Using appropriate competition experiments,
host compound TETROL [(+)-(2R,3R)-1,1–4,4-tetraphenylbutane-1,2,3,4-tetraol] was revealed
to discriminate between the isomeric methylcyclohexanone guests; the
host selectivity was in the order 2- ≫ 3- > 4- methylcyclohexanone.
Surprisingly, addition of unsubstituted cyclohexanone to these competitions
instigated a complete reversal in the host’s preference for
the alkylcyclohexanones: a selectivity order of cyclohexanone >4-
> 3- > 2-methylcyclohexanone was attained. It is proposed that
it is the significant preference of TETROL for cyclohexanone that
is responsible for the host behavior change in the presence of this
guest. As evidence, the crystal packing of a mixed complex, TETROL·(72%)cyclohexanone·(28%)4-methylcyclohexanone,
was shown to be isostructural with pure TETROL·cyclohexanone
(which differs from TETROL·4-methylcyclohexanone). The addition
of cyclohexanone to the competition experiments therefore steers crystallization
toward crystals with similar packing to that of the preferred guest
complex accounting for TETROL’s change in selectivity in the
presence of this guest. Hirshfeld surface analyses were used to demonstrate
that TETROL·cyclohexanone experiences a significantly larger
number of stabilizing O···H/H···O interactions
(15.2%) compared with the other complexes (11.5–14.4%), accounting
for the host’s overwhelming preference for this guest. Thermogravimetric
analysis further confirmed that cyclohexanone was more strongly bound
in the crystal than the other guests.
In this work, we have shown that host−guest hydrogen bonding is intricately associated with host selectivity in supramolecular systems comprising host (+)-(2R,3R)-1,1,4,4-tetraphenylbutane-1,2,3,4-tetraol (TETROL) and guests aniline, toluene, and the toluidines. Competition experiments provided the host selectivity order, p-toluidine > aniline > m-toluidine > o-toluidine > toluene and, additionally, three crystalline two-guest mixed complexes containing aniline/ o-toluidine, aniline/p-toluidine, and aniline/toluene; the overall host:guest ratio was 2:3. Crystal diffraction experiments showed the host packing to be consistently isostructural. Furthermore, only two of the three guests in the asymmetric unit are hydrogen bonded to the host; the third does not experience this interaction type even if possessing hydrogen-bonding capability (e.g., an amino functionality). In the TETROL/aniline/ o-toluidine and TETROL/aniline/toluene mixed complexes comprising both favored (aniline) and disfavored (o-toluidine or toluene) guests, it is exclusively the former that are accommodated in the two hydrogen-bonding sites, while the third location, where hydrogen bonding with the host is absent, is able to contain both guest types. However, each of the three sites in the mixed complex containing only favored guests (TETROL/aniline/p-toluidine) is able to clathrate both species. These observations explain the increased site occupancy factors of preferred guests in the crystal and hence the selectivity of the host.
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