Cold crystallization in bis[1-(3-methylphenyl)iminomethylnaphthalen-2- olato]nickel(II) studied by thermal analysis and x-ray diffraction

Article abstract of DOI:10.1246/cl.130409

Bis[1-(3-methylphenyl)iminomethylnaphthalen-2-olato]- nickel(II) was synthesized to see a potential use of this type of compounds as heat-storage materials utilizing cold crystallization. Its thermal, structural, and energetic properties were investigated by thermal analysis, X-ray diffraction, and DFT calculation. Some principles in designing complexes for this purpose are deduced.

Full text of DOI:10.1246/cl.130409

doi:10.1246/cl.130409
1040
Published on the web June 7, 2013
Cold Crystallization in Bis[1-(3-methylphenyl)iminomethylnaphthalen-2-olato]nickel(II) Studied
by Thermal Analysis and X-ray Diffraction
Katsunori Iwase,1,2 Hirohiko Houjou,3 Yasuhisa Yamamura,1 and Kazuya Saito*1
1Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8571
2DENSO CORPORATION, 500-1 Minamiyama, Komenoki-cho, Nisshin, Aichi 470-0111
3Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505
(Received April 30, 2013; CL-130409; E-mail: kazuya@chem.tsukuba.ac.jp)
Bis[1-(3-methylphenyl)iminomethylnaphthalen-2-olato]-
nickel(II) was synthesized to see a potential use of this type of
compounds as heat-storage materials utilizing cold crystalliza-
tion. Its thermal, structural, and energetic properties were
investigated by thermal analysis, X-ray diffraction, and DFT
calculation. Some principles in designing complexes for this
purpose are deduced.
Global ecology demands highly fuel-efficient vehicles such
as hybrid vehicles (HV). Although the transportation is certainly
of the primary importance for any vehicles, the comfort is also
crucial for the modern life. The improved fuel efficiency
generally accompanies the lowered temperature of exhaust heat
(200-300 °C), which is to be utilized for the improved net
Figure 1. Atomic numbering and ORTEP drawing of the
efficiency of vehicles. In this respect, the development of heat-
molecular structure in Form 1 crystal. Hydrogen atoms are
storage materials that are workable reliably and safely in this
omitted for clarity.
temperature range is strongly desired. Principal demands for
heat-storage materials are the controllability of storage and
release and the heat capacity.
(Figure 1), was synthesized7 and characterized by X-ray
crystallography on a single crystal.8 This as-synthesized crystal
is designated as Form 1 hereafter.
The phase-transition behavior of the complex was inves-
tigated utilizing a commercial differential scanning calorimeter
(DSC) (Mettler Toledo, DSC-1). Open pans of aluminum were
used under a nitrogen gas (50 mL min¹1). The scanning rate was
«15.0 °C min¹1. Transition temperatures were determined as
onset temperatures.
Figure 2 shows DSC curves with marked A-F peaks. Run 1
is the first heating process of Form 1. Form 1 melts at 241.5 °C
(A) with the enthalpy of fusion of 104 kJ mol¹1. On cooling the
liquid (run 2), it shows supercooling and finally undergoes a
glass transition (B) around 90 °C without any anomalies
suggesting crystallization. On heating (run 3), this glass exhibits
two stepped anomalies at 97.2 (C) and 149.7 °C (D). The two
anomalies are tentatively assigned to two-step softening (glass
transitions), suggesting that a softening process of translational
degrees of freedom of molecules is not closely coupled with that
of orientations. Above the second softening, the sample is in the
supercooled liquid, which relaxes to a crystalline form (the so-
called cold crystallization) at 191.4 °C (E) on further heating.
The enthalpy involved in the cold crystallization is estimated as
60 kJ mol¹1. The resultant crystal melts at 243.7 °C (F) with the
enthalpy of fusion of 79 kJ mol¹1. Since the temperature and
enthalpy of fusion are different from Form 1, the crystal
obtained after the cold crystallization (E) is referred to as
Form 2. After the second cooling (run 4), essentially the same
behavior as runs 2 and 3 was observed. The detailed thermody-
namics quantities are given in Table S1.12 Since the melting
Thermal storage materials can be grouped into two:
thermophysical1,2 and thermochemical materials.3,4 Thermo-
physical ones utilize sensible and latent heats and usually need
thermal insulation to prevent the heat losses to the environment,
except for those utilizing supercooling and cold crystallization.5
On the other hand, in thermochemical materials, chemical
reactions (breakage and formation of covalent bonds) are
utilized to reversibly store and withdraw energy without the
need for thermal insulation. Despite the high energy density and
insulation-free long-term storage, however, thermochemical
technologies have not been used in automobiles because of
needs for high temperatures (over 300 °C), which is closely
related to a large energy of a covalent bond.
The use of the change in coordination bonds, energy of
which lies in between van der Waals interaction used in a
thermophysical system and covalent bond in the thermochemical
one, would lower the working temperature while maintaining
relatively high energy density. Thermal properties and the cold
crystallization with the change in the coordination mode was
reported for a Schiff-base metal complex, bis[N-isopropyl-5,6-
benzosalicylideneaminato]nickel(II), by Wang et al.6 The aim of
this letter is to report the synthesis of a complex exhibiting cold
crystallization as the first attempt at developing heat-storage
materials of coordination complexes. The mechanism of cold
crystallization is discussed on the basis of results of thermal
analysis, X-ray diffraction, and quantum chemical calculation.
For a possible application in HVs, a high-melting point
complex with a substituted phenyl group as an N-substituent,
bis[1-(3-methylphenyl)iminomethylnaphthalen-2-olato]nickel(II)
Chem. Lett. 2013, 42, 1040-1042
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Experimental
100
Simulated
10
20
/ °
30
2
θ
Figure 3. Experimental powder diffraction patterns of Form 1
(blue line), Form 2 (red line), and glass (black line) compared
with simulated pattern based on crystal structures determined
independently.
Figure 2. DSC traces of the complex measured under nitrogen
with a scanning rate of 15.0 °C min¹1. A, F, melting; B, C, D,
glass transition; E, cold crystallization.
Amorphous nature of the glass (after B in Figure 2) was
confirmed as shown in Figure 3. Form 1 (as-synthesized crystal
before A) and Form 2 (obtained through cold crystallization E)
gave completely different diffraction patterns.
temperature of Form 2 (observed at F) is slightly higher by
ca. 2 °C than that of Form 1 (A), Form 2 is more stable
thermodynamically than Form 1 is in this temperature region.
The enthalpies involved in F in run 3, 5, and 7 were
approximately the same, implying the similar extent of cold
crystallization. If the extent was full, thermodynamic stabilities
of Forms 1 and 2 will be interchanged at a low temperature. The
interchange temperature (the expected temperature of the phase
transition) was calculated as ca. 233 °C. On the one hand, the
temperature of the cold crystallization (E) is 192 °C. Form 2 is
metastable at that temperature, accordingly. Hence, the behavior
of the cold crystallization is in accordance with Ostwald’s step
rule, which says the solid first formed from the melt is the least-
stable polymorph with the highest Gibbs energy.
As explained above, bis[1-(3-methylphenyl)iminomethyl-
naphthalen-2-olato]nickel(II) exhibits an exothermic process
upon the cold crystallization; this can be regarded as heat
storage in the first heating process and heat release in the second
or later heating process at an arbitrary moment by a thermal
stimulus. Note that the state storing heat is practically stable
below Tg µ 90 °C. The changes in the enthalpy of melting and
cold crystallization, corresponding to the heat storage capacity
of Form 2, are 79-84 and 60-62 kJ mol¹1, respectively. Since
the change in coordination bonds is not involved in this process
as described later, the latent heat utilized here relies on
van der Waals interaction.
Single-crystal pieces were picked up from the sample after
cooling the cold-crystallized sample to room temperature. After
some trials, the crystal structure of Form 2 was successfully
solved at room temperature.8 Resultant crystallographic data
reasonably explain the powder X-ray diffraction patterns for
both Forms 1 and 2, as shown in Figure 3. Therefore, we
confirm that both forms are the predominant crystals in the bulk
composition.
The peaks of the lowest angles in Forms 1 and 2 are indexed
ꢀ
as (101) and (100), respectively. If the setting of the space group
of Form 1 (P21/n) is changed to P21/c, the structural similarity
between the corresponding planes becomes clear (Figure S2).12
This suggests superior structural stability of molecular packing
within these planes. In other words, Forms 1 and 2 structurally
differ in stacking of these planes. On the other hand, the powder
X-ray pattern of the glass shows two broad peaks. The first broad
peak has the peak centered around 8.05°, which is roughly equal
to the lowest angle of Form 1 (8.05°). In contrast, the second
peak is very broad (16.0-27.0°).
For the comparison of molecular structures in Forms 1 and
2, three planes are defined: Plane P1, which serves as the
reference in the followings, is defined as that on which the five
central atoms sit. Plane P2 is the plane of the ligand except a
substituent on N1 atom. P2 is, in practice, determined as the
“best plane,” from which the sum of squares of distances of C9-
C18 atoms is the minimum. Although P2 of Form 2 (2-P2) is
almost flat (as indicated by «0.01 ¡) in terms of the distance of
atoms, the planarity of 1-P2 is worse («0.03 ¡). Plane P3 is the
best plane of the benzene ring consisting of C1-C6 atoms. P3 is
practically flat in both Forms 1 and 2.
To see structural changes associated with thermal anomalies
observed by DSC, simultaneous powder X-ray diffraction and
DSC (XRD-DSC) analysis was employed. Despite slightly
different sample environments, thermal behavior observed in
DSC (Figure 2) was essentially reproduced. The result of XRD-
DSC is presented in Figure S1 in Supporting Information.12
Now, the molecular structures can be compared. The
bending angle between 1-P1 and 1-P2 was 18.1°. 1-P3 is
Chem. Lett. 2013, 42, 1040-1042
© 2013 The Chemical Society of Japan
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1042
the flexibility in coordination geometry should be pursued in
future.
References and Notes
1
2
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Nickel(II) acetate hydrate (4.24 g, 16.0 mmol) in THF (20 mL)
and ethanol (30 mL) was mixed with N-(m-tolyl)-2-hydroxy-1-
naphthaldimine (2.00 g, 8.0 mmol) in ethanol (50 mL). The liquid
solution was heated at 70 °C. After 1 h, a platelet-shaped crystals
started appearing. We confirmed the purity of complex which is
high enough to pass 1H NMR, Karl-Fisher (water content ca.
270 ppm).
X-ray measurements of single crystals of Form 1 and Form 2
were carried out using a R-AXIS RAPID imaging plate
diffractometer with Mo K¡ radiation (0.71075 ¡) and a Rigaku
Saturn724 diffractometer using multilayer mirror monochromat-
ed Mo K¡ radiation at 20 « 1 °C, respectively. The structures
were solved by the direct method (SIR20089) and refined by the
full-matrix least-squares method on «F«.2 The non-hydrogen
atoms were refined anisotropically. Hydrogen atoms were refined
using the riding model. Absorption correction was applied using
an empirical procedure. All calculations were performed using
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Crystal Data for Form 1: C36H28N2NiO2, M = 579.33, green
platelet, crystal dimensions 0.200 © 0.090 © 0.030 mm3, mono-
clinic, space group P21/n, a = 12.076(1) ¡, b = 2.8739(5) ¡, c =
Figure 4. Molecular energy starting from the structures in
Forms 1 and 2 as a function of the twist angle of the phenyl
moiety in the molecule in 15° steps from 40 to 100°. Crosses
indicate the angles corresponding to the structures in crystals.
Diamond corresponds to the fully optimized structure.
twisted from 1-P1 by 71.2°. The corresponding angles in Form 2
are 14.7 and 81.5°, respectively. If the step distance is defined as
the vertical distance between P2 and P2¤, those of Forms 1 and 2
are 0.00 and 0.62 ¡, respectively. It is reported13 that complexes
with bulkier side groups as substituents on N1 atom exhibit
larger step distances. Hence, the substituent (3-methylphenyl
group) on the N atom turned to an effectively bulkier structure
by twisting.
The energy profiles of isolated molecules were calculated
by varying the twist angle º of the phenyl moiety (expressed by
the dihedral angle of C8-N1-C1-C2 in the molecule) at every
15° from 40 to 100°, while fixing structures of other parts at
those observed crystallographically. DFT calculation was per-
formed utilizing the Gaussian03 package at level of B3LYP/
6-311G**.14
19.484(2) ¡, ¢ = 99.593(7)°, V = 1362.8(2) ¡3, Z = 2, Dcalcd
=
1.412 g cm¹3, 9040 reflections collected, 3080 independent
(Rint = 0.0515), GOF = 1.006, R1 = 0.0511 (I > 2.00·(I)),
wR2 = 0.1368 for all reflections. Crystal Data for Form 2:
C36H28N2NiO2, M = 579.33, brown block, crystal dimensions
0.080 © 0.077 © 0.020 mm3, monoclinic, space group P21/c,
a = 9.318(4) ¡, b = 11.799(5) ¡, c = 13.690(6) ¡, ¢ =
Results of the calculation, i.e., the intramolecular stability,
are shown in Figure 4. Although the molecular energy in
Form 1 is calculated to be slightly lower than that of Form 2 by
ca. 5 kJ mol¹1, this magnitude is too small to deduce a definite
conclusion concerning the relative stability of molecules in
Forms 1 and 2. Considering thermal energy at temperatures of
present interest [R © (500 K) µ 4 kJ mol¹1], on the other hand,
the minima of the energy curves obtained for both Forms are
very shallow. This implies that molecules in melt have a wide
range of the twist angle, resulting in the ease of supercooling,
i.e., the difficulty in crystallization on cooling. In this respect, a
flexible molecular structure is preferred in designing molecules
exhibiting cold crystallization.
In conclusion, we considered the possibility of utilizing
organic metal complexes as heat-storage materials, and
we prepared bis[1-(3-methylphenyl)iminomethylnaphthalen-2-
olato]nickel(II) as a trial material. Thermal, crystallographic, and
computational analyses revealed the cold crystallization behav-
ior of the compound. Molecular flexibility enhances the ease of
supercooling, possibly leading to the occurrence of the desired
cold crystallization. Since Ostwald’s step rule practically
applies, avoiding metastable polymorphs is preferable to achieve
a large storage capacity. Considering the potential advantages,
¹3
108.253(5)°, V = 1429.4(11) ¡3, Z = 2, Dcalcd = 1.346 g cm
,
11239 reflections collected, 3272 independent (Rint = 0.0378),
GOF = 0.916, R1 = 0.0401 (I > 2.00·(I)), wR2 = 0.1054 for all
reflections.
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12 Supporting Information is available electronically on the CSJ-
Journal Web site, http://www.csj.jp/journals/chem-lett/index.
html. Crystallographic data have been deposited with Cambridge
Crystallographic Data Centre: Deposition numbers CCDC-
936032 (Form 1) and 936033 (Form 2) of bis[1-(3-methylphe-
nyl)iminomethylnaphthalen-2-olato]nickel(II). Copies of the data
can be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html (or from the Cambridge Crystallographic
Data Centre, 12, Union Road, Cambridge, CB2 1EZ, U.K.; Fax:
+44 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).
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Chem. Lett. 2013, 42, 1040-1042
© 2013 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/