Crown ether lanthanide complexes as building blocks for luminescent ternary complexes

Article abstract of DOI:10.1016/S0277-5387(02)01405-5

Lanthanide complexes of the macrocyclic ligands 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N'-diacetic acid (H₂(dacda)) and 1,4,7,10-tetraoxa-13-azacyclopentadecane-13-acetic acid (H(macma)) have been isolated and characterised. The photoph

Full text of DOI:10.1016/S0277-5387(02)01405-5

Polyhedron 22 (2003) 745ꢁ754
/
www.elsevier.com/locate/poly
Crown ether lanthanide complexes as building blocks for luminescent
ternary complexes
Steven W. Magennis a, J. Craig b, A. Gardner b, Flavia Fucassi c, Peter J. Cragg c,
Neil Robertson a, Simon Parsons a, Zoe Pikramenou b,*
a Department of Chemistry, The University of Edinburgh, King’s Buildings, West Mains Road, Edinburgh, EH9 3JJ, UK
b School of Chemical Sciences, The University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
c School of Pharmacy and Biomolecular Sciences, University of Brighton, Brighton, BN2 4GJ, UK
Received 2 September 2002; accepted 19 November 2002
Abstract
Lanthanide complexes of the macrocyclic ligands 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-N,N?-diacetic acid (H2(dacda))
and 1,4,7,10-tetraoxa-13-azacyclopentadecane-13-acetic acid (H(macma)) have been isolated and characterised. The photophysical
properties of the red and green emissive, Eu(III) and Tb(III), respectively, complexes have been elucidated. Upon addition of mono-
or bi-dentate aromatic carboxylates or dibenzoylmethide in solutions of the Eu(III) macrocycles triggering of the lanthanide
emission is observed. The formation of the ternary complex between the lanthanide macrocycle and the sensitiser is controlled by the
available coordinaton sites of the lanthanide macrocycle prior to the binding event.
# 2003 Elsevier Science Ltd. All rights reserved.
Keywords: Luminescence; Lanthanide; Macrocycles; Ternary complexes
1. Introduction
errors and tedious procedures to avoid contamination
[14]. An approach employed for direct signal response is
based on fluorescence resonance energy transfer me-
chanism (FRET) [15] where a europium cryptate plays
the role of both the light harvester and the energy donor
to an energy acceptor such as allophycocyanin [16]. We
are interested in triggering fluorescence based on
intramolecular energy transfer mechanism in order to
avoid the limited choice of applications imposed by
donor/acceptor concentration or energy levels difference
in FRET. Our design is based on using two building
blocks for lanthanide coordination, one based on a
ligand that is non-absorbing (NAL), and the other based
on a LHC (Scheme 1) [17]. Addition of the LHC to a
complex of NAL with the lanthanide will then sensitise
lanthanide luminescence by filling in the coordination
sphere of the lanthanide centre. Triggering of the
lanthanide emission by a sensitiser external to the
lanthanide complex has been successfully used for the
development of chemosensors [18] using non-covalent
interactions to bring the participating units in one
supramolecular structure; although lanthanide sensitisa-
tion has been reported in ternary lanthanide complexes
Lanthanide ions, particularly Eu3ꢀ and Tb3ꢀ, are
popular luminescent probes for the development of
fluoroimmunoassays [1ꢁ6]. Their photophysical proper-
/
ties, narrow line luminescence with long lifetimes, are
attractive in comparison with the broad luminescence of
the organic fluorescent dyes. The disadvantage of low
absorption coefficients of the lanthanides can be over-
come by the introduction of ligands that act as light-
harvesting centres (LHCs) to sensitise the lanthanide
emission [7ꢁ13]. In the homogeneous immunoassay area
/
the design of labels for direct signal response upon
interaction of an antigen/antibody remains a challenge.
Most of the methods employed so far are based on
heterogeneous fluoroimmunoassays involving indirect
detection of the lanthanide label content, by dissociation
of the lanthanide label, a method which gives increased
* Corresponding author. Tel.: ꢀ
414-4446/4403.
E-mail address: z.pikramenou@bham.ac.uk (Z. Pikramenou).
/
44-121-414-2290; fax: ꢀ44-121-
/
0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved.
doi:10.1016/S0277-5387(02)01405-5

746
S.W. Magennis et al. / Polyhedron 22 (2003) 745ꢁ754
/
a Kratos MS-50 mass spectrometer. Elemental analyses
were performed on a PerkinꢁElmer 2400 CHN elemen-
tal analyser. UVꢁVis absorption spectra were recorded
on a PerkinꢁElmer Lambda 9 spectrometer. All mea-
surements were made at room temperature (:20 8C) in
/
/
/
/
aerated solutions. Luminescence emission and excitation
spectra were recorded on a Photon Technology Inter-
national QM-1 emission spectrometer, previously de-
scribed [32]. The luminescence studies of the lanthanide
complexes were performed in Tris [tris(hydroxymethyl)-
aminomethane] buffer, 0.05 mol dmꢃ3, at pH 7.5
(25 8C), unless otherwise stated.
Scheme 1.
2.1. [Eu(dacda)]Cl
Scheme 2.
H2(dacda) [33] (0.097 g, 0.26 mmol) and NaOMe
(0.028 g, 0.53 mmol) were dissolved in methanol (3 cm3)
and EuCl3 (0.110 g, 0.30 mmol) in H2O (2 cm3) was
added dropwise to this solution. After stirring for 30
min, the solution was filtered and acetone (50 cm3)
added. The white solid that crystallised overnight was
filtered, washed with a 5% H2O/acetone mixture (1 cm3)
and dried under vacuum (0.11 g, 73%). Single crystals of
[Eu(dacda)]Cl suitable for an X-ray diffraction analysis
were grown by slow evaporation from an acetonitrile
[19,20], there is little known of their controlled forma-
tion in solutions [21ꢁ24] and in most cases is based on
/
either a synergistic coordination of the different ligands
or addition of excess of one of the ligands [25,26].
We have chosen to use macrocycles with ionisable
pendant donor groups as NALs. The macrocyclic
ligands 1,10-diaza-4,7,13,16-tetraoxacyclooctadecane-
N,N?-diacetic acid (H2(dacda)) and 1,4,7,10-tetraoxa-
13-azacyclopentadecane-13-acetic acid (H(macma))
were chosen for the role of the NAL (Scheme 2). The
pendant ionic groups greatly increase the thermody-
namic and kinetic stability of their complexes compared
with the non-functionalised analogues. This results from
the greater preorganisation of the ligand for metal
binding (i.e. it is held in a conformation close to that
required for complex formation) and the electrostatic
solution. FAB MS m/z 527 [Mꢃ
H, 5.33; N, 4.73%. C16H28ClEuN2O8×
32.04; H, 5.38; N, 4.67%).
/
Cl]ꢀ. (Found: C, 32.38;
2H2O requires C,
/
2.1.1. Crystallographic data for {[Eudacda(H2O)]Cl}n ×
/
6nH2O
C16H42ClEuN2O15, Mꢂ
group P21/n, aꢂ
/
689.93, monoclinic, space
/
13.207(4), bꢂ
908, bꢂ96.28(3)8, Vꢂ
1.728 g cmꢃ3, F(000)ꢂ
1408, Mo Ka radiation
220 K, mꢂ
2.536 mmꢃ1. The
structure was solved by Patterson methods (DIRDIF
[34] and refined by full-matrix least squares against F2
SHELXL); [35] H-atoms of the water molecules were
located in a Fourier map and refined subject to
restraints applied to OH and HH distances. R1ꢂ
0.0317 [umax 258, 3889 data F ꢀ4s (F)], wRꢂ0.0701
for 4602 independent reflections, GOFꢂ1.060.
/
12.110(4), cꢂ
/
16.681(5)
metalꢁ
pendant arms [27]. There have been a number of studies
involving Ln3ꢀ ions and dacda [28ꢁ
30]. These have
/
ligand interactions introduced by the ionisable
3
˚
˚
A, aꢂ
rcalcd
(lꢂ0.71073 A), Tꢂ
/
gꢂ
/
/
/
2651.8(13) A , Zꢂ4,
/
ꢂ
/
/
/
˚
/
/
/
been concerned with the solution formation of the
complexes and measurement of the stability constants
)
with Ln3ꢀ ions (log Kꢂ
/
10.8ꢁ
macma has only one pendant arm, its lanthanide
complexes will possess a 2ꢀ charge. It also provides
/
12.2) [31]. As the ligand
(
/
/
two fewer donor atoms than dacda; four oxygen atoms
and one nitrogen atom in the ring, and the carboxylate
group.
ꢂ
/
/
/
/
We report the isolation and characterisation of
lanthanide complexes of dacda and macma, their
luminescent properties and the triggering of lumines-
cence by the formation of ternary complexes with
LHCs.
2.2. [La(dacda)]Cl
A mixture of LaCl3×
/
7H2O (0.20 g, 0.53 mmol),
H2(dacda) (0.21 g, 0.53 mmol) and NaHCO3 (0.10 g,
1.2 mmol) was dissolved in H2O (100 cm3). This, initially
effervescent, solution was stirred for 1 h, and the initial
white precipitate that formed was filtered off and
discarded. The volume was reduced to 10 cm3 and
acetone (120 cm3) was added, resulting in the precipita-
tion of a white solid, which was isolated and identified
as the desired product (0.17 g, 56%). dH (360 MHz,
2. Experimental
1H, 13C{1H} and 31P{1H} NMR spectra were re-
corded on Bruker AC-200, 250 and 360 MHz spectro-
meters. Positive ion FAB mass spectra were recorded on
D2O) 4.11ꢁ
/
4.18 (4H, m, OCH2CH2N), 3.94ꢁ4.03 (8H,
/

S.W. Magennis et al. / Polyhedron 22 (2003) 745ꢁ
/754
747
m, OCH2CH2O), 3.72ꢁ
(4H, s, NCH2COO), 3.19ꢁ
2.79ꢁ
2.83 (4H, m, OCH2CH2N); dC{1H} (91 MHz,
/
3.77 (4H, m, OCH2CH2N), 3.49
that turned to a glass on standing (quantitative yield
1
based on H NMR spectrum). dH (250 MHz, CDCl3)
/3.27 (4H, m, OCH2CH2N),
/
8.18 (1H, br s, CO2H), 4.47 (2H, s, NCH2COOH), 4.17ꢁ
/
CDCl3) 180.6 (CO), 69.9 (OCH2CH2O), 69.1
(OCH2CH2N), 59.9 (NCH2COO), 59.1 (OCH2CH2N);
3.65 (20H, m, CH2); dC{1H} (62.9 MHz, CDCl3) 167.1
(CO2H), 70.3 (OCH2CH2O), 69.8 (OCH2CH2O), 69.6
(OCH2CH2O), 65.0 (OCH2CH2N), 55.1 (OCH2CH2N),
54.6 (NCH2CO2H).
FAB MS m/z 515 [Mꢃ
Cl]ꢀ. (Found: C, 33.69; H, 5.96;
/
N, 4.23%. C16H28ClLaN2O8×
/
2H2O×
/1/2CH3COCH3 re-
quires C, 34.13; H, 5.73; N, 4.55%).
2.5. Preparation of [Eu(macma)](NO3)2
2.3. [Tb(dacda)]Cl
Eu(NO3)3×
/
5H2O (0.219 g, 0.511 mmol) was dissolved
in dry acetonitrile (10 cm3) and heated under reflux with
triethylorthoformate (1 cm3) for 4 h, followed by the
H2(dacda) (0.077 g, 0.20 mmol) and KOH (0.024 g,
0.43 mmol) were dissolved in H2O (10 cm3) and TbCl3
(0.075 g, 0.20 mmol) in H2O (2 cm3) was added dropwise
to this solution. The volume was reduced to 3 cm3 under
reduced pressure and acetone (40 cm3) added. The white
solid that crystallised was filtered, washed with a 5%
H2O/acetone mixture (1 cm3) and dried under vacuum
(0.10 g, 85%). Single crystals of [Tb(dacda)]Cl suitable
for an X-ray diffraction analysis were grown by slow
evaporation from an acetonitrile solution. FAB MS m/z
dropwise addition of a solution of H(macma)×
/
HCl
(0.157 g, 0.494 mmol) in dry acetonitrile (4 cm3). The
solution was heated under reflux for 24 h. Diethyl ether
(20 cm3) was added and the resultant solid was washed
with diethyl ether (2ꢄ
(0.12 g, 20%). dH (200 MHz, CD3NO2) 3.37ꢁ
m, CH2), 2.89 (16H, br s, CH2); FAB MS m/z 489 [Mꢃ
NO3]ꢀ. (Found: C, 23.75; H, 4.21; N, 7.16%.
C12H22EuN3O12×3H2O requires C, 23.77; H, 4.65; N,
6.93%).
/
10 cm3) and dried under vacuum
3.70 (6H,
/
/
535 [Mꢃ
H, 4.91; N, 3.53%. C16H28ClTbN2O8×
quires C, 24.55; H, 4.91; N, 3.53%).
/
Cl]ꢀ(thioglycerol matrix). (Found: C, 24.55;
6H2O×KCl re-
/
/
/
2.6. Preparation of [Y(macma)](NO3)2
2.3.1. Crystallographic data for [Tbdacda]n ×
4nH2O
*
/
Cln ×
/
Y(NO3)3×
/
6H2O (0.201 g, 0.525 mmol) was dissolved
in dry acetonitrile (10 cm3) and heated under reflux with
triethylorthoformate (1 cm3) for 4 h, followed by the
C16H36ClN2O12Tb, Mꢂ
group Pna21, aꢂ12.634(3), bꢂ
908, Vꢂ2364.3(12) A , Zꢂ
1.806 g cmꢃ3, F(000)ꢂ
1296, Mo Ka radiation (lꢂ
220 K, mꢂ
3.166 mmꢃ1. The structure
was solved by Patterson methods (DIRDIF) and refined
by full-matrix least squares against F2
/
642.84, orthorhombic, space
14.828(2), cꢂ12.620(6)
4, rcalcd
/
/
/
dropwise addition of a solution of H(macma)×
/
HCl
3
˚
˚
A, aꢂ
/
bꢂ
/
gꢂ
/
/
/
ꢂ
/
(0.168 g, 0.529 mmol) in dry acetonitrile (3 cm3). The
solution was heated under reflux for 17 h. Diethyl ether
(20 cm3) was added and the resultant solid was washed
/
/
˚
0.71073 A), Tꢂ
/
/
with diethyl ether (2ꢄ
(0.191 g, 67%). dH (360 MHz, D2O) 3.68ꢁ
/
10 cm3) and dried under vacuum
3.97 (18H, m,
(
SHELXL). H-
/
atoms of the water molecules were not evident in
Fourier maps, and have not been placed. This is likely
owed to the lower crystal quality of this sample
compared to that of the Eu complex. Low crystal
quality is probably also to be the cause of the difficulties
experienced in applying an absorption correction; the
program SHELXA was used as a last resort, empirical
procedures having failed to yield acceptable refinement
CH2), 3.42 (4H, br s, CH2); FAB MS m/z 427 [Mꢃ
/
NO3]ꢀ.
3. Results and discussion
3.1. Characterisation of lanthanide dacda complexes
statistics (for example difference peaks of over 3 eAꢃ3 in
The crystal structure of [Eu(dacda)]Cl is shown in Fig.
1. Each Eu3ꢀ ion is ten-coordinate, and is bound to four
oxygen and two nitrogen atoms of the dacda ring, one
oxygen atom from each of the ligand’s carboxylate arms
and a water molecule. One of the carboxylate arms is
monodentate, whilst the other is 1,3-binucleating, link-
ing adjacent Eu3ꢀ centres to form a polymeric chain.
The bridging carboxylate arm from a neighbouring
ligand completes the coordination sphere of the metal.
The Eu3ꢀ ion is situated in the middle of the dacda ring,
the two carboxylate arms from the same ligand are on
one side of the ring, and the other carboxylate and water
molecule are on the other side of the ring. While it is
˚
the region of the Tb atom). R1ꢂ
2284 data F ꢀ4s (F)], wRꢂ0.0975 for 2976 indepen-
dent reflections, GOFꢂ1.019.
/
0.0412 [umax
ꢂ258,
/
/
/
/
2.4. H(macma)×
/
HCl×
/
nH2O (1B
/
n B2) [36]
/
Ethyl 1,4,7,10-tetraoxa-13-azacyclopentadecane-13-
acetate [37] (1.6 g, 5.2 mmol) was stirred at reflux with
concentrated hydrochloric acid (3 cm3) in H2O (300
cm3) for 67 h. The clear solution was cooled, the solvent
removed under reduced pressure and the compound
dried under vacuum, yielding a viscous, pale yellow oil

748
S.W. Magennis et al. / Polyhedron 22 (2003) 745ꢁ754
/
Fig. 1. Crystal structure of {Eudacda(H2O)Cl}n ×
/
6nH2O showing (a) the Eu3ꢀ coordination sphere with the atomic numbering scheme [selected
O(3B) 2.396(3), Eu(1)ꢀO(3A) 2.402(3), Eu(1)ꢀO(4A)#1 2.459(3), Eu(1)ꢀO(1W) 2.461(3), Eu(1)ꢀO(7) 2.528(3), Eu(1)ꢀ
O(16) 2.613(3), Eu(1)ꢀO(4) and CPK models of top and bottom view and (b) crystal packing diagram, the hydrogens are
˚
bond distances (A): Eu(1)ꢀ
O(13) 2.612(3), Eu(1)ꢀ
omitted for clarity.
/
/
/
/
/
/
/
/
difficult to directly assess the solution structure from
this polymeric crystal structure, it is clear that the ligand
is able to wrap around the metal, yet still leave
coordination sites free. This is important when con-
ported structure of Eu3ꢀ complexed with 1,10-diaza-
4,7,13,16-tetraoxacyclooctadecane, an analogue of
dacda with no carboxylate arms. In this structure,
[Eu(NO3)2(C12H26N2O4)](NO3), the Eu3ꢀ ion is also
ten-coordinate, bonding to the six donor atoms of the
macrocycle and to the four oxygen atoms of two
bidentate nitrates [40]. The average bond distances for
sidering ternary complex formation. The metalꢁligand
/
bond distances depend primarily on the donor atom.
The intracomplex carboxylate bonds have an average
˚
O distance of 2.40 A. These are of a similar length
Lnꢀ
/
Euꢀ
/
O(ether), Euꢀ
/
O(NO3) and Euꢀ
2.62 A, respectively. The addition of the carboxylate
arms appears to weaken the EuꢀN bonds, possibly due
/
N are 2.60, 2.47 and
˚
to those found in other complexes (e.g. the average Lnꢀ
/
O bond distance for the coordinated carboxylates in
[EuDOTA]ꢃ is 2.39 A) [38]. Bond distances from the
/
˚
to steric factors. The flexibility of the dacda ligand
allows it to optimise bonding interactions with the Ln3ꢀ
ion by adopting a polymeric structure. In fact, this type
of behaviour does not appear to be unusual for
complexes of lariat crown ethers. The structure of the
cadmium complex of macma is a tetrameric cluster,
˚
metal to ether oxygens (average 2.60 A) and nitrogen
˚
(average 2.71 A) are longer, as are the Lnꢀ
/
O distances
for the bridging carboxylate ligand and the water
˚
molecule (both are 2.46 A) as expected for lanthanide-
aza[15]crown-5 complexes [39]. It is interesting to
compare the structure of [Eu(dacda)]Cl with the re-
[Cd4(macma)4][CdCl4]2×
/3H2O, with bridging acetate

S.W. Magennis et al. / Polyhedron 22 (2003) 745ꢁ
/754
749
groups [36]. The structure of the cadmium complex with
the ligand Lꢂ1,4,7,10-tetraoxa-13-azacyclopentade-
be due to the five unique carbon environments. The
signal at d 181 can be assigned to the carboxylate
/
cane, a macma analogue without the pendant arm, is
monomeric [Cd(L)NO3)2]. Thus, the introduction of
pendant carboxylates to crown ethers can result in
significant changes to the solid-state structure of the
macrocyclic complexes formed. The crystal packing
diagram of [Eu(dacda)]Cl shows that the polymeric
chains are aligned parallel to the b axis and are
separated by hydrophilic channels of water molecules
and Clꢃ ions. There is an intermolecular network of
hydrogen bonds, involving coordinated and non-coor-
dinated water molecules, Clꢃ ions and ethylenic protons
that help to cement the structure. The [Tb(dacda)]Cl
complex is isostructural with that of the Eu3ꢀ one. The
carbon (carbon 5). From the 1Hꢁ13
C COSY NMR
/
spectrum, the 13C resonance at d 60 is assigned to the
acetate carbon (carbon 1). Similarly the multiplet at d
1
4.03 in the H NMR spectrum can be assigned as
3.94ꢁ
/
the eight diether-ethylenic protons (e.g. H4a/b), allowing
assignment of the carbon resonance at d 70 to carbon 4.
1
This leaves four H resonances and two carbon reso-
1
nances still to be assigned. The Hꢁ13
/
C COSY shows
that each of the remaining carbon resonances correlates
with two proton resonances. Based on chemical shift,
the carbon resonance at d 69, which correlates with the
proton resonances at d 4.11ꢁ
/
4.18 and 3.72ꢁ3.77, is
/
assigned to carbon 3 because it is adjacent to an ether
oxygen. Similarly, the carbon resonance at d 59.12,
only differences are the shortening of the metalꢁ
/
ligand
bond distances and the absence of a coordinated water
molecule. Both of these differences can be attributed to
the lanthanide contraction.
In an attempt to gain more information about their
solution structure, the (dacda) complexes were studied
1
by NMR spectroscopy. The H NMR spectra of both
which correlates with the proton resonances at d 3.19ꢁ
3.27 and 2.79ꢁ2.83, is assigned to carbon 2 because it is
adjacent to an amine nitrogen.
/
/
1
Upon closer inspection of the H NMR spectrum of
[La(dacda)]Cl it is evident that there are some smaller
peaks which may be due to the presence of a small
amount of an isomeric form of the complex. Such
solution behaviour is not uncommon for macrocyclic
complexes. For example, two isomers are observed in
solution in the 1H NMR spectra of [LnDOTA]ꢃ
complexes [41], though only one of these is observed
in the solid state. To investigate this possibility, 1H
NMR spectra of [La(dacda)]Cl were recorded at 600
MHz at 5, 25 and 45 8C. The small peaks appear to be
quite structured, as would be expected from an isomeric
form of [La(dacda)]Cl, although they are too weak to
attempt their proper assignment.
[Eu(dacda)]Cl and [Tb(dacda)]Cl consisted of unre-
solved, overlapping multiplets. However, NMR spectro-
scopy of the diamagnetic La3ꢀ complex was far more
1
1
revealing. The H and Hꢁ13
C COSY NMR spectra of
/
[La(dacda)]Cl in D2O are shown in Figs. 2 and 3,
respectively. There are six intense signals in the 1H
NMR spectrum. The singlet at d 3.49 is assigned to 4
equiv. acetate protons (e.g. H1a and H1b), indicating that
on the NMR timescale the complex has high symmetry.
This high symmetry is confirmed by the presence of only
five resonances in the 13C NMR spectrum, which must
Fig. 2. The 360 MHz 1H NMR spectrum of [La(dacda)]Cl at room temperature in D2O; the inset shows a labelled structure (see text for
assignments).

750
S.W. Magennis et al. / Polyhedron 22 (2003) 745ꢁ754
/
Fig. 3. The 360 MHz 1Hꢁ13
C COSY spectrum of [La(dacda)]Cl at room temperature in D2O.
/
The other feature of these spectra is the broadening of
all the resonances with a lowering of the temperature.
This corresponds to the slowing down of fluxional
processes and is in agreement with the observed 13C
NMR spectra reported for the [Y(dacda)]Cl complex
[29]. There were no significant changes in the relative
intensities of the weak and strong resonances upon
varying the temperature, as might be expected for an
equilibrium between isomers. However, these studies
were performed over a limited temperature range, and
changes may not have been readily discernible due to the
small intensity of the peaks. Although the structure of
[La(dacda)]Cl is drawn with the carboxylate arms on the
same side of the ring, the NMR data do not exclude a
structure with the arms on opposite sides of the ring.
Indeed, if isomeric forms are present, they may result
from changes in the conformation of the acetate arms.
complexation by dacda. The excitation spectrum con-
sists of sharp f ꢁf transitions, such as the strong
7F005L6 transition at 394 nm, and broad UV bands
centred at 280ꢁ290 nm, which are attributed to LMCT
transitions associated with oxidation of the amine
nitrogens. Similar broad LMCT bands are observed in
the excitation spectrum of the Eu3ꢀ complex of the 2.2.1
cryptand (1) [43].
/
/
/
The emission spectrum of [Eu(dacda)]Cl shows the
7FJ (Jꢂ
characteristic 5D00
/
/
0ꢁ
4) Eu3ꢀ transitions. The
/
luminescence lifetimes of [Eu(dacda)]Cl (1ꢄ
10ꢃ3
/
mol dmꢃ3) in H2O and D2O are 0.43 and 1.50 ms,
respectively. The monoexponential decays obtained
confirm the existence of the single species in solution.
The number of coordinated water molecules can be
calculated using equation,
qꢂA(kH O ꢃkD O
)
2
2
3.2. Luminescence spectroscopy
where Aꢂ 1.05 and 4.2 for Eu and Tb, respectively [44]
/
giving a q-value of 1.7.
If the equation which takes into account any outer
sphere water molecules [45] is used
The steady-state excitation and emission spectra for
[Eu(dacda)]Cl are shown in Fig. 4. Both spectra have
differences to those reported for the free Eu3ꢀ ion in
aqueous solution [42], which can be attributed to
qcorr ꢂA?(k ꢃk
)
D2O corr
H2O

S.W. Magennis et al. / Polyhedron 22 (2003) 745ꢁ
/754
751
1
In the H NMR spectrum of [Y(macma)](NO3)2 in
D2O, all of the protons can be accounted for by a poorly
resolved multiplet at d 3.68ꢁ3.97 and a broad singlet at
/
d 3.42, which integrate as 18 and 4 protons, respectively.
Similarly, the 1H NMR spectrum of [Eu(macma)](NO3)2
in CD3NO2 shows an unresolved multiplet at d 3.37ꢁ
/
3.70 and a broad singlet at d 2.89, which integrate as 6
and 16 protons, respectively. These spectra suggest that
fluxional processes, which are slow on the NMR time-
scale, are operative for these complexes in solution.
Both steady-state excitation and emission spectra
were recorded for solutions of [Eu(macma)](NO3)2 in
Tris buffer (Fig. 5). Complexation by macma is demon-
strated by the broad UV bands attributed to LMCT
transitions, while the emission spectrum displays a
Fig. 4. Excitation spectrum (top) and corrected emission spectrum
(bottom) of [Eu(dacda)]Cl in Tris buffer; lem 618 nm, lexc 270 nm.
ꢂ/
ꢂ/
where A?ꢂ 1.2 and 5 for Eu and Tb, respectively a qcorr-
/
value of 1.7 is also obtained. The high q value may be
attributed to the different confirmations that can be
adopted from the crown with one or two water
molecules coordinated if the carboxylate arms are on
the same or opposite sides of the crown, respectively.
This is in agreement with the previously reported data
when the complexes were formed in solution [29].
3.3. Characterisation of lanthanide macma complexes
The positive-ion electrospray mass spectrum of
[Eu(macma)](NO3)2 in H2O shows peaks at m/z 489
Fig. 5. Excitation spectrum (top) and corrected emission spectrum
and 491, corresponding to [Mꢃ
/
NO3]ꢀ, as found in the
(bottom) of [Eu(macma)]Cl in Tris buffer; lem
nm.
ꢂ/
618 nm, lexc
ꢂ/270
FAB MS spectrum.

752
S.W. Magennis et al. / Polyhedron 22 (2003) 745ꢁ754
/
relatively high intensity 5D00
nm).
/
7F4 transition (:
/695
to lanthanide is also established by excitation spectro-
scopy where the characteristic profile of coordinated
DBM absorption is observed. Electrospray mass spec-
trometry also confirmed the presence of 1:1 {[Eu(dac-
da)]:DBM} species (Table 1).
The luminescence lifetimes for [Eu(macma)](NO3)2 in
H2O and D2O were found to be 0.18 and 1.0 ms,
respectively. Monoexponential decays were obtained
which confirms the presence of a single species. The
calculated values of q and qcorr for this complex are 4.8
and 5.2, indicating five water molecules are coordinated
to the Eu3ꢀ ion. This high q-value is due to the presence
of only five ring donor atoms and one carboxylate group
on the ligand.
Picolinic and phthalic acid were also chosen as
bidentate LHCs. Addition of those acids to solutions
of [Eu(dacda)]ꢀ or [Tb(dacda)]ꢀ was performed in
aqueous solutions at pH 7.5. In both cases a plateau
of the emission intensity is observed following the
addition of the LHC attributed to the formation of a
ternary complex between the crown and the LHC. In the
case of terbium the signal enhancement with picolinic
acid is almost two hundred and fiftyfold whereas in the
case of europium is much smaller around one hundred
and seventyfold; this is expected due to the better match
of the LHC triplet state with terbium as well as the
presence of charge transfer deactivating pathways in the
case of the europium complex with carboxylates. The
titration plots for the terbium complex are presented in
Fig. 7. Although the increase of the emission intensity
slows down after 1 equiv. of the phthalate or picolinate
th saturation point suggests that 1 or 2 equiv. of
phthalate or picolinate bind to [Ln(dacda)]ꢀ. Although
there are only two coordination sites of the lanthanide
available for binding the flexibility of the two arms of
the crown together with the similarity of the carboxylate
binding site leads to a binding competition of the
carboxylate arms with the carboxylates of the LHC.
Control experiments under the same conditions of the
emission titration plots in the absence of the crown
moiety do not show saturation plateaus, confirming the
role of the crown as a building block for the ternary
formation.
3.4. Ternary complex formation with LHCs
The formation of ternary complexes of the
[Ln(dacda)]ꢀ and [Eu(macma)]2ꢀ with different LHCs
was monitored by electrospray mass spectrometry and
luminescence spectroscopy.
Initially, we chose dibenzoylmethane (DBM) as the
LHC. It has good absorption properties and it is known
to sensitise lanthanide luminescence. Upon incremental
addition of a solution of deprotonated DBM to a
solution of [Eu(dacda)]ꢀ in acetonitrile an increase of
the lanthanide emission is observed which slows down
after 1 equiv. of DBM and results in sensitisation of the
europium signal of about fiftyfold (Fig. 6). DBM was
deprotonated with triethyl amine prior to addition. The
discontinuity in the shape of the titration plot is
attributed to the complex equilibrium between the
deprotonated and bound DBM, especially due to the
possibility of formation of a six-member ring of triethyl
amine with deprotonated DBM. The observed plateau
confirms the saturation of the lanthanide binding site
with the LHC, which is not observed in the absence of
the crown ether under the same conditions. The binding
of the LHC and the energy transfer process from DBM
When [Eu(macma)]2ꢀ was used, a clear plateau at the
europium emission is observed upon addition of 2 equiv.
of picolinate and phthalate (Fig. 8). The 1:2 ternary
complex stoichiometry was expected due to the more
available coordination sites of the macma complex to
Table 1
Data of electrospray mass spectrometry analysis
Complex
ES-MS mode
m/z; assignment
[Eu(dacda)]ꢀ
positive
positive
positive
positive
positive
positive
positive
positive
positive
positive
negative
527/529, [M]ꢀ
535, [M]ꢀ
[Tb(dacda)]ꢀ
[Eu(macma)]2ꢀ
[Eu(dacda):DBM]
[Eu(dacda):PCA]
[Eu(dacda):PTA]ꢃ
[Eu(dacda):PTA]ꢃ
[Eu(dacda):BZA]
[Tb(dacda):PCA]
[Tb(dacda):PTA]ꢃ
[Tb(dacda):PTA]ꢃ
489/491 [Mꢀ
751/753, [Mꢀ
672/674, [Mꢀ
715/717 [Mꢀ
691/693 [M]ꢃ
671/673, [Mꢀ
680, [Mꢀ
Na]ꢀ
723, [MꢀNaꢀ
H]ꢀ
699, [M]ꢃ
/
NO3]ꢀ
/
H]ꢀ
/
Na]ꢀ
/
Naꢀ
H]ꢀ
/
/
Na]ꢀ
/
/
/
Fig. 6. Relative emission intensity of [Eu(dacda)]ꢀ upon addition of
10ꢃ5 M, lexc
ꢂ/350 nm. Only
PCA, picolinate; PTA, phthalate; BZA, benzoate; DBM, dibenzoyl-
methide.
DBM in CH3CN, [Eu(dacda)Cl]ꢂ
selected errors bars are shown for clarity.
/
1.1ꢄ
/

S.W. Magennis et al. / Polyhedron 22 (2003) 745ꢁ
/754
753
Scheme 3.
lack of stability of the higher species under the electro-
spray conditions.
Fig. 7. Relative emission intensity of [Tb(dacda)]ꢀ upon addition of
picolinate and phthalate in aqueous solution, [Tb(dacda)Cl]ꢂ1ꢄ
10ꢃ4 M, lexc
280 nm.
4. Conclusions
/
/
ꢂ/
We have shown that the luminescent europium(III)
and terbium(III) complexes of dacda and macma, have
available coordination sites so that the can act as
building blocks for the formation of ternary complexes
in aqueous solution (Scheme 3). Sensitisation of the
luminescence signal up to two hundred and fiftyfold is
observed by addition of simple LHCs to the lanthanide
crowns. The availability of coordination sites of the
lanthanide can be predicted by the number of water
molecules around the lanthanide and it is consistent for
most of the cases. The flexibility of the carboxylate arms
of the crowns can be a problem for the control of the
ternary complex formation. The different charge of the
crowns examined may also affect the strength of the
LHC binding and consequently the sensitisation of the
lanthanide luminescence signal. Further studies are
underway to extend the control of formation of ternary
lanthanide complexes.
Fig. 8. Relative emission intensity of [Eu(macma)]2ꢀ upon addition of
benzoate, phthalate isophthalate and picolinate in aqueous solution
[Eu(macma)(NO3)2]ꢂ
/
1ꢄ
/
10ꢃ4 M, lexc
ꢂ/280 nm.
5. Supplementary material
bind the LHC in comparison with the dacda. Isophtha-
late was also employed to compare the binding with
phthalate. It is shown that there is no saturation plateau
due to the weak binding of the monodentate carboxylate
and the possibility of forming complexes in both the
sites; although the latter is expected to occur in
conditions of higher concentrations than the ones
recorded in this experiment. Weak binding is also
observed for benzoate.
The CCDC contains the supplementary crystallo-
graphic data for this paper. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/contc/
retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ UK (fax: ꢀ
/
44-1223-336033; e-mail: deposit@ccdc.
cam.ac.uk)).
Excitation spectra of the ternary complexes of
[Ln(dacda)]ꢀ and [Eu(macma)]2ꢀ with the relevant
aromatic acids also show the characteristic peak corre-
sponding to the absorption of the LHC. Electrospray
mass spectra also show the presence of the ternary
complexes (Table 1) of the lanthanide crowns with the
aromatic acids. Species of 1:2 ternary complexes were
only detected for the macma case, probably due to the
Acknowledgements
The authors wish to thank A. Parkin for preliminary
studies. Support for this work from EPSRC (S.W.M)
and The Nuffield Foundation is gratefully acknowl-
edged.

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S.W. Magennis et al. / Polyhedron 22 (2003) 745ꢁ754
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