Thermochromic Tetrachlorocuprate(lI) An Advanced Integrated Laboratory Experiment Sunhee Choi and James A. Lamabee Middlebuty College, Middlebury, VT 05753
A thermochromic substance undergoes a color change upon heating or cooling. One such thermochromic solid is bis(diethylammonium)tetrachlorocuprate(II), [(CH&Hz)zN H ~ ] z C U CThe ~ + tetrachlorocuprate(I1) anion displays discontinuous thermochromism, which is a rapid color change over a narrow temperature range ( I ) . The color change of bright to yellow corresponds to a coordination geome- .green . try change from distorted squar-planar to distorted tetrahedral(2). There are a number of thermochromic tetrachlorocumate com~lexes(3) -. . as well as other inoreanic thermochromic compounds (4). Thermochromism is a facinating subject that is based on some fundamental principles taught in advanced undergraduate courses: entropy-driven reactions, crystal field theory, d 4 transitions, hydrogen bonding, and prediction of molecular geometry. The delicate balance between the distorted square-planar and distorted tetrahedral geometry in CuCL2- is influenced hy several factors. Ligand-ligand repulsion favors the tetrahedral geometry; the square-planar geometry is stabilized in the solid by stronger (shorter) hydrogen bonds between the chlorides &d thehiethylammonium hydrogens, and crystal field stabilization energy favors the square-planar geometry ( 2 . 3 ~ ) .It is generally accepted that the dominant factor in the thermochromic phase transition of [(CH"CH2)2NH~11CuCL is the disordering" of the diethvlammooium cation 12). This entropy increase is consistent with the weakeninzof the hvdroeen honds between the cation hvdroaens and the chlorihes & the high-temperature phase (2,34. We have developed a multi-technique experiment based on the study of [(CH3CHz)zNHz]zCuCla for our advanced integrated laboratory course. The complex is easily prepared in high purity (3,5); the thermochromic transition (in the solid phase a t 45 OC) is studied by differential scanning calorimetry (DSC); a W / v i s absorption spectrum of a solid pellet of the complex above and below the thermochromic transition is obtained to show the effect on the d 4 transitions; and the infrared spectrum of the same pellet in the NH stretching region above and below the thermochromic transition shows the change in the hydrogen bonding. The energy of the thermochromic phase transition can be determined from the DSC data. Using the UV/vis data and values from the literature, the d-4 transitions can be assigned for the low-temperature square-planar and high-temperature tetrahedral forms of CuCl2-. While this experiment uses a variety of methods anddemonstrates several concepts, the instrumentation required is widely available. Any scanning UV/vis spectrometer with a range of 400-800 nm can be used, any medium-resolution dispersive or Fourier transform infrared spectrometer can be used, and any DSC can be used. If a DSC is not available, the temperature range of the thermochromic transition can be obtained using a conventional melting point apparatus (5). ~~
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Experimental Bis(diethylammonium)tetraehlorocuprate(II) was prepared according to published procedures (5). Diethylammonium chloride (0.02 mol, 2.2 g) was dissolvedin 15mL isapropylalcoholwith gentle 774
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heating; copper(II)chloride (0.01 mol, 1.7 g) was dissolved in 3 mL absolute ethanol with gentle heating. The two hot solutions were mixed and then cooled in an ice bath. Green needle crystals were isolated by filtration and rinsed with isopropyl alcohol. The hygroscopic solid was stored in a desiccator. Samples for differential scanning calorimetry (Perkin-Elmer model DSC-1B) were prepared by melting a few crystals (515 mg) of [ C H ~ C H ~ ) ~ N H ~ in ] ~aCtared U C ~volatile sample pan (PerkinElmer). This was accomplishedby placing apan on a warm hot plate (-90 "C) and placing the crystals into the pan. After the pan was cooled and the sample solidified, it was weighed and a cover was crimped onto the pan. The cqstals are too light and fluffy to easily place 5-15 mg into the DSC pan without the melting step. The sample does have a tendency to supercool, so the sample must he allowed toequilibrate at roomtemperaturefor at least 15 min before starting the DSC experiment. Furthermore, if a few crystals are placed in a sample pan without the melting step, poor thermal contact between the crystals and the pan causes multiple peaks in the thermochromie phase transition region; however, once this sample is heated to above the melting point and cooled, a normal sharp thermachromic phase transition peak is observed during suhsewent DSC runs. A reference samole of tin (5-10 me) - was also prepared in another volatile rnmple pan. Indium can also be used as a reference material. Thp [(CH3CHr)rNHrl.CuCIrsample was temperature-scanned from 300 K to 370 K at 10 Klmin on a range of 8. The tin reference sample wan temperature scanned from 490 K to 520 K at 10 Klmin on a range of 8. Visible absorption spectra were recorded from 800 to450 nm on a Cam 118 UVIvis smrtnmeter equipped with a variahle.temperawas prepared as a KC1pellet turesample compakment. The s&& (as oooosed to KBr oellet. to avoid anv oossihle halide exchange)in & i --~~r e aoellet n hie obtained from~ilmad.The sam~le remains - ~ -~~~~ in the stainless steel dir so that if n vnrist,le-temperature rnmple compartment is not available, thenampleand die could br heated on a hot plate above the thermochromic transition (45 "C);the die and. sample will remain above this temperature long enough to obtain a spectrum. Spectra of the same sample were recorded at room temperature and at 75 'C. Infrared spectra were obtained on the same KC1 pellet of [(CHr CHz)zNHz]~CuCl,at room temperature and at 75 'C using a Mattson Cygnus 1W FTIR at 4 em-' resolution.
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Results The thermochromic transition in I(CHCH?)?NHACuCh " -is seen visibly, by DSC, by visible absorption spec&mcopy and bv infrared a b s o r ~ t i o nsoectrmcoDv. Fieure 1 shows the DSC ;can of I ( C H ~ C H Z ) ~ N H ? ] ? C U C Erst I ~ . ~peak T ~ ~at 318 K 145 "C) is the thermochromic transition durine which the complex turns from green to yellow. The peak isvery sharp, consistent with discontinuous thermochromism. The second peak at 350 K (77 OC) is the melting peak. ALso shown in Figure 1 is the melting peak of tin.'l'he heat of fusion of tin is 60.7J/g. Comparison of the area under the tin peak with that under the thermochrnmic transition peak yields a value of 10.9 kJ/mol for the distorted squarkpl&-to-distortedtetrahedral transition. The thermochromic phase transition is reversible; however, scanning the DSC from higher to lower temperature results in supercooling of the sample. If the sample was originally heated to above the melting point, supercooling was observed for both the crystallization and the thermochromic transitions. When the sample was al-
lowed to cool at 300 K for approximately 15 min hefore reheating in the DSC, duplicate results are obtained from multiple heating cycles on the same sample. Figure 2 shows the visible absorption spectrum of [(CH3CH2)2NH2]2CuCL at room temperature and at 75 OC. The room temperature spectrum shows a maximum at 769 nm; at 75 "C the maximum shifts to below 800 nm and has been reported at 1100 nm (2). Figure 3 shows the infrared spectra in the N-H stretching region of [ ( C H ~ C H ~ ) ~ N H ~ ] ~ atCroom U C Itemperature ~ and at 75 "C. A clearly resolved N-H stretching doublet at 3086 and 3030 cm-I in the room temperature spectrum merges toward a single broad peak at 3086 cm-I in the 75 "C spectrum. Dlscusslon
Hydrogen honding between the diethylammonium hydrogens and the chlorides has been found to be the dominant stabilizing factor for the planar CuCl42- in the solid state (2.3~).The NH. .C1 distances are 331 pm in planar CuCLand lengthen (weaken) to 345 pm in the higher temperature, distorted tetrahedral CUCI~~-. The infrared spectra in the N-H stretching region show the effect of decreasing hydrogen bond strength with conversion of planar CuCL2- to tetrahedral CuCIP. N-H stretching in ammonium salts shows multiple splitting with strong hydrogen honding and will collapse to a single peak in the absence of hydrogen honding (6). This is the trend observed in Figure 3; the number of N-H stretching peaks decreases (system more
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TEMPERATURE ( K ) Figwe 1. (a) Lk9C m n of 15.2 mg [(CH&H2kNH2].CuCI,, scan rate 10 Klrnin. r w e 8: (b) DSC scan of 7.5 mg tin, scan rate 10 Klmin, range 8.
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WAVELENGTH ( n m ) F i w 2. (a) Wlvls abaomtion spectrum of KC1 pellet containing [(ChCH& NH,IPUCI, at mom temperahre; (b) same but at 75 OC.
Figure 9. (a) FTlR absorption spechum of KC1 pellet containing [(CH&H.k NH2I2CUCLat mom ternperatwe; (b) same but at 75 'C.
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Number 9
September 1989
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dynamic), and they collapse to a higher energy (stronger NH bond, weaker hydrogen bonding). Both of these changes are consistent with a decrease in the strength of the hydrogen honding, although the yellow high-temperature form still has considerable hydrogen honding (3c). A more dramatic effect has been observed with the Cu-CI stretching frequencies in the far-IR (3b). The v(Cu-C1) for the lowtemperature square-planar CuCL2- are reported at 282 and 287 cm-I. These shift to 295 cm-I in the high-temperature tetrahedral form. This change is consistent with an increase in the copper-chlorine bond strength in the tetrahedral fonn, which in turn is consistent with the decreased diethylammonium hydrogen-ehlorine hydrogen bonding. The heat necessary for the dynamic disordering of the diethylammonium cation and conversion of the CuC42- from distorted square-planar to distorted tetrahedral is 10.9 kJImol. A decrease in the d-d transition energies upon going from planar CuClr2- (green) to tetrahedral CuCL2- (yellow) is expected based on crystal field theory. The 769 nm peak (13,000 em-') observed in the low-temperature square-planar pellet spectrum is assigned to the electronic transition d, dx~-yz:This transition has been assigned at 12,500 cm-' in thesmgle crystal spectrum (7). In the high-temperature distorted tetrahedral form of CuCla2-, the electronic
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transition (d9, d.2-y2) (dZy,dxz,dyJ has been assigned at 1100 nm (9,100 cm-9 in the pellet spectrum (2). This experiment involves variety of techniques, instruments, and theories and allows each student to add his or her own personal touch. For example, a student can show that the melting transition in the DSC becomes narrower with recrystallization of [(CH,&H~)~NH~]ZCUC~~, or he or she may wish to calculate the purity of his or her product quantitativelv usine this DSC transition (8).A further extension of this work cokd involve the study bf the Cu-C1 stretches in the far-IR (36) or a more complete electronic soectral analvsis using a spectrometer withnear-IR capability.
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5. Van Oon,M. J. M. . IC h m . Edue. 1988,65,&1. 6. Rao,C. N. R.: Ferraro. J. R. S p r c r m s r o ~in hommie Chomintry; Academic: Naa
