Monomethylaluminum and dimethylaluminum pyrrolylaldiminates for the ring-opening polymerization of rac-lactide: Effects of ligand structure and coordination geometry

Article abstract of DOI:10.1039/c3dt52455h

Two series of aluminum alkyl complexes supported by pyrrolylaldiminate ligands, LAlMe₂ (1a-7a) and L₂AlMe (1b-7b), were successfully synthesized and characterized by NMR spectroscopy and elemental analysis. Reactions of trimethylaluminum with the corresponding pyrrolylaldiminate ligands in the molar ratios of 1:1 and 1:2 yielded dimethylaluminum pyrrolylaldiminates (1a-7a) and monomethylaluminum pyrrolylaldiminates (1b-7b), respectively, in good yields. The structure of 3b, determined by single-crystal X-ray diffraction, displayed a distorted trigonal bipyramidal geometry with the τ value of 0.65. Upon addition of 1 equivalent of benzyl alcohol, all complexes promoted the living ring-opening polymerization of rac-lactide with a good control over molecular weights and polydispersities. Complexes 6a and 7a were found to efficiently mediate the immortal polymerization in the presence of excess equivalents of benzyl alcohol (up to 5 equivalents), as evidenced by the narrow PDI values and the good agreement between the experimental M_n values and monomer/benzyl alcohol ratios. The steric and electronic effects of the imine nitrogen substituents had a strong influence on the polymerization activities both in catalytic activity and polymer microstructure. The catalytic activity decreased as follows: 4-Me-C₆H₄ (3) > C₆H₅ (1) ≈ 4-F-C₆H₄ (2) ≈ 2-Me-C₆H₄ (5) > 4-OMe-C₆H₄ (4) ? 2-^tBu-C ₆H₄ (6) > adamantyl (7). In comparison, the catalytic activity of the monomethylaluminum complex is slightly higher than that of the dimethylaluminum counterpart. The polymerization of rac-lactide by 6b yielded heterotactically enriched polylactide (P_r = 0.60) whereas the isotactic-enriched polymer (P_m = 0.74) was obtained from 7b. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c3dt52455h

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Monomethylaluminum and dimethylaluminum
pyrrolylaldiminates for the ring-opening
polymerization of rac-lactide: effects of ligand
structure and coordination geometry†
Cite this: Dalton Trans., 2014, 43,
1348
Sittichoke Tabthong,a Tanin Nanok,a Palangpon Kongsaeree,b Samran Prabpaib and
Pimpa Hormnirun*a
Two series of aluminum alkyl complexes supported by pyrrolylaldiminate ligands, LAlMe2 (1a–7a) and
L2AlMe (1b–7b), were successfully synthesized and characterized by NMR spectroscopy and elemental
analysis. Reactions of trimethylaluminum with the corresponding pyrrolylaldiminate ligands in the molar
ratios of 1 : 1 and 1 : 2 yielded dimethylaluminum pyrrolylaldiminates (1a–7a) and monomethylaluminum
pyrrolylaldiminates (1b–7b), respectively, in good yields. The structure of 3b, determined by single-crystal
X-ray diffraction, displayed a distorted trigonal bipyramidal geometry with the τ value of 0.65. Upon
addition of 1 equivalent of benzyl alcohol, all complexes promoted the living ring-opening polymeriz-
ation of rac-lactide with a good control over molecular weights and polydispersities. Complexes 6a and
7a were found to efficiently mediate the immortal polymerization in the presence of excess equivalents
of benzyl alcohol (up to 5 equivalents), as evidenced by the narrow PDI values and the good agreement
between the experimental Mn values and monomer/benzyl alcohol ratios. The steric and electronic
effects of the imine nitrogen substituents had a strong influence on the polymerization activities both in
catalytic activity and polymer microstructure. The catalytic activity decreased as follows: 4-Me-C6H4 (3) >
C6H5 (1) ≈ 4-F-C6H4 (2) ≈ 2-Me-C6H4 (5) > 4-OMe-C6H4 (4) ≫ 2-tBu-C6H4 (6) > adamantyl (7). In compari-
son, the catalytic activity of the monomethylaluminum complex is slightly higher than that of the
dimethylaluminum counterpart. The polymerization of rac-lactide by 6b yielded heterotactically enriched
polylactide (Pr = 0.60) whereas the isotactic-enriched polymer (Pm = 0.74) was obtained from 7b.
Received 6th September 2013,
Accepted 22nd October 2013
DOI: 10.1039/c3dt52455h
www.rsc.org/dalton
microstructure via ligand modification, high Lewis acidity and
Introduction
low toxicity, although their activity is low.3
The ring-opening polymerization (ROP) of rac-lactide (rac-LA)
Over the past few decades, particular attention has been
to form polylactide (PLA) is a topic of interest in both aca- given to aluminum complexes supported by tetradentate
demic and industrial research due to the biomedical, pharma- ligands, such as Salen,4 Salan,5 and Salalen.6 These complexes
ceutical, and agriculture applications of polylactide.1 The ROP have been reported to display excellent molecular weight and
of rac-LA occurs via a coordination–insertion mechanism stereoselectivity control. Recently, the anilido–aldimine ligand,
initiated by metal alkoxides or amide complexes.2 Among the closely related to the Salen framework, has been used for the
metal-based initiators, aluminum complexes have received synthesis of dimethylaluminum complexes.7 These complexes
considerable attention because of their good control over were found to be effective initiators for the controlled ROP of
the polymerization reaction, ability to control the polymer rac-LA. For the bidentate aluminum complexes, most studies
have focused on the use of these complexes for the ROP of
ε-caprolactone.8 Only a few studies have been reported on low-
coordinate aluminum complexes bearing bidentate ligands for
the ROP of LA and other lactone monomers.9 For example,
aluminum complexes supported by bidentate phenoxy–
thioether ligands have been recently exploited to polymerize
ε-caprolactone, L-LA, and rac-LA in a controlled fashion and
the steric and electronic characteristics of the ligands were
found to have moderate influence on the polymerization
aDepartment of Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900,
Thailand. E-mail: fscipph@ku.ac.th
bDepartment of Chemistry and Center for Excellence in Protein Structure and
Function, Faculty of Science, Mahidol University, Bangkok 10400, Thailand
†Electronic supplementary information (ESI) available. CCDC 958599. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt52455h
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performance of the complexes.10 The ROP of L-LA with high aluminum complexes was demonstrated by the disappearance
conversions obtained in a short period of time was achieved by of the N–H signal of the free ligands and the appearance of
employing aminoarenethiolate aluminum complexes.11 the methyl protons bound to the aluminum in the high field
Recently, it has been found that dimethylaluminum complexes region (δ −0.26 to −0.69 ppm) of the NMR spectra, which is
stabilized by salicylaldiminate ligands promoted the living and characteristic of an Al–CH3 group (see Fig. S1–S14 in ESI†).15
well-controlled polymerization of rac-LA with various polymer The integration ratio of the signals ascribed to the imine and
microstructures, depending upon the imino substituents of the methyl protons of the aluminum methyl for complexes 1a–
the ligand framework.12
7a was 1 : 6, demonstrating the formation of the four-coordi-
Unlike the closely related salicylaldiminate ligands, the nate aluminum complexes, whereas the integration ratio of
pyrrolylaldiminate ligands have received much less attention. 2 : 3 was observed for five-coordinate aluminum counterparts
The synthesis of pyrrolylaldiminate complexes of Zn, Cr, Ti, 1b–7b.
Mg, Al, Sm and Y have been reported.13 To the best of our
Single crystals of 3b suitable for X-ray diffraction analysis
knowledge, there is no such report on the use of aluminum were grown from a saturated hexane solution at −20 °C. The
pyrrolylaldiminates for the application in the ROP of lactide molecular structure of 3b features a monomeric molecule with
and other cyclic ester monomers. Thus, herein we report the
a five-coordinate aluminum center in a geometry best
synthesis of two novel series of aluminum complexes sup- described as a distorted trigonal bipyramid with a τ value of
ported by bidentate pyrrolylaldiminate ligands. The catalytic 0.65.16 An ORTEP view and selected bond lengths and angles
performance of these complexes for the ROP of rac-LA is also are shown in Fig. 1 (see Tables S1 and S2 in ESI† for the
presented. Additionally, the effects of the structure of the crystallographic details). The pyrrolyl nitrogen atoms (N1 and
ancillary ligands and the coordination geometry around the N15) and the methyl carbon atom (C29) arrange at the equator-
metal center on the polymerization activity are discussed.
ial positions and axial positions are occupied by the two imino
Results and discussion
Synthesis and characterization of aluminum
pyrrolylaldiminate complexes 1a–7a and 1b–7b
The pyrrolylaldiminate ligands (HL1–HL7) were synthesized by
reacting pyrrole-2-carboxaldehyde with an equimolar amount
of the corresponding primary amines in ethanol with a small
amount of formic acid as a catalyst according to the literature
procedure.13c,14 As illustrated in Scheme 1, dimethylaluminum
pyrrolylaldiminate complexes (1a–7a) were prepared in good
yields (42–89%) by the stoichiometric reaction of the appropri-
ate ligands with one equivalent of AlMe3 in toluene at room
temperature. Treatment of AlMe3 with the appropriate ligands
in the molar ratio 1 : 2 in toluene at 100 °C afforded mono-
methylaluminum pyrrolylaldiminates (1b–7b) in good yields
(50–91%).
Fig. 1 Molecular structure of L12AlMe (1b). Selected bond lengths (Å)
and angles (°) are as follows: Al1–N21, 2.125(2); Al1–N7. 2162(2); Al1–N1,
1.912(2); Al1–N5, 1.908(2); Al1–C29, 1.963(3); N1–Al1–N15, 115.47(9);
N1–Al1–N21, 90.74(8); N1–Al1–N7, 80.23(8); N1–Al1–C29, 125.3(1);
N15–Al1–N21, 81.38(9); N15–Al1–N7, 91.13(9); N15–Al1–C29, 119.3(1);
N21–Al1–N7, 164.53(9); N21–Al1–C29, 97.9(1); N7–Al1–C29, 97.5(1).
1
All aluminum complexes were characterized by H and 13C
NMR spectroscopy and elemental analysis. The formation of
Scheme 1 Synthesis of pyrrolylaldiminate aluminum complexes.
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nitrogen atoms (N7 and N21). The Al–Npyrrole distances of between the number-averaged molecular weights (Mn) and
1.912(2) Å and 1.908(2) Å are longer than those in the recently monomer conversion with narrow molecular weight distri-
reported
compound
AlMe{2-[(3′,5′-Me2C3N2C6H4NvCH)- butions, as illustrated in Fig. 2 (see also Fig. S15–S27 in ESI†).
In the case of dimethylaluminum pyrrolylaldiminate com-
C4H3N]}2 [1.899(4) Å and 1.901(4) Å],17 while the Al–CH3 bond
[1.963(3) Å] is longer than that found in AlMe{2-[(3′,5′- plexes (1a–7a), a good correlation between the experimental
Me2C3N2C6H4NvCH)C4H3N]}2 [1.949(5) Å]. The Al–Nimino dis- and theoretical Mn values demonstrated that a single polymer
tances in complex 3b [2.125(2) Å and 1.162(2) Å] are longer chain was produced per aluminum center, reflecting the
than those reported in AlMe{2-[(3′,5′-Me2C3N2C6H4NvCH)- single-site nature of the active species. When two equivalents
C4H3N]}2 [2.115(3) Å and 2.112(4) Å]17 and AlCl{2-[(2′,6′- of benzyl alcohol were added, the molecular weights of polymers
iPr2C6H3NvCH)C4H3N]}2 [1.993(1) Å and 1.962(1) Å].13a
produced by 1a–5a were significantly lower than the theoretical
values and the PDI values became broader (entries 1–5,
Table 2), suggesting that benzyl alcohol acted as a chain trans-
fer agent in these catalytic systems.12b However, in the case of
complexes 6a and 7a, the immortal polymerization condi-
tion3h,12a,19 was observed as evidenced by the narrow PDI
values and the good agreement between the observed Mn
values and the monomer/alcohol ratio (entries 6 and 10,
Table 2). The immortal character of both complexes was
further investigated using excess equivalents of benzyl alcohol
Ring-opening polymerization of rac-lactide
Polymerizations of rac-LA using dimethylaluminum pyrrolyl-
aldiminate complexes (1a–7a) and monomethylaluminum
pyrrolylaldiminates (1b–7b) in the presence of 1 equivalent of
benzyl alcohol to form in situ aluminum alkoxide species were
carried out at 70 °C in toluene. The molar ratio of rac-LA to
initiator was fixed at 100 : 1 ([LA]0/[Al] = 100; [LA]0 = 0.83 M;
[Al] = 8.33 mM; Mn (theory) = 14 400). All complexes were
effective for the polymerization of rac-LA, as shown in Table 1.
The molecular weights and PDIs (Mw/Mn) were determined by
gel permeation chromatography (GPC) using the Mark–
Houwink correction factor of 0.58.18 All of the initiator systems
exhibited molecular weights in close agreement with theore-
tical values and narrow molecular weight distributions in
accord with controlled living polymerizations (entries 1–14).
For complexes 1a–5a and 1b–5b, the polymerizations pro-
ceeded to conversion over 84% within 8 hours (in the range of
84–92%) while the polymerizations using complexes 6a and 6b
reached 58% and 65% conversion, respectively, at the same
period of time. Changing the substituted phenyl group with
the adamantyl moiety (7) resulted in a significant decrease of
the catalytic activity. The polymerization time of 108 hours was
required for complexes 7a and 7b to achieve 91% and 95%
conversion, respectively. The living characteristic of the
● ○
( ) and PDI ( ) as a function of monomer
Fig. 2 Plots of PLA Mn
conversion for a rac-LA polymerization using 1a/PhCH2OH ([LA]0/[Al] =
polymerization was also demonstrated by a linear relationship 50, toluene, 70 °C).
Table 1 Polymerization of rac-lactide using complexes 1a–7a and 1b–7b in the presence of 1 equivalent of benzyl alcohola
d
e
e
Entry
Complex
Time (h)
Conversionb (%)
Mn (theory)c (g mol−1
)
Mn (GPC)d (g mol−1
)
Mw/Mn
Pr
Pm
1
2
3
4
5
6
7
8
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
8
8
8
8
8
8
8
8
8
8
8
8
108
108
86
91
92
86
90
92
90
89
84
92
58
65
91
95
12 500
13 200
13 400
12 500
13 100
13 400
13 100
12 900
12 200
13 400
8500
11 500
12 400
12 400
11 700
11 600
12 100
12 100
11 600
9800
12 100
6000
7100
14 000
13 300
1.16
1.28
1.24
1.15
1.21
1.16
1.18
1.14
1.19
1.21
1.06
1.12
1.06
1.04
0.50
0.47
0.46
0.47
0.46
0.48
0.48
0.47
0.44
0.45
0.58
0.60
0.37
0.26
0.50
0.53
0.54
0.53
0.54
0.52
0.52
0.53
0.56
0.55
0.42
0.40
0.63
0.74
9
10
11
12
13
14
9500
13 200
13 800
a [LA]0/[Al] = 100, [Al]/[PhCH2OH] = 1, [LA]0 = 0.83 M, toluene, 70 °C. b As determined via integration of the methine resonances (1H NMR) of LA
and PLA (CDCl3, 400 MHz). c Calculated by [([LA]0/[Al]) × 144.13 × conversion] + 108.14. d Determined by gel permeation-chromatography (GPC)
calibrated with polystyrene standards in THF and corrected by a factor of 0.58 for PLA. e Pm and Pr are the probability of meso and racemic
linkages between monomer units, respectively.
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Table 2 Polymerization of rac-lactide using complexes 1a–7a in the presence of benzyl alcohola
Conversionb (%)
Mn (theory)c (g mol−1
)
Mn (GPC)d (g mol−1
)
Mw/Mn
d
Entry
Complex
[LA]0 : [Al] : [PhCH2OH]
Time (h)
1
2
3
4
5
6
7
8
1a
2a
3a
4a
5a
6a
6a
6a
6a
7a
7a
7a
7a
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 2
100 : 1 : 3
100 : 1 : 4
100 : 1 : 5
100 : 1 : 2
100 : 1 : 3
100 : 1 : 4
100 : 1 : 5
12
12
12
12
12
24
12
12
12
108
24
24
24
95
97
94
98
95
95
94
94
96
95
96
96
98
7000
7100
6900
7200
7000
7000
4600
3500
2900
7000
4700
3600
2900
3700
3900
4000
2700
5000
7700
4000
3200
2800
8100
5600
4000
3200
1.93
1.94
1.98
1.75
2.44
1.10
1.05
1.05
1.03
1.10
1.14
1.14
1.10
9
10
11
12
13
a [LA]0 = 0.83 M, toluene, 70 °C. b As determined via integration of the methine resonances (1H NMR) of LA and PLA (CDCl3, 400 MHz).
c Calculated by ([([LA]0/[Al]) × 144.13 × conversion]/[PhCH2OH]) + 108.14. d Determined by gel permeation-chromatography (GPC) calibrated with
polystyrene standards in THF and corrected by a factor of 0.58 for PLA.
(up to 5 equivalents) (entries 7–9 and 11–13, Table 2). The displayed no induction period, indicating that the active
increase in the ratio of benzyl alcohol led to a decrease in the species form instantaneously by an in situ alcoholysis reaction.
molecular weight of the resulting polymer. The good degree of Thus, the polymerization proceeded according to the rate law
control over the polymerization in terms of the narrow PDI −d[LA]/dt = kapp[LA], where kapp = kp[Al]x, in which kp is the
values and the good agreement between the experimental and propagation rate constant. To determine the order in alumi-
monomer/benzyl alcohol ratios were still observed. This illus- num (x), kinetic experiments were conducted with variable
trates that the number of polymer molecules becomes larger, concentrations of the catalyst. For example, the appropriate
proportional to the number of added benzyl alcohol molecu- semi-logarithmic plots of different concentrations of dimethyl-
les.19d Taken together, these results showed that a fast revers- aluminum complex 1a are shown in Fig. 4. The linear relation-
ible exchange between dormant hydroxyl-end-capped polymer ship between ln kapp versus ln[1a] revealed that the order in the
chains/free alcohol and the active growing polymer chain co- catalyst was ca. 1.0 (1.15), indicating a first-order dependence
ordinated onto the aluminum center can take place faster than on the catalyst concentration (Fig. 5). The propagation rate
the chain propagation.19
constant (kp) of 1.07 × 10−2 s−1 mol−1 L was obtained from the
Kinetic studies of rac-LA polymerization initiated by com- gradient of the kapp versus [1a] plot, as depicted in Fig. 6.
plexes 1a–7a and 1b–7b in the presence benzyl alcohol were Therefore, the overall rate equation is −d[LA]/dt = kp[LA][1a].
carried out in toluene at 70 °C ([LA]0/[Al] = 50; [Al] = 8.33 mM; The same procedure was applied for the determination of the
[LA]0 = 0.42 M). In each case, kinetics of the first-order in overall rate law of complexes 1b, 4a, and 4b (see Fig. S34–S42
monomers were observed, as evidenced from the linear in ESI†). The order (x) in 1b was ca. 1.0 (0.98) and the kp value
relationship between ln([LA]0/[LA]t) and time (Fig. 3 and see of 1.33 × 10−2 s−1 mol−1 L was obtained, exhibiting the overall
Fig. S28–S33 in ESI†). In addition, the rac-LA polymerization
Fig. 4 Semilogarithmic plots of the rac-lactide conversion versus time
Fig. 3 Semilogarithmic plots of rac-lactide conversion versus time
in toluene at 70 °C with complex 1a/PhCH2OH as an initiator ([LA]0 =
●
■
in toluene at 70 °C with complexes 1a ( ) and 1b ( ) ([LA]0/[Al] = 50,
0.42 M: I, [Al] = 16.65 mM, [LA]0/[Al] = 25; II, [Al] = 12.49 mM, [LA]0/[Al] =
34; III, [Al] = 8.33 mM, [LA]0/[Al] = 50; IV, [Al] = 6.24 mM, [LA]0/[Al] = 67).
[Al]/[PhCH2OH] = 1, [LA]0 = 0.42 M, [Al] = 8.33 mM).
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Table 3 Kinetic results of rac-lactide polymerization using complexes
1a–7a and 1b–7b in the presence of 1 equivalent of benzyl alcohola
Entry
Complex
kapp (×10−6 s−1
)
1
2
3
4
5
6
7
8
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
89.1
110.7
88.7
115.6
113.0
144.5
72.4
95.5
83.6
107.1
24.7
34.4
9
10
11
12
13
14
15.6
19.7
Fig. 5 Plot of ln kapp versus ln [Al] for the polymerization of rac-lactide
with complex 1a/PhCH2OH as an initiator (toluene, 70 °C, [LA]0 = 0.42 M).
a [LA]0/[Al] = 50, [Al]/[PhCH2OH] = 1, [LA]0 = 0.42 M, toluene, 70 °C.
activity.5a,9h When a fluorine atom was introduced at the para-
position of the phenyl group (complexes 2a and 2b), the cata-
lytic activities of these complexes were higher than those of
complexes 4a and 4b, respectively, suggesting the enhance-
ment of the catalytic activity by a decrease of electron density
at the aluminum center.20 However, in comparison with the
para-methyl substituted complexes (3a and 3b), the catalytic
activities of the para-fluoro substituted complexes (2a and 2b)
slightly dropped but were comparable to those of the unsubsti-
tuted aluminum analogues (1a and 1b). These complicated
results were also observed for β-diketiminate aluminum com-
plexes containing fluorine and chlorine atoms at the para-posi-
tion of the phenyl moiety5a and bis(phenolato)bis(amine)
aluminum complexes with a para-bromo phenoxy substitu-
ent.21 In these cases, an electron-donating conjugated effect
was responsible for the lower activity: i.e., the lone pair in the
Fig. 6 Plot of kapp versus [Al] for the polymerization of rac-lactide with
complex 1a/PhCH2OH as an initiator (toluene, 70 °C, [LA]0 = 0.42 M).
rate equation is −d[LA]/dt = kp[LA][1b]. In the case of com- p-orbital of the halogen atom can donate electron via p–π
plexes 4a and 4b, the kp values were 1.18 × 10−2 s−1 mol−1
L
bonding to its para- and ortho-positions when it was intro-
and 1.32 × 10−2 s−1 mol−1 L, respectively. The order (x) values duced into a phenyl ring.5a Substitution of the methyl group at
for both cases were also ca. 1.0 (1.20 for 4a and 1.17 for 4b), the ortho-position of the phenyl rings (5a and 5b) resulted in a
supporting the first-order kinetics in monomer.
diminished activity when compared with the catalytic activities
The first-order rate constants (kapp) for the polymerization of the para-methyl substituted aluminum complexes (3a and
of rac-LA with complexes 1a–7a and 1b–7b were collected in 3b). The steric effect was more pronounced when the bulky
Table 3. It was found that the catalytic activity of each five- tert-butyl group was introduced at the ortho-position of the
coordinate aluminum complex was slightly higher than that of phenyl ring (complexes 6a and 6b). For example, the catalytic
its four-coordinate aluminum counterpart. This can be activity of the ortho-methyl phenoxy substituent complex 5a
ascribed to the more electrophilicity at the aluminum center was 144.5 × 10−6 s−1 (entry 6, Table 3) while the ortho-tert-butyl
due to the presence of two pyrrolylaldiminate ligands. The phenoxy substituent complex 6a exhibited the lower kapp value
results also revealed that the imino substituents exerted great of 24.7 × 10−6 s−1 (entry 11, Table 3). The observed lower
influence on the catalytic activity. The electronic factor of the activity was attributed to the steric protection at the aluminum
para-substituent of the phenyl group affected the catalytic center by the bulky tert-butyl group, which hindered the in-
activity in the order p-Me (3) > p-H (1) ≈ p-F (2) > p-OMe-C6H4 corporation of monomer into the growing polymer chain.22 In
(4). Complex 3b displayed the highest catalytic activity with the addition, replacing the substituted phenyl group with the steri-
kapp value of 144.5 × 10−6 s−1 (entry 6, Table 3) whereas cally more congested adamantyl moiety (complexes 7a and 7b)
complex 4a exhibited the lowest catalytic activity, kapp = 72.4 × led to a decrease in the catalytic activity. The lowest catalytic
10−6 s−1 (entry 7, Table 3). These results demonstrated that the activity observed in this study was obtained from complex 7a
presence of an electron-donating substituent increased the with the kapp value of 15.6 × 10−6 s−1 (entry 13, Table 3). These
electron density at the metal center, leading to a decrease in observations revealed that both electronic and steric effects in
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the imino substituent had a strong impact on the catalytic Ring-opening polymerization of ε-caprolactone
activity. For this study, the activity decreased in the order
As a result of the good performance in the polymerization of
rac-LA, all monomethyl aluminium and dimethylaluminum
pyrrolaldiminate complexes have been further applied to cata-
lyze the ROP of ε-caprolactone (ε-CL). The polymerizations
were carried out under the identical conditions for the
polymerization of rac-LA and the results are summarized in
Table 4. The molecular weights and PDIs (Mw/Mn) were deter-
mined by gel permeation chromatography (GPC) using the
Mark–Houwink correction factor of 0.56.24 It was revealed that
all complexes exhibited high activity. More than 99%
monomer conversion was achieved within 15 min in all cases.
A good agreement between the experimental and the theore-
tical Mn values and a narrow molecular weight distribution
were observed, indicating the single-site nature of these cata-
lysts. The results clearly indicate that these catalysts also
initiate the ROP of ε-CL in a controlled manner.
4-Me-C6H4 (3) > C6H5 (1) ≈ 4-F-C6H4 (2) ≈ 2-Me-C6H4 (5) >
4-OMe-C6H4 (4) ≫ 2-tBu-C6H4 (6) > adamantyl (7).
The polymer microstructure of the PLA was determined by
the inspection of the methine region of the homonuclear
decoupled 1H NMR spectra of the resultant polymers (Fig. 7
and Fig. S37–S48 in ESI†).23 Complexes 1a–5a and 1b–5b poly-
merized rac-LA to atactic polylactides with the Pm = 0.50–0.56
(entries 1–10, Table 1). Heterotactic-enriched polylactides were
produced by complexes 6a (Pr = 0.58) and 6b (Pr = 0.60) with
the ortho-tert-butyl substituent on the phenyl ring (entries 11
and 12, Table 1). Apparently, the steric hindrance at the ortho-
position of aniline derivatives had a certain effect on the
stereoselectivity. Exchanging the substituted phenyl unit with
the adamantyl group resulted in the production of isotactic-
enriched polylactide. PLA with a Pm value of 0.63 was obtained
from complex 7a (entry 13, Table 1). The use of five-coordinate
aluminum complex 7b with the N-adamantyl substituent gave
rise to isotacticity enhancement of the polymerization. The
PLA produced by 7b was isotactic with a Pm value of 0.74 (entry
14, Table 1), as evidenced by the observed strong mmm tetrad
in Fig. 7b.
Conclusions
In conclusion, the synthesis and characterization of two series
of aluminum complexes supported by pyrrolylaldiminate
ligands, LAlMe2 (1a–7a) and L2AlMe (1b–7b), were reported. All
complexes were found to be efficient catalysts for the polymer-
ization of rac-lactide and ε-caprolactone in the presence of one
equivalent of benzyl alcohol, displaying controlled and living
polymerization. The imino substituent and the number of
ligands coordinated onto the aluminum center had a strong
effect on the catalytic activity and stereoselectivity. In particu-
lar, the steric hindrance at the ortho-position of the phenyl
group of the imino substituent was detrimental to the catalytic
activity. The five-coordinate aluminum complex exhibited a
slightly higher catalytic activity than its four-coordinate
counterpart. In addition, the polymer microstructure of poly-
lactides can be varied from heterotactic-bias to isotactic-bias,
depending on the use of imino substituent. Further study will
be focused on the fine tuning of the imino substituent in
order to enhance the catalytic activity and improve the stereo-
selectivity of these groups of complexes.
Experimental section
Materials and methods
All manipulations with air- and/or water-sensitive compounds
were carried out under a dry nitrogen atmosphere using stan-
dard Schlenk and cannula techniques in oven dried glassware
or in a glove box. Toluene was distilled from Na-benzophenone
before use. Hexane and pentane were distilled from CaH2 prior
to use. Benzyl alcohol was refluxed over sodium and then
freshly distilled onto activated 4 Å molecular sieves. All sol-
vents were degassed prior to use unless stated otherwise. NMR
solvents were dried over 4 Å molecular sieves and degassed
prior to use. Aniline (99%), 4-fluoroaniline (99%), 4-methyl-
aniline (99%), 4-methoxyaniline (99%), 2-methylaniline (99%),
Fig. 7 Homonuclear decoupled 1H NMR spectra of the methine region
of PLA prepared from rac-lactide at 70 °C in toluene (500 MHz, CDCl3)
with (a) 6b/PhCH2OH and (b) 7b/PhCH2OH.
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Table 4 Polymerization of ε-caprolactone using complexes 1a–7a and 1b–7b in the presence of benzyl alcohola
Conversionb (%)
Mn (theory)c (g mol−1
)
Mn (GPC)d (g mol−1
)
Mw/Mn
d
Entry
Complex
Time (min)
1
2
3
4
5
6
7
8
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
15
15
15
15
15
15
15
15
15
15
15
15
15
15
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
10 900
11 300
11 500
10 100
10 700
10 700
11 200
10 400
11 300
10 300
10 100
11 300
11 500
11 500
1.13
1.16
1.18
1.15
1.25
1.17
1.17
1.21
1.20
1.18
1.09
1.14
1.15
1.13
9
10
11
12
13
14
a [CL]0/[Al] = 100, [Al]/[PhCH2OH] = 1, [CL]0 = 0.83 M, toluene, 70 °C. b As determined via integration of the methylene resonances (1H NMR) of
CL and PCL (CDCl3, 400 MHz). c Calculated by [([CL]0/[Al]) × 114.13 × conversion] + 108.14. d Determined by gel permeation-chromatography
(GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.56 for PCL.
2-tert-butylaniline (99%), 1-adamantylamine (97%), pyrrole-2- temperature, during which a white precipitate formed. The
carboxaldehyde (98%) and trimethyl aluminum (2.0 M solu- mixture was stirred for 5 hours and the white solid was filtered
tion in toluene) were purchased from Aldrich and used as and dried in vacuo. Yield: 0.52 g, 29%.
received. The monomer rac-lactide (Aldrich) was sublimed
three times prior to use. All other chemicals were commercially (2H, m, o-C6H5), 7.25–7.20 (3H, m, m/p-C6H5), 6.83–6.81 (1H,
1H NMR (400 MHz, CDCl3): δ 8.27 (1H, s, NCH), 7.42–7.38
4
3
available and used as received unless otherwise stated.
m, pyrrole-H), 6.71 (1H, dd, JHH = 1.4, JHH = 3.6, pyrrole-H),
1H (399.87 MHz) and 13C (100.55 MHz) nuclear magnetic 6.27 (1H, dd, 3JHH = 2.6, 3JHH = 3.6, pyrrole-H).
resonance (NMR) spectra were recorded on a Varian Unity
13C NMR (100.6 MHz, CDCl3): δ 151.8 (i-C6H5), 150.2 (NCH),
Inova 400 MHz spectrometer at 300 K. Homonuclear 130.65 (pyrrole-C), 129.2 (o-C6H5), 125.4 (pyrrole-CH), 123.5
decoupled 1H NMR experiments were performed on Bruker (p-C6H5), 120.9 (m-C6H5), 116.9 (pyrrole-CH), 110.3 (pyrrole-
Avance 500 MHz spectrometer. NMR spectra were referenced CH).
internally to the residual protio impurity peaks in the CDCl3
Anal. calcd for C11H10N2 (170.21): C, 77.62; H, 5.92; N,
solvent (δ 1H: 7.26, 13C: 77.0). The following abbreviations 16.46%. Found C, 77.73; H, 5.89; N, 16.63%.
have been used for multiplicities: s (singlet); d (doublet);
Synthesis of (4-fluorophenyl)(1H-pyrrol-2-ylmethylene)amine
t (triplet); q (quartet); sept (septet); dd (doublet of doublets); (HL2). The synthesis of HL2 was performed according to the
dt (doublet of triplets); td (triplet of doublets); m (unresolved same procedure as for HL1. Yield: 1.40 g, 70%.
multiplet); br (board). Elemental analysis data (C, H, N) were
1H NMR (400 MHz, CDCl3): δ 10.42 (1H, br s, pyrrole-NH),
obtained from a LECO Elemental Analyser CS-932H. Gel per- 8.26 (1H, s, NCH), 7.20–7.16 (2H, m, C6H4), 7.10–7.05 (2H, m,
4
meation chromatography (GPC) measurements were con- C6H4), 6.81–6.80 (1H, m, pyrrole-H), 6.71 (1H, dd, JHH = 1.4,
3
3
ducted on a Polymer Laboratories PL-GPC-220 instrument 3JHH = 3.6, pyrrole-H), 6.29 (1H, dd, JHH = 2.6, JHH = 3.6,
equipped with PLgel 5 μm MIXED-D 300 × 7.5 mm columns pyrrole-H).
and tetrahydrofuran (THF) was used as the eluent (flow rate:
13C NMR (100.6 MHz, CDCl3): δ 162.0 (ArCF), 159.6 (ArCF),
1 mL min−1 at 40 °C). The number averaged molecular weights 150.0 (NCH), 147.7 (ArCN), 130.4 (pyrrolyl-C), 123.7 (pyrrole-
(Mn) and polydispersity index (Mw/Mn) were calibrated against CH), 122.2 (ArCF), 122.2 (C6H4), 117.2 (pyrrole-CH), 116.0
polystyrene (PS) standards. To account for the difference in the (ArCF), 115.8 (C6H4), 110.4 (pyrrole-CH).
hydrodynamic volume of polystyrene and polymer, Mn values
Anal. calcd for C11H9FN2 (188.20): C, 70.20; H, 4.82; N,
of PLAs and PCLs were corrected with a Mark–Houwink factor 14.88%. Found C, 69.99; H, 4.70; N, 14.68%.
of 0.5818 and 0.56,24 respectively.
Synthesis of (4-methylphenyl)(1H-pyrrol-2-ylmethylene)-
amine (HL3). The synthesis of HL3 was performed according
to the same procedure as for HL1. Yield: 1.03 g, 53%.
Synthesis of ligands
1H NMR (400 MHz, CDCl3): δ 10.11 (1H, br s, pyrrole-NH),
8.29 (1H, s, NCH), 7.21–7.19 (2H, m, C6H4), 7.15–7.12 (2H, m,
Synthesis of phenyl(1H-pyrrol-2-ylmethylene)amine, (HL1).
The reaction was performed according to the procedure pre-
viously reported in the literature.13c,14 To a stirred solution of
pyrrole-2-carboxaldehyde (1.00 g, 10.52 mmol) in methanol
(15 mL) was slowly added aniline (0.98 g, 10.52 mmol). The
reaction mixture was stirred and a catalytic amount of formic
acid was added. The reaction mixture was stirred at room
4
C6H4), 6.82–6.80 (1H, m, pyrrole-H), 6.68 (1H, dd, JHH = 1.4,
3
3
3JHH = 3.6, pyrrole-H), 6.28 (1H, dd, JHH = 2.6, JHH = 3.5,
pyrrole-H), 2.39 (3H, s, p-CH3C6H4).
13C NMR (100.6 MHz, CDCl3): δ 149.5 (NCH), 149.2 (i-C6H4),
135.2 (p-C6H4), 130.8 (pyrrole-C), 129.8 (C6H4), 123.4 (pyrrole-
1354 | Dalton Trans., 2014, 43, 1348–1359
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CH), 120.8 (C6H4), 116.6 (pyrrole-CH), 110.2 (pyrrole-CH), 20.9
(p-CH3C6H4).
13C NMR (100.6 MHz, CDCl3): δ 169.9 (NCH), 146.1
(pyrrole-C), 124.6 (pyrrole-CH), 117.2 (pyrrole-CH), 110.4
Anal. calcd for C12H12N2 (184.24): C, 78.23; H, 6.57; N, (pyrrole-CH), 57.1 (NC(CH2)3), 42.9 (C(CH2)3), 36.4 (CH(CH2)3),
15.21%. Found C, 78.30; H, 6.53; N, 15.47%.
29.5 (CH(CH2)3).
Synthesis of (4-methoxyphenyl)(1H-pyrrol-2-ylmethylene)-
Anal. calcd for C15H20N2 (228.33): C, 78.90; H, 8.83; N,
amine (HL4). The synthesis of HL4 was performed according 12.27%. Found C, 79.09; H, 8.79; N, 12.46%.
to the same procedure as for HL1. Yield: 0.80 g, 40%.
1H NMR (400 MHz, CDCl3): δ 8.28 (1H, s, NCH), 7.21–7.178
Synthesis of complexes
Synthesis of L1AlMe2 (1a). To a stirred solution of HL1
(2H, m, C6H4), 6.94–6.91 (2H, m, C6H4), 6.85–6.83 (1H, m, pyrrole-
4
3
H), 6.65 (1H, dd, JHH = 1.4, JHH = 3.6, pyrrole-H), 6.27 (1H, dd,
3JHH = 2.6, 3JHH = 3.6, pyrrole-H), 3.83 (3H, s, p-OCH3C6H4).
(0.80 g, 4.70 mmol) in toluene (20 mL) was slowly added AlMe3
13C NMR (100.6 MHz, CDCl3): δ 151.0 (p-OCH3C6H4), 148.4 (2.35 mL of a 2.0 M solution in toluene, 4.70 mmol). The reac-
(NCH), 144.7 (i-C6H4), 130.9 (pyrrole-C), 123.0 (pyrrole-CH), tion mixture was stirred at 100 °C for 24 hours, after which the
121.940 (C6H4), 116.2 (pyrrole-CH), 114.4 (C6H4), 110.2 volatiles were removed in vacuo to yield an orange oil. The
(pyrrole-CH), 55.49 (p-OCH3C6H4).
product was obtained as a yellow crystal by sublimation under
Anal. calcd for C12H12N2O (200.24): C, 71.98; H, 6.04; N, reduced pressure at room temperature. Yield: 0.45 g, 42%.
4
13.99%. Found C, 71.13; H, 5.89; N, 13.33%.
Synthesis of (2-methylphenyl)(1H-pyrrol-2-ylmethylene)- 7.45–7.40 (2H, m, ArH), 7.35–7.31 (3H, m, ArH and pyrrole-H),
1H NMR (400 MHz, CDCl3): δ 8.36 (1H, d, JHH = 1.1, NCH),
amine (HL5). The synthesis of HL5 was performed according 7.30–7.25 (1H, m, ArH), 7.05 (1H, dd, JHH = 1.0, JHH = 3.7,
4
3
4
3
to the same procedure as for HL1. Yield: 1.13 g, 59%.
1H NMR (400 MHz, CDCl3): δ 10.74 (1H, br s, pyrrole-NH), −0.62 (6H, s, Al(CH3)2).
8.23 (1H, s, NCH), 7.34–7.28 (2H, m, C6H4), 7.23–7.18 (1H, m,
13C NMR (100.6 MHz, CDCl3): δ 153.8 (NCH), 144.1 (ArC),
C6H4), 7.06–7.03 (1H, m, C6H4), 6.72 (1H, dd, 4JHH = 1.4, 3JHH
137.2 (pyrrole-CH), 136.4 (pyrrole-C), 129.9 (ArCH), 126.6
3.6, pyrrole-H), 6.54–6.56 (1H, m, pyrrole-H), 6.26 (1H, dd, (ArCH), 121.4 (ArCH), 120.4 (pyrrole-CH), 116.0 (pyrrole-CH).
3JHH = 2.6, 3JHH = 3.6, pyrrole-H), 2.42 (3H, s, o-CH3C6H4).
Anal. calcd for C13H15N2Al (226.25): C, 69.01; H, 6.68; N,
13C NMR (100.6 MHz, CDCl3): δ 151.0 (i-C6H4), 150.2 (NCH), 12.38%. Found: C, 68.95; H, 7.09; N, 12.19%.
pyrrole-H), 6.48 (1H, dd, JHH = 1.80, JHH = 3.7, pyrrole-H),
=
131.7 (o-C6H4), 130.6 (pyrrole-C), 130.4 (C6H4), 126.9 (C6H4),
Synthesis of L2AlMe2 (2a). To a stirred suspension of HL2
125.3 (C6H4), 123.7 (C6H4), 118.4 (pyrrole-CH), 116.7 (pyrrole- (1.60 g, 8.50 mmol) in toluene (20 mL) was slowly added AlMe3
CH), 110.1 (pyrrole-CH), 17.9 (o-CH3C6H4).
Anal. calcd for C12H12N2 (184.24): C, 78.23; H, 6.57; N, tion mixture was stirred at room temperature for 24 hours,
15.21%. Found C, 78.43; H, 6.44; N, 15.56%. after which the volatiles were removed in vacuo to leave an
(4.25 mL of a 2.0 M solution in toluene, 8.50 mmol). The reac-
Synthesis of (2-tert-butylphenyl)(1H-pyrrol-2-ylmethylene)- orange oil. After recrystallization in hexane at −20 °C, pale
amine (HL6). The synthesis of HL6 was performed according yellow crystals formed. Yield: 1.31 g, 63%.
4
to the same procedure as for HL1. Yield: 1.57 g, 66%.
1H NMR (400 MHz, CDCl3): δ 8.26 (1H, d, JHH = 1.0, NCH),
1H NMR (400 MHz, CDCl3): δ 9.38 (1H, br s, pyrrole-NH), 7.34–7.32 (1H, m, pyrrole-H), 7.29–7.25 (2H, m, ArH), 7.13–7.07
4
3
4
3
8.12 (1H, s, NCH), 7.40 (1H, dd, JHH = 1.5, JHH = 7.8, C6H4), (2H, m, ArH), 7.03 (1H, dd, JHH = 0.9, JHH = 3.7, pyrrole-H),
4
3
4
4
3
7.24 (1H, td, JHH = 1.6, JHH = 7.5, C6H4), 7.16 (1H, td, JHH
1.6, JHH = 7.5, C6H4), 6.96–6.94 (1H, m, pyrrole-H), 6.84 (1H, Al(CH3)2).
=
6.46 (1H, dd, JHH = 1.1, JHH = 3.7, pyrrole-H), −0.63 (6H, s,
3
4
3
4
dd, JHH = 1.6, JHH = 7.5, C6H4), 6.68 (1H, dd, JHH = 1.3,
13C NMR (100.6 MHz, CDCl3): δ 162.4 (ArCF), 159.9 (ArCF),
3JHH = 3.6, pyrrole-H), 6.33 (1H, dd, JHH = 2.7, JHH = 3.6, 153.9 (NCH), 140.4 (ArCN), 137.3 (pyrrolyl-CH), 136.2 (pyrrole-
pyrrole-H), 1.47 (9H, s, o-C(CH3)3C6H4). C), 121.8 (ArCH), 121.8 (pyrrole-CH), 121.5 (ArCH), 116.8
13C NMR (100.6 MHz, CDCl3): δ 151.64 (i-C6H4), 148.0 (ArCH), 116.6 (ArCH), 116.1 (pyrrole-CH).
3
3
(NCH), 142.8 (o-C6H4), 131.4 (pyrrole-C), 127.1 (C6H4), 126.0
Anal. calcd for C14H14N2FAl (244.24): C, 63.93; H, 5.78; N,
(C6H4), 125.1 (C6H4), 122.6 (C6H4), 119.7 (pyrrole-CH), 115.4 11.47%. Found: C, 64.02; H, 5.41; N, 11.70%.
(pyrrole-CH), 110.4 (pyrrole-CH), 35.6 (o-C(CH3)3C6H4), 30.8
(o-C(CH3)3C6H4).
Synthesis of L3AlMe2 (3a). To a stirred suspension of HL3
(1.00 g, 5.43 mmol) in toluene (20 mL) was slowly added AlMe3
Anal. calcd for C15H18N2 (226.32): C, 79.61; H, 8.02; N, (2.71 mL of a 2.0 M solution in toluene, 5.43 mmol). The reac-
12.38%. Found C, 79.90; H, 8.04; N, 12.31%. tion mixture was stirred at room temperature for 45 min, after
Synthesis of (1-adamantyl)(1H-pyrrol-2-ylmethylene)amine which the volatiles were removed in vacuo to leave a yellow oil.
(HL7). The synthesis of HL7 was performed according to the Yield: 1.16 g, 89%.
4
same procedure as for HL1. Yield: 1.11 g, 46%.
1H NMR (400 MHz, CDCl3): δ 8.32 (1H, d, JHH = 1.1, NCH),
1H NMR (400 MHz, CDCl3): δ 8.74 (1H, br s, pyrrole-NH), 7.32–7.30 (1H, m, pyrrole-H), 7.21 (4H, s, ArH), 7.00 (1H, dd,
3
4
3
7.78 (1H, s, NCH), 6.95–7.01 (1H, m, pyrrole-H), 6.59 (1H, d, 4JHH = 0.9, JHH = 3.6, pyrrole-H), 6.45 (1H, dd, JHH = 1.8, JHH
4JHH = 1.6, pyrrole-H), 6.25 (1H, dd, JHH = 2.8, JHH = 3.2, = 3.7, pyrrole-H), 2.36 (3H, s, ArCH3), −0.63 (s, 6H, Al(CH3)2).
3
3
3
pyrrole-H), 2.16 (3H, br s, CH), 1.82 (6H, d, JHH = 2.4, NCH2),
1.57–1.69 (6H, m, CH2).
13C NMR (100.6 MHz, CDCl3): δ 153.3 (NCH), 141.6 (ArCN),
136.7 (pyrrolyl-CH), 136.7 (pyrrole-CH), 136.6 (pyrrole-C),
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136.3 (ArC), 130.4 (ArCH), 120.8 (pyrrole-CH), 120.1 (ArCH),
115.7 (pyrrole-CH), 21.0 (ArCH3), −0.49 (Al(CH3)2).
Anal. calcd for C14H17N2Al (240.28): C, 69.98; H, 7.13; N,
11.66%. Found: C, 69.86; H, 7.70; N, 11.49%.
Anal. calcd for C17H23N2Al (282.36): C, 72.31; H, 8.21; N,
9.92%. Found: C, 72.10; H, 8.62; N, 9.80%.
Synthesis of L7AlMe2 (7a). To a stirred suspension of HL7
(1.00 g, 4.38 mmol) in toluene (20 mL) was slowly added AlMe3
Synthesis of L4AlMe2 (4a). To a stirred suspension of HL4 (2.20 mL of a 2.0 M solution in toluene, 4.40 mmol). The reac-
(0.90 g, 4.49 mmol) in toluene (20 mL) was slowly added AlMe3 tion mixture was stirred at room temperature for 4 hours, after
(2.25 mL of a 2.0 M solution in toluene, 4.49 mmol). The which the volatiles were removed in vacuo to leave a white
reaction mixture was stirred at room temperature for 24 hours, solid. Recrystallization from hexane at −20 °C afforded color-
after which the volatiles were removed in vacuo to leave a less crystals. Yield: 0.762 g, 61%.
brown oil. After recrystallization in hexane at −20 °C, yellow
1H NMR (400 MHz, CDCl3): δ 8.17 (1H, s, NCH), 7.19 (1H,
3
3
3
crystals formed. Yield: 0.71 g, 62%.
dd, JHH = 1.0, JHH = 1.0, pyrrole-H), 6.84 (1H, dd, JHH = 2.5,
4
3
4
1H NMR (400 MHz, CDCl3): δ 8.27 (1H, d, JHH = 1.0, NCH), 4JHH = 1.0, pyrrole-H), 6.40 (1H, dd, JHH = 2.5, JHH = 1.0,
4
3
3
7.28–7.23 (1H, m, pyrrole-H), 6.98 (1H, dd, JHH = 0.9, JHH = 3.6, pyrrole-H), 2.22 (3H, br s, CH), 1.96 (6H, d, JHH = 2.4, NCH2),
pyrrole-H), 6.96–6.91 (2H, m, ArH), 6.44 (1H, dd, 4JHH = 1.8, 3JHH
3.6, pyrrole-H), 3.82 (3H, s, ArOCH3), −0.63 (6H, s, Al(CH3)2).
=
1.71–1.74 (6H, m, CH2), −0.64 (6H, s, Al(CH3)2).
13C NMR (100.6 MHz, CDCl3): δ 155.2 (NCH), 135.4
13C NMR (100.6 MHz, CDCl3): δ 158.4 (ArCOCH3), 152.9 (pyrrole-C), 134.0 (pyrrole-CH), 117.8 (pyrrole-CH), 114.1
(NCH), 137.4 (ArCN), 136.3 (pyrrolyl-CH), 136.2 (pyrrole-C), (pyrrole-CH), 56.9 (NC(CH2)3), 42.6 (C(CH2)3), 36.0 (CH(CH2)3),
121.4 (ArCH), 120.4 (pyrrole-CH), 115.5 (pyrrole-CH), 115.0 29.5 (CH(CH2)3).
(ArCH), 55.54 (ArOCH3), −10.2 (Al(CH3)2).
Anal. calcd for C17H25N2Al (284.38): C, 71.80; H, 8.86; N,
Anal. calcd for C14H17N2OAl (256.28): C, 65.61; H, 6.69; N, 9.85%. Found: C, 71.99; H, 8.22; N, 9.65%.
10.93%. Found: C, 65.57 H, 6.28; N, 11.06%.
Synthesis of L12AlMe (1b). To a stirred solution of HL1
Synthesis of L5AlMe2 (5a). To a stirred solution of HL5 (1.00 g, 5.88 mmol) in toluene (20 mL) was slowly added AlMe3
(0.83 g, 4.50 mmol) in toluene (20 mL) was slowly added AlMe3 (1.47 mL of a 2.0 M solution in toluene, 2.97 mmol). The reac-
(2.25 mL of a 2.0 M solution in toluene, 4.50 mmol). The reac- tion mixture was stirred at 100 °C for 3 days. The volatiles were
tion mixture was stirred at room temperature for 24 hours. The removed in vacuo to leave an orange oil which was then recrys-
mixture was then concentrated in vacuo. Pale yellow crystals tallized for hexane at −20 °C to afford the product as a yellow
were then formed at room temperature. Yield: 0.895 g, 83%.
solid. Yield: 0.54 g, 50%.
4
4
1H NMR (400 MHz, CDCl3): δ 7.96 (1H, d, JHH = 1.0, NCH),
1H NMR (400 MHz, CDCl3): δ 8.51 (2H, d, JHH = 0.9, NCH),
7.34–7.32 (1H, m, pyrrole-H), 7.29–7.17 (3H, m, ArH), 7.06–7.03 7.57–7.52 (4H, m, ArH), 7.45–7.38 (4H, m, ArH), 7.29–7.24 (2H,
4
3
(1H, m, ArH), 7.00 (1H, dd, JHH = 0.9, JHH = 3.7, pyrrole-H), m, ArH), 6.90–6.87 (4H, m, pyrrole-H), 6.25 (2H, dd, 4JHH = 2.1,
4
3
6.46 (1H, dd, JHH = 1.8, JHH = 3.6, pyrrole-H), 2.29 (3H, s, 3JHH = 3.5, pyrrole-H), −0.46 (3H, s, AlCH3).
ArCH3), −0.69 (6H, s, Al(CH3)2).
13C NMR (100.6 MHz, CDCl3): δ 152.5 (NCH), 146.2 (ArC),
13C NMR (100.6 MHz, CDCl3): δ 159.5 (NCH), 144.4 (ArCN), 136.9 (pyrrole-CH), 136.6 (pyrrole-C), 129.5 (ArCH), 126.2
136.7 (pyrrolyl-CH), 135.8 (ArC), 131.4 (ArCH), 131.1 (pyrrole- (ArCH), 121.4 (ArCH), 119.6 (pyrrole-CH), 114.3 (pyrrole-CH).
C), 126.9 (ArCH), 126.9 (ArCH), 124.0 (ArCH), 120.7 (pyrrole-
CH), 115.5 (pyrrole-CH), 18.3 (ArCH3), −10.4 (Al(CH3)2).
Anal. calcd for C14H17N2Al (240.28): C, 69.98; H, 7.13; N,
11.66%. Found: C, 70.00; H, 7.32; N, 11.68%.
Anal. Calcd for C23H21N4Al (380.42): C, 72.62; H, 5.56; N,
14.73%. Found: C, 72.75; H, 5.65; N, 14.77%.
Synthesis of L22AlMe (2b). To a stirred suspension of HL2
(1.00 g, 5.31 mmol) in toluene (20 mL) was slowly added AlMe3
Synthesis of L6AlMe2 (6a). To a stirred suspension of HL6 (1.33 mL of a 2.0 M solution in toluene, 2.66 mmol). The reac-
(0.77 g, 3.40 mmol) in toluene (20 mL) was slowly added AlMe3 tion mixture was stirred at 100 °C for 24 hours. The volatiles
(1.70 mL of a 2.0 M solution in toluene, 3.40 mmol). The reac- were removed in vacuo to leave a yellow solid which was then
tion mixture was stirred at room temperature for 24 hours, washed with hexane and dried in vacuo. Yield: 0.82 g, 74%.
4
after which the volatiles were removed in vacuo to leave a white
1H NMR (400 MHz, CDCl3): δ 8.38 (2H, d, JHH = 0.8, NCH),
solid as the product. Yield: 0.873 g, 81%.
7.46–7.40 (4H, m, ArH), 7.07–7.01 (4H, m, ArH), 6.82 (2H, dd,
4
3
1H NMR (400 MHz, CDCl3): δ 7.91 (1H, d, JHH = 1.0, NCH), 4JHH = 1.0, JHH = 3.5, pyrrole-H), 6.78–6.76 (2H, m, pyrrole-H),
4
3
4
3
7.54 (1H, dd, JHH = 1.5, JHH = 8.0, ArH), 7.34–7.33 (1H, m, 6.19 (2H, dd, JHH = 2.0, JHH = 3.5, pyrrole-H), −0.58 (3H, s,
4
3
pyrrole-H), 7.26 (1H, dt, JHH = 1.0, JHH = 7.3, ArH), 7.20 (1H, AlCH3).
4
3
4
3
dt, JHH = 1.6, JHH = 7.3, ArH), 6.98 (1H, dd, JHH = 0.9, JHH
=
13C NMR (100.6 MHz, CDCl3): δ 162.3 (ArCF), 159.8 (ArCF),
4
3
3.6, pyrrole-H), 6.95 (1H, dd, JHH = 1.6, JHH = 7.7, ArH), 6.46 152.5 (NCH), 152.5 (NCH), 142.4 (ArCN), 142.4 (ArCN), 136.8
(1H, dd, JHH = 1.8, JHH = 3.6, pyrrole-H), 1.38 (9H, s, ArC (pyrrolyl-CH), 136.5 (pyrrole-C), 122.8 (ArCH), 122.8 (ArCH),
4
3
(CH3)3), −0.64 (6H, s, Al(CH3)2).
119.8 (pyrrole-CH), 116.5 (ArCH), 116.2 (ArCH), 114.5 (pyrrole-
13C NMR (100.6 MHz, CDCl3): δ 160.8 (NCH), 144.3 (ArCN), CH).
143.8 (ArC), 136.8 (pyrrolyl-CH), 135.3 (pyrrole-C), 128.8
Anal. calcd for C23H19N4F2Al (416.40): C, 66.34; H, 4.60; N,
(ArCH), 127.3 (ArCH), 127.0 (ArCH), 126.7 (ArCH), 120.5 13.45%. Found: C, 66.31; H, 4.83; N, 13.44%.
(pyrrole-CH), 115.4 (pyrrole-CH), 36.0 (ArC(CH3)3), 33.0
(ArC(CH3)3), −9.3 (Al(CH3)2).
Synthesis of L32AlMe (3b). To a stirred suspension of HL3
(0.70 g, 3.80 mmol) in toluene (20 mL) was slowly added AlMe3
1356 | Dalton Trans., 2014, 43, 1348–1359
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4
3
(1.20 mL of a 2.0 M solution in toluene, 1.90 mmol). The reac- ArH), 6.73 (2H, dd, JHH = 1.0, JHH = 3.6, pyrrole-H), 6.00 (2H,
4
3
tion mixture was stirred at 100 °C for 24 hours. The volatiles dd, JHH = 1.9, JHH = 3.6, pyrrole-H), 5.79–5.68 (2H, m, ArH),
were removed in vacuo to leave a yellow solid which was then 1.22 (18H, s, Ar(CH3)3), −0.64 (3H, s, AlCH3).
washed with hexane and dried in vacuo. Yield: 0.56 g, 72%.
13C NMR (100.6 MHz, CDCl3): δ 160.8 (NCH), 147.5 (ArCN),
1H NMR (400 MHz, CDCl3): δ 8.50 (2H, d, 4JHH = 0.80 NCH), 143.8 (ArC), 138.5 (pyrrolyl-CH), 134.2 (pyrrole-C), 128.6
7.47–7.42 (4H, m, ArH), 7.24–7.19 (4H, m, ArH), 8.88–6.85 (4H, (ArCH), 127.7 (ArCH), 126.6 (ArCH), 126.6 (ArCH), 119.3
m, pyrrole-H), 6.25–6.22 (2H, m, pyrrole-H), 2.38 (3H, s, (pyrrole-CH), 114.4 (pyrrole-CH), 36.1 (ArC(CH3)3), 32.4 (ArC-
ArCH3), −0.49 (3H, s, AlCH3).
(CH3)3).
Anal. calcd for C31H37N4Al (492.63): C, 75.58; H, 7.57; N,
13C NMR (100.6 MHz, CDCl3): δ 151.9 (NCH), 143.7 (ArCN),
136.6 (pyrrolyl-CH), 136.1 (pyrrole-C), 130.1 (ArCH), 121.3 11.37%. Found: C, 75.62; H, 7.62; N, 11.42%.
(ArCH), 119.0 (pyrrole-CH), 114.0 (ArCH), 21.0 (ArCH3).
Synthesis of L72AlMe (7b). To a stirred suspension of HL7
Anal. calcd for C25H25N4Al (408.47): C, 73.51; H, 6.17; N, (0.80 g, 3.55 mmol) in toluene (20 mL) was slowly added AlMe3
13.72%. Found: C, 73.58; H, 6.81; N, 13.66%.
(0.89 mL of a 2.0 M solution in toluene, 1.78 mmol). The
Synthesis of L42AlMe (4b). To a stirred suspension of HL4 reaction mixture was stirred at 100 °C overnight during which
(0.86 g, 4.29 mmol) in toluene (20 mL) was slowly added AlMe3 time a white solid precipitated. Solid was isolated by filtration
(1.07 mL of a 2.0 M solution in toluene, 2.14 mmol). The reac- and dried in vacuo. Yield: 0.64 g, 57%.
tion mixture was stirred at 100 °C for 2 days. The volatiles were
removed in vacuo to leave a yellow solid which was then dd, JHH = 2.0, JHH = 2.0, pyrrole-H), 6.65 (2H, dd, JHH = 3.5,
1H NMR (400 MHz, CDCl3): δ 8.32 (2H, s, NCH), 7.07 (1H,
3
3
3
3
4
washed with hexane and dried in vacuo. Yield: 0.75 g, 79%.
4JHH = 2.0, pyrrole-H), 6.25 (2H, dd, JHH = 3.5, JHH = 2.0,
1H NMR (400 MHz, CDCl3): δ 8.43 (2H, d, JHH = 0.6, NCH), pyrrole-H), 2.20 (12H, m, NCH2), 2.11 (6H, s, CH), 1.72–1.77
4
7.51–7.46 (4H, m, ArH), 6.96–6.91 (4H, m, ArH), 6.86–6.81 (4H, (12H, m, CH2), −0.26 (3H, s, AlCH3).
m, pyrrole-H), 6.24–6.21 (2H, m, pyrrole-H), 3.82 (6H, s,
ArOCH3), −0.49 (3H, s, AlCH3).
13C NMR (100.6 MHz, CDCl3): δ 155.2 (NCH), 135.4
(pyrrole-C), 134.0 (pyrrole-CH), 117.8 (pyrrole-CH), 114.1
13C NMR (100.6 MHz, CDCl3): δ 158.1 (ArCOCH3), 151.2 (pyrrole-CH), 56.9 (NC(CH2)3), 42.6 (C(CH2)3), 36.0 (CH(CH2)3),
(NCH), 139.4 (ArCN), 136.6 (pyrrolyl-C), 136.2 (pyrrole-CH), 29.52- (CH(CH2)3).
122.5 (ArCH), 118.6 (pyrrole-CH), 114.6 (ArCH), 113.9 (pyrrole-
CH), 55.46 (ArOCH3).
Anal. calcd for C31H41N4Al (496.67): C, 74.97; H, 8.32; N,
11.28%. Found C, 74.78; H, 8.43; N, 11.47%.
Anal. calcd for C25H25N4O2Al (440.47): C, 68.17; H, 5.72; N,
12.72%. Found: C, 68.10; H, 5.93; N, 12.70%.
General polymerization procedure
Synthesis of L52AlMe (5b). To a stirred solution of HL5 In a nitrogen-filled glove box, rac-lactide (720 mg, 5.0 mmol)
(0.83 g, 4.52 mmol) in toluene (20 mL) was slowly added AlMe3 or ε-caprolactone (570 mg, 5.0 mmol) and benzyl alcohol
(1.13 mL of a 2.0 M solution in toluene, 2.26 mmol). The reac- (5.17 μL, 0.05 mmol) were placed in
a polymerization
tion mixture was stirred at 100 °C for 24 hours. The volatiles ampoule. To this ampoule was added a solution of initiator
were removed in vacuo to leave a brown oil which was then (0.05 mmol) in toluene (6.00 mL) ([monomer] : [Al] = 100 : 1).
recrystallized for hexane at −20 °C to afford the product as a The reaction was stirred for the desired reaction time at 70 °C.
white solid. Yield: 0.96 g, 72%.
At the desired reaction time, the reaction was quenched with
1H NMR (400 MHz, CDCl3): δ 7.98 (2H, d, JHH = 1.0, NCH), methanol (2–3 drops). The polymer was precipitated from
7.44–7.39 (2H, m, ArH), 7.34–7.30 (2H, m, ArH), 7.24–7.14 (4H, excess methanol, collected by filtration and dried in vacuo to a
m, ArH), 6.80–6.78 (2H, m, pyrrole-H), 6.65–6.62 (2H, m, constant mass. Conversions were determined by integration of
pyrrole-H), 6.21–6.19 (2H, m, pyrrole-H), 2.33 (3H, s, ArCH3), the monomer versus polymer methane resonances in the 1H
4
−0.35 (3H, s, AlCH3).
NMR spectrum of crude product (in CDCl3).
13C NMR (100.6 MHz, CDCl3): δ 159.3 (NCH), 147.8 (ArCN),
136.2 (pyrrolyl-CH), 135.5 (pyrrole-C), 132.1 (ArCH), 129.9
General procedure for kinetic studies
(ArCCH3), 126.8 (ArCH), 125.9 (ArCH), 124.4 (ArCH), 118.9 The polymerizations were carried out at 70 °C in a glove box.
(pyrrole-CH), 114.1 (ArCH), 19.0 (ArCH3). The molar ratio of monomer to initiator was fixed at 50 : 1. At
Anal. calcd for C25H25N4Al (408.47): C, 73.51; H, 6.17; N, appropriate time intervals, 0.5 µL aliquots were removed and
13.72%. Found: C, 73.64; H, 6.82; N, 13.70%.
quenched with methanol. The solvent was removed in vacuo
Synthesis of L62AlMe (6b). To a stirred suspension of HL6 and the percent conversion determined by 1H NMR in CDCl3.
(0.70 g, 3.09 mmol) in toluene (20 mL) was slowly added AlMe3
(0.77 mL of a 2.0 M solution in toluene, 1.55 mmol). The reac-
Crystal structure determination
tion mixture was stirred at 100 °C for 2 days. The volatiles were X-ray diffraction data of 3b were measured on a Bruker–Nonius
removed in vacuo to leave a yellow solid which was then kappaCCD diffractometer with graphite monochromated
washed with hexane and dried in vacuo. Yield: 0.69 g, 91%.
MoKα radiation (λ = 0.71073 Å) at 298 (2) K. The structure was
4
1H NMR (400 MHz, CDCl3): δ 7.89 (2H, d, JHH = 1.1, NCH), solved by direct methods with SIR97,25 and refined with full-
4
3
7.49 (2H, dd, JHH = 1.5, JHH = 8.1, 2H, pyrrole-H), 7.30–7.24 matrix least-squares calculations on F2 using SHELXL-97.26
(2H, m, ArH), 7.23–7.16 (2H, m, ArH), 7.04–6.91 (1H, br s, Crystallographic data have been deposited at the Cambridge
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Crystallographic Data Centre under the reference numbers
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Acknowledgements
This research was in part financially supported by Kasetsart
University Research and Development Institute (KURDI), the
National Science and Technology Development Agency
(NSTDA) and the Faculty of Science, Kasetsart University
(grant: ScRF-S6-2555). S. T. thanks the Graduate School, Kaset-
sart University for his scholarship supporting the publication
in an international journal. P. K. and S. P. thank Mahidol Uni-
versity and the Center for Innovation in Chemistry
(PERCH-CIC) for financial support. The National Research
University (NRU), Kasetsart University and NANOTEC Center of
Excellence, National Nanotechnology Center (NANOTEC) are
also acknowledged for equipment support. We also gratefully
acknowledge Assist. Prof. Nonlawat Boonyalai for his useful
discussion.
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Dalton Trans., 2014, 43, 1348–1359 | 1359