catena-Poly­[[di­aqua­cobalt(II)]-μ-oxalato]

Single crystals of the title compound, [Co(C₂O₄)(H₂O)₂]_n, have been prepared by hydro­thermal methods and characterized by X-ray diffraction analysis. The crystal structure consists of infinite one-dimensional chains of di­aqua­cobalt(II) units bridged by oxalate groups. These chains lie on twofold symmetry axes parallel to the b axis, and the [Co(C₂O₄)]_n system is nearly planar within experimental error. The cobalt(II) coordination polyhedra are irregular octahedra, with oxalate O atoms at the equatorial positions and water mol­ecules at the axial positions. The chains are linked by hydrogen bonds via the water mol­ecules.

S1. Comment

Polymeric divalent metal oxalates tend to form microcrystalline precipitates that are difficult to characterize by single-crystal X-ray diffraction. Rare prismatic crystals of whewellite {[Ca(C₂O₄)]·H₂O} up to 100 mm long have been discovered in Kladno, Czech Republic (Korbel & Novak, 1999), and diffraction quality crystals of humboldtine {[Fe(C₂O₄)]·2H₂O} can be found as hydrothermal deposits in coal basins.

The mineral humboldtine crystallizes in the space group C2/c and consists of one-dimensional chains of [Fe^II(C₂O₄)(H₂O)₂] units with bridging oxalate ions (Caric, 1959). One-dimensional coordination polymers are important for studying fundamental theoretical aspects of magnetism in the solid state.

Metal oxalates have also been subjected to detailed thermal analyses and the results used in establishing theoretical concepts applicable to solid-state reactions (see, for example, Coetzee et al., 1994). Two different forms of ferrous oxalate dihydrate (α and β) have been characterized (Deyrieux & Peneloux, 1969), but the α form, humboldtine, is the more common. Powder diffraction patterns of the α-oxalates [M(C₂O₄)]·2H₂O (M = Mg, Mn, Fe, Co, Ni and Zn) have been indexed in the monoclinic space group C2/c with nearly identical cell parameters. The crystal structures of α-oxalates [M(C₂O₄)]·2H₂O (M = Mn, Co, Ni and Zn) have been studied by Lagier et al. (1969) and Deyrieux et al. (1973) using powder diffraction.

The atomic coordinates of humboldtine have been used as starting values for a least-squares refinement based on fitting the calculated and observed intensity profiles of a powder diffraction pattern of microcrystalline [Mn(C₂O₄)]·two-dimensional₂O (Śledzińska et al., 1987). These results, and diffraction studies on single crystals of [Zn(C₂O₄)]·2H₂O (Bacsa, 1995), confirm that these oxalates are isostructural and isomorphous. A new hydrated phase of cobalt oxalate crystallizes in the triclinic space group P1 with similar cell parameters to the α-forms (Castillo et al., 2004). Here, we report the crystal structure of the title compound, (I), prepared by hydrothermal methods. \sch

The structure of (I) consists of infinite chains of [Co(C₂O₄)(H₂O)₂] units which lie on twofold symmetry axes parallel to the b axis (Fig. 1). The neighbouring chains are parallel but displaced by 0.787 (1) Å relative to each other along the b axis. In this arrangement, the H atoms of the water molecules are in favourable positions for donating normal hydrogen bonds to oxalate O atoms (Fig. 2). This network of hydrogen bonds is in an up-down arrangement along these chains, i.e. when an oxalate O accepts a hydrogen bond above the [Co(C₂O₄)]_n plane, the next O atom on the same side of the chain accepts a bond below the plane of the oxalate group. The separation of the Co^II atoms along the chains coincides with the repeat distance of the b axis. The closest interchain spacing is 4.9 Å, and the Co^II atoms are located on crystallographic planes with indices (200) and with a repeat distance of 5.853 (1) Å. The reflection (200) in the powder diffraction patterns of the α-oxalates is the most intense, and studies (Masuda et al., 1987) show that crystals of these oxalates have perfect cleavage planes parallel to the (100) plane.

The oxalate ions in (I) are bis-bidentate and bridge the Co atoms sideways rather than head-on, i.e. each oxalate ion forms two five-membered chelate rings rather than two four-membered rings. The O.·O distance [2.700 (3) versus 2.239 (3) Å Source of reference value?] in this arrangement is better for forming a stable octahedral coordination geometry about each Co^II atom with slight distortion.

The bridging oxalate ions and Co atoms are nearly planar within experimental error, and since the chains lie on twofold axes, the entire [Co(C₂O₄)]_n system is also planar. This planarity suggests the existence of delocalized bonding involving all the equatorial atoms. The C—C bond distance is slightly longer than that typically found in other oxalates (Cambridge Structural Database, Version?; Allen, 2002). According to the bond-valence model (Brown, 2002), this bond should be an electron-pair bond and have a distance of 1.54 Å. Electron density that would be expected to be within this bond is likely to have been transferred to the C—O and Co—O bonds, resulting in a longer bond (Mak & Zhou, 1992).

The water molecule is tilted toward atom O1 and away from atom O2 (within each chain). The O—Co—O(H₂O) coordination angles are close to the ideal octahedral values, but the O—Co—O angles within the two unique chelate rings are acute, while the remaining unique angle is obtuse. The bond angle of the carboxylate O—C—O group is 126.5 (3)°. The ideal O—C—O angle in the oxalate ion is close to 120°. This bond angle widens to accommodate the Co atom. However, this angle is rigid and distorts to a lesser extent than the coordination angles. The two pairs of Co—O equatorial bonds are the same within experimental error. The axial Co—O(H₂O) bond is almost exactly the same length as the equatorial bonds.

The structure of (I) is held together by an ordered network of coordination and hydrogen bonds. A disruption of any of these is likely to break the chains. Therefore, the crystals are small but diffract well, perhaps indicating a nearly perfectly ordered structure.

S2. Experimental

A solution of cobalt(II) chloride hexahydrate (1.2 g, 5.0 mmol) in water (10 ml) was added dropwise to an aqueous solution (10 ml) of potassium oxalate (1.2 g, 6.2 mmol). The resulting microcrystalline precipitate of [Co(C₂O₄)]·2H₂O was transferred to a 23 ml Teflon-lined autoclave. The autoclave was sealed, transferred to an oven and heated to 483 K for 12 h. Small pink crystals of (I), as well as dark-blue soluble crystals of cobalt(II) chloride, were obtained on slowly cooling the mixture to room temperature.

S3. Refinement

All H atoms were located from difference electron-density maps and refined isotropically with distances restrained. Please give brief details of restraints used.

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2001); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: X-SEED (Barbour, 2001); software used to prepare material for publication: Please provide missing information.

Figures

	Fig. 1. A view of one of the linear chains of (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small circles of arbitrary radii.
	Fig. 2. A view of part of the molecular packing of (I), projected on the ac plane, showing how adjacent chains are linked by hydrogen bonds (dashed lines) between water molecules and oxalate O atoms. Displacement ellipsoids are drawn at the 50% probability leveland H atoms are shown as small circles of arbitrary radii.

catena-Poly[[diaquacobalt(II)]-µ-oxalato]

Crystal data

[Co(C2O4)(H2O)2]	F(000) = 364
Mr = 182.98	Dx = 2.446 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C 2yc	Cell parameters from 1356 reflections
a = 11.707 (2) Å	θ = 3.5–27.4°
b = 5.4487 (10) Å	µ = 3.41 mm^−1
c = 9.6477 (19) Å	T = 298 K
β = 126.155 (8)°	Plate, pink
V = 496.89 (17) Å^3	0.11 × 0.08 × 0.03 mm
Z = 4

Data collection

Bruker SMART APEX CCD area-detector diffractometer	565 independent reflections
Radiation source: fine-focus sealed tube	542 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.020
ω and ϕ scans	θmax = 27.4°, θmin = 4.3°
Absorption correction: multi-scan (SADABS; Bruker, 2003)	h = −14→8
Tmin = 0.708, Tmax = 0.911	k = −5→7
1722 measured reflections	l = −11→12

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)] = 0.032	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.089	All H-atom parameters refined
S = 1.15	w = 1/[σ^2(Fo^2) + (0.0404P)^2 + 2.8955P] where P = (Fo^2 + 2Fc^2)/3
565 reflections	(Δ/σ)max < 0.001
50 parameters	Δρmax = 0.83 e Å^−3
2 restraints	Δρmin = −0.72 e Å^−3

Special details

Experimental. The crystals were extermely small. Approximately a hemisphere of diffraction data in reciprocal space were collected by a combination of three sets of exposures: two 180° ω scans and one 70° ϕ scan. Exposure times of 30 s per frame and scan widths of 0.5° were used throughout the data collection. Crystal decay was monitored by analyzing duplicate reflections, and found to be negligible, therefore no decay correction was applied.

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. 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 > σ(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
Co	0.5000	0.07229 (10)	0.7500	0.0092 (2)
O1	0.4127 (2)	0.3672 (4)	0.5768 (3)	0.0101 (5)
O2	0.4122 (2)	0.7782 (4)	0.5769 (3)	0.0101 (5)
O3	0.3249 (2)	0.0646 (4)	0.7572 (3)	0.0114 (5)
C1	0.4487 (3)	0.5748 (5)	0.6492 (4)	0.0087 (6)
H1	0.235 (3)	0.111 (9)	0.650 (4)	0.041 (14)*
H2	0.339 (6)	0.136 (10)	0.859 (4)	0.048 (15)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Co	0.0115 (3)	0.0058 (3)	0.0093 (3)	0.000	0.0055 (3)	0.000
O1	0.0120 (10)	0.0069 (10)	0.0091 (10)	−0.0005 (8)	0.0049 (9)	−0.0009 (8)
O2	0.0127 (11)	0.0078 (10)	0.0086 (10)	−0.0001 (8)	0.0057 (9)	−0.0007 (8)
O3	0.0109 (11)	0.0137 (12)	0.0081 (10)	0.0026 (8)	0.0047 (9)	−0.0009 (8)
C1	0.0077 (14)	0.0102 (14)	0.0091 (15)	−0.0006 (11)	0.0055 (13)	−0.0012 (10)

Geometric parameters (Å, º)

Co—O3	2.093 (2)	O2—C1	1.243 (4)
Co—O2^i	2.095 (2)	C1—C1^ii	1.571 (6)
Co—O1	2.099 (2)	O3—H1	0.98 (4)
O1—C1	1.264 (4)	O3—H2	0.98 (4)
O1^ii—Co—O1	80.07 (12)	O2^iii—Co—O1	179.81 (8)
O2^i—Co—O1	99.87 (9)	C1—O1—Co	113.44 (19)
O3—Co—O3^ii	177.70 (13)	O2—C1—O1	126.5 (3)
O3—Co—O2^i	88.96 (8)	O2—C1—C1^ii	116.97 (16)
O3—Co—O2^iii	89.28 (9)	O1—C1—C1^ii	116.52 (16)
O2^i—Co—O2^iii	80.18 (12)	Co—O3—H1	115 (3)
O3—Co—O1	90.90 (9)	Co—O3—H2	116 (3)
O3—Co—O1^ii	90.86 (9)	H1—O3—H2	113 (5)
O2^i—Co—O1	99.87 (9)
O1—C1—C1^ii—O1^ii	−1 (1)	Co—O1—C1—C1^ii	0.7 (4)
O1—C1—C1^ii—O2^ii	179.7 (2)	Co—O2^iii—C1^iii—C1^i	−0.2 (4)

Symmetry codes: (i) x, y−1, z; (ii) −x+1, y, −z+3/2; (iii) −x+1, y−1, −z+3/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H2···O2^iv	0.98 (4)	1.80 (4)	2.751 (3)	164 (5)
O3—H1···O1^v	0.98 (4)	1.83 (4)	2.772 (3)	162 (5)

Symmetry codes: (iv) x, −y+1, z+1/2; (v) −x+1/2, −y+1/2, −z+1.