Gold(I)-template catenane and rotaxane synthesis

Article abstract of DOI:10.1002/anie.200801904

(Figure Presented) Gold member: The last of the simple metal coordination geometries (linear) joins the family of metal-ligand arrangements that have been used to template the formation of mechanical bonds. Both catenanes and rotaxanes are assembled about a gold(I) template. The picture shows the molecular structure of the gold(I) catenate, viewed parallel to its N-Au-N bond.

Full text of DOI:10.1002/anie.200801904

Angewandte
Chemie
DOI: 10.1002/anie.200801904
Gold(I)-Template Synthesis
Gold(I)-Template Catenane and Rotaxane Synthesis**
Stephen M. Goldup, David A. Leigh,* Paul J. Lusby, Roy T. McBurney, and
Alexandra M. Z. Slawin
Metal ions with a range of different two- and three-dimen-
sional coordination geometries (Scheme 1a–d) have been
used as a template for the synthesis of catenanes and
rotaxanes.[1,2] The result is a rich tapestry of mechanically
interlocked ligands and complexes that have been studied
from the point of view of their electrochemistry,[3] photo-
chemistry,[4] reactivity,[5] selectivity,[3b,6] and as prototypes for
molecular machines.[7] Some of these systems can be assem-
bled under thermodynamic control,[8] others under kinetic
control.[3–6,9] Some use sophisticated ligand systems,[10] others
relatively simple ones.[8] Some feature homoleptic complex-
ation modes,[3a,8a,c,11,12] others heteroleptic ones.[9a,c,13,14] All
employ at least one multidentate ligand. Herein we report on
the first use of a one-dimensional (linear) metal–ligand
coordination geometry (Scheme 1e) to template the synthesis
of mechanical bonds.[15,16] The method requires only mono-
Scheme 1. Metal coordination geometries successfully utilized in the
dentate units on each component (we have used pyridine
rings but aryl–gold coordination motifs are also well
known[17]) and both homoleptic (suitable for homocircuit
catenanes) and heteroleptic (suitable for heterocircuit cate-
nanes and rotaxanes) complexes can be assembled. The
approach is exemplified through the gold(I)-template syn-
thesis of a catenane and a rotaxane by ring closing olefin
metathesis (RCM) macrocyclization[8a,11] protocols.
Although gold(III),with its square-planar coordination
preference,has previously been used as a template in
synthesis (to direct the assembly of aza-macrocycles[18]),to
the best of our knowledge two-coordinate gold(I) has not,
despite it being an integral part of many oligomeric and
polymeric supramolecular complexes,helicates,and organo-
metallic structures (which often feature multiple gold–gold
metal-template synthesis of catenanes and rotaxanes. a) Four-coordi-
nate (tetrahedral);[3a,4e,5a,b,6,8c,9a,b,11] b) four-coordinate (square pla-
nar);[5d,9c,13] c) five-coordinate (square-based pyramid/trigonal bipyra-
midal);[14] d) six-coordinate (octahedral);[8a,b,12] e) two-coordinate
(linear; this work).
aurophilic interactions in addition to the metal–ligand
interactions).[15,19] The attractiveness of using a linear coordi-
nation mode in synthesis lies in its simplicity and the potential
generality of a motif that requires just two monodentate
ligands to bind the metal. However,a key design question in
any metal-template catenane or rotaxane synthesis is how to
promote entwining of the ligands once they are attached to
the metal. With gold(I) this is particularly easy to achieve: two
2,6-dialkylpyridine ligands, which are readily accessible by
metal-mediated cross-couplings with 2,6-dihalopyridines,[20]
must necessarily assemble with orthogonal orientations
about the gold ion with the “arms” of each ligand pointing
over the other ligand to create the required two crossover
points. For our chosen ligand system (L1,see Scheme 2) we
also introduced aromatic rings (Scheme 1e) at positions
equivalent to those found to form efficient intercomponent
aromatic stacking interactions in benzylic amide and imine
catenanes and rotaxanes (Scheme 1b,d). The terminal func-
tional groups on each ligand should thus be oriented such that
macrocyclization reactions should favor interlocked products
over the formation of larger macrocycles,oligomers,and
polymers.
[*] Dr. S. M. Goldup, Prof. D. A. Leigh, Dr. P. J. Lusby, R. T. McBurney
School of Chemistry, University of Edinburgh
The King’s Buildings
West Mains Road, Edinburgh EH93JJ (UK)
Fax: (+44)131-650-6453
E-mail: David.Leigh@ed.ac.uk
Homepage: http://www.catenane.net
Prof. A. M. Z. Slawin
School of Chemistry, University of St. Andrews
Purdie Building
St. Andrews, Fife KY169ST (UK)
[**] We thank Dr. J. D. Crowley for useful discussions and the EPSRC
National Mass Spectrometry Service Centre (Swansea, UK) for
accurate mass data. This work was supported by the EPSRC. P.J.L. is
a Royal Society University Research Fellow. D.A.L. is an EPSRC
Senior Research Fellow and holds a Royal Society–Wolfson Research
Merit Award.
Pyridine ligand L1 was synthesized by alkylation of a
known[2d] bis-phenol (see the Supporting Information),and a
2:1 complex with gold(I),[( L1)2Au]SbF6,assembled by
treating L1 (2 equiv) with AuCl(SMe2) (1 equiv) followed
by anion exchange with AgSbF6 (Scheme 2,step a). Ring
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801904.
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Scheme 2. Gold(I)-template catenane synthesis and subsequent
chemistry. a) 1. AuCl(SMe2), acetone, 5 min; 2. AgSbF6, 5 min;
=
b) [(PCy3)2Cl2Ru CHPh], CH2Cl2, 2 d; c) 1. 1m HCl(aq.)/CH2Cl2 (1:1),
408C, 18 h; 2. H2, Pd/C, THF/EtOH (1:1), 18 h; L2 (41% from L1);
d) AuCl(SMe2), acetone, 5 min; e) AgSbF6, acetone, 5 min; >98%;
f) 1m HCl(aq.)/CH2Cl2 (1:1), 408C, 18 h or hn 400–700 nm (500 W
halogen lamp), CHCl3, 6 d; >98%.
Figure 1. 1H NMR spectra (400 MHz, CDCl3, 300 K) of a) the non-
interlocked macrocycle; b) the metal-free catenane L2; c) [(L2)AuCl];
d) gold(I) catenate [(L2)Au]SbF6; e) thread L3, and f) the metal-free
rotaxane L4. The assignments correspond to the lettering shown in
Schemes 2 and 3.
closing metathesis of [(L1)2Au]SbF6 (Scheme 2,step b)
afforded one major and two minor products,each as a
mixture of olefin diastereomers and in both metalated and
demetalated forms. To facilitate purification,the crude
product mixture was fully demetalated and the olefins
reduced before being subjected to column chromatography
to yield the major product,later confirmed to be the desired
catenane L2,in 41% yield (from L1). Also isolated were the
macrocycle resulting from simple cyclization of L1 (20%) and
a larger macrocycle (an isomer of the catenane) formed from
the 1+1 cyclization of L1 (18%). It is not clear whether the
noninterlocked products arise from unproductive macrocyc-
lization reactions on the gold(I) template (similar product
distributions have been observed in some PdII-templated
catenane syntheses[13c]) or from reactions involving uncom-
plexed L1 (a potential disadvantage of using relatively weakly
binding monodentate ligands for a template synthesis is that
unbound ligand can be present during the reaction).
Figure 2. Molecular structure[21] of H2L2(OTs)2 from X-ray structure
analysis of a single crystal grown by vapor diffusion of diisopropyl
ether into a saturated acetone solution. Carbon atoms are shown in
light blue for one macrocycle and in brown for the other; O red,
N blue, S yellow. Hydrogen bond lengths [Š] and angles [8]:
NH1···O1=NH2···O2 1.76; N1-H1-O1=N2-H2-O2 163.
Evidence supporting assignment of the major product as
[2]catenane L2 came initially from the comparison of the
1
mass spectrometry and H NMR spectroscopic data for the
products (Figure 1a and b: the major product (Figure 1b)
shows shielding effects similar to those found in other
catenanes).[8a,b,13c] Confirmation of the mechanically inter-
locked structure was obtained from X-ray crystallography of
the bis-p-toluenesulfonic acid salt (Figure 2),in which the
protonated pyridine groups lie to the outside of the catenane,
with the alkyl chains of each macrocycle buried through the
cavity of the other.[21]
Gold(I) could be re-introduced into catenane L2 in a two-
step process. Treatment with AuCl(SMe2) quantitatively
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generated the interesting complex [(L2)AuCl],the 1H NMR
spectrum of which (Figure 1c) shows inequivalent resonances
for the two macrocycles indicating that gold is coordinated to
just one ring. Subsequent addition of AgSbF6 and filtration
through celite generated [(L2)Au]SbF6,whose 1H NMR
spectrum (Figure 1d) shows only one set of macrocycle
resonances,enhanced shielding for aromatic protons H F and
HG,and reduced shielding of the alkyl-chain protons,
indicating that a significant co-conformational rearrangement
takes place for both rings to bind to the gold ion. Crystals of
the gold(I) catenate suitable for X-ray crystallography were
obtained by vapor diffusion of diisopropyl ether into a
saturated acetone solution of the complex.[21] The solid state
structure of [(L2)Au]SbF6 (Figure 3),in which the pyridine
Scheme 3. Gold(I)-template rotaxane synthesis. a) 1. AuCl(SMe2), ace-
tone, 5 min; 2. AgSbF6, acetone, 5 min; b) acetone, 5 min;
=
c) 1. [(PCy3)2Cl2Ru CHPh], CH2Cl2, 2 d; 2. 1m HCl(aq)/CH2Cl2 (1:1),
408C, 18 h; 3. H2, Pd/C, THF/EtOH (1:1), 18 h, L4 (26% from L1).
generating [(L1)Au(L3)]SbF6 (Scheme 3,steps a and b).
Subsequent RCM (Scheme 3,step c) captured the interlocked
architecture. As with the catenane synthesis,the metal was
removed and the double bond hydrogenated prior to purifi-
cation by column chromatography,which yielded metal-free
rotaxane L4 in 26% yield (from L1) together with 14% of
catenane L2 (arising from ligand-scrambling of
Figure 3. X-ray crystal structure[22] of gold(I) catenate [(L2)Au]SbF6,
À
À
À
viewed a) parallel to the N Au N bond, b) close to the axis of the N
À
Au N bond. Carbon atoms are shown in light blue for one macrocycle
and in brown for the other; O red, N blue, AuI gold, Sb grey, F green.
À
À
Selected bond lengths [Š] and angles [8]: N1 Au 2.05, N2 Au 2.06;
N1-Au-N2 175.3; closest Au···Au contact 11.31 Š.
1
[(L1)Au(L3)]SbF6). The H NMR spectrum of rotaxane L4
(Figure 1 f) shows shielding with respect to the spectra of its
noninterlocked components (macrocycle,Figure 1a,and
thread,Figure 1e) similar to that observed for catenane L2
(Figure 1b).
groups are now internal to the catenate so that both can bind
the metal ion,with the alkyl chains to the outside,shows the
close-to-linear (175.38) coordination geometry of the pyri-
dine–gold(I)–pyridine motif and some of the offset inter-
component aromatic stacking interactions introduced to aid
catenane formation. The gold ion is fully encapsulated within
the organic framework and thus prevented from making
aurophilic gold–gold interactions[19] (the shortest Au···Au
distance in [(L2)Au]SbF6 in the solid state is more than one
nanometer).
It is easy to forget that prior to Sauvageꢀs original CuI-
template synthesis[9a] the construction of mechanically inter-
locked molecules was a task of almost Herculean propor-
tions.[22] A quarter of a century on,the gold(I)-template
synthesis of [2]catenane L2 and [2]rotaxane L4 marks the last
of the simple metal coordination geometries (linear) to join
the family of metal–ligand arrangements that can direct the
formation of mechanical bonds. The ability to form both
homoleptic and heteroleptic complexes,and the simplicity of
the monodentate ligands required,suggests that gold(I) could
prove to be a particularly versatile template for the synthesis
of interlocked molecular structures.
To extend the methodology to rotaxane formation
required the assembly of a heteroleptic complex about the
gold(I) center (Scheme 3). Treatment of L1 with AuCl(SMe2)
(1 equiv),followed by AgSbF and L3,attached both the
6
macrocycle precursor and the thread to the same gold ion,
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Communications
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Experimental Section
Gold(I)-template synthesis of catenane L2: To a stirred solution of L1
(0.178 g,0.348 mmol) in acetone (5 mL) was added AuCl(SMe
)
2
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(0.0513 g,0.174 mmol). After 5 min AgSbF (0.0598 g,0.174 mmol)
6
was added and the resulting grey-blue suspension stirred for a further
5 min before filtration through a pad of celite and removal of the
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CH2Cl2 (10 mL) and 1m HCl(aq) (10 mL),and heated at 40 8C for
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Pd/C (0.070 g) added. The reaction vessel was repeatedly degassed
and purged with N2,then repeatedly degassed and purged with H 2 and
its content stirred for 18 h. The reaction mixture was then filtered
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gradient elution; silica gel) to yield catenane L2 (0.069 g,41%) as a
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Published online: July 15,2008
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Keywords: catenanes · coordination modes · gold · rotaxanes ·
template synthesis
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ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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