A Chiral Three Dimensional Potassium ( I ) / Strontium ( II ) / Chromium ( III ) Oxalato-Bridged Coordination Polymer : Synthesis , Characterization and Thermal Analysis

A new compound of general formula {[(H2O)2K(μ-H2O)Sr]@[Cr(C2O4)3]}n (1) has been synthesized in water and characterized by elemental and thermal analyses, EDX, IR and UV-Vis spectroscopies and by single crystal X-ray structure determination. Compound 1 crystallizes in the chiral space group Fdd2 of orthorhombic system with a = 14.110 (4) Å, b = 36.074 (11) Å, c =11.034 (3) Å and Z = 16. Compound 1 is a coordination polymer in which the three-dimensional lattice framework is realized by the interconnectivity between K+ cations, Sr2+ cations, aqua ligands and [Cr(C2O4)3]3− complex anions. The asymmetric unit of 1 consists of one cationic motif formally written [(H2O)2K(μ-H2O)Sr]3+ and one anionic entity, [Cr(C2O4)3]3−. The K+ and Sr2+ ions in the cationic motif are both eight-coordinate while the Cr3+ ions in the anionic complex are six-coordinate in a distorted octahedral geometry. Coulombic interactions between the ionic motifs and the three-dimensional H-bonding involving aqua ligands help to consolidate the bulk structure. Thermogravimetric analysis (TGA) shows that compound 1 is stable to heat up to ca. 80 ̊C.


Introduction
One of the current research activities in the field of materials science is unarguably the rational design and synthesis of heterometallic complexes with interesting molecular topologies, crystal packing motifs and potential applications as advanced multifunctional materials [1]- [6].In this respect, the old but evergreen oxalate dianion, C 2 O 4 2− , with its π-extended system together with its ability to transmit relatively large magnetic interactions between the metal ions that it bridges, provides unique opportunities for the discovery of unusual networks in this fascinating and challenging field [7]- [9].
In our research group, we recently have been interested in the design and construction of polymetallic oxalatebased architectures by following the so-called "complex-as-ligand approach".In this synthetic strategy, a molecular building block, the homoleptic [M III (C 2 O 4 ) 3 ] 3− tris(oxalato)metalate (III) octahedral complex is used as a ligand towards the divalent Ba 2+ cations to generate 3-D polymeric complexes of general formula, {Ba 6 (H 2 O) 17 [M III (C 2 O 4 ) 3 ] 4 }•7H 2 O (M = Cr; Fe) [10]  [11], with the ionic ratio of 3Ba II vs. 2M III .This family of complexes, indeed, has proven to be a useful precursor for the synthesis of highly versatile materials with the potential of accumulating within a single system a whole set of relevant functionalities, such as nanoscale structural features [12] [13], extended hydrogen bonding and magnetic interactions [14] [15].Furthermore, introducing chirality into these materials may give place to potential applications in research fields such as chiral spintronics or magneto-chiral dichroism [16].
In the course of a systematic search for new members of this interesting family of heterometallic complexes involving oxalate dianions, we herein report the synthesis, crystal structure and thermal behavior of a chiral three-dimensional oxalato-bridged coordination polymer, {[( (1), obtained from a metathesis reaction in water of the K 3 [Cr(C 2 O 4 ) 3 ]•3H 2 O and SrCl 2 salts with the stoichiometric ratio of 2:3.

Materials and Measurements
All reagents for the syntheses were purchased from commercial sources and used as received, except for the starting potassium salt, K 3 [Cr(C 2 O 4 ) 3 ]•3H 2 O, which was synthesized according to the literature [17].Microanalyses (carbon and hydrogen) were performed on a Vario EL (Heraeus) analyzer.The FT-IR spectrum was recorded from KBr pellets in the range of 4000 -400 cm −1 on a Perkin-Elmer spectrometer.The UV-Vis spectrum was obtained on a Perkin-Elmer Lambda 900 spectrophotometer in water at room temperature within the range 300 -700 nm.The chemical analysis was obtained using Energy Dispersive X-ray Spectroscopy (EDX), with a JDX-7000F X-ray spectrometer.TGA experiments were performed with a Mettler Toledo TGA/ SDTA 851 thermal analyzer, heated from room temperature to 600˚C under nitrogen gas with a heating rate of 10˚C/min.The melting points were measured using an Electrothermal 9100 apparatus.

Energy Dispersive X-Ray Analysis
Results from the Energy Dispersive X-ray Spectroscopy (EDX) analyses of 1 are shown in

Infrared Spectrum
The IR spectrum exhibits weak absorption bands centered at 3449 cm −1 and attributable to the well-known ν O-H vibrations of the H 2 O molecules that are coordinated to the K + and Sr 2+ sites.The band at 1700 cm −1 is assigned to ν C=O whereas those at 1419 cm −1 are attributed to ν O-C=O [24].The sharp peaks at 1277 cm −1 and 907 cm −1 may be tentatively assigned to the ν C-O and ν C-C vibrations [24], respectively.Medium to weak bands appearing in the region below 600 cm −1 may be attributed to vibrations within the coordination spheres around the metallic centers.These results are, in fact, consistent with the presence of [Cr(C 2 O 4 ) 3 ] 3− and H 2 O in the material concerned.

Thermal Analysis
The thermal behavior of 1 has been studied by thermogravimetric analysis (TGA) and differential thermal analysis (DTA) in the temperature range of 25˚C to 600˚C at a heating rate of 10˚C per minute under nitrogen atmosphere.TGA and DTA curves are depicted in Figure 2. The overall TGA curve consists of two-step weight loss process.The first step between 80˚C and 150˚C corresponds to a weight loss of 11.4% (calc.11.3%) which is due to release of the three coordinated water molecules.During the second step, beyond 150˚C, a gradual decomposition of the crystal network takes place with formation of a final residue which is proved to be a mixture of SrCrO 4 and K 2 CO 3 compounds.

Crystal Structure
The molecular structure of 1 has been determined by X-ray study, which shows that it crystallizes in orthorhombic chiral space group Fdd2 with three-dimensinal arrangement.As depicted in Figure 3, the asymmetric unit of , and one anionic [Cr(C 2 O 4 ) 3 ] 3− entity.The anionic building stone, [Cr(C 2 O 4 ) 3 ] 3− , functions as a metalloligand or else, as an internetting bridge.Hence, this internetting bridge interconnects, across its O atoms, the independent sites K, Sr1 and Cr1 into a three-dimensional polymeric lattice network.Two independent K centers (K1 and K2) are eight-coordinate both by H 2 O and oxalate ligands and are present 50% in the asymmetric unit.Atom K1 is coordinated to two water molecules (OW13 and OW14).Atom K2 is coordinated to two water molecules (OW14 and OW15).The water molecules OW13 and O14W bridge the K1 and Sr1 centers and the K1 and K2 centers, respectively.The water molecule OW15 belongs solely to the K2 center.A molecular drawing of compound 1 with complete coordination spheres of the metal atoms is shown in Figure 4 and selected geometrical parameters are presented in Table 2.As can be seen from Figure 4, the eighth coordination of the K2 center is completed by an unusual short K2 -K2 4 metallic bond of 2.214 (4) Å.All the K-O bond lengths fit well within the range of previous results [26].

Conclusion
We have isolated from aqueous solution at room temperature, a novel trinuclear heterometallic complex of composition {[(H 2 O) 2 K(µ-H 2 O)Sr]@[Cr(C 2 O 4 ) 3 ]} n (1), via a metathesis reaction of K 3 [Cr(C 2 O 4 ) 3 ]•3H 2 O with SrCl 2 .The bulk structure of 1 is a chiral, 3-D metal-organic framework held together by intermetallic linkages across oxalate and aqua oxygen bridging with extensive hydrogen-bonding interactions stabilizing the crystal packing.Thermal analysis of 1 reveals that it is stable up to ca. 80˚C.Preliminary observations from our laboratory suggest that a well conceived and systematically conducted preparative procedure may be applied generally to fabricate a whole range of homologous members of this promising family of chiral, 3-D metal(I)/metal(II)/metal(III)-oxalate polymers.O (13) -H (13A) X-ray facilities and to Priv.-Doz.Dr. Boniface P. T. Fokwa, Aachen University (Germany), for his help with EDX, thermal and elemental analyses.

Figure 1 .
It is observed in this EDX spectrum the presence of chemical elements O, K, Sr and Cr, suggesting clearly the success in the preparation of compound 1.

Figure 1 .
Figure 1.EDX analysis of compound 1 showing the nature of its different chemical elements.

Figure 3 .
Figure 3.A molecular drawing of the asymmetric unit of compound 1 shown with 50% probability ellipsoids.Atoms K1 and K2 are present 50% of the time.

Table 1 .
Crystal data and structure refinement parameters for compound 1.
(C 2 O 4 ) 3 ] 2 }•nH 2 Owith the ionic ratio of 3Sr II vs. 2CrIII, analog of the well-documented {Ba 6 (H 2 O) 17 [M III (C 2 O 4 ) 3 ] 4 }•7H 2 O (M = Cr; tric ratios of 1:1 and 1:2.As expected, the same compound 1 was obtained, different only in purity and yield, the maximum yield being 89 % with the 1:1 ratio.From the above results, it is evident that, irrespective to the stoichiometric ratio of reagents, the title compound is readily formed if K 3 [Cr(C 2 O 4 ) 3 ]•3H 2 O and SrCl 2 are combined.The possible formation mechanisms producing compound 1 are summarized in Scheme 1. Scheme 1.The possible formation mechanisms of compound 1.