Coordination of substitutionally inert phenolate ligands to lanthanide(ii) and (iii) compounds - Catalysts for ring-opening polymerization of cyclic esters

Article abstract of DOI:10.1039/b821770j

A new [ONO] tridentate phenolate ligand (H₂L^I) containing an aliphatic alcohol as a side arm has been synthesized, deprotonated and attached to lanthanide(ii) and (iii) ions, which are employed as catalysts for ring-opening polymeriz

Full text of DOI:10.1039/b821770j

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Coordination of substitutionally inert phenolate ligands to lanthanide(II) and
(III) compounds—catalysts for ring-opening polymerization of cyclic esters†
Pascal I. Binda,a Ewan E. Delbridge,*b Harmon B. Abrahamson*a and Brian W. Skeltonc
Received 4th December 2008, Accepted 26th January 2009
First published as an Advance Article on the web 23rd February 2009
DOI: 10.1039/b821770j
A new [ONO] tridentate phenolate ligand (H2LI) containing an aliphatic alcohol as a side arm has been
synthesized, deprotonated and attached to lanthanide(II) and (III) ions, which are employed as catalysts
for ring-opening polymerization of cyclic esters. In contrast to many other mono-phenolate lanthanide
compounds, these have been found to be inert to polymer incorporation during the polymerization
reactions. Three new divalent ytterbium compounds have been synthesized in high yield containing
ancillary ligands; two via a transamination reaction between [Yb(N(SiMe3)2)2(THF)2] and one
equivalent of the phenols, HOC6H2-(2,4-tBu)-6-CH2N(Me)CH2CH2OH (H2LI) or HOC6H2-(2,4-tBu)-
6-CH2N(Me)CH2CH2N(Me)CH2-6-(2,4-tBu)-C6H2OH (H2LII) in hexanes to yield [Yb(LI)]2 (1) and
[Yb(LII)]2 (2), respectively. The third divalent ytterbium compound [Yb(L2)] (3) was prepared by
treatment of [Yb(N(SiMe3)2)2(THF)2] with two equivalents of a related monoanionic ancillary phenol,
HOC6H2-(2,4-tBu)-6-CH2N(Me)CH2CH2NMe2 (HL) in hexanes. Additionally, the oxidation chemistry
of these divalent systems was explored where compound 1 was treated with silver triflate and phenol to
form corresponding heteroleptic trivalent ytterbium phenolate complexes [Yb(LI)(O3SCF3)(THF)] (4)
and [Yb(LI)(OPh)] (5), respectively. Finally, three new heteroleptic trivalent lanthanide silylamido
compounds were synthesized via a ligand exchange transamination reaction between the homoleptic
trivalent [Ln(N(SiMe3)2)3] compound and one equivalent of the new dianionic ligand (H2LI) in THF
{[La(LI)(N(SiMe3)2)(THF)2] (6); [Sm(LI)(N(SiMe3)2)(THF)] (7); [Yb(LI)(N(SiMe3)2)(THF)] (8)}. These
lanthanide(II) and (III) compounds were assessed as catalyst precursors towards the ring-opening
polymerization of both L-lactide and e-caprolactone. End-group analyses and detailed kinetics studies
{rate law: -d[LA]/dt = k[LA]1[catalyst]1} of the most efficacious lanthanum compound (6) further
corroborated the substitutionally inert characteristics of the new stationary ancillary [ONO] tridentate
dianionic ligand.
lanthanide compounds have found great utility in catalysis,14,19–22
Introduction
fluorescence,19,23–28 agriculture29 and redox chemistry.19,30 Among
Recent developments in the synthesis and applications of
these ligand systems, lanthanide compounds containing multiden-
lanthanide complexes have focused on the use of non-
tate and often multianionic phenolate and bis(phenolate) ligands
cyclopentadienyl (Cp) ligands containing multidentate an-
have recently exhibited excellent catalytic properties towards the
ring-opening polymerization (ROP) reactions of cyclic esters such
ionic ligands such as guanidinate,1–4 b-diketiminates,5–8 salen,9,10
diamides11,12 and bis(phenolates).13–18 This is primarily because
these ligands are easily tunable; a unique feature, which al-
lows for the convenient variation of the steric and electronic
properties, making them less susceptible to ligand redistribution
reactions compared with their Cp counterparts. As a result,
as lactide (LA) and e-caprolactone (e-CL).9,14,16–18,31–37 The signif-
icant chelating ability of such ligands to oxophilic lanthanides
renders these substrates substantially inert to substitution during
the polymerization reaction; an often sought after property where
stereochemical control of the polymerization of rac-lactide is
desired. As such, there is continued interest in the architecture
and binding stability of such multidentate ligands as stationary
‘spectator’ ligands in ROP research.
aChemistry Department, University of North Dakota, Grand Forks, ND
58202-9024, USA. E-mail: habrahamson@chem.und.edu; Fax: +1 701 777
2331; Tel: +1 701 777 4427
The authors’ recent studies have focused on the use of po-
tential monoanionic phenolate ligand (HL) (Fig. 1) to stabilize
lanthanide metals during ROP of cyclic esters.37 The choice of
this system was motivated by the hypothesis that monoanionic
multidentate (HL) ligands could provide an optimal balance
between ligand conformational flexibility and substitutionally
inert characteristics, if compared to a typical unidentate PhO- and
more encumbered multidentate ligands. However these previous
studies revealed unanticipated lability of L- ligand during ROP
reactions such that the multidentate, monoanionic phenolate
bThe Lubrizol Corporation, Wickliffe, OH 44092, USA. E-mail:
ewan.delbridge@lubrizol.com; Fax: +1 440 347 4713; Tel: +1 440 347 1535
cThe University of Western Australia, School of Biomedical, Biomolecu-
lar and Chemical Sciences, Chemistry M313, Crawley, 6009/, Western,
Australia. E-mail: brian.skelton@uwa.edu.au; Fax: +61 08 6488 1118;
Tel: +61 08 6488 7107
† Electronic supplementary information (ESI) available: Mass spectrum,
1H NMR spectra, ORTEP diagram, ROP mechanism and second-order
kinetics plot for L-lactide vs. time. CCDC reference numbers 711938. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/b821770j
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were synthesized via reported procedures38,39 while the new ligand
2,4-di-tert-butyl-6-{[(2¢-hydroxyethyl)-ethylamino]methyl}phenol
(H2LI) was analogously synthesized from 2,4-di-tert-butylphenol,
formaldehyde and 2-(methylamino)ethanol (Scheme 1) and fully
1
characterized via established methods (1H and 13C{ H} NMR, IR
spectroscopy, ESI mass spectrometry and elemental analysis (see
ESI Fig. S1 and S2).† These ligands were subsequently utilized
in the syntheses of divalent and trivalent lanthanide compounds,
catalyst precursors for the ROP of cyclic esters.
Scheme 1 Synthesis of ancillary ligands via Mannich condensations.
Fig. 1 Multidentate ancillary phenolate ligands.
ligand did not remain bound to the lanthanide metal center
but rather it was incorporated into the polymer, making it an
effective polymerization initiation group.37 It was rationalized
that the lanthanide metal forms only weakly dative bonds with
the hindered tertiary amine, NMe2 side arm in ligand (L-).37 In
order to circumvent this problem, we decided to rebuild and tune
the ligand architecture by substituting the poorly coordinating
NMe2 side arm for a readily deprotonatable aliphatic alcohol or
another phenol to afford two dianionic ligand variants, H2LI and
H2LII, respectively (Fig. 1). Reported herein is the synthesis and
characterization of divalent and trivalent lanthanide complexes,
supported by potentially tridentate [ONO] (H2LI) and tetradentate
[ONNO] (H2LII) dianionic phenolate ligands, and their reactivity
towards L-lactide and e-caprolactone.
Synthesis of lanthanide compounds
Synthesis of divalent ytterbium compounds. Three new divalent
ytterbium compounds 1–3 have been synthesized in high yield, via
acid–base transamination reactions employing one equivalent of
the ytterbium precursor, [Yb(N(SiMe3)2)2(THF)2] and either one
equivalent of the ligands H2LI and H2LII in hexanes to afford
[YbLI]2 (1) and [YbLII]2 (2), or two equivalents of HL to afford,
YbL2 (3) in hexanes (Scheme 2).
The divalent ytterbium compounds 1–3 have been fully char-
acterized by NMR and IR spectroscopy, elemental analysis and
melting point (see Experimental). Both H and 13C{ H} NMR
spectroscopy of compounds 1 and 2 in C7D8 revealed two
distinct ligand environments (in approximate equal ratio); likely
caused by molecular oligomerization to satisfy the coordinatively
unsaturated ytterbium atoms. However, NMR spectroscopy of
1 and 2 in the presence of a donor solvent (C4D8O), revealed
only one chemical environment consistent with a symmetrical,
coordinatively saturated, monomeric ytterbium center. This so-
lution behaviour exhibited by compounds 1 and 2 has been
well documented for ytterbium(II) compounds with coordinated
ancillary, dianionic ligands.13,18 X-Ray crystallography of deep-
red single crystals of 2, grown from a toluene–hexanes mixture
1
1
Results and discussion
Design and synthesis of stationary spectator ligands
Due to the previously reported37 lability of the ligand HOC6H2-
(2,4-tBu)-6-CH2N(Me)CH2CH2NMe2 (Fig. 1; HL) we decided
to rebuild and tune the ligand architecture by modification
of the NMe2 side arm to: (1) an aliphatic alcohol to afford a
potentially tridentate [ONO] dianionic ligand, HOC6H2-(2,4-
tBu)-6-CH2N(Me)CH2CH2OH (H2LI); and (2) another phenol to
afford a bis(phenolate) [ONNO] potentially tetradentate dianionic
ligand, HOC6H2-(2,4-tBu)-6-CH2N(Me)CH2CH2N(Me)CH2-6-
(2,4-tBu)-C6H2OH (H2LII) (Fig. 1). The ligands HL and HLII
◦
at -20 C, indeed supported a dimeric, divalent, five-coordinate
ytterbium arrangement where the fifth coordination site arises
from a bridging phenolic oxygen of the second ligand (see
Fig. 3, Table 1 and X-ray crystallographic discussion below).
t
Interestingly, agostic interactions between the Bu protons and
Scheme 2 Synthesis of ytterbium(II) and (III) ancillary phenolate compounds.
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Fig. 2 1H NMR spectrum of compound 6 in C4D8O at 70 ◦C.
Table 1 Ytterbium atom environments in compound 2
˚
Yb(1)
r/A
O(21)
O(21¢)
N(1)
N(2)
O(11)
O(21)
O(21¢) 2.3143(16)
N(1)
N(2)
2.2153(17)
2.3650(16)
127.47(6)
129.65(6)
77.31(6)
84.46(7)
142.77(6)
97.38(6)
102.88(7)
81.80(6)
125.53(6)
71.45(7)
2.537(2)
2.532(2)
˚
˚
Yb(1) ◊ ◊ ◊ Yb(1¢) 3.6393(2) A. r/A is the ytterbium–ligand atom distance;
other entries in the matrix are the angles (◦) subtended at the ytterbium
atom by the relevant atoms at the head of the row and column. Primed
atoms are generated by 1 - x, y, 3/2 - z.
Oxidation of divalent ytterbium compounds. Trivalent het-
eroleptic lanthanide complexes are often synthesized via proton
exchange reactions involving appropriate homoleptic starting
materials, such as [Ln(N(SiHMe2)2)3(THF)2], [Ln(N(SiMe3)2)3] or
[Ln(CH2Ph)3(THF)3], and ligands possessing sufficiently acidic
protons available to affect a ligand exchange.35,37,40 However,
lanthanide compounds possessing an accessible divalent oxidation
state, such as ytterbium, offer an alternate synthetic pathway to
heteroleptic complexes by reaction of the ytterbium(II) compound
with mild oxidizing agents such as AgO3SCF3, ROH (aliphatic),
PhOH, TlCp and AgPF6.13,18,30
Divalent ytterbium compounds 1 and 3 were treated with differ-
ent oxidants to obtain a variety of trivalent ytterbium compounds
4 and 5 (Scheme 2) having LLnX formulations. These motifs
represent ideal candidates for coordination/insertion catalyst
precursors for the ring-opening polymerization of cyclic esters,
where L is a stationary ancillary ligand, Ln is a lanthanide metal
and X is a labile initiating group. The divalent ytterbium dimeric
compound 1 was readily oxidized with an equivalent (based on Yb)
Fig. 3 Molecular representation of compound 2 (hydrogen atoms and the
toluene solvent molecules have been omitted). Primed atoms are generated
by the 2-fold axis.
ytterbium (see below discussion) are preserved in solution; as
indicated in the 1H NMR spectrum of 2 in C7D8 where the
agostic proton(s) are somewhat deshielded (2.3 vs. 1.8 ppm, see
ESI Fig. S3).†
1
Conversely, the H NMR spectrum of compound 3 demon-
strated a symmetric arrangement of both L ligands in either
donor or non-donor solvents; indicative of a monomeric solution
structure. Undoubtedly, steric constraints impose a monomeric
ytterbium structure since two conformationally flexible monoan-
ionic ligands (L) are required to satisfy charge, which completely
fill the coordination sphere about ytterbium.
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of silver triflate and phenol to obtain [Yb(LI)(O3SCF3)(THF)] 4
and [Yb(LI)(OPh)] 5, respectively.
but less pronounced effect, was observed for the b-THF protons.
These results imply that tridentate coordination of the stationary
ligand is likely since the coordination sphere in these compounds is
unsaturated and completed by THF coordination, suggesting that
this stationary ligand would remain bound to the metal center
during any ROP reactions.
Numerous attempts to grow X-ray quality single crystals were
thwarted such that definitive solid-state structural data was not
available. Furthermore, no mass spectral data were obtained for
these highly air- and moisture-sensitive compounds.
Compounds 4 and 5 were characterized by elemental analysis,
melting point and IR spectroscopy. NMR spectroscopic analyses
were not feasible due to the highly paramagnetic properties of
trivalent ytterbium.13,18,37 However, satisfactory elemental analyses
of 4 suggested a molecule of THF present in the formulation.
Repeated attempts to obtain X-ray quality single crystals of
these compounds were unsuccessful. However, compound 4 likely
has a similar structure to the previously reported analogous het-
eroleptic ytterbium(III) phenolate triflate compound, containing a
six-coordinate ytterbium center with quasi-octahedral geometry
comprising a bidentate coordination of the triflate ligand.13
Oxidation of compound 3 with silver triflate or tertiary butanol
yielded un-isolable products; likely resulting in a mixture of
disproportionated heteroleptic compounds.37 Despite a significant
colour change from dark green to pale yellow, indicative of
successful oxidation of Yb(II) to Yb(III), repeated attempts to
obtain analytically pure samples were unsuccessful.
X-Ray crystallography of 2
The results from the single-crystal X-ray structural determination
of 2 report a dimeric structure with bridging and terminal
phenolate ligands consistent with the [Yb(LII)]2 formulation (Fig. 3
and Table 1). The recently reported La(III) bis(phenolate),41 also
dimeric, displays structural similarities to [Yb(LII)]2, which is
expected given the larger size of lanthanum being able to accom-
modate a bridging ligand arrangement. Both elemental and the 1H
NMR spectroscopic analyses support the solid-state formulation
(see above Discussion); the latter collected in C7D8 promoting
magnetically inequivalent phenolate ligand environments in non-
donor solvents. Additionally, the solid-state structure (Fig. 3)
proposes an agostic interaction between one (tBu) group of the
ligand and the ytterbium metal [Yb(1) ◊ ◊ ◊ H(26A) (1 - x, y, 3/2 - z)
Synthesis of trivalent lanthanide phenolate compounds by ligand
exchange reactions. Access to trivalent heteroleptic lanthanide
complexes containing silylamides (as the initiating group) were
readily obtained by ligand exchange reactions between the ho-
moleptic [Ln(N(SiMe3)2)3] compounds and H2LI in THF to afford
three new lanthanide compounds: [La(LI)(N(SiMe3)2)(THF)2]
(6), [Sm(LI)(N(SiMe3)2)(THF)] (7), [Yb(LI)(N(SiMe3)2)(THF) (8)
(Scheme 3).
˚
2.71 A], which appears to be preserved in solution (see above)
also destroying the C2-symmetry of the bis(phenolate) ligand.
Agostic interactions between lanthanide metals and alkyl/aryl
hydrogen atoms are well established in the literature42 and support
the credibility of the interaction reported herein.
The dimeric structure is generated by the crystallographic 2-fold
axis along b. There are also two toluene molecules per dimer. The
central Yb2O2 ring deviates from planarity with the angle between
the two YbO2 planes being 10.41(7)◦. Each ytterbium atom is
4
five-coordinate; a h -bis(phenolate) occupying four coordination
Scheme 3 Synthesis of Ln(III) compounds via transamination reactions.
sites and the fifth obtained from a bridging phenolic oxygen of
the the other bis(phenolate) ligand (see ESI Fig. S5 for thermal
ellipsoid plot).† The ytterbium coordination environments in 2
display distorted trigonal bipyramidal geometry possessing met-
rical similarities to recently reported five-coordinate ytterbium(II)
bis(phenolate) compounds13 and as such, no detailed discussion
of bond lengths and angles are presented herein (see Table 1
and Fig. 3). As typical with these compounds, steric bulk of
the bis(phenolate) [ONNO] ligand stabilizes the somewhat low
coordination number of the ytterbium(II) center.
The lanthanide compounds 6–8 have been fully characterized
by NMR and IR spectroscopy, elemental analysis and melting
point, except for the paramagnetic ytterbium compound 8, where
no satisfactory NMR spectroscopic data were obtained (see
Experimental). NMR spectroscopic and elemental analyses data
is suggestive that the lanthanum compound, 6, contains two THF
molecules whilst the samarium analogue, 7, contains only one
THF molecule in the formulation. If these THF molecules are
coordinated to the metal center, these formulations are consistent
with the relative ionic radii of the larger La vs. the smaller Sm. The
room temperature 1H NMR spectrum of compound 6 in C4D8O
displayed broadened methylene and NMe protons that sharpened
as the temperature was raised (see Fig. 2, Experimental and ESI
Fig. S4).† Elemental analysis of compound 8 also supports the
presence of one THF molecule. The proposed formulations of
compounds 6–8 were further established by comparison of the
THF signals found in the 1H NMR spectra of compounds 6 and
7 C6D6 to those observed in an NMR spectrum of neat THF
in C6D6. A discernible downfield shift of the a-protons (from
3.53 to 3.60 and 3.56 ppm for 6 and 7, respectively) is observed
upon THF coordination to the lanthanide metal centers. A similar,
Polymerization studies using lanthanide compounds
Compounds 1–8 reported herein have been assessed as catalyst
precursors in ring–opening polymerization of cyclic esters, i.e.,
e-caprolactone and L-lactide. Polycaprolactone and polylactide
are among a growing number of ‘green’ polyesters derived from
renewable resources which continue to receive significant attention
because of their numerous applications ranging from packaging
to drug delivery materials.43–57
Recently, the authors reported the lability of the multiden-
tate monoanionic phenolate ligand (L) (Fig. 1) coordinated to
Ln(III) ions (Ln = La, Sm and Yb) during ROP of cyclic
2780 | Dalton Trans., 2009, 2777–2787
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esters and its incorporation into polymer chains (via activation
during polymerization initiation).37 The current study expands
on these initial findings via catalyst refinement by way of ligand
modification (i.e. H2LI and H2LII, Fig. 1). The principal objective
of this study was to investigate two effects: (1) How stationary
‘spectator’ ligand redevelopment would affect polymerization
dynamics (i.e., reaction kinetics, polymer composition, etc.);
(2) comparison of lanthanide(II)/(III) catalyst efficacy towards
ring-opening polymerization of cyclic esters.
The outcomes from these studies, discussed below in detail,
reveal the following: (1) the modified phenolate ligand [(LI)2-]
remains bound to the lanthanide metal center during poly-
merization reactions, in contrast to the monoanionic phenolate
ligand [(L)1-];37 (2) the more substitutionally inert (LI)2- ligand
influences catalyst activity, most notably in ytterbium catalysts;
(3) catalyst precursors containing larger lanthanide metals poly-
merize cyclic esters more rapidly than those containing smaller,
heavier lanthanides; (4) lanthanide catalyst precursors polymerize
caprolactone more readily than lactide, and (5) the rate law
(obtained via compound 6 and L-LA polymerization systems)
obeys: -d[LA]/dt = k[monomer]1[catalyst]1, which is first-order
in monomer and first-order in catalyst precursor in contrast to
the previously reported [La(L)2(N(SiMe3)2)],37 which was second-
order in lactide.
polylactide (80 ◦C) and polycaprolactone (room temperature).
Near instantaneous oxidation (observable by reaction colour
change) of the ytterbium(II) center in 3, in the presence of
monomer substrates, presumably affords a “YbIIIL2X” species,
similar in motif to the previously reported amide, where X is a
ring-opened monomer i.e. a propagating alkoxide group. Thus it
would be anticipated that if kinitiation >> kpropagation (highly plausible)
similar polymer conversions for these two catalyst systems would
be observed since initiation would not be rate-determining. Addi-
tionally, polycaprolactone polydispersity was higher for reactions
employing catalyst precursor 3 vs. 1 (1.52 vs. 1.26, respectively;
Table 3, entries 1 and 3) despite higher reaction temperatures being
employed in the latter. This is consistent with 1 comprising a more
substitutionally inert dianionic ‘spectator’ phenolate ligand which
suppresses transesterification side reactions (see below for more
detailed discussion of catalysts employing this dianionic, mono-
phenolate ligand systems).
Ytterbium(III) phenolate catalyst precursors 4, 5 and 8 containing
the dianionic phenolate ligand LI. Compounds 4, 5 and 8, the
trivalent counterparts to 1, were insufficiently active to appreciably
polymerize the monomers L-lactide or e-caprolactone within
60 min at 80 ◦C. However, longer reaction times (8–24 h),
under similar reaction conditions, did produce polylactide and
polycaprolactone, albeit at modest conversion (43–61% polymer
conversion) using compounds 5 and 8 whilst compound 4 was
totally inactive (Table 2; entries 4–5 and Table 3; entries 4–6).
Additionally, complete monomer conversion was not obtained
with compounds 5 and 8, even at extended reaction times
(days), which may simply reflect polymerization termination via
hydrolysis derived from adventitious moisture (see ESI Fig. S6).†
This lack of reactivity is reminiscent of that observed for the
divalent ytterbium catalyst precursor 1, merely reflecting a similar
restrictive coordination environment about ytterbium metal that
is less able to accommodate an incoming monomer substrate; a
necessary requirement for effective ROP via the widely proported
coordination–insertion mechanism (see ESI Fig. S6).† Moreover,
Ytterbium(II) phenolate catalyst precursors 1–3. The homolep-
tic divalent ytterbium phenolates, compounds 1–3, had difficulty
polymerizing both L-lactide and e-caprolactone (Tables 2 and 3,
respectively; entries 1–3) even at elevated temperature (80 ◦C).
However, catalyst precursor 3 displayed superior polymer conver-
sion in both lactide (80 ◦C) and caprolactone (room temperature)
polymer systems yielding 63 and 76%, respectively, whilst catalyst
precursors 1 and 2 yielded no detectable polylactide and only
47 and 45% polycaprolactone conversions (80 ◦C), respectively,
within 60 min. These polymer conversion data for 3 are compa-
rable to those reported37 for the trivalent ytterbium phenolate
amido catalyst precursor [Yb(L2)(N(SiMe3)2)] (59% for both
Table 2 Results of polymerization of L-LA in toluene at room temperature
Entry
Initiator [cat.]
[M]/[I]a
Time/min
Conv. (%)b
Mn (x 104)c
PDIc
1
2
3
4
5
6
7
8
9
LIYb 1
100
100
100
100
100
100
100
200
500
1000
100
60d
60d
60d
24 hd
24 hd
1
10
10
10
0
0
—
—
0.91
—
—
2.35
2.26
—
—
—
—
—
1.27
—
—
1.23
1.62
—
—
—
LIIYb 2
L2Yb 3
63
57
61
94
97
96
95
94
35
LIYbOPh 5
LIYbN(SiMe3)2(THF) 8
LILaN(SiMe3)2(THF)2 6
LILaN(SiMe3)2(THF)2 6
LILaN(SiMe3)2(THF)2 6
LILaN(SiMe3)2(THF)2 6
LILaN(SiMe3)2(THF)2 6
LISmN(SiMe3)2(THF) 7
10
11
10
60d
0.525
1.01
a Monomer to initiator ratio. b Conversion: determined by 1H NMR spectroscopy. c Determined by gel permeation chromatography (GPC) with calibration
to polystyrene standards. d Temperature at 80 ◦C.
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Table 3 Results of polymerization of e-CL in toluene at room temperature
Entry
Initiator [cat.]
[M]/[I]a
Time/min
Conv.b (%)
Mn(x 104)c
PDIc
1
2
3
4
5
6
7
8
9
LIYb 1
100
100
100
100
100
100
100
100
100
100
60d
60d
60
47
45
76
0
45
43
100
62
77
78
0.60
—
0.61
—
—
—
1.63
—
—
1.26
—
1.52
—
—
—
1.34
—
—
LIIYb 2
L2Yb 3
LIYbO3SCF3(THF) 4
LIYbOPh 5
60d
8 hd
8 hd
1
LIYbN(SiMe3)2(THF) 8
LILaN(SiMe3)2(THF)2 6
LISmN(SiMe3)2(THF) 7
LISmN(SiMe3)2(THF) 7
LISmN(SiMe3)2(THF) 7
10
30
60
10
0.83
1.11
a Monomer to initiator ratio. b Conversion: determined by 1H NMR spectroscopy. c Determined by gel permeation chromatography (GPC) calibrated to
polystyrene standards. d Temperature at 80 ◦C.
poor nucleophilicity of the triflate ion in 4, steric crowding
promoted by the OPh initiating group in 5 coupled with a smaller
lanthanide ion, ytterbium, also likely contribute to this lack of
catalytic activity.13,31
However, this decreased reactivity of ytterbium catalyst sys-
tems containing dianionic monophenolate LI vs. monoanionic
monophenolate L highlights the former ligand’s superior chelating
ability, a deliberately sought after design feature to be more
substitutionally inert. Possibly a larger lanthanide metal, able to
accommodate LI and a monomer substrate, may represent an ideal
ROP catalyst precursor.
indicative of transesterification37 (Tables 2 and 3). Most notable
is that longer polymerization reaction times produce broader
molecular weight distributions (Table 2, entries 6 vs. 7) consistent
with the notion that kpropagation > ktransesterification. This is a reasonable
proposition (especially for intermolecular transesterification) since
ktransesterification requires collision of two large polyester oligomers
whereas kpropagation merely requires monomer and propagating chain
collisions to occur. Extended polymerization reaction times, where
quantitative polymer conversion has already ensued, allows the
effect of slower transesterification reactions to become more
significant, thereby affecting polymer polydispersity.
Obviously robust coordination of the multidentate, dianionic
phenolate ligand (LI)2- to lanthanide(III) metals provides a uni-
form, consistent catalytic pocket, or coordination environment,
for monomer or polyester oligomers such that some of these subtle
differences in polymer characteristics are detectable.
Lanthanide(III) phenolate amide catalyst precursors, Ln = La
(6) and Sm (7) containing the dianionic phenolate ligand, LI.
Indeed, attachment of the dianionic phenolate ligand, LI to larger
lanthanide metals, such as samarium and lanthanum, drastically
increased catalytic activity towards ROP of both L-lactide and
e-caprolactone compared with the less active ytterbium catalyst
precursor (compound 8). Quantitative polymer conversion was
readily achieved with catalyst precursor 6 within 1 min at room
temperature at a monomer–catalyst ratio of 100 : 1 (Table 2; entry
6 and Table 3; entry 7). Moreover, increasing L-lactide–catalyst
6 ratios from 100 : 1 up to 1000 : 1 readily afforded complete
conversion to polylactide (the more challenging of the two cyclic
monomers to polymerize) at room temperature within 10 min
(Table 2, entries 7–10).
The samarium analogue, 7 displayed intermediate catalytic
behaviour, between the larger, highly active lanthanum and the
less active, smaller ytterbium compounds (Table 2; entry 11 and
Table 3; entries 8–10). Finally, heating (80 ◦C) the L-lactide
polymerization reaction with 7 (100 : 1) for 60 min only achieved
moderate polymer conversion (35%), whilst significantly greater
conversion to polycaprolactone (78%) was readily attained at room
temperature under identical reaction conditions.
End-group analysis via ESI-MS of polylactide obtained from
reaction of 6 with L-lactide (Table 2, entry 8) provides unequivocal
evidence that the modified tridentate, dianionic phenolate ligand
remains bound to the lanthanide metal center during polymer-
ization reactions, and the labile monodentate nucleophilic amide
group, [N(SiMe3)2] is the preferred initiating group attached to
the lanthanum metal (Fig. 4). However, polylactide cycles pre-
dominate (at least in low molecular weight fractions), which result
from intra-moleuclar, intra-chain transesterification reactions. It
is noteworthy that this mass spectrum is significantly different
from that previously reported37 for polylactide polymerized by the
analogous [La(L)2(N(SiMe3)2)] where significant quantities of L
was incorporated into the polymer.37
Finally, polymerization of rac-lactide (1 : 100 cat–monomer) in
THF or toluene by catalyst precursor 6 was performed to assess the
stereoselectivity. Unfortunately, this study indicated that there was
no significant heterotactic bias in polymer formed using catalyst
precursor 6 (Pr = 48.5% in toluene).
Polymer molecular weight distributions/compositions from cata-
lyst precursors 6 and 7. Gel permeation chromatographic (GPC)
analyses of the polylactide samples indicated formation of high
molecular weight polymer. The polydispersities of all measured
polymers are in the range 1.01–1.62; the upper ranges being
Kinetics analysis of ring-opening polymerization of L-lactide.
Detailed studies of the kinetics of the L-lactide polymerization
reaction with catalyst precursor 6 were conducted to establish
reaction order in monomer and lanthanum catalyst precursor.
2782 | Dalton Trans., 2009, 2777–2787
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Fig. 4 ESI-MS of polymer produced from (Table 2, entry 8) indicating amide end-groups and polylactide cycles.
1H NMR spectroscopy (CDCl3) was used to determine polymer
conversions (or monomer consumption). Several reactions were
performed at room temperature where initial lactide concentration
[LA]o (0.0694 M) was held constant and catalyst concentrations
(1 : 300, 0.2313 mM; 1 : 400, 0.1735 mM; 1 : 500, 0.1388 mM; 1 :
600, 0.1156 mM; 1 : 750, 0.0925 mM) were varied until at least
>85% conversion to polylactide was observed. First-order kinetics
plots of ln([LA]o/[LA]t) vs. time were linear over all concentration
ranges assessed, indicating that the polymerization proceeded with
a first-order dependence on monomer concentration (Fig. 5). As
a precautionary measure, second-order kinetic plots of (1/[LA]t
- 1/[LA]o) vs. time were plotted and, as anticipated, these were
non-linear, supporting the first-order determination plotted above
(ESI Fig. S7).†
Fig. 5 First-order kinetic plots for L-lactide vs. time at various concen-
trations of 6.
From these data and a plot of kobs vs. [6] (Fig. 6), the value of k
was determined as 0.025 M-2 s-1 (note: kobs = k[6]x, where kobs is the
pseudo-first-order rate constant and k is the overall propagation
rate constant) such that the overall kinetic rate equation was
determined: -d[LA]/dt = k[LA]1[6]1.
Furthermore, the PDIs are low, ranging from 1.19–1.63 compared
to similar lanthanide complexes of monoanionic phenolate ligands
reported previously.37
These kinetics findings are similar to those observed for the ring-
opening polymerization of lactide by zinc alkoxide and yttrium
silylamido complexes.35,58 The first-order dependence on lactide
is consistent with a mechanism which requires one molecule of
lactide monomer per active site of the catalyst.
GPC analyses of these polylactide samples obtained from kinet-
ics analyses indicate good polymerization control as evidenced by a
linear relationship between the number average molecular weight
(Mn) and the initial lactide-to-initiator ratio ([M]/[I]) (Fig. 7).
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Experimental
General
All air- or moisture-sensitive reactions were carried out under a
dry nitrogen atmosphere, employing standard Schlenk line and
drybox techniques. L-lactide and e-caprolactone were purchased
from Aldrich, purified by sublimation and distillation, respectively,
and both were stored under an inert atmosphere. Solvents were
dried over sodium/benzophenone and distilled under a nitrogen
atmosphere. Deuterated solvents were purchased from Cambridge
Isotope Laboratory, dried over sodium, deoxygenated and distilled
by vacuum transfer. HL ligand,38 H2LII ligand,39 [Ln(N(SiMe3)2)3]
(Ln = La, Sm, Yb)59 and [Yb(N(SiMe3)2)2(THF)2]60 were prepared
and purified according to the indicated literature procedure.
AgO3SCF3 and unsubstituted phenol were purchased from Acros,
stored under an inert atmosphere, and were used as received.
Melting points were obtained from sealed capillaries on a Mel-
Temp apparatus and are uncorrected. IR spectra (4000–450 cm-1)
were recorded as KBr Nujol mulls on an ATI Mattson Genesis
Fig. 6 Linear plot of kobs vs. [6] for the polymerization of lactide.
1
Series FTIR Spectrometer. All H and 13C NMR spectra were
recorded on a Bruker AVANCE-500 NMR spectrometer and
referenced to C6D5H, C7D7H or C4D7HO (1H) and C6D6, C7D8
or C4D8O (13C). Elemental analyses (sealed ampoules under inert
atmosphere) were performed by Midwest Microlab, Indianapolis,
IN. Single crystals, suitable for X-ray crystallography were sealed
under an inert atmosphere in glass capillaries. Mass spectrometry
was performed using high-resolution time of flight G1969A instru-
mentation (Agilent, Santa Clara, CA, USA). Samples (10 ppm)
were dissolved in 10% THF and introduced to the MS using
direct infusion at flow rate of 10 mL min-1 using acetic acid as
an ionization agent. The positive ionization was performed at the
voltage of 4000 V and with fragmentor set to 150 V. Nitrogen
was used as the nebulizing (25 psi) and drying (7 L min-1) gas at a
temperature of 150 ◦C. The determinations were performed within
the mass error of 10 ppm determined by the calibration prior to
and after the measurement.
Fig. 7 Polymerization of L-lactide catalyzed by 6 in toluene at r.t. after
5 min displaying relationship between Mn(obs) of polymer and initial
monomer to initiator ratio, [M]/[6] (NB: data obtained from kinetics
studies, values in parenthesis are polydispersities).
2,4-Di-tert-butyl-6-{[(2¢-hydroxyethyl)methylamino]methyl}-
phenol (H2LI) ligand
2,4-Di-tert-butylphenol (27.47 g, 133.1 mmol), 37 wt% forma-
lin (4.0 g, 130 mmol), and 2-(methylamino)ethanol (10.00 g,
133.1 mmol) were dissolved in ethanol (100 mL). The resulting
solution was heated at reflux for 16 h and then cooled to
room temperature. Solvent was removed via rotary-evaporation to
obtain pale yellow oil. Recrystallization from methanol at -20 ◦C
yielded a white powder (21 g, 53%). Mp: 96–98 ◦C. (Found: C
73.6, H 10.55, N 4.8. C18H31NO2 requires C 73.7, H 10.65, N
Conclusions
A series of divalent and trivalent lanthanide complexes containing
new tridentate [ONO] and tetradentate [ONNO] dianionic ligands
has been synthesized and characterized. The trivalent compounds
were synthesized by oxidation of the corresponding homoleptic
divalent ytterbium compounds using mild oxidizing agents such
as AgOTf and alcohols and by ligand exchange transamination
reactions. The solid-state structure of 2 supports a dimeric
divalent ytterbium complex containing five-coordinate centers;
the fifth coordination is from a bridging phenoxide. Many of
these compounds have moderate catalytic activity towards the
ring-opening polymerization of L-lactide and e-caprolactone, with
6 being an excellent initiator. Studies of the kinetics of L-LA
polymerization by 6 revealed that the modified [ONO] tridentate,
dianionic phenolate ligand (LI)2- remains bound to the lanthanide
metal center during polymerization reactions and accounted for
the first-order consumption of L-lactide and good polymerization
control.
4.8%). nmax/cm-1 3201 s, 3105 s, 1759 w, 1604 m, 1315 m, 1242 vs,
˜
1199 m, 1153 m, 1126 w, 1084 s, 1053 m, 1026 s, 929 m, 875 vs,
810 m, 760 w, 725 s, 667 w, 652 w, 578 w, 528 w, 509 w, 467 w,
t
416 w (Nujol). dH (500.1 MHz; C6D6; 298 K) 1.36 (9H, s, Bu),
t
1.70 (9H, s, Bu), 2.01 (3H, s, ArCH2NMe), 2.31 (2H, t, J 5.6,
NCH2CH2OH), 3.27 (2H, t, J 5.6, NCH2CH2OH), 3.35 (2H, s,
ArCH2N), 6.06 (1H, b, NCH2CH2OH), 7.07 (1H, s, ArH), 7.50
(1H, s, ArH), 10.76 (1H, b, ArOH). dC{H} (125.8 MHz; C6D6;
298 K) 30.4 (CMe3), 32.3 (CMe3), 34.7 (CMe3), 35.7 (CMe3),
41.8 (ArCH2NMe), 51.9 (NCH2CH2NMe2), 60.2 (ArCH2N), 62.9
(NCH2CH2NMe2), 122.3 (arom-CH), 123.5 (arom-CMe3), 124.3
2784 | Dalton Trans., 2009, 2777–2787
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(arom-CH), 135.9 (arom-CMe3), 141.2 (arom-CCH2N), 155.4
(arom-CO).
[Yb(L2)] (3). A deep orange hexanes solution (10 mL) of
[Yb(N(SiMe3)2)2(THF)2] (0.32 g, 0.50 mmol) was treated with
a colourless hexanes solution (10 mL) of HL ligand (0.32 g,
1.0 mmol) at room temperature and stirred overnight. Upon
addition, the colour changed immediately to a dark green solution
with deposition of 3 (0.38 g, 94%) as dark green crystals. Mp: 157–
160 ◦C. (Found: C 58.9, H 8.6, N 6.8. C40H70N4O2Yb requires C
[Yb(LI)]2 (1). A deep orange hexanes solution (10 mL) of
[Yb(N(SiMe3)2)2(THF)2] (0.32 g, 0.50 mmol) was treated with
a colourless hexanes solution (10 mL) of H2LI ligand (0.15 g,
0.50 mmol) at room temperature and stirred overnight. Upon
addition, the colour changed immediately to a deep red solution
with◦deposition of 1 (0.21 g, 90%) as deep red crystals. Mp: 234–
239 C. (Found: C 46.3, H 6.4, N 3.25. C36H58N2O4Yb2 requires
59.2, H 8.7, N 6.9%). nmax/cm-1: 1601 m, 1319 s, 1261 m, 1203 w,
˜
1161 m, 1099 w, 1022 w, 968 w, 879 w, 802 w, 775 w, 725 vs (Nujol).
dH (500.1 MHz; C6D6; 298 K) 1.38 (4H, b, NCH2CH2NMe2),
C 46.55, H 6.3, N 3.0%); nmax/cm-1: 1604 w, 1307 s, 1199 w, 1095
˜
t
t
1.55 (18H, s, Bu), 1.69 (6H, s, ArCH2NMe), 1.75 (18H, s, Bu),
1.81 (6H, s, NMe2), 1.98 (6H, s, NMe2), 2.28 (2H, b, ArCH2N),
2.86 (2H, b, NCH2CH2NMe2), 2.94 (2H, b, NCH2CH2NMe2),
4.10 (2H, b, ArCH2N), 7.16 (2H, s, ArH), 7.85 (2H, s, ArH). dC{H}
(125.8 MHz; C6D6; 298 K) 30.9 (CMe3), 32.9 (CMe3), 34.5 (NMe2),
36.2 (NMe2), 44.3 (CMe3), 45.8 (CMe3), 48.3 (NCH2CH2NMe2),
50.6 (NCH2CH2NMe2), 57.6 (ArCH2NMe), 66.5 (ArCH2N),
123.3 (arom-CMe3), 124.6 (arom-CH), 127.3 (arom-CH), 128.1
(arom-CMe3), 132.4 (arom-CCH2N), 166.0 (arom-CO).
s, 972 w, 929 m, 887 m, 817 m, 771 w, 725 vs, 667 w (Nujol).
dH (500.1 MHz; C4D8O; 298 K) -0.19 (3H, s, ArCH2NMe), 0.90
t
t
(9H, s, Bu), 1.14 (2H, b, NCH2CH2O), 1.29 (9H, s, Bu), 1.55
(2H, b, NCH2CH2O), 2.31 (2H, s, ArCH2NMe), 7.13 (1H, d, J
7.5 ArH), 7.19 (1H, d, J 7.5 ArH). dH (500.1 MHz; C7D8; 298
K) 0.76 (2H, s, ArCH2NMe), 1.03 (2H, s, ArCH2NMe), 1.13
(2H, b, NCH2CH2O), 1.61 (2H, b, NCH2CH2O), 1.21 (4H, b,
t
t
NCH2CH2O), 1.29 (9H, b, Bu), 1.35 (18H, s, Bu), 1.41 (3H,
t
s, ArCH2NMe), 1.47 (3H, s, ArCH2NMe), 1.79 (9H, s, Bu),
[Yb(LI)(O3SCF3)(THF)] (4). A solution of THF (15 mL) was
added at room temperature to a deep red powder of 1 (0.40 g,
0.43 mmol) and a white powder of AgO3SCF3 (0.22 g, 0.86 mmol)
and stirred for 24 h. The resulting pale yellow solution was
separated from the black silver metal and concentrated to yield
a yellow solid, which was recrystallized from a mixture of THF–
hexanes at 35 ◦C to afford 4 (0.33 g, 56%) as yellow powder. Mp:
216–219 ◦C. (Found: C 40.35, H 5.4, N 2.2. C23H37F3NO6SYb
6.72 (1H, s, ArH), 7.45 (1H, s, ArH), 8.88 (1H, s, ArH), 9.15
(1H, s, ArH); dC{H} (125.8 MHz; C4D8O; 298 K) 6.9 (CMe3),
11.9(CMe3), 14.6 (CMe3), 23.7 (CMe3), 30.1 (NCH2CH2NMe2),
30.8 (NCH2CH2NMe2), 32.7 (ArCH2NMe), 33.8 (ArCH2N),
126.2 (arom-CCH2N), 128.5 (arom-CMe3), 128.9 (arom-CMe3),
129.1 (arom-CH), 129.8(arom-CH), 155.1 (arom-CO).
[Yb(LII)]2 (2). A deep orange hexanes solution (10 mL) of
[Yb(N(SiMe3)2)2(THF)2] (0.32 g, 0.50 mmol) was treated with
a colourless hexanes solution (10 mL) of H2LII ligand (0.26 g,
0.50 mmol) at room temperature and stirred overnight. Upon
addition, the colour changed immediately to a deep red solution.
Deep red crystals of 2 (0.32 g, 92%) were recrystallized from
hexanes at -20 ◦C. Deep red single crystals of 2 suitable for X-ray
crystallography were obtained from a toluene–hexanes solution
at -20 ◦C. Mp: 200–204 ◦C. (Found: C 58.75, H 7.65, N 4.0.
requires C 40.3, H 5.4, N 2.0%). nmax/cm-1 1612 m, 1593 m, 1462
˜
vs, 1373 s, 1311 vs, 1257 vs, 1196 vs, 1080 s, 1030 vs, 972 w, 895 w,
876 m, 841 m, 802 s, 748 w, 725 w, 679 w, 644 s, 609 w, 517 w, 494
w, 463 w (Nujol).
[Yb(LI)(OPh)] (5). Phenol (0.15 g, 1.6 mmol) was added to a
deep red THF (15 mL) solution of 1 (0.46 g, 0.50 mmol) at room
temperature and stirred overnight. The solution gradually turned
into a pale yellow solution, after which time all volatiles were
removed in vacuo, to yield a yellow powder of 5 (0.38 g, 68%). Mp:
210–213 ◦C. (Found: C 52.0, H 6.5, N 2.4. C24H34NO3Yb requires
(for no THF) C 51.7, H 6.15, N 2.5%; C28H42NO4Yb requires (for
C68H108N4O4Yb2 requires C 58.7, H 7.8, N 4.0%). nmax/cm-1: 1604
˜
s, 1304 vs, 1280 vs, 1261 vs, 1203 s, 1169 s, 1130 m, 1091 s, 1076 s,
1018 vs, 926 s, 875 s, 837 vs, 806 vs, 744 s, 671 m, 644 w, 609 m,
528 s, 430 m (Nujol). dH (500.1 MHz; C4D8O; 298 K) 0.99 (18H,
t
t
s, Bu), 1.22 (18H, s, Bu), 1.81 (6H, s, ArCH2NMe), 2.08 (4H,
s, NCH2CH2N), 2.65 (4H, s, ArCH2NMe), 6.45 (2H, s, ArH),
6.93 (2H, s, ArH). dH (500.1 MHz; C7D8; 298 K) 1.47 (9H, s,
tBu), 1.55 (9H, s, tBu), 1.59 (9H, s, tBu), 1.81 (3H, s, ArCH2NMe),
1.99 (9H, s, tBu), 2.28 (3H, s, ArCH2NMe), 2.64 (1H, dd, J 11.0,
NCH2CH2N), 2.67 (1H, dd, J 11.0, NCH2CH2N), 2.82 (1H, t, J
13.1, NCH2CH2N), 3.33 (1H, t, J 13.1, NCH2CH2N), 4.13 (2H,
d, J 10.7, ArCH2N), 4.17 (2H, d, J 10.7, ArCH2N), 6.98 (1H,
d, J 2.5, ArH), 7.17 (1H, d, J 2.5, ArH), 7.55 (1H, d, J 2.5,
ArH), 7.66 (1H, d, J 2.5, ArH). dC{H} (125.8 MHz; C4D8O; 298
K) 29.9 (CMe3), 30.6 (ArCH2NMe), 32.2 (CMe3), 34.0 (CMe3),
35.6 (CMe3), 42.8 (NCH2CH2N), 51.1 (ArCH2N), 123.3 (arom-
CCH2N), 125.7 (arom-CH), 131.2 (arom-CMe3), 135.7 (arom-
CMe3), 138.1 (arom-CH), 165.8 (arom-CO). dC{H} (125.8 MHz;
C7D8; 298 K) 30.0 (CMe3), 31.9 (CMe3), 32.2 (CMe3), 32.4
(CMe3), 34.0 (CMe3), 34.1 (CMe3), 35.3 (CMe3), 35.7 (CMe3),
37.7 (ArCH2NMe), 43.7 (ArCH2NMe), 50.1 (NCH2CH2N), 57.2
(NCH2CH2N), 62.9 (ArCH2N), 64.5 (ArCH2N), 121.9 (arom-
CH), 124.9 (arom-CMe3), 138.1 (arom-CH), 132.9 (arom-CMe3),
158.6 (arom-CCH2N), 168.8 (arom-CO).
1 THF) C 53.4, H 6.7, N 2.2%). nmax/cm-1 1589 s, 1461 vs, 1377 s,
˜
1311 s, 1288 m, 1246 s, 1203 w, 1165 m, 1134 w, 1084 s, 1022 m,
926 w, 895 w, 875 w, 841 m, 810 m, 764 m, 725 w, 694 m, 656 w,
613 w, 571 w, 536 w, 490 w, 459 w (Nujol).
[La(LI)(N(SiMe3)2)(THF)2] (6). H2LI ligand (0.59g, 2.0 mmol)
in THF (20 mL) was added to a THF (20 mL) solution of
[La(N(SiMe3)2)3] (1.24 g, 2.00 mmol) at -40 ◦C via a cannula.
The resulting colourless reaction mixture was stirred gently and
warmed to room temperature (18 h) to afford a pale yellow solution
of 6. Volatiles were removed in vacuo, and the compound was
recrystallized from hexanes at 25 ◦C and dried to give 6 as a pale
yellow powder (0.75 g, 51%). Mp: 218–222 ◦C. (Found: C 52.5,
H 8.8, N 3.7. C32H63LaN2O4Si2 requires C 52.3, H 8.6, N 3.8%).
˜
n
max/cm-1 1604 w, 1307 m, 1261 s, 1199 w, 1153 m, 1088 vs, 1026
vs, 972 w, 933 w, 883 w, 802 s, 725 s (Nujol). dH (500.1 MHz;
t
C4D8O; 348 K) -0.08 (18H, s, SiMe3), 1.30 (9H, s, Bu), 1.45
(9H, s, tBu), 2.02 (3H, s, ArCH2NMe), 2.44 (4H, b, NCH2CH2O),
4.07 (2H, b, ArCH2N), 6.66 (1H, s, ArH), 6.98 (2H, s, ArH).
dH (500.1 MHz; C6D6; 323 K) 0.18 (18H, s, SiMe3), 0.87 (2H, s,
This journal is
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t
ArCH2NMe), 0.88 (3H, s, ArCH2NMe), 1.25 (9H, s, Bu), 1.41
Kinetics analyses (from ROP of L-lactide using 6) were obtained
from room temperature toluene (4.5 mL) solutions of L-LA (50 mg,
0.347 mmol) to which the appropriate amount of catalyst (1 : 300,
0.2313 mM; 1 : 400, 0.1735 mM; 1 : 500, 0.1388 mM; 1 : 600,
0.1156 mM; 1 : 750, 0.0925 mM) in toluene (0.5 mL) was added
via syringe. Aliquots (ca. 0.3 mL) (see Fig. 5–7) were taken from the
reaction mixture and evaporated to dryness until >85% polymer
conversion was attained. 1H NMR spectroscopy (CDCl3) was used
to determine % polymer conversion. GPC analyses were measured
using a Waters 1525 Binary HPLC pump coupled to two 7/8 ¥
(9H, b, tBu), 1.45 (8H, b, THF), 1.67 (1H, b, NCH2CH2O), 1.92
(1H, b, NCH2CH2O), 2.19 (1H, b, NCH2CH2O), 2.50 (1H, b,
NCH2CH2O), 3.60 (8H, b, THF), 7.00 (1H, s, ArH), 7.45 (1H, s,
ArH). dC{H} (125.8 MHz; C4D8O; 298 K) 6.9 (SiMe3), 26.5 (CMe3),
30.8 (ArCH2NMe), 38.8 (CMe3),34.3 (NCH2CH2NMe2), 36.0
(NCH2CH2NMe2), 43.9 (CMe3), 52.4 (CMe3), 64.4 (ArCH2N),
122.7 (arom-CH), 126.5 (arom CMe3), 129.2 (arom-CH), 129.7
(arom-CMe3), 136.3 (arom-CCH2N), 167.9 (arom-CO). dC{H}
(125.8 MHz; C6D6; 298 K) 3.0 (SiMe3), 7.2 (ArCH2NMe), 12.0
(CMe3), 14.7 (CMe3), 21.2 (CMe3), 23.4 (CMe3), 26.0 (THF), 32.3
(ArCH2N), 32.7 (NCH2CH2NMe2), 35.3 (NCH2CH2NMe2), 68.3
(THF), 127.9 (arom-CH), 128.1 (arom-CMe3), 128.2 (arom-CH),
128.7 (arom-CMe3), 129.0 (arom-CCH2N), 162.4 (arom-CO).
R
300 mm Styragelꢀ HR4 columns and a Waters 40 Differential
Refractometer. The eluent was HPLC grade THF running at
1 mL min-1 (sample concentration 0.1% w/v THF). Polystyrene
R
standards were used for calibration of the instrument. Breezeꢀ
software was used for peak analyses and calibration procedures.
GPC was performed on polylactide samples from kinetics studies
and 1 : 100 catalyst–monomer polymerization reactions.
[Sm(LI)(N(SiMe3)2)(THF)] (7). H2LI ligand (0.59 g, 2.0 mmol)
in THF (20 mL) was added to a THF (20 mL) solution of
[Sm(N(SiMe3)2)3] (1.26 g, 2.00 mmol) at -40 ◦C via a cannula.
The resulting reaction mixture was stirred gently and warmed to
room temperature (18 h) to afford a pale yellow cloudy solution of
7. Volatiles were removed in vacuo with the resulting pale yellow
solid being washed twice with hexanes and dried to afford 7 as
a pale yellow powder (0.73 g, 54%). Mp: 217–220 ◦C. (Found: C
50.25, H 8.0, N 4.1. C28H55N2O3Si2Sm requires C 49.9, H 8.2, N
X-Ray crystallography-structure determination
Data were collected using an Oxford Diffraction Gemini diffrac-
tometer (w-scans) with Mo Ka radiation. The structure was
solved by direct methods using Sir-9261 and, following empirical
absorption corrections, was refined with full-matrix least squares
based on F2 using SHELX-97.62 All hydrogen atoms were added
at calculated positions and refined by use of a riding model with
isotropic displacement parameters based on the displacement pa-
rameter of the parent atom. Anisotropic displacement parameters
were employed throughout for the non-hydrogen atoms.
4.15%). nmax/cm-1 1743 w, 1597 m, 1307 s, 1261 s, 1199 w, 1157 m,
˜
1076 s, 1022 s, 972 w, 933 w, 887 w, 837 m, 806 m, 771 w, 725 vs,
601 w, 524 w (Nujol). dH (500.1 MHz; C6D6; 298 K) -1.67 (18H,
t
s, SiMe3), 0.21 (9H, s, Bu), 0.30 (2H, s, ArCH2N), 0.97 (4H, b,
NCH2CH2O), 1.49 (4H, b, THF), 2.00 (9H, b, tBu), 2.32 (3H, b,
ArCH2NMe), 3.56 (4H, b, THF), 7.65 (1H, s, ArH), 8.12 (1H, s,
ArH).
Crystal refinement data
[YbLII]2 (2). C68H108N4O4Yb2.2(C7H8), M = 1575.94, red
plates, monoclinic, space group C2/c, a = 28.8197(4), b =
[Yb(LI)(N(SiMe3)2(THF)] (8). H2LI ligand (0.59 g, 2.0 mmol)
in THF (20 mL) was added to a THF (20 mL) solution of
[Yb(N(SiMe3)2)3] (1.31 g, 2.00 mmol) at -40 ◦C via a cannula. The
reaction mixture was stirred and warmed to room temperature
(18 h) to afford a pale yellow cloudy solution of 8. Volatiles
were removed in vacuo with the resulting pale yellow solid being
washed with hexanes and dried to afford 8 as a pale yellow powder,
which was later recrystallized from toluene at 35 ◦C (0.72 g, 52%).
Mp: 220–224 ◦C. (Found: C 49.0, H 7.8, N 3.4. C28H55N2O3Si2Yb
requires (for 1 THF) C 48.25, H 7.95, N 4.0%); (Found: C 51.65, H
7.3, N 3.6. C31H55N2O2Si2Yb requires (for 1 molecule of toluene)
◦
3
˚
˚
9.0269(1), c = 30.2320(5) A, b = 91.433(1) , V = 7862.47(19) A ;
Dc (Z = 4) = 1.331 Mg m-3; m(Mo Ka) = 2.413 mm-1; specimen:
0.48 ¥ 0.28 ¥ 0.15 mm; Tmin/max = 0.786/1; T 100(2) K, 2qmax
=
63.4◦, 49 880 reflections collected of which 12 724 were unique
(Rint = 0.050); R1 = 0.054, wR2 = 0.067 (all data); R1 = 0.032,
-3
˚
wR2 = 0.064 (I > 2s(I)), GOF = 0.91; |rmax| = 2.1 e A .
Acknowledgements
We are very grateful to Kirtipal Barse and Dr Edward Kolodka,
Josef Beranek and Dr Alena Kubatova at the University of North
Dakota, for assistance with GPC data and TOF MS, respectively.
Financial support from ND EPSCoR program and UND’s
chemistry department is gratefully acknowledged. The work on
TOF MS was supported by the National Science Foundation
under grant no. CHE-0216038.
C 51.9, H 7.7, N 3.9%). nmax/cm-1: 1604 w, 1458 vs, 1377 s, 1307 s,
˜
1273 s, 1242 m, 1203 w, 1165 m, 1126 w, 1080 s, 1045 m, 976 m,
914 w, 891 m, 833 m, 910 m, 771 w, 740 m, 725 m, 644 w, 605 m,
567 w, 605 w, 567 w, 528 w, 440 w (Nujol).
Typical polymerization reactions of L-LA or e-CL
The appropriate quantity of catalyst precursor (e.g. 0.00347 mmol
for L-LA 1 : 100; 0.0044 mmol for e-CL 1 : 100) in toluene
(0.5 mL) was added to a toluene (4.5 mL) solution of L-LA
(50 mg, 0.347 mmol) or e-CL (50 mg, 0.44 mmol) at the requisite
temperature in a Schlenk flask via syringe. After an appropriate
time (see Tables 2 and 3), an aliquot (ca. 0.5 mL) was taken
from the reaction mixture and evaporated to dryness. H NMR
spectroscopy (CDCl3) was used to determine the percentage of
polymer conversion.
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