New manganese (II) structures derived from 2,6-dichlorobenzoic acid: Syntheses, crystal structures and magnetism

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Abstract

One novel coordination polymer [Mn2(μ-2,6-DCBA)32-CH3CO2)2(2H2O)]·2H2O (2,6-DCBA = 2,6-dichlorobenzoato) (compound 1) has been synthesized by self-assembly of bridging ligand 2,6-dichlorobenzoic acid and manganese acetate tetrahydrate. Single crystal X-ray diffraction analysis reveals that this compound crystallizes in space group P21/c with a = 10.1547(7), b = 24.5829(2), c = 12.6606(2) Å, β = 93.707(3), V = 3153.9(3) Å3 and Z = 4. The Mn(II) ions are connected by 2,6-DCBA and acetate group in μ-bridging mode to form 1D chains. Two water molecules are in the inter-layer space forming strong hydrogen bonds originating 2D layer structure. The preparation of this compound is very sensitive to the synthesis conditions, mainly to the solution pH and solvent yielding other two compounds 2 and 3. In compound 1 Mn(II) atoms in octahedral coordination are arranged in a zig–zag chain, with a trimeric structure repeated periodically along the chain, giving two exchange parameters: J1 related to a syn–syn bond; and J2 related to a bond of type anti–anti. A theoretical model was developed and then fitted to the magnetic susceptibility data, revealing an antiferromagnetic arrangement along the chain.

Introduction

Nowadays there is an intensive research in the field of molecular magnets, in particular for that exhibiting fascinating applications such as quantum computation devices using the high temperature quantum entanglement effect [1], [2], [3], magnetic sensors and magnetic data storage [4]. These molecular magnets are structures consisting of metal ions coordinated to inorganic or organic molecules; and are capable of forming zero dimension (cluster), one-dimension (chain), two-dimension (layer) and three-dimension structures, in which strongly affects the magnetic behavior. Regarding the magnetic interest, chemists and physicists put their knowledge together to understand in a systematic way, to what extent the different structural and chemical parameters affect the sign and magnitude of the magnetic interaction [5]. With this purpose to better control material's magnetic characteristics, we have been working in rational design and preparation of functional transition metal compounds with intriguing structures and potential applications in magnetism. We have successfully prepared two copper silicate: Na2Cu5Si4O14, Na2Cu2Si4O11·2H2O [6], [7], [8], three isostructural compounds of type KNaMSi4O10 (M = Mn, Fe or Cu) [9] two copper germanates: (CaCuGeO4·H2O and BaCu2Ge3O9·H2O) [10] and recently a di-iron compound [Fe22-oxo)(C3H4N2)6(C2O4)2] [11]. As a natural extension of this work, aimed at finding novel molecular magnets and to better control their magnetic properties, we choose the manganese as transition metal because manganese compounds are expected to show interesting magnetic properties as very well documented by several authors [12], [13], [14], [15], [16], [17].

When thinking about magnetic compounds and once the metal is selected, one of the points to be seriously considered is the choice of appropriate bridging ligands, which originates substantial modifications in the magnetic exchange between the paramagnetic adjacent metal centers [18]. As part of a work initially aimed at preparing a multi-nuclear manganese (II) compounds, we choice as bridging ligand the 2,6-dichlorobenzoic acid (2,6-DCBA) which works as multidentate chelating ligand allowing multiple conformation environments: syn–syn, syn-anti, anti–anti conformations [19]. A particularly interesting case of conformation is the syn–syn originating shorter distances between metal centers. In this paper Mn(II) compounds using 2,6-DCBA bridging ligand are synthesized and their structures determined by single crystal X-ray diffraction. The magnetic properties were also investigated for one of these compounds (compound 1), showing an antiferromagnetic behavior in the system.

Section snippets

Synthesis

All reagents and chemicals were purchased from commercial sources and used without further purification. Manganese acetate tetrahydrate (99%), 2,6-dichlorobenzoic acid (99%) and ethanol absolute (99%) were obtained from Jenssen Chimica and Fischer Scientific, respectively.

Structure description of compound 1 [Mn2(μ-2,6-DCBA)32-CH3CO2)2H2O]·2H2O

Single crystal X-ray analysis indicates that the [Mn2(μ-2,6-DCBA)32-CH3CO2)2H2O]·2H2O (compound 1) crystallizes in the monoclinic system with P21/c symmetry. The asymmetric unit of this manganese compound contains: two independent manganese centers Mn(1) and Mn(2); three 2,6-DCBA; one acetate group; two water molecules connected to the Mn centers; and two extra lattice waters (Fig. 1(a)). Both Mn centers exhibit distorted octahedral geometry as shown in Fig. 1(b), surrounded by 3 oxygens from

Conclusions

In conclusion, three novel Mn(II) compounds using 2,6 DCBA ligand were synthesized and characterized. Their structures were determined by single crystal X-ray diffraction. Compound 1 with formula [Mn2(μ-2,6-DCBA)32-CH3CO2)(2H2O)]·2H2O showed an interesting 1D structure composed of zig–zag Mn chains with distances metal to metal alternating between 3.8904(4) and 3.6725(4) Å. Magnetic susceptibility of compound 1 was measured and analyzed by means of a theoretical model based on a regular

Acknowledgement

The authors acknowledge European Union, QREN, Fundo Europeu para o desenvolvimento regional (FEDER), COMPETE, FCT, Collaboration Project FCT/CAPES and CICECO (pEstc/CTM/LA001/2011 for financial support) and to Dr. J.P. de Araújo from University of Porto for magnetic measurements. C. C, J. C. G. T., S. S. P. and M. S. R. also thanks FAPERJ, CNPq, CAPES and PROPPI-UFF for financial support.

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