## 1 Introduction  
L Lignin comprises polymerized phenolic and aliphatic-hydroxyl groups, being the most abundant renewable source of aromatic polymers in nature [1, 2]. Historically, the degradation of lignin represented a major target in the pulp industry [3]. In order for the pulp to reach an acceptable brightness level, the residual lignin after the Kraft process was bleached either to remove it from the pulp or, alternatively, to decrease absorption by the residual chromophoric groups as much as possible [4]. Of all the processes involved in the manufacturing of pulp for the paper industry, the bleaching stage to degrade lignin is the step which contributes most to environmental pollution [5, 6]. Several alternative bleaching processes have been developed to date including those using chlorine-free oxidants to make this challenging process more environmentally friendly [7, 8, 9]. A general problem with bleaching the small percentages of lignin and colored chromophores in a bulk matrix of cellulose is the lack of selectivity of the oxidation reaction. Therefore, the driving force in the development of new bleaching technologies is to produce pulps with the best brightness,lowest lignin and chromophore contents, while preserving the yield and strength of the fibers by minimizing cellulose degradation. Different sites in the cellulose matrix are susceptible to attack by oxidative agents: the damage may occur within the polysaccharide chains (oxidizing carbon positions of the monomeric sugar units to carbonyl groups which give rise to alkali-labile glycosidic bonds), or alternatively the end groups (oxidizing aldehyde groups that are formed after each cleavage of the glycosidic linkage) can be targeted. In addition to lignin disposal in the pulp industry, another major goal in this field is the development of green and efficient methods to transform lignin into fuels and high value-added chemicals [10; 11; 12]. The polymeric structure of lignin provides an attractive renewable source of aromatic chemicals [13; 14]. Catalysis is regarded as a key technology to fulfil the promise of lignin valorization. Catalytic systems for the reduction of lignin generally produce bulk chemicals with reduced functionality, whereas catalytic systems for lignin oxidation produce fine chemicals with increased functionality [15; 16]. The chemical processes occurring in pulp with dioxygen and hydrogen peroxide under alkaline conditions are quite similar. When oxygen reacts with the substrate, it may be reduced to water by four successive one-electron transfer steps giving rise to intermediate products such as hydroperoxyl radicals, hydrogen peroxide, hydroxyl radicals and their organic counterparts. The reaction of the peroxo intermediates can proceed heterolytically (formation of ionic species) or homolytically (formation of radical species). Controlling these two pathways and favoring the heterolytic pathway is a key of selectivity. We previously reported several homogeneous catalysts for the oxidation of lignin model compounds and these systems were inspired by examples of efficient catalyst systems capable of performing transformations that are already found in nature [17; 18; 19; 20]. Manganese(III) Schiff base complexes have long been known to be effective catalysts for the oxygenation of both saturated and unsaturated hydrocarbons [21; 22]. Oxygen-nitrogen-nitrogen-oxygen (ONNO) Schiff base ligands with strong chelate effects are able to stabilize the manganese ion in various oxidation states since the two donor atoms of the chelated Schiff base exert two opposite electronic influences: the phenolate oxygen is a hard donor that is known to stabilize the higher oxidation state of the metal ion, whereas the imine nitrogen is a softer donor with a tendency to stabilize the lower oxidation state [23; 24].