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

• Synthesis of di(biferrocenyl)alkynes using different C,C cross-coupling reactions.
• Solid state structure of di(biferrocenyl)butadiyne revealing coplanarity of cp-rings.
• Electrochemistry showed subsequent oxidation of the individual ferrocenyl units.
• Classification of the mixed-valent compounds as class II species.

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-CCH (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 (–CC–)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 νCC band is present, while 3+ and 33+ showed two νCC stretching vibrations due to Fermi resonance. Compound 43+ exhibits only one sharp νCC frequency.

The synthesis of di(biferrocenyl)ethyne and -butadiyne using different C,C cross-coupling reactions is discussed. Electrochemical and spectroelectrochemical studies showed that the ferrocenyl groups can separately be oxidized and are II species according to Robin and Day. The solid state structure of di(biferrocenyl)butadiyne is reported.Figure optionsDownload as PowerPoint slide