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UMass Amherst CHEM 242 - Thione-Thiol Tautomerism

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Chem242. Int. Inorg. Chem. Spring, 2007 UMass-Amherst 1/4 Experiment 6 Thione-Thiol Tautomerism in Co(II) complexes * This lab has been adapted from “Microscale Inorganic Chemistry: A Comprehensive Laboratory Experience,” 1991, John Wiley and Sons, New York, NY, pg 207. Introduction This experiment demonstrates how spectroscopic and magnetic data for many transition metal complexes can be used to determine the coordination geometry of a set of ligands about the transition metal ion. You will prepare cobalt(II) complexes using the monodentate organic ligand, 2-mercapto-1-methylimidazole (MIMT) and characterize them with UV/VIS and IR spectroscopy. Two complexes will be formed. Complex A has the formula [Co(MIMT)4(NO3)2], while complex B has the formula Co(MIMT)2(NO3)2. The structure of the MIMT ligand is depicted below. MIMT is one example of a ligand that can bond to a transition metal in more than one manner. Such ligands are said to be ambidentate. This behavior arises from the fact that MIMT can exist in two tautomeric forms, namely the -thione and -thiol forms. The behavior of MIMT is very similar to that of the keto-enol tautomerism which you learned about in organic chemistry. In the thione form, MIMT coordinates to a metal through the sulfur atoms, while in the thiol form it coordinates through the imidazole ring nitrogen atom. In the two complexes prepared here, MIMT adopts the thione form. The coordination sphere in A is CoS4 while that in B is CoS2O2, with the oxygens coming from the two coordinated nitrate ions. MIMT by itself exists in the thione form. Since the C=S bond of the parent MIMT ligand has an IR stretching frequency at 745 cm-1, the spectrum for the complexes can be monitored in this region to see what happens to this bond on coordination of the sulfur to the cobalt atom. This frequency should increase if the electron density in the bond is being enhanced, and will decrease if the electron density in the bond is reduced. The UV/VIS spectra of A and B should also differ, reflecting the difference in the d-orbital splitting capability of the four sulfurs in A versus the two sulfurs and two oxygens in B.Chem242. Int. Inorg. Chem. Spring, 2007 UMass-Amherst 2/4 Procedure: CAUTION: The MIMT ligand is a toxic compound. Exercise care when weighing out your samples. Wear gloves to prevent it coming into contact with your hands. Preparation of Complex A – [Co(MIMT)4(NO3)2] In a small Erlenmeyer flask, prepare the solvent to be used for these reactions by mixing 8 ml of absolute (100%) ethanol with 0.45 ml of triethylorthoformate. We will refer to this solvent as TF-ETOH. In a 25ml round-bottomed flask equipped with a magnetic stirring bar, dissolve 228 mg (2.0 mmol) of MIMT in 2.5 ml of your TF-ETOH solvent. You may need to warm this solution to get the MIMT ligand to fully dissolve. Use a Pasteur pipette to add a separately prepared solution of 73 mg hydrated cobalt nitrate, Co(NO3)2 x 6H2O, in 2 ml TF-ETOH. Fill a drying tube with CaCl2 at the appropriate lab station, working over the plastic tub to contain CaCl2 spills. Attach a water condenser capped with your CaCl2 drying tube to the round-bottomed flask, and clamp the apparatus in a sand bath set atop a magnetic stirring hot plate. Heat the resulting mixture at reflux with stirring for about 30 min. Then use a Pasteur pipette to transfer the hot liquid to a small Erlenmeyer flask (10ml) containing a boiling stone. Reduce the volume by about 10% by heating on a sand bath IN THE HOOD, and then allow the solution to cool to room temperature. Further cooling in an ice bath should induce crystallization. Collect the resulting emerald green crystals by filtration using a Hirsch funnel and wash them with a 1ml portion of cold absolute ethanol. If this does not work, further reduce the sample volume, and try again. Using dried product, estimate your yield and measure the melting point of the product (the literature value is between 150 and 200°C). If you complete this reaction but fail to produce a solid sample of A, it is still possible to obtain an UV/VIS spectrum of the solution. Estimate your yield and measure the melting point of the product. Preparation of Complex B – (Co(MIMT)2(NO3)2) In a small Erlenmeyer flask, prepare the solvent to be used for these reactions by mixing 8 ml of ethyl acetate with 0.45 ml of triethylorthoformate. We will refer to this solvent as TF-EA. In a 25ml round-bottomed flask equipped with a magnetic stirring bar, dissolve 114 mg (1.0 mmol) of MIMT in 1.5 ml of your TF-EA solvent. You may need to warm this solution to get the MIMT ligand to fully dissolve. Use a Pasteur pipette to add a separately prepared solution of 146 mg hydrated cobalt nitrate, Co(NO3)2 x 6H2O, in 5 ml TF-EA. Fill a drying tube with CaCl2 at the appropriate lab station, working over the plastic tub to contain CaCl2 spills. Attach a water condenser capped with your CaCl2 drying tube to the round-bottomed flask, and clamp the apparatus in a sand bath set atop a magnetic stirring hot plate. Heat the resulting mixture at reflux with stirring for about 30 min. Then use a Pasteur pipette to transfer the hot liquid to a small Erlenmeyer flask (10ml) containing a boiling stone. Reduce the volume by about 10% by heating on a sand bath IN THE HOOD, and then allow the solution to cool to room temperature. Further cooling in an ice bath should induce crystallization. Collect the resulting dark blue crystals by filtration using a Hirsch funnel and wash them with a 1ml portion of cold absolute ethanol. Dry the product, calculate your yield, and measure the melting point of the product (should be between 150 and 200°C).Chem242. Int. Inorg. Chem. Spring, 2007 UMass-Amherst 3/4 Characterization of Your Product: Collect UV-VIS spectra of freshly prepared solutions of A and B, as these complexes degrade within a few minutes. Either dilute the respective mother liquor into ethanol (just before you filter off the crystals), or else prepare acetonitrile (CH3CN) solutions. These spectra should differ considerably. Compare the positions of the strongest absorption maxima in the visible region and comment on the relative values for the two complexes and the reasons for these differences. Obtain the IR spectrum of the solid product by preparing a KBr pellet, following the method suggested by your TA’s. Compare this with the spectrum for the free MIMT ligand and comment


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UMass Amherst CHEM 242 - Thione-Thiol Tautomerism

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