The electronic behavior of 14 electron-deficient aromatic systems has been determined by cyclic voltammetric techniques in both the absence and presence of Li+, Na+, and K+. The substrates include carbon- and nitrogen-pivot lariat ethers, nitroaromatic podands, and simple nitroaromatic ethers as well as an azocryptand. The carbon-pivot lariats used in this study are ethers attached at the methyl group of 2-methyl-15-crown-5 or -18-crown-6 and have the following side arms: 2-nitrophenoxy, 4-nitrophenoxy, and 2,4-dinitrophenoxy. The nitrogen-pivot lariat ethers are 2- and 4-nitrobenzyl derivatives of aza-15-crown-5 or bis(2-nitrobenzyl)-4,13-diaza-18-crown-6. As model systems, 2- and 4-nitroanisole, 2- and 4-nitrotoluene, and (methoxyoctaethoxy)nitrobenzene were also examined. Finally, 3,3’-dimethylazobenzene and the [2.2]azocryptand derived therefrom were prepared. Generally, the electrochemistry of the macrocyclic nitroaromatics was as expected for the aromatic portion of the molecule irrespective of the macroring. When cations were added to the nitroaromatic lariat ethers in which the nitro groups were sterically accessible to a ring-bound cation, new, quasi-reversible couples were observed which are attributed to intramotecular ion pairing. When two side arms were present, only one of them interacted with a ring-bound Na+. From the differences in potentials, it was found that binding of the cations was enhanced from 15- to 13000000-fold upon electrochemical reduction. With the exception of the azocryptand, binding enhancements were in the order Li+> Na+ > K+. In the azocryptand case, the nearly exact fit between the cryptand cavity and the K+ makes the enhancement greater with this cation than with Na+. As a result, both cation-binding enhancements and cation-binding selectivity are possible in these electrochemically switched systems.
ASJC Scopus subject areas
- Colloid and Surface Chemistry