Bis(imino)diphenylamido rare-earth metal dialkyl complexes: Synthesis, structure, and catalytic activity in living ring-opening ε-caprolactone polymerization and copolymerization with γ-butyrolactone

Article abstract of DOI:10.1039/c2dt31932b

Bis(imino)diphenylamido rare-earth metal dialkyl complexes [o-(2,6- ⁱPr₂-C₆H₃-NC-C₆H ₄)₂-N]Ln(CH₂SiMe₃)₂ (1: Ln = Sc; 2: Ln = Lu; 3: Ln = Y) have

Full text of DOI:10.1039/c2dt31932b

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Bis(imino)diphenylamido rare-earth metal dialkyl
complexes: synthesis, structure, and catalytic activity in
living ring-opening ε-caprolactone polymerization and
copolymerization with γ-butyrolactone†
Cite this: Dalton Trans., 2013, 42, 1278
Gaixia Du,‡a,b Yanling Wei,‡a Wei Zhang,a Yuping Dong,b Zhengguo Lin,a Huan He,a
Shaowen Zhang*a and Xiaofang Li*a
Bis(imino)diphenylamido rare-earth metal dialkyl complexes [o-(2,6-iPr2-C6H3–NvC–C6H4)2–N]Ln-
(CH2SiMe3)2 (1: Ln = Sc; 2: Ln = Lu; 3: Ln = Y) have been synthesized in good yields and structurally
characterized by elemental analysis, NMR spectroscopy, and single-crystal X-ray diffraction studies. They
serve as highly efficient single-component catalysts both for the living ring-opening ε-caprolactone
polymerization and random copolymerization with γ-butyrolactone, with the activity being dependent
on the steric hindrance around the metal center, yielding high molecular weight PCLs or P(CL-co-BL)s
with narrow molecular weight distributions.
Received 24th August 2012,
Accepted 18th October 2012
DOI: 10.1039/c2dt31932b
www.rsc.org/dalton
based on magnesium,8 calcium,9 aluminium,10 zinc,11 tita-
nium,12 zirconium,13 vanadium,14 iron,15 tin,16 lanthanides
Introduction
Biodegradable polymers have received much current interest and rare earth metals17 offers a means from control over the
because of increasing demand for plastic materials and some polymer structure to kinetic enhancement, producing PCLs
serious disadvantages of oil-based poly(olefin)s such as with high molecular weights. By comparison, low toxic rare
environmental problem, economic pressure and petroleum earth metal catalysts bearing alkoxides,18 beta-diketiminate,19
resource shortage. Poly(ε-caprolactone) (PCL), which has main bis(amidinate),20 aryloxide21 ligands usually exhibited higher
characteristics of biodegradability, biocompatibility and poten- activities than other metal catalysts in the ring opening
tial availability from renewable resource starch,1 is one of the polymerization of ε-caprolactone. However, few cases exhibit
most promising, environmentally friendly commodity plastic exceptional activity and living mode.22 Therefore, there is an
for numerous biomedical and pharmaceutical applications. impetus to develop highly efficient rare-earth metal catalysts,
Since the first synthesis of PCL was reported by more than half producing biodegradable PCL at low cost with improved
a century ago,2 extensive studies have been carried out in this property.
area. A variety of ways such as polycondensation,3 enzymatic
In view that the use of bulky ligand for metal complex
polymerization,4 cationic polymerization,5 anionic polymeriz- reduces transesterification reactions and usually yields PCL
ation,6 or coordination/insertion polymerization7 have been with narrow polydispersity in the coordination/insertion
adopt for the synthesis of PCL. Among them, the coordi- polymerization of ε-caprolactone (CL), the sterically demand-
nation/insertion polymerization catalyzed by metal complexes ing pincer ligand is considered to be good choice for the
design of new rare-earth metal catalyst. Pincer ligand rep-
resents one of the most widely utilized tridentate chelating
aKey Laboratory of Cluster Science of Ministry of Education, School of Chemistry,
ligand in organometallic chemistry. Until now, metal pincer
Beijing Institute of Technology, 5 South Zhongguancun Street, Haidian District,
complexes are widely used in olefin polymerization, catalysis
Beijing 100081, China. E-mail: xfli@bit.edu.cn, swzhang@bit.edu.cn;
and bond activation.23 Early examples of pincer ligand are
Fax: +86 10-68914780; Tel: +86 10; Fax: -68914780
bSchool of Materials Science and Engineering, Beijing Institute of Technology, 5
monoanionic PCP in which a carbanion is located in the
central site and two phosphines are placed in flank.24 Sub-
sequently, variations have been developed where the phos-
phines are replaced by thioethers and tertiary amines or the
nitrogenous donors such as pyridines are introduced at the
central position.25 Despite these extensive efforts, however, the
South Zhongguancun Street, Haidian District, Beijing 100081, China
†Electronic supplementary information (ESI) available: GPC, DSC, and NMR (1H
and 13C) spectra of representative polymer products. CCDC 857397 (1), 857396
(2), and 857398 (3). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c2dt31932b
‡G. Du and Y. Wei contributed equally to this paper.
1278 | Dalton Trans., 2013, 42, 1278–1286
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monoanionic NNN pincer ligands are less exploited.26 The
metal complexes bearing a monoanionic bis(imino)diphenyl-
amido pincer ligand have not been reported previously.27
Their catalytic potential for the olefin polymerization has
remained unexplored to date, as far as we are aware. We report
herein the synthesis and structural characterization of novel
bis(imino)diphenylamido rare-earth metal dialkyl complexes.
They serve as highly efficient single-component catalysts in the
living ring-opening polymerization of CL, with the activity
being significantly dependent on the steric hindrance around
the metal center, yielding high molecular weight PCLs with
narrow molecular weight distributions. Moreover, the random
copolymerization of CL and γ-butyrolactone (BL) can be also
carried out by these complexes, yielding high molecular
weight copolymers with BL contents up to 18 mol%.28
Fig. 1 Variable temperature 1H NMR (600 MHz, C7D8) of complex 3.
Results and discussion
The monoanionic bis(imino)diphenylamido NNN pincer
ligand was synthesized according to the literature.29 The acid–
base reaction between the rare-earth metal trialkyl complex Ln-
30
(CH2SiMe3)3(THF)2 and the bis(imino)diphenylamido pincer
ligand in toluene at 25 °C afforded in high yields the corres-
ponding bis(imino)diphenylamido rare-earth metal dialkyl
complexes
1–3
[o-(2,6-iPr2-C6H3–NvC–C6H4)2–N]Ln-
(CH2SiMe3)2 (1: Ln = Sc, 69%; 2: Ln = Lu, 72%; 3: Ln = Y, 65%)
(Scheme 1). All complexes 1–3 are soluble in THF, toluene, and
benzene. 1H NMR spectra of complexes 1–3 indicated that four
similar doublets for the isopropyl-methyl group between
0.8–2.0 ppm integrating for twenty-four protons and another
singlet for the imino-methine groups at 8.20 ppm integrating
for two protons were observed, implying a symmetry in the
chelating pincer ligand. No THF molecule which showed
signals at 1.70 and 3.50 ppm was found. Two double signals
for the methylene protons of the Ln–CH2SiMe3 groups at high
field (1: 0.04 ppm (d, 2 H, J = 11.6 Hz), 0.18 ppm (d, 2 H, J =
11.6 Hz); 2: −0.70 ppm (d, 2 H, J = 12.0 Hz), −0.59 ppm (d,
2 H, J = 12.0 Hz); 3: −0.47 ppm (d, 2 H, J = 11.2 Hz), −0.42 ppm
(d, 2 H, J = 11.6 Hz)) indicated that the metal alkyl species in
1–3 are fixed in the solution state, which could not rotate
Fig. 2 ORTEP drawings of 1 (a), 2 (b) and 3 (c) with 30% thermal ellipsoids.
The hydrogen atoms in 1, 2 and 3 and one toluene solvent molecule are
omitted for clarity.
Single crystals of complexes 1–3 suitable for an X-ray deter-
mination were grown from a concentrated toluene–hexane
mixture solution at −30 °C. The X-ray diffraction study revealed
that complexes 1–3 are isostructural and isomorphous (Fig. 2,
Table 1). Table 2 provides selected bond lengths and angles
for complex 1–3. Each of complex 1–3 (τ = 0.2–0.3) adopts a
distorted square pyramidal geometry, in which the rare-earth
metal center is boned with one monoanionic bis(imino)diphe-
nylamido unit and two trimethylsilylmethyl groups. The Ln–N
bond with the negatively charged amido nitrogen N(2) is sig-
nificantly shorter at 2.141(2)–2.282(2) Å than the Ln–N bonds
with the neutral imino nitrogens N(1) and N(3) at 2.327(2)–
2.457(2) Å. Because of the ionic radius of the metal center in a
trend of Sc (0.89 Å) < Lu (1.00 Å) < Y (1.04 Å), the bond dis-
tances of the chelating Ln–N(1), Ln–N(2), Ln–N(3), Ln–C(1) as
well as Ln–C(5) bond increase in order of 1 < 2 < 3.
1
freely. That the H NMR spectra of complex 3 had no changes
according to the temperature range 25 to −40 °C (Fig. 1) also
ascertained this conclusion. These results suggested that the
complexes 1–3 bearing bis(imino)diphenylamido pincer ligand
have large steric hindrance around metal center.
The Ln–N(2) bonds of complexes 1–3 divide the angles of
N(1)–Ln–N(3) (147.2(1)–154.0(1)°) into two almost equal parts
Scheme 1 Synthesis of bis(imino)diphenylamido rare-earth metal dialkyl
complexes.
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Table 1 Crystallographic data and structure refinement details for complex 1–3
1
2
3
Formula
fw
Cryst syst
Space group
a (Å)
b (Å)
c (Å)
C
53H74N3ScSi2
C53H74N3LuSi2
984.30
Monoclinic
P21/c
13.3938(8)
18.7363(12)
20.5867(13)
97.4890(10)
5122.2(6)
C53H74N3YSi2
898.24
Monoclinic
P21/c
13.506(1)
18.827(1)
20.695(1)
9 7.347(1)
5219.0(5)
854.29
Monoclinic
P21/c
13.661(2)
18.871(3)
20.672(3)
97.225(5)
5286.9(1)
4
Β (°)
V (Å3)
Z
4
4
Dc (Mg m−3
)
1.073
0.219
1848
1.276
2.010
2048
1.143
1.198
1920
μ (mm−1
F (000)
)
θ range
No. of reflns collected
No. of indep reflns
2.01 to 25.02°
58 700
9346 (Rint = 0.0446)
100.0%
1.53 to 25.02°
24 877
8997 (Rint = 0.0176)
99.5%
3.23 to 25.02°
24 387
9178 (Rint = 0.0259)
99.7%
Completeness to θ
Max. and min. transmn
No. of data/restraints/params
Goodness-of-fit on F2
Final R indices [I > 2σ(I)]
R indices (all data)
0.9273/0.9079
9346/0/579
1.055
R1 = 0.0446, wR2 = 0.1060
R1 = 0.0648, wR2 = 0.1232
0.302/−0.283
0.4857/0.4649
8997/0/543
1.198
R1 = 0.0204, wR2 = 0.0731
R1 = 0.0229, wR2 = 0.0870
1.039/−0.882
0.6791/0.6146
9178/0/548
1.017
R1 = 0.0329, wR2 = 0.0725
R1 = 0.0488, wR2 = 0.0782
0.293/−0.242
Largest diff. Peak and hole, e Å−3
complexes 1–3 are much shorter, only lying 0.195(2)–0.262(2) Å
out of the NNN plane. The dihedral angels of the best planes
of N(1)–Ln–N(3) and NNN are around 21.1(1)–22.3(1)°, consist-
ent with an almost planar four-membered metallacyclic ring
(NNNLn) in complexes 1–3. In contrast, the distances of C(1)
and C(5) to the NNN plane in complexes 1–3 are much longer
(about 1.207(4)–1.250(5) Å and 2.318(3)–2.540(4) Å, respect-
ively) and increase in order of 1 < 2 < 3. As a result, the steric
environment around rare-earth metal center decreases as the
follow: 1 > 2 > 3.
Table 2 Selected bond lengths [Å] and angles [°] for complex 1–3
1
2
3
Ln=
Ln–N1
Ln–N2
Ln–N3
Ln–C1
Ln–C5
Sc
Lu
Y
2.342(2)
2.141(2)
2.327(2)
2.226(2)
2.224(2)
77.5(1)
2.407(2)
2.239(2)
2.382(2)
2.345(3)
2.333(3)
75.4(1)
2.457(2)
2.282(2)
2.429(2)
2.383(2)
2.390(2)
73.8(1)
∠N1–Ln–N2
∠N2–Ln–N3
∠N1–Ln–N3
∠N2–Ln–C1
∠N2–Ln–C5
∠C1–Ln–C5
∠N1–Ln–C1
∠N1–Ln–C5
∠N3–Ln–C1
∠N3–Ln–C5
∠C9–N1–Ln
∠C35–N3–Ln
Ln–N1N2N3
C1–N1N2N3
C5–N1N2N3
C9–N1N2N3
C35–N1N2N3
∠N1LnN3–N1N2N3
∠C1LnC5–N1N2N3
∠C9–C14–N1N2N3Ln
∠C35–C40–N1N2N3Ln
77.7(1)
74.9(1)
74.6(1)
154.0(1)
112.6(1)
135.7(1)
111.7(1)
99.9(1)
97.4(1)
96.4(1)
149.2(1)
112.7(1)
134.9(1)
112.2(1)
96.4(1)
96.3(1)
102.5(1)
98.8(1)
130.9(2)
120.9(2)
0.230(2)
−2.459(4)
1.252(5)
0.766(4)
−0.861(4)
22.3(1)
147.2(1)
113.3(1)
134.2(1)
112.4(1)
103.4(1)
99.1(1)
A preliminary study showed that the complexes 1–3 alone
exhibited very high activities in the ring-opening polymeriz-
ation of ε-caprolactone (Table 3). Moreover, their activities
depended on the steric hindrance around the metal center.
The sterically demanding scandium complex 1 showed the
97.1(1)
96.3(1)
95.2(1)
lowest activities ca. 1.0 × 105 g mol−1
h
−1, which converted
124.5(1)
131.8(1)
−0.195(2)
−2.318(3)
1.207(4)
−0.793(3)
0.858(4)
21.5(1)
120.5(1)
131.5(1)
−0.262(2)
−2.540(4)
1.250(5)
−0.755(3)
0.839(3)
21.1(1)
4000 equiv. of CL in 4 h at room temperature (Table 3, entries
1–2). By contrast, the relatively bulky lutetium complex 2 exhi-
bited much higher activities up to 1.6 × 106 g mol−1 h−1 under
the same condition (Table 3, entries 3–8). It could completely
convert 12 000 equiv. of CL in 3 h, yielding high molecular
weight PCL (Mn = 310 000 g mol−1) with relatively narrow mo-
lecular weight distribution (Mw/Mn = 1.57) (Table 3, entry 8). It
is noteworthy that the less bulky yttrium complex 3 exhibited
exceptional activities up to 5.5 × 107 g mol−1 h−1 in the ring-
88.9(1)
67.4(1)
53.8(1)
87.8(1)
70.2(1)
53.0 (1)
87.7(1)
70.5(1)
52.3 (1)
N(1)–Ln–N(2) and N(2)–Ln–N(3) (73.8(1)–77.7(1)°), suggesting opening polymerization of CL (Table 3, entries 9–23). Only
2 hours was needed to completely convert 12 000 equiv. of
monomer, producing high molecular weight PCL (Mn
the N(1), N(2), N(3) and Ln atom are almost planar. These
=
angles decrease in order of 1 > 2 > 3. Whereas the angles of
C(35)–N(3)–Ln increased in a trend of 1 (114.7(1)°) < 2 920 000 g mol−1) with narrow molecular weight distribution
(Mw/Mn = 1.39) (Table 3, entry 16). The order of the gradually
increased activities from 1 to 3 is consistent with the increased
(130.9(2)°) < 3 (131.5(1)°). These results show that this pincer
ligand occupy the less steric space around the larger metal
center. The Ln, C(1), and C(5) atoms have slightly folded geo- trend of their ionic radius of the metal center. Such an “ionic
radius effect” on the catalytic activity was also observed
metry with the NNN plane. The Ln–NNN distances in
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Table 3 Polymerization of ε-caprolactone by bis(imino)diphenylamido rare-earth metal dialkyl complexes 1–3a
Act.b
Mnc (104)
Mw/Mn
Tmd (°C)
c
Entry
Cat. (Ln)
CL (mmol)
[CL]/[Cat.]
t (min)
T (°C)
Yield (%)
1
2
3
4
5
6
7
8
1(Sc)
1(Sc)
2(Lu)
2(Lu)
2(Lu)
2(Lu)
2(Lu)
2(Lu)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
10
20
10
20
30
40
50
60
5
5 + 5
10
20
30
40
50
60
40
40
40
40
40
40
40
2000
4000
2000
4000
6000
8000
10 000
12 000
1000
2000
2000
4000
6000
8000
10 000
12 000
8000
8000
8000
8000
8000
8000
8000
120
240
15
20
25
60
120
180
2
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
25
50
70
90
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
100
56
0.1
0.1
0.9
1.4
1.6
0.9
0.6
0.5
3.4
2.7
2.7
2.7
2.7
2.7
1.7
0.7
30.7
13.9
9.5
3.5
5.5
11.0
54.8
17
26
17
25
54
58
60
31
10
25
24
36
54
69
80
92
53
59
63
67
63
73
44
1.26
1.35
1.35
1.45
1.41
1.50
1.51
1.57
1.21
1.22
1.25
1.26
1.36
1.39
1.40
1.39
1.26
1.32
1.31
1.34
1.37
1.51
1.58
59
57
57
58
57
58
56
56
57
57
57
57
56
57
57
57
55
56
57
55
56
55
56
9
10e
11
12
13
14
15
16
17
18
19
20
21
22
23
5
5
10
15
20
40
120
1
3
5
15
10
5
76
86
96
100
100
100
3(Y)
3(Y)
1
a Conditions: 5 μmol of catalyst, 30 mL of toluene. b Activity in 106 g mol−1 h−1 c Determined by GPC in THF at 40 °C against polystyrene
.
standard. d Measured by DSC. e Polymerization of 1000 equiv. of CL in 30 mL of toluene for 2 min followed by the addition of 1000 equiv. of CL
for another 3 min.
Fig. 3 Plot of monomer conversions vs. molecular weights and molecular
weight distributions of the PCLs (a–e: Table 3, entries 14, 17–20).
Fig. 4 Plot of molecular weights and molecular weight distributions of the
PCLs (a: Table 3, entry 9; b: Table 3, entry 10).
previously in the rare-earth-metal-catalyzed ε-caprolactone
polymerization.31 These catalysts also behaved living nature in character. In addition, the temperature has significant influ-
the ring-opening polymerization of CL. For complex 3, with an ence on the activity. With the temperature increasing from 25
increase in the monomer conversion, the molecular weight of to 90 °C, the activity of complex 3 gradually increased from 2.7
the resultant PCL increased linearly from 530 000 to 690 000 g to 54.8 × 106 g mol−1 h−1 (Table 3, entries 14, 21–23).
mol−1, whereas the molecular weight distribution remained
The GPC curves of the resulting PCLs are all unimodal with
very narrow (Mw/Mn = 1.26–1.39) (Fig. 3, Table 3, entries 14, narrow molecular weight distributions (Mw/Mn = 1.21–1.58), in
17–20). Moreover, after complete consumption of the first 1000 consistence with the predominance of a homogeneous single-
equiv. of CL in 2 min, the polymer chain ends are still active site catalytic species. The 1H NMR spectrum of PCL (Fig. 5,
and continually concerted the second 1000 equiv. of CL to PCL complex 3, [CL]/[Y] = 100) displays the presence of an end
in 3 min (Table 3, entries 9–10). The molecular weight of the group of HOCH2– which assigns its methylene protons signal
resulting PCL increased from 100 000 g mol−1 (Mw/Mn = 1.21) at 3.64 ppm. The resonances for the other end group
to 250 000 g mol−1 (Mw/Mn = 1.22), whereas the molecular –COCH2SiMe3 derived from the coordination/insertion of CL
weight distributions remained unchanged (Fig. 4). These into metal alkyl species Ln–CH2SiMe3 are observed at 0.07
results demonstrated that the complex 3 indeed has the living (SiMe3) and 0.11 (CH2) ppm.
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Remarkably, the complexes 2 and 3 also showed good activi-
ties in the ring-opening copolymerization of ε-caprolactone
and γ-butyrolactone (BL), even though these catalysts could
Dalton Transactions
Experimental section
General methods
not promote the ring-opening polymerization of BL. Some All manipulations of air and moisture-sensitive compounds
representative results are summarized in Table 4. The complex were performed under a nitrogen atmosphere in an Mbraun
2 showed higher activity (1.4 × 105 g mol−1 h−1) than complex glovebox. Anhydrous toluene, THF and hexane were purified
3 (0.6 × 105 g mol−1 h−1) in the equal amount of CL and BL at by use of a SPS-800 solvent purification system (Mbraun), and
room temperature. Whereas the resulting poly(CL-co-BL) has dried over fresh Na chips in the glovebox. ε-Caprolactone was
less BL content (ca. 12 mol%) than that obtained by complex 3 purchased from J&K Chemical, dried over CaH2, vacuum-trans-
(BL content ca. 15 mol%) (Table 4, entries 1 and 6). Moreover, ferred, and degassed by two freeze–pump–thaw cycles prior to
the resulting copolymer by complex 2 has bad solubility in polymerization experiments. LnCl3 was purchased from Strem.
THF, as a result the molecular weight could not been obtained. LiCH2SiMe3 (1.0 M solution in pentane) was purchased from
For the complex 3, with the increased mole ratio of CL and BL Aldrich and used as received. The anionic NNN bis(imino)-
monomer from 3 : 1 and 1 : 3, the BL contents of the resulting diphenylamide pincer ligand [o-(2,6-iPr2-C6H3–NvC–C6H4)2–
copolymers also increased from 6 mol% to 18 mol% (Table 4, NH] and Ln(CH2SiMe3)3(THF)2 were synthesized according to
entries 2–7).
the literature.29,30 The deuterated solvents C6D6 (99.6 atom%
The copolymers obtained by complex 3 show good solubili- D), CDCl3 (99.8 atom% D) and C7D8 (99.6 atom% D) were
ties in THF and benzene. The 1H NMR and 13C NMR spectrum obtained from Cambridge Isotope.
suggest random distribution of the monomer moieties in the
copolymer products (Fig. 6 and 7). The GPC curves of the co-
polymers are all unimodal with moderate polydispersities (Mw/
Mn = 1.31–1.71), in consistence with the predominance of a
homogeneous single-site catalytic species. The metal tempera-
ture (Tm) of the copolymers shows a decreased tendency corre-
lation with the increased γ-butyrolactone content.
Fig. 6 1H NMR spectrum of poly(CL-co-BL).
Fig. 5 1H NMR spectrum of PCL.
Fig. 7 13C NMR spectrum of poly(CL-co-BL).
Table 4 Random copolymerization of ε-caprolactone with γ-butyrolactone by bis(imino)diphenylamido rare-earth metal dialkyl complexes 2–3a
Actb
Cont. of BLc (mol%)
Mn (103)d
Mw/Mn
Tme (°C)
d
Entry
Cat. (Ln)
CL (mmol)
BL (mmol)
CL/BL
t (h)
Yield (g)
1
2
3
4
5
6
7
2(Lu)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
3(Y)
30
45
40
30
30
20
10
30
15
20
30
10
20
30
1/1
3/1
2/1
1/1
3/1
1/1
1/3
3
3
3
3
1
1
1
2.1
3.7
1.3
0.9
3.7
1.8
0.5
1.4
2.5
0.9
0.6
7.4
3.6
1.4
12
6
7
15
6
12
18
—
—
48
52
53
48
51
49
45
81
65
85
88
106
25
1.50
1.63
1.30
1.51
1.31
1.71
a Conditions: 5 μmol of catalyst, 30 mL of toluene, 25 °C. b Activity in 105 g mol−1 h−1
.
c Determined by 1H NMR. d Determined by GPC in THF at
40 °C against polystyrene standard. e Measured by DSC.
1282 | Dalton Trans., 2013, 42, 1278–1286
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Samples of rare-earth metal complexes for NMR spectro- CHN), 11.19 (s, 1H, NH). 13C NMR (400 MHz, CDCl3, 25 °C, δ/
scopic measurements were prepared in the glovebox using ppm): 23.2 (s, CH3), 27.9 (s, CH(CH3)2), 119.5, 121.3, 122.9,
J. Young valve NMR tubes. The NMR (1H, 13C) spectra were 124.2, 124.7, 131.7, 132.1, 137.7, 144.5, 149.1 (m, ArC), 162.5
recorded on a Bruker 400 or 600 spectrometer at a suitable (s, NvC).
temperature in CDCl3, C6D6 or C7D8 as a solvent. Elemental
SYNTHESIS OF [O-(2,6-iPr2-C6H3–NvC–C6H4)2–N]SC(CH2SiMe3)2
analyses were performed on an Elementar Vario MICRO CUBE (1). A toluene solution (5 mL) of o-(2,6-iPr2-C6H3–NvC–
(Germany). The molecular weights and molecular weight dis- C6H4)2–NH (1.55 g, 2.85 mmol) was added to a hexane solution
tributions of polymerization were determined at 40 °C by gel (5 mL) of Sc(CH2SiMe3)3(THF)2 (1.29 g, 2.85 mmol) at room
permeation chromatography on a Waters 1515 HPLC using temperature. The yellow mixture was stirred at room tempera-
THF as an eluent at a flow rate of 1 mL min−1 against poly- ture for 1.5 hours. After removal of all volatiles in vacuo, the
styrene standards. The DSC measurements were performed on residue was recrystallized from hexane at −30 °C to give 1 as
1
a Q2000 (TA Co.) Differential Scanning Calorimeter at a rate of orange-yellow crystals (1.50 g, 69% yield). H NMR (400 MHz,
20 °C min−1. Any thermal history difference in the polymers C6D6, 25 °C, δ/ppm): −0.08 (s, 18H, CH2Si(CH3)3), 0.04 (d, J =
was eliminated by first heating the specimen to 300 °C, 11.60, 2H, CH2SiMe3), 0.18 (d, J = 11.60, 2H, CH2SiMe3), 0.93
cooling at 10 °C min−1 to −90 °C, and then recording the (d, 6H, CH(CH3)2), 1.11 (d, 6H, CH(CH3)2), 1.45 (d, 6H, CH-
second DSC scan.
(CH3)2), 1.75 (d, 6H, CH(CH3)2), 2.83 (m, 2H, CH(CH3)2), 4.08
SYNTHESIS OF O-(C6H4–COOCH3)2–NH. The o-(C6H4–COOH)2– (m, 2H, CH(CH3)2), 6.76–7.35 (m, 14H, ArH), 8.32 (s, 2H,
NH (5.02 g, 19.50 mmol) was added to stirring methanol CHvN,). 13C NMR (400 MHz, C6D6, 25 °C, δ/ppm): 3.8 (s, Si-
solvent (50 mL), then concentrated H2SO4 (5 mL) was dropped (CH3)3), 21.4, 22.5, 22.9, 23.4, 26.3, 26.5, 28.1, 29.1 (m, CH-
in. The reaction mixture was refluxed for 3 hours. After remov- (CH3)2, CH(CH3)2), 44.4 (s, ScCH2Si(CH3)3), 119.9, 123.4, 124.6,
ing the methanol, the reaction mixture was poured into water 125.0, 125.7, 127.4, 128.5, 129.3, 133.9, 134.7, 141.0, 141.9,
and treated by Na2CO3 solution. The solution was extracted 149.0, 150.4 (m, ArC), 167.9 (s, CvN). Anal. Calcd for
with EtOAc (2 × 20 mL) and the combined organic solvent was C53H74N3ScSi2: C, 63.50; H, 9.56; N, 4.36. Found: C, 63.35; H,
removed under reduced pressure to obtain the pure o-(C6H4– 9.88; N, 4.20.
COOCH3)2–NH as pale yellow solid (5.34 g, 96%). 1H NMR
(400 MHz, CDCl3, 25 °C, δ/ppm): 3.94 (s, 6H, CH3), 6.88–8.00 (2). A toluene solution (5 mL) of o-(2,6-iPr2-C6H3–NvC–
(m, 8H, ArH), 11.06 (s, 1H, NH). C6H4)2–NH (2.72 g, 5.00 mmol) was added to a hexane solution
SYNTHESIS OF [O-(2,6-iPr2-C6H3–NvC–C6H4)2–N]Lu(CH2SiMe3)2
SYNTHESIS OF O-(C6H4–CHO)2–NH. The o-(C6H4–COOCH3)2– (5 mL) of Lu(CH2SiMe3)3(THF)2 (2.90 g, 5.00 mmol) at room
NH (6.70 g, 23.48 mmol) was added to a suspension of LiAlH4 temperature. The yellow mixture was stirred for 0.5 hours.
(2.00 g, 52.70 mmol) in anhydrous ether (200 mL) during After removal of all volatiles in vacuo, the residue was recrystal-
1 hour. The resulting mixture was refluxed for 4 hours. After lized from hexane at −30 °C to give 2 as orange-yellow crystals
1
cooling, the reaction mixture was treated sequentially by water (3.21 g, 72% yield). H NMR (400 MHz, C6D6, 25 °C, δ/ppm):
(3 mL), NaOH (3 mL, 15%) and water (50 mL), and then fil- −0.70 (d, J = 12.00, 2H, CH2SiMe3), −0.59 (d, J = 12.00, 2H,
tered. The precipitate was extracted by boiling chloroform (4 × CH2SiMe3), −0.01 (s, 18H, CH2Si(CH3)3), 0.94 (d, 6H, CH-
100 mL) and the combined extracts and filtrate were concen- (CH3)2), 1.17 (d, 6H, CH(CH3)2), 1.41 (d, 6H, CH(CH3)2), 1.71
trated to afford the white solid o-(C6H4–CH2OH)2–NH was (d, 6H, CH(CH3)2), 2.87 (m, 2H, CH(CH3)2), 3.86 (m, 2H, CH-
afforded. The o-(C6H4–CH2OH)2–NH (5.00 g, 21.81 mmol) in (CH3)2), 6.70–7.30 (m, 14H, ArH), 8.18 (s, 2H, CHvN). 13C
anhydrous ether (100 mL) and active MnO2 (38.00 g, NMR (400 MHz, C6D6, 25 °C, δ/ppm): 4.1 (s, Si(CH3)3), 21.4,
437.08 mmol) were stirred and refluxed together for 8 hours. 22.4, 23.0, 26.1, 26.2, 28.5, 29.2, 31.9 (m, CH(CH3)2, CH-
The MnO2 was filtered off and extracted with boiling chloro- (CH3)2), 44.9 (s, LuCH2Si(CH3)3), 119.6, 124.4, 124.6, 124.7,
form (3 × 100 mL). The combined extracts and filtrate were 125.7, 127.5, 128.5, 129.3, 134.2, 135.2, 141.0, 141.7, 148.1,
concentrated to afford the o-(C6H4–CHO)2–NH as yellow crystal 151.6 (m, ArC), 169.0 (s, CvN). Anal. Calcd for C53H74N3ScSi2:
(3.29 g, 67%). 1H NMR (400 MHz, CDCl3, 25 °C, δ/ppm): C, 52.82; H, 7.95; N, 3.62. Found: C, 52.77; H, 7.85; N, 3.43.
7.02–7.71 (m, 8H, ArH), 10.02 (s, 2H, CHO), 11.34 (s, 1H, NH).
SYNTHESIS OF [O-(2,6-iPr2-C6H3–NvC–C6H4)2–N]Y(CH2SiMe3)2
SYNTHESIS OF [O-(2,6-iPr2-C6H3–NvC–C6H4)2–NH]. 2,6-iPr2- (3). A toluene solution (5 mL) of o-(2,6-iPr2-C6H3–NvC–
C6H3–NH2 (0.66 g, 3.70 mmol), o-(C6H4–CHO)2–NH (0.42 g, C6H4)2–NH (1.63 g, 3.00 mmol) was added to a hexane solution
1.85 mmol), 10 mL methanol, 2 drops of methane acid, and (5 mL) of Y(CH2SiMe3)3(THF)2 (1.48 g, 3.00 mmol) at room
5 Å molecular sieves (approximately 50% by volume) were temperature. The yellow mixture was stirred for 3.5 hours.
added to a 25 mL round bottom flask. The mixture was heated After removal of all volatiles in vacuo, the residue was recrystal-
to reflux and allowed to react overnight. The reaction mixture lized from hexane at −30 °C to give 3 as orange-yellow crystals
1
was hot filtrated. Filtrate was removed under pressure and the (1.57 g, 65% yield). H NMR (400 MHz, C6D6, 25 °C, δ/ppm):
residue was purified by flash silica gel column chromato- −0.47 (d, J = 11.20, 2H, CH2SiMe3), −0.42 (d, J = 11.60, 2H,
graphy with a petroleum ether–EtOAc mixture (15/1, v/v) to CH2SiMe3), −0.02 (s, 18H, CH2Si(CH3)3), 0.95 (d, 6H, CH-
obtain the pure product as bright yellow crystals (0.72 g, 72%). (CH3)2), 1.19 (d, 6H, CH(CH3)2), 1.41 (d, 6H, CH(CH3)2), 1.71
1H NMR (400 MHz, CDCl3, 25 °C, δ/ppm): 0.98 (d, 24H, CH3), (d, 6H, CH(CH3)2), 2.91 (m, 2H, CH(CH3)2), 3.85 (m, 2H, CH-
2.87 (m, 4H, CH(CH3)2), 7.03–7.79 (m, 14H, ArH), 8.36 (s, 2H, (CH3)2), 6.74–7.30 (m, 14H, ArH), 8.20 (s, 2H, CHvN,). 13C
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NMR (400 MHz, C6D6, 25 °C, δ/ppm): 4.0 (s, Si(CH3)3), 21.4,
22.4, 23.1, 26.0, 26.2, 28.8, 29.3, 31.9 (m, CH(CH3)2, CH-
Conclusions
(CH3)2), 38.2, 38.6 (d, Y CH2Si(CH3)3), 119.6, 124.3, 124.6, In summary, we have demonstrated that the novel bis(imino)-
124.7, 125.7, 127.5, 128.5, 129.3, 134.2, 135.4, 140.9, 141.3, diphenylamido rare-earth metal dialkyl complexes 1–3 can be
147.8, 151.9 (m, ArC), 169.0 (s, CvN). Anal. Calcd for easily prepared by reaction of the rare-earth metal trialkyl com-
C53H74N3ScSi2: C, 59.44; H, 8.95; N, 4.08. Found: C, 59.60; H, plexes with the bis(imino)diphenylamido NNN pincer ligand.
9.06; N, 4.22.
With the increased ionic radius of the metal center in a trend
A TYPICAL PROCEDURE FOR ε-CAPROLACTONE POLYMERIZATION BY COMPLEX of Sc (0.89 Å) < Lu (1.00 Å) < Y (1.04 Å), the complexes 1–3 have
3 (TABLE 3, ENTRY 14). In the glovebox, [o-(2,6-iPr2-C6H3–NvC– the steric hindrance around the metal center decreasing in
C6H4)2–N]Y(CH2SiMe3)2 (4.03 mg, 5 μmol) was dissolved in order of 1 > 2 > 3. An “ionic radius effect” on the catalytic
30 mL of toluene in a 100 ml flask that was equipped with a activity of these complexes is observed in the ring-opening
magnetic stir bar. Then ε-caprolactone (4.57 g, 40 mmol) was polymerization of ε-caprolactone. The less bulky yttrium
added immediately. The color of the mixture changed immedi- complex 3 shows exceptional activity not only in the living
ately from dark yellow to pale yellow, and the resulting pale ring-opening polymerization of CL but also in the random
yellow solution quickly became very viscous. After 20 min, no copolymerization of CL and BL, yielding high molecular
solvent was left and the flash was taken outside. The polymer- weight PCLs or P(CL-co-BL) with narrow molecular weight dis-
ization was quenched by addition of alcohol (50 mL, contain- tributions. These results indicate that the introduction of the
ing 5% butylhydroxytoluene (BHT) as a stabilizing agent). The monoanionic NNN pincer ligand into the rare-earth metal
precipitated polymer was filtered and dried under vacuum at dialkyl complex has obviously impeded the transesterification
60 °C overnight to a constant weight (4.57 g, 100% yield).
reactions and yields PCL with narrow polydispersity in the
A TYPICAL PROCEDURE FOR COPOLYMERIZATION OF ε-CAPROLACTONE WITH polymerization of ε-caprolactone. Further studies on the
γ-BUTYROLACTONE BY COMPLEX 3 (TABLE 4, ENTRY 6). In the glovebox, detailed polymerization mechanism and related transition
[o-(2,6-iPr2-C6H3–NvC–C6H4)2–N]Y(CH2SiMe3)2
(4.03
mg, metal catalysts bearing bis(imino)diphenylamido ligand are in
5 μmol) was dissolved in 30 mL of toluene in a 100 ml flask progress.
that was equipped with a magnetic stir bar. Then a mixture of
ε-caprolactone (2.28 g, 20 mmol) and γ-Butyrolactone (1.72 g,
20 mmol) was added immediately. After 60 min, the polymeriz-
Acknowledgements
ation was quenched by addition of alcohol (50 mL, containing
5% butylhydroxytoluene (BHT) as a stabilizing agent). The pre- We gratefully acknowledge the support from the National
cipitated polymer was filtered and dried under vacuum at Natural Science Foundation of China (No. 20974014,
60 °C overnight to a constant weight (1.80 g colorless polymer 21173022, 21274012), the 111 project (B07012) and Basic
was obtained).
Research Fund of Beijing Institute of Technology
(20111942018).
X-ray crystallographic studies
Notes and references
A crystal was sealed in a thin-walled glass capillary under a
microscope in the glove box. Data collections were performed
at −100 °C on a Bruker Smart-Apex CCD diffractometer with a
CCD area detector using graphite-monochromated Mo Kα radi-
ation (λ = 0.71073 Å). The determination of crystal class and
unit cell parameters was carried out by the SMART program
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refinement of the parameters. The residual electron densities
were of no chemical significance. Crystallographic data
(excluding structure factors) have been deposited with the
Cambridge Crystallographic Data Centre. CCDC 857397 (1),
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1286 | Dalton Trans., 2013, 42, 1278–1286
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