Halide anion directed assembly of luminescent pseudorotaxanes.

Article abstract of DOI:10.1039/b401900h

A series of new photo-active rhenium(i) bipyridyl based pseudorotaxane complexes is assembled via halide anion templation.

Full text of DOI:10.1039/b401900h

Halide anion directed assembly of luminescent pseudorotaxanes
David Curiel, Paul D. Beer,* Rowena L. Paul, Andrew Cowley, Mark R. Sambrook and Fridrich
Szemes
Department of Chemistry, University of Oxford, Inorganic Chemistry Laboratory, Oxford, UK OX1 3QR.
E-mail: paul.beer@chem.ox.ac.uk; Fax: +44 1865 272690; Tel: +44 1865 272632
Received (in Cambridge, UK) 9th February 2004, Accepted 24th March 2004
First published as an Advance Article on the web 22nd April 2004
A series of new photo-active rhenium(
pseudorotaxane complexes is assembled via halide anion
templation.
I
) bipyridyl based
of this macrocycle to bind halide anions was demonstrated by 1H-
NMR titration experiments. The addition of tetrabutylammonium
halide salts to acetone-d6 solutions of 6 resulted in significant
perturbations of most notably the amide and 3,3A-bipyridyl receptor
protons. WinEQNMR7 analysis of the respective titration curves
gave 1:1 receptor:anion stability constant values (Table 1) which
suggest the macrocycle strongly binds halide anions, especially
chloride and bromide.
The template synthesis of interlocked supramolecular architectures
is currently an area of intense research activity.1 The control over
the relative disposition of the interlocked molecular constituents in
the final structure is usually derivative of the residual non-covalent
interactions initially employed to construct them. Examples include
hydrogen-bonding, p-stacking, metal–ligand dative bonding and
hydrophobic interactions between species which are neutral or
cationic in nature.2 With few exceptions,3 the participation of
anions in the formation of the interlocked molecular entities has
largely been unexplored. We have recently shown that pseudor-
otaxane4 and rotaxane5 formation can be templated selectively by a
chloride anion which facilitates the interpenetration of a pyridinium
bis-amide thread through the annulus of an isophthalamide
macrocycle. Here we describe halide anion templated synthesis of
a range of new photo-active pseudorotaxanes where the threading
process can be detected by luminescence spectroscopy (Fig. 1).
1
Analogous H-NMR titration experiments with pyridinium bis-
amide halide salts 7a–c (Fig. 2) revealed evidence for pseudorotax-
ane formation. In addition to significant respective downfield and
upfield amide proton perturbations of 6 and 7, notable shifts were
observed in the receptor’s hydroquinone proton resonances, which
can be attributed to p–p interactions between the pyridinium cation
and hydroquinone rings (Fig. 3).
In acetone solution, ion-pairing between the pyridinium cation
and halide anion is very strong,4 and consequently halide
complexation at the rhenium( ) bipyridyl amide recognition site of
I
Table 1 Stability constants of 6 with halide anions in acetone-d6
The new rhenium( ) bipyridyl macrocycle 6 was prepared in four
I
steps according to Scheme 1. As in related compounds6 the ability
Anion
2
Cl2
Br2
I2
PF6
b
K [M21]a:
> 105
8 3 104
1.7 3 103
–
Anions were used as tetrabutylammonium salts;a Error < 10%.
b No evidence of complexation.
Fig. 1 Anion directed pseudorotaxane formation.
Fig. 2 Structures of ion-paired threads.
Scheme 1 (i) Et3N, CH2Cl2 (97%); (ii) H2, Pd–C, DMF, MeOH (85%); (iii)
Re(CO)5Cl, THF (85%); (iv) tetraethyleneglycol di-p-tosylate, K2CO3,
CH3CN, THF (15%).
Fig. 3 Comparison of expanded 1H-NMR spectra in acetone-d6 for 6, 7a and
a 1:1 mixture of both 6 and 7a.
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C h e m . C o m m u n . , 2 0 0 4 , 1 1 6 2 – 1 1 6 3
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

Table 2 Stability constants of 6 with 7a–d, 8–10 in acetone-d6
Thread
7a
7b
7c
7d
8
9
10
b
K [M21]a:
1.5 3 103
1.2 3 103
1.4 3 102
–
6.1 3 103
> 105
1.3 3 103
a Error < 10%. b No evidence of complexation.
6 results in the cationic pyridinium moiety forming an interlocked
assembly with the macrocycle. Interestingly no evidence of
interpenetration was seen with the hexafluorophosphate pyridinium
salt 7d which suggests threading of the pyridinium cation is
accomplished and driven by recognition of the halide anion by the
receptor.8 Stability constant determinations (Table 2) reveal
pseudorotaxane complex thermodynamic stability mirrors the
strength of halide binding by the receptor (Table 1) where chloride
and bromide pyridinium salts form the strongest pseudorotaxane
complexes.
In summary a series of new photo-active rhenium( ) bipyridyl
I
based pseudorotaxane complexes containing various pyridinium,
benzoimidazolium and guanidinium threading components have
been assembled via halide anion templation.
This research has been supported by a Marie Curie Fellowship
(D.C.) of the European Union, and we thank the EPSRC for a
postdoctoral fellowship (F.S.) and studentship (M.R.S.).
Solid state evidence for pseudorotaxane formation comes from
the X-ray structural determination† of the complex 6·7a.
The structural analysis reveals the expected interlocked product
in which the macrocycle encircles the ion-pair. The chloride anion
fits into the cleft of the pyridinium ring, forming five short X–
H…Cl(2) contacts indicative of hydrogen bonding (distances: N(3)
3.417 (0.006) Å, N(4) 3.351 (0.006) Å, N(6) 3.473 (0.013) Å, N(7)
3.281 (0.006) Å, C(42) 3.459 (0.006) Å). These may be regarded as
the vertices of a highly distorted octahedron. The macrocycle
surrounds the cation such that the two phenyl rings are approx-
imately parallel and sandwich the central pyridinium ring. Least
squares calculations show that the two phenyl rings are ~ 7 Å apart
and are parallel to within 6.6°. These two rings therefore allow
enough room for p-stacking with enclosed cation.
Notes and references
†
Single crystals were grown by slow diffusion of Et2O into CHCl3.
Crystallographic data were collected in an Enras-Nonius KappaCCD
diffractometer using graphite monocromatised Mo–Ka radiation (l
=
071073 Å). Intensity data were processed using the DENZO-SMN
package.9 The structure was solved by direct methods using the SIR92
program.10 Full matrix least-squares refinement was carried out using the
CRYSTALS program suite.11 A Chebychev polynomial weighting scheme
was applied.
Crystal data for 6+7a·CH2Cl2: T = 150 K, crystal size 0.20 3 0.20 3
0.10 mm, monoclinic, space group P21/c, a = 13.6751 (2), b = 22.3512 (2),
c = 22.6383 (3) Å, b = 105.9996 (5) °, V = 6651.47 (14) Å3, Z = 4, dcalcd
= g cm23, m = 1.990 mm21, R1 (wR2) = 0.0541 (0.0604) for the 8504
unique data with I > 3s(I) and 781 parameters.
As evidenced by 1H-NMR titration experiments it is noteworthy
that other strongly ion-paired4 nicotinamide 8, benzoimidazolium
9, and methylguanidinium 10 chloride anion salts also form strong
pseudorotaxane complexes in acetone-d6 solutions (Table 2).
Because the respective cationic species is strongly ion-paired to the
chloride counterion, halide recognition by 6 results in threading of
the cationic component through the macrocyclic cavity.
CCDC 231488. See http://www.rsc.org/suppdata/cc/b4/b401900h/ for
crystallographic data in .cif or other electronic format.
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Preliminary absorption and emission investigations reveal 6
behaves like the parent Rebipy(CO)3Cl complex. As was hoped the
addition of 7a, 8, 9 and 10 was found to affect the luminescence
spectrum of 6 with a significant enhancement in emission intensity
(Fig. 5) which may be a consequence of pseudorotaxane complex
formation increasing the rigidity6 of the receptor, disfavouring non-
radiative decay processes.
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Tomapatanaget, PNAS., 2002, 99, 4983.
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Chem. Soc., 2002, 124, 12469.
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Dent and M. Maestri, J. Am. Chem. Soc., 1997, 119, 11864; L. H.
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Fig. 4 Stick representation of the solid state structure of pseudorotaxane
6+7a.
7 M. J. Hynes, J. Chem. Soc., Dalton Trans., 1993, 311.
8 Sequential addition of one equivalent of TBACl to 6 followed by 7d
leads to pseudorotaxane formation as evidenced by 1H-NMR in acetone-
d6.
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Collected in Oscillation Mode’, Methods Enzymol., Eds C. W. Carter
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Cooper, CRYSTALS, issue 11, Chemical crystallography Laboratory,
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Fig. 5 Emission spectral variations upon titration of 6 (2 3 1025 M in
acetone) with 7a in acetone; (lexc. = 400 nm)
C h e m . C o m m u n . , 2 0 0 4 , 1 1 6 2 – 1 1 6 3
1163