Facile Synthesis of Ferrocene Biscrown Ethers and Ferrocene Cryptands: NMR Complexation Studies and the Redox‐Switched Bonding of H+ and Na+

Abstract

AbstractThe reaction of 1,1′‐ferrocenylene‐bis(methylenepyridinium) tosylate chloride (1) with aza‐x‐crown‐y (x/y = 12/4; 15/5; 18/6) gives the three novel ferrocene biscrown ethers 1,1′‐ferrocenylene‐bis(methylene‐aza‐x‐crown‐y) (x/y = 12/4, 2; 15/5, 3; 18/6, 4). The reaction of 1 with diaza‐18‐crown‐6 represents a new and simple route to the known ferrocene cryptands 1,1′‐zacyclooctadecane)] (5) and 1,1″:1′,1″‐bisferrocenylene‐bis‐[bis‐7,16‐methylene(1,4,10,13‐tetraoxa‐7,16‐diazacyclooctadecane)] (6). The solid‐state structure of 5 is described. The ferrocene bridge compresses the diaza crown ether leading to an internitrogen distance of 458 pm. The stoichiometry of Na+ and K+ complexes with 2 and 3 in CD3CN determined in NMR titration experiments suggests the formation of Na+ and K+ crown ether sandwich complexes in solution. Cyclic voltammograms of 2 and 5 in the presence of varying amounts of H+ and Na+ have been recorded. The redox potential of ferrocene is shifted by +180 mV in the case of 5 · Na+ and up to +600 mV in the case of 5 · H+H+ relative to 5, but only by + 70 mV with 2 · Na+ and 2. The protonation of 5 give rises to several isomers (exo and endo protonation) which differ significantly in their redox potential. The extremely large positive shift of the oxidation potential of the diprotonated ferrocene cryptand corresponds to an increase in the acidity of 5 · H+H+ upon oxidation by more than ten orders of magnitude and makes this system a candidate for the directed membrane transport of protons.