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Chapter 5 Crystal engineering: molecular architectures in the solid state. 5.1 Concepts, Examples and Perspectives. The assembly of individual molecular units driven by intermolecular interactions represents the central scope of supramolecular chemistry.1 Its principles can be applied in the solid state to the formation of supramolecular networks. These networks can be mono dimensional 1D, 2D or 3D and this depends on the number of translations operating at the molecular assembly core.2 The solid state is the preferred phase to develop such aim due to the fact that analysis by X-ray techniques allows accurate structural studies, but in principle molecular networks could be obtained in any type of condensed phase. This field of research which recalls scientists from many different areas is frequently called crystal engineering,3a or molecular tectonics.3b A tecton is the basic molecular building block. It is rationally designed in such a way to bear proper recognition sites able to lead to the desired organization upon association with itself or with another tecton. Again molecular recognition is the fundamental step towards complexity. A crystal solid can be viewed as the supermolecule par excellence.4 This vision replaced the old-fashioned concept for which a crystal architecture was the result of the orror vacui, i.e. the need to avoid emptiness in the crystal. The geometrical motifs and the architecture of a crystalline solid are the result of many weak interactions whose complete understanding is, still nowadays, far from completion. Indeed, any crystal structure of organic molecules cannot be predicted a priori.5 The present knowledge of covalent valence can be considered satisfactory and by the covalent approach it can reasonably be stated that every target molecule can be synthesized. In variance, the non-covalent valence still need many efforts towards understanding and rationalization. This problem is common to several other related fields. The structure-reactivity relationship in the folding of proteins is just one example. However, despite many evident difficulties, the following question made by Feyman in 1960 “What would the properties of materials be if we could really arrange the atoms the way we want them?” opened and fuelled the continuously growing area of research on crystal engineering.6 Indeed, obtaining a supramolecular assembly in the solid state means the creation of a new material which can posses new features and properties. Applications in several field of research is Chapter 5 therefore possible and highly appealing. Many examples can be found in literature showing that even if crystal prediction is very elusive and much remains in the realm of pure speculation, short term applications are not precluded and some of them are even of consummate structural beauty. Figure 1: Enatiopure bulding block 1 forms highly organized crystal structures (a) which are provided with large chiral channels (b), they can be exploited in enantioselective catalysis. The enantiopure chiral organic building block 1 is easily synthesized from D-tartaric acid.7 It coordinates to Zn2+ ions to produce a homochiral open-framework solid (referred as D-POST-1). The enantiomorphic L-POST-1 is obtained from the enantiomer of 1 and the Zn2+ ions under the same conditions. In D-POST-1, three zinc ions, held together with six carboxylate groups of the chiral ligands and a bridging oxo oxygen, form a trinuclear subunit. These trinuclear units are interconnected through coordinative bonds between the zinc ions and pyridyl groups of 1, thereby generating two-dimensional (2D) infinite layers. (Figure 1a and 1b). The average interlayer separation is 15.47 Å. Most notably, large chiral 1D channels are formed and their cross-section is best described as an equilateral triangle with a side length of 13.4 Å (Fig. 1b). Almost 50% of the total volume is void filled with water molecules. These chiral channels are responsible for the fact that this solid compound is a catalysis of transesterification reactions. In Figure 2 different rates in the ethanolysis of 2,4-dinitro-phenoxyacetate in the presence of POST-1, the pyridine N-methylated derivative and in the absence of catalyst are shown. Moderate enhancements are observed. Moreover POST-1 revealed enantioselective catalytic activity in this transesterfication reaction. Studies showed that the reaction of the latter with a large excess of racemic 1-phenyl-2-propanol in the presence of D-POST-1 or the 124 Crystal engineering. enantiomorphic L-POST-1 produces the corresponding esters with 8% enantiomeric excess in favour of S or R enantiomer, respectively. Figure 2: Ethanolysis reaction catalysed by POST-1. This enantioselectivity is modest but still noteworthy because asymmetric induction has never been observed in reactions mediated by modular porous materials. Many extended metal-organic frameworks (MOF) with rich structural, thermal, magnetic, and sorptive properties by the use of simple diazaaromatic anions have been reported. For instance neutral and flexible 3D sodalite-type MOFs of formula [Cu(2-pymo)2]n show heterogeneous solid-liquid sorption responsible for guest induced crystal-to-crystal phase transitions (pymo=pyrimidinolate).8 Figure 3: a-d) Green balls: Cu; purple balls: MNO3; green sticks: pymo N,N' bridges; e) LiNO3 tetraaquo complex. An ab initio X-ray powder diffraction (XRPD) study on the hydrated [Cu(2-pymo)2]n rhombohedral material (1R, Figure 3a) reveals its distorted 3D sodalite-type framework. This 3D framework is not rigid but, upon exposure to an aqueous methanol 125 Chapter 5 solution of MNO3 (M= NH4+, Li+, Na+, K+, Rb+ and Tl+), a transition to a cubic phase (MNO3@1C, Figure 3b), is observed. The kinetically controlled crystal-to-crystal inclusion process of the 3D framework 1R to MNO3@1C is, indeed, followed by further incorporation of MNO3 ion pairs, leading to isomorphous orthorhombic layered materials of type [Cu(2-pymo)2]n‚(MNO3)n/2 (MNO3@1O) (Figure 3c). Despite these large guest-induced structural changes, the original 1R phase can be restored. For instance, refluxing KNO3@1O or RbNO3@1O in MeOH for 6 days with 18-crown-6-ether removes the MNO3 guests, giving an empty layered [Cu(2-pymo)2]n species (1O, Figure 3d) which can be readily converted to the original 1R phase by exposing it to water. A particular of the LiNO3@1C structure reveals the Li+ ion as a tetraaquo complex (Figure 3e). Figure 4: Legenda: blue polyhedron= zinc; yellow balls= accessible space for storage. Metal-organic frameworks of composition Zn4O(BDC)3 where BDC stays for 1,4-benzenedicarboxylate (Figure 4, A) and related structures (Figure 4, B and C), form a cubic three-dimensional extended porous structure which is able to adsorb hydrogen up to 4.5 weight percent (17.2 hydrogen molecules per formula unit) at 78 K and 1.0 weight percent at room temperature and pressure of 20 bar. These materials seem to be competitive with metal hydrides which are expensive and with carbon nanotubes and similar compounds which have been beset by mixed results.9 Supramolecular architectures in the solid state can also be used as a template to induce chemical reaction as shown in the next example. The [n]ladderanes are molecule that consist of n edge-sharing (n>2) cyclobutane rings and are viewed as a molecular equivalent of a macroscopic ladder.10a Ladderanes are promising in optoelectronics devices and have been found to posses an important role in biological systems. Despite the apparent simplicity of the intermolecular photochemical dimerization of two all-trans-poly-m-enes (m=2, 3, 4…) which should occur to generate ladderanes, generally such transformations fail. A crystal engineering approach can be very useful. If a properly designed crystal architectures is created in 126 Crystal engineering. such a way to put the two polyenes in an organized solvent free environment (as in the case shown in Figure 5), the desired reaction takes place upon UV irradiation. Two crystal structures of the starting and ending material are reported in Figure 5. The conversion of the polyene into the ladderane is quantitative.10b a) b) Figure 5: Solid state conversion by UV irradiation of a polyene a) into ladderane, b). Designing molecular architectures requires great effort as already shown. However a strategy for a rational design must consider three different, consecutive steps of increasing complexity which refer to three grade of sub-structures. The first step must account for the tecton structure and its features, being it a metal center or a molecule. A metal should feature a coordination geometry directing the interacting ligands towards association in the desired geometry; a molecule has to bear binding sites properly disposed to interact, with an high grade of directionality, with the molecular partner. The geometrical features of the tecton or tectons is the first substructure to be considered. The secondary structure corresponds to the object formed by the association of tectons with each others. This assembly process can lead to rows, layers or 3D objects depending on the primary structure. The way in which secondary subunits interact with each others determines the tertiary structure. This final arrangement highly influences the properties of the crystal and it has to be taken in great consideration in the a priori design process even if it is the most elusive to prediction. Polymorphism is the risk and it should be avoided in order to obtain always the same solid with the same properties upon crystallization. In the case of a 3D 127 Chapter 5 secondary sub-unit the tertiary structure coincides with the secondary for obvious reason. Building the desired object in the solid state is very complex and subtle factors can determine big failure or success. To try to minimize the sources of errors, strong and highly directional interactions are required to produce high affinities between tectons and to avoid polymorphism. Not surprisingly H-bonds and coordinative bonds are the most used tools in crystal engineering.11 Some example are described. Figure 6: The organization in the solid state of the porphyrin in figure create cavities of 22 Å (edge to edge). In Figure 6 a porphiryn is functionalized with H-bond donor and acceptor groups and the crystal structure obtained shows interesting structural features which suggest the use of such compound as molecular sieves.12 Figure 7: Polymetal oxalate complexes that form multilayers with magnetic properties. 128 Crystal engineering. Obtaining materials with unusual magnetic properties can be achieved by the formation of successive layers of different composition. Several metal elements can be used. Their oxalate complexes can be arranged in infinite layers between which cromocenium or ferrocenium complexes intercalate. The result, shown in Figure 7, reveals a complex structure whose bulk magnetic properties can be tuned depending on the metal.13 Figure 8: High symmetry in molecular networks based on the [Re 6-(3-Q)8]2+ core (molecular formula, left and X-ray structure of two superimposing trigonal units, right). The [Re6-(3-Q)8]2+, where Q is S or Se, revealed to be a very versatile building block to build complex molecular networks.14 In Figure 8 only one over the manifold possible geometries is reported. The versatility of the building blocks along with the high synthetic accessibility of the cluster core itself provide access to many possible geometries and applications. The cluster core is inert and thus it prohibits stereochemical scrambling, ensuring a fixed geometry which limits the structural possibilities of a multicluster array upon assembly. Clearly all kinds of intermolecular forces have a role in crystal packing and can be exploited to obtain the desired geometry. Therefore, - stacking and charge transfer interactions have been proposed to be good candidates in molecular tectonics as well.15 Strong ion-ion electrostatic forces,16 along with weaker CH··· and CH···O17 and CH···X18 interactions can also give remarkable contributions to this field. Even if a rational design is the first compulsory step towards engineered crystals, serendipitous, yet extremely fascinating organizations are reported in literature. With surprise, Rissanen and co-workers found that mixing basic building blocks consisting in tetramethylresorcinarene 2 and the diquat 3 resulted in an 129 Chapter 5 intriguing nano-tubular structure held together by intra-tubolar interactions, as shown in Figure 9.19 a) b) Figure 9: a) molecular building blocks 2 and 3 and b) their supramolecular assembly (some representations). 5.2 UO2- and Al-Salophen and Salen Complexes as Building Block for Molecular Tectonics. The careful and rational design of interacting tectons, have led to very complex molecular architectures with interesting properties which have been reported so far. Sometimes even very simple tectons can display interesting geometry in the solid state, provided that they posses suitable binding sites. An interesting example is given by compound 4. It is a very simple molecule which can be dissected into two domains. The aromatic moiety is hydrophobic and its optimal large surface gives - staking. O1 H O2H O 4 C1 10C C2 C9 C3 C4 C8 C7 C5 C6 Figure 10: Tecton 4 and the molecular organization in staked layers which shows - and H-bonding interactions. Colour scheme: black, grey=C, red=O; cyan dotted lines= H-bond, blue lines=- stacking. 130 Crystal engineering. The aldehydic moieties are polar and provide donor and acceptor sites for H-bonding. The crystal structure is reported in Figure 10. The organization at the supramolecular level shows rows of molecules connected by a four center hydrogen bond between the hydroxyaldehyde moieties and several - interactions between the aromatic surfaces. Distances of the aromatic carbon atoms C3 and C5 to the closest centroid on the naphthyl moieties are 3.504 and 3.7489 Å (blue lines). The four centered hydrogen bond displays interacting oxygens (carbonyl and hydroxyl ones) 3.022 Å apart. Naphthyl-CH2 between different rows also show close contacts, distances ranging from 3.442 to 3.493 Å. Figure 11: Tecton 5 and the molecular organization in staked layers. Colour scheme: black, grey=C, red=O; blue dotted line= H-bond. When a more complex molecule is taken into account, things can change dramatically. With tecton 5 the recognition pattern displayed by 4 is absent. The four center H-bond is replaced with a four center CH···O, OH···O bond with CH···O=C, CH···OH, C=O···HOinter and C=O···HOintra distances of 3.736, 3.338, 3.093 and 2,675 Å, respectively. Salophen-Uranyl complexes have been reported to be effective receptors for hard anions in polar solvents.20 The uranyl cation complexed in a salophen unit prefers a pentagonal bipyramidal coordination, with the two oxygens at the apical positions and with both the four-coordinating sites of the salophene moiety and a guest molecule in the equatorial positions. In 1994 a crystal structure of Salophen-UO2 complex 6 with H2PO4- have been documented.21 As shown in Figure 12, the phosphate ion is coordinated to the UO2, not surprisingly (the association constant of 6 with H2PO4 was 131 Chapter 5 measured: K= 1.5x104 M-1 in MeCN:DMSO 99:1). Moreover the anion forms a doubly H-bonded bridge which connect the two salophen molecules. Thus the two phosphate molecules, connected by H-bonding, act as a spacer between the two receptor units. N Figure 12: Receptor 6 and its H2PO4- 2:2 complex in the solid state (guest P=green, O=red). N UO2 O O O O 6 In order to build infinite chains of this kind in the solid state the receptor has to be ditopic with the two binding sites pointing in opposite directions. For this purpose we synthesised salophen-UO2 complex 7 (Figure 13). This compound is formed as a complex with two pyridine molecules (present in the reaction mixture). Unfortunately the compound is soluble only in DMSO and this fact renders the crystallization process very slow. The phosphate ion can therefore act as a linear spacer upon dimerization in the solid state via H-bond. If a different spacer is considered diverse geometries, apart from the linear, could be expected as shown in Figure 14. Figure 13: Ditopic receptor 7. 1 H- and 13C-NMR spectra. Pyridine signals are denoted with an x 7 132 Crystal engineering. O N O O UO2 N N N N N UO2 O N O N N O N N UO 2 O N O N N O N N UO 2 O N UO 2 N O O UO2 N O N N N N N N O UO 2 N N O N O UO 2 UO2 O N O N N N UO 2 O N O Figure 14: Some of the possible spatial organization in the association of bi- and tri-dentate nitrogen ligands with 7 As a preliminary study the complexation between bipyridyl and the simple salophene-UO2 complexes in solution showed moderate binding in 95:5 chloroform: methanol solution. In Figure 15 is shown a 1H NMR titration experiment between the Host 0 bipyridyl two species. 9.2 9.1 N N UO2 9.0 O O C N 8.9 N 4,4'-bipyridyl 8.8 8.7 0.000 0.002 0.004 0.006 [host] Figure 15: 1H NMR titration between salophen-uranyl host C and the bipyridyl ligand. 133 Chapter 5 The data can be easily fitted with a 1:1 binding isotherm, suggesting that the second equilibrium in Scheme 1 is not favourable in these conditions (K1=400±30 M-1, =-0.56; K2 is negligible). Anyway, the binding occurs and it supports the possibility of the desired complexation in the solid state. N N N UO 2 O O N N UO 2 O N O N N UO 2 O O N UO 2 O O N N N K1 K2 N O O N UO 2 N N Scheme 1: Multiple equilibria in the association between salophen-UO2 complex C and the bipyridyl ligand. The creation of proper architecture in the solid state could lead also to the exploitation of the concave nature of the salophen-UO2 compounds. The salophen derivative 8 that we prepared does not posses suitable secondary binding sites for the recognition of organic cations due to poor electron density on the aromatic rings. Still, in the solid state it displays evident interactions with a N-Methylquinuclidinium bromide exploiting the concave structure derived by the distortion of the ligand caused by the large ionic radius of the UO22+. The uranyl oxygens may display weak Lewis basicity and interact with cationic partners. Moreover phenoxy rings on the host can be involved in cation- interactions. The X-ray structure obtained is shown in Figure 16, along with the tripeptide-NMQI complex reported by Kubik.22 A clear resemblance can be noted. However, as reported in Table 1, cation- interactions play an ancillary role in the complex formation. Figure 16: Comparison between two NMQ complexes 134 Crystal engineering. O N N U O 8 O O S C9 C13 C11 C12 C9 4.407 C10 4.007 51-O=U 3.293 52 O=U 3.407 S C11 3.726 54 O=S 3.366 C12 3.873 53 O=S 3.257 C13 4.337 57 O=S 3.865 C14 4.585 U-Br 2.784 O O Br 51C N C14 C10 A O O O O C52 C53 C54 C57 Table 1: Relevant interatomic distances in 8-NMQBr (Å). CH-O and coord. Cation··· NMQBr Figure 17: Numbering scheme for the 8-NMQBr complex Aluminium complexes are finding an increasing importance in many field of organic synthesis, especially as catalysts for polymerisation and Lewis acid promoted reduction of aldehydes and ketones.23 Moreover they posses some interesting features from a spectroscopic and structural point of view. Salen-Al complexes are the most widely used for their synthetic accessibility and inertness even in the presence of Al-C bonds. Aluminium cations are naturally electron deficient and they act as hard Lewis acids. The X-ray structure in Figure 18 shows a square-pyramidal geometry, not very common for Al3+.24 In this case the complex is a salen-Al-ethyl crystallized by slow cooling of a dry acetonitrile solution. The four donors of the tetradentate ligand are on the same plane, while the alkyl chain is on the apical position. Figure 18: Square pyramidal geometry in the X-ray structure of the salen-Al -ethyl complex. The coordination geometry can change in the presence of coordinative solvent molecule such as water or methanol. The geometry of the Al center switches from square-pyramidal to octahedral with the two solvent molecules in the apical positions while the salen ligand donors occupy the equatorial plane. Many example of salen derivatives are reported in literature. Some of them are shown in Figure 19.25 135 Chapter 5 Figure 19: Planar octahedral complexes of salen-Al derivatives a-f. All these case are summarized in the scheme 2, (right hand side). The situation is different when an alkyl group is coordinated to the metal center. In these case the protonation of alkyl group, which forms the corresponding alkane and deprotonated solvent, leads to a dimerization with two bridging solvent molecules. The process is described by the left-hand portion of Scheme 2. A crystal structure of a salen derivative complexed with methanol is reported in Figure 20.25b In this case the geometry is different compared to previous examples and the four donor atoms from the salen ligand are not on the equatorial plane anymore. R1 N R O O O O O O Al N N N R R Al R1 R1OH N X = Alkyl R= CH2-CH2, C6H4 R1= Me, H X Al N 2 R1OH O O N R X = Chloride R= CH2-CH2, C6H4 R1= Me, H O X Al N O 1/2 Scheme 2: Behavior of salen- salophen- Al complexes depending on the X ligand nature. Figure 20: doubly MeOH bridged salen derivative compound X-ray structure (left), and the chemical structure, (right) 136 OR1 OR1 Crystal engineering. Salen-Aluminium complexes spectroscopic properties have been only recently documented and they possess interesting luminescence.26 To our knowledge Salophen-Al complexes have never been synthesized. The aluminium center is a strong Lewis acid and therefore can interact with suitable Lewis bases to form organized structures in the solid state. 4,4'-bipyridyl has been chosen as a good starting point, being a ditopic ligand with the two nitrogen donor atoms pointing outwards. It should in principle be able to form infinite linear structures when associated with salophen- or salen-aluminium complexes (Fig. 21a). With this idea in mind Al-complexes 9-12 has been synthesized (Fig. 21b). Many crystallization attempts in obtaining the desired geometry failed and the only single crystal suitable to X-rays analysis resulted to be analogous to the “dimeric” complex shown in Figure 20. The structure is reported in Figure 21c. a) 9: N R N b) O X Al O X= Cl R= CH2-CH2 10: X= Me R= CH2-CH2 11: X= Cl R= C6H4 12: X= Me R= C6H4 c) Figure 21: a) Possible linear organization in the solid state of bipyridyl ligand with Al-complex (represented as a blue ellipsoid); b) salen- and salophen-Al complexes synthesized c) crystal structure of the methanol solvated dimeric species: H=white, C=dark grey, N= blue, O=red, Al= purple; ORTEP view with 50% ellipsoid probability (H in their calculated positions). 137 Chapter 5 5.3 Experimental part. Synthesis of compound 7 (Scheme 3): 0.344 g of salycilaldehyde are added to 200mg solution of 1,2,4,5-tetraaminobenzene tetrachlorohydrate and 0.23 ml of pyridine in 40ml of MeOH under argon atmosphere. The solution is refluxed for 30 minutes, whereupon solid uranyl diacetate is added (0.59 g). The solution turns deep red with the formation of abundant precipitate. The solid corresponding to 7·2C5H5N is filtered. Yield: 45%. 1H NMR (DMSO): 9.88 (s, 4H), 8.5786-8.5703 (d, 2H, pyr), 8.299 (s, 2H), 7.814-7.7693 (m, 6H), 7.6508 (t, 4H), 7.3816 (t, 4H, pyr), 7.0489-7.0323 (d, 6H), 6.745 (t, 4H). 13C NMR: 170.095, 166.981, 149.524, d 146.576, d 136.284, d 136. 155, d 136.028, d 124.184, d 123.884, d 120.720, d 116.717. Mass spectrum (ESI), m/z 2181.50, 1091.3, 1125.28 ([2M+H] +, [M+H]+, [M+Cl]- calcd for C68H45N8O16U4, C34H23N4O8U2 and C34H22N4O8U2Cl 2181.50, 1091.25 and 1125.21, respectively). H2N NH2 . H2N Scheme 3 O 4 HCl + OH 4 2 UO2(OAc)2 + NH2 N CH3OH, reflux O N N O O UO2 UO2 N N . 2 N O 7 Synthesis of 2-Hydroxy-3-(phenylsolphoxy)benzaldehyde (Scheme 4): To a suspension of NaH (0.565 g, 80% in oil), prewashed with n-pentane, in DMSO (10 mL) a solution of 2,3-dihydroxybenzaldehyde (1 g, 7.764mmol) in DMSO (5 mL) was added at 20-25°C. After 1 h of stirring, neat benzenesulphonylchloride (1.37 g, 7.764 mmol) was added, and stirring was continued for 24 h, whereupon the mixture was poured into water (50 mL) and extracted with CHC13 (3x20ml). The aqueous layer was acidified with 6 M HCl to adjust the pH to 3 and was again extracted with CHC13 (3x50 mL). The latter combined CHC13 layers were washed with 1 M HCl (2x20 mL). Column (SiO2/CHCl3) gives pure compound as a yellow solid: yield 53%; 138 Crystal engineering. H NMR (CDCl3): 10.90 (s, 1H, CHO), 9.86 (s, 1H, -OH), 7.96-7.484 (m, 7H), 7.0039 (t, 1H). mass spectrum (EI), m/z 278 (M+, calcd for C13H10O5S 278.28). 1 Synthesis of compound 8 (Scheme 5): To a refluxing solution of 2-hydroxy-3(phenylsolphoxy)benzaldehyde (0.9 g, 3.23 mmol) in methanol (50 mL) was added dropwise a solution of 1,2-benzenediamine (0.174 g, 1.615 mmol) in MeOH (15mL). After 1.5 h UO2(OAc)2 2H20 (0.693 g, 1.615 mmol) was added and reflux was maintained for 15 min whereupon the mixture was allowed to cool to room temperature overnight. The red solid formed is filtered and dried in vacuo. Yield 63%; mass spectrum (ESI), m/z 919.4, 935.24 ([M+Na]+, calcd for C32H22O10N2S2UNa 919.1; [M+K]+, calcd for 323H22O10N2UK 935.1). 1H NMR (CDCl3): 9.3534 (s, 2H, CH=N), 8.14-7.200 (m, 18H), 6.6161 (t, 2H). O O H H 1. NaH 2 eq. OH OH 2. R OH O Br SO2 O H N N UO2 O O methanol 2 OH O + H2N + NH2 R UO 2(AcO)2 O O SO2 O2S 8 Scheme 4 General synthesis of compound 9-12: To a refluxing solution of salycilaldehyde (1ml, 1.146g, 9.38 mmol) in toluene (30 ml) ethylendiammine (or orthophenylendiammine, 0.5 eq.) is added under stirring. After 1h Al(CH3)3 (or AlCl3 0.5 eq) is added with caution whereupon the solution is cooled to room temperature overnight. The solid formed is filtered and crystallized from a hot mixture MeCN:CHCl3: MeOH 10:10:1. Yields 50-70 %. 139 Chapter 5 5.4 Conclusions. The formation of highly organized structures in the solid state is the aim of the emerging field of organic-inorganic chemistry, called Molecular Tectonics. Our efforts were aimed to the exploitation of UO2- and Al-based salen and salophen complexes in the formation of crystal architectures endowed with appealing features. Up to this point no conclusion can be given, since the desired single crystals have not been obtained yet. However, new compounds that could provide interesting properties have been synthesized and the crystal structure of the first salophen-Al complex has been obtained. References: [1]: M. W. Hosseini, Coord. Chem. Rev., 2003, 240, 157-166. [2]: S. Mann, Nature, 1993, 365, 499. [3]: a) C. V. K. Sharma, Crystal Growth & Design,2002, 2, 465-474; D.Braga, F. Grepioni, Acc. Chem. Res, 2000, 33, 601-608; b) M. W. Hosseini, Cryst. Eng. Comm., 2004, 318-322; [4]: J. D. Dunitz, Pure and Appl. Chem., 1991, 63, 171-185. [5]: A. 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