基于2，5‑二溴对苯二甲酸配体的锰/钴配合物的合成、晶体结构及性质

黄瑞琴 刘 峥 王 胜 余彩莉 魏润芝 唐 群

(桂林理工大学化学与生物工程学院，广西电磁化学功能物质重点实验室，桂林 541006)

0 Introduction

Coordination polymers are usually obtained from multi‑dentate organic ligands as building templates or assembled from connecting rods with metal ions as nodes[1‑2].Among them,multi‑dentate nitrogen‑containing ligands,multi‑dentate carboxylic acid ligands,and multi‑dentate pyridine carboxylic acid ligands are the preferred ligands for the self‑assembly of organic ligands[3].The complexes have potential applications in the fields of catalysis,ion recognition and antioxidation,electrochemistry,etc.,and are one of the research hot spots in organometallic chemistry and coordination chemistry[4‑7].The synthesis of coordina‑tion polymers using ligands containing carboxyl groups is more common.In addition to structural stability,due to the multiple coordination modes and bridging modes of carboxyl‑containing ligands,the complexes are easy to obtain topological diversity and excellent properties.Ligands containing nitrogen or oxygen groups can be re‑introduced during the construction of the complexes to increase the active sites,form more coordination modes,diversify the pore channels and improve their luminescence performance[8‑10].

Among them,terephthalic acid‑like ligands are one of the important ligands for the preparation of func‑tional MOFs materials,because having bifunctional groups in thepara‑position provides the possibility of forming polymer chains of mono‑and heterometallic two‑or three‑dimensional coordination compounds,which can be used as one of the ligands for the synthe‑sis of bidentate,tridentate or tetradentate chelate com‑plexes based on the characteristics of terephthalate ion structure.The presence of emptydorbitals in transi‑tion metals allows the use of hybrid orbitals to accept electrons,resulting in stable structures of 16 or 18 elec‑trons,and thus are widely used for the preparation of complexes[11‑14].

Our group chose 2,5‑dibromoterephthalic acid(H2L1)as the primary ligand and 2,2′‑bipyridine(L2)and 1,10‑phenanthroline(L3)as the secondary ligands and obtained complexes[Mn2(L1)2(L2)2(H2O)2]n(1)and[Co2(L1)2(L3)2(H2O)2]n(2).The structures were character‑ized by single‑crystal X‑ray diffraction and IR spectros‑copy,and their thermal stability and fluorescence prop‑erties were also investigated.

1 Experimental

1.1 Instruments and reagents

H2L1(Purity：97% )was purchased from Shanghai Maclean Reagent Company;L2(Purity：99% ),L3(Purity：99% ),N,N‑dimethylformamide(DMF,AR),manganese sulfate monohydrate(AR)and cobalt nitrate hexahy‑drate(AR)were purchased from Aladdin Reagent(Shanghai)Company.

An Agilent Technologies G8910A single‑crystal diffractometer was used to determine the crystal struc‑ture.A Perkin‑Elmer 240Q elemental analyzer was used to determine the elemental contents(C,N,H).A Shimadzu FTIR‑8400 FTIR spectrometer(with KBr compression)was used to measure the IR spectra of the complexes.A UV‑5500PC double‑beam UV‑Vis spec‑trophotometer was used to determine the UV‑Vis spec‑tra of the complexes.An EDXRF‑type fluorescence spectrometer was used to determine the fluorescence spectra of the complexes.An SDT‑Q600 type synchro‑nous TG/DTG analyzer was used for thermogravimetric analysis(TGA).

1.2 Synthesis of the complexes

DMF(6 mL),H2O(4 mL),0.2 mmol(0.064 8 g)of H2L1,0.2 mmol(0.031 2 g)of L2,and 0.2 mmol(0.057 5 g)of manganese sulphate monohydrate were placed in a beaker,stirred magnetically for 30 min and then placed in a reaction kettle.The reaction was carried out in an oven at 90℃for 3 d,then cooled down to 50℃ at a rate of 10℃·h-1for 6 h and then reduced to room temperature.Massive yellow crystals of complex 1(0.014 7 g)were obtained with a yield of 43.7% (based on Mn).Elemental analysis Calcd.for C18H12Br2MnN2O5(% )：C,39.20;N,5.08;H,2.18.Found(% )：C,39.52;N,4.95;H,2.11.

Complex 2 was synthesized in a similar way to that of complex 1,using cobalt nitrate hexahydrate(0.058 2 g,0.2 mmol)reacted with H2L1(0.064 8 g,0.2 mmol)and L3(0.036 0 g,0.2 mmol).The reaction gave 0.029 7 g of red massive crystals in 51.2% yield(based on Co).Elemental analysis Calcd.for C20H12Br2CoN2O5(% )：C,41.45;N,4.84;H,2.07.Found(% )：C,41.08;N,4.75;H,2.01.

1.3 Crystal structure determination

Crystals with the sizes of 0.18 mm×0.16 mm×0.15 mm(1)and 0.19 mm×0.17 mm×0.15 mm(2)were selected and diffraction point data were collected using an Agilent Technologies G8910A single‑crystal X‑ray diffractometer with MoKαradiation(λ=0.071 073 nm)at a temperature of 293(2)K,usingφ‑ωscan[15].The raw data were reduced using the CryAlisPro program and all data were subjected to Lp factor correction and empirical absorption correction.The coarse structure was solved using the direct method in SHLEXS‑97,and then the full matrix least‑squares refinement of the non‑hydrogen atomic coordinates and their anisotropic tem‑perature factors was performed using the SHLEXL‑97 program.All hydrogen atom coordinates were obtained using theoretical hydrogenation.The relevant crystallo‑graphic data are listed in Table 1 and the main bond lengths and bond angles are listed in Table 2.

Table 1 Crystallographic data for complexes 1 and 2

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

CCDC：2034070,1;2034085,2.

Continued Table 2

2 Results and discussion

2.1 Crystal structural analysis

2.1.1 Crystal structure of complex 1

As shown in Fig.1a,each asymmetric unit of com‑plex 1 consists of two Mn2+ions,two L12-ions,two L2molecules,and two coordinated water molecules.In the coordination environment of Mn1,O5 is an oxygen atom from the coordinated water molecule,O1 and O1iare both oxygen atoms from the carboxyl group on L12-,and N1 and N2 are nitrogen atoms from L2,where the sum of the bond angles formed by O1,O1i,O5,and N2 with the central Mn1 ion is 359.74°,which is close to the ideal angle(360°),indicating that these four coordi‑nation atoms are in the equatorial plane of the tetrago‑nal bipyramidal cone and have good coplanarity.As shown in Fig.1b,the carboxyl oxygen atom O3 on L12-and N1 on L2are located in axial positions with axial bond angle O3—Mn1—N1(167.85(10)°),forming a distorted[MnO4N2]tetragonal bipyramidal geometry.The coordination environment of Mn1iis the same as that of Mn1,Mn—O(0.220 9(2)‑0.221 89(19)nm),Mn—N(0.223 8(2)‑0.224 7(2)nm)are in the normal range of bond lengths,the bond angles of O—Mn—O are in a range of 77.28(7)°‑168.84(8)°,the bond angles of N—Mn—O are in a range of 85.22(9)°‑170.69(8)°,and the bond angle of N—Mn—N is 73.41(9)°,all with‑in the normal range[16].

Fig.1 Crystal structure of complex 1：(a)ellipsoid diagram with 30% probability level;(b)coordination polyhedron diagram;(c)3D stacked diagram

With Mn2+as the metal node,L12-and L2as the coordination linkages,the Mn1 and Mn1iions are con‑nected by carboxyl oxygen atoms O1 and O1ion L12-,thus forming a fully symmetric bidentate chelate coor‑dination structural unit.The intermolecular linkages through L12-bridges form an infinitely extended 2D network‑like structure.As shown in Fig.1c,a 3D network‑like structure is formed between adjacent layers through intermolecular hydrogen bonding andπ‑πstacking.Theπ‑πstacking is between the Cg(1)ring(a ring consisting of N2i‑C5i‑C4i‑C3i‑C2i‑C1i)and the Cg(2)ring(a ring consisting ofC12i‑C14‑C13‑C12‑C14i‑C13i)between layer and layer molecules,where the distance from the Cg(1)ring to the center of the Cg(2)ring is 0.354 6 nm and the inter‑ring dihedral angle is 5.511°.The pore size of complex 1 was calculated by the tools module of Diamond software as 0.83 nm×2.05 nm.

2.1.2 Crystal structure of complex 2

As shown in Fig.2a,each asymmetric unit of complex 2 consists of two Co2+ions,two L12-ions,two L3molecules,and two coordinated water molecules.In the coordination environment of Co1,O5 comes from the water molecule,O3,O1 and O1iare the carboxyl oxygen atoms on L12-,and N1 and N2 come from L3.As shown in Fig.2b,the carboxyl oxygen atom O3 on L2-1and N1 on L3are located in the axial position with axial bond angle O3—Co1—N1(169.90(9)°),forming a dis‑torted[CoO4N2]tetragonal bipyramidal geometry.The coordination environment of Co1iis the same as that of Co1,Co—O(0.207 4(2)to 0.213 6(2)nm),Co—N(0.212 0(2)to 0.212 3(2)nm)are in the normal range of bond lengths,the bond angles of O—Co—O are in a range of 77.75(7)°‑170.36(8)°,the bond angles of N—Co—O are in a range of 83.22(9)°‑175.06(8)°,and the bond angle of N—Co—N is 78.80(10)°,all within the normal range[17].

Fig.2 Crystal structure of complex 2：(a)ellipsoid diagram with 30% probability level;(b)coordination polyhedron diagram;(c)3D stacked diagram

With Co2+as the metal node,L12-and L3as the coordination linkage,the Co1 and Co1iions are con‑nected by carboxyl oxygen atoms O1 and O1ion L12-,thus forming the coordination structural unit of the bidentate chelate.The intermolecular linkages via L12-bridges form an infinitely extended 2D network‑like structure.As shown in Fig.2c,the adjacent layers are stacked to form a 3D network‑like structure through O—H…O intermolecular hydrogen bonding andπ‑πstacking.Theπ‑πstacking is between the Cg(1)ring consisting of N2‑C10‑C9‑C8‑C7‑C11 and the Cg(2)ring consisting of C14i‑C15i‑C16i‑C14 ‑C15 ‑C16 between layer and layer molecules.The distance from the Cg(1)ring to the center of the Cg(2)ring is 0.355 8 nm and the dihedral angle between the two rings is 11.709°.The pore size of complex 2 was calculated by the tools module of Diamond software,giving 2.07 nm×1.52 nm.

2.2 IR spectroscopic analysis

The IR spectra of ligands H2L1,L2,L3,and complexes 1 and 2 were measured in a range of 4 000‑400 cm-1using a Shimadzu FTIR‑8400 infrared spec‑trometer(Fig.3).As shown in Fig.3a,the O—H stretch‑ing vibration peak for the carboxyl group of H2L1appeared at 3 438 cm-1,while this peak moved to 3 408 cm-1for complex 1.The C=O stretching vibra‑tion absorption peak for the carboxyl group of H2L1appeared at 1 709 cm-1,and this peak appeared at 1 609 cm-1for complex 1.The O—H bending vibration absorption peak of H2L1at 900 cm-1did not move after the formation of complex 1.These indicate that the manganese ion is coordinated with the carboxyl group in H2L1.L2showed a C=N bending vibration absorp‑tion peak at 1 569 cm-1,which moved to 1 609 cm-1for complex 1;the presence of the Mn—N absorption peak at 646 cm-1for complex 1 indicates that the nitrogen atom of L2is coordinated to the manganese ion[18].

Fig.3 IR spectra of complexes 1,2,and the ligands

The IR spectrum of complex 2 is shown in Fig.3b.The O—H stretching vibration peaks for the carboxyl group of H2L1shown at 3 438 and 3 093 cm-1were shown again at both 3 403 and 3 051 cm-1for complex 2,which indicates that the carboxyl group in H2L1is coordinated to the cobalt ion.The C=N stretching vibration peak for L3at 1 587 cm-1shifted to 1 608 cm-1after the formation of complex 2;the Co—N absorption peak shown at 534 cm-1for complex 2 indi‑cates that the nitrogen atom of L3undergoes coordina‑tion with the cobalt ion[19‑20].

2.3 UV‑Vis and fluorescence spectra analysis

The UV‑Vis and fluorescence spectra of complex‑es 1 and 2 and ligands H2L1,L2,and L3were measured at room temperature.The samples were dissolved in DMF at a concentration of 10 μmol·L-1as a reference solution.The UV‑Vis spectra of complexes 1 and 2 and ligands H2L1,L2,and L3are shown in Fig.4.Ligands H2L1,L2,and L3and their complexes all showed an absorption peak around 270 nm,which can be attribut‑ed to the B‑band whereπ‑π*leap occurs on the intra‑molecular heterocyclic ring according to the molecular structure analysis.

Fig.4 UV‑Vis spectra of complexes 1,2,and the ligands

The fluorescence emission spectra of complexes 1 and 2 and their ligands are shown in Fig.5.The struc‑tures of ligands H2L1,L2,L3,and complexes 1 and 2 all contain heterocyclic conjugatedπ‑bonds and are prone to fluorescence.The wavelengths of the maximum emis‑sion peaks of ligands H2L1,L2,and L3were 348,341,and 362 nm(λex=305,265,289 nm,respectively),which are attributed toπ‑π*electron leap;while the wavelengths of the maximum emission peaks of com‑plexes 1 and 2 were 355 and 365 nm(λex=322,319 nm,respectively),which were both red‑shifted com‑pared to the ligands.This may be because the energy level of the HOMO of the complex decreases and the energy level difference between the HOMO and the LUMO becomes larger after the coordination of the ligand with the metal ion;while the metallization effect elevates the HOMO energy level of the complex and the energy level difference between the HOMO and the LUMO decreases[21].The maximum emission wave‑lengths of complexes 1 and 2 were extremely similar,indicating that the fluorescence properties of both com‑plexes are mainly based on the luminescence of L2-1itself in them.The main ligand H2L1had a maximum excitation wavelength of 305 nm with a Stokes shift(Δλ)of 43 nm;the maximum excitation wavelength of complex 1 was 322 nm with a Δλof 33 nm;the maxi‑mum excitation wavelength of complex 2 was 319 nm with a Δλof 46 nm.Compared with the main ligand H2L1,the Stokes shifts of complexes 1 and 2 were smaller and the fluorescence efficiency was higher.

Fig.5 Fluorescence emission spectra of complexes 1,2,and the ligands

2.4 TGA of the complexes

The TG and differential thermogravimetry(DTG)curves of complex 1 are shown in Fig.6a.Complex 1 remained stable until 290℃,after which the skeleton began to collapse continuously as the temperature increased until about 440℃,when the skeleton collapsed completely,with a final residual rate of 12.91% .According to the common sense of coordina‑tion,the residue might be MnO,and the calculated val‑ue was 12.87% ,which was consistent with the actual residual rate.As shown in Fig.6b,complex 2 hardly decomposed before 130℃,and the first weight loss of 2.05% occurred from 130 to 175℃,which is attributed to the decomposition of the coordinated water molecule and was consistent with the calculated value of 3.10% .After that,with the increase in temperature,the skele‑ton of the complex collapsed continuously,and at about 596℃,the skeleton collapsed completely.The final residual rate was 27.27% ,and according to the com‑mon sense of coordination,the residue might be CoO(Calcd.12.94% ).The residual rate was greater than the calculated value,indicating that complex 2 was protect‑ed by N2.The thermal decomposition yielded N‑doped carbon material,which produced the carbon accumula‑tion effect.In summary,both two complexes have cer‑tain thermal stability.

Fig.6 TG and DTG curves of complexes 1 and 2

3 Conclusions

The organic 2,5‑dibromoterephthalic acid was used as the primary ligand and 2,2′‑bipyridine and 1,10‑phenanthroline as the secondary ligands,and com‑plexes[Mn2(L1)2(L2)2(H2O)2]n(1)and[Co2(L1)2(L3)2(H2O)2]n(2)were synthesized with manganese sulfate monohy‑drate and cobalt nitrate hexahydrate,respectively,using a solvothermal method.The crystal structure analysis shows that complex 1 is an infinitely extended 2D network‑like structure formed by L12-and L2coordi‑nation linking Mn2+ion,and each layer forms a 3D net‑work‑like structure under intermolecular hydrogen bonding andπ‑πstacking.Complex 2 is an infinitely extended 2D network‑like structure formed by L12-and L3coordination linking Co2+ion,and the layers are stacked under intermolecular hydrogen bonding andπ‑πstacking to form a 3D network‑like structure.Both 1 and 2 have good fluorescence properties and thermal stability.