Simple protocol for the synthesis of the asymmetric PCP pincer ligand [C6H4-1-(CH2PPh2)-3- (CH(CH3)PPh2)] and its Pd(II) derivative [PdCl{C6H3-2-(CH2PPh2)-6- (CH(CH3)PPh2)}]

Article abstract of DOI:10.1016/j.poly.2008.02.005

The asymmetric PCP pincer ligand [C₆H₄-1-(CH₂PPh₂)-3-(CH (CH₃)PPh₂)] (4) has been synthesized in a facile manner in three simple steps in high yield. Metallation of PCP pincer ligand (4) with [Pd(COD)Cl₂] affords complex [PdCl{C₆H₃-2-(CH₂PPh₂)-6-(CH (CH₃)PPh₂)}] (7) in good yield.

Full text of DOI:10.1016/j.poly.2008.02.005

Available online at www.sciencedirect.com
Polyhedron 27 (2008) 1947–1952
www.elsevier.com/locate/poly
Simple protocol for the synthesis of the asymmetric PCP pincer
ligand [C6H4-1-(CH2PPh2)-3-(CH(CH3)PPh2)] and its Pd(II)
derivative [PdCl{C6H3-2-(CH2PPh2)-6-(CH(CH3)PPh2)}]
a,
b
c
*
Ali Naghipour , Zahra Haji Ghasemi , David Morales-Morales ,
Juan M. Serrano-Becerra c, Craig M. Jensen d
a Department of Chemistry, Faculty of Science, University of Ilam, Ilam 69315, Iran
b Department of Chemistry, Azad University of Ilam, Ilam, Iran
c Instituto de Quı´mica, Universidad Nacional Auto´noma de Me´xico, Cd. Universitaria, Circuito Exterior, Coyoaca´n, 04510 Me´xico DF, Mexico
d Department of Chemistry, University of Hawaii, Honolulu, HI 96822, USA
Received 22 January 2008; accepted 6 February 2008
Available online 10 April 2008
Abstract
The asymmetric PCP pincer ligand [C6H4-1-(CH2PPh2)-3-(CH(CH3)PPh2)] (4) has been synthesized in a facile manner in three simple
steps in high yield. Metallation of PCP pincer ligand (4) with [Pd(COD)Cl2] affords complex [PdCl{C6H3-2-(CH2PPh2)-
6-(CH(CH3)PPh2)}] (7) in good yield.
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: PCP pincer ligands; PCP pincer complexes; Palladium complexes
1. Introduction
[4], oxazolines [5], phosphinites [6] and amines and imines
[7]. Moreover, the very same system can be modified to
include functional groups that enable these species to be
anchored to solid supports [8] or to allow further function-
alization to afford dendrimeric or nanostructured systems
[9]. In many cases, these complexes have been successfully
modified to include chiral motifs that have allowed the syn-
thesis of enantiomerically pure systems which have been
employed successfully in asymmetric synthesis and enantio-
selective catalysis. In addition, the bare inclusion of differ-
ent metals in the cavity of the ligands offers an endless
possibility of a very diverse chemistry according to the
metal selected (see Scheme 1).
Hence, nowadays pincer compounds of many elements
are known and their chemistries are motif of continuous
and numerous studies. Noteworthy is the fact that the
number of examples of complexes including asymmetric
pincer type ligands is limited in comparison with those of
their symmetric analogs [10]. This is partly because their
preparation is a considerable challenge, which is laborious
Pincer compounds are a group of species with very par-
ticular and interesting physical properties among which
their high thermal stability and unusual reactivities that
confer to the metal complexes they form stand out. It is
due to these characteristics of robustness and thermal sta-
bility that pincer compounds have attracted the continuous
attention of the chemistry community for multiple applica-
tions, this being particularly true in the case of homoge-
neous catalysis [1]. In the beginning, the very simple
backbone exhibited by these compounds did not anticipate
the wide variety of possible functionalizations in the main
frame of the complex. Thus, till date, these ligands have
been modified to include different donor groups such as
NHC’s heterocyclic carbenes [2], phosphines [3], thioethers
*
Corresponding author. Tel.: + 98 841 2227010; fax: + 98 841 2227061.
E-mail address: naghipour51@yahoo.com (A. Naghipour).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2008.02.005

1948
A. Naghipour et al. / Polyhedron 27 (2008) 1947–1952
- Steric and electronic modulation
via donor groups
- Chirality
- Anchoring site
- Remote electronic
modulations
R
R
D
Y
E
MmLn
D
- Cavity for metal binding
with tunable accesibility
- Sites for counter anions or
ancillary ligands
- Chirality
- Steric modulation
Scheme 1. Versatility of the pincer framework.
and requires a series of steps to introduce different groups
or donors. Moreover, complexes bearing asymmetric pin-
cer ligands have shown enhanced and in many cases mark-
edly different reactivities, such as hemilability [11].
From the above discussion, it clearly results that the
design of ligands is one of the most important aspects in
the chemistry of this kind of compounds, this being spe-
cially important in homogeneous catalysis. The ability to
simply and independently vary the steric and electronic
properties of a given ligand may provide a wealth of
opportunities to influence reactivity, stability, catalysis
and other important properties at the metal center. Hence,
following our continuous interest in the development on
pincer chemistry [12], herein we would like to report a sim-
ple protocol for the high yield synthesis of the asymmetric
ligand [C6H4-1-(CH2PPh2)-3-(CH(CH3)PPh2)] (4) and its
Pd(II) derivative [PdCl{C6H3-2-(CH2PPh2)-6-(CH(CH3)-
PPh2)}] (5).
(COD)] was synthesized according to the published proce-
dure [13].
2.2. Synthesis of {C6H4-1-(CH2Br)-3-(CH2PPh3Br)} (1)
To a stirred solution of a,a0-dibromo-m-xylene (3.70 g,
14 mmol) in THF (100 ml) was added a solution of triphen-
ylphosphine (4.3 g, 16 mmol) in THF (20 ml) at room tem-
perature and the solution allowed to reach 60 °C slowly.
The resulting white suspension was stirred overnight. The
reaction mixture was allowed to reach room temperature,
the mixture was filtered off and the remaining solid was
washed with THF (3 ꢀ 10 ml) and dried under vacuum to
1
give a pure white powder of 1 (6.3 g, 12 mmol, 85%). H
NMR (300 MHz, CDCl3): d 4.17 (s, 2H, CH2Br); 5.32 (d,
J = 14.39 Hz, CH2P); 6.97–7.27 (m, 4H, Ph); 7.53–7.73
(m, 15H, PPh3). 13C{1H} NMR (75.40 MHz, CDCl3): d
30.37 (d, JCP = 47.66 Hz, CH2P); 32.64 (CH2Br); 116.62,
117.76, 127.64, 128.83, 129.09, 130.01 (d, J = 12.1 Hz),
131.29, 131.90, 134.09 (d, J = 9.80 Hz) 134.84, 138.08.
31P{1H} NMR (121 MHz, CDCl3): d 24.22. Anal. Calc.
for C26H23Br2P (Mr = 526.24): C, 59.34; H, 4.41. Found:
C, 59.38; H, 4.45%.
2. Experimental
2.1. Materials and methods
Unless otherwise stated, all reactions were carried out
under an atmosphere of purified argon using conventional
Schlenk glassware and glovebox techniques; solvents were
dried using the established procedures and distilled under
dinitrogen and freeze–pump–thaw degassed immediately
prior to use. The 1H, 13C{1H}, 31P{1H} NMR spectra were
recorded at 300, 75.4 and 121.4 MHz, respectively at
295 K, using a Bruker Avance DRX 300 MHz NMR
spectrometer. 1H NMR and 13C{1H} NMR chemical shifts
are reported in ppm downfield from TMS. 1H NMR chem-
ical shifts are referenced to the residual hydrogen signal of
the deuterated solvents and in 13C{1H} NMR the 13C
signal of the deuterated solvents was used as a reference.
31P{1H} NMR chemical shifts are reported in ppm down-
field from H3PO4 and referenced to high frequency of 85%
H3PO4. Elemental analyses were determined on a Heraeus
CHN-O-RAPID elemental analyzer. The starting materials
a,a0-dibromo-m-xylene, chlorodiphenylphosphine, PdCl2,
CDCl3 were purchased from Aldrich Chemical Co. and
used without further purification. The complex [PdCl2-
2.3. Synthesis of {C6H4-1-(CH2Br)-3-(CH@CH2)} (2)
To a stirred solution of 1 (2.63 g, 5 mmol) in dichloro-
methane (40 ml) was added para-formaldehyde (0.27 g,
9 mmol) and KOH solution (6 ml, 50%) at room tempera-
ture, respectively and stirred for 1 h. The organic layer was
separated and the aqueous layer extracted with ether. The
combined organic extracts were washed and dried over
MgSO4. After filtration, the solution was passed through
a short plug of celite and the solvent evaporated under vac-
uum to afford a white solid. The remaining solid was
extracted with n-hexane and then the solvent evaporated
under vacuum to afford 2 as a pure colorless oil (0.79 g,
4 mmol, 80%). 1H NMR (300 MHz, CDCl3): d 4.37 (s,
2H, CH2Br), 5.18 (d, JHH = 11.1 Hz, 1H, CH2), 5.67 (d,
JHH = 17.4 Hz, 1H, CH@CH2), 6.60 (dd, JHH = 17.70 HZ,
JHH = 10.8 Hz, CH@CH2), 7.15–7.314 (m, 4H, Ar).
13C{1H} NMR (75.40 MHz, CDCl3): d 33.39 (CH2Br),
114.54 (CH@CH2), 126.17, 126.78, 128.32, 128.93, 136.13

A. Naghipour et al. / Polyhedron 27 (2008) 1947–1952
1949
(CH@CH2), 137.93, 138.02. Anal. Calc. for C9H9Br
(Mr = 197.07): C, 54.85; H, 4.60. Found: C, 54.90; H,
4.57%.
mixture is allowed to reach room temperature and the stir-
ring continued for an additional 1 h to give a clear mixture.
After this time, the resulting reaction mixture is placed into
a salt-ice bath (0 °C) and a solution of NH4Cl in water
(10 wt%, 45 ml) is carefully added to afford a colorless mix-
ture. The THF layer was separated and the aqueous layer
extracted twice with ether (2 ꢀ 15 ml). The combined
extracts were dried over MgSO4 and passed through a
short column of alumina. Finally, the solvent was removed
under vacuum to afford 4 as a colorless oil (1.39 g,
2.85 mmol, 95%). 1H NMR (300 MHZ, CDCl3): d 1.02
2.4. Synthesis of {C6H4-1-(CH2Br)-3-(CH(CH3)Br)}
(3)
To a stirred solution of 2 (0.62 g, 3.1 mmol) in dichloro-
methane (15 ml), PBr3 (0.123 ml, 1.3 mmol) was added at
room temperature and stirred for 3 h. After this time, the
solution is then washed with aqueous NaHCO3 and then
with water, the organic phase is separated and the aqueous
layer extracted twice with CH2Cl2 (2 ꢀ 10 ml). The organic
phase is dried over MgSO4 and the solvent removed by
rotary evaporation to afford 3 as a colorless oil (0.86 g,
3
3
(dd, JHP = 14.7 Hz, JHH = 7.35 Hz, 3H, CH3), 2.94 (d,
2JHP = 18 Hz, 2H, CH2P), 3.10 (m, 1H, CHP), 6.52–6.91
(m, 20H, PPh2), 7.04–7.30 (m, 4H, Ar). 13C{1H} NMR
(75.40 MHz, CDCl3): d 20.17 (d, JCP = 21.2 Hz, CHPPh2),
1
3.1 mmol, 98%). H NMR (300 MHz, CDCl3): d 1.95 (d,
36.26 (d, JCP = 16.1 Hz, CH2PPh2); 39.47 (d, JCP =
J = 6.90 Hz, 3H, CH3); 4.39 (s, 2H, CH2Br); 5.10 (q,
J = 6.90 Hz, 1H, CH), 7.15–7.36 (m, 4h, Ar). 13C{1H}
NMR (75.40 MHz, CDCl3): d 26.65 (CH3), 33.01 (CH2Br),
48.76 (CHBr), 126.83, 127.35, 128.94, 129.13, 138.13,
143.74. Anal. Calc. for C9H10Br2 (Mr = 277.98): C, 38.89;
H, 3.63. Found: C, 38.85; H, 3.65%.
13.2 Hz, CH3), 126.60 (d, J = 5.3 Hz), 127.67, 128.00,
128.15, 128.33, 128.52, 128..60, 128.69, 129.23, 129.90,
130.76, 131.34, 133.08, 133.28, 133.33, 133.39, 133.53,
133.62, 134.36, 134.64, 137.80, 137.91, 138.02, 138.49,
139.12 (d, J = 4.8 Hz), 143.96 (d, J = 8.60 Hz), Ar.
31P{1H} NMR (121 MHz, CDCl3): d ꢁ9.157 (CH2PPh2),
2.69 (CHPPh2). Anal. Calc. for C33H30P2 (Mr = 488.54):
C, 81.13; H, 6.19. Found: C, 81.20; H, 6.25% (see Scheme
2).
2.5. Synthesis of {C6H4-1-(CH2PPh2)-3-
(CH(CH3)PPh2)} (4)
To a solution of Ph2PH (1.17 g, 6 mmol) in THF (35 ml)
was added dropwise a solution of n-BuLi in hexane
(3.75 ml of 1.6 M/l hexane solution, 6 mmol) at ꢁ78 °C,
over a period of 30 min with stirring. After this time, the
reaction mixture was allowed to reach room temperature.
The resulting orange suspension was cooled to ꢁ78 °C
and a THF solution (20 ml) of 3 (0.834 g, 3 mmol) was
added dropwise with a syringe over 30 min. The resulting
2.6. Synthesis of [PdCl{C6H3-2-(CH2PPh2)-6-(CH(CH3)-
PPh2)}] (7)
To a stirred suspension of [Pd(COD)Cl2] (0.777 g,
2.72 mmol) in toluene (25 ml), a solution of ligand 4
(1.329 g, 2.72 mmol) in toluene (20 ml) was slowly added.
The resulting solution was set to reflux for 5 h. After this
time, the solution was filtered over cotton pad and pumped
_
Br
+
Br
PPh3
PPh3
THF
a) KOH,
p-HCOH
b) CH2Cl2
Br
Br
Br
(2)
(1)
PBr3
Br
PPh2
PPh2
a) HPPh2,
THF
b) NH4Cl (aq), MgSO4
n-BuLi,
Br
(4)
Scheme 2. Synthesis of the asymmetric PCP pincer ligand (4).
(3)

1950
A. Naghipour et al. / Polyhedron 27 (2008) 1947–1952
was treated with a strong base (KOH) in the presence of
PPh2
PPh2
PPh2
an excess of para-formaldehyde to afford the Witting [15]
product {C6H4-1-(CH2Br)-3-(CH@CH2)} (2). Analysis of
[Pd(COD)Cl2]
Toluene
Pd Cl
1
this compound by H NMR shows a typical pattern for a
single substituted ethylene type olefin, exhibiting two sets
of doublets for the terminal protons of the alkene at d
5.18 and d 5.67 ppm and a doublet of doublets for the
methyne group at d 6.60 ppm. A further singlet at d
4.37 ppm and signals due to the aromatic protons between
d 7.15 and 7.31 ppm are also observed in the spectrum.
Bromination of the olefin compound (2) with PBr3 gives
place to the formation of the asymmetric dibromo starting
material {C6H4-1-(CH2Br)-3-(CH(CH3)Br)} (3). This com-
pound afforded a 1H NMR spectrum where the most noto-
PPh2
(4)
(7)
Scheme 3. Synthesis of the asymmetric Pd(II)-PCP pincer complex (7).
off under vacuum to dryness. The crude solid was purified
by recrystallization from CHCl3/MeOH to afford complex
7 as a white microcrystalline powder (1.46 g, 2.31 mmol,
85%): 1H NMR (300 MHZ, CDCl3):
d 1.02 (dd,
3
3JHP = 16.04 Hz, JHH = 7.1 Hz, 3H, CH3), 3.82 (d,
J = 6 Hz, 2H, CH2PPh2), 4.00 (m, 1H, CHPPh2), 6.95–
7.04 (m, 3H, Ar), 7.26–7.82 (m, 20H, Ar). 13C{1H} NMR
(75.40 MHz, CDCl3): d 21.46 (s, CH3), 49.37 (d, J =
31.0 Hz, CH2PPh2), 46.31 (d, J = 30.4 Hz, CH(CH3)PPh2),
122.56 (d, J = 20.7 Hz), 123.54 (d, J = 22.4 Hz), 126.21,
128.52 (d, J = 24.1 Hz), 128.55, 129.98, 130.42, 130.86,
132.13 (d, J = 9.7 Hz), 132.97 (d, J = 17.3 Hz), 132.99,
134.92 (d, J = 10.86 Hz), 147.67 (d, J = 21.2 Hz),
153.87(d, J = 22.3 Hz). 31P{1H} NMR (121 MHz, CDCl3):
rious signal is a doublet located at d 1.95 ppm
corresponding to the newly formed CH3 group in one of
the arms. Complementary to this signal is a quartet at d
5.10 ppm due to the methyne group of the same arm. While
the signal corresponding to the un-substituted arm and
those due to the aromatic protons can be observed at d
4.39 and 7.15–7.36 ppm, respectively. Signals concurrent
with the previous analysis can be observed at d 26.65,
33.01 and 48.76 ppm in the 13C{1H} NMR spectrum, these
signals being assigned to the methyl, methylene and meth-
yne groups in (3), respectively. Results obtained from ele-
mental analysis are also in agreement with the proposed
formulation.
d 34.32 (d, JPP = 418 Hz, CH2PPh2), 48.57 (d, JPP
=
418 Hz, CH(CH3)PPh2). Anal. Calc. for C33H29ClP2Pd
(Mr = 629.40): C, 62.97; H, 4.64. Found: C, 63.00; H,
4.60% (see Scheme 3).
The final step leading to the synthesis of the PCP pincer
ligand involved the reaction of the lithium salt of the
diphenylphosphine with the dibromo compound (3), at a
low temperature to afford compound {C6H4-1-(CH2-
PPh2)-3-(CH(CH3)PPh2)} (4) as an air sensitive colorless
oil in excellent yield (95%). Once again signals correspond-
ing to the CH3, CH2 and CH groups can be observed in the
3. Results and discussion
The initial step for the formation of PCP pincer ligand
(4) very much resembles the initial step employed by Shaw
on his seminal report in the 1970s [14]. Thus, a,a0-dibromo-
m-xylene was treated with one equivalent of triphenylphos-
phine in THF to afford the single substituted triphenyphos-
phonium salt {C6H4-1-(CH2Br)-3-(CH2PPh3Br)} (1).
1
corresponding H NMR spectrum at d 1.02 (dd), 2.94 (d)
and 3.10 (m) ppm, respectively. The substitution of the bro-
mides for the phosphine groups can be inferred for the mul-
tiplicity observed in these signals due to the coupling with
the P nuclei and for the shift of these signals to higher field
due to the inductive effect of the phosphine moieties. As is
the case for the previous intermediates, a similar behavior
is observed in the 13C{1H} NMR. More illustrative are
the results of the analysis by 31P{1H} NMR, where two dif-
ferent signals, due to two inequivalent phosphorus nuclei
are observed; one due to the substituted arm at a lower
field (d 2.69 ppm) and the other centered at d ꢁ9.16 ppm.
It is very interesting to note that these chemical shifts agree
well when compared to the bis-disubstituted ligand {C6H4-
1,3-(CH(CH3)PPh2)} (5) reported by Venanzi [16] and
Zhang [17] and the simple PCP pincer ligand {C6H4-1,3-
(CH2PPh2)} (6) reported earlier by Venanzi [18] with only
very slight variations (Fig. 1).
1
Analysis of the solid product (1) by H reveals that the
a,a0-dibromo-m-xylene has been substituted in only one
arm. Thus, signals corresponding to two different CH2
groups in the molecule can be observed, a singlet corre-
sponding to the CH2Br located at d 4.17 ppm and a doublet
due to the presence of the CH2PPh3Br found at d 5.32 ppm.
Similar assignation can be done from the results obtained
from the analysis of the same sample by 13C{1H} NMR,
where besides the signals observed for the aromatic car-
bons in the usual region (d 116–138 ppm) signals corre-
sponding to the two different CH2 can be observed at d
32.64 and d 30.37 ppm for the CH2Br and CH2PPh3Br
moieties, respectively. Furthermore, the presence of the
phosphonium group was detected by 31P{1H} NMR, this
analysis showing a single signal at d 22.24 ppm due to the
PPh3Br group. Elemental analysis is also consistent with
the proposed formulation.
As for most of the cases, metallation of ligand (4)
proceeds in a very facile manner via the C–H activation
of the aromatic ring by [Pd(COD)Cl2] under reflux using
toluene as solvent. The white product of [PdCl{C6H3-2-
(CH2PPh2)-6-(CH(CH3)PPh2)}] (7) thus obtained was
In contrast to the procedure employed by Shaw [14],
where the phosphonium salt was treated with a mild base
to afford the corresponding phosphine, compound (1)

A. Naghipour et al. / Polyhedron 27 (2008) 1947–1952
1951
when compared to the un-substituted arm. Experiments
aimed to confirm these theories and the synthesis of other
group 10 transition metal derivatives of ligand (4) and
the study of the potential activity of complex (7) are cur-
rently underway in our laboratories.
PPh2
δ (P)= -10.1
PPh2
P1Ph2
P2Ph2
PPh2
δ (P)= 2.39
PPh2
δ (P1)= -9.16
δ (P2)= 2.69
(6)
(4)
(5)
Acknowledgements
Fig. 1. Chemical shift (d) Comparison between symmetric PCP pincer
ligands and compound (4).
We are grateful to the ILAM University and the Minis-
try of Science, Research & Technology of Iran, for finan-
cial support. J.M. S.-B. would like to thank CONACYT
for financial support. D. M.-M. would like to thank
CONACYT (Grant F58692) and PAPIIT (Grant
IN227008) for financial support.
1
analyzed by H NMR, showing signals similar to those of
the free ligand but slightly shifted to lower field, a fact that
agrees well with the metal being coordinated to the ligand.
This argument is also supported for the more notorious
shift to the lower field of the signals in the 31P{1H}
NMR spectra, passing from d ꢁ9.16 and d 2.69 ppm in
the free ligand (4) to d 34.32 and d 48.57 ppm in the metal-
lated complex (7) for the CH2PPh2 and CH(CH3)PPh2,
respectively. Moreover, the signals of the phosphorus
nuclei of compound (7) are shown as doublets, which also
accounts for the ligand to be coordinated to the metal in a
j2-P,P form. Besides the Jp2p value of 418 Hz, it is consistent
with a trans arrangement of the two phosphorus nuclei.
Once again, the chemical shifts observed for (7) agree well
when compared to their symmetric analogous [PdCl{C6H3-
2,6-(CH(CH3)PPh2)2}] (8) [16,17b] and [PdCl{C6H3-2,6-
(CH2PPh2)2}] (9) [18] (Fig. 2).
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and easy to perform methodology for the synthesis of
asymmetric PCP pincer ligands and their palladium deriv-
atives. It is important to note that the method may not be
limited to the synthesis of ligand (4) but by careful selection
of the aldehyde and reaction conditions, the methyl group
in the substituted arm can be changed for various other
substituents or even lead to the elongation of the arm thus
affording another group of potentially interesting PCP pin-
cer ligands having 5 and 6 membered rings. Moreover,
exploitation of the different reactivities of the two different
arms in compound (3) may offer the opportunity to attain
PCP pincer ligands with different donor groups, since it is
expected that the more substituted arm would react slowly
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Ph2P
Pd
Cl
PPh2
1Ph2P
Pd
Cl
P2Ph2
Ph2P
Pd
Cl
PPh2
δ (P1) = 34.3
δ (P2) = 48.6
δ (P) = 33.4
δ (P) = 46.5
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(9)
(7)
(8)
Fig. 2. Chemical shift (d) Comparison between symmetric Pd(II)-PCP
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