A triclinic polymorph of dicadmium divanadate(V)

The title compound, Cd₂V₂O₇, was obtained under hydro­thermal conditions. Different from the known monoclinic form, the new polymorph of Cd₂V₂O₇ has triclinic symmetry and is isotypic with Ca₂V₂O₇. The building units of the crystal structure are two Cd²⁺ cations, with coordination numbers of six and seven, and two V atoms with a tetra­hedral and a significantly distorted trigonal–pyramidal coordination environment, respectively. Two VO₅ pyramids share an edge and each pyramid is connected to one VO₄ tetra­hedron via a corner atom, forming an isolated V₄O₁₄⁸⁻ anion. These anions are arranged in sheets parallel to (-211) and are linked through the Cd²⁺ cations into a three-dimensional framework structure.

Related literature

For Ca₂V₂O₇, isotypic with the title compound, see: Trunov et al. (1983). For the structure of the monoclinic polymorph of Cd₂V₂O₇, see: Au & Calvo (1967). For the thermal stability of the monoclinic polymorph, see: Krasnenko & Rotermel (2010). For applications of vanadates, see: Jin et al. (2013); Valverde et al. (2012). For bond-valence analysis, see: Brown & Altermatt (1985).

Experimental

Crystal data

• Cd₂V₂O₇

• M_r = 438.68

• Triclinic,

• a = 6.5974 (2) Å

• b = 6.8994 (2) Å

• c = 6.9961 (2) Å

• α = 83.325 (1)°

• β = 63.898 (1)°

• γ = 80.145 (1)°

• V = 281.45 (1) ų

• Z = 2

• Mo Kα radiation

• μ = 10.65 mm⁻¹

• T = 296 K

• 0.29 × 0.17 × 0.12 mm

Data collection

• Bruker X8 APEX diffractometer

• Absorption correction: multi-scan (SADABS; Bruker, 2009) T_min = 0.164, T_max = 0.376

• 10113 measured reflections

• 2134 independent reflections

• 2077 reflections with I > 2σ(I)

• R_int = 0.025

Refinement

• R[F² > 2σ(F²)] = 0.016

• wR(F²) = 0.037

• S = 1.21

• 2134 reflections

• 100 parameters

• Δρ_max = 0.70 e Å⁻³

• Δρ_min = −1.53 e Å⁻³

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT; program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Supporting information

S1. Comment

Vanadate-based compounds have received a great extent of interest and still remain promising functional materials. Their structural diversity, mainly associated with the ability of vanadium to form different anions like (VO₄)^3-, (V₂O₇)^4-, (V₄O₁₂)^4-, (V₁₀O₂₈)^6-, is strongly required for catalysis applications (Jin et al., 2013; Valverde et al., 2012).

A bibliographic analysis revealed that pyrovanadates with the (V₂O₇)^4- anion can adopt different symmetries. For example, Cu₂V₂O₇, Mn₂V₂O₇ and Mg₂V₂O₇ are polymorphic and exhibit both monoclinic and triclinic varieties, the Cu-member additionally an orthorhombic form. In the case of Co₂V₂O₇, Ni₂V₂O₇, and Cd₂V₂O₇ only one monoclinic form is yet known, for Zn₂V₂O₇ two monoclinic forms are reported. Ca₂V₂O₇ and Ba₂V₂O₇ crystallize in the triclinic system, Sr₂V₂O₇ is likewise polymorphic, with triclinic and tetragonal varieties. We report here on the crystal structure determination of a new form of Cd₂V₂O₇ that was hydrothermally synthesized. In contrast to the known monoclinic polymorph (Au & Calvo, 1967) that is stable from ambient temperature to 1173 K (Krasnenko et al., 2010), this new form has triclinic symmetry and is isotypic with Ca₂V₂O₇ (Trunov et al., 1983).

The structure of the title compound is built up from two types of vanadium sites and two types of cadmium sites, each with a different oxygen coordination as shown in Fig. 1. The coordination environment of V1 is tetrahedral with V1–O distances in the range 1.6882 (13) Å - 1.7708 (13) Å. V2 is surrounded by five oxygen atoms with four V2—O distances ranging from 1.6612 (14) Å to 1.8535 (13) Å and the fifth O atom at a longer distance V2–O1 = 2.0348 (13) Å, forming a distorted trigonal V2O₅^5- pyramid. The bond valence sum calculation (Brown & Altermatt, 1985) for V1 and V2 are as expected, viz. 4.99 and 5.11 valence units, respectively. This result confirms the involvement of O1 in the V2 environment. The V2O₅^5- pyramids build a dimeric unit (V2)₂O₈^6- by sharing an edge. A corner atom (O1) of each of the pyramids is also part of a V1O₄ tetrahedron. This linkeage leads to the formation of a centrosymmetric (V₄O₁₄)^8- anion. This type of anion is rarely encountered in the crystal chemistry of pyrovanadates. Like in the monoclinic Cd₂V₂O₇ variety (Au and Calvo, 1967), (V₂O₇)^4- groups made up from two corner-sharing VO₄ tetrahedra are more commonly observed.

The two independent cadmium sites Cd1 and Cd2 are surrounded by six and seven oxygen atoms, respectively, in the form of distorted polyhedra. The Cd—O distance range from 2.2401 (13) Å to 2.5300 (13) Å for Cd1O₇ and from 2.2449 (14) Å to 2.4562 (14) Å for Cd2O₆. As shown in Fig. 2, edge-sharing CdO₆ and CdO₇ polyhedra built up sheets parallel to (211) with 8-membered open rings. Two adjacent layers are linked by the (V₄O₁₄)^8- groups into a three-dimensional framework (Fig. 3). In the monoclinic Cd₂V₂O₇ polymorph, the CdO₆ octahedra share edges and form sheets with six-membered rings that are linked by (V₂O₇)^4- groups.

Related literature

For Ca₂V₂O₇, isotypic with the title compound, see: Trunov et al. (1983). For the structure of the monoclinic polymorph of Cd₂V₂O₇, see: Au & Calvo (1967). For the thermal stability of the monoclinic polymorph, see: Krasnenko & Rotermel (2010). For applications of vanadates, see: Jin et al. (2013); Valverde et al. (2012). For bond-valence analysis, see: Brown & Altermatt (1985).

S2. Experimental

Crystals of the title compound were isolated from the hydrothermal treatment of cadmium oxide, ammonium metavanadate and zinc oxide in a molar ratio Cd:V:Zn = 11:8:1. Zinc oxide, not present in the obtained crystals, presumably acted as a mineralizing agent. The hydrothermal reaction was conducted in a 23 ml Teflon-lined autoclave, filled to 50% with distilled water and under autogeneous pressure at 493 K for four days. After being filtered off, washed with deionized water and air dried, the reaction product consists of colourless sheet-shaped crystals corresponding to the title compound.

S3. Refinement

The highest peak and the deepest hole in the final Fourier map are at 1.69 Å and 0.08 Å, from O2 and Cd2, respectively. Reflections (200) and (020) were omitted from the refinement due to large differences between observed and calculated intensities.

Computing details

Data collection: APEX2 (Bruker, 2009); cell refinement: SAINT (Bruker, 2009); data reduction: SAINT (Bruker, 2009); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: ORTEP-3 for Windows (Farrugia, 2012) and DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2010).

Figures

	Fig. 1. The coordination environment of the Cd and V sites in the crystal structure of the title compound, triclinic Cd2V2O7. Displacement ellipsoids are drawn at the 70% probability level. [Symmetry codes: (i) -x, -y + 1, -z + 1; (ii) -x + 1, -y, -z + 1; (iii) -x + 1, -y + 1, -z + 1; (iv) x - 1, y, z + 1; (v) x, y, z + 1; (vi) -x + 2, -y, -z + 1.]
	Fig. 2. Polyhedral representation of triclinic Cd2V2O7, showing the sheets of cadmium- and vanadium-oxygen polyhedra.
	Fig. 3. A three dimensional view of the crystal structure of triclinic Cd2V2O7 showing the stacking of the cadium and vanadium layers that extend parallel to (211).

Dicadmium divanadate(V)

Crystal data

Cd2V2O7	Z = 2
Mr = 438.68	F(000) = 396
Triclinic, P1	Dx = 5.176 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 6.5974 (2) Å	Cell parameters from 2717 reflections
b = 6.8994 (2) Å	θ = 3.0–33.1°
c = 6.9961 (2) Å	µ = 10.65 mm^−1
α = 83.325 (1)°	T = 296 K
β = 63.898 (1)°	Block, colourless
γ = 80.145 (1)°	0.29 × 0.17 × 0.12 mm
V = 281.45 (1) Å^3

Data collection

Bruker X8 APEX diffractometer	2134 independent reflections
Radiation source: fine-focus sealed tube	2077 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.025
ϕ and ω scans	θmax = 33.1°, θmin = 3.0°
Absorption correction: multi-scan (SADABS; Bruker, 2009)	h = −10→10
Tmin = 0.164, Tmax = 0.376	k = −10→10
10113 measured reflections	l = −10→10

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) = 0.037	w = 1/[σ^2(Fo^2) + (0.0148P)^2 + 0.2572P] where P = (Fo^2 + 2Fc^2)/3
S = 1.21	(Δ/σ)max = 0.001
2134 reflections	Δρmax = 0.70 e Å^−3
100 parameters	Δρmin = −1.53 e Å^−3

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. 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 > 2σ(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
Cd1	0.24214 (2)	0.336697 (18)	0.83258 (2)	0.00767 (4)
Cd2	0.74980 (2)	0.034380 (18)	0.75748 (2)	0.00845 (4)
V1	0.71038 (5)	0.16450 (4)	0.25864 (4)	0.00469 (5)
V2	0.22836 (5)	0.45517 (4)	0.34409 (5)	0.00542 (5)
O1	0.8612 (2)	0.3328 (2)	0.0816 (2)	0.0100 (2)
O2	0.8622 (2)	0.0439 (2)	0.3907 (2)	0.0099 (2)
O3	0.4592 (2)	0.2893 (2)	0.4363 (2)	0.0092 (2)
O4	0.6546 (2)	−0.00638 (19)	0.1233 (2)	0.0086 (2)
O5	0.2714 (2)	0.29481 (19)	0.1660 (2)	0.0093 (2)
O6	0.3839 (2)	0.6438 (2)	0.2436 (2)	0.0104 (2)
O7	−0.0496 (2)	0.5892 (2)	0.3678 (2)	0.0100 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cd1	0.00672 (6)	0.00774 (6)	0.00900 (6)	−0.00006 (4)	−0.00431 (4)	0.00047 (4)
Cd2	0.00779 (6)	0.00795 (6)	0.00808 (6)	0.00031 (4)	−0.00277 (4)	0.00069 (4)
V1	0.00423 (11)	0.00515 (11)	0.00483 (11)	−0.00033 (8)	−0.00222 (9)	−0.00007 (9)
V2	0.00472 (11)	0.00635 (12)	0.00446 (11)	−0.00089 (9)	−0.00115 (9)	−0.00065 (9)
O1	0.0089 (5)	0.0096 (5)	0.0098 (5)	−0.0031 (4)	−0.0025 (5)	0.0023 (4)
O2	0.0080 (5)	0.0124 (6)	0.0090 (5)	0.0009 (4)	−0.0044 (4)	0.0003 (4)
O3	0.0070 (5)	0.0116 (6)	0.0079 (5)	0.0021 (4)	−0.0032 (4)	−0.0013 (4)
O4	0.0110 (6)	0.0068 (5)	0.0095 (5)	−0.0012 (4)	−0.0054 (5)	−0.0014 (4)
O5	0.0128 (6)	0.0079 (5)	0.0083 (5)	0.0002 (4)	−0.0057 (5)	−0.0015 (4)
O6	0.0081 (5)	0.0079 (5)	0.0147 (6)	−0.0021 (4)	−0.0046 (5)	0.0014 (4)
O7	0.0056 (5)	0.0157 (6)	0.0064 (5)	0.0026 (4)	−0.0020 (4)	0.0004 (4)

Geometric parameters (Å, º)

Cd1—O7^i	2.2401 (13)	Cd2—O4^ii	2.4562 (14)
Cd1—O4^ii	2.2898 (13)	V1—O1	1.6882 (13)
Cd1—O6^iii	2.3083 (14)	V1—O2	1.7028 (14)
Cd1—O1^iii	2.3345 (14)	V1—O3	1.7265 (13)
Cd1—O1^iv	2.3476 (13)	V1—O4	1.7708 (13)
Cd1—O5^v	2.4043 (13)	V2—O5	1.6612 (14)
Cd1—O3	2.5300 (13)	V2—O6	1.6885 (14)
Cd2—O6^iii	2.2449 (14)	V2—O7	1.8530 (13)
Cd2—O5^ii	2.2858 (13)	V2—O7^i	1.8535 (13)
Cd2—O2^vi	2.2894 (14)	V2—O3	2.0348 (13)
Cd2—O2	2.3327 (14)	V2—V2^i	2.8482 (6)
Cd2—O4^v	2.3459 (13)
O7^i—Cd1—O4^ii	114.29 (5)	O2^vi—Cd2—O2	74.71 (5)
O7^i—Cd1—O6^iii	131.24 (5)	O6^iii—Cd2—O4^v	96.72 (5)
O4^ii—Cd1—O6^iii	83.66 (5)	O5^ii—Cd2—O4^v	75.42 (5)
O7^i—Cd1—O1^iii	85.81 (5)	O2^vi—Cd2—O4^v	102.17 (5)
O4^ii—Cd1—O1^iii	157.35 (5)	O2—Cd2—O4^v	174.49 (5)
O6^iii—Cd1—O1^iii	90.73 (5)	O6^iii—Cd2—O4^ii	81.30 (5)
O7^i—Cd1—O1^iv	76.73 (5)	O5^ii—Cd2—O4^ii	75.69 (5)
O4^ii—Cd1—O1^iv	94.46 (5)	O2^vi—Cd2—O4^ii	160.52 (5)
O6^iii—Cd1—O1^iv	149.98 (5)	O2—Cd2—O4^ii	98.40 (5)
O1^iii—Cd1—O1^iv	79.55 (5)	O4^v—Cd2—O4^ii	83.09 (5)
O7^i—Cd1—O5^v	153.49 (5)	O1—V1—O2	109.38 (7)
O4^ii—Cd1—O5^v	74.22 (5)	O1—V1—O3	107.57 (7)
O6^iii—Cd1—O5^v	73.06 (5)	O2—V1—O3	109.85 (7)
O1^iii—Cd1—O5^v	83.15 (5)	O1—V1—O4	109.71 (7)
O1^iv—Cd1—O5^v	77.60 (5)	O2—V1—O4	109.65 (7)
O7^i—Cd1—O3	62.10 (5)	O3—V1—O4	110.65 (6)
O4^ii—Cd1—O3	86.49 (5)	O5—V2—O6	114.25 (7)
O6^iii—Cd1—O3	75.19 (5)	O5—V2—O7	99.11 (7)
O1^iii—Cd1—O3	113.31 (5)	O6—V2—O7	98.25 (7)
O1^iv—Cd1—O3	134.73 (5)	O5—V2—O7^i	121.64 (7)
O5^v—Cd1—O3	144.28 (4)	O6—V2—O7^i	123.74 (7)
O6^iii—Cd2—O5^ii	156.37 (5)	O7—V2—O7^i	79.57 (6)
O6^iii—Cd2—O2^vi	116.20 (5)	O5—V2—O3	92.06 (6)
O5^ii—Cd2—O2^vi	87.38 (5)	O6—V2—O3	93.85 (6)
O6^iii—Cd2—O2	88.75 (5)	O7—V2—O3	158.40 (6)
O5^ii—Cd2—O2	99.75 (5)	O7^i—V2—O3	78.83 (6)

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