Synthesis of the Lewis Adduct of Co^II with Imidazole; Stoichiometry of Reaction

• To improve your skill in observation
• To synthesize a chemical substance and purify it by recrystallization
• To discover the stoichiometry of a chemical reaction

Chemical synthesis--the art and practice of the design and creation of molecules--is the essence of chemistry. In no other field of science does the practitioner have the ability to create the objects of his/her interest. It is this single fact that sets chemistry apart from physics, geology, and biology, in which the scientist is restricted to examining what s/he finds, rather than what s/he creates. Chemistry has an extra dimension that combines vision and art in the process of creating new substances with potentially new and exciting uses.

This experiment provides you with an opportunity to experience chemical synthesis first hand, to determine the composition of the molecule that you create, and to spectroscopically characterize the molecule (i.e., to discover what types and energies of light the molecule interacts with). The reaction that you will carry out is shown (sans stoichiometry) in equation (1).

The reaction will be carried out in ethanol, a commonly-used laboratory solvent. C₃H₄N₂ is the formula for imidazole (click to see the structure). This reaction is yet another example of a class of reaction that permeates chemistry: the donation of an electron pair by a Lewis base to a Lewis acid to form an adduct in which acid and base are joined by a covalent bond. As we have seen and will continue to see, the donation/acceptance of electron pairs is involved in a huge number of chemical reactions ranging from simple laboratory processes to the complex reactions in biological systems.

The adduct of Co²⁺ with imidazole is called a transition metal complex to indicate that it is a complex molecule involving an electron pair donor and a transition metal ion (Co²⁺). Imidazole is an important biological molecule. It occurs in the side chain of the amino acid, histidine, and very frequently donates an electron pair to a transition metal such as Cu²⁺, Fe²⁺ in enzymes and proteins.

1. In what molar ratio are the two reactants, cobalt nitrate hexahydrate and imidazole, initially mixed?
2. Choose an appropriate wavelength and use a spreadsheet to make a plot of the absorbance at this wavelength versus the mole ratio of imidazole to cobalt. What is the stoichiometry of the adduct?
3. Propose a 3-dimensional structure for the adduct, keeping in mind that the x imidazole molecules tend to arrange themselves symmetrically around the cobalt(II) ion.
4. In narrative form, discuss the infrared spectrum of the adduct. Attempt to assign observed bands to particular structural features of the adduct.
5. (Optional) What do you think happens to the cobalt-imidazole adduct in hot water? Is the material that comes out the same as the material that when in? How could you prove your answer?

• 5.75" Pasteur pipets, 5 per group
• 1 Pasteur pipet bulb
• Kimwipes
• 8 small (1 x 6 cm) test tubes
• 2-mL graduated pipet
• syringe pipet pump
• aspirator with trap
• 3 25-mL Erlenmeyer flasks
• Rubber stoppers for Erlenmeyer flasks
• small plastic wash bottle containing distilled water
• Buchner funnel
• suction flask
• filter paper
• Co(NO₃)₂.6H₂O
• Imidazole, C₃H₄N₂
• UV-visible spectrometer
• spectrometer cuvette

Safety goggles must be worn at all times in the laboratory. You will be working with solutions of cobalt nitrate and imidazole. Avoid ingestion and contact of these solutions with the skin. In the event of skin contact, immediately flush the affected area with copious amounts of water.

Record your observations in your notebook.

Synthesis. Obtain the required equipment. If necessary, wash the Erlenmeyer flasks and test tubes using brushes and Alconox detergent, then thoroughly rinse with water to remove all traces of Alconox.

Weigh 0.5 millimole (mmole) of Co(NO₃)₂.6H₂O and 204 mg of imidazole, and transfer the solids to one of the Erlenmeyer flasks. Add 7.5 mL of ethanol measured using a graduated cylinder, and swirl the mixture intermittently until all of the cobalt starting material has dissolved. Record any observations. Set the flask aside for 30 minutes. (At this point, you may want to proceed with the stoichiometric part of the experiment, and return to this point later.) Isolate the product by suction filtration, suction dry, and transfer to a small weighed vial. Determine the mass of product obtained.

Purification. Slurry a portion (not more than half) of the product with 3 mL of warm (not hot) ethanol. Observe what happens. Suction filter to remove insoluble material, and place the filtrate in an ice bath for 30-40 minutes. Suction filter to isolate the solid, wash it with 2 mL ethanol, suction dry, and vial.

Stoichiometry. Stoichiometry will be determined using Job's Method. Dry the insides of the test tubes and mark them 1-8 using a Sharpie marker. Place the tubes in order in a test tube rack. Weigh precisely 1 mmole of cobalt nitrate hexahydrate and dissolve in exactly 20 mL of ethanol in a clean and dry 25-mL Erlenmeyer flask. Weigh precisely 1 mmole of imidazole and dissolve in exactly 20 mL of ethanol in a second Erlenmeyer flask. Using the syringe pump and graduated pipet, transfer respectively 0.3, 0.4, 0.5, 0.7, 0.9, 1.2, 1.75, and 1.75 mL of cobalt solution to tubes 1-8. Then transfer respectively 3.2, 3.1, 3.0, 2.8, 2.6, 2.3, 1.75, and 0 mL of imidazole solution into tubes 1-8. Pipet 1.75 mL of ethanol into tube 8. Obtain the electronic absorption spectra of the solutions in the 8 tubes and determine the stoichiometry of the Lewis adduct from your data. (Note that the 8th solution provides you with the spectrum of ethanolic Co^II for comparison with the spectrum of the imidazole complex.)

When finished, pour all solutions into the heavy metal waste jar.

Optional Synthesis. Take a small amount of the product (perhaps 1/4 of the original yield) and slurry it with 2-3 mL of water. What do you observe? Heat the mixture in a hot water bath, making continuous careful observations. Scrape the solid from the sides and bottom of the flask with a spatula to loosen it, then collect it by suction filtration. Suction dry and transfer to a labelled vial.

Characterization. If desired, you may characterize your product using infrared spectroscopy. See your instructor for assistance.

Clean-up. When you have finished all of your work,

• Clean all glassware with Alconox and water, and shake dry.
• Rinse Pasteur pipets and graduated pipets with distilled water by inserting the nozzle of your wash bottle in the top end and squirting water through. Aspirate dry.
• Return all components to the labkit.
• Return all borrowed equipment to the instructor before leaving lab.

Discard all solutions containing cobalt in the heavy metal waste jar. Dispose of broken glassware in the plastic lined cardboard container provided for this purpose.

1. The appropriate sections of your textbook.

1. A Job's Method study was carried out for the reaction
   aA + bB ---> dD + fF

   Stock solutions of A and B containing 1 mmole per 10 mL of solution were prepared. These were used to prepare a series of 8 solutions containing varying amounts of A and B, but such that the total moles of A and B was maintained constant. The absorbance of each solution was then measured at a chosen wavelength (the same for all solutions), and the following data were obtained:

   Solution	volume A solution	volume B solution	Volume solvent	A at chosen wavelength
   1	0.3	3.2		0.12
   2	0.4	3.1
   3	0.5	3.0
   4	0.7	2.8
   5	0.9	2.6
   6	1.2	2.3
   7	1.75	1.75		0.12
   8	1.75	0	1.75	0.12

   What is the stoichiometry of the reaction?