Inclusion complexes of 18-crown-6 and (Na⁺⊂[2.2.2]cryptand) in [C-methylcalix[4]resorcinarene-H_n], n⊕=⊕0, 1

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Abstract

Reaction of 18-crown-6 and (Na⁺⊂[2.2.2]cryptand) with [C-methylcalix[4]resorcinarene-H_n], n⊕=⊕0, 1, affords novel inclusion complexes which involve hydrogen bonding; in the latter complex metal cryptand ions reside in the cavity of the calix[4]resorcinarene.

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Introduction

C-Methylcalix[4]resorcinarene 1 is a resorcinol-based macrocycle which commonly assumes a bowl-shaped conformation in organic media.¹ The curved architecture of such calix[4]resorcinarenes, and the phenolic groups on the upper rim, render them ideal for both the inclusion of appropriately sized and shaped species^2,3 as well as interaction with metal ions⁴ or hydrogen-bonding species at the periphery.⁵ A range of assemblies with defined interiors and interesting extended structures have been obtained using C-methylcalix[4]resorcinarene. For example, in the presence of 4,4′-bipyridyl, capsules based on two calix[4]resorcinarene molecules result,⁶ as is the case in the presence of propan-2-ol.⁷ These structures are both assembled through hydrogen bonding. Similarly, hydrogen-bonding interactions with water molecules are instrumental in the assembly of C-methylcalix[4]resorcinarene into a spherical hexamer.⁸ However, its assembly in the presence of curved molecules for which there is complementary size and shape with the cavity has not been explored. Suitable guests include 18-crown-6 2 and [2.2.2]cryptand (shown as its Na⁺ complex 3), and in this context we note that the similarly bowl-shaped p-sulfonatocalix[4]arene, and a resorcinarene-based cavitand, are capable of forming host–guest complexes with 18-crown-6^9–13 and [2.2.2]cryptand¹⁴ respectively. In the case of 18-crown-6 with p-sulfonatocalix[4]arene a range of structural types have been observed, including dimeric capsules,^9,10,11 clay-like bilayers¹² and helices.¹³ Herein we establish that a 1∶2.5 assembly can be obtained from a mixture of the calix[4]resorcinarene and crown ether, and that a 1∶1 assembly forms from the mono-deprotonated calix[4]resorcinarene and the cryptand as its sodium complex.

Results and discussion

The crown ether assembly C-methylcalix[4]resorcinarene(18-crown-6)_2.5(H₂O)₂4 crystallises from an ethereal solution of 1 and 2, whilst the [2.2.2]cryptand-containing assembly [Na⁺⊂[2.2.2]cryptand][methylcalix[4]resorcinarene-H](CH₃CN)₄(H₂O)₃5 crystallises as pink plates from a mixture of acetonitrile and water containing [2.2.2]cryptand, C-methylcalix[4]resorcinarene and trisodium 3,3′,3″-phosphinidyne-tris(benzenesulfonate).† Whilst repeated attempts to obtain a mass spectrum of 4 were unsuccessful, the electrospray mass spectrum of 5 shows a strong signal at m/z 399 consistent with the [Na⁺⊂cryptand] molecular ion. Electron microprobe analysis confirmed the presence of sodium in 5. Microcrystals of the compound formed from neutral [2.2.2]cryptand and calix[4]resorcinarene, 6, were also obtained by a similar method to that forming 4. Owing to the low solubility of this product, ¹H NMR spectra could not be obtained. However, electrospray mass spectrometry of 6 showed the presence of free [2.2.2]cryptand (molecular ion m/z 378), and the supermolecular ion [[2.2.2]cryptand·calix[4]resorcinarene]¹⁺ (m/z 921). The structures of 4 and 5 were determined by single crystal X-ray crystallography (Table 1).

Table 1 Details of data collection and structure refinement for C-methylcalix[4]resorcinarene(18-crown-6)_2.5(H₂O)₂4 and [Na⁺⊂[2.2.2]cryptand][C-methylcalix[4]resorcinarene-H](CH₃CN)₄(H₂O)₃ 5^a

Property	4	5
a X-Ray data were collected on a Bruker SMART CCD for 4 and an Enraf-Nonius KappaCCD for 5 with MoKα radiation (λ⊕=⊕0.71073 Å). C–H hydrogen atoms were fixed at geometrically estimated positions with a riding refinement. In 4 a carbon and two oxygen atoms in the crown ether showed some positional disorder so were modelled as disordered over two positions with occupancies of 0.6∶0.4, 0.6∶0.4 and 0.5∶0.5 respectively. With the exception of C62a, all non-hydrogen atoms were refined anisotropically. In structure 5, hydroxide hydrogen atoms and those associated with the waters within the hydrogen bonded network were refined, some with restraints on bond lengths. All non-hydrogen atoms were refined anisotropically. The cryptate showed some positional disorder and three carbon and one oxygen atoms were each modelled as disordered over two positions with occupancies 0.6∶0.4. Click b100578m.txt for full crystallographic data (CCDC 156826 & 156827).
Formula	C62H96O25	C58H85O17N6Na
M r/g mol^−1	1241.39	1161.31
Crystal system	Monoclinic	Monoclinic
Space group	P21/c	P21/n
T/K	173(1)	123(1)
a/Å	15.9136(11)	18.4036(3)
b/Å	14.6367(10)	13.2152(2)
c/Å	28.8411(19)	25.3892(4)
β/°	105.4910(10)	101.136(1)
U/Å^3	6473.70	6058.6(2)
Z	4	4
ρ calc/g cm^−1	1.274	1.273
μ/mm^−1	0.098	0.099
Data collected	39976	93672
Unique data, Rint	14297	13755
Observed data [I⊕>⊕2σ(I)]	6103	8360
Structure solution	Direct methods (SHELXS-97)	Direct methods (SHELXS-97)
Refinement (SHELXL-97)	Full-matrix on F^2	Full-matrix on F^2
Parameters	813	821
R 1 (observed data)	0.0872	0.0970
wR 2 (all data)	0.2853	0.2498
S	1.010	1.044

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The asymmetric unit of 4 consists of one calix[4]resorcinarene, two-and-a-half crown ethers and two water molecules. In the extended structure these components reside in two distinct layers [Fig. 1(a)]. One layer consists of two types of crown ethers: one uncomplexed and the other interacting with two crystallographically unique water molecules positioned on each side of the crown. These water molecules are within hydrogen-bonding distances of several crown oxygen atoms (O⋯O distances ranging from 2.79 to 3.23 Å) as well as one phenolic group of a calix[4]resorcinarene (O⋯O 2.55 and 2.63 Å). The other repeating layer consists of a mixture of calix[4]resorcinarenes and a third type of crown ether. These crowns are oriented roughly perpendicular with respect to those in the aforementioned layer. This final type of crown ether interacts with the hydroxy groups on the upper rim of a pair of calix[4]resorcinarenes via hydrogen-bonding interactions (O⋯O 2.65 and 2.66 Å). The crown ether does not reside in the cavity of the resorcinarenes, rather, two crown ether molecules reside partially within the cavity created by a pair of calix[4]resorcinarene host molecules [Fig. 1(b)].

Fig. 1 Crystal structure of C-methylcalix[4]resorcinarene(18-crown-6)_2.5(H₂O)₂4: (a) cross-section of the extended structure. Click image or 1.htm to access a 3D representation (which includes a degree of disorder); (b) hydrogen bonding between two 18-crown-6 molecules which partially reside within the cavity of each pair of resorcinarenes.

The structure of [Na⁺⊂[2.2.2]cryptand][C-methylcalix[4]resorcinarene-H](CH₃CN)₄(H₂O)₃5 is significantly different, with the formation of an infinite 2D hydrogen bonded sheet structure between calix[4]resorcinarene and water molecules in the ab plane.‡ A section of this {(calix[4]resorcinarene-H)(H₂O)₂}_∞ network is shown in Fig. 2(a). Four of the calix[4]resorcinarene hydroxy hydrogen atoms act as intramolecular hydrogen bond donors at OH⋯O distances of 1.66 to 1.99 Å. Two of the remaining hydroxy hydrogen atoms (of the same resorcinarene ring) each act as a hydrogen bond donor to a water molecule at OH⋯O distances of 1.71 and 1.85 Å (O⋯O 2.66, 2.68 Å). Charge balance requires the calix[4]resorcinarene to be mono-deprotonated and the averaged structure has half a proton on each of the two remaining phenolic oxygens sited on centres of inversion. The closest O⋯O separations between adjacent calix[4]resorcinarene molecules across these centres are 2.44 and 2.46 Å. Each of the water molecules within the network accepts a hydrogen-bonding proton from one calix[4]resorcinarene hydroxy group and donates to two adjacent calix[4]resorcinarene molecules (HOH⋯O distances 1.93–2.19 Å).

Fig. 2 Crystal structure of [Na⁺⊂[2.2.2]cryptand][C-methylcalix[4]resorcinarene-H](CH₃CN)₄(H₂O)₃5: (a) {(calix[4]resorcinarene-H)(H₂O)₂}_∞ 2D hydrogen bonded network, only hydrogens associated with H-bonding are shown for clarity. Click image or 2.htm to access a 3D representation (which includes a degree of disorder); (b) pair of {[Na⁺⊂[2.2.2]cryptand]∩(C-methylcalix[4]resorcinarene-H)} supermolecules.

The orientation of the calix[4]resorcinarene host molecules within the 2D sheet structure is the same along the b axis with but orientated alternate bowl-up, bowl-down along the a axis. Each of the bowl-shaped calix[4]resorcinarenes acts as a host molecule for a [Na⁺⊂[2.2.2]cryptand] supermolecule to form a {[Na⁺⊂[2.2.2]cryptand]∩(C-methylcalix[4]resorcinarene-H)} assembly [Fig. 2(b)], effectively a second sphere coordination of the sodium cation. Closest interactions of molecules between the networks are between the calix[4]resorcinarene methyl groups (C⋯C 3.78 Å) and [Na⁺⊂[2.2.2]cryptand] molecules associated with adjacent networks (C⋯O 3.63 Å). Solvent acetonitrile and water fill gaps in the crystal packing.

These results establish that bowl-shaped calix[4]resorcinarenes can interact in concert with either 18-crown-6 or [2.2.2]cryptand to form extended structures. Their ability to hydrogen bond via the upper rim hydroxy groups of the calix[4]resorcinarene, and the ability to prepare calix[4]resorcinarenes with other resorcinarene linkers, is likely to lead to a diverse range of supramolecular arrays.

Acknowledgements

We are grateful to the Australian Research Council for support of this work.

References

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Footnotes

† Synthesis of 4: 100 mg (378 µmol) of 18-crown-6 was added to a 2 ml ethereal solution containing 6 mg (11 µmol) of C-methylcalix[4]resorcinarene. Pale pink diamond-shaped plates grew after several weeks of slow evaporation (4 mg, 29%).

‡ Synthesis of 5: 5.2 mg (13.8 µmol) of [2.2.2]cryptand (4,7,13,16,21,24-hexaoxa-1,10-diazabicyclo[8.8.8]hexacosane) and 4.4 mg (7.7 µmol) of trisodium 3,3′,3″-phosphinidyne-tris(benzenesulfonate) were each dissolved in 0.1 mL of water and added to an acetonitrile solution (1.8 mL) which contained 5.7 mg (10.5 µmol) of C-methylcalix[4]resorcinarene. Square pink crystalline plates grew within a few hours (2 mg, 18%).

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