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Overall stability constants and thermodynamic parameters of various metal 8-quinolinethiolato complexes in nonaqueous solvents
OVERALL STABILITY CONSTANTS AND THERMODYNAMIC PARAMETERS OF VARIOUS METAL &QUINOLINETHIOLATO COMPLEXES IN NONAQUEOUS SOLVENTS YOSHITAKAMASUDA and EIICHI SEKIDO
YASUO NAKABAYASHI,
Division of Science of Materials, The Graduate
School of Science and Technology, Kobe 657, Japan
Kobe University,
(Receiued 4 June 1984)
Abstract-The
overall stability constants of various metal &quinolinethiolato complexes ([M(c&]; have been obtained by a combination of the polarography of the sodium salt of I-quinolinethioland the potentiometry with a mercury electrode based on the determination of the position of exchange equilibria of the ty& [Hg(qt),] + M*+ +[M(qt)2] +Hgs+. The stability sequence of Mn(II) to Zn(I1) complexes was in agreement with Irving-Williams series. Enthalnies and entropies of the complex formation have been also evaluated from the effect of temperature on tbk overall M = Mn, Co, Ni, Cu, Zn, Cd, Hg) in DMSO
stability constants. Enthalpies stability constants and entropies and Hg(I1) complexes, however, linearly on the softness of metal stability constant of [Zn(qt)J
of Mn(II) to Zn(I1) complexes were dominated in relation to the overall were almost the samevalues. The overallstabilityconstants of Zn(II), Cd(II), were contributed to both parameters. Enthalpies of all complexes depended ions. A nearly linear relationship was found between the logarithmic overall and the donor numbers of the solvents.
PRINCIPLE
INTRODUCTION The stability constants indicating the complex forma-
tion quantitatively have been determined by many investigators, and Martell and Sillen have reported these data systematically[l]. The stability constant is related to the free energy of reaction (AG). Because the free energy (AC) consists of enthalpy (AH) and entropy (AS), terms which frequently tend to compensate each other in dissociation processes[2], more complete information about the factors governing complex formation can be gained by determining these terms as well. These thermodynamic data have been reported for various polyamines[3], but for other types of ligands, especially those containing sulfur as donor atom, the data are sparse. The first stepwise stability constants of various metal 8-quinolinethiolato complexes ([M(qt)] ‘) have been determined spectrophotometrically~4] and potentiometricallyr5] in 50 V/V aJ aqueous dioxane. The thermodynak& parameters have also been determined calorimetricallvr61. The overall stabilitv constants and thermodyn&& parameters for the Xquinolinethiolato complexes of 1 : 2 of metal : ligand, however, have not been able to be measured because of insolubility even in 50 V/V 0/O aqueous dioxane. Accordingly, the present investigation was undertaken to determine those of 1: 2 complexes in dimethylsulfoxide. Data of the R-quinolinethiolato complexes obtained were compared to those of the I-quinolinolato complexes[6, 71, and were discussed using the conception of HSAB (Hard and Soft Acids and Bases). Effect of nonaqueous solvents on the logarithmic overall stability constants of bisl8-quinolinethiolato)zinc(II) complex was also shown. _ 347
In the polarographic measurement, the sodium salt of S-quinolinethiol(abbreviated hereafter as Naqt) in N,N-dimethylformamide solutions has been found to give a reversible oxidation wave[Sj 2Naqt + Hg +(Hg(qt)J
+ 2Na+ -t 2.5.
(1)
The equation of the polarographic oxidation wave is as follows.
where E&z+,~~ is the standard electrode potential of Hg’*/Hg; /Ilngtqt~,l,the overall stability constant of EHg(qt)L]; D, the diffusion coefficient; Cgaqt, the concentration of Naqt in the bulk solution. Measuring the half-wave potential for the oxidation wave of Naqt, the overall stability constant of [Hg(qt),] can be obtained from Equation (3) assuming equal values of the diffusion coefficients. The overall stability constant of [M(qt)J IM: Mn, Co, Ni, Cy Zn, Cd) is determined according to the method of Schmid and Reilley[9]. Consider the following equilibrium reaction. tHg(qt),]
+ M2+ &M(qt)J
+ Hg’+.
(4)
The equilibrium constant of Equation (4) is expressed
348
YASUO NAKABAYASHI,YOSHITAKA
MASUDAAND
E~ICHI SEKIDO
roform. The complexes synthesized were dried in vacua over silica gel. The solutions of metal(I1) perK = CC~~s~~~lJ~C~~2+l/tCC~~~~~~,11~C~Z+1~ chlorate were standardized by EDTA titration method. Tetraethylammonium perchlorate (TEAP) or (5) = BCM(qt),llBCHg(qt),ltetrabutylammonium perchlorate (TBAP) as the sup A mercury electrode as an indicator electrode in porting electrolyte was used. Solvents (Merck Corp.) contact with solutions containing [Hg(qt), J, M(I1) were stored over the type 4A molecular sieves. ions, and [M(qt)2] exhibits a potential corresponding to the following half-cell:
as follows:
Apparatus
As @lHgfqtlllis intent to be extremely larger than flCMcq,,,l,the equilibrium concentration of each species becomes equal to the initial one. From Equation (3) assuming equal values of the diffusion coefficients, the Netnst equation is derived E = J?&+,~$
=
+ (RT/ZF)
Eilgl',Hg+ (RTLw
In [Hg2 ‘1 1" ~CM~qrhl&-ls(st)2,
+ (RT/2F)1nC~Hg(q,)11 'C~2'/C~~(qt)tl'
(7)
Substituting Equation (3) assuming equal values of the diffusion coefficients into Equation (7), the following equation is derived E = (&,2)Nlqt - (RT/2F)
In 2/C&,
+ (RF/W
ln BcMcs,,13
+ (RTI2F)
ln C&s,,,J
. c*,4q,qqql,
(8)
is the half-wave potential for the oxidation wave of Naqt at the dme [refer to Equation (3)]; C*, the initial concentration of each species. and the electrode potential of the Measuring (F&.+tr above halfcell, the overall stability constant of [M(qt)J can be obtained from Equation (8). It is worth noting that this method need not consider the standard electrode potential, E&+,Hg and the liquid junction potential. Enthalpy (AH) and entropy (AS) of the complex formation can be determined from the temperature coefficients of the overall stablity constants
where (&dNqt
In &W&l
= - AHIRT
+ AS/R.
(9)
EXPERIMENTAL Materials The sodium salt of S-quinolinethiol(Naqt) was prepared according to the literature[lO, 111. Various metal 8-quinolinethiolato complexes ( [M(qt)z]; M = Ni, Cu, Zn, Cd) were synthesized by the method of Sekido, Fujiwara, and Masuda[12]. The formation of Mn(I1) and Co(I1) complexes was accomplished by addition of Naqt to oxygen-free solutions contained M(ClO& (M = Mn, Co). The following procedures were adopted for the formation of -[Hg(qt),J. Mercurv~II) sulfate solution (1 mmol dm-‘. 100 cm I contain&g 1 moldm-’ hydr&chbric acid was heated to 65-75°C under nitrogen, and Naqt solution (10mmoldm-3, 50% excess) containing 1 moldmm3 hydrochloric acid was added. Then sodium hydroxide solution was carefully added to precipitate the complex. After ageing for about 10 mitt, Hg(I1) complex was collected and washed with 50 cm3 of hot water at 80°C. and then was recrystallized twice from chlo-
The polarograms were recorded by a Fuso polarograph model 312. A three-electrode cell equipped with an aqueous saturated calomel electrode (see) and a platinum-wire electrode was employed. A dropping mercury electrode (dme) with a flow rate of TEAP-DMSO sol0.808 mg s- ’ in 0.1 moldme utions was used. An see which was separated from nonaqueous solutions by a TEAP-methylcellulose salt bridge served as the reference electrode[13]. This electrolytic cell used was the same configuration as the one of Yamada et al.[l4] and had a volume of ca. 20 cm3. A Soar digital multimeter, MN-536, was used to measure the current and the potential precisely. Current--potential curves were recorded on a Riken Denshi X-Y recorder model F-43P. A mercury electrode[l5] with a geometric area of 2.3cm’ as the indicator electrode was used for the potentiometric measurement. Procedure
In order to determine the standard electrode potential of Hg’+/Hg in DMSO, measurements of the potential of Hg(C10,)2 over the concentration range in 0.1 mol dm- 3 TEAP10-4-10-3 moldm-” DMSO solutions were carried out. The normal pulse polarograms of Naqt over the concentration range 0.1 moldm-’ 5x10-5-10-3moldm-3 TEAP-DMSO solutions were korded for the determination of the overall stability constant of [Hg(qt),]. In order to determine the overall stability constants of [M(qt),] (M = Mn, Co, Ni, Cu, Zn, Cd), measurements of the potentials against see of the halfcell indicated Equation (6) were carried out. The dissolved oxygen was removed by bubbling purified nitrogen gas through the sample solutions. All the measurements were made at 25.0 * 0.1 “C unless otherwise stated.
RESULTS
AND DISCUSSION
In the polarographic measurements, Naqt in DMSO solutions was found to give a well-defined oxidation wave. The limiting current was proportional to the concentration of Naqt. The plot of log I/(1, - Q2 us E yielded satisfactory straight line with the slope of 31 mV. The half-wave potential shifted cathodically 30 mV per IO-fold increase in concentration. These results indicate that Equations (2) and (3) can be applied to the analysis of the oxidation wave of Naqt. The standard electrode potential of Hg*+/Hg, V us see, was obtained from the E",,'*,H,= +0.399 Nernst equation. The logarithmic overall stability constant? log &Hg(g&]r assuming equal values of the diffusion coefficients m Equation (3) were determined
Thermodynamic Table 1. Determination constant of [Hg(qt),]
parameters
of various
metal I-quinolinethiolato
of the logarithmic overall stability in DMSO, 0.1moldm-a TEAP at 25°C
Concn. of Naqt (mol dme3)
El/Z (V “S see)
2.52
-0.822
45.1, 45.1,
-0.808 -0.799
45.0, 45.0s
I
E Fe?
CM(qtLl (M = Mn, Co, Ni, Cu, Zn, Cd), log&+{, was determined by measuring the electrode potentla s of the half-cell indicated in Equation (6) under various concentration of metal ions. As an example, the result for [Zn(qt)2] is presented in Table 2. The decrease of the concentrations of [Hg(qt),] and [Zn(qt)J is attributed to the effect of dilution in the addition of metal ions. The logarithmic overall stability constants of another complexes could be determined as well. The values obtained are shown in Table 3 and Fig. I. For the sake of comparison, the logarithmic overall stability constants of the X-quinolinolato complexes ([M(qn)J>[7] are also given in Fig. 1. The stability sequence of Mn(II) to Zn(I1) 8-quinolinethiolato complexes as well as the Squinolinolato complexes was also in agreement with the Irving-Williams series: MnZn. The difference of the overall stability constants of [Mn(qt)*] and [Mn(qn),] was the smallest, whereas that of [Cu(qt),] and [Cu(qn),] was the largest compared with that of other complexes. The fact that the overall stability constants of the O-quinolinethiolato complexes are larger than those of the 8quinolinolato complexes reported Johnston and Freiser[7] may be attributed to the presence of donor n-bonds as a result of an increase in electron density on the sulfur atom and an increase of the positive charge on the nucleus of the central atom which promotes acceptance of the unshared electron pair of the nitrogen atom in the 8-quinolinethiolato complexes. The thermodynamic parameters of the complex formation obtained from the effect of temperature on
Table 2. Determination
of the logarithmic
Concn. of Zn(I1)
Concn. of [Zn(qt),] (mmoI dm-‘)
0.910 0.903 0.896 0.8X9 0.879
b
\
20
by measuring the half-wave potential at various concentrations of Naqt. The result is given in Table 1. The logarithmic overall stability constant of
0.0992 0.295 0.488 0.678 0.958
I
1
30-
45.09
Av. = 45.1 fO.0,
(mmol dm-‘)
I
a
-0.838 -0.830
x tO-4
,
349
l”gb[Wsthl
1.02 x 10-S 5.04 x 10-’ 1.02 x 10-4 5.04 x 10-S
40
complexes
--A-
t.in
N,
co
CU
zn
Cd
Fig. I. The logarithmic overall stability constantsof metal 8quinolinethiolato ([M(q& J, (a)) and metal S-quinolinolato ([M(qn), J, (b)) complexes. (a) This work, (b) Ref. 7.
the overall stability constants of various metal Squinolinethiolato complexes are shown in Table 4 and Fig. 2. Enthalpies of Mn(I1) to Zn(I1) complexes reflected to the overall stability constants, whereas entropies of these complexes were almost the same values. The overall stability constants of Zn(II), Cd(II), and Hg(II) complexes having d lo electrons, on the other hand, were affected by both parameters. The stability sequence of these complexes were as follows: Cd c Zn K Hg. A reversal in the stability order anticipated from the softness of metal ions was observed for Zn(I1) and Table 3. The logarithmic overall stability constants of [M(qt)2j in DMSO,
0.1 mol dm-’ TEAP at 25°C
Metal ion Mn(I1) Co(I1) Ni(I1) Cu(I1) Zn(II) WII) Hg(II1
overall stability constant
TEAP at 25°C
Concn. of [Hg(qt),] (mm01 dm-‘) 0.702 0.697 0.691 0.686 0.678
16.8 26.7 28.4 34.9 24.5 24.5 45.1
of [Zn(qt)J
f f f f f f f
0.2 0.2 0.2 0.3 0.3 0.2 0.0,
in DMSO, 0.1moldm-a
E
(V usSC.?) -0.283 -0.264 -0.252 -0.241 -0.235
log BCZnlqtl,l 26.2 26.3 26.5 26.8 26.8 Av. = 26.5k 0.3
YASUO NAKABAYASHI, YOSHITAKA MASUDA AND EIICHXSEK~DO
350
Table 4. Thermodynamic parameters of [M(qt),] 0.1 moldmm3 TEAP at 25°C -AG (kJ mol-‘)
Metal ion MI@) Co(H) Ni(lI) Cu(I1) Zn(I1) Cd(H)
95.9 152 162 211 151 140
H&II)
257
in DMSO,
TAS (kJmol_‘)
-AH (k.J mol-‘) 65.4 115 117 154 92.5
30.7 37.0 45.3 56.1 58.7
110 258
29.8 -0.93,
a Fig. 3. Relationship between enthalpies of [M(qt),] DMSO and a (the softness of metal ions).
in
between metal and ligand. Yamada and Tanaka have estimated LX,the parameter indicated the softness of various metal ions[l7]. Hence, the relationship between enthalpies and a was examined. It was found that enthalpies of all complexes depended linearly on a (Fig. 3). It may be suggested that two linear lines with different slopes are attributed to different types of the
Mn
co
NI
Cu
Ln
Cd
geometric configurations of the 8-quinolinethiolato complex formed, explaining with the formation of
Hg
Fig. 2. The logarithmic
overall stability constants, enthalpies, and entropies of [M(qt)t] in DMSO, 0.1 moldm-s TEAP at 25°C.
Cd(I1) complexes. The order of enthalpies complexes, however, was given as:
of these
Zn i Cd < Hg. This order agrees with the order of the softness of metal ions. This fact indicates that the increase in the softness of metal ions is favorable for the formation of the 8-quinotinethiolato complexes. The sequence of entropies of these complexes was reversed. Therefore, it may be considered that the order of the stability is reversed for Zn(II) and Cd(l1) complexes. GQuinolinethiol
containing
sulfur
atom
is a soft
base. Therefore, the softer metal ions, the greater enthalpies reflecting directly the bonding strength
octahedral (line a) and tetrahedral (line b) complexes. The potentiometric measurement of the half-cell used zinc for M [see Equation (6)] and the polarographic measurement of Naqt were undertaken in the solvents: acetone (AC), propanediol-1,2-carbonate (PC), acetonitrile (An), ethanol (EtOH), methanol (MeOH), N&V-dimethylformamide (DMF), and dimethylsulfoxide (DMSO). The experimental data determined from Equation (8) are summarized in Table 5 together with the donor numbers (DN)[lX, 191, acceptor numbers (AN)[20] and dielectric constants (s) of the solvents. No correlation was found between the logarithmic overall stability constants of [Zn(qt)J and the dielectric constants of the solvents. Figure 4 shows the relationship between the logarithmic overall stability constants of [Zn(qt)J and the donor numbers of the solvents[ 1 S]. In a series of the solvents with similar acceptor numbers, the linear relationship existed. It is obvious that the solvent exerts a profound influence on the
Table 5. Effect of nonaqueous solvents on the logarithmic overall stability constants of [Zn(qt)J
0.1 mol dmm3 TBAP Solvent Acetone Propanediol-1,2-carbonate Acetonitrile Ethanol Methanol N,iV-dimethylformamide Dimethylsulfoxide
Abbreviation AC PC An EtOH MeOH DMF DMSO
43.9 43.2 42.8 34.4 32.9 31.0 26.5
DN
AN
E
17.0 15.1 14.1 20.0 19.0 26.6 29.8
12.5 18.3 19.3 37.1 41.3 16.0 19.3
20.7 64.4 37.5 24.6 32.7 36.7 46.7
at 25”C,
Thermodynamic
parameters of various metal Squinolinethiolato
complexes
351
Acknowledgemenls-The authors wish to thank the Ministry of Eduration for the support of this work. We are deeply indebted to Professor Emeriti, N. Tanaka, Tohoku University and Dr. A. Yamada, Technological University of Nagaoka for their helpful suggestions and warm encouragement.
50i
REFERENCES
Fig. 4. Relationship between the logarithmic overall stability constants of [Zn(qt)2] and the donor numbers of the solvents. stability of [Zn(qt)2], the overall stability constants of [Zn(qt)2] being shifted to greater values with decreasing the donor numbers of the solvents. The positive deviation from the straight line in the case of AC and the negative one in the case of EtOH and MeOtI are almost certainly due to different acceptor numbers from those of another solvents. The higher coordination bond of ligand with the complex formation and the smaller the donicity of solvent, the larger the stability of the compIex formed. The strength of coordination of ligand changes with acceptor strength of the solvent if the various side-effects of salvation can be neglected. When the ligand solvates with solvent having higher acceptor strength, the strength of the coordination of the solvent weakens. Consequently, it is confirmed that the smaller the donor and acceptor numbers of the solvents, the larger the stabitity of
EZnhW.
I. A. E. MartelI and L. G. Sillen, Stability Consrants oj Metal-Ion Complexes, Organic Part, Part II. Supplement No. 1 to Special Publication No. 17, Chem. Sot. London (1971). 2. D. J. G. Ives and P. D. Marsden, J. &em. Sot. 1965,649. 3. D. L. Wright, J. H. Holloway and C. N. Reilley. Analyt. Chem. 37, 884 (1965). 4. A. Corsini, Q, Fernando and H. Freiser, Analyt. Chem. 35, 1424 (1963). 5. D. Kealey and H. Freiser, Anal+ Chem. 38, 1577 (1966). 6. G. Gutnikov and H. Freiser, Anaiyt. Chem. 40, 39 (1968). 7. W. D. Johnston and H. Freiser, 1. Am. &em. SOC. 74, 5239 (1952). 8. Y. Nakabayashi, Y. Masuda and E. Sekido, J. electroonal. Chem. 176, 243 (1984). 9. R. W. Schmid and C. N. Reilley. J. Am. them. Sot. 78,55 13 (1956). 10. D. Kealey and H. Freiser, Talonto 13, 1381 (1966). 11. N. Nakamura and E. Sekido, Tulonta 17, 515 (1970). 12. E. Sekido, 1. Fujiwara and Y. Masuda, Tulantn 19, 479 (1972). 13. J. Broadhead and P. J. Elving, Analyt. Chem. 41. 1814 (1969). 14. A. Yamada, T. Yoshikuni, Y. Kato and N. Tanaka, Annlyt. Chim. Acta 133, 421 (1981). 15. C. N. Reilley, R. W. Schmid and D. W. Lamson, Analyt. them 30,953 (1958). 16. K. Burger, Tuianta 8, 769 (1961). 17. S. Yamada and M. Tanaka, J. inorg. nucI. Chem. 37,587 (1975). 18. Y. Gutmann, Coord. them. Rev. 18, 225 (1976). 19. K. Burger, Salvation, ionic and Complex Formation Reactions in Non-Aqueous Salvent, p. 42. Elsevier Scientific Publishing Company, Oxford (1983). 20. V. Gutmann, Electrochim. Acta 21, 661 (1976).
YASUO NAKABAYASHI,
Division of Science of Materials, The Graduate
School of Science and Technology, Kobe 657, Japan
Kobe University,
(Receiued 4 June 1984)
Abstract-The
overall stability constants of various metal &quinolinethiolato complexes ([M(c&]; have been obtained by a combination of the polarography of the sodium salt of I-quinolinethioland the potentiometry with a mercury electrode based on the determination of the position of exchange equilibria of the ty& [Hg(qt),] + M*+ +[M(qt)2] +Hgs+. The stability sequence of Mn(II) to Zn(I1) complexes was in agreement with Irving-Williams series. Enthalnies and entropies of the complex formation have been also evaluated from the effect of temperature on tbk overall M = Mn, Co, Ni, Cu, Zn, Cd, Hg) in DMSO
stability constants. Enthalpies stability constants and entropies and Hg(I1) complexes, however, linearly on the softness of metal stability constant of [Zn(qt)J
of Mn(II) to Zn(I1) complexes were dominated in relation to the overall were almost the samevalues. The overallstabilityconstants of Zn(II), Cd(II), were contributed to both parameters. Enthalpies of all complexes depended ions. A nearly linear relationship was found between the logarithmic overall and the donor numbers of the solvents.
PRINCIPLE
INTRODUCTION The stability constants indicating the complex forma-
tion quantitatively have been determined by many investigators, and Martell and Sillen have reported these data systematically[l]. The stability constant is related to the free energy of reaction (AG). Because the free energy (AC) consists of enthalpy (AH) and entropy (AS), terms which frequently tend to compensate each other in dissociation processes[2], more complete information about the factors governing complex formation can be gained by determining these terms as well. These thermodynamic data have been reported for various polyamines[3], but for other types of ligands, especially those containing sulfur as donor atom, the data are sparse. The first stepwise stability constants of various metal 8-quinolinethiolato complexes ([M(qt)] ‘) have been determined spectrophotometrically~4] and potentiometricallyr5] in 50 V/V aJ aqueous dioxane. The thermodynak& parameters have also been determined calorimetricallvr61. The overall stabilitv constants and thermodyn&& parameters for the Xquinolinethiolato complexes of 1 : 2 of metal : ligand, however, have not been able to be measured because of insolubility even in 50 V/V 0/O aqueous dioxane. Accordingly, the present investigation was undertaken to determine those of 1: 2 complexes in dimethylsulfoxide. Data of the R-quinolinethiolato complexes obtained were compared to those of the I-quinolinolato complexes[6, 71, and were discussed using the conception of HSAB (Hard and Soft Acids and Bases). Effect of nonaqueous solvents on the logarithmic overall stability constants of bisl8-quinolinethiolato)zinc(II) complex was also shown. _ 347
In the polarographic measurement, the sodium salt of S-quinolinethiol(abbreviated hereafter as Naqt) in N,N-dimethylformamide solutions has been found to give a reversible oxidation wave[Sj 2Naqt + Hg +(Hg(qt)J
+ 2Na+ -t 2.5.
(1)
The equation of the polarographic oxidation wave is as follows.
where E&z+,~~ is the standard electrode potential of Hg’*/Hg; /Ilngtqt~,l,the overall stability constant of EHg(qt)L]; D, the diffusion coefficient; Cgaqt, the concentration of Naqt in the bulk solution. Measuring the half-wave potential for the oxidation wave of Naqt, the overall stability constant of [Hg(qt),] can be obtained from Equation (3) assuming equal values of the diffusion coefficients. The overall stability constant of [M(qt)J IM: Mn, Co, Ni, Cy Zn, Cd) is determined according to the method of Schmid and Reilley[9]. Consider the following equilibrium reaction. tHg(qt),]
+ M2+ &M(qt)J
+ Hg’+.
(4)
The equilibrium constant of Equation (4) is expressed
348
YASUO NAKABAYASHI,YOSHITAKA
MASUDAAND
E~ICHI SEKIDO
roform. The complexes synthesized were dried in vacua over silica gel. The solutions of metal(I1) perK = CC~~s~~~lJ~C~~2+l/tCC~~~~~~,11~C~Z+1~ chlorate were standardized by EDTA titration method. Tetraethylammonium perchlorate (TEAP) or (5) = BCM(qt),llBCHg(qt),ltetrabutylammonium perchlorate (TBAP) as the sup A mercury electrode as an indicator electrode in porting electrolyte was used. Solvents (Merck Corp.) contact with solutions containing [Hg(qt), J, M(I1) were stored over the type 4A molecular sieves. ions, and [M(qt)2] exhibits a potential corresponding to the following half-cell:
as follows:
Apparatus
As @lHgfqtlllis intent to be extremely larger than flCMcq,,,l,the equilibrium concentration of each species becomes equal to the initial one. From Equation (3) assuming equal values of the diffusion coefficients, the Netnst equation is derived E = J?&+,~$
=
+ (RT/ZF)
Eilgl',Hg+ (RTLw
In [Hg2 ‘1 1" ~CM~qrhl&-ls(st)2,
+ (RT/2F)1nC~Hg(q,)11 'C~2'/C~~(qt)tl'
(7)
Substituting Equation (3) assuming equal values of the diffusion coefficients into Equation (7), the following equation is derived E = (&,2)Nlqt - (RT/2F)
In 2/C&,
+ (RF/W
ln BcMcs,,13
+ (RTI2F)
ln C&s,,,J
. c*,4q,qqql,
(8)
is the half-wave potential for the oxidation wave of Naqt at the dme [refer to Equation (3)]; C*, the initial concentration of each species. and the electrode potential of the Measuring (F&.+tr above halfcell, the overall stability constant of [M(qt)J can be obtained from Equation (8). It is worth noting that this method need not consider the standard electrode potential, E&+,Hg and the liquid junction potential. Enthalpy (AH) and entropy (AS) of the complex formation can be determined from the temperature coefficients of the overall stablity constants
where (&dNqt
In &W&l
= - AHIRT
+ AS/R.
(9)
EXPERIMENTAL Materials The sodium salt of S-quinolinethiol(Naqt) was prepared according to the literature[lO, 111. Various metal 8-quinolinethiolato complexes ( [M(qt)z]; M = Ni, Cu, Zn, Cd) were synthesized by the method of Sekido, Fujiwara, and Masuda[12]. The formation of Mn(I1) and Co(I1) complexes was accomplished by addition of Naqt to oxygen-free solutions contained M(ClO& (M = Mn, Co). The following procedures were adopted for the formation of -[Hg(qt),J. Mercurv~II) sulfate solution (1 mmol dm-‘. 100 cm I contain&g 1 moldm-’ hydr&chbric acid was heated to 65-75°C under nitrogen, and Naqt solution (10mmoldm-3, 50% excess) containing 1 moldmm3 hydrochloric acid was added. Then sodium hydroxide solution was carefully added to precipitate the complex. After ageing for about 10 mitt, Hg(I1) complex was collected and washed with 50 cm3 of hot water at 80°C. and then was recrystallized twice from chlo-
The polarograms were recorded by a Fuso polarograph model 312. A three-electrode cell equipped with an aqueous saturated calomel electrode (see) and a platinum-wire electrode was employed. A dropping mercury electrode (dme) with a flow rate of TEAP-DMSO sol0.808 mg s- ’ in 0.1 moldme utions was used. An see which was separated from nonaqueous solutions by a TEAP-methylcellulose salt bridge served as the reference electrode[13]. This electrolytic cell used was the same configuration as the one of Yamada et al.[l4] and had a volume of ca. 20 cm3. A Soar digital multimeter, MN-536, was used to measure the current and the potential precisely. Current--potential curves were recorded on a Riken Denshi X-Y recorder model F-43P. A mercury electrode[l5] with a geometric area of 2.3cm’ as the indicator electrode was used for the potentiometric measurement. Procedure
In order to determine the standard electrode potential of Hg’+/Hg in DMSO, measurements of the potential of Hg(C10,)2 over the concentration range in 0.1 mol dm- 3 TEAP10-4-10-3 moldm-” DMSO solutions were carried out. The normal pulse polarograms of Naqt over the concentration range 0.1 moldm-’ 5x10-5-10-3moldm-3 TEAP-DMSO solutions were korded for the determination of the overall stability constant of [Hg(qt),]. In order to determine the overall stability constants of [M(qt),] (M = Mn, Co, Ni, Cu, Zn, Cd), measurements of the potentials against see of the halfcell indicated Equation (6) were carried out. The dissolved oxygen was removed by bubbling purified nitrogen gas through the sample solutions. All the measurements were made at 25.0 * 0.1 “C unless otherwise stated.
RESULTS
AND DISCUSSION
In the polarographic measurements, Naqt in DMSO solutions was found to give a well-defined oxidation wave. The limiting current was proportional to the concentration of Naqt. The plot of log I/(1, - Q2 us E yielded satisfactory straight line with the slope of 31 mV. The half-wave potential shifted cathodically 30 mV per IO-fold increase in concentration. These results indicate that Equations (2) and (3) can be applied to the analysis of the oxidation wave of Naqt. The standard electrode potential of Hg*+/Hg, V us see, was obtained from the E",,'*,H,= +0.399 Nernst equation. The logarithmic overall stability constant? log &Hg(g&]r assuming equal values of the diffusion coefficients m Equation (3) were determined
Thermodynamic Table 1. Determination constant of [Hg(qt),]
parameters
of various
metal I-quinolinethiolato
of the logarithmic overall stability in DMSO, 0.1moldm-a TEAP at 25°C
Concn. of Naqt (mol dme3)
El/Z (V “S see)
2.52
-0.822
45.1, 45.1,
-0.808 -0.799
45.0, 45.0s
I
E Fe?
CM(qtLl (M = Mn, Co, Ni, Cu, Zn, Cd), log&+{, was determined by measuring the electrode potentla s of the half-cell indicated in Equation (6) under various concentration of metal ions. As an example, the result for [Zn(qt)2] is presented in Table 2. The decrease of the concentrations of [Hg(qt),] and [Zn(qt)J is attributed to the effect of dilution in the addition of metal ions. The logarithmic overall stability constants of another complexes could be determined as well. The values obtained are shown in Table 3 and Fig. I. For the sake of comparison, the logarithmic overall stability constants of the X-quinolinolato complexes ([M(qn)J>[7] are also given in Fig. 1. The stability sequence of Mn(II) to Zn(I1) 8-quinolinethiolato complexes as well as the Squinolinolato complexes was also in agreement with the Irving-Williams series: Mn
Table 2. Determination
of the logarithmic
Concn. of Zn(I1)
Concn. of [Zn(qt),] (mmoI dm-‘)
0.910 0.903 0.896 0.8X9 0.879
b
\
20
by measuring the half-wave potential at various concentrations of Naqt. The result is given in Table 1. The logarithmic overall stability constant of
0.0992 0.295 0.488 0.678 0.958
I
1
30-
45.09
Av. = 45.1 fO.0,
(mmol dm-‘)
I
a
-0.838 -0.830
x tO-4
,
349
l”gb[Wsthl
1.02 x 10-S 5.04 x 10-’ 1.02 x 10-4 5.04 x 10-S
40
complexes
--A-
t.in
N,
co
CU
zn
Cd
Fig. I. The logarithmic overall stability constantsof metal 8quinolinethiolato ([M(q& J, (a)) and metal S-quinolinolato ([M(qn), J, (b)) complexes. (a) This work, (b) Ref. 7.
the overall stability constants of various metal Squinolinethiolato complexes are shown in Table 4 and Fig. 2. Enthalpies of Mn(I1) to Zn(I1) complexes reflected to the overall stability constants, whereas entropies of these complexes were almost the same values. The overall stability constants of Zn(II), Cd(II), and Hg(II) complexes having d lo electrons, on the other hand, were affected by both parameters. The stability sequence of these complexes were as follows: Cd c Zn K Hg. A reversal in the stability order anticipated from the softness of metal ions was observed for Zn(I1) and Table 3. The logarithmic overall stability constants of [M(qt)2j in DMSO,
0.1 mol dm-’ TEAP at 25°C
Metal ion Mn(I1) Co(I1) Ni(I1) Cu(I1) Zn(II) WII) Hg(II1
overall stability constant
TEAP at 25°C
Concn. of [Hg(qt),] (mm01 dm-‘) 0.702 0.697 0.691 0.686 0.678
16.8 26.7 28.4 34.9 24.5 24.5 45.1
of [Zn(qt)J
f f f f f f f
0.2 0.2 0.2 0.3 0.3 0.2 0.0,
in DMSO, 0.1moldm-a
E
(V usSC.?) -0.283 -0.264 -0.252 -0.241 -0.235
log BCZnlqtl,l 26.2 26.3 26.5 26.8 26.8 Av. = 26.5k 0.3
YASUO NAKABAYASHI, YOSHITAKA MASUDA AND EIICHXSEK~DO
350
Table 4. Thermodynamic parameters of [M(qt),] 0.1 moldmm3 TEAP at 25°C -AG (kJ mol-‘)
Metal ion MI@) Co(H) Ni(lI) Cu(I1) Zn(I1) Cd(H)
95.9 152 162 211 151 140
H&II)
257
in DMSO,
TAS (kJmol_‘)
-AH (k.J mol-‘) 65.4 115 117 154 92.5
30.7 37.0 45.3 56.1 58.7
110 258
29.8 -0.93,
a Fig. 3. Relationship between enthalpies of [M(qt),] DMSO and a (the softness of metal ions).
in
between metal and ligand. Yamada and Tanaka have estimated LX,the parameter indicated the softness of various metal ions[l7]. Hence, the relationship between enthalpies and a was examined. It was found that enthalpies of all complexes depended linearly on a (Fig. 3). It may be suggested that two linear lines with different slopes are attributed to different types of the
Mn
co
NI
Cu
Ln
Cd
geometric configurations of the 8-quinolinethiolato complex formed, explaining with the formation of
Hg
Fig. 2. The logarithmic
overall stability constants, enthalpies, and entropies of [M(qt)t] in DMSO, 0.1 moldm-s TEAP at 25°C.
Cd(I1) complexes. The order of enthalpies complexes, however, was given as:
of these
Zn i Cd < Hg. This order agrees with the order of the softness of metal ions. This fact indicates that the increase in the softness of metal ions is favorable for the formation of the 8-quinotinethiolato complexes. The sequence of entropies of these complexes was reversed. Therefore, it may be considered that the order of the stability is reversed for Zn(II) and Cd(l1) complexes. GQuinolinethiol
containing
sulfur
atom
is a soft
base. Therefore, the softer metal ions, the greater enthalpies reflecting directly the bonding strength
octahedral (line a) and tetrahedral (line b) complexes. The potentiometric measurement of the half-cell used zinc for M [see Equation (6)] and the polarographic measurement of Naqt were undertaken in the solvents: acetone (AC), propanediol-1,2-carbonate (PC), acetonitrile (An), ethanol (EtOH), methanol (MeOH), N&V-dimethylformamide (DMF), and dimethylsulfoxide (DMSO). The experimental data determined from Equation (8) are summarized in Table 5 together with the donor numbers (DN)[lX, 191, acceptor numbers (AN)[20] and dielectric constants (s) of the solvents. No correlation was found between the logarithmic overall stability constants of [Zn(qt)J and the dielectric constants of the solvents. Figure 4 shows the relationship between the logarithmic overall stability constants of [Zn(qt)J and the donor numbers of the solvents[ 1 S]. In a series of the solvents with similar acceptor numbers, the linear relationship existed. It is obvious that the solvent exerts a profound influence on the
Table 5. Effect of nonaqueous solvents on the logarithmic overall stability constants of [Zn(qt)J
0.1 mol dmm3 TBAP Solvent Acetone Propanediol-1,2-carbonate Acetonitrile Ethanol Methanol N,iV-dimethylformamide Dimethylsulfoxide
Abbreviation AC PC An EtOH MeOH DMF DMSO
43.9 43.2 42.8 34.4 32.9 31.0 26.5
DN
AN
E
17.0 15.1 14.1 20.0 19.0 26.6 29.8
12.5 18.3 19.3 37.1 41.3 16.0 19.3
20.7 64.4 37.5 24.6 32.7 36.7 46.7
at 25”C,
Thermodynamic
parameters of various metal Squinolinethiolato
complexes
351
Acknowledgemenls-The authors wish to thank the Ministry of Eduration for the support of this work. We are deeply indebted to Professor Emeriti, N. Tanaka, Tohoku University and Dr. A. Yamada, Technological University of Nagaoka for their helpful suggestions and warm encouragement.
50i
REFERENCES
Fig. 4. Relationship between the logarithmic overall stability constants of [Zn(qt)2] and the donor numbers of the solvents. stability of [Zn(qt)2], the overall stability constants of [Zn(qt)2] being shifted to greater values with decreasing the donor numbers of the solvents. The positive deviation from the straight line in the case of AC and the negative one in the case of EtOH and MeOtI are almost certainly due to different acceptor numbers from those of another solvents. The higher coordination bond of ligand with the complex formation and the smaller the donicity of solvent, the larger the stability of the compIex formed. The strength of coordination of ligand changes with acceptor strength of the solvent if the various side-effects of salvation can be neglected. When the ligand solvates with solvent having higher acceptor strength, the strength of the coordination of the solvent weakens. Consequently, it is confirmed that the smaller the donor and acceptor numbers of the solvents, the larger the stabitity of
EZnhW.
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