Dinuclear metal complexes. Part 8. Effect of remote substituents on the redox potentials of mono- and di-nuclear copper(II) complexes

Abstract

A number of mono- and di-nuclear copper(II) complexes have been synthesized from the Schiff bases obtained by condensing 2-hydroxy-5-methylbenzene-1,3-dicarbaldehyde (HA) with p-H₂NC₆H₄X (X = H, Me, OMe, Cl, COMe, or NO₂) alone or in combination with 1,3-diaminopropane. Three types of Schiff bases have been prepared by condensing (a) one of the CHO groups of the dicarbaldehyde with the aromatic amines, (b) both CHO groups with the same or different arylamines, and (c) one CHO with 1,3-diaminopropane and the other with p-H₂NC₆H₄X. The e.s.r. spectra of the mononuclear complexes show the absence of tetrahedral distortion. The formal redox potentials (E_f¹, E_f²) of the complexes are influenced by the nature of X, the reduction taking place more easily with electron-withdrawing substituents. The linear free-energy relation obtained by plotting E_f¹ or E_f²vs.Σσ_p(Hammett parameters) indicates that the electron-transfer reactions are governed by inductive and resonance effects. The free energy of stabilization of the mixed-valence species, ΔG_c, also has a similar correlation with Σσ_p. The difference between E_f¹ and E_f² for the complexes with the ligands of type (c) are considerably greater than those obtained with type (b). The electrochemical behaviour of the dicopper(II) complexes of type (a) ligands shows the prevalence of adsorption, demetallation, and slow electrode kinetics. The dicationic Cu^II–Cu^II and cationic Cu^II–Cu^I species are respectively 0.59 and 0.18 V easier to reduce than the corresponding neutral mononuclear complexes.