Palladium complexes of heterobidentate ligands: Active catalysts for direct acylation of aryl halides with aldehydes via C(sp2)-H activation

Article abstract of DOI:10.1016/j.molcata.2015.07.007

Abstract Heterobidentate P-S donor ligands [P-S = {2-(methylthio) phenyl}diphenylphosphine (a) and {2-((methylthio) methyl) phenyl}diphenylphosphine (b)], and their palladium complexes of the type [Pdη²-(P-S)Cl₂] (1a, 1b) have been synthesised and characterised. Single crystal X-ray diffraction shows that in both the complexes, Pd occupies the centre of a slightly distorted square planar geometry. 1a forms a planner ring structure, whereas, the hexagonal ring of 1b bends from planarity to adjust any ring strain. Interesting differences between the complexes were observed in terms of the intermolecular forces. The catalytic activities of the synthesised complexes towards the direct acylation of aryl halides with aldehydes via C(sp²)-H activation were good to excellent. 1b shows better catalytic activity over 1a which may be attributed to the higher stability of the pentagonal ring of 1a. Aryl halides containing electron withdrawing group enhance the reaction, while electron donating substituent tend to retard the desired product formation. The difference in the bond lengths of Pd-P and Pd-S of the chelate complexes may impart hemilabile behaviour in the catalytic cycle by dissociating the weaker bond (Pd-S) to generate vacant coordination site at the metal centre and reassociate after the completion of the reaction.

Full text of DOI:10.1016/j.molcata.2015.07.007

Journal of Molecular Catalysis A: Chemical 408 (2015) 20–25
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Palladium complexes of heterobidentate ligands: Active catalysts for
direct acylation of aryl halides with aldehydes via C(sp2)-H activation
Kokil Saikia, Dipak Kumar Dutta∗
Materials Science Division, CSIR-North East Institute of Science and Technology, Jorhat, Assam 785006, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Heterobidentate P-S donor ligands [P-S = {2-(methylthio) phenyl}diphenylphosphine (a) and {2-
((methylthio) methyl) phenyl}diphenylphosphine (b)], and their palladium complexes of the type
[Pdꢀ2-(P-S)Cl2] (1a, 1b) have been synthesised and characterised. Single crystal X-ray diffraction shows
that in both the complexes, Pd occupies the centre of a slightly distorted square planar geometry. 1a
forms a planner ring structure, whereas, the hexagonal ring of 1b bends from planarity to adjust any
ring strain. Interesting differences between the complexes were observed in terms of the intermolecular
forces. The catalytic activities of the synthesised complexes towards the direct acylation of aryl halides
with aldehydes via C(sp2)-H activation were good to excellent. 1b shows better catalytic activity over
1a which may be attributed to the higher stability of the pentagonal ring of 1a. Aryl halides containing
electron withdrawing group enhance the reaction, while electron donating substituent tend to retard the
desired product formation. The difference in the bond lengths of Pd-P and Pd-S of the chelate complexes
may impart hemilabile behaviour in the catalytic cycle by dissociating the weaker bond (Pd-S) to generate
vacant coordination site at the metal centre and reassociate after the completion of the reaction.
© 2015 Elsevier B.V. All rights reserved.
Received 19 February 2015
Received in revised form 2 July 2015
Accepted 4 July 2015
Available online 14 July 2015
Keywords:
Palladium
Heterobidentate phosphine
C–H activation
Acylation
Hemilability
1. Introduction
ing the efficiency of the catalysts for generating the Pd(0) species
for the reaction cycle to start. Different monodentate and biden-
The C H bond activation has emerged as a useful tool in organic
synthesis for providing desired transformations [1–8]. Insertion of
metal into the carbon-hydrogen bonds via oxidative addition has
long been investigated, which often is the key step for majority
of transformations leading to many pioneering discoveries in the
field of catalysis [9,10]. Several transition metal catalysts based on
rhodium [11], nickel [12], ruthenium [13] etc. have been deployed
for the C H activation reactions. The aldehyde C H activation
by direct acylation of aryl halides with aliphatic aldehydes is an
interesting prospect to get alkyl-aryl ketones; which are poten-
tial intermediates in different pharmaceutical, fragrance, dye and
agrochemical industries [14–22]. Friedel-Crafts acylation has been
a traditional way of acylating arenes, which has the strict limitation
of reacting with activated benzene derivatives only [23].
tate phosphine ligands such as Q-phos, xantphos, DPEphos, dppp,
BINAP etc., were studied for the catalysis of acylation of aryl bro-
mides with aldehydes [16–18]. Bidentate ligands having wider bite
angle tend to destabilize the divalent square planner complexes
and favour zerovalent complexes [24]. Most of the C H activation
reactions have been carried out by using phosphine ligands which
are known to reduce Pd(II) to Pd(0) to initiate the oxidative addi-
tion step of the catalytic cycle. However, as per our knowledge, no
heterobidentate hemilabile P-S ligands have been reported in the
past for the catalysis of direct acylation of aromatic halides with
aliphatic aldehydes.
We chose two palladium metal complexes of heterobidentate
P-S ligand systems {2-(methylthio) phenyl}diphenylphosphine (a)
and {2-(methylthio) methyl)phenyl}diphenyl-phosphine (b) for
catalyzing C H functionalization of aliphatic aldehyde by aryl
halides to form alkyl-aryl ketone. Although palladium complexes
of these ligands have been synthesized in the past [25,26], their
catalytic activities towards C H activation reactions have not been
studied. Heterobidentate ligands like a and b tend to show hemil-
ability at the metal centre by dissociating the weaker bond during
the catalytic cycles to step up the oxidative addition [27–29].
In our continued efforts [30–39] to develop new catalysts for
Synthesis of alkyl-aryl ketones by direct acylation of aryl halides
with aliphatic aldehydes have been reported by using palladium
complexes of phosphine ligands as well as palladium nanopaticles
[16–21]. Selection of ligands plays an important role in determin-
∗
Corresponding author. Fax: +91 376 2370011.
E-mail address: dipakkrdutta@yahoo.com (D.K. Dutta).
http://dx.doi.org/10.1016/j.molcata.2015.07.007
1381-1169/© 2015 Elsevier B.V. All rights reserved.

K. Saikia, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 408 (2015) 20–25
21
different coupling reactions, herein, we report synthesis and char-
acterization of two palladium complexes of heterobidentate P-S
ligand systems, and their catalytic activity towards acylation of aryl
halides with aldehydes.
2. Experimental
2.1. General information
All the solvents were distilled under N2 prior to use. PdCl2 was
purchased from M/S Arrora Matthey Ltd., Kolkata, India. Elemental
analyses were performed on a PerkinElmer 2400 elemental ana-
lyzer. IR spectra (4000–400 cm−1) were recorded as thin film using
NaCl cell in 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. Melting points were measured
in melting point apparatus Buchi M-560.
Fig. 1. Ortep diagram of a (thermal ellipsoids are drawn at 50% probability). Hydro-
gen atoms and uncoordinatated molecules are omitted for clarity.
2.2. Synthesis of the ligands
the respective compounds with hexane. Intensity data of 1a and
1b were collected on a Bruker Smart-CCD with Mo-K␣ radiation
(ꢁ = 0.71073 Å) at 150 and 293 K respectively. The structures were
solved with SHELXS-97 and refined by full-matrix least squares on
F2 using the SHELXL-97 computer program. Hydrogen atoms were
idealised by using the riding models.
Ligands a and b were synthesized by adapting the literature
[26,40].
2.3. Analytical data for the ligands a and b
a: 1H NMR (CDCl3, ppm): ␦ 2.53 (s, 3H, CH3), ␦ 7.26, 7.78 (m,
14H, Ph). 13C NMR (CDCl3, ppm): ␦ 15.1 (CH3), ␦ 125.1-140.8 (Ph),
2.7. Catalytic activity of 1a and 1b in acylation reaction
31P{ H}NMR (CDCl3, ppm): ␦ -14.67 [s, PIII] Elemental analyses:
1
Found (Cald. for C19H17PS), C 73.92 (74.0); H 4.98 (5.56). Mass:
307.7 [m/z+].
Aryl halides (2 mmol) and aldehydes (2.4 mmol) in 1, 4 - dioxane
(20 cm3) were mixed together. The base cesium carbonate (6 mmol)
and the respective catalyst (0.004 mmol) in dichloromethane
(5 cm3) were then added to the solution and stirred at 100 ◦C. 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 characterized by 1H NMR, 13C NMR and
melting point determination followed by their comparison with the
standard literature data.
b: 1H NMR (CDCl3, ppm): ␦ 2.14 (s, 3H, CH3), ␦ 3.56 (s, 2H, CH2), ␦
7.24, 7.98 (m, 14H, Ph). 13C NMR (CDCl3, ppm): ␦ 14.9 (CH3), ␦ 40.9
1
(CH2), ␦ 127.41-143.8 (Ph), 31P{ H}NMR (CDCl3, ppm): ␦ -16.52 [s,
P
III] Elemental analyses: Found (Cald. for C20H19PS), C 73.90 (74.6);
H 5.09 (5.89). Mass: 320.5 [m/z+].
2.4. Synthesis of the complexes
PdCl2 (0.05 mmol, 8.86 mg) was dissolved in acetonitrile
(15 cm3). The ligands [0.05 mmol, 15.41 mg (a) and 16.1 mg (b)]
were dissolved in dichloromethane (10 cm3) to prepare corre-
sponding 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 1a and 1b.
3. Results and discussion
3.1. Synthesis and characterization
The ligands a and b were synthesised according to the litera-
ture methods. Elemental analyses and mass spectrometric results
demonstrate the observed molecular composition of a and b. The
1H NMR spectra of both a and b show characteristic phenylic mul-
tiplets in the region 7.24–7.98 ppm. The characteristic methylic
singlet in the range 2.14–2.51 ppm, and methylene singlet at
2.5. Analytical data for the complexes 1a and 1b
1
1a: 1H NMR (CDCl3, ppm): ␦ 7.36–7.83 (m, Ph), ␦ 2.59 (s, CH3).
3.56 ppm confirm the formation of b. 31P{ H} NMR spectra of
1
13C NMR (CDCl3, ppm): ␦ 127.7–142.2 (Ar), ␦ 15.47 (s, CH3). 31P{ H}
a and b show characteristic resonances at ␦ −14.67 and −16.52
respectively, corresponding to the trivalent phosphines. In 13C NMR
spectra, the chemical shifts due to the phenylic and methyl carbons
of both the ligands are found in the range 125.1–143.8 ppm and
14.9-15.1 ppm respectively. The appearance of singlet at 40.9 ppm
for the methylene carbon in the 13C NMR spectra further confirms
the formation of the ligand b. The ligand a is also structurally char-
acterized by single crystal X-ray diffraction, which crystallizes in
monoclinic crystal system (Fig. 1, Table 1). However, the structure
of ligand b could not be determined due to its liquid state. Palla-
dium complexes 1a and 1b were prepared by stirring acetonitrile
solution of palladium dichloride with dichloromethane solution of
ligands a and b in 1:1 mol ratio at room temperature (Scheme 1).
NMR (CDCl3, ppm): ␦ 40.83 (s) Elemental analyses: Found (Cald. for
PdC19H17C
l2PS), C 46.31 (47.01); H 5.09 (5.96). m/z: 484.63 [M+].
1b: 1H NMR (CDCl3, ppm): ␦ 7.31-7.91 (m, Ph), ␦ 2.19 (s,
CH3), ␦ 3.86 (s, CH2). 13C NMR (CDCl3, ppm): ␦ 128.4–142.8 (Ar).
31P{ H}NMR (CDCl3, ppm): ␦ 89.997 (s). Elemental analyses: Found
1
(Cald. for PdC20H19
[M+].
Cl2PS), C 47.42 (48.09); H 3.71 (3.8). m/z: 498.46
2.6. X-ray structural analysis
Single crystals of 1a and 1b suitable for X-ray crystallographic
analysis were obtained by layering a dichloromethane solution of

22
K. Saikia, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 408 (2015) 20–25
Table 1
3.2. Catalytic activity of 1a and 1b towards acylation of aryl
halides with aldehydes
Crystallographic data and structure refinement details for a, 1a and 1b.
a
1a
1b
Catalytic activity of the synthesised complexes were inves-
tigated towards the acylation of aryl halides with aldehydes at
elevated temperature. Optimization of the catalytic reaction con-
ditions was carried out by acylation of iodobenzene with pentanal
using 1a as catalyst precursor in presence of a base. The desired
ketone was obtained in small amounts when secondary amines and
KOBut were used as base, while with Cs2CO3, maximum yield up
to 76 % was observed. Use of piperidine as base in 1,4-dioxane as
solvent yields about 20 % of the desired ketone. Similarly, effect
of solvent on the reaction was significant, 1,4-dioxane yielding the
best results. Use of dimethyl formamide (DMF) and Cs2CO3 as sol-
vent and base respectively yielded 25 % product. The effects of
the combinations of different solvents with different bases were
also tested and the results are summarized in Table 2. It therefore
appears that the reaction proceeds efficiently in the presence of
Cs2CO3 as base and 1,4-dioxane as solvent.
Emphirical formula
Formula weight
Temperature (K)
ꢁ (Å)
Crystal syst.
Space group
Z
C19H17PS
308.37
150 K
0.71073
Monoclinic
P 21/C
PdCl2C19H17PS
485.67
150 K
0.71073
Monoclinic
P 21/C
PdCl2C20H19PS
499.68
293 K
0.71073
Monoclinic
P 21/C
4
4
4
a (Å)
b (Å)
11.3199 (13)
15.8299 (19)
9.5357 (11)
90
108.069(2)
90
0.288
3525
0.0384 (3256)
0.1138 (3525)
10.5589 (8)
17.2956 (12)
20.9978 (15)
90
99.799 (2)
90
1.458
7395
0.0473 (6733)
0.0950 (7395)
11.1276 (16)
10.0382 (15)
17.648(3)
90
95.995(3)
90
1.408
3843
0.0841 (3101)
0.1603 (3843)
c (Å)
˛ (◦)
ˇ (◦)
ꢂ (◦)
ꢃ (Mo K␣) mm−1
Reflections collected
R1 (observed data)
wR2 (all data)
Having established the optimized conditions, acylation of differ-
ent halides with aldehydes were tested. Good to excellent results
were obtained (Table 3). Both 1a and 1b show better catalytic activ-
ity over PdCl2, indicating the dictation of the coordinating ligands
in the catalytic performance of the complexes. The better activity
of 1b over 1a may be attributed to the higher stability of the pen-
tagonal ring of 1a. The hexagonal ring of 1b twists from planarity
to offer more space as well as a greater bite angle (Fig. 1); thereby
enhancing more probability of the oxidative addition at the metal
centre. As oxidative addition is often the first step of the catalytic
cycle [1,18], greater flexibility of 1b offers geometrical favour dur-
ing the reaction cycle than 1a. The wider bite angle of 1b tends to
Scheme 1. Synthesis of the Palladium complexes.
Elemental analyses and mass spectrometric results of the com-
plexes support the observed molecular composition of 1a and 1b.
The 31P NMR spectra of 1a and 1b show two sharp singlets at ␦ 40.83
and 89.99 ppm respectively for the corresponding metal bonded P
atoms. Both the complexes were structurally characterized by sin-
gle crystal X-ray diffraction using full-matrix least squares on F2
using the SHELXL-97 computer program (Fig. 2, Table 1) [41,42].
In both the complexes, Pd atom occupies the centre of a slightly
distorted square planar geometry formed by a P atom, a S atom
and two Cl atoms. The spatial P-S distance of 1a (3.112 Å) is almost
equal to that of the free ligand a (3.115 Å), which demonstrates
the ideal space a provides for the formation of 1a. The ligands a
and b form chelate through P-S donors with palladium centre and
show bite angles P(1)-Pd(1)-S(1) 88.42 (1a) and P(1)-Pd(1)-S(1)
94.05 (1b); for the pentagonal and hexagonal rings respectively. 1a
forms a planner ring structure, whereas, the hexagonal ring of 1b
bends from planarity to adjust any ring strain. The Pd-S {2.270 (1a)
and 2.274 Å (1b)} bonds are longer than the respective Pd P bond
lengths {2.217 (1a) and 2.239 Å (1b)} in both the complexes, imply-
ing the possibility of Pd-S bond cleavage during the catalytic cycle.
Interesting differences between the complexes were observed in
terms of the intermolecular forces. Intermolecular weak bonds
between one Cl-atom and two H-atoms were observed in both the
complexes. In addition, 1a has other intermolecular interactions
involving different atoms of the complex (see Suppoting informa-
tion).
Fig. 2. Ortep diagram of 1a and 1b (thermal ellipsoids are drawn at 50% probability).
Hydrogen atoms and uncoordinatated molecules are omitted for clarity.

K. Saikia, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 408 (2015) 20–25
23
Table 2
Optimizing conditions for acylation of iodobenzene with pentanal.
Entry
Base
Solvent
Yield (%)
1
2
3
Pyrrolidine
Piperidine
KOBut
DMF
DMF
DMF
<15
<15
<3
4
5
6
7
8
9
10
Cs2CO3
DMF
25
<5
0
<20
76
10
<5
Piperidine
Toluene
Toluene
1,4-dioxane
1,4-dioxane
Toluene
1,4-dioxane
KOBut
Piperidine
Cs2CO3
Cs2CO3
KOBut
Reactions were carried out with 1 mmol of halide and 1.2 mmol of aldehyde, 3 mmol of base and 1 mol% 1a at refluxed temperature for 16 h.
Table 3
Acylation of aryl halides with aldehydes.a
Entry
1
Halides
Aldehyde
Desired product
Yield (%)b
76 1a
79 1b
56 PdCl2
2
72 1a
76 1b
53 PdCl2
3
4
80 1a
88 1b
78 1a
80 1b
63 1a
74 1b
5
6
7
8
75 1a
81 1b
62 1a
69 1b
52 1a
59 1b

24
K. Saikia, D.K. Dutta / Journal of Molecular Catalysis A: Chemical 408 (2015) 20–25
Table 3 (Continued)
Entry
Halides
Aldehyde
Desired product
Yield (%)b
9
77 1a
82 1b
80 1a
88 1b
10
11
0 1a
0 1b
12
13
0 1a
0 1b
65 1a
78 1b
69 1a
80 1b
14
a
Reactions were carried out with 1 mmol of halide and 1.2 mmol of aldehyde, 3 mmol Cs2CO3 and 1 mol% catalyst in 1,4-dioxane at refluxed temperature for 16 h.
Isolated yields were calculated in respect of halides.
b
favour the Pd(0), which accelerates the reductive elimination step
4. Conclusions
[24]. This fastens the catalytic cycle of 1b over 1a.
Since, iodide is a better leaving group than bromide, the acy-
lation reactions (Table 3) shows the higher yields in case of
iodo-containing substrates. It appears that aryl halides containing
electron withdrawing group enhance the reaction, while electron
donating substituent tend to retard the desired product formation.
This is in accordance with the deactivating nature of the electron
withdrawing groups, which remove electron density from the para
position and make it less nucleophillic. 1H NMR spectrum of the
reaction mixture containing 1a shows a peak at −44.99 ppm, which
may be due to formation of the hydride complex during the reac-
In conclusion, we have developed two new palladium com-
plexes of heterobidentate P-S ligand systems which show good to
excellent catalytic activity towards C H activation of aliphatic alde-
hydes by the acylation of aryl halides to form alkyl - aryl ketones.
The hexagonal chelate complex (1b) shows better catalytic activ-
ity over the pentagonal one (1a) owing to the deviation of the
hexagonal ring from planarity. Cs2CO3 yields better results than
secondary amines and KOBut, when used as base. The chelating
heterobidentate ligands may show hemilabile behavior during the
catalytic cycle by dissociating the weaker Pd S bond to generate
vacant coordination site at the metal centre and reassociate after
the completion of the reaction. Mechanistic investigations of the
reaction will be the focus of our future work.
tion. As the Pd S bonds are weaker than the corresponding Pd
P
bonds, it is possible of the complexes to show hemilabile behaviour
in the catalytic cycle by dissociating the weaker bond to gener-
ate vacant coordination site at the metal centre and reassociate
after the completion of the reaction. This is worth mentioning that,
under the given reaction conditions, the desired ketone was not
obtained when benzaldehyde and 2-phenylpropanal are used as
substrates (entry 11, 12); while using 3-phenylpropanal, substan-
tial amount of the desired product was obtained (entry 13, 14).
Though we could not make a precise study of the reaction mech-
anism, the lack of activity of benzaldehyde and 2-phenylpropanal
may be due to the steric hindrance at the ␣-carbon of benzalde-
hyde and 2-phenylpropanal, which inhibit the coordination of
palladium of the complex with the carbonyl carbon, resulting the
failure to yield the desired ketone. Moreover, benzaldehyde does
not have any ␣-hydrogen to form enolate anion which may be
the nucleophile to bind to the metal centre during the reaction
cycle.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2015.07.
007
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