Complexation thermodynamics of rare earth (III) with crown ethers. 31.Calorimetric titration of light lanthanoid (III) nitrates with dibenzo-18-crown-6 in acetonitrile

Article abstract of DOI:10.1016/s0040-6031(98)00358-x

Calorimetric titrations have been performed in anhydrous acetonitrile at 25°C to give the complex stability constants (K_s) and the thermodynamic quantities for the complexation of light lanthanoid (III) nitrates (La-Gd) with 2,3,11,12-dibenzo-1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-diene (dibenzo-18-crown-6) (2). Data analyses, assuming 1 : 1 stoichiometry were successfully applied to all the light lanthanoid-crown ether combinations employed. Using the present and reported data, the complexation behaviors of (2) and 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) (1) are compared from the thermodynamic point of view. The rigid structure of (2), as compared with (1), gave the higher K_s for Nd³⁺ and Sm³⁺ among the light lanthanoid nitrates. The complex stability sequence as a function of reciprocal ionic diameter of lanthanoid showed a monotonically declining pattern for (1) except for a jump at Ce³⁺, and a characteristic peak profile at Nd³⁺ and Sm³⁺ for (2). Thermodynamically, the complexation of light lanthanoid nitrates with 18-crown-6 is mainly enthalpy-driven in acetonitrile, but the complexation with (2) is chiefly entropy-driven.

Full text of DOI:10.1016/s0040-6031(98)00358-x

Thermochimica Acta 317 (1998) 1±6
1
Complexation thermodynamics of rare earth (III) with crown ethers. 3 .
Calorimetric titration of light lanthanoid (III) nitrates
with dibenzo-18-crown-6 in acetonitrile
*
Yu Liu , Baohang Han, Zhihui Zhang, Jihui Guo, Yunti Chen
Department of Chemistry, Nankai University, Tianjin 300071, China
Received 23 October 1997; received in revised form 18 March 1998; accepted 27 March 1998
Abstract
Calorimetric titrations have been performed in anhydrous acetonitrile at 258C to give the complex stability constants (Ks)
and the thermodynamic quantities for the complexation of light lanthanoid (III) nitrates (La±Gd) with 2,3,11,12-dibenzo-
1,4,7,10,13,16-hexaoxacyclooctadeca-2,11-diene (dibenzo-18-crown-6) (2). Data analyses, assuming 1 : 1 stoichiometry were
successfully applied to all the light lanthanoid±crown ether combinations employed. Using the present and reported data, the
complexation behaviors of (2) and 1,4,7,10,13,16-hexaoxacyclooctadecane (18-crown-6) (1) are compared from the
3
‡ 3‡
thermodynamic point of view. The rigid structure of (2), as compared with (1), gave the higher K
the light lanthanoid nitrates. The complex stability sequence as a function of reciprocal ionic diameter of lanthanoid showed a
s
for Nd and Sm among
3‡
3‡
3‡
monotonically declining pattern for (1) except for a jump at Ce , and a characteristic peak pro®le at Nd and Sm for (2).
Thermodynamically, the complexation of light lanthanoid nitrates with 18-crown-6 is mainly enthalpy-driven in acetonitrile,
but the complexation with (2) is chie¯y entropy-driven. # 1998 Elsevier Science B.V.
Keywords: Complexation; Crown ether; Rare earth (III); Stability
1
. Introduction
the induced three-dimensional cavity of carbon-pivot
6-crown-5 lariat(s) shows poor cation selectivity for
1
Enormous effort has been devoted to thermody-
the light lanthanoid series [5]. More recently, we have
demonstrated that the nitrogen-pivot 16-crown-5 lariat
possesses much higher cation-binding ability and
selectivity compared with the carbon-pivot 16-
crown-5 [1]. The high cation selectivity of an aza-
16-crown-5 lariat is attributed to the entropic loss that
is minimized only when a strict size match is realized
between the ionic diameter of the lanthanoids and the
relatively ¯exible three-dimensional cavity induced
upon lariat ligation. It is interesting that Huang et al.
and La Rosa et al. [6] studied the complexation of
namic studies of cation±crown ether complexation to
elucidate the nature of the cation-binding in terms of
the enthalpy and entropy changes [1±4]. However,
most papers dealing with the complexation thermo-
dynamics have concentrated mainly on alkali, alkali-
earth, and some heavy metal salts. Less attention has
been paid to the complexation thermodynamics of
trivalent lanthanoids. We have recently shown that
*
Corresponding author. Tel: +86 22 23503625; fax: +86 22
3
‡
dibenzo-18-crown-6 with La , using different meth-
ods and counterions, giving distinctly different stabi-
2
0
3502458; e-mail: yuliu@public.tpt.tj.cn
1
See Ref. [1] for previous paper.
040-6031/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved
P I I S 0 0 4 0 - 6 0 3 1 ( 9 8 ) 0 0 3 5 8 - X

2
Y. Liu et al. / Thermochimica Acta 317 (1998) 1±6
lity constants. These results prompted us to investigate
the complexation behavior of the rigid dibenzo-
light lanthanoid nitrates behave as nonelectrolytes
in anhydrous acetonitrile solution [9].
1
8-crown-6 with light lanthanoid nitrates.
In the present study, we report our results of
2.2. Apparatus and procedure
the thermodynamic study on complexation of light
lanthanoid (III) nitrates (LnˆLa, Ce, Pr, Nd, Sm,
Eu, and Gd) with dibenzo-18-crown-6 ((2), Chart I).
The thermodynamic parameters for the complexa-
tion of light lanthanoid nitrates with (2), together
with those for 18-crown-6 (1) [6], will further
our understanding of this thermodynamically less
investigated area of crown ether chemistry [7,8].
Another point of interest is the reduced in¯uence
of the electron-withdrawing effect of the aromatic
ring upon complexation of parent crown ether with
light lanthanoid nitrates.
Calorimetric titrations were performed at atmo-
spheric pressure in a temperature-controlled water
bath maintained at 25.08C by using a TRONAC model
458 isoperibol titration calorimeter connected to a
personal computer for automated titration and data
processing [10]. The principle of the measurement and
the detailed experimental procedures have been des-
cribed previously [11±13]. Typically, a crown ether
solution (20-40 mM) in anhydrous acetonitrile was
3
� 1
continuously introduced at a rate of 0.3321 cm min
intoalanthanoidnitratesolution(1.0±2.0 mM,50.0 ml)
placed in the calorimeter. In order to obtain the net
heat of complexation (Qnet), the total apparent heat
observed (Qobs) was corrected for the dilution of titrant
(
Q ), the nonchemical contributions (QHL), including
D
agitation, heat ¯ow between the vessel and its sur-
rounding, and resistance heating by the thermistor
used, and the temperature difference between titrant
and titrate (QTC) in each run: Q ˆQ � Q � Q
�
net
obs
D
HL
QTC. A titration curve was obtained by plotting the
temperature change (measured by voltage) against the
amount of the crown-ether solution added, from which
2
. Experimental
the complex stability constant (K ) and the enthalpy
s
0
change (ÁH ) were calculated [13,14]. Reliability of
2
.1. Materials
the whole system and the calculation procedures were
doubly checked as earlier [15] by test runs using the
reported reaction systems:
Commercially available (2) (Merck) was used with-
out further puri®cation. Analytical-grade acetonitrile
was dried over calcium hydride and then distilled
fractionally to give the anhydrous solvent (conductiv-
1
(
i) the enthalpy change (ÁHˆ� 11.34 kcal mol�
at 258C) upon titration of Tris with hydrochloric
acid was consistent with that reported (� 11.35 kcal
�
7
� 1
ity <5Â10 S cm ) for calorimetry. Light lanthanoid
�
1
mol ) [16]; and
(
III) nitrates (La±Gd) were prepared by dissolving the
�
1
(
ii) log K and ÁH (1.97Æ0.05, � 6.22 kcal mol
s
corresponding oxides of 99.9% purity (Baotou Rare
Earth Chemical) in 50% aqueous nitric acid while
heating for ca. 10 min. After evaporation, the solid
residue was dehydrated with P O in vacuo for several
at 258C) for the complexation of 18-crown-6 with
potassium chloride are in good agreement with the
reported values (2.03Æ0.10, � 6.21 kcal mol� ) [17].
1
2
5
days to give a powdery product. Lanthanoid nitrate
was dissolved in anhydrous acetonitrile and re¯uxed
for 24 h over molecular sieves for further removal
of water. The concentrations of lanthanoid nitrate
solutions in acetonitrile were determined by EDTA
titration using xylenol orange as an indicator. The
conductometric measurements showed that these
3. Results and discussion
Assuming the 1 : 1 stoichiometry [18,19] for the
complexation of Ln(NO ) (LnˆLa, Ce, Pr, Nd, Sm,
3
3
Eu, and Gd) with the dibenzo-18-crown-6 (CE)
(Eq. (1)), the complex stability constant (K ) and
s

Y. Liu et al. / Thermochimica Acta 317 (1998) 1±6
3
0
the enthalpy change (ÁH ) were calculated by using
the least-squares method to minimize the U value
(
Eq. (2)) [20,21]:
KS
CE ‡ Ln „ CE Ln
(1)
(2)
m
X
0
0
2
UꢀK ; ÁH † ˆ
ꢀQ � ÁH  N †
s
t
t
tˆ1
where Q refers to the net heat of complexation
t
measured at time t in minutes and N denotes the
t
amount in moles of the complex formed at time t. N is
t
directly related to the complex stability constant Ks.
The stability constant K and the enthalpy change
s
0
ÁH for the complexation of lanthanoid nitrates with
Fig. 2. Comparison of experimental and calculated data for the
3‡
(
tinuously changing K , i.e. N , to minimize the U
2) were calculated by computer simulation by con-
calorimetric titration of complexation of (2) with Nd
.
s
t
value. For each lanthanoid±crown ether combination,
the measurement was repeated more than three times
and the U value obtained was minimized satisfactorily
4. Complex stability and cation selectivity
0
to give the optimized set of K and ÁH with standard
In order to visualize the cation-binding properties of
crown ethers (1±2), the changing pro®le of the com-
s
deviations. No serious deviation was found in the
®
tting process, verifying the 1 : 1 stoichiometry of
complexation as assumed above. The calculation of
plex stability constant (K ) is plotted as a function of
s
reciprocal ionic diameter of the lanthanoid in Fig. 3.
As can be seen from Fig. 3, the pro®le of K for (1)
0
log K and ÁH for the complexation (2) with lantha-
noid nitrates in acetonitrile was illustrated in Figs. 1
s
s
3
‡
shows a characteristic peak at Ce and gradually
declines with increasing atomic number or decreasing
3
ionic radius from Ce to Eu , probably owing to
3
‡
3‡
and 2 for La and Nd , respectively. The complex
stability constants and thermodynamic parameters
obtained are listed in Table 1. For comparison, the
thermodynamic quantities reported for the complexa-
tion with (1) in acetonitrile are also included in
Table 1.
‡
3‡
Ê
their ®tted cavity size (2.6±3.2 A) and proper donated
orientations. Although the lanthanoid diameter is
Ê
much smaller (2.12±1.88 A), it is noteworthy that
the ion-pair form of lanthanoid(III) nitrates, behaving
as nonelectrolyte in anhydrous acetonitrile, must be
much larger than the naked trivalent lanthanoids. This
result indicates that (1) possesses the stronger binding
3
‡
ability for Ce . On the other hand, the size-®t concept
has been shown to be unsuccessful in explaining the
trivalent lanthanoid ion selectivities of (2), since this
rigid framework crown ether gives higher K values for
s
3
the latter half of the light lanthanoids than Ce and
‡
3
Pr . One possible explanation for this obvious devia-
‡
tion from the size-®t concept for (2) complexation
3
‡
3‡
Ê
Ê
with Ce (2.06 A) through Gd (1.88 A) [22] is
attributed to the increasing surface charge density due
to the lanthanoid contraction and the subsequent
tighter solvation for heavier lanthanoids. This seems
reasonable, since the extensive thermodynamic stu-
dies on cation-crown ether complexation have shown
that the cation-binding ability of the crown ether
Fig. 1. Comparison of experimental and calculated data for the
3
‡
calorimetric titration of complexation of (2) with La
.

4
Y. Liu et al. / Thermochimica Acta 317 (1998) 1±6
Table 1
�
1
Complex stability constant (log K ) and thermodynamic parameters (in kJ mol ) for 1 : 1 complexation of light lanthanoid (III) nitrates with
s
a
(
1) and (2) in anhydrous acetonitrile at 25.08C
0
0
0
Crown ether
Lanthanoid
log K
s
ÁG
ÁH
TÁS
Ref.
3
‡
‡
(
(
1)
La
Ce
4.40
4.50
3.70
3.50
2.70
� 25.10
� 25.69
� 21.13
� 20.00
� 15.40
� 36.19
� 43.01
� 44.02
� 36.19
� 12.80
� 11.09
� 17.32
� 22.89
� 16.19
2.59
[6c]
[6c]
[6c]
[6c]
[6c]
3
3
‡
Pr
3
‡
‡
Nd
3
Eu
3
‡
‡
2)
La
Ce
3.32Æ0.02
2.34Æ0.06
2.54Æ0.07
3.82Æ0.05
3.84Æ0.09
3.14Æ0.08
2.91Æ0.02
� 18.95
� 13.39
� 14.52
� 21.80
� 21.88
� 17.95
� 16.61
16.07Æ0.38
35.02
23.51
41.97
33.35
18.87
20.46
20.71
This work
This work
This work
This work
This work
This work
This work
3
10.13Æ0.17
27.45Æ0.42
11.55Æ0.38
� 3.01Æ0.38
2.51Æ0.33
4.10Æ0.25
3
‡
Pr
3
‡
‡
‡
‡
Nd
3
Sm
3
Eu
Gd
3
a
Values are the averages of more than three independent measurements in this work.
containing benzo groups is lower than that for the
parent crown ether, and have demonstrated that the
reduced complex stability is due to the decreased
electron density of donor oxygens produced by the
electron-withdrawing aromatic ring. Therefore, it is
very interesting and signi®cant that the cation-binding
constants and relative cation selectivities observed for
(
2), shown in Fig. 3, do not show a regular trend and
are totally different from those of the reference com-
pound (1). There are hitherto limited data for the
complexation of lanthanoids with crown ethers to
elucidate the nature of the binding behavior. The
irregular trend of the complexation ability may be
attributable to the conformational change of the 18-
crown-6 ring upon the complexation resulting from
the attachment of two benzo groups and the lanthanoid
ion-pair form. Possessing structural rigid skeleton as
compared with parent (1), (2) gave the highest K for
s
3
‡
3‡
3‡
3‡
Sm and Nd and the lowest for Ce and Pr
among the light lanthanoid ions, eventually showing
3
‡
3‡
the highest cation selectivity for Sm and Nd
Therefore, the highly selective complexation for (2)
with Sm and Nd is attributed to the increased
molecular rigidity of crown ether, which leads to small
conformational change upon the complexation.
.
3
‡
3‡
5
. Thermodynamic parameters
In order to discuss the changing pro®le of K and the
s
relative cation selectivity from the thermodynamic
point of view, thermodynamic quantities for com-
plexation with (1) and (2) are plotted against the
s
Fig. 3. Complex stability constant (K ) as a function of reciprocal
�
1
� 1
Ê
ionic radius (r /A ) for the complexation of light lanthanoids
with (&) (1) and (*) (2) in acetonitrile at 258C.

Y. Liu et al. / Thermochimica Acta 317 (1998) 1±6
5
higher positive entropy changes (TÁS). The larger
positive entropic changes and somewhat small posi-
tive enthalpic changes for (2) complexation with most
lanthanoids are contributed to the complex stabilities
as a consequence of compensation effects. As can be
seen from Figs. 3 and 4, although the complex stabi-
3
‡
lity decreases for (2) complexation with La and
3
‡
Ce as compared with the parent (1), the relative
cation selectivities are enhanced for (2) complexation
3
‡
3‡
with Nd and Sm . Thus, (2) is shown to possess the
most advantageous structure for highly selective com-
plexation of lanthanoids.
Acknowledgements
This work was supported by National Outstanding
Youth Fund (Grant No. 29625203) and Natural
Science Foundation of China (Grant No. 29672021),
Trans-Century Training Programme Foundation
for the Talents of the State Education Commission
of China, and Natural Science Fund of Tianjin
Municipality (Grant No. 973602211), all of which
are gratefully acknowledged.
Fig. 4. Thermodynamic parameters for the complexation of light
lanthanoids (La±Gd) with (a) (1) and (b) (2) in acetonitrile at 258C.
lanthanoids in Fig. 4. As can be recognized more
0
easily from Fig. 4, the ÁH values of the complex
formation for (1) are all negative with negative entro-
3
‡
pic changes, except for the complexation with Eu
accompanying slightly positive entropic changes. This
means that these reactions are chie¯y enthalpy-driven
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0
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‡
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