黎 彧 鄒訓(xùn)重 馮安生 趙振宇

(1廣東輕工職業(yè)技術(shù)學(xué)院，廣東省特種建筑材料及其綠色制備工程技術(shù)研究中心/佛山市特種功能性建筑材料及其綠色制備技術(shù)工程中心，廣州 510300)

(2深圳信息職業(yè)技術(shù)學(xué)院智能制造與裝備學(xué)院，深圳 518172)

0 Introduction

In recent years,great interest has been focused on the design and hydrothermal syntheses of functional coordination polymers owing to their intriguing architectures and topologies,as well as potential applications in catalysis,magnetism,luminescence and gas absorption［1-10］.Up to now,a large numbers of coordination polymers have been obtained by hydrothermal methods,which are optimal for crystal growth［1,3-4,11-13］.The mechanism of the complicated reactions under hydrothermal methods remain unclear,which depends directly on the interplay of starting materials, pH value, template, and reaction temperature［14-18］.

In this regard,the selection of organic ligands is one of the most important aspects.Among a wide variety of organic ligands,various types of aromatic polycarboxylic acids have been proved to be versatile and efficient candidates for constructing diverse coordination polymers due to their rich coordination chemistry,tunable degree of deprotonation,and ability to act as H-bond acceptors and donors［15-16,18-22］.

On the basis of the above account,we selected 5-bromoisophthalic acid (H2BIPA)and investigated the influence of the reaction conditions on the structures of coordination polymers under hydrothermal conditions.

Herein,we report the syntheses,crystal structures,and magnetic properties of two Cu（Ⅱ）coordination compounds constructed from 5-bromoisophthalic acid ligand.

1 Experimental

1.1 Reagents and physical measurements

All chemicals and solvents were of AR grade and used without further purification.Carbon,hydrogen and nitrogen were determined using an Elementar Vario EL elemental analyzer.IR spectra were recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer.Thermogravimetric analysis(TGA)data were collected on a LINSEIS STA PT1600 thermal analyzer with a heating rate of 10℃·min-1.Magnetic susceptibility data were collected in a temperature range of 2～300 K with a Quantum Design SQUID Magnetometer MPMS XL-7 with a field of 0.1 T.A correction wasmade forthe diamagnetic contribution prior to data analysis.

1.2 Synthesis of[Cu(BIPA)(2,2′-bipy)(H2O)2]·H2O(1)

A mixture of CuCl2·2H2O (0.017 g,0.10 mmol),H2BIPA (0.024 g,0.10 mmol),2,2′-bipyridine (2,2′-bipy,0.016 g,0.1 mmol),NaOH(0.008 g,0.20 mmol)and H2O (8 mL)was stirred at room temperature for 15 min,and then sealed in a 25 mL Teflon-lined stainless steel vessel,and heated at 120℃for 3 days,followed by cooling to room temperature at a rate of 10℃·h-1.Blue block-shaped crystals of 1 were isolated manually,and washed with distilled water.Yield:58%(based on H2BIPA).Anal.Calcd.for C18H17BrCuN2O7(%):C 41.83,H 3.32,N 5.42;Found(%):C 42.05,H 3.34,N 4.39.IR (KBr,cm-1):3 444w,3 238m,1 593s,1543s,1476w,1420m,1354s,1248w,1174w,1091w,1 063w,1 030w,901w,763m,724m,661w,594w,546w.

1.3 Synthesis of[Cu3(μ3-BIPA)2(μ-OH)2(2,2′-bipy)2]n(2)

Synthesis of 2 was similar to 1 except using a different amount of NaOH(0.012 g,0.30 mmol).Blue block-shaped crystals of 2 were isolated manually,and washed with distilled water.Yield:38% (based on H2BIPA).Anal.Calcd.for C36H24Br2Cu3N4O10(%):C 42.26,H 2.36,N 5.48;Found(%):C 42.03,H 2.35,N 5.51.IR (KBr,cm-1):3 423w,3 055w,1 627m,1 604s,1 576m,1 555m,1 494w,1 427w,1 370m,1 326s,1 248 w,1 158w,1 119w,1 086w,1 024w,886w,767m,728m,657w,634w,546w.The compounds are insoluble in water and common organic solvents,such as methanol,ethanol,acetone and DMF.

1.4 Structure determinations

Two single crystals with dimensions of 0.26 mm×0.23 mm×0.22 mm (1)and 0.25 mm×0.18 mm×0.16 mm(2)were collected at 293(2)K on a Bruker SMART APEXⅡ CCD diffractometer with Mo Kα radiation(λ=0.071 073 nm).The structures were solved bydirect methods and refined by full matrix least-square on F2using the SHELXTL-2014 program［23］.All nonhydrogen atoms were refined anisotropically.All the hydrogen atoms were positioned geometrically and refined using a riding model.A summary of the crystallography data and structure refinements for 1 and 2 is given in Table 1.The selected bond lengths and angles for compounds 1 and 2 are listed in Table 2.Hydrogen bond parameters of compounds 1 and 2 are given in Table 3.

Table 1 Crystal data for compounds 1 and 2

CCDC:1922559,1;1922560,2.

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

Continued Table 1

Table 3 Hydrogen bond parameters of compounds 1 and 2

2 Results and discussion

2.1 Description of the structure

2.1.1 ［Cu(BIPA)(2,2′-bipy)(H2O)2］·H2O(1)

Single-crystal X-ray diffraction analysis reveals that compound 1 crystallizes in the monoclinic space group P21/c.Its asymmetric unit contains two mononuclear copper（Ⅱ）units and two lattice water molecules(Fig.1).In each ［Cu(BIPA)(2,2′-bipy)(H2O)2］unit,the Cu（Ⅱ）ions are five-coordinated and form a distorted square-pyramidal{CuN2O3}geometry with the τ parameters of 0.066 7 or 0.048 3(τ=0 or 1 for a regular square-pyramidal or trigonal-bipyramidal geometry,respectively)［24］.It is taken by a carboxylate O atom of BIPA2-,two H2O ligands,and two N donors from the 2,2′-bipy ligand.The Cu-O bonds(0.195 7(5)～0.224 2(4)nm)and the Cu-N distances(0.200 2(7)～0.202 6(6)nm)agree with literature data［3,22,25］.In 1,the BIPA2-moiety acts as a terminal ligand(modeⅠ,Scheme 1).Discrete mononuclear copper（Ⅱ）units are assembled,via the O-H…O hydrogen bonds,into a 2D H-bonded sheet(Fig.2 and Table 3).

Fig.1 Drawing of asymmetric unit of compound 1 with 30%probability thermal ellipsoids

Scheme 1 Coordination modes of BIPA2-ligands in compounds 1 and 2

Fig.2 Perspective of 2D sheet in 1

2.1.2 ［Cu3(μ3-BIPA)2(μ-OH)2(2,2′-bipy)2］n(2)

The asymmetric unit of 2 consists of two Cu（Ⅱ）ions (Cu1 with full occupancy and Cu2 with half occupancy),one μ3-BIPA2-block,one 2,2′-bipy ligand,and μ-OH-linker.As shown in Fig.3,five-coordinated Cu1 atom reveals a distorted square-pyramidal{CuN2O3}geometry with the τ parameters of 0.076 5,filled by two carboxylate O atoms from two individual μ3-BIPA2-blocks,one O atom from the μ-OH-linker,and a pair of N atoms from 2,2′-bipy ligand.The fourcoordinated Cu2 atom shows a distorted{CuN2O2}square-planar geometry,which is taken by two carboxylate O atoms from two different BIPA2-blocks and two O atoms from two individual μ-OH-linkers.The Cu-O lengths range from 0.190 2(4)to 0.264 9(4)nm,whereas the Cu-N lengths vary from 0.200 1(4)to 0.202 2(4)nm;these bonding parameters are comparable to those observed in other Cu（Ⅱ） compounds［3,22,25］.In 2,the BIPA2-block acts as a μ3-linker,and its COO-groups are monodentate or bidentate(modeⅡ,Scheme 1).The three adjacent Cu（Ⅱ）ions are bridged by means of two carboxylate groups from two different BIPA2-blocks and two μ-OH-linkers,giving rise to a trinuclear copper（Ⅱ）subunit(Fig.4).In this Cu3subunit,the Cu1…Cu2 distance is 0.344 1(4)nm.The neighboring Cu3subunits are multiply interlinked by BIPA2-blocks into a 1D chain (Fig.4),having the shortest distance of 0.985 2 nm between the adjacent trinuclear copper（Ⅱ）subunits.The intrachain (N1/C9-C13 and C2A-C7A,Cg…Cg 0.374 6(2)nm,Symmetry code:A:x+1,y,z)and interchain(N2/C14-C18 and C2B-C7B,Cg…Cg 0.352 1(2)nm,Symmetry code:B:x+1/2,-y+1/2,z+1/2)π-π stacking interactions between adjacent pyridyl planes of the 2,2′-bipy ligands and the benzene planes of BIPA2-blocks are observed(Fig.5).The chains are further extended into a 2D sheet by ππ stacking interactions(Fig.5).

Fig.3 Drawing of asymmetric unit of compound 2 with 30%probability thermal ellipsoids

Fig.4 One-dimensional chain viewed along c axis in 2

Fig.5 Two-dimensional sheet viewed along c axis in 2

2.2 TGA analysis

To determine the thermal stability of compounds 1 and 2,their thermal behaviors were investigated under nitrogen atmosphere by thermogravimetric analysis (TGA).As shown in Fig.6,compound 1 lost its one lattice and two coordinated water molecules in a range of 43～122 ℃ (Obsd.10.1%;Calcd.10.4%),followed by the decomposition at 234℃.The TGA curve of 2 reveals that compound 2 was stable up to 241℃,then was decomposed upon further heating.

Fig.6 TGA curves of compounds 1 and 2

2.3 Magnetic properties

Variable-temperature magnetic susceptibility studies were carried out on powder sample of 2 in a temperature range of 2～300 K (Fig.7).The χMT value at 300 K was 1.20 cm3·mol-1·K,which is slightly higher than the value (1.125 cm3·mol-1·K)expected for three magnetically isolated Cu（Ⅱ）centers(SCu=1/2,g=2.0).Upon cooling,the χMT value decreased to reach a plateau around 11 ～7 K with χMT value of 0.608 ～0.598 cm3·mol-1·K,and finally went down to 0.406 cm3·mol-1·K at 2 K.The plateau corresponds to the ground state (S=1/2).In the 15～300 K interval,the χM-1vs T plot for 2 obeys the Curie-Weiss law with a Weiss contant θ of-25.3 K and a Curie constant C of 1.26 cm3·mol-1·K.Although the separation between the adjacent Cu3subunits are somewhat longer,the magnetic exchange coupling mediated by spinpolarization mechanism through the intrachain and interchain π-π stacking interactions［26-28］.Because the magnetic exchange coupling through π-π stacking interactions is assumed to be weaker than the intratrinuclear interactions,the negative value of θ and the decrease in χMT should be attributed to the overall antiferromagnetic coupling between the Cu（Ⅱ）ions within the Cu3subunit.

The spin Hamiltonian appropriate for describing the magnetic properties of an isolated linear trinuclear system is given in Eq.(1):

Where J denotes the exchange parameter between the central and terminal copper（Ⅱ） ions,and J′is assumed to be zero since the distance between the two terminal Cu（Ⅱ）ions is so large(0.688 3 nm).The magnetic properties were analyzed using Eq.(2),derived from Eq.(1)for a linear trinuclear model with S=1/2［29］:

Fig.7 Temperature dependence of χMT(○)and 1/χM(□)vs T for compound 2

Where θ is a Weiss-like correction for intermolecular interactions,and Nαis temperature independent paramagnetism.Using this method,the susceptibilities for 2 above 15.0 K were simulated,and the best-fit parameters for 2 were obtained:J=-35.6 cm-1,g=2.11,θ=-0.47 K,Nα=3.60×10-4cm3·mol-1and R=5.2×10-5,where R=∑(Tobs-Tcalc)2/∑(Tobs)2.The J value of-35.6 cm-1indicates that the coupling between the adjacent Cu（Ⅱ）centers is antiferromagnetic.According to the structure of compound 2,there are two sets of magnetic exchange pathways within the trinuclear copper（Ⅱ）cores,namely,via the μ-OH-groups and μ-carboxylates bridges (Fig.4),which can be responsible for the observed antiferromagnetic exchange.

3 Conclusions

In summary,we have synthesized two Cu（Ⅱ）coordination compounds whose structures depend on the molar ratio between NaOH and H2BIPA.This work demonstrates that the molar ratio between NaOH and carboxylic acid ligand has a significant effect on the structures of Cu（Ⅱ）coordination compounds.