Structural Variation Involving Triphenylbenzylphosphonium Cation and p-Sulfonated Calixarene Anion

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ScienceDirect Materials Today: Proceedings 5 (2018) S166–S171

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Structural Variation Involving Triphenylbenzylphosphonium Cation and p-Sulfonated Calixarene Anion Irene Linga*, Alexandre N. Sobolevb a

b

School of Science, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway, 46150 Selangor, Malaysia. School of Molecular Sciences, M310, The University of Western Australia, 35 Stirling Highway, Perth, WA 6009, Australia.

Abstract Combining equimolar of p-sulfonated calix[4]arene anion and triphenylbenzylphosphonium cation along with three molar of gadolinium(III) ions affords the typical bilayer arrangement while incorporating the same phosphonium cation in excess amount induces a porous-like array in the extended structure however without the uptake of the lanthanide in the final product. Addition of butyl-methylpyrrolidinium cation as a guest molecule demonstrates consistency to recent findings where the charged molecule is selectively drawn into the cavity with the long alkyl chain directs away from the cavity. © 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of 7th Asian Conference on Colloid and Interface Science. Keywords: calixarene; phosphonium; lanthanide; self-assembly; crystallography

1. Introduction Water-soluble sulfonated calixarenes, accessible by functionalising at the upper rim of the polyphenol units are amongst other amphiphilic macromolecules that have been explored comprehensively as supramolecular tectons.[1,2] The conformationally constrained p-sulfonated calix[4]arene being the smallest molecule in the calixarene family normally assembles in a bilayer arrangement through a range of intermolecular interactions

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This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited. * Corresponding author. Tel.: +603-55146113; fax: +603-55146364. E-mail address: [email protected] 2214-7853 © 2018 The Author(s). Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and Peer-review under responsibility of 7th Asian Conference on Colloid and Interface Science.

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comprising π-stacking, hydrogen bonding and C-H···π interactions.[2,3] Occasionally the bilayer arrangement is led to comparison with clay minerals owing to the well separated hydrophobic and hydrophilic layered structures and also been compared to lipid bilayers when relates to molecules spanning the bilayer or cationic channels.[4] The versatility of the self-assembly of p-sulfonated calix[4]arene in the solid-state is demonstrated in forming supersized hydrophobic bilayers without carrying-out a covalent synthesis.[5,6] This can be achieved by controlling the interplay of calixarene and large phosphonium cations. It is interesting that the hydrophobic phosphonium cations are able to expand the hydrophobic surface and change the hydrophobic surface of the bilayers. Conversely, the phosphonium cation can serve as guest molecule having the likelihood of its phenyl ring included in the calixarene cavity and subsequently disrupts the typical modes of association for these cations notably the phenyl embraces.[7] In this paper we present the structure elucidation of four complexes (CCDC: 1576782-1576785) based on psulfonated calix[4]arene and benzyltriphenylphosphonium cation, catalysed by gadolinium(III), Gd3+ cation as a detailed study to understand the interplay of the various components and the variation in structural motifs. The synthesis of the complexes involved a systematic study accompanied by varying the molar ratio of the phosphonium cation and integrating an additional heterocyclic molecule notably the butyl-methylpyrrolidinium, Table 1. This combinatorial method demonstrated some design rules in forming multi-component complexes in the solid-state. Full synthetic details are described in Supporting Information. Table 1. Molar ratios of multi-components affording complexes 1, 2, 3 and 4

2. Results and Discussion 2.1 Up-down anti-parallel bilayer Complex 1 crystallises in the monoclinic space group P21/m, Z = 2 and the asymmetric unit comprised of one calixarene, two phosphonium cations, a disordered Gd3+ ion and a disordered chloride ion, along with some disordered and/or partially occupied water molecules. The calixarene adopts the expected almost symmetrical cone conformation although one of the rings is splayed greater than others (Table S1). The structure is heavily hydrated with water molecules that form intricate hydrogen bonded network, however the quality of data does not allow to localise the majority of H-atoms on both coordinated and free water. Two hydrogen bonded water molecules are held within the calixarene cavity are half-populated and situated close to each other, Fig. 1(a), both residing close to calixarene aromatic rings at distance consistent with the formation of a weak π-arene non-classical hydrogen bond interaction. The calixarenes are self-assembled systematically into the ubiquitous bilayer arrangement with molecular capsules comprising two adjacent calixarenes facing each other over a mirror plane, being the subunit building up the extended structure. The homoleptic octa-aqua Gd3+ cation is not coordinated to any sulfonate groups of the calixarene but resides in close proximity to a sulfonato group of the calixarene that presumably caused the ring to be splayed greater. The head-to-head organization of the two calixarenes is facilitated through secondary coordination from the aquated lanthanide sphere to calixarene sulfonate groups and hydrogen bonding from water molecules to the calixarene sulfonate groups yielding a molecular capsule size to be approx. 14 Å (Fig. 1(b)). The calixarenes in the opposite direction in the bilayers are separated by corrugated layers of phosphonium cations at the interface of calixarene hydroxyl groups (both components involved in O···H hydrogen bonding) and

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ultimately expands the hydrophobic domain. The distance from the centre of one bilayer to the centre of the neighbouring bilayer is 23 Å, with the thickness of the bilayer estimated to be 14 Å, Fig. 1(b). The benzyltriphenylphosphonium cations form columnar arrays along c-axis where each phosphonium pair is held through their concerted multiple phenyl embraces involving 2 edge-to-face interactions and 1 offset face-to-face interaction, Fig. 1(c). Every phosphonium pair is connected to each other through C-H···H-C hydrogen bonding. The phosphonium H-atoms are also involved in CH···π interaction with the aromatic rings of the calixarene and phosphonium aromatic ring has short CH···π contact with the calixarene methylene bridge. The same ternary system can also crystallise in the triclinic space group P1¯ , Z = 2, complex 2. The basic structural motif of complex 2 is similar to complex 1 with the calixarenes arranged in a linear array of molecular capsules along the bilayers, likewise linked through hydrophobic interactions involving the benzyltriphenylphosphonium cations however the head-to-head alignment is rather skewed (tilt angle relative to the principal axis is 4.78o). The partially occupied octa-aqua Gd3+ centre has larger Gd–O(H2O) distances as opposed to complex 1 and consequently afforded a marginally larger molecular capsule that is calculated to be approx. 15 Å.

Fig. 1. (a) Ball and stick representation of water molecules included in calixarene cavity along with the aquated Gd3+ forming secondary coordination one sulfonate group; (b) molecular capsule size and bilayer thickness and (c) multiple phenyl embraces involving edge-to-face interactions (blue dashed lines) and offset face-to-face interaction (red dashed line) with P···P distance at 7.93 Å.

The combination of sodium p-sulfonated calix[4]arene, benzyltriphenylphosphonium chloride, butylmethylpyrrolidinium and Gd3+ ions afforded colorless prisms of complex 3. The crystal structure was solved in the triclinic space group P1¯ , Z = 1. The asymmetric unit consists of one calixarene, with one disordered butylmethylpyrrolidinium cation (0.25 occupancy) residing in the cavity of the calixarene molecule (Fig. 2(a&b)), two phosphonium cations surrounding the calixarene and a disordered homoleptic Gd3+ cation (0.25 occupancy) that forms secondary coordination sphere through hydrogen bonding with the calixarene sulfonate groups. It is noteworthy that the calixarenes are self-assembled in a slipped bilayer arrangement with the cavity of one calixarene from one bilayer positioned directly above a calixarene sulfonate group from the opposite bilayer that deviates from the molecular capsule arrangement, Fig. 2(c). Such configuration presumably maximises the electrostatic interaction between the sulfonate groups and the polyaquated Gd3+ cations. The homoleptic octa-aqua Gd3+ ion similarly interacts with the upper rim sulfonate groups through hydrogen bonding. The distance between the interlayer is now slightly shorter relative to complex 1, thickness at 19 Å. The slipped bilayers are separated by benzyltriphenylphosphonium cations that form corrugated layers within the calixarene bilayers with the same multiple phenyl embrace manner likewise in both aforementioned complexes. The calixarene is slightly distorted from a symmetrical cone shape with two opposite 1,3-disposed aromatic rings pushed away, pinching a disordered butyl-methylpyrrolidinium cation that is distributed between two positions. The interplay of the cationic pyrrolidinium molecule with calixarene cavity is similar to previous complexes [8] based on the same components where the pyrrolidine charged head group is directed into the calixarene cavity along with the alkyl group directed away from the sulfonate groups. The complementarity of interaction of the pyrrolidinium cation with calixarene cavity relates to the size of the components that includes the multiple interactions as follows: (i) Hatoms of the pyrrolidine ring are involved in C–H···π contacts with the phenyl rings of calixarene, (ii) H-atom of the pyrrolidine ring hydrogen bonds to a sulfonate group, (iii) H-atoms of the N-methyl to a sulfonate group and (iv) H-

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atom of the N-butyl in close to a sulfonate group. We further visualised the close intermolecular interactions between the calixarene and pyrrolidinium molecules using Hirshfeld surface analysis [9]. From the analysis of the fingerprint plots (Figs. 2(d-g)), C–H···O contacts is the dominant intermolecular interactions contributing 35% and 60% of the total surface for calixarene and pyrrolidinium respectively. As for the C–H···π contacts both the calixarene and pyrrolidinium molecules involved in 20 % of the overall interaction.

Fig. 2. (a) Ball and stick representation of butyl-methylpyrrolidinium included in calixarene cavity along with the aquated Gd3+ forming secondary coordination one sulfonate group; (b) space-filling representation of the supermolecule; (c) molecular capsule size and bilayer thickness; (d) Hirshfeld surface for calixarene showing closest contacts as red spots, corresponding fingerprint plot (e) and (f) Hirshfeld surface for butyl-methylpyrrolidinium displaying the short contacts, corresponding fingerprint plot (g).

2.2 Metal-organic network On evaporation, mixture of 1:3 of sodium p-sulfonated calix[4]arene and benzyltriphenylphosphonium chloride in THF/water along with three fold of Gd3+ ions gave colorless crystals, complex 4. However, the Gd3+ ions are not taken up in the final structure. The X-ray diffraction study showed that the complex crystallized in the space group P1¯, Z = 4, with the asymmetric unit contains two calixarene molecules and eight benzyltriphenylphosphonium cations and crystalline water molecules. The overall structure is intricate, devoid of the usual divergent back-to-back bilayer arrangement. Instead of bilayer arrangement, the extended structure is composed of infinite network of calixarene chains propagate along a-axis in the crystal lattice. The calixarenes are connected in a unique way governed by (i) calixarene...calixarene π-stacking interactions and (ii) hydrogen bonding between the sulfonate groups and hydroxyl groups in opposing directions. The diameter of the chain is calculated to be approx. 10 Å. When these individual chains are self-assembled in the extended structure, they form a porous type of assembly along b-axis, Fig. 3(b). The crystal lattice is not heavily hydrated and the disordered water molecules (no H-atoms located) within the molecular solid are in close proximities with the hydrophilic segment of calixarene and the Oatoms participate in hydrogen bonding with the sulfonate groups. The high content of phosphonium cations relative to calixarene, results in the phosphonium cations dominating the interplay between the two entities. Calixarene in the cone C4v conformer has its cavity occupied by a phenyl ring of a benzyltriphenylphosphonium cation rather than water molecules as opposed to complex 1 and 2, Fig. 3(a). The phenyl ring inclusion is associated with a C–H···π interaction to the aromatic moieties of the calixarene, with two H-atoms directed towards the centroids of opposite calixarene phenyl rings. Other H-atoms of the included phosphonium molecule H-bond to the adjacent sulfonate groups. Results from the Hirshfeld surface analysis (Fig. S1) indicates that the red spots on the surfaces of both the calixarene and phosphonium molecules corresponds to all

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the short contacts mentioned above. Fingerprint plots for the interplay of the two components shows that C–H···π contacts for calixarene contributes 18 % while for phosphonium molecule contributes 33% to the overall surface. As for C–H···O interactions, calixarene contributes 46 % to the total surface while for phosphonium molecule it contributes 16 %. Careful inspection of the self-assembly involving the supermolecules (each made up of calixarene-phosphonium inclusion complex) demonstrated that two supermolecules are intertwined by two geometrically inverted phosphonium molecules through extensive hydrogen bonding from their H-atoms to adjacent calixarene sulfonate groups, aromatic ring to H-atoms of included phosphonium molecule and between the two intercalated phosphonium molecules. The supermolecules are surrounded by other independent phosphonium molecules and interact extensively in intricate hydrogen bonding involving H-atoms of the phosphonium methylene groups to calixarene methylene bridge, H-atoms of the phosphonium molecule to neighboring calixarene sulfonate groups and H-atoms of the phosphonium methylene groups to adjacent calixarene sulfonate groups.

Fig. 3. (a) Extended structure of complex 4 along a-axis, with included phosphonium molecules in blue while not included phosphonium molecules in orange; (b) Porous-like array of the calixarenes along b-axis.

3. Conclusion p-Sulfonated calix[4]arene anion shows the consistency to form the bilayer arrangement with the triphenylbenzylphosphonium cation self-embrace within the calixarene hydroxyls interface along with lanthanide ions stabilising the overall structure through extensive hydrogen bonding. Also, the host-guest interplay of the butylmethylpyrrolidinium cation with the calixarenes is similar to previous findings, which resides in calixarene cavity. In the present study, the high content of phosphonium cations relative to calixarene, can effectively perturb the common bilayer array and generate a different type of packing arrangement in the supramolecular building blocks relative to those in the conventional bilayer arrangement. Moreover, the excess of phosphonium cation can win out over the incorporation of lanthanide ions in the final structure. These new findings, coupled with other recent studies on phosphonium cations, essentially create a level of predictability of the structures. Acknowledgements The authors acknowledge the facilities, scientific and technical assistance of the Australian Microscopy & Microanalysis Research Facility at the Centre for Microscopy, Characterization & Analysis, The University of Western Australia, a facility funded by the University, State and Commonwealth Governments.

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