The magnetocaloric effect and the heat capacity of aqueous suspensions of porphyrin complexes of rare earth elements according to microcalorimetric data

Article abstract of DOI:10.1134/S0036024412030181

The magnetocaloric effect (MCE) and heat capacity during the magnetization process of (5,10,15,20-tetraphenylporphyrinato)acetatogadolinium(III), (AcO)GdTPP, and (5,10,15,20-tetraphenylporphyrinato)chlorogadolinium(III) complexes, (Cl)GdTPP, in the form of 6%-aqeous suspensions are determined by the microcalorimetric method in a range of temperatures from 278 to 318 K and magnetic fields from 0 to 1 T. It is found that MCE for all the complexes are positive, i.e., at applying a magnetic field in the adiabatic conditions temperature of a suspension of complex increases. It is established that MCE increases with an increase in magnetic induction at all temperatures and decreases with an increase in temperature at all magnetic fields. It is shown that the substitution of chloride ligand by acetate in (X)GdTPP leads to a significant increas in MCE and its temperature dependence; in the case of (Cl)GdTPP actually MCE does not depend on temperature. Relationships between magnetothermal properties and structure of the complexes are analyzed. The conclusion is argumented that the reason of changes in magnetothermal properties after the replacement of axial ligand in gadolinium complexes and complexes of lanthanides with an unsymmetrically filled f-shell is non-planar geometry of the coordination site and specific electronic properties of the central ion. It is concluded that heat capacity of the complexes slightly increases with an increase in temperature and more noticeably in the case of (AcO)GdTPP; a magnetic component of heat capacity is revealed only in (AcO)GdTPP at temperatures above 298 K, which is connected perhaps with a temperature change in the crystal lattice of the complex and influence of the magnetic properties of gadolinium ion on this change. Pleiades Publishing, Ltd., 2012.

Full text of DOI:10.1134/S0036024412030181

ISSN 0036ꢀ0244, Russian Journal of Physical Chemistry A, 2012, Vol. 86, No. 3, pp. 504–508. © Pleiades Publishing, Ltd., 2012.
Original Russian Text © V.V. Korolev, D.V. Korolev, T.N. Lomova, E.G. Mozhzhukhina, A.G. Zakharov, 2012, published in Zhurnal Fizicheskoi Khimii, 2012, Vol. 86, No. 3,
pp. 578–582.
PHOTOCHEMISTRY
AND MAGNETOCHEMISTRY
The Magnetocaloric Effect and the Heat Capacity
of Aqueous Suspensions of Porphyrin Complexes
of Rare Earth Elements According to Microcalorimetric Data
V. V. Korolev, D. V. Korolev, T. N. Lomova, E. G. Mozhzhukhina, and A. G. Zakharov
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, 153045 Russia
eꢀmail: vvk@iscꢀras.ru
Received March 14, 2011
Abstract—The magnetocaloric effect (MCE) and heat capacity during the magnetization process of
(5,10,15,20ꢀtetraphenylporphyrinato)acetatogadolinium(III), (AcO)GdTPP, and (5,10,15,20ꢀtetrapheꢀ
nylporphyrinato)chlorogadolinium(III) complexes, (Cl)GdTPP, in the form of 6%ꢀaqeous suspensions are
determined by the microcalorimetric method in a range of temperatures from 278 to 318 K and magnetic
fields from 0 to 1 T. It is found that MCE for all the complexes are positive, i.e., at applying a magnetic field
in the adiabatic conditions temperature of a suspension of complex increases. It is established that MCE
increases with an increase in magnetic induction at all temperatures and decreases with an increase in temꢀ
perature at all magnetic fields. It is shown that the substitution of chloride ligand by acetate in (X)GdTPP
leads to a significant increas in MCE and its temperature dependence; in the case of (Cl)GdTPP actually
MCE does not depend on temperature. Relationships between magnetothermal properties and structure of
the complexes are analyzed. The conclusion is argumented that the reason of changes in magnetothermal
properties after the replacement of axial ligand in gadolinium complexes and complexes of lanthanides with
an unsymmetrically filled fꢀshell is nonꢀplanar geometry of the coordination site and specific electronic propꢀ
erties of the central ion. It is concluded that heat capacity of the complexes slightly increases with an increase
in temperature and more noticeably in the case of (AcO)GdTPP; a magnetic component of heat capacity is
revealed only in (AcO)GdTPP at temperatures above 298 K, which is connected perhaps with a temperature
change in the crystal lattice of the complex and influence of the magnetic properties of gadolinium ion on this
change.
Keywords: porphyrin, complexes of rare earth elements, magnetocaloric properties, molecular structure
effect.
DOI: 10.1134/S0036024412030181
INTRODUCTION
magnetic resonance tomography [3], e.g., a conjugate
of 2ꢀ(1'ꢀhexyloxyethyl)ꢀ2ꢀdevinylpyropheophorbide
а
Studies on magnetothermal properties of metalꢀ
loporphyrins are of interest for several reasons. Some
thermodynamic properties of compounds that are the
subject of laborious calculations in the case of macroꢀ
heterocyclic molecules can be obtained in such studies
with using direct experimental measurements [1]. On
the other hand, there is a definite practical demand.
The area of applied research on the use of porphyrins
and their complexes with some metals for the treatꢀ
ment of oncological diseases has expanded considerꢀ
ably in the last 20 years [2]. The development of new
diagnostic methods for neoplasms and obtaining subꢀ
stances that allow us to combine treatment and diagꢀ
nostics are topꢀpriority fields.
with aminophenylꢀdiethyleneꢀtriamineꢀpentaacetic
acid (DTPA) coordinated by gadolinium(III). Such
complexes are efficient as well in diagnostics along
with the use in tumor therapy. Introducing additional
properties to such agents, i.e., thermal radiation under
influence of a magnetic field, enhances their diagnosꢀ
tic capability considerably. Porphyrin complexes with
ions of rare earth elements (REE) in the center of their
coordination planes are of particular interest.
Studies on introducing magnetic properties into
metalloporphyrin catalysts to simplify methods of
preparation of solid catalysts and increasing of their
catalytic activity and recyclability were also performed
in [4].
Ions of rare earth elements are already used in porꢀ
phyrin photosensitizers of active singlet oxygen, which
Investigations of the magnetothermal properties of
can gather in a tumor and be used to accumulate paraꢀ compounds from the class of porphyrins were performed
magnetic ions that are capable of emitting a signal for the first time in [1, 5]: (2,3,7,8,12,13,17,18ꢀoctaethꢀ
under the influence of an external magnetic field in ylporphyrinato)chloromanganese(III),
(Cl)MnOEP,
504

THE MAGNETOCALORIC EFFECT
505
(5,10,15,20ꢀtetraphenylporphyrinato)chloromangaꢀ a complex in CHCl3 was separated, washed with water
nese(III), (Cl)MnTPP, and their analogues with broꢀ to remove the acid, and evaporated to dryness.
mide and acetate ligands in place of chloride were
studied. The effects of structural modifications in the
manganese(III) complexes on their magnetothermal
properties were determined; these are more greater
(5,10,15,20ꢀtetraphenylporphyrinato)chlorogadoꢀ
linium(III) complex, (Cl)GdTPP. Yield 25%. Elecꢀ
tronic absorption spectrum (EAS) in chloroform on
an SFꢀ26 spectrophotometer, λmax, nm (logε)): 592
when varying peripheral substituents, as compared to
replacing a acyclic acid ligand. Further systematic
studies are needed to reveal the general relationships
between magnetothermal properties and chemical
structure of porphyrin complexes.
In this work, the magnetocaloric effect (MCE) and
the heat capacity during the magnetization processing
of highly refined particles of (5,10,15,20ꢀtetraphenylporꢀ
phyrinato)acetatogadolinium(III), (AcO)GdTPP, and
(5,10,15,20ꢀtetraphenylporphyrinato)chlorogadolinꢀ
ium(III) complexes, (Cl)GdTPP, are determined by
microcalorimetry in the range of temperatures from
278 to 318 K and magnetic fields from 0 to 1 T, and the
effect of the nature of acidoligand and metal in the
coordination center on magnetothermal properties is
examined using the preliminary data from [6].
(3.72), 555 (4.20), 512 (3.79), 485 (shoulder), 421
(5.63). IR spectrum, KBr, a Specord 75 IR spectromꢀ
eter,
H)), 1010 (
(C–N)), and 1440 (
cycles, 702, 751 ( (C–H)), 1067 and 1175 (
1475, 1580, 1595 ( (C=C)), 3050 and 3080 (
vibrations of Gd–N bonds, 252 and 421.
ν
(cm–1): vibrations of pyrrole cycles, 800 (
γ
(C–
(C–N), 1372, 1325
(C=N)); vibrations of benzene
(C–H)),
(C–H));
ν(C–C)), δ(C–H), ν
(ν
ν
γ
δ
ν
ν
(5,10,15,20ꢀtetraphenylporphyrinato)acetatogadoꢀ
linium(III) complex, (AcO)GdTPP. Yield 20%. Elecꢀ
tronic absorption spectrum (EAS) in chloroform on
an SFꢀ26 spectrophotometer (λmax, nm (logε)): 592
(3.74), 554 (4.18), 515 (3.68), 485 (shoulder), 421
(5.58). IR spectrum, KBr, a Specord 75 IR spectromꢀ
eter,
H)); 988, 1006 (
(C–N)); and 1441 (
cycles, 702, 752, (C–H)), 1070 and 1178 (
1492, 1559, 1597 ( (C=C)), 3054, and 3076 (
bond vibrations of acidoligand AcO–
1401 (νs(COO)),
2854, and 2925 ( (C–H)); vibrations of Gd–N bonds,
ν
(cm–1): vibrations of pyrrole cycles, 799 (
γ
(C–
(C–H)); 1328
(C=N)); vibrations of benzene
(C–H),
(C–H));
ν(C–C)), (ν(C–N), δ
(ν
ν
(
ν
γ
δ
ν
X
,
Gd
N
ν
N
423; vibrations of Gd–O bonds, 659 and 724.
N
N
The measurements of MCE and heat capacity of
6% aqeous suspensions of (X)GdTPP complexes were
performed on an automated unit composed of a
microcalorimeter with an isothermic shell [7]. The
measuring microcalorimeter cell together with the isoꢀ
thermic shell was placed in an interpole gap (60 mm)
of the electromagnet, that makes it possible to measure
a value of MCE and heat capacity in magnetic fields
from 0 to 1 T at temperatures from 278 to 318 K. Temꢀ
perature fluctuations in the thermostated space of the
microcalorimetric cell during a calorimetric experiꢀ
ment were 0.0002. Error of the measurements of
MCE and heat capacity is 1 and 2%, respectively.
(X)GdTPP, X = Cl, AcO
EXPERIMENTAL
Syntheses of Complexes
Complexes (X)GdTPP were obtained via the reacꢀ
tion of a macrocyclic ligand H2TPP with GdCl3 ·
6H2O of purified grade or Gd(AcO)3 · 4H2O of CP
grade (Chemically Pure) in the 1 : 4 molar ratio in
media of boiling imidazole of CP grade for 30 min.
After cooling the reaction mixture, the reaction prodꢀ
ucts were dissolved in chloroform of CP grade and
washed with water to remove imidazole. The solution
was then concentrated by distilling off chloroform
partially and placed in the column packed with alumiꢀ
num oxide of Brockmann activity grade II. The use of
chloroform yields two well separated zones, the first of
which, H2TPP, is completely washed away with the
eluent, and the second one, (X)GdTPP complex, is
strongly adsorbed in the upper part of the column.
Desorption from Al2O3 was performed using an empirꢀ
The following equations were used to determine
MCE and to calculate the specific heat capacity:
T dM
CH dT
⎛
⎞
⎠
ΔT = –ꢀꢀꢀꢀ ꢀꢀꢀꢀꢀꢀ ΔH,
(1)
⎝
H
where
Δ
Т
is MCE,
Т
is temperature, СH is heat capacꢀ
dМ dТ)H is temperaꢀ
ity in a constant magnetic field,
(
/
ture derivative of magnetization, and
the magnetic field;
ΔH
is a change in
QC
W = ꢀꢀꢀꢀꢀ – Cpm,
ΔT
(2)
ically found mixture of ethanol with 1% AcOH from where
which the complex was extracted by adding chloroꢀ amount of heat energy (J) introduced into a cell equal
form and then water in the amount which is sufficient to I2R
(where is the current intensity, A; is the
for separation of the chloroform layer. The solution of resistance of a heater, , and is time, s); is the mass
W is the calorimeter constant [8]; QC is the
τ
I
R
Ω
τ
m
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506
KOROLEV et al.
ΔT, K
1
2
0.20
0.15
0.10
0.05
3
4
5
6
8
7
0
0.4
0.8
1.2
B, T
Fig. 1. Dependences of magnetocaloric effect on magnetic induction at temperatures of (
1) 278, (2) 288, (3) 298, and (4) 313 K
for (AcO)GdTPP complex, and ( ) 278, ( ) 288, ( ) 298, and ( ) 318 K for (Cl)GdTPP complex.
5
6
7
8
of water in the calorimetric cup, g; Сp is heat capacity asymmetrical structure of coordination sites in the
of water, J/(g K), and is difference between the iniꢀ complexes.
tial and final temperatures in an experiment, K;
ΔТ
The field and temperature dependences of MCE
for the gadolinium complexes are shown in Figs. 1 and
2. Values of MCE for both the complexes are positive,
increase with an increase in the induction of a magꢀ
netic field and decrease in the case of (AcO)GdTPP
complex with temperature rise. In the case of
(Cl)GdTPP magnetocaloric effect basically does not
depend on temperature.
QC
Cp = ꢀꢀꢀꢀꢀꢀꢀꢀꢀ – ꢀꢀꢀ.
ΔTm
W
m
(3)
RESULTS AND DISCUSSION
MCE for the acetate complex is considerably
higher than for the chloride. Since the bonding
strengths of the acidoligands in (X)GdTPP are
unknown, one reason for the difference between the
two complexes might be revealed from indirect data on
the dissociation kinetics of REE complexes with porꢀ
phyrins, depending on nature of axial ligand. It is
known that the sensitivity of the reaction of dissociaꢀ
tion of the coordination center by the Ln–N bonds
toward the nature of the X– ligand differs strongly for
the analogous complexes of phthalocyanine, (X)LnPc,
and porphyrin (X)LnTPP [10–12]:
The magnetocaloric effect is the change in temperꢀ
ature of a magnetic material during its magnetization
or demagnetization in an external magnetic field in
adiabatic conditions. It appears as a result of redistriꢀ
bution of internal energy of a magnetic substance
between the system of magnetic moments of its atoms
and crystal lattice. The adiabaticity of the process is
achieved in practice by a fast change in the magnetic
field. MCE reaches its maximum value at temperaꢀ
tures of a magnetic phase transition, e.g., in the area of
the Curie temperature of ferromagnetic materials.
MCE exhibited by porphyrin highꢀspin complexes
was observed for the first time on a highꢀspin complex
of manganese(III) (Cl)MnOEP [1]. Paramagnetic
properties of porphyrin complexes of lanthanides from
the second half of the row (subgroup of gadolinium)
(X)LnPc + 2AcOH
(4)
= [(X)Ln]2+ + H2Pc + 2AcO–,
(X)LnTPP + 2AcOH
(5)
1
can be reliably characterized by their H NMR specꢀ
= Ln3+ + H2TPP + 2AcO– + X–.
tra. Typical spectra for paramagnetic substances were
obtained in [9] using the example of (Acac)LnTPP
In contrast to (X)LnPc complexes in which not
complexes (Acac is an acetylacetonate anion; Ln is only the rate of dissociation changes by eq. (4) after
lanthanide) which were synthesized in [10] by one of replacing Cl– with AcO– but also mechanism of the
the authors of this work. In the NMR spectra, the reaction (including the reaction order with respect to
orthoꢀ and methaꢀprotons of phenyl groups each emit acid), (Cl)LnTPP and (AcO)LnTPP complexes dissoꢀ
two signals with intervals of 48.2 and 6.7 ppm respecꢀ ciate either at the same rates (Tm complexes) or with
tively (a spectrum of (Acac)DyTPP), indicating the very close rates (Dy complexes). That signifies the
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THE MAGNETOCALORIC EFFECT
, K
507
Δ
T
, K
ΔT
0.20
0.15
0.10
0.05
0
0.20
0.16
0.12
0.08
0.04
0
8
1
7
6
5
2
3
4
4
3
2
1
275
285
295
305
315
T, K
275
285
295
305
315
T, K
Fig. 2. Dependences of the magnetocaloric effect on temꢀ
perature in magnetic fields of ( ) 0.25, ( ) 0.5, ( ) 0.75,
and ( ) 1.0 T for (Cl)GdTPP complex, and ( ) 0.25,
) 0.5, ( ) 0.75, and ( ) 1.0 T for (AcO)GdTPP complex.
Fig. 3. Dependences of magnetocaloric effect on temperꢀ
ature for ( ) (Cl)TmTPP, ( ) (Cl)GdTPP, ( ) (Cl)EuTPP,
1
2
3
5
1
2
3
4
and (
4
) (AcO)GdTPP complexes in a magnetic field of
(6
7
8
1.0 T.
2
3
C
p, J/(g К)
1
4
5
6.0
5.5
5.0
4.5
4.0
6
7
8
9
10
280
290
300
310
320
, K
T
Fig. 4. Dependences of specific heat capacity on temperature in magnetic fields of (
1
) 0, (2) 0.25, (3) 0.50, (4) 0.75, and (5) 1.0 T for
(AcO)GdTPP complex, and ( ) 1.0, ( ) 0.75, ( ) 0.5, (9) 0.25, and (10) 0 T for (Cl)GdTPP complex.
6
7
8
weak bonding of X– ligands in porphyrin complexes tively its
fꢀorbitals take part in a πꢀdative interaction
and the occurrence of the dissociation reaction by the
Ln–X bonds in a rapidly proceeding stage during the
dissociation of the complex’s coordination center
(Eq. (5)).
with an aromatic system in the molecule. Such an
interaction is confirmed by the presence of minor
periodicity in variations in the constant of kinetic staꢀ
bility of (X)LnTPP [13], demonstrating in particular
the relative stability of the half filled
ium ion (f7). Lowering the conditions for the Gd
ꢀinteraction will lead to the enhancement of paraꢀ
magnetic properties of the central ion, which gives rise
fꢀshell of gadolinꢀ
As follows from the above, the differences in MCE
of (Cl)GdTPP and (AcO)GdTPP can be explained by
differences in the interaction of the central paramagꢀ
netic ion with macrocyclic ligand. It is obvious that a
↔
N
π
spin state of gadolinium ion depends on how effecꢀ to changes in the MCE values. It seems that the tranꢀ
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508
KOROLEV et al.
sition from the chloride complex, (Cl)GdTPP, to the magnetothermal properties than changes in the nature
acetate counterpart in which AcO– anion is coordiꢀ of the lanthanide.
nated bidentally [13] is accompanied by the additional
displacement of the central ion from the macrocyclic
ACKNOWLEDGMENTS
plane.
In contrast to GdIII complexes, the electronic facꢀ
The work was partially supported by Basic
Research Program of No. 7 of the Presidium of Rusꢀ
sian Academy of Sciences (2011); the federal target
program Science and Science Teachers of an Innovative
Russia, 2009–2012, state contract no. 02.740.11.0106;
and the Raising the Scientific Potential of Higher
Education program of the RF Ministry of Education
and Science, contract no. 2.2.1.1/2820 (2011).
tor (
metrical in the case of lanthanide complexes with
unsymmetrically filled ꢀshells. This follows from the
πꢀdative interaction) starts to prevail over the geoꢀ
f
comparative analysis of magnetocaloric effect of comꢀ
plexes of different lanthanides with the same axial aciꢀ
doligand (Fig. 3 also presents the acetate complex for
purposes of comparison [6]). MCE values fall along
the Eu > Gd > Tm series at temperatures from 275 to
300 K despite the a more planar disposition of the lanꢀ
thanide ion in the porphyrin plane, due to the effect of
lanthanide compression in the same series. The maxiꢀ
mum value of the magnetocaloric effect for
(Cl)EuTPP complex is in good agreement with the
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