Design and Fine-Tuning Redox Potentials of Manganese(II) Complexes with Isoindoline-Based Ligands: H2O2 Oxidation and Oxidative Bleaching Performance in Aqueous Solution

Bashdar I Meena,József Kaizer

doi:10.3390/catal10040404

Abstract

A series of divalent manganese complexes [MII(HL1–6)Cl2] with the 1,3-bis(2’-Ar-imino)isoindolines (HLn, n = 1–6, Ar = pyridyl, 4-methylpyridyl, imidazolyl, thiazolyl, benzimidazolyl and N-methylbenzimidazolyl, respectively) including the previously reported ligands (HL1–2, 4–6) and complexes ([MII(HL1,5)Cl2]) have been prepared and characterized by electrochemical and spectroscopic methods. In these complexes, it was possible to control the redox potential of the metal center by varying the aryl substituent on the bis-iminoisoindoline moiety, and investigate its effect in a catalase-like reaction, and oxidative bleaching process in buffered aqueous solution. The kinetics of the dismutation of H2O2 into H2O and O2, and the oxidative degradation of morin by H2O2 were investigated in buffered water, where the reactivity of the catalysts in both systems was markedly influenced by the redox and Lewis acidic properties of the metal centers and the concentration of the bicarbonate ions. Both the catalase-like and bleaching activity of the catalysts showed a linear correlation with the MnIII/MnII redox potentials. The E1/2 spans a 561 mV range from 388 mV (Ar = benzymidazolyl) to 948 mV (Ar = 4-methylpyridyl) vs. the SCE. The amount of bicarbonate is a critical issue for the in situ formation of peroxycarbonate as a versatile oxidant, and its participation in the formation of high valent MnIV = O species.

Highlights

1,3-bis(2’-Ar-imino)isoindoline and MnCl2 .4H2 O in methanol resulted in the formation of mononuclear pentacoordinate complexes with [MnII (HLn )Cl2 ] composition (Ar = pyridyl (n = 1), 4-methylpyridyl (n = 2), imidazolyl (n = 3), thiazolyl (n = 4), benzimidazolyl (n = 5) and N-methylbenzimidazolyl (n = 6))
The free and the nondeprotonated isoindoline ligands exhibit intense bands in the 1600–1660 cm−1 region that can be assigned as coupled nonspecific νC=N vibrations which can be explained by the two different endocyclic and exocyclic position of the amino groups, and one additional weak absorption around 3300 cm−1, characteristic of a nondeprotonated isoindoline ligand [1]
Absorption maxima in the UV–vis spectra of [MnII (HL1–6 )Cl2 ] that are found between 350 and 500 nm can be assigned to the π–π* transitions of the coordinated neutral ligands, except for the lowest energy bands in the range of 450–600 nm, which can be attributed to charge transfer transitions from the manganese(II) ion to a ligand π* orbital (LUMO, MLCT; Figure 1)

Summary

Introduction

1,3-bis(2’-Ar-imino)isoindolines are pincer type ligands with a tridentate coordination mode and aromatic planarity around the metal centers [1]. As a continuity of our research, we synthesized the analogs of [MnII (HL2–4, and 6 )Cl2 ] complexes as catalysts against H2 O2 , and morin by the use of H2 O2 as an oxidant in context with the ligand modification by varying the aryl substituent on the bis-iminoisoindoline moiety and redox chemistry. These systems may serve as functional models for the MnCat enzyme, and the title complexes could serve as potential candidates for oxidative delignification and/or bleaching performance

Results and Discussion

Materials and Methods

Test Reactions of the Catalase-Like Activity

Bleaching of Morin

Conclusions