Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica Research paper A rare flattened tetrahedral Mn(II) salen type complex: Synthesis, crystal structure, biomimetic catalysis and DFT study Saikat Banerjee a , Pravat Ghorai a , Papiya Sarkar b , Anangamohan Panja c,d , Amrita Saha a, ⁎ a Department of Chemistry, Jadavpur University, Kolkata 700032, India b Chemistry and Biomimetics Group, CSIR-Central Mechanical Engineering Research Institute, M. G. Avenue, Durgapur 713209, India c Department of Chemistry, Panskura Banamali College, Panskura RS, WB 721152, India d Department of Chemistry, Gokhale Memorial Girls' College, 1/1 Harish Mukherjee Road, Kolkata 700020, India ARTICLE INFO Keywords: Mn(II) salen type complex Crystal structure Spectroscopic studies Phenoxazinone synthase like activity Theoretical calculations ABSTRACT A new flattened tetrahedral high spin Mn(II) complex (1) has been synthesized using N 2 O 4 donor Schiff base ligand. Complex 1 was characterized by X-ray diffraction and various spectroscopic techniques. For further understanding of electronic structure of the complex, DFT calculations and electrochemical studies have been performed. This is a rare example of a flattened tetrahedral Mn(II) salen type Schiff base complex. High-spin d 5 configuration of the metal center provides no crystal-field stabilization energy to the system and that is the main reason behind the significant deviation of this salen-type ligand from planarity. Notably, the propylenic linker in the ligand provides adequate flexibility so that such an uncommon binding mode of the salen type Schiff base ligand becomes possible. Complex 1 exhibits excellent catalytic property towards oxidation of o-aminophenols in aerobic condition. Detailed kinetic investigations together with the mass spectrometry studies reveal several important information relating to biomimetic catalytic activity of the present complex. 1. Introduction Manganese is the 12th most naturally occurring trace metal found in the living systems. Coordination chemistry of manganese is driven by a part of its occurrence in the active sites of several enzymes in the bio- logical systems [1–4]. For example, in photosystem-II (PS-II), manga- nese centers constitute oxygen evolving complex (OEC) which photo- lytically oxidizes water to oxygen. In the active site structures of Mn containing catalase [5–7] and peroxidase, the manganese centers are found to coordinate with N or O donor ligands [8,9]. It is clear that nature has chosen Mn in the active site of different metalloenzymes due to its rich redox properties and possibilities of presence of Mn ions in different geometries and stable oxidation states. These enzymatic ac- tivities of Mn inspired us to use its model complexes for selective oxi- dation of organic molecules. It is important to mention that synthesis of biologically-compatible, environment-friendly and energetically-effi- cient metal complexes is a challenging task for the development of new chemicals for industrial processes and subsequently facilitating the advancement of science in different fields. Oxidation process plays a crucial role in organic reaction for the synthesis of several valuable organic compounds in the fields of pharmaceuticals, agrochemicals, etc. [10–12]. Although in chemical industries mainly molecular oxygen is used as a primary oxidant, [13–17] direct oxidation of small organic molecules by molecular oxygen is still difficult because of its spin re- striction that reduces its reactivity severely with ending up of poor yield [18–21]. In this connection phenoxazinone synthase (PHS) needs spe- cial mention for its biological importance, which is a penta copper oxidase that efficiently activates molecular dioxygen at ambient con- dition to catalyze the oxidative coupling of two molecules of a sub- stituted o-aminophenol to the phenoxazinone chromophore in the final step for the biosynthesis of actinomycin D [22,23]. Actinomycin D is an aromatic heterocyclic natural product which is clinically used for treatment of choriocarcinoma, wilms tumors, rhabdomyosarcoma, and Kaposi's sarcoma [24]. So, it is important to develop metal complexes which can efficiently mimic PHS by oxidizing o-aminophenol to 2- aminophenoxazin-3-one chromophore [25]. On the other hand, Schiff base ligands are classical chelating ligands which are vigorously used to understand molecular processes occurring in biochemistry, material science, catalysis, encapsulation, activation, transport and separation phenomena, hydrometallurgy, etc. [26,27]. Their ease of synthesis and reactivity with almost all metal ions present in the periodic table make them suitable synthons for the development of coordination chemistry. Literature has witnessed rich coordination chemistry involving H 2 L(Scheme 1) ligand with reports of numerous https://doi.org/10.1016/j.ica.2019.119176 Received 27 July 2019; Received in revised form 26 September 2019; Accepted 26 September 2019 ⁎ Corresponding author. E-mail address: amritasahachemju@gmail.com (A. Saha). Inorganica Chimica Acta 499 (2020) 119176 Available online 27 September 2019 0020-1693/ © 2019 Elsevier B.V. All rights reserved. T