Two Zinc(II) Complexes with 1D Chain and 2D Layer Directed by Competitive Coordination of the Mixed Ligands: Syntheses, Crystal Structures, and Fluorescent Properties

Two new competitive coordination-directed zinc(II) complexes, [Zn2(bpp)2(na)4]n (1) and [Zn(bpp)2(nas)2]n (2) (bpp = 1,3-bi(4-pyridyl)propane, na− = 1-naphthoate, and nas− = 2-aminonaphthalene-1-sulfonate), were hydrothermally synthesized by varying carboxylate- or sulfonate-containing coligands. Structural analyses reveal that complex 1 modified by terminal na− spacers possesses a bent one-dimensional chain bridged by ditopic bpp linkers. By contrast, complex 2 with two monodentate nas− ligands exhibits a two-dimensional layered structure extended by four equatorial bpp connectors. Obviously, the increase on the dimensionality of 2 than 1 is significantly resulting from the competitive coordination of the two mixed ligands with differently tunable binding groups to variable metal polyhedra. In addition, both complexes with analogously high thermal stability display strong fluorescent emissions at room temperature resulting from the ligand-to-metal or intraligand charge-transfer, suggesting their hopeful applications as efficient fluorescent materials.


Introduction
Rational design and successful preparation of novel coordination polymers (CPs) have recently attracted considerable attention due to their intriguing structural diversities [1] and potential applications as selective catalysts, [2] gas adsorbents, [3] molecule-based magnets, [4] and fluorescent materials. [5][6][7] It is well known that the structural diversities of the reported CPs can be significantly dominated by variously tuneable factors from both the reactants or surroundings. Some external perturbations, such as temperature, pH value, solvent, template molecule, and so on, can essentially direct the types and connection manners of structural subunits and the dimensionality of the resulting CPs. Additionally, the coordination preference of the single or mixed ligands toward the metal coordination polyhedra can more importantly govern the structural novelty and unpredictability, which can further influence the structuredependent optical, electronic, or magnetic properties of the targeted CPs. For example, resulting from competitive or synergic binding of the mixed ligands to the same metal ion with variable coordination spheres, several interesting samples with different structures and the same compositions have been successively fabricated from the reaction systems of the Cu II /Co II -triazolate-sulfoisophthalate and Cu II -triazolatecitrate, [8,9,10] which exhibit interesting transformations on the structure and magnetic behaviors. Herein, to further investigate the competitive coordination ability of the mixed ligands with different binding groups to transition metal ions, two Odonor-based coligands respectively with functional carboxylate and sulfonate moieties, 1-naphthoic acid (Hna) and 2aminonaphthalene-1-sulfonate acid (Hnas), were chosen as fundamentally building blocks to self-assembly with 1,3-bi(4pyridyl)propane (bpp) and inorganic zinc(II) salts. Obviously, the particular purpose for introducing the two different coligands is that the coordination ability of the deprotonated carboxylate group to metal ion is generally considered stronger than that of organic sulfonate ligand. [11] Moreover, the binding mode of carboxylate group is more diverse and flexible than that of sulfonate moiety. Additionally, due to the free rotation of the C¡C single bond, the long aliphatic chain of bpp ligand is conformationally flexible, [12] which can easily control the coordination behavior of the mixed-ligand system. As a result, two novel bpp-based CPs, [Zn 2 (bpp) 2 (na) 4 ] n (1) and [Zn(bpp) 2 (nas) 2 ] n (2), were hydrothermally obtained and structurally characterized. Structural analyses reveal that 1 possesses a bent one-dimensional (1D) chain extended by ditopic bpp connectors, which is further assembled into an unusual triple helix-based hydrogen-bonding network. By contrast, complex 2 with two terminal nas ¡ anions exhibits a two-dimensional (2D) layer extended by flexible bpp connectors. Obviously, the flexible bpp ligand in 1 and 2 is consistently responsible for the extension of the metal ions into polymeric frameworks with 1D chain and 2D layer, and the O-donor coligands play key roles on the construction of the metal coordination polyhedron and the assembly of the higher level ordered supramolecular architecture. Additionally, the two solid samples with higher compositional stability up to 215 C display strong fluorescent emissions at room temperature, which is primarily resulting from the ligand-tometal and/or intraligand charge-transfer.

Materials and Instruments
Bpp, Hnas, and Hna were purchased from Acros and other analytical-grade starting materials were obtained commercially and used as received without further purification. Doubly deionized water was employed for the conventional synthesis. IR spectra were collected on a Nicolet IR-200 spectrometer with KBr pellets in the range 4000¡400 cm -1 .
Elemental analyses for C, H, and N were determined on a Perkin Elmer 2400 C elemental analyzer. TGA experiments were carried out on a Shimadzu simultaneous DTG-60A thermal analysis instrument with a heating rate of 10 C min ¡1 from room temperature to 800 C under a nitrogen atmosphere (flow rate 10¢mL min -1 ). Fluorescence spectra of the polycrystalline powder samples of 1 and 2 were performed on a Fluorolog-3 fluorescence spectrophotometer from Horiba Jobin Yvon at room temperature.

Crystallography
Diffraction intensities for both 1 and 2 were collected on a computer-controlled Bruker APEX-II QUAZAR diffractometer equipped with graphite-monochromated Mo-Ka radiation with a radiation wavelength of 0.71073 A by using the v-' scan technique at room temperature. The program SAINT [13] was used for integration of the diffraction profiles. Semiempirical multiscan absorption corrections were applied using SADABS program. [14] The structures were solved by direct methods and refined with the full-matrix least-squares technique using the SHEXL-97 programs. [15] Anisotropic thermal parameters were assigned to all non-hydrogen atoms. The organic hydrogen atoms were generated geometrically. The starting positions of H attached to oxygen atom were located in difference Fourier syntheses and then fixed geometrically as riding atoms. Crystallographic data and experimental details for structural analyses were summarized in Table 1. Selected bond distances and angles for 1 and 2 were  Tables 2¡3, respectively. Hydrogen-bonding parameters are included in Table 4.

Results and Discussion
Syntheses and FT-IR Spectra Bulk products of crystalline 1 and 2 were hydrothermally prepared by the reactions of inorganic Zn(II) salt and flexible bpp molecule in the presence of Hna or Hnas coligand, in which aqueous NaOH solution was used to make O-donorcontaining ligands deprotonation and further to facilitate their coordination with Zn(II) ion. Additionally, both complexes are air stable, insoluble in common organic solvents and can retain their crystalline integrity at ambient conditions for a considerable length of time.
In the IR spectra ( Figure S1), weak absorption bands located at 3431 and 3330 cm -1 for 2 should be assigned to the stretch vibrations of exocyclic amino group of nas ¡ anion. Weak absorptions appeared at 3054 (for 1) and 3041 (for 2) cm -1 could be ascribed to the C-H stretching vibrations of aromatic ring. An absence of a characteristic band at 1675 cm -1 in 1 indicates the complete deprotonation of Hna. [16] Complex 1 also gave characteristic bands at 1615, 1569, and 1420, 1361 cm -1 for the asymmetric (n as ) and symmetric (n s ) stretching vibrations of the carboxylate group. By contrast, the adsorptions at 1200, 1161, and 622 cm -1 in 2 was related to stretching and bending vibrations of the sulfonate group from the nas ¡ anion. [17] Thus, the results of IR spectra were well agreement with those of crystal structural determinations.

Descriptions of Crystal Structures
Complex 1, [Zn 2 (bpp) 2 (na) 4 ] n , crystallizes in the monoclinic P2 1 /n space group, possessing an infinite zigzag chain with hexa-and pentacoordinated Zn II ions alternately extended by flexible bpp linkers. The asymmetric unit of 1 consists of two crystallographically independent Zn(II) ions, two neutral bpp molecules, and four deprotonated na ¡ anions in two different binding modes. As shown in Figure 1, the Zn1 ion in 1 is hexa-coordinated in a distorted O 4 N 2 octahedral coordination geometry completed by two bidentate chelating carboxylate groups from two na ¡ anions and two pyridyl N donors from two crystallographically unique bpp ligands. By contrast, the unique Zn2 ion is five-coordinated to two pyridyl N donors from two neutral bpp molecules and three carboxylate O atoms from two separate na ¡ anions, adopting a severely distorted square-pyramidal geometry with Addison parameter t D 0.32. The t value is defined as an index of trigonality (t D 1) and square-pyramid (t D 0). [18] The Zn¡O and Zn¡N distances are in the region of 1.970-2.468 A (Table 2), falling into the normal range of Zn(II)-based complexes with mixed carboxylate or pyridyl ligands. [19] Four unique deprotonated na ¡ anions in 1 exhibit two different binding modes: three of them coordinate with the Zn II ions in an asymmetric bidentate chelating fashion, and the last one adopts a monodentate coordination mode to complete the coordination sphere of the Zn2 ion.
As shown in Figure 1b, both crystallographically unique bpp molecules in 1 adopt a bidentate bridging binding mode to alternately aggregate the two unique Zn(II) ions into a 1D zigzag chain with the Zn2 ion locating at a inflection point.   (Table 4), generating a scarcely observed twisted triple-chain substructure, as shown in Figure 1c. Adjacent triple-chain substructures of 1 are further hydrogen-bonded together into a higher-dimensional supramolecular network of 1.
Complex 2, [Zn(bpp) 2 (nas) 2 ] n , crystallizes in the orthorhombic Pna2 1 space group, exhibiting a coplanar 2D layer extended by flexible bpp ligands. The asymmetric unit of 2 contains one crystallographically independent Zn(II) site, two neutral bpp ligands and two monodentate nas ¡ anions for charge compensation. The unique Zn(II) ion is hexacoordinated by four equatorial N donors from the pyridyl group of four neutral bpp ligands and two axially sulfonate O atoms from deprotonated nas ¡ anions, exhibiting a slightly distorted octahedral coordination geometry with one axial Zn1¡O6 bond considerable longer by 0.4 A than those of Zn¡O and Zn¡N distances ( Table 3). The bpp entity adopts a bidentate bridging mode, and the nas ¡ anion shows a monodentate binding mode. As shown in Figure 2b, each Zn (II) ion in 2 is infinitely extended by four equatorial bpp ligands in a bidentate mode, leading to macrocyclic {Zn 4 (bpp) 4 } 2C subunit-based 2D layer. Topologically, each Zn(II) ion can be considered as a four-connected node, and the neutral bpp ligand acts as a ditopic connector. Thus, the 2D layer of 2 belongs to an infinite (4 4) sheet (Figure 2b). Adjacent 2D layers of 2 are linked together to form 3D supramolecular architecture by interlayer N¡HÁÁÁO interactions between the amino group and sulfonate moiety of nas ¡ anions (Figure 2c and Table 4).
Apparently, the increase of the dimensionality from 1D chain (for 1) to 2D layer (for 2) is structurally due to the flexible bpp ligand, and the O-donor coligands play key roles on the construction of the metal coordination polyhedron and the assembly of the higher level ordered supramolecular architecture.

Thermal Stability
TGA experiments of 1 and 2 were carried out to explore their thermal stability and the results were presented in Figure 3. Complex 1 exhibits a one-step weight-loss stage once the temperature is higher than 215 C, ascribing to the continuous decomposition of bpp ligand and deprotonated na ¡ anions. The final product of 1 above 800 C is ZnO (obsd. 14.7%, calcd. 13.4%). Analogous to 1, polymeric sample of 2 can also be thermally stable up to 215 C and is followed by a continuous weight-loss stage between 215 and 660 C. The obvious weight-loss stage is corresponding to the broken of the extended structure and the decomposition of the mixed bpp and nas ¡ ligands. The remained substance beyond 660 C is calculated to be ZnO (obsd. 8.81%, calcd. 8.98%). Thus, the two polymeric complexes with different dimensionality exhibit analogous high thermal stability.

Luminescent Properties
The emissions of the two solid-state samples were measured at room temperature, together with the free Hna and Hnas ligands for comparison. As shown in Figure 4, complex 1 exhibits one intense band at 496 nm upon excitation at 280 nm. By contrast, a strong emission at 412 nm is observed in 2 upon excitation at 315 nm. Under comparable experimental conditions, free Hna species shows a strong emission at 421 nm (λ ex D 370 nm) and the Hnas ligand gives a broad maximum at 462 nm (λ ex D 349 nm). Therefore, the observed luminescence for 1 should be ascribed to the ligand-to-metal charge transfer and the emission for 2 is due to intraligand electron-charge. The slight difference of the complexes from the free ligands is probably resulting from the deprotonation of Hna/Hnas and the coordination behavior of the na-/nas ¡ to Zn(II) ion. [20] Conclusions In summary, two fluorescent Zn(II)-bpp complexes with 1D zigzag chain and 2D coplanar layer have been hydrothermally prepared by incorporating with two different O-donor coligands. Structurally, the competitive coordination of the flexible bpp and carboxylate/sulfonate-based coligand significantly dominates the extension of the polymeric frameworks with different dimensionality. Resulting from the intraligand and ligand-to-metal charge transfer, both complexes with good thermal stability can exhibit strong emissions with variable intensities, suggesting their potential applications as luminescent materials.

Funding
This present work was supported by the Science Foundation of Shaanxi University of Technology (SLGQD13-4), the National Natural Science Foundation of China (Grants 21171129, 21173157, and 21373132), Tianjin Municipal Education Commission (2012ZD02), and the Program for Innovative Research Team in University of Tianjin (TD12-5038), which are gratefully acknowledged.

Supplementary Material
Supplemental data for this article can be accessed at the publisher's website. Crystallographic data (excluding structure factors) for the structures in this paper have been deposited with the Cambridge Crystallographic Centre, 12 Union Road, Cambridge CB21EZ, UK. Copies of the data can be obtained free of charge on quoting the depository numbers CCDC-986982 and CCDC-986983 (for 1-2) (Fax: +44-1223-336-033; E-mail: deposit@ccdc.cam.ac.uk, http:// www.ccdc.cam.ac.uk).  Two Competitive Coordination-Directed Complexes 1167