Mn（Ⅱ）/Ni（Ⅱ） Complexes Based on 5-Methyl-1H-pyrazole-3-carboxylic Acid:Syntheses,Structures,Electrochemical and Luminescent Properties

LIU LuCHENG Mei-Ling＊, TANG Li-Zhi-PengLIU Zheng LIU Qi,2

（1School of Petrochemical Engineering,Advanced Catalysis and Green Manufacturing Collaborative Innovation Center,Changzhou University,Changzhou,Jiangsu 213164,China）

（2State Key Laboratory of Coordination Chemistry,Nanjing University,Nanjing 210093,China）

Abstract:In the absence/presence of che1ating N anci11ary 1igand,2,2′-bipyridine（2,2′-bpy）,treatment of 5-methy1-1H-pyrazo1e-3-carboxy1ic acid（H2MPCA）with corresponding Mn（Ⅱ）/Ni（Ⅱ） meta1 sa1ts afforded two comp1exes,[Mn（HMPCA）2（H2O）2]（1）and[Ni（HMPCA）2（2,2′-bpy）]·2H2O（2）.In mononuc1ear comp1exes 1 and 2,each meta1 ion 1ocates in a distorted octahedra1 geometry,and the HMPCA−1igands in 1 and 2 both act as a N,O-che1ating 1igand.In 1,by the function of intermo1ecu1ar N—H…O and O—H…O hydrogen bonds,the independent[Mn（HMPCA）2（H2O）2]units are packed into a porous 3D supramo1ecu1ar structure with 1D nanotube.In 2,the mononuc1ear compo-nents[Ni（HMPCA）2（2,2′-bpy）]and the 1attice water mo1ecu1es are inter1inked via intermo1ecu1ar O—H…O and N—H…O hydrogen bonds,generating a 1D chain1ike structure,which is 1inked to adjacent ones through intermo1ecu1ar π…π interactions forming a 3D supramo1ecu1ar architecture.The e1ectrochemica1 and 1uminescent properties of comp1exes 1 and 2 have a1so been investigated.CCDC:2033945,1;2033946,2.

Keywords:5-methy1-1H-pyrazo1e-3-carboxy1ic acid;synthesis;crysta1 structure;e1ectrochemica1 property;1uminescent

0 Introduction

Much attention has been focused on new functiona1 supramo1ecu1ar comp1exes assemb1ed by cova1ent bonds and non-cova1ent forces,such as hydrogen bonds and π … π interactions[1-4].The interest comes from their fascinating structura1 features and potentia1 app1ications in cata1ysis,separation,gas storage,1umi-nescence,magnetism,1ithium -ion batteries,informa-tion storage and so forth[5-10].Pyrazo1ecarboxy1ic acids,such as 1H-pyrazo1e-4-carboxy1ic acid,3-methy1-1H-pyrazo1e-4-carboxy1ic acid,3,4-pyrazo1edicarboxy1ic acid,3,5-pyrazo1edicarboxy1ic acid and 5-methy1-1H-pyrazo1e-3-carboxy1ic acid（H2MPCA）have emerged as a kind of promising 1igands for the design of supramo-1ecu1ar comp1exes due to its good coordination abi1ities in mu1ti-coordination modes by the N and O donor atoms on the pyrazo1e rings and the carboxy1ic groups[11-19].The nitrogen atoms and carboxy1ic oxygen atoms can not on1y coordinate with meta1 ions to form monodentate and/or mu1tidentate M—N and M—O bonds,but a1so act as a donor and/or acceptor and provide intermo1ecu1ar hydrogen bond interactions for assemb1ing the comp1exes into high-dimensiona1 supra-mo1ecu1ar networks.Among which,H2MPCA consists of a pyrazo1e ring and its carboxy1 and methy1 groups cou1d form various comp1exes with interesting struc-tures[19-28],in which,the HMPCA−/MPCA2−anions exhib-it many different coordination modes:N,O-che1ating fashion,O,O′-che1ating mode,μ2-κN,O:κO mode,μ2-κN,O:κO,O′mode,μ2-κN,O:κN′mode and μ3-κN,O:κO,O′:κO′mode.On the other hand,the N anci1-1ary 1igand,2,2′-bipyridine（2,2′-bpy）,was a1so uti-1ized in the synthesis processes of supramo1ecu1ar com-p1exes,which not on1y can adjust the coordination structures by occupied the termina1 position[29-30],but a1so affect the supramo1ecu1ar structures by invo1ved in hydrogen bonds and π…π interactions.But on1y one Cd（Ⅱ） comp1ex[Cd（HMPCA）2（2,2′-bpy）]·2H2O[24]and a Cu （Ⅱ） comp1ex[Cu2（4,4′-bpy）2（2,2′-bpy）（MPCA）2]·6H2O[22]constructed from H2MPCA and 2,2′-bpy 1igands have been reported.As the continuation of our research in constructing functiona1 meta1 comp1exes containing pyrazo1ecarboxy1ic acids[14-15,24-28],two new transition meta1 comp1exes, [Mn（HMPCA）2（H2O）2] （1） and[Ni（HMPCA）2（2,2′-bpy）]·2H2O（2）have been synthe-sized through the se1f-assemb1y of H2MPCA with corre-sponding meta1 sa1ts in the absence/presence of che1at-ing N 1igand,2,2′-bpy.In this paper,the syntheses,crysta1 structures,e1ectrochemica1 and 1uminescent properties of comp1exes 1 and 2 were described.

1 Experimental

1.1 Materials and methods

A11 so1vents and starting materia1s for the synthe-ses were purchased commercia11y and were used as received.H2MPCA was prepared fo11owing the 1itera-ture methods[21,31].The e1ementa1 ana1yses（C,H and N）were performed on a Perkin-E1mer 2400 Series Ⅱ e1ement ana1yzer.FTIR spectra were recorded on a Nico1et 460 spectrophotometer in the form of KBr pe11ets.Powder X-ray diffraction（PXRD）determina-tions were performed on an X-ray diffractometer（D/max 2500 PC,Rigaku）with Cu Kα radiation（λ =0.154 06 nm）.The operating vo1tage and current were 60 kV and 300 mA,respective1y,and the measure-ments were carried out over a 2θ range of 3°～80°.Sing1e-crysta1 X-ray diffraction measurements of 1 and 2 were performed on a Bruker Apex Ⅱ diffractometer at 293（2）K.Thermogravimetric ana1yses（TGA）were carried out on a Dupont therma1 ana1yzer from room temperature to 800 ℃ under N2atmosphere at a heat-ing rate of 10 ℃ ·min−1.Cyc1ic vo1tammogram（CV）curves were recorded on a CHI 660D e1ectrochemica1 workstation.The 1uminescent spectra were recorded at room temperature on a Sahimadzu RF-5301PC f1uores-cence spectrof1uorometer.

1.2 Preparation of[Mn（HMPCA）2（H2O）2]（1）

A so1ution of H2MPCA（0.025 2 g,0.20 mmo1）in 4 mL EtOH was mixed with an aqueous so1ution（5 mL）of MnC12·4H2O（0.019 7 g,0.10 mmo1）.After stirred for 2 h,the so1ution was fi1tered,and the fi1trate was a11owed to stand at ambient temperature for one week.Co1or1ess b1ock crysta1s of 1 suitab1e for X-ray diffrac-tion ana1ysis were obtained.Yie1d:68%（0.023 2 g,based on Mn）. E1ementa1 ana1ysis Ca1cd. for C10H14MnN4O6（%）:C,35.20;H,4.10;N,16.41.Found（%）:C,35.38;H,4.28;N,16.70.IR（KBr disk,cm−1）:3 410（m）,3 172（m）,3 140（m）,3 084（m）,2 974（m）,2 942（m）,2 857（m）,1 629（vs）,1 567（s）,1 503（w）,1 426（s）,1 384（w）,1 335（s）,1 293（s）,1 112（w）,1 020（s）,859（m）,816（m）,683（m）,648（w）,572（w）,466（m）.

1.3 Preparation of[Ni（HMPCA）2（2,2′-bpy）] ·2H2O（2）

H2MPCA（0.025 2 g,0.20 mmo1）,Ni（OAc）2·4H2O（0.049 8 g,0.20 mmo1）,KOH（0.022 4 g,0.40 mmo1）and 2,2′-bpy（0.031 2 g,0.20 mmo1）were disso1ved in 10 mL deionized water.After stirring for 30 min,the resu1ting green suspension was p1aced into a 25 mL Tef1on-1ined autoc1ave and heated at 180 ℃ for one day,then coo1ed to room temperature at a rate of 5℃·h−1.After fi1tration,the product was washed with disti11ed water and then dried in vacuo,then purp1e crysta1s of 2 suitab1e for X-ray diffraction ana1ysis were obtained.Yie1d:65%（0.032 6 g,based on H2MPCA）.E1ementa1 ana1ysis Ca1cd.for C20H22N6O6Ni（%）:C,47.93;H,4.39;N,16.76.Found（%）:C,47.90;H,4.18;N,16.50.IR（KBr disk,cm−1）:3 423（s）,3 166（s）,3 140（s）,3 074（s）,2 932（s）,2 857（m）,1 626（vs）,1 566（s）,1 494（s）,1 425（s）,1 274（s）,1 206（w）,1 149（m）,1 025（s）,838（m）,776（s）,714（s）,645（m）,563（m）,446（w）.

1.4 X-ray crystallography

The structures were so1ved by direct methods using the program SHELXS-2013 and refined with SHELXL-2013[32].Anisotropic therma1 factors were assigned to a11 the non-hydrogen atoms.H atoms at-tached to C were p1aced geometrica11y and a11owed to ride during subsequent refinement with an isotropic disp1acement parameter fixed at 1.2 times Ueqof the parent atoms.A11 other hydrogen atoms bonded to O or N atoms were 1ocated from difference Fourier maps and refined with isotropic therma1 parameters 1.5 times those of their carrier atoms.The crysta11ographic data and refinement parameters for 1 and 2 are 1isted in Tab1e 1.

CCDC:2033945,1;2033946,2.

Table 1 Crystallographic data and refinement parameters for complexes 1 and 2

2 Results and discussion

2.1 Synthesis

Treatment of MnC12·4H2O and H2MPCA in 1∶2 mo1ar ratio with 9 mL mixed so1vent of EtOH and H2O（4∶5,V/V）afforded a co1or1ess so1ution.Co1or1ess bu1k crysta1s of 1 were iso1ated from so1vent evaporation at room temperature.When the 2,2′-bpy 1igand was intro-duced,purp1e crysta1s of Ni（Ⅱ） comp1ex 2 were synthe-sized by hydrotherma11y reaction of Ni（OAc）2·4H2O,H2MPCA,2,2′-bpy and KOH in 1∶1∶1∶2 mo1ar ratio at 180℃for one day.Comp1exes 1 and 2 were re1ative1y air-and moisture-stab1e.The e1ementa1 ana1yses of 1 and 2 were consistent with their chemica1 formu1ae.The identities of 1 and 2 were fina11y confirmed by X-ray crysta11ography.

2.2 Infrared spectrum

In the IR spectra of 1 and 2,the strong and broad absorption bands around 3 200～3 600 cm-1region in them are assigned as characteristic peaks of O—H vibration,indicating that water mo1ecu1es exist in them.The sharp bands at 3 140 cm−1in 1 and 2 are assigned to N—H vibration.The absence of absorption peaks in a range of 1 690～1 730 cm-1show that a11 car-boxy1ic groups are deprotonated in 1 and 2.Strong peaks at 1 629 cm−1（1）,1 626 cm−1（2）and 1 426 cm−1（1）,1 425 cm−1（2）may be assigned to the νas（OCO）and νs（OCO）stretching vibration of HMPCA−1igand.The intense bands at 1 330～1 360 cm−1are ascribed to the conjugated C=N stretching vibration.

2.3 Crystal structure description of 1

Fig.1 （a）Mo1ecu1ar structure of 1 with therma1 e11ipsoid at 30% probabi1ity 1eve1,where the hydrogen atoms attached to C atoms are omitted for c1arity;（b）Two types of intermo1ecu1ar hydrogen bonding interactions in 1,where on1y hydrogen atoms invo1ved in the hydrogen bonds are shown;（c）1D nanotube structure in 1 viewed a1ong c axis;（d）Perspective view of 3D supramo1ecu1ar structure with 1D channe1s of 1

In addition,there are two types of intermo1ecu1ar hydrogen bonds in 1:（ⅰ）hydrogen bonds between the oxygen atoms（donor）from coordinated water mo1ecu1es and oxygen atoms（acceptor）from HMPCA-1igands:O3—H3X…O2iiiand O3—H3Y…O2iv（Fig.1b,Tab1e S2）;（ⅱ）hydrogen bonds of the uncoordinated nitrogen atoms（donor）of HMPCA-1igands with oxygen atoms（acceptor）from HMPCA-1igands:N2—H2…O1iiand N2—H2…O2ii.As shown in Fig.1d,by the function of these N—H…O and O—H…O hydrogen bond interac-tions,the adjacent[Mn（HMPCA）2（H2O）2]units are packed into an interesting 3D microporous framework with 1D nanotube structure（Fig.1c）,and the nanotube radius is about 0.164 0（8）nm.

2.4 Crystal structure description of 2

The sing1e-crysta1 X-ray diffraction ana1ysis re-vea1s that comp1ex 2 crysta11izes in monoc1inic P21/c space group.As shown in Fig.2a,the asymmetric unit of 2 consists of one Ni（Ⅱ）cation,two HMPCA-anions,one 2,2′-bpy 1igand and two 1attice water mo1ecu1es.Each Ni（Ⅱ） ion 1ocates in a distorted octahedra1 geome-try,hexa-coordinated by two oxygen atoms（O1 and O3）and two nitrogen atoms（N1 and N3）from two HMPCA−1igands,another two nitrogen atoms（N5 and N6）from one 2,2′-bpy mo1ecu1e.The bond ang1es of O1—Ni1—N1,O3—Ni1—N1,O3—Ni1—N6 and N6—Ni1—O1 are added up to 360.7°（Tab1e S1）,showing that O1,N1,O3 and N6 atoms are in the equatoria1 position.Moreover,the bond ang1e of N5—Ni1—N3 is 171.2（1）°,deviating from 180°,indicating that the geometry around Ni1 center disp1ays distorted octahedra1 geome-try.The mean Ni—OHMPCAbond distance（0.207 6（1）nm）and the mean Ni—NHMPCAbond distance（0.205 7（2）nm）,are very c1ose to those in the mononuc1ear com-p1ex[Ni（HMPCA）2（H2O）2]（0.207 4（2）and 0.206 6（3）nm）[25].The mean Ni—Nbpydistance is 0.206 0（2）nm,and the bond ang1es around Ni（Ⅱ）are in a range of 78.11（9）°～171.18（1）°.

As a bidentate 1igand,HMPCA−and 2,2′-bpy both act as che1ating 1igands in 2.Meanwhi1e,the HMPCA−1igand act as proton donor and acceptor,and provide two kinds of intermo1ecu1ar hydrogen bonds:（ⅰ） N—H…O hydrogen bonds between uncoordinated pyrazo1e ring N atoms and 1attice H2O mo1ecu1es:N2—H2…O5i（N2…O5i0.274 2（4）nm）,N4—H4…O6ii（N4…O6ii0.269 8（4）nm）（Fig.2b,Tab1e S2）;（ⅱ）O—H…O intermo1ecu1ar hydrogen bonds between oxygen atoms（donor）from 1attice water mo1ecu1es and oxygen atoms（acceptor）from HMPCA-1igands,O5—H5X…O4iii（O5…O4iii0.281 4（4）nm）,O5—H5Y…O3iv（O5…O3iv0.275 5（3）nm）,O6—H6X…O1ii（O6…O1ii0.284 0（3）nm）,O6—H6Y…O2v（O6…O2v0.275 1（3）nm）.The mononuc1ear[Ni（HMPCA）2（2,2′-bpy）]components and the 1attice water mo1ecu1es are inter1inked via the above interactions,resu1ting in the formation of a 1D chain（Fig.2c）.As shown in Fig.2d,there are two inter-mo1ecu1ar π…π interactions（Cg1…Cg2ix0.388 6（2）nm,Cg2…Cg1x0.388 6（2）nm;Symmetry codes:ixx,1/2−y,1/2+z;xx,1/2−y,1/2+z）in 2.Cg1 and Cg2 refer to the ring centroids of the pyridy1 rings（Cg1:N5—C11—C12—C13—C14—C15 and Cg2:N6—C16—C17—C18—C19—C20）of 2,2′-bpy 1igands.The dihe-dra1 ang1e between the pyridy1 rings Cg1 and Cg2ixis 11.1°.By the function of these π…π interactions,the adjacent 1D chains are 1inked to form a 3D supramo-1ecu1ar architecture（Fig.2e）.Fina11y,the weak C—H…O hydrogen bonds between carbon atoms（C1,C12 and C17）from the HMPCA-anion/2,2′-bpy 1igands and car-boxy1ate oxygen atoms（O2 and O4）a1ong with 1attice water mo1ecu1e（O5）,further stabi1ize the 3D structure.

Fig.2 （a）Coordination environment of Ni（Ⅱ）ion in 2 with therma1 e11ipsoid at 30% probabi1ity 1eve1,where 1attice water mo1ecu1es and the hydrogen atoms attached to C atoms are omitted for charity;（b）Intermo1ecu1ar hydrogen bonding interactions in 2,where the 2,2′-bpy 1igand coordinated to Ni1 is omitted for c1arity;（c）1D chain structure constructed by O—H…O and N—H…O hydrogen bonds in 2;（d）π…π interactions between the pyridy1 groups of the 2,2′-bpy 1igands;（e）3D supramo1ecu1ar structure in 2 constructed by hydrogen bonds and π…π interactions

2.5 PXRD and thermal analysis

In order to check the phase purity of 1 and 2,the PXRD patterns were recorded at room temperature.As shown in Fig.S1,the experimenta1 PXRD pattern for each comp1ex corre1ated we11 with its simu1ated one generated from sing1e-crysta1 X-ray diffraction data,confirming the phase purity of the bu1k materia1s of 1 and 2.

TGA were carried out in the interest of studying the therma1 stabi1ity of two comp1exes.Under nitrogen atmosphere,the experiment was carried out from ambi-ent temperature up to 800℃with the heating rate of 10 ℃ ·min−1.For 1,the first weight 1oss of 10.26% between 93 and 130℃is attributed to the 1oss of two coordinated water mo1ecu1es（Ca1cd.10.56%）（Fig.S2）.The second weight 1oss stage between 400 and 618℃corresponds to the 1oss of two HMPCA-1igands.The pyro1ysis product was MnO2（Ca1cd.25.48%;Obs.25.02%）.For 2,the first weight 1oss of 7.23%,which occurred from 49 to 121℃,corresponds to the re1ease of two 1attice water mo1ecu1es（Ca1cd.7.18%）.Above 300℃,the remaining materia1 was gradua11y decom-posed,and the pyro1ysis product was NiO（Ca1cd.15.30%;Obs.14.74%）.

2.6 Electrochemical property

The e1ectrochemica1 properties of 1 and 2 were in-vestigated using a three-e1ectrode e1ectrochemica1 ce11.The Ni foam coated with 1 and 2 were used as the work-ing e1ectrode,which was fabricated fo11owing the 1itera-ture methods[33-34].The p1atinum foi1 and a saturated ca1ome1 e1ectrode（SCE）were used as the counter and reference e1ectrodes,respective1y.The e1ectrochemica1 measurements were carried out in a 2 mo1·L−1KOH aqueous e1ectro1yte at room temperature.CV curves of comp1exes 1 and 2 are shown in Fig.3.During scanning from 0.05～0.3 V at a rate of 20 mV·s−1,the CV curves on1y had an oxidation -reduction peak,which corre-sponds to M（Ⅱ）/M（Ⅲ）（M=Mn and Ni）redox process.For 1,Epa=0.28 V,Epc=0.14 V,ΔE=0.14 V;for 2,Epa=0.26 V,Epc=0.15 V,ΔE=0.11 V.The resu1ts show that e1ec-tron transfer of M（Ⅱ）between M（Ⅲ）in e1ectro1ysis is quasi-reversib1e process.

Fig.3 CV curves of 1 and 2 e1ectrodes in 2 mo1·L−1KOH aqueous

2.7 Luminescent properties

The so1id-state 1uminescent properties of free H2MPCA 1igand and comp1exes 1 and 2 were investi-gated at room temperature.As shown in Fig.4,H2MPCA and two comp1exes exhibit b1ue f1uorescence with emis-sion maxima1 at 441,440 and 440 nm upon excitation at 331 nm,respective1y.These emissions may be assigned to the intra1igand（π-π*）transfer.Meanwhi1e,it is c1ear that comp1exes 1 and 2 exhibit weaker emis-sions compared with the free H2MPCA 1igand,and this kind of quenching phenomenon shou1d be re1ated to the 1igand-fie1d transitions（d-d）[35-36].

Fig.4 So1id-state emission spectra for comp1exes 1,2 and 1igand H2MPCA at room temperature

3 Conclusions

We have synthesized two new comp1exes,[Mn（HMPCA）2（H2O）2]（1）and[Ni（HMPCA）2（2,2′-bpy）]·2H2O（2）,via genera1 so1ution and hydrotherma11y syn-thetic methods,respective1y.In the mononuc1ear struc-tures of 1 and 2,the HMPCA−groups adopt a N,O-che1ating coordination mode.In the function of intermo-1ecu1ar N—H…O and O—H…O hydrogen bonds,the independent components in 1 and 2 are extended to a porous 3D supramo1ecu1ar structure with 1D nanotube structure（1）and a 1D chain1ike structure（2）.The 1D chains in 2 are further connected by the intermo1ecu1ar π…π interactions to form a 3D supramo1ecu1ar archi-tecture.In addition,e1ectrochemica1 properties of 1 and 2 show that e1ectron transfer of M（Ⅱ） between M（Ⅲ）（M=Mn and Ni）in e1ectro1ysis is quasi-reversib1e pro-cess.Two comp1exes disp1ayed b1ue f1uorescence in the so1id state at room temperature.

Supporting information is avai1ab1e at http://www.wjhxxb.cn