Chemistry Reference
In-Depth Information
t
-Bu
t
-Bu
O
1.40
1.40
1.46
1.35
1.407
1.367
C
4
C
3
C
2
t
-Bu
t
-Bu
t
-Bu
t
-Bu
t
-Bu
t
-Bu
C
1
OH
O
OH
O
1.40
1.50
1.471
1.37
1.25
1.246
(a)
(b)
(c)
(d)
Figure 8.3
Bond distances in: (a) the tri-tert-butylphenoxyl radical; (b) the tri-tert-butylphenol; and (c) the
di-tert-butylquinone.The numbering used in the text is indicated in (d). ([34] Reproduced by permission of the
RoyalSocietyofChemistry.)
8.2.3 Structure of non-coordinated phenoxyl radicals
Although phenoxyl radicals have been known for over six decades, very few X-ray crystal structures
have been reported to date.
33,34
This is the consequence of the low stability of these molecules, combined
with their high solubility in most conventional solvents. Until very recently, structural aspects were only
investigated by computational methods. The first high resolution X-ray crystal structure was reported
in 2008 for the well known tri-
tert
-butylphenoxyl radical. The bond lengths observed in the tri-
tert
-
butylphenoxyl radical are compared to those of the tri-
tert
-butylphenol precursor in Figure 8.3. The C1-O
bond length in the radical is significantly shorter than in the phenol, and is quite similar to the C1
=
O bond
length in the 2,6-di-
tert
-butyl-1,4-benzoquinone.
35
The carbon - carbon bond lengths in the phenoxyl ring
are inequivalent, with a C2-C3 bond length that is shorter than the C3-C4, itself shorter than the C1-C2
bond length. This alternance compares well with that found in asymetrical quinones such as 2,6-di-
tert
-
butyl-1,4-benzoquinone and reflects the significant double bond character of the C1-O, C2-C3 and C3-C4
bonds, as expected for the canonical forms depicted in Figure 8.1.
8.2.4 UV-Vis spectroscopy
As long as 80 years ago it was reported that phenoxyl radicals exhibit an intense color, in contrast with
the precursor phenols that are usually colorless.
36,37
The first visible spectra of phenoxyl radicals were
reported in the 1950s. A feature common to most of these species is the intense bands in the 370 - 440 nm
range along with a less intense transition at higher wavelengths (600 - 800 nm), as illustrated in Figure 8.4.
The position of these bands is slightly (but significantly) dependent on the substituent's properties, as
well as the protonation state of the radical. As an example, the
λ
max
for the tri-
tert
-butylphenoxyl radical
400 nm is shifted to 419 nm in 12 M sulphuric acid (protonation of the oxygen).
38
As shown below,
metal coordination can also shift these transitions and enhance their intensities. These bands originate from
π
at
≈
* transitions, although their exact attribution has been debated, sometimes controversially, for several
years.
39
In the presence of a metal ion, complication of the UV-Vis spectra is often observed. This is
mainly due to the presence of additional bands corresponding to charge transfer (CT) and d-d transitions.
-
π
8.2.5 EPR spectroscopy
The free phenoxyl radicals are paramagnetic (S
2
) species that could be easily characterized by
EPR and related magnetic spectroscopies. From the g-tensor and the hyperfine coupling constants the
electronic distribution in the radical can be elucidated. Most of the non-coordinating phenoxyl radicals
exhibit isotropic g-tensor values ranging between 2.003 and 2.007.
40
=
1
/
Resolution of the anisotropy can be
Search WWH ::
Custom Search