Unusual Si-H bond activation and formation of cationic scandium amide complexes from a mono(amidinate)-ligated scandium bis(silylamide) complex and their performance in isoprene polymerization

Article abstract of DOI:10.1021/om3002119

Amine elimination of scandium tris(silylamide) complex Sc[N(SiHMe ₂)₂]₃(THF) with 1 equiv of the amidine [PhC(N-2,6-ⁱPr₂C₆H₃)₂]H in toluene afforded the neutral mono(a

Full text of DOI:10.1021/om3002119

Article
pubs.acs.org/Organometallics
Unusual Si−H Bond Activation and Formation of Cationic Scandium
Amide Complexes from a Mono(amidinate)-Ligated Scandium
Bis(silylamide) Complex and Their Performance in Isoprene
Polymerization
Fei Chen, Shimin Fan, Yibin Wang, Jue Chen, and Yunjie Luo*
Organometallic Chemistry Laboratory, Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, People’s Republic of
China
S
* Supporting Information
ABSTRACT: Amine elimination of scandium tris(silylamide) complex Sc[N(SiHMe2)2]3(THF) with 1 equiv of the amidine
[PhC(N-2,6-iPr2C6H3)2]H in toluene afforded the neutral mono(amidinate) scandium bis(silylamide) complex [PhC(N-
2,6-iPr2C6H3)2]Sc[N(SiHMe2)2]2 (1) in 93% isolated yield. When 1 was activated with 1 equiv of [Ph3C][B(C6F5)4] in the
presence of THF, the unexpected cationic amidinate scandium amide complex [{PhC(N-2,6-iPr2C6H3)2}ScN{SiHMe2}-
{SiMe2N(SiHMe2)2}(THF)2][B(C6F5)4] (2) was generated. Treatment of 1 with excess AlMe3 gave the Sc/Al heterometallic
methyl complex [PhC(N-2,6-iPr2C6H3)2]Sc[(μ-Me)2AlMe2]2 (3). All these complexes were well-characterized by elemental
analysis, NMR spectroscopy, and X-ray crystallography. The combination 1/[Ph3C][B(C6F5)4] in toluene showed activity
toward isoprene polymerization. Addition of excess AlMe3 to the 1/[Ph3C][B(C6F5)4] catalyst system switched the
regioselectivity of isoprene polymerization from 3,4-specific to cis-1,4-selective.
cationic indenyl-ligated rare-earth-metal amide [(2-Me-Ind)-
LnN(SiMe3)2(THF)x][B(C6F5)4] (x = 0, 2) is obtained via an
unexpected abstraction of one indenyl ligand from the
bis(indenyl) rare-earth-metal mono(amide) complex (2-Me-
Ind)2Ln[N(SiMe3)2] by organoborate, as reported by Tardif
and Kaita.5 The other is the β-diketiminate-ligated samarium
amide complex [{(2,6-iPr2C6H3)NC(CH2)CHC(CH3)N-
(2,6-iPr2C6H3)}SmN(SiMe3)2(THF)2][BPh4], which is formed
from the treatment of the dianionic β-diketiminate samarium
mono(amide) complex {(2,6-iPr2C6H3)NC(CH2)CHC(CH3)-
N(2,6-iPr2C6H3)}SmN(SiMe3)2(THF) with [HNEt3][BPh4],
reported by Shen et al. recently.6 These cationic rare-earth-
metal amide species are not obtained from their corresponding
rare-earth-metal bis(amide) complexes, and the amide groups
are constrained to the sterically hindered −N(SiMe3)2 group.
During our study on the essential role of rare-earth-metal
bis(amide) complexes as catalyst precursors in polymerization,
it was found that the Si−H σ bond in the mono(amidinate)-
ligated scandium bis(silylamide) complex [PhC(N-
INTRODUCTION
■
Cationic rare-earth-metal complexes containing reactive Ln−C
σ-bonds have received intense attention in recent years, and
these complexes have shown unique reactivity and selectivity in
organic transformation and polymerization.1 However, most of
these reported catalyst precursors, for example, [LLnR2] (L =
monoanionic ancillary ligand, Ln = rare-earth metal, R = alkyl,
benzyl, allyl, etc.), are generally difficult to handle and to store
due to ligand redistribution and thermal instability. Thus, it is
of great interest to explore less sensitive precatalyst candidates.
As far as bond disruption enthalpy, structural diversity, and
thermal stability are concerned, rare-earth-metal bis(amide)
complexes (Ln−N σ-bonded derivatives, LLn(NR′2)2) are
expected to possess reactivity comparable to that of Ln−C σ-
bonded complexes.2 Recently, we found that rare-earth-metal
bis(amide) complexes are thermally stable and highly effective
stereospecific olefin polymerization precatalysts.3,4 Even though
these complexes are highly active species in polymerization,
their novel structures have not yet been intensively studied.
Only a few cationic rare-earth-metal amide complexes have
been fully characterized, in comparison to the many well-
known cationic rare-earth-metal alkyl complexes.5,6 The
Received: March 15, 2012
Published: May 1, 2012
© 2012 American Chemical Society
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2,6-iPr2C6H3)2]Sc[N(SiHMe2)2]2 (1) could be activated by
[Ph3C][B(C6F5)4] to generate the unexpected cationic
amidinate scandium amide complex [{PhC(N-2,6-iPr2C6H3)2}-
ScN{SiHMe2}{SiMe2N(SiHMe2)2}(THF)2][B(C6F5)4] (2).
Remarkably, addition of AlMe3 to the 1/[Ph3C][B(C6F5)4]
catalyst system not only increased the polymerization activity
but also switched the regioselectivity of the polymerization
from 3,4-specific to cis-1,4-selective. Moreover, the reaction of
1 with excess AlMe3 afforded the amidinate Sc/Al hetero-
metallic methyl complex [PhC(N-2,6-iPr2C6H3)2]Sc[(μ-
Me)2AlMe2]2 (3) via amide−alkyl exchange. This demon-
strated that the active species in the binary catalyst system 1/
[Ph3C][B(C6F5)4] and in the ternary catalyst system 1/
[Ph3C][B(C6F5)4]/AlMe3 are strikingly different. Here we
wish to report these results.
RESULTS AND DISCUSSION
■
Amine elimination of the scandium tris(silylamide) complex
Sc[N(SiHMe2)2]3(THF) with 1 equiv of the amidine [PhC(N-
2,6-iPr2C6H3)2]H in toluene at 100 °C for 3 h, after workup,
afforded the mono(amidinate) scandium bis(silylamide)
complex [PhC(N-2,6-iPr2C6H3)2]Sc[N(SiHMe2)2]2 (1) in
93% isolated yield (Scheme 1). Elemental analysis, NMR
Figure 1. Molecular structure of 1. Hydrogen atoms (except those on
SiH) are omitted for clarity. Selected bond lengths (Å) and angles
(deg): N1−C1 = 1.336(5), C1−N2 = 1.347(5), Sc1−N1 = 2.158(3),
Sc1−N2 = 2.160(3), Sc1−N3 = 2.065(3), Sc1−N4 = 2.065(3); N1−
Sc1−N2 = 62.15(11), N3−Sc1−N4 = 120.42(13), N1−C1−N2 =
112.3(3).
Scheme 1. Synthesis of Mono(amidinate)-Ligated Scandium
Bis(silylamide) Complex 1
The neutral amidinate scandium bis(silylamide) complex 1
alone showed no activity toward isoprene polymerization.12
However, upon activation with 1 equiv of [Ph3C][B(C6F5)4] in
toluene at room temperature, 1 became active for the
polymerization of isoprene in a 3,4-selective fashion, and the
conversion reached 100% in 45 min at the initial feed ratio
[M]/[Sc] = 500 (Table 1, run 2). This finding is surprising,
since isoprene polymerization catalyzed by [RC(N-2,6-
Me2C6H3)2]Y[N(SiMe3)2]2/[Ph3C][B(C6F5)4] (R = Ph, Cy)
showed no activity at all.4
spectroscopy, and X-ray crystallography confirmed the
composition of 1 and showed that it is a neutral, mononuclear,
and solvent-free species. The FT-IR spectrum contains well-
resolved ν(SiH) bands around 2109 and 2070 cm−1, showing
an asymmetrical coordination of silylamide groups to the center
metal.3,7 Room-temperature H NMR spectra of 1 in C6D6
To understand the true active species with this binary catalyst
system, a stoichiometric reaction of 1 with [Ph3C][B(C6F5)4]
1
showed that the resonances of the isopropyl groups on the aryl
rings were two doublets, indicating a restricted rotation of N−
Caryl bonds. Complex 1 is thermally stable at ambient
temperature in the glovebox and soluble in THF, toluene,
and diethyl ether but sparingly soluble in aliphatic solvents such
as hexane and pentane. Single crystals of 1 suitable for X-ray
diffraction were grown from a mixture of hexane and toluene at
−30 °C. The molecular structure is shown in Figure 1. The
metal center is coordinated to a bidentate amidinate ligand
through two nitrogen atoms and two nitrogen atoms from two
silylamide groups to form a distorted-tetrahedral geometry. The
planes of the three phenyl groups on the amidinate backbone
are not coplanar with the plane formed by C1N1Sc1N2, which
reduces the delocalization of π electrons among these moieties.
The Sc−N(amidinate) bond distances in 1 (average 2.159 Å)
are shorter than those in [PhC(N-2,6-iPr2C6H3)2]Sc-
(CH2SiMe3)2(THF) (average 2.207 Å)8 and in [TolC(N-
2,6-iPr2C6H3)2]Y[N(SiHMe2)2]2(THF) (average 2.382 Å).9
Both Sc−N(SiHMe2)2 bond distances in 1 are 2.065(3) Å,
which are comparable with those in Sc[N(SiHMe2)2]3(THF)
(average 2.069 Å)10 and mono(aminopyridinate)Sc[N-
(SiHMe2)2]2 (average 2.049 Å).11
1
was carried out. The H NMR monitoring technique showed
that addition of 1 equiv of [Ph3C][B(C6F5)4] to 1 in C6D5Cl at
room temperature instantly produced a cationic scandium
amide species, with the release of Ph3CH in a 1:1 molar ratio
(not the release of Ph3C−N(SiHMe2)2). The formation of
Ph3CH during the reaction indicated that the cationic species
generated was not the anticipated product [{PhC(N-
2,6-iPr2C6H3)2}Sc−N(SiHMe2)2][B(C6F5)4]. On a preparation
scale, reaction of 1 with [Ph3C][B(C6F5)4] in a 1:1 molar ratio
in toluene at room temperature, after the volatiles were
removed and the residue washed with hexane, gave a cationic
scandium amide species whose component was identical with
that obtained from NMR-scale experiments. Unfortunately,
attempts to obtain single crystals suitable for X-ray analysis
were unsuccessful. However, addition of a small amount of
THF into a toluene solution of the cationic scandium amide
complex, after workup, afforded the THF-coordinated cationic
amidinate scandium amide complex [{PhC(N-2,6-iPr2C6H3)2}-
ScN{SiHMe2}{SiMe2N(SiHMe2)2}(THF)2][B(C6F5)4] (2) as
colorless microcrystals in 65% isolated yield (Scheme 2).
Complex 2 is thermally stable at ambient temperature for weeks
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a
Table 1. Isoprene Polymerization by Amidinate-Ligated Scandium Catalysts
b
b
c
run
complex
[M]/[Sc]
Sc/Al/B
t (min)
yield (%)
Mn (×104)
Mw/Mn
3,4/cis-1,4/trans-1,4
1
2
3
4
5
6
7
8
1
500
500
500
500
500
1000
500
500
1/0/1
1/0/1
1/0/0
1/0/0
1/5/1
1/5/1
1/0/0
1/0/1
15
45
30
15
2
58
100
0
10.54
14.71
1.76
1.61
86/13/1
89/9/2
1
2
2a
1
56
9.87
3.53
4.95
1.68
1.51
1.79
88/11/1
6/93/1
2/94/4
100
100
0
1
2
3
30
2
3
98
8.03
1.56
5/94/1
a
b
Conditions: performed in toluene; Sc 20 μmol; B = [Ph3C][[B(C6F5)4]; 1/3 (v/v) isoprene/toluene; 25 °C. Determined by GPC in THF at 40
c
1
°C against a polystyrene standard. Determined by H NMR in CDCl3.
Scheme 2. Possible Mechanism for the Generation of the Cationic Amidinate Scandium Amide Complex 2
in the glovebox and soluble in THF and chlorobenzene but
insoluble in toluene and hexane. X-ray diffraction revealed
unambiguously that 2 consists of a separated [{PhC(N-
2,6-iPr2C6H3)2}ScN{SiHMe2}{SiMe2N(SiHMe2)2}(THF)2]+
cation and [B(C6F5)4] anion (Figure 2). The metal center is
five-coordinated to a bidentate amidinate ligand, an amide
group, and two THF molecules to adopt a distorted-trigonal-
bipyramidal geometry. The amide group occupies an axial
position relative to the N1C1Sc1N2 plane. The Sc−
N(amidinate) (2.208(3) and 2.173(3) Å) and Sc−N(amide)
(2.043(3) Å) bond distances in 2 are close to those in the
neutral complex 1 (Sc1−N1 = 2.158(3), Sc1−N2 = 2.160(3),
and Sc1−N3 = 2.065(3) Å). The formation of 2 from
treatment of 1 with [Ph3C][B(C6F5)4] in the presence of THF
instead of simple silylamide (−N(SiHMe2)2) abstraction was
unexpected. A possible mechanism for the transformation of 1
to 2 may involve concerted Si−H σ-bond activation by
[Ph3C][B(C6F5)4], accompanied by silylamide migration
(Scheme 2). This is the first example of such an unusual Si−
H σ-bond activation in rare-earth-metal complexes to our
knowledge.13 It also proves indirectly that 1 could be converted
into a cationic amide species, albeit not in an expected manner.
Although the isolated THF-coordinated 2 was not active for
isoprene polymerization (Table 1, run 3), 2a showed activity
comparable with that of 1/[Ph3C][B(C6F5)4] in toluene (Table
1, run 4). Thus, the true active species with the binary catalyst
system of 1/[Ph3C][B(C6F5)4] in toluene is suggested mostly
to be a THF-free cationic amide intermediate such as
[{PhC(N-2,6-iPr2C6H3)2}ScN{SiHMe2}{SiMe2N(SiHMe2)2}]-
[B(C6F5)4] (2a), as envisioned in the proposed polymerization
mechanism (Scheme 3).
Remarkably, addition of AlMe3 to the 1/[Ph3C][B(C6F5)4]
catalyst system increased the polymerization activity and also
switched the regioselectivity of the polymerization from 3,4-
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Figure 2. Molecular structure of 2. Hydrogen atoms (except those on
SiH) and the [B(C6F5)4] anion are omitted for clarity. Selected bond
lengths (Å) and angles (deg): N1−C1 = 1.344(4), C1−N2 =
1.343(4), Sc1−N1 = 2.208(3), Sc1−N2 = 2.173(3), Sc1−N3 =
2.043(3); N1−Sc1−N2 = 61.27(10), N1−C1−N2 = 112.4(3).
Figure 3. Molecular structure of 3. Hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): N1−C1 =
1.338(4), Sc1−N1 = 2.171(3), Sc1−C18 = 2.371(5), Sc1−C19 =
2.437(4), Al1−C18 = 2.030(5), Al1−C19 = 2.070(5), Al1−C20 =
1.959(6), Al1−C21 = 1.954(5); N1−C1−N1A = 111.2(5), N1−Sc1−
N1A = 61.12(16), C18−Sc1−C19 = 86.87(18), C18−Al1−C19 =
107.44(19), C20−Al1−C21 = 117.5(3).
specific to cis-1,4-selective (Table 1, runs 5 and 6). GPC curves
indicated that the polyisoprene samples produced by this
ternary catalyst system were unimodal, indicative of single-site
polymerization behavior. To get some information on the effect
of AlMe3 on the polymerization, the reaction of 1 with excess
AlMe3 in toluene at room temperature was performed, which
gave the Sc/Al heterometallic methyl complex [PhC(N-
2,6-iPr2C6H3)2]Sc[(μ-Me)2AlMe2]2 (3) (Figure 3), a product
formed via an amide−alkyl exchange (Scheme 4).7,14 Since the
crystal structure of 3 is isomorphous with that of [PhC(N-
2,6-iPr2C6H3)2]Y[(μ-Me)2AlMe2]2 from the reaction of [PhC-
(N-2,6-iPr2C6H3)2]Y(o-CH2C6H4NMe2)2 with AlMe3,15 the
active species in this polymerization should therefore also be
a cationic Sc/Al heterobimetallic complex,16 as postulated by
Hou15 and Kempe.11,17
CONCLUSION
■
In summary, on treatment with [Ph3C][B(C6F5)4], an unusual
Si−H σ-bond activation and silylamide migration took place in
a thermally stable mono(amidinate)-ligated scandium bis-
(silylamide) complex (1) and generated an unexpected cationic
amidinate scandium amide species, which showed good
performance in the 3,4-selective polymerization of isoprene.
In addition, introduction of AlMe3 into the binary catalyst
system 1/[Ph3C][B(C6F5)4] changed the active species from a
cationic amide complex to a cationic heterometallic amidinate
Sc/Al complex, which showed high activity toward cis-1,4-
selective polymerization of isoprene. To the best of our
Scheme 3. Possible Mechanism for 3,4-Selective Polymerization of Isoprene by 1/[Ph3C][B(C6F5)4]
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Scheme 4. Synthesis of Amidinate Heterometallic Sc/Al Methyl Complex 3
1357 (m), 1251 (s), 1011 (m), 899 (s), 769 (s), 697 (s). Anal. Calcd
for C39H67N4ScSi4: C, 62.51; H, 9.03; N, 7.48. Found: C, 62.65; H,
8.98; N, 7.36.
knowledge, this is the first example of unusual Si−H σ-bond
activation in rare-earth-metal complexes and formation of a
stable cationic rare-earth-metal mono(amide) complex from a
neutral rare-earth-metal bis(amide) complex. We are currently
in the process of developing an efficient synthetic strategy for
cationic rare-earth-metal mono(amide) derivatives and explor-
ing their reactivity.
[{PhC(N-2,6-iPr2C6H3)2}ScN{SiHMe2}{SiMe2N(SiHMe2)2}][B-
(C6F5)4] (2a). A toluene solution (20 mL) of [Ph3C][B(C6F5)4] (62
mg, 70 μmol) was added dropwise into a toluene (15 mL) solution of
1 (50 mg, 70 μmmol) at room temperature. The reaction mixture was
stirred for 2 h at room temperature to give a clear solution. Removal of
the volatiles under vacuum produced a pale yellow powder. The crude
product was washed with hexane (3 × 10 mL) to remove Ph3CH and
dried (70 mg, 71%). 1H NMR (400 MHz, C6D5Cl/C6D6): δ −0.02 (d,
J = 2.8 Hz, 6H, NSiHMe2), 0.00 (s, 6H, SiMe2), 0.01 (s, 12H,
N(SiHMe2)2), 0.84 (d, J = 6.8 Hz, 12H, CHMe2), 1.06 (d, J = 6.8 Hz,
12H, CHMe2), 3.02 (sept, 4H, CHMe2), 4.57 (sept, 2H, SiHMe2),
5.36 (sept, 1H, SiHMe2), 6.51 (t, J = 7.6 Hz, 2H, Ar-H), 6.55−6.57
(m, 1H, Ar-H), 6.87 (d, J = 7.6 Hz, 4H, Ar-H), 6.91 (d, J = 7.6 Hz, 2H,
Ar-H), 6.96 (d, J = 7.6 Hz, 2H, Ar-H). Anal. Calcd for
C68H73BF20N4ScSi4: C, 57.18; H, 5.12; N, 3.92. Found: C, 57.24; H,
5.31; N, 3.88.
EXPERIMENTAL SECTION
■
Materials and Procedures. All manipulations were performed
under pure argon with rigorous exclusion of air and moisture using
standard Schlenk techniques and an argon-filled glovebox. Solvents
(toluene, hexane, and THF) were distilled from sodium/benzophe-
none ketyl, degassed by the freeze−pump−thaw method, and dried
over fresh Na chips in the glovebox. Anhydrous ScCl3 and
[Ph3C][B(C6F5)4] were purchased from Strem. AlMe3 (1.0 M in
hexane solution) was purchased from Acros and used as received.
HN(SiHMe2)2 was purchased from Fluoro Chem and used as
received. Isoprene was purchased from Acros, dried by stirring with
CaH2, and distilled before polymerization. Deuterated solvents
(CDCl3, C6D6, THF-d8, C6D5Cl) were obtained from Aldrich.
Sc[N(SiHMe2)2]3(THF)10 and [PhC(N-2,6-iPr2C6H3)2]H18 were
prepared according to the literature.
[{PhC(N-2,6-iPr2C6H3)2}ScN{SiHMe2}{SiMe2N(SiHMe2)2}-
(THF)2][B(C6F5)4] (2). A toluene solution (15 mL) of [Ph3C][B-
(C6F5)4] (0.462 g, 0.5 mmol) was added dropwise into a toluene (10
mL) solution of 1 (0.375 g, 0.5 mmol) at room temperature. The
reaction mixture was stirred at room temperature to give a clear brown
solution. After the reaction mixture was kept at room temperature for
1 h, THF (0.2 g, 3 mmol) was introduced via a pipet to give a clear
pale yellow solution. Removal of the volatiles under vacuum produced
a pale yellow powder. The crude product was washed by hexane (3 ×
10 mL) to get rid of Ph3CH. The resulting pale yellow powder was
dissolved in about 2 mL of a hexane/THF (3/1, v/v) mixture and was
kept at room temperature overnight to give 2 as colorless crystals (0.43
g, 65%). 1H NMR (400 MHz, THF-d8/C6D6): δ 0.10 (s, 6H, SiHMe2),
0.18 (d, J = 3.6 Hz, 12H, SiHMe2), 0.28 (s, 6H, SiMe2), 0.72 (br s,
12H, CHMe2), 1.15 (d, J = 5.6 Hz, 12H, CHMe2), 1.44−1.47 (m, 8H,
THF-β-H), 3.03 (br s, 4H, CHMe2), 3.55−3.58 (m, 8H, THF-α-H),
4.57 (sept, 2H, SiHMe2), 5.13 (sept, 1H, SiHMe2), 6.57 (t, J = 7.2 Hz,
2H, Ar-H), 6.65 (t, J = 7.6 Hz, 1H, Ar-H), 6.86 (d, J = 7.2 Hz, 2H, Ar-
H), 6.97 (d, J = 7.2 Hz, 4H, Ar-H), 7.06 (t, J = 7.6 Hz, 2H, Ar-H). 13C
NMR (100 MHz, THF-d8/C6D6): δ 1.8 (SiHMe2), 6.6 (SiMe2), 23.7,
25.5 (CHMe2), 25.8 (THF-β-C), 28.6 (CHMe2), 67.9 (THF-α-C),
125.1, 125.7, 126.7, 127.7, 128.5, 129.3, 129.4, 131.3 (Ar-C), 135.8,
137.7, 138.2, 140.1 (C6F5), 141.7, 142.3 (Ar-C), 147.9, 150.3 (C6F5),
180.5 (NCN). Anal. Calcd for C74H89BF20N4O2ScSi4: C, 55.04; H,
5.57; N, 3.47. Found: C, 54.86; H, 5.61; N, 3.59.
Samples of scandium complexes for NMR spectroscopic measure-
ments were prepared in the glovebox using J. Young valve NMR tubes.
NMR (1H, 13C) spectra were recorded on a Bruker AVANCE III
spectrometer at 25 °C and referenced internally to residual solvent
resonances unless otherwise stated. Carbon, hydrogen, and nitrogen
analyses were performed by direct combustion on a Carlo-Erba EA-
1110 instrument; quoted data are the average of at least two
independent determinations. FT-IR spectra were recorded on a Bruker
TENSOR 27 spectrometer. Molecular weights and molecular weight
distributions of the polymers were measured by a PL GPC 50
instrument with two Mixed-B columns at 40 °C using THF as eluent
against polystyrene standards: flow rate, 1 mL/min; sample
concentration, 1 mg/mL.
[PhC(N-2,6-iPr2C6H3)2]Sc[N(SiHMe2)2]2 (1). A toluene (10 mL)
solution of [PhC(N-2,6-iPr2C6H3)2]H (0.441 g, 1.0 mmol) was added
dropwise to a toluene solution (5 mL) of Sc[N(SiHMe2)2]3(THF)
(0.514 g, 1.0 mmol) at room temperature. The reaction mixture was
stirred at 100 °C for 3 h to give a clear pale yellow solution. Removal
of the volatiles under vacuum afforded a pale yellow powder, which
was extracted by hexane (2 × 10 mL). After hexane was removed, the
crude product 1 was obtained as a white powder (0.69 g, 93%).
Recrystallization from hexane solution at −30 °C gave 1 as colorless
block crystals (0.54 g, 72%). 1H NMR (400 MHz, C6D6): δ 0.17 (d, J
= 2.8 Hz, 24H, SiHMe2), 0.95 (d, J = 6.8 Hz, 12H, CHMe2), 1.43 (d, J
= 6.8 Hz, 12H, CHMe2), 3.58 (sept, 4H, CHMe2), 5.23 (sept, 4H,
SiHMe2), 6.59−6.64 (m, 3H, Ar-H), 7.07 (s, 6H, Ar-H), 7.25−7.27
(m, 2H, Ar-H). 13C NMR (100 MHz, C6D6): δ 2.5 (SiHMe2), 23.7,
25.2 (CHMe2), 29.0 (CHMe2), 124.4, 125.1, 127.6, 130.4, 130.7,
131.3, 141.5, 142.8 (Ar-C), 176.4 (NCN). FT-IR (KBr, cm−1): 2962
(s), 2868 (m), 2109 (m), 2070 (m), 1624 (s), 1576 (m), 1459 (s),
[PhC(N-2,6-iPr2C6H3)2]Sc[(μ-Me)2AlMe2]2 (3). A toluene (10
mL) solution of 1 (0.749 g, 1.0 mmol) was added dropwise to a
heptane solution (6 mL) of AlMe3 (6 mmol, 1.0 M) at room
temperature. The reaction mixture was stirred at room temperature for
2 h to give a clear pale yellow solution. Removal of the volatiles under
vacuum produced a pale yellow oily residue, which was extracted by
toluene (2 × 10 mL). After the side product of Me2Al[(μ-
N(SiHMe2)2]2AlMe2 was separated from the extract solution by
fractional recrystallization, the residual mother solution was dried
under vacuum. The resulting pale yellow powder was dissolved in
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Organometallics
Article
hexane, and the solution was filtered. Concentrating and cooling the
filtrate gave 3 as a white powder (0.43 g, 65%). H NMR (400 MHz,
(c) Nolan, S. P.; Stern, D.; Marks, T. J. J. Am. Chem. Soc. 1989, 111,
7844. (d) Monteil, V.; Spitz, R.; Boisson, C. Polym. Int. 2004, 53, 576.
(3) Luo, Y. J.; Feng, X. Y.; Wang, Y. B.; Fan, S. M.; Chen, J.; Lei, Y. L.
Organometallics 2011, 30, 3270.
(4) Luo, Y. J.; Fan, S. M.; Yang, J. P.; Fang, J. H.; Xu, P. Dalton Trans.
2011, 40, 3053.
(5) Tardif, O.; Kaita, S. Dalton Trans. 2008, 2531.
(6) Liu, P.; Zhang, Y.; Yao, Y. M.; Shen, Q. Organometallics 2012, 31,
1017.
1
C6D6): δ −0.01 (br s, 24H, [AlMe4]), 0.96 (d, J = 6.8 Hz, 12H,
CHMe2), 1.28 (d, J = 6.8 Hz, 12H, CHMe2), 3.51 (sept, 4H, CHMe2),
6.60−6.61 (m, 3H, Ar-H), 6.98−7.03 (m, 6H, Ar-H), 7.09−7.12 (m,
2H, Ar-H). 13C NMR (100 MHz, C6D6): δ 23.4 (CHMe2), 25.9
(CHMe2), 28.8 (CHMe2), 124.4, 126.2, 127.4, 130.3, 130.4, 141.4,
142.7 (Ar-C), 177.5 (NCN), the signal for [AlMe4] was not
observed. Anal. Calcd for C39H63Al2N2Sc: C, 71.09; H, 9.66; N, 4.25.
Found: C, 70.85; H, 9.47; N, 4.18.
(7) Anwander, R.; Klimpel, M. G.; Dietrich, H. M.; Shorokhov, D. J.;
Scherer, W. Chem. Commun. 2003, 1008.
Typical Procedure for Isoprene Polymerization. The
procedures for isoprene polymerization catalyzed by these scandium
complexes were similar, and a typical polymerization procedure is
given below. A 50 mL Schlenk flask equipped with a magnetic stirring
bar was charged in sequence with the desired amount of the scandium
complex, toluene, borate (or borate and methylaluminum) and
isoprene. The mixture was stirred vigorously at room temperature
for the desired time, during which an increase of viscosity was
observed. The reaction mixture was quenched by the addition of
ethanol and then poured into a large amount of ethanol to precipitate
the polymer, which was dried under vacuum at 60 °C and weighed.
X-ray Crystallographic Study. Suitable single crystals of
complexes 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 using Mo Kα radiation (λ
= 0.710 70 Å). The diffracted intensities were corrected for Lorentz−
polarization effects and empirical absorption corrections. The
structures were solved by direct methods and refined by full-matrix
least-squares procedures based on |F|2. All the non-hydrogen atoms
were refined anisotropically. The structures were solved and refined
using the SHELXL-97 program. CCDC 866532−866534 contain
supplementary crystallographic data for complexes 1−3. These data
can be obtained free of charge from The Cambridge Crystallographic
Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
(8) Bambirra, S.; Bouwkamp, M. W.; Meetsma, A.; Hessen, B. J. Am.
Chem. Soc. 2004, 126, 9182.
(9) Aubrecht, K. B.; Chang, K.; Hillmyer, M. A.; Tolman, W. B. J.
Polym. Sci., Part A 2001, 39, 284.
(10) Anwander, R.; Runte, O.; Eppinger, J.; Gerstberger, G.;
Herdtweck, E.; Spiegler, M. J. Chem. Soc., Dalton Trans. 1998, 847.
(11) Doring, C.; Kretschmer, W. P.; Bauer, T.; Kempe, R. Eur. J.
Inorg. Chem. 2009, 4255.
(12) For examples of diene polymerization catalyzed by rare-earth-
metal amide complexes as catalyst precursors, see: Fischbach, A.;
Anwander, R. Adv. Polym. Sci. 2006, 204, 155 and refs 2d, 3, 4, and 11.
(13) While organolanthanide-catalyzed silanolytic chain transfer
involving a Si−H σ-bond activation step has been well documented,
in contrast, the mechanism of Si−H σ-bond activation in this case has
not been reported previously to our knowledge; see: Amin, S. B.;
Marks, T. J. Angew. Chem., Int. Ed. 2008, 47, 2006.
(14) For examples of amide−alkyl exchange reactions, see:
(a) Zimmermann, M.; Anwander, R. Chem. Rev. 2010, 110, 6194.
(b) Occhipinti, G.; Meermann, C.; Dietrich, H. M.; Litlabo, R.; Auras,
F.; Tornroos, K. W.; Mossmer, C. M.; Jensen, V. R.; Anwander, R. J.
Am. Chem. Soc. 2011, 133, 6323. (c) Dietrich, H. M.; Tornroos, K. W.;
Herdtweck, E.; Anwander, R. Organometallics 2009, 28, 6739. For
examples of the formation of scandium tetramethylaluminum
complexes, see: (d) Carver, C. T.; Monreal, M. J.; Diaconescu, P. L.
Organometallics 2008, 27, 363 and refs 3 and 11.
(15) (a) Zhang, L. X.; Nishiura, M.; Yuki, M.; Luo, Y.; Hou, Z. M.
Angew. Chem., Int. Ed. 2008, 47, 2642. (b) Hong, J. Q.; Zhang, L. X.;
Yu, X. Y.; Li, M.; Zhang, Z. X.; Zhang, P. Z.; Nishiura, M.; Hou, Z. M.;
Zhou, X. G. Chem. Eur. J. 2011, 17, 2130.
(16) In the ternary catalyst system 1/[Ph3C][B(C6F5)4]/AlMe3, the
addition sequence of [Ph3C][B(C6F5)4] or AlMe3 first to 1 had little
effect on either the polymerization activity or the selectivity. However,
the isolation of a cationic species from either the treatment of 3 with
[Ph3C][B(C6F5)4] or the reaction of 2a with AlMe3 was unsuccessful.
(17) NMR spectroscopic studies showed that reaction of
mono(aminopyridinate)Sc[N(SiHMe2)2]2 with AlMe3 gave a mono(-
aminopyridinate)Sc[(μ-Me)2AlMe2]2 species; however, the isolation
of this species was not successful.11 In contrast, mono(aminopyr-
idinate)Ln[N(SiHMe2)2]2 (Ln = La, Y) reacted with AlMe3 by transfer
of aminopyridinate ligand from the rare-earth metal to the aluminum
atom; see: Doring, C.; Kempe, R. Eur. J. Inorg. Chem. 2009, 412.
(18) Ogata, S. I.; Mochizuki, A.; Kakimoto, M. A.; Imai, Y. Bull.
Chem. Soc. Jpn. 1986, 59, 2171.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures giving NMR spectra of complexes 1−3 and CIF files
giving full crystallographic data for complexes 1−3. This
material is available free of charge via the Internet at http://
pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lyj@nit.zju.edu.cn. Tel: +86-574-88130085. Fax: +86-
574-88130130.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was partly financially supported by the National
Natural Science Foundation of China (No. 21172195), the
Science and Technology Department of Zhejiang Province
(No. 2010R10020), and the State Key Laboratory of Rare Earth
Resource Utilization (No. RERU2011020). We are grateful to
Mr. Yong Zhang at Suzhou University for X-ray analysis help.
REFERENCES
■
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(2) (a) Lappert, M. F.; Protchenko, A. V.; Power, P. P.; Seeber, A. L.
Metal Amide Chemistry; Wiley: Hoboken, NJ, 2008. (b) Schumann, H.;
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