Palladium complexes of P,P and P,S type bidentate ligands: Implication in Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1016/j.jorganchem.2011.10.001

New palladium complexes of the type [PdCl₂(η²- P∩P)] (1a,1b) and [PdCl₂(η²-P∩S)] (1c,1d) have been synthesised by the reaction of PdCl₂ with P,P and P,S type bidentate ligands in 1:1 mol ratio, where,

Full text of DOI:10.1016/j.jorganchem.2011.10.001

Journal of Organometallic Chemistry 696 (2012) 4293e4297
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Journal of Organometallic Chemistry
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Palladium complexes of P,P and P,S type bidentate ligands: Implication
in SuzukieMiyaura cross-coupling reaction
*
Kokil Saikia, Biswajit Deb, Bibek Jyoti Borah, Podma Pollov Sarmah, Dipak Kumar Dutta
Materials Science Division, Council of Scientific and Industrial Research-North East Institute of Science and Technology, Jorhat 785 006, Assam, India
a r t i c l e i n f o
a b s t r a c t
New palladium complexes of the type [PdCl2(h2ePXP)] (1a,1b) and [PdCl2(h2ePXS)] (1c,1d) have been
synthesised by the reaction of PdCl2 with P,P and P,S type bidentate ligands in 1:1 mol ratio, where,
Article history:
Received 4 August 2011
Received in revised form
29 September 2011
PXP
¼
9,9edimethyl-4,5-bis(diphenylphosphanyl) xanthene {Xantphos}(a) or bis(2-diphenylphos-
phanylphenyl)ether{DPEphos}(b); PXS ¼ 9,9-dimethyl-4,5-bis(diphenyl -phosphanyl) xanthenemono-
sulfide {Xantphos(S)}(c) or bis(2-diphenylphosphanyl phenyl) ether monosulfide {DPEphos(S)}(d). The
complexes are characterized by elemental analyses, mass spectrometry, 1H, 13C and 31P NMR spectros-
copy together with the single crystal X-ray structure determination of 1a and 1d. The palladium atom in
all the complexes occupies the centre of a slightly distorted square planar environment formed by a P
atom, a P/S atom and two Cl atoms. The catalytic activities of 1ae1d investigated for SuzukieMiyaura
cross-coupling reactions at room temperature exhibit higher yield of the coupling products than cata-
lysed by PdCl2 itself. Among 1ae1d, the palladium complexes of bidentate phosphine (1a, 1b) show
higher efficacy than their monosulfide analogues (1c, 1d). However, the recycling experiments with the
catalysts for a selected coupling reaction between 4-bromobenzonitrile and phenylboronic acid exhibit
that 1c and 1d are more efficient than 1a and 1b, which may be due to the donor effect of the P,S ligands
during catalytic reaction.
Accepted 5 October 2011
Keywords:
Palladium
Bidentate
Functionalized phosphine
Cross-coupling reaction
Catalysis
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
susceptible to oxidationduringthe reaction toyieldphosphine oxides
and “palladium black”, which is catalytically inactive [19]. Other type
Palladium chemistry plays an extremely active part of coordina-
tion and organometallic chemistry: both in homogeneous as well
as heterogeneous catalysis [1e7]. Recently, palladium complexes
occupy a special position of pivotal importance in the field of catalysis
for different chemical transformations particularly in carbonecarbon
bonds formation in organic syntheses [5e8]. The palladium catalysed
SuzukieMiyaura cross-coupling reaction of arylboronic acids with
arylhalides is found to be one of the most powerful tools for syn-
thesising different bi-aryls, which constitute an important class of
compounds for pharmaceutical and agricultural chemistry [9e15].
For this type of catalytic reactions, Pd(0) acts as active site to initiate
the catalytic cycle, and the activity depends upon the ability of the
Pd(0) species to activate the carbon-halogen bond by oxidative
addition, which is further tuned by the steric and electronic proper-
ties of the ligands attached to the metal [16,17]. In this regard, phos-
phine based ligands have been widely used due to its effectiveness in
stabilizing the zero valent palladium [18]. However, phosphines are
of ligand systems like PeO, PeN, PeS and certain electron rich
phosphines have been used extensively for the SuzukieMiyaura
cross-coupling reaction [20e25]. Weakly bound ligands tend to give
high catalytic activity, but are unstable and decompose into inactive
palladium black, while catalytic cycle is blocked by strongly bound
ligands in complexes to give low catalytic activity [26]. Thus, proper
choice/design of ligands will play a crucial role to generate efficient
catalysts in such reaction. As a part of our ongoing research activity
[27e33], we have chosen two bidentate phosphine ligands, namely,
9,9-dimethyl-4,5-bis(diphenylphosphanyl)xanthene {xantphos} and
bis(2-diphenylphosphanylphenyl)ether {DPEphos} with different
ligand backbones and have made them heterobidentate by selective
oxidation of one of the phosphorus atoms with elemental sulfur. In
this paper, we report the synthesis of four new air stable Pd(II)
complexes containing symmetrical bidentate PXP and hetero-
bidentate PXS ligands and their reactivity in SuzukieMiyaura cross-
coupling reaction at room temperature. The recycling experiments of
these catalysts have also been investigated under the similar exper-
imentalconditionsin order to evaluate the effectof the ligandsduring
the catalytic cycle.
* Corresponding author. Tel.: þ91 376 2370081; fax: þ91 376 2370011.
E-mail address: dipakkrdutta@yahoo.com (D.K. Dutta).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.10.001

4294
K. Saikia et al. / Journal of Organometallic Chemistry 696 (2012) 4293e4297
2. Experimental
(CDCl3, ppm): d 23.8 (s, P), 43.34 (s, P]S) Elemental analyses:
Found (Cald. for PdC39H32P2SOCl2), C 59.24 (59.4); H 3.96 (4.06). m/
2.1. General information
z:781.9 [Mþ].
1d: IR (KBr, cmꢀ1): 590 [ (P]S)]. 1H NMR (CDCl3, ppm):
n
All the solvents were distilled under N2 prior to use. PdCl2 was
purchased from M/S Arrora Matthey Ltd., Kolkata, India. Ligands
xantphos and DPEphos, and elemental sulphur were purchased
from M/S Aldrich, USA and were used without further purification.
Elemental analyses were performed on a PerkineElmer 2400
elemental analyzer. IR spectra (4000-400 cmꢀ1) were recorded on
KBr discs with a Shimadzu IR Affinity-1 FTIR spectrophotometer. 1H,
13C and 31P NMR spectra were recorded in CDCl3 solution on
a Bruker DPX-300 spectrometer, and chemical shifts were reported
relative to SiMe4 and 85% H3PO4 as internal and external standard
respectively. Mass spectra of complexes were recorded on an
ESQUIRE 3000 mass spectrometer.
d
7.0e7.76 (m, Ph). 13C NMR (CDCl3, ppm):
d
133.4e167.3 (Ar). 31P
{1H} NMR (CDCl3, ppm):
d
20.52 (s, P), 43.91 (s, P]S) Elemental
analyses: Found (Cald. for PdC36H28P2SOCl2), C 57.24 (57.9); H 3.56
(3.7). m/z: 742.0 [Mþ].
2.5. X-ray structural analysis
Single crystals of 1a and 1d suitable for X-ray crystallographic
analysis were obtained by layering a dichloromethane solution of
the respective compounds with hexane. Intensity data of 1a and 1d
were collected on a Bruker Smart-CCD with Mo-K
a radiation
(
l
¼ 0.71073 Å) at 25 ꢁC. The structures were solved with SHELXS-
97 and refined by full-matrix least squares on F2 using the SHELXL-
97 computer program [34]. Hydrogen atoms were idealised by
using the riding models.
2.2. Synthesis of xantphos monosulfide and DPEphos monosulfide
Monosulfur functionalized xantphos and DPEphos ligands were
prepared by adopting the literature procedure [30].
2.6. Catalytic activity of 1ae1d in SuzukieMiyaura cross-coupling
reaction
2.3. Synthesis of the complexes [PdCl2(h2e PXP)] (1a,1b) and
[PdCl2(h2e PXS] (1c,1d), where PXP ¼ Xantphos(a), DPEphos(b);
PXS ¼ Xantphos(S)(c) and DPEphos(S)(d)
Solutions of aryl bromides (2 mmol) in dichloromethane
(10 cm3) and aryl boronic acids (2 mmol) in acetonitrile (20 cm3)
were mixed together. The base triethylamine (6 mmol) and the
respective catalyst (0.004 mmol) in dichloromethane (5 cm3) were
then added to the solution and stirred at room temperature. The
reactions were monitored by TLC and the products were isolated by
silica gel column chromatography using ethyl acetate and hexane
as eluents. The products were characterised by 1H NMR, mass
spectrometry and melting point determination followed by their
comparison with the standard literature data. In order to study the
recycling experiments, the organometallic residues of 1ae1d along
with the silica gel were collected from the chromatographic column
and refluxed in acetonitrile to dissolve the residue to get a solution.
The silica gel was separated by filtration and the residue was
recovered by removing the solvent in vacuum. The residue was
reused by maintaining the similar experimental conditions as
described above.
PdCl2 (0.05 mmol, 8.86 mg) was dissolved in acetonitrile
(15 cm3). The ligand [0.05 mmol, 28.90 mg (a), 26.92 mg (b),
30.53 mg (c) and 28.50 mg (d)] were dissolved in dichloromethane
(10 cm3) to prepare corresponding solutions. Both the solutions
were mixed together and stirred for 30 min at room temperature.
The solvent was removed under reduced pressure and washed with
diethyl ether. The resulting yellowish orange compounds were
recrystalized from dichloromethane and hexane to obtain the
complexes 1ae1d.
2.4. Analytical data for the complexes 1ae1d
1a: 1H NMR (CDCl3, ppm):
NMR (CDCl3, ppm): 124.7e162.2 (Ar),
CH3). 31P{1H} NMR (CDCl3, ppm):
d
7.02e7.73 (m, Ph),
d
1.67 (s, CH3). 13
33.4 (s,
C
d
d
33.0 (CMe2), d
d
22.70 (d, JP-P ¼ 56.64 Hz)
Elemental analyses: Found (Cald. for PdC39H32P2OCl2), C 61.31
3. Results and discussion
(61.91); H 4.09 (4.23). m/z: 749.63 [Mþ].
1b: 1H NMR (CDCl3, ppm):
d
7.31e7.91 (m, Ph). 13C NMR (CDCl3,
19.53 (d, JP-
¼ 68.4 Hz). Elemental analyses: Found (Cald. for PdC36H28 P2OCl2),
3.1. Synthesis and characterization of 1ae1d
ppm): d d
128.4e162.8 (Ar). 31P{1H}NMR (CDCl3, ppm):
Palladium complexes 1ae1d were prepared by stirring aceto-
nitrile solution of palladium dichloride with dichloromethane
solution of ligands aed in 1:1 mol ratio at room temperature
(Scheme 1). Elemental analyses and mass spectrometric results of
the complexes support the observed molecular composition of
P
C 60.02 (60.37); H 3.81 (3.9). m/z: 710.43 [Mþ].
1c: IR (KBr, cmꢀ1): 621 [ (P]S)]. 1H NMR (CDCl3, ppm):
6.91e7.76 (m, Ph),
126.7e164.2 (Ar),
n
d
d
d
1.62 (s, CH3). 13C NMR (CDCl3, ppm):
d
33.1 (CMe2),
d
33.7 (s, CH3). 31P{1H}NMR
P
Cl
P-X (X = P or S) / DCM
Pd
PdCl2
(Acetonitrile)
Stirring, r.t.
X
Cl
(1a-1d)
P-P :
P-S :
O
O
O
O
P
P
P
P
S
P
P
S
P
P
Ph
Ph Ph
Ph
Ph
Ph Ph
(c)
Ph
Ph
Ph Ph
Ph
Ph
Ph Ph
Ph
(a)
(b)
(d)
Scheme 1. Synthesis of complexes 1ae1d.

K. Saikia et al. / Journal of Organometallic Chemistry 696 (2012) 4293e4297
4295
Table 1
1ae1d. In the IR spectra, the positions of
at 621 and 590 cmꢀ1 are about 24 and 45 cmꢀ1 lower than those of
the corresponding free ligands [
(PeS): 645(c), 635 (d) cmꢀ1],
which reveal that the coordination to metal is through the S donor.
The 31P NMR spectra of 1a and 1b exhibit doublets at
JP-P ¼ 56.64 Hz) and 19.53 (d, JP-P ¼ 68.4 Hz) ppm, respectively
corresponding to two tertiary phosphorus bonded to the metal
centre. However, the complexes 1c and 1d show two sharp singlets,
where pentavalent phosphorus atom bonded to sulfur and the
n(PeS) bands of 1c and 1d
Crystallographic data and structure refinement details for complexes 1a and 1d.
n
1a
1d
Emphirical formula
Formula weight
Temperature (K)
PdC40H34P2OCl4
840.81
293
0.71073
Monoclinic
P 21/n
PdC37H30P2SOCl4
832.82
293
0.71073
Triclinic
d
¼ 22.70 (d,
l
(Å)
Crystal syst.
Space group
Z
a (Å)
b (Å)
P-1
2
4
12.8043(17)
21.789(3)
13.4518(19)
90
100.389(2)
90
0.911
5797
0.0982(3601)
0.1988(5782)
12.329 (4)
12.987 (4)
13.244 (4)
117.286 (5)
95.241 (5)
102.603 (5)
0.992
8958
0.0627 (6127)
0.1600 (8056)
tertiary phosphorus atom bonded to the metal resonate at
d
¼ 43.34
(1c), 43.91(1d) and 23.8 (1c), 20.52 (1d), ppm respectively. The
bands for the P and P]S atoms in 1ae1d show a significant
downfield shift relative to those of the corresponding free ligands
aed, [30,31] which is in good agreement with the chelate forma-
tion in the complexes. The 1H and 13C NMR spectra do not provide
any significant evidences about the formation of 1ae1d, however,
the data are found in the expected ranges.
c (Å)
a
b
g
m
(ꢁ)
(ꢁ)
(ꢁ)
(Mo-K
a
) mmꢀ1
Reflections collected
R1 (observed data)
wR2 (all data)
The complexes 1a and 1d are structurally determined by single
crystal X-ray diffraction (Fig. 1, Tables 1,2). The structural charac-
terisation of 1b and 1c were not possible because no suitable
crystals could be developed despite of several attempts. In all the
complexes, the Pd atom occupies the centre of a slightly distorted
square planar geometry formed by a P atom, a P/S atom and two Cl
atoms. The ligands a and d form chelate through PeP and PeS
donors with palladium centre to form wide bite angles P(1)e
Pd(1)eP(2) 101.18 (1a) and S(1)ePd(1)eP(2) 94.19 (1d). The
average PdeP bond length in 1a is 2.29 Å, whereas in 1d, it is found
to be 2.24 Å. The bond length of PdeS (2.32 Å) in 1d is longer than
that of PdeP (2.24 Å), which indicate a weaker interaction in the
former and is likely to be cleaved more easily during catalytic
reaction.
The complex 1a forms a wheel chair like conformation (Fig. 1a)
where the ligand backbone is almost perpendicular to the square
plane [P(1)Pd(1)P(2)Cl(2)Cl(1)]. The natural bite angle 111.7ꢁ of the
free xantphos ligand reduces to 101.18ꢁ {bite angle, P(1)ePd(1)e
P(2)} upon complexation. The two aromatic rings of the backbone
are not coplanar, but twisted by an angle 38.52ꢁ to adjust the
coordination environment. In the complex 1d, one of the phenyl
rings adjacent to backbone oxygen of the ligand occupies the apical
position of the palladium atom to generate a protective cover
towards the fifth coordination site (Fig. 2).
3.2. Catalytic activity of 1ae1d towards Suzuki coupling reaction
The catalytic activity of 1ae1d were evaluated in Suzu-
kieMiyaura cross-coupling reaction using different aryl bromides
and aryl boronic acids. 1ae1d show efficient catalytic activity for
the synthesis of biaryl products with isolated yields 70e98%, which
are higher than the activity of PdCl2 itself (Table 3). As shown in
Table 3, aryl bromides having electron withdrawing substituent
(eCN) gives higher yield than those containing electron donating
group (eOMe). However, the efficacy of the complexes depends on
Table 2
Selected bond lengths and bond angles for complexes 1a and 1d.
1a
Bond lengths (Å)
Pd(1)eP(1)
2.295(3)
3.075
Pd(1)eP(2)
2.289(3)
Pd(1)/O(1) (spatial)
Bond angles (ꢁ)
C(17)eO(1)eC(21)
P(1)ePd(1)eCl(2)
1d
115.07(7)
171.71(11)
P(1)ePd(1)eP(2)
101.19(11)
Bond lengths (Å)
Pd(1)eS(1)
2.321(15)
2.247(14)
P(1)eS(1)
Pd(1)/O(2) (spatial)
2.017(18)
3.12
Pd(1)eP(2)
Bond angles (ꢁ)
C(18)eO(2)eC(19)
Cl(1)ePd(1)eS(1)
Fig. 1. Ortep diagram of (a) [PdCl2(Xantphos)] (1a) and (b) [PdCl2(DPEphosS)] (1d)
(thermal ellipsoids are drawn at 50% probability). Hydrogen atoms and uncoordinated
solvent molecules are omitted for clarity.
118.96(3)
177.01(5)
Pd(1)eS(1)eP(1)
P(2)ePd(1)eS(1)
103.7(6)
94.1(5)

4296
K. Saikia et al. / Journal of Organometallic Chemistry 696 (2012) 4293e4297
Fig. 3. A brief comparison of the catalytic activity of 1ae1d before and after recycling
experiments of cross-coupling between 4-bromobenzenitrile and phenylboronic acid.
decomposition of the catalyst to palladium black and hence become
unfavourable to reuse for several runs. The formation of phosphine
oxide was substantiated by spectroscopic analysis of the organo-
metallic residues of 1a and 1b. The appearance of IR bands at
Fig. 2. Pictorial view of 1d to show the protective cover at the fifth coordination site of
the palladium centre.
n
¼ 1191(1a) and 1187(1b) cmꢀ1 and 31P NMR resonances in the
region
P]O bonds [29,35].
d
¼ 30.5 (1a) and 26.3 (1b) ppm indicate the formation of
the nature of the ligands and follows the order
1a > 1b > 1c > 1d > PdCl2. The recycling experiments of the
organometallic residues were performed by maintaining the
similar experimental conditions for a selected reaction of 4-
bromobenzonitrile and phenylboronic acid. The efficacy of 1a and
1b decreases considerably, while 1c and 1d show almost similar
activity during the second run (Fig. 3). It is evident from the
observed trend that in the initial reaction condition, the bidentate
phosphine palladium complexes (1a,1b) are much effective than
their mono sulfur functionalized analogues (1c,1d). This may be
due to the higher effectiveness of bidentate phosphine in 1a and 1b
to undergo facile generation of Pd(0) species to activate the carbon-
halogen bond through oxidative addition. However, during recy-
cling experiment, partial deactivation of the phosphine ligands take
place due to the formation of phosphine oxide. Oxygen being a hard
donor, may not stabilize the soft Pd(0), resulting in partial
On the other hand, the functionalized phosphines (PXS) in 1c
and 1d though acts slowly in the reduction of Pd(II) to Pd(0), but
proceeds smoothly during the catalytic cycle without significant
loss of activity (Fig. 3). This suggests that the ligand remains bound
to the metal centre to stabilize the catalyst and hence cycle prop-
agates with almost similar efficacy. The incomparable stability of
PXS ligands over PXP might be due to the hemilabile behaviour of
the PdeS bonds, which may undergo facile dissociation during
substrate binding followed by reassociation after product forma-
tion. In order to get some evidences of hemilability, we recovered
the organometallic residues of 1c and 1d after completion of the
reaction and termed as 1c* and 1d* respectively, and examined
their IR and 31P NMR spectra at room temperature; which indicate
the presence of a considerable amount of free PeS group [IR:
n
(PeS)
644 (1c*), 634 (1d*) cmꢀ1 31P NMR:
; d
¼ 29.5 (1c*), 40.9 (1d*) ppm]
Table 3
Catalytic efficacy of 1ae1d for SuzukieMiyaura cross-coupling reaction.
HO
PdCl2 or 1a-1d
R1
R2
R1
Br
B
R2
Acetonitrile, r.t., Et3N, 24 h
HO
Entry
R1
R2
Catalysts
Isolated Yielda (%)b
TONc
1
2
3
4
5
6
7
8
eCN
eCN
eCN
eCN
eCN
eOMe
eOMe
eOMe
eOMe
eOMe
eH
eH
eH
eH
eH
eH
eH
eH
eH
PdCl2
1a
1b
1c
1d
PdCl2
1a
1b
1c
1d
PdCl2
1a
1b
1c
1d
61.0
98.2
93.3
83.4
80.9
57.7
88.2
87.0
71.6
70.5
54.6
84.5
83.6
70.3
70.0
305
491
467
417
405
288
441
435
358
353
273
422
418
351
350
9
eH
eH
10
11
12
13
14
15
eOMe
eOMe
eOMe
eOMe
eOMe
eH
eH
eH
eH
a
Reaction conditions: 2 mmol of aryl bromide, 2 mmol of aryl boronic acid, 6 mmol of triethylamine, 0.004 mmol of catalyst, reaction time is 24 h.
Yields are calculated with respect to aryl boronic acid.
TON : Number of moles of product/Number of moles of catalyst (Pd in mole).
b
c

K. Saikia et al. / Journal of Organometallic Chemistry 696 (2012) 4293e4297
4297
efficacy of 1a and 1b decreases considerably, while 1c and 1d show
almost similar activity during the second run.
PdII
(X = P or S)
X
P
Acknowledgements
The authors are grateful to Dr. P. G. Rao, Director, CSIR-North
East Institute of Science and Technology, Jorhat, Assam, India, for
his kind permission to publish the work. The authors K. Saikia (JRF,
UGC), B. Deb (SRF, CSIR) and P. P. Sarmah (SRF, CSIR) are grateful to
the corresponding organizations for providing the fellowships.
Pd0
Pd0 P~X
X
P
or
Appendix A. Supplementary material
CCDC-813769(1a) and 801482(1d); contains the supplementary
crystallographic data for this paper. These data can be obtained free
of charge from The Cambridge Crystallographic Data Centre via
www.ccdc.cam.ac.uk/data_request/cif.
Ar Br / PPh3
Ar Br
Ar
Br
References
P~X
Ar
Br
PdII
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4. Conclusions
Four new palladium (II) complexes of the type [PdCl2(h2ꢀPXX)]
(1ae1d) (X ¼ P, S) have been synthesised and characterized by
different physicochemical methods. The complexes 1a and 1d have
been characterized by the single crystal X-ray diffraction. Palladium
atom in all the complexes occupies the centre of a slightly distorted
square planner geometry. The catalytic activities of 1ae1d have
been investigated for SuzukieMiyaura cross-coupling reactions
using different aryl bromides and aryl boronic acids at room
temperature and 1ae1d showed better catalytic activity than PdCl2.
Among 1ae1d, the bidentate phosphine palladium complexes (1a,
1b) show higher efficacy than their monosulfide analogues (1c, 1d).
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