Dedicated to Professor J. Fraser Stoddart on the occasion of his 60th birthday
The advent of supramolecular chemistry [1] has stimulated the contemporary chemist©s interest in the development of chemosensors capable of recognizing specific chemical species. [2] Calix[4]pyrroles [3] -first synthesized in the 19th Century by Baeyer [4] -contain four pyrrole±NH hydrogen-bond functionalities and have recently been studied for possible use as receptors for anionic and neutral substrates. [5] They have been used to prepare optical anion sensors [6] and anionselective high-performance liquid-chromatography (HPLC) supports. [7] However, only a few electrochemically active sensors based on calix[4]pyrroles have been reported. [8] Electrochemically active sensors, designed to permit the detection of substrates by binding-induced changes in the redox properties, are generally composed of a receptor unit, which works by the covalent association of a substraterecognition functionality, and an electrochemical-signaling capacity (redox-active unit). The redox-active tetrathiafulvalene [9] (TTF) unit can exist in three stable redox states (TTF 0 , TTF þ C, and TTF 2þ ) and for this reason TTF derivatives have found widespread use in materials chemistry. [9] Progress in synthetic TTF chemistry [9] has revolutionized the possibilities for the incorporation of TTF into macrocyclic, molecular, and supramolecular structures and has transformed complicated systems, such as TTF cyclophanes, [9] TTF catenanes, [9] and TTF rotaxanes/pseudorotaxanes [9, 10] from chemical curiosities into a vibrant area of modern-day research. In the context of electrochemically active sensors, TTF has already been used as the redox-active unit in a number of cation responsive receptors. [11] However, to our knowledge, no anion receptor incorporating TTF as the redox-active unit has been reported. We have recently developed an efficient synthesis of the parent pyrrolo[3,4-d]TTF-ring system. [12] With this building block in hand, we have prepared the first example of a single molecule in which the anion-receptor abilities of the calix[4]pyrrole system were coupled to the favorable redox properties of the TTF core through direct annulation of one TTF unit to the upper rim of the calix[4]pyrrole skeleton.
Herein, we report the synthesis of the first calix[4]pyrrole incorporating a TTF unit. Furthermore, we describe our 1 H NMR spectroscopic and electrochemical studies on the recognition abilities of this novel mono-TTF calix[4]pyrrole 4 system towards anions.
Our preparation of the mono-TTF calix[4]pyrrole 4 is outlined in Scheme 1. The tripyrrane dialdehyde 1 was prepared according to a literature procedure [13] by the condensation of pyrrole and acetone followed by a Clezy formylation. The reduction of the formyl groups with NaBH 4 / LiBr in anhydrous THF/MeOH produced the bishydroxymethyltripyrrane 2 in 76 % yield. The tripyrrane 2 was used immediately after purification by means of flash column chromatography on account of the low stability of the compound. Treatment of equal quantities of 2 and the newly developed pyrrolo TTF 3 with a catalytic amount of BF 3 ¥Et 2 O in anhydrous MeCN, gave the mono-TTF calix[4]pyrrole 4 as yellow crystals after purification by means of column chromatography (yield 21 %). [14] T ¼ 293 K, Z ¼ 4, reflections collected/unique: 9438/3824 (R int ¼ 0.0457), no observation [I > 2s(I)] 3824, parameters 257. CCDC-190370 (5 g) contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (þ 44