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Melamine induced conformational change of ethylresorcinarene in solid state |
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CrystEngComm,
Volume 2,
Issue 28,
2000,
Page 151-153
Maija Nissinen,
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摘要:
Melamine and its derivatives have been widely used as precursors for self-assembling,supramolecular aggregates, such as cyclic rosettes,1–3linear and crinkled tapes4,5and molecularstrands and ribbons6–10with the aim to investigate the molecular recognition and organisationviaweak intermolecular interactions1–10and to prepare chemical nanostructures11,12and artificial receptors for biological compounds.13The aim of our study was to crystallise melamine with ethyl resorcinarene (1) with unsubstituted hydroxyl groups inorder to get preorganised superstructure held togetherviahydrogenbonding. To our surprise, in addition to the complicated intermolecular hydrogen-bondingnetwork, the conformational change of the ethyl resorcinarene from crown toboat was observed owing to the breakage of the intramolecular hydrogen bondsbetween the adjacent hydroxyl groups. The previous studies of the conformationalproperties of resorcinarenes show that resorcinarenes with unsubstituted hydroxylgroups and methylene bridge substituents in all-cisarrangementare found exclusively in crown conformation both in solution14–17and in solid state.18–25The conformation is determined by the maximal hydrogen bonding,i.e.,usually by the hydrogen bonds between the adjacent hydroxyl groups even ifthe hydrogen bonding solvents are used.18–25To our knowledge, the only example of the resorcinarene in boat conformationin solid state is the Ag2·resorcinarene·(C6H6)2complex by Munakataet al.26in whichthe 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 tobe the only gues27The crystal structure of ethyl resorcinarene crystallised from ethanol isused as a reference structure.Ethyl resorcinarene–melamine complex1·22·EtOH was preparedby dissolving the resorcinarene into warm ethanol and adding melamine (∼1∶1molar ratio) into the solution. Water was added dropwise to dissolvethe relatively insoluble melamine and the solution was warmed. After a coupleof weeks colourless, plate-like crystals suitable for X-ray analysisformed (Table 1). Thereference crystallisation of ethyl resorcinarene from ethanol was carriedout at the ambient temperature. After a few days colourless crystals formedand were used for X-ray analysis (Table 1andScheme 1).The crystallographicnumbering of ethyl resorcinarene and two crystallographically independentmelamine molecules.Crystal data, data collection and refinementparameters for1·22·EtOH and1·3 EtOHProperty1·22·EtOH1·3 EtOHThe data for1·22·EtOHwere recorded on a Nonius Kappa CCD and for1·3 EtOH on an Enraf Nonius CAD4 diffractometer using graphite monochromatisedradiation. The structures were solved by direct methods (SHELXS-9729) and refinements, based onF2,were made by full-matrix least-squares techniques (SHELXL-9730). The hydrogen atoms were calculated to theiridealised positions and refined as riding atoms except for hydroxyl and aminohydrogens of1·22·EtOH which werelocated from the difference Fourier map. The isotropic temperature factors (1.2or 1.5 times the C temperature factor) were used for all hydrogens. Oneof the ethanol molecules of1·3 EtOH is highly disordered.FormulaC36H40O8·2 C3H6N6·C2H6OC36H40O8·3 C2H6OFormula weight899.02738.88Crystal colourColourlessColourlessDimensions/mm0.05 × 0.30 × 0.300.30 × 0.40 × 0.50Crystal systemMonoclinicTriclinicSpace groupP21/cP1&cmb.macr;a/Å11.8691(6)11.566(2)b/Å12.8337(6)12.289(2)c/Å29.238(1)14.641(2)α/°90110.62(1)β/°100.840(2)93.16(1)γ/°9094.35(2)U/Å34374.3(4)1934.6(5)Z42Dc/mg m–31.3651.268μ(MoKα),mm–10.0980.091T/K173.0(1)180.0(1)Measured reflections158697168Unique reflections71756799R1(Fo)/wR2(Fo)0.0686/0.13390.0370/0.0962When crystallised from ethanol, ethyl resorcinarene adopts the expected,slightly “pinched” crown conformation stabilised by intramolecularhydrogen 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 hydrogenbonds to the solvent ethanol [2.653(2)–2.792(2) Å]do not affect the conformation significantly even if they are of the samestrength as the intramolecular hydrogen bonds. However, when the crystallisationof ethyl resorcinarene is carried out in the presence of melamine completelydifferent kind of behaviour of the resorcinarene is observed. All intramolecularhydrogen bonds are broken and replaced by multiple intermolecular hydrogenbonds to the melamine molecules (Fig. 1b).Owing to the lack of the stabilising effect of the intramolecular hydrogenbonds the conformation is no longer a crown but more boat-like.A portion of the crystalstructures of ethyl resorcinarene crystallised from ethanol (a)and co-crystallised with melamine (b). Hydrogen bonds are shownas dashed lines and non-hydrogen bonding hydrogens are omitted for clarity.The investigation of the hydrogen bonds of the1·22·EtOH complexshows the primary and secondary type of hydrogen bonding. The primary bondsbetween the aromatic nitrogens of the melamine and hydroxyl groups of theresorcinarene are stronger [2.637(3)–2.770(3) Å]and approximately the same strength as the intramolecular hydrogen bonds andinteraction with ethanol in crown conformation. The weaker, secondary hydrogenbonds [2.943(3)–3.382(4) Å]are interactions between the amino groups of the melamine and hydroxyl groupsof the resorcinarene. The primary hydrogen bonding connects three resorcinarenesaround 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) Åreinforce the complexation and contribute to the conformational propertiesof the resorcinarene. The amino nitrogens N38 and N46 are situated betweenthe opposite hydroxyl groups resembling closely the position of the silvercations in Ag2·resorcinarene·(C6H6)2complex.26The hydrogen bonds (shownas dashed lines) connect three resorcinarenes around a pair of melaminesto 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 distancesbetween the opposite hydroxyl groups of the resorcinol rings. In Ag-complexthe closest distance is between the coordinated hydroxyl groups, being 4.09 Å.26In1·3 EtOH in crown conformation the respective distance from O6 to O18 is 7.45 Å,while in1·22·EtOH the distancesare 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 bondingcompared to the metal coordination but remarkably shorter than in crown conformation.The additional way to describe the conformation is to study the anglesbetween the aromatic rings and the least-squares plane formed by the bridgingmethylene carbons. In the reference structure the pinching of the crown isseen by the angles: C8–C13 and C22–C27 are slightly bent towardsthe 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–C13and C22–C27 deviate only by 10.4(1) and 17.14(8)°from the least-squares plane of the methylene bridges while the othertwo rings C1–C6 and C15–C20 are bent upward from the plane [84.00(7)and 75.46(8)°, respectively]. The angles indicate thatthe 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 hydrogenbonding of melamines to O25 and O27, the amino groups N40 and N48 interactwith both upward pointing hydroxyl O6 and O18 and downward bent O25 and O27,therefore lifting the resorcinol ring C22–C27 slightly up. At the oppositeresorcinol ring C8–C13 similar lifting is not observed since the primaryhydrogen bonds are the only interaction.In the structure of ethyl resorcinarene in crown conformation the intermolecularhydrogen bridges [O18⋯O25′ = 2.852(2)and O27⋯O4″ = 2.847(2) Å]connect the adjacent resorcinarene molecules to the chains, while in melaminecomplex direct hydrogen bonding contacts between the host molecules are notobserved. However, the superstructure of the1·22·EtOH complexcan be described as a pile of hydrophobic layers formed by resorcinarene-melaminechains crossing in 90° angle (Fig. 3).The hydrophilic parts of the molecules are saturated with hydrogen bonds andfacing each other, therefore making the surface of the layers hydrophobic.The ethanol molecules are filling interstice in crystal lattice by hydrogenbonding to melamine and resorcinarene [N42⋯O100* = 2.870(4)and O100⋯O253* = 2.925(4) Å].The wheel-likeclusters pack into crossing chains and furthermore into pile of hydrophobiclayers. Hydrogen atoms are omitted for clarity.In conclusion, we have observed that melamine induces the breakage of theintramolecular hydrogen bonds between the adjacent hydroxyl groups and theirreplacement by the intermolecular hydrogen bonds. The reason for this is theability 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-likepacking around a pair of melamines. The breakage of the intramolecular hydrogenbonds causes an unusual conformational change from crown to the boat-likeconformation.Ethyl resorcinarene (1)was prepared according to the literature procedure.28Due to the insolubility of the melamine the reasonable NMR spectra of thecomplex were not obtained. Detailed NMR investigations are in progress.
ISSN:1466-8033
DOI:10.1039/b006193j
出版商:RSC
年代:2000
数据来源: RSC
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