A Benzimidazole-pyridine-2,3-dicarboxylic Acid Bridged Zinc(II) Coordination Complex–crystal Structure, Quantum Chemistry and Luminescence①

WANG Ji-Jun SUN Hn-Yng LI Chun-Bi, b

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A Benzimidazole-pyridine-2,3-dicarboxylic Acid Bridged Zinc(II) Coordination Complex–crystal Structure, Quantum Chemistry and Luminescence①

WANG Jia-Juna②SUN Han-YangaLI Chuan-Bia, b②

a(130103)b(136000)

The structure of a zinc(II)coordinationcomplex (1),[C14H10N3O5Zn1.5]nor [Zn1.5(bzim)(pydc)(H2O)]n(H2pydc = pyridine-2,3-dicarboxylic acid, Hbzim = benzimidazole), has been determined by X-ray crystallography and characterized by elemental analysis, IR spectrum and luminescence.Chemical formula: C14H10N3O5Zn1.5.It crystallizes in the monoclinic system, space group21/with= 12.303(4),= 12.052(4),= 10.212(3) ?,= 104.147(4),= 1468.3(8) ?3,= 4,M= 398.30,D= 1.802 g/cm3,(000) = 800,= 2.501 mm-1and= 1.000.The 2-D network architecture of 1 is constructed from benzimidazole, zinc and pyridine- 2,3-dicarboxylic acid.The quantum-chemical calculations have been performed on ‘molecular fragments’ extracted from the crystal structure using the B3LYP method in Gaussian 09.The luminescence spectrum shows that complex 1 emits blue luminescence.

pyridine-2,3-dicarboxylic acid, benzimidazolezinc(II) complex, quantum chemistry study, luminescence spectrum;

1 INTRODUCTION

Crystal engineering of supramolecular architec- tures based on metal organic frameworks (MOFs) have been fast expanding in the last few years owing to their potential applications such as absorp- tion[1], catalysis[2], luminescence[3],.The history ofMOFs used in the field of removing the highly toxic heavy metal ions is not long but has rapid development and broad prospects[4-9].MOFs exhi- biting luminescence emission in solid state is useful, especially, the fluorescence of the zinc(II) ion containing MOFs can be used for not only selective detection and differentiating Fe(III) and Cr(VI) ions[10], but also high iodine capture and nitro- explosive detection[11, 12, 13].

Recently, many high-dimensional coordination complexes have been designed and prepared through molecular self-assembly process[14].Struc- turally, the weaker intermolecular forces, especially hydrogen bonds, play an important role in funda- mental biological processes in the field of bioche- mistry and supramolecular chemistry[15].The multicarboxylate ligands have been proved to be good candidates because they can be regarded as hydrogen-bonding accepters and hydrogen-bonding donors, dependent upon the number of deprotonated carboxylic groups.The imidazole-like ligand can coordinate to metal center, and it can even be deprotonated and act as an anion[16, 17].

The hetero-ring carboxylic acid, pyridine-2,3- dicarboxylic acid (H2pydc) is the precursor of adenine-nicotinamide dinucleotide[18].It has great potential for coordinative interactions and hydrogen bonding, and hence may result in a large diversity of supramolecular architectures.Benzimidazole (Hbzim) is a fungicide[19].As a unique ligand it shows binding affinity to transition metals[20].Moreover, stable complexes are known to be formed as a neutral molecule or act as a benzimida- zole anion (bzim)[21].

In this paper, in view of the weak water solubility of the ligand used, we adopt hydrothermal methods to carry out our experiment.We report a new coordination complex, [C14H10N3O5Zn1.50]nor [Zn1.5(bzim)(pydc)(H2O)]n(1, H2pydc = pyridine- 2,3-dicarboxylic acid, Hbzim = benzimidazole).Complex 1 exhibits a two-dimensional (2D)hydro- gen bonding network structure in theplane in the crystal lattice.

2 EXPERIMENTAL

2.1 Synthesis of complex 1

Zn(Ac)2·2H2O (0.220 g, 1 mmol),benzimidazole(Hbzim, 0.118 g, 1 mmol), pyridine-2,3-dicar- boxylic acid(H2pydc, 0.167 g, 1 mmol) and 16 mL water were mixed with stirring followed by adjusting the pH value to 8 with an aqueous solution of NaOH.Then the mixture was sealed in a 25 mL Teflon-lined stainless-steel reactor and heated at 110 ℃ for 96 h to give colorless crystals of the title complex after cooling.The C, H and N contents were determined by elemental analysis: Calcd.(%) forC14H10N3O5Zn1.5: C, 42.18; H, 2.51; N, 10.54; Zn 24.63.Found (%): C, 41.95; H, 2.47; N, 10.39; Zn 24.98.IR(KBr, cm-1) 3395-3478br-vs, 3434vs, 2115m, 2101-1834w, 1826s, 1679s, 1625m, 1605s, 1596w, 1510m, 1365m, 1263m, 839m, 783scm-1.

2.2 Structure determination and physical measurements

A colorless block crystal for 1 with dimensions of 0.32mm × 0.27mm × 0.23mm was chosen for X-ray diffraction analysis.Crystal structure measurement was performed on a Bruker SMART APEX II CCD diffractometer equipped with a graphite-monochro- matic Mo(= 0.71073 ?) radiation by using anscan mode at 292(2) K.Absorption corrections were applied with a multi-scan mode[22].A total of 12414 reflections were obtained in the rangeof 2.40≤≤26.06o, of which 2907 were independent (int= 0.0722) and 2303 observed reflections with> 2()were employed for structuredetermination and refinement.The structure was solved by direct methods with SHELXS-97[23]and refined by full-matrix least-squares techniques using SHELXL-97 program[24]within WINGX[25].All non-hydrogen atoms were refined anisotropically.All H atoms on C atoms were positioned geometri- cally and refined as riding, with C–H = 0.93 andiso(H) = 1.2eq(C).The final= 0.0501,= 0.0743 (= 1/[2(F2) + (0.0265)2+ 1.5096], where= (F2+ 2F2)/3), (Δ/)max=0.000,= 1.000, (Δ)max= 0.362 and (Δ)min= –0.464 e/?3.The FT-IR spectrum was recorded from KBr pellets in the range of 4000～400 cm–1on a Perkin-Elmer 240C spectrometer.TG was performed using a Perkin-Elmer TG-7 analyzer in nitrogen with a heating rate of 10oC/min.The luminescence spectra for the powdered solid samples were measured at room temperature and the spectra were collected with a Perkin-Elmer LS-55 fluorescence spectro- meter.Other reagents were of analytical grade.

2.3 Quantum chemistry calculation

The quantum chemistry calculation of complex 1 was performed with Gaussian09 program[26]at the B3LYP/GenECP level, the 6-31+G(d) basis set for C, H, O, N and LANL2DZbasis set for Zn.For modeling the initial guess of the title complex was obtained from the X-ray refinement data (cif).In order to save computing time, we adopt the mini- mum structural unit of the complex to make an approximate structure model in the calculation, and use the neutral Hbzim instead of the deprotonatedbzim anion and the “one hydrogen atom contained” H2pydcanion ligand (that is, Hpydc) rather than the pydc dianionfor simplifying the polymeric complex 1 to a “monomer”, andkeep the charge balance in the model at the same time.

3 RESULTS AND DISCUSSION

3.1 Crystal structure of [Zn1.5(bzim)(pydc)(H2O)]n (1)

The structure of complex 1 is described in Fig.1.It has two Zn centers, however, the coordination manners of them are different.Zn(1) is four-coor- dinated by two N and two O atoms, forming a tetrahedral coordination geometry.The two N atoms are from two different benzimidazole and the two O atoms are from two different pyridine-2,3-dicar- boxylic acids; while the Zn(2) is located at an inversion center and it is six-coordinated by two N and four O atoms, with two N and two O atoms from two different pyridine-2,3-dicarboxylic acids.The Npyridineand 2 site carboxylic O atoms from one ligand coordinated to Zn(2) in a chelated mode, while the other two O atoms are from two coor- dinated water molecules.For the Zn2-center, the two chelate O(4), N(1) atoms and two chelate O(4i), N(1i) atoms (symmetry code i:1–, –1–, 1–) are all located at the equatorial positions, while Owand Owioccupy two axial sites.This coordination manner formed an octahedral geometry (Fig.1).The bond lengths and bond angles are listed in Table 1.From Table 1, we can see that the Zn–N(bipy)distance is 2.067 ?, which agrees with the normal Zn–N(bipy)distance (2.064 ?)[27].The Zn–N(imidazole)distances are 1.986 and 1.990 ?, also in accord with the-imidazole type Zn–N(imidazole)distances (1.986 and 1.994 ?[28]).Zn···O(carboxyl)distances vary from 1.976 to 2.077 ?, which consist with the typical Zn–O(carboxyl)distance (2.010 ?[27]), and the Zn–Owbond (2.210 ?)is slightly longer than Zn–O(carboxyl), indicating that the former is stronger than the latter.With the connecting manner described above, every benzimidazole is deprotonized and connects two Zn(1) atoms, and every pyridine-2,3-dicarboxylic acid connects three Zn atoms.The two are Zn(1) atoms with each carboxyl in a single dentate coordination manner and another one is Zn(2) atom chelated by N and O.In this connecting mode, complex 1 formed a two-dimensional network[29]structure in theplane.In the network structure, there are O(w1)– H(W1A)???O(2iv) and O(w1)– H(W1B)???O(1v) (iv: 1–, –1–, –, v: 1–, –0.5+, 0.5–, in Table 2) hydrogen bonds which may stabilize[30]the structure of 1 extending into a2Dlayered supramolecular architecture (Fig.2).

Fig.1. Coordination geometry of the coordination complex 1.

(Symmetry codes for i: 1–, –1–, 1–；ii: 1–, –, 1–；iii:, 0.5–, 0.5+)

Fig.2. 2D network supramolecular structure in theplane of complex 1 and hydrogen bonds (dotted line)

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

S ymmetry transformation:i:1–, –1–, 1–; ii: 1–, –, 1–; iii:, 0.5–, 0.5+

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

Symmetry transformations used to generate the equivalent atoms: iv: 1–, –1–, –; v: 1–, –0.5+, 0.5–

3.2 Quantum chemistry study

The calculation covered 85 atoms, 1098 basis functions, 1978 primitive gaussians, 211electrons and 211electrons for the model of complex 1.As the model does not have a single electron, the spin multiplicity is 1.The total molecular energy is–2917.983 a.u., the energies of HOMO and LUMO are –0.227 and –0.112 a.u., respectively, with the Δ(ELUMO–EHOMO) value to be –0.115 a.u., which shows the complex is stable[31]in the ground state.The HOMO and LUMO are presented inFig.3, from which we can see the HOMO electron cloud is mainly located at bzim and the LUMO electron cloud at the pydc ligand.

Selected atom net charges and electronic con- figuration of the title complex at the B3LYP/(6- 31+G(d) (for C, H, N and O) and Lanl2dz (for Zn) levels are listed in Table 3.The calculation results show that electronic configurations of the central Zn atoms are 40.25, 0.3139.9840.30, 0.34, and those of O and N atoms are 21.68~1.7425.02~5.3230.0130~0.01and 21.33~1.3724.18~4.3440.01, 0.02.The valence of zinc is +2.The zinc partially obtains electrons from the pydc and bzim ligands, illustrating that the charge of the central Zn metals for complex 1 has positive charge, and it is 1.41307 for Zn(1) and 1.41897 for Zn(2).In Table 3, the net charges of the coordinated O and N atoms are all negative.The charges of O(1), O(3ii), N(2), N(3iii) around Zn(1) are –0.75115, –0.74627, –0.62411, –0.72322, and those of N(1), N(1i), O(4), O(4i), O(w1), O(w1i) around Zn(2) are –0.54178, –0.55373, –0.75661, –0.85343, –0.99498, –1.05467, respectively.

Fig.3. Molecular orbitals of complex 1

Table 3. Selected Atom Net Charges and Electronic Configuration of Complex 1 at the B3LYP/(6-31+G(d) (for C, H, N and O) and Lanl2dz (for Zn) Levels

Symmetry codes:i: 1–, –1–, 1–; ii: 1–, –, 1–; iii:, 0.5–, 0.5+

3.4 Luminescence spectra

Luminescence is an important property[32].To examine the luminescent properties of10metal complexes, the solid-state emission and excitation (inset) spectra of complex 1 at room temperature are depicted in Fig.4.The obvious emissions are observed at 466 nm (= 520 nm) for 1.To under- stand the nature of the emission spectra, the luminescence properties of the free Hbzim and H2pydc ligands under the same conditions were recorded for comparison.The Hbzim and H2pydc ligands exhibit emission bands located at 396 and 394 nm upon 570 and 560 nm excitation, respec- tively.These ligands are observed without emission bands above 460 cm-1.The emissions of complex 1 may be assigned to the metal-to-ligand charge- transfer (MLCT).

The luminescence spectrum of complex 1 exhibits a red shift in solid state, compared with that of the two ligands (H2pydc and Hbzim).The reason may be that the10metal ion [zinc(II)] coordinates to the ligands, which probably forms the back-coupling-bond between the metal and ligands, and decreases the electron transition energy of intraligand charge transfer.Otherwise, the ligand coordinated with metal ions also forms additional five-membered rings, which also increases the* conjugation length and the conformational coplanarity of the ligand, accor- dingly reduces the energy gap between theand* molecular orbitals of the ligand[33, 34].

Fig.4. Solid-state emission and excitation (inset) spectra of complex 1 and ligands at room temperature

4 CONCLUSION

The structure of a zinc(II)coordinationcomplex (1, [C14H10N3O5Zn1.50]n) has been determined by X-ray crystallography and characterized by the luminescence.The conclusions of this article are as follows: (a) It is a 2-D layered network architecture constructed from benzimidazole, zinc and pyridine- 2,3-dicarboxylic acid.(b) The calculations on ‘molecular fragments’ are extracted from the crystal structure using B3LYP method.The calculation reveals that the HOMO and LUMO orbitals are located at the bzim and pydc ligands, respectively.The atom net charges and electronic configuration are also shown.(c) The luminescence spectrum shows that complex 1 emits blue luminescence, and the luminescence mechanism of the emissions may be assigned to the metal-to-ligand charge-transfer (MLCT).

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18 January 2018;

26 April 2018 (CCDC 689913)

① Supported by the Jilin Province Science and Technology Development Plan Item (No.20140204080GX), the Project of the Education Department of Jilin Province, China (No.JJKH20180777KJ) and the Science and Technology Development Projects of(No.2017057)

Wang Jia-Jun, Tel: +86 0431 81765105.E-mail: jiajunwang@jlnu.edu.cn.Li Chuan-Bi, E-mail: li_c_b@163.com

10.14102/j.cnki.0254-5861.2011-1955