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 3.3 × 10⁷ and 2.9 × 10⁷ M⁻¹ s⁻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.