Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ?-Caprolactone

pubs.acs.org/IC

Article

Collaboration between Trinuclear Aluminum Complexes Bearing

Bipyrazoles in the Ring-Opening Polymerization of ε‑Caprolactone

Someswara Rao Kosuru,^○Feng-Jie Lai,^○Yu-Lun Chang, Chen-Yu Li, Yi-Chun Lai, Shangwu Ding,

Kuo-Hui Wu,* Hsuan-Ying Chen,* and Yung-Han Lo

Cite This: Inorg. Chem. 2021, 60, 10535 −10549

Read Online

sı

*

Metrics & More

Article Recommendations

Supporting Information

ACCESS

ABSTRACT: Trinuclear aluminum complexes bearing bipyrazoles were

synthesized, and their catalytic activity for ε-caprolactone (CL) polymer-

ization was investigated. D^Bu₂Al₃Me₅exhibited higher catalytic activity than

did the dinuclear aluminum complex L^Bu₂Al₂Me₄(16 times as high for CL

polymerization; [CL]:[D^Bu₂Al₃Me₅]:[BnOH] = 100:0.5:5, [D^Bu₂Al₃Me₅] =

10 mM, conversion 93% after 18 min at room temperature). Density

functional theory calculations revealed a polymerization mechanism in which

CL ﬁrst approached the central Al atom and then moved to an external Al.

The coordinated CL ring was opened because the repulsion of two tert-butyl

groups on the ligands pushed an alkoxide initiator on an external Al to initiate

CL. In these trinuclear Al catalysts, the central Al plays a role in monomer

capture and then collaborates with the external Al to activate CL, accelerating

polymerization.

the catalytic activities of trinuclear Al complexes for CL

polymerization were investigated.

INTRODUCTION

■

The inﬂuence of petrochemical plastics on modern society is

remarkable. However, environmental pollution caused by

discarded plastic waste must be managed¹⁻⁴because discarded

plastics do not quickly decompose naturally. To accelerate the

environmental degradation of polymers and further the goal of

a sustainable society, biodegradable poly-ε-caprolactone

(PCL)⁵has been developed. PCL-based biomaterials are

used in various ﬁelds⁶⁻¹⁶because of their biocompatibility^5,17

and permeability.¹⁸The primary method for manufacturing

PCL is the Lewis acidic catalyzed ring-opening polymerization

(ROP) of ε-caprolactone (CL).¹⁹⁻²⁸

Al catalysts are frequently used for ROP because they are

strong Lewis acids, are easy to synthesize, and have inexpensive

precursors. Ligands are essential for catalyst design because

they can inﬂuence the catalytic activity through electronic,²⁹⁻³²

steric,^31,33,34chelating,^31,35−37and dinuclear cooperation

eﬀects^29,38−40and reduce transesteriﬁcation.^41,42Recently, Al

complexes⁴³⁻⁴⁹bearing pyrazolyl-containing ligands and their

application in ROP have been reported; these complexes

exhibit moderate catalytic activity for cyclic ester polymer-

ization. Importantly, dinuclear Al complexes²⁹bearing

pyrazolide derivatives (Figure 1a) with high catalytic activities

for CL ROP have been reported. Such activities have been

ascribed to electronic and dinuclear cooperation eﬀects. To

further investigate the catalytic activity in this class of

molecules, a series of bipyrazoles and associated trinuclear Al

complexes (Figure 1b and Scheme 1) were synthesized, and

RESULTS AND DISCUSSION

■

Synthesis and Characterization of Al Complexes.

Alkane- and arene-1,3,4,6-tetraketones were prepared using

methods based on those described in the literature.^50,51

A

series of bipyrazoles was synthesized through the condensation

reaction of hydrazine hydrate and an alkane- or arene-1,3,4,6-

tetraketone (2:1 ratio). D^Bu₂Al₃Me₅, D^Ad₂Al₃Me₅, D^Pr₂Al₃Me₅,

D^tolyl₂Al₃Me₅, and D^ph₂Al₃Me₅were synthesized through the

reaction of bipyrazole ligands and AlMe₃(2:3 ratio). The

products of these ligands, including D^Cl-H and D^Br-H, are

insoluble in organic solvents, including dimethyl sulfoxide and

dimethylformamide. The formation of the bipyrazole ligands

and associated Al complexes was veriﬁed through nuclear

magnetic resonance (NMR) spectroscopy. The ¹H NMR peak

of the four methine groups of two D^Bu-Hs (at 6.30 ppm in

CDCl₃) shifted to 6.49 ppm after reaction with AlMe₃, and the

presence of ﬁve methyl groups on Al atoms was observed (at

−0.18, −0.48, and −0.99 ppm in CDCl₃; integration ratio =

2:2:1 (Figure S6)). The ligands, including D^Bu-H, D^Ad-H, D^Pr-

Received: April 19, 2021

Published: July 7, 2021

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

10535

Inorganic Chemistry

pubs.acs.org/IC

Article

Figure 1. Dinuclear and trinuclear Al complexes bearing pyrazolyl-containing ligands.

Scheme 1. Synthesis of (A) Bipyrazole Ligands and (B) Associated Al Complexes

H, D^tolyl-H, D^ph-H, D^Cl-H, and D^Br-H, are not insoluble in

CDCl₃; however, D^Ad₂Al₃Me₅, D^Pr₂Al₃Me₅, D^tolyl₂Al₃Me₅, and

Polymerization of ε-Caprolactone. The results of the

catalytic activity for CL polymerization in toluene in the

presence of benzyl alcohol (BnOH) are presented in Table 1.

The optimization of CL polymerization using D^Bu₂Al₂Me₅as a

catalyst with BnOH was investigated as shown in entries 1−3

of Table 1, and the optimal [D^Bu₂Al₃Me₅]:[BnOH] ratio was

1:5 with the greatest catalytic activity and controllability. In

comparison with L^Bu₂Al₂Me₄, D^Bu₂Al₃Me₅had a substantially

higher catalytic activity for CL polymerization ([CL]:

[D^Bu₂Al₃Me₅]:[BnOH] = 100:0.5:2.5, where [CL] = 2 M,

conversion 93% at 25 °C after 18 min; entry 2 in Table 2). The

k_obsvalue of D^Bu₂Al₃Me₅is 16 times higher than that of

L^Bu₂Al₂Me₄. Similarly, in a comparison of D^ph₂Al₃Me₅and

L^ph₂Al₂Me₄, the k_obsvalue of D^ph₂Al₃Me₅was determined to be

twice that of L^ph₂Al₂Me₄, further demonstrating that trinuclear

Al complexes have greater catalytic activity than do dinuclear

Al complexes. In addition, phenyl and tolyl groups on the 5,5′-

positions of bipyrazoles reduced the catalytic activity of

1

D^ph₂Al₃Me₅are soluble in CDCl₃. The H NMR peaks of the

two methine groups of D^Ad₂Al₃Me₅, D^Pr₂Al₃Me₅,

D^tolyl₂Al₃Me₅, and D^ph₂Al₃Me₅were at 6.37, 6.42, 6.45, and

6.58 ppm, respectively, and three sets of methyl groups with a

2:2:1 integration were also observed (Figures S8 and S9).

The molecular structure of D^Bu₂Al₃Me₅(Figure 2) was

characterized using a single-crystal X-ray diﬀraction analysis

(CCDC 1955471). The external Al atoms on each side

exhibited a distorted-tetrahedral geometry due to their

association with two methyl groups. The central Al atom has

a quadrangular-pyramidal geometry with one methyl group

and four nitrogen atoms, one from each of the four pyrazolidyl

groups. Surprisingly, the Al−C bonds of the pentacoordinated

central Al atom were shorter than those of tetracoordinated Al

atoms on either side of the complex (Table S35).

10536

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

5,5′-positions of ligands can enhance the catalytic activity of

CL polymerization. The reason was discussed in a mechanism

study. When the linear relationship between M_n,GPCand

([monomer]₀× conversion)/[BnOH] (entries 3, 4, and 11−

15 in Table 1) presented in Figure 3 was considered,

D^Bu₂Al₃Me₅as a catalyst was deemed to be highly controllable

and to have acceptable polydispersity indexes (Đs) for CL

polymerization. However, Figure 3 also revealed that only two

to three BnOHs could be initiators to initiate CL. This may be

t

ascribed to two bulky Bu groups restraining the two benzyl

alkoxide exchanges and prevent BnOH in a far position from

attacking CL.

To examine the living and immortal properties^19,52−60of

D^Bu₂Al₃Me₅in CL polymerization, 25 equiv of BnOH was

combined with 200 equiv of CL in 5 mL of toluene. After

>90% conversion, 50 equiv of CL monomers was reloaded into

the solution. This step was repeated until the solution could

not be stirred. The solution could not be stirred after 17

additions of CL (total ratio [CL]:[D^Bu₂Al₃Me₅]:[BnOH] =

1050:1:25; Table 2 and Figure 4). The results revealed that

D^Bu₂Al₃Me₅had excellent living and immortal properties in CL

polymerization. In addition, the results also revealed that at

least four-ﬁfths of BnOH can participate in the ring-opening

polymerization reaction. This proved that the free alcohol

could easily exchange with the alkoxide on an Al atom to form

a new alkoxide to initiate CL.

Kinetic Study of ε-Caprolactone Polymerization. The

kinetic data (Table S2 and Figures S2 and S3) revealed that

the polymerization rate had a ﬁrst-order dependence on the

monomer concentration. In CL polymerization, the k_obsvalue

of D^Bu₂Al₃Me₅was 0.188 min⁻¹, and the polymerization rate

was 15 times higher than that associated with L^Bu₂Al₂Me₄

(0.0123 min⁻¹). After k_obswas plotted against [D^Bu₂Al₃Me₅·

5BnOH] ([D^Bu₂Al₃Me₅·5BnOH] being a mixture of 1 equiv of

D^Bu₂Al₃Me₅with 5 equiv of BnOH), the k_prop(propagation)

value was calculated to be 15.01 (Figure S3). CL polymer-

Figure 2. Molecular structure of D^Bu₂Al₃Me₅depicted as 20%

probability ellipsoids. All of the hydrogen atoms are omitted for

clarity. The selected bond lengths (Å) of Al(1)−C(1), Al(1)−C(2),

Al(2)−C(3), Al(3)−C(4), and Al(3)−C(5) are 1.960(3), 1.969(3),

1.942(3), 1.974(3), and 1.960(3), respectively. The selected bond

angles (deg) of C(1)−Al(1)−C(2), C(4)−Al(3)−C(5), N(1)−

Al(1)−N(8), N(4)−Al(3)−N(5), N(2)−Al(2)−N(7), N(7)−

Al(2)−N(6), N(6)−Al(2)−N(3), and N(3)−Al(2)−N(2) are

123.96(12), 109.30(13), 106.01(8), 99.55(8), 89.85(8), 80.36(8),

88.19(8), and 80.32(8), respectively.

trinuclear Al complexes in comparison with that of

D^Bu₂Al₃Me₅. Moreover, the catalytic activity of L^Ad₂Al₂Me₄

was greater than those of L^Bu₂Al₂Me₄and L^Pr₂Al₂Me₄, and this

revealed that increasing the steric bulk of the groups on the

a

Table 1. ε-Caprolactone Polymerization with Al Complexes as Catalysts

b

c

d

entry

cat.; [CL]:[cat.]:[BnOH]

time (min)

conversn (%)

M_n,Cal

M_n,NMR

M_n,GPC

Đ

k_obs(error), 10³min⁻¹

e

1

D^Bu₂Al₃Me₅; 100:1:1

240

35

12

18

250

16

60

55

100

55

15

70

20

58

75

93

90

95

93

92

90

96

97

98

10700

3500

2300

4300

5400

4300

4200

5200

4200

2800

8300

4500

9200

11300

19400

5600

3500

8200

7400

7300

5300

6200

5500

5900

5800

15300

8300

14100

22300

19500

6400

3600

8700

7900

6400

5900

5200

7000

4600

5300

15300

9000

16300

26000

1.35

1.52

1.24

1.30

1.13

1.24

1.11

1.14

1.34

1.10

1.27

1.29

1.25

1.29

1.28

8.8 (1)

67.5 (1)

283.0 (16)

188.1 (32)

12.3 (4)

205.0 (30)

52.5 (1)

57.1 (1)

26.2 (4)

52.8 (1)

204.6 (33)

38.2 (9)

m

f

2

D^Bu₂Al₃Me₅; 100:1:3

g

3

D^Bu₂Al₃Me₅; 100:1:5

a

4

D^Bu₂Al₃Me₅; 100:0.5:2.5

L^Bu₂Al₂Me₄; 100:0.5:2

D^Ad₂Al₃Me₅; 100:0.5:2.5

D^Pr₂Al₃Me₅; 100:0.5:2.5

D^ph₂Al₃Me₅; 100:0.5:2.5

L^ph₂Al₂Me₄; 100:0.5:2

D^tolyl₂Al₃Me₅; 100:0.5:2.5

D^Bu₂Al₃Me₅; 100:0.75:3.75

D^Bu₂Al₃Me₅; 100:0.25:1.25

D^Bu₂Al₃Me₅; 200:1:5

a

5

a

6

a

7

a

8

a

9

a

10

11

12

13

14

15

h

i

j

k

D^Bu₂Al₃Me₅; 400:1:5

D^Bu₂Al₃Me₅; 500:1:5

m

l

a

Unless speciﬁed otherwise, the reaction was carried out in toluene with [CL] = 2 M, [CL]:[Cat.]:[BnOH] = 100:0.5:2.5, at 25 °C for entries 1, 3,

b

and 5, and [CL]:[Cat.]:[BnOH] = 100:0.5:2 for entries 2 and 4). Calculated from the molecular weight of M_w(CL) × [CL]₀/[BnOH]₀×

c

d

conversion yield + M_w(BnOH). The data were determined using ¹H NMR analysis. Obtained through gel permeation chromatography (GPC).

e

f

Values of M_n,GPCwere obtained times 0.56 for PCL. The [CL]:[Cat.]:[BnOH] ratio is 100:1:1 in toluene with [CL] = 2 M. The [CL]:[Cat.]:

[BnOH] ratio is 100:1:3 in toluene with [CL] = 2 M. The [CL]:[Cat.]:[BnOH] ratio is 100:1:5 in toluene with [CL] = 2 M. The [CL]:[Cat.]:

[BnOH] ratio is 100:0.75:3.75 in toluene with [CL] = 2 M. The [CL]:[Cat.]:[BnOH] ratio is 100:0.25:1.25 in toluene with [CL] = 2 M. The

[CL]:[Cat.]:[BnOH] ratio is 200:1:5 in toluene with [CL] = 4 M. The [CL]:[Cat.]:[BnOH] ratio is 400:1:5 in toluene with [CL] = 7 M. The

g

h

i

j

k

l

m

[CL]:[Cat.]:[BnOH] ratio is 500:1:5 in toluene with [CL] = 10 M. Did not study.

10537

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

a

Table 2. CL Polymerization with D^Bu₂Al₃Me₅for Living and Immortal Characters

b

c

d

e

d

entry

[CL]:[D^Bu₂Al₃Me₅]:[BnOH]

time (min)

conversn (%)

M_n,Cal

M_n,GPC

M_n,NMR

Đ

1

2

3

4

5

6

7

8

200:1:25

250:1:25

300:1:.5

350:1:25

400:1:25

450:1:25

500:1:25

550:1:25

600:1:25

650:1:25

700:1:25

750:1:25

800:1:25

850:1:25

900:1:25

950:1:25

1000:1:25

1050:1:25

36

47

58

75

90

93

95

90

93

92

93

91

92

93

92

95

92

95

93

1000

1200

1300

1600

1800

2000

2200

2400

2600

2900

3000

3300

3500

3800

3900

4200

4300

4400

900

1000

1200

1500

1600

1900

2100

2200

2600

2800

3000

3200

3400

3600

3800

4200

4300

4600

1500

1700

1800

2200

2500

2800

3300

3600

3800

4000

4300

4500

4700

4900

5200

5400

5600

5900

1.11

1.15

1.13

1.12

1.14

1.15

1.18

1.19

1.21

1.23

1.26

1.27

1.25

1.23

1.27

105

121

135

156

172

195

218

249

280

320

380

450

540

9

10

11

12

13

14

15

16

17

18

90

a

b

Add 50 equiv of CL in comparison with [D^Bu₂Al₃Me₅] in every entry. In general, the reaction was carried out in toluene with [D^Bu₂Al₃Me₅] =

c

d

0.02 M at 25 °C. Calculated from the molecular weight of M_w(CL) × [CL]₀/[BnOH]₀× conversion yield + M_w(BnOH). Values of M_n,GPCwere

those from GPC times 0.56. The data were determined from H NMR analysis.

e

1

withdrawing eﬀect of the external Al to the central Al via

bridging pyrazole ligands described below). Thus, a higher

catalytic activity for CL polymerization was observed.

Density Functional Theory Analysis of the CL

Polymerization Mechanism Study Using D^Bu₂Al₃Me₅as

a Catalyst. The reaction mechanism, as shown in Figures 5

and 6, was studied using density functional theory (DFT) at

the B3LYP/6-31G(d) level. The values of the free energies

obtained from the DFT calculation results are shown in Figure

7. In the mechanism study, all of the methyl groups on Al

atoms of D^Bu₂Al₃Me₅were replaced by MeO⁻ligands to form

D^Bu₂Al₃(OMe)₅(cat) because all methyl groups on Al atoms

of D^Bu₂Al₃Me₅are strong bases and can be readily replaced by

alcohol molecules under the reaction conditions.

To better understand the activity of the catalysts, the

Mulliken charges of the Al atoms of D^Bu₂Al₃Me₅and L₂Al₂Me₄

were calculated on the basis of their crystal structures, as

shown in Figure 8. The charges of the two Al atoms of

L₂Al₂Me₄were both calculated to be +0.8517, and the charges

of the central and the external Al atoms of D^Bu₂Al₃Me₅were

calculated to be +0.929 and +0.836 (average), respectively.

Therefore, the central Al of D^Bu₂Al₃Me₅has much higher

Lewis acidity than do the external Al atoms of L₂Al₂Me₄.

Therefore, CL molecules can be attracted by the central Al of

D^Bu₂Al₃Me₅, increasing the eﬀective concentration for CL

coordination and reactivity.

Figure 3. Linear plots of M_n,GPCversus ([CL]₀× conversion)/

▼

[BnOH], (blue inverted triangles , entries 3, 4 and 11−15) in Table

■

2. Black squares are theoretical values calculated with 5 equiv of

●

BnOHs as initiator, red dots indicate 3 equiv of BnOH, and green

▲

triangles indicate 2 equiv of BnOH.

ization with D^Bu₂Al₃Me₅as the catalyst exhibited the following

rate law:

d[CL]/dt = 15.01 × [CL][D^Bu₂Al₃Me₅·5BnOH]

After D^Bu₂Al₃Me₅reacts with the initiator alcohol to form

cat, three sets of MeO⁻ligands located in totally diﬀerent

environments on cat can be found. The ﬁrst set is one MeO⁻

ligand pointing up on the central Al atom. The second is two

MeO⁻ligands pointing up on the two external Al atoms (one

on each). The third is two MeO⁻ligands pointing down on the

two external Al atoms (one on each). Because the rigid

bipyrazole ligands on cat prohibit the formation of bridged

methoxide structures (the average Al···Al distance is 3.779 Å,

which is longer than 2 times the length of Al−OMe bonds,

about 3.4 Å), the anionic MeO⁻ligand cannot migrate among

Al atoms without breaking strong O−Al coordination bonds.

Dinuclear Al complexes bearing pyrazole exhibited a high

catalytic activity in CL ROP because of electronic and

dinuclear cooperative eﬀects between the two Al atoms,²⁹

with the Al−Al distance being 3.525−3.783 Å. In D^Bu₂Al₃Me₅,

the distance between the central Al(2) and the external Al(1)

was 3.785 Å and the Al(2)−Al(3) distance was 3.662 Å,

suggesting that trinuclear cooperation inﬂuenced polymer-

ization in the D^Bu₂Al₃Me₅system. In addition, in comparison

with the dinuclear Al complex L^Bu₂Al₂Me₄, the trinuclear Al

complex D^Bu₂Al₃Me₅had a stronger electronic eﬀect (Al³⁺−

Al³⁺charge inﬂuence, which comes from the direct electron-

10538

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

■

Figure 4. Linear plot of M_n,GPCvs [CL]₀× conversion/[BnOH], with Đ indicated by blue squares

.

Moreover, due to the restricted and crowded coordination

environment around external Al, the structure distortions are

impossible. The rigid N−Al−N angle (105°, average) and the

small distances (about 4 Å, average) between H atoms of the

two ^tBu groups both restrict the relocation of the Al atom and

two MeO⁻ligands to form an SP geometry. This results in the

fact that the two MeO⁻ligands are structurally isolated, and

therefore the interchange is hindered. This can explain why

only the two alkoxide ligands close to the CL receiving site are

active in the polymerization reaction. However, if free alcohols

are present in the system, ligand exchange processes assisted by

those free alcohols become a possible pathway to increase the

number of alkoxide ligands involved in the catalytic reaction.

In the catalytic cycle study of the ﬁrst mechanism (right part

of Figure 5), the calculation results demonstrate that a CL

molecule ﬁrst approached the central Al atom to form

intermediate I (Figure 9) with a free energy of 7.81 kcal

mol⁻¹. The structure of I is similar to those of D^Bu₂Al₃Me₅and

cat, except for the central Al, which has a distorted-octahedral

structure. The distance between the CL carbonyl O and the

central Al is 2.983 Å, which is longer than the sum of the single

bond covalent radii of O and Al (1.89 Å)⁶²but shorter than

the sum of the crystallographic van der Waals radii of O and Al

(3.65 Å).⁶³This indicates that the CL carbonyl O does not

directly coordinate to the central Al but is attracted by

noncovalent interactions. However, the distance between the

α-hydrogen pointing toward the plane of the complex and the

pyrazole ring is 2.901 Å, which is in the range of a C−H···π

interaction.⁶⁴The central Al intrudes into the plane formed by

the four coordinated N atoms toward the MeO⁻ligand. The

distance between the Al atom and the plane is 0.494 Å, which

is slightly shorter than that of D^Bu₂Al₃Me₅(0.604 Å) due to

the interaction between the CL carbonyl O and the central Al.

The average bond length of the four N−Al bonds of the central

Al is 1.990 Å, which is slightly longer than that of the external

Al atoms (1.965 Å).

transferred readily from the trap site to the reaction site. For

these reasons, this capturing phenomenon can dramatically

increase the collision frequency between CL molecules and a

MeO⁻initiator to increase the reaction rate. Moreover,

because of those two interactions, the sp²plane of the CL

ester group is locked, facing one MeO⁻ligand on the external

Al. This conformation locking increases the ratio of eﬀective

collision overall, facilitating the addition of the MeO⁻ligand to

the ester group of CL. These are the primary eﬀects promoting

the catalytic activity of the trinuclear complex.

The CL molecule on the central Al moves toward the MeO⁻

ligand on one external Al. The addition of MeO⁻to the

carbonyl C goes through transition state 1 (TS1). The free

energy of TS1 is 28.7 kcal mol⁻¹, and the energy barrier for this

reaction is 20.89 kcal mol⁻¹. This is a reasonable energy barrier

for room-temperature reactions.⁶¹On consideration of the

high eﬀective concentration of CL on cat and high eﬀective

collision rate, the overall reaction rate is expected to be high. In

the transition structure, the distance between the CL carbonyl

O and the central Al is 3.716 Å, and the distance between the

carbonyl O and the closer external Al is 1.941 Å, implying that

the central Al plays a role in monomer capture but does not

directly cooperate with the external Al to activate the CL

carbonyl group. The distance between the CL carbonyl C and

the methoxy O atom is 1.905 Å. The distance between the

methoxy O and the external Al is 1.912 Å. The distance

between the CL carbonyl O and the external Al is 1.941 Å.

After a transition, the central Al returns to a pentacoordinate

structure with a square-pyramidal geometry.

After MeO⁻insertion to CL, intermediate II is formed. This

intermediate has a free energy of 11.87 kcal mol⁻¹. The

structure of II resembles that of cat with one pentacoordinated

central Al atmand two tetracoordinated external Al atoms. The

distance between the CL−MeO adduct alkoxy O atom and the

Al atom on which it coordinates is 1.752 Å. This distance is

slightly longer than the lengths of the other three O−Al bonds

on the two external Al atoms (average: 1.718 Å) and that of

the central Al (1.734 Å). The longer Al−O bond between the

CL−MeO adduct and Al is expected to be caused by the

repulsion between the bulky structure of the adduct and the

In intermediate I, the CL molecule is captured onto the

pentacoordinate central Al by two noncovalent interactions to

increase the eﬀective concentration of CL in cat. Moreover,

the relatively weak noncovalent interactions allow CL to be

10539

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

Figure 5. First reaction mechanism of ring-opening polymerization of CL catalyzed by D^Bu₂Al₃(OMe)₅.

two t-Bu groups on the ligands. This is one of the driving

forces for the next step in the catalytic cycle reaction opening

the CL ring.

between the newly formed carbonyl O atom and the central Al

atom is 3.647 Å, and the distance between the newly formed

carbonyl O atom and the external Al atom is 1.935 Å. These

distances are both shorter than those of TS1, indicating that

after the CL ring is opened, due to a decrease in steric

repulsion, the carbonyl group can be more easily attracted by

the two Al atoms. The distance between the newly formed

carbonyl C atom and the newly formed alkoxy O atom is 1.857

Å, which is shorter than that of TS1, indicating that the

structure of TS2 is relatively close to that of intermediate II in

comparison to the structures between intermediate II and TS1.

Interestingly, the steric repulsion also is one important driving

The CL ring opens through transition state 2 (TS2). This

transition structure has a free energy of 33.42 kcal mol⁻¹, and

the energy barrier for this reaction is 21.55 kcal mol⁻¹.

Although this energy barrier is slightly higher than that of the

reaction through TS1, the intramolecular characteristic of this

reaction causes this step to be more favorable than the step

through TS1. In this transition structure, the newly formed

carbonyl group migrates from the external Al to the central Al

and the C−O bond in the CL ring breaks. The distance

10540

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

Figure 6. Second reaction mechanism of ring-opening polymerization of CL catalyzed by D^Bu₂Al₃(OMe)₅.

force to assist in CL ring opening. In the structure of TS2, the

shortest distance between the H atoms of ^tBu groups and those

of the CL ring is 2.451 Å, which is close to the closest possible

distance between two isolated H atoms from the point of view

of van der Waals radii.⁶³Together with thermal motion, these

two groups shall collide at high frequency to introduce a large

repulsion force between them to assist CL ring opening.

After the CL ring opens, intermediate III is formed when the

newly formed carbonyl group coordinates with the central Al

atom to form a hexacoordinate structure. The free energy of

intermediate III is 6.97 kcal mol⁻¹, which is lower than that of

intermediate I. The newly formed alkoxy O atom is bonded to

the external Al atom, and the newly formed carbonyl O atom is

bonded to the central Al atom. The distance between the

newly formed carbonyl O atom and the central Al atom is

2.900 Å, which is slightly shorter than that of intermediate I

due to the less bulky structure of the opened CL chain. The

bond length between the external Al atom and the alkoxy O

atom of the opened CL is 1.723 Å. The central Al atom also

intrudes toward the MeO⁻ligand with a distance of 0.482 Å

from the central NNNN plane of intermediate III, but the

distance is slightly shorter than that of intermediate I, because

of the stronger carbonyl O−Al interaction in intermediate III

than in intermediate I.

Finally, the carbonyl O atom of the opened CL leaves the

central Al atom to form intermediate IV. The free energy of

intermediate IV, 6.11 kcal mol⁻¹, is slightly lower than that of

intermediate III. The structure of IV is almost identical with

10541

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

Figure 7. Energy diagram of all the intermediates and transition structures. The sum of free energies of cat and CL is set as 0 kcal mol⁻¹.

structure, the central Al atom is farther from the NNNN plane

with a distance of 0.828 Å, forming a highly distorted

hexacoordinated structure. The distance between the carbonyl

O atom and central Al atom is 2.141 Å. The distance between

the CL carbonyl C atom and central Al methoxy O atom is

2.404 Å much longer than that of TS1. The distance between

the methoxy O atom and central Al atom is 1.796 Å. These

structural characteristics are more similar to CL-coordinated

intermediates than to transition structures because C−O bond

formation and the central Al atom distortion process are

coupled in this step. Distorting the geometry of the central Al

atom requires substantially more energy than that required for

overcoming the C−O bond formation in this step.

After TSc1, intermediate cI, in which the CL−MeO adduct

coordinates on the central Al atom, is formed. The free energy

of intermediate cI is 11.19 kcal mol⁻¹, which is slightly lower

than that of intermediate II because the less crowded

environment of the central Al is less crowded. This structure

resembles the structure of cat and intermediate II, with a

pentacoordinated central Al atom and two tetracoordinated

external Al atoms. The distance between the central Al and the

alkoxy O atom of the CL−MeO adduct is 1.756 Å. The

distance between the central Al and the NNNN plane is 0.584

Å, indicating that the Al returns to a stable coordination

position in this complex.

Figure 8. Mulliken charges of Al atoms in L₂Al₂Me₄and D₂Al₂Me₅.

that of cat. The newly formed carbonyl group moves from the

central Al atom toward the ^tBu groups of the bipyrazole ligand.

The distances between the carbonyl O atom and the closest

t

two H atoms on the Bu groups are 2.794 and 2.791 Å. These

distances are close to the sum of the crystallographic van der

Waals radii (∼2.75 Å) but are smaller than the sum of the

equilibrium van der Waals radii (∼3 Å), indicating that the ^tBu

groups weakly attract the carbonyl group, assisting in its

detachment from the central Al.

Another possible catalytic reaction cycle proceeds entirely

on the central Al atom, as shown in the left side of Figure 5. A

CL molecule approaches the central Al atom from the

direction of the MeO⁻ligand to directly form transition

structure c1 (TSc1) without forming a CL-coordinated

intermediate. No stable CL-coordinated intermediate has

been found in the calculations even when the structural search

is carried out from TSc1 to elongate the length between the

carbonyl C atom and the methoxy group. In this transition

The CL ring then opens through transition state c2 (TSc2).

The free energy of TSc2 is 31.62 kcal mol⁻¹, and the energy

barrier of this reaction step is 20.43 kcal mol⁻¹. This energy

Figure 9. Side-view structure (a) and top-view structure (b) of cat.

10542

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

Figure 10. Survey of the recent literature sources on CL polymerization using Al complexes at 0−30 °C.

barrier is similar to that of TS2 in the ﬁrst mechanism. In this

transition structure, the central Al atom (resembling that of

TSc1) retreats from the NNNN plane to a distance of 0.830 Å.

The distance between the newly formed carbonyl O atom and

the central Al atom is 1.899 Å. The distance between the newly

formed alkoxy O atom and the central Al atom is 1.911 Å. The

10543

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

length of the broken C−O bond in the CL ring is 1.888 Å.

This bond is very similar to those of TS1 and TS2, indicating

that this transition structure is dominated by the breaking of

the O−C bond of CL.

and the conformation lock eﬀect induced by weak interactions

generated by the central Al atom and the π-system of ligands.

EXPERIMENTAL SECTION

■

After the breakage of the C−O bond in the CL ring,

intermediate cIII is formed, in which the newly formed

carbonyl group is still coordinated on the central Al. The free

energy of cIII is 27.65 kcal mol⁻¹, substantially higher than

those of other intermediates and closer to those of the

transition structures. The increased energy is because the

central Al atom is still in a highly distorted geometry. The

distance between the newly formed carbonyl O atom and the

central Al atom is 2.068 Å. The distance between the newly

formed alkoxy O atom and the central Al atom is 1.812 Å. The

distance between the central Al atom and the NNNN plane is

0.846 Å.

Finally, the carbonyl group of the opened CL chain is

detached from the central Al atom to form intermediate cIII,

closing this catalytic cycle. The structure of intermediate cIII is

almost identical with those of cat and intermediate IV. The

free energy of intermediate cIII is 6.09 kcal mol⁻¹, which is

much lower than the free energy of cII but close to that of IV.

On the basis of our calculated results, the ﬁrst catalytic cycle

is considered the most likely mechanism for the CL ROP

reaction. First, the highest energy barrier of the ﬁrst

mechanism is 21.55 kcal mol⁻¹(II to III through TS2) and

that of the most diﬃcult reaction step (I to II through TS1) is

20.89 kcal mol⁻¹; these energy barriers are much lower than

those of the second mechanism (32.88 kcal mol⁻¹for the step

from cat and CL to intermediate cI through TSc1). Second,

the eﬀective concentration of CL is substantially increased in

the ﬁrst mechanism due to the monomer capture eﬀect of the

central Al atom on the base side of the square-pyramidal

structure. Third, the two noncovalent interactions create a

special conformational lock eﬀect on CL, increasing the

eﬀective collision rate between CL and MeO⁻. Fourth, the

bulky ^tBu groups around the external Al atom eﬀectively assist

in the ring opening of CL (TS2).

Comparison of CL Polymerization in the Literature. A

survey of recent literature of CL polymerization using Al

complexes as catalysts at 0−30 °C is illustrated in Figure 10.

An Al complex bearing a Salen ligand⁶⁵revealed the greatest

catalytic activity in CL polymerization at 0 °C. Other Al

complexes, including an Al complex⁷¹bearing aminophenolate,

a low-valent Al(II)−Al(II) complex⁸⁵bearing 1,2-bis(2,6-iPr₂-

C₆H₃)-iminoacenaphthene, an Al complex⁹⁸bearing (oxybis-

(2,1-phenylene))bis(cyclohexylamide), a ﬁve-membered Al

complex⁷⁰bearing a thiol-Schiﬀ base, and an Al complex¹⁰¹

bearing benzofurazan, as well as D^Bu₂Al₃Me₅could polymerize

CL in 15 min at room temperature. In comparison with these

Al complexes in Figure 10, the collaboration that each Al atom

performs its own functions can indeed eﬀectively improve the

catalytic activity of the Al catalyst.

Standard Schlenk techniques and a N₂-ﬁlled glovebox were used

throughout the isolation and handling of all the compounds. Solvents,

ε-caprolactone, and deuterated solvents were puriﬁed prior to use.

Deuterated chloroform and ε-caprolactone were purchased from

Acros. Benzyl alcohol, trimethylaluminum, tert-butyl methyl ketone,

diethyl oxalate, methyl phenyl ketone, methyl p-tolyl ketone, 4′-

chloroacetophenone, 4′-bromoacetophenone, and hydrazine hydrate

were purchased from SIGMA-Aldrich. ¹H and ¹³C NMR spectra were

recorded on a Varian Gemini2000-200 (200 MHz for ¹H and 50 MHz

for ¹³C) spectrometer with chemical shifts given in ppm from the

internal TMS or the center line of CDCl₃. Microanalyses were

performed using a Heraeus CHN-O-RAPID instrument. GPC

measurements were performed on a Jasco PU-2080 PLUS HPLC

pump system equipped with a diﬀerential Jasco RI-2031 PLUS

refractive index detector using THF (HPLC grade) as an eluent (ﬂow

rate 1.0 mL/min, at 40 °C). The chromatographic column was

JORDI Gel DVB 10³Å, and the calibration curve was made by

primary polystyrene standards to calculate M_n,GPC. All single-crystal X-

ray diﬀraction data were accumulated using Rigaku Oxford Diﬀraction

single-crystal X-ray diﬀractometers with Mo Kα radiation (λ =

0.71073 Å). The data collection was executed using the CrysAlisPro

1.171.41.56a program. Cell reﬁnement and data reduction were

carried out with the CrysAlisPro 1.171.41.56a program. The structure

was determined using the Olex2/ShelXL program reﬁned using full-

matrix least squares. All non-hydrogen atoms were reﬁned anisotropi-

cally, whereas hydrogen atoms were placed at calculated positions and

included in ﬁnal stage of reﬁnement with ﬁxed parameters. D^Bu-H,¹⁰⁴

D^ph-H,⁵⁰D^tolyl-H,⁵⁰and D^Cl-H⁵⁰were prepared following the

literature procedures.

Synthesis of D^Br-H. To a solution of sodium hydride (1.04 g,

0.052 mol) in dry THF was slowly added a THF solution of 4′-

chloroacetophenone (4.2 g, 0.0208 mol) at room temperature, and

the mixture was heated to 50 °C. The reaction mixture was stirred for

1 h and a THF solution of diethyl oxalate (1.53 g, 0.0104 mol) added

slowly for 1 h at 50 °C. The reaction mixture was stirred overnight

and quenched with cold water at room temperature. The aqueous

layer was separated and the pH adjusted to 3−4 using a 2 N HCl

solution and extracted with diethyl ether (3 × 50 mL). The combined

organic layers were washed with brine solution and dried over

magnesium sulfate. The organic layer was concentrated and the crude

1,6-diphenylhexane-1,3,4,6-tetraone (3.25 g, 35%) obtained was used

as such in the next step. To a solution of 0.003 mol of an appropriate

1,3,4,6-tetraketone (1.33 g, 0.003 mol) in ethanol (10 mL) was added

1 mL (excess) of a 70% water solution of hydrazine, and the mixture

was boiled for 4 h. The precipitate obtained was ﬁltered and washed

with ethanol (2 mL) and diethyl ether. The product was dried to give

the product as a white solid. Yield: 0.61 g (46%). ¹H NMR (DMSO-

d₆, 400 MHz): δ 7.74 (d, J = 8.0 Hz, 4H, m-Br-PhH), 7.55 (d, J = 8.0

Hz, 4H, o-Br-PhH), 6.806 (s, 2H, Pz-H). ¹³C NMR (CDCl₃, 400

MHz): δ 148.57 (Ar-C^Pz), 144.43 (Pz-C^Pz), 133.43, 131.58, 127.07,

120.09 (C^Ar), 102.80 (H-C^Pz). Anal. Calcd (found) for C₁₈H₁₂Br₂N₄:

C, 48.68 (48.55); H, 2.72 (2.65); N, 12.62 (12.50).

Synthesis of D^Pr-H. This compound was obtained using a method

similar to that for D^Br-H except methyl 2-butanone was used in place

1

of 4′-chloroacetophenone. Yield: 2.1 g (60%). H NMR (DMSO-d₆,

CONCLUSIONS

400 MHz): δ 6.22 (s, 2H, Pz-H), 2.91 (br, 2H, Me₂CH), 1.19 (d,

12H, (CH₃)₂CH). ¹³C NMR (CDCl₃, 400 MHz): δ 158.62 (ⁱPr-C^Pz),

146.95 (Pz-C^Pz), 99.22 (H-C^Pz), 25.62 (Me₂CH), 23.35, 22.90

(CH₃)₂CH). Anal. Calc. (found) for C₁₂H₁₈N₄: C 66.02 (66.00), H

8.31 (8.22), N 25.67 (25.55).

■

A series of trinuclear Al complexes bearing bipyrazoles were

synthesized, and their CL polymerization rates were studied.

The trinuclear Al complexes had a substantially higher (2−15

times higher k_obsvalue) CL polymerization rate. DFT

calculations revealed that the reaction is a cooperated bimetal

catalytic reaction between the pentacoordinated central Al

atom and the tetracoordinated exteral Al atom. The reactivity

is improved due to an increased eﬀective concentration of CL

Synthesis of D^Ad-H. This compound was obtained using a

method similar to that for D^Br-H except 1-adamantyl methyl ketone

1

was used in place of 4′-chloroacetophenone. Yield: 4.5 g (20%). H

NMR (DMSO-d₆, 400 MHz): δ 6.22 (s, 2H, Pz-H), 1.95, 1.81, 1.67

(br, 30H, Ad). ¹³C NMR (CDCl₃, 400 MHz): δ 164.10 (Ad-C^Pz),

10544

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

142.44 (Pz-C^Pz), 102.35 (H-C^Pz), 42.66, 36.80, 33.51, 28.48 (Ad-C).

Anal. Calcd (found) for C₂₆H₃₄N₄: C, 77.57 (77.2); H, 8.51 (8.49);

N, 13.92 (13.69).

Computational Methods. The reaction mechanism was studied

by DFT calculations at the B3LYP/6-31G(d) level.^105,106The

calculations were carried out with Gaussian 09. The calculations of

Gibbs free energy were done at 298.15 K under 1 atm. The minimum

energy stationary point and the energy saddle point were conﬁrmed

by frequency analysis with the same calculation level.

Synthesis of D^Bu₂Al₃Me₅. A mixture of D^Bu-H (0.45 g, 1.8 mmol)

and AlMe₃(11 mL, 2.0 M, 5.5 mmol) in THF (30 mL) was stirred

overnight at room temperature. Volatile materials were removed

under vacuum to give a white powder, and then hexane (40 mL) was

transferred to give a suspension. A white powder was obtained after

ﬁltering. Yield: 0.49 g (85%). ¹H NMR (CDCl₃, 400 MHz): δ 6.69 (s,

2H, Pz-H), 1.52 (s, 18H, C(CH₃)₃), −0.18, −0.47 (s, 12H,

Al^terminal(CH₃)₂), −0.99 (s, 3H, Al^center(CH₃)). ¹³C NMR (CDCl₃,

50 MHz): δ 168.23 (^tBu-C^Pz), 144.71 (Pz-C^Pz), 100.62 (H-C^Pz), 32.97

(C(CH₃)₃), 31.76 (C (CH₃)₃), 0.10, −4.29 (Al(CH₃)₂), −9.23

(Al(CH₃)). Anal. Found (calcd) for D^Bu₂Al₃Me₅, C₃₃H₅₅Al₃N₈: C,

61.47 (61.52); N, 17.38 (17.56); H, 8.60 (8.81).

ASSOCIATED CONTENT

■

sı

* Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c01192.

Polymer characterization data, details of the kinetic

study (PDF)

Synthesis of D^Pr₂Al₃Me₅. )This compound was obtained using a

method similar to that for D^Bu₂Al₃Me₅except D^Pr-H was used in

place of D^Bu-H. Yield: 0.32 g (80%). ¹H NMR (CDCl₃, 400 MHz): δ

6.37 (s, 4H, Pz-H), 3.35 (m, 4H, Me₂CH) 1.34 (m, 24H,

(CH₃)₂CH), −0.42, −0.57 (s, 12H, Al^terminal(CH₃)₂), −0.94 (s, 3H,

Al^center(CH₃)). ¹³C NMR (CDCl₃, 50 MHz): δ 164.97 (ⁱPr-C^Pz),

145.22 (Pz-C^Pz), 97.84 (H-C^Pz), 27.26 (CH(CH₃)₂), 24.38, 23.18

(CH(CH₃)₂), −6.95, −8.23 (Al(CH₃)₂), −9.99 (Al(CH₃)). Anal.

Found (calcd) for D^Pr₂Al₃Me₅, C₂₉H₄₇Al₃N₈: C, 59.17 (59.31); N,

19.03 (18.97); H, 8.05 (7.79).

Accession Codes

CCDC 1955471 contains the supplementary crystallographic

data for this paper. These data can be obtained free of charge

via www.ccdc.cam.ac.uk/data_request/cif, or by emailing

data_request@ccdc.cam.ac.uk, or by contacting The Cam-

bridge Crystallographic Data Centre, 12 Union Road,

Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

AUTHOR INFORMATION

■

Synthesis of D^Ad₂Al₃Me₅. This compound was obtained using a

method similar to that for D^Bu₂Al₃Me₅except D^Ad-H was used in

Corresponding Authors

1

place of D^Bu-H. Yield: 2.1 g (60%). H NMR (CDCl₃, 400 MHz): δ

Kuo-Hui Wu − Department of Chemistry, National Central

University, Taoyuan 32001, Taiwan, Republic of China;

Email: wukuohui@ncu.edu.tw

6.42 (s, 4H, Pz-H), 2.07, 1.74 (br, 60H, Ad), −0.28, −0.53 (s, 12H,

Al^terminal(CH₃)₂), −1.22 (s, 3H, Al^center(CH₃)). ¹³C NMR (CDCl₃, 50

MHz): δ 168.88 (Ad-C^Pz), 145.00 (Pz-C^Pz), 100.21 (H-C^Pz), 42.74,

36.52, 35.44, 28.75 (Ad-C), −1.99, −3.38 (Al(CH₃)₂), −9.55

(Al(CH₃)). Anal. Found (calcd) for D^Ad₂Al₃Me₅, C₅₇H₇₉Al₃N₈: C,

71.52 (71.89); N, 11.71 (11.58); H, 8.32 (8.10).

Hsuan-Ying Chen − Department of Medicinal and Applied

Chemistry, Drug Development and Value Creation Research

Center, Kaohsiung Medical University, Kaohsiung 80708,

Taiwan, Republic of China; Department of Chemistry,

National Sun Yat-Sen University, Kaohsiung 80424, Taiwan,

Republic of China; Department of Medical Research,

Kaohsiung Medical University Hospital, Kaohsiung 80708,

Taiwan, Republic of China; orcid.org/0000-0003-1424-

3540; Phone: +886-7-3121101-2585; Email: hchen@

kmu.edu.tw; Fax: +886-7-3125339

Synthesis of D^ph₂Al₃Me₅. This compound was obtained using a

method similar to that for D^Bu₂Al₃Me₅except D^ph-H was used in

place of D^Bu-H. Yield: 0.42 g (85%). ¹H NMR (CDCl₃, 400 MHz): δ

7.51−7.48, 7.41−7.39 (m, 20 H, Ar-H), 6.58 (s, 2H, Pz-H), −0.21,

−0.42 (s, 12H, Al^terminal(CH₃)₂), −1.97 (s, 3H, Al^center(CH₃)). ¹³C

NMR (CDCl₃, 50 MHz): δ 157.93 (Ph-C^Pz), 144.74 (Pz-C^Pz), 132.28,

129.19, 129.09, 128.52 (Ph), 101.91 (H-C^Pz), 0.08, −5.91 (Al-

(CH₃)₂), −9.83 (Al(CH₃)). Anal. Found (calcd) for D^ph₂Al₃Me₅,

C₄₁H₃₉Al₃N₈: C, 67.95 (67.88); N, 15.46 (15.51); H, 5.42 (5.33).

Synthesis of D^tolyl₂Al₃Me₅. This compound was obtained using a

method similar to that for D^Bu₂Al₃Me₅except D^tolyl-H was used in

place of D^Bu-H. Yield: 0.47 g (75%). ¹H NMR (CDCl₃, 400 MHz): δ

7.30 (d, J = 8.0 Hz, 8H, m-CH₃−PhH), 7.12 (d, J = 8.0 Hz, 8H, o-

CH₃−PhH), 6.45 (s, 4H, Pz-H), 2.29 (s, 12H, CH₃), −0.30, −2.00 (s,

12H, Al^terminal(CH₃)₂), −0.52 (s, 3H, Al^center(CH₃)). ¹³C NMR

(CDCl₃, 50 MHz): δ 157.95 (Ph-C^Pz), 144.73 (Pz-C^Pz), 139.11,

129.46, 129.18, 128.98 (Ph), 101.75 (H-C^Pz), 21.42 (CH₃Ph) 1.12,

−5.92 (Al(CH₃)₂), −9.49 (Al(CH₃)). Anal. Found (calcd) for

D^tolyl₂Al₃Me₅, C₄₅H₄₇Al₃N₈: C, 69.22 (69.11); N, 14.35 (14.36); H,

6.07 (6.01).

Authors

Someswara Rao Kosuru − Department of Medicinal and

Applied Chemistry, Drug Development and Value Creation

Research Center, Kaohsiung Medical University, Kaohsiung

80708, Taiwan, Republic of China

Feng-Jie Lai − Department of Dermatology, Chi Mei Medical

Center, Tainan, Taiwan, Republic of China; Center for

General Education, Southern Taiwan University of Science

and Technology, Tainan, Taiwan, Republic of China

Yu-Lun Chang − Department of Medicinal and Applied

Chemistry, Drug Development and Value Creation Research

Center, Kaohsiung Medical University, Kaohsiung 80708,

Taiwan, Republic of China

Chen-Yu Li − Department of Medicinal and Applied

Chemistry, Drug Development and Value Creation Research

Center, Kaohsiung Medical University, Kaohsiung 80708,

Taiwan, Republic of China

Yi-Chun Lai − Department of Medicinal and Applied

Chemistry, Drug Development and Value Creation Research

Center, Kaohsiung Medical University, Kaohsiung 80708,

Taiwan, Republic of China

General Procedures for the Polymerization of ε-Caprolac-

tone. A typical polymerization procedure was exempliﬁed by the

synthesis in entry 1 of Table 2 using complex D^Bu₂Al₃Me₅as a

1

catalyst. The polymerization conversion was analyzed by H NMR

spectroscopic studies. Toluene (5.0 mL) was added to a mixture of

the complex D^Bu₂Al₃Me₅(0.25 mmol), BnOH (1.25 mmol), and ε-

caprolactone (10 mmol) at room temperature. At indicated time

intervals, 0.05 mL aliquots were removed, trapped with CDCl₃(1

mL), and analyzed by ¹H NMR. After the solution was stirred for 150

min, the reaction mixture was then quenched by adding a drop of

isopropyl alcohol, and the polymer precipitated as a white solid when

it was poured into n-hexane (50.0 mL). The isolated white solid was

dissolved in CH₂Cl₂(5.0 mL), and water (10 mL) was used to

washed the organic solution. Volatile materials were removed under

vacuum to give a puriﬁed crystalline solid. Yield: 1.04 g (92%).

Shangwu Ding − Department of Medicinal and Applied

Chemistry, Drug Development and Value Creation Research

Center, Kaohsiung Medical University, Kaohsiung 80708,

Taiwan, Republic of China; Department of Chemistry,

10545

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

(14) Xiang, C.; Acevedo, N. C. In Situ Self-Assembled Nano-

composites from Bacterial Cellulose Reinforced with Eletrospun

Poly(lactic acid)/Lipids Nanofibers. Polymers 2017, 9, 179−191.

(15) Bhowmick, S.; Thanusha, A. V.; Kumar, A.; Scharnweber, D.;

Rother, S.; Koul, V. Nanofibrous Artificial Skin Substitute Composed

of mPEG −PCL Grafted Gelatin/Hyaluronan/Chondroitin Sulfate/

Sericin for 2nd Degree Burn Care: in Vitro and in Vivo Study. RSC

Adv. 2018, 8, 16420−16432.

National Sun Yat-Sen University, Kaohsiung 80424, Taiwan,

Republic of China

Yung-Han Lo − Department of Chemistry, Faculty of Science

and Technology, Keio University, Minato City 108-8345

Tokyo, Japan

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.1c01192

(16) Slater, B.; Wong, S. O.; Duckworth, A.; White, A. J. P.; Hill, M.

R.; Ladewig, B. P. Upcycling a Plastic Ccup: One-Pot Synthesis of

Lactate Containing Metal Organic Frameworks from Polylactic Acid.

Chem. Commun. 2019, 55, 7319−7322.

(17) Ramot, Y.; Haim-Zada, M.; Domb, A. J.; Nyska, A.

Biocompatibility and Safety of PLA and Iits Copolymers. Adv. Drug

Delivery Rev. 2016, 107, 153−162.

Author Contributions

^○S.R.K. and F.-J.L. contributed equally.

Notes

The authors declare no competing ﬁnancial interest.

(18) Carosio, F.; Colonna, S.; Fina, A.; Rydzek, G.; Hemmerlé, J.;

Jierry, L.; Schaaf, P.; Boulmedais, F. Efficient Gas and Water Vapor

Barrier Properties of Thin Poly(lactic acid) Packaging Films:

Functionalization with Moisture Resistant Nafion and Clay Multi-

layers. Chem. Mater. 2014, 26, 5459−5466.

(19) Huang, B. H.; Tsai, C. Y.; Chen, C. T.; Ko, B. T. Metal

Complexes Containing Nitrogen-Heterocycle Based Aryloxide or

Arylamido Derivatives as Discrete Catalysts for Ring-Opening

Polymerization of Cyclic Esters. Dalton Trans. 2016, 45, 17557−

17580.

ACKNOWLEDGMENTS

■

This study was supported by the Ministry of Science and

Technology of Taiwan (Grant MOST 109-2113-M-037-006)

and Kaohsiung Medical University “NSYSU-KMU JOINT

RESEARCH PROJECT” (NSYSUKMU 107-P010 and KMU-

DK109004). We thank the Center for Research Resources and

Development at Kaohsiung Medical University for instrumen-

tation and equipment support.

(20) Redshaw, C. Metallocalixarene Catalysts: Alpha-Olefin

Polymerization and ROP of Cyclic Esters. Dalton Trans. 2016, 45,

9018−30.

REFERENCES

■

(1) Mato, Y.; Isobe, T.; Takada, H.; Ohtake, C.; Kaminuma, T.

Plastic Resin Pellets as a Transport Medium for Toxic Chemicals in

the Marine Environment. Environ. Sci. Technol. 2001, 35, 318−324.

(2) Green, D. S.; Boots, B.; Blockley, D. J.; Rocha, C.; Thompson, R.

Impacts of Discarded Plastic Bags on Marine Assemblages and

Ecosystem Functioning. Environ. Sci. Technol. 2015, 49, 5380−9.

(3) Jambeck, J. R.; Geyer, R.; Wilcox, C.; Siegler, T. R.; Perryman,

M.; Andrady, A.; Narayan, R.; Law, K. L. Plastic Waste Inputs from

Land into the Ocean. Science 2015, 347, 768−771.

(4) Lamb, J. B.; Willis, B. L.; Fiorenza, E. A.; Couch, C. S.; Howard,

R.; Rader, D. N.; True, J. D.; Kelly, L. A.; Ahmad, A.; Jompa, J.;

Harvell, C. D. Plastic Waste Associated with Disease on Coral Reefs.

Science 2018, 359, 460−462.

(5) Woodruff, M. A.; Hutmacher, D. W. The Return of a Forgotten

Polymer Polycaprolactone in the 21st Century. Prog. Polym. Sci.

2010, 35, 1217−1256.

(6) Kamaly, N.; Yameen, B.; Wu, J.; Farokhzad, O. C. Degradable

Controlled-Release Polymers and Polymeric Nanoparticles: Mecha-

nisms of Controlling Drug Release. Chem. Rev. 2016, 116, 2602−63.

(7) Liu, K.; Jiang, X.; Hunziker, P. Carbohydrate-Based Amphiphilic

Nano Delivery Systems for Cancer Therapy. Nanoscale 2016, 8,

16091−16156.

(8) Palivan, C. G.; Goers, R.; Najer, A.; Zhang, X.; Car, A.; Meier,

W. Bioinspired Polymer Vesicles and Membranes for Biological and

medical applications. Chem. Soc. Rev. 2016, 45, 377−411.

(9) Park, J. Y.; Gao, G.; Jang, J.; Cho, D.-W. 3D Printed Structures

for Delivery of Biomolecules and Cells: Tissue Repair and

Regeneration. J. Mater. Chem. B 2016, 4, 7521−7539.

(10) Shukla, S. K.; Shukla, S. K.; Govender, P. P.; Giri, N. G.

Biodegradable Polymeric Nanostructures in Therapeutic Applications:

Opportunities and Challenges. RSC Adv. 2016, 6, 94325−94351.

(11) Birnin-Yauri, A. U.; Ibrahim, N. A.; Zainuddin, N.; Abdan, K.;

Then, Y. Y.; Chieng, B. W. Effect of Maleic Anhydride-Modified

Poly(lactic acid) on the Properties of Its Hybrid Fiber Biocomposites.

Polymers 2017, 9, 165−180.

(21) Wei, Y.; Wang, S.; Zhou, S. Aluminum Alkyl Complexes:

Synthesis, Structure, and Application in ROP of Cyclic Esters. Dalton

Trans. 2016, 45, 4471−85.

(22) Chang, Y. A.; Waymouth, R. M. Recent Progress on the

Synthesis of Cyclic Polymers via Ring-Expansion Strategies. J. Polym.

Sci., Part A: Polym. Chem. 2017, 55, 2892−2902.

(23) Fuoco, T.; Pappalardo, D. Aluminum Alkyl Complexes Bearing

Salicylaldiminato Ligands: Versatile Initiators in the Ring-Opening

Polymerization of Cyclic Esters. Catalysts 2017, 7, 64.

(24) Osten, K. M.; Mehrkhodavandi, P. Indium Catalysts for Ring

Opening Polymerization: Exploring the Importance of Catalyst

Aggregation. Acc. Chem. Res. 2017, 50, 2861−2869.

(25) Redshaw, C. Use of Metal Catalysts Bearing Schiff Base

Macrocycles for the Ring Opening Polymerization (ROP) of Cyclic

Esters. Catalysts 2017, 7, 165−175.

(26) Ligny, R.; Hanninen, M. M.; Guillaume, S. M.; Carpentier, J. F.

Steric vs. Electronic Stereocontrol in Syndio- or Iso-Selective ROP of

Functional Chiral Beta-Lactones Mediated by Achiral Yttrium-

Bisphenolate Complexes. Chem. Commun. 2018, 54, 8024−8031.

(27) Stirling, E.; Champouret, Y.; Visseaux, M. Catalytic Metal-

Based Systems for Controlled Statistical Copolymerisation of Lactide

with a Lactone. Polym. Chem. 2018, 9, 2517−2531.

(28) Kremer, A. B.; Mehrkhodavandi, P. Dinuclear Catalysts for the

Ring Opening Polymerization of Lactide. Coord. Chem. Rev. 2019,

380, 35−57.

(29) Kosuru, S. R.; Sun, T. H.; Wang, L. F.; Vandavasi, J. K.; Lu, W.

Y.; Lai, Y. C.; Hsu, S. C. N.; Chiang, M. Y.; Chen, H. Y. Enhanced

Catalytic Activity of Aluminum Complexes for the Ring-Opening

Polymerization of Epsilon-Caprolactone. Inorg. Chem. 2017, 56,

7998−8006.

(30) Wang, P.; Chao, J.; Chen, X. Living Ring-Opening Polymer-

ization of Epsilon-Caprolactone Catalyzed by Beta-Quinolyl-Enamino

Aluminium Complexes: Ligand Electronic Effects. Dalton Trans.

2018, 47, 4118−4127.

(31) Chang, C.-C.; Chang, Y.-C.; Lu, W.-Y.; Lai, Y.-C.; Wu, K.-H.;

Lin, Y.-F.; Chen, H.-Y. Novel Aluminum Complexes Bearing 2-

(Aminomethylene)malonate Ligands with High Efficiency and

Controllability in Ring-Opening Polymerization of ε -Caprolactone.

Eur. Polym. J. 2019, 115, 399−408.

(12) Medel, S.; Syrova, Z.; Kovacik, L.; Hrdy, J.; Hornacek, M.;

Jager, E.; Hruby, M.; Lund, R.; Cmarko, D.; Stepanek, P.; Raska, I.;

Nyström, B. Curcumin-Bortezomib Loaded Polymeric Nanoparticles

for Synergistic Cancer Therapy. Eur. Polym. J. 2017, 93, 116−131.

(13) Watts, A.; Kurokawa, N.; Hillmyer, M. A. Strong, Resilient, and

Sustainable Aliphatic Polyester Thermoplastic Elastomers. Biomacro-

molecules 2017, 18, 1845−1854.

(32) Qin, L.; Zhang, Y.; Chao, J.; Cheng, J.; Chen, X. Four- and

Five-Coordinate Aluminum Complexes Supported by N,O-Bidentate

10546

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

Beta-Pyrazylenolate Ligands: Synthesis, Structure and Application in

ROP of Epsilon-Caprolactone and Lactide. Dalton Trans. 2019, 48,

12315−12325.

(33) Huang, T. W.; Su, R. R.; Lin, Y. C.; Lai, H. Y.; Yang, C. Y.;

Senadi, G. C.; Lai, Y. C.; Chiang, M. Y.; Chen, H. Y. Improvement in

Aluminum Complexes Bearing a Schiff Base in Ring-Opening

Polymerization of Epsilon-Caprolactone: the Synergy of the N,S-

Schiff Base in a Five-Membered Ring Aluminum System. Dalton

Trans. 2018, 47, 15565−15573.

(34) Macaranas, J. A.; Luke, A. M.; Mandal, M.; Neisen, B. D.;

Marell, D. J.; Cramer, C. J.; Tolman, W. B. Sterically Induced Ligand

Framework Distortion Effects on Catalytic Cyclic Ester Polymer-

izations. Inorg. Chem. 2018, 57, 3451−3457.

(48) Huang, T.-L.; Chen, C.-T. Aluminium Complexes Containing

Pyrazolyl −Phenolate Ligands as Catalysts for Ring Opening Polymer-

ization of ε -Caprolactone. J. Organomet. Chem. 2013, 725, 15−21.

(49) Rufino-Felipe, E.; Lopez, N.; Vengoechea-Gómez, F. A.;

́

Guerrero-Ramírez, L.-G.; Munoz-Hernández, M.-A. Ring-Opening

̃

Polymerization of rac-Lactide Catalyzed by Al(III) and Zn(II)

Complexes Incorporating Schiff Base Ligands Derived from Pyrrole-

2-Carboxaldehyde. Appl. Organomet. Chem. 2018, 32, e4315−e4324.

(50) Shironina, T. M.; Igidov, N. M.; Koz’minykh, E. N.; Kon’shina,

L. O.; Kasatkina, Y. S.; Koz’minykh, V. O. 1,3,4,6-Tetracarbonyl

Compounds: IV.* Reaction of 3,4-Dihydroxy-2,4-hexadiene-1,6-

diones with Hydrazine and Arylhydrazines. Russ. J. Org. Chem.

2001, 37, 1486−1494.

(35) Chumsaeng, P.; Haesuwannakij, S.; Virachotikul, A.;

Phomphrai, K. Random Copolymerization of L-Lactide and ε -

Caprolactone by Aluminum Alkoxide Complexes Supported by

N₂O₂Bis(phenolate)-Amine Ligands. J. Polym. Sci., Part A: Polym.

Chem. 2019, 57, 1635−1644.

(36) Li, M.; Behzadi, S.; Chen, M.; Pang, W.; Wang, F.; Tan, C.

Phenoxyimine Ligands Bearing Nitrogen-Containing Second Coordi-

nation Spheres for Zinc Catalyzed Stereoselective Ring-Opening

Polymerization of rac-Lactide. Organometallics 2019, 38, 461−468.

(37) Plommer, H.; Murphy, J. N.; Dawe, L. N.; Kerton, F. M.

Morpholine-Stabilized Cationic Aluminum Complexes and Their

Reactivity in Ring-Opening Polymerization of epsilon-Caprolactone.

Inorg. Chem. 2019, 58, 5253−5264.

(38) Chen, L.; Li, W.; Yuan, D.; Zhang, Y.; Shen, Q.; Yao, Y.

Syntheses of Mononuclear and Dinuclear Aluminum Complexes

Stabilized by Phenolato Ligands and Their Applications in the

Polymerization of epsilon-Caprolactone: A Comparative Study. Inorg.

Chem. 2015, 54, 4699−708.

(39) Huang, H.-C.; Wang, B.; Zhang, Y.-P.; Li, Y.-S. Bimetallic

Aluminum Complexes with Cyclic β -Ketiminato Ligands: the

Cooperative Effect Improves Their Capability in Polymerization of

Lactide and ε -Caprolactone. Polym. Chem. 2016, 7, 5819−5827.

(40) Beament, J.; Mahon, M. F.; Buchard, A.; Jones, M. D.

Aluminum Complexes of Monopyrrolidine Ligands for the Controlled

Ring-Opening Polymerization of Lactide. Organometallics 2018, 37,

1719−1724.

(41) Duda, A.; Florjanczyk, Z.; Hofman, A.; Slomkowski, S.;

Penczek, S. Living Pseudoanionic Polymerization of e-Caprolactone.

Poly(e-capro1actone) Free of Cyclics and with Controlled End

Groups. Macromolecules 1990, 23, 1640−1646.

(42) Yu, Y.; Fischer, E. J.; Storti, G.; Morbidelli, M. Modeling of

Molecular Weight Distribution in Ring-Opening Polymerization of

L,L-Lactide. Ind. Eng. Chem. Res. 2014, 53, 7333−7342.

(43) Lian, B.; Thomas, C. M.; Casagrande, O. L.; Lehmann, C. W.;

Roisnel, T.; Carpentier, J. F. Aluminum and Zinc Complexes Based

on an Amino-Bis(pyrazolyl) Ligand: Synthesis, Structures, and Use in

MMA and Lactide Polymerization. Inorg. Chem. 2007, 46, 328−340.

(44) Milione, S.; Grisi, F.; Centore, R.; Tuzi, A. Neutral and

Cationic Heteroscorpionate Aluminum Complexes: Synthesis,

Structure, and Ring-Opening Polymerization of ε -Caprolactone.

Organometallics 2006, 25, 266−274.

(45) Chai, Z.-Y.; Zhang, C.; Wang, Z.-X. Synthesis, Characterization,

and Catalysis in ε -Caprolactone Polymerization of Aluminum and

Zinc Complexes Supported by N,N,N-Chelate Ligands. Organo-

metallics 2008, 27, 1626−1633.

(51) Storch, G.; Spallek, M. J.; Rominger, F.; Trapp, O.

Tautomerization-Mediated Molecular Switching between Six- and

Seven-Membered Rings Stabilized by Hydrogen Bonding. Chem. -

Eur. J. 2015, 21, 8939−45.

(52) Aida, T.; Inoue, S. Metalloporphyrins as Initiators for Living

and Immortal Polymerizations. Acc. Chem. Res. 1996, 29, 39−48.

(53) Ajellal, N.; Lyubov, D. M.; Sinenkov, M. A.; Fukin, G. K.;

Cherkasov, A. V.; Thomas, C. M.; Carpentier, J. F.; Trifonov, A. A.

Bis(guanidinate) Alkoxide Complexes of Lanthanides: Synthesis,

Structures and Use in Immortal and Stereoselective Ring-Opening

Polymerization of Cyclic Esters. Chem. - Eur. J. 2008, 14, 5440−8.

(54) Rosen, T.; Goldberg, I.; Navarra, W.; Venditto, V.; Kol, M.

Divergent [{ONNN}Mg-Cl] Complexes in Highly Active and Living

Lactide Polymerization. Chem. Sci. 2017, 8, 5476−5481.

(55) Wang, B.; Zhao, H.; Wang, L.; Sun, J.; Zhang, Y.; Cao, Z.

Immortal Ring-Opening Polymerization of Lactides with Super High

Monomer to Catalyst Ratios Initiated by Zzirconium and Titanium

Complexes Containing Multidentate Amino-Bis(phenolate) Ligands.

New J. Chem. 2017, 41, 5669−5677.

(56) Wilson, J. A.; Hopkins, S. A.; Wright, P. M.; Dove, A. P.

‘Immortal ’ Ring-Opening Polymerization of ω -Pentadecalactone by

Mg(BHT)₂(THF)₂. Polym. Chem. 2014, 5, 2691−2694.

(57) Ma, H.; Okuda, J. Kinetics and Mechanism of L-Lactide

Polymerization by Rare Earth Metal Silylamido Complexes: Effect of

Alcohol Addition. Macromolecules 2005, 38, 2665−2673.

(58) Bridier, B.; Lopez, N.; Perez-Ramirez, J. Molecular Under-

standing of Alkyne Hydrogenation for the Design of Selective

Catalysts. Dalton Trans 2010, 39, 8412−8419.

(59) Amgoune, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J. F.

Ring-Opening Polymerization of Lactide with Group 3 Metal

Complexes Supported by Dianionic Alkoxy-Amino-Bisphenolate

Ligands: Combining High Activity, Productivity, and Selectivity.

Chem. - Eur. J. 2006, 12, 169−179.

(60) Nuyken, O.; Pask, S. Ring-Opening Polymerization An

Introductory Review. Polymers 2013, 5, 361−403.

(61) Hsu, C.-Y.; Tseng, H.-C.; Vandavasi, J. K.; Lu, W.-Y.; Wang, L.-

F.; Chiang, M. Y.; Lai, Y.-C.; Chen, H.-Y.; Chen, H.-Y. Investigation

of the Dinuclear Effect of Aluminum Complexes in the Ring-Opening

Polymerization of ε -Caprolactone. RSC Adv. 2017, 7, 18851−18860.

(62) Pyykko, P.; Atsumi, M. Molecular Single-Bond Covalent Radii

for Elements 1 −118. Chem. - Eur. J. 2009, 15, 186−97.

(63) Batsanov, S. S. Van der Waals Radii of Elements. Inorg. Mater.

2001, 37, 871−885.

(64) Nishio, M. The CH/pi Hydrogen bond in chemistry.

Conformation, Supramolecules, Optical Resolution and Interactions

Involving Carbohydrates. Phys. Chem. Chem. Phys. 2011, 13, 13873−

13900.

(46) Silvestri, A.; Grisi, F.; Milione, S. Ring Opening Polymerization

of Lactide Promoted by Alcoholyzed Heteroscorpionate Aluminum

Complexes. J. Polym. Sci., Part A: Polym. Chem. 2010, 48, 3632−3639.

(47) Otero, A.; Lara-Sánchez, A. n.; Fernández-Baeza, J.; Alonso-

Moreno, C.; Castro-Osma, J. A.; Márquez-Segovia, I.; Sánchez-Barba,

L. F.; Rodríguez, A. M.; Garcia-Martinez, J. C. Neutral and Cationic

Aluminum Complexes Supported by Acetamidate and Thioacetami-

date Heteroscorpionate Ligands as Initiators for Ring-Opening

Polymerization of Cyclic Esters. Organometallics 2011, 30, 1507−

1522.

(65) Marlier, E. E.; Macaranas, J. A.; Marell, D. J.; Dunbar, C. R.;

Johnson, M. A.; DePorre, Y.; Miranda, M. O.; Neisen, B. D.; Cramer,

C. J.; Hillmyer, M. A.; Tolman, W. B. Mechanistic Studies of epsilon-

Caprolactone Polymerization by (salen)AlOR Complexes and a

Predictive Model for Cyclic Ester Polymerizations. ACS Catal. 2016,

6, 1215−1224.

(66) Li, Y.; Zhao, K.-Q.; Elsegood, M. R. J.; Prior, T. J.; Sun, X.; Mo,

S.; Redshaw, C. Organoaluminium Complexes of ortho-, meta-, para-

10547

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

Anisidines: Synthesis, Structural Studies and ROP of ε -Caprolactone

(and rac-Lactide). Catal. Sci. Technol. 2014, 4, 3025−3031.

(67) Hsu, C.-Y.; Tseng, H.-C.; Vandavasi, J. K.; Lu, W.-Y.; Wang, L.-

F.; Chiang, M. Y.; Lai, Y.-C.; Chen, H.-Y.; Chen, H.-Y. Investigation

of the Dinuclear Effect of Aluminum Complexes in the Ring-Opening

Polymerization of ε -Caprolactone. RSC Adv. 2017, 7, 18851−18860.

(68) Chang, M. C.; Lu, W. Y.; Chang, H. Y.; Lai, Y. C.; Chiang, M.

Y.; Chen, H. Y.; Chen, H. Y. Comparative Study of Aluminum

Complexes Bearing N,O- and N,S-Schiff Base in Ring-Opening

Polymerization of epsilon-Caprolactone and L-Lactide. Inorg. Chem.

2015, 54, 11292−11298.

(69) Lee, C.-L.; Lin, Y.-F.; Jiang, M.-T.; Lu, W.-Y.; Vandavasi, J. K.;

Wang, L.-F.; Lai, Y.-C.; Chiang, M. Y.; Chen, H.-Y. Improvement in

Aluminum Complexes Bearing Schiff Bases in Ring-Opening

Polymerization of ε -Caprolactone: A Five-Membered-Ring System.

Organometallics 2017, 36, 1936−1945.

(70) Huang, T. W.; Su, R. R.; Lin, Y. C.; Lai, H. Y.; Yang, C. Y.;

Senadi, G. C.; Lai, Y. C.; Chiang, M. Y.; Chen, H. Y. Improvement in

Aluminum Complexes Bearing a Schiff Base in Ring-Opening

Polymerization of epsilon-Caprolactone: the Synergy of the N,S-

Schiff Base in a Five-Membered Ring Aluminum System. Dalton

Trans. 2018, 47, 15565−15573.

(71) Lu, W.-Y.; Chang, Y.-C.; Lian, C.-J.; Wu, K.-H.; Chiang, M. Y.;

Chen, H.-Y.; Lin, C.-C.; Ko, B.-T. Optimization of Six-Membered ring

Aluminum Complexes in ε -Caprolactone Polymerization. Eur. Polym.

J. 2019, 114, 151−163.

(72) Huang, T.-L.; Chen, C.-T. Aluminium Complexes Containing

Pyrazolyl −Phenolate Ligands as Catalysts for Ring Opening Polymer-

ization of ε -Caprolactone. J. Organomet. Chem. 2013, 725, 15−21.

(73) Tseng, H. C.; Chiang, M. Y.; Lu, W. Y.; Chen, Y. J.; Lian, C. J.;

Chen, Y. H.; Tsai, H. Y.; Lai, Y. C.; Chen, H. Y. A Closer Look at

epsilon-Caprolactone Polymerization Catalyzed by Alkyl Aluminum

Complexes: the Effect of Induction Period on Overall Catalytic

Activity. Dalton Trans. 2015, 44, 11763−73.

(74) Alcazar-Roman, L. M.; O’Keefe, B. J.; Hillmyer, M. A.; Tolman,

W. B. Electronic Influence of Ligand Substituents on the Rate of

Polymerization of ε -Caprolactone by Single-Site Aluminium Alkoxide

Catalysts. Dalton Trans. 2003, 3082−3087.

aluminiums Supported by Bulky Heteroscorpionate Ligands. Dalton

Trans. 2015, 44, 12388−12400.

(83) Bouyahyi, M.; Grunova, E.; Marquet, N.; Kirillov, E.; Thomas,

C. M.; Roisnel, T.; Carpentier, J. F. Aluminum Complexes of

Fluorinated Dialkoxy-Diimino Salen-like Ligand: Synthesis, Struc-

tures, and Use in Ring-Opening Polymerization of Cyclic Esters.

Organometallics 2008, 27, 5815−5825.

(84) Amgoune, A.; Lavanant, L.; Thomas, C. M.; Chi, Y.; Welter, R.;

Dagorne, S.; Carpentier, J. F. Chelation Effect in Polymerization of

Cyclic Esters by Metal Alkoxides: Structure Characterization of the

Intermediate Formed by Primary Insertion of Lactide into the Al-OR

Bond of an Organometallic Initiator. Organometallics 2005, 24, 6279−

6282.

(85) Kazarina, O. V.; Gourlaouen, C.; Karmazin, L.; Morozov, A. G.;

Fedushkin, I. L.; Dagorne, S. Low Valent Al(ii)-Al(ii) Catalysts as

Highly Active epsilon-Caprolactone Polymerization Catalysts: In-

dication of Metal Cooperativity through DFT Studies. Dalton Trans.

2018, 47, 13800−13808.

(86) Wu, C.; Zhou, H.; He, M.; Su, Q.; Li, G.; Wu, Q.; Mu, Y.

Binuclear Aluminum Complexes with Amine −Imine Type Ligands

Derived from 1,3-Benzenedialdehyde: Synthesis, Structures and Their

Ccatalytic Properties in Ring-Opening Polymerization. J. Coord.

Chem. 2016, 69, 1066−1075.

(87) Wei, C.; Han, B.; Zheng, D.; Zheng, Q.; Liu, S.; Li, Z.

Aluminum Complexes Bearing Bidentate Amido −Phosphine Ligands

for Ring-Opening Polymerization of ε -Caprolactone: Steric Effect on

Coordination Chemistry and Reactivity. Organometallics 2019, 38,

3816−3823.

(88) Li, G.; Lamberti, M.; Pappalardo, D.; Pellecchia, C. Random

Copolymerization of ε -Caprolactone and Lactides Promoted by

Pyrrolylpyridylamido Aluminum Complexes. Macromolecules 2012,

45, 8614−8620.

(89) Hsiao, H.-C.; Datta, A.; Chen, Y.-F.; Chang, W.; Lee, T.-Y.; Lin,

C.-H.; Huang, J.-H. Structural Aspects and Ring Opening Polymer-

ization of ε -Caprolactone Using Mono- and Di-aluminum Com-

pounds Incorporating Bidentate Pyrrole-Morpholine Ligands. J.

Organomet. Chem. 2016, 804, 35−41.

(90) Armitage, A.; Boyron, O.; Champouret, Y.; Patel, M.; Singh, K.;

Solan, G. Dimethyl-Aluminium Complexes Bearing Naphthyl-

Substituted Pyridine-Alkylamides as Pro-Initiators for the Efficient

ROP of ε -Caprolactone. Catalysts 2015, 5, 1425−1444.

(91) Huang, C.-H.; Wang, F.-C.; Ko, B. T.; Yu, T.-L.; Lin, C. C.

Ring-Opening Polymerization of e-Caprolactone and L-Lactide Using

Aluminum Thiolates as Initiator. Macromolecules 2001, 34, 356−361.

(92) Tseng, H.-C.; Chen, F.-S.; Chiang, M. Y.; Lu, W.-Y.; Chen, Y.-

H.; Lai, Y.-C.; Chen, H.-Y. Optimizing Ring-Opening Polymerization

of ε -Caprolactone by Using Aluminum Complexes Bearing Amide as

Catalysts and Their Application in Synthesizing Poly-ε -Caprolactone

with Special Initiators and Other Polycycloesters. RSC Adv. 2015, 5,

90682−90690.

(93) Zhang, W.; Wang, Y.; Cao, J.; Wang, L.; Pan, Y.; Redshaw, C.;

Sun, W.-H. Synthesis and Characterization of Dialkylaluminum

Amidates and Their Ring-Opening Polymerization of ε -Caprolactone.

Organometallics 2011, 30, 6253−6261.

(94) Koller, J. r.; Bergman, R. G. Highly Efficient Aluminum-

Catalyzed Ring-Opening Polymerization of Cyclic Carbonates,

Lactones, and Lactides, Including a Unique Crystallographic Snapshot

of an Intermediate. Organometallics 2011, 30, 3217−3224.

(95) Wang, X.; Zhao, K.-Q.; Al-Khafaji, Y.; Mo, S.; Prior, T. J.;

Elsegood, M. R. J.; Redshaw, C. Organoaluminium Complexes

Derived from Anilines or Schiff Bases for the Ring-Opening

Polymerization of ε -Caprolactone, δ -Valerolactone andrac-Lactide.

Eur. J. Inorg. Chem. 2017, 2017, 1951−1965.

(75) Ikpo, N.; Barbon, S. M.; Drover, M. W.; Dawe, L. N.; Kerton, F.

M. Aluminum Methyl and Chloro Complexes Bearing Monoanionic

Aminephenolate Ligands: Synthesis, Characterization, and Use in

Polymerizations. Organometallics 2012, 31, 8145−8158.

(76) Wei, J.; Riffel, M. N.; Diaconescu, P. L. Redox Control of

Aluminum Ring-Opening Polymerization: A Combined Experimental

and DFT Investigation. Macromolecules 2017, 50, 1847−1861.

(77) Alkarekshi, W.; Armitage, A. P.; Boyron, O.; Davies, C. J.;

Govere, M.; Gregory, A.; Singh, K.; Solan, G. A. Phenolate

Substituent Effects on Ring-Opening Polymerization of ε -Caprolac-

tone by Aluminum Complexes Bearing 2-(Phenyl-2-olate)-6-(1-

amidoalkyl)pyridine Pincers. Organometallics 2013, 32, 249−259.

(78) Gesslbauer, S.; Hutchinson, G.; White, A. J. P.; Burés, J.;

Romain, C. Chirality-Induced Catalyst Aggregation: Insights into

Catalyst Speciation and Activity Using Chiral Aluminum Catalysts in

Cyclic Ester Ring-Opening Polymerization. ACS Catal. 2021, 11,

4084−4093.

(79) Hsieh, Y.-L.; Huang, N.; Lee, G.-H.; Peng, C.-H. Bipyridine-

Phenolate Based Aluminum Complexes Mediated Ring-Opening

Polymerization of ε -Caprolactone and Lactides with a High

Stereoselectivity. Polymer 2015, 72, 281−291.

(80) Wang, P.; Hao, X.; Cheng, J.; Chao, J.; Chen, X. Aluminum

Chelates Supported by beta-Quinolyl Enolate Ligands: Synthesis and

ROP of epsilon-CL. Dalton Trans. 2016, 45, 9088−96.

(81) Chen, C.-T.; Liao, C.-H.; Peng, K.-F.; Chen, M.-T.; Huang, T.-

L. Synthesis, Characterization and Catalytic Studies of Aluminium

Complexes Containing Sulfonamido −Oxazolinate or − Pyrazolinate

Ligands. J. Organomet. Chem. 2014, 753, 9−19.

(82) Castro-Osma, J. A.; Alonso-Moreno, C.; Lara-Sanchez, A.;

Otero, A.; Fernandez-Baeza, J.; Sanchez-Barba, L. F.; Rodriguez, A. M.

Catalytic Behaviour in the Ring-Opening Polymerisation of Organo-

(96) Chakraborty, D.; Chen, E. Y.-X. Neutral, Three-Coordinate,

Chelating Diamide Aluminum Complexes: Catalysts/Initiators for

Synthesis of Telechelic Oligomers and High Polymers. Organo-

metallics 2002, 21, 1438−1442.

(97) D’Auria, I.; Lamberti, M.; Mazzeo, M.; Milione, S.; Pellecchia,

C. Phosphido-diphosphine pincer aluminum complexes as catalysts

10548

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Inorganic Chemistry

pubs.acs.org/IC

Article

for ring opening polymerization of cyclic esters. J. Polym. Sci., Part A:

Polym. Chem. 2014, 52, 49−60.

(98) Hild, F.; Neehaul, N.; Bier, F.; Wirsum, M.; Gourlaouen, C.;

Dagorne, S. Synthesis and Structural Characterization of Various

N,O,N-Chelated Aluminum and Gallium Complexes for the Efficient

ROP of Cyclic Esters and Carbonates: How Do Aluminum and

Gallium Derivatives Compare ? Organometallics 2013, 32, 587−598.

(99) Ziemkowska, W.; Kucharski, S.; Kołodziej, A.; Anulewicz-

Ostrowska, R. Reactions of Alkylalane Diolates with Water Synthesis,

Characterisation and ε -Caprolactone Polymerisation Aactivity of

Novel Alane Benzopinacolates. J. Organomet. Chem. 2004, 689,

2930−2939.

(100) Huang, Y.-T.; Wang, W.-C.; Hsu, C.-P.; Lu, W.-Y.; Chuang,

W.-J.; Chiang, M. Y.; Lai, Y.-C.; Chen, H.-Y. The Ring-Opening

Polymerization of ε -Caprolactone and L-Lactide Using Aluminum

Complexes Bearing Benzothiazole Ligands as Catalysts. Polym. Chem.

2016, 7, 4367−4377.

(101) Chang, C.-J.; Chiu, C.-F.; Wu, K.-H.; Chang, Y.-L.; Lai, Y.-C.;

Ding, S.; Chen, H.-Y. N-heterocyclic ligand optimization for

aluminum complexes in ε -caprolactone and L-Lactide polymerization.

Polymer 2020, 202, 122572.

́

(102) Ziemkowska, W.; Kochanowski, J.; Cyranski, M. K. Synthesis,

structures and ε -caprolactone polymerization activity of aluminum

N,N ′-dimethyloxalamidates. J. Organomet. Chem. 2010, 695, 1205−

1209.

(103) Wang, Q.; Zhao, W.; He, J.; Zhang, Y.; Chen, E. Y. X. Living

Ring-Opening Polymerization of Lactones by N-Heterocyclic Olefin/

Al(C₆F₅)₃Lewis Pairs: Structures of Intermediates, Kinetics, and

Mechanism. Macromolecules 2017, 50, 123−136.

(104) Jozak, T.; Zabel, D.; Schubert, A.; Sun, Y.; Thiel, W. R.

Ruthenium Complexes Bearing N-H Acidic Pyrazole Ligands. Eur. J.

Inorg. Chem. 2010, 2010, 5135−5145.

(105) Becke, A. D. Density-Functional Thermochemistry. III. The

Role of Exact Exchange. J. Chem. Phys. 1993, 98, 5648−5652.

(106) Lee, C.; Yang, W.; Parr, R. G. Development of the Colle-

Salvetti Correlation-Energy Formula into a Functional of the Electron

Density. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−

789.

10549

https://doi.org/10.1021/acs.inorgchem.1c01192

Inorg. Chem. 2021, 60, 10535−10549

Article Doi

Collaboration between Trinuclear Aluminum Complexes Bearing Bipyrazoles in the Ring-Opening Polymerization of ?-Caprolactone

DOI: 10.1021/acs.inorgchem.1c01192

Source and publish data:

Authors:

Article abstract of DOI:10.1021/acs.inorgchem.1c01192

Full text of DOI:10.1021/acs.inorgchem.1c01192

Products guided by the article

R&D Labs maybe for 63499-70-7

Relevant to this article

Hot Product