ZHANG Rong-Hua ZHAO DanLI Fei-FeiFAN Yun-Chang LIU Bing-Guo

(Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, Henan 454000, China)

Structural Determination of a New La(III) Coordination Polymer with Pseudo-merohedral Twinning Structure①

ZHANG Rong-Hua ZHAO Dan②LI Fei-Fei②FAN Yun-Chang LIU Bing-Guo

(Department of Physics and Chemistry, Henan Polytechnic University, Jiaozuo, Henan 454000, China)

Hydrothermal reaction of benzene-1,2,4,5-tetracarboxylic (Btec) dianhydride with La2O3lead to a new organometallic compound, [La2(betc)(Hbtec)(H2O)3][HCONH(CH3)2] (I).Single-crystal X-ray diffraction analysis reveals that it crystallizes in monoclinic space group P21/n that mimics orthorhombic space group Pnma (viz.pseudo-merohedral twinning structure) with a = 7.6002(16), b = 18.954(4), c = 18.213(4) ?, β = 90.121(3)o, V = 2623.5(9) ?3, Z = 4, Mr= 907.21, Dc= 2.297 g/cm3, F(000) = 1752, μ(MoKα) = 3.32 mm-1, R = 0.0575 and wR = 0.0956.The structure of I features a 3-D framework constructed by La(III) cations, btec4-ligands, Hbtec3-ligands and coordinated water molecule.All of the La(III) cations are 10-coordinated by O atoms into 1-D infinite chains, which are further interconnected by C atoms of btec ligands to form a 3-D framework.

organometallic, btec, pseudo-merohedral twinning;

1 INTRODUCTION

In recent years, study of coordination polymers has gained great recognition as an important interface between synthetic chemistry and material science not only because of their fascinating topology and intriguing architectures, but also due to their potential applications in magnetism, luminescence, ion exchange, electronic apparatus, catalysis etc[1-6].Crystal engineering provides a powerful tool for the design and synthesis of novel interesting compounds, and the advances in theory and practice of molecular crystallography have increased the speed and scope of structure determination enormously.However, due to the complexity of chemical reactions, suitable single crystals that have a single lattice throughout the structure are usually hardly to obtain, which will greatly increase the difficulty of structure analysis.Structure twinning which is frequently observed in crystals of small molecules is a problem in crystallography because it causes superposition or overlap between reflections which are not related by symmetry[7-11].

From the viewpoint of fundamental understanding, twinning is the oriented association of two or more individuals of the same crystalline phase, in which pairs of individuals are related by a geometrical operation termed twin operation.According to the literatures of "French school"[12-14], twinning can be classified into four types: (i) merohedral twin in which the twin element belongs to the point group of the lattice but not to the point group of the crystal; (ii)pseudo-merohedral twinning in which the twin element belongs to the higher crystal system; (iii) reticular-merohedral twinning (obverse/reverse twin) in which the twin law of a rhombohedral structure parallels to the threefold axis (matrix (?1 0 0 0 ?1 0 0 0 1) in the hexagonal setting) or to a-b (matrix (0?1 0 ?1 0 0 0 0 ?1) in the hexagonal setting); (iv) non-merohedral twinning in which the twin operation does not belong to the Laue group or to the point group of the crystal.

The kind of pseudo-merohedral twinning may occur in crystals where the cell dimensions mimic those of a higher-symmetry crystal system.Common examples for pseudo-merohedral twinning are monoclinic with β ≈ 90.0o or a ≈ c with β ≈ 120o.Since the reciprocal lattices of pseudo-merohedral twinning components overlap so well in low angle regions, the twinning is difficult to detect from the diffraction pattern.Thus we may usually overestimate the symmetry leading to a poor refinement.To refine such structures better, we must find the true space group and set up the INS file to solve the structure.Then we can build a recognizable model although the refinement may be difficult and the refinement is not very good.After introducing twin law into the INS file, refinement is shown to be quite straightforward, in spite of the twinning.

In this paper, we report a new organometallic compound I, whose structure is determined from a pseudo-merohedral twinning crystal.The detailed processing of pseudo-merohedral twinning structure will be discussed in detail in this manuscript.

2 EXPERIMENTAL

2.1 Synthesis

Benzene-1,2,4,5-tetracarboxylic dianhydride (C10H2O6), N,N-dimethylformamide (DMF), La2O3and KOH were purchased from Sinopharm Chemical Reagent Co.Ltd.All chemicals of analytical grade were obtained from commercial sources and used without further purification.A mixture of C10H2O6(0.1 mmol, 0.022 g), La2O3(0.5 mmol, 0.163 g), DMF (5 mL) and water (5 mL) whose pH value was adjusted to 4 by KOH (0.1 M) was placed in a 30-mL Teflon-lined stainless steel autoclave.Then the autoclave was sealed and heated to 160 ℃under autogenously pressure for 72 h.After being slowly cooled to room temperature at a rate of 5 ℃/h, colorless prismatic crystals were recovered by filtration, washed by distilled water and air dried.

2.2 X-ray crystal determination

A suitable single crystal with dimensions of 0.20mm × 0.05mm × 0.05mm for compound I was selected for single-crystal X-ray diffraction experiments.It was mounted on a Bruker Smart APEXII CCD diffractometer equipped with a graphite-monochromatic Mo-Kα radiation (λ = 0.71073 ?) with a tube power of 20 kV and 5 mA.Intensity data were collected by the narrow frame method at 296(2) K with a scan width of 0.5° in an ω-scan method.The unit cells were determined and refined by least-squares upon the refinement of XYZ-centeroids of reflections above 2.0σ(I).Then the data were scaled for absorption using APEX II package[15].Intensities of all measured reflections were corrected for Lp and crystal absorption effects.The crystal structures of the title complexes were solved by direct methods and refined by full-matrix leastsquares fitting on F2by SHELX97[16].All nonhydrogen atoms in the structure were refined using harmonic anisotropic atomic displacement parameters (ADP).All hydrogen atoms were located at the geometrically calculated positions and refined with isotropical thermal parameters.The final R == 0.907, (Δρ)max= 1.207, (Δρ)min= –1.554 e/?3and (Δ/σ)max= 0.000.The final refined solutions obtained were checked with the programme PLATON[17].

3 RESULTS AND DISCUSSION

3.1 Structure solution

The symptoms of twinning can be recognized atthe data-processing stage and if suitably treated, the solution of the structure is possible.It is frequently observed in crystals of small molecules and is not regarded as an insoluble problem.The initial data procession for I gave an orthorhombic cell with space group Pnma.However, structure solution with orthorhombic model gave large residue peaks, high R value, and poor refinement, indicating a possibly twinning problem.

As can be seen in the selected diffraction image (Fig.S1 of supporting information), some reflections in the high angle regions are obviously split, while in the low angle regions the split is obscure.This is the typical characteristics of pseudo-merohedral twinning whose overlap of the lattices might not be perfect for all reflections like merohedral twinning.Thus we constructed the twinning model using lower symmetry of monoclinic space group P21/n.The processes of data collection, data integration, absorption, structure solution, and structure refinement were not extraordinary, except for adding“Twin law” instruction to the refinement file.For I, the twin law was determined to be (–1 0 0 0–1 0 0 0 1) which can be thought as the relation between mimic orthorhombic space group Pnma and real monoclinic space group P21/n.The BASF statement should also be added into the INS file to refine the ratio of two twinning domains.For I, the BASF parameter was refined to be 0.433, indicating the ratio of two twin domains.The final refinement converges with R (F2> 2σ(F2) = 5.8% and wR(F2) = 1.13% for 418 parameters and gives the inessential highest residual peak of 1.21 e/?3and the lowest residual peak of –1.55 e/?3, proving that our twinning structure model for I is reasonable.The important bond lengths and bond angles are listed in Table 1.

Table 1.Selected Bond Lengths (?) for I

3.2 Crystal structure description

Structure solution revealed that there are two La3+cations, one btec4-anion, one Hbtec3-anion, three coordinated water molecules and one protonated dissociative DMF molecule in the asymmetric unit.The La(1)(III) cation is 10-coordinated by seven carboxylate oxygen atoms from five btec4-or Hbtc3-ligands, and three oxygen atoms from aqua ligands to form a distorted La(1)O10polyhedron, as shown in Fig.1a.Unlike the La(1) (III) cation, as shown in Fig.1b, the La(2) (III) cation is 10-coordinated by nine carboxylate oxygen atoms from six btec4-or Hbtc3-ligands, and one oxygen from aqua ligand to form a distorted La(2)O10polyhedron.The La–O distances range from 2.432(11) to 2.892 (11) ?, which is the common value compared with those reported for other La(III) organometallic compounds[18-20].In addition, bond valence sum calculations prove that the La atoms are in the +3 oxidation state[21,22].The calculated total bond valences for La(1) and La(2) are 3.238 and 3.170, respectively.

Fig.1.Coordination environments of La(1) (III) cations (a) and La(2) (III) cations (b).Hydrogen atoms are omitted for clarity.Symmetry codes: (i) x?1/2, ?y+1/2, z?1/2; (ii) ?x, ?y+1, ?z+1; (iii) ?x+1/2, y?1/2, ?z+3/2; (iv) x?1, y, z; (v) x?1/2, ?y+1/2, z+1/2; (vi) ?x, ?y+1, ?z+2

The btec ligand has multi-coordination modes which include μn-bridging modes (n = 2～12) in complexing with metal ions and hence generate a large number of coordination configurations.For compound I, one crystallographic distinct btec has four deprotonated carboxylate groups into a btec4-anion, while the other crystallographic distinct btec has three deprotonated carboxylate groups into a Hbtc3-anion.The four carboxylate groups of btec4-anion adopts a μ2-coordination model, and the whole ligand acts as a μ6-bridge linking six La(III) cations, as shown in Fig.2a.Somewhat differently, two carboxylate groups of Hbtec3-anion adopts a μ2-coordination model, one arboxylate group of Hbtec3-anion adopts a μ1-coordination model, and the whole ligand acts as a μ5-bridge linking five La(III) cations, as shown in Fig.2b.Furthermore, when the C and H atoms are omitted, a 1-D La-O chain is emerged (Fig.3a).These 1-D layers are further integrated by C atoms of btec ligands into a 3-D framework (Fig.3b).The H-bonds play an important role in the construction of the 3-D framework, as listed in Table 2.On the other hand, there is a free protonated DMF molecule residing among the 3-D framework and join them through coulombic action to maintain the electrical neutrality.

Fig.2.Coordination environments of btec4-(a) and Hbtec3-ligands (b) Hydrogen atoms are omitted for clarity

Fig.3.(a) 1-D infinite La?O chains of compounds (I); (b) Crystal structures of (I) showing 3-D framework.Hydrogen atoms are omitted for clarity.Symmetry codes: (i) x?1/2, ?y+1/2, z?1/2; (iv) x?1, y, z

Table 2.Hydrogen-bond Geometry (?, °)

4 CONCLUSION

In summary, a new metal-organic framework compound[La2(betc)(Hbtec)(H2O)3][HCONH(CH3)2] has been obtained and structurally characterized through single-crystal X-ray diffraction analysis.Refinement of the structure is complicated by a pseudo-merohedral twinning with the twin law of (–1 0 0 0 –1 0 0 0 1).The structure of [La2(betc)(Hbtec)(H2O)3] [HCONH(CH3)2] features a 3-D framework constructed by La(III) cations, btec4-ligands, Hbtec3-ligands and aqua.La(III) cations are 10-coordinated by O atoms into 1-D infinite chains, which are further interconnected by C atoms of btec ligands to form the 3-D framework.

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10.14102/j.cnki.0254-5861.2011-0624

4 January 2015; accepted 16 March 2015 (CCDC 1049668)

① This work was supported by the National Natural Science Foundation of China (Nos.21201056, 11204067 and 21307028)

② Corresponding authors.E-mails: iamzd@hpu.edu.cn (D.Zhao) and lifeifei@hpu.edu.cn (F-F.Li)