A new type of mixed anionic framework in microporous rubidium copper vanad­yl(V) phosphate, Rb₂Cu(VO₂)₂(PO₄)₂

In dirubidium copper bis[vanadyl(V)] bis­(phosphate), Rb₂Cu(VO₂)₂(PO₄)₂, three different oxo complexes form an anionic framework. VO₅ polyhedra in a trigonal bipyramidal configuration and PO₄ tetra­hedra share vertices to form eight-membered rings, which lie in layers perpendicular to the a axis of the monoclinic unit cell. Cu atoms at centres of symmetry have square-planar coordination and link these layers along [100] to form a three-dimensional anionic framework, viz. [Cu(VO₂)₂(PO₄)₂]_∞²⁻. Inter­secting channels in the [100], [001] and [011] directions contain Rb⁺ cations. Topological relations between this new structure type and the crystal structures of A(VO₂)(PO₄) (A = Ba, Sr or Pb) and BaCrF₂LiF₄ are discussed.

S1. Comment

The rich chemistry of vanadium includes a range of coordination geometries with oxygen in crystal structures which include tetrahedral, pentahedral, trigonal pyramidal, tetragonal–pyramidal and octahedral. Combined with its variable oxidation states of 2, 3, 4 and 5, this leads to a large diversity of vanadium-containing structures. The amphoteric character of vanadium oxo complexes explains the peculiarities of its crystal chemistry: these oxo complexes can have both cation- and anion-forming functions in mineral and biological processes (Baran, 2003). As a cation (V³⁺, VO²⁺, VO₂⁺), vanadium acts like a typical transition metal, while its anionic form (VO₄^3-) resembles phosphorous in phosphates. In spite of the fact that phosphate and vanadate minerals often have isotypic crystal structures, there are no cases of isomorphous substitution between VO₄ and PO₄ tetrahedra, most certainly due to the large difference in the sizes of V⁵⁺ and P⁵⁺ ions. However, a solid solution between VO₄ and AsO₄ is known, as found in the crystal structure of the volcanic mineral coparsite, Cu₄ClO₂[(As_0.5V_0.5)O₄] (Starova et al., 1998).

Mineral and synthetic phases with complex anions and open framework structures have been intensively studied over the last two decades. Among them, vanadyl phosphates seem to be promising due to their potential applications as catalytic materials, sorbents, molecular sieves or ion-exchange materials similar to zeolites (Centi et al., 1988). As part of our investigation of these types of compounds (Massa et al., 2002; Yakubovich et al., 2006), we present here the title compound, which has a microporous structure with an open mixed framework formed by Cu, V and P oxo complexes.

The V⁵⁺ ions in the structure (Fig. 1) occupy strongly distorted five-vertex VO₅ polyhedra. The two shortest V1—O bonds (Table 1) are typical of vanadyl groups, while the three longer V1—O distances correspond to V1—O bonds where the O atoms belong to PO₄ tetrahedra. The Cu1—O distances around the square-planar Cu²⁺ cation at the centre of symmetry are 1.9072 (17) and 1.9752 (16) Å. The largest cation–oxygen distances around V1, Cu1 and P1 involve atom O2, which is shared by three polyhedra (the so-called `loop configuration'). The VO₅ polyhedra approach a trigonal-bipyramidal configuration and, together with the PO₄ tetrahedra, form mixed anionic layers parallel to the bc plane at x = 0 (Fig. 2). Alternating vertex-sharing VO₅ bipyramids and PO₄ tetrahedra form both four-membered and eight-membered rings within these layers. The vanadyl groups, V═O, are terminal. V1═O1 is parallel to the a axis, with atom O1 pointing into the inter-layer space; V1═O3 is parallel to the c axis, with atom O3 pointing into the eight-membered ring. Along the [100] direction, V/P layers alternate with layers of Rb and Cu atoms at x = 0.5 (Fig. 3), and quadrilaterals CuO₄ link the V/P layers by sharing two vertices (O4) with P1 tetrahedra and two others (O2) with P1 and V1 polyhedra. Thus, a three-dimensional anionic framework with the formula [Cu(VO₂)₂(PO₄)₂]^2- is formed. It contains channels with eight polyhedra at the circumference, as viewed along the [100] direction. Crossing channels are present in the [001] and [011] directions. The Rb atoms reside in these channels and are surrounded by ten O atoms, with Rb1—O distances ranging from 2.8427 (18) to 3.2981 (19) Å (average 3.108 Å) (Table 1); an additional atom O6ⁱ [symmetry code (i) as shown in Table 1] is 3.5044 (17) Å from atom Rb1. Including this eleventh O atom in the first coordination sphere around Rb1 gives the wrong bond-valence sum for atom O6. Bond-valence sum data, shown in Table 2, are consistent with the assumed oxidation states of V and Cu.

In crystals of vanadyl phosphates, [VO₂]⁺ cations in combination with PO₄^3- tetrahedra usually form one-dimensional (ribbons), two-dimensional (layers) or three-dimensional (framework) anions. Six different structure types based on mixed anionic one-dimensional ribbons formed by VO₅ (or VO₆) and PO₄ polyhedra have been described: A(VO₂)(HPO₄), with A = K, Rb, NH₄ or Tl (Amoros et al., 1988; Huan et al., 1991); the α-modification of (NH₄)(VO₂)(HPO₄) (Amoros & Le Bail, 1992); A₂(VO₂)(PO₄), with A = K or Na (Korthuis et al., 1993a); K₃(VO₂)₂(PO₄)(HPO₄)(H₂O) (Leclaire et al., 2002); Ba₂(VO₂)(PO₄)(HPO₄)(H₂O) (Bircsak & Harrison, 1998a) and Cd(VO₂)(PO₄)(H₂O) (Leclaire et al., 2000). Among the two-dimensional layered structures, four different types can be distinguished: K(VO₂)₂(PO₄) (Berrah et al., 1999); A(VO₂)(PO₄), with A = Ba, Sr or Pb (Kang et al., 1992; Borel et al., 2000); Ag₂(VO₂)(PO₄) (Kang et al., 1993) and (CN₃H₆)₂(VO₂)₃(PO₄)(HPO₄) (Bircsak & Harrison, 1998b). Three-dimensional mixed anionic frameworks are represented by the crystal structures of (NH₄)(VO₂)₂(PO₄)(H₂O)₃ (Wilde et al., 2000), Pb(VO₂)₂(PO₄) (Borel et al., 2000) and Pb(VO₂)(PO₄)(H₂O) (Leclaire et al., 2001).

Sometimes, additional cations along with [VO₂]⁺ may form three-dimensional mixed anionic frameworks in combination with PO₄ tetrahedra. Two compounds of this kind include Cs₂[(UO₂)(VO₂)₂(PO₄)₂](H₂O)_0.59 (Shvareva et al., 2005) and Cs₂[Ti(VO₂)₃(PO₄)₃] (Yakubovich et al., 2006). The novel crystal structure of Rb₂Cu(VO₂)₂(PO₄)₂ described here belongs to this same group of vanadyl(V) phosphates having three different oxo complexes in the anionic part of their structures.

To our knowledge, only one crystal structure among the vanadyl phosphates published so far contains [VO₂]⁺ ions participating in the formation of the cationic framework. There is a close-packed framework formed by edge-sharing LiO₆ and VO₆ octahedra in [Li₂(VO₂)](PO₄) (Korthuis et al., 1993b). This crystal structure is based on hexagonal close-packing of O atoms, in which Li and V atoms occupy octahedral voids and a fraction of the tetrahedral voids contain P atoms.

The novel crystal structure of rubidium copper vanadyl phosphate is closely related to the structures of A(VO₂)(PO₄), with A = Ba, Sr or Pb (Kang et al., 1992; Borel et al., 2000), and BaCrF₂LiF₄ (Babel, 1974), by having similar unit-cell parameters and the same space group, P2₁/c (Table 3). In bc projections of the Ba(VO₂)(PO₄) and Ba(CrF₂)(LiF₄) crystal structures (Figs. 4 and 5), one can see eight-membered windows formed by alternating octahedra (VO₆ and CrF₆) and tetrahedra (PO₄ and LiF₄) sharing vertices. These windows are topologically very similar to the windows walled in by VO₅ bipyramids and PO₄ tetrahedra in the title structure (Fig. 2). In all three structures, the eight-membered windows enclose large channels parallel to the [100] direction which contain Rb, Ba, Sr or Pb atoms. The main topological difference between these structures occurs along the a axis of their monoclinic unit cells. In the V/P layers of rubidium copper vanadyl phosphate, V bipyramids have no O atoms shared between them. Cu atoms with square-planar coordination link these V/P layers along the a axis to form the three-dimensional mixed anionic framework (Fig. 3). Sharing one vertex between two neighbouring V octahedra in barium vanadyl phosphate leads to the formation of double V/P layers alternating with Ba ions along [100] (Fig. 6). In the Ba(CrF₂)(LiF₄) structure, eight-membered ring layers are linked along the a axis through common vertices of Cr octahedra and Li tetrahedra, resulting in the three-dimensional structure (Fig. 7).

S2. Experimental

Light-blue plate crystals of Rb₂Cu(VO₂)₂(PO₄)₂ up to 0.5 mm long were formed by hydrothermal synthesis in the CuCl₂–Rb₃PO₄–V₂O₅–H₂O system (ratio 1:6:3:30) in a PTFE-lined stainless steel autoclave at a temperature of 553 K and a pressure of 7 × 10³ kPa, over a period of 20 d. The presence of Rb, Cu, V and P in the samples was confirmed by qualitative X-ray spectroscopic analysis

Computing details

Data collection: SMART (Bruker, 2001); cell refinement: SAINT (Bruker, 2002); data reduction: SAINT (Bruker, 2002); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2006); software used to prepare material for publication: publCIF (Westrip, 2008).

Figures

	Fig. 1. The main structural elements of the title compound, showing the atomic labelling scheme. Displacement ellipsoids are drawn at the 70% probability level. [Symmetry codes: (*) 1 - x, 1 - y, 1 - z; (') 1 + x, y, z; ('') -x, 1 - y, 1 - z; (''') -x, 1/2 + y, 1/2 - z.]
	Fig. 2. The Rb2Cu(VO2)2(PO4)2 crystal structure in the [100] projection.
	Fig. 3. Layers of VO5 bipyramids and PO4 tetrahedra linked by Cu atoms to form a three-dimensional anionic framework in the Rb2Cu(VO2)2(PO4)2 crystal structure.
	Fig. 4. The layer formed by VO6 octahedra and PO4 tetrahedra in the Ba(VO2)(PO4) crystal structure.
	Fig. 5. The three-dimensional microporous framework of CrF6 octahedra and LiF4 tetrahedra, with Ba atoms in channels, in the crystal structure of BaCrLiF6.
	Fig. 6. Double layers of VO6 and PO4 polyhedra in the Ba(VO2)(PO4) crystal structure, viewed along the [010] direction.
	Fig. 7. The BaCrLiF6 crystal structure in an [010] projection.

dirubidium copper bis[vanadylV)] bis(phosphate)

Crystal data

Rb2Cu(VO2)2(PO4)2	F(000) = 550
Mr = 590.31	Dx = 3.664 Mg m^−3
Monoclinic, P21/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ybc	Cell parameters from 1024 reflections
a = 4.9292 (9) Å	θ = 2.8–28.3°
b = 11.471 (2) Å	µ = 13.08 mm^−1
c = 9.4810 (17) Å	T = 100 K
β = 93.535 (3)°	Plate, light blue
V = 535.04 (17) Å^3	0.20 × 0.08 × 0.04 mm
Z = 2

Data collection

Bruker SMART CCD diffractometer	1290 independent reflections
Radiation source: fine-focus sealed tube	1221 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.028
Detector resolution: 8.33 pixels mm^-1	θmax = 28.3°, θmin = 2.8°
ω scans	h = −6→6
Absorption correction: multi-scan (SADABS; Bruker, 2000)	k = −15→15
Tmin = 0.285, Tmax = 0.600	l = −12→12
6107 measured reflections

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)] = 0.019	w = 1/[σ^2(Fo^2) + (0.025P)^2] where P = (Fo^2 + 2Fc^2)/3
wR(F^2) = 0.048	(Δ/σ)max < 0.001
S = 1.14	Δρmax = 0.52 e Å^−3
1290 reflections	Δρmin = −0.51 e Å^−3
89 parameters	Extinction correction: SHELXL97 (Sheldrick, 2008), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
0 restraints	Extinction coefficient: 0.0131 (7)

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
Rb1	0.44273 (5)	0.36203 (2)	0.10575 (3)	0.00953 (10)
V1	0.05514 (8)	0.61258 (4)	0.25006 (4)	0.00535 (11)
P1	−0.00751 (13)	0.36132 (5)	0.41537 (7)	0.00511 (15)
Cu1	0.5000	0.5000	0.5000	0.00542 (12)
O5	0.0766 (3)	0.32860 (15)	0.56944 (18)	0.0080 (4)
O1	0.3710 (4)	0.62618 (15)	0.2144 (2)	0.0118 (4)
O4	−0.3098 (3)	0.38822 (15)	0.39457 (19)	0.0075 (4)
O3	−0.0963 (3)	0.52780 (16)	0.13335 (19)	0.0102 (4)
O6	0.0735 (3)	0.26407 (15)	0.31708 (18)	0.0073 (4)
O2	0.1658 (3)	0.47068 (15)	0.37944 (18)	0.0057 (3)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Rb1	0.01080 (14)	0.00900 (15)	0.00870 (15)	−0.00279 (9)	−0.00024 (9)	0.00037 (9)
V1	0.0063 (2)	0.0049 (2)	0.0049 (2)	0.00019 (15)	0.00096 (15)	0.00022 (15)
P1	0.0059 (3)	0.0042 (3)	0.0053 (3)	0.0004 (2)	0.0007 (2)	−0.0006 (2)
Cu1	0.0053 (2)	0.0054 (2)	0.0056 (2)	0.00051 (15)	0.00021 (15)	−0.00141 (16)
O5	0.0106 (9)	0.0081 (9)	0.0053 (9)	0.0022 (7)	0.0012 (7)	0.0006 (7)
O1	0.0096 (9)	0.0095 (9)	0.0165 (10)	0.0011 (7)	0.0034 (8)	0.0043 (8)
O4	0.0060 (8)	0.0083 (8)	0.0083 (9)	0.0006 (7)	0.0012 (7)	−0.0022 (7)
O3	0.0144 (9)	0.0091 (9)	0.0071 (9)	−0.0003 (7)	−0.0001 (7)	−0.0001 (7)
O6	0.0091 (8)	0.0062 (9)	0.0069 (9)	−0.0006 (7)	0.0015 (6)	−0.0027 (7)
O2	0.0057 (8)	0.0043 (8)	0.0070 (9)	−0.0006 (7)	−0.0011 (6)	0.0004 (7)

Geometric parameters (Å, º)

Rb1—O5^i	2.8427 (18)	P1—O6	1.5225 (17)
Rb1—O4^ii	2.9436 (18)	P1—O5	1.5401 (18)
Rb1—O3^ii	2.9619 (18)	P1—O2	1.5670 (18)
Rb1—O6	3.0066 (17)	Cu1—O4^vii	1.9072 (17)
Rb1—O3^iii	3.0288 (18)	Cu1—O4^ii	1.9072 (17)
Rb1—O1	3.2266 (19)	Cu1—O2^viii	1.9752 (16)
Rb1—O1^iv	3.228 (2)	Cu1—O2	1.9752 (16)
Rb1—O2	3.2537 (17)	O5—V1^vii	1.9855 (18)
Rb1—O3	3.2910 (18)	O5—Rb1^ix	2.8427 (17)
Rb1—O1^v	3.2981 (19)	O1—Rb1^iv	3.228 (2)
Rb1—O6^i	3.5044 (17)	O1—Rb1^x	3.2981 (19)
V1—O3	1.6204 (18)	O4—Cu1^xi	1.9072 (17)
V1—O1	1.6209 (19)	O4—Rb1^xi	2.9436 (18)
V1—O6^vi	1.9435 (17)	O3—Rb1^xi	2.9619 (18)
V1—O5^vii	1.9855 (18)	O3—Rb1^iii	3.0288 (18)
V1—O2	2.0901 (17)	O6—V1^xii	1.9435 (17)
P1—O4	1.5225 (18)	O6—Rb1^ix	3.5044 (17)
O5^i—Rb1—O4^ii	114.50 (5)	O3—Rb1—O6^i	85.27 (4)
O5^i—Rb1—O3^ii	169.22 (5)	O1^v—Rb1—O6^i	100.36 (4)
O4^ii—Rb1—O3^ii	65.21 (5)	O3—V1—O1	108.86 (10)
O5^i—Rb1—O6	52.21 (5)	O3—V1—O6^vi	100.69 (8)
O4^ii—Rb1—O6	69.60 (5)	O1—V1—O6^vi	98.27 (8)
O3^ii—Rb1—O6	132.35 (5)	O3—V1—O5^vii	129.13 (8)
O5^i—Rb1—O3^iii	84.60 (5)	O1—V1—O5^vii	121.09 (9)
O4^ii—Rb1—O3^iii	146.70 (5)	O6^vi—V1—O5^vii	82.01 (7)
O3^ii—Rb1—O3^iii	101.08 (4)	O3—V1—O2	91.49 (8)
O6—Rb1—O3^iii	108.56 (5)	O1—V1—O2	88.75 (8)
O5^i—Rb1—O1	132.82 (5)	O6^vi—V1—O2	163.05 (7)
O4^ii—Rb1—O1	69.79 (5)	O5^vii—V1—O2	81.15 (7)
O3^ii—Rb1—O1	57.83 (5)	O4—P1—O6	111.15 (10)
O6—Rb1—O1	93.18 (5)	O4—P1—O5	112.10 (10)
O3^iii—Rb1—O1	77.25 (5)	O6—P1—O5	109.67 (10)
O5^i—Rb1—O1^iv	97.68 (5)	O4—P1—O2	110.60 (9)
O4^ii—Rb1—O1^iv	138.15 (5)	O6—P1—O2	106.64 (10)
O3^ii—Rb1—O1^iv	78.17 (5)	O5—P1—O2	106.43 (10)
O6—Rb1—O1^iv	149.47 (5)	O4^vii—Cu1—O2^viii	90.10 (7)
O3^iii—Rb1—O1^iv	57.22 (5)	O4^ii—Cu1—O2^viii	89.90 (7)
O1—Rb1—O1^iv	107.70 (4)	O4^vii—Cu1—O2	89.90 (7)
O5^i—Rb1—O2	95.55 (5)	O4^ii—Cu1—O2	90.10 (7)
O4^ii—Rb1—O2	52.35 (4)	P1—O5—V1^vii	130.65 (10)
O3^ii—Rb1—O2	92.35 (5)	P1—O5—Rb1^ix	115.72 (9)
O6—Rb1—O2	46.43 (5)	V1^vii—O5—Rb1^ix	113.33 (7)
O3^iii—Rb1—O2	101.08 (5)	V1—O1—Rb1	95.84 (7)
O1—Rb1—O2	47.63 (4)	V1—O1—Rb1^iv	122.20 (9)
O1^iv—Rb1—O2	153.07 (4)	Rb1—O1—Rb1^iv	72.30 (4)
O5^i—Rb1—O3	86.81 (5)	V1—O1—Rb1^x	102.21 (8)
O4^ii—Rb1—O3	99.06 (5)	Rb1—O1—Rb1^x	154.53 (6)
O3^ii—Rb1—O3	103.93 (6)	Rb1^iv—O1—Rb1^x	111.44 (5)
O6—Rb1—O3	68.78 (5)	P1—O4—Cu1^xi	125.25 (10)
O3^iii—Rb1—O3	52.97 (6)	P1—O4—Rb1^xi	116.48 (9)
O1—Rb1—O3	47.71 (5)	Cu1^xi—O4—Rb1^xi	111.54 (7)
O1^iv—Rb1—O3	109.18 (5)	V1—O3—Rb1^xi	139.84 (9)
O2—Rb1—O3	48.30 (4)	V1—O3—Rb1^iii	118.39 (9)
O5^i—Rb1—O1^v	65.55 (5)	Rb1^xi—O3—Rb1^iii	78.92 (4)
O4^ii—Rb1—O1^v	61.09 (5)	V1—O3—Rb1	93.48 (7)
O3^ii—Rb1—O1^v	107.06 (5)	Rb1^xi—O3—Rb1	103.93 (6)
O6—Rb1—O1^v	60.63 (5)	Rb1^iii—O3—Rb1	127.03 (6)
O3^iii—Rb1—O1^v	148.80 (5)	P1—O6—V1^xii	139.73 (11)
O1—Rb1—O1^v	129.623 (17)	P1—O6—Rb1	109.05 (9)
O1^iv—Rb1—O1^v	115.74 (4)	V1^xii—O6—Rb1	108.34 (7)
O2—Rb1—O1^v	91.09 (4)	P1—O6—Rb1^ix	88.14 (7)
O3—Rb1—O1^v	129.29 (5)	V1^xii—O6—Rb1^ix	91.72 (6)
O5^i—Rb1—O6^i	44.98 (4)	Rb1—O6—Rb1^ix	111.60 (5)
O4^ii—Rb1—O6^i	159.09 (4)	P1—O2—Cu1	117.19 (10)
O3^ii—Rb1—O6^i	133.84 (4)	P1—O2—V1	128.68 (10)
O6—Rb1—O6^i	93.42 (4)	Cu1—O2—V1	112.65 (8)
O3^iii—Rb1—O6^i	49.16 (5)	P1—O2—Rb1	97.35 (8)
O1—Rb1—O6^i	125.10 (4)	Cu1—O2—Rb1	98.67 (6)
O1^iv—Rb1—O6^i	56.43 (4)	V1—O2—Rb1	86.41 (6)
O2—Rb1—O6^i	123.90 (4)

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