Single-crystal X-ray diffraction study of a host–guest system comprising monofunctionalized-hydroxy pillar[5]arene and 1-octanamine

The crystal structure and supramolecular interactions between a hydroxy-functionalized pillarene and 1-octanamine guest molecule, which forms an interesting host–guest system, are reported.


Chemical context
arenes are a relatively new class of three-dimensional macrocyclic compounds having a well-defined inner cavity for guest encapsulation. Unlike cone-shaped calixarene or resorcinarene-type structures, the pillararenes have a tabular cavity, which makes them interesting molecular hosts. It is well known that pillar [5]arenes exhibit an outstanding ability to selectively bind different kinds of guest molecules and thus are valuable chemical entities in the areas of host-guest systems and molecular recognition (Ogoshi et al., 2008). The guest moieties that could be encapsulated by pillararenes include both neutral and charged guest species and the preference will be for those having long alkyl chains. Appropriate functionalization of the pillararene framework could enable efficient control over the binding properties of these macrocycles with a variety of guest species (Han et al., 2010(Han et al., , 2015Pan & Xue, 2013;Hu et al., 2016).
Chemical modification of the pillararene system could be achieved in two ways, namely cyclization of appropriately functionalized monomers or functionalization of preformed pillararenes (Al-Azemi et al., 2017). In the former, co-cyclization of pre-functionalized monomers in an appropriate feed ratio could be employed to generate pillararenes having the desired functionalities in terms of numbers and positions.
The pillar[5]rene system having one hydroxy group is interesting because this OH-function is susceptible for further chemical transformation (Al-Azemi et al., 2018). Furthermore, the OH-group in pillararenes could involve hydrogen bonding with guest molecules and/or with neighboring pillararenes, which makes them valuable compounds in ISSN 2056-9890 molecular recognition and supramolecular chemistry. We have recently reported details of the host-guest complexation between mono-hydroxy-pillar [5]arenes with long-chain alkyl alcohol guests (Al-Azemi et al., 2018). It was observed that the encapsulation characteristics of the pillar [5]arene was affected by the presence of the hydroxy group, resulting in the formation of a 1:2 complex with long-chain alkyl alcohols.

Structural commentary
The crystal structure of the inclusion complex Pil-OHÁOctNH 2 is given in Fig. 1. The mono-hydroxy-pillar[5]arene (Pil-OH) has a rigid three-dimensional macrocyclic architecture with a wide cavity having a pentagonal shape. The 1-octanamine molecule is threaded inside the pillararene cavity and one water is included in asymmetric unit, displaying strong hydrogen-bonding interactions with the amino group of the guest molecule inside the cavity and the hydroxy group on the pillararene system via O11-H11AÁ Á ÁN1 and O11-H11BÁ Á ÁO1 bonds respectively (Table 1).

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. The hydrogen atoms belonging to water, the OH fraction of the pillarene apex and the NH 2 group of 1-octanamine were found in the electron density map and freely refined. All other hydrogen atoms are placed at calculated positions and refined using a riding model: C-H = 0.95-0.99 Å with U iso (H) = 1.2U eq (C).
Cg1, Cg2, Cg3 and Cg4 are the centroids of the C1-C6, C29-C34, C15-C20 and C22-C27 rings, respectively. Symmetry codes: (ii) x À 1, y, z; (iii) x À 1, y À 1, z.      Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.