Cooperative halide, perrhenate anion-sodium cation binding and pertechnetate extraction and transport by a novel tripodal tris(amido benzo-15-crown-5) ligand

Article abstract of DOI:10.1039/a903440d

A new tripodal tris(amido benzo-15-crown-5) ligand L¹ cooperatively binds chloride, iodide and perrhenate anions via co-bound crown ether complexed sodium cations and efficiently extracts and transports the pertechnetate anion from simulated aq

Full text of DOI:10.1039/a903440d

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Cooperative halide, perrhenate anion–sodium cation binding and pertechnetate
extraction and transport by a novel tripodal tris(amido benzo-15-crown-5)
ligand
Paul D. Beer,*a Peter K. Hopkinsa and James D. McKinneyb
a Department of Chemistry, Inorganic Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, UK
OX1 3QR
b BNFL, Springfields, Preston, Lancashire, UK PR4 0XJ
Received (in Cambridge, UK) 29th April 1999, Accepted 25th May 1999
A new tripodal tris(amido benzo-15-crown-5) ligand L1
cooperatively binds chloride, iodide and perrhenate anions
via co-bound crown ether complexed sodium cations and
efficiently extracts and transports the pertechnetate anion
from simulated aqueous nuclear waste solutions via cooper-
ative ion-pair binding effects.
Ion pair recognition, the simultaneous complexation of cationic
and anionic guest species by multisite receptors, is a new,
emerging and topical field of coordination chemistry.1–6 These
heteroditopic ligands can be designed to exhibit novel cooper-
ative and allosteric behaviour whereby the binding of one
charged guest can influence, through electrostatic and con-
The anion binding properties of L1–L3 were initially
1
investigated by H NMR titration studies with Cl2, I2 and
formational effects, the subsequent coordination of the pairing
ReO42 in CDCl3 solution. The latter two anions were chosen as
ion. Such systems have potential as new selective extraction and
I2 has approximately the same size and charge density as
transportation reagents for ion pair species of environmental
TcO42, and ReO42 is an isostructural analogue. The addition of
importance. As a consequence of discharges from nuclear fuel
all three anions (as their Bu4N+ salts) to CDCl3 solutions of L1–
2
reprocessing plants the radioactive pertechnetate anion TcO4
L3 caused significant downfield perturbations of the respective
is alleged to have a detrimental effect on the environment.7
ligand’s amide protons by up to Dd = 1.41 ppm, indicating
Approaches to the removal of pertechnetate anion from waste
anion binding is taking place in the tripodal amide vicinity of the
include liquid–liquid coextraction with metal complexed crown
ligand. Analysis of the resulting titration curves with the
ethers,8 lipophilic arsonium9 and ferrocenium10 salts and p-
computer program EQNMR13 suggested 1+1 complex stoichio-
transition metal–cyclotriveratrylene derivatives.11 In order to
metry in all cases and the determined stability constant values
demonstrate, to the best of our knowledge for the first time, the
are presented in Table 1. The magnitudes of the stability
practical usage of ion-pair cooperativity for extraction of such a
constants shown in Table 1 are relatively modest, reflecting the
toxic anionic guest species, we report here the synthesis of a
neutrality of the ligands; all three ligands exhibit the largest
new tripodal tris(amido benzo-15-crown-5) ligand L1 that
stability constant with chloride. Because nuclear waste dis-
charges typically contain high concentrations of sodium cations
it was of interest to investigate the mode of binding of this
cooperatively binds halide and perrhenate anions via crown
ether complexation of sodium cations, and also efficiently
extracts and transports the pertechnetate anion from simulated
particular cationic guest to L1. Unfortunately solubility prob-
aqueous nuclear waste via cooperative ion-pair binding effects.
lems prevented use of 13C NMR titration techniques.† By
The target ligand L1 containing a tripodal tetrahedral amide
monitoring the picrate chromophore, UV-visible spectroscopic
hydrogen bond donor anion recognition site in combination
titration experiments with sodium picrate and L1 in 1+1 THF–
CH2Cl2 indicated that at high concentrations of sodium cation
with crown ether cation binding moieties was synthesised in
55% yield by condensation of tris(2-aminoethyl)amine (tren) 1
(i.e. ! 3 equiv.) unsurprisingly, one sodium cation is bound in
with 3 equiv. of 4-chlorocarbonylbenzo-15-crown-512 2 in the
each benzo-15-crown-5 moiety. However at lower concentra-
tions, the observation of a well documented bathochromic shift
presence of Et3N in CH2Cl2 (Scheme 1). Analogous synthetic
procedures using appropriate acid chlorides were used to
of the picrate chromophore to 377 nm14 indicated the additional
prepare the tripodal model ligands L2 and L3 in 30 and 29%
presence of a 1+1 charge separated sandwich complex.
yield respectively.
Interestingly, alkali metal picrate extraction experiments with
L1 (Table 2) revealed an extraction preference for Na+ > K+ >
Cs+. In order to elucidate cooperative ion-pair binding effects
Table 1 Anion stability constant data
Ka/M21
Cl2
I2
ReO4
2
Ligand
L1
L2
L3
60
75
40
30
40
20
40
40
30
a Determined in CDCl3 at 298 K, errors estimated to be @10%. [L] = 1 3
1022 M.
Scheme 1
Chem. Commun., 1999, 1253–1254
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Table 2 Extraction of metal picrates by L1
Picrate
Extractiona (%)
Na
K
Cs
80
40
15
a Aqueous phase: 1 3 1024 M metal picrate; CDCl3 phase: 1 3 1023 M L1
at 298 K.
1H NMR anion titration experiments with L1 were repeated in
the presence of 1 equiv. of sodium picrate and EQNMR
determined stability constant values are present in Table 3.
Clearly there is a significant increase in the strength of binding
for all anions in the presence of 1 equiv. of sodium cation; in the
Fig. 1 Distribution coefficients for pertechnetate extraction by L1–L3 and
benzo-15-crown-5. Organic phase: CH2Cl2, concentration of L1–L3 = 1.5
3 1022 M, benzo-15-crown-5 = 4.5 3 1022 M; aqueous phase: 100 ppm
NH4TcO4, 2.35 M NaNO3, pH 11, T = 298 K.
2
case of ReO4 the increase is greater than twenty-fold. This
positive cooperativity may be attributed to the increased
electrostatic attraction between the positively charged L1·Na+
complex and the guest anion. The complexed metal cation may
also affect the spatial arrangement of the ligand and enhance the
relative acidity of the ligand’s amide protons, leading to
stronger hydrogen bonding of the anionic guest. It is noteworthy
that the anion selectivity trend displayed by L1 is altered in the
presence of sodium. With no metal cation present, L1 binds Cl2
in preference to ReO42, however on addition of sodium cation,
noted in the NMR titration experiments. Preliminary U-tube
membrane transport investigations using the same aqueous
conditions as described previously for the extraction experi-
ments as the source of the aqueous phase revealed a six-fold
increase in the rate of transport of TcO42 by L1 (flux = 6.3 3
1028 mol h21) over transport mediated by benzo-15-crown-5
(flux = 1.0 3 1028 mol h21).
In summary, the new tripodal heteroditopic ligand L1, by
virtue of having both anion and cation recognition sites, has
been demonstrated to cooperatively bind chloride, iodide and
perrhentate anions via co-bound crown ether complexed sodium
cations, and is a more efficient extraction and carrier transport-
ing reagent for the pertechnetate anion when compared to
monotopic ligands L2, L3 and benzo-15-crown-5.
2
ReO4 is more strongly bound (Table 3), which suggests the
binding of the metal cation preorganises L1 for tetrahedral
anionic guest recognition.
Table 3 Stability constants for L1 anion binding in the presence and absence
of sodium picrate in CDCl3
Anion
K/M21
We thank BNFL for a studentship (P. K. H.) and the EPSRC
for use of the mass spectrometry service at University College,
Swansea.
Cl2
60a
520c
30a
Cl2 (+Na+)b
I2
I2 (+Na+)b
ReO4
ReO42 (+Na+)b
390c
40a
Notes and references
2
†
1H NMR titration studies with L1 and Na+ unfortunately proved too
840c
complicated to monitor, to enable binding stoichiometries to be deter-
mined.
a
b
At 298 K, errors estimated to be @10%. Titration carried out in the
c
‡ In analogous extraction experiments at neutral pH, L1 extracted 65%
presence of 1 equiv. of sodium picrate. At 298 K, errors estimated to be
@15%.
2
TcO4
.
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Encouraged by these cooperative ion pair binding results,
pertechnetate extraction experiments were carried out using
conditions that simulated nuclear waste streams. The aqueous
phase contained ammonium pertechnetate (100 ppm) and
sodium nitrate (2.35 M) and the pH was adjusted to basic
conditions (pH = 11) using NaOH. The organic phase consisted
of CH2Cl2 solutions of L1–L3 and benzo-15-crown-5 at
concentrations of 1.5 3 1022 M for the tripodal ligands and 4.5
3 1022 M for the crown ether. Equal volumes (2 ml) of each
solution were mixed, and rapidly shaken. Inductively coupled
plasma mass spectrometry (ICP-MS) and 99Tc NMR were used
to determine the concentrations of pertechnetate in the re-
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2
spective phases. L1 extracted ca. 70% TcO4 whereas benzo-
15-crown-5 achieved ca. 20% extraction after 5 min.‡ No
increase in percentage extraction occurred after this time.
Interestingly the tripodal ligands L2 and L3 containing no crown
moieties displayed respectively less than 10 and 0% extraction
2
of TcO4 even after 24 h. The distribution coefficients for
2
TcO4 extraction by all the ligands are shown in Fig. 1. The
distribution coefficient D for L1 of 2.3 represents a greater than
twenty-fold and ten-fold enhancement in extraction efficiency
when compared to L2 and benzo-15-crown-5, respectively. This
important result suggests that a crown ether cation binding site
covalently linked to a tripodal amide anion coordinating cavity
2
creates a new efficient extraction reagent for TcO4 which
operates via similar cooperative ion-pair binding effects as
Communication 9/03440D
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Chem. Commun., 1999, 1253–1254