Robert D. Peacock The University Dundee DDI 4HN, Scotland
The Preparation and Investigation of ~is(acetylacetonato)Copper(II)
Inorganic chemistry is concerned as much with the investigation of compounds as with their synthesis, and as much wit,h the interpretation of results as with their collection. We have designed an experiment in which a complex of known and comparatively simple structure is synthesized and subjected to the scrutiny of infrared and visible spectroscopy. The experiment was devcloped for a Scottish Second RSc class which had received lectures in ir spectroscopy and crystal field theory earlier in the year, but it could easily be modified (more in the interpretation of results than in the physical parts of the experiment) for more advanced students. Presented below is the laboratory script as given to the students, accompanied by notes and followed by a discussion of the project.
a hlue powder. ltecrystalliae about 1 g from methanol (at least 100 ml) on a water bath. On cooling in an ice bath his(acety1scetonato)copper(II) (C~(itcac)~) is obtained as dark hlue dichroic needles. Examine under the polarising microscope (Note 1 ). lnfrored Spectrum You will be shown how to use the ir spectrometer (Note 2). Run the spectrum of Cu(swe)s as a Nujol mull. YOUwill be given the spectrum of acetylacetone (Note 3) and that of Nujol. Comment on anv differencesbetween the soeotra of Cu(acac). and aeetylacetone Gith special reference to the ketone hands. show by examining tho ir spectrum that Cu(rtca~)~ is anhydrous. Structure ond Visible Spectrum
The structureof Cu(rt~sc)~is (Note 4)
The Experiment
Acetylacetone loses a proton when treated with strong base to give the acteylacetonate anion The molecules are packed in the crystal as indicated by the side elevation below HC HF This is a very efficient bidentate chelating ligand and forms strong complexes with metal ions. Prepomtion
Measure 2.5 ml acetylacetone into a 100-ml beaker and add the minimum amount of dilute ammonia, solution to dissolve the ketone m d give a homogeneous solution. Add this slowly, with stirring, to a. solution of 3.2 g CuS04.5&0 in about 50 ml cold water. Filter the dark blue precipitate at the pump and wash thoroughly with water until the filtrate is colorless. Wash slowly with about 100 ml acetone and finitlly with a little ether. Suck dry at the pump. The precipitate should now be in the form of
(Note that there will be little interaction between the copper and the CH groups, and so we h w e copper in an essentially squareplanar environment.) Yon will be supplied with copies of the visible (d-d) spectrum of the complex. Usink crystal field theory, explain the number of absorplion hands and draw lo scale a diagram of the energy levels, indicatirlg what arhital(s) are represented by each level, ltnd the trsnsitions giving rise to the spectrum. Stability
Some idea of the great stability of the complex e m be obtained thus. Place a little on the end of a spatula and hold it in the heat abare the Bunsen flame (or place some on a hotplate). The complex vaporizes without decomposition as is shown by the absence of any residue. Reactions
Dissolve a little of the complex in chloroform (a noncoordinating solvent for comparison), dilute HC1, concentrated ammonia, and pyridine. Explain any color changes. Notes
(1) Other preparative methods will be found in reference (1 ). The method described above, however, is quick, more or less foolproof, and gives a remarkably pure product. (2) I n our case a Perkin-Elmer 137. (3) Acetylacetoue mixed with Nujol was found to give a sharper spectrum than a film of the pure compound. (4) References (Z-4). Figure 1. Infrared spectra of bir(asetylacernnoto)coppeell~(solid line) and acetylacetone os Nujol mulls (broken line).
This work was supported in part by from The Science Research Council.
8,
Maintenance Grant
Volume 48, Number 2, February 1971
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K
K
C
~ ~ lo-? X
Figure 2. Visible.near ir spectrum of crystalline birlacetylacetonatobpper(ll1. Observed spectrum, solid line; components, broken liner.
Discussion
Our students had some experience of interpreting organic ir spectra, but had not run any themselves. Teaching them to use the Perkin-Elmer 137 and to make Nujol mulls was, therefore, an integral part of the experiment. The spectra obtained were very sharp. Figure 1 shows a comparison of the spectra of acetylacetone and Cu(acac), (both in Nujol) in the 20001400 cm-' region. That of acetylacetone has contributions from both keto and enol forms (see, e.g., reference (5)) and that of C u ( a c a ~ is ) ~fully discussed by Nalcamoto (6). All we expected the students to do was to qualitatively explain the spectra in terms of the change from C=C, C=O and C=O--H stretching frequencies to those of C-O and C=C. Many, however, were sufficiently interested to ask for an assignment of all t,he bands and undoubtedly the discussion section of Ndcamoto's paper could be given as a reference to more advanced students. The object of examining the d-d spectrum was t o discuss the origins of the bauds. We supplied, therefore, a diffuse reflectance spectrum rather than a solution one. I t can be convincingly resolved into three components (Fig. 2). The class had just been introduced to elementary crystal field theory, and we found discussion of the spectrum an ideal way of reinforcing some of the lecture material. Exact assignment of the
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bands must, of course, be done using molecular oribtal theory and is still a subject of discussion. References (7-1 1) are to recent papers in this field. If a teaching spectrometer recording down to 10,000 cm-' is available, the reactions section of the experiment can be extended by having the class run spectra of C ~ ( a c a cin ) ~chloroform, dilute HC1, concentrated ammonia, and pyridine. Acid can be shown, by comparison with the the spectrum of copper sulfate, to produce the aquo ion. In solution in ammonia or pyridine the baud shape and position changes quite noticeably and may be explained by coordination in the axial position(s). We found the best method for getting the most out of the experiment was as follows. The experiment was performed and all observations made in one lab period. The class was aslied to think about the spectra and questions before next period. Students discussed their conclusions singly or in small groups with a demonstrator. Only then were the conclusions written up. Such a system not only lets the students find out what validity their conclusions have but allows them t o be modified before being committed to paper. Acknowledgmenl
I wish to thank Dr. R. F. Jameson for encouraging t,he submission of this article and the 1968-69 Second BSc class for the enthusiasm and interest with which they received the experiment. Literature Cited (1)
PEBNELIVS.
W. C.. A N D DRYANT,D. & . , I n o ~ 8Synlh.. .
(2) S m n ~ ~ 5n ..,AND
5,105 (1957).
STONE.I
