Lanthanide complexes of ethylenediaminotetramethylene-phosphonic acid

Article abstract of DOI:10.1023/A:1019943319341

Ethylenediaminotetramethylenephosphonic acid (EDTMP, H₈L) was prepared and its complexes with some lanthanide ions (La, Eu, Gd and Sm) were isolated under various conditions. IR spectra and thermal stabilities of EDTMP and its complexes were studied. The experimental conditions of the preparation influence to the composition of the complexes were shown. In alkaline solution (pH=8) deprotonated (P(O)(O^-)₂), and in acidic solution (pH=3-4) deprotonated and partly protonated (P(O)(O^-)(OH)) and non-protonated (P(O)(OH)₂) phosphonic groups are present in the complexes. All the complexes contain coordinated water molecules. The complexes containing a protonated phosphonic group contain coordinated and hydrogen-bonded water molecules.

Full text of DOI:10.1023/A:1019943319341

Journal of Thermal Analysis and Calorimetry, Vol. 69 (2002) 427–439
LANTHANIDE COMPLEXES OF
ETHYLENEDIAMINOTETRAMETHYLENE-
PHOSPHONIC ACID
1
E. Princz , I. Szilágyi , K. Mogyorósi and I. Labádi
2
3
2
1
Romanian Academy, Timisoara Branch, Inorganic Chemistry Laboratory, Timisoara, Romania
University of Szeged, Department of Inorganic and Analytical Chemistry, Szeged, Hungary
University of Szeged, Department of Colloid Chemistry, Szeged, Hungary
2
3
(Received December 3, 2001; in revised form January 18, 2002)
Abstract
8
Ethylenediaminotetramethylenephosphonic acid (EDTMP, H L) was prepared and its complexes
with some lanthanide ions (La, Eu, Gd and Sm) were isolated under various conditions. IR spectra
and thermal stabilities of EDTMP and its complexes were studied. The experimental conditions of
the preparation influence to the composition of the complexes were shown. In alkaline solution
–
pH=8) deprotonated (P(O)(O )
(
2
), and in acidic solution (pH=3–4) deprotonated and partly
) phosphonic groups are present in the
–
protonated (P(O)(O )(OH)) and non-protonated (P(O)(OH)
2
complexes. All the complexes contain coordinated water molecules. The complexes containing a
protonated phosphonic group contain coordinated and hydrogen-bonded water molecules.
Keywords: aminophosphonates, ethylenediaminotetramethylenephosphonic acid, IR spectra,
NMR contrast agent, thermoanalytical studies
Introduction
The development of nuclear magnetic resonance (NMR) imaging techniques as a clinical
diagnostic modality has prompted the need for a new class of pharmaceuticals. These
drugs would be administrated to a patient in order to (a) enhance the image contrast be-
tween normal and diseased tissue and/or (b) indicate the status of organ function or blood
1
flow. The intensity of the H NMR images, largely composed of the NMR signal of the
water protons, is dependent on the nuclear relaxation times. Complexes of paramagnetic
transition metal and lanthanide ions, which can decrease the relaxation times of nearby
nuclei via dipolar interactions, have received most attention as potential contrast agents,
and numerous complexes of polyaminopolycarboxylic acid with these metal ions have
been studied from this respect [1–5]. The structural similarities between the
polyaminopolycarboxylic and polyaminopolyphosphonic acids (replacement of the
carboxylic acid groups by organophosphonic acid groups) generally make them more se-
lective towards cations with an enhanced coordination capacity [6]. The specific nature
1
418–2874/2002/ $ 5.00
2002 Akadémiai Kiadó, Budapest
Akadémiai Kiadó, Budapest
©
Kluwer Academic Publishers, Dordrecht

4
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PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
of such chelating agents is due to the presence of the tetrahedral phosphonic group as
compared to the planar carboxylic group and the greater polarizability of the P–O bond
[7].
In the present work, the complexes of ethylenediaminotetramethylene-
phosphonic acid (EDTMP) with lanthanide ions were isolated and their IR spectra
and thermal stabilities were studied. The aims of the work were to determine the ef-
fects of the method of preparation on the compositions of the complexes, and on the
type of binding of the water molecules, and to choose the most appropriate method of
preparation of the desired complexes.
Experimental part
Preparation of EDTMP
EDTMP was obtained via a Mannich reaction of ethylenediamine with phosphorous
acid and formaldehyde according to Eq. (1).
C H N +4H PO +4CH O=4H O+EDTMP
2
(1)
2
8
2
3
3
2
The optimum result was obtained when the reaction was carried out at low pH,
e.g. in the presence of concentrated hydrochloric acid. Concentrated HCl p.a. was
added dropwise to 0.125 mol 1,2-ethanediamine, followed slowly by 0.5 mol phos-
phorous acid dissolved in water. The mixture was refluxed for 30 min and 1 mol 38%
aqueous formaldehyde solution was added dropwise, over a 2 h period. The reaction
was refluxed for a further 3 h, and then allowed to cool and stand at room tempera-
ture. For crystallization, ethanol was added to this solution. The crystals that precipi-
tated out were filtered off and recrystallized twice from ethanol solution. The yield
was ca 60%. It was found that high acidity increased the rate of reaction and im-
proved the yield. The white crystalline material had m.p.: 213/215°C.
Preparation of complexes
Method I
EDTMP (or its Na salt) and metal nitrate or acetate were mixed in a ratio of 1:1 or 1:2,
under continuous heating. No pH adjustment was used. (After the mixing, a white
precipitate was formed immediately). The precipitate was removed from the solution
by suction, washed with ethanol, dried and stored under laboratory conditions. Lan-
thanum (samples 1 and 2) and samarium (samples 4 and 5) complexes were prepared
in this way. According to the literature [8–10], the compositions of these complexes
are LnH L×xH O (1 and 4) and Ln (H L) ×xH O (2 and 5) (H L=ligand). The water
5
2
4
4
3
2
8
contents of complexes were dependent of the storage conditions.
J. Therm. Anal. Cal., 69, 2002

PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
429
Method II
A solution of 0.02 mol EDTMP (acid form) dissolved in water was adjusted to pH=8
by the addition of NaOH solution. 0.02 mol lanthanide salt (nitrate or acetate) was
then added, the solution was heated for a short time, the pH was adjusted to pH=8
with NaOH solution and heating was continued for a further 1 h. The complex precip-
itated on the addition of absolute methanol. The solid product was washed with ace-
tone and then dried at 80°C. The complexes are very soluble in water. Lanthanum (3),
samarium (6), europium (7) and gadolinium (8) complexes were prepared in this way.
Compositions of complexes
+ 3+
Metal ion (Na and Ln ) analysis supported the compositions LnH L×xH O and
5
2
Ln (H L) ×xH O for samples 1, 2 and 4, 5 (method I), and the general formula
Na LnL×xH O for samples 3, 6, 7 and 8 (method II). All complexes contained water
4
4
3
2
5
2
molecules.
IR spectra showed a little contamination of the lanthanum (samples 1–3) and sa-
marium (sample 6) complexes with nitrate and acetate ions, respectively.
IR study
IR spectra of EDTMP and its complexes were taken in the JATE UV-VIS Laboratory
of the University of Szeged with Biorad instruments. The KBr disc method was used.
Thermal analysis
Thermal analysis was performed with a computer-controlled MOM derivatograph
(
Q-1500 D), using 50–100 mg samples. The measurements were made between room
–
1
temperature and 600, 800 or 1000°C by a dynamic technique, with a 5°C min heat-
ing rate, using ceramic crucibles and air atmosphere. The reference substance was
a-Al O . To determine the decomposition scheme, the IR spectra of the final decom-
2
3
position products were recorded.
IR study
IR spectrum EDTMP
The characteristic IR bands of EDTMP (prepared according to the above procedure)
are shown in Table 1. The observed absorption bands are assigned to the hydrogen
+
bound to the protonated quaternary nitrogen atom (ºNH ), to the OH groups, binding
to the P=O group with hydrogen bond, to the free P=O group and two different CH2
groups. It could be observed also the hydrogen bound water molecules.
Broadening of the bands could be observed in the ranges 2000–4000 and
–
1
1000–1400 cm , which can be interpreted by the presence of (many) strong hydro-
gen-bonds.
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30
PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
Table 1 Characteristic bands of EDTMP and their assignments
–
1
Band maximum (cm )
Assignment
3
3
2
2
1
1
1
8
5
402s, 3246s
n(OH, water)
+
n(ºNH )
026s, 3014s
954s, 2924vs, 2854s
767m, 2611m, 2308m
653m, broad
n
as and nsym of CH
n(OH, POH)
dHOH
2
461m, 1438m, 1413w, 1379, 1321w
261s, 1207s, 1121vs, 1008vs, 953vs
38w, 792w, 776w, 748w
73m, 532m, 487m
2
dCH or/and n(C–N)
n(P=O, P–O(H)
n(C–P)
n(OH, POH)
IR spectra of complexes
The characteristic IR bands of the investigated complexes are shown in Tables 2 and 3.
Table 2 Characteristic IR bands of complexes prepared by method I
EDTMP
1
2
4
5
Assignment
3
3
402, 3246
026, 3014
3403
3011, 3007
3379
3007
3385
3006
n(OH)
n(ºNH )
3397
+
2
767, 2611
2
2760, 2627
2345
2775, 2627
2310
–
2330
2626
n(OH, POH)
d(HOH)
308
2331
1641
1
652
1640
1658
1648
1
1
461, 1438
412, 1322
–
1436
–
1440
–
1436
1437
–
dCH
2
or/and n(C–N)
1
1
1
261, 1207
121, 1088
036, 1008
1171
1080
–
1167
1083
–
1169
1087
–
1170
1085
–
n(=O, P–O(H))
9
51
–
945
936
941
8
7
38, 792
76, 746
838
750
–
850
752
–
–
764
–
–
–
734
n(C–P)
7
42
5
5
4
74
31
87
557
–
–
552
–
–
552
463
–
551
455
–
n(P=O, P–O(H))
–
1
2
000–4000 cm range
In this wavenumber range, similarly as in the IR spectrum of EDTMP, n(OH)
was observed in the spectra of all the complexes. This band points to the presence of
water molecules in the complexes, as suggested by the elemental analysis. The
+
n(ºNH ) and nOH(PO) bands were observed only in the spectra of complexes 1, 2, 4
J. Therm. Anal. Cal., 69, 2002

PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
431
and 5 (prepared by method I), and not those complexes 3, 6, 7 and 8 (prepared by
method II), which supports the suggested compositions.
Table 3 Characteristic IR bands of complexes prepared by method II
EDTMP
3
6
7
8
Assignment
3
3
3
3
402
426
026
014
3415
3428
3419
3404
–
–
–
–
–
–
–
–
–
–
–
–
n(OH)
n(ºNH )
+
2
2
2
767
611
308
–
–
–
–
n(OH, POH)
d(HOH)
1
652
1651
1640
1648
1660
1
1
1
1
461
438
412
322
1461
1461
1
1
475
–
413
–
–
–
–
–
–
dCH
2
or/and n(C–N)
–
1
1
261
207
–
1157
–
1085
–
–
–
–
–
–
–
1
121, 1088
1
1
1110
–
–
992
–
1114
–
1054
983
–
1112
–
1055
982
–
036
008
n(P=O, P–O(H)
9
51
–
994
–
–
–
–
–
771
–
8
38
92
76
46
42
–
773
–
–
–
–
–
772
–
–
784
–
–
–
7
7
7
7
n(C–P)
–
–
5
5
4
74
31
87
588
–
–
580
–
–
581
483
–
569
475
–
n(P=O, P–O(H)
–1
1300–1800 and 700–950 cm
–
One or two bands at around 1550 and 1700 cm range support the presence of water
1
–
1
molecules in the complexes. The band at around 1650 cm points to the presence of
water molecules, bound by one coordination bond or one coordination bond and one
hydrogen bond. The band at higher wavenumber (or the shoulder on the band at
–
1
1
650 cm ) refers to the presence of a water molecule bound by hydrogen-bond only.
–
1
The absence of a band in the range 1500–1600 cm demonstrates the absence of a
water molecule bound by two coordination bonds.
The skeletal vibration (dCH or/and n(V–N) and n(C–P) bands were observed
2
for all the complexes, as in the IR spectrum of EDTMP.
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4
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PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
–
1
9
50–1300 and 400–700 cm wavenumber range
Relatively to the IR spectrum of EDTMP, the numbers of band’s maxima for the
complexes in these ranges decreased drastically. The reason for this phenomenon is
the change in the hydrogen-bonding network. In the EDTMP molecule, there are
many possibilities to form intra- and intermolecular hydrogen-bonds between the ni-
trogen atom and the HO(O)P group, and between the P=O oxygen and the P(O)OH
groups. In the complexes 3, 6, 7 and 8, the OH in the phosphonic groups is
deprotonated, so the possibility to form a hydrogen-bond via this OH group is virtu-
ally zero.
The bands in this range are assigned to the P–O bonds, the number of bands is
–
dependent on the –P(O)(OH) ), –P(O)(OH)O , –P(O)(O ) structures. In the case of
–
2
2
–
C and C symmetry of a –P(O)(O ) tetrahedron, 2 or 3 bands can be observed in the
3
v
2v
2
–
1
ranges 950–1300 and 400–700 cm . For the investigated complexes, 3 or 4 bands and
or 2 bands with shoulders were detected in these ranges, respectively, which can be
1
interpretate by the presence of two or more different –P(O)(O ) tetrahedra with dif-
ferent symmetries.
–
2
Thermal study
Thermal behaviour of EDTMP
The thermal analysis suggests the presence of »8% water content, which is approxi-
mately equivalent to 2 water molecules per EDTMP molecule in a stoichiometric
sense. The sample loses its water content in two steps (below 150°C and between 150
and 200°C); the ratio of the mass losses in two steps is 3 to 1. The mass loss is accom-
panied by an endothermic effect (Fig. 1).
Fig. 1 Thermoanalytical curves of decomposition of EDTMP
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PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
433
Between 200 and 490°C, EDTMP sample loses mass continuously, with 3 peaks
in the DTG curve, and the process is accompanied by an exothermic effect. The mass
losses are equivalent to approximately 1, 1 and 2 water molecules. The remaining
sample lost its mass accompanied by an exothermic effect, in two steps between 490
and 700°C. The suggested final product is P O (in the first step) and (PO ) in the
2
5
2 x
second step (Eq. (2)). The exact determination of the decomposition scheme is com-
plicated by the sublimation of P O at higher temperature.
2
5
EDTMP×2H O ® EDTMP×0.5H O ® EDTMP ® P O ® (PO )
2 x
(2)
2
2
2
5
The water molecules can be bound to EDTMP via hydrogen-bonds. The thermal
behaviour of the water molecules reflects the existence of a medium and a very strong
hydrogen-bond. These bonds are responsible for the broad IR band with maximum at
–
1
1653 cm . A similar IR band can be observed for hydrated inorganic compounds in
which the water molecules are coordinated to the metal ion by one coordination bond.
Thermal behaviour of complexes
From the point of view of the thermal behaviour, the investigated complexes could be
divided into two groups (A and B), consisting of the complexes prepared by method I
and II, respectively. The thermoanalytical curves characteristic of groups A and B are
shown in Figs 2 and 3. The main differences between the complexes in groups A
and B are the thermal behaviour of the water molecules, the stability of the anhydrous
complexes between 250 and 500°C and the IR spectra of the final products.
Thermal behaviour of water molecules in complexes
The complexes in groups A and B lose their water content in two (group A) or one
step (group B), in the temperature ranges 50–150, 150–250 and 50–154°C, respec-
tively, which shows the presence (or formation in the course of heating) of differently
Fig. 2 Thermoanalytical curves of the decomposition of complex 1 (decomp. type A)
J. Therm. Anal. Cal., 69, 2002

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PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
Fig. 3 Thermoanalytical curves of the decomposition of complex 6 (decomp. type B)
bonded water molecules in the complexes. Similar thermal behaviour was observed
in the case of EDTMP.
The water molecules released from the metal complexes in the range 50–150°C
are bound by hydrogen-bounding or by one coordination bond. The water molecules
released at higher temperature (between 200 and 300°C) are bound to the metal ions
by two coordination bonds or a very strong hydrogen-bond, as in the case of EDTMP.
The water molecules of different types give different IR spectra in the range
–
1
1
weak hydrogen-bonds only, with one or two coordination bonds, contain absorption
500–1800 cm . The IR spectra of compounds containing water molecules bound by
–
bands in the ranges 1700–1800, 1600–1700 and 1500–1600 cm , respectively. One
1
strong hydrogen-bond and coordination bond result in approximately the same IR
–
1
bands in the range 1600–1700 cm .
In the IR spectra of EDTMP and all the investigated complexes, a medium
–
1
strong band could be observed in the range 1600–1700 cm (Tables 1–3), which re-
flects the presence of water molecules bound by a strong hydrogen-bond (EDTMP)
or one coordination bond in all the investigated complexes and these water molecules
are released in the temperature range 50–150°C.
In EDTMP and complexes 1, 2, 4 and 5 (which contain P(OH)OH groups), there
–
1
are water molecules which give IR bands in the range 1600–1700 cm (bound by a
hydrogen-bond or one coordination bond) which are released above 150°C (150–200
and 150–250°C). A strong hydrogen-bond could be suggested between the hydro-
gen-bonded or coordinated water molecule and the free POH group, which did not
–
1
change the IR spectrum of this water molecule in the range 1600–1700 cm , but in-
creased the temperature range of water release from the solid phase. In complexes 3,
6, 7 and 8, which do not contain P(O)OH groups, all the water molecules are released
between 50 and 150°C.
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PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
435
Thermal behaviour of complexes between 250 and 500°C
The complexes containing P(O)OH groups (group A) decompose continuously with a
relatively small loss of mass, while the complexes in group B show no change in the
TG curves in this temperature range. The loss of mass for complexes A, similarly as
for EDTMP, is assigned to the release of water molecules from the P(O)OH groups.
Thermal behaviour of complexes above 500°C
The anhydrous complexes decompose in two overlapping steps above 500°C, accom-
panied by exothermic effect.
Final decomposition products of complexes
An important difference between the investigated complexes (as regards composi-
tion) is the ratio of the charges of all metal ions to the number of phosphorus atoms in
the molecule. The complexes (1, 2, 4 and 5) prepared by method I (group A) contain
3
–
4–
partly protonated ligand (H L or H L ) as complex-forming agent and the
lanthanide metal ion. The ratio of the metal ion charge to the number of phosphorus
5
4
atoms is 3 to 4 or 12 to 12. The complexes (3, 6, 7 and 8) prepared by method II
(
8
group B) contain the fully deprotonated ligand (L ) and sodium and lanthanide ions,
–
the ratio of the total metal ion charge to the number of phosphorus atoms is 8 to 4.
On the basis of both theoretical considerations and the earlier experimental ob-
servations [11–13] lanthanide and sodium phosphates, linear or cyclic
polyphosphates, could be suggested as the final decomposition products of the inves-
tigated complexes. The compositions of the different inorganic phosphates, linear
polyphosphates, cyclic metaphosphates and ratio of the total metal ion charge to the
number of phosphorus atoms (R) are shown in Table 4.
Table 4 Compositions of inorganic phosphates, linear polyphosphates, cyclic metaphosphates
and ratio of total metal ion charge to number of phosphorus atoms (R)
Composition
Name
R
Na
Na
Na
Na
Na
Na
Na
Na
3
4
5
6
3
4
5
6
PO
4
or LnPO
4
sodium or lanthanum phosphate
3:1
4:2
5:3
6:4
3:3
4:4
5:5
6:6
P
P
P
2
3
4
O
O
O
7
or Ln (P
4
2
O
7
)
3
sodium or lanthanum pyrophosphate
sodium or lanthanum tripolyphosphate
sodium or lanthanum tetrapolyphosphate
sodium or lanthanum trimetaphosphate
sodium or lanthanum tetrametaphosphate
sodium or lanthanum pentametaphosphate
sodium or lanthanum hexametaphosphate
10 or Ln
11 or Ln
5
2
(P
(P
3
4
O
O
10
11
)
)
3
(PO
(PO
(PO
(PO
3
3
3
3
)
)
)
)
3
4
5
6
2 3 3
or Ln (PO )
or Ln4/3(PO
or Ln5/3(PO
3
)
4
5
3
)
2 3 6
or Ln (PO )
It can be seen that for simple phosphates and linear polyphosphates R>1, while for
cyclic methaphosphates R=1. On the basis of R, the final products in the decomposition
J. Therm. Anal. Cal., 69, 2002

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PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
of the investigated complexes can be suggested to be cyclic lanthanide metaphosphates
2 and 5), cyclic lanthanide metaphosphates and P O (1 and 4), sodium and lanthanum
pyrophosphates (R=2) or a mixture of Na PO or LnPO and their linear polyphosphates
(
2
5
3
4
4
(
Na P O , La (P O ) , Na P O , Ln (P O ) (3, 6, 7 and 8).
To determine the compositions of the final decomposition products of the investi-
4 2 7 3 2 7 3 5 3 10 5 3 10 3
gated complexes, the IR spectra of the final products of thermal analysis of the com-
plexes and of some inorganic phosphate derivatives were recorded. The experimentally
found characteristic IR bands and some literature data are presented in Tables 5 and 6.
Table 5 Characteristic IR bands of decomposition products of complexes
1
2
4
5
3
6
7
1
1
1
35sh
313s
277s
1357s
1300s
1278s
1363s
1300s
1279s
1362s
1300s
1280s
–
–
–
–
–
–
–
–
–
–
152
–
124
090
056
017
1
1
1
173m
160m
146m
–
–
043
–
1172m
1150
–
1116
–
–
1156
–
1125
–
1172sh
1153
1146
–
–
1152
–
1129
–
1067
1022
–
–
–
1
sh
1124
–
1062
1029
988
965
1
1
1
1
–
–
1
–
–
1018
981
–
1016
–
960
1022
986
–
988
9
80
–
–
964
–
–
–
–
908
908
908
7
7
93
60
–
–
91
–
sh
775
–
715
708
805
767
750
716
700
–
–
–
734
–
–
–
–
734
–
–
–
–
734
–
762
748
712
–
6
–
–
–
–
–
–
–
–
622
618
634
–
628
618
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
576
–
–
492
–
574
558
523
492
426
575
560
522
492
428
There are many similarities and differences between the IR spectres of the final
decomposition products of the complexes prepared by methods I and II.
a. The final decompositions products of the complexes prepared by method I
–
group A of complexes) give absorption bands above 1200 cm .
1
(
–
b. In the range 950–1200 cm all the decomposition products give many bands.
1
c. The decomposition products of the complexes prepared by method II have a
–
strong absorption band at around 908 cm .
1
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PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
437
–
d. In the range 650–900 cm , many IR bands are observed for the complexes
1
prepared by method I, but only one band for the complexes prepared by method II.
–
e. Below 650 cm , only the decomposition products of complexes B furnish ab-
1
sorption bands. The linear polyphosphates have 4-4 absorption bands in each of the
–
1
–1
ranges 950–1200 and 400–700 cm and 1 band each at around 910 and 740 cm .
This behaviour is close to the case for the decomposition products of the complexes
prepared by method II, and it supports the hypothesis that the decomposition prod-
ucts of these complexes are sodium and lanthanide pyrophosphate or a mixture of
phosphate, pyrophosphate and tripolyphosphate.
Table 6 Characteristic IR bands of inorganic phosphate derivatives
4
7
-
5-
P3O10
-
P2O
(PO )
3
6
–
–
1277
–
–
117
110
–
1221
1176
1124
–
1085
–
–
1153
–
1
1
–
1095
–
984
1
023
9
95
–
9
22
903
–
–
–
758
–
878
773
721
7
34
–
–
665
611
569
503
470
–
–
–
505
497
6
37
76
94
75
5
4
4
–
Cyclic sodium hexametophosphate has a band at 1277 cm , but no band be-
1
–
1
tween 900 and 950 cm . This is characteristic of the decomposition products of the
complexes prepared by method I. These facts support the hypothesis that the decom-
position products are different cyclic metaphosphates and P O .
2
5
Composition and decomposition schemes of EDTMP and its
lanthanide complexes
On the basis of the thermoanalytical curves and IR spectra of the complexes and their
decomposition products, the following decomposition schemes can be suggested for
the complexes 1, 4 and 2, 5 and 3, 6, 7, 8 Eqs (3), (4) and (5), respectively.
LnH L×(x+y)H O ® LnH L×xH O ® LnH L ® Ln(PO ) +1/4P O
5
(3)
5
2
5
2
5
3 3
2
J. Therm. Anal. Cal., 69, 2002

4
38
PRINCZ et al.: ETHYLENEDIAMINOTETRAMETHYLENEPHOSPHONIC ACID
Ln (H L) ×(x+y)H O ® Ln (H L) ×xH O ® Ln (H L) ® 4Ln(PO )
3 3
(4)
(5)
4
4
3
2
4
4
3
2
4
4
3
LnNa L×yH O ® LnNa L ® 5/4Na P O +1/4Ln (P O )
7 3
5
2
5
4
2
7
4
2
According to the IR spectra and the thermal behaviour, two different types of
water molecules are present in EDTMP and its complexes: One of them is bound by a
hydrogen-bond or one coordination bond (EDTMP and the B complexes) and is re-
leased from the solid phase below 150°C. The second type of water molecules is
bound by a strong hydrogen-bond (probably a double hydrogen-bond in the case of
EDTMP) or by one coordination bond and a hydrogen-bond via a P(O)OH group.
This kind of water molecule is released from the solid phase between 150 and 250°C.
The TG curves of the complexes give the possibility to determine the amounts of
both kinds of water in the complexes. For the complexes prepared by methods I
and II, the mass of the sample at 250 and 150°C is considered the mass of the anhy-
drous compounds.
Conclusions
It has been shown that the pH of the solution in which the preparation is performed,
determines the compositions of the complexes and the type of binding the water mol-
ecules. If deprotonated phosphonic groups are present in the complexes, the water
molecules bind weakly to the lanthanide ion. In the presence of a protonated
phosphonic group in the complexes, the water molecules bind to the lanthanide ion in
two different ways. The OH group gives a possibility for the coordinated water mole-
cule to form a hydrogen-bond too. This phenomenon is caused by the protonation
process of the ligand. EDTMP has 8 dissociable protons, with different pK values.
The pK values, depending on the pH, determine the speciation of the ligand and con-
sequently the compositions of the complexes.
In this way, the pH of the solution may determine the type of binding of the wa-
ter molecules in the complexes in both solution and the solid phase. Different behav-
iour of these water molecules can be suggested if the complexes are used as contrast
agents in NMR relaxation experiments.
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2
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5
6
7
8
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