Solid-State Synthesis of a Thermochromic Compound

In the Laboratory
Solid-State Synthesis of a Thermochromic Compound
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Chen Changyun, Zhou Zhihua,* Zhou Yiming, and Du Jiangyan
Department of Chemistry, Nanjing Normal University, Nanjing 210097, PRC; *[email protected]
The study of thermochromism has gradually advanced
from thermochromism and its mechanism (1–3) in solution
to thermochromism in the solid state, and synthesis methods
have followed the same trend, from synthesis in solution to
synthesis with solid-state reaction. The preparation of thermochromic solids has been described by several publications in
this Journal (4–6 ); however, they mainly involved synthesis
in solution or with liquid-phase reaction. Recently, synthesis
by solid-state reaction at room or mild temperature has
attracted more and more chemists owing to its high efficiency,
energy saving, and environmentally friendly features (7 ).
Here we report the synthesis of the thermochromic solid
[(C2H 5)2NH 2]2NiCl4 by solid-state reaction at mild temperature and suggest how this could be used as a classroom
demonstration to help college or university students understand
the synthesis of thermochromic solids by solid state reaction,
the effect of crystal field splitting on color, and the strength
effect of hydrogen bonding in a solid crystal on the configuration of a compound.
Experimental Procedure
The preparation of bis(diethylammonium) tetrachloronickelate(II) is carried out by mixing of 2 moles of diethylammonium chloride and 1 mole of anhydrous nickel chloride
according to the following reaction equation:
2(C2H5)2NH2Cl + NiCl2 → [(C2H5)2NH2]2NiCl4 (1)
Accurately weigh 0.02 mol of diethylammonium chloride
and 0.01 mol of anhydrous nickel chloride. Both solids are
ground. Then the two are mixed in an agate mortar under an
infrared lamp (to prevent the reactants from absorbing
moisture) and continuously ground for about 10 minutes.
The mixture is then rapidly transferred to a weighing bottle
and is kept at 60 °C for 1 h. The product thus obtained is a
slightly hygroscopic brown-yellow solid and should be stored
in a desiccator. The thermochromic temperature of this
material is 72–73 °C as determined by DSC. The color
change is from brown-yellow to blue at the thermochromic
temperature.
As a classroom demonstration, one could put the product
in a stoppered test tube and then place the tube in a water bath
at 80 °C to observe the thermochromic phase transition. For
more convenience as a classroom demonstration, one may
mix the product with a colorless printing ink and then disperse a thin layer of the mixture on a piece of filter paper.
The filter paper thus obtained is ready for demonstration. Heat
the paper on a hot-plate and observe the thermochromism. The
colorless printing ink is chosen as a matrix because it could
prevent the product from absorbing moisture and is stable
under the experimental heating conditions. Note that the
product should not be exposed to air for more than 10 min.
Discussion
Bis(diethylammonium) tetrachloronickelate(II) is analogous
to bis(diethylammonium) tetrachlorocuprate(II). Both are
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discontinuous thermochromic materials, but they differ in
their method of synthesis and in their thermochromic mechanism. The copper complex can be simply prepared by liquidphase reaction (4), but the nickel complex can never be obtained
in this way because the complex is very unstable in solution.
The Ni(II) complex can only be prepared by solid-state
reaction (8). The solid-state synthesis described here is relatively
mild, so the experiment is suitable as a demonstration for
undergraduates in general chemistry.
While the color change for the copper complex discussed
in this Journal (4, 6 ) is due to the change from square-planar
to a much less constrained distorted tetrahedral geometry, the
color change for the analogous nickel complex is attributed
to the change from octahedral to tetrahedral geometry. With
the discussion of mechanism we help the students understand
some principles of coordination chemistry taught in inorganic
chemistry.
First, the color change from brown-yellow to blue at
thermochromic temperature for the nickel complex is associated with changes in coordination geometry, coordination
number, and crystal field splitting energy. In our case, the
structure of the complex at room temperature is postulated
to consist of NiCl6 octahedra sharing bridging chlorine atoms
and linked into a two-dimensional infinite polymeric sheet
(9, 10). Because the substituted ammonium is not quaternary,
the hydrogen bonding between the cation hydrogen and the
chlorines in NiCl42᎑ anion play a major role in maintaining
the octahedral geometry. The stronger the N–H…Cl hydrogen
bonding ability, the higher the thermochromic temperature
that is required to change the geometry.
Second, we use the following approximate calculations
for crystal field splitting energy to help the students appreciate
how color changes are related to internal structure changes.
The d–d transitions may be responsible for the color of
bis(diethylammonium)tetrachloronickelate(II). The changes
in color indicate internal structure changes in the complex.
The variation of coordination field strength is due to changes
in the coordination number of Ni(II) from a 6-coordinate
octahedral field to a 4-coordinate tetrahedral field. The following simple evaluation may illustrate the relation between the
coordination field strength and the color of the complex. In
general, the relation between crystal field splitting energy ∆ o
for an octahedral geometry and ∆ t for a tetrahedral one in
NiCl42᎑ anion (1) can be approximately expressed as
∆ t ≈ 1⁄2 ∆o
(2)
In the low-temperature phase, six chlorine ions around a
Ni2+ ion are arranged as an octahedron; the d-electron configuration of the central Ni2+ ion is t2g6eg2; the d–d transitions
absorb shorter wavelengths of visible light, so the complex
appears brown-yellow. In the high-temperature phase, four
chlorine ions around a Ni2+ ion have a tetrahedral geometry;
thus the Ni 2+ ion d-electron configuration becomes e4t24,
resulting in absorption of longer wavelengths of visible light
by d–d transitions, so the complex appears blue. The stabili-
Journal of Chemical Education • Vol. 77 No. 9 September 2000 • JChemEd.chem.wisc.edu
In the Laboratory
zation energy in the crystal field for NiCl42᎑ anion in d8 weak
field configuration can be estimated from the d-electron arrangement of the central Ni2+ ion. The stabilization energy Eso for
an octahedral environment can be calculated from
Eso = [(᎑ 2⁄5) × 6 + (3⁄5) × 2] ∆ o = ᎑(6/5)∆ o
(3)
In a similar way the stabilization energy Est for a tetrahedral environment is
Est = [(᎑ 3⁄5) × 4 + (2⁄5) × 4 ] ∆ t = ᎑(4⁄5) ∆ t
(4)
Since ∆ t ≈ (1⁄2)∆o, we find that Est ≈ ᎑ (2⁄5)∆ o. As a
result, for nickel ∆E = Eso – Est = ᎑ (4⁄5)∆ o, with ∆ o = 7300 cm᎑1
(11), so that ∆E = ᎑5840 cm᎑1 = ᎑78 kJ/mol. This result shows
that the octahedral arrangement is slightly more stable than
the tetrahedral structure. That is why the tetrahedral geometry for this complex is favored at higher temperature.
Acknowledgments
We gratefully acknowledge the kind help of Jin Anding
and Wang Jialiang and the financial support of the Natural
Science Foundation of Education Commission of Jiangsu
Province, P. R. China.
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