Phthalocyanine-based molecular paramagnets. Effect of double-decker structure on magnetothermal properties of gadolinium complexes

Article abstract of DOI:10.1016/j.jorganchem.2016.07.002

Gadolinium is useful in the room temperature magnetic refrigeration due to its large magnetocaloric effect occurring at temperatures close to room. We predict and represent known phthalocyanine complexes as a new class of paramagnets with a large magnetocaloric effect at the temperatures close to room acting due to gadolinium atom located in a coordination site of a conjugated phthalocyanine macrocycle/macrocycles. Magnetothermal properties (a magnetocaloric effect, specific heat capacity, heat released due to MCE, enthalpy/entropy change) of paramagnetic (AcO)GdPc and GdPc₂[Formula presented], Pc – phthalocyanine dianion, as the 3% water suspensions at 278–338?K in the magnetic fields of 0–1.0?T were studied for the first time in the family of these paramagnets. The study of magnetothermal properties and specific heat capacity were performed in two independent experiments by microcalorimetry and differential scanning calorimetry, respectively. Large positive MCE in (AcO)GdPc reaches a value of 0.47?K at 278?K when the magnetic field is changed from 0 to 1?T. MCE in [Formula presented] is lower by almost an order of magnitude compared with (AcO)GdPc, which indicates that the intramolecular oxidation-activated spin coupling does not prevail over the intermolecular coupling of two radical spins.

Full text of DOI:10.1016/j.jorganchem.2016.07.002

Accepted Manuscript
Phthalocyanine-based molecular paramagnets. Effect of double-decker structure on
magnetothermal properties of gadolinium complexes
V.V. Korolev, T.N. Lomova, A.G. Ramazanova, D.V. Korolev, E.G. Mozhzhukhina
PII:
S0022-328X(16)30286-8
10.1016/j.jorganchem.2016.07.002
JOM 19553
DOI:
Reference:
To appear in: Journal of Organometallic Chemistry
Received Date: 19 January 2016
Revised Date: 29 June 2016
Accepted Date: 4 July 2016
Please cite this article as: V.V. Korolev, T.N. Lomova, A.G. Ramazanova, D.V. Korolev, E.G.
Mozhzhukhina, Phthalocyanine-based molecular paramagnets. Effect of double-decker structure on
magnetothermal properties of gadolinium complexes, Journal of Organometallic Chemistry (2016), doi:
10.1016/j.jorganchem.2016.07.002.
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ACCEPTED MANUSCRIPT
Phthalocyanine-based Molecular Paramagnets. Effect of double-decker
structure on magnetothermal properties of gadolinium complexes
∗
V.V. Korolev, T.N. Lomova , A.G. Ramazanova, D.V. Korolev, E.G. Mozhzhukhina
G. A. Krestov Institute of Solution Chemistry of the Russian Academy of Sciences, Akademicheskaya
str., 1, Ivanovo, 153045, Russia
ABSTRACT: Gadolinium is useful in the room temperature magnetic refrigeration due to its large magnetocaloric
effect occurring at temperatures close to room. We predict and represent known phthalocyanine complexes as a new
class of paramagnets with a large magnetocaloric effect at the temperatures close to room acting due to gadolinium
atom located in a coordination site of a conjugated phthalocyanine macrocycle/macrocycles. Magnetothermal prop-
erties (a magnetocaloric effect, specific heat capacity, heat released due to MCE, enthalpy/entropy change) of para-
•
magnetic (AcO)GdPc and GdPc2 , Pc – phthalocyanine dianion, as the 3% water suspensions at 278 - 338 K in the
magnetic fields of 0–1.0 T were studied for the first time in the family of these paramagnets. The study of magneto-
thermal properties and specific heat capacity were performed in two independent experiments by microcalorimetry
and differential scanning calorimetry, respectively. Large positive MCE in (AcO)GdPc reachs a value of 0.47 K at
•
278 K when the magnetic field is changed from 0 to 1 T. MCE in GdPc2 is lower by almost an order of magnitude
compared with (AcO)GdPc, which indicates that the intramolecular oxidation-activated spin coupling does not pre-
vail over the intermolecular coupling of two radical spins.
KEYWORDS: gadolinium, (phthalocyaninato) complexes, magnetothermal properties, magnetocaloric effect, heat
capacity, molecular structure effect.
1. Introduction
Both the magnetizing and demagnetizing processes in materials with a large magnetocaloric ef-
fect (MCE) was established [1] can be employed for cooling. Magnetic refrigeration is a very prom-
ising alternative technology, near room temperature, to conventional gas-compression refrigerators
because of its environmentally friendly and highly efficient. The discovery of the quantum nature of
single-molecule magnets (SMMs) led to potential applications in quantum computing [2]. At last, a
few complexes with specific properties in magnetic feature have been studied as agents for hyper-
thermia [3].
Depending on the central lanthanide ion, double-decker phthalocyanine complexes, LnPc2 can
show slow magnetization relaxation as SMMs [4,5]. A larger number of coupled magnetic centers
rather than individual spins of zero-dimensional complexes are used for increasing the total spin of
the molecule in the case of transition metals of the 3d group [6,7]. The spin and orbital atomic mo-
ment in rare earths complexes, conversely, are maximized due to the strong electronic repulsion.
The effective energy barrier against magnetic relaxation, Uef of the lanthanide-based SMMs such as
the phtalocyanine double-decker family is quantitatively very similar to zero-field energy gap be-
tween the two lowest levels ꢀ [4]. In other words, Uef is determined by the ligand field around a
lanthanide ion. For example, the Uef and ꢀ values observed for [Pc2Tb]-TBA+ (TBA is tetrabu-
tylammonium) are equal to 230 cm-1 and 220 cm-1 [8,9]. In addition the effective barrier 230 cm-1 is
larger than for any 3d-based SMM to date [4,8].
That’s why the temperature ranges in which single-molecule magnet behavior of the double-
decker phthalocyanine complexes was observed were far higher than those of the transition-metal-
cluster SMMs [8]. The dominant effect of the coordination geometry of the spin carrier on the mag-
netic properties is indicated for the mixed triple-decker tetrapyrrole complexes of Dy−Dy, Y−Dy,
Dy−Y [10] and Dy-Dy [11] in which the intramolecular f–f and antiferromagnetic dipole-dipole in-
teraction respectively are possible. The porphyrin core conformation and hence the ligand
________________________
∗ Corresponding author. Tel.: +7 4932 336990; fax: +7 4932 336237.
E-mail addresses: tnl@isc-ras.ru, tnl50@mail.ru (T.N. Lomova).

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field-effect on a lanthanide ion controls the nature and magnitude of spin coupling also in the case
when it has another paramagnetic center, unpaired π-electron in the tetrapyrrole molecule. Tempera-
ture-dependent (6-300 K) magnetic susceptibility of [VO(OH2)(OEP•)]SbCl6, (OEP is β-
octaethylporphyrin dianion) was characterized from a model in which the vanadyl electron is fer-
romagnetically coupled to the electron of the porphyrin cation and two radical spins are antiferro-
magnetically coupled [12]. In this work, Jv-r, and Jr-r parameters which describe the intra- and in-
termolecular magnetic coupling are equal to 63 cm-1 and -139 cm-I, respectively.
A choosing strategy of our work for deriving a potential molecular system with specific mag-
netic properties is both a coupling of spin-containing central ion, GdIII with a ferromagnetic cou-
pling unit of an oxidized molecule and a spin density delocalization between GdIII and a readily po-
larizable aromatic macrocycle. In our recent works [13,14], magnetothermal properties of (porphy-
rinato) lanthanide(III) acetates/chlorides have been used for measuring a magnetocaloric effect for
the first time. Specific heat capacity, the magnetic component of molar heat capacity change, en-
thalpy/entropy change was in detail observed. The large and giant MCE (porphyrinato) REE com-
plexes displayed a magnetocaloric effect at temperatures close to room were obtained. General
strategy of variations in the temperature-dependent magnetic behavior through modification of the
porphyrin was provided. It can be assumed that the phthalocyanine analogs of (porphyrinato) REE
complexes have the magnetothermal properties useful in the field of room temperature magnetic
refrigeration and hyperthermia.
To further correlate magnetic behavior of lanthanide phthalocyanine with a coordination center
structure, in particular by to double-decker complex transition, we have undertaken a comparative
study of the magnetothermal properties of (phthalocyaninato) gadolinium(III) complexes of 1:1,
•
(AcO)GdPc with axial coordinated acetate and 1:2, GdPc2 oxidized due to one electron removing
(Fig. 1).
Fig. 1. Structures of (phthalocyaninato) gadolinium(III) acetate, (AcO)GdPc and oxidized (phthalocyaninato) gadolini-
•
um(III) double-decker complex, GdPc2
2. Experimental
2.1. Synthesis of the phthalocyanine complexes
(Phthalocyaninato) gadolinium(III) acetate were prepared from Li2Pc and Gd(AcO)3 4H2O
in a molar ratio 1:2.5 in boiling DMSO during 20 min by a literature method [15]. The reaction
mixture was cooled and diluted with twice the volume of water. The residue was filtered, washed
with water and air-dried. The yield was ca. 90 %. Li2Pc was synthesized from H2Pc and butyllitium.
UV-vis (DMF): λmax, nm (logarithm of the molar absorption coefficient, logε) 670 (4.28), 639
(shoulder), 605 (3.39), 338 (3.69). IR (KBr, 350 ÷ 4000)*: ν, cm-1 463 (Gd-N), 555 (δ, Pc), 617,
684 (Pc), 735, 741, 766, 1251, 1277 (C-H, Pc), 1555 (deformation vibrations, CH3), 2853, 2923,
2953 (ν, CH3), (3014, 3029, 3049, 3057, 3076) (ν, Pc), 874, 884, 1438, 1455 (isoindole ring), 1045,
1060, 1081, 1093, 1118 (combine vibrations, isoindole ring –C-H, Pc), 1112, 1140, 1158, 1185 (Pyr

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combined with C-H, Pc), 1304, 1320 (Pyr), 1405 (−С=С−N=), 1484 (−N=), 1502, 1591, 1608 (ν,
C-C, benzene), 1333 (symmetrical ν, COO, AcO), 1582 (asymmetric ν, COO, AcO). Elemental
analysis, calculated for C34H19N8O2Gd: C, 56.03; N, 15.37; H, 2.63; and Gd, 21.58 %. Found: C,
55.74; N, 15.20; H, 2.74; and Gd, 21.40 %.
•
Oxidized (phthalocyaninato) gadolinium(III) double-decker complex GdPc2 was synthe-
sized using method [16,17] by interaction of 10 g of o-phthalonitrile with lanthanide acetate at the
temperature of 560 K during 1 hour. The product blue-green in color was endured in vacuum at the
temperature of 620 K for 1 hour. Then one dissolved in DMF (150 ml) with addition of hydrazine
hydrate. The solution was filtered and undergone electrolysis with constant current in a cell with
graphite cathode and Pt anode. The crystals obtained were filtered, washed with acetone and dried
•
at the temperature of 370 K in air. UV-vis spectrum of GdPc2 in concentrated sulfuric acid have
one wide band at λmax 407 nm, log ε = 4.00 in 17.7 M H2SO4. UV-vis (CH2Cl2): λmax, nm (relative
intensity) 884.0 (0.0869), 668.0 (1), 602.0 (0.232), 459.0 (0.224), 321.0 (0.807). IR (KBr, 350 ÷
4000) (Fig. 2S): ν, cm-1 498 (Gd-N), 562 (δ, Pc), 626, 678 (Pc); 727, 740, 778, 1060, 1156, 1174,
1192 1282 (C-H, Pc), 3028, 3048, 3058, 3075 (ν, C-H, Pc), 1113, 1421, 1447, 1463 (isoindole
ring), 1319, 1365, 1400 (−С=С−N=), 1483 (−N=), 1500, 1523 (isoindole ring combined with pyr-
role ring vibrations), 1509, 1523, 1594, 1607 (ν, C-C, benzene ring). Elemental analysis, calculated
for C64H32N16Gd•: C, 65.02; N, 18.95; H, 2.73; and Gd 13.30 %. Found: C, 64.75; N, 19.08; H,
2.97; and Gd, 13.54 %.
The rate constants of complex dissociation in the acid solvents of variable composition (Eq. 1)
was measured spectrophotometrically. The experiment was carried out in a thermostatically con-
trolled glass cell at the temperature of 298 ± 0.1 K.
kobs = 1/τ ln[(A0 - A∞)/(Aτ - A∞)]
(1)
Here Aτ, A0, A∞ is absorbance observed at present time, one when τ = 0, and one when a reac-
tion had terminated, respectively.
2.2. Microcalorimetric experiment
The special calorimetric device, the isothermal-shell microcalorimeter was used for studying
magnetocaloric effects of paramagnetic complexes. Its shape and construction are developed and
presented in the work [18]. Experiment was performed over the temperature range 278 - 338 К in
magnetic fields from 0 to 1.0 T. The measuring microcalorimetric cell with the isothermal shell was
placed in the gap of the electromagnet. Switching on the magnetic field causes heating of a ferro-
magnetic material and the entire contents of the calorimeter cell and the switching off – the cooling.
Adiabatic magnetization process was achieved by rapid changes of the magnetic field. The tempera-
ture fluctuation in the thermostatically controlled space of the calorimetric cell during the calorimet-
ric experiment was ±0.0002 °C. The sensitivity of the setup was 2 10–5 K. Heat capacity in zero
field was measured by means of DSC 204 F1 Phoenix (NETZSCH). The error in MCE and
heat capacity measurements was 2%. Every experiment was repeated five times.
Amorphous samples of phthalocyanine complexes synthesized were used for suspensions in
which a solid with the average size of particles 25 ꢁm controlled with polarizing microscope Altami
____________________________
*For IR spectrum in KBr registered on the Avatar 360 FT-IR ESP spectrometer (Figure 1S), see Online Supplementary
Materials.

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Polar 312 was in the highly disperse state. A solid did not change during microcalorimetric experi-
ment on both size of particles and chemical composition. The spectrophotometric control by means
of the Agilent 8453 UV-Visible spectrophotometers of a chemical structure of complexes was used.
2.3. Calculation of the magnetothermal properties parameters
We measured a temperature change in the calorimeter, which occurred as a result of the magne-
tocaloric effect during the microcalorimetric experiment. Then we introduced a known quantity of
Joule heat into the calorimeter. Comparing a temperature rise of the calorimeter as a result of the
input of Joule heat and a temperature rise as a result of the MCE, we calculated using the formula
(2) the amount of heat QMCE, which was allocated (switching on a magnetic field) as a result of the
magnetocaloric effect of particles of a metal complex:
QMCE = QJ (ꢀT/ꢀTJ).
(2)
Here QJ – Joule heat, injected in the calorimetric experiment, ꢀTJ – the temperature change of
calorimetric system as result of injecting of Joule heat, ꢀT – the temperature change of a calorimet-
ric system as a result of a magnetic field change. The amount of heat QMCE is related to the magni-
tude of the MCE by ratio (3), which is similar to the fundamental equation of heat balance [19].
QMCE = mmСР,m ꢀTMCE
.
(3)
Here ꢀTMCE – the adiabatic temperature change as result of MCE, QMCE – heat generated in the
calorimetric experiment as a result of MCE when a magnetic field is applied; mm, СР,m – mass and
heat capacity of a magnetic material. Using the experimental values of heat QMCE, the enthalpy
change, ꢀН(enth) of a magnetic material resulting from the changes in the magnetic field may be de-
termined. The values of enthalpy changes of a magnetic material, ꢀН(enth) can be obtained by direct
calorimetric measurements. Character of dependences of the change QMCE and ꢀН(enth) on magnetic
induction and temperature are similar to that of the magnetocaloric effect changes, and these values
can be used to calculate the cooling capacity of magnetic materials.
Heat capacity in zero fields in solid of complexes was directly measured in experiment in this
work. The specific heat capacities of the magnetic particles in the suspensions obtained in [13,14]
and used here for comparison were calculated taking into account the assumption of the absence of
interaction between components of a heterogeneous system. The heat capacity in such systems is
the total of heat capacities of components in accordance with their concentrations. Thus, the specific
heat capacity of a solid phase in heterogeneous systems can be calculated from the equations (4)
and (5).
Ср(sus) = Ср(solid) ω(solid) + Ср(water) ω(water)
Ср(solid) = (Ср(sus) - Ср(water) ω(water))/ω(solid)
(4)
(5)
.
Here Ср(solid), Ср(sus) and Ср(water) are specific heat capacities of a solid phase, suspension and wa-
ter, accordingly, ω(solid) and ω(water) are mass fractions of a solid phase and water in a suspension,
accordingly. When the values of ꢀT, QMCE и СР,m are determined from the experiment, the value of
MCE (ꢀTMCE) can be calculated (Eq. 2).
The entropy values were calculated from MCE at specified values of Ср and temperature.
2.4. The reliability of the microcalorimetric method
To check the reliability of the developed method, the temperature dependence of MCE of gado-
linium was determined. The polycrystalline metallic Gd with chemical purity of 98% was used. De-

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pendences of MCE of the metallic gadolinium on magnetic induction and temperature are well stud-
ied. This metal is often used as a reference standard in the study of magnetocaloric properties and
cooling performance of magnetic materials [20-22]. Fig. 2 shows temperature dependence of MCE
in gadolinium when the magnetic field was changed from 0 to 1.0 T in comparison with the litera-
ture data [23,24]. The MCE values were obtained on the basis of equation (2). It is known that the
temperature dependence of MCE in Gd due to magnetic phase transition (ferromagnetism – para-
magnetism) has a maximum at 292 K. As seen in Fig. 2, experimental values of MCE, obtained by
the calorimetric method, are in a good agreement with the literature data.
Fig. 2. Temperature dependence of MCE in gadolinium in magnetic field 0 - 1 T: (1) experimental data from the work
[23], (2) our experimental data observed by the microcalorimetric method, (3) experimental data from the work [24],
and (4) our experimental data obtained by the microcalorimetric method, based on the heat capacity from the work [25]
To estimate the cooling capacity of highly dispersed magnetic materials, we propose the appli-
cation of specific heat QMCE/m (J/kg), which is released or absorbed as a result of MCE with the
change of a magnetic field, and which is obtained by direct calorimetric measurements. This value
for the metallic Gd is 733 J/kg at 292 K when the magnetic field changes from 0 to 1.0 T.
3. Results and discussion
3.1. Chemical structure and stability in solutions and solid
Chemical structure of (phthalocyaninato) gadolinium(III) acetate is proved due to spectral data
and analysis of the elemental composition. UV-vis spectrum of (AcO)GdPc corresponds to the liter-
ature data [15] for (phthalocyaninato) lanthanide(III) acetates. Typical UV-vis spectrum of these in
organic solvent has long-wave maximum in the region 670 – 700 nm (π→π* transition [26]) and its
oscillation component of lower intensity about 610 nm. Intensive Soret band is disposed at 330- 390
nm. In the lanthanides raw, the UV-vis spectra show low dependence from Ln and acido-ligand na-
ture [14,27,28] but vary markedly wavelength (Table 1). This differs the complexes of 1:1 from
double- (Fig. 1) and triple-deckers [15].
Table 1.
Long wave absorption band position in UV-vis spectra of (AcO)GdPc, λmax, nm
Solvent
Solvent
λmax, nm
λmax, nm
DMSO 673.4 Benzonitrile 718.4, 691.2
DMF
670.0 Acetonitrile
663.1
675.7
670.2
Benzene 677.5
Acetone 666.7
Pyridine 675.7
CHCl3
(C2H5)2O

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Vibrations of the phthalocyanine molecule fragments are reflected in IR spectrum (Experi-
mental, Figs. 1S, 2S). The signals with maximum frequents 1333 and 1582 cm-1 belong to OCO vi-
brations indicating to bidentate coordination of axial acetate. The difference in the frequency values
of valence asymmetric and symmetrical vibration of OCO, 249 cm-1 is a little greater than 225 cm–1
and lower 300 cm–1 [29-31] for the ionic complexes and for the monodentate coordination of the
carboxyl group, respectively. This relation demonstrates the axial ligand binding by means of weak
covalent predominantly ionic bonds Gd-O.
UV-vis spectra of (phthalocyaninato) lanthanide(III) double-decker complexes are mainly iden-
tical to those of monophthalocyanines, but there are a small absorption band associated to the semi-
occupied molecular orbital (SOMO), near 450 nm and an intramolecular charge-transfer band in the
•
1200–1500 nm range arising due to the radical nature of LnIIIPc2 complexes [32, 33]. A band of
•
low intensity near 890 nm is also observed [32, 34]. GdPc2 exhibit typical features of the electronic
absorption spectra in UV-visible region studied for (phthalocyaninato) rare earth double-decker
complexes with the new low-intensity absorptions appearing at 458.0 and 884.0 nm, B band at
321.0 nm and hypsochromically shifted Q band at 668.0 nm.
Transition to double-decker complex noticeable changes a location and a number of C-H
bonds, isoindole ring, pyrrole ring and −С=С−N= signals in IR spectrum. The vibrations of phthal-
ocyanine molecule, valence ones of benzene ring C-H and C-C as −N= are practically changeless.
The axial ligand signals is absent in IR spectrum. Probably, these peculiarities are determined by
not only the chemical structure differences. In addition, distortions of the macrocycle structure take
place. Nowadays there is information about structures of CeIV(Pc)2 [35], NdPc2 [36], ErIII(Pc)2 [37],
and others. Data on sandwich-type complexes the structure of which is studied by X-ray crystallog-
raphy are listed in [28]. From the results, it is seen that the structure of (phthalocyaninato) double-
decker complexes for all the investigated compounds has the same type. The metal atom is coordi-
nated by eight isoindole nitrogen atoms (N-iso) and is located between two phthalocyanine macro-
cycles. Macrocycles undergo distortion and become non-planar but saucer-shaped with benzene
rings bent outward. The distortion degree as well as M-N-iso distances and rotation of the macrocy-
cles with respect to one another are determined by nature and radius of the metal cation.
•
•
Both (AcO)GdPc and GdPc2 are stable in solutions and solid in the absence of acids. GdPc2
do not change in the temperature exposure to 400 K, whereas the first complex undergoes destruc-
tion. The complexes are showing significant difference in resistance to acids dissociating slowly in
mixed protolytic media namely ethanol – 1.5-3.5 M AcOH and 15.9-17.7 M H2SO4 in the case of
(AcO)GdPc and GdPc2 , respectively. The first order rate constants of dissociation k298K are equal
•
•
(0.61 ± 0.02) 10-3 to (AcO)GdPc in ethanol – 4.25 M AcOH and 0.12 10-4 s-1 to GdPc2 in 16.9 M
H2SO4. That is in a good agreement with the earlier results [38, 39]. Amorphous samples of the
complexes used for the preparation of water suspensions in magnetocaloric experiment were exam-
ined on stability after the stay in the magnetic field through UV-vis spectra.
3.2. Magnetotermal properties
Measurements of DC magnetic susceptibility for bis(octaethyltetraazaporphyrinato) lantha-
nide(III) under an applied field of 0.1 T [40] shown that the gadolinium(III) complex, unlike terbi-
um(III) and dysprosium(III) analogs with SMM behavior maintains a Curie’s constant value in
good agreement with that expected for only a spin (S = 7/2) contribution due to absence of spin-
orbit coupling. Nevertheless gadolinium(III) phthalocyanines display an interesting magnetic be-
havior at temperatures close to room.
The temperature dependence of the specific heat capacity in zero magnetic field for both
•
(AcO)GdPc and GdPc2 (Fig. 3) has an extreme character. The effect of magnetic field on the tem-
perature dependence of both complexes practically does not appear and therefore the temperature
dependence of specific heat capacity in a magnetic field is not given here.
•
Fig. 4 represents experimental results for MCE in (AcO)GdPc and GdPc2 in dependence on
temperature and magnetic induction. MCE values when magnetic induction starting are positive.

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MCE value increases with the increasing in the magnitude of magnetic field and non-linearly de-
crease with temperature growth exhibiting extremes in the studied ranges of temperature and mag-
netic induction. Character of a changing of MCE is largely determined by the temperature depend-
ence of the specific heat capacity (Fig. 3).
•
Fig. 3. Temperature dependence of the specific heat capacity (Cp) of (1) (AcO)GdPc and (2) GdPc2 in zero magnetic
field
MCE in (AcO)GdPc reaches the value of 0.47 K at B = 1.0 T and T = 278 K (Fig. 4). That in
the same complex with porphyrin, (AcO)GdTPP is equal to 1.45 K [14]. The temperature depend-
ence of MCE in (AcO)GdPc has minimum and maximum at 318 K and 338 K, respectively. The
•
values of the MCE in GdPc2 much less than in (AcO)GdPc. The temperature dependence of the
•
MCE in GdPc2 has a peak at 288 K, which increases with increasing magnetic induction.
•
Fig.4. Temperature dependence of MCE in (a) (AcO)GdPc and (b) GdPc2 at applied magnetic induction: (1) 1.0, (2)
0.8, (3) 0.6, (4)0.4, and (5) 0.2 T
The temperature dependences of the enthalpy change, ꢀН(enth) and of the entropy change,
ꢀS(enth) shown in Figures 5 and 6, respectively have the same character as these of MCE (Fig. 4).
•
The decrease of MCE values in the series of (AcO)GdTPP, (AcO)GdPc, GdPc2 allows a quali-
tative estimate the ratio of contributions of ferromagnetic and antiferromagnetic coupling to mag-
netic behavior of double-decker complex. Firstly, the entropy changes because of MCE (Fig. 6) are
only a few joules per kilogram of paramagnetic material. Cp value in zero magnetic field at the tem-
perature of 288 K in the series mentioned is equal to 1.19, 1.05 and 1.19 J g-1 K-1. The absence of
correlation MCE - Cp indicates zero contribution of the effects in lattice subsystem on magnetic be-
havior of the gadolinium(III) complexes. It means that MCE in ones is connected with the changes
in the electronic subsystem. As it was mentioned in Introduction, temperature-depending magnetic

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behavior of paramagnets based on 4f metal complexes is connected with slow thermally-activated
transition between spin-up and spin-down states. Ligand-field effect depends on both an electronic
system of ligands and coordination polyhedron of a central ion. As it was shown on an example of
(meso-tetraphenylporphinato) gadolinium(III), thulium(III), and europium(III) chlorides, the macro-
cyclic ligand in the tetrapyrrole complexes of gadolinium with the stabile 4f7 electronic system is
bonded without dative π interaction Gd↔N due to σ bonds. There is also an analogy in a coordina-
tion center structure of three gadolinium(III) complexes mentioned despite the difference in the co-
ordination numbers in a mono- and bistetrapyrrole complexes. A metal atom in such complexes is
displaced from N4 plane namely of approximately 1.7 [41, 42], 1.26 [43], and 1.450/1.495 [44] Ǻ
for (X)LnIIIP, (AcO)LuIIIPc, and GdIIIH(TPP)(Pc), respectively. The strength of the σ bonding in the
isostructural complexes (AcO)GdTPP, (AcO)GdPc grows in the transition to phthalocyanine com-
plex according to data [38, 45-47]. The more strongly is bonded a macrocyclic ligand the better is
conditions of the spin carrier - nuclear spins interactions being another important factor of the mag-
netic behavior of lanthanide phthalocyanines [2]. The negative contribution associated with the
growth of the strength of this interaction determines lower MCE in (AcO)GdPc. Besides that, radius
of the coordination cavity and the displacement of the Gd atom from its plane in (AcO)GdPc are
lower in comparison with porphyrin complex.
Fig.5. Temperature dependence of the enthalpy change, ꢀН(enth) in (a) (AcO)GdPc and (b) GdPc2• at applied magnetic
fields: (1) 1.0, (2) 0.8, (3) 0.6, (4)0.4, and (5) 0.2 T
Fig. 6. Temperature dependence of the entropy change, ꢀS(enth) in (a) (AcO)GdPc and (b) GdPc2• at applied magnetic
fields: (1) 1.0, (2) 0.8, (3) 0.6, (4)0.4, and (5) 0.2 T
In addition to the ligand-field splitting and interactions with nuclear spins, the oxidized π sys-
tem of one of two macrocyclic ligands having a half-occupied π orbital, SOMO can determine the

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•
magnetic properties GdPc2 . We could expect that the unpaired electron stabilizes the GdIII high-
spin state due to the promotion of the parallel alignment of the magnetic moments in oxidized
•
(bisphthalocyaninato) complex. Low MCE in GdPc2 shows that the antiferromagnetic intermolecu-
lar coupling of two radical spins as in [VO(OH2)(OEP•)]SbCl6 [12] which was discussed in Intro-
•
duction is the predominate contribution to the GdPc2 spin state. The contribution of the radical cen-
tered on the macrocyclic ligand to the overall magnetic moment is also negligible in
bis(octaethyltetraazaporphyrinato) gadolinium(III) [40] as well in its terbium and dysprosium ana-
logs. The intramolecular ferromagnetic spin coupling in metall porphyrin π-cation radicals is ration-
alized as arising from the exchange interaction of unpaired electrons in orthogonal magnetic orbitals
[48] in the case of the copper(II) complex with the sterically bulky tetramesitylporphyrin prevented
intermolecular aggregation and accompanying porphyrin core ruffling as in CuTPP π-cation radical.
The antiferromagnetic coupling in [CuTPP•]+ is rationalized in terms of spin pairing in the bonding
molecular orbital in the paper cited.
4 Conclusion
The gadolinium ion of charge 3+ retains its paramagnetic properties being in a porphy-
rins/phthalocyanines ligand field. As paramagnets, these complexes exhibit a large magnetocaloric
effect at temperatures close to room e.g. up to 1.45 and 0.47 K in (AcO)GdTPP and (AcO)GdPc,
respectively when the magnetic induction is changed from 0 to 1 T. That is only slightly lower than
one in 3d-4f clusters/homo-metallic gadolinium compounds at higher fields (2.4-3 K at ∆B of 0-7 T)
[7]. It could be employed for cooling in home and industrial refrigerators and other devices. Since
porphyrins/phthalocyanines and their complexes can selectively accumulate in a tumor of a human
body, the tetrapyrrole complexes under consideration promise to expand opportunities of hyper-
thermia in cancer diagnostics and therapy.
The advantages of lanthanide porphyrins/phthalocyanines over other paramagnets consist first-
ly in the fact that we can control their magnetothermal properties through structural molecule modi-
fication that is unlimited in the chemistry of tetrapyrroles. It was shown above that positive contri-
bution to MCE in gadolinium(III) porphyrine/phthlocyanine give a high spin of the paramagnetic
carrier and depending from a macrocycle’s electron structure a spin density delocalization between
Gd and a macrocyclic system. Negative contributions to a MCE value from both a growing interac-
tion with nuclear spins and the antiferromagnetic intermolecular coupling of two radical spins if
there are any. In addition to a written, the paramagnets based on porphyrins/phthalocyanines con-
tain only a small percentage of expensive gadolinium in comparison with the polycrystalline gado-
linium. They are soluble in organic media what allows forming from their nanoscaled molecules
nanostructures higher order. As the environmentally friendly materials, the complexes studied can
replace the toxic compounds used in vapor-compression cycle.
Acknowledgements
This work was carried out with the help of the center of the scientific equipment collective use “The
upper Volga region centre of physic-chemical research”. Synthesis was performed under grant of
the RFBR, No 15-03-00646-a.
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Online Supplementary Materials
Phthalocyanine-based Molecular Paramagnets. Effect of double-decker
structure on magnetothermal properties of gadolinium complexes
∗
V.V. Korolev, T.N. Lomova , A.G. Ramazanova, D.V. Korolev, E.G. Mozhzhukhina
Fig. 1S. IR spectrum of (AcO)GdPc in KBr

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Fig. 2S. IR spectrum of GdPc2 in KBr

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Graphic Abstract

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Phthalocyanine-based Molecular Paramagnets. Effect of double-decker
structure on magnetothermal properties of gadolinium complexes
V.V. Korolev, T.N. Lomova, A.G. Ramazanova, D.V. Korolev, E.G.
Mozhzhukhina
Magnetothermal properties of gadolinium(III) single/double-decker phthalocyanine complexes
were obtained by microcalorimetry for the first time.
Magnetocaloric effect in (AcO)GdPc reaches the value of 0.47 K at ∆B 0-1 T and T 278 K that is
only slightly lower than one in 3d-4f clusters/homo-metallic gadolinium compounds at higher
fields (0-7 T).
The intramolecular oxidation-activated spin coupling does not prevail over the intermolecular
•
coupling of two radical spins in GdPc2 .