Structural and spectroscopic investigations of europium(III) entrapped by the ethylenediaminetetra(methylenephosphonate) ligand in K12H8[Eu4(EDTMP)4] · 45H2O crystal

Article abstract of DOI:10.1016/j.poly.2008.03.002

The X-ray crystal structure of an anionic octacoordinate Eu³⁺ complex of the formula K₁₂H₈[Eu₄(EDTMP)₄] · 45H₂O, where EDTMP is the ethylenediaminetetra(methylenephosphonate) anion, hereina

Full text of DOI:10.1016/j.poly.2008.03.002

Polyhedron 27 (2008) 1942–1946
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Structural and spectroscopic investigations of europium(III) entrapped by the
ethylenediaminetetra(methylenephosphonate) ligand in K12H8[Eu4(EDTMP)4] ꢀ
45H2O crystal
*
Rafał Janicki, Anna Mondry
Faculty of Chemistry, University of Wrocław, F. Joliot-Curie 14, 50-383 Wrocław, Poland
a r t i c l e i n f o
a b s t r a c t
The X-ray crystal structure of an anionic octacoordinate Eu3+ complex of the formula K12H8[Eu4-
(EDTMP)4] ꢀ 45H2O, where EDTMP is the ethylenediaminetetra(methylenephosphonate) anion, hereinafter
referred to as I, has been determined. The crystal consists of cyclic tetrameric complex anions, in which
the surrounding of each Eu3+ ion is composed of two nitrogen atoms and six oxygen atoms from phos-
phonate groups. One of the phosphonate groups in the ligand molecule is tridentate (O:O0,O00), thus giving
rise to the formation of the tetramers. The compound was characterized by UV–Vis electronic spectros-
copy. At room and liquid nitrogen temperatures the complex shows luminescence from both 5D0 and 5D1
states, the latter one, which is very rare in Eu3+ compounds with organic ligands, is probably brought
about by the saturation of the coordination sphere with the phosphonate groups.
Article history:
Received 31 January 2007
Accepted 7 March 2007
Available online 7 April 2008
Keywords:
Europium(III)
Polyaminopolyphosphonate
EDTMP
Quadramet
Structure
Ó 2008 Elsevier Ltd. All rights reserved.
UV–Vis spectroscopy
1. Introduction
carbonate to 10.7, in this work we have intended to check on how
the change of crystallization conditions may lead to a modification
One of the important applications of lanthanide complexes with
acyclic polyaminopolyphosphonic acids is their use as therapeutic
pharmaceuticals [1]. Though the 153Sm–EDTMP complex (a sche-
matic representation of the ligand is shown in Scheme 1) known
as QUADRAMETÒ has been applied since 1997 for the pallation of
pain from metastatic bone cancer [1–4], the knowledge of the
structural relationships between Ln3+ ions and acyclic poly-
aminopolyphosphonates is still fragmentary.
of the Eu3+–EDTMP complex structure.
For this purpose, we have crystallized the Eu3+–EDTMP complex
from aqueous solution which was alkalized with KOH to pH 7.5.
Under these conditions crystals of I with good optical quality have
been obtained. The presented crystal structure is the second, to the
best of the authors’ knowledge, example of X-ray analysis of the
Eu3+–EDTMP complex.
To date only two crystal structures of these complexes have
been reported [5,6]. Our previous studies on structural and spec-
troscopic properties of the Eu3+–EDTMP system have shown that
the coordination of the Eu3+ ion in aqueous solution in the pres-
ence of the carbonate anion is similar to that found in the
[C(NH2)3]7[Eu(EDTMP)(CO3)] ꢀ 10H2O crystal [6] (denoted here as
II), namely the EDTMP ligand is bound to the Eu3+ ion through
two nitrogen atoms and four oxygen atoms from monodendate
phosphonic groups, and two oxygen atoms from the bidentate car-
bonate anion fill up the coordination environment of the metal
cation.
2. Experimental
2.1. Crystal preparation
A solution of EuCl3 was prepared from Eu2O3 (99.99% Stanford
Materials). H8EDTMP acid (1.79 g, 4.1 mmol) was neutralized with
KOH to a pH value of about 7.5 and added to an aqueous solution of
EuCl3 (4 mmol). The resulting solution was alkalized with KOH to
pH 7.5. Colourless crystals of I were formed during very slow
evaporation.
Since the latter crystals were obtained from 1:1 Eu(ClO4)3:
H8EDTMP solution, of which the pH was adjusted by guanidinium
2.2. X-ray analysis
An appropriate crystal was cut from a larger one and mounted
on a Kuma KM4 diffractometer equipped with a CCD counter.
The collected data were corrected for polarization, Lorentz and
* Corresponding author. Tel.: +48 71 375 7370; fax: +48 71 328 2348.
E-mail address: anm@wchuwr.chem.uni.wroc.pl (A. Mondry).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.03.002

R. Janicki, A. Mondry / Polyhedron 27 (2008) 1942–1946
1943
Scheme 1.
absorption, the last being calculated from the crystal habit cap-
tured from photoscans. The positions of Eu atoms were found from
Patterson maps, the rest of the non-H atoms were found from dif-
ference Fourier maps. The positions of the C-bonded hydrogen
atoms were calculated geometrically. The refinement was full-ma-
trix with all ordered non-H atoms being anisotropic. All computa-
tions were performed using SHELXS 97 and SHELXL 97 programs [7].
1
The molecular graphics was prepared with DIAMOND
.
The crystal
data and structure refinement for I are as follows: empirical formula
Fig. 1. A view of the tetrameric complex anions in I.
C
24H146Eu4K12N8O93P16
,
M = 3608.04, T = 100 K, k/Å = 0.71073,
Z = 8, a = 27.43(2) Å, c = 33.17(3) Å,
l = 2.702 mmꢁ1 qcalc = 1.920 g cmꢁ3
ꢀ
tetragonal
I4c2,
V = 24960(30) Å3,
,
,
Table 1
Selected bond lengths (Å) for I
F(000) = 14640, crystal size = 0.4 ꢂ 0.3 ꢂ 0.2 mm, h = 3–28.5°, index
ranges = ꢁ36 6 h 6 28, ꢁ36 6 k 6 35, ꢁ43 6 l 6 43, reflections col-
Eu(1)–EDTMP
Eu(2)–EDTMP
lected/unique = 85266/14973 (Rint = 0.0943). Final
R
indices
Eu1–O110
Eu1–O109
Eu1–O106
Eu1–O101
2.349(6)
Eu2–O201
Eu2–O207
Eu2–O210
Eu2–O204
2.345(6)
2.376(6)
2.301(6)
2.378(7)
[I > 2r(I)]
R(F) = 0.0562,
Rw(F2) = 0.1264
and
R(F) = 0.0793,
2.339(6)
2.348(6)
2.394(6)
Rw(F2) = 0.1341 (all data). Data completeness to 2h = 28.6°, 95.6%.
Largest difference in peak and hole 1.024 and ꢁ0.819 e Åꢁ3
.
Eu–Oav
Eu1–O1040
Eu1–O1050
2.36(3)
2.526(6)
2.408(6)
Eu–Oav
Eu2–O2020
Eu2–O2030
2.35(4)
2.397(6)
2.534(6)
2.3. Spectroscopy
Eu–Oav
Eu1–N11
Eu1–N12
2.47(8)
2.640(7)
2.683(8)
Eu–Oav
Eu2–N22
Eu2–N21
2.47(10)
2.681(8)
2.654(7)
The luminescence spectra (kexc = 394 nm), corrected for the
lamp characteristics, were measured on a SLM Aminco 500 spec-
trofluorometer at 298 and 77 K. The luminescence decay curves
were detected on a device equipped with a Hamamatsu photomul-
tiplier and a Tektronix TDS-3052 digital oscilloscope. The sample
was excited with a N2-pumped dye laser with the monitored emis-
sion at 612 nm.
Eu–Nav
Eu–Eu
2.66(3)
6.301(8)
Eu–Nav
Eu–Eu
2.67(2)
6.327(8)
both are octacoordinate and have an identical composition of the
first coordination spheres. Each Eu3+ ion is surrounded by two
nitrogen atoms and six oxygen atoms from phosphonic groups.
The coordination sphere of the Eu3+ ions may be described as a dis-
torted square antiprism. The EDTMP anion is involved in bonds
with two neighbouring Eu3+ cations. Three of the ligand phos-
phonic groups are monodendate, whereas the fourth one is
three-coordinate, i.e. one oxygen atom coordinates to Eu1 and
the two others (hereinafter called ‘bridging’) coordinate to a Eu cat-
ion generated by symmetry from Eu1. In this way four Eu(EDTMP)
entities are bonded together to create a cyclic tetramer with S4
symmetry (Fig. 1). In a similar fashion, Eu2 generates another tet-
ramer with the same symmetry. The conformations of the tetra-
mers are very similar, the difference being only minute variations
of bond lengths and angles. Inside each tetramer a hole is formed
and its diameter is 5.2 Å in both cases. The cavities host in part
potassium cations and water molecules. The whole structure is
held together by a network of complicated hydrogen bonds and
electrostatic interactions.
The IR spectra of the complexes in KBr pellets and nujol suspen-
sions were recorded in the range 50–4000 cmꢁ1 with a Bruker IF
S66 spectrometer.
Electronic absorption spectra were recorded on a Cary 500 UV–
Vis–NIR spectrophotometer at 298 K. The oscillator strengths (P)
were determined using the equation:
Z
P ¼ 4:32 ꢂ 10ꢁ9ðcdÞꢁ1 AðrÞdr
ð1Þ
where c is the concentration of the Eu3+ ion in M, d is the length of
the optical way in cm and A(r) is the absorbance as the function of
the wavenumber in cmꢁ1
.
3. Results and discussion
3.1. Structure
ꢀ
Compound I crystallizes in the tetragonal system in the I4c2
space group. There are two symmetry independent Eu3+ cations,
Selected bond lengths are presented in Table 1. In the present
structure the average Eu–O bond length equals 2.35(4) Å (where
the O atoms belong to the EDTMP molecule wrapped around the
respective Eu cation) which is shorter than the average length of
1
DIAMOND – Visual Crystal Structure Information System, CRYSTAL IMPACT, Postfach
1251, D-53002 Bonn, Germany.

1944
R. Janicki, A. Mondry / Polyhedron 27 (2008) 1942–1946
the Eu–O bonds, 2.47(9) Å, formed by the bridging O atoms belong-
ing to the neighbouring EDTMP anions. All Eu–O distances involv-
ing non-bridging O atoms are similar to those observed in II [6].
The average distance of the remaining four bridging Eu–O bonds
is 0.03 Å longer as compared to the Eu–O (bidentate carbonate an-
ion) observed in II, probably due to the greater bulkiness of the tet-
at low temperatures [11] as well as in a macrocyclic carboxylate
complex incorporated into sol–gel glass [12]), is probably brought
about by the saturation of the europium coordination sphere with
the phosphonate groups.
Although the 5D1 emission for this compound is significantly
less intense than the emission from the 5D0 state, it is observed
both at 77 K and at room temperature. The emission intensities
of the 5D0,1 ? 7FJ transitions determined from the spectra recorded
under the same measurement conditions (excitation and emission
slits, intensity of the light beam) as well as the ratios of intensities
of the 5D1 ? 7FJ transitions relative to the 5D0 ? 7FJ ones are pre-
sented in Table 2. Since the distances between the 5D1 and 5D0 lev-
els are comparable for both Eu3+–EDTMP compounds and amount
to 1744 and 1747 cmꢁ1 for I and II, respectively, one may assume
that the luminescence quenching from the 5D1 state in the latter
crystal is brought about by one or two high energy phonons which
do not occur in I. Therefore, comparison between the IR spectra
and the 7F0 ? 5D2 excitation spectra recorded at 77 K (the elec-
tron–phonon coupling is the strongest for the hypersensitive tran-
sition) of both compounds may be helpful in adjudicating which of
the phonons is responsible for the luminescence quenching (Fig. 3).
The intense bands observed in the IR spectrum of I at 1057 and
1653 cmꢁ1 may be attributed to the mP–O vibration and an overtone
rahedral phosphonate group in comparison with
a planar
carbonate anion. In I the Eu–N average distance is 0.05 Å shorter
than that in II. The shortest Eu–Eu distances are 6.301(8) Å for
Eu(1) and 6.327(8) Å for Eu(2), and are smaller than that in II
(9.998(3) Å). On average two of the phosphonic groups per EDTMP
molecule are protonated. The longest P13–O103 distance,
1.582(7) Å, may indicate that the O103 oxygen atom is protonated.
The three other protons seem to be disordered. Some of the potas-
sium ions and water molecules are disordered too.
3.2. Spectroscopic results
The low symmetry of the Eu3+ ion surroundings has been re-
flected by the luminescence and absorption spectra of the crystal.
The corrected luminescence spectra of I, presented in Fig. 2, were
measured at 298 and 77 K. The large half-width (23 cmꢁ1 at
77 K) of the symmetrical, unsplit 5D0 ? 7F0 band confirms the re-
sults obtained from the X-ray analysis, namely the Eu3+ ions are lo-
or combination bands of the mC—PO H vibrations [13,14], whereas the
3
cated in very similar coordination environments of
symmetry.
a
low
bands at 1036, 1369 and 1663 cmꢁ1 in the spectrum of II corre-
spond to mP–O, mC–O and dNH vibrations.
2
It is worth stressing that in the present crystal, luminescence
from the 5D1 state could also be recorded. This emission, which
is not common in Eu3+ compounds with organic ligands (it has
been observed for example in Eu3+ chelates with b-diketones [8–
10] and in 1:3 Eu3+:oxydiacetate single crystals of D3 symmetry
As it may be seen in Fig. 3, the vibronics originating from the mP–
oscillations are observed in the excitation spectra of both com-
pounds. Apart from that, in the spectrum of II the mC–O vibronic
component at 22813 cmꢁ1 (1369 cmꢁ1 off the zero-phonon line)
is also detected. Thus, the mC–O oscillation (with the C–O oscillators
O
Fig. 2. Emission spectra of I at room and liquid nitrogen temperatures.
Table 2
Intensity of emission bands determined for I
5D1
5D0
!
!
7 F0
7 F0
5D1
5 D0
!
!
7F1
7 F1
5 D1
5D0
!
!
7F2
7F2
5D1
?
7F0
5D1
?
7F1
5D1
?
7F2
5D0
?
7F0
5D0
?
7F1
5D0 7F2
?
T = 298 K
T = 77 K
0.0092
0.0083
0.40
0.39
0.26
0.17
2.4
1.5
28.8
21.0
154
96.7
0.0038
0.0055
0.014
0.018
0.0016
0.0018

R. Janicki, A. Mondry / Polyhedron 27 (2008) 1942–1946
1945
Fig. 3. IR spectra (upper trace) and excitation 7F0
?
5D2 spectra obtained at 77 K (lower trace) of I and II.
located nearest to the Eu3+ cation) fits well in the 5D1–5D0 gap and
is probably responsible for the radiationless decay path of the 5D1
state in the latter compound. Absence of this phonon in I may be
one of the reasons of a longer luminescence lifetime in this com-
pound than in II. The decay lifetimes with estimated error of mea-
Table 3
Oscillator strength values (P) of I determined at different orientations of the crystal
(I–III)
Transitions
I, P ꢂ 108
0.44
1.55
14.23
132.25
II, P ꢂ 108
0.69
1.72
21.62
144.62
III, P ꢂ 108
0.63
1.63
24.70
157.42
Pav ꢂ 108
0.59
1.63
20.18
144.76
7F0
7F0
7F0
7F0
?
?
?
?
5D0
5D1
5D2
5L6
surements 10%, obtained by fitting the curves to
a single
exponential function, were found as 1340 ls for the present crystal
and 1036 ls for the previously studied one [6].
The absorption spectra of I, at different orientations of the crys-
tal with respect to the incident light beam, were also measured at
room temperature (Fig. 4). The crystal is anisotropic; the intensities
of the bands, especially those of the hypersensitive 7F0 ? 5D2 tran-
sition, depend on the orientation of the crystal, whereas the ener-
gies of all the crystal field components do not change. The
orientation averaged oscillator strength values of the f–f transi-
tions of the complex are given in Table 3. One may notice that
the average oscillator strength value of the 7F0 ? 5D2 transition
for I is similar to that observed in II, which may result from the
analogous arrangement of the first coordination sphere of the
Eu3+ ion. The splitting of the f–f levels is also similar, although
the 7F0 ? 5D0,1,2 bands in the present complex are slightly (a few
cmꢁ1) red-shifted in comparison to those observed in II (Fig. 4).
The high value of the oscillator strength of the 7F0 ? 5D0 transition
as well as the large half-width of this band at 298 K (15 cmꢁ1) may
be connected to the low symmetry of the surroundings of the Eu3+
ions.
Fig. 4. Absorption spectra of the f–f transitions of I and II [6] at 298 K. For II instead of the the 7F0
?
5D0 absorption spectrum, which could not be recorded due its low
intensity, the luminescence excitation spectrum of this transition is displayed.

1946
R. Janicki, A. Mondry / Polyhedron 27 (2008) 1942–1946
4. Conclusions
Appendix A. Supplementary data
The coordination sphere of the Eu3+ ion in the studied crystal
consists of the same donor atoms (six oxygen atoms, two nitrogen
atoms) as in the previously reported structure of II [6]. However, it
should be noted that two of the coordinated oxygen atoms are of
different origin for the two crystals. In the present crystal they be-
long to a tridentate phosphonate group, giving thus rise to forma-
tion of cyclic tetrameric complex anions, whereas in the former
monomeric complex anion they come from the bidentate carbon-
ate anion.
CCDC 632399 contains the supplementary crystallographic data
for compound I. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at doi:10.1016/
j.poly.2008.03.002.
Although such differences of the Eu3+–EDTMP complex struc-
ture do not influence the oscillator strengths of the f–f transitions
significantly, they affect the luminescence lifetimes of the excited
states of the Eu3+ ion. The more rigid structure of I is a reason for
a longer (approximately 300 ls) europium 5D0 level lifetime, as
compared to that found for II. The present crystal, contrary to the
former one, also displays luminescence from both 5D0 and 5D1
states.
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Acknowledgements
We thank Dr. P. Starynowicz for the X-ray measurements and
for assistance during the structure solution. This work was sup-
ported by MNiSW N204 135 31/3127.