Chapter 6: Chapter 6: Cyclic imides containing hydroxy carboxylic acids in the syntheses of manganese(II), zinc(II) and cadmium(II) complexes
1.6 Supramolecular and host-guest chemistry of cyclic imides
These examples are all based on the use of 4-amino-1,8- naphthalimide structures, where the focus has been on the detection of cations, but the 3-amino-1,8-naphthalimide structures have also been employed in such sensing, as demonstrated elegantly by de Silva et al.96
Scheme 1.32.
The electron deficient and aromatic nature of such imides is important for face-to-face aromatic interactions and their rigid, planar structure along with the ability to be functionalized with a wide variety of side groups to tune their properties; means that their assemblies have strong, well defined directionality in space. Cyclic imides have been widely studied with respect to host–guest systems such as intercalation,99,100 foldamers,101,102 ion channels,103,104 catenanes105,106 and rotaxanes.107,108 Hydrogen bonding is the most predominant organizational tool in the construction of these supramolecular host-guest solids. Another interaction that is a recurrent motif in crystalline solids is … stacking between aromatic rings. It has long been established that the order of stability in the interaction of two - systems is -deficient – - deficient > -deficient – -rich > -rich – -rich.109 In the past few years, Reger et al. have focused on exploiting the … stacking capabilities of the strongly -deficient 1,8- naphthalimide group.110-114 They have recently reported the formation of different types of supramolecular architectures (from 1D to 3D) in a series of molecules containing a carboxylic acid and a 1,8-naphthalimide group joined by different linkers (1.58-1.63).115 Alternating hydrogen bonding of the carboxylic acids and … stacking interactions of the naphthalimide groups assembling the molecules into parallel chains that are linked into sheets by a second set of … stacking interactions (Figure 1.1).
Figure 1.1: Structure of molecules 1.58-1.63 and formation of 2D sheets in the structure of 1.59.
The role of less directional -deficient – -rich stacking interactions in the formation of extended molecular assemblies has also been shown earlier by our group. It is reported that compound N,N´-bis(glycinyl)pyromellitic diimide (1.64) forms molecular complexes (1.65- 1.68) with aromatic hydrocarbons such as anthracene, phenanthrene and perylene as well as tetrathiafulvalene.116 In these complexes, a primary self-assembly is governed by the hydrogen bonding of the carboxylic acid groups assembling the host molecules into 1D zig-zag chains that are further linked into sheets by second set of … stacking interactions between the aromatic -donor guest molecules and electron deficient -acceptor units of host molecules (Figure 1.2).
Figure 1.2: Formation of molecular complexes and weak interactions in the structure of 1.66.
Further to this, Degenhardt et al reported a pyromellitic diimide tethered aromatic carboxylic acids atropisomer (1.69) as a conformationally imprinted receptor for molecular recognition.117 When atropisomeric diacid 1.69 is heated in the presence of a ethyl adenine-9-acetate guest molecule, it rotates and coordinates with the guest molecule via hydrogen bonding. This results
Scheme 1.33.
in an isomeric shift to the guest accommodating syn conformer. This complementary conformation is stable even upon removal of guest and lowering of temperature. The imprinted
host can be returned back to its original state by heating in the absence of a guest (Scheme 1.33).
Rasberry et al developed a bispyridyl based pyromellitic diimide hydrogen bonding charge transfer (CT) receptor 1.69, to demonstrate and study the origins of the excellent selectivity of the sensor against various aromatic diols.118 Despite its low association constants of
( 101 M-1), receptor 1.69 was highly selective forming CT complexes of varying color and intensity with different phenol and naphthol guests. The CT band can simultaneously report multiple characteristics about a guest such as size, recognition ability, and electronic structure.119 The differences in the colorimetric responses are due to a combination of the abilities of the guests to form stable hydrogen-bonded complexes and the electronic structure of the guest (Scheme 1.34).
Scheme 1.34
Shinkai and co-workers have described the features of a naphthalene diimide low molecular weight gelator 1.70 with utility in sensing the seven different positional isomers of dihydroxynaphthalene at millimolar concentrations by visible colour changes (Scheme 1.35).120 Temperature-dependent spectral changes upon the addition of the dihydroxynaphthalenes indicate a mixture of … stacking and partial charge transfer interactions stabilizing the complexes. The NDI gelator 1.70 forms stable gels in some common organic solvents and shows a reversible sol–gel transition. It is important to note that solvophobic effects alone cannot explain the binding of the dihydroxynaphthalenes in the organogel matrix and hence this reorganisation process takes place.
Scheme 1.35.
Colquhoun et al reported macrocyclic receptors which are accessible by cycloimidization of an amine-functionalized aryl ether-sulfone with pyromellitic dianhydride or 1,4,5,8- naphthalenetetracarboxylic dianhydride, 1.71 and 1.72, respectively.121 These receptors bind a wide range of electron-donor substrates via -stacking donor-acceptor interactions. Addition of the -electron donor molecules such as pyrene, perylene, 2,6-dimethoxynaphthalene, tetrathiafulvalene, and pyren-1-ol to solutions of the almost colorless 1.71 and 1.72 produced intense colors assigned to intermolecular charge-transfer absorptions, and 1H-NMR spectra for equimolar ratios of receptor to substrate showed large ring-current-induced complexation shifts (Figure 1.3).
Figure 1.3: Structures of 1.71 and 1.72 and the complexes of 1.72 with perylene and pyrene guests.
Iwanaga et al reported a pyromellitic diimide based cyclophane-type macrocycles (1.73) as building blocks for the formation of tubular structures which encapsulate electron rich guest molecules like p-xylene and toluene.122 The complex 1.74 (1:2 p-xylene) is reported as
“cyclophane within cyclophane” that has been confirmed by X-ray crystallography whose driving force is a CT interaction (Scheme 1.36). In this structure, p-xylene guest molecule is arranged in a parallel fashion with the benzene ring of the facing pyromellitic diimide moiety which suggests the importance of a CT-type … interaction for the inclusion of guest molecule.
Scheme 1.36.
Self-assembly of naphthalene diimides (NDI) bearing carboxylic acid groups have also been shown to form cylindrical microstructures on the surface of solid substrates.123 The hydrogen bonding between carboxylic acid termini combined with the hydrophobic contacts between the
NDI cores is mainly responsible for the formation of these supramolecular arrays. This approach has been evolved at a molecular level into hydrogen-bonded helical organic nanotubes by utilising the chirality in a series of NDIs containing chiral -amino acid derivatives.124 The chirality of such nanotubes is determined by the constituent amino acid but is independent of the nature of the side chains. Crystal structure of amino acid derivative of NDI (1.75) reveals that amino acid side chains adopt a syn geometry with respect to the NDI plane which allows carboxylic acid groups of three different molecules to interdigitate through two strong intermolecular hydrogen bonds assembling in a hydrogen bonded nanotubular supramolecular structure, in which NDI cores are coplanar with each other, forming the walls of the nanotube (Figure 1.4).
Figure 1.4: Formation of nanotubes in the structures of 1.75.