Scandium and gadolinium complexes with aryldiimine NCN pincer ligands: Synthesis, characterization, and catalysis on isoprene and 1,5-hexadiene polymerization

Article abstract of DOI:10.1016/j.poly.2018.02.010

Several scandium and gadolinium complexes bearing tridentate aryldiimine NCN pincer ligands ^Ar[NCN^H]ScCl₂(THF)₂ (Ar = Ph (1a); Ar = 2,6-Me₂C₆H₃ (1b), Ar = 2,6-Et₂C

Full text of DOI:10.1016/j.poly.2018.02.010

Accepted Manuscript
Scandium and Gadolinium Complexes with Aryldiimine NCN Pincer Ligands:
Synthesis, Characterization, and Catalysis on Isoprene and 1,5-Hexadiene Pol-
ymerization
Teng Jia, Shu-yun Xu, Li-cheng Huang, Wei Gao
PII:
S0277-5387(18)30077-9
https://doi.org/10.1016/j.poly.2018.02.010
POLY 13045
DOI:
Reference:
Polyhedron
To appear in:
Received Date:
Accepted Date:
22 January 2018
12 February 2018
Please cite this article as: T. Jia, S-y. Xu, L-c. Huang, W. Gao, Scandium and Gadolinium Complexes with
Aryldiimine NCN Pincer Ligands: Synthesis, Characterization, and Catalysis on Isoprene and 1,5-Hexadiene
Polymerization, Polyhedron (2018), doi: https://doi.org/10.1016/j.poly.2018.02.010
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Scandium and Gadolinium Complexes with Aryldiimine
NCN Pincer Ligands: Synthesis, Characterization, and
Catalysis on Isoprene and 1,5-Hexadiene Polymerization
Teng Jia, Shu-yun Xu, Li-cheng Huang, and Wei Gao*
College of Chemistry, Jilin University, Changchun 130012, China.
To whom correspondence should be addressed. E-mail: gw@jlu.edu.cn
Fax:+86-431-8849055
Abstract
Several scandium and gadolinium complexes bearing tridentate aryldiimine NCN
pincer ligands Ar[NCNH]ScCl2(THF)2 (Ar= Ph (1a); Ar = 2,6-Me2C6H3(1b), Ar =
2,6-Et2C6H3 (1c), Ar = 2,6-iPr2C6H3 (1d), Ar = p-MeC6H4 (1e), Ar = 2,4,6-Me3C6H2
(1f)), Ar[NCNMe]MCl2(THF)2 (M = Sc, Ar = 2,6-Me2C6H3(2b); M = Gd, Ar =
2,6-Me2C6H3(3b), Ar = 2,6-iPr2C6H3 (3d)), and iPrPh[NCNMe]ScCl2(2d) were
n
synthesized via lithiation of the corresponding ligands with BuLi and subsequent
addition of ScCl3(THF)3 or GdCl3(THF)3 respectively. These complexes were well
characterized by elemental analysis and the solid structures of 1c, 1d, 1f, and 2b were
confirmed by X-ray diffraction analysis to be monomeric with N,C,N-tridentate
ligands coordinating to the metal center in a typical meridional manner. In the
presence of alkylaluminium and trityl borate, these well-defined complexes showed
moderate to high activities in isoprene polymerization affording cis-1,4 enriched
polymers (up to 98.3%). Moreover, the NCN pincer scandium complexes can also
1

catalyze the 1,5-hexandiene polymerization giving methylene-1,3-cyclopentane
enriched polymers (MCP content up to 99.3%).
Keywords: Scandium complexes; Gadolinium complexes; Isoprene polymerization;
1,5-Hexadiene polymerization
1. Introduction
Highly cis-1,4 specific polymerization of dienes have attracted considerable attention
in both academic and industry area because of the unique properties of the resulting
polymer.[1] The high cis-1,4 content polyisoprene possessing the excellent properties
such as high elasticity, abrasion resistance, low rolling resistance, cold resistance and
low hysteresis are widely used for tires and other elastic materials. After decades
research, it is well established that the regularity of the polymer is greatly dependent
on the nature of the catalyst used.[2] Since 1950s, many homogeneous catalyst systems
composed of transition metals complexes, such as Ti,[3] V,[4] Fe,[5] Co,[6] and Ni[7] and
the co-activators (aluminum alkyls or the aluminum alkyl chloride) have been
extensively explored. In the recent decades, many research efforts were devoted to
developing the rare-earth metal containing systems because of their good performance
in stereospecific polymerization of dienes,[8] in which some scandium complexes have
received especial attention due to their unique properties in the selective
polymerization of diene. For example, the cationic scandium complex bearing PNP
ligands can catalyze the isoprene polymerization giving polyisoprene with high
percentage of cis-1,4 unit.[9a] The scandium bisalkyls ligated by Cp shows high
activity and high cis-1,4 selectivity for the polymerization of isoprene.[9b] Whereas the
2

scandium bisalkyls with amidinate ligands exhibits high activity and 3,4-selectivity
for isoprene polymerization.[9c]
On the other hand, the metal complexes with bis(imino)aryl ligands, which
possess unique properties in balancing the activities and stabilities of metal centers,
have received considerable attention in homogeneous catalysis and material science.
A large amount of the transition complexes based on pincer ligands were synthesized
and investigated. The Pd,[10] Pt,[11] Rh[12] and Ir[13] complexes supported by such
ligands were synthesized and investigated in catalysis on Heck-coupling and C–C
bond formation etc reactions and material science. As part of main work in our group
for developing the catalysts for olefin or diene polymerization, we are very interested
in the NCN pincer complexes. We have reported that the lanthanide complexes and
yttrium complexes bearing aldimine-based NCN pincer ligands show high activities
and high selectivity in dienes polymerization.[14] The NCN chromium complexes
show high activity in isoprene trans-1,4 polymerization.[15] The corresponding iron
and nickel complexes show moderate activities in diene and norbornene
polymerization.[16] It was reported that the methyl groups attached to the imine
moieties in NNN bis(imino)pyridyl iron complexes showed some positive effects in
olefin polymerization and the ketimine-based catalysts exhibited higher activity than
that with aldimine-based analogues.[17] In extending the research of NCN pincer
complex and inspecting the ketimine-methyl effects of NCN pincer ligands, the
current study reports on the synthesis and characterization of scandium and
gadolinium complexes bearing aldimine or ketimine NCN pincer ligands. The
3

catalytic behaviors of these scandium and gadolinium complexes toward
polymerization of isoprene and 1,5-hexadieneare also presented.
2. Experimental
2.1. General procedures and methods
All manipulations involving air and moisture-sensitive compounds were carried out
under an atmosphere of dried and purified nitrogen using standard Schlenk or dry box
techniques. Toluene and hexane were dried over sodium/benzophenone and distilled
under nitrogen prior to use. Elemental analyses were performed on a Varian 12EL
microanalyzer. NMR spectra were recorded on a Bruker-400 NMR spectrometer at
room temperature in CDCl3, THF-d8, and C6D6. The molecular weight and molecular
weight distribution of the polymers were measured on a TOSOH HLC 8220 GPC at
40 °C using THF as eluent at a flow rate of 1.0 mL/min. Polystyrene standards were
used for calibrations. The ligands LbBr, LcBr, LdBr, and LgBr were synthesized
according to the literature.[14, 18]
2.2 ligands preparation and characterization
2.2.1. Preparation of LaBr
The ligand LaBr was synthesized in the same way as described for the synthesis of
LbBr.[14] 2-Bromoisophthalaldehyde (2.14 g, 10.0 mmol), aniline (2.30 g, 24.0 mmol)
were combined in a flask with 150 mL methanol and treated with 3 drops of formic
acid. The mixture was stirred overnight at room temperature. The resulting suspension
was filtered and washed with cold methanol (3 × 5.0 mL). The yellow solid was dried
in vacuo (3.10 g, 85%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 7.29 (t, J = 7.0 Hz, 6H,
4

N-C6H5), 7.44 (t, J = 7.8 Hz, 4H, o-N-C6H5), 7.52 (t, J = 7.7 Hz, 1H, p-Br-C6H3), 8.35
(d, J = 7.7 Hz, 2H, m-Br-C6H3), 8.99 (s, 2H, CH=N). 13C NMR (100 MHz, CDCl3): δ
121.28 (N-Ar), 126.73 (N-Ar), 127.90 (N-Ar), 129.17 (N-Ar), 129.39 (Br-Ar), 131.89
(Br-Ar), 135.59 (Br-Ar), 151.61 (Br-Ar), 159.11 (CH=N) ppm.
2.2.2. Preparation of LeBr
Following the same procedure described for the synthesis of LaBr, treatment of
2-bromoisophthalaldehyde (2.14 g, 10.0 mmol), p-methylaniline (2.56 g, 24.0 mmol),
and 3 drops of formic acid in methanol (150 mL) afforded LeBr as pale yellow solid
(3.60 g, 91%). 1H NMR (400 MHz, CDCl3, 25 °C): δ 2.40 (s, 6H, p-Ar-CH3), 7.22 (m,
J = 8.5 Hz, 8H, N-C6H4), 7.50 (t, J = 7.7 Hz, 1H, p-Br-C6H3), 8.32 (d, J = 7.7 Hz, 2H,
m-Br-C6H3), 8.99 (s, 2H, HC=N). 13C NMR (100 MHz, CDCl3, 25 °C): δ 21.22
(Ar-CH3), 121.27 (N-Ar), 127.87 (N-Ar), 129.00 (N-Ar), 130.00 (N-Ar), 131.66
(Br-Ar), 135.71 (Br-Ar), 136.73 (Br-Ar), 149.04 (Br-Ar), 158.32 (HC=N) ppm.
2.2.3. Preparation of LfBr
Following the same procedure described for the synthesis of LaBr, treatment of
2-bromoisophthalaldehyde (2.14 g, 10.0 mmol), 2,4,6-trimethylaniline (3.25 g, 24.0
mmol), and 3 drops of formic acid in 150 mL methanol afforded LfBr as yellow solid
1
(4.00 g, 90%). H NMR (400 MHz, CDCl3, 25 °C): δ 2.17 (s, 12H, o-N-C6H2-CH3),
2.31 (s, 6H, p-N-C6H2-CH3), 6.93 (s, 4H, N-C6H2Me3), 7.56 (t, J = 7.7 Hz, 1H,
p-Br-C6H3), 8.38 (d, J = 7.6 Hz, 2H, m-Br-C6H3), 8.74 (s, 2H, CH=N). 13C NMR (100
MHz, CDCl3, 25 °C): δ 18.49 (o-N-C6H2-CH3), 20.91 (p-N-C6H2-CH3), 127.14
(N-Ar), 127.99 (N-Ar), 128.47 (N-Ar), 128.99 (N-Ar), 131.44 (Br-Ar), 133.66
5

(Br-Ar), 135.78 (Br-Ar), 148.47 (Br-Ar), 162.04 (CH=N) ppm.
2.2.4. Preparation of LhBr
2,6-Diacetylphenyl bromide (3.60 g, 15.0 mmol), 2,6-diisopropylaniline (6.80 mL,
36.0 mmol), and Si(OEt)4 (8.00 mL, 36.0 mmol) were mixed in a Schlenk vessel
equipped with a still head and treated with 20 drops of H2SO4 (95%). The solution
o
was heated at 160 C for 24 h and the distillate (EtOH) was discarded. Agian,
2,6-diisopropylaniline (2.00 mL, 10.0 mmol) and 10 drops of H2SO4 (95%) were
o
added and the mixture was heated for another 24 h at 160 C. Ethanol (80 mL) and
KOH(aq) (10 mL, 2.00 M solution in H2O) were added at room temperature and the
resulting mixture was extracted with diethyl ether. The organic layer was washed with
o
brine and water, dried on MgSO4 and submitted to a Kugelrohr distillation at 160 C
for 30 min. The residue was purified by column chromatography; pale yellow solid
1
(3.40 g, 40%). H NMR (400 MHz, CDCl3, 25 °C): δ 1.24 (d, J = 6.6 Hz, 12H,
CH(CH3)2), 1.30 (d, J = 6.6 Hz, 12H, CH(CH3)2), 2.13 (s, 6H, CH3-C=N), 3.12 (m,
4H, CH(CH3)2), 7.15 (t, 2H, p-N-C6H3), 7.22 (d, J = 7.3 Hz, 4H, m-N-C6H3), 7.47 (d,
J = 7.4 Hz, 2H, p-Br-C6H3), 7.56 (t, 1H, m-Br-C6H3). 13C NMR (100MHz, CDCl3,
25 °C): δ 22.19 (N=C-CH3), 23.42 (ArCH(CH3)2), 23.62 (ArCH(CH3)2), 28.13
(ArCH(CH3)2), 116.91 (ArC), 122.87 (ArC), 123.28 (ArC), 124.21 (ArC), 128.68
(ArC), 128.97 (ArC), 136.52 (ArC), 144.79 (ArC), 144.99 (ArC), 170.25 (ArC) ppm.
2.3. Preparation and characterization of scandium and gadolinium complexes
2.3.1. Preparation of complex 1a
6

n
Under nitrogen atmosphere, BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) was
added dropwise to a solution of LaBr (0.36 g, 1.00 mmol) in 25 mL THF at –78 ºC.
o
The mixture was stirred for 1 hour then allowed to warm to –40 C and stirred for
another 2 h before the addition of ScCl3(THF)3 (0.37 g, 1.10 mmol) in one portion.
The resulting mixture was allowed to warm to room temperature gradually and stirred
for 12 h. Removal of the volatiles under reduced pressure gave brownish residue that
was extracted with toluene. Remove of toluene under vacuum afforded complex 1a as
1
yellow powder (0.46 g, 85%). H NMR (400 MHz, THF-d8, 25 °C): δ 1.77 (m, 8H,
THF), 3.62 (m, 8H, THF), 7.15 (t, J = 7.5 Hz, 1H), 7.20 (t, J = 7.4 Hz, 2H), 7.26 (d,
1H), 7.37 (t, 4H), 7.47 (d, J = 7.4 Hz, 3H), 7.60 (t, 1H), 7.63 (d, J = 7.4 Hz, 1H), 8.64
13
(s, 1H),8.73 (s, 1H). C NMR (100 MHz, THF-d8, 25 °C): δ 26.51 (THF), 68.38
(THF), 121.89 (p-N-C6H3), 123.58 (m-N-C6H3), 126.26 (p-Sc-C6H3), 129.47
(m-N-C6H3), 129.96 (m-Sc-C6H3), 131.78 (o-N-C6H3), 132.03 (o-Sc-C6H3), 143.65
(N-C), 160.25 (CH=N), 174.35 (C-Sc) ppm. Anal. Calcd for C28H31Cl2N2O2Sc(%): C,
61.89; H, 5.75; N, 5.16. Found(%): C, 61.94; H, 5.87; N, 5.10.
2.3.2. Preparation of complex 1b.
Using the same procedure described for the synthesis of 1a, treatment of LbBr (0.42 g,
n
1.00 mmol) with BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) and then in situ
addition of ScCl3(THF)3 (0.37 g, 1.10 mmol) afforded 1b as yellowish powder (0.51 g,
85%). Single crystals suitable for X-ray diffraction analysis grew from the mixture of
1
THF and hexane at –30 ºC within several days as yellow blocks. H NMR (400 MHz,
CDCl3, 25 °C): δ 1.55 (s, 8H, THF), 2.51 (s, 12H, CH3), 3.73 (s, 8H, THF), 7.03–7.11
7

(m, 6H, , N-C6H3Me2), 7.26 (t, H, p-Sc-C6H3), 7.50 (d, 2H, JH–H = 8.0 Hz, m-Sc-C6H3),
8.36 (s, 2H, CH=N). 13C NMR (100 MHz, CDCl3, 25 °C): δ 19.50 (CH3), 24.91
(THF), 71.36 (THF), 125.36 (p-N-C6H3), 126.61 (m-N-C6H3), 128.34 (p-Sc-C6H3),
130.65 (m-Sc-C6H3), 131.90 (o-N-C6H3), 141.29 (o-Sc-C6H3), 151.20 (N-C), 176.14
(CH=N), 193.74 (C-Sc) ppm. Anal. Calcd for C32H39Cl2N2O2Sc (%): C, 64.11; H,
6.56; N, 4.67. Found (%): C, 64.18; H, 6.67; N, 4.61.
2.3.3. Preparation of complex 1c
Using the same procedure described for the synthesis of 1a, treatment of LcBr (0.48 g,
n
1.00 mmol) with BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) and ScCl3(THF)3
1
(0.37 g, 1.10 mmol) afforded 1c as yellowish powder (0.52 g, 82%). H NMR (400
MHz, CDCl3, 25 °C): δ 1.18 (m, 12H, CH2CH3), 1.68 (b, 8H, THF), 2.99 (m, 8H,
CH2CH3), 3.85 (b, 8H, THF), 7.16 (m, 6H, N-C6H3), 7.27 (m, 1H, p-Sc-C6H3), 7.50 (d,
13
2H, JH–H = 5.4 Hz, m-Sc-C6H3), 8.35 (s, 2H, CH=N). C NMR (100 MHz, CDCl3,
25 °C): δ 15.50 (CH2CH3), 24.80 (CH2CH3), 25.22 (THF), 71.23 (THF), 125.78
(p-N-C6H3), 126.27 (m-N-C6H3), 126.64 (p-Sc-C6H3), 132.04 (m-Sc-C6H3), 136.61
(o-N-C6H3), 141.22 (m-Sc-C6H3), 150.40 (N-C), 175.69 (CH=N), 175.73 (C-Sc) ppm.
Anal. Calcd for C36H47Cl2N2O2Sc (%): C, 65.95; H, 7.23; N, 4.27. Found (%): C,
65.89; H, 7.32; N, 4.20.
2.3.4. Preparation of complex 1d
Using the same procedure described for the synthesis of 1a, treatment of LdBr (0.53 g,
n
1.00 mmol) with BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) and ScCl3(THF)3
1
(0.37 g, 1.10 mmol) afforded 1d as yellowish powder (0.55 g, 78%). H NMR (400
8

MHz, CDCl3, 25 °C): δ 1.10 (d, 12H, JH–H = 4 Hz, CH(CH3)2), 1.29 (d, 12H, JH–H = 4
Hz, CH(CH3)2), 1.71 (b, 8H, THF), 3.81 (m, 2H, CH(CH3)2), 3.83 (b, 8H, THF), 7.23
– 7.31 (m, 7H, Ar), 7.53 (d, 2H, JH–H = 4 Hz, Ar), 8.37 (s, 2H, CH=N). 13C NMR (100
MHz, CDCl3, 25 °C): δ 23.08 (CH(CH3)2), 25.24 (THF), 26.38 (CH(CH3)2), 27.78
(CH(CH3)2), 71.13 (THF), 123.74 (m-N-C6H3), 126.43 (p-N-C6H3), 127.14
(p-Sc-C6H3), 131.93 (m-Sc-C6H3), 140.89 (o-N-C6H3), 141.60 (o-Sc-C6H3 ), 147.81
(N-C), 175.82 (CH=N), 175.91 (C-Sc) ppm. Anal. Calcd for C40H55Cl2N2O2Sc (%): C,
67.50; H, 7.79; N, 3.94. Found (%): C, 67.43; H, 7.84; N, 3.91.
2.3.5. Preparation of complex 1e
Using the same procedure described for the synthesis of 1a, treatment of LeBr (0.39 g,
n
1.00 mmol) with BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) and subsequent
addition of ScCl3(THF)3 (0.37 g, 1.10 mmol) afforded 1e as pale yellow powder (0.48
1
g, 85%). H NMR (400 MHz, CDCl3, 25 °C): δ 1.67 (s, 8H, THF), 2.39 (s, 6H,
p-CH3), 3.82 (b, 8H, THF), 7.20 (q, J = 8.2 Hz, 6H), 7.41 (d, J = 6.6 Hz, 3H), 7.49 (d,
13
J = 7.5 Hz, 2H), 8.53 (d, 2H, CH=N). C NMR (100 MHz, CDCl3, 25 °C): δ 22.21
(CH3), 25.20 (THF), 71.60 (THF), 120.98 (p-N-C6H3), 122.06 (m-N-C6H3), 127.83
(p-Sc-C6H3), 129.94 (m-Sc-C6H3), 130.11 (o-N-C6H3), 141.15 (o-Sc-C6H3), 146.48
(N-C), 172.88 (CH=N), 172.94 (C-Sc) ppm. Anal. Calcd forC30H35Cl2N2O2Sc (%): C,
63.05; H, 6.17; N, 4.90. Found (%): C, 63.12; H, 6.22; N, 4.87.
2.3.6. Preparation of complex 1f
Using the same work-up procedure described for the synthesis of 1a, treatment of
n
LfBr (0.45 g, 1.00 mmol) with BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) and
9

1
ScCl3(THF)3 (0.37 g, 1.10 mmol) afforded 1f as orange powder (0.55 g, 87%). H
NMR (400 MHz, CDCl3, 25 °C): δ 1.52 (s, 8H, THF), 2.30 (s, 6H, p-CH3), 2.48 (s,
12H, o-CH3), 3.73(b, 8H, THF), 6.93 (s, 4H, C6H2Me3), 7.26 (t, 1H, JH–H = 7.2 Hz,
13
p-Sc-C6H3), 7.51 (d, 2H, JH–H = 7.2 Hz, m-Sc-C6H3), 8.37 (s, 2H, CH=N). C NMR
(100 MHz, CDCl3, 25 °C): δ 19.64 (CH3), 20.89 (CH3), 25.13 (THF), 71.83 (THF),
127.50 (p-N-C6H3), 129.29 (m-N-C6H3), 130.65 (p-Sc-C6H3), 131.47 (m-Sc-C6H3),
135.35 (o-N-C6H3), 140.85 (o-Sc-C6H3), 146.94 (N-C), 177.13 (CH=N) ppm. Anal.
Calcd for C34H43Cl2N2O2Sc (%): C, 65.07; H, 6.91; N, 4.46. Found (%): C, 64.92; H,
6.87; N, 4.40.
2.3.7. Preparation of complex 2b
n
Under nitrogen atmosphere, BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) was
o
added dropwise to a solution of LgBr (0.45 g, 1.00 mmol) in 25 mL THF at –78 C
o
and kept stirring for 1 h. The reaction mixture was warmed to –40 C and then
ScCl3(THF)3 (0.37 g, 1.10 mmol) was added in one portion. The resultant mixture
was allowed to warm to room temperature gradually and stirred for 12 h. Removal of
the volatiles under reduced pressure gave orange residue which was extracted with
toluene. Stripping off toluene afforded 2b as yellowish powder (0.44 g, 70%). Single
crystals suitable for X-ray diffraction analysis grew as yellow blocks from the mixture
of THF and hexane solution at –30 oC within several days. 1H NMR (400 MHz, C6D6,
25 °C): δ1.41 (b, 8H, THF), 1.79 (s, 6H, N=CCH3), 2.01 (s, 12H, o-N-C6H3(CH3)2),
3.57 (b, 8H, THF), 6.97 (t, 1H, C6H3), 7.06 (d, 4H, C6H3), 7.25 (t, 1H, C6H3), 8.11 (d,
2H, C6H3), 8.96 (s, 1H, p-Sc-C6H3). 13C NMR (100 MHz, C6D6, 25 °C): δ 16.49
10

(N-CH3), 19.68 (CH3), 25.14 (THF), 70.89 (THF), 125.74 (p-N-C6H3), 126.12
(m-Sc-C6H3), 127.30 (p-Sc-C6H3), 129.07 (m-N-C6H3), 130.89 (o-N-C6H3), 143.66
(Ar-C), 145.54 (o-Sc-C6H3), 164.50 (CH=N), 184.00 (C-Sc) ppm. Anal. Calcd for
C34H43Cl2N2O2Sc (%): C, 65.07; H, 6.91; N, 4.46. Found (%): C, 64.95; H, 6.83; N,
4.51.
2.3.8. Preparation of complex 2d
Using the same work-up procedure described for the synthesis of 2b, treatment of
n
LhBr (0.56 g, 1.00 mmol) with BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) and
ScCl3(THF)3 powder (0.37 g, 1.10 mmol) afforded 2d as yellowish powder (0.57 g,
1
78%). H NMR (400 MHz, CDCl3, 25 °C): δ 1.15 (d, J = 4.0 Hz, 12H, CH(CH3)2),
1.17 (d, J = 4.0 Hz, 12H, CH(CH3)2), 2.18 (s, 6H, N=C-CH3), 2.77 (m, 4H, CH(CH3)2),
7.10 (t, 1H, C6H3), 7.17 (d, J = 7.4 Hz, 4H, C6H3), 7.61 (t, J = 7.8 Hz, 1H, C6H3), 8.23
(d, J = 7.7 Hz, 2H, m-Sc-C6H3), 8.66 (s, 1H, p-Sc-C6H3). 13C NMR (100 MHz, CDCl3,
25 °C): δ 18.39 (N-CH3), 23.12 (CH(CH3)2), 23.38 (CH(CH3)2), 28.42 (CH(CH3)2),
123.14 (m-N-C6H3), 123.56 (p-N-C6H3), 128.72 (p-Sc-C6H3), 129.20 (m-Sc-C6H3),
136.17 (o-N-C6H3), 141.47 (o-Sc-C6H3), 146.79 (N-C), 164.64 (CH=N) ppm. Anal.
Calcd for C34H43Cl2N2Sc (%): C, 68.57; H, 7.28; N, 4.70. Found (%): C, 68.48; H,
7.37; N, 4.75.
2.3.9. Preparation of complex 3b
Using the same procedure described for the synthesis of 2b, treatment of LgBr
n
(0.447g, 1.00 mmol) with BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) and
GdCl3(THF)3 (0.53 g, 1.10 mmol) afforded 3b as yellowish powder (0.59 g, 80%).
11

IR(KBr): ν (cm–1) 3350s, 3050w, 3010w, 2950s, 1620s, 1580m, 1510w, 1450s, 1425s,
1375m, 1300w, 1225s, 1200s, 1100w, 1050w, 910w, 850w, 800w, 780m, 750s, 700m.
Anal. Calcd for C34H43Cl2GdN2O2 (%): C, 55.19; H, 5.86; N, 3.79. Found (%): C,
55.08; H, 5.78; N, 3.84.
2.3.10. Preparation of complex 3d
Using the same work-up procedure described for the synthesis of 3b,treatment of
n
LhBr (0.56 g, 1.00 mmol) with BuLi (1.60 M in hexane, 0.65 mL, 1.05 mmol) and
GdCl3(THF)3 (0.53 g, 1.10 mmol) afforded 3d as yellow powder (0.64 g, 76%).
IR(KBr): ν (cm–1) 3350s, 3045w, 2950s, 2850m, 1650s, 1575m, 1520w, 1480m,
1450m, 1375w, 1350m, 1330w, 1260m, 1250m, 1200m, 1100w, 1050w, 950w, 850w,
780m. Anal. Calcd for C42H59Cl2GdN2O2 (%): C, 59.20; H, 6.98; N, 3.29. Found (%):
C, 59.11; H, 7.02; N, 3.32.
2.4. Polymerizations
In a glove box, isoprene (2.00 mL, 20.0 mmol), 10 mL toluene and 1000 µmol
AliBu3 were added into a 15 mL flask. Then 20 µmol complex and 1 equiv of
tritylborate ([Ph3C][B(C6F5)4]) were added. After a designated time, methanol was
injected into the system to terminate the reaction. The mixture was poured into a large
quantity of methanol to precipitate the white solid of polyisoprene. The polymer was
filtered and dried in vacuum at ambient temperature to constant weight.
In a glove box, 1,5-hexadiene (2.40 mL, 20.0 mmol), 10 mL toluene and 2 mmol
AliBu3 were added into a 15 mL flask. Then 20 µmol complex and 1 equiv of
tritylborate ([Ph3C][B(C6F5)4] were added. After a designated time, methanol was
12

injected into the system to terminate the reaction. The mixture was poured into a large
quantity of methanol to precipitate the white solid of polyhexadiene. The polymer was
filtered and dried in vacuum at ambient temperature to constant weight.
2.5. Crystal structure determination
The crystals were mounted on a glass fiber using the oil drop. Data obtained with
the ω−2θ scan mode were collected on a Bruker SMART 1000 CCD diffractometer
with graphite-monochromated Mo-Kα radiation (λ = 0.71073 Å). The structures were
solved by direct methods, and refined with full−matrix least squares on F2. All
non-hydrogen atoms were refined anisotropically andhydrogen atoms were introduced
in calculated positions with the displacement factors of the host carbon atoms. All
calculations were performed using the SHELXTL crystallographic software
packages.[19]
3. Results and discussion
3.1. Synthesis and characterization of ligands and NCN complexes
3.1.1. Synthesis and characterization of ligands.
The ligands of LaBr, LeBr, and LfBr were synthesized in a similar way to that for
preparation
of
LbBr[14]
via
simple
condensation
reactions
of
2-bromoisophthalaldehyde with substituted aniline in methanol in the presence of
catalytic amount of formic acid. Filtration gave the corresponding ligands with high
purity in high yields. All these new synthesized ligands were identified by 1H NMR
and 13C NMR, in which the typical singlet resonances of 8.74~8.99 ppm assignable to
the proton of HC=N were found. Correspondingly, the resonancesat 158.32~162.04
13

ppm for HC=N were detected in 13C NMR spectroscopy. The ligand of
2,6-bis[N,N′-(2′,6′-diisopropylphenyl)ketimino]-phenyl bromide (LhBr) was prepared
following a reported method by condensation of 2,6-diacetylphenyl bromide with
1
2,6-diisopropylaniline in harsh condition.[18] LhBr was also identified by H{13C}
NMR in which the double-double resonance pattern around 1.24~1.30 ppm was
1
observed in H NMR suggesting that the methyls of isopropyl groups in LhBr are in
inequivalent chemical environments. This phenomenon is in striking contrast to that
of LdBr in which only simple doublet in high field is observed revealing that the
incorporation of methyls to the NCN frameworks may preclude the free movements
of the phenyl groups attached to the nitrogen atoms.
3.1.2. Synthesis and characterization of scandium complexes based on aldimine NCN
ligands.
Generally, there are two main strategies for preparing the aryl-based pincer
complexes. One is oxidative addition of the ligands with low valent metal precursors.
This method is an atom-economic reaction and very suitable for preparing Ni,[16,18]
Ir,[13] and Pd[10] complexes which have stable low valent precursors. The other one is
conventional salt-metathesis reaction of the ligand-salt with metal-halides. Such
method is suitable for the metals with stable oxidation states. The lanthanide[14] and
chromium[15] complexes with aldimine NCN ligands, we reported previously, were
synthesized via salt metathesis reaction in high yields. So the lithium-salt metathesis
reaction was also used to prepare the NCN pincer scandium complexes. Treatment of
LaBr with nBuLi at low temperature (–78 ºC) afforded LaLi as blood-red solution and
14

then ScCl3(THF)3 was added to this solution in one portion. After stirring overnight at
room temperature the solution was subjected to routine work-up. Complex 1a was
obtained as yellow powder in 85% yield (Scheme 1).
Scheme 1. Synthesis of the aldimine NCN pincer scandium chlorides
Following the same procedure as described for synthesis of 1a, complexes 1b, 1c,
1d, 1e, and 1f were also successfully prepared in moderate to high yield as yellow
powder. All these complexes are soluble in common polar organic solvents such as
1
THF and toluene and give well-resolved NMR spectra in CDCl3 or C6D6. H NMR
spectra of these complexes display similar topology giving singlets at 8.29～8.43 ppm
assignable to the HC=N proton which are shifted to high field when compared with
13
those of 8.74~8.99 ppm in free ligands. C NMR spectroscopy confirms further the
formation of Sc‒C bond which shows the typical downfield chemical shifts around
1
185～191 ppm. It is worth to note that the H NMR of 1d shows two doublets with
identical integration in high field indicating that the methyls of the isopropyl groups
are in different environments (only one doublet in free ligand). This phenomenon may
be ascribed to the crowded environment at the metal center arising from the bulky
ligands and two coordinated THF molecules (see below).
Single crystals of complexes 1c, 1d, and 1f suitable for X-ray diffraction study
15

were grown from THF and hexane mixed solution. Their molecular structures are
depicted in Figures 1, 2, and 3 respectively along with selected bond lengths and
angles in the captions. All the three complexes are isostructural and the monoanionic
NCN pincer ligand coordinates to the Sc3+ ion in a κC: κN: κN tridentate mode. The
Sc3+ ions are essentially coplanar with the NCN plane with the max deviation of
0.081(4) Å for 1f. The back-bone of the ligand is coplanar with the N-aryl rings
disposing the perpendicular positions against the plane. Meanwhile, the two THF
rings arrange in cis-positions parallel to the two N-aryl rings respectively. The
trans-located chloride ligands form a large Cl–Sc–Cl angle bisected by the NCN
pincer plane. The Sc–Cl bond length, ranging from 2.412(3) to 2.4569(11) Å, is
slightly
shorter
than
those
of
2.5502(14)
Å
in
in
[{(η5-C5Me5)Sc(µ-Cl)(THF)2}2][B(C6F5)4]2
and those of 2.574(1)
Å
[20]
[(η5-C5Me5)ScCl(bpy)]2.[21] Meanwhile, the N–Sc–N angle of 136.2(2)~136.46(7)° in
these complexes are much larger than those of 130.24(10)~131.49(9)° in NCN
yttrium[14] complexes as well as that of 130.80(10) in aldimine NCN gadolinium[14]
complex probably due to the short C–Sc bond which holds the Sc3+deeply into the
coordination box.
16

Figure 1.ORTEP drawing of 1c with thermal ellipsoids drawn at 30% probability
level. Solvent molecular and hydrogens are omitted for clarity. Selected bond
distances (Å) and angles deg (°): C(1)–Sc(1) = 2.249(3), N(1)–Sc(1) = 2.6092(17),
O(1)–Sc(1) = 2.3145(15), Cl(1)–Sc(1) = 2.4247(5), C(2)–C(5) = 1.457(3), C(5)–N(1)
= 1.278(3); N(1A)–Sc(1)–N(1) = 136.46(7), O(1A)–Sc(1)–O(1) = 75.39(8),
O(1)–Sc(1)–Cl(1) = 90.97(4), O(1)–Sc(1)–N(1) = 74.10(5), Cl(1)–Sc(1)–N(1) =
90.99(4), Cl(1)–Sc(1)–Cl(1A) = 177.87(4).
Figure 2. ORTEP drawing of 1d with thermal ellipsoids drawn at 30% probability
level. Hydrogens are omitted for clarity. Selected bond distances (Å) and angles deg
(°): C(1)–Sc(1) = 2.238(4), N(1)–Sc(1) = 2.623(3), N(2)–Sc(1) = 2.608(3),
O(1)–Sc(1) = 2.289(3), O(2)–Sc(1) = 2.315(3), Cl(1)–Sc(1) = 2.4569(11), Cl(2)–Sc(1)
= 2.4389(12), C(2)–C(7) = 1.447(5), C(6)–C(8) = 1.442(5), C(7)–N(1) = 1.288(4),
C(8)–N(2) = 1.286(4); N(1)–Sc(1)–N(2) = 136.49(10), O(1)–Sc(1)–O(2)= 75.82(9),
O(1)–Sc(1)–Cl(1) = 91.37(7), O(1)–Sc(1)–Cl(2) = 87.33(7), Cl(1)–Sc(1)–Cl(2) =
177.72(5).
17

Figure 3. ORTEP drawing of 1f with thermal ellipsoids drawn at 30% probability
level. Hydrogens are omitted for clarity. Selected bond distances (Å) and angles deg
(°): C(1)–Sc(1) = 2.253(7), N(1)–Sc(1) = 2.653(6), N(2)–Sc(1) = 2.507(7),
O(1)–Sc(1) = 2.310(5), O(2)–Sc(1) = 2.312(5), Cl(1)–Sc(1) = 2.412(3), Cl(2)–Sc(1) =
2.441(3), C(2)–C(8) = 1.462(10), C(6)–C(7) = 1.440(11), C(7)–N(1) = 1.283(9),
C(8)–N(2) = 1.291(9); N(1)–Sc(1)–N(2) = 136.2(2), O(1)–Sc(1)–O(2)= 74.59(18),
O(1)–Sc(1)–Cl(1) = 89.04(15), O(1)–Sc(1)–Cl(2) = 91.73(15), Cl(1)–Sc(1)–Cl(2) =
177.72(5).
Scheme 2. Synthesis of the ketimine NCN pincer complexes
3.1.3. Synthesis and characterization of complexes based on ketimine NCN ligands.
The salt-metathesis protocol was also used to synthesize the ketimine-based NCN
pincer complexes. Treatment of LgBr or LhBr with nBuLi followed by transmetallation
reaction with ScCl3(THF)3 afforded 2b and 2d respectively as yellow powder in
moderate yields (Scheme 2). 2b and 2d show similar topologies in 1H NMR spectrum
in which a sharp singlet at 8.65~8.97 ppm assignable to the para proton of parent
18

phenyl ring is observed indicative of the formation of the Sc–C bonds. In high field,
2b shows a singlet resonance at 2.01 ppm for the four o-methyls in imine moieties
indicating that the whole molecule is in C2v symmetry in the solution. It is worth to
note that 2d is totally THF free and no THF signals were found in the NMR
1
spectroscopy. This result is consistent with the elemental analysis. In addition, the H
NMR of 2d shows double-double resonance with very small coupling constant (J =4
Hz) at 1.15~1.17 ppm. These resonances can be assigned to the methyls of isopropyl
groups based on their integrations. The small coupling constant suggests that the eight
methyls of four isopropyl group are almost in the same chemical environments. It is
reasonable to deduce that the absence of THF in the molecule may free the rotations
of aryl rings in the imine moieties. More importantly, the isopropyl resonances remain
almost no change when THF was added and no further splitting is observed even in
the presence of excess THF in CDCl3. This result rules out the possibility of the labile
coordination of THF to the metal centers in the solution. The crystals of 2b suitable
for X-ray diffraction analysis were grown from THF/hexane mixed solution. The
molecular structure is depicted in Figure 4 with selected bond distances and angles in
the caption. As expected, the monoanionic ketimine NCN pincer ligand coordinates to
the central metal ion in a typical meridional manner. All the metric parameters of 2b
are comparable to those in the aldimine NCN scandium complexes. It is worth to note
that the whole molecule of 2b is slightly twisted and has C2 symmetry with the C–Sc
vector as C2 axis. The existence of ketimine methyls may push the imine phenyl rings
closer to the coordinated THF and the twist of the molecule can release the repulsion
19

between imine aryl ring and THF. Given that 2b shows only one o-methyl signal in 1H
NMR, we believe that the molecule in solution is in fluxional isomerization between
two enantiomers on the NMR time scale.[22]
Figure 4.ORTEP drawing of 2b with thermal ellipsoids drawn at 30% probability
level. Solvent molecular and hydrogens are omitted for clarity. Selected bond
distances (Å) and angles deg (°): C(1)–Sc(1) = 2.232(3), N(1)–Sc(1) = 2.5269(17),
O(1)–Sc(1) = 2.3406(15), Cl(1)–Sc(1) = 2.4404(5), C(5)–C(6) = 1.507(3), C(5)–N(1)
= 1.295(3); N(1)–Sc(1)–N(1A) = 136.45(8), O(1)–Sc(1)–O(1A)= 73.60(7),
O(1)–Sc(1)–Cl(1) = 84.08(4), Cl(1)–Sc(1)–Cl(1A) = 176.95(4).
Similarly, the gadolinium complexes with ketimine NCN pincer ligands were also
prepared in a similar way (Scheme 2.). These gadolinium complexes were well
characterized by elemental analysis and IR spectroscopy. No informative NMR
spectra were obtained due to the magnetic nature of these complexes. Attempts to
obtain the crystals suitable for X-ray diffraction analysis failed.
Table 1. Summary of crystallographic data for complexes 1c, 1d, 1f, and 2b.
1c
C44H63Cl2N2O4Sc
799.82
1d
C40H55Cl2N2O2Sc
711.72
1f
C38H51Cl2N2O3Sc
699.67
2b
C34H43Cl2N2O2Sc
627.56
Formula
F
w
crystsyst
spacegroup
a (Å)
b (Å)
c (Å)
α (deg)
β (deg)
tetragonal
P4(3)2(1)2
16.3033(10)
16.3033(10)
16.1333(13)
90
orthorhombic
Pbca
19.9214(17)
16.6290(15)
23.813(2)
90
triclinic
P–1
8.709(3)
15.119(5)
15.781(5)
113.703(5)
101.759(5)
Monoclinic
C2/c
19.9248(13)
13.1663(11)
12.4001(10)
90
90
90
107.604(3)
20

γ (deg)
v (Å3)
90
4288.2(5)
4
1.239
0.339
1712
90
7888.8(12)
8
1.199
0.356
3040
93.527(6)
1838.7(10)
2
1.264
0.383
90
3100.7(4)
4
1.344
0.444
1328
Z
Dcalcd(gcm-3)
µ (mm-1)
F(000)
744
26.04
24236
4240
26.05
42645
7795
θmax(deg)
collectedreflns
Uniqreflns
26.09
10272
7084
27.53
19682
3568
R
0.0477
1.050
0.0366
0.0808
0.1415
0.842
0.0534
0.0904
0.0947
1.002
0.1058
0.1787
0.0737
1.018
0.0423
0.0834
int
GOF
R1
wR2
Largest diff peak, hole
(e A^-3)
0.295,–0.166
0.355, –0.315
0.602,–0.437
0.33, –0.34
3.2. Polymerization of isoprene and 1,5-hexadiene.
3.2.1. Polymerization of isoprene with NCN pincer complexes
With these well-defined NCN pincer scandium and gadolinium complexes in hand,
we set out to test them as catalysts for the isoprene polymerization. The
polymerization data are summarized in Table 2. Upon activation with
trialkylaluminum, all complexes were found to be inactive towards isoprene
polymerizationand no polymer was obtained even with higher temperatures and
longer times, which may be attributed to the low Lewis acidity of the resulted alkyl
scandium complexes. Whereas when activated with AlR3/[Ph3C][B(C6F5)4], 1a•1f, 2b,
and 2d show moderate to high catalytic activity and good cis-1,4 selectivity at room
temperature. It is found that the ortho-substitutes of the ligands show some influence
on the activity of the catalysts. The conversions gradually decreas from 100% to 73.4%
with the increase of the ortho substitutes size from H to isopropyl (see entries 1•4).
This phenomena is in contrast to the NCN yttrium system, in which the ortho-ethyl
containing catalyst shows higher activity than the o-methyl and o-isopropyl
containing analogues.[14] This difference suggests that the scandium NCN pincer
systems are more sensitive to the steric effects of the ligands probably due to the
21

small ionic radius of Sc3+. Although with same ortho-methyl groups in ligands, 1f
shows relatively higher activity than 1b and a complete conversion could be achieved
in 24 hours at room temperature. This slight improvement in the activity may be due
to the electronic effect of the para methyl in 1f. It is worth to note that the
ketimine-based complexes 2b and 2d are slightly less active than the corresponding
aldimine-based complexes (see entries 2, 7; 3, 8 in Table 2). As mentioned above, the
repulsion between the methyl in ketimine moiety and the aryl ring makes the metal
center more crowded and is not good for the monomer coordination. The catalytic
activity of these scandium complexes is strongly affected by the trialkylaluminum
cocatalyst in the trend of AliBu3> AlEt3>> AlMe3. (see entries 6, 12, and 13 in Table
2). The low activities of AlMe3 catalytic system may due to instability of methyl
scandium intermediate during the polymerization. The catalytic activities of 1f also
show great dependence on the polymerizationtemperature (entries 6, 16•19). When
the polymerization was performed at –20 ºC, low activity was obtained. The monomer
conversion increased rapidly with the increase ofpolymerizationtemperaturefrom –20
ºC to 40 ºC, while further increase of the temperature to 60 ºC the activity decreased.
Meanwhile, the cis-1,4selectivity was also effected by the polymerization
temperature. The selectivity decreases from 98.0% to 88.1% with the increase of the
temperature from –20 ºC to 60 ºC. The gadolinium complexes 3b and 3d show similar
activities and conversions of 84–95% were obtained in 6 hours affording the polymers
with almost identical cis-1,4selectivity (98.3%).
Table 2. Polymerization of isoprene under various conditionsa
22

b
entry
cat
AlR3
Tp
time
(h)
Yield
(%)
M
PDI
microstructurec
(%)
n
(°C)
(×103)
cis-1,4-
94.3
92.0
92.8
83.3
88.6
94.8
86.8
95.6
98.3
98.3
92.5
–
trans-1,4
0.5
3.9
1.3
4
3,4-
1
1a
1b
1c
1d
1e
1f
AliBu3(50)
AliBu3(50)
AliBu3(50)
AliBu3(50)
AliBu3(50)
AliBu3(50)
AliBu3(50)
AliBu3(50)
AliBu3(50)
AliBu3(50)
AliBu3(50)
AlMe3(50)
AlEt3(50)
AliBu3(100)
AliBu3(200)
AliBu3(100)
AliBu3(100)
AliBu3(100)
AliBu3(100)
20
20
20
20
20
20
20
20
20
20
20
20
20
20
20
-20
0
24
24
24
24
24
24
24
24
6
100
92.0
86.0
73.4
100
100
88.1
66.0
95.0
84.0
88.1
–
6.0
1.4
2.6
2.5
2.8
2.2
3.8
3.4
1.2
2.2
1.8
1.9
–
5.1
4.1
5.9
12.7
11.4
3.4
10
2
3
6.5
9.0
4
8.0
5
11.0
32.0
6.0
0
6
1.8
3.2
1.8
0
7
2b
2d
3b
3d
1f
8
105.0
7.5
2.6
1.7
1.7
7.3
–
9
10
11
12
13
14
15
16
17
18
19
6
16.5
8.0
0
6
0.2
–
1f
24
24
24
24
24
24
12
24
–
1f
47.0
100
100
9.6
4.0
1.3
1.5
2.3
1.6
–
96.7
95.3
94.7
98.0
96.4
90.9
88.1
1.9
1.5
1.5
0
1.2
3.2
3.8
2.0
3.6
5.8
7.7
1f
23.0
20.5
95.0
–
1f
1f
1f
72.0
100
85.9
0
1f
40
5.0
1.5
3.3
1f
60
5.0
1.8
4.2
aUnless otherwise specified, the polymerization reactions: toluene (10 mL), complex (20 µmol) isoprene 20 mmol.
[Ph3C][B(C6F5)4] 20 µmol. bDetermined by gel permeation chromatography (GPC) with respect to a polystyrene
standard. cDetermined by NMR.
3.2.1. Polymerization of 1,5-hexadiene with NCN pincer complexes
All these pincer complexes were also tested as catalysts for 1,5-hexadiene
polymerization and the preliminary results are listed in Table 3. It is found that, when
activated with alkylaluminum and trityl borate the scandium complexes show low to
moderate activities in 1,5-hexadiene polymerization. Complex 1b shows the highest
activity and 64.3% conversion is obtained at room temperature. The ketimine-based
pincer scandium complexes 2b and 2d are relatively lower active under the same
condition. With 2d, only 1% conversion is obtained in 24 hours at room temperature.
The gadolinium complexes are inert for 1,5-hexadiene polymerization. It is found that
the polymerization temperature shows slight influence on the activity. With 1b as
catalyst, the conversion increasesto 71.1% when the temperature increases from 20 ºC
23

to 40 ºC. When the temperature goes up to 60 ºC, lower activity is found. Moreover,
the polymerizations carried out at low temperature give no polymers (entry 15, 16). It
is worth to note that all the polymers are MCP enriched with the MCP content ranging
from 98.2% to 99.3% at room temperature. The MCP content drops gradually to 82.6%
when the polymerization temperature goes up to 60 ºC. No molecular weight
information was available due to the lower solubility of these polymers in THF.
Table 3. Polymerization of 1,5-hexadiene under various conditions
entry
cat
AlR3
Tp
Time
Yield
(%)
MCPb
(%)
VTMb
(%)
(°C)
20
20
20
20
20
20
20
20
20
20
20
20
20
20
-20
0
(h)
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
24
1
1a
1b
1c
1d
1e
1f
AliBu3(100)
AliBu3(100)
AliBu3(100)
AliBu3(100)
AliBu3(100)
AliBu3(100)
AliBu3(100)
AliBu3(100)
AliBu3(100)
AliBu3(100)
AlMe3(100)
AlEt3(100)
AliBu3(50)
29.6
64.3
18.1
18.8
5.0
98.8
99.3
98.8
99.1
98.3
98.2
98.3
98.8
-
1.2
0.7
1.2
0.9
1.7
1.8
1.7
1.2
-
2
3
4
5
6
5.1
7
2b
2d
3b
3d
1b
1b
1b
1b
1b
1b
1b
1b
14.5
1.1
8
9
-
10
11
12
13
14
15
16
17
18
-
-
-
trace
15.1
0.7
-
-
95.6
92.6
99.2
-
4.4
7.4
0.8
-
AliBu3(200)
AliBu3(100)
AliBu3(100)
AliBu3(100)
AliBu3(100)
65.1
trace
trace
71.1
0.9
-
-
40
93.5
82.6
6.5
17.4
60
aConditions: cat, 20 µmol; [Ph3C][B(C6F5)4], 20 µmol;AliBu3, 2000µmol; toluene, 10 mL; 25 oC, 24
hours.bDetermined by NMR.
4. Conclusions
In summary, we have reported the synthesis and characterization of several
scandium and gadolinium complexes bearing aldimine (ketimine) NCN pincer ligands.
In combination with AliBu3 and [Ph3C][B(C6F5)4], these complex exhibited moderate
24

activity for isoprene polymerization affording the polyisoprene with 88.1~98.3%
cis-1,4-regularity. The NCN pincer scandium complexes/AlR3/[Ph3C][B(C6F5)4]
systems show moderate activities for 1,5-hexadiene polymerization affording the
MCP enriched ployhexadiene (MCP content up to 99.3%). Whereas the gadolinium
complexes are almost inert under the same condition.
Appendix A. Supplementary data CCDC 1818410–1818413 contains the
supplementary crystallographic data for 1c, 1d, 1f, and 2b. These data can be obtained
free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;
fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.
Acknowledgements
The authors would like to thank theNational Natural Science Foundation of China
(21574052, U1462111, and 51673078) and Jilin University (J1103302).
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28

Several scandium and gadolinium complexes supported by aldimine and ketimine
NCN pincer ligands were synthesized and used as catalysts for isoprene and
1,5-hexadiene polymerization.
29