New azobenzene crown p-tert-butylcalix[4]arenes as switchable receptors for Na+ and K+ ions: Synthesis and isomerization studies

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 o f the nitrobenzene groups by metallic zinc in an alkaline solution afforded 10 and 11 in 8% and 12%, respectively. In the second pathway, an azobenzen e containing two glycolic units was prepared prior connecting to t-butylcal ix[4]arenes. The yields from the second approach (5%, 8% for 10 and 11, res pectively) were lower than those from the former approach. Single crystals of 10 suitable for X-ray crystallography was obtained by recrystallization in methanol. Both the X-ray structure and the H-1-NMR spectrum of 10 indica ted that the stereoisomer of the azobenzene moiety was trans and the calixa rene platform was in cone conformation. H-1 NMR spectroscopy suggested that 10 underwent an observable cis-trans isomerization in CDCl3 under room lig ht and upon UV irradiation with cis:trans ratios of 33:67 and 36:64, respec tively. 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 r atio of 47:53 at -30 degrees C. H-1 NMR spectrum of 11 suggested that 11 wa s initially isolated as cis azobenzene with calix[4]arene in cone conformat ion and underwent conformational interconversion through calix[4]arene annu las in a similar fashion to 6 upon exposing to light. The complexation stud ies of 10 with picrate salts of Na+ and K+ using H-1 NMR spectroscopy sugge sted that Na+ preferred to bind the cis form of 10 while K+ preferred to bi nd the trans form. The stereoisomer of the azobenzene unit in 11 changed pa rtially from cis to trans upon complexing with K+.