DOI: 10.1002/chem.201302084

Simultaneous Presence of Both Open Metal Sites and Free FunctionalOrganic Sites in a Noncentrosymmetric Dynamic Metal–Organic Framework

with Bimodal Catalytic and Sensing Activities

Rajat Saha,[a] Biplab Joarder,[b] Anupam Singha Roy,[c] Sk. Manirul Islam,[c] andSanjay Kumar*[a]

In memory of Golam Mostafa (1962–2011)

Introduction

In the last two decades, porous metal–organic frameworks(MOFs) have emerged as a most promising functional mate-rial on account of their numerous material properties suchas gas and solvent sorption, storage, and separation.[1] In ad-dition to porosity, researchers have further tried to encom-pass other properties like magnetism, sensing, drug delivery,second-harmonic generation/nonlinear optical (SHG-NLO)activity, ferroelectricity, catalysis, and so forth, which crucial-

ly depend upon the nature of the framework as well as thepore surface.[2] Accordingly, multifunctional metal–organicframeworks (MMOFs) have gained tremendous attentionand are at the heart of current scientific advancements.[3]

Therefore the design and synthesis of MMOFs have becomeimmense challenges in crystal engineering. Crystal engineershave focused on surface functionalization to fabricate newMOFs capable of showing multiple functionalities. Surfacefunctionalization can be performed in two ways: 1) the im-mobilization of open metal sites (OMSs) and 2) the intro-duction of free functional organic sites (FOSs) within theframework.[4]

The presence of OMSs within a framework is quitecommon. The generation of OMSs through the removal ofcoordinated solvent molecules is also a rather easy process.It is well known that the presence of OMSs allows theframework to act as a Lewis acid catalyst.[5] Furthermore, ithelps to improve the adsorption capacity of the framework[6]

and is also responsible for the hysteresis phenomenon.[7] Inaddition, Chen and co-workers reported the small-moleculesensor activity of an MOF with OMSs.[8] By contrast, thepresence of free FOSs within a framework is very rareowing to their strong affinity towards metal ions and pro-

Abstract: Assimilation of open metalsites (OMSs) and free functional or-ganic sites (FOSs) with a frameworkstrut has opened up a new route forthe fabrication of novel metal–organicmaterials, thereby providing a uniqueopportunity to explore their multiplefunctionalities. A new metal–organicframework (MOF), {[Cu ACHTUNGTRENNUNG(ina)2ACHTUNGTRENNUNG(H2O)][Cu ACHTUNGTRENNUNG(ina)2ACHTUNGTRENNUNG(bipy)]·2 H2O}n (1) (ina= iso-nicotinate, bipy=4,4’-bipyridine), hasbeen synthesized and characterized.Complex 1 is crystallized in the ortho-rhombic noncentrosymmetric spacegroup Aba2 and consists of two differ-ent 2D coordination polymers, [Cu-ACHTUNGTRENNUNG(ina)2ACHTUNGTRENNUNG(H2O)]n and [Cu ACHTUNGTRENNUNG(ina)2ACHTUNGTRENNUNG(bipy)]n,with entrapped solvent water mole-

cules. Hydrogen-bonding interactionsassemble these two different 2D coor-dination layers in a single-crystal struc-ture with interdigitation of pendant4,4’-bipy from one layer into thegroove of another. Upon removal ofguest molecules, 1 undergoes a structur-al transformation in single-crystal-to-single-crystal fashion with expansion ofthe effective void space. Each metalcenter is five-coordinated and thus canpotentially behave as an OMS, and thefree pyridyl groups of pendant 4,4’-bipy

moieties and free �C=O groups can actas free FOSs. Thus, owing to presenceof both OMSs and free FOSs, theframework exhibits multifunctionalproperties. Owing to the presence ofOMSs, the framework can act asa Lewis acid catalyst as well as a small-molecule sensor material, and in a simi-lar way, owing to the presence of freeFOSs, it performs as a Lewis base cata-lyst and a cation sensor material. Fur-thermore, owing to noncentrosymmetrywith large polarity along a particulardirection, it shows strong second-har-monic generation/nonlinear optical(SHG-NLO) activity.

Keywords: catalysis · metal–organicframeworks · Lewis acids · Lewisbases · sensors

[a] R. Saha, Dr. S. KumarDepartment of Physics, Jadavpur University, JadavpurKolkata 700032 (India)Fax: (+33) 2414-6584E-mail : kumars@phys.jdvu.ac.in

[b] B. JoarderDepartment of Chemistry, IISER PunePashan, Pune 411008 (India)

[c] A. S. Roy, S. Manirul IslamDepartment of Chemistry, University of KalyaniKalyani 741235 (India)

Supporting information for this article is available on the WWWunder http://dx.doi.org/10.1002/chem.201302084.

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tons. Thus, during the self-assembly process, functional or-ganic sites bind metal ions and protons; that is, they remaincompletely blocked. These free FOSs can serve as activebase sites and thus can be used for selective Lewis basic cat-alysis.[9] Furthermore, Chen et al. have established the cationsensing activity of framework materials with free FOSs.[10]

But up to now the simultaneous presence of both OMSs andfree FOSs in a single framework has only been reportedonce.[1s]

In this paper we report an unusual simultaneous presenceof both OMSs and free FOSs in a noncentrosymmetric dy-namic MOF, {[Cu ACHTUNGTRENNUNG(ina)2ACHTUNGTRENNUNG(H2O)][Cu ACHTUNGTRENNUNG(ina)2(4,4’-bipy)]·2 H2O}n

(1; ina= isonicotinate, bipy= 4,4’-bipyridine) and establishthe multiple functional properties of the material. This isa hitherto unreported inorganic cocrystal of 2D coordinationpolymers; two different coordination layers become stabilein a framework through interdigitation. The dynamic natureof 1 has been established by solid-state structural transfor-mation in single-crystal-to-single-crystal (SC-SC) fashionand, interestingly, upon removal of disordered guest watermolecules, the framework expands the effective void space.Owing to the presence of five-coordinated CuII centers(OMSs) and free pyridyl groups (FOSs), the frameworkshows size-selective Lewis acidic catalysis and Lewis basiccatalysis with both metal-ion and solvent (small-molecule)sensing activities (Scheme 1). Owing to its noncentrosymme-try along with its high polarity, the framework shows strongSHG-NLO activity.

Results and Discussion

Crystal and supramolecular structure of complex 1: Single-crystal X-ray diffraction (SC-XRD) analysis revealed that1 is crystallized in the orthorhombic noncentrosymmetricspace group Aba2 and is a hydrated cocrystal of two differ-ent 2D metal–organic coordination polymers (type A: [Cu-ACHTUNGTRENNUNG(ina)2ACHTUNGTRENNUNG(H2O)]n ; type B: [CuACHTUNGTRENNUNG(ina)2(4,4’-bipy)]n ; Figure 1). Theasymmetric unit contains half an A-type molecule, half aB-type molecule and one disordered guest water molecule.

The A-type molecule consists of one CuII ion, two uninega-tive ina ligands, and one coordinated water molecule, where-as the B-type molecule consists of one CuII ion, two ina lig-ACHTUNGTRENNUNGands, and one coordinated 4,4’-bipy moiety (Figure S1 in theSupporting Information). In both A- and B-type molecules,CuII adopts a five-coordinated square-pyramidal geometry(for Cu1 t= 0.20, for Cu2 t=0.29). In each case (for bothCu1 and Cu2), the basal plane of the pyramid is built up bytwo pyridyl nitrogen atoms of two different ina ligands intrans mode and two carboxylato-oxygen atoms of two differ-ent ina lig ACHTUNGTRENNUNGands also in trans mode. Each metal center isbridged by ina ligands to generate 2D coordination layerswith a grid topology in the crystallographic bc plane (Fig-ure S2 in the Supporting Information). The two layers differfrom each other in the sense that 4,4’-bipy has occupied thefifth coordination site of one layer (layer B) in monodentatefashion, whereas in the other layer (layer A) this has beensatisfied by the coordination from the water molecule (Fig-ure S3 in the Supporting Information). These five-coordinat-ed metal centers act as OMSs and the free pyridyl groups ofpendant 4,4’-bipy can behave as free FOSs (Figure S4 in theSupporting Information).

The coordinated water molecules form twisted Y-pat-terned hydrogen bonds with the uncoordinated oxygenatoms of carboxylato groups of the adjacent B layer withthe formation of a supramolecular bilayer (Figure S5 in theSupporting Information). These supramolecular bilayers arepacked in a parallel fashion along the crystallographic a axisto form a three-dimensional structure (Figures S6–S8 in theSupporting Information). This 3D decoration of the overallstructure is the reason why the free pyridyl groups of 4,4’-bipy moieties of layer B are thrown into the groove of adja-cent layer A; that is, interdigitation occurs (Figure 1). Guestwater molecules are trapped in between such bilayers. Thedistance between layers A and B within the supramolecularbilayers is 5.861 � and the distance between one A layerand one B layer of two adjacent bilayers is 5.333 �. But thedistances between two A layers and two B layers are thesame (11.195 �). Supramolecular hydrogen-bonding interac-

Scheme 1. The presence of both open metal sites and free functional or-ganic sites allows the framework to act as a multifunctional material.

Figure 1. a) Asymmetric unit of 1. b) Schemetic representation of a bilay-er. c) Actual representation of 1. Guest molecules and hydrogen atomshave been removed for clarity.

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tions between A and B layers are responsible for such un-usual cocrystallization. This is the first example of an inor-ganic cocrystal of coordination polymers, which is yet to beexplored.

Thermal and PXRD analysis : The thermogravimetric analy-sis of complex 1 indicates the occurrence of four consecutivesteps (Figure S9 in the Supporting Information). In the firststep, the guest water molecules are removed within the tem-perature range of 80–150 8C. The partially dehydrated phaseis also stable up to 170 8C and the coordinated water mole-cules are lost from the framework within the temperaturerange of 170–220 8C. Consequently, after removal of coordi-nated water molecules the framework starts to decompose.

The phase purity of the as-synthesized complex was exam-ined by PXRD analysis. As synthesized compound washeated at 150 8C for 4 h to remove guest water moleculesand was then characterized by PXRD analysis. The PXRDanalysis indicates that after loss of guest water moleculesthe framework remains crystalline (2) and is different fromthe original complex 1. The PXRD pattern reveals thatupon removal of guest water molecules the peak at 7.908shifts to a lower angle of 7.728, thus indicating a slight in-crease in the interlayer separation. All PXRD patterns arepresented in Figure S10 of the Supporting Information.

Dynamic nature—Single-crystal-to-single-crystal transforma-tion : Guest-templated host frameworks undergo structuraltransformation depending upon the nature and amount ofguest molecules within the framework.[11] In most cases, sucha structural transformation upon removal of the guest mole-cules leads to a shrinking of the effective void space, but inrare cases expansion has been found to occur.[12]

Complex 1 was heated at 150 8C for 1 hour and, interest-ingly, after heating, the retention of single crystallinity wasobserved with a change of color from light blue to verydeep blue (complex 2). SC-XRD analysis indicated thatcomplex 2 crystallized in the monoclinic space group Pc(Table S1 in the Supporting Information). Due to heating,the length of the a axis decreased from 22.389 to 12.999 �and also the b angle changed from 90 to 118.858 withchange of the Z value from 4 to 2. Structural analysis alsorevealed that upon removal of guest water molecules, boththe A-type and B-type layers remain identical with respectto their coordination mode like complex 1, but the interlay-er separation between the 2D layers increases (from 5.861to 5.962 � and 5.333 to 5.424 �; Figure S11 in the Support-ing Information) through the sliding of 2D coordinationlayers along the a axis. As a result, the effective void spaceincreases by 0.3 %. Such dynamicity of this framework arisesfrom the interdigitation of the 4,4’-bipy unit of the B layerinto the groove of the A layer.[13]

Adsorption study : To measure the porosity of the evacuatedframework (2), we carried out a gas adsorption study. Itshows no uptake of N2 (8 mL g�1) and little uptake of CO2

(15 mL g�1) at 77 and 195 K, respectively. CO2 has smaller

kinetic diameter (3.3 �) and a larger quadrupole momentthan N2, so it can interact with the host framework, there-fore there is little uptake and hysteresis behavior (Fig-ure S12 in the Supporting Information).

Size-selective bicatalytic activities : Porous metal–organicframeworks are highly attractive for heterogeneous size-se-lective catalytic activities. Generally, MOFs that containopen metal sites can be used in Lewis acidic catalysis suchas oxidation, cyanosilylation, and so forth.[5,14] And MOFsthat contain Lewis basic free FOSs can be utilized for Lewisbasic catalysis such as Knoevenagel condensation reac-tions.[4c,9] The framework contains both Lewis acidic five-co-ordinated CuII centers as well as Lewis basic free pyridylgroups with free �C=O groups. Owing to the presence ofboth Lewis acidic and Lewis basic functional groups, wehave attempted to study both catalytic oxidation and Knoe-venagel condensation reactions using the evacuated frame-work (2) as catalyst.

Catalytic oxidation reactions : Catalytic oxidation reactionswere carried out in the presence of oxidant (tert-butyl hy-droperoxide (TBHP)) and in the absence of oxidant (aero-bic oxidation).

Styrene was selected as the model substrate to optimizethe catalytic oxidation reaction conditions. In this study, thereaction was carried out with 70 wt % TBHP in water as oxi-dant and acetonitrile as solvent at a constant temperature of50 8C. Samples were taken out from the reaction mixtureafter certain intervals and were analyzed by gas chromatog-raphy. Catalytic oxidation of styrene resulted in the forma-tion of benzaldehyde, styrene epoxide, and phenylacetalde-hyde (Scheme 2 and Figure S13 in the Supporting Informa-

tion). The main results are presented in Figure 2 (Table 1).A blank experiment in the absence of a framework withTBHP results in low conversion of styrene. Next, anotherexperiment was carried out with neat styrene without anysolvent. In this case, primarily benzaldehyde was obtainedwith low substrate conversion. Conversion of styrene in-creases with time and reaches a maximum of 80 % at 8 hwith an increase in the selectivity towards aldehyde. In thiscatalytic reaction, benzaldehyde predominates among theoxidation products with a selectivity of 63 % (24 % epoxideand 13 % phenylacetaldehyde). The selectivity of benzalde-hyde can be increased up to 84 % by increasing the TBHPmolar ratio but substrate conversion remains almost con-stant. Further decreasing the molar ratio of TBHP leads to

Scheme 2. Catalytic oxidation of styrene at 50 8C.

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a sharp decrease in substrate conversion but the distributionof products remains almost the same as mentioned earlier.

In the case of 4-cholorstyrene, conversion of styrene de-creases substantially to 60 % with a product distribution thatcomprises 4-chlorobenzaldehyde as the major product ac-companied by 4-chlorophenylacetaldehyde and 4-chlorostyr-ene oxide. Owing to the larger size of 4-chlorostyrene, weget a smaller conversion (60 %), and also due to the pres-ence of a chloride group, the selectivity of epoxide forma-tion is enhanced (29 %). a-Methylstyrene showed 42 % con-version with 75 % formation of acetophenone and 18 % ofpropiophenone in 8 h with TBHP (Table 1).

Catalytic aerobic oxidation reaction : Recently, aerobic oxi-dations of a series of substrates were carried out by usingdifferent MOFs as catalyst under different reaction condi-tions.[15] Herein, we have also studied the oxidation of ole-fins under aerobic conditions at 50 8C in acetonitrile(Table S6 and Figure S14 in the Supporting Information).Olefin oxidations using our framework as catalyst were car-ried out using O2 and small additives of TBHP as initiatoror by using only O2 as oxidant. In both cases, all the olefinsare selectively converted to allylic products (Scheme 2).

When O2 was used as oxidant, the conversion of styrenewas 12 % with benzaldehyde as the major product (62 % se-lectivity). The addition of TBHP (0.1 mmol) to the reactionmixture increased the conversion to 37 % with a slight in-crease in the selectivity of benzaldehyde. Similar aerobic ox-idation reactions were also carried out for 4-chlorostyreneand a-methylstyrene (Table S6 in the Supporting Informa-tion). Furthermore, a comparison of the catalytic efficiencyof this framework in the oxidation of styrene with other re-ported MOFs clearly indicates that it performs as a bettercatalyst than other reported MOFs.[15]

Catalytic Knoevenagel condensation reactions : The frame-work was also used as catalyst in the liquid-phase Knoeve-nagel reaction by studying the condensation of benzalde-hyde with malononitrile to form benzylidenemalononitrileas the probe reaction (Scheme 3).

The Knoevenagel condensa-tion reaction is an important re-action for the generation ofC�C bonds. As a greener alter-native, the Knoevenagel con-densation reaction was carriedout at room temperature usingethanol as solvent. The ratio ofaldehyde and malononitrile isan important factor in Knoeve-nagel condensation. The effectof the molar ratio of benzalde-hyde/malononitrile on the reac-tion conversions was thereforeinvestigated, and the reactions

were carried out at molar ratios of 1:0.5, 1:1, and 1:2, respec-tively. The products were characterized by GC, GC-MS, andNMR spectroscopic studies (Figures S15 and S16 in the Sup-porting Information). Experimental results showed that90 % conversion was obtained after 4 h of reaction ata molar ratio of 1:2. As expected, decreasing the benzalde-hyde/malononitrile molar ratio to 1:1 resulted in a slightdrop in the reaction rate; however, 90 % conversion couldbe achieved after 6 h (Table 2). The reaction using half anequivalent of malononitrile proceeded slowly, with only61 % conversion being observed after 6 h (Figure S17 in theSupporting Information).

Now, to investigate the effect of the catalyst amount onthe reaction conversion, the reactions were carried out atroom temperature by using a benzaldehyde/malononitrilemolar ratio of 1:1. It was observed that 82 % conversion ofbenzaldehyde was achieved after 4 h with 25 mg catalyst,

Figure 2. The time-dependent conversion of styrene and formation of thethree products A–C catalyzed by the framework at 50 8C.

Table 1. Oxidation of various olefins with TBHP catalyzed by the framework.[a]

Entry Substrate Oxidant Conv. Selectivity [%][b]

[%][b] aldehyde epoxide others

1 styrene TBHP[c] 3 79 19 traceTBHP[d] 8 64 23 13[g]

TBHP 80 63 24 13[g]

TBHP[e] 82 84 12 4[g]

TBHP[f] 36 54 30 16[g]

2 a-methylstyrene TBHP 42 75[h] 7 –18[i]

3 4-chlorostyrene TBHP 60 58 29 13[j]

[a] Reaction conditions: olefin (5 mmol), complex 1 (20 mg), acetonitrile (5 mL), O2 purged through balloon,50 8C, 8 h. [b] Conversion determined by GC and GC-MS. [c] Without catalyst. [d] Without solvent.[e] 15 mmol TBHP. [f] 5 mmol TBHP. [g] Phenylacetaldehyde. [h] Acetophenone. [i] Propiophenone.[j] 4-Chlorophenylacetaldehyde.

Scheme 3. Catalytic Knoevenagel condensation reaction.

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whereas 75 % conversion was obtained in the case of 20 mgcatalyst over the same reaction time. However, in both casesthe same amount of conversion of benzaldehyde was ach-ieved after 6 h. It was found that decreasing the catalystamount to 15 mg led to a significant drop in the reactionrate, with 73 % conversion being achieved after 6 h. As ex-pected, a lower reaction rate was observed for the reactionusing 10 mg catalyst, although the reaction afforded 54 %conversion after 6 h (Figure 3).

Cation sensor activity : Pyridyl groups and carboxylateoxygen atoms are efficient in binding cations, and hence thecation sensing activity of the framework was investigated.The evacuated framework was immersed in solutions thatcontained different metal ions, namely, Mg2+ , Ni2+ , Zn2+ ,Hg2+ , and Pb2+ , in DMF with a concentration of 0.1 mmoleach to form a metal-ion-incorporated microcrystallinesolid. The framework shows selectivity toward the Ag+ ionover the other tested cations. The selectivity and sensitivityof the framework towards Ag+ have been proved by solid-state UV/Vis and fluorescence measurements (Figure S18 inthe Supporting Information). Solid-state UV/Vis measure-ments have showed that the framework exhibits two reflec-tance bands at approximately 300 nm and approximately635 nm, which are assigned to the p–p*/n–p* transition ofthe ligand p system and the d–d transition of the framework,respectively.

For the Ag+-incorporated framework, the lower-energyband broadens drastically, whereas the higher-energy band

remains almost unaffected, and small changes were ob-served for other ions. The prominent broadening of the d–dtransition band is indicative of the distortion of the axial co-ordination position of CuII of the framework due to strongerbinding of Ag+ with the pyridyl nitrogen of pendant 4,4’-bipy. The framework shows a broad emission band at ap-proximately 405 nm upon excitation at approximately300 nm (Figure 4). A small amount of quenching was ob-served for Mg2+ , Ni2+ , Hg2+ , and Pb2+ , thus indicatingweaker interactions between the framework and these cat-ions. The Zn2+ ion shows negligible fluorescence enhance-ment. Importantly, the emission intensity of the Ag+-incor-porated framework was drastically reduced. This significantquenching in fluorescence (�1/4 times) indicates strongerbinding between the framework and the Ag+ ion.

Small-molecule sensor activity : Owing to the presence ofopen metal sites within the framework, the framework canpotentially be used as a small-molecule sensor material.Upon excitation at approximately 300 nm, the evacuated

Table 2. Knoevenagel condensation of benzaldehyde with malononitri-le.[a]

Entry Aldehyde Product t [h] Conversion [%][b]

1 6 90 (86)

[a] Reaction conditions: aldehyde (1 mmol), malononitrile (1 mmol), eth-anol (5 mL), catalyst (20 mg), RT, 6 h. [b] Determined by GC. Isolatedyields are given in parentheses.

Figure 3. Effect of catalyst amount on Knovenagel reaction conversion ofbenzaldehyde and malononitrile to benzilidinemalononitrile.

Figure 4. Fluorescence spectra of 2 with different cations.

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framework (2) shows emission maxima at approximately420 nm. This evacuated framework was soaked in differentsolvents, namely, MeOH, DMF, CH3CN, EtOH, and ace-tone. The fluorescence studies indicate that in the case ofMeOH, large quenching occurs, whereas for others a smallamount of quenching was found (Figure 5); acetone quench-ing is the lowest. It may be concluded that MeOH pene-trates into the void space and strongly binds with the metalions due to its small size and large polarity, whereas nonpo-lar acetone binds with metal centers to a lesser extent andthus quenching becomes smaller. PXRD analyses also cor-roborate these findings (Figure S19 in the Supporting Infor-mation). Upon soaking in MeOH, large changes in thePXRD pattern were noticed.

SHG-NLO activity : Owing to their high thermal stability,neutral noncentrosymmetric MOFs are considered potentialNLO-active materials. Apparently, the topological structuresof the resultant coordination polymers are mainly depen-dent on the geometry of metal centers and the binding

mode of bridging ligands. In 1, all pendant water moleculesfrom A layers and all pendant 4,4’-bipy from B layers areoriented in the same direction, which develops a large polar-ity within the framework (Figure S20 in the Supporting In-formation). Compound 1 is also crystallized in the noncen-trosymmetric space group Aba2. SHG measurements[16] re-vealed that 1 displays an SHG efficiency 10.2 times that ofKDP, the second highest among reported coordination poly-mers until now.[2b]

Conclusion

In conclusion, we have synthesized a new cocrystalline dy-namic framework that contains both open metal sites(OMSs) and free functional organic sites (FOSs) and wehave successfully established its multiple functionalities.This is the first example of an inorganic cocrystal of 2D co-ordination polymers. One layer consists of a [Cu ACHTUNGTRENNUNG(ina)2]n

square grid with a pendant water molecule, and the otherlayer consists of a similar [Cu ACHTUNGTRENNUNG(ina)2]n square grid witha pendant 4,4’-bipyridine. Hydrogen-bonding interactionsbetween these 2D layers assemble them in a single-crystalstructure, thereby leading to cocrystallization. Upon remov-al of guest water molecules, 1 undergoes SC-to-SC transfor-mation with concomitant interlayer void-space expansion;the framework is highly flexible in nature.

The five-coordinated metal centers act as open metal sitesand thus the framework shows size-selective Lewis acidiccatalytic activity for the oxidation of styrene derivatives. Iteven responds to catalytic oxidation reactions under aerobicconditions. Owing to the presence of such OMSs, it showssolvent sensor activity with selectivity and sensitivity towardsmall and polar methanol. Consequently, the free pyridylgroups that are present in the framework act as free func-tional organic sites, and thus the framework shows Lewisbasic catalytic activity for the condensation reaction of ben-zaldehyde with malononitrile, yet it also performs asa cation sensor material with selectivity and sensitivity to-wards Ag+ . Furthermore, owing to its noncentrosymmetricnature and high polarity on account of the orientation ofpendant 4,4’-bipyridyl and water from the nonpolar 2Dsheets in the same direction, the framework shows strongSHG-NLO activity. Such structural versatility toward multi-functionality that has been accomplished here is really un-precedented and also creates an innovative route toward thedesign and fabrication of new multifunctional metal–organicmaterials.

Experimental Section

General : Copper (II) nitrate hexahydrate, isonicotinic acid, and 4,4’-bi-pyridine were purchased from Merck chemical company. All other chem-icals used were of analytical reagent grade. Elemental analysis (C, H, N)was carried out using a Perkin–Elmer 240C elemental analyzer. The ther-mal analysis was carried out using a Mettler Toledo TGA/SDTA 851thermal analyzer under flow of N2 (30 mL min�1). The sample was heated

Figure 5. Fluorescence spectra of 2 with different small-solvent mole-cules.

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at a rate of 10 8C min�1 with inert alumina as a reference. IR spectra wererecorded using a Nicolet Impact 410 spectrometer between 400 and4000 cm�1 by using the KBr pellet method. Photoluminescence spectrawere collected using a Shimadzu RF-5301PC spectrophotometer. The ad-sorption isotherms of N2 at 77 K and CO2 at 195 K were measured usinga Quantachrome Quadrasorb-SI analyzer.

Synthesis of complex 1 {[Cu ACHTUNGTRENNUNG(ina)2 ACHTUNGTRENNUNG(H2O)][CuACHTUNGTRENNUNG(ina)2 ACHTUNGTRENNUNG(bipy)]·2H2O}n : Anaqueous solution of sodium isonicotinate (0.190 g, 2 mmol) and 4,4’-bipyr-idine (0.078 g, 0.5 mmol) in methanol (H2O/MeOH 2:3 v/v) was carefullylayered on an aqueous solution of copper(II) nitrate hexahydrate(0.241 g, 1 mmol). Deep blue block-shaped single crystals suitable forX-ray diffraction were grown at the junction of the two layers after oneweek. Yield: 83 %. IR: n= 3444 (br), 2926 (w), 1643 (s), 1556 (m), 1376(s), 1218 (w), 774 (m), 698 cm�1 (s); elemental analysis calcd (%) forC34H30N6O11Cu2: C 49.45, H 3.64, N 10.19; found: C 49.52, H 3.68, N10.12.

Synthesis of complex 2 {[Cu ACHTUNGTRENNUNG(ina)2 ACHTUNGTRENNUNG(H2O)][CuACHTUNGTRENNUNG(ina)2 ACHTUNGTRENNUNG(bipy)]}n : Complex 2was synthesized by heating complex 1 at 150 8C for 1 h. IR: n =3444 (br),2926 (w), 1643 (s), 1556 (m), 1376 (s), 1218 (w), 774 (m), 698 cm�1 (s); el-emental analysis calcd (%) for C34H26N6O9Cu2: C 51.64, H 3.29, N 10.63;found: C 51.68, H 3.32, N 10.60.

Crystallographic data collection and refinement : Single crystals of com-plexes 1 and 2 were mounted on a Bruker SMART CCD diffractometerequipped with a graphite monochromator and MoKa (l=0.71073 �) radi-ation. The structures were solved by the Patterson method and followedby successive Fourier and difference Fourier synthesis. Full-matrix least-squares refinements were performed on F2 using SHELXL-97 with aniso-tropic displacement parameters for all non-hydrogen atoms. The hydro-gen atoms were refined isotropically, and their locations were determinedfrom a difference Fourier map. All calculations were carried out usingSHELXL 97,[17] SHELXS 97,[18] PLATON 99,[19] ORTEP-32,[20] andWinGX system v1.64.[21]

Crystal data for 1: C34H30Cu2N6O11; orthorhombic, space group Aba2-cba ;a= 22.3890, b= 13.2170, c =11.8610 (5) �; V=3509.853 �3; Z =4; T=

293 K; R =0.0560, wR2 =0.1610, GoF =1.13.

Crystal data for 2 : C34H26Cu2N6O9; monoclinic, space group Pc ; a=

12.999, b= 11.889, c=13.000 �; b =118.858 ; V=1759.73 �3; Z=2; T=

293 K; R =0.0439, wR2 =0.1370, GoF =1.02.

CCDC-917902 (1) and -917903 (2) contain the supplementary crystallo-graphic data for this paper. These data can be obtained free of chargefrom The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Second-harmonic measurements : For SHG experiments, powder sampleswere illuminated by a Q-switched Nd:YAG laser beam of fundamentalwavelength 1064 nm (Spectra Physics, PROLAB 170, pulse width 10 ns,and repetition rate 10 Hz). The beam was passed through a couple ofhigh-energy laser mirrors and a long-pass colored glass filter before beingfocused onto a glass capillary using a converging lens of 200 mm focallength. The incoherently scattered SH photons were collected in thetransverse direction using a combination of a monochromator and a pho-tomultiplier tube. The second-harmonic signal was then sampled, aver-aged over 512 shots, and recorded using a digital storage oscilloscope.KDP powder was used as the reference material.

Experimental details for catalysis—Materials and methods : Olefins, alde-hydes, malononitrile, and TBHP (70 % in water) were purchased fromMerck Chemical Company and used without further purification. Productanalysis was performed using a Varian 3400 gas chromatograph (GC)equipped with a 30 m CP-SIL8CB capillary column and a flame ioniza-tion detector. All reaction products were identified by using a TraceDSQ II GC-MS.

Catalytic oxidation reaction : Catalytic experiments were carried out inthermostatted (50 8C) glass vessels under vigorous stirring. Typically, thereaction of styrene oxidation was started by the addition of TBHP(10 mmol) to a mixture that contained styrene (5 mmol) and complex 2(catalyst 20 mg). In the experiments with molecular oxygen, TBHP(0.1 mmol) was added to a mixture that contained 2 (20 mg) and styrene(5 mmol) and had been preliminarily blown with O2 (1 atm). A sample

was taken out periodically to monitor the reaction. The products wereanalyzed by GC and GC-MS. After the reaction, the catalyst was re-moved by filtration, washed with acetone, and dried at room temperatureovernight. A similar procedure was followed for the reusability tests.

Catalytic Knoevenagel condensation reaction : The Knoevenagel reactionbetween aromatic aldehyde and malononitrile using 2 (catalyst) was car-ried out in a magnetically stirred round-bottomed flask. A mixture of 2(20 mg) and benzaldehyde (1 mmol) was placed into a 25 mL flask thatcontained ethanol (5 mL). The reactants were stirred for 5 min to dis-perse 2 in the liquid phase. A solution of malononitrile (1 mmol) in etha-nol (1 mL) was then added, and the resulting mixture was stirred at roomtemperature for 6 h. Reaction conversion was monitored by withdrawingaliquots from the reaction mixture at different time intervals, analyzingthem by GC, and further confirming the product identity by means ofGC-MS. The catalyst was separated from the reaction mixture by simplefiltration, washed with ethanol and dichloromethane, dried undervacuum for 6 h, and reused when necessary.

Acknowledgements

R.S. is thankful to the CSIR for a research fellowship (09/096 ACHTUNGTRENNUNG(0565)2008-EMR-I). Thanks to Prof. P. K. Das and his student Mr. R. Pandey, IISc.,Bangalore, India for SHG measurements. We are grateful to Dr. D. Dasof the UGC-DAE CSR Kolkata Center for Magnetic Studies. Thanks toS. K. Ghosh, IISER Pune, Prof. S. Paul, Department of Life Science andBio-Tech., J. U., and Dr. K. Pramanik, Department of Chemistry, J. U.India, for scientific suggestions. B.J. is also thankful to CSIR for a re-search fellowship (09/936 ACHTUNGTRENNUNG(0085)/2013 EMR-I).

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Received: May 31, 2013Published online: October 23, 2013

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