NITRENO RADICALS - Nitrenes and Nitrenium Ions - page 325

electrons must be large. Thus, it can be concluded that p- 6b and o- 6b have well-

isolated spin-quartet ground states.

9.3 EPR SPECTROSCOPY ON IODOPHENYL NITRENES

Phenyl nitrenes 5 are conveniently generated by photolysis of the corresponding
phenyl azides. Since phenyl nitrenes have triplet ground state, they can easily be
characterized by Matrix-EPR spectroscopy after short irradiation times. All nitrenes
5 show three EPR transitions in a region that is characteristic of aryl nitrenes. The

signals represent the transitions x 2, y 2, and z 1 of randomly oriented triplet systems
with a large zero-field splitting parameter (ZFS) D , which generally show values in
the range of 0.8-1.1 cm 1 . The splitting of the x - and y -transitions in these systems,
however, is significantly larger than in most aryl nitrenes. Phenyl nitrenes

show
splittings of these two transitions in a range of a few hundred Gauss (e.g., 2-iodo-
3,4,5,6-tetrafluorophenyl nitrene o -

5

5b corresponding to a ZFS parameter j E /hc j¼

0.0071 cm 1

up to 4500 G for 3,5-Diiodo-2,4,6-trifluorophenyl nitrene m - 5d

0.094 cm 1 ). The 4-iodophenyl nitrenes p - ( j E /hc j¼ 5 have E -values up to j E /hc j¼

0.0120 cm 1 . Thus, phenyl nitrene ( 0.002 cm 1 ) 24

shows E -values close to zero. Fluorine substitution does not raise the E -value, as
2,3,5,6-tetraflluorphenyl nitrene (

0.9978 cm 1 , j D /hc j¼ j E /hc j <

0.0008 cm 1 ) also
has E -values close to zero. This indicates a significant influence of the iodine
substituent on the ZFS parameters. Hall et al. investigated several 4-substituted
phenyl nitrenes in methycyclohexane at 77 K and observed a—compared to the rest
of the investigated derivatives—very broad EPR spectrum for 4-iodophenyl
nitrene. 25 This was attributed to hyperfine coupling (HFC) with the iodine substitu-
ent. In our experiments in argon matrices no HFC was observed, but a very well-
resolved splitting between the x 2 and the y 2 transitions. It is well known that not only
spin-spin dipolar interactions contribute to the ZFS parameters but also spin-orbit
effects. 26,27 Since iodine is a heavy atom, a significant contribution of spin-orbit
coupling (SOC) to the ZFS parameters of the iodinated phenyl nitrenes

1.051 cm 1 ,

j D /hc j¼ j E /hc j¼

is very
likely. The effect of iodine substitution on ZFS is largest in the meta position and can
very easily be demonstrated by comparing the ZFS parameters of 3-iodo-2,4,5,6-
trifluorophenyl nitrene m -

5

1.033 cm 1 , 0.039 cm 1 ) with 3,5-

5b ( j D /hc j¼ j E /hc j¼

diiodo-2,4,6-trifluorophenyl nitrene m - , which bears an additional iodine substit-

uent in the meta position. To our knowledge, 3,5-diiodo-2,4,6-trifluorophenyl nitrene
shows the largest E -valueof all aryl nitrenes (

5d

0.094 cm 1 ).
The ZFS parameters of several iodinated phenyl nitrenes are summarized in Table 9.3.
The EPR spectrum of m-

1.011 cm 1 , j D /hc j¼ j E /hc j¼

is shown in Figure 9.3. On the other hand, D -values
strongly depend on the spin density at the nitrene center, which is influenced by
substituent effects. A strong linear correlation between the D -values of aryl
nitrenes and the spin densities at the nitrogen atoms was found by Wentrup
et al. 28 The D -values of the 4-iodophenyl nitrenes only roughly fit into this scheme,
which also indicates that the influence of the heavy iodine atom on the ZFS
parameters has to be taken into account. The strong contribution of SOC effects on

5d