Infrared spectrum of DMSO ComplexesName: Nicole LapeyrousePID: 3373674

13Fall

Abstract

Dimethylsulfoxide (DMSO) is an ambidentate ligand, which means that it has two

different sites in which an atom can bind to. The two sites that an atom can bind to on

DMSO is either the sulfur or oxygen atom. Using infrared spectroscopy the site in which

Palladium, Ruthenium, and Copper bind to on DMSO can be determined by the

absorption of the S=O bond, which absorbs at 1050 cm-1. If the absorption frequency

increases than the atom bonds to the sulfur atom and if the absorption decreases the atom

binds to the oxygen atom. According to literature values CuCl2 ⋅2 DMSO binds to the

oxygen atom and absorbs around 960 and 910 cm-1, PdCl2 ⋅ 2 DMSO binds to the sulfur

atom and absorbs around 1157-1116 cm-1, and RuCl3 ⋅ 4 DMSO binds to both the sulfur

and oxygen atom and absorbs at about 1088 and 910 cm-1.i The results that were obtain

for this experiment shows that CuCl2 ⋅2 DMSO absorbed at both 980 and 898, PdCl2 ⋅ 2

DMSO absorbs at 1113.59, and RuCl3 ⋅ 4 DMSO absorbs at 1084 and 930.

Introduction

Infrared spectroscopy is a common spectroscopic

technique and can be used to identify organic

compounds, functional groups, the determination of

molecular structures, and a variety of other general

uses. The infrared region spans the spectrum in

wavenumbers from 12,800 to 10 cm-1 and is divided

into three regions near-IR, mid-IR, and far-IR. The

absorption of infrared radiation depends on the

molecular species and this causes a transition from

one vibrational or rotational energy state to another.ii

An ambidentate ligand has two different sites for

which an atom can bind to. Some examples of

ambidentate ligands are CN-, NO2-, and S2O3

2-.iii The

site of binding for CN-is with either the lone pair on

the C- atom or the lone pair on N- group (fig. 1). For

NO2- atom the sites for binding could be the lone pair

1)

2)

3)

Figure 1: The following ambidentate ligand structures are as followed 1) CN-, 2) NO2

-, and 3) S2O32-.ii

of electrons on N- or the lone pair of electrons on the O- atom (fig. 1). The S2O32-

structure

displays two sites for which an atom could bind to either the lone pair on the S - or the

lone pair on the O- atom.

In this experiment the infrared spectroscopy is being used to determine the site in which

metals bind to DMSO. In DMSO the S=O bond absorbs at 1050 cm-1 in the infrared

region, which this absorption can be used to identify which atom the metals binds to. If

the metal binds to the sulfur atom then the absorption frequency will increase because the

metal atom is donating electrons to the sulfur atom. However, if the metal binds to the

oxygen atom the metal will form a bond with the lone pair from the oxygen and the

absorption frequency will decrease.iii

Procedures

This experiment required the preparation of three separate compounds, which were

copper(II) chloride, palladium(II) chloride, and ruthenium(III) chloride trihydrate.

The preparation of CuCl2 ⋅2 DMSO was prepared into a 10 mL

Erlenmeyer flask by dissolving 150 mg of CuCl2 in 1 mL of

ethanol. Then into the Erlenmeyer flask 250μL of DMSO was

slowly added, while the mixture was being stirred with a

magnetic stir bar. Once the DMSO was added an immediate

color change occurred resulting in a light green precipitate (Fig.

2). The precipitate was collect by using vacuum filtration and

rinsing the product twice with 500 μL of ethanol. Once the

crystals were collect the product was dried in a desiccator.

The preparation PdCl2 ⋅ 2 DMSO was prepared in a 10 mL

Erlenmeyer flask. Into the flask 1.25 mL of DMSO was

added with a magnetic stir bar. Then 135 mg of PdCl2 was

slowly add into the flask and the solution was left to sit until

it turns a dark brown (Fig.3), which takes about 2.5-3 hours.

The product was left standing until an orange crystal was

produced. Once the crystals were produced the use of

vacuum filtration was used to collect the crystals it was

rinsed twice with 500 μL of ether. The crystals were left to

dry.

Figure 2: CuCl2

⋅2 DMSO precipitate

Figure 3: PdCl2 ⋅ 2 DMSO precipitate

The preparation of RuCl2 ⋅ 4 DMSO required a simple

reflux. Into a 10 mL round bottom flask 100mg of RuCl3

⋅ xH2O was weighed. The flask to the water condenser

was attached and placed into a sand bath. The apparatus

that was used is displayed in figure 4. Heat was applied

for about 5 minutes until the solution changes color to

orange. The solution was transferred into a 10 mL beaker

and sat until crystals formed. Once the compound had

formed crystals 2 mL of acetone was added dropwise

until the formation of two layers formed. Then it was

place in an ice bath for 10 minutes. After cooling for the

allotted time the yellow crystals that had formed were

collected by the use of vacuum filtration and the crystals

were washed once with 500 μL of acetone and then

followed with 500 μL of ether.

Once the compounds had dried the melting point or the decomposition point were

determined. The samples were then analyzed by using an Infrared spectroscopy.

Results

Table 1: Results

CuCl2 RuCl2 PdCl2

Mass (g) 0.3314 0.2554 0.2425

Molecular weight (g/mol) 170.48 207.43 177.31

Theoretical Yield 0.4457 0.5639 0.3847

Actual Yield (%) 74.3625 45.2934 63.0442

Melting point (oC) 212-214 158-162 214-216

Figure 4: The apparatus used for the preparation of RuCl2 ⋅ 4 DMSO.

Figure 7: IR spectrum of CuCl2 ⋅2 DMSO

Figure 5: IR spectrum of DMSO

Figure 6: IR spectrum of RuCl2 ⋅ 4 DMSO

Figure 8: The preparation of PdCl2 ⋅ 2 DMSO

Discussion

DMSO is a dipolar aprotic solvent, which means that it does not contain O-H or N-H

bonds and have a large dipole moment. The advantage to having a dipolar aprotic solvent

is that it cannot hydrogen bonds with it self. Also the dissolution process of DMSO is an

excellent solvent for polar compounds such as: acids, alkalis, metals, and cations because

of the dipole moment. In comparison to water, which is a dipolar protic solvent, that can

solvate polar solvents and anions readily because of the hydrogen bonding. This dipolar

aprotic characteristic of DMSO is an important characteristic because it allows it to

participate in having metals bind to it at different sites. DMSO is an ambidentate ligand

and metals can bind to either the sulfur or to the oxygen atoms.iv

The metals are able to bind to DMSO because it can act as both a hard and soft base,

which this allows soft and hard acids to bind to it. Soft acids are defined generally as a

Lewis acid, an electron pair acceptor, and their size is moderately large. The

characteristics of hard acids are the opposite, which have a low polarization and have

smaller sizes. If the metal binds to the sulfur atom then the metal is a soft acid and the

absorption frequency will increase because the metal atom is donating electrons to the

sulfur atom. However, if the metal binds to the oxygen atom the metal is a hard acid and

the metal will form a bond with a lone pair from the oxygen.v The fundamental

theoretical reasons behind the hard-soft acid base (HSAB) rules are due to the orbitals.

The more closed shells that are prevalent around the nucleus the more shield it is. For the

soft acids they have more shielding versus the hard acids where they have less shielding.

In this experiment RuCl3, CuCl2, and PdCl2 were analyzed to determine the site of binding

to DMSO by the use of the IR spectra. In the IR spectra it shows that CuCl2 ⋅2 DMSO

absorbed at both 980 and 898, PdCl2 ⋅ 2 DMSO absorbs at 1113.59, and RuCl3 ⋅ 4 DMSO

absorbs at 1084 and 930. According to literature values CuCl2 ⋅2 DMSO binds to the

oxygen atom and absorbs around 960 and 910 cm-1, PdCl2 ⋅ 2 DMSO binds to the sulfur

atom and absorbs around 1157-1116 cm-1, and RuCl3 ⋅ 4 DMSO binds to both the sulfur

and oxygen atom and absorbs at about 1088 and 910 cm -1.vi The values obtain by the IR

corresponds to the literature values.

Based on where the atoms bind to DMSO (either the sulfur or the oxygen atom)

determines whether they are a hard or soft acid. The PdCl2 atom binds to the sulfur atom

indicating that it is a soft acid. As for CuCl2 it binds to the oxygen atom indicating that it

is a hard acid. However, the RuCl3 binds to both the oxygen and sulfur atom making it a

borderline acid or base. Using the data that was collected and the general idea of the

HSAB theory it is possible to predict the binding sites of Pt3+, Hg2+, Fe2+, and Zn2+ to

DMSO. The predict sites of binding would be that Pt3+, Hg2+, and Zn2+ would be soft acids

and bind to the sulfur atom while Fe2+ would bind to the oxygen indicating it as a hard

acid.

Reference

i Nakamoto, Kazuo. Infrared and Raman Spectra of Inorganic and Coordination Compounds. New York: Wiley, 1986. Print.

ii Skoog, Douglas A., F. James. Holler, and Stanley R. Crouch. Instrumental Analysis. iii Prakash, Satya. Advanced Inorganic Chemistry. New Delhi: S. Chand &, 2005. Print.iv “Metal complexes of DMSO and FT-IR spectroscopy – Linkage isomerism”v Pike, Ronald M., and Mono M. Singh. "Metal Complexes of Dimethyl Sulfoxide." Microscale

Inorganic Chemistry: A Comprehensive Laboratory Experience. By Zvi Szafran. N.p.: n.p., n.d. 218-22. Print.

vi Nakamoto, Kazuo. Infrared and Raman Spectra of Inorganic and Coordination Compounds. New York: Wiley, 1986. Print.