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 338

Figure9.11 X-raystructureof1,4-phenylenebis(1,3,2,4-dithiadiazolyl)diradical. 148

N

N

S

S

CN N

S

S

N

NC

NC N S

S

N

53

N

S

N

S

N

S

N N

S

S

N

S

S

N

S

N

54

55

Scheme 9.15

Several poly(1,3,2,4-dithiadiazolyl) radicals 49 have been explored, with phenylene-based spacers
(derived from di- and tricyanobenzenes) or single carbon spacers (from tricyanomethanide). Reduction
of the polycations leads to the polyradicals, which rearrange slowly in the solid state (or more rapidly
with heating) to the 1,2,3,5-isomers. 147,148 Banister has cleverly synthesized dithiadiazolium dications
53 - in which each of the two heterocycles is a different dithiadiazole isomer - by assembling the rings
in stepwise fashion (Scheme 9.15; Ar 1,4- and 1,3-phenylene). 149 Reduction of both rings leads to the

mixed diradicals 54 . The EPR spectra of these consist of a superposition of the spectra of each type of
dithiadiazolyl radical, suggesting very weak intramolecular radical - radical interactions. The fact that the
1,3,2,4- cations are reduced at significantly lower potentials (

=

Ar) 87,95

+ 0.3 - 0.4 V vs SCE for R = than

the corresponding 1,2,3,5- isomers ( + 0.6 - 0.7 V vs SCE) permits the selective reduction of the latter ring

to afford radical cations 55 .
Attempts to prepare the 1,3,2,4-dithiadiazolyl diradical 59 with the two rings directly attached were ulti-
mately successful, but en route produced a multitude of species of varying oxidation state and dithiadiazole
isomer. 108,115,150,151 One-electron reduction of bis(1,3,2,4-dithiadiazolium) dication 56 gives radical cation
58 , in which the radical ring has rearranged to the 1,2,3,5- isomer; this product presumably proceeds via
the bis(1,3,2,4) radical cation 57 (Scheme 9.16). The structure of 58 consists of trans -cofacial dimers

of the 1,2,3,5- radicals. Further reduction of 58 gives the 'mixed' diradical 61 , which adopts a cofacial but
centrosymmetric dimer in which each

π

π

dimer is built from the two different CN 2 S 2 rings (Figure 9.12).