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PbO
2
K
2
CO
3
∆
(RS)
2
N
20
(RS)
2
NH
(RS)
2
N
N(SR)
2
22
21
Scheme 9.7
N
S
S
N
S
S
+
N
N
N
N
N
S
S
S
S
2
N
23
h
n
H
H
S
N
N
S
+
N
S
N
S
S
S
S
25
26
H
H
24
Scheme 9.8
correct structural assignment to be 1,3,2-dithiazolidinyl radicals
24
, which are cyclic analogues of
20
.
75
EPR-active solutions of these radicals can be generated made more directly by the reaction of S
4
N
2
26
with (bi)cyclic alkenes.
76
Several different derivatives of
24
have been characterized by EPR spectroscopy
∼
(
a
(N) is consistently
13 G), but no derivatives have been isolated. These radicals reversibly dimerize in
solution; dimerization enthalpies are in the range 7 - 11 kcal/mol.
77
9.3.1.2 Delocalized thioaminyl radicals
The reaction of benzamidine with arylsulfenyl chlorides gives, depending on stoichiometry, bis- or
tris(arylthio)benzamidines
27
and
28
respectively (Scheme 9.9). Both of these species can be converted
into thioaminyl radicals
29
in which a sulfen
imide
fragment (
NSAr) is conjugated directly to the
thioaminyl radical (-N
•
-S-Ar)component.
78
These radicals are highly persistent and insensitive to
oxygen, and dimerize reversibly to give N-N bonded dimers
30
which are even more weakly bound
(BDE
=
6 - 7 kcal/mol) than simple thioaminyl radical derivatives
5
. The increase in radical stability (with
respect to dimerization) can be ascribed to delocalization over two structurally equivalent RSNR units.
The delocalization is supported by the EPR spectra of
29
which consist of a dominant five-line pattern
arising from hyperfine coupling to two equivalent nitrogen atoms (
a
(N)
∼
1G).
In related work, Miura reported the reaction of arenesulfenyl chlorides with imidoylamidines
31
to give
bis- and tris-arylsulfenyl compounds
32
and
33
, respectively.
79
Oxidation (of
32
) or photolysis (of
33
)
produces radicals
34
as long lived and oxygen-insensitive species (Scheme 9.10). EPR characterization of
the radicals, coupled with semi-empirical molecular orbital calculations, indicate that the spin is highly
delocalized, shared predominantly between the two equivalent nitrogens (
=
6
.
0
−
6
.
∼
19 % each), the central nitrogen
(
15 % each). The radicals associate reversibly into diamagnetic dimers
in solution. Interestingly, one of the dimers was reported to have a genuine N-N bond at the terminal
nitrogens based on X-ray crystallographic studies, but no crystallographic information was given.
79
∼
17 %), and both sulfur atoms (
∼
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