Redox-switched bonding in biferrocene-containing crown ethers: Complexing behavior

Two series of novel biferrocenes containing crown ethers have been prepared . The new compounds include l',l'''-bis(1,4,7,10,13-pentaoxa-16-azacyclooct adecane-16-methyl)biferrocene (7) and l',l'''-bis(1,4,7,10-tetraoxa-13-azac yclopentadecane-13-methyl)biferrocene (8). The complexing behavior toward a lkali and alkaline earth metal cations for 7 and 8 was investigated. Solid- state structures have been determined for NaPF6 and KPF6 complexes of 8. Th e reaction products of 7 and 1-(1,4,7,10,13-pentaoxa-16-azacyclooctadecane- 16-methyl)ferrocene with I-2 have also been characterized by structural det erminations, H-1 NMR, magnetic measurements, and Fe-57 Mossbauer technique. Complexation properties for 7 and 8 have been assessed by use of H-1 NMR, cyclic voltammetry measurements, and Fe-57 Mossbauer analyses. When the sys tem was studied by cyclic voltammetry, new redox couples were observed in t he presence of certain alkaline earth metal cations. For example, the elect rochemical behavior observed for 8 with Ba2+ ion is fundamentally different since two new redox couples are observed for Ba2+ concentrations within th e range 0 < [Ba2+] < 2equiv. The peak currents for the two new redox couple s increase with the concentration of Ba2+ ion until a full equivalent is ad ded; at this point, the original redox couples disappear and the new redox couples reach full development. This is quite unusual since compounds 7 and 8 do not have a well-formed cavity to facilitate the formation of internal metal cation complexes. Furthermore, the Mossbauer and UV/vis studies sugg est no direct interaction between the iron atom and metal cation. The solid -state electron transfer in mixed-valence l',l'''-bis(crown ether)biferroce nium cations was also characterized by Mossbauer spectroscopy. The features in all of the Mossbauer spectra include two doublets, one with a quadrupol e splitting (Delta E-Q) of similar to 2.2 mms(-1) (Fe(II) site) and the oth er with Delta E-Q = similar to 0.4 mms(-1) (Fe(III) site). The Mossbauer st udies indicate that the electron-transfer rates in the series of mixed-vale nce l',l"'-bis(crown ether)biferrocenium cations are less than 10(7) s(-1) in the solid state.