Specific interaction between chloroform and the pockets of triangular annulene derivatives providing symmetry carry-Oer crystallization

Article abstract of DOI:10.1002/chem.200901848

"Chemical Equation Presented" What's it got in its pockets? Symmetry carry-over crystallization of hexadehydrotribenzo[12]annulene derivatives was provided by specific complexation between the annulene and the chloroform molecule through mainly dispersion

Full text of DOI:10.1002/chem.200901848

DOI: 10.1002/chem.200901848
Specific Interaction between Chloroform and the Pockets of Triangular
Annulene Derivatives Providing Symmetry Carry-Over Crystallization
Ichiro Hisaki,*[a] Hirofumi Senga,[a] Yuu Sakamoto,[a] Seiji Tsuzuki,*[b]
Norimitsu Tohnai,[a] and Mikiji Miyata*[a]
Hexadehydrotribenzo[12]annulene derivatives (DBAs)[1]
are molecules that have the following structural characteris-
tics: 1) delocalized p-electron clouds on the conjugated
cycle and 2) a well-defined triangle geometry.[2] Because of
these aspects, DBAs have been recently investigated from
such viewpoints as optoelectronic materials[3] and building
blocks for supramolecular assemblies.[4] DBAs are also at-
tractive building blocks for crystalline superstructures.[5] In
this context, we designed C3-symmetric DBAs 1 and 2, as
well as 3 for a control (Figure 1a), because 1 and 2 are ex-
pected to undergo symmetry carry-over crystallization
(SCC)[6] through weak hydrogen bonds between the elec-
tron-donating substituents and aromatic hydrogen atoms as
acceptors (Figure 1b). Furthermore, the rotationally flexible
substituents of the DBAs can fine-tune special noncoinci-
dence when the molecules are packed into the crystals.
During this study on crystal engineering of the DBAs, we
serendipitously found exotic complexes of the DBAs with
chloroform in crystals of 1 and 2 (Figure 1c). In the com-
plexes, a hydrogen atom of the chloroform fits into the
DBAꢀs “pocket” surrounded by p electrons, which is actual-
ly another structural feature of the DBAs in addition to the
two mentioned above. Such a complex of DBA with a neu-
tral molecule has been hitherto unknown, although excellent
organometallic complexes of DBAs using the pocket were
reported by Youngs and co-workers.[7] Furthermore, the
complexation seems to play a significant role for SCC in the
present system.[8] Namely, chloroform assists the DBAs to
form 2D layered networks with C3-symmetric inclusion
[a] Dr. I. Hisaki, H. Senga, Y. Sakamoto, Dr. N. Tohnai,
Prof. Dr. M. Miyata
Department of Material and Life Science
Graduate School of Engineering
Osaka University, 2-1 Yamadaoka
Suita, Osaka 565-0871 (Japan)
Fax : (+81)6-6879-7406
E-mail: hisaki@mls.eng.osaka-u.ac.jp
miyata@mls.eng.osaka-u.ac.jp
[b] Dr. S. Tsuzuki
Research Institute of Computational Sciences
National Institute of Advanced Industrial Science and Technology
Tsukuba Ibaraki 305-8568 (Japan)
Fax : (+81)29-851-5426
Figure 1. a) Structures of DBA derivatives 1–3. b) A possible molecular
arrangement with C3 symmetry, in which electron donors interact with
positively charged aromatic hydrogen atoms. c) Complexation of chloro-
form with DBAꢀs pocket. Schematic representations of 2D (d) and 3D
(e) packing of DBA and chloroform. For 3D packing, the complex can
pile up either monomerically or dimerically.
E-mail: s.tsuzuki@aist.go.jp
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200901848.
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COMMUNICATION
space (Figure 1d), and subsequently the complexation be-
tween the chloroform and the DBA allows one layer to fit
in the adjacent ones with aligning their C3-rotational axes
(Figure 1e). Herein, we describe crystal structures of DBAs
1 and 2, roles of chloroform for SCC, and quantitative eval-
uation for the origin of attractive interactions in the com-
plexes.
Syntheses of the DBAs were performed according to
Scheme S1 in Supporting Information. Crystallographic
analysis revealed that 1 and 2 gave coplanar structure with
C3-axes, while 3, which has no electron-donating groups,
gave no coplanar but zigzag b-type[9] structure.[10]
DBA 1 crystallized into the space group of R3c with ac-
companying inclusion of chloroform in a 1:1 molar ratio as
shown in Figure 2a–d.[11] DBA molecules in green color are
arranged on the ab plane coplanar, and the molecules in
orange on the neighbor plane are inverted (Figure 2a). The
distance between the layers composed of green and orange
molecules is 2.28 ꢂ (Figure 2b). Since the substituents of 1
are not long enough to interact with the neighbors on the
same plane, they interact with the DBAs on the adjacent
À
layer; the nitrogen atoms of the pyridines form C H···N
bonds (i–iii) with the aromatic hydrogen atoms of the DBA
on the adjacent layer (N···C distance of 3.38 ꢂ and C-H-N
angle of 133.38 in Figure 2d).[12] Chloroform molecules are
laid in C3-symmetric inclusion spaces surrounded by the pyr-
idine rings staggered by 35.28 from the DBA plane, though
directional, strong interactions are not observed between
the chloroform and the pyridine rings. However, the chloro-
form forms a complex with 1 as described above. The dis-
tance between the carbon atom of the chloroform and the
DBA plane is 2.99 ꢂ (Figure 2b). Figure 2c shows a sche-
matic sectional view along the (110) plane. Chloroform mol-
ecules on the DBAs fit into the C3-symmetric cavities so-
phisticatedly to pile up the layers along the c axis with the
chloroform molecules facing to the same direction, con-
structing the 3D crystal structure.
Figure 2. Crystal structures of 1·CHCl3 (a–d) and 2·CHCl3 (e–h). a,e) Mo-
lecular arrangements in the layered motifs, where the DBAs are colored
in green and orange and chloroform in red. b,f) Complexes of the DBA
and chloroform molecule. c,g) Schematic sectional views along the (110)
plane. In the view g), one of the three chloroform molecules in the cavity
are disordered. d,h) Intermolecular interactions around the included
chloroform molecules with thermal ellipsoids at the 50% probability.
Symmetry codes for d) A: 5/3Ày, 4/3Àx, 1/6+z; B: 1Àx+y, 1Àx, z; C: 2/
3Àx+y, À2/3+y, À1/6+z; D: 1Ày, xÀy, z: E: À1/3+x, 1/3+xÀy, À1/6+
z. Symmetry codes for h) A: 1Ày, xÀy, x; B: 1Àx+y, 1Àx, z.
¯
DBA 2 crystallized into the space group of R3c with ac-
companying inclusion of chloroform in a 2:3 molar ratio as
shown in Figure 2e–h.[11] Contrary to 1, compound 2 forms a
p-stacked dimer (a pair of green and orange molecules)
with an interplanar distance of 3.19 ꢂ, probably due to
dipole interactions between the carboxylate moieties. Both
faces of the dimer are occupied by chloroform to form the
complex (Figure 2 f). The distance between the carbon atom
of the chloroform and the DBA plane in the complex is
3.29 ꢂ. The dimers are then hexagonally packed to form the
À
coplanar structure, which is stabilized by intermolecular C
H···O interactions between the hydrogen atoms in the ben-
one cavity, totally three chloroform molecules are included
on a line.
Crystallization of 2 from THF also gave a R3c crystal, a
void space of which accommodates three disordered THF
molecules (crystal 2·THF).[10] This indicates that strict C3-
symmetric guest molecule is not necessary for 2 to experi-
ence C3-SCC probably due to relatively robust network of
the DBA.[13] A solution of 1 in CH2Cl2/hexane , on the other
hand, gave a crystal of the hydrate, instead of CH2Cl2 sol-
vate, with the space group of C2/c, although the DBAs were
¯
À
zene ring and the carbonyl oxygen atoms (i: O1···H C8A,
À
À
ii: O1A···H C8B, and iii O1B···H C8 with C···O distance of
3.44 ꢂ and C-H-O angle of 147.08 in Figure 2 h).[12] The ben-
zene rings, staggered by 20.78 from the DBA plane, interact
with the chloroform (iv: C13 H···Cl1, v: C13A H···Cl1A,
and vi: C13B H···Cl1B with the C···Cl distance of 3.64 ꢂ
À
À
À
and Cl-H-C angle of 132.88). The sectional view (Figure 2 g)
shows that the complexes pile up along the c axis, and in
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I. Hisaki, S. Tsuzuki, M. Miyata et al.
[10]
¯
arranged coplanar on the (404) plane. These indicate that
chloroform molecules must contribute to C3-SCC of DBAs,
at least in the case of crystal 1·CHCl3. Chloroform molecules
can act as a template of the C3-void space. Furthermore,
specific interaction between chloroform in the C3-void space
and the C3-pocket of DBA allows the C3-rotation axes in
one layer to align with those in the adjacent layers, leading
to the construction of C3-symmetric crystals.
In an early stage of this study, we believed that the attrac-
tive interaction to form the complexes should be the electro-
static one between the acidic hydrogen atom of the chloro-
form and p-electron clouds of three acetylene units: namely,
a CH/p interaction with a hydrogen bond character. Howev-
er, Tsuzuki and co-workers suggested us another possible
candidate: dispersion force between the benzene rings of
the DBA and the chlorine atoms.[14] To certify the origin of
the affinity between the DBA core and chloroform molecule
quantitatively, we applied ab initio molecular orbital calcula-
tions for the following four model complexes (Figure 3): A)
Figure 4. Intermolecular interaction potential curves of complexes a) A,
b) B, c) C, and d) D calculated by the HF (open square) and MP2 (solid
circle) method with cc-pVTZ basis set. Small open circles in a) corre-
spond to the complexes observed in crystals 1·CHCl3 and 2·CHCl3. In b)–
d), the curves for complex A are described in gray for easy comparison.
force.[15] Figure 4b demonstrates that the HF level potential
of the complex B has no minimum, indicating that almost no
electrostatic attraction works in the complex B. The MP2
level potential of the complex B has a much shallower mini-
mum than that of the complex A. Therefore, in the com-
plex A, the chlorine atoms strongly contribute to the disper-
sion interaction.
Next, the orientation effects of the chloroform were inves-
tigated by comparing the potentials of the complex A with
those of the complexes C and D. Instead of shallow mini-
mum in the HF potential of the complex A, that of the com-
plex C has no minimum (Figure 4c), indicating that the elec-
Figure 3. Four model complexes for the ab initio computational study. R
denotes the distances between the carbon atom of the chloroform and
the DBA plane.
À
a DBA–chloroform complex in which the hydrogen atom of
the chloroform is directed into the center of the pocket, and
the chlorine atoms are located above the benzene rings (the
complexes experimentally observed in the crystals corre-
spond to the complex A); B) a DBA–methane complex in
which the chlorine atoms of the chloroform molecule are re-
placed by hydrogen atoms although the geometry is same as
the complex A; C) a DBA–chloroform complex in which
the hydrogen atom of the chloroform is facing in the oppo-
site direction to that of the complex A; and D) a DBA–
chloroform complex in which the chlorine atoms are located
over the acetylene units. The intermolecular distance (R)
between the carbon atom of the chloroform and the DBA
plane is indicated for each model in Figure 3.
Figure 4 shows the interaction energy (E) plotted versus
R at the HF and MP2 levels using the cc-pVTZ basis set. It
can be seen from Figure 4a that the HF level potential of
the complex A has a very shallow minimum, while that of
the MP2 level is deep. This indicates that the contribution of
the electrostatic interaction to the attractive force is small
and the major source of the attractive force is the dispersion
trostatic interaction is attractive when the C H bond of
chloroform points toward the DBA. The potential of the
complex D (Figure 4d) is slightly shallower than that of the
complex A. The dispersion interactions of chlorine atoms
with the benzene rings are larger than those with the triple
bonds.
The interaction energies at the potential minima are sum-
marized in Table 1. The MP2/cc-pVTZ level interaction en-
Table 1. Intermolecular interaction energies [kcalmolÀ1] of complexes A,
B, C, and D at interaction potential minimum.
[b]
[c]
[d]
[e]
R[a]
EMP2
EHF
Ecorr
Ees
A
B
C
D
2.8
3.4
3.8
3.0
À10.75
À2.26
À6.73
À8.79
5.11
1.31
6.46
3.08
À15.86
À3.57
À1.97
0.17
0.51
À1.44
À13.20
À11.87
[a] Distance [ꢂ] between the carbon atom of the chloroform and DBA
plane. [b] The MP2 level interaction energy. [c] The HF level interaction
energy. [d] The correlation interaction energy (Ecorr =EMP2ÀEHF), which
is mainly the dispersion energy. [e] The electrostatic energy, calculated as
an interaction between distributed multipoles of monomers obtained
from the MP2/6-311G** wave functions of isolated molecules.
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Research in Evolutional Area (CITY AREA) program by the Ministry
of Education, Culture, Sports, Science and Technology, Japan.
ergies (EMP2) for the complexes A–D are À10.75, À2.26,
À6.73, and À8.79 kcalmolÀ1, respectively. Complex A has
the largest (most negative) interaction energy among them,
supporting the validity of the experimentally observed struc-
tures, in which the hydrogen atom of chloroform fits into
the pocket of the DBA. The correlation interaction energy
of the complex A (Ecorr =À15.86 kcalmolÀ1) is significantly
larger than that of the complex B, indicating that the chlor-
ine atoms particularly contribute to the dispersion. The elec-
trostatic interaction (Ees) also acts as an attractive force in
the complex A, although its magnitude is smaller by a factor
Keywords: ab initio calculations
· annulenes · crystal
engineering · dispersion energy · supramolecular chemistry
[1] a) H. A. Staab, F. Graf, Tetrahedron Lett. 1966, 7, 751–757; b) I. D.
Campbell, G. Eglinton, W. Henderson, R. A. Raphael, Chem.
Commun. 1966, 87–89.
[2] a) C. S. Jones, M. J. OꢀConnor, M. M. Haley in Acetylene Chemistry
(Eds.: F. Diederich, P. J. Stang, R. R. Tykwinski), Wiley-VCH, Wein-
heim, 2005, pp. 303–385; b) U. H. F. Bunz, Y. Rubin, Y. Tobe,
Chem. Soc. Rev. 1999, 28, 107–119; c) E. L. Spitler, C. A. Johnson
II, M. M. Haley, Chem. Rev. 2006, 106, 5344–5386.
[3] a) J. J. Pak, T. J. R. Weakley, M. M. Haley, J. Am. Chem. Soc. 1999,
121, 8182–8192; b) A. Sarkar, J. J. Pak, G. W. Rayfield, M. M.
Haley, J. Mater. Chem. 2001, 11, 2943–2945; c) J. A. Marsden, J. J.
Miller, M. M. Haley, Angew. Chem. 2004, 116, 1726–1729; Angew.
Chem. Int. Ed. 2004, 43, 1694–1697; d) M. Sonoda, Y. Sakai, T.
Yoshimura, Y. Tobe, K. Kamada, Chem. Lett. 2004, 33, 972–973;
e) J. A. Marsden, J. J. Miller, L. D. Shirtcliff, M. M. Haley, J. Am.
Chem. Soc. 2005, 127, 2464–2476.
[4] Examples for molecular assemblies, see: a) H. Enozawa, M. Hasega-
wa, D. Takamatsu, K. Fukui, M. Iyoda, Org. Lett. 2006, 8, 1917–
1920; b) S. H. Seo, T. V. Jones, H. Seyler, J. O. Peters, T. H. Kim,
J. Y. Chang, G. N. Tew, J. Am. Chem. Soc. 2006, 128, 9264–9265;
c) S. H. Seo, J. Y. Chang, G. N. Tew, Angew. Chem. 2006, 118, 7688–
7692; Angew. Chem. Int. Ed. 2006, 45, 7526–7530; d) K. Tahara, S.
Furukawa, H. Uji-i, T. Uchino, T, Ichikawa, J. Zhang, W. Mam-
douh, M. Sonoda, F. C. De Schryver, S. De Feyter, Y. Tobe, J. Am.
Chem. Soc. 2006, 128, 16613–16625.
[5] a) A. J. Matzger, M. Shim, K. P. C. Vollhardt, Chem. Commun. 1999,
1871–1872; b) I. Hisaki, Y. Sakamoto, H. Shigemitsu, N. Tohnai, M.
Miyata, S. Seki, A. Saeki, S. Tagawa, Chem. Eur. J. 2008, 14, 4178–
4187; c) K. Tahara, T. Fujita, M. Sonoda, M. Shiro, Y. Tobe, J. Am.
Chem. Soc. 2008, 130, 14339–14345.
[6] One of the interesting issues for crystal engineering is how molecu-
lar symmetry effects on that of molecular aggregation motifs, and
then, hole crystals. To answer this question, G. R. Desiraju proposed
the importance of symmetry carry-over crystallization (SCC), point-
ing to C3-symmetric molecules as examples, see: a) A. Anthony,
G. R. Desiraju, R. K. R. Jetti, S. S. Kuduva, N. N. L. Madhavi, A.
Nangia, R. Thaimattam, V. R. Thallad, Cryst. Eng. 1998, 1, 1–18;
b) P. K. Thallapally, K. Chakraborty, A. K. Katz, H. L. Carrell, S.
Kotha, G. R. Desiraju, CrystEngComm 2001, 31, 1–3.
of eight than that of the dispersion forces, that is, Ecorr
.
In Figure 4a, two open circles correspond to the com-
plexes experimentally observed in crystals 1·CHCl3 and
2·CHCl3. The R value for 1·CHCl3 (2.99 ꢂ) is close to that
at the potential minimum and the R value for 2·CHCl3
(3.29 ꢂ) is slightly larger. Their EMP2 at these distances are
À10.4 and À8.8 kcalmolÀ1, respectively. These calculations
are consistent with their crystallization behaviors. Namely,
crystallization of 1 was significantly dependent on the sol-
vent and the molecular arrangement in crystal 1·CHCl3 re-
sults from specific strong interaction with chloroform.
Meanwhile, the molecular arrangement in crystal 2·CHCl3
was achieved even when THF was adopted as the solvent,
implying that interaction between the DBA and chloroform
is not significantly specific for controlling the crystal struc-
ture.
In conclusion, we described that C3-symmetric DBAs 1
¯
and 2 underwent SCC into the space groups of R3c and R3c,
respectively. In the crystals, the DBA core and chloroform
form the complex, in which a hydrogen atom of the chloro-
form molecule fits into a “pocket” of the DBA. The com-
plex is the first example for complexation between DBA
and an organic molecule. The calculations of the chloro-
form–DBA complexes show that most of the affinity was
produced by the dispersion forces between the benzene
rings of the DBA and the chlorine atoms (À15.86 kcal
molÀ1), while contribution of the electrostatic interaction
was only À1.97 kcalmolÀ1.
Finally, we suggest that the SCC of planer p-conjugated
molecules should require the following two key steps: 1)
construction of highly-symmetric layer assembly and 2)
stacking of the layer with orienting its symmetrical axes to
those of adjacent ones. In this study, the former step was ac-
complished by rational design of the DBAs and inclusion of
chloroform into the void space. The latter was done by spe-
cific interaction between chloroform and the pocket of the
DBA. However, many unsettled issues still remain concern-
ing SCC, such as what crucially controls the symmetry of
molecular assemblies. Further investigation is continuing in
our laboratory.
[7] Complexation with metals such as Ni was reported, see: a) J. D. Fer-
rara, C. Tessier-Youngs, W. J. Youngs, J. Am. Chem. Soc. 1985, 107,
6719–6721; b) J. D. Ferrara, C. Tessier-Youngs, W. J. Youngs, Orga-
nometallics 1987, 6, 676–678; c) A. Djebli, J. D. Ferrara, C. Tessier-
Youngs, W. J. Youngs, J. Chem. Soc. Chem. Commun. 1988, 548–
549; d) J. D. Ferrara, A. A. Tanaka, C. Fierro, C. A. Tessier-Youngs,
W. J. Youngs, Organometallics 1989, 8, 2089–2098; e) W. J. Youngs,
J. D. Kinder, J. D. Bradshaw, C. A. Tessier, Organometallics 1993, 12,
2406–2407.
[8] To date only three crystal structures are reported for SCC of DBAs
in reference [5a–c], although the last one has pseudo C3-symmetry.
[9] G. R. Desiraju, A. Gavezzotti, Acta. Crystallogr. Sect. B 1989, 45,
473–482.
[10] For the details of crystallographic analyses, see the Supporting Infor-
mation.
[11] Crystal data for 1·CHCl3: (C39H21N3)
19.8463(4) ꢂ, c=13.7227(3) ꢂ, a=b=908,
4680.87(16) ꢂ3, T=213 K, trigonal, space group R3c (no. 161), Z=
6, m
(CuKa)=2.929 mmÀ1, 1calcd =1.386 gcmÀ3, 15269 collected, 1903
ACHTUNGTRENNUNG
Acknowledgements
AHCTUNGTRENNUNG
This work was financially supported by a Grant-in-Aid for Scientific Re-
search and the Cooperation for Innovative Technology and Advanced
unique (Rint =0.084) reflections, the final R1 and wR2 values 0.050
[I>2.0s(I)] and 0.124 (all data), respectively. Crystal data for
Chem. Eur. J. 2009, 15, 13336 – 13340
ꢁ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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13339

I. Hisaki, S. Tsuzuki, M. Miyata et al.
2·CHCl3: (C48H30O6)
(CHCl3)6, Mr =3527.31, a=b=18.7299(5) ꢂ,
2002, 41, 48–76; c) G. R. Desiraju, Chem. Commun. 2005, 2995–
3001.
c=40.0102(14) ꢂ, a=b=908, g=1208, V=12155.5(6) ꢂ3, T=
¯
[13] Similar rhombohedral symmetry systems of chloroform solvate and
disordered solvate have been reported, see: R. Thaimattam, F. Xue,
J. A. R. P. Sarma, T. C. W. Mak, G. R. Desiraju, J. Am. Chem. Soc.
2001, 123, 4432–4445.
[14] a) S. Tsuzuki, K. Honda, T. Uchimaru, M. Mikami, K. Tanabe, J.
Phys. Chem. A 1999, 103, 8265–8271; b) S. Tsuzuki, K. Honda, T.
Uchimaru, M. Mikami, K. Tanabe, J. Am. Chem. Soc. 2000, 122,
3746–3753; c) K. Shibasaki, A. Fujii, M. Mikami, S. Tsuzuki, J.
Phys. Chem. A 2007, 111, 753–758; d) S. Tsuzuki, A. Fujii, Phys.
Chem. Chem. Phys. 2008, 10, 2584–2594.
[15] The HF calculation can evaluate the electrostatic and exchange–re-
pulsion interactions but cannot do the dispersion interaction, while
the MP2 calculation can evaluate the dispersion interaction, since
the dispersion interaction has its origin in electron correlation. Thus,
the HF calculation underestimates the attraction significantly com-
pared to the MP2 calculation when the dispersion interaction is
mainly responsible for the attraction in the complex: see refer-
ence [14].
123 K, trigonal, space group R3c (no. 167), Z=3,
mACHTNGUTRENNU(G CuKa)=
3.395 mmÀ1, 1calcd =1.445 gcmÀ3, 42736 collected, 2486 unique (Rint
=
0.130) reflections, The final R1 and wR2 values 0.097 [I>2.0s(I)]
and 0.300 (all data), respectively. CCDC-745440 (1·CHCl3) and
CCDC-745441 (2·CHCl3) contain the supplementary crystallograph-
ic 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.
À
[12] In connection with a weak hydrogen bond such as C H···X (X=O
or N), a distance between C and X atoms can range from 3.0 to
4.0 ꢂ, which is longer and more widely distributed than the distance
observed in stronger hydrogen bond such as O H···X (X=O or N)
bond. The hydrogen bond angle C-H-X (X=O or N) of the weak
hydrogen bond is also widely distributed in the range of 90 to 1808,
although Desiraju and Steiner recommend a lower limit of 1108
À
À
when accepting a C H···O geometry as a hydrogen bond. See:
a) G. R. Desiraju, T. Steiner, The Weak Hydrogen Bond in Structural
Chemistry and Biology, Oxford University Press, Oxford, 1999; b) T.
Steiner, Angew. Chem. 2002, 114, 50–80; Angew. Chem. Int. Ed.
Received: July 3, 2009
Published online: November 2, 2009
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