Ternary self-assemblies in water: Forming a pentanuclear ReLn4 assembly by association of binuclear lanthanide binding pockets with fac-Re(CO)3(dinicotinate)2Cl

Article abstract of DOI:10.1039/c3dt51705e

The self-assembly of higher order structures in water is realised by using the association of 1,3-biscarboxylates to binuclear meta-xylyl bridged DO3A complexes. Two dinicotinate binding sites are placed at a right-angle in a rhenium complex, which is shown to form a 1:2 complex with α, α′-bis(Eu·DO3A)-5-amino-m-xylene.

Full text of DOI:10.1039/c3dt51705e

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Ternary self-assemblies in water: forming a
pentanuclear ReLn assembly by association of
Cite this: DOI: 10.1039/c3dt51705e
4
binuclear lanthanide binding pockets with
Received 26th June 2013,
Accepted 24th July 2013
fac-Re(CO) (dinicotinate) Cl†
3
2
DOI: 10.1039/c3dt51705e
a
a
a
a
Leila R. Hill, Octavia A. Blackburn, Michael W. Jones, Manuel Tropiano,
a,b
a
www.rsc.org/dalton
Thomas Just Sørensen* and Stephen Faulkner*
The self-assembly of higher order structures in water is realised by formation of a pentanuclear assembly from three distinct
using the association of 1,3-biscarboxylates to binuclear meta- complexes.
xylyl bridged DO3A complexes. Two dinicotinate binding sites are
Kinetically stable lanthanide complexes derived from aza-
placed at a right-angle in a rhenium complex, which is shown to macrocycles offer a unique platform characterised by high
form a 1 : 2 complex with α,α’-bis(Eu·DO3A)-5-amino-m-xylene.
thermodynamic and kinetic stability constants. By contrast,
with kinetically labile lanthanide complexes, the speciation in
solution is extremely complex as nine-coordinate lanthanide
Self-assembly of dynamic systems in water and formation of
thermodynamically stable, physically constrained, supramole-
cular structures in aqueous solution have been studied exten-
2
−1
ions favour high dissociation rate constants (k = 1.4 × 10 s
)
d
even for polydentate ligands such as EDTA in aqueous media,
1
–7
sively in the last decade. In these studies, while labile bonds
involving main group and d-block elements have been widely
exploited, stable supramolecular assemblies that exploit
lanthanide complexation are less common.
17
−1 13
despite high association constants (K = 3 × 10
M ). Hepta-
and octadentate ligands such as DO3A and DOTA form kineti-
cally stable complexes with the lanthanides, where only one
14
complex dominates the solution speciation at any time. In a
Supramolecular lanthanide complexes are well-known.
Many helical structures whose formations are templated by
lanthanide ions have been reported, and the thermodynamic
processes underpinning such assemblies have been studied in
range of studies, we have shown that N-substituted DO3A
derivatives form kinetically robust complexes with lantha-
nides, and that linked DO3A units bridged by 3,5-aryl units
15
provide an effective binding pocket for bis-carboxylate ions.
8
great detail. Lanthanide complexes that bind carboxylate ions
Dinicotinate ions bind efficiently to such binuclear lanthanide
are becoming established as analytical tools and in selective
12
binding pockets. In this paper, we exploit and extend such
9
sensing. In our own work, we have used the association of
processes to form a self-assembled pentanuclear d–f hybrid
in which a rhenium complex with two dinicotinic acid moi-
eties forms a “corner” that links two binuclear lanthanide
complexes together.
fac-Re(CO) (dinicotinate) Cl was prepared as shown in
3 2
Scheme 1. Reaction of dimethyl dinicotinate with rhenium
mono-carboxylates to screen photo-sensitizers for lanthanide
1
0
emission, and have seen self-association in lanthanide com-
1
1
plexes with carboxylate pendant arms. Recently, we have
studied the selective binding of bis-carboxylate ions by binuc-
lear lanthanide complexes with a meta-xylyl bridged bis DO3A
1
2
ligand. Here, we report the first example of a higher order
supramolecular structure defined by lanthanide–carboxylate
interactions, using luminescence titrations to show the
pentacarbonyl chloride yielded fac-Re(CO) (dimethyldi-
3
2
nicotinate) Cl (1, see the ESI† for synthetic details). Crystals suit-
able for X-ray diffraction were obtained by slow evaporation of
a chloroform solution, and a representation of the crystal
structure is shown in Fig. 1. From this, it is clear that the
two dinicotinic ester ligands are held at right angles to one
another. Hydrolysis of the methyl esters yielded fac-Re(CO)3-
a
University of Oxford, Chemistry Research Laboratory, Mansfield Road, Oxford,
OX1 3TA, UK. E-mail: Stephen.Faulkner@chem.ox.ac.uk, TJS@chem.ku.dk
b
(
2
dinicotinate) Cl, 2.
Department of Chemistry, University of Copenhagen, Universitetsparken 5,
DK-2100 København Ø, Denmark
Both fac-Re(CO) (dimethyldinicotinate) Cl,
1
and fac-
3
2
†
Electronic supplementary information (ESI) available: Synthetic procedures
Re(CO) (dinicotinate) Cl, 2 were found to exhibit luminescence
3
2
and details of characterization, single crystal structure determination, together
with titration data and binding models can be found online. CCDC 946857.
For ESI and crystallographic data in CIF or other electronic format see DOI:
3
(
Fig. S3†), with emissive MLCT states having luminescence
lifetimes of 2.66 ns and 2.11 ns, respectively. Key photophysical
10.1039/c3dt51705e
data for 1, 2, 3·Eu and 2 : (3·Eu ) are compiled in Table 1.
2
2 2
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Fig. 2 Total emission spectra recorded following 260 nm excitation of a solu-
tion containing 5 × 10− M 2 and increasing amounts of 3·Eu
5
in 0.1 M TRIS
2
buffer at pH 9. Highlighted spectra show the emission spectrum observed in the
absence of 3·Eu (black), and illustrate the different response of MLCT and
lanthanide luminescence at intermediate and high concentrations of 3·Eu
red and blue respectively).
2
2
(
In the light of these results, we resolved to study the
assembly of 2 with α,α′-bis(Eu·DO3A)-5-amino-m-xylene, 3·Eu
2
.
Fig. 2 shows the changes in luminescence from buffered aque-
ous solutions containing 5 × 10− M of 2 with increasing con-
5
2
centrations of 3·Eu . The rhenium-containing chromophore
proved to be an effective sensitizer of europium centred emis-
sion. The rhenium centred emission and europium centred
emission intensities increase initially, and at a given point the
rhenium centred emission intensity drops where the europium
centred emission intensity continues to rise. The rhenium-
centred emission band was initially used to monitor the
formation of the assembly (depicted in Scheme 1), as the
europium bands could not be completely separated from
the tail of the rhenium emission in steady-state spectra.
Time-gated emission spectroscopy was used to separate the
short-lived rhenium emission from the much longer-
lived emission arising from the europium species, resulting
in a signal that arises solely from lanthanide luminescence.
The assembly between 2 and 3·Eu2 was monitored by
keeping the concentration of one species constant and titrat-
ing with the other species. When the concentration of 2 was
kept constant the equilibria could be followed by measuring
either rhenium centred or europium centred emission. When
the concentration of 3·Eu2 was kept constant the emission
from the high concentration of the highly luminescent 2 had
to be removed by applying a time-gate. Therefore, only the
europium centred emission could be followed. In the following
analysis four independent titrations are used (several data
series (N) are generated in each titration): a titration of 2 with
4
Scheme 1 Synthesis and self-assembly of ReLn corner.
Fig. 1 X-ray crystal structure of rhenium complex (1) with thermal ellipsoids
shown at 50% created in Ortep (3.2). Hydrogen atoms have been omitted for
clarity.
Table 1 Key photophysical data for the complexes in buffer
a
em
a
Species
M
λ
max
λ
em
τ
<τ>amp
1
2
Re
Re
267 nm
269 nm
412 nm
420 nm
2.66 ns
0.73 ns (89%)
—
3
·Eu monitoring the europium bands (1–7) following 260 nm
2
2.11 ns
3.95 ns (11%)
excitation using time-gating, a titration of 2 with 3·Eu moni-
2
3
2
2
·Eu
2
Eu
Re
Eu
292 nm
290 nm
—
580, 589, 615,
0.74 ms
—
toring the total emission (8–15, europium and rhenium) fol-
lowing 260 nm excitation, and two independent titrations of
3·Eu2 with 2 monitoring the europium bands (16–22 and
650, 700 nm
: 3·Eu
: 3·Eu
2
2
421 nm
0.67 ns (66%)
1.70 ns
—
3.74 ns (34%)
580, 589, 615,
0.72 ms
2
2
3–29) in the time-gated luminescence spectra following
60 nm excitation. The intensities of each europium band as
650, 700 nm
a
Relative error on lifetimes ±10%.
well as the intensity ratios of the hypersensitive band (ΔJ = 2)
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Table 2 Association constants obtained by fitting data from titration of 2 with
3·Eu
2 2
and 3·Eu with 2, in 0.1 M TRIS buffer at pH 9. For the overall association
constant (Ktot) the 90% confidence intervals are given
Compound
2
3·Eu
2
5 × 10−
5
M
2 × 10−5
M
Conc.
Titrant
3·Eu
9000
2500
2
2
—
—
5.8 × 10
−1
−1
−2
a
a
K
K
K
1
M
M
M
2
6
6
tot
5.2 × 10
6
6
6
6
[
5.0 × 10 –5.4 × 10 ]
[5.5 × 10 –6.0 × 10 ]
Log(Ktot
)
6.72
6.76
a
1 2
An independent fit of K and K was not possible, only open ended
1 2
confidence intervals were calculated and the value of K and K could
be interchanged and a similar fit obtained.
1 2
constant, respectively. The two association constants (K and K )
are coupled in either of the global multi-parameter fits and
only a reliable value for Ktot could be determined. One of the
1 2
global fits yielded approximate values for K and K . The
affinity constants are compiled, together with related data, in
Table 2. The observed bimolecular binding constants imply
2 2
that the second association of 3·Eu to 2 : 3·Eu is slightly less
2
Fig. 3 Datasets, and fits of these, from titrations of solutions of 3·Eu with 2
favourable than the initial binding of 3·Eu2 to 2. This is an
expected consequence of increased steric crowding as the
assembly increases in size. The equilibrium constant describ-
5
7
(top) and 2 with 3·Eu
2 0 J
(bottom); all in TRIS at pH 9, and showing the D – F
transitions and the key peak ratios.
ing the complete formation of the pentanuclear ReLn assem-
4
−2
6
6
bly was found to lie between 5.0 × 10 and 6.0 × 10 M in the
global fits (identical values was found when each titration was
treated individually).
against the other strong bands in the spectra were treated as
separate datasets.
To confirm that sensitization arose from intramolecular
processes in a supramolecular assembly rather than from
intermolecular energy transfer, the 29 datasets from the titra-
1
6
Conclusions
tions were fitted using Dynafit (see ESI† for details). The
Dynafit program was used to determine the best possible
model for the observed binding, essentially excluding other
These results demonstrate the formation of a pentanuclear
complex by self-assembly in water, mediated by the ortho-
gonally oriented dinicotinate domains. In particular, the
strength of binding observed suggests that such building
blocks will provide the means to access more complicated
architectures through self-assembly. We are currently investi-
gating this further.
The authors thank the University of Oxford for support, the
Danish Council for Independent Research, Technology and
Production Sciences (grant 10-093546) for financial support
for TJS, EPSRC (funding for MT) and the European Research
Council ((FP7/2007-2013)/ERC-Grant Agreement Number
267426) for a fellowship for OAB.
17
possible binding stoichiometries (Fig. 3). The model accounts
for the possibility of two binding events, and treats the two
dinicotinate binding sites independently, as described by the
following series of equilibria:
2
þ 3·Eu2 ¼ 2 : 3·Eu2; K1 ¼ ½2 : 3·Eu2ꢀ=½2ꢀ½3·Eu2ꢀ
2
: 3·Eu2þ 3·Eu2 ¼ 2 : ð3·Eu2Þ ;
2
K2 ¼ ½2 : ð3·Eu2Þ ꢀ=½2 : 3·Eu2ꢀ½3·Eu2ꢀ
2
2
þ 2 3·Eu2 ¼ 2 : ð3·Eu2Þ2;
2
Ktot ¼ K1 K2 ¼ ½2 : ð3·Eu2Þ ꢀ=½2ꢀ½3·Eu2ꢀ
2
It was found that it was only possible to determine the
overall binding constant Ktot for titration of 2 with 3·Eu2.
Notes and references
When 3·Eu
2
was titrated with 2, model discrimination indi-
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2
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