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R
2
R
2
R
2
R
2
N
N
N
N
H
+
N
N
N
N
R
1
N
N
N
R
1
R
1
N
N
N
R
1
H
114
115
Scheme 7.39
Schreiner
et al
. have explored heptaaza-anthracenes
114
, which are pyridine-based analogues of
113
.
176,177
These compounds are synthesized in a manner analogous to the one used to make the hex-
aazaanthracene and have similar electronic structures (ground state zwitterions, low lying triplet diradical
excited states). These derivatives are strongly solvatochromic and can be protonated (trifluoroacetic acid)
to give stable cations
115
in which the central (pyridine) nitrogen is the site of protonation (Scheme 7.39).
The influence of donor and acceptor groups on the aromatic substituents on the singlet - triplet energy
separation was explored computationally. Although the changes are modest within a series where R
1
and
R
2
are both substituted phenyl (15 - 20 kcal/mol), more dramatic changes in the electron configurational
energetics are predicted for derivatives with donor (R
1
=
NMe
2
) and acceptor (R
2
=
NO
2
) groups
attached
directly
to the polycyclic skeleton.
177
7.8 Summary
Verdazyl radicals have enjoyed steady attention over the past four and a half decades, and stand out as the
dominant species among hydrazyl-based radicals. A range of other species have received far less attention
but add to the collection of stable radicals based on the hydrazyl (R
2
NNR
•
) functional group. As such,
hydrazyl deserves to be considered as another pre-eminent “building block” for stable radicals, both in
terms of variety and stability, along with the nitroxyl (R
2
NO
•
) moiety (Chapter 5) and the thiazyl (NS
•
)
unit (Chapter 9).
References
1.
In the context of radical chemistry the term “stable” has long been used subjectively. The term “stable radical”
was originally and arbitrarily applied to any system with a sufficiently long lifetime to be observable by
“conventional” spectroscopic methods. There have been attempts to specify a “stable” radical as one which is
isolable
(see, e.g., D. Griller and K. U. Ingold,
Acc. Chem. Res.
9
, 13 - 19 (1976) and P. P. Power,
Chem. Rev.
103
, 789 - 809 (2003)) and as such distinct from
persistent
radicals, which have some degree of kinetic stability
but which cannot be isolated. Nonetheless, for the purposes of this review there is some utility in maintaining
a degree of flexibility in distinguishing between stable and persistent radicals (for example, in a strict sense
trityl would fall firmly within the “persistent” category, being a long lived but reactive radical and one which
can not be isolated as a pure compound).
2.
(a) S. Goldschmidt,
Ber. Deutsch. Chem. Gesel.
B53
, 44 - 62 (1920). (b) S. Goldschmidt and K. Euler,
Ber.
Deutsch. Chem. Gesel.
B55
, 616 - 628 (1922). (c) S. Goldschmidt, A. Wolf, E. Wolffhardt,
et al
.,
Ann. Chem.
427
, 194 - 226 (1924). (d) S. Goldschmidt and J. Bader,
Ann. Chem.
473
, 137 - 162 (1929).
3.
S. Goldschmidt and K. Renn,
Ber. Deutsch. Chem. Gesel.
B55
, 628 - 643 (1922).
4.
A. R. Forrester, J. M. Hay and R. H. Thomson,
Organic Chemistry of Stable Free Radicals
, Academic Press,
New York, 1968, 137 - 179.
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