Formation and catalytic activity of novel water soluble di[ethylenediaminetetraacetato bis(N-oxido)] lanthanides

Article abstract of DOI:10.1016/j.inoche.2013.05.012

Reaction of hydrogen peroxide with ethylenediaminetetraacetato lanthanides results in the formation of water-soluble isomorphous N-oxido ethylenediaminetetraacetato lanthanides K₅[Ln(edtaO₂) ₂] · 12H₂O [Ln = La (1), Ce (2), Nd (3), H ₄edta = ethylenediaminetetraacetic acid C₁₀H ₁₆O₈N₂] in weak basic solution, where lanthanide ions are octa-coordinated by two quardentate N-oxido edta ligands, resulting in a distorted anti-tetragonal prism. Based on the comparisons of solid and solution ¹³C NMR spectra, these compounds are fully dissociated in solution. Catalytic reaction of K₅[La(edtaO ₂)₂]·12H₂O shows 96% conversion for the reaction of pyridine to pyridine N-oxide at 70 C.

Full text of DOI:10.1016/j.inoche.2013.05.012

Inorganic Chemistry Communications 35 (2013) 9–12
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Formation and catalytic activity of novel water soluble di
[ethylenediaminetetraacetato bis(N-oxido)] lanthanides
⁎
Xue Jiang, Mao-Long Chen, Yu-Chen Yang, Zhao-Hui Zhou
State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of hydrogen peroxide with ethylenediaminetetraacetato lanthanides results in the formation of water-
soluble isomorphous N-oxido ethylenediaminetetraacetato lanthanides K5[Ln(edtaO2)2] · 12H2O [Ln = La (1),
Ce (2), Nd (3), H4edta = ethylenediaminetetraacetic acid C10H16O8N2] in weak basic solution, where lanthanide
ions are octa-coordinated by two quardentate N-oxido edta ligands, resulting in a distorted anti-tetragonal prism.
Based on the comparisons of solid and solution 13C NMR spectra, these compounds are fully dissociated in solution.
Catalytic reaction of K5[La(edtaO2)2]·12H2O shows 96% conversion for the reaction of pyridine to pyridine N-oxide
at 70 °C.
Received 11 April 2013
Accepted 9 May 2013
Available online 15 May 2013
Keywords:
Lanthanide
Ethylenediaminetetraacetate
N-oxide
© 2013 Elsevier B.V. All rights reserved.
Hydrogen peroxide
Catalysis
Peroxo lanthanides are the major species of lanthanide in neutral
solution with hydrogen peroxide [1]. They are important in the pro-
cess of catalytic reaction with rare earth oxide like hydroxylations of
benzene and selective oxidation of low alkane [2–4]. When hydrogen
peroxide exists, lanthanides are suggested to form peroxo lanthanides.
Several factors such as molar ratio of Ln: H2O2, acidity, solvent, ligand
and temperature have been found to affect the formation of peroxo
lanthanides. However, more details of isolation of the peroxo lanthanides
are limited. Only a well-defined side-on-bonded superoxo Sm(η2-O2)
is reported [5]. There are also a few lanthanide complexes containing
μ-η2:η2-peroxo group bridged with heteroligand environments [6–14].
The conversions are unknown for the peroxo group. This is probably
due to the instability of the system and the slow formation of the anions.
Moreover, photoinduced formation of lanthanide peroxide has been
discovered by the laser excitation on the Ln2O3 surface under oxygen at-
mosphere [15]. Here we have succeeded in the isolations of N-oxido edta
lanthanides in presence of excess hydrogen peroxide and edta lantha-
nides K[Ln(edta)(H2O)3] · 5H2O (Ln = La, Ce, and Nd). The later have
been also used as starting materials for the preparation of dimeric lan-
thanide citrate and malate with edta in our previous report [16]. An
interesting addition of peroxo group into less soluble edta lanthanides
forms water-soluble di-[ethylenediaminetetraacetato bis(N-oxido)]
lanthanides K5[Ln(edtaO2)2] · 12H2O, which serves as the reaction
products of Ln(η2-O2) with edta. The syntheses, infrared and 13C NMR
spectra, and structural characterizations of the three novel lanthanides
K5[Ln(edtaO2)2] · 12H2O [Ln = La (1), Ce (2), Nd (3)] are reported,
where K5[La(edtaO2)2] · 12H2O was used as catalyst for the conversion
pyridine to pyridine N-oxide.
Unlike direct reaction between metal salt with N-oxide ligand
[19–22], edta lanthanide K[Ln(edta)(H2O)3] · 5H2O and hydrogen
peroxide were used as reactants for the preparations of N-oxido edta lan-
thanides K5[Ln(edtaO2)2] · 12H2O in aqueous solution at pH 7.0 ~ 9.0
[17,18]. Under an optimal condition of pH 8, K5[Ln(edtaO2)2] · 12H2O
can be obtained in yields of 53% (La), 55% (Ce), and 69% (Nd) respectively.
The molar ratio of K[Ln(edta)(H2O)3] · 5H2O: H2O2 was maintained in 1:
5. Syntheses and conversions of the peroxo lanthanides are depicted in
Scheme 1. pH control and the concentration of hydrogen peroxide are
critical for the formation of N-oxido lanthanides.
Crystal structures of K5[Ln(edtaO2)2] · 12H2O consist of di-
[ethylenediaminetetraacetato bis(N-oxido)] lanthanide anions,
potassium cations, and lattice water molecules. The anion structure
of 1 was shown in Fig. 1. Those of 2 and 3 were shown in Figs. S1
and S2. Each lanthanide cation is octa-coordinated by two edtaO2
ligands and displays as a distorted anti-tetragonal prism as shown
in Fig. 2. Each N-oxido edta coordinates to lanthanide cation with
two N-oxide atoms and two carboxy groups.
Selected bond distances of the title compounds are listed in Table 1.
In monomer 1, the Ln\O bond distances are between 2.427(3) and
2.537(3) Å, where La\O bonds of carboxy groups are longer [La1\O1,
2.537(3); La1\O5, 2.486(3); La1\O11, 2.515(4); La1\O15, 2.500(4)
Å] than those of N-oxide [La1\O9, 2.427(3); La1\O10, 2.484(3);
La1\O19, 2.500(3); La1\O20, 2.445(3) Å]. The similar results can be
found in compounds 2 and 3. It is worth noting that, for all three com-
plexes, the N\O bond distances (N-oxide) range from 1.396(5) to
1.411(5) Å with a mean value of 1.402(5) Å. This is similar to the N\O
⁎
Corresponding author. Tel.: +86 592 2184531; fax: +86 592 2183047.
E-mail address: zhzhou@xmu.edu.cn (Z.-H. Zhou).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2013.05.012

10
X. Jiang et al. / Inorganic Chemistry Communications 35 (2013) 9–12
Scheme 1. Formation of water soluble N-oxido ethylenediaminetetraacetato lanthanides
K5[Ln(edtaO2)2]·12H2O.
distance [1.413(7) Å] as reported [21]. Based on Table S1, the Ln\O bond
distances [La\O, 2.509(4); Ce\O, 2.493(5); Nd\O, 2.458(5) Å]
decrease with the increase of atomic number. This is the result of lantha-
nide contraction.
Solid and solution 13C NMR spectra of 1 were shown in Fig. 3 and its
solution 1H NMR spectrum in Fig. S3. The solid state 13C NMR spectra of
K5[La(edtaO2)2] · 12H2O (1) give five peaks at δ = 54.2, 65.9, 70.4,
170.4 and 171.2 ppm. It is interesting to note that all of the four CH2
moieties from acetate groups occur as two single peaks at 70.4 and
65.9 ppm, which correspond to the two carboxy groups coordinated
to La, and the other two groups remain free as internal reference. Obvi-
ous downfield shift (Δδ 3.5 ppm) is observed after coordination. More-
over, the CH2 groups of N–bound methylene moiety (δ = 54.2 ppm)
are not differentiated, which confirms the presence of N\O groups in
the complex. The presence of two 13C peaks at 170.4 and 172.1 ppm
also implies that two carboxy groups coordinate to La with downfield
shift Δδ of 1.7 ppm, while the other two groups remain free. This is con-
sists with the structural analysis.
Fig. 2. Distorted anti-tetragonal configuration of La(III) cation.
1310 to 1400 cm−1 belong to νs(CO2). Moreover, oxidation of the
nitrogen atoms of the edta ligand is evidenced by the ν(N\O)
band at 900 cm−1. In free ligand, the corresponding band appears
at ~890 cm−1 [21]. Thermal analysis of compound 1 was shown
in Fig. S6. The weight loss of 6.41% below 223 °C (calcd 6.04%) cor-
responds to the loss of four crystal water molecules/formula. The
compound begins to decompose and degrade to K10La2O8 at the
higher temperature. The thermal behaviors of compounds 2 and 3
are similar.
The solution 13C NMR spectrum of K5[La(edtaO2)2] · 12H2O (1) only
shows three peaks. The chemical shift (173.0 ppm) of carboxy groups in
[edtaO2]4− is similar to that of free [edtaO2]4− (173.3 ppm). This may
indicate the full dissociation of the complex in solution. The other two
peaks at 70.5 and 59.2 ppm are ascribed to CH2 groups of acetates and
N-bound moieties respectively. Comparing the solution spectrum of 1
with free [edtaO2]4− ligand, highfield shift for CH2N (Δδ = −3.2 ppm)
and downfield shift for CH2CO2 (Δδ = 0.3 ppm) can be found. The for-
mer is due to the binary effect of pulling electrons by quaternary nitrogen
and the coordination of N-oxide.
The lanthanide edta N-oxide K5[La(edtaO2)2] · 12H2O was used for
the catalytic conversion of pyridine to pyridine N-oxide [23,24]. In order
to find suitable conditions for catalytic oxidation of pyridine, various pa-
rameters, such as amount of catalyst 1, reaction time and temperature
have been taken into considerations. Water is used as reaction media;
this is due to the solubility of pyridine and catalyst. The molar ratio of
pyridine: H2O2 was maintained in 1: 4. The reaction temperature had
a great effect on the oxidation of pyridine as shown in Fig. 4. The conver-
sion of pyridine increased with increasing temperature and reached the
maximum at 70 °C. At the same temperature, the yield of pyridine
N-oxide increased with the catalyst amount. More catalyst resulted in
the precipitation and reduced the activity of the catalyst. Fig. S7 shows
the solution 13C NMR spectra of reaction mixture with time at 70 °C,
the reaction could not be detected within 2 hours. After 9 hours,
three peaks at 150.8, 140.4 and 127.1 ppm are assigned to the signals
of pyridine, the other three peaks at 141.4, 134.9 and 129.9 ppm to
13C NMR signal of pyridine N-oxide. The conversion of pyridine
reached almost 100% after 24 hours without side product. The opti-
mal molar ratio of catalyst K5[La(edtaO2)2] · 12H2O (1) was 0.67 %
(0.1 g) with respect to pyridine, and the molar ratio of pyridine to
H2O2 was 1: 4 at 70 °C for 24 hours. Catalytic conversion of lantha-
num chloride gave 16%, while the solutions of K4(edtaO2) [21] and
K[La(edta)(H2O)3] · 5H2O shown only 7% and 4% respectively. This
shows that only edtaO24− itself can not play a catalytic conversion
role for the formation of pyridine N-oxide. Both lanthanide and
edtaO24− ions cooperatively catalyzes the conversion reaction of pyri-
dine into N-oxide. K5[La(edtaO2)2] · 12H2O (1) showed a significant
catalytic activity for the oxidation of pyridine with an yield of 96%.
New lanthanide complexes with N-oxide derivatives of ethylenedi-
aminetetraacetate have been prepared and characterized. Conversion
of N-oxide added a new reactivity for peroxo peroxo lanthanides. More-
over, K5[La(edtaO2)2] · 12H2O was a suitable catalyst for the catalytic
oxidation of pyridine in high yield.
IR spectra of K5[Ln(edtaO2)2] · 12H2O are shown in Fig. S5. Com-
paring with the free ligand at 1700 cm−1, we found significant
blue-shift for νas(CO2) at ~1620 cm−1. The bands ranging from
O3
O13
C4
O2
C2
C3
C1
O12
C14
O14
C13
O4
C11
N3
C19
C12
O11
N1
C9
O9
O1
O19
La1
C10
C20
O5
O10
C5
O20
C15
N2
O15
C6
O6
N4
O7
C8
C16
O16
C17
C7
C18
O18
O17
O8
Fig. 1. ORTEP plot of the anion of monomeric K5[La(edtaO2)2]·12H2O (1) at the 30%
probability levels.

X. Jiang et al. / Inorganic Chemistry Communications 35 (2013) 9–12
11
Table 1
Comparisons of selected bond distances (Å) in N-oxido ethylenediaminetetraacetato complexes.
Complexes
N\O (hydroxy)
M\O (carboxy)
M\O (N-oxide)
K5[La(edtaO2)2]·12H2O (1)
1.401(5), 1.399(5)
1.396(5), 1.411(5)
1.402(5)
2.537(3), 2.486(3)
2.515(4), 2.500(4)
2.509(4)
2.427(3), 2.484(3)
2.500(3), 2.445(3)
2.464(3)
Average (Å)
K5[Ce(edtaO2)2]·10H2O (2)
1.405(8), 1.414(7)
1.403(7), 1.401(7)
1.406(7)
2.518(5), 2.466(5)
2.480(6), 2.506(5)
2.493(5)
2.396(5), 2.458(5)
2.414(5), 2.472(5)
2.435(5)
Average (Å)
K5[Nd(edtaO2)2]·12H2O (3)
1.395(6), 1.415(6)
1.403(6), 1.402(7)
1.404(6)
2.486(5), 2.435(5)
2.443(5), 2.469(5)
2.458(5)
2.378(4), 2.428(4)
2.387(4), 2.454(4)
2.412(4)
Average (Å)
(gu)3[Ta(O2)2(edtaO2)]·2H2O [21]
(gu)3[Nb(O2)2(edtaO2)]·2H2O [22]
1.393(6)
1.413(5)
2.130(4), 2.136(4)
2.155(5), 2.154(5)
2.084(4), 2.099(4)
2.089(4), 2.083(5)
13C NMR of K5[La(edtaO2)2]·12H2O (1)solid
Appendix A. Supplementary material
CCDC deposition numbers for 1 ~ 3 are CCDC 897828 ~ 897830.
Copies of this information can be obtained free of charge from The
Director, CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: +44
1223 336 033; e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.
cam.ac.uk). Supplementary data to this article can be found online at
http://dx.doi.org/10.1016/j.inoche.2012.09.014. The other IR, 1H NMR
spectra and Ortep plots of peroxo lanthanides of peroxo lanthanides
are available as supplementary materials. Supplementary data associat-
ed with this article can be found, in the online version, at http://dx.doi.
org/10.1016/j.inoche.2013.05.012.
-CH2CO2(uncoord.)
172.1 170.4
65.9
54.2
-CO2
-CO2(uncoord.)
70.4
NCH2-
-CH2CO2
(1)solution
173.0
-CO2
70.5
-CH2CO2
DSS
59.2
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Acknowledgements
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[17] Preparation of K5[La(edtaO2)2] · 12H2O (1): K[La(edta)(H2O)3] · 5H2O (0.6 g,
1.0 mmol) was dissolved in water (10.0 mL). Then H2O2 (30 wt %, 0.5 mL) and
H4edta (0.3 g, 1.0 mmol) was added and stirred for 12.0 h at room temperature.
The pH value of the mixture was adjusted to 8.0 with potassium hydroxide. Colorless
crystalline product of K5[La(edtaO2)2] · 12H2O (1) was obtained after evaporation
at room temperature for one week and isolated by filtration, washed with ethanol
and dried in air. Anal. Found (Calcd): C, 20.1 (20.2); H, 3.9 (4.1); N, 4.8 (4.7). 2
and 3 are prepared similarly.
[18] Crystal data for 1:
C20H48N4K5LaO32, M = 1191.03, monoclinic, P 21/n,
a = 10.3453(2), b = 23.5084(7), c = 17.8976(5), β = 97.450(2)°, V = 4316.0(2)
Å3, T = 123 K, Z = 4, Dc = 1.833 g · cm−3, F(000) = 2416, 43873 reflections
collected, 9909 unique (Rint = 0.0430), GOF = 1.105, final R indices [I > 2σ]:
R1 = 0.0577, wR2 = 0.1294. Crystal data for 2: C20H44N4K5CeO30, M = 1159.18,
monoclinic,
P
21/n,
a = 10.3116(2),
b = 23.2884(7),
c = 17.8318(4),
β = 97.880(2)°, V = 4241.7(2) Å3, T = 173 K, Z = 4, Dc = 1.779 g · cm−3
,
[24] A. Albini, S. Pietra, Heterocyclic N-Oxide, CRC Press, Boca Raton, 1991.
F(000) = 2260, 20324 reflections collected, 9384 unique (Rint = 0.0518),
GOF = 1.269, final R indices [I > 2σ]: R1 = 0.0776, wR2 = 0.1472. Crystal data