The new one-dimensional coordination polymer catena-poly[[di­aqua­sodium(I)]-μ-oxalato-[di­aqua­iron(III)]-μ-oxalato]

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:O^1′,O^2′-[di­aqua­iron(III)]-μ-oxalato-κ⁴O¹,O²:O^1′,O^2′], [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 Na^I and one Fe^III 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 A^IM^III(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.

S1. Introduction

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.

S2. Experimental

S2.1. Synthesis and crystallization

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.

S2.2. Preliminary characterization

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.

S2.3. Refinement

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 U_iso(H) =1.5U_eq(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).

S3. Results and discussion

S3.1. Structure description

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 Na^I and one Fe^III 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 µ_1,3 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? µ_1,3 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 C_2v 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.

S3.2. Comparison study

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 [A^IM^III(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 K₂M^II(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 M^II 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 M^III oxalate phases and appears to be unique in the [A^IM^III(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 M^III 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 A^I 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 M^III 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 M^III 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.

S3.3. IR spectroscopy

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 ν_as(C═O) + ν_as(OCO). For ν_s(C—O) and ν_s(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 [C_2v?], 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.

S3.4. UV–Visible absorption

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 ⁶A_{1 g} → (⁴A_{1 g},⁴E_g) spin-forbidden transition, as expected for oxide-bridged Fe^III 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 ⁶A_{1 g} → ⁴T_{2 g} transition found for Fe ³⁺ in a weak ligand field, while the band near 990 nm corresponds to the ⁶A_{1 g} → ⁴T_{1 g} transition. The very intense transition in the visible absorption edge gives the compound its yellow colour.

S3.5. Thermal behaviour

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, NaFeO_1.66(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.

S4. Conclusion

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

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 Na^I and one Fe^III 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 µ_1,3 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? µ_1,3 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 C_2v 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 [A^IM^III(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 K₂M^II(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 M^II 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 M^III oxalate phases and appears to be unique in the [A^IM^III(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 M^III 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 A^I 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 M^III 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 M^III 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 ν_as(C═O) + ν_as(OCO). For ν_s(C—O) and ν_s(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 [C_2v?], 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 ⁶A_{1 g} → (⁴A_{1 g},⁴E_g) spin-forbidden transition, as expected for oxide-bridged Fe^III 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 ⁶A_{1 g} → ⁴T_{2 g} transition found for Fe ³⁺ in a weak ligand field, while the band near 990 nm corresponds to the ⁶A_{1 g} → ⁴T_{1 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, NaFeO_1.66(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

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

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 U_iso(H) =1.5U_eq(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

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

	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.]
	Fig. 2. Packing diagram, viewed along the c axis, showing the chains of polyhedra running along the [100] and [010] directions.
	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.)
	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.]
	Fig. 5. FT–IR vibration spectrum of [NaFe(C2O4)2(H2O)4]n, (I), obtained in pure KBr.
	Fig. 6. Thermogravimetric (black) and differential thermal analysis (blue) curves for (I).

(I)

Crystal data

[FeNa(C2O4)2(H2O)4]	Dx = 2.077 Mg m^−3
Mr = 326.94	Mo Kα radiation, λ = 0.71073 Å
Tetragonal, I41	Cell parameters from 1283 reflections
a = 9.8572 (9) Å	θ = 2.8–27.5°
c = 10.7595 (9) Å	µ = 1.55 mm^−1
V = 1045.4 (2) Å^3	T = 295 K
Z = 4	Prismatic, yellow
F(000) = 660	0.25 × 0.20 × 0.15 mm

Data collection

Bruker APEXII diffractometer	1254 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.024
CCD rotation images, thick slices scans	θmax = 30.4°, θmin = 2.8°
Absorption correction: multi-scan (SADABS; Sheldrick, 2002)	h = −14→12
Tmin = 0.636, Tmax = 0.746	k = −11→10
3032 measured reflections	l = −12→15
1475 independent reflections

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.029	H-atom parameters constrained
wR(F^2) = 0.056	w = 1/[σ^2(Fo^2) + (0.0151P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 0.98	(Δ/σ)max < 0.001
1475 reflections	Δρmax = 0.33 e Å^−3
84 parameters	Δρmin = −0.25 e Å^−3
7 restraints	Absolute structure: Flack (1983), 649 Friedel pairs; refined as an inversion twin
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.04 (2)

Special details

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 (Ų)

	x	y	z	Uiso*/Ueq
Fe1	0.0000	0.5000	0.24930 (4)	0.02613 (14)
Na2	0.5000	0.5000	−0.0057 (2)	0.0326 (3)
O1	0.3034 (2)	0.3674 (2)	0.03667 (19)	0.0370 (5)
OW1	−0.0682 (2)	0.6217 (3)	0.3906 (2)	0.0431 (6)
H1	−0.0220	0.6245	0.4558	0.065*
H2	−0.1488	0.6470	0.3991	0.065*
O2	0.1051 (2)	0.3790 (2)	0.13547 (18)	0.0328 (5)
OW2	0.3741 (2)	0.6051 (2)	−0.16968 (19)	0.0378 (6)
H3	0.2991	0.6150	−0.1344	0.057*
H4	0.3970	0.6755	−0.2071	0.057*
O3	0.3698 (2)	0.6130 (2)	0.14568 (19)	0.0336 (5)
O4	0.16580 (19)	0.6123 (2)	0.2348 (2)	0.0295 (4)
C1	0.2232 (3)	0.4232 (3)	0.1054 (2)	0.0265 (6)
C2	0.2591 (3)	0.5624 (3)	0.1659 (2)	0.0252 (6)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe1	0.0202 (3)	0.0308 (3)	0.0274 (2)	−0.0046 (3)	0.000	0.000
Na2	0.0222 (9)	0.0442 (10)	0.0312 (7)	0.0028 (8)	0.000	0.000
O1	0.0332 (13)	0.0379 (13)	0.0400 (10)	0.0005 (11)	0.0035 (9)	−0.0106 (9)
OW1	0.0322 (14)	0.0607 (16)	0.0362 (10)	0.0145 (11)	0.0010 (9)	−0.0100 (10)
O2	0.0285 (12)	0.0336 (13)	0.0362 (10)	−0.0105 (10)	0.0028 (9)	−0.0078 (9)
OW2	0.0405 (15)	0.0348 (13)	0.0380 (11)	0.0043 (10)	0.0064 (9)	0.0041 (9)
O3	0.0246 (12)	0.0331 (13)	0.0431 (10)	−0.0097 (9)	0.0063 (9)	−0.0039 (9)
O4	0.0233 (10)	0.0307 (11)	0.0344 (9)	−0.0049 (8)	0.0031 (9)	−0.0085 (8)
C1	0.0279 (16)	0.0257 (16)	0.0258 (12)	−0.0055 (12)	−0.0019 (10)	0.0003 (10)
C2	0.0250 (15)	0.0270 (15)	0.0236 (11)	−0.0050 (11)	−0.0015 (10)	0.0008 (9)

Geometric parameters (Å, º)

Fe1—O4^i	1.9803 (19)	Na2—C2	3.071 (3)
Fe1—O4	1.9803 (19)	Na2—C1^ii	3.073 (3)
Fe1—O2^i	1.999 (2)	Na2—C1	3.073 (3)
Fe1—O2	1.999 (2)	Na2—H3	2.6690
Fe1—OW1	2.050 (2)	O1—C1	1.215 (3)
Fe1—OW1^i	2.050 (2)	OW1—H1	0.8370
Na2—O3^ii	2.354 (3)	OW1—H2	0.8371
Na2—O3	2.354 (3)	O2—C1	1.284 (3)
Na2—O1^ii	2.382 (2)	OW2—H3	0.8366
Na2—O1	2.382 (2)	OW2—H4	0.8330
Na2—OW2	2.393 (3)	O3—C2	1.220 (3)
Na2—OW2^ii	2.393 (3)	O4—C2	1.279 (3)
Na2—C2^ii	3.071 (3)	C1—C2	1.559 (4)
O4^i—Fe1—O4	170.97 (14)	O1—Na2—C1^ii	141.52 (14)
O4^i—Fe1—O2^i	81.80 (8)	OW2—Na2—C1^ii	129.84 (8)
O4—Fe1—O2^i	92.64 (8)	OW2^ii—Na2—C1^ii	86.17 (7)
O4^i—Fe1—O2	92.64 (8)	C2^ii—Na2—C1^ii	29.40 (7)
O4—Fe1—O2	81.80 (8)	C2—Na2—C1^ii	113.77 (11)
O2^i—Fe1—O2	104.43 (12)	O3^ii—Na2—C1	95.63 (11)
O4^i—Fe1—OW1	96.60 (9)	O3—Na2—C1	50.45 (7)
O4—Fe1—OW1	90.11 (9)	O1^ii—Na2—C1	141.52 (14)
O2^i—Fe1—OW1	86.27 (9)	O1—Na2—C1	21.26 (7)
O2—Fe1—OW1	166.79 (8)	OW2—Na2—C1	86.17 (7)
O4^i—Fe1—OW1^i	90.11 (9)	OW2^ii—Na2—C1	129.84 (8)
O4—Fe1—OW1^i	96.60 (9)	C2^ii—Na2—C1	113.77 (11)
O2^i—Fe1—OW1^i	166.79 (8)	C2—Na2—C1	29.40 (7)
O2—Fe1—OW1^i	86.27 (9)	C1^ii—Na2—C1	134.20 (14)
OW1—Fe1—OW1^i	84.29 (13)	O3^ii—Na2—H3	164.7
O3^ii—Na2—O3	92.41 (16)	O3—Na2—H3	75.7
O3^ii—Na2—O1^ii	71.55 (9)	O1^ii—Na2—H3	118.0
O3—Na2—O1^ii	92.94 (11)	O1—Na2—H3	74.3
O3^ii—Na2—O1	92.94 (11)	OW2—Na2—H3	18.0
O3—Na2—O1	71.55 (9)	OW2^ii—Na2—H3	100.7
O1^ii—Na2—O1	157.91 (19)	C2^ii—Na2—H3	166.1
O3^ii—Na2—OW2	176.23 (14)	C2—Na2—H3	69.7
O3—Na2—OW2	91.29 (6)	C1^ii—Na2—H3	139.1
O1^ii—Na2—OW2	108.99 (9)	C1—Na2—H3	69.4
O1—Na2—OW2	87.53 (8)	C1—O1—Na2	113.41 (19)
O3^ii—Na2—OW2^ii	91.29 (6)	Fe1—OW1—H1	117.5
O3—Na2—OW2^ii	176.23 (14)	Fe1—OW1—H2	124.5
O1^ii—Na2—OW2^ii	87.53 (8)	H1—OW1—H2	114.5
O1—Na2—OW2^ii	108.99 (9)	C1—O2—Fe1	114.96 (17)
OW2—Na2—OW2^ii	85.02 (15)	Na2—OW2—H3	100.0
O3^ii—Na2—C2^ii	21.15 (7)	Na2—OW2—H4	125.2
O3—Na2—C2^ii	95.74 (12)	H3—OW2—H4	111.2
O1^ii—Na2—C2^ii	50.61 (8)	C2—O3—Na2	114.71 (18)
O1—Na2—C2^ii	113.83 (12)	C2—O4—Fe1	115.04 (18)
OW2—Na2—C2^ii	158.63 (9)	O1—C1—O2	126.1 (3)
OW2^ii—Na2—C2^ii	87.45 (7)	O1—C1—C2	120.4 (3)
O3^ii—Na2—C2	95.74 (12)	O2—C1—C2	113.5 (2)
O3—Na2—C2	21.15 (7)	O1—C1—Na2	45.32 (15)
O1^ii—Na2—C2	113.83 (12)	O2—C1—Na2	170.43 (18)
O1—Na2—C2	50.61 (8)	C2—C1—Na2	75.21 (15)
OW2—Na2—C2	87.45 (7)	O3—C2—O4	126.1 (3)
OW2^ii—Na2—C2	158.63 (9)	O3—C2—C1	119.2 (3)
C2^ii—Na2—C2	106.05 (14)	O4—C2—C1	114.7 (2)
O3^ii—Na2—C1^ii	50.45 (7)	O4—C2—Na2	168.9 (2)
O3—Na2—C1^ii	95.63 (11)	C1—C2—Na2	75.40 (15)
O1^ii—Na2—C1^ii	21.26 (7)
Na2—O1—C1—O2	174.3 (2)	O1—C1—C2—O3	−1.7 (4)
Na2—O1—C1—C2	−5.2 (3)	O2—C1—C2—O3	178.7 (2)
Fe1—O2—C1—O1	−179.1 (2)	Na2—C1—C2—O3	−5.5 (2)
Fe1—O2—C1—C2	0.4 (3)	O1—C1—C2—O4	178.9 (2)
Na2—O3—C2—O4	−172.9 (2)	O2—C1—C2—O4	−0.8 (3)
Na2—O3—C2—C1	7.7 (3)	Na2—C1—C2—O4	175.0 (2)
Fe1—O4—C2—O3	−178.8 (2)	O1—C1—C2—Na2	3.9 (2)
Fe1—O4—C2—C1	0.7 (3)	O2—C1—C2—Na2	−175.8 (2)
Fe1—O4—C2—Na2	154.8 (9)

Symmetry codes: (i) −x, −y+1, z; (ii) −x+1, −y+1, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
OW1—H1···OW2^iii	0.84	1.83	2.663 (3)	170
OW1—H2···O2^iv	0.84	2.31	3.068 (3)	151
OW2—H3···O3^v	0.84	1.87	2.680 (3)	163
OW2—H4···O4^vi	0.83	2.27	2.994 (3)	146

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.