Structure and magnetic behavior of unpredictable EE-azide bridged tetranuclear Mn(II) complex with ONO-donor hydrazone ligand and its transformation to dinuclear Mn(III) complex
Introduction
Due to wide application of manganese compounds in various fields, its coordination chemistry is an attractive research field [1]. Manganese is a versatile metal with a variety of oxidation states which plays an important role in several catalytic reactions and redox-biochemical processes like in water oxidation process as the active center of Photosystem(II) [2] and Mn-containing superoxide dismutases [3]. The active site of several metalloenzymes, metalloproteins and other biological systems contains multinuclear manganese clusters [4]. In this respect, multinuclear manganese complexes have been widely used as model systems for mimicking several biological systems [5]. Additionally, manganese ions may possess different numbers of unpaired electrons in various oxidation states. As a result, multinuclear manganese complexes usually exhibit interesting magnetic properties which make suitable precursors for studying magnetic exchange interactions and designing of new magnetic materials [6].
Employing suitable bridging groups (like halides, pseudo-halides, cyanide, etc.) together with flexible multidentate organic ligands is a useful and efficient strategy to construct multinuclear transition metal complexes [7]. Among the various bridging groups the azide anion (N3−), due to several possible coordination modes, is a versatile bridging ligand for obtaining multinuclear complexes [8]. The geometries of the organic ligands have a considerable effect on the coordination behavior and the bridging mode of the azide anion. Depending on the steric and electronic requirements, the azide anion can coordinate to the metal ions as a terminal monodentate ligand [9] or alternatively it can bind to metal ions as bridging group with common μ2-1,1-N3, μ2-1,3-N3 [10], or rare μ3-1,1,3-N3, μ3-1,1,1-N3, μ4-1,1,1,3-N3 and μ4-1,1,3,3-N3 modes [11]. Besides, the azide anion is an excellent magnetic coupler and plays an important role in the magnetic interactions between paramagnetic centers wherein the type and magnitude of interaction is mainly determined due to the coordination mode of the azide bridging group [12]. Although an increasing number of exceptions have been recently observed, generally the end-on (μ2-1,1) and end-to-end (μ2-1,3) coordination modes of azide bridging group mediate the ferromagnetic and antiferromagnetic interactions, respectively [13]. Moreover, several experimental and theoretical magneto-structural studies illustrate that some other parameters like bridging angles, dihedral angles, out-of-plane angles, metal···metal distances and metal–ligand bond lengths can also have an effect on the magnetic interactions [14].
Hydrazone derivatives are appropriate choices for preparing multinuclear metal complexes with azide bridging ligands [15]. These ligands show very high efficiency at chelating transition metal ions and their complexes are indeed attractive compounds in various fields like sensors, non-linear optics, medicine, catalytic reactions and electrochromism, wherein a change in the oxidation state of the metal is possible [16]. Due to facile keto–enol tautomerization in the 
NHC(
O) moiety, hydrazones can easily modulate the number of negative charges and the oxidation state of the metal cores by keto–enol tautomerism. The hydrazone ligands obtained from the reaction of aliphatic/aromatic acid hydrazides with 2-hydroxybenzaldehyde derivatives (Scheme 1b) are a class of well known tridentate ONO-donor Schiff base ligands which can coordinate to the metal ions as mononegative (in the keto form) [17] or dinegative (in the enol form) ligand [18]. Replacing 2-hydroxybenzaldehyde with β-diketones also leads to O,N,O-donor hydrazone ligands with similar coordination behavior (see Scheme 1c) [19]. However, the steric properties of these ligands are considerably different which can have an effect on the coordinating behavior of the bridging co-ligands.
Reviewing of the literature including Cambridge Structural Database (CSD) [20] for the reported Mn–azide complexes of hydrazones together with the consideration of our previous reports on such systems [21], we may assume that the azide ligand acts as a terminal monodentate ligand [22] or a EO-bridging group [23] in the presence of ONO-donor hydrazone ligands. To the best of our knowledge there is no report to date on EE-bridged Mn–azide complexes with tridentate hydrazone ligands. In our studies on such coordination systems, we have been encouraged by interesting observation in preparing Mn–azide complexes with ONO-donor hydrazone ligand obtained from the reaction of 3-hydroxy-2-naphthoic acid hydrazide with benzoylacetone (Scheme 2). In this paper, we report the synthesis, structure, spectroscopic studies and magnetic properties of new EE-bridged tetranuclear Mn(II) complex with a ONO-donor hydrazone ligand and its transformation in a methanol solution to dinuclear Mn(III) complex. The geometry of the hydrazone ligand, its keto–enol tautomerism, its high potential to form intramolecular and intermolecular hydrogen bonds and the flexible oxidation behavior of the manganese ion are effective parameters in this observation.
Section snippets
Materials and instrumentations
Manganese(II) chloride tetrahydrate, MnCl2·4H2O, 3-hydroxy-2-naphthoic acid hydrazide and 1-phenyl-1,3-butanedione (benzoylacetone) were purchased from Acros and used as received. Solvents of the highest grade commercially available (Merck) were used without further purification. FT-IR spectra were recorded in KBr discs with a Bruker FT-IR spectrophotometer. UV–Vis solution spectra were recorded using a thermo-spectronic Helios Alpha spectrometer. 1H and 13C NMR spectra of the ligand in DMSO-d6
Synthesis and spectroscopic studies
The desired tridentate O,N,O-donor Schiff base ligand, H2L, was synthesized by the reaction of 3-hydroxy-2-naphthohydrazide with 1-phenyl-1,3-butanedione in methanol (Scheme 2). Elemental analysis and spectroscopic studies confirmed the formation of H2L in high purity. In the FT-IR spectrum of H2L (Fig. S1) the weak broad band at about 3450 cm−1 and the band at 3270 cm−1 can be attributed to the OH and NH vibrations, respectively, involved in hydrogen bonding interactions [27]. Also, in FT-IR
Conclusion
In summary, synthesis of a new tetranuclear Mn(II) complex, [Mn4(H0.5L)4(μ1,3-N3)2(CH3OH)4] (1), and a dinuclear Mn(III) complex, [Mn2L2(μ1,1-OCH3)1.5(μ1,1-N3)0.5] (2), can be controlled by the reactions of MnCl2·4H2O, sodium azide and hydrazone ONO-donor Schiff base ligand in methanol solvent. The red crystals of complex 1 were obtained as the first and sole product of the reaction and were stable out of solvent for a long time. However, the red crystals of complex 1 were converted to brown
Acknowledgement
The authors are grateful to Imam Khomeini International University (Iran), the University of Zanjan (Iran), the National Institute of Materials Physics (through the grant of the Romanian Ministry of Research and Innovation, CCDI-UEFISCDI, project number PN-III-P1-1.2-PCCDI-2017-0871/2018) and Universidad de La Laguna (Spain) for financial support of this study.
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