Luminescence and structural properties of lanthanide complexes of Schiff bases derived from pyridoxal and amino acids

Article abstract of DOI:10.1039/b706020c

Lanthanide complexes of Schiff bases (SBs) with 1: 1 and 1: 2 (M: Lig) stoichiometric ratios were prepared by condensation of pyridoxal (PL) and aspartic acid (Asp) or l-histidine (His), respectively, in the presence of the appropriate metal chloride as a templating agent. These complexes were studied by optical spectroscopy and single crystal X-ray diffraction techniques. Crystallographic studies of 1: 1 ([Eu(PL-Asp)(H₂O) ₄](H₂O)) and 1: 2 ([Eu(PL-His)₂(H ₂O)₂]Cl(H₂O)₄) complexes show that Eu^III is eight-coordinate in both structures, in a distorted square antiprism environment formed by the phenolic oxygen of PL, the nitrogen atom of carbon-nitrogen double bond, oxygen atoms of the carboxylate groups of His or Asp, and oxygen atoms of the water molecules. The main species formed in aqueous solutions containing these SBs have been determined by analysis of the luminescence spectra, lifetimes of Eu^III excited states and vibronic interaction as well as structural features of the Eu^III coordination sphere. Possible tetradentate coordination function of SBs in aqueous solutions with relatively high concentrations as well as the potential application of the lanthanide SB complexes as new luminescence materials are discussed. The Royal Society of Chemistry.

Full text of DOI:10.1039/b706020c

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www.rsc.org/dalton | Dalton Transactions
Luminescence and structural properties of lanthanide complexes of Schiff
bases derived from pyridoxal and amino acids†‡
Lada Puntus,*a Konstantin Zhuravlev,a Konstantin Lyssenko,b Mikhail Antipinb and Irina Pekarevaa
Received 23rd April 2007, Accepted 2nd July 2007
First published as an Advance Article on the web 30th July 2007
DOI: 10.1039/b706020c
Lanthanide complexes of Schiff bases (SBs) with 1 : 1 and 1 : 2 (M : Lig) stoichiometric ratios were
prepared by condensation of pyridoxal (PL) and aspartic acid (Asp) or L-histidine (His), respectively, in
the presence of the appropriate metal chloride as a templating agent. These complexes were studied by
optical spectroscopy and single crystal X-ray diffraction techniques. Crystallographic studies of 1 : 1
([Eu(PL-Asp)(H2O)4](H2O)) and 1 : 2 ([Eu(PL-His)2(H2O)2]Cl(H2O)4) complexes show that EuIII is
eight-coordinate in both structures, in a distorted square antiprism environment formed by the phenolic
oxygen of PL, the nitrogen atom of carbon–nitrogen double bond, oxygen atoms of the carboxylate
groups of His or Asp, and oxygen atoms of the water molecules. The main species formed in aqueous
solutions containing these SBs have been determined by analysis of the luminescence spectra, lifetimes
of EuIII excited states and vibronic interaction as well as structural features of the EuIII coordination
sphere. Possible tetradentate coordination function of SBs in aqueous solutions with relatively high
concentrations as well as the potential application of the lanthanide SB complexes as new luminescence
materials are discussed.
including macrocyclic SBs,3 anhydrous SBs,4 SB complexes with
Introduction
liquid crystal properties5 etc. However, in the case of tetradentate
SBs, many of which have a −2 charge (for example, H2salen6
Lanthanide complexes of Schiff bases derived from pyridoxal and
intensively investigated),7 LnIII ions usually form complexes with
the formula Ln2L3 and exhibit less quantum yield in comparison
with LnL2 complexes.2 As a result Schiff bases derived from
pyridoxal and amino acids (Asp and His) were chosen as potential
tetradentate ligands containing chromophore groups and the
ability to form one centered lanthanide complexes.
amino acids are very attractive subjects for investigation due to
two aspects of their potential applications, namely luminescent
and biological ones. In view of the former, the luminescence of
lanthanide coordination compounds, supramolecules and poly-
mers has been widely investigated because of the strong emission
important for luminescent and laser materials.1 Owing to the
structural features of Schiff bases (SBs) their lanthanide complexes
have some advantages for luminescence research.2 First, the SB
ligands can be tetradentate which greatly stabilizes the complex
in solution relative to bi- and tridentate ligands and strongly
minimizes the probability of solvent molecule coordination (that is
especially important for water molecules) for 1 : 2 complexes which
results from the predisposition of lanthanide ions towards high
coordination numbers (CN = 8–9). Second, the high extinction
coefficients of many SB ligands in the near ultraviolet-visible range
provide effective absorption of excitation energy which promotes
more effective energy transfer to the lanthanide ion. Therefore
numerous types of lanthanide SB complexes have been prepared
A second aspect of interest concerns the biological relevance of
compounds containing metal ions and amino acids. Schiff bases
of pyridoxal 5ꢀ-phosphate (PLP) and its analogues with amino
acids have been extensively studied because of their important
biological role in the metabolism of amino acids.8 Research con-
cerning lanthanide complexes of SBs with amino acids also sheds
light on the coordination function of amino acid radicals. The
precipitation of lanthanide ions at alkali pH prevents investigation
of the coordination of amino acids whereas in complexes with
SBs lanthanide ions do not hydrolyze until pH = 10.9 Despite
the large interest in metal ion containing model systems which
reproduce some of the transformations of amino acids effected
by pyridoxal-dependent enzymes, most of the studies have been
carried out in solution, and only a limited number of Schiff
base chelates related to pyridoxal catalysis have been isolated.
Basically, the majority of studies reported so far focused either on
SBs derived from pyridoxal or on their metal complexes. There
are many papers describing crystal structures of pyridoxal SB
complexes with copper(II)10 and cobalt(III)11 while there are none
for analogous complexes with lanthanide ions according to the
best of our knowledge. On the other hand Zolin et al. have
found that complexes of the Schiff base derived from pyridoxal
and aspartic acid can be used as lanthanide shift reagents (LSR)
for investigation of derivatives of amino acids and nucleotides
in aqueous solutions.12 However the study of the luminescence
aLaboratory of Molecular Nanoelectronics, Institute of Radio Engineering
& Electronics, Russian Academy of Sciences, 11-7 Mokhovaya, Moscow,
125009, Russia. E-mail: lada_puntus@mail.ru; Fax: +7 495 2038414;
Tel: +7 916 0157310
bA. N. Nesmeyanov Institute of Organoelement Compounds, Russian
Academy of Sciences, GSP-1, 28 Vavilov Street, Moscow, 119991, Russia.
E-mail: kostya@xray.ineos.ac.ru; Fax: +7 495 1359212; Tel: +7 495
1359214
† CCDC reference numbers 644340 and 644341. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b706020c
‡ Electronic supplementary information (ESI) available: Additional tables
and pictures of X-ray data as well as luminescence data including the
crystal-field splitting of the EuIII spin–orbit levels in 1 and 2 complexes.
See DOI: 10.1039/b706020c
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properties of this system is incomplete and further work in
the LSR field was not performed due to the lack of structural
characterization. The absence of information on Schiff bases of L-
histidine, notwithstanding the biological importance of this amino
acid as a metal binding site, was noted in 1981.13
In view of this, we report here two europium complexes
([Eu(PL-Asp)(H2O)4](H2O) (1) and [Eu(PL-His)2(H2O)2]Cl(H2O)4
(2)) synthesized and structurally characterized by spectroscopic
and single crystal X-ray diffraction techniques. Combined analysis
of the luminescence, phosphorescence and vibronic spectra has
been used to describe the photophysical properties of the europium
SB complexes. The differences between the structural properties
of the complexes considered in the solid state and in solution are
discussed based on the analysis of lifetimes of EuIII excited states
and luminescence spectra.
Scheme 1 Schematic presentation of Schiff bases used.
Results and discussion
are listed in Table 2. Although complex 1 is homochiral it
crystallizes in a racemic space group (C2/c). The nitrogen atom
Synthesis and structural characterisation
=
N(2) of the C N bond as well as carbon atoms C(8), C(9), C(10)
Lanthanide complexes of Schiff bases with 1 : 1 and 1 : 2
(Ln : Lig) stoichiometric ratios (Ln = La, Eu, and Gd) were
prepared by the typical condensation reaction of pyridoxal (PL)
and racemic aspartic acid (Asp) or L-enantiomeric histidine (His),
respectively, in the presence of the appropriate metal chloride as
a templating agent (Scheme 1).14 Single crystals of the [Eu(PL-
Asp)(H2O)4](H2O) (1) and [Eu(PL-His)2(H2O)2]Cl(H2O)4 (2) com-
plexes obtained were studied by single crystal X-ray diffraction
methods (Table 1).
of the dicarboxylate moiety containing the chiral center C(9)
are disordered by two positions (Fig. S1, ESI‡). Such disorder
corresponds to superposition of two enantiomers in a ratio of
0.802(6) : 0.198(6). The total ratio of enantiomers is 1 : 1 due
to centrosymmetric packing in the crystal despite enantiomer
excess in the independent part. The europium ion possesses a
distorted square antiprism geometry and is coordinated to the
=
phenolic oxygen atom O(1), the nitrogen atom of the C N double
bond N(2), two oxygen atoms of the carboxylate groups O(5)
and O(3) and four oxygen atoms of water molecules (O(4w),
O(2w), O(3w), O(1w)). One of the antiprism bases is formed
The general view of complex 1 along with the atom numbering
scheme is shown in Fig. 1. Selected bond lengths and angles
Table 1 Summary of crystallographic data and experimental details for [Eu(PL-Asp)(H2O)4](H2O) (1) and [Eu(PL-His)2(H2O)2]Cl(H2O)4 (2) complexes†
Complex 1
Complex 2
Empirical formula
Formula weight
Crystal colour, habit
Crystal size/mm
Temperature/K
Crystal system
C12H21EuN2O11
521.27
Yellow, prism
0.14 × 0.16 × 0.23
100(2)
Monoclinic
C2/c
15.418(1)
8.4575(5)
26.016(2)
90.6985(12)
3392.1(4)
8(1)
C28H42ClEuN8O14
902.11
Yellow, plate
0.04 × 0.08 × 0.11
100(2)
Orthorhombic
C2221
7.268(2)
20.816(5)
23.403(6)
90
3540.7(16)
4(0.5)
Space group
˚
a/A
˚
b/A
˚
c/A
b/◦
3
˚
V/A
Z (Zꢀ)
F(000)
2064
2.041
37.61
1832
1.692
19.25
D
calc/g cm−1
Linear absorption, l/cm−1
T
min/Tmax
0.4783/0.6210
x (8 s exposure per frame)
2.64–29.0
99.9
0.8162/0.9270
x (50 s exposure per frame)
1.57–27.00
100.0
Scan type
h range/◦
Completeness of dataset (%)
Reflections measured
Independent reflections
Observed reflections [I > 2r(I)]
Parameters
15 841
15 989
4496 [Rint = 0.0309]
3859 [Rint = 0.1654]
3965
2833
263
236
Final R(Fhkl) : R1
0.0272
0.0587
1.063
1.019, −0.997
0.0626
0.1268
1.041
1.158, −1.508
wR2
GOF
−3
˚
Dqmax, Dqmin/e A
4080 | Dalton Trans., 2007, 4079–4088
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◦
˚
Table 2 Selected bond lengths (A) and bond angles ( ) in the crystal of [Eu(PL-Asp)(H2O)4](H2O) (1)
Eu(1)–O(1)
Eu(1)–O(3)
Eu(1)–O(5)
Eu(1)–N(2)
2.259(2)
2.389(2)
2.416(2)
2.504(3)
Eu(1)–O(1W)
Eu(1)–O(2W)
Eu(1)–O(3W)
Eu(1)–O(4W)
2.459(2)
2.413(2)
2.416(2)
2.443(2)
O(1)–Eu(1)–O(3)
135.09(8)
82.45(8)
138.44(8)
103.44(8)
78.10(9)
75.34(8)
153.74(8)
70.03(7)
77.51(8)
87.81(8)
88.26(8)
115.06(8)
75.75(8)
146.95(8)
O(3W)–Eu(1)–O(4W)
O(1)–Eu(1)–O(1W)
O(3)–Eu(1)–O(1W)
O(2W)–Eu(1)–O(1W)
O(5)–Eu(1)–O(1W)
O(3W)–Eu(1)–O(1W)
O(4W)–Eu(1)–O(1W)
O(1)–Eu(1)–N(2)
O(3)–Eu(1)–N(2)
O(2W)–Eu(1)–N(2)
O(5)–Eu(1)–N(2)
O(3W)–Eu(1)–N(2)
O(4W)–Eu(1)–N(2)
O(1W)–Eu(1)–N(2)
70.55(8)
80.68(8)
72.27(8)
143.56(8)
140.15(8)
105.92(8)
71.69(8)
O(1)–Eu(1)–O(2W)
O(3)–Eu(1)–O(2W)
O(1)–Eu(1)–O(5)
O(3)–Eu(1)–O(5)
O(2W)–Eu(1)–O(5)
O(1)–Eu(1)–O(3W)
O(3)–Eu(1)–O(3W)
O(2W)–Eu(1)–O(3W)
O(5)–Eu(1)–O(3W)
O(1)–Eu(1)–O(4W)
O(3)–Eu(1)–O(4W)
O(2W)–Eu(1)–O(4W)
O(5)–Eu(1)–O(4W)
69.33(9)
71.73(10)
121.62(10)
63.97(9)
136.22(9)
147.65(9)
81.58(10)
Fig. 1 The general view of [Eu(PL-Asp)(H2O)4](H2O) (1) with atoms
represented by thermal ellipsoids (p = 50%). The disorder is omitted for
clarity.
Fig. 2 The general view of [Eu(PL-His)2(H2O)2]Cl(H2O)4 (2) with atoms
represented by thermal ellipsoids (p = 50%).
by three water molecules (O(1w), O(3w) and O(4w)) and the
carboxylate oxygen O(3) while the second is composed of O(1),
O(2w), O(5) and N(2) atoms (Fig. S2, ESI‡). The coordination
number of the europium ion is eight. The distortion of the EuIII
coordination polyhedron in the above enantiomers is different.
For the predominant component the mean deviation for the two
=
C N double bond, the oxygen atom O(1) of pyridoxal and the
oxygen atom O(1w) of water. The europium coordination number
is eight. The symmetrically related antiprism bases are composed
of O(1), O(1w), O(3) and N(2) atoms (Fig. S2, ESI‡). The mean
displacement of atoms from the antipri◦sm base and the dihedral
˚
antiprism bases are 0.08 and 0.11 A with a dihedral angle between
them equal to 1◦. For another component the mean deviation
˚
˚
of atoms for the O(1)–O(2w)–O(5)–N(2) base is ∼0.25 A and
angle between bases are 0.10 A and 4.9 , respectively. The Eu–O
the corresponding dihedral angle is 6.2◦. The Eu–N(2) bond
bond lengths of PL-His vary in the interval 2.316(7)–2.377(7) A,
˚
˚
˚
lengths are equal to 2.504(3) and 2.545(13) A for two components
the Eu–O(1w) bond length is 2.417(6) A, and the Eu–N(2) bond
˚
of the disordered group. The Eu–O bond lengths of PL-Asp
length is 2.562(8) A. The pyridine rings of PL-His ligands are
vary in the range of 2.259(2)–2.416(2) A and the Eu–O bonds
almost parallel to each other (the dihedral angle is 4.2◦) with the
˚
of the coordinated water molecules are systematically elongated
shortest contacts C(2) · · · C(2A) and N(1) · · · N(1A) equal to 3.03
˚
˚
(2.413(2)–2.459(2) A).
and 3.24 A, respectively. Such disposition as well as shortened
The general view of the cation complex 2 along with the atom-
numbering scheme is shown in Fig. 2. Selected bond lengths
and angles are listed in Table 3. Complex 2 crystallizes in the
homochiral space group C2221 (the europium ion occupies the
special position—two-fold axis). The coordination geometry can
be described as a distorted square antiprism similar to complex
1. The EuIII is coordinated by two PL-His ligands, the oxygen
atom O(3) of the carboxylate group, the nitrogen atom N(2) of the
distances between rings serves as direct evidence of a rather strong
intramolecular p-stacking interaction.15
In summarizing the features of the molecular structures of
complexes 1 and 2 it is worth emphasizing that the bonds formed
by europium are longer in complex 2 probably owing to steric
hindrances. However, the formal charge of the ligands in the
complexes are principally different. In complex 1 the PL-Asp
ligand is characterized by a formal charge of −3 formed by
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◦
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Table 3 Selected bond lengths (A) and bond angles ( ) in the crystal of [Eu(PL-His)2(H2O)2]Cl(H2O)4 (2)
Eu(1)–O(1)
Eu(1)–O(3)
2.316(7)
2.377(7)
Eu(1)–O(1W)
Eu(1)–N(2)
2.417(6)
2.562(8)
O(1A)–Eu(1)–O(1)
O(1A)–Eu(1)–O(3Aa)
O(1)–Eu(1)–O(3A)
O(1)–Eu(1)–O(3)
O(3A)–Eu(1)–O(3)
O(1A)–Eu(1)–O(1WA)
O(1)–Eu(1)–O(1WA)
O(3)–Eu(1)–O(1WA)
O(3)–Eu(1)–O(1W)
O(1W)–Eu(1)–O(1WA)
78.7(3)
113.5(2)
141.0(2)
113.5(2)
81.0(3)
69.8(2)
137.3(2)
78.3(2)
O(1A)–Eu(1)–N(2)
O(1)–Eu(1)–N(2)
O(3)–Eu(1)–N(2)
O(1WA)–Eu(1)–N(2)
O(1W)–Eu(1)–N(2)
O(3)–Eu(1)–N(2A)
O(1WA)–Eu(1)–N(2A)
O(1W)–Eu(1)–N(2A)
N(2)–Eu(1)–N(2A)
87.4(2)
70.2(2)
65.0(3)
80.1(2)
107.3(2)
143.2(3)
107.3(2)
80.1(2)
151.2(4)
79.6(2)
150.9(3)
a Atoms with the label A are obtained from base one by the symmetry operation −x + 1, y, −z + 3/2.
the deprotonated hydroxyl group of PL and two deprotonated
carboxylate groups. In complex 2 both PL-His ligands are
characterized by a formal charge of −1 owing to the deprotonated
hydroxyl group of PL, deprotonated carboxylate group of His
and the protonated N(1) atom of pyridoxal ring. The latter
is unambiguously reflected in the pronounced increase of the
C(1)–N(1)–C(5) angle from 118.7(2) to 125.6(1.5)◦. The above
increase of the C–N–C angle upon protonation agrees well with
available structural data for similar complexes of transition metals.
Indeed according to the Cambridge Structural Database (Ver.
5.28, November 2006) in 34 complexes with protonated pyridoxal
ring the value of the corresponding C–N–C angle varies in the
range of 123.4–125.4◦. Hence complex 1 is neutral while complex 2
is a cation with a chloride as the counterion owing to the difference
in the charges of the coordinated ligand. The chloride anions
in crystal of complex 2 form H-bonds with two solvate water
˚
molecules (O · · · Cl separation varies in the range of 2.88–3.02 A)
and all cation · · · anion contacts are rather long, in particular the
˚
shortest Cl(1) · · · Eu(1) contact is as large as 7.268(5) A.
The crystal packing in both complexes is related to the presence
of hydrogen bonds and p–p interactions. Molecules of complex
1 in the crystal are assembled into centrosymmetric dimers
(Fig. 3). These pairs are held together by intermolecular stacking
interactions of moderate strength between the pyridoxal ring and
Fig. 3 The 3-dimensional H-bonded framework in the crystal of
[Eu(PL-Asp)(H2O)4](H2O) (1).
=
the C N double bond of the coordinated PL-Asp ligands with
the shortest C · · · C and C · · · N contacts equal to 3.33–3.48 and
˚
∼3.50 A, respectively. In addition these dimers are assembled into
3-dimensional frameworks by numerous O–H · · · O (O · · · O 2.68–
˚
˚
2.90 A) and O–H · · · N (O · · · N 2.77 A) hydrogen bonds.
The cation complexes 2 in the crystal are assembled into infinite
chains by intermolecular stacking interactions of strength similar
to the above mentioned intramolecular stacking interactions (the
shortest contacts C(3) · · · C(3) (−x, y, 3/2 − z) and C(5) · · · C(5)
˚
(−x, y, 3/2 − z) amount to 3.31 and 3.23 A, respectively) but
the type of cycle overlap is different in principle. Moreover the
O–H · · · N hydrogen bonds formed by water molecules coordi-
nated with the nitrogen atom of histidine can be anticipated
Fig. 4 Cationic chains interlinked by p–p stacking interactions and
O–H · · · N bonds in crystal of [Eu(PL-His)2(H2O)2]Cl(H2O)4 (2).
˚
(N(4) · · · O(3w) 2.79 A). The said chains are interlinked into the
cationic layers by rather strong CO2 · · · H–N(1) hydrogen bonds
supramolecular framework by means of N–H · · · O and O–H · · · O
hydrogen bonds. Thus the supramolecular organization in the
crystal of complex 2 formed by inter- and intramolecular stacking
interactions can be described as separated cationic layers with
anionic (H2O) · · · Cl · · · (H2O) filling (Fig. 5).
˚
(N(1) · · · O(4) is 2.682(2) A), Fig. 4. These layers are parallel to the
crystallographic bc plane and thus are perpendicular to the polar
axis 21. The solvate water molecules and (H2O) · · · Cl · · · (H2O)
anionic aggregates assemble the layers into a 3-dimensional
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Fig. 5 The crystal packing of [Eu(PL-His)2(H2O)2]Cl(H2O)4 (2) illustrat-
ing alternation of cationic layers and anionic filling.
Fig. 6 Luminescence spectra of [Eu(PL-Asp)(H2O)4](H2O) (1) (a) and
[Eu(PL-His)2(H2O)2]Cl(H2O)4 (2) (b) recorded by a low resolution spec-
trometer at 77 K.
Luminescence studies of 1 and 2 complexes
The europium complexes of Schiff bases investigated exhibit the
characteristic luminescence spectrum of the EuIII ion at both low
and ambient temperatures. Two different types of luminescence
7
7
7
general splitting of F1, F2 and F4 levels amounts to 260, 250
and 405 cm−1 and 125, 150 and 290 cm−1 for complexes 1 and
2, respectively. These values are determined by the asymmetric
charge environment of EuIII in the former complex, where two of
the three charges belong to one base of the square antiprism,
and the more symmetric charge environment of EuIII in the
latter complex (three charges belong to one base of square
antiprism and the other three charges to the other base). Such
large general splittings are also evidence that the effect of JJꢀ-
mixing has to be taken into account upon the analysis of Stark
components.17 The mono-exponential luminescence decay of the
Eu(5D0) level obtained for 1 and 2 complexes reveals the equivalent
europium environments in both complexes (Table 5). The s values
7
spectra were obtained (Fig. 6). Stark components of the 5D0 → FJ
transitions (J = 0–4) in the luminescence spectra of the europium
complexes recorded at 77 K under broad-band excitation point
to low site symmetry of EuIII. Judging from the ratio of the
5
7
integrated intensities of the D0 → FJ (J = 0, 2–6) transitions
to the 5D0
→
7F1 transition (Table 4) the site symmetry of
EuIII is slightly nearer to the inversion centre for 1 than for 2.
Intensity of the 5D0 → F2 transition, dependent on T2 (Judd–Ofelt
7
intensity parameter)16 and sensitive to the structure of lanthanide
surroundings, is 1.6 times higher for complex 2 which indicates
the lower symmetry of complex 2 compared to complex 1. A
Table 4 Relative integral intensity of 5D0–FJ (J = 0, 2–6) electronic transitions in luminescence spectra of compounds investigated upon ligand excitation
at 77 K (kexc = 365 nm)
Compounds
5D0–7F0
5D0– 7F2
5D0–7F3
5D0–7F4
5D0–7F5
5D0–7F6
[Eu(PL-Asp)(H2O)4](H2O) (1)
[Eu(PL-His)2(H2O)2]Cl(H2O)4 (2)
[Eu + (PL-Asp)] (solution s1)
[Eu + (PL-His)2] (solution s2)
0.55
0.18
0.26
0.20
5.45
8.63
6.44
6.38
0.29
0.36
0.28
0.23
3.02
5.13
2.98
2.90
0.25
0.23
0.14
0.16
0.76
1.07
0.55
0.51
Table 5 Position of 5D0–7F0 electronic transition in the luminescence spectra and lifetime (s) of the metal-centered luminescence upon ligand excitation
at 77 K (kexc = 365 nm)
Compounds
5D0–7F0/cm−1
s/ms
[Eu(PL-Asp)(H2O)4](H2O)(1)
[Eu(PL-His)2(H2O)2]Cl(H2O)4 (2)
17 259
17 212
0.22 0.02
0.75 0.03
Solutions, concentration/M
[Eu + (PL-Asp)], 3.5 × 10−3 (s1)
[Eu + (PL-Asp)], 3.5 × 10−4
[Eu + (PL-His)], 7.6 × 10−3
[Eu + (PL-His)2], 9.7 × 10−3 (s2)
17 224
17 247
17 250
17 215
0.83 0.03
0.38 0.02 (80%) 0.83 0.03 (20%)
0.33 0.02
0.42 0.02 (50%) 0.88 0.03 (50%)
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Table 6 Vibronic satellites observed in the luminescence spectra of
[Eu(PL-Asp)(H2O)4](H2O) (1) and [Eu(PL-His)2(H2O)2]Cl(H2O)4 (2) at
77 K
(0.22 0.02 and 0.75 0.03 ms) are well adjusted with four and
two water molecules coordinated, respectively.
Parts of the high resolution luminescence spectrum of complex
1 in the region of europium electronic transitions and the crystal-
field splitting of the EuIII spin–orbit levels are presented in Fig. 7
and Table S1 (ESI‡), respectively. The asymmetric two component
Frequencies for
Frequencies for
Assignment
complex 1/cm−1
complex 2/cm−1
m(M–Lig)
d(COO−)
d(CH) ring
m(C–OH)
ms(COO−)
—
745
—
—
1323, 1370
1425, 1575
1654
180
5
7
peak corresponding to the D0 → F0 transition along with the
570, 670, 740
1220
1280
1320
1435, 1585
1652
7
broadened first component of the 5D0 → F1 transition and excess
7
number of Stark components of the 5D0 → F4 transition reveal the
presence of nonequivalent luminescence centers. According to X-
=
=
m(C C, C N)
ray data the structure of complex 1 is disordered by two positions
mas(COO−)
=
one of which is the nitrogen atom of the C N bond (Fig. S1, ESI‡).
The length of the Eu–N bond is different in these enantiomers
and as a result two luminescence centers were observed. High
resolution luminescence data obtained for complex 2 revealed one
luminescence center in line with the X-ray experiment and were
used for obtaining the crystal-field splitting of the EuIII spin–orbit
levels (Fig. S1 and Table S1, respectively, ESI‡).
vibration with frequency ∼1370 cm−1 in the vibronic sideband
5
7
of the D0 → F0 transition (17 050–15 000 cm−1 region) could
be assigned to the stretching symmetrical vibration of another
carboxylate group taking into account that the PL-Asp ligand
contains two carboxylate groups having Eu–O bonds of different
length (Table 2).
5
7
5
7
Vibronic satellites of D0 → F0 and D0 → F2 transitions
found in the luminescence spectrum of complex 2 at 77 K in the
regions 17 050–15 000 and 15 100–14 600 cm−1, respectively, are
presented in Table 6 and Fig. S4 (ESI‡). Judging from the higher
number of vibronic satellites observed in this spectrum than in
the luminescence spectrum of complex 1 as well as from their
intensities, the vibronic interaction is remarkably stronger in the
former. This can be conditioned by the presence of intraligand
charge transfer originating with two negative oxygen atoms and
one protonated nitrogen atom of PL. The vibronic satellites found
indicate the monodentate coordination function of the carboxylate
groups similar to complex 1 as well as the coordination of
pyridoxal ring through the hydroxyl oxygen atom (m(C–OH) is
∼1280 cm−1). The appearance of three frequencies attributed to
d(COO−) correlates with the presence of the shortest bond between
the EuIII ion and the carboxylate group in complex 2 (2.377(7) and
˚
2.389(2)–2.417(3) A for 2 and 1 complexes, respectively).
In order to both obtain additional information on excited
states of the systems studied and to estimate the potential of
their luminescent applications the phosphorescence spectra of the
lanthanum complexes were also considered. It is well-known that
the energy transfer from ligand excited states to the resonance
states of LnIII in complexes can occur in different ways. The favored
simplified mechanism involves ligand excitation by the absorption
of ultraviolet energy to an excited singlet state, followed by energy
migration via nonradiactive intersystem crossing to a ligand triplet
state (E1 = E(S1–T)) and energy transfer from the triplet state to a
resonance state of LnIII ion, from which the emission occurs (E2 =
E(T)–E(5D0). Therefore there is no doubt that the singlet and
triplet states of the ligand play an important role in the effective
energy transfer in lanthanide systems.20
The triplet states of sodium salts of PL-Asp and PL-His are
located at 19 200 and 22 400 cm−1 at 77 K, respectively (Table 7).
The triplet energy state is affected by complexation in the LaIII
complexes and sustains a blue shift in 1 (+2100 cm−1) and a red
shift in 2 (−1500 cm−1) up to 21 300 and 20 900 cm−1, respectively.
The relatively low energy of the triplet state of the former sodium
salt and uncommon blue shift of this state upon complexation may
be explained by complicated supramolecular assemblies formed
by PL-Asp anions. The singlet state emission was observed at
Fig. 7 Parts of high resolution luminescence spectrum of [Eu(PL-Asp)-
(H2O)4](H2O) (1) in the region of europium electronic transitions at 77 K.
Vibronic interaction. Since vibronic satellites are produced by
a dependence of the 4f wavefunctions on the vibrational motions
of the bound ligands they can reveal the features of the lanthanide
coordination sphere and allow for the estimation of interaction
intensity of a lanthanide ion with ligands. IR spectra published
previously for PL-Asp14 and PL-His13 were used to assign the
satellites observed. A row of vibronic satellites associated with
7
the 5D0 → F0,2 transitions appears in the luminescence spectrum
of complex 1 at 77 K (Table 6). The stretching symmetrical
(1323 cm−1) and asymmetrical (1654 cm−1) vibrations of car-
boxylate groups are observed in the vibronic sideband of the
7
5D0 → F2 transition of the EuIII ion (15 100–14 600 cm−1 region).
Their splitting amounts to ∼320 cm−1 and corresponds to the
monodentate coordination function of carboxylate groups.18,19 The
4084 | Dalton Trans., 2007, 4079–4088
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Table 7 Energy of singlet and triplet states upon ligand excitation (kexc
365 nm)
=
S1/cm−1
T/cm−1
a
a
a
a
[PL-Asp], ss
[La(PL-Asp)], ss
[PL-His], ss
[La(PL-His)2], ss
[La(PL-Asp)], sol
[La(PL-His)2], sol
19 200
21 300
22 400
20 900
20 200
22 800
23 700
24 200
a State was not detected; ss = solid state; sol = solution.
300 K as a weak shoulder for all solid state compounds and so
the energy of this state could not be determined unambiguously.
Contrary to the solid state case the 1pp* state was detected
in aqueous solutions of the lanthanum SBs with PL-Asp and
PL-His (Table 7). The energy gap E1 is relatively small (about
1400–1830 cm−1) which points to ineffective intersystem crossing
processes. To estimate the luminescence efficiency for europium
complexes the energy gap E2 was also calculated on the basis of
the values obtained. The E2 amounts to ∼4100 and 3700 cm−1 for 1
and 2 complexes, respectively. These values are near to optimal for
the high luminescence intensity of europium complexes according
to published data21 with some advantage for complex 2. The
absence of any residual luminescence of excited ligand states for
both complexes is evidence of the effective energy transfer from
excited ligand states to EuIII resonance levels and is in the line with
this estimation.
Fig. 8 Luminescence spectra of water solutions of PL-His with two (9.7 ×
10−3 M) (a) and one (7.6 × 10−3 M) (c) molar equivalents of europium
chloride as well as solutions of PL-Asp with one molar equivalent of
europium chloride at 3.5 × 10−3 (b) and 3.5 × 10−4 M (d) at 77 K.
(iii) the position of the D0 → F0 transition at ∼17 250 cm−1.
Analysis of the latter can be useful upon the description of the
europium coordination sphere owing to the correlation between
5
7
the frequency of the 5D0
the derived nephelauxetic parameters of the coordinating atoms
obtained by Frey and Horrocks.23 Based on this correlation
the frequency shift of the 5D0
→
7F0 transition and the sum of
Luminescence studies of SB complexes in solution
Since the SB complexes studied are soluble in water their
photophysical properties were also investigated in solution. It
is worth noting that this luminescence analysis sheds light on
some features of the main EuIII species only although various
species formed in solution. The luminescence decay used for this
analysis is sufficiently informative22 except in the case where the
species formation had a very short lifetime of the excited state. The
emission spectra of water frozen solutions of europium chloride
containing one or two molar equivalents of PL-Asp (s1) or PL-His
(s2), respectively, with concentrations similar to those used in the
synthesis are given in Fig. 8. Stark splittings of EuIII electronic
transitions in both solutions are quite similar and remarkably
different from the corresponding values for complexes 1 and 2.
The values of relative integrated intensities of EuIII electronic
transitions for these solutions are also similar (Table 4). The EuIII
site symmetry is slightly nearer to the inversion centre in these
solutions than in the corresponding solid state complexes.
The luminescence decay obtained for the Eu(5D0) level in
the solution s2 (∼9.7 × 10−3 M) is a bi-exponential function
and corresponding lifetimes amount to 0.42 0.02 and 0.88
0.03 ms. This indicates the presence of at least two types of EuIII
chemical environments. To identify these species the luminescence
spectrum of the aqueous solution containing europium chloride
with one molar equivalent of PL-His (∼7.6 × 10−3 M) was
recorded (Fig. 8c). This spectrum is characterized by: (i) another
type of Stark splitting in comparison with solution s2; (ii) the
mono-exponential luminescence decay of the Eu(5D0) level with
→
7F0 transition from 172 15
to 17 250 cm−1 could be explained by the presence of both the
1 : 2 and 1 : 1 species in solutions containing two and one molar
equivalent of PL-His, respectively. These data allow the formation
of the 1 : 1 species in solution s2 to be excluded at least in a
quantity detectable by the luminescence spectroscopy. In view of
this the bi-exponential luminescence decay observed in solution
s2 seems to be determined by the distinctions in the coordination
sphere of the EuIII 1 : 2 species. The longer lifetime probably
corresponds to a EuIII environment devoid of O–H vibrations or
with one water molecule coordinated, whereas the shorter lifetime
corresponds to a metal ion bound to several water molecules.22,24
Taking into account these types of EuIII environment and its high
coordination number one can surmise the coordination of the
nitrogen atom of His. Similar coordination of His was found in
several cases.25 Hence PL-His is a tetradentate ligand in solution s2.
The luminescence spectrum corresponding to the 1 : 1 species was
not observed even upon the 30-fold dilution of solution s2. This
behaviour is probably determined by the intense intramolecular
p–p stacking which can remarkably increase the stability constant
of the 1 : 2 species26 (cf. crystallographic section) and/or by the
tetradentate coordination function of PL-His.
The luminescence spectrum of water solution s2 contains a
limited number of vibronic satellites at 77 K with: (i) frequency
700 cm−1 in the range 16 640–16 540 cm−1 assigned to d(COO−),
(ii) frequency 1390 cm−1 in the range 15 840–15 850 cm−1 assigned
to ms(COO−), and (iii) frequency 1410 cm−1 assigned to stretching
vibration of imidazolium ring as vibronic repetition of the
the corresponding lifetime amounting to 0.33
0.02 ms; and
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5D0–7F2 transition in the range 15 000-14 750 cm−1. The latter
vibronic satellites are evidence in favour of the coordination of
the His ring by the EuIII ion. This coordination leads to weakened
bonds between EuIII and ligands due to steric hindrances and as
a consequence the vibronic interaction decreases. The vibronic
satellites found also confirm the preservation of the coordination
of carboxylate groups upon the transition from solid state to
solution.
and the possibility of tetradentate coordination of both SBs lead
to similar luminescence spectra for s1 and s2 solutions.
Conclusions
Two europium SB complexes derived from pyridoxal and
amino acids, viz Eu(PL-Asp)(H2O)4](H2O) (1) and [Eu(PL-
His)2(H2O)2]Cl(H2O)4 (2), were characterized by single crystal
X-ray techniques as the first case of lanthanide SBs derived from
pyridoxal. The europium polyhedron in complexes 1 and 2 is
an antiprism with a considerable degree of distortion due to
the different nature of bonded atoms and bond length variation.
Disorder in the crystal of complex 1 was found by X-ray diffraction
and the difference between the coordination centres was apparent
Let us to return to the question of similarity of the luminescence
spectra of s1 and s2 solutions taking into account that PL-His is a
tetradentate ligand and forms mainly EuIII 1 : 2 species in solution
s2. The emission spectrum of the solution s1 (3.5 × 10−3 M)
(Fig. 8b) is quite different when compared to the luminescence of
7
the complex 1: (i) the position of the 5D0 → F0 transition is shifted
from 17 259 to 17 224 cm−1 upon the transition from complex 1 to
˚
mainly in the Eu–N bond lengths (2.504(3) and 2.536(9) A). High
7
solution s1, (ii) the relative intensity of the 5D0 → F0 transition is
resolution luminescence experiments for this complex also showed
the presence of two nonequivalent luminescence centers owing to
high efficiency of EuIII ion as a luminescence probe for the chemical
environment29 and allowed a scheme of EuIII electronic levels for
both systems considered to be obtained.
twice as small in solution, and (iii) Stark components of electronic
transitions are very different. In addition a single exponential
decay is observed for solution s1 and the corresponding lifetime
amounts to 0.83
0.03 ms (Table 5). The latter is unlikely for
the europium 1 : 1 species with a tetradentate ligand due to its
high CN. These data can be explained by an assumption about
the formation of a 1 : 2 complex [Eu(PL-Asp)2]3− as the main
species detected in solution s1. The residual europium chloride
could not be registered as another species in this case owing to
the short luminescence decay; s for europium chloride amounts
In the solid state, complex 2 forms supramolecular networks
via relatively strong intra- and intermolecular p–p stacking and
H-bonding interactions, complexes 1 are arranged in the crystal
by dimers via intermolecular stacking interaction of moderate
strength. This strong intramolecular p–p stacking interaction
promotes the formation of the EuIII 1 : 2 species and the increase
of stability constants of these species in aqueous solutions upon
the relatively high concentrations of ligands (∼10−3 M) for both
europium SBs. Moreover these SBs demonstrate the tetradentate
coordination function in solution which includes the coordination
of the nitrogen atom of histidine for the PL-His ligand. The anal-
ysis of the vibronic interaction also enables some features of the
EuIII coordination sphere in solution to be revealed. The vibrations
assigned to the carboxylate groups with monodentate coordi-
nation function were found in the solution containing PL-Asp
whereas the vibronic satellites attributed to both carboxylates and
the histidine ring were observed in solutions containing PL-His.
The analysis performed on the scheme of ligand energy levels
as well as detected lifetimes of EuIII excited states reveal the
relatively effective intramolecular energy transfer from the ligands
to europium ion. This factor together with the stability of the
EuIII 1 : 2 species in solution and good solubility of lanthanide SB
complexes in water make them possible prospective candidates for
luminescence materials for biological and medical applications.
to 0.22
0.02 ms in the solid state.27 Vibronic satellites found
in the emission spectrum of solution s1 indicate the presence of
two types of coordinated carboxylate groups and the splitting
between ms(COO−) and mas(COO−) indicates that the monodentate
coordination function of the carboxylates is preserved. In order
to provide a rationale for the above assumption, the solution
s1 was diluted 10-fold and a luminescence spectrum of another
type was observed (Fig. 8d). The behaviour of Stark components
of the EuIII electronic transitions in this spectrum is similar to
complex 1 as well as the magnitudes of general splittings of
7
7
the electronic transitions (148, 139, 384 cm−1 for F1, F2 and
7
7F4 levels, respectively). A ratio of general splitting of the F1
level to the 7F2 level smaller than 1 along with doublets in
the 5D0
→
7F2 transition point to the triangle as a model of
EuIII charge surroundings according to mathematical modelling
of the luminescence spectra of europium complexes in solutions
performed by Zolin et al.28 Therefore one can suppose that the
luminescence spectrum recorded for the diluted solution belongs
mainly to the 1 : 1 species. The bi-exponential luminescence decay
of the Eu(5D0) level and corresponding lifetimes 0.38 0.02 and
0.83 0.03 ms were observed in the diluted solution (Table 5). The
shorter lifetime, which contributes ∼80% to the decay, probably
corresponds to the 1 : 1 species with coordinated water molecules
and the longer lifetime indicates the formation of 1 : 2 species with
an EuIII environment basically devoid of O–H vibrations owing to
the tetradentate coordination function of PL-Asp. Thus the EuIII
1 : 1 and 1 : 2 species coexist in solution containing one equivalent
of PL-Asp with predominant contribution of the 1 : 1 species to
the luminescence when the concentration is ∼10−4 M. The stability
of the 1 : 2 species in diluted solution is possibly connected with
the p-stacking interaction between ligands through pyridoxal rings
similar to the case of solution s2. The formation of predominantly
EuIII 1 : 2 species in solution (with relatively high concentration)
Experimental
Synthesis
All solvents were of analytical grade and used without further
purification. Pyridoxal hydrochloride (PL), DL-aspartic acid, and
L-histidine dihydrochloride (Sigma) were used as received. The
lanthanide chlorides (Ln = La, Eu, Gd) were prepared from their
oxides in the usual way.30 The content of compounds synthesized
was confirmed by elemental analyses. Lanthanide Schiff bases of
pyridoxal with Asp and L-His were prepared by template synthesis
according to a previously described method.14 Sodium Schiff bases
of pyridoxal with Asp and L-His were prepared in a similar way
to the lanthanide case.
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Complex 1 : 1
C(9ꢀ)–C(10ꢀ); C(8)–N(2) and C(8ꢀ)–N(2ꢀ), C(9ꢀ)–N(2ꢀ) and C(9)–
N(2); C(11)–C(9ꢀ) and C(11)–C(10); C(8)–C(3) and C(8ꢀ)–C(3)
were refined to be equal for each pair using the free variables. In
addition to the above fragment the solvate water molecule (O(5w))
in 1 is also disordered by two positions with equal occupancies.
The ADP of O(5w) and O(5wꢀ) atoms were refined with EADP
instructions. In 2 analysis of the atomic displacement parameters
of the chloride anion have revealed that despite of its general
position its occupancy is equal to 0.5. Observed occupancy of the
chlorine anion coincides well with the charge of the cationic part as
well as with the intermolecular contacts of chlorine anion. Indeed
the Cl(1) form in the crystal had an unrealistically shortened
An aqueous solution (2 ml) of PL (0.0509 g; 0.25 mmol) was
prepared as well as an aqueous solution (2 ml) of Asp (0.0333 g;
0.25 mmol) with 0.33 ml of NaOH (0.04 g; 1.0 mmol) added to
obtain a pH of ∼6. These solutions were mixed to form a yellow
solution. An aqueous solution (1 ml) of EuCl3·6H2O (0.0916 g;
0.25 mmol) was added dropwise to the mixture. A small quantity
of precipitate appeared right away. The mixture was stirred for
3 h at rt. The yellow precipitate obtained the next day was
filtered, washed with 0.5 ml isopropyl alcohol and dried. Yields: La
(50 mg, 70%), Eu (116 mg, 66%), Gd (96 mg, 60%). LaC12N2O12H23
(526.23): (Found: C, 27.18; H, 4.22; N, 5.27%. Calc.: C 27.39, H
4.41, N 5.32%); EuC12N2O11H21 (521.27): (Found: C, 27.52; H,
4.16; N, 5.22%. Calc.: C 27.65, H 4.06, N 5.37%); GdC12N2O11H21
(526.56): (Found: C, 27.22; H, 3.90; N, 5.30%. Calc.: C 27.37, H
4.02, N 5.32%). Single crystals suitable for X-ray analysis were
obtained by slow diffusion of an ethanol–water solution.
˚
contact equal to 3.06 A. It should be noted that such an occupancy
of Cl(1) is not a symmetry imposed one and was observed in
the case of monoclinic space groups with the cation in a general
position. The positions of hydrogen atoms of water molecules and
amino groups were located from Fourier density synthesis while for
all other hydrogen atoms positions were calculated geometrically.
We have to emphasize that chlorine can be treated not only as
chlorine (s.o.f. 0.5) but also as oxygen or even potassium with s.o.f.
equal to 1.0. The latter is unfavorable from the point of view
of interatomic contacts formed by the atom (coordination 3 is
too unusual for potassium). Of course the main reason to treat
that atom as chlorine is the overall charge of the complexes. The
discrepancy factor (R1) can serve as an additional independent
justification of this mode. Thus for chlorine the R-factor value is
the lowest one (0.0626) while for oxygen with an s.o.f. 1.00 it is
slightly higher 0.0634.
The positions of the hydrogen atoms of the water molecules and
amino groups were located from Fourier density synthesis while for
all other hydrogen atoms positions were calculated geometrically.
Taking into account that for two positions of O(5w) only one set
of hydrogen atoms was located and any of the available AFIX
instructions is suitable, the O–H bond lengths for O(5w) and
O(5wꢀ) were refined using a DFIX instruction.
Complex 1 : 2
An aqueous solution (1 ml) of PL (0.0407 g; 0.2 mmol) was
prepared as well as an aqueous solution (1 ml) of L-His (0.0383 g;
0.2 mmol) with 0.2 ml of NaOH (0.024 g; 0.6 mmol) added
to obtain a pH of ∼6. These solutions were mixed to form a
yellow solution. An aqueous solution (0.5 ml) of EuCl3·6H2O
(0.0366 g; 0.1 mmol) was added dropwise to the mixture. Right
away the complex began to precipitate out. The mixture was
stirred for 3 h at rt. The yellow precipitate obtained the next day
was filtered, washed with 0.2 ml EtOH and dried for 2 h at
40 ◦C. Yields: La (54 mg, 55%), Eu (81 mg, 70%), Gd (64 mg,
58%). LaClC28N8O14H42 (889.05): (Found: C, 37.58; H, 4.42; N,
12.56%. Calc.: C 37.83, H 4.76, N 12.60%); EuClC28N8O14H42
(902.11): (Found: C, 37.08; H, 4.32; N, 12.20%. Calc.: C 37.28, H
4.69, N 12.42%); GdClC28N8O13H40 (889.37): (Found: C, 37.64; H,
4.36; N, 12.54%. Calc.: C 37.81, H 4.53, N 12.60%). Single crystals
suitable for X-ray analysis were obtained by a slow evaporation
method of aqueous solutions.
Crystal data and structure refinement parameters for 1 and 2
complexes are given in Table 1. All calculations were performed
using the SHELXTL software.31
X-Ray crystallography†
Luminescence measurements
X-Ray diffraction experiments were carried out with a Bruker
SMART APEX II CCD area detector, using graphite monochro-
The luminescence spectra of the compounds investigated were
measured with a LOMO DFS-12 spectrometer (high resolution)
and LOMO UM-2 monochromator (low resolution) at 300 and
77 K. DRSh-250 and 500 mercury high-pressure lamps were used
as an excitation source for these experimental set-ups, respectively.
The intense Hg 365 nm line of this lamp was isolated by UV filters.
Registration of phosphorescence spectra as well as measurements
of the luminescence decay were carried out by a low resolution
set-up. The lifetimes of europium excited states were obtained as
averages of at least three independent measurements and were
analyzed with Origin 7.0.
˚
mated Mo-Ka radiation (k = 0.71073 A) at 100 K. Reflection in-
tensities were integrated using SAINT software and the absorption
correction was applied semi-empirically using SADABS program.
The sufficiently high value of Rint for Eu(PL-His)2(H2O)6Cl (2)
is a consequence of extremely small crystal dimensions and weak
reflection power. The structures were solved by direct methods and
refined by the full-matrix least-squares against F2 in anisotropic
approximation for non-hydrogen atoms.
The analysis of Fourier density synthesis revealed that in com-
plex 1 the nitrogen atom of the C N bond as well as the carbon
=
atoms of the dicarboxy moiety (chiral center) are disordered by
two positions. Such disorder corresponds to superposition of two
enantiomers in ratios of 0.802(6) and 0.198(6) (Fig. S1, ESI‡). The
refinement of self occupancies factors (s.o.f.) for C(8), C(9), C(10)
and C(8ꢀ), C(9ꢀ) and C(10ꢀ) was carried out with constraints on the
anisotropic displacement parameters (ADP) values of each pair of
atoms (EADP instruction). The bond lengths for C(9)–C(10) and
Acknowledgements
Authors thank Prof. V. F. Zolin for chemical reagents and
suggesting the subject of the research presented. This work
is supported by Russian Science Support Foundation and the
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Russian Foundation for Basic Research (grant N 06-03-32557,
07-03-12196).
14 A. V. Dushkin, V. F. Zolin, L. G. Koreneva, E. G. Ruhadze, G. P.
Talyzenkova and V. I. Tsaruyk, Russ. J. Coord. Chem., 1975, 1, 1533–
1541.
15 E. A. Meyer, R. K. Castellano and F. Diederich, Angew. Chem., Int.
Ed., 2003, 42, 1210–1250.
16 B. R. Judd, Phys. Rev., 1962, 127, 750–761; G. S. Ofelt, J. Chem. Phys.,
1962, 37, 511–520.
References
1 M. J. Weber, in Rare Earth Lasers, North-Holland Publishing Co.,
Amsterdam, 1979, vol. 4, pp. 275–315.
17 O. J. Sovers and T. J. Yoshioka, J. Chem. Phys., 1968, 49, 4945–54; O. J.
Sovers and T. Yoshioka, J. Chem. Phys., 1968, 51, 5330–5336.
18 K. Nakamoto, in Infrared and Raman Spectra of Inorganic and
Coordination Compounds. Part A. Theory, and Applications in Inorganic
Chemistry, John Wiley Interscience Publ., New York, 1997, p. 536.
19 L. Puntus, V. Zolin and V. Kudryashova, J. Alloys Compd., 2004, 374(1-
2), 330–334.
20 N. Sabbatini, M. Guardigli and J.-M. Lehn, Coord. Chem. Rev., 1993,
123, 201–228; J.-C. G. Bunzli, J. Alloys Compd., 2006, 408-412, 934–944.
21 F. J. Steemers, W. Verboom, D. N. Reinhoudt, E. B. Vandertol and J. W.
Verhoeven, J. Am. Chem. Soc., 1995, 117, 9408–9414; J.-C. G. Bu¨nzli,
Met. Ions Biol. Syst., 2004, 42, 39–75.
2 R. D. Archer and H. Chen, Inorg. Chem., 1998, 37, 2089–2095.
3 H. C. Aspinall, Chem. Rev., 2002, 102, 1807–1850; U. Casellato, S.
Tamburini, P. Tomasin, P. A. Vigato, S. Aime and M. Botta, Inorg.
Chem., 1999, 38, 2906–2916; S. Aime, M. Botta, U. Casellato, S.
Tamburini and P. A. Vigato, Inorg. Chem., 1995, 34, 5825–5831.
4 S. A. Schuetz, V. W. Day, R. D. Sommer, A. L. Rheingold and J. A.
Belot, Inorg. Chem., 2001, 40, 5292–5295; P. Blech, C. Floriani, A.
Chiesivilla and C. Guastini, J. Chem. Soc., Dalton Trans., 1990, 3557–
3561.
5 K. Binnemans and C. Gorller-Walrand, Chem. Rev., 2002, 102, 2303–
2345.
6 J. I. Bullock and H. A. Tajmir-Riahi, J. Chem. Soc., Dalton Trans.,
1978, 36–39; N. K. Dutt and K. Nag, J. Inorg. Nucl. Chem., 1968,
30, 2779–2781; W. Nowicki and S. Zahara, Spectrosc. Lett., 1992, 25,
593–601.
7 H. Chen and R. D. Archer, Inorg. Chem., 1994, 33, 5195–5202; S.
Afshar and J. Bullock, Inorg. Chim. Acta, 1980, 38, 145; N. K. Dutt
and K. Nag, J. Inorg. Nucl. Chem., 1968, 30, 2493–2496.
8 D. E. Metzler, M. Ikawa and E. E. Snell, J. Am. Chem. Soc., 1954, 76,
648–652.
9 V. F. Zolin and L. G. Koreneva, in Rare Earth Probe in Chemistry and
Biology, ed. M. E. Zhabotinskii, Nauka, Moscow, ch. 2, pp. 205–207.
10 F. Nepveu, J.-J. Bonnet, J.-P. Laurent and M. Massol, J. Coord. Chem.,
1981, 11, 185–189; F. Nepveu and J.-J. Bonnet, Acta Crystallogr., Sect.
C: Cryst. Struct. Commun., 1983, C39, 339–341; R. E. Marsh, Acta
Crystallogr., Sect. B: Struct. Sci., 1997, B53, 317–322; I. I. Mathews
and H. Manohar, Polyhedron, 1991, 10, 2163–2169.
11 K. Jitsukawa, T. Yamamoto, H. Masuda and H. Einaga, Chem. Lett.,
1993, 1693; K. Jitsukawa, T. Yamamoto, H. Takahashi, H. Masuda
and H. Einaga, Chem. Lett., 1995, 777; A. G. Sykes, R. D. Larsen,
J. R. Fischer and E. H. Abbott, Inorg. Chem., 1991, 30, 2911–2916;
M. B. Feerrari, G. G. Fava, M. Lanfranchi, C. Pelizzi and P. Tarasconi,
J. Chem. Soc., Dalton Trans., 1991, 1951–1957.
22 P. P. Barthelemy and G. R. Choppin, Inorg. Chem., 1989, 28, 3354–
3357; G. R. Choppin and D. R. Peterman, Coord. Chem. Rev., 1998,
174, 283–299.
23 S. T. Frey and W. deW. Horrocks, Inorg. Chim. Acta, 1995, 229, 383–
390.
24 R. M. Supkowski, W. De and W. Horrocks, Inorg. Chim. Acta, 2002,
340, 44–48.
25 L. L. Koh, J. O. Ranford, W. T. Robinson, J. O. Svensson, A. L. C. Tan
and D. Wu, Inorg. Chem., 1996, 35, 6466–6472; P. S. Subramanian, E.
Suresh, P. Dastidar, S. Waghmode and D. Srinivas, Inorg. Chem., 2001,
40, 4291–4301; H. M. Dawes and T. N. Waters, J. Chem. Soc., Chem.
Commun., 1982, 1390–1391; K. Aoki and H. Yamazaki, J. Chem. Soc.,
Chem. Commun., 1984, 410–411.
26 O. Yamauchi, A. Odani and M. Takani, J. Chem. Soc., Dalton Trans.,
2002, 3411–3421.
27 L. N. Puntus, K. P. Zhuravlev, K. A. Lyssenko and V. F. Zolin, Opt.
Mater., 2007, DOI: 10.1016/j.optmat.2007.02.043.
28 V. F. Zolin and L. G. Koreneva, J. Struct. Chem., 1983, 24, 704–710.
29 J.-C. G. Bu¨nzli, in Lanthanide Probes in Life, Chemical and Earth
Sciences. Theory and Practice, ed. J.-C. G. Bu¨nzli and G. R. Choppin,
Elsevier Science Publ. B.V., Amsterdam, 1989.
30 J. F. Desreux, in Lanthanide Probes in Life, Chemical and Earth Sciences.
Theory and Practice, ed. J.-C. G. Bu¨nzli and G. R. Choppin, Elsevier
Science Publ. B.V., Amsterdam, 1989, ch. 2, pp. 43–64.
31 G. M. Sheldrick, SHELXTL-97, Version 5.10, Bruker AXS Inc.,
Madison, WI-53719, USA, 1997.
12 L. M. Vainer, V. F. Zolin, L. G. Koreneva and P. Z. Sagnaev,
Russ. J. Coord. Chem., 1975, 1, 1512–1515; V. F. Zolin and L. G.
Koreneva, J. Struct. Chem., 1980, 21, 51–55.
13 L. Casella and M. Gullotti, J. Am. Chem. Soc., 1981, 103, 6338–6347.
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