COORDINATION OF CU^2± BY MONO-AMINOALKYLOXAMIDES IN AQUEOUS SOLUTION

Abstract

Complex formation equilibria of Cu^2± complexes of N-(2-aminoethyl)(oxamide, N-3-aminopropyl)oxamide, 1,8-diamino-3,6-diazaoctane-7,8-dione and 1,10-diamino-4,8-diazaoctane-9,10-dione in aqueous solution at 25°C ± 0.1°C and I = 0.1 mol dm⁻³ (KNO₃) have been studied using potentiometric and spectrometric titrimetry. Mixed ligand titrations using 2,2′-bipyridyl as the second ligand have been added in order to obtain unambiguous results. The Cu^2± complexes of the monoalkyl substituted oxamides studied can be classified into three groups: (1) CuLH₁ and CuLH₂ complexes; these complexes have a single deprotonated oxamide group in a trans configuration; (2) a CuLH₋₃ complex; this complex has a doubly deprotonated oxamide group in a cis configuration; (3) Cu₂LH₋₂, Cu₃L₂H₋₄ and Cu₃L₂H₋₅ complexs; these polynuclear complexes have the doubly deprotonated oxamide group in a trans configuration. Deprotonation of the primary amide group in the Cu₂LH₋₂ complex of these ligands occurs before pH = 5. This unprecedented deprotonation of a primary amide group under these conditions is due to the cooperation of both strong and optimally positioned coordinating groups. The concept of amide oxygen anchoring is introduced.

Key Words:

• aminoalkyloxamides
• protonation
• stability constants
• copper(II)