Ytterbium(II) complex bearing a diaminobis(phenolate) ligand: Synthesis, structure, and one-electron-transfer and ε-caprolactone polymerization reactions

Article abstract of DOI:10.1021/ic061805w

The first divalent ytterbium complex supported by a diaminobis(phenolate) ligand, YbL(THF)₂·0.5C₇H₈ (1; THF = tetrahydrofuran), was synthesized in good yield by the amine elimination reaction of Yb[N(SiMe₃)₂]₂(THF)₂ with H₂L (L = [Me₂NCH₂CH₂N(CH ₂-OC₆H₂-3,5-Bu^t₂) ₂]) in a 1:1 molar ratio. X-ray structural determination shows complex 1 to be a THF-solvated monomer, which adopts a distorted octahedral coordination geometry around the Yb atom. Complex 1 can react with PhNCO and PhC≡CH, as a single electron-transfer reagent, to give the corresponding reduction coupling product [(YbLOCNPh)(THF)]₂·4THF (2) and the alkynide complex YbLC≡CPh(DME) (3; DME = 1,2-dimethoxyethane). Complexes 2 and 3 have been characterized by X-ray crystal structural analysis. In complex 2, the dianionic oxamide ligand resulting from the reductive coupling of two phenyl isocyanate molecules coordinates to two Yb atoms in a μ,η⁴ fashion. Complex 3 has a monomeric structure with a Yb-C(terminal phenylacetynide) bond length of 2.374(3) A. Complex 1 is also a highly efficient catalyst for ring-opening polymerization of ε-caprolactone.

Full text of DOI:10.1021/ic061805w

Inorg. Chem. 2007, 46, 958−964
Ytterbium(II) Complex Bearing a Diaminobis(phenolate) Ligand:
Synthesis, Structure, and One-Electron-Transfer and
Polymerization Reactions
E-Caprolactone
Hui Zhou,† Huadong Guo,† Yingming Yao,† Liying Zhou,† Hongmei Sun,† Hongting Sheng,†
Yong Zhang,† and Qi Shen*,†,‡
Key Laboratory of Organic Synthesis of Jiangsu ProVince, Department of Chemistry and Chemical
Engineering, Dushu Lake Campus, Suzhou UniVersity, Suzhou 215123, People’s Republic of
China, and State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, Shanghai, 200032, People’s Republic of China
Received September 22, 2006
The first divalent ytterbium complex supported by a diaminobis(phenolate) ligand, YbL(THF)2
tetrahydrofuran), was synthesized in good yield by the amine elimination reaction of Yb[N(SiMe3)2]2(THF)2 with H2L
(L
[Me2NCH2CH2N(CH2-2-OC6H2-3,5-But2)2]) in a 1:1 molar ratio. X-ray structural determination shows complex
1 to be a THF-solvated monomer, which adopts a distorted octahedral coordination geometry around the Yb atom.
Complex 1 can react with PhNCO and PhC CH, as a single electron-transfer reagent, to give the corresponding
reduction coupling product [(YbLOCNPh)(THF)]2 4THF (2) and the alkynide complex YbLC CPh(DME) (3; DME
1,2-dimethoxyethane). Complexes 2 and 3 have been characterized by X-ray crystal structural analysis. In
complex 2, the dianionic oxamide ligand resulting from the reductive coupling of two phenyl isocyanate molecules
coordinates to two Yb atoms in a C(terminal
η4 fashion. Complex 3 has a monomeric structure with a Yb
phenylacetynide) bond length of 2.374(3) Å. Complex 1 is also a highly efficient catalyst for ring-opening polymerization
of -caprolactone.
‚0.5C7H8 (1; THF )
)
t
‚
t
)
µ
,
−
ꢀ
Introduction
bulky alkyl,6 bridged diamido,7 etc., and some of lanthanide-
(II) complexes displayed a rich chemistry1,8 and a level of
reactivity even higher than that of the Cp derivatives in the
activation of dinitrogen.9
Bridged bis(phenolate) ligands have become increasingly
popular in transition-metal and lanthanide(III) chemistry.10
Over the last 20 years, the chemistry of lanthanide(II)
complexes has made great progress. The use of substituted
cyclopentadienyl (Cp)-based ligand systems, which show
unique versatility in meeting the electronic and steric
requirements necessary for stabilizing a wide variety of
complexes, has been a significant factor contributing to the
rapid development.1 During the past decades, considerable
attention has been turned to the development of lanthanide-
(II) complexes supported by ligand systems other than Cp
anions. As a result, a variety of non-Cp ligands have been
found to be suitable in stabilizing lanthanide(II) species,
including amido,2 phenoxide,3 amidinato,4 â-diketiminato,5
(2) (a) Hitchcock, P. B.; Khvostov, A. V.; Lappert, M. F.; Protchenko,
A. V. J. Organomet. Chem. 2002, 647, 198. (b) Hou, Z.; Koizumi,
T.-a; Nishiura, M.; Wakatsuki, Y. Organometallics 2001, 20, 3323.
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Chem. 1988, 27, 575.
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125, 10790. (b) Yao, Y. M.; Zhang, Y.; Zhang, Z. Q.; Shen, Q.; Yu,
K. B. Organometallics 2003, 22, 2876. (c) Avent, A. G.; Khvostov,
A. V.; Hitchcock, P. B.; Lappert, M. F. Chem. Commun. 2002, 1410.
* To whom correspondence should be addressed. E-mail: qshen@
suda.edu.cn. Tel.: (86)512-65880329. Fax: (86)512-65880305.
† Suzhou University.
‡ Chinese Academy of Sciences.
(1) (a) Evans, W. J.; Davis, B. L. Chem. ReV. 2002, 102, 2119. (b) Evans,
W. J. J. Organomet. Chem. 2002, 647, 2. (c) Evans, W. J. Coord.
Chem. ReV. 2000, 206-207, 263. (d) Evans, W. J.; Miller, K. A.;
Lee, D. S; Ziller, J. W. Inorg. Chem. 2005, 44, 4326. (e) Hou, Z.;
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958 Inorganic Chemistry, Vol. 46, No. 3, 2007
10.1021/ic061805w CCC: $37.00
© 2007 American Chemical Society
Published on Web 01/13/2007

Yb(II) Complex Bearing a Diaminobis(phenolate) Ligand
The tetradentate side-arm donor-bridged bis(phenolate) ligands,
where the bracketed atoms are those capable of binding to
the metal but the type of bond is not defined, therefore called
a hemilabile functional group, have shown great potential
applications in catalytic reactions promoted by groups 3 and
4 and lanthanide(III) metals. For example, the aminobis-
(phenolate)-based group 4 metal catalysts exhibit remarkably
high activity and may be modified to give isospecific poly-
(1-hexene).11 Alkoxyaminobis(phenolate) group 3 and lan-
thanide metal catalysts are highly active in the synthesis of
heterotactic and sydiotactic poly(lactide) from rac- and meso-
lactide, respectively,10a,b and syndiospecific poly(â-butyro-
lactone) from racemic â-butyrolactone.10c However, the
application of these tetradentate bridged bis(phenolate)
ligands in lanthanide(II) chemistry has been ignored to date.
In continuing our research on the chemistry of bridged bis-
(phenolate)lanthanide(II) complexes,3a we have studied the
synthesis of a ytterbium(II) complex with a diaminobis-
(phenolate) ligand and its reactivity. Here we report the
synthesis and characterization of a monomeric ytterbium-
(II) complex YbL(THF)2‚0.5C7H8 (1; L ) [Me2NCH2CH2N-
(CH2-2-OC6H2-3,5-But2)2]2-; THF ) tetrahydrofuran), which
is very soluble in THF and soluble in hot toluene. The
catalytic activity of complex 1 for the ring-opening polym-
erization of ꢀ-caprolactone (ꢀ-CL) is described, together with
a study of its potential application as a one-electron-transfer
reagent for the activation of organic molecules including
reactions with phenyl isocyanate to a coupling product
[(YbLOCNPh)(THF)]2‚4THF (2) and with phenylacetylene
to an alkynide complex YbLCtCPh(DME) (3; DME ) 1,2-
dimethoxyethane).
Experimental Section
General Procedures. All manipulations were performed under
an Ar atmosphere, using standard Schlenk techniques. THF, DME,
toluene, and hexane were distilled from sodium benzophenone ketyl
before use. ꢀ-CL was purchased from Acros, dried over CaH2 for
48 h, and distilled under reduced pressure. PhNCO was purchased
from Acros, dried over P2O5 for 48 h, and distilled under reduced
pressure. PhCtCH was purchased from Acros, dried over molecular
sieves for 48 h, and distilled by vacuum. The starting complexes
Yb[N(SiMe3)2]2(THF)212 and H2L13 were synthesized according to
published methods. Lanthanide metal analyses were performed by
ethylenediaminetetraacetic acid titration with a xylenol orange
indicator and a hexamine buffer.14 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 Fourier
transform IR spectrometer as KBr pellets. The melting points of
complexes were measured in sealed capillaries and were uncor-
1
rected. H NMR spectra were obtained on an INOVA 400 MHz
apparatus and referenced to benzene-d. Molecular weights and
molecular weight distributions were determined against polystyrene
standards by gel permeation chromatography (GPC) at 30 °C on a
Water 1515 apparatus with three HR columns (HR-1, HR-2, and
HR-4) using THF as the eluent. The molecular weights of polymers
were corrected by a factor of 0.56.15
YbL(THF)2‚0.5C7H8 (1). A Schlenk flask was charged with H2L
(1.45 g, 2.77 mmol), toluene (15 mL), and a stirring bar. To this
solution was added a toluene (20 mL) solution of Yb[N(SiMe3)2]2-
(THF)2 (1.76 g, 2.77 mmol), leading to the formation of a brown-
red precipitate. The suspension was stirred at room temperature
for 24 h. After centrifugation, the precipitate was dissolved in THF
(30 mL) and concentrated. The orange-red microcrystals of 1 (1.50
g, 61% based on Yb) were obtained upon crystallization at -15 °C.
Mp: 120-121 °C (dec). Elemental analysis and
1H NMR for
complex 1 with a half of toluene lost are as follows: Anal. Calcd
for YbC42H70N2O4 (840.06): C, 60.05; H, 8.40; N, 3.33; Yb, 20.60.
Found: C, 60.21; H, 8.11; N, 3.30; Yb, 20.97. 1H NMR (400 MHz,
C6D6, 25 °C): δ 6.80-7.15 (m, 4H, Ph), 3.60 (s, 4H, CH2), 3.42
(m, 8H, CH2), 2.50 (m, 4H, CH2CH2), 2.30 (s, 6H, CH3), 1.73 (m,
8H, CH2CH2), 1.35 (s, 36H, C(CH3)3)). IR (KBr, cm-1): 2956 (w),
2902 (w), 2871 (w), 1605 (m), 1474 (w), 1381 (m), 1304 (w), 1243
(m), 1196 (m), 1165 (m), 1111 (s), 1027 (m), 911 (s), 880 (m),
841 (m), 780 (s), 741 (m), 648 (s), 525 (m), 440 (m). Moreover,
the free solvent complex YbL was obtained under vacuum for
several hours. Elemental analysis and 1H NMR for complex 1 with
all solvent lost are as follows. Anal. Calcd for YbC34H54N2O2
(695.84): C, 58.69; H, 7.82; N, 4.03; Yb, 24.87. Found: C, 58.81;
(6) (a) Hou, Z.; Zhang, Y.; Nishiura, M.; Wakatsuki, Y. Organometallics
2003, 22, 129. (b) Forsyth, C. M.; Deacon, G. B. Organometallics
2000, 19, 1205. (c) Eaborn, C.; Hitchcock, P. B.; Izod, K.; Lu, Z.-R.;
Smith, J. D. Organometallics 1996, 15, 4783. (d) Hitchcock, P. B.;
Holmes, S. A.; Lappert, M. F.; Tian, S. J. Chem. Soc., Chem. Commun.
1994, 2691.
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Organomet. Chem. 2005, 690, 5078. (b) Giesbecht, G. R.; Cui, C.
M.; Shafir, A.; Schmidt, J. A. R.; Arnold, J. Organometallics 2002,
21, 3841.
(8) (a) Deng, M. Y.; Yao, Y. M.; Zhang, Y.; Shen, Q. Chem. Commun.
2004, 2742. (b) Hou, Z.; Wakatsuki, Y. Coord. Chem. ReV. 2002,
231, 1. (c) Schuktz, M.; Boncella, J. M.; Berg, D. J.; Tilley, T. D.;
Anderson, R. A. Organometallics 2002, 21, 460. (d) Edelmann, F.
T.; Freckmann, D. M. M.; Schumann, H. Chem. ReV. 2002, 102, 1851.
(e) Tardif, O.; Hou, Z.; Nishiura, M.; Koizumi, T.-a.; Wakatsuki, Y.
Organometallics 2001, 20, 4565. (f) Dube, T.; Gambarotta, S.; Yap,
G. P. A. Angew. Chem., Int. Ed. 1999, 38, 1432.
(9) (a) Ganesan, M.; Gambarotta, S.; Yap, G. P. A. Angew. Chem., Int.
Ed. 2001, 40, 766. (b) Ganesan, M.; Lalonde, M. P.; Gambarotta, S.;
Yap, G. P. A. Organometallics 2001, 20, 2443. (c) Dube, T.; Ganesan,
M.; Conoci, S.; Gambarotta, S.; Yap, G. P. A. Organometallics 2000,
19, 3716. (d) Dube, T.; Conoci, S.; Gambarotta, S.; Yap, G. P. A.;
Vasapollo, G. Angew. Chem., Int. Ed. 1999, 38, 3657.
1
H, 7.63; N, 3.87; Yb, 25.04. H NMR (400 MHz, C6D6, 25 °C):
δ 6.80-7.15 (m, 4H, Ph), 3.60 (s, 4H, CH2), 2.50 (m, 4H, CH2-
CH2), 2.30 (s, 6H, CH3), 1.35 (s, 36H, C(CH3)3)). Crystals suitable
for an X-ray diffraction analysis were obtained by crystallization
at -15 °C from THF.
[(YbLOCNPh)(THF)]2‚4THF (2). To a solution of 1 (1.85 g,
2.09 mmol) in 20 mL of THF was slowly added PhNCO (0.23
mL, 2.09 mmol). The color of the solution changed from red to
yellow immediately. The reaction was maintained for 24 h with
(10) (a) Amgoune, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J.-F.
Chem.sEur. J. 2006, 12, 169. (b) Cai, C. X.; Amgoune, A.; Lehmann,
C. W.; Carpentier, J.-F. Chem. Commun. 2004, 330. (c) Amgoune,
A.; Thomas, C. M.; Ilinna, S.; Roisnel, T.; Carpentier, J.-F. Angew.
Chem., Int. Ed. 2006, 45, 2782. (d) Yao, Y. M.; Ma, M. T.; Xu, X.
P.; Zhang, Y.; Shen, Q.; Wong, W.-T. Organometallics 2005, 24, 4014.
(e) Yao, Y. M.; Xu, X. P.; Liu, B.; Zhang, Y.; Shen, Q.; Wong, W.-
T. Inorg. Chem. 2005, 44, 5133.
(12) (a) Evans, W. J.; Drummond, D. K.; Zhang, H.; Atwood, J. L. Inorg.
Chem. 1988, 27, 575. (b) Boncella, J. M.; Anderson, R. A. Organo-
metallics 1985, 4, 205.
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Z.; Shuster, M. Organometallics 2004, 23, 5291. (b) Tshuva, E. Y.;
Goldberg, I.; Kol, M.; Goldschmidt, Z. Organometallics 2001, 20,
3017. (c) Tshuva, E. Y.; Goldberg, I.; Kol, M. J. Am. Chem. Soc.
2000, 122, 10706.
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1981, 20, 4115.
(15) Barakat, I.; Dubois, P.; Jerome, R.; Teyssie, P. J. Polym. Sci., Part A:
Polym. Chem. 1993, 31, 505.
Inorganic Chemistry, Vol. 46, No. 3, 2007 959

Zhou et al.
Table 1. Experimental Data for the X-ray Diffraction of Complexes 1-3
param
1
2
3
empirical formula
fw
C
45.50H74N2O4Yb
C106H166N6O12Yb2
2062.53
C46H69N2O4Yb
887.07
886.11
T (K)
193(2)
153(2)
153(2)
wavelength (Å)
cryst syst
space group
a (Å)
b (Å)
c (Å)
R (deg)
â (deg)
γ (deg)
V (Å 3)
0.710 70
triclinic
P1h
0.710 70
triclinic
P1h
0.710 70
monoclinic
P21/c
20.1597(18)
10.5615(8)
23.409(2)
90
114.011
90
4552.8(7)
4
9.6766(19)
14.893(3)
15.930(4)
99.946(5)
93.953(6)
94.277(4)
2247.1(8)
2
11.7955(10)
14.2426(13)
16.2769(14)
80.499(4)
76.699(4)
86.179(4)
2623.5(4)
1
Z
D
calc (Mg m-3
)
1.310
2.122
926
1.305
1.831
1080
1.294
2.095
1844
abs coeff (mm-1
F(000)
)
θ range (deg)
final R indices [I > 2σ(I)]
3.12-25.35
R1 ) 0.0300
wR2 ) 0.0739
3.17-25.35
R1 ) 0.0253
wR2 ) 0.0604
3.08-25.35
R1 ) 0.0252
wR2 ) 0.0576
stirring at room temperature. After removal of THF by vacuum, a
yellow solid was obtained. The solid was dissolved in DME with
a small amount of THF. Colorless crystals of 2 were obtained (1.36
g, 63% based on Yb) from THF (2 mL)-DME (8 mL) at room tem-
perature. Mp: 164-166 °C (dec). Anal. Calcd for Yb2C90H134N6O8-
(1774.14): C, 60.93; H, 7.61; N, 4.74; Yb, 19.51. Found: C, 60.04;
H, 7.55; N, 4.54; Yb, 19.32. IR (KBr, cm-1): 3419 (s), 2963 (w),
1728 (w), 1620 (s), 1497 (s), 1389 (s), 1258 (w), 1157 (m), 1119
(w), 887 (w), 841 (w), 748 (w), 594 (m), 432 (w). Crystals suitable
for X-ray analysis were grown in THF-DME at room temperature.
YbLCtCPh(DME) (3). To a solution of 1 (1.94 g, 2.19 mmol)
in 20 mL of THF was added PhCtCH (0.24 mL, 2.19 mmol)
slowly. The color of the solution changed from red to black
immediately. The reaction was maintained for 24 h with stirring at
room temperature. After removal of THF by vacuum, a brown solid
was dissolved in hexane with a small amount of DME, and colorless
crystals of 3 were obtained (1.15 g, 59% based on Yb) from DME
(4 mL)-hexane (4 mL) upon crystallization at 0 °C. Mp: 181-
182 °C (dec). Anal. Calcd for YbC46H69N2O4(887.07): C, 62.28;
H, 7.84; N, 3.16; Yb, 19.51. Found: C, 62.40; H, 7.95; N, 3.03;
Yb, 19.56. IR (KBr, cm-1): 3453 (m), 2954 (m), 1479 (m), 1360
(w), 1235 (s), 1213 (s), 1155 (s), 1026 (w), 877 (w), 837 (w), 757
(w), 639 (m), 555 (m), 503 (s). Crystals suitable for X-ray analysis
were grown from DME-hexane at 0 °C.
The structures were solved by direct methods and refined by
2
full-matrix least-squares procedures based on |F| . All non-H atoms
were refined anisotropically. The H atoms 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 C atoms. All of 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 the SHELEXL-97 program.
Results and Discussion
Synthesis and Structure of 1. The amine elimination
reaction was used for the synthesis of complex 1. Thus,
treatment of the ligand H2L with Yb[N(SiMe3)2]2(THF)2 in
a 1:1 molar ratio in toluene at room temperature, after
workup, afforded complex 1 as orange-red crystals in good
yield. Complex 1 was characterized by elemental analysis,
1H NMR spectral analysis, and X-ray diffraction (Scheme
1). Complex 1 loses a half of toluene under gentle evacuation.
So, satisfactory elemental analysis for complex 1 without a
half of toluene can be obtained by careful treatment of the
sample. Moreover, the crystals of complex 1 lose all of the
solvated solvent easily under vacuum to become an unsol-
vated complex YbL as an orange-red powder. In the former
X-ray Crystallography. Suitable single crystals of complexes
1-3 were sealed in a thin-walled glass capillary for determining
the single-crystal structure. Intensity data were collected on 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.
1
case, the H NMR spectrum shows signals at δ 3.42 and
1.73 for solvated THF, while no signals for THF were
observed in the latter one. Complex 1 is well soluble in THF,
soluble in hot toluene, but not soluble in cold toluene and
hexane. The solubility of complex 1 is much better than that
Scheme 1
960 Inorganic Chemistry, Vol. 46, No. 3, 2007

Yb(II) Complex Bearing a Diaminobis(phenolate) Ligand
Table 3. Selected Bond Distances (Å) and Angles (deg) for Complex 2
Yb1-O2
Yb1-O1
Yb1-O3
Yb1-O4
Yb1-N3A
2.101(2)
2.104(2)
2.233(2)
2.452 (2)
2.497(2)
Yb1-N1
Yb1-N2
O3-C35
N3-C35
C35-C35A
2.582(2)
2.657(2)
1.273(3)
1.305(3)
1.507(5)
O3-C35-N3
127.9(2)
117.1(3)
115.0(3)
124.77(16)
115.15(16)
O3-Yb1-N3A
C35-N3-C36
C36-N3-Yb1A
O2-Yb1-O1
67.54(7)
113.1(2)
131.64(16)
153.40(7)
O3-C35-C35A
N3-C35-C35A
C35-O3-Yb1
C35-N3-Yb1A
such kind of divalent complex reported. The bond lengths
of Yb-N in complex 1 are 2.554(3) and 2.603(3) Å [average
2.579(3) Å], which are shorter than 2.484(3) and 2.518(3) Å
[average 2.501(3) Å] found in a diaminobis(phenolate)-
ytterbium(III) chloride complex,10d when the difference in
the ionic radii between Yb(II) and Yb(III) is considered. (The
ion radius of Yb(II) is 0.152 Å bigger than that of Yb(III)
when Yb is six-coordinated.16)
Figure 1. Molecular structure of complex 1 with a 30% probability level.
H atoms are omitted for clarity.
Table 2. Selected Bond Distances (Å) and Angles (deg) for Complex 1
One-Electron-Transfer Reactions. 1. Reaction of 1 with
PhNCO. The samarium(II) complexes Sm(MeC5H4)2(THF)217
and Sm(ArO)2(THF)3 (ArO ) 2,6-di-tert-butyl-4-methyl-
phenoxide)18 were reported to react with PhNCO, as a single
electron-transfer agent, yielding the corresponding reduction
coupling products [Sm(MeC5H4)2OCNPh]2 (4)19a and [Sm-
(ArO)2OCNPh]2 (5).19b To understand the electron-transfer
properties of ytterbium(II) bis(phenolate), the reaction of
complex 1 with PhNCO in a molar ratio of 1:1 was examined
in THF at room temperature. The reaction went smoothly,
and the color change from red to yellow was observed as
the reaction proceeded. After workup, colorless crystals were
obtained from THF-DME in high yield at room temperature.
The crystals were characterized as complex 2 by elemental
analysis and X-ray determination (Scheme 2). Obviously,
complex 2 was formed via two one-electron reductions (2Yb-
(II) f 2Yb(III) + 2e) plus coupling from two phenyl
isocyanate molecules by two complexes 1. The reduction
coupling reaction of PhNCO by the ytterbium(II) complex
presented here is closely analogous to that by the samarium-
(II) complex.17,18 The molecular structure of complex 2 is
shown in Figure 2. Selected bond distances (Å) and angles
(deg) are listed in Table 3. There are four free THF molecules
in a unit cell. So, many free THF molecules existed in the
molecule of complex 2, making the determination of its
crystal structure as well as elemental analysis very difficult.
Satisfactory elemental analysis data can only be obtained
by very careful treatment of the sample.
Yb1-O1
Yb1-O2
Yb1-O4
2.270(2)
2.284(2)
2.414(3)
Yb1-O3
Yb1-N1
Yb1-N2
2.502(3)
2.554(3)
2.603(3)
O1-Yb1-O2
O1-Yb1-O4
O2-Yb1-O4
O1-Yb1-O3
O2-Yb1-O3
O4-Yb1-O3
O1-Yb1-N1
O2-Yb1-N1
151.16(9)
88.94(9)
84.35(9)
101.06(10)
106.20(10)
84.87(11)
80.20(8)
O4-Yb1-N1
O3-Yb1-N1
O1-Yb1-N2
O2-Yb1-N2
O4-Yb1-N2
O3-Yb1-N2
N1-Yb1-N2
119.75(10)
155.37(10)
92.46(9)
98.97(9)
169.99(10)
85.14(10)
70.23(9)
78.96(8)
of the C-bridged bis(phenolate)ytterbium(II) complexes
reported.3a
Single-crystal X-ray diffraction showed that complex 1
possesses a THF-solvated monomeric structure with a half
toluene in a unit cell (Figure 1). A six-coordinated Yb atom
adopts a distorted octahedral geometry, in which O1, N1,
O2, and O4 can be considered to occupy equatorial positions
and O3 and O5 occupy axial positions. The C-bridged
ytterbium(II) complexes were reported to have dimeric
structures.3a The difference in the solid-state structure
between complex 1 and C-bridged bis(phenolate)ytterbium-
(II) complexes should be attributed to the presence of the
coordination of N atoms on the bridge to Yb atoms, which
may provide steric protection against the formation of a
dimer.
Selected bond distances (Å) and angles (deg) for complex
1 are listed in Table 2. The Yb-OAr bond lengths of 2.270-
(2) and 2.284(2) Å and the average of 2.277(2) Å are close
to lengths of 2.201(2) and 2.349(2) Å and the average of
2.275(2) Å for C-bridged ytterbium(II) complex [Yb{CH2-
(2-OC6H2Me-4-But-6)2}(THF)(HMPA)]2.3a The values can
also compare with those found in the other ytterbium(II)
complexes with unbridged aryloxides [Yb(OAr)(µ-OAr)]2
[2.10(2) Å],3b Yb(OAr)2(THF)3 [2.202(6) Å],3c Yb(Odpp)2-
(THF)3 [2.207(3) Å],3d and [Yb(Odpp)(µ-Odpp)]2 [2.11(2)
Å], where Odpp ) OC6H3Ph2-2,6,3e when the effect of the
coordination number on the effective ionic radii is consid-
ered.16 The length of the Yb-N bond can only be compared
with those for the trivalent complexes because there is no
Complex 2 has a crystallographic C2h symmetry, with the
mirror plane containing C35, N3, O3, and Yb1 and phenyl
rings. The C2 axis bisects the C35-C35A bond. The C35-
C35A bond length of 1.507(5) Å in complex 2 is that of a
single bond. The C35-O3 and C35-N3 bond lengths, 1.273-
(16) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
(17) Evans, W. J. Polyhedron 1987, 6, 803.
(18) Qi, G.; Shen, Q.; Lin, Y. Acta Crystallogr., Sect. C 1994, 50, 1456.
(19) (a) Yuan, F. G.; Shen, Q.; Sun, J. Chem. J. Chin. UniV. 2001, 22,
1501. (b) Yuan, F. G.; Shen, Q.; Sun, J. Polyhedron 1998, 17, 2009.
(c) Evans, W. J.; Drummond, D. K.; Chamberlain, L. R.; Doedens,
R. J.; Bott, S. G.; Zhang, H.; Atwood, J. L. J. Am. Chem. Soc. 1988,
110, 4983.
Inorganic Chemistry, Vol. 46, No. 3, 2007 961

Zhou et al.
Figure 2. Molecular structure of complex 2 with a 30% probability level. H atoms are omitted for clarity.
Scheme 2
(3) and 1.305(3) Å, respectively, are consistent with multiple-
bond character delocalized over the N-C-O unit. The bond
lengths of Yb-O3 ) 2.233(2) Å and Yb-N3 ) 2.497(2) Å
are longer than those for the typical trivalent Yb-OR [Yb-
O1 ) 2.104(2) Å and Yb-O2 ) 2.101(2) Å in complex 2]
and Yb-NR2 single-bond length [Yb-NPh2 ) 2.256(3) Å
amount of DME, colorless crystals were obtained in high
yield. Single crystals suitable for X-ray diffraction studies
were obtained from hexane-DME and have been identified
as the monomeric ytterbium alkynide 3 (Scheme 3). The
molecular structure of complex 3 is shown in Figure 4. The
central Yb atom is coordinated by a bridged biphemolate
group, a terminal phenylacetynide anion, and a DME
molecule. The coordinated geometry around the Yb atom
can be described as an approximate face-capped trigonal
prism with the face-capping atom N1. The dimeric structure
in the diaminobis(phenolate)ytterbium(III) amide complex10d
]
but shorter than the corresponding dative bond lengths [Yb-
O4 ) 2.452(2) Å, Yb-N1 ) 2.582(2) Å, and Yb-N2 )
2.657(2) Å]. These bond parameters indicate that a [(PhN)-
OCCO(NPh)]2- dianion in complex 2 highly delocalizes its
electrons (Figure 3). The structural character of complex 2
is quite similar to that for complexes 4 and 5 as well as [Sm-
(C5Me5)2]2[µ-η4-(PhN)OCCO(NPh)]‚2C7H8, which was formed
by the reaction of Sm(C5Me5)2(µ-η1:η1-PhN2Ph) with CO.19c
2. Reaction of 1 with Phenylacetylene. The reaction of
Yb(C5Me5)2(OEt2) with phenylacetylene was addressed to
give the mixed-valent ytterbium phenylacetynide Yb3(C5-
Me5)4(µ-CtCPh)4 (6)20 presumably by way of prior acety-
lene coordination, while the same reaction with the divalent
samallocene afforded the trivalent phenylacetynide complex
Figure 3. Bond fashion of a dianionic ligand in 2.
t
[SmCp′2(µ-CtCPh)]2 for Cp′ ) C5H4 Bu21 and SmCp′2(Ct
CPh)(THF) for Cp′ ) C5Me5.22 To see the electron-transfer
property of the divalent ytterbium(II) complex supported by
a bridged bis(phenolate) ligand, the reaction of complex 1
with phenylacetylene has been examined. Complex 1 does
react with PhCtCH in a 1:1 molar ratio in THF at room
temperature. After concentration and treatment with a small
(20) Boncella, J. M.; Tilley, T. D.; Anderson, R. A. J. Chem. Soc., Chem.
Commun. 1984, 710.
(21) Shen, Q.; Zheng, D. S.; Lin, L.; Lin, Y. H. J. Organomet. Chem. 1990,
391, 307.
(22) Evans, W. J.; Ulibarri, T. A.; Chamberlain, L. R.; Ziller, J. W.; Alvarez,
D., Jr. Organometallics 1990, 9, 2124.
Figure 4. Molecular structure of complex 3 with a 30% probability level.
H atoms are omitted for clarity.
962 Inorganic Chemistry, Vol. 46, No. 3, 2007

Yb(II) Complex Bearing a Diaminobis(phenolate) Ligand
Scheme 3
with bridged alkynides is popular for lanthanide alkynide
complexes; in contrast, the monomeric structure is rare.22,23
The monomeric structure here may be attributed to the bulky
bis(phenolate) ligand and the coordination of DME, which
makes the formation of alkyne bridges impossible. The
ytterbium alkynide complexes are rare. To our best knowl-
edge, this is the second complex structurally characterized
to date; the first is a mixed-valent complex 6.20 So, the Yb-
(III)-C(CtCPh) bond length can only be compared with
those for other monomeric lanthanide alkynides. Selected
bond distances (Å) and angles (deg) for complex 3 are listed
in Table 4. The Yb-C35 bond length of 2.374(3) Å is
comparable to 2.419(6) Å for Sm(C5H5)2(CtCMe3)23a and
2.448(4) Å for Y[Me2Si(NCMe3)(OCMe3)]2CtCPh(THF),23b
when differences in the ionic radii are considered. Although
the value is shorter than 2.49(2) and 2.494(9) Å published
for Sm(C5Me5)2(CtCPh)(THF)22 and Sm(TpMe2)2(CtCPh),23c
respectively, the two data for Sm-C(terminal alkynide) were
regarded as unusually long. The bond length of Yb-C
[2.374(3) Å] in complex 3 is shorter than that of Yb(III)-
C(CtCPh) [2.40(2) Å] in complex 6; this is reasonable
because the bond length of Yb-C(terminal alkynide) nor-
mally is shorter than that of Yb-C(bridged alkynide). The
average bond length of Yb-OAr [2.105 (2) Å] is very close
to those found in complexes 1 and 2, when the differ-
ence in the ionic radii between Yb(II) and Yb(III) is
considered.16
Complexes 1-3 are very sensitive to air and moisture.
Complexes 2 and 3 did not provide any resolvable 1H NMR
spectra because of the strong paramagnetism of the Yb(III)
ions.
Catalytic Activity of Complex 1 for Ring-Opening
Polymerization of ꢀ-CL. To identify whether the ligand set
L can stabilize the new ytterbium (II) based catalyst, the
catalytic activity of complex 1 was tested for the ring-opening
polymerization of ꢀ-CL. The preliminary results are listed
in Table 5. The data previously reported with some other
ytterbium(II) complexes were also included for comparison.
It can be seen that complex 1 shows extremely high activity.
The polymerization was completed in 30 min at ambient
temperature when the molar ratio of [CL] to [I] (I indicates
complex 1) is 3000:1, and even when the molar ratio of [CL]
to [I] increases to 3500:1, the polymerization still gives a
yield as high as 73.7% within 30 min under the same
conditions. The activity is much higher than those found for
complexes Yb(ArO)2(THF)2 (entry 4), [Yb(MBMP)(THF)2]2
(entry 5),3a and Yb(C5H9C9H6)(THF)2 (entry 6)24 and even
comparable with that of Sm(ArO)2(THF)3 (entry 7).25
However, the degree of control of the polymerizations
performed with complex 1 is much lower than that afforded
by Sm(ArO)2(THF)3. In fact, the molecular weights of the
resulting polymers are always much lower than those
expected from the monomer-to-initiator ratio and the mo-
lecular weight distributions are broader than those obtained
with Sm(ArO)2(THF)3. These observations suggest that
transfer reactions may take place with complex 1.
Table 4. Selected Bond Distances (Å) and Angles (deg) for Complex 3
Yb1-O2
Yb1-O1
Yb1-C35
Yb1-O4
Yb1-N2
2.088(2)
2.122(2)
2.374(3)
2.459(2)
2.551(2)
Yb1-N1
Yb1-O3
C35-C36
C36-C37
2.566(2)
2.619(2)
1.211(4)
1.437(4)
Conclusion
The first ytterbium(II) complex supported by a diaminobis-
(phenolate) ligand, 1, has been synthesized in high yield by
the amine elimination reaction. Complex 1 can react with
PhNCO and phenylacetylene as a one-electron-transfer
reagent to give the corresponding complexes of 2 and 3.
Complex 1 exhibits extremely high activity for the ring-
opening polymerization of ꢀ-CL. The results indicate that
improved catalytic behavior is shown by the ytterbium(II)
complex supported by a tetradentate bis(phenolate) ligand
bearing side-arm donors.
O2-Yb1-O1
O2-Yb1-C35
O1-Yb1-C35
O2-Yb1-O4
C35-Yb1-O4
O2-Yb1-N2
O1-Yb1-N2
C35-Yb1-N2
O4-Yb1-N2
O2-Yb1-N1
O1-Yb1-N1
153.20(7)
95.56(9)
111.17(9)
79.24(7)
128.04(9)
94.13(7)
89.20(7)
79.23(9)
152.18(7)
77.58(7)
78.81(7)
O4-Yb1-N1
O2-Yb1-O3
O1-Yb1-O3
C35-Yb1-O3
O4-Yb1-O3
N2-Yb1-O3
N1-Yb1-O3
C36-C35-Yb1
C35-C36-C37
C35-Yb1-N1
82.94(6)
115.80(7)
74.85(7)
74.77(9)
62.37(6)
141.68(7)
137.46(6)
176.2(3)
179.2(3)
146.94(9)
Table 5. Polymerization of ꢀ-CL Initiated by Divalent Lanthanide Complexesa
b
entry
initiator
T (°C)
[M]/[I]
t (h)
yield (%)
Mn (×10-4
)
PDIb
ref
1
2
3
4
5
6
7
1
1
1
20
25
25
25
40
rt
1000
3000
3500
800
300
300
5 min
0.5
0.5
0.5
2
100
100
73.7
70
45
91.6
100
4.34
2.78
1.80
2.29
2.15
1.99
this work
this work
this work
this work
3a
(ArO)2Yb(THF)2
[(MBMP)Yb(THF)2]2
(C5H9C9H6)2Yb(THF)2
(ArO)2Sm(THF)2
1
1.31
62.6
1.71
1.56
24
25
25
2000
5 min
a Polymerization conditions: [M] ) 1 mol/L, with toluene as the solvent. b Measured by GPC at 30 °C in THF.
Inorganic Chemistry, Vol. 46, No. 3, 2007 963

Zhou et al.
Supporting Information Available: Crystallographic data for
1-3 and synthesis details. This material is available free of charge
via the Internet at http://pubs.acs.org.
Acknowledgment. Financial support from the National
Natural Science Foundation of China (Grants 20572077 and
20632040) is gratefully acknowledged.
IC061805W
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964 Inorganic Chemistry, Vol. 46, No. 3, 2007