Reversible anion binding in aqueous solution at a cationic heptacoordinate lanthanide centre: Selective bicarbonate sensing by time-delayed luminescence

Article abstract of DOI:10.1039/a804255a

Reversible displacement of up to two metal bound water molecules at a coordinately unsaturated, cationic lanthanide centre is signalled by increases in the luminescence intensity or lifetime of the emissive lanthanide: selective chelation of hydrogencarbo

Full text of DOI:10.1039/a804255a

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Reversible anion binding in aqueous solution at a cationic heptacoordinate
lanthanide centre: selective bicarbonate sensing by time-delayed luminescence
Rachel S. Dickins,a Thorfinnur Gunnlaugsson,a David Parker*a† and Robert D. Peacockb
a Department of Chemistry, University of Durham, South Road, Durham, UK DH1 3LE
b Department of Chemistry, University of Glasgow, Glasgow, UK G12 8QQ
Reversible displacement of up to two metal bound water
molecules at a coordinately unsaturated, cationic lanthanide
centre is signalled by increases in the luminescence intensity
or lifetime of the emissive lanthanide: selective chelation of
hydrogencarbonate is consistent with emission spectro-
scopic, ESMS and CPL measurements.
amide NH oscillators.3 In the presence of base (I = 0.14 NaCl),
the lifetime of [TbL1]3+·(OH2)2 increased by almost 50% and
the qA value fell from 1.9 to 0.4, consistent with deprotonation
and concomitant displacement of the second water molecule by
Cl2—leaving one quenching OH oscillator. The labilisation of
metal bound water molecules following deprotonation is well-
known with di- and poly-aqua metal ion species.4
Lifetimes for [TbL1]3+ in the presence of I2/Br2/Cl2 and
2
Intrigued by the prospect of using a charged luminescent
lanthanide receptor for the time-gated signalling of anion
binding (hinted at by a brief communication1), we set out to
prepare well-defined coordinately unsaturated Tb and Eu
complexes wherein stepwise displacement of quenching metal-
bound water molecules by a given anion would be signalled by
increases in the lifetime or emission intensity of the lanthanide
luminescence. Accordingly the enantiopure complexes
[EuL1]3+ and [TbL1]3+ were devised, wherein the remote chiral
centre at carbon determines the chirality of the complex in a
similar fashion to that established with the Ln complexes of L2.2
In aqueous solution, the complexes complete their coordination
number by binding two displaceable water molecules.
NO3 (pH 5.5–6.5) were virtually unchanged, whereas F2,
acetate and sulfate competed with H2O for binding and led to
the displacement of up to one of the quenching water molecules.
Most striking was the behaviour of hydrogencarbonate (pH 6.5)
and carbonate (pH 11) in which both water molecules were
displaced leading to a large change in the measured lifetime
(Table 1). This pattern of behaviour was echoed in lifetime
measurements with [EuL1]3+, suggesting formation of a che-
lated adduct with HCO32/CO322 (Scheme 1).5
Further support for the distinctive nature of the anion adduct
was provided by examination of the Eu emission spectra
wherein the hypersensitive DJ = 2 transition at 618 nm is
particularly sensitive to coordination environment.2 The in-
tensity ratio of the DJ = 2 : DJ = 1 emission bands was 4: 1
(10-fold excess of anion in D2O) for HCO32, and the other
values measured were all lower (e.g. F2, 4 : 3; H2PO42, 2 : 1). A
titration of the change in the intensity of one component of the
620 nm band as a function of added bicarbonate ([Eu·L1]3+ = 2
3 1023 m, 0–40 mm NaHCO3, 293 K, 0.1 m MES buffer,
Me
Me
Ph
O
Ph
O
HN
HN
H
H
N
Me
N
N
N
N
N
N
N
N
O
Ph
Ph
O
Ph
O
Me
N
Me
N
H
H
O
O
Table 1 Effect of added anions (10 mm complex) on the rate constants
NH
Me
NH
Me
(±10%) for depopulation of the excited states of [Eu·L1]3+ and [Tb·L1]3+
and corrected inner-sphere hydration numbers, qA
,
Ph
Ph
L1
L2
Complex/anion
kH O/ms21 kD O/ms21 Dkcorr./ms21 qAis (±0.15)
2
2
[Tb·L1]a (CF3SO3
OH2 (pH 11)
F2
)
0.84
0.59
0.61
0.87
0.84
0.74
0.66
0.57
0.55
0.54
0.53
0.39
0.45
0.36
0.39
0.42
0.42
0.37
0.37
0.41
0.45
0.46
0.39
0.08
0.19
0.42
0.36
0.26
0.23
0.14
0.08
0.03
0.01
1.91
0.39
0.93
2.06
1.76
1.27
1.13
0.69
0.39
0.15
0.05
2
Reaction of 3 equiv. of (R)-N-(2-chloroethanoyl)-2-phenyl-
ethylamine with 1-(4-methoxyphenylsulfonyl)-1,4,7,10-tetra-
azacyclododecane (Cs2CO3, DMF, 70 °C) followed by re-
ductive deprotection (Na, aq. NH3, EtOH–THF) allowed
isolation of the heptadentate, tri-N-substituted ligand L1 (mp
64 °C), following chromatography on neutral alumina (CH2Cl2
to 1% MeOH–CH2Cl2). Complexation with Eu(CF3SO3)3 or
Tb(CF3SO3)3 in anhydrous MeCN gave tripositive complexes.
I2, Br2 or Cl2
2
NO3
2
CH3CO2
22
SO4
2
H2PO4
Citrate
1
Analysis of [EuL1]3+ by H NMR spectroscopy in D2O and
2
HCO3
CO3
22c
CD3OD revealed only one set of four distinct broadened
resonances for the most shifted axial ring proton (293 K, D2O:
+27.0, +16.1, +13.5, +8.8 ppm). A low temperature EXSY
spectrum at 220 °C, clearly indicated that there were four
stereoisomeric complexes (in a ratio of ca. 3.5: 1: 0.5: 0.25)
undergoing exchange by arm rotation but not ring inversion at
this temperature.
The luminescence lifetimes for each complex were measured
in H2O and D2O, initially in the absence and presence of a
10-fold excess of a selection of halides and oxyanions (Table 1).
Such measurements allow the estimation of the number of
coordinated water molecules (qA = 2 for each in the absence of
added anions). A correction has been made in each case to
account for the quenching effect of closely diffusing OH and
[Eu·L1]d (CF3SO3
)
3.85
2.33
2.44
2.63
2.94
3.45
2.70
1.54
1.64
1.67
1.54
1.67
1.69
1.39
1.81
0.19
0.27
0.59
0.77
1.26
0.81
2.14
0.24
0.34
0.74
0.96
1.58
1.01
2
22
CO3
2b
HCO3
2
H2PO4
22
SO4
2
CH3CO2
F2
a
Corrections for Tb complexes were to allow for the effect of closely
diffusing OH oscillators (20.06 21 in Dk) and used qAis = AADkcorr. where
b
AA = 4.9. Unchanged in 0.14 m NaCl; reduces to qA = 0.25 in 50 mm
HCO32. c At pH 11. d For Eu complexes, the correction for closely diffusing
OH oscillators was 20.25 and 20.08 ms21 for each amide NH, with AA =
1.25.
Chem. Commun., 1998
1643

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H
O
2+
O
O
O
2+
H
H
H
O
O
O
–H2O
O
OH
O
+
2+
O
O
O
Scheme 1
Figs. 1 and 2), revealed an increase in the intensity of the band,
with a switching factor of six, consistent with reversible
formation of a 1 : 1 adduct. A similar titration with added
dihydrogenphosphate showed much less change, with an
enhancement factor of two in the range 0–5 mm. It is well
known that hydrogenphosphate prefers to bind in a monodentate
manner,6 while carboxylates and carbonates often chelate,
particularly to metal centres in higher coordination numbers.7
Support for the existence of such a species came from ESMS,
where in addition to the observation of an [TbL1OH]+ species at
831, peaks at 875.2 and 876.2 were apparent, together with
Fig. 3 Circularly polarised luminescence spectra for [EuL1](CF3SO3)3
(pD 6.9, 293 K, 1.0 mmol dm23 in 0.1 m MES) (upper), and in the presence
of 10 mm H2PO42 (bottom) and 40 mm HCO32 (centre). With H2PO42, the
three DJ = 1 components have alternating signs while with HCO3 the
highest energy component is absent and the signs are reversed.
m
2
437.6/438.2 ( ⁄2), whose relative abundance was in agreement
with the isotope pattern calculated for [TbL1 + HCO3]+ [calc.:
875.34 (100%) and 876.34 (51%)]. The structure of the excited
state of lanthanide complexes is probed sensitively by circularly
polarised luminescence (CPL).8 CPL spectra for [EuL1]3+ in the
0.1 m MES buffer) (Fig. 3) were measured. Distinctively
different CPL spectra were found in the presence of both
2
2
presence of CF3SO32, H2PO4 and HCO3 (at pH 6.5 in an
2
22
HCO3 and HPO4 consistent with a pronounced change in
the helicity of the ‘anion-bound’ europium excited-state
structure.
In summary, measurements of the luminescence lifetime or
intensity of emissive, coordinatively unsaturated enantiopure
lanthanide complexes in aqueous media afford a new way of
signalling the presence of selected oxyanions. Such systems
may also act as simple models for studying anion binding in
water that is relevant to the mode of action of alkaline
phosphatase and carbonic anhydrase.
We thank the EPSRC, the BBSRC, the Royal Society and the
University of Durham for support.
Notes and References
† E-mail: david.parker@durham.ac.uk
Fig. 1 Variation of luminescence intensity at 620 nm for [EuL1](CF3SO3)3
(pH 6.4 to 7.3 in 0.1 m MES buffer, 293 K, 1 mm) as a function of added
NaHCO3 in the range 1–30 mm. Emission intensities and lifetimes were
unchanged (±7%) in the presence of 140 mm NaCl and in control
experiments varying pH only (Perkin Elmer LS50B fluorimeter).
1 N. Sabbatini, M. Guardigli, J.-M. Lehn and G. Mathis, J. Alloys Compds.,
1992, 180, 363.
2 R. S. Dickins, J. A. K. Howard, C. W. Lehmann, J. M. Moloney, D.
Parker and R. D. Peacock, Angew. Chem., Int. Ed. Engl., 1997, 36, 521;
R. S. Dickins, J. A. K. Howard, J. M. Moloney, D. Parker, R. D. Peacock
and G. Siligardi, Chem. Commun., 1997, 1747.
3 D. Parker and J. A. G. Williams, J. Chem. Soc., Dalton Trans., 1996,
3613; M. P. O. Wolbers, F. C. J. M. van Veggel, J. W. Hofstraat,
F. A. J. Geurts and D. N. Reinhoudt, J. Chem. Soc., Perkin. Trans. 2,
1997, 2275.
4 D. T. Richens, The Chemistry of Aqua Ions, Wiley, Chichester, 1997.
5 In Scheme 1, chelation in principle may involve attack by bound
bicarbonate at the metal centre or nucleophilic attack by the bound OH
group at the trigonal carbonyl centre: such mechanisms may be
distinguishable by appropriately controlled isotope incorporation experi-
ments in 18OH2.
6 B. Schneider and M. Kabelac, J. Am. Chem. Soc., 1998, 120, 161.
7 C. J. Carrell, H. L. Carrell, J. Erlebacher and J. P. Glusker, J. Am. Chem.
Soc., 1988, 110, 8651.
8 J. P. Riehl and F. S. Richardson, Chem. Rev., 1986, 86, 1; E. Huskowska,
C. L. Maupin, D. Parker, J. A. G. Williams and J. P. Riehl, Enantiomer,
1997, 2, 381.
Fig. 2 Increase in the Eu emission intensity as a function of added NaHCO3
(5; 293 K, pH 6.5–7.3 in an 0.1 m MES buffer, lexc. 257 nm), NaH2PO4
(Ω) and NaHCO3 (2) in a simulated clinical background of anions
(H2PO42 0.9 mm, 2.3 mm lactate, 145 mm Cl2, 0.13 mm citrate)
Received in Cambridge, UK, 5th June 1998; 8/04255A
1644
Chem. Commun., 1998