John A. Weyh Western Washington State College Bellingham, Washington 98225
II
Preparative and Ion Exchange Studies on the Cobalt(lN)-hinodia~etatetoteSystem A laboratory experiment
The topic of inorganic coordination chemistry is an integral part of the introductory college chemistry course today and, as such, experimentation on the undergraduate level in this field is an important complement to lecture material presented in the classroom. Many experiments pertaining to coordination chemistry for use in the freshman laboratory have appeared in THIS JOURNAL and various laboratory manuals in the last ten to fifteen years. These deal generally with the preparation and analysis of various coordination compounds. Some experiments deal with the preparation of the various geometrical isomers of certain compounds, and some involve the study of selected reactions of various compounds. It is important that experiments selected for use in the introductory laboratory demonstrate fundamental chemical principles to which the student has been introduced in tha classroom. Secondly, these experiments must demonstrate and utilize modern methods of investigation and allow some insight as to how conclusions are drawn from experimental observations. The experiment described below is intended to demonstrate these principles. Two of the three possible geometrical isomers of the 1:2 (metal ion to ligand) iminodiacetato (HN(CH2COO-)2, IDA) complexes of cobalt(II1) will be prepared. The three possible isomers are shown in the figure. The cobalt(II1) system was chosen because of the relative ease of preparation of these compounds and because the compounds obtained are highly colored. I n the system presently under discussion, one of the isomers is purple, the other brown. This system has been studied by Tsuchida, et. al.,' and C ~ o k e . ~ Cobalt chloride, CoC12.6H20,is used as the source of cobalt(I1) ion, and its oxidation to cobalt(II1) is effected using hydrogen peroxide. Iminodiacetic acid is used as the source of ligand. Both preparations are carried out using the same mole ratio of metal ion to ligand to illustrate that two different compounds are
obtained from solutions containing the same ratio of reactants. Only the volume and temperature conditions vary in the two procedures. Some students may obtain a mixture of the two isomers in a reaction v e ~ s e l . ~On a small scale this is good as it demonstrates the sensitivity of the preparation to reaction conditions and points to the need of a means of separation of the two isomers. Rather than asking the students to accept a stated geometric identification for the solid compounds, they will be required to establish the geometric structure of the two isomers through experimentation. The tool to be used is anion exchange chromatography. A brief introduction to the principles of ion exchange and the nature of ion exchange resins can be given prior to the laboratory period. By noting the separation and the rates of elution of the two isomers, a preliminary assignment of geometrical structure can be made. Since both complexes have the same charge of minus one, the separation on the column must be based on the difference in polarity of the two isomers. Using molecular modelsJ4the student can determine for himself which of the three isomers will have the greatest dipole moment and hence have the greatest affinity for the resin. Molecular models clearly demonstrate the greater polarity of the cis complex and the relative non-polarity of the trans(facial) and trans(meridiona1) isomers. I t is also apparent from the models that the two trans isomers have very similar polarities; hence the elution properties of these species should be quite similar. The separation on the column is quite pronounced and based on the above polarity arguments and the separations observed, the assignment of the cis structure to the purple compound and the trans structure to the brown compound can easily be made. The problem of which trans isomer was obtained lends itself nicely to a discussion utilizing theoretical principles. This relates to bond angles and the possible straining of the C-N-C bond angle in the IDA ligand -
TRANS (FACIAL)
CIS
TRANS (MERIDIONAL)
The three possible geometrical isomers in the 1 :2 cobaIt(lll)-iminodiacetate system.
HIDAKA,J., SHIMURA, Y., A N D TSUCHIDA, R., Bull. Chem. Soc. Japan, 35, 567 (1962). D. W., Inorg. Chem., 5, 1141 (1966). COOKW, The preparation of the brown trans(facia1) isomer should be quite pure. The cis isomer preparation may be contaminated with some trans material. The author believes that it is imperative to have two or three sets of molecular models available in the laboratory so that students can clearly visualize the various geometrical configurations for this system. This is especially important in distinguishing between the trans(facia1) and trans(meridiona1) configurations. The inexpensive Benjamin/Maruzen Molecular Model Sets have been found to be quite satisfactory for this purpose. Volume 47, Number 7 0, October 7 970
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for one of the configurations. In the trans(meridiona1) complex, models show that there is a strain involved in placing the ligand on the meridional position. Models also show that such bond angle strain is not present in the trans(facia1) configuration. Because of the high energy situation involved in the strained trans(meridional) configuration, it can be argued that this isomer would be quite unstable. Hence, a reasonable assignment for the trans compound obtained would be the trans(facia1) geometrical configuration. The above assignments for this system have been verified using nmr spectroscopy. An important point for the student to realize concerning this experiment is that his experimental observations do not constitute proof of the geometrical structures of the isomers but onlv lend evidence to these assignments. This realization "by the student would open the door for the instructor to present a brief introduction of more advanced techniques which are capable of definitely answering the question of geometrical structure for these systems. The experiment can be performed in two 3-4 hr laboratory periods. During the first period the complexes are synthesized and the ion exchange column is prepared. During the second period the solid com~ o u n d sare collected and washed. and the chromatographic separation and identificatiin are performed. Experimental
Preparation of the purple isomer. To a 100 ml beaker is added 1.6 g of solid, reagent grade potassium hydroxide.5 Then 6.5 ml of water is added and the solution swirled to dissolve the solid. Next 2.0 g of iminodiacetic acid (H21DA)is added. After the acid dissolves, 1.7 g of cobaltous chloride (CoC12.6HzO) is added and dissolved and the solution cooled to approximately 12OC in an ice bath. After cooling, 5.0 ml of 15% ' 0 2 0 2 is added 0ver.a period of 2-3 min. with ~ t i r r i n g . ~ While maintaining a temperature of 12"C, the mixture is stirred periodically during the next 2-3 hr. The purple crystalline product is collected on a Buchner funnel, washed first with one 4-5 ml portion of ice cold water then with one 10 ml portion of ethanol, and allowed to dry. Approximately 1.5-2 g of solid is obtained. Preparation of the brown isomer. Solid potassium hydroxide (1.6 g) is added to a 100 ml beaker and dissolved with 28 ml of water. Next 2.0 g of iminodiacetic acid is added and dissolved, followed by the addition of 1.7 g of cobaltous chloride. The reaction mixture is heated on a water bath to 80°C and 1ml of 30% Hz02 is added.= (CAUTION! The reaction is vigorous.) After the reaction has ceased, the beaker is covered with a watch glass
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and heating is continued for approximately 45 min. The beaker and cover are then placed in the locker until the next laboratory period. The brown crystalline product is collected by filtration onto a Buchner funnel. The solid is washed first with one 5-ml portion of ice cold water then with one 10-ml portion of ethanol, and allowed to dry. Approximately 1 g of solid is obtained. Preparation of the im exchange column. The students are given the packed column containing the resin surrounded by an aqueous phase. The glass columns are prepared from 16 mm 0.d. glass tubing with drip tip added, and are approximately 25 cm in length. Glass wool is used to prevent the loss of resin. Either Amberlite CG-400 AR Grade (100-200 mesh) or Dowex 1-X8 AR Grade (100-200 mesh) anion exchange resin in the C1- form is used for the separations. Resin height in the columns is about 10 cm. During the first laboratory period the columns should be flushed with water and the liquid level reduced to 2-3 cm above the resin. NEVER allow the liquid level to fall below the level of the resin. Care should be exercised so that the surface of the resin bed is not disturbed throughout the experiment. Glass wool may be used to aid in this purpose. Isomer separation utilizing ion exchange chromatography. Take 0.025 g of the purple crystalline isomer and 0.04 g of the brown isomer and dissolve in 4-5 ml of water. With the liquid level in the column about 3 cm above the resin surface, add the prepared solution. Be careful not to disturb the surface of the resin. Allow the column to drip a t the rate of one drop every 12-15 sec. When the column is charged, rinse the resin by running 5 ml of water through the column. This may be done rapidly. Next, begin elution and development of the column using 0.1 F NaCl as the eluting agent. Fill the column with 0.1 F NaCl solution and allow it to drip a t the rate of one drop every 20-25 sec. Allow development to proceed from 11/2-2 hr. Since the complexes are high colored it is easy to follow the advancement of each down the column. From the separations obtained on the column, assign either the cis or trans structure to each complex. Conclusion
The above ex~erimenthas introduced the student to coordination chemistry and ion exchange chromatography in a highly contemporary manner. He is confronted with a problem and presented with a means of attacking it. After completing the experiment he finds he is able to describe the cobalt(II1)-iminodiacetate system utilizing experimental results, chemical principles, and sound reasoning. The weights given for potassium hydroxide correspond to solid reagent grade KOH pellets which contain approximately 15% Hz0. I t is recommended that the hydrogen peroxide be dispensed into the reaction vessel by the instructor or the laboratory assistant as a safety precaution.
