Cationic palladacycles as catalyst precursors for phenyl acetylene polymerization

Article abstract of DOI:10.1016/j.jorganchem.2011.07.044

Novel cationic palladacycles based on benzylidene-2,6- diisopropylphenylimines were prepared via C-H activation using Pd(CH ₃CN)₂Cl₂ as metal precursor. The complexes were fully characterized by IR and NMR spectroscopy, mass spectrometry and elemental analysis. The cationic palladacycles were found to be active catalysts for the polymerization of phenylacetylene producing largely trans-cisoidal PPA.

Full text of DOI:10.1016/j.jorganchem.2011.07.044

Journal of Organometallic Chemistry 696 (2011) 3527e3535
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Journal of Organometallic Chemistry
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Cationic palladacycles as catalyst precursors for phenyl acetylene polymerization
,
c
N. Mungwe a, A.J. Swarts b, S.F. Mapolie b , G. Westman
*
a Department of Chemistry, University of the Western Cape, Bellville, South Africa
DST-NRF Centre of Excellence in Catalysis (c change), Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag 1, Matieland,
b
*
7601 Stellenbosch, South Africa
c The Department of Chemical and Biological Engineering, Chalmers University of Technology, Gothenburg, Sweden
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel cationic palladacycles based on benzylidene-2,6-diisopropylphenylimines were prepared via CeH
activation using Pd(CH3CN)2Cl2 as metal precursor. The complexes were fully characterized by IR and
NMR spectroscopy, mass spectrometry and elemental analysis. The cationic palladacycles were found to
be active catalysts for the polymerization of phenylacetylene producing largely trans-cisoidal PPA.
Ó 2011 Elsevier B.V. All rights reserved.
Received 7 June 2011
Received in revised form
28 July 2011
Accepted 29 July 2011
Keywords:
Cationic palladacycles
Phenyl acetylene polymerization
Polyphenylacetylene
Benzylideneeamines complexes
1. Introduction
acyl hydrazone ligands, [9] and more recently bis(pyrazole)- and
bis(pyrazolyl)-palladium complexes [10]. Here we report the
The polymerization of substituted acetylene derivatives produce
polymers with backbone conjugation which confer interesting and
novel properties upon the polymeric material [1]. These may be
further enhanced by functionalizing the polymer backbone with
suitable substituents. Physical properties which these polymeric
materials may display include photoconductivity [2] and liquid
crystallinity [3].
synthesis and characterization of cationic palladacycles derived
from imine ligands and their application as phenylacetylene poly-
merization catalysts.
2. Experimental procedure
2.1. General
The first report of the polymerization of acetylenes was by Natta
and co-workers and involved a process mediated by a Ti/Al catalyst
system [4]. This led to the development of early transition metal
catalyst systems, particularly of group 5 and 6 metals, which
demonstrated both high activity and selectivity for acetylene
polymerization reactions [5]. The inherent air and moisture sensi-
tivity of early transition metal complexes led to the development of
late transition metal complexes capable of polymerizing acetylenes
with polar functionality and in polar reaction media. Of all the late
transition metals, rhodium complexes [6] occupy an enviable
position as mediators of acetylene polymerization reactions with
exceptionally high activity and selectivity [7].
All transformations were performed using standard Schlenk
techniques under a nitrogen atmosphere or in a nitrogen-filled glo-
vebox. Solvents were dried by distillation prior to use and all other
reagents were employed as obtained. NMR (1H: 300 and 400 MHz;
13C: 75 and 100 MHz) spectra were recorded on Varian VNMRS
300 MHz; Varian Unity Inova 400 MHz spectrometers and chemical
shifts are reported in ppm, referenced to the residual protons of the
deuterated solvents and tetramethyl silane (TMS) as internal stan-
dard. ESI-MS (positive and negative modes) analyses were performed
on either Waters API Quattro Micro or Waters API Q-TOF Ultima
instruments by direct injection of sample (Capillary voltage: 3.5 kV;
Cone voltage: 15 RF1:40; Source: 100 ꢀC; Desolvation Temp: 400 ꢀC;
Desolvation gas: 500 L/h; Cone gas: 50 L/h). FT-IR analysis was per-
formed on a Thermo Nicolet AVATAR 330 instrument, and was
recorded as neat spectra (ATR) unless otherwise specified. Melting
point determinations were performed on a Stuart Scientific SMP3
melting point apparatus and are reported as uncorrected. The
Few examples of palladium-mediated acetylene polymerization
reactions appear in literature. These include diphosphine-
palladium(II) complexes, [8] Pd(N,N,O)Cl complexes derived from
* Corresponding author. Tel.: þ27 21 8082722.
E-mail address: smapolie@sun.ac.za (S.F. Mapolie).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.07.044

3528
N. Mungwe et al. / Journal of Organometallic Chemistry 696 (2011) 3527e3535
number-average (Mn) and weight-average molecular weights (Mw) as
well as polydispersity (Mw/Mn) were determined by gel permeation
chromatography (THF, 30 ꢀC, rate - 1.0 cc/min) employing a PL mixed-
C column and polystyrene standards on an Agilent 1100 series
chromatograph equipped with an RI detector.
3JHeH 7.78 Hz, 4JHeH 1.17 Hz);
(dt,1H, H5, 3JHeH 7.92 Hz, 4JHeH 1.91 Hz);
(sept., 2H, H12, 3JHeH 6.82 Hz);
13C NMR {1H} (100 MHz, CDCl3, numbering as per Fig. 2.3):
(CH]N, C7); 148.9 (CAr, C8); 137.6 (CAr, C9); 134.6 (CAr, C2);
133.2 (CAr, C1); 132.4 (CAr, C4); 128.8 (CAr, C6); 127.8 (CAr, C3);
125.7 (CAr, C5); 124.4 (CAr, C11); 123.1 (CAr, C10); 27.9 (C12);
23.5 (C13). ESI-MS (Mþ, m/z): 345. Anal. Found: C, 66.23; H, 6.41; N,
d
7.47 (t, 1H, H4, 3JHeH 7.34 Hz);
d
d
7.37
3.00
d
7.19 (m, 3H, H10,11);
d
1.20 (d, 12H, H12,13, 3JHeH 7.02 Hz).
d
161.4
d
d
d
d
d
d
d
d
d
d
d
d
2.2. Synthesis of monofunctional imine ligands, L1eL4
2.2.1. Benzylidene-2,6-diisopropylphenylamine (L1)
4.02. Calc. for C19H22BrN: C, 66.28; H, 6.44; N, 4.07.
To
a stirring solution of 2,6-diisopropylaniline (1.8 mL,
2.2.4. Benzylidenepropylamine (L4)
9.423 mmol) in EtOH (10 mL) was added benzaldehyde (0.96 mL,
9.423 mmol). The resulting yellow solution was stirred for 24 h at
room temperature. After the allotted time the solvent was removed,
the residue obtained redissolved in dichloromethane (20 mL) and
washed with H2O (10 ꢁ 20 mL portions). The organic layer was
dried over MgSO4, filtered and the solvent removed. The yellow
residue was recrystallized from DCM/MeOH at low temperature
and was isolated as a yellow crystalline solid. Yield: 2.25 g, 90%. FT-
The same synthetic procedure as outlined above for (L1) was
employed for the synthesis of L4, using 2-bromobenzaldehyde and
n-propylamine as reagent. Yield: 1.08 g, 74%. FT-IR (
1645 cmꢂ1. 1H NMR (400 MHz, CDCl3, numbering as per Scheme 1):
8.27 (s, 1H, H7); 7.75 (m, 2H, H2,6); 7.43 (m, 3H, H3,4,5);
7.17 (m,
3H, H10,11); 3.54 (t, 2H, H8, 3JHeH 6.70 Hz); 1.68 (sext, 2H, H9, 3JHeH
7.40 Hz, 3JHeH 7.40 Hz); 0.91 (t, 3H, H10, 3JHeH 7.40 Hz). 13C NMR {1H}
(100 MHz, CDCl3, numbering as per Fig. 2.3):
160.6 (CH]N, C7);
128.4 (CAr, C2,6); 127.9 (CAr, C3,5);
11.7 (C10). ESI-MS (Mþ, m/z): 148. Anal. Found:
y, C]N):
d
d
d
d
d
d
d
d
IR (
y
, C]N): 1638 cmꢂ1 1H NMR (400 MHz, CDCl3, numbering as per
8.22 (s, 1H, H7); 7.94 (m, 2H, H2,6);
7.54 (m, 3H,
7.17 (m, 3H, H10,11); 3.00 (sept., 2H, H12 3JHeH 6.82 Hz);
1.19 (d, 12H, H12,13, 3JHeH 7.02 Hz). 13C {1H} NMR (100 MHz, CDCl3,
160.2 (CH]N, C7); 147.5 (C,Ar C8);
133.8 (CAr, C1); 131.1 (CAr, C4); 129.2 (CAr, C2,6);
126.7 (CAr, C11); 124.7 (CAr, C10); 27.96 (C12);
23.45 (C13). ESI-MS (Mþ, m/z): 266. Anal. Found: C, 85. 92; H, 8.70;
d
d
136.2 (CAr, C1);
d
130.2 (CAr, C4);
d
d
Scheme 1):
d
d
d
63.3 (C8); 23.9 (C9);
d
d
H
3,4,5);
d
d
,
C, 81.41; H, 8.82; N, 9.43. Calc. for C10H13N: C, 81.59; H, 8.90; N, 9.51.
d
numbering as per Scheme 1):
d
d
d
d
d
138.3 (CAr, C9);
128.9 (CAr, C3,5);
d
d
d
2.3. Synthesis of m-Cl palladacycles (C1eC4)
d
d
d
2.3.1. Synthesis of [PdCl(C6H4)CH]N{2,6-iPr2eC6H3}]2 (C1)
N, 5.20. Calc. for C19H23N: C, 85.99; H, 8.74; N, 5.28.
To a stirring solution of (MeCN)2PdCl2 (100 mg, 0.386 mmol) in
dichloromethane (10 mL) was added benzylidene-2,6-
diisopropylphenylamine (L1, 102 mg, 0.386 mmol) and NaOAc
(63 mg, 0.772). The reaction mixture was stirred for 18 h at room
temperature, after which the solvent was removed. The yellow solid
residue obtained was redissolved in dichloromethane (20 mL) and
filtered through celite. The solvent was removed from the filtrate and
the pale-yellow solid obtained recrystallized from dichloromethane:
2.2.2. 2-Chlorobenzylidene-2,6-diisopropylphenylamine (L2)
The same synthetic procedure as outlined above for (L1) was
employed for the synthesis of L2, using 2-chlorobenzaldehyde as
reagent. Yield: 2.46 g, 87%. FT-IR (
y
, C]N): 1624 cmꢂ1
8.76 (s, 1H, H7);
7.53 (m, 3H, H4,5,6);
7.24 (m, 3H,
3JHeH 6.82 Hz); 1.30 (d, 12H, H12,13
.
1H NMR
(400 MHz, CDCl3, numbering as per Scheme 1):
d
d
8.37 (d, 1H, H3, 2JHeH 8.58 Hz);
10,11); 3.08 (sept., 2H, H12
d
d
H
d
,
d
,
hexane. Yield: 120 mg, 79%. FT-IR (
(CDCl3, 300 MHz): 7.75 (s, 2H, CH]N);
7.11 (m, 7H, Ar. protons); 3.53 (sept., 4H, iPr-CH, JHeH 6.63 Hz);
y
.
, C]N): 1599 cmꢂ1 1H NMR
3JHeH 7.02 Hz). 13C {1H} NMR (100 MHz, CDCl3, numbering as per
d
d
7.19 (m, 7H, Ar. protons);
3
Scheme 1):
d
159.2 (CH]N, C7);
d
149.1 (CAr, C8);
133.2 (CAr, C1); d 132.2 (CAr, C4);
127.2 (CAr, C5); 124.4 (CAr, C11);
d
137.6 (CAr, C9);
129.9 (CAr, C6);
123.1 (CAr, C10);
d
d
d
i
3
i
3
d
d
d
135.9 (CAr, C2);
128.4 (CAr, C3);
27.9 (C12);
d
d
1.39 (d, 6H, Pr-Me, JHeH 6.04 Hz); d 1.14 (d, 6H, Pr-Me, JHeH
d
d
d
6.82 Hz). 13C {1H} NMR (CDCl3, 75 MHz):
d
176.21 (CH]N);
d
d
155.43
132.56
d
23.5 (C13). ESI-MS (Mþ, m/z): 301. Anal. Found: C,
(C1);
d
145.67 (o-metallated C);
d
141.48 (CAr); 133.90 (CAr);
d
76.05; H, 7.35; N, 4.63. Calc. for C19H22ClN: C, 76.11; H, 7.40; N, 4.67.
(CAr);
(CAr);
d
130.86 (CAr);
d
128.29 (CAr); 127.74 (CAr); 124.68 (CAr); 123.23
22.99, 24.45 (iPr-Me).
d
28.19 (iPr-CH);
d
2.2.3. 2-Bromobenzylidene-2,6-diisopropylphenylamine (L3)
The same synthetic procedure as outlined above for (L1) was
employed for the synthesis of L3, using 2-bromobenzaldehyde as
2.3.2. Synthesis of [PdCl(2-Cl-C6H3)CH]N{2,6-iPr2eC6H3}]2 (C2)
The same synthetic procedure as outlined above for (C1) was
employed for the synthesis of C2, using L2 as reagent. Yield:
reagent. Yield: 2.98 g, 92%. FT-IR (
(400 MHz, CDCl3, numbering as per Scheme 1):
8.28 (dd, 1H, H3, 3JHeH 7.78 Hz, 4JHeH 1.91 Hz);
y .
, C]N):1624 cmꢂ1 1H NMR
d
8.60 (s, 1H, H7);
7.66 (dd, 1H, H6,
145 mg, 85%. FT-IR (
8.14 (s, 2H, CH]N); d 7.34 (m, 2H, Ar. protons); d 7.20 (m, 4H, Ar.
y
, C]N): 1585 cmꢂ1. 1H NMR (CDCl3, 300 MHz):
d
d
d
Scheme 1. The preparation of monofunctional imine ligands, L1eL4.

N. Mungwe et al. / Journal of Organometallic Chemistry 696 (2011) 3527e3535
3529
protons);
6.43 Hz);
d
d
7.01 (m, 6H, Ar. protons);
1.39 (d, 6H, iPr-Me, 3JHeH 6.43 Hz);
174.75 (CH]N);
141.40 (CAr); 133.82 (CAr);
128.04 (CAr); 127.97 (CAr 124.89 (CAr);
28.28 (iPr-CH); 24.48 (iPr-Me); 22.94 (iPr-Me).
d
3.49 (sept., 4H, iPr-CH, 3JHeH
1.19 (d, 6H, iPr-Me,
156.38 (C1);
132.25
metallated C);
(CAr); 134.96 (CAr);
128.03 (CAr); 127.19 (CAr);
(iPreCH); 24.56 (iPr-Me);
d
141.05 (CAr);
d
140.38 (CAr);
d
138.14 (CAr);
d
135.04
d
d
130.76 (CAr);
d
130.71 (CAr); d 128.22 (CAr);
3JHeH 6.82 Hz). 13C (CDCl3, 75 MHz):
d
d
d
d
d
124.12 (CAr);
d
122.86 (CAr);
d
28.64
41.76.
d
142.88 (o-metallated C);
(CAr); 130.52 (CAr);
123.35 (CAr);
d
d
d
d
d
22.97 (iPr-Me). 31P{1H} NMR:
d
d
d
d
)
d
ESI-MS (þ, m/z): 667 [M ꢂ Cl]þ. Anal. Found: C, 63.18; H, 5.11; N,
d
d
d
d
1.92. Calc for C37H36Cl2NPPd: C, 63.22; H, 5.16; N, 1.99.
2.3.3. Synthesis of [PdCl(2-Br-C6H3)CH]N{2,6-iPr2eC6H3}]2 (C3)
The same synthetic procedure as outlined above for (C1) was
employed for the synthesis of C3, using L3 as reagent and stirring
2.4.3. Synthesis of [Pd(PPh3)(2-Br-C6H3)CH]N{2,6-iPr2eC6H3}Cl] (C7)
The same synthetic procedure as outlined above for (C5) was
employed for the synthesis of C7, using [PdCl(2-Br-C6H3)CH]N
for 6 h. Yield: 149 mg, 80%. FT-IR (
(CDCl3, 300 MHz): 8.13 (s, 2H, CH]N);
7.20 (m, 8H, Ar. protons); 6.87 (m, 2H, Ar. protons);
y
, C]N): 1585 cmꢂ1
7.34 (m, 2H, Ar. protons);
3.49 (sept.,
1.38 (d, 6H, Pr-Me, JHeH 6.60 Hz);
1.17 (d, 6H, Pr-Me, JHeH 6.60 Hz). 13C {1H} (CDCl3, 75 MHz):
174.15 (CH]N); 156.89 (C1); 142.56 (o-metallated C); 141.35
133.85 (CAr 132.45 (CAr); 130.62 (CAr); 128.35 (CAr);
127.95 (CAr); 124.73 (CAr);
24.49 (iPr-Me); 22.90 (iPr-Me).
.
1H NMR
{2,6-iPr2eC6H3}]2 (C3) as reagent. Yield ¼ 184 mg, 78%. FT-IR (
y, C]
4
d
d
N): 1606 cmꢂ1
.
1H NMR (CDCl3, 300 MHz):
d
8.61 (d, 1H, JHꢂP
d
d
d
7.78 Hz);
protons);
6.90 Hz);
d
7.69e7.76 (m, 6H, Ar. protons);
7.15e7.24 (m, 3H, Ar. protons); d 7.10 (d, 1H, JHeH
d 7.33e7.44 (m, 9H, Ar.
3
i
3
3
4H, iPr-CH, JHieH 6.31 Hz);
d
d
3
3
3
d
d
d
6.51 (t, 1H, JHeH 7.19 Hz);
d
6.42 (t, 1H, JHeH 6.90 Hz);
1.37 (d, 6H, iPr-Me,
1.23 (d, 6H, Pr-Me, JHeH 6.90 Hz). 13C {1H} NMR
(CDCl3, 75 MHz): 177.70 (CH]N); 160.69 (C1); 145.92 (o-
metallated C); 141.23 (CAr); 140.74 (CAr); 137.27 (CAr); 135.04
(CAr); 134.96 (CAr); 130.84 (CAr); 130.49 (CAr); 128.29 (CAr);
128.01 (CAr); 127.13 (CAr); 124.38 (CAr); 122.13 (CAr); 28.65
(iPr-CH); 24.58 (iPr-Me);
d
d
d
d d
3.40e3.49 (sept., 2H, iPr-CH, 3JHeH 6.90 Hz);
i
3
(CAr);
d
d
)
d
d
3JHeH 6.90 Hz);
d
d
d
123.30 (CAr);
d
28.25 (iPr-CH);
d
d
d
d
d
d
d
d
d
d
d
d
2.3.4. Synthesis of [PdCl(C6H4)CH]N{n-Pr}]2 (C4)
d
d
d
d
d
The same synthetic procedure as outlined above for (C1) was
employed for the synthesis of C4, using L4 as reagent and stirring for
d
d
22.96 (iPr-Me). 31P{1H} NMR: 41.53. ESI-
MS (þ, m/z): 711 [M ꢂ Cl]þ. Anal. Found: C, 59.40; H, 4.82; N, 1.80.
18 h. Yield: 140 mg, 70%. FT-IR (
300 MHz): 7.79 (s, 2H, CH]N);
3.58 (t., 4H, NeCH2-, 3JHeH 6.63 Hz);
3JHeH 7.34 Hz); 0.94 (t, 6H, n-Pr-Me, 3JH-H 7.04 Hz). 13C {1H} (CDCl3,
75 MHz): 173.69 (CH]N);
y
, C]N): 1638 cmꢂ1. 1H NMR (CDCl3,
Calc for C37H36BrClNPPd: C, 59.46; H, 4.85; N, 1.87.
d
d
7.01e7.49 (m, 8H, Ar. protons);
1.91 (sext., 4H, eCH2eCH2Me,
d
d
2.4.4. Synthesis of [Pd(PPh3)(C6H4)CH]N{n-Pr}Cl] (C8)
d
The same synthetic procedure as outlined above for (C5) was
employed for the synthesis of C8, using [PdCl(C6H4)CH]N{n-Pr}]2
d
d
155.01 (C1);
d
d
146.03 (o-metallated C);
124.68 (CAr); 29.69 (Pr,-
d
133.30 (CAr);
d
119.83 (CAr
)
d
d
126.96 (CAr);
11.10 (Pr,-CH3).
d
(C4) as reagent. Yield ¼ 156 mg, 89%. FT-IR (
NMR (CDCl3, 300 MHz):
8.07 (d,1H, 4JHꢂP 7.78 Hz);
6H, Ar. protons); 7.33e7.39 (m, 9H, Ar. protons);
proton); 6.90 (t, 1H, JHeH 7.34 Hz); d 6.53 (t, 1H, JHeH 7.34 Hz);
y
, C]N): 1626 cmꢂ1. 1H
7.73e7.79 (m,
7.16 (m, 1H, Ar.
NCH2-);
d
23.33 (Pr,-CH2-);
d
d
d
d
3
3
2.4. Synthesis of mononuclear palladacycles
d
3
3
d
6.39 (t, 1H, JHeH 6.82 Hz);
d
3.90 (t., 2H, N-CH2-, JHeH 6.75 Hz);
1.88 (sext., 2H,-CH2eMe, JHeH 7.14 Hz); 0.91 (t, 3H, n-Pr-Me,
174.72 (CH]N);
138.34 (CAr); 138.14
135.04 (CAr); 134.92 (CAr);
129.50 (CAr); 29.69 (-NCH2-);
42.79. ESI-
2.4.1. Synthesis of [Pd(PPh3)(C6H4)CH]N{2,6-iPr2eC6H3}Cl] (C5)
To a stirring solution of [PdCl(C6H4)CH]N{2,6-iPr2eC6H3}]2 (C1,
128 mg, 0.158 mmol) in dichloromethane (5 mL) was added triphe-
nylphosphine (83 mg, 0.316 mmol). The reaction mixture was stirred
under an inert atmosphere and at room temperature for 1 h, after
which the solvent was removed. The yellow solid residue obtained
was recrystallized from dichloromethane:Et2O. Yield: 162 mg, 77%.
d
d
3
3JHeH 6.96 Hz). 13C {1H} NMR (CDCl3, 75 MHz):
d
d
158.46 (C1);
(CAr); 135.60 (CAr);
130.70 (CAr); 130.65 (CAr);
24.09 (n-Pr-CH2-);
d
144.17 (o-metallated C);
d
d
d
d
135.36 (CAr);
d
d
d
d
d
d
d
d
11.26 (n-Pr-Me). 31P{1H} NMR:
d
MS (þ, m/z): 514 [M ꢂ Cl]þ. Anal. Found: C, 60.42; H, 4.32; N,
FT-IR (
4JHꢂP 7.78 Hz);
Ar. protons);
7.34 Hz); 6.70 (t, 1H, JHeH 7.34 Hz);
3.42e3.53 (sept., 2H, iPr-CH, JHeH 6.75 Hz);
y
, C]N): 1604 cmꢂ1. 1H NMR (CDCl3, 300 MHz):
d
8.11 (d, 1H,
2.01. Calc for C28H27ClNPPd: C, 61.10; H, 4.94; N, 2.54.
d
7.71e7.78 (m, 6H, Ar. protons);
d
7.33e7.43 (m, 9H,
3
d
7.14e7.23 (m, 3H, Ar. protons);
d
7.02 (t, 1H, JHeH
2.4.5. Synthesisof[Pd(PMe3)(2-Cl-C6H3)CH]N{2,6-iPr2eC6H3}Cl] (C9)
The same synthetic procedure as outlined above for (C5) was
employed for the synthesis of C9, using [PdCl(2-Cl-C6H3)CH]N
{2,6-iPr2eC6H3}]2 (C2) and trimethylphosphine as reagent. Yield:
3
3
d
d
6.50 (t, 1H, JHeH 7.04 Hz);
3
d
d
1.37 (d, 6H, iPr-Me,
1.21 (d, 6H, Pr-Me, JHeH 6.90 Hz). 13C {1H} NMR
177.03 (CH]N); 159.54 (C1); 145.15 (o-met-
141.48 (CAr); 140.80 (CAr); 138.20 (CAr); 135.41 (CAr);
135.04 (CAr); 131.68 (CAr); 130.83 (CAr); 130.56 (CAr); 128.84
127.92 (CAr); 126.88 (CAr); 124.09 (CAr); 122.77 (CAr);
24.51 (iPr-Me);
3JHeH 6.90 Hz);
(CDCl3, 75 MHz):
allated C);
d
i
3
d
d
d
131 mg, 80%. FT-IR (
y
, C]N): 1609 cmꢂ1. 1H NMR (CDCl3, 400 MHz):
7.24e7.28 (m, 2H, Ar. protons);
4
d
d
d
d
d
8.51 (d, 1H, JHꢂP 7.81 Hz);
d
3
d
d
d
d
d
d
7.18e7.21 (m, 3H, Ar. protons);
d
7.11 (d, 1H, JH-H 7.03 Hz);
1.69 (d, 9H,P(Me)3,
1.34 (d, 6H, Pr-Me, JHeH 7.03 Hz); 1.16 (d, 6H,
174.92 (CH]N);
141.71 (CAr); 140.79 (CAr);
129.79 (CAr); 125.47 (CAr); 123.36
22.90 (PMe3);
16.99 (iPr-
(CAr);
d
d
d
d
d
3.23e3.70 (sept., 2H, iPr-CH, 3JHeH 6.64 Hz);
d
i
3
d
28.64 (iPr-CH);
d
d
23.02 (iPr-Me). 31P{1H} NMR:
2JHeP 10.74 Hz);
d
d
d
42.41. ESI-MS (þ, m/z): 633 [M ꢂ Cl]þ. Anal. Found: C, 66.42; H, 5.54;
iPr-Me, 3JHeH 7.23 Hz). 13C NMR (CDCl3,100 MHz):
d
N, 2.04. Calc for C37H37ClNPPd: C, 66.47; H, 5.58; N, 2.10.
d
160.54 (C1);
138.65 (CAr);
d
145.32 (o-metallated);
135.21 (CAr);
d
d
d
d
d
d
d
2.4.2. Synthesisof[Pd(PPh3)(2-Cl-C6H3)CH]N{2,6-iPr2eC6H3}Cl] (C6)
The same synthetic procedure as outlined above for (C5) was
employed for the synthesis of C6, using [PdCl(2-Cl-C6H3)CH]N
(CAr);
Me);
d
122.89 (CAr);
d
28.40 (iPr-CH);
d
d
d
16.33 (iPr-Me). 31P{1H} NMR: - 4.60. ESI-MS (þ, m/z): 481
[M ꢂ Cl]þ. Anal. Found: C, 51.05; H, 5.76; N, 2.68. Calc for
{2,6-iPr2eC6H3}]2 (C2) as reagent. Yield: 182 mg, 83%. FT-IR (
y
, C]
C22H30Cl2NPPd: C, 51.13; H, 5.85; N, 2.71.
4
N): 1605 cmꢂ1
.
1H NMR (CDCl3, 300 MHz):
7.69e7.77 (m, 6H, Ar. protons); 7.33e7.44 (m, 9H, Ar.
7.15e7.23 (m, 3H, Ar. protons);
d
8.61 (d, 1H, JHꢂP
7.92 Hz);
protons);
7.92 Hz);
d
d
2.5. Synthesis of cationic mononuclear palladacycles
3
d
d 6.92 (d, 1H, JHeH
3
3
d
6.61 (t, 1H, JHeH 7.78 Hz);
d
6.38 (t, 1H, JHeH 7.04 Hz);
1.37 (d, 6H, iPr-Me,
1.23 (d, 6H, Pr-Me, JHeH 6.90 Hz). 13C {1H} NMR
(CDCl3, 75 MHz): 175.58 (CH]N); 160.21 (C1); 145.81 (o-
2.5.1. Synthesis of [Pd(MeCN)(PPh3)(C6H4)CH]N
d
3.40e3.49 (sept., 2H, iPr-CH, 3JHeH 6.90 Hz);
d
{2,6-iPr2eC6H3}]þ[B(Ar)4]ꢂ [Ar ¼ 3,5- (CF3)2eC6H3] (C10)
To a stirring solution of [Pd(PPh3)(C6H4)CH]N{2,6-iPr2eC6H3}
Cl] (C5, 50 mg, 0.075 mmol) in dichloromethane (7 mL) was added
3JHeH 6.43 Hz);
d
i
3
d
d
d

3530
N. Mungwe et al. / Journal of Organometallic Chemistry 696 (2011) 3527e3535
a solution of NaB(Ar)4 (66 mg, 0.075 mmol) in acetonitrile (3 mL).
The onset of a white precipitate was observed immediately, and the
reaction mixture was stirred at room temperature for 2 h. After the
allotted time the reaction mixture was filtered and the solvent
removed. The resulting pale yellow solid residue was recrystallized
from dichloromethane:pentane and a pale yellow solid was iso-
z): 863 [B(Ar)4]ꢂ. Anal. Found: C, 52.59; H, 2.85; N, 1.39. Calc for
C71H51BBrF24N2PPd: C, 52.76; H, 3.18; N, 1.73.
2.5.4. Synthesis of [Pd(MeCN)(PPh3)(C6H4)CH]N{n-Pr}]þ[B(Ar)4]ꢂ
[Ar ¼ 3,5e(CF3)2eC6H3] (C13)
The same synthetic procedure as outlined above for (C10) was
employed for the synthesis of C13, using [Pd(PPh3)(C6H4)CH]N{n-
lated. Yield: 101 mg, 88%. FT-IR (
(CDCl3, 300 MHz):
8.09 (d, 1H, 4JH-P 7.60 Hz);
Ar. protons, B(Ar)4); 7.41e7.56 (m, 15H, Ar. protons, PPh3);
7.19e7.28 (m, 4H, Ar. protons); 6.78
7.12 (t, 1H, 3JHeH 7.48 Hz);
(t, 1H, 3JHeH 7.92 Hz); 6.48 (t, 1H, 3JHeH 7.34 Hz);
3.35 (sept., 2H,
iPr-CH, 3JHeH 6.90 Hz); 1.28 (d, 6H, iPr-Me, 3JHeH 6.75 Hz);
1.25 (d,
6H, Pr-Me, JHeH 6.75 Hz);
1.06 (s, 3H, MeCN). 13C {1H} NMR
(CDCl3, 75 MHz): 178.15 (CH]N); 162.66 (CAr, BAr4); 162.00
(CAr, BAr4); 161.33 (CAr, BAr4); 160.67 (CAr, BAr4); 153.99 (C1);
147.67 (o-metallated C); 143.76 (CAr); 140.40 (CAr); 138.83
(CAr); 134.75 (CAr); 134.59 (CAr); 132.24 (CAr); 130.73 (CAr);
129.18 (CAr); 128.63 (CAr); 128.18 (CAr); 126.32 (CAr);
(CAr); 122.71 (CAr); 117.43 (C^N);
28.59 (iPr-CH);
Me);
22.62 (iPr-Me); 0.13 (CNeCH3). 31P{1H} NMR:
y
.
, C]N): 1616 cmꢂ1 1H NMR
d
d
7.64e7.71 (m, 12H,
Pr}Cl] (C8) as reagent. Yield: 77 mg, 73%. FT-IR (
y
, C]N): 1618 cmꢂ1
7.63e7.69
7.44e7.51 (m, 15H, Ar. protons, PPh3);
7.00 (t, 1H, 3JHeH 7.36 Hz);
6.59 (t, 1H,
.
d
1H NMR (CDCl3, 300 MHz):
d
7.99 (d, 1H, 4JHꢂP 7.48 Hz);
d
d
d
d
(m, 12H, Ar. protons, BAr4); d
d
d
d
7.28 (m, 1H, Ar. protons);
d
d
d
d
3JHeH 7.99 Hz);
d d 3.52 (t., 2H, NeCH2-,
6.34 (t, 1H, 3JHeH 6.82 Hz);
i
3
d
3JHeH 6.75 Hz);
d
1.51 (sext., 2H,-CH2eMe, 3JHeH 7.14 Hz);
d
1.22 (s,
3
d
d
d
3H, MeCN);
(CDCl3, 75 MHz):
BAr4); 161.02 (CAr, BAr4);
147.29 (o-metallated C);
(CAr); 129.29 (CAr); 127.74 (CAr);
122.64 (CAr); 117.20 (C^N); 29.49 (-NCH2-);
11.34 (n-Pr-Me);
d
0.86 (t, 3H, n-Pr-Me, JHeH 6.96 Hz). 13C {1H} NMR
d
d
d
d
177.94 (CH]N);
d 162.32 (CAr, BAr4); 161.95 (CAr,
d
d
d
d
d
d
160.29 (CAr, BAr4);
d
153.94 (C1);
d
d
d
d
d
d
134.34 (CAr);
d
132.14 (CAr); d 130.60
d
d
d
d
d
123.67
d
d
d
126.32 (CAr);
d 123.70 (CAr);
d
d
d
d
24.10 (iPr-
41.24. ESI-
d
d
d
d
24.19 (n-Pr-CH2-
d
d
d
);
d
d
0.11 (CNeCH3). 31P{1H} NMR: 40.97. ESI-MS
MS (þ, m/z): 674 [M-B(Ar)4]þ. ESI-MS (-, m/z): 863 [B(Ar)4]ꢂ. Anal.
Found: C, 55.01; H, 3.14; N, 1.43. Calc for C71H52BF24N2PPd: C, 55.47;
H, 3.41; N, 1.82.
(þ, m/z): 556 [M-B(Ar)4]þ. ESI-MS (-, m/z): 863 [B(Ar)4]ꢂ. Anal.
Found: C, 52.14; H, 2.62; N, 1.71. Calc for C62H42BF24N2PPd: C, 52.47;
H, 2.98; N, 1.97.
2.5.2. Synthesis of [Pd(MeCN)(PPh3)(2-Cl-C6H3)CH]N
2.5.5. Synthesis of [Pd(MeCN)(PMe3)(2-Cl-C6H3)CH]N
{2,6-iPr2eC6H3}]þ[B(Ar)4]ꢂ [Ar ¼ 3,5e(CF3)2eC6H3] (C11)
The same synthetic procedure as outlined above (C10) was
employed for the synthesis of C11, using [Pd(PPh3)(2-Cl-C6H3)CH]
{2,6-iPr2eC6H3}]þ[B(Ar)4]ꢂ [Ar ¼ 3,5e(CF3)2eC6H3] (C14)
The same synthetic procedure as outlined above (C10) was
employed for the synthesis of C14, using [Pd(PMe3)(2-Cl-C6H3)
CH]N{2,6-iPr2eC6H3}Cl] (C9) as reagent. Yield: 94 mg, 90%. FT-IR
N{2,6-iPr2eC6H3}Cl] (C6) as reagent. Yield: 107 mg, 91%. FT-IR (
y,
C]N): 1611 cmꢂ1
.
1H NMR (CDCl3, 300 MHz):
d
8.59 (d, 1H, JH-P
(
y
, C]N): 1613 cmꢂ1
4JH-P 7.80 Hz);
(m, 3H, Ar. protons);
.
1H NMR (CDCl3, 300 MHz):
7.64e7.71 (m, 12H, Ar. protons, B(Ar)4);
7.06 (d, 1H, 3JHeH 8.01 Hz); 6.95 (t, 1H, 3JHeH
6.62 (t, 1H, 3JHeH 7.00 Hz); 2.89 (sept., 2H, iPr-CH, 3JHeH
1.13 (s, 3H, MeCN);
0.98 (d, 6H, iPr-Me, 3JHeH 6.82 Hz);
d
8.40 (d, 1H,
6.94e7.10
4
7.60 Hz);
15H, Ar. protons, PPh3);
1H, 3JHeH 8.19 Hz); 6.70 (t, 1H, 3JHeH 7.99 Hz);
7.02 Hz);
3.34 (sept., 2H, iPr-CH, 3JHeH 6.82 Hz);
Me, 3JHeH 6.63 Hz); 1.29 (d, 6H, iPr-Me, 3JHeH 6.82 Hz);
MeCN). 13C {1H} NMR (CDCl3, 75 MHz):
178.95 (CH]N);
(CAr, BAr4); 162.02 (CAr, BAr4); 161.32 (CAr, BAr4); 160.66 (CAr
BAr4); 155.56 (C1); 146.13 (o-metallated C); 143.84 (CAr);
140.28 (CAr); 137.99 (CAr); 134.77 (CAr); 133.88 (CAr); 132.39
(CAr); 130.70 (CAr); 129.26 (CAr); 128.54 (CAr); 127.91 (CAr);
126.37 (CAr); 123.77 (CAr); 122.71 (CAr); 117.45 (C^N); 28.69
(iPr-CH); 24.21 (iPr-Me); 22.57 (iPr-Me);
NMR:
d
7.63e7.70 (m, 12H, Ar. protons, B(Ar)4);
7.21e7.30 (m, 3H, Ar. protons);
6.35 (t, 1H, 3JHeH
1.30 (d, 6H, iPr-
1.05 (s, 3H,
162.66
d
7.43e7.54 (m,
d
d
d
d
7.06 (d,
d
d
d
d
7.94 Hz);
6.99 Hz);
d
d
d
d
d
d
d
d
d
0.91 (d, 6H, iPr-Me, 3JHeH 6.62 Hz). 13C {1H} NMR (CDCl3, 75 MHz):
d
d
d
178.90 (CH]N);
160.59 (CAr, BAr4);
133.62 (CAr); 132.40 (CAr);
128.57 (CAr); 127.95 (CAr); 126.33 (CAr);
(CAr); 117.49 (C^N);
28.69 (iPr-CH); 24.21 (iPr-Me);
Me); 21.93 (PMe3); - 3.98. ESI-MS
0.07 (CNeCH3). 31P{1H} NMR:
d
162.86 (CAr, BAr4);
155.65 (C1);
130.71 (CAr);
123.71 (CAr);
22.57 (iPr-
d
162.42 (CAr, BAr4);
146.19 (o-metallated
129.28 (CAr);
122.75
d 161.02
d
d
d
,
(CAr, BAr4);
C);
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
d
0.11 (CNeCH3). 31P{1H}
(þ, m/z): 522 [M-B(Ar)4]þ. ESI-MS (-, m/z): 863 [B(Ar)4]ꢂ. Anal.
Found: C, 48.13; H, 2.91; N, 1.62. Calc for C56H45BClF24N2PPd: C,
48.54; H, 3.27; N, 2.02.
d
40.97. ESI-MS (þ, m/z): 709 [M-B(Ar)4]þ. ESI-MS (-, m/z):
863 [B(Ar)4]ꢂ. Anal. Found: C, 53.89; H, 2.94; N, 1.42. Calc for
C71H51BClF24N2PPd: C, 54.25; H, 3.27; N, 1.78.
2.6. X-ray crystallography
2.5.3. Synthesis of [Pd(MeCN)(PPh3)(2-Br-C6H3)CH]N
{2,6-iPr2eC6H3}]þ[B(Ar)4]ꢂ [Ar ¼ 3,5e(CF3)2eC6H3] (C12)
The same synthetic procedure as outlined above (C10) was
employed for the synthesis of C12, using [Pd(PPh3)(2-Br-C6H3)CH]
Single crystals of complex C2 were mounted on nylon loops and
centred in a stream of cold nitrogen at 100(2) K. Crystal evaluation
and data collection was performed on a Bruker-Nonius SMART
N{2,6-iPr2eC6H3}Cl] (C7) as reagent. Yield: 87 mg, 72%. FT-IR (
y
, C]
Apex-CCD diffractometer with Mo Ka radiation (
l
¼ 0.71073 Å).
N): 1615 cmꢂ1
7.60 Hz); 7.64e7.71 (m, 12H, Ar. protons, B(Ar)4);
15H, Ar. protons, PPh3); 7.18e7.27 (m, 3H, Ar. protons);
1H, 3JHeH 8.12 Hz); 6.70 (t, 1H, 3JHeH 7.94 Hz); 6.37 (t, 1H, 3JHeH
6.89 Hz);
3.33 (sept., 2H, iPr-CH, 3JHeH 7.04 Hz); 1.31 (d, 6H, iPr-
Me, 3JHeH 6.63 Hz); 1.30 (d, 6H, iPr-Me, 3JHeH 6.82 Hz);
1.09 (s, 3H,
MeCN). 13C {1H} NMR (CDCl3, 75 MHz):
178.89 (CH]N); 162.66
(CAr, BAr4); 162.01 (CAr, BAr4); 161.34 (CAr, BAr4); 160.68 (CAr
BAr4); 155.34 (C1); 146.06 (o-metallated C); 143.94 (CAr);
140.33 (CAr); 137.83 (CAr); 134.69 (CAr); 133.79 (CAr); 132.36
(CAr); 130.62 (CAr); 129.11 (CAr); 128.49 (CAr); 127.79 (CAr);
126.33 (CAr); 123.77 (CAr); 122.71 (CAr); 117.39 (C^N); 28.63
(iPr-CH); 24.56 (iPr-Me); 22.94 (iPr-Me);
{1H} NMR:
.
1H NMR (CDCl3, 300 MHz):
d
8.56 (d, 1H, JH-P
7.41e7.52 (m,
7.05 (d,
Data collection were recorded using scans. Data reduction and
u
4
d
d
absorption corrections were performed using SAINT [11] and
SADABS [11,12], which forms part of the APEX II software package.
The structure was solved by direct methods and refined by using
SHELXS-97 [13] and SHELXL-97 [13] within the X-Seed graphical
user interface [14]. All non-hydrogen atoms were refined aniso-
tropically by full matrix least squares calculations on F2 and all
hydrogen atoms were placed in calculated positions using riding
models.
d
d
d
d
d
d
d
d
d
d
d
d
d
,
d
d
d
d
d
d
d
d
d
d
d
d
2.7. Phenylacetylene polymerization
d
d
d
d
d
d
d
d
0.08 (CNeCH3). 31P
The cationic palladium complex (0.03 mmol) was dissolved in
dichloromethane (10 mL). Thereafter phenylacetylene (in molar
d
40.56. ESI-MS (þ, m/z): 753 [M-B(Ar)4]þ. ESI-MS (-, m/

N. Mungwe et al. / Journal of Organometallic Chemistry 696 (2011) 3527e3535
3531
ratio 1:50; 1:100 or 1:200 Pd:PA) was added. The reaction mixture
was stirred under an inert atmosphere at 60 ꢀC for the required
time. A colour-change from light yellow to orange was observed.
After the allotted time the solvent was removed and MeOH (20 mL)
added to precipitate the polymer, followed by stirring overnight.
The brown solid which had formed was filtered and dried in vauo.
The filtrate was collected, the solvent removed and the brown
residue which was obtained was dried in vacuo.
The aromatic region of the 1H NMR spectra showed resonances
typical of either 1,2- or 1,4-disubstitution. In the aliphatic region we
observed the equivalence of the Pr groups (for ligands L1eL3),
observed as a doublet integrating for twelve protons, due to free
rotation about the CeN single bond. For ligand L4 the aliphatic
region showed resonances due to the n-propyl chain. In the 13C
NMR spectra of the ligands the imine carbon resonances were
observed in the range 158e162 ppm.
i
The chloro-bridged palladacycles, C1eC4, were prepared by the
reaction of the palladium precursor bis(acetonitrile) palladium
dichloride, (MeCN)2PdCl2, with L1eL4, in the presence of excess
NaOAc as a base at room temperature (Scheme 2) and the reaction
proceeded via electrophilic CeH activation. The palladacycles were
isolated as yellow air-stable solids in 70e90% yield. Complexes C1
and C4 were found to be soluble in chlorinated organic solvents
whereas C2 and in particular C3 displayed only partial solubility in
chlorinated organic solvents.
3. Results and discussion
3.1. Synthesis of cationic palladacycles
For the benefit of the reader, all syntheses and characterization
data are outlined in Section 2. Monofunctional imine ligands,
L1eL4, were prepared by Schiff base condensation of 2,6-
diisopropylaniline and n-propylamine with various mono-
substituted aldehydes (Scheme 1). Ligands L1eL3 was isolated as
pale-yellow crystalline solids whereas ligand L4 was isolated as
a bright-yellow oil, in high yields in all cases. The ligands displayed
solubility in polar organic solvents as well as stability both in
solution and in the solid state. Ligands L1, L3eL4 have been
reported previously [15], whereas ligand L2 is novel. FT-IR analysis
indicated the formation of the desired condensation product as
In the FT-IR spectra of the complexes the imine absorption band
was observed to shift to lower wavenumbers in the range
1580e1638 cmꢂ1, in comparison to that of the free ligands. This was
indicative of the coordination of the N-atom of the imine moiety to
the metal centre which resulted in a decrease in the double bond
character of the imine functionality [17].
The 1H NMR spectra of the complexes showed an upfield shift of
the imine proton resonances in the range of
d 7.75e8.14 ppm, in
evidenced by the absence of characteristic yC
aldehyde starting material and the formation of absorption bands
corresponding to the yC of the imine products in the range
1620e1650 cmꢂ1
The imine proton resonances in the 1H NMR spectra of the
ligands were observed in the range 8.20e8.80 ppm. For ligands L2
and L3 (halide substituents) the imine resonances are shifted more
downfield in comparison to L1 and L4. This difference may be
attributed to the inductive effect of the halogen substituents [16].
]
bands of the
O
comparison to that of the free ligands. Furthermore, in the aromatic
region the resonance of the ortho-H atom of the benzylidene ring
disappeared and a complicated series of multiplets were observed
integrating for eight (as is the case for C4), twelve (as is the case for
C2 and C3) and fourteen protons (as is the case for C1) respectively.
This was indicative of the fact that cyclopalladation took place
preferentially via CeH bond activation [18]. In the aliphatic region
the methine proton resonances of the isopropyl substituents (for
]
N
.
d
Scheme 2. The preparation of neutral and cationic phosphine-substituted palladacycles.

3532
N. Mungwe et al. / Journal of Organometallic Chemistry 696 (2011) 3527e3535
Table 2
C1eC3) and the methylene protons of the n-propyl chain (for C4)
were observed to shift downfield as a result of complexation of the
ligand to the metal centre and the Me-groups of the Pr unit was
Selected bond lengths (Å) and angles (ꢀ) of complexes C2.
i
Complex
C2
split into two doublets integrating for a total of six protons each.
Pd(1)-N(1)
Pd(1)-C(6)
C(7)-N(1)
Pd(1)-Cl(1)
Pd(1)-Cl(A)
Pd(1),,Pd
2.028(5)
1.979(6)
1.288(8)
2.447(2)
2.329(2)
3.477
Analogous resonance shifts to those observed in the 1H NMR
spectra were observed when subjecting the m
-Cl complexes to 13C
NMR analysis. The imine carbon atom resonances shifted to the
high field region upon complexation and was observed in the range
d
173e177 ppm. Also, the carbon atom bound to palladium was
N(1)-Pd(1)-C(6)
N(1)-Pd(1)-Cl(1)
N(1)-Pd(1)-Cl(A)
C(6)-Pd(1)-Cl(1)
C(6)-Pd-Cl(A)
Cl(1)-Pd(1)-Cl(A)
178.2(2)
176.3(2)
96.9(2)
94.9(2)
178.2(2)
86.59(6)
observed to resonate in the range 142e146 ppm and provided
further evidence for cyclopalladation occurring [15,19].
d
Suitable crystals of complex C2 for single crytal X-ray diffraction
analysis were obtained by the slow evaporation of a dichloro-
methane: hexane mixture. The complex crystallised as a discrete
molecule with one half of the molecule in the asymmetric unit.
Crystallographic data (Table 1) as well as selected bond lengths and
angles are tabulated (Table 2). The molecule consists of two palla-
dium centres bridged by two chloride atoms. The remainder of the
coordination sphere of the metal is occupied by the Schiff base
ligand, coordinated through the benzylidene carbon and imine
nitrogen atom to form the five-membered chelate ring (Fig. 1).
The geometry around the metal centre is distorted square planar
as evidenced by the sum of the angles about the palladium centre.
The most noticeable distortion is found for the C(6)ePdeN(1) angle
of the cyclometallated ring of 81.5(2)ꢀ which is as a result of
chelation of the ligand to the metal centre. The PdeC bond length of
1.979(6) Å is somewhat shorter than the expected value based on
the sum of the covalent radii whereas the PdeN bond length of
2.028(5) Å is longer than the expected value for the sum of the van
der Waals radii [20]. This difference is due to the differing trans
influence of the two coordinating atoms. This is further demon-
strated by the difference in the PdeCl bond lengths. The PdeCl(1)
bond trans to the N-atom of 2.329(2) Å is shorter than the PdeCl
bond trans to the C-atom, which has a length of 2.447(2) Å. The
PdePd interatomic distance of 3.477 Å precludes any PdePd
interaction. All bond lengths and angles fall within the range
In the FT-IR spectrum of the mononuclear complexes, a slight
shift to higher wavenumbers of the imine absorption band was
observed, in comparison to the
m
-Cl palladacycles. This was due to
-donor ability of the phosphine ligand,
the relative increase in
s
when compared to Clꢂ and resulted in an increase in the double
bond character of the C]N bond. Also, strong absorption bands in
the range 1430e1440 cmꢂ1 and 680e690 cmꢂ1 were observed in
the FT-IR spectra of the complexes which were attributed to the
y
(CeP) and CeH out of plane bending vibrations respectively. These
observations provided further evidence that coordination of the
ligand to the metal centre had occurred [21].
In the 1H NMR spectra of the mononuclear cyclometallated
complexes, C5eC11, the imine proton resonance was observed to
shift upfield in comparison to the
m-Cl complexes, with the tri-
phenylphosphine complexes shifting more downfield than their
trimethylphosphine analogues. This was attributed to an increase
in the basicity of the trimethylphosphine ligand, which resulted in
an increase in electron density on the palladium centre. This in turn
resulted in back-donation of excess electron density onto the imine
functional group thereby shielding the imine proton from the effect
of the externally applied magnetic field [16]. Also, the imine proton
resonance was observed to be split into a doublet integrating for
observed for analogous m-Cl palladacycles [15].
The chloro-bridged palladacycles, C1eC4, were reacted with
tertiary phosphines, varying in their steric and electronic proper-
ties (Scheme 2). The phosphine-substituted palladacycles (C5 to C9)
were isolated as pale-yellow air- and moisture-stable solids in high
yields and displayed solubility in chlorinated organic solvents but
were found to be insoluble in alkanes, ethers and alcohols.
one proton in the range
d 8.00e8.70 ppm as a result of four-bond
coupling (4JHeP) of the imine proton to the phosphorus atom of
the tertiary phosphine ligand.
The bridge-splitting reaction was noted to have a remarkable
effect on the proton resonances associated with the ligand back-
bone. The H-atom ortho to the metallated carbon atom was
observed to be the most shielded, as a result of the coordination of
Table 1
Crystallographic data for complexes C2.
Complex
C2
Empirical Formula
Temperature (K)
Wavelength (Å)
Crystal System
Space Group
a [Å]
C38H42Cl4N2Pd2
100(2)
0.71073
monoclinic
P21/n
9.8189(18)
10.0246(18)
18.797(3)
90.00
91.899(2)
90.00
b [Å]
c [Å]
a
b
g
[ꢀ]
[ꢀ]
[ꢀ]
volume [Å3]
1849.2(6)
2
1.583
Z
calc density [Mg/m3]
m
(Mo K)[mmꢂ1
]
1.291
F(000)
888
crystal dimensions (mm)
0.14 ꢁ 0.10 ꢁ 0.04
goodness-of-fit on F2
1.171
Final R indices [I > 2
s/(I)]
R1 ¼ 0.0591
wR2 ¼ 0.1322
Fig. 1. Molecular structure of complex C2, drawn with 50% probability ellipsoids.
Hydrogen atoms omitted for clarity.

N. Mungwe et al. / Journal of Organometallic Chemistry 696 (2011) 3527e3535
3533
the triphenylphosphine ligand in a cis fashion relative to the met-
allated carbon atom. The phenyl protons experience anisotropic
40
30
20
10
0
shielding by the aromatic rings, resulting in an upfield shift of these
4
resonances [16,21b]. The ortho H-atom also exhibited JHeP
coupling to the P-atom of the tertiary phosphine which provided
further evidence to the cis geometry of the phosphine ligand. The
methine protons of the iPr groups of complexes C5eC7, C9 as well
as the methylene protons of the n-propyl chain (C8) were observed
to resonate in the same region as observed for complexes C1eC4
and confirmed coordination of the imine N-atom to the palladium
centre.
Analogous resonances to those of complexes C1eC4 were
observed in the 13C NMR spectra of the cleaved complexes (C5-C9)
with the imine carbon resonance observed in the range
50
100
200
d
174e178 ppm, while the o-metallated carbon resonance was
observed in the range 145e146 ppm.
In the 31P{1H} NMR spectra of complexes C5eC9 only a single
resonance was observed in the range 41e43 ppm for the PPh3-
cleaved (C5eC8) complexes and in the range
ꢂ4 to ꢂ5 ppm for the
PA:Pd ratio
d
Fig. 2. Effect of monomer concentration of PA conversion, employing catalyst C10.
d
d
PMe3-cleaved complex (C9). The coordinated tertiary phosphine
was observed to be high field shifted when compared to the free
phosphine ligand. This provided further confirmation that the
phosphine was in fact coordinated to the metal centre [22].
The phosphine-substituted palladacycles were also character-
ized by ESI-MS. The major fragment observed in the positive ion
ESI-MS spectra of the phosphine-cleaved complexes corresponded
to the fragment, [M ꢂ Cl]þ, in which the Clꢂ ion was abstracted as
a result of the ionization process [23].
The neutral mononuclear palladacycles (C5eC9) were reacted
with NaB(Ar)4 [Ar ¼ 3,5-bistrifluoromethylphenyl] in the presence
of acetonitrile as coordinating solvent to generate cationic palla-
dacycles (C10eC14, Scheme 2). The cationic palladacycles were
isolated in high yields as pale-yellow air- and moisture-stable
solids. The complexes displayed solubility in common organic
solvents but were insoluble in unsatutared hydrocarbons.
1H NMR analysis of the cationic palladacycles showed analogous
resonances to that observed for the neutral palladacycles, with the
Polymerization data showing the effect of reaction temperature
and reaction time on catalyst performance are tabulated in Table 3.
Under optimized reaction conditions cationic palladacycles
C10eC14 were efficient catalysts for the PA oligo-/poly-merization
(Entries 1e5).
At room temperature oligo-/poly-mers with low molecular
weights and relatively narrow polydispersity indices were
obtained. A mixture of cis-transoidal (65%) and trans-cisoidal (35%)
isomers was observed as determined by FT-IR and 1H NMR spec-
troscopy. 1H NMR analysis of the oligo-/polymers showed a singlet
at
bone being in a cis configuration. Additionally, broad resonances at
6.64 ppm (o-aromatic protons) and 6.95 ppm (m- and
d 5.84 ppm indicative of the vinyl protons of the polymer back-
d
d
p-aromatic protons) were observed which is indicative of an oligo-/
polymer with a regular head-to-tail structure of cis-transoidal
oligo-/poly-phenylacetylene [7a,8]. Broad resonances were also
observed in the range 7.00e7.20 ppm which can be ascribed to the
aromatic protons of the trans-cisoidal isomer [10].
Strong absorption bands at 756 and 895 cmꢂ1 were observed in
the FT-IR spectrum of the oligo-/polymers which are characteristic
of cis-transoidal oligo-/poly-phenylacetylene [24]. Furthermore,
the IR spectrum showed an additional strong absorption band at
1278 cmꢂ1 which is characteristic of the trans-cisoidal isomeric
structure. These results confirmed the formation of both
addition of a multiplet in the range
grated for a total of twelve protons as a result of the counter-ion.
Also, a singlet was observed in the aliphatic region in the range
d 7.60e7.80 ppm which inte-
d
1e1.30 ppm integrating for three protons and was ascribed to the
coordinating acetonitrile molecule.
In the 13C NMR spectrum of the cationic complexes four reso-
nances were observed in the range
assigned the B(Ar)4ꢂ ion. The Me-carbon of the acetonitrile was
observed in the range 0e1 ppm.
No significant shift of the phosphorus resonance was observed
in the 31P{1H} NMR spectrum of the cationic complexes in
comparison to the neutral complexes which confirmed that the
tertiary phosphine remained coordinated in the same fashion in the
cationic palladacycles.
ESI-MS analysis (positive and negative mode) showed peaks
corresponding to the cationic fragment as well as the anionic
fragment of which the molecule was comprised and elemental
analysis was in agreement with the proposed molecular
composition.
d 160e163 ppm which was
d
Table 3
Data for PA oligo-/polymerization catalyzed by cationic palladacycles, C10eC14.a
Entry Cat. Temp (ꢀC) Time (h) Conversion Activityb
%
Mw
PDI
1
2
3
4
5
6
7
8
C10
C11
C12
C13
C14
C10
C11
C12
C13
C14
C10
C11
C12
C13
C14
25
25
25
25
25
60
60
60
60
60
25
25
25
25
25
24
24
24
24
24
24
24
24
24
24
48
48
48
48
48
35
52
44
36
16
84
99
90
71
37
56
55
53
64
34
1767
2667
2233
1833
810
4300
5067
4600
3633
1890
2867
2833
2733
3267
1737
801 1.30
1034 1.46
929 1.47
705 1.25
45,222 1.78
638 1.16
675 1.12
712 1.17
577 1.08
59,863 1.54
2162 1.01
2226 1.01
2189 1.00
2196 1.01
55,963 1.36
9
3.2. Phenylacetylene oligo-/polymerization
10
11
12
13
14
15
The cationic palladacycles (C10eC14) were evaluated as cata-
lysts in the oligo-/poly-merization of phenylacetylene (PA). A series
of optimization catalytic runs were performed employing com-
plexC10 as catalyst (Fig. 2). An increase in the ratio of monomer to
catalyst resulted in a decrease in activity, thus a monomer/catalyst
ratio of 50:1 was employed for all subsequent catalytic runs.
a
Reaction conditions: CH2Cl2 (10 mL), PA:Pd ¼ 50:1 unless otherwise specified,
catalyst concentration 3 ꢁ10ꢂ3 M.
b
Activity ¼ g PPA/mol Pd.

3534
N. Mungwe et al. / Journal of Organometallic Chemistry 696 (2011) 3527e3535
cis-transoidal and trans-cisoidal oligo-/polyphenylacetylene at
room temperature. From the analytical data obtained it would
appear that an insertion mechanism predominates at room
temperature for our catalyst system as had been observed for Rh
catalyst systems [25].
temperature on the catalytic activity and stereoselectivity. An
increase in reaction temperature resulted in a dramatic increase in
catalytic activity with activities ranging between 1900 and 5100
while following the same trend in catalytic activity for the different
catalysts observed at room temperature. The oligo-/poly-phenyl-
acetylenes produced had slightly lower molecular weights than
those obtained at room temperature. This may be attributed to an
increase in the rate of chain termination relative to propagation at
elevated temperature. The FT-IR spectrum of the oligo-/poly-
phenylacetylene showed strong absorption bands at 1278, 980 and
915 cmꢂ1 which are characteristic of trans-cisoidal PPA [24]. 1H
NMR analysis showed broad resonances in the range
The catalytic activity, (g PPA/mol Pd) ranged between 700 and
2800, with the highest activity observed for C11 (Fig. 3). The order
of
catalytic
activity
may be
arranged
as
follows:
C11 > C12 > C13 > C10 > C14.
This result indicated that the substituent on the imino-N atom
does not have a significant effect on catalytic activity whereas
electron-withdrawing substituents at the 2-position on the cyclo-
metallated ring resulted in an increase in catalytic activity. This is as
a result of the fact that electron-withdrawing groups enhanced the
electrophilicity of the palladium centre thereby promoting mono-
mer coordination. The nature of the phosphine also seems to have
a marked effect on the efficiency of the catalyst. The precursor
containing the more basic tertiary phosphine, PMe3 (C14) showed
a much lower catalyst activity (Table 3, entries 2 and 5). This again
can be attributed to the fact that the more basic phosphine
increased the electron density on the metal thus reducing the
electrophilicity of the metal centre. This would reduce the rate of
monomer coordination and insertion. The net result is that the
PMe3 based catalyst, C14, has an activity which is about half of that
of the PPh3 analogue, C11 (entries 1 and 5, Table 3). It is also
possible that phosphine dissociation plays a role in the catalytic
process. In this case PPh3 is more likely to dissociate more easily
than PMe3. Phosphine dissociation will facilitate the coordination
of the monomer thus enhancing the rate of the reaction. This could
also explain the higher activity of the PPh3 based catalyst.
d
6.40e7.60 ppm as well as the absence of any vinylic proton
resonances which confirmed the selective formation of trans-
cisoidal oligo-PPA at higher temperature [26]. It has been proposed
previously that higher reaction temperatures lead to selective trans
monomer insertion [27].
Interestingly the PMe3 catalysts behaved differently when
compared to the PPh3 based catalysts. The former did not show
a decrease in molecular weight of the polymer obtained at elevated
temperature. Instead it shows a 30% increase in molecular weight of
the polymer at elevated temperature. It would thus appear that
even at 60 ꢀC, the rate of chain propagation still exceeds that of
chain transfer for the PMe3 catalysts.
Increasing reaction time while performing the polymerization
runs at room temperature resulted in a slight increase in catalytic
activity while significantly increasing the molecular weight of the
produced oligo-/PPA (Table 3, entry 11e15). A further improvement
in the PDI values was also observed under these reaction condi-
tions. The polyphenylacetylene produced showed IR absorption
bands and 1H NMR resonances characteristic of head-to-tail cis-
transoidal PPA.
In addition to affecting the catalyst activity, the decrease in
steric bulk of the tertiary phosphine impacts on the molecular
weight as well as on the polydispersity of the formed polymer. The
PMe3 based catalyst gives a polymer with higher molecular weight
as well as higher PDI values.
4. Conclusions
From these results it is clear that the rate of chain propagation
exceeds the rate of chain transfer when considering catalyst C14 in
comparison to catalyst C11. This is most probably due to the fact
that b-hydride transfer will be slower for a metal centre which has
a higher electron density (PMe3 complex) compared to that of
a metal centre with lower electron density (PPh3 complex). The
We have prepared and characterized air- and moisture-stable
cationic palladacycles which were active catalysts of the oligo-/
polymerization of PA. At room temperature moderate activities
were observed with a mixture of cis-transoidal and trans-cisoidal
oligo-/PPA produced, the cis-transoidal oligo-/PPA being the major
product. At higher temperature a marked increase in catalytic
activity was observed with the selective formation of trans-cisoidal
PPA. The presence of electron-withdrawing groups on the cyclo-
metallated ring served to enhance the catalytic activity of the pal-
catalyst system [Rh(OMe)(nbd)]2/PMePh2 (nbd
¼
2,5-
norbornadiene) has been shown to polymerize PA to high molec-
ular weight and slightly broader PDI values, consistent with our
results with catalyst C14 [7c].
ladacycles. Reaction time had
a marked effect on polymer
Polymerization experiments were also performed at an elevated
molecular weight, with high molecular weight polymers being
formed at longer reaction time. Even though the PPA produced by
the cationic palladacycles had low molecular weights in all cases
polymers with narrow PDI’s were obtained.
temperature (60 ꢀC, Table 3, entries 6e10) to study the effect of
6000
25 oC
Acknowledgements
60 oC
Acknowledgements go to National Research Foundation (South
Africa) and the DST/NRF COE in Catalysis (c*change) as well as the
NRF/SIDA for financial support. This work was also supported by
Research Committees of the University of the Western Cape and
Stellenbosch University.
4000
2000
Appendix A. Supplementary material
0
Crystallographic Data Centre. Copies of the data [CCDC 82836]
can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ, UK [Fax: (international) þ 44(1223)336
033, E-mail: deposit@ccdc.cam.ac.uk].
C10
C11
C12
Catalyst
C13
C14
Fig. 3. Catalyst activity for catalyst C10eC14.

N. Mungwe et al. / Journal of Organometallic Chemistry 696 (2011) 3527e3535
3535
[10] K. Li, M.S. Mohlala, T.V. Segapelo, P.M. Shumbula, I.A. Guzei, J. Darkwa, Poly-
hedron 27 (2008) 1017.
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