A polymacrocyclic oligoarylamine with a pseudobeltane motif: Towards a cylindrical multispin system

Article abstract of DOI:10.1002/anie.201206831

Spin doctor: The polymacrocyclic oligoarylamine in the picture serves as a multispin source owing to its multi-electron redox activity. As a result of its pseudobeltane structure and para-phenylenediamine bridges, the radical cation can convert between two sideslipped structures with a windshield-wiper-like motion. In contrast, the tetraazacyclophane units are rigid components in the polymacrocycle. Copyright

Full text of DOI:10.1002/anie.201206831

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DOI: 10.1002/anie.201206831
Cyclophanes
A Polymacrocyclic Oligoarylamine with a Pseudobeltane Motif:
Towards a Cylindrical Multispin System**
Daisuke Sakamaki, Akihiro Ito,* Ko Furukawa, Tatsuhisa Kato, Motoo Shiro, and
Kazuyoshi Tanaka*
Although several types of belt-shaped compounds with novel
structures have been reported over the past 30 years,[1] they
are currently receiving increasing attention in conjunction
with the synthesis of the shortest possible segments of single-
walled carbon nanotubes.[2] Nanoscaled beltlike molecules are
considered to be “cycles of cycles”, and thus they have well-
defined shapes with rigid cavities and can conceivably be used
to construct solid-state materials with nanoporous networks.
In addition, polymacrocycles with electron- (or hole-) delo-
calized (or localized) scaffolds are fascinating for potential
applications towards electron (or hole) transport and/or as
magnetic materials. In this context, oligoarylamine-based
macrocyclic spin systems are being pursued to take advantage
of the multi-electron redox properties of oligoarylamines and
the relative stability of their poly(radical cation)s.[3–5] It is well
known that strong Coulombic interactions between charged
centers in oligoarylamine-based macrocycles hinder the
generation of higher oxidation states with maximum spin
multiplicity (Coulombic penalty).[6] As we have shown
recently, however, the insertion of para-phenylenediamine
(PD) units into the molecular backbone can alleviate the
Coulombic penalty between charged triarylaminium radical
centers in oligoarylamines and lower their oxidation poten-
tials.[5]
upon two-electron oxidation.[7] Moreover, it has been dem-
onstrated that the introduction of this macrocycle into
a oligoarylamine backbone with one-dimensional connectiv-
ity can can convert the one-dimensional multispin system with
a fragile spin-coupling pathway into a robust, aligned high-
spin system.[8] Thus the polymacrocycles provided by the
tetraazacyclophanes may be an indication for the further
development of cylindrical multispin systems, which could
culminate in nanotube-like surfaces with multi-electron redox
activity. These findings led to the idea of utilizing the
tetraazacyclophane unit as a component for a belt-shaped
polymacrocyclic
oligoarylamine.
Polymacrocycle
1 (Scheme 1), which is classified as a pseudobeltane according
to Vçgtleꢀs nomenclature,[1f,9] can be viewed as a kind of
molecular belt containing six PD units connected by four
1,3,5-benzenetriyl ferromagnetic couplers,[10] and thereby the
higher oxidation states of 1 can lead to multispin systems.
Tetraaza[14]m,p,m,p-cyclophane, the smallest macrocyclic
oligoarylamine bearing the alternating meta–para linkage, is
transformed into an almost pure spin-triplet diradical dication
[*] D. Sakamaki, Dr. A. Ito, Prof. Dr. K. Tanaka
Department of Molecular Engineering
Graduate School of Engineering, Kyoto University
Nishikyo-ku, Kyoto 615-8510 (Japan)
E-mail: aito@scl.kyoto-u.ac.jp
Scheme 1. The polymacrocyclic oligoarylamine 1 with a pseudobeltane
structure and tetraaza[14]m,p,m,p-cyclophane 4 as a reference com-
pound.
ktanaka@moleng.kyoto-u.ac.jp
According to density functional theory (DFT) calcula-
tions for the model compound 1’, in which the n-butoxy
groups in 1 are replaced by hydrogen atoms for simplicity, at
the B3LYP/6-31G* level, the optimized structure is a C2h-
symmetrical structure that closely resembles the X-ray
structure of 1 (Figure 2).[11] The highest occupied molecular
orbital (HOMO) and the next to highest molecular orbital
((HO-1)MO) is largely localized on the two PD bridges
connecting two tetraazacyclophane moieties, whereas in the
next highest orbitals, (HO-2)MO and (HO-3)MO, the elec-
tron density is mainly localized on the two tetraazacyclophane
moieties (Figure 1). Moreover, as is apparent from the orbital
energy diagram, the frontier MOs from HOMO to (HO-3)-
MO are quasi-fourfold degenerate, so that a spin-quintet state
can be anticipated in the tetracation 14+.
Dr. K. Furukawa
Institute for Molecular Science
Myodaiji, Okazaki 444-8585 (Japan)
Prof. Dr. T. Kato
Institute for the Promotion of Excellence in Higher Education
Kyoto University
Yoshida-Nihonmatsu, Sakyo-ku, Kyoto 606-8501 (Japan)
Dr. M. Shiro
Rigaku Corporation, X-ray Research Laboratory
Matsubaracho 3-9-12, Akishima, Tokyo, 196-8666 (Japan)
[**] This work was supported by a Grant-in-Aid for Scientific Research
(B) (24310090) from the Japan Society for the Promotion of Science
(JSPS). D.S. thanks the JSPS Research Fellowship for Young
Scientists.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206831.
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rigidity in the molecular structure appears in the 1H and
13C NMR spectra. Whereas only one singlet 1H NMR signal is
expected for the freely rotating para-phenylenes because of
the highly symmetrical structure of 1, a multiplet signal was
observed for the corresponding protons attached to the para-
phenylenes in the tetraazacyclophane moieties. In addition,
the 13C NMR spectrum of 1 also exhibits not 16-line but 17-
line signals in the aromatic region, thus indicating that the
free rotation of the para-phenylenes in the tetraazacyclo-
phane moieties is prohibited even in solution at room
temperature. In contrast, judging from the reported solution
NMR spectra, free rotation of para-phenylene rings have
been confirmed for the tetraazacyclophanes[7a,c] and their two-
dimensionally expanded polymacrocycles.[4,5b]
As shown in Figure 2, direct information about the
structure of 1 was obtained by X-ray crystallographic analysis
of colorless needlelike single crystals grown from a mixed
solution (benzene/n-hexane) (Table S1 in the Supporting
Figure 1. Frontier MOs for 1’ and their relative energy levels at the
B3LYP/6-31G* level of theory.
The target molecule 1 was successfully prepared from
1,3,5-benzenetriamine 2[12] and p-dibromobenzene in a one-
pot manner by using the Buchwald–Hartwig cross-coupling
amination reaction[13] (Scheme 2).[14] A solution of p-dibro-
mobenzene in toluene was slowly added to a stirred mixture
of 2, P(tBu)3, [Pd(dba)2], and NaOtBu in toluene at 858C over
2 h, and the reaction mixture was stirred further at 858C. The
reaction was monitored by MALDI-MS; the spectra exhib-
ited an increase of a peak assigned not to 1 (m/z = 2715) but to
a compound with a tetraazacyclophane framework 3 (m/z =
1284). After 5 h stirring, an additional solution of p-dibro-
mobenzene in toluene was added to the stirred reaction
mixture over 2 h, and subsequently the reaction mixture was
stirred for a further 12 h at 858C. Finally, macrocyclization to
1 was confirmed by the appearance of a peak at m/z = 2715,
and purification by silica gel column chromatography
afforded 1 in 3.5% yield as a white powder. Evidence of
Figure 2. a) ORTEP representations of 1. Crystallization solvent mole-
cules (benzene and n-hexane), n-butoxy groups, and hydrogen atoms
are omitted for clarity; nitrogen atoms are colored in black; ellipsoids
are set at 50% probability; b) another view including n-hexane; c) side
views.
Information).[15] In the crystal, each molecule of 1 possesses
a crystallographic centrosymmetric point, and moreover, is
associated with two benzene molecules and 1.5 n-hexane
molecules, which penetrate the cavity (ca. 5 ꢁ ꢂ 8 ꢁ) formed
between the two tetraazacyclophane macrocycles (Figure 2b
and c). In addition, two tetraazacyclophane moieties are
sideslipped by 3 ꢁ in parallel with each other in association
with an inclination (568) of the two PD bridges.
Electrochemical analysis of 1 indicated multi-electron
redox activity originating from six PD units: quasi-two-
electron transfer (ꢀ0.138 (2e) [V vs. Fc0/+ (number of
electrons)]); two one-electron transfers (+ 0.046 (1e) and
+ 0.137 (1e)); quasi-two-electron transfer (+ 0.292 (2e));
quasi-four-electron transfer (+ 0.511 (2e), + 0.560 (1e), and
+ 0.598 (1e)); quasi-two-electron transfer (+ 0.666 (1e) and
+ 0.702 (1e)). Thus, 1 can be oxidized up to a dodecacation on
Scheme 2. Synthesis of 1. dba=trans,trans-dibenzylideneacetone.
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the cyclic and/or differential pulse voltammetric timescale
(Figure 3). Under the same conditions, the oxidation poten-
tials for the tetraazacyclophane 4 as a reference compound
are ꢀ0.01 (1e), + 0.22 (1e), + 0.54 (1e), and + 0.67 (1e).[5b]
Therefore, it is deducible that removal of the first six electrons
from 1 leads to the generation of six PD-based semiquinone
radical cations, while the removal of further six electrons
corresponds to the generation of six diamagnetic PD-based
quinone dications.
Figure 3. Cyclic voltammogram (CV) and differential pulse voltammo-
gram (DPV) of 1, measured in CH2Cl2 containing 0.1m nBu4NBF4 at
298 K (scan rate 100 mVsꢀ1). The simulated DPV[16] is drawn with
a dotted line and the oxidation potentials were estimated by the DPV
simulation (see text for details).
To gain more insight into the electronic structure corre-
sponding to each oxidation state indicated in the electro-
chemical study, we recorded changes in the absorption
spectrum upon electrochemical oxidation of 1 to 16+ by
using an optically transparent thin-layer electrochemical cell
(Figure 4). As shown in Figure 4a, the growing lower energy
bands at 1.37 eV (lmax = 904 nm) and 2.23 eV (lmax = 556 nm)
for 1+ were gradually red-shifted to 1.28 eV (lmax = 966 nm)
and 2.14 eV (lmax = 579 nm) with an isosbestic point at
3.44 eV (360 nm) upon oxidation to 12+. This observation
strongly suggests that the radical cation 1+C can be generated
by treatment with less than one equiv of an oxidant, although
the first oxidation process can be considered to be a two-
electron transfer process based on the present electrochem-
ical study. Further oxidation into 16+ was accompanied by the
continuous increase in intensity of the lowest energy band
which is assignable to the charge-resonance (CR) interva-
lence (IV) band originating from generation of the semi-
quinone radical cation in the PD unit.[17] Finally the absorb-
ance intensity reached up to about thrice that of 12+, thus
indicating that all the PD moieties were converted to the
corresponding semiquinone radical cations (Figure 4b).
When 16+ was oxidized further, the intense CR IV band
rapidly decreased together with the higher energy band at
3.01 eV (lmax = 412 nm), which had been observed for 16+
Figure 4. UV/Vis-NIR absorption spectra of the stepwise electrochem-
ical oxidation of 1 in CH2Cl2 with 0.1m nBu4NBF4 at 298 K: a) 1 to 12+
b) 12+ to 16+; c) further oxidation process from 16+
;
.
(Figure 4b), and simultaneously, a new band at 1.51 eV
(lmax = 823 nm) grew with two isosbestic points [1.44 eV
(863 nm) and 2.70 eV (460 nm)]. This new band corresponds
to the conversion from the semiquinoidal radical cation into
the diamagnetic quinoidal dication in the PD units (Fig-
ure 4c).
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The solution ESR spectrum of 1+ generated by chemical
oxidation with 0.5 equivalents of tris(4-bromophenyl)ami-
nium hexachloroantimonate (Magic Blue)[18] in CH2Cl2 at
295 K exhibited a multiplet hyperfine structure at g = 2.0029
(Figure S2a in the Supporting Information). The observed
spectrum was reasonably reproduced by the following hyper-
fine coupling (hfc) constants: aN1 = 0.250 mT (4N) and aN2
=
0.035 mT (8N), and the contribution from unresolved hydro-
gen nuclei was assumed to be incorporated in the linewidth of
the spectrum simulation (0.20 mT) (Figure S2b), indicating
that the spin density resides mainly on the two PD bridges
(Figure 5a). However, on closer inspection of the spin-density
distribution over the tetraazacyclophane moieties in 1’+
(Figure 5a) we found that no spin density is distributed on
the inner PD units, whereas a part of the spin density is on the
Figure 6. a) Temperature-dependent ESR spectra of 1+ in CH2Cl2 and
b) the simulated spectra.
The electrochemical and spectroelectrochemical studies
of 1 (Figures 3 and 4) indicate that the dicationic, tetracat-
ionic, and hexacationic species are accessible by appropriate
chemical oxidation, and therefore, their spin multiplicities at
low temperatures are of great interest in view of the
realization of cylindrical multispin systems. Polycations 12+
and 14+ were readily generated by adding 2 and 4 equivalents
of Magic Blue at 195 K in CH2Cl2. However, treatment of
1 with 6 or more equivalents of Magic Blue did not generate
16+. At this stage, we did not search for oxidizing agents
suitable for the generation of 16+. Information about the spin
multiplicities at 5 K for 12+ and 14+ were unequivocally
obtained by pulsed ESR spectroscopy detecting the electron
spin transient nutation (ESTN) signal.[20] As is apparent from
Figure 7a and Table 1, the nutation signal of 25.4 MHz can be
assigned to the spin-triplet state of 12+, judging from the
nutation signal (18.6 MHz) due to the spin-doublet impurity.
Note that no noticeable zero-field splitting was observed in
the nutation signal for the spin-triplet 12+, indicating that the
average distance between the two unpaired electrons is quite
great. In fact, it is anticipated that the quasi-degenerate
HOMO and (HO-1)MO of 1’ (Figure 1) can be regarded as
two singly occupied MOs in 1’2+, and thus, the two unpaired
electrons are mainly distributed over two PD linkers, which
are separated by 12.83 ꢁ according to the X-ray structure
(Figure 2). Concomitantly, we observed the nutation signals
of 30.6 and 36.2 MHz due to the spin-quartet state of 13+,
which is generated simply because of a slight excess of Magic
Blue. In contrast, the ESTN spectrum for the polycationic
species generated by treatment with 4 equivalents of Magic
Blue clearly demonstrated the existence of an almost pure
spin-quintet state of 14+.[21] The two nutation signals of 27.8
and 35.5 MHz indicate S = 2 spin multiplicity of 14+, since only
a very weak signal corresponding to the excited triplet state of
14+ was detected at 20.0 MHz (Figure 7b and Table 1). This
result indicates that 14+ is in a spin-quintet state at 5 K and the
excited spin-triplet state is readily accessible owing to the very
small energy spacing, and thus four unpaired electrons are
accommodated separately in the two PD bridges and the two
Figure 5. a) Spin-density distribution of 1’+ (UB3LYP/6-31G*; black:
positive spin, white: negative spin; spin isosurface value=0.0003
electron auꢀ3), and b) a windshield-wiper-like interconversion between
two sideslipped structures at a rate constant k.
outer PD units, probably originating from the sideslipped
structure of 1 (Figures 1c and 5b).[11] Indeed, these findings
were reflected in the temperature dependence of the
observed ESR spectrum (Figure 6a). The hyperfine structure
for 1+ became vague with decreasing temperature, and we
found that such a spectral change is reproducible only by
considering a windshield-wiper-like motion interconverting
two sideslipped structures with a rate constant k (Figure 5b).
The rate constants at various temperatures were determined
by spectral simulations of the observed ESR spectra by using
the ESR-EXN program on the basis of the stochastic
Liouville method (Figure 6b).[19] In the measured temper-
ature range, plots of ln(k) versus 1/T gave a linear relation-
ship, thus indicating an Arrhenius-type temperature depend-
ence [k = Aexp(ꢀDG*/kB T), where kB is the Boltzmann
constant] (Figure S3). Consequently, from the Arrhenius
plots, the barrier to thermal interconversion between two
sideslipped structures, DG*, and the prefactor A were
estimated to be 5.0 kcalmolꢀ1 and 2.3 ꢂ 1011 sꢀ1, respectively.
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extension to cylindrical multispin systems, which we are now
actively pursuing.
Received: August 23, 2012
Published online: November 20, 2012
Keywords: arylamines · cyclophanes · electrochemistry ·
.
ESR spectroscopy · radical ions
[1] Deltaphane: a) H. C. Kang, A. W. Hanson, B. Eaton, V. Boe-
kelheide, J. Am. Chem. Soc. 1985, 107, 1979; Kohnkene: b) F. H.
Kohnke, A. M. Z. Slawin, J. F. Stoddart, D. J. Williams, Angew.
Chem. 1987, 99, 941; Angew. Chem. Int. Ed. Engl. 1987, 26, 892;
Collarene: c) P. R. Ashton, N. S. Isaacs, F. H. Kohnke, A. M. Z.
Slawin, C. M. Spencer, J. F. Stoddart, D. J. Williams, Angew.
Chem. 1988, 100, 981; Angew. Chem. Int. Ed. Engl. 1988, 27, 966;
d) J. Benkhoff, R. Boese, F.-G. Klꢃrner, Liebigs Ann. 1997, 501;
Beltane: e) F. Vçgtle, A. Schrçder, D. Karbach, Angew. Chem.
1991, 103, 582; Angew. Chem. Int. Ed. Engl. 1991, 30, 575;
Pseudobeltane: f) H. Schwierz, F. Vçgtle, Synthesis 1999, 295;
Belt cyclophane: g) H. Meier, K. Mꢄller, Angew. Chem. 1995,
107, 1598; Angew. Chem. Int. Ed. Engl. 1995, 34, 1437; Picotube:
h) S. Kammermeier, P. G. Jones, R. Herges, Angew. Chem. 1996,
108, 2834; Angew. Chem. Int. Ed. Engl. 1996, 35, 2669; Cyclic
paraphenylacetylene: i) T. Kawase, H. R. Darabi, M. Oda,
Angew. Chem. 1996, 108, 2803; Angew. Chem. Int. Ed. Engl.
1996, 35, 2664; [8]Cyclacene triquinone: j) R. M. Cory, C. L.
McPhail, Tetrahedron Lett. 1996, 37, 1987; Vçgtle belt: k) F.
Vçgtle, Top. Curr. Chem. 1983, 115, 157; l) G. J. Bodwell, D. O.
Miller, R. J. Vermeij, Org. Lett. 2001, 3, 2093; m) C. Denekamp,
A. Etinger, W. Amrein, A. Stanger, M. Stuparu, A. D. Schlꢄter,
Chem. Eur. J. 2008, 14, 1628; [6.8]3Cyclacene: n) B. Esser, F.
Rominger, R. Gleiter, J. Am. Chem. Soc. 2008, 130, 6716.
[2] a) B. D. Steinberg, L. T. Scott, Angew. Chem. 2009, 121, 5504;
Angew. Chem. Int. Ed. 2009, 48, 5400; b) R. Jasti, C. R. Bertozzi,
Chem. Phys. Lett. 2010, 494, 1; c) T. Iwamoto, Y. Watanabe, Y.
Sakamoto, T. Suzuki, S. Yamago, J. Am. Chem. Soc. 2011, 133,
8354; d) T. J. Sisto, R. Jasti, Synlett 2012, 483; e) S. Schrettl, H.
Frauenrath, Angew. Chem. 2012, 124, 6673; Angew. Chem. Int.
Ed. 2012, 51, 6569; f) U. H. F. Bunz, S. Menning, N. Martꢅn,
Angew. Chem. 2012, 124, 7202; Angew. Chem. Int. Ed. 2012, 51,
7094; g) H. Omachi, Y. Segawa, K. Itami, Acc. Chem. Res. 2012,
45, 1378.
Figure 7. Two-dimensional ESTN spectra of 1 in CH2Cl2 at 5 K after
addition of a) 2 equiv and b) 4 equiv of Magic Blue.
Table 1: Spectroscopic data of the ESTN spectroscopy for 12+ and 14+
.
Species
Observed nutation frequen-
cy [MHz][a]
Transition assignment
pffiffi
12+
25.4 (wt = 2 wd)
18.6 (wd)
j1, ꢁ1i,j1, 0i
j1/2, +1/2i,j1/2,
ꢀ1/2i
doublet
impurity
quartet
impurity
pffiffi
30.6 (wq,1
=
3 wd)
j3/2, ꢁ3/2i,j3/2,
ꢁ1/2i
36.2 (wq,2 =2 wd)
j3/2, +1/2i,j3/2,
ꢀ1/2i
pffiffi
[3] a) A. Ito, Y. Ono, K. Tanaka, J. Org. Chem. 1999, 64, 8236;
b) R. J. Bushby, C. A. Kilner, N. Taylor, M. E. Vale, Tetrahedron
2007, 63, 11458; c) M. Vale, M. Pink, S. Rajca, A. Rajca, J. Org.
Chem. 2008, 73, 27; d) K. Ishibashi, H. Tsue, N. Sakai, S. Tokita,
K. Matsui, J. Yamauchi, R. Tamura, Chem. Commun. 2008, 2812.
[4] X. Z. Yan, J. Pawlas, T. Goodson III, J. F. Hartwig, J. Am. Chem.
Soc. 2005, 127, 9105.
14+
27.8 (wquintet,1
35.5 (wquintet,2
=
=
2 wt)
j2, ꢁ2i,j2, ꢁ1i
j2, ꢁ1i,j2, 0i
j1, ꢁ1i,j1, 0i
pffiffi
3 wt)
excited triplet 20.0 (wt)
[a] d=doublet, t=triplet, q=quartet.
[5] a) A. Ito, S. Inoue, Y. Hirao, K. Furukawa, T. Kato, K. Tanaka,
Chem. Commun. 2008, 3242; b) A. Ito, Y. Yanagishi, K. Fukui, S.
Inoue, Y. Hirao, K. Furukawa, T. Kato, K. Tanaka, Chem.
Commun. 2008, 6573.
tetraazacyclophane moieties, as is expected from the HOMO
to (HO-3)MOs on 1’ (Figure 1).
In summary, the newly prepared pseudobeltane-like
polymacrocyclic oligoarylamine 1 displays multi-electron
redox activity up to the electrochemical generation of
dodecacation, and moreover, the diradical dication 12+ and
tetraradical tetracation 14+ generated by chemical oxidation
were found to be in high-spin states at low temperatures. In
addition, dynamic ESR spectroscopy revealed the thermally
activated interconversion between two sideslipped structures
in radical cation 1+. The present toroidal multispin system is
thus expected to serve as a useful building block in the
[6] a) R. J. Bushby, D. R. McGill, K. M. Ng, N. Taylor, Chem.
Commun. 1996, 2641; b) R. J. Bushby, D. R. McGill, K. M. Ng,
N. Taylor, J. Chem. Soc. Perkin Trans. 2 1997, 1405. On the other
hand, for macrocyclic high-spin systems containing neutral
radical centers the Coulombic penalty does not apply; see, for
instance: carbon-centered radicals: c) A. Rajca, S. Rajca, S. R.
Desai, J. Am. Chem. Soc. 1995, 117, 806; d) K. Matsuda, N.
Nakamura, K. Takahashi, K. Inoue, N. Koga, H. Iwamura, J. Am.
Chem. Soc. 1995, 117, 5550; nitroxide radicals: e) A. Rajca, S.
Mukherjee, M. Pink, S. Rajca, J. Am. Chem. Soc. 2006, 128,
13497; f) T. Sawai, K. Sato, T. Ise, D. Shiomi, K. Toyota, Y.
12780 www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Morita, T. Takui, Angew. Chem. 2008, 120, 4052; Angew. Chem.
Int. Ed. 2008, 47, 3988; g) A. Olankitwanit, V. Kathirvelu, S.
Rajca, G. R. Eaton, S. S. Eaton, A. Rajca, Chem. Commun. 2011,
47, 6443.
[14] Details on the synthesis of 1 are given in the Supporting
Information.
[15] CCDC 879828 (1) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[16] W. Huang, T. L. E. Henderson, A. M. Bond, K. B. Oldham,
Anal. Chim. Acta 1995, 304, 1.
[17] a) A. Ishitani, S. Nagakura, Mol. Phys. 1967, 12, 1; b) B. Badger,
B. Brocklehurst, R. D. Russell, Chem. Phys. Lett. 1967, 1, 122;
c) A. V. Szeghalmi, M. Erdmann, V. Engel, M. Schmitt, S.
Amthor, V. Kriegisch, G. Nçll, R. Stahl, C. Lambert, D. Leusser,
D. Stalke, M. Zabel, J. Popp, J. Am. Chem. Soc. 2004, 126, 7834;
d) Y. Hirao, A. Ito, K. Tanaka, J. Phys. Chem. A 2007, 111, 2951;
e) A. Ito, D. Sakamaki, Y. Ichikawa, K. Tanaka, Chem. Mater.
2011, 23, 841.
[18] a) F. A. Bell, A. Ledwith, D. C. Sherrington, J. Chem. Soc. 1969,
2719; b) N. G. Connelly, W. E. Geiger, Chem. Rev. 1996, 96, 877.
[19] a) J. Heinzer, Mol. Phys. 1971, 22, 167; b) Quantum Chemistry
Program Exchange 1972, No. 209. We thank Prof. S. F. Nelsen for
a copy of this program (modified by P. A. Petillo, R. F. Ismagilov,
and Y. Hirao).
[7] a) A. Ito, Y. Ono, K. Tanaka, Angew. Chem. 2000, 112, 1114;
Angew. Chem. Int. Ed. 2000, 39, 1072; b) T. D. Selby, S. C.
Blackstock, Org. Lett. 1999, 1, 2053; c) S. I. Hauck, K. V.
Lakshmi, J. F. Hartwig, Org. Lett. 1999, 1, 2057; d) I. Kulsze-
wicz-Bajer, V. Maurel, S. Gambarelli, I. Wielgus, D. Djurado,
Phys. Chem. Chem. Phys. 2009, 11, 1362.
[8] D. Sakamaki, A. Ito, K. Furukawa, T. Kato, K. Tanaka, Chem.
Commun. 2009, 4524.
[9] The present compound is structurally akin to Gleiterꢀs p-boat
cage compounds: R. Gleiter, K. Hçvermann, J. Ritter, B. Nuber,
Angew. Chem. 1995, 107, 867; Angew. Chem. Int. Ed. Engl. 1995,
34, 789.
[10] a) Diradicals (Ed.: W. T. Borden), Wiley, New York, 1982; b) H.
Iwamura, Adv. Phys. Org. Chem. 1991, 26, 179; c) D. A.
Dougherty, Acc. Chem. Res. 1991, 24, 88; d) A. Rajca, Chem.
Rev. 1994, 94, 871; e) J. A. Crayston, J. N. Devine, J. C. Walton,
Tetrahedron 2000, 56, 7829; f) K. Yoshizawa, A. Chano, A. Ito,
K. Tanaka, T. Yamabe, H. Fujita, J. Yamauchi, M. Shiro, J. Am.
Chem. Soc. 1992, 114, 5994; g) K. R. Stickley, S. C. Blackstock, J.
Am. Chem. Soc. 1994, 116, 11576; h) K. R. Stickley, T. D. Selby,
S. C. Blackstock, J. Org. Chem. 1997, 62, 448; i) M. M. Wienk,
R. A. J. Janssen, J. Am. Chem. Soc. 1997, 119, 4492; j) K. Sato, M.
Yano, M. Furuichi, D. Shiomi, T. Takui, K. Abe, K. Itoh, A.
Higuchi, K. Katsuma, Y. Shirota, J. Am. Chem. Soc. 1997, 119,
6607, and references therein.
[20] a) J. Isoya, H. Kanda, J. R. Norris, J. Tang, M. K. Brown, Phys.
Rev. B 1990, 41, 3905; b) A. V. Astashkin, A. Schweiger, Chem.
Phys. Lett. 1990, 174, 595.
[21] As pointed out by one of the reviewers, it is important to
quantify the fraction of the generated tetracationic species of
1 in CH2Cl2 solution. As shown in the Suppoting Information
(Figure S4), we have confirmed the quantitative generation of
14+ by chemical oxidation of bis(trifluoroacetoxy)iodobenzene
(PIFA), which is one of the diamagnetic one-electron oxidizing
agents [L. Eberson, M. F. Hartshorn, O. Persson, Acta Chem.
Scand. 1995, 49, 640], in comparison with a stable monoradical
(2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO)) as a reference,
and the solution exhibited the same ESTN spectrum at 5 K
(Figure 7b). The tetracation 14+ was found to be stable in
solution under inert atmosphere at room temperature for three
months or longer. However, we failed to isolate the salt of 14+
which shows a satisfactory elemental analysis, probably due to
partial decomposition.
[11] The full geometry optimizations of 1’ and 1’+ in C2h symmetry
were performed by using the Gaussian09 program package:
Gaussain09 (RevisionC.01), M. J. Frisch et al. Detailed geo-
metrical information is given in the Supporting Information.
[12] Ng. Ph. Buu-Hoꢆ, J. Chem. Soc. 1952, 4346.
[13] a) J. P. Wolfe, S. Wagaw, J. F. Marcoux, S. L. Buchwald, Acc.
Chem. Res. 1998, 31, 805; b) J. F. Hartwig, Acc. Chem. Res. 1998,
31, 852; c) J. F. Hartwig, Angew. Chem. 1998, 110, 2154; Angew.
Chem. Int. Ed. 1998, 37, 2046; d) A. R. Muci, S. L. Buchwald,
Top. Curr. Chem. 2002, 219, 133.
Angew. Chem. Int. Ed. 2012, 51, 12776 –12781
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