Crystal growth of host compound trans-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic
acid (H1, DED) and three derivatives thereof
(H2–H4) from potential guest solvents
toluene (TOL), ethylbenzene (EB), and cumene (CUM) revealed that each
one has the ability to enclathrate one, some, or all of these guest
species. In equimolar TOL/CUM mixtures, H4 displayed
an increased affinity for TOL (86.1%) compared with H2 (62.4%), while these selectivities for this guest species were only
moderate when crystallization was from the equimolar mixed ternary
guests (56.9 and 51.1%, respectively). H1 and H3, on the other hand, formed a mixed complex only in the equimolar
TOL/EB solution, the remaining experiments furnishing a guest-free
apohost compound only. Importantly, since various procedures in the
industry yield mixtures of these alkylbenzenes, H4 was
shown to be a likely candidate for the separation of both TOL from
the TOL/CUM and EB from EB/CUM mixtures (the averaged selectivity
coefficients, K, were high, 13.7 and 12.1, respectively). SCXRD experiments
showed that EB in 3H3·EB experienced a significantly
shorter (host)C–H···π(guest) interaction
(2.50 Å) than TOL in its complex with H3 (2.87,
2.66 Å), in accordance with the higher affinity of H3 for EB relative to that for TOL. The H4·TOL and H4·EB complexes experienced analogous host···host
interactions, but host···guest contacts could not be
compared since the SQUEEZE function in PLATON was required during
the refinement of the crystal structure of H4·EB
to deal with severe guest disorder. Thermal analyses showed that the
preferred guest of H3 (EB) formed a complex with a greater
relative thermal stability compared with less favored TOL. Finally,
this investigation has shown that the size of the alkyl group bonded
to the aromatic ring of the guest species affects the selectivity
behavior of each host compound. CUM, which has the largest alkyl substituent
of the three guest species, was not ever preferred by any of the four
host compounds investigated here.