Microwave-assisted aqueous Suzuki coupling reactions catalyzed by ionic palladium(II) complexes

Article abstract of DOI:10.1007/s11243-011-9564-1

Suzuki coupling is one of the most powerful methods for the synthesis of biaryls. We have prepared and characterized a series of unsymmetrical sulfonated watersoluble Pd(II)-pyridyl imine complexes and investigated them as catalysts for the Suzuki cross-coupling reaction in water under microwave irradiation. The compounds proved to be effective catalysts. Springer Science+Business Media B.V. 2011.

Full text of DOI:10.1007/s11243-011-9564-1

Transition Met Chem (2012) 37:109–116
DOI 10.1007/s11243-011-9564-1
Microwave-assisted aqueous Suzuki coupling reactions catalyzed
by ionic palladium(II) complexes
•
M. Emre Hanhan Yetkin Senemoglu
Received: 7 October 2011 / Accepted: 9 November 2011 / Published online: 20 November 2011
Ó Springer Science+Business Media B.V. 2011
Abstract Suzuki coupling is one of the most powerful
methods for the synthesis of biaryls. We have prepared and
characterized a series of unsymmetrical sulfonated water-
soluble Pd(II)-pyridyl imine complexes and investigated
them as catalysts for the Suzuki cross-coupling reaction in
water under microwave irradiation. The compounds proved
to be effective catalysts.
oxime cabapalladacyle as a very efficient catalyst for
Suzuki coupling of aryl chlorides in water with TBAB
(tetrabutylammonium bromide) as an additive [1, 15, 16].
Zhou et al. [17] synthesized a series of water-soluble Pd(II)
diimine complexes for aqueous Suzuki coupling reactions.
Their catalysts were effective with very low catalyst
loading.
Our group has previously used diimine and pyridyl
imine ligands for olefin polymerization [18, 19]. Imine
complexes are useful catalysts because of their easily tuned
electronic and steric properties. However, the use of
hydrophilic nitrogen ligands with ionic or polar substitu-
ents (e.g. –SO3Na, –PO3Na2, CO2H, –OH) in aqueous
organometallic chemistry and catalysis has so far remained
relatively unexplored [20–24]. The lack of research effort
in this field is due to rapid hydrolysis of imine-based
ligands in aqueous solution [25]. Using catalysts with ionic
imine ligands may provide new alternatives to catalytic
processes currently performed in organic solvents.
Recently both our group [26] and Zhou et al. [17, 27]
reported the effects of complexes of palladium(II) com-
plexes with diimine and pyridyl—imine ligands on the
Suzuki and Heck reactions. In this paper, we report a
palladium catalyzed homogeneous Suzuki cross-coupling
reaction in water with TBAB as an additive under micro-
wave irradiation.
Introduction
The use of water as a solvent for homogeneous catalysis
has become an important target in green chemistry to
develop more simple, cheap, and environmentally friendly
industrial processes [1]. The Suzuki cross-coupling reac-
tion is one of the most widely used carbon–carbon bond
forming reactions [2–7]. Suzuki reactions provide a general
method for the synthesis of biaryls that are found in
polymers [8], biologically active compounds [9], ligands
[10], and various materials [8]. In this context, the use of
water-soluble transition metal complexes is of much
interest since they provide the unique advantages of
homogeneous catalysis as well as facilitating the easy
separation of the catalyst from the products [11, 12]. Pal-
ladium catalyzed cross-coupling reactions in aqueous
media catalyzed by PPh2(m-C6H4SO3K/Pd(OAc)2 have
been reported by Casalnuovo [13]. Shoughnessy et al. [14]
synthesized a series of water-soluble palladcycles for Su-
zuki reactions in water and they reached 12 reaction cycles
with more than 86% product yield. Najera et al. reported an
Experimental
All solvents were purified according to standard proce-
dures. Other reagents were used as received without further
purification. Melting points were determined with a Buchi
Melting Point B-540 apparatus and are uncorrected. FTIR
spectra were recorded on a Pelkin Elmer Spectrum 100
M. E. Hanhan (&) ꢀ Y. Senemoglu
Department of Chemistry, Science and Letters Faculty ,
Zonguldak Karaelmas University, 67100 Zonguldak, Turkey
e-mail: emrehanhan@karaelmas.edu.tr
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Transition Met Chem (2012) 37:109–116
spectrophotometer using an ATR attachment between
4,000 and 400 cm-1. NMR spectra were recorded on a
Bruker 300 MHz Ultrashield TM instrument using DMSO
as solvent and Me4Si as an internal standard. Elemental
analysis was performed on a LECO CHSN 932 instrument.
LC–MS spectra were recorded with an AGILENT LC/MSD
mass spectrometer. GC analyses were performed with an
AGILENT 6890 FID gas chromatogram equipped with an
HP-INNOWAX column.
7.79 (m, 3H, Py–H), 2.17 (s, 3H, N=C–CH3). 13CNMR
(DMSO-d6, d, ppm): 162.43 (N=C(6)–CH3), 158.05,
155.16, 147.14, 143.22, 138.27, 135.82 127.42 123.14
122.21 119.84, 18.42(N=C–CH3). LC–MS (API-ES) m/z:
298.3. Calcd. for C13H11N2NaO3S: C, 52.3; H, 3.7; N, 9.4.
Found: C, 52.0; H, 3.4; N, 10.2.
Compound 3
Yield 69%; m.p. 229 °C; yellow; FTIR (ATR, m, cm-1
)
1
Syntheses of the ligands; general procedure
1,636 (C=N), 1,195 (S=O). HNMR (DMSO-d6, d, ppm):
8.84 (s, 1H, C(6)–H=N), 8.58 (d, 1H, J = 5.2 Hz, C(4)–H),
8.09–8.18 (m, 2H, Ar–H), 7.62–7.77 (m, 3H, Py–H), 2.29
(s, 6H, Ar–CH3). 13CNMR (DMSO-d6, d, ppm): 157.14
(C(6)=N), 154.12, 148.84, 142.72, 136.72, 134.41, 131.64,
131.64, 127.19, 125.77, 123.36, 18.94(Ar–CH3). LC–MS
(API-ES) m/z: 312.5. Calcd for C14H13N2NaO3S: C, 53.8;
H, 4.2; N, 9.0. Found: C, 53.1; H, 4.8; N, 9.7.
2,6-Dialkyl anilines were sulfonated at 180–190 °C with
concentrated sulfuric acid, followed by neutralization with
NaOH to form the sodium salt according to the litera-
ture[17, 27, 28] Sulfonated pyridyl amines were prepared
by condensing either pyridinecarboxaldehyde or 2-acetyl-
pyridine with the required sulfonated aniline in toluene
using a Dean–Stark apparatus (Fig. 1). Ligands 7 and 8
were prepared by an ion exchange reaction between the
ligand 6 and excess 1-butyl-3-methylimidazolium chloride
(BmimCl) or tetrabutylammonium chloride (TBAC) in iso-
propanol according to the literature [17].
Compound 4
Yield 69%; m.p. 226 °C; pale yellow; FTIR (ATR, m, cm-1
)
1
1,642 (C=N), 1,188 (S=O). HNMR (DMSO-d6, d, ppm):
8.73 (d, 1H, J = 4.9 Hz, Py–C(4)–H, 7.78–7.32 (m, 5H, Ar
and Py–H), 2.28 (s, 6H, Ar–CH3), 2.12 (s, 3H, N=CH–CH3).
13CNMR (DMSO-d6, d, ppm): 161.83 (N=C(6)–CH3),
152.42, 147.19, 145.21, 136.55, 134.42, 133.19, 131.19,
126.13, 122.14, 116.84, 22.19(N=C–CH3). 16.22(Ar–CH3).
LC–MS (API-ES) m/z: 328.1. Calcd for C15H15N2NaO3S: C,
55.2; H, 4.6; N, 8.6. Found: C, 55.4; H, 4.9; N, 9.1.
Compound 1
Yield: 96%; m.p. 261 °C; pale yellow; FTIR (ATR, m,
1
cm-1) 1,629 (C=N), 1,188 (S=O); HNMR (DMSO-d6, d,
ppm): 8.73 (s, 1H; C(6)–H=N), 8.67 (d, 1H, J = 5.1 Hz,
C(4)–H), 7.89–7.78 (m, 4H, Ar–H), 7.72–7.64 (m, 3H, Py–
H); 13CNMR (DMSO-d6, d, ppm): 155.05 (C(6)=N),
151.32, 150.14, 145.42, 144.43, 138.51, 133.26, 131.18,
128.52, 126.93, 123.54, 120.83. LC–MS (API-ES) m/z:
284.1. Calcd. for C12H9N2NaO3S: C, 50.7; H, 3.1; N, 9.9.
Found: C, 51.1; H, 3.4; N, 9.1.
Compound 5
Yield 89%; m.p. 239 °C; yellow; FTIR (ATR, m, cm-1
)
1
1,636 (C=N), 1,179 (S=O). HNMR (DMSO-d6, d, ppm):
8.79 (s, 1H; CH=N), 8.61 (d, 1H, J = 5.2 Hz, Py–C(4)–H),
7.81–7.69 (m, 5H, Ar–H and Py–H), 2.86 (m, 2H, Ar–CH–
(CH3)2), 1.22 (d, 12H, J = 6.4 Hz, Ar–CH–(CH3)2).
13CNMR (DMSO-d6, d, ppm): 152.61 (C(6)=N), 150.14,
148.9, 143.77, 135.39, 134.11, 127.82, 122.42, 121.36,
28.16 (Ar–CH–(CH3)2), 23.19 (Ar–CH–(CH3)2). LC–MS
Compound 2
Yield: 72%; m.p. 296 °C; yellow; FTIR (ATR, m, cm-1
1,652 (C=N), 1,176 (S=O); HNMR (DMSO-d6, d, ppm):
8.75 (d, 1H, J = 5.3 Hz, C(4)–H), 8.11–7.88 (m, 4H, Ar–H),
)
1
Fig. 1 Schematic display of
ligands and complexes
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Transition Met Chem (2012) 37:109–116
111
(API-ES) 368.4. Calcd for C18H21N2NaO3S: C, 58.7; H,
5.8; N, 7.6. Found: C, 59.2; H, 5.2; N, 7.5.
(Py–C(1)), 46.77 (imidazolium-N–(CH2–CH2–CH2–CH3)4),
36.41 (imidazolium-CH3), 32.55 (imidazolium-N–(CH2–
CH2–CH2–CH3)4), 24.45 (Ar–CH–(CH3)2), 21.09 (Ar–CH–
(CH3)2), 20.77 (imidazolium-N–(CH2–CH2–CH2–CH3)4)
14.21 (imidazolium-N–(CH2–CH2–CH2–CH3)4). LC–MS
(API-ES) 500.7. Calcd for C27H38N4O3S: C, 65.0; H, 7.7; N,
11.2. Found: C, 66.0; H, 7.1; N, 11.5.
Compound 6
Yield 76%; m.p. 226 °C; yellow; FTIR (ATR, m, cm-1
)
1
1,627 (C=N), 1,188 (S=O). HNMR (DMSO-d6, d, ppm):
8.51 (d, 1H, J = 4.9 Hz, Py–C(4)–H), 7.71 (m, 2H, Ar–H),
7.56 (m, 3H, Py–H), 2.79 (m, 2H, Ar–CH–(CH3)2), 2.19 (s,
3H, N=C–CH3), 1.19 (d, 12H, J = 7.1 Hz, Ar–CH–
(CH3)2). 13CNMR (DMSO-d6, d, ppm): 151.23 (N=C–
CH3), 149.13, 145.63, 136.22, 133.19, 126.27, 122.19,
121.93, 120.14, 118.14, 117.41, 28.72 (Ar–CH–(CH3)2),
22.74 (Ar–CH–(CH3)2), 18.77 (N=C–CH3). LC–MS (API-
ES) 381.69. Calcd for C19H23N2NaO3S: C, 59.7; H, 6.1; N,
7.3. Found: C, 60.2; H, 6.8; N, 7.5.
Syntheses of Pd(II) complexes; general procedure
All the Pd(II) complexes were prepared according to the
literature method [26]. The required ligand (1–8) (1 mmol)
and Pd(MeCN)2Cl2 (1 mmol) were stirred at room tem-
perature for 12 h in THF (35 mL) under nitrogen. The
solvent was then removed under vacuum and the resulting
solid was washed with diethyl ether three times and dried
under vacuum.
Compound 7
Complex C1
Yield 88%; m.p. 198 °C; yellow; FTIR (ATR, m, cm-1
)
1
1,633 (C=N), 1,181 (S=O). HNMR (DMSO-d6, d, ppm):
8.66 (d, 1H, J = 5.1 Hz, Py–C(4)–H), 7.79–7.61 (m, 5H,
Ar–H and Py–H), 3.09 (m, 8H, N–CH2–CH2–CH2–CH3),
2.61 (s, 3H, N=CH–CH3), 2.77 (m, 2H, Ar–CH–(CH3)2);
1.64 (m, 8H, N–CH2–CH2–CH2–CH3), 1.22 (m, 8H, N–
CH2–CH2–CH2–CH3), 1.21 (d, 12H, J = 7.2 Hz, Ar–CH–
(CH3)2), 0.88 (m, 6H, N–CH2–CH2–CH2–CH3). 13CNMR
(DMSO-d6, d, ppm): 153.41 (N=C–CH3), 151.92, 147.11,
139.59, 136.53, 129.41, 125.46,124.03, 117.41, 115.36,
59.16 (N(CH2–CH2–CH2–CH3)4, 29.54 (N(CH2–CH2–
CH2–CH3)4), 24.61 (Ar–CH–(CH3)2), 21.16 (Ar–CH–
(CH3)2), 19.18 (N=C–CH3), 17.99 (N(CH2–CH2–CH2–
CH3)4), 14.66 (N(CH2–CH2–CH2–CH3)4). LC–MS (API-
ES) 602.9. Calcd for C35H59N3O3S: C, 69.8; H, 9.9; N, 6.7.
Found: C, 70.5; H, 9.1; N, 7.5.
Yield 78%. m.p. 315 °C (decomposition); red; FTIR (ATR,
1
m, cm-1) 1,548 (C=N), 1,179 (S=O). HNMR (DMSO-d6,
d, ppm): 9.11 (s, 1H, CH=N), 8.36 (d, 1H, J = 6.3 Hz, Py–
C(4)–H), 7.52 (d, 2H, J = 8.4 Hz, Ar–H), 6.98–7.31 (m, 2H,
Ar–H), 6.64–6.79(m, 3H, Py–H). 13CNMR (DMSO-d6, d,
ppm): 163.24 (C(6)=N), 156.82 (Ar–C(4)), 153.42 (Py–C(5)),
149.16 (Py–C(7)), 146.92 (Ar–C(10)), 142.71 (Py–C(2)),
137.42 (Ar–C(9)), 134.26 (Py–C(3)), 131.19 (Ar–C(10)),
129.52 (Ar–C(8)), 125.04 (Ar–C(12)), 124.42 (Ar–C(1)).
LC–MS (API-ES) m/z: 463.2. Calcd for C12H9Cl2N2
NaO3PdS: C, 31.2; H, 2.0; Cl, 15.4; N, 6.1. Found: C, 32.1;
H, 2.2; Cl, 14.9; N, 6.4.
Complex C2
Yield 76%. m.p. 249 °C (decomposition); orange; FTIR
(ATR, m, cm-1) 1,554 (C=N), 1,189 (S=O). 1HNMR
(DMSO-d6, d, ppm): 8.92 (d, 1H, J = 6.1 Hz, C4-H), 8.65
(d, 1H, J = 7.2 Hz, Ar–H), 8.33 (d, 1H, J = 7.5 Hz, Ar–
H), 7.21–7.92 (m, 5H, Ar–H and Py–H), 2.35(s, 3H,
N=CH–CH3). 13CNMR (DMSO-d6, d, ppm): 171.54
(N=C–CH3), 166.42 (Py–C(4)), 159.41 (Ar–C(7)), 158.72
(Py–C(5)), 151.99 (Ar–C(10)), 143.19 (Py–C(2)), 139.19
(Ar–C(9)), 135.41 (Ar–C(11)), 132.19 (Py–C(3)), 127.51
(Ar–C(8)), 125.13 (Ar–C(12)), 122.61 (Py–C(1)),
22.19(N=C–CH3). LC–MS (API-ES) m/z: 475.6. Calcd for.
C13H11Cl2N2NaO3PdS: C, 32.8; H, 2.3; Cl, 14.9 N, 5.9.
Found: C, 32.4; H, 2.6; Cl, 14.5; N, 5.4.
Compound 8
Yield 71%; m.p. 211 °C; pale yellow; FTIR (ATR, m,
cm-1) 1,641 (C=N), 1,189 (S=O). HNMR (DMSO-d6, d,
1
ppm): 9.04 (s, 1H, imidazolium-H), 8.52 (d, 1H,
J = 5.3 Hz, Py–C(4)–H), 8.23–7.91 (m, 3H, Py–H), 7.63
(m, 2H, Ar–H), 7.59 (s, 1H, imidazolium-H), 7.51 (s, 1H,
imidazolium-H), 4.11 (m, 2H, imidazolium-N–CH2–CH2–
CH2–CH3), 3.88 (s, 3H, imidazolium–CH3), 3.28 (m, 2H,
Ar–CH–(CH3)2), 2.17 (s, 3H, N=CH–CH3), 1.63 (m, 2H,
imidazolium-N–CH2–CH2–CH2–CH3), 1.52 (m, 2H, imi-
dazolium-N–CH2–CH2–CH2–CH3), 1.41 (d, 12H, J = 7.1,
Hz, Ar–CH–(CH3)2), 1.06 (s, 3H, imidazolium-N–CH2–
CH2–CH2–CH3). 13CNMR (DMSO-d6, d, ppm): 155.41
(N=C–CH3), 150.89, 149.15, 148.84, 141.19 (imidazoli-
um–CH), 137.28, 134.48, 132.54, 126.41, 124.12, 126.11
(imidazolium-CH), 125.77 (imidazolium-CH), 121.41
Complex C3
Yield 89%. m.p. 232 °C (decomposition); dark red; FTIR
(ATR, m, cm-1) 1,539 (C=N), 1,177 (S=O). 1HNMR
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Transition Met Chem (2012) 37:109–116
(DMSO-d6, d, ppm): 9.18 (s, 1H, CH=N), 8.91 (d, 1H,
J = 6.9 Hz, C4–H), 7.89–8.21 (m, 2H, Ar–H), 7.22–7.60
(m, 3H, Py–H), 2.79(s, 6H, Ar–CH3), 13CNMR (DMSO-d6,
d, ppm): 163.42 (C(6)=N), 159.41 (Py–C(4)), 152.71 (Ar–C(7)),
148.19 (Py–C(5)), 144.03 (Ar–C(10)), 139.19 (Py–C(2)),
138.51 (Ar–C(8)), 133.29 (Ar–C(12)), 129.54 (Py–C(3)),
127.04 (Ar–C(9)), 123.69 (Ar–C(11)), 19.31(Ar–CH3),
15.22(Ar–CH3). LC–MS (API-ES) m/z: 487.3. Calcd for
C14H13Cl2N2NaO3PdS: C, 34.3; H, 2.7; Cl, 14.5; N, 5.7.
Found: C, 34.0; H, 2.1; Cl, 14.1; N, 5.9.
126.72 (Ar–C(3)), 123.71 (Ar–C(7) and Ar–C(12)), 122.14
(Ar–C(9) and Ar–C(11), 119.71 (Py–C(3)), 118.4 (Py–
C(1)), 29.41 (Ar–CH–(CH3)2), 24.14 (Ar–CH–(CH3)2),
19.51 (N=C–CH3). LC–MS (API-ES) 561.4. Calcd for
C19H23Cl2N2NaO3PdS: C, 40.8; H, 4.1; Cl, 12.7; N, 5.0.
Found: C, 41.1; H, 3.6; Cl, 12.4; N, 4.5.
Complex C7
Yield 71%; m.p. 322 °C (decomposition); red; FTIR (ATR,
1
m, cm-1) 1,571 (C=N), 1,187 (S=O). HNMR (DMSO-d6,
Complex C4
d, ppm): 9.08 (d, 1H, J = 6.9 Hz, Py–(C4)–H), 8.21 (m,
3H, Py–H), 8.11 (m, 2H, Ar–H), 3.14 (m, 8H, N–CH2–
CH2–CH2–CH3), 2.83 (m, 2H, Ar–CH–(CH3)2), 2.74 (s,
3H, N=CH–CH3), 1.83 (m, 8H, N–CH2–CH2–CH2–CH3),
1.46 (m, 8H, N–CH2–CH2–CH2–CH3), 1.36 (d, 12H,
J = 7.3 Hz, Ar–CH–(CH3)2), 0.96 (m, 6H, N–CH2–CH2–
CH2–CH3). 13CNMR (DMSO-d6, d, ppm): 166.74 (N=C–
CH3), 157.41 (Py–C(4)), 151.92 (Ar–C(7)), 143.66 (Py–
C(5)), 140.70 (Ar–C(10)), 132.12 (Py–C(2)), 129.27 (Ar–
C(8) and Ar(C12)), 125.56 (Ar–C(9) and Ar–C(11)),
122.42 (Py–C(3)), 121.48 (Py–C(1)), 62.23 (N(CH2–CH2–
CH2–CH3)4, 31.02 (N(CH2–CH2–CH2–CH3)4), 24.03
Yield 89%. m.p. 273 °C (decomposition); red; FTIR (ATR,
m, cm-1) 1,552 (C=N), 1,182 (S=O). 1HNMR (DMSO-d6, d,
ppm): 8.88 (d, 1H, J = 6.4 Hz, Py–C(4)-H), 8.22 (d, 1H,
J = 7.7 Hz, Py–C(1)-H), 7.88–8.31 (m, 4H, Ar–H and Py–
H), 2.88 (s, 3H, Ar–CH3), 2.69(s, 3H, Ar–CH3), 2.22(s, 3H,
N=CH–CH3). 13CNMR (DMSO-d6, d, ppm): 171.22
(N=C(6)–CH3), 161.19 (Py–C(4)), 158.23 (Ar–C(7)),
152.19 (Py–C(5)), 141.13 (Py–C(4)), 140.21 (Ar–C(10)),
139.53 (Py–C(2)), 137.02 (Ar–C(8) and Ar–C(12)), 131.29
(Ar–C(9) and Ar–C(11)), 127.55 (Py–C(3)), 122.31 (Py–
C(1)), 20.18(N=C–CH3), 17.55(Ar–CH3). LC–MS (API-
ES) m/z: 503.2. Calcd for C15H15Cl2N2NaO3PdS: C, 35.8; H,
3.0; Cl, 14.1; N, 5.6. Found: C, 35.7; H, 2.8; Cl, 14.7; N, 5.4.
(Ar–CH–(CH3)2),
22.94
(Ar–CH–(CH3)2),
21.23
(N=C–CH3), 18.06 (N(CH2–CH2–CH2–CH3)4), 15.41
(N(CH2–CH2–CH2–CH3)4). LC–MS (API-ES) 836.7.
Calcd for C39H67Cl2N3O3PdS: C, 56.1; H, 8.1; Cl, 8.8; N,
5.0. Found: C, 55.5; H, 7.3; Cl, 8.5; N, 5.1.
Complex C5
Yield 71%; m.p. 311 °C (decomposition); red; FTIR (ATR,
m, cm-1) 1,592 (C=N), 1,183 (S=O). 1HNMR (DMSO-d6, d, ppm):
9.12 (s, 1H; CH=N), 8.92 (d, 1H, J = 6.4 Hz, Py–C(4)–H),
8.03 (m, 3H, Ar–H and Py–H), 7.82 (m, 2H, Py–H), 2.91
(m, 2H, Ar–CH–(CH3)2); 1.36 (d, 12H, J = 7.3 Hz,
Ar–CH–(CH3)2). 13CNMR (DMSO-d6, d, ppm): 162.24
(CH=N), 158.41 (APy–C(4)), 153.77 (Py–C(5)), 149.63
(Ar–C(10)), 139.11 (Ar–C(2)), 137.27 (Ar–C(8) and
Ar–C(12)), 132.16 (Py–C(3)), 126.14 (Ar–C(9) and
Ar–C(11)), 123.42 (Ar–C(1)), 31.91 (Ar–CH–(CH3)2),
24.51 (Ar–CH–(CH3)2). LC–MS (API-ES) 546.7. Calcd for
C18H21Cl2N2NaO3PdS: C, 39.6; H, 3.9; Cl, 13.0; N, 5.1.
Found: C, 40.4; H, 4.6; Cl, 12.6; N, 5.2.
Complex C8
Yield 79%; m.p. 296 °C (decomposition); pale red; FTIR
(ATR, m, cm-1) 1,554 (C=N), 1,182 (S=O). 1HNMR
(DMSO-d6, d, ppm): 9.34 (s, 1H, imidazolium-H), 9.12 (d,
1H, J = 7.7 Hz, Py–C(4)–H), 8.59 (m, 2H, Ar–H), 8.31
-7.92 (m, 3H, Py–H), 7.77 (s, 1H, imidazolium-H), 7.73 (s,
1H, imidazolium-H), 4.49 (m, 2H, imidazolium-N–CH2–
CH2–CH2–CH3), 4.06 (s, 3H, imidazolium–CH3), 3.57 (m,
2H, Ar–CH–(CH3)2), 2.36 (s, 3H, N=CH–CH3), 1.82 (m,
2H, imidazolium-N–CH2–CH2–CH2–CH3), 1.66 (m, 2H,
imidazolium-N–CH2–CH2–CH2–CH3), 1.49 (s, 12H, Ar–
CH–(CH3)2), 1.12 (s, 3H, imidazolium-N–CH2–CH2–CH2–
CH3). 13CNMR (DMSO-d6, d, ppm): 176.42 (N=C–CH3),
163.19 (Py–C(4)), 157.11 (Ar–C(7)), 152.11 (Py–C(5)),
143.11 (imidazolium–CH), 140.59 (Ar–C(10)), 137.53
(Py–C(2)), 135.13 (Ar–C(8) and At–C(12)), 134.21 (Ar–
C(9) and Ar–C(11)), 131.86 (Py–C(3), 129.22 (imidazoli-
um-CH), 128.14 (imidazolium-CH), 125.96 (Py–C(1)),
48.14 (imidazolium-N–(CH2–CH2–CH2–CH3)4), 37.04
(imidazolium-CH3), 34.22 (imidazolium-N–(CH2–CH2–
CH2–CH3)4), 31.96 (imidazolium-N–(CH2–CH2–CH2–
CH3)4), 29.19 (Ar–CH–(CH3)2), 23.54 (Ar–CH–(CH3)2),
20.12 (imidazolium-N–(CH2–CH2–CH2–CH3)4), 15.19
Complex C6
Yield 84%; m.p. 301 °C (decomposition); pale red; FTIR
(ATR, m, cm-1) 1,586 (C=N), 1,180 (S=O). 1HNMR
(DMSO-d6, d, ppm): 8.96 (d, 1H, J = 6.1 Hz, Py–(C(4)-
H), 8.12 (m, 2H, Ar–H), 7.89 (m, 3H, Py–H), 2.86 (m, 2H,
Ar–CH–(CH3)2), 2.33 (s, 3H, N=CH–CH3), 1.32 (d, 12H,
J = 6.9 Hz, Ar–CH–(CH3)2). 13CNMR (DMSO-d6, d, ppm):
161.22 (N=C–CH3), 155.42 (Py–C(6)), 151.77 (Py–C(4)),
142.19 (Ar–C(7)), 137.55 (Py–C(5)), 131.21 (Ar–C(10)),
123

Transition Met Chem (2012) 37:109–116
113
(imidazolium-N–(CH2–CH2–CH2–CH3)4). LC–MS (API-
ES) 676.1. Calcd for C27H38Cl2N4O3PdS: C, 48.0; H, 5.7;
Cl, 10.1; N, 8.3. Found: C, 48.1; H, 6.3; Cl, 9.9; N, 7.8.
spectra, the characteristic imine proton resonances of 1, 2,
and 5 were observed between d 8.84 and 8.73 ppm as a sharp
singlet. Diagnostic of chelating ligand coordination in these
complexes is the downfield shift in the 1HNMR spectra of the
imine signal by 0.3–0.4 ppm upon coordination to the Pd(II)
centre. Similar shifts have been observed in related com-
pounds [18, 25, 30–32]. The imine signals in the 13CNMR
spectra also show a downfield shift of 10.1–16.3 ppm upon
metal coordination. For ligands 2, 4, 6, 7, and 8, the imine is
substituted with a methyl group. The 1HNMR resonances of
these –CH3 groups were observed at 2.17, 2.19, 2.16, and
2.17 ppm, respectively. After coordination, the corre-
sponding protons in C2, C4, C6, C7, and C8 experience a
downfield shift between 0.18 and 0.10 ppm as expected [32].
General procedure for Suzuki coupling reaction
A mixture of the required complex (0.1% mol), aryl bro-
mide (1 mmol), arylboronic acid (1.3 mmol), powdered
K2CO3 (2 mmol), and water (5 mL) was placed in a Teflon
screw capped Schlenk tube equipped with a magnetic
stirrer. The reaction mixture was stirred at the desired
temperature or in a microwave oven for the required
reaction time. The mixture was then diluted with water
(15 mL) and extracted with CH2Cl2. The combined organic
phase was dried over anhydrous Na2SO4 and solvent was
evaporated under vacuum. The product was isolated by
flash chromatography. The amount of the product shown is
the average of two runs. All products were characterized by
1HNMR spectroscopy.1
Catalysis of the Suzuki reaction
To investigate the catalytic properties of these complexes,
the reaction of bromobenzene with phenylboronic acid in
aqueous phase was selected as a model reaction. K2CO3
was chosen for the base because it has been shown to be
effective in related systems [33]. Reactions were carried
out under both microwave irradiation and conventional
heating. Terabutylammonium bromide (TBAB) was used
as a phase transfer reagent for all the reactions except C7.
The results are summarized in Table 1. For the sake of
comparison, the active catalysts PdCl2 and Li2PdCl4 were
also tried under the optimized conditions. Because of their
insolubility in aqueous media, the yields for these com-
plexes were low (Table 1, entries 1,2). When water-soluble
ligand precursors were combined with ligand-free Pd spe-
cies to generate the catalyst in situ, yields were increased to
62–98% in aqueous media. The effect of methyl substitu-
ents on the reaction is clearly shown by Table 1. Com-
parison of entry 3 with entry 5, and entry 4 with entry 6
shows that catalytic activity is increased with additional
–CH3 groups. In C5 and C6 where the CH3 groups are
replaced with i-Pr groups, the catalytic activity increased to
89% yield. The effects of ionic groups were also investi-
gated. Complex C7, for which the NBu?4 group behaves as
counter-ion for SO3-, showed the highest activity with
98% conversion in 5 min under microwave irradiation
without adding TBAB. Complex C8, for which 1-butyl-3-
methylimidazolium [Bmim] was used as an additive,
showed similar activity to C7 plus TBAB. The high
activities of both complexes can be explained by the
favorable effect of bulky groups. Also, we found that
microwave heating is more efficient and faster (24-fold)
than conventional heating.
Results and discussion
Characterization of the ligands and complexes
All the ligands and complexes were characterized by a
combination of elemental analyses, LC–MS, FTIR, 1HNMR
and 13CNMR. LC–MS analyses were run in the negative ES
mode. In general for Schiff bases, the m(C=N) frequency
decreases after coordination [29]. For the free ligands,
m(C=N) was observed between 1,652 and 1,629 cm-1, while
in the complexes m(C=N) was shifted to 1,592–1,539 cm-1
,
consistent with coordination of the nitrogen. In the 1HNMR
1
(1)Biphenyl d 7.48–7.46 (m, 4H), 7.31–7.26 (m, 4H), 7.21–7.18 (m,
2H). (2) 4-Methylbiphenyl d 7.56 (d, 2H, J = 7.9 Hz), 7.41 (d, 2H,
J = 8.0 Hz), 7.33 (t, 2H, J = 7.5 Hz), 7.19 (t, 1H, J = 8.6 Hz), 7.11
(d, 2H, J = 7.4 Hz), 2.29 (s, 3H).(3) 2-Methylbiphenyl d 7.41–7.33
(m, 2H), 7.31–7.29 (m, 3H), 7.27–7.21 (m, 4H), 2.19 (s, 3H).(4)
4-acetylbiphenyl
d 8.01 (d, 2H, J = 8.1 Hz), 7.61 (d, 2H,
J = 8.8 Hz), 7.59 (d, 2H, J = 8.6 Hz), 7.55–7.34/m, 3H), 2.61 (s,
3H).(5) 4-methoxybiphenyl d 7.51–7.48 (m, 4H), 7.41 (t, 2H,
J = 6.9 Hz), 7.21 (t, 2H, J = 7.9 Hz), 6.88 (d, 2H, J = 8.9 Hz),
3.77 (s, 3H).(6) 2,6-dimethylbiphenyl d 7.51 (d, 2H, J = 6.9 Hz),
7.39 (m, 2H), 7.11 (m, 1H), 6.91 (m, 3H), 2.41 (s, 6H). (7)
4-nitrobiphenyl d 8.25 (s, 1H), 8.03 (d, 1H, J = 7.9 Hz), 7.86 (d, 1H,
J = 7.6 Hz), 7.31 - 7.24 (m, 3H), 7.19–7.11 (m, 3H).(8) 4-Methyl-40
methylbiphenyl d 7.53 (d, 4H, J = 7.7 Hz), 7.14 (d, 4H, J = 8.1 Hz).
(9) 4-Methyl-20 methylbiphenyl d 7.41 (d, 3H, J = 7.7 Hz), 7.09 (m,
5H), 2.28 (s, 6H). (10) 4-chloro-4-methoxybiphenyl d 7.49–7.43 (m,
4H), 7.32 (d, 2H, J = 8.6 Hz), 6.73 (d, 2H, J = 8.5 Hz), 3.77 (s, 3H).
(11) 4-Hidroxybiphenyl d 7.48 (d, 2H, J = 6.9 Hz), 7.36–7.29 (m,
5H), 6.71 (d, 2H, J = 7.3 Hz), 5.32 (s, 1H, br). (12) 4-acetylbiphenyl
d10.09 (s, 1 h, br), 8.14 (d, 2H, J = 7.9 Hz, 7.74 (d, 2H, J = 8.3 Hz),
7.54 (m, 2H), 7.23 (m, 3H). (13) 2-methyl-4-benzoicacid d 10.09 (s,
1H), 8.14 (d, 2H, J = 7.9 Hz), 7.74 (d, 2H, J = 8.3 Hz), 7.54 (m,
2H), 7.23 (m, 3H). (14) 4-Chlorobiphenyl d 7.55–7.32 (m, 9H).
Base optimization is a crucial consideration in these
reactions. The base generates an intermediate compound
with Pd(II) that undergoes transmetallation with boronic
acid derivatives. All the complexes were tried with several
123

114
Transition Met Chem (2012) 37:109–116
Table 1 Effect of complexes on Suzuki coupling reaction
Entry
Catalyst
Yield
80 °C
MW
1
2
3
4
5
6
7
8
9
10
PdCl2
Li2PdCl4
C1
24
19
53
67
54
72
84
88
96
91
31
28
62
71
61
77
89
89
98
93
C2
C3
C4
C5
C6
C7a
C8
Reaction conditions: 1 mmol of bromobenzene, 1.3 mmol of phenylboronic acid, 2 mol of K2CO3, 0.1 mol % catalyst, 80 °C, with conventional
heating or 850 W microwave irritation, 5 mL of H2O, 0.5 mmol TBAB. Reaction time = 2 h for conventional heating, 5 min for microwave
irradiation
a
0.5 mmol TBAB is not added
bulky groups on the Pd(0) intermediates, as reported sev-
eral times recently [35]. Sterically hindered aryl bromides
were less reactive, and these gave 66–69% yield in 2 h
with conventional heating or 72–78% yield in 5 min. with
microwave irradiation with both catalysts (Table 3, entry
6). As expected, water-soluble aryl bromides gave good
yields with both catalysts (Table 3, entries 12–17). Com-
plexes C7 and C8 were tried to couple phenyl chloride with
phenylboronic acid (Table 3, entry 16–18), but the yields
were very low and some homocoupling products were also
obtained.
Table 2 Time-dependent optimization of base for Suzuki reaction
using C1
Entry
Base
Yielda
80 °C
MW
1
2
3
3
4
5
K2CO3
Cs2CO3
NaHCO3
Na2CO3
NaOH
53
51
38
44
47
7
62
63
42
51
49
11
NEt3
Reaction conditions: 1 mmol of bromobenzene, 1.3 mmol of phen-
ylboronic acid, 0.1 mol % catalyst, 80 °C 2 h or 850 W 5 min.
microwave irritation, 5 mL of H2O, 0.5 mmol tetrabutylammonium
bromide (TBAB)
Catalyst recycling
a
The potential recyclability of the catalysts was examined
using the general experimental conditions mentioned in
the Experimental section. When conventional heating was
used, the catalysts decomposed to palladium black over
time; hence, recyclability experiments were tried only
with microwave irradiation. The reaction was carried out
in water under 850 W microwave irradiation for 5 min. In
this time period, according to the 1HNMR spectra, the
catalysts did not decompose to Pd-black. After cooling to
room temperature, the organic products were extracted
with dichloromethane and yields were determined by GC.
The aqueous phase was then used for the next reaction
cycle. Even under microwave conditions, the catalyst
slowly started to decompose to Pd-black by the second
GC yield after 2 runs
bases and in Table 2, the experimental results for complex
C1 are summarized because this complex gave the best
distinction between bases. Overall, K2CO3 and Cs2CO3
gave the best catalytic activity. KOH, Na2CO3, and
NaHCO3 were also effective but slightly inferior [34].
Hence, being cheap and effective, K2CO3 was chosen as
the base for further studies.
Table 4 summarizes the effects of complexes C7 and C8
on the Suzuki reaction in water. In general, these catalysts
were very effective when electron poor or electron-rich
aryl bromides were used (Table 3, entry 4–5). The high
activity of C7 and C8 was expected because of the effect of
123

Transition Met Chem (2012) 37:109–116
115
Table 3 Suzuki coupling of various aryl halides with arylboronic acid
Entry
X
Y
Z
Isolated yield (%)b,c
C7
Isolated yield (%)b,d
C7
C8a
C8a
1
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Br
Cl
Cl
Cl
H
H
96
90
87
84
91
69
83
87
85
79
82
84
87
81
84
89
7
91
88
84
81
88
66
77
82
79
72
78
79
82
73
79
81
5
98
97
96
93
94
78
93
94
92
94
96
92
95
94
97
91
14
16
8
93
91
92
89
92
72
96
92
91
89
94
92
91
90
91
88
11
13
8
2
4-Me
H
3
2-Me
H
4
COMe
OMe
H
5
H
6
2,6-diMe
4-NO2
4-Me
H
7
H
8
4-Me
4-Me
4-Cl
H
9
2-Me
10
11
12
13
14
15
16
17
18
19
4-OMe
4-OH
4-COOH
4-COOH
4-OH
4-COOH
4-OH
4-NO2
H
H
4-Me
4-Me
4-F
4-F
H
H
9
7
4-Me
H
8
5
Reaction conditions: 1 mmol of aryl halide, 1.3 mmol of arylboronic acid, 0.1 mol % catalyst, 5 mL of H2O
a
0.5 mmol tetrabutylammonium bromide (TBAB)
b
GC yield after 2 runs
c
80 °C 2 h
850 W 5 min, microwave irradiation
d
Table 4 Recycling of C4 and C6
C7
C8a
93
C7
C8a
65
C7
C8a
27
C7
C8a
9
C7
C8a
–
Cycles (n)/time (min)
Yield (%)b
0/5 min
98
1/5 min
72
2/5 min
39
3/5 min
16
4/5 min
4
Reaction conditions: 1 mmol of bromobenzene, 1.3 mmol of phenylboronic acid, 0.1 mol % catalyst, 850 W 5 min. microwave irradiation,
5 mL of H2O
a
0.5 mmol tetrabutylammonium bromide (TBAB)
b
GC yield after 2 runs
cycle, and after the fourth cycle, it was fully decomposed
(Table 4). The main reason for decomposition of the
catalysts is probably the ready hydrolysis of the imine
C=N bonds in water. To probe the stability of the com-
plexes, 1HNMR spectra of complexes C1–C8 were
recorded for every 30 min. in D2O. Catalysts C1 to C6
were stable in water for approximately 2 h. On the other
hand, C7 and C8 were stable for 5 h before decomposi-
tion to Pd-black, because of the stabilizing effects of the
bulky groups [35]. Hydrolysis of the Schiff base was
evidenced by the appearance of aldehyde signals in the
1HNMR spectra.
123

116
Transition Met Chem (2012) 37:109–116
11. ten Brink GJ, Arends IWCE, Sheldon RA (2000) Science
287(5458):1636–1639
Conclusion
12. Herwig J, Zimmermann B, Beller M (1999) Angew Chem Int
Edit 38(16):2372–2375
13. Casalnuovo AL, Calabrese JC (1990) J Am Chem Soc 112(11):
4324–4330
14. Shaughnessy KH, Huang RC (2006) Organometallics 25(17):
4105–4112
15. Falvello LR, Najera C, Gil-Molto J, Karlstrom S (2003) Org Lett
5(9):1451–1454
16. Najera C, Botella L (2002) Angew Chem Int Edit 41(1):179–181
17. Sun HJ, Zhou J, Guo XM, Tu CZ, Li XY (2009) J Organomet
Chem 694(5):697–702
18. Mentes A, Hanhan ME (2008) Transit Metal Chem 33(1):91–97
19. Mentes A, Sezek S, Hanhan ME, Buyukgungor O (2007) Turk J
Chem 31(6):667–676
20. Lemaire M, Fache F, Schulz E, Tommasino ML (2000) Chem
Rev 100(6):2159–2231
21. Togni A, Venanzi LM (1994) Angewandte Chemie Int Edition
Engl 33(5):497–526
Pyridyl—imine and diimine type ligands are very useful
for designing homogeneous catalysts because of their easy
tuning properties. On the other hand, the main disadvan-
tage of these ligands is their easy decomposition in water.
Here, a series of ionic asymmetric pyridyl imine ligands
and their Pd(II) complexes were synthesized. As expected,
microwave irradiation proved faster and more efficient than
conventional heating. With microwave irradiation, the
catalysts could be used for up to four cycles. Bulky groups
on the ligand increased the effectiveness of these catalysts.
All of the catalysts tolerate a broad range of functional
groups on the phenyl ring. The catalysts decomposed to
give aldehydes and Pd-black. The decomposition chemistry
of these metal complexes is currently under further
investigation.
22. Laine TV, Piironen U, Lappalainen K, Klinga M, Aitola E,
Leskela M (2000) J Organomet Chem 606(2):112–124
23. Borriello C, Ferrara ML, Orabona I, Panunzi A, Ruffo F (2000) J
Chem Soc Dalton (15):2545–2550
24. Ferrara ML, Orabona I, Ruffo F, Funicello M, Panunzi A (1998)
Organometallics 17(18):3832–3834
Acknowledgments This work was supported by Research Board of
Zonguldak Karaelmas University (BAP-2011-10-03-011 and BAP-
2011-10-03-012).
25. Kundu A, Buffin BP (2001) Organometallics 20(17):3635–3637
26. Hanhan ME (2008) Appl Organomet Chem 22(5):270–275
27. Sun HJ, Guo XM, Zhou J, Li XY (2008) J Organomet Chem
693(25):3692–3696
References
28. Kanetani F, Yamaguchi H (1981) B Chem Soc Jpn 54(10):
3048–3058
29. Chec EK, Kubiak M, Glowiak T, Ziolkowski JJ (1998) Transit
Metal Chem 23(5):641–643
30. Yang KY, Lachicotte RJ, Eisenberg R (1997) Organometallics
16(24):5234–5243
31. vanAsselt R, Elsevier CJ, Amatore C, Jutand A (1997) Organo-
metallics 16(3):317–328
1. Najera C, Botella L (2002) J Organomet Chem 663(1–2):46–57
2. Rossi R, Bellina F, Carpita A (2004) Synthesis 2004(15):2419–
2440
3. Liebscher J, Yin LX (2007) Chem Rev 107(1):133–173
4. Miyaura N, Suzuki A (1995) Chem Rev 95(7):2457–2483
5. Suzuki A (1999) J Organomet Chem 576(1–2):147–168
6. Alonso F, Beletskaya IP, Yus M (2008) Tetrahedron 64(14):
3047–3101
32. Buffin BP, Kundu A (2003) Inorg Chem Commun 6(6):680–684
33. Milstein D, Weissman H (1999) Chem Commun (18):1901–1902
34. Liu P, Zhang WZ, He R (2009) Appl Organomet Chem
23(4):135–139
7. Mignani G, Corbet JP (2006) Chem Rev 106(7):2651–2710
8. Kertesz M, Choi CH, Yang SJ (2005) Chem Rev 105(10):
3448–3481
9. Buchwald SL, Billingsley KL, Anderson KW (2006) Angew
Chem Int Edit 45(21):3484–3488
35. Thimmaiah M, Fang S (2007) Tetrahedron 63(29):6879–6886
10. Buchwald SL, Wolfe JP, Tomori H, Sadighi JP, Yin JJ (2000) J
Org Chem 65(4):1158–1174
123