Flexible and Asymmetric Ligand in Constructing Coordinated Complexes: Synthesis, Crystal Structures and Fluorescent Characterization

Flexible and asymmetric ligand L [L = 1-((pyridin-3-yl)methyl)-1H-benzotriazole], is used as a basic backbone to construct complicated metal-organic frameworks. Two new polymers, namely, [Ag2(L)2(NO3)2]n (1) and [Ag(L)(ClO4)]n (2), were synthesized and characterized by X-ray structure analysis and fluorescent spectroscopy. The complex 1 gives an “S” type double helical conformation, whereas complex 2 exhibits a 1D zigzag configuration. Different anions affect the silver coordination geometry and crystal packing topology.


Introduction
In the research of supramolecular chemistry, great interest has recently focused on crystal engineering of coordination frameworks due to their intriguing architectures, new topologies, intertwining phenomena and potential applications in microelectronics, nonlinear optics, ion exchange, molecular selection, molecular separation and recognition [1][2][3][4][5][6][7][8]. The structural motifs of coordination polymers rest on several factors, such as the central atom, the performance of the ligands, the coordinated and/or non-coordinated counter ions, the solvent systems and the reaction conditions. The choice of appropriate ligands is no doubt the key factor because it has an obvious influence on the OPEN ACCESS topologies of the coordination polymers and behavior of the molecules. Some flexible bidentate ligands, which can adopt various conformations, have be widely used to construct helixes [9][10][11][12]. So far, a number of metal complexes utilizing flexible bis (triazole), bis (benzotriazole) or dipyridyl ligands have been reported [13][14][15][16], but the symmetry of these ligands has greatly limited the novelty and variety of the configuration.
Recently, by a radical nucleophilic substitution, we obtained the hybrid heterocyclic ligand L (L = 1-((pyridin-3-yl)methyl)-1H-benzotriazole), which is a versatile N-donor in transition metal chemistry (Figure 1). From a structural point of view, it should be pointed out that (a) as a kind of angular ditopic ligand, the two exo-N atoms of its two different rings can μ 2 -bridge to two different metal atoms; (b) though the CH 2 spacer's torsion angle can be twisted only within a limited range due to the sp 3 configuration of C atom, its pyridine and benzotriazole rings bear a variable dihedral angle to meet the requirement of coordination geometries of metal ions as well as minimize steric interactions in the assembly process; (c) due to its unsymmetrical nature, L satisfies the fundamental requirements for the construction of acentric solids and NLO (non-linear optical) materials. Furthermore, we speculate that the non-coordinated counter anion's ligands may affect the polymers' structure and properties. In this paper, we have studied the interaction of L with silver nitrate and perchlorate. From our attempts, two new polymers, namely [Ag 2 (L) 2 (NO 3 ) 2 ] n (1) and [Ag (L)(ClO 4 )] n (2), were obtained as crystals suitable for single-crystal X-ray analysis.

Structure Description
The asymmetry unit of 1 consists of two independent Ag(I) cations, two L ligands, one integrated nitrate and two counteranion nitrate halves, as shown in Figure 2. The environment of every Ag center is the same tetrahedron geometry. Each Ag(1) center is coordinated by two L ligands via the In each L molecule, the sp 3 configuration of C of -CH 2 -spacer forces the L ligand to be non-linear, with the N(1)-C(7)-C(8) angle 110.2(4)° and the N(5)-C(19)-C(20) angle 111.3(5)°. Each L behaves as an exo-bidentate linker, bridging adjacent Ag(I) cations to give rise to an [-Ag(L)-] n infinite 2 1 helix strand, and such that each strand is interwind compactly by another [-Ag(L)-] n strand through the aurophilic d 10 -d 10 interactions (Ag· · · Ag distance 3.2683 Å) to form a double helical species. Unlike the standard tubular double helixes that have been reported [17][18][19], the double helical chain of 1 wriggles with an -S‖ configuration, as shown in Figure 3. This result can be attributed to the 3-position of the pyridyl ring as the metal coordination site, which limits the stretched-out direction. Each chiral double helical chain links to its adjacent symmetric-related equivalents via the weak attraction of nitrate anion, to generate a quasi 3D supramolecular architecture. In each monocrystal, the balanced packing of left (P) and right (M) chains counteracts the chirality.
A drawing of the asymmetric unit of 2 is shown in Figure 4, with selected bond distances and angles listed in Table 1. The crystal structure reveals that the Ag(I) center is coordinated with two N atoms from two different L, with the Ag-N distances 2.135 (3)   In 2, each L group also behaves as an exo-bidentate linker, bridging adjacent Ag(I) cations to form an infinite -Z‖ configuration alone the [100] direction ( Figure 5). Furthermore, each zigzag chain links to its adjacent symmetric-related equivalents via the duple π-π interactions with centroid to centroid distances 3.586(1) Å to generate a dimer. Other parameters of the π-π interactions are listed in Table 1. It is necessary to point out that these dual π-π interactions were not frequently observed in previously reported architectures [20,21].    5. The infinite -Z‖ configuration of 2 and the dual π-π interactions between the dimers. Symmetry code: A: x + 1, y + 2, z + 2; A': −x + 3, −y + 3, −z + 1; B: x + 1, y + 1, z + 2; B': −x + 3, −y + 2, −z + 1; C: x, y, z + 3; C': −x + 2, −y + 1, −z + 2.

Fluorescent Properties
Metal-organic polymer compounds with a d 10 configuration have been found to exhibit photoluminescent properties [22][23][24]. Here, we wanted to examine the photoluminescence of 1 and 2. The solid-state emission spectra of L and complexes 1 and 2 at room temperature are shown in Figure 6. It can be observed that the intense emissions occurr in the same range for the two complexes (λ ex = 350 nm, λ em = 417.5 nm for 1; λ ex = 340 nm, λ em = 419.5 nm for 2), which show a very light red-shift to that observed from L (λ ex = 340 nm, λ em = 402 nm). In d 10 -metal ions with one or two positive charges, the d-orbitals are contracted and therefore the electrons in these orbitals are much less accessible for back bonding to p-acceptor ligands. Moreover, silver cations have weak electro-accepting nature with respect to electrons from L, so these emissions can be assigned to ligand-to-metal charge transfer (LMCT) bands [25].

Materials and Methods
Benzotriazole was purchased from Acros Ltd. Company and used without further purification, the other reagents were commercially available and used as purchased. The IR spectra as KBr discs were recorded on a Magna 750 FT-IR spectrophotometer. C, H, and N analysis were determined on an Elementary Vario ELIII elemental analyzer. Fluorescent spectra were measured on an Edinburgh Instruments analyzer model FL920.

Synthesis of the Ligand
The desired L was prepared by condensation of 1H-benzotriazole with the 3-picolyl chloride in DMF at reflux 4 h in the presence of triethylamine as basic catalyst [26]. Separation of pure L was performed by chromatography on a silica gel column (eluent ethyl acetate-light petroleum 40:60).

Synthesis of [Ag(L)(ClO 4 )] n (2)
The procedure of 2 is similar to the synthesis of 1 except that AgClO 4 was used instead of AgNO 3 . Yield: 0.015 g (48% based on L); Elementary analysis: calcd. for AgClC 12

X-ray Crystallography
The single crystal X-ray diffraction measurements were carried out on a Siemens Smart CCD area detector. Intensities of reflections were measured using graphite monochromatized Mo-K radiation ( = 0.71073 Å) with ω scan mode at 293(2) K in the range of 2.11 < θ < 27.48 for 1 and 2.42 < θ < 27.48 for 2. Unit cell dimensions were obtained with least-squares refinements, and semi-empirical absorption corrections were applied using SADABS program [27]. The structure was solved by direct method [28] and non-hydrogen atoms were obtained in successive difference Fourier syntheses. The final refinements were performed by full-matrix least-squares methods on F 2 by SHELXL-97 program package [29]. For both 1 and 2, hydrogen atoms were generated geometrically and treated as riding. The crystallographic data for 1-2 are summarized in Table 2, and the selected bond distances and angles are listed in Table 3.

Conclusion
For L, the advantages of flexibility and exo-bidentate are fully demonstrated by the structural data. Two Ag(I) coordination polymers both possess 1d catenulate structure. It is even more surprising that different anions would lead to the complete diversity of the ultimate coordinated compounds' geometrical structures. However, to our disappointment, we failed to achieve some acentric solids or NLO materials as predicted. Some deeper reconstruction at the -CH 2 -spacer of the ligand is now being processed for achieving the chirality, and more tests with other metal cations (such as Pt 2+ and Pd 2+ ), counter anionic donors (such as p-toluenesulfonate and hexafluoro-phosphate), solvent systems and reaction conditions are in progress. It can be expected that many other novel asymmetry metal-organic materials will be realized.