Mn(II) and Zn(II) ions catalytic oxidation of o-phenylenediamine and characterization, X-ray crystal structure and solution study of the final products DAPH + Cl  3H 2 O and [DAPH][Zn(dipicH)(dipic)]4H 2 O Khaled Ghasemi a,⇑ , Fatemeh Ghasemi a , Ali Reza Rezvani a , Ardeshir Shokrollahi b , Masoud Refahi b , Santiago García-Granda c,⇑ , Rafael Mendoza-Meroño c a Department of Chemistry, Faculty of Science, University of Sistan and Baluchestan, P.O. Box 98135-674, Zahedan, Iran b Department of Chemistry, Yasouj University, Yasouj, Iran c Departamento de Química Física y Analítica, Universidad de Oviedo—CINN, 33006 Oviedo, Spain article info Article history: Received 4 November 2015 Accepted 5 February 2016 Available online 9 February 2016 Keywords: o-Phenylenediamine 2,6-Pyridinedicarboxylic acid 2,3-Diaminophenazine Crystal structure Solution study abstract Compound DAPH + Cl  3H 2 O(1), and complexes Mn(dipic)(H 2 O) 3 (2) and [DAPH][Zn(dipicH)(dipic)]4H 2 O (3), (dipicH 2 = 2,6-pyridinedicarboxylic acid and DAP = 2,3-diaminophenazine) were prepared from the reaction of MgCl 2 4H 2 O and ZnCl 2 with mixture of o-phenylenediamine (OPD) and 2,6-pyridinedicar- boxylic acid in water. These compounds were characterized by elemental analysis, FTIR, 1 H NMR and UV–Vis spectroscopy. The solid-state structure of [DAPH][Zn(dipicH)(dipic)]4H 2 O (monoclinic, P2 1 /c) was determined by the single crystal X-ray diffraction method. The protonation constants of dipic and DAP, in all of probability protonated forms, the equilibrium constants for the dipic–DAP proton transfer system and the stoichiometry and stability constants of binary and ternary complexes of this system with Zn 2+ and Mn 2+ ions in aqueous solution were investigated by the potentiometric pH titration method. The stoichiometries of the most complex species in solution were compared with corresponding crystalline complexes in the solid state. Ó 2016 Elsevier Ltd. All rights reserved. 1. Introduction Phenylenediamines (known as Benzenediamines) are aromatic amines with two amino groups attached to benzene. They are found in ortho, meta and para isomeric forms. The ortho form (OPD) of phenylenediamine has two amino groups at the one and two positions of the benzene ring. o-Phenylenediamine in addition to acting as monodentate [1], chelating bidentate [2] and bridging [3] ligand, plays an important role in the synthesis of other func- tional diamines, such as 2,3-diaminophenazine (DAP) [4]. 2,3-Diaminophenazine (DAP) is a heterocyclic aromatic dia- mine, which has been the subject of interest to both chemists and biochemists because of its chemical and physical properties [5]. DAP can coordinate with the metal center as a chelating biden- tate ligand [6], and it can be also found in some complexes as a dis- crete cation [7]. Synthetically, DAP is prepared by the catalyzed autosensitized or photochemical oxidation cyclization of o-phenylenediamine (OPD). Perhaps the first evidence from the oxidation of OPD mole- cules by ferric chloride and ring-forming dimerization came from Griess [8]. The oxidation by various oxidants, including, silver oxide, lead(IV) oxide, cupric chloride, cupric nitrate, cupric hydrox- ide, cupric acetate and perchlorate and by cobalt perchlorate has been also reported in literature [5,7,9–13]. Three years ago, Mei et al. [13] in an effort to prepare diamine complexes of copper from copper chloride anhydrous reaction with OPD, several structures of hydrochloride salt of this compound with different solvents has been reported. Recently, the aerobic oxidation of o-phenylenedi- amine to 2,3-diaminophenazine in the present of copper(II) com- plexes as catalyst is also reported [14]. 2,6-Pyridinedicarboxylic acid (dipicolinic acid, dipicH 2 ), is a ver- satile ligand with the ability to act as a bidentate, tridentate, and/or bridging ligand [15–19]. It can interact with metal ions in dianionic (dipic 2 ), monoanionic (dipicH  ) or neutral (dipicH 2 ) forms [20,21]. A large number of octahedral complexes containing dipi- colinic as a tridentate ligand with the first row transition metal ions such as Mn 2+ or Zn 2+ ions have been reported [22–33]. In these complexes, a single planar dipic ligand binds to metal centers by tridentate (O,N,O 0 ) chelation and other ligands such as H 2 O or pyr- idine based heterocyclic occupy the remaining three sites, thereby http://dx.doi.org/10.1016/j.poly.2016.02.005 0277-5387/Ó 2016 Elsevier Ltd. All rights reserved. ⇑ Corresponding authors. E-mail addresses: ghasemi.khaled@gmail.com (K. Ghasemi), sgg@uniovi.es, s.garciagranda@cinn.es (S. García-Granda). Polyhedron 110 (2016) 55–62 Contents lists available at ScienceDirect Polyhedron journal homepage: www.elsevier.com/locate/poly