Isoprene polymerization with aminopyridinato ligand supported rare-earth metal complexes. Switching of the regio- and stereoselectivity

Article abstract of DOI:10.1039/c0cc00297f

The highly cis-1,4, trans-1,4 and 3,4-selective polymerizations of isoprene were successfully regulated by simply modifying the auxiliary ligands, metal centres and initiation groups of rare-earth metal complexes and cocatalysts.

Full text of DOI:10.1039/c0cc00297f

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Isoprene polymerization with aminopyridinato ligand supported
rare-earth metal complexes. Switching of the regio- and stereoselectivityw
Yi Yang,ab Kui Lv,ab Lingfang Wang,ab Yang Wangab and Dongmei Cui*a
Received 9th March 2010, Accepted 18th June 2010
DOI: 10.1039/c0cc00297f
The highly cis-1,4, trans-1,4 and 3,4-selective polymerizations of
isoprene were successfully regulated by simply modifying
the auxiliary ligands, metal centres and initiation groups of
rare-earth metal complexes and cocatalysts.
Cp*2Gd[(m-Me)AlMe2(m-Me)]2GdCp*2/AlR03/[Ph3C][B(C6F5)4]
displays a high cis-1,4 selectivity (97.3%, R0 = iBu) for
the polymerization of butadiene, however its cerium analogue
switches to trans-1,4 selectivity (93.8%, R0
=
Me);3b
the binary system composed of [(PhC(NC6H4iPr2-2,6)2)Y-
(o-CH2C6H4-NMe2)2] and [Ph3C][B(C6F5)4] is 3,4-selective
for isoprene polymerization, which upon addition of AlMe3
drastically switches to cis-1,4 selective;7a the aminopyridinato-
stabilized scandium alkyl complexes [iPr3C6H2(C6H3N)-
NC6H3-iPr2]ScR2(THF) (R = CH2SiMe3, CH2Ph) are initiators
for controlled 3,4-selective isoprene polymerization, while
their amino counterparts (R = N(SiHMe2)2) exhibited high
cis-1,4 selectivity.7b However, the controllable switching
among the three kinds of selectivities (cis-1,4, trans-1,4 and
3,4) toward isoprene polymerization, has not been reported, as
far as we are aware.
The polymerization of 1,3-conjugated dienes is one of the most
important industrial processes as the resultant polymers
possess versatile properties and wide applications that depend
mainly on their microstructures, generated during the
polymerization. Therefore much effort has been devoted to
designing highly cis-1,4, trans-1,4 or 3,4 regio- and stereo-
selective catalyst systems, to prepare polydienes with precisely
tailor-made microstructures.1 The neutral Ziegler–Natta catalyst
systems and the cationic rare-earth metal alkyl or chlorido
species bearing non-cyclopentadienyl ligands, have been
extensively investigated, and some have shown distinguished
cis-1,4 selectivity.2 Despite these achievements, only recently
have lanthanocene monomethyl or monosilylamide complexes
been reported to display good cis-1,4 selectivity under the
activation of a borate salt and aluminium reagents,3 yet non-
cyclopentadienyl (non-Cp) rare-earth metal complexes bearing
mono-initiators possessing such performance, have remained
scarce. Simultaneously, some trans-1,4 selective catalyst
precursors have also been obtained, such as lanthanide tris-
or bis(allyl)s,4 lanthanide-aluminium heterometallic methyl
complexes3b,5 and neodymium tris- or bis(borohydride)s.6
However, to date, systems showing both high trans-1,4
selectivity and a living mode are rather rare.5b Regarding the
3,4-polymerization of isoprene, only few catalysts, composed
of rare-earth metal alkyl complexes attached to half-sandwich
ligands or sterically bulky NPN-tridentate ligands, are effective,7,8
as isoprene prefers to coordinate to the metal center in a cis-1,4
mode under generation of a thermally stable Z3-p-butenyl
species. Thus to initiate the 3,4-polymerization of isoprene,
steric hindrance at the metal center is requisite.
Herein we wish to report the synthesis of rare-earth metal
alkyl complexes bearing phosphine-aminopyridinyl ligands.
These complexes, in the presence of organoborate and aluminium
trialkyls, were the first monoalkyl complexes bearing non-Cp
ligands to be highly cis-1,4 selective for isoprene polymerization.
By tuning the ligand to iminophosphine-aminopyridinyl,
the resultant rare-earth metal dialkyl derivative provided
3,4-regulated polyisoprene. Strikingly replacing the metal-
alkyl species by metal-borohydride, led to high trans-1,4
polymerization in an unprecedented living mode. These results
displayed for the first time the shuttling among the three types
of selectivities.
Treatment of Ln(CH2SiMe3)3(THF)2 with 1 equiv. of
pyridyl-NHPPh2 (HL1) in toluene afforded rare-earth metal
mono(alkyl) complexes L1 Ln(CH2SiMe3)(THF) (Ln = Lu
2
(1a), Y (1b), Sc (1c)) via abstraction of the two alkyls
(Scheme 1, Fig. S1w, 1a). 2-Pyridyl-NHPPh2QNC6H3-2,6-
Me2 (HL2) was achieved in a moderate yield by Staudinger
reaction of HL1 with one equiv. of the corresponding
azide. Further acid–base reaction between HL2 and
Lu(CH2SiMe3)3(THF)2 via alkane elimination gave the lutetium
bis(alkyl)s, L2Lu(CH2SiMe3)2(THF) (2) (Scheme 1). Although
without X-ray analysis, the 1H NMR spectrum (Fig. S3w) was
informative for the formation of 2 as a THF-coordinated
bis(alkyl).8c
Meanwhile,
a more convenient method of switching
the types of selectivities for the polymerization of dienes by
simply modifying the catalytic precursors and cocatalysts,
has attracted increasing attention. Some catalyst systems
have shown shuttling between two types of selectivities
for the polymerization of dienes.3b,4b,7,9 For instance,
The unprecedented straightforward reaction between
Nd(BH4)3(THF)3 and an equimolar amount of HL2 in THF
a State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute ofApplied Chemistry, Chinese Academy of
Sciences, Changchun 130022, China. E-mail: dmcui@ciac.jl.cn;
Fax: +86-431-85262773; Tel: +86-431-85262773
b Graduate University of Chinese Academy of Sciences,
Beijing 100039, China
w Electronic supplementary information (ESI) available: Experimental
section. CCDC reference numbers 756926 (1a) and 758593 (3). For
ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c0cc00297f
generated the bis(borohydrido) neodymium complex
3
(Scheme 2). The molecular structure of 3 was confirmed by
X-ray crystallographic analysis (Fig. 1), in which both the
borohydride groups (BH4ꢀ) and the newly formed N–BH3
species coordinate to Nd3+ in the unusual Z2-H–B mode.z As
far as we are aware, few borohydrido complexesꢀare prepared
through ligand deprotonation directly by a BH4 group with
ꢁc
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Scheme 1 Synthesis of complexes 1a, 1b, 1c and 2.
Fig. 1 X-Ray structure of complex 3 with thermal ellipsoids at 35%
probability. Hydrogen atoms are omitted for clarity. Selected bond
distances (A) and angles (1): Nd–H(101) 2.45(4), Nd–H(103) 2.38(4),
Nd–H(104) 2.44(4), Nd–H(105) 2.47(4), Nd–H(108) 2.38(3),
Nd–H(109) 2.43(3), Nd–N(2) 2.517(2), Nd–N(3) 2.468(2), Nd–P
3.2116(7), Nd–O(1) 2.5291(18), Nd–O(2) 2.558(2), N(1)–B(3)
1.569(4), N(2)–Nd–N(3) 59.34(7), B(1)–Nd–B(2) 166.13(9).
extruding H2 gas, although the Z2-fashion of the BH3 group
has been once found in a lutetium borohydride complex
supported by a 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}-
ꢀ
pyrrole ligand.10 If the BH4 group is considered monodentate,
the structure of complex 3 reveals a seven coordination sphere
of the ligand around the neodymium atom, resulting in a
ꢀ
molecular weight distribution, indicative of the absence of
other polymerization active species at low temperature.3b
In contrast, when the ancillary ligand HL2, a spatially steric
derivative of HL1, was introduced to form complex 2,
remarkably, the resultant system 2/borate/AliBu3, under the
same polymerization conditions as complex 1a, provided
3,4-regulated polyisoprene. When using Brønsted acid
[PhNMe2H][B(C6F5)4] to replace [Ph3C][B(C6F5)4] to activate
2, 3,4-selectivity increased slightly to 88.0% from 87.2%
(Runs 5–6). Hence, switching from the cis-1,4 (1a–1c) to 3,4-
selectivity was successfully achieved simply by modifying steric
hindrance around the metal centre. This was different from the
previous report that selectivity switching was reached through
modification of cocatalysts7a or initiation groups.7b Ligand
modification leading to dramatically different selectivity for
isoprene polymerization is, to our knowledge, unprecedented.
Strikingly, the neodymium borohydrido complex 3 supported
by ligand HL2, could initiate highly trans-1,4 selective
polymerization of isoprene in the presence of organoborate
and MgnBu2 (97%, Run 7). However, under the same conditions
(organoborate and MgnBu2), rare-earth metal alkyl analogues
(1a, 1b, 1c and 2) were all inert to isoprene polymerization.
Considering the well-known bridging ability of the borohydride
ligand in lanthanide chemistry, the trans-1,4 stereospecificity
can be tentatively correlated to the formation of sterically
hindered borohydrido-bridged bimetallic Nd/Mg species.6
Kinetic studies demonstrated that the conversion increased
with polymerization time, which had a linear correlation with
the number-average molecular weight (Mn) of the obtained
polymer; while the molecular weight distribution remained
almost unchanged (Mw/Mn = 1.05–1.10) (Fig. 2). This
result was indicative of a single-site nature of this ternary
catalyst system that induced the remarkable living trans-1,4-
polymerization of isoprene. It is noteworthy that a narrow
PDI (1.20 or so) was not dependent on borate and temperature,
distorted pentagonal bipyramidal geometry. The two BH4
groups are trans-positioned in an almost straight line, giving a
large B(1)–Nd–B(2) angle (166.13(9)1), while the two nitrogen
atoms N(2) and N(3) and two oxygen atoms and one boron
atom B(3) occupy the equatorial positions.
All these complexes were evaluated in the catalysis of the
polymerization of isoprene. The neutral bis(aminopyridinato)
monoalkyl complexes 1a–1c, as a single component, did not
polymerize isoprene, neither did the two-component systems
of (1a–1c)/borate or (1a–1c)/AliBu3. Fortunately the homo-
geneous ternary systems (1a–1c)/borate/AliBu3 showed
versatile activities and high cis-1,4 selectivities. This is the first
example of a non-Cp lanthanide monoalkyl precursor initiating
cis-1,4 isoprene polymerization. The catalytic activity was
strongly dependent on the ionic radius of the central metal
with the trend of Sc 4 Y 4 Lu, but the trend of cis-1,4
selectivity was opposite (Table 1, Runs 1–3). This suggested
that the Lewis-acidic Sc ion favored the coordination and
insertion of a non-polar isoprene monomer. Moreover, the
influences of the type of borate, the amount of AlR3 and the
monomer-to-lanthanide ratio on catalytic performance, for
instance of 1a, are presented in Table S1.w Decrease of the
polymerization temperature (0 1C) led to a minor increase
in cis-1,4 content (97.7%, Run 4) and a relatively narrow
Scheme 2 Synthesis of complex 3.
ꢁc
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Chem. Commun., 2010, 46, 6150–6152 | 6151

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a
Table 1 Isoprene polymerization with complexes 1a, 1b, 1c, 2 and 3/borate/AliBu3 or MgnBu2
cis-1,4b (%)
trans-1,4b (%)
3,4b (%)
Mn (10ꢀ4
)
Mw/Mn
c
c
Run
Catalyst
Al or Mg
[Al]/[Ln]
T/1C
Time/h
Yield
1
2
3
4
5
6
7
8
9
10
1a + B
1b + B
1c + B
1a + B
2 + B
2 + A
3 + A
3 + A
3 + B
3 + C
AliBu3
AliBu3
AliBu3
AliBu3
AliBu3
AliBu3
MgnBu2
MgnBu2
MgnBu2
MgnBu2
10
10
10
10
10
10
1
1
1
1
20
20
20
0
15
15
15
50
50
50
4
10 min
5 min
100
99
98
82
98
100
73
80
82
78
97.2
95.6
95.0
97.7
12.8
12.0
1.3
3.8
3.2
3.1
1.0
1.3
1.5
0
1.8
3.1
3.5
2.3
87.2
88.0
1.5
2.0
1.6
8.75
16.8
23.0
11.5
3.19
3.08
4.21
4.36
4.43
4.50
2.08
2.52
3.07
1.87
1.40
1.42
1.10
1.17
1.23
1.17
8
4
4
5
6
6
6
97.2
94.2
95.2
94.2
2.7
a
General conditions: 5 mL toluene, A = [PhMe2NH][B(C6F5)4], B = [Ph3C][B(C6F5)4], C = B(C6F5)3, [Borate]/[Ln] = 1.0, [IP]/[Ln] =
c
1000. Determined by H NMR and 13C NMR. Measured by GPC calibrated with standard polystyrene samples.
b
1
and trans-1,4 selectivity decreased slightly with a temperature
increase (Runs 8–10).
Notes and references
z Crystal data for 1a: C42H47LuN4OP2Si, M = 888.84, trigonal,
In summary, we have demonstrated that rare-earth metal
alkyl or borohydrido complexes bearing aminopyridine based
ligands, in combination with proper cocatalysts generated
ternary catalyst systems, which realized successful and adjustable
shuttling among the cis-1,4 (97%), 3,4- (88%) and living trans-
1,4 (97%) regio- and stereoselectivities for the polymerization
of isoprene. The rare-earth metal monoalkyl precursors bearing
the less sterically bulky ligand provided cis-1,4 selectivity;
whilst the 3,4-selectivity could be achieved with the bis(alkyl)
analogues attached to the sterically bulky ligand; furthermore,
the catalyst system based on the borohydrido counterpart and
MgnBu2 was prone to distinguished trans-1,4 selectivity, which
might be attributed to the formation of a bimetallic inter-
mediate. These results may shed new light on catalyst design,
microstructure control and mechanism investigation.
a = 18.0407(6) A, b = 18.0407(6) A, c = 11.0329(7) A, a = 90.001,
b = 90.001, g = 120.001, V = 3109.8(2) A3, T = 187(2) K, space
group P3(2), Z = 3, Rint = 0.0341. The final R1 values were 0.0398
(I 4 2s(I)). The final wR(F2) values were 0.0886 (I 4 2s(I)). The final
R1 values were 0.0435 (all data). The final wR(F2) values were 0.0903
(all data). Crystal data for 3: C37H58B3N3NdO3P, M = 800.50,
triclinic, a = 9.5888(5) A, b = 13.5918(8) A, c = 17.0238(9) A,
a = 75.0720(10)1, b = 87.9480(10)1, g = 69.6440(10)1, V =
3
ꢀ
2006.39(19) A , T = 187(2) K, space group P1, Z = 2, Rint
=
0.0115. The final R1 values were 0.0273 (I 4 2s(I)). The final wR(F2)
values were 0.0714 (I 4 2s(I)). The final R1 values were 0.0289 (all
data). The final wR(F2) values were 0.0726 (all data).
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Fig. 2 Polymerization of isoprene (IP) with complex 3/[PhNMe2H]-
[B(C6F5)4)]/MgnBu2: molecular weight vs. conversion. Conditions:
[IP]/[Nd] = 1000, [Nd]0 = 3.33 mol mLꢀ1, 3/[PhNMe2H][B(C6F5)4)]/
MgnBu2 = 1/1/1, toluene, 15 1C.
ꢁc
This journal is The Royal Society of Chemistry 2010
6152 | Chem. Commun., 2010, 46, 6150–6152