Received 12th January 2014,Accepted 24th February 2014
DOI: 10.1039/c4gc00056k
www.rsc.org/greenchem
Aerobic homocoupling of arylboronic acidscatalysed by copper terephthalate metal–organicframeworks†
Pillaiyar Puthiaraj,a Palaniswamy Suresha,b and Kasi Pitchumani*a,b
Copper terephthalate MOF is utilised as an environmentally benign, efficient and reusable heterogeneous
catalyst to effect the aerobic homocoupling of arylboronic acids yielding the corresponding symmetrical
biphenyls under mild reaction conditions. This method tolerates various substituents present in arylboro-
nic acids such as halogens, cyano and nitro groups. The catalytic performance has been compared with
that of other copper based MOFs namely MOF-101, [Cu(pdc)2]NH2Me2, [Cu2(ndc)2ted]n and [Cu(H2L)]n as
well as with other copper salt catalysts. Sheldon test confirmed the heterogeneity of the catalyst, which
can be reused under optimized conditions with only a minor loss in its activity. A mechanism for the
homocoupling reaction is also proposed. The simplicity of catalyst preparation, its stability, substrate
selectivity, easy recovery and regeneration designate possible utilization of this catalytic system in a multi-
tude of catalytic reactions and industrial processes.
Introduction
Metal–organic frameworks (MOFs) have emerged as a hottopic in heterogeneous catalysis.1 They are composed of metalions as nodes and organic ligands as linkers, creating infinitepolymeric frameworks with regular void spaces that canaccommodate small gas and solvent molecules in their pores.2
Over the past decade, chemists have focused only on designand synthesis of new MOFs and studying their role in gasstorage, separations and sensors.3 MOFs have recentlyemerged as a particular class of functional materials owing totheir high inner surface area, tenability of pore size, chemicaltenability and topologies.4 MOFs contain a large percentage oftransition metals that can act as catalytic sites5 provided thatthey have free coordination positions not compromised in theconstruction of MOFs. These materials are expected to havemany similarities from the structural point of view with zeo-lites and related microporous solids. Consequently MOFs areattractive catalysts for organic transformations, though,however, only a relatively limited number of studies havefocussed on the investigation of the catalytic activity of theseMOFs.1a,c,6 Another advantage is, compared to conventionallyused microporous and mesoporous inorganic materials like
zeolites, these MOFs have the potential for a more flexiblerational design through control of the architecture,functionalization of the pores and substantially higher metalloading, which offers the opportunity to significantly reducethe overall amount of catalyst, provided that the internal metalcentres are accessible to the substrates.7
Aryl C–C (sp2–sp2) bond formation is an important andchallenging process in both synthetic and industrial points ofview. Symmetrical and unsymmetrical biaryls are importantstructural motifs exhibiting a wide variety of physical andchemical properties8 with versatile applications in drugs, agro-chemicals, dyes, semi-conductor and optically active ligands.9
These couplings have been achieved by palladium catalysedprocesses such as Suzuki reaction, modified Ullmann reactionand Hiyama–Kumada reaction.10 Particularly, palladium cata-lysed homocoupling of boronic acids has been extensivelyreported.11 However there are limitations in these palladiumcatalysed methods: (1) palladium is expensive and additionalligands are needed to stabilize the palladium species,11c,i,j (2)stoichiometric amount of oxidants are needed to restore thecatalytically active palladium(II) species,11g,h (3) require a baseto improve the yields,11c,i, j and (4) need of high temperature.11h
Consequently many research groups have reported the homo-coupling of boronic acids also by other metals.12 The syntheticuse of boronic acids are more common as they are more stableand less toxic than other organometallic reagents.13 Generallyhomocoupling of arylboronic acids is also very slow.14 In con-cordance with the principles of green chemistry,15 air is anideal oxidant because of its abundance, low cost, safety, lack oftoxic by-products and it is cheap for industrial applications.16
†Electronic supplementary information (ESI) available. See DOI:10.1039/c4gc00056k
aSchool of Chemistry, Madurai Kamaraj University, Madurai 625 021, IndiabCentre for Green Chemistry Processes, School of Chemistry, Madurai Kamaraj
Recently, Singh et al. reported the clay encapsulated Cu(OH)xcatalysed homocoupling of arylboronic acids,17a Xiao et al.reported the Mg–Al oxides stabilised gold nanoparticles cata-lysed homocoupling reaction17b and Sakurai et al. reported thegold nanoparticles catalysed homocoupling of arylboronicacids under acidic conditions.17c Fe3O4 nanoparticle-supportedCu(II)-β-cyclodextrin complex is also employed as a reusablecatalyst for homocoupling of arylboronic acids.18 However, pre-vious investigations of copper-mediated coupling reactions forthe synthesis of biaryls required a stoichiometric amount ofcatalyst and ligand, and a higher reaction time.12 Althoughhomogeneous catalysts have been extensively investigated forcoupling reactions, the challenges are very significant as thecatalysts are expensive, metal contamination in the reactionmixture, cannot be reused and the products are difficult to sep-arate from the reaction mixture, chances of undesired productformation due to the requirement of ligands and additionalreagents.19 In order to overcome these problems, MOFsprovide unique advantages as these heterogeneous materialsare easily prepared and are cheap. Recently, Yaghi et al.reported the Cu3(BTC)2 catalysed homocoupling of arylboronicacids with external cyclohexylamine base20 and observed thearylated Chan–Lam cross coupling by-products. Corma et al.reported the Cu(BDC) MOF as catalyst for three-componentcouplings of amines, aldehydes and alkynes.21 Recently Phanet al. reported the use of Cu(BDC) MOF as catalyst for themodified Friedlander reaction.22 To the best of our knowledge,there are no reports describing the copper terephthalate MOFcatalysed homocoupling of arylboronic acids.
In this study, we have demonstrated copper terephthalateCu(BDC) MOF as an efficient heterogeneous reusable catalysttowards the aerobic homocoupling of arylboronic acidswithout losing the crystallinity and structure of the Cu(BDC)MOF (Scheme 1). The Cu(BDC) catalysed protocol offersseveral advantages, compared to the conventional approach inthe formation of biphenyl derivatives and the observed resultsare discussed below.
Results and discussion
Herein, we report the heterogeneous copper terephthalateMOF catalysed homocoupling of arylboronic acids in air atroom temperature. To the best of our knowledge, the MOFcatalysed aerobic homocoupling of arylboronic acids has notbeen reported so far. In addition, Cu(BDC) MOF is shown asan efficient and environmentally-friendly heterogeneous, reu-sable catalyst. The various factors determining the reactivity ofboronic acids under different experimental conditions are dis-cussed below.
Initially phenylboronic acid was selected as a model sub-strate in the presence of different catalysts under various con-ditions with the aim of optimizing the yield and the results aresummarized in Table 1. All reactions were conducted at roomtemperature (RT) in air. No product was detected either in theabsence of catalyst or in an inert atmosphere (Table 1, entries1 and 32). When the reaction was conducted using Cu(BDC)MOF in DMSO, a 75% yield of homocoupling product(Table 1, entry 2) was observed. When the reaction was con-ducted using other copper sources like Cu(OAc)2, CuSO4·5H2Oand CuCl2 respectively (Table 1, entries 3–5) a lower yield ofhomocoupling products was observed (42, 18 and 31%). Whenthe amount of Cu(OAc)2, CuSO4·5H2O and CuCl2 catalysts wasreduced to 50 mol%, the homocoupled product was alsoreduced to 34, 12, 22%. When Cu(NO3)2·3H2O was used ascatalyst, no product was observed but in the presence of ter-ephthalic acid as an additive, a very low yield was noticed(Table 1, entries 6 and 7). This observation of a significantlydecreased yield with other Cu(II) sources, eliminates the possi-bility of a Cu(0)/Cu(II) cycle in the present study as the majorreaction pathway. These results indicate that the catalyticcentres might be ligated by the terephthalic acid, whichcannot be easily approached by the substrates. When the reac-tion was conducted using Cu(BDC) MOF in DMF, an excellentyield of homocoupling product (Table 1, entry 8) was obtained.One of the main reasons for the high catalytic activity ofcopper terephthalate MOF is that it is a porous material andhas a high surface area,23 which is favourable for the accessi-bility of reactants to the active metal sites of MOF. To gain aninsight into the role of the substituents, the presence ofheteroatoms and the size of the linkers, the following fouradditional MOFs namely MOF-101, [Cu(pdc)2]NH2Me2,[Cu2(ndc)2ted]n and [Cu(H2L)]n, were synthesised (see ESI† fortheir structure and characterization) and their catalytic activityin the homocoupling reaction was studied (Table 1, entries9–12). With an ortho-substituent namely bromine as inMOF-101, a small decrease in yield was noticed (71%), whichmay be attributed to steric effects. When a heteroatom ispresent in the linker as in [Cu(pdc)2]NH2Me2 MOF, thedecrease in the yield of homocoupled product was significant,probably due to the presence of an alternative binding site.With more bulkier and flexible linkers as in [Cu2(ndc)2ted]nand [Cu(H2L)]n, and the presence of more heteroatoms as in[Cu2(ndc)2ted]n and [Cu(H2L)]n, the homocoupling reactiondid not proceed at all, indicating clearly that steric effects andthe presence of heteroatoms play a very significant role in theabove reaction. It is also likely that in these cases, the linkersthemselves bind strongly to the central copper metal atomthereby affecting the overall reaction. When the reaction wasalso conducted with MOF-101, [Cu(pdc)2]NH2Me2, [Cu2(ndc)2-ted]n and [Cu(H2L)]n, MOFs containing an identical amount ofcopper species as Cu(BDC), the homocoupled product wasobtained in only 76 and 62%, and there was no reaction withthe other two MOFs (Table 1, entries 13–16). Interestingly,Cu(BDC) MOF showed no catalytic activity in non-polar sol-vents, namely toluene, xylene, DCM, DCE, dioxane, THF andScheme 1 Homocoupling of arylboronic acids.
CHCl3 for the homocoupling reaction of arylboronic acid(Table 1, entries 18–24). As the aryl substrates are more solublein nonpolar solvents, their binding to Cu(BDC) MOF is veryslow, resulting in the absence of homocoupling. When thereaction was conducted in protic polar solvents like water,methanol and ethanol, the biphenyl product was obtained inonly 8, 26 and 42% yields respectively, along with consider-able amounts of phenol (Table 1, entries 25–27). When proticsolvents were used, the conversion of the C–B bond into a C–Obond via a peroxyboronate intermediate was promoted result-ing in a considerable amount of phenol formation.17c Gener-ally, as the base can improve the reaction yield for somecatalytic systems,24 we have used different bases for this reac-tion and only 20–60% yields were observed (Table 1, entries28–31). When carried out in DMF–water (1 : 1) mixture, 68%yield of the biphenyl and 12% of phenol were observed(Table 1, entry 33).
The biaryl yield was also found to increase rapidly with anincrease in the amount of catalyst. As the amount of catalystwas increased to 100 mg, biphenyl was obtained in highestyield and the further addition of catalyst had no obviouseffects on the yield of biphenyl (Fig. 1). Thus the optimizedconditions for the homocoupling of arylboronic acid are use ofCu(BDC) MOF as a catalyst in DMF under air for 16 h at roomtemperature.
This Cu(BDC) MOF promoted aerobic homocoupling reac-tion was also successfully extended to various substituted aryl-boronic acids. As depicted in Table 2, this reaction worked verywell for a wide range of substrates with both electron-donatingand electron-withdrawing substituents. It can be concludedthat the nature of the substituent, either electron-donating orwithdrawing, did not show any significant change in theoverall yield. Both para- and meta-substituted arylboronic acidsgave homocoupling products in excellent yields (Table 2,
a Reaction conditions: phenylboronic acid (2 mmol), catalyst (100 mg), solvent (2 mL), RT. b Isolated yield. c 100 mol% of catalyst. d 155 mg ofcatalyst. e 130 mg of catalyst. f 105 mg of catalyst. g 120 mg of catalyst. hNitrogen atmosphere. i 50 mol% of catalyst; H2L = (Z)-4-((2-hydroxynaphthalen-1-yl) methyleneamino)benzene-1,3-dioic acid; ndc = 1,4-naphthalenedicarboxylic acid; ted = triethylenediamine; BDC = 1,4-benzenedicarboxylic acid.
entries 1–16). But in the case of ortho-substituted arylboronicacids only lower yields (Table 2, entries 17–19) are observed.The hetero arylboronic acids also gave only moderate yields(Table 2, entries 20 and 21), as the heteroatom is likely to bindwith the metal centre. Bulkier substrates such as naphthalene-
1-boronic acid, 2,6-difluorophenylboronic acid and 2,4,6-tri-methylphenylboronic acid did not give the correspondinghomocoupling products (Table 2, entries 22–24) as the steri-cally hindered boronic acids did not bind with MOF metalcenters. The biphenyl and their derivatives were characterisedby m.p. and NMR techniques.
To verify whether catalysis by Cu(BDC) MOF is truly hetero-geneous or due to some leached copper species present in thefiltrate, the Sheldon test was performed. The reaction wascarried out under the optimized conditions and the copper ter-ephthalate MOF catalyst was filtered from the reaction mixtureat 48% of biphenyl formation (Table 3). After removal of MOFcatalyst, the filtrate was further stirred for an additional 6 hand no further biphenyl product was observed. The absence ofmetal leaching was also confirmed from atomic absorptionspectroscopy analyses of the filtrate from the reaction mixtureand also of the filtrate from a stirred solution of Cu(BDC) inDMF under identical reaction conditions. Thus the atomicabsorption spectroscopic data clearly demonstrate thatCu(BDC) MOF is truly heterogeneous in nature.
Reusability of the Cu(BDC) MOF was also studied. Aftercompletion of the reaction, the catalyst was recovered by fil-tration and washed with ethyl acetate, heated with 2 mL offresh DMF at 100 °C for 2 h, activated under vacuum at roomtemperature for 4 h, which was subsequently reused and theresults are presented in Fig. 2. It is clear that the decrease in
Fig. 1 Dependence of yield on the amount of catalyst for homo-coupling of phenylboronic acid.
Table 2 Cu(BDC) MOF catalysed homocoupling of various arylboronicacidsa
the activity of the catalyst resulted in a minor loss of productyield even after five times of its reuse.
Comparison of the powder XRD patterns of the fresh, threeand five times reused MOF were also recorded (Fig. 3) whichclearly show that the reused catalyst exhibits a similar powderXRD pattern. The specific surface area and average porevolume for the fresh and reused Cu(BDC) MOF were measuredusing BET surface area analysis. The BET isotherms show asmall decrease in the surface area (590 m2 g−1 and 564 m2 g−1
for fresh Cu(BDC) and fifth time used Cu(BDC) respectively).The pore volume of the fresh and reused MOF is 0.268 cm3 g−1
and 0.264 cm3 g−1 from the BET analysis. These BET surfacearea analyses and powder XRD pattern results clearly demon-strate that no appreciable change in the structural integrity isobserved during/after the reuse.
To demonstrate the potential utility of this method for pre-parative purposes, the reaction was also carried out in the opti-mized reaction conditions on a 1.2 gram scale, giving 94%yields which are comparable to those obtained for a small-scale reaction (reaction condition: phenylboronic acid, 10 mLDMF, RT, 16 h).
By analogy with these reports and previous report,12a–d,18,20,34
a plausible reaction pathway is proposed as shown inScheme 2. The copper terephthalate MOF shows a largersurface area and the presence of the copper atom and itsexposed apical coordination sites might also lead to catalyticapplications.23 Terephthalate ligands are coordinated in abidentate bridging fashion to Cu(II) dimer, separated verticallyby 2.63 Å. Each Cu(II) is also coordinated to a molecule of DMFto give the Cu(II) atom a square-planar geometry. The catalyti-cally active copper present in the channel walls of MOF canpromote the homocoupling reaction of various arylboronicacids. During the homocoupling reaction, in the first step thesolvent molecule may be replaced by the arylboronic acid. Theproposed mechanism involves a Cu(I)/Cu(III) cycle. Double
transmetallation of Cu(II) with two molecules of arylboronicacid affords a Ar–Cu(II)–Ar, which undergoes air oxidation toyield a Cu(III) intermediate, reductive elimination of whichreleases the homocoupled product Ar–Ar. An alternativepathway involving a Cu(0)/Cu(II) cycle is ruled out on the basisof the control experiments discussed earlier.
With a rigid framework as in the present MOF, a bimetallicmechanism is also likely to yield the homocoupled product.
In the IR spectrum of the copper containing MOF,recorded with the reaction intermediate, the DMF carbonylstretching frequency 1663 cm−1 had disappeared and theB–OH band shift from 3480 to 3442 cm−1 peak was observed(Fig. 4). In the original catalyst, there is no peak around3442 cm−1 (Fig. 5b).
Data on reaction conditions, activity and efficiency of thevarious other catalysts employed earlier for the homocouplingof arylboronic acids are given in Table 4. Comparison of theresults indicates that our catalytic system (entry 27) exhibitsbetter catalytic activity compared to conventional catalystssuch as Cu, Pd, Fe and Au. These systems require additionalbase (entries 1, 3, 10, 11–17, 20 and 24–26), external additives(entries 3, 5, 9, 11, 12, 15 and 25), additional oxidants (entries3, 5, 9, 11, 12, 15, 25 and 26) and higher reaction time (entries2, 5, 6, 11, 20, 22, 24, 26 and 25).
Experimental sectionGeneral methods
All reactions were carried out under aerobic conditions. Allchemicals were used without further purification as commer-cially available unless otherwise noted. NMR spectra wererecorded at 500, 400 and 300 MHz (mentioned in respectiveNMR data itself) on a Bruker spectrometer. All 1H NMR and13C NMR spectra were measured in CDCl3 with TMS as theinternal standard. The powder XRD pattern of the catalystsample was measured with a SHIMADZU XD-D1 and X’PertPANalytical Diffractometer instrument using a Cu Kα radiationat room temperature. Brunauer Emmett Teller (BET) surfacearea analysis was carried out using the Quantachrome-Auto-sorb at 77 K. Elemental analysis with atomic absorption spec-trophotometry (AAS) was performed on Varian AA 240. IRspectral analyses were performed using a JASCO FT/IR-410instrument by the KBr pellet technique in the range of4000–500 cm−1. Thermogravimetric analysis (TGA) was per-formed with a TGA/Shimadzu Thermal Analyser under a nitro-gen atmosphere in the temperature range 50–570 °C in a flowof nitrogen at a heating rate of 10 °C min−1. Electrosprayionization mass spectrometry (ESI-MS) analyses were recordedin LCQ Fleet, Thermo Fisher Instruments Limited, US. ESI-MSwas performed in negative ion mode. The collision voltage andionization voltage were −70 V and −4.5 kV, respectively, usingnitrogen as atomization and desolvation gas. The desolvationtemperature was set at 300 °C.
Fig. 3 Powder XRD patterns of fresh and reused catalyst.
Cu(BDC) MOF was prepared according to the procedurereported by Tannenbaum et al.23 Equimolar quantities ofcopper nitrate trihydrate (15 mmol) and terephthalic acid(15 mmol) in 300 mL DMF were used. This solution wasplaced in a closed flask in an oven at 110 °C for 36 h. It wasobserved the blue crystals precipitated in a yield of 75% whichis comparable to that of the reported procedure.23
Characterization of copper terephthalate MOF [Cu(BDC)]
The Cu(BDC) MOF was characterized using a variety ofdifferent techniques. A very sharp peak below 15° (with 2θ of10.27) was observed on the powder X-ray diffractogram of theCu(BDC) MOF, indicating that a highly crystalline material(Fig. 3). Furthermore, the powder XRD patterns of theCu(BDC) MOF exhibited a better crystallinity as compared tothat of silica-based materials such as SBA-15, SBA-16 and
MCM-41 where broader peaks were normally observed on theirdiffractograms.25 Elemental analysis by atomic absorptionspectroscopy (AAS) indicated a copper loading of 3.87 mmolg−1. The FT-IR spectra of the Cu(BDC) MOF exhibited the pres-ence of a strong peak at 1603 cm−1, which was lower than thevalue for CvO stretching vibration observed in free carboxylicacids (1760–1690 cm−1) (Fig. 5a and b). This strong peak wasdue to the stretching vibration of carboxylate anions present inthe material. After removal of DMF, the 1603 cm−1 peak wasshifted to 1576 cm−1 and the DMF carbonyl frequency peak(1665 cm−1) disappeared (Fig. 5c). Thermogravimetric analysis(TGA) was carried out to examine the stability of the frame-work. For the as-synthesised samples, the TGA of Cu(BDC)showed a loss of ligated form of DMF molecules in the temp-erature range 160–260 °C. After 260 °C, the phase remainscompletely changed, which was stable upto over 370 °C(Fig. 6). The overall powder XRD patterns, IR spectrum and thethermal stability of the copper terephthalate MOF were ingood agreement with the literature.23
The Cu(BDC) MOF was dissolved in HNO3, diluted and ana-lysed with AAS and the results show that the copper concen-tration in Cu(BDC) MOF was found to be 3.87 mmol g−1. Tosee if there was any metal leaching, the following controlexperiment was performed. Cu(BDC) MOF (0.100 g) was addedto a mixture of phenylboronic acid (2 mmol), in DMF (2 mL)and stirred at RT for 16 h. After completion of reaction, thereaction mixture was filtered. The filtrate and reused Cu(BDC)MOF (dissolved in HNO3) were analysed by AAS. The copperconcentration in the filtrate was found to be BDL (belowdetectable limit). In the reused Cu(BDC) MOF (dissolved inHNO3), the copper concentration was found to be 3.86 mmolg−1. This result shows that no leaching of copper has takenplace from the solid to the liquid phase. This proves that thecatalyst is heterogeneous in nature.
Synthesis of MOF-101
2-Bromoterephthalic acid was prepared according to the pub-lished procedure.38 The prepared 2-bromoterephthalic acidwas characterized by ESI-MS spectrum (ESI-MS: m/z calcd forC8H5BrO4: 243.94; found, 243.00 (M − H)). MOF-101 was pre-pared according to the published procedure.39 An equimolaramount of 2-bromoterephthalic acid and Cu(NO3)2·2.5H2O inDMF in a capped vial at room temperature gave blue crystals ofthe MOF-101. The prepared MOF-101 was characterized byFT-IR and powder XRD techniques (see ESI†). The FT-IRspectra of the MOF-101 exhibited the presence of a strongpeak at 1623 cm−1, which was lower than the value for theCvO stretching vibration observed in free carboxylic acids1704 cm−1. This strong peak was due to the stretchingvibration of carboxylate anions present in the material. TheDMF solvent carbonyl stretching frequency band appears to be1666 cm−1 for a molecule that is coordinated to the coppermetal centre. These powder XRD pattern and FT-IR spectrawere in good agreement with that reported.39 Elemental analy-sis by AAS indicated a copper loading of 2.48 mmol g−1.
Fig. 4 FT-IR spectra of phenylboronic acid (a) and reaction intermedi-ate (b).
Fig. 5 FT-IR spectra of 1,4-benzenedicarboxylic acid (a), Cu(BDC) (b)and Cu(BDC) after removal of DMF (c).
Synthesis of copper pyridine-2,5-dicarboxylate MOF([Cu(pdc)2]NH2NMe2)
[Cu(pdc)2]NH2NMe2 was prepared according to the previouslyreported procedure40 and the prepared MOF was characterizedby FT-IR and powder XRD techniques (see ESI†). The FT-IRspectra of the [Cu(pdc)2]NH2NMe2 exhibited the presence of astrong peak at 1652 cm−1, which was lower than the value forCvO stretching vibration observed in free carboxylic acids1732 cm−1. This strong peak was due to the stretchingvibration of carboxylate anions present in the material. Thecarboxylate ion is coordinated to the copper metal centre.These characterization results were in good agreement withreported crystal powder XRD simulated patterns.40 Elementalanalysis by AAS indicated a copper loading of 2.92 mmol g−1.
Synthesis of [Cu2(ndc)2ted]n MOF
[Cu2(1,4-naphthalenedicarboxylate)2ted]n was prepared accord-ing to the reported procedure.41 The prepared MOF wascharacterized by FT-IR and powder XRD techniques (see ESI†).The prepared [Cu2(ndc)2ted]n was characterized by FT-IR and
Table 4 Comparison with reported catalytic systems for homocoupling of arylboronic acidsa
Entry Catalyst Additive Base Solvent OxidantTemp.(°C)
powder XRD techniques (see ESI†). The FT-IR spectra of the[Cu2(ndc)2ted]n exhibited the presence of a strong peak at1615 cm−1, which was lower than the value for CvO stretchingvibration observed in free carboxylic acids 1702 cm−1. The car-boxylate ion is coordinated to the central copper metal. Thisstrong peak was due to the stretching vibration of carboxylateanions present in the material. These results were in goodagreement with those reported.41 Elemental analysis by AASindicated a copper loading of 3.68 mmol g−1.
Synthesis of [Cu(H2L)]n
(Z)-4-((2-Hydroxynaphthalen-1-yl)methyleneamino)benzene-1,3-dioic acid (H2L) was prepared according to the reported pro-cedure.42 The prepared H2L was characterized by ESI-MS spec-trum (ESI-MS: m/z calcd for C19H13NO5: 335.08; found, 334.11(M − H)). [Cu(H2L)]n was prepared according to the reportedprocedure.42 The prepared MOF was characterized by FT-IRand powder XRD techniques (see ESI†). The FT-IR spectra ofthe [Cu(H2L)]n exhibited the presence of a strong peak at1598 cm−1, which was lower than the value for the CvOstretching vibration observed in free carboxylic acids1717 cm−1. This strong peak was due to the stretchingvibration of carboxylate anions present in the material. Thecarboxylate ion is coordinated to the copper metal centre.These characterization results were in good agreementwith the reported crystal powder XRD simulated patterns.42
Elemental analysis by AAS indicated a copper loading of3.26 mmol g−1.
General experimental procedure for the Cu(BDC) MOFcatalysed homocoupling reaction of arylboronic acid
A solution of Cu(BDC) MOF (100 mg) and arylboronic acids(2 mmol) was taken in 2 mL of DMF under an air atmosphere.After stirring at room temperature for 16 h, the mixture wasdiluted with ethyl acetate (5 mL) and then the catalyst removedby filtration, followed by solvent evaporation under reducedpressure, the resulting crude product was finally purified bycolumn chromatography on silica gel (60–120 mesh) withpetroleum ether and ethyl acetate as eluting solvent to give thedesired product upto 97% yield. The recovered catalyst wasthoroughly washed with ethyl acetate and heated with 2 mL offresh DMF at 100 °C for 2 h, activated under vacuum at roomtemperature for 4 h, which was subsequently reused.
Conclusions
In conclusion, we have demonstrated the utility of a simpleand highly efficient heterogeneous catalyst for promoting theaerobic homocoupling of arylboronic acid involving Cu(BDC)MOF as the catalyst at room temperature without any additivesand bases. The reaction employs environmentally benign airas oxidant under mild reaction conditions. The catalyst isstable, shows no metal leaching and the reused catalyst exhibi-ted only a minor loss of catalytic activity. In comparison toother catalysts, framework-immobilization confers multiple
advantages: low cost, higher stability, substrate selectivity,easily separated and recyclability. The leaching test and re-cycling experiment clearly demonstrate that the copper MOF istruly heterogeneous in nature.
Acknowledgements
PP gratefully acknowledges financial support from UniversityGrants Commission (UGC), New Delhi, for UGC-BSR-SRF.KP thanks DST, New Delhi for financial support.
References
1 (a) A. Corma, H. Garcia and F. X. L. Xamena, Chem. Rev.,2010, 110, 4606; (b) D. Farrusseng, S. Aguado and C. Pinel,Angew. Chem., Int. Ed., 2009, 48, 7502; (c) J. Y. Lee,O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen andJ. T. Hupp, Chem. Soc. Rev., 2009, 38, 1450; (d) Z. Wang,G. Chen and K. Ding, Chem. Rev., 2009, 109, 322.
2 E. Jeong, W. R. Lee, D. W. Ryu, Y. Kim, W. J. Phang,E. K. Koh and C. S. Hong, Chem. Commun., 2013, 49, 2329.
3 (a) K. Sumida, D. L. Rogow, J. A. Mason, T. M. McDonald,E. D. Bloch, Z. R. Herm, T. H. Bae and J. R. Long, Chem.Rev., 2012, 112, 724; (b) M. P. Suh, H. J. Park, T. K. Prasadand D. W. Lim, Chem. Rev., 2012, 112, 782; (c) J. R. Li,J. Sculley and H. C. Zhou, Chem. Rev., 2012, 112, 869;(d) X. Zhu, H. Zheng, X. Wei, Z. Lin, L. Guo, B. Qiu andG. Chen, Chem. Commun., 2013, 49, 1276.
4 N. Stock and S. Biswas, Chem. Rev., 2012, 112, 933.5 (a) L. Alaerts, E. Seguin, H. Poelman, F.-T. Starzyk,
P. A. Jacobs and D. E. DeVos, Chem. – Eur. J., 2006, 12,7353; (b) P. Horcajada, S. Surble, C. Serre, D. Y. Hong,Y. K. Seo, J. S. Chang, J. M. Greneche, I. Margiolaki andG. Ferey, Chem. Commun., 2007, 2820.
6 M. Yoon, R. Srirambalaji and K. Kim, Chem. Rev., 2012,112, 1196.
7 M. J. Beier, W. Kleist, M. T. Wharmby, R. Kissner,B. Kimmerle, P. A. Wright, J.-D. Grunwaldt and A. Baiker,Chem. – Eur. J., 2012, 18, 887.
8 (a) Y. Liao, W. Hung, T. Hou, C. Lin and K. Wong, Chem.Mater., 2007, 19, 6350; (b) J. Hassan, M. Sevignon, C. Gozzi,E. Schulz and M. Lemaire, Chem. Rev., 2002, 102,1359.
9 (a) J.-P. Corbet and G. Mignani, Chem. Rev., 2006, 106,2651; (b) B. Yuan, Y. Pan, Y. Li, B. Yin and H. Jiang, Angew.Chem., Int. Ed., 2010, 49, 4054; (c) F. Monnier andM. Taillefer, Angew. Chem., Int. Ed., 2009, 48, 6954;(d) B. Karimi and P. F. Akhavan, Chem. Commun., 2009,3750; (e) B. Karimi, D. Elhamifar, J. H. Clark andA. J. Hunt, Chem. – Eur. J., 2010, 16, 8047; (f ) D. Lee,J. H. Kim, B. H. Jun, H. Kang, J. Parkand and Y. S. Lee, Org.Lett., 2008, 10, 1609; (g) S. Martin and L. Buhwald, Acc.Chem. Res., 2008, 41, 1461.
10 (a) A. Fihri, M. Bouhrara, B. Nekoueishahraki, J.-M. Bassetand V. Polshettiwar, Chem. Soc. Rev., 2011, 40, 5181;(b) A. Molnarc, Chem. Rev., 2011, 111, 2251;(c) L.-C. Campeau and K. Fagnou, Chem. Soc. Rev., 2007, 36,1058.
11 (a) M. Shimizua, I. Nagao, Y. Tomioka, T. Kadowaki andT. Hiyama, Tetrahedron, 2011, 67, 8014; (b) S.-Y. Xu,Y.-B. Ruan, X.-X. Luo, Y.-F. Gao, J.-S. Zhao, J.-S. Shen andY.-B. Jiang, Chem. Commun., 2010, 46, 5864; (c) Z. Jin,S.-X. Guo, X.-P. Gu, L.-L. Qiu, H.-B. Song and J.-X. Fang,Adv. Synth. Catal., 2009, 351, 1575; (d) S. R. Cicco,G. M. Farinola, C. Martinelli, F. Naso and M. Tiecco,Eur. J. Org. Chem., 2010, 2275; (e) K. Mitsudo, T. Shiragaand H. Tanaka, Tetrahedron Lett., 2008, 49, 6593;(f ) A. Prastaro, P. Ceci, E. Chiancone, A. Boffi, G. Fabriziand S. Cacchi, Tetrahedron Lett., 2010, 51, 2550;(g) K. Mitsudo, T. Shiraga, D. Kagen, D. Shi, J. Y. Beckerand H. Tanaka, Tetrahedron, 2009, 65, 8384; (h) C. Amatore,C. Cammoun and A. Jutand, Eur. J. Org. Chem., 2008, 4567;(i) B. Mu, T. Li, Z. Fu and Y. Wu, Catal. Commun., 2009, 10,1497; ( j) A. Ciric and F. Mathey, Organometallics, 2010, 29,4785.
12 For examples of other transition-metal mediators, see: Cu:(a) N. Kirai and Y. Yamamoto, Eur. J. Org. Chem., 2009,1864; (b) B. Kaboudin, T. Haruki and T. Yokomatsu, Syn-thesis, 2011, 1, 91; (c) B. Kaboudin, Y. Abedi andT. Yokomatsu, Eur. J. Org. Chem., 2011, 6656; (d) G. Chengand M. Luo, Eur. J. Org. Chem., 2011, 2519; Mn:(e) A. S. Demir, O. Reis and M. Emrullahoglu, J. Org. Chem.,2003, 68, 578; Au: (f ) H. Tsunoyama, H. Sakurai, N. I. kuni,Y. Negishi and T. Tsukuda, Langmuir, 2004, 20, 11293;(g) C. G. Arellano, A. Corma, M. Iglesias and F. Sanchez,Chem. Commun., 2005, 1990; (h) S. Carrettin, J. Guzmanand A. Corma, Angew. Chem., Int. Ed., 2005, 44, 2242;(i) A. Primo and F. Quignard, Chem. Commun., 2010, 46,5593; ( j) A. Kar, N. Mangu, H. M. Kaiser, M. Bellerab andM. K. Tse, Chem. Commun., 2008, 386; Rh: (k) T. Volglerand A. Studer, Adv. Synth. Catal., 2008, 350, 1963;(l) K. R. Reddy, K. Rajgopal and M. L. Kantam, Catal. Lett.,2007, 114, 36.
13 (a) Z.-L. Shen, S.-Y. Wang, Y.-K. Chok, Y.-H. Xu andT.-P. Loh, Chem. Rev., 2013, 113, 271; (b) H. F. Sore, W. R. J.D. Galloway and D. R. Spring, Chem. Soc. Rev., 2012, 41,1845; (c) C. E. I. Knappke and A. J. V. Wangelin, Chem. Soc.Rev., 2011, 40, 4948.
14 (a) E. M. Campi, W. R. Jacksonand and S. M. Marcuccio,J. Chem. Soc., Chem. Commun., 1994, 2395; (b) T. Gillmannand T. Weeber, Synlett, 1994, 649; (c) Z. Z. Songand andH. N. C. Wong, J. Org. Chem., 1994, 59, 33.
15 P. T. Anastas and J. C. Warner, Green Chemistry Theory andPractice, Oxford University Press, New York, 1998.
16 (a) A. E. Wendlandt, A. M. Suess and S. S. Stahl, Angew.Chem., Int. Ed., 2011, 50, 11062; (b) Z. Shi, Y. Cui andN. Jiao, Org. Lett., 2010, 12, 2908.
17 (a) B. A. Dara, S. Singh, N. Pandey, A. P. Singh, P. Sharma,A. Lazar, M. Sharma, R. A. Vishwakarma and B. Singh,
Appl. Catal., A, 2014, 470, 232; (b) L. Wang, W. Zhang,D. S. Su, X. Meng and F.-S. Xiao, Chem. Commun., 2012, 48,5476; (c) R. N. Dhital, A. Murugadoss and H. Sakurai,Chem. – Asian J., 2012, 7, 55.
18 B. Kaboudin, R. Mostafalu and T. Yokomatsu, Green Chem.,2013, 15, 2266.
19 (a) A. W. Thomas and S. V. Ley, Angew. Chem., Int. Ed.,2003, 42, 5641; (b) J.-B. Lan, G.-L. Zhang, X.-Q. Yu,J.-S. You, L. Chen, M. Yan and R.-G. Xie, Synlett, 2004,1095; (c) M. L. Kantam, G. T. Venkanna, C. Sridhar,B. Sreedhar and B. M. Choudary, J. Org. Chem., 2006, 71,9522; (d) J. X. Qiao and P. Y. S. Lam, Synthesis, 2011, 829.
20 O. M. Yaghi, A. U. Czaja, B. Wang and Z. Lu, US Patents,2012/0130113, 2012.
21 I. Luz, F. X. L. I. Xamena and A. Corma, J. Catal., 2012, 285,285.
22 N. T. S. Phan, T. T. Nguyen, K. D. Nguyen and A. X. T. Vo,Appl. Catal., A, 2013, 464–465, 128.
23 C. G. Carson, K. Hardcastle, J. Schwartz, X. Liu,C. Hoffmann, R. A. Gerhardt and R. Tannenbaum,Eur. J. Inorg. Chem., 2009, 2338.
24 (a) R. N. Dhital, C. Kamonsatikul, E. Somsook,K. Bobuatong, M. Ehara, S. Karanjit and H. Sakurai, J. Am.Chem. Soc., 2012, 134, 20250; (b) Y. Dingyi and Z. Yugen,Green Chem., 2011, 13, 1275; (c) E. Bernoud, C. Alayrac,O. Delacroix and A.-C. Gaumont, Chem. Commun., 2011, 47,3239; (d) Y.-X. Liao, C.-H. Xing, M. Israel and Q.-S. Hu, Org.Lett., 2011, 13, 2058; (e) K. D. Hesp, R. J. Lundgren andM. Stradiotto, J. Am. Chem. Soc., 2011, 133, 5194.
25 D. Zhao, Q. Huo, J. Feng, B. F. Chmelka and G. D. Stucky,J. Am. Chem. Soc., 1998, 120, 6024.
26 J.-S. Chen, K. K. Jespersen and J. G. Khinast, J. Mol. Catal.A: Chem., 2008, 285, 14.
27 L. Zhou, Q. X. Xu and H. F. Jiang, Chin. Chem. Lett., 2007,18, 1043.
28 M. S. Wong and X. L. Zhang, Tetrahedron Lett., 2001, 42, 4087.29 K. Cheng, B. Xin and Y. Zhang, J. Mol. Catal. A: Chem.,
2007, 273, 240.30 Z. Xu, J. Mao and Y. Zhang, Catal. Commun., 2008, 9, 97.31 G. W. Kabalka and L. Wang, Tetrahedron Lett., 2002, 43, 3067.32 T. Matsuda, T. Asai, S. Shiose and K. Kato, Tetrahedron
Lett., 2011, 52, 4779.33 L. Chaicharoenwimolkul, A. Munmai, S. Chairam,
U. Tewasekson, S. Sapudom, Y. Lakliang and E. Somsook,Tetrahedron Lett., 2008, 49, 7299.
34 A. S. Demir, O. Reis and M. Emrullahoglu, J. Org. Chem.,2003, 68, 10130.
35 K. Rahme, M. T. Nolan, T. Doody, G. P. McGlacken,M. A. Morris, C. O’Driscoll and J. D. Holmes, RSC Adv.,2013, 3, 21016.
36 M. P. Sk, C. K. Jana and A. Chattopadhyay, Chem. Commun.,2013, 49, 8235.
37 J. Mao, Q. Hua, G. Xie, Z. Yao and D. Shi, Eur. J. Org.Chem., 2009, 2262.
38 X.-T. Zhang, L.-M. Fan, X. Zhao, D. Sun, D.-C. Li andJ.-M. Dou, CrystEngComm, 2012, 14, 2053.
