Structural studies on inclusion compounds and solvent sorption behavior of gradually elongated wheel-and-axle-type diol hosts featuring lateral benzo[b]thiophene units
Abstract: Based on the wheel-and-axle design strategy, a series of six new clathrate hosts featuring two di(benzo[b]thien-2-yl)hydroxymethyl units attached to both ends of a central linear building element of varying length have been synthesized and their capability to form crystalline inclusion compounds with a fixed range of organic solvents are reported. X-ray crystal structures of relevant inclusion compounds have been determined and are comparatively discussed involving structural modification of the host molecules. Organic vapor sorption behavior of the host compounds coated as solid films on the quartz crystal of a QCM device has been studied. Significant differences in the affinities towards solvent vapors dependent both on structural and polarity properties of host and solvent are observed, indicating potential application as mass sensitive materials.
Summary (2 min read)
- Aiming at applications such as separation, sensing and storage of chemical substances, crystalline inclusion compounds are very encouraging [1-4].
- In previous studies concerning this topic respective results are evident.
- Owing to their irregular shape, corresponding hosts do not pack efficiently and thus they tend to yield inclusion compounds in the presence of neutral guest molecules.
- In a modification of this proven design, wheel-and-axle type geometries have also been created bipartitely via an association of bulky carboxylic acid fragments  or using ligands of varying length for bridging the metal centers of porphyrins [8,9] or other metal containing molecules .
- Finally, compounds 1-6 have been tested as solid coatings of a quartz crystal microbalance (QCM)  to reveal their potential in organic vapor sorption and conclusions are drawn from all the findings.
Preparation of compounds
- For the preparation of 1, this involves reaction of dimethyl biphenyl-4,4'- dicarboxylate (7) with in situ generated 2-lithiobenzo[b]thiophene (obtained from benzo[b]thiophene 8 and n-butyl lithium in dry THF at -30 °C under argon).
- Analogous reaction protocols using dimethyl 4,4'-dibenzoate (9) or trimethyl 3,3',3''-(benzene-1,3,5triyl)tripropynoate (11) (prepared from 10) as ester components yielded 3 and 6, respectively.
- Differing from the literature description , the dimethyl dicarboxylate 7 was prepared from 4-phenylboronic acid (synthesized by literature procedures [16,17]) and methyl 4-iodobenzoate in the presence of Pd(PPh3)2Cl2 and NaHCO3 in water/THF following a Suzuki-Miyaura type reaction .
- Pd-catalyzed coupling reactions following the Sonogashira-Hagihara protocol  were applied to 1,1-di(benzo[b]thien-2-yl)prop-2-yne-1-ol (14) and corresponding diiodides [1,4- diiodobenzene, 4,4'-diiodobiphenyl and bis(4-iodophenyl)ethyne] to yield the respective diols 2, 4 and 5.
- Inclusion compounds of 1-6 including the studied solvents were obtained as described in the Experimental.
Capability of inclusion formation
- Previously, the authors demonstrated that the replacement of thien-2-yl groups of a new prototype host structure  by the more bulky benzo[b]thien-2-yl groups giving rise to BTh1 and BTh2 (Fig. 1) leads to a distinct enhancement of the crystalline inclusion formation with organic solvent molecules .
- Hence, representatives of the new hosts do not develop such strong limitation to include only solvents of protic and distinctly polar nature as shown for the former hosts.
- Hence, in order to provide a more profound basis for drawing conclusion extending those from the above findings, a detailed X-ray crystallographic structural study performed of selected inclusion compounds was carried out.
- But also the other inclusion compounds (2b, 4b, 4c, 5a) enable an immediate structural comparison based on the same solvent species, even if only included in the crystals of the previous hosts BTh1 and BTh2.
- For the determination of the molecular conformations of the respective hosts, the authors focused on the one hand on the dihedral angles describing the relative orientations of the two benzo[b]thiophene units and the hydroxyl function.
- On the other hand, the authors also incorporated the relative orientations of the phenylene units of the central spacer element to one another and with reference to the orientation of the OH function.
- In the structure of 1a (Fig. 4a), the benzene rings of the biphenyl unit are coplanar to each other.
- Emphasizing packing density of the different kinds of host-guest complexes (Table 4), particularly noticeable observations are as follows.
- When the quartz crystal was dipped in a 0.01M solution of 6 in EtOH and the solvent was evaporated aiming for a solvent-free and loosely packed coating, sorption measurements with the guest vapors applied before, revealed a different outcome of the experiment instead.
- Carbonyl addition reactions of benzo[b]thien-2-yl-lithium to corresponding di- or triesters in the cases of 1, 3 and 6, respectively, and Sonogashira-Hagihara cross-coupling reactions between a particular benzo[b]thien-2-yl substituted propynol (14) and corresponding spacer moieties in the cases of 2, 4 and 5 gave the new diol compounds in respective to high yields (54-96 %).
- An intensive study of the inclusion property in solid state by crystallization from a variety of solvents proved their versatile host behavior and inclusion activity.
- This is in line with findings for gradually elongated porphyrin systems [8,9].
- By way of interest, for the new compounds, high ratio of included solvent is rather common, which is especially true for the trigonal host 6.
- As before for the tendency to form crystalline inclusion compounds, the ability for QCM vapor absorption is rarely depending on the spacer length, though much more on the spacer components.
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...(Novoa et al., 1995; Katzsch et al., 2016) prevail (Table 5)....
Cites background from "Structural studies on inclusion com..."
...Compounds of this latter type are significant crystalline inclusion hosts (Katzsch et al., 2015, 2016) and important examples in the course of the development of the concept of crystal engineering (Hart et al., 1984; Bishop, 2012)....
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Q1. What have the authors contributed in "Structural studies on inclusion compounds and solvent sorption behavior of gradually elongated wheel-and-axle-type diol hosts featuring lateral benzo[b]thiophene units" ?
In this paper, it is shown that the covalently linked axis of a conventional wheel-and-axle host molecule can be replaced by a system of hydrogen bonds or by coordination of ligands generating inclusions alike.