WANG Si-Si CUI Zhi-Hua WANG Xiu-Guang GAO Dong-Zhao SUN Ya-Qiu ZHANG Guo-Ying XU Yan-Yan

(Key Laboratory of Inorganic-organic Hybrid Functional Material Chemistry (Tianjin Normal University), Ministry of Education; Tianjin Key Laboratory of Structure and Performance for Functional Molecule, College of Chemistry, Tianjin Normal University, Tianjin 300387, China)

1 INTRODUCTION

The design and construction of coordination polymers during the past decades have attracted much attention in the field of supramolecular chemistry and crystal engineering because of its intriguing structural topologies and various potential applications as catalysis, sensors, functional solid materials,host-guest chemistry, ion exchange and molecular recognition[1-3]. The structural topology of coordination polymers can be deliberately designed by the elaborate selection of coordination geometry such as the metals, the structure of spacer ligands, the counteranions and the reaction conditions. In this regard,much progress has been made on the design and synthesis of novel coordination frameworks and the relationships between their structures and properties[4-6]. Meantime, crystallization of flexible ligands with metal ions offers an exciting approach to the discovery of new coordination networks than the rigid ligands because the flexible ligands can adopt different conformations which are governed by the restraints imposed in the crystal lattice. As the 4,4?-bipyridine analogue, some flexible bipyridyl ligands including 1,2-bis(4-pyridyl)ethane (bpe) and 1,3-bis(4-pyridyl)propane (bpp) have been utilized for the construction of polymeric compounds because these ligands are convenient linkers for constructing MOFs[7–11]. As another flexible bipyridyl ligand, N,N-bis(3-pyridylmethyl)amine (bpma,Scheme 1) has similar conformations with the bpp ligand when the imine group of the bpma ligand is protonated. In our previous work, three complexes linked by bpma ligands have been reported and the TG and TT conformations have been found[12]. In this paper, we report the syntheses and crystal structures of two new cadmium(II) compounds linked by bpma ligands showing the TT conformation in complex 1 and the GG conformation in complex 2.

Scheme 1. bpma ligand

2 EXPERIMENTAL

2.1 Materials and instruments

All chemicals were of reagent grade and used without purification. Elemental analyses for carbon,hydrogen and nitrogen were carried out on a Leeman-Labs CE-440 elemental analyzer. The infrared spectra were taken on a Nicolet Avatar 370 FT-IR spectrometer in the range of 4000–400 cm–1by KBr pellet technique.

2.2 Synthesis of the complexes

The flexible bipyridyl ligand bpma was prepared using the same method with N,N-bis(2-pyridylmethyl)amine[13].

CdCl2·2.5H2O (0.1 mmol) and neutralized bpma ligand (0.1 mmol) were dissolved in 10 mL distilled water with constant stirring for 2 h. After the filtrate was allowed to stand at room temperature for 2 weeks, colorless crystals for complex 1 were grown by the slow evaporation suitable for X-ray structure analysis in a yield of 51% based on CdCl2·2.5H2O.Anal. Calcd. (%) for C12H20Cd2Cl5N3O3: C, 21.94; H,3.05; N, 6.40. Found (%): C, 21.76; H, 3.20; N, 6.35.IR (KBr disc, cm–1): 3421(br), 1606(m), 1437(s),1033(m), 813(m) and 705(m).

CdSO4·6H2O (0.1 mmol) and neutralized bpma ligand (0.1 mmol) were dissolved in 10 mL distilled water with constant stirring for 2 h. After the filtrate was allowed to stand at room temperature for 2 weeks, colorless crystals for complex 2 were grown by slow evaporation suitable for X-ray structure analysis in a yield of 39% based on CdSO4·6H2O.Anal. Calcd. (%) for C24H42Cd2Cl2N6O15S2: C, 28.39;H, 4.14; N, 8.28. Found (%): C, 28.43; H, 4.28; N,8.40. IR (KBr disc, cm–1): 3369(br), 1604(m),1441(s), 1094(m), 803(m) and 705(m).

2.3 Crystal structure determination

Two colorless block single crystals with dimensions of 0.18mm × 0.17mm × 0.13mm for complex 1 and 0.12mm × 0.11mm × 0.10mm for complex 2 were mounted on glass fibers, respectively. X-ray diffraction intensity data were collected on a Bruker APEX-II CCD diffractometer equipped with a graphite-monochromatic MoKα radiation (λ =0.71073 ?) using the φ-ω scan mode (2.10≤θ≤25.01o for 1 and 1.38≤θ≤25.01o for 2) at 173(2) K for 1 and 150(2) K for 2. For 1, a total of 5190 reflections were collected and 3558 were independent (Rint= 0.0262), of which 3443 were observed with I ＞ 2σ(I); for 2, out of the 9194 total reflections,6267 were independent (Rint= 0.0111) including 6026 observed ones with I ＞ 2σ(I). The empirical adsorption corrections by SADABS were carried out.The structures were solved by direct methods using SHELXS-97 program[14]and refined with SHELXL-97[15]by full-matrix least-squares techniques on F2.All non-hydrogen atoms were refined anisotropically,while the hydrogen atoms were located geometrically and refined isotropically. For complex 1, the final R = 0.0319, wR = 0.0822 (w = 1/[σ2(Fo2) +(0.0393P)2+ 2.2073P], where P = (Fo2+ 2Fo2)/3), S= 1.059, (Δ/σ)max= 0.002, (Δρ)max= 1.734 and(Δρ)min= –1.324 e/?3; for 2, the final R = 0.0178,wR = 0.0451 (w = 1/[σ2(Fo2) + (0.0208P)2+1.4899P], where P = (Fo2+ 2Fc2)/3), S = 1.050,(Δ/σ)max= 0.002, (Δρ)max= 0.423 and (Δρ)min=–0.466 e/?3. Selected bond lengths and bond angles are listed in Tables 1 and 2, respectively.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for Complex 1

Table 2. Selected Bond Lengths (?) and Bond Angles (°) for Complex 2

3 RESULTS AND DISCUSSION

3.1 Crystal structure of complex 1

Complex {[Cd2(Hbpma)Cl5(H2O)]·2H2O}n1 has a one-dimensional double chain structure comprised of tetranuclear Cd(II) clusters bridged by the protonated bpma ligands and solvated water mole- cules.In the tetranuclear Cd(II) cluster, there are two symmetric units and independent unit contains two Cd(II) ions (Cd(1), Cd(2)), five chlorine anions, one Hbpma ligand and one coordinated water. As depicted in Fig. 1, two Cd(II) ions are both six-coor-dinated with the distorted octahedral geometry. Cd(1)is coordinated by two chlorine anions (Cl(3), Cl(2B)(B: –x, –y+3, –z+1)) in the axial sites, while the equatorial positionsare occupied by one pyridine nitrogen atom (N(3A) (A: x–1, y+1, z)) from a bpma ligand and three chlorine anions (Cl(1), Cl(2) and Cl(4B)). Cd(2) is coordinated by one oxygen atom(O(1)) of a water molecule and a chlorine anion(Cl(4)) in the axial sites, while the equatorial positionsare occupied by one pyridine nitrogen atom(N(1)) from the other one bpma ligand and three chlorine anions (Cl(1), Cl(2) and Cl(5)). The bond lengths of Cd(1)–N(3A), Cd(2)–N(1) and Cd(2)–O(1)fall in the range of 2.3240(18)–2.3593(19) ?,whereas the Cd–Cl bond distances vary from 2.5128(10) to 2.8731(9) ?. Meantime, Cd(1), Cd(2)and their symmetric equivalents (Cd(1B), Cd(2B))are connected by chlorine anions leading to the tetranuclear Cd(II) cluster, in which the distances of Cd(1)–Cd(2) and Cd(1)–Cd(2B) are 3.9253(5) and 4.1571(5) ?, respectively. Further-more, each bpma ligand bridges two tetranuclear Cd(II) clusters to form a one-dimensional double chain as shown in Fig. 2, in which the separation of two Cd(II) ions by bpma ligand is 12.5900(7) ?. It is noting that the imine groups of bpma ligands are all protonated and only TT conformation is found in this complex due to the thermodynamical stability. In complex 1, there exist abundant intermolecular hydrogen bonding interactions between the imine groups of bpma ligands, chlorine anions and water molecules. The hydrogen bond lengths and bond angles are listed in Table 3. Thus a three-dimensional network is formed through the hydrogen bonding interactions and the packing arrangement is shown in Fig. 3.

Table 3. Hydrogen Bond Lengths (?) and Bond Angles (°) for Complex 1

Fig. 2. View of the one-dimensional double chain structure.H atoms and solvated water molecules were omitted for clarity

Fig. 3. A perspective view of the three-dimensional crystal packing in complex 1 along the a direction. Dotted lines indicate hydrogen bonding interactions

3.2 Crystal structure of complex 2

Complex [Cd(Hbpma)Cl(H2O)2]2·2SO4·3H2O 2 contains two independent binuclear cadmium structures, free sulfate anions and solvated water molecules. In the centrosymmetric binuclear structure,two Cd(II) ions are bridged by both Hbpma ligands and two chlorine anions as shown in Fig. 4. In the coordination environment, two unique Cd(II) ions(Cd(1), Cd(2)) are both six-coordinated in the distorted octahedral geometry with only different bond lengths and bond angles. The equatorial positions of the Cd(II) ion are coordinated by two chlorine anions and two water molecules, while the axial sites are occupied by two pyridine nitrogen atoms from two bpma ligands. The Cd–N and Cd–O bond lengths fall in the range of 2.2934(13)– 2.3319(16)? and the Cd–Cl distances vary from 2.5957(5) to 2.6344(5) ?, respectively. Meantime, two Cd(II)ions are connected by both Hbpma ligands and two chlorine anions leading to the binuclear structure, in which the distances of Cd(1)– Cd(1A) (A: –x+2, –y,–z) and Cd(2)–Cd(2B) (B: –x, –y+1, –z+1) are 3.6804(1) and 3.6560(1) ?, respec- tively. It is noting that the imine groups of bpma ligands are all protonated and only GG conformation is found in this complex. However, the separations of two metal ions by bpma ligands with the GG conformation are much less than the ones with the TT and TG conformations[12]. In complex 2, the binuclear structures are linked through π-π stacking interactions between the adjacent pyridyl rings of bpma ligands leading to a one-dimensional framework as shown in Fig. 5, in which the distances between two parallel pyridyl rings are 3.8077(12) and 3.5532(12) ?. In addition, a three-dimensional network is also formed through the hydrogen bonding interactions between the imine groups of bpma ligands and water molecules and the packing arrangement is shown in Fig.6. The hydrogen bond lengths and bond angles are listed in Table 4.

Table 4. Hydrogen Bond Lengths (?) and Bond Angles (°) for Complex 2

Fig. 4. Coordination environment of Cd(II) ions in complex 2 showing the atom labeling scheme(Symmetry codes: A: –x+2, –y, –z, B: –x, –y+1, –z+1). H atoms, free sulfate anions and solvated water molecules were omitted for clarity

Fig. 5. One-dimensional framework of complex 2 based on π-π stacking interactions (indicated by dashed lines). H atoms, free sulfate anions and solvated water molecules have been omitted for clarity

Fig. 6. A perspective view of the crystal packing in complex 2 along the a direction.Dotted lines indicate hydrogen bonding interactions

(1) Ockwig, N. W.; Delgado-Friederichs, O.; O’Keeffee, M.; Yaghi, O. M. Reticular chemistry: occurrence and taxonomy of nets and grammar for the design of frameworks. Acc. Chem. Res. 2005, 38, 176–182.

(2) Ye, Q.; Wang, X. S.; Zhao, H.; Xiong, R. G. Highly stable olefin-Cu(I) coordination oligomers and polymers. Chem. Soc. Rev. 2005, 34, 208–225.

(3) Sun, D. F.; Ma, S. Q.; Ke, Y. X.; Collins, D. J.; Zhou, H. C. An interweaving MOF with high hydrogen uptake.J. Am. Chem. Soc. 2006, 128, 3896–3897.

(4) Batten, S. R.; Robson, R. Interpenetrating nets: ordered, periodic entanglement. Angew. Chem. Int. Ed. 1998, 37, 1460–1494.

(5) Wen, L. L.; Dang, D. B.; Duan, C. Y.; Li, Y. Z.; Tian, Z. F.; Meng, Q. J. 1D helix, 2D brick-wall and herringbone, and 3D interpenetration d10metal-organic framework structures assembled from pyridine-2,6-dicarboxylic acid. Inorg. Chem. 2005, 44, 7161–7170.

(6) Wang, R. H.; Han, L.; Jiang, F. L.; Zhou, Y. F.; Yuan, D. Q.; Hong, M. C. Three novel cadmium(II) complexes from different conformational 1,1?-biphenyl-3,3?-dicarboxylate. Cryst. Growth Des. 2005, 5, 129–135.

(7) Zang, S. Q.; Su, Y.; Li, Y. Z.; Lin, J. G.; Duan, X. Y.; Meng, Q. J.; Gao, S. Four 2D metal-organic networks incorporating Cd-cluster SUBs:hydrothermal synthesis, structures and photoluminescent properties. CrystEngComm. 2009, 11, 122–129.

(8) Feng, X.; Wang, L. Y.; Shi, X. G.; Zhao, J. S.; Sun, Q. A dicyanamido-bridged zinc(II) complex based on the bis(4-pyridyl)ethane with a three-dimensional framework. Chin. J. Struct. Chem. 2010, 29, 1567–1573.

(9) Wang, G. H.; Li, Z. G.; Jia, H. Q.; Hu, N. H.; Xu, J. W. Topological diversity of coordination polymers containing the rigid terephthalate and a flexible N,N?-type ligand: interpenetration, polyrotaxane and polythreading. Cryst. Growth Des. 2008, 8, 1932–1939.

(10) Han, L.; Zhou, Y.; Zhao, W. N.; Li, X.; Liang, Y. X. Assembly of metal-organic frameworks with helical layer: from 2D parallel interpenetrated layer to 3D self-penetrating network. Cryst. Growth Des. 2009, 9, 660–662.

(11) Fan, L. Q.; Wu, J. H.; Lin, J. M.; Huang, Y. F. Synthesis, crystal structure and characterization of a new coordination polymer from the self-assembly of CoCl2salt and flexible ligand 1,3-bis(4-pyridyl)propane. Chin. J. Struct. Chem. 2010, 29, 1489–1494.

(12) Cui, Z. H.; Wang, S. S.; Wang, X. G.; Gao, D. Z.; Sun, Y. Q.; Zhang, G. Y.; Xu, Y. Y. Syntheses, crystal structures and magnetic properties of three transition metal complexes based on a flexible dipyridyl ligand and oxalate. Z. Anorg. Allg. Chem. 2012, 638, 669–674.

(13) Larsen, S.; Michelsen, K.; Pedersen, E. A new dimeric Cr(IlI) complex with one bidentate sulfate and two hydroxide ions.Acta Chem. Scand. A 1986, 40, 63–76.

(14) Sheldrick, G. M. SHELXS-97∶ Program for the Solution of Crystal Structures. University of G?ttingen, Germany 1997.

(15) Sheldrick, G. M. SHELXL-97∶ Program for the Refinement of Crystal Structures. University of G?ttingen, Germany 1997.