Synthesis and characterization of dialkylaluminum amidates and their ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1021/om2008343

The stoichiometric reactions of N-(2-methylquinolin-8-yl) (R)amides (L1-L8; L1, R = Ph; L2, R = p-FPh; L3, R = p-ClPh; L4, R = p-(MeO)Ph; L5, R = o-MePh; L6, R = p-MePh; L7, R = Me; L8, R = CF₃) with Me₃Al afforded the corresponding dimethylaluminum amidate complexes [Me ₂AlL] (C1-C8). The treatment of N-(2-methylquinolin-8-yl)picolinamide (L9) with 1 or 2 equiv of Me₃Al formed Me₂AlL9 (C9) or Me₂AlL9·Me₃Al (C10), respectively; meanwhile, the stoichiometric reaction of L9 with iBu₃Al gave iBu₂AlL9 (C11). All organoaluminum amidate complexes were fully characterized by ¹H/¹³C NMR spectroscopy and elemental analysis, and the unambiguous structures of complexes C2, C4, C9, and C11 were further determined by single-crystal X-ray diffraction. With the assistance of 1 equiv of BnOH, all dialkylaluminum amidate complexes showed appreciable activities toward the ring-opening polymerization of ε-caprolactone and produced polycaprolactones with narrow polydispersity; the nature of the active species was also investigated.

Full text of DOI:10.1021/om2008343

Article
pubs.acs.org/Organometallics
Synthesis and Characterization of Dialkylaluminum Amidates and
Their Ring-Opening Polymerization of ε-Caprolactone
Wenjuan Zhang,*,† Youhong Wang,† Ji Cao,‡ Lin Wang,† Yi Pan,‡ Carl Redshaw,*,§ and Wen-Hua Sun*,†
†Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, People's Republic of China
‡College of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, People's Republic of China
§School of Chemistry, University of East Anglia, Norwich NR4 7TJ, U.K.
S
* Supporting Information
ABSTRACT: The stoichiometric reactions of N-(2-methylquinolin-8-yl) (R)amides (L1−
L8; L1, R = Ph; L2, R = p-FPh; L3, R = p-ClPh; L4, R = p-(MeO)Ph; L5, R = o-MePh; L6,
R = p-MePh; L7, R = Me; L8, R = CF3) with Me3Al afforded the corresponding
dimethylaluminum amidate complexes [Me2AlL] (C1−C8). The treatment of N-(2-
methylquinolin-8-yl)picolinamide (L9) with 1 or 2 equiv of Me3Al formed Me2AlL9 (C9)
or Me2AlL9·Me3Al (C10), respectively; meanwhile, the stoichiometric reaction of L9 with
iBu3Al gave iBu2AlL9 (C11). All organoaluminum amidate complexes were fully
1
characterized by H/13C NMR spectroscopy and elemental analysis, and the unambiguous
structures of complexes C2, C4, C9, and C11 were further determined by single-crystal X-
ray diffraction. With the assistance of 1 equiv of BnOH, all dialkylaluminum amidate
complexes showed appreciable activities toward the ring-opening polymerization of ε-
caprolactone and produced polycaprolactones with narrow polydispersity; the nature of the
active species was also investigated.
Chart 1. Different Coordination Models for Amidate
Aluminum Complexes
INTRODUCTION
■
Alkylaluminum compounds, which tend to be highly air
sensitive, are very important reagents widely employed in
organic synthesis,1 as cocatalysts in olefin polymerization,2 and
as initiators in the ring-opening polymerization (ROP) of cyclic
esters3 or copolymerization of CO2 and epoxides.4 Recently,
the reactivity and catalytic behavior of alkylaluminum
complexes have been extensively reported.5 Using the multi-
coordination features of amide compounds, we have synthe-
sized several series of titanium amidates, which act as effective
precatalysts in ethylene (co)polymerization,6 and now we
consider the aluminum compounds derived from this ligand set.
Indeed, there are only a few reports of alkylaluminum amidates:
for example, the first aluminum amidates of the type
[Me2Al(RNC(O)R)]2 possessing eight-membered cyclic struc-
tures on the basis of spectroscopic studies were reported by
Wade7 and Lappert.8 Furthermore, alkylaluminum amidates as
the dimeric compounds {Me2Al[η2-tBuNC(R)(μ2-O)]}2 (I;
Chart 1) or 8-membered (II; Chart 1) or trimeric 12-
membered cyclic aluminum compounds (III; Chart 1) have
been reported.9 Interestingly, an aluminum amidate complex
was proposed as an intermediate for transamidations.10 With
regard to the reactivity of common trialkylaluminums, the
stoichiometric reaction of AlEt3 with the sulfonyl amine
ArNH(SO2-p-Tol) (Ar = 2,6-iPr2C6H3; Tol = 4-MeC6H4)
yielded the dimeric aluminum species [ArN(SO2-p-Tol)AlEt2]2
(IV), possessing an 8-membered cyclic compound structurally
similar to that of II.11 The varied coordination modes available
provided different compounds: for example, the stoichiometric
Received: September 5, 2011
Published: November 4, 2011
© 2011 American Chemical Society
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reaction of Me3Al with 2-methoxybenzanilide in n-hexane gave
a dinuclear aluminum complex (VI; Chart 1),9 and the reaction
of AlR3 with anthranilic acid produced the dimeric carboxylates
V,12a−c which were used to explain the reaction mechanism of
amino acids with aluminum.12d
methyl groups attached to the aluminum centers, while the N−
H signals (single peak around 10.4−10.9 ppm) of the free
ligands disappeared. Furthermore, the resonances from −6.0 to
−9.6 ppm in the 13C NMR spectra of C1−C10 confirmed the
formation of the Al−CH3 bond.
In the case of titanium amidate compounds,6 such amidates
have adopted monodentate features6a−c and η,μ 2-OCN
coordination,6d in which the different coordination features
affected the structure and catalytic behavior of their complexes.6
Given our previous success in using aluminum precatalysts in
the ring-opening polymerization of ε-caprolactone,13 we
explore herein the reaction of alkylaluminum with quinoline
amides and isolated the highly sensitive alkylaluminum
amidates in high yields, which serve as precatalysts for the
ring-opening polymerization of ε-caprolactone. To the best of
our knowledge, this is the first time such alkylaluminum
amidates have been reported as precatalysts in the ring-opening
polymerization of ε-caprolactone. In the presence of benzyl
alcohol, the alkylaluminum amidates exhibited high efficiency
and produced polymers with narrow polydispersity, indicating
their adaptability for controlling polycaprolactones in terms of
both molecular weight and polydispersity.
Single crystals of complexes C2, C4, C9, and C11 were
obtained from their toluene/n-heptane solutions at −30 °C.
The molecular structure of dimethylaluminum 4-fluoro-N-(2-
methylquinolin-8-yl)benzamidate (C2) is illustrated in Figure
RESULTS AND DISCUSSION
■
Figure 1. ORTEP drawing of C2 with thermal ellipsoids drawn at the
30% probability level. Hydrogen atoms are omitted for clarity.
1. Synthesis and Characterization of Dialkylaluminum
Quinolylamidates (C1−C11). The ligands L1−L9 were
prepared according to the method given in our previous
work.6d The stoichiometric reactions of L1−L9 with 1 equiv of
AlMe3 in toluene at −30 °C formed the corresponding
mononuclear dialkylaluminum quinoline amidates C1−C9,
while the treatment of L9 with 2 equiv of AlMe3 gave the
dinuclear methylaluminum N-(2-methylquinolin-8-yl)-
picolinamidate (C10) (Scheme 1). Reaction of L9 with 1
1, and selected bond lengths and bond angles are given in Table
1. The Al center is coordinated to two N atoms, and the NCO
group does not adopt the η2 coordination model as observed in
their titanium analogues.6d The geometry at aluminum is best
described as distorted tetrahedral, as evidenced in the bond
angles N(2)−Al(1)−C(19) = 115.13(10)° and C(18)−Al(1)−
N(1) = 104.98(9)°. The Al−C bond distances in C2 (Al(1)−
C(19) = 1.958(2) Å, Al(1)−C(18) = 1.973(2) Å) and the
Al(1)−N(1) bond (1.9808(19) Å) are typical, while the
Al(1)−N(2) bond length (1.924(2) Å) is much shorter than
1.9808(19) Å (Al(1)−N(1)), indicating the typical character-
istics of a σ bond. The dihedral angle between the quinoline
plane and aryl plane is 60.83°.
Scheme 1. Synthesis of the Dialkylaluminum Complexes
C1−C11
The molecular structure of dimethylaluminum 4-methoxy-N-
(2-methylquinolin-8-yl)benzamide (C4; Figure 2) possesses a
geometry very similar to that of C2, and selected bond lengths
and bond angles are collected in Table 1. Small differences are
observed between the Al−C bond lengths of C4 (1.962(3),
1.968(2) Å) and those of C2 (1.958(2), 1.978(2) Å), and also
in the bond lengths Al−N2imide = 1.920(2) Å and Al−N1quin
=
1.982(2) Å in C4 versus those of C2 (1.924(2), 1.9808(19) Å).
Due to the stronger donor ability of the −OMe group on the
benzene ring, the bond distances of C11−C12aryl and C11−
C12aryl are shorter than those of C2 possessing the F
substituent, while the bond distance of N2−C11imide was
elongated. The dihedral angle between the quinoline plane and
the aryl plane was larger in C4 (68.71°) versus that observed
for C2 (60.83°).
By introducing a pyridine group at the amidate, a
coordination model similar to that of the N−CO group
was obtained, featuring an Al−N σ-bond in C9. However, there
was a substantial change in the coordination geometry, and
Figure 3 reveals that Al is now, in addition to the methyl
groups, coordinated by three nitrogen atoms such that the
geometry can be best described as distorted trigonal
bipyramidal. Selected bond lengths and angles are also collected
equiv of iBu3Al afforded [iBu2AlL9] (C11). All aluminum
1
complexes were characterized by H/13C NMR spectroscopy
1
and elemental analysis. Comparison of the H NMR spectra of
C1−C10 with those of the corresponding ligands L1−L9
revealed that additional resonances appeared in the high-field
region (δ −0.50 to −0.90 ppm), which are attributed to the
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Table 1. Selected Bond Lengths and Bond Angles
C2
C4
C9
C11
Bond Lengths (Å)
1.920(2)
Al−N2imide
Al−N1quin
Al−N1Py
Al−CMe
Al−CMe
N2−C11imide
C11−C12aryl
C11−O1imide
1.924(2)
1.952(2)
2.134(2)
2.121(3)
1.987(3)
1.989(3)
1.356(4)
1.506(4)
1.226(3)
1.940(2)
1.9808(19)
1.982(2)
2.135(2)
2.112(3)
1.992(2)
1.994(3)
1.353(3)
1.513(3)
1.232(3)
1.958(2)
1.973(2)
1.367(3)
1.502(3)
1.234(3)
1.962(3)
1.968(2)
1.375(3)
1.494(3)
1.232(3)
Bond Angles (deg)
119.68
C−Al−C
118.73
116.19
84.75
121.27(13)
111.7(2)
123.82(11)
111.2(2)
78.25(9)
22.47
Nim−Cim−CAr
Nim−Al−NQuin
∠mean(quin, aryl)
115.18
84.61
79.02(9), 79.72(10)
13.25
60.83
68.71
Either treatment of C9 with 1 equiv of AlMe3 or the reaction
of L9 with 2 equiv of AlMe3 in toluene produced the same
binuclear aluminum complex C10, in which the N of the amide
formed a σ-bond with one Al; meanwhile, the O atom of the
amide coordinated with AlMe3 via an O-donating type bond.
However, recrystallization of complex C10 failed in obtaining
the dimetallic complex but instead afforded single crystals of
C9, indicating the weak nature of the donating bonding of the
O of the amide with AlMe3 in C10. In our previous work,13b
the molecular structure of a dinuclear aluminum compound
revealed the presence of a coordinated AlMe3 at the oxygen
atom via dative bonding. More importantly, the compound
C10 was confirmed by 1H/13C NMR measurements and
1
elemental analysis. In the H NMR of C9, there is only one
Figure 2. ORTEP drawing of C4 with thermal ellipsoids drawn at the
30% probability level. Hydrogen atoms are omitted for clarity.
resonance attributed to the methyl group, which appeared at δ
−0.70 ppm (s, 6H); in contrast, two resonances for the methyl
groups were observed at δ −0.68 (s, 6H) and −0.72 ppm (s,
9H) in the spectrum of C10.
Figure 4 reveals that the molecular structure of diisobuty-
laluminum N-(2-methylquinolin-8-yl)picolinamide (iBu2AlL9,
Figure 3. ORTEP drawing of C9 with thermal ellipsoids drawn at the
30% probability level. Hydrogen atoms are omitted for clarity (two
independent molecules are included, only one structure is listed).
in Table 1 for comparison. It can be seen that the bond
distances of Al−N2imide, Al−N1quin, and Al−C (1.952(2),
2.134(2), 1.987(3) and 1.989(3) Å) are much longer than
those observed in C2 and C4, illustrating the effect of the
pyridine group on the amidate. The pyridyl plane is also almost
coplanar with the quinolyl plane, with a dihedral angle of
13.25°, which is much smaller than those of C2 (60.83°) and
C4 (68.71°). These large differences are expected to have a
significant effect on the catalytic behavior of these complexes
for the ROP of ε-caprolactone, and this will be discussed in the
catalytic discussion (section 2).
Figure 4. ORTEP drawing of C11 with thermal ellipsoids drawn at the
30% probability level. Hydrogen atoms are omitted for clarity.
C11) possesses a geometry similar to that of C9, and selected
bond lengths and bond angles are collected in Table 1. Small
differences are observed between the Al−C bond lengths of
C11 (1.992(2), 1.994(3) Å) versus those of C2 (1.987(3),
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1.989(3) Å), also in the bond lengths Al−N2imide = 1.940(2) Å,
Al−N1quin = 2.135(2) Å, and Al−N1Py = 2.112(3) Å in C11
compared with those of C9 (Al−N2imide = 1.952(2) Å, Al−
N1quin = 2.134(2) Å, Al−N1Py = 2.121(3) Å). Due to the
greater hindrance associated with the iBu group around the Al
center, the Al−C bond length in C11 is much longer than that
in C9, whereas the Al−N2imide bond length in C11 is much
shorter than that of C9.
2. Catalytic Behavior toward Ring-Opening Polymer-
ization (ROP) of ε-Caprolactone (ε-CL). Aluminum com-
pounds are often reported as efficient catalysts for ring-opening
polymerization (ROP) of cyclic esters.5 The catalytic behavior
of C1−C11 was explored toward the ROP of ε-CL, which is
the first example of the use of aluminum amidates for ring-
opening polymerization. Generally these complexes exhibited
good activity for the ROP of ε-CL, and the detailed
investigations for the optimization of the conditions were
conducted by employing C2 as the initiator, the results for
which are collected in Table 2.
For example, only a trace of polymer was obtained at room
temperature over 30 min (run 1), whereas high conversion of
92.3% was achieved at 80 °C over 10 min (run 10, Table 2).
When the polymerization time was increased from 5 to 60 min
(runs 9−13, Table 2), the molecular weight increased with
amplified conversion rate, while the molecular weight
distribution remained fairly constant (1.19−1.30), consistent
with good control for this polymerization process.
As high-molecular-weight polyesters possess better mechan-
ical properties for subsequent utilization, high-molecular-weight
PCL is an attractive target.15 An increase of the monomer/Al
ratio often leads to higher molecular weights; however, this is
usually to the detriment of the monomer conversion rate. Here,
we also investigated the effect of the ε-CL/Al molar ratio on
the catalytic behavior, and the results are given in Table 2 (runs
10, 14, 16, and 19−21). When the mole ratio CL:Al is increased
from 250 to 2500, the molecular weight increased from 2.88 ×
104 to 11.85 × 104 with little change of molecular weight
distribution (1.22 to 1.35), but the conversion rate significantly
decreased, producing polymers with lower molecular weight
than the calculated Mn values. This may be due to bulky chain
transfer properties that resulted in an increase of chain
termination when the concentration of monomer increased.16
It was generally observed that all PCLs obtained in the
presence of benzyl alcohol possessed narrow distributions (Mw/
Mn = 1.05−1.35) with unimodal characteristics (Table 2),
consistent with a single-site active species.
We also investigated the behavior of the other complexes
herein toward the ring-opening polymerization of ε-CL, and the
results are collected in Table 3. Generally, the amidate
aluminum complexes with aryl substituents showed good
catalytic activity with high conversion (>90%). The substituent
on the aryl ring greatly affected the catalytic activity, with more
strongly electron donating substituents leading to higher
conversions. Such results were similar to literature re-
ports,5k,l,15e and the activity order C4 (p-OMe) > C6 (p-Me)
> C5 (o-Me) > C1 > C3 (p-Cl) > C2 (p-F) was observed. The
effect on the molecular weight of the polymer ((2.88−3.66) ×
104) and on the distribution (PDI 1.22−1.38) was less
pronounced. When the substituent on the amidate was changed
to an alkyl group, the catalytic activity decreased substantially
with lower conversion rates (<90%) and lower molecular
weight (runs 6 and 7, Table 3). When the substituent on the
amidate was replaced with a pyridine group, the activity
decreased rapidly and the conversion decreased to 63.9%, while
the molecular weight of the obtained polymer was much lower
than those obtained by C1−C5 (run 8), and the binuclear
complex C10 exhibited slightly higher activity than did the
mononuclear aluminum complex C9. Diisobutylaluminum N-
(2-methylquinolin-8-yl)picolinamide (iBu2AlL9, C11) showed
much higher activity toward the ring-opening polymerization of
ε-CL than did C9, which may be attributed to the greater steric
hindrance of iBu, which could be beneficial for the existence of
a living species. For comparison, blank experiments were also
conducted, and the results showed that without benzyl alcohol
trimethylaluminum showed poor activity for ring-opening
polymerization of ε-CL, even at high temperature (run 13).
In the presence of 1 equiv of benzyl alcohol, it can initiate the
ring-opening polymerization of ε-CL with considerable activity
but produced a polymer with much lower molecular weight and
with rather broader distribution (run 14). When the amount of
BnOH was increased to 2 or 3 equiv, only trace polymer was
obtained (runs 15 and 16). The active species in the reaction of
a
Table 2. ROP of ε-CL by C2/BnOH
b
run complex CL:Al:BnOH T/°C t/min yield/mg (%) 10−4Mn Mw/Mn
1
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
C2
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
500:1:1
500:1:1
1000:1:1
1000:1:1
1000:1:1
1500:1:1
2000:1:1
2500:1:1
250:1:0
250:1:0
20
20
20
20
40
40
60
60
80
80
80
80
80
80
80
80
80
80
80
80
80
20
20
30 trace
2
60 322 (56.1)
90 510 (88.9)
720 574 (100)
30 485 (84.5)
60 515 (89.7)
30 530 (92.3)
60 557 (97.0)
0.75
0.82
0.94
2.61
2.74
2.95
1.05
1.07
1.21
1.14
1.24
1.16
3
4
5
6
7
8
9
5
430 (74.9)
2.16
2.88
3.19
3.69
3.97
2.93
5.32
6.19
8.28
9.71
9.61
10.62
11.85
1.19
1.22
1.28
1.29
1.30
1.23
1.29
1.24
1.29
1.35
1.28
1.30
1.35
10
11
12
13
14
15
16
17
18
19
20
21
22
23
10 541 (92.3)
20 558 (97.2)
30 563 (98.1)
60 574 (100)
10 1058 (92.2)
30 1120 (97.6)
10 1940 (84.5)
30 2220 (96.7)
60 2250 (98.0)
10 2762 (80.2)
10 3361 (73.2)
10 3510 (61.1)
30 trace
90 416 (72.5)
5.18
1.19
a
b
Conditions: 20 μmol of cat.; 1.0 M ε-CL toluene solution. GPC data
in THF vs polystyrene standards, using a correcting factor of 0.56.14
The precatalyst C2 showed efficiency in the ROP of ε-
caprolactone in both the presence and absence of benzyl
alcohol (BnOH) (Table 2, runs 1−4 and 23),5n,13a though
lower activity was observed without the benzene alcohol and
such conditions also produced polymers with a still narrow
molecule distribution PDI value. Thus, the aluminum complex,
in the presence of benzyl alcohol, exhibited better control for
the ROP of ε-CL. As a consequence, detailed investigations of
complex C2 have been carried out by using BnOH as
coinitiator (Table 2).
Table 2 displays results for both the elevation of the
temperature (runs 1, 5, 7, and 12) and the variation of the
polymerization time (runs 2−4 and 9−13), which resulted in
higher molecular weight polymer and higher conversion rate.
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a
Table 3. ROP of ε-CL by C1, C3−C11/BnOH
b
b
run
complex
CL:Al:BnOH
T/°C
t/min
yield/mg (%)
10−4Mn
Mw/Mn
1
C1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
250:1:1
500:2:2
250:2:1
500:2:1
250:1:1
250:1:0
250:1:1
250:1:2
250:1:3
250:1:2
250:1:5
250:1:10
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
80
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
10
552 (96.2)
546 (95.1)
574 (100)
553 (96.3)
565 (98.4)
499 (86.9)
485 (84.5)
367 (63.9)
802 (69.9)
440 (76.7)
1010 (88.0)
466 (81.2)
52 (8.9)
3.55
3.66
3.39
3.49
1.36
1.38
1.31
1.38
2
C3
3
C4
4
C5
5
C6
6
C7
2.84
2.54
2.93
1.28
2.84
5.71
1.21
1.20
1.07
1.10
1.25
1.20
7
C8
8
C9
9
C10
C10
C10
C11
AlMe3
AlMe3
AlMe3
AlMe3
C5
10
11
12
13
14
15
16
17
18
19
5.60
1.53
1.41
2.97
288 (50.2)
trace
trace
568 (99.0)
431 (75.1)
trace
1.65
0.98
1.61
1.64
C5
C5
80
a
b
Conditions: 20 μmol of cat.; 1.0 M ε-CL toluene solution. GPC data in THF vs polystyrene standards, using a correcting factor of 0.56.14
AlMe3 with 1−3 equiv of BnOH, proposed as [(BnO)2AlMe]n
derivatives in the literature,17 could explain the differing
polymerization results, through the formation of other
aggregated aluminum alkoxides, which are inactive for the
ROP of ε-CL. Also, the polymerization behavior when L2 was
employed as the initiator was also investigated under the same
conditions, and the results showed that it cannot initiate the
ring-opening polymerization of ε-CL in either the absence or
presence of benzyl alcohol. Use of increased amounts of BnOH
gives polymers with lower molecular weights and broader
distributions (runs 4 and 17−19, Table 3), suggesting that the
BnOH works as the chain transfer reagent. Similar results were
also observed using C10/BnOH (runs 9 and 11, Table 3).
3. Mechanism of Ring-Opening Polymerization of ε-
Caprolactone by Aluminum Amides. As reported else-
where in the literature for numerous systems, aluminum
alkoxides are assumed to be the active species in the ring-
opening polymerization of ε-caprolactone.18 In this work, some
experiments were conducted to investigate the mechanism in
more detail. First, the 1H NMR spectra of the resultant polymer
was measured to analyze the end group of the polymer, and the
results showed a signal for −OCH2Ph (δ 5.10 ppm), thereby
providing evidence for an active species possessing an Al−
OCH2Ph group. Attempts to isolate an intermediate such as
L(Me)Al−OCH2Ph by treatment of C2 with 1 equiv of benzyl
alcohol in toluene for 12 h failed, and the NMR spectra of the
residue showed two kinds of resonances in the region for aryls;
in particular, a broad peak appeared at δ 10.79 ppm. At the
same time, the signals for the methyl group were the same as
those of the free ligand. The above results were consistent with
the recovery of the free ligand in this reaction. In addition,
there were some broad peaks at around δ 4.93−5.04 and −1.11
to −1.29 ppm, indicative of unreacted C2; according to the
around δ 5.1 and −0.83 to −1.29, which were assigned to the
presence of unidentified compounds AlMex(OCH2Ph)3−x
,
which are plausibly acting as initiators in the polymerization
of ε-CL in a fashion similar to our previous results.13b The
ligand was probably released within the reaction of the amidate
alunmium complexes with BnOH, which is consistent with the
observation by the Chen and Roesky groups.17,19 In order to
1
further investigate the mechanism, the H NMR of C6 with
BnOH and ε-CL (the ratio is 1:1:10) after heating at 80 °C
showed that there is still a peak for −NHC(O)− at δ 10.76
ppm and two kinds of resonances for PhCH2O and quin-Me,
but also with strong signals for the polymer. For comparison,
blank experiments with the free ligand and AlMe3 were
conducted for the ROP of ε-CL. Therefore, the ligand itself
could not initiate the ROP of ε-CL, and AlMe3 showed only
very low activity in the absence of BnOH (run 13, Table 3).
When 1 equiv of BnOH was employed as initiator for AlMe3,
considerable activity could be achieved with 50.2% conversion
yield (run 14, Table 3), giving a polymer showing broad
molecular distribution. However, using more BnOH (2 or 3
equiv), the catalytic systems produced only trace amounts of
polymer (runs 15 and 16, Table 3). In contrast, the
polymerization by complexes C1−C11/BnOH showed high
activity and produced polymers with narrow molecular
distributions. These results are further evidence of the
advantage of using aluminum complex precatalysts in the
ROP of CL, which can afford single-site active species for PCLs
with narrow molecular weight distributions. Possibly, any
ligands released could coordinate with the alkylaluminum to
some extent and act as active species to initiate the
polymerization. Though the true active species remains
unknown, the ligands used herein have played an important
role with regard to the formation of the active species, which
efficiently initiated the ROP polymerization of ε-CL in a
controlled manner. These complicated species, which underpin
this catalytic process, are deserving of further detailed
investigations.
1
integration of H NMR spectra, the ratio of free ligand and
unreacted C2 was about 1.7:1. A similar reaction of C5 with an
additional 1 equiv of BnOH afforded similar results with a 1.7:1
ratio of ligand L5 to unreacted C5. However, the reaction of C5
with 2 equiv of BnOH completely formed the ligand L5, as
confirmed by NMR spectroscopy; there were some broad peaks
6257
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CH3). 13C NMR: 172.3, 156.2, 141.3, 140.5, 138.9, 129.0, 126.1, 122.3,
120.0, 118.5, 26.2, 23.5. Anal. Calcd for C12H9F3N2O: C, 56.70; H,
3.57; N, 11.02. Found: C, 56.61; H, 3.59; N, 11.15.
CONCLUSIONS
■
A series of dialkylaluminum compounds (C1−C11) containing
quinoline amides were synthesized and characterized by
1H/13C NMR spectroscopy and elemental analysis. The
molecular structures of the highly sensitive dialkylaluminum
compounds C2, C4, C9, and C11 were confirmed by single-
crystal X-ray crystallography. This is the first time the ROP of
ε-CL employing aluminum amidates has been reported. All
dialkylaluminum amidates exhibited good to high activities in
the ROP of ε-CL in the presence of BnOH. The substituents at
the amide groups greatly affected the catalytic behavior of their
aluminum complexes, and higher activity was achieved by
introducing a more strongly electron donating group. With the
assistance of benzyl alcohol, the alkylaluminum complexes
easily decomposed to the free ligands together with observed
aluminum alkoxides; the mixtures of aluminum compounds
could efficiently initiate the ring-opening polymerization of ε-
CL.
Synthesis of Complexes C1−C11. Synthesis of Dimethylalu-
minum N-(2-Methylquinolin-8-yl)benzamidate, [Me2AlL1] (C1). To
a stirred solution of N-(2-methylquinolin-8-yl)benzamide (0.262 g, 1.0
mmol) in toluene (15.0 mL) was added 1.0 mL (1.0 mmol) of AlMe3
solution (1.0 M solution in toluene) dropwise at −30 °C. The slurry
was warmed slowly to room temperature and was stirred for 3 h, and
the solution became clear. Following concentration to 5 mL in vacuo,
10 mL of n-heptane was added, and the solution was placed in the
freezer (−30 °C) to afford C1 as a yellow powder. Yield: 0.261 g
(82.1%). 1H NMR (CDCl3): δ 8.95 (d, 1H, J = 7.82, Qin H), 8.36 (d,
1H, J = 8.41, Qin H), 7.76 (d, 2H, J = 7.11, Qin H), 7.67 (t, 1H, J =
8.02, Qin H), 7.45 (m, 5H, Ar H), 2.81 (s, 3H, Py CH3), −0.87 (s, 6H,
Al(CH3)2).13C NMR (CDCl3): δ 174.6, 156.5, 141.2, 140.8, 140.6,
138.8, 130.2, 128.8, 128.4, 127.2, 126.5, 123.1, 122.1, 119.3, 22.8, −8.9.
Anal. Calcd for C19H19AlN2O: C, 71.68; H, 6.02; N, 8.80; Found: C,
71.22; H, 6.13; N, 8.73.
Synthesis of Dimethylaluminum 4-Fluoro-N-(2-methylquinolin-
8-yl)benzamidate, [Me2AlL2] (C2). The synthesis of C2 was carried
out by the same procedure as for C1, except 4-fluoro-N-(2-
1
EXPERIMENTAL SECTION
methylquinolin-8-yl)benzamide was used. Yield: 0.293 g (87.1%). H
■
NMR (CDCl3): δ 8.89 (d, 1H, J = 7.88, Qin H), 8.38 (d, 1H, J = 8.44,
Qin H), 7.77 (m, 2H, Qin H), 7.69 (t, 1H, J = 8.04, Qin H), 7.52 (s,
1H, J = 8.08, Ar H), 7.45 (d, 1H, J = 8.44, Ar H), 7.15 (t, 2H, J = 8.68,
Ar H), 2.83 (s, 3H, Qin CH3), −0.84 (s, 6H, Al(CH3)2). 13C NMR
(CDCl3): 174.2, 165.5, 163.0, 157.0, 141.7, 139.2, 130.0, 129.9, 129.3,
126.9, 123.4, 123.1, 115.8, 115.6, 23.3, −8.6. Anal. Calcd for
C19H18AlFN2O: C, 67.85; H, 5.39; N, 8.33. Found: C, 67.56; H,
5.61; N, 8.24.
Synthesis of Dimethylaluminum 4-Chloro-N-(2-methylquinolin-
8-yl)benzamidate, [Me2AlL3] (C3). Using the same procedure as for
C1, dimethylaluminum 4-chloro-N-(2-methylquinolin-8-yl)-
benzamidate, [Me2AlL3] (C3), was isolated as a yellow solid. Yield:
0.286 g (81.2%). 1H NMR (CDCl3): δ 8.90 (d, 1H, J = 7.32, Qin H),
8.38 (d, 1H, J = 7.56, Qin H), 7.69 (m, 3H, Qin H), 7.51 (m, 2H, Ar
H), 7.44 (m, 2H, Ar H), 2.83 (s, 3H, Qin CH3), −0.84 (s, 6H,
Al(CH3)2). 13C NMR (CDCl3): δ 173.9, 165.3, 163.1, 157.2, 141.3,
139.5, 130.0, 129.8, 129.3, 127.2, 123.9, 123.0, 115.2, 115.3, 23.3, −8.6.
Anal. Calcd for C19H18AlClN2O: C, 64.68; H, 5.14; N, 7.94. Found: C,
64.81; H, 5.17; N, 7.79.
General Considerations. All reactions were performed using
standard Schlenk techniques in an atmosphere of high-purity nitrogen
or glovebox techniques. Toluene, n-heptane, and THF were dried by
refluxing over sodium and benzophenone, distilled under nitrogen, and
stored over activated molecular sieves (4 Å) for 24 h in a glovebox
prior to use. C6D6 was dried over activated 4 Å molecular sieves.
CH2Cl2 and CDCl3 were dried over CaH2 for 48 h, distilled under
nitrogen, and stored over activated molecular sieves (4 Å) in a
glovebox prior to use. AlMe3 and iBu3Al were purchased from Aldrich
and used as received. The ligands N-(2-methylquinolin-8-yl)-
benzamide (L1−L6), N-(2-methylquinolin-8-yl)acetamide (L7 and
L8), and N-(2-methylquinolin-8-yl)picolinamide (L9) were synthe-
sized according to the literature procedures.6d Elemental analyses were
performed using a PE2400II Series (Perkin-Elmer Co.). 1H NMR
spectra and 13C NMR spectra were recorded on a Bruker DMX-400
1
(400 MHz for H, 100 MHz for 13C) instrument. All spectra were
obtained in the solvent indicated at 25 °C, unless otherwise noted, and
chemical shifts are given in ppm and are referenced to SiMe4 (δ 0.00,
1H, 13C). The GPC measurements were performed on a setup
Synthesis of Dimethylaluminum 4-Methoxy-N-(2-methylquino-
lin-8-yl)benzamidate, [Me2AlL4] (C4). Using the same procedure as
for C1, dimethylaluminum 4-methoxy-N-(2-methylquinolin-8-yl)-
benzamidate, [Me2AlL4] (C4), was isolated as a yellow solid. Yield:
0.255 g (73.2%). 1H NMR (CDCl3): δ 8.84 (d, 1H, J = 7.92, Qin H),
8.36 (d, 1H, J = 8.40, Qin H), 7.76 (d, 2H, J = 8.12, Qin H), 7.67 (t,
1H, J = 8.04, Qin H), 7.47 (d, 1H, J = 8.18, Ar H), 7.43 (d, 1H, J =
8.45, Ar H), 6.97 (d, 2H, J = 8.13, Ar H), 3.88 (s, 3H, OCH3), 2.82 (s,
3H, Qin CH3), −0.83 (s, 6H, Al(CH3)2). 13C NMR (CDCl3): δ 175.2,
156.8, 141,4, 140.9, 139.1, 129.3, 129.1, 129.0, 128.2, 127.5, 126.8,
125.3, 123.3, 113.8, 55.4, 23.1, −8.6. Anal. Calcd for C20H21AlN2O2: C,
68.95; H, 6.08; N, 8.04. Found: C, 68.72; H, 6.11; N, 8.15.
Synthesis of Dimethylaluminum 2-Methyl-N-(2-methylquinolin-
8-yl)benzamidate, [Me2AlL5] (C5). Using the same procedure as for
C1, dimethylaluminum 2-methyl-N-(2-methylquinolin-8-yl)-
benzamidate, [Me2AlL5] (C5), was isolated as a yellow solid. Yield:
0.283 g (85.2%). 1H NMR (CDCl3): δ 8.36 (d, 1H, J = 8.42, Qin H),
7.66 (t, 1H, J = 7.93, Qin H), 7.48 (d, 2H, J = 8.30, Qin H), 7.43 (d,
1H, J = 8.45, Qin H), 7.33 (d, 1H, J = 7.88, Ar H), 7.25 (m, 3H, Ar H),
2.81 (s, 3H, Py CH3), 2.41 (s, 3H, Ar CH3), −0.90 (s, 6H, Al(CH3)2).
13C NMR: 175.1, 156.7, 141.5, 141.0, 140.0, 139.1, 136.0, 130.6, 129.5,
consisting of a Waters 515 HPLC pump, a Waters 2414 refractive
index detector, and a combination of Styragel HT-2, HT-3, and HT-4,
the effective molar mass ranges of which are 100−10 000, 500−30 000
and 5000−600 000, respectively. THF was used as the eluent (flow
rate 1 mL min−1, at 40 °C). Molecular weights and molecular weight
distributions were calculated using polystyrene as the standard.
Synthesis of N-(2-Methylquinolin-8-yl)acetamide (L7). To a
stirred toluene solution (50 mL) of 2-methylquinolin-8-amine (1.58 g,
10 mmol) was added 0.72 g (12 mmol) acetic acid at room
temperature. The mixture was stirred for 15 min and heated to 80 °C.
Trichlorophosphine (0.545 g, 4.0 mmol) was added slowly through a
dropping funnel over a period of 15 min. After reflux for an additional
6 h, the solvent was removed by vacuum evaporation. N-(2-
Methylquinolin-8-yl)acetamide (L7) was purified by column chroma-
tography (silica gel, petroleum ether−ethyl acetate 5:1), and 2.16 g
(8.3 mmol) of L7 was obtained (yield 83.0%).1 H NMR: 9.32 (d, 1H, J
= 7.56, Qin H), 8.02 (d, 1H, J = 8.38, Qin H), 8.00 (s, 1H, NH), 7.44
(m, 2H, Qin H), 7.31 (d, 1H, J = 8.38, Qin H), 2.74 (s, 3H, Qin CH3),
2.36 (s, 3H, COCH3). 13C NMR: 173.6, 156.5, 142.5, 138.6, 138.1,
129.9, 126.59, 123.8, 123.6, 120.5, 27.1, 25.5. Anal. Calcd for
C12H12N2O: C, 71.98; H, 6.04; N, 13.99. Found: C, 71.82; H, 6.09;
N, 14.12.
129.4, 127.1, 126.9, 125.6, 121.6, 119.3, 23.2, 19.6, −9.2. Anal. Calcd
for C20H21AlN2O: C, 72.27; H, 6.37; N, 8.43. Found: C, 72.15; H,
6.42; N, 8.52%.
Synthesis of Dimethylaluminum 4-Methyl-N-(2-methylquinolin-
8-yl)benzamidate, [Me2AlL6] (C6). Using the same procedure as for
C1, dimethylaluminum 4-methyl-N-(2-methylquinolin-8-yl)-
benzamidate, [Me2AlL6] (C6), was isolated as a yellow solid. Yield:
Synthesis of 2,2,2-Trifluoro-N-(2-methylquinolin-8-yl)-
acetamide (L8). By using the same procedure as L7, 2,2,2-
trifluoro-N-(2-methylquinolin-8-yl)acetamide (L8) was isolated as a
1
yellow solid. Yield: 2.03 g (80.1%). H NMR: 9.45 (d, 1H, J = 7.38,
Qin H), 8.36 (d, 1H, J = 7.45, Qin H), 8.02 (s, 1H, NH), 7.65 (m, 1H,
Qin H), 7.25(s, 1H, Qin H), 7.12 (m, 1H, Qin H), 2.38 (s, 3H, Qin
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Table 4. Crystal Data and Refinement Details for C2, C4, C9, and C11
C2
C19H18AlFN2O
C4
C20H21AlN2O2
C9
C36H36Al2N6O2
C11
C24H30AlN3O
empirical formula
formula wt
cryst color
temp (K)
wavelength (Å)
cryst syst
space group
a (Å)
336.33
348.37
638.67
403.49
colorless
173(2)
0.710 73
triclinic
colorless
colorless
colorless
173(2)
173(2)
173(2)
0.710 73
0.710 73
0.710 73
orthorhombic
Pna21
monoclinic
monoclinic
P1
P21/c
P21/c
̅
7.3526(15)
8.7401(17)
13.854(3)
74.85(3)
8.9394(18)
13.006(3)
15.405(3)
9.907(2)
14.858(3)
90
b (Å)
26.659(5)
17.275(4)
c (Å)
7.5551(15)
14.339(3)
α (deg)
90
90
β (deg)
88.40(3)
94.03(3)
90.54
90
γ (deg)
79.40(3)
90
90
90
V (Å3)
844.5(3)
1796.1(6)
3221.5(11)
2267.6(8)
4
Z
2
4
4
−3
Dcalcd (Mg m
)
1.323
1.288
1.317
1.182
−1
μ (mm
F(000)
)
0.138
0.128
0.134
0.108
352
736
1344
864
cryst size (mm)
θ range (deg)
limiting indices
0.50 × 0.45× 0.18
2.46−27.51
−7 ≤ h ≤ 9
−10 ≤ k ≤ 11
−17 ≤ l ≤ 18
9341
0.76 × 0.47 × 0.46
2.41−26.38
−11 ≤ h ≤ 11
−33 ≤ k ≤ 33
−9 ≤ l ≤ 9
18 344
0.44 × 0.38 × 0.29
1.57−27.48
−16 ≤ h ≤ 13
−22 ≤ k ≤ 22
−17 ≤ l ≤ 18
28 296
0.40 × 0.25 × 0.20
2.74−27.48
−19 ≤ h ≤ 14
−5 ≤ k ≤ 12
−13 ≤ l ≤ 19
6255
no. of rflns collected
no. of unique rflns
Rint
3868
3667
7347
3662
0.0398
0.0466
0.0555
0.0298
completeness to θ (%)
goodness of fit on F2
final R indices (I > 2σ(I))
R indices (all data)
largest diff peak, hole (e Å−3
99.2
99.8
99.4
99.1
1.100
1.143
1.235
1.073
R1 = 0.0632, wR2 = 0.1803
R1 = 0.0682, wR2 = 0.1852
0.363, −0.276
R1 = 0.0638, wR2 = 0.1721
R1 = 0.0656, wR2 = 0.1738
0.422, −0.407
R1 = 0.0804, wR2 = 0.1758
R1 = 0.0873, wR2 = 0.1801
0.360, −0.395
R1 = 0.0444, wR2 = 0.1180
R1 = 0.0454, wR2 = 0.1190
0.252, −0.264
)
0.283 g (85.2%). 1H NMR (CDCl3): δ 8.90 (d, 1H, J = 7.72, Qin H),
8.39 (d, 1H, J = 8.40, Qin H), 7.69 (m, 3H, Ar H), 7.52 (d, 1H, J =
7.56, Qin H), 7.45 (d, 1H, J = 8.40, Ar H), 7.28 (d, 2H, J = 7.80, Ar
H), 2.83 (s, 3H, Qin CH3), 2.44 (s, 3H, Ar CH3), −0.85 (s, 6H,
Al(CH3)2). 13C NMR: 175.5, 165.2, 163.5, 156.9, 141.1, 139.8, 130.0,
129.9, 129.8, 127.2, 123.6, 123.2, 115.7, 115.3, 23.3, 19.8, −8.9. Anal.
Calcd for C20H21AlN2O: C, 72.27; H, 6.37; N, 8.43. Found: C, 72.20;
H, 6.30; N, 8.55.
Synthesis of Dimethylaluminum N-(2-Methylquinolin-8-yl)-
acetamidate, [Me2AlL7] (C7). Using the same procedure as for C1,
dimethylaluminum N-(2-methylquinolin-8-yl)acetamide, [Me2AlL7]
(C7), was isolated as a yellow solid. Yield: 0.283 g (85.2%). 1H
NMR (CDCl3): 8.49 (d, 1H, J = 7.32, Qin H), 8.38 (s, 1H, J = 7.14,
Qin H), 7.52 (m, 3H, Qin H), 2.88 (s, 3H, Qin CH3), 2.44 (s, 3H,
COCH3), −0.50 (s, 6H, Al(CH3)2). 13C NMR: 174.9, 157.1, 142.0,
139.5, 138.8, 129.5, 126.9, 123.3, 122.1, 120.3, 27.0, 23.3, −9.6. Anal.
Calcd for C14H17AlN2O: C, 65.61; H, 6.69; N, 10.93. Found: C, 65.55;
H, 6.61; N, 10.85.
(83.5%). 1H NMR (CDCl3): δ 9.40 (d, 1H, J = 7.80), 8.54 (d, 1H, J =
4.92), 8.47 (d, 1H, J = 7.84), 8.23 (d, 1H, J = 8.37), 8.10 (t, 1H, J =
7.65), 7.66 (d, 1H, J = 4.86), 7.62 (d, 1H, J = 7.89), 7.51 (d, 1H, J =
7.91), 7.41 (d, 1H, J = 8.38), 2.97 (s, 3H), −0.70 (s, 6H). 13C NMR
(CDCl3): δ 165.1, 156.4, 149.9, 144.2, 139.8, 139.7, 139.0, 138.7,
128.1, 126.5, 126.4, 124.0, 122.8, 120.3, 120.2, 23.1, −6.0. Anal. Calcd
for C18H18AlN3O: C, 67.70; H, 5.68; N, 13.16. Found: C, 67.82; H,
5.77; N, 13.12.
Synthesis of Dimethylaluminum N-(2-Methylquinolin-8-yl)-
picolinamidate Trimethylaluminum [AlMe2L9·AlMe3] (C10). To a
stirred solution of N-(2-methylquinolin-8-yl)picolinamide (L9; 0.263
g, 1.0 mmol) in toluene (15.0 mL) was added 2.0 mL (2.0 mmol) of
AlMe3 solution (1.0 M solution in toluene) dropwise at −30 °C. The
slurry was warmed slowly to room temperature and was stirred for 3 h,
and the solution became clear. Following concentration to 5 mL in
vacuo, 10 mL of n-heptane was added, and the solution was placed in
the freezer (−30 °C) to afford C10 as a yellow powder. Yield: 0.314 g
(80.1%). 1H NMR (CDCl3): δ 9.30 (d, 1H, J = 6.72), 8.65 (d, 1H, J =
7.94), 8.59 (d, 1H, J = 5.00), 8.30 (d, 1H, J = 8.42), 8.18 (t, 1H, J =
7.77), 7.75 (t, 1H, J = 5.26), 7.69 (m, 2H), 7.48 (d, 1H, J = 11.94),
3.00 (s, 3H), −0.68 (s, 6H), −0,72 (s, 9H). 13C NMR (CDCl3): δ
164.3, 157.7, 147.3, 145.1, 140.5, 139.4, 139.2, 136.5, 128.1, 127.7,
126.7, 125.2, 124.7, 124.0, 123.4, 23.4, −6.0, −6.7. Anal. Calcd for
C21H27Al2N3O: C, 64.44; H, 6.95; N, 10.74. Found: C, 64.56; H, 6.82;
N, 10.52.
Synthesis of Dimethylaluminum 2,2,2-Trifluoro-N-(2-methylqui-
nolin-8-yl)acetamidate, [Me2AlL8] (C8). Using the same procedure
as for C1, dimethylaluminum 2,2,2-trifluoro-N-(2-methylquinolin-8-
yl)acetamidate, [Me2AlL8] (C8), was isolated as a yellow solid. Yield:
1
0.283 g (85.2%). H NMR (CDCl3): 9.40 (d, 1H, J = 7.28, Qin H),
8.27 (s, 1H, Qin H), 7.38 (m, 3H, Qin H), 2.34 (s, 3H, Qin CH3),
−0.56 (s, 6H, Al(CH3)2). 13C NMR: 174.1, 156.4, 141.6, 140.8, 138.5,
129.4, 126.7, 122.9, 120.5, 118.5, 27.5, 23.0, −9.6. Anal. Calcd for
C14H14AlF3N2O: C, 54.20; H, 4.55; N, 9.03. Found: C, 54.31; H, 4.50;
N, 9.01.
Synthesis of Dimethylaluminum N-(2-Methylquinolin-8-yl)-
picolinamidate, [Me2AlL9] (C9). Using the same procedure as for
C1, dimethylaluminum N-(2-methylquinolin-8-yl)benzamidate,
[Me2AlL9] (C9), was isolated as a yellow solid. Yield: 0.267 g
Synthesis of Diisobutylaluminum N-(2-Methylquinolin-8-yl)-
picolinamidate, [iBu2AlL9] (C11). To a stirred solution of N-(2-
methylquinolin-8-yl)picolinamide (L9; 0.263 g, 1.0 mmol) in toluene
(15.0 mL) was added 2.0 mL (2.0 mmol) of AliBu3 solution (1.0 M
solution in toluene) dropwise at −30 °C. The slurry was warmed
slowly to room temperature and was stirred for 3 h, and the solution
became clear. Following concentration to 5 mL in vacuo, 10 mL of n-
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heptane was added, and the solution was placed in the freezer (−30
Aluminum, Gallium, Indium and Thallium; Downs, T. J., Ed.; Chapman
and Hall: London, 1993; pp 322−371.
1
°C) to afford C11 as a yellow powder. Yield: 0.344 g (85.2%). H
NMR (CDCl3): δ 9.49 (d, 1H, J = 7.86), 8.57 (d, 1H, J = 4.98), 8.45
(d, 1H, J = 7.79), 8.22 (d, 1H, J = 8.36), 8.10 (t, 1H, J = 7.64), 7.65 (d,
1H, J = 4.85), 7.61 (m, 1H), 7.47 (d, 1H, J = 8.07), 7.40 (d, 1H, J =
8.37), 3.00 (s, 3H), 1.07 (m, 2H), 0.55 (d, 6H, J = 6.44), 0.42 (d, 6H, J
= 6.40), 0.17 (d, 2H, J = 7.16), 0.12 (d, 2H, J = 6.72). 13C NMR
(CDCl3): δ 165.5, 156.5, 151.2, 144.6, 140.4, 139.9, 139.4, 138.9,
128.5, 126.7, 126.1, 124.0, 122.8, 120.4, 120.1, 28.4, 28.3, 27.8, 27.0,
24.0, 23.5, 19.5. Anal. Calcd for C24H30AlN3O: C, 71.44; H, 7.49; N,
10.41. Found: C, 71.52; H, 7.40; N, 10.53.
́ ̂
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Ring-Opening Polymerization (ROP) of ε-Caprolactone
(CL). Typical polymerization procedures in the presence of benzyl
alcohol (Table 2, run 10) are as follows. A toluene solution of C2
(0.020 mmol, 1.0 mL of toluene) and BnOH (0.020 mmol) were
added into a Schlenk tube in the glovebox at room temperature. The
solution was stirred for 2 min, and then ε-caprolactone (5.0 mmol)
along with 3.44 mL of toluene was added to the solution. The reaction
mixture was then placed into an oil bath preheated to 80 °C, and the
solution was stirred for the prescribed time (10 min). The
polymerization mixture was then quenched by addition of an excess
of glacial acetic acid (0.2 mL) into the solution, and the resultant
solution was then poured into methanol (200 mL). The resultant
polymer was then collected on filter paper and was dried in vacuo.
Crystal Structure Determinations. Single crystals of C2, C4,
C9, and C11 suitable for X-ray structural analysis were obtained from
chilled toluene/n-heptane solution. With graphite-monochromated
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́
Mo Kα radiation (λ = 0.710 73 Å), 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. The structures were solved by direct methods
and refined by full-matrix least squares on F2. All hydrogen atoms were
placed in calculated positions. Structure solution and refinement were
performed by using the SHELXL-97 package.20 Details of the X-ray
structure determinations and refinements are provided in Table 4.
(6) (a) Liu, S.; Sun, W.-H.; Zeng, Y.; Wang, D.; Zhang, W.; Li, Y.
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Organometallics 2010, 29, 732−741. (e) Wang, D.; Liu, S.; Zeng, Y.;
Sun, W.-H. Organometallics 2011, 30, 3001−3009.
ASSOCIATED CONTENT
■
S
* Supporting Information
CIF files giving X-ray crystal structural data for C2, C4, C9, and
C11. This material is available free of charge via the Internet at
http://pubs.acs.org.
(7) Jennings, J. R.; Wade, K.; Wyatt, B. K. J. Chem. Soc. A 1968, 21,
2535.
AUTHOR INFORMATION
■
(8) Holder, J. R.; Lappert, M. F. J. Chem. Soc. A 1968, 21, 2004.
(9) Huang, B.-H.; Yu, T.-L.; Huang, Y.-L.; Ko, B.-T.; Lin, C.-C. Inorg.
Chem. 2002, 41, 2987−2994.
(10) Hoerter, J. M.; Otte, K. M.; Gellman, S. H.; Stahl, S. S. J. Am.
Chem. Soc. 2006, 128, 5177−5183.
(11) Zhao, J.; Song, H.; Cui, C. Organometallics 2007, 26, 1947−
1954.
(12) (a) Ski, J. L.; Justyniak, I.; Zachara, J.; Tratkiewicz, E.
Organometallics 2003, 22, 4151−4157. (b) Meisters, A.; Mole, T. Aust.
J. Chem. 1974, 27, 1665−1672. (c) Gibson, V. C.; Redshaw, C.; White,
A. J. P.; Williams, D. J. Chem. Commun. 2001, 79−80. (d) Redshaw,
C.; Elsegood, M. R. J.; Holmes, K. E. Angew. Chem., Int. Ed. 2005, 44,
1850−1853.
(13) (a) Shen, M.; Zhang, W.; Nomura, K.; Sun, W.-H. Dalton Trans.
2009, 38, 9000−9009. (b) Shen, M.; Huang, W.; Zhang, W.; Hao, X.;
Sun, W.-H.; Redshaw, C. Dalton Trans. 2010, 39, 9912−9922. (c) Sun,
W.−H.; Shen, M.; Zhang, W.; Huang, W.; Liu, S.; Redshaw, C. Dalton
Trans. 2011, 40, 2645−2653.
(14) Save, M.; Schappacher, M.; Soum, A. Macromol. Chem. Phys.
2002, 203, 889−899.
Corresponding Author
*C.R.: Tel: +44 (0)1603 593137. Fax: +44 (0)1603 592003. E-
mail: carl.redshaw@uea.ac.uk. W.-H.S.: Tel: +86-10-62557955.
Fax: +86-10-62618239. E-mail: whsun@iccas.ac.cn.
ACKNOWLEDGMENTS
■
This work was supported by National Natural Science
Foundation of China (No. 20904059) and the MOST 863
program (No. 2009AA033601). C.R. wishes to thank the
EPSRC for an overseas travel grant (EP/H031855/1).
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