Crystal Growth of Host Compound DED and Derivatives in Alkylbenzenes

Crystal growth of host compound <i>trans</i>-9,10-dihydro-9,10-ethanoanthracene-11,12-dicarboxylic acid (<b>H1</b>, <b>DED</b>) and three derivatives thereof (<b>H2</b>–<b>H4</b>) 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, <b>H4</b> displayed an increased affinity for TOL (86.1%) compared with <b>H2</b> (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). <b>H1</b> and <b>H3</b>, 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, <b>H4</b> 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 3<b>H3</b>·EB experienced a significantly shorter (host)­C–H···π­(guest) interaction (2.50 Å) than TOL in its complex with <b>H3</b> (2.87, 2.66 Å), in accordance with the higher affinity of <b>H3</b> for EB relative to that for TOL. The <b>H4</b>·TOL and <b>H4</b>·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 <b>H4</b>·EB to deal with severe guest disorder. Thermal analyses showed that the preferred guest of <b>H3</b> (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.