Copper(II) and Copper(I) Complexes with an Open-Chain N₄ Schiff Base Ligand Modeling CuZn Superoxide Dismutase:  Structural and Spectroscopic Characterization and Kinetics of Electron Transfer

The structure of the complex [Cu^II(PuPy)](ClO₄)₂ (PuPy = L = 1,8-bis(2-pyridyl)-2,7-diazaoctadiene-1,7) and the structure of the corresponding copper(I) complex were determined. In Cu^IIL(ClO₄)₂, a model compound with CuZnSOD activity, the unit Cu^IIL²⁺ has a tetrahedrally distorted square-planar N₄ coordination geometry. The copper(I) complex with L was found to be dimeric, (Cu^IL)₂(ClO₄)₂·DMF (DMF = N,N-dimethylformamide). The binuclear unit (Cu^IL)₂²⁺ has a helical structure with two ligands L bridging the two copper atoms to provide tetrahedral N₄ coordination of each copper(I). In solutions of (Cu^IL)₂(ClO₄)₂·DMF, solvent-dependent dissociation occurs according to D ⇌ 2M (D = (Cu^IL)₂²⁺; M = Cu^ILS_x⁺; S = solvent). Stopped-flow spectrophotometry was used to determine the rate constants for the dissociation of the dimer D (k_M) and dimerization of the monomer M (k_D) for S = acetonitrile and DMF. Equilibrium constants K_dim = k_M/k_D were determined spectrophotometrically. In aqueous solution, the oxidation of the dimer (Cu^IL)₂²⁺ by Co^III(NH₃)₅Cl²⁺ and cis- and trans-Co^III(en)₂Cl₂⁺ follows a second-order rate law, rate = k_ox[(Cu^IL)₂²⁺][Co(III)]. Data for rate constant k_ox and for the activation parameters ΔH^⧧ and ΔS^⧧ are presented. In DMF, the oxidation of (Cu^IL)₂²⁺ by Co^III(NH₃)₅Cl²⁺ occurs via the monomer Cu^IL(DMF)_x⁺ and the dissociation of (Cu^IL)₂²⁺ becomes rate-controlling. The reduction of Cu^IIL²⁺ by Ru^II(edta)H₂O^2- was found to be too fast to be resolved by stopped-flow spectrophotometry. The kinetic results are discussed mechanistically in terms of the redox switch aspects of the system.