The coupled TG-MS investigations of lanthanide(III) nitrate complexes with hexamethylenetetramine

Article abstract of DOI:10.1023/B:JTAN.0000046116.43443.36

New transition metal compounds of the general formula Ln(NO ₃)₃·2[N₄(CH₂) ₆]·nH₂O, where Ln = La, Nd, Sm, Gd, Tb, Dy, Er, Lu, and n = 7-12, were obtained. The compounds and the gases evolve

Full text of DOI:10.1023/B:JTAN.0000046116.43443.36

Journal of Thermal Analysis and Calorimetry, Vol. 78 (2004) 525–534
THE COUPLED TG-MS INVESTIGATIONS OF
LANTHANIDE(III) NITRATE COMPLEXES WITH
HEXAMETHYLENETETRAMINE
*
M. Zalewicz and A. Trzesowska
Institute of General and Ecological Chemistry, ˜ódü University of Technology, ¨ekomskiego 116,
90-924 ˜ódü, Poland
Abstract
3 3 4 2 6 2
New transition metal compounds of the general formula Ln(NO ) ·2[N (CH ) ]·nH O, where Ln =
La, Nd, Sm, Gd, Tb, Dy, Er, Lu, and n = 7–12, were obtained. The compounds and the gases evolved
in the course of their thermal decomposition were characterised by thermogravimetric analysis. The
measurements were carried out in air and argon environment in order to compare the intermediate
products, final products and the mechanism of the thermal decomposition. The combined TG-MS
system was used to identify the main volatile products of thermal decomposition and fragmentation
processes of the obtained compounds.
Keywords: hexamethylenetetramine, lanthanide complexes, mass spectroscopy,
thermogravimetry
Introduction
The lanthanides (Ln) chemistry is a field of the science, which has developed rapidly
in the recent years because of their huge and versatile applications [1–5]. Their im-
portant feature is a relatively steady variations of properties across the group, which
results from the ‘lanthanide contraction’ (the lanthanides have large ionic radii,
which decrease continually with the increasing of the element atomic number). This
affects the coordination numbers (CN) and the coordination geometry of their com-
n+
plexes. The CN of a Ln ion is very rarely six, and higher coordination numbers
(7–12) appear to dominate. The most common ligands existing in lanthanide com-
pounds are these ones containing oxygen atom.
The present work is a continuation of our study on properties of lanthanide com-
pounds with hexamethylenetetramine (HMTA) [6–14]. The aim was to obtain lanthanide
nitrates complexes with HMTA in the solid state. The TG-MS analysis was performed
on complexes in order to obtain information about their thermal stability and propose the
scheme for their thermal decomposition in the air and argon atmosphere.
*
Author for correspondence: E-mail: zalewicz@p.lodz.pl
1
388–6150/2004/ $ 20.00
Akadémiai Kiadó, Budapest
©
2004 Akadémiai Kiadó, Budapest
Kluwer Academic Publishers, Dordrecht

5
26
ZALEWICZ, TRZESOWSKA: LANTHANIDE(III) NITRATE COMPLEXES
Experimental
Materials, synthesis and analysis of the complexes
The following reagents were used for the syntheses: analytically pure
hexamethylenetetramine (Fluka AG), 99% pure lanthanide oxides and cerium car-
bonate (Aldrich and Fluka AG), concentrated HNO (Chempur).
3
The lanthanide complexes of higher hydration state were prepared by crystallisation
from water solutions. The samples of lanthanide oxides were dissolved in possible small-
est amount of nitric acid (1:1) (the salt of cerium nitrate was prepared by dissolving the
cerium carbonate in HNO ). The obtained solutions were cooled and then mixed with
3
cooled saturated aqueous solutions of hexamethylenetetramine (molar ratio 1:2). The
mixtures were placed in a refrigerator and left to crystallise at 5°C. The crystals were fil-
tered and dried in air. The content of lanthanides was determined by complexometric ti-
tration using EDTA vs. xylenol orange [15].
Physical measurement
The thermal analysis was carried out in a TG/DTA-SETSYS-16/18 thermoanalyser
coupled with ThermoStar (Balzers) mass spectrometer. The samples (9.5–14.8 mg)
–
1
were heated in corundum crucibles up to 1000°C at the heating rate 10°C min in air
and argon atmosphere. The products of decomposition were calculated from TG
curves. The temperature ranges were determined by means of thermoanalyser Data
Processing Module [16]. The thermogravimetric analysis in argon atmosphere of the
neodymium salt was not carried out because of the rapidly proceeding decomposition
process in air atmosphere, which caused the contamination of the apparatus.
Results and discussion
Thermal decomposition in air atmosphere
The thermoanalytical curves TG, DTA, DTG of lanthanum decahydrate salt decom-
position are presented in Fig. 1a. The decomposition is multi-stage process. The first
endothermic step of thermal decomposition occurs within the temperature range
o
6
1–180 C. The initial mass loss is attributed to the removal of water molecules. The
DTA curve indicates that it is multi-stage dehydration process, but the overlapping
peaks make it impossible to distinguish individual steps. The mass spectrum of the
thermal decomposition of lanthanide nitrates complexes with HMTA shows the ion
+
current signals m/z=17, 18, which correspond to the molecular mass of OH and
+
H O . The succeeding stages are exothermic. The next step of decomposition is char-
acterised by strong, narrow peaks on the DTA, DTG curves and the biggest mass
2
o
loss. According to the detected gaseous products, in the range 180–400 C the decom-
position of the nitrates, hexamethylenetetramine molecules and removal of water
molecules begins simultaneously. Such high temperature of the dehydration can indi-
cate that some oxygen atom of the water molecules (m/z=16, 17, 18 corresponds to
J. Therm. Anal. Cal., 78, 2004

ZALEWICZ, TRZESOWSKA: LANTHANIDE(III) NITRATE COMPLEXES
527
Fig. 1 TG, DTG and DTA curves of a – La(NO
b – Er(NO ) ·2[N (CH ) ]·10H O (in argon)
3
)
3
·2[N
4 2 6 2
(CH ) ]·9H O (in air) and
3 3 4 2 6 2
+
O , OH and H O ) must be coordinated directly to the lanthanum ion. The exother-
+
+
2
mic effect connected with combustion of organic ligand is so strong that it can mask
the endothermic effect attached with the decomposition of the nitrates and dehydra-
tion process. The main volatile products include ammonium (m/z=17) and formalde-
hyde (m/z=30), probably as a result of HMTA decomposition. The hydrated
hexamethylenetetramine decomposes into ammonia and formaldehyde by heating
o
over 60 C [17]. According to the Guo et al. [18] one of the HMTA decomposition
product may be the hydrazine but the MS peaks with m/z=32, which corresponds to
+
the N H , does not appear for investigated compounds. The signal with m/z=44 can
2
4
+
be attributed to CO , which comes from the HMTA decomposition. This is sup-
2
+
ported by the fact that the parallel peak for CO (m/z=28) is observed in the mass
spectrum for the thermal decomposition in argon. The nitrates decompose gradually
with releasing nitrogen oxide (m/z=30), dinitrogen oxide (m/z=44) and probably oxy-
gen (m/z=16). These oxides were reported as products of transition metal hydrated
nitrates thermal decomposition in previous works [19–21]. The MS spectra do not
J. Therm. Anal. Cal., 78, 2004

5
28
ZALEWICZ, TRZESOWSKA: LANTHANIDE(III) NITRATE COMPLEXES
display the presence of other typical nitrate decomposition products such as NO or
2
HNO . Even if NO is formed, the existence of NO instead of NO is observed proba-
3
2
2
bly due to the fragmentation process of NO to NO or following reaction NO + O →
2
2
NO + O takes place. According to MaÓecki et al. [20] the amount of nitrogen releas-
2
ing during heating in the form of the nitric acid does not exceed 5% of the total nitro-
gen. The thermal decomposition intermediate products may include La(NO ) (OH)
and LaO(NO ) respectively in the two next steps. The presence of such hydroxo- and
3
2
3
oxonitrates is observed in previous works [22, 23] concerning the thermal decompo-
sition of transition metal hydrated nitrates. Creating of LnOX (X = CO , Cl) com-
pounds was detected during thermal decomposition of lanthanide complexes
3
[
(
(
24–28]. During heating over 400°C the solid decomposes with forming the La O
2 3
o
final product) at 725°C. The two gaseous product were liberated over 400 C, the NO
m/z=30) and the N O (m/z=44) as a result of thermal decomposition of hydro- and
2
oxo-species. The previous investigations of the bromide salts of lanthanides(III) with
HMTA [7], especially the results of X-ray powder studies, confirm the final forming
of lanthanide oxide.
The mechanisms of the thermal decomposition in air environment of all
lanthanide nitrates complexes with HMTA differ from each other in the temperature
ranges, the number of stages and the values of mass loss. However they have a few
features in common: in the first stage the dehydration occurs, in the temperature
range 240–270°C the rapid decomposition of hexamethylenetetramine molecule
(
LnO(NO ) and the final product in all cases is lanthanide oxide. The thermal decom-
deamination process) takes place, the intermediate products are Ln(NO ) (OH),
3 2
3
position data are collected in Table 1. The temperatures of the oxides formation are
high and change in the lanthanide series from 550°C (Tb O ) to 725°C (La O ).
4
7
2
3
These temperatures are similar to these found during thermal analysis of lanthanide
complexes [24, 29].
Thermal decomposition in argon atmosphere
The thermal decompositions of lanthanide nitrates complexes with HMTA were car-
ried out in argon atmosphere in order to eliminate the reaction of principle volatile
products with air constituents. The TG, DTA, DTG curves of erbium decahydrate salt
decomposition in argon environment are presented in Fig. 1b. The complex is stable
up to 65°C. The decomposition is multi-stage. In the range 66–190°C the water mole-
cules are released from the outer and inner coordination sphere. In the DTA curve
such dehydration process appears as an endothermic peaks. The MS spectrum dis-
+
+
plays the presence of OH (m/z=17) and H O (m/z=18) in this step of decomposition
2
process. All succeeding steps are exothermic. According to the detected gaseous
o
products, in the range 190–420 C the decomposition of the nitrates,
hexamethylenetetramine molecules and removal of water molecules begins simulta-
+
+
neously. The HMTA decomposes to NH (m/z=17), HCOH (m/z=30). The signal
3
+
+
with m/z=28 can be attributed to N2 CO or CH N , which come also from the
+
2
HMTA decomposition. There are the ion current signals m/z=17, 18, which corre-
J. Therm. Anal. Cal., 78, 2004

ZALEWICZ, TRZESOWSKA: LANTHANIDE(III) NITRATE COMPLEXES
529
J. Therm. Anal. Cal., 78, 2004

5
30
ZALEWICZ, TRZESOWSKA: LANTHANIDE(III) NITRATE COMPLEXES
J. Therm. Anal. Cal., 78, 2004

ZALEWICZ, TRZESOWSKA: LANTHANIDE(III) NITRATE COMPLEXES
531
+
+
+
spond to the molecular mass of OH and H O . The nitrates decompose to NO
2
+
+
o
(
compound decomposes gradually. In the MS spectrum only one strong signal for
m/z=30), N O (m/z=44) and probably O (m/z=16). During heating over 420 C the
2
+
+
+
m/z=28 is observed. It may correspond to N CO or CH N . Over this temperature
2
2
+
there are also two small signals for m/z=16, 44, which correspond to O and N O . It
+
2
suggests that the decomposition of nitrate ions and hexamethylenetetramine mole-
o
cules still takes place. The product which forms at 1000 C is Er(NO ) (OH).
3
2
The thermoanalytical curves show a different pattern of decomposition in argon at-
mosphere (Table 2), distinct from the decomposition pattern in the air and different for
each lanthanide compound. For example, in case of lutetium compound decomposition,
the dehydration process starts at 40°C and the Lu
O forms at 860°C. However, in the
2 3
+
range 40–190°C the MS spectrum displays the presence of OH (m/z=17) and H O
+
2
(m/z=18) among the volatile products next to the broad peak for m/z=28, which may cor-
responds to N CO or CH N . It may suggest that the HMTA molecule starts to decom-
+
+
+
2
2
pose at the lower temperature, probably because organic compound was weaker bounded
in the crystal lattice. The complexes of lanthanum, gadolinium, terbium and lutetium de-
Fig. 2 Gaseous products of a – La(NO
b – Nd(NO ·2[N (CH ]·9H O the thermal decomposition in air (heating rate
0°C/cyc)
3 3 4 2 6 2
) ·2[N (CH ) ]·10H O and
3
)
3
4
2
)
6
2
1
J. Therm. Anal. Cal., 78, 2004

5
32
ZALEWICZ, TRZESOWSKA: LANTHANIDE(III) NITRATE COMPLEXES
Fig. 3 Gaseous products of a – Er(NO
b – Lu(NO ·2[N (CH ]·12H
rate 10°C/cyc)
3
)
3
·2[N
O the thermal decomposition in argon (heating
4 2 6 2
(CH ) ]·10H O and
3
)
3
4
2
)
6
2
compose to the lanthanide oxides while the samarium, dysprosium and erbium com-
pound form the hydroxo- and oxonitrates at 1000°C.
Conclusions
New lanthanide compounds of the general formula Ln(NO ) ·2[N (CH ) ]·nH O,
3
3
4
2 6
2
where Ln = La, Nd, Sm, Gd, Tb, Dy, Er, Lu, and n = 7–12, were obtained and charac-
terised by thermogravimetric analysis in conjunction with evolved gases. The mea-
surements were carried out in air and argon environment in order to determine the in-
termediate and final products, to establish the terms of the dehydration reaction and
to compare the main volatile products of the thermal decomposition.
The mechanisms of the thermal decomposition in air environment of all lanthanide
nitrates complexes with HMTA are similar to each other while the schemes for their ther-
mal decomposition in argon atmosphere are different. In all cases of decomposition car-
ried out in air, the dehydration process takes place in the initial stages and the compounds
decompose probably via Ln(OH) NO and LnO(NO ) to the lanthanide oxides. In the
2 3 3
J. Therm. Anal. Cal., 78, 2004

ZALEWICZ, TRZESOWSKA: LANTHANIDE(III) NITRATE COMPLEXES
533
case of decomposition carried out in argon the only common feature is the initial removal
of water molecules. The decomposition of the nitrates and HMTA molecules is
multi-stage combined process and generally it succeeds at higher temperatures, which
makes it impossible to distinguish precisely the stages and the intermediate products
from the TG curves. The complexes of lanthanum, gadolinium, terbium and lutetium de-
compose to the lanthanide oxides while the samarium, dysprosium and erbium com-
pound form the hydroxo- and oxonitrates at 1000°C.
It is worth stressing that the maxima of strong, narrow, exothermic peaks on the
DTA curves corresponding to the biggest mass loss are shifted to the side of higher
temperatures in case of decomposition carried out in argon. For instance, the DTA
peak (air) occurs at 249°C for the gadolinium salt and at 236°C for the dysprosium
salt. When the decomposition occurs in argon atmosphere the gaseous products can
not react with oxygen and the DTA peak occurs at 270°C and 250°C respectively. It
indicates that the simultaneous decomposition of nitrate ions and organic molecules
takes place earlier in air environment. Taking into account the fact of existence of the
+
+
+
signals for m/z=44, which correspond to N , CO or CH N in the MS spectra in ar-
2
2
gon at higher temperatures, it can be assumed that the decomposition
hexamethylenetetramine molecules takes place over 500°C.
According to the X-ray investigations [30], there is the change in the composition
of the lanthanide coordination sphere, adopted polyhedron and coordination number
with changing the size of Ln ions. The inner coordination spheres of the lanthanum and
neodymium [9] salts consist of two nitrate ions and six water molecules. The complexes
of samarium and gadolinium include one nitrate ion and seven water molecules in the in-
ner coordination sphere. In the inner coordination sphere of the dysprosium, erbium and
lutetium complexes, there are only water molecules, probably due to the steric effects.
The decrease of the ionic radius leads to the narrowing of the coordination sphere and en-
3+
hances the interaction of the Ln –OH bonds. The mechanisms of the thermal decompo-
2
sition in air environment of all lanthanide nitrates complexes with HMTA are the same
besides their different conformation. This can be explained by the fact that the HMTA
molecules and the nitrate ions may migrate from the outer coordination sphere to the in-
ner coordination sphere of the complex and bond directly to the Ln ion after the dehydra-
tion process, due to absence of the steric hindrance caused by the water molecules. This
is consistent with the presence of the absorption peaks from the vibration of the Ln-N
−
HMTA) and O-Ln-O (NO ) bonds in the infrared spectra of the complex of lower
(
hydration state (n = 0.5–3.5) [14].
3
*
* *
This work was supported financially by the KBN grant No. 7-T09B-110-21.
References
1
2
D. Carmona, M. P. Lamata and L. A. Oro, Coord. Chem. Rev., 200 (2000) 717.
D. M. Epstein, L. L. Chappell, H. Khalili, R. M. Supkowski, W. D. Horrocks and J. R. Mor-
row, Inorg. Chem., 39 (2000) 2130.
J. Therm. Anal. Cal., 78, 2004

5
34
ZALEWICZ, TRZESOWSKA: LANTHANIDE(III) NITRATE COMPLEXES
3
4
5
6
7
8
9
J. Feng, G. Sun, F. Pei and M. Liu, J. Inorg. Biochem., 92 (2002) 193.
J. L. Sessler and R. A. Miller, Biochem. Pharmacol., 59 (2000) 733.
U. Ulusoy and J. E. Whitley, Nutr. Res., 19 (1999) 675.
M. Zalewicz, Termochim. Acta, 149 (1989) 133.
M. Zalewicz, Termochim. Acta, 171 (1990) 131.
M. Zalewicz, Monatsh. Chem., 121 (1990) 249.
M. Zalewicz and T. J. Bartczak, J. Crystallogr. Spectrosc. Res., 23 (1993) 417.
1
1
1
1
1
1
0 G. Oczko and M. Zalewicz, J. Mol. Struct., 407 (1997) 63.
1 M. Zalewicz, Monatsh. Chem., 128 (1997) 431.
2 M. Zalewicz and B. GoliÕski, J. Chem. Crystalogr., 28 (1998) 879.
3 T. J. Bartczak, R. Kruszynski, A. Trzesowska and M. Zalewicz, Chem. Listy, 97 (2003) 636.
4 M. Zalewicz, Monatsh. Chem., in press.
5 A. I. Busiew, W. G. Tipcova and W. M. Ivanov, Analytical Chemistry of Rare Elements, WNT
Warsaw, Polish Translation, 1982.
1
1
6 Data processing Module. Copyright © 1994-1998 SETARAM - FRANCE; Version 1.4.
7 E. M. Smolin and L. Rapoport, The chemistry of heterocyclic compounds. s-Triazines and
derivatives, Interscience Publishers: New York, 1959, Vol. 13.
1
1
8 Y. C. Guo, S. R. Luan, Y. R. Chen, X. S. Zang, Y. Q. Jia, G. Q. Zhong and S. K. Ruan,
J. Therm. Anal. Cal., 68 (2002) 1025.
9 A. MaÓecki, R. Gajerski, S. ˜abuí, B. Prochowska-Klisch and K. T. Wojciechowski, J. Therm.
Anal. Cal., 60 (2000) 17.
20 A. MaÓecki, B. MaÓecka, R. Gajerski and S. ˜abuí, J. Therm. Anal. Cal., 72 (2003) 135.
21 K. T. Wojciechowski and A. MaÓecki, Termochim. Acta, 331 (1999) 73.
22 Z. D. Zivkovic, D. T. Zivkovic and D. B. Grujicic, J. Therm. Anal. Cal., 53 (1998) 617.
23 K. Wieczorek-Ciurowa and A. J. Kozak, J. Therm. Anal. Cal., 58 (1999) 647.
24 W. Brzyska, A. Tarnawska, A. Twardowska and E. Wiëniewska, J. Therm. Anal. Cal.,
6
8 (2002) 123.
2
2
2
2
2
3
5 W. Ferenc and A. Walków-Dziewulska, J. Therm. Anal. Cal., 70 (2002) 949.
6 W. Ferenc, B. Bocian and M. Chudziak, J. Therm. Anal. Cal., 58 (1999) 639.
7 W. Ferenc and B. Bocian, J. Therm. Anal. Cal., 74 (2003) 521.
8 W. Ferenc and B. Bocian, J. Therm. Anal. Cal., 55 (1999) 671.
9 A. Kula, J. Therm. Anal. Cal., 68 (2002) 957.
0 T. J. Bartczak, R. Kruszynski, A. Trzesowska and M. Zalewicz, in preparation.
J. Therm. Anal. Cal., 78, 2004