Binuclear rare-earth-metal alkyl complexes ligated by phenylene-bridged β-diketiminate ligands: Synthesis, characterization, and catalysis toward isoprene polymerization

Article abstract of DOI:10.1021/om400105t

Deprotonation of m-phenylene-bridged bis(β-diketiminate) ligands (PBDI^iPr-H₂ = [2,6-ⁱPr₂C ₆H₃NHC(Me)C(H)C(Me)N]₂-(m-phenylene); PBDI ^Et-H₂ = [2,6-Et₂C₆H ₃NHC(Me)C(H)C(Me)N]₂-(m-phenylene); PBDI ^Me-H₂ = [2,6-Me₂C₆H ₃NHC(Me)C(H)C(Me)N]₂-(m-phenylene)) by rare-earth-metal tris(alkyls) Ln(CH₂SiMe₃)₃(THF)₂ (Ln = Y, Lu, Sc) gave a series of rare-earth-metal bis(alkyl) complexes: PBDI ^iPr-[Y(CH₂SiMe₃)₂] ₂(THF)₂ (1), PBDI^Et-[Ln(CH₂SiMe ₃)₂]₂(THF)_n (2a, Ln = Y, n = 2; 2b, Ln = Lu, n = 2; 2c, Ln = Sc, n = 1), and PBDI^Me-[Y(CH ₂SiMe₃)₂]₂(THF)₂ (3). All these complexes were fully characterized by NMR spectroscopy, X-ray diffraction, and elemental analyses, adopting binuclear structures with the two rare-earth-metal ions taking trans positions versus the phenyl ring. Complexes 1, 2a,b, and 3 coordinate two solvated THF molecules, while the scandium complex 2c incorporates only one THF molecule, owing to the steric crowding. Upon activation with 2 equiv of organoborate, the yttrium systems showed higher catalytic activity toward isoprene polymerization in comparison to those based on lutetium, and the scandium system was less active. Addition of aluminum alkyls to the above binary systems accelerated dramatically the polymerization rate irrespective of the central metal type through scavenging impurities in the systems and abstracting the solvated THF molecules in the precursors. The resultant polyisoprene had higher 3,4-regularity (20% vs 5%) as well as higher molecular weights in comparison with the mononuclear systems, which might be attributed to the steric bulky effect of the binuclear systems.

Full text of DOI:10.1021/om400105t

Article
pubs.acs.org/Organometallics
Binuclear Rare-Earth-Metal Alkyl Complexes Ligated by Phenylene-
Bridged β‑Diketiminate Ligands: Synthesis, Characterization, and
Catalysis toward Isoprene Polymerization
Lei Li,†,‡ Chunji Wu,† Dongtao Liu,† Shihui Li,*,† and Dongmei Cui*,†
†State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences,
Changchun 130022, People’s Republic of China
‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China
S
* Supporting Information
ABSTRACT: Deprotonation of m-phenylene-bridged bis(β-
diketiminate) ligands (PBDIiPr-H2 = [2,6-iPr2C6H3NHC(Me)-
C(H)C(Me)N]2-(m-phenylene); PBDIEt-H2 = [2,6-
Et2C6H3NHC(Me)C(H)C(Me)N]2-(m-phenylene); PBDIMe-
H2 = [2,6-Me2C6H3NHC(Me)C(H)C(Me)N]2-(m-phenyl-
ene)) by rare-earth-metal tris(alkyls) Ln(CH2SiMe3)3(THF)2
(Ln = Y, Lu, Sc) gave a series of rare-earth-metal bis(alkyl)
complexes: PBDIiPr-[Y(CH2SiMe3)2]2(THF)2 (1), PBDIEt-
[Ln(CH2SiMe3)2]2(THF)n (2a, Ln = Y, n = 2; 2b, Ln = Lu,
n = 2; 2c, Ln = Sc, n = 1), and PBDIMe-[Y(CH2SiMe3)2]2(THF)2 (3). All these complexes were fully characterized by NMR
spectroscopy, X-ray diffraction, and elemental analyses, adopting binuclear structures with the two rare-earth-metal ions taking
trans positions versus the phenyl ring. Complexes 1, 2a,b, and 3 coordinate two solvated THF molecules, while the scandium
complex 2c incorporates only one THF molecule, owing to the steric crowding. Upon activation with 2 equiv of organoborate,
the yttrium systems showed higher catalytic activity toward isoprene polymerization in comparison to those based on lutetium,
and the scandium system was less active. Addition of aluminum alkyls to the above binary systems accelerated dramatically the
polymerization rate irrespective of the central metal type through scavenging impurities in the systems and abstracting the
solvated THF molecules in the precursors. The resultant polyisoprene had higher 3,4-regularity (20% vs 5%) as well as higher
molecular weights in comparison with the mononuclear systems, which might be attributed to the steric bulky effect of the
binuclear systems.
mononuclear Grubbs catalysts.5 Baar et al. prepared the linked
pyridylimine binuclear Pd(II) complexes, in which the two Pd
INTRODUCTION
■
Due to the unique reactivities and selectivities of multinuclear
centers appeared to act essentially independently and exhibited
metalloenzymes, bi- or oligonuclear organometallic complexes
the same activity and stereoregulation as the mononuclear
have obtained increasing attention, which are anticipated to
analogues.6 Pd(II) derivatives bearing pyrazole-based dinucleat-
lead to unique substrate activation modes and to novel
ing ligands with appended imine functions were synthesized by
reactivity patterns on the basis of the possible cooperative
Meyer, displaying performances rather similar to those of the
effects between adjacent active metal centers. Recent years have
Brookhart α-diimine catalysts.7 Casalino designed a Pd(II)
witnessed an impressive progress in the development of
binuclear olefin polymerization catalysts.1 Marks and co-
binuclear complex employing a bis-chelating ligand based on
the [1,4]dioxocino[6,5-b:7,8-b′]dipyridine moiety, which was
workers reported that phenylene-, siloxane-, and polymethy-
lene-linked metallocene and half-metallocene early-transition-
active for alternating styrene and CO copolymerization.8 The
binuclear phenoxyiminato Ni catalysts generally exhibit higher
metal binuclear complexes can provide higher molecular weight
thermal stability due to the increased steric congestion around
and short branch polyolefins and incorporate more comonomer
or bulkier α-olefins in the copolymerization with ethylene
the metal centers.9 Those having electron-withdrawing groups
developed by Wehrmann and Mecking10 gave higher activity,
which was attributed to an intrinsically higher rate of
propagation rather than a more efficient activation of the
catalyst precursor (pyridine dissociation). Agapie reported that
the syn late-transition-metal nickel binuclear complex bearing
versus their monometallic counterparts.2 Salata and Marks
reported that the linked phenoxyiminato Ti and Zr complexes
gave an activity ∼8× that of mononuclear catalysts for ethylene
polymerization and enhanced α-olefin enchainment.3 Solan’s
group synthesized bis(imino)pyridine Ni(II) complexes as
ethylene oligomerization catalysts,4 while binuclear phenox-
yiminato catalysts with different types of linkages have also
been reported, albeit with lower activity in comparison with the
Received: February 7, 2013
© XXXX American Chemical Society
A
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Scheme 1. Syntheses of Bimetallic Complexes 1−3
the terphenyl ligand appended with phenoxy and imine donors
was more durable to amines than its anti isomer and the
monomeric counterparts, thus providing solid proof for the
cooperative effect of the proximity of the two metal ions.11 In
contrast, reports about rare-earth-metal-based binuclear com-
plexes are scarce and their catalytic behavior has been less
explored.12
On the other hand, increasing efforts have been devoted to
the design of more efficient catalysts for the polymerization of
1,3-conjugated dienes with desired microstructures and proper-
ties to afford polyisoprene/polybutadiene, among the most
significant and widely used rubbers.13 The homogeneous rare-
earth-metal-based catalytic systems have been demonstrated to
be superior by providing high cis-1,4, trans-1,4 ,or 3,4- (1,2-)
regulated polydienes depending on the ancillary ligand
frameworks and metal ionic radii; thus, the lanthanocene
aluminates and the alkyl-bridged lanthanide carboxylates,
monocyclopentadienyl (Cp) rare-earth-metal complexes, and
NCN and NPN multidentate non-Cp rare-earth-metal
complexes have been reported.14−18 Despite the significant
achievements obtained to date, few reports have been related to
employing rare-earth-metal binuclear precursors.19
Our group has been pursuing the exploration of new ancillary
ligands to support rare-earth-metal cationic active species for
specific selective (co)polymerizations of the conjugated dienes
to access polydiene materials with tailor-made micro-
structures.13e,15f,i,l,n−q Among these ligands, β-diketiminates
can be prepared swiftly and modified easily, although the
attached rare-earth-metal complexes are not as stable during the
generation of active species.15m Herein, we wish to report that
m-phenylene-bridged bis(β-diketiminates), initially employed
by Harder et al. to stabilize zinc bimetallic moieties to initiate
expoxide and CO2 copolymerization,20 were used for the first
time to stabilize rare-earth-metal bis(alkyl) species. Moreover,
their catalytic performances under the proper activation of
aluminum alkyls and organoborate for isoprene polymerization
were also examined.
(CH2SiMe3)3(THF)2 with PBDIEt-H2 gave the lutetium and
scandium analogues PBDIEt-[Lu(CH2SiMe3)2]2(THF)2 (2b)
and PBDIEt-[Sc(CH2SiMe3)2]2(THF) (2c), respectively. 1H
NMR spectroscopic analysis revealed that complexes 1−3 show
similar resonance topologies. No amino proton resonances (δ
13.19, 12.59, 12.52) from the ligands are observed, suggesting
the completeness of the reaction; the yttrium methylene groups
Y−CH2SiMe3 appear as a doublet due to yttrium and hydrogen
coupling around δ −0.50 (2JY−H = 2.8 Hz) for 1, δ −0.49 (2JY−H
= 2.0 Hz) for 2a, and δ −0.48 (2JY−H = 2.8 Hz) for 3; the
lutetium methylene groups Lu−CH2SiMe3 in 2b and the
scandium methylene groups Sc−CH2SiMe3 in 2c give singlet
resonances. Meanwhile the phenylene-bridged bis(β-diketimi-
nato) backbones display a similar unitary set of resonances in
all these complexes, and the alkyl substituents on the N-aryl
rings are nearly indistinguishable, implying rapid rotation and
ring flip of the β-diketiminato backbones and the symmetric
molecular structures in the solution state. The solid-state
structures of complexes 1−3 were established by single-crystal
X-ray diffraction analysis, as shown in Figures 1 and 2 for
complexes 1 and 2c (Figures S1−S3 (Supporting Information)
for 2a,b and 3, respectively). All of these complexes are
analogous, adopting binuclear structures. For the yttrium and
lutetium complexes, each metal ion bonds to a N,N-chelating β-
RESULTS AND DISCUSSION
■
Synthesis and Characterization of β-Diketiminato
Binuclear Complexes. To construct the binuclear structure
envisioned above, bis(β-diketiminate) ligands with the different
substituents PBDIiPr-H2, PBDIEt-H2 and PBDIMe-H2 were
prepared by following a modified literature procedure.20
Metalation of these ligands by a 2-fold molar mass of
Y(CH2SiMe3)3(THF)2 was carried out readily at room
temperature to afford the corresponding bis(alkyl) complexes
PBDIiPr-[Y(CH2SiMe3)2]2(THF)2 (1), PBDIEt-[Y-
(CH2 SiMe3 )2 ]2 (THF)2 (2a), and PBDIM e -[Y-
(CH2SiMe3)2]2(THF)2 (3) in good yields (Scheme 1).
Treatment of Lu(CH2 SiMe3 )3 (THF)2 and Sc-
Figure 1. (top) X-ray structure of complex 1 with 30% probability
thermal ellipsoids. Hydrogen atoms and solvents are omitted for
clarity. (bottom) Asymmetric structure of the two β-diketiminato
backbones linked by a phenylene ring in 1.
B
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average Y(2)−N = 2.341 Å vs average Y−N = 2.369 Å; average
Y(1)−C = 2.398 Å and average Y(2)−C = 2.392 Å vs average
Y−C = 2.384 Å; N−Y(1)−N = 79.13(13)° and N−Y(2)−N =
77.50(14)° vs N−Y−N = 77.52(10)°; C−Y(1)−C =
113.89(1)° and C−Y(2)−C = 110.74(1)° vs C−Y−C =
112.59(13)°). The two β-diketiminato−metal units take trans
positions against the phenyl plane due to the steric repulsion of
the four alkyl moieties (Figure 1, bottom). Interestingly, the
precise coordination geometries around the two metal centers
are quite different. For Ln(1) the five atoms on the β-
diketiminate unit are virtually coplanar (the largest deviations
from the NCCCN plane defined by these atoms are 0.069,
0.021, 0.019, and 0.051 Å for 1, 2a,b, and 3, respectively), while
for Ln(2) the β-diketiminato backbone is highly distorted (the
largest deviations from the plane are 0.157, 0.101, 0.07, and
0.099 Å for 1, 2a,b, and 3, respectively). Furthermore, the metal
ions coordinate to the β-diketiminate units in different fashions,
as evidenced by the fact that Ln(1) sits slightly out of the
NCCCN plane (0.685, 0.192, 0.310, and 0.457 Å for 1, 2a,b,
and 3, respectively), rendering it coordinated edge-on, while
Ln(2) is situated significantly out of the plane (1.717, 0.917,
0.985, and 1.045 Å for 1, 2a,b, and 3, respectively), resulting in
its face-on coordination (Table S1 (Supporting Information)).
It deserves special comment that the large displacement off the
backbone plane in 1 is rarely seen in β-diketiminato lanthanide
complexes,22 which might be attributed to the most sterically
encumbered substituents on the ancillary ligand in complex 1
among the series. These geometry characteristics are reflected
by the corresponding bond lengths and bond angles. In
complex 1, the N(1)−C(2) (1.336(7) Å), C(2)−C(3)
(1.393(7) Å), C(3)−C(4) (1.395(7) Å), and N(2)−C(4)
Figure 2. X-ray structure of complex 2c with 30% probability thermal
ellipsoids. Hydrogen atoms and solvents are omitted for clarity.
diketiminate unit, two alkyl moieties, and a THF molecule,
generating a trigonal-bipyramidal geometry similar to that in
the mononuclear complexes.15m,21 Thus, typical bond lengths
and bond angles around the metal centers in the binuclear
complex 3 are comparable to those found in the mononuclear
counterpart 4 (Chart 1)15m (average Y(1)−N = 2.361 Å and
Chart 1
a
Table 1. Polymerization of Isoprene by Using Various Catalyst Systems
c
microstructure (%)
b
d
e
e
run
cat.
[AlR3]/ [cat.] [B]/ [cat.]
time (min) yield (%) cis-1,4 trans-1,4 3,4-
Mn(th) (×104) Mn(exp) (×104) Mw/Mn
f
f
1
2
3
4
5
6
7
8
9
10
1−3
1−3
1
10 (iBu)
0
1 (TB)
2 (TB)
2 (TB)
2 (TB)
2 (TB)
2 (TB)
1 (TB)
2 (TB)
3 (TB)
2 (TB)
2 (TB)
2 (TB)
2 (TB)
2 (TB)
1 (TB)
1 (HNB)
2 (TB)
2 (TB)
2 (TB)
2 (TB)
2 (HNB)
600
600
200
200
200
200
200
10
5
trace
trace
100
100
88.6
41.1
100
100
100
100
100
100
100
100
100
100
60
0
0
86.6
83.5
76.5
70.6
83.2
84.1
83.5
82.3
83.1
78.3
71.6
70.1
79.7
94.1
96.3
83.5
83.2
83.9
83.3
82.8
0.3
2.6
2.4
4.7
1.3
1.1
0.8
1.0
1.0
1.1
2.9
3.0
1.1
2.2
0.5
1.0
1.1
3.5
1.8
1.1
13.1
13.9
21.1
24.7
15.5
14.8
15.7
16.7
15.9
20.6
25.5
26.9
19.2
3.7
3.4
3.4
3.0
1.4
3.4
6.8
3.4
6.8
3.4
3.4
3.4
3.4
3.4
3.4
2.0
3.4
3.4
0.35
3.4
3.4
21.3
19.9
41.3
4.7
1.3
1.3
1.8
1.2
1.3
1.9
1.4
2.0
1.6
1.5
1.6
1.6
2.2
1.4
2.1
1.6
1.7
2.0
1.6
1.6
2a
2b
2c
3
0
0
0
0
22.3
17.5
13.1
23.1
10.8
15.1
15.2
12.9
14.1
6.4
1
10 (iBu)
10 (iBu)
10 (iBu)
10 (iBu)
10 (iBu)
10 (iBu)
10 (iBu)
10 (iBu)
5 (iBu)
10 (iBu)
20 (iBu)
40 (iBu)
10 (Me)
10 (Et)
10 (iBu)
1
1
10
5
g
11
1
12
13
14
15
2a
2b
2c
3
5
5
5
5
h
16
h
4
5
17
4
5
3.2
4.2
18
19
20
21
22
1
5
100
100
10.4
100
100
15.5
15.7
12.6
14.9
16.1
9.9
1
5
9.4
1
5
14.5
11.4
34.3
1
5
1
5
a
b
Polymerization conditions: 10 °C; toluene (5.0 mL); cat. (10 μmol); [IP]/[cat.] = 1000. Abbreviations: TB, [Ph3C][B(C6F5)4]; HNB,
c
d
[PhNHMe2][B(C6F5)4]. Determined by 1H NMR and 13C NMR spectroscopy in CDCl3. Mn(th) = 1000 × ([cat.]/[B]) × yield × 68.
e
f
g
Determined by GPC in THF at 40 °C against a polystyrene standard. Polymerization performed at 60 °C. Polymerization performed in 10 mL of
h
toluene. Conditions: cat. (20 μmol); [IP]/[cat.] = 500.
C
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bond distances (1.334(7) Å) are intermediate between the
corresponding single-bond and double-bond distances, indicat-
ing that the electrons are largely delocalized over the NCCCN
backbone attached to Y(1); in contrast, N(4)−C(15)
(1.288(7) Å) and C(13)−C(14) (1.402(7) Å) are shorter
than N(3)−C(13) (1.335(7) Å) and C(14)−C(15) (1.445(8)
Å), respectively, suggesting a more localized distribution of
electrons within the NCCCN backbone attached to Y(2).
Moreover, the face-on coordination in 1 drags the Y(2) ion
closer to the backbone carbon atom C(13), rendering a Y(2)−
C(13) distance of 2.909(5) Å, indicative of a weak interaction,
which might also contribute to the shortest distance between
the two yttrium ions among its yttrium congeners (Y(1)−Y(2)
= 7.735 Å for 1 vs 8.127 Å for 2a vs 8.104 Å for 3). In the
smallest scandium complex 2c, the more twisted Sc(2) metal
center is four-coordinate, remaining free of a THF base, while
the slightly twisted Sc(1) metal center is additionally
coordinated with a THF molecule (as in the case of the
above yttrium and lutetium complexes). The variance in
coordination mode renders the N(3)−Sc(2)−N(4)
(86.03(16)°) angle larger than N(1)−Sc(1)−N(2)
(83.79(15)°) by 2.24° (the largest variation among the series),
probably compensating for the coordination-number degener-
ation. The Sc−N, Sc−C, and Sc−O bond distances fall in the
normal single-bond regions in the literature,23 while a slightly
closer proximity of the two scandium ions (Sc(1)−Sc(2) =
7.955 Å) in 2c is mainly ascribed to the smaller metal ionic size.
According to previous reports,2 metal−metal spatial
proximity and the steric bulk in a binuclear complex may
incur notable concerted effects of the two metal centers and
influence its catalytic behavior. Thus, the polymerization of
isoprene (IP) by using these bimetallic precursors was
tentatively investigated.
Catalysis on Isoprene Polymerization. All of the
bimetallic complexes activated with 10 equiv of AliBu3 or 1
equiv of [Ph3C][B(C6F5)4] alone did not induce visible
polymerization, even at elevated temperature (Table 1, runs 1
and 2). When the amount of [Ph3C][B(C6F5)4] was increased
to 2 equiv, so that both metal centers were cationized, gradual
consumption of isoprene could be observed (Table 1, runs 3−
7). The yttrium- and lutetium-based systems showed similar
activities, while the scandium complex 2a was less active,
probably due to the crowded environment around the smaller
Sc3+ center providing fewer chances for isoprene monomer
coordination. This might explain to some degree why the larger
yttrium (ionic radius 1.040 Å) complexes 1, 2a, and 3 exhibited
higher cis-1,4 selectivity (84%−87%, runs 3, 4, and 7) in
comparison with their lutetium and scandium counterparts
(Lu3+ at 0.977 Å, 77%; Sc3+ at 0.885 Å, 71%; runs 5 and 6), as
the cis-η4 coordination of isoprene monomer that incurs cis-1,4
regularity favors a large coordination environment.15g,24 In
particular, the scandium complex 2c provided much higher 3,4-
selectivity, ∼25% (run 6), in accord with the crowding
environment around the smallest Sc3+ ion center and its
lower coordination number that allowed isoprene η2 coordina-
tion. When both aluminum alkyls and organoborate cocatalysts
were employed, the polymerization activity was accelerated
significantly. In the presence of 10 equiv of AliBu3 when 1 equiv
of [Ph3C][B(C6F5)4] was added to abstract the alkyl group in
complex 1, that is, only one of the two yttrium centers was
cationized to become an active site (Y+) while the other center
remained neutral (Y) and inert, the polymerization could be
completed in 10 min in comparison with 600 min without
AliBu3 (run 2 vs 8); when the borate loading was increased to
an equimolar amount of the metal, both yttrium centers
became cationic (2Y+),25 resulting in more rapid polymer-
ization (run 9); on further increase in borate loading, which
might arouse a dicationic Y2+ (no metal−carbon σ bond for the
monomer insertion) and a cationic yttrium center Y+, no
obvious change in activity was observed, as shown in run 10 in
comparison with run 8. Correspondingly, the molecular weight
of polyisoprene obtained from the 2Y+ system was smaller with
narrow polydispersity in comparison with those from Y + Y+
and Y2+ + Y+ systems. Addition of aluminum alkyls to the
catalyst systems significantly increasing activity has been well
documented, as they play the role of impurity scavenger,
transform (alkylating) active species, and adjust the molecular
weight of the product as a chain transfer agent. In this system,
we believed that aluminum alkyls first acted as an impurity
scavenger. This can be proved by the fact that the catalytic
efficiency increased upon addition of aluminum alkyls, as
evidenced by the decrease in the molecular weight of the
resultant polyisoprene in comparison with those of binary
systems without aluminum alkyls, in which some active species
were consumed by impurities (runs 8−15 vs 3−7). Second, the
aluminum alkyls abstracted the coordinated THF molecule in
precursors to give unsaturated THF-free active species, allowing
coordination and insertion of monomers, which was the main
reason for the aluminum alkyls accelerating the polymerization
rate.15h,i,n,17e,26 Third, aluminum alkyl might participate in
polymerization, since changing the type of aluminum alkyl
affected the activity in the order AliBu3 = AlEt3 ≫ AlMe3
(Table 1, runs 9, 21, and 20), which might be attributed to the
formation of a less active aluminate species between the
sterically less demanding AlMe3 and the cationic rare-earth-
metal moiety.27 In the meantime, slight drops of 1,4-regularity
and molecular weight of the resultant polyisoprene and
broadening of the molecular weight distributions were
observed. Adding an excess amount of AliBu3, such as 20 and
40 equiv, respectively, led to further decrease of the molecular
weight with a negligible change of regularity (Table 1, runs 18
and 19), albeit not inverse to the amount of AliBu3,15n
suggesting that it induced chain transfer polymerization but in a
nonliving fashion.28 The possible cooperative binuclear effects
of these systems were reflected by their different selectivities in
comparison to the mononuclear β-diketiminato yttrium system
reported by us previously, which gave high cis-1,4 polyisoprene
(94.1%, 96.3%) under the same conditions (runs 16 and
17).15m This could be attributed to the steric bulkiness of
binuclear complexes, which provided more chances for η2
coordination of polyisoprene, resulting in higher 3,4-regulated
polyisoprene. The binuclear catalytic systems usually show
steric influences, as reported in systems of ethylene polymer-
ization.2 This steric effect might also contribute to higher
molecular weight polyisoprene, as the active metal centers were
wrapped.2b,5,10,29 Thus, when the polymerization was per-
formed at dilute catalyst and monomer concentrations, the
molecular weight of the isolated polyisoprene decreased
significantly (run 11).
CONCLUSION
■
The series of binuclear rare-earth-metal bis(alkyl) complexes
1−3 have been successfully synthesized and well-defined by
introducing bis(β-diketiminate) ligands bridged by the rigid m-
phenylene group, [2,6-R2C6H3NHC(Me)C(H)C(Me)N]2-(m-
phenylene). Due to the steric repulsion of the Ln−alkyl
D
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157.16 (CN), 162.63 (CN). Anal. Calcd for C36H46N4: C,
moieties, in the solid state, two β-diketiminato−metal units of
all binuclear complexes take trans positions against the phenyl
ring. In solution, both the Ln−alkyl groups and the β-
diketiminate ligands are fluxional and indistinguishable on the
NMR time scale at room temperature. Upon activation with 2
equiv of organoborate, these binuclear rare-earth-metal alkyl
complexes exhibit moderate catalytic activity for isoprene
polymerization, while further addition of aluminum alkyls
accelerates the polymerization rate significantly and arouses an
increment of catalytic efficiency by excluding the coordinated
THF molecules and impurities in the system. The binuclear
active species show relatively lower cis-1,4-selectivity and
increased 3,4-selectivity and give polyisoprene with much
higher molecular weight in comparison with the mononuclear
analogue, which might be attributed to the steric effect of the
two adjacent active metal centers. The copolymerization of
conjugated dienes and α-olefins catalyzed by binuclear rare-
earth-metal alkyl complexes are in progress.
80.85; H, 8.67; N, 10.48. Found: C, 80.91; H, 8.82; N, 10.27.
1
PBDIMe-H2. H NMR (400 MHz, CDCl3, 25 °C): δ 1.66 (s, 6H,
CH3CN), 2.05 (s, 6H, CH3CN), 2.12 (s, 12H, CH3), 4.86 (s, 2H,
CH3CNCH), 6.58 (t, 4JH−H = 1.8 Hz, 1H, CHN‑aryl), 6.63 (dd, 3JH−H
=
4
3
7.9 Hz, JH−H = 1.8 Hz, 2H, CHN‑aryl), 6.90 (t, JH−H = 7.9 Hz, 2H,
3
3
CHN‑aryl), 7.02 (d, JH−H = 7.9 Hz, 4H, CHN‑aryl), 7.14 (t, JH−H = 7.9
Hz, 1H, CHN‑aryl), 12.52 (s, 2H, NH). 13C NMR (400 MHz, CDCl3,
25 °C): δ 18.54 (CH3), 20.85 (CH3CN), 20.93 (CH3CN), 96.27
(CH3CNCH), 117.13 (CHN‑aryl), 117.78 (CHN‑aryl), 123.76 (CHN‑aryl),
127.88 (CHN‑aryl), 128.47 (CHN‑aryl), 129.10 (CHN‑aryl), 130.66
(CHN‑aryl), 144.94 (CN), 145.69 (CN), 156.78 (CN), 162.84
(CN). Anal. Calcd for C32H38N4: C, 80.29; H, 8.00; N, 11.70.
Found: C, 80.43; H, 8.06; N, 11.51.
Synthesis and Characterization of Binuclear Complexes.
PBDIiPr-[Y(CH2SiMe3)2]2(THF)2 (1). To a stirred solution of the ligand
PBDIiPr-H2 (0.590 g, 1.0 mmol) in hexane (10 mL) was added
Y(CH2SiMe3)3(THF)2 (0.990 g, 2.0 mmol) in hexane (5 mL) at 25
°C, and the mixture was stirred for 0.5 h. Then the solution was
concentrated to half volume and cooled to −30 °C. Yellow single
1
crystals of complex 1 were isolated after 1 day (0.791 g, 70%). H
2
NMR (400 MHz, C6D6, 25 °C): δ −0.50 (d, JY−H = 2.8 Hz, 8H,
EXPERIMENTAL SECTION
3
■
CH2SiMe3), 0.22 (s, 36H, CH2SiMe3), 1.14 (d, JH−H = 6.8 Hz, 12H,
3
CH(CH3)2), 1.31 (d, JH−H = 6.8 Hz, 12H, CH(CH3)2), 1.33 (br s,
General Methods. All reactions were carried out under a dry and
oxygen-free argon atmosphere by using Schlenk techniques or under a
nitrogen atmosphere in a glovebox. All solvents were purified from an
8H, THF-β-CH2), 1.65(s, 6H, CH3CN), 2.18 (s, 6H, CH3CN), 3.23
3
(sept, JH−H = 6.8 Hz, 4H, CH(CH3)2), 3.64 (br s, 8H, THF-α-CH2),
5.16 (s, 2H, CH3CNCH), 6.58 (t, 4JH−H = 1.9 Hz, 1H, CHN‑aryl), 7.02
1
MBraun SPS system. H and 13C NMR spectra were recorded on a
3
4
(dd, JH−H = 7.9 Hz, JH−H = 1.9 Hz, 2H, CHN‑aryl), 7.09 (s, 6H,
CHN‑aryl), 7.14 (t, 3JH−H = 7.9 Hz, 1H, CHN‑aryl). 13C NMR (400 MHz,
C6D6, 25 °C): δ 4.55 (CH2SiMe3), 24.15 (CH3CN), 24.59
(CH(CH3)2), 24.71 (CH3CN), 25.05 (THF-β-CH2), 25.34 (CH-
(CH3)2), 34.94 (CH2SiMe3), 35.34 (CH2SiMe3), 70.08 (THF-α-CH2),
96.80 (CH3CNCH), 119.43 (CHN‑aryl), 119.95 (CHN‑aryl), 124.35
(CHN‑aryl), 126.22 (CHN‑aryl), 127.94 (CHN‑aryl), 128.18 (CHN‑aryl),
130.00 (CHN‑aryl), 142.32 (CHN‑aryl), 144.43 (CN), 151.09 (CN),
160.81 (CN), 167.22 (CN). Anal. Calcd for C64H112N4O2Si4Y2:
C, 61.02; H, 8.96; N, 4.45. Found: C, 59.95; H, 8.86; N, 4.29.
PBDIEt-[Y(CH2SiMe3)2]2(THF)2 (2a). Following the procedure
described for 1, the reaction of PBDIEt-H2 (0.427 g, 0.8 mmol) with
2 equiv of Y(CH2SiMe3)3(THF)2 (0.792 g, 1.6 mmol) afforded 2a as
yellow crystals (0.654 g, 68%) after crystallization from hexane at −30
Bruker AV400 spectrometer. The molecular weight and molecular
weight distribution of the polymers were measured by TOSOH HLC
8220 GPC at 40 °C using THF as eluent (the flow rate was 0.35 mL/
min) against polystyrene standards. Elemental analyses were
performed at the National Analytical Research Centre of the
Changchun Institute of Applied Chemistry (CIAC). Isoprene was
dried over CaH2 with stirring for 48 h and distilled under vacuum
30
before use. Ln(CH2SiMe3)3(THF)2 was prepared according to the
literature. [Ph3C][B(C6F5)4] and [PhNHMe2][B(C6F5)4] were
synthesized following the literature procedures.31 The microstructure
1
of polyisoprene was determined by H NMR and 13C NMR spectra.
X-ray Crystallographic Studies. Crystals for X-ray analysis were
obtained as described in the preparations. The crystals were
manipulated in a glovebox. Data collections were performed at
−86.5 °C on a Bruker SMART APEX diffractometer with a CCD area
detector, using graphite-monochromated Mo Kα radiation (λ =
0.71073 Å). The determination of crystal class and unit cell parameters
was carried out using the SMART program package. The raw frame
data were processed using SAINT and SADABS to yield the reflection
data file. The structures were solved by using the SHELXTL program.
Refinement was performed on F2 anisotropically for all non-hydrogen
atoms by the full-matrix least-squares method. The hydrogen atoms
were placed at calculated positions and were included in the structure
calculations without further refinement of the parameters. The
crystallographic data and structure refinement details of complexes
1−3 are summarized in Table S2 (Supporting Information).
1
2
°C. H NMR (400 MHz, C6D6, 25 °C): δ −0.49 (d, JY−H = 2.0 Hz,
3
8H, CH2SiMe3), 0.22 (s, 36H, CH2SiMe3), 1.20 (t, JH−H = 7.5 Hz,
12H, CH2CH3), 1.25 (br s, 8H, THF-β-CH2), 1.52 (s, 6H, CH3CN),
2.20 (s, 6H, CH3CN), 2.52−2.67 (m, 8H, CH2CH3), 3.44 (br s, 8H,
THF-α-CH2), 5.11 (s, 2H, CH3CNCH), 6.92 (s, 1H, CHN‑aryl), 6.99
(s, 6H, CHN‑aryl), 7.29−7.31 (m, 2H, CHN‑aryl), 7.36−7.40 (m, 1H,
CHN‑aryl). 13C NMR (400 MHz, C6D6, 25 °C): δ 4.55 (CH2SiMe3),
13.92 (CH2CH3), 23.53 (CH3CN), 25.12 (CH2CH3), 25.22 (THF-β-
CH2), 25.78 (CH3CN), 35.59 (CH2SiMe3), 35.99 (CH2SiMe3), 70.53
(THF-α-CH2), 98.93 (CH3CNCH), 122.62 (CHN‑aryl), 123.58
(CHN‑aryl), 125.40 (CHN‑aryl), 126.06 (CHN‑aryl), 127.94 (CHN‑aryl),
128.18 (CHN‑aryl), 130.62 (CHN‑aryl), 138.08 (CHN‑aryl), 146.28 (C
N), 151.30 (CN), 165.26 (CN), 165.90 (CN). Anal. Calcd for
C60H104N4O2Si4Y2: C, 59.87; H, 8.71; N, 4.65. Found: C, 59.47; H,
8.52; N, 4.51.
Synthesis and Characterization of Ligands. The synthesis of
PBDIiPr-H2 was carried out according to the procedure by Harder et
al.20b The syntheses of PBDIEt-H2 and PBDIMe-H2 were carried out
following a similar procedure, but 2,6-Et2C6H3NH2 and 2,6-
Me2C6H3NH2 were used, respectively, instead of 2,6-iPr2C6H3NH2.
PBDIEt-[Lu(CH2SiMe3)2]2(THF)2 (2b). Following the procedure
described for 1, the reaction of PBDIEt-H2 (0.427 g, 0.8 mmol) with
2 equiv of Lu(CH2SiMe3)3(THF)2 (0.929 g, 1.6 mmol) afforded 2b as
yellow crystals (0.744 g, 67%) after crystallization from hexane at −30
The yields of these ligands were moderate (PBDIEt-H2, 44%; PBDIMe
H2, 39%).
-
1
3
1
PBDIEt-H2. H NMR (400 MHz, CDCl3, 25 °C): δ 1.15 (t, JH−H
=
°C. H NMR (400 MHz, C6D6, 25 °C): δ −0.65 (s, 8H, CH2SiMe3),
7.5 Hz, 12H, CH2CH3),, 1.68 (s, 6H, CH3CN), 2.05 (s, 6H, CH3CN),
2.38−2.58 (m, 8H, CH2CH3), 4.85 (s, 2H, CH3CNCH), 6.56 (t, 4JH−H
= 1.9 Hz, 1H, CHN‑aryl), 6.61 (dd, 3JH−H = 7.9 Hz, 4JH−H = 1.9 Hz, 2H,
0.22 (s, 36H, CH2SiMe3), 1.18 (br s, 8H, THF-β-CH2), 1.20 (t, 3JH−H
 7.5 Hz, 12H, CH2CH3), 1.51 (s, 6H, CH3CN), 2.18 (s, 6H,
CH3CN), 2.55−2.69 (m, 8H, CH2CH3), 3.39 (br s, 8H, THF-α-CH2),
5.10 (s, 2H, CH3CNCH), 6.92 (s, 1H, CHN‑aryl), 6.99 (s, 6H, CHN‑aryl),
7.28−7.30 (m, 2H, CHN‑aryl), 7.35−7.39 (m, 1H, CHN‑aryl). 13C NMR
(400 MHz, C6D6, 25 °C): δ 4.73 (CH2SiMe3), 13.83 (CH2CH3), 23.80
(CH3CN), 25.02 (CH2CH3), 25.21 (THF-β-CH2), 26.14 (CH3CN),
42.57 (CH2SiMe3), 70.82 (THF-α-CH2), 99.32 (CH3CNCH), 122.96
(CHN‑aryl), 124.11 (CHN‑aryl), 125.41 (CHN‑aryl), 125.98 (CHN‑aryl),
3
CHN‑aryl), 7.01−7.09 (m, 6H, CHN‑aryl), 7.14 (t, JH−H = 7.9 Hz, 1H,
CHN‑aryl), 12.59 (s, 2H, NH). 13C NMR (400 MHz, CDCl3, 25 °C): δ
14.49 (CH2CH3), 20.96 (CH3CN), 20.99 (CH3CN), 25.05
(CH2CH3), 96.12 (CH3CNCH), 116.76 (CHN‑aryl), 117.53 (CHN-aryl),
124.31 (CHN‑aryl), 126.09 (CHN‑aryl), 128.47 (CHN‑aryl), 129.12
(CHN‑aryl), 136.67 (CHN‑aryl), 144.09 (CN), 145.27 (CN),
E
dx.doi.org/10.1021/om400105t | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
127.94 (CHN‑aryl), 128.18 (CHN‑aryl), 130.46 (CHN‑aryl), 138.09
(CHN‑aryl), 146.76 (CN), 151.51 (CN), 165.99 (CN), 166.63
(CN). Anal. Calcd for C60H104N4O2Si4Lu2: C, 52.38; H, 7.62; N,
4.07. Found: C, 51.99; H, 7.42; N, 3.99.
ACKNOWLEDGMENTS
■
This work was partially supported by the National Natural
Science Foundation of China for projects No. 20934006,
51073148, 51021003, 201104074, and 201104072 and the
Ministry of Science and Technology of China for projects No.
2009AA03Z501 and 2011DF50650.
PBDIEt-[Sc(CH2SiMe3)2]2(THF) (2c). Following the procedure
described for 1, the reaction of PBDIEt-H2 (0.534g, 1.0 mmol) with
2 equiv of Sc(CH2SiMe3)3(THF)2 (0.902 g, 2.0 mmol) afforded 2c as
a yellow powder (0.762 g, 73%). Pale yellow crystals suitable for X-ray
analysis were grown from a mixture of hexane and toluene (5/1 v/v) at
−30 °C. 1H NMR (400 MHz, C6D6, 25 °C): δ 0.10 (br s, 8H,
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3
CH2SiMe3), 0.14 (s, 36H, CH2SiMe3), 1.21 (t, JH−H = 7.5 Hz, 12H,
CH2CH3), 1.32 (m, 4H, THF-β-CH2), 1.50 (s, 6H, CH3CN), 2.05 (s,
6H, CH3CN), 2.54−2.71(m, 8H, CH2CH3), 3.54 (m, 4H, THF-α-
CH2), 5.08 (s, 2H, CH3CNCH), 7.01 (s, 1H, CHN‑aryl), 7.05 (s, 6H,
CHN‑aryl), 7.25−7.27 (m, 2H, CHN‑aryl), 7.28−7.31 (m, 1H, CHN‑aryl).
13C NMR (400 MHz, C6D6, 25 °C): δ 3.82 (CH2SiMe3), 14.26
(CH2CH3), 23.26 (CH3CN), 24.64 (CH3CN), 25.16 (CH2CH3),
25.48 (THF-β-CH2), 43.91 (CH2SiMe3), 69.74 (THF-α-CH2), 99.54
(CH3CNCH), 122.94 (CHN‑aryl), 123.10 (CHN‑aryl), 126.34 (CHN‑aryl),
126.57 (CHN‑aryl), 127.94 (CHN‑aryl), 128.18 (CHN‑aryl), 130.84
(CHN‑aryl), 137.98 (CHN‑aryl), 144.41(CN), 149.75 (CN),
165.35 (CN), 167.29 (CN). Anal. Calcd for C56H96N4OSi4Sc2:
C, 64.45; H, 9.27; N, 5.37. Found: C, 64.01; H, 9.04; N, 5.25.
PBDIMe-[Y(CH2SiMe3)2]2(THF)2 (3). Following the procedure de-
scribed for 1, the reaction of PBDIMe-H2 (0.478 g, 1.0 mmol) with 2
equiv of Y(CH2SiMe3)3(THF)2 (0.990 g, 2.0 mmol) afforded 3 as
yellow crystals (0.809 g, 71%) after crystallization from hexane at −30
1
2
°C. H NMR (400 MHz, C6D6, 25 °C): δ −0.48 (d, JY−H = 2.0 Hz,
8H, CH2SiMe3), 0.22 (s, 36H, CH2SiMe3), 1.20 (br s, 8H, THF-β-
CH2), 1.48 (s, 6H, CH3CN), 2.15 (s, 12H, CH3), 2.19 (s, 6H,
CH3CN), 3.39 (br s, 8H, THF-α-CH2), 5.10 (s, 2H, CH3CNCH),
6.78−6.82 (m, 2H, CHN‑aryl), 6.86−6.88 (m, 4H, CHN‑aryl), 6.97 (s,
1H, CHN‑aryl),7.30−7.32 (m, 2H, CHN‑aryl), 7.37−7.41 (m, H,
CHN‑aryl). 13C NMR (400 MHz, C6D6, 25 °C): δ 4.55 (CH2SiMe3),
19.37 (CH3), 23.20 (CH3CN), 25.25 (THF-β-CH2), 25.83 (CH3CN),
35.59 (CH2SiMe3), 35.72 (CH2SiMe3), 36.12 (CH2SiMe3), 70.33
(THF-α-CH2), 99.07 (CH3CNCH), 122.90 (CHN‑aryl), 124.13
(CHN‑aryl), 124.88 (CHN‑aryl), 127.94 (CHN‑aryl), 128.17 (CHN‑aryl),
128.84 (CHN‑aryl), 130.68 (CHN‑aryl), 133.01 (CHN‑aryl), 147.27 (C
N), 151.12 (CN), 165.42 (CN), 165.72 (CN). Anal. Calcd for
C56H96N4O2Si4Y2: C, 58.61; H, 8.43; N, 4.88. Found: C, 58.19; H,
8.33; N, 4.61.
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Isoprene Polymerization. A detailed polymerization procedure is
described as follows (Table 1, run 9). Under a nitrogen atmosphere, a
toluene solution (2 mL) of [Ph3C][B(C6F5)4] (18.4 mg, 20 μmol) was
added to a toluene solution (3 mL) of complex 1 (12.6 mg, 10 μmol)
in a 25 mL flask. Then 10 equiv of AliBu3 (0.1 mL, 100 μmol, 1.0 M in
toluene) was added with stirring after a few minutes. Upon the
addition of 1000 equiv of isoprene (1 mL, 0.01 mol), polymerization
was initiated and carried out for 5 min. The reaction mixture was
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at 50 °C to a constant weight (0.68 g, 100%).
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, tables, and CIF files giving crystallographic data and
structure refinement details 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: dmcui@ciac.jl.cn (D.C.); shihui-li@ciac.jl.cn (S.L.).
Fax: (+86) 431 85262774. Tel: +86 431 85262773.
Notes
(15) For examples of cis-1,4 polymerization of isoprene/butadiene:
The authors declare no competing financial interest.
(a) Kaita, S.; Yamanaka, M.; Horiuchi, A. C.; Wakatsuki, Y.
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dx.doi.org/10.1021/om400105t | Organometallics XXXX, XXX, XXX−XXX

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Article
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3
3
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dx.doi.org/10.1021/om400105t | Organometallics XXXX, XXX, XXX−XXX