Novel hemi-labile pyridyl-imine palladium complexes: Synthesis, molecular structures and reactions with ethylene

Article abstract of DOI:10.1016/j.poly.2011.07.004

Ligands (2-pyridyl-2-furylmethyl)imine, (L1), (2-pyridyl-2-thiophenemethyl) imine (L2), and (2-pyridyl-2-thiopheneethyl)imine (L3) were synthesized by condensation reactions and obtained in good yields. Reactions of L1-L3 with either [PdClMe(cod)] or [PdCl₂(cod)] gave the corresponding monometallic palladium(II) complexes 1-5 in very good yields. Molecular structures of complexes 1, 4 and 5 indicated that the ligands are bidentate and coordinate to the palladium metal through the imine and pyridine nitrogen atoms. When complexes 3-5 were treated with NaBAr₄, cationic species, 3a, 4a, and 5a were produced which catalyzed polymerization of ethylene though with very low activities. ¹H NMR spectroscopy studies showed that these cationic species were very stable in solution. DFT calculations showed high ethylene coordination barriers to the cationic species 3a, 4a and 5a.

Full text of DOI:10.1016/j.poly.2011.07.004

Polyhedron 30 (2011) 2574–2580
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Novel hemi-labile pyridyl-imine palladium complexes: Synthesis, molecular
structures and reactions with ethylene
a
William M. Motswainyana a, Stephen O. Ojwach b, Martin O. Onani a, , Emmanuel I. Iwuoha ,
⇑
James Darkwa c
a Chemistry Department, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa
b Department of Chemistry, Maseno University, P.O. Box 333, Maseno 40105, Kenya
c Chemistry Department, University of Johannesburg, Auckland Park Kingsway Campus, Johannesburg 2006, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
Ligands (2-pyridyl-2-furylmethyl)imine, (L1), (2-pyridyl-2-thiophenemethyl)imine (L2), and (2-pyridyl-
2-thiopheneethyl)imine (L3) were synthesized by condensation reactions and obtained in good yields.
Reactions of L1–L3 with either [PdClMe(cod)] or [PdCl2(cod)] gave the corresponding monometallic pal-
ladium(II) complexes 1–5 in very good yields. Molecular structures of complexes 1, 4 and 5 indicated that
the ligands are bidentate and coordinate to the palladium metal through the imine and pyridine nitrogen
atoms. When complexes 3–5 were treated with NaBAr4, cationic species, 3a, 4a, and 5a were produced
which catalyzed polymerization of ethylene though with very low activities. 1H NMR spectroscopy stud-
ies showed that these cationic species were very stable in solution. DFT calculations showed high ethyl-
ene coordination barriers to the cationic species 3a, 4a and 5a.
Received 13 June 2011
Accepted 3 July 2011
Available online 23 July 2011
Keywords:
Hemi-lability
Pyridyl-imine
Palladium complexes
Molecular structures
Ethylene
Ó 2011 Elsevier Ltd. All rights reserved.
DFT calculations
1. Introduction
or polymerization process. However, it has been shown that more
electrophilic metal centres give rise to unstable catalysts leading to
The development of nitrogen-donor late transition metal
catalysts as olefin oligomerization and polymerization catalysts
has witnessed significant growth since the discovery of Brookhart
rapid decomposition of the active species [3,4]. On the other hand,
use of electron-rich ligands produce stable catalysts but exhibit
very low activities [5,6]. One approach to the design of stable
and more active olefin oligomerization and polymerization
catalysts has been through the use of ‘‘hemi-labile ligands’’ first
introduced by Jeffrey and Rauchfuss in 1979 [7]. The role of the
hemi-labile donor group is to stabilize the active cationic species,
hence improving stability of the catalysts. The crucial interplay in
using hemi-labile ligands is the balance between the donor ability
of the hemi-labile group and the incoming monomer. It is essential
that the incoming monomer be more strongly coordinating than
the hemi-labile group in order to gain access to the metal centre
[2b,c].
Several examples of ethylene oligomerization and polymeriza-
tion catalysts that contain hemi-labile ligands have been re-
ported in literature. For example, palladium complexes of
hemi-labile P^N^O ligands [8] have been shown to give active
or inactive ethylene oligomerization catalysts depending on the
donor strength of the O–R functionality (Eq. (1)). While catalysts
bearing methoxy group show low activity (86 mg product after
24 h), no catalytic activity was observed for those containing
an O–H group.
and co-workers [1] that
a-diimine nickel and palladium
complexes produce active catalysts for ethylene polymerization
reactions. To date several reviews have been written on late tran-
sition-metal olefin oligomerization and polymerization catalysis
[2]. From these literature reports, it is evident that catalyst
performance (activity and selectivity) in these reactions can be
regulated through ligand modification as well as changing the
identity of the metal centre.
While catalyst selectivity towards the formation of either oligo-
mers or polymers has generally been understood to be controlled
by steric factors of the ligand backbone [2a], understanding the
balance between catalyst activity and stability still remains a dark
horse. Generally, catalysts containing more electrophilic metal
centre tend to be more active owing to the ease of olefin coordina-
tion to the vacant metal site, a crucial step in olefin oligomerization
⇑
Corresponding author. Tel.: +27 219593050; fax: +27 219593055.
E-mail address: monani@uwc.ac.za (M.O. Onani).
0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.poly.2011.07.004

W.M. Motswainyana et al. / Polyhedron 30 (2011) 2574–2580
2575
Ph
Ph
Pd
7.92 (d, 1H, J = 8.0, py); 13C NMR (200 MHz, CDCl3) d 142.29,
110.33, 107.79, 151.74; 163.67; 56.67; 154.36, 149.36, 124.83,
136.45, 121.35; Anal. Calc. for C11H10N2O: C, 70.95; H, 5.41; N,
15.04. Found: C, 70.58, H, 5.24, N, 14.88%.
Ph
Ph
Pd
P
N
P
N
Me
Me
O
Me
40 bar
OMe
2.2.2. (2-Pyridyl-2-thiophenemethyl)imine (L2)
Compound L2 was synthesized following the procedure de-
scribed for L1 using (2-pyridinecarboxaldehyde (0.51 g, 4.76 mmol)
and 2-thiophenemethylamine (0.54 g, 4.76 mmol). Light yellow oil
MeO
OMe
was obtained. Yield = 0.89 g (92%). IR (Nujol cmꢁ1);
m
(C@N) 1648,
(C–S–C) 1314, 1H NMR (200 Hz, CDCl3) d 7.78 (t, 1H, J = 5.0, thio-
ð1Þ
m
phen); 7.27 (t, 1H, J = 6.0, thiophen); 7.02 (d, 1H, J = 7.2, thiophen);
5.07 (s, 2H, –CH2); 8.48 (s, H-8, –CH@N); 8.67 (d, 1H, J = 3.8, py);
7.37 (t, 1H, J = 6.6, py); 8.07 (d, 1H, J = 8.0, py); 8.11 (d, 1H, J = 7.6,
py). 13C NMR (200 MHz, CDCl3) d 58.89, 121.39, 124.91, 125.37,
126.30, 126.89, 136.53, 141.17; 149.38, 154.36, 163.05. Anal. Calc.
for C11H10N2S: C, 65.32; H, 4.98; N, 13.85. Found: C, 65.12; H, 5.06;
N, 14.09%.
It is believed that the non-labile phenolate group coordinates to
the metal centre without disassociation as required for an active
catalyst and hence blocks ethylene coordination. However, these
catalysts showed high stability in solution, presumably due to
the stabilizing role of the phenolate group. In our current attempt
to develop both active and stable ethylene oligomerization and
polymerization catalysts, we report the synthesis of new palladium
complexes of imino-pyridine ligands containing potential hemi-la-
bile thiophene or furan groups. Molecular structures of these palla-
dium complexes and attempts to use them as ethylene
oligomerization or polymerization catalysts are discussed.
2.2.3. (2-Pyridyl-2-thiopheneethyl)imine (L3)
This compound was also prepared according to the procedure
used for L1 using 2-pyridinecarboxaldehyde (0.38 g, 3.56 mmol)
and 2-thiopheneethylamine (0.45 g, 3.56 mmol). Light yellow oil
was obtained. Yield = 0.74 g (92%). IR (Nujol cmꢁ1);
m
m
(C@N) 1650,
2. Experimental
(C–S–C) 1336, 1H NMR (200 Hz, CDCl3) d 7.32 (dd, 1H, J = 5.8, thio-
2.1. Materials and methods
phen); 6.95 (t, 1H, J = 7.0, thiophen); 7.50 (t, 1H, J = 5.4, thiophen);
3.21 (t, 2H, J = 6.4 = N–CH2); 3.91 (t, 2H, J = 6.8, –CH2); 8.31 (s, 1H, –
CH@N); 8.65 (d, 1H, J = 4.2, py); 7.46 (t, H, J = 7.2, py); 7.92 (d, 1H,
J = 7.6, py); 7.98 (t, 1H, J = 7.4, py); 13C NMR (200 MHz, CDCl3) d
30.60, 61.40, 120.53, 124.10, 125.21, 125.36, 126.83, 136.98,
141.97; 149.37, 153.98, 162.56. Anal. Calc. for C12H12N2S: C,
66.63; H, 5.59; N, 12.95. Found: C, 66.77; H, 5.76; N, 12.57%.
All reactions were carried out under nitrogen atmosphere using
standard Schlenk techniques. Dichloromethane and hexane were
refluxed and distilled from calcium hydride (CaH2) while diethyl
ether was dried over sodium wire and benzophenone. Methanol
was dried from magnesium. Starting materials, [PdClMe(cod)]
and [PdCl2(cod)] were prepared following literature methods [9].
The following materials, anhydrous magnesium sulfate, 2-pyri-
dinecarboxaldehyde, 2-furylmethylamine, 2-thiopheneethylamine,
tetramethyltin, 1,5-cyclooctadiene, palladium(II)chloride and 2-
thiophenemethylamine were purchased from Sigma–Aldrich and
used without any further purification. The NMR experiments were
done on a Varian XR200 MHz spectrometer. Chemical shifts are gi-
ven in ppm while all coupling constants are reported in Hz. IR spec-
tra in solution were recorded on a Perkin–Elmer Spectrum 100
Series FT-IR instrument using Nujol mulls on NaCl plates. Elemen-
tal analyses was performed on Server 1112 Series Elemental Ana-
lyzer. Melting points were recorded on open capillaries using
SMP10 melting point apparatus.
2.2.4. Dichloro-[(2-pyridyl-2-furylmethyl)imine]palladium (II) (1)
To a solution of [PdCl2(cod)] (0.10 g, 0.35 mmol) in CH2Cl2
(20 mL) was added a solution of L1 (0.07 g, 0.35 mmol) in CH2Cl2
(5 mL). The solution was stirred for 6 h to give a light yellow pre-
cipitate. The precipitate was filtered to obtain a light yellow solid.
Recrystallization from a mixture of CH2CN: hexane solution affor-
ded single crystals suitable for X-ray analysis. Yield = 0.11 g (88%),
mp: 173 °C. IR (Nujol cmꢁ1); (C–O–C) 1301, 1H NMR
m(C@N) 1597, m
(200 MHz, DMSO) d 6.84 (t, 1H, J = 4.0, furan); 5.56 (d, 1H, J = 3.2,
furan); 5.48 (t, 1H, J = 2.4, furan); 4.01 (s, 2H, –CH2); 7.56 (s, 1H,
–CH@N); 7.91 (d, 1H, J = 5.2, py); 6.67 (t, 1H, J = 2.4, py); 7.15 (d,
1H, J = 5.4, py); 7.29 (t, 1H, J = 6.0, py); 13C NMR (200 MHz, DMSO)
d 53.47, 111.10, 111.52, 128.95, 141.39, 143.89, 147.77, 150.24;
155.62, 172.31. Anal. Calc. for C11H10Cl2N2PdO: C, 36.34; H, 2.77;
N, 7.71. Found: C, 36.78; H, 2.70; N, 7.50%.
2.2. Synthesis of ligands and palladium complexes
2.2.1. (2-Pyridyl-2-furylmethyl)imine (L1)
To a solution of 2-pyridinecarboxaldehyde (0.20 g, 1.89 mmol)
and anhydrous magnesium sulfate (1.00 g) in methanol (15 mL)
at 0 °C was added dropwise a solution of 2-furylmethylamine
(0.18 g, 1.89 mmol) in methanol (15 mL). The reaction was allowed
to proceed at room temperature for 12 h. After the reaction period,
the light orange mixture was filtered and solvent removed under
vacuum to obtain the crude product. The crude product was
washed with water (10 mL) and the organic material extracted
with dichloromethane (2 ꢀ 10 mL) and dried over anhydrous mag-
nesium sulfate. The solvent was then removed to afford compound
L1 as a light brown oil. Yield = 0.33 g (93%). IR (Nujol cmꢁ1);
2.2.5. Dichloro-[(2-pyridyl-2-thiophenemethyl)imine]palladium (II)
(2)
Complex 2 was prepared in a similar manner to complex 1.
[PdCl2(cod)] (0.10 g, 0.35 mmol) and L2 (0.07 g, 0.35 mmol). Yel-
low solid. Yield = 0. 11 g (85%), mp: 171 °C. IR (Nujol cmꢁ1);
m
(C@N) 1598, m
(C–S–C) 1306, 1H NMR (200 Hz, DMSO) d 6.94 (t,
1H, J = 7.0, thiophen); 6.66 (d, 1H, J = 5.2, thiophen); 6.16 (d, 1H,
J = 4.8, thiophen); 4.28 (s, 2H, –CH2); 7.79 (s, 1H, –CH@N); 8.01
(d, 1H, J = 5.2, py); 6.37 (d, 1H, J = 6.8, py); 7.23 (d, 1H, J = 7.6,
py); 7.40 (t, 1H, J = 7.4, py); 13C NMR (200 MHz, DMSO) d 54.74;
127.07, 127.40; 128.76, 128.80, 136.93, 141.20, 150.03, 155.60,
172.21. Anal. Calc. for C11H10Cl2N2PdS: C, 34.80; H, 2.66; N, 7.38.
Found: C, 35.98; H, 2.70; N, 7.60%.
m
(C@N) 1649, m
(C–O–C) 1325, 1H NMR (200 MHz, CDCl3): d 7.60
(t, 1H, J = 7.6, furan); 6.22 (d, 1H, J = 7.2, furan); 6.16 (d, 1H,
J = 3.0, furan); 4.71 (s, 2H, –CH2); 8.30 (s, 1H, –CH@N); 8.51 (d,
1H, J = 4.6, py); 7.19 (dd, 1H, J = 6.2, py); 7.88 (d, 1H, J = 7.8, py);

2576
W.M. Motswainyana et al. / Polyhedron 30 (2011) 2574–2580
2.2.6. Chloromethyl-[(2-pyridyl-2-furylmethyl)imine]palladium (II)
(3)
J = 5.6, thiophen). 13C NMR (200 MHz, CDCl3) d: 29.73; 61.56,
30.58; 124.77, 125.31, 126.54, 128.07, 127.24, 127.77; 138.53;
141.42, 149.64, 166.44. Anal. Calc. for C13H15ClN2PdS: C, 41.84; H,
4.05; N, 7.51. Found: C, 41.65; H, 4.25; N, 7.23%.
To a solution of [PdClMe(cod)] (0.04 g, 0.16 mmol) in 1:2 mix-
ture of CH2Cl2/Et2O (15 mL) was added a solution of L1 (0.03 g,
0.16 mmol) CH2Cl2 (2 mL). The yellow solution was stirred for 5 h
at room temperature to produce a light yellow precipitate. Recrys-
tallization from CH2Cl2:hexane gave 3 as an analytically pure
yellow solid. Yield = 0.06 g (98%), mp: 132 °C. IR (Nujol cmꢁ1);
2.3. Molecular structures of 1, 4 and 5
Single-crystal X-ray diffraction data for compounds 1, 4 and 5
were collected on a Bruker KAPPA APEX II DUO diffractometer
m
(C@N) 1590, m
(C–O–C) 1291, 1H NMR (200 Hz, CDCl3) d 4.97 (s,
2H, CH2); 8.16 (s, 1H, –CH@N); 1.11 (s, 3H, Pd–Me); 9.07 (d, 1H,
J = 5.2, py); 7.94 (t, 1H, J = 7.6, py); 7.45 (t, 1H, J = 6.8, furan);
7.62 (t, 1H, J = 5.4, py); 6.47 (t, 1H, J = 4.6, furan); 7.23 (t, 1H,
J = 5.0, furan), 6.53 (d, 1H, J = 4.4, py). 13C NMR (200 MHz, CDCl3)
d: 29.70; 54.67; 111.07, 111.78, 125.85; 128.20, 138.48, 141.20,
143.86, 149.62, 166.61. Anal. Calc. for C12H13ClN2PdO: C, 42.01;
H, 3.82; N, 8.16. Found: C, 42.21; H, 3.50; N, 8.25%.
using graphite-monochromated Mo K
The crystal structure was solved by direct methods using SHELXS
97 [10] and refined by full-matrix least-squares methods based
on F [10] using SHELXL-97 [10] and using the graphics interface pro-
gram X-SEED [11,12]. The programs X-SEED and POV-RAY [12] were both
used to prepare molecular graphic images.
a radiation (v = 0.71073 Å).
-
2.4. Preparation of compound 5a its 1H NMR spectroscopy study
2.2.7. Chloromethyl-[(2-pyridyl-2-thiophenemethyl)imine]palladium
(II) (4)
To a mixture of 5 (0.10 g, 0.24 mmol) and NaBAr4 in an NMR
tube was added CDCl3 (1.00 ml) to give 5a. The solution was sha-
ken and 1H NMR spectrum of the solution acquired at various
intervals for 7 days. 1H NMR of 5a after 15 min (200 Hz, CDCl3)
d: 1.11 (s, 3H, Pd–CH3); 3.20 (t, 2H, J = 5.6, CH2); 3.92 (t, 2H,
J = 4.4, CH2); 7.07–8.47 (m, 7H, thiophene and py); 7.50 (s, 4H,
BAr4), 7.71 (s, 8H, BAr4); 8.01 (s, 1H, H-imine).
The complex was prepared following the method described for
3
using [PdClMe(cod)] (0.05 g, 0.20 mmol) and L2 (0.04 g,
0.20 mmol). Recrystallization from CH2Cl2:hexane solution affor-
ded yellow single crystals suitable for X-ray analysis. Yield = 0.07 g,
0.19 g (95%). mp: 129 °C. IR (Nujol cmꢁ1);
m(C@N) 1591, m(C–S–C)
1309, 1H NMR (200 Hz, CDCl3) d 5.16 (s, 2H, CH2); 8.16 (s, 1H, –
CH@N); 1.12 (s, 3H, Pd–Me); 9.06 (d, 1H, J = 5.4, py); 8.56 (d, 1H,
J = 3.8, py); 7.07 (dd, 1H, J = 4.6, thiophen); 7.23 (t, 1H, J = 4.2,
py); 7.35 (dd, 1H, J = 7.0, thiophen); 7.59 (t, 1H, J = 7.2, thiophen);
7.94 (d, 1H, J = 7.2, py). 13C NMR (200 MHz, CDCl3) d: 29.70,
56.37, 125.90, 127.10, 127.46, 128.18, 129.13, 1135.66, 38.49,
148.84, 149.62, 160.74, 166.28. Anal. Calc. for C12H13ClN2PdS: C,
40.13; H, 3.65; N, 7.80. Found: C, 39.99; H, 3.23; N, 7.43%.
3. Results and discussion
3.1. Synthesis of the ligands and metal complexes
Compounds L1–L3 were prepared by condensation reactions of
2-pyridinecarboxaldehyde with the appropriate (furylalkyl)amine
or (thiophenylalkyl)amine (Scheme 1). The compounds were iso-
lated as light brown oils in quantitative yields. Reactions of L1–
L3 with either [PdCl2(cod)] or [PdMeCl(cod)] afforded the corre-
sponding complexes, 1–5, in moderate yields (Scheme 1).
IR spectra of the ligands generally exhibited strong absorption
bands at around 1650 cmꢁ1, typical of an imine functionality
[13–16]. Coordination of L1–L3 to the palladium atom could be de-
duced from the IR spectra of complexes 1–5, which showed typical
lower absorption bands between 1590 and 1598 cmꢁ1 [17] com-
pared to the respective ligands. 1H NMR spectra of the compounds
also provided a useful tool for their elucidation. For instance, sin-
glet peaks between 8.36 and 8.76 ppm in L1–L3 were indicative
2.2.8. Chloromethyl-[(2-pyridyl-2-thiopheneethyl)imine]palladium
(II) (5)
Complex 5 was prepared in a similar manner to 3 using
[PdClMe(cod)] (0.05 g, 0.19 mmol) and L3 (0.04 g, 0.19 mmol).
The light yellow solid was recrystallized from CH2Cl2:hexane solu-
tion to give single crystals suitable for X-ray analysis. Yield = 0.06 g
(90%). mp: 126 °C. IR (Nujol cmꢁ1);
m(C@N) 1590, m(C–S–C) 1296,
mp: °C, 1H NMR (200 Hz, CDCl3) d 3.37 (s, 2H, @N–CH2); 3.99 (s,
2H, –CH2); 7.79 (s, 1H, –CH@N); 1.08 (s, 3H, Pd–Me); 9.09 (d, 1H,
J = 5.8, py); 7.91 (d, 1H, J = 4.2, py); 7.62 (t, 1H, J = 4.2, thiophen);
7.49 (t, 1H, J = 4.8, py); 7.15 (d, 1H, J = 5.2, thiophen); 6.85 (t, 1H,
n
X
N
n
+
H
NH2
N
X
N
O
Et2O
PdClY(cod)
n
N
N
X
Pd
Cl
X = O, Y = Cl, n = 1 ( 1); X = S, Y = Cl, n = 1 ( 2)
Y
X = O, Y = Me, n = 1 ( 3); X = S, Y = Me, n = 1 ( 4)
X = S, Y = Me, n = 2 ( 5)
Scheme 1.

W.M. Motswainyana et al. / Polyhedron 30 (2011) 2574–2580
2577
of the presence of the imine protons [18]. In addition, singlet at
4.83 and 5.00 ppm were assigned to the CH2 linker protons in L1
and L2, respectively [19]. 1H NMR spectra of complexes 1–5
showed upfield shifts of the imine protons in the region of 7.5–
8.16 ppm. In the 13C NMR spectra, the imine carbons in 1–5 ap-
peared downfield between 166.00 and 173.00 ppm compared to
162.00 ppm in the free imine ligands. Zhang et al. reported a sim-
ilar trend in related imino-pyridyl palladium complexes where
they observed a downfield shift from 167.00 to 165.4 ppm in the
free imine compounds [13].
angle for N(1)–Pd(1)–Cl(2) of (90.59(5)°) in 1 is slightly higher than
the angle N(1)–Pd(1)–Cl(1) of (88.81(6)°) observed in 4. The bond
angles for N(1)–Pd(1)–N(2) of 80.5(2)° in 1, 79.10(6)° in 4 and
79.33(6)° in 5 are all statistically similar and compare well with
bond angles in related 2-methoxycarbonyl-6-iminopyridine palla-
dium compounds which averaged 79.4(2)° [13]. The average Pd–
N(2) bonds lengths of 2.044(4) Å in 1, 4 and 5 compare well with
bond lengths of 2.045(2) Å reported for the unconjugated diimine
palladium complexes [20]. The differences between Pd(1)–Cl(1)
and Pd(1)–Cl(2) bond lengths in 1 reflects the stronger trans influ-
ence of the pyridyl group compared to the imine group. This is con-
sistent with the observations of Doherty et al. [21]. In general, the
Pd–Cl distances in the three complexes are normal, averaging
2.290(13) Å. This value is in good agreement with the Pd–Cl bond
distance of 2.298(15) Å averaged for 491 Pd complexes as reported
in the Cambridge Structural Database (CSD) [22]. The Pd(1)–N(1)
bond length of 2.1322(16) Å in 4 is significantly longer than the
Pd(1)–N(2) bond length of 2.0509 (16) Å; this difference reflects
the greater trans influence of the methyl group compared to the
chloride ligand [23]. Similar trend is also observed in complex 5.
The Pd–CH3 bond length in 4 was reported as 2.0366(18) Å, and
compares well with the bond lengths of 2.005(12) Å observed in
other related palladium complexes [22].
3.2. Molecular structures of complexes 1, 4 and 5
Single crystals of complexes 1, 4 and 5 suitable for X-ray anal-
yses were grown by slow diffusion of hexane into a dichlorometh-
ane solution at room temperature. Crystallographic data and
structural refinement parameters are given in Table 1 while se-
lected bond lengths and angles are contained in Table 2. Molecular
structures of complexes 1, 4 and 5 are shown in Figs. 1–3. The
geometry around the palladium atom in the three complexes (1,
4 and 5) could be described as distorted square planar. For in-
stance, the bond angles around the Pd metal atoms of N(1)–
Pd(1)–N(2) (79.10(6)°) in 1 and N(8)–Pd(1)–N(1) (80.5(2)°) in 4
show significant deviations from planarity. The reported bond
3.3. Reactions of complexes 3–5 with ethylene
The chloromethyl palladium complexes (3–5) were investigated
for their ability to oligomerize or polymerize ethylene using Na-
BAr4 (Ar = 3,5-(CF3)C6H3) as the activator. In a typical experiment,
approximately 0.05 mmol of the respective complex was reacted
with an equivalent amount of NaBAr4 in CH2Cl2 (80 mL). Ethylene
pressure of 40 bar, time of 2 h and temperature of 30 °C was em-
ployed (Scheme 2). After the reaction time, the volatile compo-
nents were trapped using liquid nitrogen and analyzed for the
presence of dimers (butenes). The GC chromatogram did not show
the presence of any volatile oligomers. Then, excess ethylene was
vented off and the reaction quenched by addition of a small
amount of MeOH. The solvent was then removed under reduced
pressure to give a white residue (Yield = 0.08 g). 1H NMR spectrum
of the product showed signature peaks of low density polyethylene
at 0.87 and 1.25 ppm [1]. From the 1H NMR spectrum, the degree of
branching in the polymer was calculated to contain 28 carbons
atoms/1000 carbon atoms. In addition, the 1H NMR spectrum also
showed signals at 7.48 and 7.68 ppm corresponding to the BAr4
protons. Attempts to optimize the catalytic conditions to improve
the product yields were made. For instance, catalyst loading was
increased by 10-fold to 0.5 mmol and longer reaction time of 5 h
was employed. However, all these variations did not improve the
activity of the catalysts and negligible amounts of products were
obtained. Interesting though, no palladium black was observed,
indicating the absence of catalyst decomposition, thus ruling out
the possibility of catalyst instability. Therefore, this poor activity
of catalysts derived from complexes 3–5 could be due to; one, poor
electrophilicity of the Pd metal centre which hinders ethylene
coordination, and two, stronger binding affinity of the furan or thi-
ophene groups, which blocks ethylene coordination.
Table 1
Crystal data and structure refinement parameters for complexes 1, 4 and 5.
Parameter
1
4
5
Formula
Formula weight
T (K)
Wavelength (Å)
Crystal system
Space group
a (Å)
C
11H10Cl2N2OPd C22H13ClN2PdS C23H15ClN2PdS
363.51
100(2)
0.71073
monoclinic
P21/c
7.1568(9)
21.642(3)
8.2697(10)
90
108.788(5)
90
1212.6(3)
4
359.5
100(2)
0.71073
monoclinic
P21/c
8.7860(2)
8.8009(2)
9.5884(2)
88.2940(10)
63.3111(10)
77.8350(10)(2) 90
645.76(2)
2
373.18
100(2)
0.71073
monoclinic
P21/c
9.7013(13)
17.425(2)
8.5427(12)
90
b (Å)
c (Å)
a
(°)
b (°)
(°)
107.723(4)
c
Volume (Å3)
Z
1375.6(3)
4
1.802
1.677
DCalcd (mg/m3)
Absorption coefficient
1.991
1.95
1.847
1.78
(mmꢁ1
F (0 0 0)
)
712
356
744
Final R indices (R1)
R indices all data (R1)
Reflections collected
Completeness to theta
(%)
0.1316
0.0526
9213
99.4
0.020
0.050
12 231
99.6
0.1404
0.1631
23 515
98.8
Goodness of Fit (GOF)
0.982
1.05
1.106
on F2
Largest difference peak 0.411 and
and hole (e Åꢁ3
ꢁ0.261
2.923 and
ꢁ0.567
2.944 and
ꢁ1.908
)
Table 2
Selected bond lengths and angles for complexes 1, 4 and 5.
Since our initial objective in this ligand design was to use the
furan or thiophene donor groups as hemi-labile motifs to stabilize
the active intermediate species, we further investigated the
propensity of these groups to stabilize the active species and their
subsequent displacement by an incoming ethylene monomer
(Scheme 2). This was done by 1H NMR spectroscopic studies to
determine half-lives of the cationic compounds 3a, 4a and 5a
(see section 2.4) and Density Functional Theory calculations [24]
to evaluate their ethylene coordination barrier. Table 3 gives
selected 1H NMR signals, half-lives and thermodynamic data of
1 X = Cl(1)
4 X = Cl(1)
5 X = C(13)
Bond lengths (Å)
Pd(1)–N(1)
Pd(1)–N(2)
Pd(1)–X
2.033(2)
1.983(2)
2.3191(7)
2.028(5)(5)
1.985(5)
2.3121(17)
2.137(4)
2.057(16)
2.079(5)
Bond angles (°)
N(1)–Pd(1)–N(2)
N(1)–Pd(1)–X
N(2)–Pd(1)–X
80.5(2)
93.8(1)
95.1(1)
79.10(6)
96.06(4)
95.1(1)
79.33(16)
174.15(6)
95.30(7)

2578
W.M. Motswainyana et al. / Polyhedron 30 (2011) 2574–2580
Fig. 1. Molecular structure of 1 shown with 50% probability ellipsoids.
Fig. 2. Molecular structure of 4 shown with 50% probability ellipsoids. The thiophene molecule is severely distorted.
Fig. 3. Molecular structure of 5 shown with 50% probability ellipsoids. Hydrogen atoms are ommitted for clarity.

W.M. Motswainyana et al. / Polyhedron 30 (2011) 2574–2580
2579
BAr4
BAr4
N
N
Pd
NaBAr4
N
N
X
Me
N
Pd
n
Pd
CH2Cl2
40 bar
Me
N
Me
n
Cl
n
X
X
X = O, n = 1 (3a)
X = S, n = 1 (4a)
X = S, n = 2 (5a)
Scheme 2.
(5a) were obtained (Table 3). In an earlier study by Morokuma
et al. [25], ethylene coordination barriers for the -diimine palla-
dium catalysts of
H = ꢁ29.4 kcal/mol were re-
Table 3
Selected 1H NMR signals, half-lives (t1/2
barriers to the cationic complexes 3a–5a.
)
and energies of ethylene coordination
Energies
a
D
G = ꢁ18.4 and
D
1H NMR (CDCl3, ppm)
ported. These values are significantly more negative than those
obtained for our complexes, 3a, 4a and 5a. It is therefore conceiv-
able that complexes 3–5 gave poor activities due to the difficulty of
ethylene in displacing the coordinated furan or thiophene groups.
Compound
Pd–Me
–CH2–
C@N–H
t1/2 (h)a
D
Hb
D
Gb
3a
4a
5a
1.07
1.12
1.11
5.05
5.23
3.20, 3.92
8.11
7.98
8.03
84
96
72
ꢁ16.53
ꢁ13.18
18.24
ꢁ5.48
ꢁ2.34
ꢁ8.26
Typical
DG values of ethylene coordination to the vacant metal
centres of active ethylene polymerization catalysts are known to
be between ꢁ30 and ꢁ60 kcal/mol [26]. A recent publication by
Ojwach et al. [27] on the tridentate bound bis(pyrazolyl)pyridine
palladium complexes gave ethylene coordination barriers of
a
Determined by 1H NMR spectroscopy using BAr4 protons as internal standard.
Calculated by Density Functional Theory at the B3LYP/LAN2DZ level of theory.
b
Units, kcal/mol.
D
H = +1.7 kcal/mol and DG = +12.6 kcal/mol. These tridentate
bis(pyrazolyl)pyridine palladium complexes are completely inac-
tive towards ethylene oligomerization or polymerization [27]. In
another related work, Darkwa and co-workers reported that
[PdCl2{(R2pz-CO)2py}] can be activated to afford active catalysts
for the polymerization of ethylene [28]. The carbonyl linker group
in these complexes makes the Pd–Npz bonds weaker resulting in
dissociation of one of the coordinated pyrazolyl units to allow eth-
ylene to coordination to the palladium metal centre and undergo
subsequent insertion to form polyethylene. The lack of increased
catalytic activity even when complexes 3–5 were subjected to high
ethylene pressures of 40 atm therefore points to the strength of the
Pd–O(furan) or Pd–S(thiophene) bonds in 3a, 4a and 5a relative to the
binding affinity of ethylene monomer.
compounds 3a, 4a and 5a. Fig. 4 shows the optimized geometries
of 4a and its corresponding ethylene coordinated complex. The
identity of the cationic compounds, 3a, 4a and 5a could be easily
deduced from their respective 1H NMR spectra obtained after the
addition of NaBAr4. For example, in 5a, the signal corresponding
to the Pd-Me protons was observed at 1.11 ppm as compared to
1.05 ppm in 5. Similarly, upfield shifts were observed for the ethyl-
ene linker at 3.20 and 3.92 ppm for 5a relative to the signals at 3.32
and 4.03 ppm in 5. More conspicuous change was observed in the
signal of the furan proton at 8.47 ppm compared to the downfield
signal at 8.62 ppm in complex 5.
From Table 3, it was evident that these cationic complexes were
stable, with half-lives of 84, 96 and 72 h for 3a, 4a and 5a,
respectively. This confirms the stabilities of the catalysts obtained
from these complexes and the absence of any palladium black
4. Conclusion
formation. Ethylene coordination barriers of
H = ꢁ16.53 kcal/mol (3a), G = ꢁ2.34 kcal/mol;
kcal/mol (4a) and H = ꢁ18.24 kcal/mol
G = ꢁ8.26 kcal/mol;
D
G = ꢁ5.48 kcal/mo;
D
D
D
H = ꢁ13.18
The pyridyl-imine compounds containing furan or thiophene
hemi-labile groups react with palladium metal precursors to
D
D
Fig. 4. Optimized geometries of 4a (a) and its corresponding ethylene complex (b) computed at the B3LYP/LANL2DZ level of theory.

2580
W.M. Motswainyana et al. / Polyhedron 30 (2011) 2574–2580
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Gertenbach (University of Stellenbosch) for solving the crystal
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CCDC 828159, 828093 and 828094 contain the supplementary
crystallographic data for compounds 1, 4 and 5. These data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/
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