由不同長度柔性雙咪唑配體構(gòu)筑的鎳( II)配位聚合物

屠長征，繆嬌嬌，李俊莉，陳　廣，楊玉亭

(曲靖師范學院化學化工學院，云南曲靖655011)

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由不同長度柔性雙咪唑配體構(gòu)筑的鎳( II)配位聚合物

屠長征，繆嬌嬌，李俊莉，陳廣，楊玉亭1*

(曲靖師范學院化學化工學院，云南曲靖655011)

摘要:利用2，5-吡啶二酸分別與兩種具有不同長度的柔性雙咪唑配體，在同樣的水熱條件下合成了兩例結(jié)構(gòu)完全不同的鎳( II)配位聚合物Ni2( 2，5-pdc)2( L1)2( H2O)2·6H2O ( 1)，Ni( 2，5-pdc) ( L2) ( H2O)·H2O ［2，5-pdc = 2，5-吡啶二酸，L1= 1，4-二( 1-咪唑基)丁烷，L2= 1，6-二( 1-咪唑基)己烷］( 2)．配合物1是三維4-連Cd-SO4拓撲網(wǎng)格，Schl?fli符號為( 65·8)．而配合物2是由一維螺旋鏈［Ni( L2) ］n和一維［Ni( 2，5-pdc) ］n鏈構(gòu)筑而成的環(huán)-環(huán)相扣的一維鏈狀結(jié)構(gòu)．

關(guān)鍵詞:鎳( II)配合物;配位聚合物;晶體結(jié)構(gòu);拓撲;雙咪唑配體

The construction and synthesis of coordination polymers ( CPs) has recently attracted considerable attention，owing to their versatile framework topologies as well as their potential applications as functional materials in molecular magnetism，catalysis，gas sorption and fluorescent sensing［1－5］．It is well known that the factors，such as temperature，solvent，reactant concentration，pH value，coordination geometries of the central metals，configurations and flexibility of the organic ligands，all play important roles in the progress of constructing coordination polymers［6－7］．Therefore，in order to obtain the target frameworks with specific structures and functions，it is important to regulate the synthesis conditions and design the organic ligands．The flexible bis-imidazole ligands with different -( CH2)n－( n = 1，2，4，5，6，8) spacers have been proven to be suitable N-containing ligands for generating novel CPs［8－18］．Furthermore，recent studies have shown that the flexibility and length of the N-donor ligands play an significant role in the assembling of CPs withcharming structure［19］．To date，a large number of CPs based on flexible bis-( imidazole) ligands have been constructed．Our group has prepared several transition metal CPs based on the flexible bis-imidazole ligands with various -( CH2)n-spacers and different polycarboxylates，whichexhibit significant properties［11－17］．However，in the literature，the counter anions usually focus on the carboxylate-containing ligands，such as 1，3-benzenedicarboxylate，1，3，5-benzenetricarboxylate，1，2，4，5-benzenetetracarboxylate，and so on［18］．In contrast，the N-donor aromatic multicarboxylate ligands，such as pyridine-2，3-dicarbox-ylic acid ( 2，3-pdc)，pyridine-3，5-dicarboxylic acid ( 3，5-pdc)，and pyridine-2，5-dicarboxylic acid ( 2，5-pdc)，have rarely been reported［10－11］．

In this context，we chose pyridine-2，5-dicarboxylate ( 2，5-pdc) as the organic counter anion and two bis-( imidazole) -containing ligands，1，1'-( 1，4-butanediyl) bis ( imidazole) ( L1)，1，1'-( 1，6-hexanediyl) bis( imidazole) ( L2)，as the auxiliary ligands to combine with the metal ions．Fortunately，two new complexes namely，Ni2( 2，5-pdc)2( L1)2( H2O)2· 6H2O ( 1)，Ni( 2，5-pdc) ( L2) ( H2O)·H2O ( 2) were successfully obtained and characterized．Notably，1 and 2 are stable in the solid state upon extended exposure to air，and they have poor solubility in common organic solvents at room temperature，such as alcohol，acetonitrile，DMF，DMA and DMSO．Both 1 and 2 have been characterized by infrared spectroscopy，elemental analysis and thermogravimetric analysis．

1　Experimental

1．1 Materials and instruments

All the chemical reagents were purchased commercially and were used as received without further purification．The ligands L1and L2were synthesized according to the literature procedures［20］．The IR spectra were recorded as KBr pellets with a Nicolet Magna-FTIR 560 spectrometer in the 4 000－400 cm－1region．Elemental analyses for C，H and N were performed with a Perkin-Elmer 240 analyzer．Thermogravimetric analyses ( TGA) were performed with a Shimadzu TGA-50H TG analyzer in the range of 24－800℃under a nitrogen flow at a heating rate of 5℃/min for all measurements．

1．2Synthesis of Ni2( 2，5-pdc)2( L1)2( H2O)2· 6H2O( 1)

A mixture of Ni( NO3)26H2O ( 0．15 g，0．5 mmol)，2，5-pdc ( 0．084 g，0．5 mmol)，L1( 0．10 g，0．5 mmol) and water ( 6 mL) was placed in a Teflon-lined stainless steel vessel and the pH was adjusted to 6－6．5 by addition of dilute NaOH solution ( 1．0 mol/L)．The mixture was sealed and heated at 140℃for 72 h and then cooled to room temperature at a rate of 5℃/h．Green crystals were obtained in 68% yield based on Ni．Elemental analysis ( %) : calcd．C 42．00，H 5．18，N 14．41; found: C 42．04，H 5．14，N 14．50．Selected IR data ( KBr，ν) : 3 453 ( m)，2 950 ( vs)，1 611 ( s)，1 525( vs)，1 378 ( s)，1 359 ( s)，1 279 ( vs)，1 107 ( s)，1 086 ( s)，941 ( m)，766 ( m)，622 ( m) cm－1．

1．3 Synthesis of Ni( 2，5-pdc) ( L2) ( H2O)·H2O

The reaction conditions are similar to those described for 1 except that L2was used instead of L1．Green crystals were obtained in 62% yield based on Ni．Elemental analysis ( %) : calcd．C 47．73，H 5．27，N 14．65，found: C 47．78，H 5．21，N 14．63．Selected ( KBr，ν) : 3 509 ( m)，3 385 ( m)，1 634 ( s)，1 603 ( s)，1 560 ( vs)，1 524 ( vs)，1 465 ( vs)，1 448 ( s)，1 243 ( vs)，1 175( s)，1 109 ( vs)，1 084 ( s)，1 036 ( s)，946 ( s)，842 ( m)，815 ( m)，773 ( m) cm－1．

1．4 Crystal structure determination

Suitable single crystal of the title adduct was selected and mounted in air onto thin glass fibers．Accurate unit cell parameters of complexes 1 and 2 were determined by a least-squares fit of 2θ values，and intensity data were measured on a Bruker SMART area detector with Mo-Kαradiation (λ= 0．071 073 nm)．The intensity was corrected for Lorentz and polarization effects as well as for empirical absorption based on multi-scan technique［21］．The structure was solved by direct method and refined by full-matrix least-squares fitting on F2by SHELX-97［22］．All non-hydrogen atoms were refined anisotropically，the hydrogen atoms of organic ligand were located by geometrically．Table 1 shows the crystallographic crystal data of 1 and 2．Selected bond distances and angles are listed in Tables 2 and 3．CCDC 1061878 and 1061879 contain the supplementary crystallographic data for this paper．The data can be obtained free of charge from the CambridgeCrytallographic Data Centre via www．ccdc．cam．a(chǎn)c．uk/data request/cif．

Table 1　Crystallographic data for 1 and 2

Table 2　Selected bond lengths and bond angles for 1

Table 3　Selected bond lengths and bond angles for 2

2　Results and discussion

2．1 Structural description of Ni2( 2，5-pdc)2( L1)2( H2O)2·6H2O ( 1)

Single-crystal X-ray diffraction analysis reveals that complex 1 crystallizes in the triclinic crystal system and space group P1．The asymmetric unit of 1 contains two crystallographic independent Ni( II) ions，two fully deprotonated 2，5-pdc ligands，two ancillary L1ligands，two coordinated water molecules and six lattice water molecules( Fig．1a)．The Ni( 1) ion shows distorted octahedral geometry formed by two imidazole N atoms of two L1ligands ( Ni( 1)－N( 6) = 0．207 5( 4) nm，Ni( 1)－N( 7) = 0．207 4( 4) nm)，two carboxyl O atoms of two 2，5-pdc ligands ( Ni( 1)－O( 4) = 0．205 7( 3) nm，Ni( 1)－O( 8) = 0．206 3( 3) nm)，and two coordinated water molecules ( Ni( 1)－O( 9) = 0．208 4( 3) nm，Ni( 1)－O( 10) = 0．211 7( 3) nm)．The Ni( 2) ion is also six-coordinated by two O atoms and two N atoms from two 2，5-pdc anions，and two N atoms of two L1ligands to give a { NiN4O2} distorted octahedron．The bond distances for Ni( 2)－N and Ni ( 2)－O are in the ranges of 0．208 2( 3)－0．210 6( 4) nm and 0．206 4( 3)－0．206 7( 3) nm，respectively．The Ni－O/N bond lengths are comparable to other related Ni-based complexes［23］．

The completely deprotonated 2，5-pdc anion in 1 adopts only one kind of coordination mode，in which one oxygen atom of the 2-position carboxylate participates in chelating one Ni( II) ions with pyridyl nitrogen atom and the other 2-position oxygen atom is uncoordinated．The 5-position carboxylate adopts monodentate bridging (μ1－η1∶η0) mode，bridging two Ni( II) ions with a Ni…Ni separation of 0．723 4 nm．Thus，the Ni( II) ions are connected by 2，5-pdc anions to form 1D wavelike ［Ni( 2，5-pdc) ］nchains running along the c axis ( Fig．S1)．The L1ligand displays a trans configuration with a dihedral angle of 8．90°between the two imidazole rings．The trans－L1ligands link the neighboring Ni( 1) ions and Ni( 2) ions，respectively，to form the 1D ［Ni( L1) ］nchains with the Ni…Ni distance of 1．181 9 nm ( Fig．1b)，which are further linked by the 1D chains built from the 2，5-pdc anions，generating a complicated 3D framework ( Fig．1c)．And the void space is occupied by free water molecules，which are stabilized by typical O－H…O hydrogen bonds．

At the sight of topology，the final framework can be defined as a uninodal 4-connected CdSO4net with the Schl?fli symbol of ( 65·8)，by denoting the Ni( 1) and Ni( 2) atoms to four-connected nodes and 2，5-pdc and L1ligands as spacers．( Fig．1d)．

2．2Structural description of Ni( 2，5-pdc) ( L2) ( H2O)·H2O ( 2)

When the L2ligand was incorporated into the reaction system instead of the L1ligand，an infinite 1D complex was obtained．Single crystal X-ray diffraction analysis indicates that 2 crystallizes in monoclinic crystal system and P21/c space group．The asymmetric unit contains one Ni( II) ion，one 2，5-pdc anion，one L2ligand，as well as one lattice water molecule．The crystallographically independent Ni( II) ion is six-coordinated by one pyridine N1 atom from 2，5-pdc anion ( Ni ( 1)－N( 1) = 0．206 2( 2) nm)，two imidazole nitrogen atoms from two L2ligands ( Ni ( 1)－N ( 2) = 0．207 9( 2) nm，Ni( 1)－N( 4) = 0．208 7( 2) nm)，two carboxyl oxygen atoms from two 2，5-pdc anions ( Ni( 1)－O( 1) = 0．206 44( 18) nm，Ni( 1)－O( 4) = 0．210 17( 19) nm)，and one coordination water molecule ( Ni( 1)－O( 5) = 0．208 24( 19) nm)，exhibiting a distorted octahedral geometry { NiN3O3} ( Fig．2a)．

In 2，the 2，5-pdc ligand adopts the same coordination mode with complex 1．The neighboring Ni( II) ions were linked by the carboxyl groups of 2，5-pdc to construct a 1D ［Ni( 2，5-pdc) ］wavelike chain with the Ni…Ni distance of 0．855 1 nm ( Fig．S2)．The bidentate L2ligands display a cis configuration with the dihedral angle of 80．18°between the two imidazole rings．It is notable that the cis-L2ligand links Ni( II) ions，generating 1D ［Ni( L2) ］nhelical chains along the b direction ( Fig．2b)．The ［Ni( 2，5-pdc) ］nchains and ［Ni( L2) ］nchains are combined by Ni ( II) ions to give the 1D circle-connecting-circle chain ( Fig．2c)．

As shown in Fig．S3，the adjacent 1D double chains are extended to a 2D supramolecular network by the weak hydrogen bonds between lattice water molecules，coordinated-water molecules and carboxyl oxygen atoms ( O( 6)－H( 6A)…O( 3)，O( 6)－H( 6B) …O( 2)，O( 5)－H( 5A)…O( 1)，O( 5)－H( 5B) …O( 3) )．

Fig．1 ( a) Coordination environment of Ni( 1) and Ni( 2) ions in 1．The hydrogen atoms are omitted for clarity，( b) View of 1D wavelike chain formed by L1ligands and Ni( 1) ( or Ni( 2) ) ions，( c) View of the 3D network，( d) ( 4，4) -connected topological net of complex 1

Fig．2 ( a) Coordination environments of Ni( II) ions in 2 ( symmetry codes: #1－x+1，y－1/2，－z+1/2; #2－x+1，y+1/2，－z+1/2)，( b) View of the 1D ［Ni( L2) ］nhelical chain in 2，( c) The 1D circle-connecting-circle chain of 2

2．3 Thermogravimetric analysis

Complexes 1 and 2 are stable at ambient conditions，and thermogravimetric experiments were performed to explore their thermal stabilities ( Fig．3)．For 1，the first weight loss occurs in the range 160 to 270℃，which is attributed to the loss of six free water molecules per formula ( obsd 11．16%，calcd 11．11%)，followed by a plateau region ranging from 270 to 360℃，and consecutive decompositions suggest the total destruction of the framework．The TGA of 2 shows the loss of free water and coordinated molecules at 80－170 ℃( obsd 7．48%，calcd 7．53%) and after that，the further weight loss is attributed to the decomposition of complex 2．

Fig．3 The TGA diagram of 1 and 2

3　Conclusion

In summary，we have successfully isolated two new Ni( II) complexes with distinct structural features based on the assembly of flexible bis-imidazole ligands and pyridine-2，5-dicarboxylic acid．Complex 1 exhibits 3D 4-connected CdSO4type topology with the Schl?flisymbol of ( 65·8)，while complex 2 possesses a 1D circle-connecting-circle chain structure．The structural analysis indicates that the spacer of flexible bis-imidazole ligands can greatly affect the motif of complex．

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