A R T I C L E S
Diallo et al.
rigid dendrimers are well-known,10 there is no report of rigid
ferrocene-containing dendrimers, presumably because of the
synthesis problems related to their insolubility. In star-shaped
systems, the number of branches is reduced, for instance to six
in arene-cored stars. This reduced number of redox sytems and
their localized positions at the rigid branch termini should allow
us to better understand the parameters governing heterogeneous
electron transfers with electrodes than those in dendritic redox
systems in which the terminal redox centers are numerous and
located on flexible tethers. Recently, we have reported the
synthesis and electrochemistry of hexa(ferrocenylethynyl) ben-
zenes.11 Now we report our investigations of the synthesis of
rigid ferrocenyl-terminated arene-cored stars and the comparison
of their electron-transfer properties, especially the stabilization
of mixed valences and the implication of the electrostatic factor.
Geiger’s group has shown that, inter alia, when up to four
ferrocene redox centers are equivalent around a small core,
cyclic voltammograms using a supporting electrolyte that
contains a perfluorinated anion can be split into several waves
due to reduced ion pairing.12 In the present work, we are using
such a perfluorinated anion in order to provide the optimized
separation beween the CV waves of the redox centers. In this
way, it should be possible to investigate electrostatic effects
among the redox centers of the various star systems and
tentatively rationalize the comparison between flexible and rigid
stars in terms of electron-transfer mechanisms and electrostatic
effects. A few arene-centered star molecules containing terminal
ferrocenyl groups are known in which the tethers are flexible,
and the length of the tethers and lack of conjugation in these
compounds result in the independency of the redox centers that
give a single CV wave as in ferrocenyl-terminated dendrimers.13
However, the extraordinary hexaferrocenylbenzene synthesized
by the Vollhardt group shows an electrochemical behavior that
seemingly results from the extreme bulk and distortion rather
than from extended electronic communication among the six
ferrocenyl groups.14
Results
Synthesis of Hexa(ferrocenylethynyl)arene Complexes. The
chemistry of hexa(ethynyl)benzene derivatives has been pio-
neered by the Vollhardt group in the 1980s15 and further
developed by this group and by Rubin’s group in the 1990s.16
Then, the very useful Negishi reaction17 was introduced by
Tobe’s group18 for the syntheses of a series of these compounds
and used with cyclopentadienylruthenium compounds by Carella
et al.19 The mono-, bis-, and tris(ferrocenylethynylation) of
1,3,5-tribromobenzene was reported by the groups of Long and
Zanello20 using Sonogashira coupling.21 Various polyferroce-
nylarene complexes have also been reported.22 Long and Zanello
observed that their three compounds above gave only one cyclic
voltammetry (CV) wave, which was taken into account by a
lack of electronic communication between the redox centers.20
We, however, met with failure while attempting to extend this
Sonogashira coupling of ethynylferrocene to hexabromobenzene.
Alternatively, we found that Negishi coupling of ferrocenyl-
ethynyl zinc chloride with hexabromobenzene provided a
suitable route to hexa(ferrocenylethynyl)benzene (Scheme 1).
The reaction produced the dimer di(ferrocenyl)butadiyne20
along with 1 (Figure 1), which requires the use of an excess of
the zinc reagent. It also produced 7% of penta(ferrocenylethy-
nyl)benzene, 2, resulting from debromination. Such dehaloge-
nation of hexahalogenobenzenes upon reactions of organome-
tallic reagents is well-known;13,22-24 it is probably due to side
electron transfer from the organozinc reagent to the bulky
penta(ferrocenylethynyl)bromobenzene intermediate.24 The solu-
bility of 1 is very low, although it could be characterized by
MALDI-TOF mass and Mo¨ssbauer spectrocopy, CV, and
elemental analysis. Other related star complexes in which the
ethynylferrocenyl groups contains a differently substituted free
cyclopentadienyl-type ring have been synthesized in the same
way starting from ethynylpentamethylferrocene (Scheme 1),
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