Synthesis and characterization of aluminum and zinc complexes supported by pyrrole-based ligands and catalysis of the aluminum complexes toward the ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1016/j.jorganchem.2011.04.028

Reaction of quinolin-8-amine with 1H-pyrrole-2-carbaldehyde or 5-tert-butyl-1H-pyrrole-2-carbaldehyde catalyzed by HCO₂H forms N-((1H-pyrrol-2-yl)methylene)quinolin-8-amine (≡ HL, 3a) or N-((5-tert-butyl-1H-pyrrol-2-yl)methylene)quinolin-8-amin

Full text of DOI:10.1016/j.jorganchem.2011.04.028

Journal of Organometallic Chemistry 696 (2011) 2746e2753
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis and characterization of aluminum and zinc complexes supported
by pyrrole-based ligands and catalysis of the aluminum complexes toward
the ring-opening polymerization of 3-caprolactone
*
Shuo Qiao, Wen-An Ma, Zhong-Xia Wang
CAS Key Laboratory of Soft Matter Chemistry, Joint Laboratory of Green Synthetic Chemistry and Department of Chemistry, University of Science and Technology of China, Hefei,
Anhui 230026, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Reaction of quinolin-8-amine with 1H-pyrrole-2-carbaldehyde or 5-tert-butyl-1H-pyrrole-2-carbalde-
hyde catalyzed by HCO2H forms N-((1H-pyrrol-2-yl)methylene)quinolin-8-amine (h HL, 3a) or N-((5-
tert-butyl-1H-pyrrol-2-yl)methylene)quinolin-8-amine (h HL0, 3b). Treatment of 3a and 3b respectively
with AlMe3 or AlEt3 in toluene affords corresponding aluminum complexes LAlMe2 (4a), L0AlMe2 (4b)
and LAlEt2 (4c). Reaction of 3a and 3b with an equivalent of ZnEt2 in toluene generates L2Zn and L02Zn,
respectively. A related compound N-((1H-pyrrol-2-yl)methylene)-2-(3,5-dimethyl-1H-pyrazol-1-yl)ben-
zenamine (h HL00, 7) was prepared by reaction of 2-(3,5-dimethyl-1H-pyrazol-1-yl)benzenamine with
1H-pyrrole-2-carbaldehyde in the presence of HCO2H. Reaction of 7 with AlMe3 gives L002AlMe (8), and
with ZnEt2 yields L002Zn (9). All new compounds were characterized by NMR spectroscopy and elemental
analysis. The structures of complexes 4b, 5b and 8 were additionally characterized by single crystal X-ray
diffraction analyses. Complexes 4ae4c, and 8 were proved to be active catalysts for the ring-opening
Received 27 January 2011
Received in revised form
16 April 2011
Accepted 26 April 2011
Keywords:
Aluminum
Zinc
Complex
Synthesis
Polymerization
3-Caprolactone
polymerization (ROP) of
3-caprolactone (3-CL) in the presence of BnOH. The kinetic study of the poly-
merization reactions catalyzed by 4a and 8 was performed.
Ó 2011 Elsevier B.V. All rights reserved.
1. Introduction
good catalytic activity for the ring-opening polymerization of 3-CL
[26e31]. We wished to further investigate the catalysis of
aluminum and zinc complexes supported by other heterocyclic-
containing multidentate ligands. Hence, we designed two classes
of pyrrole-based N,N,N-tridentate ligands. One involves 8-quinolyl
moiety and the other contains pyrazolyl moiety. We synthesized Al
or Zn complexes bearing these ligands and found the aluminum
Aliphatic polyesters such as poly(3-caprolactone) (PCL), poly-
lactide (PLA) and their copolymers are the most important
synthetic biodegradable polymers. These polymers have attracted
great interest for their applications in the medical field [1e11].
Among the polymers, PCL shows specific advantages in some
applications [12e16]. For example, PCL is ideally suitable for long-
term drug delivery due to its slow degradation in comparison to
other polymers [16]. The major way to synthesize the polymers is
the ring-opening polymerization of cyclic esters catalyzed by metal
complexes [4e11,17e25]. A number of excellent catalysts have been
reported for the polymerization. However, it is still of interest to
find new catalysts for the synthesis of well-defined polyesters with
precisely controlled end group functionalities. Recently, we
synthesized some aluminum and zinc complexes supported by
heterocyclic-containing multidentate ligands such as N,N-, N,O-,
N,N,N- and N,N,O-ligands and found some of the complexes to have
complexes to be efficient catalysts in the ROP of 3-CL. Herein we
report the results.
2. Results and discussion
2.1. Synthesis of aluminum and zinc complexes
Reaction of quinolin-8-amine with 1H-pyrrole-2-carbaldehyde
(2a) or 5-tert-butyl-1H-pyrrole-2-carbaldehyde (2b) in toluene in
the presence of formic acid and 4 Å molecular sieves affords
N-((1H-pyrrol-2-yl)methylene)quinolin-8-amine (h HL, 3a) and
N-((5-tert-butyl-1H-pyrrol-2-yl)methylene)quinolin-8-amine (h
HL0, 3b), respectively, in good yields (Scheme 1). Treatment of 3a
and 3b with AlMe3 or AlEt3 in toluene at room temperature
generates corresponding aluminum complexes LAlMe2 (4a),
* Corresponding author. Tel.: þ86 551 3603043; fax: þ86 551 3601592.
E-mail address: zxwang@ustc.edu.cn (Z.-X. Wang).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2011.04.028

S. Qiao et al. / Journal of Organometallic Chemistry 696 (2011) 2746e2753
2747
geometry of the aluminum coordination sphere can be best
described as a distorted trigonal bipyramid with the imido nitrogen
atom (N2) and two methyl carbon atoms (C19 and C20) occupying
the equatorial positions and the quinolyl nitrogen atom (N1) and the
pyrrolyl nitrogen atom (N3) occupying the axial positions. The N2,
C19, C20 and Al atoms are approximatelycoplanar (the total angles of
the trigonalplaneare 358.01ꢀ). However, the N1, Al, andN3atoms are
not in a line due to strain of the ligand [N1eAleN3 ¼ 155.30(13)ꢀ].
The AleN1distanceof2.262(3) Å islongerthan correspondingthat in
[Al(Me2){OC(Me)]CHC(Me)]NC9H6N}] [2.1448(19) Å] [31]. The
AleN2 distance of 1.970(3) Å is a little shorter than the AleNimine
distance in [Al(Me2){OC(Me)]CHC(Me)]N-C9H6N}] [1.9956(19) Å].
The AleN3 distance of 2.096(3) Å is longer than corresponding those
found in ½AlMe2f2 ꢁ ð20; 60 ꢁ Pr2i C6H3N]CHÞ ꢁ 5 ꢁ ButC4H2Ngꢂ
[1.923(4) Å] [32] and [AlCl{2-(Me2NCH2C4H3N}2] [1.896(3) and
1.906(3) Å, respectively] [33].
The molecular structure of complex 5b is depicted in Fig. 2 along
with selected bond lengths and bond angles. In the structure, the
coordination geometry of the Zn atom is a distorted octahedron.
The N2, N4, N5, and N6 atoms are approximately coplanar, the
torsion angle of N6eN2eN4eN5 being 0.9ꢀ, while the Zn atom is
out of the plane. The N(1)eZn(1)eN(3) angle of 153.14(17)ꢀ shows
that the arrangement of the atoms has a little derivation from a line.
The ZneNquinoline distances of 2.339(5) Å and 2.411(5) Å are longer
than corresponding that in ½ZnðMeÞfCHð8 ꢁ C9H6NÞPðPri2Þ]NButgꢂ
[2.1651(17) Å] [29]. The Zn-Npyrrole distances of 2.138(4) Å and
2.126(5) Å are also longer than corresponding that in [Zn(But){2-
(PriN]CHC4H3N}] [1.996(2) Å] [34].
H+
N
+
R
CHO
N
NH
N
H
N
NH2
R
3a R = H
3b R = Bu
2a R = H
1
t
t
2b R = Bu
AlR1
ZnEt2
3
N
N
R
Zn
N
N
R
N
N
N
Al
N
N
R1
R1
R
5a R = H
5b R = Bu
4a: R = H, R1 = Me
t
4b: R = Bu , R1 = Me
t
4c: R = H, R1 = Et
Scheme 1. Synthesis of complexes LAlR2 (R ¼ Me, Et), L0AlMe2, L2Zn and L20 Zn.
L0AlMe2 (4b) and LAlEt2 (4c). Reaction between 3a or 3b and an
equiv of ZnEt2 always gives L2Zn (5a) or L02Zn (5b) instead of
expected LZnEt or L0ZnEt. Attempts to prepare LZnCl or L0ZnCl
through reaction of lithiated 3a or 3b with ZnCl2 were unsuccessful,
yielding L2Zn and L02Zn, respectively. Each of compounds 3a and 3b
gave satisfactory elemental analysis. The NMR spectra are consis-
tent with their respective molecular structure. The aluminum and
zinc complexes were characterized by elemental analyses and NMR
spectroscopy. The 1H and 13C NMR spectra of 4ae4c display AlMe or
AlCH2 signals at low frequency regions. The other signals are
consistent with the ligand structures. Each of the NMR spectra of 5a
and 5b exhibits one set of ligand signals, implying that the two
ligands have the same coordination mode.
Synthesis of pyrazole-containing ligand and its aluminum and
zinc complexes is shown in Scheme 2. Reaction of 2-(3,5-dimethyl-
1H-pyrazol-1-yl)benzenamine (6) with 1H-pyrrole-2-carbaldehyde
(2a) in refluxed toluene in the presence of formic acid and 4 Å
The structures of complexes 4b and 5bwere furthercharacterized
by single-crystal X-ray diffraction. Complex 4b crystallizes with two
molecules in the asymmetric unit (Fig. 1, one molecule was pre-
sented). The central aluminum atom is five-coordinated. The
Fig. 2. ORTEP drawing of complex 5b (30% probability thermal ellipsoids. Toluene
molecules are omitted). C(34)0, C(35)0, and C(36)0, which represent alternative orien-
tations of C(34), C(35), and C(36) in the disordered t-butyl group, have been omitted
for the sake of clarity. Selected bond lengths (Å) and angles (ꢀ): Zn(1)eN(1) 2.138(4),
Zn(1)eN(2) 2.057(4), Zn(1)eN(3) 2.339(5), Zn(1)eN(4) 2.126(5), Zn(1)eN(5) 2.062(4),
Zn(1)eN(6) 2.411(5), N(1)eZn(1)eN(2) 80.22(18), N(1)eZn(1)eN(3) 153.14(17), N(1)e
Zn(1)eN(4) 105.35(16), N(1)eZn(1)eN(5) 113.76(16), N(1)eZn(1)eN(6) 94.27(16),
Fig. 1. ORTEP drawing of complex 4b (30% probability thermal ellipsoids). C(16)0,
C(17)0, and C(18)0, which represent alternative orientations of C(16), C(17), and C(18) in
the disordered t-butyl group, have been omitted for the sake of clarity. Selected bond
lengths (Å) and angles (ꢀ): Al(1)eC(19) 1.959(3), Al(1)eC(20) 1.961(4), Al(1)eN(1)
2.262(3), Al(1)eN(2) 1.970(3), Al(1)eN(3) 2.096(3), N(2)eC(10) 1.320(4), N(1)eAl(1)-
N(2) 74.40(12), N(1)eAl(1)-N(3) 155.30(13), N(2)eAl(1)-N(3) 81.03(13), C(19)eAl(1)-
C(20) 127.28(18), C(19)-Al(1)eN(1) 89.76(13), C(20)-Al(1)eN(1) 90.54(14), C(19)e
Al(1)eN(2) 114.75(15), C(20)eAl(1)eN(2) 115.98(15), C(19)eAl(1)eN(3) 102.96(15),
C(20)eAl(1)eN(3) 98.06(15), C(10)eN(2)eAl(1) 114.0(2), C(6)eN(2)eAl(1) 122.9(2),
C(11)eN(3)eAl(1) 108.3(3).
N(2)eZn(1)eN(3)
73.11(19),
N(2)eZn(1)eN(4)
118.47(17),
N(2)eZn(1)eN(5)
153.68(17), N(2)eZn(1)eN(6) 85.60(16), N(3)eZn(1)-N(4) 90.39(16), N(3)eZn(1)eN(5)
89.88(16), N(3)eZn(1)-N(6) 80.75(15), N(4)eZn(1)eN(5) 80.64(18), N(4)eZn(1)eN(6)
150.82(17), N(5)eZn(1)eN(6) 71.67(18).

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S. Qiao et al. / Journal of Organometallic Chemistry 696 (2011) 2746e2753
Me
Me
Me
Me
N
N
Me
+
H+
N
N
CHO
N
N
H
H
N
NH2
6
2a
7
AlMe3
ZnEt2
Fig. 3. ORTEP drawing of complex 8 (30% probability thermal ellipsoids. Benzene
molecule is omitted). Selected bond lengths (Å) and angles (ꢀ): Al(1)eN(1) 1.899(4),
Al(1)eN(5) 1.901(4), Al(1)eC(33) 1.949(5), Al(1)eN(2) 2.115(3), Al(1)eN(6) 2.112(4),
N(1)eAl(1)eN(5) 104.89(16), N(1)eAl(1)eC(33) 124.78(18), N(5)eAl(1)eC(33)
130.32(19), N(1)eAl(1)eN(6) 94.86(15), N(5)eAl(1)eN(6) 81.00(15), C(33)eAl(1)eN(6)
94.46(18), N(1)eAl(1)eN(2) 81.01(15), N(5)eAl(1)eN(2) 91.70(15), C(33)eAl(1)-N(2)
94.96(17), N(2)eAl(1)eN(6) 170.45(16).
N
N
Me
N
Me
N
Me
N
N
Al
N
Me
N
N
N
Zn
N5) and the carbon atom of methyl group in equatorial positions.
The N1, N5, C33 and Al atoms are coplanar, the total angles of the
trigonal plane being 359.99ꢀ. The N2, Al1, and N6 atoms are
approximately in a line, the angle of N(2)eAl(1)eN(6) being
170.45(16)ꢀ. The AleNpyrrole distances of 1.899(4) Å and 1.901(4) Å
are shorter than corresponding AleN distances in complex 4b
[2.096(3) Å] and in ½AlMe2f2 ꢁ ð20; 60 ꢁ Pr2i C6H3N]CHÞ ꢁ 5 ꢁ But
C4H2Ngꢂ [1.923(4) Å] [32], but are comparable to corresponding
those in [AlCl{2-(Me2NCH2C4H3N}2] [1.896(3) and 1.906(3) Å,
respectively] [33]. The AleNimine distances of 2.115(3) Å and
2.112(4) Å are longer than those found in complex 4b [1.970(3) Å]
and in ½AlMe2f2 ꢁ ð20; 60 ꢁ Pr2i C6H3N]CHÞ ꢁ 5 ꢁ ButC4H2Ngꢂ
[1.9956(19) Å] [32], but are still in the normal range for Al-imine
complexes.
N
N
Me
Me
N
N
N
N
Me
Me
9
8
Scheme 2. Synthesis of L020AlMe and L200Zn.
molecular sieves gives an imine 7 (h HL00) in 65% yield. Reaction of
7 with Me3Al affords L002AlMe (8). We can not obtain L00AlR2 through
changing proportion of the reactants. Treatment of 7 with Et2Zn
produces L002Zn (9). Attempts to prepare L00ZnEt by changing reac-
tion conditions such as the ratio of the reactants, reaction
temperature and solvents were unsuccessful. Compounds 7e9
were characterized by 1H and 13C NMR spectroscopy and elemental
analyses. Each of the 1H NMR spectra gives appropriate ligand
signals. In the 1H NMR spectrum of complex 8 AleMe signal was
2.2. Catalyzed ring-opening polymerization of
3-caprolactone
The ring-opening polymerization of
3-CL catalyzed by
complexes 4ae4c and 8 in the presence of benzyl alcohol was
carried out and the results are listed in Table 1. Each of the
complexes is an active catalyst toward the ROP of 3-CL at the
also observed at
d
ꢁ0.21 ppm. The NMR spectra of complex 9
exhibit one set of ligand signals, implying that the two ligands
adopt the same coordination mode in the solution. Single crystals of
complex 9 suited for X-ray diffraction analysis can not be obtained.
We guess the central zinc atom may be four-coordinated due to the
steric hindrance. Complex 8 was additionally characterized by
single-crystal X-ray diffraction. The ORTEP drawing is presented in
Fig. 3, along with selected bond lengths and angles. Complex 8 is
monomeric in the solid state and the aluminum atom is five-
coordinated. The nitrogen atoms of the pyrazolyl groups do not
coordinate to the central metal. The coordination geometry is
a distorted trigonal bipyramid with two imido nitrogen atoms (N2
and N6) in axial positions and two pyrrolyl nitrogen atoms (N1 and
elevated temperature. The polymerization catalyzed by 4a/BnOH
was performed at 70 ꢀC. The monomer conversion reached 93% in
105 min. At the higher temperature the reaction went to comple-
tion in shorter time (entry 2 in Table 1). Complex 4c exhibits a little
higher catalytic activity than complex 4b. The catalytic activity of
each of 4ae4c is comparable to those of N,N,O-chelate methyl-
aluminium 8-quinolinolates [35]. It was also noted that complexes
4ae4c lead to higher polymer molecular weights than calculated
ones. This may be due to partial decomposition of the active cata-
lysts during the reaction at the high reaction temperatures.
Complex 8/BnOH exhibits much higher activity than complexes
4ae4c/BnOH systems. At 60 ꢀC 8/BnOH system leads to 90%
Table 1
Ring-opening polymerization of 3
-CL catalyzed by complexes 4ae4c and 8 in the presence of benzyl alcohol.a
e
Entry
Cat.
[M]0/[Al]0/[BnOH]0
Temp. (ꢀC)
Time (min.)
Conv. (%)b
Mn (GPC)c
Mn (Calcd)d
Yield (%)
PDI
1
2
3
4
5
4a
4a
4b
4c
8
200:1:1
200:1:1
199:1:1
200:1:1
204:1:1
70
100
100
100
60
105
75
60
60
2
93
98
85
94
90
54900
e
21300
22500
19400
21600
21100
90
94
82
90
87
1.17
e
36900
48000
25700
1.41
1.30
1.20
a
All polymerizations were carried out in toluene; [CL]0 ¼ 2 M.
b
c
Measured by 1H NMR spectra.
Obtained from GPC analysis and calibrated against polystyrene standard, multiplied by 0.56 [37,38].
d
e
Calculated from the molecular weight of
Obtained from GPC analysis.
3-CL times the conversion of monomer and the ratio of [CL]0/[BnOH]0 plus the molecular weight of BnOH.

S. Qiao et al. / Journal of Organometallic Chemistry 696 (2011) 2746e2753
2749
6.0
4.8
3.6
2.4
1.2
0.0
6.0
4.8
3.6
monomer conversion within 2 min, exhibiting comparable activity
to that of N,N-dialkylaniline-arylamidoaluminum/BnOH [36]. The
determined molecular weight by GPC closely matches the theo-
retical value, showing the catalytically active species to be stable at
the reaction temperature. The higher catalytic activity of complex 8
than complexes 4ae4c is attributed to difference of electronic effect
of the auxiliary ligands. In complex 8 two anionic ligands coordi-
nate to the aluminum center and this provides a proper electronic
environment at the metal center for the catalysis.
PDI
2.4
1.2
0.0
In order to establish reaction order in monomer and metal
concentration, kinetic studies of 3-CL polymerization catalyzed by
complexes 4a and 8 in the presence of BnOH were performed. Plots
of ln([CL]0/[CL]) versus time using each catalyst exhibits a perfect
linear relationship (Fig. 4), which indicates that the polymerization
proceeds with first-order dependence on monomer concentration.
The reaction rate remains constant with reaction time, indicating
a constant number of active sites throughout the polymerization.
This implies that the polymerizations catalyzed by 4a and 8,
respectively, are controlled. A further indicator of controlled poly-
merization is the linear relationship of molecular weight with
conversion in each catalytic reaction, along with relatively low PDI
values (Figs. 5 and 6). In addition, it was noted the existence of CH2
signals of BnO in the 1H NMR spectra of the polymers. This shows
that the polymerizations are initiated through the insertion of BnO
64
72
80
88
96
Conversion / %
Fig. 5. Plots of PCL Mn (- obtained from GPC analysis) and polydispersity (:, Mw/Mn)
as function of
-CL conversion using complex 4a at 70 ꢀC
a
3
[M]0:[Al]0:[BnOH]0 ¼ 200:1:1, [M]0 ¼ 2 M.
3.0
2.5
2.0
1.5
1.0
0.5
3.0
2.5
2.0
1.5
1.0
0.5
group to
For the sake of comparison, we also investigated catalysis of
complex 4a toward the ROP of -CL in the absence of BnOH and in
3-CL.
3
the presence of two equiv of BnOH. In the absence of BnOH complex
PDI
4a showed a very poor catalytic activity. It leads to about 6%
monomer conversion at 70 ꢀC in 105 min when 200 equiv of
3-CL
were used. In the presence of two equiv of BnOH complex 4a
exhibited a little higher catalytic activity than that using one equiv
BnOH (Table 2). The monomer conversion reaches 95% in 60 min at
70 ꢀC. However, the molecular weights of the polymers determined
by 1H NMR spectroscopy are about half of the theoretical values
(Table 2, entries 1e5). This is attributed to that all the added alcohol
molecules contribute to the immortal polymerization. Thus, one of
the aluminum methyl groups in 4a was replaced by OBn through
reaction 4a with one equiv of BnOH, and the resultant benzylox-
yaluminum complex catalyzes the ring-opening polymerization.
During the polymerization an exchange takes place between the
growing polymer chains and the free alcohol, which generates PCL
45
50
55
60
Conversion / %
Fig. 6. Plots of PCL Mn (- obtained from GPC analysis) and polydispersity (:, Mw/Mn)
as function of -CL conversion using complex
at 20 ꢀC
[M]0:[Al]0:[BnOH]0 ¼ 200:1:1, [M]0 ¼ 2 M.
a
3
8
and
a new benzyloxyaluminum molecule (Scheme 3) [39].
However, it is possible that the two methyl groups in 4a were
replaced by two OBn groups through reaction of 4a with two equiv
of BnOH and two polymer chains grow simultaneously at the metal
center [36]. A well linear relationship of the concentration change
of the monomer versus time supports the polymerization to be
controlled (Fig. 7).
3.0
2.4
1.8
1.2
0.6
0.0
Table 2
Ring-opening polymerization of
3-CL catalyzed by complex 4a in the presence of
2 equiv of benzyl alcohol.a
Entry
Time (min.)
Conv. (%)b
Mn (NMR)b
Mn (Calcd)c
1
2
3
4
5
30
45
60
75
90
57
84
95
98
99
6300
10100
12400
13600
14500
13100
19050
21800
22600
22700
0
28
56
84
112
140
a
The polymerizations were carried out in toluene at 70 ꢀC; [CL]0 ¼ 2 M, [CL]0/
Time / Min.
[Al]0/[BnOH]0 ¼ 200:1:2.
b
Fig. 4. Plots of ln([M]0/[M]) versus time for the polymerization of
3
-CL catalyzed by 4a
Obtained from NMR analysis.
Calculated from the molecular weight of 3-CL times the conversion of monomer
c
(:) and 8 (-). Conditions: Solvent: toluene; polymerization temperature: 70 ꢀC for 4a
and 20 ꢀC for 8; [M]0/[Al]/[BnOH]0 ¼ 200:1:1, M]0 ¼ 2 M.
and the ratio of [CL]0/[BnOH]0 plus the molecular weight of BnOH.

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S. Qiao et al. / Journal of Organometallic Chemistry 696 (2011) 2746e2753
Cambridge Isotope Laboratories, were degassed and stored over 4 Å
molecular sieves (CDCl3) or Na/K alloy (C6D6). -Caprolactone,
O
OBn
Me
3
LAl
Me
O
C
O
OBn
n
+
n
LAl
purchased from Acros Organics, was stirred over CaH2 for 24 h and
distilled under vacuum. All other chemicals were obtained from
commercial vendors. NMR spectra were recorded on a Bruker
av300 spectrometer at the ambient temperature. The chemical
shifts of 1H and 13C NMR spectra were referenced to internal
solvent resonances or TMS. Elemental analysis was performed on
an Elementar Vario EL-III instrument. Gel permeation chromato-
graph (GPC) measurements were performed on a Waters 150C
instrument equipped with UltraStyragel columns (103, 104, and
105 Å) and 410 refractive index detector, using monodispersed
polystyrene as calibration standard. THF was used as eluent at
a flow rate of 1 ml/min.
O
10
11
OBn
BnOH
H
O
C
O
OBn
n
+
11
LAl
Me
O
OBn
LAl
O
C
O
OBn
+ m
LAl
O
m
Me
Scheme 3. [4a/BnOH]-mediated “immortal” ROP of
3
-CL.
4.2. Synthesis of compounds
5
4.2.1. Synthesis of N-((1H-pyrrol-2-yl)methylene)quinolin-8-amine
(3a)
4
3
2
1
0
A mixture of quinolin-8-amine (2.00 g 13.87 mmol), 1H-pyrrole-
2-carbaldehyde (1.33 g, 13.99 mmol), 4 Å molecular sieves (10 g),
toluene (15 ml) and five drops of HCOOH was stirred overnight at
room temperature. Molecular sieves were filtered and washed with
CH2Cl2. Solvents were removed from the filtrate by rotatory evap-
oration. The residue was dissolved in n-hexane. The solution was
concentrated to yield white powder of 3a (2.10 g, 68%), m.p.
179e180 ꢀC. Anal. Calcd. for C14H11N3: C, 76.00; H, 5.01; N, 18.99.
Found: C, 75.83, H, 5.13; N, 18.88%. 1H NMR (CDCl3):
d 6.33
(t, J ¼ 3.2 Hz, 1H, Ar), 6.72e6.75 (m, 1H, Ar), 7.04 (s, 1H, Ar), 7.29
(d, J ¼ 7.2 Hz, 1H, Ar), 7.44 (dd, J ¼ 4.2, 8.1 Hz, 1H, Ar), 7.53
(t, J ¼ 7.5 Hz,1H, Ar), 7.65 (d, J ¼ 8.1 Hz,1H, Ar), 8.18 (m, 1H, Ar), 8.42
(s, 1H, N]CH), 8.95 (m, 1H, Ar). 13C NMR (CDCl3):
d 1110.44, 116.92,
15
30
45
60
75
90
118.20, 121.54, 123.34, 124.67, 127.02, 136.20, 149.95, 151.71.
Time / Min.
4.2.2. Synthesis of N-((5-tert-butyl-1H-pyrrol-2-yl)methylene)
quinolin-8-amine (3b)
Fig. 7. Plots of ln([M]0/[M]) versus time for the polymerization of 3-CL catalyzed by 4a.
Conditions: Solvent: toluene; polymerization temperature: 70 ꢀC; [M]0/[Al]/
[BnOH]0 ¼ 200:1:2, M]0 ¼ 2 M.
The same procedure as for 3a was used to prepare 3b. Thus,
a mixture of quinolin-8-amine (2.00 g, 13.87 mmol), 5-tert-butyl-
1H-pyrrole-2-carbaldehyde (2.11 g, 13.95 mmol), 4 Å molecular
sieves (10 g), toluene (15 ml) and five drops of HCOOH was stirred
overnight at room temperature. Molecular sieves were filtered and
washed with CH2Cl2. Solvents were removed from the filtrate by
rotatory evaporation. The residue was dissolved in n-hexane. The
solution was concentrated to give a pale yellow solid of 3b (2.78 g,
72%), m.p. 126e128 ꢀC. Anal. Calcd. for C18H19N3: C, 77.95; H, 6.90;
3. Conclusions
We have synthesized and characterized aluminum and zinc
complexes supported by two classes of pyrrole-based ligands. In
the presence of BnOH the aluminum complexes 4ae4c and 8 are
active catalysts for the ROP of 3-CL. Complex 8 exhibits much higher
catalytic activity than 4ae4c. Kinetic studies reveal that each of the
polymerization reactions catalyzed by complexes 4a and 8 follows
first-order kinetics in the concentration of monomer and gives
polymers with relatively narrow molecular weight distributions,
which indicate that the polymerizations proceed in a controlled
manner.
N, 15.15. Found: C, 77.78; H, 7.02; N, 15.05. 1H NMR (CDCl3):
d 0.86
(s, 9H, But), 5.97 (dd, J ¼ 4.5, 8.4 Hz, 1H, Ar), 6.30 (d, J ¼ 3.9 Hz, 1H,
Ar), 6.63 (d, J ¼ 8.1 Hz, 1H, Ar), 6.81e6.86 (m, 1H, Ar), 6.91e6.98 (m,
2H, Ar), 7.15 (d, J ¼ 6.9 Hz, 1H, Ar), 7.89 (dd, J ¼ 1.8, 4.5 Hz, 1H, Ar),
8.37 (s, 1H, N]CH). 13C NMR (C6D6):
d 31.23, 111.82, 114.17, 121.29,
122.33, 124.67, 128.03, 130.00, 136.48, 146.02, 146.87.
4. Experimental
4.2.3. Synthesis of LAlMe2 (4a)
To a stirred solution of 3a (0.221 g, 1 mmol) in toluene (10 ml)
was added dropwise AlMe3 (0.5 ml, a 2.3 M solution in hexane,
1.15 mmol) at room temperature. The mixture was stirred overnight
at that temperature. Solvent was removed under vacuum. The
residue was dissolved in Et2O and then filtered. The filtrate was
concentrated under vacuum to yield colorless crystals of 4a (0.22 g,
79%), m.p. 197e198 ꢀC. Anal. Calcd. for C16H16N3Al: C, 69.30; H,
5.82; N, 15.15. Found: C, 69.15; H, 5.88; N, 15.02. 1H NMR (C6D6):
4.1. General remarks
All air- or moisture-sensitive manipulations were performed
under nitrogen atmosphere using standard Schlenk and vacuum
line techniques. Solvents were distilled under nitrogen over sodium
(toluene), sodium/benzophenone (n-hexane, THF and Et2O) or CaH2
(CH2Cl2) and degassed prior to use. Compounds 2a [40], 2b [41] and
6 [42] were prepared according to the procedures described in
literature. R3Al (R ¼ Me, Et) and Et2Zn were purchased from Alfa-
Aesar and used as received. CDCl3 and C6D6, purchased from
d
ꢁ0.01 (s, 6H, Me), 6.66e6.71 (m,1H, Ar), 6.87 (d, J ¼ 7.5 Hz,1H, Ar),
7.01 (d, J ¼ 8.4 Hz, 1H, Ar), 7.09e7.14 (m, 2H, Ar), 7.40 (d, J ¼ 8.4 Hz,
1H, Ar), 7.74 (s, 1H, Ar), 7.98 (s, 1H, N]CH), 8.51 (d, J ¼ 8.4 Hz, 1H,

S. Qiao et al. / Journal of Organometallic Chemistry 696 (2011) 2746e2753
2751
Ar). 13C NMR (C6D6):
128.59, 129.36, 131.15, 134.38, 137.96, 140.05, 145.64, 145.68, 147.86.
d
ꢁ8.13, 106.71, 123.62, 126.34, 127.88, 127.94,
11.57 mmol), 4 Å molecular sieves (10 g), toluene (30 ml) and five
drops of HCOOH was refluxed for 6 h. The resulting mixture was
cooled to room temperature and then filtered. The molecular
sieves were washed with CH2Cl2 and the combined organic phase
was distilled to dryness by rotatory evaporation. The residue was
dissolved in Et2O. Concentration of the solution gave colorless
crystals of 7 (2.08 g, 74%), m.p. 171e172 ꢀC. Anal. Calcd. for
C16H16N4: C, 72.70; H, 6.10; N, 21.20. Found: C, 72.31; H, 6.23; N,
4.2.4. Synthesis of L0AlMe2 (4b)
Complex 4b was synthesized using the same procedure as for
4a. Thus, reaction of 3b (0.277 g, 1 mmol) with AlMe3 (0.5 ml,
a 2.3 M solution in hexane, 1.15 mmol) in toluene (10 ml) at room
temperature afforded, after recrystallizing from Et2O, colorless
crystals of complex 4b (0.17 g, 51%), m.p. 225e226 ꢀC. Anal. Calcd.
for C20H24N3Al,0.2Et2O: C, 71.74; H, 7.53; N, 12.07. Found: C, 71.79;
21.26. 1H NMR (CDCl3):
d 2.03 (s, 3H, Me), 2.28 (s, 3H, Me), 5.90 (s,
1H, Ar), 6.26 (s, 1H, Ar), 6.62 (s, 1H, Ar), 6.93 (s, 1H, Ar), 7.15 (d,
H, 7.35; N, 12.06. 1H NMR (C6D6):
d
ꢁ0.33 (s, 6H, Me), 1.35 (s, 9H,
J ¼ 7.8 Hz, 1H, Ar), 7.23e7.28 (m, 1H, Ar), 7.40 (d, J ¼ 7.5 Hz, 2H,
But), 6.30e6.35 (m, 2H, Ar), 6.50 (d, J ¼ 7.8 Hz, 1H, Ar), 6.64
(d, J ¼ 8.4 Hz, 1H, Ar), 6.73e6.79 (m, 2H, Ar), 7.03e7.06 (m, 1H, Ar),
Ar), 8.11 (s, 1H, N]CH). 13C NMR (CDCl3):
d 11.66, 13.65, 105.15,
110.54, 116.75, 119.83, 123.18, 125.72, 128.86, 129.66, 133.18,
140.87, 143.62, 148.30, 150.79.
7.54 (s, 1H, N]CH), 8.07e8.09 (m, 1H, Ar). 13C NMR (C6D6):
d
ꢁ3.07,
31.85, 35.17, 111.51, 117.10, 120.64, 122.67, 125.94, 129.18, 136.92,
138.17, 139.66, 141.20, 145.92, 146.14, 168.76.
4.2.9. Synthesis of [L002AlMe] (8)
To a stirred solution of 7 (0.264 g, 1 mmol) in toluene (10 ml)
was added AlMe3 (0.44 ml, a 2.3 M solution in hexane, 1.01 mmol)
at room temperature and then stirred overnight. Solvent was
removed under vacuum. The residue was dissolved in Et2O and
filtered. Concentration of the filtrate afforded colorless crystals of
8 (0.18 g, 63%), m.p. 248e249 ꢀC. Anal. Calcd. for C33H33N8Al$Et2O:
C, 69.14; H, 6.74; N, 17.43. Found: C, 68.83; H, 6.86; N, 17.78. 1H
4.2.5. Synthesis of LAlEt2 (4c)
Complex 4c was synthesized using the same procedure as for 4a.
Thus, reaction of 3a (0.221 g, 1 mmol) with AlEt3 (0.66 ml, a 1.82 M
solution in hexane, 1.2 mmol) in toluene (10 ml) yielded colorless
crystalline solid of 4c (0.26 g, 85%), m.p. 145e146 ꢀC. Anal. Calcd. for
C18H20N3Al: C, 70.80; H, 6.60; N, 13.76. Found: C, 70.52; H, 6.75; N,
13.59. 1H NMR (C6D6):
d
0.47e0.71 (m, 4H, CH2), 1.44 (t, J ¼ 8.1 Hz,
NMR (C6D6):
d
ꢁ0.21 (s, 3H, AlMe), 1.11 (t, J ¼ 6.9 Hz, Et2O), 1.81 (s,
6H, Me), 6.68e6.72 (m, 2H, Ar), 6.88 (d, J ¼ 7.5 Hz, 1H, Ar), 7.02
(d, J ¼ 8.1 Hz, 1H, Ar), 7.10e7.15 (m, 2H, Ar), 7.42 (d, J ¼ 8.1 Hz, 1H,
Ar), 7.79 (s, 1H, Ar), 8.00 (s, 1H, N]CH), 8.55 (d, J ¼ 4.8 Hz, 1H, Ar).
6H, Me), 2.31 (s, 6H, Me), 3.26 (q, J ¼ 6.9 Hz, Et2O), 5.72 (s, 2H, Ar),
6.17 (s, 2H, Ar), 6.63 (d, J ¼ 2.7 Hz, 2H, Ar), 6.80 (t, J ¼ 7.5 Hz, 2H,
Ar), 6.92 (t, J ¼ 8.1 Hz, 2H, Ar), 6.96 (s, 2H, Ar), 7.26 (d, J ¼ 7.8 Hz,
2H, Ar), 7.48 (s, 2H, Ar), 7.74 (d, J ¼ 8.1 Hz, 2H, Ar). 13C NMR
13C NMR (C6D6):
123.16, 129.27, 137.55, 142.58, 146.92, 148.54, 164.18.
d
ꢁ13.85, 11.00, 112.11, 118.43, 121.61, 122.61,
(C6D6):
d
ꢁ9.63, 11.57, 14.06, 15.88, 66.21, 107.25, 116.01, 122.20,
126.14, 127.15, 130.10, 131.31, 132.85, 138.15, 141.02, 143.38, 150.34,
157.80.
4.2.6. Synthesis of L2Zn (5a)
To a stirred solution of 3a (0.221 g, 1 mmol) in toluene (15 ml)
was added dropwise ZnEt2 (1.24 ml, a 0.875 M solution in hexane,
1.09 mmol) at 0 ꢀC. The resulting mixture was warmed to ambient
temperature and stirred for 16 h. Solvent was removed and the
residue was dissolved in Et2O. The resultant solution was filtered
and the filtrate was concentrated to generate yellow crystals of 5a
(0.21 g, 83%), m.p. 217e218 ꢀC. Anal. Calcd. for C28H20N6Zn: C,
66.48; H, 3.98; N, 16.61. Found: C, 66.71; H, 4.07; N, 16.42. 1H NMR
Single crystals for X-ray diffraction were obtained by recrystal-
lization the sample in benzene.
4.2.10. Synthesis of [ZnL002] (9)
To a stirred solution of 7 (0.264 g, 1 mmol) in toluene (10 ml)
was added dropwise ZnEt2 (1.2 ml, a 0.875 M solution in hexane,
1.05 mmol) at 0 ꢀC. The mixture was warmed to room tempera-
ture and stirred overnight. Solvent was removed in vacuo and the
residue was dissolved in Et2O. The resulting solution was filtered
and the filtrate was concentrated to form colorless crystals of 9
(0.22 g, 74%), m.p. 178e179 ꢀC. Anal. Calcd. for C32H30N8Zn: C,
64.92; H, 5.11; N, 18.93. Found: C, 64.75; H, 5.33; N, 18.73. 1H NMR
(C6D6):
d
5.90 (dd, J ¼ 4.2, 8.1 Hz, 2H, Ar), 6.22 (d, J ¼ 3.6 Hz, 2H, Ar),
6.67 (d, J ¼ 8.1 Hz, 2H, Ar), 6.80e6.87 (m, 4H, Ar), 6.92e7.00 (m, 4H,
Ar), 7.14 (d, J ¼ 7.5 Hz, 2H, Ar), 7.76e7.80 (m, 2H, Ar), 8.45 (s, 2H, N]
CH). 13C NMR (C6D6):
d 111.99, 115.98, 121.74, 121.95, 121.16, 126.03,
129.67, 129.94, 136.58, 139.61, 141.33, 142.15, 146.90, 147.82.
(C6D6): d 1.59 (s, 6H, Me), 2.13 (s, 6H, Me), 5.59 (s, 2H, Ar), 6.49
(dd, J ¼ 1.8, 3.6 Hz, 2H, Ar), 6.69 (t, J ¼ 7.8 Hz, 2H, Ar), 6.77 (t,
J ¼ 7.5 Hz, 2H, Ar), 6.84 (d, J ¼ 3.6 Hz, 2H, Ar), 6.95 (d, J ¼ 8.1 Hz,
2H, Ar), 7.04 (d, J ¼ 7.5 Hz, 2H, Ar), 7.09 (s, 2H, Ar), 7.56 (s, 2H, N]
4.2.7. Synthesis of L02Zn (5b)
The synthesis of complex 5b was carried out using a similar
procedure to that described for complex 5a. Thus, a mixture of 3b
(0.277 g, 1 mmol), ZnEt2 (1.25 ml, a 0.875 M solution in hexane,
1.09 mmol) and toluene (20 ml) was stirred at room temperature
for 16 h. The resulting solution was filtered and the filtrate was
concentrated to afford yellow crystals of 5b$2PhCH3 (0.19 g, 47%),
m.p. 124e125 ꢀC. Anal. Calcd. for C36H36N6Zn$2PhCH3: C, 74.84; H,
6.53; N, 10.47. Found: C, 75.02; H, 6.51; N, 10.47. 1H NMR (C6D6):
CH). 13C NMR (C6D6):
d 11.85, 14.19, 107.12, 115.30, 121.51, 124.49,
125.33, 129.17, 129.55, 133.10, 138.56, 138.79, 141.23, 144.78,
150.56, 158.03.
4.3. X-ray crystallography
Single crystals of complexes 4b, 5b and 8 were respectively
mounted in Lindemann capillaries under nitrogen. Diffraction data
were collected at 298(2) K on a Bruker Smart CCD area-detector
d
1.19 (s, 18H, tBu), 6.32 (dd, J ¼ 4.2, 8.1 Hz, 2H, Ar), 6.64
(d, J ¼ 3.9 Hz, 2H, Ar), 6.97 (d, J ¼ 8.1 Hz, 2H, Ar), 7.16e7.21 (m, 2H,
Ar), 7.27 (d, J ¼ 8.1 Hz, 2H, Ar), 7.31 (d, J ¼ 3.9 Hz, 2H, Ar), 7.49
(d, J ¼ 7.8 Hz, 2H, Ar), 8.22e8.24 (m, 2H, Ar), 8.70 (s, 2H, N]CH).
with graphite-monochromated Mo Ka radiation (
l
¼ 0.71073 Å).
Unit-cell dimensions were obtained with least-squares refinement.
Data collection and reduction were performed using the SMART
software [43]. Absorption corrections were applied using the
SADABS program [44]. The structures were solved by direct
methods using SHELXS-97 [45] and refined against F2 by full-
matrix least-squares using SHELXL-97 [46]. Hydrogen atoms were
placed in calculated positions. Crystal data and experimental
details of the structure determinations are listed in Table 3.
13C NMR (C6D6):
d 31.00, 34.14, 111.59, 113.94, 121.05, 122.10,
124.44, 126.03, 127.84, 128.90, 129.67, 136.25, 139.50, 145.83,
146.64, 166.41.
4.2.8. Synthesis of HL00 (7)
A mixture of 2-(3,5-dimethyl-1H-pyrazol-1-yl)benzenamine
(2.00 g, 10.68 mmol), 1H-pyrrole-2-carbaldehyde (1.10 g,

2752
S. Qiao et al. / Journal of Organometallic Chemistry 696 (2011) 2746e2753
Table 3
Details of the X-ray structure determinations of complexes 4b, 5b and 8.
4b
5b$2PhMe
8$C6H6
Formula
fw
C20H24N3Al
333.40
C50H52N6Zn
802.35
C39H39N8Al
646.76
Crystal system
Space group
a (Å)
b (Å)
c (Å)
Orthorhombic
P2(1)2(1)2(1)
8.6567(9)
16.7879(16)
26.157(2)
90
90
90
3801.3(6)
8
1.165
Triclinic
P ꢁ 1
Triclinic
P ꢁ 1
13.3250(14)
13.6230(16)
14.468(2)
108.526(2)
106.686(2)
107.5660(10)
2150.5(5)
2
8.6826(12)
15.3608(16)
15.5650(18)
69.6580(10)
77.430(2)
79.682(2)
1887.6(4)
2
a
b
g
(deg)
(deg)
(deg)
V (Å3)
Z
Dcalcd (g/cm3)
F (000)
1.239
848
0.612
1.138
684
0.091
1424
0.112
m
(mmꢁ1
)
q
range for data collecn (deg)
1.44e25.01
19914
6711(0.0587)
0/489
1.64e25.01
11493
7488(0.0515)
0/618
1.42e25.00
10038
6552(0.0373)
0/438
No. of reflns collected
No. of indep reflns (Rint
Restraints/params
)
Goodness of fit on F2
1.000
1.032
1.056
Final R indices [I > 2
s
(I)]a
R1 ¼ 0.0481
wR2 ¼ 0.0751
R1 ¼ 0.1110
wR2 ¼ 0.0879
0.133 and ꢁ0.237
R ¼ 0.0679
wR2 ¼ 0.1416
R1 ¼ 0.1551
wR2 ¼ 0.1644
0.524 and ꢁ0.379
R1 ¼ 0.0757
wR2 ¼ 0.1966
R1 ¼ 0.1539
wR2 ¼ 0.2305
0.928 and ꢁ0.423
R indices (all data)
Largest diff peak and hole [e Åꢁ3
]
P
P
P
P
R1 ¼
jjFoj ꢁ jFcjj= jFoj; wR2 ¼ ½ wðFo2 ꢁ Fc2Þ2= wðFo4Þꢂ1=2
.
a
4.4. Polymerization of
4ae4c and 8
3-CL catalyzed by aluminum complexes
html, or from the Cambridge Crystallographic Data Center, 12
Union Road, Cambridge CB2 1EZ, UK; fax: t44-1223-336033; or
e-mail: deposit@ccdc.cam.ac.uk.
A typical polymerization procedure is exemplified by the
synthesis of PCL using complex 4a as a catalyst in the presence of
benzyl alcohol (Table 1, entry 2). Complex 4a (0.042 g, 0.151 mmol)
and toluene (15 ml) were added successively into a Schlenk tube.
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We thank the National Natural Science Foundation of China
(Grant no. 20972146) and the Foundation for Talents of Anhui
Province (Grant no. 2008Z011) for financial support. We also thank
Prof. D.-Q. Wang for the crystal structure determinination and
Ms. W.-Q. Lu for the GPC analysis.
Appendix A. Supplementary data
CCDC 806572-806574 contain the supplementary crystallo-
graphic data for complexes 4b, 5b and 8. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.

S. Qiao et al. / Journal of Organometallic Chemistry 696 (2011) 2746e2753
2753
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