Toolbox of Nonmetallocene Lanthanides: Multifunctional Catalysts in Group-Transfer Polymerization

Article abstract of DOI:10.1021/acs.inorgchem.7b01261

Herein, we present a fundamental study of isostructural 2-methoxyethylamino-bis(phenolate)-lanthanide complexes [(ONOO)^RM(X)(THF)] (M = Lu, Y; R = ^tBu, CMe₂Ph, X = CH₂TMS, collidine; THF = tetrahydrofuran; TMS =

Full text of DOI:10.1021/acs.inorgchem.7b01261

Article
pubs.acs.org/IC
Toolbox of Nonmetallocene Lanthanides: Multifunctional Catalysts in
Group-Transfer Polymerization
Friederike Adams, Martin R. Machat, § Peter T. Altenbuchner, Johannes Ehrmaier,§
†
‡
,‡
†,‡
†,‡
,†,‡
Alexander Po
̈
thig, Tolga N. V. Karsili, and Bernhard Rieger*
†
§
‡
WACKER-Lehrstuhl fu
Research Center, Technische Universitat
̈
r Makromolekulare Chemie, Chair of Theoretical Chemistry, and Department Chemie & Catalysis
̈
̈ ̈
Munchen, 85748 Garching bei Munchen, Germany
*
S Supporting Information
ABSTRACT: Herein, we present a fundamental study of isostructural
-methoxyethylamino-bis(phenolate)-lanthanide complexes
2
R
t
[
(ONOO) M(X)(THF)] (M = Lu, Y; R = Bu, CMe Ph, X =
2
CH TMS, collidine; THF = tetrahydrofuran; TMS = trimethylsilyl) for
2
rare-earth metal-mediated group-transfer polymerization (GTP). This
analysis includes the differentiation of electron-donating and non-
donating vinyl monomers and two metal centers with regard to the
ionic radius (yttrium and lutetium). In addition, highly nucleophilic
alkyl initiators are compared with electron-donating heteroaromatic
initiators. Our examinations include the impact of these parameters on
the activity, initiator efficiency, and tacticity of the obtained polymers.
Density functional theory calculations and proposed catalyst structure
determinations via X-ray analysis support these investigations. This
facilitates the selection of the best metal and initiator combination to
tBu
address efficient and stereospecific polymerization of a broad range of Michael monomers. [(ONOO) Lu(X)(THF)] shows the
−1
highest activity of 2220 h (normalized turnover frequency) for the polymerization of 2-vinylpyridine due to the higher Lewis-
3
acidity of lutetium. Through C(sp )−H bond activation, catalysts with higher initiator efficiency in N,N′-dimethylacrylamide
tBu
(
DMAA) and diethylvinylphosphonate polymerization were synthesized. Remarkably, [(ONOO) Y(collidine)(THF)] was
capable of stereospecifically polymerizing DMAA to highly isotactic poly(DMAA) (P = 0.94). Overall, the kinetics studies reveal
m
a living-type GTP mechanism for all of the tested catalysts, enabling precise molecular-weight predeterminations with narrow
molecular weight distributions (Đ ≤ 1.06).
INTRODUCTION
■
One of the core challenges in polymer synthesis is the creation
of highly complex and well-defined structures from simple
molecules. To synthesize polymers for high-tech applications, it
is necessary to accurately influence the mechanical and thermal
properties via changes in the microstructure or molecular
weight. Therefore, the availability of catalysts for the precise
synthesis of these polymers is an essential requirement. The
living character and high activity of rare-earth metal-mediated
group-transfer polymerization (REM-GTP) are two advantages
of this polymerization method. Beyond that, molecular weights
are precisely controllable, and narrow molecular weight
distributions are observed. Currently, different types of
Michael-type polar vinyl monomers have been homopolymer-
1
−8
Figure 1. REM-GTP of Michael-type oxygen-, phosphorus-, and
nitrogen-containing vinyl monomers.
ized and copolymerized via REM-GTP (Figure 1).
The
possibility of using different phosphorus and nitrogen-
containing polar vinyl monomers leads to different function-
alities, which enables access to various fields for applica-
good activities for various monomers without inducing
13,14
1
,9−12
tacticity.
Recently, two different approaches toward highly
tion.
REM-GTP with a symmetric 2-aminoalkoxy-bis(phenolate)-
yttrium trimethylsilylmethyl complex (1a) is one of the first
examples of a nonmetallocene system. This catalyst showed
Received: May 19, 2017
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b01261
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Article
stereospecific polymerization of prochiral 2-vinylpyridine
are not possible with trivalent lanthanides. This σ-bond
metathesis was first shown by Watson et al. in 1983 via the
activation of isotopically labeled methane, benzene, and
(
2VP) with nonmetallocene lanthanides have been established
6,15,16
by the groups of Rieger and Lu.
As lanthanocenes show
27,28
insufficient polymerization activity and stereospecificity for
pyridine with a cyclopentadienyl-lutetium-methyl complex.
various monomers, the scope of bis(phenolate)lanthanide
11,13
complexes is further broadened by this contribution.
A
RESULTS AND DISCUSSION
■
systematic study on the activity, initiator efficiency, and
stereospecificity (with reference to three different parameters)
is presented herein. The experimental investigations are
substantiated with detailed density functional theory (DFT)
calculations and proposed catalyst structure determinations via
X-ray analysis. For the first parameter, a distinction was made
between electron-donating and nonelectron-donating mono-
mers. For the second parameter, changes in the metal center
Synthesis and Characterization of Alkyllanthanide
Complexes. The symmetrically substituted 2-methoxyethyla-
mino-bis(phenolate) ligand (L1) was synthesized via a
modified Mannich reaction of 2,3-di-tert-butylphenol with 2-
methoxyethylamine in a formaldehyde solution. The
asymmetric ligand (L2) was only accessible through a multistep
reaction. The final nucleophilic substitution reaction of the
cumyl-substituted 2-methoxyethylaminophenolate with the tert-
butyl-substituted phenol-methylenebromide led to the desired
2
9
(from yttrium to lutetium) were performed with regard to the
15
influence of the ionic radius on the polymerization. The last
parameter was generated by the introduction of a new
heteroaromatic electron-donating initiator. This initiator was
then compared with highly basic alkyl initiators. Two different
types of initiators were investigated, as different initiation
mechanisms can occur. Strongly basic alkyl initiators can be
inefficient if they favor deprotonation of an acidic α-CH and do
not initiate polymerization via nucleophilic attack (see Figure
ligand in moderate yield (see Supporting Information).
R
For the synthesis of (ONOO) Ln(CH TMS)(THF) (1a−
2
3a) (Ln = Y, Lu; TMS = trimethylsilyl; THF = tetrahydrofur-
an) complexes, 1 equiv of the respective ligand was reacted with
1 equiv of the precursor, Ln(CH TMS) (THF) , in a mixture
2
3
3
of toluene/pentane at 0 °C and stirred overnight at room
temperature (Supporting Information). As yttrium complexes
1a and 3a were isolated in moderate yields (43 to 47%),
lutetium analogue 2a was synthesized in good yields of up to
75%. The monomeric structure of every complex was verified
1
3,17
2).
To eliminate side reactions and the slow deprotonation-
mediated initiation, heteroaromatic initiators can be used
instead.
1
15,30
via H NMR spectroscopy.
C−H Bond Activation. Initially, we tested the reactivity of
complexes 1a−3a toward sym-collidine (Scheme 1). Time-
1
resolved H NMR experiments of the reaction were performed
at room and elevated temperatures (60 and 80 °C, respectively)
to investigate the tendency to activate sym-collidine. These
experiments revealed strongly varying ambitions for the
catalysts to undergo C−H bond activation (Figure 2 and
Supporting Information, Figures S1−S3).
For all complexes, only very slow activation was observed at
room temperature. Increased temperatures led to significant
changes in the reactivity, and catalyst 1a was transformed to
complex 1b in the presence of 1 equiv of 2,4,6-trimethylpyr-
1
idine within 4 h at 60 °C (see Figure 3). H NMR examinations
Figure 2. Initiation pathways for REM-GTP of DAVP via
deprotonation (A) or nucleophilic transfer (B).
revealed that after the addition of sym-collidine (Figure 3,
spectrum 2: 6.77, 2.47, and 2.24 ppm), the signals for the
CH TMS initiator of complex 1a decreased (comparison with
spectrum 1). The concurrent increase of the tetramethylsilane
2
To introduce these heteroaromatic initiators, C−H bond
1
2,18−26
activation can be utilized.
Lanthanide complexes are
signal (SiMe : 0 ppm) indicated the ongoing metathesis
4
able to activate compounds such as 2,4,6-trimethylpyridine
reaction. In the same manner, two new signals arose in the
aromatic region, representing the binding of collidine. Two
signals in the aliphatic region show the two new methyl groups
(spectrum 4). The reaction was straightforward, and no side
(
sym-collidine) via σ-bond metathesis. This type of C−H bond
activation is a common way to access C−H bond cleavage,
because oxidative addition and reductive elimination reactions
Scheme 1. C−H Bond Activation of Catalysts 1a−3a with sym-Collidine in Toluene to Obtain Complexes 1b−3b
B
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1
Figure 3. H NMR kinetic experiments for the C−H bond activation of sym-collidine with complex 1a in C D . (1) Complex 1a. (2) Immediately
6
6
after addition of sym-collidine. (3) 40 min; 60 °C. (4) 4 h; 60 °C. (5) Complex 1b.
reactions occurred, which enabled us to scale up the catalyst
synthesis of 1b in toluene. Purification was performed via
recrystallization from pentane at −30 °C (spectrum 5). The
interaction between the metal center and the initiator via an
(1b−3b) (Scheme 1). All C−H bond-activated compounds
were characterized via NMR spectroscopy and elemental
analysis. In the case of complex 1b, X-ray diffraction analysis
was used to analyze the proposed complex structure.
1
13
activated CH22 group was verified by H- and C NMR
Structure Analysis. Crystals of complex 1b were grown
through recrystallization from pentane at −30 °C. Against all
expectations, and in contrast to the NMR spectra, the complex
crystallized as two independent molecules without tetrahy-
drofuran as a coordinating solvent (see Figure S4, Supporting
Information). As a consequence, the yttrium is coordinated in a
sevenfold manner and not in the generally favored octahedral
coordination sphere. Nevertheless, the 2-methoxy-bis-
(phenolate) ligand coordinated in the same way as in complex
spectroscopy. J-Coupling (J = 2.9 Hz) of the CH signal at
2
1
2
.80 ppm in the H NMR spectrum (spin = 1/2, leading to a
splitting as a doublet) and a splitting of the carbon atom at 52.5
1
13
ppm ( J = 6.0 Hz) in the C NMR spectra underscored the
successful C−H bond activation.
Compound 2a was more reluctant in its reactivity toward
sym-collidine; nevertheless, quantitative C−H bond activation
could be achieved after 9 d at 80 °C. As the 4f electrons have
no contribution to the binding valence orbitals, lutetium can be
seen as an isoelectronic equivalent to yttrium. Therefore, the σ-
bond metathesis was investigated to determine the influence of
the effective ionic radius. Lutetium is more Lewis-acidic/
electrophilic owing to its higher effective nuclear charge, leading
32
1a. The solid-state structure of one independent molecule of
complex 1b is shown in Figure 4. The three Y−O bonds are
equatorial, whereas the nitrogen is ordered axial to the yttrium
center. Owing to the missing tetrahydrofuran, which
coordinates equatorial to the yttrium in 1a, the bond angles
between the yttrium and the oxygen atoms differ in these two
compounds and are larger in 1b. Fundamental for the
performed C−H bond activation is the coordination of the
(4,6-dimethylpyridine-2-yl)-methyl initiator to the metal center.
31
to a stronger binding of the nucleophilic CH TMS initiator.
2
This led to a prolonged reaction time for C−H bond activation.
These observations and suggestions were in accordance with
hard-soft acid-base (HSAB) theory. Complex 2b was isolated in
good yields via recrystallization in pentane. No formation of
side products was observed during the activation with
compound 1a and 2a, which highlighted the remarkable
stability of the catalysts. Only complex 3a slowly decomposed
at elevated temperatures. Hence, complex 3b was generated
through the reaction of 3a with sym-collidine at room
temperature over 6 d in moderate yield. In all reactions, the
Lu complexes were generated in higher yields than the Y
analogues due to the higher insolubility in pentane. For all C−
H bond-activated compounds, recrystallization in pentane
The Y−N bond is shorter than the Y−C bond; thus, the
α
initiator is primarily coordinated via the Y−N moiety.
The C−C bond lengths in the aromatic ring were all similar
and lay between double- and single-bond lengths (1.362[1.364]
to 1.423[1.424] Å; Figure 4), as might be expected for aromatic
systems (bond lengths for the second independent molecule
3
are given in parentheses). Remarkably, the C(sp )−H activated
methyl−methyl bond (C −C
) had a reduced length of
3
4,α
35,ipso
1.400[1.399] Å. Consequently, double-bond character was
predominant, and the activated methyl group was conjugated in
the aromatic system. One C−C bond of the heteroaromatic
afforded the desired complexes with the general structure
tBu,R
(
ONOO)
Ln((4,6-dimethylpyridin-2-yl)methyl))(THF)
system (1.423[1.424] Å, Figure 4, C3 −C
) was even
36,ortho
5,ipso
C
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delocalization and allylic binding motif (see Figure S5 and
Table S3, Supporting Information). Whereas the benzyl ligand
in [Y(CH Ph) (THF) ] was primary bond via the Y−C , the
2
3
3
α
collidine ligand in 1b is coordinated via Y−N binding. The
longer Ln−C bond of 1b and the shorter C −C bond
α
α
ipso
(
higher double bond character) indicated a more pronounced
multihapto binding due to the higher charge delocalization (see
3
Table 1). The η -allylic binding in 1b was also confirmed by the
higher phenyl tilt (80.12° vs 117.17°) and the shorter Ln−C
ipso
bond. The same parameters can be used to compare the
collidine structures regarding the impact of the metal size. As
we were not able to isolate crystals of 2b, we used DFT
calculations instead. Harder et al. stated that within
benzyllanthanide(III) complexes, some parameters do not
33
vary significantly along the lanthanide series. Nevertheless,
we compared the DFT-calculated structures fully aware that
only slight trends might be observed. Overall, these calculations
predicted a higher end-on coordination in place of a side-on
coordination, as found in the solid-state structure of 1b (see
Table 1). Less allylic bindings were calculated for both metals,
Figure 4. ORTEP-style representation of 1b. Hydrogen atoms and the
second independent molecule are omitted for clarity. Ellipsoids with
as indicated by the longer Ln−C bonds and the broader
ipso
5
0% probability. Selected bond lengths (Å) and angles (deg): Y(1)−
C
−C −Ln angles. Within these calculations, the same trends
ipso
α
O(1), 2.1130(15); Y(1)−O(2), 2.1318(16); Y(1)−O(3), 2.3641(16);
Y(1)−N(1), 2.5385(19); Y(1)−N(2), 2.3950(19); Y(1)−C(34),
as in the series of different benzyl lanthanides were
33−35
observed.
Because of the higher Lewis acidity of lutetium,
2
.608(2); Y(1)−C(35), 2.740(2); N(2)−C(35), 1.386(3); N(2)−
a higher charge localization is provoked. The C −C bond
α
ipso
C(39), 1.361(3); C(34)−C(35), 1.400(4); C(35)−C(36), 1.423(3);
C(36)−C(37), 1.362(4); C(37)−C(38), 1.413(4); C(38)−C(39),
elongates on account of weaker charge delocalization. As
lutetium polarizes the negative charge more than yttrium, the
1
.366(3); C(39)−C(41), 1.493(3); C(37)−C(40), 1.508(4); O(1)−
Y(1)−O(2), 119.74(6); O(1)−Y(1)−O(3), 123.02(6); O(2)−Y(1)−
O(3), 99.09(6); N(1)−Y(1)−N(2), 167.93(6); O(1)−Y(1)−C(34),
allylic binding motif becomes weaker. As a result, the Ln−C
ipso
bond was longer, and the C −C −Ln angle was broader. The
ipso
α
9
1.17(7); N(2)−Y(1)−C(34), 55.94(7).
less distinct multihapto binding with decreasing ion size is also
shown by the Ln−C bond shortening.
α
Group-Transfer Polymerization. The isolated catalysts
slightly longer than the activated methyl group, indicating the
high charge delocalization of the latter group. The tilt of the
phenyl ring was calculated with the help of the C −Y−C
(
1a−3b) were employed in the GTP of vinyl monomers
diethylvinylphosphonate (DEVP), N,N′-dimethylacrylamide
DMAA), and 2VP to evaluate the general activities, initiator
efficiencies, and the microstructures of the isolated polymers.
Poly-2-vinylpyridine (P2VP) is a polymer for high-performance
applications due to its pH-dependent solubility. Several
examples of block copolymers containing hydrophobic 2VP
α
ipso
(
angle. The value of 80.12° and a relatively short Y−C bond
ipso
(
2.740[2.715] Å) indicated binding of the collidine occurred via
3
an allylic motif. From these findings, the η -(C,C,N)-aza-allylic
binding to the yttrium center was evident. This coordination
2
1
type is an intermediate between an η -alkyl-amine and an η -
amido-olefin coordination, as Teuben and co-workers inves-
are known in the literature where micelle formation occurs due
20
36−43
tigated in bis(alkoxysilylamido)-yttrium-pyridyl complexes.
to their amphiphilic character.
Therefore, our attention
The two other methyl groups possessed single-bond character
with bond lengths of 1.493[1.496] and 1.508[1.502] Å;
therefore, they were not integrated into the conjugated system.
As the deprotonated collidine species can be seen as a
benzyl-type ligand, 1b was compared to the literature-known
complex [Y(CH Ph) (THF) ] with respect to the charge
was directed to enhancements in 2VP polymerization. First, the
initiation mechanisms of the C−H bond-activated complexes
were elucidated by end-group analysis of oligomeric 2VP. This
was produced by reacting catalyst 1b with 10 equiv of 2VP and
monitoring the reaction via NMR spectroscopy. Electrospray
ionization mass spectrometry (ESI-MS) analysis of the
2
3
3
a
Table 1. REM-GTP Polymerization Results of Catalyst 1a−3b of 2VP
4
b
4
c
d
TOF* [h−1]
e
f
entry
[cat]
time [min]
conversion [%]
Mn,calc (1 × 10 ) [g/mol]
Mn,exp (1 × 10 ) [g/mol]
Đ
I
Pm
1
3
1
2
3
4
5
6
1a
2a
3a
1b
2b
3b
90
130
120
60
99
99
99
99
99
99
2.1
2.3
2.2
2.0
2.0
2.0
2.3
7.4
3.9
4.1
4.9
4.7
1.01
1.06
1.00
1.01
1.06
1.06
0.99
0.30
0.71
0.42
0.43
0.42
1110
1750
440
0.55
0.56
0.57
0.54
0.54
0.56
1
5
1080
2220
470
45
180
a
1
Reactions performed with [2VP] = 2.7 mmol, [2VP]/[Cat] = 200/1, at 25 °C in 2 mL of solvent, conversions determined by H NMR
b
c
spectroscopy and Mn,expd determined by GPC-MALS. Mn,calc from Mn,calc = M × (([M]/[Cat]) × conversion). Polydispersity calculated from
Mw,expd/M
centers. P is the probability of meso linkages between monomer units and was determined by C NMR spectroscopy of the quaternary carbon
atom (Supporting Information).
d
e
as determined by GPC-MALS. I = Mn,calc/Mn,exp at the end of the reaction. TOF* = TOF/I; normalized TOF for the active metal
n,expd
f
13
m
15,44
D
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Figure 5. (left) Catalytic activity of catalysts 1a/b and 2a/b (Y = line; Lu = dashed; CH TMS = black; sym-collidine (coll) = gray; catalyst: 135
2
μmol, 2VP: 27 mmol, toluene: 20 mL, T = 25 °C) measured via the aliquot method. (right) Linear growth of the absolute molecular weight (Mn)
(
determined by GPC-MALS) as a function of monomer conversion (gravimetrically determined).
−
1
respective oligomers showed signals corresponding to n × M2
most active catalysts (TOF* = 1750−2220 h ) in the
polymerization of 2VP while maintaining accurate control
over the molecular masses (Đ = 1.06). Owing to the higher
Lewis-acidity of lutetium, the stronger polarization of the
coordinating monomer in the propagation mechanism may be
decisive for the higher activity. The stronger binding of the
VP
+
Mcollidine with either H+ or Na+ as the charge carrier
Supporting Information, Figure S6). The initiating groups
(
were clearly visible in the ESI-MS; a nucleophilic transfer
reaction of the initiator via monomer insertion into a Y−C
bond is apparent (Figure 2). The evidence of a nucleophilic
transfer reaction for the initiation of 2VP polymerization with
CH TMS initiator in compound 2a (vide supra) was reflected
2
13
1
a has already been reported in a previous study. On the basis
in the initiator efficiency, which was lower in comparison to the
respective yttrium compounds. In contrast to the yttrium
complexes, the change in initiator from 2a to 2b was a suitable
approach for improving the initiator efficiency and led to an
increase of ∼45%. As the tacticity of P2VP is influenced by an
enantiomorphic site control mechanism, the change of metal
of the observed mechanism, these findings were further
elucidated via DFT calculations (vide infra).
To determine conversions and kinetic parameters of the
polymerization, aliquots were taken at regular time intervals
during polymerization. The conversion was gravimetrically
calculated. Absolute molecular masses and mass distributions
were measured by gel-permeation chromatography multiangle
light scattering (GPC-MALS). The polymerization of 2VP with
all 2-methoxyethylamino-bis(phenolate) catalysts proceed in a
living fashion, as indicated by the narrow polydispersities (1.00
15
has no effect on the tacticity.
Recently, we reported on nontoxic phosphorus-containing
poly(dialkylvinylphosphonate)s (DAVPs) in combination with
P2VP as advanced micellar systems for drug-release applica-
26,45
tions.
On the basis of these results, our study also included
≤
Đ ≤ 1.06) and good agreement between the experimentally
the detailed investigation of DEVP polymerization (alkyl chain
= ethyl) with the above-mentioned catalysts. Complexes 1a−
3a, with an alkyl initiator, all showed relatively low initiator
efficiency in the polymerization of DEVP (Supporting
Information, Table S4). In the case of DEVP, basic alkyl
initiators are inefficient due to deprotonation of the acidic α-
CH of the first monomer (see Figure 2). Especially, complex 2a
demonstrates a low initiator efficiency, leading to a time-shifted
and slower initiation, resulting in an increase of the molecular
mass distribution (Đ = 1.48−1.50). Initiation via the collidine
initiator led to an increase in efficiency up to 80%. The
polydispersity (Đ = 1.03−1.14) of the polymers was improved
as slow initiation via deprotonation was eliminated. In contrast
to 2VP polymerization, the two metal species showed similar
tendencies when the initiator was changed. In addition, the
activity and initiator efficiency of the lutetium species seemed
to be lower than for the yttrium catalysts.
determined and theoretically expected M values (Table 1).
n
The plots of conversion against the absolute molecular mass
reveal a linear relationship between Mn and conversion,
underlining the living fashion of the polymerization (Figure 5
and Supporting Information, Figures S9, S10, S12, and S13). If
the conversion is plotted against time, the polymerizations can
be compared with regard to the activity of the different
catalysts. Catalysts 1a and 3a were tested in the GTP of 2VP in
a previous contribution from our group, and we had already
shown that the increased steric demand of the bis(phenolate)
ligand led to a decrease in catalyst activity and initiator
efficiency (Table 1, Entries 1 and 3). In both cases, only atactic
P2VP was obtained. As expected, changing the initiator to
sym-collidine did not impact the normalized turnover
frequencies (TOF*), since the activity is solely influenced by
the metal center and the ligand, which are both involved in the
propagation mechanism (Table 1, Entries 4 and 6).
15
Poly(DMAA) (PDMAA) is of special interest regarding the
The introduction of the collidine initiator led to a decrease in
initiator efficiency of the complexes from 99% (1a)/71% (3a)
to 42% (1b/3b); therefore, no improvement was found for the
yttrium-based bis(phenolate)-mediated 2VP polymerization.
Remarkably, the change of the metal center to lutetium
increased the catalytic activity; complexes 2a and 2b were the
application as stimuli-responsive materials (micelles, hydrogels,
46−51
and nanocomposites).
The polymerization of DMAA was
investigated with regard to catalyst activity and the micro-
structure of PDMAA. Because of the exothermic nature of the
polymerization, the reaction was performed at low temper-
atures (−78, −50, and −20 °C). When refrigerated monomer
E
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a
Table 2. REM-GTP Polymerization Results of Catalyst 1a−3a and 1b for DMAA
b
c
e
4
4
f
g
entry
[cat]
TAdd [°C]
TPolym [°C]
conversion [%]
Mn,calc (1 × 10 ) [g/mol]
Mn,exp (1 × 10 ) [g/mol]
Đ
I
Pm
1
2
3
4
5
1a
1a
1a
1a
2a
2a
2a
2a
3a
1b
−78
−50
−20
0
−50
−50
−20
0
99
99
99
99
99
94
99
99
99
99
1.9
2.1
1.9
2.0
2.1
1.9
2.0
2.0
2.3
2.0
18.4
24.4
8.5
1.42
1.49
1.66
2.12
1.17
1.34
1.69
2.22
1.30
1.06
0.10
0.09
0.22
0.47
0.04
0.04
0.11
0.26
0.15
0.30
0.91
0.91
0.89
0.83
0.56
0.56
4.3
−78
−78
−15
0
−20
−20
−15
0
57.7
48.3
18.8
7.5
d
6
7
8
9
−78
−78
−50
−78
15.1
6.6
0.90
10
0.94
1
a
Reactions performed with [DMAA] = 2.15 mmol (−35 °C), [DMAA]/[Cat] = 200/1, in 4.5 g of dichloromethane, conversions determined by H
NMR spectroscopy, and M
the reaction mixture at addition of monomer. Estimated temperature when polymerization occurred; observed through the increase in viscosity and
end of stirring. Quenching after 3 min; first time for the nonquantitative conversion of the slowest catalyst. M
cat]) × conversion). I = M /Mn,exp at the end of the reaction. P is the probability of meso-linkages between monomer units and was
determined by high-temperature H NMR spectroscopy in DMSO-d at 140 °C.
b
determined by GPC-MALS. Several attempts to polymerize DMAA with catalysts 2a and 3a failed. Temperature of
c
n,expd
d
e
from Mn,calc = M × (([DMAA]/
n,calc
f
g
[
n,calc
m
1
6
Figure 6. (A) Carbonyl region of the 13C NMR spectra (126 MHz, CDCl , 25 °C) of PDMAA prepared with catalyst 1b. (B) Backbone methylene
3
1
52
signal in the H NMR spectra (300 MHz, deuterated dimethyl sulfoxide (DMSO-d ), 140 °C) of PDMAA prepared with catalyst 1b. (C) Carbonyl
6
region of the 13C NMR spectra (126 MHz, CDCl , 25 °C) of PDMAA prepared with catalyst 2a. (D) Backbone methylene signal in the H NMR
1
3
5
2
spectra (300 MHz, DMSO-d , 140 °C) of PDMAA prepared with catalyst 2a.
6
was added to the reaction mixture at a certain temperature, and
no polymerization initiation was observed, the mixture was
slowly warmed. The end of polymerization was apparent
through the increased viscosity and the immediate end of
stirring. Kinetic measurements were not possible. At 0 °C the
reaction was not controllable. The alkyl-yttrium complexes 1a
and 3a showed no activity at −78 °C; warming to −50 °C led
to an immediate reaction but with low initiator efficiencies
lutetium (complex 2a) was expressed by a very low initiator
efficiency, since the polymerization did not start until warming
to −20 °C. As a consequence, it was possible to quench the
polymerization before quantitative conversion. A conversion of
94% in 3 min for complex 2a showed the high activity of this
catalyst and all of the other tested catalysts. In comparison to
previous studies on coordinative-DMAA polymerization, these
52−54
bis(phenolate)lanthanides were highly active.
We searched
(
Table 2, entries 1 and 9). Because of a delayed and
for initiators that were also active at −78 °C to overcome the
problem of high molecular mass distributions and the low
initiator efficiency for the yttrium complexes. The introduction
of sym-collidine led to an improvement for DMAA polymer-
ization (Table 2, entry 10) since the initiator efficiency was
improved, facilitating a polymerization at −78 °C. Because of
this fact, a very low polydispersity of 1.06 was observed,
demonstrating the high suitability of complex 1b for the
polymerization of DMAA.
spontaneous reaction, as well as the rapidly rising viscosity of
the reaction mixture, the molecular mass distributions were
higher than expected for a living-type polymerization
mechanism (Đ ≤ 1.49). Reactions were also performed at
higher temperatures to investigate the influence of temperature
on initiator efficiency and tacticity. These polymerizations
indeed demonstrated a higher initiator efficiency, as more
catalytic centers were active at higher temperatures; however, a
higher molecular mass distribution, up to 2.22 (Table 2, entries
The polymerization of DMAA was also studied in regard to
the resulting microstructures. Figure 6 depicts the carbonyl
region of the 13C NMR spectra and the backbone methylene
3
, 4, 7, and 8), was found. Polymerizations that were gradually
warmed from −78 °C, in comparison with those performed
directly at elevated temperatures, showed similar results, which
attests the reproducibility of the results (Table 2, entries 1 and
1
signal in the high-temperature (HT) H NMR spectra of
PDMAA prepared with catalysts 1b and 2a. Assignment of the
m- and r-dyad of the methylene proton was performed
2
). The stronger binding of the alkyl initiator to electrophilic
F
DOI: 10.1021/acs.inorgchem.7b01261
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 7. Reaction path for the initiation process: catalyst 1a (black) and 1b (red) with 2VP and DMAA.
analogously to the method of Kakuchi et al. For the triad
distribution of the carbonyl signal, assignment was performed
ability of those systems on the additional vinyl monomers,
53,54
beyond acrylamides, was not investigated.
In comparison,
52,55
analogously to the method of San Roman
higher initiator efficiencies were found for the heteroaromatic
compound 1b; therefore, polymerization was possible at −78
́
et al.
For DMAA,
catalyst 1b is a suitable generalist for DMAA polymerizations.
This catalyst combines high activity and increased initiator
efficiency with a low polydispersity and high polymer
isotacticity.
°C. This was reflected by the increasing percentage for meso-
linkages in the polymer. Catalyst 1b produced PDMAA with
Density Functional Theory Calculations on the
Initiation Process. DFT calculations at the B3LYP level of
theory, coupled with the LANL2DZ basis set, were used to
uncover the mechanistic proceedings during the initiation
process of GTP with 2VP (electron-donating monomer) and
DMAA (nonelectron-donating monomer). These calculations
were performed to assess the feasibility of the initiation reaction
for the two different initiators (alkyl initiator (1a) vs collidine
(1b)). Gas-phase geometry optimization of 1a, 1b, DMAA, and
2VP gave the starting-point geometries for the ensuing reaction
path (Figure 7) and reference energies. Then, the monomers
were chelated to the isolated complexes (THF served as the
exchange molecule) as the first step of initiation, resulting in
the highest isotacticity (P = 0.94). This value was determined
m
1
as the proportion of m-dyad in the methylene region of the H
NMR spectrum. Complexes 1a and 3a, which were active at
−
50 °C, both produced PDMAA with slightly lower isotacticity
Pm = 0.91 and 0.90, respectively). The slight increase in steric
(
demand of ligand L2 in 3a did not affect the tacticity. On
account of this, it was obvious that the lutetium complex (2a),
which was only active at −20 °C, would produce PDMAA with
a lower tacticity. Figure 6 shows that only atactic polymer was
obtained. To analyze if this decrease in tacticity was only caused
by the increase in temperature, polymerizations with 1a were
performed at −20 and 0 °C. The temperature rise led, in fact,
to a decrease in isotacticity, but the catalyst still produced
isotactic PDMAA (Pm = 0.89). Remarkably, not only the
temperature but also the metal center has an impact on the
tacticity. Because of these findings, different mechanisms for
determining the stereoselectivity were expected for polymer-
izations with 2VP (enantiomorphic site control) and DMAA
13
structures I −I . The chelated structures were optimized
a
d
again to obtain the respective coordination energies. Finally, the
products II −II generated via a nucleophilic transfer reaction
a
d
were calculated to gain insights into the relative energies of the
product with respect to the reactants and the isolated systems.
The electronic energies associated with the reaction path are
depicted in Figure 7 and refer to the minimum-energy
geometry of the isolated monomers and catalysts. The stepwise
profiles of the initiation process reaction sequence for DMAA
and 2VP are depicted in black for catalyst 1a and red for
catalyst 1b. It is evident for DMAA that initiation with either
(chain-end control).
Radical and anionic polymerizations are only able to produce
PDMAA with moderate isotacticity ([mm] = 54−81). Chiral
ansa-zirconocenium catalysts, developed by Chen et al.,
exclusively produce isotactic PDMAA; however, the applic-
G
DOI: 10.1021/acs.inorgchem.7b01261
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 8. Optimized structures IIa−d after nucleophilic attack of the initiator to the monomer (light blue = yttrium; red = oxygen; purple = nitrogen;
yellow = silicon; gray = carbon).
catalyst leads to a net favorable driving force for nucleophilic
completely detached from the metal center (Figure 8). As
known, 2VP is a strong electron donor, and when used in
tandem with an electron-donating molecule like the pyridine
moiety in 1b, an overload of electron density to the metal
center is provoked. This overload in electron density weakens
the binding energy of the collidine moiety, which explains the
detachment from yttrium in the optimized geometry. The
transfer (II = −84 kJ/mol; II = −139 kJ/mol) confirming our
c
d
experimental observations. Both catalysts were able to initiate
the polymerization, and cooling was necessary to avoid thermal
desorption and to ensure isothermal conditions. Whereas both
catalysts show a thermodynamically favored process, the
calculations indicate that the initial coordination step, as well
as the nucleophilic transfer reaction, are energetically more
preferred using collidine as an initiator (catalyst 1b). This was
geometry I of the initial coordination step already confirmed
b
dissociation of one electron-donating molecule (Figure 9).
manifested in lower-energy products Id and II . These
d
computations were in good agreement with the experimental
findings. The experiments of DMAA with catalyst 1a could be
performed at −50 °C, whereas the reactions with catalyst 1b
had to be cooled to −78 °C for a controlled reaction due to the
significantly stronger exothermic nature of the reaction.
Attempts at raising the temperatures led to the evaporation
of the solvent, as the heat dissipation was not sufficient.
Polymerizations with 2VP were conducted at room temper-
ature, since the reaction was less exothermic. Again, this was
supported by the calculations, which indicated a slightly
endothermic process, which was associated with the coordina-
tion step (I = +11 kJ/mol; I = +16 kJ/mol). Consequently,
a
b
Figure 9. Geometry of I (light blue = yttrium; red = oxygen; purple =
nitrogen; gray = carbon).
b
13
reactions at −78 °C were not successful. The calculated
energy differences between the initiation of 2VP with 1a and 1b
show only small differences. Contrary to DMAA initiation, 1a
seems to be slightly energetically favored for initiating 2VP
polymerization (II = −21 kJ/mol; II = −6 kJ/mol). Again,
Because of the coordination of the electron-donating
collidine initiator, coordination of the 2VP via the nitrogen
atom is disfavored. This led to the detachment of the monomer
from yttrium. It appears that high electron density/charge
localization on the nucleophilic carbon atom of the initiator
(predominant in alkyl initiators) is essential for nucleophilic
attack of barely coordinated 2VP. The combination of an
electron-donating nitrogen atom and reduced electron density
on the activated CH2 group of collidine (due to charge
delocalization) was not sufficient for efficient nucleophilic
transfer to 2VP. These assumptions were in accordance with
the experimental findings, as initiation with collidine was less
efficient than initiation via an alkyl species.
We were interested in the initiator efficiencies of 1a and 1b
in IPOx polymerization (Supporting Information, Table S5) to
verify our examinations on the validity for all nitrogen-
coordinating Michael monomers. The results underline our
postulation, as they were very similar to 2VP polymerization
results. High initiator efficiency was observed for alkyl initiators
(I = 0.93), whereas less-efficient initiation was predominant
with the heteroaromatic collidine initiator (I = 0.47).
a
b
this is in accordance with the experimental results, in which 1a
shows a higher initiator efficiency. We were able to calculate
two transition states for initiation with complexes 1a and 1b
(
I →II and I →II ). In both cases, the transition states were
b b c c
∼
1 eV above the reactant energy levels (I →II = 1.2 eV; I →II
b b c c
0.9 eV). Given the stabilization gained by the initial monomer
=
coordination (barrierless, since this represents a long-range
attraction) and the zero-point energy, the energy barrier is not
expected to hinder the reactivity as also confirmed by the
experiment. Attempts in optimizing all other transition states
along the nucleophilic transfer path proved challenging and
were limited by convergence problems as well as high
computational costs.
As the experimental finding shows a less-efficient initiation in
2VP polymerization with collidine than with the alkyl initiator,
we were interested in the electronic and steric properties of the
optimized geometries (Ia−d and IIa−d). By comparing structures
II and II (after nucleophilic transfer of the respective initiator
a
b
to 2VP) it seemed that the collidine moiety of IIb was
H
DOI: 10.1021/acs.inorgchem.7b01261
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Table 3. Comparison and Advantages of the Best Suitable Catalysts According to the Respective Monomer
monomer
metal center
Lu
initiator
N-coordinating
VP
O-coordinating
Y
+
CH TMS
collidine
advantage of catalyst
2
2
+
+
high initiator efficiency
high activity
+
+
+
+
a
DEVP
+
+
improved initiator efficiencies
improved Đ
DMAA
+
high activity
high isotacticity of PDMAA
improved Đ
a
2
b shows just a slightly broader Đ and initiator efficiency in comparison to 1b.
To show that the opposite trend is evident in DMAA
polymerization, the isolated structures IIb and IIc were
compared (see Figure 8). An eight-membered cycloaddition-
assumed for DEVP polymerization while neglecting the
electronic effects. For DMAA polymerization, yttrium catalyst
1b was highly active, and the resulting polymer microstructure
exhibited high isotacticity (Pm = 94%) while maintaining a
narrow molecular mass distribution. In summary, this study
facilitates the selection of the ideal initiator and metal
combination for obtaining the most efficient and stereospecific
polymerization system for a broad range of polar vinyl
monomers. Table 3 summarizes the most suitable catalysts
for each monomer type studied.
type nucleophilic transfer reaction of I involving the CH2
d
substituent of collidine leads to the formation of an aliphatic
chain (II ). The N−Y bond was not completely cleaved after
d
the addition process, since the pyridine moiety remained
coordinated to the metal center. In comparison, the CH TMS
2
moiety of structure II shows a complete detachment from the
c
Y atom and is solely bound via the oxygen atom of DMAA.
This can be understood by considering that the CH TMS
2
moiety in 1a is a poor electron donor when compared with the
pyridine moiety of 1b and, thus, has weaker binding energy. In
addition, DMAA itself has electron-withdrawing character, and
electron density overload is not caused by initiation with
electron-donating collidine. The coordination of DMAA
simultaneously with the attached electron-donating collidine
does not provoke an electron overload at the metal center,
neither in I nor in II . Therefore, the calculation results
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b01261.
■
*
S
Experimental section, further analytic data, DFT
calculations, X-ray data (CCDC 1547971), and polymer
characteristics (PDF)
d
d
underline our experimental findings and enable a comparison
between the different initiators, metal centers, and monomers
during the initiation process.
Accession Codes
CCDC 1547971 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
request@ccdc.cam.ac.uk, or by contacting The Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge CB2
CONCLUSION
In this study, REM-GTP with isostructural 2-aminoalkoxybis-
phenolate)yttrium and lutetium complexes 1a−3b was
■
(
1
EZ, UK; fax: +44 1223 336033.
examined. This study showed moderate-to-exceptionally high
catalyst activities with various polar monomers. Kinetic studies
in 2VP polymerization revealed that all of the tested catalyst
systems followed a living-type mechanism, allowing precise
molecular weight control with very narrow molecular-weight
distributions. A heteroaromatic molecule was efficiently
introduced as an initiator via C−H bond activation. Altogether,
these structure variations made it possible to tune the activity,
initiator efficiency, and the resulting microstructures of 2VP,
AUTHOR INFORMATION
Corresponding Author
*E-mail: rieger@tum.de.
■
ORCID
̈
Alexander Pothig: 0000-0003-4663-3949
Bernhard Rieger: 0000-0002-0023-884X
Author Contributions
DEVP, and DMAA polymerization. Lutetium catalysts
tBu
The manuscript was written through contribution of all authors.
All authors have given approval to the final version of the
manuscript.
[
(ONOO) Lu(X)(THF)] (2a/2b), with a higher effective
nuclear charge, demonstrated the highest activity for GTP of
VP. Bis(phenolate)yttrium complexes with heteroaromatic
2
Notes
initiators showed decreased efficiency in polymerization with
electron-donating monomers (2VP and IPOx), because
electron density overload was provoked at the yttrium center.
Computational studies underline that the coordination of
nitrogen-donating monomers (containing a free electron pair)
is hindered with yttrium catalysts when an electron-donating
initiator is used. Since all of the other monomers were
nonelectron-donating systems, the change of initiator via C−H
bond activation with 2,4,6-collidine led to an enhancement in
initiator efficiency. Deprotonation of the acidic α-CH is avoided
with heteroaromatic initiators, which, until now, was solely
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
F.A. thanks the Bavarian State Ministry of Environment and
Consumer Protection for financial support within BayBiotech
research network. J.E. acknowledges support by the Interna-
tional Max Planck Research School of Advanced Photon
Science. We thank T. Helbich, S. Vagin, A. Kronast, and B.
Soller for the valuable discussions. The authors thank D. Mayer
and B. Rehl for proofreading of the manuscript.
I
DOI: 10.1021/acs.inorgchem.7b01261
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Inorganic Chemistry
Article
(
17) Salzinger, S.; Soller, B. S.; Plikhta, A.; Seemann, U. B.;
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DOI: 10.1021/acs.inorgchem.7b01261
Inorg. Chem. XXXX, XXX, XXX−XXX