Electronic Journal of Polish Agricultural Universities (EJPAU) founded by all Polish Agriculture Universities presents original papers and review articles relevant to all aspects of agricultural sciences. It is target for persons working both in science and industry,regulatory agencies or teaching in agricultural sector. Covered by IFIS Publishing (Food Science and Technology Abstracts), ELSEVIER Science - Food Science and Technology Program, CAS USA (Chemical Abstracts), CABI Publishing UK and ALPSP (Association of Learned and Professional Society Publisher - full membership). Presented in the Master List of Thomson ISI.
2008
Volume 11
Issue 2
Topic:
Agricultural Engineering
ELECTRONIC
JOURNAL OF
POLISH
AGRICULTURAL
UNIVERSITIES
Ciesielski W. , Tomasik P. 2008. METAL COMPLEXES OF XANTHAN GUM, EJPAU 11(2), #25.
Available Online: http://www.ejpau.media.pl/volume11/issue2/art-25.html

METAL COMPLEXES OF XANTHAN GUM

Wojciech Ciesielski1, Piotr Tomasik2
1 Institute of Chemistry and Environmental Protection, Jan Długosz University in Częstochowa, Poland
2 Department of Chemistry, University of Agriculture in Cracow, Poland

 

ABSTRACT

Interactions of selected paramagnetic transition metal salts [Ni(II), Co(II), Cu(II), Fe(III), Mn(II)] with xanthan gum are described. The conductivity, DSC/TG/DTG measurements, and EPR measurements produced evidence for the formation of Werner-type complexes of metal central atoms with the polysaccharide ligand. The carboxylic groups of xanthan gum were preferably involved in the ligation of the central atoms. Cu(II), Co(II) and Fe(III) ions were most readily coordinated by xanthan gum whereas Mn(II) cations were coordinated most randomly. Sometimes, the copper central atom resided in dimeric form.

Key words: clathrates, hydrocolloids, macro ligands, polysaccharides, thermal analysis, Werner complexes.

INTRODUCTION

After almost century lasting studies of interactions of polysaccharide hydrocolloids with metal ions, little is known about polysaccharide – metal complexes and impact of coordination to metal ions upon properties of those hydrocolloids [1,14]. Such studies involved mostly glycoproteins and/or anionic saccharides in which the amino group nitrogen atom and oxygen atoms of the ionising groups (for instance, in hyaluronic acid [2]) were the coordination sites for metal atoms [3,12,13,17,18,20,23]. In spite of a large number of the hydroxyl groups in polysaccharide molecules, low electron density at the potential oxygen atom donors results in a moderate number of those groups substituting water molecules in the coordination sphere of the central metal atoms [16,22,25].

Recently, a series of papers was published on metal complexes of starches [8,9,11,12], amylose and amylopectins [10], cellulose [21], and three carrageenans [5,6,7]. In our former studies [10] anionic potato amylopectin and, in general ligation of metal atoms with potato starch was described. The anionic phosphate and the hydroxyl groups which participated in ligation provided formation of clathrates with water molecules trapped therein. Similarly, κ-, ι- and λ-carrageenans formed clathrates with selected central metal atoms in corresponding their complexes.

Xanthan gum is a microbial product of unique properties in stabilisation of hydrocolloids and other water-based systems. It is a mixture of two polysaccharides. Structure (I) represents a component constituting approximately 40% of xanthan gum. The dominating component has the mannopyranosyl unit as the terminal group in the side chain [4].

Structure I.
Carboxylic groups in the side chain provide anionic properties of the gum. The macromolecule forms a five-fold helical structure. Low salt concentration and elevated temperature favored a disorder of this structure whereas a higher salt concentration stabilized the ordered conformation.

Xanthan gum is soluble in cold and hot water, stable over a broad range of pH values [15]. Resistance to its desiccation is a key property for its practical applications in food industry and for non-nutritional purposes, i.e. as a biodegradable component of drilling mud [24].

It seemed likely, that the formation of the Werner-type metal complexes of the xanthan gum macro-ligand with central metal atom would additionally increase the water-holding ability of that polysaccharide and additionally stabilised the macroligand and water trapped in it. In order to prove this concept complexes of that gum with Co(II), Cu(II), Fe(III), Mn(II), and Ni(II), metal ions were synthesised and their properties recognised.

METHODS

Materials. Xanthan gum was purchased from Fluka (Switzerland). CuCl2·2H2O, Cu(NO3)2·3H2O, Cu(OCOCH3)2·3H2O, CoCl2·6H2O, Co(NO3)2·6H2O, Co(OCOCH3)2·4H2O, FeCl3·H2O, MnCl2·4H2O, Mn(OCOCH3)2·4H2O, NiCl2·6H2O, Ni(NO3)2·6H2O, and Ni(OCOCH3)2·4H2O, all of analytical grade, were purchased from POCh Gliwice (Poland).

Formation of complexes. A 7 w/w% of aqueous suspension of the polysaccharide was prepared on one-hour heating with agitation at 90°C followed by cooling. So pre-treated gum (0.5 g of dry residue) was blended with 0.1 M aqueous solution of anhydrous salt (10 cm3) and agitated for 24 h at room temperature. Preparations, which freely settled on the bottom of the vessel were collected and dried at 50°C.

EPR spectra. EPR spectra were recorded for powdered samples in the X-band region (ν = 9.5 GHz, λ = 3.2 cm) at room temperature. Diphenyl picrylhydrazide (DPPH) was taken as the g-factor (g = 2.0036). The apparatus was manufactured by Politechnika Wrocławska (Poland). The spectral curves were processed using the 2.8 b MicroCal Origin program.

Thermogravimetry (TG), differential thermogravimetry (DTG) and differential scanning calorimetry (DSC). Thermal DSC-TG-DTG analysis was carried out with the NETZSCH STA-409 simultaneous thermal analyser calibrated with standard indium, tin, zinc, and aluminium of the 99.99% purity. Samples (approximately 0.020 g) were heated in corundum crucibles with non-hermetic lids. Corundum was the standard. The heating was performed in the air at 20-500°C with the 5 K·min-1 temperature rate increase. Measurements were duplicated. They provided the ±0.5°C reading precision. The weight of the solid residue after heating to 500°C was recorded for estimation of the metal ions coordinated by the xanthan gum.

Conductivity measurements. Conductivity measurements were performed at room temperature on aqueous suspension of xanthan gum (0.5 g of dry residue in 10 cm3 of water), salt solutions (0.1 M calculated for anhydrous salt) and aqueous blends of xanthan gum with salts (0.5 g of dry residue in 10 cm3 of 0.1 M aq. salt solution). Results were proven stable within 24 hours. Measurements were run in triplicates. Conductometer (inoLab, Pol-Eko-Aparatura, Poland) working at 1000 Hz provided precision of ±1% of recorded value.

Computer simulations. Using the ZINDO/1 method with Cache 5.0 (evaluation version) the heat of formation of complexes was computed for several potential coordination spheres and coordination sites preferred structures of metal complexes was determined. Computations were performed with Pentium 4 3.2 GHz HT and 1 GB RAM computer.

RESULTS AND DISCUSSION

Undoubtedly, xanthan gum coordinated to metal ions as documented by increase in differences of conductivity (18-49%) caused on admixture of xanthan gum to aqueous salt solutions (Table 1). Conductivity is obviously governed by concentration and viscosity of solutions. The latter obstructs the mobility of ions in solution. The concentration effect in presented studies was controlled by maintaining proportions of solution components constant throughout measurements. However, viscosity of the solutions was dependent not only on concentration of admixed salts but also on efficiency of their interactions with xanthan gum. Viscosity of aqueous solutions of metal salts depends to a great extent on size of ions. Large cations and anions significantly decrease the viscosity [19]. Cations under study in this project had negligibly different effect upon viscosity whereas difference in size of anions appeared essential. The metal ions with the chloride counter ion, the smallest among anions under this study, were the most efficiently coordinated. The effect of the nitrate counter ion upon the formation of complexes was irregular. The most pronounced, 58.1% decrease in the conductivity of the solution was observed for cupric nitrate. Simultaneously, the 37.2% decrease in the conductivity of nickelous nitrate was on the level typical for the majority of salts tested. The decrease in the conductivity in the case of cobaltous nitrate was the lowest among recorded in this study. The acetate counter ion only slightly disturbed efficient coordination of xanthan gum to the metal ions. Thus, results of the conductivity measurements at least qualitatively point to coordination of xanthan gum to metal ions.

Table 1. Conductivity of aqueous suspensions of xanthan guma and its metal complexes, mS·cm-1
 

Salt

Complex b

Co(II)

Chloride

16.58

-40.40

Nitrate(V)

17.69

-18.10

Acetate

20.01

-35.50

Cu(II)

Chloride

24.51

-49.00

Nitrate(V)

23.58

-58.10

Acetate

21.54

-22.80

Fe(III)

Chloride

2.56

-32.60

Mn(II)

Chloride

23.54

-43.40

Acetate

18.95

-41.60

Ni(II)

Chloride

14.79

-49.30

Nitrate(V)

18.25

-37.20

Acetate

19.46

-43.60

aConductivity of xanthan gum is 0.554 mS·cm-1.
bGiven as increase in conductivity resulting from complexation, %.

Inspection of the results of the electron paramagnetic resonance studies (Table 2) confirmed the formation of complexes of metal ions with the xanthan gum ligand. The coordination sphere around the cation always changed because of degeneration of orbitals. In several spectra, the interaction of unpaired spin localised on the metal cation with other atoms generated a splitting of signals.

Table 2. g-Factor in EPR spectra of salts and complexes with xanthan gum
 

Salt

Complex

Co(II)

Chloride

2.2448

2.0219 II; 2.2641

Nitrate(V)

2.2423

1.992; 2.0311

Acetate

2.0011

2.0026; 2.0505; 2.1994

Cu(II)

Chloride

2.1073

2.0016;.2.2106

Nitrate(V)

2.3148 II; 2.0946

2.1310

Acetate

2.0231 II; 2.1652

2.0012; 2.1672; 4.312

Fe(III)

Chloride

2.2109

1.9926; 2.1261

Mn(II)

Chloride

0.9884

2.3126

Acetate

2.0125

2.1726; A = 102G; A1 = 121G

Ni(II)

Chloride

1.9876

1.9926; A = 162G

Nitrate(V)

2.2201

2.1792; 2.0021

Acetate

2.2452

1.8002

In its complexes, the Co(II) central atom had the d7 configuration. In all spectra, the delocalisation of unpaired electron from the Co(II) ion towards the ligand could be seen. The cobalt cation had the high spin density. It was likely that the magnetic induction caused a coupling of the dx2-y2 and dxy orbitals. However, in the spectrum of the complex with Co(OCOCH3)2 the shift of g-factor was low. It might suggest a weak delocalisation of unpaired electron suggesting a square planar arrangement of ligands around the central atom.

In the spectra of Ni(II) complexes with gelatinised xanthan gum splitting of g-factor was observed. It might suggest asymmetry in arrangement of equatorial ligands. A low shift of g-factor in the spectrum of the complex with NiCl2 suggested that the complex might be square planar in contrast to other complexes of that metal showing a remarkable shift of that factor and, therefore, considered as octahedral species.

The spectral parameters pointed to the high-spin character of the Fe(III) and Mn(II) central atoms in complexes with xanthan gum. The unpaired electron of the central atoms was shifted to the ligand. A large shift of g-factor in complexes resulted from the cancellation of degeneration of the dxz and dyz orbitals.

g-Factor for the Cu(II) cation with the nitrate and chloride counter ions informed that the complexes were monomeric and that the unpaired electron was localised in the dx2-y2 orbital. There was an octahedral symmetry around central atom with a small Jahn-Teller distortion. In the case of acetate, value of g-factor (= 4.312) pointed to dimeric central atom in the complex. The acetate anion, probably, was included into the inner coordination sphere and the complex might be square planar. The effects of the metal atoms on the degeneration of orbitals could be arranged in the following orders:

They showed the lack of any regularity in the arrangement of these orders. Different structure of the inner coordination sphere could be charged for it.

The thermogravimetric (TG), differential thermogravimetric, and differential scanning calorimetric studies essentially contributed to the knowledge of the structure of metal complexes of xanthan gum. Table 3 presents shortened thermal characteristic of xanthan gum and its metal complexes ranging from room temperature to the point of decomposition of the complex, respectively.

Table 3. Thermal studies of xanthan gum and its metal complexes

Salt

Temp. range

TGA

DTGa

DSC

Metal contentd

weight lossb

slope

complex

salt

temp.

∆H°

 

°C

%

tg α

°C

°C

°C

J·g-1

%

None

25-82

16

 

she

       
 

82-124

30

0.33

         
 

124-135

70

3.64

127

 

129.5

-189.9

 
 

135-235

71.5

0.01

         
 

235-290

90

0.34

246

 

245.4

-92.4

 
       

266

 

260.2

-121.4

 
           

281.5

-111.1

 

Co(II)

Chloride

25-245

0

   

107, 147, 190

     
 

245-275

40

1.33

259

 

252.8

-132.4

 
           

271.8

+81.3

 
 

275-350

46.5

0.09

   

284.7

-45.7

 
 

350-400

50

0.07

   

387.0

+71.8

64.40

Nitrate(V)

25-79

3

     

46.1

+28.2

 
 

79-98

9

0..55

88

94

91.8

+256.4

 
 

98-113

16

0.47

         
 

113-134

44

1..30

127

114

115.9

+187.6

 
           

128.4

+901.2

 
 

134-151

53

0..53

         
 

151-156

60

0..33

153

 

155.7

+272.1

 
 

156-178

64

0..38

183

       
 

178-191

69

0..38

         
 

191-202

73

0..36

         
 

202-225

80

0..38

213

 

216.2

+309.7

 
 

225-377

85

0..03

 

248, 268

     
 

377-400

87

0.09

       

11.04

Acetate

25-225

0

   

129

     
 

225-350

40

0..31

262, 313

273, 332

53.3

+28.2

 
           

263.1

+3.2

 
           

324.1

+18.2

 
 

350-400

48

0.16

sh

 

463.0

+117.2

79.66

Cu(II)

Chloride

25-63

0

           
 

63-121

4

0.07

98

117

97.5

+82.1

 
 

121-148

22.5

0.69

131

 

137.0

+392.8

 
 

148-175

26.5

0.15

         
 

175-187

32

0.46

182

 

183.2

+141.2

 
 

187-231

47

0.34

209

 

214.5

+167.2

 
 

231-350

52

0.04

         
 

350-400

54

0.035

   

370.2

+211.8

71.23

Nitrate

25–124

13

0.18

         
 

124–127

70

1.90

125

 

125.4

-482.7

 
 

127–400

72

0.01

 

179, 249, 259

175.8

-11.8

 
           

279.4

-292.4

58.33

Acetate

25–71

0

     

56.3

+117.2

 
 

71–121

10

0.20

97

 

100.5

+80.4

 
 

121–140

21

0.58

129

 

130.7

+110.1

 
 

140–170

28

0.23

177

152, 274

163.5

+6.2

 
           

168.9

+5.4

 
 

170–195

45

0.68

         
 

195–358

53

0.05

   

207.8

-13.4

 
 

358–400

56

0.07

       

69.85

Fe(III)

Chloride

25–59

0

     

57.1

+254.1

 
 

59–120

5

0.11

90

115

92.1

+58.1

 
 

120–137

22

0.60

129

 

130.3

+382.7

 
 

137–165

26.5

0.16

   

165.4

+82.2

 
 

165–177

45.5

1.58

   

177.4

+5.8

 
 

177–400

53

0.07

 

200, 348

203.5

+18.2

67.86

Mn(II)

Chloride

25–96

2

0.03

         
 

96–102

17

2.50

100

85

100.7

+147.2

 
 

102–120

29

0.67

 

129

112.1

+158.7

 
 

120–129

44

1.67

121

 

123.1

+169.2

 
 

129–154

51

0.28

 

150

     
 

154–175

56

0.30

162

 

163.7

+58.4

 
 

175–196

57

0.05

         
 

196–204

61

0.30

202

 

202.6

+62.8

 
 

204–282

71

0.14

248, 268

       
 

282–335

77

0.13

         
 

335–400

88

0.17

   

355.2

-1921.8

48.22

Acetate

25–55

0

           
 

55–90

13

0.44

80

88

82.4

+252.1

 
 

90–106

20

0.37

         
 

106–125

29

0.47

117

116

121.3

+192.2

 
 

125–135

33

0.40

         
 

135–150

54

1.40

141

 

137.9

+381.1

 
 

150–223

60

0.08

         
 

223–310

74

0.16

267

       
 

310–346

83

0.25

320

313

327.4

-841.1

 
 

346–400

85

0.04

381

     

23.64

Ni(II)

Chloride

25-98

13

0.30

         
 

98-126

33

0.71

 

102

108

+2721.8

 
 

126-144

47

0.93

132

       
 

144-238

58

0.12

175

       
 

238-271

82

0.72

251

 

253.6

-271.8

 
 

271-400

82

0.0

 

327,360

   

48.71

Nitrate(V)

25-75

4

0.08

         
 

75-125

20

0.32

93

110

95.5

+497.4

 
 

125-136

24

0.36

124

       
 

136-196

36.5

0.21

 

149

     
 

196-220

48

0.48

219

 

204.0

+18.2

 
 

220-221

50

2.00

         
 

221-260

69

0.49

240

 

234.3

-378.4

 
 

260-400

69

0.0

 

387

300.0

-251.0

77.60

Acetate

25-55

0

           
 

55-121

10

0.15

82

141

89.3

+492.1

 
 

121-219

10

0.0

         
 

219-299

55

1.22

281

       
 

299-400

73

0.18

 

311

334

-141.1

44.07

aMain peaks are denoted with heavy print.
bValues report a total weight loss from origin.
cNegative values denote exothermic processes whereas positive values point to endothermic processes.
dPercentage of the total metal ion in the solution.
eA shoulder.

Xanthan gum processed in the manner necessary for a preparation of metal complexes decomposed as follows (Table 3, Fig. 1).The TG-line declined immediately after heating was applied. This step of tg α = 0.33 lasted up to 124°C and only a flat shoulder on the DTG line additionally characterized this step. A fast weight loss (tg α = 3.64) followed from 124°C. It was slightly exothermic. Corresponding DTG peak had its minimum at 127.2°C and the DSC maximum was located at 129°C. Although the temperature region of this process suggested that it might be associated with the loss of water included on precipitation of the gum from its solution, the exothermic character of this step suggested that the water desorption might be accompanied by another process. It was likely that on precipitation, the gum formed cages with trapped water. It prevented the gum from taking a typical five-fold helical structure. After water desorption the gum turned into this favoured conformation.

These two steps of the weight loss were followed by a plateau ranging up to 231.8°C after which further decomposition took place with a moderate rate (tg α = 0.33). It was over at 290°C with a total 91% weight loss. The temperature region of this step was slightly higher than tat of the main decomposition step of the commercial sample of the polysaccharide.

Fig. 1. The TG/DTG/DSC diagram for xanthan gum

A majority of investigated complexes showed a spectacular ability of holding water in its structure and this ability was assigned to the formation of clathrates. It should be noted that this is also a property of xanthan gum itself. However, inspection of Table 3 showed that complexes with Co(NO3)2, NiCl2, and MnCl2 clathrated over 40% of water whereas precipitated xanthan gum held only 30% of water. Complexes with CuCl2, Cu(OCOCH3)2 FeCl3, Mn(OCOCH3)2 and Ni(NO3)2 included less water than precipitated xanthan gum. The pattern of evolution of water from the complexes with Cu(NO3)2 and Ni(OCOCH3)2 (Fig. 2) suggested that these complexes contained only surface sorbed water.

Fig. 2. The TG/DTG/DSC diagram for the xanthan gum – Cu(NO3)2 complex

Complexes with CoCl2 and Co(OCOCH3)2 were anhydrous (Fig. 3) and thermogram for the complex of Co(NO3)2 (Fig. 4) with several peaks related to water loss was typical for complex clathrates.

Fig. 3. The TG/DTG/DSC diagram for the xanthan gum – Co(OCOCH3)2 complex

Fig. 4. The TG/DTG/DSC diagram for the xanthan gum – Co(NO3)2 complex

In terms of the thermal stability expressed as a point of thermal decomposition of anhydrous complexes (TG line), only complexes of cobaltous and nickelous chlorides and acetates were more stable than free ligand. The coordination of the ligand to cupric nitrate and acetate decreased the rate of decomposition of the ligand expressed as the slope of the TG line in the corresponding region (tg α). Coordination to Co(OCOCH3)2 and MnCl2, CuCl2 and Ni(NO3)2 only slightly influenced the decomposition rate whereas in the other cases a strong acceleration of decomposition was observed. The decomposition of complexes with cobaltous and nickelous chlorides, and cobaltous nitrate proceeded exothermically as free ligand did but the other complexes decomposed endothermically showing that products of decomposition did not form any complexes and/or compounds with salts and/or products of their thermal decomposition. These differences could not be explained as the result of the octahedral and square planar structures of the inner coordination sphere of the complex but certainly the coordination of the ligand had an impact to the ability of the formation of clathrates with involvement of the intermolecular hydrogen bonds. In Table 3, there is also the yield of the metal ions bound to xanthan gum. These figures are given as the percent of the metal atom bound in respect to the total number of metal ions in solution. One might see that Cu(II), Co(II) and Fe(III) ions were the most readily coordinated by xanthan gum, although the role of the counter ion could also be seen. Thus, the nitrate(V) ion inhibited the coordination of the Co(II) ion. Generally, there is no correlation of the level of the ion bonding and changes in the electric conductivity due to coordination.

Computer simulations were carried out to recognise favourable sites of coordination and composition of the inner coordination spheres of particular complexes. Thus, heat of formation was calculated for particular complexes assuming that the coordination number of the Co(II), Mn(II), and Fe(III) central metal atoms was 6 and the coordination number of the Cu(II) and Ni(II) central atoms was 4. The number of the water molecules in the inner coordination spheres varied between 0 and 6 and 0 and 4, respectively. Simultaneously, the carboxylic group of the central glucopyranose unit, the carboxylic group in the terminal, side-chain, pyruvated mannopyranose unit, and the 2-hydroxyl group of that unit were considered as the ligating sites.

Table 4. Calculations of the heat of formation for complexes of xanthan gum with metal ions

Heat of formation, ∆H, kJ·mol-1a
Structure of coordination sphere (xanthan gum +H2O)

Ion

0+4

0+6

1+3

1+5

2+2

2+4

3+1

3+3

4+0

4+2

5+1

6+0

Cl-

Co

 

-786

 

23

 

-421* (-438)**

 

321

 

-121

652#

1383

Cu

-695

 

134

 

-234* (-242)**

 

643

 

-198

     

Fe

 

-954

 

57

 

-365* (-399)**

 

612

 

-143

632#

 

Mn

 

-964

 

78

 

-429* (-398)**

 

549

 

-118

298#

939

Ni

-731

 

-23

 

-321* (-342)**

 

362

 

-173

     

NO3-

Co

 

-517

 

12

 

-363* (-401)**

 

213

 

-287

689#

783

Cu

-412

 

11

 

-198* (-214)**

 

754

 

-101

     

Fe

 

-695

 

43

 

-432* (-493)**

 

345

 

-265

563#

743

Mn

 

-556

 

98

 

-421* (-445)**

 

215

 

-139

756#

859

Ni

-459

 

-21

 

-203* (-189)**

 

864

 

-45

     

CH3COO-

Co

 

-541

 

143

 

-487* (-567)**

 

116

 

-154

654#

902

Cu

-542

 

-5

 

-141* (-143)**

 

548

 

-124

     

Fe

 

-695

 

162

 

-379* (-439)**

 

286

 

-111

968#

432

Mn

 

-556

 

98

 

-384* (-399)**

     

-119

396#

745

Ni

-654

 

-18

 

-176* (-201)**

 

287

346

-118

     
The results denoted with “*”, “**”, and # relate to complexes with involvement of the glucopyranosyl carboxylic group, mannopyranosyl carboxylic group, and 2-hydroxyl group in the mannopyranosyl unit, respectively.

One might see that except the cases with MnCl2 and Ni(OAc)2 where the coordination to the glucopyranosyl carboxylic group was energetically favoured, the involvement of the carboxylic group in the terminal mannopyranose unit provided the more stable complexes than the involvement of the carboxylic group in the central glucopyranose unit. Other potential coordination sites under consideration did not provide relatively stable complexes. The complexes with two xanthan gum ligands and two water molecules were the most stable in case of the central metal ions with the coordination number 4, and complexes with two xanthan gum ligands and four water molecules were preferred in case of the central metal atoms with the coordination number 6.

Fig. 5. Optimized structure of the complex of Cu2+ ion
(a blue point) with one mer of xanthan gum

Table 5. Bond lengths and angles for selected, energetically favored optimized structures of metal ions coordinated to xanthan gum

Bond lengths for tetrahedral complexes (1)a

 

O1 – M

O2 – M

O3 – M

O4 – M

Cu + H2O

1.93

1.93

1.93

1.93

Cu + xanthan gum

1.87

2.11

2.13

2.07

Ni + H2O

1.91

1.91

1.91

1.91

Ni + xanthan gum

1.90

2.02

2.06

2.10

Bond angles for tetrahedral complexes (2)a

 

O1 – M – O2

O1 – M – O3

O1 – M – O4

Cu + H2O

109.47

109.47

109.47

Cu + xanthan gum

118.74

123.12

119.84

Ni + H2O

109.41

109.41

109.41

Ni + xanthan gum

120.45

119.27

124.98

 

O2 – M – O3

O2 – M – O4

O3 – M – O4

Cu + H2O

109.47

109.47

109.47

Cu + xanthan gum

78.82

72.15

124.53

Ni + H2O

109.41

109.41

109.41

Ni + xanthan gum

81.73

74.92

119.65

Bond lengths for octahedral complexes (3)a

Metal ion (M) and ligand

O1 – M

O2 – M

O3 – M

O4 – M

O5 – M

O6 – M

Co + H2O

1.89

1.89

1.89

1.89

1.89

1.89

Co + xanthan gum

2.17

2.10

2.04

1.92

2.07

1.91

Mn + H2O

1.90

1.90

1.90

1.90

1.90

1.90

Mn + xanthan gum

2.14

2.08

2.02

2.00

2.09

1.95

Fe + H2O

1.92

1.92

1.92

1.92

1.92

1.92

Fe + xanthan gum

2.09

2.10

2.03

1.98

2.01

1.93

Bond angles for octahedral complexes (4)a

Metal ion (M) and ligand

O1 – M – O2

O1 – M – O3

O1 – M – O4 (H2O)

Co + H2O

89.10

90.00

90.10

Co + xanthan gum

48.65

71.98

87.52

Mn + H2O

90.00

90.00

90.00

Mn + xanthan gum

52.40

73.25

85.34

Fe + H2O

90.00

90.00

90.00

Fe + xanthan gum

47.92

69.37

89.73

Metal ion (M) and ligand

O1 – M – O5

O1 – M – O6 (H2O)

O2 – M – O3

Co + H2O

90.00

180.00

90.00

Co + xanthan gum

92.37

172.83

51.27

Mn + H2O

90.00

180.00

90.00

Mn + xanthan gum

95.13

178.21

49.23

Fe + H2O

90.00

180.00

90.00

Fe + xanthan gum

90.92

174.98

51.35

Metal ion (M) and ligand

O2 – M – O4 (H2O)

O2 – M – O5

O2 – M – O6

Co + H2O

180.00

90.00

90.10

Co + xanthan gum

119.75

97.55

149.10

Mn + H2O

180.00

90.00

90.00

Mn + xanthan gum

111.72

97.23

132.43

Fe + H2O

180.00

90.00

90.00

Fe + xanthan gum

124.76

100.23

140.21

Metal ion (M) and ligand

O3 – M – O4 (H2O)

O3 – M – O5

O3 – M – O6 (H2O)

Co + H2O

90.00

180.00

90.00

Co + xanthan gum

114.42

146.69

111.19

Mn + H2O

90.00

180.00

90.00

Mn + xanthan gum

121.43

141.83

108.71

Fe + H2O

90.00

180.00

90.00

Fe + xanthan gum

109.28

149.32

112.92

Metal ion (M) and ligand

O4 (H2O) – M – O5

O4 (H2O) – M – O6 (H2O)

O5 – M – O6 (H2O)

Co + H2O

90.00

90.00

90.00

Co + xanthan gum

89.27

91.23

83.21

Mn + H2O

90.00

90.00

90.0

Mn + xanthan gum

91.83

90.47

79.23

Fe + H2O

90.00

90.00

90.0

Fe + xanthan gum

87.41

91.92

84.11

Fig. 5 presents optimized structure of the coordination of one xanthan gum mer to the Cu2+ central metal ion. Optimized structures of complexes of other metal ions closely resemble that presented in Fig. 5. There were only slight differences in some bond lengths and angles as shown in Table 5.

CONCLUSIONS

  1. Xanthan gum coordinated to Co(II), Cu(II), Fe(II), Mn(II), and Ni(II) ions to form the Werner-type complexes.

  2. Ligation of the central atom involved preferably COO- groups of xanthan gum.

  3. The complexes were mainly octahedral but square planar complexes were also formed.

  4. The coordination contributed to the formation of clathrates in which water molecules were trapped.


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Accepted for print: 17.04.2008


Wojciech Ciesielski
Institute of Chemistry and Environmental Protection,
Jan Długosz University in Częstochowa, Poland
Armii Krajowej 13/15, 42-201 Częstochowa, Poland
Phone: (+48) 34 361 49 18 ext. 153
email: wc@ajd.czest.pl

Piotr Tomasik
Department of Chemistry,
University of Agriculture in Cracow, Poland
Balicka Street 121, 30-149 Cracow, Poland
phone/fax: (+48 12) 662 43 35
email: rrtomasi@cyf-kr.edu.pl

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