Coordination abilities of three donor groups ligand, DL-2-amino-3-phosphonopropionic acid, in copper(II) heteroligand complexes with polyamines: Implications for metal ions coordination in aqueous solution

Abstract

The stability constants and coordination modes of the copper(II) heteroligand complexes Cu2+–L1–L2 formed by ethylenediamine (en), diethylenetriamine (dien), N,N,N′,N″,N″-pentamethyldiethylene triamine (Me5dien) (=ligand L1) and DL-2-amino-3-phosphonopropionic acid (β-Asp(P)=ligand L2) have been determined in an aqueous solution (25.00°C and I=0.2moldm−3 KCl) by pH-metric, spectrophotometric and EPR methods. Heteroligand complexes with [Cu(L1)(β-Asp(P))]− stoichiometry are formed in all studied Cu2+–L1–L2 ternary systems. Additionally, in the system with en and dien the protonated [Cu(L1)(H–β-Asp(P))] species is postulated. The results indicate the tridentate coordination mode of β-Asp(P) and the tetragonal geometry for the [Cu(en)(β-Asp(P))]− species. If ligand L1 is dien or Me5dien, five-coordinated heteroligand copper(II) complexes are supported. For the [Cu(dien)(β-Asp(P))]− complex the geometry slightly deviated from square pyramidal is postulated, whereas for the [Cu(Me5dien)(β-Asp(P))]− species somewhat stronger geometry distortion is observed. In both complexes of tridentate amines, β-Asp(P) reveals similar coordination ability as α-Ala and coordinates to the copper(II) ion in the amino acid manner with the formation of five-membered chelate ring.