Introduction

     The nature of dimethyl sulfoxide (DMSO, (CH₃)₂SO) as a monodentate ligand is explored in this set of experiments.^1,2  DMSO can ligate metal atoms by bonding in one of two possible ways: through the oxygen atom or through the sulfur atom (Figure 1).  The method of bonding in the complex can be determined with the use of infrared spectroscopy, and the nature of the ligand and its complexes can be explored using molecular modeling software from CAChe Scientific (see appendix of this manual).

Figure 1. Resonance structures of DMSO.

     The infrared spectrum of dimethyl sulfoxide shows an absorption peak at 1050 cm^-1, which arises from the sulfur-oxygen bond.  If a Lewis acid bonds to the sulfur atom, the contribution of resonance structure 1 decreases, and the SO stretching frequency increases. Conversely, if a Lewis acid bonds to the oxygen atom, the contribution of resonance structure 2 decreases, and the SO stretching frequency decreases.³

Experimental

The preparation of Cu(DMSO)₂Cl₂  (approximate time required: 30 minutes)

     300 mg of copper(II) chloride is placed into a 25 mL Erlenmeyer flask equipped with a magnetic stirring bar.  A calibrated Pasteur pipet or syringe is used to add 2.0 mL of absolute ethanol to the flask.  The mixture is stirred until all of the copper(II) chloride has dissolved.  0.5 mL of DMSO  is then added slowly with an automatic deliver pipet or syringe.  An immediate exothermic reaction will occur, yielding a light green precipitate.  The mixture should be stirred for five minutes.

     The product is collected by suction filtration using a Hirsch funnel.  The crystals should be washed with cold ethanol (two 1.0 mL portions) and air-dried on a piece of filter paper placed on a watch glass.  The crystals can then be weighed for a percent yield calculation.  Determine the melting point and obtain an infrared spectrum of the product in chloroform solution.  If the product is insoluble in chloroform, prepare a Nujol mull.  Also obtain an infrared spectrum of DMSO.  For your report, determine the absorption frequency of the S-O band for both the complex and DMSO solvent.

The preparation of Ru(DMSO)₄Cl₂ (approximate time required: 2.5 hours)

     0.30 g RuCl₃·3H₂O is placed in a 10 mL round-bottomed flask equipped with a magnetic stirring bar.  A water condenser is attached to the flask and the apparatus is placed on a heating mantle set on a magnetic stirring hot plate.  Bubble nitrogen gas through 1.0 mL of DMSO placed in a graduated cylinder.  Add the degassed DMSO to the round-bottomed flask, and heat to reflux with continuous stirring.  Allow the mixture to boil for five minutes only.  Do not overheat.  The mixture will turn an brown-orange color when the reaction is complete.  The solution is cooled and transferred, using a Pasteur filter pipet, to a 25 mL Erlenmeyer flask.  Yellow crystals may separate out at this point; if this is the case, the crystals can be collected as stated below.  Otherwise, the volume of the solution is reduced to approximately 0.5 mL by passing a gentle stream of nitrogen gas over the warmed liquid.

     To isolate the product, 20.0 mL of dry, reagent grade acetone is slowly added to form two phases.  The mixture is cooled in an ice bath.  Yellow crystals should form upon standing for 10-15 minutes.  Collect the crystals by suction filtration using a Hirsch funnel.  The crystals are then washed with 500 mL of acetone , followed by 500 mL diethyl ether.  The crystals are then weighed to calculate a percent yield.  Determine the melting point and obtain an infrared spectrum to locate the absorption frequency of the S-O band.

Discussion

     Compare the three infrared spectra of DMSO, Cu(DMSO)₂Cl₂ and Ru(DMSO)₄Cl₂ that you obtained, to determine the type(s) of bonding that occurs for the DMSO ligands to the corresponding metal atom.  You should observe one band for the copper-DMSO complex and two bands for the ruthenium-DMSO complex (Figure 2).  In your report, you should compare the experimental data to theoretical CAChe data (see below), as well as literature data available in the library.³

Figure 2. Structure of Ru(DMSO)₄Cl₂ (without H atoms), minimized using Mechanics in CAChe.  Note that three DMSO molecules are S-bonded (coordinate covalent bonds indicated by arrows), while one is O-bonded.⁴

CAChe Component

     The CAChe molecular modeling system allows one to investigate the DMSO ligand and the transition metal complexes that it can form.  The software allows the construction and optimization of structures and the generation of theoretical infrared spectra.  The general approach is as follows:

• Build the DMSO ligand in the Editor.  Either resonance structure can be used (See Figure 1).
  
• Optimize the geometry of the molecule using Mechanics or, preferably, MOPAC.
  
• Using resonance structure 1, and the Editor, attach the simple Lewis acid AlCl₃ via a coordinate covalent bond to the S atom of DMSO.  Minimize the geometry and perform a molecular orbital calculation to determine the effect of donating a pair of electrons from the S atom of the molecule.   What happens to the bond angles in the DMSO ligand?  What changes in partial charges and bond orders are observed?  How should the bond order changes effect the infrared spectrum?
       Now attach the Lewis acid to the O atom of DMSO.  Again, speculate on the effect of the DMSO changes on these properties.
  
•  Use one of the molecular orbital calculation tools ("Tabulator", Extended Hückel or MOPAC) to generate the molecular orbitals for DMSO.  Color the electron density surface according to the frontier density option-electrophilic attack (which will generate a file labeled .FonD).  Of sulfur or oxygen, which atom is predicted to be more open to electrophilic attack?
  
• If MOPAC is available, generate the vibrational spectrum of DMSO.  Use the Visualizer application to view the molecule and its spectrum.  Compare the calculated and experimental spectra.  (The frequency at which the S-O absorption occurs should not be the same as that obtained in the experimental spectrum; this discrepancy is expected when creating spectra with the MOPAC application.⁵  One can still compare, however, changes in calculated frequencies that occur when DMSO is bonded to a Lewis acid.)
  
• Again, if MOPAC is available, calculate the vibrational spectra of the S-bonded and O-bonded Lewis acid-DMSO complexes you just built.  Does the S-O stretching frequency, or the C-S stretching band energy, change as expected?

References

1. This experiment was adapted from Microscale Inorganic Chemistry, by Z. Szafran, R. M. Pike, and M. M. Singh, John Wiley: New York, 1991 and from a laboratory experiment developed by John C. Kotz of SUNY at Oneonta.

2. Reynolds, W. R. "Dimethyl Sulfoxide in Inorganic Chemistry", Progress in Inorganic Chemistry, S. J. Lippard, Ed., Interscience: New York, 1970, volume 12, 1.

3. Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed., John Wiley: New York, 1986, pp. 269-272.  On hold for Chem445 in Science Library.  (Science Library, Call Number: QD96 .I5N33 1986)

4. Mercer, A.; Trotter, J. J. Chem. Soc. Dalton Trans. 1975, 2480.

5. See the section entitled, "Limitations in vibrational spectra generated by MOPAC," in the CAChe MOPAC Guide.

Back to CHEM445 Main Page