Kinetics and Mechanism of Ligand Substitution in Binuclear Copper(II) Complexes as Model Compounds for Hemocyanin

Abstract

A series of binuclear. hydroxyl-bridged copper(II) complexes 1-10 with the general formula [Cu2(LR2D2)(OH)](C104)2 was prepared and characterized. The mono basic, five- or seven-dentate ligands HLR2D2 were obtained either by reduction of the Schiff bases derived from the condensation of 2.6-diformyl-4-methylphenol and the amines H2N-D (complex 1: D = -(CH2,), -NH2,; 2: -(CH2)2-NMe2; 3: -(CH2-NEt2,: 4: -(CH2)3-NEt2; 5: -(CH2)2-4-imidazolyl; 6: -CH2, -2“-pyridyl; “7: -(CH2)2-2-pyridyl) or by reacting 2.6-di-(chloromethyl)-4-methylphenol with 2-[(2-methylamino)ethyl]pyridine (complex 8). N, N, N', N'-tetraethyldiethylenetriamine (complex 9) or bis[2-(2-pyridyl)ethyl]amine (complex 10) under HC1 elimination. The UV/VIS spectra of complexes 1-8 with two four-coordinate copper centers and of 9 and 10 with two five-coordinate copper centers are discussed. An X-ray structure analysis of complex 6 (= [Cu2(LR2D2)(OH)](C104)2 with R = H and D = -CH2-2-pyridyl) was carried out. 6 crystallizes in the orthorhombic space group Pbca and the two copper centers have practically co-planar, doubly O-bridged cis-N202 coordination geometries with one perchlorate anion being weakly coordinated to each copper. Stopped-flow spectrophotometry was used to study the kinetics of copper removal from [Cu2(LR2D2)(OH)]2+ with EDTA in the pH range 8.5-6.6 at 20 C and I = 0.2 M (NaCl4). The reaction is first-order in both complex and EDTA, the rate law being: rate = k|complex][EDTA]total. At pH 8.0 second-order rate constants k range from 40.3 x 103' M 1 s1(complex 6) to 3.72*10-2 M-1s-l (complex 9). It follows from the pH dependence of k that both species H2EDTA2” and HEDTA3” react independently with [Cu2(LR2D2)(OH)]2+, the reactivity of H2, EDTA2 being considerably greater than that of HEDTA3'. The experimental facts support the operation of an associative (A) mechanism with the removal of the first copper being rate-controlling. The kinetic effects of the donor group D and substituent R are attributed to steric and/or electronic interactions. © 1990, Walter de Gruyter. All rights reserved.