Synthesis and structural characterization of divalent ytterbium complexes supported by β-diketiminate ligands and their catalytic activity for the polymerization of methyl methacrylate

Article abstract of DOI:10.1021/om030136g

Three divalent ytterbium complexes supported by a β-diketiminate ligand were synthesized for the first time. The mixed-ligand ytterbium chlorides (CH₃C₅H₄)[(DIPPh)₂nacnac]YbCl (1), (C₉H₇)[(DIPPh)₂nacnac]YbCl (2), and (ArO)[(DIPPh)₂nacnac]YbCl(THF) (3) ((DIPPh)₂nacnac = N,N-diisopropylphenyl-2, 4-pentanediimine anion, ArO = 2,6-di-tert-butyl-4-methylphenoxo), which were obtained in high yield by using [(DIPPh)₂nacnac] YbCl₂(THF)₂ as precursor, were reduced by Na/K alloy in THF to afford the corresponding divalent species (CH₃C₅H₄)[(DIPPh)₂nacnac]Yb(THF) (4), (C₉H₇)[(DIPPh)₂nacnac]Yb(THF) (5), and {(ArO)-[(DIPPh)₂nacnac]Yb(THF)}(THF) (6), respectively. Complexes 2-6 were well characterized, and crystal structures of complexes 4-6 were determined. These divalent complexes exhibit good catalytic activity for the polymerization of methyl methacrylate.

Full text of DOI:10.1021/om030136g

2876
Organometallics 2003, 22, 2876-2882
Syn th esis a n d Str u ctu r a l Ch a r a cter iza tion of Diva len t
Ytter biu m Com p lexes Su p p or ted by â-Dik etim in a te
Liga n d s a n d Th eir Ca ta lytic Activity for th e
P olym er iza tion of Meth yl Meth a cr yla te
Yingming Yao,† Yong Zhang,† Zhenqin Zhang,† Qi Shen,*,†,‡ and Kaibei Yu§
Department of Chemistry and Chemical Engineering, 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 Chengdu Institute of Organic Chemistry, Chinese Academy of
Sciences, Chengdu 610041, People’s Republic of China
Received February 27, 2003
Three divalent ytterbium complexes supported by a â-diketiminate ligand were synthesized
for the first time. The mixed-ligand ytterbium chlorides (CH3C5H4)[(DIPPh)2nacnac]YbCl
(1), (C9H7)[(DIPPh)2nacnac]YbCl (2), and (ArO)[(DIPPh)2nacnac]YbCl(THF) (3) ((DIPPh)2nacnac
) N,N-diisopropylphenyl-2, 4-pentanediimine anion, ArO ) 2,6-di-tert-butyl-4-methylphe-
noxo), which were obtained in high yield by using [(DIPPh)2nacnac]YbCl2(THF)2 as precursor,
were reduced by Na/K alloy in THF to afford the corresponding divalent species
(CH3C5H4)[(DIPPh)2nacnac]Yb(THF) (4), (C9H7)[(DIPPh)2nacnac]Yb(THF) (5), and {(ArO)-
[(DIPPh)2nacnac]Yb(THF)}(THF) (6), respectively. Complexes 2-6 were well characterized,
and crystal structures of complexes 4-6 were determined. These divalent complexes exhibit
good catalytic activity for the polymerization of methyl methacrylate.
In tr od u ction
In recent years, the use of â-diketiminates as sup-
porting ligand systems in both main and transition
metal coordination chemistry has attracted considerable
attention,7 since their steric and electronic properties
can be easily tuned by an appropriate choice of starting
Over the past few decades, great progress has been
made in the chemistry of divalent organolanthanide
complexes. Divalent organolanthanide complexes ex-
hibit a particularly rich reaction chemistry based on
their strong reductive property,1 and a variety of com-
plexes have been synthesized and widely used in organic
synthesis and polymer chemistry.2-6 The majority of
this chemistry has been done utilizing pentamethylcy-
clopentadienyl or functionalized substituted-cyclopen-
tadienyl as ancillary ligands.
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* Corresponding author. Fax: (86)512-65112371. Tel: (86)512-
65112513. E-mail: qshen@suda.edu.cn.
† Suzhou University.
‡ Shanghai Institute of Organic Chemistry.
§ Chengdu Institute of Organic Chemistry.
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10.1021/om030136g CCC: $25.00 © 2003 American Chemical Society
Publication on Web 06/13/2003

Divalent Ytterbium Complexes
Organometallics, Vol. 22, No. 14, 2003 2877
Sch em e 1
materials used in their synthesis, and they can coordi-
nate metal centers in different bonding models. More-
over, some of these complexes exhibit exciting reactivity;
for example, they show high activity for the polymeri-
zation of ethylene8 and lactide9 and the copolymeriza-
tion of carbon dioxide and epoxide.10 However, the
utilization of this ligand system in the synthesis of
divalent lanthanide derivatives still remains relatively
poorly explored. To our best knowledge, there is only
one paper concerning the synthesis and structural
characterization of divalent lanthanide complexes uti-
lizing â-diketiminates as ancillary ligands, in which the
â-diketiminates act as novel dianionic ligands.11 Now,
we have become interested in studying the synthesis
and reactivity of organolanthanide complexes supported
by â-diketiminate ligands12 and find that the bulky
â-diketiminate group (DIPPh)2nacnac ((DIPPh)2nacnac
) N,N-2,6-diisopropylphenyl-2,4-pentanediimine anion)
is an ideal ligand for the synthesis of mixed-ligand
divalent lanthanide complexes. Here, we describe the
synthesis and characterization of the divalent ytterbium
complexes utilizing this ligand system and the catalytic
activities of these complexes for the polymerization of
methyl methacrylate (MMA).
Resu lts a n d Discu ssion
Syn th esis of Tr iva len t Ytter biu m â-Dik etim in a te
Com p lexes. We previously reported that the bulky
â-diketiminate [(DIPPh)2nacnac]- is an ideal ligand for
synthesis of the mixed-ligand trivalent lanthanide
derivatives, and (CH3C5H4)[(DIPPh)2nacnac]YbCl (1)
can be conveniently synthesized in high yield using
[(DIPPh)2nacnac]YbCl2(THF)2 as precursor.12a Thus,
freshly prepared [(DIPPh)2nacnac]YbCl2(THF)2 in situ
was used to synthesize the â-diketiminate ytterbium
derivatives. [(DIPPh)2nacnac]YbCl2 reacted with 1 equiv
of C9H7K in THF; after workup, the black microcrys-
tals were obtained as final products which were iden-
tified to be (C9H7)[(DIPPh)2nacnac]YbCl (2) by elemen-
tal analyses and IR spectrum. We propose that com-
plex 2 exists as a monomer according to the structure
of the analogous complex 1, which was identified by
structure determination. When [(DIPPh)2nacnac]YbCl2
was treated with 1 equiv of ArONa (ArO ) 2,6-di-tert-
butyl-4-methylphenoxo) in THF, the desired mixed-
ligand aryloxo ytterbium complex (ArO)[(DIPPh)2nacnac]-
YbCl(THF) (3) was isolated as red microcrystals in high
yield (Scheme 1). In the IR spectra of complexes 2 and
3, there are strong absorptions near 1550 and 1530
cm-1, which were consistent with partial CdN double
bond character. These complexes are well soluble in
THF and DME.
Syn th esis of Diva len t Ytter biu m â-Dik etim in a te
Com p lexes. Reduction of trivalent lanthanide com-
plexes by alkali metals is a popular method to synthe-
size divalent lanthanide complexes. The reactions of
complexes 1-3 with a slight excess of Na/K alloy in
THF smoothly produce the divalent â-diketiminate
ytterbium complexes (CH3C5H4)[(DIPPh)2nacnac]Yb-
(THF) (4), (C9H7)[(DIPPh)2nacnac]Yb(THF) (5), and
{(ArO)[(DIPPh)2nacnac]Yb(THF)}(THF) (6), respectively
(Scheme 2). Recrystallization from concentrated toluene
solution gave 4-6 as black crystals in good yield.
Usually, the synthesis of monomethylcyclopentadienyl
or mono-unsubstituted indenyl divalent lanthanide
complexes was difficult due to disproportionation. Com-
plex 4 is the first example of a monomethylcyclopenta-
dienyl heteroleptic divalent lanthanide complex. Up to
now, there has been only one example of an indenyl-
containing heteroleptic divalent lanthanide complex,
[(C9H7)YbI(DME)2], in the literature,13 and complex 5
is the second one.
(8) (a) Feldman, J .; McLain, S. J .; Parthasarathy, A.; Marshall, W.;
Calabrese, J . C.; Arthur, S. D. Organometallics 1997, 16, 1514. (b)
Vollmerhaus, R.; Rahim, M.; Tomaszewski, R.; Xin, S.; Taylor, N. J .;
Collins, S. Organometallics 2000, 19, 2161. (c) Gibson, V. C.; Newton,
C.; Redshaw, C.; Solan, G.; White, A. J . P.; Williams, D. J . Eur. J .
Inorg. Chem. 2001, 1895. (d) MacAdams, L. A.; Kim, W. K.; Liable-
Sands, L. M.; Guzei, I. A.; Rheingold, A. L.; Theopold, K. H. Organo-
metallics 2002, 21, 952. (e) Hayes, P. G.; Piers, W. E.; McDonald, R. J .
Am. Chem. Chem. 2002, 124, 2132.
(9) (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G. W.
J . Am. Chem. Soc. 1999, 121, 11583. (b) Dove, A. P.; Gibson, V. C.;
Marshall, E. L.; White, A. J . P.; Williams, D. J . Chem. Commun. 2001,
283. (c) Chisholm, M. H.; Huffman, J . C.; Phomphrai, K. J . Chem. Soc.,
Dalton Trans. 2001, 222. (d) Chamberlain, B. M.; Cheng, M.; Moore,
D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J . Am. Chem. Soc.
2001, 123, 3229. (e) Chisholm, M. H.; Gallucci, J .; Phomphrai, K. Inorg.
Chem. 2002, 41, 2785.
(10) (a) Cheng, M.; Lobkovsky, E. B.; Coates, G. W. J . Am. Chem.
Soc. 1998, 120, 11018. (b) Cheng, M.; Moore, D. R.; Reczek, J . J .;
Chamberlain, B. M.; Lobkovsky, E. B.; Coates, G. W. J . Am. Chem.
Soc. 2001, 123, 8738. (c) Allen, S. D.; Moore, D. R.; Lobkovsky, E. B.;
Coates, G. W. J . Am. Chem. Soc. 2002, 124, 14284. (d) Moore, D. R.;
Cheng, M.; Lobkovsky, E. B.; Coates, G. W. Angew. Chem., Int. Ed.
2002, 41, 2599. (e) Eberhardt, R.; Allmendinger, M.; Luinstra, G. A.;
Rieger, B. Organometallics 2003, 22, 211.
(11) Avent, A. G.; Khvostov, A. V.; Hitchcock, P. B.; Lappert, M. F.
Chem. Commun. 2002, 1410.
(12) (a) Yao, Y. M.; Zhang, Y.; Shen, Q.; Yu, K. B. Organometallics
2002, 21, 819. (b) Yao, Y. M.; Luo, Y. J .; J iao, R.; Shen, Q.; Yu, K. B.;
Weng, L. H. Polyhedron 2003, 22, 441.

2878 Organometallics, Vol. 22, No. 14, 2003
Yao et al.
Sch em e 2
F igu r e 1. Molecular structure of complex 4 drawn with
50% probability ellipsoids (hydrogen atoms omitted for
clarity). Each disordered THF ligand is shown in two
possible orientations.
Complexes 4-6 are all very sensitive to air and
moisture. They have good solubility in THF and DME
and moderate solubility in toluene. These complexes
gave satisfactory elemental analyses, and their infrared
spectra showed absorptions characteristic of the â-dike-
tininate ligand. The diamagnetic nature of divalent Yb
allowed characterization of these complexes by NMR
spectroscopy. The structural analyses reveal that the
â-diketininate acts as a monoanionic spectator ligand
in these divalent ytterbium complexes (see below). This
is quite different from those in {Yb[Me3SiNC(C6H4Ph-
4)CHC(C6H4Ph-4)NSiMe3(µ-Li(THF))]2}(THF) and {Yb-
[Me3SiNC(Ph)CHC(Ph)NSiMe3(µ-Li(THF))]2}, in which
the â-diketininates act as dianionic ligands, although
they are also obtained by the reduction of trivalent
â-diketininate ytterbium complexes.11
F igu r e 2. Molecular structure of complex 5 drawn with
50% probability ellipsoids (hydrogen atoms omitted for
clarity).
Molecu la r Str u ctu r es. The molecular structures of
complexes 4-6 were characterized by X-ray diffraction.
All of them have solvated monomeric structure. Figures
1-3 show their ORTEP drawings, and their selected
bond parameters are listed in Tables 1-3, respectively.
The coordination geometries around ytterbium atoms
in complexes 4 and 5 are similar, which resemble that
in complex 1.12a The coordination sphere of the Yb center
is composed of the two nitrogen atoms of the chelating
â-diketiminate anion, a methylcyclopentadienyl (inde-
nyl) group, and an oxygen atom of the THF molecule;
the formal coordination number of the central metal is
six. The coordination geometry of the ytterbium atom
can be described as slightly distorted pseudo-tetrahedral
if the methylcyclopentadienyl (indenyl) group is re-
garded to occupy one coordination site with the ring
center. However, the coordination mode of the â-diketim-
inate ligand in complexes 4 and 5 is different from that
in complex 1. In complexes 4 and 5, the Yb-C(13, 14,
15) (Yb-C(11, 12, 13) for 5) distances are quite long,
which are well beyond values considered indicative of
π bonding. Thus, there is only purely σ bond between
ytterbium and the â-diketiminate ligand, and the
â-diketiminate ligand adopts an η2-coordination mode,
while the â-diketiminate ligand adopts an η4-coordina-
F igu r e 3. Molecular structure of complex 6 drawn with
50% probability ellipsoids (hydrogen atoms omitted for
clarity).
tion mode in complex 1.12a This is also different from
those in {Yb[Me3SiNC(C6H4Ph-4)CHC(C6H4Ph-4)NSiMe3-
(µ-Li(THF))]2}(THF) and {Yb[Me3SiNC(Ph)CHC(Ph)-
NSiMe3(µ-Li(THF))]2}, in which the â-diketiminate
ligands can be regarded as being η5-bonded to the
central metal.11 The bond distances within the backbone
of the â-diketiminate ligand fall in the range of corre-
sponding distances of single and double bonds, which
(13) Trifonov, A. A.; Kirillov, E. N.; Dechert, S.; Schumann, H.;
Bochkarev, M. N. Eur. J . Inorg. Chem. 2001, 3055.

Divalent Ytterbium Complexes
Organometallics, Vol. 22, No. 14, 2003 2879
Ta ble 1. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lex 4
among the bulky substituents on the arene rings of the
ArO group and the â-diketiminate ligand. The coordi-
nate geometry can be best described as a distorted
tetrahedron.
Yb-N(1)
Yb-N(2)
Yb-O
Yb-C(30)
Yb-C(31)
Yb-C(32)
Yb-C(33)
2.380(4)
2.362(4)
2.382(4)
2.679(6)
2.689(7)
2.683(7)
2.688(6)
Yb-C(34)
2.701(6)
2.688(6)
1.305(6)
1.321(6)
1.420(7)
1.401(7)
Yb-C(av)
N(1)-C(13)
N(2)-C(15)
C(13)-C(14)
C(14)-C(15)
The average Yb-N bond distance of 2.388(4) Å is
comparable with the corresponding values in complexes
4 and 5. But these distances are apparently longer than
those in {Yb[Me3SiNC(C6H4Ph-4)CHC(C6H4Ph-4)NSiMe3-
(µ-Li(THF))]2}(THF) and {Yb[Me3SiNC(Ph)CHC(Ph)-
NSiMe3(µ-Li(THF))]2}, although there are two bulky
â-diketiminate ligands coordinated to the central metal
in the latter.11 The â-diketiminate ligand in complex 6
is an N,N′-bonded chelate, and as expected, there is
electron delocalization within the backbone of the
â-diketiminate ligand. The bond distance of Yb-O(Ar)
(2.179(3) Å) is slightly longer than those values found
in (ArO)2Yb(THF)2 (2.137(9) Å) and (ArO)2Yb(Et2O)2
(2.154(8) Å),5a and it is also slightly longer than the
terminal Yb-O(Ar) bonds found in [(ArO)Yb(µ-OAr)]2
(2.10(2) and 2.08(2) Å)19 if the effect of coordination
number on ionic radii is considered.14 It is reasonable
to ascribe the difference in bond parameters among
these complexes to the increased steric congestion in
the former due to the replacement of one ArO group by
a bulky â-diketiminate ligand. Also reflecting the
sterically congested feature in complex 6, the angle of
Yb(1)-O(1)-C(1) (163.0(3)°) is smaller than those of
Yb-O-C (168-171°) found for the terminal ArO ligands
N(1)-Yb-N(2)
N(1)-Yb-O
N(2)-Yb-O
81.25(15)
101.27(14)
102.68(15)
N(1)-C(13)-C(14)
N(2)-C(15)-C(14)
C(13)-C(14)-C(15)
124.3(5)
125.1(5)
132.0(5)
reflects substantial electron delocalization. This re-
sembles those in trivalent â-diketiminate lanthanide
complexes.
The â-diketiminate ligand in complex 4 is sym-
metrically coordinated to the ytterbium atom, with a
variation in Yb-N bond lengths of 0.018 Å (2.380(4) and
2.362(4) Å, respectively) and an average Yb-N bond
length of 2.371(4) Å. The Yb-C(ring) distances range
from 2.679(6) to 2.701(6) Å, giving the average
Yb-C(ring) distance of 2.688(6) Å. These values are ca.
0.13 Å longer than those in complex 1,12a which is in
accordance with the difference in ionic radii between
Yb(II) and Yb(III) species.14 The Yb-C(ring) distance
is still comparable to the average Yb-C(ring) distances
in divalent ytterbocene complexes such as (CH3C5H4)2-
Yb(DME)15 and [(CH3C5H4)2Yb(THF)]n.16 The THF mol-
ecule in complex 4 exhibits disorder between C37 and
C38 atoms; however, the Yb-O bond length of 2.382(4)
Å is still observed. The N1-Yb-N2 angle of 81.25(15)°
is comparable to that in complex 1.12a
19
in [(ArO)Yb(µ-OAr)]2 and in (ArO)2Yb(Et2O)2.5a The
bond distance of O(1)-C(1) (1.343(5) Å) is apparently
shorter than the single C-O bond length, reflecting
substantial electron delocalization from oxygen into the
aromatic ring.
In complex 5, the average Yb-N bond distance of
2.360(3) Å is comparable with that in complex 4. The
Yb-C(ring) distances range from 2.669(5) to 2.762(3)
Å. Indenyl hapticities can be determined conveniently
by calculating the slip value ∆M-C,17 which is found
to be 0.026 Å in complex 5. Thus, the bonding mode of
the metal atom with the indenyl ligand might be
described as η5-mode. This coordination mode resembles
those in some divalent indenyl lanthanide complexes,3d,e
but it is different from that in rac-[O(CH2CH2C9H6)2]-
YbN(SiMe3)2, which is partially slipped toward η3- from
η5-mode.18 The average Yb-C(ring) distance of 2.713-
(4) Å is comparable with the corresponding values in
complex 4 and other divalent indenyl lanthanide com-
plexes if the difference in ionic radii is considered.3
An X-ray analysis has revealed that complex 6 is a
solvated monomeric divalent ytterbium complex, which
bears mixed ArO and â-diketiminate ligands. This
complex has a low coordination number, and the central
ytterbium atom is four-coordinated by two nitrogen
atoms from the â-diketiminate ligand and two oxygen
atoms from ArO group and a THF molecule, respec-
tively. This may be attributed to the steric repulsion
To our best knowledge, complexes 4-6 represent the
first examples of a structurally characterized mixed-
ligand lanthanide(II) species containing a â-diketimi-
nate ligand. Lappert et al. previously mentioned that
YbI2 reacted with 2 equiv of [(Me3Si)NC(Ph)CHC(Ph)N-
(SiMe3)]K to give a homoleptic divalent complex, but no
structure was reported.20 Recently, two ionic â-diketimi-
nate ytterbium(II) complexes, {Yb[Me3SiNC(C6H4Ph-4)-
CHC(C6H4Ph-4)NSiMe3(µ-Li(THF))]2}(THF) and {Yb-
[Me3SiNC(Ph)CHC(Ph)NSiMe3(µ-Li(THF))]2}, were re-
ported, in which only â-diketiminate ligands coordinated
to the central metal.11
P olym er iza tion of MMA by Diva len t Ytter biu m
Com p lexes. It has been reported that divalent lantha-
nocece complexes are effective single-component initia-
tors for MMA polymerization.15,21 To elucidate the effect
of ancillary ligands on the activity of divalent lanthanide
complexes, we tested the catalytic activity of complexes
4-6 for MMA polymerization. Preliminary results re-
veal that these complexes as single-component catalysts
can initiate the polymerization of MMA in toluene as
shown in Table 4. The conversions are in the range
78-100% at -20 °C for 0.5 h in the case of 0.5 mol %
initiator concentration ([M]/[I] ) 200). The polymeriza-
tion of MMA with complex 4 or 6 gives a relatively high
(14) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
(15) J iang, T.; Shen, Q.; Lin, Y. H.; J in, S. C. J . Organomet. Chem.
1993, 450, 121.
(16) Zinnen, H. A.; Pluth, J . T.; Evans, W. J . J . Chem. Soc., Chem.
Commun. 1980, 810.
(17) (a) ∆M-C ) 1/2[(Yb-C1 + Yb-C5) - (Yb-C2 + Yb-C4)];
∆M-C values close to zero indicate little distortion from η5 hapticity,
whereas values above ca. 0.5 Å indicate nearly η3 hapticity (ref 17b).
(b) Westcott, S. A.; Kakkar, A.; Stringer, G.; Taylor, N. J .; Marder, T.
B. J . Organomet. Chem. 1990, 394, 777.
(18) Qian, C. T.; Zou, G.; Chen, Y. F.; Sun, J . Organometallics 2001,
20, 3106.
(19) van den Hende, J .; Hitchcock, P. B.; Holmes, S. A.; Lappert,
M. F. J . Chem. Soc., Dalton Trans. 1995, 1435.
(20) Hitchcock, P. B.; Holmes, S. A.; Lappert, M. F.; Tian, S. J . Chem.
Soc., Chem. Commun. 1994, 2691.
(21) (a) Yasuda, H.; Yamamoto, H.; Yamashita, M.; Yokota, K.;
Nakamura, A.; Miyaki, S.; Kai, Y.; Kanehisa, N. Macromolecules 1993,
26, 7134. (b) Boffa, L. S.; Novak, B. M. Macromolecules 1994, 27, 6993.

2880 Organometallics, Vol. 22, No. 14, 2003
Yao et al.
Ta ble 2. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lex 5
forming a trivalent radical anion. Subsequent dimer-
ization of the radical anions gives the final actual
bisinitiator.21b
Yb-N(1)
Yb-N(2)
Yb-O(1)
Yb-C(1)
Yb-C(2)
Yb-C(3)
Yb-C(4)
2.363(3)
2.358(3)
2.383(3)
2.731(4)
2.669(5)
2.687(5)
2.719(4)
Yb-C(5)
2.762(3)
2.713(4)
1.319(5)
1.332(5)
1.412(5)
1.416(5)
Yb-C(av)
N(1)-C(11)
N(2)-C(13)
C(11)-C(12)
C(12)-C(13)
Con clu sion
In summary, three new soluble mixed-ligand divalent
ytterbium complexes supported by â-diketiminate ligands
were successfully synthesized by direct reduction of the
corresponding mixed-ligand trivalent ytterbium chlo-
rides with Na/K alloy without ligand redistribution, and
their crystal structures were determined. These com-
plexes represent the first examples of a structurally
characterized mixed-ligand lanthanide(II) species con-
taining a â-diketiminate ligand. Moreover, these diva-
lent ytterbium complexes can serve as single-component
catalysts for the polymerizations of MMA.
N(1)-Yb-N(2)
N(1)-Yb-O(1)
N(2)-Yb-O(1)
80.9(1)
97.6(1)
102.58(11)
N(1)-C(11)-C(12)
N(2)-C(13)-C(12)
C(11)-C(12)-C(13)
124.4(3)
124.5(3)
131.0(3)
Ta ble 3. Selected Bon d Len gth s (Å) a n d An gles
(d eg) for Com p lex 6
Yb(1)-N(1)
Yb(1)-N(2)
Yb(1)-O(1)
Yb(1)-O(2)
O(1)-C(1)
2.374(4)
2.402(4)
2.179(3)
2.321(3)
1.343(5)
N(1)-C(17)
N(2)-C(19)
C(17)-C(18)
C(18)-C(19)
1.333(6)
1.332(5)
1.406(6)
1.407(6)
Exp er im en ta l Section
N(1)-Yb(1)-N(2)
80.97(13) N(2)-Yb(1)-O(2)
98.94(14)
163.0(3)
O(1)-Yb(1)-N(1) 127.24(12) Yb(1)-O(1)-C(1)
The chemistry described below was performed under pure
argon with exclusion of air and moisture by Schlenk tech-
niques. Solvents were dried and freed of oxygen by refluxing
over Na or sodium benzophenone ketyl and distilled under
argon prior to use. Methyl methacrylate was dried over CaH2
for 4 days and stored over 3 Å molecular sieves under argon
after distillation. Anhydrous YbCl323 and [(DIPPh)2nacnac]Li8a
were prepared according to the literature methods. (CH3C5H4)-
[(DIPPh)2nacnac]YbCl (1) was prepared as previously re-
ported.12a ArONa was obtained by the reaction of ArOH with
sodium in THF. C9H7K was prepared by the reaction of C9H8
with potassium in THF.
O(1)-Yb(1)-N(2) 140.30(12) N(1)-C(17)-C(18) 125.1(4)
O(1)-Yb(1)-O(2) 100.51(13) N(2)-C(19)-C(18) 124.7(4)
N(1)-Yb(1)-O(2) 102.39(13) C(17)-C(18)-C(19) 131.9(4)
conversion even when using 0.2 mol % initiator concen-
tration ([M]/[I] ) 500). However, for complex 5, the
conversion is still very low even when the polymeriza-
tion time is prolonged to 3 h under the same polymer-
ization conditions. This may be attributed to the de-
struction of the extremely sensitive indenyl lanthanide
complex by the contaminating H2O and/or oxygen in the
system.21a The living polymerization of MMA via diva-
lent lanthanocenes as an initiator was previously
reported.21a However, the polymerization in the present
case is not well controlled. The polydispersity of the
resultant polymers is rather broad.
Melting points were determined in sealed argon-filled
capillaries and are uncorrected. Metal analyses were carried
out using complexometric titration. Carbon, hydrogen, and
nitrogen analyses were performed by direct combustion on a
Carlo Erba-1110 instrument; quoted data are the average of
1
at least two independent determinations. The H NMR spectra
The ancillary ligand has great effect on the activity
of divalent lanthanide complexes for MMA polymeriza-
tion. Replacing one cyclopentadienyl group by a â-di-
ketiminate ligand results in a decrease of its activity.
The conversion is as high as 95% using (MeC5H4)2Yb-
(DME) as an initiator at -40 °C, and the conversion is
still 70% even when the polymerization temperature is
decreased to -78 °C.15 However, complexes 4-6 are
inactive for MMA polymerization when the polymeri-
zation temperature decreases to -40 °C under the same
polymerization conditions. The tacticity of the resultant
poly(MMA) was determined with reference to the re-
ported triad.22 The coordination environment of the
catalyst also has influence on the tacticity of the
resultant polymers. The atactic poly(MMA) is obtained
were obtained in C6D6 solution for lanthanide complexes and
in CDCl3 for polymers on a Unity Varian-400 spectrometer,
and the 13C NMR spectra were obtained in C6D6 solution on a
Bruker APX-300 spectrometer. The IR spectra were recorded
on a Nicolet-550 FTIR spectrometer as KBr pellets. Molecular
weight and molecular weight distributions were determined
against a polystyrene standard by gel permeation chromatog-
raphy (GPC) on a Waters 1515 apparatus with three HR
columns (HR-1, HR-2, and HR-4). THF was used as an eluent
at 30 °C.
(C9H7)[(DIP P h )2n a cn a c]YbCl (2). To a slurry of anhy-
drous YbCl3 (1.63 g, 5.83 mmol) in about 40 mL of THF was
slowly added a solution of [(DIPPh)2nacnac]Li (18.6 mL, 5.83
mmol) in toluene/hexane at room temperature. After YbCl3
disappeared completely, a THF solution of C9H7K (1.14 mL,
5.83 mmol) was added slowly. The mixture was stirred at room
temperature for another 48 h, and then the precipitate was
removed from the reaction mixture by centrifugation. THF was
completely removed in a vacuum, and toluene was added to
extract the product. The precipitate was removed again by
centrifugation. Black crystals were obtained from a concen-
trated toluene solution at -20 °C for 4 days (3.34 g, 77.3%).
Mp: 245-247 °C. Anal. Calcd for C38H48ClN2Yb: C, 61.57; H,
6.53; N, 3.78; Yb, 23.34. Found: C, 61.33; H, 6.24; N, 3.58;
Yb, 23.51. 1H NMR (C6D6, ppm): -49.80 (s, 2H), -17.29 (t,
8H), -15.19 (d, 5H), 1.73 (m, 5H), 4.94 (s, 6H), 15.01 (s, 2H),
22.02 (d, 4H), 23.65 (s, 2H), 25.50 (t, 6H), 39.45 (d, 2H), 40.82
(s, 6H). IR (KBr, cm-1): 3062(w), 2962(vs), 2928(s), 2868(s),
1622(vs), 1552(vs), 1528(vs), 1461(s), 1437(s), 1388(s), 1363-
for complexes 4-6. But the highly syndiotactic poly-
21a
(MMA) can be obtained using (C5Me5)2Yb(THF)2
or
(MeC5H4)2Yb(DME)15 as an initiator under the same
polymerization temperature.
GPC analyses showed that the molecular weights for
the polymers obtained are at least twice the values
calculated depending on the molar ratio of monomer to
initiator. These results support the supposition that the
polymerization mechanism by divalent lanthanide com-
plexes proceeds via the formation of a trivalent bisini-
tiator. The Yb(II) complex acts as a reductant and
undergoes one-electron transfer to a molecule of MMA,
(22) Ferugusan, R. C.; Overnall, D. W. Polym. Prepr. (Am. Chem.
Soc., Div. Polym. Chem.) 1985, 26, 182.
(23) Taylor, M. D.; Carter, C. P. J . Inorg. Nucl. Chem. 1962, 24,
387.

Divalent Ytterbium Complexes
Organometallics, Vol. 22, No. 14, 2003 2881
Ta ble 4. P olym er iza tion of MMA In itia ted by Com p lexes 4, 5, a n d 6a
tacticity (%)
mr
b
entry
initiator
[M]/[I]
conv (%)
Mn × 10-4 c
Mw/Mn
mm
rr
1
2
3
4
5c
6
7
8
4
4
5
5
5
6
6
200
500
200
500
500
200
500
200
78.9
74.1
78.5
0
12.7
100
63.8
0
6.26
6.09
5.16
3.08
3.15
2.96
24.9
31.2
12.7
41.7
47.1
27.5
33.4
21.7
59.8
2.39
5.99
3.99
4.42
2.08
2.94
21.5
12.5
14.8
30.9
23.3
26.0
47.6
64.2
59.2
[(DIPPh)2nacnac]Li
a
b
Polymerization conditions: in toluene; -20 °C; 0.5 h; solvent/monomer ) 2 (v/v). Measured by GPC calibrated with standard
polystyrene samples. c 3 h, solvent/monomer ) 1 (v/v).
Ta ble 5. Cr ysta llogr a p h ic Da ta for Com p lexes 4, 5, a n d 6
4
5
6
formula
fw
C
39H56N2OYb
C42H56N2OYb
777.96
C52H80N2O3Yb
954.26
741.90
T (K)
290(2)
193(2)
193(2)
cryst syst
space group
a (Å)
b (Å)
c (Å)
monoclinic
P2(1)/n
10.198(2)
17.576(3)
20.701(4)
90
90.84(2)
90
3710.0(12)
4
monoclinic
P21/c
17.683(2)
11.130(1)
20.150(2)
90
105.698(2)
90
3817.5(7)
4
triclinic
P1h
9.510(4)
12.673(6)
20.547(9)
93.348(10)
92.645(11)
96.75(2)
2451.5(19)
2
R (deg)
â (deg)
γ (deg)
V (Å3)
Z
Dcalcd (g cm-3
)
1.328
2.550
1528
1.353
2.482
1600
1.293
1.949
1000
µ (mm-1
F(000)
)
cryst size (mm)
θmax (deg)
no. of collected reflns
no. of unique reflns
no. of obsd reflns
no. of variables
GOF
0.58 × 0.48 × 0.12
0.65 × 0.45 × 0.30
0.60 × 0.30 × 0.30
25.0
27.5
27.5
7432
41 442
27 264
6234 (Rint ) 0.042)
3939 [I > 2.0σ(I)]
408
9070 (Rint ) 0.034)
7010 [I > 2.0σ(I)]
471
11 009 (Rint ) 0.039)
8924 [I > 2.0σ(I)]
603
0.842
0.97
0.99
R
0.0354
0.036
0.037
Rw
0.0698
0.051
0.047
(s), 1325(s), 1276(s), 1178(s), 1096(m), 1068(w), 1019(m), 933-
(m), 786(m), 759(s), 692(m).
3.07-3.54 (m, 8H, THF and CH3CHCH3), 4.52 (s, 1H, CHd
C(CH3)N), 5.95∼6.21 (m, 4H, CH3C5H4), 6.52∼7.31 (m, 6H, Ph).
13C NMR (C6D6, ppm): 23.05, 24.35, 25.01, 28.14, 32.03, 38.97,
68.56, 92.88, 94.36, 109.37, 120.05, 123.55, 124.64, 126.56,
140.94. IR (KBr, cm-1): 3063(m), 2962(vs), 2868(s), 1660(m),
1623(s), 1551(s), 1515(m), 1461(s), 1436(s), 1394(s), 1363(s),
1312(m), 1256(m), 1173(m), 1107(m), 1044(m), 938(w), 784-
(m), 758(s). Crystals suitable for X-ray crystal structure studies
were obtained by recrystallization from THF/toluene solution
at -5 °C for a week.
(C9H 7)[(DIP P h )2n a cn a c]Yb(THF ) (5). The synthesis of
complex 5 was carried out as described for complex 4, but a
THF solution of 2 (1.23 g, 1.66 mmol) was used in place of the
solution of 1. Black crystals were obtained from a concentrated
toluene solution at -20 °C for 4 days (1.00 g, 78.0%). Mp: 220-
222 °C (dec). Anal. Calcd for C42H56N2OYb: C, 64.83; H, 7.27;
N, 3.60; Yb, 22.24. Found: C, 64.53; H, 7.19; N, 3.85; Yb, 22.45.
1H NMR (C6D6, ppm): 1.24 (d, 24H, CH3), 1.53-1.70 (m, 10H,
CH3 and THF), 3.07-3.54 (m, 8H, THF and CH3CHCH3), 4.56
(s, 1H, CHdC(CH3)N), 6.45-7.20 (m, 13H, Ph and C9H7). 13C
NMR (C6D6, ppm): 20.71, 23.04, 24.45, 25.07, 28.64, 53.09,
69.75, 93.58, 94.35, 96.71, 118.09, 119.56, 120.63, 123.55,
124.73, 125.81. IR (KBr, cm-1): 3061(w), 2961(vs), 2926(s),
2867(s), 1624(vs), 1552(vs), 1461(s), 1438(s), 1380(m), 1363-
(s), 1325(s), 1276(s), 1175(s), 1102(w), 1057(w), 1035(w), 935-
(m), 786(m), 757(s), 702(m). Crystals suitable for X-ray crystal
structure studies were obtained by recrystallization from a
toluene solution at room temperature for 3 days.
(Ar O)[(DIP P h )2n a cn a c]YbCl(THF ) (3). The synthesis of
complex 3 was carried out as described for complex 2, but a
THF solution of ArONa (1.56 mmol) was used in place of the
solution of C9H7K. Red crystals were obtained from a tol-
uene solution (1.16 g, 81.2%). Mp: 153-156 °C (dec). Anal.
Calcd for C48H72ClN2O2Yb: C, 62.83; H, 7.91; N, 3.05; Yb,
18.86. Found: C, 62.52; H, 7.92; N, 3.07; Yb, 18.72. 1H
NMR (C6D6, ppm): -24.67 (br, 4H), -9.96 (s, 3H), -0.82 (br,
3H), 0.93 (d, 2H), 1.25 (d, 21H), 1.42 (s, 16H), 1.59 (d, 3H),
1.71 (s, 4H), 2.29 (s, 4H), 3.03 (s, 3H), 3.35 (s, 4H), 4.92 (d,
4H), 12.54 (s, 1H). IR (KBr, cm-1): 3061(w), 2960(vs), 2927-
(s), 2867(s), 1635(s), 1621(vs), 1550(vs), 1531(vs), 1463(s), 1439-
(s), 1396(m), 1362(s), 1325(s), 1277(s), 1225(m), 1175(s), 1152-
(m), 1120(w), 1073(w), 932(m), 865(m), 789(s), 759(s), 685(w),
642(w).
(CH3C5H4)[(DIP P h )2n a cn a c]Yb(THF ) (4). To a THF so-
lution of (CH3C5H4)[(DIPPh)2nacnac]YbCl (2.1 g, 2.98 mmol)
was added Na/K alloy in 1.2 molar ratio. The mixture was
stirred at room temperature, and the color of the solution
gradually changed to black. After stirring for 48 h, the
precipitate was removed from the reaction mixture by cen-
trifugation. THF was completely removed in a vacuum, and
toluene was added to extract the product. The precipitate was
removed again by centrifugation. Black crystals were obtained
from a concentrated THF solution at -20 °C for a week (1.88
g, 85.0%). Mp: 188-189 °C (dec). Anal. Calcd for C39H56N2-
OYb: C, 63.13; H, 7.62; N, 3.78; Yb, 23.32. Found: C, 63.05;
H, 7.56; N, 3.73; Yb, 23.52. 1H NMR (C6D6, ppm): 1.23 (d, 24H,
CH3), 1.53-1.70 (m, 10H, CH3 and THF), 2.64 (s, 3H, CH3),
{(Ar O)[(DIP P h )n a cn a c]Yb(THF )}(THF ) (6). The syn-
thesis of complex 6 was carried out as described for complex
4, but a THF solution of 3 (2.50 g, 2.72 mmol) was used in

2882 Organometallics, Vol. 22, No. 14, 2003
Yao et al.
place of the solution of 1. Black crystals were obtained from a
concentrated toluene solution at -20 °C for 3 days (2.18 g,
84.1%). Mp: 141 °C (dec). Anal. Calcd for C52H80N2O3Yb: C,
65.44; H, 8.47; N, 2.94; Yb, 18.13. Found: C, 65.28; H, 8.36;
X-r a y Str u ctu r e Deter m in a tion . Suitable single crystals
of complexes 4, 5, and 6 were each sealed in a thin-walled glass
capillary for single-crystal structure determination. Intensity
data were collected on a Siemens P4 diffractometer in ω scan
mode (for 4) or a Rigaku Mercury CCD in ω-2θ (for 5 and 6)
using Mo KR radiation (λ ) 0.71073 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 5.
The crystal structures of these complexes were solved by
direct methods and expanded by Fourier techniques. All the
non-hydrogen atoms were refined anisotropically. Hydrogen
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 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.
1
N, 2.98; Yb, 18.24. H NMR (C6D6, ppm): 1.08-1.24 (m, 42H,
CH3), 1.53-1.70 (m, 14H, CH3 and THF), 2.59 (s, 3H, CH3),
3.07-3.35 (m, 12H, THF and CH3CHCH3), 4.55 (s, 1H, CHd
C(CH3)N), 6.00 (s, 2H, Ph), 6.82-7.31 (m, 6H, Ph).13C NMR
(C6D6, ppm): 20.73, 23.72, 24.47, 25.33, 28.39, 69.51, 93.60,
94.37, 96.71, 118.12, 119.55, 120.65, 123.36, 123.88, 124.75,
125.83. IR (KBr, cm-1): 3060(w), 2961(vs), 2925(s), 2869(s),
1623(s), 1552(vs), 1531(s), 1462(s), 1437(s), 1395(m), 1363(s),
1326(s), 1275(s), 1175(m), 1065(w), 1044(m), 861(w), 789(m),
758(s). Crystals suitable for X-ray crystal structure studies
were obtained by recrystallization from a THF/toluene solution
at -5 °C for a few days.
P olym er iza tion of Meth yl Meth a cr yla te In itia ted by
Diva len t La n th a n id e Com p lexes. A typical example of
polymerization is given. To a toluene solution (1 mL) of methyl
methacrylate (1 mL, 9.35 mmol) was added at once a toluene
solution (1 mL) of complex 4, 5, or 6 (0.0467 mmol) with
vigorous magnetic stirring at the desired temperature. After
the polymerization was carried out for a fixed time, 5 mL of
ethanol containing 2% HCl solution was added to terminate
the reaction, and then the viscous solution mixture was poured
into a large excess of petroleum ether to induce the precipita-
tion of the polymer. The polymer was washed with petroleum
ether three times and dried at 30 °C in a vacuum.
Ack n ow led gm en t. Financial support from the Chi-
nese National Natural Science Foundation, Department
of Education of J iangsu Province, is gratefully acknowl-
edged.
Su p p or tin g In for m a tion Ava ila ble: Tables of X-ray
diffraction data for complexes 4, 5, and 6. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM030136G