In the Laboratory
Preparation and Thermochromic Properties of Copper(II)-N,N-Diethylethylenediamine Complexes Ai-Li Cui, Xi Chen, Long Sun, Jing-Zhi Wei, Jin Yang, and Hui-Zhong Kou* Department of Chemistry, Tsinghua University, Beijing 100084, P. R. China *
[email protected] This experiment illustrates the thermochromism of compounds at the undergraduate level. Such compounds demonstrate reversible color change upon heating or cooling. For inorganic compounds, this transition is usually due to change in the crystalline phase, in coordination geometry around a metal ion, or in the number of solvent molecules in the coordination sphere (1). Synthesis of thermochromic coordination compounds of Cat2[MCl4] (Catþ = diethylammonium, dimethylammonium) and M2HgI4 [M = Cu(I), Ag(I)] has been reported in this Journal (2-6). It is noteworthy that most experiments or demonstrations deal with transition-metal halides. In this undergraduate laboratory experiment, the syntheses of thermochromic chelate compounds [Cu(deen)2](NO3)2 and [Cu(deen)2](BF4)2 (deen = N,N-diethylethylenediamine) are performed (Scheme 1). The solids are stable and change color from red to violet at transition temperature of 145 and 15 °C, respectively. Such phenomena (7) can easily be observed. The red and the violet phases for [Cu(deen)2](BF4)2 are practically isostructural to the perchlorate compound [Cu(deen)2](ClO4)2 (8, 9) that also displays similar thermochromic behavior. Structural determination of [Cu(deen)2](ClO4)2 shows that the red-violet color change corresponds to the structural change of the CuN4 coordination configuration from square planar to tetrahedral (vide post). By comparing the structures of [Cu(deen)2](ClO4)2, students learn the origin of the thermochromic property of [Cu(deen)2](BF4)2. The time required for the experiment is 3-4 h. This experiment is suitable for first-year undergraduates majoring in chemistry. Experimental Details This experiment contains two parts: synthesis and thermochromism tests. All chemicals are of analytical grade and are commercially available. The preparation experiments are conducted in fume hood at room temperature. The precursor Cu(BF4)2 can be prepared in situ by reacting an excess quantity of solid cupric carbonate basic, CuCO3 3 Cu(OH)2, with tetrafluoroboric acid (40% in water) (10, 11). The filtrate containing Cu(BF4)2 is subsequently used to react with N,N-diethylethylenediamine. Preparation of [Cu(deen)2](BF4)2 An excess of CuCO3 3 Cu(OH)2 (0.830 g, 3.75 mmol) is added with stirring to 40% aqueous HBF4 (2.2 g, ca. 10 mmol) in a 100 mL beaker. After the releasing of CO2 has ceased, ethanol (30 mL) is added to the solution. The unreacted solid CuCO3 3 Cu(OH)2 in the reaction mixture is removed by filtration. To the light blue filtrate, an ethanolic solution (30 mL) of
_
Scheme 1. Synthesis of the [Cu(deen)2]X2 Complexes
N,N-diethylethylenediamine (1.16 g, 10 mmol) is added dropwise with stirring. Stirring is continued for an additional 5 min. A violet microcrystalline solid precipitates from the solution after standing at room temperature (28 °C) for about 5 min. The solid is collected by suction filtration and then washed twice with diethyl ether (5 mL) to remove any absorbed byproducts. The solid is placed on filter paper, air-dried for 30 min, and then weighed. Preparation of [Cu(deen)2](NO3)2 Cu(NO3)2 3 3H2O (1.2 g, 5 mmol) is dissolved in absolute ethanol (40 mL) in a 150 mL beaker, giving a clear blue solution. Ethanolic solution (10 mL) of N,N-diethylethylenediamine (1.16 g, 10 mmol) is added to the blue solution dropwise with stirring. The color of the reaction mixture turns blue-violet. A red solid precipitates from the solution after standing for about 10 min. The solid is collected by suction filtration, and washed twice with diethyl ether (5 mL). The solid is placed on filter paper, air-dried for 30 min, and then weighed. Thermochromism of [Cu(deen)2]X2 (X = BF4-, NO3-) The solids undergo color changes at certain temperatures when heated or cooled. The exact transition temperature should be determined by heating the solid using a platform-type melting point apparatus. A small quantity, few milligrams, of [Cu(deen)2](BF4)2 or [Cu(deen)2](NO3)2 solid are heated on a melting point apparatus. Students record the temperatures at which the color change occurs. Note that, if the ambient temperature is higher than the transition temperature (∼25 °C), that is, [Cu(deen)2](BF4)2 is in the violet form at room temperature, this procedure should be omitted. A small quantity of violet [Cu(deen)2](BF4)2 solid is placed in a test tube. The solid is chilled in an ice water bath until the
_
r 2010 American Chemical Society and Division of Chemical Education, Inc. pubs.acs.org/jchemeduc Vol. 88 No. 3 March 2011 10.1021/ed100103a Published on Web 12/17/2010
_
Journal of Chemical Education
311
In the Laboratory
color change occurs. After the test tube is removed from the ice water bath, the color reverses to the original color within a few minutes. Alternatively, if the ambient temperature is low and [Cu(deen)2](BF4)2 exists in the red form, the red solid can be heated using a hair dryer or heat gun or by immersing in a hot water bath at 80 °C. After the color change is completed, the test tube is removed from water bath and the color is observed. The process can be repeated many times. A small quantity of red [Cu(deen)2](NO3)2 is placed in a test tube. The test tube is heated in an oven at 146 °C for 35 min. The test tube is removed and the violet color is observed. The solid is allowed to cool on the bench, but the violet color remains as the transition to the red color takes nearly 2 months at room temperature. Hazards The diamine is corrosive and causes irritation to skin and eyes. Breathing diamine will cause coughing and short breath and may damage the lungs. It is harmful if inhaled or in contact with skin. HBF4 is harmful by inhalation, in contact with skin, or swallowed. Copper nitrate is a strong oxidizer and causes irritation to the eyes, skin, and respiratory tract. CuCO3 3 Cu(OH)2 causes irritation to the eyes, skin, and respiratory tract. Ethanol and diethyl ether are flammable and harmful if inhaled or absorbed through skin. Results and Discussion Bidentate diamine ligands can coordinate to transition-metal ions in a chelating mode to form stable [M(L)x]nþ (x = 1-3) species. In the case of N,N-diethylethylenediamine, reaction with CuX2 (X- = NO3-; BF4-) in a molar ratio of 2:1 gives rise to compounds [Cu(deen)2]X2. The reaction can be performed at room temperature in a nonaqueous solution, for example, ethanol. The synthesis is simple, and students can learn a nonaqueous synthetic method for the preparation of coordination compounds, especially the use of appropriate solvents to isolate the products. The use of ethanol is crucial for the preparation, and the volume of the solvent used can significantly affect the yield of the products. It is suggested that this effect should be investigated in the lab during the preparation of the tetraborate compound. Both solids show thermochromic properties where they change color from red to violet (Figure 1). The transition temperatures of [Cu(deen)2](BF4)2 and [Cu(deen)2](NO3)2 occur at 15 and 145 °C, respectively. Because of the low transition temperature of the tetrafluoroborate compound, the isolated solid may be violet or red in color depending on the ambient temperature. Structural analysis of the isostructural [Cu(deen)2](ClO4)2 shows that subtle structural variation is responsible for the color change (8). At low temperature, the Cu(II) coordination geometry can be described as ideal planarity with four nitrogen atoms of the two coordinating deen ligands in a square planar arrangement. At high temperature, the CuN4 coordination plane slightly deviates from planarity with the distortion of backbone alkyl groups in N,N-diethylethylenediamine (Figure 2). However, the very weak axial coordination contributes little to the color change. The reversibility of thermochromic property for the two compounds is different. The red tetrafluoroborate reverses to the violet form in 1 min when warmed on the bench,
312
Journal of Chemical Education
_
Vol. 88 No. 3 March 2011
_
Figure 1. The color change of [Cu(deen)2](BF4)2 (upper) and [Cu(deen)2](NO3)2 (lower).
Figure 2. The different coordination configuration of Cu(II) in [Cu(deen)2](BF4)2: the structure on the left is the red form and the structure on the right is the violet form.
whereas it takes nearly 2 months for the violet nitrate compound to turn red. Acknowledgment We acknowledge the financial support from the National Natural Science Foundation of China (20953006) and Ministry of Science Technology, China through 973-project (2002CB613301). We thank Joseph F. Chiang (State University of New York) for proof reading the English language of this manuscript. Literature Cited 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
Day, J. H. Chem. Rev. 1988, 68, 649. Van Oort, M. J. M. J. Chem. Educ. 1988, 65, 84. Hughes, J. G. J. Chem. Educ. 1998, 75, 57. Chen, C.; Zhou, Z.; Zhou, Y.; Du., J. J. Chem. Educ. 2000, 77, 1206. Choi, S.; Larrabee, J. A. J. Chem. Educ. 1989, 66, 774. Bukleski, M.; Petrusevski, V. M. J. Chem. Educ. 2009, 86, 30. Fabbrizzi, L.; Michcloni, M.; Paoletti, P. Inorg. Chem. 1974, 13, 3019. Grenthe, I.; Paoletti, P.; Sandstrom, M.; Glikberg, S. Inorg. Chem. 1979, 18, 2687. Naumov, P.; Sakurai, K.; Asaka, T.; Belik, A. A.; Adachi, S.-i.; Takahashi, J.; Koshihara, S.-y. Inorg. Chem. 2006, 45, 5027. Narayanan, B.; Bhadbhade, M. M. J. Coord. Chem. 1998, 46, 115. Lever, A. B. P.; Mantovani, E.; Donini, J. C. Inorg. Chem. 1971, 10, 2424.
Supporting Information Available Lab Documentation containing experimental procedure for the students, student handouts and instructor notes. This material is available via the Internet at http://pubs.acs.org.
pubs.acs.org/jchemeduc
_
r 2010 American Chemical Society and Division of Chemical Education, Inc.
