摘要:

Despite the recent surge in crystal design and engineering, there are no successful methods for predicting the solid state 3-D structures assembled from small organic building blocks arranged through non-covalent binding motifs.1,2The discovery of molecular building blocks, that self-assemble in a predetermined fashion, is therefore vital for understanding solid-state supramolecular architectures. This is not merely an intellectual challenge, it is also crucial for the development of nanoscale devices, such as artificial transmembrane ion channels3and nonlinear optical (NLO) materials.4Many self-assembled structures have been observed and characterised3,5and one of the most interesting and studied structures is that of the helix,6as it occurs throughout nature. Yet, to our knowledge, no helical structure built upon hydrogen bonding between hydroxy groups and ketones has been reported. The use of the rigid bicyclo[2.2.2]octane framework allows a distinct sterical disposition of the coordinating moieties, whose coordinates can only be slightly distorted by the crystal lattice forces. We herein report the first example of a helical supramolecular solid-state structure, assembled from bicyclo[2.2.2]octane building blocks using the intermolecular hydrogen bond between a hydroxy and a keto groups as the non covalent binding motif.During our work with reduction of the diketone1with baker's yeast7andCandida tropicalis8we noticed that compound (−)-2was an amorphous solid, unsuitable for X-ray crystallographic analysis. However, compound (+)-3easily produced crystals suitable for X-ray crystallographic analysis (Table 1). The absolute configurations of (−)-2and (+)-3are as shown inScheme 1,i.e.(1R,4S,5S)8and (1S,4R,6S),9respectively.The major products from reduction of1with baker's yeast [(−)-2] andCandida tropicalis[(+)-3].Crystal data and structure refinement for (+)-3·tBuOMeData collected with MoKα radiation on a SMART CCD system using a rotating anode. The absolute configuration of (+)-3was determined by CD spectroscopy. Clickherefor full crystallographic data (CCDC 179474).Empirical formulaC8H12O2Formula weight140.18Crystal systemorthorhombicSpace groupP212121(No. 19)a/Å6.4365(1)b/Å6.4769(1)c/Å17.637(4)V/Å3735.3(3)T/K293(2)Z4Dc/g cm−31.266μ/mm−10.089Unique reflections1974Reflections (I> 2σ(I))1184R0.0539wR0.1535When examining the X-ray crystal structure of (+)-3, we found that a supramolecular helical structure was formed. The stereoview of the crystal structure (Fig. 1) shows how (+)-3self-assembles,viaintermolecular hydrogen bonds into an infinite supramolecular helix. The alcohol hydrogen of one molecule is bonded to the ketone oxygen of another with an O–O distance of 2.85 Å,10forming a left-handed helical coil. A full turn of the helix contains two keto-alcohols (Fig. 2) generating a helical pitch of 6.44 Å.Stereoview showing the helical H-bonding network of (+)-3. All hydrogens were removed except those involved in the helix forming network for clarity. Click image orhereto acces a 3D representation.Stereoview showing one turn of the helix motif. Click image orhereto access a 3D representation.Helical superstructures formed by hydrogen bonds between small organic building blocks often have additional stabilising forces, in addition to the hydrogen bond, such as favourable intermolecular π–π interactions. However, in this case, no such interactions are possible.Interestingly, in the recently published crystal structure of4(Fig. 3) De Santiset al.noted intermolecular hydrogen bonds, but did not explicitly mention the supramolecular arrangement.11On re-examining the X-ray crystallographic data, we found indeed that the intermolecular hydrogen bonds assembled4in a helical superstructure (Fig. 3).Stereoview showing the helical H-bonding network of4. All hydrogens were removed except those involved in the helix forming network for clarity.The hydroxy hydrogen of one molecule is bonded to the carbonyl oxygen of another, with an O–O distance of 2.80 Å. A full turn of this left-handed helix contains two keto-alcohol molecules, with a helical pitch of 6.36 Å. Also in the superstructure of4, other stabilising intermolecular interactions, such as π–π interactions, are absent.The dihedral angle, defined as outlined (Fig. 4), between the C–O bonds of the ketone and the alcohol groups in (+)-3is 89°, whereas in4this angle is −31.2°. Despite the different dihedral angles, the supramolecular structures formed with (+)-3and4share similar attributes other than the helical structure; O–O distances are 2.85 and 2.80 Å, two molecules are required for a full turn of the helix and the helical pitch is 6.44 and 6.36 Å for (+)-3and4, respectively.The differences in dihedral angle between the C–O bonds of the carbonyl and the alcohol groups in (+)-3(+89°) and4(−31.2°). For clarity all of the hydrogens except those involved in the supramolecular structure have been removed.The similar dihedral angle between the C–O bonds of (−)-2and4imply that an intermolecular hydrogen bond may be present for (−)-2as well as4.11This suggests that (−)-2may also form a supramolecular coil, similar to that of4. However, it is unclear why (−)-2does not form crystalline material suitable for X-ray analysis. It is possible that the weak aliphatic interactions of the steriodal structure in4act as further stabilising forces, resulting in crystalline material. It would be interesting to see if other, minor, modifications attached to thebicyclic framework of (−)-2may further stabilise the supramolecular structure and help develop crystalline material.In summary we have shown that hydrogen bonds between carbonyl groups and hydroxy groups in bicyclo[2.2.2]octane derivatives may be used for supramolecular helix formation. The need of additional weak stabilising forces, such as π–π interactions, are not necessary. Despite the large difference of the dihedral angle between the C–O bonds of the ketones and the hydroxy groups in (+)-3and4they both form similar supramolecular helices in the solid state.

ISSN:1466-8033    

DOI:10.1039/b202761e

出版商:RSC    

年代:2002

数据来源: RSC