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

Treatment of Fe(eta(5)-C5H4CH2OH)(2) (1) with two equivalents of ClC(O)R (2) (a, R=2-(C4H3O)-C-c; b, R = 2-(C4H3S)-C-c; c, R = 2-(C4H3S)-C-c); d, R =3-(C4H3S)-C-c) produced the corresponding ferrocenylmethyl carboxylates Fe(eta(5)-C5H4CH2OC(O)R)(2) (3a-d), while the reaction of FcCH(2)OLi (Fc = Fe(eta(5)-C5H5)(eta(5)-C5H4)) with 2,5-(ClC(O))(2)-(C4H2X)-C-c (5) (a, X = O; b, X = S; c, X = Se) in a 2:1 molar ratio gave 2,5-(FcCH(2)OC(O))(2)-(C4H2X)-C-c (6a-c). Compounds 3a-d and 6a-c were characterized by elemental analysis, NMR (H-1 and C-13{H-1}) 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 P2(1)/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 [(NBu4)-Bu-n][B(C6F5)(4)] as the supporting electrolyte. It was found that for 3a, a somewhat higher Fc/Fc(+) redox potential (E-0') 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. (C) 2018 Elsevier Ltd. All rights reserved.