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(a)
(b)
(c)
Figure 9.13
X-ray structures of 1,3,2-dithiazolyl radical dimers: (a) 4,5-bis(trifluoromethyl) derivative,
166
(b)
70
,
161
and(c)
69
.
165
does pyridyl-fused radical
70
161
(Figure 9.13b) and the radical cation of
80
.
167
There is substantial variation
in the structural specifics of the dimers: the interplanar separation in the 4,5-dicyano derivative is 3.14 A but
substantially longer in
80
+
(which exists as two different polymorphs, with average S-S distances of 3.35
and 3.26 A). The two rings are substantially tilted in the bis(trifluoromethyl) structure, such that the N-N
intradimer distance (2.86 A) is shorter than the S-S distances (average 3.17 A). The benzo-fused radical
69
is one of two derivatives to dimerize centrosymmetrically, with average
d
(S-S) of 3.17 A (Figure 9.13c),
165
the other being
78
(the latter of which crystallizes with two radicals in the asymmetric unit - one of which
adopts the centrosymmetric structure with
d
(S-S)
3.23 A, while the other remains unassociated).
160
Other
derivatives remain monomeric in the solid state (
73, 76, 78, 80
and the methyl-substituted benzo-fused
radical
81
168
). Finally, several dithiazolyls have temperature-dependent solid state structures, consisting of
a “high temperature” phase of unassociated radicals and a “low temperature” phase in which the radicals
form cofacial
∼
π
dimers. These temperature-dependent structures are discussed further in Section 9.4.3.1.
Me
S
N
S
81
Selenium analogues of 1,3,2-dithiazolyls are all but unknown. Attempts to prepare the 1,3,2-diselenazolyl
radicals
82
and
83
were explored by reduction of the corresponding diselenazolium cations, but the radicals
were too unstable to isolate; the major species isolated in both instances was the tetraselenocine
84
.
169
Se
Se
F
3
C
R
R
Se
Se
N
N
R
R
Se
Se
F
3
C
Se
Se
82
83
84
9.3.5 1,2,3-Dithiazolyl radicals
Development of the 1,2,3-dithiazolyl radical system took place in two distinct stages. Mayer reported their
generation and identification by EPR spectroscopy,
170,171
while in the past decade Oakley has devised
new synthetic strategies and new derivatives which are amenable to more comprehensive characterization.
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