An outer-sphere ligand for uranyl carbonate

Article abstract of DOI:10.1039/b309733a

A novel supramolecular host for the uranyl carbonate complex has been designed and synthesized. The modified cyclodextrin host binds uranyl carbonate in water with a stability of 253 M^-1.

Full text of DOI:10.1039/b309733a

An outer-sphere ligand for uranyl carbonate†
Anthony R. Prudden, Nathan R. Lien‡ and Jason R. Telford‡*
Department of Chemistry, University of Iowa, Iowa City, IA, USA 52242-1294.
E-mail: jason-telford@uiowa.edu; Fax: 01 319-335-1270; Tel: 01 319-353-1971
Received (in Columbia, MO, USA) 12th August 2003, Accepted 19th November 2003
First published as an Advance Article on the web 8th December 2003
A novel supramolecular host for the uranyl carbonate complex
has been designed and synthesized. The modified cyclodextrin
one face of the torus, thus providing a moderately preorganized
binding site for the uranyl complex.
host binds uranyl carbonate in water with a stability of 253 M21
.
The interaction of per-en-a-Cyd with the substrate, uranyl
1
carbonate, was determined by H NMR titration. These titrations
The reactivity of a metal ion is controlled by inner- and outer-
sphere influences. In metalloproteins, Nature provides inner-sphere
coordination by the amino acids and cofactors capable of binding a
metal, and outer-sphere influences are a result of (usually) protein
secondary and tertiary structure. In all instances characterized to
date, proteins recognize specific small inorganic substrates, such as
phosphate or iron complexes, by providing a binding site with a
spatial arrangement of weak interactions, hydrogen bonds or
hydrophobic patches complementary to their substrate.1 These
weak interactions contribute to an overall stability constant large
enough to remove the substrate from solution. In many cases, there
is no exchange of ligands (replacement of a water by a protein-
based ligand, for example), but rather the inorganic complex is
recognized in toto.
The same features that Nature uses to mediate recognition can
also be incorporated into smaller host architectures.2 Defined
synthetic molecular recognition agents have potential non-bio-
logical applications in remediation technology, biological or
environmental monitoring, and the development of novel catalysts
based on the operational principles of enzymes.3 Most examples of
synthetic molecular recognition target organic or oxyanion sub-
strates, since their coordination environment and molecular
interactions can easily be predicted or modelled.4 Metal complexes,
on the other hand, comprise a widely variable set of substrates since
they have much more diverse coordination preferences. The design
of a receptor for a given metal complex must include a topological
consideration of the substrate, including symmetry, hydrogen bond
donor/acceptor preferences and van der Waals interactions.5
In addition to its relevance as an environmental hazard, the
uranyl carbonate ion [UO2(CO3)3]42 is well-suited structurally as a
target guest molecule for our designs.6 The complex is charged,
with a symmetric (D3h) coordination environment and potential
hydrogen bond acceptors decorating the ion in the equatorial
plane.7
were carried out in D2O under conditions of constant pH (8.5) and
ionic strength (0.1 M Na2CO3–NaHCO3 buffer). In several trials,
aliquots of a standard solution of uranyl carbonate (0.1015 M, 0.1
M carbonate buffer) were added to a buffered 1–10 mM solution of
per-en-a-Cyd.
Fig. 2 shows a typical titration curve based on the 1H NMR shifts
of the ethylenediamine protons with increasing uranyl carbonate
concentration. While these protons are broad in the absence of
uranyl-guest, the signals sharpen and shift upfield at higher
complex concentrations. Small, though significant shifts are also
seen with the cyclodextrin C1, C4, C5, C6 and C6A protons. These
data were fit to yield a 1 : 1 binding constant of 253 (22) M21. We
see no evidence for higher stoichiometries by MS or NMR.
In an aqueous system under the conditions of these titrations, the
first coordination sphere of the uranyl ion is certainly carbonate.†
The solution equilibria with water and hydroxide are complex, and
involve polynuclear species, but modelling of the system indicates
that water/hydroxide cannot compete effectively with carbonate as
a ligand under these conditions, as the formation constant of uranyl
ion with carbonate is log b130 = 21.0.9 Nitrogen donors are even
weaker ligands for the metal ion and amine groups are unable to
compete with even water as a ligand.10 However, the amino group
is an excellent hydrogen bond donor.
The complexation induced chemical shifts (CIS) of protons
adjacent to the nitrogens (ethylenediamine backbone) and inside
the cavity of the cyclodextrin derivative are sensitive to uranyl
binding. An upfield shift is observed for protons H1 and H4, which
are oriented to the interior of the cyclodextrin cavity, and for the
ethylene protons, but no change is seen for H2 or H5, which are
oriented toward the outside of the torus. The shifts of the C4 proton,
as well as the ethylene protons support the proposed model of an
encapsulated metal complex rather than a simple electrostatic
interaction.11
It is important to note that the per-en-a-Cyd forms an inclusion
complex with the substrate in aqueous solution. This is in contrast
to most synthetic hydrogen-bond mediated host–guest complexes
which must be characterized in non-polar media, since water is an
We have designed a host for uranyl carbonate based on these
features. The architectural scaffold of the receptor is provided by a
cyclodextrin (CyD, Fig. 1). The torus is an appropriate size ( ~ 12
Å) to encapsulate the guest uranyl complex ( ~ 8 Å diameter in the
equatorial plane). The CyD was functionalized† by substituting
ethylenediamine groups for the primary hydroxyls to give per-
(6-deoxy-6-ethylenediamine)-a-cyclodextrin (per-en-a-Cyd, Fig.
1).8 The amino groups form an array of hydrogen bond donors on
† Electronic supplementary information (ESI) available: spectroscopic
characterization of per-(6-deoxy-6-ethylenediamine)-a-cyclodextrin, 2D
NMR data, fit of the H1 proton to the binding isotherm, calculated species
distribution of uranyl ion vs. pH in seawater and in the presence of per-
(6-deoxy-6-ethylenediamine)-a-cyclodextrin. See http://www.rsc.org/
suppdata/cc/b3/b309733a/
‡ A.R.P. and N.R.L. are both undergraduate researchers. J.R.T. would like
to acknowledge a U. Iowa, Carver Foundation Research Grant. N.R.L.
would like to thank the NSF for the opportunity to participate in a summer
research experience for undergraduates through the University of Iowa,
Center for Global and Regional Environmental Research, http://
www.cgrer.uiowa.edu
Fig. 1 Synthetic scheme used in constructing the molecular host.
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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

effective competitor for hydrogen bonding interactions. Natural
systems, such as proteins, may have much higher affinities for
substrates like phosphate, but these proteins are generally highly
preorganized for a substrate. In an equilibrium speciation model of
seawater (pH 8–8.3) and freshwater (pH 7–8), the interaction of
per-en-a-Cyd with uranyl carbonate is strong enough to complex
70–75% of aqueous uranyl carbonate ion based on this thermody-
namic model.† Although there are several examples of hydrogen
bonding interactions with uranyl species, to our knowledge, this is
the first example of entirely outer-sphere coordination of a uranyl
carbonate complex.12
Notes and references
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Fig. 2 (Top) 1H NMR spectra of the ethylenediamine proton region and
trend in chemical shifts during titration. (Bottom) Data (3) and fit to a
1 : 1 isotherm (—) with Kassoc = 253 (22) M21
.
C h e m . C o m m u n . , 2 0 0 4 , 1 7 2 – 1 7 3
173