The V–P–O system has received considerable attention, not only because of its application to catalysis, but also due to its rich and impressive structural chemistry associated with the ability of vanadium to have tetrahedral, square-pyramidal and octahedral coordination environments in various oxidation states. The introduction of pyridinium cations into the V–P–O system leads to rather complicated structures (Huang
et al., 2001
![[triangle]](/corehtml/pmc/pmcents/rtrif.gif)
; Luan
et al., 2001
). Several structures have also been reported with the introduction of organic ammonium cations into the V–P–O system (Zhang
et al., 1995
; Soghomonian
et al., 1996
; Luan
et al., 2003
). A few structures with introduction of inorganic cations into the P—V—O system have also been reported (Soghomonian
et al., 1998
; Khan
et al., 1996
; Tian & Wu, 2002
). In the present paper, we report a new compound, Ba
5Cl
4(H
2O)
8(PVO
5)
8, in which the oxovanadium phosphate framework is templated by an unusual cationic barium chloride hydrate cluster of composition Ba
5Cl
4(H
2O)
8.
Ba
5Cl
4(H
2O)
8(VPO
5)
8 crystallizes in the orthorhombic space group
Cmca with a unit cell consisting of four asymmetric units. The coordination environments of the V, P and Ba atoms are shown in Fig. 1. Each of the two independent V sites in the asymmetric unit exhibits a distorted square-pyramidal [VO
5] geometry. The basal positions are defined by O-atom donors from four adjacent phosphate groups and the apical O atoms (O1 and O2) are coordinated to adjacent Ba
2+ cations. Based on the stoichiometry of the compound and assuming normal oxidation states for Ba, Cl, P and O, the oxidation states of vanadium are V
IV and V
V in a ratio of 3:1,
i.e. two of the eight V atoms are V
V and six are V
IV. Bond-valence sum calculations (Brese & O’Keeffe, 1991
) for V1 and V2 give values of 4.42 and 4.35, respectively. The mean valence, 4.385, is reasonably close to 4.25 for V
IV:V
V = 3:1. Similar mixed bond-valence sums for V atoms have been reported in other oxovanadium phosphates (Zhang
et al., 1999
; Le Fur
et al., 2001
). Atoms Ba1, Ba2, Cl1, Cl2 and O11–O14 are situated on the (100) mirror plane, while atom Ba3 is on the intersection of the (100) plane and the twofold axis parallel to
a. Atom P1 is on the twofold axis parallel to
b and atom P3 is on the twofold axis parallel to
a. All other atoms,
i.e. V1, V2, P2 and O1–O10, are on general positions.
Atoms Ba1 and Ba2 are both 11-coordinate, with four phosphate O atoms, two vanadyl oxide atoms, three water molecules and two bridging chloride anions. Atom Ba3 is ten-coordinate, with four phosphate O atoms, four water molecules and two bridging chloride anions. The Ba—O bond lengths range from 2.725 (4) to 3.086 (7) Å, while the O—Ba—O angles span the range 46.77 (10)–180°. The Ba—Cl bond lengths vary from 3.225 (2) to 3.362 (2) Å, with the Cl—Ba—Cl angles in the range 131.61 (5)–180°.
There are two kinds of layer in the structure, namely anionic oxovanadium phosphate (VPO
5) and cationic barium chloride hydrate, Ba
5Cl
4(H
2O)
8. The VPO
5 layers consist of a checkerboard pattern of corner-sharing VO
5 square pyramids and PO
4 tetrahedra in the
bc plane, and are similar to those found in Na
3V
2O
2F(PO
4)
2 (Massa
et al., 2002
) (Fig. 2). In the layers of barium chloride hydrate, clusters of three Ba
2+ cations are linked
via additional bridging of the chloride anions and water molecules to form a two-dimensional network in the
bc plane (Fig. 3). To our knowledge, this barium chloride hydrate structural unit is unprecedented. Adjacent VPO
5 and Ba
5Cl
4(H
2O)
8 layers are connected by Ba—O bonds involving both phosphate and vanadyl O atoms of the anionic layers, and the layers alternate along the
a axis to generate the three-dimensional structure (Fig. 4).
