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Complexes of sulphur-containing ligands—II
Z inorg, nucl. (.'hem. Vol. 41, pp. 1629-1633 Pergamon Press Ltd., 1979. Printed in Great Britain
COMPLEXES OF SULPHUR-CONTAINING LIGANDS--II BINARY A N D T E R N A R Y C O M P L E X E S O F D - P E N I C I L L A M I N E A N D L - C Y S T E I N E WITH NICKEL(II) A N D ZINC(II) IONS IMRE SOVA.GO,ARTHUR GERGELY,* BI~LAHARMANand TAMA,S KISS Institute of Inorganicand AnalyticalChemistry,Lajos Kossuth University, H-4010 Debrecen, Hungary
(Received 5 September 1978; received[or publication 16 May 1979) Abstract--Complexes of nickel(II) and zinc(II) with L-cysteine and D-penicillaminewere investigated by pHmetric, spectrophotometricand magneticmeasurements. The formation of polynuclearspecies M3,Mwas proved in the case of L-cysteine. Both ligands can also form protonated complexes.The concentrationof mixed species is higher in the systems containingthe zinc(II) ion. Mixed ligand complexeswere not identifiedin the nickel(II)-Dpenicillamine-glycinesystem. With L-histidineor histamineas ligand _B,the formation of mixed ligandcomplexes was detected in spite of the different structures of the parent complexes. The stability constants of the mixed complexes of zinc(II) closely correspondedto statistical expectation. INTRODUCTION Widespread investigations have long been performed on the complex-forming properties of D-penicillamine and L-cysteine, which are of outstanding importance biologically and therapeutically[l]. Earlier results were summarized in a review published in 1972 by McAuliffe and Murray [2]. The study of the interaction of copper(I[) and D-penicillamine lies at the centre of these investigations; this system is complicated by the fact that complex formation and redox reactions take place simultaneously. The possible pathways and the products formed in this system were reported earlier[3]. A deeper understanding of this type of chemical reaction, however, requires extension of the investigations to complexes of other. transition metal ions (particularly nickel(II) and zinc(ll) ions) which are not susceptible to redox reactions. The first investigations in this field were those of Lenz and Martell[4] and Kuchinskas and Rosen[5], who determined the stability constants of the transition metal complexes with cysteine and D-penicillamine. However, their data can be considered only as approximative, because the calculations were made on the assumption merely of parent complexes MA and MA2. The formation of polynuclear species in systems containing cysteine and nickel(II) or zinc(II) ions was first proved by Perrin and Sayce [6]. In addition, they found that the equilibrium relations of the complexes of D-penicillamine can be described by the formation of mononuclear complexes. On the other hand Srivastava et a/.[7] concluded in good agreement with other investigators[8, 9] that the complexes NiA2 are planar diamagnetic compounds. On the basis of thermodynamic and spectroscopic measurements, the former authors [7] reported that at a high ligand/metai ion ratio the formation of polynuclear species is negligible in the nickel(II)--cysteine system too. Recently, however, Corrie et a/.[10] also assume the formation of polynuclear complexes in the zinc(II)-D-penicillamine system. *Author for correspondence.
As stated the equilibrium studies on the parent complexes of cysteine and D-penicillamine are somewhat contradictory, while the mixed ligand complexes of sulphur-containing amino acids have barely been investigated. According to the CD measurements of Chang and Martin[9], there is no mixed ligand complex formation in the nickei(II)-cysteine-ethylenediamine system. This is in good agreement with the observations of Kida et a/.[ll] that mixed ligand complexes are not formed when one of the parent complexes is octahedral and the other is planar. The complexes of cadmium(II), mercury(II) and iead(II) with D-peniciilamine and other sulphur-containing ligands were investigated by Sugiura and Tanaka[12]. Their results on the effect of the sulphur donor atom on the mixed ligand complex formation, however, can scarcely be generalized, because they used very similar types of ligands. The aim of the present work was to study mixed ligand complex formation between sulphur-containing amino acids (L-cysteine and D-peniciUamine) and other types of ligands (glycine, histamine and L-histidine) and nickel(II) and zinc (II) ions. Because of the contradictions in the equilibrium data on the parent complexes, a structural and thermodynamic investigation of the binary systems was also needed. EXPERIMENTAL D-penicillamine (Fluka), L-cysteine hydrochloride monohydrate (Merck),L-histidinehydrochloridemonohydrate,histamine dihydrochlorideand glycine (p,a. reagents of Reanal) were used for the experiments. Before use, the reagents were further purified by recrystallizationfrom ethanol-wateror from acetonewater. NiCl2 and ZnCI2 stock solutions were prepared from p.a. Reanal reagents, and their concentrations were determined by gravimetry via the oxinates. The concentrations of the stock solutions of hydrochloric acid and potassium hydroxide were measured pH-metrically as described previously[13]. All measurements were carried out at 25-+0.1°C; the ionic strength was adjusted to 0.2 M with KCI. The determination of stability constants was performed as published earlier[13]. A Radiometer priM-64 digital pH-meter 1629
1630
I. SrvAGraaL
with G202B glass and K401 calomel reference electrodes was used for the measurements. The pH-metric examinations were evaluated by the method previously described[14]. The parent and mixed ligand complexes of the nickel(II) ion were investigated by spectrophotometric and magnetic measurements. Visible and u.v. spectra of the complexes were recorded in aqueous solutions with a Beckman Acta M-IV spectrophotometer. The resultant magnetic moments of samples containing the nickel(lI) ion were measured in aqueous solutions using the NMR method proposed by Lfliger and Scheffold[15]. These measurements were performed with a Jeol MH-100 NMRspectrometer. RESULTS AND DISCUSSION The pH-metric equilibrium investigations were carried out at various metal ion/ligand ratios for the complexes of both metal ions ( 1 0 - 3<- CM < 10-2M; and 4.10-3< CA < 10-2 M): 150-200 exPerimental data were available for the evaluation in every system. In contrast to the results of some other earlier investigations[7, 8, 16], our titration curves could be described by the assumption of the protonated and polynuclear complexes: when the species listed in Table 1 were taken into account, the average difference between the measured and calculated titration curves was less than 0.008 cm 3. It can be concluded from the data in Table 1 that our results show a similar pattern to those of Perrin and Sayce[6]. The formation of polynuclear species is characteristic of the complexes of cysteine, but protonated complexes can be formed with penicillamine too. However, there are some slight differences between our equilibrium data and those obtained by the former authors[6]. According to our results, particularly the polynuclear complexes with a composition of M3A4 were formed. The concentration of the species M2A3 was considerably less than 5% in all cases, and the evaluation of the experimental data was hardly influenced by neglecting this species. In addition, on the basis of the data in Table 1, the concentration of the protonated complexes is very high in the case of the zinc(II) ion, and the protonated polynuclear complex Zn3A4H is also formed in high concentration, in good agreement with the observations of Corrie et a/.[10]. The complicated equilibrium relations of these systems are illustrated in Fig. 1 for the example of the zinc(II)--cysteine system. According to Fig. 1, the number of complexes formed in these systems is very high, and therefore magnetic and spectrophotometric measurements were also carried out to prove the formation of polynuclear complexes. In the
5o
o~ 40
~
Zn3A4HZnA~-
30
/V 5.0
5.5
6.0
6.5
pH
Fig. 1. Concentration distribution of the complexes formed in the zinc(II)-L-cysteine system. Cz,(.) = 5.10-3 M; Coy,= 10-2 M. course of these investigations, the resultant magnetic moments and the absorbances of the solutions containing nickel(II) ion and l)-penicillamine or cysteine at a metal ion: ligand ratio of 1:2 were measured as a function of the concentration of potassium hydroxide added to the samples. The variation in the absorbance values measured at 470 NM is shown in Fig. 2, while Fig. 3 presents the variation of the resultant magnetic moments of the solutions. It can be seen from Fig. 2 that the absorbance values pass through a maximum in the nickel(II)--cysteine system. This can be explained by taking into account the very intense charge-transfer band near the d-d transitions. The energy of the charge-transfer transition will be decreased in the sulphur-bridged polynuclear complexes, which contain even more easily excitable sulphur atoms. Therefore, the charge-transfer band will in part be built upon the d-d transitions of the nickel(II) ion. On
Table 1. Stability constants of the parent complexes of L-cysteine and D-penicillamine with nickel(II) and zinc(II) ions. t = 25°; I = 0.2 M(KCI) nickel(II) D-penicillamine log/3~o log/3~20
-22.92+ 0.02
log/~111
log/3t2t log fll22 log/3230 log fl34o log ~341
--
27.06-+0.05 ---
[MpAqH,] *//pqr= [M]p[A]q[H]r
-
-
--
zinc(II) L-cysteine
I)-penicillamine
8.7-+0.10 19.61 -+0.02 14.87-+0.04 24.02-+0.05 -30.3 -+0.10 44.51 -+0.02 --
9.66-+0.05 19.39-+0.02 14.80-+0.05 25.23-+0.05 30.65-+0.05 --
-
--
L-cysteine 8.2-+0.10 18.05-+0.02 14.76-+0.05 24.43-+0.05 29.93-+0.05 29.20-+0.I0 42.11 + 0.02 49.01 -+0.02
Complexes of sulphur-containingligands--lI
1631
o L cysteme + D pemclllamine
14
12 A47o I 0
+
0.8 O6 04 02 I
I
I
I
20
I
40
I
I
I
,SO
I
I
80
I00
KOH%
Fig. 2. Variation of the absorbances of the solutions as a function of the concentration of added potassium hydroxide. Cs.m = 5.10 SM; Gcw = Cp. = 10 2M.
'°°Io+
o L - cysteine
+ D- penicillomine
to
\ o +
:=L 6 0
40
o~+ °
20
I
I
I
I0
20
30
1
40
I
I
I
I
I
50
60
70
80
90
O '~111
I00
KOH %
Fig. 3. Variation of the resultant magnetic moments of the solutions as a function of the concentration of added potassium hydroxide. CN,m= 2.10 2 M; C,.,.s= Cp,, = 4.10 2 M. the other hand, Fig. 2 shows that the variation of the spectral data can be characterized by a saturation curve in the nickel(II)-D-penicillamine system. It can be concluded from this that the concentration of the polynuclear species is very low in this system. These findings are confirmed by the magnetic measurements. For the complexes of cysteine, the decrease of the resultant magnetic moment is higher than in the nickel(II)-D-penicillamine system, which is characterized by a linear decrease in accordance with the formation of the species NiA2. This can be interpreted by the greater metal ion consumption of NisA4 species, and proves the diamagnetic character of the polynuclear complexes. Consequently, the equilibrium, spectral and magnetic measurements showed an obvious difference between the complex-forming capabilities of cysteine and Dpenicillamine. Polynuclear complexes are formed with cysteine in all cases, but can be detected only by a very sensitive spectrophotometric method in the case of D-
penicillamine. This phenomenon may be explained by structure (1), suggested by Jicha and Busch[17] for complexes of cysteamine.
N
M N
s
S
/ \ J M
N
M
S
s
N
I
In (I) the methyl substituents on the/3-carbon atom of D-penicillamine tend towards the central metal ion, so the formation of complexes M3A4 is hindered for steric reasons. The mixed ligand complexes were investigated with D-penicillamine as ligand ,_A,and glycine, histamine or histidine as ligand B_. Hence, it was possible to study the
1632
I. SrvAGOet
effect of the amino acids and imidazole-N donor atom on the stabilities of the mixed complexes. The equilibrium data on mixed ligand complexes involving D-penicillamine are given in Table 2. It is clear from Table 2 that equilibrium evaluation of the nickel(II)-D-penicillamine-glycine system was possible without the formation of a mixed ligand complex. This is in good agreement with the observation of Chang and Martin[9], who did not find mixed complex formation in the nickel(II)-cysteine-ethylenediamine system. Since the amino acids and simple diamines form octahedral complexes with the nickel(II) ion[18, 19], the absence of the species NiAB can be explained by "Kida's rule"[ll]. On the other hand, the equilibrium data concerning the mixed ligand complexes of histamine and histidine seem to be in contradiction with this rule. As Fig. 4 shows, the mixed ligand complexes are formed in low concentration, but the measured and calculated titration curves could be fitted on the assumption of the species NiAB. The possibility of the formation of mixed ligand complexes was also proved by the magnetic measurements detailed in Table 3. In addition to the experimental data, this Table also contains the values calculated on the basis of the stability constants, assuming that glycine and histidine form octahedral parent complexes. The data on the glycine complexes reveal that the calculated and measured values are in good agreement when there is no mixed ligand complex formation. At the same time, in the nickel(II)-D-penicillamine-L-histidine system the experimental data can be described by the formation of diamagnetic mixed ligand complexes. This phenomenon can be interpreted by the stronger ligand fields of histamine and histidine [20]. In addition, an octahedral-planar equilibrium in solutions of the parent complexes[21] can also play an important role in the formation of the species NiAB. The octahedral form is probably predominant in the parent complexes of histamine and histidine. However, a very small amount of the planar form can make mixed ligand complex formation possible with another planar complex. It is clear from Table 2 that the stability constants of mixed complexes of the zinc(II) ion generally correspond to statistical expectation. This can be explained in that structural effects are not involved in the complex formation, as a consequence of the d '° electronic configuration. Thus, the stabilities of the mixed iigand complexes are influenced by other factors, especially the properties of the donor atoms [22] and steric effects [23]. In this respect, the data concerning the three different B Table 2. Stability constants of mixed ligand complexes of Dpenicillamine with nickel(II) and zinc(II) ions. t=25°; I = 0.2 M(KCI) nickel(lI)
zinc(II)
ligand _B glycine histidine histamine
Iog/3m
Alog/3m
logflm
AIog/3m
-18.5± 0.2 17.0±0.2
--0.9 -0.7
13.51 -+0.05 15.14± 0.05 14.61±0.05
-0.33 -0.11 0.13
[MAB] tim [M]IA]IB] =
I A log tim = log fl etxl ~t - ~(Iog f12 + log flz ~ + log 4)
al.
60
I|Niz+ /
~NiB +
50
40
Z 20
I0
4.0
4.5
5.0
5.5
6.0
pH
Fig. 4. Concentration distribution of the complexes formed in the nickel(II)-D-penicillamine-L-histidine system. CNitm= Cpa = 5.10-3; Chd = 10-2M. Table 3. Percentage of paramagnetic centres in nickel(lI)-Dpenicillamine-histidine(or glycine)systems Compound Ni(II):D-penicillamine:
# calc %
# expt %
/~m = 0 Iog/~m= 18.5
histidine = 1 : 1 : I
Ni(ll): D-penicillamine: histidine= 1: 1 : 2 Ni(ll): D-penicillamine: glycine= 1: I: 1
46
51
44
63 tim = 0
59 --
58 51
51
ligands seem to support the view that penicillamine complexes of type MA promote the coordination of aromatic nitrogen donors as compared with the amino acids. In addition, the formation of the mixed ligand complexes is also indicated by the stability data on the corresponding parent complexes. The protonated complex MA2H, which is formed in both nickel(II)-D-penicillamine and zinc(II)-D-penicillar,line systems, must contain at least two types of the ambidentate ligands, bonded in different manners. In the pH region in question (4 < pH < 8) the carboxylic group cannot be protonated, so both an (S, N) and an (S, O) or an (N, O) bonded ligand are coordinated to the metal in the species MA2H. The amount of the protonated complex is 5-10% in the nickei(II)-D-penicillamine system, and about 30% in the zinc(II)-D-penicillamine system. Thus, in the ease of the zinc(II) ion the possibility of the mixed bonding is much more considerable, which is accordance with the higher stability constants of the corresponding mixed ligand complexes.
Complexes of sulphur-containing ligands--II
REFERENCES 1. O. Szazukin and S. M. Navarin, Antibiotiki. 6, 562 (1%5). 2. C. A. McAuliffe and S. G. Murray, Inorg. Chim. Aeta Rev. 103, (1972). 3. A. Gergely and I. S6v~ig6, Bioinorg. Chem. 9, 47 (1978). 4. G. R. Lenz and A. E. Martell, Biochemistry. 3, 745 (1%4). 5. E. J. Kuchinskas and Y. Rosen, Archly. Biochem. and Biophys. 97, 370 (1%2). 6. D. D. Perrin and I. G. Sayce, J. Chem. Soc. (A), 53 (1%8). 7. S. K. Srivastava, E. V. Rajn and H. B. Mathur, J. lnorg. Nucl. Chem. 35, 253 (1973). 8. J. H. Ritsma and F. Jellinek, Recueil. 91,923 (1972). 9. J. W. Chang and R. B. Martin, J. Phys. Chem. 73, 4277 (1%9). 10. A. M. Corrie, M. D. Walker and D. R. Williams, J. Chem. Soc. Dalton Trans. 1012 (1976). 11. S. Kida, N. Maki and R. Tsuchida, Bull. Chem. Soc. Japan. 32,573 (1959). 12. Y. Sugiura and H. Tanaka, Chem. Pharm. Bull. 18, 746 (1970).
1633
13. I. Nagyp~il, A. Gergely and E. Farkas, J. lnorg. Nucl. Chem. 36, 699 (19741. 14. I. Nagypfil, Acta Chim. Acad. Sci. Hung. 82, 29 I1974t. 15. J. L61iger and R. Scheffold, J. Chem. Educ. 49, 646 11972/. 16. Y. Sugiura, A. Yokoyama and H. Tanaka, Chem. Pharm. Bull. 18, 693 (1970). 17. D. C. Jicha and D. H. Busch, lnorg. Chem. 1,872 11%2~. 18. I. S6vS.g6, A. Gergely and J. Posta, Acta Chim. Acad. Sci. Hung. 85, 153 (1975). 19. P. Paoletti, L. Fabrizzi and R. Barbucci, lnorg. Chim. Acta Rev. 7, 43 (1973). 20. I. S6v~.g6, A. Gergely and Y. Kiss, J. Chem. Soc. Dalton Truns, 964 (1978). 21. L. D. Pettit and J. L. M. Swash, L Chem. Soc. Dalton Trans. 697 (1977). 22. H. Sigel, Structural Aspects o[ Mixed Ligand Complex Formation in Solution; in Metal Ions in Biological Systems (Edited by H. Sigel), Vol. 2. p. 64. Marcel Dekker, New York (1973). 23. I. S6v~ig6 and A. Gergely, lnorg. Chim. Acta 20, 27 (1971:,).
COMPLEXES OF SULPHUR-CONTAINING LIGANDS--II BINARY A N D T E R N A R Y C O M P L E X E S O F D - P E N I C I L L A M I N E A N D L - C Y S T E I N E WITH NICKEL(II) A N D ZINC(II) IONS IMRE SOVA.GO,ARTHUR GERGELY,* BI~LAHARMANand TAMA,S KISS Institute of Inorganicand AnalyticalChemistry,Lajos Kossuth University, H-4010 Debrecen, Hungary
(Received 5 September 1978; received[or publication 16 May 1979) Abstract--Complexes of nickel(II) and zinc(II) with L-cysteine and D-penicillaminewere investigated by pHmetric, spectrophotometricand magneticmeasurements. The formation of polynuclearspecies M3,Mwas proved in the case of L-cysteine. Both ligands can also form protonated complexes.The concentrationof mixed species is higher in the systems containingthe zinc(II) ion. Mixed ligand complexeswere not identifiedin the nickel(II)-Dpenicillamine-glycinesystem. With L-histidineor histamineas ligand _B,the formation of mixed ligandcomplexes was detected in spite of the different structures of the parent complexes. The stability constants of the mixed complexes of zinc(II) closely correspondedto statistical expectation. INTRODUCTION Widespread investigations have long been performed on the complex-forming properties of D-penicillamine and L-cysteine, which are of outstanding importance biologically and therapeutically[l]. Earlier results were summarized in a review published in 1972 by McAuliffe and Murray [2]. The study of the interaction of copper(I[) and D-penicillamine lies at the centre of these investigations; this system is complicated by the fact that complex formation and redox reactions take place simultaneously. The possible pathways and the products formed in this system were reported earlier[3]. A deeper understanding of this type of chemical reaction, however, requires extension of the investigations to complexes of other. transition metal ions (particularly nickel(II) and zinc(ll) ions) which are not susceptible to redox reactions. The first investigations in this field were those of Lenz and Martell[4] and Kuchinskas and Rosen[5], who determined the stability constants of the transition metal complexes with cysteine and D-penicillamine. However, their data can be considered only as approximative, because the calculations were made on the assumption merely of parent complexes MA and MA2. The formation of polynuclear species in systems containing cysteine and nickel(II) or zinc(II) ions was first proved by Perrin and Sayce [6]. In addition, they found that the equilibrium relations of the complexes of D-penicillamine can be described by the formation of mononuclear complexes. On the other hand Srivastava et a/.[7] concluded in good agreement with other investigators[8, 9] that the complexes NiA2 are planar diamagnetic compounds. On the basis of thermodynamic and spectroscopic measurements, the former authors [7] reported that at a high ligand/metai ion ratio the formation of polynuclear species is negligible in the nickel(II)--cysteine system too. Recently, however, Corrie et a/.[10] also assume the formation of polynuclear complexes in the zinc(II)-D-penicillamine system. *Author for correspondence.
As stated the equilibrium studies on the parent complexes of cysteine and D-penicillamine are somewhat contradictory, while the mixed ligand complexes of sulphur-containing amino acids have barely been investigated. According to the CD measurements of Chang and Martin[9], there is no mixed ligand complex formation in the nickei(II)-cysteine-ethylenediamine system. This is in good agreement with the observations of Kida et a/.[ll] that mixed ligand complexes are not formed when one of the parent complexes is octahedral and the other is planar. The complexes of cadmium(II), mercury(II) and iead(II) with D-peniciilamine and other sulphur-containing ligands were investigated by Sugiura and Tanaka[12]. Their results on the effect of the sulphur donor atom on the mixed ligand complex formation, however, can scarcely be generalized, because they used very similar types of ligands. The aim of the present work was to study mixed ligand complex formation between sulphur-containing amino acids (L-cysteine and D-peniciUamine) and other types of ligands (glycine, histamine and L-histidine) and nickel(II) and zinc (II) ions. Because of the contradictions in the equilibrium data on the parent complexes, a structural and thermodynamic investigation of the binary systems was also needed. EXPERIMENTAL D-penicillamine (Fluka), L-cysteine hydrochloride monohydrate (Merck),L-histidinehydrochloridemonohydrate,histamine dihydrochlorideand glycine (p,a. reagents of Reanal) were used for the experiments. Before use, the reagents were further purified by recrystallizationfrom ethanol-wateror from acetonewater. NiCl2 and ZnCI2 stock solutions were prepared from p.a. Reanal reagents, and their concentrations were determined by gravimetry via the oxinates. The concentrations of the stock solutions of hydrochloric acid and potassium hydroxide were measured pH-metrically as described previously[13]. All measurements were carried out at 25-+0.1°C; the ionic strength was adjusted to 0.2 M with KCI. The determination of stability constants was performed as published earlier[13]. A Radiometer priM-64 digital pH-meter 1629
1630
I. SrvAGraaL
with G202B glass and K401 calomel reference electrodes was used for the measurements. The pH-metric examinations were evaluated by the method previously described[14]. The parent and mixed ligand complexes of the nickel(II) ion were investigated by spectrophotometric and magnetic measurements. Visible and u.v. spectra of the complexes were recorded in aqueous solutions with a Beckman Acta M-IV spectrophotometer. The resultant magnetic moments of samples containing the nickel(lI) ion were measured in aqueous solutions using the NMR method proposed by Lfliger and Scheffold[15]. These measurements were performed with a Jeol MH-100 NMRspectrometer. RESULTS AND DISCUSSION The pH-metric equilibrium investigations were carried out at various metal ion/ligand ratios for the complexes of both metal ions ( 1 0 - 3<- CM < 10-2M; and 4.10-3< CA < 10-2 M): 150-200 exPerimental data were available for the evaluation in every system. In contrast to the results of some other earlier investigations[7, 8, 16], our titration curves could be described by the assumption of the protonated and polynuclear complexes: when the species listed in Table 1 were taken into account, the average difference between the measured and calculated titration curves was less than 0.008 cm 3. It can be concluded from the data in Table 1 that our results show a similar pattern to those of Perrin and Sayce[6]. The formation of polynuclear species is characteristic of the complexes of cysteine, but protonated complexes can be formed with penicillamine too. However, there are some slight differences between our equilibrium data and those obtained by the former authors[6]. According to our results, particularly the polynuclear complexes with a composition of M3A4 were formed. The concentration of the species M2A3 was considerably less than 5% in all cases, and the evaluation of the experimental data was hardly influenced by neglecting this species. In addition, on the basis of the data in Table 1, the concentration of the protonated complexes is very high in the case of the zinc(II) ion, and the protonated polynuclear complex Zn3A4H is also formed in high concentration, in good agreement with the observations of Corrie et a/.[10]. The complicated equilibrium relations of these systems are illustrated in Fig. 1 for the example of the zinc(II)--cysteine system. According to Fig. 1, the number of complexes formed in these systems is very high, and therefore magnetic and spectrophotometric measurements were also carried out to prove the formation of polynuclear complexes. In the
5o
o~ 40
~
Zn3A4HZnA~-
30
/V 5.0
5.5
6.0
6.5
pH
Fig. 1. Concentration distribution of the complexes formed in the zinc(II)-L-cysteine system. Cz,(.) = 5.10-3 M; Coy,= 10-2 M. course of these investigations, the resultant magnetic moments and the absorbances of the solutions containing nickel(II) ion and l)-penicillamine or cysteine at a metal ion: ligand ratio of 1:2 were measured as a function of the concentration of potassium hydroxide added to the samples. The variation in the absorbance values measured at 470 NM is shown in Fig. 2, while Fig. 3 presents the variation of the resultant magnetic moments of the solutions. It can be seen from Fig. 2 that the absorbance values pass through a maximum in the nickel(II)--cysteine system. This can be explained by taking into account the very intense charge-transfer band near the d-d transitions. The energy of the charge-transfer transition will be decreased in the sulphur-bridged polynuclear complexes, which contain even more easily excitable sulphur atoms. Therefore, the charge-transfer band will in part be built upon the d-d transitions of the nickel(II) ion. On
Table 1. Stability constants of the parent complexes of L-cysteine and D-penicillamine with nickel(II) and zinc(II) ions. t = 25°; I = 0.2 M(KCI) nickel(II) D-penicillamine log/3~o log/3~20
-22.92+ 0.02
log/~111
log/3t2t log fll22 log/3230 log fl34o log ~341
--
27.06-+0.05 ---
[MpAqH,] *//pqr= [M]p[A]q[H]r
-
-
--
zinc(II) L-cysteine
I)-penicillamine
8.7-+0.10 19.61 -+0.02 14.87-+0.04 24.02-+0.05 -30.3 -+0.10 44.51 -+0.02 --
9.66-+0.05 19.39-+0.02 14.80-+0.05 25.23-+0.05 30.65-+0.05 --
-
--
L-cysteine 8.2-+0.10 18.05-+0.02 14.76-+0.05 24.43-+0.05 29.93-+0.05 29.20-+0.I0 42.11 + 0.02 49.01 -+0.02
Complexes of sulphur-containingligands--lI
1631
o L cysteme + D pemclllamine
14
12 A47o I 0
+
0.8 O6 04 02 I
I
I
I
20
I
40
I
I
I
,SO
I
I
80
I00
KOH%
Fig. 2. Variation of the absorbances of the solutions as a function of the concentration of added potassium hydroxide. Cs.m = 5.10 SM; Gcw = Cp. = 10 2M.
'°°Io+
o L - cysteine
+ D- penicillomine
to
\ o +
:=L 6 0
40
o~+ °
20
I
I
I
I0
20
30
1
40
I
I
I
I
I
50
60
70
80
90
O '~111
I00
KOH %
Fig. 3. Variation of the resultant magnetic moments of the solutions as a function of the concentration of added potassium hydroxide. CN,m= 2.10 2 M; C,.,.s= Cp,, = 4.10 2 M. the other hand, Fig. 2 shows that the variation of the spectral data can be characterized by a saturation curve in the nickel(II)-D-penicillamine system. It can be concluded from this that the concentration of the polynuclear species is very low in this system. These findings are confirmed by the magnetic measurements. For the complexes of cysteine, the decrease of the resultant magnetic moment is higher than in the nickel(II)-D-penicillamine system, which is characterized by a linear decrease in accordance with the formation of the species NiA2. This can be interpreted by the greater metal ion consumption of NisA4 species, and proves the diamagnetic character of the polynuclear complexes. Consequently, the equilibrium, spectral and magnetic measurements showed an obvious difference between the complex-forming capabilities of cysteine and Dpenicillamine. Polynuclear complexes are formed with cysteine in all cases, but can be detected only by a very sensitive spectrophotometric method in the case of D-
penicillamine. This phenomenon may be explained by structure (1), suggested by Jicha and Busch[17] for complexes of cysteamine.
N
M N
s
S
/ \ J M
N
M
S
s
N
I
In (I) the methyl substituents on the/3-carbon atom of D-penicillamine tend towards the central metal ion, so the formation of complexes M3A4 is hindered for steric reasons. The mixed ligand complexes were investigated with D-penicillamine as ligand ,_A,and glycine, histamine or histidine as ligand B_. Hence, it was possible to study the
1632
I. SrvAGOet
effect of the amino acids and imidazole-N donor atom on the stabilities of the mixed complexes. The equilibrium data on mixed ligand complexes involving D-penicillamine are given in Table 2. It is clear from Table 2 that equilibrium evaluation of the nickel(II)-D-penicillamine-glycine system was possible without the formation of a mixed ligand complex. This is in good agreement with the observation of Chang and Martin[9], who did not find mixed complex formation in the nickel(II)-cysteine-ethylenediamine system. Since the amino acids and simple diamines form octahedral complexes with the nickel(II) ion[18, 19], the absence of the species NiAB can be explained by "Kida's rule"[ll]. On the other hand, the equilibrium data concerning the mixed ligand complexes of histamine and histidine seem to be in contradiction with this rule. As Fig. 4 shows, the mixed ligand complexes are formed in low concentration, but the measured and calculated titration curves could be fitted on the assumption of the species NiAB. The possibility of the formation of mixed ligand complexes was also proved by the magnetic measurements detailed in Table 3. In addition to the experimental data, this Table also contains the values calculated on the basis of the stability constants, assuming that glycine and histidine form octahedral parent complexes. The data on the glycine complexes reveal that the calculated and measured values are in good agreement when there is no mixed ligand complex formation. At the same time, in the nickel(II)-D-penicillamine-L-histidine system the experimental data can be described by the formation of diamagnetic mixed ligand complexes. This phenomenon can be interpreted by the stronger ligand fields of histamine and histidine [20]. In addition, an octahedral-planar equilibrium in solutions of the parent complexes[21] can also play an important role in the formation of the species NiAB. The octahedral form is probably predominant in the parent complexes of histamine and histidine. However, a very small amount of the planar form can make mixed ligand complex formation possible with another planar complex. It is clear from Table 2 that the stability constants of mixed complexes of the zinc(II) ion generally correspond to statistical expectation. This can be explained in that structural effects are not involved in the complex formation, as a consequence of the d '° electronic configuration. Thus, the stabilities of the mixed iigand complexes are influenced by other factors, especially the properties of the donor atoms [22] and steric effects [23]. In this respect, the data concerning the three different B Table 2. Stability constants of mixed ligand complexes of Dpenicillamine with nickel(II) and zinc(II) ions. t=25°; I = 0.2 M(KCI) nickel(lI)
zinc(II)
ligand _B glycine histidine histamine
Iog/3m
Alog/3m
logflm
AIog/3m
-18.5± 0.2 17.0±0.2
--0.9 -0.7
13.51 -+0.05 15.14± 0.05 14.61±0.05
-0.33 -0.11 0.13
[MAB] tim [M]IA]IB] =
I A log tim = log fl etxl ~t - ~(Iog f12 + log flz ~ + log 4)
al.
60
I|Niz+ /
~NiB +
50
40
Z 20
I0
4.0
4.5
5.0
5.5
6.0
pH
Fig. 4. Concentration distribution of the complexes formed in the nickel(II)-D-penicillamine-L-histidine system. CNitm= Cpa = 5.10-3; Chd = 10-2M. Table 3. Percentage of paramagnetic centres in nickel(lI)-Dpenicillamine-histidine(or glycine)systems Compound Ni(II):D-penicillamine:
# calc %
# expt %
/~m = 0 Iog/~m= 18.5
histidine = 1 : 1 : I
Ni(ll): D-penicillamine: histidine= 1: 1 : 2 Ni(ll): D-penicillamine: glycine= 1: I: 1
46
51
44
63 tim = 0
59 --
58 51
51
ligands seem to support the view that penicillamine complexes of type MA promote the coordination of aromatic nitrogen donors as compared with the amino acids. In addition, the formation of the mixed ligand complexes is also indicated by the stability data on the corresponding parent complexes. The protonated complex MA2H, which is formed in both nickel(II)-D-penicillamine and zinc(II)-D-penicillar,line systems, must contain at least two types of the ambidentate ligands, bonded in different manners. In the pH region in question (4 < pH < 8) the carboxylic group cannot be protonated, so both an (S, N) and an (S, O) or an (N, O) bonded ligand are coordinated to the metal in the species MA2H. The amount of the protonated complex is 5-10% in the nickei(II)-D-penicillamine system, and about 30% in the zinc(II)-D-penicillamine system. Thus, in the ease of the zinc(II) ion the possibility of the mixed bonding is much more considerable, which is accordance with the higher stability constants of the corresponding mixed ligand complexes.
Complexes of sulphur-containing ligands--II
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13. I. Nagyp~il, A. Gergely and E. Farkas, J. lnorg. Nucl. Chem. 36, 699 (19741. 14. I. Nagypfil, Acta Chim. Acad. Sci. Hung. 82, 29 I1974t. 15. J. L61iger and R. Scheffold, J. Chem. Educ. 49, 646 11972/. 16. Y. Sugiura, A. Yokoyama and H. Tanaka, Chem. Pharm. Bull. 18, 693 (1970). 17. D. C. Jicha and D. H. Busch, lnorg. Chem. 1,872 11%2~. 18. I. S6vS.g6, A. Gergely and J. Posta, Acta Chim. Acad. Sci. Hung. 85, 153 (1975). 19. P. Paoletti, L. Fabrizzi and R. Barbucci, lnorg. Chim. Acta Rev. 7, 43 (1973). 20. I. S6v~.g6, A. Gergely and Y. Kiss, J. Chem. Soc. Dalton Truns, 964 (1978). 21. L. D. Pettit and J. L. M. Swash, L Chem. Soc. Dalton Trans. 697 (1977). 22. H. Sigel, Structural Aspects o[ Mixed Ligand Complex Formation in Solution; in Metal Ions in Biological Systems (Edited by H. Sigel), Vol. 2. p. 64. Marcel Dekker, New York (1973). 23. I. S6v~ig6 and A. Gergely, lnorg. Chim. Acta 20, 27 (1971:,).