Self-assembly of [2]rotaxane exploiting reversible Pt(II)-pyridine coordinate bonds

Article abstract of DOI:10.3390/90500278

A dinuclear self-assembled cationic macrocycle based on Pt(II)-N(pyridine) coordinative bonds and having competitive triflate anions, as metal counterions, is used in the construction of [2]rotaxane and [2]pseudorotaxane architectures assisted by hydrogen bonding. The kinetic lability of the Pt(II)-N(pyridine) coordinative bond controls the dynamics of the [2]rotaxane.

Full text of DOI:10.3390/90500278

Molecules 2004, 9, 278–286
molecules
ISSN 1420-3049
http://www.mdpi.org
Self-assembly of [2]Rotaxane Exploiting Reversible Pt(II)-
†
Pyridine Coordinate Bonds
Pablo Ballester,* Magdalena Capó, Antoni Costa, Pere M. Deyà, Antoni Frontera and Rosa
Gomila
Departament de Química, Universitat de les Illes Balears, 07122 Palma de Mallorca, Illes Balears,
Spain. Tel. (+34) 971-172547, Fax: (+34) 971-174326.
*
†
Author to whom correspondence should be addressed; e-mail pablo.ballester@uib.es
Dedicated to Professor R. Mestres on the occasion of his 65th birthday. It was my privilege to obtain
the Ph.D. degree working on the reactivity of carboxylic acid lithium dienolates under his
supervision. I shall always be indebted to him for introducing me to chemical research (P.B.).
Received: 22 January 2004 / Accepted: 25 January 2004 / Published: 30 April 2004
Abstract: A dinuclear self-assembled cationic macrocycle based on Pt(II)-N(pyridine)
coordinative bonds and having competitive triflate anions, as metal counterions, is used in
the construction of [2]rotaxane and [2]pseudorotaxane architectures assisted by hydrogen
bonding. The kinetic lability of the Pt(II)-N(pyridine) coordinative bond controls the
dynamics of the [2]rotaxane.
Keywords: Self-assembly, [2]rotaxane, coordinative bond.
Introduction
Self-assembly is an attractive approach for the construction of organized interlocked molecular
compounds. Tetralactam macrocycles have been widely used as the wheel component for the
templated synthesis of interlocked molecules such as rotaxanes and pseudorotaxanes [1]. Rotaxanes
can exhibit dynamic features such as shuttling [2] or pirouetting [3] and there is hope to use these
interlocked molecules and their dynamic features in developing artificial molecular machines [3,4].

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279
The use of noncovalent interactions in the synthesis of rotaxanes does not necessarily to be confined
to the stabilization of the template where the linear molecule is threaded through the plane of the
macrocycle. In fact, the replacement of one or more covalent bonds, either in the macrocycle or in the
linear molecule, by reversible controllable weak bonds will afford more readily accessible rotaxanes,
which will display more diverse motional modes associated with their reversible assembly
disassembly. Among possible weak reversible bonds, the metal-ligand interaction or coordinate bond is
very suitable for this purpose because both strength and geometry are tunable with the selection of the
right combination of metal and ligand. Based on these ideas, rotaxanes in which coordinate bonds are
used for attaching stoppers to the linear molecule [5] or for the construction of the macrocycle unit
[
6,7] have been recently reported. The macrocycles used in these two later examples contain reversible
coordination bonds N-Os(VI) [6] and N-Zn(II) [7], and in both cases the metal lacks of non-coordinate
metal counter ions. Here we describe an example of non-covalent self-assembly of a [2]rotaxane like
complex with a macrocycle containing reversible coordinate N-Pt(II) bonds and featuring non
coordinate metal counter ions (triflate) which may compete with the template threading process based
on the formation of intermolecular hydrogen bonds.
Results and Discussion
The dinuclear macrocycle 2, the macrocycle component of the rotaxane, was quantitatively self-
assembled through Pt(II)-N coordination bonds [8] from a mixture of a 1:1 of cis-Pt-(dppp)(OTf) and
2
bispyridyl ligand 1. The ethoxy group of the bispyridyl ligand was introduced in order to increase the
solubility of 1 in organic solvents. The macrocycle 2 contains a square cavity in which four hydrogen
donors (NHs) converge by virtue of internal (pyridine N⋅⋅⋅H-N amide) intramolecular hydrogen bonds
[9].
Scheme 1. Self-assembly of macrocyle 2 and formation of [2]rotaxane 2·4 and
2]pseudo rotaxane 2·3.
[

Molecules 2004, 9
280
During the course of our studies a similar macrocycle containing Pd(II) metal centers has been
described and its X-ray structure solved [10]. Due to the similarity of our system with those reported
[
6,7] and since the efficient constructions of [2]rotaxanes require a high affinity between the wheel and
the axle molecules, we decided to use axle molecules containing an adipamide unit. When macrocycle
2
(4mM) and four equivalents of the adipamide axle 3 were mixed in CDCl at room temperature the
3
1
H-NMR spectrum immediately showed a slight downfield shift of the NH resonances of the self-
assembled macrocycle 2, indicating the formation of an hydrogen bonded-complex even in the
presence of competing triflate anions. Synthetic receptors for anions are based on the amide H-bond
donor properties. Particularly, a luminiscent rhenium (I) 2,6-pyridinecarboxamide, N, N´-di-4-pyridyl-
4
5
-1
based receptor has been recently described to bind anions with binding constants as high as 10 -10 M
in CH Cl solution [11]. We were worried that due to the structural resemblance to the binding sites of
, the triflate anions acting as metal counter ions will inhibit the hydrogen-bonded complex formation
2
2
2
between 2 and 3. Upon cooling the solution of 2 and 3 at 273 K a new set of signals assigned to the 2•3
complex could be seen (Figure 1).
1
Figure 1. H-NMR spectra (300 MHz) in CDCl
3
of a) 2 (4mM), b) 2 (4mM) + 3 (16
mM) at 293 K, c) 2 (4mM) + 3 (16 mM) at 273 K.

Molecules 2004, 9
281
1
The H-NMR spectrum of the 2•3 complex is clearly different from that of the free components.
The amide NH and the proton of the dimethylpyridyl are shifted downfield, the first as a result of
hydrogen bonding to the adipamide carbonyl and the second due to its diamagnetic anisotropy. In
addition, the hydrogens α and β to the carbonyls in the adipamide 3 (H and H in Figure 1) appear
a
b
very far upfield at δ = 0.242 and δ = - 0.341 ppm, respectively, compared to δ = 2.195 and δ = 1.636
ppm for free 3.
The upfield shift of the H and H signals strongly suggests that the adipic moiety of 3 is inserted
a
b
3
1
into the cavity of 2. The P-NMR spectrum of the mixture at 273 K also reveals the existence of two
different phosphorus signals at δ = -15.16 ppm for the 2•3 complex and δ = -13.98 ppm for the free
1
31
macrocycle 2. On the basis of NMR integration ( H, P) of several stock solutions an average
-1
association constant was calculated to be Ka(2+3 ⇔ 2•3) = 20 ± 2M . Jeong has recently reported that the
association constants for tetralactam macrocycles, derived from pyridine 2,6-carboxamide, binding
adipamides were highly sensitive to the substituents at the para position of the pyridine ring [12].
Electron donating groups (NMe , OMe) decrease the binding affinities. Even so, the calculated
2
-1
association constant Ka(2+3 ⇔ 2•3) = 20 ± 2M is one order of magnitude lower than the reported values
#
in the literature for similar systems. The energy barrier ∆G for the dissociation process of the 2•3
-
1
complex is 14 kcal mol , based on the coalescence temperature (290 K) [13] of the NH signals of the
wheel component 2, in complete agreement with the literature values for similar systems [6]. Taken
together, these results suggest the existence of a competitive binding of the wheel component 2
between the axle molecule 3 and the triflate anions.
The pseudorotaxane geometry for the 2•3 complex was confirmed using an adipamide based axle
molecule 4, which posses 4-[tris-(4-tert-butylphenyl)-phenoxyl] stopper end groups. The axle 4, with
1
31
its bulkier stoppers, showed a different behaviour from 3 in the H-NMR and P-NMR spectra. First,
after addition of axle 4 no immediate changes were observed in any of the resonances of the self-
assembled macrocycle 2. It was necessary to heat the CDCl solution of 2 (4 mM) and 4 (4
3
equivalents) during 48 h at 50º C in an oil bath to achieve equilibrium between the 2•4 complex and its
components. Second, with axle 4 and after the equilibration time it was possible to observe even at
room temperature sharp and widely separated signals for the 2•4 complex, very similar to those
observed for the 2•3 complex relative to free 3 at 273 K. This is indicative of slow exchange on the
NMR time scale between complex 2•3 and its components. The NHs are downfield shifted from δ =
9
0
.50 ppm to δ = 10.10 ppm, and the H
.247 and δ = -0.236 ppm. The P-NMR spectrum showed a new signal at δ = -15.26 ppm
a
and H
b
hydrogen atoms of the axle 4 shifted upfield to δ =
3
1
corresponding to the [2]rotaxane complex 2•4 and the signal for the free macrocycle 2 at δ = -14.07
ppm (Figure 2). No coalescence could be observed up to the temperature limit of the solvent (320 K).
#
Thus, although it was not possible to determine the corresponding ∆G for the exchange process here,
-1
clearly it must be significantly higher than 15.8 kcal mol consistent with a higher kinetic barrier to
exchange introduced by the stoppering. The binding constant between wheel 2 and axle 4 measured at
1
31
room temperature by H and P integration after the equilibration period was estimated to be Ka(2+4
-1
⇔
2•4) = 18 ± 2 M . The magnitude of this value indicates that the apparent association constant is only
slightly affected by the size of the end groups.

Molecules 2004, 9
282
1
Figure 2. H-NMR (300 MHz) spectra in CDCl at 293 K of 2 (4mM) + 4 (16mM)
3
3
1
showing free and bound species. Inset corresponding P-NMR (121 MHz).
Figure 3. ESI-MS in acetone of 2·4 [2]rotaxane showing the peak corresponding to the
-
2+
molecular ion [2·4 (CF
distribution.
3
SO
3
)
2
] . a) Observed isotopic distribution. b) Calculated isotopic
Further evidence for the existence of the [2]rotaxane 2•4 in solution came from electrospray mass
spectroscopy (ESI-MS). Ions corresponding to the di-cation of the 2•4 complex, resulting from the loss
of two triflate counter ions, were observed with a mass-to-charge ratio (m/z) of 1834.7319 (cald.
1
834.7279) (Figure 3).

Molecules 2004, 9
283
1
On the one hand, the observed changes in the kinetics of the formation and also in the H-NMR
dynamic behavior of the 2•3 and 2•4 complexes can be interpreted as evidences for the existence of
different exchange pathways leading to complex formation. On the other hand, the similarities
observed for the chemical shift changes (proton and phosphorous) upon formation of both hydrogen
bonded complexes and for its stability constants strongly suggest a very close geometry. Thus, the 2•3
complex forms a [2]pseudorotaxane where the barrier to exit or entry the wheel component 2 by the
axle 3 is only controlled by the formation or disruption of the hydrogen bonds which hold together the
axle and the wheel, while the 2•4 complex forms a [2]rotaxane stabilized by the same set of hydrogen
bonds but where exit or entry of the axle 4 requires, due to the bulky stoppers attached at its ends, the
opening of the wheel component 2 by breaking the coordinative bonds (Scheme 2).
Scheme 2. Schematic pathways involved in the exchange process between the free and
the bound species for the [2]rotaxane. Due to the bulky stoppers of 4 the only operative
pathway for the formation and dissociation of the 2·4 complex requires the disruption of a
coordinative bond. The slipping process which is the main pathway for the exchange
process in the 2·3 complex is inhibited due to the bulky stoppers.
It is well known that Pt(II) complexes with nitrogen ligands have high affinities and consequently
are kinetically stable on the NMR time scale. The fact that the dissociation of the highly stable self-
assembled macrocycle 2, based on Pt(II)-pyridine coordinative bonds, controls the exchange dynamics
for the 2•4 complex explains its kinetic stability on the NMR time scale. For this reason, we can
1
observe a different set of signals for free and bound 2 and 4 at room temperature on the H-NMR and
3
1
P NMR spectrum (Figure 2)
Conclusions
In summary we have demonstrated that self-assembled macrocycles based on Pt(II)-N(pyridine)
coordinative bonds and featuring competitive triflate anions, as metal counter ions, can be used for the
effective construction of [2]rotaxane and [2]pseudorotaxane architectures assisted by hydrogen
bonding formation. The kinetic lability of the Pt-N(pyridine) coordinative bond controls the exchange
dynamics of the assembly when bulky substituents attached at the ends of the axle molecule inhibit the
slipping process for its formation. The dynamic (kinetic) change observed on the NMR time scale and
the similarity in the association constants (thermodynamic) and chemical shifts for the two complexes
2
·3 and 2·4 have been used as direct evidences to assign the complexes’ architectures.

Molecules 2004, 9
284
Acknowledgments
We thank the MCYT (BQU-2002-04651) for financial support, and Prof. K-S. Jeong for providing
a sample of the axle 4.
Experimental
General
1
31
H- and P-NMR spectra were obtained using a Bruker AMX 300 NMR and were recorded at 300
and 121 MHz respectively. All reagents and chemicals were obtained from the Aldrich Chemical
Company (Spain) and were used as received unless otherwise noted. Methylene chloride was distilled
from CaH under an argon atmosphere. Compound 5 was prepared according to literature procedures
2
[14].
N,N’-Bis(3,5-dimethyl-4-pyridinyl)-4-ethoxy-2,6-pyridinedicarboxamide (1).
A solution of 4-amino-3,5-lutidine (285 mg, 2.3 mmol) in CH
ethylamine (420µL, 2.4 mmol) were added to a solution of 4-ethoxy-2,6-pyridinedicarbonyl dichloride
200 mg, 0.8 mmol) in CH Cl (10 mL) cooled to 0 ºC. The resulting solution was stirred under an
argon atmosphere at room temperature for 2 h and subsequently heated at reflux for 2h. The reaction
mixture was diluted with CH Cl (10mL), washed twice with saturated NaHCO solution and once
with brine, dried over anhydrous MgSO , filtered and evaporated in vacuo. The solid residue was
2
Cl (5 mL) and N,N-diisopropyl-
2
(
2
2
2
2
3
4
washed with diethyl ether and recrystallized from CH
2
Cl /EtOAc to yield 1 as a white solid (190 mg,
2
1
5
1
6
6%). H-NMR (CD
2H), 1.50 (t, 3H, J = 6.5 Hz); C-NMR (CD
5.4, 15.9, 14.7; HRMS (ESI) for C23
3
Cl) δ: 9.49 (s, NH, 2H), 8.32 (s, 4H), 7.98 (s, 2H), 4.31 (q, 2H, J = 6.5), 2.26 (s,
1
3
3
Cl) δ: 168.7, 161.5, 150.6, 149.7, 142.1, 130.3, 112.7,
+
H
24
N
5
O
3
(MH ): Calcd 420.2036; Found 420.2047.
Macrocycle 2
Compound 5 (9.1 mg, 1 mmol) was slowly added as a solid to a solution of 1 (4.19 mg, 1 mmol) in
1
CD
3
Cl (500µL). H-NMR (CD
3
Cl) δ: 9.74 (s, NH, 4H), 8.38 (s, 8H), 7.81 (s, 4H), 7.66 (brs, 16H),
7
.42 (brs, 24H), 4.23(q, J = 6.5 Hz, 4H), 3.30 ( brs, 8H), 2.17( brs, 4H), 1,89 (s, 24H), 1.45(t, J = 6.5
3
1
1
195
19
Hz, 6H); P{ H}NMR (CD Cl) δ: -14.01 (s, Pt satellite, JPt-P = 3044 Hz); F-NMR (CD Cl) δ:
3
3
+
2+
-
77.30; MS (ESI) m/z 2500.3127 [M-OTf] , 1175.7456 [M-2OTf] .
Hexane-1,6-dioic acid bis(octylamide) (3)
A suspension of adipic acid (0.3 g, 2 mmol) in thionyl chloride (4 mL) containing a catalytic
amount of triphenylphosphine was heated at reflux until a clear solution was obtained (~ 2 h). The

Molecules 2004, 9
285
excess thionyl chloride was removed and the residue dissolved in dry CH Cl (10 mL). To this
2
2
solution octylamine (680 µL, 4.1 mmol) and triethylamine (600 µL, 4.3 mmol) were added. The
reaction mixture was stirred overnight, then washed successively with aqueous HCl (5%), NaOH
solution (10%) and brine, dried over Na SO , filtered and concentrated. The residue was purified by
2
4
1
column chromatography (5% CH Cl /MeOH) to give 3 as a white solid (320 mg, 43%). H-NMR
2
2
(
0
7
CD Cl) δ: 5.62 (s, 2H), 3.24 (m, 4H), 2.19 (m, 4H), 1.65 (m, 4H), 1.49 (t, 4H, J=5.8), 1.27 (m, 20H),
3
-1
.88 (m, 6H); IR (KBr) cm : 3313, 1632; Elemental analysis: calcd (%) for C22
1.69, H 12.03, N 7.60; found C 71.42, H 12.20, N 7.62.
H N
44 2
2
O (368.59): C
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