Synthesis of an Air-Stable Pd(0) Catalyst Bearing Donor and Acceptor Phosphine Ligands

Article abstract of DOI:10.1021/acs.organomet.9b00646

We synthesized a new Pd(0) complex bearing π-acceptor ligands and a σ-donor ligand and confirmed that it exhibits high air stability and catalytic activity in direct arylation reactions. Moreover, stoichiometric reactions of the complex revealed that the σ-donor ligand does not dissociate from the Pd center.

Full text of DOI:10.1021/acs.organomet.9b00646

Communication
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Synthesis of an Air-Stable Pd(0) Catalyst Bearing Donor and
Acceptor Phosphine Ligands
Ryota Sato, Takaki Kanbara,* and Junpei Kuwabara*
Tsukuba Research Center for Energy Materials Science (TREMS), Graduate School of Pure and Applied Sciences, University of
Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japan
S
* Supporting Information
ABSTRACT: We synthesized a new Pd(0) complex bearing π-
acceptor ligands and a σ-donor ligand and confirmed that it exhibits
high air stability and catalytic activity in direct arylation reactions.
Moreover, stoichiometric reactions of the complex revealed that the
σ-donor ligand does not dissociate from the Pd center.
ransition-metal-catalyzed cross-coupling reactions are
effective methods to construct C−C or C−heteroatom
T
bonds.1−3 Various precursors and ligands of Pd catalysts have
been developed and used in many types of coupling reactions.
Bi- or zerovalent Pd complexes are used as Pd catalyst
precursors.4 Since Pd(II) complexes have high air stability,
they can be easily handled and stored in an air atmosphere
(Figure 1a). However, the process of forming a catalytically
active Pd(0) species causes decomposition of substrates or
ligands at the initial stage of cross-coupling reactions.5−7 To
solve this problem, many efforts have been devoted to the
development of Pd(II) complexes that can be smoothly
reduced to Pd(0) active species without the decomposition of
substrates or ligands.8−10 Because Pd(0) complexes have the
same oxidation state as catalytically active species, they do not
cause decomposition reactions (Figure 1b). However, Pd(0)
complexes are generally unstable in air.11 Multidentate ligands
have been studied to provide air stability to Pd(0)
complexes.12−14 Recentry, Soos and co-workers have devel-
́
oped an air-stable Pd(0) complex, Superstable Pd catalyst
(Pd[P{3,5-(CF3)2C6H3}3]3 = SSPd), which bears three π-
acceptor phosphine ligands (Figure 1c).15 The ligands decrease
the electron density of the Pd center to suppress the oxidation
of Pd.16 In catalytic reactions, strongly electron donating
phosphine (σ-donor) ligands with steric bulkiness are preferred
to achieve a high catalytic activity.17 Therefore, we designed a
new Pd(0) catalyst precursor bearing a σ-donor ligand to
impart catalytic activity and a π-acceptor ligand to provide air
stability (Figure 1d). During a catalytic reaction, the π-acceptor
ligands may selectively dissociate from the Pd center because
of their weak coordination ability in comparison with that of
the σ-donor ligand. Herein, we synthesize a novel Pd(0)
complex and determine its stability and catalytic activity for a
direct C−H arylation reaction.3
Figure 1. Various Pd catalyst precursors.
The target Pd(0) complex was synthesized from SSPd
through a ligand exchange reaction with tricyclohexylphos-
phine (PCy3) in 68% yield (Scheme 1). The reaction of SSPd
Received: September 30, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00646
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
Scheme 1. Synthesis of Complex 1
with 1 equiv of PCy3 afforded a mixture of complex 1 and
residual SSPd. The quantitative formation of complex 1 was
observed in the reaction with 2 equiv of PCy3 or an excess of
PCy3. The structure of complex 1 might be thermodynamically
stable in terms of steric and electronic aspects.
Doublet and triplet signals with an integral value of 2:1 were
observed through 31P NMR spectroscopy (Figure S2). These
signals indicate that the Pd(0) complex possesses two types of
phosphine ligands in a 2:1 ratio. To obtain detailed structural
information, a single-crystal X-ray analysis was conducted
(Figure 2).
Figure 3. Accelerated aging test (in air, 60 °C).
center of complex 1 and imparted high air and thermal stability
similar to those of SSPd.
Subsequently, complex 1 was used as a catalyst for direct C−
H arylation reactions.19 We optimized a loading amount of the
Pd(0) complex (Table 1). The reactions with 2 and 0.5 mol %
Table 1. Direct Arylation of 2-Thiophenecarbonitrile with
4-Bromoanisole by Complex 1
a
entry
X
yield (%)
1
2
3
2
0.5
0.25
87
87 (83)
60
a
1
The yield was determined by H NMR analyses of a crude product
with ferrocene as an internal standard. The value in parentheses is the
isolated yield.
of complex 1 afforded efficient product formation (entries 1
and 2). Meanwhile, 0.25 mol % of Pd loading did not give full
conversion of the reaction (entry 3). Therefore, 0.5 mol %
(entry 2) was considered as the optimal catalyst loading.
To compare catalytic activities, the time conversion of the
reaction with the Pd(0) catalysts was investigated (Figure 4).
Figure 2. ORTEP drawing of complex 1 with thermal ellipsoids at the
30% probability level. All hydrogen and disordered fluorine atoms are
omitted for clarity. Selected bond lengths (Å): Pd1−P1 = 2.3565(8),
Pd1−P2 = 2.2856(8), Pd1−P3 = 2.3565(8). Selected bond angles
(deg): P1−Pd1−P2 = 119.16(3), P1−Pd−P3 = 126.71(3), P2−Pd1−
P3 = 113.31(3).
The structure with a σ-donor phosphine (PCy3) and two π-
acceptor phosphines (P{3,5-(CF3)2C6H3}3) is consistent with
the 31P NMR spectroscopy results. The Pd1−P1 bond
(2.3565(8) Å) was longer than those of Pd1−P2 and Pd1−
P3 (2.2856(8) and 2.3565(8) Å). The long Pd1−P1 bond is
presumably due to the presence of bulky cyclohexyl groups in
PCy3.18 In addition, back-donation from Pd to the π-acceptor
phosphines may result in short Pd1−P2 and Pd1−P3 bonds.
To prove our concept, the air stability of complex 1 was
evaluated and compared with that of other Pd(0) complexes.
Complex 1 did not decompose in air at room temperature for
1 month, which was confirmed by 31P NMR spectroscopy
(Figure S3). This observation verifies that complex 1 has good
air stability, which enables its storage and handling in air.
Accelerated aging tests were conducted to evaluate air stability
under harsh conditions (in air at 60 °C; Figure 3). Because
Pd(PCy3)2 turned black after 1 h, Pd(PCy3)2 was decomposed
under the tested conditions (Figure S4). In contrast, complex 1
and SSPd showed no color change. The π-acceptor ligand
P{3,5-(CF3)2C6H3}3 decreased the electron density of the Pd
Figure 4. Time conversions of direct arylation reaction by various
Pd(0) catalyst precursors. Conditions: Pd 0.0025 mmol, 4-
bromoanisole 0.50 mmol, 2-thiophenecarbonitrile 1.0 mmol, PivOH
0.15 mmol, K2CO3 0.75 mmol, toluene 1.7 mL, temperature 100 °C.
1
The yield was determined by H NMR analysis of a crude product
with ferrocene as an internal standard.
B
DOI: 10.1021/acs.organomet.9b00646
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
The Pd(PCy3)2 system showed good catalytic activity in the
initial stage. However, the catalytic activity decreased during
the reaction. The relatively low catalyst loading (0.5 mol %)
resulted in the recognizable decomposition of the catalyst.
Although the reactions with complex 1 and SSPd showed
induction periods, these catalytic systems showed high catalytic
activity in the middle to late stages of the reactions. Because
the catalytic activity of complex 1 was better than that of SSPd,
PCy3 in complex 1 can be considered to provide a positive
effect on the catalytic reaction.
Table 2. Direct Arylation Reaction by Complex 1
A stoichiometric reaction of complex 1 was conducted to
determine the reason for the high catalytic activity of complex
1.20 An oxidative addition reaction is the first step of the direct
arylation reaction (Scheme S1). The model reaction of the
oxidative addition reaction afforded a Pd(II) complex bearing
PCy3 and P{3,5-(CF3)2C6H3}3 ligand (complex 2 in Scheme
2). An ORTEP drawing of complex 2 showed that one of the
Scheme 2. Synthesis of Complex 2
a
1
The yield was determined by H NMR analyses of a crude product
with ferrocene as an internal standard.
higher than that of other known Pd(0) complexes. A
stoichiometric model reaction reveals that the σ-donor ligand
imparts the high catalytic performance of complex 1. The
strategy of combining “stabilization” and “activation” gives a
stimulus for catalyst chemistry.
π-acceptor ligands is selectively dissociated and PCy3 remains
on the Pd center in the oxidative addition reaction (Figure 5).
This result supports our hypothesis that PCy3 remains on the
Pd center during the catalytic reaction and provides good
catalytic activity.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.organomet.9b00646.
■
S
Experimental details, crystallographic data, and NMR
spectra (PDF)
Accession Codes
CCDC 1950053−1950054 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 5. ORTEP drawing of complex 2 with thermal ellipsoids at the
30% probability level. All hydrogen atoms and incorporated molecules
are omitted for clarity. Selected bond lengths (Å): Pd1−P1 =
2.354(2), Pd1−P2 = 2.322(2), Pd1−I1 = 2.7032(8), Pd1−C1 =
2.019(9). Selected bond angles (deg): P1−Pd1−P2 = 178.99(8), P1−
Pd−I1 = 95.12(6), P2−Pd1−I1 = 85.89(5), I1−Pd1−C1 = 171.5(3).
AUTHOR INFORMATION
Corresponding Authors
*E-mail for T.K.: kanbara@ims.tsukuba.ac.jp.
■
*E-mail for J.K.: kuwabara@ims.tsukuba.ac.jp.
ORCID
Furthermore, we tested the substrate scope in direct
arylation under the optimized conditions (Table 2). The
reactions with the catalyst (0.5 mol %) afforded the products
in good to moderate yields (entries 1−4). The yield can be
further improved by optimization of the reaction conditions for
each substrate.
In conclusion, we synthesized a novel air-stable Pd(0)
complex bearing π-acceptor and σ-donor ligands. The Pd(0)
complex is stable in air at 60 °C and serves as a good catalyst
for direct arylation reactions even with a small amount of Pd
loading. Additionally, complex 1 shows a catalytic activity
Takaki Kanbara: 0000-0002-6034-1582
Junpei Kuwabara: 0000-0002-9032-5655
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the Chemical Analysis Center of University
of Tsukuba for the measurements of NMR spectra, X-ray
analysis, and elemental analysis. The authors are grateful to
Prof. M. Ichinohe and Prof. K. Takeuchi for lending their
C
DOI: 10.1021/acs.organomet.9b00646
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Communication
̈
(18) Bunten, K. A.; Chen, L.; Fernandez, A. L.; Poe, A. J. Cone
Angles: Tolman’s and Plato’s. Coord. Chem. Rev. 2002, 233−234, 41−
51.
expertise on X-ray analysis. This study was supported by a
Grant-in-Aid for Scientific Research (17K05973, 17H03063)
and Scientific Research on Innovative Areas (17H05141) from
JSPS.
(19) Schipper, D. J.; Fagnou, K. Direct Arylation as a Synthetic Tool
for the Synthesis of Thiophene-Based Organic Electronic Materials.
Chem. Mater. 2011, 23, 1594−1600.
(20) Yamamoto, A.; Ozawa, F.; Kawasaki, N.; Okamoto, H.;
Yamamoto, T. Mechanisms of Double and Single Carbonylation
Reactions of Aryl Iodides Catalyzed by Palladium Complexes To Give
A-Keto Esters and Esters. Organometallics 1987, 6, 1640−1651.
REFERENCES
■
(1) Johansson Seechurn, C. C. C.; Kitching, M. O.; Colacot, T. J.;
Snieckus, V. Palladium-Catalyzed Cross-Coupling: A Historical
Contextual Perspective to the 2010 Nobel Prize. Angew. Chem., Int.
Ed. 2012, 51, 5062−5085.
(2) Li, H.; Johansson Seechurn, C. C. C.; Colacot, T. J.
Development of Preformed Pd Catalysts for Cross-Coupling
Reactions, beyond the 2010 Nobel Prize. ACS Catal. 2012, 2,
1147−1164.
(3) Campeau, L. C.; Stuart, D. R.; Fagnou, K. Recent Advances in
Intermolecular Direct Arylation Reactions. Aldrichimica Acta 2007,
40, 35−41.
(4) Alberico, D.; Scott, M. E.; Lautens, M. Aryl-Aryl Bond
Formation by Transition-Metal-Catalyzed Direct Arylation. Chem.
Rev. 2007, 107, 174−238.
(5) Kuwabara, J.; Sakai, M.; Zhang, Q.; Kanbara, T. Mechanistic
Studies and Optimisation of a Pd-Catalysed Direct Arylation Reaction
Using Phosphine-Free Systems. Org. Chem. Front. 2015, 2, 520−525.
(6) Ozawa, F.; Kubo, A.; Hayashi, T. Generation of Tertiary
Phosphine-Coordinated Pd(0) Species from Pd(OAc)2 in the
Catalytic Heck Reaction. Chem. Lett. 1992, 21, 2177−2180.
(7) Bao, Z.; Chan, W. K.; Yu, L. Exploration of the Stille Coupling
Reaction for the Syntheses of Functional Polymers. J. Am. Chem. Soc.
1995, 117, 12426−12435.
(8) Biscoe, M. R.; Fors, B. P.; Buchwald, S. L. A New Class of Easily
Activated Palladium Precatalysts for Facile C-N Cross-Coupling
Reactions and the Low Temperature Oxidative Addition of Aryl
Chlorides. J. Am. Chem. Soc. 2008, 130, 6686−6687.
(9) Bruno, N. C.; Tudge, M. T.; Buchwald, S. L. Design and
Preparation of New Palladium Precatalysts for C-C and C-N Cross-
Coupling Reactions. Chem. Sci. 2013, 4, 916−920.
(10) Gioria, E.; del Pozo, J.; Martínez-Ilarduya, J. M.; Espinet, P.
Promoting Difficult Carbon−Carbon Couplings: Which Ligand Does
Best? Angew. Chem., Int. Ed. 2016, 55, 13276−13280.
(11) Fantasia, S.; Nolan, S. P. A General Synthetic Route to Mixed
NHC-Phosphane Palladium(0) Complexes (NHC = N-Heterocyclic
Carbene). Chem. - Eur. J. 2008, 14, 6987−6993.
(12) Aizawa, S. I.; Majumder, A.; Yokoyama, Y.; Tamai, M.; Maeda,
D.; Kitamura, A. Air-Stable, Recyclable, and Regenerative Phosphine
Sulfide Palladium(0) Catalysts for C-C Coupling Reaction. Organo-
metallics 2009, 28, 6067−6072.
(13) Majumder, A.; Gupta, R.; Mandal, M.; Babu, M.; Chakraborty,
D. Air-Stable Palladium(0) Phosphine Sulfide Catalysts for Ullmann-
Type C-N and C-O Coupling Reactions. J. Organomet. Chem. 2015,
781, 23−34.
(14) Sluijter, S. N.; Warsink, S.; Lutz, M.; Elsevier, C. J. Synthesis of
Palladium(0) and -(II) Complexes with Chelating Bis(N-Heterocyclic
Carbene) Ligands and Their Application in Semihydrogenation.
Dalton. Trans. 2013, 42, 7365−7372.
́
(15) Jakab, A.; Dalicsek, Z.; Holczbauer, T.; Hamza, A.; Papai, I.;
́
́
Finta, Z.; Timari, G.; Soos, T. Superstable Palladium(0) Complex as
an Air-and Thermostable Catalyst for Suzuki Coupling Reactions. Eur.
J. Org. Chem. 2015, 2015, 60−66.
(16) Korenaga, T.; Ko, A.; Uotani, K.; Tanaka, Y.; Sakai, T.
Synthesis and Application of 2,6-Bis(Trifluoromethyl)-4-Pyridyl
Phosphanes: The Most Electron-Poor Aryl Phosphanes with
Moderate Bulkiness. Angew. Chem., Int. Ed. 2011, 50, 10703−10707.
(17) Barrios-Landeros, F.; Carrow, B. P.; Hartwig, J. F. Effect of
Ligand Steric Properties and Halide Identity on the Mechanism for
Oxidative Addition of Haloarenes to Trialkylphosphine Pd(0)
Complexes. J. Am. Chem. Soc. 2009, 131, 8141−8154.
D
DOI: 10.1021/acs.organomet.9b00646
Organometallics XXXX, XXX, XXX−XXX