Angewandte
Chemie
D1 ¼ ꢀDd þ Dexch
ð1aÞ
ð1bÞ
ꢀ268C and ꢀ788C, respectively, resulted in almost complete
disappearance of the diradical. In the absence of iodine and
oxygen, EPR spectra of 2 in 2-MeTHF show only slight
changes in intensity upon sequential warming to ꢀ26, 0, and
228C. This remarkable persistence of 2 prompted us to
explore the isolation of the diradical.
D2 ¼ Dd=3 þ Dexch=3
For dilute solutions of 2, which behave largely like a
monomeric S = 1 diradical, another pair of outermost EPR
bands separated by 2 j Dd j appears as pentuplets (Figure 2a),
which arise from 14N-hyperfine coupling with two nitrogen
atoms in the S = 1 diradical.[9] Therefore, the largest compo-
nents of the zero-field splitting tensor (D) and hyperfine
tensor (A) are parallel to each other, and to the 2pp orbitals on
the nitrogen atoms (2pz in Figure 2a), thus indicating that
diradical 2 has an approximately planar diazapentacene
backbone, as inferred from the X-ray crystal structure of
diamine 6. This parallel orientation of the largest components
for the D and A tensors in 2 is qualitatively different
compared to the perpendicular orientation in the recently
reported S = 1 planar nitroxide diradical derived from
diamine 3,[20] thus precluding the possibility that 2 is a
nitroxide diradical.
The orientation of the largest component of the D tensor
in 2 indicates that the magnetic dipole–dipole interactions are
strongest along the z axis,[21] with the strength qualitatively
related to the spectral width 2 j Dd j (Figure 2a). This
observation suggests that the spin density is most pronounced
along the z axis in 2, and even more so in the S = 2 state of 22,
which has an approximately centrosymmetric p-dimer-like
structure (Figure S54 in the Supporting Information). Thus,
the magnetic dipole–dipole interactions in the S = 2 state of 22
are expected to be even stronger along the z axis, therefore
leading to the relatively large spectral width 6 j D2 j (Fig-
ure 2a), which both enables the spectroscopic detection of the
S = 2 state and provides evidence for a structure of the dimer.
Based upon values of D2 and Dd, and assuming that these
values have the same signs, we can estimate from Equa-
tion (1) that j D1/hc j is on the order of 1 ꢀ 10ꢀ3 cmꢀ1, which is
smaller than j Dd/hc j , as qualitatively expected for the more
spin-dilute S = 1 state of the dimer. This approximate value is
reasonably consistent with the value of j D1/hc j ꢁ 1.8 ꢀ
10ꢀ3 cmꢀ1, which was obtained from numerical simulations
of EPR spectra (Figure 2a and Figure S13 in the Supporting
Information). These results provide further support for the
structure of 22. The association constant of 22 is Kassoc ꢁ 3 ꢀ
102 mꢀ1 at 132 K (DG ꢁ ꢀ1.5 kcalmolꢀ1),[22] estimated from
numerical simulations of EPR spectra for three samples of
0.5–4.0 mm 2 in 2-MeTHF.
We devised a stringent procedure for the isolation of
diradical 2. The procedure involves generation of 30–65 mg of
diradical, followed by precipitation and filtration at low
temperature in a Schlenk vessel under an inert atmosphere.
After diradical 2 was generated from the diamine as described
above, the solvent was removed under vacuum at ꢀ788C.
Then, methanol was added by vacuum transfer in order to
dissolve salts and any excess iodine, and to precipitate the
diradical. Because of its reactivity towards methanol above
ꢀ788C, the dark blue solid aminyl diradical was filtered at low
temperature to minimize decomposition. The solid sample
(7:3 diradical/monoradical) could be stored in a glovebox at
ꢀ208C for seven months, during which time only slight
decomposition occurred. At room temperature, the concen-
tration of the S = 1 diradical decreased to 60% after five days
(Figure S21 in the Supporting Information).
The properties of 2, that is, a solid diradical that is
persistent at room temperature, facilitate the magnetic
measurements that provide an estimated DEST value with
significantly greater precision compared to low-temperature
measurements in solution. In particular, the cT versus T plot
for solid 2 is flat from 30 K to 290 K, which is the highest
temperature of the SQUID measurement (Figure S22 in the
Supporting Information), thus indicating that DEST ꢂ 2 kcal
molꢀ1. This limiting value is in qualitative agreement with the
value of DEST ꢁ 7 kcalmolꢀ1 calculated for diradical 1 using
the UB3LYP/6-31(d) level of theory.[23,24]
The isolation of diradical 2 also allows a more in-depth
study of kinetic stability of the aminyl diradical and the
derived monoradical. The stability of 2 in 2-MeTHF was
monitored by EPR spectroscopy at room temperature. The
first-order decay kinetics of diradical and monoradical
corresponds to a half-life of about 3 and 7 hours, respectively
1
(Figure 3); after several days, H NMR spectra showed that
the isolated sample was predominantly diamine 6 (Figur-
es S23–S29 in the Supporting Information). These results
suggest that the process of conversion of 2 to 6 proceeds by a
hydrogen abstraction mechanism, which is facilitated by the
One of the possible structures of 22 is modeled using a
simplified centrosymmetric dimer without long alkyl chains,
that is, 12, for which the geometry of S = 2 state is optimized
using the standard density functional level of theory
(UB3LYP/6-31G(d)).[23] This model structure possesses a
relatively long plane-to-plane distance (5.73 ꢁ) between the
diazapentacene backbones of bulky S = 1 diradicals; this
distance is consistent with the weak exchange coupling
between the diradicals (Figure S53 in the Supporting Infor-
mation).
Figure 3. Decay kinetics of diradical and monoradical in 2-MeTHF at
*
294 K ( : experimental values, c: numerical fit). The four-parameter
fit to the first-order decay kinetics (R2 =0.9999) gives rate constants
k1 =6.8ꢀ10ꢀ5 and k2 =2.9ꢀ10ꢀ5 sꢀ1 (additional details are given in
Figures S23–S31 in the Supporting Information).
EPR spectroscopy of 2 in 2-MeTHF shows that the
diradical is inert toward an excess of iodine and oxygen at
temperatures as low as ꢀ1088C, however a brief warming to
Angew. Chem. Int. Ed. 2010, 49, 5459 –5462
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5461