MOLECULAR RECOGNITION OF ANIONIC SUBSTRATES.
CRYSTAL STRUCTURE OF THE SUPRAMOLECULAR INCLUSION COMPLEX OFTRANS-3,3'-AZODIBENZOATE DIANION WITH A BIS- INTERCALAND RECEPTOR MOLECULE.

M. Cesario¹, P. Cudic², J.P. Vigneron², J.M. Lehn² , T. Prangé³

¹Cristallochimie, I.C.S.N., CNRS, 91198 Gif/Yvette,Cedex France.
²Laboratoire de Chimie des Interactionsmoléculaires, CNRS UPR 285. College de France, 11 Place MarcelinBerthelot, 75231 Paris Cedex 05., France.
³LURE, Université Paris-Sud, Bat . 209 D, 91405Orsay, Cedex, France.

The water soluble macrocyclic bis-intercaland receptor1.4H+, containing two acridine subunits linked by two positively chargedoxy-bis- ethylamine binding sites, has been shown to bind flat organicanions in aqueous solution with high affinity. In particular it binds trans -3,3'-azodibenzoic acid (logKs=5.4) more strongly than cis3-3'-azodibenzoic acid (log Ks=4.0). Taking advantage of this observationthe photochemical cis <=> trans isomerization of 3,3'-azodibenzoic acid in aqueous solution was studiedin the presence of 1.4H⁺. It was shown that the composition ofthe mixture in the photostationary states was modified in favour of the trans isomer 2. It was also shown that 1.4H⁺ catalyzed the thermalisomerization of cis -3,3'-azodibenzoic acid to the transisomer.

Scheme 1

In an attempt to interpret these results, the complexbetween 1.4H⁺ and trans-3,3'-azodibenzoic acid wascrystallized and its structure determined.

As only very small crystals were obtained, they wererecorded at the Synchrotron beam line W32 located at LURE-DCI (Orsay),using a MAR Research image-plate system.

The complex crystallizes in triclinic cell , P-1, with a =10.606(4) , b = 18.842(7), c = 19.483(8) A, a = 71.27(3)°,b = 85.49(3)^o, g = 84.08(3)^o, Z = 2, with one ligand and two anionic species in the asymetric unit.

An original host-guest association is found in thesupramolecular structure. One trans -3,3'-azodibenzoate dianion is partially intercalated between the twoacridine units of the macrocyclic receptor, at van der Waals contactdistance of 3.5 A. The second moiety of the dianion tilted of 35° with respect to the inserted part of the substrate extends out of thecavity of the macrocycle.

The second charged guest, with a less distorted backbone (10° between the two benzoic planes) anchors the macrocyclic framework outside,perpendicularly to the flat units. In the crystal packing, these"external" dianions are stacked by p-interactions with aninterplanar spacing of 3.5 A.

The whole structure includes numerous water molecules inchannels, building a strong hydrogen-bond network between the ligand, thedianions and the solvent.

Scheme 2

The study of the crystal structure of this supramolecularspecies resulting from the binding of anionic subtances by a bis-acridinemacrocycle receptor, reveals that both intercalative ( p -stacking ) and electrostatic interactions contribute to the complexationstability.