Azobenzene Crown Calix[4]arenes in Na + /K + Switchable Receptors Bull. Korean Chem. Soc. 2000, Vol. 21, No. 9 867 New Azobenzene Crown p-tert-Butylcalix[4]arenes as Switchable Receptors for Na + and K + ions: Synthesis and Isomerization Studies Bongkot Pipoosananakaton, Mongkol Sukwattanasinitt, Nongnuj Jaiboon, Narongsak Chaichit, † and Thawatchai Tuntulani * Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand † Department of Physics, Faculty of Science, Thammasat University at Rangsit, Pathumthani 12121, Thailand Received June 19, 2000 Two new azobenzene crown ether calix[4]arenes, 10 and 11, were synthesized by two pathways. In the first pathway, two ethoxy nitrobenzene groups were attached to t-butylcalix[4]arenes in a 1,3 position. Subsequent reduction of the nitrobenzene groups by metallic zinc in an alkaline solution afforded 10 and 11 in 8% and 12%, respectively. In the second pathway, an azobenzene containing two glycolic units was prepared prior connect- ing to t-butylcalix[4]arenes. The yields from the second approach (5%, 8% for 10 and 11, respectively) were lower than those from the former approach. Single crystals of 10 suitable for X-ray crystallography was ob- tained by recrystallization in methanol. Both the X-ray structure and the 1 H-NMR spectrum of 10 indicated that the stereoisomer of the azobenzene moiety was trans and the calixarene platform was in cone conformation. 1 H NMR spectroscopy suggested that 10 underwent an observable cis-trans isomerization in CDCl 3 under room light and upon UV irradiation with cis:trans ratios of 33:67 and 36:64, respectively. Compound 6 which was the precursor of 11 showed fluxional behavior and was found to have mixed conformations of cone and partial cone with a ratio of 47:53 at -30 °C. 1 H NMR spectrum of 11 suggested that 11 was initially isolated as cis azobenzene with calix[4]arene in cone conformation and underwent conformational interconversion through calix[4]arene annulas in a similar fashion to 6 upon exposing to light. The complexation studies of 10 with picrate salts of Na + and K + using 1 H NMR spectroscopy suggested that Na + preferred to bind the cis form of 10 while K + preferred to bind the trans form. The stereoisomer of the azobenzene unit in 11 changed partially from cis to trans upon complexing with K + . Introduction Large and complex molecular systems can be constructed utilizing various synthetic techniques. Attaching functional moieties into specific molecular frameworks can result in interesting molecular or supramolecular devices. 1 Azoben- zenes make up an interesting class of compounds that exhibit photoresponsive properties. They have been incorpo- rated into a number of supramolecular frameworks to pro- duce ionophores for transports and photo-switchable recep- tors. 2 Calix[n]arenes have been of particular interest as molecular platforms for synthesizing selective ionophores in inclusion complexation. 3 Yamada et al. have shown that an azobenzene-capped calix[4]arene possessed a higher bind- ing ability to metal ions such as Na + , K + , Ni 2+ , Cu 2+ , Zn 2+ , Cd 2+ and Al 3+ in the trans form than in cis isomer. 4 Later, Vicens and colleagues have synthesized a series of trans and cis azobenzene crown calix[4]arenes. 5 From the crystal structure, an azobenzene calix[4]arene containing one gly- colic unit existed in cis isomer. 6 Azobenzene crown calix[4]- arenes that contained more than one glycolic unit exhibited allosteric properties. Changes have been observed in cis/ trans ratio from 70/30 for the free ligand to 80/20, 80/20, 75/ 25 and 85/15 for K + , Rb + , Cs + and NH 4 + , respectively. 7 We are interested in constructing a switchable molecular system which can selectively bind Na + or K + to mimic the biological Na + /K + ion pump. 8 According to a report by Swa- ger and coworkers, it was found that bithiophene calix[4]- arenes containing 6 ethereal oxygen donors can bind Na + and K + to different extents. 9 We therefore synthesized two azo- benzene crown ether calix[4]arenes (10 and 11) in which the azo benzene unit was connected to the glycolic chains of the calix[4]crown and studied their isomerization and inclusion properties. It is anticipated that azobenzene crown ether calix[4]arenes will form complexes with both Na + and K + , and the binding abilities can then be switchable by external stimuli. Experimental Section Materials. All materials and solvents were standard analy- tical grade, purchased from Fluka, J. T. Baker or Merck, and used without further purification. Commercial grade sol- vents such as acetone, dichloromethane and methanol were distilled and stored over 4 Å molecular sieves. Acetonitrile was dried according to the standard technique. 10 Chromato- graphic separations were performed on silica gel columns (kieselgel 60, 0.063-0.200 mm, Merck). Thin layer chroma- tography (TLC) were carried out using silica gel plates (kie- selgel 60 F 254 , 1 mm, Merck). p-tert-Butylcalix[4]arene, 1, 11 and 26,28-dimethoxy-p-tert-butylcalix[4]arene, 2, 12 were prepared according to methods described in the literature. Unless otherwise noted, all reactions were carried out under nitrogen.