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-II(ClO4)2, a model compound with CuZnSOD activity, the unit (CuL2+)-L-II has a tetrahedrally distorted square-planar N-4 coordination geometry. The copper(I) complex with L was found to be dimeric, ((CuL)-L-I)(2)(ClO4)(2).DMF (DMF = N,N-dimethylformamide). The binuclear unit ((CuL)-L-I)(2)(2+) has a helical structure with two ligands L bridging the two copper atoms to provide tetrahedral N-4 coordination of each copper(I). In solutions of ((CuL)-L-I)(2)(ClO4)(2).DMF, solvent-dependent dissociation occurs according to D reversible arrow 2M (D = ((CuL)-L-I)(2)(2+); M = (CuLSx+)-L-I; S = solvent). Stopped-flow spectrophotometry was used to determine the rate constants for the dissociation of the dimer D (kM) 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 ((CuL)-L-I)(2)(2+) by Co-III(NH3)(5)Cl2+ and cis- and trans-Co-III(en)(2)Cl-2(+) follows a second-order rate law, rate = k(ox)[((CuL)-L-I)(2)(2+)][Co(III)]. Data for rate constant k(ox) and for the activation parameters Delta H double dagger and Delta S-double dagger are presented. In DMF, the oxidation of ((CuL)-L-I)22+ by Co-III(NH3)(5)Cl2+ occurs via the monomer (CuL)-L-I(DMF)(x)(+) and the dissociation of ((CuL)-L-I)(2)(2+) becomes rate-controlling. The reduction of (CuL2+)-L-II by Ru-II(edta)H2O2- 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.