Bis(β-diketiminate) rare-earth-metal borohydrides: Syntheses, structures, and catalysis for the polymerizations of L-lactide, ε-caprolactone, and methyl methacrylate

Article abstract of DOI:10.1021/om3005299

Reaction of LnCl₃ (Ln = Y, Yb) with 2 equiv of NaL ^2,6-ipr2_Ph (L^2,6-ipr2_Ph = [(2,6-ⁱPr₂C₆H₃)NC(Me)CHC(Me)N(C ₆H₅)]^-) afforded the chlorides (L ^2,6-ipr2_Ph)₂YCl (1) and (L ^2,6-ipr2_Ph)₂YbCl (2). Crystal structure analysis revealed 2 to be the unsolvated monomer. Treatment of the chlorides 1 and 2 with NaBH₄ in a 1/1 molar ratio in THF led to the preparation of the monoborohydrides (L^2,6-ipr2_Ph)₂LnBH ₄ (Ln = Y (3), Yb (4)) in good yields. Reaction of LnCl₃ (Ln = Y, Yb) with 2 equiv of NaL^2-Me (L^2-Me = [N(2-MeC₆H₄)C(Me)]₂CH^-) in THF, followed by treatment with 1 equiv of NaBH₄, afforded the monoborohydrides (L^2-Me)₂LnBH₄ (Ln = Y (5), Yb (6)). Complexes 3-6 were fully characterized, including X-ray crystal structure analyses. Complexes 3-6 are isostructural. The central metal in each complex is ligated by two β-diketiminate ligands and one η³-BH ₄^- group in a distorted trigonal bipyramid. Complexes 3-6 were found to be highly active in the ring-opening polymerization of l-lactide (l-LA) and ε-caprolactone (ε-CL) to give polymers with relatively narrow molar mass distributions. The activity depends on both the central metal and the ligand (Y > Yb and L^2,6-ipr2_Ph > L ^2-Me). The best control over the molar mass was found for complex 6. The M_n(obsd) values (M_n = the number-average molar mass) of the resulting PCL are in good agreement with M_n(calcd), with a ratio of monomer to 6 of up to 1000. The polymerization kinetics of l-LA in THF at 20 °C by complex 6 displays a first-order dependence on the monomer concentration. Notably, the binary 6/ⁱPrOH system exhibited an immortal nature and proved able to quantitatively convert 10 000 equiv of l-LA with up to 200 equiv of ⁱPrOH per metal initiator. All the obtained PLAs showed monomodal, narrow distributions (M_w/M _n = 1.06-1.11), with the M_n values decreasing proportionally with an increasing amount of ⁱPrOH. Complex 4 can also initiate the polymerization of methyl methacrylate (MMA) at -40 °C with high activity, affording the PMMA with 83.3% syndiotacticity.

Full text of DOI:10.1021/om3005299

Article
pubs.acs.org/Organometallics
Bis(β-diketiminate) Rare-Earth-Metal Borohydrides: Syntheses,
Structures, and Catalysis for the Polymerizations of L‑Lactide,
ε‑Caprolactone, and Methyl Methacrylate
Xiaodong Shen,† Mingqiang Xue,† Rui Jiao,† Yong Ma,† Yong Zhang,† and Qi Shen*,†,‡
†Key Laboratory of Organic Synthesis of Jiangsu Province, College of Chemistry, Chemical Engineering and Materials Science,
Soochow University, Suzhou 215123, People’s Republic of China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: Reaction of LnCl3 (Ln = Y, Yb) with 2 equiv of
NaL2,6‑ipr2 (L2,6‑ipr2 = [(2,6-iPr2C6H3)NC(Me)CHC(Me)N-
Ph
(C6H5)]−P)h afforded the chlorides (L2,6‑ipr2Ph)2YCl (1) and
(L2,6‑ipr2Ph)2YbCl (2). Crystal structure analysis revealed 2 to
be the unsolvated monomer. Treatment of the chlorides 1 and 2
with NaBH4 in a 1/1 molar ratio in THF led to the preparation
of the monoborohydrides (L2,6‑ipr2Ph)2LnBH4 (Ln = Y (3), Yb
(4)) in good yields. Reaction of LnCl3 (Ln = Y, Yb) with 2 equiv
of NaL2‑Me (L2‑Me = [N(2-MeC6H4)C(Me)]2CH−) in THF,
followed by treatment with 1 equiv of NaBH4, afforded the
monoborohydrides (L2‑Me)2LnBH4 (Ln = Y (5), Yb (6)). Complexes 3−6 were fully characterized, including X-ray crystal
structure analyses. Complexes 3−6 are isostructural. The central metal in each complex is ligated by two β-diketiminate ligands
−
and one η3-BH4 group in a distorted trigonal bipyramid. Complexes 3−6 were found to be highly active in the ring-opening
polymerization of L-lactide (L-LA) and ε-caprolactone (ε-CL) to give polymers with relatively narrow molar mass distributions.
The activity depends on both the central metal and the ligand (Y > Yb and L2,6‑ipr2Ph > L2‑Me). The best control over the molar
mass was found for complex 6. The Mn(obsd) values (Mn = the number-average molar mass) of the resulting PCL are in good
agreement with Mn(calcd), with a ratio of monomer to 6 of up to 1000. The polymerization kinetics of L-LA in THF at 20 °C by
complex 6 displays a first-order dependence on the monomer concentration. Notably, the binary 6/iPrOH system exhibited an
i
“immortal” nature and proved able to quantitatively convert 10 000 equiv of L-LA with up to 200 equiv of PrOH per metal
initiator. All the obtained PLAs showed monomodal, narrow distributions (Mw/Mn = 1.06−1.11), with the Mn values decreasing
i
proportionally with an increasing amount of PrOH. Complex 4 can also initiate the polymerization of methyl methacrylate
(MMA) at −40 °C with high activity, affording the PMMA with 83.3% syndiotacticity.
C5Me5,2a,d,f,3c tetradentate bisphenolate,2c,h bridged bisamidi-
INTRODUCTION
■
nate,5a guanidinate,5b etc.
There is continuing interest in the synthesis of aliphatic
polyesters due to their biodegradable and biocompatible
β-Diketiminate ligands, as one of the most important non-
cyclopentadienyl monoanionic ligands, have proven to be useful
characteristics and wide applications in the medical and
in organometallic chemistry of rare-earth metals, due to their
materials fields.1 Controlled ring-opening polymerization of
easily accessed and tunable steric and electronic effects.6
cyclic esters is a convenient route for the synthesis of aliphatic
polyesters with predictable molar mass, low polydispersity
indices, and control over end groups.
Various rare-earth-metal derivatives supported by mono-β-
diketiminate ligands have been well documented.7 However,
their application in rare-earth-metal complexes supported by
two monoanionic β-diketiminate ligands as single-site initiators
has been limited until now. Although a series of bis(β-
Rare-earth-metal borohydrides have been known to be
valuable active species for the polymerization of cyclic esters,2
MMA,3 and even dienes4 in reactions together with metal alkyl
diketiminate) rare-earth-metal halides L′2LnX (L′ = {[N(Ph)-
complexes. Consequently, significant efforts over the past few
C(Me)]2CH}, X = Br, Ln = Sm, Gd; L′ = {[N(SiMe3)C-
decades have been made toward the design and synthesis of
well-characterized rare-earth-metal monoborohydrides by use
of appropriate ancillary ligands as single-site initiators in the
ring-opening polymerization of cyclic esters.2b−d,f,h,3c,5 Remark-
able advances have been reported with bulky ligands such as
(Ph)]2CH}, X = Cl, Ln = Ce, Pr, Nd, Sm, Yb; L′ =
[N(SiMe3)C(Ph)]CHC(But)], X = I, Ln = Tm) were
Received: June 13, 2012
Published: August 15, 2012
© 2012 American Chemical Society
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successfully prepared,8 the further transformation of {[N-
(SiMe3)C(Ph)]2CH}CeCl with LiCH(SiMe3)2 did not afford
the corresponding monoalkyl complex; Ce[CH(SiMe3)2]2{[N-
(SiMe3)C(Ph)]2CH} was obtained instead.8b It is suggested
that Ce[CH(SiMe3)2]{[N(SiMe3)C(Ph)]2CH}2 would be too
sterically hindered. As a result, only two yttrium complexes,
YL′2(N[SiHMe3]2)9 and [Y(L1)2OtBu],10 have been synthe-
sized and structurally characterized up to date. Furthermore, no
catalytic activity was found for these kinds of complexes.
Very recently we became interested in the reaction chemistry
of β-diketiminate rare-earth-metal complexes.11 In a continu-
ation of our work, we focus our attention now on the synthesis
of rare-earth-metal derivatives supported by bis(β-diketiminate)
ligands, with the aim of exploring the potential of these kind of
rare-earth-metal complexes as single-site initiators in the
controlled ring-opening polymerization of cyclic esters.
The same reactions with a THF solution of NaL2‑Me did not
afford the isolable monochloride (L2‑Me)2LnCl (Ln = Y, Yb),
but an oil was obtained instead as the product. The oil products
prevented further characterization. Fortunately, the further
transformation of them with NaBH4 went smoothly with the
formation of the monoborohydride complexes of Y and Yb in
desired yields (see below), indicating the oil products were the
monochlorides (L2‑Me)2YCl and (L2‑Me)2YbCl.
Complex 2 crystallizes with two THF molecules and one
toluene molecule of solvation: 2·THF·0.5(toluene). The
molecular structure of 2 is shown in Figure 1 with selected
bond distances and angles. The molecular structure of 2 is quite
similar to those reported for L2NdCl8b and L2YCl.10 The
coordination geometry around the Yb atom is a distorted
trigonal bipyramid formed by four nitrogen atoms from two
L2,6‑ipr2 ligands and one chlorine atom with the two nitrogen
Ph
In this contribution, we report the synthesis of the bis(β-
atoms of N(1) and N(1A) occupying axial sites (the angle of
N(1)−Yb(1)−N1A = 171.2(2)°) and the three atoms N(2),
N(2A), and Cl(1) at the equatorial positions (the sum of the
angles of N(2)−Yb(1)−Cl(1), Cl(1)−Yb(1)−N(2A), and
N(2A)−Yb(1)−N(2) is 360.04(10)°). The bond parameters
in 2, including Yb−N and Yb−Cl bond distances and the bond
angles around the Yb atom, are comparable with those found in
the analogues reported previously,8b,10,14 when the differences
in ion radii among them are considered.
Syntheses and Molecular Structures of Bis(β-diketi-
minate) Rare-Earth-Metal Monoborohydrides
(L2,6‑ipr2Ph)2LnBH4 (Ln = Y (3), Yb (4)) and (L2‑Me)2LnBH4
(Ln = Y (5), Yb (6)). The metathesis reaction of 1 with NaBH4
was then tried to see whether the monoborohydride could be
prepared. The reaction of 1 with 1 equiv of NaBH4 was carried
out in a THF solution at 60 °C. After the mixture was stirred
for 5 days, the monoborohydride complex (L2,6‑ipr2Ph)2YBH4
(3) was isolated as pale yellow crystals upon crystallization
from toluene solution in good yield (Scheme 2). By the same
procedure, the Yb complex (L2,6‑ipr2Ph)2YbBH4 (4) was also
prepared in the desired yield as red crystals (Scheme 2).
Although an isolable monochloride with L2‑Me ligands,
(L2‑Me)2LnCl (Ln = Y, Yb), could not be obtained as
mentioned above, treatment of the reaction solution of LnCl3
and 2 equiv of NaL2‑Me with 1 equiv of NaBH4 in THF led to
the isolation of the corresponding monoborohydride complexes
(L2‑Me)2YBH4 (5) and (L2‑Me)2YbBH4 (6) as pure crystals
(Scheme 2).
diketiminate) rare-earth-metal chlorides (L2,6‑ipr2Ph)2LnCl
(L2,6‑ipr2 = [(2,6-iPr2C6H3)NC(Me)CHC(Me)N(C6H5)]−,
Ph
Ln = Y (1), Yb (2)) and their transformation with NaBH4 to
the monoborohydride complexes (L2,6‑ipr2Ph)2LnBH4 (Ln = Y
(3), Yb (4)) and (L2‑Me)2LnBH4 (L2‑Me = [N(2-MeC6H4)C-
(Me)]2CH−, Ln = Y (5), Yb (6)) and the molecular structures
of complexes 2−6. The catalytic behavior of these new
borohydrides toward ε-CL and L-LA polymerizations, including
an “immortal” nature in the presence of HOiPr with up to 200
PAL per metal active center and catalytic activity in the
polymerization of MMA, is also presented.
RESULTS AND DISCUSSION
■
Syntheses of Bis(β-diketiminate) Rare-Earth-Metal
Chlorides (L2,6‑ipr2Ph)2LnCl (Ln = Y (1), Yb (2)) and the
Molecular Structure of Complex 2. The β-diketiminate
proligands L2,6‑ipr2PhH and L2‑MeH were prepared by the
published methods.12 Reaction of YCl3 with a THF solution
of NaL2,6‑ipr2Ph, which was formed in situ by the reaction of
L2,6‑ipr2PhH with NaH, in a 1/2 molar ratio at 60 °C was
conducted first. After workup the monochloride (L2,6‑ipr2Ph)2YCl
(1) was isolated as yellow crystals in good yield. The analogous
Yb complex, (L2,6‑ipr2Ph)2YbCl (2), was also prepared by the
same procedure without difficulty (Scheme 1).
Scheme 1. Syntheses of 1 and 2
Complexes 3−6 were characterized by standard analytical/
spectroscopic techniques. The IR spectra of 3−6 exhibited
strong absorptions near 1550 and 1510 cm−1, which were
consistent with the partial CN character of the β-
diketiminate ligands,13 and the expected two strong absorptions
diagnostic of the η3-bridging borohydride stretching vibration
and terminal borohydride stretching vibration in the range of
15
2300−2200 cm−1. A resolvable H NMR spectrum for 4 and
6 could not be measured owing to paramagnetism of Yb. In the
1H NMR spectra of 3 and 5 the signals at 0.97−1.16 ppm for 3
and 1.07−1.33 ppm for 5 assigned to the BH4 ligand are
observed clearly.
Complexes 3, 4, and 6 were isolated as the solvates
3·0.5(toluene), 4·0.5(toluene), and 6·2THF, respectively. X-
ray diffraction analyses showed that complexes 3−6 are
isostructural and all are five-coordinate monomers (Figure 2
gives the ORTEP plot of complex 3, and crystal data are given
in Table 1), reflecting that both ligands are sterically more
1
The elemental analyses of 1 and 2 are consistent with their
formulas. The IR spectra exhibited strong absorptions near
1547 and 1516 cm−1, indicative of the partial CN character
of the β-diketiminate ligands.13 1H NMR of 1 revealed signals
assigned to the L2,6‑ipr2Ph ligand. The formation of 2 was further
confirmed by a single-crystal structure analysis. An attempt to
determine the molecular structure of 1 was unsuccessful, due to
the poor quality and weak diffraction of the crystals.
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Figure 1. ORTEP diagram of complex 2 (one of the two independent molecules) showing the atom-numbering scheme. Thermal ellipsoids are
drawn at the 20% probability level. Hydrogen atoms and the free toluene and THF molecules are omitted for clarity. Selected bond distances (Å)
and angles (deg): Yb(1)−N(1) = 2.295(5), Yb(1)−N(2) = 2.256(5), Yb(1)−Cl(1) = 2.501(2); N(1)−Yb(1)−N(1A) = 171.2(2), N(2)−Yb(1)−
Cl(1) = 126.82(13), Cl(1)−Yb(1)−N(2A) = 126.82(13), N(2)−Yb(1)−N(2A) = 106.4(3). Symmetry transformations used to generate equivalent
1
1
atoms: (#1) −x + /2, y, −z + /2.
176.9(6)° for 4, 170.39(19)° for 5, and 168.45(16)° for 6
(Table 2). The average Ln−N bond distances of 3−6 are
2.340(4), 2.282(4), 2.337(4), and 2.302(3) Å, respectively,
which are comparable with each other when the differences in
ionic radii among these metals are considered.
Scheme 2. Syntheses of Complexes 3−6
The N−C and C−C bond distances of each anionic
NCCCN unit in the β-diketiminate ligand in each complex
are almost consistent (Table 2), reflecting the electron
delocalization within the anionic NCCCN unit. The Ln−B
distances (2.524(8) Å for 3, 2.471(9) Å for 4, 2.544(10) Å for
5, 2.480(8) Å for 6) are quite comparable to the corresponding
values found in the monoborohydride complexes of rare-earth
metals with a η3-BH4 group but are significantly shorter than
those with a η2-BH4 group.15c,16 The average Ln−H bond
distances of complexes 3−6 are 2.27(8), 2.40(11), 2.27(6), and
2.16(8) Å, respectively, which are comparable with those for
the complexes reported.15c Thus, the boron atoms in 3−6 are
linked to the rare-earth-metal center via the three bridging
hydrogen atoms.
Ring-Opening Polymerization of L-LA. The catalytic
behaviors of 3−6 for polymerization of L-LA were assessed at
room temperature with a molar ratio of monomer to initiator of
1000 ([M]0 = 1.0 M). The results are given in Table 3. An
almost complete conversion of the monomer was achieved after
4 min for complexes 3 (93%) and 5 (96%), while an 82% yield
for complex 4 and 73% yield for complex 6 were obtained. The
slightly lower activity for Yb compared to that for Y may be
attributed to the small ionic radius of Yb. The borohydride
demanding than the C5Me5 ligand.2a For clarity the molecular
cores of complexes 4−6 are shown in Figure 3.
Each central metal in complexes 3−6 is ligated by one BH4
−
group and two β-diketiminate ligands. The coordination
polyhedron around the five-coordinate metal center can be
described as a distorted trigonal bipyramid with two nitrogen
atoms from two ligands respectively occupying the axial sites in
an distorted setup to form an N(1)−Ln(1)−N(3) angle of
177.10(15)° for 3, and N(1)−Ln(1)−N(1A) angles of
complexes with L2,6‑ipr2 are more active than the complexes
Ph
with L2‑Me (Table 3, entries 1−4). The polymerization can
proceed either in THF or in toluene (Table 3, entries 4 and 6).
A control experiment with the chloride 1 was carried out. Even
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Figure 2. ORTEP diagram of complex 3 showing the atom-numbering scheme. Thermal ellipsoids are drawn at the 20% probability level. Hydrogen
atoms and the free toluene molecule are omitted for clarity. Symmetry transformations used to generate equivalent atoms: (#1) −x, −y + 1, −z + 1.
Table 1. Crystallographic Data for Complexes 2−6
param
empirical formula
2·C4H8O·0.5C7H8
3·0.5C7H8
4·C4H8O
5
6·2C4H8O
C53.50H70ClN4OYb
993.63
C49.50H66BN4Y
816.78
C50H71BN4OYb
927.96
C38H47BN4Y
659.52
C46H63BN4O2Yb
886.85
formula wt
temp (K)
cryst syst
space group
cryst size (mm)
a (Å)
293(2) K
monoclinic
P2/n
223(2)
223(2)
223(2)
orthorhombic
Fdd2
223(2)
triclinic
monoclinic
C2
monoclinic
C2/c
̅
P1
̅
0.80 × 0.80 × 0.50
23.123(5)
8.9946(18)
24.820(5)
90
0.36 × 0.26 × 0.10
9.0620(6)
12.4085(12)
20.938(2)
86.514(6)
81.914(5)
82.017(6)
2306.4(3)
2
0.40 × 0.20 × 0.20
22.0087(17)
8.4890(4)
16.4327(13)
90
0.50 × 0.35 × 0.10
22.685(5)
38.486(6)
8.1013(13)
90
0.80 × 0.60 × 0.45
21.2509(17)
12.8271(10)
16.7357(13)
90
b (Å)
c (Å)
α (deg)
β (deg)
94.149(5)
90
129.7710(10)
90
90
105.687(2)
90
γ (deg)
V (Å3)
90
5148.4(18)
4
2359.7(3)
2
7073(2)
8
4394.2(6)
4
Z
Dcalcd. (mg cm−3
)
1.282
1.176
1.306
1.239
1.341
abs coeff (mm−1
)
1.907
1.300
2.020
1.680
2.168
F(000)
2056
870
964
2776
1828
no. of collected/unique rflns (R(int)) 36 764/9316 (0.0507) 19 887/8513 (0.0700) 6122/3895 (0.0283) 5600/2851 (0.0558) 10 849/4060 (0.0262)
no. of data/restraints/params
goodness of fit on F2
final R1 (I > 2σ(I))
wR2 (all data)
9316/19/538
1.151
8513/15/466
1.061
3895/11/264
1.026
2851/4/212
0.920
4060/13/246
1.078
0.0540
0.0870
0.0314
0.0470
0.0336
0.1192
0.2134
0.0669
0.0670
0.0870
after 10 min, no ROP was observed (Table 3, entry 5). This
confirms that the M−BH4 unit in each complex is the only
active site. The molar mass distributions of the polymers
formed with these borohydride complexes range from 1.33 to
1.44 (Table 3, entries 1−4). The best control over the molar
mass was found for complex 6. The Mn(obsd) values of PLAs
obtained with complex 6 either in THF or in toluene are in
good agreement with the Mn(calcd) fvalues or one chain
growing per metal center (Table 3, entries 4, 6, and 7). In
contrast, the Mn(obsd) values of the polymers obtained with
complexes 3−5 are somewhat lower than those calculated. This
can be attributed to transesterification reactions.
The polymerization kinetics of L-LA with complex 6 at a
molar ratio of monomer to 6 of 1000 was further investigated at
20 °C in THF. The conversion increases with time, and a first-
order dependence on the monomer concentration, without any
induction period, was observed (Figure 4). The number-
average molar mass (Mn) of the polymers increased linearly
with the monomer conversion, while the values of Mw/Mn
stayed relatively narrow (Mw/Mn = 1.31−1.38) (Figure 5),
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either at 60 °C or at room temperature (Table 4, entries 2 and
3). Remarkably, the binary 6/iPrOH system was able to
quantitatively convert 10 000 equivalents of L-LA with up to
200 equiv of chain-transfer agent per metal initiator. All the
i
obtained PLAs at various PrOH/6 ratios (from 6 to 200)
showed monomodal, narrow distributions (Mw/Mn = 1.06−
1.11) with Mn values decreasing proportionally with increasing
i
amounts of PrOH.
The results indicated that the β-diketiminate ligands used
here appear to be suitable ligands for stabilizing the Ln−
alkoxide active species; thereby, an excess amount of iPrOH did
not lead to deactivation but behaved as a chain transfer agent,
in particular, arousing a living chain transfer polymerization to
give PLAs with a chain end-capped by a OH group. To the best
of our knowledge, metal catalysts which allow “immortal” ring-
opening polymerization of LA, that is, to grow as many as 200
polymer chains per metal center, remain quite uncommon.17
The resulting polymer with an end group of OH was further
Figure 3. ORTEP drawing of the molecular core of complexes 4−6.
1
1
confirmed by H NMR spectroscopy. The H NMR spectrum
of PLAs obtained at [L-LA]0/[iPrOH]0/[6]0 = 500/50/1 in
CDCl3 showed a multiplet resonance at 5.03 ppm and a
doublet of doublets (or a quartet) resonance at 1.22 ppm
assigned to the isopropoxide chain end −OCH(CH3)2 a quartet
resonance around 4.33 ppm and a doublet resonance around
1.47 ppm indicative of the −CH(CH3)OH chain end, and a
quartet resonance at 5.14 ppm and a doublet resonance around
1.56 ppm attributed to {−C(O)CH(CH3)O−}n.
Ring-Opening Polymerization of ε-CL. The catalytic
activities of complexes 3−6 for ring-opening polymerization of
ε-CL were evaluated at room temperature in toluene. All
complexes are highly active. The activities depend on both the
metal and the ligand with the same active sequences as those
for the polymerization of L-LA (Table 5, entries 1−4). The
systems with complexes 3−6 gave polymers with Mw/Mn values
ranging from 1.29 to 1.48. The best molar mass control was
found for complex 6. Decreasing the polymerization temper-
ature from 20 to 10 °C led to a greater deviation between the
Mn(obsd) and Mn(calcd) values (for one chain growing per
metal center) of the resulting polymers. This may be attributed
to a slow initiation step in comparison to propagation. Indeed,
which demonstrated that the polymerization in this system
occurs in a living manner.
The end groups of the oligomer of L-LA, which was prepared
by the polymerization of L-LA using complex 6 as the initiator
in a 1/10 molar ratio and quenched by wet n-hexane, were
determined by 1H NMR. The 1H NMR spectrum exhibited the
quartet characteristic of a −CH(Me)OH terminal group at δ
4.33 ppm. The −CH(Me)OH group was formed by hydrolysis
of the metal−alkoxide bond. An additional resonance at δ 4.16
ppm assigned to the CH2OH end group2e,h was also observed.
The results indicate that the polymerization proceeds by a
classical coordination/insertion mechanism with an initial ring
opening through acyl−oxygen bond cleavage, as described
previously for the catalyst systems with rare-earth-metal
borohydride complexes.2e,h
To address the possibility of achieving immortal polymer-
ization with these systems by introducing several equivalents of
a chain-transfer agent, polymerizations with complex 6 were
conducted in the presence of isopropyl alcohol (iPrOH).
Representative results are summarized in Table 4. Immortal
i
polymerization with complex 6 and PrOH could be obtained
Table 2. Selected Bond Lengths (Å) and Angles (deg) for Complexes 3−6
3
4
5
6
Bond Lengths
Ln(1)−N(1)
Ln(1)−N(2)
Ln(1)−N(3)
Ln(1)−N(4)
Ln−N(av)
Ln(1)−B(1)
Ln−H(av)
C(2)−C(3)
C(3)−C(4)
N(1)−C(2)
N(2)−C(4)
2.382(5)
2.293(4)
2.371(5)
2.315(4)
2.340(4)
2.524(8)
2.27(8)
Ln(1)−N(1)
Ln(1)−N(2)
Ln(1)−N(1A)
Ln(1)−N(2A)
Ln−N(av)
Ln(1)−B(1)
Ln−H(av)
C(2)−C(3)
C(3)−C(4)
N(1)−C(2)
N(2)−C(4)
2.312(3)
2.252(4)
2.312(3)
2.252(4)
2.282(4)
2.471(9)
2.40(11)
1.417(9)
1.396(8)
1.368(13)
1.347(7)
2.365(3)
2.309(4)
2.365(3)
2.309(4)
2.337(4)
2.544(10)
2.27(6)
2.325(3)
2.278(3)
2.325(3)
2.278(3)
2.302(3)
2.480(8)
2.16(8)
1.421(8)
1.387(9)
1.325(7)
1.342(8)
1.395(7)
1.391(7)
1.328(6)
1.334(5)
1.394(6)
1.397(6)
1.327(5)
1.325(5)
Bond Angles
N(1)−Ln(1)−N(3)
N(2)−Ln(1)−N(4)
N(2)−Ln(1)−B(1)
B(1)−Ln(1)−N(4)
177.10(15)
102.95(16)
124.5(3)
N(1)−Ln(1)−N(1A)
N(2)−Ln(1)−N(2A)
N(2)−Ln(1)−B(1)
B(1)−Ln(1)−N(2A)
176.9(6)
170.39(19)
103.2(2)
168.45(16)
108.31(16)
125.85(8)
125.85(8)
106.3(2)
126.83(11)
126.83(11)
128.39(10)
128.39(10)
132.4(3)
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a
Table 3. Polymerization of L-LA by 3−6
b
c
d
entry
initiator
solvent
[M]0/[I]0
time (min)
yield (%)
10−4Mn(obsd)
10−4Mn(calcd)
Mw/Mn
1
2
3
4
5
6
7
3
4
5
6
1
6
6
toluene
toluene
toluene
toluene
toluene
THF
1000
1000
1000
1000
1000
1000
1000
4
4
93
82
96
73
<3
80
96
12.01
10.45
12.66
10.03
13.42
11.76
13.76
10.57
1.44
1.35
1.35
1.33
4
4
10
4
10.47
13.32
11.54
13.86
1.33
1.34
THF
8
a
b
c
Conditions: [L-LA] = 1.0 M, temp 20 °C. Yield = (weight of polymer obtained)/(weight of monomer used). Measured by SEC calibrated with
d
standard MMA samples. Mn(calcd) = ([M]/[I]) × 144 × (polymer yield).
monomer to 6. All polymerizations were complete within 2
min, giving polymers with Mn(obsd) values in good agreement
with Mn(calcd) values and low polydispersities (Mw/Mn
=
1.15−1.17) (Figure 6), indicating that complex 6 can serve as a
single-site initiator initiating the controlled ring-opening
polymerization of ε-CL in the present cases.
1
The H NMR spectra of the oligomer of ε-CL obtained by
complex 6 display, in addition to the polymer chain peaks, a
triplet resonance at 3.64 ppm, assigned to the −CH2OH chain
end which was formed by hydrolysis of the metal−alkoxide
bond. No other chain end signals were observed, and there is
no evidence of a −C(O)OH linkage. This supports the view
that both chain ends of the polymers prepared from initiator 6
are hydroxyl groups, and the ring-opening process occurs by a
coordination insertion mechanism via an oxygen−acyl bond
cleavage.2a,d,f,19
Polymerization of MMA. Rare-earth-metal borohydride
complexes are well-known to be active initiators for MMA
polymerization.3 Thereby the polymerization of MMA by
complex 4, as an example, was examined. The preliminary
results are shown in Table 6. Complex 4 showed high activity in
toluene, giving the polymers in quantitative yield at 0 °C within
2 h at a MMA/4 molar ratio of 1000 (entry 3, Table 6). The
polymerization can still give the polymer in 85% yield, with
even the MMA/4 molar ratio increasing to 1500 (entry 4,
Table 6). The activity depends greatly on the reaction
temperature. The polymer yields increased with a decrease in
Figure 4. Plot of ln([M]0/[M]) versus time for the polymerization of
L-LA by 6. Conditions: [M]0/[I]0 = 1000, [L-LA]0 = 1.0 M, in THF, 20
°C.
raising the reaction temperature to 40 °C led to the
improvement of the controllability of the system with complex
6, as judged by the resulting polymer with narrower Mw/Mn
(1.16; entry 6, Table 5). Toluene is a better solvent compared
to THF (entries 6 and 7, Table 5). The polymerizations of ε-
CL with complex 6 were further investigated at various ratios of
Figure 5. Relationship between conversion and Mn and Mw/Mn for the polymerization of L-LA by 6. For the conditions, see Figure.4.
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a
Table 4. Immortal Ring-Opening Polymerization of L-LA in the Presence of a Binary 6/iPrOH System
b
c
d
entry
[6]/[iPrOH]/[L-LA]
temp (°C)
time (h)
yield (%)
10−4Mn(obsd)
10−4Mn(calcd)
Mw/Mn
1
2
3
1/6/1000
60
60
25
60
60
60
60
1
1
1
1
1
2
2
99
99
93
99
99
99
99
2.51
1.89
2.04
0.15
0.74
1.38
0.62
2.38
2.14
2.01
0.14
0.71
1.43
0.71
1.11
1.08
1.04
1.09
1.08
1.06
1.07
1/20/3000
1/20/3000
1/50/500
e
4
e
5
1/100/5000
1/100/10000
1/200/10000
e
6
e
7
a
b
c
Conditions: in THF; [L-LA] = 1.0 M. Yield = (weight of polymer obtained)/(weight of monomer used). Measured by SEC calibrated with
standard polystyrene samples and corrected with a coefficient of 0.58.18 Mn(calcd) = ([M]/[OH]) × 144 × (polymer yield). [L-LA] = 2.0 M.
d
e
a
Table 5. Polymerization of ε-CL by 3−6
b
c
d
entry
initiator
solvent
T (°C)
t (min)
yield (%)
10−4Mn(obsd)
10−4Mn(calcd)
Mw/Mn
1
2
3
4
5
3
4
5
6
6
6
6
toluene
toluene
toluene
toluene
toluene
toluene
THF
20
20
20
20
10
40
40
3
3
99
96
98
57
65
90
93
10.65
9.50
11.29
10.96
11.17
6.53
1.48
1.47
1.35
1.29
1.38
1.16
1.34
3
9.87
3
6.56
12
2
8.04
7.45
e
6
e
10.23
12.48
10.53
7
4.5
10.63
a
b
c
Conditions: [M]0/[I]0 = 1000/1,VCL/VSol = 1/10. Yield = (weight of polymer obtained)/(weight of monomer used). Measured by SEC
d
e
calibrated with standard MMA samples. Mn(calcd) = ([M]/[I]) × 114 × (polymer yield). VCL/VSol = 1/20.
attributed to the occurrence of a side reaction concomitant with
the polymerization reaction, leading to the destruction of the
active species, which was often found in the polymerization
systems with rare-earth-metal amide and alkyl complexes.20 The
polymerization initiated by 4 at 0 °C afforded PMMA with a
syndiotacticity of ∼80% (entries 3−5, Table 6). The
syndiotacticity of PMMA obtained at −40 °C increases to
83.3% (entry 6). The molar mass distributions of the resulting
polymers range from 1.50 to 2.65, indicating the present
polymerization is out of control.
CONCLUSIONS
■
The monoborohydrides 3−6 were synthesized by the metha-
thesis reactions of monochlorides with 1 equiv of NaBH4 and
structurally characterized. All monoborohydrides can serve as
highly active initiators for ring-opening polymerizations of ε-CL
and L-LA to give polymers with relatively narrow molar mass
distributions. The polymerizations of both ε-CL and L-LA with
complex 6 proceed in a controlled manner. Interestingly, the
initiator 6 appears to be well suited for achieving immortal
polymerization of L-LA by addition of isopropyl alcohol as a
chain transfer agent, up to 200 equiv of HOiPr per metal
initiator. Complex 4 can also initiate MMA polymerization at
Figure 6. Mn vs [M]0/[I]0 for the polymerization of ε-CL by 6.
Conditions: Vε‑CL/Vsol = 1/20, in toluene, 40 °C.
temperature from 40 to 0 °C and then decreased when the
temperature was decreased to −40 °C (entries 1, 3, and 5,
Table 6). The highest activity was obtained at 0 °C. The lower
activity at 40 °C, in comparison to that at 0 °C, can be
a
Table 6. Polymerization of MMA Catalyzed by 4
d
tacticity (%)
b
c
entry
[M]0/[I]0
T (°C)
t (min)
yield (%)
10−4Mn
Mw/Mn
mm
mr
rr
1
2
3
4
5
6
1000
600
40
0
120
120
120
120
120
240
8.1
>99.0
>99.0
85.1
12.13
42.66
52.49
67.59
72.09
17.29
1.45
1.80
1.81
1.54
1.50
2.65
2.4
24.5
73.1
1000
1500
2500
1000
0
1.3
∼0
∼0
21.4
21.2
20.6
15.7
77.3
78.8
79.4
83.3
0
0
54.1
−40
26.2
1.0
a
b
c
Conditions: in toluene, VMMA/Vsol = 1/2. Yield = (weight of polymer obtained)/(weight of monomer used). Measured by SEC calibrated with
d
standard MMA samples. 1H NMR spectra in CDCl3 at 25 °C.
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Organometallics
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concentrated to dryness and then 4 mL of THF was added.
Crystallization at room temperature gave complex 4 as orange crystals
(1.21 g, 45%). Anal. Calcd for C46H62BN4Yb (855.45): C, 64.63; H,
7.31; N, 6.55; Yb, 20.24. Found: C, 64.08; H, 7.41; N, 6.52; Yb, 19.11.
IR (KBr, cm−1): 2336 (m), 2294 (w), 2227 (w), 1625 (s), 1600 (s),
1556 (vs), 1507 (s), 750 (s).
−40 °C, giving PMMA with 83.3% syndiotacticity content.
Further study on the application of diketiminate ligands in the
synthesis of single-site rare-earth-metal catalysts is proceeding
in our laboratory.
(L2‑Me)2YBH4 (5). A THF solution of NaL2‑Me (14.82 mL, 0.452 M),
which was formed by reaction of L2‑MeH with NaH in THF, was added
to a slurry of anhydrous YCl3 (3.35 mmol) in THF (about 20 mL) at
room temperature. The reaction mixture was stirred at 60 °C for 24 h.
Then NaBH4 (0.17 g, 4.47 mmol) was added. The reaction mixture
was stirred at 60 °C for 5 days. After the undissolved portion was
removed by centrifugation, the solution was concentrated to dryness
and then 3 mL of DME was added. Crystallization at room
temperature gave complex 5 as yellow crystals (0.91 g, 41.2%). Anal.
Calcd for C38H46BN4Y (658.51): C, 69.31; H, 7.04; N, 8.51; Y, 13.50.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were performed under a
purified argon atmosphere using standard Schlenk techniques. The
ligands were synthesized by the published procedure.12 Solvents were
degassed and distilled from sodium benzophenone ketyl before use.
Rare-earth-metal analyses were performed by ethylenediaminetetra-
acetic acid (EDTA) titration with a xylenol orange indicator and a
hexamine buffer.21 Element analyses were performed by direct
combustion using a CarloErba EA-1110 instrument. The IR spectra
were recorded with a Nicolet-550 FTIR spectrometer as KBr pellets.
1H NMR spectra were measured on an INOVA-400 MHz apparatus.
The number-average molar masses (Mn) and molar mass distributions
(Mw/Mn) were determined against a MMA standard by size exclusion
chromatography (SEC) on a PL-50 apparatus, and THF was used as
an eluent at a flow rate of 1.0 mL min−1 at 40 °C.
1
Found: C, 69.05; H, 6.88; N, 8.24; Y, 13.11. H NMR (400 MHz,
C6D6) δ 7.02 (d, 4H, ArH), 6.92 (m, 8H, ArH), 6.83 (t, 4H, ArH),
4.81 (d, 2H, −CH−), 2.01 (s, 12H, Ar−CH3), 1.58 (d, 12H, −CH3),
1.07−1.33 (m, 4H, BH4) ppm. IR (KBr, cm−1): 2342 (m), 2294 (w),
2226 (w), 1627 (s), 1604 (m), 1558 (s), 1508 (m), 744 (s).
(L2‑Me)2YbBH4·2C4H8O (6). This complex was prepared in a manner
similar to that used for the preparation of 5, but NaL2‑Me (15.31 mL,
0.452 M), YbCl3 (3.46 mmol), and NaBH4 (0.18 g, 4.74 mmol) were
used instead. After the undissolved portion was removed by
centrifugation, the solution was concentrated to dryness and then 6
mL of THF was added. Crystallization at room temperature gave
complex 6 as red crystals (1.48 g, 57.5%). Anal. Calcd for
C38H46BN4Yb (743.32): C, 61.46; H, 6.24; N, 7.54; Yb, 23.30.
Found: C, 60.82; H, 6.28; N, 7.45; Yb, 23.14. IR (KBr, cm−1): 2350
(m), 2293 (m), 2226 (w), 1628 (s), 1604 (m), 1555 (s), 1509 (m),
743 (s).
General Procedure for the Polymerization of ε-CL and L-LA.
The procedures for the polymerization of ε-CL and L-LA initiated by
complexes 3−6 are similar, and a typical polymerization procedure is
given below. To a stirred solution of ε-CL or L-LA in THF was added
a THF solution of the initiator using a syringe. The polymerization
mixture was stirred for a definite time at the desired temperature and
then quenched with an ethanol solution containing a small amount of
hydrochloric acid (5%). The polymer was precipitated from ethanol,
washed with ethanol three times, and dried under vacuum.
X-ray Crystallography. Suitable single crystals of complexes 2−6
were sealed in thin-walled glass capillaries. Intensity data were
collected with a Rigaku Mercury CCD area detector in ω scan
mode by using Mo Kα radiation (λ = 0.710 70 or 0.710 75 Å). The
diffracted intensities were corrected for Lorentz and polarization
effects, and empirical methods were used to correct for absorption.
Details of the intensity data collection and crystal data are given in
Table 1. 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 hydrogen atoms in
these complexes were all generated geometrically (C−H bond lengths
fixed at 0.95 Å), assigned appropriate isotropic thermal parameters,
and allowed to ride on their parent carbon atoms. All the H atoms
were held stationary and included in the structure factor calculation in
the final stage of full-matrix least-squares refinement. The structures
were solved and refined by using the SHELXL-97 program.22
Synthesis of Complexes 1−6. (L2,6‑ipr2Ph)2YCl (1). A THF
solution of NaL2,6‑ipr2 (28.7 mL, 0.353 M), which was formed by
Ph
reaction of L2,6‑ipr2PhH with NaH in THF, was added to a slurry of
anhydrous YCl3 (0.99 g, 5.07 mmol) in THF (about 20 mL) at room
temperature. The reaction mixture was stirred at 60 °C for 24 h. After
the undissolved portion was removed by centrifugation, the yellow
solution was concentrated to dry and then about 30 mL of toluene was
added. Crystallization at room temperature afforded 1 as pale yellow
crystals (1.95 g, 49%). Anal. Calcd for C46H58ClN4Y (790.341): C,
69.82; H, 7.39; N, 7.08; Y, 11.23. Found: C, 69.32; H, 7.21; N, 6.54; Y,
11.01. 1H NMR (400 MHz, C6D6): δ 7.13−6.81 (m, 16H, ArH),
6.52−6.49 (d, 4H, Ar−CH(CH3)2), 4.96 (s, 2H, CH), 1.81−1.62 (m,
12H, −CH3), 1.18−1.11 (m, 24H, Ar−CH(CH3)2) ppm. IR (KBr,
cm−1): 1744 (m), 1705 (m), 1547 (m), 1516 (m), 1462 (w), 1339
(w), 1223 (w), 1196 (w), 952 (m), 779 (s), 740 (s), 680 (m), 644
(m), 556 (m), 525 (s), 444 (s), 420 (vs).
(L2,6‑ipr2Ph)2YbCl·C4H8O·0.5C7H8 (2). A THF solution of NaL2,6‑ipr2
Ph
(17.43 mL, 0.460 M), which was formed by reaction of L2,6‑ipr2PhH with
NaH in THF, was added to a slurry of anhydrous YbCl3 (1.12 g, 4.01
mmol) in THF (about 20 mL) at room temperature. The reaction
mixture was stirred at 60 °C for 24 h. After the undissolved portion
was removed by centrifugation, the brown solution was concentrated
to dry and then about 0.5 mL of THF and 20 mL of toluene were
added. Crystallization at room temperature afforded yellow crystals of
2 (1.09 g, 31%). Anal. Calcd for C46H58ClN4Yb (875.37): C, 63.11; H,
6.68; N, 6.40; Yb, 19.77; Found: C, 62.71; H, 6.42; N, 6.14; Yb, 19.25.
IR (KBr, cm−1): 1794 (m), 1744 (s), 1705 (s), 1678 (s), 1647 (s),
1616 (w), 1516 (vs), 1462 (m), 1423 (m), 1396 (w), 1339 (w), 1196
(w), 1018 (w), 976 (w), 880 (m), 780 (s), 648 (s), 598 (s), 540 (vs),
482 (vs), 428 (s).
(L2,6‑ipr2Ph)2YBH4·0.5C7H8 (3). A THF solution of (L2,6‑ipr2Ph)2YCl
(3.33 g, 4.21 mmol) was added to a solution of NaBH4 (0.16 g, 4.21
mmol) in THF (about 20 mL) at room temperature. The reaction
mixture was stirred at 60 °C for 5 days. After the undissolved portion
was removed by centrifugation, the solution was concentrated to
dryness and then 4 mL of toluene was added. Crystallization at room
temperature gave complex 3 as pale yellow crystals (0.85 g, 38%).
Anal. Calcd for C46H62BN4Y (770.41): C, 71.68; H, 8.11; N, 7.27; Yb,
ASSOCIATED CONTENT
1
■
11.54. Found: C, 70.90; H, 8.14; N, 6.96; Yb, 10.85. H NMR (400
MHz, C6D6) δ 7.14−7.06 (m, 6H, ArH), 7.02 (t, 4H, ArH), 6.90 (t,
2H, ArH), 6.85 (d, 4H, ArH), 4.85 (s, 2H, −CH−), 3.19 (m, 4H, Ar−
CH(CH3)2), 1.69 (s, 6H, −CH3), 1.67 (s, 6H, −CH3), 1.15 (d, 12H,
Ar−CH(CH3)2), 1.10 (d, 12H, Ar−CH(CH3)2), 0.97−1.16 (m, 4H,
BH4) ppm. IR (KBr, cm−1): 2355 (m), 2294 (m), 1631 (s), 1549 (m),
1501 (m), 746 (vs).
S
* Supporting Information
CIF files giving X-ray crystallographic data for complexes 2−6.
This material is available free of charge via the Internet at
http://pubs.acs.org.
(L2,6‑ipr2Ph)2YbBH4·C4H8O (4). Complex 4 was prepared in a manner
similar to that used for the preparation of 3, but (L2,6‑ipr2Ph)2YbCl (2.73
g, 3.12 mmol) and NaBH4 (0.17 g, 4.47 mmol) were used. After the
undissolved portion was removed by centrifugation, the solution was
AUTHOR INFORMATION
■
Corresponding Author
*Fax: +86-512-65880305. E-mail: qshen@suda.edu.cn.
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Article
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are grateful to the National Natural Science Foundation of
China (Grants 21132002 and 20972107) and the Project
Funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions for financial support.
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