Crystal structure of a mono-bridged calix[4]arene

The structure of the title compound, consists of a tert-butylcalix[4]arene with a five-carbon bridge connecting two proximal phenolic O atoms, and with a bromopentoxy chain on one of the remaining phenolic O atoms.


Chemical context
Calixarenes are macrocyclic molecules made up of phenol and methylene units, and are useful as host molecules and as building blocks for larger systems. (Ikeda & Shinkai, 1997;Gutsche, 2008). Calix[4]arenes exist in four well-defined conformations, and conformational interconversion (by rotation around the methylene bridges) is inhibited when the phenolic oxygen atoms are alkylated with sufficiently large groups (Ikeda & Shinkai, 1997). Calix[4]arenes in the cone conformation, which are tetra-O-alkylated with bulky groups, generally adopt a flattened conformation (flattened or pinched cone, approximate C 2v symmetry) in the solid state; in solution they experience conformational mobility between flattened cones (Conner et al., 1991;Arduini et al., 1995Arduini et al., , 1996bDrew et al., 1997;Hudrlik et al., 2007Hudrlik et al., , 2013Hailu et al., 2012Hailu et al., , 2013. Rigidified cone calixarenes (approximate C 4v symmetry) have been prepared by forming diethylene glycol ether bridges between proximal phenolic oxygen atoms (Arduini et al., 1995). In an effort to make a rigid cone calix[4]arene, we sought a strategy that would enable bridging of the phenolic oxygen atoms by the reactions of a calix[4]arene with 1,5dibromopentane. The reaction, using K 2 CO 3 in CH 3 CN, gave a mixture consisting primarily of a bis-calixarene and a monobridged calixarene . In the present work, the X-ray crystal structure of the mono-bridged calixarene, the title compound, is described.

Structural commentary
The structure consists of a flattened-cone calix[4]arene having a five-carbon bridge joining two proximal phenolic oxygen atoms, and a bromopentoxy chain attached to one of the remaining oxygen atoms. The molecule (Fig. 1) has a relatively rigid framework with a semi-flexible bridge and a flexible side chain. The molecule is inherently chiral, but crystallizes in a centrosymmetric space group; therefore both enantiomers have to be present in the unit cell in equal amounts. However, the molecule is disordered such that both enantiomers involving the conformation adopted by the bridging atoms are present in the asymmetric unit. In one of the two enantiomers, the bridging group links O3 and O2, and O3 and O4 in the other. The flexible side chain is disordered over three conformations. The diagrams show only the major component for the disordered regions.
The flattening of the calixarene cone could be observed by comparing distances between para carbon atoms of opposite phenolic rings. The distance between C4 and C27 is 5.698 (5) Å , while that between C16 and C38 is 9.390 (6) Å . The structure of a cone calix[4]arene is frequently described (Arduini et al., 1996b;Drew et al., 1997) using the dihedral angles of the phenol rings with the plane of the bridging methylene groups (C11, C22, C33, and C44). For the title compound, the aromatic rings attached to O2 and O4 are inclined outward, making fold angles of 136.2 (1) and 133.0 (1) , respectively, while those attached to O1 and O3 are almost perpendicular to this plane, making dihedral angles of 83.27 (9) and 105.46 (9) , respectively.
The fold angles reported here for the title compound are similar to those reported for other flattened cone calixarenes as referenced above. The joining of two proximal phenolic oxygen atoms by one five-carbon bridge does not appear to prevent flattening of the cone structure in the title compound. By contrast, a calix[4]arene having both sets of proximal phenolic oxygen atoms joined by five-atom bridges (diethylene glycol derivatives) (and with a simple guest) had equivalent fold angles of about 115-118 (Arduini et al., 1996a).
In the molecule there are several weak intramolecular C-HÁ Á ÁO interactions (Table 1). In addition, there is a weak intramolecular C-HÁ Á ÁBr interaction.

Supramolecular features
The bromine atoms in the disordered bromopentoxy chain also participate in weak intermolecular interactions, which link the molecules into loosely associated dimers. Other than that, there are no close contacts between molecules nor are there any significant intermolecular or intramolecularinteractions, possibly as a result of the conformation adopted

Synthesis and crystallization
The synthesis of the title compound was reported in the literature . Crystals for X-ray diffraction were obtained as follows. Approximately 10 mg of the white powdered solid compound was dissolved in a minimum amount of dichloromethane. The solution was filtered into a micro beaker and then methanol was added dropwise (final volume ratio about 4:1 methanol: dichloromethane). The beaker was covered loosely to allow slow evaporation of solvent. After a number of days, crystals suitable for X-ray analysis were obtained.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. There is considerable disorder in this molecule. One of the t-butyl groups is disordered over two conformations with occupancies of 0.527 (5)  Packing diagram for the title compound, viewed along the b axis. each are constrained to the usual tert-butyl geometry. The bromopentoxy chain is disordered over three conformations with occupancies of each conformer constrained to values of 0.418, 0.332 and 0.250 (total occupancy 1.000) which are similar to values of 0.417 (1), 0.331 (1) and 0.249 (1) obtained using the SAME command in SHELXL2014 (Sheldrick, 2008). The five-carbon bridge connecting two proximal phenolic oxygen atoms is disordered over two conformations with occupancies of 0.537 (7) and 0.463 (7), such that one conformer links O2 and O3 while the other conformer links O3 and O4 and each conformer is constrained to have similar metric parameters as above. All hydrogen atoms attached to carbon atoms were refined using a riding model with idealized geometries (C-H = 0.95-0.98 Å with U iso (H) = 1.5U eq (C) for methyl H atoms and = 1.2U eq (C) for other H atoms).