Phenylacetylene polymerisation mediated by cationic cyclometallated palladium(ii) complexes bearing benzylidene 2,6-diisopropylphenylamine and its derivatives as ligands

Article abstract of DOI:10.1039/C8DT02728E

A series of novel cationic palladacycle complexes bearing benzylidene-2,6-diisopropylphenylamine derivatives as ligands and with the general formula [Pd(MeCN)(L)(R-C₆H₃)CH═N{2,6-ⁱPr₂-C₆H₃}]

Full text of DOI:10.1039/C8DT02728E

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ISSN 1477-9226
PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
framework
a Zr-metal–organic
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ARTICLE
Phenylacetylene polymerisation mediated by cationic
cyclometallated palladium (II) complexes bearing
benzylidene 2,6-diisopropylphenylamine and its derivatives
as ligands
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
M. Cassiem. Joseph,a Andrew J. Swarts b and Selwyn F. Mapolie *a
A series of novel cationic palladacycle complexes bearing benzylidene-2,6-diisopropylphenylamine derivatives as ligands
and with the general formula [Pd(MeCN)(L)(R-C6H3)CH=N{2,6-iPr2-C6H3}][B(3,5-(CF3)2-C6H3)] (R = H, Cl, Br, F, OMe and L =
1,3,5-triaza-7-phosphaadamantane (PTA), tricyclohexylphosphine (PCy3) and triphenylphosphine (PPh3) were prepared and
characterized by a range of analytical techniques. These cationic palladacycle complexes were found to be active
precatalysts for the polymerisation of phenylacetylene. The meta-substituent on the cyclometallated ring was found to
have a marked effect on the catalyst performance in that complexes, which contained electron-withdrawing substituents,
were found to be the most active in the polymerisation process. Furthermore, increasing the steric bulk of the phosphine
ligand led to the production of higher molecular weight polyphenylacetylenes (PPA). Polymerisation reactions performed
at 25 °C gave a mixture of both cis-transoidal and trans-cisoidal PPA while trans-cisoidal PPA was selectively produced at
elevated temperatures (60 °C). Preliminary mechanistic studies demonstrated that polymerisation proceeds via
dissociation of the C,N-chelating ligand, and involves the formation of an iminium cationic fragment.
moisture sensitivity of early transition metal complexes has
necessitated the development of more robust late transition
metal complexes capable of polymerising acetylenes
containing polar functionalities and the ability to conduct
Introduction
The polymerisation of substituted acetylenes has attracted
steady attention over the years due to the unique properties
of resulting polymeric material. With their unique π-
conjugated backbone, these polymers display several desirable
properties which include gas permeability,1,2 magnetic
polymerisations in polar reaction media.8,9 Cationic
palladacycle complexes have gained increasing attention in
recent years especially in the field of catalysis.10 They can be
regarded as potential model compounds for proposed active
species postulated to be present in typical catalytic cycles for
several organic transformations involving unsaturated
hydrocarbons. One of the key factors that influences the
catalytic activity of these type of cationic precatalysts is the
presence of a highly electrophilic metal centre. In addition, the
presence of a weakly coordinating solvent moiety facilitates
the stabilization of the active catalyst. However, the
coordination of the solvent needs to be weak enough to allow
the incoming monomer access to the active centre in order to
initiate the catalytic transformation. Herein, we report the
synthesis and characterization of several novel cationic
palladacycle complexes bearing different meta-substituents
and their application as catalyst precursors in the
polymerisation of phenylacetylene. Specifically, we sought to
establish the role that i) the meta-substituents of the C,N-
chelate ligand, ii) the electronic and steric properties of the
phosphine ligand and iii) the solvent properties has on the
polymerisation productivity and polymer properties. We also
susceptibility3 and nonlinear-optical properties,4 electrical-
5
and photo-conductivity.6,7 Of particular interest amongst this
class of polymers are polyphenylacetylenes (PPAs) which
exhibit excellent stability in air and show high solubility in
common organic solvents. Early transition metal catalyst
systems, particularly those of group 5 and 6 metals are known
to demonstrate both high activity and selectivity for acetylene
polymerisation reactions. However the inherent air and
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report spectroscopic evidence for imine dissociation en route spectroscopy of the cationic palladacycles confirmed successful
to the active catalyst, facilitated by the formation of an coordination of the acetonitrile ligand to the palladium metal
iminium moiety.
centre. This is indicated by a singlet assigned to the methyl protons
of CH3CN in the upfield region at δ 1.44-1.38 ppm (for complexes
C11-C15 and C21-C25) and δ 1.05-1.04 ppm (for complexes C31-
C35). The difference in the chemical shift of the imine proton
resonance (H7, Figure S8b†) in the 1H NMR spectra between the
neutral palladacycles and their cationic counterparts is discernible
but the changes are not particularly large. Analogous resonances to
the previously synthesized neutral palladacycles were observed in
the 13C {1H} NMR spectra of all cationic palladacycle complexes.
Thus for example the imine carbon resonance (C7, Figure S8b†)
shifted slightly downfield in the region δ 179.1-172.5 ppm and
Results and Discussion
Synthesis of cationic mononuclear palladacycle complexes
The cationic palladacycle complexes, C11-C15; C21-C25 and
C31-35, were prepared by the reaction of the corresponding neutral
palladacycles, C8-C10, C16-C20 and C26-C30 with sodium
tetrakis[3,5- bis(trifluoromethyl)-phenyl] borate in the presence of
acetonitrile (Scheme 1).
3
appears as a doublet due to a heteronuclear JC-P coupling to the
phosphorus atom of the phosphine ligand. The 31P {1H} NMR
spectra showed
a
well-defined singlet in the region
δ 48.2-41.0 ppm for the PCy3- and PPh3 cationic analogues and in
the region δ -46.3-(-48.1) ppm for the PTA cationic complexes. The
presence of only one phosphorus resonance confirms that only one
isomer is present. ESI-MS spectra recorded in both the positive and
negative ion mode were consistent with the proposed structures
for cationic palladacycle complexes C11-C15, C21-C25 and C31-C35
(see Figures s25, s41 and s48†).
We obtained single crystals suitable for X-ray diffraction
analysis of complexes C15 and C25. The molecular structures are
depicted in Figures 1 and 2. Crystallographic data for C15 and C25
are presented in Table S1† while selected bond lengths, bond
angles and torsion angle are presented in Table S2†.
The molecular structures of the cationic complexes exhibit a
slightly distorted square-planar geometry around the palladium
centre in which the coordination sphere of the metal is occupied by
the N-benzylidene bound bidentate ligand (N^C) as well as the
phosphine ligand. The fourth coordination site is occupied by the
weakly coordinating acetonitrile molecule.
Scheme 1: Synthetic methodology for the preparation of phosphine
substituted cationic palladacycles bearing different meta-substituents.
Several of the neutral mononuclear palladacycles and their µ-
Cl bridged dimeric precursors have been previously reported11 and
are thus not discussed here. For completeness their preparation
and characterisation data are given in the Supplementary
Information section.
Figure 1: Molecular structure of complex C15 with atomic numbering, drawn
at 50% probability ellipsoids. Counterion and all hydrogen atoms are
omitted for clarity.
The cationic complexes were isolated as off-white solids in
high yields (80-91 %) and displayed solubility in chlorinated
solvents, diethyl ether and alcohols, but were insoluble in
unsaturated hydrocarbons. The cationic palladacycles containing
the PTA and PCy3 auxiliary ligand are all novel, whereas the PPh3
complexes C31-C33 have previously been reported.11a NMR
The orientation of the phosphine ligands remain trans relative
to the imine nitrogen upon coordination of the acetonitrile ligand.
The Pd-N(1) bond length for complexes C15 and C25 are 2.0975(2) Å
and 2.0995(2) Å respectively and are within the range observed for
analogous endocyclic complexes.17
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architecture of the catalyst. This is most likely due to the fact that
electron-withdrawing substituents enhance the electrophilicity of
the metal centre, hence promoting monomer coordination. Masuda
et al.18 for example studied the polymerisation of phenylacetylene
using
rhodium
zwitterionic
complexes
containing
tetrafluorobenzobarrelene ligands and found that the electron
donating methyl groups on the tetrafluorobenzobarrelene
ultimately increases electron density on metal centre, which
subsequently reduces the ability of the monomer to coordinate.
Figure 2: Molecular structure of complex C25 with atomic numbering, drawn
at 50% probability ellipsoids. Counterion and hydrogen atoms are omitted
for clarity.
Phenylacetylene polymerisation
Phenylacetylene polymerisation reactions were initially
performed in tetrahydrofuran as solvent, employing a monomer to
catalyst ratio of 50:1 at a reaction temperature of either 25 °C or
60 °C. The polymerisation products were characterized by FTIR and
1H NMR spectroscopy as well as gel permeation chromatography
(GPC).
Figure 3: Effect of palladacycles bearing different meta-substituents and
auxiliary phosphine ligands on the conversion. Reaction conditions:
PA:Pd = 50:1; [Pd] = 3mM; 60 °C; 48 hours; THF (10 mL). Conversions were
determined by the total mass of the crude product as a percentage of the
monomer used.
H
active catalyst
n
Complex C13, which contains a bromo substituent in the meta-
position does not follow the expected inductive effect trend. One
would have expected that this catalyst would show similar activity
to that of the chloro analogue C12 since both the bromo and chloro
substituents have very similar electronegativities. Similar trends are
observed in the analogous PCy3 and PPh3 complexes. It should be
noted that we observed that C13, appears to have a lower stability
in solution when compared to the other complexes tested in this
study, resulting in significant decomposition over time. We have
previously found that the neutral precursor to this complex, C8, was
similarly also quite unstable in solution when tested as an
anticancer agent.19 This instability may be attributed to the larger
size of the bromo substituent in the meta position which results in a
larger steric interaction with the ortho imine group. This would
impact negatively on the coordination ability of the ortho-imine
group, resulting in a weaker Pd-N bond and hence an unstable
complex.
n
Ph
Scheme 2: A general scheme for the polymerisation of phenylacetylene.
Effect of the nature of catalyst precursors on the polymerisation of
phenylacetylene
The polymerisation of phenylacetylene (Scheme 2) was
performed using cationic palladacycles (C11-C15, C21-C25, C31-
C35). These complexes differ from each other with regards to the
nature of the meta-substituents on the cyclometallated ring as well
as the type of the auxiliary phosphine ligand coordinated to the
metal with the latter differing in its basicity and steric properties.
Figure
3 summarizes the activity of the catalyst precursors
evaluated in the polymerisation of phenylacetylene. The catalyst
activities are summarized in terms of the effect on the
polymerisation of the meta-substituents on the cyclometallated
ring as well as the nature of the phosphine ligand viz. PTA, PCy3 and
PPh3.
Moderate to high conversions were obtained for both the PCy3
and PPh3 analogues of the PTA complexes with the highest
conversion observed for C24 and C34. It is clear that the nature of
the tertiary phosphine ligand has a marked effect on the activity of
the catalyst. The basicity and cone angles of the tertiary phosphine
ligands can be arranged in the following order: PCy3 > PPh3 > PTA.
The catalysts containing the phosphine with the largest cone angle
and basicity,PCy3 (C21-C25), generally gave the lowest conversion
when compared to their PPh3 and PTA analogues. (Figure 3). The
observed trend may be attributed to the combination of both the
steric and electronic properties of the ligated phosphine, with a
In general, under the optimized conditions employed (PA:Pd =
50:1; [Pd]
= 3mM; 60 °C; 48 hours; THF), the PTA-based
palladacycles gave high conversions with the highest conversion of
95 % observed for C14, the PTA based complex with the fluoro
substituent in the meta position. Turnover-numbers (TON) and
productivity values for reactions performed with the various
catalysts are shown in Table 1. The results for the PTA based
catalysts highlight the effect of the different meta-substituents on
the catalytic activity. Essentially higher activities were observed
when electron-withdrawing substituents are incorporated into the
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H
P
Ph H
Ph
P
H
Ph H
Ph
larger steric effect and stronger donating properties decreasing the
electrophilicity of the metal and hindering monomer approach.
P
P
H
Ph
Ph H
Ph H
Table 1: TON and GPC data pertaining to polymerisation reactions
a
performed using the cationic palladacycle complexes
.
trans-cisoidal
cis-transoidal
d
Entry Catalyst
TONb
Productivity c
Mw
PDI
1
2
3
4
5
6
7
8
C11
C12
C13
C14
C15
C21
C22
C23
C24
C25
C31
C32
C33
C34
C35
4400
4533
2853
4850
4203
3770
3810
3353
4160
3547
3953
4360
3080
4447
3857
41.3
42.6
26.8
45.6
39.5
35.4
35.8
31.5
39.1
33.3
37.1
41.0
28.9
41.8
36.2
2440
2515
2362
2656
2557
4157
3957
3934
4410
4041
2710
2797
2341
2875
2465
1.18
1.23
1.16
1.20
1.16
1.16
1.25
1.38
1.19
1.22
1.26
1.35
1.28
1.19
1.31
P
polymer chain
Figure 4: cis-transoidal and trans-cisoidal isomers of polyphenylacetylene.26
The groups highlighted in blue belong to the same phenylacetylene
monomer.
Polymerisation of phenylacetylene using single solvents and a
mixture of CH2Cl2 and MeCN
9
10
11
12
13
14
15
The polymerisation of phenylacetylene was further probed at
both 25 °C and 60 °C in different solvents to investigate the effect of
the reaction medium on the polymerisation (Table 2). For these
experiments, the PTA-based fluoro complex, C14, which was earlier
shown to be the most active catalyst, was employed as a model
catalyst. At 25 °C the nature of the solvent had less of an impact
than was the case at the higher temperature of 60 °C. However, at
both temperatures THF was found to be the most effective solvent
for the polymerisation reactions (entries 1 and 5, Table 2). As
shown in the proposed reaction pathway (Scheme 3), a crucial step
is the dissociation of acetonitrile to produce an open coordination
site on the metal. A coordinating solvent such as THF can stabilize
the coordinatively unsaturated metal complex increasing catalyst
lifetime, leading to enhanced overall catalyst activity over time. This
is not possible for non-coordinating solvents such as CH2Cl2 and
toluene hence the lower activity in these solvents. Another factor
that influences the catalyst reactivity is the polarity of the solvent.
Solvents of low polarity such as toluene and CH2Cl2 will favour the
cationic metal complex existing as a close-contact ion pair with the
BArF counter ion while in the more polar THF, a solvated ion-pair
will be present. Monomer coordination will be easier in the latter
situation thus facilitating the polymerisation reaction. Comparing
the reactions performed in THF with those conducted in another
coordinating solvent, MeCN, it is noted that in the latter case, the
conversion is much lower. It is expected that in THF, the
dissociation of the coordinated acetonitrile occurs faster when
a
Reaction conditions: PA:Pd = 50:1; [Pd] = 3mM; 60 °C; 48 hours; THF (10 mL).b
c
d
Turnover number (TON) = g product/mol Pd. Productivity = g product/g Pd. Weight
average molecular weight of methanol insoluble product. GPC analysis performed at
room temperature and measured against polystyrene standards.
This ultimately decreases the overall rate of phenylacetylene
polymerisation.
In addition to controlling the activity of the catalysts, the
nature of the phosphine also influenced the molecular weight of
the polymers produced. Catalysts containing the more sterically
hindered phosphine (larger cone angle), PCy3 (C21-C25) produced
higher molecular weight polymers (entries 6-10, Table 1) relative to
the catalysts with less sterically hindered phosphines (entries 1-5,
11-15, Table 1). Incorporating steric bulk into the catalyst design
has previously been shown to promote chain propagation in
phenylacetylene polymerisation reactions using late transition
metal complexes as catalysts.20 Regardless of the catalyst precursor
used, reactions performed at 60 °C selectively produced
trans-cisoidal PPA. This was confirmed by spectroscopy (Figure S2†
and Figure S3†). The selective formation of the trans-cisoidal PPA
stereoisomer at elevated temperatures has been observed
previously using other catalytic systems.21 Employing the best compared to reactions conducted in pure acetonitrile due to the
high concentration of acetonitrile present in the latter case. This
would assist the coordination of the incoming monomer when using
THF as solvent.
catalyst, C14, the polymerisation was also conducted at room
temperature. This resulted in a TON of 3016 compared to 4850 at
60 °C. The polymers isolated at 25° generally had higher molecular
weights and lower polydispersity indices (entries 1-4, Table 2) than
the material isolated at 60°C (entries 5-10, Table 2), which is due to
an increase in chain termination relative to chain propagation at
higher temperatures.22 At the lower temperature of 25 °C, mixtures
of cis-transoidal and trans-cisoidal polymers, as determined
spectroscopically, with high molecular weights were obtained
(Figures S3a and S3b†, Figure 4).23-25 The activity of C14 was found
to be comparable to previously reported palladium catalyst systems
bearing various N-donor ligands.20,26,28
The effect of binary solvents was also investigated by
performing polymerisations at 25 °C in mixtures of 9:1 and 7:1
CH2Cl2/MeCN respectively. This resulted in low conversion but
yielded high molecular weight polymers with narrow polydispersity
indices (entries 1-2, Table 2). Increasing the proportion of
acetonitrile in the solvent mixture led to a slightly lower conversion,
however the molecular weight of the polymers increased by almost
1000 Da. These results suggest that increasing the concentration of
acetonitrile leads to increased competition between the
phenylacetylene monomer and the acetonitrile solvent for the
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vacant site on the metal centre. This results in a lower rate of were not successful. We suspect that this is due the short life-time
polymerisation. On the other hand, the catalyst lifetime is improved of this species, which undergoes rapid transformation in the
due to the higher stability of the solvated species, which promotes
polymer chain growth over time resulting in higher molecular
weight polymers. When reactions were performed at 60 °C using
the binary solvent system (entries 9-10, Table 2), a significant
increase in conversion was observed with slightly lower molecular
weight polymers with higher polydispersity indices when compared
to the reactions at 25 °C.
following step in the proposed mechanism. The σ-alkynyl-hydride
species undergoes reductive elimination in which the imine moiety
of the C,N chelating ligand and the hydride combine to produce
species B containing an uncoordinated iminium moiety. Signs of the
1
formation of such a species is seen when closely examining the H
NMR spectrum recorded after 48 hours reaction time (Figure 5 and
Figure 6). A signal appearing around δ 9.41 ppm was assigned to
the iminium moiety present in species B. This resonance is in the
range observed for analogous zwitterionic Pt(II) complexes bearing
C,N-chelating ligands, δ 11-16 ppm, in which the bridging proton of
the iminium moiety interacts with the platinum centre.35 The slight
discrepancy (∼ δ 9 ppm vs δ 11-16 ppm) may be due to the lack of a
M-H interaction in our catalyst system, which is a pronounced
feature for the reported Pt(II) complexes. Attempts to characterise
species B crystallographically is in process, with no success thus far.
In order to confirm the feasibility of the protonation of the imine
nitrogen, we carried out a separate NMR experiment in which the
free ligand is treated with dilute HCl (Scheme 4). Here we observed
the signal for the protonated imine at a slightly higher chemical
shift viz. δ 10.31 ppm (Figure S5†). It should however be pointed
Table 2: Catalytic data pertaining to cationic palladacycle C14.a
T
(°C)
Conv
(%)
Productiv
ity c
d
Entry
Solvent
TONb
Mw
PDI
28.3
20.5
23.0
22.5
45.6
29.9
23.3
19.3
36.5
33.4
1
25
25
25
25
60
60
60
60
60
60
THF
CH2Cl2
9:1
45
24
38
36
95
62
49
40
76
70
3016
2183
2450
2390
4850
3180
2480
2050
3883
3550
4175
3615
5944
6879
2656
1133
1046
1249
4190
4429
1,03
1,22
1,02
1,03
1,20
1,47
1,51
1,35
1,46
1,55
2
3e
4e
5
7:1
THF
6
CH2Cl2
toluene
MeCN
9:1
7
8
9e
10e
out that in this case we are dealing with
a completely
7:1
uncoordinated imine ligand while in the case of species B, one end
of the chelating C,N-ligand is still coordinated to the metal via the C
terminus. An alternative reaction that the above-mentioned species
A can be involved in, is the reductive elimination in which the C end
of the C,N chelate combines with the hydride ligand to produce
a Reaction conditions: PA:Pd = 50:1; [Pd] = 3mM; 48 hours; 10 mL solvent. Conversions
were determined by the total mass of the crude product as a percentage of the
b
c
monomer used. Turnover number (TON) = g product/mol Pd. Productivity = g
product/g Pd.
d
Weight average molecular weight of methanol insoluble product. GPC analysis
e
1
performed at room temperature and measured against polystyrene standards.
species C. Our H NMR results seems to indicate that this does not
Solvent mixture CH2Cl2:MeCN.
occur as the signals for the aromatic ring protons attached to the
carbons adjacent to the carbon under consideration remains
unchanged. Species B is then thought to undergo ligand exchange
of coordinated acetonitrile with the phenylacetylene monomer,
resulting in the weakly coordinating acetonitrile dissociating
(Scheme 3). This is confirmed by the appearance of a signal in the
Possible mechanism for the polymerisation of phenylacetylene using
catalyst, C14
In an attempt to probe the mechanism through which these
cationic
palladium
complexes
mediate
phenylacetylene
polymerisation, we conducted a NMR experiment in which we
reacted complex C14 with a slight excess of phenylacetylene in
THF-d8 as solvent. In analysing the NMR spectra (Figures 5 and 6)
obtained during the course of the reaction, we tried to correlate
any identifiable species with intermediates proposed for one of the
most commonly accepted mechanisms for phenylacetylene
polymerisation, viz. the insertion mechanism. The alternative
pathway viz. the metathesis mechanism was not considered given
that the generation of the requisite active species, a metal-carbene,
is not possible for our palladacycle-based catalyst precursors. The
primary steps of the mechanism proposed are depicted in Scheme
3. The insertion mechanism has previously been observed for
several rhodium complexes used in the polymerisation of
phenylacetylene.29–32 It is proposed that using the palladacycles
reported here as pre-catalysts, we initially have oxidative addition
of phenylacetylene to the cationic palladium (II) precursor to
produce a σ-alkynyl Pd (IV) hydride species, A. The existence of
such Pd (IV) σ-alkynyl species have been reported in the
literature.33,34 Unfortunately, attempts to detect a metal hydride
1 H NMR spectrum at
δ 1.93 ppm with the concomitant
disappearance of a signal at δ 1.84 ppm assigned to the coordinated
acetonitrile (Figure 6). Subsequent to acetonitrile dissociation we
have phenylacetylene coordination in a η2-fashion followed by 2,1-
insertion of the alkyne into the Pd-alkynyl σ-bond. Sequential
alkyne coordination and insertion steps promotes chain
propagation. It should be noted that from the available evidence
chain
growth
occurs
via
insertion
into
Pd-C(alkynyl) bond and not the Pd-C(aryl) bond. Further evidence
for the dissociation of the imine moiety of the C,N chelating ligand
is the fact that the two doublets around ~ δ 1.25 ppm assigned to
the methyl resonances of the isopropyl substituents disappears
with time and is gradually replaced a doublet around δ 1.15 ppm.
The latter correlates well with a situation in which the imine moiety
1
is uncoordinated (free). This resembles what is observed for the H
NMR spectrum of the free benzylidene-2,6-diisopropylphenylamine
ligand in THF-d8 where a single doublet for the methyl resonances
of the isopropyl is also observed. In addition to the signal for the
isopropyl methyl resonances changing, a new signal assigned to the
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methine proton of the isopropyl group appears as the reaction
progresses over time. From the NMR experiment, it is apparent that
the tertiary phosphine ligand is not directly involved in the
mechanism, as the signals for the tertiary phosphine remain largely
unchanged. Thus, it would appear that phosphine dissociation is not
a prerequisite for the reaction to occur. The results obtained from
the 1H NMR experiment over time are corroborated by 13C NMR
Cl
F
F
H
H
HCl
N
HN
R
R
R = 2,6-diisopropylaniline
1
Scheme 4: Protonation of the imine
spectroscopy (Figure 7). As was observed in the H NMR spectra,
the corresponding 13C NMR spectra also shows the consumption of
free phenylacetylene as evidenced by the disappearance of the
resonances at δ 84.0 and δ 79.0 ppm. These are assigned to the α
and β carbons of the alkyne monomer. Simultaneously a new signal
at δ 123.8 ppm appears which is assigned to a Pd-alkynyl species.
Such species have previously been reported in literature.36-37
moiety (for L4) to yield the iminium ligand.
Figure 5: Variable time 1H NMR spectrum (in the region δ 9.50 – 5.50 ppm)
in d8-THF (recorded at 25 °C) of palladacycle C14 at (a) before
phenylacetylene addition; (b) after phenylacetylene addition; (c) 24 hours;
(d) 48 hours and (e) 72 hours. The solvent signal in all spectra was
normalized to 1. Inset shows zoom-in region of the vinylic proton.
Scheme 3: Proposed phenylacetylene mechanism mediated by cationic
palladacycles.
1
Figure 6: Variable time H NMR spectrum (in the region δ 5.00 – 0.80 ppm)
in d8-THF (recorded at 25 °C) of palladacycle C14 at (a) before
phenylacetylene addition; (b) after phenylacetylene addition; (c) 24 hours;
(d) 48 hours and (e) 72 hours. The solvent signal in all spectra was
normalized to 1.
Furthermore, a new methine signal associated with the
isopropyl units of an uncoordinated imine was also observed at δ
29.0 ppm. Dissociation of the acetonitrile ligand is further
evidenced by the appearance of a peak at
∼ δ 30.50 ppm. Over the
course of the reaction, we observe the appearance of signal around
δ 133.4 ppm assigned to the vinylic carbons of the growing polymer
chain.
6 | J. Name., 2012, 00, 1-3
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ARTICLE
Experimental section
Materials and methods
All transformations were carried out under an inert
atmosphere of dry nitrogen using a dual vacuum/nitrogen line and
standard Schlenk techniques, unless specified otherwise. All
reagents were acquired from Sigma-Aldrich, Merck and Alfa-Aesar
and used without further purification. Solvents were obtained from
Merck and Kimix and purified by a Pure SolvTM micro solvent
purifier fitted with activated alumina columns. The palladium
precursor, bis(acetonitrile) palladium(II)chloride, was prepared
using a literature method.38 CCDC 1568394 for complex C15 and
CCDC 1568393 for complex C25 contains the supplementary
crystallographic data for this paper. This data can be obtained free
Figure 7: Variable time 13C NMR spectrum in d8-THF (recorded at 25 °C) of
palladacycle C14 at (a) before phenylacetylene addition; (b) 6 hours; (c) 30
hours and (d) 54 hours. The solvent signal in all spectra was normalized to 1. of charge from The Cambridge Crystallographic Data Centre via
Inset shows zoom-in region of the Pd-alkynyl species.
www.ccdc.cam.ac.uk/data_request/cif.
Conclusions
Phenylacetylene polymerisation reactions
We have prepared and characterised
a series of cationic
All catalytic reactions were performed using a Radleys 12-
stage carrousel parallel reactor equipped with a gas distribution
system. The cationic palladacycle complex (30µmol) was dissolved
in either a pure solvent or solvent mixture under inert atmosphere
using standard Schlenk techniques. Phenylacetylene (50:1, PA/Pd)
was added to the stirring solution and the reaction mixture was
heated to 25 °C or 60 °C. The total reaction volume was 10 mL with
a catalyst concentration of 3mM. The reaction was allowed to stir
for the required time under a nitrogen atmosphere. A gradual
colour change from light yellow to dark yellow to orange then to
dark brown was observed. After the allotted time, the solvent and
excess phenylacetylene were removed in vacuo and the brown
residue was re-dissolved in a minimum amount of CH2Cl2. The
polymer was precipitated out of solution by adding methanol (15
mL), yielding a yellow-orange solid. The solid was stirred in
palladacycles bearing different meta-substituents on the
cyclometallated ring as well as auxiliary tertiary phosphine ligands,
varying in their basicity and cone angle, coordinated to the metal
(C11-C15, C21-C25 and C31-C35). These palladacycles were
evaluated as catalyst precursors in the polymerisation of
phenylacetylene, during which the effect of various structural and
experimental parameters were evaluated. The initial screening of
the pre-catalysts found that the meta-substituent on the
palladacycle ring and the basicity and cone angle of the auxiliary
phosphine has the most significant impact on productivity.
Decreasing the electron-withdrawing ability of the meta-
substituent, thereby decreasing the electrophilicity of the metal led
to a decrease in productivity. The exception to this was complexes
containing the Br-substituent in the meta position. This anomaly
was attributed to the steric bulk of the Br-substituent, which
increases the steric strain associated with the imine moiety, leading methanol for 18 hours, filtered and dried under vacuum. The
to a weakening of the N-Pd bond. The combined effect is a decrease
in the stability of the Br-containing cationic palladacycle pre-
catalysts. Similarly, increasing the basicity and cone angle of the
phosphine ligand decreased catalyst productivity, due to the
combination of a less electrophilic and sterically accessible metal
centre. A mixture of both cis-transoidal and trans-cisoidal PPA were
produced at room temperature with moderate molecular weights
while trans-cisoidal PPA was selectively produced when the
reactions were performed at slightly higher temperatures (60 °C). In
addition, increasing the steric bulk of the phosphine ligand led to
higher molecular weight PPA being produced. Our results
established that the optimum degree of polymerisation was
achieved using a single, coordinating solvent such as THF, which
enhances the stability of the active species. An initial investigation
of the mechanism by which polymerisation occurs for this catalyst
system provided evidence that the process proceeds via an alkyne
insertion into a M-C (alkyne) bond and that initial dissociation of the
imine moiety of the C,N chelating unit, facilitated by reductive
elimination to generate an iminium ion, plays a key role in the
polymerisation process.
solvent was removed from the filtrate in vacuo yielding a brown
residue, which was dried under vacuum. Both the methanol soluble
and methanol insoluble fractions were subjected to GPC analysis.
All catalytic reactions were performed in duplicate.
Variable time NMR experiments
Cationic palladacycle C14 (17µmol) was dissolved in
deuterated THF (0.75 mL) and placed in a NMR tube. Two
equivalences of phenylacetylene were added and NMR spectra
were collected at 25 °C over a 72 hour period. Photos were taken
(Fig. S4†) to highlight the gradual colour change of the catalytic
reaction over time.
Synthesis of µ-Cl bridge palladacycles, C4 and C5
.
Complexes C4 and C5 were prepared according to a general
procedure in literature.11 See supplementary information for
experimental details.
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J. Name., 2013, 00, 1-3 | 7
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ARTICLE
Journal Name
17 K. J. Keuseman, I. P. Smoliakova and V.
Organometallics, 2005, 24, 4159.
V Dunina,
Synthesis of neutral palladacycles, C6-C10, C16-C20 and C26-C30
.
18 N. Onishi, M. Shiotsuki, F. Sanda and T. Masuda,
Macromolecules, 2009, 42, 4071.
19 A. Blackenberg, Preparation and characterization of novel
palladacycles and their evaluation as anticancer agents, Ph.D
Thesis, Stellenbosch University, 2016.
The neutral complexes were prepared by following a well
reported literature protocol. Synthetic details and characterization
data are given in the supplementary information section.
Synthesis of cationic palladacycles, C11-C15, C21-C25 and C31-C35
.
20 K. Li, M. S. Mohlala, T. V. Segapelo, P. M. Shumbula, I. Guzei
and J. Darkwa, Polyhedron, 2008, 27, 1017.
21 M. Marigo, N. Marsich and E. Farnetti, J. Mol. Catal. A Chem.,
2002, 187, 169.
22 K. Li, G. Wei, J. Darkwa and S. K. Pollack, Macromolecules,
2002, 35, 4573.
23 J. M. Sibanyoni, G. B. Bagihalli and S. F. Mapolie, J.
Organomet. Chem., 2012, 700, 93.
24 A. Sen and T. Lai, Organometallics, 1982, 1, 415.
The synthesis of the cationic palladacycles were generally
prepared by removal of the chloride ligand using Na[B(ArF)4] as
chloride abstractor. The general procedure using C11 as an example
as well as the detailed characterization data of all the other cationic
complexes prepared are discussed in the Supplementary
Information section.
25 J. G. Rodríguez, A. Lafuente and R. Martín-Villamil, J. Polym.
Sci. Part A Polym. Chem., 2005, 43, 1228.
26 Z. Ke, S. Abe, T. Ueno and K. Morokuma, J. Am. Chem. Soc.,
2011, 133, 7926.
Conflicts of interest
There are no conflicts to declare.
27 P. Pelagatti, M. Carcelli, C. Pelizzi and M. Costa, Inorganica
Chim. Acta, 2003, 342, 323.
28 S. O. Ojwach, I. A. Guzei, J. Darkwa and S. F. Mapolie,
Polyhedron, 2007, 26, 851.
29 Y. Kishimoto, P. Eckerle, T. Miyatake, T. Ikariya and R. Noyori,
J. Am. Chem. Soc., 1994, 116, 12131.
Acknowledgements
The authors acknowledge the financial support by the National
Research Foundation (NRF) and the DST-NRF Centre of Excellence in
Catalysis, c*change, South Africa.
30 Y. Kishimoto, P. Eckerle, T. Miyatake, M. Kainosho, A. Ono, T.
Ikariya and R. Noyori, J. Am. Chem. Soc., 1999, 121, 12035.
31 Y. Misumi and T. Masuda, Macromolecules, 1998, 31, 7572.
References
1
Y. Hayakawa, M. Nishida, A. Okumura, M. Matsui and H.
Muramatsu, Polym. Bull., 1992, 28, 293.
32 M. Casado, A. Fazal and L. Oro, Arab. J. Sci. Eng., 2013, 38
1631.
,
2
T. Masuda, E. Isobe and T. Higashimura, J. Am. Chem. Soc.,
1983, 105, 7473.
33 A. Canty, T. Rodemann, B. Skelton and A. White,
Organometallics, 2006, 25, 3996.
34 M. Sharma, A. Ariafard, A. Canty, B. Yates, M. Gardinera and
R. Jones, Dalton Trans., 2012, 41, 11820.
3
4
F. Rossitto and P. Lahti, Macromolecules, 1993, 26, 6308.
J. Le Moigne, A. Hilberer and C. Strazielle, Macromolecules,
1992, 25, 6705.
35 (a) I. C. M. Wehman-Ooyevaar, D. M. Grove, P. De Vaal, A.
Dedieu and G. Van Koten, Inorg. Chem., 1992, 31, 5484; (b) I.
C. M. Wehman-Ooyevaar, D. M. Grove, H. Kooijman, P. Van
der Sluis, A. L. Spek and G. Van Koten, J. Am. Chem. Soc.,
1992, 114, 9916.
5
C. K. Chiang, C. R. Fincher, Y. W. Park, A. J. Heeger, H.
Shirakawa, E. J. Louis, S. C. Gau and A. G. MacDiarmid, Phys.
Rev. Lett., 1977, 39, 1098.
A. J. Heeger, Angew. Chem. Int. Ed, 2001, 40, 2591.
A. G. MacDiarmid, Angew. Chem. Int. Ed, 2001, 40, 2581.
R. Schrock, S. Luo, N. Zanetti and H. Fox, Organometallics,
1994, 13, 3396.
6
7
8
36 Y. Kim, K. Osakada and A. Yamamoto, J. Organomet. Chem.,
1993, 452, 247.
37 Y. Kim, S. Lee, S. Lee, S. Il, M. S. Lim and S. W. Lee, Inorganica
Chim. Acta, 2005, 358, 650.
9
T. Masuda, K. Hasegawa and T. Higashimura,
Macromolecules, 1974, 7, 728.
38 F. R. Hartley, S. G. Murray and C. A. McAuliffe, Inorg. Chem.,
1979, 18, 1394.
10 (a) D. A. Alonso and C. Nájera, Chem. Soc. Rev., 2010, 39
2891;
,
(b) M. Albrecht, Chem. Rev., 2010, 110, 576; (c) C. L. Chen, Y.
H. Liu, S. M. Peng and S. T. Liu, J. Organomet. Chem., 2004,
689, 1806.
11 (a) N. Mungwe, A. J. Swarts, S. F. Mapolie and G. Westman, J.
Organomet. Chem., 2011, 696, 3527; (b) A. Blanckenberg, S.
Aliwaini, S.W. Kimani, A. van Niekerk, A. Neumann-Mufweba,
S. Prince, S.F. Mapolie, J. Organomet. Chem., 2017, 851, 68;
12 J. Long, H. Gao, K. Song, F. Liu, H. Hu, L. Zhang, F. Zhu and Q.
Wu, Eur. J. Inorg. Chem., 2008, 4296.
13 D. Srimani and A. Sarkar, Tetrahedron Lett., 2008, 49, 6304.
14 J. Albert, M. Gomez, J. Granell and J. Sales, Organometallics,
1990, 9, 1405.
15 J. Ruiz, N. Cutillas, F. López, G. López and D. Bautista,
Organometallics, 2006, 25, 5768.
16 R. B. Bedford, C. S. J. Cazin, M. B. Hursthouse, M. E. Light and
V. J. M. Scordia, Dalton Trans., 2004, 3864.
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Table of contents entry
-B(Ar)4
F
F
Pri
H
N
Ph H
Ph H
Ph
Pri
PTA
Pd
MeCN
N
MeCN Pd PTA
n
n
H
Ph
H
Ph
cis-transoidal
trans-cisoidal
Ph
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