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In the title metal–organic framework (MOF), [La(C8H8N2O6)(C2O4)0.5(H2O)]n, the LaIII cation is coordinated by eight O atoms in a square anti­prismatic configuration. Each LaIII cation is connected to adjacent LaIII cations by bridging 2,5-dioxopiperazine-1,4-diacetate (PODC2−) and oxalate (lying about an inversion centre) ligands, generating two-dimensional grid layers. The layers are further linked via the carboxyl­ate groups of the PODC2− ligands in synsyn and synanti modes, resulting in a three-dimensional framework with a short Schläfli vertex notation of {47.63}{47.67.8}.

Supporting information

cif

Crystallographic Information File (CIF) https://doi.org/10.1107/S0108270112048184/fg3274sup1.cif
Contains datablocks I, global

mol

MDL mol file https://doi.org/10.1107/S0108270112048184/fg3274Isup2.mol
Supplementary material

hkl

Structure factor file (CIF format) https://doi.org/10.1107/S0108270112048184/fg3274Isup3.hkl
Contains datablock I

CCDC reference: 925249

Comment top

The design and synthesis of metal–organic frameworks (MOFs) have received much attention in recent years, because of their aesthetically interesting structures and potential applications in adsorption, separation, catalysis, luminescence, magnetism and nonlinear optics (Ma & Zhou, 2006; Lin et al., 2010; Chen et al., 2010; Kurmoo, 2009). Although a great variety of MOFs with diverse topologies have been obtained, those based on lanthanide ions are still relatively rare. This lack of examples may be attributable to the high coordination-number requirement of these 4f metal ions, as well as their flexible coordination geometry (Zhao et al., 2008). Therefore, the effective construction of lanthanide-based MOFs (LMOFs) is still a challenge. According to the philosophy of the hard–soft acid–base theory, the ligands used by LMOFs usually involve multicarboxylates and N/O mixed ligands, such as terephthalic acid and pyridine-2,6-dicarboxylic acid (Reineke et al., 1999; Ghosh & Bharadwaj, 2003). Nonetheless, to the best of our knowledge, LMOFs based on cyclopeptide ligands are very rare (Kong et al., 2009; Zhuang et al., 2010).

Utilizing 2,5-dioxopiperazine-1,4-diacetatic acid (H2PODC) as a ligand, we previously reported the synthesis and structure of a series of LMOFs (Zhuang et al., 2010). As an expansion of our studies, we herein report a new LMOF, the title compound, (I), employing mixed ligands [H2PODC and oxalic acid (H2ox)], and focus on its synthesis, structure and thermostability.

Complex (I) crystallizes in the monoclinic space group P2/n. X-ray analysis reveals that the asymmetric unit contains one LaIII cation, one PODC2- ligand, half an ox2- ligand (lying about an inversion centre) and one aqua ligand. As shown in Fig. 1, the coordination environment of the LaIII cation can be viewed as a square antiprism, featuring contributions by four carboxylate O atoms from PODC2- ligands, two carboxylate O atoms from ox2- ligands, one carbonyl O atom of a PODC2- ligand and two O atoms of the aqua ligands. The LaIII—O bond lengths [2.4378 (17)–2.5758 (18) Å] are in agreement with those of pure H2PODC-based compounds, for example [La(PODC)1.5(H2O)].2H2O, (II) (Zhuang et al., 2010). As shown in Fig. 1, the two carboxylate groups of the PODC2- ligand feature syn–syn and synanti coordination modes and one carbonyl group takes part in coordination. This was also found in (II). However, in a similar compound obtained in situ, viz. [Dy2(PODC)(ox)2(H2O)2] (Kong et al., 2009), the PODC2- ligand exhibits a different coordination mode from that of (I), owing to the different lanthanide cation. Furthermore, the number of PODC2- ligands around the LaIII cation in (I) is five, which is less than that of pure H2PODC-based LaIII MOFs, e.g. (II). In addition, two hydrogen bonds (see Table 1) are found in the three-dimensional network; according to the classification of Steiner (2002), they are moderate hydrogen bonds.

Inspection of the three-dimensional structure of (I) shows that each LaIII cation is connected to adjacent LaIII cations by bridging PODC2- and ox2- ligands, generating two-dimensional grid layers (see Fig. 2a). These layers are further linked via the carboxylate groups of the PODC2- ligands in syn–syn (O6—C8—O5)and syn–anti (O1—C1—O2) modes (see Fig. 2b), resulting in a three-dimensional framework, as shown in Fig. 3. The PODC2- ligand serves as a five-connecting node, while the LaIII cation is a six-connecting node. Generally, the short Schläfli vertex notation of the net can be represented as {47.63}{47.67.8}, as indicated by the software TOPOS (Blatov, 2006).

In conclusion, to the best of our knowledge (I) features a new topology and exhibits a different structure to that obtained in situ (Kong et al., 2009). The difference can be ascribed to two aspects: (i) the ratio of PODC2- and ox2- ligands in the two compounds; and (ii) the lanthanide cation, i.e. a light rare earth element (La) for (I), but heavy rare earth elements (Dy, Ho and Yb) for the compounds reported by Kong et al. (2009).

Related literature top

For related literature, see: Blatov (2006); Chen et al. (2010); Ghosh & Bharadwaj (2003); Kong et al. (2009); Kurmoo (2009); Lin et al. (2010); Ma & Zhou (2006); Reineke et al. (1999); Steiner (2002); Zhao et al. (2008); Zhuang et al. (2010).

Experimental top

2,5-Dioxopiperazine-1,4-diacetic acid (H2PODC) was prepared according to our previously reported methods (Kong et al., 2009). Oxalic acid dihydrate (0.032 g, 0.25 mmol) and H2PODC (0.12 g, 0.50 mmol) were dissolved in water (10 ml) and subjected to ultrasonic treatment for 10 min. La(NO3)3.6H2O (0.22 g, 0.50 mmol) was added to the mixture. The pH was adjusted slowly to about 4–5 with 1.0 mol l-1 sodium hydroxide solution. The solution was then sealed in a 25 ml Teflon-lined Parr bomb at 423 K for 3 d and then cooled to room temperature at a rate of 5 K h-1. White crystals of (I) were obtained in 42% yield (based on H2PODC). Analysis, calculated (found) for C9H10LaN2O9 (%): C 25.16 (25.45), N 6.52 (6.88), H 2.33 (2.07). IR spectrum for (I) (KBr, ν, cm-1 ): 493 (m), 703 (w), 796 (m), 960 (m), 1187 (w), 1267 (w), 1286 (w), 1313 (m), 1387 (w), 1426 (w), 1485 (w), 1639 (s), 2937 (w).

Refinement top

All H atoms were generated geometrically and allowed to ride on their parent atoms in riding-model approximations, with C—H = 0.99 Å and Uiso(H) = 1.2Ueq(C) for methyl H atoms, and with O—H = 0.85 Å and Uiso(H) = 1.5Ueq(O) for water H atoms.

Computing details top

Data collection: PROCESS-AUTO (Rigaku, 1998); cell refinement: PROCESS-AUTO (Rigaku, 1998); data reduction: CrystalClear (Rigaku, 1999); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: DIAMOND (Brandenburg, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2010).

Figures top
Fig. 1. The coordination environment of the LaIII cation in (I), showing the atom-labelling scheme. Displacement ellipsoids are drawn at the 30% probability level. [Symmetry codes: (i) -x, -y + 1, -z + 1; (ii) -x + 1/2, y, -z + 1/2; (iii) x + 1/2, -y, x + 1/2; (iv) x + 1/2, -y + 1, z + 1/2; (v) x, y - 1, z.]

Fig. 2. (a) A view of the structure of (I), showing the two-dimensional network viewed along the b axis. (b) A view showing how the two-dimensional network is linked by carboxylate groups in the b direction, with only the carboxylate groups shown for simplicity. [Symmetry codes: (i) x, y + 1, z; (ii) x + 1/2, -y + 1, z + 1/2; (iii) x, y - 1, z.]

Fig. 3. A schematic representation of the topological motif of (I).
poly[diaquabis(µ5-2,5-dioxopiperazine-1,4-diacetato)(µ2- oxalato)dilanthanum(III)] top
Crystal data top
[La(C8H8N2O6)(C2O4)0.5(H2O)]F(000) = 828
Mr = 429.10Dx = 2.290 Mg m3
Monoclinic, P2/nMo Kα radiation, λ = 0.71073 Å
a = 13.022 (3) ÅCell parameters from 2856 reflections
b = 4.9269 (10) Åθ = 3.5–27.5°
c = 19.420 (4) ŵ = 3.48 mm1
β = 92.54 (3)°T = 173 K
V = 1244.7 (4) Å3Block, colourless
Z = 40.30 × 0.10 × 0.05 mm
Data collection top
Rigaku R-AXIS RAPID
diffractometer
2305 independent reflections
Radiation source: fine-focus sealed tube2228 reflections with I > 2σ(I)
Graphite monochromatorRint = 0.037
ω scansθmax = 25.5°, θmin = 3.5°
Absorption correction: analytical
(Alcock, 1970)
h = 1513
Tmin = 0.421, Tmax = 0.845k = 55
9542 measured reflectionsl = 2323
Refinement top
Refinement on F2Primary atom site location: structure-invariant direct methods
Least-squares matrix: fullSecondary atom site location: difference Fourier map
R[F2 > 2σ(F2)] = 0.020Hydrogen site location: inferred from neighbouring sites
wR(F2) = 0.052H-atom parameters constrained
S = 1.07 w = 1/[σ2(Fo2) + (0.0313P)2 + 0.6462P]
where P = (Fo2 + 2Fc2)/3
2305 reflections(Δ/σ)max = 0.001
190 parametersΔρmax = 0.92 e Å3
0 restraintsΔρmin = 1.27 e Å3
Crystal data top
[La(C8H8N2O6)(C2O4)0.5(H2O)]V = 1244.7 (4) Å3
Mr = 429.10Z = 4
Monoclinic, P2/nMo Kα radiation
a = 13.022 (3) ŵ = 3.48 mm1
b = 4.9269 (10) ÅT = 173 K
c = 19.420 (4) Å0.30 × 0.10 × 0.05 mm
β = 92.54 (3)°
Data collection top
Rigaku R-AXIS RAPID
diffractometer
2305 independent reflections
Absorption correction: analytical
(Alcock, 1970)
2228 reflections with I > 2σ(I)
Tmin = 0.421, Tmax = 0.845Rint = 0.037
9542 measured reflections
Refinement top
R[F2 > 2σ(F2)] = 0.0200 restraints
wR(F2) = 0.052H-atom parameters constrained
S = 1.07Δρmax = 0.92 e Å3
2305 reflectionsΔρmin = 1.27 e Å3
190 parameters
Special details top

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 (Å2) top
xyzUiso*/Ueq
La10.230949 (9)0.30366 (3)0.462681 (6)0.01058 (8)
O10.17268 (14)0.1398 (4)0.00689 (8)0.0186 (4)
O20.14481 (13)0.5137 (3)0.05406 (9)0.0195 (4)
O30.10580 (14)0.3500 (4)0.09680 (10)0.0218 (4)
O40.12635 (18)0.6344 (6)0.30450 (12)0.0434 (6)
O50.16005 (16)0.5553 (4)0.35673 (10)0.0280 (4)
O60.21154 (13)0.9843 (4)0.36714 (9)0.0231 (4)
O70.11893 (14)0.6514 (4)0.51960 (11)0.0221 (4)
O80.03796 (14)0.7761 (3)0.54948 (10)0.0187 (4)
N10.03579 (17)0.2771 (4)0.15563 (11)0.0129 (4)
N20.02225 (19)0.6916 (4)0.24984 (12)0.0181 (5)
C10.13570 (19)0.2642 (5)0.04486 (12)0.0133 (5)
C20.07713 (18)0.1004 (5)0.10108 (12)0.0154 (5)
H2A0.12400.03410.12090.019*
H2B0.02020.00030.08050.019*
C30.05291 (17)0.4024 (5)0.14654 (12)0.0137 (5)
C40.1062 (2)0.3585 (6)0.20841 (14)0.0184 (5)
H4A0.12390.19500.23510.022*
H4B0.17050.42450.18490.022*
C50.09306 (19)0.6067 (6)0.19807 (13)0.0192 (5)
H5A0.15510.53020.22210.023*
H5B0.11470.76990.17270.023*
C60.06865 (19)0.5736 (5)0.25856 (13)0.0212 (5)
C70.0628 (2)0.8989 (5)0.29710 (13)0.0213 (5)
H7A0.00670.96300.32580.026*
H7B0.08621.05540.26990.026*
C80.1521 (2)0.8015 (5)0.34452 (13)0.0171 (6)
C90.02240 (18)0.6237 (5)0.52002 (13)0.0164 (5)
O1W0.18441 (13)0.1122 (4)0.58007 (8)0.0204 (4)
H1WA0.14560.02520.57410.031*
H1WB0.16090.19570.61430.031*
Atomic displacement parameters (Å2) top
U11U22U33U12U13U23
La10.00843 (10)0.01532 (12)0.00802 (11)0.00177 (4)0.00075 (6)0.00119 (4)
O10.0200 (9)0.0260 (8)0.0098 (8)0.0085 (8)0.0010 (7)0.0024 (7)
O20.0234 (9)0.0188 (9)0.0158 (9)0.0049 (7)0.0031 (7)0.0008 (7)
O30.0167 (9)0.0311 (9)0.0182 (10)0.0009 (8)0.0076 (8)0.0032 (8)
O40.0300 (12)0.0717 (15)0.0298 (13)0.0055 (12)0.0144 (10)0.0302 (12)
O50.0353 (11)0.0231 (10)0.0246 (10)0.0029 (9)0.0084 (9)0.0073 (8)
O60.0214 (9)0.0246 (9)0.0227 (9)0.0039 (8)0.0066 (7)0.0117 (8)
O70.0100 (9)0.0190 (8)0.0372 (11)0.0009 (7)0.0021 (8)0.0113 (8)
O80.0120 (9)0.0187 (9)0.0255 (11)0.0006 (7)0.0030 (8)0.0059 (7)
N10.0122 (10)0.0178 (10)0.0085 (11)0.0001 (8)0.0003 (8)0.0017 (8)
N20.0199 (12)0.0211 (12)0.0132 (12)0.0022 (8)0.0016 (10)0.0053 (8)
C10.0087 (12)0.0210 (12)0.0104 (12)0.0008 (10)0.0025 (10)0.0001 (10)
C20.0190 (12)0.0146 (12)0.0125 (11)0.0001 (10)0.0020 (9)0.0019 (9)
C30.0127 (11)0.0186 (12)0.0098 (11)0.0040 (10)0.0009 (9)0.0028 (9)
C40.0136 (12)0.0277 (12)0.0142 (13)0.0009 (11)0.0049 (10)0.0041 (11)
C50.0161 (12)0.0255 (13)0.0160 (12)0.0039 (11)0.0006 (10)0.0020 (11)
C60.0179 (12)0.0309 (14)0.0149 (12)0.0019 (11)0.0002 (10)0.0050 (11)
C70.0249 (13)0.0182 (13)0.0199 (13)0.0037 (11)0.0102 (11)0.0067 (10)
C80.0190 (13)0.0235 (15)0.0087 (12)0.0062 (10)0.0010 (11)0.0022 (9)
C90.0115 (11)0.0170 (11)0.0207 (13)0.0009 (10)0.0009 (10)0.0016 (11)
O1W0.0245 (9)0.0219 (8)0.0155 (9)0.0082 (8)0.0097 (7)0.0065 (7)
Geometric parameters (Å, º) top
La1—O6i2.4374 (17)N1—C31.328 (3)
La1—O3ii2.4746 (19)N1—C21.456 (3)
La1—O2iii2.5143 (17)N1—C41.462 (3)
La1—O72.5353 (18)N2—C61.336 (4)
La1—O52.5407 (19)N2—C51.456 (3)
La1—O8iv2.5443 (19)N2—C71.457 (3)
La1—O1W2.5645 (17)C1—C21.534 (3)
La1—O1v2.5752 (18)C1—La1vii3.138 (2)
La1—C1iii3.138 (3)C2—H2A0.9900
O1—C11.255 (3)C2—H2B0.9900
O1—La1vi2.5752 (18)C3—C51.497 (3)
O2—C11.248 (3)C4—C61.506 (4)
O2—La1vii2.5143 (17)C4—H4A0.9900
O3—C31.238 (3)C4—H4B0.9900
O3—La1ii2.4746 (19)C5—H5A0.9900
O4—C61.229 (3)C5—H5B0.9900
O5—C81.239 (3)C7—C81.528 (3)
O6—C81.254 (3)C7—H7A0.9900
O6—La1viii2.4374 (17)C7—H7B0.9900
O7—C91.265 (3)C9—C9iv1.546 (5)
O8—C91.245 (3)O1W—H1WA0.8500
O8—La1iv2.5443 (19)O1W—H1WB0.8500
O6i—La1—O3ii76.50 (7)C6—N2—C7121.6 (2)
O6i—La1—O2iii144.10 (6)C5—N2—C7114.4 (2)
O3ii—La1—O2iii75.94 (6)O2—C1—O1124.1 (2)
O6i—La1—O7136.54 (6)O2—C1—C2117.6 (2)
O3ii—La1—O7131.60 (6)O1—C1—C2118.3 (2)
O2iii—La1—O779.35 (6)O2—C1—La1vii49.53 (12)
O6i—La1—O571.15 (6)O1—C1—La1vii74.95 (14)
O3ii—La1—O582.29 (7)C2—C1—La1vii164.95 (16)
O2iii—La1—O5126.60 (6)N1—C2—C1111.1 (2)
O7—La1—O580.09 (7)N1—C2—H2A109.4
O6i—La1—O8iv76.17 (6)C1—C2—H2A109.4
O3ii—La1—O8iv146.76 (6)N1—C2—H2B109.4
O2iii—La1—O8iv136.29 (6)C1—C2—H2B109.4
O7—La1—O8iv63.88 (6)H2A—C2—H2B108.0
O5—La1—O8iv71.23 (7)O3—C3—N1121.6 (2)
O6i—La1—O1W114.75 (6)O3—C3—C5118.1 (2)
O3ii—La1—O1W133.68 (6)N1—C3—C5120.2 (2)
O2iii—La1—O1W70.99 (6)N1—C4—C6116.8 (2)
O7—La1—O1W72.40 (6)N1—C4—H4A108.1
O5—La1—O1W143.87 (6)C6—C4—H4A108.1
O8iv—La1—O1W75.78 (6)N1—C4—H4B108.1
O6i—La1—O1v70.12 (6)C6—C4—H4B108.1
O3ii—La1—O1v76.43 (6)H4A—C4—H4B107.3
O2iii—La1—O1v81.37 (6)N2—C5—C3116.2 (2)
O7—La1—O1v139.22 (6)N2—C5—H5A108.2
O5—La1—O1v139.09 (6)C3—C5—H5A108.2
O8iv—La1—O1v111.08 (6)N2—C5—H5B108.2
O1W—La1—O1v67.42 (6)C3—C5—H5B108.2
O6i—La1—C1iii149.48 (6)H5A—C5—H5B107.4
O3ii—La1—C1iii72.98 (7)O4—C6—N2124.0 (3)
O2iii—La1—C1iii22.19 (6)O4—C6—C4116.7 (2)
O7—La1—C1iii68.65 (6)N2—C6—C4119.2 (2)
O5—La1—C1iii104.70 (7)N2—C7—C8113.9 (2)
O8iv—La1—C1iii132.38 (6)N2—C7—H7A108.8
O1W—La1—C1iii86.83 (6)C8—C7—H7A108.8
O1v—La1—C1iii101.88 (6)N2—C7—H7B108.8
C1—O1—La1vi139.82 (16)C8—C7—H7B108.8
C1—O2—La1vii108.27 (15)H7A—C7—H7B107.7
C3—O3—La1ii154.49 (17)O5—C8—O6126.2 (2)
C8—O5—La1131.05 (17)O5—C8—C7118.5 (2)
C8—O6—La1viii140.52 (18)O6—C8—C7115.3 (2)
C9—O7—La1121.40 (15)O8—C9—O7125.9 (2)
C9—O8—La1iv120.37 (15)O8—C9—C9iv118.3 (3)
C3—N1—C2118.5 (2)O7—C9—C9iv115.8 (3)
C3—N1—C4122.8 (2)La1—O1W—H1WA109.5
C2—N1—C4116.8 (2)La1—O1W—H1WB128.7
C6—N2—C5123.7 (2)H1WA—O1W—H1WB105.1
Symmetry codes: (i) x, y1, z; (ii) x+1/2, y, z+1/2; (iii) x+1/2, y+1, z+1/2; (iv) x, y+1, z+1; (v) x+1/2, y, z+1/2; (vi) x1/2, y, z1/2; (vii) x1/2, y+1, z1/2; (viii) x, y+1, z.
Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O1W—H1WA···O7i0.851.942.678 (2)145
O1W—H1WB···O4iv0.851.862.703 (3)172
Symmetry codes: (i) x, y1, z; (iv) x, y+1, z+1.

Experimental details

Crystal data
Chemical formula[La(C8H8N2O6)(C2O4)0.5(H2O)]
Mr429.10
Crystal system, space groupMonoclinic, P2/n
Temperature (K)173
a, b, c (Å)13.022 (3), 4.9269 (10), 19.420 (4)
β (°) 92.54 (3)
V3)1244.7 (4)
Z4
Radiation typeMo Kα
µ (mm1)3.48
Crystal size (mm)0.30 × 0.10 × 0.05
Data collection
DiffractometerRigaku R-AXIS RAPID
diffractometer
Absorption correctionAnalytical
(Alcock, 1970)
Tmin, Tmax0.421, 0.845
No. of measured, independent and
observed [I > 2σ(I)] reflections
9542, 2305, 2228
Rint0.037
(sin θ/λ)max1)0.606
Refinement
R[F2 > 2σ(F2)], wR(F2), S 0.020, 0.052, 1.07
No. of reflections2305
No. of parameters190
H-atom treatmentH-atom parameters constrained
Δρmax, Δρmin (e Å3)0.92, 1.27

Computer programs: PROCESS-AUTO (Rigaku, 1998), CrystalClear (Rigaku, 1999), SHELXS97 (Sheldrick, 2008), DIAMOND (Brandenburg, 2008), SHELXL97 (Sheldrick, 2008) and publCIF (Westrip, 2010).

Hydrogen-bond geometry (Å, º) top
D—H···AD—HH···AD···AD—H···A
O1W—H1WA···O7i0.851.942.678 (2)145
O1W—H1WB···O4ii0.851.862.703 (3)172
Symmetry codes: (i) x, y1, z; (ii) x, y+1, z+1.
 

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