Molecular tweezers as synthetic receptors: molecular recognition of electron-deficient aromatic and aliphatic substrates.
Syntheses and supramol. properties of the mol. tweezers 1 and 2 (I; n = 2, 1, resp.) , contg. naphthalene and benzene spacer units, resp., are described. They selectively bind electron-deficient arom. and aliph. substrates, for example di- and tetracyanobenzenes, 1,4-dinitrobenzene, p-benzoquinone, 7,7,8,8-tetra-cyano-p-quinodimethane (TCNQ), 1,2,4,5-tetrafluorobenzene, acetonitrile, and malononitrile. They form stable complexes with the cationic substrate N-methylpyrazinium iodide that are sol. in chloroform. A quant. investigation using NMR titrn. and solid-liq. extn. shows that the naphthalene-spaced tweezer 1 forms stronger complexes with arom. and quinoid substrates than the benzene-spaced tweezer 2 (DDG=1.5+-1 kcal mol-1), whereas the aliph. substrates are only complexed by receptor 2. Force-field calcns. (AMBER*) and single-crystal structure analyses reveal that 1 has an almost ideal geometrical topol. for the complexation of arom. substrates, while complexation of these substrates by the smaller receptor 2 requires expansion of the tweezer tips by about 2 .ANG.. This causes an extra strain energy in 2 of 1 - 2 kcal mol-1. According to semiempirical AM1 calcns., the electrostatic potential surfaces (EPSs) of mol. tweezers 1 and 2 are surprisingly neg. on the concave sides of the mols. and, hence, complementary to those of the electron-deficient substrates.
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