The Synthesis and Characterization of Stable Radicals Containing the Thiazyl (SN) Fragment and Their Use as Building Blocks for Advanced Functional Materials - Stable Radicals: Fundamentals and Applied Aspects of Odd-Electron Compounds - page 331

R

38

A large number of 1,2,3,5-dithiadiazolyl radicals have been structurally characterized by X-ray crys-
tallography. Several different structure types involving radical dimers have been noted (Figure 9.6). The
most common motif is the cis -cofacial dimer structure (Figure 9.6a), in which the two rings are directly
superimposed and nearly parallel. The sulfur - sulfur distances between rings within the dimer (dashed lines
in Figure 9.6a) are typically 3.0 - 3.1 A. There are approximately 20 examples of dithiadiazolyl dimers of
this type; nearly all are aromatic 92,94,98 - 101

or heteroaromatic 102,103

derivatives, or have R groups which are

H, 104,105 Cl, 106 F, 107 CN 108 ). Several derivatives adopt a twisted structure (Figure 9.6b)
in which two rings are rotated by approximately 90 ◦ with respect to each other. The lone S-S contact is
the shortest one between rings and is typically between 2.8 and 3.1 A. This structure is adopted by radicals
for which the size of R prohibits the cis -cofacial structure from forming for steric reasons (e.g., R

small (e.g., R =

=

CH 3 , 109 Br, F, Cl) 106,107,110 and one derivative

with an aromatic substituent. 111 There are two possible trans -type dimer structures. One of these has the
two rings slipped along their molecular long axis relative to one another, so that they only interact via
the sulfur atoms ( trans -antarafacial, Figure 9.6c), analogous to the structure of the S 3 N 2 radical cation
dimers (Section 9.2.3). There are only two examples of this dimer type; one (R

CF 3 , 88 Me 2 N, 93 adamantyl 84 ) but also the halogeno (R =

2,4,6-CF 3 ) 3 C 6 H 2 -) 112 =

3-cyanophenyl) 92 does not. The other trans associative
mode, also relatively rare, is trans -cofacial (Figure 9.6d), in which the two rings overlap exclusively via
S-N contacts which range from 3.09 to 3.26 A. 113 - 116

has considerable steric bulk while the other (R =

Finally, there are a small number of derivatives

(a) (c)

(b) (d)

Figure 9.6 Examples of structural classes of π dimers of 1,2,3,5-dithiadiazolyl radicals 35 . (a) cis-cofacial

phenyl), 98 CF 3 ), 88 3-cyanophenyl), 92

(R = (b) twisted (R = (c) trans-antarafacial (R = (d) trans-cofacial (R =

4-iodophenyl). 113