Synthesis and supramolecular structures of molecular clips

The syntheses of the novel dimethylene-bridged clips 4a-e and 5b are reported. They selectively bind electron-deficient neutral and cationic aromatic substrates comparable to the tetramethylene-bridged tweezers 1 and 2. The geometry of the noncovalently bound complexes with 4b-d as receptors derived from the single-crystal structure analyses is, however, different from that of the complexes with 2 as receptor. In clip complexes the plane of the included aromatic substrate molecule is orientated almost parallel to the naphthalene sidewalls of the clip, whereas in the tweezer complex the substrate is orientated parallel to the central arene spacer-unit. TCNB 19 as substrate is placed inside the cavity of the hydroquinone clip 4c in solution as well as in the cocrystal. In contrast it was found for the cocrystal with the diacetate clip 4b that the TCNB 19, is placed between the naphthalene side-wall of two different clip molecules whereas in solution 19 is included into the cavity of 4b. Finally, 19 forms a (1:2) complex with dinaphthonorbornadiene 20 in solution as well as in the crystalline state. The findings reported here are instructive for the understanding of the weak; noncovalent binding forces particularly the arene-arene interaction. (C) 2001 Elsevier Science Ltd. All rights reserved.