A New Family of Indoaniline-Derived Calix[4]arenes: Synthesis and Optical Recognition Properties as a Chromogenic Receptor1

Abstract

A new family of indoaniline-derived calix[4]arenes has been synthesized for the purpose of developing a new chromogenic receptor. A condensing reaction of calix[4]arene (1) with 4-(diethylamino)-2-methylaniline hydrochloride (2) in the presence of an oxidizing agent under alkaline conditions affords mono- (3), 1,2-bis- (4), 1,3-bis- (5), and tetrakisindoaniline-derived (6) calix[4]arenes after careful column chromatography. Compound 3 is crystallized from a CHCl₃−MeOH solution, and the crystal structure was determined by X-ray analysis. The crystal is monoclinic, space group P2₁/n , Z = 4, a = 19.507(6) Å, b = 18.591(6) Å, c = 8.524(2) Å, β = 94.69(2)°. The final R value for 2406 reflections of F_o > 3σ(F_o) is 0.085. A unique intramolecular hydrogen-bonding network involving the carbonyl oxygen of indoaniline for 3 implied that the quinone carbonyl group as an acceptor of the chromophore can easily be subjected to an electrostatic interaction in the lower rim. Indeed, 1,3-bis(indoaniline)-derived 2,4-bis((ethoxycarbonyl)methoxy)calix[4]arene 7, prepared by the reaction of 5 with ethyl bromoacetate in the presence of NaH, is capable of undergoing an efficient ion−dipole interaction between the binding cation and the two quinone carbonyl groups of the chromophores, so that a selective Ca²⁺-induced pronounced color change (wavelength change > 100 nm) occurs with an association constant on the order of 10⁶ in 99% EtOH, making 7 of potential use as an optical sensor for Ca²⁺ detection. The IR and NMR studies have indicated that Ca²⁺ is encapsulated in the cavity made by the distally located OCH₂CO₂ groups on the lower rim of the cone-shaped calix[4]arene segment. Interestingly, however, the shape of the cavity in which Ca²⁺ has been encapsulated does not have a C₂ axis of symmetry, as inferred from the ¹H−¹H COSY experiment. On the other hand, 1,2-bis(indoaniline)-derived analogue 8 shows no response with metal ions, which can be interpreted to mean the absence of a cavity for encapsulation on the lower rim.