Tetra­aqua­cobalt(II) bis­[vanadyl(IV) phosphate], [Co(H₂O)₄][VO(PO₄)]₂

The structure of [Co(H₂O)₄][VO(PO₄)]₂ is composed of [VO(PO₄)] layers and interlayer tetrahydrated Co²⁺ ions. Alternating VO₅ square pyramids and PO₄ tetrahedra share O-atom vertices, thus forming the vanadyl phosphate layers. Two vanadyl oxo groups from neighbouring layers are coordinated to each Co atom in a trans fashion, with Co-O distances of 2.157 (4) Å, thus generating a three-dimensional framework structure.

S1. Comment

Vanadium phosphate systems are of interest because of their catalytic properties and their diverse structural characteristics (Zubieta, 1994). To date, the V/P/O phases have been shown to possess a wide variety of structural types as result of the different coordination geometries of vanadium (e.g. octahedra, square pyramids, trigonal bipyramids and tetrahedra; Pope & Müller, 1991) and the different linkages between vanadium polyhedra and phosphate tetrahedra. Some examples of these phases are AVOPO₄ (Haushalter, 1994), A[VOPO₄]₂ (Grandin et al., 1992) and A_0.5VOPO₄.nH₂O (Kang et al., 1991), where A is an alkali or alkaline earth metal cation. Further structural diversity can be accomplished by introducing other inorganic and organic cations into this system (Duan et al., 2003). In the present paper, we report the hydrothermal synthesis and crystal structure of a new member of the A/V/P/O class of solids (A is a transition metal cation), viz. [Co(H₂O)₄][VO(PO₄)₂], (I).

The crystal structure of (I) consists of vanadium phosphorous oxide layers, with hydrated [Co(H₂O)₄]²⁺ units located in the interlayer positions (Fig. 1). The coordination spheres around the V and Co atoms in the asymmetric unit of (I) are shown in Fig. 2. The V atom exhibits a square-pyramidal geometry involving a vanadyl oxo group with a short V—O bond distance of 1.624 (4) Å and four basical O atoms with an equivalent V—O distance of 1.9959 (16) Å. Valence-sum calculations (Brown & Altermatt, 1985) resulted in a value of 3.897 for the V site, which is very close to the ideal value of 4 for V^IV. The coordination octahedron around the Co atom has D_{4 h} symmetry. The four equatorial O atoms from four water molecules have Co—O bond lengths of 2.064 (3) Å, and the two axial O atoms from the vanadyl oxo atoms of two adjacent vanadium phosphorous oxide layers each have a Co—O distance of 2.157 (4) Å.

A view perpendicular to one of the vanadium phosphorous oxide layers is illustrated in Fig. 3. The layer is a four-connected net of corner-sharing VO₅ square pyramids and PO₄ tetrahedra. Each VO₅ pyramid shares its four basical O atoms with four different PO₄ groups. Along the [110] direction, the vanadyl O atoms alternately point up and down relative to the layer. Two neighbouring layers are aligned so as to order the oxo groups of the vanadyl sites directly towards one another along the c axis, and then the two corresponding vanadyl oxo atoms are coordinated to a Co atom in a trans fashion, such that a linear V═O—Co—O═V group is formed (Fig. 1). Therefore, the structure of (I) can be described as vanadyl phosphate layers connected through Co—O covalent interactions into a three-dimensional open framework.

Compound (I) belongs to the family of compounds with the general composition A_0.5[VOPO₄].nH₂O (n = 2 or 1.5), which structures can be topologically derived from the parent layered compound VOPO₄·2H₂O (Tietz, 1981). The layer in VOPO₄·2H₂O is constructed via the alternation of vanadium octahedra and phosphate tetrahedra. The introduction of a mono- or divalent metal cation not only leads to the partial or full reduction of V⁵⁺ to V⁴⁺ but also affects the way that the layers are stacked relative to one another and the arrangement of water molecules between the layers, so as to accommodate the second metal ions. While Na, K, Ca, Sr (Kang et al., 1991) and Cu (Zhang et al., 1995) compounds of the family are layered, the structures of A_0.5[VOPO₄]·1.5H₂O (A = Ni and Pb; Lii & Mao, 1992; Tietz, 1981) exhibit three-dimensional architectures built from V^IVO₆ octahedra, PO₄ tetrahedra and AO₆ octahedra. In contrast, the three-dimensional network of (I) is fused by V^IVO₅ square pyramids, PO₄ tetrahedra and CoO₆ octahedra through sharing corners, and the stacking of the vanadyl phosphate layers in (I) can be viewed as the relative shifting of neighbouring parent layers in VOPO₄·2H₂O along the [110] direction by (a+b)/2.

S2. Experimental

Compound (I) was hydrothermally synthesized under autogenous pressure. A mixture of NH₄VO₃, CoCl₂·6H₂O, H₃PO₄ (85%), Na₂MoO₄·6H₂O, hexamethylene tetramine and H₂O in the molar ratio of 2:1:1.5:1:0.5:492 was sealed in a 17 ml Teflon-lined autoclave and heated at 433 K for 90 h. After the reaction mixture was cooled slowly to room temperature at a rate of 8 K h⁻¹, green plate-like crystals of (I) were obtained. The crystals were filtered off, washed with distilled water and dried in air (yield 45%, based on vanadium). The pH of the system increased from 5.2 before heating to 6.3 at the end of the reaction. The IR spectrum of (I) exhibits a strong band at 995 cm⁻¹, which was attributed to ν(V—O), and features at 1118 and 1027 cm⁻¹, which are related to PO₄. The weight loss of (I) in the range 538–663 K is 15.89%, in agreement with the calculated removal of the water molecules associated with the Co²⁺ cations (15.85%). Analysis calculated for CoH₈O₁₄P₂V₂: P 13.62, V 22.40, Co 12.96%; found: P 13.72, V 22.28, Co 12.91%.

S3. Refinement

H atoms were found in a difference Fourier map and their positions were fixed at calculated positions during refinement, with O—H distances of 0.829 Å and with a common isotropic displacement parameter [U_iso(H) = 0.05 Ų].

Computing details

Data collection: MSC/AFC Diffractometer Control Software (Molecular Structure Corporation, 1987); cell refinement: TEXSAN (Molecular Structure Corporation, 1987); data reduction: TEXRAY in TEXSAN; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL/PC (Sheldrick, 1990); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A representation of the structure of (I) along the a axis, showing the vanadyl phosphate layers fused by VO5 square pyramids and PO4 tetrahedra. These layers are connected via Co—O covalent interactions into a three-dimensional framework structure. The stripped and open circles denote cobalt and water O atoms, respectively, and H atoms have been omitted for clarity.
	Fig. 2. The asymmetric unit in the structure of (I), with ellipsoids at the 50% probability level, showing the coordination environment around the phosphorous and metal atoms. [Symmetry codes: (i) −x, −y, −z; (ii) y, −x, −z; (iii) −y, x, z; (iv) −x, −y, z; (v) y, −x, z; (vi) −y + 1/2, x − 1/2, −z + 1/2; (vii) −x + 1, −y, z; (viii) y + 1/2, −x + 1/2, −z + 1/2.]
	Fig. 3. A view perpendicular to one of the vanadium oxide layers in (I), showing the connectivities between the VO5 square pyramids and the PO4 tetrahedra.

Tetraaquacobalt(II) bis[vanadyl(IV) phosphate]

Crystal data

[Co(H2O)4][VO(PO4)]2	Dx = 2.789 Mg m^−3
Mr = 454.81	Mo Kα radiation, λ = 0.71069 Å
Tetragonal, I4/m	Cell parameters from 80 reflections
Hall symbol: -I 4	θ = 5–15°
a = 6.307 (1) Å	µ = 3.58 mm^−1
c = 13.615 (3) Å	T = 296 K
V = 541.58 (17) Å^3	Plate, green
Z = 2	0.15 × 0.10 × 0.10 mm
F(000) = 446

Data collection

Rigaku AFC-5R diffractometer	337 independent reflections
Radiation source: fine-focus sealed tube	318 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.033
Detector resolution: 10.0 pixels mm^-1	θmax = 27.5°, θmin = 3.0°
ω–2θ scans	h = 0→8
Absorption correction: ψ scan (North, Phillips & Mathews, 1968)	k = 0→8
Tmin = 0.656, Tmax = 0.699	l = 0→17
2204 measured 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)] = 0.031	H-atom parameters not refined
wR(F^2) = 0.087	w = 1/[σ^2(Fo^2) + (0.0335P)^2 + 0.8319P] where P = (Fo^2 + 2Fc^2)/3
S = 1.02	(Δ/σ)max = 0.001
337 reflections	Δρmax = 0.63 e Å^−3
27 parameters	Δρmin = −0.87 e Å^−3
0 restraints	Extinction correction: SHELXL97, Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.0092 (18)

Special details

Experimental. crystal coated in epoxy glue

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	0.0000	0.0000	0.0000	0.0178 (4)
V	0.0000	0.0000	0.27769 (6)	0.0053 (3)
P	0.5000	0.0000	0.2500	0.0060 (4)
O1	0.0000	0.0000	0.1584 (3)	0.0133 (8)
O2	0.3037 (3)	0.0162 (3)	0.31822 (13)	0.0095 (4)
O3	0.1756 (5)	0.2762 (5)	0.0000	0.0246 (7)
H1	0.1733	0.3512	0.0500	0.050*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Co	0.0195 (4)	0.0195 (4)	0.0142 (6)	0.000	0.000	0.000
V	0.0042 (3)	0.0042 (3)	0.0075 (6)	0.000	0.000	0.000
P	0.0041 (4)	0.0041 (4)	0.0099 (7)	0.000	0.000	0.000
O1	0.0154 (10)	0.0154 (10)	0.0092 (18)	0.000	0.000	0.000
O2	0.0055 (7)	0.0119 (8)	0.0112 (8)	−0.0003 (5)	0.0013 (7)	−0.0007 (7)
O3	0.0391 (18)	0.0220 (14)	0.0127 (12)	−0.0081 (12)	0.000	0.000

Geometric parameters (Å, º)

Co—O3	2.064 (3)	V—O2^iii	1.9959 (16)
Co—O3^i	2.064 (3)	V—O2^iv	1.9959 (16)
Co—O3^ii	2.064 (3)	V—O2^v	1.9959 (16)
Co—O3^iii	2.064 (3)	P—O2^vi	1.5511 (17)
Co—O1	2.157 (4)	P—O2^vii	1.5511 (17)
Co—O1^i	2.157 (4)	P—O2^viii	1.5511 (17)
V—O1	1.624 (4)	P—O2	1.5511 (17)
V—O2	1.9959 (16)	O3—H1	0.8290
O3—Co—O3^i	180.0	O2—V—O2^iii	85.62 (3)
O3—Co—O3^ii	90.0	O1—V—O2^iv	106.05 (6)
O3^i—Co—O3^ii	90.0	O2—V—O2^iv	147.90 (11)
O3—Co—O3^iii	90.0	O2^iii—V—O2^iv	85.62 (3)
O3^i—Co—O3^iii	90.0	O1—V—O2^v	106.05 (6)
O3^ii—Co—O3^iii	180.0	O2—V—O2^v	85.62 (3)
O3—Co—O1	90.0	O2^iii—V—O2^v	147.90 (11)
O3^i—Co—O1	90.0	O2^iv—V—O2^v	85.62 (3)
O3^ii—Co—O1	90.0	O2^vi—P—O2^vii	111.01 (7)
O3^iii—Co—O1	90.0	O2^vi—P—O2^viii	106.43 (13)
O3—Co—O1^i	90.0	O2^vii—P—O2^viii	111.01 (7)
O3^i—Co—O1^i	90.0	O2^vi—P—O2	111.01 (7)
O3^ii—Co—O1^i	90.0	O2^vii—P—O2	106.43 (13)
O3^iii—Co—O1^i	90.0	O2^viii—P—O2	111.01 (7)
O1—Co—O1^i	180.0	V—O1—Co	180.0
O1—V—O2	106.05 (6)	P—O2—V	126.66 (11)
O1—V—O2^iii	106.05 (6)	Co—O3—H1	118.2
O2^vi—P—O2—V	−49.78 (15)	O2^iii—V—O2—P	−113.20 (13)
O2^vii—P—O2—V	−170.69 (15)	O2^iv—V—O2—P	172.25 (12)
O2^viii—P—O2—V	68.40 (16)	O2^v—V—O2—P	97.71 (14)
O1—V—O2—P	−7.75 (12)

Symmetry codes: (i) −x, −y, −z; (ii) y, −x, −z; (iii) −y, x, z; (iv) −x, −y, z; (v) y, −x, z; (vi) −y+1/2, x−1/2, −z+1/2; (vii) −x+1, −y, z; (viii) y+1/2, −x+1/2, −z+1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H1···O2^ix	0.83	1.98	2.803 (2)	169

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