Molecular tweezers as synthetic receptors: molecular recognition of cationic substrates; an insight into the mechanism of complexation

Abstract

245Aromatic molecules play an important role in many areas of biological and supramolecular chemistry [1] and their noncovalent interactions with other aromatic units (π-π-or arene-arene interaction) [2] and positively charged ions (cation-π interaction) [3] are of particular importance in the formation of superstructures [2][3][4] The design of efficient synthetic receptors with the ability to selectively bind substrates in solution requires precise control of both their electronic and topological properties. Besides the frequently used cyclic receptors of the cyclophane-type [5] non-cyclic receptors with cavities of flexible size proved to be very effective [6]. We have recently reported on the synthesis and some supramolecular properties of the hydrocarbon compounds 1a and 1b, which due to their ability to selectively bind electron-deficient aromatic and aliphatic substrates as well as organic cations can be regarded as molecular tweezers [7]. Here we report on the binding properties of the benzene-spaced molecular tweezers 1a towards various aliphatic and aromatic cations in organic solution and attempts to use this binding motive for the template synthesis of mechanically interlocked supermole- Molecular Tweezers as Results and DiscussionDue to the ribbon-type concave topology the five benzene units of molecular tweezer 1a define a cavity in which a substrate can be bound by multiple noncovalent interaction. The magnetic anisotropy of these benzene units makes the 1 H NMR spectroscopy to be a very sensitive probe to detect the complexation of substrate molecules bound inside the cavity of 1a [7]. From the results obtained in binding studies with small electrondeficient aliphatic substrates (CH 3 CN, CH 2 (CN) 2 , CH 3 NO 2 ) [7a] and intramolecular complexation of flexible aliphatic sidechains [7b] we already know that the cavity of 1a has an almost ideal size for the binding of methyl or methylene groups. Because of the very electron-rich character of the tweezer's concave side [9] we supposed that ammonium ions also should be suitable substrates due to multiple cation-π interactions with the host 1a.Due to the different solubility properties of the hydrocarbon 1a and ionic substrates only CDCl 3 and mixtures of CDCl 3 and (CD 3 ) 2 CO (v/v 1:1 or 1:2) can be used as solvents. Most of the primary ammonium ions, however, are nearly insoluble in CDCl 3 or CDCl 3 / (CD 3 ) 2 CO mixtures even as PF 6 -salts. Therefore, we performed 1 H NMR bindings studies only with the better soluble secondary ammonium salts, di-n-butylamguest molecule enters the tweezer's cavity through the open sides of the host, a second mechanism is available in which the substrate enters the cavity through a gap emerging after a substantial spreading of the tweezer's tips by at least of about 280 pm. n 1 (n = 0) 2 (n = 1) 1a (n = 0) 1b (n = 1)