Comment

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, reports on 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 to 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-diacetic 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 and structure.

Complex (I) crystallizes in the monoclinic space group P2/n. The asymmetric unit contains one La^III cation, one PODC^2- ligand, half an ox^2- ligand (lying about an inversion centre) and one aqua ligand. As shown in Fig. 1, the coordination environment of the La^III cation can be viewed as a square antiprism, featuring contributions by four carboxylate O atoms from four different PODC^2- ligands, two carboxylate O atoms from an ox^2- ligand, one carbonyl O atom of a fifth PODC^2- ligand and one O atom from an aqua ligand. The La^III-O bond lengths [2.4374 (17)-2.5752 (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 PODC^2- ligand feature syn-syn and syn-anti coordination modes and one carbonyl group takes part in coordination. This was also found in (II). However, in a similar compound obtained previously, [Dy₂(PODC)(ox)₂(H₂O)₂] (Kong et al., 2009), the PODC^2- ligand exhibits a different coordination mode from that of (I), owing to the different lanthanide cation. Furthermore, the number of PODC^2- ligands around the La^III cation in (I) is five, which is less than that of pure H₂PODC-based La^III 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 La^III cation is connected to adjacent La^III cations by bridging PODC^2- and ox^2- ligands, generating two-dimensional grid layers (see Fig. 2a). These layers are further linked via the carboxylate groups of the PODC^2- 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 PODC^2- ligand serves as a five-connecting node, while the La^III 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 previously (Kong et al., 2009). The difference can be ascribed to two aspects: (i) the ratio of PODC^2- and ox^2- 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).

Experimental

2,5-Dioxopiperazine-1,4-diacetic acid (H₂PODC) was prepared according to our previously reported method (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 was heated at 423 K for 3 d and cooled to room temperature at a rate of 5 K h^-1. Colourless 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, , 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).

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 U_iso(H) = 1.2U_eq(C) for methyl H atoms, and O-H = 0.85 Å and U_iso(H) = 1.5U_eq(O) for water H atoms.

Data collection: PROCESS-AUTO (Rigaku, 1998); cell refinement: PROCESS-AUTO; 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 and publCIF (Westrip, 2010).

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Supplementary data for this paper are available from the IUCr electronic archives (Reference: FG3274 ). Services for accessing these data are described at the back of the journal.

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