Stability Constants of Ternery Complexes of Transition Metal(Ii) Ions with Aspartic Acid and Glutamic Acid as Primary Ligands and Levodopa and Methyldopa as Secondary Ligands

Introduction

Recently, there has been considerable interest in the study of binary, ternary and quaternary complexes by pH-metric method^1-3. The ligand levodopa (LDP) is well known  for its use in neurotransmission process⁴ and the treatment of Parkinson disease⁵.  The ligand methyldopa (MDP) is an antihypersensetive agent which has a depressant action on the central nervous system^6-8.

Ternary complexes of  Ni (II) and Cu (II) with nicotinic acid as primary ligand and imidazoles, benzimidazole, histamine and L-histidine as secondary ligands have been studied by Nair and Neekantan⁹. Patil and Mhaske¹⁰ have studied the stability constants of Mn (II), Co (II), Ni (II), Cu (II) and Zn (II) with nitrilotriacetic acid and Iminodiacetic acid as primary ligands and levodopa and methyldopa as secondary ligands potentiometrically. Nigam and coworkers¹¹ have studied the ternary complexes of Mn (II), Co (II), Ni (II), Cu (II) and Zn (II) using ASP as primary ligand and thymine as secondary ligand potentiometrically. Ternary complexes of Cu (II) using ASP and GLU as primary ligands have been reported potentiometrically by Pandeya and Patel¹².Kalshetti and coworkers¹³ have studies binary and ternary complexes of Co (II), Ni (II), Cu (II) and Zn (II). Mixed ligand complexes of transition metal(II) ions with N-(2-hydroxybenzylidene)-2,3-dimethylaniline as primary ligand and N-(2-hydroxy-1-naphthylidene)-4-chloroaniline as secondary ligand has been studied by Mapari and Mangaonkar¹⁴. Solution equilibria of ternary systems involving transition metal ions, hydroxamic acids, and bioligands have been studied by Khalil and Mahmoud¹⁵.

In this paper the stability constants of ternary complexes of Mn (II), Co (II), Ni (II), Cu (II) and Zn (II) ions with ASP and GLU as primary ligands and LDP and MDP as secondary ligands at 302 ±0.5 K and at fixed ionic strength, m = 0.1M KNO₃ using modified form of Irving-Rossotti pH-metric technique¹⁶ in aqueous medium have been studied.

Experimental

The ligand LDP was obtained from Loba Chemie and MDP was Merind Limited.  These ligands were  used as such. Carbonate free sodium hydroxide solution was prepared by standard method¹⁷. All other solutions were prepared in doubly distilled water.

The pH-metric measurements were carried out by using Elico digital pH-meter model L-120 with combined glass-calomel electrode with an accuracy of ± 0.01 of pH unit at 302 ± 0.5 K. The pH-meter was standardized against 0.05 M potassium hydrogen phthalate solution in acid medium and 0.01M borax solution in alkaline medium.

For determination of proton-ligand stability constant of the secondary ligand and the metal-ligand stability constants of ternary complexes, the following set of solutions were prepared and titrated against standard alkali solution.

Ternary Systems

i)    9.6 X 10^-3M HNO₃

ii)    9.6 X 10^-3M HNO₃ + 1.0 X 10^-3M secondary ligand

iii)  9.6 X 10^-3M HNO₃ + 1.0 X 10^-3M primary ligand +1.0 X 10^-3M metal ion

iv)     9.6 X 10^-3M HNO₃ + 1.0 X 10^-3M primary ligand +1.0 X 10^-3M metal ion + 1.0 X 10^-3M secondary ligand

The ionic strength was maintained constant (0.1M) by adding required volume of 1M KNO₃. The ratio of metal (M): primary ligand (A): secondary ligand (L) was maintained at 1: 1: 1 in each of the ternary system.

Results and Discussion

Proton-Ligand Stability Constants: The plots of volume of alkali (NaOH) against pH-meter readings were used to evaluate the proton-ligand stability constants of LDP and MDP. The deviation between free acid titration curve and secondary ligand titration curve was used to evaluate the formation functions . The proton-ligand formation curves were then obtained by plotting the values of  versus pH-meter readings. From the graphs, the values of
were evaluated by half integral method (method A) and point wise calculation method (method B) and presented in Table 1.

Table 1: Proton-Ligand Stability Constants of LDP and MDP

Ligand
	Method (A)	Method (B)	Method (A)	Method (B)	Method (A)	Method (B)
LDP	9.94	9.94	8.77	8.78	18.71	18.72
MDP	10.83	10.82	9.25	9.21	20.08	20.03

Metal – Ligand Stability Constants of Ternary Complexes

The metal – ligand stability constants of the ternary complexes were evaluated assuming that the formation of hydrolyzed products, polynuclear complexes, hydrogen and hydrogen bearing complexes were absent. An examination of titration curves indicates that complex formation has taken place in the solution on the following grounds:

The ternary complex titration curves show the displacement with primary complex titration curves. The horizontal distance was measured between acid curve and the secondary ligand curve (V₂ – V₁) and subtracted through the horizontal distance between ternary complex curves and primary complex titration curves (V₄ – V₃) show the positive difference which proves the earlier released of protons in the formation of ternary complexes.
The hydrolysis of the metal ions was suppressed and precipitation did not result.

The values of  vary from 0 to 1.0, thus confirming the formation of 1:1:1 mixed ligand complexes. The values of log and log have been evaluated from the formation curves ( vs. pL). At = 0.5 in the formation curve, pL = log K. The metal- ligand stability constant of LDP as secondary ligand and ASP and GLU as primary ligand are presented in Table 2.

Table 2: Metal – Ligand Stability Constants of Ternary Complexes

System
	Mn (II)		Co (II)		Ni (II)		Cu (II)		Zn (II)
	A	B	A	B	A	B	A	B	A	B
[M (II) – (ASP) (LDP)]	4.80	4.82	5.72	5.73	5.45	5.48	9.01	8.96	6.12	6.11
[M (II) – (GLU) (LDP]	4.74	4.75	6.06	6.05	6.10	6.15	9.68	9.65	6.85	6.83
[M (II) – (ASP) (MDP)]	8.67	8.70	10.20	10.21	9.64	9.66	12.07	12.05	9.42	9.41
[M (II) – (GLU) (MDP)]	8.12	8.14	9.45	9.47	9.40	9.41	12.04	12.03	9.71	9.67

The Irving-Williams order^{18, 19}of stability constants was observed in ternary complexes which is

Mn (II) < Co (II) <Ni (II) <Cu (II) > Zn (II)

This sequence of stability of complexes with respect to metal ion is due to deceasing atomic radius and increasing the second ionization potential.

Acknowledgement

The authors are thankful to UGC and Principal, P. N, College, Pusad for providing necessary facilities.

References

1. A. E. Martell and R. M. Smith, Critical Constants-1: Amino acids, New York (1974).
2. H. Sigel, Metal Ions in Biological Systems-2, Marcel-Dekker, Inc., New York (1973).
3. M. T. Beck, Chemistry of Complex Equilibria, Van Nostrand, New York, pp.174-190 (1970).
4. A Carlson, J. Neur. Trans. Suppl., 19, 153 (1983).
5. R.J. Sundberg and R. B. Martin, Chem. Rev., 74,471 (1974).
6.  The Pharmaceutical Codex, 11^th Edition, The Pharmaceutical Press, London (1979).
7. Clarke’s Isolation and Identification of Drugs, 2^nd Edition, The Pharmaceutical Press, London (1986).
8. Remington’s Pharmaceutical Science, 17^th Edition, Mack Publishing Co., Easton, Pennsylvania (1985).
9. M.S. Nair and M. A. Neelkantan, Indian J. Chem., 38A, 1307(1999).
10. A. B. Patil and T. H. Mhaske, Asian J. Chem., Vol. 13(4), pp. 1544-1548 (2001).
11. N. B. Nigam, P. C. Sinha, M. Gupta and M. N. Srivastava, Indian J. Chem., 22A, 818(1983).
12. K.B. Pandeya and R. N. Patel, Indian J. Chem., 30A, 193 (1991).
13. B.M.Kalshetty, S.S.Suvarnakhandi, B.I.Karlatti and M.B.Kalashetti, International Journal of Applied Biology and Pharmaceutical Technology,2(2),386(2011).
14. A. K. Mapari and K. V. Mangaonkar, International Journal of ChemTech Research, Vol. 3, No.2, pp 802-805 (2011).
15. M. M. Khalil and R. K. Mahmoud, J. Chem. Eng. Data,52(2), pp789-797(2010).
16. H.Irving and H. S. Rossotti, J. Chem. Soc., 76, 2904(1954).
17. A.I. Vogel, A Text Book of Quantitative Analysis, Longman, London, pp.241 (1961).
18. H.Irving and R.P.J. Williams, Nature,162,746(1948)
19. H.Irving and R.P.J. Williams, J. Chem. Soc., 76, 3192(1953).

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