Melamine induced conformational change of ethyl resorcinarene in solid state

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Abstract

When ethyl resorcinarene (1) and melamine (2) are co-crystallised, all intramolecular hydrogen bonds keeping the resorcinarene in crown conformation are broken causing an unexpected conformational change to boat, and a highly ordered hydrogen bonded network is formed.

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Melamine and its derivatives have been widely used as precursors for self-assembling, supramolecular aggregates, such as cyclic rosettes,^1–3 linear and crinkled tapes^4,5 and molecular strands and ribbons^6–10 with the aim to investigate the molecular recognition and organisation via weak intermolecular interactions^1–10 and to prepare chemical nanostructures^11,12 and artificial receptors for biological compounds.¹³

The aim of our study was to crystallise melamine with ethyl resorcinarene (1) with unsubstituted hydroxyl groups in order to get preorganised superstructure held together via hydrogen bonding. To our surprise, in addition to the complicated intermolecular hydrogen-bonding network, the conformational change of the ethyl resorcinarene from crown to boat was observed owing to the breakage of the intramolecular hydrogen bonds between the adjacent hydroxyl groups. The previous studies of the conformational properties of resorcinarenes show that resorcinarenes with unsubstituted hydroxyl groups and methylene bridge substituents in all-cis arrangement are found exclusively in crown conformation both in solution^14–17 and in solid state.^18–25 The conformation is determined by the maximal hydrogen bonding, i.e., usually by the hydrogen bonds between the adjacent hydroxyl groups even if the hydrogen bonding solvents are used.^18–25 To our knowledge, the only example of the resorcinarene in boat conformation in solid state is the Ag₂· resorcinarene · (C₆H₆)₂ complex by Munakata et al.²⁶ in which the coordination of silver to hydroxyl groups causes the conformational change. In our solid state studies of the ethyl resorcinarene (1) with various, nitrogen containing organic guests melamine was observed to be the only guest facilitating such behaviour.²⁷ The crystal structure of ethyl resorcinarene crystallised from ethanol is used as a reference structure.

Ethyl resorcinarene–melamine complex 1·2₂· EtOH was prepared by dissolving the resorcinarene into warm ethanol and adding melamine (∼1∶1 molar ratio) into the solution. Water was added dropwise to dissolve the relatively insoluble melamine and the solution was warmed. After a couple of weeks colourless, plate-like crystals suitable for X-ray analysis formed (Table 1). The reference crystallisation of ethyl resorcinarene from ethanol was carried out at the ambient temperature. After a few days colourless crystals formed and were used for X-ray analysis (Table 1 and Scheme 1).

Scheme 1 The crystallographic numbering of ethyl resorcinarene and two crystallographically independent melamine molecules.

Table 1 Crystal data, data collection and refinement parameters for 1·2₂· EtOH and 1· 3 EtOH^a

Property	1·22· EtOH	1· 3 EtOH
a The data for 1·22·EtOH were recorded on a Nonius Kappa CCD and for 1· 3 EtOH on an Enraf Nonius CAD4 diffractometer using graphite monochromatised radiation. The structures were solved by direct methods (SHELXS-97^29) and refinements, based on F^2, were made by full-matrix least-squares techniques (SHELXL-97^30). The hydrogen atoms were calculated to their idealised positions and refined as riding atoms except for hydroxyl and amino hydrogens of 1·22· EtOH which were located from the difference Fourier map. The isotropic temperature factors (1.2 or 1.5 times the C temperature factor) were used for all hydrogens. One of the ethanol molecules of 1· 3 EtOH is highly disordered.
Formula	C36H40O8· 2 C3H6N6· C2H6O	C36H40O8· 3 C2H6O
Formula weight	899.02	738.88
Crystal colour	Colourless	Colourless
Dimensions/mm	0.05 × 0.30 × 0.30	0.30 × 0.40 × 0.50
Crystal system	Monoclinic	Triclinic
Space group	P21/c	P1̄
a/Å	11.8691(6)	11.566(2)
b/Å	12.8337(6)	12.289(2)
c/Å	29.238(1)	14.641(2)
α/°	90	110.62(1)
β/°	100.840(2)	93.16(1)
γ/°	90	94.35(2)
U/Å^3	4374.3(4)	1934.6(5)
Z	4	2
Dc/mg m^–3	1.365	1.268
μ(MoKα), mm^–1	0.098	0.091
T/K	173.0(1)	180.0(1)
Measured reflections	15869	7168
Unique reflections	7175	6799
R1(Fo)/wR2(Fo)	0.0686/0.1339	0.0370/0.0962

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When crystallised from ethanol, ethyl resorcinarene adopts the expected, slightly “pinched” crown conformation stabilised by intramolecular hydrogen bonds between the adjacent hydroxyl groups [O4⋯O13 = 2.786(2), O20⋯O11 = 2.763(2), O25⋯O6 = 2.763(2) and O27⋯O18 = 2.921(2) Å] (Fig. 1a). Additional intermolecular hydrogen bonds to the solvent ethanol [2.653(2)–2.792(2) Å] do not affect the conformation significantly even if they are of the same strength as the intramolecular hydrogen bonds. However, when the crystallisation of ethyl resorcinarene is carried out in the presence of melamine completely different kind of behaviour of the resorcinarene is observed. All intramolecular hydrogen bonds are broken and replaced by multiple intermolecular hydrogen bonds to the melamine molecules (Fig. 1b). Owing to the lack of the stabilising effect of the intramolecular hydrogen bonds the conformation is no longer a crown but more boat-like.

Fig. 1 A portion of the crystal structures of ethyl resorcinarene crystallised from ethanol (a) and co-crystallised with melamine (b). Hydrogen bonds are shown as dashed lines and non-hydrogen bonding hydrogens are omitted for clarity.

The investigation of the hydrogen bonds of the 1·2₂· EtOH complex shows the primary and secondary type of hydrogen bonding. The primary bonds between the aromatic nitrogens of the melamine and hydroxyl groups of the resorcinarene are stronger [2.637(3)–2.770(3) Å] and approximately the same strength as the intramolecular hydrogen bonds and interaction with ethanol in crown conformation. The weaker, secondary hydrogen bonds [2.943(3)–3.382(4) Å] are interactions between the amino groups of the melamine and hydroxyl groups of the resorcinarene. The primary hydrogen bonding connects three resorcinarenes around a pair of melamines to wheel-like clusters [O13*⋯N37 = 2.737(3), O11*⋯N45 = 2.637(3) Å, O4⋯N39 = 2.727(3), O6⋯N47 = 2.693(3), O25**⋯N41 = 2.770(3), O27**⋯N43 = 2.729(3) Å] (Fig. 2). Secondary hydrogen bonds N38⋯O4 = 3.282(4), N40⋯O25** = 3.041(4), N40⋯O6** = 2.943(3), N46⋯O6 = 3.079, N46⋯O18 = 3.382(4), N48⋯O27** = 3.004(4) and N48⋯O18** = 3.139(3) Å reinforce the complexation and contribute to the conformational properties of the resorcinarene. The amino nitrogens N38 and N46 are situated between the opposite hydroxyl groups resembling closely the position of the silver cations in Ag₂· resorcinarene · (C₆H₆)₂ complex.²⁶

Fig. 2 The hydrogen bonds (shown as dashed lines) connect three resorcinarenes around a pair of melamines to wheel-like clusters. Stick (a) and VDW presentation (b). Non-hydrogen bonding hydrogens are omitted for clarity.

The conformation of the resorcinarenes can be described by the distances between the opposite hydroxyl groups of the resorcinol rings. In Ag-complex the closest distance is between the coordinated hydroxyl groups, being 4.09 Å.²⁶ In 1· 3 EtOH in crown conformation the respective distance from O6 to O18 is 7.45 Å, while in 1·2₂· EtOH the distances are O6⋯O18 = 5.96 Å and O4⋯O20 = 6.12 Å, i.e, longer than in Ag-complex owing to the weakness of the hydrogen bonding compared to the metal coordination but remarkably shorter than in crown conformation.

The additional way to describe the conformation is to study the angles between the aromatic rings and the least-squares plane formed by the bridging methylene carbons. In the reference structure the pinching of the crown is seen by the angles: C8–C13 and C22–C27 are slightly bent towards the plane of the methylene carbons [42.68(3) and 40.12(5)°, respectively] compared to 64.39(4) and 67.01(5)° deviation of C1–C6 and C15–C20. In melamine complex the C8–C13 and C22–C27 deviate only by 10.4(1) and 17.14(8)° from the least-squares plane of the methylene bridges while the other two rings C1–C6 and C15–C20 are bent upward from the plane [84.00(7) and 75.46(8)°, respectively]. The angles indicate that the shape of the boat conformation is not symmetrical but somewhat distorted. The reason for this is the unsymmetrical hydrogen bonding of the melamines (Fig. 1b). In addition to the primary hydrogen bonding of melamines to O25 and O27, the amino groups N40 and N48 interact with both upward pointing hydroxyl O6 and O18 and downward bent O25 and O27, therefore lifting the resorcinol ring C22–C27 slightly up. At the opposite resorcinol ring C8–C13 similar lifting is not observed since the primary hydrogen bonds are the only interaction.

In the structure of ethyl resorcinarene in crown conformation the intermolecular hydrogen bridges [O18⋯O25′ = 2.852(2) and O27⋯O4″ = 2.847(2) Å] connect the adjacent resorcinarene molecules to the chains, while in melamine complex direct hydrogen bonding contacts between the host molecules are not observed. However, the superstructure of the 1·2₂· EtOH complex can be described as a pile of hydrophobic layers formed by resorcinarene-melamine chains crossing in 90° angle (Fig. 3). The hydrophilic parts of the molecules are saturated with hydrogen bonds and facing each other, therefore making the surface of the layers hydrophobic. The ethanol molecules are filling interstice in crystal lattice by hydrogen bonding to melamine and resorcinarene [N42⋯O100* = 2.870(4) and O100⋯O25³* = 2.925(4) Å].

Fig. 3 The wheel-like clusters pack into crossing chains and furthermore into pile of hydrophobic layers. Hydrogen atoms are omitted for clarity.

In conclusion, we have observed that melamine induces the breakage of the intramolecular hydrogen bonds between the adjacent hydroxyl groups and their replacement by the intermolecular hydrogen bonds. The reason for this is the ability of melamine to act simultaneously as hydrogen bond acceptor and donor, as well as the suitable form and the size of the melamine for tight, wheel-like packing around a pair of melamines. The breakage of the intramolecular hydrogen bonds causes an unusual conformational change from crown to the boat-like conformation.

Ethyl resorcinarene (1) was prepared according to the literature procedure.²⁸ Due to the insolubility of the melamine the reasonable NMR spectra of the complex were not obtained. Detailed NMR investigations are in progress.

Acknowledgements

We thank Prof. Dr Volker Böhmer, University of Mainz, Germany, for valuable comments. MN and EW gratefully acknowledge the financial support of the Finnish Ministry of Education.

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