Syntheses,Crystal Structures and Photoluminescent Properties of Two Co（Ⅱ）/Cu（Ⅰ） Coordination Polymers Based on Bis(imidazol)Ligands

Abstract:Two coordination polymers,{[Co(bib)3](ClO4)2}n(1)and{[Cu3(bix)4.5](ClO4)3}n(2),(bib=1,4-bis(imidazol-1-yl)-butane,bix=1,4-bis(imidazol-1-ylmethyl)benzene)have been hydrothermally synthesized by using Co(ClO4)2·6H2O/Cu(ClO4)·6H2O,1,4-bis(imidazolyl-1-yl)butane/1,4-bis(imidazolyl-1-yl)benzene and 4,4′-(1,3-phenylenebis(methyleneoxy))dibenzoic acid(H2pmda)as raw materials and structurally characterized by elemental analysis,IR spectrum,fluorescence spectrum,single-crystal and powder X-ray diffraction.Complex 1 crystallizes in trigonal system,space group R3 with a=b=1.393 37(5)nm,c=1.740 54(13)nm,V=2.926 5(3)nm3,Mr=828.59,Dc=1.410 g·cm-3,F(000)=1 293,μ=0.639 mm-1and Z=3.The final R1=0.061 1 and wR2=0.193 7 for 1 274 observed reflections with I＞2σ(I).Complex 2 belongs to trigonal system too,space group P3 with a=b=2.334 41(15)nm,c=0.715 11(9)nm,V=3.374 9(5)nm3,Mr=1 561.28,Dc=1.536 g·cm-3,F(000)=1 602,μ=1.131 mm-1and Z=2.The final R1=0.043 9 and wR2=0.109 0 for 3 167 observed reflections with I＞2σ(I).Structural analyses reveal that complex 1 shows 2D network structure with 36-membered ring and complex 2 possesses 2D network structure with 84-membered ring.The intermolecular hydrogen bonding or π-π stacking interactions extend the complexes 1 and 2 into supramolecular architectures.Furthermore,we also studied the fluorescence properties of the complexes.CCDC:1950200,1;1950184,2.

Keywords:hydrothermal synthesis;crystal structure;Co（Ⅱ） complex;Cu（Ⅰ） complex;luminescence

0 Introduction

The design and synthesis of metal-organic coordination polymersrelied on theselection of ligands and metal ions has become a very attractive research field.This is motivated not only by the intriguing structural diversity but also by the demand of applying functional materials into the fields of catalysis,porosity,magnetism,luminescence and nonlinear optics[1-3].In general,grids with various sizes and shapes can be synthesized by choosing suitable single metalions and organic ligands such as carboxylates and N-donor groups[4-6].Self-assembly is a complex process,highly influenced by many factors,such as the coordination geometry of metal ions,the nature of organic ligands,solvent system,temperature,pH value of the solution,the ratio between metal salt and ligand,the templates and the counter anions[7-14].These factors have been well-studied,but there are other forces such as hydrogen-bonding,π-π interactions,metal-metal interactions that can greatly influence the supramolecular topology and its dimensionality[15-17].Therefore,these considerations made us investigate new coordination structures with 4,4′-(1,3-phenylenebis(methyleneoxy))dibebzoic acid(H2pmda),and bridging ligands(bib/bix).In this manuscript,we reported the syntheses,crystal structures,IR and fluorescence properties of two new complexes,namely {[Co(bib)3](ClO4)2}n(1)and{[Cu3(bix)4.5](ClO4)3}n(2).Complexes 1 and 2 all exhibit intense purple luminescence,and appears to be good candidates for novel hybrid inorganic-organic photoactive materials.

1 Experimental

1.1 General procedures

All reagents were purchased commercially and used without further purification.Elemental analyses(C,H and N)were measured on a Vario EL（Ⅲ）Elemental Analyzer.IR spectrum was recorded in a range of 4 000～400 cm-1on a Nicolet 6700 spectrometer using a KBr pellet.The fluorescence spectra wereperformed on an F-7000 photospectrometer(Hitachi,High-Tech,Science,Japan).The powder X-ray diffraction (PXRD)studies were performed with a Bruker D8 Discover instrument(Cu Kα radiation,λ=0.154 184 nm,U=40 kV,I=40 mA)over the 2θ range of 5°～50°at room temperature.

1.2 Synthesis

{[Co(bib)3](ClO4)2}n(1):H2pmda(0.075 6 g,0.2 mmol),Co(ClO4)2·6H2O(0.2 mmol,0.075 g),bib(0.038 g,0.2 mmol)and 18 mL H2O were mixed,and the pH value of the mixture was adjust to about 8 with 40%NaOH.Then it was sealed in a Teflon-lined stainless steel vessel,heated to 120℃for 7 days,and followed by slow cooling to room temperature.Pink block crystals were obtained.Unfortunately,H2pmda ligand did not participate in the coordination.Yield:34%.Anal.Calcd.for C30H42Cl2CoN12O8(%):C,43.49;H,5.11;N,20.29.Found(%):C,43.00;H,4.99;N,19.95.IR (cm-1):3 126m,2 952w,1 657w,1 605w,1 521s,1 465w,1 443w,1 404w,1 373m,1 303w,1 280m,1 236s,1 096s,936m,870w,834m,772m,743w,729w,667m,623m.

{[Cu3(bix)4.5](ClO4)3}n(2):H2pmda(0.075 6 g,0.2 mmol),Cu(ClO4)2·6H2O(0.2 mmol,0.08 g),bix(0.048 g,0.2 mmol)and 18 mL H2O were mixed,and the pH value of the mixture was adjust to about 6 with 40%NaOH,Then it was sealed in a Teflon-lined stainless steel vessel,heated to 120℃for 7 days,and followed by slow cooling to room temperature.Pale yellow block crystals were obtained.Unfortunately,H2pmda ligand did not participate in the coordination.Yield:23%.Anal.Calcd.for C63H63Cl3Cu3N18O12(%):C,48.46;H,4.07;N,16.15.Found(%):C,48.01;H,3.71;N,15.85.IR (cm-1):3 448w,3 128m,2 948w,1 655w,1 618w,1 521s,1 450m,1 430w,1 401w,1 351w,1 302w,1 279m,1 237s,1 181w,1 126w,1 081s,1 023w,978w,942w,869w,824w,815w,764w,742m,714s,653m,624s,517w,436w.

1.3 Structure determination

Single-crystal X-ray diffraction data for 1 with dimensions of 0.45 mm ×0.35 mm ×0.34 mm and 2 with dimensions of 0.30 mm×0.22 mm×0.18 mm were recorded on a Bruker D8 QUEST CMOS diffractometerwith graphite-monochromated Mo Kα radiation(λ=0.071 073 nm)at 293 K.The structures were solved with the direct method of SHELXS-97 and refined with full-matrix least-squares techniques using the SHELXL-97 program[18-19].The non-hydrogen atoms of the complexes were refined with anisotropic temperature parameters.The hydrogen atoms attached to carbons were generated geometrically.Selected bond lengths and bond angles are listed in Table 1.

Table 1 Selected bond lengths(nm)and bond angles(°)for 1 and 2

CCDC:1950200,1;1950184,2.

2 Results and discussion

2.1 Description of the structure

Single crystal X-ray analysis reveals that complex 1 crystalizes in trigonal system with P3 space group.The asymmetric unit of complex 1 is composed of onesixth crystallographically Co（Ⅱ）ion,half bib ligand,and one-third free ClO4-(Fig.1).Each Co（Ⅱ）ion is sixcoordinated and attach to six nitrogen atoms from six different bib ligands,forming the cation[Co(bib)3]2+,while the two ClO4-anions play the role of balancing the charge.The geometry around each Co（Ⅱ）center is slightly distorted octahedron coordination sphere because the angles in a range of 89.18(9)°～180.00(9)°.The Co-N bond distance is 0.218 2(2)nm and similar to those in analogous complexes[20-21].

Fig.1 View of coordination environment(at 30%probability level)of Co（Ⅱ）ion of 1

In complex 1,the bib ligand takestransconformation bridging mode with a dihedral angle between the two imidazole rings of 0°.As depicted in Fig.2,three Co（Ⅱ）ions are bridged by six nitrogen atoms of bib ligandstogivecrystallographically isosceles triangle trimers with Co…Co distance of 1.411 9 nm,which are further connected to a twodimensional network with a 36-memberedring,and to the best of our knowledge,this structure is relatively rare.Single crystal X-ray diffraction reveals that[Co(bib)3]2+and ClO4-units form 3D supramolecular architectures(Fig.3)through two hydrogen bonds:C(1)-H(1)…O(2)(C(1)…O(2)0.323 5(17)nm)and C(3)-H(3)…O(1)(C(3)…O(1)0.315 6(8)nm),undoubtedly stabilizing the 3D structure of complex 1.

Fig.2 View of 2D network structure with 36-membered ring of complex 1

Fig.3 Three-dimensional supramolecular structure of 1 through hydrogen bonds

A single-crystal X-ray diffraction study reveals that complex 2 crystallizes in trigonal system with R3 space group and features a 2D network structure.The coordination environment of Cu（Ⅰ）ion in 2 is shown in Fig.4.The Cu（Ⅰ）ion is three-coordinated by three nitrogen atoms from three different bix ligands to furnish a slightly distorted plane triangle coordination architecture,which has been reported relatively rarely[22].The bond distances of Cu-N in complex 2 are in a range of 0.196 4(3)～0.197 4(2)nm and the coordination angles around the Cu（Ⅰ）ion are in a range of 119.819(14)°～119.990(3)°,which further proves that the plane triangle structure of Cu（Ⅰ）ion.It is worth pointing out that Cu（Ⅱ）ion took place redox reaction,and turned into Cu（Ⅰ）ion,which may be the magic of hydrothermal reactions.

Fig.4 View of coordination environment(at 30%probability level)of Cu（Ⅰ）ion of 2

In the crystal structure of complex 2,the bix ligand adopts a trans-conformation bridging mode with a dihedral angle between two imidazole rings of 28.81°and link the Cu（Ⅰ） ions to form a twodimensional network structure with 84-memberedring,as shown in Fig.5.

Fig.5 Two-dimensional network structure with 84-membered ring along c axis of complex 2

Hydrogen bonding interactions arefrequently chief in the synthesis of supramolecular structure.Single crystal X-ray reveals that[Cu3(bix)4.5]3+and ClO4-units form 3D supramolecular architectures through two hydrogen bonds:C(12)-H(12A)…O(1)(C(12)…O(1)0.334 9(7)nm)and C(20)-H(20A)…O(4)(C(20)… O(4)0.348 4(5)nm).In addition,there are π-π interactions (Fig.6)in complex 2 between imidazole rings of bix ligands.The centroid-to-centroid distance between adjacent ring is 0.366 82(17)nm for N(3)C(12)N(4)C(13)C(14)and N(5)C(15)C(16)N(6)C(17)(Symmetry code:y,-x+y,1-z)imidazole rings.The perpendicular distance is 0.338 23(12)nm and the dihedral angle is 0.00(17)°.Thus,through hydrogen bondsand π-π interactions,a three-dimensional supramolecular architecture is formed and plays an important role in stabilizing complex 2.

Fig.6 View of π-π interactions in 2

To investigate whetherthe analyzed crystal structure is truly representative of the bulk materials,X-ray powder diffraction (PXRD)analysis has been performed for the complex at room temperature(Fig.7).The main peak positions observed are in good agreement with the simulated ones.Although minor differences can be found in the positions,widths,and intensitiesofsome peaks,the bulk synthesized materials and analyzed crystal can still be considered as homogeneous.The differences may be due to the preferred orientation of the powder samples[23-24].

Fig.7 PXRD patterns of complexes 1 and 2

2.2 Photoluminescent properties

Metal-organic coordination polymers and conjugated organic linker have been studied because of their fluorescent properties and potential applications as fluorescent-emitting materials,chemical sensors and electroluminescent displays[25].Therefore,in the present work,the photoluminescent properties of bib,bix,complexes 1 and 2 have been investigated in the solid state at room temperature,as depicted in Fig.8.The free ligands bib and bix showed photoluminescence with the emission maximum at 408 nm(λex=348 nm)and 450 nm (λex=350 nm),respectively,which can be assigned to intraligand(π→π*)transition[26].Compared with the free bib ligand,a wide range of the emission with maximum peaks at ca.438 nm upon excitation at ca.350 nm for complex 1 was observed,which was red-shift.This emission band can be tentatively attributed to a ligand-to-metal charge transfer(LMCT)[27].While the emission peak of complex 2(λem=434 nm,λex=350 nm)was blue-shift as compared to the free ligand bix.Therefore,this emission band may be assigned as ligand-to-ligand charge transfer(LLCT)transitions[28-29].The fluorescence emissions of the complexes make them potentially useful photoactive materials.

Fig.8 Solid-state emission spectra of the ligands,complexes 1 and 2 at room temperature

3 Conclusions

In general,we have prepared two new complexes with bib/bix ligands.The bib/bix ligandsadopt bridging mode,and link neighboring Co（Ⅱ）/Cu（Ⅰ） ions to generate 2D network structure.Furthermore,the 3D supramolecular architectures are formed by hydrogen bonding or π-π interactions.They all exhibit intense purple luminescence.These materials will give new impetusto the construction ofnovelfunctional material with potentially useful physical properties.