For cage 1, the strongest binding guests were carbon tetrachloride > cyclohexane > toluene > o-xylene > cyclopentane, and the proportion of bound guests increased from 25% (8/32) after 1 day to 69% (22/32) after 7 days, indicating that guest uptake was slow. Separations of hydrocarbons from crude oil and alkenes from alkanes have been labelled as two of the seven chemical separations crucial to future prosperity. In this MOC host-guest assay, o-xylene⊂cage 1 was observed but no encapsulation occurred for the p-or m-isomers; t-butanol⊂cage 1 was also observed but the n-and s-isomers exhibited no binding, and preferential binding of an alkane over an alkene was observed with cyclohexane⊂cage 1 versus cyclohexene, with the latter again showing no binding. After studying the host:guest behavior of cage 1, we then turned our attention to comparing the binding of a subset of guests (toluene, n-octane, carbon tetrachloride, and o-xylene) in the structurally analogous cages 2 and 3 (Figure ). Cage 1 and cage 2 differ only through their counterions, and cage 2 and cage 3 only differ based on whether the pyridyl ring is methylated or not, but computationally, the MOC cores are similar in size, suggesting cages 1-3 have similar cavity sizes (Table ). Therefore, any changes in binding properties between the cages may be attributed to the effect of the counterion or formylpyridine methylation. Electrostatics can govern host-guest behavior, where a counterion that is larger than the host void space may be necessary in order to preserve guest binding capability. The bis(trifluoromethylsulfonyl)imide (triflimide, -NTf2) counterion has been previously chosen for MOCs as a non-or weakly-coordinating counterion, and in the literature it was inferred to be too large to fit inside of cage 1. In contrast, the smaller size of the tetrafluoroborate (BF4) -counterion would allow possible occupation of the internal cavity, which may compete for binding with guest molecules. Nonetheless, the same general trend is observed for cage 1 and cage 2 where the binding was observed as follows: carbon tetrachloride > toluene > o-xylene > n-octane. One difference between binding was found within tolu-ene⊂cage 2, where binding was slower (not observed after 1 day but was for 7 days) and weaker by an order of magnitude than for toluene⊂cage 1. In contrast, the association constant for carbon tetrachloride⊂cage 2 > carbon tetrachloride⊂cage 1, however, this should be interpreted with care due to the singlepoint derivation of these Ka values. However, for cage 3, the binding trends differ as follows: carbon tetrachloride > toluene > n-octane, with no binding of o-xylene observed. In addition, binding of toluene and n-octane in cage 3 was slower and only apparent after 7 days, although n-octane had a higher preliminary Ka than for both cage 2 and cage 1. This trend may indicate that methylation of the tetrahedral MOC may slow guest encapsulation, along with competitive binding from the tetrafluoroborate (BF4) -counterion. As binding was slow, a longer time was required for the host-guest complexes to equilibrate and for encapsulation to occur.