Tailored Strength Neighboring Group Interactions Switch Polymerization to Dimerization Catalysis

Article abstract of DOI:10.1021/acscatal.9b00129

A combined experimental and theoretical study elucidates how Ni···O neighboring group interactions drastically switch catalytic properties toward ethylene. A range of salicylaldiminato complexes with aryloxy groups in the 2,6-position of the N-phenyl grou

Full text of DOI:10.1021/acscatal.9b00129

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Article
Tailored strength neighboring group interactions
switch polymerization to dimerization catalysis
Eva Schiebel, Stefano Santacroce, Laura Falivene, Inigo
Göttker-Schnetmann, Lucia Caporaso, and Stefan Mecking
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.9b00129 • Publication Date (Web): 21 Mar 2019
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ACS Catalysis
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Tailored strength neighboring group interactions switch
polymerization to dimerization catalysis
Eva Schiebel†, Stefano Santacroce‡, Laura Falivene§, Inigo Göttker-Schnetmann†, Lucia Caporaso*‡,
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and Stefan Mecking*†
†Chair of Chemical Materials Science, Department of Chemistry, University of Konstanz, 78457 Konstanz, Germany
‡ Dipartimento di Chimica e Biologia, Università di Salerno, Via Papa Paolo Giovanni II, I-84084 Fisciano, Italy
§ Physical Sciences and Engineering Division, Kaust Catalysis Center (KCC), King Abdullah University of Science and Technology
(KAUST), Thuwal 23955-6900, Saudi Arabia
homogeneous nickel(II) catalysts; ethylene insertion polymerization; secondary interactions; mechanism; density functional
theory
ABSTRACT: A combined experimental and theoretical study elucidates how Ni∙∙∙O neighboring group interactions drastically
switch catalytic properties towards ethylene. A range of salicylaldiminato complexes with aryloxy groups in 2,6-position of the N-
phenyl group were found to dimerize and oligomerize ethylene to butenes and branched oligomers (C4, C6, C8, C10…) in pressure
reactor experiments, while corresponding reference catalysts with arylmethylene groups yield linear polyethylene with Mn 100.000 g
mol-1. While both types of catalysts consume ethylene with similar high activities (105 turnovers h-1), the rate of ß-hydride elimination
(BHE) is much increased for the case of aryloxy substitution. DFT studies show that formation of the relevant cis-agostic complex
from which BHE occurs by displacement of ethylene from the cis-alkyl olefin complex is promoted by an Ni∙∙∙O interaction. This
low energy pathway renders chain transfer competitive with insertion chain growth. The resulting Ni∙∙∙O intermediate is rather stable
and similar in energy to key species of catalysis (ß-agostic and alkyl olefin complexes) but barely not yet an energetic sink that would
impede catalysis.
stable toward water and compatible with certain vinyl monomers.7,
INTRODUCTION
9-12
In catalytic insertion polymerizations a control of chain growth vs.
chain transfer is paramount to achieve desired polymer molecular
R
R
Y
R
N
O
weights and material properties. The relevant pathways can involve
cocatalysts and chain transfer reagents, but especially depend on
the active transition metal site itself. This is most pronounced for
late transition metal catalysts, which are generally prone to
ß-H elimination (BHE), a key step of chain transfer.1
R
Ni
X
N
O
H
Ni
O
branching & transfer
N
-
Ni
py
Me
N
Ni
O
Late transition metal polymerization catalysts are outstanding in
their synthetic scope in that they can tolerate heteroatom-
containing polar substrates and solvents. Further, they can provide
a unique branching microstructure. These arise from ‘chain
walking’ by repeated sequences of BHE and olefin reinsertion.2 To
use the potential of such catalysts, a control of the underlying
pathways is required. Notably, both a suppression of BHE as well
as its promotion are of practical interest. A complete suppression
enables the formation of ultra high molecular weight strictly linear
polyethylene (PE), which is formed in a desirable non-entangled
state.3-4 On the other hand, hyperbranched oligomer materials have
potential as lubricants or surface modifiers.5-6
R
N
O
R
Ni
linear chain growth
insertion
H
R = -CF3, -SF5, -NO2
-Me, -OMe
Scheme 1. N-Terphenyl salicylaldiminato Ni(II) catalysts and
mechanism of weak neighboring group interactions’ promoting
chain transfer and branch formation.
Despite their rather remote position, substituents on the distal rings
of the aforementioned terphenyl motif effect the products of
catalysis dramatically.13-14 With electron-withdrawing substituents,
high molecular weight linear PE is formed while with electron-
donating substituents under otherwise identical conditions
hyperbranched low molecular weight polymers are obtained. This
has been traced to a weak interaction of the distal aryl rings with
the Ni(II) center, which promotes decoordination of ethylene and
hereby favors BHE and consequently chain transfer and branch
formation.15 Only with electron-rich substituted aryls this weak
An instructive case with regard to these general issues are
N-terphenyl salicylaldiminato Ni(II) catalysts (Scheme 1).7-8
Salicylaldiminato Ni(II) catalysts in general stand out in being
highly active for ethylene polymerization and at the same time
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interaction is operative. Notably, also for other late transition metal
To allow a fine tuning of the anticipated weak interactions, three
phenols with electron withdrawing or donating substituents were
used. The obtained nitro benzenes were reduced according to a
reported procedure21 and used without further purification in the
following condensation step. The anilines were reacted with either
1
2
3
4
5
6
7
8
catalyst structures the role of substituents’ attractive ‘secondary’
interactions with substrates and the active site is being revealed.16-
18
This recently gained understanding provokes the question how
stronger such neighbouring group interactions affect catalysis, that
is promote certain pathways strongly without shutting down
activity due to blocking of the active site. We now show that
appropriately placed additional O-donors hit this spot.
3,5-diiodosalicylaldehyde
or
2-hydroxy-3-(9-
anthryl)benzaldehyde in an acid-catalyzed reaction in methanol.
The obtained salicylaldimines are poorly soluble in methanol,
enabling the generation of analytically clean products by washing
with solvent. All six different salicylaldimines were reacted with
[(tmeda)NiMe2] in the presence of pyridine to give the oxygen
bridged complexes 4 in good yields.
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RESULTS AND DISCUSSION
The anilines for the methylene bridged complexes were
synthesized based on a procedure by Knochel et. al.22 (Scheme 3).
To this end, organozinc compounds with different substitution
patterns were prepared and used directly (after filtration) in a C-C
coupling reaction with 2,6-dibromoaniline. The substituted anilines
were condensed with 3,5-diiodosalicylaldehyde or 2-hydroxy-3-(9-
anthryl)benzaldehyde in toluene. The salicylaldimines were
obtained after purification by column chromatography. The
complexes were obtained by reacting the salicylaldimines with
[(tmeda)NiMe2] and pyridine in benzene (for full characterization
data cf. the Supporting Information, SI).
As a potentially coordinating additional donor, we introduced
-OAryl groups in the 2,6 potition of the well known
salicylaldimnato motif (Figure 1). The phenoxy substitution itself
is expected to have a minor impact on the electronic properties on
the N-donor as given by the Hammet constant.19
R1
R1
R1
R1
O
Me
Pyr
Pyr
Me
N
Ni
Ni
N
O
O
R1
R1
O
R2
R2
NH2
Br
Br
NH2
Zn,
LiCl
R1
R1
R1
R1
R1
R1
R3
R3
Br
ZnBr LiCl
4
8
Pd(OAc)2
SPhos
,
R1
R1
3: R1=CH3; 91 %
R1
R1
3: R1=CH3, 63 %
5-CH
5-H
Figure 1: Structures of the complexes 4 and 8 prepared in this
work.
6-CH
6-H
: R1=H; 97 %
: R1=H, 64 %
3: R1=CF3; 90 %
5-CF
3: R1=CF3, 41 %
6-CF
conversions given.
As a reference, another set of catalysts bearing non-coordinating
methylene groups instead of the oxygen atoms were targeted. A
structurally related, but sterically more crowded ligand structure
was reported by Chen and coworkers.20 These catalysts with a
dibenzhydryl moiety were highly active within the first ten minutes
of polymerization and produce high molecular weight PE with a
moderate degree of branching.20
R1
R1
R3
R2
O
Pyr
Me
N
OH
N
Ni
OH
R2
O
[(tmeda)NiMe2],
R1
pyridine
R1
R3
R2
R1
R1
R3
R1
R1
Synthesis and characterization of the complexes.
7-CH3-I2
: R1=CH3, R2, R3=I, 94 %
: R1=CH3, R2, R3=I, 63 %
: R1= H, R2, R3=I, 65 %
8-CH3-I2
8-H-I2
7-H-I2
7-CF3-I2
: R1= H, R2, R3=I, quantitative
: R1=CF3, R2, R3=I, 97 %
: R1=CF3, R2, R3=I, 53 %
: R1=CH3, R2=9-anthryl, R3=H, 53 %
: R1=H, R2=9-anthryl, R3=H, 63 %
Synthesis. 2,6-difluoronitrobenzene was substituted in
a
8-CF3-I2
7-CH3-ant
: R1=CH3, R2=9-anthryl, R3=H, 92 %
8-CH3-ant
8-H-ant
nucleophilic aromatic substitution with modified phenols (Scheme
7-H-ant
7-CF3-ant
: R1=H, R2=9-anthryl, R3=H, quantitative
: R1=CF3, R2=9-anthryl, R3=H, 92 %
2).21
: R1=CF3, R2=9-anthryl, R3=H, 57 %
8-CF3-ant
1. NaH
Scheme 3. Synthesis of methylene bridged complexes.
2.
NO2
OH
F
F
NO2
NH2
Pd/C,
5 bar H2
Properties. The electron density on the Ni-atom is directly
correlated to the catalytic properties.15 To this end, forward peak
potentials obtained by cyclic voltammetry (CV) measurements of
the Ni(II)/Ni(III) pair are a probe for the electronic properties of the
complexes (Table 1).
R1
O
O
R1
R1
O
O
R1
2
R1
R1
R1
R1
1=CH3, 88 %
1=H, 86 %
R1
R1
1=CH3, 78 %
1=H, quantitative
R
1=CF3, 89 %
2-CF3:
1-CH3
1-H:
:
2-CH3
:
R
R
2-H:
R
R
1=CF3, 86 %
1-CF3
:
R
Table 1: Forward peak potentials from cyclic voltammetry
measurements.
R1
R1
R3
R2
O
OH
R2
O
Me
[(tmeda)NiMe2],
pyridine
pyr
Ni
O
O
H
R3
#
complex
bridge
R1
E [mV]a
N
N
R1
O
R1
O
O
R1
R2
1
2
3
4
5
6
7
4-CH3-I2
4-H-I2
O
O
CH3
H
-98
-65
192
7b
n.d.c
283
189
R1
R1
R1
1=CH3, R2, R3=I, quantitative
1=H, R2, R3=I, 81 %
R3
1=CH3, R2, R3=I, 95 %
1=H, R2, R3=I, 92 %
3-CH3-I2:
4-CH3-I2:
R
R
3-H-I2:
4-H-I2:
R
R
1=CF3, R2, R3=I, 58 %
R
1=CF3, R2, R3=I, quantiative
3-CF3-I2:
4-CF3-I2:
R
4-CF3-I2
4-CH3-ant
4-H-ant
O
CF3
CH3
H
3-CH3-ant:
4-CH3-ant:
R
1=CH3, R2=9-anthryl, R3=H, 76 %
1=H, R2=9-anthryl, R3=H, 66 %
R
1=CH3, R2=9-anthryl, R3=H, 94 %
1=H, R2=9-anthryl, R3=H, 92 %
3-H-ant:
4-H-ant:
R
R
O
3-CF3-ant:
4-CF3-ant:
R
1=CF3, R2=9-anthryl, R3=H, 63 %
R
1=CF3, R2=9-anthryl, R3=H, 91 %
Scheme 2. Synthesis of oxygen bridged complexes.
O
4-CF3-ant
8-CH3-I2
O
CF3
CH3
CH2
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8
8-H-I2
8-CF3-I2
8-CH3-ant
8-H-ant
CH2
CH2
CH2
CH2
CH2
H
228
1
2
3
4
9
CF3
CH3
H
356
113
139
308
10
11
12
5
8-CF3-ant
CF3
6
7
8
9
aDetermined from cyclic voltammetry against [Cp*2Fe]/
[Cp*2Fe]+ (Cp*
= pentamethylcyclopentadienyl) in DCM,
NBu4PF6 as electrolyte and a sweep rate of 400 mV s-1, bno CV in
DCM possible due to composition, measured in THF, ccomplex in
both DCM and THF not stable enough for CV measurements.
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The forward peak potentials are comparable to structurally related
Ni(II) salicylaldiminato complexes bearing a N-terphenyl group.15
The oxidation is only partially reversible at a scan rate of 400
mV s-1. Even at a scan rate of 2000 mV s-1 the process is not fully
reversible, which indicates a highly instable Ni(III) species. The
forward peak potentials of the oxygen bridged complexes are lower
when compared to their methylene bridged counterparts (e.g E =
-98 mV for 4-CH3-I2 vs. E = 189 mV for 4-CH3-I2). This
difference is particularly pronounced for the complexes with diiodo
substitution on the salicyl ring. Additionally, the forward peak
potentials are highly dependent on the remote substituent R1: The
more electron-donating R1, the higher the electron density at the
Ni-atom (e.g. E = 192 mV for 4-CF3-I2 vs. E = -98 mV for 4-CH3-
I2). This effect can be seen for both the oxygen- and the methylene
bridged complexes and indicates a transfer of the electron density
of the remote substituents to the Ni-atom. Interestingly, the effect
of R1 is higher for the oxygen bridged complexes compared to the
methylene bridged complexes. This indicates a more efficient
transfer of the electron density to the Ni-atom. According to crystal
structures (see below) the lone electron pairs of the oxygen atoms
are located above and below the Ni-atom. The electron density of
the oxygen atoms is expected to be influenced by R1 and
consequently this could impact the forward peak potentials of the
Ni-atom via the lone electron pairs. This mechanism is not possible
for the methylene bridged complexes.
Figure 2: ORTEP (50% probability ellipsoids, hydrogens are
omitted for clarity) plots of structures determined by X-ray
diffraction of 4-CH3-ant (top) and 4-H-ant (bottom). The distance
between the oxygen atoms of the N-aryl moiety and the Ni-center
is given.
X-ray crystallography. Crystals suitable for x-ray crystallography
could be grown for four anthryl substituted complexes. For the
diiodo substituted complexes either no crystals were obtained or, in
case of one diiodo substituted complex 4-CF3-I2, yielded in a
decomposition product: [NiI2(pyr)2] (see SI). This agrees with a
generally limited stability of the oxygen bridged complexes with
diiodo substitution on the salicyl ring in solution. The
decomposition product is tetrahedral and therefore presumably
paramagnetic and not detectable in NMR experiments.
The Ni-atom is coordinated nearly square-planar with the Ni-Me
group coordinated trans to the oxygen atom of the
salicylaldiminato group. The oxygen atoms on the N-aryl ring (O2
and O3) are located above and below the coordination plane of the
metal center. The distance between the oxygen atoms and the Ni-
atom ranges between 3.49 Å and 3.91 Å. The closest distance is
larger compared to the sum of the van der Waals (vdW) radii
determined by Bondi (3.15 Å)23, but smaller than the vdW radii
determined by Batsanov (3.55 Å)24 or found by an empirical study
(3.9 Å)25. Within this range of given vdW radii, an interaction
between the oxygen atoms and the Ni atom might be possible,
especially as the lone electron pairs point in the direction of the Ni-
atom. These indications from the solid state structures of the
catalyst precursors are indeed corroborated by theoretical studies
of the intermediates of catalysis (vide supra).
Regarding the oxygen bridged complexes, crystals suitable for x-
ray crystallography were obtained for 4-CH3-ant and 4-H-ant by
layering a saturated solution of the complexe in benzene with
pentane (Figure 2).
For the methylene bridged complexes, solid state structures for
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8-CH3-ant and 8-CF3-ant were obtained (Figure 3).
Ethylene Polymerization Experiments
1
2
3
4
5
6
7
Oxygen bridged catalysts. All oxygen bridged catalysts 4 are active
towards ethylene in pressure reactor experiments (Table 2).
Productivities determined from the ethylene uptake amounts to
1.3 g and 17.6 g, depending on the catalyst and the polymerization
temperature. In general, the catalysts with anthryl substitution are
more stable, especially at higher temperatures, and therefore more
productive compared to the catalysts with diiodo substitution.
Products from butenes to oligomers with up to 1200 g mol-1 were
obtained. Catalysts with R1 = H or CH3 (4-CH3-I2, 4-H-I2, 4-CH3-
ant and 4-H-ant) oligomerize ethylene to low molecular weight
products from C4 to roughly C22 (Figure 4).
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Figure 4: GC-trace of the crude oligomerization mixture with 4-H-
ant at 50 °C, 40 bar ethylene, t = 30 min. The obtained gaseous C4
products are largely lost upon venting the reactor.
The volatile products were removed, and the remaining non volatile
products further analyzed. The oily products show no detectable
melting or glass transition point. The molecular weight determined
by NMR is roughly 300 g mol-1. The oligomers are moderately
branched (between 53 and 81 branches per 1000 C atoms), which
corresponds to roughly one branch per chain. The branching
increases with increasing polymerization temperature. The
branches are mainly methyl branches, but ethyl and and propyl
branches are also present (for a detailed microstructure analysis see
SI).
Figure 3: ORTEP (50% probability ellipsoids, hydrogens are
omitted for clarity) plots of structures determined by X-ray
diffraction of 8-CH3-ant (top) 8-CF3-ant (bottom).
The benzylic rings are located above and below the metal center.
The structure in solution was elucidated by 2D NOESY NMR
spectroscopy (see SI). This shows that the methylene group is also
located in spatial proximity to the Ni-Me group. The methylene
group is close enough to the Ni center to allow a Ni…CH2
interaction, whereas a Ni…aryl interaction is unlikely due to the
large distance between the Ni atom and the distal aryl rings.
Table 2: Oligomerization results with oxygen bridged catalyst precursors 4.
#
precatalyst
t
T
yield
(mass
flow)
[g]a
TON
[103]b
TOF
non-volatile products
[103]c
Bf
[min]
[°C]
yield [g]
Mn [g
Tm
mol-1]d
[°C]e
g
g
g
g
g
1
2
3
4
5
4-CH3-I2
4-CH3-I2
4-CH3-I2
4-H-I2
60
30
30
60
30
30
50
70
30
50
5.4
3.2
1.3
2.4
2.5
19.3
11.4
4.6
19.3
22.9
9.3
0.65
0.61
0.17
1.43
1.69
293
331
282
281
303
/
/
/
/
/
61
72
81
56
61
8.6
8.6
4-H-I2
8.9
17.9
4
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g
6
4-H-I2
4-CF3-I2
4-CF3-I2
4-CF3-I2
4-CH3-ant
4-CH3-ant
4-CH3-ant
4-H-ant
30
30
30
30
30
30
30
30
30
30
30
30
30
70
30
50
70
30
50
70
30
50
70
30
50
70
1.3
1.4
2.0
1.5
3.3
7.4
6.6
4.3
7.9
7.6
1.6
8.0
17.6
4.6
9.3
0.73
0.33
1.82
1.06
1.10
2.43
1.50
0.71
3.60
3.20
1.21
6.72
14.4
267
1022
810
896
386
275
290
338
321
321
1380
895
791
/
70
21
27
35
51
55
69
42
51
50
16
17
23
1
2
3
4
5
6
7
8
7
5.0
7.1
10.0
14.3
10.7
23.6
52.9
47.1
30.7
56.4
54.3
11.4
57.1
125.7
56, 88
56, 88
67
8
9
5.35
11.8
26.4
23.5
15.4.
28.2
27.1
5.7
g
10
11
12
13
14
15
16
17
18
/
g
/
g
/
g
/
9
g
4-H-ant
/
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
g
4-H-ant
/
4-CF3-ant
4-CF3-ant
4-CF3-ant
103
97
28.6
62.9
57, 90
Oligomerization conditions: 100 mL of toluene, 40 bar of ethylene, n(cat.) = 10 µmol, 1000 rpm. aethylene consumption determined by
mass flow. bin units of mol C2H4 (mol of Ni)-1from ethylene consumption as determined by mass flow. cin mol C2H4 x mol-1[Ni] x h-1 from
ethylene consumption as determined by mass flow. dDetermined by 1H-NMR spectroscopy comparing the integrals of the unsaturated end
groups with the integral of the backbone. eTm determined by DSC, second heating cycle. fBranches per 1000 C atoms, determined by 13C-
NMR spectroscopy (inverse gated decoupled). gNo melting or glass transition point detectable.
Catalysts with R1 = CF3 or H oligomerize ethylene to solid, waxy
products with a detectable melting point. The molecular weight lies
between 800 g mol-1 and 1200 g mol-1 and the oligomers are
moderately branched. The molecular weight decreases with
increasing temperature, whereas the branching increases with
increasing temperature. This correlation is expected, as with
increasing temperature, β-H elimination is increased, which is the
key step for both chain transfer and branching.
These findings indicate that chain walking and chain transfer is
rather frequent for the oxygen bridged catalysts 4, and especially
prominent for the catalyst with R1 = CH3 or H.
Methylene bridged catalysts. The complexes 8 are also highly
active in ethylene polymerization experiments but yield entirely
different products (Table 3). All catalysts produced polymers with
molecular weights between 32 kg mol-1 and 449 kg mol-1 and low
branching densities (between 1.0 branches and 11.5 branches per
1000 C atoms).
Table 3: Polymerization results with methylene bridged precatalysts 8.
#
pre-catalyst
n (cat)
[µmol]
T
yield
[g]
TON
[103]a
TOF
Mn [kg
mol-1]c
PDIc
Tm
Χ
[%]d
Be
[°C]d
[°C]
[103
h-1]b
1
2
8-CH3-I2
8-CH3-I2
8-H-I2
10
10
10
10
10
10
10
10
5
30
60
30
60
30
60
30
60
60
30
60
30
60
1.9
3.5
6.8
12.5
12.0
3.3
13.6
25.0
24.1
6.5
224
57
1.6
1.9
1.6
1.6
1.4
4.1
1.3
n.d.
1.5
1.3
1.6
1.3
1.6
126
116
127
116
133
124
134
117
122
134
125
133
129
53
51
58
54
59
60
52
56
53
53
52
57
57
3.6
11.5
4.1
15.3
2.1
6.5
1.4
n.d.
7.3
1.0
6.2
1.8
3.8
3
0.9
127
42
4
8-H-I2
3.4
5
8-CF3-I2
8-CF3-I2
8-CH3-ant
8-CH3-ant
8-CH3-ant
8-H-ant
1.8
6.4
12.9
10.2
48.6
75.7
90.0
55.7
95.7
27.9
111.4
192
32
6
1.4
5.1
7
8f
6.8
24.3
37.9
45.0
27.9
47.9
13.9
55.7
465
n. d.
123
449
131
307
161
10.6
6.3
9
10g
11
12
13
10
10
10
10
7.8
8-H-ant
13.4
3.9
8-CF3-ant
8-CF3-ant
15.6
Polymerization conditions: 100 mL of toluene, 40 bar of ethylene, t = 30 min, 1000 rpm. ain units of mol C2H4 (mol of Ni)-1. bin mol C2H4
c
x mol-1[Ni] x h-1. Determined by GPC versus linear polyethylene standards at 160 °C; PDI = Mw/Mn dCrystallinity determined by DSC,
second heating cycle. eBranches per 1000 C atoms, determined by 13C-NMR spectroscopy (inverse gated decoupled). fTemperature control
lost, temperature rose to 90 °C, stopped after 8.5 min, new experiment with half of the catalyst loading performed (line 9). gpressurized once
to 40 bar; pressure after 30 min: 38 bar.
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The microstructure of the polymers is determined by the reaction
isomer. The latter is formed from the aforementioned intermediate
1-Dat-C through displacement of the Ni∙∙∙O interaction by an
agostic one. From 1--C branch formation and chain transfer occur.
Notably, the Ni∙∙∙aryl interactions assisting ethylene decoordination
in N-Terphenyl-catalysts are considerably weaker, and unlike the
oxygen-coordinated 1-Dat-C the Ni∙∙∙aryl intermediate is high in
energy compared to the other species involved.13
1
2
3
4
5
6
7
8
temperatures, higher temperatures favoring branch formation.
Additionally catalysts with anthryl substitution on the
salicylaldiminato ring produce more linear polyethylene with a
higher molecular weight when compared to the catalysts with diodo
substitution. Other than for the O-bridged complexes 4, for the
methylene bridged catalysts no significant influence of the distal
aryl rings’ substituents (R1) are observed.
By comparison, for the reference system 8-CH3-I2 no such assisted
two-step pathway is available, neither an interaction of the
methylene bridge nor the distal aryl ring with Ni is available, and
1--C forms directly from 2-Coor-C with a corresponding energy
barrier.
R1
X = O
X = CH2
oligomerization
polymerization
limited impact
on molecular
weight
R1
9
X
Me
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
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40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Pyr
Ni
The key transition state for the olefin displacement with 4-CH3-I2
system is a five coordinated complex (Figure 7a) with the leaving
ethylene occupying an apical position and the oxygen coordinating
in the ligand plane (O…Ni distance 2.17 Å). The following
intermediate (Figure 7b) exhibits a planar quadrant coordination
geometry with the oxygen weekly coordinated to the metal at a
distance of 1.98 Å.
N
O
R1
X
R2
R1
R3
affects activity and stability
Figure 5: Summary of impact of catalyst structure on properties.
In summary, the catalytic properties (Figure 5) revealed a strong
propensity of the oxygen-bridged catalysts 4 for β-H elimination,
resulting in formation of low molecular weight branched products.
This strongly contrasts with the properties of the reference system
(8).
Density functional theory calculations.
To rationalize the effect of the bridge on the catalytic properties,
two exemplary catalysts (4-CH3-I2 and 8-CH3-I2) were studied by
DFT methods.
For Ni(II) salicylaldiminato complexes in general insertion chain
growth occurs from the alkyl olefin complex with the growing
chain cis to the salicylaldiminato oxygen. Alternatively, from this
species ethylene can be decoordinated to form a cis ß-agostic
complex providing an entry to further branching and chain transfer
pathways.2, 15, 26-27
Figure 7. Optimized structures of a) TS-2decoorDat and b) 1-Dat-C
for the system 4-CH3-I2. Distances are in Å. Carbon atoms of
monomer and growing chain are highlighted in green.
GTol
[kcal mol-1
]
TS-2decoor
TS-2ins
12.3
12.3
12.5
TS-2decoorDat
10.7
TScoord
10.0
TS-2openDat
9.3
It is worth noting that the alternative displacement of ethylene by
the η2-coordination of the distal aryl ring to the metal center
requires a higher energy barrier (of about 21 kcal mol-1, see SI).
10
5
-
9.2
2-Coor-C
5.0
+
H
N
O
-
Ni
In addition to this Ni∙∙∙O interaction, the bridge also increases the
flexibility of the ligand framework. This is reflected by low energy
barriers for the isomerization process (1-β-T to 1-β-C) for both
systems (12.5 and 10.7 for 8-CH3-I2 and 4-CH3-I2, respectively).
This flexibility releases steric pressure especially in the crowded
1--C
branch
1.2
0.8
formation
--T
1-Dat-C
-1.2
N
0
- 5
Ni
O
0.2
0.0
chain
0.0
transfer
N
X
Ni
O
H
X
N
O
Ni
8-CH3-I2 (X = CH2)
4-CH3-I2 (X = O)
five coordinated transition states TS-2decoorDat and TS-2decoor
.
N
Ni
H
- 10
- 15
O
Comparing the overall energetics of the key steps for both catalysts
8-CH3-I2 and 4-CH3-I2, the latter features the lowest
isomerization barrier (10.7 kcal mol-1, cf. TS-2decoorDat) due to the
oxygen assisted decoordination of ethylene. This renders formation
of the cis agostic complex, responsible for branch formation and
chain growth, competitive with chain growth. This agrees with the
experimentally observed formation of ethylene dimers and low
molecular weight oligomers, vs. high molecular weight linear
polymer with 8-CH3-I2.
--T
-13.3
linear chain growth
-15.0
Figure 6. Linear chain growth pathway and two potential pathways
for the decoordination of ethylene to form 1-β-C from 1-β-T. Free
energies (ΔGTol in kcal mol-1).
For the oxygen-bridged complex studied (4-CH3-I2),
decoordination of ethylene from the cis alkyl-olefin complex (2-
Coor-C) occurs through displacement of the olefin by the oxygen
lone pair to form the intermediate 1-Dat-C (Figure 6 and Figure 7,
for the full study cf. the SI). This intermediate is rather stable but
barely not yet an energetic sink impeding catalysis, being similar
in energy to the 1--T starting point and the corresponding 1--C
The ethylene decoordination rate determining transition state is 1.6
kcal mol-1 lower than the insertion transition state (see again Figure
6) with the oxygen-coordinated intermediate 1-Dat-C being only
1.2 kcal mol-1 more stable than 1--T and not blocking the active
site. These results agree with the experimental observation that
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ACS Catalysis
system 4-CH3-I2displays a high catalytic activity towards ethylene
but produces highly branched ethylene oligomers.
Financial support by the DFG (Me 1388/14-1) is gratefully
acknowledged. We thank Steffen Oßwald for assistance with the
CV measurements and Lars Bolk for DSC and GPC measurements.
1
2
3
CONCLUSION
REFERENCES
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4
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AUTHOR INFORMATION
Corresponding Author
*stefan.mecking@uni-konstanz.de
*lcaporaso@unisa.it
11. Radlauer, M. R.; Buckley, A. K.; Henling, L. M.; Agapie, T.,
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Ethylene by Dinickel Bisphenoxyiminato Catalysts. J. Am. Chem.
Soc. 2013, 135, 3784-3787.
Notes
The authors declare no competing financial interest.
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Olefin Homo- and Copolymerization: Relationships between
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ASSOCIATED CONTENT
Supporting Information
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Active and Robust Neutral Nickel(II) Complexes. Angew. Chem.
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Detailed experimental procedures, NMR spectra, crystallographic
data including CCDC numbers, complete polymerization
experiment information, analysis of the oligomerization
polymerizations and computational details (PDF).
The Supporting Information is available free of charge on the ACS
Publications website.
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(κ2-N,O)-Salicylaldiminato
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