Poly­[di­aqua-μ₄-oxalato-di-μ₆-phos­phato-tetrazinc]

The structure of the title compound, [Zn₄(C₂O₄)(PO₄)₂(H₂O)₂]_n, which was synthesized under hydro­thermal conditions, consists of zinc phosphate layers joined by bridging oxalate ligands to generate a three-dimensional framework. An extended zinc phosphate layer lies parallel to the ab plane and within this layer there are helical chains, composed of ZnO₆ octahedra and ZnO₅ square pyramids, that run parallel to the b axis and coincide with the 2₁ screw element. The oxalate groups sit on crystallographic inversion centers.

S1. Comment

Inorganic–organic hybrid materials based on phosphate and oxalate ligands are of considerable interest because of the rich structural chemistry of these materials and their established or potential applications in sorption and separation, heterogeneous catalysis, and ion exchange (Cheetham et al., 1999; Ekambaram & Sevov, 2000; Halasyamani et al., 1997; Lin et al., 2001). Research into metal–phosphate–oxalate systems has generated numerous structures containing C₂O₄²⁻ and PO₄³⁻ ions, including the phosphatooxalates of vanadium (Do et al., 2000; Do et al., 2001; Tsai et al., 1999), manganese (Lethbridge et al., 2000; Lethbridge et al., 2004), iron (Choudhury et al., 2000; Lethbridge & Lightfoot, 2000; Choudhury et al., 1999), aluminium (Kedarnath et al., 2000; Lightfoot & Lethbridge, 1999; Rajic et al., 2003), gallium (Choi & Lachgar, 2002; Lii & Chen, 2000), indium (Huang & Lii, 1998), molybdenum (Lee & Wang, 1999) and zinc (Fu et al., 2003; Neeraj et al., 2001). The aim of our work is to pursue new open frameworks that combine the rigidity of the oxalate ligand with the thermal stability of the phosphate groups. In the course of this work, we have isolated a new zinc phosphatooxalate, Zn₄[(PO₄)₂(C₂O₄)(H₂O)₂], (I), the structure of which is reported here. The two zinc phosphatooxalates cited above, [NH₃(CH₂)₃NH₃][Zn₆(PO₄)₄(C₂O₄)] (Neeraj et al., 2001) and {Na₂[Zn(C₂O₄)_1.5H₂PO₄]·2(H₂O)}_n (Fu et al., 2003), possess different structure-directing entities. Those two compounds consist of macro-anionic frameworks, with the guest templating agents intercalated between the inorganic layers or located within channels in the structure. The structure of the title compound provides an interesting example of a three-dimensional zinc phosphatooxalate with a neutral framework.

The structure of (I), Fig. 1, has two crystallographically independent zinc sites within a zinc phosphate layer. Atom Zn1 is surrounded by a distorted octahedron formed by one doubly bridging O atom and five µ₃-O atoms, with Zn1—O bond distances in the range 1.994 (2)–2.320 (2) Å and cis O—Zn1—O bond angles in the range 65.72 (8)–97.01 (9)°. Atom Zn2 is five-coordinated by three µ₃-O atoms, one doubly bridging O atom and one terminal water molecule, with Zn2—O bond distances ranging from 1.947 (2)–2.346 (2) Å and cis O—Zn2—O bond angles ranging from 84.24 (9)–107.18 (10)°. The PO₄ tetrahedron acts as a multiple bridge, with three µ₃-O atoms and one doubly bridging O atom, and with P1—O bond distances in the range 1.530 (2)–1.552 (2) Å and O—P1—O bond angles in the range 102.81 (12)–113.44 (13)° (Table 1). The linkage of ZnO₆, ZnO₅ and PO₄ polyhedra generates two-dimensional zinc phosphate layers parallel to the ab plane. The PO₄ tetrahedron shares a common edge with the ZnO₆ octahedron.

The oxalate molecule sits on an inversion center and possesses unexceptional geometrical parameters. It exhibits bisbidentate coordination to atom Zn1 – that is, it chelates two symmetry-related congeners of Zn1 – and bismonodentate coordination to atom Zn2. The oxalate ligand thus acts as a spacer, or short pillar, between adjacent two-dimensional zinc phosphate layers, thus linking the latter into a three-dimensional architecture (Fig. 2). The ac projection of the structure reveals a small amount of space between the oxalate moieties. Small channels along b accommodate the ZnOH₂ groups. Hydrogen bonding is present between the water molecules and the framework O atoms [H1···O4ⁱ = 1.85 (5) Å and H2···O5ⁱⁱ = 1.91 (5) Å; see Table 2 for further details and symmetry codes].

One unique structural feature of the title compound is the presence of an extended two-dimensional Zn—O—Zn– net, in which two distinct types of rings, labelled I and II in Fig 3(a), each with four metal atoms, share edges (Fig. 3a). Interestingly, the layer is further characterized by the presence of helical chains composed of ZnO₆ octahedra and ZnO₅ square pyramids. The central axis of each helical chain is a twofold screw axis along the crystallographic b axis. Fig. 3(b) shows a helical Zn—O chain.

S2. Experimental

In a typical synthesis, a mixture of ZnO (0.16 g), H₃PO₄ (0.2 ml, 85 wt%), H₂C₂O₄·2H₂O (0.606 g) and H₂O (5.0 ml) in a molar ratio of 1.0:1.5:2.4:141 was sealed in a Teflon-lined steel autoclave, heated at 433 K for 3 d and then cooled to room temperature. The resulting plate-like crystals were recovered by filtration, washed with distilled water and dried in air (72% yield based on zinc). The elemental analysis of the bulk product is consistent with the reported stoichiometry. Analysis found: C 4.08%, H 0.62%; calculated: C 4.17%, H 0.70%. IR (KBr pellet, cm⁻¹): 3395 (s), 1644 (s), 1364 (m), 1320 (s), 1158 (m), 1052 (s), 1004 (s), 955 (m), 822 (m), 625 (m), 497 (m). The initial thermogravimetric analysis, performed under a flowing N₂ atmosphere in the range 313–873 K with a heating rate of 10 K min⁻¹, shows a two-step weight loss for the sample. The initial weight loss, between 393 and 443 K, corresponds to the release of water ligands (observed: 6.18%; expected: 6.26%). The second step, occurring between 503 and 743 K, is assigned to the removal of oxalate ligands. The weight loss (12.29%) is slightly less than the calculated value (12.52%). The title compound exhibits intense photoluminescence upon photoexcitation at 310 nm, which was measured with an Edinburgh FLS920 analytical instrument. The intense emissions shown at 470 nm may be assigned as ligand-to-metal charge transfer (LMCT).

S3. Refinement

The two water H atoms were clearly visible in difference density maps. They were refined freely [OW1—H1 = 0.87 (5) Å and OW1—H2 =0.85 (6) Å] with individual isotropic displacement parameters.

Computing details

Data collection: CrystalClear (Rigaku Corporation, 2000); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: SHELXTL (Siemens, 1994); software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. A view of the coordination environments of the Zn and P atoms. [Symmetry codes: (A) 1 − x, −y, 1 − z; (B) −0.5 + x, 0.5 − y, 0.5 + z; (C) x, −1 + y, z; (D) 0.5 − x, −0.5 + y, 1.5 − z; (E) 1.5 − x, −0.5 + y, 1.5 − z.]
	Fig. 2. A view along the b axis, showing the zinc phosphate layers separated by oxalate ligands. [Zn: medium gray; PO4: tetrahedra; C: light gray; O: white; H: small white.]
	Fig. 3. (a) A ball-and-stick view of the two-dimensional Zn—O—Zn connectivity in the ab plane. (b) A space-filling plot of the helical chains composed of ZnO6 octahedra and ZnO5 square pyramids along c axis. [Zn: medium gray; O: white.]

Poly[diaqua-µ₄-oxalato-di-µ₆-phosphatotetrazinc]

Crystal data

[Zn4(C2O4)(PO4)2(H2O)2]	F(000) = 556.0
Mr = 575.47	Dx = 3.613 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.7107 Å
Hall symbol: -P 2yn	Cell parameters from 1080 reflections
a = 7.8752 (6) Å	θ = 2.6–25.0°
b = 4.7971 (2) Å	µ = 9.36 mm^−1
c = 14.0735 (6) Å	T = 293 K
β = 95.835 (7)°	Plate, colorless
V = 528.92 (5) Å^3	0.15 × 0.10 × 0.10 mm
Z = 2

Data collection

Mercury CCD diffractometer	932 independent reflections
Radiation source: Rotating Anode	860 reflections with I > 2σ(I)
Graphite Monochromator monochromator	Rint = 0.024
Detector resolution: 14.6306 pixels mm^-1	θmax = 25.0°, θmin = 2.9°
dtprofit.ref scans	h = −9→9
Absorption correction: multi-scan (CrystalClear; Rigaku Corporation, 2000)	k = −5→5
Tmin = 0.338, Tmax = 0.392	l = −11→16
2971 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)] = 0.023	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.066	All H-atom parameters refined
S = 1.01	w = 1/[σ^2(Fo^2) + (0.047P)^2] where P = (Fo^2 + 2Fc^2)/3
932 reflections	(Δ/σ)max < 0.001
108 parameters	Δρmax = 0.49 e Å^−3
0 restraints	Δρmin = −0.85 e Å^−3

Special details

Experimental. Photoluminescence was measured with an Edinburgh FLS920 analytical instrument.

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
Zn1	0.59286 (5)	0.02674 (8)	0.69456 (3)	0.01057 (17)
Zn2	0.29578 (5)	0.02375 (8)	0.86606 (3)	0.01410 (17)
P1	0.49638 (11)	0.47727 (17)	0.80332 (6)	0.0086 (2)
O1	0.4934 (3)	0.7968 (5)	0.80199 (15)	0.0116 (5)
O2	0.3795 (3)	0.3433 (5)	0.72028 (16)	0.0151 (5)
O3	0.6741 (3)	0.3599 (5)	0.78817 (15)	0.0130 (5)
O4	0.4363 (3)	0.3643 (5)	0.89588 (16)	0.0133 (5)
O5	0.6375 (3)	0.2265 (5)	0.56664 (16)	0.0161 (5)
O6	0.5693 (3)	0.2002 (5)	0.40820 (16)	0.0146 (5)
C1	0.5599 (4)	0.1228 (7)	0.4932 (2)	0.0119 (7)
OW1	0.2942 (3)	−0.1580 (6)	0.99234 (17)	0.0179 (6)
H1	0.377 (6)	−0.254 (10)	1.022 (3)	0.040 (13)*
H2	0.278 (8)	−0.025 (10)	1.030 (5)	0.061 (19)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Zn1	0.0133 (3)	0.0104 (3)	0.0083 (3)	−0.00024 (14)	0.00247 (18)	−0.00043 (14)
Zn2	0.0162 (3)	0.0149 (3)	0.0110 (3)	−0.00427 (15)	0.00030 (19)	−0.00074 (14)
P1	0.0106 (4)	0.0079 (5)	0.0077 (4)	0.0001 (3)	0.0020 (4)	0.0007 (3)
O1	0.0160 (11)	0.0067 (12)	0.0128 (11)	−0.0006 (9)	0.0052 (9)	−0.0013 (9)
O2	0.0178 (11)	0.0097 (13)	0.0166 (12)	0.0017 (9)	−0.0044 (10)	0.0001 (10)
O3	0.0116 (11)	0.0107 (12)	0.0172 (12)	0.0009 (9)	0.0039 (9)	−0.0024 (10)
O4	0.0182 (11)	0.0125 (13)	0.0101 (11)	−0.0020 (10)	0.0058 (9)	0.0005 (10)
O5	0.0201 (12)	0.0160 (13)	0.0119 (12)	−0.0055 (10)	0.0008 (9)	−0.0022 (10)
O6	0.0195 (12)	0.0146 (13)	0.0097 (12)	−0.0032 (10)	0.0020 (9)	0.0012 (10)
C1	0.0122 (15)	0.0093 (16)	0.0145 (17)	0.0045 (14)	0.0029 (12)	0.0006 (14)
OW1	0.0237 (14)	0.0163 (15)	0.0141 (12)	0.0063 (11)	0.0032 (11)	0.0027 (11)

Geometric parameters (Å, º)

Zn1—O3^i	1.994 (2)	P1—O3	1.543 (2)
Zn1—O1^ii	2.087 (2)	P1—O2	1.552 (2)
Zn1—O5	2.100 (2)	O1—Zn1^vi	2.087 (2)
Zn1—O3	2.128 (2)	O1—Zn2^vi	2.169 (2)
Zn1—O6^iii	2.129 (2)	O2—Zn2^vii	1.947 (2)
Zn1—O2	2.320 (2)	O3—Zn1^viii	1.994 (2)
Zn2—O2^iv	1.947 (2)	O5—C1	1.250 (4)
Zn2—OW1	1.981 (2)	O6—C1	1.261 (4)
Zn2—O4	1.994 (2)	O6—Zn1^iii	2.129 (2)
Zn2—O1^ii	2.169 (2)	O6—Zn2^ix	2.346 (2)
Zn2—O6^v	2.346 (2)	C1—C1^iii	1.534 (7)
P1—O4	1.530 (2)	OW1—H1	0.87 (5)
P1—O1	1.533 (2)	OW1—H2	0.85 (6)
O3^i—Zn1—O1^ii	96.22 (9)	O4—P1—O3	110.58 (13)
O3^i—Zn1—O5	93.18 (9)	O1—P1—O3	112.10 (13)
O1^ii—Zn1—O5	165.87 (9)	O4—P1—O2	106.48 (14)
O3^i—Zn1—O3	90.63 (6)	O1—P1—O2	113.44 (13)
O1^ii—Zn1—O3	93.43 (8)	O3—P1—O2	102.81 (12)
O5—Zn1—O3	97.01 (9)	P1—O1—Zn1^vi	122.05 (13)
O3^i—Zn1—O6^iii	111.01 (9)	P1—O1—Zn2^vi	120.51 (12)
O1^ii—Zn1—O6^iii	88.76 (9)	Zn1^vi—O1—Zn2^vi	111.95 (10)
O5—Zn1—O6^iii	78.00 (8)	P1—O2—Zn2^vii	127.91 (15)
O3—Zn1—O6^iii	157.90 (9)	P1—O2—Zn1	90.29 (11)
O3^i—Zn1—O2	156.30 (9)	Zn2^vii—O2—Zn1	132.25 (11)
O1^ii—Zn1—O2	84.36 (9)	P1—O3—Zn1^viii	132.30 (14)
O5—Zn1—O2	91.26 (9)	P1—O3—Zn1	98.03 (11)
O3—Zn1—O2	65.72 (8)	Zn1^viii—O3—Zn1	126.29 (11)
O6^iii—Zn1—O2	92.68 (8)	P1—O4—Zn2	108.91 (13)
O2^iv—Zn2—OW1	107.18 (10)	C1—O5—Zn1	114.7 (2)
O2^iv—Zn2—O4	147.03 (10)	C1—O6—Zn1^iii	113.9 (2)
OW1—Zn2—O4	103.05 (10)	C1—O6—Zn2^ix	121.7 (2)
O2^iv—Zn2—O1^ii	90.60 (9)	Zn1^iii—O6—Zn2^ix	122.41 (10)
OW1—Zn2—O1^ii	102.86 (10)	O5—C1—O6	126.6 (3)
O4—Zn2—O1^ii	95.30 (9)	O5—C1—C1^iii	117.2 (4)
O2^iv—Zn2—O6^v	84.24 (9)	O6—C1—C1^iii	116.2 (4)
OW1—Zn2—O6^v	86.97 (10)	Zn2—OW1—H1	126 (3)
O4—Zn2—O6^v	84.52 (8)	Zn2—OW1—H2	104 (4)
O1^ii—Zn2—O6^v	169.92 (8)	H1—OW1—H2	104 (5)
O4—P1—O1	111.03 (13)

Symmetry codes: (i) −x+3/2, y−1/2, −z+3/2; (ii) x, y−1, z; (iii) −x+1, −y, −z+1; (iv) −x+1/2, y−1/2, −z+3/2; (v) x−1/2, −y+1/2, z+1/2; (vi) x, y+1, z; (vii) −x+1/2, y+1/2, −z+3/2; (viii) −x+3/2, y+1/2, −z+3/2; (ix) x+1/2, −y+1/2, z−1/2.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
OW1—H1···O4^x	0.87 (5)	1.85 (5)	2.698 (3)	164 (4)
OW1—H2···O5^v	0.85 (6)	1.91 (5)	2.676 (4)	149 (6)

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