Elsevier

Tetrahedron

Volume 59, Issue 2, 6 January 2003, Pages 175-185
Tetrahedron

Self-association and self-assembly of molecular clips in solution and in the solid state

https://doi.org/10.1016/S0040-4020(02)01480-1Get rights and content

Abstract

Clip molecules based on diphenylglycoluril form well-defined dimeric structures in chloroform solution and in the solid state. In solution the dimerization process is based on favourable π–π interactions and cavity filling effects. A combination of favourable π–π interactions and crystal packing forces determine the self-assembly of clips in the solid state. The geometry that the clip molecules adopt in solution and in a series of X-ray crystal structures is compared with favourable geometries predicted by molecular modelling calculations.

Introduction

The self-assembly of relatively simple building blocks is a topic of great interest in modern supramolecular chemistry, since many biological systems are constructed by hierarchical self-assembly processes.1., 2. These processes can lead to complex multicomponent superstructures of nanomeric size, both in the case of the natural systems and the synthetic mimics.3 However, detailed insight into the translation of the properties that are encoded in the building blocks to form assemblies of a particular shape or size is still predominantly lacking. The construction of structures in the solid state using self-assembly can be seen as an extension of the formation of aggregates in solution. Although the process of gaining control over structures in the solid state is very complex,4., 5. significant help can be gathered from the understanding of the crystal packing of organic molecules. The design and construction of well-defined and predictable solid-state structures has become a new research area in its own right. It can be foreseen that crystal engineering,6., 7., 8., 9. as the research area has become known, can have numerous applications in the development of electronic devices.10 It has already been shown that by using self-assembly techniques large organic assemblies can be constructed,11., 12., 13. with hydrogen bonding often being the predominant tool applied. Numerous hydrogen bonded networks and discrete nanoscopic aggregates have been obtained by the self-assembly of building blocks with complementary hydrogen bond donor and acceptor functions.14., 15., 16. A somewhat different strategy is to use molecules which can act as both a hydrogen bond donor and an acceptor, and in this way molecular networks with zeolitic properties,17 stacked columns,18., 19., 20. interpenetrating molecular ladders,21 molecular capsules22., 23., 24. and a variety of designed organic structures in the solid state have been assembled.25 In relatively few cases, however, well-defined self-assembled materials have been constructed based on less directional π–π and electrostatic interactions.26., 27., 28. The ‘Molecular Meccano’ work of Stoddart, the construction of catenanes and rotaxanes based on donor–acceptor complexation, is an excellent example of this strategy.29., 30.

In our group we have been investigating clip-shaped receptor molecules of type 1 (Chart 1) which are capable of selectively binding dihydroxybenzenes.31 Binding of these guests is based on hydrogen bonding, π–π interactions, and a ‘cavity effect’.32., 33. It is only recently that a further aspect of these receptor molecules has revealed itself: in solution34., 35., 36., 37., 38. as well as in the solid state39., 40., 41. they can form head-to-head dimeric structures, in which the cavity of one molecule is filled by one of the side-walls of its dimeric partner and vice-versa. In this paper we will discuss in detail factors that determine this self-association process in solution and in the crystalline state.

Section snippets

Self-association in solution

Careful study of the 1H NMR spectra of molecular clip 1a (Chart 1) in CDCl3 revealed that upon dilution or an increase in temperature, the resonances of the aromatic side-wall and methoxy protons exhibited considerable downfield shifts. Since for this molecule no conformational changes were expected, a self-association process was proposed involving dimerization of the receptor cavities. To study this in more detail, an NMR dilution titration was carried out in which a plot of the chemical

Conclusions

In Table 3 the important distances and offsets between aromatic rings found in the solid-state structures of clip molecules are summarized. Comparison of the X-ray geometries with the optimal geometries calculated using the Hunter and Sanders model shows that there is reasonable agreement between the calculations and the experimental results. Although the Hunter and Sanders model is relatively simple, it is useful in calculating the optimum geometries and relative interaction energies between

General

The syntheses of the molecules discussed in this paper have been previously described.54 Dimerization constants were determined by diluting the samples from their maximum solubility (varying from 10 to 40 mM) to their minimum concentrations required for detection of signals by 1H NMR (approximately 0.1 mM). In general the chemical shifts of the side-wall protons were followed as function of the concentration of the clips. The obtained curves were fitted using the following equation:δobsm+md

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    Present address: Faculty of Chemistry, Institute for Molecular Chemistry, Homogeneous Catalysis, University of Amsterdam, Nieuwe Achtergracht 166, 1018 WV, Amsterdam, The Netherlands.

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