Rare-earth alkyl complexes supported by formamidinate ligands: Synthesis, structure, and catalytic activity for isoprene polymerization

Article abstract of DOI:10.1039/c4dt00242c

A series of rare-earth metal monoalkyl complexes supported by N,N′-di(2,6-dialkylphenyl)formamidinate ligand (L)₂RECH ₂SiMe₃·thf [L¹ = HC(N-2,6-Me ₂C₆H₃)₂, RE = Y (1), L² = HC(N-2,6-ⁱPr₂C₆H₃)₂, RE = Y (2), Er (3), Dy (4), Sm (5), and Nd (6)] were synthesized by alkyl elimination reaction or by salt metathesis reaction in good yields. All complexes were characterized by elemental analyses, FT-IR spectroscopy and single crystal X-ray diffraction. In combination with [Ph₃C][B(C ₆F₅)₄] and alkylaluminium, these complexes displayed a good activity towards isoprene polymerization to give polyisoprenes with high molecular weight (M_n > 10⁴) and narrow molecular distribution (PDI < 2.0). The influence of alkylaluminium, central metal, temperature, sequence of addition of alkylaluminium and [Ph ₃C][B(C₆F₅)₄] on the polymerization of isoprene was studied. It was interesting to find that addition of the cocatalysts sequence has a great influence on the regioselectivity of the polymerization. High 1,4-regioselectivity polymerizations of isoprene (as high as 98%) were observed when the catalysts were added in the order [RE]/[alkylaluminum]/[borate]. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4dt00242c

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Rare-earth alkyl complexes supported by
formamidinate ligands: synthesis, structure, and
catalytic activity for isoprene polymerization†
Cite this: Dalton Trans., 2014, 43,
6842
Liping Guo,a Xiancui Zhu,a Shuangliu Zhou,*a Xiaolong Mu,a Yun Wei,a
Shaowu Wang,*a,b Zhijun Feng,a Guangchao Zhanga and Baojia Denga
A series of rare-earth metal monoalkyl complexes supported by N,N’-di(2,6-dialkylphenyl)formamidinate
ligand (L)2RECH2SiMe3·thf [L1 = HC(N-2,6-Me2C6H3)2, RE = Y (1), L2 = HC(N-2,6-iPr2C6H3)2, RE = Y (2),
Er (3), Dy (4), Sm (5), and Nd (6)] were synthesized by alkyl elimination reaction or by salt metathesis reaction
in good yields. All complexes were characterized by elemental analyses, FT-IR spectroscopy and single
crystal X-ray diffraction. In combination with [Ph3C][B(C6F5)4] and alkylaluminium, these complexes dis-
played a good activity towards isoprene polymerization to give polyisoprenes with high molecular weight
(Mn > 104) and narrow molecular distribution (PDI < 2.0). The influence of alkylaluminium, central metal,
temperature, sequence of addition of alkylaluminium and [Ph3C][B(C6F5)4] on the polymerization of iso-
prene was studied. It was interesting to find that addition of the cocatalysts sequence has a great influence
on the regioselectivity of the polymerization. High 1,4-regioselectivity polymerizations of isoprene (as high
as 98%) were observed when the catalysts were added in the order [RE]/[alkylaluminum]/[borate].
Received 23rd January 2014,
Accepted 20th February 2014
DOI: 10.1039/c4dt00242c
www.rsc.org/dalton
the carbon atom. In contrast to other amidinate ligands, the
formamidinate has a low tension of the N–C–N moiety and is
Introduction
As an alternative to cyclopentadienyl, amidinates have been more flexible in stereochemistry. The chemistry of formamidi-
widely used in the field of coordination and organometallic nate supported alkaline and alkaline earth metal complexes
chemistry because their steric and electronic properties can be has been reported.3 The formamidinate incorporated rare-
easily modulated by the substituents on C and N atoms.1 Over earth metal complexes were recently reported through reaction
the past few decades, different kinds of amidinate ligands of atomic metal, lanthanide(II) intermediate or Ln(AlMe4)3
have been successfully employed in organo rare-earth metal (Ln = La, Nd, Y, and Lu).4
chemistry for the synthesis and characterization of a series of
Monoamidinate rare-earth metal bisalkyls5 and bisami-
highly reactive species.2 Rare-earth metal amides,2a,b alkyls,2e des2a,6 have been found to be good precatalysts for olefin
and hydrides2c,d with amidinate as supporting ligands have polymerization. Herein, we would like to report the synthesis
been described and some of them have performed a high cata- and characterization of a series of rare-earth metal monoalkyl
lytic activity in organic transformation and polymerization complexes supported by formamidinate as well as their per-
reaction. In order to improve the stability of those complexes formance as precatalysts for polymerization of isoprene.
and to gain a full range of species, most of the amidinates
applied in rare-earth metal chemistry are decorated with bulky
groups, such as phenyl,2a,b cyclohexyl,2a and terphenyl2g on
Results and discussion
Synthesis and characterization of bis(formamidinate)
rare-earth metal alkylates
aLaboratory of Functionalized Molecular Solids, Ministry of Education, Anhui
Laboratory of Molecule-Based Materials, School of Chemistry and Materials Science,
Alkyl elimination reactions of RE(CH2SiMe3)3(thf)X with
Anhui Normal University, Wuhu, Anhui 241000, P. R. China.
2 equiv. of formamidinates 2,6-(CH3)2C6H3NvCH–NHC6H3-
(CH3)2-2,6 (HL1) or 2,6-(iPr)2C6H3NvCH–NHC6H3(iPr)2-2,6
(HL2) in hexane at room temperature for three hours, after
work-up, afforded the corresponding bis(formamidinate) rare-
earth metal alkyls [HC(N-2,6-R2C6H3)2]2RECH2SiMe3·thf (R =
E-mail: swwang@mail.ahnu.edu.cn, slzhou@mail.ahnu.edu.cn;
Fax: +86-553-3883517; Tel: +86-553-5910015
bState Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Shanghai 200032, P. R. China. E-mail: swwang@mail.ahnu.edu.cn
†Electronic supplementary information (ESI) available. CCDC 982911–982916.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c4dt00242c
i
CH3, RE = Y (1), R = Pr, RE = Y (2), Er (3), Dy (4), Sm (5) in
6842 | Dalton Trans., 2014, 43, 6842–6847
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located on the backbone carbon (NCN carbon). The represen-
tative molecular structures of 1 and 2 are shown in Fig. 1 and
2, respectively. Selected bond parameters are listed in Table 1.
The two C–N bond lengths from the same formamidinate
ligand are similar and fall in the intervals of the typical single
bond and double bond lengths (e.g., C(13)–N(1) = 1.313(4) Å
and C(13)–N(2) = 1.323(4) Å in 2) indicating that the negative
charges of anionic formamidinates are delocalized on the
NCN cores. The length of the Y–Calkyl bond in 2 (2.376 Å) is
slightly longer than that found in 1 (2.371 Å); this might have
i
resulted from the steric effect of the bulky Pr group, but they
Scheme 1 Synthesis of rare-earth metal alkyl complexes 1–6.
are both shorter than the length of Y–C (2.427(19) Å) in
64% to 73% yields (Scheme 1). This kind of complex can also
be prepared by one-pot salt metathesis reaction of anhydrous
LnCl3 with 2 equiv. of formamidinate lithium L2Li (formed by
in situ treatment of formamidinate HL2 with 1 equivalent of
n-BuLi in THF at room temperature) followed by treatment of
1 equivalent of LiCH2SiMe3 in THF at room temperature for
three hours. A THF coordinated complex [HC(N-2,6-iPr2C6H3)2]2-
NdCH2SiMe3·thf (6) formulated the same as those of 2–5 was
isolated (Scheme 1).
All complexes are extremely sensitive to air and moisture
and soluble in hexane, toluene and THF. They were character-
ized by elemental analyses, FT-IR spectroscopy and single
crystal X-ray diffraction. FT-IR spectroscopy of these complexes
are devoid of any N–H stretching around 3300 cm−1 and
showed a strong absorption of CvN stretch at approximately
1650 cm−1 (1) and 1664 cm−1 (2–6) indicating the delocalized
double bond of the N–C–N linkage.4 Complexes 1 and 2 were
also characterized by NMR spectroscopic techniques.
Fig. 2 The representative molecular structure of complexes 2–6. Hydrogen
atoms are omitted for clarity.
Table 1 Selected bond lengths (Å) and bond angles (°) of complexes
1–6
1
2
3
X-ray diffraction revealed that all bis(formamidinate) rare-
earth metal alkyl complexes 1–6 adopted the same coordi-
nation environment, involving one carbon of the alkyl group,
one oxygen atom of the coordinated THF molecule and four
nitrogen atoms. Both formamidinate ligands in those com-
plexes are κ2-bonded to the metal ion through two nitrogen
donor atoms. The coordinated geometry of these complexes
can be described as a distorted tetrahedron if the formamidi-
nate ligand can be considered to be a single-point donor
Ln(1)–N(1)
Ln(1)–N(2)
Ln(1)–N(3)
Ln(1)–N(4)
2.388(3)
2.411(3)
2.400(3)
2.378(3)
2.394(3)
2.371(4)
2.396(2)
2.390(2)
2.443(2)
2.475(3)
2.391(3)
2.425(3)
2.376(3)
2.406(2)
1.313(4)
1.323(4)
1.303(4)
1.319(4)
56.18(8)
55.51(9)
130.29
2.365(6)
2.439(6)
2.423(6)
2.367(5)
2.398(6)
2.365(8)
2.402(5)
1.323(9)
1.332(9)
1.322(8)
1.335(9)
56.5(2)
Ln(1)−Nav
Ln(1)–Calkyl
Ln(1)−O(1)
C(13)−N(1)
C(13)−N(2)
C(38)−N(3)
C(38)−N(4)
N(1)–Ln(1)–N(2)
N(3)–Ln(1)–N(4)
C(13)–Ln(1)–C(38)
56.44(10)
56.40(10)
56.65(19)
130.34
4
5
6
Ln(1)–N(1)
Ln(1)–N(2)
Ln(1)–N(3)
Ln(1)–N(4)
2.4051(19)
2.4776(17)
2.3936(19)
2.4539(17)
2.4326(18)
2.375(2)
2.4224(17)
1.326(3)
1.306(3)
1.324(3)
1.318(3)
55.39(6)
56.02(6)
2.519(4)
2.456(4)
2.437(4)
2.494(4)
2.477(4)
2.388(6)
2.482(4)
1.305(6)
1.321(7)
1.321(6)
1.316(6)
54.39(14)
54.94(13)
130.42
2.550(3)
2.482(3)
2.467(3)
2.519(3)
2.505(3)
2.440(4)
2.519(3)
1.307(4)
1.319(4)
1.315(4)
1.314(4)
53.74(9)
54.33(9)
130.39
Ln(1)−Nav
Ln(1)–Calkyl
Ln(1)−O(1)
C(13)−N(1)
C(13)−N(2)
C(38)−N(3)
C(38)−N(4)
N(1)–Ln(1)–N(2)
N(3)–Ln(1)–N(4)
C(13)–Ln(1)–C(38)
Fig. 1 Molecular structure of complex 1. Hydrogen atoms are omitted
for clarity.
130.76
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Y(CH2SiMe3)3(thf)3.9 The lengths of Y–Calkyl in 1 and 2 fall into
Table 2 Isoprene polymerization by the 3/borate/AlR3 systema
the intervals of distances typical for six-coordinated alkyl
a
c
AlR3
Entry Borateb [equiv.]
Time Conv Mn
[min] [%]
yttrium
complexes
[(iPr)2TACN(CH2)2NtBu]Y(CH2SiMe3)2
[10−4
]
PDIc 1,4/3,4d
(2.476(5) and 2.421(7) Å),7a [Me3SiNC(Ph)N(CH2)3NC(Ph)NSiMe3]-
YCH(SiMe3)2(thf) (2.410(8) Å),7b and Ap′ YCH2SiMe3(thf)
1
2
3
4
5
6
7
[B]C
[B]C
[B]C
[B]C
[B]N
[B]C
[B]C
—
360
—
98
—
—
—
2
AlMe3 (10) 360
14.1
7.3
12.4
12.3
10.9
12.2
2.0
1.9
1.8
1.8
1.7
1.8
66/34
47/53
39/61
37/63
34/66
50/50
(2.342(5) Å) (Ap′-H = 2,6-(diisopropylphenyl)-[6-(2,6-dimethyl-
phenyl)pyridin-2-yl]amine).7c For complexes 2, 4, 5, and 6,
which were crystallized in the same space group and crystal
system, the length of RE–Calkyl increased with the increase of
the size of the central metal, and is in good accord with the
lanthanide contraction (Table 1). The angles between the two
bulky formamidinates in terms of the nonbonding angle
C(13)–Ln–C(38) in 2–5 are almost equal (e.g., 130.29° for 2;
130.34° for 3; 130.76° for 4; 130.42° for 5) and they are larger
than that in the less bulky formamidinates (e.g., C(9)–Y1–C(26)
122.73° for 1). The N(1)–Y(1)–N(2) and N(3)–Y(1)–N(4) bite
angles (56.44(10)° and 56.40(10)° for 1; 56.18(8)° and 55.51(9)°
for 2) are similar to C6H4-1,2-{NC(tBu)N(2,6-Me2C6H3)}2-
Y(CH2SiMe3)(thf) (55.00(5)° and 55.84(5)°).8 For compound 6,
the N(1)–Nd(1)–N(2) and N(3)–Nd(1)–N(4) bite angles
(53.74(9)° and 54.33(9)°) are all slightly smaller than those
found in [HC(N-2,6-iPr2C6H3)2]2NdBr(thf)4a (54.77(14)° and
55.03(14)°) as a result of the steric effect of the bulky
CH2SiMe3 group.
AlEt3 (10)
AliBu3 (10)
AliBu3 (10)
AliBu3 (5)
AliBu3 (20)
360
30
30
30
30
98
100
100
99
100
a Conditions: (Ln = Er, 10 μmol); isoprene–toluene = 1 : 10 (vol/vol);
T = 20 °C; Er–borate–IP = 1 : 1 : 1000; equiv. = AlR3/Er (molar ratio).
b [Borate]: [B]C
= [Ph3C][B(C6F5)4]; [B]N = [PhNMe2H][B(C6F5)4].
c Determined by means of GPC against polystyrene standards in THF at
30 °C. d Determined by 1H and 13C NMR spectroscopy.
with 98% conversion in 360 min with a ratio (63 : 34) of 1,4- to
3,4-regioselectivity at 20 °C. AlEt3 has a similar activity to
AlMe3 but with
a decreased 1,4-regioselectivity and an
increased 3,4-regioselectivity (Table 2, entry 3). Under the
same conditions, using AliBu3 instead of AlMe3 or AlEt3,
almost 100% conversion with a more improved 3,4-regioselec-
tivity was observed in 30 min (Table 2, entries 2–5), indicating
the steric effect of alkylaluminum on the regioselectivity of
these homogeneous catalytic systems. When the amount of
AliBu3 was reduced to 5 equivalents, 66% 3,4-regioselectivity
can be reached (Table 2, entry 6). The borates [Ph3C][B(C6F5)4]
and [PhNMe2H][B(C6F5)4] showed little effect on the polymeri-
zation activity and regioselectivity when AliBu3 was used
(Table 2, entries 4–5).
It is interesting to find that adjustment of the addition
sequence of borate and AlR3 has a dramatic effect on the
regioselectivity of polymerization. Addition of 1000 equivalents
of isoprene to the system of 3 mixed with 10 equivalents of
AlMe3 and 1 equivalent of [Ph3C][B(C6F5)4] in a sequence of
alkylaluminum and borate produced polyisoprene with a 99%
conversion having 98% 1,4-regioselectivity and high molar
weight Mn = 44.5 × 104 in 150 min at 20 °C (Table 3, entry 1).
Using AlEt3 instead of AlMe3 in the above catalytic system, the
1,4-regioselectivity of polymers dramatically decreased from
98% to 80% (Table 3, entry 2). However, using AliBu3 instead
of AlMe3 resulted in decrease of 1,4-regioselectivity of
polymers (Table 3, entry 3). This homogeneous catalytic
system showed high catalytic activities when the polymeri-
zation reaction was run at elevated temperature. For example,
the monomers were almost completely transferred (96% and
98%) when the temperatures were increased to 50 °C and
70 °C in short times (45 min and 35 min). To date, few cataly-
tic systems have been known to be so stable at such a high
temperature.2c,d However, the molar weight of the resulting
polyisoprene and 1,4-regioselectivity decreased and the molar
weight distribution became wide (Table 3, entries 5–6). This
might be attributed to the combined effects of the increased
number of active species and rapid chain transfer reactions at
high temperatures. It is also found that decreasing the
[M]/[Ln] ratio has a little effect on the molecular weight,
Isoprene polymerization
Cationic alkyl species of rare-earth metals (group 3 and lantha-
nide) have been intensively investigated over the past several
years because of their crucial importance in catalytic olefin
polymerization and unique activities that differ from those of
group 4 metal complexes. The cationic species can be gener-
ated conveniently by the reaction of neutral alkyl precursors
with an equimolar amount of a strong Lewis acid. The com-
plexes of rare-earth metal mono(alkyl) supported by bis-
[(7-(N-2,6-diisopropylphenyl)iminomethyl)indole] showed no
activity for the polymerization of isoprene with the combi-
nation of aluminum alkyls and borates; this phenomenon was
ascribed to the absence of a metal alkyl initiator after dealkyla-
tion by the borate [Ph3C][B(C6F5)4].5a The bis(formamidinate)
rare-earth metal monoalkyl complex 3 was tested as an
initiator for the polymerization of isoprene (IP) with the acti-
i
vation of aluminum alkyls (AlR3, R = Me, Et, Bu) and borates
B(C6F5)3, [Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4]. Complex 3
alone was inactive to the polymerization of isoprene, which
was still inert by the reaction with one equivalent of [Ph3C]-
[B(C6F5)4] or [PhNMe2H][B(C6F5)4], respectively (Table 2, entry
1). Upon addition of the third component aluminum alkyls,
homogeneous catalytic systems were generated and showed a
i
good activity. Various AlR3 (R = Me, Et, Bu) and borates have
been tested to this ternary catalytic system. The results are
summarized in Table 2. The polymerization activities of the
system were significantly dependent on both AlR3 and the
aluminum-to-lanthanide ratio. For example, addition of 1000
equivalents of isoprene to a mixture of 3 mixed with 1 equi-
valent of [Ph3C][B(C6F5)4] and 10 equivalents of AlMe3 in a
sequence of borate and alkylaluminum afforded polyisoprene
6844 | Dalton Trans., 2014, 43, 6842–6847
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Table 3 Isoprene polymerization by bis(formamidinate) rare-earth metal alkyls/AlR3/boratea
Entry
Precatalyst
[M]/[RE]
AlR3
Borateb
Temp [°C]
Time [min]
Conv [%]
Mn c [10−4
]
PDIc
1,4/3,4d
1
2
3
4
5
6
7
8
3
3
3
3
3
3
3
3
3
1
2
4
5
6
6
6
1000
1000
1000
1000
1000
1000
500
2000
3000
1000
1000
1000
1000
1000
1000
1000 + 1000
AlMe3
AlEt3
[B]C
[B]C
[B]C
[B]N
[B]C
[B]C
[B]C
[B]C
[B]C
[B]C
[B]C
[B]C
[B]C
[B]C
[B]C
[B]C
20
20
20
20
50
70
20
20
20
20
20
20
20
20
20
20
150
240
360
150
45
99
82
99
99
96
98
100
99
98
99
99
99
Trace
96
97
44.5
52.4
59
59
27
22
44
57
50
31.4
15
25
—
26
13
19
1.5
1.5
1.3
1.4
1.8
1.9
1.3
1.3
1.5
2.2
2.2
1.8
—
98/2
80/20
24/76
30/70
90/10
81/19
96/4
86/14
76/24
72/28
94/6
93/7
—
90/10
92/8
92/8
AliBu3
AliBu3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AlMe3
AliBu3
AliBu3
35
150
200
250
180
180
330
720
240
45
9
10
11
12
13
14
15
16e
1.7
1.8
1.7
45 + 45
95
a Conditions: Ln (10 μmol); isoprene–toluene = 1 : 10 (vol/vol); Ln–Borate–AlR3 = 1 : 1 : 10 (molar ratio). b [Borate]: [B]C = [Ph3C][B(C6F5)4]; [B]N
=
[PhNMe2H][B(C6F5)4]. c Determined by means of GPC against polystyrene standards in THF at 30 °C. d Determined by 1H and 13C NMR
spectroscopy. e 1 mL IP was added first followed by addition of 1 mL IP in 10 mL toluene after 45 min.
molecular weight distribution and 1,4-regioselectivity (Table 3,
entry 7). Increasing the [M]/[Ln] ratio influenced the 1,4-regio-
Conclusions
selectivity significantly, but molecular weight and molecular In summary, a series of rare-earth metal alkylates formulated
i
distribution changed slightly (Table 3, entries 8 and 9). Other as (HC(N-2,6-R2C6H3)2)2RECH2SiMe3·thf (R = CH3, Pr) were
catalysts 2, 4, and 6 displayed a similar catalytic activity to give synthesized and fully characterized. In combination with alkyl-
polymers having a high 1,4-regioselectivity (≥90%) and narrow aluminum and borate, these complexes displayed a good cata-
molecular weight distribution under the same conditions lytic activity on polymerization of isoprene. It is found that
except for 5 (Table 3, entries 11–14); however, the produced addition of the cocatalysts sequence has a great influence on
polyisoprenes have low molecular weight. Using complex 1 as the regioselectivity of the polymerization. High 1,4-regioselec-
a precatalyst, polyisoprene having a low 1,4-regioselectivity tivity (as high as 98%) polymerizations were observed when
(72%) was obtained (Table 3, entry 10). It is surprisingly found the catalysts were added in the order [RE]/[alkylaluminum]/
that on addition of AliBu3 instead of AlMe3 to the catalytic [borate].
system 3 or 6, respectively, different regioselectivities of poly-
merizations were observed and addition of AliBu3 to catalyst 3
followed by addition of [Ph3C][B(C6F5)4] or [PhNMe2H]-
Experimental section
General procedure
[B(C6F5)4] produced a polymer with a low 1,4-regioselectivity
(Table 3, entries 3, 4); however, addition of AliBu3 to the cata-
lyst 6 system followed by addition of [Ph3C][B(C6F5)4] produced
a polymer with 1,4-regioselectivity as high as 92% (Table 3,
comparison of entry 3 and entry 15). The results might be
attributed to the ionic radii difference between Er3+ and Nd3+;
the larger neodymium catalyst can hold more sterically bulky
AliBu3 that favors the 1,4-regioselectivity. In order to estimate
the ratio between the cis-1,4-PIP and trans-1,4-PIP, we used
13C NMR spectroscopy to characterize the product of entry 15.
The 13C NMR spectroscopy results indicated a ratio of 36%
(cis-1,4), 55% (trans-1,4), and 9% (3,4-)polymers (see ESI†),
which was in accordance with the 1H NMR spectroscopy
All syntheses and manipulations of air- and moisture-sensitive
materials were performed under a dry argon and oxygen-free
atmosphere, using standard Schlenk techniques or a glovebox.
All solvents were refluxed and distilled over sodium benzo-
phenone ketyl under argon prior to use unless otherwise
noted. [Ph3C][B(C6F5)4] and [PhNMe2H][B(C6F5)4] were pur-
chased from STREM. AlMe3, AlEt3, and AliBu3 were purchaed
from Sigma-Aldrich, and used as received. Isoprene was pur-
chased from TCI, dried with CaH2, and distilled before
polymerization. Formamidinates HL1 and HL2 were prepared
according to literature methods.10 Elemental analysis data
were obtained on a Perkin-Elmer Model 2400 Series II elemen-
tal analyzer. 1H NMR and 13C NMR spectra for analyses of
compounds were recorded on a Bruker Model AV-300 NMR
(92% 1,4-regioselectivity). Catalyst
activity when the monomer was fractionally added
producing polymers with high 1,4-regioselectivity and
6 also displayed high
a
1
spectrometer (300 MHz for H; 75.0 MHz for 13C) in C6D6 for
narrow molecular weight distribution, but the molecular
weight did not increase dramatically. Different borates slightly
influence the regioselectivity of the catalytic system (Table 3,
entries 3 and 4).
rare-earth metal complexes and in CDCl3 for polyisoprene.
Chemical shifts (δ) were reported in ppm. J values are reported
in Hz. IR spectra were recorded on a Shimadzu Model
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FTIR-8400s spectrometer (KBr pellet). Gel permeation chrom- Dy(CH2SiMe3)3(THF)2 (0.312 g, 0.548 mmol) in hexane
atography (GPC) analyses of the polymer samples were carried (30.0 mL) for 3 h at room temperature. Colorless crystals 4
out at 30 °C using THF as an eluent on a Waters-2414 instru- were obtained in 70% yield after recrystallization from the con-
ment and calibrated using monodispersed polystyrene stan- centrated hexane solution at 0 °C (0.39 g). M.p.: 145 °C.
dards at a flow rate of 1.0 mL min−1
.
νmax/cm−1: 2960 (w), 2866 (m), 1662 (w), 1587 (s), 1456 (m),
1234 (s), 1180 (s), 1001 (s), 798 (s), 667 (s). Anal. calcd
for C58H89N4DyOSi: C, 66.41; H, 8.55; N, 5.34. Found: C, 66.45;
Synthesis of [HC(N-2,6-Me2C6H3)2]2Y(CH2SiMe3)(THF) (1)
To
a
hexane (15.0 mL) solution of Y(CH2SiMe3)3(THF)2 H, 8.55; N, 5.29.
(0.343 g, 0.69 mmol) was added a hexane (15.0 mL) solution of
Synthesis of [HC(N-2,6-iPr2C6H3)2]2Sm(CH2SiMe3)(THF) (5)
HL1 (0.348 g, 1.38 mmol) at room temperature. The mixture
was stirred for 3 h at room temperature and the color of the A similar method for preparing complex 1 was used. Complex
solution gradually changed from colorless to faint yellow. The 5 was prepared by reaction of HL2 (0.192 g, 0.492 mmol) with
solvent was evaporated under reduced pressure. The residue Sm(CH2SiMe3)3(THF)3 (0.155 g, 0.246 mmol) in hexane
was extracted with hexane (2 × 8.0 mL). The extractions were (30.0 mL) for 3 h at room temperature. Yellow crystals 5 were
combined and concentrated to about 10.0 mL. Colorless crys- obtained in 64% yield after recrystallization from the concen-
tals 1 were obtained at 0 °C (0.36 g, 71% yield). M.p.: 132 °C. trated hexane solution at 0 °C (0.16 g). M.p.: 149 °C.
δH (C6D6, ppm): 7.69 (s, 2H, NC(H)vN), 6.92–6.99 (m, 12H, νmax/cm−1: 2960 (w), 2866 (m), 1664 (w), 1587 (s), 1456 (m),
C6H3), 3.61 (s, 4H, thf), 2.32 (s, 24H, CH3), 0.98 (s, 4H, thf), 1361 (s), 1180 (s), 1020 (s), 798 (m), 669 (s), 599 (s). Anal. calcd
0.40 (s, 9H, SiCH3), 0.16 (s, 2H, CH2Si). δC (C6D6, ppm): 170.1, for C58H89N4OSiSm: C, 67.19; H, 8.65; N, 5.40. Found: C,
147.0, 131.1, 122.8, 122.4, 68.4, 38.2, 23.5, 18.9, 18.5, 3.3. 67.22; H, 8.68; N, 5.36.
νmax/cm−1: 3155 (s), 2918 (s), 1649 (m), 1587 (m), 1556 (s), 1466
(m), 1367 (s), 1201 (w), 1091 (s), 1033 (s), 758 (s). Anal. calc.
Synthesis of [HC(N-2,6-iPr2C6H3)2]2Nd(CH2SiMe3)(THF) (6)
for C42H57N4OSiY: C, 67.18; H, 7.65; N, 7.46. Found: C, 66.97; n-BuLi (2.0 mL, 3.0 mmol, 1.5 M in hexane) was added drop-
H, 7.66, N, 7.28.
wise to a THF (15.0 mL) solution of HL2 (1.177 g, 3.0 mmol) at
room temperature. After 30 min, the reaction mixture was
transferred to a THF slurry of NdCl3 (0.38 g, 1.5 mmol) at
Synthesis of [HC(N-2,6-iPr2C6H3)2]2Y(CH2SiMe3)(THF) (2)
A similar method for preparing complex 1 was used. Complex room temperature. The mixture was stirred for 1 h to afford a
2 was prepared by reaction of HL2 (0.38 g, 1.96 mmol) with clear solution, to which LiCH2SiMe3 (1.8 mL, 1.5 mmol,
Y(CH2SiMe3)3(THF)2 (0.486 g, 0.98 mmol) in hexane (30.0 mL) 0.82 M in hexane) was added via a pipette. The resulting
for 3 h at room temperature. Colorless crystals 2 were obtained mixture was stirred at room temperature for about 2 h and
in 72% yield after recrystallization from the concentrated then the solvent was removed under reduced pressure to give
hexane solution at 0 °C (0.68 g). M.p.: 143 °C. δH (C6D6, ppm): an oily crude product. The crude product was extracted by
8.09 (s, 2H, NC(H)vN), 7.18–7.24 (m, 12H, C6H3), 3.75 (s, 4H, hexane (2 × 10 mL). The extractions were combined and con-
i
i
thf), 3.64 (m, 8H, CH, Pr), 1.34 (s, 48H, CH3, Pr), 1.18 (s, 4H, centrated to about 10 mL. The blue crystals 6 were obtained at
thf), 0.35 (s, 9H, SiMe3), 0.05 (s, 2H, CH2Si). δC (C6D6, ppm): 0 °C (1.01 g, 65% yield). M.p.: 147 °C. νmax/cm−1: 2960 (w),
168.7, 143.3, 142.1, 124.1, 122.7, 122.5, 67.9, 67.6, 38.2, 27.3, 2866 (m), 1660 (w), 1587 (s), 1456 (m), 1264 (s), 1180 (s),
24.2, 24.1, 23.5, 2.9. νmax/cm−1: 2960 (w), 2926 (m), 1664 (w), 1010 (s), 798 (m), 665 (s). Anal. calcd for C58H89N4NdOSi: C,
1587 (s), 1454 (m), 1361 (s), 1330 (s), 1180 (s), 862 (s), 798 (s). 67.59; H, 8.70; N, 5.44. Found: C, 67.62; H, 8.71; N, 5.41.
Anal. calcd for C58H89N4OSiY: C, 71.42; H, 9.20; N, 5.74.
Crystal structure analyses of 1–6
Found: C, 71.38; H, 9.21; N, 5.71.
A suitable crystal of complexes 1–6 was each mounted in a
sealed capillary. Diffraction was performed on a Bruker
Synthesis of [HC(N-2,6-iPr2C6H3)2]2Er(CH2SiMe3)(THF) (3)
A similar method for preparing complex 1 was used. Complex SMART APEX II CCD area detector diffractometer using
3 was prepared by reaction of HL2 (0.56 g, 1.434 mmol) with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å),
Er(CH2SiMe3)3(THF)2 (0.411 g, 0.717 mmol) in hexane temperatures 173(2) K for 1–5 and 293(2) K for 6, and the φ
(30.0 mL) for 3 h at room temperature. Pink crystals 3 were and ω scan technique. An empirical absorption correction was
obtained in 73% yield after recrystallization from the concen- applied using the SADABS program.11 All structures were
trated hexane solution at 0 °C (0.55 g). M.p.: 145 °C. solved by direct methods, completed by subsequent difference
νmax/cm−1: 2960 (w), 2926 (m), 1664 (w), 1587 (s), 1454 (m), Fourier syntheses, and refined anisotropically for all non-
1361 (s), 1330 (s), 1180 (s), 1001 (s), 798 (s), 667 (s). Anal. calcd hydrogen atoms by full-matrix least-squares calculations based
for C58H89N4ErOSi: C, 66.11; H, 8.51; N, 5.32. Found: C, 66.07; on F2 using the SHELXTL program package.12 The hydrogen
H, 8.50; N, 5.28.
atom coordinates were calculated with SHELXTL by using an
appropriate riding model with varied thermal parameters. The
residual electron densities were of no chemical significance.
Synthesis of [HC(N-2,6-iPr2C6H3)2]2Dy(CH2SiMe3)(THF) (4)
A similar method for preparing complex 1 was used. Complex But in the structure of 3, two large residual peaks are very
4 was prepared by reaction of HL2 (0.43 g, 1.096 mmol) with close to the central metal with distances of 0.84 Å and 0.70 Å,
6846 | Dalton Trans., 2014, 43, 6842–6847
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respectively, so they were treated as noise. Selected bond
lengths and angles are compiled in Table 1, and crystal data
and details of the data collection and structure refinements
are given in Tables S1 and S2 in ESI.†
CCDC 982911–982916 for 1–6 contain the supplementary
crystallographic data for this paper.
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Isoprene polymerization
A typical polymerization procedure for the isoprene polymeri-
zation catalyzed by these complexes is given below. A 50 mL
Schlenk flask, equipped with a magnetic stirring bar, was
charged the desired amount of toluene, the rare earth metal
complex, alkyl aluminium, borate, and isoprene. The mixture
was stirred vigorously for a fixed time during which an
increase of viscosity was observed. The reaction mixture was
quenched by the addition of methanol, and poured in a large
amount of methanol to precipitate the polymer and then dried
under vacuum and weighed.
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Acknowledgements
This work was co-supported by the National Natural Science
Foundation of China (grant no. 21372010, 21202002, and
21172003), the National Basic Research Program of China
(2012CB821600), and grants from the Ministry of Education
(20103424110001).
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This journal is © The Royal Society of Chemistry 2014
Dalton Trans., 2014, 43, 6842–6847 | 6847