New bis(allyl)(diketiminato) and tris(allyl) lanthanide complexes and their reactivity in the polymerization of polar monomers

Article abstract of DOI:10.1021/om050309x

The reaction of LnCl₃(THF)_N (Ln = La, n = 4; Ln = Y, Sm, Nd, n = 3) with 3 equiv of allylMgCl in THF/1,4-dioxane followed by crystallization in 1,4-dioxane/toluene proceeds in very high yields to give the tris(η³-allyl) complexes [Laη³-C₃H ₅)₃(κ¹dioxane)]₂(μ-dioxane) and [Ln(η³-C₃H₅)₃(μ-dioxane) ]_infin; (Ln = Y, 1; Sm, 2; Nd). Improved one-pot syntheses for the tris(η³-allyl) lanthanum and neodymium compounds are reported. The crystal structures of 1 and 2 show that the compounds consist of distorted square-pyramidal lanthanide centers in linear polymer chains. The reaction of [η³-C₃H₅)₃(κ¹- dioxane)]₂(μ-dioxane) and [Ln(η³-C ₃H₅(μ-dioxane)]_infin; with the diketimine ligand 2-(2,6-diisopropylphenyl)amino-4-(2,6-diiso-propylphenyl)imino-2-pentene (BDI-H) in THF at 60°C generates cleanly the β-diketiminato complexes Ln(η³-C₃H₅)₂{κ ²-HC(Me-CNAr)₂} (Ar = 2,6-C₆H₃ ⁱPr₂; Ln = La, 3; Y, 4; Sm, 5; Nd, 6) with propene elimination. The crystal structure of 5 shows a distorted tetrahedral SmN ₂(allyl)₂ core, with the samarium atom positioned 1.438(4) A out of the C₃N₂ ligand plane. All complexes are highly effective single-component catalysts for the ring-opening polymerization of ε-caprolactone and rac-lactide. Polymer end group analysis shows that the polymerization process is initiated by allyl transfer to monomer.

Full text of DOI:10.1021/om050309x

3792
Organometallics 2005, 24, 3792-3799
New Bis(allyl)(diketiminato) and Tris(allyl) Lanthanide
Complexes and Their Reactivity in the Polymerization of
Polar Monomers
Luis F. Sa´nchez-Barba,† David L. Hughes,† Simon M. Humphrey,‡ and
M. Bochmann*,†
Wolfson Materials and Catalysis Centre, School of Chemical Sciences and Pharmacy,
University of East Anglia, Norwich, NR4 7TJ, U.K., and University Chemical Laboratory,
University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K.
Received April 20, 2005
The reaction of LnCl3(THF)n (Ln ) La, n ) 4; Ln ) Y, Sm, Nd, n ) 3) with 3 equiv of
allylMgCl in THF/1,4-dioxane followed by crystallization in 1,4-dioxane/toluene proceeds in
very high yields to give the tris(η3-allyl) complexes [La(η3-C3H5)3(κ1-dioxane)]2(µ-dioxane)
and [Ln(η3-C3H5)3(µ-dioxane)]∞ (Ln ) Y, 1; Sm, 2; Nd). Improved one-pot syntheses for the
tris(η3-allyl) lanthanum and neodymium compounds are reported. The crystal structures of
1 and 2 show that the compounds consist of distorted square-pyramidal lanthanide centers
in linear polymer chains. The reaction of [La(η3-C3H5)3(κ1-dioxane)]2(µ-dioxane) and [Ln(η3-
C3H5)3(µ-dioxane)]∞ with the diketimine ligand 2-(2,6-diisopropylphenyl)amino-4-(2,6-diiso-
propylphenyl)imino-2-pentene (BDI-H) in THF at 60 °C generates cleanly the â-diketiminato
i
complexes Ln(η3-C3H5)2{κ2-HC(MeCNAr)2} (Ar ) 2,6-C6H3 Pr2; Ln ) La, 3; Y, 4; Sm, 5; Nd,
6) with propene elimination. The crystal structure of 5 shows a distorted tetrahedral
SmN2(allyl)2 core, with the samarium atom positioned 1.438(4) Å out of the C3N2 ligand
plane. All complexes are highly effective single-component catalysts for the ring-opening
polymerization of ꢀ-caprolactone and rac-lactide. Polymer end group analysis shows that
the polymerization process is initiated by allyl transfer to monomer.
Introduction
complexes5a and allyl-bridged oligonuclear complexes5b,c
are highly active single-component catalysts for the
polymerization of methyl methacrylate, butadiene,6 and
cyclic esters. Allyl complexes of early lanthanides based
on the [Flu-CMe2-Cp]2- ligand for the syndiospecific
polymerization of styrene have also been reported
recently.7
Given the recent interest in non-cyclopentadienyl
ligand environments in group 3 metal chemistry,8 and
more specifically in the use of the â-diketiminato
(“nacnac”) donor framework,9 we became interested in
exploring the synthesis of nacnac-based mixed-ligand
lanthanide allyl complexes. These ligands have several
attractive features. â-Diketiminato ligands can play a
role similar to cyclopentadienyls, the steric and elec-
tronic properties of â-diketiminato ligands can be readily
altered through an appropriate choice of substituents,
and they can coordinate to the metal center in different
Lanthanide catalysts (including group 3) are being
widely employed in coordination polymerization to give
a range of polymers or copolymers and can work without
conventional aluminum alkyl additives to afford materi-
als that do not contain traces of toxic metals.1,2 In this
context, lanthanide allyl complexes3 have been shown
to be excellent precatalysts. For example, Taube et al.4
reported the synthesis of neutral tris(η3-allyl)lanthanide
complexes [La(η3-C3H5)3(κ1-dioxane)]2(µ-dioxane) and
[Nd(η3-C3H5)3(µ-dioxane)]∞, which catalyze the polym-
erization of butadiene in toluene with high 1,4-trans-
stereroselectivity. Other allyl complexes have been used
in the polymerization of polar monomers; for example,
we showed recently that ansa-bis(allyl) lanthanide
* Corresponding author. E-mail: M.Bochmann@uea.ac.uk.
† University of East Anglia.
‡ University of Cambridge.
(5) (a) Woodman, T. J.; Schormann, M.; Hughes, D. L.; Bochmann,
M. Organometallics 2003, 22, 3028. (b) Woodman, T. J.; Schormann,
M.; Bochmann, M. Organometallics 2003, 22, 2938. (c) Woodman, T.
J.; Schormann, M.; Hughes, D. L.; Bochmann, M. Organometallics
2004, 23, 2972.
(6) Woodman, T. J.; Schormann, M.; Hughes, D. L.; Bochmann, M.
Isr. J. Chem. 2002, 42, 283.
(7) Kirillov, E.; Lehmann, C. W.; Razavi, A.; Carpentier, J. J. Am.
Chem. Soc. 2004, 126, 12240.
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Ittel, S. D.; Johnson, L. K.; Brookhart, M. Chem. Rev. 2000, 100, 1169.
(c) Yasuda, H. J. Polym. Sci. Polym. Chem. Ed. 2001, 39, 1955. (d)
Bonnet, F.; Barbier-Baudry, D.; Dormond, A.; Visseaus, M. Polym. Int.
2002, 35, 986.
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2002, 102, 1851.
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mann, H. J. Orgamomet. Chem. 1996, 513, 49. (b) Taube, R.; Langlotz,
J.; Sieler, J.; Gelbrinch, T.; Tittes, K. J. Orgamomet. Chem. 2000, 597,
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10.1021/om050309x CCC: $30.25 © 2005 American Chemical Society
Publication on Web 06/08/2005

Bis(allyl)(diketiminato) Lanthanide Complexes
Organometallics, Vol. 24, No. 15, 2005 3793
bonding modes ranging from purely σ to a combination
of σ and π donation.10 High-activity lanthanide polym-
erization catalysts incorporating nacnac ligands have
been reported. In particular, Piers et al. have developed
the chemistry of organo-scandium complexes supported
by â-diketiminato ligands11 and described scandium and
yttrium alkyl compounds with related bulky chelating
ligands.12 The syntheses of scandium and yttrium
complexes bearing moderately bulky 2-iminophenolato
ligands have been reported,13 and heteroleptic sa-
marium,14 neodymium,14 and ytterbium15 complexes
and lanthanide chlorides16 containing â-diketiminato
ligands have also recently been made. In contrast, to
the best of our knowledge, no examples of allyl-
lanthanides supported by â-diketiminato ligands are
known. Generally, mixed-ligand lanthanide compounds
(L)LnR2 are difficult to synthesize, due to the tendency
of these types of compounds to undergo ligand redistri-
bution, dimerization, or oligomerization reactions or to
retain additional donor molecules in the coordination
sphere.
of the complex [Nd(η3-C3H5)3(µ-dioxane)]∞ starting from
NdI3(THF)3 and the corresponding allyl Grignard re-
agent. In our case, we used commercially available
metal chlorides in one-pot reactions. The tris(η3-allyl)
complexes [La(η3-C3H5)3(κ1-dioxane)]2(µ-dioxane) and
[Ln(η3-C3H5)3(µ-dioxane)]∞ were easily obtained at room
temperature after 15 h, followed by several recrystal-
lizations at -26 °C to give yields of >80% (Scheme 1).
1
The H and {1H}13C NMR spectra of the new Y and
Sm compounds 1 and 2 in THF-d8 at room temperature
display one set of signals for the η3-allyl groups, showing
the equivalence between Hsyn and Hanti and a single
sharp signal for the 1,4-dioxane molecule displaced by
THF-d8. The compounds are extremely air and moisture
sensitive, highly soluble in THF or 1,4-dioxane, and
insoluble in light petroleum, toluene, or diethyl ether.
They decompose in dichloromethane.
Whereas single crystals of 1 suitable for X-ray dif-
fraction crystallography were readily grown from 1,4-
dioxane/toluene at -26 °C, the isolation of crystals for
the samarium derivative 2 was complicated by the
extreme sensitivity and relative instability of this
compound. The smallest crystals decomposed almost
immediately when removed from the Schlenk tube
under nitrogen, whereas the bigger ones degraded on
the diffractometer during the course of the acquisition
of the intensity data. Suitable crystals of 2 were eventu-
ally obtained by recrystallization from a 1:8 mixture of
1,4-dioxane/toluene after several days at -26 °C. The
difference in stability between the yttrium and sa-
marium tris(η3-allyl) complexes was also evident by the
fact that 2 decomposed in the inert atmosphere of the
glovebox over a period of a few months, while 1
remained unchanged under identical conditions.
Compounds 1 and 2 are isostructural (Figure 1); the
bond lengths and angles are very similar in the two
molecules; they resemble Taube’s neodymium ana-
logue.4a,d Selected dimensions are listed in Table 1. The
structures confirm the presence of three η3-allyl groups
coordinated to the metal center and bridging 1,4-dioxane
molecules linking the Ln(η3-C3H5)3 units to form linear
polymeric chains. The 1,4-dioxane ligands are arranged
about centers of symmetry, and the Ln atoms lie on
2-fold symmetry axes; thus the allyl ligands of C(41-
43) and C(41′-43′) are related.
We report here the synthesis and structures of novel
tris(η3-allyl)lanthanides, their facile conversion into
bis(η3-allyl)(nacnac) complexes, and their reactivity as
single-component catalysts for the ring-opening polym-
erization of ꢀ-caprolactone and rac-lactide.
Results and Discussion
Syntheses. The reaction of LnCl3(THF)n (Ln ) La,
n ) 4; M ) Y, Sm, Nd, n ) 3.5) in a mixture of THF/
1,4-dioxane (4:1) with 3 equiv of allylmagnesium chlo-
ride affords the corresponding tris(η3-allyl) complexes
[La(η3-C3H5)3(κ1-dioxane)]2(µ-dioxane) and [Ln(η3-C3H5)3-
(µ-dioxane)]∞ (Ln ) Y, 1; Sm, 2; Nd) in very good yields
as yellow, orange, red, and green crystals, respectively.
Qualitative tests showed the absence of halide. The
lanthanum and neodymium tris(η3-allyl) compounds
were reported before by Taube.4a However, in that
synthesis the tetra(allyl) metalates [Li(µ-dioxane)1.5][Ln-
(η3-C3H5)4] (Ln ) La, Nd) were used as starting materi-
als, followed by the abstraction of allyllithium with BEt3
to afford a dimeric compound for the former and a linear
coordination polymer for the latter. An alternative
strategy was reported4c a few years ago for the synthesis
(10) (a) Parks, J. E.; Holm, R. H. Inorg. Chem. 1968, 7, 1408. (b)
Wright, R. J.; Power, P. P.; Scott, B. L.; Kiplinger, J. L. Organometallics
2004, 23, 4801.
The yttrium and samarium atoms have a distorted
square-pyramidal coordination pattern, with alternating
orientations along the chains. The O-M-O′ angles,
across the square base of the pyramid, are 155.1(2)° and
154.72(10)° in 1 and 2. There are slight differences in
the two complexes in the arrangements of the η3-allyl
ligand containing C(31-33), which is disordered about
the 2-fold symmetry axis. In 1, C(31) and C(33′) have
been resolved into two overlapping atoms with centers
about 0.25 Å apart; however, refinement was not totally
satisfactory and both C-C bond lengths in this ligand
were constrained at 1.39(1) Å. In 2, the atoms C(31) and
C(33′) were found to be essentially coincident and were
refined satisfactorily as a single-site atom; the variation
of C-C bond lengths in this ligand suggests however
that, assuming the delocalization across the ligand is
equal, the two atoms are not perfectly coincident. The
corresponding ligand in the Nd analogue appears less
well-resolved.4a,d
(11) (a) Hayes, P. G.; Piers, W. E.; Lee, L. W. M.; Knight, L. K.;
Parvez, M.; Elsegood, M. R. J.; Clegg, W. Organometallics 2001, 20,
2533. (b) Hayes, P. G.; Piers, W. E.; McDonal, R. J. Am. Chem. Soc.
2002, 124, 2132. (c) Hayes, P. G.; Piers, W. E.; Parvez, M. J. Am. Chem.
Soc. 2003, 125, 5622. (d) Knight, L. K.; Piers, W. E.; Fleurat-Lessard,
P.; Parvez, M.; McDonald, R. Organometallics 2004, 23, 2087. (e)
Hayes, P. G.; Piers, W. E.; Parvez, M. Organometallics 2005, 24, 1173.
(f) Basuli, F.; Tomaszewski, J.; Huffman, J. C.; Mindiola, D. J.
Organometallics 2003, 22, 4705.
(12) (a) Emslie, D. J. H.; Piers, W. E.; McDonald, R. J. Chem. Soc.,
Dalton Trans. 2002, 293. (b) Emslie, D. J. H.; Piers, W. E.; Parvez,
M.; McDonald, R. Organometallics 2002, 21, 4226. (c) Emslie, D. J.
H.; Piers, W. E.; Parvez, M. Dalton Trans. 2003, 2615. (d) Hayes, P.
G.; Welch, G. C.; Emslie, D. J. H.; Noack, C. L.; Piers, W. E.; Parvez,
M. Organometallics 2003, 22, 1577.
(13) Lara-Sa´nchez, A.; Rodr´ıguez, A.; Hughes, D. L.; Schormann,
M.; Bochmann, M. J. Organomet. Chem. 2002, 663, 63.
(14) Bonnet, F.; Visseaux, M.; Barbier-Baudry, D.; Vigier, E.;
Kubicki, M. M. Chem. Eur. J. 2004, 10, 2428.
(15) Yao, Y.; Xue, M.; Luo, Y.; Zhang, Z.; Jiao, R.; Zhang, Y.; Shen,
Q.; Wong, W.; Yu, K.; Sun, J. J. Organomet. Chem. 2003, 678, 108.
(16) Zhang, Z.; Yao, Y.; Zhang, Y.; Shen, Q.; Wong, W. Inorg. Chem.
Act. 2004, 357, 3173.

3794 Organometallics, Vol. 24, No. 15, 2005
Sa´nchez-Barba et al.
Scheme 1
Table 1. Selected Interatomic Distances (Å) and
Angles (deg) for 1 and 2
The allyl ligand of C(41-43) was fully resolved in both
complexes and shows more reliable dimensions. Here,
the Y-C distances are in the range 2.638(7)-2.676(7)
Å and the Sm-C distances are slightly longer at
2.678(4)-2.731(5) Å. All the C-C bonds have essentially
the same length, the mean value is 1.375(4) Å, and the
C-C-C angles are 126.4(7)° and 126.8(4)° in 1 and 2,
respectively.
[Y(η3-C3H5)3(µ-C4H8O2)]n, 1
[Sm(η3-C3H5)3(µ-C4H8O2)]n, 2
Y-O(1)
2.507(4)
2.76(4)
2.656(11) Sm-C(32)
2.661(17) Sm-C(33)
2.638(7)
2.650(7)
2.676(7)
1.390(10) C(31)-C(32)
1.399(10) C(32)-C(33)
1.389(11) C(41)-C(42)
1.364(11) C(42)-C(43)
Sm-O(1)
2.577(2)
2.725(4)
2.715(7)
2.725(4)
2.678(4)
2.705(4)
2.731(5)
1.281(12)
1.428(12)
1.369(6)
1.379(6)
154.72(10)
104.9(3)
100.0(3)
129.48(11)
92.73(10)
78.40(10)
100.3(2)
Y-C(31)
Sm-C(31)
Y-C(32)
Y-C(33)
Y-C(41)
Sm-C(41)
Sm-C(42)
Sm-C(43)
Y-C(42)
The crystal structure packing diagram of 1 and 2
(Figure 2) shows the linear polymeric chains, with the
η3-allyl groups in alternating orientations and the
bridging 1,4-dioxane ligands in chair conformation.
Compounds 1, 2, and the tris(η3-allyl)lanthanum and
neodymium analogues serve as convenient starting
materials for the synthesis of mixed-ligand complexes.
Thus stirring with diketimine (BDI-H) in THF at 60 °C
for 15 h gives high yields of the first examples of bis-
(η3-allyl)(BDI) lanthanide complexes, Ln(η3-C3H5)2{κ2-
Y-C(43)
C(31)-C(32)
C(32)-C(33)
C(41)-C(42)
C(42)-C(43)
O(1)-Y-O(1′)
O(1)-Y-C(32′)
O(1)-Y-C(32)
O(1)-Y-C(33)
O(1)-Y-C(42)
O(1)-Y-C(42′)
C(32)-Y-C(42)
155.1(2)
106.1(4)
98.4(4)
O(1)-Sm-O(1′)
O(1)-Sm-C(32′)
O(1)-Sm-C(32)
O(1)-Sm-C(33)
128.3(3)
92.63(19) O(1)-Sm-C(42)
78.84(18) O(1)-Sm-C(42′)
100.6(4)
C(32)-Sm-C(42)
C(32)-Sm-C(42′) 120.6(2)
C(42)-Sm-C(42′) 139.0(2)
i
HC(MeCNC6H3 Pr2-2,6)2} (Ln ) La, 3; Y, 4; Sm, 5; Nd,
C(32)-Y-C(42′) 119.4(4)
C(42)-Y-C(42′) 139.9(4)
6), as yellow, orange, red, and green microcrystalline
solids, respectively (Scheme 1).
The new compounds 3-6 were characterized by NMR
spectroscopy (1H, {1H}13C, NOESY-2D, HETCOR, for
diamagnetic species) and elemental analysis. In all
cases, the 1H spectra in benzene-d6 at room temperature
display one set of signals for the η3-allyl groups and the
â-diketiminato ligand. Complexes 3 and 4 show two
different doublets for the Hanti and Hsyn atoms with two
different JHH coupling constants, as well as a multiplet
for the central CH proton. The BDI ligands show in all
cases one signal for the Me and CH groups, but
restricted rotation about the N-Caryl bonds gives rise
Figure 1. View of a fragment of the polymer complex
[Y(η3-C3H5)3(µ-C4H8O2)]∞, 1. Hydrogen atoms have been
omitted for clarity. Thermal ellipsoids are drawn at the
50% probability level. The samarium analogue 2 is iso-
structural.

Bis(allyl)(diketiminato) Lanthanide Complexes
Organometallics, Vol. 24, No. 15, 2005 3795
Table 2. Selected Interatomic Distances (Å) and
Angles (deg) for 5
Sm(1)-N(1)
Sm(1)-N(2)
Sm(1)-C(30)
Sm(1)-C(31)
Sm(1)-C(32)
Sm(1)-C(33)
Sm(1)-C(34)
Sm(1)-C(35)
N(1)-C(1)
2.372(3)
2.369(3)
2.606(4)
2.692(4)
2.702(4)
2.612(4)
2.674(4)
2.636(4)
1.327(4)
1.421(5)
1.402(5)
1.340(4)
1.344(7)
1.389(8)
1.381(7)
1.373(7)
N(1)-Sm(1)-N(2)
N(1)-Sm(1)-C(31)
N(1)-Sm(1)-C(34)
N(2)-Sm(1)-C(31)
N(2)-Sm(1)-C(34)
C(31)-Sm(1)-C(34)
Sm(1)-N(1)-C(1)
Sm(1)-N(1)-C(6)
C(1)-N(1)-C(6)
Sm(1)-N(2)-C(3)
Sm(1)-N(2)-C(18)
C(3)-N(2)-C(18)
N(1)-C(1)-C(2)
82.92(10)
111.66(15)
110.67(14)
129.07(15)
114.93(13)
105.05(17)
110.1(2)
126.5(2)
119.9(3)
C(1)-C(2)
109.1(2)
C(2)-C(3)
N(2)-C(3)
C(30)-C(31)
C(31)-C(32)
C(33)-C(34)
C(34)-C35)
128.1(2)
119.6(3)
124.1(3)
C(1)-C(2)-C(3)
C(2)-C(3)-N(2)
C(30)-C(31)-C(32)
C(33)-C(34)-C(35)
132.9(3)
124.5(3)
127.1(6)
123.9(4)
The two Sm-N bond lengths are closely similar (2.372(3)
and 2.369(3) Å). The allyl fragment opposite the N
atoms is η3-bonded, with three very similar Sm-C bond
lengths [Sm(1)-C(33) ) 2.612(4) Å, Sm(1)-C(34) )
2.674(4) Å, Sm(1)-C(35) ) 2.636(4) Å]. Bonding of the
second allyl moiety is less symmetrical, with Sm(1)-
C(30, 31, 32) distances of 2.606(4), 2.692(4), and 2.702(4)
Å.
Figure 2. Projection of the linear polymer chains down
the b-axis in complex 1; complex 2 is essentially identical.
Polymerization Studies. Complexes 1-6 were tested
for the ring-opening polymerization of ꢀ-caprolactone
(CL) and rac-lactide (rac-LA). In principle, polymeriza-
tion can be envisaged to occur by one of two mecha-
nisms: either the transfer of the nucleophilic alkyl
ligand to the monomer, with formation of a metal
alkoxide-propagating species (Scheme 2, path A), or ring
opening and propagation via a cationic (activated chain
end, ACE) mechanism (path B).
Complexes 1-6 act as highly active single-component
catalysts for the polymerization of ꢀ-caprolactone (CL)
to give high molecular weight polymers (Table 3). A
variety of polymerization conditions were explored. The
tris(η3-allyl) complexes 1 and 2 initiated very rapid
polymerization of CL at room temperature (entries 1
and 4), and 2 gave complete conversion of 200 equiv of
CL in 90 s, with a productivity of more than 9 × 105 g
PCL (mol Sm)-1‚h-1. The polymerization was well
controlled and gave high molecular weight polymer with
narrow polydispersity (Mw ) 51 000, Mw/Mn ) 1.4). For
catalyst 1, productivity and molecular weight decreased
on cooling, and at -20 °C only traces of polymer were
found. By contrast, complex 2 was still very active at
-20 °C and reached a conversion of 47% after 3.5 min,
with a polydispersity suggestive of living behavior (Mw/
Mn ) 1.1) (entry 5). The dimeric complex [La(η3-C3H5)3-
(κ1-dioxane)]2(µ-dioxane) proved to be an extremely
active catalyst, with productivities of more than 4 × 106
g PCL (mol La)-1‚h-1 at room temperature (entry 6).
This activity was maintained at -20 °C, with Mw/Mn )
1.1.
Figure 3. Molecular structure of Sm(η3-C3H5)2{HC(MeCN-
2,6-iPr2C6H3)2}, 5. Thermal ellipsoids are drawn at the 30%
probability level.
to two separate doublets for the isopropyl methyl
groups. The 13C spectra in benzene-d6 display one set
of signals for the η3-allyl groups and the â-diketiminato
ligand. The compounds do not retain donor solvent
molecules in the metal coordination sphere. There is a
clear NOE (nuclear Overhauser effect) signal in the
NOESY-2D spectrum between the Hanti and only one
iPr group in the BDI ligand, which confirms the re-
stricted rotation about the N-Caryl bonds and the
proximity between the two different ligands in the same
molecule. This experiment eliminates the alterna-
tive formulation of the compounds as the ion pairs,
[Ln{HC(MeCNAr)2}2]+[Ln(η3-C3H5)4]-.
The geometry of these compounds was confirmed by
the single-crystal X-ray structure of complex 5 (Figure
3). Selected bond lengths and angles are collected in
Table 2. The samarium center exhibits a distorted
tetrahedral geometry, in which the N(1) and N(2) occupy
two positions and the η3-allyl groups the other two
positions.
The diketiminato-bis(allyl) complexes 3-6 also showed
very high CL polymerization activity. For instance, 3
gave complete conversion within 15 s at 0 °C, with a
productivity of >6.7 × 105 g PCL (mol La)-1‚h-1. The
polymer had a high molecular weight, while the poly-
dispersity was broader (Mw ) 144 000, Mw/Mn ) 1.8).
At -20 °C the polymerization also proceeded more
The samarium atom lies 1.438(4) Å above the C3N2
ligand plane. This displacement is significantly larger
than that observed in the related five-coordinated
dialkyl yttrium12d compounds (0.819(5) Å) incorporating
a similar chelating ligand, or an organo-scandium11a,b
compound (1.232(3) Å) containing a bulky nacnac ligand.

3796 Organometallics, Vol. 24, No. 15, 2005
Sa´nchez-Barba et al.
Scheme 2
Table 3. Polymerization of E-Caprolactone Catalyzed by Complexes 1-6a
conv (%)b
prodc
Mw
Mw/Mn
d
d
entry
initiator
temp [°C]
time [min]
yield [g]
1
Y(allyl)3 1
20
0
-20
20
-20
20
-20
0
1.3
2
1.23
0.74
traces
2.03
0.97
2.06
1.49
2.03
1.64
1.52
0.74
1.90
1.49
2.03
2.03
0.46
2.03
61
36
615
246
38 000
25 000
1.3
1.2
2
1
3
1
3.5
1.5
3.5
0.3
0.3
0.2
0.5
0.5
3
4
Sm(allyl)3 2
99
47
99
73
99
81
75
36
94
73
99
99
23
99
902
184
51 000
23 000
59 000
28 000
144 000
52 500
53 000
151 000
141 000
72 000
92 000
78 000
20 500
29 500
1.4
1.1
1.4
1.1
1.8
1.2
1.2
1.8
2.0
1.2
1.6
1.5
1.2
1.8
5
2
e
6
La(allyl)3
4161
3010
6766
2186
2026
164
e
7
La(allyl)3
8
La(allyl)2(DBI) 3
9
3
-20
-50
20
10
11
12
13
14
15
16
17
3
3f
3f
20
20
63
Y(allyl)2(DBI) 4
0
1.5
1.5
1.5
1.5
1
662
Sm(allyl)2(DBI) 5
20
902
5
5
0
902
-50
0
204
Nd(allyl)2(DBI) 6
1353
a Polymerization conditions: 90 µmol of initiator, ꢀ-CL/initiator ) 200. b Percentage conversion of the monomer (weight polymer recovered
× 100/weight of monomer). c kg polymer (mol Ln)-1‚h-1
.
d Determined by GPC relative to polystyrene standards. e [La(η3-C3H5)3(η1-
C4H8O2)]2(µ-C4H8O2). f 90 µmol of initiator, ꢀ-CL/initiator ) 5000.
slowly and led to a decrease in polydispersity (Mw
52 500, Mw/Mn ) 1.2). This catalyst was still active even
at -50 °C.
)
Complexes 1-6 were examined for the production of
polylactide (PLA).19b,20 Although a number of zinc and
magnesium â-diketiminato complexes have been re-
ported21 to polymerize lactide with high stereocontrol
and well-controlled living chain propagation, relatively
few examples of organo-lanthanides22 with related
chelating ligands are available.
Complexes 1-6 proved to be very active catalysts for
the polymerization of rac-lactide (LA) under mild condi-
tions (20-40 °C) (Table 4). A variety of polymerization
conditions were explored. The tris(η3-allyl) complexes
1 and 2 initiate rapid polymerization of rac-lactide at
In these tests the polymer molecular weights were
limited by the monomer/initiator ratio of 200:1. Increas-
ing this ratio 25-fold gave polymers with significantly
higher molecular weights (Mw > 105) (entries 11 and
12). End group analysis showed that the polymer con-
tained -CH2CHdCH2 termini, proof that the polymer-
ization follows the nucleophilic route (Scheme 2, path
A) and is initiated by the transfer of an allyl ligand to
the monomer, with formation of a metal alkoxide-
propagating species.17
The nature of the metal affects the catalytic activity,
which under the present polymerization conditions
decreases in the order 3 . 6 > 5 > 4, in agreement with
the decrease in ionic radii (La > Nd > Sm > Y). A
similar dependence was found in lanthanocene-based
systems.18 Although a number of organo-lanthanides6,19
have been shown to polymerize ꢀ-caprolactone, only few
show good activity at very low temperatures as in this
case (entries 10 and 16).
(19) See for example: (a) Agarwal, S.; Mast, C.; Dehnicke, K.;
Greiner, A. Macromol. Rapid Commun. 2000, 21, 195, and references
therein. (b) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. J. Chem.
Soc., Dalton Trans. 2001, 2215. (c) Arnt, S.; Okuda, J. Chem. Rev. 2002,
102, 1953. (d) Yao, Y.; Luo, Y.; Chen, J.; Zhang, Z.; Zhang, Y.; Shen,
Q. J. Organomet. Chem. 2003, 679, 229. (e) Cheng. J.; Cui, D.; Chen.
W.; Hu, N.; Tang, T.; Huang, B. J. Organomet. Chem. 2004, 689, 2646.
(20) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater.
2000, 12, 1841.
(21) (a) Cheng, M.; Attygalle, A. B.; Lobkovsky, E. B.; Coates, G.
W. J. Am. Chem. Soc. 1999, 121, 11583. (b) Chamberlain, B. M.; Cheng,
M.; Moore, D. R.; Ovitt, T. M.; Lobkovsky, E. B.; Coates, G. W. J. Am.
Chem. Soc. 2001, 123, 3229. (c) Coates, G. W. J. Chem. Soc., Dalton
Trans. 2002, 467. (d) Chisholm, M. H.; Huffman, J. C.; Phomphrai, K.
J. Chem. Soc., Dalton Trans. 2001, 222. (e) Chisholm, M. H.; Gallucci,
J.; Phomphrai, K. Inorg. Chem. 2002, 41, 2785.
(22) (a) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121,
4072. (b) Aubrecht, K. B.; Chang, K.; Hillmyer, M. A.; Tolman, W. B.
J. Polym. Sci. Part A: Polym. Chem. 2001, 39, 248.
(17) (a) Yong, T. M.; Holmes, A. B.; Tailor, P. L.; Robinson, J. N.;
Segal, J. A. J. Chem. Soc., Chem. Commun. 1996, 863. (b) Bolig, A.
D.; Chen, E. Y. X. J. Am. Chem. Soc. 2001, 123, 7943.
(18) Yamashita, M.; Takemoto, Y.; Ihara, E.; Yasuda, H. Macro-
molecules 1996, 29, 1798.

Bis(allyl)(diketiminato) Lanthanide Complexes
Organometallics, Vol. 24, No. 15, 2005 3797
Table 4. Polymerization of rac-Lactide Catalyzed
by Complexes 1-6a
temp time yield
convb prodc Mw
Mw/Mn
d
d
entry initiator [°C]
[h]
[g]
1
2
3
4
5
6
7
8
1
1
1
1
1
2
2
2
2
2
2
e
20
40
40
40
40
20
20
20
20
20
40
20
40
40
40
40
40
40
40
40
40
40
40
40
40
1
1
traces
0.16
16
40
58
76
2.28 18 000
2.48 19 000
2.36 20 000
2.31 20 000
1.1
1.2
1.2
1.2
2.3 0.40
3.5 0.58
4.7 0.76
1
traces
2.5 0.05
4.5 0.64
5
64
79
86
23
56
63
58
70
16
35
51
65
77
51
46
61
68
78
0.28 22 500
2.03 27 500
1.88 29 000
1.63 30 000
3.28 29 000
5.33 25 000
8.18 25 500
7.53 21 500
4.34 25 500
2.28 18 500
2.17 19 000
2.08 21 000
1.97 24 500
1.89 27 000
7.28 18 500
2.85 28 000
7.92 39 000
4.22 39 500
3.18 42 000
1.2
1.3
1.3
1.4
1.4
1.3
1.3
1.2
1.3
1.2
1.2
1.3
1.3
1.4
1.2
1.2
1.3
1.3
1.4
9
6
0.79
7.5 0.86
0.23
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
1
Figure 4. Conversion of rac-LA as a function of time,
catalyzed by 4 at 40 °C (THF, [LA]/[Y] ) 100).
1.5 0.56
1.1 0.63
1.1 0.58
2.3 0.70
e
3
3
4
4
4
4
4
4f
5
6
6
6
Increasing the reaction time at 20 °C to several hours
did, however, result in significant polymer formation.
Evidently at that temperature there was an induction
period of up to 2.5 h. No such induction period was
observed at 40 °C, as is exemplified in Figure 4 for the
yttrium BDI catalyst 4.
1
0.16
2.3 0.35
3.5 0.51
4.7 0.65
5.8 0.77
1
0.51
2.3 0.46
1.1 0.61
2.3 0.68
3.5 0.78
These observations suggest the existence of a pre-
activation step, possibly due to the presence of traces
of protic impurities. Consistent with that assumption,
the addition of 1 equiv of HOiPr to the precatalyst
resulted in a significant activity increase, with the
conversion rising from 16 to 51% under otherwise
identical conditions. The polymer characteristics were
unchanged.
a Polymerization conditions: 70 µmol of initiator, rac-LA:
initiator ) 100. b Percentage conversion of the monomer (weight
polymer recovered × 100/weight of monomer). c kg polymer (mol
e [{La(η3-C3H5)3(η1-C4H8O2)}2(µ-C4H8O2)]. f Addition of HOiPr to
the precatalyst in a ratio of 1:1.
Ln)-1‚h-1 d Determined by GPC relative to polystyrene standards.
.
As seen in the case of ꢀ-caprolactone polymerization,
the effect of the metal radius on the catalytic activity
can also be observed, La g Nd > Sm > Y (entries 15,
17, 22, and 24). End group analysis of the polymers by
1H NMR spectroscopy showed that, as for PCL, the
polymerization was initiated by nucleophilic attack of
40 °C (entries 2 and 11); 1 gives 16% of conversion of
100 equiv of rac-lactide after 1 h, with a productivity of
>2.3 × 103 g PLA (mol Y)-1‚h-1. The product was a
medium molecular weight polymer with very narrow
polydispersity (Mw ) 18 000, Mw/Mn ) 1.1). On the other
hand, when 1 and 2 were tested at room temperature,
no polymer was obtained after 1 h polymerization,
whereas the analogous La complex [La(η3-C3H5)3(κ1-
dioxane)]2(µ-dioxane) showed good activity (entry 12).
1
allyl on lactide. The homonuclear decoupled H NMR
spectra in the methine region of the PLA (CDCl3, 298
K, 500 MHz) revealed that, unlike zinc and magnesium
BDI complexes, catalysts 3-6 are not stereoselective
Table 5. Crystal Data and Summary of Data Collection and Refinement Details for 1, 2, and 5
1
2
5
formula
C
13H23O2Y
C13H23O2Sm
C35H51N2Sm
cryst size (mm)
0.28 × 0.07 × 0.05
0.14 × 0.05 × 0.02
0.16 × 0.14 × 0.12
fw
300.2
361.7
650.13
cryst syst
monoclinic
C2/c (no. 15)
14.0919(5)
7.7544(3)
monoclinic
C2/c (no. 15)
14.1965(3)
7.8233(2)
triclinic
space group
P1h (no. 2)
a, Å
10.9089(3)
b, Å
12.3956(3)
c, Å
13.6632(6)
90
112.552(2)
90
13.6478(4)
90
111.5527(10)
90
13.9731(4)
R, deg
65.0710(18)
78.7228(14)
73.3055(12)
1635.17(8)
â, deg
γ, deg
V, Å3
1378.87(9)
4
1409.79(6)
4
Z
2
Dcalcd, Mg/m3
1.446
1.704
1.320
µ, mm-1
4.215
4.151
1.819
F(000)
624
716
674
no. of indep reflns
no. of obsd reflns (I > 2σi)
no. of data/restraints/parameters
goodness of fit
final R indices (all data)
final R indices (obsd data)
weighting params A, Ba
1576 (Rint ) 0.075)
1424
1576/2/83
1.073
R1 ) 0.068, wR2 ) 0.152*
R1 ) 0.059, wR2 ) 0.146a
0.0808, 8.37
1.87, -1.36
close to Y atom
1621 (Rint ) 0.052)
1488
7428 (Rint ) 0.0424)
6436
7428/0/343
1.076
1621/0/79
1.078
R1 ) 0.034, wR2 ) 0.056b
R1 ) 0.026, wR2 ) 0.054
0.0184, 0.64
0.88, -1.09
close to Sm atom
R1 ) 0.0522, wR2 ) 0.0836c
R1 ) 0.0401, wR2 ) 0.0782
0.0154, 2.97
1.08, -0.56
close to allyl group of C(30-32)
largest diff peak and hole (e‚Å-3
)
location of largest diff peak(s)
a Reflection weighting scheme: w ) [σ2(Fo2) + (AP)2 + BP]-1 with P ) (Fo + 2Fc )/3.
2
2

3798 Organometallics, Vol. 24, No. 15, 2005
Sa´nchez-Barba et al.
min and was calibrated using eight monodisperse polystyrene
standards in the range 580-483 000 Da.
and give atactic PLA, most probably as the result of the
larger ionic radius, which leads to more flexible and
sterically less congested active centers. Similar behavior
has been recently reported for calcium complexes of the
type (BDI)Ca(N(SiMe3)2)2‚THF in comparison with the
corresponding magnesium analogues.23
Preparation of [Y(η3-C3H5)3(µ-C4H8O2)]∞ (1). To a solu-
tion of YCl3(THF)3.5 (1 g, 2.43 mmol) in THF (100 mL) was
added dropwise a solution of (allyl)MgCl in THF (16.2 mL, 0.45
M, 7.29 mmol) at room temperature, followed by 1,4-dioxane
(25 mL). The initial pale orange solution became dark orange
over time. The resulting suspension was stirred at room
temperature for 15 h. Filtration and removal of the volatiles
left an orange oil, which was extracted with 1,4-dioxane (10
mL). Finally, toluene was added (60 mL), and the mixture was
left to crystallize at -26 °C to give 1 as an orange microcrys-
talline solid, yield 0.55 g (1.82 mmol, 82.2%). Anal. Calcd for
C13H23O2Y: C, 51.96; H, 7.66. Found: C, 51.88; H, 7.66. 1H
NMR (THF-d8, 20 °C): δ 6.23 (m, 3 H, CH2CHCH2), 2.34 (d,
JHH ) 9.6 Hz, 12 Hsyn,anti, CH2CHCH2), 3.54 (s, 8 H, µ-C4H8O2).
13C NMR (THF-d8, 20 °C): δ 149.5 (CH2CHCH2), 56.9
(CH2CHCH2), 67.9 (µ-C4H8O2).
Conclusions
The tris(η3-allyl) lanthanides [La(η3-C3H5)3(η1-diox-
ane)]2(µ-dioxane) and [Ln(η3-C3H5)3(µ-dioxane)]∞ (Ln )
Y, 1; Sm, 2; Nd) are readily accessible in one-pot
reactions from LnCl3(THF)n with (allyl)MgCl, in almost
quantitative yields. In the solid state, complexes 1 and
2 exist as linear coordination polymers containing
distorted square-pyramidal lanthanide centers with
alternating relative orientations. The reaction of the
tris(η3-allyl) lanthanides with BDI-H generates cleanly
â-diketiminato compounds of the type Ln(η3-C3H5)2(BDI)
(Ln ) La, 3; Y, 4; Sm, 5; Nd, 6), the first such mixed-
ligand lanthanide allyl compounds. The structure of 5
shows a distorted tetrahedral coordination geometry in
the solid state. The allyl ligands are sufficiently nucleo-
philic for attack on cyclic esters, so that these com-
pounds can act as highly effective single-component
catalysts for the ring-opening polymerization of ꢀ-
caprolactone and rac-lactide and, with suitable pre-
cautions, without the need of an activator or scavenger.
Not unexpectedly, the polymerization of LA is slower
than that of CL but offers good control and gives
polymers with polydispersities as narrow as Mw/Mn )
1.1 under mild conditions.
Preparation of [Sm(η3-C3H5)3(µ-C4H8O2)]∞ (2). Following
the procedure described for 1, a solution of SmCl3(THF)3.5 (1
g, 2.11 mmol) in THF (100 mL) was treated with (allyl)MgCl
(14.1 mL, 6.34 mmol) at room temperature, with addition of
1,4-dioxane (25 mL). The color changed from orange to red over
time. Workup gave 2 as a red microcrystalline solid, yield 0.59
g (1.63 mmol, 82.9%). Anal. Calcd for C13H23O2Sm: C, 43.17;
H, 6.36. Found: C, 43.12; H, 6.44.
Preparation of La(η3-C3H5)2{HC(MeCNC6H3 Pr2-2,6)2}
i
(3). To a yellow solution of [La(η3-C3H5)3(η1-C4H8O2)]2(µ-
C4H8O2) (1 g, 1.26 mmol) in THF (60 mL) was added dropwise
at room temperature a solution of the diketimine HC(MeCd
NAr)(MeCNHAr) (Ar ) 2,6-iPr2C6H3) (1.06 g, 2.53 mmol) in
THF (20 mL). The mixture was warmed to 60 °C and stirred
for 15 h, during which time the solution became pale yellow.
After removing the volatiles, the residue was washed with cold
petroleum (10 mL). Finally, adding THF (20 mL) and cooling
to -26 °C afforded 3 as a pale yellow microcrystalline solid,
yield 1.12 g (1.75 mmol, 69.1%). Anal. Calcd for C35H51N2La:
C, 65.84; H, 7.99; N, 4.38. Found: C, 65.68; H, 7.81; N, 4.41.
1H NMR (benzene-d6, 20 °C): δ 7.27 (s, 6 H, Ho,m,p), 6.41 (m, 2
Experimental Section
General Procedures. All manipulations were performed
under nitrogen, using standard Schlenk techniques. Solvents
were predried over sodium wire (toluene, light petroleum,
THF, diethyl ether) or calcium hydride (dichloromethane) and
distilled under nitrogen from sodium (toluene), sodium-
potassium alloy (light petroleum, bp 40-60 °C), sodium-
benzophenone (THF, diethyl ether), or calcium hydride (dichlo-
romethane). Deuterated solvents were stored over activated
4 Å molecular sieves and degassed by several freeze-thaw
cycles. LnCl3(H2O)6 and rare earth metal powders were
purchased (Aldrich), and anhydrous LnCl324 and â-diketimines25
were prepared according to the literature procedures. LnCl3-
(THF)n was prepared by Soxhlet extraction of LnCl3 with THF
for several days. ꢀ-Caprolactone was dried by stirring over
fresh CaH2 for 48 h, distilled under reduced pressure, and
stored over activated 4 Å molecular sieves. rac-Lactide was
sublimed twice, recrystallized from THF, and finally sublimed
again prior to use. NMR spectra were recorded using a Bruker
Avance DPX-300 spectrometer. 1H NMR spectra (300.1 MHz)
were referenced to the residual solvent protons of the deuter-
ated solvent used. 13C NMR spectra (75.5 MHz) were refer-
enced internally to the D-coupled 13C resonances of the NMR
solvent. Gel permeation chromatography (GPC) measurements
were performed on a Polymer Laboratories PL-GPC-220
instrument equipped with a PLgel 5 Å Mixed-C column, a
refractive index detector, and a PD2040 light-scattering detec-
tor. The GPC column was eluted with THF at 40 °C at 1 mL/
3
H, CH2CHCH2), 4.91 (s, H, CHâ), 3.29 (d, JHH ) 8.8 Hz, 4
3
H
syn, CH2CHCH2), 2.80 (sep, JHH ) 6.8 Hz, 4 H, CH(CH3)2),
2.52 (d, 3JHH ) 15.5 Hz, 4 Hanti, CH2CHCH2), 1.62 (s, 6 H, Me),
3
3
1.19 (d, JHH ) 6.8 Hz, 12 H, CH(CH3)2), 1.08 (d, JHH ) 6.8
Hz, 12 H, CH(CH3)2). 13C NMR (benzene-d6, 20 °C): δ 160.7
(CH2CHCH2), 142.4-123.6 (aryl), 118.3 (Câ), 93.5 (CHâ), 69.0
(CH2CHCH2), 28.1 CH(CH3)2, 27.3 (Me), 25.1-20.4 (CH(CH3)2).
Preparation of Y(η3-C3H5)2{HC(MeCNC6H3 Pr2-2,6)2}
i
(4). Following the procedure described for 3, an orange solution
of 1 (1 g, 3.33 mmol) in THF (60 mL) was reacted with
HC(MeCdNAr)(MeCNHAr) (Ar ) 2,6-iPr2C6H3) (1.39 g, 3.33
mmol) in THF (20 mL) at room temperature. Suitable workup
gave 4 as a pale orange microcrystalline solid, yield 1.23 g (2.09
mmol, 62.7%). Anal. Calcd for C35H51N2Y: C, 71.44; H, 8.67;
N, 4.76. Found: C, 71.23; H, 8.52; N, 4.78. 1H NMR (benzene-
d6, 20 °C): δ 7.21 (s, 6 H, Ho,m,p), 6.80 (m, 2 H, CH2CHCH2),
3
4.97 (s, H, CHâ), 3.47 (d, JHH ) 8.6 Hz, 4 Hsyn, CH2CHCH2),
2.73 (sep, 3JHH ) 6.8 Hz, 4 H, CH(CH3)2), 2.46 (d, 3JHH ) 15.2
Hz, 4 Hanti, CH2CHCH2), 1.58 (s, 6 H, Me), 1.16 (d, 3JHH ) 6.8
Hz, 12 H, CH(CH3)2), 1.06 (d, 3JHH ) 6.8 Hz, 12 H, CH(CH3)2).
13C NMR (benzene-d6, 20 °C): δ 163.6 (CH2CHCH2), 141.3-
121.3 (aryl), 117.2 (Câ), 93.4 (CHâ), 68.7 (CH2CHCH2), 28.1
CH(CH3)2, 26.9 (Me), 24.1-19.9 (CH(CH3)2).
Preparation of Sm(η3-C3H5)2{HC(MeCNC6H3 Pr2-2,6)2}
i
(5). Following the procedure described for 3, a red solution of
2 (1 g, 2.76 mmol) in THF (60 mL) was reacted with HC(MeCd
NAr)(MeCNHAr) (Ar ) 2,6-iPr2C6H3) (1.15 g, 2.76 mmol) in
THF (20 mL) at room temperature. After stirring for 15 h at
60 °C volatiles were removed and the residue was washed with
cold light petroleum (10 mL). Finally, THF was added (20 mL)
and the mixture left to crystallize at -26 °C to give 5 as a
pale red microcrystalline solid, yield 1.23 g (2.09 mmol, 62.7%).
(23) Chisholm, M. H.; Huffman, J. C.; Phomphrai, K. Inorg. Chem.
2004, 43, 6717.
(24) Taylor, M. D.; Carter, C. P. J. Inorg. Nucl. Chem. 1962, 24,
387.
(25) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W.
J.; Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514.

Bis(allyl)(diketiminato) Lanthanide Complexes
Organometallics, Vol. 24, No. 15, 2005 3799
Anal. Calcd for C35H51N2Sm: C, 64.60; H, 7.84; N, 4.30.
Found: C, 64.60; H, 7.84; N, 4.30.
Polymerization Procedures. Polymerizations of ꢀ-capro-
lactone (CL) were carried out under nitrogen in flame-dried
round-bottom flasks equipped with a magnetic stirrer. In a
typical procedure, the initiator was dissolved in the appropri-
ate amount of solvent, and the temperature equilibration was
ensured by stirring the solution for 15 min on a water bath.
ꢀ-CL was injected, and polymerization times were measured
from that point. Polymerizations were terminated by addition
of acetic acid (5% vol) in methanol. The polymers were
precipitated into excess methanol, filtered, dissolved in THF,
reprecipitated with methanol, and dried in vacuo to constant
weight.
Polymerizations of rac-lactide (LA) were performed under
nitrogen in flame-dried round-bottom flasks. In the glovebox,
the required amounts of rac-lactide and initiator were weighed
out into separate Schlenk tubes. These were then attached to
the vacuum line, and the initiator and the monomer were
dissolved in the appropriate amounts of solvent. Temperature
equilibration was ensured by stirring the flasks for 15 min on
a water bath, followed by the injection of the initiator into the
monomer solution. Polymerizations were stopped by injecting
a solution of acetic acid (5% vol) in methanol. Polymers were
precipitated in methanol, filtered, dissolved in THF, repre-
cipitated in methanol, and dried in vacuo to constant weight.
i
Preparation of Nd(η3-C3H5)2{HC(MeCNC6H3 Pr2-2,6)2}
(6). The compound was made from [Nd(η3-C3H5)3(µ-C4H8O2)]∞
(1 g, 2.81 mmol) and HC(MeCdNAr)(MeCNHAr) (Ar ) 2,6-
iPr2C6H3) (1.17 g, 2.81 mmol), to give 6 as a pale green
microcrystalline solid, yield 1.09 g (1.69 mmol, 60.1%). Anal.
Calcd for C35H51N2Nd: C, 65.29; H, 7.92; N, 4.35. Found: C,
65.35; H, 7.87; N, 4.33.
X-ray Crystallography. Crystals coated with dry Nujol or
perfluoropolyether were mounted on a glass fiber and fixed in
the cold nitrogen steam (T ) 180(2) K). Intensity data were
collected on a Nonius Kappa CCD diffractometer equipped
with a graphite monochromator (Mo KR, λ ) 0.71073 Å). Data
were processed using the DENZO/SCALE PACK programs26
and absorption corrections applied in SORTAV.27 The struc-
tures were determined by direct methods in the SHELXS
programs and refined by full-matrix least-squares methods,
on F2 values, in SHELXL.28 Compounds 1 and 2 are isostruc-
tural and, in both, the allyl group of C(31-33) is disordered
about a 2-fold symmetry axis, in overlapping sites; in 1, C(31)
was refined isotropically and both C-C bond lengths in this
ligand were constrained; in 2, C(31) and C(33′) are essentially
superimposed and were refined as a single, anisotropic atom.
The remaining non-hydrogen atoms (in all complexes) were
refined with anisotropic thermal parameters. Hydrogen atoms
were included in idealized positions, and their Uiso values set
to ride on the Ueq values of the parent carbon atoms. Scattering
factors for neutral atoms were taken from the literature.29
Computer programs were run on a DEC-AlphaStation 200
4/100 in the Biological Chemistry Department, John Innes
Centre.
Acknowledgment. L.F.S. thanks the European
Commission for a Marie Curie Fellowship (contract
MEIF-CT-2003-500476). We are grateful to Dr. Juan
Ferna´ndez of the Facultad de Qu´ımicas, Universidad
de Castilla-La Mancha, for homonuclear decoupled 1H
NMR spectra of poly(lactide) samples, and Dr. J. E.
Davies of the Department of Chemistry, University of
Cambridge, for the measurement and processing the
diffraction intensities for compounds 1 and 2.
(26) Otwinowki, Z.; Minor, W. Methods Enzymol. 1997, 276, 307.
(27) Blessing, R. H. J. Appl. Crystallogr. 1997, 30, 421.
(28) Sheldrick, G. M. SHELX-97, Programs for crystal structure
determination (SHELXS) and refinement (SHELXL); University of
Go¨ttingen: Go¨ttingen, Germany, 1997.
(29) International Tables for X-ray Crystallography: Kluwer Aca-
demic Publishers: Dordrecht, The Netherlands, 1992; Vol. C, pp 193,
219, 500.
Supporting Information Available: Full listing of crys-
tallographic details for compounds 1, 2, and 5. This material
is available free of charge via the Internet at http:/pubs.acs.org.
OM050309X