Chen Zilu, Liang Fupei, Hu Ruixiang, Liang Hong, Yu Kaibei ^#
(Department of Chemistry, Guangxi Normal University, Guilin 541004, China; ^#Analysis Center, Chengdu Branch of Chinese Academy of Science, Chengdu 610041, China)

Received Sep. 3, 2000; Supported by the National Natural Science Foundation of China (No. 29761002) and the Ten, Hundred or Thousand Distinguished Person Foundation of Guangxi, China.

Abstract Treatment of Y(pic)₃ with 1, 10'-phenanthroline and 4, 4'-bipyridine in acidic environments yields a kind of polymer assembled by non-covalent weak interactions. It was characterized by single crystal X-ray diffraction which shows that a novel supramolecular unit of [(phenH)(4, 4'-bpyH)(phen)](pic)₂ ( 4, 4'-bpy: 4, 4'-bipyridine, phen: 1, 10'-phenanthroline, pic^-: picric anion ) was formed via hydrogen bonds among one 4, 4'-bipyridine molecule and two 1, 10'-phenanthroline molecules and p-p stacking effect between pyridyl ring from 4, 4'-bpy and phenyl ring from picric anions, then a 2D brick-wall structure is constructed by p-p stacking effect among different units in the form of head-to-tail overlap.
Keywords picric anion, phen, 4, 4'-bpy, hydrogen bond, p-p stacking effect

1. INTRODUCTION
All chemists are familiar with intramolecular covalent bonds, but less familiar with the concepts of intermolecular non-covalent bonds between molecules and/or ions^[1]. Indeed, the non-covalent interactions such as p-p stacking effect, hydrogen bonds and host-guest interactions play an important role in the formation of supramolecules and crystalline form. So the nature of non-covalent bond gets to the center of interest in organic or inorganic chemistry and leads to the rapid development in this new field.^[2-7] A series of concepts such as self-assembly, supramolecular synthesis and crystal engineering are followed hand at heel.^[8] This provides reliable theoretical basis and practical synthetic routes for the design of new type functional materials demonstrating special characters such as non-linear optical property, superconductivity and ferro-magnetism.^[5-6]
    4, 4'-Bipyridine is an excellent rigid bridging ligand and liable to have some weak intermolecular interactions such as hydrogen bonding and p-p stacking effect with other molecules. Many structures have been done on the coordination of 4, 4-bipyridine to metals, but less done on the non-covalent weak interactions of 4, 4-bipyridine with other molecules. We have synthesized a polymer assembled non-covalent weak interactions and found that 4, 4'-bipyridine has other two kinds of non-covalent bridging modes ( hydrogen bridging mode, p-p stacking bridging mode ), which have been reported in the literature.

2. EXPERIMENTAL
2.1 Materials and methods
All starting chemical reagents were commercially available with analytic grade except that 4, 4'-bipyridine with reagent grade. Rare earth picrate was prepared according to a literature method^[9]. Elemental analyses ( C, H, N ) were done on a German EL-CHNS-O elemental analyser. Infrared spectrum was measured with American Nicolet 5DXB FT-IR spectrophotometer on KBr disk.
2.2 Synthesis of the complex
Y(pic)₃·5H₂O ( 0.5 mmol, 0.4319g ) was dissolved in CH₃CN-H₂O ( 1:1 V/V, 10 ml ) at 50°C, then   CH₃CN-H₂O (1:1 V/V, 10 ml ) solution containing  phen ( 1 mmol, 0.1982g ) and CH₃CN-H₂O (1:1V/V, 10 ml) solution containing   4, 4'-bipyridine (0.5 mmol, 0.0781g ) were added stepwise with stirring, yielding a small amount of precipitate. 20h later, the reaction mixture was filtered and the filtrate was laid aside to evaporate for one month, yellow needle crystals were obtained. A single crystal suitable for X-ray analysis was obtained by redissolving the yellow needle crystals in absolute ethanol and evaporating the solvent at room temperature for one month. Anal. Found: C, 56.68%; H, 3.10%; N, 17.24%. Calc. for C₄₆H₃₀N₁₂O₁₄: C, 56.77%; H,3.07%; N,17.28%.
2.3 Crystallography
Crystal data were collected on Siemens P4 diffractometer using graphite-monochromated Mo Ka radiation ( l = 0.071073 nm ). The intensities of reflections were measured in the w scanning mode at 296(2) K. Cell Parameters of the complex were determined from 25 reflections in the q range of 2.15° -25.00° . An empirical absorption correction was applied to the original data set. The structure was solved using direct methods and refined by a full-matrix least-squares on F² technique using the SHLXL-97 program package. Hydrogen atoms of the ligands were generated geometrically. a = 0.73370(10) nm, b = 1.03390(10) nm， c = 1.5195(2) nm,  a= 75.990(10)° , b = 84.780(10)° , g = 70.160(10)°, V = 1.0519(2）nm³, Z = 1, D_c = 1.539 g·cm ³, m = 0.118 mm^-1, F(000) = 502, R1 = 0.0467, wR2 = 0.1143, W = 1 / [ s²F₀² + ( 0.0685 P )² ], (D/s)_max = 0.000, Dr_min = -366 e/nm³, Dr_max = 588 e/nm³.

3. RESULTS AND DISCUSSION
3.1 Synthesis of the complex   
The reactions among yttrium picrate, 1, 10-phenanthroline and 4, 4'-bipyridine in CH₃CN-H₂O ( 1:1 ) gave some precipitates. The crystals obtained from the filtrate were found to be soluble in solvents such as CH₃CN, CH₃COCH₃ and CH₃CH₂OH, but very difficult to dissolve in H₂O. Suitable crystals were not easy to obtain for single crystal X-ray diffraction. The result of X-ray diffraction indicates that there are some protonized 4, 4'-bpy existing in the compound. This is caused by the yttrium picrate which was not thoroughly purified. The solution of yttrium picrate in CH₃CN-H₂O ( 1:1 ) has the pH value of 4. In this acidic environment, some 4, 4'-bipyridine molecules are easily protonized.

Fig. 1 Crystal structure of [ ( phenH ) ( 4,4'-bpyH ) ( phen )] (pic)₂, showing the atom numbering scheme.

3.2 Structure of [ ( phenH ) ( 4,4'-bpyH ) ( phen )] (pic)₂
The crystal structure consists of centro-symmetric supramolecular units assembled from 4, 4'-bipyridine, 1, 10'-phenanthroline and picric anions via intermolecular weak interactions. In this supramolecular unit, 4, 4'-bpy molecule acts as two kinds of weak bridging function. One is hydrogen bonding bridge , that is to say, one 4, 4'-bpy molecule bridges two 1, 10-phenanthroline molecules via H-bond of the type of N-H ^... N ( N₁ ^... N₂: 0.2716 nm, N₁-H_1N ^... N₂: 164(5)°, N₂-H_2N ^... N₁: 167(5)° ). The other is that one 4, 4'-bpy molecule bridges two picric anions via p-p stacking effect between pyridyl ring of the 4, 4'-bpy molecule and phenyl ring of the picric anion. The phenyl rings, which are parallel to the pyridyl rings of the 4, 4'-bpy, overlap the nearest pyridyl rings with the separation of 0.3316nm, indicating significant p-p stacking effect. Two noncoplanar pyridyl rings of the 4, 4'-bpy molecule are parallel to each other with the separation of 0.00279nm, different from the reported cases^[10-12] in which two pyridyl rings of the 4, 4'-bpy molecule are coplanar or twisted with a certain dihedral angle. One pyridyl ring of the 4, 4'-bpy molecule and the nearest 1, 10-phenanthroline molecule are non-coplanar with a dihedral angle of 42.6°. The packing of such units in the ac plane is dominated by p-p interactions between phen molecules in the form of head-to-tail overlap with the separation of 0.3561nm, which is comparable to the reported cases^[12-14]. So formed a two-dimensional brick-wall structure in the ac plane. Between layers only exist electrostatic force and Van der Waals force.

Fig. 2 Two-dimensional brick-wall structure in the ac plane

    According to the above and literature results, four kinds of bridging mode for 4, 4'-bpy are summarized here. One is coordination bridging mode ( M-N(4, 4'-bpy)N-M )^[14-15], another two are hydrogen bonding bridge ( M-H₂O ^... N(4, 4'-bpy)N ^... H₂O-M )^[16] and p-p stacking bridging mode described in this paper. And the fourth is an unsymmetric bridging mode containing coordination bond and hydrogen bond simultaneously ( M-N(4, 4'-bpy)N ^... H₂O-M )^[17].

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