A spiro-fused triarylaminium radical cation with a triplet ground state

Article abstract of DOI:10.1002/anie.200390244

Novel spirobis(radical cation) 1²⁺ was prepared to examine the magnetic interaction between the two radical centers. The dication 1²⁺ was found to be a robust triplet diradical on the basis of the ESR spectrum, magnetic susceptibilit

Full text of DOI:10.1002/anie.200390244

Angewandte
Chemie
Stable Spirobis(radical cation)
A Spiro-Fused Triarylaminium Radical Cation
with a Triplet Ground State**
Akihiro Ito, Masashi Urabe, and Kazuyoshi Tanaka*
The spiro structural motif has been occasionally revisited in
the field of molecule-based materials science. For example,
several spiro compounds were recently investigated for
constructing metallic charge-transfer salts with unique
three-dimensional structures[1,2] and molecular metals com-
posed of a single molecular component.[3] With regard to
molecule-based magnetism, Dougherty et al. elaborately
examined the possibility of generating 1,4,6,9-spiro[4.4]nona-
tetrayl.[4,5] Recently, Frank et al. reported the synthesis of a
spiro-fused bis(nitronyl nitroxide) biradical and the magnetic
interaction between the two radical centers.[6] From the
viewpoint of magnetic interaction, the spin preference in
spirobiradicals is determined by the balance between two
contributions: 1) the two perpendicular p systems, which
favor ferromagnetic interaction resulting from the degener-
acy of the magnetic orbital derived from the two ring systems,
and 2) so-called spiroconjugation,[7] which can lift the degen-
eracy of the two magnetic orbitals and thus lead to a singlet
spin state. In fact, the biradical reported in ref. [3]exhibited a
weak antiferromagnetic interaction, probably because of the
second contribution. Two nitronyl nitroxide radicals incorpo-
rated in the two subunits of a spiro compound can be regarded
as localized spin centers. The further apart the nitroxide
centers are in the molecule, the weaker their ferromagnetic
interaction becomes. In contrast, triarylaminium cations have
more delocalized spin distributions than the nitroxide radi-
cal,[8] and this ensures stronger exchange interaction between
two radical centers. Furthermore, it is well known that all-
para-substituted triarylamines often give thermally and
chemically robust radical cations.[8] Here we report the
synthesis of the novel spiro-fused triarylamine 1 and the
magnetic properties of its bis(radical cation) 12+.
Figure 1. Schematic diagram of molecular interaction between two per-
pendicular subunits in a spiro compound. A and S denote antisymmet-
ric andsymmetric, respectively, with regardto the planes a and b.
In Figure 1, the fragment MOs of the mutually perpen-
dicular p systems are classified by symmetry with respect to
two planes a and b. By consideration of symmetry it can be
deduced that the two fragment MOs can interact with each
other only when they are antisymmetric (AA) to both
planes.[7] The fragment MOs of a triarylamine such as 1
indeed have AA symmetry, as illustrated in Figure 2. Hence,
the resulting HOMO for 1 is expected to be doubly
degenerate. In fact, CAS(2,2)/6-31G* MO calculations[9] on
the dication of the unsubstituted model compound of 1
revealed degeneracy of two singly occupied MOs (SOMOs).
OMe
Me
O
AA
AA
LUMO
MeO
N
OMe
N
Si
O
Me
OMe
1
[*] Prof. K. Tanaka, Dr. A. Ito, M. Urabe
Department of Molecular Engineering
Graduate School of Engineering
Kyoto University
Sakyo-ku, Kyoto 606-8501 (Japan)
Fax: (+81)75-771-0172
HOMO
E-mail: a51053@sakura.kudpc.kyoto-u.ac.jp
AS
SA
[**] This work was supportedby a Grant-in-Aidfor Scientific Research
(C) from Japan Society for the Promotion of Science (JSPS).
Numerical calculations were partly carriedout at the Supercom-
puter Laboratory of the Institute for Chemical Research of Kyoto
University.
Figure 2. Schematic drawing of the frontier MOs for 1. Methoxy
groups are omittedfor clarity.
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In addition, the singlet state is 167 Jmolꢀ1 higher in energy
than the corresponding triplet state. Although the calculated
Mulliken spin density is mainly concentrated on the two N
atoms, it is delocalized over the benzene rings to some extent.
The distance between the two N atoms was estimated to be
6.4 Š from the optimized structure.
The synthesis of 1 (Scheme 1) started with diarylamine 2.
As a facile method to construct a spiro structure, we selected
the method of Gilmann et al.,[10] in which treatment of silicon
tetrachloride with N-alkyl-2,2’-dilithiodiphenylamine gives a
spirobis(dihydrophenazasiline) in high yield. In terms of
retrosynthetic strategy, the route starting from 2,2’-dibromo-
trianisylamine 4 is more convenient. However, dilithiated 4
and SiCl4 did not react, probably owing to the buttressing
effect[11] of the bulky N-aryl group. However, when N-benzyl-
2,2’-dibromodianisylamine (5) was employed, the corre-
sponding spiro compound 6 was obtained in 60% yield.
Finally, after removal of benzyl protecting groups, a Pd0-
catalyzed amination reaction[12] afforded the target molecule
1 in high yield.[13]
The redox behavior of 1, in which two oxidizable subunits
are connected by a spiro atom, is of interest, as it will give
information on the spiroconjugation between the two sub-
units and hence the intramolecular magnetic interaction in
12+. The electrochemical properties of 1 were investigated by
cyclic voltammetry (CV) at room temperature in acetonitrile
(Figure 3). Compound 1 displayed two reversible one-elec-
tron oxidation waves at formal potentials of 0.30[14] and 0.46 V
versus Fc/Fc+, and this indicates that the first oxidation yields
a monocation stabilized by spin delocalization over two
trianisylamine units. The separation of the two oxidation
potentials (160 mV) indicates that spiroconjugation between
two subunits is weak but not negligible.
Figure 3. Cyclic voltammogram of 1 in MeCN at 258C (scan rate
100 mVsꢀ1).
The reversibility of the two oxidation processes encour-
aged us to prepare a salt of the dication of 1. Taking into
account the observed potentials, we oxidized 1 with antimony
pentachloride in CH2Cl2. Dry diethyl ether was added to the
resulting blue solution at ꢀ788C, and the hexachloroantim-
onate salt was isolated as a greenish blue powder.[15] In
contrast to the comparatively high oxidation potentials of 1,
the isolated salt was quite stable in the solid state even at
ambient temperature in air for days.[16] Although we have not
yet obtained single crystals suitable for X-ray structural
analysis, elemental analysis strongly suggested that the
isolated salt has the composition 1(SbCl6)2.[15,17]
To gain information on whether 12+ has an open-shell
electronic structure, we recorded the EPR spectrum of
1(SbCl6)2 in a frozen CH2Cl2 matrix (Figure 4). The distinct
fine structure attributed to spin-triplet species was detected
together with the forbidden resonance resulting from DmS =
MeO
OMe
MeO
OMe
MeO
OMe
2 equiv BTMABr3
CH3OC6H4I
N
H
N
H
N
K2CO3, CH2Cl2/MeOH (5/2), RT
CuI, K2CO3, ∆
Br
Br
Br
Br
(99%)
3
2
OMe
MeO OMe
Si
(60%)
4
MeO
OMe
1) nBuLi, Et2O, 0˚C
2) SiCl4, Et2O, reflux
Ph
1) nBuLi, 0˚C 2) SiCl4, reflux
N
N
N
Ph
Et2O
Br
Br
Ph
5 (80%)
MeO OMe
6 (60%)
MeO OMe
Si
MeO OMe
CH3OC6H4Br, Pd(OAc)2, (o-biphenyl)(tBu)2P, NaOtBu
HN
NH
MeO
N
Si
N
OMe
toluene, reflux
MeO OMe
MeO OMe
(97%)
1
7 (67%)
Scheme 1. Synthetic route for 1. BTMA=Benzyltrimethylammonium.
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Angewandte
Chemie
Figure 5. Plot of cMT versus T for 1(SbCl6)2 at 500 G. The solidline
represents the best theoretical fit [Eq. (2)] to the data.
Figure 4. EPR spectra of 1(SbCl6)2 in CH2Cl2 at 123 K. Inset: The
forbidden DMS =ꢁ2 resonance at 123 K.
1
2 N g2mB
ꢀ
ꢁ
cM
¼
ð2Þ
2 J
B T
kB ðTꢀqÞ
3 þ exp ꢀ k
ꢁ 2. This clearly indicates that 12+ is a bis(radical cation). The
zero-field splitting constants were determined from the
DmS = ꢁ 1 transitions to be D = 5.5 mT and E = 0.7 mT by
spectral simulation. The midpoints of three pairs of reso-
nances did not coincide, and this indicates anisotropy of the g
factor: gxx ꢂ gyy = 2.0022, gzz = 2.0034 (the z axis is in the
direction connecting the two nitrogen atoms). The average
spin–spin distance was estimated to be 8.0 Š within the point-
dipole approximation.[18] The discrepancy between the value
of 8.0 Š and the distance between the two N atoms estimated
from the aforementioned MO calculations on the unsubsti-
tuted model compound (6.4 Š) clearly shows deviation from
the simple dipole approximation for 12+. For spin-delocalized
diradical molecules, Equation (1) can be adopted[19] where rij
corresponding to the modified Bleany–Bowers singlet–triplet
model for a diradical[20] under a weak Weiss mean field (Weiss
constant q). The exchange coupling J is defined by a
Heisenberg Hamiltonian for a diradical [Eq. (3)]where
h ¼ ꢀ2 J S1 S2 ðS1 ¼ S2 ¼ 1=2Þ
ð3Þ
positive J indicates ferromagnetic interaction between two
radical centers. The optimized values were estimated to be
J = 1.12 cmꢀ1 (1.6 K) and q = ꢀ4.3 K. The ferromagnetic J
for 12+ is in contrast to the antiferromagnetic J (ꢀ4.00 cmꢀ1
(= ꢀ5.8 K)) for the spirobis(nitronyl nitroxide) in ref. [3]
In summary, we have prepared a spiro compound with
high-spin correlation. To the best of our knowledge, this is the
first example of a stable organic spirobis(radical cation). It is
of great interest to extend 1 to polymeric systems, and further
work is in progress in this direction.
r2ijꢀ3 z2
X
3
D ¼ g2b2
4
ij 1i1j
ð1Þ
r5ij
ij
Received: June 14, 2002
Revised: November 4, 2002 [Z19531]
is the distance between atoms i and j, ri and rj are the spin
densities on atom i in one dianisylaminium moiety and on
atom j on the other, and the z axis is in the direction
connecting the two N atoms. On the basis of the B3LYP/6-
31G* calculations on the dication of the unsubstituted model
compound of 1, the D value is estimated to be 6.0 mT. This
indicates that the spin densities are to some extent delocalized
over each dianisylaminium moiety. Furthermore, the temper-
ature dependence of the signal intensity corresponding to the
DmS = ꢁ 2 transition exhibited Curie-like behavior, which
suggests that 12+ has a triplet ground state.
[1]P. Maslak, M. P. Augustine, J. D. Burkey, J. Am. Chem. Soc. 1990,
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[4]L. McElwee-White, W. A. Goddard III, D. A. Dougherty, J. Am.
Chem. Soc. 1984, 106, 3461 – 3466.
Corroboration for ferromagnetic interaction between two
trianisylaminium radical cations was provided by magnetic
susceptibility measurements in the range between 4 and 300 K
(Figure 5). The cMT value was almost constant at
0.83 emuKmolꢀ1 between room temperature and 100 K. This
value is larger than that expected for two uncoupled spins of
1/2 (0.75 emuKmolꢀ1) and suggests high-spin correlation
even at higher temperature. Furthermore, the cMT value
increased gradually with decreasing temperature, reached a
maximum value of 0.85 emumolꢀ1 K at 18 K, and finally
decreased. The measured data best fit to Equation (2)
[5]L. McElwee-White, D. A. Dougherty, J. Am. Chem. Soc. 1984,
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Angew. Chem. Int. Ed. 2003, 42, No. 8
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Communications
Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin, M. C. Strain,
Redox-Switchable Lithium Trap
O. Farkas, J. Tomasi, V. Barone, M. Cossi, R. Cammi, B.
Mennucci, C. Pomelli, C. Adamo, S. Clifford, J. Ochterski, G. A.
Petersson, P. Y. Ayala, Q. Cui, K. Morokuma, D. K. Malick,
A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. Cioslowski, J.
V. Ortiz, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.
Komaromi, R. Gomperts, R. L. Martin, D. J. Fox, T. Keith, M. A.
Al-Laham, C. Y. Peng, A. Nanayakkara, C. Gonzalez, M.
Challacombe, P. M. W. Gill, B. G. Johnson, W. Chen, M. W.
Wong, J. L. Andres, M. Head-Gordon, E. S. Replogle, J. A.
Pople, Gaussian, Inc., Pittsburgh, PA, 1998.
[1.1]Diborataferrocenophane: A Highly Efficient
Li+ Scavenger**
Matthias Scheibitz, Rainer F. Winter, Michael Bolte,
Hans-Wolfram Lerner, and Matthias Wagner*
In various cases, electrophilic substitution reactions of
ferrocene are known to proceed via precomplexation of the
iron atom by the electrophile.[1] Moreover, direct iron-to-
metal bonding appears to influence the complexation behav-
ior of certain ferrocene-based redox-switchable cryptands,[2]
as well as the properties of catalytically active 2-metalla[3]-
ferrocenophanes (metal = TiIV, PdII, PtII).[3] The interaction of
ferrocene with Li+ was studied theoretically by Ugalde
et al.,[4] who located two minima on the energy surface. In
the lower energy structure, the lithium cation is h5-coordi-
nated on top of one of the cyclopentadienyl rings (I;
Scheme 1). The second minimum structure II is 8 kcalmolꢀ1
higher in energy and has the Li+ ion bonded laterally to the
iron atom. We recently reported the synthesis and structural
characterization of a ferrocene/gallium(i) cation complex with
essentially the same structural motif as I.[5] Here we report on
the isolation of a ferrocene/lithium complex that provides
experimental evidence for the existence of structure II.
When a slurry of 1,1’-dilithioferrocene (1)[6] in hexane is
treated with a solution of 1,1’-bis(dimethylboryl)ferrocene (2)
in THF,[7] the cyclic dinuclear aggregate 32ꢀ is formed in good
yield (Scheme 1). X-ray quality crystals of [3-Li]Li([12]crown-
4)2 were grown by gas-phase diffusion of diethyl ether into a
solution of the crude material in THF after addition of
[12]crown-4.
[10]D. Wittenberg, H. A. McNince, H. Gilman, J. Am. Chem. Soc.
1958, 80, 5418 – 5422.
[11]For examples, see M. S. Newman, Effects in Organic Chemistry,
Wiley, New York, 1956; L. N. Ferguson, The Modern Structural
Theory of Organic Chemistry, Prentice-Hall, Englewood Cliffs,
NJ, 1963.
[12]a) M. Beller, Angew. Chem. 1995, 107, 1436 – 1437; Angew.
Chem. Int. Ed. Engl. 1995, 34, 1316; b) J. F. Hartwig, Synlett 1997,
329; c) J. F. Hartwig, Angew. Chem. 1998, 110, 2154 – 2177;
Angew. Chem. Int. Ed. 1998, 37, 2046 – 2067; d) J. P. Wolfe, S.
Wagaw, J.-F. Marcoux, S. L. Buchwald, Acc. Chem. Res. 1998, 31,
805; e) J. F. Hartwig, Acc. Chem. Res. 1998, 31, 852.
1
[13]Physical data for 1: White solid; H NMR (400 MHz, CDCl3):
d = 7.30 (d, 4H), 7.17 (d, 4H), 6.90 (d, 4H), 6.75 (dd, 4H), 6.47
(d, 4H), 3.94 (s, 6H), 3.64 ppm (s, 12H); 13C NMR (75 MHz,
CDCl3): d = 158.9, 152.5, 145.2, 136.83, 132.30, 119.5, 118.0,
117.3, 116.8, 116.1, 55.5 ppm; 13C NMR (75 MHz, C6D6): d =
159.4, 153.7, 146.0, 137.5, 132.8, 119.3, 119.0, 118.7, 117.6, 116.4,
55.0, 54.9 ppm; elemental analysis (%): calcd for C42H38N2O6Si:
C 72.60, H 5.51, N 4.03; found: C 72.41, H 5.44, N 3.97.
+
[14]The oxidation potential of trianisylamine was 0.16 V vs Fc/Fc
under the same conditions. This discrepancy of the first oxidation
potential between 1 and trianisylamine probably results from a
difference in conformation of the trianisylamine moieties. In 1,
the propellerlike conformation can be hardly adopted because of
its spiro structure.
[15]Compound 1 (69 mg, 0.1 mmol) was dissolved in dry dichloro-
methane and stirred at ꢀ788C under argon. SbCl5 (0.5 ml, 1m in
CH2Cl2) was added to the solution. After 10 min, the resulting
blue solution was poured into dry diethyl ether. The precipitate
was washed with dry diethyl ether to provide 1(SbCl6)2 (110 mg,
81%) as a greenish blue solid. 1(SbCl6)2: elemental analysis (%):
calcd for C42H38Cl12N2O6Sb2Si: C 36.99, H 2.81, N 2.05, Cl 31.19;
found: C 36.99, H 2.71, N 2.05, Cl 29.61.
The 11B NMR spectrum of [3-Li]Li([12]crown-4)2 reveals
one signal at d(11B, [D8]THF) = ꢀ21.8 ppm, which testifies to
[*] Prof. Dr. M. Wagner, Dipl.-Chem. M. Scheibitz, Dr. H.-W. Lerner
Institut für Anorganische Chemie
[16]The isolated solid sample was dissolved in CH 2Cl2, and the ESR
signal intensity was compared to that of a sample oxidized in situ
at the same concentration. The two signals gave the same fine
structure with almost the same intensity.
Johann Wolfgang Goethe-Universität Frankfurt
Marie-Curie-Strasse 11, 60439 Frankfurt/Main (Germany)
Fax: (+49)69-798-29260
E-mail: matthias.wagner@chemie.uni-frankfurt.de
[17]Example of oxidation with SbCl in CH2Cl2: H. Bock, A.
5
Dr. R. F. Winter
Rauschenbach, K. Ruppert, Z. Havlas, Angew. Chem. 1991, 103,
706 – 708; Angew. Chem. Int. Ed. 1998, 37, 714 – 716.
[18]N. Hirota, J. Am. Chem. Soc. 1967, 89, 32 – 41.
[19]K. Mukai, A. Sogabe, J. Chem. Phys. 1980, 72, 598 – 601.
[20]B. Bleaney, K. D. Bowers, Proc. R. Soc. London Ser. A 1952, 214,
451 – 456.
Institut für Anorganische Chemie
Universität Stuttgart
Pfaffenwaldring 55, 70569 Stuttgart (Germany)
Dr. M. Bolte+
Institut für Organische Chemie
Johann Wolfgang Goethe-Universität Frankfurt
Marie-Curie-Strasse 11, 60439 Frankfurt/Main (Germany)
[+] X-ray crystallography
[**] We wish to thank Prof. Dr. M. U. Schmidt (Universität Frankfurt) for
helpful discussions. M.W. is grateful to the Deutsche Forschungs-
gemeinschaft (DFG) for financial support. M.S. wishes to thank the
Fonds der Chemischen Industrie (FCI) and the Bundesministerium
für Bildung und Forschung (BMBF) for a PhD grant. R.W.
acknowledges support by the Stiftung Volkswagenwerk.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
924
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