Further insight into the HAA reactivity of complex 9 was gained by monitoring its decay in different chlorinated solvents (Figs. ). While all observed decomposition profiles followed the first order kinetics (Figure ), the decay rates (Table , column 2) did not show any clear correlation with neither the number of C-H bonds per solvent molecule, nor the corresponding homolytic bond dissociation energy (BDE) values (Table , columns 3 and 4). In fact, complex 9 was found to be significantly more stable in DCE (Figure , red circles), compared to both DCM and CHCl3, despite having the highest number of C-H bonds per molecule and a C-H BDE between (and very similar to) the corresponding values of the other two solvents. On the other hand, an interesting trend between the decay half-lives and the spatial characteristics of the solvent molecules was noted. The t1/2 values measured in DCM and CHCl3 seem proportional to their approximate VDW molecular volumes (Table , column 5). In DCE, however, the decay rate is ca. 3 times slower than what might be expected based on its molecular volume, which is only slightly larger than that of CHCl3. Yet, while DCM and CHCl3 are roughly spherical molecules, the elongated shape of the DCE molecule imposes an additional limitation on its HAA reaction with the Mn(IV)oxo core of 9. When in the most common anti conformation, the longest dimension of the DCE molecule (dVDW = 7.9 Å) is nearly twice longer than the average diameter of the cavity aperture defined by its aromatic walls (dVDW = 3.9 Å). As a result, to enter the cavity for the reaction with the Mn(IV)-oxo moiety, the DCE molecule must be aligned along its longest axis. This limits its possible spatial orientations to just one out of three (i.e., along the x, y, and z axes), which might explain why the HAA reaction with DCE occurs approximately 3 times slower than could be expected based on its molecular volume alone. Following the decay studies in chlorinated solvents, we decided to test whether a similar size selectivity will also be observed with substrates known to undergo HAA by Mn(IV)oxo complexes -specifically, 1,4-cyclohexadiene (CHD), 9,10dihydroanthracene (DHA), and triphenylmethane (TPM). Since complex 9 was found to be most stable in DCE, this solvent was chosen as the reaction medium (Figure ).