A new polymorph of magnesium oxalate dihydrate

In the asymmetric unit of the title compound, catena-poly[[diaqua­magnesium(II)]-μ-oxalato], [Mg(C₂O₄)(H₂O)₂]_n, there is one Mg atom in an octa­hedral coordination with site symmetry 222, a unique C atom of the oxalate anion lying on a twofold axis, an O atom of the anion in a general position and a water O atom at a site with imposed twofold rotation symmetry. The Mg²⁺ ions are ligated by water mol­ecules and bridged by the anions to form chains that are held together by O—H⋯O hydrogen bonds. The structure of the title compound has already been reported in a different space group [Lagier, Pezerat & Dubernat (1969). Rev. Chim. Miner. 6, 1081–1093; Levy, Perrotey & Visser (1971). Bull. Soc. Chim. Fr. pp. 757–761].

Related literature

For related literature, see: Basso et al. (1997); Caric (1959); Deyrieux et al. (1973); Echigo et al. (2005); Huang & Mak (1990); Lagier et al. (1969); Le Page (1987); Lethbridge et al. (2003); Levy et al. (1971); Neder et al. (1997); Schefer & Grube (1995); Tazzoli & Domeneghetti (1980); Vanhoyland, Bouree et al. (2001); Vanhoyland, Van Bael et al. (2001).

Experimental

Crystal data

• [Mg(C₂O₄)(H₂O)₂]

• M_r = 148.36

• Orthorhombic, F d d d

• a = 5.3940 (11) Å

• b = 12.691 (3) Å

• c = 15.399 (3) Å

• V = 1054.1 (4) ų

• Z = 8

• Mo Kα radiation

• μ = 0.29 mm⁻¹

• T = 290 K

• 0.30 × 0.20 × 0.15 mm

Data collection

• Rigaku AFC-7R diffractometer

• Absorption correction: ψ scan (Kopfmann & Huber, 1968) T_min = 0.915, T_max = 0.962

• 1110 measured reflections

• 483 independent reflections

• 321 reflections with I > 2σ(I)

• R_int = 0.054

• 3 standard reflections every 150 reflections intensity decay: 1.1%

Refinement

• R[F² > 2σ(F²)] = 0.034

• wR(F²) = 0.110

• S = 0.97

• 483 reflections

• 27 parameters

• All H-atom parameters refined

• Δρ_max = 0.89 e Å⁻³

• Δρ_min = −0.48 e Å⁻³

Table 1
Hydrogen-bond geometry (Å, °)

D—H⋯A	D—H	H⋯A	D⋯A	D—H⋯A
O2—H2⋯O1^i	0.84 (3)	1.97 (2)	2.761 (1)	158 (2)
Symmetry code: (i) .

Data collection: Rigaku/AFC Diffractometer Control Software (Rigaku, 1994); cell refinement: Rigaku/AFC Diffractometer Control Software; data reduction: Rigaku/AFC Diffractometer Control Software; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 1999); software used to prepare material for publication: SHELXL97.

Supporting information

S1. Comment

Oxalates are of considerable interest because many of them are natural minerals and in addition, the oxalate anion can adopt different coordination modes to bind metals to form infinite chains, sheets and networks, leading to the rich structural chemistry. For instance, in the system of MO (M = alkali-earth metal)–H₂C₂O₄–H₂O, at least eight phases have been structurally characterized, including Mg(C₂O₄).2H₂O (Lagier et al., 1969; Levy et al., 1971), Ca(C₂O₄).H₂O (Echigo et al., 2005), Ca(C₂O₄).2.375H₂O (Tazzoli & Domeneghetti, 1980), Ca(C₂O₄).3H₂O (Basso et al., 1997), SrH(C₂O₄)(C₂O₄)_0.5 (Vanhoyland, Van Bael et al., 2001), SrH(C₂O₄)(C₂O₄)_0.5.H₂O (Vanhoyland, Bouree et al., 2001), Ba(C₂O₄).H₂O (Huang & Mak, 1990), and Ba(C₂O₄).3.5H₂O (Neder et al., 1997). Among them, Mg(C₂O₄).2H₂O (which we call α-Mg(C₂O₄).2H₂O later) was reported to have one-dimensional (1D) magnesium oxalate chains, Ca(C₂O₄).3H₂O has a layered structure, and others contain three-dimensional metal oxalate frameworks. During our exploratory syntheses of novel hydrated borate materials, we have unexpectedly obtained a new Mg(C₂O₄).2H₂O polymorph [called β-Mg(C₂O₄).2H₂O in this work], (I), as a byproduct. It crystallizes in a space group different from those of other oxalates of similar stoichiometry. We describe its synthesis and crystal structure here for the first time.

The title structure contains Mg²⁺ cations, [C₂O₄]^2- anions, and H₂O molecules as the fundamental structural building units (Fig. 1). The anions are bridged by octahedral Mg²⁺ centers to generate a 1D infinite polymeric chain, and H₂O molecules are located on the two sides of the chains and coordinated to the Mg²⁺ centers to complete the octahedral coordination sphere (Fig. 2). The [Mg(C₂O₄)(H₂O)₂] chains extend along the [100] direction, and are held together via O—H···O hydrogen bonds (Table 1).

The Mg atom occupies one crystallographically distinct octahedral site with site symmetry 222. Each Mg²⁺ is coordinated by six O atoms, four of which are from two oxalate ions and the others from two H₂O molecules. The Mg—O distances are very reasonable when compared with those observed in Mg(NO₃)₂.6H₂O, where octahedrally coordinated Mg²⁺ is also found (Schefer & Grube, 1995). The unique C atom of the anion lies on a 2-fold axis and an O-atom on a general position. The unique O atom of H₂O lies on a 2-fold axis. The oxalate ion is nearly planar, with a mean deviation of 0.0134 Å, and the bond geometries of [C₂O₄]^2- are in accord with those observed in other oxalate compounds (Lethbridge et al., 2003).

In the previously reported oxalates, no one is exactly isotypic with the title compound. Several compounds including M(C₂O₄).2H₂O (M = Mg, Fe, Co, Ni, Zn) (Levy et al., 1971; Caric, 1959; Deyrieux et al., 1973) also contain topologically identical [M(C₂O₄)(H₂O)₂] chains, but crystallize in the monoclinic space group C2/c. An examination of positional parameters of these compounds using the program MISSYM (Le Page, 1987) did not show potential additional symmetry. In fact, the space group C2/c of α-Mg(C₂O₄).2H₂O as well as the isostructural analogs is a "translationengleiche" subgroup (index 2) of the group Fddd adopted by β-Mg(C₂O₄).2H₂O. The lattice vectors of α-Mg(C₂O₄).2H₂O (a₁, a₂ and a₃) are related to those of its β-form (a, b and c) in the following manner: a₁ = b, a₂ = a, and a₃ = -0.5b - 0.5c. The other compound, Mn(C₂O₄).2H₂O, was also reported to exist in two forms. The α-phase (Deyrieux et al., 1973) is isostructural with α-Mg(C₂O₄).2H₂O, while the β-phase crystallizes in the space group P2₁2₁2₁ (Lethbridge et al., 2003). The crystal structure of β-Mn(C₂O₄).2H₂O also consists of chains of oxalate-bridged Mn²⁺ centers, but MnO₆ octahedra in these chains are interconnected through sharing O corners and each oxalate ion acts as a tri-dentate ligand. This is different from the situation in other members of the M(C₂O₄).2H₂O family of compounds, where MO₆ octahedra are separated from each other and the oxalate ions act as tetra-dentate ligands. It is the difference in the coordination modes of the oxalate ions that is responsible for the structural versatility of M(C₂O₄).2H₂O.

Related literature

For related literature, see: Basso et al. (1997); Caric (1959); Deyrieux et al. (1973); Echigo et al. (2005); Huang & Mak (1990); Lagier et al. (1969); Le Page (1987); Lethbridge et al. (2003); Levy et al. (1971); Neder et al. (1997); Schefer & Grube (1995); Tazzoli & Domeneghetti (1980); Vanhoyland, Bouree et al. (2001); Vanhoyland, Van Bael et al. (2001).

S2. Experimental

β-Mg(C₂O₄).2H₂O was first obtained from a hydrothermal reaction in an attempt to prepare novel hydrated borates. For the preparation of MgB₆O₁₀, a stoichiometric mixture of MgO and B₂O₃ was heated at 873 K for two weeks with several intermediate re-mixings and the resulting product was identified to be the pure phase of MgB₆O₁₀ based on the powder XRD analysis. A 0.300 g (3.376 mmol) sample of MgB₆O₁₀, 3 ml pyridine, 0.5 ml 14.5 M (65%) HNO₃, and 0.5 ml H₂O were sealed in an 15-ml Teflon-lined autoclave and subsequently heated at 453 K for one week, then cooled slowly to room temperature. The product consisted of colorless, block-like crystals with the largest having dimensions of 0.6 × 0.6 × 0.8 mm³ in pale yellow mother liquor. The final pH of the reaction system was about 1.0. The crystals were isolated in about 30% yield (based on Mg) by washing the reaction product with deionized water and anhydrous ethanol followed by drying with anhydrous acetone. X-ray structural analysis indicated that the formula of this compound may be Mg(C₂O₄).2H₂O. It is unclear how the oxalate groups are formed.

Subsequently, a separate set of experiments was conducted, in which the starting materials were: 0.2718 g (6.7403 mmol) MgO, 0.8497 g (6.7400 mmol) H₂(C₂O₄).2H₂O, and 3 ml H₂O, and the heating and isolation procedures were the same as those described above. The reaction resulted in pure colorless crystals. The powder XRD pattern of the ground crystals in this experiment was in good agreement with that calculated from the single-crystal data of Mg(C₂O₄).2H₂O from the former experiment, confirming that the same phase had been obtained.

S3. Refinement

H-atom positions were located in a difference Fourier map and all associated parameters were refined freely.

Computing details

Data collection: Rigaku/AFC Diffractometer Control Software (Rigaku, 1994); cell refinement: Rigaku/AFC Diffractometer Control Software (Rigaku, 1994); data reduction: Rigaku/AFC Diffractometer Control Software (Rigaku, 1994); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ATOMS (Dowty, 1999); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).

Figures

	Fig. 1. The coordination geometry of Mg1 in (I) with displacement ellipsoids drawn at the 50% probability level. Symmetry codes: (i) 3/4 - x, 3/4 - y, z; (ii) x, 3/4 - y, 3/4 - z; (iii) 3/4 - x, y, 3/4 - z; (iv) 7/4 - x, 3/4 - y, z; (v) 7/4 - x, y, 3/4 - z; (vii) -1 + x, 3/4 - y, 3/4 - z; (viii) -1 + x, y, z.
	Fig. 2. The crystal structure of (I) projected approximately along the [100] direction (a) as well as the single chain of [Mg(C2O4)(H2O)2] (b); the H2···O1 contacts are shown as dashed lines.

catena-poly[[diaquamagnesium(II)]-µ-oxalato]

Crystal data

[Mg(C2O4)(H2O)2]	F(000) = 608
Mr = 148.36	Dx = 1.870 Mg m^−3
Orthorhombic, Fddd	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -F 2uv 2vw	Cell parameters from 25 reflections
a = 5.3940 (11) Å	θ = 13.0–19.6°
b = 12.691 (3) Å	µ = 0.29 mm^−1
c = 15.399 (3) Å	T = 290 K
V = 1054.1 (4) Å^3	Block, colourless
Z = 8	0.30 × 0.20 × 0.15 mm

Data collection

Rigaku AFC-7R diffractometer	321 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.054
Graphite monochromator	θmax = 32.5°, θmin = 4.2°
2θ/ω scans	h = 0→8
Absorption correction: ψ scan (Kopfmann & Huber, 1968)	k = 0→19
Tmin = 0.915, Tmax = 0.962	l = 0→23
1110 measured reflections	3 standard reflections every 150 reflections
483 independent reflections	intensity decay: 1.1%

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.034	Hydrogen site location: difference Fourier map
wR(F^2) = 0.110	All H-atom parameters refined
S = 0.97	w = 1/[σ^2(Fo^2) + (0.0683P)^2] where P = (Fo^2 + 2Fc^2)/3
483 reflections	(Δ/σ)max < 0.001
27 parameters	Δρmax = 0.89 e Å^−3
0 restraints	Δρmin = −0.48 e Å^−3

Special details

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

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > 2sigma(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
Mg1	0.3750	0.3750	0.3750	0.0163 (2)
C1	0.8750	0.3750	0.32406 (10)	0.0153 (3)
O1	0.66783 (15)	0.37630 (11)	0.28779 (5)	0.0202 (3)
O2	0.3750	0.53689 (11)	0.3750	0.0343 (4)
H2	0.399 (6)	0.578 (2)	0.3335 (16)	0.050 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Mg1	0.0117 (4)	0.0237 (4)	0.0136 (4)	0.000	0.000	0.000
C1	0.0149 (6)	0.0199 (6)	0.0110 (6)	−0.0001 (8)	0.000	0.000
O1	0.0145 (4)	0.0342 (5)	0.0120 (4)	0.0012 (4)	−0.0014 (3)	−0.0018 (4)
O2	0.0631 (11)	0.0228 (7)	0.0169 (6)	0.000	0.0086 (10)	0.000

Geometric parameters (Å, º)

Mg1—O2^i	2.0546 (15)	Mg1—O1	2.0734 (9)
Mg1—O2	2.0546 (15)	C1—O1	1.2494 (11)
Mg1—O1^i	2.0734 (9)	C1—O1^iv	1.2494 (11)
Mg1—O1^ii	2.0734 (9)	C1—C1^v	1.569 (3)
Mg1—O1^iii	2.0734 (9)	O2—H2	0.84 (3)
O2^i—Mg1—O2	180.0	O2^i—Mg1—O1	90.45 (4)
O2^i—Mg1—O1^i	89.55 (4)	O2—Mg1—O1	89.55 (4)
O2—Mg1—O1^i	90.45 (4)	O1^i—Mg1—O1	99.26 (5)
O2^i—Mg1—O1^ii	89.55 (4)	O1^ii—Mg1—O1	80.75 (5)
O2—Mg1—O1^ii	90.45 (4)	O1^iii—Mg1—O1	179.09 (8)
O1^i—Mg1—O1^ii	179.09 (7)	O1—C1—O1^iv	126.89 (14)
O2^i—Mg1—O1^iii	90.45 (4)	O1—C1—C1^v	116.56 (7)
O2—Mg1—O1^iii	89.55 (4)	O1^iv—C1—C1^v	116.56 (7)
O1^i—Mg1—O1^iii	80.75 (5)	C1—O1—Mg1	113.06 (8)
O1^ii—Mg1—O1^iii	99.26 (5)	Mg1—O2—H2	128.7 (19)

Symmetry codes: (i) −x+3/4, −y+3/4, z; (ii) x, −y+3/4, −z+3/4; (iii) −x+3/4, y, −z+3/4; (iv) −x+7/4, −y+3/4, z; (v) −x+7/4, y, −z+3/4.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2···O1^vi	0.84 (3)	1.97 (2)	2.761 (1)	158 (2)

Symmetry code: (vi) x−1/4, y+1/4, −z+1/2.