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The oxalate dianion is one of the most studied ligands and is capable of bridging two or more metal centres and creating inorganic polymers based on the assembly of metal polyhedra with a wide variety of one-, two- or three-di­mensional extended structures. Yellow single crystals of a new mixed-metal oxalate, namely catena-poly[[di­aqua­sodium(I)]-μ-oxalato-κ4O1,O2:O1′,O2′-[di­aqua­iron(III)]-μ-oxalato-κ4O1,O2:O1′,O2′], [NaFe(C2O4)2(H2O)4]n, have been synthesized and the crystal structure elucidated by X-ray diffraction analysis. The compound crystallizes in the noncentrosymmetric space group I41 (Z = 4). The asymmetric unit contains one NaI and one FeIII atom lying on a fourfold symmetry axis, one μ2-bridging oxalate ligand and two aqua ligands. Each metal atom is surrounded by two chelating oxalate ligands and two equivalent water mol­ecules. The structure consists of infinite one-dimensional chains of alternating FeO4(H2OW1)2 and NaO4(H2OW2)2 octa­hedra, bridged by oxalate ligands, parallel to the [100] and [010] directions, respectively. Because of the cis configuration and the μ2-coordination mode of the oxalate ligands, the chains run in a zigzag manner. This arrangement facilitates the formation of hydrogen bonds between neighbouring chains involving the H2O and oxalate ligands, leading to a two-dimensional framework. The structure of this new one-dimensional coordination polymer is shown to be unique among the AIMIII(C2O4)2(H2O)n series. In addition, the absorption bands in the IR and UV–Visible regions and their assignments are in good agreement with the local symmetry of the oxalate ligand and the irregular environment of iron(III). The final product of the thermal decomposition of this precursor is the well-known ternary oxide NaFeO2.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S2053229616002953/ly3026sup1.cif
Contains datablock I

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S2053229616002953/ly3026Isup2.hkl
Contains datablock I

pdf

Portable Document Format (PDF) file https://doi.org/10.1107/S2053229616002953/ly3026sup3.pdf
Figures: Projection structure Polyhedra chains DRX powder UV-Visible spectrum

CCDC reference: 1454394

Introduction top

The solid-state coordination chemistry of oxalate compounds continues to receive much attention, with new structure types being reported for binary oxalate (Bacsa et al., 2005), ternary oxalate (Kherfi et al., 2011, 2013; Zhang et al., 2011) and hybrid metallic oxalate compounds (Farkasová et al., 2014; Chen et al., 2015). Moreover, the intense activity in the elaboration of new coordination polymers resulting from the combination of two or more metal ions with oxalate ligands, including open-framework compounds, is due to their various properties, such as magnetic (Hursthouse et al., 2004), catalytic (Eddaoudi et al., 1999; Seo et al., 2000) and ion exchange (Crespi Caramella et al., 1999), and potential applications which stimulate further investigations for elaborating strategies to discover new families of metal–organic materials. Increasing inter­est has been paid to porous and microporous materials with open-framework structures displaying zeolitic properties (Yaghi et al., 1996; Audebrand et al., 2004; Mahé & Audebrand, 2006). Furthermore, double oxalate compounds are well known for their use as precursors in the synthesis of highly active ternary oxides resulting from their thermal decomposition (Prasad, 2003). The oxalate dianion is one of the most studied ligands and it is capable of bridging two or more metal centres and creating inorganic polymers based on the assembly of metal polyhedra with a wide variety of one-, two- or three-dimensional extended structures (Robin & Fromm, 2006; Bian et al., 2004; Chen et al., 2015; Kolitsch, 2004; Kherfi et al., 2011, 2013; Zhang et al., 2011) in which it adopts a large range of coordination modes. Moreover, as a bridging ligand, it is used to transmit magnetic coupling inter­actions between metal ions along one-dimensional chains. Among the oxalate-based compounds combining an alkali metal (A) and a trivalent element (M) with the general formula AM(C2O4)2(H2O)n.xH2O (Bataille & Louër, 1999; Bataille et al., 2000; Kolitsch, 2004; Bélombé et al., 2006; Zhang et al., 2011), only a few crystal structures involving sodium have been reported in the literature, namely with M = In (Bulc et al., 1983), Y (Bataille & Louër, 1999) and Yb (Chapelet-Arab et al., 2006). As a continuation of work on mixed oxalate-based compounds with a tri- or bivalent element and A = Rb or Cs (Kherfi et al., 2011, 2013), we now extend our activity on coordination polymers involving small size alkali metal. A new open-framework coordination polymer has been prepared, [NaFe(C2O4)2(H2O)4]n, (I), and its crystal structure elucidated from single-crystal diffraction. The results of IR and UV–Visible spectroscopic characterizations and the thermal behaviour of this precursor are also presented. Additionally, this novel structure is compared to that of oxalate-based compounds with the same general chemical formula.

Experimental top

Synthesis and crystallization top

The preparation of the title compound [NaFe(C2O4)2(H2O)4]n, (I), was carried out in aqueous solvent at room temperature under an air atmosphere. The starting materials, Na2CO3 and Fe (NO3)3·9H2O, taken in equal stoechiometric amounts (0.2 mmol), were dissolved in deionized water and H2C2O4·2H2O (0.05 M, 20 ml) was added dropwise. The yellow solution which formed was stirred for several hours under reflux at 353 K and kept at 313 K for evaporation. The yellow powder obtained after two weeks was dissolved in water and submitted to a new evaporation. This operation was repeated to favour the growth of crystals. The precipitation is thus total and leads to yellow prismatic single crystals which are washed with di­ethyl ether for further analysis.

Preliminary characterization top

Powder X-ray diffraction data were collected on a PANalytical-X'Pert PRO X-ray powder diffractometer at room temperature. The 2θ scan range was 5–60°, with a step size of 0.02°. The diffraction pattern of the sample shows good agreement with that calculated from the single-crystal diffraction data, indicating that the product is a pure phase (see Fig. S3 in the Supporting information). The sample was prepared by dispersing a small amount of the powdered compound in KBr (99.99% purity) and compacted it to form a pellet for IR analysis. The IR spectrum was recorded on a Nicolet Avatar 330 F T–IR spectrophotometer in the range 4000–400 cm−1. The absorbance measurements were carried out on a powdered sample of pure (I) using a Specord 200 Plus spectrophotometer, equipped with an integration sphere. The electronic spectrum is recorded on visible and near IR region, between 380 and 1000 nm (see Fig. S4 in the Supporting information). The thermogravimetric (TG) and differential thermal analysis (DTA) curves were recorded on a PerkinElmer STA 6000 instrument under the following conditions: temperature range 303–1173 K and a heating rate of 10 K min−1 under a nitro­gen atmosphere.

Refinement top

Crystal data, data collection and structure refinement details are summarized in Table 1. The H atoms of the water molecules are located in a difference Fourier map but was found to be unstable during the refinement process. Thus, the O—H distances were restrained to 0.85 (1) Å and the H···H separations were restrained to 1.39 (2) Å, with Uiso(H) =1.5Ueq(O) for all H atoms. For the determination of the absolute structure, the instructions BASF and TWIN were introduced to refine the Flack factor by using 649 Friedel pairs, calculated from PLATON (Spek, 2009).

Results and discussion top

Structure description top

The title compound crystallizes in the tetra­gonal system and the conditions of extinction are consistent with the I41 noncentrosymmetric space group. The asymmetric unit contains one NaI and one FeIII atom, both lying on a fourfold symmetry axis, one oxalate dianion and two water molecules (H2Ow1 and H2Ow2), leading to the chemical formula NaFe(C2O4)2(H2O)4.

The geometry of both metal atoms is o­cta­hedral and the structure is built from [M(C2O4)2(H2O)2] (M = Fe, Na) o­cta­hedra, bridged alternately by oxalate groups. As shown in Fig. 1, the two Fe and Na atoms are coordinated with two equivalent water molecules (H2Ow1 for Fe1 and H2Ow2 for Na2) and four O atoms from two bidentate oxalate ligands, all the ligands being in a cis arrangement. In each o­cta­hedral environment, the oxalate ligands chelate the Na and Fe atoms through an O-atom donor of each COO group, thus displaying a classical η4 chelation. Within the Fe o­cta­hedron, the Fe—O distances [1.9803 (19) to 2.050 (2) Å] are close to the values observed in oxalate-based compounds (Armentano et al., 2005; Sheu et al., 2006), the longer distances involving the water O atoms. The Fe o­cta­hedron is distorted, as indicated by the dispersion observed on the bond-angle values, which range from 81.80 (8) to 104.43 (12)°. The equatorial plane is defined by two O4, one Ow1 and one O2 atom, with a deviation from the calculated mean plane of 0.1243 (8) Å. The two resulting five-membered rings [Fe1—O2—C1—C2—O4 and Fe1—O2i—C1i—C2i—O4i; symmetry code: −x, −y + 1, −z] are not perpendicular, with the observed dihedral angle between their planes being 75.60 (4)°.

The Na polyhedron is found to be highly distorted, with a greater dispersion in the values of the bond angles between 71.55 (9) and 108.99 (9)° (Table 2), while the bond lengths vary from 2.354 (3) to 2.393 (3) Å and are comparable to those reported for other mixed-oxalate compounds with the same geometry (Chapelet-Arab et al., 2006). This distortion is noticeable for the bond angle of 158.63 (9)° involving the two axial O1 atoms being so far from the value of 180° in a regular o­cta­hedron. The equatorial plane is defined by two O3 and two Ow2 atoms, and the deviation from the calculated mean plane is 0.0141 (12) Å. The two resulting five-membered rings [Na2—O1—C1—C2—O3 and Na2—O1ii—C1ii—C2ii—O3ii; symmetry code: −x + 1, −y + 1, −z] are nearly perpendicular, with a dihedral angle between their planes of 83.33 (5)°.

The oxalate ligand connects the two different metal atoms through the conventional µ1,3 conventional bridge. Hence, it is tetra­dentate and displays an unusual µ2-coordination mode, reported however in oxalatophosphates iron (Sheu et al., 2006) and in anhydrous ternary (II) oxalates (Hursthouse et al., 2004). [Contradictory statement? µ1,3 conventional bridge/unusual µ2-coordination mode. I don't think the coordination mode is unusual] The C—O distances and the bond angles are similar to the values found in bimetallic compounds with bis-chelating oxalates. The oxalate group is planar, as shown by the mean atomic deviation from the least-squares plane of 0.00121 (21) Å and the dihedral angle of 1.2 (4)° between the two carboxyl­ate functions. These geometric features are consistent with a C2v local symmetry for the C2O42− ligand, as predicted from the presence of four C—O stretching bands in the IR spectrum (Scott et al., 1973). The two coordinated water molecules are present to complete the coordination sphere of each metal atom. More importantly, they are involved in the formation of hydrogen bonds which help to stabilize the crystal structure (Table 3).

The structure of this new mixed oxalate is built from a one-dimensional arrangement of metal atoms and oxalate ligands, supported by a hydrogen-bond network. It consists of FeO6 and NaO6 polyhedra connected by bis-chelating oxalate ligands to form infinite chains running, respectively, along the [100] and [010] directions. Fig. 2 shows the one-dimensional chains giving rise to a packing of inter­crossing chains. The resulting framework exhibits tunnels parallel to the c axis with an elliptic cross section as shown in Fig. 3. Because the oxalate ligands have a cis arrangement, the observed chains run in a zigzag manner. Due to the tetra­gonal symmetry, neighbouring zigzag chains are displaced by a/2 (or b/2) relative to each other along the [100] and [010] directions. The difference in the orientation of the Fe and Na o­cta­hedra contributes to the shape of the chains, which display equal bond angles of 121.786 (1)° (Na···Fe···Na = Fe···Na···Fe) and a Fe···Na separation of 5.6410 (4) Å. The inter­chain Fe···Na separation is slightly less between two adjacent and parallel files? [5.5891 (4) Å], while the shortest M···M distance (M = Fe or Na) observed is 5.6148 (4) Å. However, the shortest inter­metallic distance occurs between two different atoms belonging to perpendicular files? [Na···Fe = 5.3256 (4) Å]. It should be noted that the M···M separation within the same chains coincides exactly with the length of the a (or b) unit-cell parameter [9.8572 (9) Å], calculated using DIAMOND (Brandenburg & Putz, 2008).

Finally, the structure may be considered as being based on an isosceles-like FeNaFe (or NaFeNa) triangle, a geometrical motif which is repeated infinitely along the two perpendicular directions. In such an organization, the H atoms of the water molecules are in favourable positions for donating hydrogen bonds to oxalate O atoms, as well as to water O atoms belonging to adjacent and perpendicular files? (Table 3 and Fig. 4). Indeed, two independent water molecules belonging to two different chains contribute in a strong hydrogen bonds, involving atom H1 and atom Ow2 as acceptor. On the other hand, the O3 atom of the organic ligand accepts a strong hydrogen bond from H2Ow2, involving the H3 atom (Fig. 4b). Weaker inter­actions are also present between water molecules (Ow1 and Ow2) and oxalate anions via the O2 and O4 atoms. All these inter­actions act as a link between the parallel and perpendicular chains, thus stabilizing a two-dimensional open-framework structure. All these specific and geometrical features prevent the extention of the motifs into a two- or three-dimensional network.

Comparison study top

In mixed oxalate-based compounds, ambidentate oxalate is considered as an attractive building block because of its ability to link two or more metal centres, leading to the formation of chains, layers or three-dimensional structures. In the open-framework polymer reported here, (I), the oxalate ligand has been found to act as a linker between two different metal atoms only, leading to a one-dimensional architecture. To the best of our knowledge, no one-dimensional network has been reported for bimetallic oxalate polymers of the type [AIMIII(C2O4)2(H2O)n].xH2O (n = 2, 4; x = 0 to 4) and the µ2-coordination mode is rarely encountered in such compounds. Nevertherless, Hursthouse and coworkers (Hursthouse et al., 2004) have pointed out a similar coordination mode in anhydrous K2MII(C2O4)2, where an anionic [M(C2O4)3]4− complex is present and contributes to the formation of zigzag chains of metal atoms through oxalate ligands acting in a bis-chelating mode. On the contrary, such a µ2-coordination mode is usually observed in monometallic oxalate compounds, for instance, [Co(C2O4)2(H2O)2], which is isotypic with the well known humboldtine structure (Bacsa et al., 2005). This later structure exhibits parallel chains of MII atoms bridged by trans and planar oxalate ligands, and the resulting one-dimensional chains are linear, held together by a hydrogen-bond network.

The title coordination polymer, [NaFe(C2O4)2(H2O)4]n, is not isostructural with either the reported sodium and MIII oxalate phases and appears to be unique in the [AIMIII(C2O4)2(H2O)n].xH2O family. It seems inter­esting, therefore, to compare its structural features with those of previously reported mixed oxalate compounds with the same formula. Indeed, a large variety of structural architectures has been reported where the role of the metal coordination environments, as well as the multilinker character of the ligand, are important for the construction of the metal–organic frameworks. Thereby, when MIII is an o­cta­hedrally coordinated transition metal, the structural feature is obviously the configuration of the complex anion [M(C2O4)2(H2O)2] in this series. With two coplanar oxalate ligands in trans positions, linear chains of alternating eight-coordinated AI ions (CN = 8) in regular o­cta­hedral environments are realised in isostructural ACr(C2O4)2(H2O)2 complexes (space group C2/m) (Bélombé et al., 2006), leading to a two-dimensional layered structure, for instance, with A = Rb (Kolitsch, 2004). A different structure type (P2/c) with the same connectivity of building units has also been reported in the hydrated homologues incorporating a monovalent ion of greater size (CN = 8 to 10), in which µ-oxide bridges are also present (Stranger et al., 1988; Grey et al., 1985). The RbCr(C2O4)2(H2O)2 compound affords an example of a unique three-dimensional structure in this series (P2/n). It differs from the isomer described previously (Kolitsch, 2004) with respect to the coordination sphere of the Rb cation (CN = 9) and the cis arrangement of the complex anion. Accordingly, the structure consists rather of shared polyhedra via vertices and edges, forming a packing stabilized by µ6 and µ5-coordination modes of the ligands and supplementary µ-oxide bridges from H2O molecules (Kherfi et al, 2011).

In addition, various structure types have been evidenced when MIII is a rare earth, Y or a IIIb metallic element. We note mainly the quadratic prototype phase KY(C2O4)2·4H2O (Bataille et al., 1999), which is representative of a family of compounds with A = K or Rb and M = In, Y and Yb (Zhang et al., 2009; Audebrand et al., 2004; Zhang et al., 2011). It exhibits a three-dimensional open framework built from zigzag chains of both eight-coordinated metal atoms bonded to bis-chelating ligands. The four bidentate oxalate ligands around the MIII atom, as well as the µ4 bridge of the organic ligands, are favourable for inducing high dimensionality. A change in the dimensionality is obtained for related compounds with Cs and ammonium ions in a monoclinic structure, the water molecules lying between the layers (Bataille et al., 2000; Trombe et al., 2001)

However, it is more relevant to place the investigated complex in the series of mixed oxalate compounds with a sodium atom, although very few compounds with the same formula have been reported and they differ already from the coordination sphere of the trivalent cation. The first is NaIn(C2O4)2·2H2O, for which the structure has been reported, surprisingly as isostructural with the ammonium compound, but without any structural details (Bulc et al., 1983). The three-dimensional structure is hexagonal and an eight-coordinated In atom was indicated by the authors. A new structural type is evidenced with a noncentrosymmetrical monoclinic space group for M = Y or Yb (Bataille & Louër 1999; Chapelet-Arab et al., 2006). The coordination sphere of the M atom is not the same and the layered structure is characterized as mainly a two-dimensional arrangement of MO9 polyhedra linked to each other by the usual bis-chelating oxalates. The connection between the layers is assumed by dimeric units of edge-shared Na o­cta­hedra and the µ4-coordination mode of the oxalate linkers allows the extension into three directions.

IR spectroscopy top

An assignment of all the IR frequencies has been made by a comparison with related compounds in which depronated oxalate ligands are present. Fig. 5 shows the IR spectrum of (I). A strong and broad band in the range 3650–3100 cm−1, attributed to ν(OH) stretching vibrations of coordinated water molecules, reflects the presence of hydrogen bonds (Nakamoto, 2009). The presence of the oxalate ligand is proved by several bands in the region 1680–1200 cm−1, due to the absorption of the COO functional groups. Thus, the strong absorption near 1670 cm−1 is assigned to νas(CO) + νas(OCO). For νs(C—O) and νs(COO), a strong and fine band at 1394 cm−1 followed by a split localized at 1268 and 1251 cm−1. Additionally, the bands corresponding to the deformation vibrations of the functional groups appear at 899 and 804 cm−1. In the overall pattern, the frequency values indicate that the oxalate groups act as chelating and bridging ligands. The oxalate bonding to metal ions is shown by the presence of weaker bands near 540 and 480 cm−1 due to ν(M—O) stretching combined with ring deformation (Vlad et al., 2008; Martak et al., 2009; Palacios et al., 2011). According to Scott et al. (1973), the molecular symmetry of the ligand oxalate can be deduced from the number of COO vibration bands in binuclear complexes when it is coordinated in a tetra­dentate mode. The local symmetry of the C2O42− ligand in a cis configuration is likely C2v [C2v?], as expected from the existence of four IR-active ν(CO) stretching modes whether the ligand is planar or not (Scott et al., 1973; Gouteron, 1976; Del Arco et al., 2003; Nakamoto, 2009). This result agrees with the results of the X-ray analysis.

UV–Visible absorption top

The absorption spectrum obtained in the visible-near IR region is shown in Fig. S4 (see Supporting information). The title compound displays absorption bands similar to those of the previously investigated K3Fe(C2O4)3·3H2O complex (Lehmann, 1970). In particular, it shows a strong shoulder at 410 nm and two much weaker bands around 620 and 990 nm, typical of iron(III) in a distorted o­cta­hedral arrangement, according to the author. The charge-transfer band (275 nm) and ligand transition band (230 nm) are not recorded. Using ligand field theory, the first transition can be ascribed to the 6A1 g (4A1 g,4Eg) spin-forbidden transition, as expected for oxide-bridged FeIII from an optical study carried out in various clays and minerals (Sherman & Waite, 1985; Cloutis et al., 2006). The weak band near 615 nm is typical of the 6A1 g 4T2 g transition found for Fe 3+ in a weak ligand field, while the band near 990 nm corresponds to the 6A1 g 4T1 g transition. The very intense transition in the visible absorption edge gives the compound its yellow colour.

Thermal behaviour top

Thermal analysis has been used for a long time to identify oxalate, whose decomposition is established to give an equimolar ratio of carbon monoxide and carbon dioxide and to form binary or ternary oxides (Dollimore, 1987; Genčova & Šiftar, 1997). The chemical formula of the precursor suggested by its thermal analysis agrees with the formula NaFe(C2O4)2(H2O)4.

The recorded thermogravimetric (TG) and differential thermal analysis (DTA) curves (Fig. 6) show a continuous weight loss with several and successive stages until ~753 K and two well defined plateaus at 743 and 873 K. The overall mass loss of approximately 65.20% is consistent with the theoretical value of 66.04%, thus corresponding to the following decomposition scheme:

NaFe(C2O4)2(H2O)4 NaFeO2 + 4H2O + 2CO + 2CO2

Indeed, the brown residue is attributed to the ternary oxide NaFeO2 (PDF2 82–1495), as confirmed by X-ray analysis after heating at 923 K for 24 h.

As it can be seen on the TG and DTA curves, the precursor is stable up to \sim 323 K, where the dehydration starts up to ~443 K in a two-step process with equal ratio, suggesting that the four water molecules are not held in the crystal lattice in an identical manner, the two more weakly H2Ow2 bonded to sodium release the lattice slowly until ~383 K. The weight loss of 21.6% confirms the departure of the four water molecules, in agreement of the calculated value of 22.06%. The anhydrous oxalate complex obtained is not stable and decomposes immediately to carbonates in two steps between ~433 and 603 K. The departure of two CO molecules per mole of precursor is confirmed by the weight losses measured (~17.20%) being in agreement with those calculated (17.12%). The exothermic effect caused by decomposition to iron carbonate and sodium carbonate has not been observed, indicating a likely formation of a double carbonate salt at the end of this stage.

The last stage takes place from 603 K and involves a measured mass loss of 26.40% (theoretical value = 26.90%), due to the departure of two CO2 molecules. It occurs with successive pseudo-steps, suggesting the formation of oxocarbonates as inter­mediate phases observed generally in the thermal decomposition of oxalate-based alkali metals (Bataille et al., 2000; Audebrand et al., 2004; Mahé & Audebrand, 2006) and two plateaus at ~753 and ~873 K. The former, with an observed weight loss of 22.35% (the theoretical value is 22.41%, corresponding to 5/3 of CO2), agrees with the suggested formulated compound, NaFeO1.66(CO3)0.33, as a possible inter­mediate phase, while the second one corresponds to the formation of the well known ternary oxide NaFeO2 (PDF2 82–1495), confirmed by the elimination of the remaining CO2 molecule. Further investigations using diffraction RX at high temperature would confirm the mechanism of thermal degradation suggested below for the synthesized precursor.

Conclusion top

A novel coordination polymer, [NaFe(C2O4)2(H2O)4]n, has been synthesized and its crystal structure elucidated. The structure exhibits new structural features which distinguishes it from sodium-oxalate-based compounds of the same general formula, containing a trivalent element, in particular a one-dimensional topology. The comparison study has demonstrated that a resultant framework is influenced by many factors, such as the metal coordination geometry and the metallic environment of the oxalate ligand. Acting as a multilinker ligand and simultaneity in the presence of a large trivalent cation, the arrangement allows the propagation of the structural motifs in two or three directions. A lower dimensionality is preferred for the title compound because the lower coordinence of both metallic elements and the µ2-coordination mode of the ligand prevents the expansion of the structure in two or three dimensions. However, the role of coordinated H2O is not significant for building dense networks and can act as bridge between two metals as observed in above examples. Supplementary characterizations have shown a good agreement with structural results. These novel results offer the opportunity to continue our investigations to discover new coordination polymers involving alkali atoms of smaller size and trivalent element. Indeed, a crystallographic study of a monoclinic complex with lithium is in progress and seems to confirm the previous remarks. Furthermore, exchanges, zeolitic properties and physical properties of surface are envisaged further.

Structure description top

The solid-state coordination chemistry of oxalate compounds continues to receive much attention, with new structure types being reported for binary oxalate (Bacsa et al., 2005), ternary oxalate (Kherfi et al., 2011, 2013; Zhang et al., 2011) and hybrid metallic oxalate compounds (Farkasová et al., 2014; Chen et al., 2015). Moreover, the intense activity in the elaboration of new coordination polymers resulting from the combination of two or more metal ions with oxalate ligands, including open-framework compounds, is due to their various properties, such as magnetic (Hursthouse et al., 2004), catalytic (Eddaoudi et al., 1999; Seo et al., 2000) and ion exchange (Crespi Caramella et al., 1999), and potential applications which stimulate further investigations for elaborating strategies to discover new families of metal–organic materials. Increasing inter­est has been paid to porous and microporous materials with open-framework structures displaying zeolitic properties (Yaghi et al., 1996; Audebrand et al., 2004; Mahé & Audebrand, 2006). Furthermore, double oxalate compounds are well known for their use as precursors in the synthesis of highly active ternary oxides resulting from their thermal decomposition (Prasad, 2003). The oxalate dianion is one of the most studied ligands and it is capable of bridging two or more metal centres and creating inorganic polymers based on the assembly of metal polyhedra with a wide variety of one-, two- or three-dimensional extended structures (Robin & Fromm, 2006; Bian et al., 2004; Chen et al., 2015; Kolitsch, 2004; Kherfi et al., 2011, 2013; Zhang et al., 2011) in which it adopts a large range of coordination modes. Moreover, as a bridging ligand, it is used to transmit magnetic coupling inter­actions between metal ions along one-dimensional chains. Among the oxalate-based compounds combining an alkali metal (A) and a trivalent element (M) with the general formula AM(C2O4)2(H2O)n.xH2O (Bataille & Louër, 1999; Bataille et al., 2000; Kolitsch, 2004; Bélombé et al., 2006; Zhang et al., 2011), only a few crystal structures involving sodium have been reported in the literature, namely with M = In (Bulc et al., 1983), Y (Bataille & Louër, 1999) and Yb (Chapelet-Arab et al., 2006). As a continuation of work on mixed oxalate-based compounds with a tri- or bivalent element and A = Rb or Cs (Kherfi et al., 2011, 2013), we now extend our activity on coordination polymers involving small size alkali metal. A new open-framework coordination polymer has been prepared, [NaFe(C2O4)2(H2O)4]n, (I), and its crystal structure elucidated from single-crystal diffraction. The results of IR and UV–Visible spectroscopic characterizations and the thermal behaviour of this precursor are also presented. Additionally, this novel structure is compared to that of oxalate-based compounds with the same general chemical formula.

Powder X-ray diffraction data were collected on a PANalytical-X'Pert PRO X-ray powder diffractometer at room temperature. The 2θ scan range was 5–60°, with a step size of 0.02°. The diffraction pattern of the sample shows good agreement with that calculated from the single-crystal diffraction data, indicating that the product is a pure phase (see Fig. S3 in the Supporting information). The sample was prepared by dispersing a small amount of the powdered compound in KBr (99.99% purity) and compacted it to form a pellet for IR analysis. The IR spectrum was recorded on a Nicolet Avatar 330 F T–IR spectrophotometer in the range 4000–400 cm−1. The absorbance measurements were carried out on a powdered sample of pure (I) using a Specord 200 Plus spectrophotometer, equipped with an integration sphere. The electronic spectrum is recorded on visible and near IR region, between 380 and 1000 nm (see Fig. S4 in the Supporting information). The thermogravimetric (TG) and differential thermal analysis (DTA) curves were recorded on a PerkinElmer STA 6000 instrument under the following conditions: temperature range 303–1173 K and a heating rate of 10 K min−1 under a nitro­gen atmosphere.

The title compound crystallizes in the tetra­gonal system and the conditions of extinction are consistent with the I41 noncentrosymmetric space group. The asymmetric unit contains one NaI and one FeIII atom, both lying on a fourfold symmetry axis, one oxalate dianion and two water molecules (H2Ow1 and H2Ow2), leading to the chemical formula NaFe(C2O4)2(H2O)4.

The geometry of both metal atoms is o­cta­hedral and the structure is built from [M(C2O4)2(H2O)2] (M = Fe, Na) o­cta­hedra, bridged alternately by oxalate groups. As shown in Fig. 1, the two Fe and Na atoms are coordinated with two equivalent water molecules (H2Ow1 for Fe1 and H2Ow2 for Na2) and four O atoms from two bidentate oxalate ligands, all the ligands being in a cis arrangement. In each o­cta­hedral environment, the oxalate ligands chelate the Na and Fe atoms through an O-atom donor of each COO group, thus displaying a classical η4 chelation. Within the Fe o­cta­hedron, the Fe—O distances [1.9803 (19) to 2.050 (2) Å] are close to the values observed in oxalate-based compounds (Armentano et al., 2005; Sheu et al., 2006), the longer distances involving the water O atoms. The Fe o­cta­hedron is distorted, as indicated by the dispersion observed on the bond-angle values, which range from 81.80 (8) to 104.43 (12)°. The equatorial plane is defined by two O4, one Ow1 and one O2 atom, with a deviation from the calculated mean plane of 0.1243 (8) Å. The two resulting five-membered rings [Fe1—O2—C1—C2—O4 and Fe1—O2i—C1i—C2i—O4i; symmetry code: −x, −y + 1, −z] are not perpendicular, with the observed dihedral angle between their planes being 75.60 (4)°.

The Na polyhedron is found to be highly distorted, with a greater dispersion in the values of the bond angles between 71.55 (9) and 108.99 (9)° (Table 2), while the bond lengths vary from 2.354 (3) to 2.393 (3) Å and are comparable to those reported for other mixed-oxalate compounds with the same geometry (Chapelet-Arab et al., 2006). This distortion is noticeable for the bond angle of 158.63 (9)° involving the two axial O1 atoms being so far from the value of 180° in a regular o­cta­hedron. The equatorial plane is defined by two O3 and two Ow2 atoms, and the deviation from the calculated mean plane is 0.0141 (12) Å. The two resulting five-membered rings [Na2—O1—C1—C2—O3 and Na2—O1ii—C1ii—C2ii—O3ii; symmetry code: −x + 1, −y + 1, −z] are nearly perpendicular, with a dihedral angle between their planes of 83.33 (5)°.

The oxalate ligand connects the two different metal atoms through the conventional µ1,3 conventional bridge. Hence, it is tetra­dentate and displays an unusual µ2-coordination mode, reported however in oxalatophosphates iron (Sheu et al., 2006) and in anhydrous ternary (II) oxalates (Hursthouse et al., 2004). [Contradictory statement? µ1,3 conventional bridge/unusual µ2-coordination mode. I don't think the coordination mode is unusual] The C—O distances and the bond angles are similar to the values found in bimetallic compounds with bis-chelating oxalates. The oxalate group is planar, as shown by the mean atomic deviation from the least-squares plane of 0.00121 (21) Å and the dihedral angle of 1.2 (4)° between the two carboxyl­ate functions. These geometric features are consistent with a C2v local symmetry for the C2O42− ligand, as predicted from the presence of four C—O stretching bands in the IR spectrum (Scott et al., 1973). The two coordinated water molecules are present to complete the coordination sphere of each metal atom. More importantly, they are involved in the formation of hydrogen bonds which help to stabilize the crystal structure (Table 3).

The structure of this new mixed oxalate is built from a one-dimensional arrangement of metal atoms and oxalate ligands, supported by a hydrogen-bond network. It consists of FeO6 and NaO6 polyhedra connected by bis-chelating oxalate ligands to form infinite chains running, respectively, along the [100] and [010] directions. Fig. 2 shows the one-dimensional chains giving rise to a packing of inter­crossing chains. The resulting framework exhibits tunnels parallel to the c axis with an elliptic cross section as shown in Fig. 3. Because the oxalate ligands have a cis arrangement, the observed chains run in a zigzag manner. Due to the tetra­gonal symmetry, neighbouring zigzag chains are displaced by a/2 (or b/2) relative to each other along the [100] and [010] directions. The difference in the orientation of the Fe and Na o­cta­hedra contributes to the shape of the chains, which display equal bond angles of 121.786 (1)° (Na···Fe···Na = Fe···Na···Fe) and a Fe···Na separation of 5.6410 (4) Å. The inter­chain Fe···Na separation is slightly less between two adjacent and parallel files? [5.5891 (4) Å], while the shortest M···M distance (M = Fe or Na) observed is 5.6148 (4) Å. However, the shortest inter­metallic distance occurs between two different atoms belonging to perpendicular files? [Na···Fe = 5.3256 (4) Å]. It should be noted that the M···M separation within the same chains coincides exactly with the length of the a (or b) unit-cell parameter [9.8572 (9) Å], calculated using DIAMOND (Brandenburg & Putz, 2008).

Finally, the structure may be considered as being based on an isosceles-like FeNaFe (or NaFeNa) triangle, a geometrical motif which is repeated infinitely along the two perpendicular directions. In such an organization, the H atoms of the water molecules are in favourable positions for donating hydrogen bonds to oxalate O atoms, as well as to water O atoms belonging to adjacent and perpendicular files? (Table 3 and Fig. 4). Indeed, two independent water molecules belonging to two different chains contribute in a strong hydrogen bonds, involving atom H1 and atom Ow2 as acceptor. On the other hand, the O3 atom of the organic ligand accepts a strong hydrogen bond from H2Ow2, involving the H3 atom (Fig. 4b). Weaker inter­actions are also present between water molecules (Ow1 and Ow2) and oxalate anions via the O2 and O4 atoms. All these inter­actions act as a link between the parallel and perpendicular chains, thus stabilizing a two-dimensional open-framework structure. All these specific and geometrical features prevent the extention of the motifs into a two- or three-dimensional network.

In mixed oxalate-based compounds, ambidentate oxalate is considered as an attractive building block because of its ability to link two or more metal centres, leading to the formation of chains, layers or three-dimensional structures. In the open-framework polymer reported here, (I), the oxalate ligand has been found to act as a linker between two different metal atoms only, leading to a one-dimensional architecture. To the best of our knowledge, no one-dimensional network has been reported for bimetallic oxalate polymers of the type [AIMIII(C2O4)2(H2O)n].xH2O (n = 2, 4; x = 0 to 4) and the µ2-coordination mode is rarely encountered in such compounds. Nevertherless, Hursthouse and coworkers (Hursthouse et al., 2004) have pointed out a similar coordination mode in anhydrous K2MII(C2O4)2, where an anionic [M(C2O4)3]4− complex is present and contributes to the formation of zigzag chains of metal atoms through oxalate ligands acting in a bis-chelating mode. On the contrary, such a µ2-coordination mode is usually observed in monometallic oxalate compounds, for instance, [Co(C2O4)2(H2O)2], which is isotypic with the well known humboldtine structure (Bacsa et al., 2005). This later structure exhibits parallel chains of MII atoms bridged by trans and planar oxalate ligands, and the resulting one-dimensional chains are linear, held together by a hydrogen-bond network.

The title coordination polymer, [NaFe(C2O4)2(H2O)4]n, is not isostructural with either the reported sodium and MIII oxalate phases and appears to be unique in the [AIMIII(C2O4)2(H2O)n].xH2O family. It seems inter­esting, therefore, to compare its structural features with those of previously reported mixed oxalate compounds with the same formula. Indeed, a large variety of structural architectures has been reported where the role of the metal coordination environments, as well as the multilinker character of the ligand, are important for the construction of the metal–organic frameworks. Thereby, when MIII is an o­cta­hedrally coordinated transition metal, the structural feature is obviously the configuration of the complex anion [M(C2O4)2(H2O)2] in this series. With two coplanar oxalate ligands in trans positions, linear chains of alternating eight-coordinated AI ions (CN = 8) in regular o­cta­hedral environments are realised in isostructural ACr(C2O4)2(H2O)2 complexes (space group C2/m) (Bélombé et al., 2006), leading to a two-dimensional layered structure, for instance, with A = Rb (Kolitsch, 2004). A different structure type (P2/c) with the same connectivity of building units has also been reported in the hydrated homologues incorporating a monovalent ion of greater size (CN = 8 to 10), in which µ-oxide bridges are also present (Stranger et al., 1988; Grey et al., 1985). The RbCr(C2O4)2(H2O)2 compound affords an example of a unique three-dimensional structure in this series (P2/n). It differs from the isomer described previously (Kolitsch, 2004) with respect to the coordination sphere of the Rb cation (CN = 9) and the cis arrangement of the complex anion. Accordingly, the structure consists rather of shared polyhedra via vertices and edges, forming a packing stabilized by µ6 and µ5-coordination modes of the ligands and supplementary µ-oxide bridges from H2O molecules (Kherfi et al, 2011).

In addition, various structure types have been evidenced when MIII is a rare earth, Y or a IIIb metallic element. We note mainly the quadratic prototype phase KY(C2O4)2·4H2O (Bataille et al., 1999), which is representative of a family of compounds with A = K or Rb and M = In, Y and Yb (Zhang et al., 2009; Audebrand et al., 2004; Zhang et al., 2011). It exhibits a three-dimensional open framework built from zigzag chains of both eight-coordinated metal atoms bonded to bis-chelating ligands. The four bidentate oxalate ligands around the MIII atom, as well as the µ4 bridge of the organic ligands, are favourable for inducing high dimensionality. A change in the dimensionality is obtained for related compounds with Cs and ammonium ions in a monoclinic structure, the water molecules lying between the layers (Bataille et al., 2000; Trombe et al., 2001)

However, it is more relevant to place the investigated complex in the series of mixed oxalate compounds with a sodium atom, although very few compounds with the same formula have been reported and they differ already from the coordination sphere of the trivalent cation. The first is NaIn(C2O4)2·2H2O, for which the structure has been reported, surprisingly as isostructural with the ammonium compound, but without any structural details (Bulc et al., 1983). The three-dimensional structure is hexagonal and an eight-coordinated In atom was indicated by the authors. A new structural type is evidenced with a noncentrosymmetrical monoclinic space group for M = Y or Yb (Bataille & Louër 1999; Chapelet-Arab et al., 2006). The coordination sphere of the M atom is not the same and the layered structure is characterized as mainly a two-dimensional arrangement of MO9 polyhedra linked to each other by the usual bis-chelating oxalates. The connection between the layers is assumed by dimeric units of edge-shared Na o­cta­hedra and the µ4-coordination mode of the oxalate linkers allows the extension into three directions.

An assignment of all the IR frequencies has been made by a comparison with related compounds in which depronated oxalate ligands are present. Fig. 5 shows the IR spectrum of (I). A strong and broad band in the range 3650–3100 cm−1, attributed to ν(OH) stretching vibrations of coordinated water molecules, reflects the presence of hydrogen bonds (Nakamoto, 2009). The presence of the oxalate ligand is proved by several bands in the region 1680–1200 cm−1, due to the absorption of the COO functional groups. Thus, the strong absorption near 1670 cm−1 is assigned to νas(CO) + νas(OCO). For νs(C—O) and νs(COO), a strong and fine band at 1394 cm−1 followed by a split localized at 1268 and 1251 cm−1. Additionally, the bands corresponding to the deformation vibrations of the functional groups appear at 899 and 804 cm−1. In the overall pattern, the frequency values indicate that the oxalate groups act as chelating and bridging ligands. The oxalate bonding to metal ions is shown by the presence of weaker bands near 540 and 480 cm−1 due to ν(M—O) stretching combined with ring deformation (Vlad et al., 2008; Martak et al., 2009; Palacios et al., 2011). According to Scott et al. (1973), the molecular symmetry of the ligand oxalate can be deduced from the number of COO vibration bands in binuclear complexes when it is coordinated in a tetra­dentate mode. The local symmetry of the C2O42− ligand in a cis configuration is likely C2v [C2v?], as expected from the existence of four IR-active ν(CO) stretching modes whether the ligand is planar or not (Scott et al., 1973; Gouteron, 1976; Del Arco et al., 2003; Nakamoto, 2009). This result agrees with the results of the X-ray analysis.

The absorption spectrum obtained in the visible-near IR region is shown in Fig. S4 (see Supporting information). The title compound displays absorption bands similar to those of the previously investigated K3Fe(C2O4)3·3H2O complex (Lehmann, 1970). In particular, it shows a strong shoulder at 410 nm and two much weaker bands around 620 and 990 nm, typical of iron(III) in a distorted o­cta­hedral arrangement, according to the author. The charge-transfer band (275 nm) and ligand transition band (230 nm) are not recorded. Using ligand field theory, the first transition can be ascribed to the 6A1 g (4A1 g,4Eg) spin-forbidden transition, as expected for oxide-bridged FeIII from an optical study carried out in various clays and minerals (Sherman & Waite, 1985; Cloutis et al., 2006). The weak band near 615 nm is typical of the 6A1 g 4T2 g transition found for Fe 3+ in a weak ligand field, while the band near 990 nm corresponds to the 6A1 g 4T1 g transition. The very intense transition in the visible absorption edge gives the compound its yellow colour.

Thermal analysis has been used for a long time to identify oxalate, whose decomposition is established to give an equimolar ratio of carbon monoxide and carbon dioxide and to form binary or ternary oxides (Dollimore, 1987; Genčova & Šiftar, 1997). The chemical formula of the precursor suggested by its thermal analysis agrees with the formula NaFe(C2O4)2(H2O)4.

The recorded thermogravimetric (TG) and differential thermal analysis (DTA) curves (Fig. 6) show a continuous weight loss with several and successive stages until ~753 K and two well defined plateaus at 743 and 873 K. The overall mass loss of approximately 65.20% is consistent with the theoretical value of 66.04%, thus corresponding to the following decomposition scheme:

NaFe(C2O4)2(H2O)4 NaFeO2 + 4H2O + 2CO + 2CO2

Indeed, the brown residue is attributed to the ternary oxide NaFeO2 (PDF2 82–1495), as confirmed by X-ray analysis after heating at 923 K for 24 h.

As it can be seen on the TG and DTA curves, the precursor is stable up to \sim 323 K, where the dehydration starts up to ~443 K in a two-step process with equal ratio, suggesting that the four water molecules are not held in the crystal lattice in an identical manner, the two more weakly H2Ow2 bonded to sodium release the lattice slowly until ~383 K. The weight loss of 21.6% confirms the departure of the four water molecules, in agreement of the calculated value of 22.06%. The anhydrous oxalate complex obtained is not stable and decomposes immediately to carbonates in two steps between ~433 and 603 K. The departure of two CO molecules per mole of precursor is confirmed by the weight losses measured (~17.20%) being in agreement with those calculated (17.12%). The exothermic effect caused by decomposition to iron carbonate and sodium carbonate has not been observed, indicating a likely formation of a double carbonate salt at the end of this stage.

The last stage takes place from 603 K and involves a measured mass loss of 26.40% (theoretical value = 26.90%), due to the departure of two CO2 molecules. It occurs with successive pseudo-steps, suggesting the formation of oxocarbonates as inter­mediate phases observed generally in the thermal decomposition of oxalate-based alkali metals (Bataille et al., 2000; Audebrand et al., 2004; Mahé & Audebrand, 2006) and two plateaus at ~753 and ~873 K. The former, with an observed weight loss of 22.35% (the theoretical value is 22.41%, corresponding to 5/3 of CO2), agrees with the suggested formulated compound, NaFeO1.66(CO3)0.33, as a possible inter­mediate phase, while the second one corresponds to the formation of the well known ternary oxide NaFeO2 (PDF2 82–1495), confirmed by the elimination of the remaining CO2 molecule. Further investigations using diffraction RX at high temperature would confirm the mechanism of thermal degradation suggested below for the synthesized precursor.

A novel coordination polymer, [NaFe(C2O4)2(H2O)4]n, has been synthesized and its crystal structure elucidated. The structure exhibits new structural features which distinguishes it from sodium-oxalate-based compounds of the same general formula, containing a trivalent element, in particular a one-dimensional topology. The comparison study has demonstrated that a resultant framework is influenced by many factors, such as the metal coordination geometry and the metallic environment of the oxalate ligand. Acting as a multilinker ligand and simultaneity in the presence of a large trivalent cation, the arrangement allows the propagation of the structural motifs in two or three directions. A lower dimensionality is preferred for the title compound because the lower coordinence of both metallic elements and the µ2-coordination mode of the ligand prevents the expansion of the structure in two or three dimensions. However, the role of coordinated H2O is not significant for building dense networks and can act as bridge between two metals as observed in above examples. Supplementary characterizations have shown a good agreement with structural results. These novel results offer the opportunity to continue our investigations to discover new coordination polymers involving alkali atoms of smaller size and trivalent element. Indeed, a crystallographic study of a monoclinic complex with lithium is in progress and seems to confirm the previous remarks. Furthermore, exchanges, zeolitic properties and physical properties of surface are envisaged further.

Synthesis and crystallization top

The preparation of the title compound [NaFe(C2O4)2(H2O)4]n, (I), was carried out in aqueous solvent at room temperature under an air atmosphere. The starting materials, Na2CO3 and Fe (NO3)3·9H2O, taken in equal stoechiometric amounts (0.2 mmol), were dissolved in deionized water and H2C2O4·2H2O (0.05 M, 20 ml) was added dropwise. The yellow solution which formed was stirred for several hours under reflux at 353 K and kept at 313 K for evaporation. The yellow powder obtained after two weeks was dissolved in water and submitted to a new evaporation. This operation was repeated to favour the growth of crystals. The precipitation is thus total and leads to yellow prismatic single crystals which are washed with di­ethyl ether for further analysis.

Refinement details top

Crystal data, data collection and structure refinement details are summarized in Table 1. The H atoms of the water molecules are located in a difference Fourier map but was found to be unstable during the refinement process. Thus, the O—H distances were restrained to 0.85 (1) Å and the H···H separations were restrained to 1.39 (2) Å, with Uiso(H) =1.5Ueq(O) for all H atoms. For the determination of the absolute structure, the instructions BASF and TWIN were introduced to refine the Flack factor by using 649 Friedel pairs, calculated from PLATON (Spek, 2009).

Computing details top

Data collection: APEX2 (Bruker, 2012); cell refinement: APEX2 (Bruker, 2012); data reduction: SAINT (Bruker, 2012); program(s) used to solve structure: SHELXT2014 (Sheldrick, 2015a); program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015b); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg & Putz, 2008); software used to prepare material for publication: WinGX (Farrugia, 2012).

Figures top
[Figure 1] Fig. 1. The connectivity in [NaFe(C2O4)2(H2O)4]n, (I), showing displacement ellipsoids at the 50% probability level and the atom labelling. [Symmetry code: (i) −x, −y + 1, z; (ii) −x + 1, −y + 1, z.]
[Figure 2] Fig. 2. Packing diagram, viewed along the c axis, showing the chains of polyhedra running along the [100] and [010] directions.
[Figure 3] Fig. 3. Projection view showing polyhedra bridged by oxalate units along the [100] and [010] directions and the elliptical cross sections [please clarify]. (Colour key for polyhedra: Na violet and Fe yellow.)
[Figure 4] Fig. 4. (a) The structure of [NaFe(C2O4)2(H2O)4]n, (I), projected along the c axis, showing the parallel and perpendicular files [please clarify] connected by hydrogen bonds (dashed lines). (b) Details of the hydrogen-bond geometry. [Symmetry codes: (iii) y − 1/2, −x + 1, z + 3/4; (iv) −y, x + 1/2, z + 1/4; (v) y − 1/2, −x + 1, z − 1/4; (vi) −x + 1/2, −y + 3/2, z − 1/2.]
[Figure 5] Fig. 5. FT–IR vibration spectrum of [NaFe(C2O4)2(H2O)4]n, (I), obtained in pure KBr.
[Figure 6] Fig. 6. Thermogravimetric (black) and differential thermal analysis (blue) curves for (I).
(I) top
Crystal data top
[FeNa(C2O4)2(H2O)4]Dx = 2.077 Mg m3
Mr = 326.94Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I41Cell parameters from 1283 reflections
a = 9.8572 (9) Åθ = 2.8–27.5°
c = 10.7595 (9) ŵ = 1.55 mm1
V = 1045.4 (2) Å3T = 295 K
Z = 4Prismatic, yellow
F(000) = 6600.25 × 0.20 × 0.15 mm
Data collection top
Bruker APEXII
diffractometer
1254 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tubeRint = 0.024
CCD rotation images, thick slices scansθmax = 30.4°, θmin = 2.8°
Absorption correction: multi-scan
(SADABS; Sheldrick, 2002)
h = 1412
Tmin = 0.636, Tmax = 0.746k = 1110
3032 measured reflectionsl = 1215
1475 independent reflections
Refinement top
Refinement on F2Secondary atom site location: difference Fourier map
Least-squares matrix: fullHydrogen site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.029H-atom parameters constrained
wR(F2) = 0.056 w = 1/[σ2(Fo2) + (0.0151P)2]
where P = (Fo2 + 2Fc2)/3
S = 0.98(Δ/σ)max < 0.001
1475 reflectionsΔρmax = 0.33 e Å3
84 parametersΔρmin = 0.25 e Å3
7 restraintsAbsolute structure: Flack (1983), 649 Friedel pairs; refined as an inversion twin
Primary atom site location: structure-invariant direct methodsAbsolute structure parameter: 0.04 (2)
Crystal data top
[FeNa(C2O4)2(H2O)4]Z = 4
Mr = 326.94Mo Kα radiation
Tetragonal, I41µ = 1.55 mm1
a = 9.8572 (9) ÅT = 295 K
c = 10.7595 (9) Å0.25 × 0.20 × 0.15 mm
V = 1045.4 (2) Å3
Data collection top
Bruker APEXII
diffractometer
1475 independent reflections
Absorption correction: multi-scan
(SADABS; Sheldrick, 2002)
1254 reflections with I > 2σ(I)
Tmin = 0.636, Tmax = 0.746Rint = 0.024
3032 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.029H-atom parameters constrained
wR(F2) = 0.056Δρmax = 0.33 e Å3
S = 0.98Δρmin = 0.25 e Å3
1475 reflectionsAbsolute structure: Flack (1983), 649 Friedel pairs; refined as an inversion twin
84 parametersAbsolute structure parameter: 0.04 (2)
7 restraints
Special details top

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Refinement. Refined as a 2-component inversion twin.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å2) top
xyzUiso*/Ueq
Fe10.00000.50000.24930 (4)0.02613 (14)
Na20.50000.50000.0057 (2)0.0326 (3)
O10.3034 (2)0.3674 (2)0.03667 (19)0.0370 (5)
OW10.0682 (2)0.6217 (3)0.3906 (2)0.0431 (6)
H10.02200.62450.45580.065*
H20.14880.64700.39910.065*
O20.1051 (2)0.3790 (2)0.13547 (18)0.0328 (5)
OW20.3741 (2)0.6051 (2)0.16968 (19)0.0378 (6)
H30.29910.61500.13440.057*
H40.39700.67550.20710.057*
O30.3698 (2)0.6130 (2)0.14568 (19)0.0336 (5)
O40.16580 (19)0.6123 (2)0.2348 (2)0.0295 (4)
C10.2232 (3)0.4232 (3)0.1054 (2)0.0265 (6)
C20.2591 (3)0.5624 (3)0.1659 (2)0.0252 (6)
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
Fe10.0202 (3)0.0308 (3)0.0274 (2)0.0046 (3)0.0000.000
Na20.0222 (9)0.0442 (10)0.0312 (7)0.0028 (8)0.0000.000
O10.0332 (13)0.0379 (13)0.0400 (10)0.0005 (11)0.0035 (9)0.0106 (9)
OW10.0322 (14)0.0607 (16)0.0362 (10)0.0145 (11)0.0010 (9)0.0100 (10)
O20.0285 (12)0.0336 (13)0.0362 (10)0.0105 (10)0.0028 (9)0.0078 (9)
OW20.0405 (15)0.0348 (13)0.0380 (11)0.0043 (10)0.0064 (9)0.0041 (9)
O30.0246 (12)0.0331 (13)0.0431 (10)0.0097 (9)0.0063 (9)0.0039 (9)
O40.0233 (10)0.0307 (11)0.0344 (9)0.0049 (8)0.0031 (9)0.0085 (8)
C10.0279 (16)0.0257 (16)0.0258 (12)0.0055 (12)0.0019 (10)0.0003 (10)
C20.0250 (15)0.0270 (15)0.0236 (11)0.0050 (11)0.0015 (10)0.0008 (9)
Geometric parameters (Å, º) top
Fe1—O4i1.9803 (19)Na2—C23.071 (3)
Fe1—O41.9803 (19)Na2—C1ii3.073 (3)
Fe1—O2i1.999 (2)Na2—C13.073 (3)
Fe1—O21.999 (2)Na2—H32.6690
Fe1—OW12.050 (2)O1—C11.215 (3)
Fe1—OW1i2.050 (2)OW1—H10.8370
Na2—O3ii2.354 (3)OW1—H20.8371
Na2—O32.354 (3)O2—C11.284 (3)
Na2—O1ii2.382 (2)OW2—H30.8366
Na2—O12.382 (2)OW2—H40.8330
Na2—OW22.393 (3)O3—C21.220 (3)
Na2—OW2ii2.393 (3)O4—C21.279 (3)
Na2—C2ii3.071 (3)C1—C21.559 (4)
O4i—Fe1—O4170.97 (14)O1—Na2—C1ii141.52 (14)
O4i—Fe1—O2i81.80 (8)OW2—Na2—C1ii129.84 (8)
O4—Fe1—O2i92.64 (8)OW2ii—Na2—C1ii86.17 (7)
O4i—Fe1—O292.64 (8)C2ii—Na2—C1ii29.40 (7)
O4—Fe1—O281.80 (8)C2—Na2—C1ii113.77 (11)
O2i—Fe1—O2104.43 (12)O3ii—Na2—C195.63 (11)
O4i—Fe1—OW196.60 (9)O3—Na2—C150.45 (7)
O4—Fe1—OW190.11 (9)O1ii—Na2—C1141.52 (14)
O2i—Fe1—OW186.27 (9)O1—Na2—C121.26 (7)
O2—Fe1—OW1166.79 (8)OW2—Na2—C186.17 (7)
O4i—Fe1—OW1i90.11 (9)OW2ii—Na2—C1129.84 (8)
O4—Fe1—OW1i96.60 (9)C2ii—Na2—C1113.77 (11)
O2i—Fe1—OW1i166.79 (8)C2—Na2—C129.40 (7)
O2—Fe1—OW1i86.27 (9)C1ii—Na2—C1134.20 (14)
OW1—Fe1—OW1i84.29 (13)O3ii—Na2—H3164.7
O3ii—Na2—O392.41 (16)O3—Na2—H375.7
O3ii—Na2—O1ii71.55 (9)O1ii—Na2—H3118.0
O3—Na2—O1ii92.94 (11)O1—Na2—H374.3
O3ii—Na2—O192.94 (11)OW2—Na2—H318.0
O3—Na2—O171.55 (9)OW2ii—Na2—H3100.7
O1ii—Na2—O1157.91 (19)C2ii—Na2—H3166.1
O3ii—Na2—OW2176.23 (14)C2—Na2—H369.7
O3—Na2—OW291.29 (6)C1ii—Na2—H3139.1
O1ii—Na2—OW2108.99 (9)C1—Na2—H369.4
O1—Na2—OW287.53 (8)C1—O1—Na2113.41 (19)
O3ii—Na2—OW2ii91.29 (6)Fe1—OW1—H1117.5
O3—Na2—OW2ii176.23 (14)Fe1—OW1—H2124.5
O1ii—Na2—OW2ii87.53 (8)H1—OW1—H2114.5
O1—Na2—OW2ii108.99 (9)C1—O2—Fe1114.96 (17)
OW2—Na2—OW2ii85.02 (15)Na2—OW2—H3100.0
O3ii—Na2—C2ii21.15 (7)Na2—OW2—H4125.2
O3—Na2—C2ii95.74 (12)H3—OW2—H4111.2
O1ii—Na2—C2ii50.61 (8)C2—O3—Na2114.71 (18)
O1—Na2—C2ii113.83 (12)C2—O4—Fe1115.04 (18)
OW2—Na2—C2ii158.63 (9)O1—C1—O2126.1 (3)
OW2ii—Na2—C2ii87.45 (7)O1—C1—C2120.4 (3)
O3ii—Na2—C295.74 (12)O2—C1—C2113.5 (2)
O3—Na2—C221.15 (7)O1—C1—Na245.32 (15)
O1ii—Na2—C2113.83 (12)O2—C1—Na2170.43 (18)
O1—Na2—C250.61 (8)C2—C1—Na275.21 (15)
OW2—Na2—C287.45 (7)O3—C2—O4126.1 (3)
OW2ii—Na2—C2158.63 (9)O3—C2—C1119.2 (3)
C2ii—Na2—C2106.05 (14)O4—C2—C1114.7 (2)
O3ii—Na2—C1ii50.45 (7)O4—C2—Na2168.9 (2)
O3—Na2—C1ii95.63 (11)C1—C2—Na275.40 (15)
O1ii—Na2—C1ii21.26 (7)
Na2—O1—C1—O2174.3 (2)O1—C1—C2—O31.7 (4)
Na2—O1—C1—C25.2 (3)O2—C1—C2—O3178.7 (2)
Fe1—O2—C1—O1179.1 (2)Na2—C1—C2—O35.5 (2)
Fe1—O2—C1—C20.4 (3)O1—C1—C2—O4178.9 (2)
Na2—O3—C2—O4172.9 (2)O2—C1—C2—O40.8 (3)
Na2—O3—C2—C17.7 (3)Na2—C1—C2—O4175.0 (2)
Fe1—O4—C2—O3178.8 (2)O1—C1—C2—Na23.9 (2)
Fe1—O4—C2—C10.7 (3)O2—C1—C2—Na2175.8 (2)
Fe1—O4—C2—Na2154.8 (9)
Symmetry codes: (i) x, y+1, z; (ii) x+1, y+1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
OW1—H1···OW2iii0.841.832.663 (3)170
OW1—H2···O2iv0.842.313.068 (3)151
OW2—H3···O3v0.841.872.680 (3)163
OW2—H4···O4vi0.832.272.994 (3)146
Symmetry codes: (iii) y1/2, x+1, z+3/4; (iv) y, x+1/2, z+1/4; (v) y1/2, x+1, z1/4; (vi) x+1/2, y+3/2, z1/2.

Experimental details

Crystal data
Chemical formula[FeNa(C2O4)2(H2O)4]
Mr326.94
Crystal system, space groupTetragonal, I41
Temperature (K)295
a, c (Å)9.8572 (9), 10.7595 (9)
V3)1045.4 (2)
Z4
Radiation typeMo Kα
µ (mm1)1.55
Crystal size (mm)0.25 × 0.20 × 0.15
Data collection
DiffractometerBruker APEXII
Absorption correctionMulti-scan
(SADABS; Sheldrick, 2002)
Tmin, Tmax0.636, 0.746
No. of measured, independent and
observed [I > 2σ(I)] reflections
3032, 1475, 1254
Rint0.024
(sin θ/λ)max1)0.713
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.029, 0.056, 0.98
No. of reflections1475
No. of parameters84
No. of restraints7
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.33, 0.25
Absolute structureFlack (1983), 649 Friedel pairs; refined as an inversion twin
Absolute structure parameter0.04 (2)

Computer programs: APEX2 (Bruker, 2012), SAINT (Bruker, 2012), SHELXT2014 (Sheldrick, 2015a), SHELXL2014 (Sheldrick, 2015b), ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg & Putz, 2008), WinGX (Farrugia, 2012).

Selected geometric parameters (Å, º) top
Fe1—O4i1.9803 (19)Na2—C1ii3.073 (3)
Fe1—O2i1.999 (2)O1—C11.215 (3)
Fe1—OW1i2.050 (2)O2—C11.284 (3)
Na2—O3ii2.354 (3)O3—C21.220 (3)
Na2—O1ii2.382 (2)O4—C21.279 (3)
Na2—OW2ii2.393 (3)C1—C21.559 (4)
Na2—C2ii3.071 (3)
O4i—Fe1—O4170.97 (14)O1ii—Na2—O1157.91 (19)
O4i—Fe1—O2i81.80 (8)O3ii—Na2—OW2176.23 (14)
O4—Fe1—O2i92.64 (8)O1ii—Na2—OW2108.99 (9)
O2i—Fe1—O2104.43 (12)O3ii—Na2—OW2ii91.29 (6)
O4i—Fe1—OW196.60 (9)O1ii—Na2—OW2ii87.53 (8)
O2i—Fe1—OW186.27 (9)OW2—Na2—OW2ii85.02 (15)
O4i—Fe1—OW1i90.11 (9)O1—C1—O2126.1 (3)
O2i—Fe1—OW1i166.79 (8)O1—C1—C2120.4 (3)
OW1—Fe1—OW1i84.29 (13)O2—C1—C2113.5 (2)
O3ii—Na2—O392.41 (16)O3—C2—O4126.1 (3)
O3ii—Na2—O1ii71.55 (9)O3—C2—C1119.2 (3)
O3—Na2—O1ii92.94 (11)O4—C2—C1114.7 (2)
Symmetry codes: (i) x, y+1, z; (ii) x+1, y+1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
OW1—H1···OW2iii0.841.832.663 (3)169.8
OW1—H2···O2iv0.842.313.068 (3)150.6
OW2—H3···O3v0.841.872.680 (3)162.8
OW2—H4···O4vi0.832.272.994 (3)145.7
Symmetry codes: (iii) y1/2, x+1, z+3/4; (iv) y, x+1/2, z+1/4; (v) y1/2, x+1, z1/4; (vi) x+1/2, y+3/2, z1/2.
 

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