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.
; Luan et al.
). Several structures have also been reported with the introduction of organic ammonium cations into the V–P–O system (Zhang et al.
; Soghomonian et al.
; Luan et al.
). A few structures with introduction of inorganic cations into the P—V—O system have also been reported (Soghomonian et al.
; Khan et al.
; Tian & Wu, 2002
). In the present paper, we report a new compound, Ba5
, in which the oxovanadium phosphate framework is templated by an unusual cationic barium chloride hydrate cluster of composition Ba5
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 [VO5
] 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 Ba2+
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 VIV
in a ratio of 3:1, i.e.
two of the eight V atoms are VV
and six are VIV
. 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 VIV
= 3:1. Similar mixed bond-valence sums for V atoms have been reported in other oxovanadium phosphates (Zhang et al.
; Le Fur et al.
). 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.
Figure 1 The asymmetric unit and the complete coordination environments of the V, Ba and P atoms in the structure of Ba5Cl4(H2O)8(VPO5)8. Displacement ellipsoids are drawn at the 50% probability level. [Symmetry codes: (i) −x, − + (more ...)
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 (VPO5
) and cationic barium chloride hydrate, Ba5
. The VPO5
layers consist of a checkerboard pattern of corner-sharing VO5
square pyramids and PO4
tetrahedra in the bc
plane, and are similar to those found in Na3
(Massa et al.
) (Fig. 2). In the layers of barium chloride hydrate, clusters of three Ba2+
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 VPO5
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).
A view of the VPO5 layers along the a axis. The VO5 square pyramids are larger and the PO4 tetrahedra are smaller.
A view of the Ba5Cl4(H2O)8 layers along the a axis.
The three-dimensional structure of Ba5Cl4(H2O)8(VPO5)8, viewed along the b axis. The layers parallel to the bc plane can be seen to stack along the a axis.