Synthesis of Lewis Acidic, Aromatic Aminotroponiminate Zinc Complexes for the Ring-Opening Polymerization of Cyclic Esters

Article abstract of DOI:10.1021/acs.inorgchem.8b01060

Three novel aminotroponiminate (ATI) zinc complexes I-III (I = [(Ph₂)ATI]Zn-N(SiMe₃)₂, II = [(C₆H₃-2,6-C₂H₅/Ph)ATI]Zn-N(SiMe₃)₂, and III = [(C₆H<

Full text of DOI:10.1021/acs.inorgchem.8b01060

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis of Lewis Acidic, Aromatic Aminotroponiminate Zinc
Complexes for the Ring-Opening Polymerization of Cyclic Esters
Sebastian Kernbichl,† Marina Reiter,† Daniel H. Bucalon,†,‡ Philipp J. Altmann,§ Alexander Kronast,†
and Bernhard Rieger*,†
†Wacker-Lehrstuhl fu
85748 Garching bei Mu
̈
r Makromolekulare Chemie, Catalysis Research Center, Technische Universitat Munchen, Lichtenbergstraße 4,
nchen, Germany
̈
̈
̈
‡Departamento de Química, Universitade Federal de Sao
̃
Carlos, Via Washington Luiz, km 235, 13565-905 Sao
̃
Carlos, SP, Brazil
§
̈
̈
̈
Catalysis Research Center, Technische Universitat Munchen, Ernst-Otto-Fischer Straße 1, 85748 Garching bei Munchen, Germany
S
* Supporting Information
ABSTRACT: Three novel aminotroponiminate (ATI) zinc complexes I−III
(I = [(Ph2)ATI]Zn−N(SiMe3)2, II = [(C6H3-2,6-C2H5/Ph)ATI]Zn−
N(SiMe3)2, and III = [(C6H3-2,6-CH(CH3)2/Ph)ATI]Zn−N(SiMe3)2)
were synthesized and tested in the ring-opening polymerization of the
lactones β-rac-butyrolactone (BBL) and rac-lactide (LA). The ligands, with
two of them literature unknown, were readily obtained via a three-step
synthesis from tropolone. Forming a five-membered metallacycle with zinc,
the complexes were further structurally examined via single-crystal X-ray
analysis and compared with that of the established, 6-ringed β-diiminate
(BDI) complex IV ([CH(CMeNPh)2]Zn−N(SiMe3)2). The influence of the
varying metallacycle ring size on the polymerization was evaluated. In situ IR
measurements indicate a higher catalytic activity of the novel ATI complexes
I−III for BBL compared with the BDI system IV. The activity and degree of
control were further improved by an in situ generated alkoxy initiating group
generated after the addition of 2-propanol. An enhanced initiator efficiency allowed the synthesis of polymers with controlled
molecular weights and narrow polydispersities. Furthermore, II and III exhibited a high activity in the ring-opening
polymerization of rac-LA. Hereby, reaction time and initiator efficiency could also be optimized at a higher temperature or by
the addition of 2-propanol.
lactide via ring-opening polymerization. The access via ROP
enables the production of polymers with high molecular
weights, narrow dispersities, and stereoselectivity.7 Pioneering
works using chiral salen aluminum complexes in the polymer-
ization of rac-LA showed high isoselectivity.8 Since then, a
variety of ligands were coordinated to different central metals
to realize either hetero- or isotactic-enriched PLA.9 Figure 1
gives an overview of catalysts used in ROP of rac-LA starting
with a β-diiminate zinc complex4c producing heterotactic-
enriched PLA and ending with the most active and isoselective
systems using zinc, yttrium, or indium.
Because the development of new and efficient initiators for
this type of polymerization is still an ongoing challenge, the
catalytic behavior of a novel type of ligands, the amino-
troponimines (ATIHs), was investigated.
Aminotroponimines are a well-known class of ligands
initially discovered in 1960. After the insertion of tetrafluoro-
ethylene into cyclopentadiene, the fluorinated cycloheptadiene
is subsequently converted into the corresponding ATIHs via
INTRODUCTION
■
Polyesters, particularly poly(hydroxybutyrate) (PHB) and
poly(lactide) (PLA), can be produced by the ring-opening
polymerization (ROP) of lactones. These aliphatic polyesters
represent a valuable group of polymers with a great range of
thermo-mechanical properties along with a renewable origin of
quite a number of cyclic esters.1 PHB, initially identified in
Bacillus megaterium by M. Lemoigne, serves as a bacterial
storage material.2 In nature, the polymer is produced in its
strictly isotactic form; however, controlling the microstructure
via synthetic approaches has been attempted for decades. Ring-
opening polymerization of β-rac-butyrolactone (BBL) offers
the most promising way of controlling the microstructure with
different metals such as aluminum,3 zinc,4 chromium,5 and
yttrium.6 Thereby, β-diiminate (BDI) zinc complexes4d by
Coates et al., yielding atactic PHB, and amino-alkoxy-
bis(phenolate) yttrium complexes6a by Carpentier et al., giving
a syndiotactic microstructure, were introduced as highly active
catalysts. In contrast to PHB, the synthesis of PLA is
industrially already more applied, making it the leading
bioderived polymer. It is usually obtained from lactic acid
through condensation reaction or, more promising, from
Received: April 17, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b01060
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Figure 1. Active complexes for ROP of rac-LA with a focus on zinc as central metal.4c,9b,c,d,g
Figure 2. BDI zinc complexes established in polymerization catalysis by Coates (left); introduction of electron-withdrawing groups at the pentane
backbone (center); realization of a five-membered metallacycle by synthesis of aminotroponiminate complexes (right).14,15a
condensation reaction with primary amines.10 Deprotonated
ATIHs act as anionic, bidentate ligands with a delocalization of
the negative charge over the 7-membered ring, giving a 10π-
electron backbone. A wide range of main group, transition, and
f-block elements11 have been introduced as central metal, but
Roesky et al. were the first to bring aminotroponiminate (ATI)
ligands into the coordination sphere of zinc.12 A vast variety of
ATI ligands with different amines were converted to zinc
complexes and tested in intramolecular hydroamination
reactions.13 However, to the best of our knowledge, they
have not been used in ROP.
Recently, we showed that BDI complexes bearing two
electron-withdrawing trifluoromethyl groups in the pentane
backbone show enhanced activity in the ring-opening
polymerization of lactones when compared with BDI-ZnEt.14
This can be attributed to an increased Lewis acidity at the
metal center. Thus far, the influence of substituted anilines, the
pentane backbone, and the initiating group on the catalytic
activity was systematically investigated.4c,d,15 Nevertheless, the
6-membered ring structure around the zinc center was not
modified yet. It is expected that changing the ring size will have
a decisive influence in terms of catalytic activity of the
corresponding complexes (Figure 2).
In this work, we report on the synthesis of three novel,
aromatic aminotroponiminate complexes I−III. Two of the
ligands have not been previously reported. The complexes
were structurally compared with the BDI model complex IV
via single-crystal X-ray diffraction (SC-XRD), and the catalytic
activity in ring-opening polymerization of BBL and rac-LA was
examined.
RESULTS AND DISCUSSION
■
Synthesis. Aromatic aminotroponimine ligands were
obtained via a 3-step synthesis starting from tropolone. First,
trifluoromethanesulfonyl was introduced as a good leaving
group for the following cross-coupling. The palladium-
catalyzed Buchwald−Hartwig-like coupling converts the
triflatotropone to the aminotropones 1a−3a. This readily
works for sterically hindered anilines substituted in the 2,6-
position.16 After activation with Meerwein’s salt, the resulting
B
DOI: 10.1021/acs.inorgchem.8b01060
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Scheme 1. Synthesis Route toward Aminotroponimine Ligands 1b−3b
Scheme 2. Complex Synthesis with Ligands 1b−3b and Zn(NTMS2)2
vinylogous ether was converted by aminolysis to the respective
aminotroponimines (1b−3b).13 Thus, bidentate, N-donor
ligands bearing two different rests (2: R = CH2CH3; 3: R =
CH(CH3)2) at the 2,6-positions of one of the anilines were
obtained (Scheme 1). In order to realize symmetrical, 2,6
disubstituted ATIHs, the aim was to introduce another triflate
leaving group at the aminotropone 2a and subsequently couple
this with a 2,6 disubstituted aniline. The reduction of the
carbonyl unit with lithium diisopropylamide was unsuccessful,
most likely, because of the conjugated system at the amide.
Therefore, only unsymmetrical ATIHs were accessible via this
synthesis route.
BDI complexes bearing bis(trimethylsilyl)-amido (NTMS2)
initiators coordinated to the zinc center show high activities in
the ring-opening polymerization of lactones.17 Therefore, 1b−
3b were treated with Zn(NTMS2)2 to obtain ATI zinc amido
complexes. Recently, Roesky et al. reported the complexation
of aminotroponimines with ZnMe2 and ZnEt2. Depending on
the substituents of the amines, either the desired complex with
the general formula [(R2)ATI]Zn-Alkyl (R = cyclohexyl, 1-
ethylpropyl) or the corresponding homoleptic complex
[(R2)ATI]2Zn (R = Ph, Me, iPr) was obtained.20 Complex-
ation of the symmetric, unsubstituted ligand 1b with
Zn(NTMS2)2 at room temperature resulted in the formation
of two different products (Scheme 2): The target structure I
was only formed to an amount of 50%, whereas the homoleptic
byproduct constituted the remaining 50%. Metal−organic
complexes used in polymerization catalysis usually bear a
nucleophilic group that serves as an initiator for the ring-
opening step. Although there are some systems of homoleptic
complexes which are active in polymerization, it is assumed for
ATI-based complexes that an initiating group is necessary.1d,18
Hence, I′, in contrast to I, might be inactive in ROP of
lactones (tested for BBL polymerization, result shown in Table
1, entry 13). In order to overcome this high amount of
homoleptic byproduct I′, different temperatures were applied
and the influence on the complexation behavior was
investigated (Table S1): Performing the complexation reaction
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Scheme 3. Synthesis of β-Diiminate Zinc Complex IV. Reaction of BDI Ligand 4a with the Zinc Precursor
Figure 3. ORTEP style representation of I (left, CCDC 1837008) and IV (right, CCDC 1837010) with ellipsoids drawn at a 50% probability level.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): I. Zn1−N1 1.9659(16), Zn1−N2 1.9707(17), Zn1−N3
1.8653(16), N1−Zn1−N2 82.41(7), Zn1−N1−C6 123.67, and Zn1−N2−C14 124.19; IV. Zn1−N1 1.9460(16), Zn1−N2 1.9530(16), Zn1−N3
1.8832(16), N1−Zn1−N2 98.59(7), Zn1−N1−C6 117.73, and Zn1−N2−C10 119.46.
at 100 °C resulted in the formation of 59% homoleptic
complex I′. Lower temperatures, in this case 0 °C, result again
for 16 h (Scheme 3). Recrystallization from toluene at −35 °C
afforded colorless crystals that were suitable for X-ray
diffraction.
1
in an equal 50/50 mixture of I and I′. H NMR spectroscopy
Figure 3 shows the ORTEP style representations of I and
IV. Both metal cores adopt a trigonal-planar geometry. Because
I and IV are iminate-derived ligands without any substituents
at the anilines, the 5- and 6-ringed structures can be directly
compared to investigate the influence of the ring size on the
molecular structure. The N1−Zn1−N2 angle was 82.41(7)° in
the 5-ringed structure and 98.59(7)° in the 6-membered ring.
The angle Zn1−N1−C6 also differs (123.67(5) for I and
117.73(4) for IV), indicating a larger space in I for a possible
monomer coordination. A shortening of the Zn1−N3 bond
was observed for the ATI−Zn−NTMS2 I (1.8653(16)) with
respect to the BDI−Zn−NTMS2 IV (1.8832(16)).
Crystals suitable for SC-XRD were obtained via diffusion of
pentane into a saturated solution of II and III in dichloro-
methane. Their molecular structures are depicted in Figure 4.
The substituents in the 2,6-position of the aniline showed little
influence over the molecular structure compared with I. Again,
a trigonal-planar coordination geometry and very similar Zn1−
N3 distances (I. 1.8653(16) Å, II. 1.865(4) Å, and III.
1.8631(17) Å) were observed. The substituted phenyl moieties
were almost perpendicular to the coordination plane.
still shows the presence of unreacted ligand 1b. Thus, a
temperature of 25 °C for 24 h was chosen as the best condition
for the complexation reaction of 1b. Treatment of ligands 2b
and 3b, bearing substituents at the 2,6-positions of the aniline,
with the zinc precursor led to the exclusive formation of the
desired structures [(C6H3-R2/Ph)ATI]Zn−NTMS2 II−III.
This clearly shows that the steric demand of the ligand plays
a decisive role in the complexation reaction.
The mixture of I and I′ was separated based on the different
crystallization behavior: Diffusion of pentane into a saturated
solution of I and I′ in dichloromethane at −35 °C selectively
led to the crystallization of I′, whereas I remained in solution.
Thus, aminotroponiminate complexes derived from aromatic
anilines with the general formula [Ph2ATI]Zn−initiator were
isolated for the first time. Crystals suitable for single-crystal X-
ray diffraction analysis were obtained via recrystallization of a
saturated solution of I in pentane at −35 °C.
It is of interest to compare the molecular structure and the
activity of the novel ATI complexes with the established BDI
complexes. Via acid-catalyzed condensation reaction of aniline
and acetylacetone, BDI ligand 4a was obtained19 and readily
converted to the respective zinc−NTMS2 complex IV by
stirring the ligand 4a with Zn(NTMS2)2 in toluene at 60 °C
Polymerization Results. The catalytic activity of I−IV
was tested in the polymerization of BBL and rac-LA under
D
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a
Table 1. Ring-Opening Polymerization of BBL Using Catalysts I−IV
b
c
d
e
f
entry
cat.
T [°C]
time [h]
conv. [%]
Mn,calc [kg/mol]
Mn,exp [kg/mol]
Đ
Pm
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
I
II
60
60
60
25
80
60
60
60
60
25
60
60
60
60
60
8
15
8
20
3.5
7
3
8
8
20
14
4.5
20
12
16
>99
>99
>99
4
>99
95
98
97
96
7
61
93
2
1
76
52
52
52
2
52
49
5
17
33
3.6
31
48
n.d.
n.d.
39
129
75
122
36
116
53
7.5
56
65
29
58
49
1.22
1.28
1.16
1.22
1.15
1.05
1.02
1.13
1.51
1.33
1.78
1.07
n.d.
55
56
55
53
54
54
53
54
53
54
55
55
n.d.
n.d.
47
III
III
III
III
III
III
III
III
III
I
g
h
i
j
k
gl
,
l
h
I′
I′
IV
n.d.
n.d.
84
m
n.d.
1.05
a
All polymerizations were performed with nLA = 17.4 mmol in a BBL/cat. ratio of 600 in 5.0 g of toluene in an autoclave with in situ IR monitoring
under an argon atmosphere; polymerizations in entries 7−10 were performed in preheated glasses with magnetic stirring under anargon
b
c
1
atmosphere, with the equivalents of BBL indicated. Conversion determined by H NMR spectroscopy. Mn,calc [kg/mol] = 0.01·conv.·86 g/mol·
d
e
f
equiv. Mn,exp determined by GPC in THF vs polystyrene standards. Đ = Mw/Mn. Determined by 13C NMR spectrum of the carbonyl C atom of
PHB (Figure S5). Not precipitated. 1.0 equiv of iPrOH was added. 10 equiv of iPrOH was added. 200 equiv of BBL was used. 400 equiv of
BBL was used. THF as solvent. 2.0 equiv of iPrOH was added.
g
h
i
j
k
l
m
Figure 4. ORTEP style representation of II (left, CCDC 1837009) and III (right, CCDC 1837007) with ellipsoids drawn at a 50% probability
level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): II. Zn1−N1 1.970(4), Zn1−N2 1.961(4), Zn1−N3
1.865(4), N1−Zn1−N2 82.61(16), Zn1−N1−C6 123.53, and Zn1−N2−C14 125.05; III. Zn1−N1 1.9547(17), Zn1−N2 1.9553(17), Zn1−N3
1.8631(17), N1−Zn1−N2 82.80(7), Zn1−N1−C6 122.75, and Zn1−N2−C14 124.48.
different conditions (Tables 1 and 2). The polymerization of
BBL was conducted with a catalyst/monomer ratio of 1:600 at
60 °C in an autoclave with in situ IR monitoring. The three
ATI-based complexes I−III were active initiators for the ring-
opening of BBL, and the ligands had a different influence on
the activity of the respective catalysts.
According to in situ IR spectroscopy (Figure 5), complex I
(Table 1, entry 1) showed the highest activity of all the
catalysts used with an induction period of about 30 min. The
obtained polymer has a molecular weight of 129 kg/mol and a
narrow polydispersity index (Đ) of 1.22. The more sterically
demanding complexes II and III (Table 1, entries 2 and 3) also
exhibited good activity along with a molecular weight of 75 kg/
mol using II and 122 kg/mol with III. Different conditions
such as temperature, addition of an external alcohol,
concentration, and THF as solvent were tested using complex
III. At first, polymerizations at 25 °C (Table 1, entry 4) and 80
°C (Table 1, entry 5) were performed. Activity and initiator
efficiency at room temperature were low for III, leading to 4%
conversion and a molecular weight of 36 kg/mol. Higher
temperatures resulted in increased activity of III and the same
initiator efficiency as for 60 °C. A plot of the in situ IR
spectroscopy versus reaction time for the three temperatures is
shown in Figure S1. The influence of adding an external
alcohol, in this case, 1.0 equiv of 2-propanol, on complex III
1
was studied via H NMR spectroscopy. After 10 min reaction
time, −OiPr is bonded to the metal center, creating an alkoxy
initiating group by a release of HNTMS2 (see Figure S2). This
in situ formed catalyst was subsequently tested in ROP of BBL
at 60 °C (Table 1, entry 6), revealing a higher activity
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12 h at 60 °C. This clearly demonstrates that no active
complex could be formed by the addition of 2-propanol.
Once the behavior of the ATI complexes in the polymer-
ization of BBL was evaluated, they were compared with the
BDI complex model system, in this case, exemplary with
complex IV: Both I and IV have the same initiating group and
were synthesized from unsubstituted anilines, allowing a direct
comparison of the influence of the 5- vs 6-membered ring
around zinc. BDI complex IV showed lower activity in
polymerization compared with I, yet the molecular weight (84
kg/mol) and Đ (1.05) were still in a good range. There may be
various steric and electronic effects involved; thus, the reason
for the higher activity is difficult to address. According to the
molecular structures of I and IV, revealing a larger Zn1−N1−
C6 angle for I compared to IV (I. 123.67(5)°, IV. 117.73(4)°),
steric reasons seem to be more decisive rather than electronic
effects.
As an example, catalyst II was tested in the ring-opening
polymerization reaction to assess the possible living-type
character (Figure 6). The conversion−time plot is shown on
the left. Molecular weights, as well as the polydispersity indices
as a function of the conversion, show a linear increase,
indicating a living-type character of the polymerization.
Coates et al. investigated the influence of the initiating group
of BDI zinc complexes in the ROP of LA. In that work,
−N(SiMe3)2 substituted complexes show slightly lower activity
than the −OiPr substituted analogue but maintained a good
control of the molecular weight with a moderate Đ of 2.95.4b
Catalysts I−IV were also tested in the ROP of rac-LA under
different conditions (Table 2).
Figure 5. Polymerization of BBL with catalysts I−IV under the same
conditions monitored by in situ IR spectroscopy (νC=O, PHB = 1750
cm−1).
compared to III without the addition of iPrOH (Figure S3).
Most importantly, the initiator efficiency of the in situ formed
alkoxy group is higher, enabling the synthesis of controlled
molecular weights and narrow polydispersities. This worked
also for 10 equiv of iPrOH, producing PHB with 7.5 kg/mol
(Table 1, entry 7). By variation of the monomer-to-catalyst
ratio, polymers with different chain lengths were accessible in
still good conversions (Table 1, entries 8 and 9). Polymer-
ization experiments using THF as coordinating solvent (Table
1, entries 10 and 11) were successful, although lower
conversions were observed compared to toluene as solvent.
The use of THF did not influence the stereoselectivity of the
reaction. In all polymerizations, atactic PHB was obtained. As
mentioned, complex I showed an induction time of about 30
min. This can be overcome by adding 1.0 equiv of iPrOH prior
to monomer addition (Table 1, entry 12). An immediate start
of the polymerization as well as controlled molecular weights
could be observed. The homoleptic complex I′ with the
general formula [(Ph2)ATI]2Zn was also tested (Table 1, entry
13) in the polymerization. I′ did not show significant activity,
demonstrating that the catalyst requires a nucleophilic leaving
group to start the ring-opening of the monomer. Complex I′
was treated with 2.0 equiv of iPrOH to check if an active
complex can be formed by the addition of an alcohol. No
conversion to PHB could be observed after a reaction time of
Both II and III (Table 2, entries 2 and 3) were active
initiators at room temperature, producing PLA in high
conversion and very high molecular weight, whereas catalyst
I had a very poor activity, resulting in a low conversion of 12%
after 16 h. The obtained molecular weights indicate a rather
low initiator efficiency of II and III at room temperature. It can
also be concluded that ATI complexes with a higher steric
demand of the ligand had better activity in the rac-LA
polymerization. To get a closer insight why complex I showed
such a low activity, the stability of I and III was investigated in
different solvents (toluene-d8 and dichloromethane-d2, 15 min
1
and 12 h) via H NMR spectroscopy and 1H DOSY NMR. In
both solvents, I and III remained stable and showed a single
diffusion coefficient for all resonances. Hence, the reason for
this activity difference might be caused by the steric demand of
the aniline rests. The substituted complexes II and III likely
enable a better coordination of rac-LA, thus accelerating the
■
Figure 6. Polymerization of BBL with catalyst II. Plot of PHB conversion (%) vs time (h) (left), and PHB molecular weight
(Mn, exp vs
▼
polystyrene standard in THF) and polydispersity index
as a function of conversion (right).
F
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a
Table 2. Ring-Opening Polymerization of rac-LA Using Catalysts I−IV
b
c
d
e
f
entry
cat.
T [°C]
time [h]
conv. [%]
Mn,calc [kg/mol]
Mn,exp [kg/mol]
Đ
Pr
g
1
I
25
25
25
40
25
40
40
40
25
40
25
16
16
16
8
1.5
1.5
16
16
16
8
12
91
97
97
90
97
99
94
62
86
94
3.5
26
28
28
26
28
56
84
18
25
27
7.5
168
125
41
26
30
1.28
1.63
1.89
1.69
1.23
1.74
2.05
1.83
2.43
1.81
1.43
n.d.
50
54
60
50
60
55
57
47
53
62
2
3
4
II
III
II
II
II
II
II
II
II
IV
h
5
h
6
i
7
68
j
8
k
111
109
70
9
k
10
11
16
101
a
All polymerizations were performed, except otherwise indicated, with nLA = 0.8 mmol in a rac-LA/cat. ratio of 200 in 2.65 mL of dichloromethane
b
c
d
under an argon atmosphere. Conversion determined by1H NMR spectroscopy. Mn,calc [kg/mol] = 0.01·conv.·144 g/mol·equiv. Mn,exp
determined by GPC in THF vs polystyrene standards; Mn,exp values are corrected with a 0.58 factor for PLA. Đ = Mw/Mn. Determined by homo-
decoupled H NMR spectroscopy considering the methine region of PLA (Figure S6). Not precipitated. 1.0 equiv of iPrOH was added. 400
equiv of rac-LA was used. 600 equiv of rac-LA was used. THF as solvent.
e
f
g
h
i
1
j
k
monomer ring-opening step. Due to a low initiator efficiency,
polymerizations using II both at higher temperature and with
the addition of an external alcohol were performed. Molecular
weights are determined by GPC in THF and corrected with a
factor of 0.58.1d,18 Increasing the temperature from 25 to 40
°C (Table 2, entry 4) already enhanced the initiator efficiency
by a factor of 4 to a molecular weight of 41 kg/mol. The
addition of 2-propanol decisively influenced the reaction time
and the obtained molecular weights: Polymerizing rac-LA at
room temperature in the presence of 1.0 equiv of iPrOH
(Table 2, entry 5) led to full conversion within 1.5 h,
compared to 16 h when no external alcohol was used. The
obtained molecular weights are in good range with the
theoretical values. Performing the polymerization at 40 °C in
the presence of 2-propanol (Table 2, entry 6) led to the same
reaction time of 1.5 h and a comparable control over the
molecular weight. Additionally, polymerization experiments
with different monomer-to-initiator ratios (Table 2, entries 7
and 8) and with tetrahydrofuran as coordinating solvent
(Table 2, entries 9 and 10) were conducted. Because both the
substituents at the ligand and the nature of the solvent
influence stereoselectivity, the polymer microstructure was
complexes I−III in the production of PHB than the BDI
model system IV. This clearly demonstrates that the ring size
of the metal core decisively influences the catalytic behavior of
the complexes. The bulkier complexes II−III were also highly
active initiators in the ring-opening polymerization of rac-LA.
For both monomers, an optimization of the reaction
conditions regarding reaction time, concentration, influence
of the solvent, and the addition of an external alcohol was
carried out. The addition of 2-propanol generated a zinc-alkoxy
initiator group showing higher activity and initiator efficiency
in the ring-opening polymerization. This allowed the synthesis
of polymers with controlled molecular weights and narrow
polydispersities. Further modifications of the ligands’ structure
may improve the activity and the stereospecificity of the
polymerization.
EXPERIMENTAL SECTION
■
General. All reactions containing air- and/or moisture-sensitive
compounds were performed under an argon atmosphere using
standard Schlenk or glovebox techniques. All chemicals, unless
otherwise stated, were purchased from Aldrich and used as received.
Dry toluene, dichloromethane, and pentane were obtained from an
MBraun MB-SPS-800 solvent purification system. 2-Propanol was
dried over molecular sieves.
1
examined via homo-decoupled H NMR spectroscopy. It has
been reported that increased heterotacticity can be observed
for complexes bearing bulkier ligands.1d,18 This trend was also
observed for the ATI complexes II−III, although Pr is
generally lower when compared to BDI complexes. Complex
II exhibited higher heterotacticity when a higher temperature
(40 °C) was applied. The use of THF had no decisive
influence on the tacticity of the PLA. BDI model catalyst IV
also allowed the polymerization of rac-LA in 16 h with a 94%
yield (Mn = 101 kg/mol, Đ = 1.43). The living-type character
was assessed for rac-LA polymerization with III (Figure S7).
β-Butyrolactone was treated with BaO to remove crotonic acid
1
contaminants. After checking the purity via H NMR spectroscopy,
BBL was dried over calcium hydride and distilled prior to
polymerization. NMR spectra were recorded on Bruker AVIII-300
and AVIII-500 Cryo spectrometers. 1H NMR spectroscopic shifts are
reported in ppm relative to tetramethylsilane and calibrated to the
residual proton signal of the deuterated solvent. The deuterated
solvents were obtained from Aldrich and dried over 3 Å molecular
sieves. The following abbreviations are used to describe the peak
patterns when appropriate: s (singlet), d (doublet), dd (doublet of
doublets), t (triplet), m (multiplet), and br (broad). In situ IR
measurements were performed under an argon atmosphere using an
ATR-IR MettlerToledo system with mechanical stirring. Gel
permeation chromatography analysis was performed with a Varian
PL-GPC 50 using THF (HPLC grade) with 0.22 g L−1 2.6 di-tert-
butyl-4-methylphenol and a flow rate of 1 mL/min at 40 °C.
Polystyrene standards were used for calibration. Molecular weights of
PLA were corrected with a Mark−Houwink factor of 0.58.20
Elemental analysis was performed at the microanalytic laboratory of
the Department of Inorganic Chemistry at the Technical University of
Munich. Mass spectra were recorded with an Agilent Technologies
Mass Hunter Spectrometer (EI, 70 eV). Single-crystal X-ray
CONCLUSION
■
Three novel zinc complexes I−III were synthesized from
aminotroponimine ligands, two of which were literature
unknown. The ligands were obtained via a three-step synthesis
starting from tropolone. The molecular structures were
compared with the established β-diiminate model system IV
via SC-XRD. Catalysts I−IV were tested in the ring-opening
polymerization of the cyclic esters, rac-BBL and rac-LA. In situ
IR spectroscopy revealed a higher activity for the ATI
G
DOI: 10.1021/acs.inorgchem.8b01060
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
crystallography was performed at the SCXRD laboratory of the
Catalysis Research Center.
K): δ [ppm] = 152.8, 151.0, 144.9, 141.9, 137.6, 133.6, 133.5, 129.6,
126.5, 125.1, 124.0, 122.9, 121.3, 115.4, 113.1, 24.8, 14.6. Anal. Calcd
for C23H24N2: C, 84.11; H, 7.37; N, 8.53. Found C, 83.83; H, 7.60; N,
8.24. MS (EI, 70 eV): 329.2 m/z [M + H].
Ligands. 2-Triflatotropone. This compound was synthesized
according to a modified literature procedure.21 2-Hydroxycyclohep-
ta-2,4,6-trienone (tropolone) (2.50 g, 20.5 mmol, 1.00 equiv),
triethylamine (2.17 g, 21.5 mmol, 1.05 equiv), and N-phenyltriflimide
(7.68 g, 21.5 mmol, 1.05 equiv) were dissolved in dry dichloro-
methane and stirred for 48 h at r.t. After extraction with water (20
mL) and dichloromethane (2 × 20 mL), the main product was
concentrated in vacuo and purified via silica gel column chromatog-
raphy (hexane/ethyl acetate = 2:1, TLC Rf = 0.3) yielding a brownish
oil (86%). 1H NMR (500 MHz, CDCl3, 298 K): δ (ppm) = 9.07 (br
N-Phenyl-2-(2,6-diisophenylamino)troponimine 3b. 1H NMR
(500 MHz, CDCl3, 298 K): δ (ppm) = 9.16 (br s, 1H, NH), 7.46
(t, J = 7.9 Hz, 2H), 7.28−7.17 (m, 6H), 6.93 (d, J = 11.0 Hz, 1H),
6.77 (dd, J = 10.7 Hz, J = 9.6 Hz, 1H), 6.69 (dd, J = 10.7 Hz, J = 9.6
Hz, 1H) 6.31 (t, J = 9.6 Hz, 1H), 6.19 (d, J = 10.7 Hz, 1H), 2.97−
2.89 (m, 2H), 1.20 (dd, J = 11.1 Hz, 6.9 Hz, 12H). 13C NMR (126
MHz, CDCl3, 298 K): δ [ppm] = 151.3, 145.4, 142.6, 140.0, 133.5,
133.5, 129.6, 125.7, 123.9, 123.8, 122.8, 121.3, 115.1, 113.9, 28.5,
24.2, 23.5. Anal. Calcd for C25H28N2: C, 84.23; H, 7.92; N, 7.86.
Found C, 84.24; H, 8.29; N, 7.57. MS (EI, 70 eV): 357.3 m/z [M +
H].
s, OH), 7.42−7.22 (m, 4H, HAr), 7.07 (t, J = 10.2 Hz, 1H, HAr). 13
C
NMR (126 MHz, CDCl3, 298 K): δ [ppm] = 178.5 (s), 156.4 (s),
141.2 (s), 137.6 (s), 136.4 (s), 130.7 (s), 128.6 (s), 121.4 (1JCF = 323
Hz).
N-(Phenylimino)pent-2-en-2-yl)aniline (4a). The ligand was
synthesized by a modified literature procedure.19 A solution of 2,4-
pentadione (5.00 g, 49.9 mmol, 1.00 equiv) and aniline (9.18 g, 100
mmol, 2.00 equiv) was treated with concentrated hydrochloric acid
(4.90 g, 49.9 mmol) at 0 °C. After stirring the mixture for 24 h, the
precipitate was filtered, washed with hexane, and dissolved in a
solution of CH2Cl2 (4 mL), H2O, and triethylamine (10.0 mL). The
Aromatic 2-Aminotropones (1a−3a). In a preheated Schlenk
flask, 2-triflatotropone (1.20 g, 4.72 mmol, 1.00 equiv), cesium
carbonate (2.15 g, 6.61 g, 1.40 equiv), rac-2,2′-bis-
(diphenylphosphino)-1,1′-binaphthalene (29.4 mg, 47.2 μmol, 0.01
equiv), tris(dibenzylideneacetone)dipalladium (21.6 mg, 23.6 μmol,
0.005 equiv), and the respective aniline (6.14 mmol, 1.30 equiv) were
dissolved in toluene and heated to 90 °C for 24 h.16 The crude
product was filtered, concentrated in vacuo, and purified via silica gel
chromatography (hexane/ethyl acetate = 20:1, TLC Rf = 0.2).
1
crude product was recrystallized from ethanol. H NMR (500 MHz,
CDCl3, 298 K): δ (ppm) = 12.7 (s, 1H, NH), 7.29 (m, 4H, HAr), 7.06
(t, J = 7.4 Hz, 2H, HAr), 6.97 (d, J = 8.5 Hz, 4H, HAr), 4.89 (s, 1H,
CH), 2.01 (s, 6H, CH3).
1
2-(Phenylamino)tropone 1a. H NMR (500 MHz, CDCl3, 298
K): δ (ppm) = 8.84 (br s, 1H, NH), 7.53−7.19 (m, 9H, HAr), 6.88
(dd, J = 7.1 Hz, J = 8.9 Hz, 1H, HAr).
Complexes. The ligands 1b−3b (0.610 mmol, 1.00 equiv) were
dissolved in a Schlenk flask in 10 mL of toluene. Zn(NTMS2)2 (0.670
mmol, 1.10 equiv) was added to the solution and stirred for 3 h
(ligand 3a for 24 h in the case of 0 °C/25 °C and 72 h at 100 °C) at
room temperature; then the solvent was removed in vacuo. Complexes
II and III were obtained by recrystallization in CH2Cl2/pentane at
−35 °C. Complexation of 1b yielded 50% homoleptic and 50%
heteroleptic complex. Recrystallization of a saturated solution in
CH2Cl2/pentane resulted in the exclusive crystallization of the
homoleptic complex, whereas the heteroleptic one remains in solution
and can be successfully separated.
2-(2,6-Diethylphenylamino)tropone 2a. 1H NMR (500 MHz,
CDCl3, 298 K): δ (ppm) = 8.47 (br s, 1H, NH), 7.36−7.23 (m, 5H,
HAr), 7.07 (t, J = 10.3 Hz, 1H, HAr), 6.73 (t, J = 8.9 Hz, 1H, HAr), 6.26
(d, J = 10.3 Hz, 1H, HAr), 2.55−2.42 (m, 4H, −CH2−), 1.14 (t, J =
7.6 Hz, 6H, −CH3). 13C NMR (126 MHz, CDCl3, 298 K): δ [ppm] =
176.5, 155.6, 142.1, 137.5, 136.7, 133.9, 129.8, 128.4, 127.0, 123.6,
110.2, 24.7, 14.9. Anal. Calcd for C17H19NO: C, 80.60; H, 7.56; N,
5.53. Found C, 80.67; H, 7.64; N, 5.46. MS (EI, 70 eV): 253.2 m/z
[M+].
2-(2,6-Diisopropylphenylamino)tropone 3a. 1H NMR (500
MHz, CDCl3, 298 K): δ (ppm) = 8.42 (br s, 1H, NH), 7.41−7.26
(m, 5H, HAr), 7.07 (t, J = 10.3 Hz, 1H, HAr) 6.73 (t, J = 9.8 Hz, 1H,
HAr), 6.27 (d, J = 10.3 Hz, 1H, HAr) 2.94−2.85 (m, 2H, −CH−), 1.13
(dd, J = 20.3 Hz, J = 6.9 Hz, 6H, −CH3). 13C NMR (126 MHz,
CDCl3, 298 K): δ [ppm] = 176.4, 156.5, 146.9, 137.7, 136.4, 132.4,
130.0, 128.9, 124.4, 123.8, 110.7, 28.6, 24.6, 23.5. Anal. Calcd for
C19H16N2: C, 81.10; H, 8.24; N, 4.98. Found C, 81.10; H, 8.47; N,
4.74. MS (EI, 70 eV): 281.2 m/z [M+].
N,N′-Disubstituted Aminotroponimines (1b−3b). Triethy-
loxonium tetrafluoroborate (0.47 g, 2.48 mmol, 1.05 equiv) was
dissolved in a preheated flask in dry dichloromethane and added to a
solution of the respective 2-aminotropone 2a−c (2.36 mmol, 1.00
equiv) and dichloromethane. After stirring for 3 h at r.t., aniline (1.10
g, 11.4 mmol, 5.00 equiv) was added and the resulting solution was
stirred for 48 h. The crude product was extracted with a NaOH
solution (1M, 10 mL) and dichloromethane (2 × 10 mL). A dark
brownish oil was obtained after silica gel column chromatography
(hexane/ethyl acetate = 30:1, TLC Rf = 0.6).
I. 1H NMR (300 MHz, CDCl3, 298 K): δ [ppm] = 7.46 (t, J = 9.6
Hz, 4H, HAr), 7.22 (t, J = 7.3 Hz, 2H, HAr), 7.14 (dd, J = 7.4 Hz, J =
1.1 Hz, 4H, HAr), 7.07 (m, 3H, HAr), 6.72 (m, 2H, HAr), −0.18 (s,
18H, Si−CH3). 13C NMR (126 MHz, CDCl3, 298 K): δ [ppm] =
160.3, 159.7, 149.4, 148.1, 135.2, 134.8, 129.9, 129.8, 124.8, 124.5,
124.1, 122.3, 117.6, 116.2. Anal. Calcd for C25H33N3Si2Zn: C, 60.40;
H, 6.69; N, 8.45. Found C, 60.09; H, 6.79; N, 8.59.
II. 1H NMR (500 MHz, CDCl3, 298 K): δ (ppm) = 7.46 (t, J = 7.9
Hz, 2H, HAr), 7.25−7.00 (m, 9H, HAr), 6.57 (t, J = 9.2 Hz, 1H, HAr),
6.53 (d, J = 11.2 Hz, 1H, HAr), 2.55−2.37 (m, 4H, −CH2−), 1.15 (t, J
= 7.6 Hz, 6H, −CH3), −0.25 (s, 18H, Si−CH3). 13C NMR (126
MHz, CDCl3, 298 K): δ [ppm] = 159.5, 159.0, 147.8, 143.7, 137.2,
135.1, 135.0, 129.7, 126.2, 125.2, 125.0, 124.7, 121.8, 117.2, 116.7,
23.9, 14.0, 4.65. Anal. Calcd for C29H41N3Si2Zn: C, 62.96; H, 7.47; N,
7.60. Found C, 62.63; H, 7.43; N, 7.30.
1
III. H NMR (500 MHz, CDCl3, 298 K): δ (ppm) = 7.46 (t, J =
7.9 Hz, 2H, HAr), 7.26−6.97 (m, 7H, HAr), 7.07 (t, J = 10.7 Hz, 1H,
HAr), 6.99 (t, J = 10.5 Hz, 1H, HAr), 6.55 (t, J = 9.4 Hz, 2H, HAr), 2.87
(m, 2H, CH), 1.22 (d, J = 6.8 Hz, 6H, CH3), 1.04 (d, J = 6.8 Hz, 6H,
CH3), −0.23 (s, 18H, Si−CH3). 13C NMR (126 MHz, CDCl3, 298
K): δ [ppm] = 160.7, 159.0, 147.6, 142.6, 142.3, 135.2, 134.8, 129.8,
125.9, 125.1, 124.8, 124.4, 122.0, 118.5, 116.9, 28.4, 25.4, 24.3, 4.78.
Anal. Calcd for C31H45N3Si2Zn: C, 64.06; H, 7.80; N, 7.23. Found C,
63.96; H, 7.79; N, 6.96.
N-Phenyl-2-(phenylamino)troponimine 1b. 1H NMR (500 MHz,
CDCl3, 298 K): δ (ppm) = 9.18 (br s, 1H, NH), 7.40 (t, J = 7.9 Hz,
4H), 7.16−7.12 (m, 6H), 6.84 (d, J = 10.4 Hz, 2H, HAr), 6.73 (t, J =
10.3 Hz, 2H, HAr), 6.34 (t, J = 9.2 Hz, 1H). 13C NMR (126 MHz,
CDCl3, 298 K): δ [ppm] = 152.0, 145.3, 133.6, 129.6, 124.1, 122.7,
122.2, 115.0. Anal. Calcd for C19H16N2: C, 83.79; H, 5.92; N, 10.29.
Found C, 83.95; H, 6.00; N, 10.23. MS (EI, 70 eV): 273.2 m/z [M +
H].
Ligand 4a (0.68 mmol, 1.0 equiv) was dissolved in 5.0 mL of
toluene. Zn(NTMS2)2 (0.82 mmol, 1.2 equiv) was added to the
solution and stirred for 20 h at 80 °C; then the solvent was removed
in vacuo. Complex IV was obtained via recrystallization in toluene at
1
N-Phenyl-2-(2,6-diethylphenylamino)troponimine 2b. H NMR
(500 MHz, CDCl3, 298 K): δ (ppm) = 9.10 (br s, 1H, NH), 7.41 (t, J
= 7.8 Hz, 2H), 7.18−7.13 (m, 6H), 6.88 (d, J = 10.9 Hz, 1H), 6.73
(dd, J = 10.6 Hz, J = 9.5 Hz, 1H), 6.66 (dd, J = 10.6 Hz, J = 9.5 Hz,
1H), 6.28 (t, J = 9.5 Hz, 1H), 6.14 (d, J = 10.6 Hz, 1H), 2.53−2.39
(m, 4H), 7.54 (t, J = 7.5 Hz, 6H). 13C NMR (126 MHz, CDCl3, 298
1
−35 °C. IV. H NMR (500 MHz, CDCl3, 298 K): δ (ppm) = 7.33
(m, 4H, HAr), 7.15 (t, J = 7.4 Hz, 2H, HAr), 6.98 (d, J = 8.2 Hz, 4H,
HAr), 4.91 (s, 1H, CH), 1.95 (s, 6H, CH3), −0.33 (s, 18H, Si−CH3).
13C NMR (126 MHz, CDCl3, 298 K): δ [ppm] = 167.8, 148.7, 128.9,
H
DOI: 10.1021/acs.inorgchem.8b01060
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
́
125.2, 124.7, 96.2, 23.9, 4.53. Anal. Calcd for C23H35N3Si2Zn: C,
58.15; H, 7.43; N, 8.84. Found C, 58.18; H, 7.28; N, 8.69.
Coordenac
̧
a
̃
o de Aperfeic
̧
oamento de Pessoal de Nivel
Superior (Process no. 88881.131708/2016-01).
General Polymerization Procedure for BBL. The polymer-
izations performed in the in situ IR autoclave were performed in the
following way: The corresponding catalyst I−IV (1.00 equiv) was
dissolved in dry toluene (5.0 mL) and placed in a syringe. BBL was
stored in a second syringe. The two filled syringes were transported to
an in situ IR autoclave with mechanical stirring. The autoclave was
pretempered to a certain temperature under an argon atmosphere,
and the solution of the catalyst in toluene and BBL were transferred
into the reactor. After full conversion, an aliquot was taken to
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank Rike Adams for valuable discussions and
Christian Jandl and Alexander Pothig for crystallographic
̈
advice.
1
determine the conversion via H NMR spectroscopy. The amount of
toluene was reduced under vacuum prior to precipitation of the
polymer in 100 mL of pentane/diethyl ether (1:1). The polymer-
izations without IR monitoring were carried out in glass vials with
magnetic stirring under an argon atmosphere under the same
conditions as in the autoclave. In the case of using 2-propanol as
external alcohol, the catalyst was dissolved in toluene and treated with
the respective amount of iPrOH. This solution was stirred for 10 min
prior to monomer addition. Conversion was determined via 1H NMR
spectroscopy, and the polymers were precipitated in 100 mL of
pentane/diethyl ether (1:1).
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1
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́
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■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01060.
In situ IR spectroscopy plots, NMR data for micro-
1
structure determination of PHB and PLA, H DOSY
NMRs, GPC data, and SC-XRD data (PDF)
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Accession Codes
CCDC 1837007−1837010 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
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emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: rieger@tum.de. Tel: +49-89-289-13570. Fax: +49-89-
289-13562.
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ORCID
Bernhard Rieger: 0000-0002-0023-884X
Author Contributions
M.R. substantially contributed to this work during her time as a
co-worker of the Wacker-Lehrstuhl fu
Chemie. D.H.B. did a 6-month part of his Ph.D. at the
Wacker-Lehrstuhl fur Makromolekulare Chemie.
Funding
Financial support for D.H.B.’s research at the Wacker-
Lehrstuhl for Makromolekulare Chemie was provided by the
Molnar, F.; Urban, T.; Bernhart, V.; Preishuber-Pflugl, P.; Rieger, B.
̈
Variably Isotactic Poly(hydroxybutyrate) from Racemic β-Butyrolac-
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DOI: 10.1021/acs.inorgchem.8b01060
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Inorganic Chemistry
Article
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DOI: 10.1021/acs.inorgchem.8b01060
Inorg. Chem. XXXX, XXX, XXX−XXX