Syntheses and structures of tris-β-diketiminate lanthanide complexes and their high activity for ring-opening polymerization of ε-caprolactone and L-lactide

Article abstract of DOI:10.1002/ejic.200900313

Metathesis of {[(4-XC₆H₄)NC(Me)]₂CH}Li with anhydrous lanthanide trichlorides (LnCl₃) in a 3:1 molar ratio yielded a series of tris-β-diketiminate complexes with the general formula [LnL₃^X]

Full text of DOI:10.1002/ejic.200900313

FULL PAPER
DOI: 10.1002/ejic.200900313
Syntheses and Structures of Tris-β-Diketiminate Lanthanide Complexes and
Their High Activity for Ring-Opening Polymerization of ε-Caprolactone and
L
-Lactide
Mingqiang Xue,[a] Rui Jiao,[a] Yong Zhang,[a] Yingming Yao,[a] and Qi Shen*[a]
Keywords: Synthetic methods / Lanthanides / Lactones / Ring-opening polymerization / Metathesis
Metathesis of {[(4–XC6H4)NC(Me)]2CH}Li with anhydrous
lanthanide trichlorides (LnCl3) in a 3:1 molar ratio yielded a
series of tris-β-diketiminate complexes with the general for-
mula [LnLX3 ] [X = Cl, LCl: Ln = Pr (1), Nd (2), Sm (3); X = H,
L: Ln = Nd (4); X = Me, LMe: Ln = Nd (5)]. Reaction of sodium
salt of β-diketiminate with anhydrous LnCl3 in a 1:1 molar
ratio in thf afforded the corresponding β-diketiminate lantha-
nide dichlorides [LClPrCl2(thf)2] (6), [LMeNdCl2(thf)2] (7), and
1–5 were found to be very active single-component initiators
for the ring-opening polymerization of ε-caprolactone (ε-CL)
and L-lactide (L-LA) under mild conditions, giving polymers
with high molecular weight and moderate polydispersities,
whereas complexes 6–8 displayed no activity under the same
polymerization conditions. The high catalytic activity shown
by tris-β-diketiminate lanthanide complexes may be attrib-
uted to the activated Ln–β-diketiminate bond caused by the
crowded coordination sphere around the central metal.
iPr2
[LiPr NdCl2(thf)2] {L
= [(2,6-iPr2C6H3)NC(Me)]2CH–} (8),
2
respectively. Each of these complexes was characterized, and
the molecular structures of complexes 1–5 and 8 were deter- (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
mined by X-ray single-crystal structure analysis. Complexes
Germany, 2009)
crowded tris-β-diketiminate lanthanide complex. In this re-
gard, we are beginning to study the synthesis of tris-β-diket-
iminate lanthanide complexes and their reactivity.
Introduction
β-Diketiminate anions, the alternatives to cyclopen-
tadienyl ligands, have widely been used in coordination
chemistry and organometallic chemistry of lanthanide met-
als, as they can act as versatile spectator ligands by virtue
of their strong metal–ligand bonds and their exceptional
and tunable steric demands by variation of the substitu-
ents.[1,2] A variety of derivatives, such as alkylides,[3] al-
lylides,[4] amides,[2h,2k] borohydrides,[5] alkoxides,[6] and so
on, was synthesized and found to serve as highly active ini-
tiators in homogeneous catalyses. In these catalytic reac-
tions, β-diketiminate groups act as inert ancillary ligands to
stabilize an active species. In fact, the Ln–β-diketiminate
bond in β-diketiminate lanthanide dichloride was proven to
be inactive or much less active in the ring-opening poly-
merization of ε-caprolactone.[2h,2k]
Tris-β-diketiminate lanthanide complexes are rare; only
two kinds of complexes have been reported to date: one is
the complex bearing the β-diketiminate with a neighboring,
fused six-membered heterocyclic ring and the other is that
with N,N-diphenyl β-diketiminate ligands.[8] An attempt to
synthesize tris-β-diketiminate lanthanide complexes with
the bulky β-diketiminate group resulted in a series of novel
complexes containing a “normal” and a deprotonated li-
gand through ligand deprotonation induced by steric de-
mand.[2b] In this paper, we synthesized a series of tris-β-
diketiminate lanthanide complexes by using various β-di-
ketiminate anions (Scheme 1), including [LnLC3 l] {LCl = [4-
ClPhNC(Me)]2CH–; Ln = Pr (1), Nd (2), Sm (3)}, [NdL3]
{L = [PhNC(Me)]2CH–} (4), and [NdL3Me] {LMe = [4-
MePhNC(Me)]2CH–} (5), and tested their catalytic activity
for ring-opening polymerization of ε-caprolactone and -
lactide. The corresponding β-diketiminate lanthanide di-
chlorides, [LClPrCl2(thf)2] (6), [LMeNdCl2(thf)2] (7), and
Recently, it was explored by the Evans group that the
normally inert (C5Me5)– group in the sterically crowded
[(C5Me5)3Ln] shows remarkable sterically induced reacti-
vity.[7] We are interested in understanding the chemical be-
havior of the Ln–β-diketiminate bond in a sterically
iPr2
[LiPr NdCl2(thf)2] {L
= [(2,6-iPr2Ph)NC(Me)]2CH–} (8),
2
were also prepared for comparison with their catalytic ac-
tivity. It was found that tris-β-diketiminate lanthanide com-
plexes showed extremely high activity for ring-opening po-
lymerization of ε-caprolactone and -lactide, whereas the
corresponding dichlorides were inactive. The much higher
activity of these tris-β-diketiminate lanthanide complexes
compared to their dichlorides might be attributed to
[a] Key Laboratory of Organic Synthesis, Jiangsu Province, College
of Chemistry, Chemical Engineering and Materials Science,
Suzhou University,
Suzhou, 215123, P. R. China
Fax: +86-512-65880305
E-mail: qshen@suda.edu.cn
4110
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Lanthanide Complexes and Their Activity for Ring-Opening Polymerization
1
“steric-induced activation”. Here we would like to report did not provide any resolvable H NMR spectra; the reso-
these results. The effect of central metals and β-diketiminate nances are broad and shifted due to the strong paramag-
ligands was also discussed.
netic nature of the lanthanide ion.
All the complexes are moderately sensitive to air and
moisture, and the crystals can be exposed to air for a few
hours without apparent decomposition, but the color of the
solution changes immediately when the solution is exposed
to air. They are freely soluble in donor solvents such as
tetrahydrofuran (thf) and dimethoxyethane (dme) and mod-
erately soluble in toluene and sparingly soluble in hexane.
Scheme 1.
Synthesis and Characterization of [LClPrCl2(thf)2] (6),
[LMeNdCl2(thf)2] (7), and [LiPr2NdCl2(thf)2] {LiPr2 = [(2,6-
iPr2)PhNC(Me)]2CH–} (8)
Results and Discussion
The corresponding β-diketiminate lanthanide dichlorides
were also synthesized for comparison. Generally, the me-
tathesis reaction of LnCl3 with lithium salts often yields an
ate complex of β-diketiminate lanthanide dichloride with
LiCl, whereas the same reaction with sodium salts gives the
dichloride without NaCl conveniently. Thus, reaction of an-
hydrous lanthanide trichlorides with in situ formed sodium
salts of β-diketiminate in a 1:1 molar ratio in thf, after
workup, gave the corresponding β-diketiminate lanthanide
dichlorides, [LClPrCl2(thf)2] (6), [LMeNdCl2(thf)2] (7), and
Synthesis and Characterization of [LnLC3 l] {LCl
=
[4-ClPhNC(Me)]2CH–; Ln = Pr (1), Nd (2), Sm (3)},
[NdL3] {L = [PhNC(Me)]2CH–} (4), and [NdL3Me
{LMe = [4-MePhNC(Me)]2CH–} (5)
]
The β-diketiminate LCl was chosen as the ligand and a
series of complexes with different lanthanide metals [LnL
Cl] [Ln = Pr (1), Nd (2), Sm (3)] were synthesized in desired
3
yields by metathesis reaction of LnCl3 with the lithium salt
of LCl in a 1:3 molar ratio (Scheme 2).
[LiPr NdCl2(thf)2] (8) (Scheme 3), respectively, which were
2
characterized by elemental analyses and IR spectroscopy
and X-ray structural analysis for 8.
Scheme 3.
Scheme 2.
Complex [LSmCl2(thf)2] was synthesized according to a
literature procedure.[10] An attempt to determine the molec-
ular structures of 6 and 7 was unsuccessful because of the
decay of diffracted intensity of the crystals during data col-
lection. Complexes 6–8 are very soluble in thf and moder-
ately soluble in dme.
To examine the influence of the electronic effects of the
ligand on the reactivity of tris-β-diketiminate lanthanide
complexes, the β-diketiminates with a Me electron-donating
group and no substituent at the para-position on the phenyl
ring, LMe and L, were also used as ligands. Thus, the same
reaction of NdCl3 was conducted with these β-diketiminate
lithium salts. The reactions went smoothly, after workup, to
yield the corresponding complexes [NdL3] (4) and [NdLMe
(5) as blue crystals in good yields upon crystallization
]
3
Molecular Structures of Complexes 1–5 and 8
(Scheme 2). Elemental analysis revealed that all these com-
The molecular structures of complexes 1–5 and 8 were
plexes consist of three β-diketiminate ligands around the further determined by X-ray diffraction. Their molecular
metal center. The IR spectra of complexes 1–5 exhibited structures are shown in Figures 1, 2, 3, and 4. Selected bond
strong absorptions near 1551 and 1528 cm–1, which were lengths and angles and the crystallographic data are sum-
consistent with partial C=N character.[9] These complexes marized in Tables 1, 2, and 5.
Eur. J. Inorg. Chem. 2009, 4110–4118
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M. Xue, R. Jiao, Y. Zhang, Y. Yao, Q. Shen
FULL PAPER
Figure 1. ORTEP diagram of complexes 1, 2, 3 (Ln = Pr 1, Nd 2,
Sm 3) showing atom-numbering scheme. Thermal ellipsoids are
drawn at the 10% probability level. Hydrogen atoms are omitted
for clarity.
Figure 3. ORTEP diagram of complexes 5 showing atom-number-
ing scheme. Thermal ellipsoids are drawn at the 10% probability
level. Hydrogen atoms are omitted for clarity.
Figure 4. ORTEP diagram of complexes 8 showing atom-number-
ing scheme. Thermal ellipsoids are drawn at the 30% probability
level. Hydrogen atoms are omitted for clarity.
Figure 2. ORTEP diagram of complexes 4 showing atom-number-
ing scheme. Thermal ellipsoids are drawn at the 30% probability
level. Hydrogen atoms are omitted for clarity.
monomer, in which a six-coordinate neodymium ion is co-
ordinated by one β-diketiminate ligand, two chlorine atoms,
Complexes 1–5 are isostructural. They have unsolvated and two oxygen atoms from two thf molecules forming a
monomeric structure with a six-coordinate lanthanide distorted octahedron. The molecular structure of 8 is quite
metal ligated by six nitrogen atoms of three chelating biden- similar to those of monomeric β-diketiminate–lanthanide
tate β-diketiminate ligands. The coordination geometry dichlorides reported.[2g,2k,10,11]
around the lanthanide ion can be described as a distorted
The C–N distances within the chelating β-diketiminate
octahedron. Four nitrogen atoms, N2, N3, N4, and N6, ligands for all the complexes (see Tables 1 and 2) are ap-
from three β-diketiminate ligands can be considered to oc- proximately equivalent and are significantly shorter than
cupy equatorial positions within the octahedron about the C–N single bond lengths and longer than the general
lanthanide ion [the sum of these bond angles is 360.53(16)° double bond lengths, indicating that the π-electrons in these
for 1, 360.52(8)° for 2, 360.48(7)° for 3, 360.34(11)° for 4, complexes are delocalized within the N–C–C–C–N frag-
and 360.4(10)° for 5], and the other two nitrogen atoms, N1 ment. The quite long distances between the lanthanide cen-
and N5, occupy axial positions [the angle of N1–Ln1–N5 ter and the carbon atoms of the backbone of the β-diketim-
is 171.14(16)° for 1, 171.70(8)° for 2, 172.30(7)° for 3, inate ligands reveal that all the β-diketiminate ligands in
170.41(11)° for 4, and 171.57(10)° for 5]. The molecular these complexes only act as an N,NЈ-bonded chelate anion.
structures are similar to those for the analogous
The values of the bite angle of N–Ln–N in tris-β-diket-
Ln[{PhNC(Me)}2CH]3 (Ln = Sm and Gd) complexes re- iminate lanthanide complexes 1–5 are comparable to each
ported previously.[8a] Complex 8 is a two-thf-solvated other (Table 1). However, these angles are more acute than
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Eur. J. Inorg. Chem. 2009, 4110–4118

Lanthanide Complexes and Their Activity for Ring-Opening Polymerization
Table 1. Selected bond lengths (Å) and bond angles (°) for complexes 1–5.
1 (Ln = Pr)
2 (Ln = Nd)
3 (Ln = Sm)
4 (Ln = Nd)
5 (Ln = Nd)
Ln1–N1
Ln1–N2
Ln1–N3
Ln1–N4
Ln1–N5
Ln1–N6
N1–C2
N2–C4
2.508(5)
2.493(5)
2.536(5)
2.485(5)
2.479(5)
2.522(5)
1.338(8)
1.332(8)
1.387(9)
1.396(9)
74.89(16)
74.12(16)
71.57(16)
171.14(16)
96.10(16)
102.90(16)
87.41(16)
2.500(3)
2.486(3)
2.520(2)
2.474(2)
2.473(2)
2.507(2)
1.335(4)
1.331(4)
1.401(5)
1.396(5)
75.56(8)
74.78(8)
72.14(8)
171.70(8)
95.82(8)
102.15(8)
87.77(8)
2.473(2)
2.458(2)
2.487(2)
2.446(2)
2.448(2)
2.475(2)
1.334(4)
1.329(3)
1.396(4)
1.396(4)
76.46(7)
75.89(7)
73.15(7)
172.30(7)
95.62(7)
101.25(7)
87.72(7)
2.499(3)
2.481(3)
2.495(3)
2.484(3)
2.487(3)
2.491(3)
1.322(5)
1.325(5)
1.411(6)
1.406(6)
74.91(11)
74.03(11)
74.66(11)
170.41(11)
99.53(11)
101.99(11)
84.79(11)
2.475(3)
2.503(3)
2.475(3)
2.510(3)
2.499(3)
2.492(3)
1.328(4)
1.324(5)
1.395(5)
1.401(5)
72.52(10)
74.28(10)
75.58(10)
171.57(10)
101.36(9)
97.44(10)
87.32(9)
C2–C3
C3–C4
N1–Ln1–N2
N3–Ln1–N4
N5–Ln1–N6
N1–Ln1–N5
N2–Ln1–N3
N4–Ln1–N6
N2–Ln1–N6
Table 2. Selected bond lengths (Å) and bond angles (°) for complex a single-component initiator to yield polymers with high
8.
molecular weights and moderate polydispersities (Table 3).
For example, the polymerization of CL gave complete con-
version of 4000 equivalents of CL within 30 min at room
temperature in toluene (Table 3, Entry 2). The very high ac-
tivity was still observed when the polymerization proceeded
even at 0 °C (Table 3, Entry 4). The high activity shown by
complex 1 encouraged us to further screen complexes 2–5
with varied central metals and ligands. As shown in Table 3,
all the tris-β-diketiminate lanthanide complexes can serve
as highly active initiators for ring-opening polymerization
of CL, yielding polymers with high molecular weight and
moderate molecular weight distributions (Mw/Mn) ranging
from 1.38 to 1.89. The activity depended strongly on the
central metals, and the active trend of Sm Ͻ Nd Ͻ Pr,
which is consistent with the sequence of ionic radii, was
observed (Table 3, Entries 1, 5, and 9).
Nd1–N1
Nd1–Cl1
Nd1–O1
N1–C2
2.459(4)
2.6531(18)
2.528(4)
1.328(6)
1.399(8)
75.81(13)
174.45(13)
90.71(17)
98.73(11)
80.38(11)
100.60(11)
81.00(11)
155.03(6)
Nd1–N2
Nd1–Cl2
Nd1–O2
N2–C4
2.444(4)
2.6357(18)
2.540(6)
1.326(6)
1.388(7)
99.41(13)
166.52(17)
94.06(17)
102.98(10)
84.03(15)
96.98(10)
80.84(14)
C2–C3
C3–C4
N1–Ln1–N2
N1–Nd1–O1
N1–Nd1–O2
N2–Nd1–Cl2
O1–Nd1–Cl2
N2–Nd1–Cl1
O1–Nd1–Cl1
Cl2–Nd1–Cl1
N2–Nd1–O1
N2–Nd1–O2
O1–Nd1–O2
N1–Nd1–Cl2
O2–Nd1–Cl2
N1–Nd1–Cl1
O2–Nd1–Cl1
the corresponding angles in mono-β-diketiminate lantha-
nide complexes. For example, the average bond angle of N–
Nd–N in 2 is 74.16(8)°, whereas the values are 76.9(2)° for
[LMe NdCl(thf)(µ-Cl)2Li(thf)] {L
= [(2,6-Me2Ph)NC-
Me2
2
(Me)]2CH–}[2g] and 75.81(13)° for 8; the value is 75.17(7)°
for 3, whereas it is 79.5(4)° for [LSmCl2(thf)2].[10] Hence,
the Ln–N bond lengths in complexes 1–5 are longer than
those in [LЈLnX2(thf)2] (LЈ = β-diketiminate ligand; X =
anion) complexes. For example, the average bond length of
Sm–N is 2.465(2) Å for 3, which is 0.135 Å longer than
2.33(1) Å in [LSmCl2(thf)2];[10] the average Nd–N bond
length 2.492(3) Å for 2, 4, and 5 is 0.059 Å and 0.040 Å
In comparison, the catalytic activity of mono-β-diket-
iminate lanthanide dichlorides 6–8 and [LSmCl2(thf)2] were
tested for ring-opening polymerization of ε-CL under the
same reaction conditions. The results indicated that these
complexes were inactive: even when the molar ratio of
monomer to complex was decreased to 100 and the poly-
merization time prolonged to 180 min no polymer was ob-
tained (Table 3, Entries 20–23). From the comparative poly-
merization results and the bond angles and bond length
data between the two kinds of complexes, it can be sup-
posed that the high catalytic activity shown by tris-β-diket-
iminate lanthanide complexes may be attributed to the acti-
vation of the Ln–β-diketiminate bond induced by steric de-
longer than 2.433(7) Å in [LMe NdCl(thf)(µ-Cl)2Li(thf)]
[2g]
2
and 2.452(4) Å in complex 8, respectively, although these
structurally characterized mono-β-diketiminate neodymium
complexes contain a more bulky β-diketiminate ligand.
Catalytic Activity of Complexes 1–8 and [LSmCl2(thf)2] for
Ring-Opening Polymerization of ε-Caprolactone (CL) and
L-Lactide (LA)
To examine the reactivity of the Ln–β-diketiminate bond mand of the three β-diketiminate ligands.
in complexes 1–5, the polymerization of CL with complex The electronic factor of the β-diketiminate ligand has a
1 as the initiator was first tested. It was found that complex great effect on the catalytic activity of tris-β-diketiminate
1 showed extremely high activity at room temperature as lanthanide complexes. Complex 2 containing a ligand with
Eur. J. Inorg. Chem. 2009, 4110–4118
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M. Xue, R. Jiao, Y. Zhang, Y. Yao, Q. Shen
FULL PAPER
Table 3. Polymerization of ε-caprolactone initiated by complexes 1–8.[a]
M
n,theo(10–4)
Entry
Initiator
[M]/[I][b]
T (°C)
t (min)
Yield (%)[c]
M
n,exp(10–4)[e]
PDI
[d]
1
2
3
4
5
6
7
8
1
1
4000
4000
2000
2000
4000
2000
2000
2000
4000
1500
1000
500
2000
2000
1000
1000
2000
1000
500
25
25
25
0
25
25
0
15
30
15
30
15
15
20
30
60
60
60
30
15
30
15
60
15
15
5
64
100
100
100
39
100
70
100
0
90
100
100
54
100
100
90
0
29.2
45.6
22.8
22.8
17.8
22.8
16.0
22.8
–
10.3
11.4
5.70
12.3
22.8
11.4
10.3
–
27.7
41.6
21.9
29.7
16.8
20.7
21.8
27.5
–
10.6
9.68
5.05
11.2
19.7
12.5
16.6
–
1.59
1.81
1.77
1.85
1.38
1.84
1.56
1.81
–
1.48
1.78
1.68
1.42
1.89
1.77
1.43
–
1
1
2
2
2
2
0
9
3
25
25
25
25
25
25
25
0
25
25
25
25
25
25
25
10
11
12
13
14
15
16
17
18
19
20
21
22
23
3
3
3
4
4
4
4
5
5
97
100
0
0
0
11.1
5.70
–
–
–
11.6
7.78
–
–
–
1.69
1.73
–
–
–
5
6
100
100
100
100
180
180
180
180
7
8
X[f]
0
–
–
–
[a] Polymerization conditions: in toluene; ε-CL = 0.82 molL–1. [b] [M]/[I] = [monomer]/[initiator]. [c] Yield = weight of polymer obtained/
weight of monomer used. [d] Mn,theo = ([CL]/[I])ϫ114.14ϫ(polymer yield). [e] Measured by GPC calibrated with standard polystyrene
samples and corrected with the coefficient of 0.56. [f] X = [LSmCl2(thf)2].
an electron-withdrawing group, Cl, showed the highest ac- plete conversion was achieved in thf in 15 min at 65 °C
tivity, whereas complex 5 bearing a ligand with an electron- when the molar ratio of monomer to initiator ([M]/[I]) was
donating group, Me, exhibited the lowest activity. For ex- 500 (Table 4, Entry 1). A decrease in the ionic radii of the
ample, 2 gave complete conversion in the case of [M]/[I] = metals resulted in a marked decrease in the catalytic ac-
2000 at 25 °C in 15 min (Table 3, Entry 6), whereas only tivity. For example, by using complex 1 as the initiator, 90%
54% conversion (Table 3, Entry 13) and no polymer yield could be obtained within 5 min at 40 °C when the mo-
(Table 3, Entry 17) were obtained for complexes 4 and 5, lar ratio of monomer to initiator was 300 (Table 4, Entry 4),
respectively. The conversion was still 100% even when the whereas the yield was 68% within 15 min for 2 (Table 4,
temperature was decreased to 0 °C with 2 as the initiator Entry 6) and only 38% for 3 even when the polymerization
(Table 3, Entry 8). Therefore, a decrease in the electron den- time was prolonged to 60 min (Table 4, Entry 9). Therefore,
sity of the ligand around the metal center was beneficial the active order for central metals is Sm Ͻ Nd Ͻ Pr. A
to the increase in the activity of the Ln–N bond for ε-CL similar active trend was also observed when bis(allyl)(diket-
polymerization in the present systems. This may be because iminato)lanthanide complexes[4] or the β-ketoiminate rare-
the electron-withdrawing substituents on the ligand make earth metal aryloxido complexes[12] were used as the cata-
the central metal more electron-deficient, which favors the lysts for lactide polymerization.
coordination of the monomer.
Similarly, mono-β-diketiminate lanthanide dichlorides 6–
All of the polymers obtained have high molecular 8 and [LSmCl2(thf)2] were inactive for -LA polymerization
weights. Although a roughly linear relationship between under the same polymerization conditions (thf as solvent,
monomer-to-initiator ratio and molecular weight can be -LA = 1 molL–1, 40 °C). Even when the molar ratio of
observed, the molecular weight distributions of the re- monomer to initiator was decreased to 100 and the poly-
sulting polymers are relatively broad (from 1.38 to 1.89). merization time extended to 180 min, still no polymeriza-
These results indicate that these polymerization systems are tion occurred (Table 4, Entries 14–17). The results once
not well controlled.
again indicated that the Ln–β-diketiminate bond in a tris-
The catalytic behavior of complexes 1–8 for -LA poly- β-diketiminate lanthanide complex is much more active
merization was also examined, and the preliminary results than that in a less bulky dichloride, which is consistent with
are summarized in Table 4. Complexes 1–5 proved to be the data of Ln–N bond lengths for the two kinds of com-
highly active initiators for -LA polymerization. The cata- plexes.
lytic activity also depended strongly on the size of the cen-
In general, by using the ligands with better electron-do-
ter metal and the electronic factor of the β-diketiminate li- nating ability, one can expect to hamper the coordination of
gands. Praseodymium complex 1 has been found to be the the monomer and to slow polymerization process down.[13a]
most active catalyst among these complexes: almost com- Gibson and co-workers showed that modification of the
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Lanthanide Complexes and Their Activity for Ring-Opening Polymerization
Table 4. Polymerization of -lactide initiated by complexes 1–8.[a]
M
n,theo(10–4)
Entry
Initiator
[M]/[I][b]
T (°C)
t (min)
Yield (%)[c]
M
n,exp(10–4)[e]
PDI
[d]
1
2
3
4
5
6
7
8
1
1
500
500
500
300
300
300
300
300
300
200
300
300
200
100
100
100
100
65
40
20
40
20
40
40
20
40
40
40
40
40
40
40
40
40
15
15
15
5
15
15
30
30
60
60
30
30
5
98
84
44
90
80
68
98
50
38
84
86
58
81
0
7.06
6.05
3.17
3.89
3.46
2.94
4.23
2.16
1.64
2.42
3.72
2.51
2.33
–
12.0
23.5
19.4
24.6
34.6
17.3
13.6
20.7
3.64
16.5
24.3
13.1
26.2
–
1.49
1.53
1.38
1.61
1.43
1.36
1.42
1.35
1.24
1.64
1.41
1.23
1.37
–
1
1
1
2
2
2
9
3
10
11
12
13
14
15
16
17
3
4
5
5
6
180
180
180
180
7
0
0
0
–
–
–
–
–
–
–
–
–
8
X[f]
[a] Polymerization conditions: thf as solvent; -LA = 1.0 molL–1. [b] [M]/[I] = [monomer]/[initiator]. [c] Yield = weight of polymer ob-
tained/weight of monomer used. [d] Mn,theo = ([LA]/[I])ϫ144.13ϫ(polymer yield). [e] Measured by GPC calibrated with standard poly-
styrene samples and corrected with the coefficient of 0.58. [f] X = [LSmCl2(thf)2].
were no peaks for the isopropyl or β-diketiminate groups
detected for both oligomers of ε-CL and -LA, but the
1
peaks of polyCL or polyLA in the H NMR spectra were
observed. The same situation was also found for systems
with homoleptic lanthanides benzimidinates,[13a] lantha-
nides guanidinates complexes,[16] and chiral heterobimetal-
lic complexes.[17] Normally, no end-group detection was at-
supporting ligand by introduction of electron-withdrawing
tributed to the formation of cyclic polymers through intra-
molecular attack of the Ln–O bond in an active species to
the N-bonded acyl carbon atom.[13a,16,17] Therefore, the for-
mation of cyclic polymers and a similar mechanism as those
supposed for the above systems were suggested in our case.
As the initial step of the polymerization, ε-CL coordinated
to the central metal, which was then followed by nucleo-
philic attack by one of the β-diketiminate nitrogen atoms at
the carbonyl carbon atom of the lactone. Subsequently, acyl
bond cleavage occurred with the formation of an alkoxide
active species. The cyclic polymer might be formed through
intramolecular attack of the Ln–O bond to the N-bonded
acyl carbon atom.
substituents furnishes a more active aluminium catalyst for
the polymerization of lactide.[13b] In the present cases, a
similar effect of the electronic factor of the β-diketiminate
ligand on the catalytic activity was also observed. The
active sequence is 2 Ͼ 4 Ͼ 5, which is consistent with the
electron-deficiency of the ligand (Table 4, Entries 7, 11, and
12).
It was noticed that no correlation could be found be-
tween the calculated and the GPC measured Mn. In order
to get the correct Mn value of PLA the experimental value,
which is obtained from the GPC traces by using polystyrene
standards, has to be multiplied by 0.58.[14] All the number-
averaged molecular weights corrected by the coefficient 0.58
of the polymers produced are far greater than theoretical
ones. This may be due to faster propagation related to ini-
tiation,[15] or not all of the complexes participated in the
polymerization as active initiators. As expected, raising the
polymerization temperature led to an increase in the poly-
merization rate. For example, if the polymerization was per-
formed at 65 °C, 98% conversion could be achieved in
15 min, whereas only 84 and 44% were obtained at 40 and
Another possibility for no end-group detection may be
because the molecular weights of the oligomers obtained
1
are too large to be detected by H NMR spectroscopy.
Conclusions
A series of sterically demanding tris-β-diketiminate lan-
20 °C (Table 4, Entries 1–3), respectively. Moreover, the re- thanide complexes were successfully synthesized in good
sulting polymers at 65 °C have the molecular weights more yield by a salt metathesis reaction, and their structural fea-
close to theoretical ones in comparison with those obtained tures were determined by X-ray diffraction. Tris-β-diket-
at 40 °C or at 20 °C.
iminate lanthanide complexes were first proven to be highly
The oligomerizations of ε-CL and -LA were conducted active initiators for the polymerization of ε-caprolactone
with [PrLC3 l] under the condition of [M]/[I] = 10 and then and -lactide. On the basis of the fact that no activity of
terminated by isopropyl alcohol. The resulting polymers mono-β-diketiminate lanthanide dichlorides was observed
1
were examined by H NMR spectroscopic analysis. There under the same polymerization conditions, the high cata-
Eur. J. Inorg. Chem. 2009, 4110–4118
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4115

M. Xue, R. Jiao, Y. Zhang, Y. Yao, Q. Shen
FULL PAPER
mixture was stirred at 40 °C for 48 h, the solvents were stripped off
in vacuo, and toluene was added to extract the product. The dis-
solved portion was removed by centrifugation, and the yellow-
green supernatant was then concentrated and cooled to 0 °C to give
yellow-green crystals of 1 in 75% yield (2.46 g, 2.25 mmol). M.p.
220–225 °C (decomp.). C51H45Cl6N6Pr (1095.54): calcd. C 55.91, H
4.14, N 7.67, Pr 12.86; found C 55.71, H 4.34, N 7.62, Pr 12.96.
lytic activity shown by tris-β-diketiminate lanthanide com-
plexes might be attributed to the activation of Ln–N bonds
induced by the steric demand of the three β-diketiminate
ligands. Further study on the reactivity of sterically
crowded tris-β-diketiminate lanthanide complexes is ongo-
ing in our laboratory.
IR (KBr): ν = 3419 (s), 2964 (m), 2925 (m), 1636 (vs), 1551 (vs),
˜
1482 (vs), 1443 (s), 1382 (vs), 1281 (s), 1189 (s), 1096 (m), 1011 (w),
926 (w), 849 (m), 803 (w), 741 (w), 679 (w), 641 (w), 586 (w), 502
(w) cm–1.
Experimental Section
General Procedures: All manipulations were performed under a
purified argon atmosphere by using standard Schlenk techniques.
Solvents were degassed and distilled from sodium benzophenone
ketyl prior to use. ε-Caprolactone was purchased from Acros, dried
with CaH2 for 48 h, and then distilled under reduced pressure. -
Lactide was recrystallized twice with dry toluene. LLi[18a] and
[NdLC3 l] (2): To a slurry of anhydrous NdCl3 (0.95 g, 3.79 mmol) in
thf (ca. 30 mL) was slowly added a solution of LClLi (11.4 mmol)
in toluene/hexane (4:1, 35 mL) at room temperature. The reaction
mixture was stirred at room temperature for 48 h, the solvents were
stripped off in vacuo, and toluene was added to extract the product.
The dissolved portion was removed by centrifugation, and the pale-
blue supernatant was then concentrated and cooled to –10 °C to
give pale-blue crystals of 2 in 80% yield (3.35 g, 3.05 mmol). M.p.
182–184 °C (decomp.). C51H45Cl6N6Nd (1098.87): calcd. C 55.74,
H 4.13, N 7.65, Nd 13.13; found C 55.96, H 4.08, N 7.75, Nd
LiPr Li
were prepared according to a literature procedure. All
[18b]
2
glassware for the polymerization was dried in an oven before use.
Lanthanide analyses were performed by EDTA titration with a
xylenol orange indicator and a hexamine buffer.[19] Carbon, hydro-
gen, and nitrogen analyses were performed by direct combustion
with a Carlo–Erba EA-1110 instrument. The IR spectra were re-
corded with a Nicolet-550 FTIR spectrometer as KBr pellets. 1H
NMR spectra were obtained in CDCl3 for ligands by using a Unity
Inova-400 spectrometer. The uncorrected melting points of crystal-
line samples were determined in sealed argon-filled capillaries.
Molecular weights and molecular weight distributions were deter-
mined against polystyrene standards by gel permeation chromatog-
raphy (GPC) at 30 °C with a Waters 1515 apparatus with three HR
columns (HR-1, HR-2, and HR-4) by using thf as the eluent.
13.10. IR (KBr): ν = 3442 (s), 2987 (m), 2925 (m), 2856 (w), 1636
˜
(vs), 1551 (vs), 1482 (vs), 1443 (s), 1382 (vs), 1281 (s), 1189 (s),
1088 (m), 1011 (m), 834 (v), 841 (m), 764 (m), 679 (w), 633 (w),
579 (w), 509 (w) cm–1.
[SmLC3 l] (3): The synthesis of complex 3 was carried out in the same
way as that described for complex 2, but SmCl3 (0.90 g, 3.50 mmol)
was used instead of NdCl3. Orange-yellow microcrystals of 3 were
obtained from a concentrated toluene solution in 82% yield (3.17 g,
2.87 mmol). M.p. 178–180 °C (decomp.). C51H45Cl6N6Sm
(1104.98): calcd. C 55.44, H 4.10, N 7.61, Sm 13.61; found C 55.77,
LClH: A mixture of p-chloroaniline (31.9 g, 0.25 mol), 2,4-pentane-
dione (12.7 mL, 12.4 g, 0.12 mol), and p-toluenesulfonic acid
(21.3 g) in toluene (350 mL) was heated at reflux for 24 h in a
Dean–Stark apparatus. The toluene was then decanted off, and the
solid residue was treated with diethyl ether (250 mL), water
(200 mL), and Na2CO3·10H2O (53 g). After stirring for 25 min, the
ether layer was separated and dried with MgSO4, and the solvent
was removed in vacuo. The residue was dried in vacuo (10–2 bar)
at 100 °C for 6 h to remove any remaining free p-chloroaniline, and
the residue was crystallized from hexane to give 27 g (70%) of
H 4.17, N 7.82, Sm 13.39. IR (KBr): ν = 3442 (s), 2925 (m), 2856
˜
(w), 1628 (s), 1551 (s), 1482 (s), 1436 (m), 1382 (s), 1219 (vs), 1158
(vs), 1096 (m), 1027 (w), 841 (m), 803 (w), 733 (w), 633 (w), 548
(w), 509 (m) cm–1.
[NdL3] (4): To a slurry of anhydrous NdCl3 (1.00 g, 4.00 mmol) in
thf (ca. 30 mL) was slowly added a solution of LLi (12.00 mmol)
in toluene/hexane (4:1, 30 mL) at room temperature. The reaction
mixture was then stirred at 40 °C for 48 h, the solvents were
stripped off in vacuo and toluene was added to extract the product.
The dissolved portion was removed by centrifugation and the pale-
blue solution was then concentrated and cooled to –10 °C to give
pale-blue crystals in 75% yield (2.68 g, 3.00 mmol). M.p. 191–
194 °C (decomp.). C51H51N6Nd (892.22): calcd. C 68.66, H 5.76,
N 9.42, Nd 16.17; found C 68.77, H 5.95, N 9.17, Nd 16.02. IR
1
LClH. M.p. 86–87 °C. H NMR (400 MHz, CDCl3): δ = 12.43 (s,
1 H, NH), 7.11 (d, 4 H, ArH), 6.87 (d, 4 H, ArH), 5.21 (s, 1 H, β-
CH), 1.98 (s, 6 H, α-CH3) ppm. C17H16Cl2N2 (319.23): calcd. C
63.96, H 5.05, N 8.78; found C 63.77, H 5.02, N 8.80.
LMeH: A mixture of p-methylaniline (26.8 g, 0.25 mol), 2,4-pen-
tanedione (12.7 mL, 12.4 g, 0.12 mol), and p-toluenesulfonic acid
(21.3 g) in toluene (350 mL) was heated at refluxed for 24 h in a
Dean–Stark apparatus. The toluene was then decanted off, and the
solid residue was treated with diethyl ether (250 mL), water
(200 mL), and Na2CO3·10H2O (53 g). After stirring for 25 min, the
ether layer was separated and dried with MgSO4, and the solvent
was removed in vacuo. The residue was dried in vacuo (10–2 bar)
at 100 °C for 6 h to remove any remaining free p-methylaniline, and
the residue was crystallized from hexane to give 27.8 g (80%) of
(KBr): ν = 3064 (s), 2362 (m), 1636 (vs), 1559 (vs), 1490 (vs), 1436
˜
(m), 1366 (s), 1227 (s), 1158 (s), 1073 (w), 1027 (m), 903 (w), 803
(m), 749 (m), 695 (m), 634 (w), 509 (m) cm–1.
[NdLM3 e] (5): The synthesis of complex 5 was carried out in the
same way as that described for complex 4, but LMeLi (6.04 mmol)
was used instead of LLi. Complex 5 was obtained as pale-blue
crystals at –5 °C in 2 d in 65% yield (1.27 g, 1.30 mmol). M.p. 230–
233 °C (decomp.). C57H63N6Nd (976.37): calcd. C 70.12, H 6.50,
N 8.61, Nd 14.77; found C 70.08, H 6.71, N 8.40, Nd 14.50. IR
1
LMeH. M.p. 73–74 °C. H NMR (400 MHz, CDCl3): δ = 12.67 (s,
(KBr): ν = 3427 (s), 3025 (m), 2995 (m), 2925 (m), 2863 (m), 1891
˜
1 H, NH), 7.10 (d, 4 H, ArH), 6.85 (d, 4 H, ArH), 4.85 (s, 1 H, β-
CH), 2.31 (s, 6 H, p-CH3), 1.98 (s, 6 H, α-CH3) ppm. C19H22N2
(278.40): calcd. C 81.97, H 7.96, N 10.06; found C 81.77, H 7.90,
N 10.11.
(w), 1628 (vs), 1551 (vs), 1513 (vs), 1436 (s), 1366 (s), 1281 (s), 1189
(s), 1104 (w), 1027 (m), 841 (m), 810 (m), 756 (m), 602 (m), 525
(m) cm–1.
[LClPrCl2(thf)2] (6): A thf (20 mL) solution of LClH (1.32 g,
4.13 mmol) was added dropwise to a NaH suspension (20 mmol) in
thf (20 mL) at room temperature. The reaction mixture was stirred
overnight, and then the mixture was filtered. The transparent
[PrLC3 l] (1): To a slurry of anhydrous PrCl3 (0.74 g, 3.00 mmol) in
thf (ca. 20 mL) was slowly added a solution of LClLi (9.00 mmol)
in toluene/hexane (4:1, 20 mL) at room temperature. The reaction
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Lanthanide Complexes and Their Activity for Ring-Opening Polymerization
orange-yellow solution was then slowly added to a slurry of anhy-
drous PrCl3 (1.02 g, 4.13 mmol) in thf (20 mL). The color of the
solution gradually changed to pale green, and a colorless gel-like
precipitate formed gradually. The mixture was stirred at room tem-
perature for 48 h, and the precipitate was removed by centrifuga-
tion. To the thf solution was added dme (5 mL), and then the solu-
tion was concentrated in vacuo. Green crystals were obtained from
a concentrated thf/dme solution (15 mL) at room temperature in
70% yield (1.95 g, 2.89 mmol). M.p. 160–163 °C (decomp.).
C25H31Cl4N2O2Pr (674.25): calcd. C 44.53, H 4.63, N 4.15, Pr
18.56; found C 57.00, H 7.12, N 3.70, Nd 18.75. IR (KBr): ν =
˜
3412 (s), 2961 (s), 2928 (m), 2868 (m), 2360 (m), 2341 (m), 1623
(vs), 1553 (vs), 1487 (m), 1464 (m), 1439 (m), 1380 (m), 1363 (m),
1324 (m), 1277 (m), 1221 (m), 1175 (m), 1101 (m), 1058 (m), 1023
(m), 953 (w), 759 (w), 668 (w) cm–1. Crystals suitable for X-ray
diffraction analysis were obtained by slow cooling of a hot toluene/
dme solution.
Typical Procedure for the Polymerization Reaction: Polymerizations
of -lactide (-LA) were performed under an atmosphere of argon
in flame-dried round-bottom flasks. To a stirred solution of -LA
(0.50 g, 3.47 mmol) in thf (1.47 mL) was added a thf solution
(2.00 mL) of complex 1 (12.71 mg, 0.0116 mmol, [LA]/[Pr] = 300:1,
[LA] = 1.00 molL–1). Temperature equilibration was ensured by
stirring the flasks for 15 min in an oil bath thermostatted at 40 °C,
followed by the injection of the initiator into the monomer solu-
tion. Polymerizations were stopped by injecting a solution of HCl
(5 vol.-%) in ethanol. Polymers were precipitated in ethanol, fil-
tered, washed with ethanol, and then dried at 40 °C for 24 h in
vacuo to constant weight.
21.90; found C 44.83, H 4.50, N 4.28, Pr 21.75. IR (KBr): ν = 3380
˜
(s), 2987 (s), 2871 (m), 1628 (s), 1605 (s), 1536 (vs), 1489 (vs), 1443
(m), 1382 (m), 1312 (s), 1095 (m), 1034 (m), 834 (m), 712 (m) cm–1.
[LMeNdCl2(thf)2] (7): A thf (20 mL) solution of LMeH (1.06 g,
3.80 mmol) was added dropwise to a NaH suspension (18 mmol) in
thf (20 mL) at room temperature. The reaction mixture was stirred
overnight, and then the mixture was filtered. The transparent yel-
low solution was then slowly added to a slurry of anhydrous NdCl3
(0.95 g, 3.80 mmol) in thf (20 mL). The color of the solution grad-
ually changed to pale blue, and a colorless gel-like precipitate
formed gradually. The mixture was stirred overnight at room tem-
perature, and the precipitate was removed by centrifugation. The
solvent was removed under reduced pressure, and the residue was
extracted with toluene (50 mL). To the toluene solution was added
dme (3 mL), and then the solution was concentrated to about
20 mL. Pale-blue microcrystals were obtained at room temperature
in a few days (1.57 g, 65% based on Nd). M.p. 165–168 °C (de-
comp.). C27H37Cl2N2NdO2 (636.75): calcd. C 50.93, H 5.86, N
4.40, Nd 22.65; found C 51.20, H 5.78, N 4.50, Nd 22.52. IR (KBr):
The procedure for the polymerization of ε-caprolactone was sim-
ilar, and a typical polymerization procedure is given below. A 50-
mL Schlenk flask, equipped with a magnetic stirring bar, was
charged with a solution of initiator in toluene. To this solution
was added the desired amount of ε-caprolactone by syringe. The
contents of the flask were then stirred vigorously at the desired
temperature for a fixed time, during which time an increase in vis-
cosity was observed. The reaction mixture was quenched by the
addition of 1  HCl/ethanol solution and then poured into ethanol
to precipitate the polymer, which was dried in vacuo and weighed.
ν = 3445 (m), 2986 (s), 2966 (m), 2924 (m), 1632 (m), 1558 (vs),
˜
1485 (vs), 1439 (m), 1381 (m), 1312 (s), 1289 (m), 1090 (m), 837
X-ray Crystallography: Suitable single crystals of complexes 1–5
and 8 were sealed in a thin-walled glass capillary for determining
the single-crystal structure. Intensity data were collected with a
Rigaku Mercury CCD area detector in ω scan mode by using Mo-
Kα radiation (λ = 0.71070 Å). The diffracted intensities were cor-
rected for Lorentz polarization effects and empirical absorption
corrections. Details of the intensity data collection and crystal data
are given in Table 5. The structures were solved by direct methods
(m), 718 (m) cm–1.
[LiPr NdCl2(thf)2] (8): The synthesis of complex 8 was carried out
2
in the same way as that described for complex 7, but LiPr H (1.47 g,
2
3.51 mmol) was used instead of LMeH. Blue-green microcrystals
were obtained from a toluene/dme solution at 0 °C in 72%
yield (1.96 g, 2.52 mmol). M.p. 182–185 °C (decomp.).
C37H57Cl2N2NdO2 (776.99): calcd. C 57.20, H 7.39, N 3.61, Nd
Table 5. Crystallographic data for complexes 1–5 and 8.
1
2
3
4
5
8
Empirical formula
Formula mass
Temperature (K)
Crystal system
Space group
Crystal size (mm)
a (Å)
C51H45N6Cl6Pr
1095.54
223(2)
monoclinic
P21/n
C51H45N6Cl6Nd
1098.87
193(2)
monoclinic
P21/n
C51H45N6Cl6Sm
1104.98
193(2)
monoclinic
P21/n
C51H51N6Nd
892.22
223(2)
monoclinic
P21/c
C57H63N6Nd
976.37
223(2)
monoclinic
P21/n
C37H57Cl2N2NdO2
776.99
223(2)
orthorhombic
Pbca
0.29ϫ0.21ϫ 0.12 0.37ϫ0.32ϫ0.16 0.45ϫ0.43ϫ0.17 0.75ϫ0.38ϫ0.20 0.60ϫ0.42ϫ0.20 0.80ϫ0.69ϫ0.40
12.2362(15)
19.433(2)
20.928(2)
100.176(3)
4898.1(10)
4
12.2368(11)
19.401(2)
20.921(2)
100.235(3)
4887.8(8)
4
12.2461(9)
19.3441(13)
20.9205(16)
100.344(3)
4875.3(6)
4
18.7300(19)
10.6620(9)
22.747(2)
104.159(2)
4404.5(7)
4
12.6186(14)
19.3441(18)
21.108(2)
99.411(2)
5082.9(9)
4
20.5053(18)
15.6086(14)
24.136(2)
90
7725.0(12)
8
b (Å)
c (Å)
β (°)
V (Å3)
Z
Dcalcd. (gcm–3)
1.486
1.364
1.493
1.432
1.505
1.575
1.346
1.220
1.276
1.063
1.336
1.514
µ (mm–1)
F(000)
2216
2220
2228
1836
2028
3224
θ range (°)
Reflns. collected
3.01–25.35
47281
3.01–25.35
48029
3.01–25.35
47755
3.06–25.35
41462
3.12–25.35
49299
3.02–25.35
71617
Reflns. observed [IϾ2σ(I)] 7105
7917
8165
6534
7968
6430
Data/restraints/parameters 8952/0/584
8918/0/584
1.134
0.0347
8905/0/584
1.120
0.0287
8040/0/530
1.151
0.0512
9294/0/590
1.170
0.0450
7067/10/403
1.146
0.0569
0.1110
Goodness-of-fit on F2
R1 [IϾ2σ(I)]
1.172
0.0706
0.1229
wR2 (all data)
0.0708
0.0669
0.0872
0.0845
Eur. J. Inorg. Chem. 2009, 4110–4118
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4117

M. Xue, R. Jiao, Y. Zhang, Y. Yao, Q. Shen
FULL PAPER
and refined by full-matrix least-squares procedures based on |F|2.
All non-hydrogen atoms were refined anisotropically. The hydrogen
atoms in these complexes were all generated geometrically (C–H
bond lengths fixed at 0.95 Å), assigned appropriate isotropic ther-
mal parameters, and allowed to ride on their parent carbon atoms.
All H atoms were held stationary and included in the structure
factor calculation in the final stage of full-matrix least-squares re-
finement. The structures were solved and refined by using
SHELEXL-97 program. CCDC-725486 (for 1), -725487 (for 2),
-725488 (for 3), -725489 (for 4), -725490 (for 5), and -725491 (for
8) contain 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.
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Received: April 5, 2009
Published Online: August 11, 2009
4118
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