Reactions of co-ordinated ligands. Hydrolysis of tetramethyl ethylenediaminetetra-acetate and its copper(II) complexes

Abstract

Tetramethyl ethylenediaminetetra-acetate, Me₄edta, interacts with copper(II) ions to give the complex [Cu(Me₄edta)]²⁺ in which probably two ester groups of the ligand are bonded to the metal ion in conjunction with the two nitrogen donors. Base hydrolysis of two ester groups occurs in the pH range 4·1–5·4 with k_OH= 3·23 × 10⁵ and 7·02 × 10⁴ I mol^–1 s^–l at 25 °C and I= 0.lM. These constants apply to the hydrolysis of [Cu(Me₄edta)]²⁺ and [Cu(Me₃edta)]⁺, respectively. Hydrolysis by water also occurs in this pH range. Hydrolysis of the second pair of ester groups takes place in the pH range 7·2–9·0 with K_OH= 1·54 × 10³ and 7·47 × 10 I mol^–1 s^–1 at 25 °C. Base hydrolysis of the first ester groups of [Cu(Me₄edta)]²⁺ is ca. 10⁵ times faster than in the free ligand. The observed rate acceleration, and the results of solution i.r. studies, are consistent with a weak interaction between the metal ion and the carbonyl group of the ester. Base hydrolysis involves nucleophilic attack by ‘ external ’ hydroxide rather than attack by a hydroxide ion co-ordinated to Cu^II. The preparation of sodium salts of the partial esters Me₃edta^–, Me₂edta^2–, and Meedta^3– is described.