Synthesis of γ-amidine-functionalized dianionic β-diketiminato lanthanide amides and trianionic β-diketiminato Na/Sm heterobimetallic complexes and their reactivity in polymerization of l-lactide

Article abstract of DOI:10.1039/c3dt52706a

The readily accessible dianionic β-diketiminato lanthanide amido complexes LnLN(SiMe₃)₂(THF) (L = {(2,6-ⁱPr ₂C₆H₃)NC(CH₂)CHC(CH ₃)N(2,6-ⁱPr₂C₆H₃)} ^2-) show an unprecedented reactivity toward carbodiimides. The reaction with N,N′-dicyclohexylcarbodiimide (DCC) led via [4 + 2] cycloaddition to γ-amidine-functionalized dianionic β-diketiminato lanthanide amido complexes, LnL¹N(SiMe₃)₂ (L¹ = {[(NHC₆H₁₁)(NC₆H ₁₁)C]HC[C(CH₂)N(2,6-ⁱPr₂C ₆H₃)]₂}^2-, Ln = Sm(1), Yb(2), Y(3), Gd(4)). Conversion of a mixture of SmLN(SiMe₃)₂(THF) and NaN(SiMe₃)₂ with carbodiimide furnished the heterobimetallic complexes of Sm/Na with a novel amidinate-functionalized trianionic β-diketiminate ligand, [Na(DME)₂](μ-L ²)[SmN(SiMe₃)₂] (L² = {[C(N ⁱPr)₂]HC[C(CH₂)N(2,6-ⁱPr ₂C₆H₃)]₂}^3-, DME = dimethoxyethane) (5) for N,N′-diisopropylcarbodiimide (DIC) and [Na(DME)₃]⁺[SmL³N(SiMe₃) ₂]^- (L³ = {[C(NCy)₂]HC[C(CH ₂)N(2,6-ⁱPr₂C₆H₃)] ₂}^3-) (6) for DCC. Molecular structures of complexes 1-6 were determined by an X-ray single crystal structure analysis. Complexes 1-4 were found to be highly active initiators of the ring-opening polymerization (ROP) of l-lactide (l-LA). The activity depended on the central metal with the increasing sequence of Yb < Y < Gd < Sm. Notably, the binary 1/BnOH (benzyl alcohol) system exhibited an immortal nature and proved able to convert 2000 equivalents of l-LA with up to 100 equivalents of BnOH per initiator. All the polylactides (PLAs) obtained showed monomodal, narrow molar mass distributions (M_w/M_n = 1.08-1.13) with the M _n (average number molar mass) decreasing with increasing amount of BnOH proportionally. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3dt52706a

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PAPER
Synthesis of γ-amidine-functionalized dianionic
β-diketiminato lanthanide amides and trianionic
β-diketiminato Na/Sm heterobimetallic
complexes and their reactivity in polymerization
of L-lactide†
Cite this: Dalton Trans., 2014, 43,
5586
Peng Liu, Hongxia Chen, Yong Zhang, Mingqiang Xue, Yingming Yao and Qi Shen*
The readily accessible dianionic β-diketiminato lanthanide amido complexes LnLN(SiMe3)2(THF) (L =
{(2,6-iPr2C6H3)NC(CH2)CHC(CH3)N(2,6-iPr2C6H3)}2−) show an unprecedented reactivity toward carbo-
diimides. The reaction with N,N’-dicyclohexylcarbodiimide (DCC) led via [4 + 2] cycloaddition to
γ-amidine-functionalized dianionic β-diketiminato lanthanide amido complexes, LnL1N(SiMe3)2 (L1
=
{[(NHC6H11)(NC6H11)C]HC[C(CH2)N(2,6-iPr2C6H3)]2}2−, Ln = Sm(1), Yb(2), Y(3), Gd(4)). Conversion of a
mixture of SmLN(SiMe3)2(THF) and NaN(SiMe3)2 with carbodiimide furnished the heterobimetallic com-
plexes of Sm/Na with a novel amidinate-functionalized trianionic β-diketiminate ligand, [Na(DME)2](µ-L2)-
[SmN(SiMe3)2] (L2 = {[C(NiPr)2]HC[C(CH2)N(2,6-iPr2C6H3)]2}3−, DME = dimethoxyethane) (5) for N,N’-di-
isopropylcarbodiimide (DIC) and [Na(DME)3]+[SmL3N(SiMe3)2]− (L3 = {[C(NCy)2]HC[C(CH2)N(2,6-iPr2C6H3)]2}3−
)
(6) for DCC. Molecular structures of complexes 1–6 were determined by an X-ray single crystal structure
analysis. Complexes 1–4 were found to be highly active initiators of the ring-opening polymerization
(ROP) of L-lactide (L-LA). The activity depended on the central metal with the increasing sequence of
Yb < Y < Gd < Sm. Notably, the binary 1/BnOH (benzyl alcohol) system exhibited an “immortal” nature
and proved able to convert 2000 equivalents of L-LA with up to 100 equivalents of BnOH per initiator.
Received 28th September 2013,
Accepted 3rd January 2014
All the polylactides (PLAs) obtained showed monomodal, narrow molar mass distributions (Mw/Mn
=
DOI: 10.1039/c3dt52706a
1.08–1.13) with the Mn (average number molar mass) decreasing with increasing amount of BnOH
www.rsc.org/dalton
proportionally.
[H(OEt2)2]+[B(C6F5)4]−,2g,i,j N2CHSiMe3,2j and BrCH2CH2Br,2e
and [4 + 2] cycloaddition of the CuC bond of terminal alkynes
Introduction
The dianionic β-diketiminate ligand in its metal complexes as well.2k The striking reactivity makes them very useful in
has been found to have a dipolar character,1 which leads to a organometallic chemistry as readily available organometallic
variety of remarkable transformations with various substrates. intermediates.
Examples include activation of N–H bonds of ammonia and/or
Recent investigations revealed that dianionic β-diketimina-
the secondary amine HN(C6F5)2,1c,2a–c nucleophilic addition with tolanthanide complexes are facilely prepared by either
several reagents, such as RH (R = OH, PhO, C6F5O, PhCO2,1c,2d reduction of certain β-diketiminato ytterbium complexes3 or
or OTf1b), Me3SiOTf,2e–g M–Cl (M = Si or Ge),2h B(C6F5)3, deprotonation by the use of an external strong base4 and self-
deprotonation of the β-diketiminate ligand in sterically hin-
dered lanthanide complexes.5 Moreover, the dianionic β-diketi-
minatothulium complex was reported to be reactive and its
protonation can be achieved by using an appropriate
ammonium tetraarylborate, leading to the formation of salts
containing a homoleptic bis-β-diketiminatothulium cation.5a
More recently, we have succeeded in the synthesis
of a series of dianionic β-diketiminatolanthanide amides
LnLN(SiMe3)2(THF) by the reaction of β-diketiminatolanthanide
Key Laboratory of Organic Synthesis of Jiangsu Province, Department of Chemistry
and Chemical Engineering, Dushu Lake Campus, Soochow University,
Suzhou 215123, People’s Republic of China. E-mail: qshen@suda.edu.cn;
Fax: +86-512-65880305
†Electronic supplementary information (ESI) available. CCDC 976986–976990.
For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt52706a
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dichlorides with NaN(SiMe3)2 via sterically induced deprotona-
Complex 1 is very sensitive to air and moisture. It gave a sat-
tion, and a cationic β-diketiminatosamarium amide via proto- isfactory elemental analysis, consistent with the formula. IR
nation of SmLN(SiMe3)2(THF) by [HNEt3][BPh4].6a We have spectra show the band at 1646 cm−1 assigned to the group of
also demonstrated that SmLN(SiMe3)2(THF) is highly reactive NvC from the CyNCNCy. However, the resolved 1H NMR spec-
towards aromatic nitrile and ketene imine. The reaction with trum could not be obtained as the paramagnetic Sm(III). The
C6H5CN (4-CH3OC6H4CN), and (C6H5)2CvCvNtBu, respect- identity of complex 1 was further confirmed by an X-ray struc-
ively, resulted in the formation of the corresponding modified ture determination.
ˉ
dianionic β-diketiminatosamarium amides. In these com-
Complex 1 crystallizes in triclinic P1. The molecular struc-
plexes the skeleton of the original dianionic β-diketiminato ture is shown in Fig. 1, and the selected bond lengths and
ligand is entirely modified.6b The peculiar reactivity of SmLN- angles are listed in Table 1. Complex 1 consists of a bicyclo-
(SiMe3)2(THF) prompted us to probe the application of LnLN- [2,2,2]octane-like core with a LnC5N2 skeleton, and the γ-C
(SiMe3)2(THF) as useful precursors in organolanthanide atom and Sm metal as bridgehead atoms. The center Sm atom
chemistry.
is bound to three nitrogen atoms from the newly formed
Carbodiimides are important substrates for metal-based ligand L1 and one –N(SiMe3)2 group. Thus the original dianio-
reactivity studies. Various reaction modes, such as insertion nic β-diketiminate ligand L has undergone modification and
reaction into Ln–N and Ln–C bonds, reductive coupling reac-
tion etc., were realized upon the metal complexes and carbodi-
imide used, which result in the formation of metal complexes
with various skeletons.7 Herein we report the reactions of
LnLN(SiMe3)2(THF) with carbodiimides that show a novel reac-
tion mode, distinguishing from that found in the cases with
nitrile and ketene imine reported,6b and the formation of both
unpredictable amides 1–4 and Na/Sm heterobimetallic com-
plexes 5 and 6. The reactivity of complexes 1–4 in ROP of L-LA
and an “immortal” polymerization with complex 1 and BnOH
are also described.
Results and discussion
Reaction of LnLN(SiMe3)2(THF) with N,N′-
dicyclohexylcarbodiimide(DCC): synthesis and molecular
structure of complexes 1–4
Addition of a hexane solution of DCC into a red n-hexane solu-
tion of SmLN(SiMe3)2(THF)6a in a molar ratio of 1 : 1 at room
Fig. 1 ORTEP diagram of complexes 1–4 showing atom-numbering
scheme.
temperature resulted in a rapid color change to pale yellow.
After workup, yellow crystals were isolated in 60% yield upon
crystallization from n-hexane solution. The crystals were fully
characterized to be complex 1 (Scheme 1).
Table 1 Selected bond lengths (Å) and bond angles (°) for complexes
1–4
Bond lengths
1
2
3
4
Ln1–N1
Ln1–N2
Ln1–N3
Ln1–N4
N1–C2
N2–C4
C1–C2
C2–C3
C3–C4
C4–C5
C3–C36
C36–N4
C36–N5
N4–C36–N5
N1–Ln1–N2
N1–Ln1–N4
N2–Ln1–N4
2.388(3)
2.251(4)
2.308(4)
2.364(4)
1.338(5)
1.383(5)
1.399(6)
1.539(5)
1.533(6)
1.334(6)
1.543(6)
1.335(6)
1.328(5)
123.6(4)
88.76(12)
78.27(11)
85.44(13)
2.184(3)
2.209(3)
2.193(3)
2.314(3)
1.388(4)
1.391(4)
1.340(5)
1.538(5)
1.548(5)
1.333(5)
1.544(5)
1.304(4)
1.351(4)
124.0(3)
95.39(11)
83.63(10)
87.59(10)
2.248(2)
2.209(2)
2.237(2)
2.352(2)
1.384(3)
1.380(3)
1.334(4)
1.543(3)
1.544(3)
1.340(3)
1.536(3)
1.307(3)
1.354(3)
123.3(2)
93.59(7)
81.68(7)
86.57(7)
2.284 (2)
2.239(2)
2.275(2)
2.3811(2)
1.380(3)
1.381(3)
1.344(3)
1.545(3)
1.542(3)
1.343(3)
1.538(3)
1.311(3)
1.345(3)
123.37(2)
92.64(6)
80.87(6)
85.57(6)
Scheme 1 The synthesis of complexes 1–4.
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the Sm and the γ-C atom are now in a non-planar LnN2C3 six- obtained, as paramagnetic Yb(III) and Gd(III). The identity of
membered ring. The four-coordinate Sm atom adopts a dis- complexes 2–4 was confirmed by an X-ray structural determi-
torted tetrahedral geometry.
nation. Complex 2 isolated as a solvate 2·0.5(n-hexane). Com-
ˉ
The bonds of C1–C2 (1.399(6) Å) and C4–C5 (1.334(6) Å) plexes 2–4 all crystallize in triclinic P1. The molecular
both belong to a double bond, whereas the bonds of C2–C3 structures of complexes 2–4 are shown in Fig. 1 as they and
(1.539(5) Å) and C3–C4 (1.533(6) Å) are typical of single bonds, complex 1 are isostructural. The selected bond lengths and
indicating the newly formed L1 is an amidine-functionalized angles are listed in Table 1.
dianionic β-diketiminate ligand rather than an amidinate-
In each complex the bonds of C1–C2 and C4–C5 both
functionalized monoanionic β-diketiminate ligand. Thus, the belong to a double bond whereas the bonds of C2–C3 and C3–
Sm1–N4 bond is a dative one. The Sm1–N2 bond length of C4 are typical of single bonds (Table 1). These bond para-
2.251(4)
Å
is comparable to that found in SmLN- meters demonstrate that the formed new ligand L1 in 2–4 is a
(SiMe3)2(THF).6a The Sm1–N1 bond (2.388(3) Å) is much modified dianionic β-diketiminate ligand as in the case of
longer, even 0.024 Å longer than the dative Sm1–N4 bond. complex 1. The bond lengths of N1–C2 and N2–C4 in 2–4
This may be because of the existence of the sterically hindered range from 1.380–1.391 Å indicating a delocalized CH2–C–N
amidine group at the γ-C atom. The bond length of C3–C36 unit. The delocalized property of the chemical bonds in both
(1.543(6) Å) is typical of a single bond, whereas the C–N bond of the CH2vC–N units led to the observation of two singlets
1
lengths (N4–C36, 1.335(6) Å; N5–C36, 1.328(5) Å) in the NCN for the protons of CH2 groups in the H NMR spectrum of 3.
unit are shorter and almost equal, indicating a typically The bond lengths of Ln1–N1 (2.207(3) Å for 2, 2.248(2) Å for 3
delocalized amidino fragment. The Sm1–N3 (2.308(4) Å) and 2.2841(17) Å for 4) and Ln1–N2 (2.176(3) Å for 2, 2.209(2)
bond length is comparable with that in the parent complex Å for 3 and 2.2749(17) Å for 4) are well comparable with each
SmLN(SiMe3)2(THF).6a
other and the values can also be compared to those of their
The formation of complex 1 is somewhat surprising. The parent complexes reported.6a The bond lengths of Ln1–N4 in
unpredictable complex 1 may be formed by [4 + 2] cyclo- complexes 2–4 are in the range of a dative bond and are con-
addition reaction of SmLN(SiMe3)2(THF) with DCC occurred at sistent with the difference in ionic radii of the center metals.
γ-C atom, followed by H-shift (Scheme 1). To the best of our
However, some definitely unusual bond parameters for 1
knowledge the present reaction mode has not been found in (Ln = Sm), compared to 2–4 (Ln = Yb, Y and Gd) were noticed.
the reaction chemistry of a dianionic β-diketiminate lantha- For example, the Ln1–N4 bond in 1 is about 0.045 Å shorter,
nide derivative with an unsaturated small molecule. The [4 + 2] implying a stronger donor character for N4 in 1. The latter is
cycloaddition may be promoted by the electrophilic character consistent with pronounced elongation of the N4–C36 bond
of the lanthanide metal center and the nucleophilic character (1.335(6) Å) compared to those of the N4–C36 bond in 2–4
of the endocyclic γ-C atom in the LnN2C3 ring.
(1.304(4) Å in 2; 1.307(3) Å in 3; 1.311(3) Å in 4). The C1–C2
Complex 1 represents the first example of γ-C modified di- bond is about 0.05 Å longer and the N1–C2 bond is 0.05 Å
anionic β-diketiminate stabilized lanthanide derivative formed shorter than those found in 2–4.
by a direct reaction of a lanthanide metal complex with an
The bond lengths of Ln1–N3 are 2.193(3) Å for 2, 2.237(2) Å
organic molecule. To see the generality of the procedure, the for 3 and 2.2749(17) Å for 4, which are consistent with that in
same reaction with the analogous Yb, Y and Gd complexes 1, when the differences in ionic radius among these metals are
were conducted. All the reactions went smoothly and the color considered. The values are slightly longer than that in the
change from dark green to purple for Yb and from orange to parent LnLN(SiMe3)2(THF).6a This may be because of the
yellow for Y and Gd was observed. From these resulting solu- bulkier ligand L1 in complexes 2–4 compared to the L ligand
tions, the analogous complexes LnL1N(SiMe3)2 (Ln = Yb(2), in LnLN(SiMe3)2(THF).
Y(3), Gd(4)) were isolated as purple crystals for complex 2,
yellow crystals for complexes 3 and 4 in moderate yields (70% the reaction of complex 1 with [HNEt3][BPh4], as the case with
for 2, 62% for 3, 60% for 4) (Scheme 1).
its parent complex,6a was not successful and complex 1 was
An attempt to prepare a corresponding cationic complex by
All the bulk materials gave satisfactory elemental analyses. recovered completely, reflecting complex 1 is not as active as
The room-temperature 1H NMR spectrum of complex 3 in its parent complex.
C6D6 showed that the resonances of the four methine groups
Reaction of a mixture of SmLN(SiMe3)2(THF) and NaN(SiMe3)2
with carbodiimide: synthesis and molecular structures of
complexes 5 and 6
from the isopropyl groups on the phenyl rings were two
septets, suggesting restricted rotation about the N-dianionic
β-diketiminate bond. The existence of two methylene groups
on the ligand backbone is confirmed by the spectroscopic It was unexpected that treatment of the n-hexane solution of
observation of two singlets at 4.04 and 3.49 ppm. The two sing- SmLN(SiMe3)2(THF) with an equivalent of N,N′-diisopropyl-
lets, not the geminal doublets, observed for the protons of carbodiimide (DIC) solution in n-hexane at room temperature
CH2 groups may be attributed to the delocalized property of did not result in an isolable solid state, but instead a tar
the chemical bonds in both of the CH2vC–N units as con- was obtained. Thus, the difficulty was met in the further
firmed by the bond parameters determined by X-ray structural purification and characterization of the product. To see
1
analysis. No resolved H NMR spectrum for 2 and 4 could be whether an addition of NaN(SiMe3)2 into the reaction of
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Scheme 2 The synthesis of complexes 5 and 6.
SmLN(SiMe3)2(THF) with DIC could catch the “product” of [4 + 2]
cycloaddition, the reaction of a mixture of SmLN(SiMe3)2(THF)
and NaN(SiMe3)2 with DIC at a 1 : 1 molar ratio was then con-
ducted at room temperature. Indeed, the reaction afforded
yellow crystals in 30% yield upon crystallization from a
mixture of DME and hexane. The crystals were characterized to
be the heterobimetallic complex 5 (Scheme 2). The rather low
yield of complex 5 could be attributed to the high solubility of
complex 5 even in n-hexane. Complex 5 is sensitive to air and
moisture, but is thermally stable. The molecular structure of
complex 5 was determined by an X-ray crystal structure analysis.
Single crystals of complex 5 suitable for X-ray diffraction
analysis were obtained by recrystallization from a mixture of
Fig. 2 ORTEP diagram of complex
5
showing atom-numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level.
Hydrogen atoms and iPr substituents of L2 are omitted for clarity.
Selected bond lengths (Å)and angles (°): Sm(1)–N(1) 2.299(8), Sm1–N2
2.270(8), Sm1–N3 2.315(7), Sm1–N4 2.317(8), Na1–N5 2.458(9), N1–C2
1.377(12), C2–C3 1.563(13), C3–C4 1.552(13), C4–N2 1.385(11), C4–C5
1.350(13), C3–C30 1.566(13), C30–N4 1.354(11), C30–N5 1.315(12), N1–
Sm1–N2 92.8(3), N1–Sm1–N3 84.7(3), N2–Sm1–N3 88.6(3), O1–Na1–
O3 146.7(3), O2–Na1–N5 135.5(3), N5–Na1–O4 128.6(3), O2–Na1–O4
92.0(3).
DME and n-hexane at −10 °C. The molecular structure of anionic N3CN4 fragment. Thus, the newly formed anionic ami-
complex 5 is shown in Fig. 2 with the selected bond lengths dinate group coordinates to both metals of Sm and Na as a
and angles. Complex 5 is an anionic heterobimetallic complex bidentate species.
consisting of a bicyclo[2,2,2]octane-like core with a LnC5N2
The Na atom is ligated by one nitrogen atom of the ligand
skeleton and a Na ion coordinated to the formed amidinate L2 and four oxygen atoms from the two DME molecules. The
group. The Sm atom and the γ-C atom locate at bridgeheads of Na atom is five coordinate and adopts a distorted-trigonal–
the bicycle. The central Sm atom is coordinated by three nitro- bipyramidal geometry.
gen atoms from the newly formed trianionic ligand L2 and one
Replacement of DIC by DCC in the above reaction led to the
–N(SiMe3)2 group. The coordinated geometry around the four- color change from red to yellow. However, crystallization from
coordinated Sm atom can be described as a distorted tetrahe- the resulting solution did not give the analogue, but a discrete
dron. The lengths of Sm1–N1 (2.299(8) Å) and Sm1–N2 bonds ion-pair complex 6 (Scheme 2). The molecular structure of
(2.270(8) Å) are almost consistent with the corresponding complex 6 was determined by X-ray crystallography technique.
values in SmLN(SiMe3)2(THF).6b The lengths of C1–C2 and C4– The skeleton of the solid state structure of complex 6 can be
C5 bonds are 1.343(13) Å and 1.350(13) Å, respectively, indicat- clearly observed as shown in Fig. 3. The detailed bond para-
ing that both are double bonds. The C2–C3 (1.563(13) Å) and meters could not be obtained because of the poor quality of
C3–C4 (1.552(13) Å) bonds both belong to single bonds. The the crystals. Complex 6 consists of an disconnected anion
length of the Sm1–N3 bond (2.315(7) Å) is almost consistent [L3SmN(SiMe3)2]− and a cation [Na(DME)3]+. The center Sm
with that of the Sm1–N1 bond. These bond parameters indi- atom in the anion is coordinated by one L3 and one amido
cate that L2 is a trianionic ligand. The length of the Na1–N4 group being four coordinate. The coordinate geometry can be
bond (2.458(9) Å) is much longer than those found in sodium described as a distorted tetrahedron, which is similar to that of
amides reported and that of the Sm1–N3 bond, implying that complex 6. Efforts to isolate fine single-crystals were unsuccessful.
the Na1–N4 bond is a dative one. The C3–C30 bond (1.566(13)
ROP of L-LA by complexes 1–4
Å) is typical of a single bond. The N4–C30 bond is shorter than
a C–N single bond, whereas the N3–C30 bond is longer than a
Polyesters, such as poly(lactide), poly(glucol), poly(ε-caprolac-
CvN double bond, indicating a significantly π-delocalized tone), and related copolymers, have wide applications in the
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a stationary spectator ligand (L1). Thus, complexes 1–4 were
evaluated for the ROP of L-LA in toluene at room temperature
with a molar ratio of monomer to initiator of 2000 ([M]0
=
1.0 M). Representative results are summarized in Table 2. Of
the catalysts investigated, complex 1 (entry 3) was the most
effective with a 90% yield of PLA within 1.5 min. The resulting
PLA had the Mn as high as 25.7 × 104 with Mw/Mn of 1.37.
Complex 4 was also effective, but was less active than 1 (entry
10). Complex 2 showed the lowest activity and a 10% yield of
PLA was obtained (entry 4). Even with elongation of the
polymerization time to 60 min, only 47% yield could be
obtained (entry 6). The polymerization with 3 gave a 25% yield
within 1.5 min and a 96% yield after 60 min (entries 7–9). The
active sequence of 2 < 3 < 4 < 1 observed here is consistent
with the increasing order of ion radii of metals. Such an active
order is often observed in the systems with lanthanide metal
initiators.10 Polymerization in THF is less active than that in
toluene (Table 2, entry 15). This may be because of the com-
petitive coordination of THF.
Fig. 3 Plot of ln([M]0/[M]) versus time for the polymerization of L-LA by
1. Conditions: [M]0/[I]0 = 3000, [L-LA]0 = 1.0 M, in toluene, 25 °C.
medical field as biodegradable surgical sutures or as a delivery
Complexes 5 and 6 are much less active than complex 1
medium for controlled release of drugs due to their biodegrad- (entries 11–14).
able, biocompatible, and permeable properties.8 The ROP of
The polymerization of L-LA with complex 1 proceeded
cyclic esters promoted notably (but not only) by single-site smoothly in toluene at the molar ratios of monomer-to-
metal-based initiators/catalysts, is definitely the most useful complex 1 ranging from 1000 to 3000 (entries 1–3, 16–19). The
route for the synthesis of aliphatic polyesters with predictable Mn(obsd) of the resulting PLAs increased almost linearly with
molecular weight and low polydispersity index.9
the ratio and were in close agreement with the Mn(calcd)
The complexes 1–4 present new candidates for studying the values calculated for one chain growing per metal center.
ROP of cyclic esters. This is because each of them contains a All PLAs obtained in the present polymerizations showed a
highly electropositive metal, an active group of –N(SiMe3)2 and unimodal molar mass distribution as revealed by SEC (size
Table 2 Polymerization of L-LA by complexes 1–6a
Mn (calcd)c
Mn (obsd)d
Run
Initiator
[M]0/[I]0/[BnOH]
t (min)
Yieldb (%)
(104 g mol−1
)
(104 g mol−1
)
PDI
1
2
3
4
5
6
7
8
1
1
1
2
2
2
3
3
3
4
5
5
6
6
1
1
1
1
1
1
1
1
1
1
1
1000 : 1 : 0
1500 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2000 : 1 : 0
2500 : 1 : 0
2500 : 1 : 0
3000 : 1 : 0
3000 : 1 : 0
2000 : 1 : 2
2000 : 1 : 5
2000 : 1 : 10
2000 : 1 : 20
2000 : 1 : 50
2000 : 1 : 100
1.5
1.5
1.5
1.5
30
60
1.5
30
60
1.5
1.5
20
1.5
10
1.5
1.5
30
3
95
95
90
10
20
47
25
75
96
70
—
25
—
33
78
60
96
30
80
98
99
99
98
98
96
13.7
20.5
25.9
2.88
5.77
13.5
7.21
21.6
27.7
20.2
—
15.3
22.3
25.7
3.46
6.80
12.3
6.89
18.7
30.2
20.8
—
1.45
1.38
1.37
1.46
1.39
1.42
1.43
1.30
1.42
1.42
—
9
10
11
12
13
14
15e
16
17
18
19
20f
21f
22f
23f
24f
25f
7.21
—
5.89
—
1.35
—
9.51
22.5
21.6
34.6
13.0
34.6
14.1
5.71
2.85
1.41
0.57
0.28
7.42
21.2
20.1
31.6
15.7
32.7
14.8
5.30
2.94
1.36
0.59
0.27
1.45
1.46
1.41
1.42
1.43
1.33
1.13
1.12
1.08
1.13
1.09
1.11
50
1.5
3
15
20
30
30
a General polymerization conditions: in toluene, [LLA] = 1 mol L−1. T = 25 °C. b Yield: weight of polymer obtained/weight of monomer used. c Mn
value calculated from the relation: molecular weight of M × conv. × [M]/[Ln]. d Determined by GPC against polystyrene standards in THF,
multiplied by 0.58.12 e THF as solvent. f [LLA] = 2 mol L−1, time is not optimized.
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exclusion chromatography) calibration and had moderate a chain-transfer agent, detailed polymerizations with complex
molar mass distributions (Mw/Mn 1.37–1.46, entries 1–3 and 1 and BnOH were conducted. Representative results are sum-
16–19); inflecting the polymerization with complex 1 has a marized in Table 2. Immortal polymerization with complex 1
limited transesterification reaction.
and BnOH could be realized at room temperature (Table 2,
It should be noted that complex 1 is much more active than entries 20–25). Remarkably, the binary 1/BnOH system was
the previously reported monoanionic β-diketiminatolantha- able to almost quantitatively convert 2000 equivalents of L-LA
nide complexes.11 This is probably due to the more open with up to 100 equivalents of BnOH per metal initiator. The
coordination sphere around the center metal by the L1 ligand obtained PLAs at various BnOH/1 ratios (from 2 to 100)
compared to those by the two mixture ligands of monoanionic showed monomodal, narrow molar mass distributions (Mw/Mn
β-diketiminato and an another ancillary ligand, and/or by two = 1.08–1.13) with Mn values decreasing proportionally with
monoanionic β-diketiminato ligands.11
increasing amounts of BnOH. Thus, the amidino-modified
The polymerization kinetics of L-LA with complex 1 at a dianionic β-diketiminato ligand L1 appears to be a good ligand
molar ratio of monomer to 1 of 3000 was further investigated for stabilizing the oxophilic samarium center, preventing ir-
at room temperature in toluene. The conversion increases with reversible decomposition in the presence of a large excess of
time, and a first order dependence on the monomer concen- free alcohol, thereby initiating a living chain transfer polymeri-
tration was observed (Fig. 3). The Mn of the polymers increased zation to give the PLAs with a chain end caped by the OH
linearly with the monomer conversion, while the values of Mw/ group.
Mn remained relatively narrow (Mw/Mn = 1.31–1.43) (Fig. 4),
The end-group of the resulting PLA was measured by 1H
which demonstrated that the polymerization in this system NMR. The results (see ESI†) revealed that the resulting PLA
occurs in a living manner. The end groups analysis of the macromolecular chain is capped with the OH group at one
oligomer of L-LA, which was prepared from the polymerization end, and with the PhCH2O– group at the other end (PhCH2O–
of L-LA using complex 1 in a 1/10 molar ratio and quenched by is overlapped by the methyl group in the polymer chain at
wet n-hexane, were determined by 1H NMR. The 1H NMR spec- about 5.12 ppm and at 2.62 ppm for the OH group).
trum (see ESI†) exhibited the quartet characteristic of a –CH-
(Me)OH terminal group at δ 4.35 ppm. The –CH(Me)OH group
was formed by hydrolysis of the metal–silylamido bond. An
Conclusion
additional resonance at δ 4.18 ppm assigned to the CH2OH
end group12 was also observed. The results indicate that the Reaction of LnLN(SiMe3)2(THF) with DCC led to the formation
present polymerization could be supposed to proceed by a of complexes 1–4 with a γ-amidine-functionalized dianionic
coordination insertion mechanism.
β-diketiminato ligand, which represents a novel reaction mode
The polymerization with complex 1 in the presence of two of LLnN(SiMe3)2(THF) in reaction with unsaturated substrates.
equivalents of BnOH gave the PLA with narrower molar mass The heterobimetallic complexes 5 and 6 with a γ-amidinate-
distribution (Mw/Mn 1.13, entry 20, compared to that obtained functionalized trianionic β-diketiminato ligand could also be
by the system without BnOH Mw/Mn 1.37, entry 3) and the Mn accessed by reaction of a mixture of SmLN(SiMe3)2(THF) and
close to the calculated one, indicating the binary system of NaN(SiMe3)2 with carbodiimides. Complexes 1–4 were highly
1/BnOH had a living character.
active in the ROP of L-LA. Notably, “immortal” polymerization
To address the possibility of achieving immortal polymeri- could be achieved using a binary system of complex 1 and
zation using these systems by introducing several equivalents of BnOH, demonstrating the modified L1 ligand was suitable for
designing well defined single-site lanthanide catalyst.
Experimental section
General procedures
All manipulations were performed under a purified argon
atmosphere using standard Schlenk techniques. Solvents were
degassed and distilled from sodium benzophenone ketyl
before use. Anhydrous LnCl3 was prepared according to the lit-
erature procedure.13 Complexes LnLN(SiMe3)2(THF) were pre-
pared by the methods reported.6a Elemental analyses were
performed by direct combustion using a Carlo-Erba EA 1110
instrument. The IR spectra were recorded on a Nicolet-550
FT-IR spectrometer as KBr pellets. 1H NMR and 13C NMR
spectra were recorded on an INOVA-400 MHz apparatus and
processed using MestReNova software. The number-average
molar masses (Mn) and molar mass distributions (Mw/Mn)
Fig. 4 Relationship between conversion and Mn and Mw/Mn for the
polymerization of L-LA by 1. For the conditions, see Fig. 3.
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were determined against a polystyrene standard by size exclu- Found: C, 61.79; H, 8.19; N, 7.28%. IR (KBr, cm−1): 3445(s),
sion chromatography (SEC) on a PL-50 apparatus, and THF 2961(s), 2929(vs), 2854(m), 2360(m), 1645 (s), 1546(m), 1514(m),
was used as an eluent at a flow rate of 1.0 mL min−1 at 40 °C.
1457(w), 1381(m), 1159(w), 934(w), 763(w), 680(m).
[Na(DME)2]{µ-L2}[SmN(SiMe3)2] (5). Into a stirred solution
of SmLN(SiMe3)2(THF) (0.89 g, 1.00 mmol) was added a
Synthesis of complexes (1–6)
SmL1N(SiMe3)2 (1). Into a stirred red solution of SmLN- toluene solution of NaN(SiMe3)2 (5 mL, 1.00 mmol) at room
(SiMe3)2(THF) (0.89 g, 1.00 mmol) in n-hexane (30 mL) was temperature. The mixture was stirred for two hours. Into this
added dropwise a THF solution of DCC (2 mL, 1.00 mmol) at reaction solution was added a toluene solution of DIC (2 mL,
room temperature. During this period, the color of the reaction 1.00 mmol). During this period, the color of the reaction
mixture changed from red to yellow. The reaction mixture was mixture was observed to change from red to yellow. The reac-
stirred for another four hours. The solvent was partly removed tion mixture was stirred for another four hours. Removing the
under vacuum, and the remaining solution was kept at room solution by centrifugation to dry and extracting the yellow resi-
temperature for crystallization. The yellow crystals of 1 suitable dues with n-hexane led to a yellow solution. Into this solution
for X-ray crystal structure analysis were obtained after several was added a small amount of DME for crystallization at
days (0.56 g, 60%). Anal. calcd for C48H79N5Si2Sm (933.70): C, −10 °C. Complex 5 was isolated as yellow crystals after several
61.81; H, 8.54; N, 7.51. Found: C, 62.00; H, 8.26; N, 7.42%. IR days. Yield: 0.32 g (30%). Anal. calcd for C50H91N5NaO4Si2Sm
(KBr, cm−1): 3446(m), 2961(w), 2929(m), 2350(vs), 1646(m), (1055.8): C, 56.83; H, 8.62; N, 6.63. Found: C, 57.00; H, 8.36; N,
1546(m), 1514(m), 1457(w), 1381(m), 1159(w), 934(w), 762(w), 6.42%. IR (KBr, cm−1): 2960(m), 2878(w), 2180(m), 1627(m),
680(m).
1545(m), 1378(m), 1221(vs), 1164(vs), 927(w), 759(w), 631(m),
YbL1N(SiMe3)2·0.5(n-hexane) (2·0.5(n-hexane)). The syn- 508(m).
thesis of complex 2·0.5(n-hexane) was carried out in the same
[Na(DME)3]+[SmL3N(SiMe3)2]− (6). Into a stirred solution of
way as that described for complex 1; but YbLN(SiMe3)2(THF) SmLN(SiMe3)2(THF) (0.89 g, 1.00 mmol) was added a toluene
(0.92 g, 1.00 mmol) and DCC (2 mL, 1.00 mmol) were used. solution of NaN(SiMe3)2 (5 mL, 1.00 mmol) at room tempera-
Violet crystals of 2 suitable for X-ray crystal structure analysis ture. The mixture was stirred for two hours, then a toluene
were obtained at room temperature (0.68 g, 70%). Anal. calcd solution of DCC (2 mL, 1.00 mmol) was added. During this
for C51H87N5Si2Yb (999.48): C, 61.23; H, 8.71; N, 7.01. Found: period, the color of the reaction mixture changed from red to
C, 61.08; H, 8.60; N, 7.02. IR (KBr, cm−1): 3446(s), 2960(s), yellow. The reaction mixture was stirred for another four
2929(vs), 2854(m), 2360(m), 1646 (s), 1546(m), 1514(m), hours. Removing the solution by centrifugation led to yellow
1457(w), 1381(m), 1159(w), 934(w), 762(w), 680(m).
solids. The solids were extracted with n-hexane and a small
YL1N(SiMe3)2 (3). The synthesis of complex 3 was carried amount of DME was then added. The solution was kept at
out in the same way as that described for complex 1; but YLN- −10 °C for crystallization. Complex 6 was isolated as yellow
(SiMe3)2(THF) (0.83 g, 1.00 mmol) and DCC (2 mL, 1.00 mmol) crystals after several days. Yield: 0.44 g (36%). C54H101N5NaO6-
were used. Yellow crystals of 3 suitable for X-ray crystal struc- Si2Sm (1226.054): C, 52.90; H, 8.30; N, 5.71. Found: C, 53.40;
ture analysis were obtained from n-hexane (0.54 g, 62%). 1H H, 8.42; N, 5.52%. IR (KBr, cm−1): 2960(m), 2878(w), 2180(m),
NMR (400 MHz, C6D6): δ 7.48–7.54 (m, 6 H, 2,6-iPr2C6H3), 4.78 1626(m), 1545(m), 1378(m), 1220(vs), 1164(vs), 928(w), 759(w),
3
(s, 1 H, γ-CH), 4.45–4.46 (d, JHH = 4 Hz, 1 H, –NH), 3.80 (s, 2 631(m), 509(m).
H, NCCH2), 3.60–3.67 (m, 3 H, CHMe2+–CH of Cp), 3.25 (s, 2
General procedure for the polymerization of L-LA
H, NCCH2), 3.17 (sept, 3JHH = 7 Hz, 2 H, CHMe2), 3.00 (m, 1 H,
–CH of Cp), 1.67–1.84 (m, 6 H, CH2 of Cp), 1.56–1.57 (d, 3JHH
=
The procedures for the polymerization of L-LA initiated by
3
4 Hz, 6 H, CHMe2), 1.52–1.54 (d, JHH = 4 Hz, 6 H, CHMe2), complexes 1–4 are similar, and a typical procedure is given
3
3
1.39–1.41 (d, JHH = 4 Hz, 6 H, CHMe2), 1.37–1.38 (d, JHH
=
below. To a stirred solution of L-LA in toluene was added a
4 Hz, 6 H, CHMe2), 1.15–1.23 (m, 6 H, CH2 of Cp), 0.89–1.07 toluene solution of the initiator using a syringe. The polymeri-
(m, 6 H, CH2 of Cp), 0.03 (s, 18 H, SiMe3); 13C NMR (101 MHz, zation mixture was stirred for a definite time at the desired
C6D6): 163.5, 155.5, 154.7, 153.0, 147.3, 144.9, 124.9, 124.8, temperature and then quenched with an ethanol solution con-
124.7, 84.8, 57.0, 56.1, 52.6, 33.8, 30.0, 28.1, 27.0, 26.8, 26.6, taining a small amount of hydrochloric acid (5%). The
26.5, 25.9, 25.6, 25.3, 24.7, 24.3. Anal. calcd for C48H79N5Si2Y polymer was precipitated from ethanol and washed with
(872.26): C, 66.17; H, 9.14; N, 8.04. Found: C, 65.98; H, 9.12; N, ethanol three times and dried under a vacuum and weighed.
8.10. IR (KBr, cm−1): 3446(s), 2960(s), 2929(vs), 2854(m),
A typical procedure for immoral polymerization is given below
2360(m), 1646 (s), 1546(m), 1513(m), 1457(w), 1380(m), 1159(w),
934(w), 762(w), 681(m).
To a stirred solution of L-LA and BuOH in toluene was added a
GdL1N(SiMe3)2 (4). The synthesis of complex 4 was carried toluene solution of the initiator using a syringe. The polymeri-
out in the same way as that described for complex 1; but GdLN- zation mixture was stirred for a definite time at the desired
(SiMe3)2(THF) (0.90 g, 1.00 mmol) and DCC (2 mL, 1.00 mmol) temperature and then quenched with an ethanol solution con-
were used. Yellow crystals of 4 suitable for X-ray crystal struc- taining a small amount of hydrochloric acid (5%). The solvent
ture analysis were obtained from n-hexane (0.56 g, 60%). Anal. was removed under vacuum and the polymer was dried under
calcd for C48H79N5Si2Gd (940.60): C, 61.36; H, 8.48; N, 7.45. vacuum and weighed.
5592 | Dalton Trans., 2014, 43, 5586–5594
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X-Ray crystallographic structure determinations
Chem., 2009, 48, 7645; (k) S. S. Yao, C. Wüllen van and
M. Driess, Chem. Commun., 2008, 5393.
Crystals of complexes 1–6 suitable for X-ray diffraction were
sealed in a thin-walled glass capillary filled under argon. Dif-
fraction data were collected on an Agilent Xcalibur CCD area
detector in the ω scan mode using Mo Kα radiation (λ =
0.71070 Å) for complexes 2–4 and on a Rigaku Saturn CCD
area detector in the ω scan mode using Mo Kα radiation (λ =
0.71075 Å) for complexes 1, 5 and 6. The diffracted intensities
were corrected for Lorentz-polarization effects and empirical
absorption corrections. Collection and crystal data are given in
ESI.† The structures were solved by direct methods and refined
by full-matrix least-squares procedures based on |F|2. All of the
non-hydrogen atoms were refined with anisotropic displace-
ment coefficients. The hydrogen atoms in these complexes
were all generated geometrically, assigned appropriate isotro-
pic thermal parameters, and allowed to ride on their parent
carbon atoms. All of the hydrogen atoms were held stationary
and included in the structure factor calculations in the final
stage of full-matrix least-squares refinement. The structures
were solved and refined using SHELXL-97 programs. CCDC
976986 (for 1), 976987 (for 2), 976988 (for 3), 916989 (for 4),
976990 (for 5) contain the supplementary crystallographic data
for this paper.
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Acknowledgements
We are grateful to the National Natural Science Foundation of
China (grant no. 20972107 and 21132002), a project funded by
the Priority Academic Program Development of Jiangsu Higher
Education Institutions and a project funded by Soochow Uni-
versity (grant no. Q420400113) for financial support.
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5594 | Dalton Trans., 2014, 43, 5586–5594
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