NC Palladacycles and C,C-chelating phosphorus ylide complexes: Synthesis, X-ray characterization, and comparison of the catalytic activity in the Suzuki-Miyaura reaction

Article abstract of DOI:10.1080/00958972.2016.1151876

Six secondary amine palladacycles bearing monodentate ligands (1a, 2a), 1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane (dppp) containing bridging and bidentate ligands (1b, 2b-d), and four C,C-type phosphorus ylide complexes containing thiourea (tu) (3a), phenyl isothiocyanate (4a), and bridging and terminal azide groups (5 and 5a) have been synthesized. Resulting complexes have been characterized by elemental analyses, IR, ¹H-, ¹³C{¹H}-, and ³¹P{¹H}-NMR spectroscopy with single crystal X-ray structure determination of 1a and 2a. The Pd in 1a and 2a occupies the center of a slightly distorted square planar environment formed by C_aryl, N_amine, N_pyridine, and Cl. The catalytic efficiency of complexes showed that in most cases, amine palladacycles display better catalytic activities than the phosphorus ylide Pd(II) complexes. Comparison between bidentate and bridging dppe complexes showed that dppe-bridged dimer 2d has higher catalytic activity than dppe bidentate complex.

Full text of DOI:10.1080/00958972.2016.1151876

Journal of Coordination Chemistry
ISSN: 0095-8972 (Print) 1029-0389 (Online) Journal homepage: http://www.tandfonline.com/loi/gcoo20
NC Palladacycles and C,C-chelating phosphorus
ylide complexes: Synthesis, X-ray characterization
and comparison of the catalytic activity in the
Suzuki-Miyaura reaction
Kazem Karami, Mahboubeh Hosseini-Kharat, Zahra Shirani-Sarmazeh, Reza
Zahedi-Nasab, Corrado Rizzoli & Janusz Lipkowski
To cite this article: Kazem Karami, Mahboubeh Hosseini-Kharat, Zahra Shirani-Sarmazeh,
Reza Zahedi-Nasab, Corrado Rizzoli & Janusz Lipkowski (2016): NC Palladacycles and C,C-
chelating phosphorus ylide complexes: Synthesis, X-ray characterization and comparison of
the catalytic activity in the Suzuki-Miyaura reaction, Journal of Coordination Chemistry, DOI:
10.1080/00958972.2016.1151876
To link to this article: http://dx.doi.org/10.1080/00958972.2016.1151876
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Date: 22 February 2016, At: 05:36

Publisher: Taylor & Francis
Journal: Journal of Coordination Chemistry
DOI: http://dx.doi.org/10.1080/00958972.2016.1151876
NC Palladacycles and C,C-chelating phosphorus ylide complexes: Synthesis,
X-ray characterization and comparison of the catalytic activity in the
Suzuki-Miyaura reaction
KAZEM KARAMI*†, MAHBOUBEH HOSSEINI-KHARAT†, ZAHRA SHIRANI-SARMAZEH†,
REZA ZAHEDI-NASAB†, CORRADO RIZZOLI‡ and JANUSZ LIPKOWSKI§
†Department of Chemistry, Isfahan University of Technology, Isfahan, 84156/83111, Iran
‡Department of Chemistry, University of Parma, I-43124 Parma, Italy
§Institute of Physical Chemistry, Polish Academy of Sciences, Kasprzaka 44/52, 01-224 Warsaw, Poland
Six secondary amine palladacycles bearing monodentate ligands (1a, 2a) and
1,2-bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diphenylphosphino)propane (dppp)
containing bridging and bidentate ligands (1b, 2b-d), and four C,C-type phosphorus ylide
complexes containing thiourea (tu) (3a), phenyl isothiocyanate (4a), bridging and terminal azide
groups (5 and 5a) have been synthesized. Resulting complexes have been characterized by
elemental analyses, IR, 1H-, 13C{1H}- and 31P{1H}-NMR spectroscopy with single crystal X-ray
structure determination of 1a and 2a. The Pd in 1a and 2a occupy the center of a slightly
distorted square planar environment formed by Caryl, Namine, Npyridine and Cl. The catalytic
efficiency of complexes showed that in most cases amine palladacycles display better catalytic
activities than the phosphorus ylide Pd(II) complexes. Comparison between bidentate and
bridging dppe complexes showed that dppe bridged dimer 2d has higher catalytic activity than
dppe bidentate complex.
Keywords: Amine palladacycle; Phosphorus ylide; Suzuki reaction; Catalysis
1. Introduction
Palladium-catalyzed Suzuki–Miyaura (SM) coupling reaction is one of the most efficient
methods for synthesis of biaryl and alkene derivatives that are structural components of natural
*Corresponding author. Email: karami@cc.iut.ac.ir
1

products, agrochemicals, pharmaceuticals, and polymers [1-10]. This reaction is the coupling of
organic electrophiles, like aryl and alkenyl halides, and organoboron compounds. The features of
the coupling are high product yield, non-toxic boronic acid byproduct, and high functional group
tolerance [11-14].
For SM reactions, the proposed mechanism involves initial oxidative addition of the
halide to a Pd(0) catalytic species to form a Pd(II) species, transmetalation of the organometallic
reagent by palladium and reductive elimination of the product from this species to regenerate the
active Pd(0) catalyst [15]. Palladacycles are often investigated catalyst precursors in particular
due to their facile synthesis, thermal stability, and structural versatility [16, 17]. Since the first
report on the use of palladacycle for the SM reaction by Beller and coworkers [18], they have
emerged as a promising family of organometallic catalyst precursors for carbon-carbon coupling
reactions. A variety of phosphorus, nitrogen, sulfur and oxygen derived palladacycles were
presented with high TONs in different carbon-carbon and carbon-hetero atom bond forming
reactions [19, 20]. Several dimeric chloro-bridged palladacycles derived from aromatic and
aliphatic oximes from benzaldehyde, acetophenone, benzophenone, pinacolone and
acetylferrocene have been reported and their catalytic activities checked in different cross-
coupling processes [21].
The role of a palladacycle is to feed an active Pd(0) species to the reaction mixture [19].
Pd(0) can be directly introduced to the reaction mixture or generated from a Pd(II) complex.
Relative instability of Pd(0) complexes is one of the reasons why Pd(II) precatalysts are used to
generate catalytically active Pd species in situ [22, 23]. The ability of palladacycles to undergo
slow release of active Pd(0) helps to stop unwanted inactivation processes, the nucleation and
growth of large inactive Pd metal particles [23].
Phosphine based ligands have been widely used due to effectiveness in stabilizing the
zero valent palladium [24]. The larger steric demand of phosphine ligand can promote formation
of active palladium species and, therefore, lead to faster reaction [25, 26].
The coordination chemistry of phosphorus-palladium complexes is diverse and includes
mono or bis compounds, mono or dinuclear palladacycles and heteroligand systems. Among the
several classes of phosphine ligands described for Pd-catalyzed SM reactions, we became
interested in phosphorus ylide ligands. However, most phosphorus-based ligands are toxic and
sensitive to moisture or air, and are also known to increase side reactions, thereby lowering the
2

yield of desired cross-coupled products. Phosphine degradation typically involves an oxidative
addition of the phosphine ligand which deactivates the metal center towards further catalysis.
Nitrogen based ligands, such as amines, imines and pyridines, are much more robust in
general, but they are much less effective as ligands for low valent late transition metals and have
received less attention in Pd catalyzed cross-coupling compared to phosphine ligands. Amines
tend to coordinate weakly to palladium in low stability complexes. Bidentate amines provide
stable complexes, but are much less electron donating than phosphines. They stabilize high
valent metal ions and are not as sensitive to oxidation as phosphorus or sulfur based ligands.
Given the lower cost and toxicity of nitrogen based ligands, there has been renewed interest in
using these types of ligands [27]. Use of nitrogen based ligands such as N-heterocyclic carbenes,
amines, oximes, amides and azo compounds have attracted considerable attention for SM cross-
coupling [28], which drew our attention to study the catalytic activity of amine palladacycles in
homogenous media to overcome some drawbacks of traditional phosphine ligands.
In continuation of our ongoing program of design, synthesis and application of
cyclopalladated complexes [29-31], we report the synthesis and characterization of new
secondary amine and phosphorus ylide palladacycles. Comparison of the catalytic activity of
various palladacycles including mononuclear amine and ylide complexes and bidentate and
bridging diphosphine palladacycles was performed in the SM reactions.
2. Experimental
2.1. General
Starting materials and solvents were purchased from Merck and Aldrich and used without
purification. Infrared spectra were recorded on a FT-IR JASCO 680 spectrophotometer from
4000−400 cm−1 using KBr pellets. NMR spectra were measured on a Bruker spectrometer at
400.13 MHz (1H), 100.61 MHz (13C) and 161.97 MHz (31P) using standard pulse sequences at
298 K. Elemental analyses were performed on a Leco, CHNS-932 apparatus. Conversions were
monitored using an Agilent 6890N gas chromatograph equipped with a capillary HP-5+ column,
based on aryl halides. The column properties were: 30 m long, 0.32 mm in inner diameter, and
0.25 μm film thickness. The starting dimers 1-4 [32, 33] and compound 2d (figure 1) [34] were
also obtained using procedures described earlier.
3

2.2. General procedure for the synthesis of [Pd{(C,N)-C6H4CH2NH(R)}Cl(L)] (R = t-Bu, L =
Py (1a), R = Et, L = Me3Py (2a))
To a suspension of 1 (0.3 g, 0.5 mmol) or 2 (0.34 g, 0.5 mmol) in CH2Cl2 (15 mL) was added
pyridine (80 μL, 1 mmol) or 2,4,6-trimethylpyridine (131 μL, 1 mmol). The resulting solution
was stirred for 8 h and then filtered through a plug of MgSO4. The filtrate was concentrated to
ca. 2 mL, and n-hexane (15 mL) was added to precipitate 1a or 2a as pale yellow solids, which
were collected and air dried.
(1a): Yield: 62%. IR (KBr, cm−1): ν(NH) = 3212, ν(C=N) = 1621. 1H NMR (DMSO-d6,
ppm): δ = 1.16−1.26 (m, 9H, t-Bu), 3.91 (br s, Ha, CHaH), 4.28-4.33 (br s, Hb, CHbH), 5.49 (d,
H6, C6H4), 5.61 (br s, 1H, NH), 6.54 (t, 1H, C6H4), 6.79−6.94 (m, 2H, C6H4), 7.15−7.29 (m, 5H,
HAr, Py); 13C{1H} NMR (DMSO-d6, ppm): δ = 20.21, 26.90, 27.22, 30.71, 57.41 (Caliphatic),
120.12, 121.05, 123.50, 123.81, 124.32, 129.52, 152.01, 157.60, 158.12 (Caromatic). Anal. calcd.
for C16H21N2ClPd: C, 50.14; H, 5.52; N, 7.31. Found: C, 50.12; H, 5.55; N, 7.30%.
(2a): Yield: 73%. IR (KBr, cm−1): ν(NH) = 3136, 3117, ν(C=N) = 1621. 1H NMR
(CDCl3, ppm): δ = 1.27 (t, 3H, CH3, 3JHH = 7.2 Hz), 2.35 (m, 3H, CH3), 2.49−2.51 (m, 2H, CH2),
2.87 (s, 3H, CH3), 3.25 (s, 3H, CH3), 3.91 (d, Ha, CHaH, 2JHH = 16 Hz), 4.28-4.33 (dd, Hb, CHbH,
2JHH = 14.8 Hz, 3JHH = 5.2 Hz), 5.49 (d, 1H, C6H4, 3JHH = 7.6 Hz), 5.61 (br s, 1H, NH), 6.54 (t,
1H, C6H4), 6.79-6.94 (m, 2H, C6H4), 7.15 (br s, 1H, HAr, Me3Py), 7.30 (br s, 1H, HAr, Me3Py);
13C{1H} NMR (CDCl3, ppm): δ = 20.29, 26.85, 27.21, 29.81, 56.30, 57.42 (Caliphatic), 120.10,
121.05, 123.50, 123.82, 124.30, 129.50, 152.12, 157.60, 158.12 (Caromatic). Anal. calcd. for
C17H23N2ClPd: C, 51.39; H, 5.83; N, 7.05. Found: C, 51.12; H, 5.75; N, 7.05%.
2.3. General procedure for the synthesis of [Pd{(C,N)-C6H4CH2NH(R)(L-L)]Cl (R = t-Bu, L-L
= dppe (1b), R = Et, L-L = dppe (2b), R = Et, L-L = dppp (2c))
To a suspension of 1 (0.30 g, 0.5 mmol) or 2 (0.34 g, 0.5 mmol) in CH2Cl2 (15 mL) was added a
solution of dppe (0.39 g, 1 mmol) or dppp (0.41g, 1 mmol) in CH2Cl2. The resulting solution was
stirred for 1 h and then filtered through a plug of MgSO4. The filtrate was concentrated to
ca. 2 mL, and n-hexane (15 mL) was added to precipitate 1b, 2b or 2c as white solids, which
were collected and air dried.
(1b): Yield: 73%. IR (KBr, cm−1): ν(NH) = 3280. 1H NMR (CDCl3, ppm): δ = 1.19 (m,
9H, t-Bu), 2.78 (m, 1H, CHaH), 3.80−3.92 (m, 4H, CH2 (dppe)), 4.32 (m, 1H, CHbH), 5.35 (br s,
4

1H, NH), 6.05 (br s, H6, C6H4), 6.21−6.91 (m, 3H, C6H4), 7.42−7.65 (m, 20H, Ph);
13C{1H} NMR (CDCl3, ppm): δ = 15.22, 17.50, 27.10, 29.82, 30.75, 55.20, 57.62 (Caliphatic),
120.80, 124.45, 124.82, 128.50, 128.65, 128.75, 128.80, 129.00, 129.20, 129.32, 129.45, 129.82,
130.00, 130.40, 130.85, 131.75, 131.90, 132.00, 132.45, 132.50, 132.60, 133.60, 134.65, 134.82,
136.55, 148.70, 153.35 (Caromatic); 31P{1H} NMR (CDCl3, ppm): δ = 40.5 (d, 1P, 3JPP = 24.3 Hz),
60.5 (d, 1P, 3JPP = 24.3 Hz). Anal. calcd. for C37H40NP2PdCl: C, 63.25; H, 5.73; N, 1.99. Found:
C, 63.23; H, 5.71; N, 1.98%.
(2b): Yield: 74%. IR (KBr, cm−1): ν(NH) = 3280. 1H NMR (CDCl3, ppm): δ = 1.27 (t,
3H, CH3), 2.78 (m, 1H, CHaH), 2.84−3.01 (m, 2H, CH2), 3.22 (m, 1H, CHbH), 3.80−3.92 (m,
4H, CH2 (dppe)), 4.70 (br s, 1H, NH), 6.36 (br s, H6, C6H4), 6.39−6.95 (m, 3H, C6H4), 7.28−7.92
(m, 20H, Ph); 13C{1H} NMR (CDCl3, ppm): δ = 15.40, 17.55, 28.80, 30.25, 57.55 (Caliphatic),
120.50, 123.45, 124.30, 128.50, 128.75, 128.85, 129.00, 129.10, 129.30, 129.50, 129.75, 130.05,
130.15, 130.45, 130.50, 130.75, 131.30, 131.95, 132.15, 132.47, 132.55, 132.70, 133.40, 134.15,
135.50, 147.25, 152.35 (Caromatic); 31P{1H} NMR (CDCl3, ppm): δ = 40.7 (d, 1P, 3JPP = 24.3 Hz),
60.5 (d, 1P, 3JPP = 24.3 Hz). Anal. calcd. for C35H36NP2PdCl: C, 62.33; H, 5.38; N, 2.08. Found:
C, 62.31; H, 5.38; N, 2.07%.
(2c): Yield: 78%. IR (KBr, cm−1): ν(NH) = 3269. 1H NMR (CDCl3, ppm): δ = 1.17 (t,
3H, CH3), 2.69 (m, 1H, CHaH), 2.78−3.15 (m, 2H, CH2), 3.25 (m, 1H, CHbH), 3.68−3.80 (m,
6H, CH2 (dppp)), 4.54 (br s, 1H, NH), 6.27 (br s, H6, C6H4), 6.35−6.95 (m, 3H, C6H4), 7.25−7.92
(m, 20H, Ph); 13C{1H} NMR (CDCl3, ppm): δ = 15.20, 17.80, 21.85, 27.20, 30.15, 57.45
(Caliphatic), 120.10, 123.45, 124.50, 128.50, 128.65, 128.75, 129.00, 129.20, 129.50, 129.75,
130.05, 130.25, 130.45, 130.55, 130.60, 131.47, 131.85, 132.05, 132.45, 132.55, 132.85, 133.57,
133.85, 134.20, 135.65, 147.10, 152.55 (Caromatic); 31P{1H} NMR (CDCl3, ppm): δ = 23.5 (br s),
33.2 (br s). Anal. calcd. for C36H38NP2PdCl: C, 62.80; H, 5.56; N, 2.03. Found: C, 62.76; H,
5.55; N, 2.03%.
2.4. Synthesis of [Pd{κ2(C,C)-C6H4PPh2C(H)CO(C6H4-NO2-4)}Cl(tu)] (3a)
To a suspension of 3 (0.05 g, 0.045 mmol) in chloroform (15 mL) was added solution of thiourea
(0.006 g, 0.09 mmol) in methanol (2 mL). The resulting solution was stirred at room temperature
for 6 h and then filtered through a plug of MgSO4. The filtrate was concentrated to ca. 2 mL, and
n-hexane (15 mL) was added to precipitate a yellow solid, which was collected and air dried.
5

Yield: 68%. IR (KBr, cm−1): ν(NH2) = 3390–3164. 1H NMR (DMSO-d6, ppm): δ = 5.38 (br s,
1H, CHP), 7.18−8.07 (m, 18H, HAr), 8.20−8.35 (m, 4H, NH2); 13C{1H} NMR (DMSO-d6, ppm):
δ = 38.91 (CHP), 128.98, 129.11, 129.38, 132.59, 132.74, 133.11, 133.78, 133.83, 133.97
(Caromatic), 178.09 (CS), 196.20 (CO); 31P{1H} NMR (DMSO-d6, ppm): δ = 17.40 (s, CHP). Anal.
Calc. for C27H23ClN3O3PPdS: C, 50.48; H, 3.60; N, 6.54; S, 4.99. Found: C, 50.45; H, 3.58; N,
6.53; S, 4.98%.
2.5. Synthesis of [Pd{κ2(C,C)-C6H4PPh2C(H)CO(C6H4-NO2-4)}Br(Ph-NCS)] (4a)
To a suspension of 4 (0.12 g, 0.1 mmol) in CH2Cl2 (15 mL) was added phenyl isothiocyanate
(24 μL, 0.2 mmol). The resulting solution was stirred at room temperature for 20 h and then
filtered through a plug of MgSO4. The filtrate was concentrated to ca. 2 mL, and n-hexane
(15 mL) was added to precipitate a yellow solid, which was collected and air dried. Yield: 74%.
IR (KBr, cm−1): ν(CN) = 2092, ν(CO) = 1632, ν(P-C) = 853, ν(CS) = 721. 1H NMR (CDCl3,
ppm): δ = 4.99 (br s, 1H, CHP), 7.05−8.07 (m, 23H, HAr); 13C{1H} NMR (CDCl3, ppm): δ =
38.82 (CHP), 123.69, 126.21, 128.50, 128.91, 129.09, 129.34, 132.56, 132.63, 133.07, 133.74,
133.81, 133.94, 140.03 (Caromatic), 137.13 (CN), 176.57 (CS), 197.43 (CO); 31P{1H} NMR
(CDCl3, ppm): δ = 19.56 (br s, CHP). Anal. Calc. for C33H24BrN2O3PPdS: C, 53.13; H, 3.24; N,
3.75; S, 4.29. Found: C, 53.12; H, 3.22; N, 3.74; S, 4.28%.
2.6. Synthesis of [Pd{κ2(C,C)-C6H4PPh2C(H)CO(C6H4-NO2-4)}(μ-N3)]2 (5)
To a suspension of 3 (0.1 g, 0.09 mmol) in methanol (15 mL) was added NaN3 (0.01 g,
0.18 mmol) and the resulting mixture stirred for 12 h at room temperature. A yellow precipitate
formed which was filtered and washed with water and then air dried to give 5. Yield: 70%. IR
(KBr, cm−1): ν(CO) = 1633, ν(N=N=N) = 2052, ν(P-C) = 853. 1H NMR (CDCl3, ppm): δ = 4.32
(br s, 1H, CHP, min.), 4.37 (br s, 1H, CHP, maj.), 7.23−8.07 (m, 36H, HAr, both); 13C{1H} NMR
(CDCl3, ppm): δ = 38.81 (CHP, min.), 38.88 (CHP, maj.), 128.81, 128.98, 129.09, 129.17,
129.30, 129.41, 132.32, 132.48, 132.62, 132.79, 132.95, 133.07, 133.58, 133.65, 133.76, 133.83,
133.89, 133.98, (Caromatic, both), 199.21 (CO, min.), 201.79 (CO, maj.); 31P{1H} NMR (CDCl3,
ppm): δ = 17.60, 17.65, 18.47, 18.77. Anal. Calc. for C52H38N8O6P2Pd2: C, 54.51; H, 3.34; N,
9.78. Found: C, 54.50; H, 3.32; N, 9.76%.
6

2.7. Synthesis of [Pd{κ2(C,C)-C6H4PPh2C(H)CO(C6H4-NO2-4)}N3(Py)] (5a)
To a suspension of 5 (0.18 g, 0.16 mmol) in CH2Cl2 (15 mL) was added pyridine (24 μL,
0.32 mmol). The resulting solution was stirred at room temperature for 20 h and then filtered
through a plug of MgSO4. The filtrate was concentrated to ca. 2 mL, and n-hexane (15 mL) was
added to precipitate a yellow solid, which was collected and air dried. Yield: 65%. IR (KBr,
cm−1): ν(CO) = 1632, ν(N=N=N) = 2032, ν(P-C) = 853. 1H NMR (DMSO-d6, ppm): δ = 5.15 (br
s, 1H, CHP, min.), 5.00 (br s, 1H, CHP, maj.), 7.23−8.29 (m, 46H, (C6H4)PPh2 + (C6H4)NO2 +
Py, both); 13C{1H} NMR (DMSO-d6, ppm): δ = 38.83 (CHP, min.), 38.91 (CHP, maj.), 120.81,
121.06, 122.94, 123.14, 128.78, 128.91, 129.08, 129.19, 129.28, 129.39, 129.43, 129.57, 132.40,
132.58, 132.63, 132.73, 133.08, 133.19, 133.55, 133.63, 133.74, 133.80, 133.91, 133.99
(Caromatic, both), 198.33 (CO, min.), 200.31 (CO, maj.); 31P{1H} NMR (DMSO-d6, ppm): δ =
17.71 (s, CHP, maj.), 21.53 (s, CHP, min.). Anal. Calc. for C31H24N5O3PPd: C, 57.11; H, 3.71;
N, 10.74. Found: C, 57.10; H, 3.71; N, 10.72%.
2.8. General procedure for SM reaction
A round bottom flask was charged with aryl halide (0.5 mmol), phenylboronic acid (0.75 mmol),
base (1.5 mmol), catalyst (0.01 mmol) and solvent (7 mL). The mixture was heated for the time
and at the temperature given in figures and tables in section 3.3, using the base and catalyst
therein indicated. The products were characterized by GC.
2.9. Crystallography
X-ray diffraction experiments were done at 100 K with the use of an Agilent SuperNova single
crystal diffractometer (Mo Kα radiation). Analytical numeric absorption correction using a
multifaceted crystal model based on expressions derived by Clark and Reid was made [35]. The
structures were solved by direct methods using the SHELXS97 program and refined with the use
of SHELXL (Sheldrick 2008). Hydrogens were added in calculated positions as riding on their
respective carbons during the refinement.
7

3. Results and discussion
3.1. Synthesis and spectroscopy
Schemes 1-3 illustrate the synthesis of complexes, and the labeling assigned to the methylene
protons of benzylamine. All complexes were isolated as air and moisture-stable crystalline solids
in moderate to good yields (62-78%). They were also fully characterized by various methods
such as elemental analysis, IR and NMR spectroscopy and single crystal X-ray crystallography.
The elemental analysis results are in agreement with the calculated values.
Scheme 1. (i) Py or Me3Py, CH2Cl2, room temperature, 8 h; (ii) dppe, CH2Cl2, room temperature,
1 h; (iii) dppp, CH2Cl2, room temperature, 1 h.
8

Scheme 2. (i) Complex 3, CHCl3, tu, 6 h; (ii) Complex 4, CH2Cl2, Ph-NCS, 20 h.
9

Scheme 3. (i) NaN3, MeOH, room temperature,12 h; (ii) Py, CH2Cl2, 20 h.
In 1H NMR spectra of 1a and 2a, the methylene protons have AB patterns, indicating
they are situated in different environments. Moreover, the H3–H5 aromatic protons of phenyl
rings show similar patterns, but H6 (H ortho to the Pd-C bond) is significantly shifted to lower
frequency (5.49 ppm (1a) and 6.54 ppm (2a)) because of the anisotropic shielding from the
pyridine ring [36]. In IR spectra of 1b, 2b and 2c, P−C (C6H5) and Pd−P bond stretches were
observed from 1102-1104 cm−1 and 693-696 cm−1, respectively, which demonstrated
coordination of phosphines to palladium. In addition, P−C (five-membered ring) stretching
vibration, which is also sensitive to the coordination, shifted to lower frequency with respect to
free dppe or dppp, suggesting some removal of the electron density of the P−C bond [37]. In
each complex, two signals appeared for P-C bonds due to different trans effects of amino
nitrogen and arylic carbon [38].
In 1H NMR spectra of 1b, 2b and 2c, the methylene protons are also nonequivalent and
resonate at different positions. In accord with the presence of aromatic rings in the diphosphine
ligand close to H6, these protons are shifted upfield in relation to the free ligand. H6 of the
complexes was virtually coupled with phosphorus of the PPh2 groups and afforded broad signals.
10

Similar behavior has been already reported for related complexes [39]. The 31P{1H} NMR
spectra of 1b and 2b exhibited two doublets, as expected for the structures, indicating that both
phosphorus atoms of the diphosphine ligand are coordinated to palladium, and are nonequivalent.
Coordination of tu in 3a was shown by the appearance of additional IR bands at 3390–3164 cm-1
(ν N–H) and 1620 cm-1 (δ NH2) [40]. 1H NMR data also support the cleavage of 3 into the
monomeric complex 3a. For 3a, signals of the chelating C,C-phosphorus ylide were observed at
δ 7.18−8.07 (m, 18H, HAr) and 5.38 (br s, 1H, CHP), whereas the appearance of broad signals at
δ 8.20 and 8.35 ppm clearly indicated coordination of tu [40, 41]. The downfield shift of NH2
signals when compared with that of the free ligand (δ 7.06) [42] could be associated to reduction
in the electron density around nitrogen, suggesting Pd–S bond formation. The reactivity of 4
toward phenyl isothiocyanate is shown in scheme 2. When 4 was treated with a stoichiometric
amount of phenyl isothiocyanate under mild conditions (8 h, room temperature), cleavage of the
bromide bridge took place to form mononuclear 4a. In the IR spectrum of 4a, three bands for the
S-bonded coordination of phenyl isothiocyanate were observed. The S-bonded coordination was
evidenced by the presence of CN and CS bond stretches at 2092 and 721 cm-1 [43]. Several
bands of low intensity were observed near 466 cm-1 assigned to δ (NCS). These results strongly
imply S-bonded coordination of phenyl isothiocyanate, according with the anti-symbiotic
behavior of the Pd(II) center [44]. The 1H and 31P{1H} NMR spectra of 4a show broad and sharp
signals for P=C(H) groups at δ 5.00 and 19.56 ppm, respectively. The appearance of single
signals for P=C(H) groups in 1H and 31P{1H} NMR spectra indicates the presence of only one
isomer. Azido-bridged phosphorus ylide 5 was synthesized by metathesis of the corresponding
dimer 3. The formation of μ-azido bridged palladacycle was confirmed by a characteristic stretch
at 2052 cm-1 assigned to the bridging N3 group [45]. 31P{1H} NMR spectrum of 5 shows the
presence of four signals at δ 17.60, 17.65, 18.47 and 18.77 ppm (figure 2). Complex 5 is
dinuclear, obtained as a mixture of geometric isomers (cis and trans), with two chiral carbons in
each isomer. A signal is observed for each one of the four diastereoisomers: cis (RR/SS),
cis (RS/SR), trans (RR/SS) and trans (RS/SR). This behavior has already been seen for other
dinuclear phosphorus ylide complexes [33, 46]. Treatment of 5 with pyridine in 1:2 molar ratio
at room temperature produced yellow 5a. In the FT-IR spectrum of 5a, a strong stretching
vibration was observed at 2032 cm-1 due to the terminal azide, which was shifted to lower
11

wavenumber compared to the bridging azide group present in the starting material (2052 cm-1)
[47], indicating that pyridine cleaved cyclopalladated azide-bridged dimer 5.
The 1H and 31P{1H} NMR spectroscopic data show that 5a was obtained as a mixture of
geometric isomers (a) and (b) depicted in figure 3. Their 1H and 31P{1H} NMR spectra show two
signals for P=C(H) groups, assigned to a fast equilibrium between the cis and trans isomers or a
-
dynamic activity for exchange of pyridine and N3 in solution [48-50].
3.2. Molecular structures of 1a and 2a
1a and 2a have been characterized in the solid phase by single crystal X-ray diffraction. Suitable
crystals were obtained by slow diffusion of n-hexane into a CH2Cl2 solution of complex. ORTEP
views of the complexes are shown in figure 4. Relevant crystallographic data and structure
refinement details are listed in table 1. Selected bond lengths and angles are listed in table 2. For
both complexes, coordination to the metal is provided by a chloride, two trans-arranged
nitrogens from the organic ligands and the aryl-carbon. The sum of angles about Pd are 361.20°
(1a) and 361.15° (2a), with the distortion most noticeable in the somewhat reduced “bite” angle
of the metallated moiety. The requirements of the five-membered rings force the C−Pd−Namine
bond angles to 82.5°. The square-planar coordination geometry of Pd(II) is slightly tetrahedrally
distorted, and deviation of Pd from the Cl, N1, Cpalladate, Namine planes is 0.085 Å. The distortion
from ideal square planar geometry is also indicated by the values of the bond angles subtended at
the Pd center (table 2).
In 1a, the dihedral angle between benzene and pyridine is 65.77°; in 2a, the benzene and
pyridine rings are oriented almost perpendicularly with dihedral angle of 88.84(6)°. The
PdNamine bond lengths are longer than the sum of the covalent radii of Pd and N(sp2) (2.011 Å)
[51]. In addition, for both complexes the Pd–Namine and Pd–Npyridine distances are not significantly
different. The Pd–Cpalladate distances (1.9831(17) Å for 1a and 1.9792(18) Å for 2a) are shorter
than the predicted value of 2.081 Å (based on the sum of the covalent radii for C(sp2) and Pd,
0.771 and 1.31 Å, respectively) [51] but fall in the usual range of values observed for five- and
six-membered palladacycles.
In the crystal structures of both complexes, adjacent molecules related by a two-fold
rotation axis are linked into dimeric units by pairs of N–H...Cl hydrogen bonds (figure 5). Also,
CH→π and weak π→π interactions are observed, helping to stabilize the structures.
12

3.3. SM cross-coupling reactions
The catalytic applicability of amine palladacycles was evaluated for efficiency towards SM
reactions of a variety of aryl bromides with phenylboronic acid. 4a was used in preliminary
testing to optimize reaction conditions (temperature, nature of base, solvent, time). The coupling
of bromobenzene and phenylboronic acid was chosen as the model reaction and all reactions
were carried out under aerobic conditions. A study of solvent effects on the activity of 4a
revealed that the reaction proceeds well in methanol (figure 6). This may be due to the better
solubility of the reagents and easier reduction of Pd(II) to Pd(0) [29]. DMF and acetonitrile
afforded moderate and low coupled product yields. A possible explanation could be that
acetonitrile and DMF are basic solvents and could compete with the surrounding ligands in
coordinating to the palladium center and the latter complexes could be less active than the former
ones [52]. Furthermore in DMF, which is less basic than acetonitrile due to conjugation, and
therefore less available for coordination, the reaction proceeds to 50% yield of the product in 2 h,
whereas in acetonitrile only 33% yield is obtained in the same period. Other solvents such as
acetone and THF gave very low coupled product yields (30%, 17%).
Among the bases, Na2CO3 and K3PO4·3H2O gave high and comparable yields (table 3).
The reaction time and temperature were optimized as 2 h and 85 °C, respectively. A comparative
study of catalytic activity involving 1a, 1b, 2a-d, and 3a-5a was carried out to determine the
effects of palladium precursor as well as ligand substituents using conditions optimized for 4a.
As inferred by table 4, the efficacy of the complexes follows the order 1a = 2a = 4a > 2c > 1b >
2d > 2b > 5a > 3a. The obtained results (tables 4 and 5) indicate that the monodentate amine
complexes are more active than their bidentate counterparts. This may be due to monodentate
ligand coordination promoting transmetalation by making the metal center less sterically
encumbered [53]. Comparison between bidentate dppe and dppp complexes show that the bite
angle of the phosphine has a significant impact on the reactivity of the palladium(0) species.
Larger bite angles usually result in faster dissociation of phosphine ligands because of steric
crowding [54].
Generally, small ring chelates from phosphines give poor catalysts; increasing the size of
the ring tends to increase activity. In fact, the increased ring size apparently increases the lability
of the phosphine, causing it to behave more like a monodentate ligand [54-56].
13

Of the three complexes containing phosphorus ylide ligand, 4a, bearing the phenyl
isothiocyanate substituent gave the best result, ascribed to the increase in steric hindrance of the
substituent, leading to an increase in the rate of elimination [57]. As shown in table 4, 3a
containing thiourea has the lowest catalytic activity under the given conditions.
The catalytic activities of 1a, 1b, 2a, 2b, 2d, 4a and 5a were then evaluated for SM cross
coupling reaction. We set out to investigate different aryl bromides and phenylboronic acid as
substrates under the optimized conditions determined in the preliminary study for 4a. The results
are given in table 5. 1a and 2a show promising catalytic activity in the reactions of both electron-
withdrawing and electron-donating groups on the aryl bromides. 1b, 4a and 5a showed low and
good to excellent catalytic activity in the reactions of 2-bromopyridine and 2-bromothiophene
with phenylboronic acid, respectively.
Coupling products of the sterically hindered 2-bromo-p-xylene with phenylboronic acid
using these catalysts were obtained in good to moderate yields, respectively. The influence of the
halide nature was found to be similar to that observed in cross-coupling reactions of aryl
bromides with phenylboronic acid as expected. Electron-deficient aryl bromides such as
4-bromonitrobenzene afforded excellent yields.
With coordinating substrates (table 5, entries 31-34), bidentate 1b and 2b gave low yields
compared to 1a and 2a. The high reactivities of 1a and 2a are more likely due to facile oxidative
addition. Because of less steric hindrance, they tend to fill the remaining vacant sites.
Generally, amine palladacycles have better catalytic activity than ylide complexes. This
difference probably arises from different rates of Pd(0) release under the given conditions [58].
We employed phosphorus ylide complexes containing metalated chelating (soft ylidic and arylic
carbon) ligand with good thermal stabilities [37] in the SM reactions. Thus, it is likely that the
rate of Pd(0) release from the ylide complexes is slower than the amine palladacycles.
The key step in the proposed mechanism for coupling is slow release of low-ligated Pd(0)
species which is active in the catalytic process. Catalytically efficient palladacycles are those in
which the release of the Pd(0) species is neither too fast nor too slow. The former is
characteristic of palladacycles with poor thermal stability while the latter is observed for
thermally robust palladacycles [19, 22, 58].
The catalytic applicability of bridging and bidentate dppe complexes, 2b and 2d, was also
evaluated for the SM reaction with aryl bromides and chlorides. As can be seen (tables 5 and 6),
14

the rate of coupling utilizing 2d for each reaction was higher than that observed when 2b was
used. Hence, generally 2d was more effective than 2b in all reactions especially in couplings of
less reactive aryl chlorides with phenylboronic acid at low reaction time. Complex resulting from
coordination with the chelating ligand is much more thermodynamically stable. Thus, for Pd(II)
of the palladacyclic complexes containing bidentate dppe, which forms a five-membered chelate
ring with metal center, are difficult to reduce to the catalytically active Pd(0) [59]. Also,
dissociation of the dinuclear dppe bridge complex into the mononuclear units containing
dangling diphosphine [60] is entropically favored and makes the reduction process easier. The
high solubility of the dppe bridged dimer 2d facilitates reduction of Pd(ΙΙ) to Pd(0), providing a
simpler mechanism to the catalytic cycle.
4. Conclusion
Some new air- and moisture-stable Pd(II) coordination compounds of the C,N-type palladacyclic
complexes (1a, 1b, 2a-d) and C,C-type complexes bearing phosphorus ylide functionalities
(3a, 4a, 5 and 5a) have been synthesized. The characterization of all the complexes was
accomplished by different spectral methods. 1a and 2a were characterized by single crystal
X-ray diffraction. The catalytic efficiency of complexes showed that in most cases amine
palladacycles display better catalytic activities than the phosphorus ylide Pd(II) complexes. Also,
comparison between dppe complexes indicated that dppe bridged dimer 2d is more active than
bidentate dppe complex 2b.
Appendix A. Supplementary material
CCDC 965889 and 951138 contain the supplementary crystallographic data for 1a and 2a,
respectively. 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.
Acknowledgements
Funding of our research from the Isfahan University of Technology (IUT) is gratefully
acknowledged.
15

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18

Table 1. Crystallographic data and structure refinement details for 1a and 2a.
1a
2a
Empirical formula
C16H21ClN2Pd
383.20
C17H23ClN2Pd
Formula weight
397.22
T/K
100(1)
294(2)
Crystal system
Monoclinic
P21/c
Monoclinic
Space group
C2/c
ɑ /Å
9.8000(11)
10.9130(11)
15.2450(16)
90
11.3891(18)
b/Å
17.347(3)
c/Å
18.369(3)
90
α/
106.069(11)
90
106.428(2)
β/
90
γ/
V/Å3
1566.7(3)
4
3480.9(10)
Z
8
μ (mm-1)
1.35
1.22
Dcal/Mg m-3
1.625
776
1.516
F(000)
1616
3.3–27.8
11.2–22.3
ϴ Range/
Independent reflections
Data / restraints / parameters
Goodness-of-fit on F2
Final R indices
R indices (all data)
3586
3808
3586 / 0 / 185
1.713
3808 / 0 / 198
1.113
R1 = 0.0194, wR2 = 0.0434
R1 = 0.0214, wR2 = 0.0439
R1 = 0.0208, wR2 = 0.0547
R1 = 0.0224, wR2 = 0.0558
19

Table 2. Selected bond lengths (Å) and angles (°) for 1a and 2a.
1a
2a
Atoms
Bond lengths
2.4260(5)
2.0762(14)
2.0461(14)
1.9831(17)
1.407(2)
1.497(2)
1.501(2)
Bond angles
82.4(9)
95.9(8)
92.5(3)
90.4(5)
167.3(8)
116.6(8)
Atoms
Bond lengths
2.4289(5)
2.0698(15)
2.0722(14)
1.9792(18)
1.402(2)
1.488(3)
1.491(3)
Bond angles
82.97(7)
93.44(7)
Pd1−Cl1
Pd1−N1
Pd1−N2
Pd1−C3
C2−C3
C1−C2
C1−N1
Pd1—Cl1
Pd1—N1
Pd1—N2
Pd1—C9
C4—C9
C3—C4
C3—N1
C3−Pd1−N1
C3−Pd1−N2
N2−Pd1−Cl1
N1−Pd1−Cl1
C3−Pd1−Cl1
N1−Pd1−N2
C9−Pd1−N1
C9−Pd1−N2
N2−Pd1−Cl1
N1−Pd1−Cl1
N1—Pd1—N2
C9−Pd1−Cl1
90.40(4)
93.34(5)
175.91(6)
174.02(5)
20

Table 3. Effect of base, temperature and time on the SM reaction of bromobenzene with
phenylboronic acida.
Entry
Base
Temp. (°C)
Time (min)
Yield (%)
1
2
3
4
5
6
7
8
Na2CO3
K3PO4·3H2O
K2CO3
85
85
85
85
r.t
120
120
120
120
120
120
60
100
95
56
18
56
78
12
76
Et3N
Na2CO3
Na2CO3
Na2CO3
Na2CO3
50
r.t
85
60
a Reaction conditions: bromobenzene (0.5 mmol), phenylboronic acid (0.75 mmol),
catalyst 4a (0.01 mmol), base (1.5 mmol), MeOH (7 mL).
21

Table 4. Effect of different complexes on the SM reaction of bromobenzene with phenylboronic acida.
Entry
Catalyst
Base
Yield (%)
1
Na2CO3
K3PO4·3H2O
Na2CO3
Na2CO3
Na2CO3
NaOAc
Et3N
100
96
1a
1a
1b
2a
2a
2a
2a
2a
2a
2b
2b
2b
2c
2d
3a
4a
5a
2
3
92
4
100
82b
88
5
6
7
70
8
K3PO4•3H2O
K2CO3
100
100
86
9
10
11
12
13
14
15
16
17
Na2CO3
Na2CO3
NaOAc
Na2CO3
Na2CO3
Na2CO3
Na2CO3
Na2CO3
84c
10
93
90
42
100
69
a Reaction conditions: bromobenzene (0.5 mmol), phenylboronic acid (0.75 mmol),
catalyst (0.01 mmol), base (1.5 mmol), MeOH (7 mL), 2 h, 85 °C. b Time (1 h, r.t).
c Solvent: dmf
22

Table 5. The SM reaction of aryl bromides with phenylboronic acida.
Entry Aryl bromide
Catalyst
Product
Yield (%)
1
2
3
4
5
6
7
95b
92
95
78
81
94
95
1a
1b
2a
2b
2d
4a
5a
Br
8
9
10
11
12
13
14
94b
84
86
71
85
70
65
1a
1b
2a
2b
2d
4a
5a
H3CO
Br
H3CO
15
16
17
18
19
20
21
90b
90
95
71
82
100
74
1a
1b
2a
2b
2d
4a
5a
Br
S
S
22
23
24
100
96
95
83
90
95
85
1a
1b
2a
2b
2d
4a
5a
O2N
Br
25
O2N
26
27
28
NO2
NO2
29
30
77
87
2b
2d
Br
23

31
32
33
34
35
36
37
93b
30
86
21
75
23
20
1a
1b
2a
2b
2d
4a
5a
N
Br
N
38
39
85
69
85
73
92
54
45
1a
1b
2a
2b
2d
4a
5a
Br
CH3
40
41
42
43
44
CH3
CH3
H3C
a Reaction conditions: aryl bromide (0.5 mmol), phenylboronic acid (0.75 mmol), catalyst (0.01 mmol),
Na2CO3 (1.5 mmol), MeOH (7 mL), 2 h, 85 °C. b Time (1 h)
24

Table 6. Catalytic activity of 2b and 2d in the SM reaction of aryl chlorides with phenylboronic acida.
Entry Aryl chloride
Catalyst
Product
Yield (%)
1
2
10
21
2b
2d
Cl
Cl
3
4
8
45
2b
2d
5
6
18
41
2b
2d
O2N
O2N
Cl
7
8
20
45
2b
2d
aReaction conditions: aryl chloride (0.5 mmol), phenylboronic acid (0.75 mmol), catalyst (0.01 mmol), Na2CO3
(1.5 mmol), MeOH (7 mL), 2 h, 85 °C.
25

Figure 1. Dppe bridged 2d.
26

Figure 2. The four separate signals for the P=C(H) group of the chelating C,C-phosphorus ylide
moiety in the 31P{1H} NMR spectrum of 5.
27

Figure 3. Possible isomers for 5a.
28

Figure 4. ORTEP diagrams for 1a (top) and 2a (bottom) with ellipsoids drawn at the 50%
probability level.
29