Experiment 9: Synthesis and Isolation of Optical Isomers of a Cobalt (III) Compound CH3500: Inorganic Chemistry, Plymouth State University

Experiment 9: Synthesis and Isolation of Optical Isomers of a Cobalt (III) Compound CH3500: Inorganic Chemistry, Plymouth State University Adapted from GS Girolami, TB Rauchfuss, RJ Angelici, "Experiment 14: Optical Resolution of Co(en) 3 3+," Synthesis and Technique in Inorganic Chemistry: A Laboratory Manual, 3 rd ed, University Science Books: Sausalito, CA (1999). Introduction: Isomers are molecules that have the same chemical formula but different structure. Several different types of isomers are possible, depending on how the atoms are arranged and what makes two related isomers different (Figure 1). Figure 1: Types and characteristics of Isomers Enantiomers, or "optical isomers", are mirror images of one another. Enantiomerism is possible only in molecules that do not contain improper rotational symmetry (S n ). Note that, because S 1 is equivalent to a mirror plane, and S 2 is equivalent to inversion, enantiomers necessarily have neither a mirror plane nor an inversion center. In inorganic chemistry, a common type of type of coordination compound with enantiomers is a metal center octahedrally coordinated by three bidentate ligands. Such compounds fall into the D 3d point group, which you can verify from the Character Tables does not contain a mirror plane, inversion center, or any other improper rotations. The optical isomers of such compounds are given the designations Λ and Δ for the structural left and right handed isomers, respectively. A mixture of two enantiomers (known as a racemic mixture) can be very difficult to separate, because enantiomers typically have the same chemical and physical properties, including reactivities, solubilities, melting points, etc. Indeed, there are generally only two differences between two optical isomers: they will rotate polarized light in opposite directions, and they will interact / react differently with other optical isomers (an analogy is the way two people's right hands fit differently than one right and one left). Although it is not much, these two properties are enough to allow separation and analysis of enantiomers. One possible way of separating one enantiomer from another is by crystallizing it with another chiral molecule or ion. Because the two enantiomers will interact differently with the second chiral species, their solubilities may be sufficiently different to selectively precipitate only one of the enantiomers. Tartaric acid is a chiral natural product (in fact, it is a product of wine making and is known to chefs as "cream of tartar). The (+)enantiomer of tartaric acid is the natural isomer and is designated in the Cahn- Ingold-Prelog system as the R,R-isomer. The deprotonated form, (+)tartarate (C 4 H 4 O 6 2-, Figure 1) can be produced by reacting (+)tartaric acid with a strong base and makes a convenient and inexpensive chiral anion for resolving cationic enantiomers. Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 1

A B Figure 1: (R,R)-(+)-tartarate. A) 3-D drawing. B) Fischer projection. The electromagnetic waves of light emitted from most light sources oscillate in all planes perpendicular to the transverse axis. That is, if you could see the oscillations of a beam of photons aimed straight at you, you would see the waves randomly oriented in all 360. Light can be "polarized" so that all the electromagnetic waves oscillate in only one plane by passing it through a polarizing filter. Polarized light that is then passed through an enantiomer will be seen to rotate. One of the enantiomers in the pair will rotate the light to the right (dextrorotatory, designated 'd' or '+') and the other to the left (levorotary, designated 'l' or '-'). Note that there is no correlation between the structural designations (Λ and Δ) and the rotational designations ('d' or 'l'), and in fact, the degree and direction of optical rotation is dependent upon the wavelength of the light used in the analysis. Nonetheless, the measurement of the rotation of polarized light is a useful analysis for identifying enantiomers and determining their purity. The angle that a specific enantiomer will rotate light of a given wavelength (λ) is its "specific rotation" ([α] λ ). A "polarimeter" measures the "rotation" (α meas ) of a sample, which is related to the specific rotation by the concentration (c, in g/ml), path length (l, in dm or 0.1m), and optical purity (x, in decimal fraction, where '1' means only one of the enantiomers is present, and 0.5 means a racemic mixture with equal portions of both (+) and (-) enantiomers) of the sample (Equation 1): Eqn 1: [ ] = meas x l c In this lab, you will prepare a racemic mixture of tris(ethylenediamine) cobalt (III), separate the enantiomers by selective crystallization with (+)tartarate, and determine the specific rotation of both enantiomers with a polarimeter. Safety Considerations: Do NOT dispense volatile organic liquids outside the hood. Perform titrations that will involve organic chemicals under the hood. Adding solid sodium hydroxide to aqueous solutions produces a lot of heat. Take precautions. Properly dispose of organic solvents (e.g. ether) in the Organic Waste container. Properly dispose of cobalt-containing solutions in the Inorganic Waste container Procedure This lab will take place over the course of two days. The initial synthesis will be done during a normally scheduled lecture section, and the follow-up syntheses and analyses will be done during the normally scheduled lab section the next day. Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 2

Prelab 1. Draw the Δ and Λ isomers of the coordination complex [Co(en) 3] 3+ 2. Calculate the molecular masses of the intermediates and final products: [Co(en) 3]Cl 3 ½NaCl 3H 2O, [Co(en) 3][tart]Cl 5H 2O, and [Co(en) 3]I 3 H 2O. A. Synthesis of tris(ethylenediamine) cobalt(iii) chloride, [Co(en) 3 ]Cl 3 ½NaCl 3H 2 O. 1. Accurately weigh 1.2 g of CoCl 2 6H 2 O and dissolve it in 5 ml of water in a 50 ml beaker while stirring with a magnetic stir bar. 2. After the cobalt salt is fully dissolved, add 2.7g of ethylenediamine dihydrochloride (H 2 NCH 2 CH 2 NH 2 2HCl). The solution will turn cloudy pink. 3. With a watchglass ready, add 1.6 g NaOH. Quickly put the watchglass on and stir until all the NaOH dissolves (about 5 min). The solution will turn bluish. 4. Add 5 ml of 3% H 2 O 2 and boil for at least 5 min. The solution will turn dark orange during this process and crystals may begin to form. If a lot of crystals form rapidly, add a few squirts of water and continue heating the solution. The goal is to have nearly all the compound dissolved in the smallest volume of liquid at a temperature near boiling. 5. Remove the beaker from the heat, remove the stir bar, allow the solution to cool for 5-10 min, and then place the beaker in ice for about 20 min. Mix some NaCl into the ice to lower its temperature and get a higher yield of crystals. 6. Set up a vacuum filtration apparatus with a solvent trap. 7. Filter the crystals in a 15 ml glass-fritted funnel. 8. Rinse the crystals twice with ~3 ml of ethanol that has been chilled on ice, then with ~3 ml of diethyl ether that has been chilled on ice. Allow the vacuum pump to run for a few minutes to dry the crystals. Dispose of the filtrate. 9. Label and weigh a clean 20 ml beaker. Transfer the crystals to the beaker and re-weigh. B. Precipitation of the [(+)Co(en) 3 ][(+)tart]cl 5H 2 O salt The following procedure is based on using 1.0 g of tris(ethylenediamine) cobalt(iii) chloride from part A. Use ALL of your product and scale the quantities of other reagents and solvents accordingly. Based on 1.0 gram of product, the amounts to use are 1.0 g [Co(en) 3 ]Cl 3 ½NaCl 3H 2 O. 0.23 g NaOH 0.43 g (+)tartaric acid 1. To the 20 ml beaker containing your product from part A, add a stir bar and just enough water to dissolve the solid (~ 3mL). 2. Add (+)tartaric acid and stir for 1 min. 3. With a watchglass in hand, add NaOH. Cover quickly with the watchglass, stir, and gently heat until all the solids are dissolved. Do NOT boil! If all the solids do not dissolve, add small volumes of water and continue heating until they do. 4. Remove the beaker from the heat, remove the stir bar, and allow the solution to cool for a few minutes until you can comfortably hold the beaker. A few crystals may form during this time, but if a large amount crashes out, add a little more water and heat to redissolve. 5. Cover the beaker with parafilm and place it in a safe, dark place to sit until the next lab period. Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 3

SECOND LAB PERIOD C. Isolation [(+)Co(en) 3 ]I 3 H 2 O 1. Label a clean beaker "(-)". Decant the supernantant from the overnight crystallization into this beaker. Cover with Parafilm and set aside (or give to a lab partner to start working on part D). Because you selectively precipitated the (+) enantiomer, the supernantant should be enriched in the (-) enantiomer. You will precipitate and purify this one in part D. 2. Set up a vacuum filtration apparatus with a solvent trap. Use a 25 ml vacuum flask to receive the filtrate and a small plastic fritted funnel to catch the crystals. 3. Transfer the crystals onto the filter funnel and rinse with a small amount (<3 ml) of cold 1:1 water:acetone, then with a small amount (<3 ml) of cold acetone. Allow the vacuum pump to run for a few minutes to dry the crystals. Dispose of the filtrate. Weigh the crystals in a clean 10 ml beaker. Break up the crystals with a stirring rod or spatula. 4. This step uses 0.45 g of NaI based on an assumed product mass of 0.25 g. Scale the quantity of NaI based on the actual mass of your product in Step 3. Dissolve your product in the minimal amount of water (~2 ml). Use a small stir bar if available, otherwise a stirring rod or just swirling will do. Add 5 drops of 10M NaOH then the NaI. Heat gently for no more than 5 min until all the products are dissolved. Do NOT come even close to boiling or you will racemize your enantiomer! Allow the solution to cool for a few minutes, then chill in an ice bath for about 10 min. 5. Filter the crystals onto a fritted funnel. Rinse twice with less than 1 ml of chilled NaI solution that will be provided. Rinse with a small amount of chilled ethanol and then a small amount of chilled acetone. Allow the vacuum pump to run for a few minutes to dry the crystals. Dispose of the filtrate. Transfer the crystals to a clean, labeled, pre-weighed vial. Record the final weight. This is your [(+)Co(en) 3 ]I 3 H 2 O. D. Isolation [(-)Co(en) 3 ]I 3 H 2 O 1. This step uses 1.2 g of NaI based on an assumed mass of 1.0g [Co(en) 3 ]Cl 3 ½NaCl 3H 2 O from part A. Scale the quantity of NaI based on the actual mass of your product in part A. To the supernantant from Step C1, add 5 drops of 10M NaOH and then the NaI. If any solids form, heat gently for no more than 5 min and add a minimal amount of water to just barely dissolve everything. Do NOT come even close to boiling or you will racemize your enantiomer! Allow the solution to cool for a few minutes, then chill in an ice bath for about 10 min. 2. Filter the crystals onto a fritted funnel. Rinse twice with less than 1 ml of chilled NaI solution that will be provided. Allow the vacuum pump to run for a few minutes to dry the crystals. Dispose of the filtrate. Transfer the crystals to a 10 ml beaker. This product is almost certainly impure, containing both the (+) and (-) enantiomers. 3. Isolate the (-) enantiomer in the following way: Warm a small beaker of water to ~50 C. Add about 4 ml of this warm water to the beaker with the impure crystals. Swirl for a minute, then quickly filter out the undissolved solid into a clean 25 ml vacuum flask. The solid contains the impurity, the filtrate should contain pure (-) enantiomer. 4. Transfer the filtrate to a 10 ml beaker. Add a couple drops of 10M NaOH and then 0.5g NaI. Gently heat and add water as needed to just barely dissolve any solids. 5. Allow the solution to cool for a few minutes, then chill in an ice bath for about 10 min. Filter the crystals onto a fritted funnel. Rinse twice with less than 1 ml of chilled NaI solution that will be Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 4

provided. Rinse with <1mL of chilled ethanol and then <1 ml of chilled acetone. Allow the vacuum pump to run for a few minutes to dry the crystals. Dispose of the filtrate. Transfer the crystals to a clean, labeled, pre-weighed vial. Record the final weight. This is your [(-)Co(en) 3 ]I 3 H 2 O. E. Analysis of both enantiomers Use the following procedures to determine the specific rotation for both the (+) and (-) enantiomer. 1. Dissolve all of your product in the minimal amount of water. Quantitatively transfer the solution using a disposable glass pipette into a 10 ml volumetric flask and fill to the line with water. 2. Take your solution to the polarimeter and measure the rotation using the 1dm path length cell. Analysis (Lab Notebook) The following must be completed in your lab notebook before you can turn in your Notebook Report: 1. Calculate the % yield of the two [Co(en) 3 ]I 3 H 2 O enantiomer products. 2. Calculate the specific rotation of the two [Co(en) 3 ]I 3 H 2 O enantiomer products. Assume the optical purity of each is 100%. Lab Report including Conclusions and Discussion Your lab report is due by lecture on Wednesday. Your report must be handed in BOTH electronically and in hard copy form. See the document "InorgChem-LabReportGuide.pdf" on the course website for guidelines on writing your report. In addition to the Analysis performed in your notebook, include the following in your report (place them in the most relevant sections, not as a series of questions and answers): 1. With consideration of the two specific rotations you calculated for the two enantiomers, comment on their optical purities. 2. Given that it was the specific rotations of the [Co(en) 3 ] 3+ enantiomers we wanted to determine, and you had isolated the (+) enantiomer after part B, why did you have to precipitate the iodide salt of the (+) enantiomer before you could measure its optical rotation? Copyright Plymouth State University and Jeremiah Duncan. May be distributed freely for education purposes only. 5