A 3D interlocked structure from a 2D template: Structural requirements for the assembly of a square-planar metal-coordinated [2]rotaxane

Article abstract of DOI:10.1002/anie.200353622

One step beyond: The synthesis of a three-dimensional interlocked molecular architecture (a rotaxane) from a two-dimensional square-planar metal template (see scheme) has been achieved. A combination of steric and electronic factors direct the synthesis in the third dimension, either by promoting or preventing interlocking.

Full text of DOI:10.1002/anie.200353622

Communications
Rotaxane Synthesis
A 3D Interlocked Structure from a 2D Template:
Structural Requirements for the Assembly of a
Square-Planar Metal-Coordinated [2]Rotaxane**
Anne-Marie Fuller, David A. Leigh,* Paul J. Lusby,
Iain D. H. Oswald, Simon Parsons, and
D. Barney Walker
The starting point for the revolution in catenane and rotaxane
synthesis that occurred during the last part of the 20th century
was the realization by Sauvage and co-workers that metal–
ligand coordination geometries could fix molecular fragments
in three-dimensional space such that they were predisposed to
form mechanically interlocked architectures through macro-
[1]
cyclization or “stoppering”reactions.
Efficient synthetic
methods to rotaxanes were subsequently developed based on
four- (tetrahedral)[2], five- (trigonal bipyramidal and square
pyramidal)[3] and, most recently, six-coordinate (octahedral)[4]
metal templates (Figure 1).[5] One of the benefits of using
specific coordination motifs for such assemblies is that the
resulting interlocked ligands often do not permit other metal
geometries in their binding site, which can consequently be
exploited either to lock a metal in an unusual geometry for its
oxidation state[6] or to bring about large-amplitude “shut-
tling”of the ligand components. [3,7] Here we show that three-
dimensional interlocked architectures can also be assembled
from two-dimensional coordination templates by using steric
and electronic restrictions to direct the synthesis in the third
[*] A.-M. Fuller, Prof. D. A. Leigh, Dr. P. J. Lusby, I. D. H. Oswald,
Dr. S. Parsons, D. B. Walker
School of Chemistry, University of Edinburgh
The King’s Buildings, West Mains Road, Edinburgh EH9 3JJ (UK)
Fax: (+44)131-667-9085
E-mail: David.Leigh@ed.ac.uk
[**] We thank E. R. Kay for assistance with the graphics and crystallog-
raphy data. 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.
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DOI: 10.1002/anie.200353622
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Figure 1. Exploiting transition-metal–
ligand geometries in the synthesis of
mechanically interlocked architectures:
a) tetrahedral, b) square pyramidal and
trigonal bipyrimidal, c) octahedral, and
d) square-planar coordination motifs.
Scheme 1. Reagents and conditions: a) Pd(OAc)2, CH3CN, 76%; b) CHCl3, 508C; L2: 63%, L3: 96%, L4: 0%, L5: 97%; c) 1. Grubbs catalyst
(0.1 equiv), CH2Cl2; 2. H2, Pd/C, THF, H2L6: 98%, L3+L7: 63%, L5+L7: 69% (over 2 steps); d) KCN, MeOH, CH2Cl2, 208C, 1 h and then 408C,
0.5 h, 97%.
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Communications
dimension. The resulting [2]rotaxane is the first example of a
mechanically interlocked ligand that forms a four-coordinate
square-planar metal complex.[8]
The square-planar [2]rotaxane ligand design consists of a
tridentate benzylic amide macrocycle and a monodentate
thread (Figure 1d). The macrocycle incorporates a 2,6-
dicarboxyamidopyridine unit to exploit the palladium chemis-
try recently developed[9] by Hirao and co-workers. The thread
contains a pyridine donor group substituted with appropri-
ately bulky stoppers in either the 2,6- or 3,5-positions. It was
envisioned that in the key intermediate, [Pd(L1)(L2–L5)] (see
Scheme 1), the geometry of the precursor to the macrocycle
(previously used to direct hydrogen bond assembly pro-
cesses[10]) would promote intercomponent p-p stacking, thus
encouraging the pyridine donor of the thread to bind the
metal ion orthogonally to the N3 ligand (complimenting the
normally preferred orientation[11]) and directing the asssem-
bly in the third dimension. A series of readily available
threads L2–L5 was investigated during the study.
The rotaxane synthesis was carried out according to
Scheme 1. Treatment of H2L1 with Pd(OAc)2 in acetonitrile
smoothly generated a complex [Pd(L1)(CH3CN)] in which
the fourth coordination site of the metal is occupied by a
normally labile acetonitrile molecule. Nevertheless, displace-
ment of the acetonitrile by bis-ester pyridine ligand L4 was
unsuccessful (see below). However, simple combination of L5
or either of the bis-ether pyridine threads (L2 and L3) with
[Pd(L1)(CH3CN)] in either dichloromethane or chloroform
gave the desired complexes [Pd(L1)(L5/L2/L3)] in 97, 63 and
96% yields, respectively. The 1H NMR spectrum of
[Pd(L1)(L2)] is shown in Figure 2c. Comparison with the
spectra of [Pd(L1)(CH3CN)] and H2L1 (Figure 2b and 2a,
respectively) shows features clearly indicative of metal
coordination (the absence of HC and shifts in HA and HB)
and the anticipated aromatic stacking between the tridentate
and monodentate ligands (particularly HE and HF). Similar
chemical shift differences were observed for [Pd(L1)(L3)]
and [Pd(L1)(L5)], however, ring closing olefin metathesis
(RCM) followed by hydrogenation (Scheme 1, step c) of the
three complexes produced very different results. Whilst
cyclization of [Pd(L1)(L2)] gave the corresponding [2]rotax-
ane [Pd(L6)] in 77% yield following hydrogenation of the
olefin, no [2]rotaxane was produced from RCM of either
[Pd(L1)(L3)] or [Pd(L1)(L5)], the only products in each case
being the free macrocycle and thread. Why does only one of
the four threads direct rotaxane synthesis in the desired
manner?
Figure 2. 1H NMR spectra (400 MHz, 9:1 CDCl3:CD3CN, 298 K) of
a) H2L1; b) [Pd(L1)(CH3CN)]; c) [Pd(L1)(L2)]; d) [2]rotaxane [Pd(L6)].
The lettering refers to the assignments in Scheme 1.
The 1H NMR spectra of the [2]rotaxane ([Pd(L6)],
Figure 2d), mass spectrometric analysis, and the preserved
association of the organic fragments upon demetalation,
unambiguously confirmed the interlocked structure. In addi-
tion to the loss of the terminal alkene protons, some subtle
differences in the 1H NMR spectrum of [Pd(L6)] compared to
[Pd(L1)(L2)] (Figure 2c) indicates that some rearrangement
of the ligands does occur on formation of the rotaxane. Single
crystals of [Pd(L6)] suitable for X-ray crystallography[12] were
grown by slow cooling of a warm, saturated solution of the
[2]rotaxane in acetonitrile. The solid-state structure
(Figure 3) shows the interlocked architecture and the
Figure 3. X-ray crystal structure of rotaxane [Pd(L6)] showing a) stag-
gered and b) side-on views.[12] Carbon atoms of the macrocycle are
shown in light blue and those of the thread in yellow; oxygen atoms
are red, nitrogen dark blue, palladium gray. Selected bond lengths [Š]:
Pd-N15 1.95, Pd-N25 1.86, Pd-N32 2.04, Pd-N59 2.02; other selected
distance [Š]: N15-N25 3.81; macrocycle bite angle [8]: N59-Pd-N32
160.0. The macrocycle is disordered over two similar sites (50:50), but
one is omitted for clarity together with the hydrogen atoms.
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Chemie
pseudo-square-planar geometry (the N3 bite angle is 160.08)
around the palladium center. The p stacking between the
macrocycle and the pyridine ring of the thread so apparent in
hydrogen-bonding interactions between the thread and the
macrocycle are “switched on”by the demetalation/protona-
tion procedure. It appears that the amide groups of H2L6
simultaneously hydrogen bond to the pyridine groups in both
the macrocycle and thread.
1
solution from the H NMR shifts is significantly offset in the
solid state (see the side-on view, Figure 3b). The co-con-
formation adopted by the macrocycle and thread in the crystal
structure of the rotaxane clearly illustrates why RCM of the
complexes formed with the 3,5-disubstituted threads
([Pd(L1)(L3)] and [Pd(L1)(L5)]) can lead to uninterlocked
products; even with both fragments attached to the metal,
cyclization of L1 can readily occur without encircling a 3,5-
substituted pyridine thread. Similarly, the conformation of the
thread suggests a possible reason for the lack of reactivity of
the 2,6-bis-ester thread L4 towards [Pd(L1)(CH3CN)]. In the
crystal structure the electron density of the ether oxygen
atoms of the thread is directed away from the occupied dz2
orbital lobes which lie above and below the plane of the
square-planar geometry at the d8 palladium center. Chelation
of L4 to [Pd(L1)] has to occur orthogonally for steric reasons.
Such an arrangement would force electron density from the
ester carbonyl groups into this high-energy space.
In conclusion, we have described methodology for assem-
bling a three-dimensional interlocked molecular architecture
from a two-dimensional metal template. A combination of
steric and electronic factors direct the synthesis in the third
dimension, either promoting or preventing interlocking. The
resulting [2]rotaxane is the first example derived from a
square-planar-coordinated metal center and completes the
series of mechanically interlocked ligands for common
transition-metal geometries initiated by Sauvage and co-
workers in 1983.
Received: December 29, 2003 [Z53622]
Keywords: coordination modes · palladium · rotaxanes ·
.
template synthesis
Demetalation of [Pd(L6)] with potassium cyanide
(Scheme 1, step d) generates the free [2]rotaxane H2(L6) in
97% yield, thus confiming that the coordination bonds are
not required to stabilize the interlocked architecture once it is
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Figure 4. 1H NMR spectra (400 MHz, CDCl3, 298 K) of a) macrocycle;
b) demetalated [2]rotaxane H2L6; c) thread L2. The lettering refers to
the assignments in Scheme 1.
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[12] [Pd(L6)]: C121H141.5N8.5O6Pd, Mr = 1917.33, yellow block, crystal
size 0.47 ” 0.31 ” 0.27 mm3, triclinic, space group P1, a =
¯
15.7840(10),
84.9850(10),
b = 15.9967(10),
b = 71.8430(10),
c = 22.8029(14) Š,
g = 80.5460(10)8,
a =
V=
5392.5(6) Š3, Z = 2, 1calcd = 1.193 MgmÀ3
;
MoKa radiation
(graphite monochromator, l = 0.71073 Š), m = 0.231 mmÀ1, T=
150(2) K. 34043 data (15412 unique, Rint = 0.0331, 2.06 < q <
23.268), were collected on a Brucker SMART CCD diffractom-
eter using narrow frames (0.58 in w), and were corrected
semiempirically for absorption and incident beam decay. Data
beyond 0.9 Š were weak and were not used for refinement. The
structure was solved by direct methods (SIR92)[13] and refined by
full-matrix least-squares against F2.[14] Phenyl groups were
constrained to be rigid hexagons and hydrogen atoms were
placed in calculated positions. The whole macrocyclic compo-
nent, inclusive of the Pd atom, is disordered (50:50) over two
positions. The alkyl chain is further disordered and the refine-
ment of this portion of the structure was controlled by
application of restraints to both 1,2 and 1,3 distances and use
of a common isotropic displacement parameter for all C atoms
forming the chain. The only disorder present in the thread is in
two of the tert-butyl groups. That based on C244 is rotationally
disordered (70:30) with the central carbon atom (C244) fully
occupied and the two alternative sets of methyl positions related
À
by a 608 rotation about the C244 C243 bond. All atoms in the
groups were refined with anisotropic displacement parameters
(adps), those of “opposite”C atoms being constrained to be
equal. The tert-butyl group attached to C263 is disordered
(70:30) over two positions and was refined isotropically.
Similarity restraints were applied to chemically equivalent 1,2
and 1,3 distances in both disordered tert-butyl groups. In the
latter stages of refinement there was still significant unassigned
electron density associated with diffuse solvent. This density was
modeled by using the procedure of van der Sluis and Spek,[15]
comprising 199 electrons per unit cell, and corresponds to
approximately 4.5 MeCN solvent molecules per asymmetric
unit; values of Mr, F(000), m etc. have been calculated on this
assumption. wR = {ꢀ[w(F2oÀFc2)2]/ꢀ[w(F2o)2]}1/2 = 0.2527, conven-
tional R = 0.0806 for F values of 15412 reflections with F2o >
2sF2o), S = 1.115 for 885 parameters. Residual electron density
extremes were 1.03 and À0.94 eŠÀ3. CCDC-229418 contains the
supplementary crystallographic data for this paper. These data
can be obtained free of charge via www.ccdc.cam.ac.uk/conts/
3918
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