A simple general ligand system for assembling octahedral metal-rotaxane complexes

Article abstract of DOI:10.1002/anie.200353186

Full Metal Jacket: Simple stirring of a five component mixture at room temperature is sufficient to assemble a wide range of octahedrally coordinated [2] metallorotaxanes under thermodynamic control (see picture).

Full text of DOI:10.1002/anie.200353186

Angewandte
Chemie
Communications
In their Communication on the following pages, D. A. Leigh and co-
workers describe a general ligand system for rotaxane complexes of
transition-metal ions that prefer octahedral coordination—a rare
coordination mode for rotaxanes. Simple mixing of the components
at room temperature is sufficient to assemble a broad range of
octahedrally coordinated [2]metallorotaxanes in high yields.
Angew. Chem. Int. Ed. 2004, 43, 1217
DOI: 10.1002/anie.200353186
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1217

Communications
Rotaxanes
macrocyclization of
a
bis-2-nitrobenzenesulfonamide
(NsNH) derivative with 2,6-dibromomethylpyridine to give
the protected macrocycle in 67% yield. Cyclization of the
analogous Boc-protected diamine proceeded with a low yield
A Simple General Ligand System for Assembling
Octahedral Metal–Rotaxane Complexes**
Louise Hogg, David A. Leigh,* Paul J. Lusby,
Alessandra Morelli, Simon Parsons, and
Jenny K. Y. Wong
Coordination complexes in which rotaxanes act as
ligands for transition-metal atoms are amongst the
most celebrated examples of mechanically interlocked
molecular level architectures.[1] This is not only because
coordination chemistry makes possible a rich diversity
of structures, but also because the metal atom can be
locked in unusual environments for subsequent electro-
chemical,[2] photochemical[3] and catalysis[4] studies.
Efficient synthetic methods have been developed for
rotaxanes based on tetrahedral and trigonal-bipyrimidal
metal complexes by using the metal-bis-phenanthroline
synthon pioneered in Strasbourg.[1a,b,5] Herein we
describe a general ligand system for rotaxane complexes
of ions that prefer octahedral coordination—the com-
monest ligand geometry amongst transition metals, but
up to now a rare[6] coordination mode for rotaxanes.
Simple mixing of the components at room temperature
is sufficient to assemble a broad range of octahedrally
coordinated [2]metallorotaxanes in excellent yields. The
reactions have few, if any, by-products and proceed
under thermodynamic control in the absence of a
catalyst or any other external reagents.
Scheme 1. Five-component self-assembly of octahedral metal(ii)[2]rotaxanes,
[M(L1L2)](ClO4)2.Reagents and conditions: a) 1,10-dibromodecane, K 2CO3,
NaI, butanone, reflux, 18 h, 83%, b) LiAlH4, THF, 0–608C, 3 h, 92%, c) 2-
nitrobenzenesulfonyl chloride (NsCl), NEt3, CH2Cl2, 18 h, 93%, d) 2,6-dibro-
momethylpyridine, K2CO3, butanone, reflux, 18 h, 67%, e) mercaptoacetic
acid, LiOH, DMF, 24 h, 80%.
Catenanes have previously been synthesized around
octahedral metal templates by employing macrocycles
containing tridentate 2,6-diiminopyridine chelating units.[7]
This system is not well-suited to forming rotaxanes, however,
because thread–thread–metal and macrocycle–macrocycle–
metal (catenate) complexes can form in competition with the
desired thread–macrocycle–metal assembly. Replacement of
the macrocycle imine ligand set by nonlabile amine groups
removes the possibility of forming catenates and introduces a
structural asymmetry that can potentially be tailored to favor
rotaxane formation under dynamic exchange conditions.[8]
After exploring several unsuccessful designs, we inves-
tigated the chemistry of macrocycle L1, which is conveniently
prepared on a multigram scale in five steps from readily
available materials (Scheme 1). The key step to L1 is the
(< 20%) and routes based on ring-closing olefin metathesis to
form the C10 chain also proved uncompetitive. The use of
aniline, rather than benzylamine, in the thread was designed
to destabilize the dithread–metal complex with respect to the
desired interlocked structure (see below).
Octahedral metal–rotaxane formation was achieved by
sequential treatment of L1 with Zn(ClO4)2·6H2O (0.8 equiv),
2,6-diformylpyridine (1 equiv) and (p-aminophenyl)tris(p-
tert-butylphenyl)methane (2 equiv), Scheme 1. Remarkably,
after 24 h at room temperature no metal-containing species
other than the zinc(ii)[2]rotaxane was evident by either
1H NMR or electrospray mass spectrometry and the analyti-
cally pure [Zn(L1L2)](ClO4)2 rotaxane was isolated in 92%
yield by simply washing the crude product with diethyl ether.
The generality of the reaction was explored by using divalent
metal ions both across and down the periodic table with
[*] L.Hogg, Prof.D.A.Leigh, Dr.P.J.Lusby, A.Morelli, Dr.S.Parsons,
Dr.J.K.Y.Wong
!
School of Chemistry
University of Edinburgh
The King's Buildings, West Mains Road
Edinburgh EH9 3JJ (United Kingdom)
Fax: (+44)131-667-9085
respect to zinc (i.e., Mn Zn and Zn!Hg). Pleasingly, each
of [M(L1L2)](ClO4)2 (M = MnII, CoII, NiII, CuII, CdII, HgII)
could be efficiently prepared by using the procedure in
isolated yields ranging from 73 to 99% (Scheme 1). In all
cases no other metal-containing species could be detected[9]
after 24 h, which suggests near-quantitative formation of the
interlocked metal-rotaxane complex. Formation of
[Fe(L1L2)](ClO4)2 required a longer reaction time and
gentle heating (CH2Cl2/CH3CN, N2, 408C, 2 weeks) and
E-mail: david.leigh@ed.ac.uk
[**] This work was supported by the European Union Future and
Emerging Technology Program MechMol and the EPSRC.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
1218
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200353186
Angew. Chem. Int. Ed. 2004, 43, 1218 –1218

Angewandte
Chemie
resulted in a lower yield of rotaxane (57%). The sluggish rate
of reaction is characteristic of the slow rate of ligand exchange
of low spin d6 metal complexes, but a potentially useful
feature of the slower dynamics is that FeII therefore locks the
rotaxane architecture in a particularly kinetically stable form.
The nonparamagnetic metal–rotaxane complexes all have
similar 1H NMR spectra; those of the zinc(ii)[2]rotaxane
[Zn(L1L2)](ClO4)2 and macrocycle L1 are shown in
Figure 1.[10] The shielding of the HC and HD protons of the
benzyl rings of the macrocycle and several protons of the
thread indicate that extensive intercomponent p stacking
occurs in solution. Single crystals of [Cd(L1L2)](ClO4)2
suitable for investigation by X-ray crystallography were
obtained by slow vapor diffusion of diethyl ether into a
solution of the rotaxane in acetonitrile.[11] The crystal
structure (Figure 2) confirms the interlocked molecular
architecture, the pseudooctahedral geometry of the cadmiu-
m(ii) ion, and shows p stacking of both the macrocycle
benzylic rings with the pyridyl unit and an imine group of the
thread.
Figure 2. X-ray crystal structure of [Cd(L1L2)](ClO4)2.[11] Carbon atoms
of the macrocycle, L1, are shown in light blue and those of the thread,
L2, in yellow; oxygen atoms are red; nitrogen dark blue; chlorine
green; cadmium grey.Hydrogen atoms and a molecule of acetonitrile
are omitted for clarity.Selected bond lengths [Š]: Cd-N2 24. 0, Cd-N5
2.30, Cd-N11 2.40, Cd-N44 2.52, Cd-N47 2.26, Cd-N53 2.38; other
selected interatomic distances [Š]: N2-N11 4.52, N5-N47 4.52, N44-
N53 4.59; ligand bite angles [8]: N2-Cd-N11 141.5, N44-Cd-N53 139.2.
The mechanism of the rotaxane-forming reaction provides
insight into the reasons for the effectiveness of the ligand
assembly. When L1 is treated with Zn(ClO4)2·6H2O (CH2Cl2/
CH3CN, room temperature) followed by the preformed
thread, L2, electrospray mass spectrometry shows that
within 10 minutes the thread has extracted the zinc(ii) ion
from the macrocycle to form the dithread complex,
[Zn(L2)2](ClO4)2 in > 95% yield (Scheme 2). The
[Zn(L2)2](ClO4)2 species is then quantitatively converted to
the rotaxane [Zn(L1L2)](ClO4)2 over 24 h.[12] Whilst the
Figure 1. Partial 1H NMR spectra (400 MHz, CD3CN, 298 K) of a) macrocycle L1 b) zinc(ii)[2]rotaxane [Zn(L1L2)](ClO4)2 c) demetallated, reduced,
rotaxane L1H4L2.
Angew. Chem. Int. Ed. 2004, 43, 1218 –1218
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1219

Communications
both solution (NMR spectroscopy) and the solid state (X-ray
crystal studies). We believe these favorable secondary inter-
actions are important for the thermodynamic stability of the
rotaxane over the other possible products of the reaction.
The 2,6-diminopyridyl motif imparts high kinetic stability
in metal-coordinated interlocked structures. Tetraimine met-
al(ii) catenates are not demetallated by Na2EDTA (EDTA =
ethylenediaminetetracetate), which required reduction to the
more labile tetraamine catenates for the metal atom to be
extracted.[7] The [M(L1L2)](ClO4)2 rotaxanes, which contain
a combination of imine and amine N donors, do react with
excess Na2EDTA when heated (10 equiv, CH3CN/MeOH,
608C, 0.5 h) to remove the metal. However, without the
stabilization provided by metal coordination the rotaxane
decomposes through imine bond exchange and only free
macrocycle and thread are observed experimentally
(Scheme 2, path a). If the rotaxane imine bonds are reduced
beforehand ([Zn(L1L2)](ClO4)2, 10 equiv NaBH4, CH3CN/
MeOH, D, 1.5 h), however, treatment with Na2EDTA
(10 equiv, CH3CN/MeOH, 608C, 0.5 h) gives the demetal-
lated, reduced, rotaxane L1H4L2 (88% yield) with no
1
evidence of dethreading (Scheme 2, path b). The H NMR
of L1H4L2 is shown in Figure 1c. The downfield shift in the
resonances of the benzyl groups with respect to
[Zn(L1L2)](ClO4)2 indicates that p stacking with the thread
is less pronounced in the demetallated rotaxane in which
there are no coordination bonds to organize the geometry of
the components.
In conclusion, we have developed a general ligand system
for the efficient assembly of [2]rotaxanes around octahedral
metal ions. The five component self-assembly reaction
produces rotaxanes under true thermodynamic control in
excellent yields without the need for large excesses of
reagents, subsequent derivatization to stabilize the rotaxane
architecture, chromatography or any other complicated
purification processes. The system is remarkable in terms of
its simplicity and expands both the range and geometry of
metal ions that can be readily encapsulated within rotaxane
structures.
Scheme 2. Mechanism of formation and reactivity of [Zn(L1L2)](ClO4)2.
Rapid formation of Zn(L2)2 is followed by quantitative conversion to
Zn(L1L2) under thermodynamic control.Demetallation of the rotaxane
by using Na2EDTA occurs both with (b) and without (a) prior reduction
of the imine groups.
Experimental Section
Typical example of octahedral metal(ii)[2]rotaxane formation,
[Zn(L1L2)](ClO4)2: Zinc(ii) perchlorate hexahydrate (0.127 g,
0.342 mmol) in acetonitrile (5 mL), 2,6-pyridinedicarboxaldehyde
(0.055 g, 0.410 mmol) in acetonitrile (10 mL), and p-aminophenyl-
tris(p-tert-butylphenyl)methane (0.413 g, 0.820 mmol) in dichloro-
methane (10 mL) were added sequentially over five-minute periods
to a solution of L1 (0.200 g, 0.410 mmol) in acetonitrile (10 mL). The
resulting solution was stirred at room temperature for 24 h, after
which the solvent was removed under reduced pressure, the crude
residue dissolved in acetonitrile (30 mL), filtered, and finally the
solvent removed under reduced pressure. The crude residue was
washed with diethyl ether (30 mL) for 10 min, isolated by filtration
and dried in air to give [Zn(L1L2)](ClO4)2 as a bright yellow solid
(0.582 g, yield = 92%). Mp 2668C (decomp). 1H NMR (400 MHz,
CD3CN, 298 K): d = 1.32 (s, 54H, C(CH3)3), 1.48–1.68 (bm, 12H,
alkyl), 1.79 (m, 4H, OCH2CH2), 3.84 (br, 4H, OCH2), 4.06–4.55 (br,
8H, pyCH2NHCH2Ar; py = pyridyl), 6.29 (d, J = 8.6 Hz, 4H, macro-
cycle ArH), 6.49 (d, J = 8.6 Hz, 4H, macrocycle ArH), 6.96 (d, J =
8.6 Hz, 4H, thread ArH), 7.16 (d, J = 8.6 Hz, 12H, thread ArH), 7.27
reversible nature of imine bond formation accounts for the
dynamics of the system, the reasons for the metal–rotaxane
complex being the thermodynamic product rather than the
dithread–metal complex are rather more subtle. In fact, imine
N donors often form stronger coordination bonds than the
corresponding amines,[13] which leads to dithread–metal
complexes being thermodynamically favored in other ligand
systems we investigated. However, the use of aniline rather
than, for example, benzylamine groups in the thread does not
allow geometries in which the dithread–metal complex can
form attractive intercomponent p-stacking interactions, such
as those observed in the rotaxane between the benzyl groups
of the macrocycle and the extended p system of the thread in
1220
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
Angew. Chem. Int. Ed. 2004, 43, 1218 –1218

Angewandte
Chemie
(d, J = 8.6 Hz, 4H, thread ArH), 7.37 (m, 14H, thread ArH plus
macrocycle pyridyl H), 7.65 (d, J = 7.8 Hz, 2H, thread pyridyl H), 7.96
(t, J = 7.8 Hz, 1H, macrocycle pyridyl H), 8.11 (t, J = 7.8 Hz, 1H,
[6] D. Pomeranc, D. Jouvenot, J.-C. Chambron, J.-P. Collin, V. Heitz,
J.-P. Sauvage, Chem. Eur. J. 2003, 9, 4247 – 4254.
[7] D. A. Leigh, P. J. Lusby, S. J. Teat, A. J. Wilson, J. K. Y. Wong,
Angew. Chem. 2001, 113, 1586 – 1591; Angew. Chem. Int. Ed.
2001, 40, 1538 – 1543.
thread pyridyl H), 8.57 ppm (s, 2H, thread HC N); 13C NMR
=
(100 MHz, CDCl3, 298 K): d = 25.6, 28.4, 28.5, 29.4, 31.5, 34.5, 52.2,
55.3, 63.9, 67.4, 114.1, 120.8, 123.0, 124.1, 124.3, 124.6, 127.0, 128.5,
130.6, 141.5, 141.9, 143.3, 143.4, 145.9, 148.9, 149.0, 155.0, 158.2,
158.7 ppm; IR (KBr pressed pellet): n˜ = 3465, 2960, 2865, 1611, 1582,
1513, 1464, 1395, 1363, 1251, 1180, 1109, 1089, 1017, 840, 823, 637, 625,
[8] a) S. J. Cantrill, S. J. Rowan, J. F. Stoddart, Org. Lett. 1999, 1,
1363 – 1366; b) S. J. Rowan, J. F. Stoddart, Org. Lett. 1999, 1,
1913 – 1916; c) P. T. Glink, A. I. Oliva, J. F. Stoddart, A. J. P.
White, D. J. Williams, Angew. Chem. 2001, 113, 1922 – 1927;
Angew. Chem. Int. Ed. 2001, 40, 1870 – 1875; d) M. J. Gunter, N.
Bampos, K. D. Johnstone, J. K. M. Sanders, New J. Chem. 2001,
25, 166 – 173; e) S. J. Rowan, S. J. Cantrill, G. R. L Cousins,
J. K. M. Sanders, J. F. Stoddart, Angew. Chem. 2002, 114, 938 –
993; Angew. Chem. Int. Ed. 2002, 41, 898 – 952; f) K. D. John-
stone, N. Bampos, J. K. M. Sanders, M. J. Gunter, Chem.
Commun. 2003, 1396 – 1397; g) R. G. E. Coumans, J. A. A. W.
Elemans, P. Thordarson, R. J. M. Nolte, A. E. Rowan, Angew.
Chem. 2003, 115, 674 – 678; Angew. Chem. Int. Ed. 2003, 42, 650 –
654; h) A. F. M. Kilbinger, S. J. Cantrill, A. W. Waltman, M. W.
Day, R. H. Grubbs, Angew. Chem. 2003, 115, 3403 – 3407;
Angew. Chem. Int. Ed. 2003, 42, 3281 – 3285; i) J. S. Hannam,
T. J. Kidd, D. A. Leigh, A. J. Wilson, Org. Lett. 2003, 5, 1907 –
1910; j) Y. Furusho, T. Oku, T. Hasegawa, A. Tsuboi, N. Kihara,
T. Takata, Chem. Eur. J. 2003, 9, 2895 – 2903; k) M. Horn, J.
Ihringer, P. T. Glink, J. F. Stoddart, Chem. Eur. J. 2003, 9, 4046 –
4054.
582 cmꢀ1
;
LRESI-MS:
m/z = 830
[Zn(L1L2)]2+
,
1759
[Zn(L1L2)](ClO4)+; HRFAB-MS (3-NOBA matrix): m/z =
12
1657.97065
(calcd
for
C
111
13CH132N6O264Zn
[(L1L2)Zn],
1657.97363).
Received: October 29, 2003 [Z53186]
Published Online: January 27, 2004
Keywords: coordination modes · rotaxanes · self-assembly ·
.
template synthesis · thermodynamic control
[1] a) J.-P. Sauvage, C. Dietrich-Buchecker, G. Rapenne in Molec-
ular Catenanes, Rotaxanes and Knots (Eds.: J.-P. Sauvage, C.
Dietrich-Buchecker), Wiley-VCH, 1999; b) J.-P. Collin, C.
Dietrich-Buchecker, P. Gaviæa, M. C. Jimenez-Molero, J.-P.
Sauvage, Acc. Chem. Res. 2001, 34, 477 – 487; For complexes in
which metal coordination forms part of the macrocycle in a
rotaxane see: c) K.-S. Jeong, J. S. Choi, S.-Y. Chang, H.-Y.
Chang, Angew. Chem. 2000, 112, 1758 – 1761; Angew. Chem. Int.
Ed. 2000, 39, 1692 – 1695; d) S.-Y. Chang, J. S. Choi, K.-S. Jeong,
Chem. Eur. J. 2001, 7, 2687 – 2697; e) S.-Y. Chang, K.-S. Jeong, J.
Org. Chem. 2003, 68, 4014 – 4019; f) S.-Y. Chang, H.-Y. Jang, K.-
S. Jeong, Chem. Eur. J. 2003, 9, 1535 – 1541; For complexes in
which metal coordination forms part of the thread in a rotaxane
see: g) K.-M. Park, D. Whang, E. Lee, J. Heo, K. Kim, Chem.
Eur. J. 2002, 8, 498 – 508; For complexes in which metal
coordination forms the stoppers in a rotaxane see: h) R. B.
Hannak, G. Färber, R. Konrat, B. Kräutler, J. Am. Chem. Soc.
1997, 119, 2313 – 2314; i) J.-C. Chambron, J.-P. Collin, J.-O.
Dalbavie, C. O. Dietrich-Buchecker, V. Heitz, F. Odobel, N.
SolladiØ, J.-P. Sauvage, Coord. Chem. Rev. 1998, 178–180, 1299 –
1312; j) A. J. Baer, D. H. Macartney, Inorg. Chem. 2000, 39,
1410 – 1417; k) S. J. Loeb, J. A. Wisner, Chem. Commun. 1998,
2757 – 2758; l) G. J. E. Davidson, S. J. Loeb, N. A. Parekh, J. A.
Wisner, J. Chem. Soc. Dalton Trans. 2001, 3135 – 3136; For
polyrotaxanes based on metal coordination polymers see:
m) S. R. Batten, R. Robson, Angew. Chem. 1998, 110, 1558 –
1595; Angew. Chem. Int. Ed. 1998, 37, 1460 – 1494; n) A. J.
Blake, N. R. Champness, P. Hubberstey, W.-S. Li, M. A. With-
ersby, M. Schröder, Coord. Chem. Rev. 1999, 183, 117 – 138; o) K.
Kim, Chem. Soc. Rev. 2002, 31, 96 – 107.
[9] Reactions were monitored by electrospray mass spectrometry in
all cases and by 1H NMR spectroscopy for systems not contain-
ing paramagnetic metals.
[10] The L1 and L2 resonances in [Zn(L1L2)](ClO4)2] were distin-
guished by a combination of COSY and ROESY experiments.
[11] [Cd(L1L2)](ClO4)2·2 MeCN·1.5 Et2O:
Mr = 2098.82, yellow block, crystal size 0.15 ” 0.12 ” 0.10 mm,
monoclinic Cc, a = 31.636(2), b = 22.4269(14), c =
C124H153CdCl2N8O11.5,
22.2668(15) Š, b = 133.7570(10)8, V= 11410.8(13) Š3, Z = 4,
1calcd = 1.222 Mgmꢀ3; MoKa radiation (graphite monochromator,
l = 0.71073 Š), m = 0.300 mmꢀ1, T= 150(2) K. 36017 data (22989
unique, Rint = 0.03170, 1.27 < q < 28.968), were collected on a
Bruker SMARTApex CCD diffractometer by using narrow
frames (0.38 in w), and were corrected semiempirically for
absorption and incident beam decay. The structure was solved by
direct methods and refined by full-matrix least-squares on F2
values of all data (G. M. Sheldrick, SHELXTL manual, Siemens
Analytical X-ray Instruments, Madison WI, USA, 1994, version
5) to give wR = {ꢀ[w(F2oꢀFc2)2]/ꢀ[w(F2o)2]}1/2 = 0.1536, conven-
tional R = 0.0627 for F values of 22989 reflections with F2o >
2sF2o), S = 1.048 for 1371 parameters. Residual electron density
extremes were 1.12 and ꢀ0.89 eŠꢀ3. Hydrogens were added in
calculated positions and constrained to a Riding model. CCDC-
224059 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge via
www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB21EZ, UK; fax: (+ 44)1223-336-033; or deposit@
ccdc.cam.ac.uk).
[2] a) N. Armaroli, V. Balzani, J.-P. Collin, P. Gaviæa, J.-P. Sauvage,
B. Ventura, J. Am. Chem. Soc. 1999, 121, 4397 – 4408; b) L.
Raehm, J.-M. Kern, J.-P. Sauvage, Chem. Eur. J. 1999, 5, 3310 –
3317.
[3] a) J.-C. Chambron, A. Harriman, V. Heitz, J.-P. Sauvage, J. Am.
Chem. Soc. 1993, 115, 6109 – 6114; b) M. Andersson, M. Linke,
J.-C. Chambron, J. Davidsson, V. Heitz, L. Hammarström, J.-P.
Sauvage, J. Am. Chem. Soc. 2002, 124, 4347 – 4362; c) N.
Watanabe, N. Kihara, Y. Furusho, T. Takata, Y. Araki, O. Ito,
Angew. Chem. 2003, 115, 705 – 707; Angew. Chem. Int. Ed. 2003,
42, 681 – 683.
[12] Under identical experimental conditions the reduced thread,
H4L2, does not afford [Zn(L1H4L2)](ClO4)2, thus precluding the
possibility of a “slippage” mechanism.
[13] G. J. P. Britovesk, V. C. Gibson, S. Mastroianni, D. C. H. Oakes,
C. Redshaw, G. A. Solan, A. J. P. White, D. J. Williams, Eur. J.
Inorg. Chem. 2001, 431 – 437. The greater stability of octahedral
metal–imine complexes, compared with metal–amine com-
plexes, is also illustrated by the facile demetallation of
amine—but not imine—octahedral metal catenates.[7]
[4] P. Thordarson, E. J. A. Bijsterveld, A. E. Rowan, R. J. M. Nolte,
Nature 2003, 424, 915 – 918.
[5] a) C. Wu, P. R. Lecavalier, Y. X. Shen, H. W. Gibson, Chem.
Mater. 1991, 3, 569 – 572; b) J.-C. Chambron, V. Heitz, J.-P.
Sauvage, J. Chem. Soc. Chem. Commun. 1992, 1131 – 1133.
Angew. Chem. Int. Ed. 2004, 43, 1218 –1221
www.angewandte.org
ꢀ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1221