Palladacycle containing nitrogen and selenium: highly active pre-catalyst

for the Suzuki–Miyaura coupling reaction and unprecedented conversion

into nano-sized Pd17Se15w

Gyandshwar Kumar Rao, Arun Kumar, Jahangeer Ahmedz and Ajai Kumar Singh*

Received 22nd April 2010, Accepted 21st June 2010

DOI: 10.1039/c0cc01075h

Newly synthesized, air and moisture insensitive palladacycle

[PdCl(L–H)] (L = (C6H5)(2-HOC6H4)CHNH(CH2)3SePh;

a (N, Se, C�) ligand) shows high catalytic activity for the

Suzuki–Miyaura coupling reaction of phenylboronic acid with

aryl/heteroaryl chlorides/bromides (TON for aryl chlorides up

to 9200) and is converted to B8 nm size particles of Pd17Se15

(probably the real catalyst).

Pd(II) complexes are commonly used as catalysts for the

Suzuki–Miyaura cross-coupling reaction, which is among the

most important methods1a,b known for sp2–sp2 C–C coupling.

Generally it is believed that Pd(0) is formed from these

complexes during catalysis.1c When Pd species with some

additives are used for such cross-coupling reactions, they

apparently promote the formation of Pd(0).1d–f However, a

debate on this issue is still on and it is uncertain with many

Pd-complexes whether they are true catalytic species or

pre-catalysts only.1g,h The majority of Pd-based catalysts or

pre-catalysts known for Suzuki–Miyaura cross-couplings are

very sensitive to air and/or moisture.1g,2 Therefore, the design

of efficient catalysts producing high yields under aerobic

conditions continues to be an important challenge. The Pd(II)

complexes of chalcogenated Schiff bases3 and some other S- or

Se-containing ligands4 have emerged as a family of air stable,

moisture insensitive and efficient catalysts, which can deal with

this challenge for the coupling of aryl bromides and iodides,

but most of them3a–d,4,5 fail to transform aryl chlorides

effectively, which are the cheapest and most readily available

among the aryl halides. Out of the chalcogen ligand-containing

Pd-catalysts3e,f,6 some function well for electron-deficient aryl

chloride substrates but not for electron-rich ones. For the

coupling of aryl chlorides and electron-rich aryl bromides,

Pd-complexes of bulky and electron-rich phosphines,1e,f,2b,c,7

carbenes,8 and palladacycles4a,c,6a,9 have been found to be

among the efficient catalysts reported to date. The catalytic

efficiency of a palladacycle depends on the nature of other

donor groups present in the ligand. The electronic properties9–12

of the donor atoms in the side arms of pincer complexes have a

powerful influence on the selectivity and reactivity of the

pincer ligand-based palladium catalyst. The donor properties

of selenium13a have made Pd-complexes of organoselenium

ligands efficient catalysts for C–C coupling reactions.13b However,

very little is known as to whether palladacycles of organo-

selenium ligands are true catalytic species or pre-catalysts.

Thus we have synthesized 1 and explored the Suzuki–Miyaura

coupling catalyzed by it in detail. The unprecedented formation

of nanosize (B8 nm) particles of composition Pd17Se15 occurs

while carrying out the catalytic reaction and these appear to be

the real catalyst. The formation of the selenide of palladium is

found for the first time in the Suzuki–Miyaura coupling

reaction. The results are described in this communication.

Interestingly L and 1, prepared as given in Scheme 1, are

stable under ambient conditions. 1 has a distorted square

planar geometry at Pd with Se and C(aryl) disposed trans to

each other (Fig. 1). The Schiff base3d precursor of L binds in

an (O, N, Se) mode with Pd(II). This (C�, N, Se) bonding mode

of L in 1, leaving the OH group uncoordinated, may be an

outcome of preferable ortho-palladation of the ligand. The

complex 1 crystallizes with 1 mole of B(OH)3 (See Section S5.1

in ESIw) generated from sodium borohydride.

The catalytic activity of 1 in the Suzuki–Miyaura coupling

was tested by reacting bromobenzene with phenylboronic acid

in N,N-dimethylformamide in the presence of 0.001 mol% of 1

and K2CO3 as a base. The reaction after 18 h at 110 1C gave

the product biphenyl in 95% yield. The corresponding turnover

number (TON) was 95 000. On using NaOAc or Na2CO3 as a

base in place of K2CO3, the yield of the coupling product of

bromobenzene with phenylboronic acid was reduced (70 and

56% respectively). Subsequently the Suzuki–Miyaura coupling

of phenylboronic acid with a variety of aryl/heteroaryl halides,

including activated and deactivated ones (Table 1 and Table

S3.1 in ESIw), was examined using 1 as the catalyst.

The results reflect that the goal of carrying out the Suzuki–

Miyaura coupling of aryl chlorides can be successfully

achieved when 1 is used as catalyst even in the case of species

that are deactivated. For example 4-chloroanisole is converted

to 4-methoxybiphenyl in 83% yield (TON B8300) within 28 h

Scheme 1 Synthesis of palladacycle 1.

Department of Chemistry, Indian Institute of Technology Delhi,New Delhi-110016, India. E-mail: aksingh@chemistry.iitd.ac.in,ajai57@hotmail.com; Fax: + 91 11 26581102;Tel: + 91 11 26591379w Electronic supplementary information (ESI) available: Synthesisand full characterization of L and 1; general procedures for theSuzuki reactions; data for X-ray structure of 1 (CCDC 773377), fullcharacterization of Pd17Se15 nanoparticle, 1H and 13C1H NMRspectra of known compounds. For ESI and crystallographic data inCIF or other electronic format see DOI: 10.1039/c0cc01075hz Contributed to nanoparticle analysis.

5954 | Chem. Commun., 2010, 46, 5954–5956 This journal is �c The Royal Society of Chemistry 2010

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using 0.01 mol% of 1 as catalyst. The deactivated species

4-haloaniline and 4-halophenol have also been successfully

coupled (yields up to B88%). More results are given in

Table 1 and Section S3.1 (ESIw). When 4-chloronitrobenzene

and 4-fluorobenzaldehyde (1: 0.01/0.1 and 2 mol% respectively)

are used as substrates using DMF as a solvent, substitution

reactions of aryl halides leading to the N,N-dimethylaminated

products (yield B100%) take place. However, in DMA, 40%

of the product is the N,N-dimethylaminated one and the

remaining 60% is a cross-coupled product in the case of

4-chloronitrobenzene but in the case of 4-fluorobenzaldehyde,

65% of the product is N,N-dimethylaminated, while the

remaining 35% is cross-coupled (biphenyl-4-carboxaldehyde).

When MeOH is used as a solvent, only cross-coupled product

is obtained without any significant amount of N,N-dimethyl-

amination. The species HNMe2 generated by the decomposition

of DMF may be the source13c of NMe2 required for the

N,N-dimethylamination of aryl halides. The cross-coupling

reactions of heteroaryl bromides and chlorides have also been

successfully catalyzed by 1. In the coupling of 2-bromothiophene

with phenylboronic acid, 2-phenylthiophene (yield B67%)

and biphenyl (yield B33%) were both obtained. The latter

appears to be formed by homocoupling of phenylboronic acid.

Its formation in Suzuki–Miyaura couplings is not in our

knowledge and probably it is the first report. It is interesting

to compare 1 with known and efficient catalysts for aryl

chlorides. The catalytic efficiency of 1 is better or comparable

to most of the other efficient phosphine- or carbene-based

Pd-catalysts,7d–f,8 and better than all known best Pd-catalysts,

which have chalcogen donors3f,6a (as yield and TON values are

superior). There are only a few reports7a–c,14 in which %

yields/conversions of aryl chlorides appear somewhat better

than those reported here and coupling has been carried out in

aqueous media at temperatures of 25–100 1C. In such cases the

very strong base Cs2CO3 and sterically demanding phosphine

ligands (in a N2 atmosphere) have been used. However,

catalytic species are not fully understood in many of these

cases due to their in situ generation.

The palladacycle 1 decomposes in the course of the

Suzuki–Miyaura coupling reaction. The resulting product

was characterized by SEM-EDX, and after annealing by

powder X-ray diffraction, HR-TEM (Fig. 2) and TEM-EDX.

These studies revealed that first uniform and monodispersed

spherical shaped nanoparticles of average size B8 nm,

formulated as Pd17Se15, (composition established on the basis

of matching of its powder XRD pattern with that of the

known standard phase of the same composition; see S4 in

ESIw) were formed (Fig. S4.1 in ESIw) and aggregated to

particles of size B20 nm after annealing. The catalytic activity

of these aggregated Pd17Se15 nanoparticles (B20 nm) for

the Suzuki–Miyaura coupling reaction was examined.

4-Bromonitrobenzene and 4-bromoanisole react smoothly

with PhB(OH)2 in DMF in the presence of these aggregates

(0.5 mol%) and K2CO3 as a base. Reaction for 10 h at 110 1C

results in the products 4-methoxybiphenyl and 4-nitrobiphenyl

in 89 and 92% yield respectively. On reacting 4-chloronitro-

benzene and 4-chloroanisole with PhB(OH)2 in the presence of

Pd17Se15 the products of C–C coupling were not isolated in

good yields. The aggregation appears to decrease the catalytic

efficiency of Pd17Se15 nanoparticles in comparison to those

generated in situ during the C–C coupling reaction, which

convert aryl chlorides more efficiently. Overall this supports

Fig. 1 ORTEP diagram of 1; selected bond lengths (A): Pd(1)–C(18)

1.973(5); Pd(1)–N(1) 2.056(4); Pd(1)–Cl(1) 2.325(16); Pd(1)–Se(1)

2.528(11). Selected bond angles (1): C(18)–Pd(1)–N(1) 82.00(19);

C(18)–Pd(1)–Cl(1) 94.93(16); C(18)–Pd(1)–Se(1) 178.03(14);

N(1)–Pd(1)–Se(1) 97.62(12); Cl(1)–Pd(1)–Se(1) 85.61(6);

N(1)–Pd(1)–Cl(1) 174.14(18).

Table 1 Suzuki–Miyaura coupling reactions catalyzed by 1a

Entry Aryl/heteroaryl halide Mol% Pd T/h Yieldb (%) TON

1c 1-Chloro-4-nitrobenzene 0.01 36 81 81002 1-Bromo-4-nitrobenzene 0.001 15 80 80 0003 4-Chlorobenzonitrile 0.01 36 85 85004 4-Bromobenzonitrile 0.001 13 86 86 0005 4-Chlorobenzaldehyde 0.01 27 80 80006 4-Bromobenzaldehyde 0.001 15 85 85 0007 4-Chlorobenzophenone 0.01 37 75 75008 4-Bromobenzophenone 0.001 10 91 91 0009 4-Chlorobenzoic acid 0.01 39 89 8900

10 4-Bromobenzoic acid 0.001 12 93 93 00011 3-Chlorobenzoic acid 0.01 32 87 870012 3-Bromobenzoic acid 0.001 10 91 91 00013 Chlorobenzene 0.01 36 92 920014 Bromobenzene 0.001 18 95 95 00015 4-Chlorotoluene 0.01 27 89 890016 4-Bromotoluene 0.001 10 92 92 00017 4-Chloroanisole 0.01 28 83 830018 4-Bromoanisole 0.001 25 84 84 00019 4-Chloroanilined 0.1 38 68 68020 4-Bromoaniline 0.1 25 88 88021 4-Chlorophenold 0.1 48 55 55022 4-Bromophenol 0.1 28 64 64023 2-Chloropyridine 0.01 30 79 790024 2-Bromopyridine 0.001 24 60 60 00025 3-Chloropyridine 0.01 22 85 850026 3-Bromopyridine 0.001 16 93 93 00027 4-Chloropyridine 0.01 23 83 830028 4-Bromopyridine 0.001 13 94 94 00029 2-Bromothiophened 0.001 18 95 95 00030 3-Bromoquinoline 0.001 26 93 93 00031 5-Bromopyrimidine 0.001 22 91 91 000

a Reaction conditions: 1.0 equiv. of aryl or heteroaryl halide,

1.3 equiv. of phenylboronic acid, and 2 equiv. of base (K2CO3),

solvent: aqueous DMF and temperature of bath 110 1C. b Isolated

yield (average of two runs) after column chromatography. c The

solvent used was MeOH. d The product was a mixture of cross-

coupled product and biphenyl.

This journal is �c The Royal Society of Chemistry 2010 Chem. Commun., 2010, 46, 5954–5956 | 5955

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the proposition that 1 acts as a pre-catalyst which generates

nanosize Pd17Se15, probably the true catalytic species. This is

supported by comparable % conversions noticed when the

progress of reactions catalyzed with 1/Pd17Se15 with time was

monitored by 1H NMR spectroscopy (details given in S4.1:

ESIw; Table S4.1 and Fig. S4.7).

In summary pre-catalyst 1 is efficient for the C–C coupling

of aryl bromides and chlorides (having substituents: –NO2,

–CN, –NH2, –OH, –OCH3, –CHO, –COCH3, –COOH) including

heteroaryl halides with phenylboronic acid. The activation

of aryl fluorides with 2 mol% of 1 is also possible. The

unprecedented observation that Pd17Se15 nanoparticles appear

to be the real catalyst has been made. The thermal stability,

aerobic and moisture insensitivity are additional advantages of

pre-catalyst 1.

This work was supported by the Department of Science and

Technology, New Delhi-110016 (India) and the Council of

Scientific and Industrial Research (India).

Notes and references

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Fig. 2 High resolution TEM image of Pd17Se15 obtained from 1 after

annealing at 450 1C.

5956 | Chem. Commun., 2010, 46, 5954–5956 This journal is �c The Royal Society of Chemistry 2010

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