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CrystEngComm, 2010, 12, 260–269

Hydrothermal syntheses,crystal structures and luminescent properties of zinc(II)coordination polymers constructed by bifunctional tetrazolate-5-carboxylate ligands?

Mei-Feng Wu,ab Fa-Kun Zheng,*a A-Qing Wu,a Yan Li,a Ming-Sheng Wang,a Wei-Wei Zhou,ab Feng Chen,ab Guo-Cong Guo*a and Jin-Shun Huang a

Received18th May2009,Accepted24th August2009

First published as an Advance Article on the web7th September2009

DOI:10.1039/b909754f

Two bifunctional1H-tetrazolate-5-carboxylate ligands with different?exibilities,H2tza

(1H-tetrazolate-5-acetic acid)and H2tzf(1H-tetrazolate-5-formic acid),were employed in the

construction of zinc(II)complexes in the presence/absence of secondary ligands such as2,20-bipy and

4,40-bipy.Three tza coordination polymers and two tzf dinuclear complexes,namely[Zn(tza)(H2O)]n

(1),[Zn3(tza)2Cl2(2,20-bipy)2(H2O)2]n(2),[Zn2(tza)2(4,40-bipy)]n(3),[Zn(tzf)(H2O)3]2$2H2O(4)and

[Zn(tzf)(2,20-bipy)(H2O)]2$H2O(5),were hydrothermally synthesized and structurally characterized by

single-crystal X-ray diffraction.Polymer1is a3-D two-fold interpenetrating diamond-like network

with tetranuclear[Zn(COO)]4clusters as tertiary building units(TBUs)and m3-k N3:k O2:k O1,N1

bridging tza as linkers.Polymer2presents a2-D structure formed by the linkage of1-D Zn-carboxylate

[Zn(COO)]n helical chains and m3-k N4:k O2:k O1,N1bridging tza.In3,2-D(4,4)nets are built up with

dinuclear[Zn(COO)]2clusters and m3-k N4:k O2:k O1,N1bridging tza,which are pillared through4,40-

bipy to gain a3-D self-penetrating LB-1(446108)topology.In dinuclear complexes4and5,two Zn(II)

atoms are bridged by a tzf ligand in a m2-k N2:k O1,N1fashion.The results denote that tetrazolate-5-

carboxylate ligands can adopt variable coordination modes in the formation of the complexes,and

different Zn-carboxylate aggregates can serve as tertiary building units(TBUs).The effects of the

nature of tetrazolate-5-carboxylate ligands and secondary ligands,and hydrothermal reaction

conditions on the structural topologies of the obtained complexes have been investigated.The

photoluminescent properties and thermal stabilities of1–5have also been discussed.

Introduction

Exploration on the syntheses and properties of novel coordination polymers has been attracting more and more intense interest from chemists,not only for functional material purposes,1but also for their fascinating structures.2However,it would still be a challenge for a long time to design and construct metal–organic frameworks with the desired topologies and properties,because the molecular architectures of coordination polymers are affected by many factors,such as the ligand nature,3the choice of secondary ligand, reaction conditions,especially reaction temperature and pH value.4Among many in?uential factors,organic bridging ligands which contain adjustable?exibility and connectivity information, play a crucial role in the construction and structural tuning of coordination polymers.5The introduction of secondary ligands can also result in structural and functional diversity.6For example,secondary N,N0-ligands can manipulate the structural frameworks in either a terminal or bridging coordination mode.6c Among various organic ligands,polydentate ligands comprising multiple coordination sites,such as N-heterocyclic tetrazolate group7and carboxylate group,8have been extensively investigated and proved to be a rational choice to synthesize coordination polymers possessing intriguing structural varieties and potential applications.However,the investigation of bifunctional tetrazo-late-carboxylate ligands with combined tetrazolate and carboxylate groups remains less developed.9–11Especially, carboxylate-introduced5-substituted tetrazolate(shortened as tetrazolate-5-carboxylate)ligands with four N and two O poten-tial electron-donating centers can act as excellent ligands and exhibit diverse coordination modes.10,11Notably,carboxylate groups on the ligands may be conductive to the formation of discrete multinuclear clusters and in?nite building blocks by M–COO–M linkage,while tetrazolate groups may have the ability to link these clusters to an intricate extended network.Further-more,the spacers between tetrazolate and carboxylate groups can be tunable in components,?exibilities and conjugated extent, which makes the structures and properties of the formed polymers more versatile.Finally,the introduction of secondary ligands into the reaction system may generate some interesting architectures. Recently,we employed the simplest tetrazolate-5-carboxylate,

a State Key Laboratory of Structural Chemistry,Fujian Institute of

Research on the Structure of Matter,Chinese Academy of Sciences,

Fuzhou,Fujian,350002,P.R.China.E-mail:zfk@https://www.sodocs.net/doc/d012720423.html,;Fax:

(+86)591-8371-4946;Tel:+86-591-83704827

b Graduate school,Chinese Academy of Sciences,Beijing,100039,P.R.

China

?Electronic supplementary information(ESI)available:Additional

structural?gures of2–4,PXRD patterns,excitation spectra and

luminescent decay curves and IR spectra for1–https://www.sodocs.net/doc/d012720423.html,DC reference

numbers733077–733081.For ESI and crystallographic data in CIF or

other electronic format see DOI:10.1039/b909754f

PAPER https://www.sodocs.net/doc/d012720423.html,/crystengcomm|CrystEngComm

1H -tetrazolate-5-formic acid (H 2tzf),in assembly of coordina-tion polymers,and reported the structure,nonlinear optical effect and luminescent properties of a 3-D diamond-like [Zn(Htzf)2]n .11a

In this context and as an extension of our research,we selected two structurally related tetrazolate-5-carboxylate ligands with different ?exibilities,H 2tza (1H -tetrazolate-5-acetic acid)and H 2tzf obtained by in -situ hydrolysis of their respective ester (Scheme 1),as organic linkers to construct new metal–organic frameworks.With the introduction of bidentate chelating 2,20-bipy and bridging 4,40-bipy,three tza coordination polymers with in?nite networks and two tzf dinuclear complexes,[Zn(tza)(H 2O)]n (1),[Zn 3(tza)2Cl 2(2,20-bipy)2(H 2O)2]n (2),[Zn 2(tza)2(4,40-bipy)]n (3),[Zn(tzf)(H 2O)3]2$2H 2O (4)and [Zn(tzf)(2,20-bipy)(H 2O)]2$H 2O (5),were synthesized by modi-fying hydrothermal reaction conditions including reactant ratio,reaction temperature,pH value and secondary ligand.Three coordination modes of tetrazolate-5-carboxylate are adopted (modes I–III Scheme 2),of which modes I and II are ?rst observed.Interestingly,different discrete [Zn(COO)]n (n ?2,4)cluster units and in?nite [Zn(COO)]n chains can serve as tertiary building units (TBUs)in tza coordination polymers.As for tzf ligand containing N-donors in the b position,hydrothermal decarboxylation phenomena have been found in complexes formation.Luminescent properties and thermogravimetric analyses of 1–5have also been investigated in this research.

Experimental

Materials and Instruments

All chemicals were obtained from commercial sources and used without further puri?cation.Na 2tza and Na 2tzf used in the

analyses of photoluminescent spectra were prepared by the reaction of their respective ester,1H -tetrazolate-5-ethyl acetate (Htzea)and 1H -tetrazolate-5-ethyl formate (Htzef),in a dilute aqueous solution of NaOH.The elemental analyses were measured on an Elementar Vario EL III microanalyzer.The FT-IR spectra were obtained on a Perkin-Elmer Spectrum using KBr disks in the range 4000–400cm à1.Photoluminescent anal-yses were performed on an EI920?uorescence spectrometer.Thermal analyses were made on a Netzsch STA 449C Jupiter under an N 2atmosphere with the sample heated in an Al 2O 3crucible at a heating rate of 10K min à1.Powder X-ray diffrac-tion data were collected using Rigaku Dmax2500PC powder diffractometer for 1–3and X’Pert Pro powder diffractometer for 4and 5both with Cu K a radiation and 5#2q #60 .Single crystal X-ray diffraction was carried out by a Rigaku Mercury CCD/AFC7R diffractometer.Syntheses of 1–5

[Zn(tza)(H 2O)]n (1).A mixture of ZnCl 2(0.3mmol),Htzea (0.3mmol)and NaOH (0.4mmol)in an aqueous solution (5mL)was sealed into a 25mL poly(tetra?uoroethylene)-lined stainless steel container under autogenous pressure and then heated at 160 C for 3d and cooled to 30 C at a rate of 2.5 C h à1.Colorless block crystals suitable for X-ray analyses were obtained,washed with distilled water and dried in air.Yield:33%(based on Zn)for 1.Anal.Calcd for C 3H 4N 4O 3Zn:C,17.20;H,1.92;N,26.75%.Found:C,17.42;H,1.82;N,26.63%.IR (KBr pellet,cm à1):3621s,3496w,3079w,1581vs,1500s,1452vs,1407s,1376vs,1284s,1251s,1141m,941m,810s,740s,567m.

[Zn 3(tza)2Cl 2(2,20-bipy)2(H 2O)2]n (2).A mixture of ZnCl 2(0.3mmol),Htzea (0.3mmol),NaOH (0.4mmol)and 2,20-bipy (0.3mmol)in aqueous solution (5mL)was sealed into a 25mL poly(tetra?uoroethylene)-lined stainless steel container under autogenous pressure and then heated at 160 C for 3d and cooled to 30 C at a rate of 2.5 C h à1.Colorless prism crystals suitable for X-ray analyses were obtained,washed with distilled water and dried in air.Yield:9%(based on Zn)for 2.Anal.Calcd for C 26H 24Cl 2N 12O 6Zn 3:C,35.99;H,2.79;N,19.37%.Found:C,35.67;H,2.73;N,19.12%.IR (KBr pellet,cm à1):3394s,3104m,3083m,1604vs,1573s,1457s,1442vs,1384s,1315m,1249m,1081m,1024m,767s,736s,653m,582m.

[Zn 2(tza)2(4,40-bipy)]n (3).The procedure was the same as that for 2except that 2,20-bipy was replaced by 4,40-bipy.Yield:23%(based on Zn)for 3.Anal.Calcd for C 16H 12N 10O 4Zn 2:C,35.65;H,2.24;N,25.98%.Found:C,35.83;H,2.18;N,25.69%.IR (KBr pellet,cm à1):3423b,3064w,3000w,1639vs,1612s,1454s,1421s,1265m,1224m,1076m,808s,723m,636m,599w.[Zn(tzf)(H 2O)3]2$2H 2O (4).The procedure was the same as that for 1with Htzef in place of Htzea,and the reaction temperature was adjusted to 80 C.Yield:34%(based on Zn)for 4.Anal.Calcd for C 4H 16N 8O 12Zn 2:C,9.63;H,3.23;N,22.46%.Found:C,9.77;H,3.09;N,22.22%.IR (KBr pellet,cm à1):3357b,1637vs,1492vs,1415s,1338vs,1230s,1186m,1089m,1066m,844s,680b.If a smaller quantity of NaOH (0.2mmol)was added,a 3D-diamond-like [Zn(Htzf)2]n 11a with

the

Scheme 1The formation of 1H -tetrazolate-5-carboxylic

acid.

carboxylate group uncoordinated was obtained.When the reaction temperature was raised above120 C and the NaOH amount was simultaneously changed from0.2to0.4mmol,the decarboxylated product[Zn(HCN4)2],12was obtained.

[Zn(tzf)(2,20-bipy)(H2O)]2$H2O(5).The procedure was the same as that for2except that Htzea was substituted by Htzef, and the reaction temperature was adjusted to80 C.Yield:11% (based on Zn)for5.Anal.Calcd for C24H22N12O7Zn2:C,39.97; H,3.07;N,23.30%.Found:C,39.99;H,3.04;N,23.19%.IR (KBr pellet,cmà1):3270b,3114s,1616vs,1472vs,1440s,1402s, 1328vs,1311s,1025m,765s,734m,651m,410m.Notably,when the reaction temperature was increased above120 C,the decarboxylated product[Zn(HCN4)2],12was obtained.And when 4,40-bipy instead of2,20-bipy was added following the same procedure as5,the1-D polymer[ZnCl2(4,40-bipy)]n13was always obtained at different reaction temperatures(80–160 C)and pH values(with the NaOH amount ranging from0.2to0.4mmol). Single crystal structures determination

Single crystals of1–5suitable for X-ray analyses were mounted at the apex of a glass?ber for data collection.Data collections were performed at293(2)K on a Rigaku AFC7R diffractometer for1and on a Rigaku Mercury CCD diffractometer for2–5, both diffractometers being equipped with a graphite-mono-chromated Mo K a radiation source(l?0.71073A?).There was no evidence of crystal decay during data collection.The intensity data sets were collected with the u scan technique and reduced by CrystalClear software.14The structures were solved by the direct methods and re?ned by full-matrix least-squares techniques. Non-hydrogen atoms for1–5were located by difference Fourier maps and subjected to anisotropic re?nement.Hydrogen atoms of lattice water molecules were located by difference Fourier maps and those of coordinated water molecules were calculated in idealized positions,and all hydrogen atoms of water molecules were re?ned with O–H distances restrained to a target value of 0.85A?and U iso(H)?1.5U eq(O).The remaining hydrogen atoms were calculated in idealized positions and allowed to ride on their parent atoms.Approximate anisotropic thermal re?ne-ment was applied to individual carbon atoms from2,20-bipy in2 and4,40-bipy in3.All calculations were performed by the Siemens SHELXTL version5package of crystallographic soft-ware.15Pertinent crystal data and structural re?nement results and selected bond distances and angles for1–5are listed in Table 1and Table S1,?respectively.

Results and discussions

Syntheses

In order to investigate the in?uence of the ligand?exibility, secondary ligand and reaction conditions on the architecture of coordination polymers,we chose two tetrazolate-5-carboxylate ligands with different molecular backbones and?exibilities with the incorporation of different secondary ligands,and a modi?ed reactant ratio,reaction temperature and pH value.The synthetic route is shown in Scheme3.1H-tetrazolate-5-carboxylate ligands were generated by in-situ hydrolysis of their respective ester and NaOH,which could simultaneously adjust the pH value of the reactant mixture.For the Htzea reaction system with the same ZnCl2,Htzea and NaOH molar ratio of0.3:0.3:0.4at the same reaction temperature(160 C),polymers1–3with different topologies were obtained in the presence/absence of2,20-bipy and 4,40-bipy.The results indicate that different secondary ligands have a big effect on the formation of tza coordination polymers. However,the effects on the?nal product of tzf coordination compounds are more complicated.The reaction mixture of ZnCl2,Htzef and NaOH in the reactant ratio of0.3:0.3:0.4 with a higher pH value at80 C produced dinuclear4,while the reactant ratio of0.3:0.3:0.2at a lower pH value gave protonated[Zn(Htzf)2]n,which was previously obtained from the reaction of H2tzf and ZnCl2at room temperature.11a It is worthy of note that decarboxylation of tzf was observed at the higher

Table1Crystallographic data and structural re?nements for1–5

12345

Formula C3H4N4O3Zn C26H24Cl2N12O6Zn3C16H12N10O4Zn2C4H16N8O12Zn2C24H22N12O7Zn2 Formula mass209.47867.58539.10499.03721.28

Space group P4(2)/n Fdd2P2(1)/c P-1P-1

a/A?12.7076(15)22.118(3)13.515(3)7.153(4)9.396(3)

b/A?12.7076(15)44.164(7) 6.7465(13)7.752(4)11.094(3)

c/A?7.7157(17) 6.6646(7)10.667(2)8.775(4)13.567(4)

a/ 90909097.048(3)83.399(9)

b/ 909091.33(3)112.728(4)81.828(7)

g/ 909090108.627(4)74.782(9)

V/A?31246.0(3)6510.1(15)972.4(3)408.1(4)1346.3(7)

Z88212

D c/g cmà3 2.233 1.770 1.841 2.031 1.779

m/mmà1 3.899 2.418 2.517 3.020 1.853

Total re?ns1313119046394284510119

Unique re?ns11383596175914874881

R int0.05430.05820.04030.01970.0402

GOF 1.0480.979 1.076 1.017 1.030

R1a[I>2s(I)]0.03270.05420.04350.02250.0379

wR2b(all data)0.09170.13750.10680.05930.0886

a R

(F o-F c)/

F o.b wR2?[

w(F o2–F c2)2/

w(F o2)2]1/2.

reaction temperature.When the reaction temperature was raised above120 C in a limited extent of pH values with a reactant ratio of ZnCl2,Htzef and NaOH varying from0.3:0.3:0.2to 0.3:0.3:0.4,the decarboxylated product[Zn(HCN4)2]was formed,which was previously obtained by a hydrothermal reaction of Zn(OAc)2and tetrazole.12The hydrothermal decar-boxylation of carboxylic acid containing N-donors in the b position has been well documented,16which is?rst encountered for tzf ligand.Regretfully,the attempts to synthesize Zn(II) complex with mixed ligands of tzf and4,40-bipy failed at different reaction temperatures and reactant ratios,and1-D polymer [ZnCl2(4,40-bipy)]n was always produced,which was previously prepared by directly reacting4,40-bipy with ZnCl2.13Based on the aforementioned facts,reaction conditions such as reaction temperature,pH value and secondary ligand exert a synergic effect on the formation of tzf coordination compounds. Structural descriptions and discussions

[Zn(tza)(H2O)]n(1).The single-crystal X-ray structural anal-yses reveal that1features a3-D net built up by the fusion of tetranuclear[Zn(COO)]4cluster units and tetrazolate groups. Polymer1possesses an asymmetric unit consisting of one zinc atom,one tza ligand,and one coordinated water molecule (Fig.1a).The coordination environment around Zn is a slightly distorted[ZnO3N2]trigonal bipyramid with the Zn–O distances being in the normal range of1.991(2)–2.139(2)A?and Zn–N distance being 2.028(2)and 2.061(2)A?,where two nitrogen donor atoms[N1C,N3D]and two oxygen donor atoms[O1C, O2]belong to three different symmetry-related tza ligands.The remaining site is occupied by one coordinated water molecule.In 1,each tza ligand exhibits a m3-k N3:k O2:k O1,N1coordination mode(mode I in Scheme2),and acts as a tetradentate linker to connect three Zn atoms,with a syn–anti con?guration of the carboxylate group and a m2-1,3N mode of the tetrazolate group, along with O1and N1atoms being in a chelated fashion.

Although numerous coordination styles of tetrazolate-5-carboxylate ligands have been found,10,11the unusual coordina-tion mode of tza ligand in1is?rst observed,which favors the formation of a new structure with an interesting topology.

The most striking structural feature in1is that tetranuclear [Zn(COO)]4cluster units are interlinked together via the m2-1,3N tetrazolate groups to generate a novel3-D net(Fig.1b).Four Zn atoms are bridged alternately by four carboxylate groups to form a sixteen-membered cyclic[Zn(COO)]4ring as a tertiary building unit(TBU).In the tetranuclear cluster unit,four Zn atoms are not in a plane,but arrange in a distorted tetrahedron. Notably,coordination polymers containing such a

tetranuclear Fig.1Polymer1:(a)The coordination environment around the Zn atom and coordination mode of tza ligand.Symmetry codes are de?ned in Table S1.(b)A3-D network constructed by the linkage of[Zn(COO)]4 cluster units as TBUs and tetrazolate groups as‘‘double-bridges’’.The H atoms and coordinated water molecules are omitted for clarity.(c) Two-fold interpenetrated3-D diamond-like topology.The black balls represent the TBUs of[Zn(COO)]4clusters and the lines represent the ‘‘double-bridges’’.

[Zn(COO)]4TBU are scarce,17,18in some of which four Zn atoms display a coplanar array.18d The tetranuclear cluster units are linked together by tetrazolate groups,each of which coordinates two Zn centers from different tetranuclear cluster units in a m2-1,3N mode.As shown in Fig.1b,each tetranuclear cluster unit is surrounded by eight tetrazolate groups,each pair of which connecting two tetranuclear cluster units to form‘‘double-bridges’’.19As a result,each tetranuclear cluster unit is linked by four‘‘double-bridges’’,relating to four adjacent tetranuclear cluster units.Thus,if each‘‘double-bridge’’is considered as a linker,20and each tetranuclear cluster unit is regarded as a four-connected node,with each node lying at the vertex of an approximate tetrahedron,1de?nes a two-fold interpenetrating diamond-like topology(Fig.1c).The formation of such an interesting net for1constructed by tetranuclear[Zn(COO)]4 cluster units as TBUs and tetrazolate groups as‘‘double-bridges’’may be ascribed to the unusual coordination mode of the tza ligand.

[Zn3(tza)2Cl2(2,20-bipy)2(H2O)2]n(2).When secondary ligand 2,20-bipy was introduced into the reaction system of ZnCl2,Htzea and NaOH,polymer2with a new2-D framework was produced. As shown in Fig.2a,there are two crystallographically indepen-dent Zn centers with four-coordinated Zn1and six-coordinated Zn2in2.The Zn1atom,lying on a twofold axis,is coordinated by two symmetry-related tetrazolate nitrogen atoms(N4,N4A)and two symmetry-related chlorine atoms(Cl1,Cl1A)to furnish a distorted tetrahedral coordination geometry with the angles varying from102.13(10)to112.33(4) .The bond distance of Zn1–Cl1is2.273(1)A?,which is comparable with those of similar complexes with bond distances ranging from2.196(2)to2.2509(8) A?.21The Zn2atom adopts a distorted octahedral coordination geometry by two nitrogen atoms(N5,N6)of2,20-bipy ligand and two oxygen atoms(O1,O1W)in the almost equatorial plane,as well as O2B and N1from two symmetry-related tza ligands at the axial sites with the O2B–Zn2–N1angle of168.22(6) .The Zn–O distances are in the range of2.107(2)–2.156(1)A?,as well as Zn–N distances2.002(2)–2.145(2)A?.Each carboxylate group from tza ligand in2bridges two symmetry-related Zn2centers with the

same syn–anti mode as that in1.However,unlike in1,the tetrazolate group in2presents a m2-1,4N mode rather than a m2-1,3N mode,to bond Zn1and Zn2centers via N1and N4 atoms[Zn1/Zn2?6.100(1)A?],along with O1and N1atoms chelating to the Zn2center.So another unusual coordination mode m3-k N4:k O2:k O1,N1of tetrazolate-5-carboxylate ligands in2(mode II in Scheme2)is found.

Interestingly,syn–anti carboxylate groups from tza ligands link Zn2atoms in an alternate manner to form left-handed helical[Zn(COO)]n in?nite chains as TBUs,rather than a tetra-nuclear[Zn(COO)]4cluster units in1,extending along the c axis with the helical pitch being6.6646(7)A?as shown in Fig.2b.The similar[Zn(COO)]n in?nite chains were widely investigated in numerous zinc(II)polymers with varied carboxylate-containing ligands.22The helical chains arrange along the a direction to produce a chiral sheet,and the Zn2centers between adjacent chains link to each other through V-shaped tetrazolate-Zn1-tet-razolate bridging units with an angle Zn2–Zn1–Zn2of 101.059(4) .The adjacent sheets,made up of right-handed helical chains of opposite chirality,are stacked parallel to each other in an–ABAB–sequence to give the whole achiral structure,as depicted in Fig.S1.?The generation of the2-D structure in2may be attributed to the templating role of chelating2,20-bipy,as well as the versatile coordination modes of tza ligand.

[Zn2(tza)2(4,40-bipy)]n(3).When4,40-bipy was used instead of 2,20-bipy,polymer3with an unusual3-D self-penetrating structure differing from those of1and2,was obtained.It is worthy of note that there is no coordinated or lattice water molecule in3,although it was synthesized by a hydrothermal reaction.There are one zinc atom,one tza ligand and half a4,40-bipy molecule in an asymmetric unit of3(Fig.3a).The whole4,40-bipy molecule was organized through an inversion center.The coordination geometry around the Zn atom can be described as a slightly distorted trigonal bipyramid.As illus-trated in Fig.3a,N1,N4C and O2A atoms from three different symmetry-related tza ligands constitute a basal plane,while the axial positions are occupied by O1atom of the carboxylate group and N5atom of4,40-bipy with the axial angle O1–Zn1–N5of 172.37(7) ,deviating the ideal angle of180 .The Zn–O

distances Fig.2Polymer2:(a)The coordination environment around the Zn atom and coordination mode of tza ligand.Symmetry codes are de?ned in Table S1.(b)View of the left-handed helical chains extending along the c axis and the2-D sheet network.The Cl atoms,2,20-bipy and coordi-nated water molecules are omitted for clarity.

are in the normal range of 1.977(2)–2.217(2)A

?,as well as Zn–N distances 2.030(2)–2.143(2)A

?.Each tza ligand connects to three symmetry-related Zn centers,adopting the same coordination mode m 3-k N4:k O2:k O1,N1as in 2(mode II in Scheme 2),with a m 2-1,4N mode of tetrazolate group and a syn–anti con?guration

of the carboxylate group,which functions one oxygen atom for chelating and the other for bridging.

To shed light on the network constructed by carboxylate-Zn clusters and tetrazolate group linkers,we ?rst omit the 4,40-bipy bridging secondary ligands.As shown in Fig.3b,the carboxylate groups from tza ligands link Zn atoms in a syn–anti mode to give an eight-membered cyclic ring of a centrosymmetric dimeric unit [Zn(COO)]2as TBU,differing from that in 1and 2.And each unit,as a four-connecting node,relates to four equivalent ones through four m 2-1,4N tetrazolate groups as linkers.As a result,a 2-D layer network parallel to the bc plane is formed,displaying a (4,4)-connected topology.Then these layers,constructed by Zn atoms and tza ligands,are further pillared through 4,40-bipy,resulting in an unusual 3-D six-connected self-penetrating LB-1(446108)topology illustrated in Fig.3c.In contrast to most six-connected nets based on the (4,4)-connected sheet:pcu (41263),roa (446108)and rob (48668),the existing examples for such self-penetrating LB-1net are limited.23The intricate 3-D in?nite network is depicted in Fig.S2.?It is well known that 4,40-bipy as an organic bridging linker in constructing new coordination polymers with speci?c structures and properties have been widely investigated.24With this intention we successfully applied it into our case as a pillared connector to link adjacent 2-D layers into a ?nal 3-D structure,which further con?rms the key role of a rigid secondary ligand in mediating and constructing coordi-nation polymers.More signi?cantly,this offers a feasible way for us to rationally modify the structures and geometries of tetrazolate-5-carboxylate coordination polymers with an introduction of rigid secondary ligands.

[Zn(tzf)(H 2O)3]2$2H 2O (4).Single-crystal X-ray diffraction analysis revealed that 4is an isolated dinuclear structure with each asymmetric unit containing one Zn atom,one tzf ligand,three coordinated water molecules,and one lattice water mole-cule.The repetition of the Zn atoms through an inversion centre gives rise to a dinuclear Zn2unit,as shown in Fig.4a.In 4,the coordination geometry of the Zn atom can be described as a distorted octahedron [ZnN 2O 4]with an axial angle O2W–Zn1–O3W of 175.82(8) .The tzf ligand acts as a tridentate linker to chelate one Zn atom and bridges the other Zn atom in a m 2-k N2:k O1,N1coordination mode (mode III in Scheme 2).In contrast to modes I and II,the tetrazolate group in 4presents a m 2-1,2N mode and one oxygen atom from the carboxylate group remains uncoordinated.This coordination mode is similar to that reported recently.10g In 4,two Zn atoms are bridged by tetrazolate groups from two symmetry-related tzf ligands to form one six-membered ring (Zn1–N1–N2–Zn1A–N1A–N2A).Similar six-membered rings were previously found in other tet-razolate-bridged Zn complexes.25The formation of an isolated dinuclear unit for 4could be ascribed to the nature of the rigid tzf ligand as well as the formation of a Zn 2N 4six-membered ring.So far there have only been a few reports concerning dinuclear complexes bridged by tetrazolates.26

On the other hand,there are plenty of O–H /O/O–H /N hydrogen bonds as listed in Table S2,?resulting in an intricate supramolecular structure.For convenience,we ?rst consider O–H /O hydrogen bonds involved with the lattice water mole-cule O4W,which leads to a 2-D layer (Fig.4b);and then a self-assembly of 2-D layers by means of O1W /O1

hydrogen

Fig.3Polymer 3:(a)the coordination environment around the Zn atom and coordination mode of tza ligand.Symmetry codes are de?ned in Table S1.(b)View of the 2-D sheet network constructed by [Zn(COO)]2TBUs and tetrazolate groups.(c)Schematic representation of a 3-D topology built by 2-D (4,4)-connected nets and 4,40-bipy linkers.The black balls represent [Zn(COO)]2TBUs.The pink and green lines represent tetrazolate groups and 4,40-bipy molecules,respectively.

bonding interaction affords a 3-D supramolecular network (Fig.4c).In fact,intermolecular O1W /O2,O4W /N3and O3W /N4hydrogen bonds,together with intramolecular O3W /O2,coexist in the ?nal supramolecular 3-D network (Fig.S3).?No signi?cant aromatic stacking is observed.[Zn(tzf)(2,20-bipy)(H 2O)]2$H 2O (5).Another isolated neutral dinuclear complex 5was obtained when 2,20-bipy was introduced into the reaction mixture of ZnCl 2,Htzef and NaOH.In 5,the asymmetric dinuclear unit is composed of two Zn atoms,two tzf ligands,two 2,20-bipy and two coordinated water molecules (Fig.5a).The con?guration of the dinuclear core in 5is similar to that in 4.The main difference is that one chelating 2,20-bipy

molecule in 5replaces two coordinated water molecules to accomplish Zn coordination.The tzf ligand exhibits the same coordination mode as that in 4.There also exist extensive hydrogen bonds,which connect the dinuclear units to give a 2-D supramolecular network (Fig.5b).Full details of the hydrogen-bonding geometries are given in Table S2.?No signi?cant aromatic stacking is either observed.

It’s well known that the utilization of metal clusters or inor-ganic chains as building blocks in place of single metal ions has provided a promising pathway toward the generation of novel coordination polymers.27,28In polymers 1–3,three different Zn-carboxylate moieties as TBUs are found in spite of the same syn–anti fashion of the carboxylate group:sixteen-membered cyclic tetranuclear clusters [Zn(COO)]4,in?nite helix chains [Zn(COO)]n and eight-membered cyclic dinuclear clusters [Zn(COO)]2.This greatly enriches the architectures and proper-ties of metal–organic frameworks.Owing to four N and two O binding centers,tetrazolate-5-carboxylate ligands display versa-tile coordination modes,which provides the possibility of constructing novel coordination polymers with desirable struc-tures and properties.Two new coordination modes of tetrazo-late-5-carboxylate ligands are ?rst observed in this study (Modes I and II in Scheme 2).Obviously,mode I with the tetrazolate group being a m 2-1,3N mode can more effectively minimize the steric hindrance than mode II,in which the tetrazolate group adopts a m 2-1,4N mode.The tza ligand adopts mode I in 1but presents mode II in 2and 3.Thus,regardless of the

secondary

Fig.4Complex 4:(a)molecular structure of 4.Symmetry codes are de?ned in Table S1.(b)A perspective view of a 2-D supramolecular architecture in 4via O4W /O1and O4W /O2W hydrogen bonds.(c)Schematic representation of 2-D /3-D supramolecular architecture via O1W /O1hydrogen bonds in 4

Fig.5Complex 5:(a)molecular structure of 5with lattice water molecules omitted for clarity.(b)A perspective view of a 2-D supramo-lecular network formed by hydrogen bonds in 5.

ligands2,20-bipy and4,40-bipy,1has higher dimensional topology(3-D),while2and3exhibit a lower dimensional one (2-D).On the other hand,two different kinds of rigid secondary ligands were used to enrich the diversities of polymeric archi-tectures.A2-D net is formed by the linkage of Zn and tza in2,in which2,20-bipy merely exhibits a terminal chelation mode.In3, the connection of Zn and tza constructs a2-D net,which is further joined by linear bridging4,40-bipy to generate a self-penetrating3-D framework.To the best of our knowledge,this is the?rst instance to consciously synthesize coordination polymers by incorporating different rigid secondary ligands into the?ex-ible tza system with the aim to enrich their frameworks,although different rigid secondary ligands were introduced into the tzf case lately.10g It should be noted that the molecular backbone of tza ligand accompanying the centered sp3carbon atom between tetrazolate and carboxylate groups,which endows tza ligand with the capability of properly distorting to meet the steric requirement upon metal coordination.Therefore,despite the same coordination fashion of the tza ligand,the TBUs and molecular frameworks of2and3are different from each other. Compared with the tza ligand,the tzf ligand exhibits a m2-k N2:k O1,N1coordination fashion(mode III in Scheme2) in4and5,which implies that the tza ligand possesses more ?exibility than the tzf ligand in assembling coordination poly-mers.These observations suggest that the?exibilities of tetra-zolate-5-carboxylate and the coordination tendencies of secondary ligands play important roles in tuning molecular frameworks.

Luminescence properties

The photoluminescence of d10metal complexes has been attracting intensive research interest,owing to their potential applications in chemical sensors,photochemistry and electrolu-

minescent(EL)displays.29To examine the photoluminescent properties of tetrazolate-5-carboxylate coordination polymers/ complexes,the excitation and emission spectra of1–5as well as Na2tza and Na2tzf in the solid-state were investigated at room temperature(Fig.S4?and Fig.6),and the luminescent decay curves of1–5were further examined by nanosecond(ns)laser system at room temperature(Fig.S5).?Compounds1–5exhibit strong blue photoluminescence with emission maxima at424nm (s?4.75ns),404nm(s?7.02ns),409nm(s?7.20ns),428nm (s?3.84ns)and401nm(s?5.34ns)upon excitation at352, 322,318,338and324nm,respectively.The sodium salts of tet-razolate-5-carboxylate,Na2tza and Na2tzf,display photo-luminescent emission with maximum at414nm(l ex?338nm) and424nm(l ex?356nm),respectively.By comparing the locations and pro?les of their emission peaks,the photo-luminescent mechanism of1–5is tentatively attributed to intra-ligand transitions mainly through their tetrazolate rings.30The results suggest that these complexes may be good blue light emitting candidate materials.

Thermogravimetric analyses

There was no evidence of crystal decay during X-ray data collection for all compounds,which implies they are stable at ambient conditions.Thus,thermogravimetric analyses(TGA)experiments for1–5were performed to investigate their thermal stability,and the TGA curves were provided in Fig.7.The results show that the weight loss of8.55%in the temperature range of 166–316 C and4.32%in the temperature range of166–316 C for1and2(calculated:8.6%and4.15%)respectively,reveals evacuation of the coordinated water molecules.Then the decomposition of the tza ligand,with the framework

collapse, Fig.6The emission spectra in the solid state:(a)1–3and Na2tza;(b)4,5 and Na2

tzf.

Fig.7TGA curves for1–5.

occurred upon heating to325and260 C,for1and2,respec-tively.As for3,the TGA curve displays a weight loss of46.52% from320to500 C,corresponding to the pyrolysis of one tza ligand(calculated:46.77%),and then the framework continued to crash above630 C.It is notable that metal–organic coordi-nation polymers with such high thermal stability as3are unusual,31,11b which may be the consequence of no coordinated aqua or lattice water molecule in3.With regard to4,the?rst weight loss of29.09%from70to220 C corresponds to the release of six aqua and two lattice water molecules(calculated: 28.86%).With that,the remaining substance was decomposed starting at260 C.The TGA curve of5displays a similar char-acter to that of4.The?rst weight loss of7.57%from90to195 C is consistent with the release of two aqua and one lattice water molecules(calculated:7.49%),and then the rapid pyrolysis of the framework took place above245 C.

Conclusions

In summary,hydrothermal syntheses,crystal structures and luminescent properties of three tza coordination polymers with in?nite networks and two tzf dinuclear complexes have been reported.In the tza polymers,three different Zn-carboxylate aggregates,[Zn(COO)]2and[Zn(COO)]4cluster units and [Zn(COO)]n chains,serve as TBUs and tza ligand exhibits two new coordination modes.The results show that the nature of the tza and tzf ligands as well as2,20-bipy and4,40-bipy secondary ligands,and the reaction conditions play an important role in governing the molecular frameworks of1–5.Furthermore, emission spectra demonstrate that the tza/tzf complexes might be good candidates for ef?cient luminescent materials,mainly owing to intraligand n–p*or p–p*transitions of their tetrazolate rings. Acknowledgements

This work was?nancially supported by973Program (2006CB932900and2007CB936703),the National Nature Science Foundation of China(20871115)and Fujian Province (A0420002).

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2010年12月大学英语四级听力真题MP3下载

Section A 短对话 (11~18) 11. M: Oh my god! The heat is simply unbearable here. I wish we’ve gone to the beach instead. W: Well, with the museums and restaurants in Washington I’ll be happy here no matter what the temperature. Q：What does the woman mean? 12. M: How’s the new job going? W: Well, I’m learning a lot of new things, but I wish the director would give me some feedback. Q：What does the woman want to know? 13. M: Can you help me work out a physical training program John? W: Sure, but whatever you do be careful not to overdo it. Last time I had two weeks’worth of weight-lifting in three days and I hurt myself. Q: What does the man suggest the woman do? 14. M: I have an elderly mother and I’m worried about her going on a plane. Is there any risk?

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