Di(biferrocenyl)ethyne and -butadiyne: Synthesis, properties and electron transfer studies

Di(biferrocenyl)ethyne (3) and -butadiyne (4) have been prepared by Negishi, Eglinton and Sonogashira C,C cross-coupling reactions using bfc-I (1) and bfc-Ctriple bond; length of mdashCH (2) (bfc = 1:1″-biferrocenyl) as starting materials. Compound 4 was structurally analyzed by single crystal X-ray diffraction studies. The individual ferrocenyl units are all coplanar and anti-parallel oriented. Electrochemical measurements showed that all four ferrocenyl units can reversibly be oxidized. The electrochemical characteristics of these molecules represent a combination of the properties of biferrocene and the appropriate ferrocenyl analogs diferrocenyl ethyne and diferrocenyl butadiyne, respectively. While the dicationic oxidation state of both compounds showed characteristics of a charge transfer within the biferrocenium units, the tricationic form allows electron transfer through the (–Ctriple bond; length of mdashC–)n (n = 1, 2) functionalities. The observed inter-valence charge transfer (IVCT) interaction of 3 and 4 in any mixed-valent oxidation state allowed the characterization of these species as class II systems according to Robin and Day. In situ IR spectroscopy of 3 and 4 showed that within 3n+ and 4n+ (n = 0, 2, 4) no νCtriple bond; length of mdashC band is present, while 3+ and 33+ showed two νCtriple bond; length of mdashC stretching vibrations due to Fermi resonance. Compound 43+ exhibits only one sharp νCtriple bond; length of mdashC frequency.