Selecting topology and connectivity through metal-directed macrocyclization reactions: A square planar palladium [2]catenate and two noninterlocked isomers

Article abstract of DOI:10.1021/ja053005a

We report the synthesis of a [2]catenate using a square planar palladium(II) template, together with two isomers of the interlocked structure: a single tetradentate macrocycle that adopts a "figure of eight" conformation to encapsulate the metal and a com

Full text of DOI:10.1021/ja053005a

Published on Web 08/17/2005
Selecting Topology and Connectivity through Metal-Directed
Macrocyclization Reactions: A Square Planar Palladium
[2]Catenate and Two Noninterlocked Isomers
Anne-Marie L. Fuller,† David A. Leigh,*,† Paul J. Lusby,†
Alexandra M. Z. Slawin,‡ and D. Barney Walker†
Contribution from the School of Chemistry, UniVersity of Edinburgh, The King’s Buildings, West
Mains Road, Edinburgh EH9 3JJ, United Kingdom, and the School of Chemistry, UniVersity of
St. Andrews, Purdie Building, St. Andrews, Fife, KY16 9ST, United Kingdom
Received May 8, 2005; E-mail: David.Leigh@ed.ac.uk
Abstract: We report the synthesis of a [2]catenate using a square planar palladium(II) template, together
with two isomers of the interlocked structure: a single tetradentate macrocycle that adopts a “figure of
eight” conformation to encapsulate the metal and a complex in which the two macrocycles of the catenane
are not interlocked. The three isomers can each be selectively formed depending on how the building
blocks are assembled and cyclized. Olefin metathesis of both building blocks while they are attached to
the metal gives the single large macrocycle in 77% yield. Cyclizing the monodentate unit prior to attaching
both ligands to the metal gives the [2]catenate in 78% yield. Preforming the tridentate macrocycle produces
a complex in two atropisomeric formssthreaded and nonthreadedsin a 2:3 ratio, which do not interconvert
in dichloromethane at room temperature over 7 days. RCM of the nonthreaded atropisomer affords the
complex with two noninterlocked macrocyclic ligands; RCM of the threaded atropisomer generates the
topologically isomeric [2]catenate. Heating the acyclic atropisomers in acetonitrile provides a mechanism
for their interconversion via ligand exchange, allowing the threaded:nonthreaded ratio to be varied from
2:3 to 8:1. All three fully ring-closed complexes were characterized unambiguously by 1H NMR spectroscopy
and X-ray crystallography. As far as we are aware, this is the first time such a set of three formal topological
and constitutional isomers has been described.
success,2c,10 and it is only recently11 that efficient synthetic routes
based on octahedral coordination have been developed. There
Introduction
The use of transition metal ions to direct the synthesis of
interlocked architectures remains among the most efficient
strategies available.1-8 In addition to exploiting reversible
coordination chemistry to deliver high yields of thermodynami-
cally privileged catenanes incorporating metals in their ring
frameworks,2 catenatessmetal complexes of interlocked organic
macrocyclic ligandsscan be formed through metal template
macrocyclization reactions. However, while the use of tetra-
hedral copper(I) geometry to hold bidentate ligands in an
orientation suitable for subsequent interlocking macrocyclization
reactions has been extensively developed over a 20 year
period,3-8 the transposition of this basic concept to other
coordination modes has been slow to develop. Although Sokolov
alluded to the possibility of using octahedral ions to template
catenane synthesis as early as 1973,9 attempts to prepare
interlocked architectures in this way initially met with limited
is also a single example12 of five-coordinate zinc(II) being used
to direct the formation of a [2]catenate and a remarkable crown
ether-threaded organometallic catenate templated about a mag-
nesium atom that also forms part of one ring.13
At first sight, the use of a template strategy to produce
interlocked macrocyclic ligands for metals with a square planar
coordination geometry might appear somewhat counter-intuitive.
Square planar coordination obviously involves a 2D donor set,
while interlocking of the two rings requires two crossing points
(one positive, one negative14) necessitating control over the
nature of covalent bond formation in the third dimension.
However, by using a tridentate-monodentate ligand combina-
tion, the relative orientation of the ligands coordinated to the
metal can be varied (formally by rotation about the monodentate
ligand-metal bond), and therefore, in principle, systems can
be designed to extend above and below the plane of the square
planar coordination mode and direct a subsequent ring closure
reaction. Indeed, there are several examples15,16 of the orthogonal
alignment of organic fragments using such a “3 + 1” donor set
of ligands, and we recently found that it was possible to exploit
this effect to form a mechanically interlocked structure (Scheme
1).17 Through a combination of electronic and steric factors,
the square planar palladium holds the monodentate 2,6-
† University of Edinburgh.
‡ University of St. Andrews.
(1) For reviews on interlocked molecules assembled about transition metal
templates, see: (a) Sauvage, J.-P.; Dietrich-Buchecker, C. Molecular
Catenanes, Rotaxanes and Knots; Wiley-VCH: Weinheim, Germany, 1999.
(b) Hubin, T. J.; Busch, D. H. Coord. Chem. ReV. 2000, 200-202, 5-52.
(c) Collin, J.-P.; Dietrich-Buchecker, C.; Gavin˜a, P.; Jimenez-Molero, M.
C.; Sauvage, J.-P. Acc. Chem. Res. 2001, 34, 477-487. (d) Menon, S. K.;
Guha, T. B.; Agrawal, Y. K. ReV. Inorg. Chem. 2004, 24, 97-133. (e)
Cantrill, S. J.; Chichak, K. S.; Peters, A. J.; Stoddart, J. F. Acc. Chem.
Res. 2005, 38, 1-9.
9
12612
J. AM. CHEM. SOC. 2005, 127, 12612-12619
10.1021/ja053005a CCC: $30.25 © 2005 American Chemical Society

A Square Planar Palladium [2]Catenate
A R T I C L E S
Scheme 1. Palladium(II)-Directed Synthesis of a [2]Rotaxane17
dimethyleneoxypyridine thread orthogonal to a bis-olefin-
terminated tridentate benzylic amide macrocycle precursor such
that cyclization by ring closing olefin metathesis (RCM) results
in a [2]rotaxane in 77% yield.
However, extending this strategy to catenane synthesis is not
straightforward. Even though oligomer and polymer formation
could be minimized by metal chelation of the acyclic building
blocks, a double macrocyclization strategy could produce three
different isomeric products (1-3, Scheme 2), and even preform-
ing one of the rings prior to attaching the building blocks to
the metal could still afford either the interlocked (1) or
noninterlocked (3) complex. Therefore we explored several
potential routes to a square planar coordination [2]catenate,
varying the sequence that the rings were cyclized and whether
or not the building blocks were attached to the metal prior to
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macrocyclization. Remarkably, it proved possible to find
synthetic routes to each of the isomers 1-3 shown in Scheme
2.
Results and Discussion
Route I: Simultaneous Metal-Directed Olefin Metathesis
of L1 and L2. The first route investigated was the possible
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(8) For knots assembled around tetrahedral four-coordinate Cu(I) templates,
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(9) (a) Sokolov V. I. Usp. Khim. 1973, 42, 1037-1059; Russ. Chem. ReV.
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N. V.; Arion, V. B.; Burgess, J. Template Synthesis of Macrocyclic
Compounds; Wiley-VCH: Weinheim, Germany 1999; and ref 1b.
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39, 5169-5172.
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9
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A R T I C L E S
Fuller et al.
Scheme 2. Schematic Representation of the Three Isomers that
Result from Synchronous or Simultaneous Metal-Directed
Cyclization of Two Acyclic Building Blocksa
differences to L1PdL2, most notably in the region of 2.5-5.5
ppm, indicating that some change in orientation of the building
blocks had occurred upon RCM. Treatment of the complex with
potassium cyanide (Scheme 3, iv) gave a single organic
1
compound (confirmed by mass spectrometry and H NMR),
ruling out the possibility that the product of the RCM was the
two noninterlocked ring isomer, 3; that is, the product of Route
I could not be L4PdL5.
The 1H NMR spectrum (Figure 2c) of the demetalated
structure did not show the shielding effects characteristic of
benzylic amide macrocycle interlocked systems, the chemical
shifts being virtually identical to the independently prepared
free ligands, H2L4 and L5 (Figure 2a and d, respectively). This
suggested that the ligand isomer formed from Route I was
probably H2L3, the single large macrocycle (2) resulting from
intercomponent metathesis between the L1 and L2 olefin groups.
This assignment was confirmed when single crystals suitable
for X-ray crystallography were grown from slow cooling of a
hot, saturated acetonitrile solution of the complex formed by
Route I. The solid state structure (Figure 3) shows the
tetradentate 58-membered single macrocycle satisfying the
square planar coordination geometry of the palladium, with the
tridentate 2,6-dicarboxamidopyridine moiety held orthogonally
to the monodentate 2,6-bis(oxymethylene)pyridine group. This
results in twisting of the macrocycle, providing a single
crossover point in a distinctive figure-of-eight conformation
(Figure 3b). Although rare, this shape is not unique among
coordination complexes of large flexible macrocycles, with other
examplessbased on octahedral metalssreported by the groups
of Sauvage (a 58-membered hexadentate bis-terpy macrocycle
bound to iron(II))10 and Busch (a 40-membered ring coordinat-
ing to nickel(II) via 2,6-diiminopyridine groups).19 In all cases,
the metal ion provides the crossover point, imposing helical
chirality on what would otherwise be intrinsically achiral
macrocycles (for L3Pd, both enantiomers are observed in the
a Homodimerization is avoided by using one tridentate ligand (blue) and
one monodentate ligand (orange) and a metal with a preferred square planar
coordination geometry. Other possible reaction products could potentially
include knots (although knotted isomers of 1-3 are easily precluded by
limiting the size of the building blocks) and higher cyclic, catenated and
knotted oligomers and polymers resulting from the condensation of more
than just one of each building block (minimized by carrying out the
cyclization reactions at high dilution).
double macrocyclization of the tridentate (H2L1) and mono-
dentate (L2) building blocks, with both ligands already coor-
dinated to a square planar metal, L1PdL2 (Scheme 3, Route I).
The monodentate ligand L2 was prepared in two steps from
4-hydroxybenzyl alcohol (see Supporting Information) and when
combined with the known L1Pd(CH3CN) complex in dichlo-
romethane, this provided the tetra-terminal olefin complex,
1
1
L1PdL2, in 93% yield (Scheme 3, ii). H NMR confirms the
unit cell). Re-examination of the H NMR spectrum (Figure
exchange of the acetonitrile ligand for L2 (Figure 1a,b; note
the upfield shift of the aromatic resonances of L1 (HE and HF)
by 0.8 and 0.2 ppm, respectively, due to stacking with the
pyridine group of L2).
Subjecting L1PdL2 to RCM using the first-generation
Grubbs’ olefin metathesis catalyst,18 followed by hydrogenation
of the resulting internal double bond (Scheme 3, iii), gave a
single species in 75% isolated yield for the two steps. FAB mass
spectrometry confirmed that the complex had the molecular
weight (m/z ) 1110, MH+) necessary to be one of the isomers
1-3, and the 1H NMR spectrum (Figure 1c) showed significant
1c) reveals that the ring conformation of L3Pd is conserved
from the solid state to solution. The low (C2) symmetry results
in the individual protons of the HD, Hc, and Hd methylene groups
(HD, HD′, Hc, Hc′, Hd, and Hd′) being held in diastereotopic
environments; the magnitude of the splittings following their
proximity to the crossover point (HD and HD′ are split by nearly
2.5 ppm, Hc and Hc′ by 1.0 ppm, and Hd and Hd′ by just 0.4
ppm). Interestingly, the aromatic region of L3Pd is virtually
identical to that of L1PdL2 (Figure 1b), suggesting that while
the alkyl residues may be mobile, the rest of the acyclic
precursor complex is well-organized for intercomponent olefin
metathesis.
(12) Hamann, C.; Kern, J.-M.; Sauvage, J.-P. Inorg. Chem. 2003, 42, 1877-
1883.
Since the attempted simultaneous double macrocyclization
strategy had given rise to intercomponent bond formation, we
sought to exclude this possibility20 by preforming one or the
other of the rings prior to the second, metal-directed, cyclization
reaction (Scheme 3, Routes II and III).
(13) Gruter, G.-J. M.; de Kanter, F. J. J.; Markies, P. R.; Nomoto, T.; Akkerman,
O. S.; Bickelhaupt, F. J. Am. Chem. Soc. 1993, 115, 12179-12180.
(14) Flapan, E. A Knot Theoretic Approach to Molecular Chirality. In Molecular
Catenanes, Rotaxanes and Knots; Sauvage, J.-P., Dietrich-Buchecker, C.,
Eds.; Wiley-VCH: Weinheim, Germany, 1999; pp 7-35.
(15) (a) Kickham, J. E.; Loeb, S. J. Inorg. Chem. 1994, 33, 4351-4359. (b)
Kickham, J. E.; Loeb, S. J.; Murphy, S. L. Chem.sEur. J. 1997, 3, 1203-
1213.
Route II: Metal-Directed RCM of L2. Tridentate macro-
cycle H2L4 was prepared in 57% yield by treatment of 2,6-
(16) Hamann, C.; Kern, J.-M.; Sauvage, J.-P. Dalton. Trans. 2003, 3770-3775.
(17) (a) Fuller, A.-M.; Leigh, D. A.; Lusby, P. J.; Oswald, I. D. H.; Parsons, S.;
Walker, D. B. Angew. Chem., Int. Ed. 2004, 43, 3914-3918. (b) Furusho,
Y.; Matsuyama, T.; Takata, T.; Moriuchi, T.; Hirao, T. Tetrahedron Lett.
2004, 45, 9593-9597. (c) Leigh, D. A.; Lusby, P. J.; Slawin, A. M. Z.;
Walker, D. B. Angew. Chem., Int. Ed. 2005, 44, 4557-4564.
(18) (a) Schwab, P.; France, M. B.; Ziller, J. W.; Grubbs, R. H. Angew. Chem.,
Int. Ed. Engl. 1995, 34, 2039-2041. (b) Grubbs, R. H.; Miller, S. J.; Fu,
G. C. Acc. Chem. Res. 1995, 28, 446-452.
(19) Vance, A. L.; Alcock, N. W.; Busch, D. H.; Heppert, J. A. Inorg. Chem.
1997, 36, 5132-5134.
(20) At least as long as the RCM reaction is not under thermodynamic control.
However, metathesis of internal olefins is generally much slower than that
of terminal olefins. See, for example: Kidd, T. J.; Leigh, D. A.; Wilson,
A. J. J. Am. Chem. Soc. 1999, 121, 1599-1600.
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A Square Planar Palladium [2]Catenate
A R T I C L E S
Scheme 3 a
a Reagents and conditions: (i) Pd(OAc)2, CH3CN, 1 h, 76%; (ii) CH2Cl2, 1 h, 93%; (iii) a. first-generation Grubbs’ catalyst (Cy3P)2Cl2RuCHPh, 0.2
equiv, CH2Cl2, 20 h; b. H2, Pd-C, THF, 4 h, 75% (over two steps); (iv) KCN, MeOH, CH2Cl2, 20 °C, 1 h and then 40 °C, 0.5 h, 94%; (v) Pd(OAc)2,
CH3CN, 1 h, 93%; (vi) CH2Cl2, 1 h, a 3:2 ratio of L4Pd(exo-L2):L4Pd(endo-L2), isolated in 89% yield; (vii) a. first-generation Grubbs’ catalyst (0.1 equiv),
CH2Cl2, 22 h; b. H2 (50 bar), Pd EnCat, THF, 18 h; c. KCN, MeOH, CH2Cl2, 20 °C, 1 h and then 40 °C, 0.5 h, H2L4, L5, and H2L6 (25% over three steps);
(viii) part (v) and then CH2Cl2, 1 h, 78%; (ix) CH2Cl2, 1 h, 69%; (x) a. first-generation Grubbs’ catalyst (0.1 equiv), CH2Cl2, 22 h; b. H2, Pd-C, THF, 4
h, 78% (over two steps); (xi) KCN, MeOH, CH2Cl2, 20 °C, 1 h and then 40 °C, 0.5 h, 97%; (xii) Pd(OAc)2, CH3CN, 60 °C, 4 h, 85%; (xiii) Pd(OAc)2,
CH3CN, 60 °C, 4 h, 79%; (xiv) KCN, MeOH, CH2Cl2, 20 °C, 1 h and then 40 °C, 0.5 h, 98%; (xv) KCN, MeOH, CH2Cl2, 20 °C, 1 h and then 40 °C, 0.5
h, 95%.
pyridinedicarbonyl dichloride with the appropriate diamine under
high dilution conditions (see Supporting Information). Subse-
quent complexation with palladium(II) acetate afforded L4Pd-
(CH3CN) (93%, Scheme 3, v). Threading of L2 through the
cavity of L4Pd(CH3CN) via the substitution of the coordinated
acetonitrile was attempted by simple mixing of the two in
dichloromethane at room temperature (Scheme 3, vi). While
mass spectrometry confirmed the ligand exchange, the 1H NMR
spectrum of the crude product was unexpectedly complex.
Closer inspection of the thin layer chromatograph of the reaction
mixture revealed two products with very similar Rf values in a
ratio of approximately 2:3. Despite their proximity, these proved
amenable to separation by preparative thin-layer chromatography
on silica gel-coated plates (CH2Cl2:MeOH, 98.5:1.5 as eluent).
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A R T I C L E S
Fuller et al.
Figure 1. 1H NMR spectra (400 MHz, 19:1 CD2Cl2:CD3CN, 298 K) of
(a) L1Pd(CH3CN), (b) L1PdL2, and (c) L3Pd. The lettering refers to the
assignments shown in Scheme 3.
Figure 3. X-ray crystal structure of “figure-of-eight” complex L3Pd grown
by slow cooling of a warm, saturated solution of the complex in acetonitrile.
(a) Side-on and (b) top views. Carbon atoms originating from L1 are shown
in light blue, those from L2 in yellow. Oxygen atoms are red, nitrogen
dark blue, palladium gray. Selected bond lengths [Å]: Pd-N2 2.015, Pd-
N5 1.941, Pd-N11 2.027, Pd-N41 2.052; tridentate fragment bite angle
[°]: N2-Pd-N11 161.0.
Figure 2. 1H NMR spectra (400 MHz, CDCl3, 298 K) of (a) H2L4, (b)
H2L6, (c) H2L3, and (d) L5. The lettering refers to the assignments shown
in Scheme 3.
The isolated complexes gave indistinguishable fragmentation
patterns by electrospray ionization mass spectrometry, the
molecular mass ion suggesting they were both isomers of
1
L4PdL2. However, the H NMR spectra exhibited important
differences between the two products. When compared to the
spectra of the starting materials, the minor isomer (lower Rf,
Figure 4d) showed significant shielding of the L4 benzyl rings
(HE and HF), indicative of aromatic stacking with the pyridine
group of L2. In contrast, the major isomer (higher Rf, Figure
4b) showed no evidence of stacking interactions but a greater
degree of complexity in both the L2 signals (two sets of
nonequivalent Hb, Hc, Hd, He, and Hf resonances) and the HD
methylene groups (an AB system with a 2.5 ppm separation
between the two proton environments) of L4. From this, we
tentatively assigned the two products as “threaded” (minor
Figure 4. 1H NMR spectra (400 MHz, CD2Cl2, 298 K) of (a) L4PdL5,
(b) L4Pd(exo-L2), (c) L2, (d) L4Pd(endo-L2), (e) L1PdL5, and (f) L6Pd.
The lettering refers to the assignments shown in Scheme 3.
isomer) and “threaded” (minor isomer) atropisomers,21 L4Pd-
(exo-L2) and L4Pd(endo-L2), respectively (Scheme 3).
Intriguingly, CPK models suggested that cyclization of L2
could occur with each atropisomer of L4PdL2, suggesting that
two compounds of identical connectivity but different confor-
mations22 are predisposed to form topological isomeric products
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12616 J. AM. CHEM. SOC. VOL. 127, NO. 36, 2005

A Square Planar Palladium [2]Catenate
A R T I C L E S
upon RCMsthe [2]catenate, 1, and the analogous noninter-
locked double macrocycle structure, 3. Sure enough, treatment
of the 2:3 mixture of atropisomers with Grubbs’ catalyst in CH2-
Cl2, followed by hydrogenation and demetalation (Scheme 3,
vii (three stepssthe mixtures of topological and olefin isomers
preventing the isolation of pure products until the end of the
reaction sequence)), afforded three products: macrocycles H2L4
and L5 arising from complex 3, and a further compound, which
mass spectrometry confirmed to be a different isomer of H2L3,
in 25% overall yield for the three steps.23
1H NMR spectroscopy of the new compound in CDCl3
(Figure 2b) revealed shielding of most resonances compared to
the free macrocycles H2L4 (Figure 2a) and L5 (Figure 2d)
characteristic of interdigitation, suggesting that it was indeed
the [2]catenand H2L6. The exception to the upfield trend in
shifts was the amide protons (HC), which were shifted signifi-
cantly downfield (ca. 1.3 ppm) in the catenand compared to
those of H2L4, indicative of a significant hydrogen-bonding
interaction between the amide protons of one ring and the
pyridine nitrogen of the other. In contrast, the analogous amide
protons in the 58-membered free macrocycle H2L3 occur at 8.11
ppm in CDCl3 (Figure 2c), only slightly downfield of their
position in H2L4 (7.88 ppm, Figure 2a), meaning that little
intramolecular hydrogen bonding is occurring in the large
flexible macrocycle. Why is this internal hydrogen bonding
absent when it is so clearly present in the mechanically bonded
isomer H2L6? First, the size and nature of the solvent-exposed
surfaces of the compact [2]catenand structure and the large,
relatively open, macrocycle must be very different, making
desolvation of the amide and pyridine residues in the catenane
a significantly less energetically costly process. Second, with
the 2,6-bis(oxymethylene)pyridine and 2,6-pyridinecarboxamide
groups on different components, the macrocycles in H2L6 can
orientate themselves for inter-residue hydrogen bonding with
little more than the loss of a single rotational degree of freedom.
In contrast, alignment of the groups to enable a similar
interaction within H2L3 would significantly restrict the number
of conformations accessible by the alkyl chains in the large
flexible ring, the resulting losses in degrees of freedom raising
the energy of the hydrogen-bonded structure. This unusual
orthogonal double bifurcated bis-pyridine hydrogen-bonding
interaction is also observed in the related [2]rotaxane system.17
Reintroduction of Pd(II) into the free ligand systems (H2L3
f L3Pd, Scheme 3, xii; H2L6 f L6Pd, Scheme 3, xiii; H2L4
f L4Pd(CH3CN) + L5 f L4PdL5, Scheme 3, v, viii)
proceeded smoothly in each case, the last two providing pure
samples of complexes formed previously as intermediates during
each pathway of the Route II syntheses. ESI mass spectrometry
initially identified the formation of L4PdL5, confirming that
both macrocycles could simultaneously bind to palladium
Figure 5. X-ray crystal structure of noninterlocked double macrocycle
complex L4PdL5 grown from a saturated solution of the complex in acetone.
(a) Side-on and (b) top views. Carbon atoms of the L4 macrocycle are
shown in light blue, and those of the L5 macrocycle in yellow; oxygen
atoms are red, nitrogen dark blue, and palladium gray. Selected bond lengths
[Å]: N2-Pd 2.032, N5-Pd 1.943, N11-Pd 2.032, N41-Pd 2.077;
tridentate fragment bite angle [°]: N2-Pd-N11 160.8.
corroborated this result and showed significant similarity (a
diastereotopic environment for HD, and two sets of signals for
each Hb, Hc, Hd, He, Hf, and Hg) to the presumed nonthreaded
isomer of L4PdL2 (Figure 4b). These spectral features are
consistent with orthogonal binding of the rings to the square
planar geometry metal, with neither macrocycle being able to
pass through the cavity of the other nor rotate (at least not
rapidly on the NMR time scale in the case of the monodentate
ligand) about the plane of the square planar coordination
geometry. This means both macrocycles are perforce desym-
metrized in the plane that they coordinate to the metal, that is,
top from bottom in L4 and left from right in L5 (as L4PdL5 is
depicted in Figure 5a).
Pleasingly, the pure samples of L4PdL5 and L6Pd both
provided single crystals suitable for structure elucidation by
X-ray crystallography (Figure 5 and Figure 6, respectively).
Between them, the X-ray crystal structures of L3Pd, L4PdL5,
and L6Pd confirm not only the identity of the metal complexess
a unique set of topological (L4PdL2 and L6Pd) and constitu-
tional (L3Pd and L4PdL5/L6Pd) isomerssbut also the structural
assignments of the free ligands inferred by the earlier 1H NMR
and mass spectrometry analysis.
1
without being interlocked. H NMR spectroscopy (Figure 4a)
(21) The term “atropisomerism” technically refers to conformers which can be
isolated as separate chemical species as a result of restricted rotation about
a single bond [IUPAC Compendium of Chemical Terminology 2nd ed.;
1997]. We stretch this definition slightly in applying it to L4Pd(endo-L2)
and L4Pd(exo-L2), which are isolable as a result of restricted rotation about
various single bonds in particular ligand orientations.
(22) The term “co-conformation” is generally used to refer to the relative
positions of mechanically interlocked components with respect to each other
[Fyfe, M. C. T.; Glink, P. T.; Menzer, S.; Stoddart, J. F.; White, A. J. P.;
Williams, D. J. Angew. Chem., Int. Ed. Engl. 1997, 36, 2068-2070].
However, since the two components in L4Pd(endo-L2) and L4Pd(exo-
L2) are connected by a continuous sequence of covalent and coordination
bonds, in this case “conformation” is a sufficient descriptor.
Atropisomer-Specific Synthesis of Different Topological
Isomers. Although RCM (and subsequent hydrogenation and
demetalation) of the mixture of the two L4PdL2 atropisomers
gives a ratio of isolated interlocked to noninterlocked products
similar to the starting atropisomer ratio, it does not necessarily
follow that one atropisomer leads solely to one product. The
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A R T I C L E S
Fuller et al.
Route III: Metal-Directed RCM of L1. Finally, we
investigated the product distribution arising from preforming
the monodentate macrocycle and applying metal-directed cy-
clization of the tridentate ligand (Scheme 3, Route III).24 L1Pd-
(CH3CN) and L5 were stirred together in dichloromethane at
room temperature (Scheme 3, ix) and, in contrast to the
analogous step in Route II, reacted to give a single product rather
1
than a mixture of atropisomers. The H NMR spectrum of
L1PdL5 (Figure 4e) suggests the two ligands are threaded; the
upfield shift of HE and HF compared to similar protons in the
nonthreaded L4PdL5 and L4Pd(exo-L2) complexes (Figure 4a
and b, respectively), indicating π-stacking of the benzyl groups
of L1 with the pyridine unit of L5. It is difficult to distinguish
between whether L1PdL5 can exist as threaded/nonthreaded
atropisomers but is formed solely as the (exo-L1)PdL5 isomer,
or rather threaded and nonthreaded forms of L1PdL5 are in
equilibrium with the threaded conformation being thermody-
namically preferred by several kcal mol-1. In any event, RCM
of L1PdL5 and subsequent hydrogenation (Scheme 3, x)
afforded exclusively the [2]catenate, L6Pd, in 78% yield, making
this route both synthetically efficient and completely selective
for the mechanically interlocked topological isomer.
Conclusions
A [2]catenate and the isomeric single macrocycle and double
macrocycle metal complexes can each be efficiently assembled
about a palladium(II) template via RCM. The order in which
the tridentate and monodentate ligand cyclization reactions and
coordination steps are performed determines the outcome of the
synthetic pathway, providing selective routes to each of the three
topological and constitutional isomers. In one case, preforming
the tridentate macrocycle followed by its coordination along
with the acyclic monodentate ligand to the Pd produces threaded
and nonthreaded atropisomers. These can be isolated and, while
the individual forms are stable in dichloromethane, they can be
interconverted in a coordinating solvent through ligand ex-
change. Each atropisomer was shown to be a true intermediate
to a different topological product, meaning that, in this reaction,
the choice of solvent can determine whether the [2]catenate is
formed or its noninterlocked isomer. It is remarkable to see how
topology and connectivity can be selected so exquisitelysin
three different formssusing just one set of organic building
blocks and a metal atom with a two-dimensional coordination
geometry.
Figure 6. X-ray crystal structure of palladium[2]catenate L6Pd grown by
slow cooling of a warm, saturated solution of the complex in acetonitrile.
(a) Side-on and (b) top views. Carbon atoms of the L4 macrocycle are
shown in light blue, and those of the L5 macrocycle in yellow; oxygen
atoms are red, nitrogen dark blue, and palladium gray. Selected bond lengths
[Å]: Pd-N2 1.934, Pd-N5 2.041, Pd-N11 2.036, Pd-N41 2.079;
tridentate fragment bite angle [°]: N2-Pd-N11 160.2.
interconversion of the two L4PdL2 atropisomers in solution was
1
investigated by H NMR spectroscopy. The initial threaded:
nonthreaded ratio of ca. 2:3 remained unchanged over 7 days
in CD2Cl2 at room temperature. Similarly, spectra of pure
samples of each of the compounds were invariant under these
conditions, demonstrating that the atropisomers are kinetically
stable at room temperature in a noncoordinating solvent.
However, addition of ∼10% CD3CN to either of the pure
atropisomer solutions or the 2:3 mixture led to a gradual change
in the threaded:nonthreaded ratio, increasing to ca. 7:3 after 4
days. This suggests that the threaded isomer is thermodynami-
cally favored, and that atropisomer interconversion can take
place via the dissociation of L2 from either form of L4PdL2,
the vacant coordination site being temporarily filled by a
molecule of CD3CN. Accordingly, the reaction between L2 and
L4Pd(CH3CN) was repeated in refluxing acetonitrile. After
several hours, 1H NMR spectroscopy showed a 7:3 ratio of the
threaded:nonthreaded forms of L4PdL2. Heating over 2 days
increased the ratio to 8:1, after which no further change
occurred.
Experimental Section
Selected Spectroscopic Data for the Set of Three Constitutional
and Topological Isomers. L3Pd: 1H NMR (400 MHz, CD2Cl2, 298
K): δ ) 1.09-1.84 (m, 32H, alkyl-H), 2.70 (d, 2H, J ) 14.4 Hz,
HD′), 3.64 (m, 4H, HG), 4.00-4.13 (m, 8H, Hg + Hc′ + Hd′), 4.39 (d,
2H, J ) 10.6 Hz, Hd), 5.04-5.14 (m, 4H, Hc + HD), 6.38 (d, 4H, J )
8.6 Hz, HE), 6.49 (d, 4H, J ) 8.6 Hz, HF), 6.84 (d, 4H, J ) 8.6 Hz,
RCM of single atropisomer samples of L4PdL2 in CH2Cl2
did, indeed, generate a single new species in each case. The
product of RCM of the presumed L4Pd(endo-L2) complex was
hydrogenated to give solely the [2]catenate, L6Pd; the product
of RCM of presumed L4Pd(exo-L2) proved unstable to hydro-
genation,23 so the metal was removed instead (KCN, MeOH,
CH2Cl2, 20 °C, 1 h and then 40 °C, 0.5 h), liberating two
different macrocycles, H2L4 (96%) and the unsaturated olefin
analogue of L5 (93%).
(23) The hydrogenation conditions employed in Routes I and III led to a complex
mixture when applied to the products of metathesis in Route II. Not only
was significant decomplexation of the somewhat strained noninterlocked
double macrocycle complex observed but also the resulting free mono-
dentate ligand, L5, was degraded, presumably by hydrogenolysis of the
benzyl ether moieties. Although the use of Pd EnCat did not prevent the
ligand decomplexation during the hydrogenation step, it significantly
reduced the degradation of the free monodentate macrocycle [Bremeyer,
N.; Ley, S. V.; Ramarao, C.; Shirley, I. M.; Smith, S. C. Synlett 2002,
1843-1844].
(24) Leigh, D. A.; Lusby, P. J.; Slawin, A. M. Z.; Walker, D. B. Submitted for
publication.
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A Square Planar Palladium [2]Catenate
A R T I C L E S
Hf), 7.20 (d, 4H, J ) 8.6 Hz, He), 7.45 (d, 2H, J ) 7.8 Hz, Hb), 7.76
(d, 2H, J ) 7.8 Hz, HB), 7.88 (t, 1H, J ) 7.8 Hz, Ha), 8.08 (t, 1H, J
) 7.8 Hz, HA). 13C NMR (100 MHz, CD2Cl2, 293 K): δ ) 23.7, 25.2,
25.5, 27.5, 27.5, 27.5, 28.6, 29.5, 48.5, 67.1, 68.1, 71.8, 73.2, 114.0,
114.8, 121.2, 124.8, 128.5, 128.7, 129.5, 132.8, 139.1, 140.7, 152.8,
158.1, 158.6, 160.6, 171.1. HRMS (FAB, NOBA): Calcd for C62H75N4O8-
Pd [M + H]+ 1109.46169. Found 1109.46275. L4PdL5: 1H NMR (400
MHz, CD2Cl2, 298 K): δ ) 0.81-1.74 (m, 32H, alkyl-H), 2.50 (d,
2H, J ) 14.8 Hz, HD′), 3.34-3.44 (m, 4H, Hd′ + Hc′), 3.78-4.02 (m,
6H, Hg′ + HG′ + HG), 4.09 (t, 2H, J ) 6.3 Hz, Hg), 4.72 (s, 2H, Hd),
5.08 (d, 2H, J ) 14.8 Hz, HD), 5.67 (s, 2H, Hc), 6.64-6.84 (m, 12H,
Hf ′ + HF + HE + Hf), 6.92 (d, 1H, J ) 7.8 Hz, Hb′), 6.99 (d, 2H, J )
8.6 Hz, He′), 7.12-7.22 (m, 3H, Hb + He), 7.38 (t, 1H, J ) 7.8 Hz,
Ha), 7.83 (d, 2H, J ) 7.8 Hz, HB), 8.13 (t, 1H, J ) 7.8 Hz, HA). 13C
NMR (100 MHz, CD2Cl2, 298 K): δ ) 25.6, 25.9, 25.9, 28.5, 29.0,
29.1, 29.3, 29.4, 29.4, 29.6, 29.8, 29.9, 49.2, 67.8, 67.9, 68.3, 72.1,
72.8, 73.4, 76.3, 114.8, 114.9, 115.4, 122.0, 122.7, 125.1, 128.3, 129.9,
130.3, 130.5, 131.4, 133.0, 138.4, 141.3, 153.0, 158.2, 158.7, 159.6,
160.6, 161.9, 171.6. LRMS (ESI) m/z ) 1132 [M + Na]+. L6Pd: 1H
NMR (400 MHz, CD2Cl2, 293 K): δ ) 1.04-1.50 (m, 24H, alkyl-H),
1.60 (m, 4H, alkyl-H), 1.70 (m, 4H, alkyl-H), 3.60 (s, 4H, HD), 3.81
(t, 4H, J ) 6.3 Hz, HG), 3.89 (t, 4H, J ) 6.3 Hz, Hg), 4.36-4.42 (m,
8H, Hd + Hc), 5.94 (d, 4H, J ) 8.6 Hz, HF), 6.10 (d, 4H, J ) 8.6 Hz,
HE), 6.63 (d, 4H, J ) 8.6 Hz, Hf), 7.05 (d, 2H, J ) 7.8 Hz, Hb), 7.15
(d, 4H, J ) 8.6 Hz, He), 7.60 (t, 1H, J ) 7.8 Hz, Ha), 7.76 (d, 2H, J
) 7.6 Hz, HB), 8.13 (t, 1H, J ) 7.6 Hz, HA). 13C NMR (100 MHz,
CD2Cl2, 293 K): δ ) 25.8, 26.0, 28.5, 28.7, 28.9, 29.2, 30.0, 30.1,
49.4, 67.1, 67.7, 69.6, 72.9, 114.5, 114.8, 121.2, 124.5, 128.2, 128.6,
131.7, 132.9, 138.8, 140.6, 153.1, 157.4, 159.5, 160.0, 171.1. HRMS
(FAB, NOBA): Calcd for C62H75N4O8 Pd [M + H]+ 1109.46169.
Found 1109.46152.
Acknowledgment. This work was supported by the European
Union Future and Emerging Technology program, MechMol,
and the EPSRC.
Supporting Information Available: Experimental procedures
and spectroscopic data for all new compounds, and thermal
ellipsoids for the crystal structures of L3Pd, L4PdL5, and L6Pd.
This material is available free of charge via the Internet at
http://pubs.acs.org.
JA053005A
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J. AM. CHEM. SOC. VOL. 127, NO. 36, 2005 12619