Palladium-Catalyzed Ethylene/Methyl Acrylate Copolymerization: Moving from the Acenaphthene to the Phenanthrene Skeleton of α-Diimine Ligands

Article abstract of DOI:10.1021/acs.organomet.9b00308

The development of efficient homogeneous catalysts for the synthesis of functionalized polyolefins is a challenging topic. Palladium(II) complexes with α-diimine ligands having a phenanthrene skeleton and 2,6-disubstituted aryl rings (Ar-BIP) were synthes

Full text of DOI:10.1021/acs.organomet.9b00308

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Cite This: Organometallics XXXX, XXX, XXX−XXX
Palladium-Catalyzed Ethylene/Methyl Acrylate Copolymerization:
Moving from the Acenaphthene to the Phenanthrene Skeleton of
α‑Diimine Ligands
Anna Dall’Anese, Vera Rosar, Luca Cusin,†,⊥ Tiziano Montini, Gabriele Balducci,
†
†,∥
†,‡
†
Ilaria D’Auria, Claudio Pellecchia,§ Paolo Fornasiero,
§
†,‡
Fulvia Felluga, and Barbara Milani*
†
,†
†
Dipartimento di Scienze Chimiche e Farmaceutiche, Universita
̀
di Trieste, Via Licio Giorgieri 1, 34127 Trieste, Italy
INSTM Trieste Research Unit and ICCOM-CNR Third Parties Research Unit, University of Trieste, Via Licio Giorgieri 1, 34127
Trieste, Italy
‡
§
Dipartimento di Chimica e Biologia “A. Zambelli”, Universita
S Supporting Information
̀
di Salerno, via Giovanni Paolo II 132, I-84084 Fisciano (SA), Italy
*
ABSTRACT: The development of efficient homogeneous
catalysts for the synthesis of functionalized polyolefins is a
challenging topic. Palladium(II) complexes with α-diimine
ligands having a phenanthrene skeleton and 2,6-disubstituted
aryl rings (Ar-BIP) were synthesized, characterized, and tested
as precatalysts in the copolymerization of ethylene with
methyl acrylate. The direct comparison with analogous
complexes having the corresponding α-diimines with an
acenaphthene skeleton (Ar-BIAN) was performed. X-ray
characterization in the solid state and NMR analysis in
solution of both neutral [Pd(Ar-BIP)(CH )Cl] and mono-
3
cationic [Pd(Ar-BIP)(CH )(NCCH )][PF ] complexes in-
3
3
6
dicate that the Ar-BIP ligands have a higher Lewis basicity and
are more strongly coordinated to the metal center in comparison to the Ar-BIAN counterparts. Therefore, the Pd(Ar-BIP)
cationic complexes can be regarded as electron-rich metal cations. In addition, they create a higher steric congestion around
palladium in comparison to Ar-BIAN, regardless of the substituents on the aryl rings. The monocationic species generate active
catalysts for the ethylene/methyl acrylate copolymerization leading to copolymers with M values up to 37000 and a content of
n
polar monomer of 5.3 mol %. A detailed study of the catalytic behavior points out that Pd(Ar-BIP) catalysts show a good affinity
for the polar monomer, have a good thermal stability, and favor the cleavage of the catalyst resting state, leading to copolymers
with M values higher than those of the macromolecules produced with the corresponding Pd(Ar-BIAN) under the same
w
reaction conditions. NMR characterization of the produced copolymers points out that the polar monomer is inserted both at
the end of the branches and into the main chain, with an enchainment more selective than that achieved when the
copolymerization is carried out in dichloromethane. In situ NMR investigations allowed us to detect relevant intermediates of
the catalytic cycle and shed light on the nature of possible deactivated species.
INTRODUCTION
acrylate (MA), yielding the real copolymer with the polar
monomer inserted into the branches of the macromolecule. A
few years later, Pugh reported the in situ generated Pd(II)
■
Functionalized polyolefins are highly desirable materials that
are currently produced by industrial processes based on radical
reactions, which suffer from low cost efficiency, high energy
consumption, and a poor control over the macromolecular
structure. The direct, controlled, homogeneously catalyzed
copolymerization of ethylene with polar vinyl monomers
represents the most powerful and environmentally friendly
approach to overcome these limits.
The breakthrough in the field is represented by Brookhart’s
catalytic system based on Pd(II) complexes with α-diimine
catalytic system based on a phosphino-sulfonate (P−O) ligand
3
(
Chart 1). The two systems show different activity in the
production of ethylene/methyl acrylate copolymers having
different molecular weight, amount of incorporated polar
monomer, and microstructures: branched macromolecules
with the polar monomer at the end of the branches are
obtained when the Pd(N−N) system is applied, whereas linear
macromolecules with the polar monomer in the main chain are
the product of the Pd(P−O) catalyst.
1
(
(
(
N−N) molecules characterized by either the acenaphthene
Ar-BIAN) or the 1,4-diaza-1,3-butadiene skeleton (Ar-DAB)
2
Chart 1). This catalytic system was the first able to
Received: May 8, 2019
copolymerize ethylene with acrylic esters, such as methyl
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.9b00308
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Chart 1. General Structures of Pd Catalysts with Different Ligands
These two catalytic systems remained the only ones to be
investigated for almost two decades, and only recently have
other ancillary ligands been considered (Chart 1), such as
camphyl or two phenyls or substituted phenyl rings have been
1
7
considered (L8 and L9 in Chart 3). The catalytic
performances of the relevant palladium complexes remarkably
depend on the ligand skeleton. The catalysts with the ligand
having the aryl-substituted skeleton showed a great drop in
both catalytic activity and incorporation of MA, whereas the
camphyl α-diimine based catalyst afforded high-molecular-
weight ethylene/MA copolymers with high incorporation of
MA, even though with lower TOF with respect to the
Brookhart type catalyst. The introduction of methoxy groups
on the acenaphthene skeleton (L10 in Chart 3) resulted in
Pd(II) catalysts with activity higher than that of the Brookhart
type catalyst, yielding the copolymer with a higher molecular
4
bisphosphine monoxide (BPMO), N-heterocyclic carbene-
5
quinolinolate (IzQO), and N-heterocyclic carbene-phosphine
6
oxide (NCPO) ligands, phosphonic diamide phosphines
7
8
(
PDAP), and amine-imine derivatives.
Even though DFT calculations indicated palladium as the
most suitable transition metal to obtain catalysts for the
9
ethylene/polar vinyl monomer copolymerization, in the last
1
0
two years a few examples based on Ni(II) appeared.
With regard to Pd(II) catalysts with α-diimine ligands, most
of the studies deal with the variation of the substituents on the
aryl rings. As a few examples to be mentioned, we consider a
cyclophane-based α-diimine (L1 in Chart 2), whose
corresponding catalyst has an enhanced lifetime leading to a
copolymer with a higher content of MA with respect to the
18
weight. Finally, an α-diimine with a substituted dibenzo-
19
barrelene skeleton was used to synthesize both palladium(II)
and nickel(II) complexes. The latter were demonstrated to be
catalysts for the copolymerization of ethylene with methyl 10-
1
1
acyclic analogue, a dinuclear catalyst based on a macrocyclic
α-diimine (L2 in Chart 2) that leads to a copolymer with the
polar monomer located both in the main chain and at the end
10a
undecenoate (L11 in Chart 3).
In this work we have studied the catalytic behavior of
organometallic palladium(II) complexes based on α-diimine
ligands featuring a phenanthrene skeleton (Ar-BIP). The
ligands of choice have the aryl rings substituted in 2,6-positions
with either a methyl or an isopropyl group, whereas the
molecule with the simple phenyl was considered as a reference
compound (ligands 1−3 in Chart 4).
The coordination chemistry of ligands 1−3 to Pd(II) has
been investigated in detail, pointing out remarkable differences
with respect to that of the corresponding Ar-BIAN molecules.
1
2
of the branches, di- or monophenylmethyl substituted α-
diimines (L3 in Chart 2), whose corresponding catalysts are
highly robust and lead to ethylene/MA copolymers with high
13
molecular weight and low branching density, the correspond-
ing rotationally restricted pentiptycenyl N-aryl-substituted
diimine, whose Pd(II) catalyst shows an enhanced comonomer
1
4
incorporation (L4 in Chart 2), and the bulky 8-p-
tolylnaphthyl-substituted α-diimine (L5 in Chart 2), whose
relevant Pd complex catalyzes the living copolymerization of
15
Neutral, [Pd(Ar-BIP)(CH )Cl], and monocationic, [Pd(Ar-
ethylene with methyl acrylate. Some of us introduced Pd
catalysts based on nonsymmetrical α-diimines with either a
BIAN or a DAB skeleton (L6 and L7 in Chart 2). Depending
on the substituents of the aryl rings, better-performing catalysts
are obtained with respect to those having the corresponding
3
BIP)(CH )(NCCH )][PF ], complexes were studied. The
3
3
6
latter were applied as precatalysts in the ethylene/methyl
acrylate copolymerization reaction and the catalytic behavior
compared to that of the catalysts having the corresponding Ar-
BIAN. In situ NMR investigations about the reactivity of the
monocationic complexes with methyl acrylate allowed us to
shed light on the possible catalyst deactivation forms.
16
symmetrical ligands.
In contrast, studies about the variations of the ligand
backbone are much more limited. α-Diimines with either a
B
DOI: 10.1021/acs.organomet.9b00308
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Article
Chart 2. Examples of Reported α-Dimine Ligands
Chart 3. α-Diimines with Different Backbones
Chart 4. α-Diimines under Investigation: (a) Ar-BIP
Molecules; (b) Already Reported α-Diimine
RESULTS AND DISCUSSION
Synthesis and Characterization of Ar-BIP Ligands and
the Relevant Pd(II) Complexes. Ar-BIP molecules were
reported for the first time in 1996 by Elsevier and have been
■
60%) as solids of orange (1, 2) or red (3) color, were
characterized by mass spectrometry and NMR spectroscopy in
solution. In analogy to Ar-BIAN molecules, also for Ar-BIPs
E,Z, E,E, and Z,Z isomers are possible depending on the
relative configuration of the aryl rings with respect to the C
N imine bonds (Chart 5). In agreement with the literature, in
20
recently reconsidered as ancillary ligands for catalysts for ring-
21
opening polymerization of rac-lactides, ethylene polymer-
2
1b
21c
ization, and isoprene polymerization.
1
Of the three different synthetic methodologies present in the
literature, only that based on the reaction of 9,10-
phenanthrenequinone with the desired aniline, at room
the H NMR spectra of 1 and 2, recorded in CD Cl at 298 K,
2
2
the number of signals and their integration indicated the
2
1b,c,22
presence of the nonsymmetrical E,Z isomer,
3 the symmetrical Z,Z isomer was present.
whereas for
2
0,21
temperature in the presence of TiCl , resulted in success in
This represents
4
2
2
our hands. Ligands 1−3, isolated in moderate yields (40−
the first main difference with respect to Ar-BIANs: these latter
C
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Chart 5. E,E, E,Z, and Z,Z Isomers for Ar-BIP Ligands
of the neutral complexes is the coordination of Ar-BIPs to
palladium.
Single crystals suitable for X-ray analysis of 2a and 3a were
obtained by slow diffusion of n-hexane into a dichloromethane
solution at 277 K (Figure 1).
The structural analysis indicates that the coordination of the
Ar-BIP ligands to the Pd center decreases their configurational
freedom due to the constraint of a square-planar geometry.
This is reflected by the substantial decrease of the NCCN and
CCCC torsion angles as well as the NCC angles of the
diiminic fragment upon coordination (Table 1).
species are always present in solution as a mixture of E,Z and
E,E isomers, in equilibrium at a slow rate on the NMR time
scale. The Z,Z isomer has never been observed up to
1
6a,c,20,23
The solid-state structure of complex 2a can be compared
now.
with that of its Ar-BIAN counterpart, [Pd(L12)(CH )Cl]
3
Ligands 1−3 were used to synthesize the corresponding
neutral, [Pd(Ar-BIP)(CH )Cl] (1a−3a), and monocationic,
25
(
L12a). In complex 2a both Pd−N bond distances are
shorter than those in L12a (2.064(1) and 2.122(1) Å in 2a vs
.088(6) and 2.203(5) Å in L12a), indicating a stronger
3
[
Pd(Ar-BIP)(CH )(NCCH )][PF ] (1b−3b), organometallic
3
3
6
2
Pd(II) complexes. The synthetic procedure applied to obtain
the neutral derivatives, 1a−3a, was based on the well-known
substitution reaction of cyclooctadiene with α-diimine on the
coordination of the Ar-BIP ligand with respect to the Ar-BIAN
molecule. This trend is reflected by the value of the Pd−C
bond length, which is longer in 2a (2.087(2) Å) than in L12a
(2.039(6) Å). Another evident difference is in the degree of
planarity of the diiminic moiety, the Ar-BIAN ligand being
almost perfectly planar, while the Ar-BIP molecule is markedly
twisted (Table 1 and Figure 2a). A greater strain in the Ar-BIP
derivative is also indicated by the NCC angles, which are
nearly ideal for L12a, while they are substantially lower than
palladium precursor [Pd(cod)(CH )Cl] (cod = 1,5-cis,cis-
3
2
4
cyclooctadiene). However, unlike the facile synthesis of
organometallic Pd(II) complexes with the corresponding Ar-
BIANs, namely [Pd(Ar-BIAN)(CH )Cl], much longer reac-
3
tion times (up to 15 days) and a greater excess of ligands with
respect to palladium (up to 15 equiv) were required to obtain
the neutral complexes 1a−3a (Scheme 1).
1
2
20° for 2a (Table 1). As a consequence, the N−Ph bonds in
Long reaction times were previously reported for the
synthesis of analogous palladium(II) complexes with either
the 8-p-tolylnaphthyl- or 2,6-bis(diphenylmethyl)-4-methyl-
a are bent away from the diazo ligand in comparison to L12a.
This effect is measured by the angle between the N−Ph bond
and the line passing through the Pd atom and bisecting the
opposite C−C bond (Figure 2b): the values for each N−Ph
bond in 2a and L12a are 80.3, 80.8° and 88.1, 86.7°,
respectively.
Both neutral and monocationic complexes were charac-
terized in solution by NMR spectroscopy recording the spectra
13a,15
substituted Ar-BIAN or Ar-DAB imines,
due to the
bulkiness of the ligand. In the case under investigation, the
difficulties in the coordination to palladium(II) of Ar-BIP
molecules might be ascribed to two main factors. (i) There is a
lack in solution of the E,E species: that is, the geometrical
isomer required for coordination of Ar-BIP to the metal center
as a chelating ligand. Therefore, an isomerization process has
to occur before coordination. For Ar-BIANs in solution the
in CD Cl at 263 K and in CDCl at 298 K, respectively. In the
2
2
3
1
H NMR spectra the number of signals and their integration
23c
isomerization from the E,Z to E,E isomer is very easy, but
apparently this is not the case for Ar-BIPs. (ii) The structure of
ligands 1−3 in the solid state points out that, unlike the Ar-
BIANs, both the phenanthrene skeleton and the iminic
fragment NCCN are not planar; for the latter the torsion
are in agreement with the coordination of the ligand in a
nonsymmetric chemical environment. The singlet of the Pd−
CH fragment falls between 0.78 and 0.41 ppm for the neutral
3
complexes and between 0.88 and 0.50 ppm for the
1
monocationic derivatives. In the H NMR spectra of L12a
21c 22
20
angles are 44.3(4), 45.0(1), and 44.7(5)° for 1, 2, and 3,
and [Pd(L12)(CH
)(NCCH
3
3
)][PF
6
] (L12b), the Pd−CH
3
respectively (Table 1). This torsion angle must remarkably
decrease for the coordination of Ar-BIP as a chelating ligand to
palladium, leading to a square-planar complex. In addition, for
ligands 1 and 2, the steric hindrance of the substituents in
ortho positions of the aryl rings might also disfavor the
coordination. Nevertheless, by following our synthetic method-
ology, complexes 1a−3a were isolated as dark purple solids in
good to high yields (70−84%). The monocationic derivatives
moiety resonates at 0.61 and 0.73 ppm, respectively, which are
at higher frequencies than the same signals for 2a and 2b (0.32
and 0.42 ppm). According to our hypothesis that this signal
might be considered as a good probe for assessing the electron
16,26
density donated by the ligand to the metal center,
the
variation in its chemical shift moving from Ar-BIAN to Ar-BIP
complexes indicates that 2 has a higher Lewis basicity in
comparison to L12. This is in agreement with the trend of the
Pd−N bond distances measured in the solid state (Figure 1)
and might be related to the lack of conjugation between the
1
b−3b were obtained in high yield from the neutral precursors
24b
1
a−3a by following the reported synthetic procedure with
20
no difficulties, confirming that the bottleneck in the synthesis
two CN bonds in the free ligand. These trends suggest
Scheme 1. Synthesis of Pd(II) Complexes
D
DOI: 10.1021/acs.organomet.9b00308
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Table 1. NCC Angles and NCCN and CCCC Torsion Angles (in Degrees, As Defined by the Schematic Drawing) for Free and
a
Coordinated 2 and 3 Ligands
compd
N(1)−C(2)−C(3)
N(2)−C(3)−C(2)
N(1)−C(2)−C(3)−N(2)
C(1)−C(2)−C(3)−C(4)
2
2
L12a
3
3
115.8
115.1(1)
118.4
117.2
114.3(1)
126.11
113.6(1)
119.0
126.33
115.1(1)
45.0(1)
22.0(2)
4.1
44.7(5)
21.3(1)
41.3
29.6(2)
0.7
47.4
32.9(2)
a
a
a
For comparison purposes, the data for the Ar-BIAN counterpart of complex 2a, L12a, have also been included; standard deviations (where
available) are given in parentheses.
Figure 1. ORTEP drawings (50% probability ellipsoids) for complexes (a) 2a and (b) 3a. Selected bond lengths (Å) and angles (deg): 2a, Pd(1)−
N(1) 2.064(1), Pd(1)−N(2) 2.122(1), Pd(1)-C 2.087(2), Pd(1)−Cl(1) 2.2821(5), N(1)−Pd(1)−N(2) 77.38(5), C(1)−Pd(1)−Cl(1) 85.64(6),
N(1)−Pd(1)−C(1) 99.41(7), N(2)−Pd(1)−Cl(1) 98.73(4); 3a, Pd(1)−N(2) 2.043(1), Pd(1)−N(1) 2.171(1), Pd(1)−C(1) 2.035(1), Pd(1)−
Cl(1) 2.2924(5), N(2)−Pd(1)−N(1) 76.99(4), C(1)−Pd(1)−Cl(1) 86.29(4), C(1)−Pd(1)−N(2) 96.08(5), N(1)−Pd(1)−Cl(1) 100.93(3).
Figure 2. (a) Average planes through the Pd and the four atoms directly bonded to it in complex 2a and L12a to evidence the different degrees of
planarity of the diiminic ligand. (b) Schematic drawings showing the bending effect on the N−Ph bonds in 2a in comparison to L12a.
that Ar-BIPs are stronger ligands than Ar-BIANs, and, in
analogy with the study on the Pd(PDAP) ligand, the Pd(Ar-
BIP) cationic complexes can be considered as electron-rich
metal cations.
Ethylene/Methyl Acrylate Copolymerization. Com-
plexes 1b−3b were tested as precatalysts for the ethylene/
methyl acrylate copolymerization (Scheme 2 and Table 2) by
carrying out the reaction in 2,2,2-trifluoroethanol at mild
reaction conditions of temperature and ethylene pressure and
with no addition of any cocatalyst. The same reaction
conditions were applied for the complex with ligand L12, in
order to have a direct comparison of data.
7
E
DOI: 10.1021/acs.organomet.9b00308
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Scheme 2. Ethylene/Methyl Acrylate Copolymerization
However, whereas for the Ar-DAB catalysts the TON
decreases for both comonomers on going from the
isopropyl-substituted ligand to the methyl derivative, for
the Ar-BIP catalysts an increase in the TON of the polar
monomer is found. Precatalysts 1b and 2b are also found
active in ethylene homopolymerization (Table 2, runs 5 and
2b
6), but the increase in the productivity with respect to the
ethylene/MA copolymerization is not as pronounced as that
reported in the literature for Ar-DAB catalysts (where the
2a
difference is of 1 order of magnitude). It is accepted that the
low productivity in the copolymerization with respect to
ethylene homopolymerization is due to the formation of the
stable six-membered palladacycle (species C′′ in Scheme 3),
which inhibits the subsequent alkene insertion and represents
Before workup, a sample for GC-MS was taken in order to
analyze the presence of alkene oligomers. The catalytic
products were isolated as brown oils and characterized by
NMR spectroscopy after removal of the volatile fraction at
reduced pressure (Figure S45−S54). The molecular weight of
the produced macromolecules was determined by GPC
analysis.
2b
the catalyst resting state. Therefore, the small difference in
productivity observed for the Ar-BIP catalysts in the two
polymerizations suggests a lower stability of the metallacycle
C′′, which easily cleaves, favoring the growth of the polymer
chain. This is in agreement with the higher Lewis basicity of
the Ar-BIP ligands that makes the palladium center less
oxophilic, disfavoring the formation of the Pd−O bond in C′′.
In addition, in the literature the Ar-DAB catalysts are applied
for the ethylene homopolymerization in dichloromethane,
whereas here the Ar-BIP derivatives are tested in trifluor-
oethanol, a solvent never used for polyolefin synthesis. Due to
the remarkably different nature of the two solvents, it cannot
be ruled out that trifluoroethanol plays a role in determining
catalyst productivity.
A remarkable effect of the N-donor ligand on the catalytic
behavior is observed, on comparison of both the phenan-
threne-based ligands bearing different substituents on the aryl
ring and ligands with a different backbone (Table 2). Inside
the series of complexes with Ar-BIP ligands, 3b is found to be
inactive toward the copolymerization reaction, yielding only a
mixture of butenes (Table 2, run 3). Complexes with the
ortho-substituted Ar-BIP, 1b and 2b, generate active catalysts
leading to the formation of ethylene/MA copolymers. An
increase in the productivity is achieved on going from the
catalyst with the methyl-substituted Ar-BIP, 2, to that with the
isopropyl-substituted ligand, 1, and the highest value of
productivity is reached with precatalyst 1b (almost 21 kg of
CP/mol of Pd, Table 2, run 1). The different substitution on
the aryl rings also affects the content of inserted polar
monomer, the molecular weight, and the branching degree of
the copolymers: the first feature remarkably decreases on going
from the catalyst with 2 to that with 1, whereas the second
parameter increases of three times following the same order
The direct comparison with the catalytic behavior of L12b
points out that the Ar-BIAN catalyst is more productive than
the active species with the corresponding Ar-BIP 2 and yields a
copolymer with a higher content of polar monomer, but with 3
times lower molecular weight value and a slightly higher
branching degree (Table 2, runs 2 and 4). The lower content
of inserted polar monomer in the macromolecules produced
with 2b might be related to the additional bulkiness around
palladium created by the phenanthrene skeleton, as evidenced
by the X-ray analysis (Figure 2a). This higher steric congestion
might be also responsible for the higher molecular weight of
the produced copolymers. In addition, as recently reported for
and an M value of 33000 is reached, together with a decrease
n
of branching degree (Table 2, runs 1 and 2). In both cases no
formation of alkene oligomers was observed, as expected for
catalysts having hindered apical positions around the metal
1
k,27
7
center.
Despite the different reaction conditions, the trends
the Pd-PDAP catalysts, also the higher Lewis basicity of Ar-
observed on moving from 2b to 1b are similar to those
BIP ligands with respect to the corresponding Ar-BIAN
derivatives can play a positive role in enhancing the length of
polymer chains.
reported for Ar-DAB catalysts and are mainly related to the
2
b
different steric hindrances around the catalytic center.
a
Table 2. Ethylene/Methyl Acrylate Copolymerization: Effect of the Ancillary Ligand
d
TON
.
b
c
e
f
,
ch
run
precat.
yield (mg)
g of CP/mol of Pd
MA (mol %)
E
MA
M (M /M )
alkenes/esters
Bd
83
n
w
n
1
2
3
4
1b
2b
3b
L12b
1b
436.4
156.6
20781
7443
0.3
2.4
734
248
2.21
6.08
33000 (1.54)
10000 (1.60)
none
none
C4
none
none
none
103
379.3
452.1
181.7
18062
21529
8652
4.4
564
767
308
26.0
3200 (2.06)
23000 (2.66)
7800 (1.80)
111
90
114
g
5
g
6
2b
a
Reaction conditions unless noted otherwise: precatalyst [Pd(CH )(NCCH )(N−N)][PF ], n = 2.1 × 10−5 mol, VTFE = 21 mL, VMA = 1.130
3
3
6
Pd
b
mL, [MA]/[Pd] = 594, T = 308 K, P
palladium. Calculated by H NMR spectroscopy on isolated product. Turnover number = moles of substrate converted per mole of catalyst (E =
= 2.5 bar, t = 18 h. Isolated yield, productivity as g of CP/mol of Pd = grams of copolymer per mole of
ethylene
c
1
d
e
−1
ethylene). Determined by GPC at 30 °C, using THF as the solvent, an eluent flow rate of 1 mL min , and 19 narrow polystyrene standards as the
f
g
−5
h
reference. Determined by GC/MS. Reaction conditions: n = 2.1 × 10 mol, VTFE = 21 mL, T = 308 K, Pethylene = 2.5 bar, t = 18 h. Bd = degree
Pd
of branching as branches per 1000 carbon atoms.
F
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Scheme 3. Accepted Mechanism for the Reaction of Pd(α-diimine) Complexes with MA
The influence of catalytic parameters, such as temperature,
ethylene pressure, methyl acrylate to palladium ratio, and
reaction time, has been evaluated for precatalysts 1b and 2b, in
order to optimize the system. As a general observation, in the
investigated ranges complex 2b is more affected than 1b by the
parameter variations.
together with a decrease in the molecular weight (Table S2).
This effect on MW indicates that the increase in productivity is
related to an increase in the number of produced macro-
molecules rather than in their length, thus suggesting that the
decrease in the propagation to chain transfer rate ratio is due to
an increase in the rate of the chain transfer step. The influence
of temperature on the productivity represents a main difference
with respect to the classical Ar-BIAN-based catalysts, whose
The temperature influence was investigated in the range
2
98−318 K (Figure 3 and Table S2). The catalytic
productivity remarkably drops at 318 K due to the
performances of 1b, in terms of productivity, content of
inserted polar monomer, and molecular weight, are almost
unaffected by the temperature variation, whereas for 2b a clear
increase in both productivity and content of MA is observed,
2a,16a
decomposition to inactive palladium metal.
The improved
thermal stability of the Ar-BIP catalysts is in agreement with
both the stronger coordinating capability of Ar-BIP ligands
with respect to Ar-BIANs and the increase in the bulkiness of
the ligand. The latter is a strategy already successfully applied
in the Ni- and Pd-catalyzed ethylene homopolymerizatio-
10a,11a,15,17
n
and in one example of ethylene/MA copoly-
merization, where the increase in the content of inserted MA
was achieved together with a decrease in the productivity when
13a
the reaction temperature was raised from 293 to 333 K.
The effect of the ethylene pressure was studied in the range
.5−7.0 bar. The increase in the ethylene pressure has no effect
2
on the performance of the catalyst obtained from 1b (Table
S3, runs 1−3), whereas for 2b the productivity almost doubles
when the ethylene pressure is increased from 2.5 to 5.0 bar
together with a drop in the content of the polar monomer
(
Table S3, run 4 vs run 5). A further increase in ethylene
pressure up to 7 bar results in a decrease in the productivity
Table S3). A similar effect of the ethylene pressure on the
Figure 3. Ethylene/methyl acrylate copolymerization: effect of
(
temperature. Precatalyst: [Pd(Ar-BIP)(CH )(CH CN)][PF ] (Ar-
3
3
6
productivity was observed by us in the ethylene/MA
BIP = 1, 2). Reaction conditions: see Table 2. Filled symbols indicate
productivity data and open symbols the content of MA in mol %: 1b
cooligomerization catalyzed by Pd complexes with the
16a
(
●, ○), 2b (▲, Δ).
nonsymmetric BIAN L6, also carried out in trifluoroethanol.
G
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For the catalysts under investigation the effect is more
pronounced and it might be related to the fact that, as
reported in Table 2, the Ar-BIP complexes are not good
catalysts for ethylene homopolymerization.
The effect of methyl acrylate to palladium ratio was
investigated by increasing the amount of the polar monomer
in the range [MA]/[Pd] = 594−1782 and by keeping constant
the amount of precatalyst. In agreement with the data on Ar-
2
DAB catalysts, the content of the inserted polar monomer
linearly increases with MA concentration (Figure 4 and Table
Figure 5. Ethylene/methyl acrylate copolymerization: effect of
reaction time. Precatalyst: [Pd(Ar-BIP)(CH )(CH CN)][PF ] (Ar-
3
3
6
BIP = 1, 2). Reaction conditions: see Table 2. Filled symbols indicate
productivity data and open symbols mol % of MA: 1b (●, ○), 2b (▲,
Δ).
indicating that the increase in productivity is associated with
the formation of more polymer chains rather than longer
macromolecules.
Two selected catalytic experiments were performed by using
a mass flow control device to measure the ethylene
consumption (Figure 6). The catalysts obtained from 1b and
Figure 4. Ethylene/methyl acrylate copolymerization: effect of [MA]/
[
=
Pd] ratio. Precatalyst: [Pd(Ar-BIP)(CH )(CH CN)][PF ] (Ar-BIP
1, 2). Reaction conditions: see Table 2. Filled symbols indicate
3 3 6
2
b show similar kinetic profiles: (i) an induction period of 90 s
is observed; (ii) the rate of ethylene uptake is the same up to
40 s; (iii) afterward the catalyst with 2 slows down, whereas
productivity data and open symbols mol % of MA: 1b (●, ○), 2b (▲,
Δ).
2
the catalyst with 1 is very active up to 2 h. When the ethylene
consumption is monitored for 18 h, the deactivation of catalyst
with ligand 1 becomes evident, while the catalyst with ligand 2
remains active.
S4). A concomitant decrease in the M values is observed
n
(
Table S4). The effect on the productivity is different
depending on Ar-BIP: for precatalyst 1b a decrease in
productivity is found due to a drop in ethylene turnover
number; instead for 2b an increase in the productivity
associated with an increase in the values of TON for both
comonomers is initially observed, followed by a decrease for a
further increase in the [MA]/[Pd] ratio (Table S4). Unlike the
The characterization of the isolated catalytic product was
1
13
performed by H and C NMR spectroscopy in CDCl at
3
1
room temperature. The signals in the H NMR spectra were
identified by comparison with literature data
2a,3,16a,29
and
assigned as follows: the singlet at 3.66 ppm to the methoxy
group, the signals between 2.50 and 1.50 ppm to allylic protons
and to methylenic groups closer to the ester functionality, the
broad signal centered at 1.25 ppm to the methylenic and
methinic moieties of ethylene units, and the signals between
1.0 and 0.6 ppm to methyl groups at the end of branches
(Figures S45 and S47). The signals due to the vinylic protons
are not observed in the spectrum of the copolymer obtained
with precatalyst 1b, and signals of very low intensity are
present in the spectrum of the copolymer produced with 2b, in
agreement with the polymeric nature of the catalytic product.
2
Ar-DAB catalysts, for both precatalysts the turnover number
for MA increases with the MA concentration, thus suggesting a
higher affinity for the polar monomer of the Ar-BIP catalysts
than of the Ar-BIAN derivatives, and this might be related to
the higher Lewis basicity of Ar-BIP, supporting the positive
effect of having electron-rich metal cationic catalysts. This is
also in agreement with DFT calculations showing that anionic
28
Pd(II) catalysts are highly tolerant toward polar monomers.
The catalytic behavior of precatalysts 1b and 2b was also
assessed over time, in the range t = 6−96 h, by carrying out the
1
3
catalytic experiments at P = 2.5 bar and T = 308 K. Precatalyst
In the C NMR spectra of the copolymers obtained with
either 2b or L12b (Table 2, run 2, and Table S5, run 11), in
addition to the typical peaks of the methylenic and methyl
groups, two overlapped signals for the methoxy group (51.36
and 51.52 ppm) and one peak at 45.94 ppm are present. For
E
1
b is active up to 48 h, whereas 2b is still working up to 96 h,
reaching a productivity value of almost 25 and 13 kg of CP/
mol of Pd, respectively (Figure 5 and Table S5). In all of the
catalytic tests no formation of inactive palladium black was
evident, indicating that catalyst deactivation follows a different
mechanism than the decomposition to palladium metal,
implying ligand dissociation. This is in agreement with the
structural features of the complexes that indicate Ar-BIP
molecules as stronger ligands to palladium than Ar-BIANs.
The increase in productivity with reaction time is related to
the increase in turnover numbers for both comonomers, even
though a slight decrease in the content of the inserted polar
monomer is measured at longer reaction times for copolymers
produced with 2b (Figure 5 and Table S5). The molecular
weight increases during the first 18 h, leveling off afterward,
1
13
the latter the H, C HSQC spectrum indicates its methinic
nature. Three signals are present for the carbonyl group at
174.37, 174.68, and 177.17 ppm. By comparison with the
spectra reported in the literature for the ethylene/MA
copolymers obtained with the Pd(Ar-DAB) and the dinuclear
2
a,12
Pd(α-diimine) catalysts,
the peak at 174.37 ppm is
attributed to the acrylate units inserted at the end of the
branches as the −CH −CH −C(O)OCH fragment. In
2
2
3
agreement with the assignment of the ethylene/MA copoly-
12
mers obtained with the Pd(L2) catalyst, the signals at 177.17
and 45.94 ppm unambiguously indicate that the polar
H
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Figure 6. Ethylene/methyl acrylate copolymerization: kinetic profiles. Precatalyst: [Pd(Ar-BIP)(CH )(CH CN)][PF ] (Ar-BIP = 1 (red curve), 2
3
3
6
(
0
blue curve)). Reaction conditions: see Table 2. (top) Ethylene uptake for 18 h and (bottom) magnification of ethylene uptake in the time range
−1 h.
monomer is inserted into the main polymer chain (Figure 7).
Inspection of the 13C NMR spectrum of the copolymer
obtained with L12b in CH Cl , instead of TFE, shows the
both at the end of the branches and into the main chain, with a
remarkable preference for the first kind of enchainment.
Whereas the beneficial effect of the fluorinated alcohol on
the productivity was reported by some of us in the ethylene/
MA cooligomerization catalyzed by Pd complexes with
2
2
presence of the same signals plus other two peaks for the
carbonyl, at 177.23 and 167.07 ppm, respectively, that
1
6b,c
30
nonsymmetric α-diimines,
the solvent effect on the
according to the literature can be attributed to the polar
monomer inserted at the beginning of the copolymer chain as
CH −CH−(CH COOCH )−CH − fragment and at the end
copolymer microstructure is evidenced here for the first time.
It might be related to the fact that, thanks to its higher
coordinating capability with respect to dichloromethane,
trifluoroethanol favorably competes for the fourth coordination
site on palladium that is necessary for β-hydrogen elimination
to occur. Therefore, the latter is disfavored and, as a
consequence, both termination and chain-walking processes
are inhibited, leading to a more selective enchainment of MA.
This hypothesis is in agreement with the molecular weight
3
2
3
2
of the chain as the unsaturated moiety −CHCH−COOCH
3
(
Figure 7c). For the latter the methoxy peak at 3.69 ppm in the
1
H NMR spectrum is also observed (Figure S55). Thus, the
NMR analysis indicates that when the copolymerization is
carried out in trifluoroethanol the polar monomer is inserted in
a more selective way than in dichloromethane and it is found
I
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Figure 7. 13C NMR spectra (CDCl , 298 K) of ethylene/methyl acrylate copolymers synthesized with (a) 2b in TFE, (b) L12b in TFE, and (c)
3
L12b in CH Cl : (left) carbonyl region; (right) methoxy and methinic region.
2
2
value, M , of the copolymers synthesized in the two solvents
The established mechanism for the reaction of the Pd(α-
diimine) precatalysts with MA is based on the migratory
n
with L12b. The M values estimated by integration of the
n
1
signals in the H NMR spectra are 3200 and 2500 for the
insertion of the polar monomer into the Pd−CH bond taking
3
copolymers obtained in trifluoroethanol and dichloromethane,
respectively.
place with either a primary or a secondary regiochemistry, the
latter being favored and followed by the chain-walking process
that leads to the intermediate C′′, the catalyst resting state
In the 13C NMR spectrum of the copolymers obtained with
b only the signals related to the polar monomer inserted at
2b,16a
1
(Scheme 3).
The occurring pathway is recognized by the
1
the end of the branches are evident (Figure S46).
H NMR signals of the typical intermediates. The analysis of
1
Mechanistic Investigation. The mechanism of the
ethylene/MA copolymerization reaction catalyzed by Pd(II)
α-diimine complexes is well established and based on three
migratory insertion reactions: that of ethylene after the
insertion of ethylene itself or of methyl acrylate and that of
the H NMR spectra related to the reactivity of 1b, 2b, and 3b
with MA points out some differences both among the three
complexes and between them and the behavior of Pd(Ar-
BIAN) and Pd(Ar-DAB) precatalysts.
1
In particular, in the H NMR spectrum of the reaction of 3b
2b
the polar monomer after the insertion of ethylene. It is
known from the literature that on the Pd(α-diimine) catalysts
the migratory insertion of ethylene is very fast. In the present
catalytic system the reactivity of 1b with ethylene was
investigated by bubbling the gaseous monomer into a 10
recorded after 1 min from the addition of MA the signals of 3b,
the five- and the six-membered metallacycles B′′ and C′′ are
present. The evolution with time indicates that these species
are progressively consumed, leading to the formation of methyl
crotonate, D′ and A′, which are already observed after 15 min
of reaction and become the main species after 3 h. These data
indicate that β-H elimination is fast and that the insertion of
1
mM solution of the complex in CD Cl at 298 K. In the H
2
2
NMR spectrum recorded after 6 min from the bubbling, the
formation of polyethylene is evident, together with the almost
complete consumption of ethylene and traces of the remaining
palladium precursor (Figure S58), thus confirming also for the
Pd(Ar-BIP) complexes the typical reactivity of the Pd(α-
MA into the Pd−CH bond is not highly regioselective (Figure
3
S61).
As far as the reactivity of 2b is concerned, the signals of the
metallacycles B′′, C′′, and A′ are observed after 5 min together
with those of 2b. These intermediates are the main species,
together with methyl crotonate and traces of 2b, at 3 h time.
These data indicate that β-H elimination is slower than for 3b
2
b,16a
diimine) systems.
The reactivity toward methyl acrylate was studied by adding
equiv of the polar monomer to a 10 mM solution of 1b, 2b,
2
and 3b in CD Cl , at 298 K. The three precatalysts show
and confirm that the insertion of MA into the Pd−CH bond is
2
2
3
remarkably different reaction rates: for complex 3b all of the
precatalyst is consumed after 3 h, whereas for 1b and 2b at the
same reaction time 29% and 86% have reacted, respectively
not highly regioselective (Figure S60). Therefore, unlike the
2b,16a
Pd(Ar-BIAN) and Pd(Ar-DAB) precatalsyts,
for 3b and
2b the migratory insertion reaction of MA into the Pd−CH
3
(
Figures S59−S61). Unlike what has been reported for other
bond is not regiospecific.
16a
Pd(α-diimine) complexes,
the reaction of 2b follows a
Finally, in agreement with the slow reaction rate, in the
spectra of 1b the signals of methyl crotonate and C′′ appear
only after 3 h. No peaks due to the other metallacycles are
second-order kinetics (Figure S62b), whereas for 1b and 3b a
proper kinetic profile is difficult to model (Figure S62a,c).
J
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Figure 8. ORTEP drawings (thermal ellipsoids at the 50% of probability level) for complexes (a) dimer 1c and (b) trimer 2c. Selected bond
lengths (Å): 1c, Pd(1)−N(11) 1.969(21), Pd(1)−N(12) 1.968(2), Pd(1)−O(31) 2.013(2), Pd(1)−O(41) 2.007(2), Pd(2)−N(21) 1.970(2),
Pd(2)−N(22) 1.968(2), Pd(2)−O(32) 2.017(2), Pd(2)−O(42) 2.014(2); 2c, Pd(1)-N 1.976(3), Pd(1)-N 1.973(3), Pd(1)−O(22) 2.024(2),
Pd(1)−O(12) 2.021(2).
Figure 9. Fragments cut out from the structures of (a) 2b, (b) trimer 2c, and (c) dimer 1c. They put in evidence the decreased twisting of the Ar-
BIP ligand on passing from the mononuclear to the multinuclear molecules.
31
observed. These data indicate that on 1b the migratory
insertion reaction of the polar monomer is very slow and it is
regiospecific (Figure S59).
two metal centers. To the best of our knowledge, these are
the first structurally characterized examples of Pd complexes of
this type.
In the spectra of the reactions of 1b and 2b in addition to
the signals discussed above, new signals for the substituents on
the aryl rings are clearly evident after 15 and 5 min,
respectively. Their assignments are in agreement with the X-
ray analysis in solid state of crystals formed upon addition of n-
hexane to these solutions kept at 277 K for 2 weeks.
The crystals obtained from the reaction of 1b with MA
correspond to a dimeric species, 1c, in which two tetrahedral
The combination of the NMR data about the reactivity of
complexes 1b, 2b, and 3b with MA and the X-ray analysis
suggests that the formation of the multinuclear species might
represent the deactivation pathway for the Ar-BIP catalysts and
their formation is relevant when the catalytic reaction slows
down.
CONCLUSION
■
2−
PO3F ions bridge two Pd atoms (Pd−Pd distance 5.227(2)
Å; Figure 8a). The coordination of each Pd is completed by
the Ar-BIP ligand 1. The crystals obtained from the reaction of
In summary, we have now investigated organometallic
palladium(II) complexes with α-diimines having a phenan-
threne skeleton and 2,6-disubstituted aryl rings (Ar-BIP). The
2
b with MA correspond to a trimeric species, 2c, in which two
synthesis of the neutral complexes [Pd(Ar-BIP)(CH )Cl] was
3
phosphate groups keep together three Pd moieties, each
completed by one Ar-BIP ligand 2 (Figure 8b). Probably due
to the increased rigidity of the molecule, the twisting of the Ar-
BIP ligand is considerably reduced in 2c with respect to the
corresponding mononuclear complex 2b, as shown in Figure 9.
The Ar-BIP conformation in 1c is very similar to that in 2c
not as trivial as that typically applied to obtain the same
compounds with the corresponding Ar-BIANs. The reasons lie
in the lack in solution of the E,E isomer, in the remarkable
distortion from planarity of the phenanthrene backbone, and in
the steric hindrance of the substituents in ortho positions on
the aryl rings. From the neutral derivatives the cationic [Pd(Ar-
BIP)(CH )(CH CN)][PF ] compounds were obtained with
(Figure 9): in this case, no crystal structure is available for the
3
3
6
corresponding 1b complex to make a direct comparison.
no difficulties.
The bridging ions, PO3F2 and PO4 , are probably
−
3−
The characterization both in solution and in the solid state
of the two series of complexes points out that Ar-BIP ligands
have a higher Lewis basicity than the corresponding Ar-BIAN
counterparts and are more strongly bonded to the metal
center. In particular, the monocationic derivatives can be
regarded as electron-rich metal cations.
−
originated by the partial and full hydrolysis of PF6
counterions, respectively. In general, this is not a very common
reaction pathway; only a few reports can be found in the
literature about metal complexes in which PO3F2 acts as a
bridging ligand and even fewer where this anion connects only
−
K
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The cationic compounds generated efficient catalysts for the
ethylene/methyl acrylate copolymerization under mild reaction
conditions of temperature and pressure, leading to the
corresponding copolymers. A detailed investigation of the
catalytic behavior was carried out highlighting the main
differences with catalysts having the Ar-BIANs. In particular,
the Pd(Ar-BIP) active species show a good affinity toward the
polar monomer, have a good thermal stability, lead to
copolymers of higher molecular weight, and favor the cleavage
of the six-membered metallacycle, recognized as the resting
state of the catalytic cycle. All of these catalytic findings
correlate well with the electron-rich metal cationic feature of
the active species.
Fulvia Felluga: 0000-0001-8271-408X
Barbara Milani: 0000-0002-4466-7566
Present Addresses
∥
V.R.: Morgarternstrasse 11, 3014 Bern, Switzerland.
̀
L.C.: Universita di Bologna, Dipartimento di chimica
⊥
“
Giacomo Ciamician”, via Selmi 2, Bologna, Italy.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
Notes
The authors declare no competing financial interest.
A detailed NMR characterization of the produced macro-
molecules probes evidence that the polar vinyl monomer is
inserted both at the end of the branches and into the main
chain with a more selective enchainment of that achieved when
the copolymerization is carried out in dichloromethane. This
aspect is currently under investigation.
This study points out the importance of the ligand skeleton
in the design of catalysts for the target reaction and
demonstrates that moving from the acenaphthene to the
phenanthrene backbone has a remarkable influence on both
the coordination chemistry and catalysis. In addition, it
represents the first time that this copolymerization has been
carried out in a fluorinated solvent, highlighting its beneficial
effect, in particular on the copolymer microstructure. These
results prompt us to investigate the catalytic performances of
old and new catalysts in trifluoroethanol.
ACKNOWLEDGMENTS
This work was financially supported by MIUR (PRIN 2015,
No. 20154X9ATP_005), Universita
Finanziamento di Ateneo per progetti di ricerca scientifica;
FRA 2018), INSTM Consortium and ICCOM-CNR.
Fondazione Beneficentia Stiftung is gratefully acknowledged
for cofinancing the fellowship to A.D.A. Fondazione CRTrieste
is gratefully acknowledged for the generous donation of a
Varian 500 MHz spectrometer. BASF Italia is acknowledged
for a generous donation of [Pd(OAc) ]. Dr. Nicola Demitri
and the staff of the Elettra Synchrotron in Trieste are gratefully
■
̀
degli Studi di Trieste
(
2
acknowledged for assistance in collecting X-ray data.
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