valence bond and spin-parity1,3a,10 models, a low-spin ground
state was observed in 6 and 7. Intermediate 4 follows
analogous behavior, surrendering the aromaticity of its
benzene rings to form an extra bond in a quinonoidal
cumulated structure (Scheme 2).
Scheme 2. Electron Pairing in 4 To Give a Diiminediyl
Structure with Bond Formation and Electron Pairing in 5 To
Give Quintet and Singlet States by Weak Spin-Pairing
Interaction
Figure 3. Baseline-corrected ESR spectrum from photolysis of
diazide 3 in 2-methyltetrahydrofuran at 76 K, ν0 ) 9.3734 GHz.
(d) Intensity data for peaks at 320 (4) and 2640 G (]). R ) radical,
N ) mononitrene, Q ) quintet 5. A spectral simulation is shown
below plot (c) for S ) 2, g ) 2.003, D/hc ) 0.287 cm-1, E/hc )
0.000 cm-1
.
diiminediyl 4 with parameters S ) 1, g ) 2.003, D/hc )
0.0822 cm-1, E/hc = 0.0 cm-1. The intensities of the triplet
biradical peaks all decreased with decreasing temperature
and disappeared below 30 K, showing the triplet to be an
excited state. The intensity versus temperature behavior is
consistent with a singlet to triplet gap of 200-30 cal/mol.
The ESR spectrum derived from photolysis of 3 is
substantially different. At 77 K, a triplet xy-transition
attributable to the mononitrene of partially photolyzed 3 is
observed that is consistent with |D/hc| ) 0.850 cm-1, |E/
hc| e 0.002 cm-1. Under no conditions did we observe a
triplet biradical set of peaks analogous to those of 4. Instead,
we observed a set of peaks at 470, ∼630, 2640, and 6000 G
(weak), which all reversibly disappeared as the temperature
was reduced below 30 K. The thermal behavior shows that
the spectrum belongs to a thermally populated excited state.
The observed spectrum fits very well to a quintet state
spectrum simulated by the eignfield method7 with parameters
S ) 2, g ) 2.003, D/hc ) 0.287 cm-1, and E/hc e 0.002
cm-1. We believe that this is quintet dinitrene 5, which does
not have a predominantly diiminediyl structure.
The difference between the ESR spectral behaviors of 4
and 5 is striking. Excited-state biradical triplet diiminediyl-
type spectra such as that assigned to 4 have readily been
observed in through-conjugated systems with substantial
conjugation lengths,4c-e,8,9 such as 6 and 7. The zfs param-
eters in these systems are sizable due to5 a dominant one-
center contribution to the D-tensor at the hypovalent nitrogen,
where the localized σ-electron and partial spin from a
delocalized π-electron interact strongly. In accordance with
Intermediate 5 acts as if its nitrene centers interact only
weakly, without sufficient pairing of nitrene π-electrons to
form an extra π-bond and a cumulated, diiminediyl structure.
The change does not appear to be due to inherent differences
in the interacting mononitrenessthe zfs parameters of the
mononitrenes from partial photolyses of 2 and 3 are
remarkably similar. But, the energy of the extra bond formed
in a quinonoidal form of 5 is apparently insufficient to make
up for the de-aromatization of the central phenylene ring.
Thus, 5 seems best described as a dinitrene, with individual
nitrene sites weakly coupled by spin polarization effects to
give an ESR silent singlet ground state lying below an ESR
active quintet dinitrene state (Scheme 2). The exchange
between the nitrene sites is insufficient to give an extra
π-bond in a biradical structure.
Wasserman11 and Itoh12 have detailed a dipolar model
describing the ESR zfs tensor behavior of high spin states
that are comprised of individual carbene or nitrene sites that
retain a large one-center interaction between localized
σ-electrons and delocalized π-electrons. The basic relation
between the zfs tensors of a quintet state (DQ) and two
interacting triplet sites that comprise it (DT) is given8 in eq
1.
(7) (a) Teki, Y. Ph.D. Thesis, Osaka City University, Osaka, Japan, 1985.
(b) Teki, Y.; Takui, T.; Yagi, H.; Itoh, K.; Iwamura, H. J. Chem. Phys.
1985, 83, 539. (c) Sato, K., Ph.D. Thesis, Osaka City University, Osaka,
Japan, 1994. (d) Takui, T. In Molecular Magnetism in Organic-Based
Materials; Lahti, P. M., Ed.; Marcel Dekker: New York, 1999.
(8) (a) Minato, M.; Lahti, P. M. J. Phys. Org. Chem. 1993, 6, 483. (b)
Ling, C.; Lahti, P. M. J. Am. Chem. Soc. 1994, 116, 8784. (c) Minato, M.;
Lahti, P. M. J. Am. Chem. Soc. 1997, 119, 2187.
If the one-center two-electron interaction in the triplet sites
is largesas it is in nitrenes and (to a lesser extent) in
(10) For more about parity models, see citations and an overview in:
Lahti, P. M. In Molecule-Based Magnetic Materials. Theory, Techniques,
and Applications; Turnbull, M. M., Sugimoto, T., Thompson, L. K., Eds.;
American Chemical Society: Washington, DC, 1996; Vol. 644, pp 218ff.
(9) Nimura, S.; Kikuchi, O.; Ohana, T.; Yabe, A.; Kaise, M. Chem. Lett.
1996, 125.
Org. Lett., Vol. 5, No. 12, 2003
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