Special Issue | October 2014 87

BARC NEWSLETTERFounder’s DaySYNTHESIS OF PALLADIUM THIOLATE/SELENOLATE

COMPLEXES AS CATALYSTS FOR SUZUKI C–C COUPLING REACTIONS

Sandip DeyChemistry Division

Abstract

The new hybrid ligands dimethylaminoalkyl dichalcogenide and 4,4’-dipyridyldiselenide were developed and

explored the complex chemistry of platinum group metals. A variety of complexes of the type [PdX(EÇN)]n (X =

Cl, OAc; E = S, Se; EÇN = ECH2CH2NMe2, ECH2CH2CH2NMe2), [PdCl(4-SeC5H4N)(PR3)2] and [PdCl(4-SeC5H4N)(PR3)]n

were synthesized. The complex [PdCl(4-SeC5H4N)(PR3)]n exists in bi- and tri-nuclear forms. These complexes were

characterized by multinuclear NMR spectroscopy in solution and single crystal X-ray diffraction analyses in solid

state. The catalytic activity of palladium complexes in Suzuki C–C cross coupling reactions has been investigated.

Shri Sandip Dey is the recipient of the DAE Scientific & Technical Excellence Award for the year 2012

Introduction

The palladium catalyzed C–C cross-coupling reaction

has become the most powerful and versatile tool

for the synthesis of pharmaceuticals, agrochemicals

and advanced materials both on laboratory and

industrial scale1,2. The importance of this general

class of reactions was recognized by the awarding

of the Nobel Prize in Chemistry in 2010 to Richard

Heck, Ei-ichi Negishi and Akira Suzuki. Catalyst

design has remained of paramount importance for

understanding various catalytic processes as well

as for improving the activity of catalyst system. The

excellent Pd catalyst systems developed initially

with Group 5 compounds such as bulky or electron

rich phosphine ligands and later with nitrogen-

containing ligands or N-heterocyclic carbenes, oxime

and imine. Most of these types of catalysts are very

air, moisture sensitive and difficult to prepare and to

recover. Although compounds containing sulfur was

neglected as potential homogeneous catalysts due to

the extended belief of sulfur to be a catalyst poison,

palladium complexes derived from chalcogen ligands

(S, Se, Te) have recently emerged as highly effective

catalysts and are air and moisture stable. Palladium

complexes containing S, Se and Te in palladacycles,

carbenes, Schiff bases and ether type ligands have

been developed as highly effective phosphine-free

palladium catalysts in Suzuki and Heck reactions3-5.

The palladium chalcogenolate complexes owing to

the stronger Pd–ER linkage may offer an attractive

potential in C–C coupling reactions. However their

utility as a catalyst in C–C coupling reactions remained

rather unexplored due to their tendency to polymerize

leading to insolubility/ sparingly solubility in organic

solvents.

We have designed and developed the hybrid and

hemilabile N,N-dimethylalkyldichalcogenide6 and

4,4’-dipyridyldiselenide7 ligands which can not only

yield low nuclearity complexes and but also enhance

complex stability and solubility. Hemilabile ligands

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88 Special Issue | October 2014

BARC NEWSLETTERFounder’s Dayhave an advantage as they can protect one or more

coordination sites that are formed during catalytic

reaction and are also capable of stabilizing the active

catalytic species/ reactive intermediates. Employing

these ligands the chemistry of palladium and platinum

complexes was explored. The catalytic activity of

several palladium complexes were evaluated in various

coupling reactions such as Suzuki7,8, Carbonylative

Suzuki9 and Sonogashira10 reactions. A few recent

results are briefly discussed here.

Discussion

1. (i) Syntheses of dimethylaminoalkylchalcogenolate complexes of palladium(II)

Reactions of the ligands (EÇN)2 with Na2PdCl4 yielded

sparingly soluble products [PdCl(EÇN)]n (E = S or Se; n/

EÇN = 3/ECH2CH2NMe2 (1) and 2/ECH2CH2CH2NMe2

(3)). Treatment of [PdCl(EÇN)]n with silver acetate

in acetone-methanol mixture at room temperature

afforded corresponding acetato-complexes, [Pd(OAc)

(EÇN)]n (2 and 4) (Scheme 1). Complex 4 can also

be prepared by a redistribution reaction between

[Pd(OAc)2]3 and [Pd(ECH2CH2CH2NMe2)2]6. The 1H

NMR spectra of 2 displayed a resonance at ~ 2.0

ppm attributable to acetato group. The 13C{1H} NMR

spectra of 2 exhibited a signal at δ ~ 177.3 ppm due to

carbonyl carbon. The mass spectra of 2 did not show

molecular ion peaks, but fragmentation ion peaks with

isotopic pattern consistent with Pd3 species suggesting

trimeric nature of 2. The TG analysis showed that these

complexes are stable up to 200 0C.

The molecular structures of dimeric

[PdX(ECH2CH2CH2NMe2)]2 and trimeric

[PdX(ECH2CH2NMe2)]3 (X = Cl or OAc) were established

by single crystal X-ray diffraction analysis. In all these

structures both the ligands 2-dimethylaminoethyl

chalcogenolate and 3-dimethylaminopropyl

chalcogenolate act as chelating-bridging mode.

The chloride and acetate ligands are bonded as

monodentate terminal groups to

palladium center.

(ii) Catalysis reactions

The coupling reaction of 4-iodotoluene

and phenyl boronic acid was studied

in the presence of all the palladium

complexes as catalysts (0.1 mol%) (Table

1). The yields of biaryls under the same

reaction conditions varied between 63-

98% depending on the nature of catalyst.

It is evident from Table 1 that the binuclear

palladium complexes (3 and 4) are more

active than the trinuclear derivatives (1

and 2). Within the series the acetato

complexes (2 or 4) are superior than the

corresponding chloro compounds (1 or

3). The activity of the sulfur derivatives

within the series was marginally higher

than the corresponding selenolate

complexes.Scheme 1: Syntheses of palladium chalcogenolate complexes.

Special Issue | October 2014 89

BARC NEWSLETTERFounder’s Day

2. (i) Syntheses of 4-pyridylselenolate complexes of palladium(II)

4-Pyridylselenolate ligand has been employed to

design various motifs ranging from mononuclear

to trinuclear Pd complexes with the general

formula trans-[PdX(4-SeC5H4N)(PPh3)2] (X = Cl (5)

or SeC5H4N (6)) and cis-[Pd(4-SeC5H4N)2(dppe)].

Reactions of [Pd2Cl2(µ-Cl)2(PR3)2] with two

equivalents of Na-4-SeC5H4N gave complexes of

composition [PdCl(4-SeC5H4N)(PR3)]N (R = Et (7a),

Ph (7b)) which were isolated as trimer/dimer

containing bridging selenolate ligands. The 31P and 77Se NMR spectra of 7a show single resonances in

solution but 7bexhibit two such resonances. One

is attributed to a trinuclear species while the other

to the dimer (Scheme 2). Both di- and trimeric forms

could be separated and structurally characterized. The

crystal structure of [PdCl(SeC5H4N)(PPh3)]2 (7bY) is a

centrosymmetric dimer with a sym-trans configuration.

The two palladium atoms are held together by two

bridging selenolate groups. The two pyridyl groups of

the bridging selenolate adopt an anti configuration.

The complex [PdCl(4-SeC5H4N)(PEt3)]3 (7a) represent an

example of neutral nearly triangular shaped molecule.

The pyridyl ring is almost perpendicular to the metal

square plane.

Entry ‘Pd’ catalyst Yield (%)1. [PdCl(SCH2CH2NMe2)]3 (1a) 872. [PdCl(SeCH2CH2NMe2)]3 (1b) 633. [PdCl(SCH2CH2CH2NMe2)]2 (3a) 964. [PdCl(SeCH2CH2CH2NMe2)]2 (3b) 935. [Pd(OAc)(SCH2CH2NMe2)]3 (2a) 926. [Pd(OAc)(SeCH2CH2NMe2)]3 (2b) 717. [Pd(OAc)(SCH2CH2CH2NMe2)]2 (4a) 988. [Pd(OAc)(SeCH2CH2CH2NMe2)]2 (4b) 92

Table 1: Suzuki-Miyaura cross-coupling of aryl halide with arylboronic acid.

S. No. R Complex mol% Pd Time (hrs)

Yield (%)

TON

1. 4-CH3 5 0.1 8 27 2702. 4-CH3 5 0.1 20 39 3903. 4-CH3 5 0.3 8 40 1334. 4-CH3 5 0.5 8 80 1605. 4-CH3 7b 0.1 8 28 2806. 4-NO2 5 0.1 8 91 9107. 4-NO2 7b 0.1 8 97 9708. 4-NO2 7b 0.01 8 62 62009. 2-CHO 5 0.01 6 82 820010. 2-CHO 7b 0.01 6 84 840011. 4-COCH3 5 0.01 12 14 140012. 4-COCH3 7b 0.01 12 29 2900

Table 2: Suzuki-Miyaura cross-coupling of aryl bromide with phenylboronic acid.

(ii) Catalysis reactions

Both 5 and 7b were examined as catalysts

in Suzuki cross-coupling reactions of

arylbromide and arylboronic acid to yield

biaryls (Table 2). Catalysts are tolerant to

various functional groups, such as CHO,

NO2, COCH3, etc. Cross-coupling yielded

products almost quantitatively when electron

deficient arylbromides were used (Table 2,

entries 6-7). High yields were also obtained

(> 82%) when the catalyst concentration was Scheme 2: Dimers and trimers of complexes [PdCl(4-SeC5H4N)(PR3)]n (7)

90 Special Issue | October 2014

BARC NEWSLETTERFounder’s Daylowered by ten times in the case of electron-deficient

arylbromides (Table 2, entries 9-10). The catalytic

activity of monomeric complex 5 and trimeric-dimeric

complex 7b are comparable as both gave similar yields

of biaryls under similar reaction conditions. As it is

commonly believed that catalytically active species are

generally monomeric which indirectly also supports

the trimer-dimer-monomer equilibrium of 7b prior to

catalyst initiation.

Acknowledgements

Dilip Paluru, K. V. Vivekananda, A. Wadawale, N.

Bhuvanesh and V. K. Jain are acknowledged for

collaborations.

References

1. Negishi, E. Handbook of organopalladium

chemistry for organic synthesis, Ed. John Wiley &

Sons, Inc., 2002.

2. Wu, X.-F.; Neumann, H.; Beller, M. Chem. Rev.

2013, 113, 1-35.

3. Yao, Q.; Kinney, E.P.; Zheng, C. Org. Lett. 2004, 6,

2997-2999.

4. Kumar, S.; Rao, G.K.; Kumar, A.; Singh, M.P.; Singh,

A.K. Dalton Trans. 2013, 42, 16939–16948.

5. Yuan, D.; Huynh, H.V. Organometallics 2010, 29,

6020–6027.

6. Dey, S.; Jain, V.K.; Chaudhury, S.; Knoedler, A.;

Lissner, F.; Kaim, W. J. Chem. Soc., Dalton Trans.

2001, 723-728.

7. Vivekananda, K.V.; Dey, S.; Wadawale, A.;

Bhuvanesh, N.; Jain, V.K. Dalton Trans. 2013, 42,

14158–14167.

8. Paluru, D.K.; Dey, S.; Wadawale, A.; Jain, V.K. J.

Organomet. Chem. 2013, 728, 52-56.

9. Paluru, D.K.; Dey, S.; Chaudhuri, K.R.; Khedkar,

M.V.; Bhanage, B.M.; Jain, V.K. Tetrahedron Lett.

2014, 55, 2953-2956.

10. Khairnar, B.J.; Dey, S.; Jain, V.K.; Bhanage, B.M.

Tetrahedron Lett. 2014, 55, 716–719.

Fig. 1: Molecular structure of [PdCl(4-SeC5H4N)(PPh3)]2 (7bY)

Conclusions

The hybrid and hemilabile ligands dimethylaminoalkyl

dichalcogenide and 4,4’-dipyridyldiselenide have been

designed and developed. Using these ligands the

chemistry of palladium and platinum complexes was

explored. The nuclearity of complexes [PdX(EÇN)]n is

influenced by the chain length of the ligand. The non-

chelating ligand 4-pyridylselenolate has a promising

potential in constructing multinuclear complexes.

The palladium complexes have been shown to serve

as catalyst in Suzuki C–C coupling reactions. Catalytic

activity of [PdX(EÇN)]n follow the trend: OAc > Cl; S

>Se; binuclear > trinuclear.