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As featured in:

See Ajai Kumar Singh et al., Chem. Commun., 2013, 49, 7483.

Showcasing research from

Professor Ajai Kumar Singh’s laboratory,

Department of Chemistry, Indian Institute of

Technology Delhi (IITD), New Delhi, India

Graphene oxide grafted with Pd17Se15 nano-particles generated

from a single source precursor as a recyclable and effi cient

catalyst for C–O coupling in O-arylation at room temperature

A recyclable catalyst has been designed from graphene oxide by

grafting Pd17Se15 nano-particles prepared in a single shot from a

Pd–Se ligand complex. It is 1 mol % with respect to Pd and catalyzes

C–O coupling between ArX and phenol at

room temperature (yield ≤ 94%).

This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 7483--7485 7483

Cite this: Chem. Commun.,2013,49, 7483

Graphene oxide grafted with Pd17Se15 nano-particlesgenerated from a single source precursor as arecyclable and efficient catalyst for C–O coupling inO-arylation at room temperature†

Hemant Joshi, Kamal Nayan Sharma, Alpesh K. Sharma, Om Prakash andAjai Kumar Singh*

The Pd17Se15 nanoparticles, synthesized for the first time from a single

source precursor [Pd(L)Cl2] {L = 1,3-bis(phenylselenyl)propan-2-ol} and

grafted onto graphene oxide, show high catalytic activity in C–O

coupling between aryl/heteroaryl chlorides/bromides and phenol at

room temperature (Pd loading 1 mol%; yield up to 94%).

Nanocatalysis, using nanoparticles stabilized with diverseagents and implanted on a solid surface or dispersed within apolymeric system, has become very attractive. Numerous reviewarticles1,2 published during the past decade have covered itextensively. The nanocatalysts are favourable compared totraditional ones due to their enhanced surface to volume ratio,resulting in more catalytically active sites. Among the nano-catalysts, use of palladium nanoparticles3 (NPs) has expandedconsiderably in the last decade. Various types of NPs of Pd havebeen developed for catalysis. Highly branched palladiumnanostructures have been synthesized for efficient hydro-genation of nitrobenzene to aniline.3a Novel palladium hollowspheres synthesized using silica spheres as templates havebeen successfully applied as recyclable heterogeneous catalystsin Suzuki cross coupling reactions.3b Shape-controlled Pdnanoparticles supported on powder alumina have been syn-thesized for selective hydrogenation of butadiene to butene.3c

There are reports in which the role of in situ generated Pdchalcogenide NPs in catalysis4 has been suggested. But to ourknowledge, use of pre-formed palladium chalcogenide NPsimmobilized on a solid surface, i.e. silica, alumina, polymeror graphene oxide (GO), as a heterogeneous catalyst forcoupling reactions is not known. Graphene oxide can form

intercalation compounds with cationic molecules because of itsfunctional groups such as carboxylic and hydroxyl groupspresent on the surface.5 Graphene oxide also exhibits highchemical, mechanical and thermal stability as well as surfacearea desirable for two-dimensional (2-D) support layers fornanoparticles used as heterogeneous catalysts.6 To date, therehave been only a few examples of GO-based materials used incatalysis.7 Mastalir et al. have reported high catalytic activityand selectivity of the GO-nano-Pd system in liquid-phase hydro-genation of alkynes.7b,c Ion exchange of GO with a Pd complexand its subsequent reduction to nano-sized crystallites by H2

have resulted in a catalyst that has surpassed the activity ofcommercially available ‘supported Pd catalysts’ such as Pd onactivated carbon (Pd/C) and Pd graphimet. The latter is a ratheran uncommon catalyst synthesized by slow intercalation ofPdCl2 into graphite in a chlorine atmosphere with subsequentreduction to Pd nanoparticles.8 The role of GO as an activehydrogen donor in palladium catalyzed Ullmann reaction hasbeen reported.9 The Pd NPs immobilized on partially reducedGO have been reported for catalysis of Suzuki–Miyauracoupling reactions.10 Palladium selenide, Pd17Se15, nanoparticlessynthesized via a single source precursor (SSP) route andimmobilized on GO have been found, for the first time, to bea very efficient and recyclable catalyst for C–O coupling reac-tions between aryl/heteroaryl chlorides/bromides and phenol.Herein we report the designing of this novel system and itsapplications as a catalyst in C–O coupling. The importantcatalysts for C–O coupling are based on copper11 andpalladium.12 The pioneering contributions were from theresearch groups of Buchwald and Hartwig.13 The palladiumor copper salt and a ligand together make the catalyst. A ligand-free catalytic system has also been reported.14 However,a recyclable catalyst working at room temperature has not beenreported to our knowledge. The present catalytic system isunique and the first to have both these qualities.

For designing the GO–Pd17Se15 catalyst, we have synthesized aligand 1,3-bis(phenylselenyl)propan-2-ol (L) which forms a complex[Pd(L)Cl2] (1) with Pd(II) (see ESI† and Scheme S1 for synthesis details).

Department of Chemistry, Indian Institute of Technology Delhi, New Delhi 110016,

India. E-mail: aksingh@chemistry.iitd.ac.in, ajai57@hotmail.com;

Fax: +91-01-26581102; Tel: +91-011-26591379

† Electronic supplementary information (ESI) available: NMR spectra, crystal dataand refinement parameters, bond lengths and angles, SEM–EDX images, PXRD,TEM images and structures with non-covalent interactions. CIF for 1. CCDC933048. For ESI and crystallographic data in CIF or other electronic format seeDOI: 10.1039/c3cc42608d

Received 10th April 2013,Accepted 3rd May 2013

DOI: 10.1039/c3cc42608d

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7484 Chem. Commun., 2013, 49, 7483--7485 This journal is c The Royal Society of Chemistry 2013

L and 1 show characteristic selenium-77 NMR spectra15 (seeESI† for 1H and 13C{1H}). Complex 1 was authenticated by singlecrystal X-ray analysis. Its ORTEP diagram is shown in Fig. 1.More details of the structure are given in ESI† (see pages 10–12;Tables T1 and T2, ESI†). Complex 1 upon thermolysis intrioctylphosphine (TOP) at 195 1C resulted in Pd17Se15 NPs,which were further treated with graphene oxide (GO) at roomtemperature to form the catalyst GO–Pd17Se15 (see ESI;†Scheme S1). Its TEM images are shown in Fig. 2 along withthose of GO and Pd17Se15 NPs.

As a model reaction, the coupling of 4-bromobenzaldehydewith phenol was studied. The reaction conditions were opti-mized (see Table 1). The control experiments and the effect ofvarying base, solvent and temperature [110 1C (selected becausemost of the C–O couplings are reported between 80–100 1C) toroom temperature] on conversions suggest that the combi-nation of K2CO3 (as base) and DMSO (as a solvent) gives themost efficient O-arylation (see Table 1, entry 9) at room tem-perature (94% yield in just 1 h). Upon increasing the reactiontemperature to 100 1C the yield goes up by 2% only (entry 10 inTable 1).

To understand the scope and generality of GO–Pd17Se15

NPs in promoting O-arylation, coupling reactions of a variety

of structurally divergent bromo and chlorobenzenes having awide range of functional groups as substrates were studied forC–O coupling and the results are summarized in Table 2.

Fig. 1 ORTEP diagram of 1 with 30% probability ellipsoids; hydrogen atomsand solvent molecules are omitted for clarity; bond distances (Å): Se(1)–Pd(1)2.389(16), Se(2)–Pd(1) 2.386(14), Cl(1)–Pd(1) 2.327(2) Cl(2)–Pd(1) 2.321(2).

Fig. 2 TEM image of Pd17Se15 NPs (a and b), GO (c) and GO–Pd17Se15 NPs (d).

Table 1 Screening of base and solvents for O-arylation of 4-bromobenzalde-hyde with phenola

S.no. Catalyst Base Solvent Time (h) Yieldb (%)

1c 1 K2CO3 DMSO 12 o102c Pd17Se15 K2CO3 DMSO 12 403c GO K2CO3 DMSO 12 04d GO–Pd17Se15 Cs2CO3 DMF 12 615d GO–Pd17Se15 Et3N DMSO 12 326d GO–Pd17Se15 K2CO3 DMF 8 847d GO–Pd17Se15 K2CO3 DMSO 8 948d GO–Pd17Se15 K2CO3 DMSO 4 949d GO–Pd17Se15 K2CO3 DMSO 1 9410e GO–Pd17Se15 K2CO3 DMSO 1 96

a Reaction conditions: aryl bromide, 1.0 mmol; phenol, 1.2 equiv.;K2CO3, 2.0 equiv.; GO–Pd17Se15; 1.0 mol% Pd; DMSO 4 ml. b Isolatedyield. c 110 1C. d Room temperature. e 100 1C.

Table 2 C–O coupling reactions of aryl/heteroaryl halide catalyzed withGO–Pd17Se15 NPsa

Entry no. Aryl/heteroaryl halide Product Yieldb (%)

1 4-Bromobenzaldehyde 942 4-Chlorobenzaldehyde 91

3 4-Bromoacetophenone 924 4-Chloroacetophenone 83

5 1-Bromo-4-nitrobenzene 896 1-Chloro-4-nitrobenzene 82

7 4-Bromobenzonitrile 918 4-Chlorobenzonitrile 84

9 2-Bromobenzaldehyde 7610 2-Chlorobenzaldehyde 61

11 2-Bromoacetophenone 7212 2-Chloroacetophenone 59

13 Bromobenzene 8714 Chlorobenzene 73

15 4-Bromotoluene 7616 4-Chlorotoluene 64

17 4-Bromoanisole 7418 4-Chloroanisole 61

19 2-Bromopyridine 9020 2-Chloropyridine 81

21 4-Bromopyridine 8822 4-Chloropyridine 83

a Reaction conditions: aryl halide, 1.0 mmol; phenol, 1.2 equiv.; K2CO3,2.0 equiv.; GO–Pd17Se15: 1.0 mol% Pd; DMSO, 4 ml, room temperature,1 h–3 h. b Isolated yield.

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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 7483--7485 7485

It is observed that the coupling efficiency follows the orderArBr > ArCl. Arylhalides having electron-donating (ED) groups(Table 2, entries 15–18) show less reactivity in comparison tothose having electron-withdrawing (EW) groups (Table 2,entries 1–8). Further when EW groups are at a position orthoto halo groups, the yields are less in comparison with thosesubstrates having a substituent at the para position (Table 2,entries 9–12). The reactions of phenol with heteroaryl halides(Table 2, entries 19–22) also gave good yields. When a sub-stituent was present at a position meta to the halo group, therewas no conversion at room temperature.

Recyclability of the catalyst was studied for the couplingreaction giving 4-phenoxybenzaldehyde at room temperaturewith 1.0 mol% of Pd. The first two runs of the reaction becamealmost complete in 40 minutes. Thereafter, catalytic activityslightly dropped and for the next two runs comparable conver-sions were observed in 90 minutes. The dramatic drop inconversion (to 19%) was observed in the fifth run (Table 3).The GO–Pd17Se15 NPs were found to deteriorate (see ESI,†Fig. S6 and S7 for TEM images) after four runs, resulting inpoor conversion in the fifth one.

The C–O coupling in the present case appears to proceedthrough oxidative addition of ArX to Pd(0) of NPs, as involve-ment of Pd(0) species in such coupling has been reportedearlier.12a,16 There have been more reports on copper salt basedC–O coupling than on Pd species based reactions. In most ofthe cases a ligand12c,13c was added to the Cu or Pd salt. Thusthere remains an element of ambiguity not only about realcatalytic species but also regarding the one which generates it.In the present case atleast the pre-catalyst system is wellauthenticated, which releases Pd(0), most likely a real catalyst.Most of the catalytic systems require a temperature >80 1C.Room temperature conversions are rare13f and the present catalystis novel in this sense. Palladium loading required in the case ofGO–Pd17Se15 NPs is less in comparison to those reported for mostof the known catalytic systems (2–5 mol%).12c,13e

In summary, we have designed Pd17Se15 NPs for the firsttime from a single source precursor. They were grafted onto GOresulting in a very efficient catalyst for C–O coupling reactions ofaryl halides with phenol. The catalyst has been found to berecyclable, and conversions in the case of a variety of substrateshave been found to be good in its presence at room temperature.

This work was supported by projects from the Department ofScience and Technology, New Delhi-110016, India, and theCouncil of Scientific and Industrial Research (CSIR), New Delhi,India. CSIR and University Grants Commission (UGC) areacknowledged for JRF/SRF awards to HJ, KNS, AS and OP.

Notes and references1 (a) A. Eppler, G. Rupprechter, L. Guczi and G. A. Somorjai, J. Phys.

Chem. B, 1997, 101, 9973; (b) A. Balanta, C. Godard and C. Claver,Chem. Soc. Rev., 2011, 40, 4973; (c) A. Fihri, M. Bouhrara,B. Nekoueishahraki, J. M. Basset and V. Polshettiwar, Chem. Soc.Rev., 2011, 40, 5181; (d) M. Kralik and A. Biffis, J. Mol. Catal. A:Chem., 2001, 177, 113; (e) J. M. Thomas, B. F. G. Johnson, R. Raja,G. Sankar and P. A. Midgley, Acc. Chem. Res., 2003, 36, 20;( f ) M. Stratakis and H. Garcia, Chem. Rev., 2012, 112, 4469.

2 (a) J. S. Bradley, Clusters Colloids, 1994, 459; (b) H. Boennermann,G. Braun, W. Brijoux, R. Brinkman, A. S. Tilling, S. K. Schulze andK. Siepen, J. Organomet. Chem., 1996, 520, 143; (c) A. B. R. Mayer,Polym. Adv. Technol., 2001, 12, 96.

3 (a) J. Watt, S. Cheong, M. F. Toney, B. Ingham, J. Cookson, P. T.Bishop and R. D. Tilley, ACS Nano, 2010, 4, 396; (b) S. W. Kim,M. Kim, W. Y. Lee and T. Hyeon, J. Am. Chem. Soc., 2002, 124, 7642;(c) L. Piccolo, A. Valcarcel, M. Bausach, C. Thomazeau, D. Uziob andG. Berhault, Phys. Chem. Chem. Phys., 2008, 10, 5504.

4 (a) G. K. Rao, A. Kumar, J. Ahmed and A. K. Singh, Chem. Commun.,2010, 46, 5954; (b) K. N. Sharma, H. Joshi, V. V. Singh, P. Singh andA. K. Singh, Dalton Trans., 2013, 42, 3908; (c) K. N. Sharma, H. Joshi,A. K. Sharma, O. Prakash and A. K. Singh, Organometallics, 2013,32, 2443.

5 Y. Matsuo and T. Nakajima, Carbon, 1994, 32, 469.6 (a) G. M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth and

R. Mulhaupt, J. Am. Chem. Soc., 2009, 131, 8262; (b) Y. Q. He,N. N. Zhang, Y. Liu, J. P. Gao, M. C. Yi, Q. J. Gong and H. X. Qiu,Chin. Chem. Lett., 2012, 23, 41.

7 (a) G. I. Titelman, S. V. Karamanenko, Y. N. Novikov,E. V. Gorozhankin and E. Z. Golosman, USSR Patent, SU 1806005,1993; (b) A. Mastalir, Z. Kiraly, A. Patzko, I. Dekany andP. L’Argentiere, Carbon, 2008, 46, 1631; (c) A. Mastalir, Z. Kiraly,M. Benko and I. Dekany, Catal. Lett., 2008, 124, 34.

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Synthesis, ed. E.-i. Negishi, J. Wiley & Sons Inc., New York, 2002,p. 1097.

Table 3 Recycling experiment with GO–Pd17Se15 NPsa

Entry no Run Conversiond (%)

1b 1 1002b 2 963c 3 754c 4 535c 5 19

a Reaction conditions: aryl bromide, 1.0 mmol; phenol, 1.2 equiv.;K2CO3, 2.0 equiv.; GO–Pd17Se15, 1.0 mol% Pd; DMSO 4 ml; roomtemperature. b 40 min. c 90 min. d Yield of 94% is equivalent to 100%conversion.

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