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π
π
anion (6
electrons), showing aromaticity and antiaromaticity, respectively
(Figure 3.58a). The frontier molecular orbitals of the cyclopentadienyl radical calculated by the
H uckel-MO method are shown in Figure 3.58b. This radical has no NBMO in contrast to the phenalenyl
system, and thus an unpaired electron resides in one of the two degenerate HOMOs. Quantum chemical
calculation indicates an extensive spin delocalization on the whole molecular skeleton without spin
polarization.
In general, the stabilities of cyclopentadienyl radicals are not high. The pristine radical
82,83d,f,g
(including
monodeuterated
83c
-
g
and pentadeuterated
84
), monoalkyl,
83a
-
d,f,g
mono(alkylsilyl),
83b,d
pentahalogenated
(fluorine, chlorine, bromine),
85
pentamethyl,
86
and pentakis(methoxycarbonyl)
87
derivatives are only stud-
ied spectroscopically in a degassed solution, in the gaseous phase, or in a matrix. Pentaaryl derivatives
possess relatively high stabilities than those of the above derivatives.
88
Sitzmann prepared and isolated a pentaisopropyl derivative in 1991, and reported the first X-ray crystal
structure analysis of a cyclopentadienyl radical (Figure 3.59).
89
The yellow - green air-sensitive crystals
were obtained by sublimation in high vacuum. Solution EPR measurements at 293 K yielded hyperfine split-
tings due to the methyl and the methyne protons and the methyl carbon nuclei (Figure 3.59b). Interestingly,
temperature-dependent EPR spectra indicate that hfcc attributable to the methyne protons decreases sub-
stantially with decreasing temperature, while hfcc due to the methyl protons does not change (Figure 3.59b).
This change in hfcc is ascribable to the temperature-dependent conformational change of the isopropyl
groups. In the crystal, the cyclopentadienyl ring system possesses a planar structure with equivalent C-C
bond lengths (1.40 - 1.41 A) (Figure 3.59c). The radical forms a slipped stack 1-D columnar structure with
a tilt angle of about 23
◦
electrons) or a cation (4
(Figure 3.59d). Probably due to the steric hindrance of the isopropyl groups,
planes is 5.28 A, indicating no electronic communication within the structure.
Electrochemical measurements in acetonitrile show a reversible reduction process from the neutral radical
to the anion (
the distance between the
π
1.91 V vs. Fc/Fc
+
) and a quasi-reversible oxidation process from the neutral radical to the
−
0.58 V vs. Fc/Fc
+
).
In 1996, Rubin prepared and isolated a pentakis(trialkylsilylethynyl)cyclopentadienyl radical (shown
below) in the process of the synthetic study of a metallocene derivative.
90
This neutral radical is sta-
ble enough to purify by silica gel column chromatography and survives in air for weeks in the solid
state and for days even in a solution state, showing a sharp contrast to those of the pentaaryl and the
cation (
+
(a)
(b)
(c)
(d)
Figure3.59
(a)Chemicalstructure,(b)EPRspectrainatoluenesolution,(c)X-raycrystalstructure,and(d)1-D
columnar structure of pentaisopropylcyclopentadienyl radical in which there is no electronic communication.
(b,cReprintedwithpermissionfrom[89b].Copyright1993AmericanChemicalSociety.)
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