Using remote substituents to control solution structure and anion binding in lanthanide complexes

Article abstract of DOI:10.1002/chem.201303183

A study of the anion-binding properties of three structurally related lanthanide complexes, which all contain chemically identical anion-binding motifs, has revealed dramatic differences in their anion affinity. These arise as a consequence of changes in the substitution pattern on the periphery of the molecule, at a substantial distance from the binding pocket. Herein, we explore these remote substituent effects and explain the observed behaviour through discussion of the way in which remote substituents can influence and control the global structure of a molecule through their demands upon conformational space. Peripheral modifications to a binuclear lanthanide motif derived from α,α′-bis(DO3 Ayl)-m-xylene are shown to result in dramatic changes to the binding constant for isophthalate. In this system, the parent compound displays considerable conformational flexibility, yet can be assumed to bind to isophthalate through a well-defined conformer. Addition of steric bulk remote from the binding site restricts conformational mobility, giving rise to an increase in binding constant on entropic grounds as long as the ideal binding conformation is not excluded from the available range of conformers.

Full text of DOI:10.1002/chem.201303183

DOI: 10.1002/chem.201303183
Using Remote Substituents to Control Solution Structure and Anion Binding
in Lanthanide Complexes
Manuel Tropiano,[a] Octavia A. Blackburn,[a] James A. Tilney,[a] Leila R. Hill,[a]
Matteo P. Placidi,[b] Rebecca J. Aarons,[b] Daniel Sykes,[a] Michael W. Jones,[a]
Alan M. Kenwright,[c] John S. Snaith,[d] Thomas Just Sørensen,*[a, e] and
Stephen Faulkner*[a]
Abstract: A study of the anion-binding
properties of three structurally related
lanthanide complexes, which all con-
tain chemically identical anion-binding
motifs, has revealed dramatic differen-
ces in their anion affinity. These arise
as a consequence of changes in the sub-
stitution pattern on the periphery of
the molecule, at a substantial distance
from the binding pocket. Herein, we
explore these remote substituent ef-
fects and explain the observed behav-
iour through discussion of the way in
which remote substituents can influ-
ence and control the global structure of
for isophthalate. In this system, the
parent compound displays considerable
conformational flexibility, yet can be
assumed to bind to isophthalate
through a well-defined conformer. Ad-
dition of steric bulk remote from the
binding site restricts conformational
mobility, giving rise to an increase in
binding constant on entropic grounds
as long as the ideal binding conforma-
tion is not excluded from the available
range of conformers.
a
molecule through their demands
upon conformational space. Peripheral
modifications to a binuclear lanthanide
motif derived from a,a’-bisACTHUNTGRENNG(U DO3Ayl)-
m-xylene are shown to result in dra-
matic changes to the binding constant
Keywords: anions · conformational
space · lanthanides · luminescence ·
remote effects
Introduction
Reetz has shown that directed evolution can be mediated
through changes in protein sequence far from an active
site,[1] whereas Clayden has observed stereochemical induc-
tion transmitted over considerable distances in large organic
molecules.[2]
Here, we use simple lanthanide-containing building blocks
to prepare a series of related multinuclear lanthanide com-
plexes, all of which feature the same lanthanide-containing
isophthalate binding motif, to explore the role of remote
substituent effects in coordination chemistry. We find that
the association constant can be influenced by remote sub-
stituents to a degree in which an increase of two orders of
magnitude is observed.
Herein, we show that by controlling the available conforma-
tional space through using remote substituents, it is possible
to influence the affinity of lanthanide-containing hosts for
anionic guests. Such ideas of remote influence on behaviour
already have some currency in other fields. For instance,
[a] Dr. M. Tropiano, Dr. O. A. Blackburn, J. A. Tilney, L. R. Hill,
Dr. D. Sykes, Dr. M. W. Jones, Dr. T. J. Sørensen,
Prof. Dr. S. Faulkner
Chemical Research Laboratory, Oxford University
12 Mansfield Road, Oxford OX1 3TA (UK)
E-mail: Stephen.Faulkner@chem.ox.ac.uk
Lanthanide ions have been widely used in optical applica-
tions and magnetic-resonance imaging for a generation.[3]
Gadolinium complexes have been, and are, widely used as
contrast agents in clinical magnetic-resonance imaging, ex-
ploiting the high paramagnetism and rapid water exchange
associated with gadolinium centres. In a biological context,
the long-lived emission from f–f transitions from lanthanide
ions such as Eu3+ and Tb3+, can be exploited in lumines-
cence imaging and assay, offering very low limits of detec-
tion when combined with time-gating methods that separate
the lanthanide-centred emission from the short-lived auto-
fluorescence and scattering arising from biological sam-
ples.[4,5]
[b] Dr. M. P. Placidi, Dr. R. J. Aarons
School of Chemistry, University of Manchester
Oxford Road, Manchester M13 9PL (UK)
[c] Dr. A. M. Kenwright
Department of Chemistry, University of Durham
South Road, Durham DH1 3LE (UK)
[d] Dr. J. S. Snaith
School of Chemistry, University of Birmingham
Edgbaston, Birmingham B15 2TT (UK)
[e] Dr. T. J. Sørensen
Nano-Science Center and Department of Chemistry
University of Copenhagen
Universitetsparken 5, DK-2100 København Ø (Denmark)
Fax : (+45)35320214
E-mail: TJS@chem.ku.dk
Application of lanthanides in vivo requires the use of ki-
netically stable lanthanide complexes to avoid complications
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201303183.
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FULL PAPER
arising from the presence of free lanthanide (as lanthanide
release has been implicated in the onset of nephrogenic sys-
temic fibrosis).[6] Their large size and high ionic character
means that kinetically stable lanthanide complexes must be
derived from rigid ligands, typically azamacrocycles bearing
hard donor atoms, such as oxygen and nitrogen. In such sys-
tems, the arrangement of donor atoms around the lantha-
nide is defined in large part by the structure of the ligand.
For example, the lanthanide complexes of 1,4,7,10-tetraaza-
cyclododecane-1,4,7,10-tetraacetic acid (DOTA) and its de-
rivatives (e.g., the DOTA tetra-amides) have been widely
studied owing to the importance of [GdDOTA]À (DOT-
AREM) as a magnetic-resonance imaging (MRI) contrast
medium.[3d] These complexes exhibit exceptional kinetic ro-
bustness due to the rigidity of the cyclen backbone, to which
four donor acetate groups are appended.
a consequence of arm rotation, though the stability of
DO3A analogues can vary with the nature of the appended
donor groups.[10]
The kinetic robustness of these complexes means that
they are stable to a wide variety of organic reactions/trans-
formations; hence, they serve as versatile building blocks for
a diverse range of syntheses.[11] We and others have used de-
rivatives of DOTA and DO3A complexes with lanthanides
to access more complicated architectures, such as heterome-
tallic lanthanide complexes, d–f hybrids and multi-modal
imaging agents.[12] Such architectures can be achieved only
when complexes are kinetically stable—otherwise lantha-
nide exchange results in intractable mixtures of products
under thermodynamic control.
There are many circumstances in which thermodynamic
control can be exploited to advantage.[13] In supramolecular
self-assembly and dynamic combinatorial chemistry, reversi-
ble interactions between species have been widely exploited
in the construction of very large molecular assemblies. In
the field of lanthanide chemistry, the assembly of lanthanide
helicates is probably the classic example, and the formation
of triple helicate assemblies has been explored extensive-
ly.[14] We have taken a different approach, and shown that
kinetically robust binuclear lanthanide complexes can form
stable self-assemblies with 1,3-dicarboxylate dianions, even
in phosphate-buffered saline solutions.[15] Although helicates
dissociate and associate reversibly, leaving the potential for
kinetic trapping of released lanthanide, the use of robust
hepta-dentate binding pockets ensures that the lanthanide
remains complexed throughout an experiment. These kineti-
cally stable complexes can be considered as a molecular
entity, rather than a typical metal complex under thermody-
namic control.
DOTA and DOTTA are octa-dentate ligands and exhibit
isomerism when binding to lanthanides: Scheme 1 shows
how both square anti-prismatic (SAP) and twisted square
Results
Scheme 2 shows how the lanthanide complex 1, a,a’-bis-
AHCTUNGTREG(NNUN EuDO3A)-m-xylene (m-bis(europium 1,4,7,10-tetraazacy-
clododecane-1,4,7-triacetate-10-methylene)-benzene), can be
derivatized to prepare the related structures 2 and 3, which
are accessed by peptide and Ugi coupling methodologies, re-
spectively. The versatility of particularly the multicompo-
nent Ugi reaction gives rise to considerable potential for de-
rivatisation.[16] All three of these molecules contain the same
m-xylyl-bridged binuclear europium complex.[17] In the case
of 1, this fragment has already been shown to bind to iso-
phthalate in aqueous buffers and methanol/water solvent
mixtures.[15a,b] The approach used to achieve the synthesis of
these compounds relies on the kinetic stability of the com-
plexes 1 and 4, which has already been amply demonstrated
through their use in preparing lanthanide arrays.[11b,18] It
should also be noted that 2 and 3 contain a terbium ion,
bound in a stable domain in the lower part of the structure
as drawn. We envisaged that this different metal centre
would provide a reporter group that is not itself involved in
anion binding—because the octa-dentate binding pocket in
Scheme 1. 3D representation of the coordination sphere of lanthanides in
DOTA ligands in solution; X is the ninth, capping, ligand position. Un-
capped structures of hepta- and octa-dentate lanthanide complex of
cyclen-based ligands with carboxylate arms. The isomers of DOTA,
DOTTA, and various derivatives are illustrated.
anti-prismatic (TSAP) geometries can be adopted.[7] Both
types of complex are robust and can interconvert through
arm rotation, which is relatively facile but slow on the NMR
timescale, or ring inversion, which is slower still. For
[EuDOTA]À at 298 K, the rates for arm rotation and ring
inversion are 78 and 35 sÀ1, respectively.[8] The hepta-dentate
ligand DO3A (1,4,7,10-tetraazacyclododecane-1,4,7-triacetic
acid) displays similar behaviour,[9] forming stable complexes
in which exchange between SAP and TSAP isomers is much
more facile, with fast exchange on the NMR timescale as
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T. J. Sørensen, S. Faulkner et al.
Figure 1. a) Titration of a 3ꢁ10À6 moldmÀ3 solution of 2 with isophthalic
acid in methanol. The curves correspond to each of the well-resolved
bands with excitation at 275 nm. b) Titration of a 3ꢁ10À6 moldmÀ3 solu-
tion of 2 with isophthalate in methanol exciting at 275 nm. c) Titration of
a 5ꢁ10À5 moldmÀ3 solution of 3 with isophthalate in methanol exciting at
275 nm. The inset shows the DJ=1 and the DJ=2 peaks with intensity
normalised to the DJ=1 peak.
Scheme 2. Synthesis of complexes 2 and 3 by peptide and Ugi coupling.
which the terbium is bound leaves insufficient space for co-
ordination of an anion (further information, spectroscopic
and synthetic details can be found in the Supporting Infor-
mation).
and 3 can be found in the Supporting Information, and the
association constants (Ka) determined for these systems are
compiled in Table 1. For titrations with isophthalate, equi-
molar concentrations of isophthalic acid and lithium hydrox-
ide were dissolved in the titrant solution. The association
constant of isophthalic acid was found to be 39400mÀ1,
which is several orders of magnitude weaker than the bind-
ing of isophthalate (see Table 1).
Having prepared the complexes, we studied their iso-
phthalate binding properties in methanolic solution. A 10À5
or 10À6 m solution of the lanthanide complex in methanol
was made. This stock was used to prepare a 10À3 m solution
of isophthalic acid. In this way, the concentration of the lan-
thanide complex remains constant during the titration. The
stock solution (2 mL) was placed in a cuvette, and the lumi-
nescence spectrum was recorded. An aliquot, ranging from
10 to 100 mL of the titrant, was added, and the luminescence
spectrum was recorded anew. The titration was continued
until the binding isotherm reached a plateau. A representa-
tive binding isotherm is shown in Figure 1, arising from a ti-
tration of isophthalic acid to complex 2. The isotherms ob-
tained from titrations of isophthalate with complexes 1, 2
Table 1. Association constants (Ka) of complexes 1–3 with isophthalate
in methanol. The concentration of the complex was 10À5 m. See the Sup-
porting Information for details regarding the determination of Ka. The
asymmetric 99%-confidence intervals are given in brackets.
Complex
Ka (isophthalate) [mÀ1
]
1
2
3
1.18ꢁ105 [1.13ꢁ105–1.24ꢁ105]
2.0ꢁ106 [1.0ꢁ106–5.5ꢁ106]
1.0ꢁ107 [9.7ꢁ106–1.1ꢁ107]
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Solution Structure and Binding in Lanthanide Complexes
FULL PAPER
Figure 1 shows the changes in the emission spectra of
complexes 2 and 3 upon addition of isophthalate. In these
complexes, emission was observed from both the terbium
centre (5D4–7Fn) and the europium centres (5D0–7Fn). Both
complexes clearly respond to isophthalate, but the observed
properties of the two complexes are very different. In com-
pound 2, there are dramatic changes to the intensities of
both terbium and europium emission bands. However, in 3,
the changes observed to the terbium centred emission are
very small, whereas the europium emission changed dramat-
ically with increasing isophthalate concentration.
The binding constants were obtained by fitting the ob-
served luminescence data by using Dynafit.[19] The associa-
tion constants for binding of isophthalate by each of the sys-
tems 1–3 are shown in Table 1. Further discussion of the fit-
ting methodology and the Dynafit software used can be
found in the Supporting Information and in Reference [19].
From the values shown in Table 1, it is clear that the affinity
for isophthalate in methanol changes by two orders of mag-
nitude between 1 and 3.
in 1 that arise as a consequence of possible rotation about
À
À
the C C and C N single bonds that link the two macrocy-
cles to the aryl ring. In fact, many more possible conformers
exist: a multitude will arise as a consequence of the rotation
just mentioned, whereas it should also be noted that both
square anti-prismatic and twisted square anti-prismatic geo-
metries are to be expected for each of the europium com-
plexes.[7]
However, from all these possible conformers, only a few
will bind effectively to isophthalate. In Scheme 3, sample
conformers I–IV cannot bind isophthalate, whereas confor-
mer V is pre-organised for binding. Thus, the interaction be-
tween host complex and anionic guest will result in a pertur-
bation to the multitude of equilibria that govern the struc-
ture in solution, and will result in a consequent loss of entro-
py.
Although the 16-membered ring formed by the binding of
isophthalate to 1 seems improbable when considered from
the standpoint of traditional supramolecular chemistry
(Scheme 3), we have amply demonstrated that the binding
does occur, and that it involves both europium centres and
both carboxylic acid residues.[15] We feel that this point can
be used to highlight the fact that f-block chemistry is, in
many ways, set apart from our general understanding of the
rest of the periodic table.
In more detail, if we consider these possible conforma-
tional equilibria as allowing each macrocycle to occupy
a multitude of positions, it is possible to define a volume of
space that each might occupy close to the aryl ring. This is
represented in 2D cartoon form in Scheme 4A for 1. It can
be seen from the deeply shaded red area that there is
a space that both europium domains might occupy. Because
this is clearly impossible, it follows that the position of one
metal centre will influence that of the other.
Addition of remote substituents will further restrict the
available conformational space. Thus, in the peptide-coupled
system 2, the terbium domain also impinges upon the con-
formational freedom of the europium domains (and thus
Discussion
We successfully synthesised two lanthanide-containing archi-
tectures by coupling of DO3A mono-amide building blocks
with 1, demonstrating the versatility and stability of kineti-
cally stable lanthanide complexes. The lanthanide complexes
1–3 contain an identical binuclear binding pocket, which can
bind isophthalate strongly. We found association constants
ranging over two orders of magnitude from 1ꢁ105 mÀ1 for 1,
through 2ꢁ106 mÀ1 for 2 to 1ꢁ107 mÀ1 for 3 by using a robust
luminescence approach to determine binding isotherms.
It is necessary to rationalise the observed trends in bind-
ing for the three complexes studied. To do so, we must
begin by considering the nature of the isophthalate binding
domain, which is identical for complexes 1–3. Scheme 3 il-
lustrates a simplified view of the conformational possibilities
Scheme 3. Illustrating the wide variety of conformations available to m-xylyl bridged bis-DO3A compounds, and showing that only a small range of con-
formers can bind effectively to an isophthalate guest.
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T. J. Sørensen, S. Faulkner et al.
tropy, when an increase in bind-
ing of two orders of magnitude
can be readily gained by reduc-
ing the entropy loss by remote
substituents. We are currently
working towards other systems,
in which the loss of entropy is
reduced by other means.
Conclusion
These results clearly show the
importance of considering the
whole structure of the molecule
when designing
a
binding
cavity. By doing so, it is possible
to both enhance and inhibit the
binding of a guest. Our results
should inform the design of
hosts for
a wide variety of
anions, and we are currently
working on the use of alterna-
tive binuclear domains that will
permit us to extend our ap-
proach to ions of biological in-
terest. In this we have been
aided by both the conforma-
tional and rotational properties
of organic ligand frameworks,
and by the absence of direction-
ality in the bonds between lan-
thanide and associated donor
atoms. Both these features are
essential in controlling the
Scheme 4. Simplified 2D representation, showing how the steric demand from remote substituents can restrict
the available conformational space in a) complex 1, b) complex 2, c) complex 3 and d) complex 5. Areas of
heavy shading show where two groups will compete to occupy the same space, restricting conformational mo-
bility and coupling their motion.
dictates the structure of the binding pocket to some degree).
It is clear that this restriction increases the affinity of the
complex for isophthalate. As such, we may infer that the
conformation that binds effectively to anions is unaffected
by this bulky group, and that the increased (pre-) organisa-
tion of the structure results in a reduced loss of entropy
upon binding.
Furthermore, in complex 3, the tertiary amide is now sub-
stituted with an additional bulky fragment, which will re-
strict rotation further, giving rise to the largest observed
binding constant (while still leaving the preferred binding
conformation unaffected).
It is perhaps worth noting that such structural variations
can have unpredictable consequences. The tetranuclear tri-
metallic complex 5, which we reported recently, is closely re-
lated to the other structures described here.[18] However, it
has no detectable affinity for isophthalate. From this, we
may deduce that it is possible to go too far, and that restric-
tion of conformational freedom can actually exclude the
preferred binding conformation. Furthermore, we must note
that the strength of the binding of isophthalate to the euro-
pium-binding pocket must be large to overcome the lost en-
properties of the host species. In particular, the lanthanide-
binding domains of an organic ligand must bind irreversibly
to a lanthanide while still retaining flexibility within the
ligand structure if this approach is to pay dividends.
Herein, we have shown that a delicate balance exists be-
tween excluding unwanted degrees of conformational free-
dom while not excluding the desired binding conformer.
Such pre-organisation can be achieved by linking building
blocks together, and potentially allows a small variety of
lanthanide-containing motifs for anion recognition to be op-
timised for a wide range of guests.
These results have shown what can be achieved by tuning
the binding for one guest. However, it is clear that a binding
site that contains a flexible binding motif is likely to bind to
different guests by favouring different conformers. It should
therefore be possible to tune the selectivity of a single flexi-
ble host to a range of guests. Indeed, this is already ob-
served in biology and in the directed evolution of enzyme
hosts. It remains to investigate whether it is possible to do
so with simpler coordination domains.
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Solution Structure and Binding in Lanthanide Complexes
FULL PAPER
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Received: August 12, 2013
Published online: November 7, 2013
Chem. Eur. J. 2013, 19, 16566 – 16571
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