Chemistry Reference
In-Depth Information
N
3
N
N
(H)F
F(H)
(H)F
F(H)
(H)F
F(H)
(H)F
F(H)
(H)F
F(H)
(H)F
F(H)
I
I
p
-
6
p
-
7
p
-
5
N
(H)F
N
(H)F
F(H)
F(H)
+
(H)F
(H)F
F(H)
F(H)
I
I
p
-
8
p
-
9
SCHEME 9.12.
Synthesis of nitreno radicals
p-6
.
conditions, the formation of
p-
6
competes with the rearrangement to the ketenimines
p-
with one or two fluorine atoms in the
ortho
positions are stable
enough toward rearrangement to produce nitreno radicals
p-
9
. Only nitrenes
p-
5
in yields high enough
to allow their spectroscopic detection. Thus, nitreno radical
p-
6
6i
could not be
detected either by IR or by the more sensitive EPR spectroscopy.
The formation of nitreno radicals
p-
6
is most efficient in argon at 4 K, whereas at
10 K or in neon at 4 K the yields are much lower. In several instances, the nitreno
radicals could be detected in argon at 10 K only by EPR, but not by IR spectroscopy,
whereas at 4 K clear IR signals were obtained. This is probably related to the
immobilization of radical pairs (
p-
and the iodine atom) in the matrix cages. A slight
increase of the temperature results in an efficient recombination of the radical pair
under formation of
p-
6
. This behavior is characteristic of radical pairs produced by
photolysis of matrix-isolated precursors. The radical pair is formed in a matrix cage,
and annealing leads to rapid thermal radical recombination. Nitrenes
p-
5
and the
other intermediates do not show this behavior (no reaction with N
2
under the
conditions of matrix isolation), which allows to assign signals of the nitreno radicals
p-
5
even if the yield is low. Thus, there is no way to prove the carbene character of the
nitreno radicals with matrix IR spectroscopy. The effect of argon versus neon
matrices is not understood, one can only speculate that the different shape of matrix
cages in these rare gases influences the stability of the radical pairs.
The theoretical studies with 4-dehydrophenyl nitrene have shown that the lowest-
energy doublet state of
p-
6
has the same electronic configuration as the quartet
ground state. The geometries of both states are quite similar and show only minor
bond length alternation, mostly reflected in a 0.028 A
6b
N bond in the
doublet geometry. Similar results are found for the geometries and infrared spectra of
the nitreno radicals
p-
longer C
. In Figure 9.7 , the geometries of the
4
A
2
state of quartet
6b-i
and the
2
A
2
state of doublet
p-
p-
6c
6c
are shown as typical examples for nitreno
