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 325

Ph

Ph

Ph H

S Ar

N Ar C 6 H 6 N

N Ar

S

S

+

80 ° C

Ph

Ph Ph

Ph Ph

5b Ar = 4-NO 2 C 6 H 4
5c Ar = 3,5-Cl 2 C 6 H 3

6b
6c

11

Scheme 9.4

N R' N R'

N R'

R S R S

R S

Scheme 9.5

hyperfine coupling to ortho and para N-phenyl protons 49,55 ; (4) semi-empirical calculations, in conjunction
with EPR spectral analyses, lead to a picture of the spin distribution which places the highest spin density
on nitrogen ( ∼ 0.50 - 0.60 for R = alkyl, ∼ 0.4 for R = aryl) and relatively small but still substantial spin

aryl). 47,51 The first two
resonance structures in Scheme 9.5 are typically invoked to describe the delocalization of spin onto sulfur;
structure 5c is less often depicted, but reflects the (partial) S-N

on sulfur ( ∼ 0.25 - 0.30 when the NR substituent is alkyl, ∼ 0.15 -0.20 when NR =

π bond (with the odd electron in an S-N

π * orbital, the overall S-N bond order is 1.5).

The bond order formulation described above is supported by several thioaminyl radical X-ray crystal
structures, for which the S-N bond length is consistently between 1.60 and 1.63 A - significantly shorter
than S-N single bonds, for example, in sulfonamides for which bond length is closer to 1.70 A. All
of the crystallographically characterized thioaminyls have electron-deficient aromatic substituents on sul-
fur (nitrophenyl, chlorophenyl, pyridyl, etc.) and bulky N-aromatic groups containing two ortho -phenyl
substituents. 54,56,57 All of these structures have similar structural features, namely: (i) a trans -coplanar dis-
position of the nitrogen and sulfur substituents (the CNSC torsion angle is close to 180 ◦ ); (ii) an N-phenyl
ring which is essentially coplanar with the N-S bond; and (iii) flanking ortho phenyl groups which are
twisted with respect to the central N-phenyl ring. Figure 9.5 shows the structure of one representative
example.
The reactivity of thioaminyl radicals has received sporadic attention. The reactions of one derivative
with several phenols have been reported. 58

These reactions are fast (10 - 30 minutes) and a number of

Figure9.5 X-raystructureofN-(2-pyridylthio)-(2,4,6-triphenylphenyl)aminyl. 57