Znorganica Chimica Acts, I55 (1989) 281-287 281

Complexes of Aminophosphonates. II. Transition Metal Complexes of Aminophosphonic Acid Analogues of Aspartic Acid and Glutamic Acid

TAMAS KISS*, ETELKA FARKAS

Department of Inorganic and Analytical Chemistry, Lajos Kossuth University, H-4010 Debrecen, Hungary

and HENRYK KOZEOWSKI

Institute of Chemistry, University of Wroclnw, JO-383 Wroclirw, Poland

(Received February 4,1988; revised June 24,1988)

Abstract

The stoichiometries and stability constants of the proton, cobalt(II), nickel(H), copper(I1) and zinc(II) complexes of 2-amino-3-phosphonopropionic acid @-P-Asp), 3-amino-3-phosphonopropionic acid (a-P- Asp) and 2-amino4-phosphonobutanoic acid (7-P- Glu) have been determined pH-metrically at 25 “C and at an ionic strength of 0.2 mol drnm3 (KCl).

From the stability data and the spectral param- eters of the complexes, it has been established that, similarly as for the aminocarboxylate analogues, the metal ion bonding mode is basically bidentate with P-Glu and tridentate with P-Asp. However, the more basic character of the -POs’- group than that of the -COO- group leads to an enhanced formation of various protonated complexes. P0s2-/COO-substitu- tion in the a-position results in a more marked ambi- dentate character of P-Asp in the copper(I1) com- plexes than does that in the &position.

Introduction

As analogues of natural amino acids, aminophos- phonic acids are of considerable interest because of their occurrence in many living organisms and because of their biological activity.

We have recently reported results on complexes of simple aminophosphonic acids with several 3d transi- tion metal ions [ 11. The differences between the complex-forming properties of aminophosphonates and their aminocarboxylate analogues were explained in terms of the differences in basicity, charge and size of the -POs2- and -COO- groups. In the present work, the study has been extended to more complex, potentially tridentate ligands containing both -POa*-- and -COO- groups besides an amino group: 2-amino-3-phosphonopropionic acid (P-P-Asp), 3-

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amino-3-phosphonopropionic acid (a-P-Asp) and 2-amino4-phosphonobutanoic acid (y-P-Glu).

These ligands are formal analogues of aspartic acid (Asp) and glutamic acid (Glu), which form a variety of complexes with transition metal ions [2]. From potentiometric and spectral studies with transition metal ions, it is clear that Asp readily forms com- plexes involving a tridentate bonding mode, while Glu coordinates to metal ions only in a bidentate way, via the amino and a-carboxylate groups [2].

The aim of the present work was to establish the stoichiometries, stability constants and bonding modes of the species formed in the systems con- taining cobalt(II), nickel(H), copper@) and zinc(I1) with the following tridentate aminophosphonocar- boxylic acids: a-P-Asp, P-P-Asp and y-P-Glu. In this way we can achieve a better comparison of the complex-forming properties of aminophosphonates and aminocarboxylates, for in these ligands all three donor groups are present in the same molecule.

Experimental

The aminophosphonic acids were obtained by the method described in ref. 3. In the case of 4-amino4- phosphonobutanoic acid (ru-P-Glu), considerable pyroglutamization took place and the pure com- pound could not be isolated either by recrystalliza- tion or by other purification methods. Hence, its complex-formation reactions were not studied. The purities and the exact concentrations of the solutions of the other aminophosphonates were determined pH-metrically by the method of Gran [4]. The con- centrations of the metal chloride stock solutions were measured gravimetrically via precipitation of the oxinates.

The stability constants of the proton and the metal complexes of the ligands were determined by pH-metric titration of 5 cm3 samples. The concentra- tion of the ligand in the samples was 4 X 10e3 mol

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Complexes of Aminophosphonates

stability of a seven-membered chelate ring) suggests a lower extent of axial coordination for P-P-Asp too, the spectral change is highly indicative of some extent of P-P-Ala-like coordination in the equatorial plane. This bonding mode, however, is rather unfavoured (see the stability sequence above), and thus the equilibrium is presumably shifted mainly in the direc- tion of the upper arrow. It is worth mentioning that Khazaeli and Viola [ 131 suggested a similar bonding mode for 0-phosphono-L-serine (which is the /3-phos- phoric acid derivative of Asp) with copper(H).

The stepwise deprotonation processes of complex CUE of B-P-Asp overlap each other considerably (P&xI(HA), = 6.7, pK&A,.r = 6.9) and differ from the pKPo,n- of the free ligand, which likewise sug- gests, that these deprotonation processes are ac- companied by a structural rearrangement of the coor- dination sphere of the metal complex.

Conclusions

The differences in basicity, charge, electron- releasing effect and size of the phosphonate and car- boxylate groups result in the following differences in the complex-forming properties of Asp and Glu and their phosphonic acid analogues.

(1) The more basic character of the -PO,‘-group leads to the formation of various protonated 1: 1 and 1:2 complexes, partly involving monodentate coor- dination of the ligand via the phosphonate group or aminocarboxylate-like coordination with protonated -POSH group(s).

(2) For glutamic acid, POs2-/COO- substitution in the r-position does not significantly change the binding ability of the ligand. The bonding mode remains bidentate, of (NH2, COO-) type, although coordination of the second or the third ligand [in the case of nickel(H)] is less favoured for electrostatic reasons.

(3) For aspartic acid, the POs2-/COO- substitu- tion in any position does not generally change the tri- dentate (NH2, COO-, POa2-) coordination of the ligands, although the electrostatic and steric effects hinder the coordination of the second ligand with all metal ions.

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(4) For the copper(I1) complexes, similarly as with the aminocarboxylate Asp, axial coordination occurs with the phosphonate derivatives, although its extent seems to be less for the complexes of P-P-Asp.

(5) There is a possibility of ambidentate (amino- carboxylate or aminophosphonate) coordination of the ligands, which results in the co-existence of species with the same stoichiometric composition, but with different bonding modes. This is more marked when POs2-/COO- substitution is in the (Y- position.

Acknowledgements

This work was supported financially by the Hungarian Academy of Sciences, Project OTKA 240/ 86, and by the Polish Academy of Sciences, Project 01.12. The authors thank Mrs A. Gnczy for valuable assistance in the experimental work.

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