Experimental and theoretical studies of magnetic exchange in silole-bridged diradicals

Article abstract of DOI:10.1002/chem.200501280

Five bis(tert-butylnitroxide) diradicals connected by a silole (7a-d) or a thiophene (12) ring as a coupler were studied. Compound 12 crystallizes in the orthorhombic space group Pna2₁ with a = 20.752(5), b = 5.826(5), and c = 34.309(5) A. X-ra

Full text of DOI:10.1002/chem.200501280

FULL PAPER
DOI: 10.1002/chem.200501280
Experimental and Theoretical Studies of Magnetic Exchange in Silole-
Bridged Diradicals
[
a]
[a]
[b]
[c, f]
Nans Roques, Philippe Gerbier,* Ulrich Schatzschneider, Jean-Pascal Sutter,
[e]
[
c]
[d]
[d]
Philippe Guionneau, JosØ Vidal-Gancedo, Jaume Veciana, Eva Rentschler, and
Christian GuØrin
[
a]
Abstract: Five bis(tert-butylnitroxide)
diradicals connected by a silole (7a–d)
or a thiophene (12) ring as a coupler
were studied. Compound 12 crystallizes
in the orthorhombic space group Pna21
with a = 20.752(5), b = 5.826(5), and
c = 34.309(5) Š. X-ray crystal struc-
ture determination, electronic spectros-
copy, variable-temperature EPR spec-
troscopy, SQUID measurements and
DFT computations (UB3LYP/6-31+
G*) were used to study the molecular
conformations and electronic spin cou-
pling in this series of molecules.
Whereas compounds 7b, 7c, and 7d
are quite stable both in solution and in
the solid state, 7a and 12 undergo a
partial electronic rearrangement to
both a diamagnetic quinonoidal form
and a monoradical species owing to the
fact that they correspond to the open
form of a p-conjugated KekulØ struc-
ture. In the solid state, magnetic meas-
urements indicate that the diradicals
are all antiferromagnetically coupled,
as expected from their topology. These
interactions are best reproduced by
means of a “Bleaney–Bowers” model
evidence of a populated triplet state
was found for diradicals 7a and 12.
Similarities between the DET–S and J
values (DET–S = ꢀ2 J) clearly show the
intramolecular origin of the observed
antiferromagnetic interaction. Analyses
of the data with a “Karplus–Conroy”-
type equation enabled us to establish
that the silole ring, as a whole, allows a
more efficient magnetic coupling of the
two nitroxide radicals attached to its
2,5-positions than the thiophene ring.
This superiority probably originates
from the nonaromaticity of the silole
which thus permits a better magnetic
interaction through it. DFT calcula-
tions also support the experimental re-
sults, indicating that the magnetic ex-
change pathway preferentially involves
the carbon p system of the silole.
ꢀ
1
that gives values of J = ꢀ142.0 cm
for 7a, ꢀ1.8 cm for 7b, ꢀ1.3 cm for
ꢀ1
ꢀ
1
ꢀ1
ꢀ
1
7c, ꢀ4.2 cm for 7d, and ꢀ248.0 cm
for 12. The temperature dependence of
the EPR half-field transition in frozen
dichloromethane solutions is consistent
with singlet ground states and thermal-
ly accessible triplet states for diradicals
7b, 7c, and 7d with DE
3.48, 2.09, and 8 cm , respectively. No
values of
T–S
ꢀ
1
Keywords: density functional calcu-
lations · EPR spectroscopy · mag-
netic interactions · nitroxide diradi-
cals · siloles
[
a] Dr. N. Roques, Dr. P. Gerbier, Prof. C. GuØrin
Laboratoire de Chimie MolØculaire et Organisation du Solide
UniversitØ Montpellier 2, C.C.007
[e] Prof. E. Rentschler
Institut für Anorganische Chemie und Analytische Chemie
Johannes-Gutenberg-Universität Mainz
Place E. Bataillon, 34095 Montpellier Cedex 5 (France)
Fax : (+33)467-143-852
E-mail: gerbier@univ-montp2.fr
Duesbergweg 10–14, 55128 Mainz (Germany)
[
f] Dr. J.-P. Sutter
Present address: Laboratoire de Chimie de Coordiantion du CNRS
UniversitØ Paul Sabatier, 205 route de Narbonne
31077 Toulouse (France)
[
b] Dr. U. Schatzschneider
Institut für Pharmazie und Molekulare Biotechnologie
Abteilung Chemie, Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 364, 69120 Heidelberg (Germany)
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author. It includes
UB3LYP/6-31G-optimized geometries for radicals 7a–d, CIF file and
crystal packing of 12 in the ac plane, selected bond lengths for selected
benzenoid and quinonoid structures, bond length variations in 2,5-di-
phenylthiophenes and p-terphenoquinones for the adjacent phenyl
ring and the central thiophene ring.
[
c] Dr. J.-P. Sutter, Dr. P. Guionneau
Institut de Chimie de la Mati›re CondensØe de Bordeaux
CNRS UPR 9048, UniversitØ Bordeaux 1
8
7, Av. Dr. Schweitzer, 33608 Pessac (France)
[
d] Dr. J. Vidal-Gancedo, Prof. J. Veciana
Institut de Ci›nca de Materials de Barcelona - CSIC, Campus de la
UAB, 08193 Bellaterra (Spain)
Chem. Eur. J. 2006, 12, 5547– 5562
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5547

Introduction
Organosilicon-based molecules have attracted much atten-
tion because of their unique optoelectronic properties and
their importance in applications such as photoresists, photo-
conductors, nonlinear optical materials, and light-emitting
[
1–4]
devices.
from the unusual types of conjugation that are encountered
Such interesting properties mainly originate
[5]
in polysilanes (s
A
C
H
T
R
E
U
N
G
(SiSi)-type), in compounds in which oligo-
silanylene units are alternating with carbon p systems (s-
[6]
A
C
H
T
R
E
U
N
G
(SiSi)-p type, Figure 1b), and in compounds in which sila-
Figure 2. HOMO and LUMO orbitals (6-31G* level) of 2,3,4,5-tetraphen-
yl-1,1-dimethylsilole (1) showing the s*–p* conjugation in the excited
state.
Figure 1. Different types of conjugation between two aromatic rings
through organosilicon units. a) Through-space overlap of the aromatic p-
orbitals in the butterfly-shaped diarylsilane family. b) s(SiSi)–p conjuga-
tion in the diaryldisilane family. c) s*(Si)–p* conjugation in the planar di-
arylsilane family.
A
H
R
U
G
of choice to build highly efficient light-emitting layers for
electroluminescent devices.
[2,9–12]
Bearing in mind the remarkable properties of silicon-con-
taining molecules, several works in the field of molecular
magnetism have been devoted to the syntheses and charac-
terization of model compounds based on paramagnetic cen-
ters (organic radicals or paramagnetic metal ions) linked by
nylene units are alternating with carbon p systems (s*(Si)-
[7]
p* type, Figure 1c). In general, the attachment of a silicon
atom to a p system is not an innocent act since its presence
induces appreciable electronic perturbations, which have
been exploited by organic chemists for a long time. Among
others, the Birch reduction of aromatic rings may be ade-
quately directed by organosilicon substituents since they sta-
bilize transient radical anions on the carbon atom to which
[3,9,13–20]
organosilicon units.
The connection of spin-bearing
moieties through silicon-containing units was achieved in
such a way that the magnetic interaction may take place
through the molecular skeleton via the set of conjugated
bonds available in such systems following two different ap-
proaches involving either the use of silanylene or disilan-
[8]
they are bound.
A
H
R
U
G
Another interesting example of the electronic perturba-
tion induced by the presence of a silicon atom is found in
five-membered heteropentacycles: while furan (X = O),
pyrole (X = NR), and thiophene (X = S) derivatives are
1) the silicon atom, when it is part of a single pathway
spacer, can act as a magnetic coupler, 2) as in other hetero-
A
H
R
U
G
change depends on the connectivity of the paramagnetic
centers, and finally 3) the strength of the magnetic interac-
tion strongly depends on the orbital overlap between the si-
lanylene or disilanylene coupler and the spin-bearing p sys-
tems. However, while several studies of systems with para-
magnetic centers connected through the 2,5-positions of het-
eropentacycles, such as thiophene, pyrrole or furan, have
been reported, none of them were dedicated to siloles until
we briefly described the synthesis and properties of the
silole-based diradical 7d (Scheme 1) in a preliminary com-
munication. Our objectives were: 1) to study the silole as a
colorless, silacyclopentadiene or silole (X = SiR ) deriva-
2
tives are highly colored compounds because of the lowest
HOMO–LUMO gap of the series. This characteristic origi-
nates from an unusual low-lying LUMO level associated
with s*–p* conjugation (Figure 1c) arising from the interac-
tion between the s* orbital of the two exocyclic s bonds on
the silicon atom and the p* orbital of the butadiene moiety,
as exemplified in Figure 2 for 2,3,4,5-tetraphenyl-1,1-di-
[7]
A
C
H
T
R
E
U
N
G
methylsilole (1). Consequently, siloles have a high electron
affinity and the fast electron mobility makes them molecules
5548
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Magnetic Exchange in Diradicals
FULL PAPER
Scheme 1. Silole- and thiophene-bridged diradicals.
magnetic coupler, to determine what kind of magnetic inter-
action is mediated through it, and the role of the silicon
atom in the magnetic exchange when it is incorporated into
such a kind of ring; 2) to study the photoexcited state of the
silole coupler, and 3) to try to use it as an access to photoex-
cited high-spin states following the strategy described by
state and in dilute solutions, and the experimental data has
been complemented by DFT calculations to shed some light
on magnetic interactions mediated by this peculiar silicon-
containing organometallic unit.
[21–23]
Teki et al. in the case of the anthracene coupler.
Com-
Results and Discussion
pound 7d, which belongs to the pseudo-disjoint diradical
class, allowed us to partially address the last two points: the
silole coupler possesses a photoexcited triplet state, but the
Synthesis: 2,5-Diaryl-3,4-diphenylsiloles were prepared as
outlined in Scheme 2 by an adaptation of the general proce-
[24]
[2,25]
diradical did not show any high-spin photoexcited state.
dure described by Tamao and Yamaguchi.
It involves the
However, the very weak intramolecular antiferromagnetic
interaction observed in this compound, mainly originating
from its doubly disjoint character, prevented us from clearly
determining whether the silole ring mediates magnetic inter-
actions and whether the magnetic exchange pathway in-
volves the silicon atom or not since the spin-density delocal-
ization on the silole ring is very low in these systems.
To complete the study of the silole ring as a magnetic
relay and to determine the role of the silicon atom in the
magnetic exchange in such a five-membered coupler, we
present herein a series of diradicals incorporating either the
silole or the thiophene ring as a coupler, in which the rela-
tive position of the nitroxide radicals on the pendant aryl
substituents (para versus meta, Scheme 1) has been changed
to increase the spin density on the silole bridge. The synthe-
sis as well as the structural and electronic characterization
of this series of diradicals is presented here in detail. Their
magnetic behavior has been investigated both in the solid
intramolecular reductive cyclization of bis(phenylethynyl)di-
methylsilane (2) followed by a Pd-catalyzed crosscoupling
[26,27]
reaction
quately functionalized aryl bromide. The arylhydroxyl-
amines 4a–d were synthesized by a stepwise procedure start-
between organozinc derivative 3 and an ade-
AHCTREUNG
ing from aryl dibromides that were first monolithiated with
n-butyllithium at low temperature and then treated with 2-
[28,29]
methyl-2-nitrosopropane.
Because the following step in-
volves reagent 3, which is sensitive to the slightly acidic
NOH groups, all hydroxylamine derivatives had to be pro-
tected with trimethylsilyl (TMS) groups, giving compounds
5a–d, before they could be used in the reaction. The TMS
protecting group was chosen because it is easily removed
during the hydrolysis step without the need to use a nucleo-
philic agent that might lead to unwanted ring-opening reac-
tions of the silole ring. The subsequent Pd-catalyzed cross-
coupling reaction between 3 and 5a–d followed by hydroly-
sis afforded bishydroxylamines 6a,b, and d in good yields.
Chem. Eur. J. 2006, 12, 5547– 5562
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5549

P. Gerbier et al.
Scheme 2. i) 4 equiv Np/Li, THF; 4 equiv [ZnCl
2
A
H
R
U
3
SiCl, 3 equiv Et
3
2 3 2 2
N, THF; iii) [PdCl (PPh ) ], THF; iv) 0.1m HCl; v) Ag O,
A
T
E
G
CH Cl ; vi) TMEDA, nBuLi, tBuNO, Et O; vii) 0.1m HCl; viii) Ag
2
2
2
2
2
2
.
Unfortunately, owing to its low stability, bishydroxylamine
c could not be obtained under such synthetic conditions.
amine, and oxidation. The crude nitroxides were all purified
over aluminum oxide or silica gel.
6
The silole derivatives 6a,b, and d were further oxidized to
the corresponding tert-butylnitroxide and nitronyl nitroxide
diradicals with freshly prepared silver oxide as the oxidizing
agent. As mentioned above, the low stability of 6c forced us
to synthesize diradical 7c by a slightly different approach
that involved the coupling reaction of 3 with excess dibro-
monaphthalene (to avoid the formation of silole-naphtha-
lene oligomers) to afford dibromosilole 6’. Compound 7c
was obtained in good yields from a one-pot reaction that in-
cluded lithiation, intermediate formation of the bishydroxyl-
Compound 12 was synthesized and studied as a model
compound for comparison. The synthesis of symmetrically
disubstituted thiophene-bridged diradicals followed an adap-
tation of the procedure previously described by Takahashi
et al. This involves the successive Pd-catalyzed crosscoupling
[30]
reaction of arylbromides with thienylzinc chloride.
As
outlined in Scheme 3 for diradical 12, the first step involves
the cross-coupling reaction of thienylzinc chloride with the
tert-butyldimethylsilyl
A
H
R
U
G
yl)-N-tert-butylhydroxylamine (4a-TBDMS) to afford 8 in
Scheme 3. i) nBuLi, THF; ZnCl
HF, THF; vi) PbO , CH Cl
2
; ii) 4a-TBDMS, [PdCl
2
A
H
R
U
G
3
)
2
], THF; iii) nBuLi, THF; [ZnCl
2
A
H
R
U
2 3 2
(tmeda)]; iv) 4a-TBDMS, [PdCl (PPh ) ], THF; v) aq.
A
H
R
N
2
2
2
.
5550
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Chem. Eur. J. 2006, 12, 5547– 5562

Magnetic Exchange in Diradicals
FULL PAPER
7
3% yield. Organozinc chloride 9, prepared from lithiated 8
Table 1. Selected bond lengths [Š], angles and torsion angles [8] for 7d.
[
a]
and ZnCl , was allowed to react with 4a-TBDMS as in the
X-ray
structure
Optimized
geometry
CSD data
2
first step to give 10 in 33% yield. Removal of the protecting
TBDMS with HF in aqueous THF quantitatively yielded
bishydroxylamine 11, which was subsequently oxidized with
lead dioxide in dichloromethane to afford diradical 12 as
black needle-shaped crystals upon evaporation of the sol-
vent with a 67% yield.
NꢀO
NꢀC
(CH
1.283
1.500
1.418
1.324
1.522
1.428
–
–
–
A
H
R
U
G
3 3
)
A[b]
NꢀPh
A
CꢀC in Ph
1.366 to 1.3971.392 to 1.412
1.480
1.362 to 1.391
1.498
1.854
1.873
1.354
1.501
8.8
–
A
Ph ꢀsilole
1.480
1.398 to 1.409
1.494
1.916
1.914
1.369
1.520
2.0
–
–
–
B
CꢀC in Ph
B
Ph ꢀsilole
SiꢀCH
1.869
1.876
1.358
1.497
–
–
–
–
–
–
3
Geometries of the silole-bridged diradicals 7a–d: The deter-
mination of the conformational preferences of these diradi-
cals is of utmost importance for the understanding of their
magnetic behavior. Because crystals suitable for an X-ray
structure determination could only be obtained for 7d (Fig-
SiꢀC2/C5
C=C
C C
ꢀ
B
B
B
Ph ꢀC=CꢀPh
A
Ph ꢀCꢀCꢀPh
4.2
6.0
A
[19]
Ph ꢀCꢀSiꢀCH
CꢀC=C
3
50.9 and ꢀ74.2
54.9 and ꢀ71.9
ure 3a), we turned to density functional theory (DFT) cal-
116.0
116.1
C=CꢀSi
107.9
108.4
108.4
109.8
CH
CH
3
ꢀSiꢀCH
3
ꢀSiꢀC2/C5
109.7and 118.4 109.8 and 111.5
–
–
3
A
Ph ꢀNꢀC
A
H
R
U
G
)
126.5
116.2
117.3
47.2
126.5
116.7–
116.8
53.2
55.7–
85.9
3
3
A
Ph ꢀNꢀO
C
A
H
R
U
G
3
)
3
ꢀNꢀO
–
–
A
B
siloleꢀPh
60.4
(
CH
3
ꢀSiꢀCH
3
)ꢀsilole 82.5
–
–
A
Ph ꢀ
A
H
R
U
G
17.0
14.3
[
a] Average data for 14 independent molecules in 12 structures, only
structures not containing coordinated transition metals were considered.
b] See Scheme 1 for the labeling of the aromatic rings.
[
other purely organic siloles obtained from the CSD data-
[35]
base. Geometrical parameters of 7d, obtained from X-ray
analysis are, in general, very well reproduced, including the
bond and torsion angles. The well-known over-estimation of
bond lengths by density functional methods, especially mani-
fest in the carbon–heteroatom bonds, does not exceed 3.3%
in any case.
The bond lengths and angles of the silole moiety and
phenyl rings in the other diradicals are very similar to those
of 7d. Moreover, the geometrical parameters of the radical
substituents are in the range of the data reported for other
tert-butylnitroxide radicals. Therefore, this discussion will
focus on the torsion angles between the aromatic spin-bear-
ing units, the phenyl rings, and the central silole group,
which are collected in Table 2. Coordinates of the minimum
geometries for 7a to 7d are included in the Supporting In-
formation (Tables S1 to S4). These torsion angles should de-
termine the degree of delocalization/spin polarization of the
Figure 3. a) X-ray structure of 7d. b) Optimized UB3LYP/6-31G geome-
try of 7c.
culations with the UB3LYP functional to obtain information
about the molecular conformations for the rest of the diradi-
[31–33]
cals.
Owing to the size of the molecules, geometry opti-
mizations without symmetry constraints were performed
with the 6-31G basis set to the standard convergence criteria
[
a]
[34]
Table 2. Torsion angles [8] in diradicals 7.
as implemented in Gaussian98. Such calculations were fol-
*
7a
7b
7c
7d (opt.) 7d (X-ray)
lowed by single-point runs using a 6-31+G basis with tight
A[b]
B
silole Ph
ꢀ
41.1
58.759.1
)ꢀsilole 85.2
41.3
72.9/71.0 53.2
53.8/54.0 55.7 60.4
47.2
convergence and the ultrafine integration grid to obtain ac-
curate energies and spin densities. Structurally characterized
silole-bridged diradical 7d served as a benchmark to test
how well the experimentally determined geometry is repro-
duced by the calculations. Some relevant bond lengths,
angles, and torsion angles are collected in Table 1 and are
siloleꢀPh
(
CH
3
ꢀSiꢀCH
3
85.5
87.7
85.9
82.5
17.0
–
A/C
Ph
ꢀ
A
H
R
U
G
3.1/2.9 6.4/6.5 74.3/76.5 14.3
A
Ph ꢀC
N
2
–
–
–
33.6
–
–
–
–
3
A
C
Ph ꢀPh
–
[
a] Only one entry if both values are identical. [b] See Scheme 1 for the
also compared to the mean values for the core SiC ring of
labeling of the aromatic rings.
4
Chem. Eur. J. 2006, 12, 5547– 5562
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5551

P. Gerbier et al.
[
36]
[37]
unpaired electrons from the radical moieties onto the silole
core of the molecule.
All compounds have a propeller-like arrangement of the
four phenyl rings, as found in the crystal structure of 7d
tures SUSNEZ
and FEJRUH. The torsion angles be-
tween the nitroxide moiety and the phenyl ring are 20.98
and 3.28 for molecule A and 23.18 and 1.88 for molecule B,
respectively. While the first torsion angle values are not un-
(
Figure 3a) and also reported for other tetraphenyl-substi-
common in phenyl-substituted nitroxide radicals (see
[2,19]
[38,39]
tuted siloles,
while the two methyl substituents on the sil-
above), the second ones are unusual,
and closer to the
icon atom are nearly perpendicular to the mean plane of the
range of values reported for benzoquinonimine-N-oxide de-
[40,41]
SiC ring. Therefore, the molecular geometries approach C
rivatives (3.38).
To check whether this structure reflects
4
2
symmetry, even though the optimizations were performed
without any constraints. The torsion angles of the nonsubsti-
tuted phenyl rings at the 3- and 4-positions of the central
silole ring are larger than those of the radical-bearing
phenyl rings in the 2- and 5-position owing to the larger
steric interactions with two neighboring rings in the former
as compared with just one for the latter ring. An exception
is found for diradical 7c, in which the larger size of the aro-
matic groups, caused by the additional fused rings D, enfor-
ces a torsion angle between the naphthyl and silole ring
which is 20—-308 larger than in the others, leading to an
almost perpendicular arrangement of the two rings (Fig-
ure 3b). The same degree of steric hindrance is also seen be-
tween the naphthyl and nitroxide groups in 7c, leading to
large torsion angles of 74.38 and 76.58, while in the cases of
either a benzenoid or a quinonoid form in the crystal lattice,
the 2,5-diphenylthienyl bond lengths for 12, crystal struc-
[36]
[37]
tures SUSNEZ,
FEJRUH (benzenoid forms),
[42]
[43]
VIZBUB and QIPQAH (quinonoid forms) were com-
pared (see Figure S2 and Table S5 in the Supporting Infor-
mation). Interestingly, the bond lengths in the thiophene
ring are not greatly affected by the structural modifications
that accompany the transformation from a benzenoid form
to a quinonoid form. More pronounced is the elongation of
the C3ꢀC4 and C4ꢀC5 bonds in the adjacent phenyl rings
that reaches ꢁ10% and the concomitant shortening of the
C2ꢀC3 and C5ꢀC6 bonds (ꢁ5%). Therefore, since the
bond lengths found for compound 12 are closer to the range
of values measured for benzenoid forms, we assume that the
diradical probably adopts this structure in the crystal.
7
a and 7b, the phenyl ring and radical-bearing units are
almost coplanar, and only a slight deviation from planarity
is found in 7d with torsion angles of 17.08 and 14.38 for the
X-ray and optimized geometries, respectively. The torsion
angle between both phenyl rings in the biphenyl unit of 7b
Electronic absorption spectra: The UV/Vis spectra of freshly
prepared diradicals and their parent hydroxylamines were
measured in chloroform. The resulting data are summarized
in Table 3, which also contains the data on silole 1 for com-
[35]
is 33.68.
Geometry of the thiophene-bridged diradical 12: Crystals of
2 obtained by slow evaporation of a dichloromethane solu-
Table 3. UV/Vis absorption spectral data for dihydroxylamines 6 and di-
radicals 7.
[
a]
1
tion were subjected to X-ray diffraction analysis at 150 K.
Compound
1
6a
7a
6b
7b
6c
7c
6d
7d
Compound 12 crystallizes in the orthorhombic crystal
[b]
p–p*Ar–NO
p–p*silole
n–p*NO
–
359
–
–
366
–
300
420
448
–
391
–
323
400
451
–
351
–
296
363
450
–
360
–
285
362
450
system in the Pna2 space group (Figure 4). The crystal lat-
1
ꢀ
3
[
3
a] 10 m solutions in CHCl . [b] Transition lmax values given in nm.
parison. For all silole derivatives, the spectra are dominated
by a broad absorption at lꢁ350–420 nm that is characteris-
[2]
tic of the p–p* transition originating from the silole ring.
The diradicals are characterized by two additional absorp-
tion bands at lꢁ300 nm and 450 nm attributed to the Ar–
NO p–p* and NꢀO n–p* transitions, respectively. From an
examination of the peak positions of the p–p* transitions
Figure 4. ORTEP view of 12 (50% probability).
(
Table 3), it follows that these transitions are affected by
three factors: 1) by the nature of the aryl groups in the 2,5-
positions (phenyl versus naphthyl), 2) by the electron-with-
drawing effects induced by the substituents, and 3) by the
substituent position relative to the silole ring. Therefore, the
conversion of the NOH groups to NOC leads to a bathochro-
mic shift of the silole ring p–p* transition in every case. A
minor red-shift of 2 nm is also observed for diradicals 7d
with the nitroxide groups in a meta orientation with respect
to the silole ring. Although they are connected in a para ori-
entation, which should promote larger electronic effects of
tice structure consists of pairs of independent molecules A
and B forming an herringbone pattern along the b axis (see
Figure S1 in the Supporting Information). An examination
of the molecular structure shows that the mean plane of the
terminal phenyl rings is tilted relative to the central thio-
phene ring by 5.08 and 8.58 for molecule A, and 11.88 and
7.28 for molecule B, respectively. These values, as well as
the bond lengths of the p system, are in the range of those
reported for the related 2,5-diarylthiophenes crystal struc-
5552
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Chem. Eur. J. 2006, 12, 5547– 5562

Magnetic Exchange in Diradicals
FULL PAPER
the substituents, 7b and 7c only show a moderate red-shift
of 9 and 12 nm, respectively, compared to the 54 nm of di-
A
C
H
T
R
E
U
N
G
radical 7a, which thus exhibits the largest shift in the series.
Such differences might be ascribed to the different torsion
angles as revealed by the molecular structures (Table 2).
Thus, changing a biphenyl (7b) to a naphthyl (7c) ring does
not seem to markedly affect the overall electronic effects in-
duced by the oxidation of the hydroxyl groups because the
combination of torsion angles and distances lowers p-conju-
gation. Diradical 12, with a lower steric hindrance and
larger conjugation, however, shows more drastic changes
upon oxidation. The unique sharp absorption band observed
at 344 nm for the bishydroxylamine, experiences a large red-
shift to yield a very broad absorption at 391 nm, whereas
three new absorption bands appear at 444, 479, and 551 nm.
These modifications strongly suggest that diradical 12 under-
goes an electronic reorganization in solution (vide infra).
The structural stability of the silole diradicals was also
checked by UV/Vis spectroscopy. With the exception of 7a,
the spectra of all the siloles remained identical for more
than two weeks in solution and for several months in the
solid state. In contrast, four new absorption bands at 309,
Figure 5. Room-temperature EPR spectra (in CH
a) freshly dissolved and b) after three days in solution.
2 2
Cl ) of diradical 7a:
Table 4. Apparent hyperfine coupling constants [Gauss] and diradical pu-
rities [%] obtained by simulation of EPR spectra of freshly prepared
dilute solutions.
Diradical
7a
7b
7c
7d
12
a
N
5.65
90
5.81
95
6.84
979789
6.59
5.97
diradical purity
364, 401, and 570 nm appear in the spectrum of 7a when the
diradical remains in solution for one day. These new absorp-
tion bands are similar to those observed for 12, indicating
that both diradicals experience the same electronic reorgani-
zation. Actually, a similar behavior was observed for diradi-
cals that correspond to an open-shell resonant form of a
case, the exchange coupling parameter J is substantially
larger than the nitrogen hyperfine coupling constant (jJj@
a ). In accordance with the UV/Vis data (vide supra), the
N
spectra of degassed solutions of diradicals 7b–d remain un-
changed for at least two weeks when stored in the dark,
while the spectra of 7a and 12 recorded after one day show
several modifications indicative of a structural/electronic re-
arrangement in solution. Such modifications include an al-
teration of the relative peak intensities of the quintet and
the appearance of a new hyperfine structure in the two
outer lines and the central line that might be attributable to
the concomitant presence of the diradical and a monoradical
that have similar chemical structures. As previously de-
[44]
closed-shell structure.
Basically, diradicals that are con-
nected through the para position of a phenyl ring of a more
extended p system quickly undergo an electronic rearrange-
ment of their backbone leading to diamagnetic quinonoidal
[
30,40,41,45–48]
structures.
quinonoidal structures are characterized by p–p* transitions
From the spectroscopic point of view,
+
ꢀ
[41,49]
at about 320, 404 (C=N -O ),
and 550 nm (terpheno-
quinone). The data strongly suggests that diradicals 7a and
2 undergo an electronic rearrangement to adopt quinonoi-
[
30]
1
[46]
dal structures, as previously found in related systems.
A
H
R
U
G
species derived from bisnitroxides in solution might origi-
nate from the abstraction of an hydrogen atom from the sol-
vent by the reactive bisnitrones that result from the quino-
noidal rearrangement of the parent diradicals (Scheme 4).
As a result, a monoradical hydroxylamine is generated,
Electron paramagnetic resonance spectra: EPR spectra of
the diradicals in degassed dichloromethane solutions were
measured in the temperature range 4–298 K. At 298 K, the
spectra of siloles 7a–d and 12 (Figure 5a) show a well-re-
solved symmetrical pattern con-
sisting of five lines with intensi-
ties close to 1:2:3:2:1 attributed
to hyperfine coupling between
two equivalent nitrogen nuclei.
The values found for the appa-
rent hyperfine coupling con-
stants with the
N
atom
(
Table 4) are, as expected, one
half of the related constant
found for monoradical deriva-
tives (vide infra). Moreover, the
spectra of such bisnitroxide di-
A
C
H
T
R
E
U
N
G
radicals indicate that, in each Scheme 4. Quinonoidal rearrangement of diradical 7a.
Chem. Eur. J. 2006, 12, 5547– 5562
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5553

P. Gerbier et al.
[
19]
which gives rise to the additional structured signals that are
observed in the EPR spectrum overlapped to the diradical
signal. For 7a, no further evolution is noticed in the EPR
spectra after three days and the resulting pattern can be
fully simulated as a mixture of 83% diradical of 7a and
groups is slightly smaller. This result is in agreement with
the large torsion angles that are found for 7c (Table 2) that
disrupts p conjugation and decreases spin delocalization.
Decreasing the temperature below 80 K allows the obser-
vation of a signal at a field of ꢁ1715 G that is attributed to
the DMS = 2 transition in the thermally populated triplet
state of diradicals 7b–d. This half-field signal is not observed
for 7a and 12, even when large signal amplification and high
concentrations are used. On further lowering of the temper-
1
7% of a para-substituted phenyl-tert-butylnitroxide mono-
radical with the following hyperfine coupling constants: aN
11.4 G, a 2.08 G and aHmeta 0.85 G (Fig-
=
=
=
Hortho
[50]
ure 5b). Similar hyperfine coupling constants were found
for diradical 12 after three days in solution. From the simu-
lated spectra, a mixture of ꢁ25% of monoradical and 75%
of diradical was obtained. Interestingly, this composition re-
mains almost constant until the disappearance of the EPR
signal is observed after a period of three weeks in solution.
EPR spectra of freshly prepared diradicals 7a–d and 12 in
glassy matrices at 100 K gave broad signals that show a fine
structure. These signals correspond to the intermolecular
DMS = 1 transition of diradicals and originate from the
weak dipolar coupling of the two unpaired electrons
ature, a pronounced increase in the intensity of the DM =
S
1 transition is noticed for diradicals 7b–d, whereas a differ-
ent behavior is observed for 7a and 12. For the latter com-
pounds, the signal intensity decreases to a minimum at 35 K
for 7a and at 50 K for 12, and then slightly increases down
to the lowest temperatures attainable. It is worth noting that
the increase of the DM = 1 signal at very low temperatures
is more pronounced with aged samples. Since the intensity
of the DM = 1 transition is proportional to the molar para-
magnetic susceptibility of the sample, this behavior might
originate from the simultaneous presence of either the di-
S
S
(
Figure 6). The determination of zero-field splitting (ZFS)
A
H
R
U
G
place between the unpaired electrons, and the monoradical
species; as suggested by the isotropic room temperature
spectrum (vide supra).
To determine the nature and the strength of magnetic
coupling between the spin-bearing moieties in diradicals
7
b–d, the intensity of the DM = 2 transition was measured
S
as a function of the temperature between 4 and 30 K in
frozen dichloromethane. In each case, as the temperature
Figure 6. Typical EPR spectrum obtained in frozen dichloromethane at
K for diradicals 7a,c,d, and 12.
4
was decreased, the DM = 2 signals, attributed to the triplet
S
state, increased in intensity. A plot of the dependence of the
intensity of this signal versus the reciprocal of the absolute
temperature (Curie plot) is given in Figure 7for diradical
[51]
parameters from the simulation of the DMS = 1 signals
could only be carried out for diradicals 7a,c,d, and 12
Table 5) because dipolar interactions in diradical 7b are
(
S
Table 5. EPR parameters used for the simulation of the DM = +1 sig-
[
a]
nals of diradicals in frozen solution at 100 K.
Diradical
g
x
g
y
g
z
D
A
H
R
N
[Gauss]
E
A
H
R
U
G
7
7
7
1
a
c
d
2
2.005
2.005
2.008
2.005
2.005
2.005
2.005
2.005
2.005
2.005
2.002
2.005
30
12.5
16
0
0
0
0
31
[
a] In dichloromethane glass.
weak compared to the hyperfine coupling constants. The
average interspin distances were estimated to be 10 Š for
diradicals 7a and 12 and 13 Š for diradical 7c using the
point-dipole approximation. A comparison of the average
interspin distances with the distance between the two NO
groups obtained from the DFT-optimized geometries
Figure 7. Temperature dependence of the EPR signal intensities of the
DM
S
= 2 transition for diradical 7b in frozen dichloromethane. The
dashed curve shows the evolution of the signal using the spin-pair Blea-
ney–Bowers model (see text). The dotted line shows the variation of the
signal expected for uncorrelated spins.
[52]
(
NO···NO: 15.3 Š for 7a and 12, and 15.2 Š for 7c) leads to
the conclusion that the spin delocalization through the aro-
matic rings is much more effective in 7a and 12 than in 7c
or 7d. Moreover, the D parameter of 7c is even smaller
than that of 7d in which the distance between the NO
7b. The observed deviation of the intensity from the Curie
Law strongly suggests that diradicals 7b–d exist in a singlet
ground state with an accessible thermally populated triplet
state.
5554
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Chem. Eur. J. 2006, 12, 5547– 5562

Magnetic Exchange in Diradicals
FULL PAPER
To evaluate the singlet–triplet energy gap, DET–S/k , the
B
Curie plots for diradicals 7b–d were analyzed with the Blea-
ney–Bowers model [Eq. (1)], which describes the magnetic
[53]
behavior of isolated diradicals. The best fit of the experi-
mental data to Equation (1), where C is a constant to fit the
sample intensity, and DET–S is the singlet–triplet energy gap,
indicates that the energy separation between the accessible
thermally populated triplet state and the singlet ground
ꢀ1
state are 3.5, 2.1, and 5.6 cm for diradicals 7b, 7c, and 7d,
respectively.
ꢀ
ꢃ
C
1
IESR ¼ T
ꢁ
ꢂ
DETꢀS
kBT
ð1Þ
3
þ exp
[54]
Magnetic susceptibility measurements:
The magnetic
properties of diradicals 7a–d and 12 were also investigated
on polycrystalline samples in the temperature range of 2 to
300 K. The temperature dependence of the molar magnetic
susceptibility, c , for compounds 7a, 7c, and 12 is given in
M
Figure 8 in the form of a plot of c T versus T since they are
M
representative of the two types of magnetic behavior that
are observed for this series of diradicals. For diradicals 7b–
3
ꢀ1
d, the c T value of ꢁ0.63 cm Kmol remains constant
M
from 300 K down to ꢁ20 K and a slight decrease at lower
temperatures is observed. Such behavior is indicative of
very weak antiferromagnetic interactions between the spin
carriers. In contrast, c T values of 0.33 cm Kmol and
.28 cm Kmol were found at 300 K for compounds 7a and
3
ꢀ1
M
3
ꢀ1
0
1
2, respectively. Such values are well below the expected
1
contribution of two noncorrelated
S
=
=
2
spins
3
ꢀ1
(
0.75 cm Kmol ). Moreover, the c T values rapidly de-
M
crease as the temperature is lowered, finally reaching a pla-
3
ꢀ1
teau value of 0.06 cm Kmol
0
below 28 K for 7a and
3
ꢀ1
.10 cm Kmol below 55 K for 12. For these two com-
pounds, rather strong antiferromagnetic interactions are op-
erative between the two radical units. In the lower tempera-
ture domain, the observation of non-zero c T values is as-
M
cribed to the presence of a minor fraction of monoradical
species in the samples that result from the structural/elec-
tronic rearrangement and hydrogen abstraction of 7a and
Figure 8. Temperature dependence of the experimental (&) and calculat-
ed (c) c T behavior for a) 7a, b) 7c, and c) 12.
M
1
2, as shown by the EPR and the UV/Vis studies described
above.
The magnetic behavior of compounds 7b–d has been ana-
lyzed by a modified dimer model with the spin Hamiltoni-
[
53,55]
an.
The corresponding expression for c T is given by
M
ꢀ1
the experimental data yielded J = ꢀ1.8 cm with f = 0.84
for 7b, J = ꢀ1.3 cm with f = 0.81 for 7c, and J =
4.2 cm and f = 0.96 for 7d. The introduction of a Weiss
Equation (2):
ꢀ
1
ꢀ1
ꢀ
ꢀ 2
ꢃ
2
2
Ng b
1
constant, q, in the analytical expression did not significantly
modify the resulting parameters. These exchange parameters
are very similar to the singlet–triplet gaps (DET–S = ꢀ2 J)
obtained from the glassy matrix EPR studies and, therefore,
they can be attributed to an intramolecular interaction
taking place between the two radical units.The magnetic be-
havior for compounds 7a and 12 has also been analyzed by
a dimer model, but including a paramagnetic contribution
cMT ¼ f
ð2Þ
kB
3 þ expðꢀ2J=k TÞ
B
where J represents the exchange parameter, g is the isotrop-
ic LandØ constant, b is the Bohr magneton, and kB is the
Boltzmann constant. A purity factor, f, was introduced to ac-
count for the possible presence of diamagnetic fractions
[56]
caused by the instability of these diradicals. The best fit to
Chem. Eur. J. 2006, 12, 5547– 5562
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5555

P. Gerbier et al.
1
arising from monoradical S = = species. The rather low
The spin density on the spin-bearing units is as expected,
with a large positive spin density on the NO groups. The
spin density then spreads out onto the adjacent phenyl ring
A (see Scheme 1) on account of the direct delocalization of
the unpaired electron onto the phenyl ring in the latter, and
2
values observed for c T at 300 K suggest the presence of a
M
nonmagnetic fraction and, therefore, the ratios of di- and
monoradicals have been taken as relative to the whole
sample. The expression used to analyze the experimental
data is given by Equation (3):
[57,58]
follows the sign alternation principle.
The only excep-
tion is found for naphthyl-substituted 7c, in which the large
NO–naphthyl torsion angles of 74.38 and 76.58 prevent de-
localization of the unpaired electron from the NO group,
therefore leading to spin densities on the A ring of the
naphthyl group that are one order of magnitude smaller
than in the other tert-butylnitroxides 7a, 7b, and 7d, while
the fused ring D carries an even smaller spin density. Fur-
thermore, in the biphenyl-substituted compound 7b, only
ring C, directly connected to the NO group, carries a spin
density comparable to 7a and 7d, while on ring A, the spin
density is smaller by a factor of ꢁfive owing to the Ph–Ph
torsion angle of 33.68, which prevents further delocalization
of the unpaired electron from ring C to ring A. This diradi-
cal is another example of conformational effects modulating
the spin density distribution in this series of compounds.
The spin density on the silicon atom is small (<0.01) in
all cases and the sign systematically depends on the position
(meta versus para) where the spin-bearing moiety is attach-
ed to the aromatic ring. It also correlates with the amount
of spin density on the phenyl ring A attached to the central
silole ring. In cases with little spin density on ring A, either
because it only originates from
ꢀ
ꢃ
ꢀ
ꢃ
2
2
2 2
2
Ng b
Ng b
cMT ¼ a
þ b
ð3Þ
k ½3 þ expðꢀ2J=k TÞ
2k
B
B
B
where a and b refer to the molar fraction of di- and mono-
radical species, respectively. The best fit of the experimental
data, taking g = 2, for 7a yielded an exchange parameter
ꢀ1
J = ꢀ142Æ0.3 cm with a sample composition in magnetic
species of 51% of diradical and 15% of monoradical. For
compound 12, the resulting parameter was: J = ꢀ248Æ
ꢀ
1
1
2
cm with a sample composition of 39% of diradical and
5% of monoradical.
Theoretical calculation of spin densities and exchange pa-
rameters: The atomic spin densities calculated in single-
point runs at the UB3LYP/6-31+G* level are collected in
Table 6 for tert-butylnitroxides 7a–d. Spin densities on the
tert-butyl group are not included because they are very
small and comparable to values observed both experimen-
tally or deduced by calculation for other radicals of the
[
38,57–59]
same kind.
Table 6. Mullikan atomic spin densities for 7a–d calculated at the UB3LYP/6-31+G* level.
spin polarization or owing to
conformational constraints that
[
a]
[b]
[b]
Atom
7a
7b
7c
7d
7d
Optimized
geometry
Optimized
geometry
Optimized
geometry
X-ray
structure
Optimized prevent efficient delocalization
geometry
of the unpaired electron (7b
N
O
0.3757/0.3747
0.4714/0.4728
0.3719/0.3785
0.4809/0.4765
0.4304/0.4326
0.5206/0.5235
0.4017
0.4764
0.4020
0.4789
and 7c), the spin density on the
silicon atom is very small
C
C4 in Ph
ꢀ0.1200/ꢀ0.1185
(<0.002). In contrast, in radicals
C
C
C3/C5 in Ph
C2/C6 in Ph
0.1352/0.1320
7
a and 7d, which have larger
0.1264/0.1285
ꢀ0.0631ꢀ0.0687
spin densities on phenyl ring A,
the spin density on the Si atom
is larger by a factor of three to
five (7a: 0.0054, 7d: ꢀ0.0051
(X-ray), ꢀ0.0061 (optimized)).
The spin density on the methyl
substituents on the silicon atom
and the 3- and 4-phenyl rings B
is very small and approaches
zero in all compounds. Interest-
ingly, no clear correlation be-
tween the sign and magnitude
of the spin density and confor-
mational preference can be
drawn for the carbon atoms of
the silole ring, especially in
those cases with no imposed
symmetry (7a–d). In general,
there is little unpaired spin on
the carbon atoms (<0.01 in
almost all cases); however, the
ꢀ
0.0712/ꢀ0.0671
C
C1 in Ph
C4 in Ph
C3/C5 in Ph
0.1175/0.1177
A
ꢀ0.1306/ꢀ0.1293
ꢀ0.0240/ꢀ0.0226
ꢀ0.0233/ꢀ0.0320
0.0349/0.0307
0.0165/0.0307
ꢀ0.0111/ꢀ0.0085
ꢀ0.0070/ꢀ0.0063
0.0204/0.0152
0.0073/0.0059
ꢀ0.0053/ꢀ0.0041
0.0073/0.0057
ꢀ0.0026/ꢀ0.0021
0.0017
0.1461
ꢀ0.0588
ꢀ0.1351
0.1196
0.1270
ꢀ0.0607
0.1634
ꢀ0.0652
ꢀ0.1487
0.1238
0.1346
ꢀ0.0696
A
A
0.1257/0.1261
0.0226/0.0220
0
.1306/0.1325
0.0222/0.0220
C2/C6 in Ph
ꢀ0.0679/ꢀ0.0687
ꢀ0.0127/ꢀ0.0138
ꢀ0.0123/ꢀ0.0121
0.0232/0.0219
ꢀ
0.0653/ꢀ0.0650
A
C1 in Ph
C7in Ph
C8 in Ph
C9 in Ph
C10 in Ph
Si
0.1228/0.1181
D
D
D
D
0.0054
0.0012
ꢀ0.0051
0.0008
0.0069
ꢀ0.0061
0.0005
0.0093
SiꢀCH
ꢀ0.0013/ꢀ0.0001
0.0277/ꢀ0.0062
0.0125/0.0395
ꢀ0.0001/0.0000
ꢀ0.0025/ꢀ0.0003
0.0045/0.0062
ꢀ0.0009/0.0016
ꢀ0.0001/0.0000
ꢀ0.0001/0.0146
0.0084/ꢀ0.0218
0.0001/0.0008
3
SiloleꢀC2/C5
SiloleꢀC3/C4
ꢀ0.0085
ꢀ0.0081
ꢀ0.0010
0.0002
ꢀ0.0059
0.0035/
ꢀ0.0002
ꢀ0.0002
B
C1 in Ph
ꢀ0.0060/ꢀ0.0062
0.0003
B
C2/C6 in Ph
0.0044/0.0042
0.0008/0.00070.0000/0.0000
ꢀ0.0001
ꢀ0.0043
ꢀ0.0006/
0.0028
0
.0039/0.0038
0.0007/0.0007
ꢀ0.0004/ꢀ0.0003
ꢀ0.0002/ꢀ0.0001
0.0007/0.0006
ꢀ0.0002/0.0001
B
C3/C5 in Ph
ꢀ0.0019/ꢀ0.0018
0.0001/0.0000
0.0002/0.0000
ꢀ0.0001/0.0001
ꢀ
0.0013/ꢀ0.0012
B
C4 in Ph
0.0032/0.0033
0.0004
[
a] See Scheme 1 for the labeling scheme. [b] Only one value is reported because the two halves of the mole-
cule are identical on account of to its C symmetry.
2
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Chem. Eur. J. 2006, 12, 5547– 5562

Magnetic Exchange in Diradicals
FULL PAPER
magnitude and sign are in some cases different for the C2/
C5 and C3/C4 atoms, even though all molecules possess
tems usually display singlet ground states with low-lying ex-
cited multiplet states. On the other hand, the attachment of
two para-phenylnitrene units provide a connection through
spin-bearing sites that leads to localized quinonoidal diradi-
cal systems because they actually correspond to an open-
shell form of a KekulØ p-conjugated molecule (Scheme 5b).
With respect to this classification, we will divide the mole-
cules described herein into two main categories. Diradical
7d will be classified in the pseudo-disjoint category and the
others will be classified into the non-pseudo-disjoint one.
Obviously, in all cases, antiferromagnetic interactions be-
tween the paramagnetic centers are expected both from a
valence-bond and spin-polarization point of view
(Scheme 6). However, a careful examination of their mag-
near-C symmetry. This might be caused by subtle differen-
2
ces in the conformations of the two halves of the molecules
or by a slight contamination of the triplet states by close-
ˆ
2
lying higher spin states, as is apparent in the deviation of S
from the expected value of 2.0 (7a: 2.0292, 7b: 2.0284, 7c:
2.0100, 7d: 2.0211 (X-ray), 2.0215 (optimized)).
To test if the computational methods used are able to cor-
rectly reproduce both the sign and magnitude of the ex-
change parameter J determined experimentally for 7d, the
energies of the broken-symmetry singlet and high-spin trip-
let states of 7d were determined at the X-ray structure coor-
dinates and optimized geometry with high accuracy
*
(
UB3LYP, 6-31+G basis set, tight convergence, ultrafine
ꢀ
1
grid). The values calculated for J of ꢀ0.55 cm (X-ray) and
ꢀ1
ꢀ
0.39 cm (optimized) from the S–T gap without spin pro-
ˆ
ˆ
1
jection using a Hamiltonian of H = ꢀ2JS SL are somewhat
2
ꢀ
1
smaller than the experimental values (J = ꢀ4.2 cm ,
ꢀ
1
SQUID, microcrystalline powdered sample and ꢀ5.6 cm ,
EPR, frozen solution); however, they do confirm the weak
antiferromagnetic coupling mediated by the silole ring in
7
d.
Magnetic coupling mechanism in silole-bridged diradicals:
There are two main issues associated with the experimental
results described here: first, how do nitroxide radicals inter-
act magnetically when linked by a silole ring? Second, what
is the role of the silicon atom in mediating the magnetic in-
teraction when incorporated in the silole ring? With respect
to the first issue, some work has recently been dedicated to
the study of systems with paramagnetic centers connected
through the 2,5-positions of heteropentacycles such as
furans, pyrroles and thiophenes, since these systems with
non-alternant conjugation may lead the way to a large
number of intramolecular magnetic exchange possibili-
Scheme 6. Expected magnetic interaction in silole-bridged diradicals on
the basis of the spin-polarization mechanism. Grey circles depict regions
with positive spin density.
netic properties will be useful to address the second ques-
tion concerning the role of silicon in the mediation of mag-
netic interactions in the silole coupler.
As mentioned above, the pseudo-disjoint category only in-
cludes diradical 7d, which displays weak intramolecular an-
tiferromagnetic interactions in the solid state as well in a
[19]
frozen solution. At first glance, the nearly planar confor-
mation around the silicon atom in the silole linker accounts
for the different behavior with respect to the flexible dini-
trene 13 (Scheme 7) for which no magnetic interaction was
[48,60–62]
ties.
Among them, the thiophene ring has been shown
to act more efficiently than the benzene ring itself since it is
more electron-rich and it is sterically less demanding than
[61]
the latter. The influence of the spin-bearing sites connect-
ed to five-membered rings has also been extensively studied.
For instance, the connection of two meta-phenylnitrene
units leads to systems termed as pseudo-disjoint because
spin-bearing units are connected to the central ring through
sites bearing minimal spin density (Scheme 5a). These sys-
Scheme 7.
[63]
observed. Though the spin density on the silicon atom is
quite low in all cases (Table 6), this observation suggests
that it might be involved in the exchange interaction path-
way. Since the exchange interaction can either be mediated
by the p system of the silole ring or by a s pathway involv-
ing the silicon atom, the calculations were repeated for the
Scheme 5.
Chem. Eur. J. 2006, 12, 5547– 5562
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5557

P. Gerbier et al.
singlet and triplet states of model compound 14 in which the
silicon atom was replaced by a sp -hybridized carbon atom,
the silole ring and the thiophene ring intrinsically act as an-
tiferromagnetic couplers with the same efficiency. The pres-
ence of the additional phenyl rings at the 3,4-positions of
the silole ring for 7a induces torsion angles for the spin-
bearing units that are responsible for the low value of the
exchange parameter compared to 12. On the other hand,
the increased stability of 12 probably originates from the ar-
omatic character of the thiophene compared to the silole
unit. Indeed, as shown for phenyl-conjugated KekulØ-type
3
while otherwise retaining the X-ray structure without geom-
ꢀ
1
etry relaxation. A value of J = ꢀ0.54 cm was obtained,
which is almost identical to the one calculated for the
parent compound. Though the energy differences are very
small, we feel that they can accurately be reproduced within
a series of compounds and give some evidence that the ex-
change interaction is mainly mediated by the p system of
[57,64,65]
[65]
the silole ring.
dinitrenes, the tendency to maintain the aromaticity tends
With regard to the second category (non-pseudo-disjoint),
two classes of molecules, named here as Class A and
Class B, can be distinguished both from magnetic measure-
ments and bonding considerations.
to favor the diradical structure over the quinonoidal one.
Class B diradicals: The second class of compounds, which in-
cludes diradicals 7b and 7c, also corresponds to the open-
shell resonant form of KekulØ p-conjugated molecules. The
main difference between these siloles and 7a is that they
have been designed to disfavors a quinonoidal rearrange-
ment, either by inserting or by fusing an additional phenyl
ring to the structure. The magnetic studies revealed weak
antiferromagnetic interactions for this class of diradicals
Class A diradicals: The first class of compounds includes di-
A
C
H
T
R
E
U
N
G
radicals 7a and 12 that correspond to the open-shell reso-
[66]
nant form of a classical KekulØ p-conjugated molecules.
In such systems, the spin coupling is often sufficient to
induce pairing of the single electrons leading to closed-shell
structures. This is indeed the case with 7a and 12, which ex-
hibit rather strong antiferromagnetic interactions and expe-
rience a structural/electronic rearrangement to the corre-
sponding quinonoidal structure. The experimental exchange
ꢀ
1
with an exchange parameter of J = ꢀ1.8 cm and J =
ꢀ1
ꢀ1.3 cm for 7b and 7c, respectively. This can be attributed
to both the large separation (7b) and the large torsion
angles (7c) between the spin bearing units. It is therefore
not surprising to find very small values of spin density on
the silicon atom by DFT calculations [Eq. (6)].
ꢀ
1
parameter for these diradicals is very large (J = ꢀ142 cm
ꢀ1
for 7a and J = ꢀ248 cm for 12), thus underlining a strong
dependence of the exchange interactions on the nature of
the central heterocycle. At least three factors may be in-
voked to explain such a difference: conformation, spin den-
sity, and heteroatom effect. Conformational and spin density
effects are already known to play an important role in the
magnetic exchange modulation for TMM-type diradi-
2
2
0
av
J ¼ A cos ½ꢀ Šcos ½ꢀ Š þ B
ð6Þ
av
Since the distance between the radical centers is very similar
in diradicals 7a and 7c, we have checked if the exchange
modulation observed for these diradicals is governed by
conformational considerations. Modification of Equation (4)
to take account of the average torsion angle between the
[64,67]
cals.
It has been shown that the exchange coupling pa-
rameter can be closely correlated to the average side-ring
torsion angles (f ) via a “Karplus–Conroy-type” equation
phenyl ring and the tBuNO group (f’ ) [Eq. (6)] allows the
av
av
[Eq. (4)], where the A term is related to the coupler and the
A
A
H
R
U
G
A
H
R
U
G
spin density on the spin-containing group and the B term
corresponds to the through-space antiferromagnetic interac-
tion.
A
A
H
R
U
G
ACHTREUNG
A
H
R
U
G
ACHTREUNG
ratio is equal to 94.7. For this class of diradical, too, the ex-
change mechanism appears to mainly involve the p system
of the silole ring.
2
J ¼ A cos ½f Š þ B
ð4Þ
av
If we assume that the through-space antiferromagnetic inter-
action is negligible (vide supra), and that the spin density
carried by the side groups is similar for both compounds
Conclusion
(
see magnetic measurements above), then the A(7a):A(12)
A
H
R
U
G
ratio should give an estimate of the ability of the central
heterocycle to mediate antiferromagnetic interactions
To study the ability of the silole ring to act as a magnetic
coupler and to determine the role of the silicon atom in the
magnetic exchange in such molecules, we have synthesized
and investigated the magnetic properties of a series of dirad-
icals in which the two spin-bearing units are linked by a
non-alternant silole ring. The thiophene-coupled diradical
[Eq. (5)].
2
Að7 aÞ
Að12Þ
Jð12Þcos ½f ð7 aފ
av
¼
ð5Þ
2
Jð7 aÞcos ½f ð12ފ
av
1
2 was also synthesized to provide a comparison with this
For average torsion angles of 41.18 for 7a and 12.38 for 12,
we find a value of 0.96 for the A(7a):A(12) ratio, whereas
the J(7a):J(12) ratio is 0.57. This result indicates that both
well-known heteropentacycle. While compounds 7b, 7c, and
7d are quite stable in solution and in the solid state, 7a and
12 undergo partial electronic rearrangement to both a dia-
AHCTREUNG
ACHTREUNG
5558
www.chemeurj.org
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 5547– 5562

Magnetic Exchange in Diradicals
FULL PAPER
1
3
1
magnetic quinonoidal form and a monoradical species on
account of the fact that they correspond to the open-shell
form of a p-conjugated KekulØ structure. Geometry optimi-
zations of silole-bridged diradicals at the UB3LYP/6-31G
level accurately reproduce the experimental geometry avail-
able for comparison. Spin densities calculated with the
J
(
1
1
A
H
R
U
G
A
T
U
G
3
,
d
=
2
9
3
ꢀ1
360 cm (NꢀO); MS (FAB+): m/z: 316 [M+H] .
-[N-tert-butyl-N-(trimethylsilyloxy)amino]-4-bromobiphenyl (5b): This
compound was prepared starting from 1-[N-tert-butyl-N-(hydroxyl-
1
[
28]
A
H
R
U
G
in a manner similar to the synthesis of
compound 5a (90%). H NMR (CDCl , 298 K): d = 0.03 (s, 9H), 1.16 (s,
H), 7.30 (d, J(H,H) 8 Hz, 2H), 7.56–7.60 ppm (m, 6H); C{ H}
NMR (CDCl , 298 K): d = 0.25, 26.63, 61.25, 121.44, 125.86, 128.84,
32.17, 136.46, 140.32, 151.29 ppm; Si NMR (CDCl
*
1
larger 6-31+G basis set and the same functional compare
9
A
T
E
N
=
quite well with other reported calculations. Such spin densi-
ties along with the structural data obtained from the geome-
try optimizations, show how different mechanisms give rise
to the spin density distribution (direct delocalization versus
spin polarization) combined with conformational constraints
determine the spin density at the core of the molecules.
Thus, the connection across the 2,5-positions of spin-bearing
units leads to intramolecular antiferromagnetic interactions
that are modulated by the conformation of the molecules
leading to singlet ground states. The weak antiferromagnetic
exchange interaction between the two radical moieties,
which is found experimentally in compound 7d, could also
be confirmed by theoretical calculations. An exchange pa-
rameter unaltered by substitution of silicon by carbon in a
model compound suggests that the interaction mainly pro-
ceeds via the p-system of the silole ring. Finally, analyses of
the data with a “Karplus–Conroy”-type equation allowed us
to establish that the silole ring, as a whole, leads to a stron-
ger exchange interaction between the two nitroxide radicals
attached to its 2,5-positions than the thiophene ring. This in-
creased efficiency probably originates from the non-aroma-
ticity of the silole thus improving the magnetic interaction
through it.
3
1
3
, 298 K): d =
21.85 ppm; IR (CHCl ): n˜ = 1361 cm (NꢀO); MS (FAB+): m/z: 392
3
+
[M+H] .
1-[N-tert-butyl-N-(trimethylsilyloxy)amino]-4-bromonaphthalene
This compound was prepared starting from 1-[N-tert-butyl-N-(hydroxyl-
amino]-4-bromonaphthalene 4c in a manner similar to the synthesis of
, 298 K): d = 0.04 (s, 9H), 1.10 (s,
(H,H) = 8 Hz,
1H), 8.53–8.56 ppm (m, 1H); Si NMR (CDCl , 298 K): d = 21.94 ppm;
): n˜ = 1361 (NꢀO); MS (FAB+): m/z: 366 [M+H] .
-[N-tert-butyl-N-(trimethylsiloxy)amino]-3-bromobenzene (5d): This
compound was prepared starting from 1-[N-tert-butyl-N-(hydroxyl-
amino]-3-bromobenzene 4d in a manner similar to the synthesis of com-
AHCTREUNG
1
compound 5a (95%). H NMR (CDCl
9
H), 5.46 (s, 1H), 7.79 (d, J(H,H) = 8 Hz, 1H), 8.18 (d, J
A
H
R
U
G
2
9
IR (CHCl
3
1
AHCTREUNG
1
3
pound 5a (89%). H NMR (CDCl , 298 K): d = 0.02 (s, 9H), 1.13 (s,
3
, 298 K): d = 0.19,
26.54, 61.31, 121.47, 124.17, 128.07, 127.93, 128.95, 153.18 ppm; Si NMR
13
1
9
H), 7.49–7.12 ppm (m, 4H); C{ H} NMR (CDCl
2
(
CDCl
3
, 293 K): d = 22.51 ppm; IR (CHCl
): n˜ = 1361 (NꢀO); MS
+
(
FAB+): m/z: 316 [M+H] .
Silole 6a: A mixture of lithium (0.2 g, 29 mmol) and naphthalene (3.71 g,
9 mmol) in THF (30 mL) was stirred at room temperature under argon
for 5 h to form a deep green solution of lithium naphthalenide. To the
mixture was added bis(phenylethynyl)dimethylsilane (2, 2 g, 7.7 mmol) in
THF (20 mL). After stirring for 10 min, the reaction mixture was cooled
2
to 08C and [ZnCl
7.82 g, 29 mmol) was added as a solid to form organozinc derivative 3.
After stirring for an hour at room temperature, a solution of 5a (4.82 g,
5.3 mmol) in THF (20 mL) and [PdCl (PPh ] (0.28 g, 0.4 mmol) were
added successively. The mixture was heated under reflux and stirred for
4 h. After hydrolysis with HCl (1m), the mixture was extracted with
Et O. The combined organic layers were washed with brine, dried over
MgSO , and concentrated. The resulting residue was subjected to column
chromatography (neutral aluminum oxide, pentane/dichloromethane
0:20) to give 2.68 g (4.57mmol) of 6a (56%). M.p. 1988C (decomp);
2
A
H
R
U
G
(
1
2
Experimental Section
2
4
Materials and methods: All reactions were routinely carried out under
argon using standard Schlenck techniques. Solvents were distilled prior to
use. THF was dried over sodium/benzophenone and distilled under
argon. All commercial reagents were used as received. Bis(phenyleth-
8
1
H NMR ([D
H), 6.81–6.87 (m, 8H), 7.00–7.05 ppm (m, 10H); C{ H} NMR
([D ]DMSO, 298 K): d
ꢀ2.67, 26.83, 60.18, 124.60, 127.14, 128.33,
28.41, 130.22, 135.48, 139.84, 141.02, 149.29, 153.81 ppm; Si NMR
([D ]DMSO, 298 K): d 7.46 ppm; IR (CCl ): n˜ 3589 (O–H),
): lmax (loge): 366 nm (1347, p!p*
silole); HRMS (FAB+): m/z: calcd for C38 Si [M+H] : 589.3250;
found 589.3198; anal. calcd for C38 Si: C 77.51, H 7.53; N, 4.76, Si
.77; found: C 77.55, H 7.59; N, 4.82, Si 4.51.
6
]DMSO, 298 K): d = 0.51 (s, 6H), 1.13 (s, 18H), 5.37(s,
3
2
[
68]
=
A
C
H
T
R
E
U
N
G
ynyl)dimethylsilane was obtained by the reaction of dimethyldichloro-
6
2
9
1
silane and phenylethynyllithium, which was prepared from nBuLi and
1
13
29
=
phenylacetylene in ether. H, C, and Si NMR spectra were recorded
on a Bruker Advance 200DPX spectrometer, the FT-IR spectra on a
Thermo Nicolet Avatar320 spectrometer, the UV/Vis spectra on a Seco-
mam Anthelie instrument, and the MS spectra on a Jeol JMS-DX300
spectrometer in a m-nitrobenzyl alcohol matrix. Elemental analyses were
performed at the Service Central de Microanalyse of the CNRS, Vernai-
son (France). The ESR spectra were recorded on X-band Bruker Elexsys
spectrometer. Magnetic measurements down to 2 K were carried out in a
Quantum Design MPMS-5S SQUID susceptometer. All magnetic investi-
gations were performed on polycrystalline samples. The molar suscepti-
bility was corrected for the sample holder and for the diamagnetic contri-
6
ꢀ
1
1
359 cm (NꢀO); UV/Vis (CCl
4
44 2 2
H N O
4
Silole 6b: This compound was prepared starting from compound 5b in a
manner similar to the synthesis of compound 6a (78%). M.p. 1728C
]DMSO, 298 K): d = 0.53 (s, 6H), 1.16 (s, 18H),
6.88–6.98 (m, 4H), 6.99–7.08 (m, 8H), 7.20–7.42 (m, 10H), 7.52–7.58 ppm
1
(decomp); H NMR ([D
6
1
3
1
(m, 6H); C{ H} NMR ([D
6
]DMSO, 298 K): d = ꢀ2.86, 26.43, 61.15,
125.25, 125.26, 126.30, 126.82, 127.37, 128.56, 129.90, 130.24, 132.27,
[
55,69]
29
bution of all atoms by means of Pascalꢁs tables.
-[N-tert-butyl-N-(trimethylsiloxy)amino]-4-bromobenzene (5a): An
excess of triethylamine (21 mL, 150 mmol) was added to a solution of 1-
132.37, 132.61, 137.54, 138.88, 139.47, 141.51, 154.69 ppm; Si NMR
(
1
[D
359 cm (N-O); UV/Vis (CCl
silole); HRMS (FAB+): m/z: calcd for C H N O Si [M+H] : 741.3877;
6
]DMSO, 298 K):
d
=
8.18 ppm; IR (CCl
4
): n˜ = 3590 (O-H),
1
ꢀ
1
4
): lmax (loge) = 391 nm (2.106, p!p*
[
29]
[
N-tert-butyl-N-(hydroxylamino]-4-bromobenzene
(4a,
12.2 g,
found 741.3875; elemental analysis (%) calcd for C50
52 2 2
H N O Si: C 81.04,
H 7.07, N, 3.78, Si 3.79. found: C 80.95, H 7.09, N, 3.85, Si 3.67.
5
0 mmol) in THF (100 mL). Chlorotrimethylsilane (19 mL, 150 mmol) in
THF (50 mL) was added to the reaction mixture. After the mixture had
been stirred at 458C for 20 h, the solvents were evaporated to yield a res-
idue that was treated with pentane (200 mL) and then filtered. The fil-
Silole 6d: This compound was prepared starting from compound 5d in a
manner similar to the synthesis of compound 6a (63%). M.p. 1308C
1
trate was evaporated in vacuum to yield 5a as an orange oil (14.1 g,
(decomp); H NMR ([D
6
]DMSO, 298 K): d = 0.48 (s, 6H), 0.97(s, 18H),
1
13
1
8
9%). H NMR (CDCl
3
, 298 K): d = 0.01 (s, 9H), 1.10 (s, 9H), 7.16 (d,
5.20 (s, 2H), 6.76–6.83 (m, 8H), 7.00–7.14 ppm (m, 10H); C{ H} NMR
Chem. Eur. J. 2006, 12, 5547– 5562
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
5559

(
[D
6
]DMSO, 298 K): d
=
ꢀ3.21, 26.72, 59.92, 80.03, 122.57, 124.94,
A
H
E
U
G
3 4
[Pd(PPh ) ] (0.058 g) were added to a solution of thienylzinc chloride
P. Gerbier et al.
[
30]
1
1
7
25.49, 127.06, 127.315, 127.93, 128.35, 129.07, 130.18, 135.38, 139.04,
39.39, 141.99, 151.29, 154.30 ppm; Si NMR ([D
.82 ppm; IR (CCl
(0.01 mol) in dry diethyl ether (20 mL). The mixture was then stirred at
508C for 16 h, quenched with a saturated NH Cl solution and extracted
with diethyl ether. The combined extracts were dried over MgSO . Evap-
oration of the solvents and chromatography of the residue (silica gel,
2
9
6
]DMSO, 298 K): d =
4
ꢀ
1
): n˜ = 3589.7(O ꢀH), 1363.2 cm (NꢀO); UV/Vis
4
4
(
CHCl
3
): lmax (loge) = 360 nm (4.83, p!p* silole); HRMS (FAB+):
+
1
m/z: calcd for C38
H
45
N
2
O
2
Si [M+H] : 589.3250; found 589.3243; elemen-
pentane) afforded 8 as a white waxy solid (73%). M.p. 568C; H NMR
tal analysis calcd (%) for C38
found: C 77.47, H 7.58, N 4.74, Si 4.56.
H
44
N
2
O
2
Si: C 77.51, H 7.53, N 4.76, Si 4.77;
(CD Cl , 298 K): d = ꢀ0.02 (brs, 6H), 0.97(s, 9H), 1.18 (s, 9H), 7. 10
2
2
1
2
(dd,
J(H,H) = 3.4 Hz, J (H,H) = 3.6 Hz, 1H), 7.25–7.47 (m, 4H),
A
H
R
U
G
0
13
1
Silole 6’: This compound was prepared by a Pd -catalyzed crosscoupling
7.53 ppm (d, J(H,H) = 8.6 Hz, 2H); C{ H} NMR (CD Cl , 298 K): d =
A
H
R
U
G
2 2
reaction between 1,4-dibromonaphtalene (6 equiv) and organozinc deriv-
ꢀ4.50, 18.25, 26.38, 61.31, 122.52, 124.41, 125.96, 131.20, 143.29,
29
ative 3 in a manner similar to the synthesis of compound 6a (40%). M.p.
152.18 ppm; Si NMR (CDCl
3
, 298 K): d = 23.95 ppm; IR (KBr): n˜ =
1
ꢀ
ꢀ
2
28–2308C; H NMR ([D
6
]DMSO, 298 K): d = 0.30 (s, 6H), 6.78–6.94
3103, 3070 (aromatic C H), 2990–2852 (aliphatic C H), 1600, 1575 (C=
ꢀ1
C), 1387cm
(N
ꢀ
O); HRMS (FAB+): m/z: calcd for C H NOSSi
20 31
(
m, 10H), 7.54–7.68 (m, 8H), 8.08–8.12 (m, 2H), 8.23–8.28 ppm (m, 2H);
1
3
1
+
C{ H} NMR ([D
6
]DMSO, 298 K): d = ꢀ3.66, 126.05, 126.59, 126.83,
[M] : 361.1896; found 361.1904.
1
1
1
27.40, 127.49, 127.52, 127.84, 129.62, 130.11, 130.50, 132.35, 134.31,
38.42, 139.58, 143.41, 155.90 ppm; Si NMR (([D ]DMSO, 298 K): d =
6
Bis[N-tert-butyl-N-(tert-butyl-dimethylsilyloxy)amino-4-phenyl]-2,5-thio-
phene (9): A 1.33m solution of nBuLi in hexanes (5.3 mL, 7.03 mmol)
was added dropwise to a stirred solution of 8 (2.48 g, 6.86 mmol) in dieth-
yl ether (25 mL) at 08C. The mixture was stirred for 1 h, and the resulting
2
9
1.63 ppm; UV/Vis (CHCl
3
): lmax (loge) = 351 nm (4.975, p!p* silole);
+
HRMS (FAB+): m/z: calcd for C38
H
28Br
2
Si [M] : 670.0327; found
6
70.0349; elemental analysis calcd (%) for C38
Si 4.18; found: C 67.98, H 4.09, Si 4.03.
Silole diradical 7a: To a solution of 6a (0.77 g, 1.31 mmol) in CH
40 mL) was added freshly prepared Ag O (0.64 g, 2.73 mmol). The mix-
2
H28Br Si: C 67.87, H 4.20,
2
solution was added to [ZnCl (tmeda)] (1.766 g, 6.99 mmol) dissolved in
A
H
R
U
THF (10 mL) at 08C. The mixture was allowed to warm gradually to
room temperature to afford the zinc derivative 9 as a light yellow solu-
2
Cl
2
(
2
A
C
H
T
R
E
U
N
G
3 4
tion. A solution of 4a-TBDMS (2.45 g, 6.86 mmol) and [Pd(PPh ) ]
ture was stirred for 45 min at 08C and filtered. The solvent was removed
under vacuum and the resulting deep red solid was purified by column
chromatography (neutral aluminum oxide, pentane/dichloromethane
(0.040 g, 35 mmol) in THF (30 mL) was added to the zinc derivative 9
and the reaction mixture was stirred for 18 h at 508C. After the usual
workup and silica gel chromatography (pentane), 10 was recovered as a
1
8
1
0:20) to give 7a (0.6 g, 1.01 mmol; 78%). M.p. 1518C; IR (CCl
4
): n˜ =
white solid (33%). M.p. 130–1318C; H NMR (CD Cl , 298 K): d
=
2
2
ꢀ
1
359 cm (NꢀO); UV/Vis (CHCl
): lmax (loge) = 300 (4.28, p!p* aryl-
ꢀ0.03 (s, b, 12H), 0.98 (s, 18H), 1.17 (s, 18H), 7.31 (s, 2H), 7.57 (d, J-
(H,H) 8.5 Hz, 4H), 7.37 ppm (d, J(H,H) 8.5 Hz, 4H); C{ H}
NMR (CD Cl , 298 K): d = ꢀ4.49, 18.24, 26.30, 26.35, 61.43, 123.83,
3
1
3
1
nitroxide), 420 (1.365, p!p* silole), 448 nm (n!p* NꢀO); HRMS
A
H
R
U
G
=
A
H
R
U
G
=
+
(
fab+): m/z: calcd for
C
38
H
44
N
2
O
2
Si [M+2H] : 588.3172; found
2
2
2
9
5
88.3168.
2 2
124.67, 126.01, 131.09, 143.29, 151.11 ppm; Si NMR (CD Cl , 298 K): d
= 23.95 ppm; IR (KBr): n˜ = 3071, 3028 (aromatic C H), 2980–2856 (ali-
ꢀ
Silole diradical 7b: This compound was prepared starting from com-
pound 6b in a manner similar to the synthesis of compound 7a (78%).
ꢀ
1
phatic C
ꢀ
H), 1600, 1575 (C=C), 1387cm (NꢀO); UV/Vis (CHCl ): l
3
max
ꢀ
1
M.p. 211–2138C; IR (CCl
4
): n˜ = 1354 cm (NꢀO); UV/Vis (CHCl
3
):
(loge) = 348 nm (4.519, p!p* thiophene); HRMS (FAB+): m/z: calcd
+
l
max (loge) = 323 (5.351, p!p* arylnitroxide), 400 (5.412, p!p* silole),
for C36H
58
N
2
O
2
SSi
2
[M] : 638.3758; found 638.3760.
4
51 nm (n!p* NꢀO); HRMS (FAB+): m/z: calcd for C50
H
52
N
2
O
2
Si
Bis[N-tert-butyl-4-phenylhydroxylamine]-2,5-thiophene (11): A 22.6m
+
[
M+2H] : 740.3798; found 740.3787.
aqueous solution of HF (0.13 mL, 3 mmol) was added to a stirred solu-
tion of 10 (0.798 g, 1.25 mmol) in THF (10 mL) at room temperature.
After 1 h, the reaction mixture was evaporated under a vacuum to yield
Silole diradical 7c: To a suspension of silole 6’ (0.37g, 0.55 mmol) in di-
ethyl ether (100 mL) was added 0.41 mL (2.57mmol) of N,N,N’,N’-tetra-
methylethylenediamine. The mixture was cooled to ꢀ788C, and a 2m sol-
ution of n-butyllithium in hexane (1.3 mL, 2.57mmol) was then added
under argon. The mixture was stirred for 90 min, warmed to room tem-
perature, stirred for a further 90 min, and then was cooled to 08C. A sol-
ution of 2-methyl-2-nitrosopropane (340 mg, 3.8 mmol) in diethyl ether
the bishydroxylamine as a white solid (99%). M.p. 1048C (decomp);
1
H NMR ([D
6
]DMSO, 298 K): d = 1.14 (s, 18H), 7.29 (d, J
(H,H) = 8.5 Hz, 4H), 8.50 ppm (s, b,
]DMSO, 298 K): d = 26.82, 60.91, 125.06, 125.14,
125.47, 130.43, 142.81, 150.76 ppm; IR (KBr): n˜ = 3230 (OꢀH), 3029 (ar-
A
C
H
T
R
E
U
N
G
(H,H) =
8.5 Hz, 4H), 7.47 (s, 2H), 7.61 (d, J
ACHTREUNG
1
3
1
2H); C{ H} NMR ([D
6
ꢀ
1
(
10 mL) was added, and the mixture was stirred overnight at ambient
omatic CꢀH), 2971–2870 (aliphatic CꢀH), 1600, 1542 (C=C), 1387cm
temperature under argon. It was then treated with saturated aqueous am-
monium chloride solution (10 mL). The organic layers were combined
and concentrated under reduced pressure. Addition of pentane yielded a
white precipitate of hydroxylamine that was washed several times with
pentane to yield the crude product. The hydroxylamine was sensitive to
both moisture and air, so it was used directly to prepare the nitroxide.
The white solid was dissolved in freshly distilled dichloromethane
(NꢀO); UV/Vis (CHCl
3
): lmax (loge)
phene); HRMS (FAB+): m/z: calcd for C24
=
348 nm (4.519, p!p* thio-
+
H
30
N
2
O
2
S [M] : 410.2028;
found 410.1965; elemental analysis calcd (%) for C H N O S: C 70.21,
2
4
30
2
2
H 7.36, N 6.82, S 7.81; found: C 70.25, H 7.40, N 6.79, S 7.80.
Bis[N-tert-butylaminoxyl-4-phenyl]-2,5-thiophene (12): Further oxidation
with PbO (2-fold excess) in dichloromethane at room temperature for
h gives a deep red solution that was filtered and evaporated under a
vacuum. The resulting deep-red solid was crystallized by diffusion of
hexane in dichloromethane to give 12 as black needle-shaped crystals
2
2
(
10 mL) and freshly prepared Ag
2
O (700 mg, 3 mmol) was added. The
O was removed
mixture was stirred for 60 min at 08C, and the solid Ag
2
by filtration. The filtrate was concentrated under reduced pressure and
chromatographed (silica gel, ethyl acetate/pentane 3:2) to yield silole 7c
(
45%). M.p. 1048C (decomp); IR (KBr): n˜ = 3069, 3028 (aromatic Cꢀ
ꢀ1
H), 2997–2853 (aliphatic CꢀH), 1601, 1580 (C=C), 1349 cm (NꢀO); lmax
4
as a reddish orange solid (37%). M.p. 129–132 8C; IR (CCl ): n˜ =
(
loge) = 322 (4.167, p!p* arylnitroxide, terphenoquinone), 391 (4.477,
p!p* thiophene), 444 (4.167), 479 (4.072), 551 nm (3.875, p!p* terphe-
noquinone); elemental analysis calcd (%) for C24 S: C 70.56, H
.91, N 6.86, S 7.85; found: C 70.13, H 7.11, N 6.81, S 7.88. A single crys-
ꢀ
1
1
357cm (NꢀO); UV/Vis (CHCl
): lmax (loge) = 293 (5.304, p!p* ar-
3
ylnitroxide), 363 (5.096, p!p* silole), 456 nm (n!p* NꢀO); HRMS
28 2 2
H N O
+
(
fab+): m/z: calcd for
C
46
H
48
N
2
O
2
Si [M+2H] : 688.3485; found
6
6
88.3476.
3
tal of approximate dimensions 0.40”0.03”0.03 mm was mounted on a
Nonius k-CCD diffractometer with MoKa radiation (0.71069 Š) and
cooled to 150 K. The diffracted intensities were collected within the
range 3.55<q<24.718. The structural determination by direct methods
and the refinement of atomic parameters based on full-matrix least-
Silole diradical 7d: This compound was prepared starting from com-
pound 6d in a manner similar to the synthesis of compound 7a (78%).
ꢀ
1
M.p. 124–1278C; IR (CCl
l
4
): n˜ = 1363.2 cm (NꢀO); UV/Vis (CHCl
3
):
max (loge) = 283 (5.31, p!p* arylaminoxide), 361 nm (5.01, p!p*
+
silole); HRMS (FAB+): m/z: calcd for
C
38
H
45
N
2
O
2
Si [M+3H] :
2
[70]
squares on F were performed with the SHELX-97programs.
tions of hydrogen atoms were all calculated. Results: 2[C24
= 20.752(5), b = 5.826(5), c = 34.309(5) Š, V = 4148(4) Š , 1calcd
1.308, system orthorhombic, space group Pna2 , 99.8% completeness to
The posi-
5
89.3250; found 589.3251.
H N O S ]: a
28 2 2 1
3
N-tert-butyl-N-(tert-butyl-dimethylsilyloxy)amino-4-phenyl-2-thiophene
=
[
29]
(
8): A solution of 4a-TBDMS (3.58 g, 0.01 mol) in THF (20 mL) and
1
5560
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 5547– 5562

Magnetic Exchange in Diradicals
FULL PAPER
q = 24.708, 24363 collected reflections of which 7020 unique (Rint
=
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1
Received: October 14, 2005
Revised: February 8, 2006
Published online: May 8, 2006
1
5562
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ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2006, 12, 5547– 5562