Synthesis, reactivity, and characterization of amine bis(phenolate) lanthanide complexes and their application in the polymerization of ε-caprolactone

Article abstract of DOI:10.1021/om050296n

A series of amine bis(phenolato)lanthanide methyl and amido complexes were synthesized by general salt metathesis reaction in this paper. The reaction of anhydrous lanthanide trichlorides with 1 equiv of sodium amine bis(phenolate) LNa₂ [L = Me₂NCH₂CH₂N{CH ₂-(2-O-C₆H₂-Bu^t₂-3,5)} ₂] in THF at room temperature gave the amine bis(phenolate) lanthanide chlorides LLnCl(THF) (Ln = Yb (1), Ln = Er (2)). Complexes 1 and 2 can be used as starting materials to prepare amine bis(phenolate) lanthanide derivatives. Complexes 1 and 2 reacted with lithium methyl and lithium amide, respectively, to give the corresponding lanthanide methyl and amido products LLnMe(THF) (Ln = Yb (3), Ln = Er (4)) and LLnNPh₂(THF) (Ln = Yb (5), Ln = Er (6)). These complexes were well characterized by elemental analyses and IR spectra. The definitive molecular structures of complexes 1, 2, 3, 5, and 6 were provided by single-crystal X-ray analyses. Reactivity studies of complexes 3, 5, and 6 have shown them to be efficient initiators for the ring-opening polymerization of ε-caprolactone.

Full text of DOI:10.1021/om050296n

4014
Organometallics 2005, 24, 4014-4020
Synthesis, Reactivity, and Characterization of Amine
Bis(phenolate) Lanthanide Complexes and Their
Application in the Polymerization of E-Caprolactone
Yingming Yao,*,† Mengtao Ma,† Xiaoping Xu,† 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 April 18, 2005
A series of amine bis(phenolato)lanthanide methyl and amido complexes were synthesized
by general salt metathesis reaction in this paper. The reaction of anhydrous lanthanide
trichlorides with 1 equiv of sodium amine bis(phenolate) LNa2 [L ) Me2NCH2CH2N{CH2-
(2-O-C6H2-But2-3,5)}2] in THF at room temperature gave the amine bis(phenolate) lanthanide
chlorides LLnCl(THF) (Ln ) Yb (1), Ln ) Er (2)). Complexes 1 and 2 can be used as starting
materials to prepare amine bis(phenolate) lanthanide derivatives. Complexes 1 and 2 reacted
with lithium methyl and lithium amide, respectively, to give the corresponding lanthanide
methyl and amido products LLnMe(THF) (Ln ) Yb (3), Ln ) Er (4)) and LLnNPh2(THF)
(Ln ) Yb (5), Ln ) Er (6)). These complexes were well characterized by elemental analyses
and IR spectra. The definitive molecular structures of complexes 1, 2, 3, 5, and 6 were
provided by single-crystal X-ray analyses. Reactivity studies of complexes 3, 5, and 6 have
shown them to be efficient initiators for the ring-opening polymerization of ꢀ-caprolactone.
Introduction
variety of heteroatom-bridged phenoxo lanthanide de-
rivatives and to subtle reactions in this chemistry.5 For
example, biphenolate rare-earth-metal amido complexes
were active catalysts for the hydroamination/cyclization
of aminoalkynes and aminoalkenes;5e alkoxy-amino bis-
(phenolate) lanthanide alkyl and amido complexes can
initiate the ring-opening polymerization of lactide in a
controlled manner.5f To systematically study the effect
of a bridged bis(phenolate) Ln fragment on the reactivity
of a Ln-X bond (X ) C, N, etc.), the synthesis of bridged
bis(phenolate) lanthanide alkyl and amido complexes
Over the past decade, significant efforts to explore
ligands other than the traditional ancillary ligand bis-
(cyclopentadienyl) set in organometallic chemistry have
led to the fruitful design of new nonmetallocene com-
plexes.1 Of these new alternatives, bridged bis(pheno-
late) ligands have received increasing attention as
ancillary ligands, because they, as dianionic ligands,
have the advantage of avoiding ligand redistribution
reactions and can easily be prepared in one step from
simple reagents, which thus allows for the systematic
variation of the steric and electronic properties of the
bisphenolic portion. Moreover, many of these kinds of
complexes of transition and main group metals have
been found to be active catalytic precursors for the
polymerization of olefins and R-olefins in the presence
of cocatalysts, and efficient initiators for the polymer-
ization of some polar monomers.2-4 In recent years, the
application of bridged bis(phenolate) anions as ancillary
ligands in group 3 and lanthanide chemistry led to a
(2) (a) Fokken, S.; Spaniol, T. P.; Kang, H. C.; Massa, W.; Okuda,
J. Organometallics 1996, 15, 5069. (b) Takeuchi, D.; Nakamura, T.;
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Chem. Soc. 2001, 121, 7973. (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) Anthis, J. W.; Filippov, I.; Wigley, D. E. Inorg.
Chem. 2004, 43, 716.
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schmidt, Z. Chem. Commun. 2000, 379. (b) Tshuva, E. Y.; Goldberg,
I.; Kol, M.; Goldschmidt, Z. Inorg. Chem. 2001, 40, 4263. (c) Tshuva,
E. Y.; Groysman, S.; Goldberg, I.; Kol, M.; Goldschmidt, Z. Organo-
metallics 2002, 21, 662. (d) Groysman, S.; Goldberg, I.; Kol, M.;
Goldschmidt, Z. Organometallics 2003, 22, 3793. (e) Chen, C. T.;
Huang, C. A.; Huang, B. H. Dalton Trans. 2003, 3799. (f) Wolff, F.;
Lorber, C.; Choukroun, R.; Donnadieu, B. Inorg. Chem. 2003, 42, 7839
(g) Velusamy, M.; Palaniandavar, M.; Gopalan, R. S.; Kulkarni, G. U.
Inorg. Chem. 2003, 42, 8283. (h) Groysman, S.; Goldberg, I.; Kol, M.;
Genizi, E.; Goldschmidt, Z. Organometallics 2004, 23, 1880.
(4) (a) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem. Soc. 2000,
122, 10706. (b) Balsells, J.; Carroll, P. J.; Walsh, P. J. Inorg. Chem.
2001, 40, 5568. (c) Chan, M. C. W.; Tam, K. H.; Pui, Y. L.; Zhu, N. J.
Chem. Soc., Dalton Trans. 2002, 3085. (d) Hormnirun, P.; Marshall,
E. L.; Gibson, V. C.; White, A. J. P.; Williams, D. J. J. Am. Chem. Soc.
2004, 126, 2688.
* To whom correspondence should be addressed. Fax: (86)512-
65112371. Tel: (86)512-65112513. E-mail: yaoym@suda.edu.cn;
qshen@suda.edu.cn.
† Suzhou University.
‡ Shanghai Institute of Organic Chemistry.
§ University of Hong Kong.
(1) For reviews see: (a) Gibson, V. C.; Spitzmesser, S. K. Chem. Rev.
2003, 103, 283. (b) Piers, E. W.; Emslie, D. J. H. Coord. Chem. Rev.
2002, 233-234, 131. (c) Bailey, P. J.; Pace, S. Coord. Chem. Rev. 2001,
214, 91. (d) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew.
Chem., Int. Ed. 1999, 38, 428.
10.1021/om050296n CCC: $30.25 © 2005 American Chemical Society
Publication on Web 07/01/2005

Amine Bis(phenolate) Lanthanide Complexes
Organometallics, Vol. 24, No. 16, 2005 4015
Scheme 1
is meaningful. Generally, the metathesis reaction of
bridged bis(phenolate) lanthanide halides with lithium
alkyls or amides, like that of bis(cyclopentadienyl)
lanthanide halides,6 should be a convenient method for
the synthesis of these complexes. Surprisingly, although
a lot of amine bis(phenolate) lanthanide alkyl and amido
complexes have been synthesized in the literature, the
methods that are used to synthesize these lanthanide
derivatives are relatively limited. Almost all of these
complexes are prepared by protonolysis reactions using
bulky trisalkyl- or trisamido-lanthanide complexes, such
as Ln(CH2SiMe3)3(THF)2, Ln[N(SiMe3)2]3, or Ln[N(Si-
Me2H)2]3(THF)2, as starting materials.5c-g There is only
one paper in the literature that deals with the synthesis
and reactivity of bridged bis(phenolate) lanthanide
chlorides. Unfortunately, the yttrium and scandium
chlorides supported by the tetradentate diamino-bis-
(phenolate) ligands O2tBuNN′ [O2tBuNN′ ) (2-C5H4N)-
t
CH2N{CH2-(2-OC6H2 Bu2-3,5)2}], which were synthe-
sized by the metathesis reactions of YCl3 or ScCl3 with
Na2O2tBuNN′, were reported to be inactive for many
halide substitution reactions.5i
Recently, we became interested in studying the
synthesis and reactivity of lanthanide complexes that
are supported by bulky bridged bis(phenolate) ligands.5h,j
We found that lanthanide chlorides supported by the
amine bis(phenolate) ligand Me2NCH2CH2N{CH2-(2-O-
C6H2-But2-3,5)}2 can be conveniently prepared in a high
yield and that these lanthanide chlorides can be used
as useful precursors to the synthesis of the correspond-
ing lanthanide derivatives by simple metathesis reac-
tions. Furthermore, the amine bis(phenolate) lanthanide
methyl and amido complexes are efficient initiators for
the ring-opening polymerization of ꢀ-caprolactone. Here
we report these results.
of one amine bis(phenolate) ligand, one chlorine atom,
and one coordinated THF molecule at the metal center.
Complexes 1 and 2 were stable enough when stored in
a sealed flask, but their exposure to air led to a rapid
color change with obvious deliquescence.
Complexes 1 and 2 are good precursors for the
preparation of well-defined amine bis(phenolate) lan-
thanide derivatives by simple metathesis reaction. They
reacted smoothly with 1 equiv of MeLi in THF after
careful workup to afford the amine bis(phenolate) lan-
thanide methyl complexes LLnMe(THF) (Ln ) Yb (3),
Er (4)) in moderate isolated yields at low temperature,
which were identified by elemental analyses and, in the
case of complex 3, single-crystal X-ray diffraction study.
The direct reactions of complexes 1 and 2 with Ph2NLi
that was prepared in situ were similar to those of MeLi,
except that the reactions were performed at room
temperature. Well-defined amine bis(phenolate) lan-
thanide amido complexes LLnNPh2(THF) (Ln ) Yb (5),
Er (6)) can be conveniently prepared in THF and
isolated from concentrated toluene solution in high
isolated yields. Complexes 5 and 6 were confirmed by
elemental analyses and structure determination (Scheme
1).
Complexes 3-6 are extremely sensitive to air and
moisture. The crystals decompose in a few minutes
when they are exposed to air, but neither the crystals
nor the solution showed any sign of decomposition after
several months when stored under argon. The lan-
thanide methyl complexes are moderately soluble in
donor solvents, such as THF and DME, and slightly
soluble in toluene and benzene, but insoluble in hexane.
The corresponding amido complexes are freely soluble
in THF and moderately soluble in toluene. These
complexes did not provide any resolvable 1H NMR
spectra, due to the strong paramagnetism of the lan-
thanide ions.
Results and Discussion
Synthesis and Reactivity of Amine Bis(pheno-
late) Lanthanide Complexes. Following the addition
of 1 equiv of LNa2 [L ) Me2NCH2CH2N{CH2-(2-O-C6H2-
But2-3,5)}2], which was prepared by the reaction of NaH
and LH2 in THF, to a suspension of anhydrous lan-
thanide trichlorides in THF, the color of the solution
gradually changed to yellow (for Yb) or pink (for Er).
After workup, novel amine bis(phenolate) lanthanide
chlorides with general formula LLnCl(THF) (Ln ) Yb
(1), Er (2)) were obtained in a high isolated yield as
analytically pure light yellow or pink crystals from a
concentrated toluene solution, as shown in Scheme 1.
Elemental analyses revealed that the complex consists
(5) (a) Skinner, M. E. G.; Tyrell, B. R.; Ward, B. D.; Mountford, P.
J. Organomet. Chem. 2002, 647, 145. (b) Arnold, P. L.; Natrajan, L.
S.; Hall, J. J.; Bird, S. J.; Wilson, C. J. Organomet. Chem. 2002, 647,
205. (c) Ma, H. Y.; Spaniol, T. P.; Okuda, J. Dalton Trans. 2003, 4770.
(d) Cai, C. X.; Toupet, L.; Lehmann, C. W.; Carpentier, J. F. J.
Organomet. Chem. 2003, 683, 131. (e) Gribkov, D. V.; Hultzsch, K. C.;
Hampel, F. Chem. Eur. J. 2003, 9, 4796. (f) Cai, C. X.; Amgoune, A.;
Lehmann, C. W.; Carpentier, J. F. Chem. Commun. 2004, 330. (g)
Kerton, F. M.; Whitewood, A. C.; Willans, C. Dalton Trans. 2004, 2237.
(h) Deng, M. Y.; Yao, Y. M.; Shen, Q.; Zhang, Y.; Sun, J. Dalton Trans.
2004, 944. (i) Boyd, C. L.; Toupance, T.; Tyrrell, B. R.; Ward, B. D.;
Wilson, C. R.; Cowley, A. R.; Mountford, P. Organometallics 2005, 24,
309. (j) Xu, X. P.; Ma, M. T.; Yao, Y. M.; Zhang, Y.; Shen, Q. Eur. J.
Inorg. Chem. 2005, 676. (k) O’shaughnessy, P. N.; Knight, P. D.;
Morton, C.; Gillespie, K. M.; Scott, P. Chem. Commun. 2003, 1770.
(6) Schumann, H.; Meese-Marktscheffel, J. A.; Esser, L. Chem. Rev.
1995, 95, 865.
These results are quite different from the cases of the
diamio-bis(phenolate) yttrium and scandium chlorides,
tBu
[Y(O2tBuNN′)(µ-Cl)(py)]2 and Sc(O2tBuNN′)Cl(py) [O2
-
t
NN′ ) (2-C5H4N)CH2N{CH2-(2-OC6H2 Bu2-3,5)2}], in

4016 Organometallics, Vol. 24, No. 16, 2005
Yao et al.
Table 1. Details of the Crystallographic Data and Refinements for Complexes 1, 2, 3, 5, and 6
1‚0.75toluene
2‚0.75toluene
3
5
6
empirical formula
fw
temp (K)
λ (Å)
C
43.25H68ClN2O3Yb
C43.25H68ClErN2O3
866.71
C
39H65N2O3Yb
C50H72N3O3Yb
936.15
C50H72ErN3O3
930.37
193(2)
0.7107
monoclinic
P21/c
872.49
193(2)
782.97
193(2)
0.7107
monoclinic
C2/c
193(2)
193(2)
0.7107
monoclinic
C2/c
0.7107
monoclinic
C2/c
0.7107
monoclinic
P21/c
cryst syst
space group
a (Å)
37.9817(12)
14.6308(4)
36.1234(12)
118.8420(10)
17583.8(9)
16
38.036(3)
14.6408(7)
36.156(3)
118.8470(10)
17636.2(19)
16
48.357(5)
11.9624(9)
36.189(4)
130.326(1)
15959(3)
16
10.7360(6)
22.4429(13)
19.8725(12)
100.018(2)
4715.2(5)
4
10.7883(12)
22.409(2)
19.903(2)
100.151(3)
4736.4(9)
4
b (Å)
c (Å)
â (deg)
V (Å3)
Z
density (Mg/m3)
1.318
1.306
1.303
1.319
1.305
abs coeff (mm-1
F(000)
θmax (deg)
)
2.225
2.001
2.379
2.025
1.814
7240
7208
6512
1948
1940
27.48
27.48
27.48
27.48
27.48
no. of reflns collected
no. of unique reflns
no. of reflns obsd
no. of params refined
R [I > 2σ(I)]
65 454
20 024
17 819
944
89 420
85 385
11 895
18 195
842
52 220
52 137
10 818
10 818
529
20 113
10 780
17 930
10 780
936
529
0.0425
0.0884
1.108
0.0421
0.0375
0.0808
1.162
0.0411
0.0386
0.0798
1.149
Rw
0.0941
0.0842
GOF on F2
1.123
1.159
Table 2. Selected Bond Lengths (Å) and Angles
(deg) for 1 and 2
1a
1b
2a
2b
Ln(1)-O(1)
Ln(1)-O(2)
Ln(1)-O(3)
Ln(1)-N(1)
Ln(1)-N(2)
Ln(1)-Cl(1)
C(1)-O(1)
C(7)-O(2)
2.116(3)
2.106(3)
2.310(3)
2.484(3)
2.518(3)
2.093(3)
2.086(3)
2.318(3)
2.484(3)
2.468(3)
2.116(3)
2.112(3)
2.331(3)
2.520(3)
2.492(3)
2.145(3)
2.125(3)
2.330(3)
2.510(3)
2.544(3)
2.5363(10) 2.5289(11) 2.5513(12) 2.5571(11)
1.337(4)
1.327(5)
1.339(5)
1.334(5)
1.337(5)
1.332(5)
1.336(5)
1.335(5)
C(1)-O(1)-Ln(1) 134.8(2)
C(7)-O(2)-Ln(1) 140.9(2)
O(3)-Ln(1)-N(2) 174.28(2)
O(1)-Ln(1)-N(1) 80.18(10)
N(1)-Ln(1)-O(2) 80.01(10)
O(2)-Ln(1)-Cl(1) 100.42(8)
N(1)-Ln(1)-Cl(1) 164.45(8)
136.7(2)
143.5(3)
137.0(3)
144.2(3)
134.5(2)
141.2(2)
167.44(12) 167.50(12) 174.72(11)
81.57(10)
79.26(10)
99.21(8)
80.38(11)
78.42(11)
100.12(8)
165.91(8)
79.54(10)
79.11(10)
101.34(8)
163.65(8)
166.31(8)
Figure 1. ORTEP diagram of complex 1 showing atom-
numbering scheme. Thermal ellipsoids are drawn at the
25% probability level. Hydrogen atoms are omitted for
clarity.
O(1)-Ln(1)-O(2) 155.58(11) 156.82(11) 154.82(11) 153.99(11)
which the substitution of the chloride with a range of
lithium or magnesium alkyls or LiN(SiMe3)2 was
unsuccessful.5i The difference between the amine bis-
(phenolate) ligands [Me2NCH2CH2N{CH2-(2-O-C6H2-
But2-3,5)}2] and O2tBuNN′ is that the sidearm donor in
the former is dimethylamido, while pyridine in the later.
These results reveal that the sidearm donor has a
significant effect on the reactivity of amine bis(pheno-
late) lanthanide complexes.
Crystal Structure Analyses. Although many amine
bis(phenolate) lanthanide complexes have been synthe-
sized, few have been structurally characterized.5d,f,g,i To
provide complete structural information for these novel
amine bis(phenolate) species, single-crystal X-ray struc-
tural investigations were carried out for complexes 1,
2, 3, 5, and 6. Details of the intensity data collection
and the crystal data are given in Table 1.
Crystals of complexes 1 and 2 that were suitable for
an X-ray structure determination were obtained from
concentrated toluene solution at room temperature.
Both complexes 1 and 2 crystallize with two crystallo-
graphically independent but chemically similar mol-
ecules (1a, 1b; 2a, 2b) in the unit cell. The selected bond
lengths and angles are provided in Table 2 for both
molecules. ORTEP diagrams of molecules 1a and 2a are
depicted in Figures 1 and 2, respectively. Complexes 1
Figure 2. ORTEP diagram of complex 2 showing atom-
numbering scheme. Thermal ellipsoids are drawn at the
20% probability level. Hydrogen atoms are omitted for
clarity.
and 2 are isostructural, and both have a monomeric
structure. Like other amine bis(phenolate) lanthanide
complexes,5d,f the sidearm amido group was found to
bind to the metal center in the solid state. The central
metal atom is coordinated 6-fold by two oxygen atoms

Amine Bis(phenolate) Lanthanide Complexes
Organometallics, Vol. 24, No. 16, 2005 4017
Table 3. Selected Bond Lengths (Å) and Angles
(deg) for 3, 5, and 6
3a
3b
5
6
Ln(1)-O(1)
Ln(1)-O(2)
Ln(1)-O(3)
Ln(1)-N(1)
Ln(1)-N(2)
Ln(1)-C(35)
Ln(1)-N(3)
C(1)-O(1)
2.119(2)
2.131(2)
2.325(3)
2.518(3)
2.536(3)
2.440(4)
2.124(2)
2.130(2)
2.330(2)
2.513(3)
2.546(3)
2.442(3)
2.115(2)
2.112(2)
2.338(2)
2.516(3)
2.509(3)
3.044(4)
2.256(3)
1.335(4)
1.342(4)
2.135(2)
2.134(2)
2.350(2)
2.549(3)
2.527(3)
3.035(3)
2.279(3)
1.340(4)
1.339(4)
1.333(7)
1.341(4)
1.338(4)
1.333(4)
C(7)-O(2)
C(1)-O(1)-Ln(1)
C(7)-O(2)-Ln(1)
137.8(2)
135.5(2)
136.6(2)
138.5(2)
141.0(2)
141.6(2)
141.3(2)
142.0(2)
O(3)-Ln(1)-N(2) 173.23(11) 175.90
159.73(10) 159.38(9)
O(1)-Ln(1)-N(1) 79.89(9)
N(1)-Ln(1)-O(2) 80.41(9)
81.14(9)
79.21(9)
79.71(9)
79.34(9)
79.03(8)
78.60(8)
N(1)-Ln(1)-C(35) 160.58(12) 159.04(11)
N(1)-Ln(1)-N(3)
169.67(11) 168.17(11)
154.35(10) 154.06(10) 154.76(9) 153.51(9)
O(1)-Ln(1)-O(2)
O(2)-Ln(1)-C(35) 103.56(12) 102.56(11)
C(35)-Ln(1)-O(1) 100.48(12) 101.86(11)
O(2)-Ln(1)-N(3)
Figure 3. ORTEP diagram of complex 3 showing atom-
numbering scheme. Thermal ellipsoids are drawn at the
30% probability level. Hydrogen atoms are omitted for
clarity.
103.41(11) 104.69(10)
N(3)-Ln(1)-O(1)
99.71(11)
100.09(10)
and two nitrogen atoms from the amine bis(phenolate)
ligand, one THF molecule, and one chlorine atom in a
distorted octahedron. The overall coordination geometry
is similar to that of Sc(O2tBuNN′)Cl(py),5i but different
from those of [(O2tBuNN′)Y(µ-Cl)(py)]25i and [(O2tPeNN′)-
Gd(µ-Cl)(THF)]2 [O2tpeNN′ ) Me2NCH2CH2N{CH2-(2-
O-C6H2-Pet2-3,5)}2],5g in which metal centers are seven-
coordinate. In complexes 1 and 2, O(1), Cl(1), O(2), and
N(1) can be considered to occupy equatorial positions
within the octahedron about the lanthanide center. O(3)
and N(2) occupy axial positions, and the O(3)-Ln-N(2)
angle is slightly distorted away from the idealized
position of 180° to 174.5(2)° for complex 1a and 167.5-
(1)° for complex 2a.
2,6)4],11 [Na(diglyme)2][Er(OC6H3Pri2-2,6)4] (diglyme )
bis(2-methoxyethyl) ether),12 and (ArO)2ErCl(THF)2,7
when the effect of the coordination number on the
effective ionic radius is considered. The Er-Cl bond
length of 2.551(1) Å in complex 2 is slightly longer than
that of (ArO)2ErCl(THF)2.7 The average C-O bond
length of the bis(phenolate) ligand is 1.334(5) Å, which
is similar to that of complex 1 (1.332(4) Å), but appar-
ently shorter than the single-bond length, which reflects
a substantial electron delocalization of oxygen into the
aromatic rings.
Crystals of complex 3 that were suitable for an X-ray
structure determination were obtained from a concen-
trated toluene solution at -15 °C. The unit cell of
complex 3 contains two half-molecules of LYbMe(THF).
The selected bond lengths and angles are provided in
Table 3 for both molecules. No significant differences
were observed between the two crystallographically
independent molecules (3a and 3b), and for the purposes
of this discussion only those that contain Yb(1) will be
considered. An ORTEP of molecule 3a is depicted in
Figure 3. Complex 3 has a monomeric structure. The
overall molecular geometry of complex 3 is somewhat
similar to that of complex 1, with the main difference
being that one methyl group instead of a chlorine atom
coordinates with the ytterbium atom. The coordination
geometry is also similar to that of the yttrium atom in
[MeOCH2CH2N{CH2-(2-O-C6H2-But2-3,5)}2]Y(CH2SiMe3)-
(THF).5d The Yb-O and Yb-N bond lengths in complex
3 are rather similar to those that were observed in
complex 1 and [MeOCH2CH2N{CH2-(2-O-C6H2-But2-
3,5)}2]Y(CH2SiMe3)(THF).5d The Yb(1)-C(35) bond length
of 2.440(4) Å compares well with the Ln-C bond length
in the other lanthanide alkyl complexes, such as
[MeOCH2CH2N{CH2-(2-O-C6H2-But2-3,5)}2]Y(CH2SiMe3)-
(THF) (2.431(6) Å),5d [MeY(THF)6][BPh4]2 (2.418(3)
Å),13a and (C5Me5)2LnCH(SiMe3)2 (2.468(7) Å),13b but
significantly longer than the value observed in (C5H5)2-
YbMe(THF) (2.362(11) Å).13c
The Yb-O(Ar) bond lengths of 2.119(5) and 2.102(5)
Å in complex 1 are comparable to the corresponding
5i
values in [Y(O2tBuNN′)(µ-Cl)(py)]2 and [(O2tPeNN′)Gd-
(µ-Cl)(THF)]2,5g when differences in ionic radii are
considered. The Yb-Cl bond length of 2.536(1) Å in
complex 1 is in agreement with the data found in the
other ytterbium chlorides, (ArO)2YbCl(THF)2 (2.4770-
(9) Å),7 Yb(ArO)Cl2(THF)3 (2.555(2) Å) (ArO ) OC6H2-
2,6-But2-4-Me),8 YbCl3(THF)3 (2.52(7) Å),9 and CpYbCl2-
(THF)3 (2.594(3) Å),10 when the difference in coordination
number is considered. The Yb(1)-N(2) bond length of
2.517(5) Å is in accordance with the corresponding
5i
values in [Y(O2tBuNN′)(µ-Cl)(py)]2 and [(O2tPeNN′)Gd-
(µ-Cl)(THF)]2,5g and the Yb(1)-N(1) bond length of
2.482(4) Å is apparently shorter than the bond lengths
that were observed in the aforementioned amine bis-
(phenolate) lanthanide complexes.
The Er-O(Ar) bond lengths of 2.116(3) and 2.112(3)
Å in complex 2 are comparable to the corresponding
bond lengths in complex 1 when the difference in ionic
radii is considered. However, these Er-O(Ar) bond
lengths are slightly shorter than those for the other
aryloxo erbium complexes, such as K[Er(OC6H3Pri2-
(7) Zhang, L. L.; Yao, Y. M.; Luo, Y. J.; Shen, Q.; Sun, J. Polyhedron
2000, 19, 2243.
(8) Yao, Y. M.; Shen, Q.; Sun, J. Polyhedron 1998, 17, 519.
(9) Qian, C. T.; Wang, B.; Deng, D. L.; Xu, C.; Sun, X. Y.; Ling, R.
G. Chin. J. Struct. Chem. 1993, 12, 18.
(11) Clark, D. L.; Gordon, J. C.; Huffman, J. C.; Vincent-Hollis, R.
L.; Watkin, J. G.; Zwick, B. D. Inorg. Chem. 1994, 33, 5903.
(12) Deacon, G. B.; Feng, T.; Junk, P. C.; Skelton, B. W.; White, A.
H. J. Chem. Soc., Dalton Trans. 1997, 1181.
(10) Adam, M.; Li, X. F.; Oroschin, W.; Fischer, R. D. J. Organomet.
Chem. 1985, 296, C19.

4018 Organometallics, Vol. 24, No. 16, 2005
Yao et al.
Figure 5. ORTEP diagram of complex 6 showing atom-
numbering scheme. Thermal ellipsoids are drawn at the
30% probability level. Hydrogen atoms are omitted for
clarity.
Figure 4. ORTEP diagram of complex 5 showing atom-
numbering scheme. Thermal ellipsoids are drawn at the
30% probability level. Hydrogen atoms are omitted for
clarity.
of 3.067(5) Å for η3-diketiminate bonding in (CH3C5H4)-
[(DIPPh)2nacnac]YbCl,15 2.986(6) and 3.180(9) Å for
bridging η2-C5H5 bonding in (C5Me5)2Sm(µ-C5H5)Sm(C5-
Me5)2,18 2.814(4)-3.148(6) Å for chelating η6-, η1-Ph-
Yb bonding in [Yb(Odpp)3]2 (Odpp ) 2,6-diphenylphe-
nolate),19 and 2.986(6) and 3.053(6) Å for chelating Yb-
η-Ph intramolecular interaction in (C5Me5)2YbNPh2.14
Variations in the Ln-N-C angles are associated with
the existences of agostic interactions in these complexes.
The angles that are associated with the nearby carbon
atoms (Ln(1)-N(3)-C(35)) are 109.3(2)° in complex 5
and 107.8(2)° in complex 6, and the other angles (Ln-
(1)-N(3)-C(41)) are 136.9(3)° and 138.5(3)°, respec-
tively; the C(35)-N(3)-C(41) angles slightly deviated
from 120° to 113.6(3)° in these two complexes. The
O(3)-Ln(1)-N(2) bond angles of 159.73(10)° in complex
5 and 159.38(9)° in complex 6 are apparently more
obtuse than the corresponding bond angles in complexes
1-3. It is reasonable to ascribe the difference in bond
parameters to the increased steric congestion in com-
plexes 5 and 6 that results from the replacement of the
chlorine atom or methyl group by a bulky diphenyla-
mido group.
Ring-Opening Polymerization of E-Caprolac-
tone. The aliphatic polyesters, such as poly(ꢀ-caprolac-
tone) (PCL) and poly(lactide) (PLA), and their copoly-
mers have found wide application in the medical field
as biodegradable surgical sutures or as a delivery
medium for controlled release of drugs due to their
biodegradable, biocompatible, and permeable proper-
ties.20 The major method that is used to synthesize these
polymers is the ring-opening polymerization of lactones/
lactides and functionally related compounds (eq 1).
Many kinds of organometallic and coordination com-
plexes have been reported to be efficient initiators for
the ring-opening polymerization of lactones/lactides. All
the systems give the polymers with both high molecular
Suitable crystals of complexes 5 and 6 for X-ray
structure determination were obtained from a concen-
trated toluene solution at room temperature. Their
molecular diagrams are depicted in Figures 4 and 5, and
their selected bond lengths and angles are provided in
Table 3. Complexes 5 and 6 are isostructural, and both
have monomeric structures. The metal center in these
two complexes is coordinated by three oxygen atoms and
three nitrogen atoms to form a slightly distorted octa-
hedron. The overall molecular geometry of complexes 5
and 6 is similar to that observed in [MeOCH2CH2N-
{CH2-(2-O-C6H2-But2-3,5)}2]Y[N(SiMe2H)](THF)5d and
in complexes 1-3.
In complexes 5 and 6, the Ln-O and Ln-N(amido)
bond lengths lie within the range of the corresponding
Ln-O(phenoxy) and Ln-N(amido) bond lengths found
in complexes 1, 2, and 3. The Ln-N(Ph) bond lengths
of 2.256(3) Å in complex 5 and 2.279(3) Å in complex 6
are comparable to the corresponding values that have
been reported for (CH3C5H4)2YbNPh2(THF), (C5Me5)2-
YbNPh2,14 (CH3C5H4)[(DIPPh)2nacnac]YbNPh2 ((DIPPh)2-
nacnac ) N,N-diisopropylphenyl-2,4-pentanediimine
anion),15 DanipYb[N(SiMe3)2] and DanipYb[N(SiHMe2)2]2
(Danip ) 2,6-di(o-anisol)phenyl),16 and [Na(THF)2(µ-η5:
η5-CH3C5H4)2Er(NPh2)2]n.17 It is worth noting that there
is an agostic interaction of the R carbon atom of the
diphenylamido group with the central metal atom. The
Ln-C(35) bond lengths are 3.044(4) Å in complex 5 and
3.035(3) Å in complex 6, which are comparable to those
(13) (a) Arndt, S.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed.
2003, 42, 5075. (b) Den Haan, K. H.; De Boer, J. L.; Teuben, J. H.;
Spek, A. L.; Kojic-Prodic, B.; Hays, G. R.; Huis R. Organometallics
1986, 5, 1726. (c) Evans, W. J.; Dominguez, R.; Hanusa, T. P.
Organometallics 1986, 5, 263.
(14) Wang, Y. R. Shen, Q.; Xue, F.; Yu, K. B. J. Organomet. Chem.
2000, 598, 359.
(15) Yao, Y. M.; Zhang, Y.; Shen, Q.; Yu, K. B. Organometallics 2002,
21, 819.
(16) Rabe, G. W.; Presse, M. Z.; Riederer, F. A.; Yap, G. P. A. Inorg.
Chem. 2003, 42, 3527.
(18) Evans, W. J.; Ulibarri, T. A. J. Am. Chem. Soc. 1987, 109, 4292.
(19) Deacon, G. B.; Nickle, S.; Mackinnon, P. I.; Tiekink, E. R. T.
Aust. J. Chem. 1990, 43, 1245.
(17) Wang, Y.; Shen, Q.; Xue, F.; Yu, K. Organometallics 2000, 19,
357.
(20) Fujisato, T.; Ikada, Y. Macromol. Symp. 1996, 103, 73.

Amine Bis(phenolate) Lanthanide Complexes
Organometallics, Vol. 24, No. 16, 2005 4019
Table 4. Ring-Opening Polymerization of
analogous lanthanide amido complexes, the difference
in catalytic activity between erbium and ytterbium
complexes was also observed. The yield reaches 52% in
1 h for complex 6 when the molar ratio of monomer to
initiator is increased to 3000:1, while no polymer is
obtained at the same molar ratio even when the polym-
erization time is prolonged to 6 h for complex 5. The
observed increasing order, Er > Yb, is in agreement
with the order of ionic radii. This may be because the
larger ionic radii result in the greater opening of the
metal coordination sphere in the vicinity of the σ ligand,
which makes the insertion of ꢀ-caprolactone into Ln-N
bonds easier. The molecular weight distributions of the
resulting polymers range from 1.45 to 2.58, and de-
creasing of the polymerization temperature for the
methyl complex led to almost no change of the molecular
weight distribution. Normally, the broader molecular
weight distribution may mainly contribute to the trans-
esterification reaction during the polymerization pro-
cess.24 These results indicate that the transesterification
reaction during the polymerization of ꢀ-caprolactone in
the present systems cannot be effectively suppressed.
E-Caprolactone by 3, 5, and 6a
b
entry initiator T/°C [M]/[I] t/h Mn( × 104) Mw/Mn yieldc
1
3
3
3
3
3
5
5
5
5
5
5
5
6
6
6
-10
0
25
25
200 1.2
200
200 0.8
500
1.30
0.84
0.93
5.05
2.76
1.73
1.96
3.03
4.60
9.64
1.47
1.62
1.59
1.49
1.45
1.89
1.69
1.71
1.94
2.58
37
71
99
99
33
92
92
95
97
99
64
0
2
2
3
4
9
5
25 1000 30
6
25
54
25
200 30 s
200 30 s
500 30 s
7
8
9
25 1000 1 min
25 2000 1 min
25 2500 4 h
25 3000 6 h
25 1000 1 min
25 2000 1 min
25 3000 1 h
10
11
12
13
14
15
3.36
6.28
1.84
1.93
95
96
52
a
General polymerization conditions: in toluene, solvent/
monomer ) 4 (v/v). b Measured by GPC calibrated with standard
polystyrene samples. c Yield: weight of polymer obtained/weight
of monomer used.
weights and high yields.21 Among these initiators, the
structurally well-defined catalyst systems are especially
interesting, as they afford the possibility of understand-
ing the catalyst structure/reactivity relationships.21a
Conclusion
In summary, we have successfully synthesized a
series of new amine bis(phenolate) lanthanide methyl
and amido complexes. Their structure features have
been determined by X-ray diffraction study. Using
amine bis(phenolate) L2- [L ) Me2NCH2CH2N{CH2-(2-
O-C6H2-But2-3,5)}2] as an ancillary ligand, the amine
bis(phenolate) lanthanide chlorides can be prepared in
high yield. These lanthanide chlorides are useful pre-
cursors for the synthesis of amine bis(phenolate) lan-
thanide methyl and amido complexes by simple salt
metathesis reaction. The catalytic behavior of these
amine bis(phenolate) lanthanide methyl and amido
complexes for ring-opening polymerization of ꢀ-capro-
lactone was also studied. The lanthanide amido com-
plexes showed extremely high activity for ꢀ-caprolactone
polymerization. Further studies on the synthesis of
lanthanide complexes that are supported by amine bis-
(phenolate) ligands bearing different sidearm donors
and the effect of the sidearm donor on their reaction
chemistry are in process in our laboratory.
The catalytic behavior of complexes 3, 5, and 6 for
the ring-opening polymerization of ꢀ-caprolactone was
examined, and the preliminary results are summarized
in Table 4. It can be seen that all of the complexes are
efficient initiators for the polymerization of ꢀ-caprolac-
tone in toluene, and the resulting polymers have a high
molecular weight and relatively narrow molecular weight
distributions (Mw/Mn). The lanthanide amido complexes
showed extremely high activity compared with the
lanthanide methyl complex. For example, for the amido
complexes 5 and 6, nearly quantitative yield is obtained
within 1 min at 25 °C when the molar ratio of monomer
to initiator is 2000:1, and even when the amount of the
initiator decreases to [CL]:[I] ) 3000:1, as in the case
of 6, the polymerization still gives a 52% yield within 1
h. However, for the methyl complex 3, the yield is only
33% after 30 h; even the molar ratio of monomer to
initiator is 1000:1. The catalytic activity of complexes
5 and 6 is much higher than those found in the systems
with amine bis(phenolato)lanthanide amides reported.5g
For example, few complexes showed activity for this
polymerization; moreover, the yield reaches 100% in 1
h only when the loading of the initiator is as high as
1:100 (molar ratio of initiator to monomer). In compari-
son with the analogous methyl and amido complexes
supported by cyclopentadienyl ligands, the activity of
the amido complexes here is also much higher than that
of lanthanocene amides,22 while almost the same reac-
tivity was found for the methyl complex.23 For the
Experimental Section
General Considerations. All of the manipulations were
performed in a purified argon atmosphere using standard
Schlenk techniques. The solvents were degassed and distilled
from sodium benzophenone ketyl under argon prior to use.
Ligand LH225 and anhydrous LnCl326 were prepared according
to the procedures that are recommended in the literature.
ꢀ-Caprolactone was purchased from Acros, dried by CaH2 for
48 h, and then distilled under reduced pressure. All of the
other reagents were purchased from Aldrich and used as
received without further purification. The uncorrected melting
points were determined in sealed Ar-filled capillary tubes.
Lanthanide analyses were carried out by complexometric
titration, and chloride analysis was performed using the
Volhard method. Carbon, hydrogen, and nitrogen analyses
(21) 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.
(22) Shen, Q.; Yao, Y. M. J. Organomet. Chem. 2002, 647, 180.
(23) Yamashita, M.; Takemoto, Y.; Ihara, E.; Yasuda, H. Macro-
molecules 1996, 29, 1798.
(24) Evans, W. J.; Katsumata, H. Macromolecules 1994, 27, 2330.
(25) Tshuva, E. Y.; Goldberg, I.; Kol, M. Organometallics 2001, 20,
3017.
(26) Tayler, M. D.; Carter, C. P. J. Inorg. Nucl. Chem. 1962, 24,
387.

4020 Organometallics, Vol. 24, No. 16, 2005
Yao et al.
were preformed 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 molecular weight
and molecular weight distributions were determined against
polystyrene standards by gel permeation chromatography
(GPC) at 30 °C with a Waters 1515 apparatus with three HR
columns (HR-1, HR-2, and HR-4) using THF as the eluent.
LYbCl(THF) (1). A solution of LH2 (2.63 g, 5.01 mmol) in
THF (20 mL) was added dropwise to a NaH suspension (10.02
mmol) in THF at room temperature. The stirring was contin-
ued for 14 h, and the mixture was then filtered. The resulting
pale yellow solution was added to a suspension of YbCl3 (1.40
g, 5.01 mmol) in THF (20 mL). After stirring the solution
overnight at room temperature, the precipitation was sepa-
rated from the reaction mixture by centrifugation. The solvent
was removed in a vacuum, and the residue was extracted with
toluene. Light yellow crystals were obtained from toluene
solution at room temperature in a few days (3.21 g, 79.5%).
Mp: 180-182 °C (dec). Anal. Calcd for C38H62ClN2O3Yb: C,
56.81; H, 7.78; N, 3.49; Cl, 4.41; Yb, 21.54. Found: C, 57.11;
H, 7.32; N, 3.62; Cl, 4.52; Yb, 21.42. IR (KBr pellet, cm-1): 2959
(s), 2905 (s), 2866 (s), 1767 (m), 1601 (m), 1562 (m), 1474 (s),
1416 (m), 1389 (m), 1362 (m), 1250 (m), 1165 (m), 1107 (m),
1022 (m), 880 (m), 841 (m), 806 (m), 775 (m), 733 (m), 532 (s).
LErCl(THF) (2). Complex 2 was prepared by a procedure
that was analogous to the procedure for complex 1, except
anhydrous ErCl3 (1.66 g, 6.07 mmol) was used instead of YbCl3.
Pink crystals were obtained from concentrated toluene solution
at room temperature in a few days (4.0 g, 82.6%). Mp: 178-
180 °C (dec). Anal. Calcd for C38H62ClErN2O3: C, 57.22; H,
7.83; N, 3.51; Cl, 4.44; Er, 20.97. Found: C, 57.41; H, 7.91; N,
3.43; Cl, 4.36; Er, 20.80. IR (KBr pellet, cm-1): 2959 (s), 2904
(s), 2870 (s), 1624 (m), 1531 (m), 1481 (s), 1416 (m), 1389 (m),
1362 (m), 1227 (s), 1157 (s), 1045 (m), 879 (m), 841 (m), 779
(m), 637 (m), 556 (m), 505 (s).
LYbMe(THF) (3). A Schlenk flask was charged with
complex 1 (4.02 g, 5.00 mmol) and THF (20 mL). The solution
was cooled to -15 °C, and MeLi (4.8 mL, 5.00 mmol) was added
by syringe. The reaction mixture was kept at -15 °C for 1 h
and was slowly warmed to room temperature and stirred
overnight. After removing the volatiles in a vacuum, the yellow
residue was extracted with toluene, and LiCl was removed by
centrifugation. Complex 3 was obtained as yellow crystals by
cooling the solution to -5 °C (1.94 g, 49.6%). Mp: 168-170
°C (dec). Anal. Calcd for C39H65N2O3Yb: C, 59.82; H, 8.37; N,
3.58; Yb, 22.10. Found: C, 60.15; H, 7.93; N, 3.57; Yb, 22.01.
IR (KBr pellet, cm-1): 2959 (s), 2905 (s), 2870 (s), 1632 (m),
1539 (m), 1544 (m), 1466 (s), 1416 (m), 1362 (m), 1228 (s), 1196
(s), 1045 (m), 987 (m), 926 (m), 799 (m), 725 (m), 637 (s), 556
(m), 505 (s).
removed in a vacuum, the residue was extracted with toluene,
and LiCl was removed by centrifugation. Yellow crystals were
obtained from concentrated toluene solution at room temper-
ature after 2 days (5.66 g, 80.0%). Mp: 164-166 °C (dec). Anal.
Calcd for C50H72N3O3Yb: C, 64.15; H, 7.75; N, 4.49; Yb, 18.48.
Found: C, 64.28; H, 7.46; N, 4.19; Yb, 18.26. IR (KBr pellet,
cm-1): 2959 (s), 2901 (s), 2866 (s), 1597 (s), 1526 (m), 1474
(s), 1416 (m), 1389 (m), 1362 (m), 1308 (s), 1238 (m), 1169 (m),
1026 (m), 879 (m), 837 (m), 748 (s), 690 (m).
LErNPh2(THF) (6). Complex 6 was prepared by a proce-
dure that was analogous to the procedure for complex 5, except
complex 1 (5.24 g, 6.57 mmol) was used instead of complex 2.
Light pink crystals were obtained from concentrated toluene
solution at room temperature in a few days (4.65 g, 76.1%).
Mp: 167-169 °C (dec). Anal. Calcd for C50H72ErN3O3: C,
64.55; H, 7.80; N, 4.52; Er, 17.98. Found: C, 64.26; H, 8.04;
N, 4.34; Er, 18.05. IR (KBr pellet, cm-1): 2955 (s), 2905 (s),
2866 (s), 1597 (s), 1478 (s), 1416 (m), 1362 (m), 1304 (s), 1238
(m), 1203 (m), 1165 (m), 1026 (m), 879 (m), 837 (m), 748 (s),
691 (m).
A Typical Polymerization Procedure. The polymeriza-
tions were carried out under dry argon atmosphere. The
general procedure was as follows: a 20 mL Schlenk flask that
was equipped with a magnetic stirring bar was charged with
an appropriate amount of the lanthanide complex and toluene,
depending upon the monomer-to-initiator ratio. To this solu-
tion was added 0.6 mL of ꢀ-caprolactone by a syringe. The
contents of the flask were then stirred vigorously at a fixed
temperature, during which time the mixture became very
viscous, which thus disrupted the stirring. The reaction
mixture was quenched by the addition of methanol solution
that contained 5% HCl and was then poured into ethanol to
precipitate the polymer, which was dried in a vacuum and
weighed.
X-ray Crystallography. Suitable single crystals of com-
plexes 1-3, 5, and 6 were sealed in a thin-walled glass
capillary to determine the single-crystal structure. Intensity
data were collected with a Rigaku Mercury CCD area detector
in ω scan mode using Mo KR radiation (λ ) 0.71070 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
2
by full-matrix least-squares procedures based on |F| . All of
the non-hydrogen atoms were refined anisotropically. The
hydrogen atoms in these complexes were all generated geo-
metrically (C-H bond lengths fixed at 0.95 Å), were assigned
appropriate isotropic thermal parameters, and were allowed
to ride on their parent carbon atoms. All of the H atoms were
held stationary and were included in the structure factor
calculation in the final stage of full-matrix least-squares
refinement. The structures were solved and refined using the
SHELXL-97 program.
LErMe(THF) (4). Complex 4 was prepared by a procedure
that was analogous to the procedure for complex 3, except
complex 2 (4.26 g, 5.34 mmol) was used instead of complex 1.
Pink microcrystals were obtained from concentrated toluene
solution at -5 °C in a few days (1.60 g, 38.6%). Mp: 172-174
°C (dec). Anal. Calcd for C39H65ErN2O3: C, 60.27; H, 8.43; N,
3.60; Er, 21.52. Found: C, 60.18; H, 8.17; N, 3.22; Cl, 4.41;
Er, 21.54. IR (KBr pellet, cm-1): 2956 (s), 2906 (s), 2867 (s),
1627 (m), 1602 (m), 1548 (m), 1470 (s), 1415 (m), 1363 (m),
1229 (s), 1194 (s), 1049 (m), 985 (m), 881 (m), 837 (m), 634 (s),
506 (s).
LYbNPh2(THF) (5). A Schlenk flask was charged with Ph2-
NH (1.28 g, 7.56 mmol), THF (25 mL), and a stirring bar. The
solution was cooled to 0 °C, and n-BuLi in hexane (6.30 mL,
7.56 mmol) was added by syringe. The solution was then slowly
warmed to room temperature and stirred for 1 h. This solution
was added to complex 1 in 20 mL of THF, and the solution
was stirred overnight at room temperature. The solvent was
Acknowledgment. The authors are grateful to the
Chinese National Natural Science Foundation (20472063
and 20272040) and the Key Laboratory of Organic
Synthesis of Jiangsu Province for financial support. This
work is also supported by the University of Hong Kong
(W.T.W.).
Supporting Information Available: Tables of X-ray
diffraction data for 1, 2, 3, 5, and 6. This material is available
free of charge via the Internet at http://pubs.acs.org
OM050296N