Phenoxytriamine complexes of yttrium: Synthesis, structure and use in the polymerization of lactide and ε-caprolactone

Article abstract of DOI:10.1039/b600939e

Reaction of the phenoxytriamine proligands 2,4-dimethyl-6-bis(2- (diethylamino)ethyl)aminomethlyphenol (HL¹) and 2,4-di-tert-butyl-6- bis(2-(diethylamino)ethyl)aminomethylphenol (HL²) with Y[N(SiMe ₂H)₂]₃

Full text of DOI:10.1039/b600939e

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www.rsc.org/dalton | Dalton Transactions
Phenoxytriamine complexes of yttrium: synthesis, structure and use in the
polymerization of lactide and e-caprolactone†
Ian Westmoreland and John Arnold*
Received 20th January 2006, Accepted 30th May 2006
First published as an Advance Article on the web 15th June 2006
DOI: 10.1039/b600939e
Reaction of the phenoxytriamine proligands 2,4-dimethyl-6-bis(2-(diethylamino)ethyl)amino-
methlyphenol (HL1) and 2,4-di-tert-butyl-6-bis(2-(diethylamino)ethyl)aminomethylphenol
(HL2) with Y[N(SiMe2H)2]3(THF)2 in pentane gave the momomeric complexes L1Y[N(SiMe2H)2]2 (1)
and L2Y[N(SiMe2H)2]2 (2). X-Ray structural analysis of 2 shows a 5-coordinate yttrium center. The
complexes 1 and 2 catalyze the ring opening polymerization of D-L-lactide and e-caprolactone leading
to narrow product polydispersities under mild conditions.
manner reminiscent of the G 4 ansa-metallocenes.17,18 Bridged
bis(phenolate) ligands have found a variety of applications within
lanthanide chemistry,19–26 and porphyrin rings have also been
extensively employed as dianionic ligands.27–30
Introduction
Organolanthanide complexes supported by cyclopentadienyl (Cp)
ligands have been demonstrated as effective precatalysts for a
variety of homogeneous processes.1–5 Lanthanide elements lend
themselves to complexation by hard sigma donors, since they
are highly acidic and form hard M3+ ions. However, an ideal
ligand system must also possess enough steric bulk to prevent
unwanted ligation by solvent, complex dimerisation, aggregation,
and ligand redistribution, without compromising the intended cat-
alytic activity of the system. While multidentate ligands are often
utilized to prevent ligand redistribution—particularly those with
donors that do not engage in bridging bonding modes—the formal
charge of the ancillary is also of paramount importance. Recent
ligand design has sought to supplant the formerly ubiquitous Cp
fragment, resulting in a deluge of new complexes.6–9
Perhaps more desirable are comparatively understudied com-
plexes of the form LMX2, (III) where L is also monoanionic. These
complexes are advantageous to catalysis because they contain a
pair of metal–organo bonds which offer themselves for catalysis;
for the case of the lanthanide bis(oxazolinates), generation my
be effected simply by adjusting reagent stiochiometry.14 Perhaps
a more obvious strategy is to increase the ligand bulk, as
demonstrated for the amidinates with yttrium31 and scandium.32
We have also previously described a bulky guanidinate-supported
lanthanum bis(alkoxide) which is a moderately successful initiator
for the ring opening polymerization (ROP) of lactide.33
Poly(lactic acid)s (PLAs) are the subject of much current interest
since their biodegradable and biocompatible nature is ideal for a
range of ecologically friendly applications to the medical, agricul-
tural and packaging industries.34–39 The availability of monomer
feedstocks from renewable resources has given PLAs an increasing
prominence in the market place.40 While a variety of lanthanide
alkoxide initiators have shown a high activity for ROP reaction,41–44
recent developments have led to new discrete complexes based
on a range of metals.45–47 The b-diketiminate anion48 has been
used independently by Coates et al.49,50 and Chisholm et al.51,52
to prepare efficient zinc-based initiators for PLA production,
and Shen et al.53 have recently performed the reaction with
lanthanide complexes of the form III supported by these ligands.
Chisholm et al. have described similar catalytic activity manifested
by tris(pyrazolato) borate-supported complexes of magnesium,54
and calcium,55,56 although bulkier ligands are required to elicit
catalytically active complexes of yttrium.57,58
One approach is to prepare monomeric complexes of the
form L2MX, (I) where L is bidentate, monoanionic ancillary, M
is a G 3 or lanthanide element, and X is an alkoxy, alkyl,
or amido coligand. Examples of such systems include benza-
midinato complexes,10,11 and the related guanidinate-supported
compounds.12 Mixed N, O donor sets include a topographically
similar chelating siloxy amido framework,13 and the C2-symmetric
bis(oxazolinates).14 A second class of compounds of the form
LMX (II) exists where L is a dianionic ancillary. Many such
examples have been prepared by the linking together of two
monoanionic amidinate15 or amionopyridinate16 fragments in a
Department of Chemistry, University of California, Berkeley and the Chem-
ical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley,
CA, 94720-1460
† Electronic supplementary information (ESI) available: Additional crystal
and NMR data. See DOI: 10.1039/b600939e
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The addition of neutral pendant arms is a convenient means
of increasing denticity, and modifying metal/ligand stoichiom-
etry, as demonstrated for the lanthanides with amidinate,59 b-
diketiminate60 and triazacyclononane-derived moieties.61 The ad-
dition of pendant donors to bis(phenolate) ligands has produced
efficient lanthanide based ROP initiators62 and a recent report has
described a new class of mono(phenolate) ligands in which a pair of
neutral pendant arms are used to support G 4 complexes.63 Inoue
and Matyjaszewski have used a similar phenoxytriamine ligand
HL1 to form non-coligand bearing Cu(I) complexes which exhibit
moderate catalytic activity for the polymerization of acrylate,
but monoanionic ancillaries with two neutral donors have yet
to appear in lanthanide chemistry.64 We envisaged that lanthanide
complexes of the form LLnX2 supported by such ligands would
have excellent potential as initiators for the ROP reaction.
Treatment of HL1 with lithium hydride in THF at low temper-
ature is low yielding, the salt LiL1 being obtained in only 37% as
colorless prisms after recrystallisation from pentane (Scheme 2).
We were able to increase the productivity slightly by affecting
n
deprotonation with either LiN(SiMe3)2 or BuLi, which afford
crystalline product in respectively 47% and 48% recovery. Carrying
out the reaction with either sodium or potassium hydride in THF
gives slightly higher yields of NaL1 or KL1 respectively. We were
unable to access LiL2 via any of the methods used to prepare
LiL1, and deprotonation with NaH was also failed to give NaL2.
However, we were ultimately able to obtain KL2 in 58% yield via
reaction of the proligand with potassium hydride.
Results and discussion
Synthesis of proligands and complexes
The potentially monoanionic proligands HL1 (R = Me) was pre-
pared in quantitative yield from 2,4-dimethylphenol, N,N,Nꢀ,Nꢀ-
tetraethyldiethylenetriamine and para-formaldehyde in methanol,
via a two step method, modified from the literature procedure
(Scheme 1).64 We found that an increased reaction time for
the first step gave an improvement in the yield of the final
product. A concomitant increase in the purity of isolated material
rendered chromatographic purification unnecessary. The tert-
butyl-substituted analogue HL2 was also prepared via this method
from the appropriate phenol.
Scheme 2
The 1H NMR spectrum of LiL1 recorded in d8-toluene at
298 K shows a complex series of resonances in the region 2.0–
3.0 ppm. A pair of singlets are observed at 2.35 and 2.40 ppm
arising from the two aromatic methyl groups, while the remaining
signals are attributed to the 14 terminal and bridging methylene
protons—the benzyl protons nitrogen manifest as a broad singlet
at d = 3.77. These broad resonances are indicative of a fluxional
solution environment perhaps involving rapid exchange of the
pair of pendant arms. Cooling of the sample to 273 K leads to a
broadening of all the methylene resonances; further cooling giving
increasingly broad signals. By 193 K the benzyl signal appears as
a pair of broad singlets downfield at d = 4.30 and 4.53 ppm;
presumably the slowing of exchange processes renders these
protons inequivalent (see below). The remainder of the aliphatic
region contains a series of broad overlapping resonances at this
temperature. Warming the sample to its original temperature gives
an identical spectrum. Further heating through the range 303–
343 K results in similar resonance broadening to that observed
at low temperatures. By 363 K the benzyl protons resonant as
a sharp singlet at d = 3.75 ppm, and the remaining methylene
protons manifest as a broad peak centered at d = 2.55 ppm (see
ESI† for details). Similar spectra were recorded for NaL1 and KL1.
Our initial attempts to access complexes of the form LLnCl2
were based on the assumption that salt metathesis of either
YCl3(THF)3 or LnCl3 with isolated ML1 (M = Li, Na, K) or
KL2 would be reasonably straightforward. However this approach
failed to yield any tractable products when performed in THF
either at ambient temperature or 65 ◦C. When carried out in
refluxing toluene over a period of several days we were similarly
unable to isolate LLnCl2. The reaction of HL with Ln[N(SiMe3)2]3
(Ln = Y, La) was not a viable route to LLn[N(SiMe3)2]2
under various conditions. Treatment of either proligand with
Y(CH2SiMe3)3(THF)2 also failed to yield tractable products.
We were slightly surprised by these results, however it has been
shown that the introduction of sterically demanding ligands to
lanthanide complexes can depend more on the precise nature
Scheme 1
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of the f element precursor than on the new ligand itself.65,66
This is born out by the result of the reaction of HL1 with the
less sterically congested silylamide Y[N(SiMe2H)2]3(THF)2 which
provides facile access to L1Y[N(SiMe2H)2]2 (1) as an off-white
oil in 69% yield; the complex L2Y[N(SiMe2H)2]2 (2) is similarly
obtained as a yellow microcrystalline solid from HL2 in 56% yield
(Scheme 1).
Molecular structure of L2Y[N(SiMe2H)2]2 (2)
Single crystals of 2 were obtained as colorless prisms upon
prolonged cooling of a pentane solution to −35 ◦C, and the
structure determined by X-ray diffraction (Table 1). The com-
pound crystallizes in the chiral monoclinic space group P21 and
the primitive unit cell contains two independent molecules (Fig. 1).
Three of the available four donor atoms in the phenoxytriamine
fragment form bonds with the yttrium atom in 2 and the 5-
coordinate center is completed by a pair of N-bound silylamido
Fig. 1 Molecular structure of 2 with thermal ellipsoids drawn at the 30%
probability level.
coligands. The geometry at yttrium is best described as distorted
from square-based pyramidal. Three of the basal positions are
occupied by the aryl oxygen, the central nitrogen, one of the two
diethylamino pendant donors which hang from the former; one of
the silylamido coligands takes up the fourth basal while the second
such coligand occupies the apical station. The second pendant
arm does not coordinate to the metal. A similar situation has been
observed in an aryl siloxide-bridged complex of yttrium where one
of a pair of dimethylamido donors is bound to the lanthanide and
a second such donor is not coordinated.67
Table 1 Experimental data for the X-ray diffraction study of 2
Empirical formula
Formula weight
Color
C35H78N5Si4YO
786.28
Colorless
Plates
Habit
Dimensions/mm
System
0.17 × 0.15 × 0.10
Monoclinic
10.023(1)
18.636(2)
12.370(1)
101.087(1)
2267.4(4)
P21
˚
a/A
˚
b/A
˚
The coordinated arm bonds to the metal with a N(3)–Y(1)
c/A
b/◦
Volume/A
˚
distance of 2.596(7) A, which falls within the range 2.28–2.62
3
˚
observed for N–Y bonds as does the N(1)–Y(1) contact of 2.607(5)
Space group
A between the central nitrogen and yttrium (Table 2).31,57,61,68–70 The
˚
Z
2
latter interaction forms a N(1)–Y(1)–N(3) angle of 70.8(2)◦ with
the pendant amido, and completes a 6-membered ring formed
with the phenoxy oxygen atom, the pertinent features of which
Calculate density/g cm−3
F000
1.152
852.00
20
2.72
Exposure time/s
l/mm−1
˚
Temperature/◦C
−160
are the O(1)–Y(1) bond distance of 2.055(5) A, and the acute
◦
N(1)–Y(1)–O(1) bond angle of 76.2(2)◦. The pair of silylamido
ligands form a N(4)–Y(1)–N(5) angle of 115.2(2)◦ at the metal, and
have slightly different N–Y bond lengths of 2.221(6) and 2.307(6)
2hmax
/
49.5
Total reflections
10290
Independent reflections
No. observations (I > 3.00r(I))
Reflection/parameter ratio
Residuals: R; Rw; Rall
Goodness of fit
6857 (Rint = 0.035)
3958
˚
A for the basal and apical ligands respectively. There are subtle
9.56
0.066; 0.059; 0.066
1.08
differences in the Y–Si inter-atomic distances also, the Y(1)–Si(2)
˚
and Y(1)–Si(4) measurements of ca. 3.15 and 3.24 A respectively
Flack parameter
0.017(8)
being significantly shorter than the comparable Y(1)–Si(1) and
◦
˚
Table 2 Selected interatomic distances (A) and bond angles ( ) for 2
Y(1)–O(1)
Y(1)–N(1)
Y(1)–N(3)
Y(1)–N(4)
Y(1)–N(5)
Si(1)–N(4)
Si(2)–N(4)
Si(3)–N(5)
Si(4)–N(5)
Y(1)–Si(1)
Y(1)–Si(2)
Y(1)–Si(3)
Y(1)–Si(4)
Y(1)–H(75)
Y(1)–H(76)
Y(1)–H(77)
Y(1)–H(78)
2.055(5)
2.607(5)
2.596(7)
2.221(6)
2.307(6)
1.705(6)
1.691(7)
1.692(6)
1.707(6)
3.478(8)
3.159(2)
3.541(7)
3.245(3)
3.425(2)
2.852(5)
3.547(4)
2.827(5)
O(1)–Y(1)–N(1)
O(1)–Y(1)–N(4)
N(1)–Y(1)–N(3)
N(1)–Y(1)–N(5)
N(3)–Y(1)–N(5)
N(4)–Si(1)–C(30)
C(30)–Si(1)–C(31)
N(4)–Si(2)–C(29)
N(5)–Si(3)–C(34)
C(34)–Si(3)–C(35)
N(5)–Si(4)–C(33)
N(4)–Si(1)–H(78)
C(31)–Si(1)–H(78)
C(28)–Si(2)–H(77)
N(5)–Si(3)–H(76)
C(35)–Si(3)–H(76)
C(32)–Si(4)–H(75)
76.2(2)
93.6(2)
70.8(2)
103.2(2)
105.6(2)
115.1(3)
107.4(4)
117.3(4)
114.0(4)
109.1(4)
115.7(4)
106.9
O(1)–Y(1)–N(3)
O(1)–Y(1)–N(5)
N(1)–Y(1)–N(4)
N(3)–Y(1)–N(4)
N(4)–Y(1)–N(5)
N(4)–Si(1)–C(31)
N(4)–Si(2)–C(28)
C(28)–Si(2)–C(29)
N(5)–Si(3)–C(35)
N(5)–Si(4)–C(32)
C(32)–Si(4)–C(33)
C(30)–Si(1)–H(78)
N(4)–Si(2)–H(77)
C(29)–Si(2)–H(77)
C(34)–Si(3)–H(76)
N(5)–Si(4)–H(75)
C(33)–Si(4)–H(75)
141.1(2)
101.4(2)
141.5(2)
99.5(2)
115.2(2)
113.1(4)
114.2(3)
109.0(4)
112.0(4)
116.4(4)
106.5(4)
106.9
107.0
105.1
107.2
106.9
105.0
105.1
107.2
105.8
105.5
105.9
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˚
series of resonances between 3.30 and 2.10 ppm. Two sets of methyl
resonances arise from the terminal groups on the donor arms silyl-
bound groups on the coligands, and manifest themselves as broad
singlets at 0.83 and 0.33 ppm respectively. The remainder of the
spectrum is completed by tert-butyl signals at 1.72 and 1.38 ppm
and the solvent. These features are consistent with the system
being in rapid exchange on the NMR timescale.
Y(1)–Si(3) distances of 3.47 and 3.54 A. The silyl hydrogen atoms
H(76) and H(78) were located from the difference map, and sit at
Y–H distances of 2.852(5) and 2.827(5) A respectively. Although
this is probably too far from the metal center to suggest there is
additional stabilization of yttrium by the type of b(SiH) agostic
interactions observed by Anwander et al., the distances between
˚
the lanthanide and the other pair of silyl protons H(75) and H(77)
are significantly greater at 3.425(2) and 3.547(4) A respectively.
66
˚
Cooling to 273 K leads to a sharpening in the silyl proton
signal, and a splitting of the methylene resonances into nine broad,
overlapping peaks. The terminal methyl groups appear as two
unequal resonances at 0.98 and 0.78 which integrate in the ratio 1 :
3. Further cooling to 253 K evokes additional splitting in the larger
such resonance, and by 233 K, a set of peaks of relative integral
1 : 2 : 1 is observed in this region. The silyl-proton signal dissolves
to a pair of resonances of equal intensity but differing shape over
this temperature range, and this splitting pattern is replicated in
the benzyl signal. At 253 K the silylmethyl groups give a pair of
overlapping signals, which, on further cooling separate to a pair of
signals of relative intensity 1 : 3. The methylene region undergoes
a general broadening on initial cooling, followed by sharpening;
NMR Characterization of L2Y[N(SiMe2H)2]2 (2)
Variable temperature 1H NMR spectra of 2 recorded in d8-toluene
are shown in Fig. 2. At 293 K a broad resonance with some
fine structure is observed at 5.08 ppm, arising from the four silyl
protons. A second broad signal at 4.29 ppm is attributed to the
benzyl CH2, while the remaining methylene signals give a complex
1
we also know from H–1H correlation spectra recorded over the
same temperature range that resonances in this region undergo
considerable chemical shift changes on cooling making direct in-
ferences difficult (see ESI†). Further cooling leads to a broadening
of all signals, and coalescence of all non-methylene resonances
is observed at 203 K. Reheating the sample to 298 K gives a
spectrum identical to that originally recorded. Increasing the
temperature to 313 K sharpens the spectrum, apart from in
the methylene region where there is some broadening. At 353 K the
methylene region is resolved into four separate signals of relative
integral 1 : 1 : 3 : 1. The system is presumably in the fast exchange
regime under these conditions.
The exchange processes apparent from the NMR spectra can
almost certainly be attributed to exchange process involving the
two pendant donor arms. The pendant arm methyl signals at
233 K can be explained as follows: as the rate of exchange
is slowed, the pair of methyl groups on the coordinated arm
(inequivalent by since there is no mirror plane in the complex)
are exchanging at a rate slower than the NMR timescale. This is
in the slow exchange regime at 233 K, hence the appearance of
two singlets; comparable exchange between methyl groups on the
non-coordinated donor is presumably significantly faster, and a
single resonance of appropriate intensity results.
The situation is similar for the Si–H signals. We can see from the
essentially distorted square-based pyramidal crystal structure of 2
(Fig. 1) that the two silylamido ligands are inequivalent in the solid
state. The apical ligand forms reasonably similar angles via yttrium
with each of the atoms at basal positions, three of these angles
fall within the range 101–106◦ and the fourth between the two
silyamido ligands is not much larger at 115◦. The basal coligand
however exhibits amuch wider range ofangles:93–141◦, the largest
forming at yttrium with the phenoxy oxygen. It is our belief that
the latter infers a significant trans effect on the basal amido. The
1
result is evident in the H NMR spectrum at 233 K where we
attribute the sharper downfield signal at ca. 5.1 ppm to the in the
silyl protons on the basal ligand, and the broader upfield resonance
to its apical counterpart, our reasoning being that exchange is fast
about the weaker Y–N bond. The same effect is also responsible
for the silyl methyl signals, where faster exchange in one ligand
Fig. 2 Variable temperature 1H NMR spectra of 2.
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