Ferrocenyl Iron as a Donor Group for Complexed Silver in Ferrocenyldimethyl[2.2]cryptand: A Redox-Switched Receptor Effective in Water

Three novel ferrocene-crown derivatives have been prepared along with their previously known amide precursors. The new compounds include 1,1′-(1,4,10,13-tetraoxa-7,16-diazacyclooctadecane-7,16-dimethyl)ferrocene (2), N,N′-bis(ferrocenylmethyl) diaza-18-crown-6 (4), mp 114-116 °C, and bis[N,N′-bis(cyclopentadienidylmethyl)-4,13-diaza-18-crown-6]diiron (6), mp 104-106 °C. Solid-state structures have been obtained for 2·H₂O (mp 101-104 °C) and for the Na⁺C104^- (mp 208-215 °C dec) and Ag⁺ClO_4^- (mp 209-210 °C dec) complexes of 2. Comparison of these structures shows that the iron atom of ferrocene is a donor ligand for Ag+ but not Na+ (the Fe-Ag+ distance is ≈1 Å shorter than for Fe-Na⁺). Stability constants have been obtained in either CH₃OH or CH₃CN with Li⁺, Na⁺, K⁺, Ca²⁺, and Ag⁺ cations. Complexation properties for these compounds have also been assessed by use of fast atom bombardment mass spectrometry. This technique reveals interactions that were not apparent when other analytical methods were applied to these compounds. When the system was studied by electrochemical methods, a new redox couple at more positive potential was observed in the presence of certain alkali, alkaline earth, and transition metal cations. Evidence is presented that cation binding by 2 involves coordination of the cation within the cryptand cavity in contrast to related amides such as 1. Thus alkali metal redox switching, previously unknown in ferrocenyl amides, is apparent. Evidence is presented for the first example of redox switching of silver ion binding. Determination of the stability constant for the complexation of 2 with the metal cations studied shows a direct correlation with the cyclic voltammetry. The large Ag⁺ binding constant and electrochemistry both confirm a direct, stabilizing Ag-Fe interaction. This interaction, previously unknown for kinetically dynamic systems, is confirmed by both ¹H NMR and UV-visible spectroscopy. ²³Na NMR studies of compound 2 in the presence of Na⁺ show two peaks in the NMR spectrum, corresponding to free and bound Na⁺. This behavior is usually associated with cryptands having oxygen donors and suggests that even sodium ions, when bound, are largely encapsulated by compound 2. This strong interaction permits the first observation of redox switching involving Ag⁺ in aqueous solution.