α-Diimine–Palladium Complexes Incorporated in Vinylic-Addition Polynorbornenes: Synthesis and Catalytic Activity

Article abstract of DOI:10.1002/ejic.201700323

α-Diimine polymeric ligands have been synthesized using the bicyclic norbornane structure, present in vinylic-addition polynorbornene (VA-PNB). The VA-PNB–diimine ligands have been prepared by functionalization of the copolymer obtained by Ni-catalyzed polymerization of norbornene and norbornenylcarbonate. Immobilized palladium complexes of the type VA-PNB–diimine–PdX₂ have been prepared, and their catalytic activity has been tested. The trifluoroacetato complex (X = CF₃COO) can be used as a recyclable precatalyst in the Suzuki reaction. It is the source of minute amounts of homogeneous palladium active species, which carry out the catalysis with high turnover numbers. The recovered polymeric complex can be reused several times with no significant loss of activity. The polymeric analogue to Brookhart's catalyst, VA-PNB–diimine–PdMeCl, can also polymerize ethylene, although it is less active than its monomeric counterparts.

Full text of DOI:10.1002/ejic.201700323

A Journal of
Accepted Article
Title: α-Diimine-Palladium Complexes Incorporated in Vinylic Addition
Polynorbornenes: Synthesis and Catalytic Activity
Authors: Jesús Ángel Molina de la Torre and Ana Carmen Albéniz
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10.1002/ejic.201700323
European Journal of Inorganic Chemistry
FULL PAPER
α-Diimine-Palladium Complexes Incorporated in Vinylic Addition
Polynorbornenes: Synthesis and Catalytic Activity
Jesús A. Molina de la Torre,[a] and Ana C. Albéniz*[a]
b)
a)
Abstract: α-Diimine polymeric ligands have been synthesized using
the bicyclic norbornane structure, present in vinylic addition
polynorbornene (VA-PNB). The VA-PNB-diimine ligands have been
prepared by functionalization of the copolymer obtained by the Ni-
catalyzed polymerization of norbornene and norbornenylcarbonate.
Immobilized palladium complexes of the type VA-PNB-diimine-PdX2
have been prepared and their catalytic activity tested. The
trifluoroacetato complex (X = CF3COO) can be used as a recyclable
precatalyst in the Suzuki reaction. It is the source of minute amounts
of homogeneous palladium active species that carry out the catalysis
with high turnover numbers. The recovered polymeric complex can
be reused several times with no significant loss of activity. The
polymeric analogue to Brookhart´s catalyst, VA-PNB-diimine-
PdMeCl, can also polymerize ethylene, although it is less active than
their monomeric counterparts.
iPr
iPr
iPr
N
N
N
N
BAr4
Pd
iPr Me
L
Ar = 3,5-(CF3)2C6H3
Figure 1: a) An example of Brookhart’s type catalyst. b) Cyclophane-based
diimine.
incorporation of the polar monomer (around 10%). Interestingly,
the modification of the diimine ligand architecture has proved to
be a useful approach for improvement,[3] and, for example, the
use of cyclophane-based Pd(II) diimine catalysts increases the
amount of polar monomer in the final copolymer (Figure 1b).[4]
In the last years, we have been using vinylic addition
polynorbornenes (VA-PNBs, Figure 2, a) as supports of
catalysts or reagents in several organocatalytic,[5] and palladium-
catalyzed processes.[6,7] In contrast to the materials resulting
from ring opening metathesis polymerization of norbornene
Introduction
α-Diimines (or 1,4-diazadienes) are a particular class of Schiff
base ligands that have been used for a long time. They
experienced a surge in the 90s when Brookhart and coworkers
discovered that cationic diimine complexes of nickel or palladium
with bulky non-coordinating anions are excellent catalysts for
olefin polymerization.[1] The use of late transition metals with α-
diimine ligands (Figure 1a) allowed the synthesis of highly
branched low density polyethylene or other poly(α-olefins), thus
expanding the type of microstructure and properties of
polyethylene that can be obtained by metal-catalyzed
polymerization. Mechanistic studies to have a better knowledge
of the reaction pathway were carried out and new types of
catalysts were tested soon after.[2] These studies showed that
bulky diimine substituents are crucial to ensure polymer
formation. Palladium and nickel diimine complexes were also
tested in the insertion copolymerization of ethylene with polar
olefins, such as acrylates or vinyl acetate. These late transition
metals are less oxophilic than traditional metallocene-type
catalysts formed with early transition metals, and therefore less
susceptible to poisoning with most polar olefins. However,
complete control of the incorporation of both polar and non-polar
monomers remains a challenge. Brookhart’s catalysts that are
very active in the polymerization of ethylene usually give
copolymers of ethylene and polar olefins with low yields and low
(ROMP-PNB, Figure 2, b),[8 these VA-PNB materials have an
]
aliphatic backbone that keeps the bicyclic structure of
norbornene. This cyclic structure is ideal to accommodate a
diimine fragment, as has been shown for comparable [2.2.1]
bicyclic molecules such as norbornane,[9] or camphor,[10,11] and
we decided to incorporate this ligand in the structure of VA-
PNBs as shown in Figure 2 (c). With this approach, we reasoned
that if the steric hindrance of the polymeric structure is to have
an influence on the ligand, and as a result in the behavior of the
catalysts derived from it, this would be larger than that of a
pendant ligand bound to the polymer skeleton with a tether.
Immobilization of diimine ligands on solid supports has been
tried a few times in the context of developing catalysts for olefin
polymerization. Most cases show diimines attached to inorganic
12
]
[ 13 ]
solid supports such as SiO2,[
MgCl2
or clays like
montmorillonite.[ 14 ] Activated multiwalled carbon nanotubes
(MWNTs) have also been used to support diimines covalently
forming palladium and nickel complexes tested in olefin
polymerization.[ 15 ] In the case of polymer-supported diimines
fewer examples can be found and most of them involve
Merrifield type resins.[12b,d,16] Jin and Zhang described nickel-
diimine catalysts with pendant allyl groups that, when reacted
with ethylene, were incorporated in the polyethylene chain (self-
supported).[17]
Although olefin polymerization has been their most
remarkable application, palladium-diimine complexes are also
useful catalysts in different C-C cross-coupling reactions such as
the Heck reaction,[18] Kumada, Negishi or Stille reactions,[19] the
Suzuki reaction,[20] as well as carbonylation processes.[21]
[a]
IU CINQUIMA/Química Inorgánica.
Universidad de Valladolid
47071 Valladolid, Spain.
E-mail: albeniz@qi.uva.es.
http://gircatalisishomogenea.blogs.uva.es/
Supporting information for this article is given via a link at the end of
the document.
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10.1002/ejic.201700323
European Journal of Inorganic Chemistry
FULL PAPER
O
c)
OH
ii)
O
i)
b)
a)
+
O
O
2
1
O
OH
O
a
b
iii)
iv)
Ar
n
n
O
N
n
2 NH2Ar
v)
iPr
iPr
N
ROMP-PNB
VA-PNB
N
R
4
3
O
N
Ar =
Ar
R
VA-PNB-diimine
Figure 2: a) Vinylic addition polynorbornene (VA-PNB); b) Ring opening
metathesis polynorbornene (ROMP-PNB); c) VA-PNB incorporated α-diimine.
Scheme 1. Synthetic route to diimine 4. Reaction conditions and reagents: i)
Toluene, 180 ˚C, 20 h; ii) NaOH (aq); iii) (CF3CO)2O, DMSO, –78 ˚C, 2 h; iv)
NEt3, –78 ˚C to room temperature; v) MeOH/HCOOH.
Both the ability of this type of ligand to support palladium in
different oxidation states and its utility in coupling reactions has
been demonstrated.[22] However, the effort to develop supported
catalysts that can be recycled,[23] has not been extended to
diimine palladium complexes and only a report of a silica-
anchored catalyst of this type in the Suzuki reaction has been
disclosed. The catalyst can be reused four times before
undergoing an important loss of activity.[24]
Thus, encouraged by the stability that the aliphatic and
robust backbone of VA-PNB has shown in former catalytic
applications,[5-7] and its suitable bicyclic structure to support a
chelating diimine fragment, we report here on the preparation of
such VA-PNB-diimine ligand (Figure 2, c) and the performance
of the palladium complexes derived from it as catalyst in the
polymerization of olefins and as recyclable catalyst in Suzuki
couplings.
diimine since condensation of 3 with 2,4-diisopropylaniline in a
mixture of methanol and formic acid led to 4,[2b] albeit in a low
yield due to the high solubility of the diimine. Once the
precursors of the diimine were synthesized we studied which
one showed the best results in the copolymerization with
norbornene. The diimine 4 was not tested because it is a good
ligand for Ni, capable of substituting SbPh3 in the polymerization
catalyst [Ni(C6F5)2(SbPh3)2]. Complexes [Ni(C6F5)2L2] show very
low activity in the polymerization of norbornene when L has a
higher coordinating ability that SbPh3.[28]
Copolymerization experiments of norbornene with carbonate
1 were initially conducted using a 1:1 ratio of monomers in
CH2Cl2 in the presence of 1% mol of the Ni-catalyst (Scheme 2).
The mixture was stirred for 24 h at room temperature and
copolymer 5 (VA-PNB-NBCO3) was obtained as a white powder
after precipitation in MeOH. Copolymerization reactions with the
diol 2 or the diketone 3 were carried out in the same way to form
copolymers 6 and 7 respectively (Table 1, entries 1-3). Only 1
afforded a copolymer in moderate yield for the copolymerization
reaction (Table 1, entry 1). Polymer 5 was soluble and its
Results and Discussion
Synthesis of the vinylic addition polynorbornene precursors.
In order to obtain diimine VA-PNBs of the type shown in Figure
2c, it is necessary to copolymerize norbornene with a substituted
norbornene that, once incorporated in the polymer, can be
transformed easily into the diimine ligand. In contrast to ROMP,
the vinylic addition polymerization of functionalized norbornenes
is not straightforward. Many catalysts which are useful in the VA
polymerization of norbornene, shown a dramatic decrease of
activity when substituted norbornenes are used, and this is
especially serious for polar O- or N-containing substituents. Our
group has obtained good results in the polymerization of
bromoalkyl or bromoaryl substituted norbornenes,[25] as well as
stannylated norbornenes,[7a] using [Ni(C6F5)2(SbPh3)2] as
catalyst. Thus, we decided to test this complex in the
polymerization of a series of substituted norbornenes that could
be intermediates in the synthesis of the norbornane-diimine
fragment.
Norbornene derivatives 1-4 were synthesized as shown in
Scheme 1. No examples of norbornene bearing diimines can be
found in the literature, and we followed the method reported by
Kobayashi et al. for the synthesis of 3.[26] The first step is a
Diels-Alder reaction of cyclopentadiene and vinylene
carbonate,[27] which gives 1 in high yield as a mixture of isomers
endo:exo = 97:3. Subsequent hydrolysis and a Swern oxidation
of the diol 2, afforded the diketone 3. This is a viable route to the
1
composition could be determined by H NMR by comparison of
the integrals of the protons bound to C2 and C3 of the carbonate
and the broad signal associated to the aliphatic protons,
showing a ratio NB/1 (a/b) = 16.8. The infrared spectrum of the
polymer showed a strong band corresponding to the ν(C=O st)
IR absorption at 1809 cm-1, typical of carbonates.
The copolymerization of monomers 2 and 3 yielded poorly
(Table 1, entries 2 and 3). Both polymers were insoluble and
their composition could not be determined. IR spectra showed a
broad band at 3400 cm-1 characteristic of ν(O-H st) for 6 and a
weak band at 1746 cm-1 ν(C=O st) for 7, characteristic of a
carbonyl moiety. According to these results the synthesis of the
diimine polymer needs to be carried out from polymer 5 by
postpolymerization functionalization.
The composition of polymer 5 obtained as shown in Table 1,
entry 1, indicates that the reactivity of norbornene is much
higher than that of monomer 1 (cf. a/b = 16.8 vs. NB:1 = 1:1 in
the monomer feed). In fact, the homopolymerization of 1 does
1
not occur as shown by H NMR monitorization of the reaction of
1 and the catalyst [Ni(C6F5)2(SbPh3)2] in CDCl3 for 72 hours. This
is not unexpected considering the low reactivity of O-substituted
norbornenes and the fact that this catalyst proved to be useless
before in the polymerization of other O-functionalized
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10.1002/ejic.201700323
European Journal of Inorganic Chemistry
FULL PAPER
obtain a polymer with moderate incorporation of the carbonate
monomer and good yield (Table 1, entry 7). A slower addition of
norbornene (3 hours) led to a higher incorporation of 1 but to a
significant decrease of the yield (Table 1, entry 8). While
studying the 1H-NMR spectra of polymer 5 we observed in some
experiments the presence of a small olefinic signal at 5.7 ppm,
specially in those polymers synthesized with slow addition of
norbornene. This olefinic signal is not a product of the ring
opening polymerization of norbornene (signals around 5.3-5.2
ppm) and it is not observed in other vinylic addition copolymers
of norbornene. We are currently studying the origin of this
unsaturation in the polymer structure. Hydrogenation of the
polymer with p-toluenesulfonylhydrazide led to its total
disappearance without modification of the carbonate moiety (see
supporting information), and this procedure, if needed, ensures
the presence of an aliphatic backbone in 5, with the carbonate
group as the only reactive moiety.
Functionalization of VA-PNB-NBCO3 (5) to synthesize a VA-
PNB supported diimine. We chose as starting material for the
synthesis of the VA-PNB-diimine, polymer 5 of composition a/b =
5.7, synthesized according to entry 7 (Table 1). It has a
molecular weight Mw = 3.19 x 104 and a moderate polydispersity
(Mw/Mn = 1.75). The polymer shows a strong ν(C=O st) IR
absorption band at 1809 cm-1 as well as characteristic signals in
solid state 13C CP-MAS NMR at 161 and 85 ppm corresponding
to C8 (carbonate) and C2 and C3 respectively. The synthetic
route followed to the diimine polymer was analogous to that
used with the monomeric compounds and it is shown in Scheme
3. Hydrolysis of 5 was carried out using a solution of NaOH in a
mixture H2O:MeOH to facilitate the contact between the polymer
and the base. Although the polymer remained undissolved in
that mixture it got completely soaked and the reaction goes to
completion. The resulting polymer VA-PNBNB(OH)2 (6) is
insoluble but can be characterized by IR spectroscopy and solid
state NMR. The ν(C=O st) band at 1809 cm-1 typical of
carbonates, visible in 5, disappears completely and a new broad
signal ν(O-H st) at 3405 cm-1 appears. 13C CP-MAS NMR of 6
shows a signal at 76 ppm associated with the carbon atoms
[Ni]
+
a
O
b
CH2Cl2
n
O
O
1
O
5
O
[Ni] = [Ni(C6F5)2(SbPh3)2]
O
Scheme 2. Copolymerization reaction of 1 and norbornene.
Table 1. Copolymerization of norbornene (NB) with O-functionalized
norbornenes (NBO), precursors of the diimine.a
Entry
1d
NBO
NB:NBO:[Ni]
50:50:1
Polymer (Yield, %)b
5 (54)
Composition, a/bc
1
2
3
1
1
1
1
1
1
16.8
-
2d
50:50:1
6 (8)
3d
50:50:1
7 (17)
-
4d
100:100:1
1000:1000:1
50:50:1
5 (30)
25.2
58.2
4.8
5.7
10
5d
5 (1.5)
6e
5 (44)
7e
100:50:1
150:50:1
150:50:1
5 (73)
8e
5 (67)
9f
5 (38)
6.4
a) Reactions performed under nitrogen atmosphere, in CH2Cl2 at 25 ˚C for 24
h. b) Yields are referred to the total monomer mass. c) Ratio of monomers in
the copolymer (a/b = NB/NBO) determined by 1H-NMR. d) [Ni(C6F5)2(SbPh3)2]
was added to a solution of both monomers in CH2Cl2. e) Norbornene (5.9 M in
CH2Cl2) was added dropwise to a mixture of 1 and the catalyst for 1 h. f)
Norbornene (2.7 M in CH2Cl2) was added dropwise to a mixture of 1 and the
catalyst for 3 h.
norbornenes like 5-norbornene-2-carboxaldehyde.[28] However,
unlike 1, the coordinating ability of the oxygen in that monomer
is strong enough to avoid copolymerization with norbornene. In
order to reach better yields and higher incorporation of the
functionalized monomer, additional polymerization reactions
were performed in different conditions. The results show that the
catalyst loading affects not only the yields but also the
incorporation of the carbonate monomer. The lower the amount
of catalyst, the lower the presence of 1 in the copolymer (Table
1, entries 1, 4 and 5). Since no homopolymerization of 1 was
observed, we decided to test the slow addition of the more
reactive norbornene to a solution of 1 and [Ni(C6F5)2(SbPh3)2] in
CH2Cl2 in order to increase the incorporation of the carbonate in
polymer 5. As can be seen in Table 1, when norbornene was
slowly added over a period of 1 hour, this protocol leads to a
higher amount of carbonate in the copolymer, although a small
reduction of yield is also observed (Table 1, entries 1 and 6).[29]
We studied the influence of the monomer feed ratio when using
this methodology (Table 1, entries 6-8). The best balance
between yield and functionalization was obtained when the
monomer ratio was NB:1 = 2:1 and this mixture allowed us to
i)
ii)
a
a
b
b
n
n
O
5
HO
O
OH
6
O
O
O
a
iii)
iv)
b1
b2
a
iPr
b
n
iPr
n
N
N
O
7
O
8
iPr
iPr
Scheme 3. Synthesis of polymer 8. Reaction conditions: i) NaOH (aq)/MeOH;
ii) (CF3CO)2O, DMSO, –78 ˚C, 2 h; iii) NEt3, –78 ˚C to room temperature; iv)
2,6-diiopropylaniline (neat), MW, 200 ˚C, 4 h. MeOH/HCOOH.
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10.1002/ejic.201700323
European Journal of Inorganic Chemistry
FULL PAPER
bound to the hydroxyl groups (C2 and C3) and there are no
traces of those signals observed in 5 at 161 and 85 ppm,
characteristic of the carbonate group. Swern oxidation of 6 was
performed in the same conditions used for diol 2. The infrared
spectrum of the resulting 7 shows a strong absorption at 1755
cm-1 corresponding to the ketone moiety ν(C=O st). In this case
the solid state 13C CP-MAS NMR spectrum shows no signal
around 200 ppm in the ketone region, but the diol signal at 76
ppm has disappeared. The final step in the synthesis of the VA-
PNB incorporated diimine is the condensation of 7 with 2,6-
diisopropylaniline. Unlike the former steps, when this reaction
was attempted in similar conditions to those described in the
synthesis of 4, only a small amount of imine was formed. We
tried different methods and reaction conditions and the best
results were obtained when polymer 7 was heated in neat
aniline at 200 ºC for 4 hours in a microwave oven (Scheme 3).
The resulting polymer 8 (VA-CopNBNB(C=N-Ar)2) was soluble in
chlorinated solvents and the amount of diimine formed could be
determined by 1H NMR, by comparing the integral of the
aromatic protons to the broad signal that comprehends all the
backbone protons. 46% of aniline was incorporated into the
polymer (a:b1:b2 = 5.7:0.46:0.54, Scheme 3), giving a diimine
content of 0.570 mmol per gram of polymer. The IR spectrum of
8 still shows a signal at 1755 cm-1 ν(C=O st) with lower intensity
than in 7, along with a new signal at 1677 cm-1 corresponding to
the ν(C=N st). The 13C CP-MAS NMR spectrum shows a signal
at 175 ppm for the imine carbons as well as signals between
157-124 ppm associated with the aromatic carbons and at 30
ppm typical of the methyl groups of the aryl substituents.
O
O
VA-PNB
Ar
[PdCl2COD]
N
a
9
N
Pd
Cl
b1
b2
Ar
iPr
n
Me
iPr
[Pd(CF3CO2)2]
N
N
VA-PNB
= Ar
Ar
8
iPr
N
iPr
iPr
10
N
Pd
OCOCF3
Ar
OCOCF3
iPr
Scheme 4. Synthesis of VA-PNB-diimine supported palladium complexes.
polyethylene is soluble and can be collected after removing the
filtrate. The recovered catalyst was reused in an ethylene
polymerization reaction under the same conditions. A polymer
with a similar size and structure was obtained but the activity of
the catalyst was clearly lower (3.5 g/ mmol Pd).
We also tested the copolymerization of ethylene and methyl
acrylate (MA), a combination of interest that has been shown to
benefit, as far as higher incorporation of polar monomer in the
copolymer is concerned, from bulky axial diimine ligands.[4] This
experiment could assess the influence of the polymeric
backbone on the steric features of the diimine fragment. A
copolymerization of ethylene (6 atm) and MA (molar ratio MA:Pd
= 5800:1) was carried out and we found that two different
polymerizations were taking place. A polyethylene with no MA
was obtained (1.75 g/ mmol Pd) as well as a copolymer MA-
ethylene with a low content of polyethylene (75% mol MA, 1.31
g/ mmol Pd), and the yield of both processes was very low. The
high amount of MA in the copolymer suggests a radical
polymerization mechanism, which can be initiated by palladium
Use of VA-PNB-NB(C=N-Ar)2 as ligand in olefin
polymerization.
Substitution
of
cyclooctadiene
in
[PdMeCl(COD)] led to [{VA-PNB-NB(C=N-Ar)2}PdClMe]) (9), a
precursor of a cationic methylpalladium complex similar to those
used by Brookhart et al. in olefin polymerization (Scheme 4).
Polymer 9 has a palladium content of 8.1 mg Pd/ g polymer,
15% of the maximum Pd-content if complete coordination to the
diimine fragment had occurred. Due to the small amount of
palladium incorporated in 9, clear differences can be seen
neither in the IR spectrum nor in the NMR spectra when
comparing 9 and 8.
Polymer 9 was used as a precatalyst in the polymerization of
ethylene in a similar way to that described by Brookhart et al.[1a]
We generated the cationic complex in situ by treating the
palladium complex with the sodium salt of the bulky-non
coordinating anion [B[3,5-(CF3)2C6H3]4]− (BARF). The results
show that the supported complex 9 is a less active catalyst
(about 10.9 g/ mmol Pd) than the analogous monomeric
catalysts (about 450 g/mmol Pd).[1a] The polyethylene obtained
has a similar structure to those obtained with Brookhart’s
catalysts. It has a high molecular weight (Mw = 2.97 x 104) and a
polydispersity of 2.4. Analysis of its 1H-NMR spectrum shows
that it is a highly branched polyethylene, with 91 branches per
1000 carbon atoms (see Figure S1, supporting information).[2d]
After the polymerization reaction, the separation of the
supported catalyst can be carried out removing the solvent and
washing the resulting solid with pentane. In this solvent, 9 is
insoluble and can be easily separated by filtration whereas
complexes.[
Thus, either monomer polymerizes by its
preferred mechanism and no control of the copolymerization
process can be achieved.
30
]
The polymerization results show that the palladium
coordination sphere attained using polymer 8 as a ligand is
analogous to that in monomeric molecular catalysts, as shown
by the similar size and structure of the polyethylene obtained.
Unfortunately, the polymer backbone does not introduce any
advantageous steric feature useful in non-polar/polar olefin
copolymerization processes.
Use of VA-PNB-NB(C=N-Ar)2 as ligand in the Suzuki reaction.
The ability of polymer 8 to anchor palladium complexes useful
as catalysts in C-C coupling reactions was tested using the
Suzuki reaction. A new polymeric palladium complex [{VA-PNB-
NB(C=N-Ar)2}Pd(OOCCF3)2] (10), was synthesized by reaction
of Pd(OOCCF3)2 with 8 (Scheme 4). The palladium loading in 10
was 29.6 mg Pd/ g polymer, 58% yield considering the
maximum possible content that could be introduced.
Trifluoromethylacetate, instead of the most common acetate,
was chosen because the characteristic signals of the CF3
moieties in infrared spectroscopy and 19F NMR would allow us
an easier characterization of the polymer. Polymer 10 is
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10.1002/ejic.201700323
European Journal of Inorganic Chemistry
FULL PAPER
insoluble in common solvents. Its IR spectrum shows two bands
at 1182 and 1146 cm-1 characteristic of the trifluoromethyl group.
Furthermore, the band at 1677 cm-1, corresponding to the ν(C=N
st) in 8, is partially overlapped by another band at 1686 cm-1,
associated with the ν(CO2 asym) absorption of the acetate. This
ν(CO2 asym) resonance has a high value, higher than the parent
The activity of polymer 10 decreases in the two first recycling
experiments (Table 2, entries 2, 3), as there is an increase of the
reaction times. However the reaction time stays almost constant
(around 30 min) from the fourth use on, reaching high yields in
every case (Table 2, entries 4, 5). Leaching measurements
showed that some palladium is being released to the solution. In
the first run 4.3 % of the initial amount of the palladium added as
supported catalyst was leached, as determined by ICP-MS, but
in the next runs leaching was around 0.4-0.2 %. The higher
value in the first use indicates that it is possible that some of the
palladium trifluoroacetate precursor used in the synthesis of 10
is trapped in the polymer backbone, not coordinated to the
diimine moiety, and gets released in the first catalytic use.
Although a transformation of the nature of the palladium diimine
complex on the polymer after the first reaction is possible, we do
not observe important changes in the SEM images of the
polymer before or after the catalytic use (see Figure S3,
supporting information). We have reported before the behavior
of polymeric complexes where palladium is coordinated to VA-
PNB-N-heterocyclic carbenes; in that case the nature of the
palladium moiety changes dramatically after the first Suzuki
reaction and palladium aggregates can be clearly seen by
electron microscopy. In contrast to the system described here,
the activity of those polymers steadily decreases in every run.[6]
In order to determine if the actual active catalytic species was
acting homogenous or heterogeneously, we performed a hot
filtration test in the sixth reuse cycle of the catalyst in the Suzuki
reaction with 4-bromobenzotrifluoride. After running the reaction
at 80 ºC for 20 minutes, the hot mixture was filtered. 19F NMR
spectrum at that point showed an 80% yield. When the filtrate
was allowed to react other 30 minutes at 80 ˚C the yield
increased to 99%. This points to a soluble palladium species as
the active catalyst and, as a result, a homogeneous catalytic
system where the polymer is acting as a reservoir of a small
amount of palladium. The determination of the amount of
palladium in solution in this experiment by ICP-MS showed that
0.26% of the initial amount added is leached, similar to the other
recycling experiments. Thus, the amount of catalyst bringing
about the reaction is equivalent to a 0.0026% molar amount of
added catalyst (26 mol ppm) and a TON in the reaction close to
4x104 in each cycle (TOF close to 5x104 TONs/h). Considering
the catalyst can be reused with little or no loss of activity after
the third cycle, the cumulative TON can reach very high values
and makes polymer 10 a useful catalyst precursor playing the
role of a source of very active species in solution.
palladium trifluoroacetate, which is consistent with
a
monodentate coordination fashion.[31] 19F MAS NMR shows a
signal at -73 ppm slightly shifted from the signal of the starting
complex Pd(CF3CO2)2 (-76 ppm). The 13C CP MAS NMR
spectrum shows new signals at 167 ppm, corresponding to the
carbon in the carboxylate group, and a quartet for the CF3 with
an average chemical shift of 120 ppm and a coupling constant
1JC-F = 280 Hz.
The catalytic activity of polymer 10 was tested using the
reaction of 4-bromobenzotrifluoride and phenylboronic acid as a
model one. This reaction, shown in Equation 1 (R = CF3), was
complete in 15 min (Table 2, entry 1). Polymer 10 was also used
with other para-substituted aryl bromides such as 4-
bromotoluene or 4-bromoanisole. These haloarenes, less
activated towards oxidative addition to Pd(0), required longer
times but high yields were also obtained (Table 2, entries 6, 7).
In order to evaluate the performance of the catalyst and its
recyclability the reaction in Eq. 1 (R = CF3) was monitored by in
situ infrared spectroscopy. The ν(C-Br) IR absorption band at
1012 cm-1 of 4-bromobenzotrifluoride was not overlapped with
the bands of the solvent or other reagents and its intensity
variation was recorded with time, indicating the progress of the
reaction (Figure S2, supporting information). When no variation
of the band was observed for several minutes the heating was
stopped and, after cooling, the yield was determined by 19F NMR
spectroscopy. The same procedure was applied in the four
recycling experiments collected in Table 2 (entries 2-5).
10 (1 mol% Pd)
2 Cs2CO3
B(OH)2
+1.5
Br
R
(1)
R
CH3CN:H2O
80 ˚C
R = CF3,11; Me,12; OMe,13
Table 2. Suzuki reactions catalyzed by 10.[a]
Time
(min)
Crude
yield (%)
Entry
ArBr
Cycle
1
2
3
4
5
6
7
p-CF3-C6H4Br
p-CF3-C6H4Br
p-CF3-C6H4Br
p-CF3-C6H4Br
p-CF3-C6H4Br
p-Me-C6H4Br
p-OMe-C6H4Br
1
2
3
4
5
1
1
15
20
25
30
30
60
120
11 (97)[b]
11 (98)[b]
11 (96)[b]
11 (98)[b]
11 (99)[b]
12 (95)[c]
13 (91)[c]
Conclusions
Vinylic addition polynorbornenes provide a suitable backbone to
introduce a diimine fragment, leading to polymeric diimine
ligands that can be coordinated to palladium. The polymer
backbone does not introduce any remarkable steric feature that
can be of use in the Pd-catalyzed polymerization of ethylene or
copolymerization of this olefin with polar alkenes. However, it is
a good support of palladium complexes that have proved to be
excellent catalyst precursors in cross coupling reactions. The
[a] Reaction conditions shown in Eq. 1. [b] Crude yields determined by 19
NMR. [c] Crude yields determined by 1H NMR.
F
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10.1002/ejic.201700323
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis of NBCO3 (1). A Schlenk flask with J Young Teflon tap was
charged with vinylene carbonate (25 g, 290.5 mmol), dicyclopentadiene
(6.401 g, 48.42 mmol), hydroquinone (0.01 g, 0.0908 mmol) and toluene
(8.0 mL). The reaction was heated at 180 ºC for 20 hours. After removing
the volatiles under vacuum at 65 ºC a pale brown solid was obtained (the
excess of dienophile can be recovered from the volatiles in this step).
The solid was dissolved in CH2Cl2 (15 mL) and crystallized with cold
hexane (60 mL). Then it was filtered, washed with cold hexane (3 x 10
mL) and air dried. The product was obtained as a pale yellow solid (13.43
g, 91% yield). It is a mixture of isomers endo:exo = 97.6:2.4. 1 endo: 1H
NMR (300.13 MHz, δ, CDCl3): 6.21 (m, 2H, H5, H6), 4.98 (m, 2H, H2, H3),
3.27 (m, 2H, H1, H4), 1.75 (d, J = 10.4 Hz, 1H, H7), 1.26 (d, J = 10.4 Hz,
1H, H7’). 13C{1H} NMR (75.4 MHz, δ, CDCl3): 155.59 (s, C8), 134.35 (s, C5,
C6), 78.96 (s, C2, C3), 45.57 (s, C1, C4), 42.55 (s, C7). 1 exo: 1H NMR
(300.13 MHz, δ, CDCl3): 6.1 (m, 2H, H5, H6), 4.55 (m, 2H, H2, H3), 3.13
(m, 2H, H1, H4), 1.84 (m, 1H, H7), 1.7 (m, 1H, H7’). 13C{1H} NMR (75.4
MHz, δ, CDCl3): 155.59 (s, C8), 135.95 (s, C5, C6), 78.81 (s, C2, C3),
45.57 (s, C1, C4), 40.94 (s, C7). MS (EI) m/z (relative intensity): 152 (0.5)
[M]+, 107 (2), 79 (34), 77 (24), 66 (100), 65 (13).
actual catalysis is homogeneous and the Pd-loaded polymer is a
source of a very small amount of palladium active species in
solution (about 25 mol ppm of Pd; TOF close to 5x104 TONs/h).
The polymer precatalyst can be reused without significant loss of
activity, so the number of cumulative TONs is very high.
A homogeneous catalysis by metal leaching is a common
scenario when using supported palladium catalysts, probably
more common than previously anticipated.[23c, 32 ] It is very
important to evaluate this point, but it does not make the metal-
loaded materials useless. On the contrary if, as it is the case of
the example described here, the dosage of soluble palladium
species remains very small, steady, and active upon reuse, the
supported precatalyst is a convenient, easy to use source of
catalyst. It becomes equivalent to use extremely active
homogeneous palladium species each time, without the need of
precise quantity measurement, and comparing well as far as
TONs reached.
Synthesis of NB(OH)2 (2). 1 (3.650 g, 24 mmol) was dissolved in a
solution of NaOH in H2O (1.0 M, 50 mL, 50 mmol). The mixture was
stirred at room temperature for 5 hours and the product was extracted
with Et2O (5 x 15 mL). The organic phase was washed with saturated
NaHCO3 aqueous solution (20 mL), water (20 mL) and dried over MgSO4.
The solvent was removed and the product was obtained as a white solid.
(2.658 g, 88% yield). 2 was obtained as a mixture of isomers in a ratio
endo:exo = 97.6:2.4 that parallel the starting material (1). 2 endo: 1H
NMR (400.13 MHz, δ, CDCl3): 6.26 (m, 2H, H5, H6), 4.18 (m, 2H, H2, H3),
3.02 (m, 2H, H1, H4), 2.25 (br, 2H, -OH), 1.51 (d, J = 9.7 Hz, 1H, H7), 1.22
(d, J = 9.7 Hz, 1H, H7’). 13C{1H} NMR (100.6 MHz, δ, CDCl3): 135.04 (s,
C5, C6), 70.92 (s, C2, C3), 47.59 (s, C1, C4), 41.81 (s, C7). 2 exo: 1H NMR
(400.13 MHz, δ, CDCl3): 6.04 (m, 2H, H5, H6), 3.71 (m, 2H, H2, H3), 2.7
(m, 2H, H1, H4), 2.25 (br, 2H, -OH), 1.89 (d, J = 9.2 Hz, 1H, H7), 1.62 (d, J
= 9.2 Hz, 1H, H7’). 13C{1H} NMR (100.6 MHz, δ, CDCl3): 136.29 (s, C5,
C6), 68.80 (s, C2, C3), 47.85 (s, C1, C4), 42.11 (s, C7). MS (EI) m/z
(relative intensity): 126 (0.6) M]+, 116 (100), 101 (25), 79 (8), 77 (8), 67
(50), 66 (76), 60 (29).
Experimental Section
General Methods
1H, 13C and 19F NMR spectra were recorded using Bruker AV-400 and
Agilent MR-500 instruments. Chemical shifts (δ) are reported in ppm and
referenced to SiMe4 (1H, 13C) or CFCl3 19F). All NMR spectra were
(
recorded at 293 K in deuterated solvents or, in the case of the catalytic
reactions, in protic solvents using an acetone-d6 capillary. The solid state
NMR spectra were recorded at 293 K under magic angle spinning (MAS)
in a Bruker AV-400 spectrometer using a Bruker BL-4 probe with 4mm
diameter zirconia rotors spinning at 8 kHz. 13C CP MAS NMR spectra
were measured at 100.61 MHz and recorded with proton decoupling
(tppm), with a 90º pulse length of 4.5 µs and a contact time of 3 ms and
recycle delay of 3 s. The 13C NMR spectra were referred to glycine (CO
signal at 176.1 ppm). 19F MAS NMR spectra were recorded at 376.5 MHz
with a 90º pulse length of 5.5 µs. 19F NMR chemical shifts are in ppm
relative to external CFCl3. IR spectra were recorded on a Perkin-Elmer
Synthesis of NB(C=N-Ar)2, Ar = 2,6-diisopropylphenyl (4). Diketone 3
(0.2853 g, 2.336 mmol) and 2,6-diisopropylaniline (0.8283 g, 4.672
mmol) were dissolved in a mixture of MeOH (2.5 mL) and formic acid
(0.25 mL). The mixture was stirred at room temperature for 24 hours.
Volatiles were removed and MeOH (1.0 mL) was added and cooled to -
78 ºC. The solid formed was filtered and air dried. (0.1164 g, 11% yield).
1H NMR (400.13 MHz, δ, CDCl3): 7.19-7.07 (m, 6H, H10-H12, H10’-H12’),
FT/IR SPECTRUM FRONTIER spectrophotometer with CsI
+ ATR
diamond accessory. The palladium content of the polymers and leaching
measurements were determined by ICP–MS, using Agilent 7500i
equipment; the samples were dissolved in HNO3 (65%) using an ETHOS
SEL Milestone microwave oven. In situ IR spectra were recorded with a
ReactIR 15 equipped with
a transmission fiber of 6.3 mm AgBr
6.36 (m, 2H, H5, H6), 3.27 (m, 2H, H1, H4), 2.95 (qq, J = 6.8 Hz, 2H, H14
,
FiberConduit and a probe DiComp with diamond sensor. Size exclusion
chromatography (SEC) was carried out using a Waters SEC system on a
three-column bed (Styragel 7.8x300 mm columns: 50-105, 5x103-5x105
and 2x103-4x106 Da) and a Waters 410 differential refractometer. SEC
samples were run in CHCl3 at 313 K and calibrated to polystyrene
standards. Controlled addition of norbornene in the copolymerization
reaction was done with a Thermo Scientific Orion M365 Sage Syringe
Pump. Solvents were dried prior to use and stored under nitrogen. The
reagents used in the synthesis of the monomers and catalytic reactions
were purchased from Aldrich, Alfa-Aesar and Acros. [Ni(C6F5)2(SbPh3)2],
[PdMeCl(COD)],[33] [Pd(CF3CO2)2][34] and NB(C=O)2 (3)[26] were prepared
according to the literature procedures. Compounds 1 and 2 have been
prepared with slight modifications of the procedure described by
Neumman et at.[27,35]
H14’), 2.70 (spt, J = 6.8 Hz, 2H, H15, H15’), 2.32 (d, J = 9.6 Hz, 1H, H7),
1.97 (d, J = 9.6 Hz, 1H, H7’), 1.23, 1.22, 1.19, 1.15 (four d, J = 6.8 Hz,
24H, H16-H19, H16’-H19’). 13C{1H} NMR (100.6 MHz, δ, CDCl3): 168.27 (s,
C2, C3), 146.13 (s, C8, C8’), 137.44 (s, C5, C6), 136.70, 136.53 (2 s, C9,
C9’, C13, C13’), 124.04, 122.82, 122.76 (three s, C10- C12 C10’-C12’), 48.9 (s,
C7), 46.08 (s, C1, C4), 28.31, 27.91 (2 s, C14, C14’, C15, C15’), 23.78, 23.70,
23.46, 23.15 (4 s, C16-C19, C16’-C19’). MS (EI) m/z (relative intensity): 440
(1) [M]+, 397 (13), 253 (100), 238 (26), 207 (14), 188 (46), 186 (31), 157
(11), 146 (19), 130 (14), 117 (7), 91 (12), 66 (87).
Syntheses of functionalized VA-polynorbornenes.
Synthesis of copolymer VA-PNB-NBCO3 (5). Method A (Table 1, entry
1): In a Schlenk flask under a nitrogen atmophere 1 (0.49 g, 3.22 mmol)
and norbornene (0.85 mL, 3.80 M in CH2Cl2, 3.22 mmol) were dissolved
in CH2Cl2 (5 mL). To this mixture, a solution of [Ni(C6F5)2(SbPh3)2]
(0.0708 g, 0.0644 mmol) in CH2Cl2 (5 mL) was slowly added.
Synthesis of norbornene-diimine precursors.
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10.1002/ejic.201700323
European Journal of Inorganic Chemistry
FULL PAPER
11
12
Synthesis of VA-PNBNB(C=N-Ar)2, Ar = 2,6-diisopropylphenyl (8).
Copolymer 7 (1 g, 1.527 mmol diketone) and 2,6-diisopropylaniline
(11.28 g, 63.62 mmol) were placed in a 30 mL microwave reaction vessel.
The mixture was heated to 200 ºC and stirred for 4 hours in the
microwave oven. The reaction mixture was then poured into MeOH (100
mL) and stirred for 3 hours at room temperature. The solid was filtered,
washed with MeOH (3 x 20 mL) and CH3CN (3 x 20 mL) and dried in a
vacuum oven at 35 ºC for 24 hours. The polymer is a pale orange powder
(1.14 g, 77%, 45% incorporation of aniline; 0.570 mmol of diimine/g
polymer). IR (neat), cm-1: 1755 ν(C=O st), 1677 ν(C=N st). 1H NMR
(500.15 MHz, δ, CDCl3): 7.3-6.7 (br, 6H, H10, H11, H12), 3.2-0.3 (br,
CH(CH3)2, CH(CH3)2 polyNB). 13C NMR (125.72 MHz, δ, CDCl3): 146 (br,
C8), 135-133 (br, C9, C13), 125-121 (br, C10, C11, C12) 54-50 and 48-45 (br,
C5, C6, C2’, C3’), 44-38 (br, C1, C1’, C4, C4’), 37-34 (br, C7, C7’), 33-28 (br,
C5’, C6’), 28 (br, CH(CH3)2), 24-21 (br, CH(CH3)2).
5'
7'
4'
19
7
6'
10
9
15
18
13
1'
17
8
14
16
3'
2'
6
5
N
a
6
b
1
7
2
5
3
4
4
18'
1
3
15'
N
2
16'
14'
17'
19'
8'
13'
O
O
8
12'
9'
10'
O
n
11'
5
4
Figure 3. Examples of the numbering scheme for NB-derived compounds.
After stirring 24 h at room temperature the viscous solution was poured
into MeOH (30 mL). The resulting polymer was filtered, washed with
MeOH (5 x 5 mL) and air dried. The product was obtained as a white
solid (0.4278 g, 54% yield). a/b = 16.8 (0.58 mmol –CO3/g polymer).36
Direct syntheses of copolymers 6 and 7 (Table 1) were carried out
following this method. They are insoluble in common organic solvents.
Method B (Table 1, entry 7): To a solution of 1 (4 g, 26.29 mmol) in
CH2Cl2 (20 mL) a mixture of [Ni(C6F5)2(SbPh3)2] (0.5778 g, 0.5258 mmol)
and triphenylstibine (0.074 g, 0.2103 mmol) in CH2Cl2 (15 mL) was
added. Then norbornene (5.89 M in CH2Cl2, 8.9 mL, 52.58 mmol) was
added in a controlled flow of 0.15 mL / min for 1 h. The resulting reaction
mixture was stirred at room temperature for 24 hours. After this time it
was poured into MeOH (250 mL) where a solid appeared, which was
filtered, washed with MeOH (5 x 20 mL) and air dried. The copolymer is a
white powder. (6.491 g, 73% yield). a/b = 5.7 (1.45 mmol –CO3/g
polymer).36
Synthesis of polymer-supported diimine-palladium complexes.
Synthesis of [{VA-PNB-NB(C=N-Ar)2}PdClMe]) (9). [PdClMeCOD]
(0.0344 g, 0.1296 mmol) was added to a solution of copolymer 8 (0.25 g,
0.1426 mmol diimine) in CH2Cl2 (20 mL). The reaction was stirred for 24
hours at room temperature. After that time the mixture was poured into
acetone (100 mL). The solid was filtered, washed with acetone (4 x 20
mL) and air dried. The polymer was obtained as a pale brown solid
(0.1892 g, 70% yield). Analysis ICP-MS Pd 8.080 mg Pd/g. The NMR
spectra of 9 are indistinguishable from those of 8. No characteristic
signals of the complex could be observed.
Synthesis of [{VA-PNB-NB(C=N-Ar)2}Pd(OOCCF3)2] (10). Palladium(II)
trifluoroacetate (0.0431 g, 0.1296 mmol) was added to a solution of
copolymer 8 (0.2500 g, 0.1426 mmol diimine) in CH2Cl2 (20 mL). The
reaction was stirred for 24 hours at room temperature and poured into
acetone (100 mL). The solid was filtered, washed with acetone (4 x 20
mL) and air dried. The polymer was obtained as a brown solid insoluble
in common solvents (0.2776 g, 95% yield). Analysis ICP-MS Pd 29.58
mg Pd/g. IR (neat), cm-1: 1755 ν(C=O st, polymer ketone), 1686 ν(COO
st as), 1677 ν(C=N st), 1182 and 1146 ν(CF3 st). 13C CP-MAS NMR
(100.61 MHz): 175 (br, C2, C3), 168 (br, CF3CO2), 155-126 (br, aromatic),
120 (q, CF3CO2, 1JC-F = 280 Hz), 63-18 (br, CH(CH3)2, CH(CH3)2, polyNB).
19F MAS NMR (376.50 MHz): -72.9 (br, CF3).
5 (a/b = 5.7). Mw (Daltons) = 3.19 x 104. Mw /Mn = 1.7. IR (neat), cm-1:
1809 ν(C=O st), 1127 ν(C-O st as), 1085 ν(C-O-C st as). 1H NMR
(500.15 MHz, δ, CDCl3): 5.1-4.3 (br, 2H, H2, H3), 3-0.3 (br, 16H). 13C
NMR (125.72 MHz, δ, CDCl3): 155 (br, C8), 79 (br, C2, C3), 54-50 and 48-
45 (br, C5, C6, C2’, C3’), 44-38 (br, C1, C1’, C4, C4’), 37-34 (br, C7, C7’), 33-
28 (br, C5’, C6’).
Synthesis of VA-PNBNB(OH)2 (6). Copolymer 5 (5.6 g, 8.174 mmol of
carbonate) was dissolved in THF (120 mL). Tetrabutylammonium
hydroxide (1 M solution in MeOH, 81.74 mL, 81.74 mmol) was added and
the reaction was refluxed for 9 hours. After cooling to room temperature
the mixture was poured into a mixture of MeOH:H2O (1:1, v/v, 300 mL)
and stirred overnight. The solid was filtered, washed with MeOH (5 x 20
mL) and dried in a vacuum oven at 35 ºC for 24 hours. The polymer is a
white powder barely soluble in common solvents (5.255 g, 98% yield). IR
(neat), cm-1: 3240 ν(O-H st), 1110 and 1076 ν(C-O st). 13C CP-MAS
NMR (100.61 MHz): 74-65 (br, C2, C3), 63-18 (br, polyNB).
Polymerization reactions with 9. Polymerization of ethylene.
A
Fischer-Porter flask with 9 (0.06 g, 4.56 x 10-3 mmol Pd) and NaB[3,5-
(CF3)2C6H3]4 (0.0041 g, 4.56 x 10-3 mmol), was cooled at -78 ºC. Upon
cooling CH2Cl2 (4.0 mL) was added and the system was evacuated and it
was purged with ethylene. The pressure was then raised to 2.5 atm and
the mixture was stirred for 24 h while the temperature rose to room
temperature. After that time, the pressure was released and the volatiles
were removed. The solid obtained was extracted with pentane (5 x 5 mL).
The undissolved solid was separated by filtration and the solution was
evaporated to dryness. Polyethylene was obtained as a white solid (54
mg, 10.9 g/ mmol Pd). Mw (Daltons) = 3.19 x 104. Mw/ Mn = 2.4. ). 1H
NMR (500.15 MHz, δ, CDCl3): 1.53-1.03 (br, CH2), 0.93-0.80 (br, CH3).
91 branches/1000 carbon atoms.37
Synthesis of VA-PNBNB(C=O)2 (7). A two-necked round-bottom flask
under a nitrogen atmosphere was charged with copolymer 6 (5 g, 7.586
mmol diol), CH2Cl2 (200 mL) and cooled to -78 ºC. To another Schlenk
flask containg a stirring mixture of dimethyl sulfoxide (7.112 g, 91.03
mmol) and CH2Cl2 (50 mL) at -78 ºC, was added trifluoroacetic anhydride
(15.93 g, 75.86 mmol) dropwise. Ten minutes later this solution was
added slowly via cannula to the polymer suspension at -78 ºC. After 4
hours triethylamine (15.35 g, 151.7 mmol) was added. The reaction was
stirred for 3 hours and allowed to warm up to room temperature. CH2Cl2
was removed to about half the initial volume and the mixture was poured
into MeOH:H2O (3:1, v/v, 400 mL) and stirred overnight. The polymer
was filtered, washed with a mixture of MeOH:H2O (1:1, v/v, 5 x 20 mL),
then MeOH (5 x 20 mL) and it was dried in a vacuum oven at 35 ºC for
24 hours. The polymer was obtained as a yellowish powder (4.440 g,
89% yield). IR (neat), cm-1: 1755 ν(C=O st). 13C CP-MAS NMR (100.61
MHz): 63-18 (br, polyNB).
Copolymerization of ethylene with methyl acrylate. A Fischer-Porter
flask was protected from light and charged with 9 (0.06 g, 4.56 x
10-3 mmol Pd) and NaB[3,5-(CF3)2C6H3]4 (0.0041 g, 4.56 x 10-3 mmol).
The system was cooled to -78 ºC and, after being evacuated and refilled
with ethylene. Methyl acrylate (2.242 g, 26.4 mmol) and CH2Cl2 (2.1 mL)
were added and the pressure of ethylene was elevated to 6.0 atm. After
20 h the pressure was released and the mixture was evaporated to
remove the solvent and the unreacted methyl acrylate. The residue was
extracted with pentane (5 x 5 mL) to separate the polyethylene formed
This article is protected by copyright. All rights reserved.

10.1002/ejic.201700323
European Journal of Inorganic Chemistry
FULL PAPER
(8.0 mg, 1.75 g/ mmol Pd). The remaining solid, insoluble in pentane,
was extracted with acetone (5 x 5mL) to separate the copolymer of
ethylene and methyl acrylate, which was obtained as a colorless gum-like
solid (6.0 mg, 1.31 g/ mmol Pd, 75% mol content of MA). In addition to
the typical methyl, methylene and methyne signals of polyethylene new
signals can be observed. 1H NMR (400.15 MHz, δ, CDCl3): 3.66 (s, 6H,
OCH3), 2.3 (br, 2H, CH(CO2CH3)s, CH(CO2CH3)i),* 1.9 (a, 1H, CHHi),*
1.69 (a, 2H, CH2s), 1.5 (a, 1H, CHHi).* 13C{1H} NMR (100.6 MHz, δ,
CDCl3): 174.9 (s, CO2CH3), 51.7 (s, OCH3), 41.3 (m, CH(CO2CH3)). * i =
isotactic, s = syndiotactic.
López-Fernández, N. Carrera, A. C. Albéniz, P. Espinet,
Organometallics, 2009, 28, 4996-5001; i) F. S. Liu, H. B. Hu, Y. Xu, L.
H. Guo, S. B. Zai, K. M. Song, H. Y. Gao, L. Zhang, F. M. Zhu, Q. Wu,
Macromolecules, 2009, 42, 7789-7796; j) C. Chen, S. Luo, R. F. Jordan,
J. Am. Chem. Soc. 2010, 132, 5273-5284; k) L. Guo, H. Gao, Q. Guan,
H. Hu, J. Deng, J. Liu, F. Liu, Q. Wu, Organometallics, 2012, 31, 6054-
6062.
[3]
[4]
[5]
a) S. Takano, D. Takeuchi, K. Osakada, N. Akamatsu, A. Shishido,
Angew. Chem. Int. Ed. 2014, 53, 9246-9250; b) S. Dai, S. Zhou, W.
Zhang, C. Chen. Macromolecules, 2016, 49, 8855-8862.
a) C. S. Popeney, D. H. Camacho, Z. Guan, J. Am. Chem. Soc. 2007,
129, 10062-10063; b) C. S. Popeney, Z. Guan, J. Am. Chem. Soc.
2009, 131, 12384-12393.
General procedure for the Suzuki reactions followed by in situ IR
(Table 2, entry 1). Polymer 10 0.03 g, 0.0084 mmol Pd), phenylboronic
acid (0.154 g, 1.26 mmol) and cesium carbonate (0.547 g, 1.68 mmol)
were introduced in a pear shaped flask with three necks. It was equipped
with the IR probe in the central neck, a gas inlet with stopcock and a
septum. Then, in a nitrogen atmosphere, a mixture of CH3CN:H2O (4.0
mL, 3:1; v/v) was added and the mixture was introduced in a heated bath
at 80 ºC. After 5 min, 4-bromobenzotrifluoride (0.189 g, 0.840 mmol) was
added and the reaction was immediately monitored by IR, flowing the
decrease of the ν(C-Br) band at 1012 cm–1. When no further decrease of
the IR absorption intensity was observed, the reaction was cooled down
to room temperature and checked by 1H NMR and 19F NMR. The
polymer was then filtered and washed with a mixture of CH3CN:H2O (5 x
2.0 mL, 3:1; v/v). It was stored for further use. In the experiments
collected in Table 2 (all of them carried out in the same way described
above), the filtrate and solvents employed to wash the polymer were
combined and used to determine the amount of Pd leached by ICP-MS
(see supporting information for details). In an additional experiment (7th
cycle), the collected filtrate (crude yield 98%) and solvents used to wash
the polymer were combined and evaporated to c.a. 8 mL. A solution of
Na2CO3(aq) (20 mL) was added and the mixture was extracted with Et2O
(20 mL). The organic phase was washed with water (3 x 10 mL), dried
with MgSO4 and evaporated to dryness. This procedure afforded pure p-
CF3-C6H4Ph (0.176 g, 94% yield).
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We would like to thank the financial support of the Spanish
MINECO (DGI, grant CTQ2013-48406-P and CTQ2016-80913-
P), the MEC (FPU fellowship to JAMT) and the Junta de Castilla
y León (grant VA302U13).
Keywords: supported catalysts• polynorbornene • palladium •
diimine • cross-coupling
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this reason characterization data are included here.
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a ratio of monomers
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10.1002/ejic.201700323
European Journal of Inorganic Chemistry
FULL PAPER
Entry for the Table of Contents
FULL PAPER
Vinylic addition polynorbornene
embraces α-diimine ligands,
incorporated in the polycyclic
backbone of the polymer, and leads to
palladium complexes that can be used
as recyclable catalysts in cross-
coupling reactions.
Jesús A. Molina de la Torre and Ana C.
Albéniz*
VA-PNB
n
Page No. – Page No.
Ar
N
X
N
Y
Ar
Pd
α-Diimine-Palladium Complexes
Incorporated in Vinylic Addition
Polynorbornenes: Synthesis and
Catalytic Activity
Br
R
R
+
B(OH)2
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