Syntheses, Spectroelectrochemical Studies, and Molecular and Electronic Structures of Ferrocenyl Ene-diynes

The readily available complex 1,1-dibromo-2-ferrocenylethylene
provides a convenient entry point for the preparation of a wide range
of cross-conjugated 1,1-bis­(alkynyl)-2-ferrocenylethenes through simple
Pd(0)/Cu­(I)-mediated cross-coupling reactions with 1-alkynes. The
ferrocene moiety in compounds of the general form FcCHC­(CCR)2 is essentially electronically isolated from the cross-conjugated
π system, as evidenced by IR and UV–vis spectroelectrochemical
experiments and quantum chemical calculations. In contrast to the
other examples which give stable ferrocenium derivatives upon electrochemical
oxidation, the aniline derivatives [FcCHC­(CCC6H4NH2-4)2]+ and
[FcCHC­(CCC6H4NMe2-4)2]+ proved to be unstable on the time scale
of the spectroelectrochemical experiments, leading to passivation
of the electrode surface over time. There is no significant thermodynamic
stabilization of the radical anion [FcCHC­(CCC6H4NO2-4)2]− relative to the neutral and dianionic analogues, although the dianion
[FcCHC­(CCC6H4NO2-4)2]2– could be studied as a relatively chemically
stable species and is well described in terms of two linked nitrophenyl
radicals. The capacity to introduce a relatively isolated point charge
at the periphery of the cross-conjugated π system appears to
make these complexes useful templates for the construction of electrochemically
gated quantum interference transistors.