Unsymmetrical palladium PCN pincer complexes containing pyrazolyl aminophosphine ligands

Article abstract of DOI:10.1016/j.poly.2014.11.004

Two unsymmetrical palladium pincer complexes containing pyrazolyl aminophosphine ligands have been prepared and characterized. Their solid structures were determined by X-ray single crystal diffraction studies. They were evaluated in the Suzuki coupling r

Full text of DOI:10.1016/j.poly.2014.11.004

Accepted Manuscript
Unsymmetrical Palladium PCN Pincer Complexes Containing Pyrazolyl Ami-
nophosphine Ligands
Earl Cook, Kaoruko Iwasaki, Jason D. Masuda, Aibing Xia
PII:
S0277-5387(14)00709-8
DOI:
http://dx.doi.org/10.1016/j.poly.2014.11.004
POLY 11051
Reference:
Polyhedron
To appear in:
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Accepted Date:
19 September 2014
6 November 2014
Please cite this article as: E. Cook, K. Iwasaki, J.D. Masuda, A. Xia, Unsymmetrical Palladium PCN Pincer
Complexes Containing Pyrazolyl Aminophosphine Ligands, Polyhedron (2014), doi: http://dx.doi.org/10.1016/
j.poly.2014.11.004
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Unsymmetrical Palladium PCN Pincer Complexes Containing Pyrazolyl
Aminophosphine Ligands
Earl Cook a, Kaoruko Iwasaki b, Jason D. Masuda b, and Aibing Xia a,*
a Department of Chemistry, Mount Saint Vincent University, Halifax, NS, B3M 2J6, Canada
b Department of Chemistry, Saint Mary’s University, Halifax, NS, B3H 3C3, Canada
* Corresponding author (e-mail: aibing.xia@msvu.ca).
Abstract
Two unsymmetrical palladium pincer complexes containing pyrazolyl aminophosphine ligands
have been prepared and characterized. Their solid structures were determined by X-ray single
crystal diffraction studies. They were evaluated in the Suzuki coupling reactions of
phenylboronic acid and various aryl chlorides.
Keywords: palladium complexes, pincer type ligands, unsymmetrical ligands,
aminophosphines, the Suzuki reaction
1. Introduction
In the past four decades, metal complexes based on pincer ligands have played an important
role in organometallic synthesis and catalysis. Their steric and electronic features can be fine-
tuned at the donor sites as well as the metal centers to afford desirable properties and activities.
The chemistry of pincer complexes is a topic of frequent reviews and continues to attract
extensive research interest [1-8]. In the pincer complexes, the most commonly studied ligands
are the symmetrical types (e.g. PCP and NCN ligands), which contain identical donors at the two
ortho-positions in the central aromatic ring. In comparison, the unsymmetrical pincer ligands are
not as common, partly due to the difficulties in their preparation. However, they have attracted
increasing attention in recent years due to their bonding versatility and catalytic applications [2,
3, 8-11]. The unsymmetrical ligands contain different bonding sites and interact with the metal
center differently and may exhibit unique reactivity that contributes to improved catalytic
activities over their symmetrical analogues [12, 13]. For example, when both soft (e.g. P) and
hard donors (e.g. N) are present, the pincer ligands may potentially exhibit hemilabile properties
which could facilitate substrate interaction with the metal center and stabilize the intermediate in
the catalytic cycles [14-18]. We have recently reported a series of palladium and nickel
complexes containing unsymmetrical iminophosphinite PCN ligands [19, 20]. Herein we wish to
describe the preparation, characterization and catalytic application of novel PCN pincer
palladium complexes containing pyrazolyl and aminophosphino groups.
2. Results and discussion
2.1. Synthesis
The synthetic routes of the pyrazolyl aminophosphine ligands and their palladium complexes
were outlined in Scheme 1. First, 3-nitrobenzyl bromide reacted with 3,5-dimethylpyrazole in

toluene to afford 3,5-Dimethyl-1-[(m-nitrophenyl)methyl]-1H-pyrazole (1) as pale yellow
solids. It should be noted that 3,5-dimethylpyrazole was also employed as the base to trap
hydrogen bromide released from the coupling reaction, which proved to be more effective than
some commonly used organic bases such as triethylamine and p-N,N-dimethylaminopyridine
(DMAP). The reduction of compound 1 with iron powder and glacial acetic acid in ethanol,
followed by neutralization with sodium hydroxide and extraction with dichloromethane, afforded
m-[(3,5-Dimethyl-1H-pyrazol-1-yl)methyl]aniline (2) as white crystalline solids.
Toluene
O2N
+
N
NH
1
N
O2N
N
Cl
HN
1. Fe, AcOH
2. NaOH
1. PR2Cl, DMAP
2. (PhCN)2PdCl2
R2P
Pd
N
H2N
N
Cl
2
N
N
R = Ph (3a) or iPr (3b)
Scheme 1. Synthesis of unsymmetrical palladium pincer complexes.
i
The reaction of compound 2 with chlorophosphines (PR2Cl, where R = Ph, Pr) in the
presence of DMAP afforded the corresponding aminophosphines, which were used directly,
without further purification, to react with bis(benzonitrile)dichloropalladium(II). The targeted
palladium pincer complexes were obtained as light yellow (3a) or off-white (3b) solids. Both
complexes were found to be air- and moisture-stable and can be handled and recrystallized in air.
They are thermally robust and can be heated well above 200 °C before decomposition occurs.
They are moderately soluble in dichloromethane and chloroform but only sparsely soluble in
other commonly used organic solvents such as toluene and THF.
2.2. Characterization
All compounds were analyzed by microanalysis and the results matched the calculated
1
values. They were further characterized by H and 13C NMR spectroscopy and FT-IR
spectroscopy. In addition, the 31P spectra of the two palladium pincer complexes were collected.

Compound 1 showed a singlet at 5.32 ppm for the methylene (CH2) group between the phenyl
1
and pyrazolyl rings in the H spectrum. The peak was shifted upfield to 5.13 ppm in the
reduction product (2) due to the presence of the electron donating NH2 group, which gave a
singlet at 3.55 ppm in the 1H spectrum. The presence of the amino group was confirmed by the
new absorption peaks around 3300 cm-1 in its IR spectrum. The methylene peak was further
shifted to 4.38 (3a) and 4.99 ppm (3b) upon coordination to palladium in the pincer complexes.
The two nonequivalent methyl groups in the pyrazolyl moiety appeared at 2.19/2.26 ppm in 1.
They were little changed in 2 but showed significant downfield shifts to 2.35/2.62 (3a) and
2.33/2.65 ppm (3b) in the palladium complexes in which one pyrazolyl nitrogen atom was
bonded to the metal center.
Figure 1. Molecular structure of 3b with 50% probability ellipsoids
Singe crystals suitable for X-ray diffraction analysis were obtained through recrystallization
in chloroform (3a) and dichloromethane/hexane (3b), respectively. The molecular structures and
bonding parameters of the two complexes are similar. A representative structure of 3b is shown
in Figure 1 and the structure of 3a can be found in the supplementary data. Selected bond
distances and angles are listed in Table 1. Crystal data and structure refinement data are listed in
Table 2. The structure shows a distorted square-planar geometry featuring two palladacycles,
one five-membered and the other six-membered. The sum of the bond angles surrounding the Pd
ion is 360.48(7)°. The bond angles of P-Pd-C (81.35°), N-Pd-C (90.14°), P-Pd-Cl (92.46°) and
N-Pd-Cl (96.53°) are in a narrower range than the corresponding angles (79.5 to 103.06°) in a
closely related palladium pyrazolyl aminophsophine complex which contains two five-
membered palladacycles and the non-substituted pyrazolyl group is directly connected to the
phenyl ring [21]. They are comparable to the corresponding values (80.55° to 97.74°) in the
recently reported palladium pyrazolyl phosphinite complexes which also features both a five-
membered and a six-membered palladacycles [22]. The bond lengths of Pd-C (2.002 Å), Pd-N
(2.134 Å), Pd-P (2.208 Å), and Pd-Cl (2.414 Å) are comparable to those in the palladium PCN
pincer complexes reported recently [21-26]. Hydrogen bonding between Cl and the amino group
(HN) was observed with a Cl-N distance of 3.268(3) Å. The hydrogen bonding gives rise to a
one-dimensional chain with a zig-zag structure (Figure 2).

Figure 2. One-dimensional zig-zag chain of 3b from a Cl…H-N hydrogen bond. Other
hydrogen atoms have been omitted for clarity.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for 3b
Pd1-C1
Pd1-N1
Pd1-P1
Pd1-Cl1
C1-Pd1-N1
C1-Pd1-P1
P1-Pd1-Cl1
N1-Pd1-Cl1
N1-Pd1-P1
C1-Pd1-Cl1
2.002(3)
2.134(2)
2.2079(7)
2.4140(8)
90.14(10)
81.35(8)
92.46(3)
96.53(7)
167.03(7)
172.94(8)
Table 2. Crystal data and structure refinement for 3b
CCDC Number
Formula
Formula weight
Space group
a (Å)
b (Å)
c (Å)
α (deg)
1019334
C18H27ClN3PPd
458.25
P2(1)/n
10.1706(11)
18.8644(19)
10.1864(10)
90°
β (deg)
91.8530°
γ (deg)
90°
1953.4(3)
4
V (Å3)
Z
Dcalc (g/cm3)
1.558

µ (mm-1)
F(000)
1.173
936
θ for data collection 2.16 to 26.00°
No. of total reflns 13937
No. of unique reflns 3831
0.0284
R1 [I > 2σ(I)]
wR2 [I > 2σ(I)]
R1 [all data]
0.0607
0.0387
0.0649
wR2 [all data]
2.3. Catalytic studies
The efficiency of the palladium pincer complexes was evaluated in the Suzuki coupling
reaction between phenylboronic acid and aryl chlorides, an important organic C-C bond
formation reaction in organic synthesis [27]. The use of aryl chlorides as the substrates over
their bromide and iodide analogues is highly desirable due to their wide availability and low cost,
despite their relative inertness [28]. Results from the catalytic study are listed in Table 3. The
coupling reactions were performed according to the procedures described in the Experimental
Section. The yields of the Suzuki reactions varied greatly with different solvents and bases. It
was found that the combination of dioxane as the solvent and K3PO4 as the base gave the best
yield for complex 3b when p-chloronitrobenzene was used in the screening studies (Table 3,
entry 3). Under similar conditions, complex 3a gave a much lower yield (Table 3, entry 6).
Similar patterns were also observed when p-chlorotoluene (a neutral aryl chloride) and p-
chloroanisole (a deactivated aryl chloride) were used (Table 3, entries 7-10). The higher
efficiency of 3b with the isopropyl substituent over 3a (the phenyl analogue) can be attributed to
the stronger electron donating ability of the alkyl groups over the aryls that is beneficial in the
catalytic processes [28], which was also observed in our previous studies [19, 20].
Pd complex
R
Cl +
B(OH)2
R
Ph
Base
Solvent
Table 3. Suzuki coupling reactions of aryl chloridesa
Entry
Pd complex
R
Base
Solvent
Toluene
Dioxane
Dioxane
DMF
Toluene
Dioxane
Dioxane
Dioxane
Dioxane
Dioxane
Yield (%)b
NO2
NO2
NO2
NO2
NO2
NO2
CH3
CH3
OCH3
OCH3
K2CO3
K2CO3
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
K3PO4
27
79
96
40
60
65
18
58
trace
41
1
2
3
4
5
6
7
8
9
10
3b
3b
3b
3b
3b
3a
3a
3b
3a
3b

aReaction conditions: 1.0 mmol aryl halide, 1.5 mmol phenyl boronic acid, 2.0 mmol base, 1.0 mol% palladium
complex, 5 mL solvent, 18 h. bGC yields based on aryl halides (average of two runs).
2.4. Conclusion
In summary, unsymmetrical palladium pincer complexes containing novel pyrazolyl
aminophosphines were synthesized from readily available starting materials under mild
conditions. The palladium complexes are air- and moisture-stable and also thermally robust.
They showed modest to excellent activity in the Suzuki coupling reaction of phenylboronic acid
and various aryl chlorides. We are investigating the employment of the unsymmetrical ligands
in other transition metal (e.g. Ni) complexes and will report the results in due course.
3. Experimental
3.1.General procedures
All experiments were performed under an argon atmosphere using standard Schlenk line or
glovebox techniques unless otherwise specified. All anhydrous solvents were purchased from
Sigma-Aldrich Canada Ltd. and stored over 4Å molecular sieves prior to use.
Bis(benzonitrile)dichloropalladium (II), 3-nitrobenzyl bromide, chlorodiphenylphosphine,
chlorodiisopropylphosphine, and 3,5-dimethylpyrazole were purchased from Sigma-Aldrich
Canada Ltd. 4-N,N-dimethylaminopyridine (DMAP) and phenylboronic acid were purchased
from Alfa Aesar (a John Matthey company). 1H, 13C, and 31P NMR spectra were recorded on a
Bruker Avance 500 spectrometer and referenced to SiMe4 (1H, 13C) and 85% H3PO4 (31P).
Infrared spectra were collected on a Nicolet Avatar 330 FT-IR spectrometer.
Gas
chromatograms were obtained using an Agilent 6890 GC. Melting points were recorded on a
Mel-Temp (Electrothermal) Apparatus and were uncorrected. Elemental analyses were
performed by Guelph Chemical Laboratories in Guelph, Ontario, Canada.
3.2. Synthesis of 3,5-Dimethyl-1-[(m-nitrophenyl)methyl]-1H-pyrazole (1)
3,5-dimethylpyrazole (1.92 g, 20.0 mmol) and 3-nitrobenzyl bromide (2.16 g, 10.0 mmol)
were dissolved in toluene (20 mL). The pale yellow solution was refluxed for 18 hr and some
white solids were formed. The mixture was cooled to room temperature. It was filtered to afford
a clear yellow filtrate. The volatiles were removed under a reduced pressure. Recrystallization
from dichloromethane/hexanes at 0°C afforded white crystalline solids. Yield: 1.63 g (70%).
Mp 70-73 °C. IR (KBr, cm-1) υ: 3073 (s), 2918 (m), 1918 (m), 1819 (m), 1618 (m), 1528 (s),
1481 (m), 1383 (m), 1348 (s), 1282 (m), 1162 (m), 1142 (m), 1096 (m), 1078 (m), 1028 (m), 907
1
(m), 887 (m), 803 (m), 732 (s), 713 (s), 669 (s). H NMR (500.13 MHz, CDCl3, ppm) δ: 2.19 (s,
3H, CH3), 2.26 (s, 3H, CH3), 5.32 (s, 2H, CH2), 5.91 (s, 1H, pz), 7.39 (d, J = 7.8 Hz, 1H, Ph),
7.49 (t, J = 7.8 Hz, 1H, Ph), 7.94 (s, 1H, Ph), 8.12 (m, 1H, Ph). 13C (125.77 MHz, CDCl3, ppm)
δ: 11.04, 13.45, 51.49, 106.03, 121.47, 122.58, 129.74, 132.65, 139.28, 139.50, 148.33, 148.42.
Anal. Calcd. for C12H13N3O2: C 62.33, H 5.67, N 18.17%. Found: C 62.02, H 5.86, N 17.89%.
3.3. Synthesis of m-[(3,5-Dimethyl-1H-pyrazol-1-yl)methyl]aniline (2)

Compound 1 (0.230 g, 1.00 mmol) was dissolved in anhydrous ethanol (20 mL). Iron
powder (0.250 g, 4.50 mmol) and glacial acetic acid (0.37 mL, 6.5 mmol) were added. The
mixture was refluxed under argon for 18 hr and cooled to room temperature. It was neutralized
with 1.0 M NaOH (100 mL). CH2Cl2 (3 x 10 mL) was used to extract the product. The
combined organic extracts were washed with distilled water (3 x 10 mL) and dried over
magnesium sulfate. Removal of volatiles after filtration under a reduced pressure afforded a
clear light yellow liquid that solidified upon addition of hexanes (5 mL). Yield: 0.355 g (70%).
Mp 80-83 °C. IR (KBr, cm-1) υ: 3405 (m), 3330 (s), 3223 (m), 3017 (s), 2962 (s), 2926 (m),
1909 (m), 1820 (m), 1636 (s), 1606 (s), 1552 (s), 1494 (s), 1460 (s), 1429 (s), 1380 (m), 1305
1
(m), 1261 (m), 1155 (m), 1096 (w), 1028 (s), 994 (m), 849 (m), 768 (w), 688 (m), 430 (m). H
NMR (500.13 MHz, CDCl3, ppm) δ: 2.16 (s, 3H, CH3), 2.26 (s, 3H, CH3), 3.55 (bs, 2H, NH2),
5.13 (s, 2H, CH2), 5.85 (s, 1H, pz), 6.38 (s, 1H, Ph), 6.49 (d, J = 7.8 Hz, 1H, Ph), 6.57 (d, J = 7.8
Hz, 1H, Ph), 7.08 (t, J = 7.8 Hz, 1H, Ph). 13C (125.77 MHz, CDCl3, ppm) δ: 11.15, 13.48,
52.57, 105.49, 113.14, 114.26, 116.81, 129.54, 138.48, 139.36, 146.71, 147.33. Anal. Calcd. for
C12H15N3: C 71.61, H 7.51, N 20.88%. Found: C 71.38, H 7.71, N 21.09%.
3.4. Synthesis of the palladium pincer complexes (3a and 3b).
General procedure: a solution of chlorophosphine (0.500 mmol) in toluene (5 mL) was
added dropwise to a solution of compound 2 (0.500 mmol) and p-N,N-dimethylaminopyridine
(DMAP) (0.060 g, 0.50 mmol) in toluene (10 mL). The resultant cloudy mixture was stirred at
room temperature for 18 hr. The mixture was then filtered to produce a clear pale yellow filtrate.
Bis(benzonitrile)dichloropalladium(II) (0.192 g, 0.500 mmol) was added to the solution. The
resulting mixture was refluxed for 18 h. It was cooled to room temperature, filtered, and washed
with toluene (2 mL).
3a (R = Ph). Yield: 0.188 g (71%). Mp 280 °C (dec). IR (KBr, cm-1) υ: 3261 (m), 3049
(s), 1608 (m), 1476 (s), 1435 (s), 1387 (m), 1308 (s), 1258 (m), 1101 (m), 1016 (m), 743 (m),
1
689 (s), 516 (m), 489 (m). H NMR (500.13 MHz, CDCl3, ppm) δ: 2.35 (s, 3H, CH3), 2.62 (s,
3H, CH3), 3.76 (s, 1H, NH), 5.56 (s, 2H, CH2), 5.87 (s, 1H, pz), 6.12 (m, 1H, Ph), 6.60 (m, 1H,
Ph), 6.76 (m, 1H, Ph), 7.38 (m, 5H, Ph), 7.49 (m, 5H, Ph). 13C (125.77 MHz, CDCl3, ppm) δ:
11.96, 14.95, 52.65, 120.37, 121.18, 128.61, 128.72, 131.36, 132.78, 136.89, 142.10. 31P NMR
(202.47 MHz, CDCl3, ppm) δ: 40.99. Anal. Calcd. for C24H23ClN3PPd: C 54.77, H 4.40, N
7.98%. Found: C 55.01, H 4.58, N 7.73%.
3b (R = iPr). Yield: 0.103 g (44%). Mp 265 °C (dec). IR (KBr, cm-1) υ: 3262 (m), 3052 (s),
2957 (w), 1890 (m), 1810 (m), 1712 (s), 1550 (s), 1435 (s), 1350 (s), 1283 (s), 1161 (m), 1028
(m), 986(m), 932 (s), 831 (s), 884 (s), 780 (s), 758 (s), 717 (s), 675 (s), 551 (m), 496 (s), 432 (m).
1H NMR (500.13 MHz, CDCl3, ppm) δ: 1.25 (dd, J = 7.1, 15.4 Hz, 6H, CH(CH3)2), 1.45 (dd, J =
7.1, 15.4 Hz, 6H, CH(CH3)2), 2.33 (s, 3H, CH3), 2.45 (m, 2H, CH(CH3)2), 2.65 (s, 3H, CH3),
3.86 (s, 1H, NH), 4.99 (s, 2H, CH2), 5.84 (s, 1H, pz), 6.54 (m, 2H, Ph), 6.86 (m, 1H, Ph). 13C
(125.77 MHz, CDCl3, ppm) δ: 11.58, 15.00, 17.15, 18.34, 27.91, 28.14, 53.63, 106.83, 108.77,
108.90, 117.69, 125.80, 138.14, 139.16, 151.28, 155.84. 31P NMR (202.47 MHz, CDCl3, ppm)
δ: 135.49. Anal. Calcd. for C18H27ClN3PPd: C 47.18, H 5.94, N 9.17%. Found: C 47.19, H 6.21,
N 8.89%.
3.5. X-ray structures

Crystals of 3a and 3b were obtained through slow-evaporation of the chloroform and
dichloromethane/hexane solutions, respectively. They were attached to the tip of a 300 µm
MicroLoop with paratone-N oil. Measurements were made on a Bruker APEXII CCD equipped
diffractometer (30 mA, 50 mV) using monochromated Mo Kα radiation (λ = 0.71073 Å) at 125
K. The initial orientation and unit cell were indexed using a least-squares analysis of a random
set of reflections collected from three series of 0.5° ω-scans, 10 seconds per frame and 12 frames
per series, that were well distributed in reciprocal space. For data collection, four ω-scan frame
series were collected with 0.5° wide scans, 10 second frames and 366 frames per series at
varying φ angles (φ = 0°, 90°, 180°, 270°). Cell refinement and data reduction were performed
with the Bruker SAINT software, which corrects for beam inhomogeneity, possible crystal
decay, Lorentz and polarization effects [29, 30]. A multi-scan absorption correction was applied
(SADABS [31]). The structure was solved using direct methods (SHELXS-2014 [32]) and
refined using a full-matrix least-squares method on F2 with SHELXL-2014 [32]. The non-
hydrogen atoms were refined anisotropically. Hydrogen atoms bonded to carbon were included
at geometrically idealized positions and were not refined.
3.6. General procedures for the Suzuki reactions
Phenylboronic acid (1.5 mmol), an aryl chloride (1.0 mmol), a base (2.0 mmol), and complex
3a or 3b (1.0 mol%) were added to a 25 mL Schlenk flask. A solvent (10 mL) was then added
and the mixture was heated to refluxing for 18 hr with stirring. The mixture was then cooled to
room temperature and filtered. The volatiles were removed from the filtrate under a reduced
pressure. The organic product was extracted with diethyl ether (3 x 10 mL) and analyzed on an
Agilent 6890 GC-FID instrument. The GC yields were calculated based on unreacted aryl
chloride and calibrated relative to standards containing aryl chlorides and their biphenyl
products. Yields are based on the average of two runs.
Supporting information available
CCDC 1019333 and 1019334 contain the supplementary crystallographic data for this paper.
These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,
or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgement
We thank Mount Saint Vincent University, the Natural Sciences and Engineering Research
Council of Canada (NSERC), and the Canada Foundation for Innovation (CFI) for financial
support.
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[28] A.F. Littke, G.C. Fu, Angew. Chem., Int. Ed. 41 (2002) 4176-4211.
[29] APEX2, Bruker AXS Inc., Madison, Wisconsin, USA, 2008.
[30] SAINT, Bruker AXS Inc., Madison, Wisconsin, USA, 2008.
[31] SADABS, Bruker AXS Inc., Madison, Wisconsin, USA, 2009.
[32] G.M. Sheldrick, Acta Cryst. A64 (2008) 112-122.

Two unsymmetrical palladium pincer complexes containing pyrazolyl aminophosphine ligands
have been prepared and characterized. The solid structures of the palladium complexes were
determined by X-ray single crystal diffraction studies. The complexes were evaluated in the
Suzuki coupling reactions of phenylboronic acid and various aryl chlorides.