The influence of an ethynyl spacer on the electronic properties in 2,5-ferrocenyl-substituted heterocycles

A series of binuclear complexes based on five-membered heterocycles featuring alkynyl ferrocenyl groups, 2,5-(FcC equivalent to equivalent to C)(2)-(C4H2E)-C-c (E = NPh, 3a; S. 3b; O, 3c), has been synthesized using the Sonogashira C,C cross-coupling protocol. The molecular structures of 3a and 3c in the solid state are discussed. The influence of the ethynyl unit on the electronic and (spectro)electrochemical properties of these compounds is shown by cyclic voltammetry (CV), square wave voltammetry (SWV) and in situ UV-Vis-NIR and IR spectroscopy. The high Delta E-p values (140-204 mV) of the single reversible redox events indicate that two individual oxidation processes take place in a close potential range. Nevertheless, the NIR measurements indicate weak and broad intervalence charge transfer (IVCT) absorptions for the mixed-valent monocationic species 3a-c(+). Compared to 2,5-Fc(2)-(C4H2E)-C-c (E = NPh, S, O) the absorptions for the IVCT transitions are shifted hypsochromically, showing that the charge transfer process is promoted at higher excitation energies, due to larger metal-metal distances in 3a-c. The occurrence of IVCT absorptions demonstrates that the 2,5-diethynylferrocenyl heterocycles are class II systems according to Robin and Day. (C) 2014 Elsevier Ltd. All rights reserved.