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XAFS Study of the Local Structure of Some
Lanthanoid(III) Complexes
Susumu Sudoh1, Takafumi Miyanaga2*, and Ryo Miyamoto1
1
Department of Frontier Materials Chemistry, Faculty of Science and Technology, Hirosaki University, Hirosaki,
Aomori 036-8561, Japan
2
Department of Advanced Physics, Faculty of Science and Technology, Hirosaki University, Hirosaki, Aomori 0368561, Japan
Abstract. Two types of lanthanoid(III) complexes were synthesized: type I complexes - Ln(III) (Ln = Sm, Eu, Tb, Dy)
anthrarufinate complexes using anthrarufin (1,5-dihydroxy-9,10-anthraquinone) as the ligands, and type II complexes Ln(III)-transition(d-bloch) metal(II) bi-nuclear complexes. The local structures of these complexes were studied by
EXAFS spectroscopy. We found that there is a good linear correlation between the ionic radii of Ln(III) and the Ln-O
distances for the type I complexes, and for type II complexes the interatomic distances between Gd and coordinated
oxygen atoms of the bi-nuclear complex are shorter than those of the Gd mononuclear complex.
Keywords: EXAFS, Lanthanoid complexes,
PACS: 61.10.Ht
INTRODUCTION
EXPERIMENT AND DATA ANALYSES
The local structures of a series of the lanthanoid,
Ln(III), chloranilate and bromanilate complexes have
been studied by EXAFS (extended X-ray absorption
fine structure) spectroscopy [1-6], IR spectroscopy and
thermal analysis [7]. From the thermal analysis and
temperature dependent EXAFS studies, interesting
results obtained are that coordinated or lattice waters
are found in these complexes, and that the atomic
distances between Ln(III) and coordinated oxygens
depend on the coordination number or the Ln(III) ionic
radius. The compounds with Ln(III) have also been
studied as useful materials, such as superconductors,
NMR shift reagents, and catalysts for many types of
chemical synthesizes. In the electronic configurations,
Ln(III) are classified as inner transition (f-block)
metals, and f-electrons are considered to rarely take
part in the chemical bond due to the strong shielding
by 5s and 5p electrons which are located in outer shell
regions. But recently the magnetic interaction between
f- and d-electrons has been discussed in the literature
[8].
Here we have studied the local structures of two
types of complexes by EXAFS spectroscopy. The first
type of complexes (type I) are the Ln(III) ( Sm, Eu,
Tb, Dy) anthurufinates; this type of complexes are
newly synthesized complexes. The second type of
complexes (type II) are the Ln(III)-transition(d-block)
metal(II) bi-nuclear complexes.
Type I Complex: Anthurafin Ln(III) complexes
were synthesized from the following reaction.
2LnCl3·nH2O + 3C14O4H8 + 6NaOH(aq) →
Ln2(C14O4H6)3·nH2O + 6NaCl(aq)
(1),
where lanthanoide trichloride (0.003 mol) and
anthrurafin
(1,5-dihydroxy-9,10-anthraquinone)
(0.01mol) were dissolved in alkaline solution with pH
= 13, and heated about one hour at 100 ºC. The
reaction mixture was cooled and left to rest about 12
hours. After this, the precipitate was filtered, rinsed
completely by hot water and dried. The complex
structures are given in Fig. 1.
O
OH2
Ln
O
O
O
Ln
O
O
O
Ln
O
OH2
FIGURE 1. The local structure of the Ln(III) anthrarufinate
complex.
Type II Complex: The Gd(III)-Ni(II) bi-nuclear
complex
of
N,N’-ethylene
bis
(3-hydroxy
salicylaldimine), with the ligand abbreviated as
H4osalen, was synthesized by a one step template
reaction [9,10]. A methanol solution (2 ml) of NiCl2
results cannot distinguish these two possibilities for
the coordination of Ln(III) anthrarufinate complexes.
6
(a)Sm
(k)
k22 χ (k)
4
(b)Eu
χ2
(c)Tb
k
(0.2 g) and GdCl3·6H2O (0.56 g) was added to 60 ml
methanol with 2,3-dihydroxybenzaldehyde (0.41 g).
After the drop-wise addition of ethylene diamine (0.09
g), the reactant solution was refluxed for about three
hours, concentrated to 30 ml and by adding 30 ml of
water , precipitates were obtained. The precipitates
filtered from the solution were successively rinsed
with water, methanol and diethyl ether. The Gd(III)
mononuclear complex was also synthesized by a
similar method. The complex structures are given in
Fig. 2 and the complexes are abbreviated as (a) Gd(III)
osalen and (b) Gd(III)-Ni(II) osalen, respectively.
(d)Dy
0
2
4
6
k/
Å-1 8
10
12
k/Å-1
FIGURE 3. The k2-weighted EXAFS χ(k) spectra for
Ln(III) anthrarufinate complexes.
1
0.8
(b)
FIGURE 2. Schematic models for the structures of (a)
Gd(III) osalen and (b) Gd(III)-Ni(II) osalen complexes.
EXAFS Spectra: The Ln(III) LIII-edge X-ray
absorption spectra were obtained at BL-10B and BL7C of Photon Factory in KEK. The energy and current
of the storage ring was 2.5 GeV and 250-400 mA,
respectively. The X-ray absorption spectra were
recorded in transmission mode using ionization
chamber detectors.
The EXAFS function, χ(k), was extracted from the
absorption spectra and was Fourier transformed by the
program of XANADU code [11]. In order to obtain the
structural parameters, the EXAFS function was fitted
by non-linear least-square method [4-6].
RESULTS AND DISCUSSION
Type I Complex: The k2-weighted EXAFS χ (k)
spectra of the type I complexes are given in Fig. 3. The
corresponding Fourier transforms (FT) of the
complexes for the k-range from 3 to 12 Å-1. We found
one peak clearly in the range of 2.0 to 2.5 Å that
represented back-scattering from oxygen atoms of the
coordinated anthrarufin carbonyl groups. The atomic
distances and the coordination numbers are shown in
Table 1. The relationship between the ionic radii of
Ln(III) and the r(Ln-O) distance obtained by EXAFS
analysis is shown in Fig.5. It is interesting to see that
a good linear relationship was obtained. These type I
complexes are considered to have two kinds of
complex structures ( Fig. 1). One is a flat sheet linked
by Ln(III), with two waters coordinated to the
lanthanoid. Another is a three-dimensional net work
seen for the Ln(III) chloranilate and bromanilate
complexes [6]. Unfortunately the present EXAFS
(a)Sm
|FT(r)| / arb.unit
(a)
0.6
(b)Eu
0.4
(c)Tb
0.2
(d) Dy
0
0
2
Å
4
6
8
rr/Å
/
FIGURE 4. FTs of EXAFS for Ln(III) anthrarufinate
complexes.
2.52
2.5
2.48
A2.46
/r
2.44
2.42
2.4
2.38
104
105
106
107
108
109
ionic radii/(pm )
110
111
FIGURE 5. The relationship between ionic radii and
interatomic distance.
TABLE 1.
Structural parameters of Ln(III) and
coordinating oxygen atoms for the anthrarufin Ln(III)
complexes.
Complex
r/Å
N
σ /Å
Sm(III)
2.51
6.5
0.10
Eu(III)
2.48
6.8
0.10
Tb(III)
2.43
6.9
0.11
Dy(III)
2.39
6.8
0.11
The error is ±0.02 Å for r and ±10% for N.
Type II Complex: The k2-weighted EXAFS χ(k)
spectra of the Gd(III)-Ni(II) bi-nuclear and Gd(III)
mono-nuclear complexes are given in Fig. 6. The
corresponding FTs of two Gd(III) complexes for the krange from 3 to 12 Å-1 are shown in Fig. 7. For Gd
osalen we found one apparent peak in the range of 2.03.5 Å that is responsible for the back-scattering from
the oxygen atoms numbered in Fig. 2.
Gd osalen, the Gd-O(1,2) distance is longer than that
of Gd-Ni osalen, i.e., the ligand-chelating cage is
compressed by the addition of Ni metal into the Gd
complex.
0.6
4
3
|FT(r)|
0.4
kk22 χ (k)
(k)
2
Gd osalen
Gd osalen
χ
0.2
1
0
Gd-Ni osalen
Gd-Ni osalen
-1
-2
0
0
2
Å
4
r/
6
8
r/Å
5
Å
10
-1
-1
kk/Å
/
FIGURE 6. The k2-weighted EXAFS χ(k) spectra for
Gd(III) mono-nuclear and Gd(III)-Ni(II) bi-nuclear
complexes.
For Gd-Ni osalen, on the other hand, we see that
there are two peaks in the range of 2.0-4.0 Å; the first
intense peak is from oxygen atoms being similar to
those in Gd osalen, and the second peak is due to Ni
and Gd atoms.
In the second peak, a small
contribution from carbon atoms is also included. We
need more precise information, however, in order to
conclude the fourth peak’s origin.
Table 2 shows the structural parameters for the Gd
mononuclear complex. In this r-range there is no
carbon contribution. In our model, 2 oxygen atoms are
coordinated to Gd atoms at a distance of 2.47 Å,
therefore about 5.5 water molecules are coordinated to
Gd in this complex.
TABLE 2. Structural parameters of Gd and coordinating
oxygen atoms for Gd osalen complex.
r/A
N
σ/ A
Gd-O(1,2)
2.38
2.0
0.07
Gd-O(3,4)
2.47
7.5
0.14
Table 3 shows the results for the Gd-Ni bi-nuclear
complex. In order to obtain convergence in the curvefitting calculations, the values of N for Gd-Gd had to
be fixed as 2. The Gd-O(1,2) distances for Gd-Ni
osalen is shorter (2.34 Å) than that for Gd osalen (2.38
Å, Table 2). This change is reliable within the error.
The contraction of bond distance is observed only for
the short Gd-O(1,2); the interatomic distances of GdO(3,4) for these two complexes are almost the same
(~2.46 Å).
The coordination number of Gd-Ni is 1.1±0.1 and
this is the value expected from the model (Fig.2). In
FIGURE 7. FTs of EXAFS for the Gd(III) mono-nuclear
and Gd(III)-Ni(II) bi-nuclear complexes.
TABLE 3. Structural parameters of Gd and coordinating
atoms for Gd-Ni osalen complex.
r/Å
N
σ /Å
Gd-O(1,2)
2.34
2
0.07
Gd-O(3,4)
2.46
5.9
0.13
Gd-Ni
3.49
1.1
0.07
Gd-Gd
3.71
2*
0.12
Gd-C
3.74
4*
0.09
*These values are fixed according to the model.
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