Rare earth complexes of phenoxy-thioether ligands: Synthesis and reactivity in the ring opening polymerization of cyclic esters

Article abstract of DOI:10.1039/c3dt00066d

The potential of the phenoxy-thioether moieties as ancillary ligands in the coordination chemistry of group 3 metals was explored. The amine elimination reactions between yttrium and scandium amido precursors {M [N(SiHMe ₂)₂]₃ (THF)_n} with the proligands {4,6-tBu₂-OC₆H₂-2-CH₂S(2-R-C ₆H₄)}H [R = H (L₁-H), CH₃ (L ₂-H), Br (L₃-H), CF₃ (L₄-H)], bearing different substituents at the ortho position of the thiophenol aryl ring, were studied. The outcome of aminolysis reactions gave different results depending on the structure of the proligand and the reaction conditions. Heteroleptic scandium and yttrium amido complexes with general formula {(L) ₂M [N(SiHMe₂)₂] (THF)_n} were prevailingly obtained from phenoxy-thioether proligands L₁-H and L₄-H. In contrast, homoleptic yttrium complexes bearing three phenoxy-thioether ligands were favored with L₂-H and L₃-H. The activities of all the synthesized complexes toward the ring-opening polymerization of ε-caprolactone and l- and rac-lactide were investigated, also in combination with an alcohol as an external chain transfer agent. Polyesters with controlled molecular parameters (M_n, end groups) and low polydispersities were obtained. The monoinsertion adduct, produced by the reaction of {(L₁)₂ScN(SiHMe₂)₂} and 1 equiv. of ε-caprolactone, was isolated proving that a coordination-insertion mechanism of ring-opening polymerization was operative. In the polymerization of rac-lactide, yttrium complexes exerted a high degree of stereocontrol producing heterotactic polylactides (P_r up to 0.91).

Full text of DOI:10.1039/c3dt00066d

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Page 1 of 33
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Rare Earth Complexes of PhenoxyꢀThioether Ligands: Synthesis and
Reactivity in the Ring Opening Polymerization of Cyclic Esters.
Mina Mazzeo,* Roberta Tramontano, Marina Lamberti, Alessia Pilone, Stefano Milione and Claudio
Pellecchia*
Department of Chemistry and Biology, University of Salerno. Iꢀ84084, via Ponte don Melillo,
Fisciano, Salerno, Italy.
Corresponding authors: mmazzeo@unisa.it, cpellecchia@unisa.it
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Abstract
The potential of the phenoxyꢀthioether moieties as ancillary ligands in the coordination chemistry of
group 3 metals was described. The amine elimination reactions between yttrium and scandium
amido precursors {M [N(SiHMe2)2]3 (THF)n } with the proligands {4,6ꢀtBu2ꢀOC6H2ꢀ2ꢀCH2S(2ꢀRꢀ
C6H4)}H [R = H (L1ꢀH), CH3 (L2ꢀH), Br (L3ꢀH), CF3 (L4ꢀH)] bearing different substituents at the
ortho position of the thiophenol aryl ring, were studied. The outcome of aminolysis reactions gave
different results depending on the structure of the proligand and the reaction conditions.
Heteroleptic scandium and yttrium amido complexes with general formula {(L)2M [N(SiHMe2)2]
(THF)n} were prevailingly obtained from phenoxyꢀthioether proligands L1ꢀH and L4ꢀH. On the
contrary, homoleptic yttrium complexes bearing three phenoxyꢀthioether ligands were favored with
L2ꢀH and L3ꢀH. The activities of all the synthesized complexes toward the ringꢀopening
polymerization of εꢀcaprolactone and Lꢀ and racꢀlactide were investigated, also in combination with
an alcohol as external chain transfer. Polyesters with controlled molecular parameters (Mn, end
groups) and low polydispersities were obtained. The monoinsertion adduct, produced by the
reaction of {(L1)2ScN(SiHMe2)2} and 1 equiv of εꢀcaprolactone, was isolated proving that a
coordinationꢀinsertion mechanism of ringꢀopening polymerization was operative. In the
polymerization of racꢀlactide, yttrium complexes exerted high degree of stereocontrol producing
heterotactic polylactide (Pr up to 0.91).
Introduction
Aliphatic polyesters derived from renewable resources have recently attracted considerable
attention as biocompatible and biodegradable alternatives to conventional oilꢀbased polymers.1ꢀ4
Among them, poly(εꢀcaprolactone) (PCL) and poly(lactide) (PLA) are thermoplastic polymers that
efficiently combine the filmꢀbarrier properties of polyesters with the mechanical performance of
polyolefins and thus they have been successfully employed in biomedical and pharmaceutical
applications such as controlled drug delivery, medical implants and scaffolds for tissue
engineering.5ꢀ7 More recently, the ability to obtain polyesters with better defined microstructures
and tunable chemical and physical properties broadened the range of their possible applications to
different industrial areas such as packaging and fibers.8
2

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Ringꢀopening polymerization (ROP) of cyclic esters by singleꢀsite metalꢀbased catalysts is the most
efficient synthetic approach to obtain polymers with controlled molecular weight, narrow
molecular weight distribution and desired tacticity.9, 10 Among the large variety of metal complexes
investigated as initiators, group 3 metal complexes have been shown to be particularly interesting
because of their very high activity, low toxicity and good control.11
Homoleptic alkoxides and aryloxides of group 3 and lanthanide metals are very active initiators for
the ringꢀopening polymerization of lactones and lactides but they are frequently rather difficult to
characterize and poorly soluble, showing, in some cases, low control of the polymerization
reactions.12ꢀ14 These polymerizations have some living character, but large molecular weight
distributions of polymers are obtained as a consequence of frequent transesterification reactions. To
overcome these limitations heteroleptic complexes of the type [LnMX], where L is an ancillary
ligand and X is an initiating group have been developed. Sterically demanding ligands added to
metal centre generally prevent the coordination of the growing chains to the metal and, therefore,
reduce inter and intramolecular transesterification reactions allowing a more efficient control on the
molecular weights of the polymeric products.15ꢀ32
Recently, several group 3 metal complexes bearing sterically encumbered ligands have been
synthesized and studied for the controlled ROP of ε-CL and lactide.33ꢀ36 They exhibit high
polymerization rates and some of them are able to exert tacticity control.37ꢀ42Among these, group 3
metal complexes with phenoxyꢀbased ligands represent the most abundant examples. Due to the
hard Lewis acidic nature of the group 3 metal complexes, phenoxide donors tend to be matched
with relatively hard firstꢀrow nitrogenꢀbased donors.43ꢀ48
In recent studies new coordination environments have been explored, and there is a growing
evidence that softer secondꢀrow donors may offer beneficial stabilization of highly reactive metal
centres.49,50
Okuda et al. reported that rare earth metal complexes bearing 1,ωꢀdithiaalkanediylꢀbridged
bis(phenolato) ligands efficiently initiate the ringꢀopening polymerization of lactides.51 By variation
of the ligand architecture, moderate to high heteroselectivity for the ROP of racꢀLA have been
obtained.36,52
We recently reported the synthesis of a new class of bidentate phenoxyꢀthioether ligands for the
synthesis of group 4 metal53 and aluminum complexes.54 Aiming at extending the use of these
ligands to the coordination chemistry of group 3 metals, in this paper we report the synthesis and
coordination modes of scandium and yttrium amide complexes whit phenoxyꢀthioether ligands. The
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obtained complexes, characterized by NMR analysis, were tested as initiators in the ringꢀopening
polymerization of the cyclic esters, such as ꢀcaprolactone and lactides (Lꢀ and rac).
Results and Discussion
Synthesis and characterization of the metal complexes.
Since group 3 metals are highly Lewis acidic, their complexes are prone to ligation by coordinating
solvents or to the formation of dimeric structures. As a consequence of this tendency to aggregate,
low energy pathways for ligands redistribution around the metal centre are frequently observed.55
Thus the design of an opportune coordinative environment by varying the steric and electronic
properties of the ancillary ligands is necessary to form heteroleptic stable complexes. Significant
differences are often observed in the coordination chemistry at the metal centre as consequence of
minimal variations of the ligand structure.
To explore the potential of the phenoxyꢀthioether moieties as ancillary ligands in the coordination
chemistry of group 3 metals, the amine elimination reactions between the proꢀligands L1ꢀ4ꢀH (see
scheme 1) and the silylamido metal precursors {M [N(SiHMe2)2]3 (THF)n } (M = Sc, n = 2; M = Y,
n = 3) were studied.
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R
THFn
S
{M[N(SiHMe2)2]3(THF)n}
M
tBu
O
N(SiHMe2)2
2
R
S
1-a M = Sc n=0 R= H
1-b M = Y n=1 R= H
4-b M = Y n=1 R= CF3
tBu
tBu
OH
tBu
L1-H; R=H
L2-H; R=CH3
L3-H; R=Br
L4-H; R=CF3
R
{Y[N(SiHMe2)2]3(THF)3}
S
Y
tBu
O
3
2-b R= CH3
3-b R= Br
tBu
Scheme 1. Structures of L1ꢀ4–H protio ligands and synthesis of the group 3 metal complexes.
Interestingly, the outcome of aminolysis reactions gave different results depending on the structure
of the proligand.
Scandium complex 1ꢀa: synthesis and characterization.
Heteroleptic bis(phenoxythioether) scandiumꢀamido complex 1ꢀa was prepared by direct aminolysis
of the scandium silylamide precursor {Sc [N(SiHMe2)2]3(THF)2 } with two equivalents of the
proligand L1ꢀH (Scheme 1) at room temperature, equally in benzene or in hexane solution. Removal
of the volatiles under reduced pressure yielded a white solid that was washed with a minimum
amount of cold hexane to obtain the product in high yield (>90%). It showed a very high stability in
aromatic and aliphatic solvent solution at room temperature and, as a solid compound, it could be
stored at room temperature for prolonged periods without any decomposition.
Complex 1ꢀa was characterized by multinuclear NMR spectroscopy and elemental analysis, which
indicated a pentacoordinate metal centre bearing two phenolato ligands and one
bis(dimethylsilyl)amido group
.
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At room temperature, the 1H NMR spectrum of 1ꢀa in C6D6 showed a single set of signals for the
two coordinated ligands, a single doublet for methyl protons and one septet for the proton of the
SiHMe2 amido fragment. The chemical shift for the SiꢀH (δ 4.73 ppm) proton, shifted upfield with
respect to the chemical shift in the {Sc [N(SiHMe2)2]3(THF)2} (δ 5.11 ppm), suggested the presence
of a β(SiꢀH) agostic interaction with the scandium centre. The coordination of the sulphur neutral
donor of the chelating ligand to the metal centre was suggested by the downfield shift of the SCH2
protons (4.23 ppm) observed in complex 1ꢀa compared to the corresponding signal of the free
ligand (see Table 1). The SCH2 protons appear as a single sharp resonance indicating a symmetry
higher than that conceivable for a pentacoordinate complex with such a coordination environment,40
suggesting a fast fluxional process on the NMR timescale (e.g. intramolecular rearrangements
and/or monomer/dimer equilibration).
To assess the presence of symmetric dimeric structures in solution, complex 1ꢀa was analyzed by
2D DOSYꢀ1H NMR measurements (Diffusion Ordered SpectroscopY).56 This NMR technique
provides information of the translational diffusion coefficient of the molecular species in solution
and it has been successfully used for characterizing various organometallic systems57ꢀ59 and
supramolecular structures in solution.60, 61
The PGSE (Pulsed field Gradient SpinꢀEcho) method assesses translational motion of the analyte in
solution through determination of the diffusion coefficient D, a quantity inversely related to its
effective hydrodynamic volume. The hydrodynamic radium, determined for 1ꢀa from the 2D DOSY
experiment (rH = 7.16 Å), was compared with the molecular radium (r′= 6.87 Å) estimated from DFT
optimized structure by measuring the lengths between centers of distant hydrogen atoms on the
molecular periphery. The experimental values of D and rH and the calculated ones for complex 1ꢀa
are listed in Table S1 of the Supporting Information. A good agreement was observed, supporting
the hypothesis of the monomeric nature of 1ꢀa in solution. This result suggests that the high
symmetry observed in solution at room temperature for complex 1ꢀa is related to intramolecular
rearrangements.
Despite several attempts, we could not obtain single crystals of complex 1ꢀa suitable for an Xꢀray
structure analysis. To propose reasonable structures and explain the fluxional behavior suggested
for this complex by the NMR analysis, DFT calculations were performed.
For a monomeric pentacoordinate complex, such as scandium complex 1ꢀa binding two nonꢀ
symmetrical bidentate ligands and one monodentate ligand, different isomers are possible (Scheme
2). In the optimized structure of diastereoisomer A, the Sc atom adopts a distorted trigonalꢀ
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bipyramidal coordination geometry in which the axial positions are occupied by two sulphur atoms
and the equatorial positions are occupied by two oxygen atoms and the amido ligand. The ScꢀO
(1.94 Å, 1.95 Å) and ScꢀS (2.77 Å, 2.79 Å) bond lengths can be favorably compared with the
corresponding distances in related Sc complexes featuring OSSO bisphenolato ligands.51 This
indicates that the quantum chemical methods used faithfully model the overall geometry for this
class of complexes. Any attempt to obtain minimum energy structures for diastereoisomers B and D
led to diastereoisomer A. Attempts to obtain minimum energy structure for diastereoisomer C led to
a highly distorted tetrahedral species in which one OS ligand employs only the anionic oxygen
donor to bind the metal centre. The formation of this species is thermodynamically unfavored due to
the high internal and free energies (these values, in gas phase and benzene, are reported in Table
S2). Minimum energy structure for diastereoisomer E was successfully located by DFT
calculations, the freeꢀenergy difference with respect to diasteroisomer A is 4.2 kcalmolꢀ1 in favor of
isomer A. The small energy difference between A and E suggests that these isomers can be
simultaneously present in solution.
S
Sc
S
O
Sc
O
S
Sc
O
O
S
O
S
X
X
X
O
S
A
B
C
X
X
S
Sc
O
S
S
Sc
S
O
O
O
D
E
Scheme 2. Possible stereoisomers for Sc complex 1ꢀa. For the chiral structures A–C, only one of
the two enantiomers is shown.
1H NMR monitoring from 20 °C to ꢀ80 °C for 1ꢀa, in methylene dichlorideꢀd2 solution,
was performed to get more information on this issue (see figures S1 and S2 in the Supporting
Information). Decreasing the temperature of the sample below room temperature, the spectra
broaden and become more complex, effectively indicating the existence of a fluxional process.
Despite the complexity of some areas of the spectra, valuable information was obtained from a
careful analysis of these results. At 193 K the presence of two patterns of signals with different
intensities indicated the coexistence of two species at a ratio 4:1. For each species, two signals for
the methyl protons of the amido group, two signals for the methyl protons of the tertꢀbutyl groups
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and an AB pattern for the protons of the bridging methylene were observed. The described patterns
are consistent with the symmetries predictable for stereoisomers A and E in scheme 2, (C2 and CS
symmetry, respectively). Taking into account the results of DFT calculations we propose the
existence of an equilibrium between these isomers. The interconversion between these species
should occur by the moving of the sulphur donors from axial to equatorial positions.
Yttrium complexes: synthesis and characterization.
Treatment of proligands L1ꢀ4ꢀH with the yttrium precursor {Y [N(SiHMe2)2]3(THF)3} resulted in
different products depending on the structure of the proligand and on the reaction conditions. In all
cases, an indication that the proligands behave as bidentate ligands, i. e. the neutral sulphur atoms
are coordinated to the metal centre, comes from the downfield shift of the signals of the protons of
the methylene bridge with respect to those of the corresponding free ligands; this is consistent with
a decrease in electron density due to coordination at the metal centre. These values are reported in
Table 1.
Table 1. Chemical shift values for the methylene group SCH2 in the free ligands and in the
synthesized yttrium complexes.
LnꢀH
3.76
3.73
3.77
3.72
(Ln)2YNR2
(Ln)3Y
n = 1
n = 2
n = 3
n = 4
4.11
ꢀ
ꢀ
4.05
4.09
ꢀ
ꢀ
4.17; 4.36
Spectroscopic studies and elemental analyses for yttrium complex 1ꢀb were coherent with a
structure within the metal centre was coordinated to two phenolato ligands, one
bis(dimethylsilyl)amido group and one THF molecule. The interaction between the coordinated
THF and the metal centre resulted quite strong: in fact solvent dissociation was not achieved, even
after evaporation in vacuo for prolonged times.
The 1H NMR spectrum of 1ꢀb in C6D6 recorded at room temperature showed a single set of signals
for the two coordinated ligands, equivalent on the NMR timescale, as well as one doublet for
methyl protons and one septet for the proton of the SiHMe2 amido fragment. The resonances
relative to the protons of the coordinated THF molecule at 3.63 ppm and 1.28 ppm resulted shifted
downfield with respect to the free THF. The presence of a weak
β
(SiꢀH) agostic interaction is
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evidenced by an upfield shift of the SiꢀH proton (δ 4.73 ppm) compared to the chemical shift (δ
5.00 ppm) of the yttrium precursor.62, 63 The peak of the SCH2 protons was broad. Raising the
temperature to 50 °C, all peaks resulted sharp, highlighting the existence of a fluxional behavior
also for complex 1ꢀb. Decreasing the temperature of the sample below room temperature, resulted
in complex spectra which could not provide further information.
For 1ꢀb different stereoisomers are plausible (Scheme 3). DFT calculations were performed
to propose reasonable structures.
THF
Y
THF
Y
S
Y
O
S
Y
O
O
Y
O
S
Y
S
S
S
O
S
S
THF
N
N
O
S
O
S
THF
N
THF
N
S
O
O
O
O
THF
S
O
N
N
J
I
F
G
H
K
Scheme 3. Representative stereoisomers for the yttrium complex 1ꢀb.
In the optimized structures of the diasteroisomer F, the yttrium atom binds two bidentate OS
ligands, one amido group and one coordinated THF molecule adopting a distorted octahedral
geometry. The THF is located cis to amido ligand and trans to one of the sulphur atoms; the two
oxygen donors of the ligand are arranged trans to each other (OꢀYꢀO 151.8°). The Y–N and YꢀO
bond lengths are comparable to the average bond lengths in similar sixꢀcoordinated Y complexes
with OSSO bisphenolato ligands.52 The two Y–S bond lengths differ from each other by about 0.22
Å, with an elongated Y–S bond trans to the amido group. This can be ascribed to a stronger
electronꢀdonating ability of the amido group that weakens the opposite Y–S bond.
Minimum energy structures for the diastereoisomers GꢀK were successfully located by DFT
calculations as well. The corresponding internal and free energies in gas phase and benzene are
reported in Table S2. Diasteroisomers F and G have very similar energies and can be considered as
the most stable species. Diasteroisomers H and I show slightly higher energies with respect to F.
The diasteroisomers J and K are thermodynamically less favored.
It is reasonable to presume that diasteroisomers F and G are present in solution. The number of
possible isomers could be even higher as the reversible THFꢀdissociation cannot be ruled out. This
would lead to a five coordinated species that can adopt different configurations. All these species
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could rapidly rearrange in solution at room temperature leading to the observation of an unique
species with an average high symmetry.
This is in line with the complicated scenario showed by VT NMR analysis and with numerous
literature examples describing fluxional phenomena for rare earth metal complexes,64, 65 even more
frequently observed when a coordinating solvent is included in the coordination sphere.
The reaction between yttrium amide and the proligand L2ꢀH produced a mixture of three products:
the homoleptic trisꢀphenolate complex 2ꢀb (Scheme 1) and two heteroleptic amide yttrium
stereoisomers bearing two phenoxyꢀbased ligands, an amide group and a THF molecule. The
composition of the mixture depends on the reaction conditions, but in all cases the homoleptic
complex was achieved as the prevailing product with a yield ranging from 60%, when the reaction
was performed in THF, to 67% when hexane or benzene was used as reaction solvent. The mixture
composition did not change increasing the temperature until 70 °C or recording the NMR spectrum
in more coordinating solvents such as THFꢀd8. Several attempts to separate the components of the
mixture allowed to isolate exclusively the homoleptic complex 2ꢀb as pure compound.
Likewise, the reaction between two equivalents of the L3ꢀH proligand and the amide yttrium
precursor, performed in hexane or in THF, furnished the homoleptic trisꢀphenolate complex 3ꢀb as a
single product (Scheme 1).
Reasonably, the monoamido complexes first formed, gradually react with the residual proligands
furnishing the homoleptic products. This behavior could be possibly attributed to the instability of
the low coordinated monoamido complexes, already observed for Schiff base group 3 metal
complexes.45
Next, the homoleptic yttrium complexes 2ꢀb and 3ꢀb were purposely prepared by reaction of the
yttrium precursor with three equivalents of the opportune proligand in benzene solution at room
temperature, and characterized by NMR spectroscopy and elemental analysis.
1
The H NMR spectra of the homoleptic complexes 2ꢀb and 3ꢀb suggest a high symmetry for these
species. The coordinated ligands appear equivalent giving rise to a unique pattern of resonances
consistent with an octahedral C3ꢀsymmetric structure resulting from the κ2ꢀO,S coordination of the
three ligands at the metal centre.
Different results were obtained by using proligand L4ꢀH. In this case an asymmetric heteroleptic
amide yttrium complex was achieved as the prevailing product (92%). The component present in
minor percentage (about 8% ) was a symmetric homoleptic species. The composition of the mixture
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did not change after treatment at higher temperature (50 °C). For complex 4ꢀb β(SiꢀH) agostic
interaction with the yttrium centre was not observed.
The behavior of these four phenoxyꢀthioether proligands in the coordination chemistry of yttrium
complexes was tentatively rationalized as follows. As monoanionic bidentate ligands they tend to
saturate the electronic and coordinative requests of the metal by forming homoleptic yttrium
complexes. In the case of complex 1ꢀb the β(SiꢀH) agostic interaction would stabilize the
heteroleptic monoamido complex hampering the chelation of a third ligand. Similarly, for complex
4ꢀb a YꢀF interaction66 would led to the same result, in this latter case the size of the CF3
substituents could reasonably play a role as well.
Unfortunately, several attempts to grow single crystals suitable for Xꢀray analysis were
unsuccessful, due to the extreme solubility of the compounds even in nonꢀpolar solvents.
Ringꢀopening polymerization of εꢀ caprolactone.
The reactivity of new bis (phenoxyꢀthioether) scandium and yttrium amide complexes (1ꢀa, 1ꢀb and
4ꢀb) in the ringꢀopening polymerization of εꢀCL was studied (scheme 4). Polymerization screenings
were performed, under a nitrogen atmosphere, in a toluene or THF solution of εꢀCL and the proper
metal complex. The polymers, precipitated from the reaction solution by addition of hexane, were
analyzed by NMR and gel permeation chromatography (GPC).
The main results of the polymerization studies are summarized in Table 2.
O
O
O
O
*
*
Scheme 4. Ringꢀopening polymerization of εꢀcaprolactone
The scandium complex 1ꢀa revealed to be an efficient initiator for the ROP of ꢀCL
showing a strong dependence of the reactivity on the reaction conditions such as temperature and
solvent. At high temperature (70°C) 1ꢀa allowed a quantitative conversion of 500 equivalents of
monomer in about 5 mins (run 1, Table 2) whereas one hour was required when the polymerization
was performed at 25 °C (run 2, Table 2).
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The yttrium complex 1ꢀb resulted a more active initiator for the polymerization of εꢀCL than the
related scandium complex 1ꢀa (cf runs 2 and 3, Table 2). The monomer was polymerized rapidly
and quantitatively at 25 °C in toluene in 30 minutes. This is consistent with literature data that
report the complexes of larger/bigger rare earth metals normally show higher activities.51, 67ꢀ69 This
behavior may be due to the increase in ionic radii from Sc → Y and to the consequent less steric
crowding created by the two stationary ligands around the active metal centre, which would favour
advantage the coordination of the incoming εꢀcaprolactone monomer, a necessary step required
prior to ringꢀopening.
Lower catalytic activities were observed when the polymerization runs were performed by
using THF as the solvent (runs 4 and 5, Table 2). In this case after 1 hour a conversion of 30 % was
observed and the quantitative consume of 500 equivalents of monomer was achieved only after 4h.
This decrease of activity can probably be accounted for the competing role of THF versus the
monomer in the coordination to the oxophilic metal centre, thus ultimately resulting in a slower
propagation process. This solvent effect has been observed for several initiators although an
opposite effect was reported for the dithiaalkanediylꢀbridged bis(phenolate)70 and the alkoxyꢀamino
bridged bis(phenolate) rareꢀearth metal amido systems.71
The GPC analyses of the polymers obtained by both complexes 1ꢀa and 1ꢀb showed
monomodal distributions of the molecular weights and polydispersity indexes (Mw/Mn) moderately
narrow ranging from 1.31 to 2.08. The numberꢀaverage molecular weights (MnGPC) increased
monotonously with the monomer/initiator ratio and the monomer conversion (see Figure S3 of
Supporting Information) but were systematically higher than those calculated on the assumption
that a single PCL chain is produced per metal centre. These are among the highest molecular
weights observed for rareꢀearth metal εꢀcaprolactone polymerization.72ꢀ75
These observations suggest that only a fraction of the metal complexes was involved in the
polymerization reaction, most likely as a result of an inefficient initiation by the poor nucleophilic
amide group.76 In fact, it is commonly observed that amido initiators such as ꢀN(SiMe3)2, and ꢀ
N(SiHMe2)2 are less efficient initiating groups for the ROP of cyclic esters at room temperature
than alkoxide derivatives. Thelateer are expected to be better initiators due to reduced steric
hindrance and higher nucleophilicity of alkoxide initiating groups. Alkoxide initiators can be
generated in situ by alcoholysis of amido complexes with alcohols. At the same time, the alcohol
present in excess can convert the growing chains into “dormant” hydroxyl endꢀcapped
(macro)molecules allowing the growing of new chains on the metal centres. If the exchange
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equilibria between “active” growing chains (i.e. coordinated to the metal centre) and the “dormant”
chains are rapid enough compared to the initiation and subsequent propagation steps, an efficient
control on the molecular weights is obtained.77
Table 2. RingꢀOpening Polymerization of εꢀCL initiated by complexes 1ꢀa, 1–b and 4ꢀb
aRun Cat iPrOH Solvent Temp Time bConv cMnth dMnGPC dMw/Mn
(eq)
(°C) (min) (%) (x 103) (x 103)
1
2
ꢀ
ꢀ
Tol
Tol
Tol
THF
THF
Tol
Tol
Tol
Tol
Tol
70
25
25
25
25
25
25
25
25
25
5
100
100
100
30
57.1
57.1
57.1
17.1
40.0
16.8
14.0
5.7
207.6
242.2
150.5
50.8
311.8
14.1
13.7
6.7
1.74
1.82
2.08
1.31
1.90
1.07
1.06
1.07
1.08
1.09
1ꢀa
1ꢀa
1ꢀb
1ꢀa
1ꢀb
1ꢀa
1ꢀb
1ꢀb
1ꢀb
4ꢀb
60
30
60
30
10
10
15
2
3
4
ꢀ
ꢀ
5
70
6
2
59
7
4
100
100
87
8
10
2
e9
e10
99.3
87.4
2
2
98
111.8 121.6
aAll reactions were carried out with in 4 mL of solvent, [Ι] 0= 3.12 mM, and [εꢀCL]0/[I]0=500. bConversion of εꢀCL as
c
determined by 1H NMR spectral data. Experimental Mn (in gmolꢀ1) and Mw/Mn (PDI) values were determined by GPC
in THF using polystyrene standards and corrected using the factor of 0.56. d Mnth (in gmolꢀ1) =114,14 x ([εꢀCL]0/[I+
iPrOH ]0) x conversion εꢀCL. eToluene = 6 mL; [CL]/[Y] = 1000.
Polymerization experiments were conducted by adding different equivalents of alcohol (runs 6ꢀ8,
Table 2). All the obtained PCLs showed unimodal molecular weight distributions with very narrow
polydispersity indexes (Mw/Mn = 1.06 – 1.09) and Mn values inversely proportional to the quantity
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of added isopropanol. As expected, all metal sites resulted active in this case as testified by the good
agreement between the experimental molecular weight values and the theoretical ones.
The activities of complexes 1ꢀb and 4ꢀb in the polymerization of ꢀCL were compared under
optimized reaction conditions (runs 9 and 10, Table 2). Complex 4ꢀb showed the highest activity
with turnover frequencies (TOF) of 28500 hꢀ1. The introduction of an electronꢀwithdrawing group
on the phenyl ring bound sulphur atom resulted in a beneficial effect on the catalytic activity likely
due to the increase of the electrophilicity of the metal. 47
Mechanism and kinetic analyses of ringꢀopening polymerization of εꢀcaprolactone by 1ꢀa and
1ꢀb.
To get more information about the mechanism of polymerization of εꢀCL promoted by this class of
complexes the reaction between complex 1ꢀa and 1 equivalent of monomer was followed by NMR
spectroscopy. The Sc–caprolactone adduct was formed quantitatively confirming that the reaction
proceeds via the wellꢀestablished coordinationꢀinsertion mechanism involving the labile silylamide
ligand as the initiating group and the cleavage of acyl-oxygen bond of the monomer. This
experiment also established that both phenolate ligands remain bound, as spectator ligands, to the
reactive metal centre during the polymerization reaction.
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1
Figure 1. H NMR spectrum (400 MHz, C6D6, 298 K) of the 1aꢀCL adduct formed by addition of
1 equiv. of ꢀCL to a solution of 1ꢀa.
We also determined the hydrodynamic volume of the propagating specie 1aꢀCL by DOSY NMR
experiments.78 The latter indicated that the propagating species, formed by reaction of 1ꢀa and 1
equivalent of εꢀcaprolactone, has a monomeric structure in aromatic solvents, as observed for the
scandium precursor 1ꢀa, ruling out the hypothesis of aggregation phenomena of the active species
occurring during the polymerization.
1H NMR analysis of a low molecular weight PCL sample obtained by carrying out a polymerization
experiment in the presence of 10 equivalents of isopropanol (run 8, Table 2) disclosed the presence
of isopropyl ester end groups (ꢀCOOCH(CH3)2; 1.28 ppm), generated via insertion of the monomer
unit into the YꢀO CH(CH3)2 bond, and hydroxyl end groups (CH2CH2OH; 3.62 ppm), generated by
hydrolysis of the growing chain (see figure S7 of Supporting Information). These data suggest that,
in the presence of isopropanol, the terminal alkoxide OiPr group is the only initiating moiety
therefore a coordinationꢀinsertion mechanism should be operative also under these polymerization
conditions.
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Detailed kinetic studies on 1ꢀa and 1ꢀb were used to determine the reaction order in monomer. The
percentage of conversion of monomer was evaluated by 1H NMR spectroscopy and was monitored
until at least 90% conversion to polycaprolactone was achieved.
3500
3000
2500
2000
1500
1000
500
0
0
200
400
600
800
1000
1200
1400
time (sec)
Figure 2. Pseudoꢀfirstꢀorder kinetic plots for ROP of εꢀcaprolactone promoted by complexes 1ꢀa
(○) and 1ꢀb (●). Pseudoꢀfirstꢀorder rate constants are kapp = (2.38 ± 0.09) × 10ꢀ3 sꢀ1 R = 0,9938 and
kapp = (1.61 ± 0.03) ×10ꢀ2 sꢀ1 R=0.99883 respectively. ([I]0 = 6.0 × 10ꢀ3 M; [ ꢀCL]0/[I]0= 100;
C7D8 as solvent; T= 298 K).
The reaction kinetics featured a pseudoꢀfirstꢀorder dependence in εꢀCL concentration, as reported in
Figure 3: the semilogarithmic plots of ln([εꢀCL]0/[ε−CL]t) vs. time were linear with a slope of kapp
(2.38 ± 0.09) × 10ꢀ3 sꢀ1 and (1.61 ± 0.03)×10ꢀ2 sꢀ1 for 1ꢀa and 1ꢀb respectively.
=
A kinetic study on complex 1ꢀa in THF as solvent was also performed. The reduced productivity
showed by 1ꢀa in the presence of a more coordinating was expressed by the lower kapp value (8.52 ±
0.32.) ×10ꢀ3 minꢀ1 (see figure S4 of Supporting Information).
Ringꢀopening polymerization of Lꢀ and rac−lactide
Complexes 1ꢀa, 1ꢀb and 4ꢀb were also tested as initiators for the ROP of Lꢀ and racꢀlactide (scheme
5) under a variety of experimental conditions. Polymers were characterized by NMR and GPC.79
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Scheme 5. Ringꢀopening polymerization of Lꢀ and racꢀlactide
Table 3. RingꢀOpening Polymerization of Lꢀ and racꢀLA by 1ꢀa ,1ꢀb and 4ꢀb.
aRun cat
Mon Solvent Time Temp Yield bMnGPC cMnth bPDI dPr
(h)
(°C)
(%)
(×103) (×103)
(%)
11
12
13
14
LꢀLA
LꢀLA
Tol
Tol
24
25
30
8.2
13.2
3.0
8.6
1.40
1.56
1.42
ꢀ
1ꢀa
1ꢀa
1ꢀb
1ꢀb
1
24
2
70
25
25
25
25
43
83
70
98
91
9.3
ꢀ
ꢀ
LꢀLA
Tol
2.4
racꢀLA
THF
THF
THF
17.1
3.1
20.2
2.8
1.78 0.78
1.06 0.78
1.06 0.91
e15 1ꢀb racꢀLA
16 racꢀLA
1
2
28.4
26.2
4ꢀb
aAll reactions were carried with [I]0 = 5 mM and [LA] 0 = 1 M in 2 mL of solvent. bExperimental Mn (corrected using
factor 0.58) and PDI values were determined by GPC analysis in THF using polystyrene standards. cCalculated Mn of
PLA (in gmolꢀ1) =144,14 x ([LA]/[I+ iPrOH]) x conversion LA. dPr is the probability of racemic linkages as
determined by 1H NMR homodecoupling experiments. e [iPrOH]0/[1b]0 =10.
Representative data are listed in Table 3 and show that the polymerization results were dependent
on the ligand structure and the nature of the metal.
The scandium complex 1ꢀa resulted poorly active in the ROP of Lꢀlactide, promoting the
conversion of about 50 equiv of monomer within 24 hour at 25 °C. As expected, at higher
temperatures, the polymerization proceeded at a larger rate (cf. runs 11 and 12, Table 3).
Compared with the scandium complex 1ꢀa, the yttrium complex 1ꢀb displayed higher activities (cf.
runs 11 and 13, Table 3) in toluene solution.
All polymerizations proceeded in a controlled fashion, leading to polymers with monomodal and
quite narrow molecular weight distributions (Mw/Mn = 1.4−2.1) . At variance with the
polymerization of ꢀCL, the experimentally determined values for the numberꢀaveraged molecular
weight well fit with those calculated on the basis of initial monomerꢀtoꢀinitiator ratio and
conversion, even in the absence of alcohol. The more efficient control of LA polymerization
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Is reasonably due to the lower propagation rate.
Kinetics studies showed that polymerizations of LꢀLA by 1ꢀa and 1ꢀb follow a first order
dependency on the concentration of monomer with kapp = (1.05 ± 0.06) ×10ꢀ2 minꢀ1 and kapp
=
(5.92.± 0.04) × 10ꢀ2 minꢀ1 respectively (see Figures S5 and S6 of the Supporting Information).
A higher activity was observed when the polymerization experiment was performed in THF (run
14, Table 3). The usual trend observed in the ROP of lactide is that reaction times are shorter in
apolar, nonꢀcoordinating solvents, such as toluene, than in THF. The opposite trend showed by
complex 1ꢀb is observed much more seldom.72
In the presence of 10 equiv of isopropanol the activity of the catalytic system increased further (cf
runs 14 and 15, Table 3). 1H NMR spectroscopic analysis of a lowꢀmolecularꢀweight sample of
PLLA, obtained by using 20 equiv of LꢀLA and 4ꢀb/iPrOH = 1:5, show PLA chains with
exclusively iPrOC(O) and HOCHꢀ(CH3)CO endꢀgroups, thus supporting the transfer process of an
effective “immortal” polymerization (see Figure S8 of Supporting Information). The ESIꢀMS
analysis of the obtained low molecular weight sample confirmed that only linear oligomers of the
type Hꢀ[OCH(CH3)C(=O)]2nꢀOCH(CH3)2 were formed. Less intense peaks consistent with linear
oligomer chains with carboxylic acid end group, presumably arising from the hydrolysis of Hꢀ
[OCH(CH3)C(=O)]2nꢀ OH were also observed. No cyclic oligomers were detectable (see figure S9
of Supporting Information).
Comparing the activity showed by the yttrium complexes 1ꢀb and 4ꢀb (cf runs 14 and 16) emerges
that complex 4ꢀb showed the highest activity (cf runs 14 and 16) in agreement with the behavior
observed in the polymerization of ꢀCL.
The microstructure analysis of the obtained polymers revealed that the ligand sphere exerts a
significant influence on the tacticity of the polymer chains. Both complexes showed substantial
heterotactic selectivity with a maximum Pr of 0.91 observed for the yttrium complex 4ꢀb (Figure 3).
This more efficient control should be due to the steric demand of the orthoꢀsubstituent in the
thioether unit.
The stereochemistry of racꢀlactide enchainment was evaluated by comparing the integrals in the
homonuclear decoupled 1H NMR spectrum, at the tetrad level, with calculated values by using
Bernoullian statistic (see Table 4). The good agreement observed confirms that the stereocontrol is
due to a chain end control mechanism, whereby the stereochemistry of the last inserted LA in the
growing chain induces the selection of the incoming enantiomeric monomer.
The thermal analysis (DSC) of this sample revealed a Tg of 49.9 °C ( see Figure S10 of Supporting
Information)
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Figure 3. Methine region of the homonuclear decoupled 1H NMR spectrum (400 MHz, CDCl3, 298
K) of the PLA sample obtained by complex 4ꢀb in THF at 25 °C (run 16 in Table 3).
Table 4. Tetrad probabilities based on Bernoullian Statistic (Th) for a Pr of 0.91 and experimental
values (Exp) as obtained by NMR analysis.
Tetrad
Formula
Exp
Th
2
[mmm]
Pm +PrPm/2
0.05
0.05
[mmr]
[rmm]
[rmr]
PrPm/2
0.04
0.04
0.42
0.45
0.04
0.04
0.41
0.46
PrPm/2
Pr2/2
[mrm]
(Pr2+PrPm)/2
Reactivity of homoleptic yttrium complexes 2ꢀb and 3ꢀb
Homoleptic complexes 2ꢀb and 3ꢀb were tested as catalysts for the ringꢀopening polymerization of
εꢀcaprolactone and racꢀlactide. Different reaction conditions were explored, some representative
results are reported in Table 5.
Both complexes resulted active in the ROP of the mentioned cyclic esters.
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In the polymerization of εꢀcaprolactone, the performance of complex 3ꢀb was strongly dependent
on the reaction conditions, such as the temperature. At high temperature (70 °C), a conversion of
450 equivalents of monomer in one hour (run 17, Table 5) was observed. At room temperature a
modest activity was detected, significantly lower than that showed by complex 2ꢀb under the same
reaction conditions (cf runs 18 and 19, Table 5).
Table 5. Ring Opening Polymerization of ꢀCL by 2ꢀb and 3ꢀb
c
aRun Cat iPrOH Temp Time bYield MnGPC dMnth cMw/Mn
(°C) (min) (%) (x 103) (x 103)
17
18
19
20
21
22
ꢀ
ꢀ
70
25
25
70
70
70
60
180
60
87
57
193.7
166.6
277.3
70.2
49.8
32.6
39.9
28.3
11.3
5.6
1.51
1.98
1.44
2.02
1.73
1.75
3ꢀb
3ꢀb
2ꢀb
3ꢀb
3ꢀb
3ꢀb
ꢀ
70
2
60
100
100
100
5
60
20.6
10
60
14.8
aAll reactions were carried out with in 4 mL of solvent, [Ι] 0=12.5 ꢁmol, and [εꢀCL]0/[I]0=500. bConversion of εꢀCL as
c
determined by 1H NMR spectral data. Experimental Mn (in gmolꢀ1) and Mw/Mn (PDI) values were determined by GPC
in THF using polystyrene standards and corrected using the factor of 0.56. d Mnth (in gmolꢀ1) =114,14 x ([εꢀCL]0/[I
+iPrOH]0) x conversion εꢀCL.
The polymers obtained by homoleptic complexes (runs 17ꢀ19, Table 5) showed molecular weights
higher than those predicted theoretically with monomodal distributions (PDI= 1.44ꢀ2.02).This is
coherent with a slow initiation step as a consequence of the fact that for homoleptic complexes the
first insertion of εꢀCL should occur on the phenoxyꢀyttrium bond of one of the coordinated ligands.
Additional polymerization experiments were performed in the presence of different equivalents of
isopropanol. Under the explored conditions, the molecular weights of the obtained polymers (entries
20ꢀ22) were still higher than those theoretically expected, but a progressive decrease of molecular
masses with the increase in the amount of alcohol was observed.
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The complexes were also tested in the polymerization of racꢀlactide. No activity was observed
when the polymerization reactions were performed at room temperature. Differently, at 70 °C, in
toluene, both complexes result active allowing a quantitative conversion of 100 equivalents of
monomer in three hours. The obtained polymers have monomodal molecular weight distributions
with polydispersivity indexes of 1.49 and 1.37 for 2ꢀb and 3ꢀb respectively. As already observed for
the polymerization of εꢀcaprolactone, the polymers have molecular masses higher than those
predicted theoretically as a consequence of a pour efficiency of the initiation reactions. NMR
analysis of the produced PLAs revealed a predominantly heterotactic microstructure with Pr of 0.72
and 0.73 for 2ꢀb and 3ꢀb respectively.
Conclusions
A series of yttrium and scandium complexes supported by monoanionic phenoxyꢀthioether ligands
have been prepared by a classical amine elimination reaction of silylamido metal precursors. With
the aim to investigate the influence of this new ligand framework in the coordination chemistry of
group 3 metals a series of proligands with different substituents on the ortho position of the
thiophenol ring have been synthesized and tested.
The synthesis and stability of monoamido yttrium complexes supported by two phenoxyꢀthioether
ligands are largely influenced by the ligand structure. Heteroleptic monoamido yttrium complexes
have been obtained when additional charge transfers from the ligands toward the metal centre are
possible, e.g. β(SiꢀH) agostic interactions; otherwise homoleptic complexes bearing three phenoxy
thioether ligands are obtained.
The heteroleptic scandium and yttrium complexes are efficient initiators in ꢀcaprolactone, Lꢀ
lactide and racꢀlactide polymerization. The reactivity resulted strongly dependent on the
polymerization reactions (temperature and solvent), the nature of the metal and the structure of the
phenoxy based ligands anchored to the reactive centre.
The presence of an excess alcohol as an initiator/chain transfer agent accelerated the reactions and a
effective “immortal” polymerizations were achieved.
Furthermore, the polymerization of racꢀlactide initiated by bis (phenoxyꢀthioether) yttrium amide
complexes gave heterotactic enriched polymers. The heterotactic stereocontrol operates by means
of a chainꢀend control mechanism and appears to be enhanced by the presence of the
trifluoromethyl ortho substituent on the aromatic ring bound to sulphur atom.
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Experimental section
General Considerations: Materials and Methods
All manipulations of airꢀ and/or waterꢀsensitive compounds were carried out under dry nitrogen
atmosphere using a Braun Labmaster drybox or standard Schlenk line techniques. Glassware and
vials used in the polymerization were dried in an oven at 120 °C overnight and exposed to vacuumꢀ
nitrogen cycle three times.
Hexane and THF (Carlo Erba) were purified by distillation from sodium benzophenone ketyl.
Toluene (Carlo Erba) was purified by distillation from sodium. Anhydrous dimethylformamide,
anhydrous yttrium trichloride and scandium trichloride (Aldrich) were used as received. All
deuterated solvents were dried using molecular sieves. Lactide (Aldrich) was purified by
crystallization from dry toluene and then stored over P2O5. εꢀcaprolactone (Aldrich) was dried with
CaH2 for 24 hours at room temperature and then distilled under reduced pressure. All other
chemicals were commercially available and used as received unless otherwise stated.
Instruments and Measurements.
The NMR spectra were recorded on Bruker Avance 400 spectrometer (1H, 400. MHz; 13C, 100.62
MHz; 31P 161.97 MHz) at 25 °C, unless otherwise stated. Deuterated solvents were purchased from
Cambridge Isotope Laboratories, Inc. and degassed and dried over activated 3 Å molecular sieves
prior to use. Chemical shifts (δ) are listed as parts per million and coupling constants (J) in hertz. 1H
NMR spectra are referenced using the residual solvent peak at δ 7.16 for C6D6 and δ 7.27 for
CDCl3. 13C NMR spectra are referenced using the residual solvent peak at δ 128.39 for C6D6 and
δ 77.23 for CDCl3. 19F NMR spectra are referenced to external CFCl3.
Variableꢀtemperature 1H NMR experiments were performed with a Bruker Avance 400 (operating
at 400.13 MHz) in CD2Cl2 using NMR tubes equipped with J Young valves. The molecular weights
(Mn and Mw) and the molecular mass distribution (Mw/Mn) of polymer samples were measured by
GPC at 30 °C, using THF as solvent, flow rate of eluent 1 mL/min, and narrow polystyrene
standards as reference. The measurements were performed on a Waters 1525 binary system
equipped with a Waters 2414 RI detector using four Styragel columns (range 1000ꢀ1,000,000 Å).
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Every value was the average of two independent measurements. It was corrected using the factor of
0.58 for polylactide and 0.56 for polycaprolactone according to the literature. 80ꢀ82
Elemental analyses were performed in the microanalytical laboratory of the institute.
Synthesis of the proligands and complexes
The phenoxyꢀthioether proligands L1ꢀ4ꢀH were synthesized according to previously published
procedures. 53,54
Complex 1ꢀa. Into a stirred solution containing {Sc [N(SiHMe2)2]3(THF)2} (0.436 g, 0.7 mmol) in
benzene (4 mL), a solution of the ligand precursor (0.489 g, 1.5 mmol) in benzene (6 mL) was
added dropwise. The solution was stirred for 2 hour at room temperature. The solvent was removed
1
under vacuum forming a white solid that was pure according H NMR and elemental analysis.
(0.328 g, 50 % yield).
1H NMR (400.13 MHz, C6D6, 298 K): δ 7.44 (d, J = 4 Hz, 2H, oꢀH ꢀAr), 7.30 (d, J = 8 Hz, 4H, Hꢀ
Ar),. 6.85 (m, 6H, mꢀHꢀAr, pꢀHꢀAr), 6.68 (s, 2H, HꢀAr), 5.42 (m, 2H, SiꢀH), 4.23 (s, 4H, CH2),
1.64 (s, 18H, CCH3), 1.25 (s, 18H, CCH3), 0.26 (d, 12H, SiꢀCH3).
13C {1H} NMR (100.62 MHz, C6D6, 298 K): δ 160. 2 q (C–O), 139.4 q (C–S), 136.44 q, 133.1 q,
126.1 (CH), 123.9 (CH), 122.2 q, 35.46 (CCH3), 34.2 (CCH3), 31.8 (CCH3), 30.6 (CCH3), 2.6
(CH3Si).
Elemental analysis calcd (%) for C46H68NO2S2ScSi2: C, 66.38; H, 8.24; N, 1.68; S, 7.71 found: C
,66.13; N, 1.78; H, 8.03; S, 7.22.
Complex 1ꢀb
Into a stirred solution containing {Y[N(SiHMe2)2]3(THF)3} (0.496 g, 0.7 mmol) in hexane (4 mL),
was added dropwise a solution of the ligand precursor (0.489 g, 1.4 mmol) in hexane (6 mL). The
solution was stirred for 20 hour at room temperature. The solvent was removed under vacuum
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1
forming a white solid that was pure according H NMR and elemental analysis. (0.614 g, 88 %
yield).
1H NMR (400.13 MHz, C6D6, 298 K): δ 7.49 (d, J = 3 Hz, 2H, HꢀAr). 7.02 (m, 4H, HꢀAr), 6.8 (d, J
= 3 Hz, 2H, HꢀAr), ), 6.81 (m, 6H, HꢀAr), 4.72 (m, 2H, SiꢀH), 4.11 (s, 4H, CH2), 3.63 (s, 4H, Oꢀ
CH2 of THF), 1.68 (s, 18H, CCH3), 1.30 (s, 18H, CCH3), 0.31 (d, J = 4 Hz; 12H, SiꢀCH3).
13C{1H} NMR (100.62 MHz, C6D6, 298 K): δ 152.2 q (C–O), 142.4 q (C–S), 137.3 q, 134.5 q, 131.7
(CH), 129.0 (CH), 127.3, 125.8, 123.8, 122.3 q, 37.7 (CH2), 35.3 (CCH3), 34.2 (CCH3), 33.0
(CCH3), 31.0 (CCH3), 0.5 (CH3Si).
Elemental analysis calcd (%) for C50H76NO3S2Si2Y: C, 63.32; H, 8.08; N 1.48; S, 6.76; found: C,
63.21 H, 8.43; N 1.36; S, 6.12.
Products obtained by reaction of {Y[N(SiHMe2)2]3(THF)3} and L2ꢀH
Into a stirred solution containing {Y[N(SiHMe2)2]3(THF)3} (91.97 mg, 1.46.10ꢀ4 mol) in benzene (4
mL), was added dropwise a solution of the L2ꢀH ligand precursor (0.100 g, 2.92.10ꢀ4 mol) in
benzene (6 mL). The solution was stirred for 2 hour at room temperature. The solvent was removed
1
under vacuum forming a white solid that was analyzed by H NMR spectroscopy. (0.114g, 80 %
yield).
1
The H NMR spectrum revealed a complex mixture of three different products: two stereoisomers
monoamide yttrium complexes and the homoleptic yttrium complex (2ꢀb) with three coordinated
ligands as major component.
1H NMR (400.13 MHz, C6D6, 298 K). First isomer: δ 7.52 (d, 2H, HꢀAr), 7.17 (d, 2H, J = 7.5 Hz,
HꢀAr), 6.92ꢀ6.74 (m, 4H, HꢀAr), 6.68ꢀ6.72 (m, 6H, HꢀAr), 5.20 (m, 2H, SiꢀH), 4.26 (s, 4H, CH2),
2.43 (s, 6H, CH3), 1.80 (s, 18H, CCH3), 1.28 (s, 18H, CCH3), 0.38 (d, 6H, J = 2.5 Hz, SiꢀCH3).
Second isomer: δ 7.49 (d, 2H, HꢀAr), 6.92ꢀ6.74 (m, 6H, HꢀAr), 6.68ꢀ6.72 (m, 4H, HꢀAr), 4.94 (m,
2H, SiꢀH), 4.18 (s, 4H, CH2), 2.18 (s, 6H, CH3), 1.69 (s, 18H, CCH3), 1.27 (s, 18H, CCH3), 0.11 (d,
6H, J = 2.5 Hz, SiꢀCH3). For the homoleptic species see below.
Because of the complexity of the obtained mixture the elemental analysis data was not performed.
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Complex 2ꢀb
Homoleptic complex 2ꢀb was purposely prepared by reaction of the metal precursor with three
equivalents of the L2ꢀH proligand. Into a stirred solution containing {Y[N(SiHMe2)2]3(THF)3}
(91.97 mg, 1.46.10ꢀ4 mol) in benzene (4 mL), was added a solution of the ligand precursor (0.152 g,
4.38.10ꢀ4 mol) in benzene (6 mL). The solution was stirred for 2 hour at room temperature. The
1
solvent was removed under vacuum forming a white solid that was analyzed by H NMR
spectroscopy and elemental analysis. (0.147g, 91 % yield).
4
1H NMR (400.13 MHz, C6D6, 298 K): δ 7.53 (d, J = 2.5 Hz, 3H, ArꢀH), 7.12 (d, 3H, J = 7.5 Hz,
ArꢀH), 6.84 (d, 3H, 4J = 2.5 Hz, ArꢀH), 6.65 (m, 6H, ArꢀH), 6.62 (m, 3H, ArꢀH), 4.05 (s, 6H, CH2),
1.75(s, 27 H, CCH3), 1.74 (s, 9 H, CH3), 1.35(s, 27 H, CCH3).
13C{1H} NMR (100.62 MHz, C6D6, 298 K): δ 156.4 (q, CꢀO), 141.5 (q, CꢀS), 135.3 (q), 131.7 (CHꢀ
Ar), 131.2 (CH), 130.1 (CH), 129.5 q, 127.4 (CH), 122.9 (CH), 125.7 q, 124.6 (CH), 118.5 q, 37.2
(CH2), 35.2 (CCH3), 34.3 (CCH3), 33.2 (CH3), 32.4 (CCH3), 31.6 (CCH3).
Elemental analysis calcd (%) for C66H87O3S3Y: C, 71.19; H, 7.88; S, 8.64; found: C 71.26; H, 7.79;
S, 8.35.
Complex 3ꢀb
In a roundꢀbottom flasks, equipped with a magnetic stirrer, a solution of the proligand L3ꢀH (0.80 g,
2.73 mmol) in 10 mL of hexane was prepared under nitrogen atmosphere. A solution of yttrium
amide {Y[N(SiHMe2)2]3(THF)3} (0.41g, 0.91 mmol) in 20 mL of hexane was added dropwise at
room temperature. The mixture was stirred for 3 hours, then the solvent was removed in vacuum
and the residue was crystallized from hexane affording 0.31g of a white powder (yield 67%).
4
1H NMR (400.13 MHz, C6D6, 298 K): δ 7.49 (d, J = 2.5 Hz, 3H, HꢀAr), 7.25 (d, 3H, HꢀAr), 7.16
(d, 3H, HꢀAr), 6.88 (d, 3H, HꢀAr), 6.57 (t, 3H, HꢀAr), 6.48 (t, 3H, HꢀAr), 4.09 (s, 6H, CH2), 1.73 (s,
27 H, CCH3)., 1.30 (s, 27 H, CCH3),
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13C{1H} NMR (100.62 MHz, C6D6, 298 K): δ 156.9 (q, CꢀO), 140.5 (q, CꢀS), 139.4 (q, CꢀBr), 133.7
(CH), 131.2 (CH), 130.5 (CH), 129.5 q, 128.4 (CH), 125.9 (CH), 125.6 (CH), 125.1 q, 119.8 q, 38.5
(CH2), 35.8 (CCH3), 34.6 (CCH3), 32.2 (CCH3), 30.7 (CCH3).
Elemental analysis calcd (%) for C63H78Br3O3S3Y: C, 57.85; H, 6.01; S, 7.35; found: C ,57.13; H,
5.63; S, 7.32
Complex 4ꢀb
Into a stirred solution containing {Y[N(SiHMe2)2]3(THF)3} (79.4 mg, 1.26.10ꢀ4 mol) in benzene (4
mL), was added dropwise a solution of the ligand precursor (0.100 g, 2.52.10ꢀ4 mol) in benzene (6
mL). The solution was stirred for 2 hour at room temperature. The solvent was removed under
vacuum forming a white solid that was pure according 1H NMR and elemental analysis. (0.113g, 93
% yield).
1H NMR (400.13 MHz, C6D6, 298 K): δ7.46 (d. 1H, HꢀAr); 7.44 (d, 1H, HAr),. 7.38 (m, 2H, HꢀAr),
7.31 (m, 2H, HꢀAr), 7.02 (d, 1H, HꢀAr), 6.84 (d, 1H, HAr), 6.68 (m, 3H, HꢀAr), 6.60 (m, 1H, Hꢀ
Ar), 5.09 (m, 1H, SiꢀH), 5.12 (m, 1H, SiꢀH), 5.09 (m, 1H, SiꢀH), 4.17 (s, 2H, CH2), 3.88 (m, 4H,
THF), 1.75 (s, 9H, CCH3), 1.60 (s, 9H, CCH3), 1.34 (m, 4H, THF), 1.31 (s, 9H, CCH3), 1.30 (s, 9H,
CCH3), 0.24 (d, 6H, SiꢀCH3), 0.23 (d, 6H, SiꢀCH3).
19F NMR (376 MHz, C6D6, 298 K): δ ꢀ60.81 (s. 3F, CF3); ꢀ61.16 (s. 3F, CF3).
13C{1H} NMR (100.62 MHz, C6D6, 298 K): δ 160.4 (q, CꢀO), 160.1 (q, CꢀO), 138.8 (q, CꢀS), 138.7
(q, CꢀS), 139.9 q, 139.6 q, 134.1 q, 134.0 q 133.3 (CH), 133.1 (CH), 126.9 (CH), 126.8 (CH), 126.5
(q, 2 C), 125.8 (CH), 125.6 (CH), 124.4 (CH), 124.1 (CH), 123.2 q, 122.8 q, ,39.8 (2, C, CF3),
35.6 (CH2), 35.5 (CH2), 34.2 (CCH3), 31.9 (CCH3), 32.24 (CCH3), 30.8(CCH3), 30.6 (CCH3), 25.2
(CCH3), 0.6 (CH3Si), 0.5 (CH3Si).
Elemental analysis calcd (%) for C52H74F6NO3S2Si2Y: C, 57.60; H, 6.88; N, 1.29; S, 5.91; found C,
57.26; H, 6.15; N, 1.34;S, 5.71.
εꢀCaprolactone Polymerizations. In a typical polymerization, a magnetically stirred reactor vessel
(10 cm3) was charged sequentially with a solution of precatalyst (12.5 ꢁmol in 4 mL of
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dry toluene) and monomer (6.25 mmol). Subsequently, 0.025 mL of a solution 0.1 M of isopropanol
in toluene (2.5 ꢁmol) was added. The mixture was thermostatized at the required temperature and,
after the required polymerization time, poured into hexane. The precipitated polymer was recovered
by filtration and dried at 40 °C in a vacuum oven. The polymer was characterized by NMR
spectroscopy and GPC analysis. 1H NMR (CDCl3, 25 °C): δ = 1.34 (m, 2H, ꢀCH2ꢀ), 1.62 (m, 4H, ꢀ
CH2ꢀ), 2.29 (t, 2H, ꢀCH2C(O)Oꢀ), 4.04 (t, 2H, ꢀCH2OC(O)ꢀ), 3.62 (t, 2H, ꢀCH2OH), 3.65 (s, 3H, ꢀ
C(O)ꢀOCH3). 13C NMR (CDCl3, 25 °C): δ = 24.7, 25.7, 28.5, 34.3, 64.3 (ꢀOCO(CH2)5ꢀ), 51.7 (ꢀ
C(O)OCH3), 62.7 (ꢀCH2OH), 173.7 (ꢀCOOꢀ).
Lactide Polymerizations. In a typical polymerization, a magnetically stirred reactor vessel (20
cm3) was charged sequentially with the monomer (racꢀ or Lꢀlactide, 0.288 g, 2.0 mmol), the
precatalyst (10 ꢁmol), and 2 mL of dry toluene. Subsequently, 0.20 mL of a solution 0.1 M of
methanol in toluene (20 ꢁmol) was added. The mixture was thermostatized at the required
temperature. After the required polymerization time, an aliquot of the crude material was sampled
by pipette and quenched in wet CDCl3 to evaluate the yields. Conversions were determined by
1
integration of the monomer vs polymer methine resonances in the H NMR spectrum of crude
product (in CDCl3). The precipitated polymer was recovered by filtration and dried at 40 °C in a
vacuum oven. The polymer was purified by redissolving in CH2Cl2 and precipitating from rapidly
1
stirring methanol. The polymer was characterized by NMR spectroscopy and GPC analysis. H
NMR (CDCl3, 25 °C): δ = 1.56 (m, 6H, ꢀCHCH3ꢀ), 3.79 (s, 3H, ꢀC(O)OCH3), 5.18 (m, 2H, ꢀ
CHCH3ꢀ). 13C NMR (CDCl3, 25 °C): δ = 16.8 (ꢀC(O)OCHCH3ꢀ), 69.2 (ꢀC(O)ꢀ OCHCH3ꢀ), 169.5,
169.8 (ꢀCOOꢀ).
Lactide Polymerizations by homoleptic tris(phenoxy thioether) yttrium complexes 2ꢀb and 3ꢀb
A typical procedure for lactide polymerization catalyzed by complexes 2ꢀb and 3ꢀb is herein
described. In a Braun Labmaster glovebox, a 10ꢀmL vial was charged with a solution of metal
initiator (10 mmol) in anhydrous toluene (0.5 mL) and the resulting solution was rapidly added to a
solution 1 mM of racꢀLA (0.214 g, 1.00 mmol, 100 equiv in toluene (1.5 mL). The mixture was
immediately stirred with a magnetic stir bar at 70 °C for three hours. An aliquot of the crude
material was sampled by pipette and quenched in wet CDCl3. The monomer conversion was
determination by 1HNMR analysis.
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Kinetic Experiments . In a typical experiment carried out in a Braun Labmaster glovebox, initiator
solution, from a stock solution in tolueneꢀd8, was injected into a in Teflonꢀvalved J. Young NMR
tube loaded with the monomer dissolved in suitable amount of tolueneꢀd8 as dry solvent. The
sample was thermo stated at the required temperature. The polymerization reaction was monitored
via 1H NMR analysis
The characteristic chemical shift for each monomer in deuterated toluene is 3.90 (q, CH; lactide),
and 3.99 (m, CH2; εꢀcaprolactone). The characteristic chemical shift for each polymer in deuterated
toluene is 5.07 (q, CH; polylactide), and 3.47 (t, CH2; polyꢀεꢀcaprolactone)
DOSYꢀNMR experiments details.
The measurement of diffusion has been carried out by observing the attenuation of the NMR
signals during a pulsed field gradient experiment using the double stimulated echo pulse
sequence.83, 84 In particular, 2D DOSY PGSE NMR spectra were performed 1ꢀa and 1ꢀaꢀCL adduct
(C6D6, 0.012 M) on a Bruker Avance 400 spectrometer at 300 K without spinning.
Tetrakismethylsilylsilane (TMSS; rH (hydrodynamic radius) ≈ rvdW (van der Waals radius) = 4.28
Å) was adds as internal standard. The most intense signals were investigated. The dependence of
the resonance intensity (I) on the gradient strength (g) is described by the following equation:
I=I0exp{ꢀDγ2g2δ2(ꢂꢀδꢃ3)}
where I is the observed intensity (attenuated signal intensity), I0 is the reference intensity
(unattenuated signal intensity), D is the diffusion coefficient, γ is the nucleus gyromagnetic ratio, g
is the gradient strength, δ is the gradient duration, and ꢂ is the diffusion delay. The parameters δ
and ꢂ were kept constant during the experiments, whereas g varied from 2 to 95% in 16 steps. The
values of ꢂ were 1800 and 2000 ꢀs for 1ꢀa and 1ꢀa ꢀ(CL) respectively.
A nonlinear regression on I and g2 data was performed to obtain the coefficients D for both
the samples and the corresponding internal standard signals (Dsample and DTMSS, respectively). The
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following expression (on the basis of the Stokes–Einstein equation) was applied and numerically
resolved to get the hydrodynamic radius of each sample
Dsample / DTMSS = cTMSSrHTMSS /csamplerH
sample
The coefficients csample and cTMSS can be estimated from the semiempirical equation.85
x
2.234
cx = 6 /(1+0.695rsolv/rH
)
where x is the sample or TMSS, and rsolv ≈ van der Waals radius of the solvent (2.7 Å for C6D6).
The diffusion coefficients (D) can be related to the hydrodynamic radius (r) of the species in
solution by the StokesꢀEinstein equation where T is the temperature, η is the viscosity of the
solvent, r is the hydrodynamic radius of the molecule or assembly, and K is the Boltzmann constant
StokesꢀEinstein equation D= KT/ 6πηr
Hydrodynamic volumes were calculated from the respective radii: VH = 4/3π(rH)3.
Computational Details
All calculations were performed using the Gaussian 03 program package.86 The complexes were
energy minimized without symmetry constraints at the BP86 level, that is, by employing the
exchange and correlation functionals of Becke87 and Perdew,88,89 respectively. The standard 6ꢀ
31G(d) basis set was used for all the main group atoms. Sc and Y were described by Lanl2DZ
together with the Lanl2DZ effective core potential (ECP). Stationary point geometries were
characterized as local minima on the potential energy surfaces. The absence of imaginary
frequencies verified that structures were true minima at their respective levels of theory. The
relative energies were corrected for vibrational zeroꢀpoint energies (ZPE, not scaled). The Gibbs
free energy change (ꢂG) are thermodynamically corrected to 298 K.
To save computational resources, we have simplified the structures of 1ꢀa and 1ꢀb replacing the tꢀ
butyl groups with hydrogen atoms and the N(SiHMe2) 2 group with the N(SiH3)2 group.
Solvent effects have been estimated in singleꢀpoint calculations on the gas phase optimized
structures, based on the polarizable continuous solvation model IEFPCM as implemented in
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