Synthesis and characterization of heterobimetallic oxo-bridged aluminum-rare earth metal complexes

Article abstract of DOI:10.1021/ic2010584

Reaction of the aluminum hydroxide LAl(OH)[C(Ph)CH(Ph)] (1, L = HC[(CMe)(NAr)]₂, Ar = 2,6-iPr₂C₆H₃) with Y(CH₂SiMe₃)₃(THF)₂ yielded the oxo-bridged heterobimetallic yttrium dialkyl complex LAl[C(Ph)CH(Ph)](μ-O) Y(CH₂SiMe₃)₂(THF)₂ (2). Alkane elimination reaction of 2 with 2-(imino)pyrrole [NN]H ([NN]H = 2-(ArN=CH)-5-tBuC₄H₂NH) afforded the yttrium monoalkyl complex LAl[C(Ph)CH(Ph)] (μ-O)Y(CH₂SiMe₃)[NN](THF) ₂ (5). Alternatively, 5 can be prepared in high yield by reaction of 1 with [NN]Y(CH₂SiMe₃)₂(THF)₂ (3). The analogous samarium alkyl complex LAl[C(Ph)CH(Ph)](μ-O)Sm(CH ₂SiMe₃)[NN](THF)₂ (6) was prepared similarly. Reactions of 5 and 6 with 1 equiv of iPrOH yielded the corresponding alkoxyl complexes 7 and 8, respectively. The molecular structures of 3, 6, and 8 have been determined by X-ray single-crystal analysis. Complexes 2, 3, 5, 7, and 8 have been investigated as lactide polymerization initiators. The heterobimetallic alkoxyl 8 is highly active to yield high molecular weight (M_n = 6.91 × 10⁴) polylactides with over 91% conversion at the lactide-to-initiator molar ratio of 2000.

Full text of DOI:10.1021/ic2010584

ARTICLE
pubs.acs.org/IC
Synthesis and Characterization of Heterobimetallic Oxo-Bridged
AluminumÀRare Earth Metal Complexes
Jingjun Hao,† Jianfeng Li,† Chunming Cui,*,† and Herbert W. Roesky‡
†State Key Laboratory and Institute of Element-Organic Chemistry, Nankai University, Tianjin 300071
‡People’s Republic of China; Institute of Inorganic Chemistry, University of Goettingen, 37077 Goettingen, Germany
S Supporting Information
b
ABSTRACT: Reaction of the aluminum hydroxide LAl(OH)[C(Ph)CH(Ph)] (1, L = HC[(CMe)-
(NAr)]2, Ar = 2,6-iPr2C6H3) with Y(CH2SiMe3)3(THF)2 yielded the oxo-bridged heterobimetallic
yttrium dialkyl complex LAl[C(Ph)CH(Ph)](μÀO)Y(CH2SiMe3)2(THF)2 (2). Alkane elimination
reaction of 2 with 2-(imino)pyrrole [NN]H ([NN]H = 2-(ArNdCH)-5-tBuC4H2NH) afforded the
yttrium monoalkyl complex LAl[C(Ph)CH(Ph)] (μÀO)Y(CH2SiMe3)[NN](THF)2 (5). Alterna-
tively, 5 can be prepared in high yield by reaction of 1 with [NN]Y(CH2SiMe3)2(THF)2 (3). The
analogous samarium alkyl complex LAl[C(Ph)CH(Ph)](μÀO)Sm(CH2SiMe3)[NN](THF)2 (6) was prepared similarly. Reac-
tions of 5 and 6 with 1 equiv of iPrOH yielded the corresponding alkoxyl complexes 7 and 8, respectively. The molecular structures
of 3, 6, and 8 have been determined by X-ray single-crystal analysis. Complexes 2, 3, 5, 7, and 8 have been investigated as lactide
polymerization initiators. The heterobimetallic alkoxyl 8 is highly active to yield high molecular weight (Mn = 6.91 Â 104)
polylactides with over 91% conversion at the lactide-to-initiator molar ratio of 2000.
’ INTRODUCTION
rare-earth metal complexes generally display remarkable high
activity.8 It has been recently shown that several lanthanide com-
plexes initiate the polymerization of rac-lactide in stereospecific
manners.9 However, oxo-bridged AlÀLn complexes have not been
explored as ROP catalysts. Recent studies have revealed that aluminum
hydroxides can function as building blocks for the synthesis of
oxo-bridged heterometallic complexes.4,5,10 We and others have
previously reported several routes to prepare organoaluminum
hydroxides that are stabilized by the bulky β-diketiminato ligand
L (HC[(CMe)(NAr)]2, Ar = 2,6-iPr2C6H3).4a,11 Herein, we
report on the synthesis and characterization of novel hetero-
bimetallic AlÀOÀLn alkyls and aloxides, prepared by alkane
elimination reactions of LAl(OH)(CPhCHPh) (1) with Ln
(Ln = Y, Sm) alkyls. In addition, lactide polymerization reactions
with these complexes have also been investigated.
Rare earth metal complexes incorporating non-Cp type of
ligands have attracted much attention since their interesting
structural features and applications in molecular catalysis such as
alkene and lactide polymerization reactions and intramolecular
hydroamination of alkenes.1 A number of bulky organic ligands
have been employed for the stabilization of well-defined rare earth
metal alkyls, amides, and alkoxides. In polymerization reactions,
aluminum alkyls and aluminoxanes are frequently used as cocatalysts.2
Consequently, the studies on the reactions of rare earth metal
complexes with aluminum complexes have been extensively
performed for the understanding polymerization mechanism.3
An alternative approach for the investigation of oxo-bridged
aluminumÀgroup 4 metal complexes indicated that these com-
pounds are not only excellent for the polymerization reactions
with low Al/Zr ratios but also display interesting structural
features as indicated by the large AlÀOÀM angles and short
MÀO bond lengths.4 Inspired by these successful results, we are
interested in the synthesis, characterization, and lactide poly-
merization behaviors of oxo-bridged aluminumÀrare earth metal
(AlÀOÀLn) complexes. The known oxo-bridged AlÀOÀLn
complexes are very limited and only confined to those containing
Cp2Ln and β-diketiminato-Ln fragments.5 Bimetallic AlÀOÀLn
complexes possessing a reactive LnÀE σ-bond (E = CR3, NR2,
OR), to the best our knowledge, have not been reported so far.
Ring-opening polymerization (ROP) of lactide with well-
defined metal complexes has been one of the recent focuses for
the preparation of biodegradable polylactides.6 Both rare-earth
metal and aluminum complexes have been investigated as
catalysts for the ROP. By incorporating suitable ancillary ligands,
aluminum alkoxides exhibit relatively low rate but well-controlled
stereochemistry over the polymerization reactions.7 In contrast,
’ EXPERIMENTAL SECTION
All operations were carried out under an atmosphere of dry argon or
nitrogen using modified Schlenk line and glovebox techniques. All
solvents were freshly distilled from sodium and degassed immediately
prior to use. Elemental analyses were carried out on an Elemental Vario
EL analyzer. The 1H and 13C NMR spectroscopic data were recorded on
a Varian Mercury Plus 400 and Bruker AV400 spectrometers. Infrared
spectra were recorded on a Bio-Rad FTS 6000 spectrometer. The mole-
cular weight and the dispersity of the polymers were measured by GPC
analysis. LAl(η2-C2Ph2),12 aryliminopyrrole [NN]H,13 Y(CH2SiMe3)3-
14
(THF)2 were synthesized according to published procedures, and
Sm(CH2SiMe3)3(THF)3 was prepared similarly.
Received: January 25, 2011
Published: July 15, 2011
r
2011 American Chemical Society
7453
dx.doi.org/10.1021/ic2010584 Inorg. Chem. 2011, 50, 7453–7459
|

Inorganic Chemistry
ARTICLE
Synthesis of LAl(OH)[C(Ph)CH(Ph)] (1). To a solution of LAl-
(η2-C2Ph2) (3.1 g, 5.0 mmol) in diethyl ether (100 mL) at À78 °C was
added a solution of water (0.09 mL, 5.0 mmol) in diethyl ether (50 mL).
The orange solution immediately turned to pale yellow. The mixture was
warmed to room temperature and stirred for additional 4 h. The volatiles
were removed under vacuum, and the remaining solid was extracted with
n-hexane (50 mL). It was filtered, the filtrate was concentrated and
stored in a freezer at À40 °C overnight to give white crystals of 1 (2.63 g,
82%). Mp: 166À168 °C. 1H NMR (C6D6, 400 MHz): δ 0.65 (s, 1H,
OH), 1.08 (t, J = 6.4 Hz, 12H, CHMe2), 1.16 (d, J = 6.8 Hz, 6H,
CHMe2), 1.32 (d, J = 6.8 Hz, 6H, CHMe2), 1.59 (s, 6H, β-CMe), 3.39
(sept, 2H, J = 6.80 Hz, CHMe2), 3.75 (sept, 2H, J = 6.80 Hz, CHMe2),
5.00 (s, 1H, γ-CH), 6.31 (d, 2H, J = 7.6 Hz, Ar-H), 6.36 (s, 1H,
CdCHPh), 6.74À6.97 (m, 10H, Ar-H), 7.17À7.27 (m, 6H, Ar-H). 13C
NMR (C6D6, 100 MHz): δ 23.40 (β-Me), 24.36, 24.59, 24.93, 25.29
(CHMe2), 27.85, 29.19 (CHMe2), 98.62 (γ-C), 123.98, 124.32 (Ar),
125.26 (C=CHPh), 126.26, 126.67, 127.42, 127.92, 128.49, 129.63,
139.23, 140.6,2 141.48, 143.57 (Ar), 147.95 (Al-C), 170.27 (NC).
IR(KBr, cmÀ1): 3482 (m, OH). Anal. Calcd for C43H53AlN2O
(640.39): C, 80.59; H, 8.34; N, 4.37. Found: C, 80.53; H, 8.30; N, 4.48.
Synthesis of LAl[C(Ph)CH(Ph)](μÀO)Y(CH2SiMe3)2(THF)2
(2). To a n-hexane solution (60 mL) of Y(CH2SiMe3)3(THF)2 (0.98 g,
2.0 mmol) was added a solution of 1 (1.28 g, 2.0 mmol) in n-hexane
(60 mL) at À78 °C. The mixture was warmed to 0 °C and stirred for
additional 4 h. The solution was concentrated (ca. 20 mL) and stored in
123.43, 127.53 (Ar), 137.12 (5-pyr), 142.07, 149.09 (Ar), 165.12 (2-pyr),
171.60 (NC); the SmÀCH2 13C NMR signal could not be located. Anal.
Calcd for C37H67N2O2Si2Sm (778.47): C, 57.09; H, 8.67; N, 3.60.
Found: C, 56.54; H, 8.70; N, 3.28.
Synthesis of LAl[C(Ph)CH(Ph)](μÀO)Y(CH2SiMe3)[NN](THF)2
(5). To solution (20 mL) of 3 (0.72 g, 1.00 mmol) in toluene was added
a solution of 1 (0.64 g, 1.00 mmol) in toluene (20 mL) at room tem-
perature. After it was stirred for 4 h, volatiles were removed under
vacuum. The remaining solid was dissolved in THF/n-hexane (1:1) and
stored at À40 °C overnight to give 5 as colorless crystals (1.12 g, 86%).
Mp: 138À141 °C. 1H NMR(C6D6, 400 MHz): δ À0.92, À0.88 (dd, 2JHH
=
11.2 Hz, 2JYH = 3.2 Hz, 1H, CH2SiMe3), 0.04, 0.08 (dd, 2JHH = 11.2 Hz,
2JYH = 4.0 Hz, 1H, CH2SiMe3), 0.30 (s, 9H, CH2SiMe3), 0.81 (d, J =
6.4 Hz, 3H, CHMe2), 0.95 (d, J = 6.4 Hz, 3H, CHMe2), 1.01 (d, J = 7.2 Hz,
3H, CHMe2), 1.04 (d, J = 6.4 Hz, 6H, CHMe2), 1.10 (d, J = 6.4 Hz 6H,
CHMe2), 1.14 (d, J = 6.4 Hz, CHMe2), 1.37 (d, J = 6.8 Hz, 3H, CHMe2),
1.39 (br, 8H, THF), 1.46 (d, J = 6.4 Hz, 3H, CHMe2), 1.55 (d, J = 6.8 Hz,
3H, CHMe2), 1.63 (s, 3H, β-Me), 1.64 (d, J = 6.8 Hz, 3H, CHMe2), 1.73
(s, 3H, β-Me), 1.85 (s, 9H, CMe3), 2.96 (m, 1H, CHMe2), 3.07À3.22
(m, 2H, CHMe2), 3.50 (m, 2H, CHMe2), 3.57 (br, 8H, THF), 4.44 (m,
1H, CHMe2), 5.21 (s, 1H, γ-CH), 6.28 (s, 1H, CdCH), 6.42À6.44 (d,
4H, Ar-H), 6.46 (d, J = 3.6 Hz, 2H, pyr-H), 6.62À6.70 (m, 3H, Ar-H),
6.75À6.78 (t, 1H, Ar-H), 6.84À6.89 (m, 3H, Ar-H), 6.91 (d, J = 3.6 Hz,
2H, pyr-H), 6.95À7.03 (m, 3H, Ar-H), 7.07À7.09 (m, 1H, Ar-H),
7.17À7.22 (m, 2H, Ar-H), 7.27À7.35 (m, 2H, Ar-H), 7.65 (s, 1H,
CHdN). 13C NMR (C6D6, 100 MHz): δ 4.49 (SiMe3), 21.94, 22.97
(CHMe2), 23.97, 24.40, 24.57, 24.64 (CHMe2), 24.77, 25.13, 25.24
(CHMe2), 25.70 (THF), 25.82 (CHMe2), 26.76, 26.95 (β-Me), 27.76,
28.02, 28.33, 28.43, 28.47, 28.59, 29.38, 30.82 (CHMe2), 31.43
(YÀCH2SiMe3), 32.38 (CMe3), 34.66 (CMe3), 67.85 (THF), 101.55
(γ-C), 110.51 (pyr), 123.29 (pyr), 123.87, 124.03, 124.27, 124.60, 124.86,
125.69, 125.75 (Ar), 126.16 (CdCHPh), 127.06, 127.27, 128.56,
129.76, 133.80 (Ar), 139.60 (5-pyr), 141.52, 141.69, 142.80, 142.96,
143.14, 144.37, 144.67, 144.86, 145.44, 146.10 (Ar), 149.39 (AlÀC),
162.42 (2-pyr), 165.76 (CHdN), 169.62, 169.99 (NC). Anal. Calcd for
a freezer at À40 °C overnight to give 2 0.5C6H14 as colorless crystals
3
(1.60 g, 76%). Mp: 125 °C (dec.). 1H NMR (C6D6, 400 MHz): δ À0.90
(s, 4H, CH2SiMe3), 0.33 (s, 18H, CH2SiMe3), 1.06 (d, J = 4.2 Hz, 12H,
CHMe2), 1.17 (d, J = 6.4 Hz, 6H, CHMe2), (s, 8H, THF), 1.29 (br, 14H,
THF and β-Me), 1.41 (d, J = 6.4 Hz, 6H, CHMe2), 3.49 (br, 12H, THF
and CHMe2), 4.88 (s, 1H, γ-CH), 6.88À7.59 (m, 16H, Ar-H). 13C
NMR (C6D6, 100 MHz): δ 5.10 (s, SiMe3), 24.11 (CHMe2), 24.56 (d,
J = 6.0 Hz, CH2SiMe3), 25.48 (THF), 26.14, 26.31 (β-Me), 27.77, 28.66
(CHMe2), 69.84 (THF), 100.70 (γ-C), 124.29, 124.69 (Ar), 126.10
(C=CHPh), 126.81, 127.40, 127.81, 130.24, 130.77, 140.47, 140.80,
142.98, 144.90, 145.63 (Ar), 147.70 (Al-C), 170.81 (NC). Anal. Calcd
C76H108AlN4O3SiY 0.5C6H14 (1312.76): C, 72.28; H, 8.83; N, 4.27.
3
for C59H90AlN2O3Si2Y 0.5C6H14 (1090.50): C, 68.29; H, 8.97; N, 2.57.
Found: C, 71.83; H, 8.75; N, 4.18.
3
Found: C, 67.64; H, 8.85; N, 2.38.
Synthesis of LAl[C(Ph)CH(Ph)](μÀO)Sm(CH2SiMe3)[NN]-
(THF) (6). Six was prepared similarly as described for 5 and obtained
as pale-yellow crystals (1.1 g, 85%). Mp: 172 °C (dec). 1H NMR (C6D6,
400 MHz): δ À5.52 (br, m, 1H, CHMe2), À3.33 (br, s, 9H, CMe3),
À2.88 (br, m, 1H, CHMe2), À1.33 (s, 3H, β-Me), À0.45 (s, 3H, CHMe2),
0.27 (s, 3H, CHMe2), 0.67 (br, s, 9H, CH2SiMe3), 0.81 (br, s, 3H,
CHMe2), 0.95 (br, 4H, THF), 1.10 (t, 1H, Ar-H), 1.32 (d, J = 6.4 Hz, 3H,
CHMe2), 1.45 (t, J = 5.2 Hz, 6H, CHMe2), 1.64 (d, J = 6.4 Hz, 3H,
CHMe2), 1.70 (d, J = 6.0 Hz, 3H, CHMe2), 1.92 (s, 3H, β-Me), 2.62 (br,
3H, CHMe2), 2.88 (br, s, 3H, CHMe2), 3.04 (d, J = 4.0 Hz, 3H, CHMe2),
3.35 (br, m, 4H, THF), 4.02 (m, 1H, pyr-H), 4.50 (m, 2H, Ar-H), 4.61
(br, m, 1H, Ar-H), 5.14 (d, J = 6.4 Hz, 1H, γ-H), 6.14 (t, 1H, Ar-H), 6.55
(m, 1H, pyr-H), 6.69 (br, s, 3H, CHMe2), 6.79 (s, 2H, CHMe2), 6.91
(t, 1H, Ar-H), 7.05À7.12 (m, 5H, Ar-H), 7.45À7.61 (m, 7H, Ar-H), 7.70
(s, 2H, CHMe2), 7.81 (m, 1H, Ar-H), 8.28 (s, 1H, CdCHPh), 8.39 (s,
1H, CHdN), 9.45 (br, s, 1H, CH2SiMe3), 10.60 (br, s, 1H, CH2SiMe3).
13C NMR (C6D6, 100 MHz): δ 2.82 (SiMe3), 19.61(CHMe2), 23.78,
24.27, 24.44, 24.62, 24.68 (CHMe2), 24.80, 25.22 (β-Me), 25.41 (THF),
27.40, 28.21, 29.00, 29.19, 29.26 (CHMe2), 30.34(CMe3), 31.17
(CHMe2), 31.55 (CMe3), 68.52 (THF), 101.37 (γ-C), 108.66 (pyr),
121.19 (pyr), 122.98, 123.70, 124.12, 125.05, 125.28, 125.39, 125.97,
126.39 (Ar), 127.39 (CdCHPh), 129.66, 130.46 (Ar), 135.95 (5-pyr),
140.36, 141.00, 143.94, 144.19, 144.52, 144.64, 145.17, 145.25, 146.19,
146.25 (Ar), 148.51(AlÀC), 164.69 (2-pyr), 168.45 (CHdN), 170.23,
170.73 (NC); the SmÀCH2 13C NMR signal could not be located. Anal.
Synthesis of[NN]Y(CH2SiMe3)2(THF)2 (3). To a solution (60 mL)
of Y(CH2SiMe3)3(THF)2 (0.98 g, 2.0 mmol) in n-hexane was added a
solution of [NN]H (0.62 g, 2.0 mmol) in 20 mL of n-hexane at À78 °C.
The mixture was allowed to warm to room temperature and stirred
overnight. It was concentrated to ca. 5 mL, and then THF (3 mL) were
added. The solution was stored at À40 °C overnight to give colorless
crystals of 3 (1.1 g, 75%). Mp: 101À103 °C. 1H NMR (C6D6, 400 MHz):
δ À0.19 (s, 4H, CH2SiMe3), 0.27 (s, 18H, CH2SiMe3), 0.96 (d, J = 6.6 Hz,
6H, CHMe2), 1.14 (d, J = 6.8 Hz, 6H, CHMe2), 1.26 (br, 8H, THF), 1.69
(s, 9H, CMe3), 3.11 (m, 2H, CHMe2), 3.58 (br, s, 8H, THF), 6.40 (d, J =
3.6 Hz, 1H, pyr-H), 6.81 (d, J = 3.6 Hz, 1H, pyr-H), 6.95À7.02 (m, 3H,
Ar-H), 7.51(s, 1H, NdCH). 13CNMR(C6D6, 100 MHz): δ4.46 (SiMe3),
22.65 (CHMe2), 25.42 (THF), 25.97, 28.71 (CHMe2), 32.29 (CMe3),
34.51 (CMe3), 37.47 (d, JYC = 52.3 Hz, YÀCH2), 69.11 (THF), 110.88
(pyr), 123.72 (pyr), 126.01, 126.37 (Ar), 134.27 (5-pyr), 141.91, 148.38
(Ar), 161.94 (2-pyr), 166.14 (NC). Anal. Calcd for C37H67N2O2Si2Y
(717.02): C, 61.98; H, 9.42; N, 3.91. Found: C, 61.51; H, 9.51; N, 3.62.
Synthesis of [NN]Sm(CH2SiMe3)2(THF)2 (4). 4 was prepared
similarly as described for 3 and obtained as orange crystals (0.84 g, 54%).
Mp: 84À86 °C. 1H NMR (C6D6, 400 MHz): δ À4.43 (s, 9H, CMe3),
0.31 (s, 6H, CHMe2), 0.70 (br, 8H, THF), 1.00 (s, 18H, CH2SiMe3),
1.26 (s, 6H, CHMe2), 1.66 (m, 2H, CHMe3), 2.53 (br, 8H, THF), 5.50
(m, 2H, Ar-H), 5.82 (m, 1H, Ar-H), 6.61 (m, 1H, pyr-H), 8.74 (m, 1H,
pyr-H), 9.02 (br, s, 4H, CH2SiMe3). 13C NMR (C6D6, 100 MHz):
δ 3.13 (SiMe3), 21.56 (CHMe2), 24.30, 25.64 (CHMe2), 26.49 (THF),
27.37 (CMe3), 30.18 (CMe3), 67.95 (THF), 109.02 (pyr), 121.63 (pyr),
Calcd for C72H100AlN4O2SiSm 0.5C6H14 (1302.10): C, 69.18; H, 8.28;
3
N, 4.30. Found: C, 68.72; H, 8.35; N, 4.06.
7454
dx.doi.org/10.1021/ic2010584 |Inorg. Chem. 2011, 50, 7453–7459

Inorganic Chemistry
ARTICLE
Synthesis of LAl[C(Ph)CH(Ph)](μÀO)Y(OiPr)[NN](THF)2 (7).
To a toluene solution (20 mL) of 5 (1.32 g, 1.00 mmol) was added a
solution of iPrOH (0.066 g, 1.10 mmol) in 5 mL of toluene at room
temperature. After it was stirred for 10 h, volatile materials were removed
under vacuum. The remaining solid was dissolved in n-hexane. The
solution was stored at À40 °C for 24 h to give 7 as colorless crystals (0.70 g,
58%). Mp: 131À133 °C. 1H NMR (C6D6, 400 MHz): δ 0.86À0.90 (t,
6H, J = 6.8 Hz, CHMe2), 0.95 (d, 3H, J = 6.8 Hz, CHMe2), 1.04 (d, 3H,
J = 6.8 Hz, CHMe2), 1.10 (t, J = 5.6 Hz, 12H, CHMe2), 1.14 (d, 3H, J =
7.6 Hz, CHMe2), 1.18 (d, 3H, J = 6.8 Hz, CHMe2), 1.22 (br, 4H, THF),
1.31 (d, 3H, J = 6.4 Hz, CHMe2), 1.34 (d, 3H, J = 7.6 Hz, CHMe2), 1.57
(d, 3H, J = 6.8 Hz, OCHMe2), 1.61 (s, 3H, β-CMe), 1.69 (s, 3H,
β-CMe), 1.74 (d, 3H, J = 6.4 Hz, OCHMe2), 1.82 (s, 9H, CMe3), 3.13
(m, 2H, CHMe2), 3.42 (m, 1H, CHMe2), 3.57 (br, 4H, THF), 3.73 (m,
2H, CHMe2), 4.37 (m, 1H, CHMe2), 4.61 (sept, 1H, OCHMe2), 5.15
(s, 1H, γ-CH), 6.12 (s, 1H, CdCH), 6.46À6.48 (d, 2H, Ar-H), 6.52
(br, s, 1H, pyr-H), 6.68 (s, 4H, Ar-H), 6.70 (br, s, 1H, pyr-H), 6.74À7.06
(m, 9H, Ar-H), 7.21À7.37 (m, 4H, Ar-H), 7.66 (s, 1H, NdCH). 13C
NMR (C6D6, 100 MHz): δ 21.89 (CHMe2), 23.76, 24.22, 24.54,
25.21(CHMe2), 25.66 (THF), 26.38, 27.03 (β-Me), 27.69, 27.90,
27.97, 28.21, 28.45, 28.50, 28.65, 29.17, 29.43 (CHMe2 and OCHMe2),
31.76 (CMe3), 34.66 (CMe3), 67.99(THF), 71.05 (OCHMe2), 100.14
(γ-C), 110.19 (pyr), 123.26 (pyr), 123.53, 123.82, 124.46, 125.28,
125.51 (Ar), 127.05 (CdCHPh), 127.49, 127.62, 129.29, 133.85 (Ar),
139.83 (5-pyr), 140.41, 141.70, 142.62, 142.68, 144.11, 144.22, 145.00,
145.57, 146.06 (Ar), 149.53 (AlÀC), 162.35 (2-pyr), 165.34(HCdN),
Table 1. Crystallographic Data for 3, 6, and 8
3
6 C7H8
8 0.5C6H14
3
3
formula
C37H67N2O2-
Si2Y
717.02
113(2)
P121/n1
10.4948(4)
18.2644(8)
21.1763(8)
90.00
94.849(3)
90.00
4044.6(3)
4
C79H108AlN4O2- C77H111AlN4O3-
SiSm
1351.11
113(2)
Pbca
40.130(8)
18.800(4)
19.107(4)
90.00
90.00
90.00
14415(5)
8
1.245
Sm
1317.02
113(2)
fw
T (K)
space group
a (Å)
b (Å)
c (Å)
P-1
10.926(2)
15.241(3)
22.931(5)
88.87(3)
78.41(3)
70.49(3)
3521.3(12)
2
1.242
1398
1.092
0.0572, 0.1482
0.0649, 0.1562
R (deg)
β (deg)
γ (deg)
V (Å3)
Z
d
calcd (g/cm3)
1.178
F(000)
1544
5720
GOF
1.066
1.041
R1, wR2 (I > 2σ(I)) 0.0576, 0.1170 0.0645, 0.1459
R1, wR2 (all data) 0.0832, 0.1272 0.1010, 0.1627
XÀray Structural Determination. All intensity data were col-
lected with a Bruker SMART CCD diffractometer using graphite
monochromated Mo KR radiation (λ = 0.71073 Å). The structures
were solved by direct methods and refined by full-matrix least-squares
on F2. Hydrogen atoms were considered in calculated positions. All
nonhydrogen atoms were refined anisotropically. Crystals of 3, 6, and 8
suitable for X-ray analysis were obtained from THF/n-hexane (1:1),
toluene, and n-hexane at room temperature, respectively (Table 1).
169.34, 169.49 (NC). Anal. Calcd for C74H104AlN4O3Y 0.5C6H14
3
(1256.62): C, 73.60; H, 8.90; N, 4.46. Found: C, 73.18; H, 8.84; N, 4.23.
Synthesis of LAl[C(Ph)CH(Ph)](μÀO)Sm(OiPr)[NN](THF) (8).
Similar procedure as described for 7 was employed for the synthesis of 8.
8 was obtained as pale-yellow crystals (0.66 g 52%). Mp: 137À139 °C.
1H NMR (C6D6, 400 MHz): δ À5.23 (br, 1H), À4.77 (br, 1H), À2.88
(s, 9H, CMe3), À1.05(s, 3H), À0.95 (d, 3H, J = 4.8 Hz, CHMe2), À0.64
(s, 3H), À0.07 (br, 3H), 0.60 (s, 3H), 1.10 (t, 3H, J = 7.6 Hz, CHMe2),
1.24 (d, 3H, J = 6.0 Hz, CHMe2), 1.50 (br, 4H, THF), 1.63 (d, 3H, J =
6.0 Hz, CHMe2), 1.81 (d, 3H, J = 6.0 Hz, CHMe2), 2.02 (s, 3H), 2.30
(br, 3H), 2.68 (br, 3H), 2.74 (d, 3H, J = 2.8 Hz, CHMe2), 3.07 (d, 3H, J =
2.8 Hz, CHMe2), 3.77 (m, 1H, CHMe2), 3.99 (m, 1H, CHMe2), 4.11
(br, 4H, THF), 4.50 (br, 2H, CHMe2), 5.06 (d, 1H, J = 7.2 Hz), 5.83 (br,
1H, CHMe2), 6.02 (t, 1H, J = 7.6 Hz,), 6.52 (d, 1H, J = 7.6 Hz,), 6.58 (s,
1H), 7.06 (t, 4H, J = 7.2 Hz), 7.30 (m, 1H, Ar-H), 7.55 (m, 3H, Ar-H),
7.75 (d, 1H, J = 6.8 Hz), 8.52 (br, 2H), 8.72 (br, 1H), 10.46 (br, 1H). 13C
NMR (C6D6, 100 MHz): δ 19.81, 21.69 (CHMe2), 23.47, 13.53, 24.46,
24.51, 24.78, 24.83, 25.13, 25.36 (CHMe2), 25.59 (THF), 26.74, 27.97
(β-Me), 28.64, 28.70, 29.25, 30.15, 30.50, 30.62, 30.68 (CHMe2 and
OCHMe2), 30.89 (CMe3), 31.70 (CMe3), 69.65 (br, THF), 70.34 (SmÀ
OCHMe2), 99.76 (γ-C), 107.84 (pyr), 120.84 (pyr), 122.32, 123.28,
124.43, 124.72, 124.78, 125.45, 125.79 (Ar), 127.13 (CdCHPh), 129.71,
130.40, 135.37 (Ar), 140.00 (5-pyr), 140.83, 142.60, 143.23, 143.94,
144.35, 144.58, 145.23, 145.39, 146.73 (Ar), 148.70 (AlÀC), 163.90
(2-pyr), 164.83 (CHdN), 169.07, 170.65 (NC). Anal. Calcd for
’ RESULTS AND DISCUSSION
Synthesis and Characterization. The stable aluminum hy-
droxide LAl(OH)[C(Ph)CH(Ph)] (1) was prepared in high yield
by controlled hydrolysis of LAl(η2-C2Ph2) (L = HC[(CMe)-
(NAr)]2, Ar = 2,6-iPr2C6H3) in diethyl ether. A singlet at
δ 0.65 ppm in the 1H NMR spectrum of 1 and a broad absorp-
tion centered at 3482 cmÀ1 in the IR spectrum indicated the
formation of the aluminum hydroxide.4a,11a,15
Reaction of LAl(OH)[C(Ph)CH(Ph)] with yttrium trialkyl
Y(CH2SiMe3)3(THF)2 in n-hexane at 0 °C yielded the oxo-
bridged heterobimetallic yttrium dialkyl 2 in moderate yield
(Scheme 1). Complex 2 is stable at room temperature under an
inert atmosphere but crystals of 2 were gradually decomposed
above 40 °C in several hours. Complex 2 has been characterized
by 1H, 13C NMR, IR spectroscopic methods, and elemental analysis.
The methylene hydrogen atoms of YÀCH2SiMe3 resonate at
δ À0.90 ppm in the proton NMR spectrum, which is slightly high-
field shifted in comparison with that of Y(CH2SiMe3)3(THF)2
(δ À0.70 ppm). It has been demonstrated that monoligated
lanthanide dialkyls could be used for the generation of highly
active cationic alkyl species. However, our attempts to prepare well-
defined cationic yttrium monoalkyl species from 2 with 1 equiv of
[HNMe3][BPh4] or [Ph3C][B(C6F5)4] were unsuccessful.
As the thermal stability of 2 is not satisfactory for further
investigation its chemistry, it is expected that the introduction of
suitable ligand frameworks would modify the properties of this
class of bimetallic complexes. As a continuation of our investigation
on the bulky pyrrolylaldiminato ligand [NN]H ([2-(ArNdCH)-
5-tBuC4H2NH]) in rare earth metal chemistry,13,16 the new lantha-
nide dialkyls incorporating this ligand have been prepared as pre-
cursors for the preparation of AlÀOÀLn bimetallic complexes.
Thus, reaction of [NN]H with 1 equiv of Ln(CH2SiMe3)3-
(THF)n (Ln = Y, n = 2; Ln = Sm, n = 3) in n-hexane at room
C74H104AlN4O3Sm 0.5C6H14 (1318.07): C, 70.16; H, 8.49; N, 4.25.
3
Found: C, 69.67; H, 8.23; N, 4.07.
Polymerization of rac-Lactide. A typical procedure for poly-
merization of rac-lactide was performed in 100 mL Schlenk flasks. To a
mixture an appropriate amount of rac-lactide and catalyst (0.02 mmol)
was added 15 mL of toluene at room temperature. After polymerization
reaction, all volatiles were removed under vacuum and the viscous solid
was washed twice with 20 mL of methanol. The remaining solid was dissolved
in a small amount of CH2Cl2, and then n-hexane was added to incipient
precipitation (CH2Cl2/n-hexane, 1:4 v/v). The precipitated polymer
was collected and dried under vacuum for 24 h. The molecular weight
and the dispersities of the resulting polymer were determined by GPC.
The 1H NMR spectra of polylactides samples were recorded in CDCl3.
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Inorganic Chemistry
ARTICLE
Scheme 1. Synthesis of 1À6
Scheme 2. Synthesis of 7 and 8
than that of O2ÀY1 (2.394(2) Å). The Y1ÀC22 and Y1ÀC26
bond lengths (2.412(3) and 2.436(3) Å) are within the reported
range for yttrium alkyls.17 The pyrrolide ligand adopts an η2
coordination with NÀYÀN angle of 71.71(8)°, which is comparable
to those found in the known pyrrolide yttrium complexes.13,16
The oxo-bridged heterobimetallic monoalkyls 5 and 6 can be
obtained in high yields by reaction of 1 with 1 equiv of the pyrrolide
dialkyls 3 and 4. Alternatively, 5 can be prepared by the reaction
of 2 with [NN]H. In the 1H NMR spectrum of 5, the methylene
proton resonances for the CH2SiMe3 group can be found at
δ À0.90 and 0.06 ppm (2JHH = 11.2 Hz, 2JYH = 3.2 and 4.0 Hz).
Reactions of 5 and 6 with 1 equiv of isopropanol afforded the
corresponding heterobimetallic alkoxyl complexes 7 and 8, respec-
Figure 1. ORTEP drawing of 3 with thermal ellipsoids at 30% prob-
ability. Hydrogen atoms and iPr groups on aryl have been omitted for
clarity. Selected bond lengths (Å) and angles (deg): Y1ÀN2 2.391(3),
Y1ÀO2 2.394(2), Y1ÀC22 2.412(3), Y1ÀC26 2.436(3), Y1ÀN1
2.529(2), Y1ÀO1 2.534(2), N2ÀY1ÀO2 148.95(8), N2ÀY1ÀC22
99.81(10), O2ÀY1ÀC22 89.72(9), O2ÀY1ÀC26 86.80(10), C22À
Y1ÀC26 100.35(11), N2ÀY1ÀN1 71.71(8), O2ÀY1ÀO1 91.94(7),
C22ÀY1ÀO1 178.25(9).
1
tively, in good yields as shown in Scheme 2. The H NMR
temperature yielded the expected monoligated dialkyls [NN]Ln-
(CH2SiMe3)2(THF)2 (Ln = Y (3), Sm (4)). Compounds 3 and
4 have been fully characterized by multiple NMR spectroscopy
and elemental analysis. The methylene hydrogen resonance
(δ À0.19 ppm) of 3 falls in the typical range for this type of
yttrium alkyls.
The molecular structure of 3 has been determined by an X-ray
single-crystal analysis, which is shown in Figure 1 with selected
bond parameters. Complex 3 is monomeric and the yttrium atom
is six-coordinate with two nitrogen atoms, two THF molecules,
and two CH2SiMe3 groups. The geometry around the yttrium
atom can be best described as a slightly distorted octahedron as
indicated by the almost linear O1ÀY1ÀC22 angle (178.25(9)°).
The O1ÀY1 (2.534(2) Å) bond length is longer (ca. 0.14 Å)
spectrum of 7 displays a new septet centered at δ 4.61 ppm, the
typical resonance for OCHMe2 group.18 The 1H NMR spectra of
the samarium complexes 4, 6, and 8 display abnormal broad
resonances due to their paramagnetic nature.
The molecular structures of the oxo-bridged samariumÀ
aluminum bimetallic complexes 6 and 8 have been determined
by X-ray single-crystal analysis, which are shown in Figures 2 and
3. The samarium centers in 6 and 8 are five-coordinate and can be
best described as a distorted square-pyramidal geometry. The
central oxygen atom links the samarium and aluminum atoms with
an approximate linear SmÀOÀAl angle (178.2(2)° in 6, 174.2(2)°
in 8).TheSm1ÀO1 bond distances of 2.101(3) Å in 6and 2.140(4) Å
in 8 are comparable to those observed in [(C5Me5)2Sm]2(μÀO)
(2.094(1) Å),19 [(C5Me4iPr)2Sm]2(μÀO) (2.116(3) Å) and
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Inorganic Chemistry
ARTICLE
Table 2. Polymerization of rac-Lactide with 2, 3, 5, 7, and 8a
entry cat. time (h) [LA]/[Cat] additive conv (%) Mn (104) PDI
1
2
2
2
3
3
5
5
5
7
8
8
8
8
8
15
5
125
125
125
125
125
250
125
500
125
250
500
1000
2000
none
HMDS
none
low
98
69
100
98
45
98
99
99
99
99
99
91
n.d.
n.d.
2.23
1.85
3
15
5
bimodel bimodel
4
HMDS
none
4.18
1.41
n.d.
1.71
1.95
n.d.
5
5
6
15
1.5
0.5
0.5
0.5
0.5
0.5
1.0
none
7
HMDS
none
0.94
2.78
1.92
1.42
1.60
4.82
6.91
2.11
2.32
1.63
2.16
1.70
1.60
1.72
8
9
none
10
11
12
13
none
none
none
none
a General conditions: 20 °C, 15 mL of toluene, [Cat] = 0.0013 mol/L;
Figure 2. ORTEP drawing of 6 with thermal ellipsoids at 30% prob-
ability. Hydrogen atoms and iPr groups on aryl have been omitted for
clarity. Selected bond lengths (Å) and angles (deg): Sm1ÀO1 2.101(3),
Sm1ÀN2 2.424(4), Sm1ÀO2 2.454(3), Sm1ÀC22 2.473(6), Sm1ÀN1
2.588(4), Al1ÀO1 1.711(4), Al1ÀN3 1.917(5), Al1ÀN4 1.930(5), Al1À
C59 1.991(5), O1ÀSm1ÀN2 109.52(14), O1ÀSm1ÀO2 90.20(13),
O1ÀSm1ÀC22 121.26(16), N2ÀSm1ÀC22 103.27(16), O2ÀSm1À
C22 85.70(15), O1ÀSm1ÀN1 130.93(14), N2ÀSm1ÀN1 69.56(14),
O1ÀAl1ÀN3 111.5(2), O1ÀAl1ÀN4 109.20(18), N3ÀAl1ÀN4
94.9(2), O1ÀAl1ÀC59 112.4(2), Al1ÀO1ÀSm1 178.2(2).
HMDS = 1,1,1,3,3,3-hexamethyldisilazane (0.0065 mol/L); the conver-
1
sions were determined by the H NMR spectra; the values of PDI
were obtained by GPC calibrated with standard polystyrene sample;
n. d. = not determined.
aluminumÀlanthanide species (1.690(6) to 1.766(6) Å).5 The
Sm1ÀO3 bond distance of 2.095(3) Å in 8 is comparable to
those reported for samarium alkoxyl complexes.21 Compounds 6
and 8 represent the first structurally characterized oxo-bridged
AlÀLn bimetallic rare earth alkyls and alkoxides. The existence of
the LnÀCR3 and LnÀOR in these complexes may allow to
investigate the insertion and σ-bond metathesis chemistry of
these complexes and apply these complexes to polymerization
reactions.
Ring-Opening Polymerization (ROP) of rac-Lactide. For
comparison, the mononuclear aluminum and yttrium complexes
1 and 3 were examined as initiators for the ROP of lactide in the
first place. The aluminum hydroxide 1 showed very low catalytic
activity within the temperature range from 25 to 70 °C while the
yttrium dialkyl complex 3 is modestly active for the ROP reaction
with ca. 69% conversion in 15 h at room temperature as shown in
Table 2 (entry 3). The results for the polymerization reactions
with the bimetallic alkyl complexes 2 and 5 are given in Table 2
(entries 1, 5, and 6). Yttrium dialkyl 2 exhibits low activity at
ambient temperature, whereas yttrium monoalkyl 5 bearing the
pyrrolide NN ligand shows relatively high reactivity with the
formation of low molecular weight polylactides at the lactide
to initiator molar ratio of 125. However, the increase of the molar
ratio to 250 led to only ca. 45% conversion under the same
conditions.
The catalytic activities of the bimetallic complexes could be
improved by adding 1,1,1,3,3,3-hexamethyldisilazane (HMDS)
as a chain transfer agent. The results for the polymerization
reactions with 2, 3, or 5 in the presence of HMDS are
summarized in Table 2 (entries 2, 4, and 7). The substantial
improvements for the polymerization activity have been achieved
for dialkyl complexes 2 and 3. The addition of HMDS into the
polymerization systems may result in the activation of lactide via
Figure 3. ORTEP drawing of 8 with thermal ellipsoids at 30% prob-
ability. Hydrogen atoms and iPr groups on aryl have been omitted for
clarity. Selected bond lengths (Å) and angles (deg): Sm1ÀO1 2.140(4),
Sm1ÀO2 2.442(3), Sm1ÀO3 2.095(3), Sm1ÀN4 2.474(4), Sm1ÀN3
2.634(5), Al1ÀO1 1.697(4), Al1ÀN2 1.938(4), Al1ÀN1 1.944(4), Al1À
C36 2.011(5), O3ÀSm1ÀO1 118.23(14), O3ÀSm1ÀO2 88.58(12),
O1ÀSm1ÀO2 95.41(13), O3ÀSm1ÀN4 99.93(13), O1ÀSm1ÀN4
109.88(13), O2ÀSm1ÀN4 144.72(14), O3ÀSm1ÀN3 107.96(14),
O1ÀSm1ÀN3 132.88(12), O2ÀSm1ÀN3 76.17(14), N4ÀSm1ÀN3
68.62(13), O1ÀAl1ÀN2 112.76(18), O1ÀAl1ÀN1 108.98(19), N2À
Al1ÀN1 93.63(17), O1ÀAl1ÀC36 116.3(2), N2ÀAl1ÀC36 110.8(2),
N1ÀAl1ÀC36 112.18(18), Al1ÀO1ÀSm1 174.2(2).
the formation of the O HÀN hydrogen bond between the
3 3 3
lactide carbonyl oxygen atom and the HMDS hydrogen atom or
elimination of SiMe4 to give the corresponding rare earth metal
amides. In the latter case, the lanthanide amide group normally
shows relatively high nucleophilicity to lactide compared to
lanthanide alkyl groups.22
[(C5Me5)2Sm(NC5H5)]2(μÀO) (2.151(2) Å).20 The Al1ÀO1
bond distances of 1.711(4) Å in 6 and 1.697(4) Å in 8 fall in the
range of the values found in known oxo-bridged heterobimetallic
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Inorganic Chemistry
ARTICLE
Metal alkoxides have been widely used for the ROP of lactides
and, in general, display a good activity and well-controlled
polymerization behaviors.18c,23 Thus, bimetallic alkoxides 7 and 8
were employed for the polymerization reactions. The results are
summarized in Table 2 (entries 8À13). As expected, 7 and 8 are
highly active initiators for the ROP reactions in toluene at room
temperature. Almost quantitative conversions (99%) with 7 and
8 have been observed in 30 min for the monomer-to-metal molar
ratio at 500. Furthermore, polylactides obtained with 8 exhibits
relatively narrow molecular weight distribution (PDI = 1.70,
entry 11). Therefore, the polymerization reactions of 8 were
further investigated by the increase of monomer-to-metal molar
ratio (Table 2, entries 9À13). As can be seen from Table 2,
complex 8 is extremely active for the ROP of rac-lactide. With
increase of the molar ratio below 1000, almost complete con-
versions were observed in 30 min and the high conversion (91%)
in 1 h was found even at the molar ratio of 2000. As far as we
known, no well-defined lanthanide alkoxides have been reported
to show such a high conversion at the high molar ratio at room
temperature.18b,24 The only comparable example is an alkoxide-
bridged bis(amidinate) ytterbium complex reported by Shen in
2009,24a which can promote a complete conversion of 1500 equiv
of L-lactide in 10 min at 100 °C. However, at the molar ratio
of 2000, the conversion of the monomer with the ytterbium
complex was significantly dropped to no more than 29%. No
comparable aluminum alkoxides have been known to show such
high activity.7 For 8, with the increase of monomer-to-metal
molar ratio, the molecular weights (Mn) are not increased
accordingly within the ratio of 500 but noticeably increased at
high molar ratios. For all of these catalysts, atactic polymers were
produced based on the NMR analysis.
’ ACKNOWLEDGMENT
We are grateful to the Natural National Science Foundation of
China (20972074 and 20725205). H.W.R. thanks the Deutsche
Forschungsgemeinschaft for support of this work.
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polymerization reactions as indicated by the very low activity
of the aluminum hydroxide 1 and increased polymerization
activity with the LnÀOiPr complexes. However, the en-
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AlÀO ligand.
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’ CONCLUSIONS
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The oxo-bridged heterobimetallic aluminumÀlanthanide
complexes have been synthesized and characterized. The oxo-
bridged bimetallic complexes feature an almost linear AlÀOÀLn
geometry. The catalytic behaviors of the complexes for rac-
lactide polymerization were investigated, in which heterobime-
tallic alkoxyl complex 8 is a highly active catalyst for ring-opening
polymerization of rac-lactide. The high activity observed with the
bimetallic alkoxides encourages us to design new bimetallic
complexes for realization of stereospecific polymerization of
lactides.
’ ASSOCIATED CONTENT
S
Supporting Information. CIF files for 3, 6 and 8. This
b
material is available free of charge via the Internet at http://
pubs.acs.org.
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’ AUTHOR INFORMATION
Corresponding Author
*E-mail: cmcui@nankai.edu.cn.
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