Research Article Synthesis, Characterization, and Catalytic Studies of Mn(III)-Schiff Base-Dicyanamide Complexes: Checking the Rhombicity Effect in Peroxidase Studies Manuel R. Bermejo, 1 Rocío Carballido, 2 M. Isabel Fernández-García, 2 Ana M. González-Noya, 1 Gustavo González-Riopedre, 2 Marcelino Maneiro, 2 and Laura Rodríguez-Silva 2 1 Department of Inorganic Chemistry, Faculty of Chemistry, Universidade de Santiago de Compostela, 15782 Santiago de Compostela, Spain 2 Department of Inorganic Chemistry, Faculty of Sciences, Universidade de Santiago de Compostela, 27002 Lugo, Spain Correspondence should be addressed to Manuel R. Bermejo; manuel.bermejo@usc.es Received 2 February 2017; Accepted 4 July 2017; Published 16 August 2017 Academic Editor: Albert Demonceau Copyright © 2017 Manuel R. Bermejo et al. Tis is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Te condensation of 3-methoxy-2-hydroxybenzaldehyde and the diamines 1,2-diphenylendiamine, 1,2-diamine-2-methylpropane and 1,3-propanediamine yielded the dianionic tetradentate Schif base ligands N,N  -bis(2-hydroxy-4-methoxybenzylidene)-1,2-di- phenylendiimine (H 2 L 1 ), N,N  -bis(2-hydroxy-4-methoxybenzylidene)-1,2-diamino-2-methylpropane (H 2 L 2 ) and N,N  -bis(2- hydroxy-4-methoxybenzylidene)-1,3-diaminopropane (H 2 L 3 ) respectively. Te organic compounds H 2 L 1 and H 2 L 2 have been characterized by elemental analysis, IR, 1 H and 13 C NMR spectroscopies and mass spectrometry electrospray (ES). Te crystal structure of H 2 L 2 in solid state, solved by X-ray crystallography, is highly conditioned in the solid state by two N-H∙∙∙N intramolec- ular interactions. Te synthesis of three new manganese(III) complexes 1–3, incorporating these ligands, H 2 L 1 –H 2 L 3 , and dicyanamide (DCA), is reported. Te complexes 1–3 have been physicochemically characterized by elemental analysis, IR and paramagnetic 1 H NMR spectroscopy, ESI mass spectrometry, magnetic moment at room temperature and conductivity measure- ments. Complex 1 has been crystallographically characterized. Te X-ray structure shows the self-assembly of the Mn(III)-Schif base-DCA complex through -aquo bridges between neighbouring axial water molecules and also by - stacking interactions, establishing a dimeric structure. Te manganese complexes were also tested as peroxidase mimics for the H 2 O 2 -mediated reaction with the water-soluble trap ABTS, showing complexes 1-2 relevant peroxidase activity in contrast with 3. Te rhombicity around the metal ion can explain this catalytic behaviour. 1. Introduction Schif base ligands stabilize diferent metal ions in solution to yield metal complexes with a variety of properties and applications. For instance, one of these ligands, the chelating salen, is known by the ability to signifcantly decrease the Mn(III)/Mn(II) redox potentials, and the resulting com- plexes constitute suitable systems to catalyse multiple redox reactions such as asymmetric epoxidation of unfunctional- ized olefns, catalase reaction, water photolysis, Diels-Alder cycloaddition, enantioselective cyclopropanation of styrenes and ring opening of epoxides [1–8]. Over the past years, a variety of applications of this type of complexes has also been reported, including a broad range of biological activities (antibacterial, antifungal, anticancer, antioxidant, anti-infammatory, etc.) [9–13]. During our search for new metal catalysts contain- ing salen-type ligands, we have observed a better cat- alytic behaviour when the substrate molecule can be easily coordinated by the complex and this is favoured when the catalyst has either a vacancy in the coordina- tion sphere or a labile ligand [14–17]. To achieve this, Hindawi Journal of Chemistry Volume 2017, Article ID 5465890, 10 pages https://doi.org/10.1155/2017/5465890