The development of phosphinoamine-Pd(II)-imidazole complexes: Implications in room-temperature Suzuki-Miyaura cross-coupling reaction

Article abstract of DOI:10.1002/aoc.3029

The reaction of N-methylimidazole (N-MeIm) and N-butylimidazole (N-BuIm) with the complexes [PdCl₂(PPh₂py-P,N)] and [PdCl ₂(PPh₂Etpy-P,N)] in the presence of NH₄PF ₆ under N₂ at

Full text of DOI:10.1002/aoc.3029

Full Paper
Received: 20 December 2012
Revised: 4 June 2013
Accepted: 4 June 2013
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3029
The development of phosphinoamine–
Pd(II)–imidazole complexes: implications in
room-temperature Suzuki–Miyaura cross-
coupling reaction
Geetika Borah*, Devajani Boruah, Gayatri Sarmah,
‡
Saitanya K. Bharadwaj and Utpal Bora
The reaction of N-methylimidazole (N-MeIm) and N-butylimidazole (N-BuIm) with the complexes [PdCl2(PPh2py–P,N)] and
[PdCl2(PPh2Etpy–P,N)] in the presence of NH4PF6 under N2 at room temperature afforded four new cationic Pd(II) complexes
[PdCl(PPh2py–P,N)(N-MeIm)](PF6) (1), [PdCl(PPh2py–P,N)(N-BuIm)](PF6) (2), [PdCl(PPh2Etpy–P,N)(N-MeIm)](PF6) (4) and
[PdCl(PPh2Etpy-P,N)(N-BuIm)](PF6) (5) in good yields, where PPh2py is 2-(diphenylphosphino)pyridine and PPh2Etpy is
2-{2-(diphenylphosphino)ethyl}pyridine). The complexes were fully characterized. The catalytic activities of these com-
plexes were investigated for Suzuki–Miyaura cross-coupling reactions at room temperature. Complex 2 exhibited excel-
lent activity compared to other analogs. Copyright © 2013 John Wiley & Sons, Ltd.
Supporting information may be found in the online version of this article.
Keywords: palladium(II) cationic complex; imidazole; Suzuki–Miyaura cross-coupling
cross-coupling reactions, including Suzuki–Miyaura.[24] Moreover,
Introduction
other N–N and P–N type ligands have also been used in the
Suzuki–Miyaura reaction.[25] However, to the best of our knowledge
The coordination chemistry of platinum group metals occupies a
special position in the field of catalysis for different chemical
transformations.[1–8] Among the various chemical transformations
triggered by platinum group metals, palladium-catalyzed
Suzuki–Miyaura reaction is vital and widely used process in
chemistry as it generates various bio-active natural products,
agrochemicals, pharmaceuticals, polymers, etc.[9–11] The catalytic
cycle of the Suzuki–Miyaura reaction involves activation of C–X
bond by Pd(0) or Pd(II) and the efficiency of the catalytic system
has been achieved by changing the ligand environment around
palladium.[12–14] Along with other ligand environments, significant
efforts have been also made in designing novel catalytic systems
of Pd(0) or Pd(II) complexes with phosphine-based ligands like
tertiaryphosphines, hemilabile-type phosphines, sterically
crowded biphenyl-type phosphines, and imidazole- and
imidazolium-functionalized phosphines.[15–21] Although complexes
containing such ligands often show excellent activity, moisture sen-
sitivity,[22] and the requirement for high temperature and
undesirable solvents such as dimethyl formamide, N-
methylpyrrolidine and dimethoxyethane,[23] are some of the
major drawbacks. Therefore the development of an eco-friendly
and technologically feasible newer approach for designing
highly active catalytic systems in environmentally preferred
solvents for the Suzuki–Miyaura cross-coupling reaction is
highly desirable.
no phosphinoamine–Pd(II)–imidazole type complexes have been
synthesized and investigated for their catalytic efficiency in
Suzuki–Miyaura reactions. Herein, we wish to report the synthesis
and characterization of four new cationic palladium(II) complexes
1, 2, 4 and 5 containing both phosphinoamine and imidazole
ligands. The catalytic efficiency of the synthesized complexes in
Suzuki–Miyaura cross-coupling reactions is also reported here.
Experimental
General Information
PdCl2, tetrabutylammonium perchlorate (TBAP) and 1,5-
cyclooctadiene (COD) were purchased from Acros Chemicals.
The ligands 2-(diphenylphosphino)pyridine (PPh2py) and
2-[2-(diphenylphosphino)ethyl]pyridine)]
(PPh2Etpy) were
purchased from Aldrich (USA). NH4PF6, N-methylimidazole,
N-butylimidazole were procured from Fluka and used without
further purification. The precursor complexes [PdCl2(COD)][26]
*
Correspondence to: Geetika Borah, Department of Chemistry, Dibrugarh
University, Dibrugarh 786004, Assam, India. E-mail: geetikachem@yahoo.co.in
Cheap, commercially available imidazoles are an important class
of N-donor ligands with strong σ-donor ability and weak π-acceptor
capability. Several air-stable Pd(II)-imidazole complexes have been
developed and documented to be efficient catalysts in various
‡Current address: Department of Chemical Science, Tezpur University, Tezpur
784028, Assam, India
Department of Chemistry, Dibrugarh University, Dibrugarh 786004, Assam, India
Appl. Organometal. Chem. (2013)
Copyright © 2013 John Wiley & Sons, Ltd.

G. Borah et al.
and [PdCl2(PPh2py-P,N)][27] were synthesized according to the
published procedure. Another previously reported precursor
complex [PdCl2(PPh2Etpy-P,N)] (3) was prepared by using [PdCl2
(COD)] instead of [PdCl2(PhCN)2].[28] The solvents used were of
analytical grade and distilled and dried over 4 Å molecular sieves.
All reactions were carried out under nitrogen atmosphere.
The melting points of the complexes were determined using
Buchi B450 melting point apparatus. Elemental analyses were
performed by Elementar Vario EL III Carlo Erba 1108. IR spectra
(4000–250°cmꢀ1) were recorded in KBr on a Shimadzu Prestige-21
FT-IR spectrophotometer. UV–visible spectra were recorded in
dichloromethane /acetonitrile using a 1°cm3 quartz cell in the
range 800–200 nm on Shimadzu-Graphicord UV-1700 spectrome-
ter. The conductivity of the complexes was measured in
C
21H20N3P2ClF6Pd: C, 39.81%; H, 3.16%; N, 6.64%. Found: C,
39.30%; H, 3.02%; N, 6.34%; Λ = 84 Ωꢀ1 cm2 molꢀ1; IR (KBr, cmꢀ1):
3106 (m), 3054 (m), 2925 (m), 2867 (m), 1568 (m) νpy(C¼N), 843 (s)
ν
ν
P–F, 557 (m) δP–F–P, 536 (m), 514 (m) νPd–py, 432 (w) νPd–P, 355 (s)
Pd–Cl, 272 (m) νPd–Im; UV–visible: λmax (CH2Cl2, nm): 229, 269 and
344. H NMR (CDCl3, δ ppm): 3.50 (s, 3H, CH3), 7.06 (s, br, 1H, H5′),
7.40–7.48 (m, 10 H, Ph), 7.60 (s, br, 1H, H4′), 7.88–7.79 (m, 2H, H3,5),
8.15 (s, 1H, H2′), 8.24 (m, 1H, H4) and 8.75(d, J = 4.0 Hz, 1H, H6);
13C NMR (CDCl3, δ ppm): 33.4 (C–CH3), 120.10 (C5), 120.28 (C3),
128.68 (2C, C–Ph), 129.40 (4C, C–Ph), 129.51 (2C, C–Ph), 133.58
(4C, C–Ph), 134.24 (C4), 139.27(C4′), 139.31(C5′), 141.25(C6),
152.34(C2′), 158.23(C2); 31P{1H}NMR (CDCl3, δ ppm): ꢀ9.98 (s,1P)
and ꢀ175.60 (spt, PFꢀ6 , JPF = 712 Hz). ESI/MS (m/z): 656 [M+Na]+.
1
dichloromethane–acetonitrile (10ꢀ2 M) by using
a digital
Synthesis of [PdCl(PPh2py–P,N)(N-BuIm)](PF6) (2)
conductivity bridge, type ELICO-CM-180 at 25°C. The 1H, 13C
and 31P{1H}NMR spectra were recorded in CD3CN/CDCl3
solutions operating at 400.13, 100.62 and 161.98 MHz respectively
on a Bruker Advance II 400 NMR spectrometer and chemical
shifts were reported relative to tetramethylsilane for 1H, 13C
NMR spectra and 85% aqueous H3PO4 for 31P{1H}NMR spectra
Complex 2 was prepared as described for complex 1, using
N-BuIm (0.038 g, 0.301 mmol), [PdCl2(PPh2py–P,N)] (0.130 g,
0.295 mmol) and NH4PF6 ( 0.049 g, 0.301 mmol) to yield the
product as a reddish-brown solid. Yield: 90% (0.179 g), m.p. 68°
C; Anal. Calcd for C24H26N3P2ClF6Pd: C, 42.60%; H, 3.85%; N,
6.21%, Found: C, 42.10%; H, 3.45%; N, 6.12%. Λ = 89 Ωꢀ1 cm2
molꢀ1. IR (KBr, cmꢀ1): 3108 (m), 3054 (m), 2928 (m), 2865 (m),
1
as standard. The H NMR assignments were done with respect
to the labeling chart (Scheme 1). The electrospray ionization
(ESI) (+) mass spectra were recorded on a Waters ZQ-4000 mass
spectrometer in CH3CN/CHCl3. A cyclic voltammetry study of the
complexes was performed in acetonitrile solution with a CH In-
strument (Model 600C) using platinum as working electrode
and Ag/AgCl as the reference electrode with 0.1 M TBAP as the
1570 (m) νpy(C¼N), 843 (s) νP–F, 557 (m), 536 (s), 515 (m) νPd–N
,
430 (m) νPd–P, 351 (s) νPd–Cl, 278 (ms) νPd–Nim; UV–visible: λmax
(CH2Cl2, nm): 228 and 270; 1H NMR (CDCl3, δ ppm): 0.88–0.95
(m, 5H, H9′,8′), 1.29–1.33 (m, 2H, H7′), 3.88–3.94 (m, 2H, H6′), 6.84
(s, br, 1H, H5′), 7.60 (s, br, 1H, H4′), 7.40–7.48 (m, 10H, Ph),
7.87–7.79 (m, 2H, H3,5), 8.21 (s, 1H, H2′) 8.25 (m, 1H, H4) and
8.75(d, J = 4.4Hz, 1H, H6); 13C NMR (CDCl3, δ ppm): 13.47 (C9′),
19.24 (C8′), 32.51 (C7′), 48.03 (C6′), 120.23 (C5), 120.38 (C3), 128.13
(2C,C–Ph), 129.35 (4C, C–Ph), 129.12(2C, C–Ph), 131.98(4C, C–Ph),
132.84 (C4′), 138.95 (C5′), 139.21 (C4), 140.87 (C6), 153.04 (C2′),
159.23 (C2); 31P{1H}NMR (CDCl3, δ ppm): ꢀ9.76 (s,1P) and ꢀ176.57
ppm (spt, PFꢀ6 , JPF = 712 Hz ); ESI/MS (m/z): 554 [M+NaꢀPF6]+.
supporting electrolyte at different scan rates 30–200 mV s ꢀ1
.
Synthesis of [PdCl(PPh2py–P,N)(N-MeIm)](PF6) (1)
To a suspension of [PdCl2(PPh2py–P,N)] (0.13 g, 0.295 mmol) in
20 ml dichloromethane, NH4PF6 ( 0.05 g, 0.301 mmol ) was added
and stirred at room temperature for 2 h under N2, resulting a
wine-red mixture, to which a solution of N-MeIm (0.025 g,
0.301 mmol) in dichloromethane was added with constant
stirring. The color of the solution changed sharply from
wine-red to brown. The resulting mixture was stirred for 14 h
at room temperature. The white precipitate of NH4Cl was
separated by filtration and the filtrate was evaporated in vacuo.
The sticky residue was washed with petroleum ether and
recrystallized from dichloromethane to yield the product as a
brown solid. Yield: 80% (0.15 g); m.p. 60°C; Anal. Calcd for
Synthesis of [PdCl2(PPh2Etpy–P,N)] (3)
To a solution of [PdCl2(COD)] (0.29 g, 1.018 mmol) in 20 ml
dichloromethane, a solution of PPh2Etpy (0.296 g, 1.016 mmol)
in 15 ml dichloromethane was added drop by drop. The reaction
mixture was stirred under nitrogen at room temperature for 18 h,
during which the color of the mixture changed from yellow-orange
to pale-yellow. The volume was reduced to 2 ml and 15 ml Et2O was
added under vigorous stirring to precipitate out the product. The
solid mass was separated by filtration, washed with Et2O several
times and dried in a desiccator to collect the product as a
pale-yellow solid. Yield: 93% (0.444 g, 0.947 mmol). These analytical
data are in accordance with those reported in the literature.[28]
Synthesis of [PdCl(PPh2Etpy–P,N)(N-MeIm)](PF6) (4)
A
solution of N-MeIm (0.020 g, 0.241 mmol) in 10 ml
dichloromethane was added drop-wise to a well-stirred suspension
of [PdCl2(PPh2Etpy–P,N)] (0.11 g, 0.235 mmol) and NH4PF6
(0.039 g, 0.239 mmol) in dichloromethane. The resulting mixture
was stirred at room temperature for 16 h under N2. After that the
yellow mixture was separated from the white precipitate of
NH4Cl by filtration. The filtrate was evaporated to dryness in
vacuo. The sticky residue was washed with petroleum ether
and recrystallized from dichloromethane to obtain the product
as a pale-yellow solid. Yield: 83% (0.129 g), m.p. 128 °C; Λ = 83
Scheme 1. Numbering of H and C-atom positions in pyridyl, imidazole
and alkyl group.
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2013)

Pd(II)-catalysed Suzuki–Miyaura cross-coupling reaction
Ωꢀ1 cm2 molꢀ1, Anal. Calcd for C23H24N3P2ClF6Pd: C, 41.75%; H,
3.63%; N, 6.35%; Found: C, 41.43%; H, 3.33%; N, 6.12%; IR (KBr,
cmꢀ1): 3106 (m), 3054 (m), 2925 (m), 2867 (m), 2780 (m), 1610
(m) νpy(C¼N), 841 (s) νP–F, 559 (s), 534 (m), 516 (w) νPd–N, 449(w)
General Information Concerning Catalytic Experiments
Suzuki–Miyaura cross-coupling reactions were carried out under
aerobic conditions at room temperature. The progress of the
reactions was monitored by thin-layer chromatography (TLC)
using aluminium-coated TLC plates (Merck) under UV light. The
products were purified by column chromatographic technique
using silica gel (60-120 mesh). The various products separated
were characterized by melting point, 1H and 13C NMR and mass
spectral data and compared with the authentic samples.
νPd–P, 355 (s) νPd–Cl, 272 (m) νPd–Nim; UV–visible: λmax (CH2Cl2,
1
nm): 230, 270 and 346; H NMR (CDCl3, δ ppm): 2.75 (s, br, 2H,
H8), 3.37 (s, 3H, CH3), 4.26–4.29 (m, 2H, H7), 6.45–6.48 (m, 2H,
H3,5), 6.86 (s, br, 1H, H5′), 7.16 (s, br, 1H, H4′), 7.38–7.42 (m, 4H,
Ph), 7.71–7.76 (m, 6H, Ph), 7.84–7.87 (m, 1H, H4), 8.42 (s, 1H, H2′)
and 9.11 (s, br, 1H, H6); 13C NMR (CDCl3, δ ppm): 30.95 (C–CH3),
33.40 (C7), 33.42 (C8), 120.12 (C5) 120.18 (C3), 129.04 (C4),
129.10 (2C,C–Ph), 129.21 (4C,C–Ph), 129.47(2C,C–Ph), 133.14
(4C,C–Ph), 139.23 (C4′), 139.31 (C5′), 140.15 (C6), 151.08 (C2′),
158.72 (C2); 31P{1H} NMR (CDCl3, δ ppm): 14.38 (s), ꢀ160.85 (spt,
PFꢀ6 , JPF = 712 Hz). ESI/MS (m/z): 516 [MꢀPF6]+.
General Procedure for Suzuki–Miyaura Reactions of Aryl Halides
A 50 ml round-bottom flask was charged with a mixture of aryl
halide (0.5 mmol), aryl boronic acid (0.55 mmol), base (1.5 mmol),
ethanol (2 ml) and palladium complex (1.5 mol%) and the
mixture was stirred at room temperature. After completion, the
reaction mixture was diluted with water (20 ml) and extracted
with ether (3 × 20 ml). The combined extract was washed with
brine (2 × 20 ml) and dried over Na2SO4. After evaporation of
the solvent under reduced pressure, the residue was
chromatographed (silica gel–ethyl acetate-hexane, 1:9) to obtain
the desired products. The products were confirmed by compar-
Synthesis of [PdCl(PPh2Etpy–P,N)(N-BuIm)](PF6) (5)
Complex 5 was synthesized following a similar procedure as that
for complex 4, using N-BuIm (0.030 g, 0.238 mmol), [PdCl2
(PPh2Etpy–P,N)] (0.11 g, 0.235 mmol) and NH4PF6 (0.039 g,
0.239 mmol). During stirring, the color of the solution changed
gradually from yellow to wine-red. After thorough washing with
petroleum ether a blood-red sticky compound was obtained,
which was recrystallized from dichloromethane. Yield: 79%
(0.130 g); m.p. 68 °C. Λ = 88 Ωꢀ1 cm2 molꢀ1. Anal. Calcd for
1
ing the melting point, H and 13C NMR and mass spectral data
with authentic samples (supporting information, Table S1).
Results and Discussion
C
26H30N3P2ClF6Pd: C, 44.32%; H, 4.26%; N, 5.97%; Found: C,
44.13% ; H, 4.03%; N, 5.71%; IR (KBr, cmꢀ1): 3108 (m), 3054 (m),
Synthesis and Characterization of [PdCl(PPh2py–P,N)
(N-MeIm)](PF6) (1)
2928 (m), 2865 (m), 1608 (m) νpy(C¼N), 842 (s) νP–F, 557 (s), 532
(m), 504 (w) νPd–N, 445 (m) νPd–P, 376 (s) νPd–Cl, 290 (ms) νPd–Nim
;
UV–visible: λmax (CH2Cl2, nm): 230, 268 and 321; 1H NMR (CDCl3,
δ ppm): 0.86–0.94 (m, 5H, H9′,8′), 1.25–1.27 (m, 2H, H7′), 2.71–2.77
(m, 2H, H8), 3.63 (t, J = 8.0, 2H, H6′), 4.20–4.24 (m, 2H, H7), 6.43–
6.47 (m, 2H, H3,5), 6.50 (s, br, 1H, H5′), 7.07 (s, br, 1H, H4′), 7.44–
7.51 (m, 4H, Ph), 7.69–7.76 (m, 6H, Ph), 7.77–7.82 (m, 1H, H4), 8.03
(s, 1H, H2′) and 9.00 (s, 1H, H6); 13C NMR (CDCl3, δ ppm): 13.45 (C9′–
CH3), 19.43 (C8′–CH2), 19.62 (C7′–CH2), 32.28 (C7–CH2), 32.68 (C8–
CH2), 48.33 (C6′–CH2), 120.10 (C5), 124.20 (C3), 128.68 (2C,C–Ph),
129.40 (4C,C–Ph), 129.51 (2C,C–Ph), 132.18 (4C,C–Ph), 133.04 (C4),
139.01 (C4′), 139.51 (C5′), 141.05 (C6), 152.98 (C2′), 159.83 (C2); 31P{1H}
NMR (CDCl3, δ ppm): 14.51 (s, PPh2), ꢀ160.79 (spt, PFꢀ6 , JPF = 712
Hz). ESI/MS (m/z): 577 [M+NH4ꢀPF6]+.
The reaction of neutral palladium complex [PdCl2(PPh2py–P,N)]
with N-MeIm on stirring in dichloromethane at room temperature
for 14 h under N2 led to the formation of [PdCl(PPh2py–P,N)
(N-MeIm)](PF6)(1) with 80% yield (Scheme 2). The elemental
analyses and ESI mass spectrum of the complex were in agreement
with the proposed formulation. The mass spectrum of complex 1
demonstrated a molecular ion peak of moderate intensity at
m/z = 656 [M+Na]+. The TGA data are also in agreement with
the proposed molecular formula of the complex. The TGA
of the complex exhibits major weight loss 73.8% (calcd 77.6%)
in the temperature range 43–338 °C, attributed to the removal
of PPh2py, N-MeIm and PFꢀ6 ion of the complex. The IR spectrum
Scheme 2. Synthesis of complexes 1, 2, 3, 4 and 5. Reagents and reaction conditions: (i) PPh2py, CH2Cl2, 0.5h, room temperature (r.t.), N2; (ii) NH4PF6, MeIm,
CH2Cl2, 14 h, r.t., N2; (iii) NH4PF6, BuIm, CH2Cl2 , 14 h, r.t., N2; (iv) PPh2Etpy, CH2Cl2, 18 h, r.t., N2; (v) NH4PF6, MeIm, CH2Cl2, 16 h, r.t., N2; (vi)
NH4PF6, BuIm, CH2Cl2, 16 h, r.t., N2.
Appl. Organometal. Chem. (2013)
Copyright © 2013 John Wiley & Sons, Ltd.
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G. Borah et al.
of 1 exhibited the characteristic peak for ν(Pd–N), ν(Pd–P) and
ν(Pd–Cl) vibrations, similar to that of the precursor complex
[PdCl2(PPh2py–P,N)][27,28] with a small shift. In the far-IR region
a new band at 272 cmꢀ1 could be assigned to Pd–Nimidazole
stretching vibration. The UV–visible spectrum of compound 1
in dichloromethane displayed three bands at 229, 269 and
344 nm, the former two presumably due to intraligand π→π*,
while the third one is due to n→π*[29–31] transitions with some
contribution of Clꢀ→Pd2+ charge transfer transition. Compared to
the starting compound [PdCl2(PPh2py–P,N)] (394 nm), the band for
n→π* transition was blue shifted, indicating a change of the ligand
environment about the Pd2+ ion.[32,33] The 31P{1H}NMR spectrum of
1 displayed a singlet at δ = ꢀ9.98 ppm. Compared with the neutral
parent compound [PdCl2(PPh2py–P,N)] (δ = ꢀ9.15 ppm), complex 1
shows an upfield shift of 0.83 ppm, indicating the formation of a
new complex. An upfield resonance in 1 is known to be diagnostic
of a four-membered chelate complex.[34,35] The N-MeIm ligand
has good electron-donating ability and its coordination might
increase the electron density on the metal center due to which
the degree of back-donation from metal to aminophosphine
ligand (dπ–pπ) increases in comparison with precursor
complex.[36] The enhanced back-donation might cause an upfield
shift of 0.83 ppm in 1 relative to the parent compound. The 31P
resonance of the PFꢀ6 counterion for complex 1 is observed at
UV–visible spectrum, complex 2 also displays two bands at
270 nm and 228 nm for intra-ligand π→π* and one at 342 nm
with a blue shift for n→π* transitions.[29–31] The 31P{1H}NMR
study (Fig. S1, supporting information) shows a singlet at δ =
9.76 ppm which was shifted upfield with respect to the neutral
parent compound [PdCl2(PPh2py–P,N)] (δ = ꢀ9.15 ppm). The
PFꢀ6 ion resonance for complex 2 was observed at δ = ꢀ176.57
1
ppm with JPF = 712 Hz.[37] The H NMR spectrum of complex 2,
in addition to aromatic protons, shows a singlet at δ = 8.21 ppm
for H2′ of N-BuIm (Scheme 1), which is shifted downfield com-
pared to free ligand. The electrochemical study of complex 2
revealed one irreversible reduction at ꢀ1.39 V (Δ Ep = 670 mV), sim-
ilar to complex 1, attributed to Pd(II)/Pd(0) redox couple.[38–40]
The shift of this reduction to a more negative value compared
to complex 1 might be due to higher electron-donating ability
of N-BuIm relative to N-MeIm ligand. The oxidation at +1.32 V
is assumed to lead to the formation of a Pd(IV).[40] The reduction
at ꢀ0.95 V with no corresponding oxidation during anodic
sweep clearly suggested a PPh2py-based process.
Synthesis and Characterization of [PdCl(PPh2Etpy–P,N)(N-
MeIm)](PF6) (4)
The reaction of N-MeIm with [PdCl2(PPh2Etpy–P,N)] (3) in
dichloromethane on stirring at room temperature for 16 h under
N2 atmosphere afforded complex 4 with 83% yield. Similar to
complexes 1 and 2, the elemental analyses and mass spectrum
are consistent with the proposed composition. The IR spectrum
revealed a slight shift of ν(Pd–N), ν(Pd–P) and ν(Pd–Cl) bands
compared to [PdCl2(PPh2Etpy–P,N)]. A new band in the far-IR
region at 272 cmꢀ1 could be assigned to ν(Pd–Nimidazole). The
UV–visible spectrum displays intra-ligands π→π* and n→π*
and are shifted towards blue compared to precursor complex
3, indicating a change in the ligand environment. The 31P{1H}
NMR spectrum showed a singlet at δ = 14.38 ppm, assignable to
the 31P nuclei of the PPh2Etpy. The signal associated with 31P
nuclei of the PPh2Etpy exhibited a significant upfield shift in
complex 4 in comparison to the other reported complexes,[42,43]
which might be due to an increase in electron density at the
metal center on coordination of N-MeIm ligand. Enhanced
electron density at the metal center increases the degree of back-
donation from metal to PPh2Etpy ligand. The signal for 31P nuclei
of the PFꢀ6 ion was detected as a septet at δ = ꢀ160.85 ppm with
1
δ = ꢀ175.60 ppm with JPF = 712 Hz.[37] The H NMR spectrum of
the complex exhibits a singlet at δ = 8.15 ppm due to the H2′
(Scheme 1) proton of N-MeIm with 0.75 ppm downfield shift
compared to free N-MeIm, clearly indicating coordination of
the imidazole through the N(3′) atom. In addition, two singlets
at δ = 7.06 ppm and δ = 7.60 ppm could be assigned to H4′ and
H5′ of N-MeIm, respectively. Complex 1 in its cyclic voltammogram
in acetonitrile exhibited one irreversible reductive response at
ꢀ1.18 V (Δ Ep = 510 mV). This feature could be assigned to the
Pd(II)/Pd(0) redox couple,[38–40] since complexes of π-acceptor
ligands undergo two-electron reduction with the stabilization of
Pd(0).[41] Extremely high peak-to-peak separation value
indicated that the redox process is quite non-Nernstian.
Moreover, a reduction peak was observed at ꢀ0.80 V with no
anodic response and may be assigned to PPh2py-based reduction.
During positive potential scan the oxidation at +1.52 V with no
peak reversal is assumed to lead to the formation of a Pd(IV),
accompanied by an increase in coordination number from 4 to
6.[40] Compared to the precursor complex [PdCl2(PPh2py–P,N)]
(Epc = ꢀ0.75 V) the reduction was shifted to a more negative
value, which may be due to the presence of electron-donating
N-MeIm ligand in 1.[38]
1
JPF = 712Hz.[37] In the H NMR spectrum two singlets at δ = 8.42
and 3.37 ppm with downfield shift are attributed to H2′ and N–CH3
protons. The 13C NMR spectrum shows characteristic resonances
for the aromatic, aliphatic as well as imidazole carbon. TGA also
supported the assigned formulation. Electrochemical study of
complex 4 demonstrates an irreversible reductive event at
ꢀ1.29 V (Δ Ep = 750 mV), which is shifted to a more negative
potential compared to starting compound [PdCl2(PPh2Etpy–P,N)]
(3) (Epc = ꢀ0.78 V). This feature is consistent with Pd(II)/Pd(0)
redox couple.[38–40] A ligand-based reduction was also observed
at ꢀ0.83 V with no peak reversal.
Synthesis and Characterization of [PdCl(PPh2py–P,N)
(N-BuIm)](PF6) (2)
Treatment of [PdCl2(PPh2py-P,N)] with N-BuIm on stirring in
dichloromethane at room temperature for 14 h under N2 led to
the formation of [PdCl(PPh2py–P,N)(N-BuIm)](PF6) (2) with 90%
yield (Scheme 2). Similar to complex 1, the elemental analyses
and mass spectrum are in good agreement with the above
composition. The mass spectrum of complex 2 shows a molecular
ion peak at 554, which correspond to [M+NaꢀPF6]+ ions. TGA
shows major weight loss of 75.8% (cald78.9%) in the temperature
range 178–394°C, consistent with the loss of PPh2py, BuIm and
PFꢀ6 ion, respectively. The IR spectrum in KBr shows a new band
at 278 cmꢀ1, consistent with Pd–Nimidazole vibration. Similar to
complex 1, all other bands are observed as expected.[37] In the
Synthesis and Characterization of [PdCl(PPh2Etpy–P,N)
(N-BuIm)](PF6) (5)
Complex 5 was prepared in a manner analogous to 4, where
N-BuIm reacted at 1:1 molar ratio with [PdCl2(PPh2Etpy–P,N)]
(3) to form [PdCl(PPh2Etpy–P,N)(N-BuIm)](PF6) (5) in good yield
(79%). The elemental analyses and mass spectrum are consistent
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Copyright © 2013 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. (2013)

Pd(II)-catalysed Suzuki–Miyaura cross-coupling reaction
with the assigned formulation. The fragmentation pattern of the
complex clearly indicates its mononuclear nature. TGA also
supports the above formulation. In the far-IR region, the band
at 290 cmꢀ1 is attributed to ν(Pd–Nimidazole). The presence of
PFꢀ6 ion was confirmed by the observation of ν(P–F) and δ(PFP)
vibrations at 842 and 557 cmꢀ1, respectively.[37] Similar to the
in the coupling reaction proceeded with a good yield of desired
hetero-coupling product (Table 1, entry 1), accompanied by a
trace amount of homodiaryl compound as a byproduct.
Formation of homodiaryl as side product in the Suzuki–Miyaura
reaction is very common. The use of complex 2 afforded the
desired product in improved yield within a shorter reaction time
compared to 1 (Table 1, entry 2). However, the use of complexes
4 and 5 significantly reduced the yield of the product even after
prolong reaction time (Table 1, entries 3 and 4). However, none
of the palladium metal complex was found to be effective when
aryl chloride was used as coupling partner. In order to compare
the efficiency of the Pd–imidazole complexes (1, 2, 4, 5) with
complex 3, the above coupling reaction was carried out under
identical experimental conditions. But the use of complex 3
under our reaction conditions gave the desired cross-coupling
product only in 40% isolated yield accompanied by a significant
amount of homodiaryl compound as a by-product (Table 1, entry
5), which provides an additional advantage of Pd–imidazole
complexes reported herein.
From the above study, complex 2 was found to be most
effective as catalyst and therefore further studies on the
Suzuki–Miyaura coupling reaction have been carried out in the
presence of complex 2. Results obtained from screening of
different bases and solvents are incorporated in Table 2.
Potassium carbonate has been found to be the most efficient
base in ethanol (Table 2, entry 1). Besides K2CO3, the reaction
can tolerate other inorganic bases such as Na2CO3, Cs2CO3 and
Na3PO4.12H2O, and gave almost comparable yields of the
cross-coupling product (Table 2, entries 2–5). However, the yield
was dramatically decreased with triethylamine (Table 2, entry 6).
Moreover, complex 2 was found to show better activity in protic
complexes 1,
2 and 4, complex 5 also demonstrated
intra-ligand π→π* (268 nm) and n→π* (321 nm) transitions in
its UV–visible spectrum. Compared to [PdCl2(PPh2Etpy–P,N)] (3)
(λmax, n→π* = 346 nm ), n→π* transition was shifted towards
blue due to the change of ligand environment about the Pd2+
ion. The 31P{1H}NMR spectrum of 5 ( Fig. S2, supporting information)
consisted of a singlet at δ = 14.51 ppm and a septet at δ =ꢀ160.79
ppm (JPF = 712 Hz ), corresponding to the 31P nuclei of the PPh2Etpy
and PFꢀ6 ion, respectively. The signal associated with 31P nuclei of
the PPh2Etpy ligand also exhibited a significant upfield shift in
complex 5 as in 4, attributable to the increase of electron density
at the metal center on coordination of N-BuIm ligand. The
resonances for H4′ and H5′ protons of N-BuIm were detected as
a singlet at δ = 7.07 and 6.50 ppm respectively. Moreover, the
presence of characteristic peaks for the aromatic, aliphatic as
well as imidazole carbon further substantiated formation of
complex 5. The electrochemical behavior of complex 5 showed
similar features to the above complexes. An irreversible
reductive event at ꢀ1.35 V (Δ Ep = 630 mV) could be ascribable to
Pd(II)/Pd(0) redox couple.[38–40] The shift of this event to a more
negative potential compared to precursor complex [PdCl2
(PPh2Etpy-P,N)] (3) (Epc = ꢀ0.78 V ) and the complex 4 may be
attributed to the presence of more electron-donating N-BuIm
ligands. Two reductive responses at ꢀ0.42 and ꢀ0.92 V with no
corresponding anodic peak may be assigned to ligand-based
reduction. During anodic sweep oxidation at +1.47 V with no
cathodic peak is likely to be due to the formation of Pd(IV).[40]
Table 2. Studies on the effects of different bases and solvents
Suzuki–Miyaura Cross-Coupling Reactions using Complexes
1, 2, 4 and 5 as Catalyst
The effectiveness of complexes 1–5 has been studied by carrying out
the model reaction of 4-bromonitrobenzene with phenylboronic
acid in ethanol using K2CO3 under aerobic conditions at room
temperature. The results are summarized in Table 1. The newly
synthesized palladium complexes are found to be effective in
the Suzuki–Miyaura reaction; however, significant variations in
yields have been noticed. Application of complex 1 as catalyst
Entry
Solvent
Base
Time (h) Yield (%)a
1
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
K2CO3
5
24
24
24
24
24
24
24
24
24
24
24
24
14
4
99
90
2
Na2CO3
NaHCO3
Na3PO4.12H2O
Cs2CO3
Et3N
3
50
4
95
5
87
Table 1. Suzuki–Miyaura cross-coupling reactions with various catalystsa
6
30
7
Water
THF
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
K2CO3
—
Trace
30
8
9
MeCN
Hexane
DMF
50
10
11
12
13
14
15
16b
17c
45
50
Entry
Catalyst (complex)
Time
Yield (%)b
i-PrOH
i-PrOH:H2O (1:1)
EtOH
85
85
1
2
3
4
5
1
2
4
5
3
22
5
97
99
50
45
40
—
EtOH
K2CO3
K2CO3
K2CO3
99
24
22
24
EtOH
4
85
EtOH
4
64
aIsolated yield.
a0.5 mmol 4-bromonitrobenzene and 0.55 mmol arylboronic acid
were used.
bIsolated yields.
b1 mol% catalyst was used.
cPdCl2 (1.5 mol%) was used as catalyst.
Appl. Organometal. Chem. (2013)
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G. Borah et al.
solvents, although significant variations in yield have been
noted with aprotic solvents, and both polar and non-polar
(Table 2, entries 8–11) gave comparatively lower yields. The
lowest yield (30%) was obtained with THF (Table 2, entry 8). The
coupling reactions did not proceed in the absence of base (Table 2,
entry 14). The amount of catalyst required for efficient activity has
also been scrutinized and 1.5mol% of complex 2 was found to be
the optimum amount for the Suzuki–Miyaura cross-coupling
reaction. The yield of the product was reduced to 85% when we
used 1 mol% of the catalyst (Table 2, entry 16) under the same
reaction conditions. In order to demonstrate the better catalytic
efficiency of complex 2, reaction with 1.5 mol% PdCl2 was carried
out under the same reaction conditions, and it was found that the
yield of cross-coupling product was dramatically reduced to 64%,
accompanied by the formation of a significant amount of
homodiaryl compound as byproduct (Table 2, entry 17).
To evaluate the scope and limitations of the current procedure,
reactions of a wide array of electronically diverse aryl bromides
with different arylboronic acids were examined using complex
2 and the results are shown in Table 3. The aryl bromides with
electron- withdrawing and donating substituents underwent
coupling reactions with phenylboronic acid effectively to afford
the desired biaryls in excellent yields (87–99%). The catalytic
system is equally effective for electronically diversified arylboronic
acids. Thus our present result is quite significant as the desired
biaryls could be achieved at room temperature.
As far as the reaction mechanism is concerned, the actual
reason for the efficiency of complex 2 is not clear. However, it
was reported that dissociation of ligand in the reaction medium
produces palladium(0) colloids or nanoparticles and the palladium(0)
nanoparticle is the active catalytic species for the Suzuki–Miyaura
reaction.[24e] With these aspects in mind, we have also investigated
the nature of the catalytic species after completion of the reaction.
Transmission electron microscopic images of the catalyst after
reaction clearly show the formation of Pd(0) nanoparticles
(Figs S1 and S2, see supporting information). We believe that
the weaker Pd–imidazole bond dissociates easily in the reaction
medium and forms stable palladium(0) nanoparticles which might
be stabilized by imidazole. Therefore, the different catalytic activities
of the complexes (1, 2, 4 and 5) might depend on the rate of ligand
dissociation to produce the real catalytic species. The steric strain
of complexes 1 and 2 might enhance the rate of ligand disso-
ciation and formation of palladium nanoparticles. Moreover, the
stabilization effect of the n-butyl side chain in complex 2 is
comparatively greater than that of n-methyl side chain, thereby
forming more stable Pd(0) nanoparticles, and hence complex 2
is more efficient for the Suzuki–Miyaura cross-coupling reaction.
Conclusion
In summary, four new cationic phosphinoamine–Pd(II)–imidazole
complexes (1, 2, 4 and 5) have been synthesized and characterized
by different physicochemical methods. The catalytic activities of
all the complexes have been investigated for Suzuki–Miyaura
cross-coupling reactions of aryl halides with aryl boronic acids
at room temperature. Among the complexes, complex 2
exhibited highest efficacy, while 4 and 5 were found to have
poor activity. A range of electronically diverse aryl bromides
underwent the coupling reactions in good to excellent yields
at room temperature with complex 2 as catalyst. Use of ethanol
as solvent is one of the most important advantages of the
present catalytic system.
Supporting Information
Supporting information may be found in the online version of
this article.
Acknowledgements
The authors are grateful to the University Grants Commission,
New Delhi, for financial support [(F. No. 36-75/2008(SR)].
Ms D. Boruah thanks the University Grants Commission,
New Delhi for a ‘Research Fellowship in Science for Meritorious
Students’ (RFSMS scheme). Financial assistance from the Department
of Science and Technology, New Delhi, is also acknowledged
(No. SR/FT/CS-098/2009). The authors are also thankful to UGC
New Delhi for the Special Assistance Programme (UGC-SAP)
and the Department of Science and Technology for financial
assistance under DST-FIST program to the Department of
Chemistry, Dibrugarh University. The services of SAIF, NEHU,
Shillong, are also gratefully acknowledged for recording
1H and 31P{1H}NMR and mass spectra.
Table 3. Suzuki–Miyaura cross-coupling reactions of various aryl
bromides with different arylboronic acids
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