Phenalenyls, Cyclopentadienyls, and Other Carbon-Centered Radicals - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 134

pentaisopropyl derivatives. Electrochemical measurement of the radical in dichloromethane revealed a
lower oxidation potential from the neutral radical to the cation ( 0.15 V vs. Fc/Fc + ) than that of pentaiso-

propyl derivative, being attributable to the significant propargylic and allenic resonance stabilization by
the silylethynyl moieties.

+

TIPS

TIPS TIPS

TIPS TIPS

TIPS = Si( i -Pr) 3

In 1997, Janiak reported the crystal structure of pentaphenylcyclopentadienyl radical (Ziegler's radical)
(Figure 3.60). 88g Although the analysis data are, unfortunately, of low quality because of the disorder of
the phenyl moieties, it turns out that the cyclopentadienyl ring possesses a planar structure.
In 2004, Kitagawa and Komatsu prepared and isolated a cyclopentadienyl radical annelated with two
homoadamantene skeletons (Figure 3.61a), and reported the X-ray crystal structure of this radical at
100 K (Figures 3.61b,c). 91 The radical survives only for a few hours in air in the solid state, but remains
unchanged for months under vacuum. In a solution state, the radical is highly air-sensitive and slowly
abstracts hydrogen atom from solvents. The substantial bond alternations are found in the C-C bonds of
the cyclopentadienyl ring, and indicate a localization of the unpaired electron, being in agreement with the
geometry calculated by the DFT method. EPR spectra in a solution state suggest a rapid equilibrium between
the two allylic states. From these results, the radical can be viewed as an allyl radical analogue incorporated
into the five-membered ring system. Cyclic voltammetry in dichloromethane shows a reversible oxidation
wave at 1.78 V (vs. Fc/Fc + ), indicating that

the radical is more easily oxidized and reduced than the pentaisopropyl derivative. The lowering of the
oxidation potential represents a cation stabilizing effect of

0.30 V (vs. Fc/Fc + ) and an irreversible reduction wave at + −

σ - π conjugation induced by the annelation of

the homoadamantene skeletons.

C42

C41

C52

C32
C31

C4

C5

C51

C3 C1

C11

C2

C12

C22

C21

(a) (b)

Figure 3.60 (a) Chemical structure and (b) X-ray crystal structure of pentaphenylcyclopentadienyl radical.
(bReprintedwithpermissionfrom[88g].Copyright1997AmericanChemicalSociety.)