Aluminum Complexes Bearing Bidentate Amido-Phosphine Ligands for Ring-Opening Polymerization of ?-Caprolactone: Steric Effect on Coordination Chemistry and Reactivity

Article abstract of DOI:10.1021/acs.organomet.9b00502

The aluminum complexes LAlMe₂ (Al1-Al5: LH = 2,6-(R¹)₂-4-R²-C₆H₂NHCH₂C₆H₄-2-PPh₂; Al1, R¹ = H, R² = H; Al2, R¹ = Me, R² = H; Al3, R¹ = ⁱPr, R² = H; Al4, R¹ = Ph₂CH, R² = ⁱPr; Al5, R¹ = Cl, R² = H) have been synthesized and characterized by elemental analysis and ¹H, ¹³C, and ³¹P NMR. NMR analysis in solution reveals an interesting hemilabile coordination of the soft P donor. The molecular structures of Al2-Al4 were defined by X-ray diffraction studies, showing a distorted-tetrahedral geometry around the aluminum center in all structures. Careful comparison of these crystal structures suggested that different substituents on the ligands could lead to unignorable changed coordination environments around the Al center, thus affecting their catalytic properties. In the presence of BnOH, complexes Al1-Al5 efficiently catalyzed the ring-opening polymerization (ROP) of ?-caprolactone (?-CL) with high conversions in a controlled manner, and high molecular weights (M_n up to 118.6 kg mol^-1) of polycaprolactones (PCLs) were readily prepared. Immortal polymerizations by Al4 having bulky Ph₂CH groups were also studied with up to 20 equiv of alcohols and 2000 equiv of monomers, without sacrificing polymerization control.

Full text of DOI:10.1021/acs.organomet.9b00502

Article
pubs.acs.org/Organometallics
Cite This: Organometallics XXXX, XXX, XXX−XXX
Aluminum Complexes Bearing Bidentate Amido−Phosphine
Ligands for Ring-Opening Polymerization of ε‑Caprolactone: Steric
Effect on Coordination Chemistry and Reactivity
Chuanzhi Wei, Binghao Han, Dejuan Zheng, Quande Zheng, Shaofeng Liu,* and Zhibo Li*
Key Laboratory of Biobased Polymer Materials, Shandong Provincial Education Department; College of Polymer Science and
Engineering, Qingdao University of Science and Technology, Qingdao 266042, People’s Republic of China
S
* Supporting Information
ABSTRACT: The aluminum complexes LAlMe2 (Al1−Al5: LH = 2,6-
(R1)2-4-R2-C6H2NHCH2C6H4-2-PPh2; Al1, R1 = H, R2 = H; Al2, R1 =
Me, R2 = H; Al3, R1 = iPr, R2 = H; Al4, R1 = Ph2CH, R2 = iPr; Al5, R1 =
Cl, R2 = H) have been synthesized and characterized by elemental
analysis and 1H, 13C, and 31P NMR. NMR analysis in solution reveals an
interesting hemilabile coordination of the soft P donor. The molecular
structures of Al2−Al4 were defined by X-ray diffraction studies, showing
a distorted-tetrahedral geometry around the aluminum center in all
structures. Careful comparison of these crystal structures suggested that
different substituents on the ligands could lead to unignorable changed
coordination environments around the Al center, thus affecting their
catalytic properties. In the presence of BnOH, complexes Al1−Al5
efficiently catalyzed the ring-opening polymerization (ROP) of ε-
caprolactone (ε-CL) with high conversions in a controlled manner, and high molecular weights (Mn up to 118.6 kg mol−1)
of polycaprolactones (PCLs) were readily prepared. Immortal polymerizations by Al4 having bulky Ph2CH groups were also
studied with up to 20 equiv of alcohols and 2000 equiv of monomers, without sacrificing polymerization control.
coordination environments by introducing various substituents
or heteroatoms into the coordinated ligands can considerably
enhance the catalytic performances, including the activity and
selectivity. Therefore, various bidentate or multidentate ligands
containing N, O, or S donors have been synthesized and
examined in this field.38 However, aluminum complexes with
ligands containing P donors have rarely been explored.39−42
Recently, our group reported that a phosphine−imine
compound, which combined the soft P donor and the hard
N donor in one molecule and constructed an unsymmetric PN
bidentate ligand, was promising for designing metal-based
catalysts.43 The corresponding chromium complexes showed
intriguing catalysis toward ethylene polymerization. With these
studies as inspiration, the neutral phosphine−imine ligand was
reduced to a phosphine−amine compound, which could serve
as a monoanionic amido−phosphine ligand by deprotonation.
The corresponding aluminum complexes are reported herein
and used as catalysts for ROP reactions. In the presence of
BnOH, Al complexes are highly efficient for the ROP of ε-CL
and produce PCLs with high MWs (up to 118.6 kg mol−1).
The X-ray diffraction of Al complexes and kinetics of
polymerization reveal that different substituents on the ligands
lead to distinguishing coordination environments around the
Al center and thus different catalytic performances. Moreover,
1. INTRODUCTION
The synthesis of aliphatic polyesters, e.g. polycaprolactone
(PCL) and poly(lactic acid) (PLA), is a topic of wide interest
in both academic and industrial research owing to their
important applications in agriculture, packaging, and bio-
medical fields.1,2 In comparison to condensation polymer-
ization, the ring-opening polymerization (ROP) of cyclic esters
is a convenient methodology for the synthesis of well-defined
polyesters with a catalyst and/or an initiator, including metal-
based catalysts,3−5 enzymes,6 organocatalysts,7−12 and dual
catalysts.13−16 In particular, this method could produce
polyester with high molecular weights, i.e. ultrahigh-molec-
ular-weight PCL (UHMWPCL, Mn > 50 kg mol−1), which is
quite desirable as a potential scaffold material for tissue
engineering.17
There is no doubt that the metal-based catalysts are
dominant in the polymerization catalysis area, given their
varying combinations of ligands and metals. In the past
decades, a variety of aluminum,18−25 magnesium and zinc,26−28
iron, titanium, and zirconium,29−31 and rare-earth-metal32−36
complexes have been employed for the preparation of
polyesters. Among these, Al complexes ligating ancillary
ligands have gained considerable attention and have become
promising candidates for ROP reactions due to their high
activity, relatively low toxicity, and good control of the
molecular weight (MW) and dispersity (Đ = Mw/Mn) of the
resulting polyesters.37 In particular, attentive tuning of the
Received: July 26, 2019
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of PN Ligands LH1−LH5 and Al Complexes Al1−Al5
Scheme 2. Possible Mechanism for the Dynamic Exchange of the Phosphine Group at the Al Center
the polymerizations using an Al complex with bulky Ph2CH
groups exhibit a feature of immortal catalysis in the presence of
excess alcohol.
copy (1H, 13C, and 31P; Figures S4−S18) and elemental
analysis, which were consistent with the chemical structure of
1
LAlMe2. In the H NMR spectrum of Al4 (Figure S13), in
comparison to that of the corresponding ligand LH4 (Figure
2. RESULTS AND DISCUSSION
S1), a new additional resonance at −0.76 ppm (doublet, JPH
=
4.2 Hz) is present and is assigned to the methyl groups on the
Al center (Al−CH3).47,48 In the meantime, the N−H signal
(broad resonance at 3.24 ppm) of LH4 disappears as expected.
Note that there is only one singlet for the benzylic methylene
2.1. Synthesis and Characterization of the Amido−
Phosphine Ligands and Aluminum Complexes. The
ligands LH1−LH5 shown in Scheme 1 were prepared
according to previous reports.44,45 The condensations of 2-
(diphenylphosphino)benzaldehyde and different amines can
easily occur to form imine intermediates (1−4). Unfortunately,
2,6-dichloroaniline, having electron-withdrawing substituents,
cannot react directly with 2-(diphenylphosphino)benzaldehyde
under these conditions. Therefore, 2,6-dichloroaniline was first
reacted with trimethylaluminum (AlMe3) to form an amino-
alane, which was an effective reagent for converting the
aldehyde into an imine (5), because the Al−O bond is much
stronger than the Al−N bond.46 Next, LiAlH4 was employed as
the reduction agent for the transformation from imine
intermediates (1−5) to amine products LH1−LH5 with
good yields (81−90%). The structures of LH1−LH5 were
confirmed by elemental analysis and NMR spectroscopy. In
the 1H NMR spectrum of LH4 (Figure S1), the resonances at
4.05 ppm for CH2−NH and 3.24 ppm for CH2−NH clearly
illustrated the amine structure of the ligand.
1
protons (ArCH2−N) in the H NMR spectra of Al complexes
(Figures S4, S7, S10, S13, and S16). This observation is quite
beyond our expectation that these methylene protons (Scheme
2, Ha and Hb in A and C) should be diastereotopic and two
doublets normally exist if a stable coordination state (see
below for molecular structures) is maintained in solution. On
the basis of these results, we believe that in solution there
should be the existence of a fast equilibrium between four-
coordinated species (Scheme 2, A and C) and three-
coordinated species (Scheme 2, B) as shown in Scheme 2.
Therefore, Ha and Hb (Scheme 2) are equivalent on the NMR
time scale, and the soft donor P atom shows a hemilabile
coordination feature.49 This also explains the fact that there is
1
only one singlet for Ph2CH (6.09 ppm) in the H NMR
spectrum of Al4 (Figure S13) at 20 °C, which is similar to the
case for the ligand LH4 (one singlet at 6.17 ppm in Figure S1).
Due to the existence of an equilibrium, the aryl−N bond of
complex Al4 could rotate in solution, although the bulky
hindrance derived from the o-Ph2CH groups exists in the Al
The ligands reacted with 1 equiv of AlMe3 in toluene to give
Al dimethyl complexes Al1−Al5 in high yields (Scheme 1).
The Al complexes were also characterized by NMR spectros-
B
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complex. This phenomenon is different from other Al
complexes with bidentate coordination. Previously, we
reported a quinolone−amine-based Al complex, which even
i
having less bulky groups, i.e. Pr, showed restricted rotation of
aryl−N bonds.50 It must be noted that a fast conformation
change of the six-membered metallacycle ring reported in
previous research could also lead to the equivalent benzylic
protons, which cannot be ruled out at this moment.51,52
The coordination structures of Al2−Al4 were further
determined by X-ray diffraction. Figures 1−3 show the
Figure 3. ORTEP drawing of the molecular structure of Al4.
Ellipsoids are given at the 50% probability level. Hydrogen atoms and
an uncoordinated toluene molecule are omitted for clarity. Selected
distances (Å) and angles (deg): Al1−N1 1.836(3), Al1−P1
2.4926(16), Al1−C63 1.940(5), Al1−C64 1.969(5); N1−Al1−P1
90.27(11), N1−Al1−C62 116.84(18), N1−Al1−C63 117.06(19),
P1−Al1−C62 108.93(15), P1−Al1−C63 105.31(18), C62−Al1−C63
114.4(2).
covalent bonds between Al and N. Complexes Al2 and Al4
have the same coordination features (Figures1 and 3) as those
of complex Al3. On the basis of these results, it can be
speculated that complexes Al1 and Al5 should have the same
coordination features. However, careful comparison of the
bond lengths and angles in Al2−Al4 reveals some intriguing
coordination characteristics. For example, with an increase in
the substituent size on the N−phenyl ring (Al2 < Al3 < Al4),
the Al−N bond length increases, while the Al−P bond length
decreases. Meanwhile, Al4 with the greatest steric hindrance
shows the smallest angle defined by two Al−C bonds (C62−
Al1−C63 114.4(2)°), in comparison to those in Al2 (C20−
Al1−C21 118.3(3)°) and Al3 (C13−Al1−C14 115.17(11)°).
These results indicate that different substituents on the ligands
could lead to different coordination environments around the
Al center. Although these differences are small, they are
unignorable and would further affect the catalytic properties in
ROP reactions (vide infra).
Figure 1. ORTEP drawing of the molecular structure of Al2.
Ellipsoids are given at the 50% probability level. Hydrogen atoms are
omitted for clarity. Selected distances (Å) and angles (deg): Al1−N1
1.819(5), Al1−P1 2.535(2), Al1−C20 1.948(6), Al1−C21 1.959(6);
N1−Al1−P1 90.31(14), N1−Al1−C20 115.8(3), N1−Al1−C21
113.7(2), P1−Al1−C20 112.1(2), P1−Al1−C21 101.97(18), C20−
Al1−C21 118.3(3).
2.2. Ring-Opening Polymerization of ε-CL. The
catalytic performances of complexes Al1−Al5 for ε-CL
polymerization were examined and compared. The corre-
sponding results are summarized in Table 1.
Figure 2. ORTEP drawing of the molecular structure of Al3.
Ellipsoids are given at the 50% probability level. Hydrogen atoms are
omitted for clarity. Selected distances (Å) and angles (deg): Al1−N1
1.8238(19), Al1−P1 2.5304(11), Al1−C13 1.948(3), Al1−C14
1.956(2); N1−Al1−P1 89.60(6), N1−Al1−C13 118.02(10), N1−
Al1−C14 116.33(10), P1−Al1−C13 108.42(8), P1−Al1−C14
104.52(8), C13−Al1−C14 115.17(11).
The complex Al4 without an alcohol was completely inactive
(Table 1, run 1), and the addition of an initiator, e.g. BnOH,
was essential for the ROP reaction. The experiment for Al4
with 1 equiv of BnOH in CDCl3 was carried out and
1
monitored by H NMR in situ (Figure S19). In Figure S19, a
new signal at 4.72 ppm for AlOCH2Ph appears, while the
signal at −0.96 ppm for AlMe has a decreased integration in
comparison to that in Al4 (Figure S13). These results indicate
the nature of the active site should be an Al-OBn species when
BnOH was employed as an initiator. With BnOH as an
initiator, Al1−Al5 were highly active for the ROP of ε-CL at
room temperature (Table 1 runs 2−6). Apparently, the
activities of Al1−Al5 complexes (conversions of 75−95%
under identical conditions) were affected by the coordinated
ligands in the order Al4 > Al1 > Al5 > Al2 > Al3. Kinetic
studies (Figure 4) reveal that Al4 (kobs = 0.153 min−1)
polymerized ε-CL at a rate 2.3 times faster in comparison to
Al3 (kobs = 0.066 min−1), highlighting the effect of substituents
on the catalytic activity of Al complexes. Generally, the steric
molecular structures, and the selected bond lengths and angles
are given in the captions, respectively. In the molecule of Al3
(Figure 2), the coordination geometry at the Al center is a
distorted tetrahedron, as evidenced by the following bond
angles (deg): N1−Al1−P1 89.60(6), N1−Al1−C13
118.02(10), N1−Al1−C14 116.33(10), P1−Al1−C13
108.42(8), P1−Al1−C14 104.52(8), and C13−Al1−C14
115.17(11). This result is consistent with other Al complexes
with bidentate coordination.53 The Al1−N1 bond distance is
1.8238(19) Å, suggesting formation of a typical ionic covalent
bond between Al and N atoms. This distance is significantly
shorter in comparison to Al−N bonds for phenoxyimine−Al or
Salen−Al complexes (>1.95 Å),53,54 which had coordination
C
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a
Table 1. ROP of ε-CL Using Al1−Al5/BnOH
b
c
d
d
run
cat.
[CL]0:[Al]0:[BnOH]0
T (°C)
time (min)
conversn (%)
Mn,cal (kg mol−1
)
Mn,GPC (kg mol−1
)
Đ
1
2
3
4
5
6
7
8
Al4
Al1
Al2
Al3
Al4
Al5
Al1
Al2
Al3
Al4
Al5
Al4
Al4
Al4
Al4
Al4
Al4
Al4
Al1
100:1:0
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
200:1:1
500:1:1
2000:1:1
200:1:2
500:1:5
1000:1:10
2000:1:20
2000:1:20
25
25
25
25
25
25
70
70
70
70
70
70
70
110
70
70
70
70
70
20
20
20
20
20
20
10
10
10
10
10
15
30
15
15
30
93
79
75
95
82
>99
56
63
>99
>99
96
>99
91
10.7
9.1
8.7
11.0
9.5
11.4
6.5
12.4
11.4
10.1
13.3
12.6
13.1
8.3
1.14
1.20
1.19
1.14
1.20
1.33
1.41
1.41
1.21
1.81
1.41
1.32
1.53
1.18
1.13
1.13
1.10
1.41
9
7.3
8.7
10
11
12
13
14
15
16
17
18
19
11.4
11.4
22.0
56.5
207.6
10.7
11.2
10.4
10.4
14.4
14.6
26.1
51.2
118.6
12.9
13.8
11.8
11.5
3.5
93
97
90
90
120
180
180
26
2.7
a
b
c
Conditions: 20 μmol of Al, BnOH as initiator, [CL]0 = 1 M in toluene. Determined by 1H NMR. Mn,cal = MCL × ([CL]:[Al]) × ([Al]:[BnOH])
d
× conversion + MBnOH
. GPC data in THF versus polystyrene standards, using a correction factor of 0.56 for PCL.
peak of the GPC trace for PCL resulting from Al1 (Figure S20,
black curve) indicates the presence of side reactions during
ROP. In contrast, a negligible shoulder peak was observed for
other samples prepared by Al2−Al5.
As expected, increasing the reaction temperature led to
higher polymerization activity. For example, a quantitative
conversion (>99%; Table 1, run 10) was obtained within 10
min by Al4 at 70 °C, while the same catalyst achieved a
conversion of 95% within 20 min at room temperature (Table
1, run 5). However, the steric effect on the activity of Al1−Al5
at 70 °C (Table 1 runs 7−11) was similar to that observed at
room temperature.
PCLs having high MWs usually exhibit high mechanical
properties, which is important for specific applications
including packing and tissue engineering.57 To prepare PCLs
with different MWs, the Al:BnOH ratio was fixed at 1:1 and
ROP with 100−2000 equiv of monomers was carried out
(Table 1, runs 10 and 12−14). Almost 500 equiv of ε-CL was
converted to high and controlled MW PCL within 30 min at
70 °C (Table 1 run 13; Mn = 51.2 kg mol−1). When the
CL:BnOH ratio was increased further to 2000, it is remarkable
that nearly 2000 equiv of ε-CL monomers was polymerized
within 15 min at 110 °C. The Mn value of the produced PCL is
over 100 kg mol−1, although it is lower in comparison to the
theoretical value (207.6 kg mol−1; Table 1 run 14). This
deviation of MW was only observed when the value of Mn was
very high, presumably as a consequence of the occurrence of
chain transfers. Similar phenomena were also constantly
observed in previous reports.39,57 Therefore, with an increase
in the ε-CL:BnOH ratios from 100:1 to 2000:1, PCLs having
different MWs, especially ultrahigh-MW PCLs (Table 1 run
14; Mn = 118.6 kg mol−1), were easily prepared. The GPC
curves are shown in Figure 5.
Figure 4. Kinetic plots of the ε-CL conversions versus the reaction
time: Al1, kobs = 0.129 min−1, black; Al2, kobs = 0.075 min−1, red; Al3,
kobs = 0.066 min−1, purple; Al4, kobs = 0.153 min−1, blue; Al5, kobs
=
0.086 min−1, green.
effects influence the catalytic activity in two opposite manners:
the bulky steric effect would reduce the coordination capability
of the monomer and decrease the activity; on the other hand,
this bulky hindrance could also stabilize the intermediate active
species and thus increase the activity. The overall steric effect
on the catalyst activity is a cumulative result of these two
manners. This is the possible reason for the unusual
observation that both Al1 and Al4, having the smallest and
largest substituents, respectively, are more active than Al2 and
Al3 with intermediate substituents. On the other hand, the
electronic effect of the ligand on the catalytic activity is not
significant as in the Salen-Al system, in which the introduction
of an electron-withdrawing Cl group dramatically increases the
activity.55 In comparison to NN or NO bidentate coordinated
Al complexes, such as quinoline−amine−Al and phenoxyi-
mine−Al,25,50,56 the amido−phosphine−Al complexes re-
ported in this study showed higher activity under similar
conditions. The GPC traces of the resulting PCLs are shown in
Figure S20 with narrow distributions (Đ = 1.14−1.20),
indicating controlled polymerizations. The small shoulder
Immortal ring-opening polymerization (iROP) has attracted
considerable attention in the past few decades, given its
advantages of low catalyst loading and high productivity.58,59
For the typical iROP, excess alcohol (relative to catalyst) is
used as both the initiator and chain transfer agent (CTA). The
D
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obtained. The current catalytic system allows the preparation
of PCLs with ultrahigh MWs of up to 118.6 kg mol−1.
Introducing bulky Ph2CH groups into the molecule of the Al
catalyst could benefit the ROP reactions, and thus Al4 showed
immortal catalysis toward the ROP of ε-CL in the presence of
excess BnOH.
4. EXPERIMENTAL SECTION
4.1. General Considerations. All moisture- and oxygen-sensitive
reactions and compounds were treated using standard Schlenk
techniques or glovebox techniques under an atmosphere of high-
purity nitrogen. Toluene, n-hexane, and THF were purified by purging
with dry nitrogen, followed by passing through columns of activated
alumina. CDCl3 dried over CaH2 was vacuum-transferred prior to use.
Anhydrous BnOH and AlMe3 were purchased from TCI and used as
received. ε-CL was purchased from Aladdin Co. and stirred with
CaH2 for 24 h and then distilled under reduced pressure and stored
over activated 4 Å molecular sieves in a glovebox. All other chemicals
were purchased from commercial suppliers and used without further
purification unless otherwise noted. NMR spectra were recorded on a
Bruker AVANCE NEO 400 MHz NMR spectrometer (400 MHz for
1H NMR, 100 MHz for 13C NMR, and 125 MHz for 31P NMR).
Elemental analyses were performed on a Flash EA 1112 micro-
analyzer. Matrix-assisted laser desorption/ionization time-of-flight
mass spectroscopy (MALDI-TOF MS) analyses were conducted on a
Bruker BIFLEX III MS spectrometer equipped with a 337 nm
nitrogen laser. The polymer was dissolved in chloroform, trans-2-[3-
(4-tert-butylphenyl)-2-methyl-2-propenylidene]malononitrile
(DCTB) was used as the matrix, and sodium chloride was used as the
cationizing agent. The gel permeation chromatography (GPC)
measurements were performed on a Agilent HPLC system equipped
with a Model 1260 Hip degasser, a Model 1260 Iso pump, and a
Model 1260 differential refractometer detector using THF as the
mobile phase at a flow rate of 1.0 mL/min at 40 °C. The molecular
weights and dispersities (Đ = Mw/Mn) were calculated using
polystyrene standards with narrow molecular weight distributions as
references. The ligands C6H5NHCH2C6H4-2-PPh2 (LH1),44 2,6-
M e 2 C 6 H 3 N H C H 2 C 6 H 4 - 2 - P P h 2 ( L H 2 ) , 4 5 a n d
2,6-iPr2C6H3NHCH2C6H4-2-PPh2 (LH3)45 were synthesized accord-
ing to the methods reported in the literature.
4.2. Synthesis of Ligands. 4.2.1. Synthesis of 2,6-(Ph2CH)2-
4-iPrC6H2NHCH2C6H4-2-PPh2 (LH4). The phosphine−imine precursor
2,6-(Ph2CH)2-4-iPrC6H2NCHC6H4-2-PPh2 was prepared using the
literature procedure.43 2,6-(Ph2CH)2-4-iPrC6H2NCHC6H4-2-PPh2
(1.48 g, 2.00 mmol) and LiAlH4 (0.19 g, 5.00 mmol) were added to
THF (60 mL) under N2. After it was stirred for 48 h at room
temperature, the solution was quenched with water (0.19 mL) and
10% NaOH (0.19 mL). The filtrate was dried under reduced pressure,
and then the resulting solid was washed with methanol and dried
under vacuum to give LH4 as a white solid (1.31 g, 1.77 mmol, 88%).
1H NMR (CDCl3): δ 7.33−7.11 (m, 25 H), 7.04 (d, J = 7.0 Hz, 8 H),
6.83 (dd, J = 7.4 and 4.5 Hz, 1 H), 6.64 (s, 2 H), 6.17 (s, 2 H), 4.05
(br, 2 H), 3.24 (br, 1 H), 2.61 (hept, J = 6.7 Hz, 1 H), 0.97 (d, J = 6.9
Hz, 6 H) ppm. 13C NMR (CDCl3): 144.62, 144.36, 143.43, 143.09,
138.80, 136.78 (d, JPC = 10.2 Hz), 135.50 (d, JPC = 13.7 Hz), 133.99,
133.80, 129.81, 129.53 (d, JPC = 5.7 Hz), 129.38, 128.76, 128.66 (d,
JPC = 6.9 Hz), 128.26, 127.73, 127.39, 126.20, 53.79, 53.57, 51.29,
33.55, 24.02 ppm. 31P NMR (CDCl3): −16.28 ppm. Anal. Calcd for
C54H48NP: C, 87.42; H, 6.52; N, 1.89. Found: C, 87.23; H, 6.36; N,
1.57.
Figure 5. GPC curves of PCLs obtained by Al4 with different [CL]0:
[BnOH]0 ratios (Table 1, runs 10 and 12−14).
other characteristic of iROP is that the MWs of the resulting
polymers are independent of the catalyst loading and are only
determined by the molar ratio of converted monomer to
alcohol. In the current work, with the ratio of ε-CL and BnOH
kept constant (100:1), when the loading of Al4 was decreased
from 1/100 to 1/2000 (Table 1, runs 10 and 15−18), high
conversions (over 90%) were achieved within 3 h. The MWs
remained controlled and matched the theoretical values well,
indicating that Al4 is a promising immortal catalyst for ROP
reactions. The chain structures of the PCLs obtained in the
presence of 1 and 20 equiv of alcohol (Table 1, runs 5 and 18)
were carefully characterized by MALDI-TOF MS, and the
spectra are shown in Figure S21. Both of the MS spectra
exhibit only one series of ion peaks, which is separated by m/z
114 and corresponds to linear PCL with BnO/H chain ends
(Mn = 144.14 n + 108.14 (BnOH) + 23.0 (Na+) (g mol−1)).
No cyclic polymers and PCLs with other chain ends are
observed, suggesting well-controlled immortal ROPs using Al4
as a catalyst. On the other hand, when Al1 was used for
immortal catalysis under identical conditions, only a 26%
conversion of monomer was obtained (Table 1, run 19, and
Figure S22). In comparison to the high conversion obtained by
Al4 (Table 1, run 18, and Figure S23), these results suggest the
introduction of bulky Ph2CH groups could increase the
catalytic performance for immortal ROP reactions.
3. CONCLUSION
Aluminum alkyl complexes Al1−Al5 with bidentate amido−
phosphine ligands having different steric and electronic effects
were synthesized and well characterized. It is surprising that
only one singlet for the benzylic methylene protons was
observed in the 1H NMR spectra of the Al complexes,
suggesting the hemilabile coordination of the P donor in
solution. A comparison of the molecular structures of Al2−Al4
reveals that different substituents on the ligands cause
unignorable differences in coordination environments around
the Al center, although the coordination geometries are all
tetrahedrons. These differences, of course, affect the catalytic
properties toward ROP reactions. In the absence of an alcohol
as initiator, Al complexes are completely inactive for the ROP
of ε-CL. In contrast, Al1−Al5 complexes are highly efficient
with BnOH as an initiator and can rapidly convert hundreds
and even thousands of equivalents of ε-CL into PCLs with
high conversions in a controlled manner. GPC and MALDI-
TOF MS analyses suggest that the ROP reactions are well
controlled, considering the MWs and end groups of PCL
4.2.2. Synthesis of 2,6-Cl2C6H3NHCH2C6H4-2-PPh2 (LH5). To a
2,6-dichloroaniline (1.62 g, 10 mmol) solution in toluene (20 mL)
was added trimethylaluminum (5 mL, 2.0 M in toluene) slowly
through a syringe at room temperature, and then the mixture was
heated to reflux for 6 h. After the solution was cooled to room
temperature, 2-Ph2PC6H4CHO (2.90 g, 10 mmol) was added. The
mixture was heated to reflux for another 6 h. The reaction mixture was
cooled to room temperature and hydrolyzed with 5% aqueous NaOH
solution. The organic product was extracted with ethyl acetate and
E
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Article
dried over MgSO4, and the solvent was evaporated off. 2,6-
Cl2C6H3NCHC6H4-2-PPh2 was obtained by washing with meth-
anol as a white solid (3.40 g, 7.9 mmol, 79% yield). 1H NMR
(CDCl3): δ 9.05 (d, J = 7.0 Hz, 1 H), 8.36−8.23 (m, 1 H), 7.41 (t, J =
7.5 Hz, 1 H), 7.33 (t, J = 7.5 Hz, 1 H), 7.28−7.12 (m, 12 H), 6.88
(dd, J = 6.9 and 4.6 Hz, 1 H), 6.83 (t, J = 8.1 Hz, 1 H) ppm. 13C
NMR (CDCl3): δ 7.45−7.37 (m, 2 H), 7.36−7.29 (m, 8 H), 7.20−
7.00 (m, 14 H), 6.94−6.87 (m, 1 H), 6.85 (d, J = 7.2 Hz, 4 H), 6.81−
6.76 (m, 4 H), 6.70 (s, 2 H), 6.20−6.21 (m, 1 H), 6.09 (s, 2 H), 3.84
(s, 2 H), 2.62 (hept, J = 6.9 Hz, 1 H), 0.99 (d, J = 6.9 Hz, 6 H), −0.76
(d, JPH = 4.2 Hz, 6 H) ppm. 13C NMR (CDCl3): 151.12, 150.97,
150.73 (d, JPC = 4.5 Hz), 146.60, 145.64, 144.51, 142.44, 142.26,
135.35, 133.72 (d, JPC = 12.6 Hz), 130.74, 130.55, 129.78 (d, JPC
=
NMR (CDCl3): 164.12 (d, JPC = 27.0 Hz), 146.20, 138.32 (d, JPC
=
13.2 Hz), 129.59, 129.23, 129.06, 128.93 (d, JPC = 9.2 Hz), 128.63,
128.57, 128.50, 128.25, 128.13, 127.67 (d, JPC = 8.0 Hz), 126.40,
126.34, 126.08, 125.34 (d, JPC = 6.4 Hz), 124.93, 124.64, 59.61, 51.15,
33.29, 23.88, −8.71 (d, JPC = 21.9 Hz) ppm. 31P NMR (CDCl3):
−26.81 ppm. Anal. Calcd for C56H53AlNP: C, 84.29; H, 6.69; N, 1.76.
Found: C, 84.02; H, 6.51.49; N, 1.88.
20.0 Hz), 137.69 (d, JPC = 17.9 Hz), 134.86 (d, JPC = 9.4 Hz), 133.06
(d, JPC = 20.0 Hz), 132.43, 130.82, 128.18, 127.98, 127.66 (d, JPC
=
7.2 Hz), 127.11, 126.85 (d, JPC = 4.3 Hz), 124.91, 123.77 ppm. 31P
NMR (CDCl3): −15.64 ppm. 2,6-ClC6H3NCHC6H4-2-PPh2 (2.16
g, 5.00 mmol) and LiAlH4 (0.57 g, 15.00 mmol) were added to THF
(60 mL) under N2. After it was stirred for 48 h at room temperature,
the solution was quenched with water (0.60 mL) and 10% NaOH
(0.60 mL). The filtrate was dried under reduced pressure, and then
the resulting solid was washed with methanol and dried under vacuum
4.3.5. Synthesis of Al5. Using the method described for Al1, Al5
1
was obtained as a yellow powder (0.45 g, 0.92 mmol, yield 92%). H
NMR (CDCl3): δ 7.51−7.36 (m, 10 H), 7.32 (t, J = 7.4 Hz, 1 H),
7.22−7.18 (m, 3 H), 7.11 (t, J = 7.5 Hz, 2 H), 6.75 (t, J = 8.0 Hz, 1
H), 4.43 (s, 2 H), −0.65 (d, JPH = 4.1 Hz, 6 H, Al−Me) ppm. 13C
NMR (CDCl3): 150.23 (d, JPC = 13.9 Hz), 149.67 (d, JPC = 3.4 Hz),
135.38 (d, JPC = 22.1 Hz), 133.98 (d, JPC = 12.6 Hz), 130.74, 130.57,
129.49, 129.18, 128.99 (d, JPC = 9.2 Hz), 128.40, 126.82 (d, JPC = 5.5
Hz), 126.40, 57.31 (d, JPC = 4.0 Hz), −9.61 (d, JPC = 22.3 Hz) ppm.
31P NMR (CDCl3): −25.41 ppm. Anal. Calcd for C27H25AlCl2NP: C,
65.87; H, 5.12; N, 2.84. Found: C, 65.54; H, 5.01; N, 2.55.
1
to give LH5 as a white solid (1.76 g, 4.05 mmol, 81%). H NMR
(CDCl3): δ 7.54−7.49 (m, 1 H), 7.42−7.24 (m, 11 H), 7.21−7.18
(m, 3 H), 6.91 (dd, J = 7.2 and 4.2 Hz, 1 H), 6.75 (t, J = 8.0 Hz, 1 H),
4.70 (br, s, 2 H), 4.45 (br, s, 1 H) ppm. 13C NMR (CDCl3): 143.84
(d, JPC = 24.0 Hz), 142.58, 136.41 (d, JPC = 10.0 Hz), 135.78 (d, JPC
=
14.1 Hz), 134.07 (d, JPC = 19.7 Hz), 133.64, 129.26, 128.97, 128.90,
128.79, 128.72, 127.78, 126.74, 121.91, 49.57 (d, JPC = 23.4 Hz) ppm.
31P NMR (CDCl3): −15.76 ppm. Anal. Calcd for C25H20Cl2NP: C,
4.4. ROP of ε-CL. A general procedure for polymerization in the
presence of benzyl alcohol (Table 1, run 2) is given as follows:
polymerization was performed in a 25 mL flame-dried Schlenk tube
interfaced to a dual-manifold Schlenk line. One reactor was charged
with Al1 (0.020 mmol) and 1 mL of toluene in the glovebox, while
the other reactor was charged with BnOH (0.020 mmol), ε-CL (2.0
mmol), and 1 mL of toluene. The reactors were sealed, taken out of
the glovebox, and immersed in an oil bath at a predetermined
temperature. After equilibration at the desired polymerization
temperature for 10 min, the polymerization was initiated by rapid
addition of the Al1 solution into the BnOH and ε-CL mixture via a
syringe. After the desired period of time (20 min), the mixture was
quenched by a few drops of glacial acetic acid. A small amount of the
solution was transferred to another Schlenk tube, and all of the
volatiles were removed under vacuum. The residues were dissolved in
68.82; H, 4.62; N, 3.21. Found: C, 68.45; H, 4.51; N, 3.10.
4.3. Synthesis of Aluminum Complexes. 4.3.1. Synthesis of
Al1. To a stirred toluene solution (30 mL) of C6H5NHCH2C6H4-2-
PPh2 (LH1) (0.73 g, 2.00 mmol) was added AlMe3 (1 mL, 2 M in
toluene). The mixture was stirred at 80 °C for 12 h and dried under
reduced pressure. The residue was washed with 10 mL of n-hexane to
give a white powder (0.69 g, 1.64 mmol, yield 82%). 1H NMR
(CDCl3): δ 7.66 (t, J = 6.3 Hz, 1 H), 7.47 (t, J = 7.5 Hz, 1 H), 7.45−
7.41 (m, 2 H), 7.39−7.32 (m, 4 H), 7.30−7.20 (m, 5 H), 7.10 (t, J =
8.3 Hz, 1 H), 7.02 (dd, J = 8.5 and 7.3 Hz, 2 H), 6.79 (d, J = 8.1 Hz, 2
H), 6.49 (t, J = 7.2 Hz, 1 H), 4.52 (s, 2 H), −0.50 (d, JPH = 4.3 Hz, 6
H) ppm. 13C NMR (CDCl3): 154.29, 149.89 (d, JPC = 14.1 Hz),
135.81, 133.60 (d, JPC = 12.9 Hz), 131.30, 130.74, 130.38 (d, JPC = 9.5
Hz), 129.13, 129.03, 128.85, 128.51, 128.19, 127.51 (d, JPC = 5.6 Hz),
127.11, 126.80, 115.05, 52.05, −7.92 (d, JPC = 21.1 Hz) ppm. 31P
NMR (CDCl3): −26.27 ppm. Anal. Calcd for C27H27AlNP: C, 76.58;
H, 6.43; N, 3.31. Found: C, 76.50; H, 6.22; N, 3.18.
1
CDCl3 for H NMR characterization to determine the conversion.
The rest of the solution was poured into methanol (100 mL) to
precipitate the polymer. The resultant polymer was then collected by
filtration and dried in a vacuum oven at 30 °C to a constant weight.
4.5. Crystal Structure Determinations. Single crystals of Al2−
Al4 were grown by diffusion of n-hexane into their toluene solutions
slowly at room temperature. X-ray diffraction studies for Al2−Al4
were performed on a Rigaku RAXIS Rapid IP diffractometer
(graphite-monochromated Mo Kα radiation). Cell parameters were
obtained by global refinement of the positions of all collected
reflections. Intensities were corrected for Lorentz and polarization
effects and empirical absorption. Using Olex2, the structures were
solved with XS and refined with ShelXL.60 Crystal data for Al2−Al4
are summarized in Table S1 in the Supporting Information. CCDC
reference numbers 1894247, 1894248, and 1894249 are for
complexes Al2−Al4, respectively.
4.3.2. Synthesis of Al2. Using the method described for Al1, Al2
1
was obtained as a yellow powder (0.81 g, 1.80 mmol, yield 90%). H
NMR (CDCl3): δ 7.56−7.49 (m, 2 H), 7.49−7.43 (m, 4 H), 7.43−
7.36 (m, 4 H), 7.31 (t, J = 7.4 Hz, 1 H), 7.22 (t, J = 7.4 Hz, 1 H),
7.09−7.01 (m, 2 H), 6.99 (d, J = 7.4 Hz, 2 H), 6.84 (t, J = 7.4 Hz, 1
H), 4.15 (s, 2 H), 1.98 (s, 6 H), −0.67 (d, JPH = 3.7 Hz, 6 H) ppm.
13C NMR (CDCl3): 152.46, 150.98 (d, JPC = 13.4 Hz), 137.80,
134.92, 134.03 (d, JPC = 12.7 Hz), 130.95, 130.80, 129.15 (d, JPC = 9.1
Hz), 128.89, 128.82, 128.59, 128.08, 127.40, 127.11, 127.01 (d, JPC
=
5.7 Hz), 122.61, 57.49, 19.59, −9.18 (d, JPC = 21.3 Hz) ppm. 31P
NMR (CDCl3): −25.01 ppm. Anal. Calcd for C29H31AlNP: C, 77.14;
H, 6.92; N, 3.10. Found: C, 77.05; H, 6.71; N, 2.99.
4.3.3. Synthesis of Al3. Using the method described for Al1, Al3
1
was obtained as a yellow powder (0.45 g, 0.88 mmol, yield 88%). H
NMR (CDCl3): δ 7.53−7.49 (m, 2 H), 7.48−7.44 (m, 4 H), 7.39 (t, J
= 8.4 Hz, 4 H), 7.30 (t, J = 7.4 Hz, 1 H), 7.22 (t, J = 7.4 Hz, 1 H),
7.13 (d, J = 6.7 Hz, 1 H), 7.10 (d, J = 6.5 Hz, 1 H), 7.06−7.01 (m, 3
H), 4.22 (s, 2 H, CH2−N), 3.33 (hept, J = 6.9 Hz, 2 H, CHMeMe),
1.07 (d, J = 6.9 Hz, 6 H, CHMeMe), 0.80 (d, J = 6.9 Hz, 6 H,
CHMeMe), −0.72 (d, JPH = 3.9 Hz, 6 H, Al−Me) ppm. 13C NMR
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.9b00502.
(CDCl3): 150.70, 150.56, 149.53 (d, JPC = 4.1 Hz), 148.22 (d, JPC
=
1.2 Hz), 135.26, 133.68 (d, JPC = 12.8 Hz), 130.76, 130.57, 129.44 (d,
JPC = 9.5 Hz), 129.10, 128.98 (d, JPC = 9.0 Hz), 128.80, 127.11,
127.01 (d, JPC = 5.6 Hz), 126.83, 123.51, 123.22, 59.71 (d, JPC = 3.7
Hz), 28.06, 25.09, 24.36, −9.88 (d, JPC = 21.5 Hz) ppm. 31P NMR
(CDCl3): −25.94 ppm. Anal. Calcd for C33H39AlNP: C, 78.08; H,
7.74; N, 2.76. Found: C, 77.98; H, 7.49; N, 2.65.
NMR spectra of Al complexes, characterization of
polymer materials, additional polymerization results,
and crystallographic data for Al2−Al4 (PDF)
Accession Codes
CCDC 1894247−1894249 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
4.3.4. Synthesis of Al4. Using the method described for Al1, Al4
1
was obtained as a yellow powder (0.72 g, 0.90 mmol, yield 90%). H
F
DOI: 10.1021/acs.organomet.9b00502
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(14) Kan, Z.; Luo, W.; Shi, T.; Wei, C.; Han, B.; Zheng, D.; Liu, S.
Facile Preparation of Stereoblock PLA From Ring-Opening Polymer-
ization of rac-Lactide by a Synergetic Binary Catalytic System
Containing Ureas and Alkoxides. Front. Chem. 2018, 6, 547.
(15) Ji, H.; Wang, B.; Pan, L.; Li, Y. S. Lewis pairs for ring-opening
alternating copolymerization of cyclic anhydrides and epoxides. Green
Chem. 2018, 20, 641−648.
(16) Naumann, S.; Scholten, P. B.; Wilson, J. A.; Dove, A. P. Dual
Catalysis for Selective Ring-Opening Polymerization of Lactones:
Evolution toward Simplicity. J. Am. Chem. Soc. 2015, 137, 14439−
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free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for S.L.: shaofengliu@qust.edu.cn.
*E-mail for Z.L.: zbli@qust.edu.cn.
■
ORCID
(17) Leong, N. L.; Kabir, N.; Arshi, A.; Nazemi, A.; Jiang, J.; Wu, B.
M.; Petrigliano, F. A.; McAllister, D. R. Use of ultra-high molecular
weight polycaprolactone scaffolds for ACL reconstruction. J. Orthop.
Res. 2016, 34, 828−835.
(18) Mandal, M.; Luke, A. M.; Dereli, B.; Elwell, C. E.; Reineke, T.
M.; Tolman, W. B.; Cramer, C. J. Computational Prediction and
Experimental Verification of ε-Caprolactone Ring-Opening Polymer-
ization Activity by an Aluminum Complex of an Indolide/Schiff-Base
Ligand. ACS Catal. 2019, 9, 885−889.
(19) Huang, H.-C.; Li, Z.-J.; Wang, B.; Chen, X.; Li, Y.-S. Synthesis
of lactide/ε-caprolactone quasi-random copolymer by using rationally
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Shaofeng Liu: 0000-0002-1230-0946
Zhibo Li: 0000-0001-9512-1507
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
Financial support by the NSFC (No. 21871157), the 111
Project (No. D17004), the Major Program of Shandong
Province Natural Science Foundation (No. ZR2017ZB0105),
the Shandong Province Natural Science Foundation (No.
ZR2018JL007), and the Taishan Scholars Program (No.
tsqn20161031) is gratefully acknowledged.
(20) Zhang, Q.; Zhang, W.; Solan, G. A.; Liang, T.; Sun, W.-H.
Bimetallic aluminum 5,6-dihydro-7,7-dimethyl quinolin-8-olates as
pro-initiators for the ROP of ε-CL; probing the nuclearity of the
active initiator. Polymers 2018, 10, 764.
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DOI: 10.1021/acs.organomet.9b00502
Organometallics XXXX, XXX, XXX−XXX