Catalytic asymmetric synthesis of Pt- and Pd-PCP pincer complexes bearing a para-N pyridinyl backbone

Article abstract of DOI:10.1016/j.jorganchem.2018.03.010

A diastereoselective catalytic asymmetric hydrophosphination reaction was performed using a palladacycle catalyst to produce chiral PCP phosphine ligand with high dr (95:5) and excellent enantioselectivity (>99% ee). Subsequent facile metalation of the ch

Full text of DOI:10.1016/j.jorganchem.2018.03.010

Journal of Organometallic Chemistry 862 (2018) 22e27
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Catalytic asymmetric synthesis of Pt- and Pd-PCP pincer complexes
bearing a para-N pyridinyl backbone
Esther Hui Yen Wong, Yu-Xiang Jia, Yongxin Li, Sumod A. Pullarkat**, Pak-Hing Leung*
Division of Chemistry & Biological Chemistry, School of Physical & Mathematical Sciences, Nanyang Technological University, 637371, Singapore
a r t i c l e i n f o
a b s t r a c t
Article history:
A diastereoselective catalytic asymmetric hydrophosphination reaction was performed using a palla-
dacycle catalyst to produce chiral PCP phosphine ligand with high dr (95:5) and excellent enantiose-
lectivity (>99% ee). Subsequent facile metalation of the chiral ligand with Pt and Pd metal salts yielded
the desired pincer complexes. The protonated form of these pincer complexes was also synthesised, and
their structures were confirmed through the use of X-ray crystal diffraction and NMR spectroscopy
analyses.
Received 27 November 2017
Received in revised form
27 February 2018
Accepted 8 March 2018
Available online 10 March 2018
© 2018 Published by Elsevier B.V.
Keywords:
PCP pincer complexes
Para-N pyridinyl backbone
Catalytic synthesis
Asymmetric hydrophosphination
Metal-carbon bond
1. Introduction
density and reactivity of the metal centre could be more effectively
influenced via N-coordination of the free nitrogen atom in a
Ever since the synthesis of pincer complexes first appeared in
the works of Moulton and Shaw in 1976 [1] and van Koten et al. in
1978 [2], many other research groups had begun working on the
synthesis and application of chiral and non-chiral pincer complexes
[3]. Today, pincer complexes are commonly used in catalysis, mo-
lecular recognition, material science and supramolecular chemistry
[4]. One main factor for their extensive research and use till now is
the ability to tune the electronic properties of the metal centre
through modifications of the different sites of the ligand scaffold.
For example, there were reports by van Koten and his co-workers
that the reactivity of the metal centre was affected by para sub-
stituents on the backbone of NCN pincer complexes through their
inductive and/or resonance effects [5]. Furthermore, such modifi-
cations do not affect the binding mode at the metal centre greatly.
In 1996, Milstein et al. had also suggested that the electron
palladium-PCP pincer complex of a 3,5-lutidine based phosphine
ligand [6]. With such a convenient method to fine tune the reac-
tivity of the metal centre, one would think that more research will
be done in that direction. However, till date, only two other pub-
lications [7] were found to further investigate the effects of N-co-
ordination on the metal centre. Moreover, the pincer complexes
that were mentioned in these papers did not contain any chiral
centres, which otherwise could potentially form a new class of
easily tuneable pincer catalysts for the synthesis of chiral
compounds.
Herein, we wish to report the synthesis and characterization of
platinum(II) and palladium(II) chiral PCP pincer complexes incor-
porating a para-N-pyridinyl backbone. Even though there were
concerns that the free nitrogen atom would affect the selectivity of
the ligand as well as the coordination mode, results show that the
heteroatom do not play a part in deciding the final configuration of
the chiral ligand. The synthesis of these novel chiral pincer com-
plexes would also serve as a stepping stone for future studies and
modifications on the catalytic application for this class of chiral
para-N-pyridinyl pincer complexes.
* Corresponding author. Division of Chemistry & Biological Chemistry, School of
Physical
Singapore.
** Corresponding author. Division of Chemistry & Biological Chemistry, School of
Physical Mathematical Sciences, Nanyang Technological University, 637371,
Singapore.
E-mail addresses: sumod@ntu.edu.sg (S.A. Pullarkat), pakhing@ntu.edu.sg
(P.-H. Leung).
& Mathematical Sciences, Nanyang Technological University, 637371,
&
https://doi.org/10.1016/j.jorganchem.2018.03.010
0022-328X/© 2018 Published by Elsevier B.V.

E.H. Yen Wong et al. / Journal of Organometallic Chemistry 862 (2018) 22e27
23
2. Experimental
2.4. General procedure for the synthesis of free phosphine ligand
3,3'-(pyridine-3,5-diyl)bis(3-(diphenylphosphino)-1-phenylpropan-
1-one) 1
2.1. General information
All reactions involving the use of air-sensitive compounds were
conducted under a nitrogen atmosphere. All chemicals and solvents
were used as received commercially without further purification.
The solvents used for the reactions were degassed whenever
necessary and dried according to literature procedures [8]. The
Eyela Low Temp. Pairstirrer PSL-1800 was used for reactions con-
ducted at ꢀ80 ꢁC. Column chromatography was done on Silica Gel
60 (Merck). The determination of melting points was conducted on
the SRS Optimelt Automated Melting Point System, SRS MPA100
with melting points of compounds determined in open capillary
tubes and their values uncorrected. Optical rotation was performed
on the Jasco P-1030 Polarimeter using cells of length 0.1 dm at 20 ꢁC
and a wavelength of 589 nm. The HRMS analysis was done on the
Water Q-Tof Premier MS machine. The Bruker AV 300, 400 and 500
NMR machines were used to obtain the 1H, 13C and 31P NMR
Dienone 4 (33.9 mg, 0.1 mmol, 1.0 equiv.) in either toluene or
CH2Cl2 (9.5 mL) was added into a toluene or CH2Cl2 (2 mL) solution
of Ph2PH (42.8 mg, 0.23 mmol, 2.3 equiv.) and chiral catalyst 5
(0.0063 g, 0.01 mmol, 10mol%). The reaction was stirred for 10 min
at room temperature and then cooled to ꢀ80 ꢁC. NEt3 (27
mL,
0.2 mmol, 2.0 equiv.) in solvent (0.5 mL) was subsequently added in
dropwise and the reaction continued to stir at ꢀ80 ꢁC for 24 h.
When the reaction had completed, the mixture was warmed to
room temperature and was subsequently metallated to give the
various pincer complexes.
2.5. General procedure for the synthesis of pincer complex 6a
To a toluene solution of the directly synthesised ligand 1
(0.1 mmol, 1.0 equiv.), PtCl2(NCCH3)2 (0.1 mmol, 1.0 equiv.) was
added into the reaction mixture. The reaction was heated to 90 ꢁC
and stirred for 4 h. The organic layer was washed with saturated
NaHCO3 (1 ꢂ 25 mL) and water (2 ꢂ 25 mL), dried over magnesium
sulfate, filtered and evaporated to dryness. The crude product was
purified via repeated precipitation using a mixture of CH2Cl2 and
ethyl acetate to give the desired complex as a white solid.
spectra. All chemical shifts were reported in
d ppm, referencing
chloroform-d (
d
¼ 7.26) for 1H NMR, chloroform-d (
d
¼ 77.2) for 13
C
NMR, and an external 85% H3PO4 standard (
d
¼ 0.0) for 31P{1H}
NMR. Diamine 2 [9], catalyst 5 [10] and 1-phenyl-2-(triphenyl-
phosphoranylidene)ethenone [11] were prepared according to
literature methods.
(R,R)-6a 48%. [
a
]
¼ ꢀ283ꢁ (c 0.6, CH2Cl2); 1H NMR (500 MHz,
D
CDCl3):
d
3.06e3.30 (m, 4H, PCHCH2), 5.08 (m, 2H, PCH), 7.19e8.18
2.2. Synthesis of pyridine-3,5-dicarbaldehyde 3
(m, 32H, Ar); 31P{1H} NMR (121 MHz, CDCl3):
d
49.4 (JPt-
¼ 2902 Hz); 13C NMR (101 MHz, CDCl3):
d 43.4 (s, 2C, CH2), 44.1
P
Diamide 2 (1.00 g, 3.95 mmol) was dissolved in dry THF (20 mL)
and cooled to ꢀ78 ꢁC under N2 protection. A solution of diisobu-
tylaluminium hydride (8.30 mL, 1 M in toluene) was added drop-
wise to the amide solution and the reaction was monitored by thin
layer chromatography until no more starting material was
observed. The reaction was carefully quenched using cold water
(0.2 mL), 15% NaOH (0.2 mL) and then water (1.6 mL), and was left
stirring for 15 min. The reaction was allowed to warm up to room
temperature and magnesium sulfate was added with stirring for
another 15 min. The mixture then passed through celite and
washed with ethyl acetate (20 mL x 3). The filtrate was dried under
vacuum and purified over silica gel (ethyl acetate: hexane ¼ 1: 1),
yielding a pale-yellow solid (0.29 g, 54%). M.p. 95e97 ꢁC; 1H NMR
(vt, 2C, PCH), 125.2e148.6 (40C, Ar), 162.3 (s, JPt-C ¼ 956 Hz, 1C,
C
ipso), 196.2 (s, 2C, COPh); HRMS (ESI) calcd for C47H38NO2P2Pt
[(M ꢀ Cl)þ] 906.2026, found 906.2006.
2.6. General procedure for the synthesis of pincer complex 6b
To a CH2Cl2 solution of the directly synthesised ligand 1
(0.1 mmol, 1.0 equiv.), PdCl2(NCCH3)2 (0.1 mmol, 1.0 equiv.) was
added into the mixture and the reaction was stirred at room tem-
perature for 20 hThe organic layer was washed with saturated
NaHCO3 (1 ꢂ 25 mL) and water (2 ꢂ 25 mL), dried over magnesium
sulfate, filtered and evaporated to dryness. The crude product was
purified over silica gel (CH2Cl2: ethyl acetate: hexane ¼ 15: 6: 14) to
give the desired product as a white solid.
(300 MHz, CDCl3):
d
8.56 (t, 1H, J ¼ 2.1 Hz, NCHCCH), 9.25 (d, 2H,
J ¼ 2.0 Hz, NCHCCH), 10.2 (s, 2H, NCHCCHO); 13C NMR (75 MHz,
(R,R)-6b 68%. [
CDCl3):
a
]
¼ ꢀ482ꢁ (c 1.0, CH2Cl2); 1H NMR (300 MHz,
D
CDCl3): d 131.4 (s, 2C, CCHO), 136.0 (s, 1C, CCHC), 155.5 (s, 2C, NCH),
d
3.18 (m, 4H, PCHCH2), 5.01 (m, 2H, PCH), 7.18e8.24 (m,
189.5 (s, 2C, CHO); HRMS (ESI) calcd for C7H6NO2 [(M þ H)þ]
32H, Ar); 31P{1H} NMR (161 MHz, CDCl3):
d
47.7 (s); 13C NMR
136.0399, found 136.0399.
(75 MHz, CDCl3): 44.4 (vt, 2C, CH245.4 (vt, 2C, PCH), 128.0e148.4
d
(40C, Ar), 170.0 (s, 1C, Cipso), 196.7 (vt, 2C, COPh); HRMS (ESI) calcd
for C47H39NO2ClPdP2 [(M þ H)þ] 852.1179, found 852.1204.
2.3. Synthesis of (2E,2'E)-3,3'-(pyridine-3,5-diyl)bis(1-phenylprop-
2-en-1-one) 4
2.7. General procedure for the synthesis of pincer complexes 7
Dialdehyde 3 (0.53 g, 3.93 mmol) and 1-phenyl-2-(triphenyl-
phosphoranylidene)-ethanone (3.14 g, 8.25 mmol) were dissolved
in CH2Cl2 (70 mL) and stirred for 20 h under reflux. Upon cooling,
the reaction mixture was evaporated to dryness and methanol
(50 mL) was added. The mixture was then filtered and the solid
collected was washed with cold methanol (20 mL x 3) to afford the
product as a pale yellow crystalline solid (1.21 g, 91%). M.p.
Complex 6 (0.1 mmol, 1.0 equiv.) was dissolved in a saturated
solution of hydrochloride in chloroform (5 mL) and stirred for
30 min. Afterwards, the solution was evaporated to dryness to give
the final product as a white solid.
26
(R,R)-7a >99%.
(300 MHz, CDCl3):
[
a]
¼ ꢀ129ꢁ (c 0.62, CH2Cl2); 1H NMR
D
d
3.11e13.35 (m, 4H, PCHCH2), 5.15 (m, 2H, PCH),
221e223 ꢁC (dec.); 1H NMR (300 MHz, CDCl3)
d
7.52e7.66 (m, 6H,
7.23e8.29 (m, 32H, Ar). 31P{1H} NMR (121 MHz, CDCl3):
d
49.4 (JPt-
Ar), 7.67 (d, 2H, J ¼ 15.9 Hz, HCCHCOPh), 7.84 (d, 2H, J ¼ 15.9 Hz,
¼ 2818 Hz); 13C NMR (121 MHz, CDCl3):
d 42.8 (s, 2C, CH2), 43.9
P
HCCHCOPh), 8.08 (m, 4H, Ar), 8.16 (t, 1H, J ¼ 2.1 Hz, NCHCCH), 8.88
(vt, 2C, PCH), 124.8e151.1 (40C, Ar),173.7 (s, JPt-C ¼ 999 Hz,1C, Cipso),
(d, 2H, J ¼ 1.8 Hz, NCHCCH); 13C NMR (75 MHz, CDCl3):
d
124.8 (s,
194.8 (s, 2C, COPh).
26
2C, CHCHCOPh), 128.6e137.6 (15C, Ar), 140.0 (s, 2C, CHCHCOPh),
150.8 (s, 2C, NCH), 189.6 (s, 2C, COPh); HRMS (ESI) calcd for
(S,S)-7b 98%. [
CDCl3):
a
]
¼ þ302ꢁ (c 0.65, CH2Cl2); 1H NMR (400 MHz,
D
d
3.25 (br, 4H, PCHCH2), 5.11 (br, 2H, PCH), 7.30e38.41 (m,
C
23H18NO2 [(M þ H)þ] 340.1338, found 340.1337.
32H, Ar); 31P{1H} NMR (161 MHz, CDCl3): 47.7 (s); 13C NMR
d

24
E.H. Yen Wong et al. / Journal of Organometallic Chemistry 862 (2018) 22e27
(100 MHz, CDCl3):
126.3e151.4 (40C, Ar), 186.9 (s, 1C, Cipso), 195.0 (m, 2C, COPh).
d
43.7 (m, 2C, CH2), 45.3 (vt, 2C, PCH),
SHELXL-2014/6 refinement program. The structures were solved by
direct methods to locate the heavy atoms, followed by difference
maps for the light, non-hydrogen atoms. Hydrogen atoms were
placed in calculated positions and refined with a riding model. The
absolute configuration of the chiral complexes was determined
unambiguously by using the Flack parameter [12].
2.8. Crystallographic studies of (R,R)-7a and (S,S)-7b
Crystal data for these 2 complexes and a summary of the crys-
tallographic analyses are given in the electronic supporting infor-
mation. Diffraction quality crystals were mounted onto quartz
fibres for analysis. X-ray data were collected at 103 K on a Bruker X8
CCD diffractometer, using Mo-Ka radiation, with the Bruker SAINT
software package using a narrow-frame algorithm. Data were
processed and corrected for absorption effects with SADABS.
Structural solution and refinement were carried out with the
3. Results and discussion
Scheme 1 illustrates the pathway for the synthesis of the chiral
PCP free phosphine ligand (R,R)-1. Diamide 2 was first reduced to
dialdehyde 3. The subsequent addition of the acetophenyl groups to
3 via the Wittig reaction gave a 91% yield of dienone 4. (R,R)-1 was
Scheme 1. Synthesis of the chiral PCP free phosphine ligand (R,R)-1.
Scheme 2. Synthesis of the chiral Pt- and Pd-PCP pincer complexes.

E.H. Yen Wong et al. / Journal of Organometallic Chemistry 862 (2018) 22e27
25
then synthesised from dienone 4 according to the methodology
described previously [10].
enriched ligand. The reaction mixture was purified via column
chromatography and moderate yields of 6 were obtained (48% for
6a and 68% for 6b).
To our delight, the synthesis of (R,R)-1 gave us high dr (95:5) and
>99% ee under the optimal condition that was established in the
above-mentioned methodology. This result suggests that the
presence of the free nitrogen on the pyridine backbone did not have
much impact on the dr and ee of the synthesised ligand, since these
values were similar to those reported for the normal aryl backbone
containing ligands [10].
Next, we proceeded to synthesise two different types of chiral
PCP pincer complex 6 using two different metal salts, namely
PtCl2(NCCH3)2 and PdCl2(NCCH3)2 (Scheme 2). The metalation
process took place with the use of the crude enantiomerically
It should be noted that the enantiomeric forms of both com-
plexes 6 and 7 can be equally prepared from their respective pincer
ligands. For example, complex (S,S)-6 and (S,S)-7 were prepared
from ligand (S,S)-1. Attempts to crystallise the neutral complexes 6
for X-ray crystal diffraction analysis were not successful. Interest-
ingly, the cationic pincer complexes 7 could be easily crystallised
when they were synthesised from the reaction with 6 and hydro-
chloric acid. Figs.1 and 2 illustrate the molecular structures of (R,R)-
7a and (S,S)-7b. Selected bond lengths and angles of these two
complexes are also listed out in Table 1.
Fig. 1. X-ray crystal structure and the absolute stereochemistry of the protonated Pt-PCP pincer complex (R,R)-7a. The chloride counter anion and all other hydrogen atoms (except
those on the chiral carbon centre) were omitted for clarity.
Fig. 2. X-ray crystal structure and the absolute stereochemistry of the protonated Pd-PCP pincer complex (S,S)-7b. The chloride counter anion and all other hydrogen atoms (except
those on the chiral carbon centre) were omitted for clarity.

26
E.H. Yen Wong et al. / Journal of Organometallic Chemistry 862 (2018) 22e27
Table 1
Selected bond lengths [Å] and angles [ꢁ] of complexes (R,R)-7a and (S,S)-7b.
(R,R)-7a (M ¼ Pt)
(S,S)-7b (M ¼ Pd)
M-C
M-Cl
M-P
M-P
C-M-Cl
P-M-P
1.995 (7)
2.377 (2)
2.281 (2)
2.276 (3)
178.2 (2)
169.1 (2)
2.033 (14)
2.363 (4)
2.310 (5)
2.285 (5)
179.1 (5)
166.7 (3)
170.0 and 186.9 ppm for the Pd complexes). This suggests that the
change of an atom at the para position of the aryl backbone from
carbon to nitrogen could influence the electron density around the
metal centre, which was similar to the observations made by Mil-
stein et al. [6]. Additionally, as seen from the JPt-C coupling con-
stants in Table 2, a stronger coupling was observed when the
nitrogen is at the para position instead.
Table 2
Selected chemical shiftsa
(
dCipso) and coupling constants (1JPt-C) of the ipso carbon of
(R,R)-benzylic-PCP pincer complexes, (R,R)-6 and (R,R)-7.
1
M ¼ Pt
M ¼ Pd
dCipso (ppm)
1JPt-C (Hz)
151.5b
158.3c
936d
e
Notably, the chemical shifts in the 31P{1H} NMR spectra of 6 and
7 were almost identical (49.4 ppm for 6a/7a and 47.7 ppm for 6b/
7b). However, when the Pt-P coupling constants in the 31P{1H}
NMR spectra of 6a and 7a were analysed, the difference became
more apparent (2902 Hz vs 2818 Hz). The observed difference in
the coupling constant values suggest that the additional presence
of the proton could possibly affect the strength of the interaction
between M and P.
(R,R)-benzylic-PCP
dCipso (ppm)
162.3
956
170.0
e
1JPt-C (Hz)
The potential for the use of complex 6 as a chiral catalyst could
be seen in the asymmetric hydrophosphination reaction of trans-
chalcone. Preliminary result showed that the use of (R,R)-6b with
K2CO3 as the base gave us 35% ee of the (R)-conformation of the
tertiary phosphine with a yield of 62% without further optimization
(Scheme 3).
(R,R)-6
dCipso (ppm)
1JPt-C (Hz)
173.7
999
186.9
e
(R,R)-7
a
All values were recorded in CDCl3.
Value as obtained from reference [10].
Value as obtained from reference [13].
Value as obtained from reference [14].
4. Conclusion
b
c
The successful synthesis of chiral Pt- and Pd-PCP pincer com-
plexes involved two main processes. Firstly, the synthesis of the
chiral PCP free phosphine ligand, through a catalytic asymmetric
hydrophosphination reaction, was performed. This process yielded
free ligands that had high dr and ee. Secondly, the facile cyclo-
metalation of the chiral ligand with Pt and Pd metal salts gave rise
to our desired product. Furthermore, it was demonstrated that the
pyridinyl nitrogen could be bonded to a proton to form variants of
the chiral pincer complexes. Initial observations suggest that
replacing the para atom of the aryl backbone with nitrogen could
significantly decrease the electron density found in the metal
centre and affect the coupling strength between the ipso carbon, M
and P. As an extension to the current work, our group is now
working to 1) further investigate the effects of the nitrogen atom on
the electron density of the metal centre and 2) synthesise similar
complexes with different functional groups attached to the nitro-
gen atom and to the ketone side arms. With the synthesis of these
alternative complexes, we hope to further fine-tune the reactivity
of and the stereoselectivity around the metal centre; and use these
d
In both complexes, it is observed that the geometry around the
metal centre is a distorted square planar, which is also supported by
the bond angles C-M-Cl and P-M-P. Additionally, the phenyl rings
on the phosphorus atoms in both complexes are noted to be
puckered in opposite directions. Consequently, the rings on each P
are positioned in a pseudo-axial-pseudo-equatorial manner.
We also employed the 13C NMR spectroscopy to help us better
understand these complexes in the liquid state. Particular attention
was paid to the chemical shifts and coupling constants (where
applicable) of the ipso carbon, i.e. the carbon atom of the aryl
backbone that is directly bonded to the metal centre, between the
benzylic analogue, 6 and 7 (refer to Table 2). For both 6 and 7, it was
noted that the chemical shifts of the ipso carbon were significantly
more downfield for the pyridinyl-backbone based complexes as
compared to the benzylic-backbone based ones (151.5 ppm [10] vs.
162.3 and 173.7 ppm for the Pt complexes and 158.3 ppm [13] vs.
Scheme 3. Synthesis of the (R)-conformation of the tertiary phosphine using (R,R)-6b as the catalyst.

E.H. Yen Wong et al. / Journal of Organometallic Chemistry 862 (2018) 22e27
27
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Acknowledgement
We are grateful to Nanyang Technological University, MoE Tier I
RG108/15 for supporting this research and the research scholarship
to Y.-X. J.
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