Wang Chongchen, Wang Peng
(School of Environmental and Energy Engineering, Beijing University of Civil Engineering and Architecture, Beijing 100044, China)

Abstract[Ce(SO (HO)]·HO (1) and 2,2′-bipyridine-4,4′-dicarboxylic acid (2), were hydrothermally synthesized, and single crystal X-ray diffractionrevealed that compound (1) crystallizes in monoclinic, space group C2/c with =1.5745(2) nm, = 0.96286(12) nm, =1.03574(17) nm,=120.000(2) and =4, and compound (2) crystallizes in Orthorhombic crystal system, space group Fdd2 with =3.7808(8) nm, =1.4620(3) nm, =0.36627(7) nm, and =8. The two-dimensional layer of compound (1) was composed of cerium(III) centers, coordination water molecules and[SO^2- ligands.The cerium(III) center is nine-coordinated by seven oxygen atoms from [SO^2- units and two oxygen atoms from two coordination water molecules and the [SO^2- ligands display two different coordination modes. The strong hydrogen bonding interactions between the coordination water molecules, lattice water molecules and [SO^2- units contribute to further link the 2-D layers into 3-D framework.The result given by thermogravimetric analysis revealed that compound (1) was thermally stable up to 150C. While the compound (2) remained the typical bond distances and bond angles, and 2-D layer is constructed by O1—H…N1 hydrogen bond.

There is considerable current interest in the synthesis and characterization of multi-dimensional coordination polymers because of their applications. Multi-dentate ligands, which are used as building blocks in constructing the coordination polymers are quite important in the crystal engineering of the supramolecular architecture organized by coordinate covalent or hydrogen bonding interaction. 2,2′-bipyridine-4,4′-dicarboxylic acid (bpdc) is a potential bridging ligand in various coordination models as a result of its multi-functional groups. The synthesis, structure and properties on the coordination compound constructed by bpdc and transition and lanthanide metal were investigated ^[1-7], which revealed that bpdc is versatile organic ligand due to its diverse coordination modes of carboxylate groups. Encouraged by the previous works by others, the hydrothermal reaction betweenCe(NO·6HO, 2,2′-bipyridine-4,4′-dicarboxylic acid and AgSCN designed and carried out. The results gave two compounds, one was two-dimensional inorganic compound [Ce(SO (HO)]·HO, and the other is the organic 2,2′-bipyridine-4,4′-dicarboxylic acid ligand.

1 EXPERIMENTAL
Reagent grade chemicals were used as received without further purification. IR spectra were recorded on a Perkin Elmer FT-IR Spectrum Spectrophotomer (Spectrum 100) in region of 4004000 cm^-1 using KBr pellets. TGA was performed from room temperature to 700C in a N stream at a flow rate of 10C·min^-1 on a ZRY-1P TGA system using-Al as reference material.
1.1 Synthesis of [Ce(SO (HO)]·HO and 2,2′-bipyridine-4,4′-dicarboxylic acid
Colorless block-shaped crystals of [Ce(SO (HO)]·HO (1) and colorless block-like crystals of 2,2′-bipyridine-4,4′-dicarboxylic acid (2) were obtained by hydrothermal reaction of Ce(NO·6HO (0.0434 g), 2,2′-bipyridine-4,4′-dicarboxylic acid (0.0244g), AgSCN (0.0165g)and deionized water (15 mL) in a 23 mL polytetrafluoroethylene (PTFE)-lined stainless steel autoclave reactor at 433 K for 120 h followed by slow cooling to room temperature. The colorless block-like crystals of compound (1) and (2) are so different to separate them from each other, and the yields are ca. 46% and 27% for compound (1) and (2) respectively based on input Ce(NO·6HO and 2,2′-bipyridine-4,4′-dicarboxylic acid.
1.2X-ray crystallography
Crystals of compound (1) and (2) suitable for single-crystal X-ray diffraction was selected for the experiment. The intensity data of the title compound were collected at room temperature (293 K) on a Rigaku R-axis Rapid IP Area Detector diffractometer byOscillation scan technique using graphite- monochromatized Mo K_αradiation (=0.071073 nm). For (1), the total reflections of 6371 were measured, unique 1562 with (int)=0.050 within the limits 3.11<27.54, and for (2), the total reflections of 4399 were measured, unique 673 with (int)=0.135 within the limits 3.52<27.50.The structure was solved using direct method with SHELXS-97 ^[8] and refined by full-matrix least-squares on with anisotropic thermal parameters for all non-hydrogen atoms. Hydrogen atoms were geometrically fixed to allow riding on the parent atoms to which they are attached. All calculations were performed using SHELXTL-97 program package ^[9]. For (1), the final full-matrix least-squares refinement gave = 0.0238, wR =0.0564. The highest and lowest residual peaks in the final difference Fourier map are 100.8 and -180.6 e/nm, respectively. The crystal belongs to monoclinic crystal system, space group C2/c with =1.5745(2) nm, =0.96286(12) nm, =1.03574(17) nm,=120.000(2) and =4. For (2), the final full-matrix least-squares refinement gave = 0.0484, wR =0.0999. The highest and lowest residual peaks in the final difference Fourier map are 52.8 and -56.7e/nm, respectively. The crystal belongs to Orthorhombic crystal system, space group Fdd2 with =3.7808(8) nm, =1.4620(3) nm, =0.36627(7) nm, and =8.

Table 1 Bond lengths and angles for compound (1)

Bond length/nm	Bond angle/(	Bond angle/(
Ce(1)-O(3)#1 2.426(3) Ce(1)-O(5)#2 2.445(3) Ce(1)-O(1) 2.457(3) Ce(1)-O(2)#3 2.458(3) Ce(1)-O(4)#2 2.488(3) Ce(1)-O(8) 2.549(3) Ce(1)-O(6)#4 2.558(3) Ce(1)-O(7) 2.619(3) Ce(1)-O(5) 2.807(3)	O(3)#1-Ce(1)-O(5)#2 82.89(10) O(3)#1-Ce(1)-O(1) 138.86(10) O(5)#2-Ce(1)-O(1) 77.64(9) O(3)#1-Ce(1)-O(2)#3 99.49(10) O(5)#2-Ce(1)-O(2)#3 142.91(10) O(1)-Ce(1)-O(2)#3 76.73(9) O(3)#1-Ce(1)-O(4)#2 78.02(10) O(5)#2-Ce(1)-O(4)#2 75.06(9) O(1)-Ce(1)-O(4)#2 129.44(9) O(2)#3-Ce(1)-O(4)#2 141.89(10) O(3)#1-Ce(1)-O(8) 73.39(10) O(5)#2-Ce(1)-O(8) 139.81(10) O(1)-Ce(1)-O(8) 140.00(10) O(2)#3-Ce(1)-O(8) 74.17(10) O(4)#2-Ce(1)-O(8) 68.66(10) O(3)#1-Ce(1)-O(6)#4 143.60(10) O(5)#2-Ce(1)-O(6)#4 121.27(9) O(1)-Ce(1)-O(6)#4 76.50(10)	O(2)#3-Ce(1)-O(6)#4 77.64(9) O(4)#2-Ce(1)-O(6)#4 82.55(10) O(8)-Ce(1)-O(6)#4 70.94(10) O(3)#1-Ce(1)-O(7) 73.41(10) O(5)#2-Ce(1)-O(7) 73.14(10) O(1)-Ce(1)-O(7) 66.39(10) O(2)#3-Ce(1)-O(7) 72.22(10) O(4)#2-Ce(1)-O(7) 139.26(10) O(8)-Ce(1)-O(7) 127.19(10) O(6)#4-Ce(1)-O(7) 136.30(10) O(3)#1-Ce(1)-O(5) 139.64(9) O(5)#2-Ce(1)-O(5) 69.59(9) O(1)-Ce(1)-O(5) 63.91(9) O(2)#3-Ce(1)-O(5) 120.35(8) O(4)#2-Ce(1)-O(5) 66.94(9) O(8)-Ce(1)-O(5) 109.52(9) O(6)#4-Ce(1)-O(5) 51.68(8) O(7)-Ce(1)-O(5) 122.41(9)

Symmetry transformations used to generate equivalent atoms: #1 x,-y+1,z-1/2; #2 -x+1,-y+1,-z+1; #3 -x+3/2,y-1/2,-z+3/2; #4 -x+1,y,-z+3/2; #5 -x+3/2,y+1/2,-z+3/2; #6 x,-y+1,z+1/2.

2 RESULTS AND DISCUSSION
2.1 Thesynthesisofcompound (1) and (2) 
The hydrothermal reaction between Ce(NO·6HO, AgSCN and 2,2′-bipyridine-4,4′- dicarboxylic acid failed to produce the organic-inorganic hybrid compound containing cerium centers and 2,2′-bipyridine-4,4′-dicarboxylic acid ligands, but gave a novel inorganic compound and the organic ligand itself. More experiments to test the role of AgSCN, but the results revealed that the title compounds were failed toobtain without AgSCN. More tries to replace AgSCN with Ag(SO) also failed to harvest the title compounds. It was thought that the hydrolysis of SCN results into the occurrence of [SO^2- under the extreme hydrothermal condition.

Fig. 1 The asymmetric unit of compound (1)

2.2 Thecrystal structure ofcompound (1)  
The framework of the compound (1) is constructed by CeO tetradecahedra and SO tetrahedra. The existence of [SO^2- can be proved by the strong character peak at 1090 cm^-1. The cerium center is nine-coordinated by five oxygen atoms from four [SO^2- anions via mono-coordination mode, two oxygen atoms from a [SO^2- unit by chelating mode, and two oxygen atoms from two water molecules, as shown in Fig.1. There are no apparent difference between the bond distance for the Ce-O([SO^2-) and Ce-O(HO), ranging from 0.2426 to 0.2619 nm, except for the Ce-O(5) 0.2807 nm. The bond angle of O-Ce-O ranges from 51.68 to 142.91, as listed in Table 1, which are similar to other cerium sulfate reported ^[10-12]
In the title compound, the [SO^2- anions show two different coordination modes to link the cerium centers, as illustrated in Fig. 2. The [S1O^2- is tetra(mono-dentate) ligand to join four different cerium centers. While the [S2O^2- acts as hexa-dentate ligand to link four cerium ions, in which the O(5) and O(6A) are coordinated to one cerium center in chelating mode, and the O(5) is also linked to another cerium via mono-dentate mode. In the title compound, the [SO^2- units play the role of multi-dentate ligands, and also to balance the charge of the whole compound.

Fig. 2 The coordination modes of [SO^2- units in (1)

Table 2 Hydrogen bonds (nm and ) in the crystal structure of compound (1)

D-H (D-H) (H..A) <DHA (D..A) A ( Symmetry transformation)

O7-H7B 0.850 2.209 157.82 3.014 O1 [ x, -y+1, z-1/2 ] O7-H7B 0.850 2.650 117.84 3.138 O8 [ -x+3/2, -y+1/2, -z+1 ] O7-H7B 0.850 2.973 131.42 3.592 S1 [ x, -y+1, z-1/2 ] O7-H7C 0.850 2.480 131.48 3.109 O3 [ -x+3/2, y-1/2, -z+3/2 ] O7-H7C 0.850 2.652 146.57 3.394 O9 [ -x+3/2, y-1/2, -z+3/2 ] O8-H8B 0.850 2.014 170.77 2.856 O9 [ x, -y+1, z-1/2 ] O8-H8C 0.850 2.095 167.37 2.930 O9 [ -x+1, y-1, -z+3/2 ] O9-H9C 0.850 2.162 163.60 2.987 O4 [ -x+1, y, -z+3/2 ] O9-H9C 0.850 2.803 152.14 3.577 S1 [ -x+1, y, -z+3/2 ] O9-H9D 0.850 2.167 163.76 2.993 O6 [ x, y+1, z ] O9-H9D 0.850 2.458 115.81 2.930 O8 [ -x+1, y+1, -z+3/2 ] O9-H9D 0.850 2.924 135.75 3.582 S2 [ x, y+1, z ]

Among the structure of [Ce(SO 4HO]·HO, rich hydrogen bonding interactions are formed betweenhe coordination water molecules, [SO^2- ligands and lattice water molecules, as listed in Table 2, which linked the adjacent 2-D inorganic layers into 3-D frameworks, as illustrated in Fig. 3.

Fig. 3 A packing crystal structure of (1) viewed along axis, including the rich hydrogen bonding interactions.

2.3 The crystal structure of compound (2)       
The crystal structure of 2,2′-bipyridine-4,4′-dicarboxylic acid, as shown in Fig. 4, in which the two pyridyl rings in the same 2,2′-bipyridine-4,4′-dicarboxylic acid are not in the same plane. The 2-D framework of the title compound built from 2,2′-bipyridine-4,4′-dicarboxylic acid molecules linked by O1—H…N1 hydrogen bond, as listed in Table 3. The reported crystal structure crystilized in monoclinic with space group of Cc. Our present work under room temperature (293K) is quite different from the previously work^[5]. There is a very real possibility that the crystal undergoes some sort of phase transition on cooling to 173K that lowers the symmetry from orthorhombic Fdd2 to monoclinic Cc.

Fig. 4The asymmetric unit of (2)

Table 3 Hydrogen bonds (nm and ) in the crystal structure of compound (2)

D-H (D-H) (H..A) <DHA (D..A) A ( Symmetry transformation)

O1-H1B 0.82 1.85 166 2.652(4) N1 [ x, y+1/2, z-1/2 ]

2.4The thermal properties ofcompound (1)
In order to study the thermal stability of the title compound, the thermaogravimetric analysis was carried out in nitrogen. The result showed (Fig. 4) that from room temperature to ca. 150 C, 2.80% weight loss is approximately the weight of one lattice water molecule (cal. 2.73%). From 150 C to 350 C, 11.5% weight loss corresponds to the loss of four coordination water molecules (cal. 10.9%). Above 650 C, the material lost weight up to 11.9 % equivalent to the loss of SO (cal. 12.1%). The residual of the thermal treatment is CeO(SO, and the existence of SO unit can be proved by the character peak at 1130 cm^-1

Fig. 5 TG curve of the compound (1)