full conversion into the corresponding phosphine oxide upon reaction with 9, as confirmed by another quantitative P NMR measurement (b). This proved that large substrate molecules are still able to react with 9, provided that their shape allows access to the reactive metal-oxo core within its cavity.