Carbon-bridged bis(phenolato)lanthanide alkoxides: Syntheses, structures, and their application in the controlled polymerization of ε-caprolactone

Article abstract of DOI:10.1021/ic050022s

A convenient method for the synthesis of lanthanide alkoxo complexes supported by a carbon-bridged bis(phenolate) ligand 2,2′-methylenebis(6- tert-butyl-4-methylphenoxo) (MBMP^2-) is described. The reaction of (C₅H₅)₃Nd with MBMPH₂ in a 1:1 molar ratio in THF gave the bis(phenolato)lanthanide complex (C₅H ₅)Nd(MBMP)(THF)₂ (1) in a nearly quantitative yield. Complex 1 further reacted with 1 equiv of 2-propanol in THF to yield the bis(phenolato)-lanthanide isopropoxide [(MBMP)₂Nd(μ-Opr ⁱ)(THF)₂]₂ (2) in high yield. Complex 2 can also be synthesized by the direct reaction of (C₅H₅) ₃Nd with MBMPH₂ in a 1:1 molar ratio and then with 1 equiv of 2-propanol in situ in THF. Thus, the analogue bis(phenolato)lanthanide alkoxides [(MBMP)₂Ln(μ-OR)(THF)₂]₂ [R = Prⁱ, Ln = Yb (3); R = Me, Ln = Nd (4), Yb (5); R = CH₂Ph, Ln = Nd (6), Yb (7)] were obtained by the reactions of (C₅H ₅)₃Ln (Ln = Nd, Yb) with MBMPH₂ and then with 2-propanol, methanol, or benzyl alcohol, respectively. The ytterbium complex {[(MBMP)₂Yb(THF)₂]₂(μ-OCH ₂Ph)(μ-OH)} (8) was also isolated as a byproduct. The single-crystal structural analyses of complexes 1-3 and 8 revealed that the coordination geometry around lanthanide metal can be best described as a distorted tetrahedron in complex 1 and as a distorted octahedron in complexes 2, 3, and 8. A O-H...Yb agostic interaction was observed in complex 8. Complexes 2-7 were shown to be efficient catalysts for the controlled polymerization of ε-caprolactone.

Full text of DOI:10.1021/ic050022s

Inorg. Chem. 2005, 44, 5133−5140
Carbon-Bridged Bis(phenolato)lanthanide Alkoxides: Syntheses,
Structures, and Their Application in the Controlled Polymerization of
-Caprolactone
E
Yingming Yao,*,† Xiaoping Xu,† Bao Liu,† Yong Zhang,† Qi Shen,*,†,‡ and Wing-Tak Wong§
Key Laboratory of Organic Synthesis of Jiangsu ProVince, Department of Chemistry and Chemical
Enginering, Suzhou UniVersity, Suzhou 215006, People’s Republic of China, State Key Laboratory
of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of
Sciences, Shanghai 200032, People’s Republic of China, and Department of Chemistry,
UniVersity of Hong Kong, Pokfulam Road, Hong Kong, People’s Republic of China
Received January 7, 2005; Revised Manuscript Received April 19, 2005
A convenient method for the synthesis of lanthanide alkoxo complexes supported by a carbon-bridged bis(phenolate)
ligand 2,2
-methylenebis(6-tert-butyl-4-methylphenoxo) (MBMP2-) is described. The reaction of (C5H5)3Nd with
′
MBMPH2 in a 1:1 molar ratio in THF gave the bis(phenolato)lanthanide complex (C5H5)Nd(MBMP)(THF)2 (1) in a
nearly quantitative yield. Complex 1 further reacted with 1 equiv of 2-propanol in THF to yield the bis(phenolato)-
lanthanide isopropoxide [(MBMP)2Nd(
direct reaction of (C5H5)3Nd with MBMPH2 in a 1:1 molar ratio and then with 1 equiv of 2-propanol in situ in THF.
µ
-OPri)(THF)2]2 (2) in high yield. Complex 2 can also be synthesized by the
Thus, the analogue bis(phenolato)lanthanide alkoxides [(MBMP)2Ln(
µ
-OR)(THF)2]2 [R
)
Pri, Ln
)
Yb (3); R
)
)
Me, Ln Nd (4), Yb (5); R CH2Ph, Ln Nd (6), Yb (7)] were obtained by the reactions of (C5H5)3Ln (Ln
)
)
)
Nd, Yb) with MBMPH2 and then with 2-propanol, methanol, or benzyl alcohol, respectively. The ytterbium complex
[(MBMP)2Yb(THF)2]2( -OCH2Ph)( -OH) (8) was also isolated as a byproduct. The single-crystal structural analyses
of complexes 1 3 and 8 revealed that the coordination geometry around lanthanide metal can be best described
as a distorted tetrahedron in complex 1 and as a distorted octahedron in complexes 2, 3, and 8. A O
agostic interaction was observed in complex 8. Complexes 2 7 were shown to be efficient catalysts for the controlled
polymerization of -caprolactone.
{
µ
µ
}
−
−H‚‚‚Yb
−
ꢀ
Introduction
lactone1d,f-h and the corresponding group IV metal complexes
can catalyze the polymerization of R-olefins and the specific
polymerization of styrene in the presence of cocatalysts,2a-k
as well as the living anionic polymerization of ꢀ-caprolactone.2l
However, the application of bridged bis(phenolates) in
lanthanide chemistry has been quite limited, although some
of these complexes have been found to be efficient initiators
In recent years, the application of bridged bis(phenols) as
ancillary ligands in main group and transition metal coor-
dination chemistry has received considerable attention.1,2 This
is because bridged bis(phenols), as dianionic ligands, have
the advantage of avoiding ligand redistribution reactions and
providing a stereochemically rigid framework for the metal
center that could affect stereospecific transformations. Fur-
thermore, some of these complexes have shown interesting
catalytic activity. For example, the bridged bis(phenolato)-
aluminum complexes can catalyze effectively the controlled
polymerization of lactide,1a,b propylene oxide,1e and ꢀ-capro-
(1) For recent examples of main group metals, see: (a) Hormnirun, P.;
Marshall, E. L.; Gibson, V. C.; White, A. J. P.; Williams, D. J. J. Am.
Chem. Soc. 2004, 126, 2688. (b) Majerska, K.; Duda, A. J. Am. Chem.
Soc. 2004, 126, 1026. (c) Shueh, M.-L.; Wang, Y.-S.; Huang, B.-H.;
Kuo, C.-Y.; Lin, C.-C. Macromolecules 2004, 37, 5155. (d) Chen,
C.-T.; Huang, C.-A.; Huang, B.-H. Macromolecules 2004, 37, 7968.
(e) Braune, W.; Okuda, J. Angew. Chem., Int. Ed. 2003, 42, 64. (f)
Alcazar-Roman, L. M.; O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W.
B. Dalton Trans. 2003, 3082. (g) Chen, H. L.; Ko, B. T.; Huang, B.
H.; Lin, C. C. Organometallics 2001, 20, 5076. (h) Liu, Y. C.; Ko, B.
T.; Lin, C. C. Macromolecules 2001, 34, 6196. (i) Ko, B. T.; Wu, C.
C.; Lin, C. C. Organometallics 2000, 19, 1864. (j) Ko, B. T.; Chao,
Y. C.; Lin, C. C. Inorg. Chem. 2000, 39, 1463. (k) Ko, B. T.; Lin, C.
C. Macromolecules 1999, 32, 8296.
* Authors to whom correspondence should be addressed. E-mail:
yaoym@suda.edu.cn (Y.Y.); qshen@suda.edu.cn (Q.S.). Fax: (86)512-
65112371 (Y.Y.). Tel.: (86)512-65112513 (Y.Y).
† Suzhou University.
‡ Chinese Academy of Sciences.
§ University of Hong Kong.
10.1021/ic050022s CCC: $30.25
Published on Web 06/02/2005
© 2005 American Chemical Society
Inorganic Chemistry, Vol. 44, No. 14, 2005 5133

Yao et al.
was purchased from Acros, dried over CaH2 for 48 h, and distilled
under reduced pressure. 2-Propanol, methanol, and benzoic alcohol
were dried with small amount of sodium and distilled before use.
MBMPH2 was commercially available. Lanthanide analyses were
performed by EDTA titration with an xylenol orange indicator and
a hexamine buffer.6 Carbon and hydrogen analyses were performed
by direct combustion with a Carlo-Erba EA-1110 instrument. The
IR spectra were recorded with a Nicolet-550 FT-IR spectrometer
as KBr pellets. The uncorrected melting points of crystalline samples
in sealed capillaries (under argon) are reported as ranges. Molecular
weights and molecular weight distributions were determined against
polystyrene standards by gel permeation chromatography (GPC)
at 30 °C with a Water 1515 apparatus with three HR columns (HR-
1, HR-2, and HR-4) using THF as the eluent.
(C5H5)Nd(MBMP)(THF)2 (1). A THF solution (25 mL) con-
taining MBMPH2 (1.12 g, 3.30 mmol) was slowly added to a THF
solution of (C5H5)3Nd (40 mL, 3.30 mmol). The reaction mixture
was stirred for 1 h at 40 °C, and the solvent was removed under
vacuum. Toluene (80 mL) was added to extract the product, and
pale-blue microcrystals were obtained in a nearly quantitative yield
(2.15 g) from 10 mL of toluene at -10 °C. Mp: 188-190 °C (dec).
Anal. Calcd for C36H51O4Nd: C, 62.48; H, 7.43; Nd, 20.84.
Found: C, 62.04; H, 7.38; Nd, 20.58. IR (KBr, cm-1): 2955 (s),
2916 (s), 2870 (s), 1605 (m), 1458 (vs), 1389 (s), 1362 (s), 1250
(s), 1204 (s), 1130 (m), 1045 (m), 914 (w), 864 (m), 729 (m). The
crystals suitable for an X-ray diffraction analysis were obtained
from the concentrated THF solution.
[(MBMP)Nd(µ-OPri2)(THF)2]2 (2). Method A. To a THF
solution (35 mL) of complex 1 (1.51 g, 2.18 mmol) was added
2-propanol (0.16 mL, 2.18 mmol) by syringe. The reaction mixture
was stirred overnight at room temperature, and the solvent was
removed under vacuum. THF (60 mL) was added to extract the
product, and pale-blue microcrystals were obtained from the
concentrated THF solution (20 mL) at room temperature (1.28 g,
86% based on Nd). Mp: 166-168 °C (dec). Anal. Calcd for
C68H106O10Nd2: C, 59.53; H, 7.79; Nd, 21.02. Found: C, 59.12;
H, 7.53; Nd, 20.47. IR (KBr, cm-1): 2955 (s), 2917 (m), 2870
(m), 1605 (w), 1462 (s), 1435 (s), 1385 (m), 1253 (s), 1138 (w),
1049 (w), 860 (w), 790 (w). The crystals suitable for an X-ray
diffraction analysis were obtained by slow cooling of a hot toluene
solution.
Method B. A THF solution (25 mL) of MBMPH2 (1.19 g, 3.45
mmol) was slowly added to a THF solution of (C5H5)3Nd (42 mL,
3.45 mmol) at 40 °C. After the solution was stirred for 1 h at 40
°C, 2-propanol (0.26 mL) was added by syringe. The mixture was
stirred overnight, and then the solvent was evaporated under reduced
pressure. THF was added to extract the product, and pale-blue
microcrystals were obtained from the concentrated THF solution
(1.94 g, 82%).
for the ring-opening polymerization of ꢀ-caprolactone and
lactide.3
Recently, we became interested in studying the synthesis
and reactivity of lanthanide complexes that are supported
by the bulky bridged bis(phenolate) ligands. In our earlier
work, the new divalent lanthanide complexes based on
carbon-bridged bis(phenolate ligands), 2,2′-methylenebis(6-
tert-butyl-4-methylphenoxo) (MBMP2-) and 2,2′-ethylidenebis-
(4,6-di-tert-butylphenoxo) (EDBP2-), were synthesized and
found to be active for the ring-opening polymerization (ROP)
of ꢀ-caprolactone and cyclic carbonate. However, these
complexes showed poor solubility even in THF, as a result
of which the polymerizations were not well-controlled.3i As
is well-known for systems with divalent complexes as the
precatalyst, the real active species are the trivalent lanthanide
complexes, which are formed from the single-electron redox
reaction of divalent lanthanide complexes with monomer in
situ.4 If the formation of the central species is rate-limiting,
the polymerization will not be well-controlled. Hence, we
tried to synthesize the corresponding trivalent bis(phenolato)-
lanthanide alkoxides and examined their catalytic behavior
in the ROP of ꢀ-caprolactone. We now report that the new
trivalent alkoxolanthanide complexes based on the carbon-
bridged bis(phenolate) ligand MBMP2- can be synthesized
by a proton exchange reaction using (C5H5)3Ln as starting
materials and these complexes can efficiently initiate the
controlled polymerization of ꢀ-caprolactone.
Experimental Section
General Procedures. All manipulations were performed under
argon, using the standard Schlenk techniques. THF and toluene were
distilled from sodium benzophenone ketyl before use. (C5H5)3Ln
was prepared according to a literature procedure.5 ꢀ-Caprolactone
(2) For recent examples of transition metals, see: (a) Groysman, S.;
Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z.; Shuster, M.
Organometallics 2004, 23, 5291. (b) Cuomo, C.; Strianese, M.;
Cuenca, T.; Sanz, M.; Grassi, A. Macromolecules 2004, 37, 7469. (c)
Groysman, S.; Goldberg, I.; Kol, M.; Genizi, E.; Goldschmidz, Z.
Organometallics 2004, 23, 1880. (d) Capacchione, C.; Proto, A.;
Ebeling, H.; Mu¨lhaupt, R.; Mo¨ller, K.; Spaniol, T. P.; Okuda, J. J
Am. Chem. Soc. 2003, 125, 4964. (e) Proto, A.; Capacchione, C.;
Venditto, V.; Okuda, J. Macromolecules 2003, 36, 9249. (f) Gonzalez-
Maupoey, M.; Cuenca, T.; Frutos, L. M.; Castano, O.; Herdtweck, E.
Organometallics 2003, 22, 2694. (g) Groysman, S.; Goldberg, I.; Kol,
M.; Genizi, E.; Goldschmidz, Z. Organometallics 2003, 22, 3013. (h)
Tshuva, E. Y.; Groysman, S.; Goldberg, I.; Kol, M.; Goldschmidt, Z.
Organometallics 2002, 21, 662. (i) Toupance, T.; Dubberley, S. R.;
Hees, N. H.; Tyrrell, B. R.; Mountford, P. Organometallics 2002, 21,
1367. (j) Tshuva, E. Y.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Chem.
Commun. 2001, 2120. (k) Tshuva, E. Y.; Goldberg, I.; Kol, M.;
Goldschmidt, Z. Organometallics 2001, 20, 3017. (l) Takeuchi, D.;
Nakamura, T.; Aida, T. Macromolecules 2000, 33, 725.
(3) (a) Schaverien, C. J.; Meijboom, N.; Orpen, A. G. J. Chem. Soc., Chem.
Commun. 1992, 124. (b) Gribkov, D. V.; Hultzsch, K. C.; Hampel, F.
Chem.sEur. J. 2003, 9, 4796. (c) Arnold, P. L.; Natrajan, L. S.; Hall,
J. J.; Bird, S. J.; Wilson, C. J. Organomet. Chem. 2002, 647, 205. (d)
Skinner, M. E. G.; Tyrell, B. R.; Ward B. D.; Mountford, P. J.
Organomet. Chem. 2002, 647, 145. (e) Natrajan, L. S.; Hall, J. J.;
Blake, A. J.; Wilson, C.; Arnold, P. L. J. Solid State. Chem. 2003,
171, 90. (f) Ma, H. Y.; Spaniol, T. P.; Okuda, J. Dalton Trans. 2003,
4770. (g) Cai, C. X.; Toupet, L.; Lehmann, C. W.; Carpentier, J. F. J.
Organomet. Chem. 2003, 683, 131. (h) Cai, C. X.; Amgoune, A.;
Lehmann, C. W.; Carpentier, J. F. Chem. Commun. 2004, 330. (i)
Deng, M. Y.; Yao, Y. M.; Shen, Q.; Zhang, Y.; Sun, J. Dalton Trans.
2004, 944. (j) Kerton, F. M.; Whitwood, A. C.; Willans, C. E.; Dalton
Trans. 2004, 2237.
[(MBMP)Yb(µ-OPri2)(THF)2]2 (3). The synthesis of complex
3 was carried out in the same way as that described for complex 2
(method B), but (C5H5)3Yb (3.05 mmol) was used instead of
(C5H5)3Nd. A 1.77 g amount of orange-yellow microcrystals was
obtained from a concentrated THF solution (12 mL) (81% based
on Yb). Mp: 109-111 °C (dec). Anal. Calcd for C68H106O10Yb2:
C, 57.13; H, 7.47; Yb, 24.21. Found: C, 57.43; H, 7.13; Yb, 23.83.
IR (KBr, cm-1): 2955 (s), 2915 (s), 2871 (s), 1605 (w), 1468 (s),
1441 (s), 1389 (m), 1232 (m), 1132 (w), 861 (m), 769 (w). The
crystals suitable for an X-ray diffraction analysis were obtained
by slowly cooling the hot THF solution.
(5) Birmingham, J. M.; Wilkinson, G. J. Am. Chem. Soc. 1956, 78, 42.
(6) Atwood, J. L.; Hunter, W. E.; Wayda, A. L.; Evans, W. J. Inorg. Chem.
1981, 20, 4115.
(4) Evans, W. J.; Katsumata, H. Macromolecules 1994, 27, 2330.
5134 Inorganic Chemistry, Vol. 44, No. 14, 2005

Carbon-Bridged Bis(phenolato)lanthanide Alkoxides
Table 1. Crystallographic Data for Complexes 1-3 and 8
param
1‚THF
2‚2THF
3‚2THF
8‚2THF
formula
fw
T (K)
C40H59NdO5
764.11
193(2)
C76H122Nd2O1 2
1516.27
193(2)
C76H122O12Yb2
1573.82
193(2)
C77H116O12Yb 2
1579.78
193(2)
cryst syst
monoclinic
P21/c
monoclinic
P21/n
monoclinic
P21/n
triclinic
Pˆı
space group
unit cell
a (Å)
b (Å)
c (Å)
8.7082(3)
25.238(1)
35.48(6)
90
90.755(1)
90
7798.2(5)
8
1.302
13.723(5)
17.247(6)
16.279(6)
90
93.800(4)
90
3844(2)
2
1.310
13.335(1)
17.830(1)
15.772(1)
90
92.996(4)
90
3744.7(5)
2
1.396
14.4866(3)
15.2484(7)
20.279(1)
87.215(5)
70.035(4)
66.052(4)
3826.6(3)
2
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
D
calcd (g cm-3
)
1.371
2.485
1628
µ (mm-1
F(000)
)
1.371
3192
1.390
1588
2.539
1628
cryst size (mm)
max (deg)
collcd reflcns
unique reflcns
obsd reflcns [I g 2.0σ(I)]
no. of variables
0.48 × 0.34 × 0.25
0.51 × 0.25 × 0.20
0.55 × 0.50 × 0.25
0.40 × 0.38 × 0.15
θ
25.35
27.48
27.48
25.35
66 315
14 215
12 919
846
30 774
8784
5931
36 833
8538
8170
38 052
13 950
11 453
791
442
391
GOF
R
wR
1.148
0.0436
0.0914
1.004
0.0660
0.1340
1223
0.0414
0.0899
1.112
0.0443
0.0907
[(MBMP)Nd(µ-OMe)(THF)2]2 (4). To a THF solution of
(C5H5)3Nd (31 mL, 2.54 mmol) was slowly added at 40 °C a THF
solution (20 mL) of MBMPH2 (0.86 g, 2.54 mmol). After the
solution was stirred for 1 h at 40 °C, methanol (0.10 mL) was added
by syringe. The mixture was stirred overnight at room temperature,
and then centrifugation was used to remove the solution. Complex
4 was obtained as pale-blue microcrystals. The second crop was
isolated from the concentrated THF solution (25 mL) (1.55 g, 93%
based on Nd). Mp: 182-184 °C (dec). Anal. Calcd for C64H98O10-
Nd2: C, 58.41; H, 7.51; Nd, 21.92. Found: C, 58.29; H, 7.23; Nd,
21.72. IR (KBr, cm-1): 2955 (s), 2916 (s), 1628 (w), 1466 (s),
1435 (s), 1385 (m), 1254 (m), 1049 (m), 860 (m).
[(MBMP)Yb(µ-OMe)(THF)2]2 (5). The synthesis of complex
5 was carried out in the same way as that described for complex 4,
but (C5H5)3Yb was used instead of (C5H5)3Nd. A 1.82 g amount
of orange-yellow microcrystals was obtained from the concentrated
THF solution (20 mL) (85% based on Yb). Mp: 128-130 °C (dec).
Anal. Calcd for C64H98O10Yb2: C, 55.96; H, 7.19; Yb, 25.20.
Found: C, 55.46; H, 6.98; Yb, 24.83. IR (KBr, cm-1): 2956 (s),
2916 (s), 1627 (w), 1466 (s), 1434 (s), 1385 (m), 1252 (m), 1050
(m), 861 (m).
[(MBMP)Nd(µ-OCH2Ph)(THF)2]2 (6). The synthesis of com-
plex 6 was carried out in the same way as that described for complex
2, but benzyl alcohol was used instead of 2-propanol. A 1.42 g
amount of pale-blue microcrystals was obtained from the concen-
trated THF solution (18 mL) (83% based on Nd). Mp: 165-167
°C (dec). Anal. Calcd for C76H106O10Nd2: C, 62.18; H, 7.28; Nd,
19.65. Found: C, 61.74; H, 6.96; Nd, 19.82. IR (KBr, cm-1): 2954
(s), 1616 (m), 1476 (s), 1444 (s), 1379 (m), 1269 (s), 1033 (s), 879
(m).
[(MBMP)Yb(µ-OCH2Ph)(THF)2]2 (7). The synthesis of com-
plex 7 was carried out in the same way as that described for complex
3, but benzyl alcohol was used instead of 2-propanol. A 1.38 g
amount of orange-yellow microcrystals was obtained from the
concentrated THF solution (15 mL) (80% based on Yb). Mp: 133-
135 °C (dec). Anal. Calcd for C76H106O10Yb2: C, 59.83; H, 7.00;
Yb, 22.68. Found: C, 59.53; H, 6.77; Yb, 22.35. IR (KBr, cm-1):
2954 (s), 1615 (w), 1476 (s), 1443 (s), 1378 (m), 1269 (s), 1032
(s), 879 (m).
{[(MBMP)Yb(THF)2]2(µ-OCH2Ph)(µ-OH)} (8). During syn-
thesis of complex 7, a small amount of orange crystals (complex
8) was isolated before the THF was removed. IR (KBr, cm-1): 3383
(s), 2955 (s), 1620 (m), 1435 (s), 1385 (m), 1270 (s), 1165 (m),
1035 (s), 887 (m), 698 (m).
Typical Procedure for the Polymerization Reaction. The
procedures for the polymerization of ꢀ-caprolactone initiated by
complexes 2-7 were similar, and a typical polymerization proce-
dure is given below. A 50 mL Schlenk flask, equipped with a
magnetic stirring bar, was charged with a solution of initiator in
toluene. To this solution was added desired amount of ꢀ-caprolac-
tone by syringe. The contents of the flask were then stirred
vigorously at 50 °C for desired time, during which time an increase
in viscosity was observed. The reaction mixture was quenched by
the addition of 1 M HCl-ethanol solution and then poured into
methanol to precipitate the polymer, which was dried under vacuum
and weighed.
X-ray Crystallography. Suitable single crystals of complexes
1-3 and 8 were sealed in a thin-walled glass capillary for
determining the single-crystal structure. Intensity data were collected
with a Rigaku Mercury CCD area detector in ω scan mode using
Mo KR radiation (λ ) 0.710 70 Å). The diffracted intensities were
corrected for Lorentz polarization effects and empirical absorption
corrections. Details of the intensity data collection and crystal data
are given in Table 1.
The structures were solved by direct methods and refined by
2
full-matrix least-squares procedures based on |F| . All the non-
hydrogen atoms were refined anisotropically. The hydrogen atom
on the hydroxyl group (H(6)) in complex 8 was located according
to the difference Fourier map and was refined isotropically. The
other hydrogen atoms in these complexes were all generated
geometrically (C-H bond lengths fixed at 0.95 Å), assigned
appropriate isotropic thermal parameters, and allowed to ride on
their parent carbon atoms. All the H atoms were held stationary
and included in the structure factor calculation in the final stage of
full-matrix least-squares refinement. The structures were solved and
refined using SHELEXL-97 (for complexes 1, 3, and 7) and
CRYSTALS (for complex 2) programs.
Inorganic Chemistry, Vol. 44, No. 14, 2005 5135

Yao et al.
Scheme 1
Results and Discussion
complexes 4 and 5 precipitated immediately from THF
solution when methanol was added to the reaction systems.
Among these complexes, the solubility of the ytterbium
compounds is larger than that of the corresponding neody-
mium species. These complexes gave satisfactory elemental
analyses (C, H, and Ln), and their IR spectra showed the
characteristic absorptions of the aryloxo group. However,
Synthesis and Characterization of Carbon-Bridged Bis-
(phenolato)lanthanide Complexes. Since the mono-bis-
(phenolato)lanthanide chlorides (MBMP)LnCl(THF)x, the
potential precursors for the synthesis of bis(phenolato)-
lanthanide derivatives, cannot be prepared by the general salt
metathesis reaction of anhydrous LnCl3 with MBMPNa2 in
a 1:1 molar ratio,7 we tried to synthesize the neutral bis-
(phenolato)lanthanide complexes by a proton exchange
reaction. MBMPH2 in THF was added to the THF solution
of (C5H5)3Nd in a 1:1 molar ratio at 40 °C. The color of the
solution changed immediately from purple-blue to yellow-
green. After workup, blue microcrystals were obtained from
the toluene solution. The composition of complex 1 was
established as (C5H5)Nd(MBMP)(THF)2 by elemental analy-
ses (C, H, and Nd) and single-crystal structure determination
(Scheme 1). Complex 1 reacted with 1 equiv of 2-propanol
in THF to give complex 2, after workup, as pale-blue crystals
(Scheme 1). Furthermore, complex 2 can also be synthesized
by the direct reaction of (C5H5)3Nd with MBMPH2 in a 1:1
molar ratio and then with 1 equiv of 2-propanol in situ.
Therefore, the carbon-bridged bis(phenolato)lanthanide alkoxo
complexes [(MBMP)Ln(µ-OR)2(THF)2] (R ) Pri, Ln ) Yb
(3); R ) Me, Ln ) Nd (4), Yb (5); R ) CH2Ph, Ln ) Nd
(6), Yb (7)) can be conveniently prepared by a proton
exchange reaction using (C5H5)3Ln as starting materials
(Scheme 1). During the synthesis of complex 7, a small
amount of {[(MBMP)Yb(THF)2]2(µ-OCH2Ph)(µ-OH)} (8)
was concomitantly isolated as orange crystals as a byproduct.
The formation of complex 8 contributed to the presence of
trace water in the reaction mixture (Scheme 1). Complexes
2, 3, 6, and 7 are moderately soluble in THF and slightly
soluble in toluene, while complexes 4 and 5 are slightly
soluble in THF. Due to their limited solubility in THF,
1
these complexes did not provide any resolvable H NMR
spectra, due to the strong paramagnetism of the central metal
ions. The definitive structures of complexes 1-3 and 8 were
determined by single-crystal structure analysis (see below).
We proposed that complexes 4-7 exist as a dimer in the
solid state according to the structures of the analogous
complexes 2 and 3. Attempts to determine their definitive
structures were unsuccessful due to solvent loss and the
deterioration in crystal quality.
Crystal Structure Analyses. Crystals of complex 1
suitable for an X-ray structure determination were obtained
from the concentrated THF solution. Complex 1 crystallizes
with two crystallographically independent but chemically
similar molecules (1a,b) in the unit cell; the selected bond
lengths and angles of 1a are provided in Table 2. An ORTEP
of molecule 1a is depicted in Figure 1. Complex 1 has a
monomeric structure; neodymium atom is coordinated by a
cyclopentadienyl group, two oxygen atoms from one MBMP
group, and two oxygen atoms from two THF molecules. The
coordination number of the central metal is 7, and the
coordination geometry at the neodymium atom can be best
described as a distorted trigonal bipyramid if the cyclopen-
tadienyl group is considered to occupy one coordination site.
The Nd-C(Cp) bond lengths range from 2.738(5) to
2.822(5) Å, giving an average of 2.784(6) Å, which is
comparable well with that in (C5H5)NdCl2(THF)3.8 The Nd-
O(Ar) bond lengths of 2.165(3) and 2.181(3) Å are compa-
rable with those in [(ArO)3Nd(THF)] (ArO ) 2,6-di-tert-
butyl-4-methylphenolate) (2.176 Å),9 [Nd(Odpp)3(DME)]
(7) Xu, X. P.; Ma, M. T.; Yao, Y. M.; Zhang, Y.; Shen, Q. Eur. J. Inorg.
Chem. 2005, 676.
5136 Inorganic Chemistry, Vol. 44, No. 14, 2005

Carbon-Bridged Bis(phenolato)lanthanide Alkoxides
Table 2. Selected Bond Lengths and Bond Angles in Complexes 1, 2,
3, and 8
1a
2
3
8
Ln(1)-O(1)
Ln(1)-O(2)
Ln(1)-O(3)
Ln(1)-O(3A)
Ln(1)-O(4)
Ln(1)-O(5)
Ln(1)-O(6)
Ln(1)-C(24A)
Ln(1)-H(6)
C(1)-O(1)
2.165(3)
2.181(3)
2.476(3)
2.228(5)
2.203(5)
2.329(5)
2.392(4)
2.582(5)
2.556(5)
2.131(3)
2.108(3)
2.226(3)
2.262(3)
2.422(3)
2.435(3)
2.109(3)
2.091(3)
2.332(4)
2.525(3)
2.360(4)
2.268(3)
2.211(4)
3.101(7)
3.080(4)
2.71(6)
1.351(6)
1.335(6)
1.330(5)
1.332(5)
1.342(8)
1.347(8)
1.349(5)
1.348(5)
C(7)-O(2)
O(1)-Ln(1)-O(2)
O(3)-Ln(1)-O(4)
O(4)-Ln(1)-O(5)
O(5)-Ln(1)-O(6)
85.92(12)
75.40(12)
96.1(2)
96.8(2)
76.592)
99.82(10)
98.00(10) 171.99(13)
75.17(10)
96.34(14)
94.95(13)
69.97(14)
151.7(3)
Ln(1)-O(1)-C(1) 151.9(3)
Ln(1)-O(2)-C(7) 151.7(3)
141.8(4)
154.9(4)
141.0(2)
151.6(2)
151.9(3)
Figure 1. ORTEP diagram of complex 1 showing atom-numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level. Hydrogen
atoms are omitted for clarity.
(2.191 Å) (Odpp ) 2,6-diphenylphenolate),10 [Nd(Odpp)3-
(THF)2] (2.190 Å),11 [Nd(Odpp)3(THF)] (2.193 Å),11 and Nd-
(Odpp)3 (2.169 Å)11 but slightly shorter than those in
[Na(THF)6][Nd(ArO)4] (2.230 Å)9 and K[Nd(OC6H3Pri2-2,6)
(2.211 Å).12 The C-O distances of the phenolate ligands of
1.330(5) and 1.332(5) Å, respectively, are apparently shorter
than the single bond length, reflecting substantial electron
delocalization from the oxygen into the aromatic rings. The
bite angle O(1)-Nd-O(2) of 85.9(1)° for the bis(phenolate)
ligand is comparable with those found in [La{1,1′-(2-OC6H2-
tBu2-3,5)2}{CH(SiMe3)2}(THF)3] (88.1(3)°)3a and [Ln(1,1′-
(2-OC6H-tBu-3-Me2-5,6)2){N(SiHMe2)2}(THF)]2 (Ln ) Y
(88.79(6)°), La (88.83(7)°)).3b The Nd(1)-O(1)-C(1) and
Nd(1)-O(2)-C(7) bond angles are 151.9(3) and 151.7(3)°,
respectively, which are similar to the corresponding bond
angles in K[Nd(OC6H3-iPr2-2,6)4].12
Crystals of complexes 2 and 3 suitable for an X-ray
structure determination were obtained by slow cooling of
the hot toluene solution. Their molecular structural diagrams,
which are centrosymmetric, are shown in Figures 2 and 3
with their selected bond lengths and bond angles listed in
Table 2. Complexes 2 and 3 are isostructural. The structure
shows a dimeric feature containing a Ln2O2 core bridging
through the oxygen atoms of the isopropoxy groups. The
two bridging oxygen atoms and two lanthanide atoms are
exactly coplanar as required by the crystallographic sym-
metry. The central metal atom is six-coordinated by one bis-
(phenolate) ligand, two isopropoxy groups, and two THF
molecules in a distorted octahedron. O(1) and O(3)-O(5)
can be considered to occupy equatorial positions within the
octahedron about the lanthanide center with ΣO-Ln-O )
360.0° for complex 2 and 359.5° for complex 3. O(2) and
O(3A) occupy axial positions, and the O(2)-Ln-O(3A)
Figure 2. ORTEP diagram of complex 2 showing atom-numbering
scheme. Thermal ellipsoids are drawn at the 10% probability level. Hydrogen
atoms are omitted for clarity.
angle is distorted away from the idealized 180° to 162.4(2)°
for complex 2 and 164.4(1)° for complex 3.
In complexes 2 and 3, the average Nd-O(Ar) and Yb-
O(Ar) bond lengths of 2.215(6) and 2.119(3) Å, respectively,
are in accordance with the corresponding bond lengths
observed in complex 1 and those (aryloxo)lanthanide com-
plexes mentioned above when the difference in ionic radii
is considered. Two isopropoxy groups are unsymmetrically
coordinated to the central metal atoms with the deviation of
0.063 Å for complex 2 and 0.036 Å for complex 3,
respectively. The Ln-O(Pri) bond lengths in complexes 2
and 3 are comparable with the bond lengths of the bridging
Ln-O(R) bonds reported in [(C5H5)2Lu(µ-OCH2CH2CH2-
CH2Ph)]2,13 [(Me3CC5H4)2Ce(µ-OPri)]2,14 and [(C5H5)2Yb-
(µ-OPr)]2,15 when the difference in metallic ion radii is
(8) Yang, G.; Fan, Y. G.; Jin, Z. S.; Xing, Y.; Chen, W. Q. J. Organomet.
Chem. 1987, 322, 57.
(9) Zhang, L. L.; Yao, Y. M.; Luo, Y. J.; Shen, Q.; Sun, J. Polyhedron
2000, 19, 2243.
(10) Deacon, G. B.; Feng, T.; Junk, P. C.; Skelton, B. W.; White, A. H.
Chem. Ber. 1997, 130, 851.
(11) Deacon, G. B.; Feng, T.; Skelton, B. W.; White, A. H. Aust. J. Chem.
1995, 48, 741.
(12) Clark, D. L.; Gordon, G. C.; Huffman, J. C.; Vincent-Hollis, R.;
Watkin, J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 5903.
(13) Stults, S. D.; Anderson, R. A.; Zalkin, A. Organometallics 1990, 9,
1623.
(14) Schumann, H.; Palamidis, E.; Loebel, J. J. Organomet. Chem. 1990,
384, C49.
Inorganic Chemistry, Vol. 44, No. 14, 2005 5137

Yao et al.
Figure 4. ORTEP diagram of complex 8 showing atom-numbering
scheme. Thermal ellipsoids are drawn at the 30% probability level. Carbon
atoms of THF molecules (O3, O4, O9, and O10) and hydrogen atoms are
omitted for clarity.
Figure 3. ORTEP diagram of complex 3 showing atom-numbering
scheme. Thermal ellipsoids are drawn at the 10% probability level. Hydrogen
atoms are omitted for clarity.
group are symmetrically coordinated to the ytterbium atoms
with a maximum deviation of 0.01 Å.
considered. It is worth noting that there is an electostatic
interaction of the R-carbon atom of one isopropoxy group
with the central metal atom. The Ln-C(24A) bond lengths
are 3.101(7) Å for complex 2 and 3.080(4) Å for complex
3, respectively, which are comparable to those of 3.067(5)
Å for η3-diketiminate bonding in (CH3C5H4)[(DIPPh)2-
nacnac]YbCl ((DIPPh)2nacnac ) N,N-diisopropylphenyl-2,4-
pentanediimine anion),16 2.986(6) and 3.180(9) Å for bridg-
ing η2-C5H5 bonding in (C5Me5)2Sm(µ-C5H5)Sm(C5Me5)2,17
and 2.814(4) to 3.148(6) Å for chelating η6-,η1-Ph-Yb
bonding in [Yb(Odpp)3]2 (Odpp ) 2,6-diphenylphenolate),18
respectively. The bite angles for O(1)-Ln-O(2) (96.1(2)°
for complex 2, 99.8(1)° for complex 3) are slightly larger
than those found in [Ln(1,1′-(2-OC6H-tBu-3-Me2-5,6)2)-
{N(SiHMe2)2}(THF)]23b and [La{1,1′-(2-OC6H2-tBu2-3,5)2}-
{CH(SiMe3)2}(THF)3].3a The Ln(1)-O(2)-C(7) bond angles
are slightly larger than the Ln(1)-O(1)-C(1) angle but
compare well with those in complex 1. The more acute Ln-
(1)-O(1)-C(1) bond angle can be attributed to the existence
of Ln-C(24A) electrostatic interaction.
The molecular structural diagram of complex 8 is shown
in Figure 4, and its selected bond lengths and bond angles
are listed in Table 2. Complex 8 is a dinuclear complex with
both ytterbium atoms coordinated by a MBMP2- group and
two THF molecules and bridged by one benzyloxy and one
hydroxyl group. Each ytterbium atom is six-coordinated in
a distorted octahedron, with two oxygen atoms from bis-
(phenolate) and two bridging oxygen atoms forming the
equatorial plane and the two oxygen atoms from the THF
molecules occupying the axial positions. Unlike those in
complexes 2 and 3, the benzyloxy group and the hydroxyl
The averaged Yb-O(Ar) and Yb-O(Bn) bond length of
2.100(3) and 2.268(3) Å are similar to the corresponding
bond lengths in complex 3. The Yb-O(H) bond length of
2.211(4) Å is in accordance with that found in other
organolanthanide hydroxides.19 The most remarkable feature
of the structure of complex 8 is that there is remote O-H‚
‚‚Yb interaction involving the hydroxide hydrogen atom. This
Yb(1)-H(6) bond length of 2.71(6) Å is significantly longer
than the Ln-H bond length in most of the organolanthanide
hydrides,20-23 but it accords with the Lu(1)-H(2) of 2.70-
(5) Å in [(C5Me4SiMe3)LuH2]4,23 Y(1)-H(16e) of 2.935(6)
t
Å in (C5Me5)Y(OC6H3 Bu2)CH(SiMe3)2,24 Lu-H(191) of
2.74(9) Å in [Me2Si(C5H4)(C5Me4)]LuCH(SiMe3)2,25 and Lu-
(1)-H(2) of 2.66(2) Å in rac-[Me2Si(2-Me-Benz-Ind)2]LuN-
(SiHMe2),26 in which the C(Si)-H‚‚‚Ln agostic interaction
was considered indubitably to exist.
Controlled Polymerization of ꢀ-Caprolactone Catalyzed
by Complexes 2-7. The aliphatic polyesters, such as poly-
(ꢀ-caprolactone) (PCL) and poly(lactide) (PLA), and their
copolymers have received wide application in the medical
field as biodegradable surgical sutures or as a delivery
medium for controlled release of drugs due to their biode-
gradable, biocompatible, and permeable properties.27 The
(19) (a) Evans, W. J.; Hozbor, M. A.; Bott, S. G.; Robinson, G. H.; Atwood,
J. L. Inorg. Chem. 1988, 27, 1990. (b) Schumann, H.; Loebel, J.;
Pickardt, J.; Qian, C. T.; Xie, Z. W. Organometallics 1991, 10, 215.
(c) Yao, Y. M.; Zhang, K. D.; Yu, L. B.; Shen, Q.; Lei, Y. Y.; Sun,
J. Chin. J. Struct. Chem. 2003, 22, 143.
(20) Evans, W. J.; Meadows, J. H.; Wayda, A. L.; Hunter, W. E.; Atwood,
J. L. J. Am. Chem. Soc. 1982, 104, 2008.
(21) Evans, W. J.; Meadows, J. H.; Wayda, A. L.; Hunter, W. E.; Atwood,
J. L. J. Am. Chem. Soc. 1982, 104, 2015.
(22) Yao, Y. M.; Song, S. P.; Shen, Q.; Hu, J. Y.; Lin, Y. H. Chin. Sci.
Bull. 2004, 49, 1578.
(15) Wu, Z. Z.; Xu, Z.; You, X. Z.; Zhou, X. G.; Jin, Z. S. Polyhedron
1992, 11, 2673.
(16) Yao, Y. M.; Zhang, Y.; Shen, Q.; Yu, K. B. Organometallics 2002,
21, 819.
(17) Evans, W. J.; Ulibarri, T. A. J. Am. Chem. Soc. 1987, 109, 4292.
(18) Deacon, G. B.; Nickle, S.; Mackinnon, P. I.; Tiekink, E. R. T. Aust.
J. Chem. 1990, 43, 1245.
(23) Tardif, O.; Nishiura, M.; Hou, Z. M. Organometallics 2003, 22, 1171.
(24) Klooster, W. T.; Brammer, L.; Schaverien, C. J.; Budzelaar, P. H. M.
J. Am. Chem. Soc. 1999, 121, 1381.
(25) Stern, D.; Sabat, M.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 9558.
(26) Eppinger, J.; Spiegler, M.; Hieringer, W.; Herrmann, W. A.; Anwander,
R. J. Am. Chem. Soc. 2000, 122, 3080.
(27) Fujisato, T.; Ikada, Y. Macromol. Symp. 1996, 103, 73.
5138 Inorganic Chemistry, Vol. 44, No. 14, 2005

Carbon-Bridged Bis(phenolato)lanthanide Alkoxides
Scheme 2
Table 3. Polymerization of e-Caprolactone Initiated by Complexes
2-7a
Mn (10-4
run initiator (molar ratio) t (h) conversn (%) calcdb obsdc PDI
)
[M]0/[I]0
1
2
3
4
5
6
7
8
9d
2
2
3
3
3
3
3
3
3
4
4
5
5
6
6
7
7
400
600
400
400
400
400
400
400
400
400
600
400
400
400
600
400
400
1
1
1
2
4
7.5
14
26
73.5
1
1
4
8
1
1
4
100
100
15
31
51
4.56
6.84
0.68
1.41
2.32
4.01
4.42
4.56
2.28
4.56
6.84
2.42
3.78
4.56
6.84
2.23
3.56
3.65 1.26
5.23 1.14
0.65 1.19
1.19 1.13
1.76 1.23
2.95 1.16
3.19 1.27
3.77 1.29
1.61 1.18
5.75 1.25
6.94 1.29
1.89 1.17
2.56 1.25
4.71 1.16
6.40 1.25
1.71 1.18
2.21 1.22
Figure 5. Polymerization of ꢀ-caprolactone initiated by complex 3 in
toluene at 50 °C. The relationship between the number-averaged molecular
weight (Mn) and the conversion is shown.
88
97
significant effect on the polymerization. Using neodymium
alkoxo complexes as initiators, the polymerization completed
in 1 h at [M]0/[I]0 ) 400, while it was completed after 14 h
when using ytterbium alkoxo complexes as the initiator under
the same polymerization conditions. The active trend is
consistent with the increasing order of their ionic radii. The
effect of the alkoxo groups in these complexes on the
polymerization was not observed. All of these alkoxo
lanthanide complexes show a similar catalytic behavior. The
polymerization temperature affects the reaction rate but not
the molecular weight and PDIs of the result polymers at same
conversion. The polymerizations at higher temperature (runs
3-7) were faster than that at room temperature (run 9) using
complex 3 as the initiator, whereas the molecular weight and
PDI of the polymer obtained were almost the same as those
of the polymer that was obtained at 17 °C under the same
conversions (runs 5 and 9). The main feature for the present
initiators is that polymerization with them can proceed at a
broad range of reaction temperatures from 17 to 50 °C in a
controlled fashion, compared with the systems using lan-
thanocene alkyls or alkoxides as initiators.30
100
50
100
100
53
10
11
12
13
14
15
16
17
83
100
100
49
8
78
a Polymerization conditions: toluene as solvent, Vsol./V[M] ) 4:1, T )
50 °C. b Mn(calcd) × (Mw of ꢀ-CL) × [ꢀ-CL]0/[Ln] × (polymer yield).
c Determined by GPC analysis in THF calibrated with standard poly(styrene).
d T ) 17 °C.
major method that is used to synthesize these polymers is
the ring-opening polymerization (ROP) of lactones/lactides
and functionally related compounds (Scheme 2). Many kinds
of organometallic and coordination complexes have been
reported to be efficient initiators for the ROP of lactones/
lactides, giving polymers with both high molecular weights
and high yields.28,29 Among them, the controlled polymeri-
zation systems are especially interesting, since they afford
the possibility of tailoring the composition and the structure
of the polymer obtained.28a
The catalytic behavior of complexes 2-7 for the ROP of
ꢀ-caprolactone was examined. The preliminary results are
summarized in Table 3. It can be seen that all the complexes
are efficient initiators of the controlled polymerization of
ꢀ-caprolactone in toluene. All the polymers obtained with
these initiators have narrow molecular weight distributions
(PDIs) (<1.30). These initiators showed high activity (even
for [M]0/[2]0 ) 600, the polymerization still can proceed
smoothly) and produced PCL with a molecular weight of
5.23 × 104 and a PDI of 1.14. The central metal ion has
To elucidate the controlled character of the polymerization,
the relationship between the number-averaged molecular
weight (Mn) and the conversion of the polymerization in
toluene at 50 °C using complex 3 as the initiator was
measured as shown in Figure 5. A linear relationship between
Mn and conversion exists, while the PDIs remained un-
changed (runs 3-7). These results indicated that obviously
all of the polymerization processes are controlled.
Conclusion
We have shown that lanthanide alkoxo complexes sup-
ported by carbon-bridged bis(phenolate) ligand MBMP2- can
be conveniently synthesized using (C5H5)3Ln as starting
materials by a proton exchange reaction. (C5H5)3Ln reacted
with MBMPH2 in a 1:1 molar ratio to give the mixed-ligand
complex (C5H5)Ln(MBMP) in an almost quantitatively yield,
which can be used as a precursor to synthesize the corre-
sponding alkoxolanthanide complexes by a further reaction
with 1 equiv of alcohol. These alkoxolanthanide complexes
are well-characterized, and three of them are structurally
characterized. The coordination geometries around the central
metals in these alkoxolanthanide complexes are similar. It
(28) For recent reviews see: (a) O’Keefe, B. J.; Hillmayer M. A.; Tolman,
W. B. J. Chem. Soc., Dalton Trans. 2001, 2215. (b) Agarwal, S.; Mast,
C.; Dehnicke, K.; Greiner, A. Macromol. Rapid Commun. 2000, 21,
195.
(29) For recent examples, see: (a) References 1a-d, 1f-h, 2l, and 3f-j.
(b) Kubies D.; Pantoustier, N.; Dubois, P.; Rulmont, A.; Jerome, R.
Macromolecules 2002, 35, 3318. (c) Takashima, Y.; Nakayama, Y.;
Watanabe, K.; Itono, T.; Ueyama, N.; Nakamura, A.; Yasuda, H.;
Harada, A. Macromolecules 2002, 35, 7538. (d) Guillaume, S. M.;
Schappacher, M.; Soum, A. Macromolecules 2003, 36, 54. (e)
Agarwal, S.; Xie, X. L. Macromolecules 2003, 36, 3545. (f) Ray, W.
C., III; Grinstaff, M. W. Macromolecules 2003, 36, 3557. (g) Martin,
E.; Dubois, Ph.; Jerome, R. Macromolecules 2003, 36, 5934. (h)
Martin, E.; Dubois, Ph.; Jerome, R. Macromolecules 2003, 36, 7094.
(i) Ling, J.; Zhu, W. P.; Shen, Z. Q. Macromolecules 2004, 37, 758.
(30) Yamashita, M.; Takemoto, Y.; Ihara, E.; Yasuda, H. Macromolecules
1996, 29, 1798.
Inorganic Chemistry, Vol. 44, No. 14, 2005 5139

Yao et al.
is interesting to note that the O-H‚‚‚Yb agostic interaction
was observed in complex 8. Furthermore, it was found that
these alkoxolanthanide complexes can catalyze the controlled
polymerization of ꢀ-caprolactone. Controlled polymerization
that is catalyzed by structurally well-defined metal complexes
is especially interesting, because they afford the possibility
of understanding the catalyst structure/reactivity relationships.
Studies on the synthesis and reactivity of other carbon-
bridged bis(phenolato)lanthanide complexes, such as aryl-
oxides, amides, and alkyls, are in process in our laboratory.
Acknowledgment. Financial support from the National
Natural Science Foundation of China (Grants 20472063 and
20272040) and the Key Laboratory of Organic Synthesis of
Jiangsu Province is gratefully acknowledged. This work is
also supported by the University of Hong Kong (W.-T.W.).
Supporting Information Available: Tables of X-ray diffraction
data for complexes 1-3 and 8 in CIF format. This material is
available free of charge via the Internet at http://pubs.acs.org.
IC050022S
5140 Inorganic Chemistry, Vol. 44, No. 14, 2005