Crystal structure of a supramolecular lithium complex of p-tert-butylcalix[4]arene

The crystal structure of a supramolecular lithium complex of p-tert-butylcalix[4]arene has been determined and analyzed. Different from the majority of calixarene–alkali metal complexes, which are formed by direct coordination of the metal cation to the calixarene hydroxy groups, this complex is stabilized by an interplay of weak interactions involving the methanol molecules surrounding the metal, giving rise to a second-sphere coordination supramolecular assembly.


Chemical context
Calixarenes are synthetic macrocyclic compounds that are composed of phenol rings, linked with methylene groups at linking positions (Gutsche, 1998). They are versatile molecules for the inclusion of organic and/or inorganic compounds into their flexible cavities and for the coordination of organic/metal ions in molecular recognition phenomena and host-guest chemistry (Vicens & Bö hmer, 1991). The coordination chemistry of alkali metal cations, involving conventional calixarenes (and their corresponding functionalized derivatives) as ligands, has been intensively investigated in the past years, as a possible method of selective extraction of this class of cations using calixarenes as extractant. At the same time, the X-ray analysis of alkali metal complexes with p-tertbutylcalix[4]arene in the crystalline state has been reported (Bock et al., 1995;Davidson et al., 1997;Dü rr et al., 2006;Gueneau et al., 2003;Guillemot et al., 2002;Hamada et al., 1993;Hanna et al., 2002Hanna et al., , 2003Harrowfield et al., 1991;Lee et ISSN 2056-9890 al., 2009. In the majority of cases, the alkali metal complexes of p-tert-butylcalix [4]arene in the solid state show direct coordination of the metal ions to the oxygen atoms belonging to the calixarene hydroxy groups at the lower rim, with the resulting crystal structures stabilized by weak interactions with the lattice solvent molecules. In the present paper, we report a different type of Li complex with p-tert-butylcalix [4]arene, in which no direct coordination of the metal to the oxygen atoms of the calixarene hydroxy groups takes place. The lithium cation is instead surrounded by four methanol solvent molecules, which are in turn connected to the host molecule via a series of hydrogen bonds, playing a significant role in the formation of the supramolecular assembly. Fig. 1 shows the molecular structure of the complex [Li(CH 3 OH) 4 ] + Á(calix[4]arene À )]ÁCH 3 OH, consisting of one mono-deprotonated calix [4]arene unit in a cone conformation, one methanol molecule included in the cavity, and one Li cation coordinated to four methanol molecules. The positive charge of the methanol-lithium complex naturally dictates that the calixarene is in a mono-anionic form. The conformation of the macrocycle is stabilized by intramolecular hydrogen bonding involving one deprotonated -O À and three -OH groups at the lower rim, as shown in Table 1. The geometrical parameters of the cone conformer are given in Table 2, which reports the angle between the mean plane passing through the oxygen atoms O1, O2, O3 and O4, and the four mean planes passing through the aromatic walls (plane A: C1-C6/O1; plane B: C7-C12/O2; plane C: C13-C18/O4; plane D: C19-C24/O3). From these values, it is possible to notice that the two neighboring aromatic rings (C1-C6 and C7-C12) are slightly outward with respect to the other two adjacent aromatic moieties. Selected bond distances and angles for the tetrakis(methanol)-lithium complex are reported in Table 3.

Supramolecular features
The relevant feature of the title complex is that the lithium cation is not directly coordinated to the hydroxy groups of the lower rim of the calix[4]arene host. On the contrary, the interaction of the [Li(CH 3 OH) 4 ] + complex with the macrocycle in the asymmetric unit is mediated by the methanol molecule embedded in the cavity, which acts as hydrogenbond acceptor for a methanol molecule (C48-O8) coordinated to the lithium cation ( Fig. 2 and Table 1). Moreover, the coordinated methanol molecules of [Li(CH 3 OH) 4 ] + further contribute to the stabilization of the complex in the structure, interacting with two other adjacent calixarene molecules through hydrogen bonds and C-HÁ Á Á interactions, as illustrated in Fig. 3 and Table 1. In particular, three of the coordinated methanol molecules (C45-O5, C47-O7 and C46-O6), act as hydrogen-bond donors towards the hydroxy groups at the lower rim of the macrocycle, namely O1 i , O3 i and O4 ii , respectively [symmetry codes: (i) Àx + 3 2 , y + 1 2 , Àz + 3 2 ; (ii) x + 1 2 , Ày + 1 2 , z + 1 2 ]. In addition, the fourth coordinated methanol molecule C48-O8 interacts with the aromatic-electrons of a calixarene ii via a C-HÁ Á Á interaction. The C48Á Á ÁC17 ii and C48-H64Á Á ÁC17 ii distances are 3.603 (4) and 2.628 Å , respectively, with a C48-H64Á Á ÁC17 ii angle of 173.3 (8) .

Figure 2
Hydrogen bonds (blue dotted lines) involving the p-tert-butylcalix[4]arene anion, the methanol molecule included in the cavity, and the [Li(CH 3 OH) 4 ] + complex belonging to the asymmetric unit. The centroid of aromatic the ring, Cg1, is represented as a blue sphere. The H atoms of the calixarene host have been omitted for clarity.
In all the cases reported, the alkali metals interact with the calix[4]arene molecules through the hydroxy groups at the lower rim. The only exception is the complex with cesium, JIVKEE, in which the bare cation is placed well inside the cavity, on the quaternary axis passing through the macrocycle. The metal is involved in a polyhapto coordination with the four phenolate rings of the calix[4]arene, on which the negative charge is delocalized (Harrowfield et al., 1991). This coordination mode is probably possible due to the dimensions of Cs + , which matches the cavity in size. In the case of lithium, the cationic radius is much smaller, hence a direct cavitycation interaction is less favoured, and the metal is either coordinating the hydroxy oxygen atoms, or forming a secondsphere coordination supramolecular complex, like in the title compound.

Refinement details
Crystal data, data collection and structure refinement details are summarized in Table 4. The C-bound H atoms were placed in calculated positions and refined using a riding model: C-H = 0.95-0.98 Å with U iso (H) = 1.5U eq (C-methyl) and 1.2U eq (C) for other H atoms. H atoms on O atoms were located in the difference-Fourier map and refined with U iso (H) = 1.5U eq (O).   Data collection: APEX2 (Bruker, 2006); cell refinement: SAINT (Bruker, 2006); data reduction: SAINT (Bruker, 2006); program(s) used to solve structure: SHELXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012); software used to prepare material for publication: Yadokari-XG (Kabuto et al., 2009) and Mercury (Macrae et al., 2008). 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. Refinement. Refinement was performed using all reflections. The weighted R-factor (wR) and goodness of fit (S) are based on F 2 . R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt).