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 319

π π π

-conjugated six-membered ring with ten electrons; the corresponding isovalent (10 ) hydrocarbon

would be the exceedingly reactive benzene tetra-anion.
The collective properties of the thiazyl moiety are the foundation behind an incredibly diverse array
of structure types which have open shell configurations and yet qualify as persistent or stable. Moreover,
as a consequence of the differences between the elements noted above, NS- and NO-containing radical
chemistries are entirely different from one another. Thiazyl radicals have been actively studied since the
1970s and are now well established from both fundamental and applied (materials) perspectives. This
chapter has as its central focus the many classes of stable neutral radicals built from the thiazyl (SN)
unit. The synthesis, electronic, molecular structures, and reactivity of different thiazyl radical types is the
central focus; a separate section introducing the reader to the various uses of these radicals in advanced
materials design is also presented.
The scope of thiazyl radical chemistry has grown tremendously over the past 30 years. The field is
now very large and there are many general and specific reviews relevant to this chapter. Reviews on spe-
cific aspects of thiazyl radicals are cited in the appropriate sections. For general aspects of -conjugated

sulfur - nitrogen compounds, readers are directed to the older reviews of (among many others) Heal, 7
Gleiter, 8 Chivers, 6 and Oakley. 5 The recent topic by Chivers provides the most up-to-date coverage
of sulfur - nitrogen chemistry. 9 Among more topic-specific reviews, the electron paramagnetic resonance
(EPR) spectroscopy of thiazyl radicals was reviewed some 20 years ago by Preston and Sutcliffe 10 ;the
electrochemical characterization of cyclic thiazyl radicals (among other species) was compiled in 2000 by
Boere, 11 and the relatively new area of thiazyl radical coordination chemistry has been recently reviewed
by Preuss. 12 Kaszynski has compared the ability of ab initio and DFT computational methods to predict
a variety of spectroscopic and structural parameters for thiazyls. 13

π

9.2 Radicals based exclusively on sulfur and nitrogen

So-called “binary” sulfur - nitrogen compounds (those only containing sulfur and nitrogen) were historically
one of the more structurally intriguing - and difficult to understand - classes of main group systems.
Many compounds of this type are thermally unstable and shock sensitive, features which arise from the
thermodynamic driving force of N 2 and S 8 formation. Furthermore, reactions of sulfur - nitrogen compounds
often involve extensive skeletal rearrangements which proved challenging to understand from a mechanistic
perspective. Nonetheless, this field has matured considerably in the past three decades, to the point where
the geometric and electronic structures and reactivity patterns of sulfur - nitrogen compounds are now
reasonably well understood. 9 Within the context of radical species, the few stable binary sulfur - nitrogen
radicals are quite different in nature from their nitrogen oxide counterparts. The sections below cover
the structurally important SN radicals, although some do not qualify as stable. Boere and Chivers have
compiled calculated and (where available) EPR spectroscopic characterization of a wide range of binary
SN compounds, both neutral and charged. 14

9.2.1 NS • and SNS •
The NS • and SNS • radicals, sulfur analogues of the ubiquitous NO • and • NO 2 , are mentioned briefly
here even though they are not stable radicals in any practical sense. The electrochemical reduction of
the thionitrosonium cation [NS] + is irreversible, leading to the S 3 N 2 • + radical cation and other radical-
containing species, indicating that radical associations and rearrangements take place upon reduction. 15
A variety of spectroscopic and computational studies have been reported for the NS • 16 and SNS • 17,18
radicals. Interestingly, condensed phase studies (Ar matrix, cryogenic temperatures) on SNS • indicate the
presence of an asymmetric isomer, NSS • . 17

Thionitrosyl complexes [M]-NS, though less well developed