Ferrocenylmethyl-functionalized 5-membered heterocycles: Synthesis, solid-state structure and electrochemical investigations

Abstract

Treatment of Fe(η5-C5H4CH2OH)2 (1) with two equivalents of ClC(O)R (2) (a, R = 2-cC4H3O; b, R = 2-cC4H3S; c, R = 2-cC4H3Se; d, R = 3-cC4H3S) produced the corresponding ferrocenylmethyl carboxylates Fe(η5-C5H4CH2OC(O)R)2 (3a–d), while the reaction of FcCH2OLi (Fc = Fe(η5-C5H5)(η5-C5H4)) with 2,5-(ClC(O))2-cC4H2X (5) (a, X = O; b, X = S; c, X = Se) in a 2:1 molar ratio gave 2,5-(FcCH2OC(O))2-cC4H2X (6a–c). Compounds 3a–d and 6a–c were characterized by elemental analysis, NMR (1H and 13C{1H}) and IR spectroscopy. The molecular structures of 3a,b,d in the solid state were determined by single crystal X-ray structure analysis. Compound 3a crystallizes in the monoclinic space group P21/c, while 3b,d crystallize in the triclinic space group P-1¯. The ester groups and the heteroatoms are in an anti arrangement with respect to each other. Cyclic voltammetry measurements for 3a–d and 6a–c show reversible electrochemical processes (Fc/Fc+) between 165 and 176 mV for 3a–d, and 94 and 116 mV for 6a–cb, using [NnBu4][B(C6F5)4] as the supporting electrolyte. It was found that for 3a, a somewhat higher Fc/Fc+ redox potential (E0′) is observed when compared with the more electron-rich systems 3b,c,d. The molecular electronic structures of the title compounds were additionally investigated by DFT calculations, revealing different degrees of HOMO–LUMO energy gaps within the series, due to a lowering of the LUMO energy, depending on the nature of the heterocyclic ring.