Group 3 metal complexes supported by tridentate pyridine- and thiophene-linked bis(naphtholate) ligands: Synthesis, structure, and use in stereoselective ring-opening polymerization of racemic lactide and β-butyrolactone

Article abstract of DOI:10.1039/b920283h

Scandium, yttrium and lanthanum amido complexes supported by tridentate bis(ortho-silyl-substituted naphtholate)-donor ligands ({ONO^SiPh3} ^2- and {ONO^SitBuMe2}^2-, donor = 2,6-pyridine; {OSO^SiPh3}

Full text of DOI:10.1039/b920283h

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www.rsc.org/dalton | Dalton Transactions
Group 3 metal complexes supported by tridentate pyridine- and
thiophene-linked bis(naphtholate) ligands: synthesis, structure, and use in
stereoselective ring-opening polymerization of racemic lactide and
b-butyrolactone†
Ekaterina Grunova, Evgueni Kirillov,* Thierry Roisnel and Jean-Franc¸ois Carpentier*
Received 29th September 2009, Accepted 25th November 2009
First published as an Advance Article on the web 7th January 2010
DOI: 10.1039/b920283h
Scandium, yttrium and lanthanum amido complexes supported by tridentate bis(ortho-silyl-substituted
2-
2-
2-
naphtholate)-donor ligands ({ONOSiPh3
}
and {ONOSitBuMe2} , donor = 2,6-pyridine; {OSOSiPh3} ,
donor = 2,5-thiophene) have been prepared in high yields (72–96%) by reaction of the corresponding
pro-ligand {OZOSiR3}H2 and Ln[N(SiHMe2)2]3(THF)n precursor. The solid-state structures of
{ONOSiPh3}La[N(SiHMe2)2](THF) (3), {ONOSitBuMe2}Ln[N(SiHMe2)2](THF) (Ln = Sc, 4; Y, 5) and
{OSOSiPh3}Ln[N(SiHMe2)2](THF) (Ln = Sc, 7; La, 9) have been determined by single-crystal X-ray
diffraction studies. In all five complexes, the naphtholate rings twist in the same direction from the
plane of the pyridine or thiophene linker, to give rise to Cs-symmetric (non crystallographic) structures.
Compounds 1–9 are single-site initiators for the ring-opening polymerization (ROP) of racemic lactide
(rac-LA) at 20 ◦C, affording poly(lactides)s (PLAs) with relatively narrow polydispersities and
molecular weights in good agreement with calculated values. When carried out in THF, the
polymerizations afforded heterotactic-enriched PLAs (Pr up to 0.93), while atactic polymers are formed
in toluene. Compounds 1–3 and 7–9, having o-SiPh3 substituents on the naphtholate rings, are also
active for the ROP of racemic b-butyrolactone at 20–50 ◦C, to form syndiotactic-enriched
poly(3-hydroxybutyrate)s (PHBs) (Pr up to 0.87) when using toluene as the solvent, whereas atactic
polymers were obtained in THF. The nature of the metal center (Sc, Y, La), the central linker in the
ligand framework (pyridine, thiophene), and the ortho-silyl substituent (SiPh3, SiMe2tBu) significantly
affect the degree of stereocontrol in those polymerizations.
with group 3 (lanthanides)7 and also to a lesser extent group 4
metals.8
Introduction
2-
Phenolates (aryloxides) combined with other donor atoms (N,
S, O…) are common multidentate ligands for early transition
metals,1 which have met with much success in the development
of post(non)-metallocenes for polymerization catalysis. Discrete
group 3–6 metal complexes bearing various bidentate,2 tridentate,3
and tetradentate4 phenolate-based ligands have demonstrated high
performances (i.e., catalytic activity, degree of control, stere-
oselectivity) in the oligomerization/polymerization of ethylene
and a-olefins.5 More recently, this class of ligands, particularly
the bis(phenolate)s, has been investigated in the ring-opening
polymerization (ROP) of cyclic esters, such as lactide, for the
preparation of biocompatible polymers,6 mostly in combination
Of particular interest are tridentate ligands {OZO} where a
rigid flat central donor of the “L”-type is flanked by two phenolate
rings, because such “LX2”-type ligands can confer various sym-
metries at the metal center. For instance, bis(phenolate) ligands
having a 2,6-pyridine, 2,5-furan or 2,5-thiophene linker have been
shown to bind to iron(III), copper(II), aluminium(III),9 and Ti(IV)3f
in a C2 (rac) fashion, while they bind to zirconium(IV)3d,e,f and
boron(III)10 in a Cs (meso) or C1 fashion. Static or dynamic control
over these diverse symmetries is of great interest in polymerization
catalysis, as it may eventually give rise to polymers with stereo-
regular microstructures.5,6 On this principle, we have recently pre-
pared sterically demanding silyl ortho-substituted tridentate 2,6-
2-
bis(naphtholate)pyridine ({ONOSiR3} , SiR3 = SiPh3, SiMe2tBu)
2-
and 2,5-bis(naphtholate)thiophene ({OSOSiPh3} ) ligands and
studied their corresponding group 4 metal (Ti, Zr, Hf) complexes.11
An anticipated key feature of these ligands was the possibility for
firm stereoselective coordination to the metal center provided by
the non-coplanar orientation of the bridging heterocyclic and ad-
jacent naphthoxy groups, due to steric repulsion between protons
at meta and 8-positions12 of these moieties, respectively. In fact,
X-ray diffraction studies revealed that, in the solid state, the latter
group 4 metal complexes adopt either rac-like or meso-like binding
of the ligand, while VT NMR spectroscopy studies in toluene
Catalysis and Organometallics, UMR 6226 Sciences Chimiques de
Rennes, CNRS-University of Rennes 1, 35042, Rennes, Cedex, (France).
E-mail: jean-francois.carpentier@univ-rennes1.fr, evgueni.kirillov@univ-
rennes1.fr; Fax: (+33)(0)223-236-939
† Electronic supplementary information (ESI) available: cif files for crystal
structure determinations of complexes 3–5, 7, 9 and 10, ORTEP views
and main bond distances and angles for complexes 3, 5 and 9, and
representative 1H NMR and 13C NMR spectra of PLA and PHB. CCDC
reference numbers 752787, 752788, 752789. 752790, 752791 and 752792.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/b920283h
This journal is The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6739–6752 | 6739
©

Scheme 1 Examples of common geometries in {OZOSiR3}M complexes and corresponding dynamic processes.
solution indicated that such complexes exist as rac and meso
Results and discussion
stereoisomers that interconvert relatively easily (activation param-
eters: DH‡ = 12.9(7)–13.4(8) kcal mol-1 and DS‡ = -3(1) to -7(1)
cal mol-1 K-1) (Scheme 1). Yet, these systems showed disappointing
catalytic performance in propylene polymerization (upon activa-
tion with MAO), leading at best to atactic oligomers with poor
Preparation of scandium, yttrium and lanthanum complexes
supported by tridentate bis(naphtholate)-donor ligands
Scandium, yttrium and lanthanum complexes were pre-
pared by amine elimination in reaction of the correspond-
ing tris(dimethylsilyl)amido metal group 3 precursors and
bis(naphthol)-donor pro-ligands {OZOSiR3}H2 (Scheme 2). The
activity.
2-
This naphthoxy-based {OZOSiR3
}
ligand platform is also of
11
potential interest to group 3 metal chemistry and related ROP
catalysis of chiral (racemic) cyclic esters, for several reasons: First,
such chelating ligands should accommodate metal centers with
quite different ionic radii; this should allow studying in more detail
the impact of the latter parameter, as well as the influence of hard
(Z = N) vs. soft (Z = S) donating heteroatom and ortho-silyl
groups over the polymerization course (activity, control). Such
data are still necessary because no clear rationale exists yet to
account for the reported behavior of group 3 metal catalysts
in ROP of cyclic esters.6 Second, yet much more challenging,
if complexes with different symmetries (e.g., with a C2-, Cs-
symmetric ligand) are attainable and if a reasonably easy dynamic
interconversion between such geometrically stable isomers, e.g. rac
and meso occurs, original multiblock microstructures (varying by
the nature or degree of tacticity) could be accessed via “oscillating”
polymerization.
reactions proceeded cleanly at room temperature in benzene or
toluene solutions, as evidenced by NMR monitoring. Complexes
1–5 and 7–9 were isolated in 72–96% yields as pale yellow
microcrystalline powders, which are air-sensitive and readily
soluble in most usual organic solvents. Complex 6 was generated
in situ for further use in catalysis (vide infra), but not isolated.
A coordinated THF molecule was systematically observed by 1H
and 13C NMR spectroscopy for all complexes. This observation,
in line with the mononuclear structures in the solid state for such
amido group 3 metal compounds (vide infra) and the unlikely
bridging modes of the bulky N(SiHMe2)2 and {OZOSiR3
}
lig-
-
2-
ands, suggests that {OZOSiR3}Ln[N(SiHMe2)2](THF) compounds
are mononuclear and five-coordinate in benzene or toluene
solution. The chemical shifts for the SiH (d 4.29–4.76 ppm at
298 K in benzene-d6), which are shifted upfield compared to the
chemical shift in the corresponding [Ln{N(SiHMe2)2}3(THF)2]
precursors (d 4.94–5.02 ppm), argue against a significant b(Si–H)
agostic interaction with the metal center in solution (vide infra).14
As mentioned in the Introduction section, these
bis(naphtholate)-donor ligands can achieve, in principle, a
variety of binding geometries, two of them being most common
(C2, Cs; Scheme 3),3f,11,15 that should be distinguishable by NMR
Accordingly, we describe herein the coordination chemistry
2-
of {OZOSiR3
}
ligands with group 3 metals (Ln = Sc, Y,
La). The performance of the amido compounds {OZOSiR3}-
Ln[N(SiHMe2)2](THF) in the stereoselective ROP of
racemic lactide and racemic-b-butyrolactone13 is also
reported.
Scheme 2
6740 | Dalton Trans., 2010, 39, 6739–6752
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The Royal Society of Chemistry 2010
©

existence, on the NMR time scale, of a single species with a Cs-
symmetrically bound ligand15 in toluene in the temperature range
223–338 K (Fig. 1 and 2). No additional resonances, in particular
in the aromatic region, were observed that could account for the
presence [within experimental accuracy] of a second stereoisomer.
Relevant features of these VT NMR spectra include, for both
compounds, one sharp singlet resonance [b] for the methyl
hydrogens of the SitBuMe2 groups in the high temperature
spectra which decoalesced to give two sharp singlet resonances
in the low temperature spectra. At the same time, as best seen in
yttrium compound 5: the a-THF methylene hydrogens appear as
a [broadened] singlet [e] in the high temperature spectrum and
decoalesced in two well-resolved resonances [e/e¢] at 233 K.16
We tentatively interpret those dynamic phenomena, which are
reversible upon increasing or decreasing the temperature, in
relation with rotation of the coordinated THF molecule (and
fluxionality within the latter cycle). Site exchange (of THF and
the amide ligands, which would imply THF dissociation) seems
unlikely as this would generate species in different environments
Scheme 3 Geometries observed in {OZOSiR3}Ln[N(SiHMe2)2](THF)
complexes.
spectroscopy. Such analysis is, however, hampered in the present
case by the complicated pattern of resonances associated with the
naphtholate rings in the 1H NMR spectra, especially when SiPh3
substituents are present. More informative data were obtained
from variable-temperature NMR experiments on the complexes
bearing SitBuMe2 substituents at the naphtholate rings. In fact,
the 1H NMR data for both 4 and 5 are consistent with the
1
that should be distinguished by H NMR at low temperature,
which is not the case (vide supra).
Fig. 1 VT-1H NMR spectra (500 MHz, toluene-d8) of {ONOSitBuMe2}Sc[N(SiHMe2)2](THF) (4); bottom, 223 K; top, 338 K. (a) SiHMe2; (b) SitBuMe2;
(c) SitBuMe2; (d) b-THF; (e) a-THF; (f) SiHMe2 (* refers to residual solvent resonances).
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6739–6752 | 6741
©

Fig. 2 VT-1H NMR spectra (500 MHz, toluene-d8) of {ONOSitBuMe2}Y[N(SiHMe2)2](THF) (5); bottom, 233 K; middle, 296 K; top, 338 K. (a) SiHMe2;
(b) SitBuMe2; (c) SitBuMe2; (d) b-THF; (e) a-THF; (f) SiHMe2 (* refers to residual solvent resonances).
Solid state structures of scandium, yttrium and lanthanum
complexes supported by tridentate bis(naphtholate)-donor ligands
All five complexes show a distorted trigonal-bipyramidal ge-
ometry at the metal center, which is five-coordinated by the
{OZOSiR3
}
2- ligand, the THF molecule and the dimethylsilylamido
Determination of the binding modes of the bis(naphtholate)-
donor ligands in these complexes is more straightforward from
single-crystal X-ray diffraction studies, although these observa-
tions in the solid state may not necessarily reflect the actual
geometries in solution. Crystals suitable for X-ray diffraction stud-
ies were successfully grown from concentrated benzene solution
at room temperature for compounds 3–5, 7, and 9. The main
crystallographic details are reported in Table 1. The molecular
structures of 4 and 7, and selected bond distances and angles for
these compounds are given in Fig. 3 and 4, respectively. Structures
and geometric factors for the other compounds are quite similar
and available as ESI.†
group. Yet, b-agostic interactions between one of the Si–H of
the latter amido group and the metal center are evidenced
˚
in all complexes by close M ◊ ◊ ◊ H(Si) contacts (2.818–3.129 A)
˚
and M ◊ ◊ ◊ Si(H) contacts (3.134–3.370 A) (Table 2). These agostic
interactions are confirmed by the observation of a more ob-
tuse M–Si–N bond angle (102.4(5)–116.4(2)◦). These interactions
remain, however, overall weak; this is attested by the Si–N–
Si bond angles of 122.1(2)–130.6(2)◦ that are slightly larger
than the angle of 120◦ for ideal sp2 hybridization and that
fall into the upper range of values reported for Si–N–Si angles
(119.93–129.58◦).17
6742 | Dalton Trans., 2010, 39, 6739–6752
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The Royal Society of Chemistry 2010
©

Table 1 Summary of crystal and refinement data for compounds 3–5, 7, 9 and 10 (all data were collected at 100 K)
3
4·2.5(C6H6)
5·2.5(C6H6)
7·4(C6H6)
9·0.5(C6H14)
10·4(C6H6)
Empirical formula C69H65LaN2O3Si4
C45H65N2O3ScSi4· C45H65YN2O3Si4·
C68H64NO3SScSi4·
C68H64LaNO3SSi4· C126H100La2N2O9Si4·
2.5(C6H6)
1034.58
2.5(C6H6)
1078.53
4(C6H6)
0.5(C6H14))
1269.62
4(C6H6)
Formula weight
Crystal system
Space group
1221.5
Triclinic
P-1
1445.01
Triclinic
P-1
2488.69
Orthorhombic
Pcab
Monoclinic
P21/n
Monoclinic
P21/n
Monoclinic
C2/c
˚
a/A
11.5427(13)
13.2146(15)
21.910(3)
72.404(6)
88.235(6)
76.123(7)
3089.5(7)
2
11.2092(6)
23.4248(12)
22.6197(11)
90
94.018(3)
90
11.2947(9)
23.4235(17)
22.7434(17)
90
93.324(4)
90
12.3960(7)
15.9382(10)
21.8083(14)
103.867(4)
98.581(4)
106.022(3)
3912.1(4)
2
38.9355(14)
15.5687(6)
22.9647(8)
90
113.803(2)
90
22.4713(9)
29.6425(13)
39.1800(14)
90
90
90
˚
b/A
˚
c/A
a (◦)
b (◦)
g (◦)
3
˚
Volume/A
5924.7(5)
4
1.160
6006.9(8)
4
1.193
12736.5(8)
8
1.324
26098.0(18)
8
1.264
Z
Density, Mg m-3
m/mm-1
1.313
1.227
0.233
1528
0.816
1260
0.248
2220
1.093
2292
0.826
5256
0.741
10192
F(000)
Crystal size/mm
q range, deg
Limiting indices
0.20 ¥ 0.05 ¥ 0.04
2.53 to 21.42
-14 ≤ h ≤ 14,
-17 ≤ k ≤ 15,
-28 ≤ l ≤ 28
14021
0.52 ¥ 0.30 ¥ 0.25
3.48 to 27.48
-14 ≤ h ≤ 13,
-30 ≤ k ≤ 30,
-29 ≤ l ≤ 29
63479
0.42 ¥ 0.38 ¥ 0.21
3.44 to 27.47
-14 ≤ h ≤ 14,
-23 ≤ k ≤ 30,
-29 ≤ l ≤ 29
52124
0.16 ¥ 0.10 ¥ 0.06
3.42 to 27.48
-16 ≤ h ≤ 15,
-20 ≤ k ≤ 20,
-28 ≤ l ≤ 27
43820
0.28 ¥ 0.17 ¥ 0.14
3.41 to 27.48
-50 ≤ h ≤ 35,
-20 ≤ k ≤ 20,
-29 ≤ l ≤ 29
67600
0.20 ¥ 0.20 ¥ 0.065
3.41 to 27.45
-29 ≤ h ≤ 24, -37 ≤
k ≤ 38, -27 ≤ l ≤ 50
Reflec. collected
Rint
Reflec. unique
[I > 2s(I)]
Data/restraints/
param.
151425
0.1008
29729
0.1200
9954
0.0538
13476
0.0534
13601
0.1075
17816
0.0618
14447
14021/0/702
1.201
13476/0/651
1.122
13601/0/691
1.03
17816/0/917
1.058
14447/2/699
1.020
29729/477/1024
1.032
Goodness-of-fit
on F2
R1 [I > 2s(I)]
(all data)
0.1307(0.1753)
0.2464 (0.2678)
2.444 and -4.305
0.0635 (0.0754)
0.1121 (0.1174)
0.528 and -0.688
0.0413 (0.0713)
0.0848 (0.0943)
0.735 and -0.591
0.1013 (0.1762)
0.1621(0.1917)
0.545 and -0.577
0.0426 (0.0693)
0.0900 (0.1018)
1.605 and -0.996
0.0828 (0.1255)
0.1829 (0.2032)
1.984 and -1.388
wR2 [I > 2s(I)]
(all data)
Largest diff. e A-3
yttrium (5, 56.5 and 60.4◦). As indicated by the minimal difference
between the twist angles, the structural features of the naphtholate
rings in scandium complexes {ONOSitBuMe2}Sc[N(SiHMe2)2](THF)
(4, 52.4 and 55.0◦) and {OSOSiPh3}Sc[N(SiHMe2)2](THF) (7, 51.9
and 53.3◦) are very similar. Yet, as illustrated in Fig. 3 and 4, the
pyridine and thiophene linkers coordinate quite differently in those
complexes: the thiophene ring is almost perpendicular to the M–S
vector (dihedral angle: 7, 88.3◦; 9, 73.9◦), while the pyridine ring
is only slightly deviated from the M–N vector (dihedral angle: 3,
156.6◦; 4, 152.8◦; 5, 152.5◦). Similar features were observed for
group 4 metal complexes of bis(phenolate)-donor ligands.3f
The selective (at least in the solid state) Cs binding mode of
these bis(naphtholate)-donor ligands with group 3 metals is in line
with the Cs binding mode of analogous CMe3 and CEt3 ortho-
substituted bis(phenolate)-donor ligands (donor = pyridine, thio-
phene, furan) observed by Bercaw et al. in a series of tantalum(V)
complexes.19 On the other hand, it is in contrast with the C2 or
C1 binding mode15 of the latter bis(phenolate)-donor ligands3f
and present bis(naphtholate)-donor ligands11 in five-coordinate
dibenzyl titanium(IV), zirconium(IV) and hafnium(IV) complexes.
Only for six-coordinate group 4 metal complexes, which adopt
a distorted octahedral geometry, those bis(phenolate)-donor
and bis(naphtholate)-donor ligands gave rise to a Cs binding
mode.3d,e,f,11 These observations indicate that the binding mode
of this type of LX2 ligand is not simply controlled either by the
˚
Table 2 Close contacts (A) and angles (deg) attesting of Si–H ◊ ◊ ◊ M
agostic interactions in compounds 3–5, 7 and 9
Compound
M ◊ ◊ ◊ H(Si)
M ◊ ◊ ◊ Si
M–Si–N
Si–N–Si
119.3(7)
123.5(1)
130.1(2)
122.1(2)
130.6(2)
3
4
5
7
9
2.818
4.240
3.095
3.912
2.966
4.040
3.023
3.428
3.129
3.278
3.210
3.737
3.213
3.264
3.247
3.443
3.134
3.336
3.370
3.474
102.4(5)
119.2(7)
116.4(1)
119.9(1)
111.4(1)
116.7(2)
112.6(2)
125.3(2)
111.8(1)
117.6(2)
In all five complexes, the naphtholate rings twist in the same
direction from the plane of the linker to give rise to Cs-symmetric
(non crystallographic) structures. The degree of twisting, as
measured by the dihedral angle between the M–O bonds and the
plane of the linker, ranges from 49 to 84◦. In some complexes,
this degree of twisting may differ considerably within the two
naphtholate rings. This difference in the twist angles is especially
marked for the lanthanum complexes (3, 49.2 and 63.3◦; 9, 73.3
and 84.0◦). This might just reflect the more pronounced influence
of crystal packing around a large and therefore less sterically
hindered metal center.18 For a given ligand, i.e., {ONOSiPh3} , the
2-
twist angles slightly increase from scandium (4, 52.4 and 55.0◦) to
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6739–6752 | 6743
©

Fig.
3
ORTEP drawings of the structure of {ONOSitBuMe2}Sc-
Fig.
4
ORTEP drawings of the structure of {OSOSiPh3}Y-
[N(SiHMe2)2](THF) (4) (50% ellipsoid probability;
H atoms are
[N(SiHMe2)2](THF) (7) (50% ellipsoid probability;
H atoms are
˚
omitted for clarity). Selected bond lengths (A) and angles (deg.):
Sc(1)–O(1), 2.0218(15); Sc(1)–O(2), 2.0067(15); Sc(1)–O(3), 2.1852(16);
Sc(1)–N(1), 2.3094(18); Sc(1)–N(61), 2.052(2); O(2)–Sc(1)–O(1),
125.51(6); O(2)–Sc(1)–N(61), 116.21(7); O(1)–Sc(1)–N(61), 118.23(7);
O(2)–Sc(1)–O(3), 87.13(6); O(1)–Sc(1)–O(3), 86.44(6); N(61)–Sc(1)–O(3),
99.33(7); O(2)–Sc(1)–N(1), 82.22(6); O(1)–Sc(1)–N(1), 82.91(6);
N(61)–Sc(1)–N(1), 104.12(7); O(3)–Sc(1)–N(1), 156.54(6).
˚
omitted for clarity). Selected bond lengths (A) and angles (deg.):
S(1)–Sc(1), 2.7758(14); Sc(1)–O(1), 1.997(3); Sc(1)–O(2), 2.008(3);
Sc(1)–O(3), 2.124(3); Sc(1)–N(2), 2.027(4); O(1)–Sc(1)–O(2), 143.37(13);
O(1)–Sc(1)–N(2), 106.27(15); O(2)–Sc(1)–N(2), 105.54(15); O(1)–
Sc(1)–O(3), 93.18(13); O(2)–Sc(1)–O(3), 98.04(13); N(2)–Sc(1)–O(3),
101.18(14); O(1)–Sc(1)–S(1), 76.03(9); O(2)–Sc(1)–S(1), 74.91(9);
N(2)–Sc(1)–S(1), 113.66(12); O(3)–Sc(1)–S(1), 145.12(10).
size of the metal center18 or its oxidation state. The coordination
number of the metal center is another obvious parameter, the
ligand adapting its binding mode upon the space available in the
coordination sphere.
Overall, the M–O and M–Z bond distances in complexes 3–5,
7, and 9 are unexceptional and compare well with those observed
in related group 3 metal complexes supported by phenolate-donor
type ligands7,20 or analogous bis(trifluoromethyl)alkoxide-donor
type ligands.21 Variations in M–O and M–Z bond distances within
this series of complexes essentially reflect the difference in ionic
radius between Sc, Y and La centers.18 Not enough data are
available to allow for a thorough study of the effect of changing the
silyl substituent and/or the donor linker. Yet, comparison between
Upon attempting to grow single crystals of amide compound 3,
small amounts of crystals differently shaped from the main prod-
ucts were isolated. Though the quality of these crystals was always
poor (see Table 1), X-ray diffraction revealed these crystals to be
the corresponding hydroxo complex 10, arising from adventitious
hydrolysis (Scheme 4). As illustrated in Fig. 5, this compound
˚
3 and 9 shows shorter (by ca. 0.03–0.07 A) M–O(THF) and M–
N(SiHMe2) bonds for the bis(naphtholate)thiophene system. This
observation indicates a more electrophilic metal center in 9 than
in 3, possibly due to the weaker interaction of the metal center
with the sulfur compared to the nitrogen donor.3f
Scheme 4
6744 | Dalton Trans., 2010, 39, 6739–6752
This journal is
The Royal Society of Chemistry 2010
©

These twist angle values fall in the middle of the range observed
for the parent complex 3 (49.2 and 63.3◦, vide supra).
Preliminary studies on ring-opening polymerization of rac-Lactide
and rac-b-butyrolactone
Group 3 metals and lanthanide complexes modified by an-
cillary ligands have received much attention in recent years
for the controlled ring-opening polymerization (ROP) of cyclic
esters such as lactide (LA) and, to a much lesser extent,
b-butyrolactone (BBL).6,7,13,17 Activity (molmonomer/molmetal.h), pro-
ductivity (molmonomer/molmetal), degree of control/“livingness”, and
stereoselectivity in the case of chiral monomers, depend crucially
on ancillary ligands that define the sterics and electronics around
the active metal center. Among the many systems investigated so
far, a few of them have shown really valuable performances. This is
notably the case of some group 3 metal complexes supported by tri-
and tetradentate bis(phenolate) ligands which have been shown to
induce significant heterotacticity and syndiotacticity in the ROP of
rac-LA and rac-b-BBL, respectively.7 We were therefore interested
in evaluating the catalytic performances of the new compounds
1–9 that possess a potentially active nucleophilic amido group for
initiation of the ROP process and original ligand platforms.
The ability of the {OZOSiR3}Ln[N(SiHMe2)2](THF) complexes
to promote the ROP of rac-LA was first examined (Scheme 5).23
Representative results are summarized in Table 3. All complexes
proved to be active at room temperature, enabling the conversion
of 100–500 equiv of rac-LA within a few hours at most. The
reactions proceeded significantly faster in toluene than in THF
(compare entries 2/3, 4/9, 8/11, 16/17). This observation can
probably be accounted for by the competing role of THF vs rac-LA
for coordination onto the metal center. All the PLAs formed under
those conditions had unimodal, although broadened molecular
distributions (Mw/Mn = 1.32–1.90) and experimental molecular
weights (determined by SEC and corrected by a factor of 0.58,
due to the use of PS standards)24,25 quite close to the values
calculated on the assumption of the growth of one macro-
molecular chain per metal.26 For a given experiment, monitoring
showed that the molecular weight increases with conversion in
a perfectly linear relationship (entries 4–5, 6–8, 12–13) (Fig. 6).
Also, increasing the rac-LA loading from 100 to 500 equiv
led to proportionally higher molecular weights PLAs (at high
conversion) (see, e.g., entries 9/11). These data illustrate the overall
good degree of control over the polymerization provided by the
{OZOSiR3}Ln[N(SiHMe2)2](THF) systems.
Fig.
5
ORTEP drawing of the structure of [{ONOSiPh3}La-
(THF)(H2O)(m-OH)2La(H2O){ONOSiPh3}]
(10)
(50%
ellipsoid
probability;
H atoms and SiPh3 groups are omitted for clarity).
˚
Selected bond lengths (A) and angles (deg.): La(1)–O(1), 2.327(4);
La(1)–O(2), 2.368(4); La(1)–O(51), 2.489(4); La(1)–O(52), 2.450(5);
La(1)–O(40), 2.663(5); La(1)–O(3), 2.671(4); La(1)–N(1), 2.718(4);
La(2)–O(2B), 2.352(3); La(2)–O(1B), 2.362(4); La(2)–O(51), 2.383(4);
La(2)–O(52), 2.430(4); La(2)–O(40B), 2.699(6); La(2)–N(1B), 2.706(5);
O(1)–La(1)–O(2), 108.46(14); O(1)–La(1)–O(52), 143.63(13); O(2)–La(1)–
O(52), 94.92(14); O(1)–La(1)–O(51), 81.01(14); O(2)–La(1)–O(51),
152.83(14); O(52)–La(1)–O(51), 66.51(15); O(1)–La(1)–O(40), 99.18(17);
O(2)–La(1)–O(40), 127.97(18); O(52)–La(1)–O(40), 86.97(19); O(51)–
La(1)–O(40), 72.95(19); O(1)–La(1)–O(3), 73.34(15); O(2)–La(1)–O(3),
70.67(14);
O(52)–La(1)–O(3),
142.28(15);
O(51)–La(1)–O(3),
136.05(15); O(40)–La(1)–O(3), 76.6(2); O(1)–La(1)–N(1), 72.85(14);
O(2)–La(1)–N(1), 70.31(14); O(52)–La(1)–N(1), 90.20(15); O(51)–La(1)–
N(1), 89.32(14); O(40)–La(1)–N(1), 161.67(19); O(3)–La(1)–N(1),
115.30(14).
adopts in the solid state a dimeric structure with m-bridging
hydroxy groups. In addition to the latter hydroxy groups, both
La centers are coordinated by the tridentate {OZOSiPh3
}
ligand
2-
and one water molecule. One La center is thus six-coordinate,
whereas the second one is seven-coordinate due to the presence
of an additional coordinated THF molecule. The La–O bond
˚
distances (2.383(4)–2.489(4) A) and O–La–O angles (66.5(1)–
68.5(1)◦) involving the bridging hydroxy groups (O(51,52)) are
comparable to those observed in the other rare examples reported
Interestingly, we observed that the ROP of 500 equiv of rac-LA
could be performed using 5 equiv of iPrOH (vs 3), leading to a PLA
with accordingly decreased molecular weight (compare entries
10–13). This experiment corresponds to a so-called “immortal”
polymerization in which the added alcohol acts as a transfer
agent.7e,27 Dormant hydroxy-end-capped polymer chains exchange
˚
of dimeric hydroxy-bridged La complexes (2.385–2.421 A and
63.05–67.92◦, respectively).22 Both {OZOSiPh3
}
ligand units are
2-
coordinated with the naphtholate rings in a Cs fashion with quite
similar twist angles [i.e., the dihedral angle between the M–O
bonds and the plane of the pyridine] of 55.0, 55.1, 55.8 and 56.3◦.
Scheme 5
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6739–6752 | 6745
©

Table 3 Ring-opening polymerization of racemic lactide promoted by {OZOSiR3}Ln[N(SiHMe2)2](THF) complexesa
Time b/min
Conv. c(%)
Mn,calc d/103 g mol-1
Mn, exp e/103 g mol-1
Mw/Mn
Prf
e
Entry
Comp.
[LA]/[Ln]
Solvent
1
2
3
4
5
6
7
8
1
100
100
100
100
100
500
500
500
100
500
500
500
500
100
100
100
100
100
100
100
100
100
THF
THF
toluene
THF
THF
THF
THF
THF
toluene
toluene
toluene
toluene
toluene
THF
THF
THF
toluene
THF
THF
700
60
3
40
60
30
60
360
2
30
100
71
100
87
96
25
42
74
100
41
88
14.4
10.2
14.4
12.5
13.8
18.0
30.3
53.3
14.4
29.5
63.4
12.3g
13.5g
14.4
14.4
11.7
14.4
14.4
14.4
14.4
14.4
14.4
16.2
8.9
1.55
1.32
1.52
1.43
1.52
1.39
1.55
1.90
1.68
1.51
1.74
1.36
1.52
1.61
1.52
1.43
1.71
1.42
1.82
1.47
1.51
1.65
0.65
0.90
0.50
nd
0.88
nd
0.87
nd
0.68
nd
2
2
10.0
12.7
13.1
23.1
38.2
49.8
17.5
34.2
51.1
10.4g
12.7g
17.1
17.3
14.1
16.7
13.1
14.0
11.2
11.6
12.2
3
3
3
3
3
9
3
10
11
12g
13g
14
15
16
17
18
19
20
21
22
3
3
60
30
60
nd
nd
nd
3
85
94
3
4
700
700
360
30
700
700
700
700
700
100
100
81
100
100
100
100
100
100
0.93
0.84
0.50
0.50
0.66
0.68
0.50
0.75
0.50
5
6h
6h
7
8h
8h
9
toluene
THF
toluene
9
a General conditions: [rac-LA] = 1.0 mol L-1, T = 20 ◦C. b Reactions times were not necessarily optimized. c Conversion of lactide as determined by 1H
NMR on the crude reaction mixture. d Mn values calculated considering one polymer chain per metal-center from the relation: Mn,calc = conv ¥ [LA]/[Ln] ¥
144. e Experimental Mn and Mw/Mn values determined by GPC in THF vs. PS standards and corrected with 0.58 factor. f Pr is the probability of racemic
linkage, as determined by 1H NMR homodecoupling experiments. g Polymerizations carried out in the presence of 5 equiv of iPrOH vs. Ln. h The complex
was preliminary prepared from the in situ reaction of the pro-ligand and tris(amido) precursor.
Most interestingly, homo-decoupled 1H NMR spectroscopy
showed that some of the PLAs formed with these systems had a
significantly heterotactic-enriched microstructure. The probability
of racemic linkage (Pr) ranged from 0.50 (atactic) up to 0.93. As we
observed with tetradentate bis(phenolato)-lanthanide systems,7b-e
the stereoselectivity strongly depended on the nature of the
solvent: almost all the PLAs produced in toluene had an atactic
microstructure whereas those obtained in THF were almost
always heterotactic. The degree of heterotacticity was also quite
different among the series of compounds 1–9, and appears to
be affected by the nature of the metal center (Sc, Y, La), the
central linker in the ligand framework (pyridine, thiophene),
and the ortho-silyl substituent (SiPh3, SiMe2tBu) as well. The
bis(naphtholate)-thiophene framework is less efficient than that
based on 2,6-pyridine; one can speculate that the [anticipated]
Fig. 6 Relationship between Mn of PLAs and conversion of rac-LA
(Table 3, entries 4–5, 6–8).
weaker interaction of the soft S (as compared to the harder N)
atom induces a larger flexibility of the ligand and, as a result, a less
sterically rigid coordination sphere around the active metal center.
Within the series of {ONOSiMe2tBu}-Ln complexes 4–6, the tacticity
decreases with increasing ionic radius18 (metal, Pr: Sc, 0.93; Y,
0.84; La, 0.50). The same trend was observed with tetradentate
bis(phenolato)-lanthanide systems,7b-e and was proposed to reflect
a better contribution of sterically constrained metal centers
on the chain-end stereocontrol.28 The aforementioned “ionic
radius/heterotacticity” trend is, however, not observed within the
series of {ONOSiPh3}-Ln complexes 1–3, since the smaller Sc led
to a modest Pr value of 0.65 whereas the small Y and large La
afforded about the same high heterotacticity (Pr = 0.88–0.90).
The less reactive rac-BBL could also be polymerized to
afford syndiotactic-enriched poly(3-hydroxybutyrate)s (PHBs)
(Scheme 6). Representative results are summarized in Table 4.
with the active alkoxy-type polymer chain coordinated onto
Ln, enabling the growth of more (i.e., 5) than one polymer
chain per metal center. The polydispersity, at least similar to
that observed using no iPrOH, and the good match between
experimental and calculated molecular weights establish that this
transfer reaction (exchange between dormant and active polymer
chains) proceeds significantly faster than propagation.7e,27 Analysis
by 1H NMR spectroscopy in CDCl3 of the low molecular weight
PLAs obtained under such conditions showed clearly the existence
of HOCH(CH3)CO- (broadened quartet at d 4.35 ppm) and
C(O)OCH(CH3)2 (doublet at d 1.25 ppm for the methyl groups;
the resonance for the OCHMe2 at d ca. 5.1 ppm mostly overlaps
with those of PLA) end-groups, thus supporting the transfer
process.7e,25
6746 | Dalton Trans., 2010, 39, 6739–6752
This journal is
The Royal Society of Chemistry 2010
©

Table 4 Ring-opening polymerization of racemic b-butyrolactone promoted by {OZOSiR3}Ln[N(SiHMe2)2](THF) complexesa
Time b/min
Conv. c(%)
Mn,calc d/103 g mol-1
Mn, exp e/103 g mol-1
Mw/Mn
Prf
e
Entry
Comp.
[BBL]/[Ln]
Solvent
1g
1
1
2
2
2
2
2
3
3
3
7
8
8
9
9
100
100
100
300
100
300
100
100
100
100
100
100
100
300
300
toluene
toluene
toluene
THF
1000
1000
300
700
300
700
30
100
2
15
1000
1000
1000
1000
360
30
62
100
30
73
70
100
80
24
62
40
2.2
5.3
8.6
7.7
6.3
18.1
8.6
6.9
2.1
5.3
3.4
5.2
7.7
25.8
23.7
5.9
7.1
1.69
1.32
1.12
1.41
1.17
1.29
1.32
1.25
1.12
1.38
1.45
1.35
1.22
1.35
1.38
0.48
0.48
0.76
0.53
0.57
0.78
0.87
0.53
0.86
nd
0.60
0.52
0.67
0.46
0.55
2g,h
3
40.7
18.4
31.3
20.3
17.8
11.1
2.6
7.0
3.2
14.2
11.0
23.5
18.6
4
5
THF
6
toluene
toluene
THF
toluene
toluene
toluene
THF
toluene
THF
toluene
7g,h
8
9
10
11g,h
12
13
14
15
20
30
100
92
a General conditions unless otherwise stated: [rac-BBL] = 3.0 mol L-1, T = 20 ◦C. b Reactions times were not necessarily optimized. c Conversion of
lactide as determined by 1H NMR on the crude reaction mixture. d Mn values calculated considering one polymer chain per metal-center from the relation:
Mn,calc = conv ¥ [BBL]/[Ln] ¥ 74. e Experimental Mn (uncorrected) and Mw/Mn values determined by GPC in THF vs. PS standards. f Pr is the probability
of racemic linkage, as determined by 13C NMR. g Reaction carried out at 50 ◦C. h Polymerizations carried out in the presence of 1 equiv of iPrOH vs. Ln.
significantly syndiotactic-enriched microstructure, with Pr values7f
ranging from 0.48 (i.e., atactic) up to 0.87. As exemplified for
complexes 2, 3 and 9, the use of toluene as solvent led always to
higher stereoselectivities than with THF; i.e., the opposite trend
than that observed for the ROP of rac-LA (vide supra) but a
trend consistent with our previous observations in the ROP of
Scheme 6
rac-BBL using tetradentate bis(phenolate)-Ln systems.7d,f In line
with the results for the ROP of rac-LA, the bis(naphtholate)-
2-
thiophene framework {OSOSiPh3
}
led to lower stereoselectivities
At first, it must be noted that compounds 4–6 did not show
(7-9, Pr = 0.55–0.67) than that based on 2,6-pyridine {ONOSiPh3} ,
especially the Y and La complexes (2, 3, Pr = 076–0.87 and 0.86,
respectively). The latter levels of syndiotacticity are just below
those we achieved with tetradentate bis(phenolate)-Ln systems
(the most stereoselective systems reported to date; Pr = up to
0.94),7d,f and range among the highest reported thus far for the
ROP of rac-BBL.
2-
any activity under the conditions investigated (THF, toluene,
20–50 ◦C). Apparently, the inertness of this series of complexes
seems to arise from the presence of SiMe2tBu ortho-substituents,
although the exact reason still remains obscure. The {OZOSiPh3}-
Ln compounds 1–3 and 7–8 are active in the ROP of rac-BBL
at room temperature, but an increase of the reaction temperature
to 50 ◦C did prove useful in some cases to reduce the reaction
time. This was particularly the case for scandium complexes 1
and 7, which are quite sluggish (entries 1, 2, 11). In those cases,
the addition of 1 equiv of iPrOH (vs Ln), to generate in situ
the corresponding {OZOSiPh3}Sc-isopropoxide species,7b-d enabled
faster reactions, suggesting a relatively slow initiation with amido
precursors (compare entries 1/2). Lanthanum complexes 3 and 9
showed the highest reaction rates. Similarly to the ROP of rac-
LA, the ROP of rac-BBL with these systems proceeded faster in
toluene than in THF.
Most of the resulting PHBs had experimental molecular weights
(determined by SEC, uncorrected) in the range 4.1–28.1 103 g
mol-1, which are in good agreement with the theoretical values
[calculated assuming the growth of one polymer chain per metal
center].29 Molecular weights larger than those calculated were
observed in some cases (entries 3, 5, 7, 12), apparently when
low [rac-BBL]/[Ln] ratios were used; exact reasons for these
discrepancies remain unclear at this time but one may speculate
that the initiation efficiency is low under such conditions. The
molecular weight distributions were unimodal and somewhat
narrower (Mw/Mn = 1.12–1.69) than those observed for PLAs,
indicative of the controlled character of the polymerizations.
Detailed 13C NMR analysis7f revealed that some PHBs had a
Conclusions
In conclusion, we have readily prepared in high yields a series of
scandium, yttrium and lanthanum amido complexes supported by
tridentate bis(ortho-silyl-substituted naphtholate)-donor ligands,
via the reaction of the corresponding pro-ligand {OZOSiR3}H2 and
Ln[N(SiHMe2)2]3(THF)n precursor. Those complexes all adopt a
Cs-symmetric ground state structure in the solid state, as observed
for compounds 1–3 and 7–9, and in toluene solution as well.
Compounds 1–9 are single-site initiators for the ROP of rac-LA
at 20 ◦C, affording PLAs with relatively narrow polydispersities
and molecular weights in good agreement with calculated values.
Atactic polymers are formed in toluene but, when carried out in
THF, the polymerizations afforded heterotactic-enriched PLAs
(Pr up to 0.93). Immortal polymerization, with good control
over the number and molecular weight of polymer chains, can
be performed upon addition of isopropanol as a transfer agent.
Compounds 1–3 and 7–9, having o-SiPh3 substituents on the
naphtholate rings, are also active for the ROP of rac-BBL at
20–50 ◦C, to form syndiotactic-enriched PHBs (Pr up to 0.87)
when using toluene as the solvent, whereas essentially atactic
This journal is The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6739–6752 | 6747
©

polymers were obtained in THF. The degree of stereocontrol
in those polymerizations of rac-LA and rac-BBL is significantly
affected by the nature (and ionic radius) of the metal center
(Sc, Y, La), the central linker in the ligand framework (pyridine,
thiophene), and the ortho-silyl substituent (SiPh3, SiMe2tBu).
However, no clear trends for predicting a property have been
identified thus far. The inactivity of complexes 4–6 in the ROP
of rac-BBL, at least under the conditions investigated, strikes at
the real influence of the ortho-silyl substituents installed on the
naphtholate rings. Those first results suggest that improvements
in terms of activity (and productivity as well, via immortal
ROP), and stereoselectivity are accessible via further tuning of
the substituents installed on the naphtholate rings.
molecular masses (Mn) and polydispersity index (Mw/Mn) of the
polymers were calculated with reference to a universal calibration
vs. polystyrene standards. Mn values of PLAs were corrected with
a factor of 0.58 to account for the difference in hydrodynamic
volumes of polystyrenes.24 Mn values of PHBs are uncorrected.
The microstructure of PLAs was determined by homodecoupling
1H NMR spectroscopy at 20 ◦C in CDCl3 with a Bruker AM-
500 spectrometer. The microstructure of PHBs was determined by
analysis of the carbonyl region of 13C NMR spectra recorded at
25 ◦C in CDCl3 on a Bruker Avance AM-500 spectrometer.3d,f
{ONOSiPh3}Sc[N(SiHMe2)2](THF) (1)
A Schlenk flask was charged with {ONOSiPh3}H2 (0.051 g,
0.060 mmol) and Sc[N(SiHMe2)2]3(THF) (0.030 g, 0.060 mmol),
and benzene (ca. 5 mL) was vacuum transferred in at -78 ◦C. The
reaction mixture was gently warmed to room temperature and
stirred overnight. Filtration of the clear solution and evaporation
of volatiles in vacuum left 1 as a pale yellow microcrystalline
powder (0.057 g, 84%). 1H NMR (500 MHz, benzene-d6, 298 K):
Experimental
General considerations
All manipulations were carried out under purified argon using
standard Schlenk techniques or in a glove box. Toluene and ben-
zene were freshly distilled under argon from Na/benzophenone
and were condensed under vacuum prior to use. Deuter-
ated solvents (>99.5% D, Eurisotop) were freshly distilled or
vacuum-transferred from Na/K alloy under argon and degassed
prior to use. Pro-ligands {OZOR}H2 (Z = N; R = SiPh3,
SiMe2tBu; Z = S, R = SiPh3),11 Y(N(SiHMe2)2)3·(THF),14a
La(N(SiHMe2)2)3·(THF)2,14a and Sc(N(SiHMe2)2)3·(THF)14a were
synthesized according to the reported procedures. rac-LA
(Aldrich) was recrystallized from dry methanol and toluene, and
sublimed under vacuum at 50 ◦C. rac-BBL (TCI) was freshly
distilled from CaH2 under argon and degassed thoroughly by
freeze–vacuum–thaw cycles.
3
d 8.20 (d, J = 8.5, 2H, Haro), 8.16 (s, 2H4, Haro), 7.93–8.03
(m, 12H, Haro), 7.46 (m, 6H, Haro), 7.22-7.31 (m, 18H, Haro),
3
3
7.13 (d, J = 7.0, 2H, Haro), 6.91 (t, J = 7.0, 1H, H3p), 4.45
(m, 2H, SiHMe), 2.96 (m, 4H, a-CH2, THF), 0.69 (m, 4H, b-
CH2, THF), -0.12 (d, J = 2.1, 12H, SiHMe2). 13C{ H} NMR
(125 MHz, benzene-d6, 298 K): d 164.0, 155.7, 145.0, 137.0, 136.7,
136.4, 136.0, 129.3, 128.7, 128.0, 127.3, 127.6, 127.1, 126.4, 124.0,
122.2, 117.6 (Caro), 70.1 (a-CH2, THF), 24.2 (b-CH2, THF), 2.5
(SiHMe2). Anal. calcd. for C69H65N2O3ScSi4: C, 73.50; H, 5.81; N,
2.48. Found: C, 73.1; H, 5.4; N, 2.4.
3
1
{ONOSiPh3}Y[N(SiHMe2)2](THF) (2)
NMR spectra of complexes were recorded in TeflonTM-valved
NMR tubes on◦Bruker AC-200, AC-300 and AM-500 spec-
This compound was prepared following the same procedure as
that described above for 1, starting from {ONOSiPh3}H2 (0.079 g,
0.090 mmol) and Y[N(SiHMe2)2]3(THF) (0.050 g, 0.090 mmol)
in benzene (5 mL). Compound 2 was obtained as a pale yellow
microcrystalline powder (0.089 g, 84%). 1H NMR (500 MHz,
1
trometers at 20 C unless otherwise stated. H and 13C NMR
chemical shifts (see Scheme 7) were determined using residual
solvent resonances and are reported vs. SiMe4. Assignment of
signals was made from 2D 1H-1H COSY and 1H-13C HMQC
and HMBC NMR experiments (see Scheme 5 for numbering of
ligand hydrogens). 19F chemical shifts were determined by external
reference to an aqueous solution of NaBF4. All coupling constants
are given in Hertz. Elemental analyses (C, H, N) were performed
using a Flash EA1112 CHNS Thermo Electron apparatus and
are the average of two independent determinations. Molecular
weights of PLAs and PHBs were determined by size exclusion
chromatography (SEC) at room temperature in THF on a Polymer
Laboratories PL-GPC 50 plus apparatus (PLgel 5 mm MIXED-
C 300 ¥ 7.5 mm columns, 1.0 mL min-1 flow rate, RI and
Dual angle LS (PL-LS 45/90) detectors). The number average
3
benzene-d6, 298 K): d 8.19 (s, 2H, H4), 8.04 (d, J = 7.1, 2H,
3
H5), 7.96-8.00 (m, 12H, Haro), 7.47 (d, J = 7.1, 2H, H8), 7.40
3
3
(t, J = 7.1, 2H, H7), 7.36 (d, J = 7.7, 2H, H2p), 7.23-7.27 (m,
18H, Haro), 7.12 (d, 3J = 7.1, 2H, H6), 6.94 (t, 3J = 7.1, 1H, H3p),
4.31 (sept, 3J = 2.8, 2H, SiHMe), 2.81 (br m, 4H, a-CH2, THF),
0.86 (br m, 4H, b-CH2, THF), -0.11 (d, 3J = 3.0, 12H, SiHMe2).
1
13C{ H} NMR (125 MHz, benzene-d6, 298 K): d 163.2, 156.7,
144.8, 136.9, 136.7, 136.6, 129.2, 128.4, 128.3, 128.1, 128.0, 127.5,
127.4, 127.3, 123.7, 122.0, 117.4 (Caro), 69.1 (a-CH2, THF), 24.7
(b-CH2, THF), 2.5 (SiHMe2). Anal. calcd. for C69H65N2O3Si4Y: C,
70.74; H, 5.59; N, 2.39. Found: C, 70.2; H, 5.1; N, 2.4.
{ONOSiPh3}La[N(SiHMe2)2](THF) (3)
This compound was prepared following the same procedure as
that described above for 1, starting from {ONOSiPh3}H2 (0.100 g,
0.110 mmol) and La[N(SiHMe2)2]3(THF)2 (0.077 g, 0.11 mmol)
in benzene (5 mL). Compound 3 was obtained as a pale yellow
microcrystalline powder (0.130 g, 91%). Crystals of 3 suitable for
X-ray diffraction analysis were prepared by prolonged crystalliza-
tion from a concentrated benzene solution at room temperature.
1H NMR (500 MHz, benzene-d6, 298 K): d 8.24 (s, 2H, H4),
8.03–7.94 (m, 14H, Haro), 7.50 (d, 3J = 7.7, 2H, Haro), 7.39 (m,
Scheme
ligands.
7
Numbering scheme of 1,1-bis(naphtholate)-1¢,1¢-pyridine
6748 | Dalton Trans., 2010, 39, 6739–6752
This journal is
The Royal Society of Chemistry 2010
©

2H, Haro), 7.31 (d, 3J = 7.7, 2H, Haro), 7.29–7.19 (m, 18H, Haro),
{ONOSitBuMe2}Y[N(SiHMe2)2](THF) (5)
3
3
7.17 (m, 2H, Haro), 6.98 (t, J = 7.7, 1H, H3p), 4.29 (sept, J =
This compound was prepared following the same procedure as that
described above for 1, starting from {ONOSitBuMe2}H2 (0.063 g,
0.110 mmol) and Y[N(SiHMe2)2]3(THF) (0.060 g, 0.110 mmol)
in benzene (5 mL). Compound 5 was obtained as a pale yellow
microcrystalline material (0.083 g, 85%). Crystals of 5 suitable for
X-ray diffraction analysis were prepared by prolonged crystalliza-
tion from a concentrated benzene solution at room temperature.
1H NMR (500 MHz, benzene-d6, 298 K): d 8.34 (s, 2H, H4), 7.86
(d, 3J = 7.5, 2H, H5), 7.79 (br d, 3J = 8.0, 2H, H8), 7.36 (t, 3J =
8.0, 2H, H7), 7.28 (m, 2H, H6), 7.19 (br d, 3J = 7.7, 2H, H2p), 6.97
(t, 3J = 7.7, 1H, H3p), 4.76 (br m, 2H, SiHMe), 4.01 (br m, 4H, a-
CH2, THF), 1.31 (br m, 4H, b-CH2, THF + 18H, SiMe2tBu), 0.78
(br s, 6H, SiMeMetBu), 0.71 (br s, 6H, SiMeMetBu), 0.03 (br s,
2.8, 2H, SiHMe), 3.07 (br m, 4H, a-CH2, THF), 1.07 (br m, 4H,
1
b-CH2, THF), -0.11 (d, 3J = 2.8, 12H, SiHMe2). 13C{ H} NMR
(125 MHz, benzene-d6, 298 K): d 162.1, 156.6, 144.3, 138.5, 137.0,
136.9, 132.6, 129.3, 129.2, 128.3, 128.0, 127.9, 127.8, 127.3, 123.8,
121.9, 118.3 (Caro), 68.3 (a-CH2, THF), 24.9 (b-CH2, THF), -2.5
(SiHMe2). Anal. calcd. for C69H65LaN2O3Si4: C, 67.85; H, 5.36; N,
2.29. Found: C, 67.3; H, 5.4; N, 2.3.
{ONOSitBuMe2}Sc[N(SiHMe2)2](THF) (4)
This compound was prepared following the same procedure
as that described above for 1, starting from {ONOSitBuMe2}H2
(0.046 g, 0.090 mmol) and Sc[N(SiHMe2)2]3(THF) (0.040 g,
0.090 mmol) in benzene (5 mL). Compound 4 was obtained as
a pale yellow microcrystalline material (0.067 g, 72%). Crystals
of 4 suitable for X-ray diffraction analysis were prepared by
prolonged crystallization from a concentrated benzene solution
at room temperature. 1H NMR (500 MHz, benzene-d6, 298 K): d
8.36 (s, 2H4), 8.01 (br d, 3J = 8.2, 2H, H5), 7.86 (br d, 3J = 7.9, 2H,
H8), 7.39 (br t, 3J = 7.9, 2H, H7), 7.29 (m, 4H, H6+2p), 6.92 (br t, 3J =
7.7, 1H, H3p), 4.63 (br m, 2H, SiHMe), 4.49 (br m, 4H, a-CH2,
THF), 1.73 (br m, 4H, b-CH2, THF), 1.27 (s, 18H, SiMe2tBu),
0.71 (s, 6H, SiMeMetBu), 0.68 (s, 6H, SiMeMetBu), -0.20 (br s,
1
12H, SiHMe2). 13C{ H} NMR (125 MHz, benzene-d6, 298 K): d
163.6, 140.4, 136.9, 136.0, 130.5, 128.8, 128.3, 128.1, 126.9, 126.7,
123.3, 121.7 (Caro), 70.9 (a-CH2, THF), 27.7 (C(CH3)3), 24.9 (b-
CH2, THF), 17.9 (C(CH3)3), 3.0 (SiMe2tBu), 2.8 (SiHMe2).. VT
1H NMR: 1H NMR (500 MHz, toluene-d8, 338 K): d 8.16 (s, 2H4),
3
3
7.70 (dd, J = 7.9 and 1.5, 2H5), 7.65 (br dd, J = 8.3 and 1.1,
2H8), 7.22 (ddd, 3,3,4J = 8.3, 6.8 and 1.5, 2H7), 7.19 (dd, 3J = 7.9
and 0.6, 2H2p), 7.12 (ddd, 3,3,4J = 7.9, 6.8 and 1.1, 2H6), 7.11 (t,
3J = 7.9, 1H3p), 4.57 (br m, 2H, SiHMe), 3.96 (br m, 4H, a-CH2,
THF), 1.44 (br m, 4H, b-CH2, THF), 1.16 (s, 18H, SiMe2tBu),
1
0.60 (s, 12H, SiMe2tBu),–0.15 (br d, J = 2.1, 12H, SiHMe2). H
1
12H, SiHMe2). 13C{ H} NMR (125 MHz, benzene-d6, 298 K): d
NMR (500 MHz, toluene-d8, 313 K): d 8.18 (s, 2H4), 7.71 (dd,
3J = 7.9, 1.3, 2H5), 7.63 (br dd, 3J = 8.1 and 1.1, 2H8), 7.23 (ddd,
3,3,4J = 8.1, 6.6, and 1.3, 2H7), 7.14 (ddd,3,3,4J = 7.9, 6.6, and 1.1,
2H6), 7.13 (m, 2H2p), 7.05 (br m, 1H3p), 4.64 (br m, 2H, SiHMe),
3.92 (br m, 4H, a-CH2, THF), 1.32 (br m, 4H, b-CH2, THF), 1.19
(s, 18H, SiMe2tBu), 0.67 (br s, 6H, SiMeMetBu), 0.61 (br s, 6H,
163.7, 140.6, 136.6, 135.4, 128.7, 128.3, 128.1, 128.0, 127.9, 126.97,
123.8, 122.2 (Caro), 71.3 (a-CH2, THF), 27.6 (C(CH3)3), 25.2 (b-
CH2, THF), 17.9 (C(CH3)3), 2.8 (SiMe2tBu), 2.2 (SiHMe2). VT
1
1H NMR: H NMR (500 MHz, toluene-d8, 338 K): d 8.17 (s,
2H4), 7.87 (br d, 3J = 8.3, 2H5), 7.70 (br d, 3J = 7.9, 2H8), 7.26 (m,
4H6+2p), 7.16 (br t, 3J = 7.9, 2H7), 7.05 (br m, 1H3p), 4.51 (br m, 2H,
SiHMe), 4.34 (br m, 4H, a-CH2, THF), 1.72 (br m, 4H, b-CH2,
THF), 1.13 (s, 18H, SitBuMe2), 0.58 (s, 12H, SitBuMe2),–0.31 (s,
12H, SiHMe2). 1H NMR (500 MHz, toluene-d8, 313 K): d 8.20 (s,
1
SiMeMetBu),–0.05 (br s, 12H, SiHMe2). H NMR (500 MHz,
toluene-d8, 296 K): d 8.19 (s, 2H4), 7.72 (dd, J = 7.7 and 1.5,
3
2H5), 7.64 (br s, 2H8), 7.24 (ddd, 3,3,4J = 8.3, 6.8, and 1.5, 2H7),
7.15 (ddd, 3,3,4J = 1.1, 6.8, and 7.7, 2H6), 7.11 (br m, 2H2p), 7.06 (br
m, 1H3p), 4.66 (br m, 2H, SiHMe), 3.91 (br m, 4H, a-CH2, THF),
1.31 (br m, 4H, b-CH2, THF), 1.20 (s, 18H, SiMe2tBu), 0.70 (br s,
6H, SiMeMetBu), 0.62 (br s, 6H, SiMeMetBu),–0.01 (br s, 12H,
SiHMe2). 1H NMR (500 MHz, toluene-d8, 263 K): d 8.23 (s, 2H4),
7.73 (br d, 3J = 7.9, 2H5), 7.73 (br m, 2H8), 7.26 (br m, 2H7), 7.16
(br m, 4H6+2p), 6.88 (br tr, 3J = 7.9, 1H3p), 4.51 (br m, 2H, SiHMe),
3.69 (br m, 4H, a-CH2, THF), 1.33 (br m, 4H, b-CH2, THF), 1.24
(s, 18H, SiMe2tBu), 0.63 (br s, 12H, SiMe2tBu), -0.27 (br s, 12H,
SiHMe2). 1H NMR (500 MHz, toluene-d8, 233 K): d 8.27 (s, 2H4),
7.77 (m, 3H), 7.53 (m, 1H), 7.27 (m, 3H), 7.11 (br s, 2H), 6.78
(m, 2H), 4.53 (br m, 2H, SiHMe), 4.15 (br m, 2H, a-CH2, THF),
3.66 (br m, 2H, a-CH2, THF), 1.35 (br m, 4H, b-CH2, THF),
1.25 (s, 18H, SiMe2tBu), 0.64 (br s, 6H, SiMeMetBu), 0.62 (br s,
6H, SiMeMetBu), -0.24 (br s, 12H, SiHMe2). Anal. calcd. for
C45H65N2O3Si4Y: C, 61.19; H, 7.42; N, 3.17. Found: C, 60.8; H,
8.1; N, 3.0.
3
3
2H4), 7.88 (br d, J = 8.2, 2H5), 7.71 (br d, J = 7.9, 2H8), 7.27
(br t, J = 7.9, 2H7), 7.22 (br d, J = 7.7, 2H2p), 7.17 (br t, J =
8.2, 2H6), 6.98 (br t, 3J = 7.7, 1H3p), 4.50 (br m, 2H, SiHMe), 4.33
(br m, 4H, a-CH2, THF), 1.71 (br m, 4H, b-CH2, THF), 1.51 (s,
18H, SitBuMe2), 0.59 (s, 12H, SitBuMe2),–0.32 (s, 12H, SiHMe2).
1H NMR (500 MHz, toluene-d8, 298 K): d 8.21 (s, 2H4), 7.89 (br
d, 3J = 8.3, 2H5), 7.72 (br d, 3J = 7.7, 2H8), 7.27 (br t, 3J = 7.7,
2H7), 7.19 (m, 4H6+2p), 6.94 (br t, 3J = 7.9, 1H3p), 4.49 (br s, 2H,
SiHMe), 4.40 (m, 4H, a-CH2, THF), 1.75 (m, 4H, b-CH2, THF),
1.16 (s, 18H, SitBuMe2), 0.16 (br s, 12H, SitBuMe2),–0.30 (br s,
12H, SiHMe2). 1H NMR (500 MHz, toluene-d8, 273 K): d 8.24 (s,
3
3
3
3
3
2H4), 7.90 (br d, J = 8.3, 2H5), 7.73 (br d, J = 7.7, 2H8), 7.29
(br t, J = 7.7, 2H7), 7.20 (br t, J = 8.3, 2H6), 7.17 (m, 2H2p),
6.86 (br t, 3J = 7.7, 1H3p), 4.48 (br m, 2H, SiHMe + 4H, a-CH2,
THF), 1.78 (br m, 4H, b-CH2, THF), 1.19 (s, 18H, SitBuMe2),
0.63 (s, 6H, SitBuMeMe), 0.59 (s, 6H, Si tBuMe(Me), -0.32 (s,
12H, SiHMe2). 1H NMR (500 MHz, toluene-d8, 223 K): d 8.28 (s,
2H4), 7.93 (d, 3J = 8.6, 2H5), 7.75 (d, 3J = 8.6, 2H8), 7.80 (m, 2H6),
7.22 (m, 2H7), 7.11 (m, 2H2p), 6.74 (t, 3J = 7.8, 1H3p), 4.52 (sept,
3J = 3.0, 2H, SiHMe), 4.45 (m, 4H, a-CH2, THF), 1.65 (m, 4H,
b-CH2, THF), 1.22 (s, 18H, SitBuMe2), 0.65 (s, 6H, SitBuMeMe),
0.62 (s, 6H, SitBuMeMe), -0.30 (d, J = 3.0, 12H, SiHMe2). Anal.
calcd. for C45H65N2O3ScSi4: C, 64.40; H, 7.81; N, 3.34. Found: C,
63.8; H, 8.2; N, 3.4.
3
3
NMR-scale generation of {ONOSitBuMe2}La[N(SiHMe2)2](THF)
(6)
Complex 6 was generated in situ from the 1 : 1 reaction of
{ONOSitBuMe2}H2 with La[N(SiHMe2)2]3(THF)2. The 1H NMR
spectrum of the reaction mixture was quite complicated and could
not be unambiguously assigned.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6739–6752 | 6749
©

{OSOSiPh3}Sc[N(SiHMe2)2](THF) (7)
Crystal structure determination of complexes 3–5, 7, 9 and 10
Suitable crystals for X-ray diffraction analysis of 3–5, 7, 9 and
10 were obtained by crystallization of purified products from
concentrated benzene solutions (see the Experimental section and
body text). Diffraction data were collected at 100 K using a Bruker
APEX CCD diffractometer with graphite-monochromated Mo-
This compound was prepared following the same procedure as
that described above for 1, starting from {OSOSiPh3}H2 (0.068 g,
0.090 mmol) and Sc[N(SiHMe2)2]3(THF) (0.040 g, 0.090 mmol)
in benzene (5 mL). Compound 7 was obtained as a pale yellow
microcrystalline powder (0.076 g, 75%). Crystals of 7 suitable for
X-ray diffraction analysis were prepared by prolonged crystalliza-
tion from a concentrated benzene solution at room temperature.
1H NMR (500 MHz, benzene-d6, 298 K): d 8.35 (d, 3J = 8.4, 2H,
˚
Ka radiation (l = 0.71073 A). A combination of w and f
scans was carried out to obtain at least a unique data set. The
crystal structures were solved by direct methods using the SIR97
program,30 remaining atoms were located from difference Fourier
synthesis followed by full-matrix least-squares refinement based
on F2 (programs SHELXS-97 and SHELXL-97)31 with the aid of
the WINGX program.32 Many hydrogen atoms could be found
from the Fourier difference analysis. Carbon- and oxygen-bound
hydrogen atoms were placed at calculated positions and forced to
ride on the attached atom. The hydrogen atom contributions were
calculated but not refined. All non-hydrogen atoms were refined
with anisotropic displacement parameters. The locations of the
largest peaks in the final difference Fourier map calculation as
well as the magnitude of the residual electron densities were of no
chemical significance. In 4 and 5, one tBu group and one Si(2)-
Me group were found to be disordered and accordingly modelled.
Crystals of 10 were found to contain lattice disordered solvent
molecules (benzene), all of which could not be satisfactorily
modelled and were removed using the SQUEEZE procedure33
implemented in the PLATON package.34 Crystal data and details
of data collection and structure refinement for the different
compounds are given in Table 1. Crystallographic data are also
available as cif files (see the ESI†).
3
Haro) 8.06 (s, 2H, H4), 7.81-7.83 (m, 12H, Haro), 7.53 (t, J =
7.6, 2H, H7), 7.49 (s, 2H, thiophene), 7.43 (d,3J = 7.6, 2H, H8),
3
7.26-7.30 (m, 18H, Haro), 7.15 (t, J = 7.7, 2H, H6), 4.34 (sept,
3J = 2.9, 2H, SiHMe), 2.82 (m, 4H, a-CH2, THF), 0.69 (m, 4H,
3
1
b-CH2, THF), 0.05 (d, J = 3.2, 12H, SiHMe2). 13C{ H} NMR
(125 MHz, benzene-d6, 298 K): d 168.8, 143.7, 141.4, 136.6, 136.5,
134.9, 133.5, 129.2, 129.1, 128.3, 128.1, 128.0, 127.6, 125.5, 123.4,
122.3, 115.4 (Caro), 71.6 (a-CH2, THF), 24.4 (b-CH2, THF), 2.8
(SiHMe2). Anal. calcd. for C68H64NO3SScSi4: C, 72.11; H, 5.70;
N, 1.24. Found: C, 71.8; H, 6.1; N, 1.2.
{OSOSiPh3}Y[N(SiHMe2)2](THF) (8)
This compound was prepared following the same procedure as
that described above for 1, starting from {OSOSiPh3}H2 (0.064 g,
0.070 mmol) and Y[N(SiHMe2)2]3(THF) (0.040 g, 0.070 mmol)
in benzene (5 mL). Compound 8 was obtained as a pale yellow
microcrystalline powder (0.065 g, 81%). 1H NMR (500 MHz,
3
benzene-d6, 298 K): d 8.08 (s, 2H, H4), 7.94 (d, J = 8.4, 2H,
Haro), 7.85-7.87 (m, 12H, Haro), 7.55 (s, 2H, thiophene), 7.49 (t,
3
3J = 7.8, 2H), 7.42 (d, J = 7.8, 2H, Haro), 7.27-7.31 (m, 18H,
3
3
Haro), 7.13 (t, J = 7.8, 2H), 4.55 (sept, J = 2.9, 2H, SiHMe),
2.85 (m, 4H, a-CH2, THF), 0.68 (m, 4H, b-CH2, THF), 0.14 (d,
Typical procedure for rac-LA and rac-BBL polymerization
1
3J = 2.9, 12H, SiHMe). 13C{ H} NMR (125 MHz, benzene-d6, 298
In a glovebox, a Schlenk flask was charged with a solution of
complex 1 (7.0 mg, 5.8 mmol) in THF or toluene (0.20–0.60 mL).
To this solution, the monomer (0.58 mmol, 100 equiv vs. La; rac-
LA: 82.6 mg, rac-BBL: 49.3 mg) was rapidly added under vigorous
stirring at 20 ◦C. Small aliquots of the reaction mixture were
periodically sampled with pipette for determining the conversion
by 1H NMR spectrometry. After the desired time, the reaction was
quenched by adding acidic methanol (ca. 1 mL of a 1.2M HCl
solution in CH3OH) and the polymer was precipitated with excess
methanol (ca. 3 mL). The supernatant solution was removed with
pipette and the polymer was dried under vacuum to constant
weight.
K): d 171.8, 149.6, 144.2, 136.8, 136.7, 136.4, 135.6, 132.4, 129.2,
128.4, 128.1, 127.6, 126.9, 126.6, 122.6, 121.8, 112.1 (Caro), 70.6
(a-CH2, THF), 24.4 (b-CH2, THF), 3.4 (SiHMe2). Anal. calcd.
for C68H64NO3SSi4Y: C, 69.42; H, 5.48; N, 1.19. Found: C, 68.8;
H, 5.37; N, 1.2.
{OSOSiPh3}La[N(SiHMe2)2](THF) (9)
This compound was prepared following the same procedure as
that described above for 1, starting from {OSOSiPh3}H2 (0.165 g,
0.180 mmol) and La[N(SiHMe2)2]3(THF)2 (0.127 g, 0.180 mmol)
in benzene (10 mL). Compound 9 was obtained as a pale
yellow microcrystalline powder (0.224 g, 96%). Crystals of 9
suitable for X-ray diffraction analysis were prepared by prolonged
Acknowledgements
1
crystallization from a benzene solution at room temperature. H
NMR (500 MHz, benzene-d6, 298 K): d 8.08 (s, 2H, H4), 7.94
This work was financially supported by Agence Nationale de la
Recherche (grant ANR-06-BLAN-0213). JFC gratefully thanks
the Institut Universitaire de France for a Junior IUF fellowship
(2005–2009).
3
(d, J = 8.3 Hz, 2H, Haro), 7.89 (m, 12H, Haro), 7.52 (s, 2H,
thiophene), 7.49 (m, 2H, Haro), 7.42 (d, 3J = 8.3, 2H, Haro), 7.33
3
(m, 18H, Haro), 7.13 (m, 2H, Haro), 4.55 (sept, J = 2.8, 2H,
SiHMe), 2.70 (br m, 4H, a-CH2, THF), 0.76 (br m, 4H, b-CH2,
1
THF), 0.19 (d, 3J = 2.8, 12H, SiHMe2). 13C{ H} NMR (125 MHz,
References
benzene-d6, 298 K): d 173.0, 151.8, 143.8, 136.6, 136.5, 136.1,
132.0, 129.1, 129.0, 128.1, 127.9, 126.9, 126.6, 122.7, 121.5, 111.6
(Caro), 69.1 (a-CH2, THF), 24.5 (b-CH2, THF), 3.1 (SiHMe2).
Anal. calcd. for C68H64LaNO3SSi4: C, 66.59; H, 5.26; N, 1.14.
Found: C, 66.8; H, 5.57; N, 1.0.
1 For reviews on alkoxo- and aryloxo-metal chemistry, see: (a) D. C.
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6750 | Dalton Trans., 2010, 39, 6739–6752
This journal is
The Royal Society of Chemistry 2010
©

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and J. Kress, Organometallics, 2000, 19, 2944; (b) K. Takaoki and
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15 Note that the C2 and Cs symmetry refers here to the coordination
mode of the ligand, but not to the complex because of the position of
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16 As revealed by significant broadening of the a-THF hydrogens at
intermediate temperatures (ca. room temperature), the same dynamic
phenomena take place in scandium compound 4, but the resonances
for the diastereotopic a-THF hydrogens appear incidentally isochronic
in this case.
This journal is
The Royal Society of Chemistry 2010
Dalton Trans., 2010, 39, 6739–6752 | 6751
©

17 A. Alaaeddine, C. M. Thomas, T. Roisnel and J.-F. Carpentier,
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18 Ionic radii (in A) for six-coordinated metal centers: Sc3+, 0.75; Y3+, 0.90;
˚
La3+, 1.17; Ti4+, 0.61: Zr4+, 0.72; Hf4+, 0.71; Ta5+, 0.64. R. D. Shannon,
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23 In some cases, for convenience, in situ combinations of the pro-ligand
{OZOSiR3}H2 with the tris(amido) precursor were investigated instead
of isolated compounds. We independently checked for 2 and 3 that the
corresponding in situ systems lead to similar polymerization results.
24 I. Barakat, P. Dubois, R. Jerome and P. Teyssie, J. Polym. Sci., Part A:
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29 The comparison of theoretical with experimental Mn values must be
done with caution since the latter values are uncorrected and might be
overestimated. However, no reliable correction factor accounting for
the difference in hydrodynamic radius between PHBs and polystyrene
standards has been reported to our knowledge. SEC analysis of higher
molecular weight PHBs is known to be problematic. See reference 7f
and references cited therein.
25 For low molar mass PLAs generated in the presence of isopropanol,
1
determination of the number-average molar mass by H NMR [from
the relative intensity of the signals of the methyl hydrogens of the PLA
chains to the methyl hydrogens of the C(O)OCH(CH3)2 end-groups]
gave values in close agreement with those determined by SEC analysis.
26 MALDI-ToF-MS analysis of some PLA samples recovered at high
monomer conversions (>95%) showed distributions containing both
even-membered and odd-membered oligomers, with peaks separated
by 72 Da. This unambiguously established that transesterification
processes take place in these systems, especially in the later stages of
the polymerization.
30 A. Altomare, M. C. Burla, M. Camalli, G. Cascarano, C. Giacovazzo,
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27 (a) For the pioneering work of Inoue in the immortal polymerizations
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34 A. L. Spek, Acta Crystallogr., 1990, A46, C-34.
6752 | Dalton Trans., 2010, 39, 6739–6752
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The Royal Society of Chemistry 2010
©