Three Complexes Constructed Using 2,2′-Oxybis(benzoic acid)and N-Donor Ligands:Syntheses,Structures and Fluorescent Properties

TANG Long FU Yu-Hao WANG Yi-Tong WANG Huan-Huan WANG Ji-Jiang HOU Xiang-Yang WANG Xiao

(Yan′an University Key Laboratory of New Energy&New Functional Materials,Shaanxi Key Laboratory of Chemical Reaction Engineering,College of Chemistry and Chemical Engineering,Yan′an University,Yan′an,Shaanxi 716000,China)

Abstract:Three new complexes[Zn(2,2′-oba)(bipy)2]·2H2O(1),[Pb(2,2′-oba)(phen)]n(2)and{[Pb(2,2′-oba)(bimbp)]·H2O}n(3)(2,2′-H2oba=2,2′-oxybis(benzoic acid),bipy=2,2′-bipyridine,phen=1,10-phenanthroline,bimbp=4,4′-bis(imidazolyl)biphenyl)were synthesized by hydrothermal reactions and characterized by single-crystal X-ray diffraction,thermogravimetric analyses,IR spectroscopy and elemental analysis.Complex 1 is a 0D structure,which is extended to a 2D supramolecular framework through aromatic π-π stacking interactions and strong intermolecular O-H…O hydrogen bonds.Complex 2 shows a 1D chain structure,which is expanded to a 2D layer structure through aromatic π-π stacking interactions.Complex 3 appears a 1D double chain structure,which is further extended to a 2D supramolecular framework through strong intermolecular O-H…O hydrogen bonds.Moreover,the solid-state fluorescent properties of 1～3 have also been investigated.According to the crystal structures,the TDDFT/6-31G(d)approach was applied to study the photoluminescence emission of 1,and the calculated results are in good agreement with the experimental results.CCDC:1973760,1;1973771,2;1973772,3.

Keywords:complex;crystal structure;fluorescent properties;DFT studies

0 Introduction

Rational design and synthesis of coordination polymers(CPs)is currently of significant interest not merely due to the diverse network topology but mainly due to these extended systems playing a significant role in catalysis,chirality,luminescence,magnetism,nonlinear optics,adsorption,and separation[1-4].As is well known,applications of CPs highly depend on their structures,so design and construction of the CPs with desired structures and functions are extremely essential.There are many factors that influence the diversity of structures,such as central metal ions,organic ligands,metal-ligand ratio,solvents,temperature,pH value,and other factors[5-8],which have been validated and summarized.Among them,the critical factor for the construction of CPs is the rational choice of organic building block.In our strategy,multidentate O-or N-donor ligands have been employed in the construction of CPs[9-11].So many polycarboxylate ligands are often employed as bridging ligands to construct CPs,due to their extension ability both in covalent bonding and in supramolecular interactions[12-14].In contrast to polycarboxylate ligands,the flexible ether-oxygen dicarboxyl-ate ligands have been employed in the construction of CPs.

The oxybis(benzoic acid)(H2oba)is a kind of typical flexible dicarboxylic acid ligands,such as 2,2′-oxybis(benzoic acid),2,4-oxybis(benzoic acid)and 4,4′-oxybis(benzoicacid),has also been reported[15-17].Wealso employ 2,2′-oxybis(benzoic acid)and N-donor ligands to synthesize some complexes,such as[Zn(2,2′-oba)(4,4′-bipy)],[Zn(2,2′-oba)(bpe)0.5(H2O)],[Cu(2,2′-oba)(phen)],[Ni2(2,2′-oba)2(bpe)],[Co(2,2′-oba)(2,2′-bipy)(H2O)2][Co(2,2′-oba)(4,4′-bimbp)1.5]·1.5H2O[18-20],among which[Zn(2,2′-oba)(4,4′-bipy)]shows a 3D metal-organic framework with a uninodal 4-connected 65.8dmpnetwork.We continued our investigation and chose 2,2′-H2oba as a bridging ligand to react with Zn（Ⅱ）/Pb（Ⅱ） metal ions,and three new complexes were obtained,namely[Zn(2,2′-oba)(bipy)2]·2H2O(1),[Pb(2,2′-oba)(phen)]n(2)and{[Pb(2,2′-oba)(bimbp)]·H2O}n(3)(bipy=2,2′-bipyridine,phen=1,10-phenanthroline,bimbp=4,4′-bis(imidazolyl)biphenyl).The syntheses,crystal structures and properties of three complexes are presented and discussed.

1 Experimental

1.1 Materials and chemical analysis

The ligands 2,2′-H2oba,bipy,bimbp and phen were purchased from Jinan Henghua Sci.&Technol.Co.,Ltd.;all other reagents and solvents employed were commercially available and used without further purification.Elemental analyses were performed with a Perkin-Elmer 2400 CHN Elemental analyzer.Infrared spectra on KBr pellets were recorded on a Nicolet 170SX FT-IR spectrophotometer in a range of 400～4 000 cm-1.TG analyses were conducted with a Nietzsch STA 449C micro analyzer under atmosphere at a heating rate of 5℃·min-1.The fluorescence spectra were studied using a Hitachi F-7100 fluorescence spectrophotometer at room temperature.All calculations have been processed in Gaussian 09 package[21].

1.2 Synthesis of[Zn(2,2′-oba)(bipy)2]·2H2O(1)

A mixture of 2,2′-H2oba(0.1 mmol,0.025 8 g),bipy(0.1 mmol,0.015 6 g),Zn(NO3)2·6H2O(0.1 mmol,0.029 7 g)and H2O(10 mL)was stirred evenly and heated in a 23 mL Teflon-lined autoclave at 140℃for 4 days,followed by slow cooling(5℃·h-1)to room temperature.The resulting mixture was washed with H2O,and colorless block crystals of 1 were collected and dried in air.Yield:53%(based on Zn).Elemental anal-ysis Calcd.for C34H28N4O7Zn(%):C 60.95,H 4.21,N 8.36;Found(%):C 62.56,H 4.32,N 8.76.IR(KBr,cm-1):3 384(s),3 044(m),1 625(s),1 548(s),1 446(w),1 396(vs),1 234(m),1 174(s),796(s),737(w),681(vs).

1.3 Synthesis of[Pb(2,2′-oba)(phen)]n(2)

A mixture of Pb(OAc)2·3H2O(0.1 mmol,0.037 9 g),2,2′-H2oba(0.1 mmol,0.025 8 g),phen(0.1 mmol,0.019 8 g)and H2O(10 mL)was stirred for 30 min in air.The mixture was transferred to a 23 mL Teflon reactor and kept at 140℃for 5 days under autogenously pressure,and then cooled to room temperature at a rate of 5℃·h-1.Colorless block crystals of 2 were obtained(Yield:48% based on Pb).Elemental analysis Calcd.for C26H16N2O5Pb(%):C 48.52,H 2.51,N 4.35;Found(%):C 48.29,H 2.34,N 4.53.IR data(KBr,cm-1):1 646(vs),1 589(s),1 443(m),1 378(s),1 226(m),1 137(s),876(m),787(m),664(w).

1.4 Synthesis of{[Pb(2,2′-oba)(bimbp)]·H2O}n(3)

Complex 3 was prepared with the method for 2 by using bimbp(0.1 mmol,0.028 6 g)instead of phen.Col-orless crystals of 3 were obtained(Yield:56% based on Pb).Elemental analysis Calcd.for C32H24N4O6Pb(%):C 50.06,H 3.15,N 7.30;Found(%):C 50.27,H 3.34,N 7.36.IR(KBr,cm-1):3 354(m),1 607(vs),1 573(s),1 506(s),1378(s),1237(w),1187(s),872(m),779(m),668(m).

1.5 X-ray crystallographic studies

Diffraction intensities for the three complexes were collected at 293 K on a Bruker SMART 1000 CCD diffractometer employing graphite-monochromated MoKαradiation(λ=0.071 073 nm).A semi-empirical absorption correction was applied using the SADABS program[22].The structures were solved by direct methods and refined by full-matrix least-squares onF2using the SHELXS 2014 and SHELXL 2014 programs,respectively[23-24].Non-hydrogen atoms were refined anisotropically,and H atoms bonded to C atoms were placed in calculated positions and water H atoms were refined in a riding mode.The crystallographic data for complexes 1～3 are listed in Table 1,and selected bond lengths and angles are listed in Table S1(Supporting information).

CCDC:1973760,1;1973771,2;1973772,3.

Table 1 Crystal data and structural refinement summary of the complexes 1～3

2 Results and discussion

2.1 Description of the structures

2.1.1 Crystal structure of[Zn(2,2′-oba)(bipy)2]·2H2O(1)

Single crystal X-ray diffraction analysis suggests that compound 1 consists of one Zn（Ⅱ） ion,one 2,2′-oba anions,two bipy molecules and two free water molecules.Each Zn（Ⅱ）center is six-coordinated by four pyridyl nitrogen donors from two bipy ligands and two oxygen atoms coming from one 2,2′-oba ligand(Zn-N/O 0.210 7(3)～0.224 3(2)nm),forming a distorted ZnN4O2octahedral geometry(Fig.1).The O/N-Zn-O/N bond angles are in a range of 59.27(10)°～173.81(14)°.One carboxylate group of 2,2′-oba ligand is not coordinated,and another one adoptsμ1-η1-η1chelating mode to link one Zn（Ⅱ）ion(mode A in Scheme S1).Two phenyl rings are severely bent with a dihedral angle of 77.53°.The 2,2′-oba ligand and two bipy ligands chelate with Zn（Ⅱ） to form a 0D structural unit.Through aromaticππstacking interactions between two bipy ligands(centroid-to-centroid distance:0.372 0 nm),the adjacent units are bridged to form a 1D chain running alonga-axis(Fig.2).Due to the strong intermolecular O-H…O hydrogen bonds(between free water molecule and carboxylate O atoms of two 2,2′-oba anions with the O6-H6A/H6B…O4/O4 distance of 0.283 9 nm/0.270 6 nm),the adjacent 1D chains are further extended to produce a 2D supramolecular framework(Fig.3).

Fig.1 Coordination environment of Zn（Ⅱ）ion of compound 1

Fig.2 One-dimensional chain of 1

2.1.2 Crystal structures of[Pb(2,2′-oba)(phen)]n(2)

Fig.3 Two-dimensional supramolecular structure of 1

Single-crystal X-ray analysis reveals that compound 2 shows an infinite 1D chain structure.Compound 2 is made up of the Pb（Ⅱ） ion,2,2′-oba ligand and phen ligand,and each Pb（Ⅱ）ion is coordinated to five oxygen atoms of three 2,2′-oba ligands and two nitrogen atoms of one phen ligand,forming a distorted pentagonal biconical geometry,as shown in Fig.4.The Pb-N/O bond lengths are in a range of 0.258 6(5)～0.284 1(6)nm,and the O/N-Pb-O/N bond angles are in a range of 63.2(2)°～145.2(2)°.As compared to compound 1,the carboxylic groups of 2,2′-oba ligand have two coordination modes:one carboxylate group adoptsμ2-η1-η2bridging mode to link two Pb（Ⅱ） ions,and another one adoptsμ2-η2-η0bridging mode to link two Pb（Ⅱ）ion(mode B in Scheme S1),resulting in a 1D chain structure(Fig.5).All phen ligands bristle out from the two sides of the chain,and further through aromaticπ-πstacking interactions between two phen ligands (centroid-to-centroid distance:0.356 6 nm),the adjacent chains are expanded to a 2D layer structure,as shown in Fig.6.

Fig.4 Coordination environment of Pb（Ⅱ）ion in compound 2

Fig.5 One-dimensional chain of 2

Fig.6 Two-dimensional layer structure of 2

2.1.3 Crystal structures of{[Pb(2,2′-oba)(bimbp)]·H2O}n(3)

Single-crystal X-ray analysis reveals that the asymmetric unit of 3 contains one Pb（Ⅱ） ion,one 2,2′-oba dianion,one bimbp ligand and one free water molecule.Each Pb（Ⅱ）ion is coordinated to three oxygen atoms of two 2,2′-oba ligands and two nitrogen atoms of two bimbp ligands,forming a distorted triangular biconical geometry,as shown in Fig.7.The Pb-N/O bond lengths are in a range of 0.229 1(4)～0.287 5(5)nm,and the O/N-Pb-O/N bond angles are in a range of 78.84(15)°～163.59(18)°.In complex 3,the coordination mode of 2,2′-oba ligand is different from those of 1 and 2,(μ1-η1-η0andμ1-η1-η1bridging mode,mode C in Scheme S1).The neighboring Pb（Ⅱ） ions are linked by 2,2′-oba ligands to generate a loop shaped unit,and through bimbp ligand bridging,the adjacent units are connected to produce a 1D double chain structure(Fig.8).Furthermore,through strong intermolecular O-H…O hydrogen bonds(between free water molecule and carboxylate O atoms of two 2,2′-oba anions with the O6-H6A/H6B…O2/O5 distance of 0.284 6 nm/0.285 1 nm),the adjacent 1D chains are further extended to produce a 2D supramolecular framework(Fig.9).

Fig.7 Coordination environment of Pb（Ⅱ）ion in compound 3

Fig.8 One-dimensional double chain structure of 3

Fig.9 Two-dimensional layer structure of 3

2.2 Thermogravimetric analysis

To study the thermal stability of 1～3,thermogravimetric(TG)analyses were performed on polycrystalline samples under a nitrogen atmosphere with a heating rate of 10 ℃·min-1(Fig.S1～S3).The TG curve of 1 showed two weight loss steps.The first weight loss in a range of 70～100 ℃ (Obsd.3.4%,Calcd.5.37%)is assignable to the loss of free water.The second weight loss of 87.8% in a temperature range of 220～560 ℃corresponds to the decomposition of the 2,2′-oba and bipy ligands(Calcd.84.87%).The TG curve of 2 suggested that no weight loss were observed until 230℃,above which,significant weight loss(Obsd.68.2%)occurred and ended at about 580℃,indicating the complete decomposition of2,2′-oba and phen ligands(Calcd.67.81%).TG curve of 3 showed the first weight loss in a range of 70～90 ℃ (Obsd.2.6%,Calcd.2.35%)is assignable to the loss of free water.The second weight loss of 71.5% in a temperature range of 210～540 ℃ corresponds to the decomposition of 2,2′-oba and bimbp ligands(Calcd.70.67%).

2.3 Photoluminescence properties

The luminescent emission spectra of 1～3 were examined in the solid state at room temperature and are shown in Fig.10.The main emission peaks of free H2oba and bimbp ligand appeared at 378 nm(λex=334 nm)and 401 nm(λex=340 nm),which can be assigned to the intra-ligandπ-π*transitions[25].The bipy ligand exhibited two emission bands at 418 and 442 nm upon excitation at 389 nm.Complex l showed the main emission peak at 442 nm(λex=380 nm),which is similar to that of bipy ligand,probably owing to bipy ligandbased charge transfer[26-27].Complex 2 had an intense emission peak at 350 nm(λex=324 nm),showing a blue shift in comparison with those of free H2oba and phen ligand(λem=384 nm,λex=350 nm),which is attributed to ligand to metal charge transfer(LMCT)[28-29].Complex 3 exhibited one weaker emission peak at 385 nm upon excitation at 334 nm,having a red shift ofca.7 nm rel-ative to that of H2oba ligand,which is attributed to H2oba ligand-based charge transfer.By comparing the emission spectra of 1～3 and ligands,we can conclude that the enhancement of luminescence in 1～3 may be attributed to the ligation of ligand to the metal center,which effectively increases the rigidity and reduces the loss of energy by radiationless decay[30].

Fig.10 Solid state luminescent emission spectra of complexes 1～3 and ligands

2.4 TDDFT calculations of compound 1

In order to reveal the photo luminescence emission of compound 1,we chose the structural unit from its single crystal X-ray diffraction data set and carried out its theoretical calculation.Based on the optimized geometries(with no constraints during the optimizations),time-dependent density functional(TDDFT)cal-culations were performed at the B3LYP level with 6-31G(d)basis set for C,H,N and O atoms,and effective core potentials basis set LanL2DZ for Zn atoms,employing the Gaussian 09 suite of programs package.The characteristics of HOMO and LUMO of compound 1 are shown in Fig.11.Based on the TDDFT energy level and MO analyses,S0-S1transitions of compound 1 is mainly associated with the transitions from the corresponding HOMO to LUMO,in which the electron-density distribution of HOMO is totally resided at the coordinatingπ-orbital of one bipy ligand with the energy of-0.149 61 Hartree;however,the electron-density popu-lation of LUMO locates atπ-orbital of other bipy ligand and the energy of LUMO was calculated to be-0.106 77 Hartree.Complexation of ligands with Zn（Ⅱ）ions usually reduces significantly the energy gaps between LUMOs and HOMOs[31].The energy difference of compound 1 between LUMO and HOMO was 0.042 84 Hartree(1.165 eV),and this is small enough to allow the charge transfer from HOMO to LUMO.According to this observation,it is proposed that the essence of the photoluminescence of compound 1 could be assigned to the ligand to ligand charge transfer.This cal-culation result is in good agreement with the experimental observations.

Fig.11 HOMO and LUMO of compound 1

3 Conclusions

In summary,three complexes have been synthesized by the self-assembly of M（Ⅱ）(M=Zn and Pb)salts with 2,2′-H2oba and auxiliary N-donor Ligands.Assemblies of these complexes generate two types of diverse frameworks:one 0D structure and two 1D chains.Moreover,the solid-state fluorescent properties of 1～3 have also been investigated.According to the crystal structures,the TDDFT/6-31G(d)approach was applied to study the photoluminescence emission of 1,the calcu-lated results are in good agreement with the experimental results.

Supporting information is available at http://www.wjhxxb.cn