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 1‘,1‘ ‘‘-bis(1,4,7,10,13-pentaoxa-16-azacyclooctadecane-16-methyl)biferrocene ( 7 ) and 1‘,1‘ ‘‘-bis(1,4,7,10-tetraoxa-13-azacyclopentadecane-13-methyl)biferrocene ( 8 ). The complexing behavior toward alkali and alkaline earth metal cations for 7 and 8 was investigated. Solid-state structures have been determined for NaPF 6 and KPF 6 complexes of 8 . The 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 determinations, 1 H NMR, magnetic measurements, and 57 Fe Mössbauer technique. Complexation properties for 7 and 8 have been assessed by use of 1 H NMR, cyclic voltammetry measurements, and 57 Fe Mössbauer analyses. When the system was studied by cyclic voltammetry, new redox couples were observed in the presence of certain alkaline earth metal cations. For example, the electrochemical behavior observed for 8 with Ba 2+ ion is fundamentally different since two new redox couples are observed for Ba 2+ concentrations within the range 0 < [Ba 2+ ] < 2equiv. The peak currents for the two new redox couples increase with the concentration of Ba 2+ ion until a full equivalent is added; 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 Mössbauer and UV/vis studies suggest no direct interaction between the iron atom and metal cation. The solid-state electron transfer in mixed-valence 1‘,1‘ ‘‘-bis(crown ether)biferrocenium cations was also characterized by Mössbauer spectroscopy. The features in all of the Mössbauer spectra include two doublets, one with a quadrupole splitting (Δ E Q ) of ∼2.2 mms - 1 (Fe(II) site) and the other with Δ E Q = ∼0.4 mms - 1 (Fe(III) site). The Mössbauer studies indicate that the electron-transfer rates in the series of mixed-valence 1‘,1‘ ‘‘-bis(crown ether)biferrocenium cations are less than 10 7 s - 1 in the solid state.