Syntheses and Crystal Structures of Two New Ag(I)Complexes with Betaine Derivative: Structure Changes via Metal-to-ligand Ratio①

HUANG Yong-Qing WAN Yi CHENG Hi-Di ZHAO Yue

a (College of Chemical and Environmental Engineering,Shandong University of Science and Technology, Qingdao 266590, China)

b (Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry,School of Chemistry and Chemical Engineering, Nanjing National Laboratory of Microstructures, Nanjing University, Nanjing 210093, China)

1 INTRODUCTION

Recent advances in coordination polymers have led to the synthesis of various one- (1D), two- (2D)and three-dimensional (3D) coordination frameworks, which have a wide range of potential applications, including gas adsorption, fluorescence,chemical sensing, heterogeneous catalysis, and so on[1-6]. However, the rational design and synthesis of coordination frameworks with specific structures and properties still remain a long-term challenge because many synthetic conditions, for instance temperature, solvent, counter ion, pH value of the solution and metal-to-ligand ratio, etc. can influence the final structure[7-12]. Therefore, a great number of efforts are still required for us to understand the self-assembly of coordination polymers, although a lot of related work, including our own, has been reported in the past decades[13-15].

On the other hand, carboxylate ligands as the linkers have been proved to be efficient building block due to their various coordination modes and good coordination capacities[16,17], which provide a high likelihood for the construction of different dimensional coordination polymers. Similarly, as the metal nodes, the silver(I) centre can supply different coordination numbers ranging from 2 to 6 and geometry from linear to octahedron, leading to the wide applications in the creation of varied structures[18-20]. Additionally, the potential Ag··Ag interaction can also affect the structures of the resulting coordination complexes[21]. Based on the above reason, we select an inner-salt-type betaine derivative 1-carboxymethylpyridium-3-carboxylate(L, Scheme 1) as the linker to react with silver nitrate, investigating the influence of metal-toligand ratio on the final structure. Herein, we report the syntheses and crystal structures of two layered silver(I) complexes {Ag2(L)(NO3)(H2O)}n(1) and{Ag(L)}n·nH2O (2), as shown in Scheme 2.

Scheme 1. Coordination modes of ligand L in complexes 1 (left) and 2 (right) (M = Ag)

Scheme 2. Synthesis of silver(I) complexes 1 and 2 with L ligand and AgNO3

2 EXPERIMENTAL

2. 1 Materials and measurements

All commercially available chemicals were of reagent grade and used as received without further purification. The ligand L was synthesized according to the method reported previously[22]. Elemental analyses of C, H and N were determined on a Perkin-Elmer 240C Elemental Analyzer. Infrared spectra were measured on a Nicolet 380 FT-IR spectrometer in the region of 400～4000 cm-1using KBr pellets.

2. 2 Synthesis of {Ag2(L)(NO3)(H2O)}n (1)

A mixture of AgNO3(67.9 mg, 0.4 mmol) and L(22.02 mg, 0.1 mmol) in 3 mL water was stirred for 10 min at room temperature and then 1 mL CH3CN was added dropwise. The resulting filtrate was left undisturbed in a dark place at room temperature.After two weeks, the colorless needle-like crystals were obtained in 47% yield. Anal. Calcd. (%) for C8H8Ag2N2O8: C, 20.19; H, 1.69; N, 5.89. Found(%): C, 20.17; H, 1.72; N, 5.82. IR (KBr, cm-1):3074 (w, v Ar–H), 3026 (w), 2947(w), 1660 (vs,vasymCO2), 1615 (vs, vasymCO2), 1582 (m), 1492 (m,vsymCO2), 1380 (vs), 1217 (m), 1204 (s), 949 (w),768 (w), 716 (m).

2. 3 Synthesis of {Ag(L)}n·nH2O (2)

The synthesis procedure of 2 is similar to that for 1, except that the amount of AgNO3was cut in half.Yield: 34%. Anal. Calcd. (%) for C8H8AgNO5: C,31.40; H, 2.63; N, 4.58. Found (%): C, 31.29; H,2.76; N, 4.53. IR (KBr, cm-1): 3077 (w, v Ar–H),1660 (s, vasymCO2), 1613 (v s, vasymCO2), 1558 (m),1494 (w, vsymCO2), 1374 (vs), 1217 (m), 1112 (s),1085 (s), 769 (w), 718 (m), 625 (m).

2. 4 Crystal structure determination

Suitable single crystals with approximate dimensions of 0.17mm × 0.15mm × 0.14mm for 1 and 0.69mm × 0.27mm × 0.22mm for 2 were used for X-ray diffraction analyses. The X-ray diffraction measurements for complexes 1 and 2 were performed on the Bruker Smart Apex CCD diffractometer and Rigaku R-axis Spider IP diffractometer,respectively, with graphite-monochromatized Mo-Kα radiation (λ = 0.71073 ?) at 296 and 293 K,respectively. All the structures were solved by direct methods using the SHELXS-97 program and refined by full-matrix least-squares techniques on F2with the SHELXL-97 program[23,24]. Anisotropic thermal parameters were applied to all the nonhydrogen atoms. The water H atoms were located in a difference Fourier map. The other hydrogen atoms were inserted at the idealized positions and refined using a riding model isotropically. All calculations were carried out using the SHELXTL crystallographic software package[25].

For complex 1, a total of 5377 reflections were collected and 2036 were independent (Rint= 0.0273).The final R = 0.0320 and wR = 0.0831 for 2036 observed reflections with I ＞ 2σ(I) and R = 0.0329 and wR = 0.0838 for all data. As for 2, a total of 8012 reflections were collected and 2011 were independent (Rint= 0.0492). The final R = 0.0283 and wR = 0.0684 for 2011 observed reflections with I ＞ 2σ(I) and R = 0.0351 and wR = 0.0751 for all data. Selected bond lengths and bond angles of the two title complexes are listed in Table 1, and the relevant data for hydrogen bonding interactions are given in Table 2.

Table 1. Selected Bond Lengths (?) and Bond Angles (°) for 1 and 2

Table 2. Hydrogen Bond Lengths (?) and Bond Angles (°) for 1 and 2

3 RESULTS AND DISCUSSION

Crystallization of L with AgNO3with metal-toligand ratio 4:1 afforded the infinite 2D undulated network based on dinuclear Ag(I) units in 47%yield. X-ray crystallographic analysis revealed that 1 crystallizes in space group P21/c. The asymmetric unit contains two Ag(I) ions, one L ligand, one nitrate anion and one coordinated water molecule.The ORTEP view of the local coordination environment around the Ag(I) centers is shown in Fig. 1. Obviously, the two Ag(I) ions have different coordination environments and geometries. Each Ag(1) ion adopting a triangular coordination geometry is surrounded by three carboxylic oxygen atoms from three different L ligands with the bond angles around Ag(1) falling in the range of 81.42(11)～173.69(12)° (Table 1) and lies in the plane defined by O(3), O(1A) and O(4B) with deviation of 0.1113 ?, while the Ag(2) center is coordinated with two L ligands via carboxylic oxygen atoms (Ag(2)–O(4) = 2.316(3) ? and Ag(2)–O(2A) = 2.363(3) ?), one water molecule(Ag(2)–O(5) = 2.498(4) ?) and one nitrate anion(Ag(2)–O(6) = 2.557(4) ?) to furnish a distorted tetrahedral coordination geometry.

Fig. 1. Coordination environment of Ag(I) ions in complex 1 with atomic numbering scheme.The hydrogen atoms were omitted for clarity

It is noteworthy that the Ag(1)(I) and Ag(2)(I)ions are bis(carboxylate-O,O’)-bridged to form a dinuclear [Ag2(O2CR)2] unit with the Ag··Ag distance of 2.9790(8) ?, which is less than the van der Waals radii of two silver atoms (3.44 ?),implying the presence of Ag··Ag interactions[26].Such [Ag2(O2CR)2] units are linked by L ligand with head-to-tail arrangement to generate 1D helical chains running parallel to the b direction, which are further extended into a chiral undulated 2D network along the a axis via the linkage of each silver(I)center to a carboxylic group of the adjacent dimer,generating rhombic [Ag2O2] units with the Ag··Ag separation of 3.7931(8) ?, as found in the previously reported complex [{Ag2(L1)2(NO3)(H2O)}n](NO3)n(L1= 4-dimethylaminopyridinioacetate), as shown in Fig. 2[27].

On the other hand, each L ligand acts as a μ5-bridge connecting five Ag(I) ions, in which two carboxylate groups take the μ3-η1:η2and μ2-η1:η1bridging modes, respectively. As depicted in Fig. 2,the pyridine ring plane of L ligands of different helical chains is parallel to each other along the a axis with the shortest centroid-to-centroid distance of 5.0809(14) ?, beyond the scope of π-π interaction[28]. The adjoining chiral 2D layers are further stacked via Ag··O weak interaction with the separation of 2.8123(6) ?, as well as O–H··O hydrogen bonding (Table 2) to form the 3D structure in an ··ABAB·· repeating mode (Fig. 3),which implies the chirality of the neighboring layers is opposite. In other words, the layers with opposite chirality are stacked up alternately. Thus, complex 1 crystallizes as intracrystal racemate, which can be confirmed by the achiral space group P21/c. In addition, the coordinated nitrate anions and water molecules locate in the voids between the 2D layers.

By analyzing the crystal data, we notice that in complex 1 one nitrate anion and one water molecule occupy two coordination sites of the Ag(2)(I) ion via relatively weak coordination interaction, which may be replaced by L ligand via relatively strong Ag–Ocarboxylatecoordination interaction by increasing the ligand-to-metal ratio. Based on the above reason,the same reaction, except for the metal-to-ligand of 2:1, was carried out under the same reaction conditions, investigating the influence of metal-toligand ratio on the structure of complexes. Fortuna-tely, a new complex 2 was obtained. X-ray crystallographic analysis revealed that 2 also crystallizes in space group P21/c. The asymmetric unit contains only one Ag(I) ion, one L ligand and one lattice water molecule. A view of the local coordination geometry around the Ag(I) ion is depicted in Fig. 4.The Ag(I) ion is coordinated by four O atoms from four different L ligands, generating a distorted

Fig. 2. 2D chiral undulated network of 1 along the ab plane. The hydrogen atoms, water molecules and nitrate anions were omitted for clarity.Color code: Ag(I) purple, C black, N bule and O red

Fig. 4. Coordination environment of Ag(I) ions in 2 with 30% probability ellipsoids. Hydrogen atoms and water molecules have been omitted for clarity

In contrast to the μ5-bridging mode of ligand L in 1, the completely deprotonated L ligand in 2 takes a μ4-(η1:η2):(η0:η1) coordination mode to connect four Ag(I) ions with the same L-shaped conformation along the c axis, resulting in the formation of a 2D structure (Fig. 6). It is intriguing that two Ag(I) ions and two L ligands form an 18-membered M2L2cycle, which are alternately arranged on both sides of the 1D ··Ag–Ag·· chain with the dihedral angle of 103.7°. In another word, the M2L2cycles are first linked into 1D tube by 1D ··Ag–Ag·· chain, which is further assembled into a 2D tetrahedral geometry. All Ag–O bond lengths are in the normal range (Table 1)[29,30]. Obviously, the weakly coordinated nitrate anion and water molecule have been replaced. As observed in 1, the ligand-supported Ag··Ag interactions also exist in 2 with the Ag··Ag distance of 3.1123(5) ?, which assembles the adjacent Ag(I) ions into the 1D zigzag chain along the b axis, as shown in Fig. 5.structure in the ··ABAB·· repeating fashion. The 2D structures are further packed into a 3D structure(Fig. 7), in which the lattice water molecules are stabilized via the O–H··O hydrogen bonds (Table 2).

Fig. 3. Stacking diagram of 1 along the a axis with weak Ag··O interactions indicated by dotted lines,showing the opposite chirality of neighbouring layers. Color code: Ag(I) purple, C black, N bule and O red

Fig. 5. 1D zigzag ··Ag–Ag·· chain along the b axis. Color code: Ag(I) purple

4 CONCLUSION

A systematic study of the influence of metal-toligand ratio on the silver(I) complexes with 1-carboxymethylpyridinium-3-carboxylate inner salt (L)has been carried out and two new Ag(I) complexes 1 and 2 with unique 2D structures were obtained,which crystallize in the same space group P21/c.The results reveal the new structure can be obtained through the replacement of weakly coordinated solvent molecules or counter anions by the bridging ligand, which can be implemented via the adjustment of metal-to-ligand ratio. We anticipate this strategy is helpful for the synthesis of new coordination frameworks with specific structures and properties.

Fig. 6. View of the 2D layer structure of 2 consisting of 1D tubes (left) and schematic representation of the 2D layer, in which ligands L are simplified as ‘L’ configuration (right). Color code: Ag(I) purple, C black, N bule and O red

Fig. 7. View of the 3D packing diagram of 2. The dashed lines represent the hydrogen bonds.Color code: Ag(I) purple, C black, N bule and O red

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