Calixarene-Tetracyanoethylene Complexes. On the Selective Complexation with Calix[4]arene Conformers

The complexation behaviors of O-alkylated p-t-butylcalix[n]arenes (1_nR), calix[n]arenes (2_nR), and their noncyclic analogs were investigated in dichloromethane. All compounds formed a 1:1 complex with TCNE, indicating that complexation with the first TCNE suppresses further complexation with the second TCNE. The λ_max values for the CT complexes with 1₆R and 1₈R (496-498 nm) were comparable with those for the noncyclic analogs, whereas the λ_max values for the CT complexes with 1₄R shifted to longer wavelengths (525-567 nm): the order (from longer to shorter wavelength) for 1₄Prⁿ is 1,3-alternate > partial cone > 1,2-alternate > cone. The association constants (K) for 1₄R were small in comparion to the λ_max. It was shown on the basis of 1H NMR measurements that "out" - 1₄R (four alkyl groups are turned outward) is sterically-stable but cannot accept TCNE because of steric crowding on the benzene ring whereas "in" -1₄R 1₄R (one alkyl group is turned inward) is sterically-unstable but can provide a room to accept TCNE. In 1₄R, therefore, the complex formation occurs in conjugation with "out"-to-"in" displacement. This is the first example for the selective CT complexation with calix[4]arene conformers.