A spin-frustrated cobalt(II) carbonate pyrochlore network

The crystal structure of the cobalt(II) carbonate-based compound cobalt(II) dicarbonate tris­odium chloride, Co(CO₃)₂Na₃Cl, grown from a water–ethanol mixture, exhibits a three-dimensional network of corner-sharing {Co₄(μ₃-CO₃)₄} tetra­hedral building blocks, in which the Co^II centres define a pyrochlore lattice and reside in a slightly distorted octa­hedral Co(O–CO₂)₆ environment. The space outside the hexa­gonal framework defined by these inter­linked groups is occupied by Na⁺ and Cl⁻ ions. Anti­ferromagnetic coupling between adjacent Co^II centres, mediated by carbonate bridges, results in geometric spin frustration which is typical for pyrochlore networks. The Co and Cl atoms reside on the special position , one Na atom on position 2 and a carbonate C atom on position 3.

S1. Comment

Geometrically frustrated magnetic materials continue to be a focal topic in magnetism, motivated by the multitude of magnetic phenomena they exhibit, ranging from spin-glass behaviour to phase transitions and quantum fluctuations (Ramirez, 1994; Greendan, 2001). Competing antiferromagnetic exchange interactions between localized spin centres can be realised within several classes of compound. Typically, extended (one- to three-dimensional) polymeric spin structures are at the heart of such studies, for example planar Kagomé lattices of regular triangles and hexagons (Schweika et al., 2007). Interest in spin frustration has also spawned efforts to recreate these phenomena in molecular (quasi-zerodimensional) systems (Kögerler et al., 2010). Next to spin-frustrated magnetic molecules, our interest concerns purely inorganic coordination networks that contain no hydrogen. These can thus be subjected to neutron diffraction studies, in order to study the magnetic phenomena associated with spin frustration, without the need for prior deuteration. In this context, we found that the presence of multidentate ligands can indeed induce the formation of network compounds in which magnetic transition metal cations are linked by comparatively simple and hydrogen-free inorganic bridging ligands (Fielden & Kögerler, 2009). We herein demonstrate that, under solvothermal conditions (433 K), small anions such as carbonate can compete with chelating ligands (here, 1,10-phenanthroline) to yield a three-dimensional cobalt(II) carbonate framework structure, the voids of which are filled with Na⁺ and Cl^- ions (Fig. 1). Single crystals of the resulting compound, Co(CO₃)₂Na₃Cl, were grown from a water–ethanol mixture and characterized at 123 K, and exhibit cubic (Fd3) symmetry.

Importantly for the magnetochemistry, the octahedrally coordinated Co^II centres exhibit large single-ion magnetic anisotropy, which renders the spin centres Ising behaviour at low temperatures. The spin centres define a cubic pyrochlore lattice of corner-sharing regular Co₄ tetrahedra arranged around hexagons, forming interconnected stacks of Kagome layers (Fig. 2).

The Co^II centres reside in slightly distorted octahedral CoO₆ coordination environments, with Co—O bond lengths of 2.0950 (9) Å, and O—Co—O bond angles alternating between 86.08 (3) and 93.92 (3)°. The nearest-neighbour Co—Co distances (i.e. the edge lengths of the Co₄ tetrahedra) are 4.9483 (2) Å. Each carbonate group coordinates to, and bridges, three Co sites and the carbonate planes are coplanar with the respective faces of the Co₄ tetrahedra, with an interplanar distance of 1.2256 (19) Å (Fig. 3).

Preliminary low-field magnetic susceptibility studies of Co(CO₃)₂Na₃Cl reveal pronounced ligand-field and spin-orbit coupling effects, assessed using the simulation package CONDON 2.0 (Speldrich et al., 2011), as well as dominant antiferromagnetic coupling between the Co^II spin centres. Here, nearest-neighbour antiferromagnetic coupling, mediated by bridging carbonate anions [Co—O—C bond angles 132.07 (9)°], results in a highly geometrically frustrated material (Bramwell & Harris, 1998). Preliminary susceptibility data for the title compound also indicate a gradual spin-glass transition between 3.0 and 4.5 K, where the fluctuating disorder required for the spin-glass state is supposed to stem from dynamic disorder of the Na⁺ and Cl^- sublattice.

Related literature

For related literature, see: Bramwell & Harris (1998); Fielden & Kögerler (2009); Greendan (2001); Kögerler et al. (2010); Ramirez (1994); Schweika et al. (2007); Speldrich et al. (2011).

S2. Experimental

A mixture of CoCl₂.6H₂O (5.9 g, 0.025 mol), 1,10-phenanthroline (10.0 g, 0.05 mol), Na₂CO₃ (2.5 g, 0.025 mol), ethanol (72 ml) and water (6 ml) was stirred in air for 10 min and then transferred to a 110 ml PTFE-lined autoclave, which was heated to 433 K for 96 h, followed by cooling to room temperature at a rate of 5 K h^-1. The resulting pink polycrystalline product was washed with large amount of ethanol/water (yield 3.0 g, 42% based on Co). Elemental analysis, calculated (found): Na 24.4 (24.8), Co 20.8 (22.3), Cl 12.4% (12.1%); Na and Co determined by ICP–OES and chloride by ion-exchange chromatography). Spectroscopic analysis: IR (KBr pellet, ν, 4000–370 cm^-1): 3506 (m), 3387 (m), 2947 (w), 2854 (m), 2625 (w), 2509 (m), 1435 (s), 1414 (s), 872 (s), 708 (m).

S3. Refinement

The systematic absences in the diffraction data were consistent for the stated space group. The position of almost all atoms were found by direct methods. The remaining atoms were located in an alternating series of least-squares cycles on difference Fourier maps. All atoms were refined in a full-matrix anisotropic approximation.

Computing details

Data collection: SMART (Bruker, 2002); cell refinement: SMART (Bruker, 2002); data reduction: SAINT (Bruker, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. The unit-cell of the title compound, emphasizing the CoO6 coordination polyhedra as transparent octahedra (blue in the electronic version of the journal; also Co light blue, O red, C black, Na dark green and Cl light green).
	Fig. 2. A section of the pyrochlore lattice assembled from Co4 tetrahedra. The regular hexagons around which the tetrahedra are arranged are emphasized by thick light-coloured lines (purple in the electronic version of the journal).
	Fig. 3. The structure of a {Co4(CO3)4} tetrahedron, the building block of the pyrochlore sublattice of the title compound.

cobalt(II) dicarbonate trisodium chloride

Crystal data

Co(CO3)2Na3Cl	Mo Kα radiation, λ = 0.71073 Å
Mr = 283.37	Cell parameters from 4453 reflections
Cubic, Fd3	θ = 2.5–28.2°
a = 13.9959 (5) Å	µ = 3.07 mm^−1
V = 2741.59 (17) Å^3	T = 123 K
Z = 16	Plate, pink
F(000) = 2192	0.14 × 0.14 × 0.08 mm
Dx = 2.746 Mg m^−3

Data collection

Bruker SMART CCD area-detector (APEXII given in exptl_special_details) diffractometer	294 independent reflections
Radiation source: fine-focus sealed tube	288 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.022
ϕ and ω scans	θmax = 28.2°, θmin = 2.5°
Absorption correction: multi-scan (SADABS; Sheldrick, 1996)	h = −18→18
Tmin = 0.67, Tmax = 0.74	k = −18→18
7452 measured reflections	l = −18→18

Refinement

Refinement on F^2	0 restraints
Least-squares matrix: full	Primary atom site location: structure-invariant direct methods
R[F^2 > 2σ(F^2)] = 0.016	Secondary atom site location: difference Fourier map
wR(F^2) = 0.047	w = 1/[σ^2(Fo^2) + (0.0207P)^2 + 6.4264P] where P = (Fo^2 + 2Fc^2)/3
S = 1.30	(Δ/σ)max < 0.001
294 reflections	Δρmax = 0.24 e Å^−3
22 parameters	Δρmin = −0.67 e Å^−3

Special details

Experimental. An X-ray quality crystal was selected under ambient conditions and covered with Paratone oil. The crystal was mounted and centred in the X-ray beam using a video camera.The crystal evaluation and data collection were performed on an APEXII CCD diffractometer with a detector-to-crystal distance of 5 cm. The initial cell constants were obtained from three series of ω scans at different starting angles. Each series consisted of 30 frames collected at intervals of 0.3 in a 10 range about ω with the exposure time of 10 s per frame. The obtained reflections were successfully indexed by an automated indexing routine built into the APEX2 program package. The final cell constants were calculated from a set of strong reflections from the actual data collection.

The data were collected using the full sphere routine by collecting four sets of frames with 0.3 scans in ω with an exposure time 10 s per frame. This data set was corrected for Lorentz and polarization effects. The absorption correction was based on a fit of a spherical harmonic function to the empirical transmission surface as sampled by multiple equivalent measurements using SADABS software (Sheldrick, 1996).

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

	x	y	z	Uiso*/Ueq
Co1	0.0000	0.5000	0.5000	0.00521 (16)
Na1	0.1250	0.34929 (5)	0.6250	0.00880 (19)
O1	−0.02419 (6)	0.64680 (6)	0.51647 (6)	0.0073 (2)
C1	0.03277 (8)	0.71723 (8)	0.53277 (8)	0.0058 (4)
Cl1	0.2500	0.2500	0.5000	0.0119 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Co1	0.00521 (16)	0.00521 (16)	0.00521 (16)	−0.00023 (7)	−0.00023 (7)	−0.00023 (7)
Na1	0.0083 (3)	0.0088 (3)	0.0093 (3)	0.000	−0.0011 (3)	0.000
O1	0.0068 (4)	0.0059 (4)	0.0094 (4)	−0.0015 (3)	−0.0005 (3)	−0.0005 (3)
C1	0.0058 (4)	0.0058 (4)	0.0058 (4)	0.0014 (4)	−0.0014 (4)	0.0014 (4)
Cl1	0.0119 (2)	0.0119 (2)	0.0119 (2)	0.00387 (15)	−0.00387 (15)	−0.00387 (15)

Geometric parameters (Å, º)

Co1—O1	2.0950 (9)	Na1—Cl1	2.8377 (4)
Co1—Na1	3.2512 (5)	Na1—Co1^iii	3.2512 (5)
Na1—O1^i	2.3528 (11)	Na1—Na1^iv	3.5358 (2)
Na1—O1^ii	2.4319 (9)	O1—C1	1.2882 (9)
O1—Co1—O1^v	86.08 (3)	O1^viii—Na1—Na1^iv	43.23 (2)
O1—Co1—Na1^v	111.68 (3)	O1^i—Na1—Na1^iv	125.13 (3)
O1^vi—Co1—Na1^v	46.23 (3)	O1^ix—Na1—Na1^iv	60.42 (2)
O1—Co1—Na1^vi	133.77 (3)	O1^ii—Na1—Na1^iv	119.27 (3)
O1^vii—Co1—Na1^vi	48.40 (3)	Cl1^iii—Na1—Na1^iv	136.27 (2)
Na1^v—Co1—Na1^vi	114.119 (8)	Cl1—Na1—Na1^iv	51.465 (8)
O1—Co1—Na1^i	68.32 (3)	Co1^iii—Na1—Na1^iv	57.060 (4)
O1^i—Co1—Na1^i	131.60 (3)	Co1—Na1—Na1^iv	114.31 (2)
O1^ii—Co1—Na1^i	111.68 (2)	O1^viii—Na1—Na1^x	96.74 (2)
Na1^vi—Co1—Na1^i	65.881 (8)	O1^i—Na1—Na1^x	74.30 (3)
O1^viii—Na1—O1^i	82.37 (5)	O1^ix—Na1—Na1^x	41.50 (2)
O1^viii—Na1—O1^ix	73.39 (4)	O1^ii—Na1—Na1^x	138.11 (3)
O1^i—Na1—O1^ix	104.60 (3)	Cl1^iii—Na1—Na1^x	51.465 (8)
O1^ix—Na1—O1^ii	177.42 (6)	Cl1—Na1—Na1^x	136.27 (2)
O1^viii—Na1—Cl1^iii	147.84 (3)	Co1^iii—Na1—Na1^x	57.059 (4)
O1^i—Na1—Cl1^iii	83.72 (2)	Na1^iv—Na1—Na1^x	101.01 (2)
O1^ix—Na1—Cl1^iii	82.34 (2)	Na1^iv—Na1—Na1^vi	77.78 (3)
O1^ii—Na1—Cl1^iii	98.93 (2)	Na1^x—Na1—Na1^vi	168.32 (2)
O1^viii—Na1—Cl1	83.72 (2)	O1^viii—Na1—Na1^v	125.13 (3)
Cl1^iii—Na1—Cl1	121.35 (3)	C1—O1—Co1	132.07 (9)
O1^viii—Na1—Co1^iii	40.01 (2)	C1—O1—Na1^vii	116.78 (5)
O1^i—Na1—Co1^iii	77.84 (3)	Co1—O1—Na1^vii	93.76 (3)
O1^ix—Na1—Co1^iii	40.10 (2)	C1—O1—Na1^ii	119.08 (10)
O1^ii—Na1—Co1^iii	137.37 (3)	Co1—O1—Na1^ii	91.50 (4)
Cl1—Na1—Co1^iii	108.525 (4)	Na1^vii—O1—Na1^ii	95.27 (3)
O1^ii—Na1—Co1	40.10 (2)	O1^xi—C1—O1^xii	119.969 (6)
Cl1—Na1—Co1	108.525 (4)	Na1—Cl1—Na1^iv	77.070 (16)
Co1^iii—Na1—Co1	99.10 (2)	Na1—Cl1—Na1^xiii	102.930 (16)

Symmetry codes: (i) z−1/2, x+1/2, y; (ii) −x, −y+1, −z+1; (iii) −x+1/4, y, −z+5/4; (iv) −z+1, x+1/4, y+1/4; (v) −z+1/2, −x+1/2, −y+1; (vi) −y+1/2, −z+1, −x+1/2; (vii) y−1/2, z, x+1/2; (viii) −z+3/4, x+1/2, −y+5/4; (ix) x+1/4, −y+1, z+1/4; (x) y−1/4, −z+1, x+3/4; (xi) z−1/2, −x+3/4, −y+5/4; (xii) −y+3/4, −z+5/4, x+1/2; (xiii) z−1/2, −x+1/4, −y+3/4.