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)](ClO4)2 was prepared and characterized. The mono basic, five- or sevendentate 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)2-NH2; 2: -(CH2)2-NMe2; 3: -(CH2)2-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), Ν,Ν,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)](ClO4)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-N2O2 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 (NaClO4). 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 -1s-1 (complex 6) to 3.72x 10-2 M-1s-1 (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 H2EDTA2- 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.