The Relative Impact of Ligand Flexibility and Redox Potential on the Activity of Cu Superoxide Dismutase Mimics

Abstract

Two copper(II) complexes, [Cu(salbn)] and [Cu(py2bn)(OAc)]ClO4, formed with the Schiff-base ligands 1,4-bis(salicylidenamino)butane (H2salbn) and 1,4-bis(pyridin-2-ylmethyleneamino)butane (py2bn), have been prepared and characterized in solid state and in solution, and their ability to catalyse the dismutation of O2•− has been evaluated in homogeneous medium and immobilized in a mesoporous matrix. The crystal structures show that [Cu(salbn)] possesses a distorted square-planar geometry, while [Cu(py2bn)(OAc)]ClO4 adopts a cis-distorted octahedral geometry. The two complexes experience structural changes in solution, and different spectroscopies were used to examine them. Moreover, their redox potentials are strongly affected by the solvent. In water, the complexes exist as [Cu(salbn)(H2O)] and [Cu(py2bn)(H2O)]2+ with Cu(II)/Cu(I) reduction potential at -361 mV and -229 mV, respectively, well different from redox potentials measured in acetonitrile. Although with a more unfavourable redox potential, [Cu(salbn)(H2O)] reacts with O2•− faster than [Cu(py2bn)(H2O)]2+, with catalytic rate constants of 3.3x107 and 2.9x107 M-1s-1, respectively, at pH = 7.8. Both complexes exhibit higher superoxide dismutase activity than the analogues with a shorter central alkyl chain. The observed catalytic rates essentially correlate with the ligand flexibility, rather than with the redox potential, which is also supported by the slower O2•− dismutation rate when the complexes are immobilized by encapsulation into the channels of well-ordered mesoporous SBA-15 silica where the pore modifies the complex structures and restraints the ligand rearrangement.