Chloroyttrium 2-(1-(arylimino)alkyl)quinolin-8-olate complexes: Synthesis, characterization, and catalysis of the ring-opening polymerization of ε-caprolactone

Article abstract of DOI:10.1021/om300778g

Stoichiometric reactions of YCl₃(THF)₃ with potassium 2-((arylimino)methyl)quinolin-8-olates or 2-(1-(arylimino)ethyl)quinolin-8- olates in THF solution gave the mononuclear LYCl₂(DMSO)₂ complexes 1-5 in the presence of DMSO and a representative dinuclear complex 6 in the absence of DMSO. All yttrium complexes were fully characterized by NMR measurements and elemental analysis, and the crystal structures of complexes 1 and 4-6 were determined by single-crystal X-ray diffraction. The structures indicate coordination number seven around the yttrium center and pentagonal bipyramidal geometries. The complexes all feature diapical YCl₂ moieties and one tridentate organic ligand in the equatorial plane. Upon reaction of the yttrium precatalysts 1-6 with LiCH₂Si(CH ₃)₃ alone or with LiCH₂Si(CH₃) ₃ together with BnOH, the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) occurred with high efficiency. Depending on conditions, the ROP of ε-CL produced polycaprolactone with narrow molecular distribution and in a living manner. Theoretical studies of the chlorine/CH₂SiMe₃ and Me₃SiCH₂/BnO ligand exchange reactions suggest that the replacement of the apical ligands can proceed without significantly affecting the equatorial ligands. These results suggest that one of the apical Y-CH₂SiMe₃ bonds within the LY(CH₂SiMe₃)₂ intermediate catalyzes the polymerization in the BnOH-free process. Most polymers generated by BnOH-assisted catalysis possess M_n values that are similar to M _n,cal values based on Y-OBn, suggesting that one apical Y-OBn bond of the diapical LY(OBn)(CH₂SiMe₃) intermediate catalyzes most or all of the ring polymerization of ε-CL.

Full text of DOI:10.1021/om300778g

Article
pubs.acs.org/Organometallics
Chloroyttrium 2‑(1-(Arylimino)alkyl)quinolin-8-olate Complexes:
Synthesis, Characterization, and Catalysis of the Ring-Opening
Polymerization of ε‑Caprolactone
Wenjuan Zhang,† Shaofeng Liu,† Wenhong Yang,† Xiang Hao,† Rainer Glaser,*,‡ and Wen-Hua Sun*,†
†Key Laboratory of Engineering Plastics and Beijing National Laboratory for Molecular Science, Institute of Chemistry, Chinese
Academy of Sciences, Beijing 100190, China
‡Department of Chemistry, University of Missouri, Columbia, Missouri 65211, United States
S
* Supporting Information
ABSTRACT: Stoichiometric reactions of YCl3(THF)3 with
potassium 2-((arylimino)methyl)quinolin-8-olates or 2-(1-
(arylimino)ethyl)quinolin-8-olates in THF solution gave the
mononuclear LYCl2(DMSO)2 complexes 1−5 in the presence of
DMSO and a representative dinuclear complex 6 in the absence of
DMSO. All yttrium complexes were fully characterized by NMR
measurements and elemental analysis, and the crystal structures of
complexes 1 and 4−6 were determined by single-crystal X-ray
diffraction. The structures indicate coordination number seven
around the yttrium center and pentagonal bipyramidal geometries.
The complexes all feature diapical YCl2 moieties and one tridentate
organic ligand in the equatorial plane. Upon reaction of the yttrium precatalysts 1−6 with LiCH2Si(CH3)3 alone or with
LiCH2Si(CH3)3 together with BnOH, the ring-opening polymerization (ROP) of ε-caprolactone (ε-CL) occurred with high
efficiency. Depending on conditions, the ROP of ε-CL produced polycaprolactone with narrow molecular distribution and in a
living manner. Theoretical studies of the chlorine/CH2SiMe3 and Me3SiCH2/BnO ligand exchange reactions suggest that the
replacement of the apical ligands can proceed without significantly affecting the equatorial ligands. These results suggest that one
of the apical Y−CH2SiMe3 bonds within the LY(CH2SiMe3)2 intermediate catalyzes the polymerization in the BnOH-free
process. Most polymers generated by BnOH-assisted catalysis possess Mn values that are similar to Mn,cal values based on Y−
OBn, suggesting that one apical Y−OBn bond of the diapical LY(OBn)(CH2SiMe3) intermediate catalyzes most or all of the ring
polymerization of ε-CL.
contrast, yttrium(III) isopropoxide and related heterobimetallic
isopropoxides showed high efficiency for polymerization of ε-
CL and produced polymers with broad molecular distributions
(PDI = 1.4−1.8).6 Among the heterobimetallic isopropoxides,
Sn[Y(OiPr)4]2 performed better than Y[Al(OiPr)4]3 and
Y[Sn(OiPr)3]3. Moreover, controlled polymerization of ε-CL
could be achieved by well-defined alkoxo-bridged di- and
triyttrium(III) complexes.7 For example, tris(phenoxy) yt-
trium/2-propanol systems were employed for the ROP of ε-
CL and formed PCL with controlled molecular weight and
narrow polydispersity, and moreover some phenoxy yttrium
compounds exhibited good controllability for copolymerization
of esters.8 In the literature, however, it was also reported that
Y(OMe)(C5Me5)2(THF) showed no reaction in the ROP of ε-
CL9 and that immobilized yttrium isopropoxides on silica
INTRODUCTION
■
Polyesters including polylactide (PLA), polyglycolide (PGL),
and polycaprolactone (PCL) are considered “green materials”
because they are biocompatible, readily biodegradable, and
easily recyclable. Because of these attractive characteristics, the
polyesters are used in biomedical and pharmaceutical
applications such as drug delivery receptacles, medical bone
screws and pins, absorbable surgical sutures, and matrix
material for tissue engineering.1 One convenient method for
the synthesis of polyesters consists of the ring-opening
polymerization (ROP) of cyclic esters using metal complexes
as catalysts or initiators,1c,f,2 in which tin and aluminum
compounds have been widely used as initiators.3 At the same
time, rare earth metal complexes have become attractive
catalysts for the ROP of ε-CL due to their low toxicity, their
rich and diverse coordination chemistry, and their high
reactivity.4
produced only PCL with lower molecular weights (Mn
=
1900).10 In contrast, the amide Y((N(SiMe3)2)3 and
11
tris(amidate)yttrium complexes,12a−c activated by alcohols,
The triflate salts of some rare earth metals (Sc, Y, Nd, Er)
were explored as possible initiators of the ROP of ε-CL, and
these metals showed low catalytic activity and resulted in
polymers with relatively low molecular weights.5 In sharp
Received: August 12, 2012
Published: November 5, 2012
© 2012 American Chemical Society
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acted as highly effective initiators of the ROP of ε-CL and
produced PCL with narrow molecular distribution, and these
observations suggested yttrium alkoxides as active species.11
Moreover, some alkylyttrium compounds such as LY-
(CH2SiMe3)2 (L: N-(8-quinolinyl)-2-(iminomethyl)anilide)
caused the ROP of ε-CL to proceed in a living fashion.13a
Interestingly, bis(phosphino)carbazolide rare earth metal
complexes promoted the copolymerization of ε-CL with
diene.14 In addition to experimental observation of borohydride
yttrium complexes as initiators for the ROP of lactone, the
mechanism of this reaction was explored with DFT
calculations.15 These achievements illustrate the high potential
of rare earth metal complexes for their applications in ROP
catalysis, and it is now a significant goal to find suitable ligands
to finely tune the catalytic behavior of their complexes.
Due to their high coordination numbers, rare earth metals
were often reported as aggregated (multinuclear) compounds16
or half-metallocene alkyl complexes.17 Multidentate organic
ligands allow for finely tuning the sizes and electronic
properties of the substituents, and these ligands stabilize rare
earth metal alkyl compounds.18 Recently, we prepared 2-
(arylimino)quinolin-8-ols and explored their potential as
multidentate ligands. Aluminum complexes with these ligands
showed high activity in the ROP of ε-CL,19 and their titanium
complexes acted as precatalysts for ethylene (co)-
polymerization.20 Subsequently, we have been exploring the
scope of rare earth metal complexes of these ligands.
donating solvent,21 and obtained a clear purple solution.
Removing the solvent under vacuum, the residue was extracted
with CH2Cl2 (40 mL) plus drops of DMSO, and the filtrate was
concentrated to a volume of about 5 mL solution. Adding 40
mL of Et2O precipitated a red solid (1) in high yield (86%).
The compound LYCl2(DMSO)2 was characterized by NMR
spectroscopy and elemental analysis. Compounds 2−5 were
also synthesized in high yields and fully characterized.
1
Comparison of the H NMR spectra of the complexes with
the corresponding free ligand shows that the OH resonances
around 8.2 ppm disappeared as the M−O bonds formed.
The stoichiometric reaction of potassium 2-((2,6-
dimethylphenylimino)methyl)quinolin-8-olate with
YCl3(THF)3 in DMSO-free solution led to a red suspension
and resulted in the formation of an orange compound (6) in
1
46% yield. Its H NMR spectrum showed two sets of ligand
peaks with a 1:1 integration ratio, and X-ray diffraction analysis
confirmed 6 to be a binuclear complex (vide infra).
2. Crystal Structure Analyses. Single crystals of 1 and 4−
6 suitable for X-ray structural analysis were grown from
dichloromethane/n-heptane (1, 4, 5) or THF/n-heptane (6)
solutions at room temperature. Their selected structural
parameters are collected in Table 1, and ORTEP diagrams
are displayed in Figures 1−4. We first describe the characteristic
structural motifs of these complexes in qualitative terms, and
subsequently we will examine some structural relationships
quantitatively.
Herein the synthesis and characterization of the title
complexes are reported along with the results of studies of
their catalytic behavior in the ROP of ε-CL. Electronic structure
theory was also employed to study the putative complexes
generated from the precatalysts by chlorine/alkyl and alkyl/
alkoxide ligand exchange reactions.
The X-ray structure analyses show monoligated complexes
with coordination number 7 for yttrium, and complexes 1, 4,
and 5 possess very similar geometries around yttrium (Scheme
2). Each yttrium atom within all of the yttrium complexes is
coordinated by a tridentate 2-(arylimino)quinolin-8-oate
N,N,O-ligand in a meridional fashion. For example, in complex
1 shown in Figure 1, the Y metal center is coordinated by the
organic tridentate ligand in a meridional mode, by two trans-
diaxial Cl atoms, and by two cis-diequatorial O(DMSO) atoms
to generate the 7-fold coordination. The decahedron spanned
by the ligands around Y is that of a distorted pentagonal
bipyramid. The deviation of Y1 from the equatorial plane is
0.052 Å. The two chelating rings formed by N1, C8, C10, and
N2 and by N1, C9, C1, and O1 are essentially coplanar. The
crystal structures of 4 and 5 (Figures 2 and 3) show the same
coordination motif as in 1, and the structural parameters agree
closely (Table 1).
While the crystal structures of 1 and 4 contain just one
molecule in their unit cells, the crystals of 5 contain two
independent molecules in the unit cell, and these are numbered
5a and 5b in Table 1. The examination of 5a and 5b presents
an opportunity to examine the sensitivity of the structural
parameters to crystal-packing effects, and two parameters stand
out. Comparing the values of 5a and 5b, one finds that the
angles ∠(Clax−Y1−Clax’) differ by about 5° and the aryl twists
∠(Cim−Nim−Ci−Co) = τ differ by about 2.5°. Hence, the
differences between the ∠(Clax−Y1−Clax’) angles of 176.73(4)°
(1), 176.61(4)° (4), 168.36(4)° (5a), and 171.18(4)° (5b)
should not be overinterpreted. The aryl twists τ are 93.3° (1),
97.5° (4), 93.3° (5a), and 95.9° (5b), and the situation is
similar. The point here is that the twists in all of the Schiff bases
place the aryl plane more or less perpendicular to the imine
plane, and the exact twist angle is less important because of the
small curvature of this internal coordinate.
RESULTS AND DISCUSSION
■
1. Synthesis and Characterization of Yttrium Com-
plexes 1−6. The 2-((arylimino)alkyl)quinolin-8-ol derivatives
were prepared according to our reported procedure.18 Employ-
ing the procedure for the synthesis of LTiCl3 complexes,18 2-
((2,6-dimethylphenylimino)methyl)quinolin-8-ol was deproto-
nated with an equivalent amount of potassium hydride in THF,
and the potassium salt was used in situ to react with an
equivalent amount of YCl3(THF)3 (Scheme 1). However, we
observed some red suspended fine precipitate, which was likely
due to aggregation. To increase the solubility of the product, we
added several drops of dimethyl sulfoxide (DMSO), a strongly
Scheme 1. Synthesis of Yttrium Complexes 1−6
The binuclear complex 6 does look rather different from 1, 4,
and 5 at first sight, but its structure in fact shares many of the
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Table 1. Selected Structural Parameters of the Crystal Structures of Yttrium Complexes
parameter
Y−Npy
Y−Nim
Y−O(L)
Y−O(L′)
Y−O(sol)
1
4
5a
5b
6.1a
6.1b
6.2a
6.2b
2.429(4)
2.653(4)
2.238(3)
2.452(4)
2.665(4)
2.241(3)
2.454(4)
2.657(4)
2.262(4)
2.440(4)
2.647(4)
2.248(4)
2.438(6)
2.619(6)
2.323 (4)
2.246(4)
2.397(5)
2.440(5)
2.604(5)
2.316(4)
2.263(4)
2.355(5)
2.442(5)
2.605(5)
2.380(4)
2.318(4)
2.463(5)
2.626(6)
2.373(4)
2.312(4)
2.296(3)
2.296(3)
2.283(4)
2.292(4)
2.296(4)
2.617(2)
2.623(2)
2.307(3)
2.314(3)
2.275(4)
Y1−Clax
Y1−Clax’
Y1−Cleq
2.6245(13)
2.6378(13)
2.6324(14)
2.6312(14)
2.6033(17)
2.6459(18)
2.5490(18)
2.7149(17)
2.541(2)
2.704(2)
2.560(2)
2.807(2)
2.575(2)
168.98(5)
96.63(7)
88.19(7)
128.63(14)
66.48(14)
63.81(16)
120.02
2.5448(19)
2.772(2)
2.5791(18)
164.24(6)
97.86(7)
88.25(7)
129.89(17)
66.15(16)
63.95(18)
118.65
Clax−Y1−Clax’
Clax−Y−Cleq
Clax’−Y−Cleq
O1−Y1−N2
O1−Y1−N1
N1−Y1−N2
∠(R1−Cim−Nim)
∠(Cim−Nim−Ci)
∠(Cim−Nim−Ci−Co)
176.73(4)
176.61(4)
168.36(5)
173.99(6)
175.08(6)
173.90(7)
131.71(12)
68.46(12)
63.47(12)
119.98
130.81(11)
68.28(12)
62.64(12)
125.45
131.07(13)
68.54(14)
62.53(13)
126.05
132.01(14)
69.03(15)
63.04(14)
123.51
130.50(19)
67.03(19)
64.2(2)
118.89
130.44(15)
67.86(16)
63.90(18)
119.21
116.50
116.63
118.22
118.84
117.27
116.72
115.35
116.25
93.32
97.47
93.32
95.86
110.73
111.73
97.55
102.89
angle of about 100°. Third, the last remaining equatorial
position of the other yttrium (Y1 in Figure 4) is occupied by
the O-donor solvent molecule THF. The Y1···Y2 distance of
3.5402(11) Å is similar to those in Y3(μ3,η2-
OC2H4OMe)2(μ2,η2-OC2H4OMe)2(μ2,η1-OC2H4OMe)-
(acac)4.22
As with 5, the unit cell of 6 contains two symmetry-
independent dinuclear complexes, 6a and 6b. Each complex
contains two yttrium centers with LYCl2 (6.1) or LYCl3 (6.2)
moieties, respectively. The pertinent structural parameters of
the four LYCln moieties are listed in Table 1 together with the
averages for the LYCl2 and LYCl3 moieties, and these data
reveal several significant features. To begin with, there exists a
pronounced difference in the Y−Clax distances: In 1, 4, and 5,
all d(Y−Clax) values fall in the narrow range 2.60−2.64 Å. In 6,
however, the d(Y−Clax’) values involving a shared chlorine are
much longer and more varied (2.70−2.81 Å), while the d(Y−
Clax) bonds to an unshared chlorine are shorter (2.54−2.60 Å).
Note in particular that the shared chlorine is significantly
farther from the YCl3 center (2.79 Å) than from the YCl2
center (2.71 Å). In 6, there also are equatorial chloride ligands,
and we note that the d(Y−Cleq) bonds (≈2.58 Å) are slightly
longer than the d(Y−Clax) bonds (≈2.55 Å).
The fusion of two pentagonal bipyramids via a shared O2Cl
face generates a trigonal bipyramidal Y2O2Cl core cluster. The
two μ2-O atoms are part of the organic ligands of the LYCl2 and
LYCl3 moieties, respectively. In Table 1, d(Y−O(L)) refers to
the Y−O distance between yttrium and the phenoxide-O of
ligand L, which also coordinates this yttrium with the N atoms
of the very same L ligand. With reference to the d(Y−O(L))
values of the mononuclear complexes (2.24−2.26 Å), it is
observed that the d(Y−O(L)) values in 6 are longer (2.32−
2.38 Å), as expected. Of interest is the fact that the d(Y−
O(L′)) bond lengths vary significantly depending on whether
the additional coordination is made to an electron-deficient
YCl2 (6.1) or to an electron-rich YCl3 (6.2) metal center; the
former are ∼2.25 Å and the latter are ∼2.31 Å, respectively.
In all of the complexes, whether mononuclear or binuclear,
the distance between Y and the quinoline-N (2.43−2.46 Å) is
consistently ∼0.2 Å shorter than the distance between Y and
the imino-N (2.60−2.66 Å). The same structural phenomenon
Figure 1. ORTEP drawing of complex 1 with thermal ellipsoids at the
30% probability level. Hydrogen atoms are omitted for clarity.
characteristic features of the mononuclear complexes (Scheme
2): (a) Each yttrium is coordinated by one organic, tridentate
ligand L by way of one O and two N atoms in a mer
arrangement. (b) The coordination number of each yttrium is
7, and each yttrium is located at the center of a trigonal
bipyramid. (c) The apical positions are occupied by Cl atoms.
The distinguishing features between 1, 4, and 5 and the
binuclear complex 6 concern the occupations of the remaining
two equatorial positions, which are occupied by DMSO solvent
molecules in the mononuclear complexes. It is the first
distinguishing feature that the two LYCl2 moieties are
combined such that their phenoxide-O atoms coordinate to
both yttrium atoms, and this mode of LYCl2 interaction fills
one of the two available equatorial positions. Second, the two
pentagonal bipyramids share one apical chlorine atom (Cl5 in
Figure 4), and this feature is important for several reasons. It is
one immediate consequence that one chloride becomes
available to occupy the last remaining equatorial site around
one yttrium (Y2 in Figure 4). It is another important
consequence of the shared apical Cl ligand that the base planes
of the pentagonal bipyramids are tilted and they enclose an
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Figure 2. ORTEP drawing of complex 4 with thermal ellipsoids at the 30% probability level. Hydrogen atoms are omitted for clarity.
Scheme 2. Structural Motifs in Yttrium Complexes 1−5
(left) and 6 (right)
was observed in the titanium analogues bearing the same
ligands.20 In the dimeric aluminum analogues,19 the imino-N is
too far away from the Al center to allow for direct bonding.
2. Ring-Opening Polymerization of ε-Caprolactone.
Rare earth halides, depending on the reaction parameters,
showed verified catalytic activities in the ROP of ε-CL that are
low23 or good.24 Previous reports on the use of alkylyttrium
complexes as initiators for the polymerization of cyclic
esters12,14 showed that the preparation of yttrium alkyl
complexes was challenging because of critical problems with
their isolation.25 Fortunately, both of these problems have now
been overcome because the complexes 1−6 can be purified
without any problem. As mentioned before, our group
developed a series of trimetallic yttrium complexes that could
initiate the high-efficiency ring-opening polymerization of ε-CL
with or without alcohols.12c Regarding the catalytic property by
the title complexes, only a trace amount of polymer was
observed in the catalytic system of 1/BnOH, even with
variations of the reaction temperature up to 90 °C and reaction
times of up to 2 h. 1H NMR measurement indicated no
reaction of the yttrium complex with BnOH and suggested a
high stability of the Y−Cl bonds.
Figure 3. ORTEP drawing of complex 5 with thermal ellipsoids at the
30% probability level. Hydrogen atoms are omitted for clarity. The
unit cell contains two independent molecules with similar structures,
and only one is shown.
Since Y−OBn intermediates are considered to be the active
species for ring-opening polymerization of ε-CL and inspired
by the NaBH4 initiation reported by the Guillaume group,26 we
sought to replace the chloride ligands of complexes 1−6 by
reaction with 2 equivalents of the initiator LiCH2Si(CH3)3 to
form the catalysts for the ROP of ε-CL in situ. After initiation
with LiCH2Si(CH3)3, the catalytic systems show high catalytic
activities for ring-opening polymerization of ε-CL.
Figure 4. ORTEP drawing of complex 6 with thermal ellipsoids at the
30% probability level. Hydrogen atoms are omitted for clarity. There
are two independent molecules in the unit cell, and one of them is
illustrated.
In order to produce PCLs with narrow molecular weight
distributions, it is common practice to employ catalytic
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a
Table 2. ROP of ε-CL by 1−6 Initiated by LiCH2Si(CH3)3/BnOH
b
b
entry
compd
CL:Y:BnOH
t [min]
mg/conv (%)
Mn × 10−4
PDI
Mn,cal × 10−4
1
2
3
1
1
1
1
1
1
1
1
1
1
2
3
4
5
6
500:1:1
500:1:1
500:1:1
500:1:1
500:1:1
250:1:1
1000:1:1
2000:1:1
500:1:1
500:1:0
500:1:1
500:1:1
500:1:1
500:1:1
500:1:1
5
10
15
15
15
30
30
30
30
30
30
30
30
30
30
291 (50.7)
408 (71.0)
478 (83.3)
561 (97.7)
570 (99.3)
275 (95.8)
1117 (97.3)
2190 (95.4)
547 (95.3)
515 (89.7)
565 (98.4)
560 (97.6)
542 (94.4)
556 (96.9)
544 (94.8)
2.83
3.93
4.70
5.84
5.95
3.16
11.8
22.9
5.79
6.57
5.84
5.66
5.20
6.36
1.15
1.18
1.19
1.35
1.37
1.27
1.33
1.37
1.21
1.68
1.14
1.19
1.09
1.28
1.21
2.89
4.05
4.75
5.62
5.70
2.76
c
4
d
5
6
7
11.2
22.0
5.48
8
9
10
11
12
13
14
15
5.16
5.66
5.62
5.42
5.58
5.46
5.14
a
b
Conditions: 10 μmol of Y, 20 μmol of LiCH2Si(CH3)3, 1.0 M ε-CL toluene solution, 20 °C. GPC data in THF vs polystyrene standards, using
correction factor 0.56.28 40 °C. 60 °C.
c
d
amounts of alcohols.4,27 In addition to the screening for
optimum polymerization conditions by complex 1 with 2
equivalents of LiCH2Si(CH3)3, 1 equivalent of BnOH was
added in the successful catalytic system. The results are
tabulated in Table 2 (entries 1−9).
Complex 1 was mixed with 2 equivalents of LiCH2Si(CH3)3
for 30 min to ensure complete alkylation. One equivalent of
BnOH and the prescribed amount of monomer ε-CL were
added consecutively to initiate the ROP. According to entries
1−3 and 9 in Table 2, a linear relationship was observed
(Figure 5) between the monomer conversions and the Mn
distributions. Nevertheless, most of the resultant PCLs
possessed narrow polydispersities with unimodal characteristics.
Changing the ε-CL/Y ratios from 250 to 2000 (entries 6−9,
Table 2), the molecular weights of the resultant polymers
increase approximately linearly with increasing concentration of
ε-CL (Figure 6). This observation further supports our
Figure 6. Plots of Mn values vs CL/Y molar ratio in the ROP of ε-CL
with 1 as pre-catalyst (entries 6−9, Table 2).
proposal that the living polymerization happened within the
catalytic system. Moreover, entries 1−3 and 9 in Table 2 show
first-order dependence of the polymerization rate on the
monomer concentration (Figure 7). The plots in Figures 6 and
7 also suggest that the active species during the ROP of ε-CL
are not scavenged. All of these results support our proposal of
the living polymerization of ROP of ε-CL in accordance with
the results obtained with dialkyl yttrium complex.12a
The polymerization in the presence of 2 equivalents of
LiCH2Si(CH3)3 without any BnOH also produced the PCLs,
and this result indicates that Y−CH2SiMe3 bonds can catalyze
the ring-opening polymerization of ε-CL. Note, however, that
this alcohol-free polymerization is not well behaved and
produces PCLs with molecular weights much higher than
Mn,cal and with broader PDI values (entry 10, Table 2). The
comparatively lower conversion (cf. entry 9, Table 2) indicated
either a different type of active species in the absence of BnOH
Figure 5. Plot of Mn vs monomer conversion in the ROP of ε-CL
initiated with 1 as pre-catalyst (entries 1−3 and 9, Table 2).
values with narrow molecular weight distributions (1.15−1.21).
This observation suggests that the polymerization happens in a
living manner, although the molecular weight distributions of
the polymers became slightly broader with increasing reaction
time. Assuming that the active species started from the Y−O
bond, the Mn values of the polymer products were quite close
to the calculated values Mcal. The result indicates “single-site”
catalytic behavior and is consistent with the performances by
dialkyl yttrium complexes or other rare earth metal complex-
es.12a,16 Higher reaction temperatures (entries 4, 5, Table 2)
resulted in better conversion and produced PCLs with higher
molecular weights with somewhat broader molecular weight
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complexes by introduction of the organic ligand into complexes
with Y−CH2SiMe3 bonds also were tried, that is, by adding 1
equivalent of Y(CH2SiMe3)3(THF)2 into a solution of the
ligand L (path B). These attempts also failed.13b The presumed
products formed by paths A and B are thought to be
coordination isomers, and the experiments suggest that only
one isomer is catalytically active while the other is inert.12a,16a
We think that the products of path A are diapical species and
that these diapical species are catalytically active. On the other
hand, this suggests that dialkyl complexes formed by path B
contain two Y−CH2SiMe3 moieties in a nondiapical arrange-
ment.
3. Theoretical Study of Chlorine/Alkyl and Alkyl/
Alkoxide Ligand Exchange Reactions. The described
experimental observations suggest that the chemistry of this
catalytic system involves the following steps in situ. First, alkyl
yttrium complexes are generated by successive reactions of the
yttrium dichloride complexes with 2 equivalents of LiCH2SiMe3
(eqs 1 and 2) and, second, the dialkyl yttrium complexes are
reacted with 1 equivalent of benzyl alcohol per yttrium to
generate the yttrium monoalkyl monoalkoxide complex (eq 3),
the presumed catalytically active species.
Figure 7. Plots of ln[[CL]/[CL]0] vs time in the ROP of ε-CL with 1
as pre-catalyst (entries 1−3 and 9, Table 2).
or only partial formation of the common active species.
Consequently, the yttrium complexes 2−6 were explored with
stoichiometric amounts of LiCH2Si(CH3)3, one for each
chloride, and in the presence of 1 equivalent of BnOH for
every yttrium equivalent (1 equiv of BnOH for complexes 2−5,
2 equiv of BnOH for complex 6).
LYCl2(sol)2 + LiCH2SiMe3
All catalytic systems exhibited very high activity for
ROP(CL), demonstrated high degrees of conversion (94.4−
98.4%), and produced PCLs with narrow molecular distribu-
tions (1.09−1.28). The nature of the organic ligands showed
some influence on the catalytic activity of their complexes, but
no clear distinctions are manifest between aldimine and
ketimine complexes and/or based on the aryl substitution
patterns. However, we note that the PCL produced with 5 has a
higher molecular weight and broader molecular distribution
than PCL produced by the other catalysts. Moreover, the
extremely similar activities and polymer characteristics of
mononuclear complex 3 and dinuclear complex 6 strongly
suggest that both catalysts lead to the same active species.
The 1H NMR analysis of the resultant polymer (run 9, Table
2) showed the typical signal of O−CH2Ph (δ 5.10 ppm),
suggesting that the Y−OCH2Ph bond of the active species
initiates the polymerization. On the other hand, the narrower
distribution of polymers indicated the better controllability by
this system with BnOH compared to the BnOH-free process.
As mentioned above, most polymers possess Mn values that are
similar to Mn,cal based on Y−OBn, and we interpret this
observation to show that only the Y−OBn bonds of the
LY(OBn)(CH2SiMe3) intermediate catalyze the ring polymer-
ization of ε-CL, while the Y−CH2SiMe3 bond does not. A
similar observation was made by the Okuda’s group, in which
the initiation of ε-caprolactone ROP by the rare earth cationic
[YMe(BH4)(THF)5]+ occurred by both the borohydride and
the methyl group. When the borohydride was replaced by a
more electron-donating group of NMe2, the trans-effect
resulted in a higher kinetic facility on the methyl side.15a In
addition, the polymerization by the in situ catalytic system
comprised of 1 and 1 equivalent of LiCH2SiMe3, either with or
without 1 equivalent of BnOH produced only trace amounts of
polymer. Therefore the monoalkyl monochloro complexes and
the monochloro monoalkoxide complexes are inactive in the
ring-opening polymerization of ε-CL.
→ LYCl(CH2SiMe3)(sol)2 + LiCl
(1)
LYCl(CH2SiMe3)(sol)2 + LiCH2SiMe3
→ LY(CH2SiMe3)2(sol)2 + LiCl
(2)
LY(CH2SiMe3)2(sol)2 + PhCH2OH
→ LY(CH2SiMe3)(OCH2Ph)(sol)2 + TMS
(3)
Two equivalents of LiCH2SiMe3 were employed to replace
both chloride ligands with carbanions. Unfortunately, all
experimental attempts to isolate the dialkyl yttrium complexes
failed irrespective as to whether the reactions of complexes 1−5
with 2 equivalents of LiCH2Si(CH3)3 were performed at room
or low temperature.29
We employed density functional theory (DFT) to investigate
the structural feasibility of the Cl/CH2SiMe3 exchange using
the organic ligand L1 of complex 1, and the B3LYP/SDD
optimized structures are shown in Figure 8.
The hybrid density functional method30,31 B3LYP was
employed in conjunction with the SDD basis set.32 The
optimized structure M1 shows essentially the same character-
istic features that are observed in complex 1 in the crystal
structure (Figure 8, top row). As can be seen, packing effects in
crystals of 1 merely alter the conformations of one ortho-
methyl group and of one DMSO ligand. The structures of M2
and M3 show that both the first and the second Cl/CH2SiMe3
exchanges in the apical positions, respectively, can proceed
without major effects on the equatorial ligands. The carbanions
are placed above the arene plane of the organic ligand, the
planes defined by one Si atom and the C atoms of two of its
methyl groups are almost parallel to the arene plane, and the
orientations of the two carbanions in M3 are markedly different
relative to the organic ligand. Roughly speaking, the carbanion
in M2 and one of the carbanions in M3 (the one shown on top
in Figure 8) are bonded such that one methyl group is placed
above and between Nim and Npy and one methyl group is
placed above and between Npy and OL1. This type of carbanion
placement (type A) requires avoidance of steric hindrance by
The catalytic system generated in situ by reaction of 1 with 2
equivalents of LiCH2SiMe3 (path A) did exhibit high catalytic
activity. We tried to isolate the dialkyl species but without
success. The preparation and isolation of dialkylyttrium
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Figure 8. Molecular models of the B3LYP/SDD-optimized structures of complexes M1−M4.
the ortho-alkyl group in the hemisphere occupied by the
carbanion and the entire N-aryl group is pushed slightly, but
markedly into the hemisphere on the other side of the organic
ligand. In contrast, the second carbanion in M3 is bonded such
that one methyl group is placed over Npy and another methyl
group is placed over OL1 (type B). It is reasonable to expect the
dialkyl complex to be fluxional and that the two nonequivalent
alkyl ligands would equilibrate fast even at low temperature.
The packings of the crystal structures of 1 and 4−6
prominently feature parallel stacked arene−arene interactions
between the organic ligands of neighboring complexes. Such
crystal packing no longer would be possible with the dialkyl
complex M3, and it is plausible that it is this feature that has
prevented the crystallization of the dialkyl yttrium complexes to
date. In the monoalkyl complex M2, one arene face of the
organic ligand remains accessible to intermolecular arene−
arene interactions, and monoalkyl complexes of this type might
be more amenable to crystallization and X-ray structure
analysis. However, trials to monitor the reaction of the complex
with 1 equivalent of LiCH2SiMe3 could not determine any main
product.
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Considering that the alkyl groups in M3 are structurally not
equivalent, we optimized two structures, M4a and M4b,
respectively, in which either the type A or the type B carbanion,
respectively, was exchanged by the alkoxide ligand BnO−. Both
initial structures were constructed such that the phenyl moiety
of the BnO− ligand had the opportunity to engage in
intramolecular arene−arene interactions. Nevertheless, the
optimized structure of neither M4a nor M4b features arene−
arene interactions. The benzyl group is placed between the
DMSO ligands in M4a, and one Cortho−H bond involving of
the BnO ligand points directly at the proximate ODMSO (2.916
Å). In M4b, the respective Cortho−H bond of the BnO ligand
points directly at the phenoxide-O of L1 (2.605 Å). The apical
Y−OBn bond lengths are 2.216 Å (M4a) and 2.167 Å (M4b)
and are considerably shorter than the Y−O(L1) bond lengths
of 2.347 Å (M4a) and 2.350 Å (M4b). The apical Y−
CH2SiMe3 bond lengths in the dialkyl complex M3 are 2.543 Å
(type A) and 2.569 Å (type B), and the Y−CH2SiMe3 bond
lengths in M4 are very similar, at 2.574 Å (M4a, type B) and
2.542 Å (M4b, type A). Structure M4a is 2.9 kcal/mol more
stable than M4b.
The vibrational spectra of M1−M4 were computed, and they
are provided as Supporting Information along with overlays of
the computed IR spectra of M1 and M2, of M1 and M3, and of
M3 and M4a. The Cl/CH2SiMe3 exchange leads to a weak
additional band in the low-energy CH stretching region and,
most characteristically, to a strong additional band around 930
cm−1 (two CH2SiMe3 bending modes) in M3. The Me3SiCH2/
BnO exchange leaves the 930 cm−1 band essentially in place,
while the feature’s intensity obviously is reduced. Most
characteristically, the reaction of M3 to M4 is accompanied
by the appearance of two bands around 1100 and 1130 cm−1
(BnO bending modes) in a region without strong features of
M3.
intermediate dialkyl complex of the BnOH-free system or based
on the unique Y−OBn bond of the monoalkyl monoalkoxide
complex of the BnOH-assisted polymerizations, respectively.
The analysis of the BnOH-free process suggests that only one
of the apical Y−CH2SiMe3 bonds of the LY(CH2SiMe3)2
intermediate catalyzes the polymerization. In the BnOH-
assisted process, it is the apical Y−OBn bond of the diapical
LY(OBn)(CH2SiMe3) intermediate that catalyzes most or all of
the ring-opening polymerization, whereas the reactivity of the
remaining apical Y−CH2SiMe3 bond in the LY(OBn)-
(CH2SiMe3) intermediate apparently cannot compete.
EXPERIMENTAL SECTION
■
1. General Procedures. All reactions were performed using
standard Schlenk techniques in an atmosphere of high-purity nitrogen
or glovebox techniques. Toluene, n-heptane, and THF were dried by
refluxing over sodium and benzophenone, distilled under nitrogen, and
stored over activated molecular sieves (4 Å) for 24 h in a glovebox
prior to use. CDCl3, DMSO, and C6D6 were dried over activated 4 Å
molecular sieves. CH2Cl2 was dried over CaH2 for 48 h, distilled under
nitrogen, and stored over activated molecular sieves (4 Å) in a
glovebox prior to use. YCl3(THF)3 was purchased from Aldrich and
used as received. Ligands were synthesized according to our reported
procedures.19,20 Elemental analyses were performed using a PE2400II
Series (Perkin-Elmer Co.). 1H and 13C NMR spectra were recorded on
1
a Bruker DMX-400 (400 MHz for H, 100 MHz for 13C) instrument.
All spectra were obtained in the solvent indicated at 25 °C unless
otherwise noted, and chemical shifts are given in ppm and are
1
referenced to SiMe4 (δ 0.00, H, 13C). The GPC measurements were
performed on a set consisting of a Waters-515 HPLC pump, a Waters
2414 refractive index detector, and a combination of Styragel HT-2,
HT-3, and HT-4, the effective molar mass ranges of which are 100−10
000, 500−30 000, and 5000−600 000, respectively. THF was used as
the eluent (flow rate: 1 mL min−1, at 40 °C). Molecular weights and
molecular weight distributions were calculated using polystyrenes as
standards.
2. Synthesis of Yttrium Complexes 1−6. Synthesis of 1. To a
stirred solution of 2-((2,6-dimethylphenylimino)methyl)quinolin-8-ol
(0.145 g, 0.50 mmol) in dried THF (20 mL) at room temperature was
added KH (0.020 g, 0.50 mmol). The mixture was allowed to stir for 2
h, and a red solution was obtained. At −30 °C, YCl3(THF)3 (0.206 g,
0.50 mmol) was added, and the resultant mixture was allowed to stirr
for an additional 24 h at ambient temperature. The residue, obtained
by removing the solvent under vacuum, was extracted with CH2Cl2
(40 mL) plus drops of DMSO. The filtrate was concentrated in vacuo
to reduce the volume to 5 mL, and then Et2O (40 mL) was add to
precipitate a red solid. 1 was obtained as a red solid in 86% yield
CONCLUSION
■
The mononuclear compounds 1−5 and the dinuclear complex
6 bearing 2-((arylimino)alkyl)quinolin-8-olate ligands were
synthesized and fully characterized. Single-crystal X-ray
diffraction analysis established the molecular structures of 1
and 4−6 and demonstrates coordination number 7 around
yttrium. Upon reaction with stoichiometric amounts of
LiCH2Si(CH3)3 (1 equivalent for each chlorine) and of
BnOH (1 equivalent for each yttrium), all of these yttrium
precatalysts were shown to produce active catalysts that are
highly efficient for the ring-opening polymerization of ε-
caprolactone with a living behavior of polymerization. In most
cases, the Mn values of the polymers resulting from the BnOH-
assisted polymerization agreed very closely with the calculated
values Mn,cal based on one Y−OBn, and this result suggests that
the conversions of the precatalysts to the active species are
essentially quantitative.
Theoretical studies of the chlorine/CH2SiMe3 and
Me3SiCH2/BnO ligand exchange reactions suggest that the
replacement of the apical ligands can proceed while leaving the
equatorial ligands largely unaffected. The computed structure of
the dialkyl complex M3 provides a plausible explanation for the
difficulties encountered with the crystallization of the dialkyl
yttrium complexes to date: The structure of M3 impedes
intermolecular arene−arene packing interactions, which are
thought to be important for crystal packing.
1
(0.254 g, 0.43 mmol). H NMR (400 MHz, CDCl3, 25 °C, TMS): δ
3
8.53 (s, 1H; CH), 8.43 (d, J(H,H) = 8.0 Hz, 1H; Qin-H), 7.78 (d,
3
3J(H,H) = 7.8 Hz, 1H; Qin-H), 7.73 (d, J(H,H) = 7.8 Hz, 1H; Qin-
3
H), 7.64 (t, J(H,H) = 7.9 Hz, 1H; Qin-H), 7.29−7.26 (m, 1H; Qin-
H), 7.17 (d, 3J(H,H) = 7.8 Hz, 2H; Ar-H), 7.14−7.10 (m, 1H; Ar-H),
2.64 (s, 6H; CH3), 2.55 (s, 12H; DMSO-CH3). 13C NMR (CDCl3,
100 MHz, 25 °C, TMS): δ 168.0, 166.6, 146.1, 145.8, 142.9, 138.9,
132.9, 131.3, 130.0, 127.8, 125.3, 122.7, 112.2, 109.9, 39.5, 21.0. Anal.
Calcd for C22H27Cl2N2O3S2Y: C, 44.68; H, 4.60; N, 4.74. Found: C,
44.22; H, 4.85; N, 4.31.
Synthesis of 2. Using the same procedure for the synthesis of 1,
replacing the ligand with 2-((2,6-diethylphenylimino)methyl)quinolin-
8-ol (0.152 g, 0.50 mmol), 2 was obtained as a red solid in 76% yield
1
(0.238 g, 0.38 mmol). H NMR (400 MHz, CDCl3, 25 °C, TMS): δ
3
8.35 (s, 1H; CH), 8.29 (d, J(H,H) = 8.3 Hz, 1H; Qin-H), 7.56 (d,
3J(H,H) = 8.3 Hz, 1H; Qin-H), 7.46 (t, 3J(H,H)= 7.9 Hz, 1H; Qin-H),
7.18 (t, 3J(H,H) = 7.5 Hz, 1H; Ar-H), 7.10 (d, 3J(H,H) = 7.5 Hz, 2H;
Qin-H), 6.92 (d, 3J(H,H) = 8.0 Hz, 1H, Ar-H), 6.78 (d, 3J(H,H) = 7.8
Hz, 1H, Ar-H), 2.89−2.80 (m, 4H; CH2), 2.61 (s, 12H, DMSO-CH3),
1.10 (t, 3J(H,H) = 7.8 Hz, 6H; CH3). 13C NMR (CDCl3, 100 MHz, 25
°C, TMS): δ 168.1, 167.0, 150.1, 145.8, 143.2, 139.5, 135.6, 133.4,
The Mn values of the polymers formed agree well with Mn,cal
values calculated based on one Y−CH2Si(CH3)3 bond of the
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131.7, 126.5, 125.6, 123.0, 112.9, 110.4, 40.0, 24.1, 15.6. Anal. Calcd
for C24H31Cl2N2O3S2Y: C, 46.53; H, 5.04; N, 4.52. Found: C, 46.15;
H, 5.01; N, 4.42.
3. ROP of ε-CL. Typical polymerization procedures in the presence
of benzyl alcohol (entry 10, Table 2) are as follows. A toluene solution
of 1 (0.010 mmol, 1.0 mL toluene) and LiCH2Si(CH3)3 (0.020 mmol)
was added into a Schlenk tube in the glovebox at room temperature.
The solution was stirred for 30 min, and then BnOH (0.010 mmol)
and ε-caprolactone (5.0 mmol) along with 3.44 mL of toluene were
added to the solution. The reaction mixture was then placed into a
water bath at the desired temperature (20 °C), and the solution was
stirred for the prescribed time (30 min). The polymerization mixture
was then quenched by addition of an excess of glacial acetic acid (0.2
mL) into the solution, and the resultant solution was then poured into
methanol (200 mL). The resultant polymer was then collected on
filter paper and dried in vacuo. Additional ROP of ε-CL was carried out
by a similar procedure to that described above.
4. Crystal Structure Determinations. Single crystals of 1 and 4−
6 suitable for X-ray structural analysis were grown from dichloro-
methane/n-heptane or THF/n-heptane solutions at room temper-
ature. Single-crystal X-ray diffraction studies for 4 and 5 was carried
out on a Rigaku Saturn 724+ CCD diffractometer with graphite-
monochromated Mo Kα radiation (λ = 0.71073 Å), and those of 1 and
6 were carried out on a Rigaku MM007-HF Saturn 724+CCD
diffractometer with confocal mirror monochromated Mo Kα radiation
(λ = 0.71073 Å). Cell parameters were obtained by global refinement
of the positions of all collected reflections. Intensities are corrected for
Lorentz and polarization effects and empirical absorption. The
structures were solved by direct methods and refined by full-matrix
least-squares on F2. All non-hydrogen atoms were refined anisotropi-
cally. Structure solution and refinement are performed by using the
SHELXL-97 package.33 Crystal data and details are shown in the
Supporting Information.
Synthesis of 3. Using the same procedure for the synthesis of 1,
replacing the ligand with 2-((2,6-dimethyl-4-methylphenylimino)-
methyl)quinolin-8-ol (0.159 g, 0.50 mmol), 3 was obtained as a red
solid in 88% yield (0.279 g, 0.44 mmol). 1H NMR (400 MHz, CDCl3,
3
25 °C, TMS): δ 8.38 (s, 1H; CH), 8.34 (d, J(H,H) = 8.0 Hz, 1H;
3
3
Qin-H), 7.62 (d, J(H,H) = 8.0 Hz, 1H; Qin-H), 7.71 (d, J(H,H) =
3
8.0 Hz, 1H; Qin-H), 7.55 (t, J(H,H) = 7.9 Hz, 1H; Qin-H), 7.29−
7.26 (m, 1H; Qin-H), 6.98 (s, 2H; Ar-H), 3.09−3.04 (m, 2H; CH2),
2.89−2.81 (m, 2H; CH2), 2.64 (s, 12H; DMSO-CH3), 2.29 (s, 3H;
CH3), 1.13−1.27 (m, 6H; CH3). 13C NMR (100 MHz, CDCl3, 25 °C,
TMS): δ 167.9, 166.5, 147.2, 145.4, 142.6, 139.0, 134.9, 134.3, 132.9,
131.2, 126.5, 123.0, 112.4, 110.1, 39.5, 23.6, 21.1, 15.3. Anal. Calcd for
C25H33Cl2N2O3S2Y: C, 47.40; H, 5.25; N, 4.42. Found: C, 47.12; H,
5.08; N, 4.22.
Synthesis of 4. Using the same procedure for the synthesis of 1,
replacing the ligand with 2-(1-(2,6-dimethylphenylimino)ethyl)-
quinolin-8-ol (0.145 g, 0.50 mmol), 4 was obtained as a red solid in
1
80% yield (0.241 g, 0.40 mmol). H NMR (400 MHz, CDCl3, 25 °C,
3
3
TMS): δ 8.53 (d, J(H,H) = 8.3 Hz, 1H; Qin-H), 8.43 (d, J(H,H) =
8.3 Hz, 1H; Qin-H), 8.02 (d, 3J(H,H) = 8.0 Hz, 1H; Qin-H), 7.88 (d,
3J(H,H) = 7.8 Hz, 1H; Qin-H), 7.57 (t, J(H,H) = 7.8 Hz, 1H; Qin-
3
H), 7.12−7.07 (m, 2H; Ar-H), 6.85 (d, 3J(H,H) = 7.7 Hz, 1H; Ar-H),
2.59 (s, 12H; DMSO-CH3), 2.40 (s, 3H; CH3), 2.34 (s, 6H; CH3). 13
C
NMR (100 MHz, CDCl3, 25 °C, TMS): δ 167.7, 167.1, 149.5, 146.8,
143.6, 140.1, 137.5, 133.3, 131.1, 126.9, 123.9, 123.1, 113.5, 110.1,
40.6, 18.4, 17.8. Anal. Calcd for C23H29Cl2N2O3S2Y: C, 45.63; H, 4.83;
N, 4.63. Found: C, 45.41; H, 5.03; N, 4.36.
5. Electronic Structure Computations. The DFT computations
were performed with the quantum-mechanical software Gaussian
0934on the SGI Altix system (BX2 NUMA architecture machine with
64 1.5 GHz Intel Itanium2 processors and 128 GB of shared memory)
of the Research Supporting Computing facility at the University of
Missouri.
Synthesis of 5. Using the same procedure for the synthesis of 1,
replacing the ligand with 2-(1-(2,6-diisopropylphenylimino)ethyl)-
quinolin-8-ol (0.173 g, 0.50 mmol), 5 was obtained as a red solid in
1
84% yield (0.272 g, 0.42 mmol). H NMR (400 MHz, DMSO-d6, 25
°C, TMS): δ 8.68 (d, 3J(H,H) = 8.6 Hz, 1H; Qin-H), 8.21 (d, 3J(H,H)
= 8.5 Hz, 1H; Qin-H), 7.57 (t, 3J(H,H) = 7.9 Hz, 1H; Qin-H), 7.25−
3
7.13 (m, 4H; Qin-H and Ar-H), 6.76 (d, J(H,H) = 7.8 Hz, 1H; Ar-
ASSOCIATED CONTENT
* Supporting Information
3
■
H), 3.03 (sept, J(H,H) = 6.5 Hz, 2H; CH), 2.35 (s, 3H; CH3), 1.18
3
3
S
(d, J(H,H) = 6.4 Hz, 6H; CH3), 1.01 (d, J(H,H) = 6.6 Hz, 6H;
CH3). 13C NMR (DMSO-d6, 100 MHz): δ 173.4, 166.5, 147.0, 145.3,
142.3, 139.6, 139.0, 132.6, 131.0, 125.6, 123.7, 121.1, 112.7, 110.1,
26.9, 24.7, 24.4, 19.9. Anal. Calcd for C27H37Cl2N2O3S2Y: C, 49.02; H,
5.64; N, 4.23. Found: C, 48.65; H, 5.29; N, 4.13.
The crystal data and processing parameters for 1 and 4−6 and
their CIF file giving X-ray crystal structural data. Tables with
computed energies and thermochemical parameters, Cartesian
coordinates of optimized structures, and computed IR spectra.
These materials are available free of charge via the Internet at
http://pubs.acs.org.
Synthesis of 6. To a stirred solution of 2-((2,6-diethyl-4-
methylphenylimino)methyl)quinolin-8-ol (0.318 g, 1.00 mmol) in
dried THF (30 mL) at room temperature was added KH (0.040 g,
1.00 mmol). The mixture was allowed to stir for 2 h, and a red solution
was obtained. At −30 °C, YCl3(THF)3 (0.412 g, 1.00 mmol) was
added, and the resultant mixture was allowed to stir for an additional
24 h at ambient temperature. The residue, obtained by removing the
solvent under vacuum, was extracted with CH2Cl2 (3 × 20 mL). The
combined filtrates were concentrated in vacuo to reduce the volume to
5 mL, and then Et2O (40 mL) was add to precipitate an orange solid
(0.236 g, 0.23 mmol, yield 46%). 1H NMR (400 MHz, CDCl3, 25 °C,
AUTHOR INFORMATION
Corresponding Authors
■
*(R.G.) Tel: +1(573) 882-0331. Fax: +1(573) 882-2754. E-
mail: GlaserR@missouri.edu. (W.-H.S.) Tel: +86(10)
62557955. Fax: +86(10)62618239. E-mail: whsun@iccas.ac.cn.
*
Notes
3
TMS): δ 8.85 (s, 1H; CH), 8.74 (s, 1H; CH), 8.66 (d, J(H,H) = 8.3
The authors declare no competing financial interest.
3
Hz, 1H; Qin-H), 8.60 (d, J(H,H) = 8.3 Hz, 1H; Qin-H), 8.13 (d,
3
3J(H,H) = 8.2 Hz, 1H; Qin-H), 8.07 (d, J(H,H) = 8.2 Hz, 1H; Qin-
ACKNOWLEDGMENTS
This work was supported by NSFC No. 20904059 and the
International Exchange Program No. 21110102017.
H), 7.54 (t, 3J(H,H) = 7.9 Hz, 1H; Qin-H), 7.46 (t, 3J(H,H) = 8.3 Hz,
1H; Qin-H), 7.09−7.6 (m, 2H; Qin-H), 6.99 (s, 2H; Ar-H), 6.97 (s,
■
3
3
2H, Ar-H), 6.72 (d, J(H,H) = 7.8 Hz, 1H; Qin-H), 6.44 (d, J(H,H)
= 7.8 Hz, 1H; Qin-H), 4.08−3.95 (m, 4H; CH2), 3.02−2.95 (m, 2H;
CH2), 2.78−2.64 (m, 4H; CH2), 2.37 (s, 3H; CH3), 2.35 (s, 3H;
CH3), 2.30−2.20 (m, 2H, CH2), 1.91−1.89 (m, 4H; CH2), 1.24−1.13
(m, 12 H; CH3). 13C NMR (CDCl3, 100 MHz): δ 168.0, 167.5, 166.8,
166.4, 147.0, 146.9, 145.8, 145.3, 143.0, 142.7, 139.6, 138.9, 135.0,
134.6, 134.4, 134.2, 133.0, 132.6, 131.2, 131.1, 126.6, 126.0, 123.2,
122.5, 112.4, 112.3, 110.0, 109.9, 55.0, 23.8, 23.5, 21.1, 20.6, 15.3, 14.9,
13.9. Anal. Calcd for C46H50Cl4N4O3Y2: C, 53.82; H, 4.91; N, 5.46.
Found: C, 53.53; H, 4.88; N, 5.13.
DEDICATION
■
Dedicated to Professor Christian Bruneau on the occasion of
his 60th birthday.
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