Synthesis, characterization, and catalytic activities of rare-earth metal complexes with iminopyrrolyl ligands

Article abstract of DOI:10.1016/j.inoche.2011.04.015

A series of rare-earth metal complexes with substituted iminopyrrolyl ligands were prepared and characterized. Reactions of [(Me₃Si) ₂N]₃Ln(μ-Cl)Li(THF)₃ with 3 equiv. of 2-(2,6-Et₂C₆H₃N CH)C₄H₃NH (1a) in toluene generated tris-iminopyrrolyl rare-earth metal complexes with formula [2-(2,6-Et₂C₆H₃N CH)C₄H ₃N]₃Ln(THF)_n [n = 0, Ln = Y (2a), Eu (2b); n = 1, Ln = Nd (2c), Sm (2d)] in moderate yields. Treatments of [(Me ₃Si)₂N]₃Ln(μ-Cl)Li(THF)₃ with 3 equiv. of 2-(2,4,6-Me₃C₆H₂N CH)C ₄H₃NH (1b) in toluene afforded tris-iminopyrrolyl rare-earth metal complexes with formula [2-(2,4,6-Me₃C ₆H₂N CH)C₄H₃N]₃Ln(THF) [Ln = Y (3a), Sm (3b), Eu (3c)]. The catalytic properties of all rare-earth metal complexes on the ring-opening polymerization of ε-caprolactone have been studied. The results represent rare example of the pyrrolyl ligands as initiators for ε-caprolactone polymerization.

Full text of DOI:10.1016/j.inoche.2011.04.015

Inorganic Chemistry Communications 14 (2011) 1196–1200
Contents lists available at ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Synthesis, characterization, and catalytic activities of rare-earth metal complexes
with iminopyrrolyl ligands
Shuangliu Zhou a, Chengwei Yin a, Hui Wang a, Xiancui Zhu a, Gaosheng Yang a, Shaowu Wang a,b,
Laboratory of Functionalized Molecular Solids, Ministry of Education, Anhui Laboratory of Molecule-Based Materials, Institute of Organic Chemistry,
⁎
a
School of Chemistry and Materials Science, Anhui Normal University, Wuhu, Anhui 241000, China
b
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A series of rare-earth metal complexes with substituted iminopyrrolyl ligands were prepared and characterized.
Reactions of [(Me3Si)2N]3Ln(μ-Cl)Li(THF)3 with 3 equiv. of 2-(2,6-Et2C6H3N CH)C4H3NH (1a) in toluene
generated tris-iminopyrrolyl rare-earth metal complexes with formula [2-(2,6-Et2C6H3N CH)C4H3N]3Ln(THF)n
[n=0, Ln=Y (2a), Eu (2b); n=1, Ln=Nd (2c), Sm (2d)] in moderate yields. Treatments of [(Me3Si)2N]3Ln(μ-Cl)
Li(THF)3 with 3 equiv. of 2-(2,4,6-Me3C6H2N CH)C4H3NH (1b) in toluene afforded tris-iminopyrrolyl rare-earth
metal complexes with formula [2-(2,4,6-Me3C6H2N CH)C4H3N]3Ln(THF) [Ln=Y (3a), Sm (3b), Eu (3c)]. The
catalytic properties of all rare-earth metal complexes on the ring-opening polymerization of ε-caprolactone have
been studied. The results represent rare example of the pyrrolyl ligands as initiators for ε-caprolactone
polymerization.
Received 23 February 2011
Accepted 15 April 2011
Available online 23 April 2011
Keywords:
Rare-earths
Catalyst
Iminopyrrolyl
Ring-opening polymerization
ε-Caprolactone
© 2011 Elsevier B.V. All rights reserved.
The ring-opening polymerization (ROP) of lactones is a convenient
method for the synthesis of biodegradable polyesters such as poly(ε-
caprolactone) (PCL), which has been attracting considerable attention
due to its biocompatibility, biodegradability, and good drug penetrability
[1]. Various catalysts including alkali metals [2], alkaline earth metals [3],
aluminum [4], zinc [5], tin [6], early transition metals [7], and lanthanides
[8] have been developed for the ROP of ε-caprolactone. Among these,
rare-earth metal catalysts have received a great interest due to their high
activities and capabilities of giving polymers with high molecular
weights in narrow polydispersity. A diversity of structurally well-defined
rare-earth metal catalysts, such as divalent [9] and trivalent lanthanide
metallocene complexes [10] and trivalent lanthanide alkoxides [11], are
typical examples of the rare-earth metal initiators for living polymer-
ization of lactones.
In our previous study, we have demonstrated that the lanthanide(II)
complexes with substituted indenyl ligands and ethylenebis(indenyl)
lanthanide(III) amides showed high catalytic activities and good
controlled polymerization for the ROP of ε-caprolactone [12]. As tunable
ancillary ligands, pyrrolyl-based ligands have shown particular versa-
tility in organolanthanide chemistry as possible alternatives to the
traditionally used cyclopentadienyl-based systems, owing to η5/k1
bonding capability of the pyrrolyl moiety [13]. The rare-earth metal
alkyl complexes bearing an η5- or η1-coordinated pyrrolide moiety
exhibited a high activity toward polymerization of methyl methacrylate
[14], isoprene and lactide [15] have been reported. Hou et al. reported
that the rare-earth metal aminobenzyl complexes with η5-coordinated
pyrrolide showed a high catalytic activity and stereocontrol for the
polymerization of styrene, however, η1-coordinated pyrrolyl rare-earth
metal aminobenzyl complexes exhibited no catalytic activity for the
styrene polymerization [16]. Mashima and co-workers also demon-
strated that the rare-earth metal amides with pyrrolyl ligands showed
high catalytic activities on the ROP of ε-caprolactone, but homoleptic
tris(pyrrolyl) yttrium complexes had no activity, suggesting that a Y\N
(pyrrolyl) bond could not initiate the ε-caprolactone polymerization
[17]. We herein report the synthesis, characterization, and structure
of the rare-earth metal complexes with the iminopyrrolyl ligands
and their catalytic activities on the ROP of ε-caprolactone. The results
represent rare examples of the ROP of ε-caprolactone initiated by
pyrrolyl groups.
The tris-iminopyrrolyl rare-earth metal complexes [2-(2,6-Et2C6
H3N CH)C4H3N]3Ln(THF)n [n=0, Ln=Y (2a), Eu (2b); n=1, Ln=Nd
(2c), Ln=Sm (2d)] and [2-(2,4,6-Me3C6H2N CH)C4H3N]3Ln(THF)
[Ln=Y (3a), Sm (3b), Eu (3c)] were prepared in good yields by
reactions of [(Me3Si)2N]3Ln(μ-Cl)Li(THF)3 with 2-(2,6-Et2C6H3N CH)
C4H3NH (1a) and 2-(2,4,6-Me3C6H2N CH)C4H3NH (1b) (Scheme 1).
The complexes are sensitive to air and moisture, they have a good
solubility in either polar solvents or nonpolar solvents. All complexes
were fully characterized by IR and elemental analyses. The complexes
2a and 3a were also characterized by 1H NMR spectra analyses. The 1H
NMR spectra of complexes 2b, 2c, 2d, 3b and 3c were not informative
due to lack of locking signal for the paramagnetic property of the
complexes. Therefore, the structures of the complexes 2a–2d, 3a and
3b were determined by single-crystal X-ray analyses [18].
⁎
Corresponding author at: Laboratory of Molecule-Based Materials, Institute of
Organic Chemistry, School of Chemistry and Materials Science, Anhui Normal
University, Wuhu, Anhui 241000, China. Fax: +86 553 5910027.
E-mail address: swwang@mail.ahnu.edu.cn (S. Wang).
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.04.015

S. Zhou et al. / Inorganic Chemistry Communications 14 (2011) 1196–1200
1197
Ar
N
N
N
toluene
100 oC
Ln
N
N
H
+ [(Me3Si)2N]3Ln( -Cl)Li(THF)3
Ar
Ar
N
N
N
Ar
(THF)n
Ar = 2,6-Et2C6H3, n = 0, Ln = Y(2a), Eu(2b);
Ar = 2,6-Et2C6H3, n = 1, Ln = Nd(2c), Sm(2d)
Ar = 2,4,6-Me3C6H2, n = 1, Ln = Y(3a), Sm(3b), Sm(3c)
Scheme 1. Synthesis of lanthanide complexes with iminopyrrolyl ligands.
X-ray analyses showed that all complexes were a monomeric
structure and the central metal was supported by three iminopyrrolyl
ligands. Each iminopyrrolyl ligand was coordinated to rare-earth
metal in η1:κ1 mode. No η5-bonding mode of the pyrrolyl ring has
been observed. The complexes 2a (as a representative showed in
Fig. 1) and 2b adopted a six-coordinate octahedral geometry, and
three pyrrolyl ligands located in almost perpendicular positions
coordinate to the central metal in an η1:κ1-N,N bidentate fashion. The
molecular structures of complexes 2c (as a representative showed in
Fig. 2) and 2d adopted a distorted pentagonal bipyramidal geometry
containing three iminopyrrolyl ligands and solvated THF molecules,
which have a symmetric face passing through one pyrrolyl cycle of
pyrrolide ligand and oxygen atom of THF, indicating that the size of
rare-earth metal ions has effects on the coordination environment of
the metal, large rare-earth metal ions gave high coordination
complexes. In our cases, the large Nd and Sm ions gave the THF
solvated structures. The molecular structures of complexes 3a (as a
representative showed in Fig. 3) and 3b were a distorted pentagonal
bipyramidal geometry containing three iminopyrrolyl ligands and
solvated THF molecules. In comparison with complexes 2a–2d, the
ligand in complexes 3a–3c is smaller than the ligand in complexes 2a–
2d, so there is a solvated THF in cases of 3a–3c. The selected bond
lengths and angles are listed in Table 1.
From Table 1, we can see that the bond distances between the
metal and the chelate nitrogen of the pyrrolyl ring Ln-Nav(pyr) are
shorter than those of the metal and the imino nitrogen Y–Nav(imino)
in all complexes 2a–2d, and 3a–3b. The average Y–N distance of the
metal and the nitrogen of the pyrrolyl ring (2.357(6) Å) found in 2a is
comparable to the corresponding values of 2.403(6) Å found in 2b,
2.483(4) Å found in 2c, 2.455(4) Å found in 2d, if the difference of the
metal ionic radii is considered [19]. The average Y–N distance of the
metal and the nitrogen of the pyrrolyl ring (2.428(7) Å) found in 3a is
longer than that of 2.357(6) Å found in 2a, due to the combination
effects of steric effects and different coordination numbers of the
central metal. The average Y–N distance between the metal and the
nitrogen of the pyrrolyl ring (2.357(6) Å) found in 2a is slightly longer
than those of 2.325(2) Å found in Y[2-(2,6-iPr2C6H3N CH)C4H3N]3
[15a], 2.340(2) Å found in tris-(pyrrolyl) yttrium complex Y(An2-
pyr)3[16], and 2.339(6) Å found in tris-(pyrrolyl) yttrium complex
Y(p-Tol2-pyr)3[16]. Similarly, the average Y–N distance between the
Fig. 1. Molecular structure of [2-(2,6-Et2C6H3N CH)C4H3N]3Y (2a). Hydrogen atoms are omitted for clarity.

1198
S. Zhou et al. / Inorganic Chemistry Communications 14 (2011) 1196–1200
complex 2d (O–Sm–N(4), 153.63(12)°), but is slightly smaller than
those of 3a (O–Y–N(2), 160.4(2) °), and 3b (O–Sm–N(4), 159.78
(10)°).
The complex 2a as a single-component catalyst for ε-caprolactone
polymerization was systematically examined in toluene and THF
under the different conditions (Table 2). From Table 2, it was found
that complex 2a exhibited good catalytic activities on the ROP of
ε-caprolactone in toluene and THF. The polymerization initiated by
complex 2a was carried out at 60 °C to reach almost complete
conversion in 0.5 min in either THF or toluene. The results were
different from the ROP of ε-caprolactone catalyzed by the η1:κ1:η1-N,
N,N tridentate pyrrolyl yttrium complexes Y(An2-pyr)3 and Y(p-Tol2-
pyr)3[17], which were less active than the yttrium amides towards
ROP of lactide. The results also represented rare examples of the ring-
opening polymerization of ε-caprolactone initiated by the pyrrolyl
groups. The pyrrolyl group of rare-earth metal complex inserted into
the coordinated ε-caprolactone to initiate the polymerization to give
the alkoxyl group linked intermediate, which then propagated to give
polymers (see Scheme S1 in supplementary materials). The molecular
weight of the resulting PCL increased with the increase of monomer-
to-initiator ratio. It was also found that the molecular weights of the
polymers generally increase as the polymerization temperatures
increase, indicating that chain propagation process be favored at
relatively high temperatures. The relatively narrow molecular weight
distributions of PCL were also observed using the complex 2a as a
catalyst.
Fig. 2. Molecular structure of [2-(2,6-Et2C6H3N CH)C4H3N]3Nd⋅OC4H8 (2c). Hydrogen
atoms are omitted for clarity.
To examine the steric effect of the substituent on the iminopyrrolyl
ligands and the effect of the central metals on the catalytic activities of
the complexes, complexes 2b–2d, 3a–3c toward ROP of ε-caprolac-
tone were also examined, results are listed in Table 3. Experimental
results showed that all complexes efficiently initiated the polymer-
ization of ε-caprolactone. In comparison, the molecular weights of
PCL, obtained from the complexes 3a–3c, were higher than those
obtained from the complexes 2a–2d, indicating the steric effect of
ligands on the chain propagation process. In order to further explore
the application of the good controllable catalysts 3a–3c, We examined
the complexes 3a–3c as a single-component catalyst on ε-caprolac-
tone polymerization in toluene and THF at 30 °C (Table 3, entries 15–
23). Results showed that complexes 3a–3c also had high catalytic
activities on ROP of ε-caprolactone at 30 °C. It was noteworthy that
metal and the imino nitrogen (2.438(6) Å) found in 2a is shorter than
those of 2.472(6) Å found in 2b, 2.582(4) Å found in 2c, and 2.562(4)
Å found in 2d. The average Y–N(imino) distances of 2.438(6) Å found
in 2a is shorter than those of 2.530(6) Å found in 3a, and 2.484(2) Å
found in Y[2-(2,6-iPr2C6H3N CH)C4H3N]3 [15a].
The N–Y–N angles (68.90(19)°,71.58(17)° and 71.92(19)°) in
complex 2a are comparable to those found in Y[2-(2,6-iPr2C6H3N CH)
C4H3N]3 (N–Y–N 71.35(8)°, 71.26(8)°, 71.11(8)°) [15a] and complex
2b (N–Eu–N, 67.42(19)°, 70.05(19)°, 70.63(19)°). The acute N–Ln–N
angles of THF solvated tris-iminopyrolyl lanthanide complexes in 2c
(average 67.26(12)°), 2d (average 67.88(13)°), 3a (average 68.57
(20)°), and 3b (average 68.52(11)°) are comparable. The O–Nd–N(3)
angle (153.94(11)°) in complex 2c is comparable to that found in
Fig. 3. Molecular structure of [2-(2,4,6-Me3C6H2N CH)C4H3N]3Y·OC4H8 (3a). Hydrogen atoms are omitted for clarity.

S. Zhou et al. / Inorganic Chemistry Communications 14 (2011) 1196–1200
1199
Table 1
Selected bond length (Å) and bond angle (°).
2a
2b
2c
2.637(3)
2.484(2)
2.480(4)
2d
2.614(3)
2.458(3)
2.449(4)
3a
3b
2.552(3)
2.450(3)
2.438(3)
2.448(3)
Ln-O
Ln-N(1)
Ln-N(3)
Ln-N(5)
2.532(6)
2.409(6)
2.452(7)
2.423(7)
2.351(6)
2.352(5)
2.367(5)
2.409(6)
2.399(5)
2.401(5)
Ln-N(1A)
Ln-Nav(pry)
Ln-N(2)
Ln-N(4)
Ln-N(6)
2.484(2)
2.483(4)
2.600(2)
2.545(4)
2.458(3)
2.455(4)
2.583(3)
2.520(4)
2.357(6)
2.513(5)
2.407(5)
2.394(6)
2.403(6)
2.545(5)
2.444(5)
2.426(6)
2.428(7)
2.497(6)
2.564(6)
2.529(6)
2.445(3)
2.566(3)
2.518(3)
2.580(3)
Ln-N(2A)
2.600(2)
2.582(4)
67.43(8)
66.92(12)
2.583(3)
2.562(4)
67.95(9)
67.75(13)
Ln-Nav(imino)
N(1)–Ln–N(2)
N(3)–Ln–N(4)
N(5)–Ln–N(6)
N(1A)–Ln–N(2A)
N(1)–Ln–N(3)
N(3)–Ln–N(6)
N(4)–Ln–N(5)
O–Ln–N(1)
O–Ln–N(2)
O–Ln–N(3)
O–Ln–N(4)
O–Ln–N(5)
O–Ln–N(6)
2.438(6)
2.472(6)
2.530(6)
69.1(2)
68.7(2)
68.4(2)
2.555(3)
68.90(19)
71.58(17)
71.92(19)
67.42(19)
70.05(19)
70.63(19)
68.53(11)
69.08(10)
67.94(11)
67.43(8)
67.95(9)
83.8(2)
83.98(19)
92.52(17)
84.2(2)
84.1(2)
93.27(18)
84.31(6)
77.32(8)
153.94(11)
139.14(12)
77.37(9)
84.11(7)
138.62(13)
153.63(12)
130.5(2)
160.4(2)
76.7(3)
86.3(2)
83.7(2)
80.3(2)
76.26(11)
87.11(10)
131.11(10)
159.78(10)
83.09(10)
80.57(10)
the molecular weight distributions of the polymers obtained using
complexes 3b and 3c as catalysts were relatively narrow (ranging
from 1.04 to 1.06, Table 3, entries 7, 13, 17, 19, and 22–23), exhibiting
the “living” character of the polymerization process using catalysts 3b
and 3c. Furthermore, altering the monomer-to-initiator ratio ([M]/
[cat]), the complex 3c still showed high catalytic activities on ROP of
ε-caprolactone (Table 3, entries 21–24). The molecular weights of the
polymers increased as the monomer-to-initiator ratio increased, and
the molecular weight distributions of the polymers were relatively
narrow ranging from 1.04 to 1.07 except for the monomer-to-initiator
ratio of 1000:1. These results implied the “living” character of the
polymerization process using complex 3c as a catalyst at a monomer-
to-initiator ratio of 500:1 and 250:1.
In conclusion, a series of rare-earth metal complexes with tris-
iminopyrroly ligands were prepared through silylamine elimination
reaction of the rare-earth metal amides [(Me3Si)2N]3Ln(μ-Cl)Li(THF)3
with neutral imino-substituted pyrrole compounds. The X-ray
diffraction analyses of the complexes indicated that the iminopyrrolyl
ligands were coordinated to rare-earth metal in η1:κ1 mode. All rare-
earth metal complexes incorporating iminopyrroly ligands showed
high catalytic activities towards the ROP of ε-caprolactone to afford
polymers with a high molecular weight and a narrow molecular
weight distribution. The results represent rare examples of the ROP of
ε-caprolactone initiated by pyrrolyl group.
Acknowledgments
Financial support for this work from the National Natural Science
Foundation of China (20832001, 20802001, 21072004), and grants from
the Ministry of Education (20103424110001) and Anhui province
(TD200707, 2007Z016) is acknowledged.
Table 3
Ring-opening polymerization ofε-caprolactone catalyzed bycomplexes 2a–2d and 3a–3ca.
b
Entry Cat. Solvent
Tp
Time
Mn
×
Mw/Mn
Conv Activity×
(%)
(°C) (min)
10−4b
10−6c
1
2
3
4
5
6
7
8
2a
2b
2c
2d
3a
3b
3c
2a
2b
2c
2d
3a
3b
3c
3a
3b
3c
3a
3b
3c
3c
3c
3c
3c
Toluene 60
0.5
0.5
0.5
0.5
5
5.61
6.01
5.16
3.02
11.8
11.8
8.60
6.27
5.05
4.57
4.33
12.1
8.1
12.4
8.60
7.10
6.70
10.3
6.50
8.10
7.2
1.46
1.41
1.52
1.85
1.59
1.62
1.05
1.35
1.51
1.63
1.63
1.58
1.06
1.57
1.21
1.04
1.06
1.60
1.05
1.06
1.07
1.59
1.07
1.62
98
6.73
6.71
6.81
6.71
0.67
3.35
0.62
6.63
6.76
6.79
6.86
0.57
3.04
0.66
0.67
3.39
0.51
0.31
0.65
1.27
3.30
0.54
3.3
Toluene 60
Toluene 60
Toluene 60
Toluene 60
Toluene 60
Toluene 60
97
N99
98
98
98
91
96
98
N99
N99
83
89
97
99
99
75
89
95
99
99
79
99
98
1
5
THF
THF
THF
THF
THF
THF
THF
60
60
60
60
60
60
60
0.5
0.5
0.5
0.5
5
1
5
5
1
5
10
5
10
5
5
5
5
9
Table 2
10
11
12
13
14
15
16
17
18
19
20
21d
22e
23d
24e
Ring-opening polymerization of ε-caprolactone catalyzed by complex 2aa.
b
Entry [M]/[Cat] Solvent Tp
Time
Mn
×
Mw/Mn
Conv Activity×
(%)
(°C) (min) 10−4b
10−6c
1
2
3
4
5
6
7
8
250:1
250:1
250:1
500:1
500:1
500:1
250:1
250:1
250:1
500:1
500:1
500:1
THF
THF
THF
THF
THF
THF
Toluene 60
Toluene 30
Toluene
60
30
0
60
30
0
0.5
2
15
0.5
2
10
0.5
3
10
0.5
2
10
5.26
3.21
3.12
6.27
4.97
2.92
2.79
3.67
3.07
5.61
3.07
3.49
1.47
1.87
1.81
1.35
1.56
1.75
1.89
1.78
1.88
1.46
1.87
1.79
N99
98
84
96
87
66
99
90
79
98
91
71
3.43
0.84
0.23
6.63
1.49
0.22
3.39
0.32
0.14
6.73
1.57
0.25
Toluene 30
Toluene 30
Toluene 30
THF
THF
THF
THF
THF
30
30
30
30
30
11.8
7.6
11.7
9
0
Toluene 60
Toluene 60
10
11
12
Toluene 60
Toluene 30
Toluene
13.4
a
Conditions: [M]/[Cat]=500:1, ε-CL/solvent(v/v)=1:3. Tp: polymerization temperature.
Obtained from GPC analysis and calibrated by polystyrene standard.
Activity: g PCL/mol cat·h.
[M]/[Cat]=250:1.
[M]/[Cat]=1000:1.
0
b
c
a
Conditions: ε-CL/solvent(v/v)=1:3, Tp: polymerization temperature.
Obtained from GPC analysis and calibrated by polystyrene standard.
Activity: g PCL/mol cat·h.
b
c
d
e

1200
S. Zhou et al. / Inorganic Chemistry Communications 14 (2011) 1196–1200
(d) R.R. Gowda, D. Chakraborty, V. Ramkumar, Eur. J. Inorg. Chem. (2009)
Appendix A. Supplementary material
2981–2993.
[8] (a) Y.Q. Shen, Z.Q. Shen, Y.F. Zhang, K.M. Yao, Macromolecules 29 (1996)
8289–8295;
CCDC-761437(3b), 761438(3a), 761439(2b), 761440(2c), 761441
(2d) and 761442(2a) contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge from the
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif. Supplementary data associated with this article can
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