VF(C₄O₄)(H₂O)₂: a new layer type of diaqua­vanadium(III) fluoride squarate

Single crystals of the title compound, poly[diaqua-μ-fluoro-μ-squarato-vanadium(III)], [V(C₄O₄)F(H₂O)₂]_n, have been prepared hydro­thermally at 423 K. This compound is shown to have a layer-type structure. The V and F atoms are localized on the symmetry centers 2a and 2d, respectively, of the monoclinic system (space group P2₁/n) and they form infinite chains which are bridged by the squarate group acting as a bidentate ligand in a trans position. Two water mol­ecules complete the octa­hedral coordination of vanadium(III). The three-dimensionality is ensured by a strong O—H...O hydrogen bond.

S1. Comment

Several metal squarates have been reported in the literature. The squarate ligand with its fourfold C═O functionality not only is a potential multiple acceptor of hydrogen bonds but also can be involved either in polydentate (Lee et al., 1996, and references therein; Weiss et al., 1986; Neeraj et al., 2002) or in chelating (Robl & Weiss, 1987; Robl et al., 1987; Trombe et al., 1990) coordination modes depending principally of the metal size and the temperature of formation of the compound. The coordination chemistry of the vanadium squarates is less developed than those of other transition metals. However, original compounds, such as [V(OH)(H₂O)₂(C₄O₄)]₂·2H₂O (Brouca-Cabarrecq et al., 2004), [V(OH)(H₂O)(C₄O₄)]₂ and [V(OH)(C₄O₄)]₂·4H₂O (Lin & Lii, 1997), have been synthesized by the hydrothermal method. These three compounds differ by their respective dimensionalities, viz. zero-, two- and three-dimensional, respectively. The first compound is a dimer where the squarate group exhibits µ₂ coordination in a cis position. For the second and the third compounds, the squarate group acts as a bridging ligand in µ₃ and µ₄ coordination modes, respectively. Within these compounds, the V atom has an oxidation state of +3, as a result of the powerful reducing property of the squarate ligand in the synthesis conditions.

As a part of our continuing investigations on the V₂O₅/H₂C₄O₄ system, we have hydrothermally tried on this system the fluoride route using either HF or HBF₄ (see Experimental) (Mohanu, 2005). By using the last fluoride reactant at 423 K for 3 d, a new compound, VF(H₂O)₂(C₄O₄), (I), has been isolated as single crystals. This compound crystallizes in the monoclinic system, space group P2₁/n. The V and F atoms are localized on the symmetry centers 2a and 2d, respectively, and they form infinite chains, running in the x direction (Fig. 1), the V—F distance being equal to 1.8895 (2) Å (Table 1). To complete the octahedral geometry, each V^III site bonds to two squarate O atoms and to two water molecules, at slightly distances than the V—F bond length (Table 1). The V^III octahedron is quite regular; the angles do not deviate from a right angle by more of 3.17°. The V^III oxidation state assignment is consistent with a valence sum calculation (Brown & Altermatt, 1985) that gives a value of 3.12, and with the charge requirement of the material. Furthermore, such an oxidation state agrees well with the experimental magnetic moment (2.51 µB, versus 2.55 µB for VF₃; Pascal, 1958). These V—F chains are bridged by a squarate ligand in µ₂ coordination in the trans position, leading to a layer parallel to the (010) plane. The C—C bond lengths of the squarate ligand are equal within the s.u. values (Table 1) and the corresponding C—C—C angles are within 0.76° of a right angle. The slight difference in the C—O bond lengths is a consequence of the fact that atom O1 is only bound to the V atom, while O2 is involved only in hydrogen bonds. However, these values are normal for a squarate ligand (Lee et al., 1996) and in particular they indicate a delocalization of the π electrons in the aromatic cycle (West, 1980). Within this layer, two adjacent squarate groups are parallel and separated by a distance of 3.416–3.779 Å. Owing to the aromatic character of the squarate species, such distances are indicative of van der Waals interactions in the interval separating two adjacent squarates. That interaction strengthens the cohesion of the layer. The H atoms of the water molecule (O3) gives two types of strong hydrogen bonds towards atom O2 in different symmetry positions (Fig. 2 and Table 2); the stronger one, which involves atom H3A, relates two adjacent layers ensuring the three-dimensionality and the other one, which involves atom H3B, strengthens once more the cohesion of the layer. The formula of this compound, VF(H₂O)₂(C₄O₄), is similar to [V(OH)(H₂O)₂(C₄O₄)]₂·2H₂O (Brouca-Cabarrecq et al., 2004), [V(OH)(H₂O)(C₄O₄)]₂ and [V(OH)(C₄O₄)]₂·4H₂O (Lin & Lii, 1997) with the hydroxy groups replaced by a fluoride ion. However, in these three compounds, two V atoms share an edge of the hydroxy groups and their changes of dimensionality (zero-, one- [two- above] and three-dimensional) are only due to the coordination mode of the squarate ligand (µ₂, µ₃ and µ₄, respectively). This new layered compound is, strictly speaking, a hybrid material, featuring vanadium–fluoride chains connected by an organic ligand.

S2. Experimental

Brown single crystals of the title compound were obtained from an aqueous suspension (5 ml) of V₂O₅ (0.182 g), H₂C₄O₄ (0.342 g) and diluted HBF₄ (3 mM) in the molar ratio 1:3:3. These reactants were sealed in a teflon-lined (about 15 ml) steel bomb (autogeneous pressure) and heated at 423 K for 3 d. After cooling, the precipitate was filtered off, washed with distilled water and dried at room temperature. The value of the final pH was about 1.0–1.4. The X-ray powder pattern of the whole material was consistent with the theoretical diffractogram of the single-crystal. Under the same conditions, using HF leads to the layer [V(OH)(H₂O)(C₄O₄)]₂ already reported by Lin & Lii (1997), and a final pH value ranging from 2.5 to 3.0. These pH differences appear a salient factor in the formation of the different phases. The use of HF instead of HBF₄ increases the concentration of [OH]^- by a factor of about ten. For a high fluoride level, at 473 K for 3 d, the V₂O₅, H₂C₄O₄ and HF system in the molar ratio 1:1:6 gives V₂F₆·H₂O as single crystals, a compound already known (Barthelet et al., 2002).

S3. Refinement

Direct methods associated with SHELXL97 (Sheldrick, 1997) quickly revealed the sites of heavy atoms, leading to a rough chemical formula VFO₂(C₄O₄). Analysis of the vanadium coordination showed that the charge of the V atom was 3+, implying that the electric balance was not ensured. From a difference Fourier synthesis it was then possible to determine two H atoms, localized on a water molecule (O3). The H atoms were placed in positions determined from the difference map, and were allowed to ride on atom O3 with a U_iso(H) value equal to 1.2U_eq(O3). Refinement of the positional and anisotropic displacement parameters gave a very good R value, the formula V³⁺F(H₂O)₂(C₄O₄) being well established, with Z= 2 units per cell.

Computing details

Data collection: COLLECT (Nonius 1998); cell refinement: DENZO-SMN (Otwinowski & Minor, 1997); data reduction: DENZO-SMN; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII (Burnett & Johnson, 1996); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view on to the (010) plane (slightly inclined) of the structure of (I). Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) -x + 1, -y + 1, -z; (iv) x - 1, y, z; (v) -x + 1, -y + 1, -z + 1.]
	Fig. 2. A view, along the x direction, of adjacent layers connected by only hydrogen-bond interactions. [Symmetry codes: (ii) x, y, 1 + z; (iii) 1/2 + x, 1/2 - y, 1/2 + z.]

poly[diaqua-µ-fluoro-µ-squarato-vanadium(III)]

Crystal data

[V(C4O4)F(H2O)2]	F(000) = 216
Mr = 218.01	Dx = 2.229 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2yn	Cell parameters from 1121 reflections
a = 3.7790 (4) Å	θ = 6.1–32.0°
b = 11.2070 (8) Å	µ = 1.54 mm^−1
c = 7.8410 (7) Å	T = 295 K
β = 102.019 (9)°	Platelet, brown
V = 324.80 (5) Å^3	0.60 × 0.40 × 0.03 mm
Z = 2

Data collection

Nonius KappaCCD area-detector diffractometer	1121 independent reflections
Radiation source: fine-focus sealed tube	709 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.080
ψ and ω scans	θmax = 32.0°, θmin = 6.1°
Absorption correction: multi-scan (SORTAV; Blessing, 1995)	h = −5→5
Tmin = 0.512, Tmax = 0.956	k = −16→16
5487 measured reflections	l = −8→11

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)] = 0.035	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.075	H-atom parameters constrained
S = 0.95	w = 1/[σ^2(Fo^2) + (0.0314P)^2] where P = (Fo^2 + 2Fc^2)/3
1121 reflections	(Δ/σ)max < 0.001
58 parameters	Δρmax = 0.42 e Å^−3
0 restraints	Δρmin = −0.49 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 > 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
V	0.5000	0.5000	0.5000	0.01408 (14)
F	1.0000	0.5000	0.5000	0.0295 (4)
O1	0.3863 (4)	0.43415 (13)	0.25861 (16)	0.0193 (3)
O2	0.3009 (4)	0.32002 (14)	−0.11716 (19)	0.0283 (4)
C1	0.4523 (5)	0.47278 (17)	0.1173 (3)	0.0145 (4)
C2	0.4097 (5)	0.41845 (18)	−0.0537 (3)	0.0160 (4)
O3	0.5368 (4)	0.33348 (12)	0.5817 (2)	0.0228 (3)
H3A	0.6400	0.2814	0.5305	0.027*
H3B	0.4550	0.2992	0.6634	0.027*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
V	0.0219 (2)	0.0134 (2)	0.0077 (2)	0.0006 (2)	0.00485 (16)	0.0002 (2)
F	0.0211 (8)	0.0368 (11)	0.0320 (10)	−0.0004 (8)	0.0085 (7)	−0.0010 (9)
O1	0.0299 (8)	0.0215 (8)	0.0081 (7)	−0.0066 (6)	0.0077 (6)	−0.0017 (6)
O2	0.0526 (10)	0.0199 (8)	0.0160 (8)	−0.0159 (7)	0.0158 (7)	−0.0056 (6)
C1	0.0159 (9)	0.0169 (11)	0.0109 (9)	0.0000 (7)	0.0033 (7)	−0.0006 (7)
C2	0.0198 (9)	0.0178 (11)	0.0112 (9)	−0.0029 (7)	0.0052 (7)	−0.0007 (8)
O3	0.0404 (8)	0.0153 (8)	0.0174 (8)	0.0050 (6)	0.0167 (6)	0.0024 (6)

Geometric parameters (Å, º)

V—F	1.8895 (2)	C1—C2	1.451 (3)
V—O3	1.9687 (14)	C1—C2^i	1.454 (3)
V—O1	1.9932 (13)	O3—H3A	0.8484
O1—C1	1.262 (2)	O3—H3B	0.8577
O2—C2	1.244 (2)
F—V—O3	89.84 (4)	C2—C1—C2^i	90.76 (16)
F—V—O1	91.02 (4)	O2—C2—C1	134.85 (18)
O3—V—O1	86.83 (6)	O2—C2—C1^i	135.9 (2)
C1—O1—V	131.58 (13)	V—O3—H3A	120.4
O1—C1—C2	131.56 (18)	V—O3—H3B	131.0
O1—C1—C2^i	137.68 (19)	H3A—O3—H3B	108.5

Symmetry code: (i) −x+1, −y+1, −z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H3B···O2^ii	0.86	1.94	2.696 (2)	146
O3—H3A···O2^iii	0.85	1.82	2.6525 (19)	169

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