Hemilabile imino-phosphine palladium(II) complexes: Synthesis, molecular structure, and evaluation in Heck reactions

Article abstract of DOI:10.2478/s11696-013-0530-6

The ligands 2-(diphenylphosphino)benzyl-(2-thiophene)methylimine (V) and 2-(diphenylphosphino) benzyl-(2-thiophene)ethylimine (VI) were prepared from 2-(diphenylphosphino)benzaldehyde and thiophene amines with very good yields. An equimolar reaction of V and VI with either PdCl₂(cod) (cod = cyclooctadiene) or PdClMe(cod) afforded palladium(II) complexes I-IV. The molecular structure of II was confirmed by X-ray crystallography. The coordination geometry around the palladium atom exhibited distorted square planar geometry at the palladium centre. Complexes I, II, and IV were evaluated as catalysts for Heck coupling reactions of iodobenzene with methyl acrylate under mild reaction conditions; 0.1 mole % catalyst, Et₃N base, MeCN reflux for 8 h, 80 C; isolated yield on a 10 mmol scale with catalyst I (64 %), II (68 %), and IV (58 %). They all exhibited significant activities.

Full text of DOI:10.2478/s11696-013-0530-6

Chemical Papers
DOI: 10.2478/s11696-013-0530-6
ORIGINAL PAPER
Hemilabile imino-phosphine palladium(II) complexes: synthesis,
molecular structure, and evaluation in Heck reactions
a
b
aWilliam M. Motswainyana, Martin O. Onani*, Roger A. Lalancette,
cPaul K. Tarus
aChemistry Department, University of the Western Cape, Private Bag X17, 7535 Bellville, South Africa
bCarl A. Olson Memorial Laboratories, Department of Chemistry, Rutgers University, 07102 Newark, NJ, USA
cDepartment of Chemistry and Biochemistry, University of Eldoret, P.O. Box 1125-30100, Eldoret, Kenya
Received 29 June 2013; Revised 23 November 2013; Accepted 23 November 2013
The ligands 2-(diphenylphosphino)benzyl-(2-thiophene)methylimine (V ) and 2-(diphenylphos-
phino)benzyl-(2-thiophene)ethylimine (VI) were prepared from 2-(diphenylphosphino)benzaldehyde
and thiophene amines with very good yields. An equimolar reaction of V and VI with either
PdCl2(cod) (cod = cyclooctadiene) or PdClMe(cod) afforded palladium(II) complexes I–IV. The
molecular structure of II was confirmed by X-ray crystallography. The coordination geometry around
the palladium atom exhibited distorted square planar geometry at the palladium centre. Complexes
I, II, and IV were evaluated as catalysts for Heck coupling reactions of iodobenzene with methyl
acrylate under mild reaction conditions; 0.1 mole % catalyst, Et3N base, MeCN reflux for 8 h,
80◦C; isolated yield on a 10 mmol scale with catalyst I (64 %), II (68 %), and IV (58 %). They all
exhibited significant activities.
c
ꢀ 2013 Institute of Chemistry, Slovak Academy of Sciences
Keywords: imino-phosphine, palladium, Schiff-base molecular structures, Heck reaction
Introduction
of the ligand 2-(diphenylphosphino)benzaldehyde has
led to the preparation of a number of bidentate imino-
phosphine ligands (Hoots et al., 1982; Garralda, 2005);
their preparation is versatile because a wide range of
amines and aldehydes of varying steric bulk are ei-
ther available commercially or can be easily synthe-
sised (Doherty et al., 2002; Chen & Yeh, 2011; An-
tonaroli & Crociani, 1998; Mogorosi et al., 2011). The
present attempt to develop new ligands which would
effectively promote the conversion of aryl halides into
Heck products reports on the synthesis, characterisa-
tion, and test results of imino-phosphine palladium(II)
complexes in standard Heck coupling reactions.
The original development of new methods for
the synthesis of mixed soft and hard donor P—N
ligand systems is well documented in a review of
pyridylphosphines (Newkome, 1993). These types of
ligands have been shown to be important in metal-
catalysed carbon–carbon reactions such as the Heck
and Suzuki coupling (Beletskaya & Cheprakov, 2000;
Scrivanti et al., 2005; Nobre & Monteiro, 2009). The
phosphorus and nitrogen atoms in an imino-phosphine
ligand provide a unique reaction to their metal com-
plex due to their hemilabile property (Espinet &
Soulantica, 1999; Motswainyana et al., 2011). This
hemilability of the ligands affords a unique reversible
protection of the coordination site, an important prop-
erty of the compounds which leads to improved cat-
alytic activities (Motswainyana et al., 2011). The use
Experimental
All reactions were carried out under a nitrogen at-
mosphere using a dual vacuum/nitrogen line and stan-
*Corresponding author, e-mail: monani@uwc.ac.za

ii
W. M. Motswainyana et al./Chemical Papers
Table 1. Characterisation data of imino-phosphine compounds
wi(calc.)/%
wi(found)/%
Yield
M.p.
Compound
Formula
Mr
C
H
N
%
86
85
78
75
92
90
◦C
230
208
158
142
–
I
C24H20Cl2NPPdS
C25H22Cl2NPPdS
C25H23ClNPPdS
C26H25ClNPPdS
C24H20NPS
562.79
576.81
542.37
556.40
385.46
399.49
51.22
50.98
52.06
52.28
55.36
55.08
56.13
55.98
74.78
75.08
75.16
74.99
3.58
3.24
3.84
3.76
4.27
4.52
4.53
4.45
5.23
5.14
5.55
5.58
2.49
2.65
2.43
2.49
2.58
2.81
2.52
2.32
3.63
3.89
3.51
3.31
II
III
IV
V
VI
C25H22NPS
–
dard Schlenk techniques unless stated otherwise. Sol-
vents (Sigma–Aldrich, USA) were dried and purified
by heating at reflux under nitrogen in the presence of
a suitable drying agent. The PdCl2(cod) (cod = cy-
clooctadiene) and PdClMe (cod) were prepared follow-
ing methods detailed in the literature (Wiedermann
et al., 2006; Salo & Guan, 2003). All the reagents and
starting materials were purchased from Sigma–Aldrich
and were used without further purification.
obtain a light brown oil. The yield given in Ta-
ble 1 for this ligand (and for the other ligands
also) is the total mass recovered from this evapo-
ration. All of the ligands isolated by this procedure
were pure, as assessed by 1H NMR and 31P NMR,
and by elemental analyses. The ligands may be
stored for extended periods of time by excluding oxy-
gen.
1
The H NMR (200 MHz) and 13C NMR (50 MHz)
Preparation of 2-(diphenylphosphino)benzyl-
(2-thiophene)ethylimine (VI)
spectra were recorded on a Varian (USA) XR 200 MHz
spectrometer in our laboratories. The 31P NMR spec-
tra were recorded on a Varian XR200 MHz spectrom-
eter at the University of Cape Town (South Africa).
Chemical shifts are given in δ relative to internal stan-
dard (TMS). IR spectra in solution were recorded with
a Perkin–Elmer (USA) Spectrum 100 Series FT-IR in-
strument using nujol mulls on NaCl plates. Elemental
analysis was performed on Server 1112 Series (Univer-
sity of Cape Town) elemental analyser. The melting
point of the synthesised compounds was determined
in open capillaries using SMP10 melting point appa-
ratus. Single crystal diffraction experiments were per-
formed in the Carl Olson laboratories, Rutgers Uni-
versity (Newark, NJ, USA).
The ligand was prepared according to the pro-
cedure described for V . 2-(diphenylphosphino)benz-
aldehyde (299 mg, 1.029 mmol) and 2-thiopheneethyl-
amine (135 mg, 1.063 mmol) were used.
Synthesis of complexes
Preparation of complex I
A solution of V (47 mg, 0.121 mmol) in CH2Cl2
(2 mL) was added drop-wise to a solution of PdCl2
(cod) (36 mg, 0.126 mmol) in CH2Cl2 (15 mL). The
reaction was stirred under reflux for 6 h, resulting in
a yellow precipitate. The precipitate was filtered to
afford a light yellow solid. The precipitated complex
I (and also the precipitate for the other ligands II–
IV ) was the catalyst used in the experiments shown
in Table 1.
Synthesis of ligands
Preparation of 2-(diphenylphosphino)benzyl-
(2-thiophene)methylimine (V)
2-thiophenemethylamine (116 mg, 1.028 mmol)
was added drop-wise to a mixture of 2-(diphenylphos-
phino)benzaldehyde (293 mg, 1.009 mmol) and an-
hydrous magnesium sulphate (800 mg) in CH2Cl2
(20 mL). The reaction was stirred at ambient tem-
perature for 18 h, resulting in a light brown mix-
ture. The mixture was filtered, followed by evap-
oration of the solvent under reduced pressure to
Preparation of complex II
The complex was prepared according to the proce-
dure described for I. PdCl2(cod) (48 mg, 0.170 mmol)
and VI (65 mg, 0.163 mmol) were used. Yellow
crystals suitable for X-ray analysis were obtained
when the solid was re-crystallised from a mixture of
CH2Cl2/hexane (ϕr = 1 : 6).

W. M. Motswainyana et al./Chemical Papers
iii
Fig. 1. Preparation of imino-phosphine palladium(II) complexes I–IV.
Preparation of complex III
water (50 mL) and extracted with CH2Cl2 to give a
pure product which was confirmed by H NMR spec-
1
A solution of V (52 mg, 0.134 mmol) in CH2Cl2
(2 mL) was added to a solution of PdClMe(cod)
(35 mg, 0.131 mmol) in CH2Cl2/Et2O (15 mL; ϕr =
1 : 2). The reaction mixture was stirred at ambient
temperature for 8 h, resulting in a white precipitate.
The precipitate was filtered to afford a white solid.
White crystals suitable for X-ray analysis were ob-
tained when the solid was recrystallised from a mix-
ture of CH2Cl2/hexane (ϕr = 1 : 3) (Onani et al.,
2010).
troscopy. Mercury drop experiments were performed
in duplicate by adding two drops of elemental mer-
cury to the reaction described above and the prod-
uct (trans- methyl cinnamate) was also analysed by
1H NMR spectroscopy (1H NMR (200 MHz, CDCl3),
—
δ: 7.83 (s, 1H, HC C), 7.56–7.51 (m, 2H, aromatic),
—
7.37–7.34 (m, 3H, aromatic), 6.27 (d, 1H, J = 15.2 Hz,
—
—
C
CH), 3.63 (s, 3H, CH ).
3
Molecular structure determination
Preparation of complex IV
Single crystals of complex II suitable for X-
ray crystallography were grown by slow diffusion of
hexane into a CH2Cl2 solution of the complex at
4◦C. X-ray diffraction data for the compound were
collected on a Bruker (Germany) KAPPA APEX II
DUO diffractometer using graphite-monochromated
The complex was prepared according to the pro-
cedure described for III. The PdClMe(cod) (37 mg,
0.141 mmol) and VI (56 mg, 0.140 mmol) were used
as starting materials. White crystals suitable for X-
ray analysis were obtained when the solid was recrys-
tallised from a mixture of CH2Cl2/hexane (ϕr = 1 : 3)
(Motswainyana et al., 2012).
˚
CuKα radiation (χ = 0.71073 A). The crystal struc-
ture was resolved by direct methods using SHELX
(Sheldrick, 2008) and refined by full-matrix least-
squares methods based on F2 (Sheldrick, 2008) using
SHELX (Sheldrick, 2008) and using the graphics inter-
face program ORTEP-3 for Windows (Farrugia, 1999,
1997).
Catalytic experiments
A dry 100 mL capacity Schlenk tube equipped with
a magnetic stirrer bar was charged with iodobenzene
(10 mmol), methyl acrylate (11 mmol), and triethy-
lamine (10 mL). The catalyst (0.01 mmol) was dis-
solved in 10 mL of CH3CN and transferred quantita-
tively to the reaction vessel. The reaction mixture was
Results and discussion
Synthesis of ligands and metal complexes
◦
heated under stirring at 80 C. Samples were taken
The reaction of 2-(diphenylphosphino)benzalde-
hyde with an equivalent amount of thiophene amine
in CH2Cl2 at ambient temperature afforded very good
yields of ligands V and VI (Fig. 1). These com-
at regular intervals and analysed by GC to deter-
mine the percentage conversions. The coupling prod-
uct was isolated by pouring the reaction mixture into

iv
W. M. Motswainyana et al./Chemical Papers
Table 2. Spectral data of imino-phosphine compounds
Compound
Spectral data
—
I
IR, ν˜/cm−1: 1629 (C N), 1435 (P—Ph), 1304 (C—S—C)
—
1
—
—
H NMR (DMSO-d6), δ: 8.79 (s, 1H, –N CH), 7.99 (dd, 1H, J = 1.8 Hz, phenyl), 7.90–7.16 (m, aromatic and
heterocycle), 7.04 (t, 1H, J = 4.8 Hz, thiophene), 6.94 (t, 1H, J = 3.0 Hz, thiophene), 5.67 (d, 2H, J = 2.8 Hz,
—
–N CH2)
—
31P NMR (CDCl3), δ: 35.0
—
II
III
IV
V
IR, ν˜/cm−1: 1628 (C N), 1435 (P—Ph), 1306 (C—S—C)
—
1
—
H NMR (DMSO-d6), δ: 8.55 (d, 1H, J = 5.2 Hz, –N CH), 7.97–6.56 (m, 4H, aromatic and heterocycle), 4.53 (t,
2H, J = 2.8 Hz, –CH2), 3.08 (t, 2H, J = 3.0 Hz, –N CH2)
31P NMR (CDCl3), δ: 35.1
—
—
—
IR, ν˜/cm−1: 1632 (C N), 1438 (P—Ph), 1296 (C—S—C)
—
—
1
—
—
H NMR (DMSO-d6), δ: 8.20 (d, 1H, J = 4.2 Hz, –N CH), 7.44–6.90 (m, 4H aromatic and heterocycle), 5.61 (s,
—
2H, –N CH2 aliphatic), 1.53 (s, 3H, Pd—Me (CH3))
—
31P NMR (CDCl3), δ: 37.0
IR, ν˜/cm−1: 1636 (C N), 1435 (P—Ph), 1307 (C—S—C)
—
—
1
—
—
H NMR (DMSO-d6), δ: 7.78 (d, 1H, J = 4.6 Hz, –N CH), 7.50–6.14 (m, 4H, aromatic and heterocycle), 4.43 (s,
—
2H, –CH2), 3.25 (s, 2H, –N CH2), 1.46 (s, 3H, Pd—Me (CH3))
—
31P NMR (CDCl3), δ: 36.1
IR, ν˜/cm−1: 1635 (C N), 1434 (P–Ph), 1313 (C—S—C)
—
—
1
—
—
H NMR (CDCl3), δ: 9.01 (d, 1H, J = 5.4 Hz, –CH N), 8.07 (t, 1H, J = 2.6 Hz, phenyl), 7.58 (s, 1H, thiophene),
7.43 (s, 1H, phenyl), 7.36 (d, 1H, J = 2.0 Hz, phenyl), 7.33 (t, 1H, J = 2.2 Hz, phenyl), 6.92 (dd, 1H, J = 3.4 Hz,
—
thiophene), 6.73 (dd, 1H, J = 3.4 Hz, thiophene), 4.84 (s, 2H, –N CH2)
—
13C NMR (CDCl3), δ: 161.0, 160.5 (CH imine), 141.7 (CH heterocycle), 139.4–126.7 (CH aromatic), 127.5, 125.0,
124.5, 59.0 (CH2 aliphatic)
31P NMR (161.9 Hz, CDCl3), δ: –14.0
—
VI
IR, ν˜/cm−1: 1637 (C N), 1434 (P—Ph), 1339 (C—S—C)
—
1
—
—
H NMR (CDCl3), δ: 8.81 (d, 1H, J = 4.6 Hz, –CH N-), 7.71 (t, 1H, J = 3.2 Hz, phenyl), 7.05 (d, 1H, J = 2.8 Hz,
phenyl), 6.98 (d, 1H, J = 2.8 Hz, phenyl), 6.80 (d, 1H, J = 3.4 Hz, phenyl), 6.69 (dd, 1H, J = 2.2 Hz, thiophene),
—
6.46 (s, 1H, thiophene), 3.70 (t, 2H, J = 6.8 Hz, –N CH2), 2.94 (t, 2H, J = 3.0 Hz, –CH2)
—
13C NMR (CDCl3), δ: 160.7, 160.3 (CH imine), 142.4–126.6 (CH aromatic), 139.7, 136.5, 124.9, 123.4, (CH hetero-
cycle), 62.6, 31.3 (CH2 aliphatic)
31P NMR (CDCl3), δ: –13.6
—
pounds are susceptible to oxidation (Antonaroli &
Crociani, 1998; Shirakawa et al., 1997). The imino-
phosphine palladium(II) complexes I–IV were pre-
pared from equimolar reactions of V and VI with ei-
ther PdCl2(cod) or PdClMe(cod) under reflux for 6 h,
resulting in a yellow precipitate with up to 75 % yields.
Complexes III and IV are reported for the first time,
at least to the best of our knowledge, while the ligands
and complexes I and II have been independently pre-
pared by another group (Mogorosi et al., 2011). All
the compounds were characterised by 1H NMR and
31P NMR spectroscopy, IR, and elemental analyses
(Tables 1 and 2).
in the region δ 8.81–9.01 due to (HC N), confirming
—
a Schiff base condensation reaction. The doublet is a
characteristic of an imine proton coupling to phospho-
rus (Ruelke et al., 1996). The 31P NMR spectra of the
ligands showed an upfield singlet at around δ –13.9
which is a characteristic of uncoordinated phosphorus
(Mogorosi et al., 2011). Coordination of the ligands to
the metal centre was confirmed in the IR spectra of
complexes I and II, which showed an absorption band
between 1625 cm−1 and 1628 cm−1 typical of coor-
dinated imines (Nobre & Monteiro, 2009; Mogorosi
et al., 2011). However, complexes III and IV did not
show any significant shift in absorption frequency from
their corresponding free imines, probably due to the
presence of the methyl group bound to the palladium
centre, which would inductively increase the electron
density on the metal centre. The general pattern in
the 1H NMR spectra of the complexes revealed an up-
field shift in the imine protons compared with their
corresponding ligands, indicating coordination. In the
31P NMR spectra, the complexes showed a character-
istic phosphorus singlet further downfield in the region
of δ 32.4–37.2, which is a typical confirmation of co-
ordinated phosphorus (Doherty et al., 2002; Mogorosi
In the IR spectra of the ligands, imine formation
was confirmed by the absorption band observed at
1635 cm−1 and 1637 cm−1 for V and VI, respectively.
The other absorption band observed at approximately
1434 cm−1 confirmed the presence of uncoordinated
(P—Ph) phosphorus (Park & Hendra, 1968). The IR
spectra of the ligands did not show an infrared ab-
sorption band due to (P—O) at around 1185 cm−1
,
an indication that the ligands did not form phosphine
oxide (Baldwin & Washburn, 1965). The 1H NMR
spectra of the ligands showed a characteristic doublet

W. M. Motswainyana et al./Chemical Papers
v
Table 3. Crystallographic data and refinement for II
Crystallographic data/dimension Value
Empirical formula
Mr
Temperature/K
Crystal system
Space group
C25H22Cl2NPPdS – H2O
594.78
100
monoclinic
P21/n
˚
Unit cell dimensions/A
a
b
c
9.8933(3)
18.8451(5)
13.4998(4)
94.892(2)
2507.74(13)
4
β/◦
3
˚
V /A
Z
Dcal/(Mg m−3
F (000)
)
1.575
1200
Crystal size/mm
Final R indices (R1)
0.14 × 0.07 × 0.06
R1 = 0.053, wR2 = 0.157
4.0 to 71.4
1.05
Theta range for data collection/◦
Goodness-of-fit on F2
Largest diff. peak and hole/(e A−3) 3.14 and –0.92
˚
nar coordination geometry around the metal centre.
The bond angles Cl(1)—Pd(1)—Cl(2) (90.84 (6)◦)
and P(1)—Pd(1)—N(1) (85.68(16)◦) describe the dis-
torted square planar geometry.
˚
The Pd(1)—C1(1) bond length of 2.2885(16) A
is in good agreement with the average Pd—Cl bond
˚
distance of 2.298(15) A for known palladium com-
plexes (Allen, 2002). A detectable trans influence is
taking place in the compound since the Pd(1)—C1(2)
˚
bond length of 2.3643(16) A is significantly longer
than the average value, which reflects the stronger
trans-influence of the diphenylphosphino group com-
pared to an amine (Doherty et al., 2002). We pre-
viously reported on the molecular structures of III
and IV depicting the geometry of these compounds.
(Onani et al., 2010; Motswainyana et al., 2012). The
Fig. 2. X-ray crystal structure of complex II.
et al., 2011; Reddy et al., 2000).
˚
average bond lengths Pd(1)—N(1) of 2.058(6) A and
˚
Single crystal X-ray diffraction studies
Pd(1)—P(1) of 2.2179(16) A compare well with the
literature values (Reddy et al., 2000).
The molecular structure of II, shown in Fig. 2, was
confirmed by X-ray crystallography. The two atoms
near the carbon atoms in the thiophene ring are rep-
resentations of the disorder of C-23 and C-24 and are
denoted as C-23A and C-24A (with their associated H
atoms). The ratio of the disorder is 64 : 36(9). That
means that C-23 and C-24 are there 64 % of the time
(with an error of 9 %), and C-23A and C-24A are there
36 % of the time.
The crystallographic data and refinement residu-
als are summarised in Table 3 while selected bond
lengths and bond angles are summarised in Table 4.
The structure crystallises in the monoclinic space
group P21/n. The ligands are tetra-coordinated via
the P and N atoms and two chloride anions to the
palladium atom, generating a distorted square pla-
Heck coupling reactions
Complexes I, II, and IV were screened towards
the standard Heck coupling reaction of iodobenzene
with methyl acrylate (Fig. 3) using CH3CN as a
solvent, Et3N as base, and a temperature of 80◦C.
Highly polar solvents in Heck reactions are neces-
sary for high reaction rates, and polar solvents such
as dimethylacetamide (DMA), N-methylpyrrolidinone
(NMP), or dimethylformamide (DMF) at high tem-
peratures have supported high activities (Scrivanti
et al., 2005; Ojwach et al., 2007; Pelagatti et al.,
2005). However, reactions involving CH3CN as a sol-
vent have yielded equally good results (Smith &
Mapolie, 2004). The mole ratio Pd/PhI/alkene/Et3N

vi
W. M. Motswainyana et al./Chemical Papers
Table 4. Selected bond lengths and bond angles for complex II
Bond angle/◦
˚
Type of bond
Bond length/A
Type of bond
Pd(1)—N(1)
Pd(1)—P(1)
Pd(1)—Cl(1)
Pd(1)—Cl(2)
2.058(6)
N(1)—Pd(1)—P(1)
N(1)–Pd(1)–Cl(1)
P(1)—Pd(1)—Cl(1)
N(1)—Pd(1)—Cl(2)
P(1)—Pd(1)—Cl(2)
Cl(1)—P(1)—Cl(2)
85.68(16)
179.52(17)
93.84(6)
89.64(16)
174.17(6)
90.84(6)
2.2179(16)
2.2885(16)
2.3643(16)
Table 5. Heck coupling reactionsa of iodobenzene with methyl acrylate catalysed with complexes I, II, and IV
Time
h
Amount of catalyst
mole %
Conversionb
%
Yield
Selectivityc
%
Entry
Complex no.
TONb
TOFb
1
2
3
4
5d
6
7
8
9
I
I
8
24
8
24
24
24
24
8
0.10
0.10
0.10
0.10
0.10
0.05
0.01
0.10
0.10
79
84
83
90
88
69
62
72
76
64
69
68
75
75
52
40
58
60
790
840
830
900
880
690
620
720
760
99
35
104
38
37
29
26
90
32
80
83
83
85
85
80
80
80
82
II
II
II
II
II
IV
IV
24
a) Reaction conditions: PhI (10 mmol), methyl acrylate (11 mmol), Et3N (10 mmol), 80◦C, CH3CN (10 mL); b) determined by
GC; c) percentage of E-diastereomer; d) mercury drop test; TON – turnover number; TOF – turnover frequency. Internal standard;
mesitylene.
Fig. 3. Heck coupling reaction of iodobenzene with methyl acrylate.
(1 : 1000 : 1100 : 1000) was maintained in all the cat-
alytic experiments. A summary of the catalytic results
is given in Table 5. Aryl iodides are the preferred sub-
strates in Heck reactions because of their lower C—
X bond strength compared to the aryl bromides and
chlorides (Grushin & Alper, 1994). The bond strength
makes the aryl iodides more susceptible to oxidative
addition to the catalytic species, a step viewed as an
initiation stage for a Heck reaction (Scrivanti et al.,
2005). Historically, phosphine complexes have been ex-
tensively used as catalysts in Heck coupling reactions
despite their air sensitivity, because they provide ther-
mally robust catalysts (Pelagatti et al., 2005).
The complexes successfully catalysed the arylation
of methyl acrylate, giving good yields after 24 h (Ta-
ble 5, Entries 2 and 4 and 9). Complex II was observed
to be the most active, giving a 68 % yield after 8 h,
and eventually achieving 75 % after 24 h. The ma-
jor coupling product was observed to be trans-methyl
cinnamate, as confirmed by 1H NMR spectroscopy.
There was a good selectivity of up to 85 % towards
the formation of trans-methyl cinnamate. Although
the reactions were performed under mild reaction con-
ditions in acetonitrile, high substrate conversions and
yields were achieved, which could be explained only
by the hemilability of the imino-phosphine ligands
(Motswainyana et al., 2011). The influence of the thio-
phene ring in the catalytic activity of the complexes
was noted (Table 5, Entries 2 and 4). Higher conver-
sions and yields were obtained with catalyst II, pos-
sibly due to the presence of the thermo-flexible ethy-
lene linker of the thiophene imine which imparts a
hemilabile property (Jeffrey & Rauchfuss, 1979; Mar-
son et al., 2009). The effect of catalyst concentration
on the performance of the catalysts was also inves-
tigated using catalyst II (Table 5, Entries 6 and 7).
The catalytic results showed that smaller amounts of
the catalyst were capable of effectively catalysing the
◦
arylation of methyl acrylate at 80 C (Cui & Zhang,
2005). At a low catalyst loading of 0.01 mole %, cat-
alyst II was still active and returned a yield of 40 %.
Mercury drop experiments showed no significant dif-

W. M. Motswainyana et al./Chemical Papers
vii
ference in conversion between the experiments with or
without mercury (Table 5, Entry 5), thereby exclud-
ing the formation of Pd(0) which could influence the
catalytic results. Structurally, the prepared complexes
could show a potential in the activation of more chal-
lenging substrates such as bromo- and chloroarenes.
Chloroarenes are the most inexpensive in the group.
Unfortunately, the high C—Cl bond strength com-
pared with C—Br and C—I bonds precludes oxidative
addition, a first step in catalytic coupling reactions.
This makes the coupling of such substrates far more
challenging. Hence, there is currently much interest in
the synthesis of catalysts that are able to activate aryl
chloride substrates at ever lower catalyst loading (Her-
rmann et al., 1995; Ahrens et al., 2006). A change in
the catalytic reaction media to non-aqueous ionic liq-
uids instead of common organic solvents is reported
to provide superior Heck vinylation of chloroarenes,
opening an avenue for further investigations of these
compounds (B¨ohm & Herrmann, 2000).
Antonaroli, S., & Crociani, B. (1998). Preparation and reac-
tions of palladium(0)-olefin complexes with iminophosphine
ligands. Journal of Organometallic Chemistry, 560, 137–146.
DOI: 10.1016/s0022-328x(98)00472-0.
Baldwin, R. A., & Washburn, R. M. (1965). Organometal-
lic azides. I. Preparation and reactions of diarylphosphinic
azides 1a. Journal of Organic Chemistry, 30, 3860–3866.
DOI: 10.1021/jo01022a063.
Beletskaya, I. P., & Cheprakov, A. V. (2000). The Heck reac-
tion as a sharpening stone of palladium catalysis. Chemical
Reviews, 100, 3009–3066. DOI: 10.1021/cr9903048.
B¨ohm, V. P. W.,
& Herrmann, W. A. (2000). Nonaque-
ous ionic liquids: Superior reaction media for the cat-
alytic Heck-vinylation of chloroarenes. Chemistry – A Eu-
ropean Journal, 6, 1017–1025. DOI: 10.1002/(sici)1521-
3765(20000317)6:6<1017::aid-chem1017>3.0.co;2-8.
Chen, C. S., & Yeh, W. Y. (2011). Synthesis of a new tetraden-
tate bis(imino-phosphine) ligand and its complexation with
copper(I) ions. Inorganica Chimica Acta, 370, 456–459. DOI:
10.1016/j.ica.2011.02.015.
Cui, Y. C., & Zhang, L. (2005). Polyvinyl chloride–polyethyl-
ene–polyamine supported palladium complexes as high ef-
ficient and recyclable catalysts for Heck reaction. Journal
of Molecular Catalysis A: Chemistry, 237, 120–125. DOI:
10.1016/j.molcata.2005.04.050.
Doherty, S., Knight, J. G., Scanlam, T. H., Elsegood, M. R.
J., & Clegg, W. (2002), Iminophosphines: synthesis, for-
mation of 2,3-dihydro-1H-benzo[1,3]azaphosphol-3-ium salts
and N-(pyridin-2-yl)-2-diphenylphosphinoylaniline, coordi-
nation chemistry and applications in platinum group cat-
alyzed Suzuki coupling reactions and hydrosilylations. Jour-
nal of Organometallic Chemistry, 650, 231–248. DOI: 10.
1016/s0022-328x(02)01203-2.
Espinet, P., & Soulantica, K. (1999). Phosphine-pyridyl and
related ligands in synthesis and catalysis. Coordination
Chemistry Reviews, 193–195, 499–556. DOI: 10.1016/s0010-
8545(99)00140-x.
Conclusions
In conclusion, imino-phosphine ligands and com-
plexes were successfully synthesised and characterised
by various spectroscopic techniques. Evaluation of the
activity of complexes I, II, and IV towards Heck cou-
pling of iodobenzene with methyl acrylate was carried
out under mild reaction conditions and low catalyst
loading. The conversion results showed the complexes
to be active, with good selectivity towards the forma-
tion of trans-methyl cinnamate. The higher activities
of the complexes suggest that the imino-phosphine lig-
ands are hemilabile.
Farrugia, L. J. (1997). ORTEP-3 for Windows – a version
of ORTEP-III with
a Graphical User Interface (GUI).
Journal of Applied Crystallography, 30, 565–566. DOI:
10.1107/s0021889897003117.
Acknowledgements. We are grateful for the financial sup-
port received from the University of the Western Cape Sen-
ate Research, Government of Botswana and National Research
Foundation (Thuthuka).
Farrugia, L. J. (1999). WinGX suite for small-molecule single-
crystal crystallography. Journal of Applied Crystallography,
32, 837–838. DOI: 10.1107/s0021889899006020.
Garralda, M. A. (2005). o-(Diphenylphosphino)benzaldehyde:
a versatile ligand and a useful hemilabile ligand precursor.
Comptes Rendus Chimie, 8, 1413–1420. DOI: 10.1016/j.crci.
2005.03.009.
Supplementary data
Grushin, V. V.,
& Alper, H. (1994). Transformations of
Crystallographic data for the structural analysis
have been deposited with the Cambridge Crystal-
lographic Data Centre, CCDC no. 874818 for com-
pound II. Copies of this information may be ob-
tained free of charge from The Director, CCDC, 12
Union Road, Cambridge, CB21EZ, UK (fax: +44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
chloroarenes, catalyzed by transition-metal complexes.
Chemical Reviews, 94, 1047–1062. DOI: 10.1021/cr00028a
008.
¨
Herrmann, W. A., Brossmer, C., Ofele, K., Reisinger, C. P.,
Priermeier, T., Beller, M., & Fischer, H. (1995). Palladacycles
as structurally defined catalysts for the heck olefination of
chloro- and bromoarenes. Angewandte Chemie International
Edition, 34, 1844–8148. DOI: 10.1002/anie.199518441.
Hoots, J. E., Rauchfuss, T. B., Wrobleski, D. A., & Knachel,
H. C. (1982). Substituted triaryl phosphines. In J. P. Fackler
(Ed.), Inorganic synthesis (Vol. 21, pp. 175–179). New York,
NY, USA: Wiley. DOI: 10.1002/9780470132524.
References
Ahrens, S., Zeller, A., Taige, M., & Strassner, T. (2006). Exten-
sion of the alkane bridge in BisNHC-palladium-chloride com-
plexes. Synthesis, structure, and catalytic activity. Organo-
metallics, 25, 5409–5415. DOI: 10.1021/om060577a.
Jeffrey, J. C., & Rauchfuss, T. B. (1979). Metal complexes of
hemilabile ligands. Reactivity and structure of dichlorobis(o-
(diphenylphosphino)anisole)ruthenium(II). Inorganic Chem-
istry, 18, 2658–2666. DOI: 10.1021/ic50200a004.
Allen, F. H. (2002). The cambridge structural database: a quar-
ter of a million crystal structures and rising. Acta Crystallo-
graphica B, 58, 380–388. DOI: 10.1107/s0108768102003890.
Marson, A., Ernsting, J. E., Lutz, M., Spek, A. L., van Leeuwen,
P. W. N. M., & Kamer, P. C. J. (2009). A novel hemilabile
calix[4], quinoline-based P ,N-ligand: coordination chemistry

viii
W. M. Motswainyana et al./Chemical Papers
and complex characterisation. Dalton Transactions, 2009,
621–633. DOI: 10.1039/b814469a.
Pelagatti, P., Carcelli, M., Costa, M., Ianelli, S., Pellizi, C.,
& Rogolino, D. (2005). Heck reaction catalyzed by pyridyl-
imine palladium(0) and palladium(II) complexes. Journal
of Molecular Catalysis A: Chemical, 226, 107–110. DOI:
10.1016/j.molcata.2004.09.049.
Mogorosi, M. M., Mahamo, T., Moss, J. R., Mapolie, S. F.,
Slootweg, J. C., Lammertsma, K., & Smith, G. S. (2011).
Neutral palladium(II) complexes with P ,N Schiff-base lig-
ands: Synthesis, characterization and catalytic oligomerisa-
tion of ethylene. Journal of Organometallic Chemistry, 696,
3585–3592. DOI: 10.1016/j.jorganchem.2011.07.042.
Motswainyana, W. M., Ojwach, S. O., Onani, M. O., Iwuoha,
E. I., & Darkwa, J. (2011). Novel hemi-labile pyridyl-imine
palladium complexes: Synthesis, molecular structures and
reactions with ethylene. Polyhedron, 30, 2574–2580. DOI:
10.1016/j.poly.2011.07.004.
Reddy, K. R., Surekha, K., Lee, G. H., Peng, S. M., & Liu, S. T.
(2000). Palladium(II) complexes with phosphorus−nitrogen
mixed donors. Efficient catalysts for the Heck reaction.
Organometallics, 19, 2637–2639. DOI: 10.1021/om000089h.
Ruelke, R. E., Kaasjager, V. E., Wehman, P., Elsevier, C. J.,
van Leeuwen, P. W. N. M., & Vrieze, K. (1996). Stable palla-
dium(0), palladium(II), and platinum(II) complexes contain-
ing a new, multifunctional and hemilabile phosphino-imino-
pyridyl ligand: Synthesis, characterization, and reactivity.
Organometallics, 15, 3022–3031. DOI: 10.1021/om9509047.
Salo, E. V., & Guan, Z. (2003). Late-transition-metal complexes
with bisazaferrocene ligands for ethylene oligomerization.
Organometallics, 22, 5033–5046. DOI: 10.1021/om034051r.
Scrivanti, A., Bertoldini, M., Matteoli, U., Beghetto, V., An-
tonaroli, S., Marini, A., & Crociani, B. (2005). Highly effi-
cient Heck olefin arylation in the presence of iminophosphine-
palladium(0) complexes. Journal of Molecular Catalysis A:
Chemical, 235, 12–16. DOI: 10.1016/j.molcata.2004.12.007.
Sheldrick, G. M. (2008). A short history of SHELX. Acta Crys-
tallographica A, 64, 112–122. DOI: 10.1107/s0108767307043
930.
Shirakawa, E., Yoshida, H., & Takaya, H. (1997). An iminophos-
phine-palladium catalyst for cross-coupling of aryl halides
with organostannanes. Tetrahedron Letters, 38, 3759–3762.
DOI: 10.1016/s0040-4039(97)00747-8.
Smith, G. S., & Mapolie, S. F. (2004). Iminopyridyl-palladium
dendritic catalyst precursors: evaluation in Heck reactions.
Journal of Molecular Catalysis A: Chemical, 213, 187–192.
DOI: 10.1016/j.molcata.2003.12.010.
Wiedermann, J., Mereiter, K., & Kirchner, K. (2006). Palladium
imine and amine complexes derived from 2-thiophenecarb-
oxaldehyde as catalysts for the Suzuki cross-coupling of aryl
bromides. Journal of Molecular Catalysis A: Chemical, 257,
67–72. DOI: 10.1016/j.molcata.2006.04.009.
Motswainyana, W. M., Onani, M. O.,
& Lalancette, R.
A. (2012). Chlorido[2-({[2-(diphenylphosphanyl)benzylidene]
amino}methyl)thiophene-κ2 N,P ]methylpalladium(II). Acta
Crystallographica E, 68, m339. DOI: 10.1107/s160053681200
7295.
Newkome, G. R. (1993). Pyridylphosphines. Chemical Reviews,
93, 2067–2089. DOI: 10.1021/cr00022a006.
Nobre, S. M., & Monteiro, A. L. (2009). Pd complexes of
iminophosphine ligands: A homogeneous molecular catalyst
for Suzuki–Miyaura cross-coupling reactions under mild con-
ditions. Journal of Molecular Catalysis A: Chemical, 313,
65–73. DOI: 10.1016/j.molcata.2009.08.003.
Ojwach, S. O., Westman, G., & Darkwa, J. (2007). Substi-
tuted (pyridinyl)benzoazole palladium complexes: Synthesis
and application as Heck coupling catalysts. Polyhedron, 26,
5544–5552. DOI: 10.1016/j.poly.2007.08.033.
Onani, M. O., Motswainyana, W. M., Iwuoha, E. I., Darkwa,
J., & Lalancette, R. A. (2010). Chlorido{N-[2-(diphenylphos-
phanyl)benzylidene]-2-(2-thienyl)ethanamine-κ2 N,P }
methylpalladium(II) dichloromethane hemisolvate. Acta
Crystallographica E, 66, m688. DOI: 10.1107/s160053681001
7824.
Park, P. J. D., & Hendra, P. J. (1968). The vibration spec-
trum of trimethylphosphine-d9. Spectrochimica Acta Part A:
Molecular Spectroscopy, 24, 2081–2087. DOI: 10.1016/0584-
8539(68)80268-5.