Nitroxide Radicals: Properties, Synthesis and Applications - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 207

their EPR parameters to the polarity of the embedding medium 41,46,47

make them ideal guests to investigate

various supramolecular assemblies. 31,241

5.7.1 Interaction of nitroxides with cyclodextrins
Rassat 242 et al . were the first to show that EPR spectroscopy is well suited to study the association of
nitroxides with cyclodextrins. When nitroxide 99 (Scheme 5.55) was used as a guest for β -cyclodextrin

( -CD) species showed separated high field EPR lines. The

spectrum with the smaller nitrogen hyperfine splitting (a N ) was assigned to the complexed nitroxide, which
inside the

β -CD) in water, the free and included ( 99 @ β

-CD cavity resides in a less polar environment than pure water. On the other hand, on using
TEMPO as guest, no clear distinction between the free and included species can be observed, the formation
of the host - guest being evidenced only by an increase of the EPR high-field line width.
Kotake and Janzen studied a series of nitroxide@

β

-CD complexes. 243 They were the first to show that
a nitroxide radical with more than one relatively bulky functional group can form two distinct host - guest
complexes (Scheme 5.55). Using a high-pressure EPR system, Sueishi et al . 243f established that the external
pressure either increases or decreases the equilibrium constant of group-in complexes, depending on the
sizes of the included group relative to that of the cyclodextrin cavity.
Lucarini et al . 31,244 studied the association of benzyl tert -butyl nitroxide, 100 (Scheme 5.56), with

β

α -, β -,

and -cyclodextrin. In the presence of cyclodextrins they observed additional lines on the EPR spectrum

of 100 that were assigned to the radical included in the host, in equilibrium with the free species. The
significantly smaller nitrogen hyperfine coupling (Table 5.4) found with

γ

-CD with respect to water
suggests that the N-O group is deeply included in the cavity of the CDs. On the other hand, with

β -and γ

α

-CD,

2 A) internal diameter than 6 A) and 4 A), the very similar which has a smaller ( ≈ 5 . β -CD ( ≈ 6 . γ -CD ( ≈ 8 .

N

O

N

O

t -butyl-in

b - CD phenyl-in

O

N
O
99

Scheme 5.55

O

N

O N

O

100 101

Scheme 5.56