The effect of denticity on the electrochemistry and oxygenation kinetics of polydentate Schiff base complexes of manganese

Manganese(II) and (III) complexes of potentially bidentate and tridentate Schiff base ligands have been prepared. The ligands were prepared from substituted salicylaldehyde or pyridine-2-carboxaldehyde and amines with hydrocarbon or alkylpyridyl substituents. The electrochemistry and the oxygenation kinetics of these and similar tetradentate, pentadentate, and hexadentate complexes have been studied.

The electrochemistry of the majority of the complexes involves the Mn(III)/Mn(II) couple. However, varying the solvent shows that electron transfer is often accompanied by slow changes in the number of solvent molecules coordinated to the metal or changes in the actual denticity of the ligand.

Activation energies and entropies for the reactions with 0₂ show that a large number of parameters influence the rate of reaction. Primary among these is competition between 0₂, solvent molecules, and donor atoms from the ligands for coordination sites on the metal. However, the reactions were all (with one exception) found to be first order in both complex and 0₂, implying that the slow step is formation of a Mn(III)-superoxo complex. The exception was with complexes of the tetradentate Mn(SALC_n) type, where a simple rate law could not be fitted. This was explained by either steric hindrance or polymerization of the complex due to the flexibility imparted by the long polymethylene chain in the tetradentate ligand.