Relationships between structure and spectroscopic properties of Nd 3+ ethylenediaminetetramethylenephosphonates and ethylenediaminetetraacetates

Article abstract of DOI:10.1002/ejic.201300089

The structural and spectroscopic properties of the Nd³⁺ compounds [C(NH₂)₃]₇[Nd(EDTMP)(CO ₃)]·10H₂O, K₁₇H₃[Nd ₄(EDTMP)₄]·36H₂O, [C(NH ₂)₃][Nd(EDTA)(H₂O)₃] and Na[Nd(EDTA)(H₂O)₃]·5H₂O are presented (H₄EDTA = ethylenediaminetetraacetic acid, H₈EDTMP is a phosphonic acid analogue of H₄EDTA). The obtained monomeric [Nd(EDTMP)(CO₃)]^7- and tetrameric [Nd₄(EDTMP) ₄]^20- structures, in which bidentate carbonate and tridentate bridged phosphonate coordination patterns appear, are exceptional. The use of different countercations has allowed us to assess their role in crystal formation and their influence on the spectroscopic properties of the investigated crystals. The countercations slightly change the geometry of [Nd(EDTA)(H₂O)₃]^- and some subtle modifications of the geometry of the carboxylic groups is observed. These changes are discussed in the context of hypersensitive transition intensities. The intensities of the f-f transitions in all studied crystals were determined and analyzed basing on the Judd-Ofelt theory. Luminescence of the Nd³⁺ ions in the NIR region could be observed solely in the phosphonate complexes. The luminescence quantum yields were calculated from the luminescence lifetimes and the Judd-Ofelt parameters. The Φ value for the rigid tetramer [Nd ₄(EDTMP)₄]^20- is twice as large (7 %) as that for the monomer [Nd(EDTMP)(CO₃)]^7- (3.5 %). The influence of different countercations on the structural and spectroscopic properties of Nd³⁺ complexes with ethylenediaminetetraacetic acid (EDTA) and its phosphonate analogue ethylenediaminetetramethylenephosphonic acid (EDTMP) are discussed. The absence of water molecules in the inner coordination spheres of the Nd³⁺ ions in the phosphonates is a reason for the efficient near-IR (NIR) f-f luminescence of the Nd³⁺-EDTMP crystals. Copyright

Full text of DOI:10.1002/ejic.201300089

FULL PAPER
DOI:10.1002/ejic.201300089
Relationships Between Structure and Spectroscopic
Properties of Nd3+ Ethylenediaminetetramethylene-
phosphonates and Ethylenediaminetetraacetates
Rafał Janicki[a] and Anna Mondry*[a]
Keywords: Lanthanides / Structure elucidation / Luminescence / Phosphonates
The structural and spectroscopic properties of the Nd3+ com-
pounds [C(NH2)3]7[Nd(EDTMP)(CO3)]·10H2O, K17H3[Nd4-
(H2O)3]– and some subtle modifications of the geometry of
the carboxylic groups is observed. These changes are dis-
cussed in the context of hypersensitive transition intensities.
The intensities of the f–f transitions in all studied crystals
were determined and analyzed basing on the Judd–Ofelt
theory. Luminescence of the Nd3+ ions in the NIR region
could be observed solely in the phosphonate complexes. The
luminescence quantum yields were calculated from the lumi-
nescence lifetimes and the Judd–Ofelt parameters. The Φ
value for the rigid tetramer [Nd4(EDTMP)4]20– is twice as
large (7%) as that for the monomer [Nd(EDTMP)(CO3)]7–
(3.5%).
(EDTMP)4]·36H2O,
[C(NH2)3][Nd(EDTA)(H2O)3]
and
Na[Nd(EDTA)(H2O)3]·5H2O are presented (H4EDTA = ethyl-
enediaminetetraacetic acid, H8EDTMP is a phosphonic acid
analogue of H4EDTA). The obtained monomeric
[Nd(EDTMP)(CO3)]7– and tetrameric [Nd4(EDTMP)4]20–
structures, in which bidentate carbonate and tridentate
bridged phosphonate coordination patterns appear, are ex-
ceptional. The use of different countercations has allowed us
to assess their role in crystal formation and their influence on
the spectroscopic properties of the investigated crystals. The
countercations slightly change the geometry of [Nd(EDTA)-
scarce,[12–15] in contrast to their carboxylic analogues. The
significantly larger size of the tetrahedral phosphonic group
compared to the planar carboxylic group as well as its
greater negative charge and basicity[16] are the reasons why
phosphonate ligands can form more diverse structures with
metal cations than carboxylate ligands.[9–11,17–19]
Introduction
There is great interest in lanthanide complexes with car-
boxylic and phosphonic polyamino acids because of their
potential applications in many areas of science. For exam-
ple, the luminescent, magnetic and radioactive properties of
lanthanide ions in these compounds are useful in medicine
for diagnosis as well as in therapy.[1–5] One of those com-
pounds, a complex of radioactive 153Sm with EDTMP
(H8EDTMP = ethylenediaminetetramethylenephosphonic
acid) is used to relieve the pain caused by bone metasta-
ses.[6] Although the complex has been applied in clinical
radiotherapy since 1997, its uptake mechanism by meta-
static bones is unclear.[7,8] Therefore, it seems that a detailed
investigation of the structural properties of Ln–EDTMP
compounds may be helpful and could shed more light on
the mechanism of drug action.
Our earlier structural studies on Ln3+ tetraphosphon-
ates[12–15] have revealed that the composition and geometry
of the inner sphere of the Ln3+ ion is strongly dependent
on synthesis conditions (e.g., pH, different countercations).
Therefore, in this paper we would like to report the struc-
tural and spectroscopic properties of Nd3+ complexes with
EDTMP obtained under different experimental conditions
(pH of stock solutions, different countercations). For com-
parison, the physicochemical properties of Nd complexes
with the carboxylic analogue of EDTMP, ethylenediamine-
tetraacetic acid (EDTA), will be also presented.
Although many structural studies of lanthanide com-
pounds with simple mono- and bisphosphonates have been
reported,[9–11] the crystal structures of lanthanide com-
plexes with polyamino polyphosphonic acids are rather
Results and Discussion
Crystal Structures
[a] Faculty of Chemistry, University of Wrocław,
14 F. Joliot-Curie, 50-383 Wrocław, Poland
Fax: +48-71-328-23-48
The crystal structures of [C(NH2)3]7[Nd(EDTMP)(CO3)]·
10H2O (I), K17H3[Nd4(EDTMP)4]·36H2O (II), [C(NH2)3]-
[Nd(EDTA)(H2O)3] (III) and Na[Nd(EDTA)(H2O)3]·5H2O
(IV) are briefly described. The EDTMP and EDTA ligands
E-mail: anna.mondry@chem.uni.wroc.pl
Homepage: http://www.chem.uni.wroc.pl/
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201300089.
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coordinate to the Nd3+ ions in a similar way, namely,
These data indicate that the equilibria between various
through four O and two N atoms. The remaining coordina- [Ln(HnCDTMP)]3–n species differ significantly for light and
tion sites are occupied by bidentate carbonate anions in I, heavy lanthanides. The structures of the studied complex
two O atoms from an adjacent tridendate phosphonate anions are presented in Figure 1, and the Nd–L bond
group in II and three water molecules in III and IV. In this lengths are listed in Table 1. The data included in Table 1
way, the coordination number of the Nd3+ ion is eight in demonstrate that for eight-coordinate Nd–EDTMP com-
the Nd–EDTMP complexes and nine in the Nd–EDTA plexes, the average Nd–O bonds are shorter and the Nd–N
complexes. Notably, in spite of the same pH (ca. 7.5) for bonds are longer than those of the corresponding Nd–
the syntheses of II and the previously studied Eu–EDTMP EDTA compounds. Compounds I, III and IV contain mo-
complex,[14] the tetrameric [Ln4(EDTMP)4]20– units bind nomeric complex anions, whereas II contains a cyclic
different numbers of protons to yield H3[Nd4(EDTMP)4]17– [Nd4(EDTMP)4]20– tetramer with a cavity in the centre.
and H8[Eu4(EDTMP)4]12– anions. This may indicate that
This highly negative anionic entity in II (Figure 2) shows
the Nd–EDTMP and Eu–EDTMP species are differently some rigidity, which is manifested in the Nd–O bond
protonated in aqueous solution at pH ca. 7.5. Such a con- lengths, that is, those inside the tetramer (Nd–O12i) are
clusion can be drawn from the stability constants deter- shorter than the outer ones (Nd–O10ii).
mined potentiometrically for a series of Ln3+ complexes
Owing to the high negative charge of [Nd4-
with CDTMP [H8CDTMP = trans-cyclohexane-1,2-di- (EDTMP)4]20–, the cavity hosts H+ and K+ cations and
amine-N,N,NЈ,NЈtetrakis(methylenephosphonic acid)].[12] water molecules. One of the K+ cations is located in the
Figure 1. Structures of the studied complex anions. For II, approximately a quarter of the tetramer is shown. Only symmetry independent
atoms are labelled. [a] Isostructural with [C(NH2)3]7[Eu(EDTMP)(CO3)]·10H2O.[13] [b] Isostructural with [C(NH2)3][Eu(EDTA)-
(H2O)3].[13] [c] Isostructural with Na[Nd(EDTA)(H2O)3]·5H2O.[20]
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Table 1. Nd–L and Nd–Nd bond lengths [Å] in Nd–EDTMP (I, II) and Nd–EDTA (III, IV).
I
II
III
IV
Nd–O8
Nd–O1
Nd–O10
Nd–O6
Nd–Oav
2.385(2)
2.407(2)
2.407(2)
2.438(2)
2.409(21)
Nd–O11
Nd–O1
Nd–O9
Nd–O6
Nd–Oav
2.405(4)
2.384(5)
2.426(5)
2.421(4)
2.409(19)
Nd–O3
Nd–O4
Nd–O1
Nd–O2
Nd–Oav
2.412(3)
2.420(3)
2.467(3)
2.482(3)
2.445(34)
Nd–O5
Nd–O1
Nd–O3
Nd–O7
Nd–Oav
2.442(3)
2.434(3)
2.457(3)
2.483(3)
2.454(21)
Nd–O22
Nd–O23
2.467(2)
2.458(2)
Nd–O12i[a]
Nd–O10ii
2.410(5)
2.545(4)
Nd–OW1
Nd–OW2
Nd–OW3
Nd–OWav
2.468(3)
2.500(3)
2.581(3)
2.516(58)
Nd–OW1
Nd–OW2
Nd–OW3
Nd–OWav
2.510(3)
2.590(3)
2.474(3)
2.525(59)
Nd–Oav
2.463(6)
Nd–Oav
2.478(95)
Nd–N1
Nd–N2
Nd–Nav
2.715(3)
2.768(3)
2.742(37)
Nd–N1
Nd–N2
Nd–Nav
2.739(5)
2.707(5)
2.723(22)
Nd–N1
Nd–N2
Nd–Nav
2.648(3)
2.735(4)
2.692(62)
Nd–N1
Nd–N2
Nd–Nav
2.674(3)
2.715(3)
2.694(29)
Nd···Nd
10.056
6.438
7.806
6.117
Symmetry codes: 1 – y, x, 1 – z.
centre of the tetramer and, in this way, stabilizes the struc-
ture of the complex. The [Nd4(EDTMP)4]20– tetramers
form columns and are held together by a network of hydro-
gen bonds and electrostatic interactions.
Inside each tetramer, a hole of diameter ca. 5.2 Å is
formed. The distances between the tetrameric units are ca.
12 Å, and the neighbouring rods are ca. 15 Å away (see
Scheme 1).
Notably, all the phosphonic O atoms form hydrogen
bonds with [C(NH2)3]+ ions in I, whereas only some of
them interact electrostatically with K+ cations in II. Owing
to these differences, as well as the partial protonation of
2–
–PO3 groups in II, the P–O distances are more diverse in
II than in I (Table 1S).
A carbonate anion in I is also involved in a network of
hydrogen bonds with some of the H2O molecules and
[C(NH2)3]+ cations. As is shown in Scheme 2, the C20–O21
and C20–O23 bond lengths are very similar, whereas the
C20–O22 bond is longer, probably because of the relatively
strong hydrogen bond formed between O22 and OW3
[2.679(5) Å]. This, in turn, brings about partial withdrawal
Figure 2. View of the tetrameric [Nd4(EDTMP)4]20– anion in II.
The shorter inner Nd–O12i bonds are marked in red, and the
longer outer Nd–O10ii bonds are marked in blue.
Scheme 1. Crystal packing of II and the rods formed by the tetramers.
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of the electron density from the C20–O22 bond and reduces ν(NH) and ν(CH) oscillations. The presence of H2O mole-
the polarizability of the O22 atom.
cules coordinated to the Nd3+ ions in the Nd–EDTA com-
plexes is the reason for the redshift of the ν(OH) bands (ca.
30 cm–1) of III and IV in comparison with those of I and
II.
The most characteristic bands for the studied com-
pounds are located in the range 870–1770 cm–1. This spec-
tral region includes bands ascribed to ν(P–O), ν(C–O),
δ(CH2), δ(OH2) and δ(NH2) vibrations. The symmetric and
asymmetric ν(P–O) vibrations in I and II are located at
870–1250 cm–1. The splitting and the shape changes of the
ν(P–O) bands reflect the variety of P–O bonds found in the
crystals. The simple structure of these bands in I is caused
by the similarity of the P–O bond lengths, whereas the com-
plex pattern in II is caused by the different coordination
modes of the phosphonic groups as well as their proton-
ation. The ν(C–O) vibrations located at 1363 and 1462 cm–1
in I confirm a bidentate coordination mode of the carbon-
ate anion, as was shown previously.[21] The δ(NH2) vi-
brations from guanidinium cations centred at 1664 cm–1 in
I partly cover the medium-intensity δ(OH2) band. The band
of the latter oscillations is clearly seen in the spectrum of
II.
Scheme 2. Geometry of the Ln–O(CO32–) unit in I.
The influence of the countercations on the C–O lengths
in the carboxylic groups is particularly well demonstrated
in III. The coordination modes of the carboxylic groups in
III and IV are presented in Scheme 3.
In the spectrum of III, the band between 1500–
1775 cm–1, which is attributed to the asymmetric ν(C–O)
and δ(NH2) vibrations, is split into seven components. In
this compound, two of the four carboxylic groups are en-
gaged in hydrogen bonds with guanidinium cations. This is
one of the reasons for the larger diversification of C–O
bond lengths in III than in IV (Table 2S and Figure 1S),
which in turn results in the intricate pattern of this band.
A similar relationship may be found for the symmetric
ν(C–O) band, which is located at 1280–1360 cm–1.
Scheme 3. Coordination modes of the carboxylic groups in Nd–
EDTA complexes.
The elongation of the C–O (C8–O3 and C5–O4) bonds
in III [type (b), Table 1S] is remarkable because of the si-
multaneous interaction of the C–O group with Nd3+ and
[C(NH2)3]+ cations. In particular, elongation of these bonds
in III is a reason for the shortening of the Nd–O bonds
(Nd–O3 and Nd–O4, Table 1). This effect is imperceptible
for the pair of Nd3+ and Na+ cations in IV, as it is illus-
trated in Scheme 4.
Electronic Spectroscopy
The electronic absorption spectra were measured at room
and liquid helium temperatures in the spectral range 200–
2600 nm. Important information may be gained by analysis
of the f–f transitions observed in this spectral region. The
crystal field (CF) splitting of 4f manifolds together with the
oscillator strengths of f–f transitions gives some insight into
the Nd–ligand interactions. It is interesting to recognize
how the various Nd3+ coordination environments influence
the above-mentioned spectral properties.
Scheme 4. C–O bond lengths in III (A) and in IV (B).
The larger difference between the C–O bond lengths in
III than in IV may indicate a more uniform charge distribu-
tion in the –CO2 groups of the latter compound. This, in
turn, makes the coordinated O atoms in IV somewhat more
polarizable.
Crystal Field Splitting
–
4
4
The energies of the CF components of the I9/2, I15/2
,
4
2
4F3/2, F9/2 and H11/2 multiplets, obtained from the low-
temperature spectra, are presented in Table 3S.
The number of CF components is in agreement with the
number predicted from group theory and indicates the low
symmetry of the studied complexes. The CF splitting of the
IR Spectroscopy
The IR spectra of the Nd compounds were recorded in 4f levels is in general larger for the phosphonate complexes
the range 450–4000 cm–1 (Figure 1S, Table 2S). The intense than for the carboxylate complexes. This is probably associ-
bands in the spectral range 2750–3550 cm–1 with ν
≈
ated with the changes in the Nd–O bond lengths.
˜
max
3400 cm–1 are attributed to ν(OH) vibrations from water
Of the f–f transitions observed in the absorption spectra
molecules, protonated phosphonic groups in II as well as to of the Nd3+ compounds, 4I9/2 Ǟ2P1/2 is one of the most
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2
suitable for the study of Nd–ligand interactions. The excited
The energies of the P1/2 level of the phosphonate com-
2P1/2 state is a Kramer’s doublet and does not split in the plexes I and II differ and are redshifted in comparison to
ligand field of any symmetry. Therefore, the 4I9/2 Ǟ2P1/2 those of the carboxylate complexes; for the latter, the re-
transition is used to study the nephelauxetic effect[22] and spective energies are identical. Hence, the variation of the
for determination of the splitting of the ground I9/2 state. 2P1/2 energies observed for the phosphonate complexes may
4
The spectra of this transition recorded at 298 and 4.2 K are be attributed to the presence of different coligands in I and
shown in Figure 3.
II. The bidentate carbonate anion is more symmetrically
bonded to the Nd3+ ion in I (Nd–O22, Nd–O23; Table 1,
Scheme 2) than the tridendate bridging phosphonic group
in II (Nd–O12i, Nd–O10ii; Table 1). For this reason, the
O12i atom in the latter interacts with the Nd3+ cation more
strongly than O10ii does. Such structural inequivalence
gives rise to a bathochromic shift (32 cm–1) of the
4I9/2 Ǟ2P1/2 transition relative to that of I. These data may
also suggest that there is a small covalent contribution to
the undoubtedly ionic Nd–ligand interaction.
From the low-temperature spectra (Figure 3), it is clear
that the half-widths (Δν1/2) of the 4I9/2 Ǟ2P1/2 bands at
4.2 K differ significantly. As the distance between neigh-
bouring Nd ions is the shortest in IV, the observed slight
broadening of this band in relation to that in III may be
attributed to ion-pair interactions.[23] A significant broaden-
4
ing of the I9/2 Ǟ2P1/2 band in II is mainly caused by the
outer-sphere disorder of the H+ and K+ cations, as well as
some of the water molecules.
Analysis of f–f Transition Intensities
The oscillator strengths of all f–f bands in the spectral
range 320–950 nm are collected in Table 2. Although there
are also f–f transitions above 950 nm in the spectra of the
Nd3+ ions, their intensities are not included as the
4
4I9/2 Ǟ4I15/2 and I9/2 Ǟ4I13/2 bands are superimposed with
overtone and/or combination bands from organic ligands
and H2O molecules.[24]
The intensities of all f–f transitions decrease as the tem-
perature is reduced from 298 to 4.2 K (Table 2); this is
caused by thermal depopulation of the higher CF levels of
4
Figure 3. Absorption spectra of the I9/2 Ǟ2P1/2 transition. Δν1/2
=
7, 16, 6 and 9 cm–1 for I, II, III and IV, respectively; Δν1/2 is the
4
half-width of the I9/2 Ǟ2P1/2 band at 4.2 K.
the ground I9/2 state. Moreover, the oscillator strengths of
4
Table 2. Oscillator strengths (P) of f–f transitions and Ωλ parameter values for crystals of I–IV. The P values with most influence on the
Ω2 and Ω6 values are in italic.
2I9/2
Ǟ
I
II
III
IV
Pϫ108
(298 K)
Pϫ108
(4.2 K)
Pϫ108
(298 K)
Pϫ108
(4.2 K)
Pϫ108
(298 K)
Pϫ108
(4.2 K)
Pϫ108
(298 K)
Pϫ108
(4.2 K)
4D3/2,4D5/2,2I11/2, 4D1/2,2L15/2 854.5
–
4.3
893.8
7.3
32.7
144.8
622.3
1808.1
18.5
900.2
9.3
34.4
157.4
566. 8
≈1480
25.3
1231.7
11.4
47.5
282.5
960.7
1308.3
21.0
–
–
1133.8
8.9
28.5
247.5
873.3
1645.0
21.5
5.6
2D5/2
2P1/2
–
20.1
14.0
18.3
264.9
467.6
≈1450
15.2
28.0
≈600
≈700
76.9
11.2
149.7
550.2
942.0
15.4
46.2
314.3
381.0
53.3
16.0
252.6
348.9
563.9
15.8
30.8
345.2
364.3
88.5
2K15/2,2G9/2, (2D,2F)3/2,4G11/2 136.2
2K13/2,4G7/2,4G9/2
4G5/2,2G7/2
2H11/2
539.5
1789.6
15.4
4F9/2
56.0
53.8
56.2
96.6
84.0
4F7/2,4S3/2
4F5/2,2H9/2
4F3/2
561.9
522.4
104.9
637.8
618.5
176.9
555.3
≈610
199.4
1204.9
1054.1
310.4
1078.2
1013.4
329.1
Ω2 ϫ1020 [cm2]
Ω4 ϫ1020 [cm2]
Ω6 ϫ1020 [cm2]
rmsϫ107
5.12Ϯ0.23
3.30Ϯ0.21
4.15Ϯ0.30
8.03
5.03Ϯ0.36
3.52Ϯ0.32
4.94Ϯ0.47
4.82
1.98Ϯ0.78
4.69Ϯ0.68
9.49Ϯ1.01
18.0
3.94Ϯ0.53
4.96Ϯ0.63
9.71Ϯ0.90
12.5
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4
the most intense bands (4I9/2Ǟ4G5/2,2G7/2, I9/2 Ǟ4F7/2,4S3/2
The Ωλ intensity parameters[27,28] are also included in
and 4I9/2 Ǟ4F5/2,2H9/2) are reduced more drastically in I and Table 2. The sets of Ωλ are a useful tool for comparison of
III than in II and IV. This is probably because of the dif- f–f intensities for different systems. For example, they may
ferent interactions of organic and inorganic countercations be applied to find relationships between spectral and struc-
([C(NH2)3]+ in I and III and K+ and Na+ in II and IV) tural properties of lanthanide complexes.
with the anionic complexes. A similar tendency was also
found for Nd–TTHA (H6TTHA = triethylenetetraamine- and Ω6 parameters may reflect the same Nd3+ coordination
N,N,NЈ,NЈЈ,NЈЈЈ,NЈЈЈ-hexaacetic acid) compounds.[25,26]
environments in III and IV, whereas significant variations
For the Nd–EDTA crystals, the constant values of the Ω4
The intensity of the hypersensitive transition in the Ω2 values may indicate the influence of the ligand
4I9/2 Ǟ4G5/2,2G7/2 is particularly affected by variations in polarizability mechanism on this parameter. Such interpret-
the immediate environment of the Nd3+ ion; therefore, re- ation is consistent with the study by Mason and Tranter on
semblance of the band shapes may reflect similar coordina- lanthanide tetrakis(dithiocarbamates).[29,30] They showed
tion environments of the Nd3+ ions.
The replacement of a CO3 by a PO3 group in by 66, 17 and 0.1% for λ = 2, 4, 6, whereas the correspond-
Nd–EDTMP crystals gives rise to comparably shaped ing CF contributions increase from 5 to 57 to 98.5%,
4I9/2 Ǟ4G5/2,2G7/2 bands and slight changes in the CF peak respectively.
that the ligand polarizability contributes to the Ωλ values
2–
2–
positions, whereas similar coordination environments of the
For the Nd–EDTMP crystals (I and II), for which the
Nd3+ cations in the Nd–EDTA complexes (III, IV) result symmetry of the Nd3+ environment is nearly the same, the
4
in similar CF energies of the I9/2 Ǟ4G5/2,2G7/2 transition oscillator strengths of the hypersensitive transition as well
(Figure 4). Although the countercations ([C(NH2)3]+, Na+) as the respective Ω4 and Ω6 values are comparable. There-
in the latter crystals do not perturb the CF energies, they fore, it is puzzling that coligands with different oxygen atom
do influence the intensity of the hypersensitive transition polarizabilities (αC–O = 3.85 Å3, αP–O ≈ 1.4 Å3)[31] give sim-
(Table 2).
ilar Ω2 parameter values in I and II.
It seems that such coincidence of Ω2 values is rather acci-
dental by taking into account the results obtained for the
Nd–EDTA crystals as well as for the more complicated
Nd–TTHA system.[25,26] A possible explanation is as fol-
lows. In Nd–EDTA and Nd–TTHA compounds, the guan-
idinium countercation, contrary to the Na+ one, is engaged
in hydrogen bonds with some O atoms, coordinated to the
Nd3+ cation, which results in some reduction of electron
density in the C–O bonds and decreases the polarizability
of the respective O atoms, as was discussed in the Crystal
Structure section.
Therefore, assuming a similar influence of the guanid-
2–
inium cation on the O atoms from the CO3 anion in I,
one may expect that the replacement of [C(NH2)3]+ by an
inorganic cation should result in an increase in the Ω2 pa-
rameter. An inspection of the spectra (Figure 5) and Ωλ pa-
rameters for the Nd–EDTMP and Nd–EDTMP–CO3 com-
plexes in aqueous solutions (Table 3) together with those
4
Figure 4. Absorption spectra of the hypersensitive I9/2 Ǟ4G5/2,2G7/2
transition for I–IV.
This change may be induced by a network of strong H
bonds as is the case for guanidinium crystal III, contrary
to the electrostatic interactions induced by the Na+ ions in
IV. This, in turn, may suggest that the observed intensity
changes of the hypersensitive transition are affected by the
polarizability variations of the O atoms, as was discussed in
the Crystal Structures section.
4
Figure 5. Absorption spectra of the I9/2 Ǟ2P1/2 (A) and hypersen-
4
sitive I9/2 Ǟ4G5/2
,
2G7/2 (B) transitions for crystals of I and Nd–
EDTMP complexes at various pH values.
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for crystals of I (Table 2) is helpful to assess the influence CH groups from the ligand and solvents. This is one of the
of coligands on the Ωλ parameter. The solutions of Nd– reasons for low luminescence intensity and short lifetimes
EDTMP at pH 9.1 and 10.7 were adjusted with NaOH, and (a few μs) in these compounds, especially in solutions.
the Nd–EDTMP–CO3 solution at pH = 10.7 was adjusted
with [C(NH2)3]2CO3 (I was obtained under similar experi- luminescence spectra could be recorded for the phos-
By exciting Nd3+ ions at 514.5 nm, the near-IR (NIR)
mental conditions).
phonate crystals, only. The emission spectra of I and II are
presented in Figure 6.
Table 3. Oscillator strengths (P؋108) of the f–f transitions and Ωλ
parameter values for Nd–EDTMP in solutions at different pH val-
ues.
2I9/2
Ǟ
pH = 10.7
[C(NH2)3]2CO3 NaOH
pH = 10.7
pH = 9.1
NaOH
4D3/2,4D5/2,2I11/2
4D1/2,2L15/2
2D5/2
,
865.5
809.9
762.9
4.6
30.1
148.7
5.5
30.7
146.6
5.2
31.6
143.8
2P1/2
2K15/2,2G9/2
,
(2D,2F)3/2,4G11/2
2K13/2,4G7/2, 4G9/2 629.2
499.2
1005.6
14.5
51.7
734.8
748.3
199.8
2.09Ϯ0.32
3.52Ϯ0.29
6.73Ϯ0.42
5.84
500.5
1013.5
14.8
54.4
736.2
751.9
198.9
2.27Ϯ0.36
3.29Ϯ0.33
6.81Ϯ0.47
6.50
4G5/2,2G7/2
2H11/2
2251.9
15.6
57.5
672.6
703.5
4F9/2
4F7/2,4S3/2
4F5/2,2H9/2
4F3/2
166.7
Ω2 ϫ1020 [cm2]
Ω4 ϫ1020 [cm2]
Ω6 ϫ1020 [cm2]
rmsϫ107
7.85Ϯ0.26
3.70Ϯ0.23
6.05Ϯ0.33
6.00
Figure 6. Emission spectra (λex = 514.5 nm) of Nd–EDTMP crys-
tals.
The remarkable agreement between the spectra of I and
The relative intensities of the 4F3/2 Ǟ4IJЈ bands are
the Nd–EDTMP–CO3 solution (Figure 5) may suggest that stronger in II than in I. Based on the determined Ωλ values
the coordination geometry of [Nd(EDTMP)(CO3)]7– spe- (Table 2), the spontaneous emission probabilities
cies in solution is similar to that found in I.
A(4F3/2Ǟ4IJЈ), luminescence branching ratios β(4F3/2 Ǟ4IJЈ)
However, the Ω2 and Ω6 values are higher for the and the radiative lifetimes were calculated and are given in
[Nd(EDTMP)(CO3)]7– complex in solution than those for Table 4, in which the τexp and quantum yield (Φ) are also
crystals of I (Table 2). This should not be surprising, as the included.
[Nd(EDTMP)(CO3)]7– species in solution are in dynamic
equilibrium and the determined Ωλ parameters are
Table 4. Luminescence parameters for the studied crystals.
averaged.
It is worth checking how variously polarizable coligands
will affect the values of the Ωλ parameter. The hydroxy spe-
cies described by the following simplified reaction are in
equilibrium in Ln–EDTMP solutions at pH above 9 alka-
lized by NaOH:[13,32]
I
II
III
IV
Ω4/Ω6
0.80
824
1027
209
0.71
898
1187
246
0.50
1301
2118
434
0.51
1362
2178
445
A(4F3/2 Ǟ4I9/2) [s–1]
A(4F3/2 Ǟ4I11/2) [s–1]
A(4F3/2 Ǟ4I13/2) [s–1]
A(4F3/2 Ǟ4I15/2) [s–1]
9
11
20
20
β(4F3/2 Ǟ4I9/2
)
0.398
0.497
0.101
0.004
483
18
3.7
0.384
0.507
0.105
0.004
427
31
7.2
0.336
0.547
0.112
0.005
258
–
–
0.340
0.544
0.111
0.005
250
–
–
β(4F3/2 Ǟ4I11/2
β(4F3/2 Ǟ4I13/2
β(4F3/2 Ǟ4I15/2
τR [μs]
)
)
)
[Nd(EDTMP)(H2O)2]5– + OH– ↔
↔ [Nd(EDTMP)(H2O)(OH)]6– + H2O
From DFT calculations, the symmetry of these species as
well as that of [Nd(EDTMP)(CO3)]7– are substantially the
same.[33,34] The Ω4 values determined for all studied solu-
tions and crystals of I are constant, whereas the Ω6 values
are similar only for the solutions. This may suggest that the
CF potential mostly contributes to the Ω6 parameter. In
τexp [μs]
Φ [%]
For Nd3+ ions, the luminescence parameters determined
from the Judd–Ofelt analysis depend on the Ω4 and Ω6 val-
ues only, as the square matrix elements U(2) for the
4F3/2 Ǟ4IJЈ transitions are equal to zero. The calculated
spontaneous emission coefficients A(4F3/2 Ǟ4IJЈ) and
branching ratios β(4F3/2 Ǟ4IJЈ) suggest that Nd–EDTA
complexes should be better NIR emitters than Nd–
turn, different coligand polarizabilities (αH O = 1.442 Å3,
2
αOH– = 2.03 Å3, αCO 2– = 3.85 Å3)[31,35] mainly influence the
3
Ω2 parameter values.
Luminescence Properties
4
The F3/2 Ǟ4IJЈ (JЈ = 9/2, 11/2, 13/2, 15/2) emission of EDTMP crystals.
Nd3+ ions in complexes with organic ligands is usually
However, the determined rates (1/τR) do not include non-
quenched by the high-energy vibrations of OH, NH and radiative processes, which are more effective for the carb-
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oxylates owing to the presence of three water molecules in
the first coordination sphere of the Nd3+ ions.
The use of different organic {[C(NH2)3]+} and inorganic
(K+/Na+) countercations has enabled us to observe their
More effective luminescence quenching in I than in II is role in crystal formation as well as their influence on the
caused by the presence of CO vibrations from the coordi- spectroscopic properties of the investigated crystals. The ge-
nated carbonate anion as well as high-energy ν(NH) oscilla- ometry of the Nd–EDTA complex in both crystals is practi-
tions from the guanidinium cations. The energy of the cally the same, as indicated by the X-ray data and the CF
ν(CO) vibrations is about 400 cm–1 higher than that from splitting of the 4f manifolds. Therefore, the observed inten-
the PO group (Table 2S). For II, τexp is probably partially sity changes of the hypersensitive transition in the spectra
reduced by ion-pair interactions, as the Nd–Nd distance is of III and IV are attributed to ligand polarizability varia-
much shorter in II (6.438 Å) than in I (10.056 Å).
tions. Indeed, some of the C–O bonds in III are signifi-
The Judd–Ofelt analysis of the absorption spectra, to- cantly elongated owing to the simultaneous interactions of
gether with the experimentally determined lifetimes, al- CO groups with Nd3+ and guanidinium cations. Such
lowed us to calculate luminescence quantum yields for the elongation results in greater separation of the electron cloud
studied crystals; it should be stressed that it is rare to use around the shifted O atom from the rest of the carboxylic
such a method to determine Φ for NIR-luminescent lantha- group. This, in turn, results in a decrease of the atom polar-
nide complexes with organic ligands.
izability. In this way, it is possible to trace the impact ex-
The obtained quantum yield is two times higher for II erted by fine modifications of the structure on the hypersen-
than for I and probably should be associated with the rigid- sitive transition intensities. The influence of ligand polariz-
ity of the tetrameric [Nd4(EDTMP)4]20– units. To the best ability and symmetry on the Ω2 and Ω6 values, respectively,
of our knowledge, the Φ value of 7.2% for II is one of the was also shown for Nd–EDTMP complex solutions and
highest reported to date for Nd3+ chelates.[36]
crystals.
It is also notable that evaluation of radiative lifetimes for
The calculated Ωλ parameters enabled us to retrieve the
Nd3+ compounds with organic ligands is not straightfor- spontaneous emission coefficients A(4F3/2 Ǟ4IJЈ) and
ward; it is usually assumed that the τR value is constant and branching ratios β(4F3/2 Ǟ4IJЈ). These data suggested that
taken from the literature (τR = 270 μs for Y2O3:Nd).[37] Our the Nd–EDTA crystals should be better NIR emitters than
results suggest that this crude assumption is unjustified, as the Nd–EDTMP complexes. Therefore, the luminescence
the τR values strongly depend on the chemical environment properties of the crystals were examined. Unfortunately,
of the Nd3+ cation.
owing to the presence of three water molecules in the vicin-
ity of the metal cation in the EDTA complexes, the NIR
emission is efficiently quenched by ν(OH) oscillations. The
lack of water molecules in the nearest environment of the
Nd3+ ions in the EDTMP crystals is a reason for the good
Conclusions
In this paper, a detailed analysis of the physico-optical luminescence properties of crystals of I and II. To check the
properties of [C(NH2)3]7[Nd(EDTMP)(CO3)]·10H2O, efficiency of the luminescence processes in both phosphonic
K17H3[Nd4(EDTMP)4]·36H2O, [C(NH2)3][Nd(EDTA)- compounds, the quantum yields were estimated. These pa-
(H2O)3] and Na[Nd(EDTA)(H2O)3]·5H2O was performed. rameters were determined based on the Judd–Ofelt param-
Both ligands, EDTMP and EDTA, coordinate to the Nd3+ eters. The Φ value calculated for II (7.2%) is approximately
ion through two N and four O atoms. The remaining coor- twice as high as that determined for I and is one of the
dination sites are occupied by two O atoms from a biden- highest reported to date for Nd3+ chelates. This result sug-
2–
2–
tate CO3 or tridendate PO3 group in the Nd–EDTMP gests that polymeric coordination compounds have better
complexes (I and II) and three H2O molecules in the Nd– luminescent properties than monomers. It was also shown
EDTA complexes (III and IV). The coordination number that the radiative luminescence lifetime for Nd3+ com-
of the Nd3+ ions in the Nd–EDTMP complexes (CN = 8) pounds with organic ligands depends on the Nd3+ environ-
is one less than that in the Nd–EDTA compounds (CN = ment.
9). The complex anions in I, III and IV are monomeric,
whereas cyclic [Nd4(EDTMP)4]20– tetramers are formed in
II. Inside each tetramer, a hole is formed, which is filled
with water molecules, as well as H+ and K+ cations. The
cations stabilize the highly negative supramolecular units.
Monomeric and tetrameric coordination patterns are
very rare for lanthanide complexes with polyphosphonic li-
Experimental Section
Crystal Syntheses: Crystals of the Nd–EDTMP complexes were
prepared as follows. Equimolar quantities of Nd2O3 (99.9%,
MERCK; 4 mmol) and solid H8EDTMP (95%, ABCR; 8 mmol)
were mixed together and heated at 90 °C (Ϯ5 °C) for ca. 3 d. Next,
the mixtures were alkalized with [C(NH2)3]2CO3 (Aldrich) or KOH
(POCH) to adjust the pH of the solutions to 10.7 or 7.5, respec-
tively. After slow evaporation of these aqueous solutions (ca.
25 mL), violet crystals of [C(NH2)3]7[Nd(EDTMP)(CO3)]·10H2O
(I) and K17H3[Nd4(EDTMP)4]·36H2O (II) were obtained.
2–
gands. Moreover, the coordination of a CO3 anion to a
highly negative lanthanide chelate, such as that found in I,
is also very uncommon. In such chelate compounds, H2O
and/or OH– coligands usually complete the inner spheres of
the metal cations.[12] The bidentate coordination mode of
the carbonate anion in I is caused by a good fit of this ion
to the coordination geometry of the [Nd(EDTMP)]5– unit.
Crystals of the Nd–EDTA complexes were prepared in the follow-
ing way. A stock solution of NdCl3 was obtained by dissolving
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Nd2O3 (99.9%, MERCK) in 2 m HCl. The metal concentration was
determined complexometrically by using xylenol orange as the indi-
cator. To the aqueous solution of NdCl3 (2 mmol), a solution of
H4EDTA (2 mmol) was added. The resulting solution was alkalized
with [C(NH2)3]2CO3 (Aldrich) or NaOH (POCH) to pH 10.5. Dur-
ing slow evaporation of the solution (ca. 25 mL), crystals of
[C(NH2)3][Nd(EDTA)(H2O)3] (III) and Na[Nd(EDTA)(H2O)3]·
5H2O (IV) formed.
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Spectroscopic Measurements: IR spectra of the complexes in KBr
pellets and nujol suspensions were measured in the range 50–
4000 cm–1 with a Bruker FTIR IFS66 spectrometer. Electronic ab-
sorption spectra were recorded with a Cary 500 UV/Vis/NIR spec-
trophotometer equipped with an Oxford Optistat continuous flow
helium cryostat for crystal measurements at 4.2 K. Luminescence
spectra in the NIR region were measured at room temperature with
X-ray Analyses: Suitable single crystals were cut from larger ones,
mounted in a Kuma KM4 diffractometer equipped with a CCD
detector and measured at 100 K (I and II) and room temperature
(III and IV). The structures were solved routinely by using a Pat-
terson synthesis. The C- and N-bonded hydrogen atoms were
placed in positions calculated from the geometry, and most of those
bonded to O atoms were found from difference Fourier maps. The
final refinements were anisotropic for all non-H atoms. The compu-
tations were performed with the SHELXS97[38] and SHELXL97[39]
programs, and the molecular graphics were prepared with XP-In-
teractive Molecular Graphics.[40]
a
Dongwoo (model DM711) scanning monochromator with
750 mm focal length equipped with an InGaAs detector. The Nd3+
ions in the studied crystals were excited by an Ar ion laser op-
erating at λ = 514.5 nm. Luminescence decay curves were recorded
with a Tektronix (model TDS 3052) digital oscilloscope. Excitation
was provided by a continuum optical parametric oscillator (OPO)
pumped by the third harmonic of an YAG:Nd laser (λ = 355 nm).
Calculations: The intensities of f–f transitions were determined in
terms of oscillator strengths (P) by numerical integration of the
corresponding absorption bands by using the formula [Equa-
tion (1)]
Selected Crystal Data for I: C14H74NdN23O25P4, M = 1240.80, mo-
noclinic, space group P21/n, T = 100 K, Z = 4, a = 12.428(4), b =
16.615(6), c = 24.066(9) Å, β = 101.58(3)°, V = 4868(3) Å3, μ =
1.30 mm–1, D = 1.682 g/cm3, F(000) = 2556, crystal size =
0.3ϫ0.4ϫ0.25 mm, θ = 3–28.5°, index ranges: –16 Յ h Յ 12, –21
Յ k Յ 22, –30 Յ l Յ 32, reflections collected/unique = 31261/11207
(Rint = 0.0728). Final R indices [IϾ2σ(I)] R(F) = 0.0420, Rw(F2) =
0.0581 and R(F) = 0.0936, Rw(F2) = 0.0638 (all data), data com-
pleteness to 2θ = 28.5°, 92.1%, largest diff. peak and hole 0.557
and –0.720 eÅ–3.
(1)
in which c is the concentration of the Nd3+ ion in m, d is the length
of the optical path in cm and A(ν) is the absorbance as a function
˜
of the wavenumber ν in cm–1.
˜
The experimental P values were used to calculate the phenomeno-
logical Ωλ intensity parameters according to the Judd–Ofelt rela-
tion [Equation (2)]
Selected Crystal Data for II: C6H30.75K4.25N2NdO21P4, M =
901.37, tetragonal, space group P42, T = 100 K, Z = 4, a =
¯
21.7012(6), b = 12.5981(6), V = 5933.0(4) Å3, μ = 1.30 mm–1, D =
2.018 g/cm3, F(000) = 3596, crystal size = 0.15ϫ0.13ϫ0.2 mm, θ
= 3–28.5°, index ranges: –16 Յ h Յ 12, –21 Յ k Յ 22, –30 Յ l Յ
32, reflections collected/unique = 39254/7147 (Rint = 0.0755). Final
R indices [I Ͼ 2σ (I)] R(F) = 0.0451, Rw(F2) = 0.1183 and R(F) =
0.1211, Rw(F2) = 0.0532 (all data), data completeness to 2θ = 28.5°,
95.71%, largest diff. peak and hole 1.333 and –1.758 eÅ–3.
(2)
in which PED is the oscillator strength of the electric dipole transi-
tion, χ = (n2 + 2)2/9n, n is the refractive index, m is the electron
Selected Crystal Data for III: C11H24NdN5O11, M = 546.64, mono-
clinic, space group P21/c, T = 293 K, Z = 4, a = 11.270(5), b =
mass, c is the speed of light, ν is the wavenumber of the band
˜
8.662(4), c = 18.736(9) Å, β = 90.09(4)°, V = 1829.0(15) Å3, μ = maximum in cm–1, h is the Planck constant, J is the ground state
2.908 mm–1, D = 1.985 g/cm3, F(000) = 1092, crystal size =
quantum number, ͗ψЈJЈʈU(λ)ʈψJ͘ are the reduced matrix elements
0.4ϫ0.17ϫ0.1 mm, θ = 3–28.5°, index ranges: –16 Յ h Յ 12, –21 of the respective unit tensor operator U(λ). The ͗ʈU(λ)ʈ͘2 =U(λ) val-
Յ k Յ 22, –30 Յ l Յ 32, reflections collected/unique = 15796/4472
(Rint = 0.0227). Final R indices [I Ͼ 2σ (I)] R(F) = 0.0299, Rw(F2)
= 0.0819 and R(F) = 0.0829, Rw(F2) = 0.0352 (all data), data com-
pleteness to 2θ = 28.5°, 93.4%, largest diff. peak and hole 1.770
and –1.740 eÅ–3.
ues were calculated and tabulated by Carnall et al.[41]
A measure of the accuracy of the fit intensity parameters was given
by the root mean square deviation (rms) of the experimental and
calculated [from Equation (2)] oscillator strengths by the following
Equation (3)
Selected Crystal Data for IV: C10H28NdN2O16Na, M = 599.63, or-
thorhombic, space group Fdd2, T = 293 K, Z = 4, a = 19.5430(3),
b
= 35.4950(5), c = 12.0585(2) Å, V = μ =
8364.7(2) Å3,
2.584 mm–1, D = 1.904 g/cm3, F(000) = 4816, crystal size =
0.2ϫ0.31ϫ0.16 mm, θ = 3–25°, index ranges: –22 Յ h Յ 23, –42
Յ k Յ 42, –14 Յ l Յ 14, reflections collected/unique = 24708/3679
(Rint = 0.0242). Final R indices [I Ͼ 2σ (I)] R(F) = 0.0197, Rw(F2)
= 0.0521 and R(F) = 0.0524, Rw(F2) = 0.0206 (all data), data com-
pleteness to 2θ = 25.1°, 99.6%, largest diff. peak and hole 1.15 and
–0.70 eÅ–3.
(3)
in which n is the number of absorption bands and 3 is the number
of fitting parameters.
The calculated Ωλ intensity parameters were used to evaluate the
radiative transition rates A(ψJ,ψЈJЈ) for electric dipole transitions
CCDC-920543 (for I), -920541 (for II), -920544 (for III) and between an excited state and the lower lying levels, luminescence
-920542 (for IV) contain the supplementary crystallographic data
branching ratios β(ψJ,ψЈJЈ) and radiative decay time τR from Equa-
for this paper. These data can be obtained free of charge from The
tions (4), (5) and (6)
Eur. J. Inorg. Chem. 2013, 3429–3438
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Received: January 22, 2013
Published Online: May 29, 2013
Eur. J. Inorg. Chem. 2013, 3429–3438
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