Volumetric properties of zinc (II) chloride in N,N-dimethylacetamide and dimethylsulfoxide

Journal of Molecular Liquids 177 (2013) 252–256

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Volumetric properties of zinc (II) chloride in N,N-dimethylacetamide and dimethylsulfoxide Dorota Warmińska ⁎, Hanna Koziel, Wacław Grzybkowski Department of Physical Chemistry, Chemical Faculty, Gdańsk University of Technology, Narutowicza 11/12, 80-233 Gdańsk, Poland

a r t i c l e

i n f o

Article history: Received 27 June 2012 Received in revised form 1 October 2012 Accepted 2 November 2012 Available online 19 November 2012

a b s t r a c t Molar volumes of zinc (II) chlorocomplexes in N,N-dimethylacetamide (DMA) and dimethylsulfoxide (DMSO) at 298.15 K have been evaluated from the density data obtained for mixtures of Zn (ClO4)2 and ZnCl2 of the same molarities. The results indicate that all zinc (II) chlorocomplexes in N,N-dimethylacetamide are four-coordinated while in dimethylsulfoxide a change in geometry during complexation of the second chloride anion is observed. © 2012 Elsevier B.V. All rights reserved.

Keywords: Chloro complexation Apparent molar volume Density Transition metal perchlorates Transition metal chlorides N,N-dimethylacetamide (DMA) Dimethylsulfoxide

1. Introduction Molar volumes of electrolytes are fundamental thermodynamic quantities which are the object of increasing practical and theoretical interest. However, current knowledge concerning electrolytes in organic solvents is limited to relatively simple systems. Most attention has been given to solutions of 1:1 salts, while polyvalent unsymmetrical electrolytes of the 1:2 and 1:3 charge types in organic systems constitute only a small fraction of all investigated systems, and there are very few papers describing the volumetric properties of systems in which complexation of a metal ion occurs [1,2]. Kawaizumi determined partial molar volumes of bipiridine and phenantroline complexes of copper (II) and zinc (II) in water [3]. Molar volumes of zinc (II) thiocyanato complexes in N,N-dimethylformamide have been reported by Umebayashi and coworkers, and the respective volume changes of complexation were evaluated [4]. In previous papers from our laboratory, the apparent molar volumes of divalent transition metal perchlorates and halides in dimethylsulfoxide (DMSO), N,N-dimethylformamide (DMF), N, N-dimethylacetamide (DMA), triethylposphate (TEP), acetonitrile and methanol were reported [5–10]. Limiting values of the partial molar volumes of perchlorates were obtained, split into respective ionic contributions and discussed in terms of structural factors related to the nature of the metal ions. Effects observed in solutions of

⁎ Corresponding author. Tel.: +48 583471410; fax: +48 583412694. E-mail address: [email protected] (D. Warmińska). 0167-7322/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molliq.2012.11.003

chlorides and bromides were explained in terms of complex formation [7,8,11]. In this paper, we report results on zinc (II) chlorocomplexes in dimethylsulfoxide and N,N-dimethylacetamide. Distinct and significant volume changes upon complexation are expected for this particular metal ion. They are related to the desolvation of the metal ion resulting from an essential change of structure and geometry of the coordination sphere of zinc (II) ion. The respective coordination number of the metal ion is reduced. It changes from six for the octahedrally solvated zinc (II) to four for the pseudotetrahedral and tetrahedral chlorocomplexes of this cation. It has been shown previously that the predominant chlorocomplexes of zinc (II) existing in DMSO and DMA solutions of zinc (II) chloride are tetrahedral tetrachlorocomplexes ZnCl42−, pseudotetrahedral trichlorospecies, i.e. the ZnCl3DMSO− and ZnCl3DMA− anions, and electrically neutral pseudotetrahedral ZnCl2(DMSO)2 and ZnCl2(DMA)2 dichlorocomplexes. It seems logical that the solutions should contain corresponding cations in amounts implied by the material and charge balance. The cationic complexes of zinc (II) present in DMSO solutions are the six-coordinated Zn(DMSO)62+ and ZnCl(DMSO)5+. The formation and presence of the four-coordinated ZnCl(DMSO)3+ species cannot be a priori excluded. Analogous chlorocomplexes of zinc (II) are assumed for the solution of zinc (II) in DMA. However, it should be noted that formation of the tetrahedral species seems to be more distinct for Zn (II)–DMA system than for the respective Zn (II)–DMSO solutions. This is due to the steric hindrance destabilizing the six-coordinated complexes and favoring the four-coordinated species. The effect should be very distinct in the case of the studied systems because of the well known inclination of zinc (II) towards tetrahedral geometry.

D. Warmińska et al. / Journal of Molecular Liquids 177 (2013) 252–256

The solutions of zinc (II) chloride, irrespectively the nature of the solvent, have to be considered as a mixture of the electrolytes. In one limiting case, ZnCl2 solution may be treated as the solution of the complex electrolyte consisting of the ZnL62+ type cation (L denotes the solvent molecule) and the ZnCl42− anion. The opposite limiting case are the ionic forms corresponding to complete lack of complexation. In this case, zinc (II) chloride exists in solution as the simplest electrolyte consisting of the ZnL62+ cation and chloride anions. Thus, real solutions may be considered as mixtures of the following series of consecutive complexes of zinc (II): ZnL62+, ZnClL5+, ZnClL3+, ZnCl2L2, ZnCl3L − and ZnCl42−. The series can be rearranged and presented as the mixture of the electrolytes, two of which were described above. This was demonstrated previously for the solutions of cobalt (II) salts in acetonitrile and N,N-dimethylformamide on the basis of spectrophotometric and conductometric data [12–14]. It has been shown by Umebayashi [4] that for a constant ionic medium, apparent molar volumes display additivity, and average apparent molar volumes of mixture of the electrolytes can be described by simple equations of the form  ¼ ∑x V V Φ i Φ;i

ð1Þ

i

where VΦ,i are the apparent molar volumes of the species being formed in the solution, independent of composition of the medium, and xi are molar fractions of the species calculated on the solvent-free basis. Thus, it seems logical that the apparent molar volume of the mixture of electrolytes is an average value and can be calculated using the following formula  ¼ 1000ðd0 −dÞ þ V Φ ∑ ci d0 i

∑ xi Mi i

d0

ð2Þ

where d and d0 denote the density of the solution and solvent, respectively, ci and xi are the concentration and molar fraction of the i-th species, and Mi is its molecular mass. Molar volumes of zinc (II) thiocyanato complexes have been evaluated by Umebayashi et al. using the data obtained by densitometric titration in N,N-dimethylformamide [4]. Their measurements were carried out in the solution containing an inert supporting electrolyte, i.e. tetraethylammonium perchlorate. The constant ionic medium was used to keep the apparent molar volumes, as well as the activity coefficients, constant. Finally, densitometric measurements were performed for solutions containing zinc (II) thiocyanato complexes, i.e. the solution of Zn(ClO4)2 was titrated with a solution of (C2H5)4NSCN in the presence of the supporting electrolyte. The obtained results were interpreted in terms of the existence of the following electrolytes [Zn(SCN)]ClO4, (C2H5)4N[Zn(SCN)3] and [(C2H5)4N]2[Zn(SCN)4]. The obtained respective molar volumes were used to determine the final results, i.e. volume changes in N, N-dimethylformamide. The respective data obtained for Zn(ClO4)2, (C2H5)4NClO4 and (C2H5)4NSCN were also employed in the calculations. In the present work, we deal with a system which seems to be somewhat simpler. In order to avoid the problems related to the presence of ‘foreign’ electrolytes, i.e. (C2H5)4NClO4 and/or (C2H5)4NSCN, in the system under investigation, we decided to study the densities of the mixtures of Zn(ClO4)2 and ZnCl2. In these mixtures, the Cl −/Zn2+ ratio ranges from zero for Zn(ClO4)2 to 2 for ZnCl2. It has been shown that mono-, di, tri- and tetrachlorocomplexes of zinc (II) are formed in N, N-dimethylacetamide as well as in dimethylsulfoxide. However, the formation of tetrachlorocomplexes of zinc (II) was observed at Cl−/Zn2+ ratio values much higher than 2. The equilibrium concentrations of chlorocomplexes of zinc (II) were evaluated using the values of formation constant reported by Suzuki et al. [15,16].

253

2. Experimental The DMA-solvated metal perchlorates and chlorides were obtained from the corresponding hydrates (all Aldrich, >0.9999) by dissolving them in N,N-dimethylacetamide. The next step was the removal of any excess solvent, as well as the products of dehydration, under reduced pressure at elevated temperature. On cooling, crystalline solids were obtained. These were recrystallized at least twice from anhydrous solvent. The same procedure was used in order to obtain DMSO-solvated perchlorates and chlorides of the transition metals studied. The stock solutions of metal salts were obtained by dissolution of the solids in anhydrous solvent. The stock solutions were analyzed for the respective metals by standard EDTA titration. At least twelve determinations were performed in each case and the relative standard deviations were smaller than ± 0.1%. Solutions for measurements were prepared by weighed dilution of the corresponding stock solutions. All operations involving the handling of anhydrous materials were performed in dry boxes. N,N-dimethylacetamide (DMA) and dimethylsulfoxide (DMSO) Fluka, puriss, ≥0.995, mass fraction of H2O b 1 ⋅ 10−4, were dried with a 0.4 nm molecular sieve. The densities of the solutions were measured using an Anton Paar DMA 5000 densimeter with a thermostate system based on a Peltier unit with a repeatability of 5.0·10 −6 g cm −3 and an uncertainty better than 3.5·10 −5 g cm −3. The temperature was kept constant at 298.15 K with an accuracy of 0.01 K. Before each measurement series, the accuracy of the density measurements and the purity of the solvent were verified by measuring its density at 298.15 K. Density values of (0.936228 ± 0.000009) g cm −3 for N,N-dimethylacetamide and (1.095264 ± 0.000004) g cm −3 for dimethylsulfoxide were found in the present study while literature values vary from 0.936076 g cm −3 to 0.93659 g cm −3for DMA and 1.095271 g cm −3 to 1.0972 g cm −3 for DMSO [17–20]. 3. Results and discussion The density data obtained for the mixtures of Zn(ClO4)2 and ZnCl2 in N,N-dimethylacetamide and dimethylsulfoxide at 298.15 K are presented in Table 1 of the supplementary material. Two series of data were obtained for N,N-dimethylacetamide. As is seen, the total metal concentration was practically constant for each of these series. It varied from 0.04756 to 0.04759 mol dm − 3 for the first series and from 0.2018 to 0.1967 mol dm − 3 for the second one. Thus, the series can be considered as mixtures of the two salts of zinc (II) of the same molarities and the total metal concentrations correspond to ionic strength amounting to values of 0.14 and 0.60 mol dm − 3, respectively. Only one series of the mixtures of Zn(ClO4)2 and ZnCl2 of the same molarities was investigated for dimethylsulfoxide solutions. The total metal concentration varied from 0.1361 to 0.1395 mol kg−1. The variation corresponds to an average ionic strength amounting to the value of 0.41 mol dm −3. The values of average apparent molar volumes of the mixed solute were calculated using the equation:  ¼ V Φ

xZnðClO4 Þ2 MZnðClO4 Þ2 þ xZnCl2 MZnCl2 1000ðd0 −dÞ  þ d0 d0 cZnðClO4 Þ2 þ cZnCl2

ð3Þ

where d and d0 denote the density of the solution and solvent, respectively, and cZnðClO4 Þ2 and cZnCl2 are the molar concentrations of the respective salts of zinc (II). The symbols xZnðClO4 Þ2 and xZnCl2 are molar fractions related to the composition of the solutes. Fig. 1 presents the resulting values of the average apparent molar volumes vs. the composition of the mixtures for all the series studied, i.e. the respective values are plotted against the value of the Cl -/Zn 2 +

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D. Warmińska et al. / Journal of Molecular Liquids 177 (2013) 252–256

1.0

60

0.8

mole fraction

65

50

0.6

0.4

1 45

0.2

40

1.0

35

0.8

3 2 0.5

mole fraction

VΦ / cm3mol-1

55

DMA 0

30 0.0

0.5

1.0

1.5

2.0

cCl− / cZn2+ Fig. 1. Average apparent molar volumes of zinc (II) perchlorate–zinc (II) chloride mixtures vs. the ratio of concentration of chloride and zinc (II) ions in DMA at I = 0.14 M (■) and I= 0.60 M (□) and DMSO at I= 0.41 M (◆) at 298.15 K.

1.0

1.5

2.0

DMSO 0 1

0.6

0.4

0.2

3 2

0.0 0.0

ratio changing from 0 for Zn(ClO4)2 to the highest value of 2 for ZnCl2. Inspection of Fig. 1 shows that the substitution of ZnCl2 for Zn(ClO4)2 in the mixtures results in marked changes in the values of average apparent molar volumes. The effect is especially distinct for the dimethylsulfoxide solutions. As is seen, substitution of ZnCl2 for Zn(ClO4)2 brings about a marked decrease of the average apparent molar volumes, indicating an essential change in the solvation of the zinc (II) cation. The changes observed for the N,N-dimethylacetamide are less significant. However, the effect is also distinct and the observed variation seems to be easy to explain in terms of complex formation. The next step of our study involved calculations of the equilibrium concentrations of the species present in the studied systems. The following species were taken into account: Zn2+, ZnCl+, ZnCl2,ZnCl3−, Cl− and ClO4−. The presence of the tetrachlorocomplex of zinc (II) in the investigated systems was ruled out for N,N-dimethylacetamide as well as for dimethylsulfoxide solutions. The calculations were performed using the GENST computer program and utilizing literature data for the formation constants of the consecutive chlorocomplexes of zinc (II) [15,16]. Fig. 2 presents the species distribution curves obtained for chlorocomplexes of zinc (II) in DMA and DMSO at 298.15 K. The knowledge of the equilibrium concentrations of the individual species enables us to calculate the values of relative densities of the studied solutions which are defined using the equation Δd exp ¼ 1000ðd−d0 Þ

ð4Þ

The relationship between the values of relative density and the concentrations of each species can be calculated using the simple equation Δdcalc ¼ ∑ Mi ci −∑ VΦi ci d0 i

i

ð5Þ

0.5

1.0

1.5

2.0

cCl− / cZn2+ Fig. 2. Species distribution for chlorocomplexes of zinc (II) in DMA and DMSO at 298.15 K. The number represents n within [ZnCln](2−n)+.

and using a multiparameter regression technique in order to determine the values of apparent molar volumes of the chlorocomplexes of zinc (II). Literature values of the limiting apparent molar volumes 0 of Cl− and ClO4− anions were used: VΦ (Cl−) = 11.4 cm3 ⋅ mol−1 for − 3 −1 0 0 DMSO and VΦ(Cl ) = 3.0cm ⋅ mol for DMA solution; VΦ (ClO4−) = 0 43.52 cm3 ⋅ mol−1 for DMSO and VΦ (ClO4−) = 37.5 cm3 ⋅ mol−1 for DMA solution [5,21–23]. The apparent molar volume of perchlorate anion was assumed to be independent of the ionic strength of the solution. Due to its size (r≅ 250 pm) and low charge density, the ClO4− anion is weakly solvated and the influence of ionic strength of the medium on its electrostriction volume may be omitted. Moreover, the effect seems to be much smaller than the changes due to chlorocomplex formation and desolvation. The calculations were performed using values of the apparent molar volumes of the solvated cations determined in our previous studies [5,8]. The value of − 27.22 cm3 ⋅ mol−1 was used for VΦ(Zn2+) in DMSO solutions of ionic strength of 0.41 mol ⋅ dm−3 while the values of − 36.07 cm3 ⋅ mol−1 and − 31.75 cm3 ⋅ mol−1 were used for VΦ(Zn2 +) in DMA solutions for the values of ionic strength of 0.14 and 0.60 mol ⋅ dm −3, respectively. The final results of the calculations, i.e. the values of the apparent molar volumes for ZnCl+, ZnCl2 and ZnCl3− complexes, are shown in Table 1. The molecules of the solvent are taken into account in the respective chemical formulas. Thus, the data obtained for the DMSO solutions correspond to the series þ

2þ

−

ZnðDMSOÞ6 →ZnClðDMSOÞ5 →ZnCl2 ðDMSOÞ2 →ZnCl3 ðDMSOÞ

where Mi and VΦi are molecular masses and apparent molar volumes of the species present in the solutions. The final calculations involved minimizing the values of the error-square sum

of the chlorocomplexes of zinc (II), while the data obtained for the DMA solutions are related to the following sequence of the chlorocomplexes:

 2 Y ¼ ∑ Δdexp −Δdcalc

It should be noted however, that the tetrahedral Zn(DMA)42 + solvated cations are the predominant (ca. 70%) species present in DMA

ð6Þ

2þ

þ

−

ZnðDMAÞ4 →ZnClðDMAÞ3 →ZnCl2 ðDMAÞ2 →ZnCl3 ðDMAÞ

D. Warmińska et al. / Journal of Molecular Liquids 177 (2013) 252–256 Table 1 The molar volumes of chlorocomplexes of zinc (II) in DMA and DMSO, VΦ, and their standard deviation, S, at 298.15 K. Complex

Complex

Table 2 The volume changes for the consecutive complex formation in the Zn–Cl system in DMA and DMSO at 298.15 K. Reaction

DMA I = 0.14 M

ZnCl(DMA)3+ ZnCl2(DMA)2 ZnCl3(DMA)−

255

I = 0.60 M

VΦ/ cm3 ⋅ mol−1

S/ cm3 ⋅ mol−1

VΦ / cm3 ⋅ mol−1

S/ cm3 ⋅ mol−1

8.4 32.1 50.5

0.1 0.1 0.4

7.0 32.9 45

0.1 0.2 1.3

DMSO

DMA ΔV(ZnCl+) ΔV(ZnCl2) ΔV(ZnCl3−)

ΔV/cm3 ⋅ mol−1 46 21.4 12

DMSO ΔV(ZnCl+) ΔV(ZnCl2) ΔV(ZnCl3−)

9.1 40.4 −1.5

I = 0.41 M VΦ/ cm3 ⋅ mol−1 ZnCl(DMSO)5+ ZnCl2(DMSO)2 ZnCl3(DMSO)−

S/ cm3 ⋅ mol−1

−6.5 45.3 55.2

0.04 0.01 0.03

is observed for chlorocomplex formation in dimethylsulfoxide solution. As is seen, the essential difference between the series is related to the volume changes corresponding to formation of the ZnCl2 formal complex, i.e. to the transformation þ

−

ZnCl þ Cl →ZnCl2 solutions of Zn(ClO4)2. The octahedral Zn(DMA)62 + hexa-solvates are also present and the properties of the system are related to the configurational equilibrium: 2þ

2þ

ZnðDMAÞ6 ↔ZnðDMAÞ4 þ 2DMA Examination of the data listed in Table 1 shows that the values of molar volumes of the chlorocomplexes of zinc (II) in N, N-dimethylacetamide are very close, irrespective of the high difference in ionic strength. This suggests and/or confirms the validity of the assumptions and simplifications involved in the calculations. It is obvious that the derived values of the molar volumes are not the thermodynamic quantities. However, they can be considered as a good approximation of the limiting molar volumes of the ZnCl +, ZnCl2 and ZnCl3− complexes:

and the structural change around the zinc (II) ion upon complexation of the second chlorides. The sequence observed for the consecutive complex formation in N, N-dimethylacetamide solutions seems to be related to liberation of the solvent molecules upon complexation 2þ

−

þ

ZnðDMAÞ4 þ Cl →ZnClðDMAÞ3 þ DMA þ

−

ZnClðDMAÞ3 þ Cl →ZnCl2 ðDMAÞ2 þ DMA −

−

ZnCl2 ðDMAÞ2 þ Cl →ZnCl3 ðDMAÞ þ DMA The observed effect is enhanced by the structural change around the zinc (II) related to the reaction 2þ

−

þ

ZnðDMAÞ6 þ Cl →ZnClðDMAÞ3 þ 3DMA

0

VΦ ðcomplexÞ≈VΦ ðcomplexÞ The derived values of the molar volumes were used to calculate the volume changes for consecutive complex formation defined as       þ 0 þ 0 2þ 0 − ΔV ZnCl ¼ VΦ ZnCl −VΦ Zn −VΦ ðCl Þ

where the liberation of the solvent molecules is accompanied by a change of the coordination number of the central metal ion. The zinc (II) ion is strongly solvated, with six solvent molecules in dimethylsulfoxide subjected to electrostrictive strain. The volume change observed for the complexation of the first chloride anion is

for the reaction Zn 2 + + Cl − → ZnCl +,   0 0 þ 0 − ΔVðZnCl2 Þ ¼ VΦ ðZnCl2 Þ−VΦ ZnCl −VΦ ðCl Þ

¼

0 − 0 0 − VΦ ðZnCl3 Þ−VΦ ðZnCl2 Þ−VΦ ðCl Þ

for the reaction ZnCl2 + Cl − → ZnCl3−. The resulting volume changes corresponding to the formation of consecutive chlorocomplexes of zinc (II) in N,N-dimethylacetamide and dimethylsulfoxide are summarized in Table 2 and presented in Fig. 3. Inspection of the data shows that the volume changes obtained for chlorocomplex formation in N,N-dimethylacetamide solution form the series:   þ ΔV ZnC1

〉

ΔV ðZnC12 Þ 〉

− ΔV ðZnC13 Þ

ΔV / cm3.mol-1

− ΔVðZnCl3 Þ

50 40

DMA

30 20 10

ΔV / cm3.mol-1

for the reaction ZnCl + + Cl − → ZnCl2,

60

50

1

  þ ΔV ZnC1

〈

3

40

DMSO

30 20 10 0 -10 1

while the sequence:

2

2

3

n −

ΔV ðZnC12 Þ 〉 ΔV ðZnC13 Þ

Fig. 3. Volume changes of stepwise formation of chlorocomplexes of zinc (II) ion in DMA (■) and DMSO (●).

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D. Warmińska et al. / Journal of Molecular Liquids 177 (2013) 252–256

due to the liberation of a DMSO molecule and the change in the eletrostriction effect which is related to the reduced charge −

2þ

þ

ZnðDMSOÞ6 þ Cl →ZnClðDMSOÞ5 þ DMSO A major modification of the structure is observed upon complexation of the second chloride anion. The change in geometry is accompanied by the liberation of three solvent molecules þ

−

ZnClðDMSOÞ5 þ Cl →ZnCl2 ðDMSOÞ2 þ 3DMSO and the volume of the liberated molecules is significantly larger than the volume of the molecules bound to the central metal ion. The volume change of the last step of complexation observed in the studied systems, i.e. for the reaction −

Moreover, results obtained for solutions with different ionic strengths in DMA confirm the validity of the assumptions and simplifications involved in the calculations. Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.molliq.2012.11.003. Acknowledgments The authors wish to express their sincere thanks to Dr. R. Pastewski for his help during the calculations and for the stimulating discussions. References [1] [2] [3] [4]

−

ZnCl2 ðDMSOÞ2 þ Cl →ZnCl3 ðDMSOÞ þ DMSO

[5]

is related to the liberation of one DMSO molecule.

[6] [7]

4. Conclusion

[8]

The results obtained in the present study show that the nature of chloro complexation of zinc (II) ions in N,N-dimethylacetamide is quite different from that in dimethylsulfoxide. This is due to the solvation steric effect. The derived values of molar volumes and volume changes of complexation indicate beyond doubt that all zinc (II) chlorocomplexes in N,N-dimethylacetamide are four-coordinated while in dimethylsulfoxide a change in geometry during complexation is observed. The relatively high value of the volume change accompanying the formation of the second chlorocomplex reflects geometry change from octahedral to tetrahedral structure around the central metal ion in DMSO. The main result of our study is the finding of the sequence:   þ ΔV ZnC1

〉

−

ΔV ðZnC12 Þ 〉 ΔV ðZnC13 Þ

[9] [10] [11] [12] [13] [14] [15] [16] [17] [18]

for N,N-dimethylacetamide and:   þ ΔV ZnC1

〈

[19] −

ΔV ðZnC12 Þ 〉 ΔV ðZnC13 Þ

for dimethylsulfoxide. Analogous sequences were observed for the variation of the formation of constant of metal complexes with halides [15,16].

[20] [21] [22] [23]

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