Diamido-ether actinide complexes as initiators for lactide ring-opening polymerization

Article abstract of DOI:10.1021/om300993r

The synthesis and characterization of a series of new diamido-ether actinide(IV) alkoxide complexes are reported. Addition of 2 equiv of LiO ⁱPr to [^tBuNON]ThCl₅Li₃·DME ([^tBuNON]^2- = [(^tBuNSiMe₂) ₂O]^2-) in toluene gives [^tBuNON]Th(O ⁱPr)₃Li·DME (1-DME). Recrystallization of 1-DME from diethyl ether gives [^tBuNON]Th(OⁱPr) ₃Li·Et₂O (1-Et₂O). The addition of 2 equiv of LiOⁱPr to {[^tBuNON]UCl₂}₂ gives {[^tBuNON]U(OⁱPr)₂}₂ (2). If KO^tBu is used instead, the product is {[^tBuNON]U(O ^tBu)}₂ (3). The reaction of 2 equiv of KO^tBu with [iPr₂Ph NCOCN]ThCl₂·DME ([iPr₂Ph NCOCN]^2- = [(2,6-ⁱPr₂PhNCH₂CH ₂)₂O]^2-) gives [iPr₂Ph NCOCN]Th(O^tBu)₂ (4), while addition of 1 equiv of KO ^tBu to [iPr₂Ph NCOCN]UCl₃Li·THF results in [iPr₂Ph NCOCN]U(O^tBu)Cl (5). Complexes 1-5 as well as the previously reported diamido actinide dialkyl complexes [^tBuNON] An(CH₂SiMe₃)₂ (An = Th (6), U (7)) and [ ^iPr2PhNCOCN]An(CH₂SiMe₃)₂ (An = Th (8), U (9)) were evaluated as initiators for the ring-opening polymerization (ROP) of l-lactide (l-LA) and racemic lactide (rac-LA). It was established that complexes 1-8 were capable of producing poly(l-LA) (PLLA) under mild conditions within relatively short periods of time. Diisopropoxide complex 2 polymerized up to 500 equiv of l-LA in 90 min at 30 C, with moderate to good control over the molecular weight features (values of M_w/M_n with this complex are typically 1.5 or below). The hydrocarbyl complexes 6-8 also proved active under the same conditions, with 7 showing a similar ability to control the ROP with monomer loadings up to 500 equiv. PLLAs prepared with initiators 1-5 and 6-8 have been characterized by end-group analysis via NMR and MALDI-TOF MS, which indicate clearly that the diamido ligand does not participate as an initiating group in the ROP process and that the alkoxide and alkyl moieties, respectively, are the only initiating groups. When it was applied to mediate the ROP of rac-LA, 2 yielded heterotactically enriched PLA (P_r values up to 0.73 in THF).

Full text of DOI:10.1021/om300993r

Article
pubs.acs.org/Organometallics
Diamido-Ether Actinide Complexes as Initiators for Lactide Ring-
Opening Polymerization
Cassandra E. Hayes,† Yann Sarazin,‡ Michael J. Katz,§ Jean-Francois Carpentier,‡,*
̧
and Daniel B. Leznoff†,*
†Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, British Columbia, Canada V5A 1S6
‡Institut des Sciences Chimiques de Rennes, UMR 6226 CNRS, Universite
France
́
de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex,
§Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois, United States 60208-3113
S
* Supporting Information
ABSTRACT: The synthesis and characterization of a series of
new diamido-ether actinide(IV) alkoxide complexes are
reported. Addition of 2 equiv of LiOiPr to [tBuNON]-
ThCl5Li3·DME ([tBuNON]2− = [(tBuNSiMe2)2O]2−) in
toluene gives [tBuNON]Th(OiPr)3Li·DME (1-DME). Recrys-
tallization of 1-DME from diethyl ether gives [tBuNON]Th-
(OiPr)3Li·Et2O (1-Et2O). The addition of 2 equiv of LiOiPr to
{[tBuNON]UCl2}2 gives {[tBuNON]U(OiPr)2}2 (2). If KOtBu
is used instead, the product is {[tBuNON]U(OtBu)}2 (3). The reaction of 2 equiv of KOtBu with [iPr PhNCOCN]ThCl2·DME
2
([iPr PhNCOCN]2− = [(2,6-iPr2PhNCH2CH2)2O]2−) gives [iPr PhNCOCN]Th(OtBu)2 (4), while addition of 1 equiv of KOtBu to
2
2
[
iPr PhNCOCN]UCl3Li·THF results in [iPr PhNCOCN]U(OtBu)Cl (5). Complexes 1−5 as well as the previously reported
2
2
diamido actinide dialkyl complexes [tBuNON]An(CH2SiMe3)2 (An = Th (6), U (7)) and [iPr2PhNCOCN]An(CH2SiMe3)2 (An =
Th (8), U (9)) were evaluated as initiators for the ring-opening polymerization (ROP) of L-lactide (L-LA) and racemic lactide
(rac-LA). It was established that complexes 1−8 were capable of producing poly(L-LA) (PLLA) under mild conditions within
relatively short periods of time. Diisopropoxide complex 2 polymerized up to 500 equiv of L-LA in 90 min at 30 °C, with
moderate to good control over the molecular weight features (values of Mw/Mn with this complex are typically 1.5 or below).
The hydrocarbyl complexes 6−8 also proved active under the same conditions, with 7 showing a similar ability to control the
ROP with monomer loadings up to 500 equiv. PLLAs prepared with initiators 1−5 and 6−8 have been characterized by end-
group analysis via NMR and MALDI-TOF MS, which indicate clearly that the diamido ligand does not participate as an initiating
group in the ROP process and that the alkoxide and alkyl moieties, respectively, are the only initiating groups. When it was
applied to mediate the ROP of rac-LA, 2 yielded heterotactically enriched PLA (Pr values up to 0.73 in THF).
based complexes that show good performance for such ROPs
are also available.4
INTRODUCTION
■
A large number of efficient oxophilic metal-based complexes
have been reported over the past two decades for the ring-
opening polymerization (ROP) of cyclic esters such as ε-
caprolactone, L and racemic lactide (L-LA, rac-LA) and racemic
β-butyrolactone.1 Many of these catalysts display high control
over the polymerization parameters, and a number of them
have been reported to facilitate the production of highly
stereoregular polylactide (PLA) by stereocontrolled ROP of
rac-LA. A range of different ligand frameworks that support
these metal catalysts have been described. Ancillary ligands
based on anionic phenoxide donors further supported by
neutral, potentially hemilabile donors such as amines and ethers
have been very popular in these studies.2 Nitrogen-based
ligands in the form of β-diketiminates or weakly donating
sulfonamides and phosphinamides also proved to be highly
effective frameworks for metal complexes that facilitated the
controlled ROP of lactides.3 Some reports of strictly amide-
In comparison with the large amount of work that has been
done targeting lanthanide-based ROP catalysts,1 reports of the
use of actinide complexes for the ROP of cyclic esters are very
scarce.5 Generally speaking, actinide metals exhibit a number of
properties that make them good candidates for use in catalysis:
namely, access to an enlarged coordination sphere, the
possibility of f-orbital participation, and access to a number
of oxidation states.6 There has indeed been considerable work
in the advancement of actinide-based catalysts in general, as
highlighted in several recent reviews.6b,d,7 Until recently,
actinide alkoxide complexes were not expected to be viable
for the catalysis of organic transformations due to the high
oxophilicity of the actinide centers; however, several reports
Special Issue: Recent Advances in Organo-f-Element Chemistry
Received: October 22, 2012
Published: February 11, 2013
© 2013 American Chemical Society
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have now shown that actinide-based alkoxide complexes are in
fact active in a variety of catalytic cycles.5,8 In particular,
Cp*2AnMe2 (An = Th, U) and [U(NEt2)3][BPh4] have shown
activity as catalysts for the ROP of ε-caprolactone and L-LA.5a
In another related example, Cp*2ThMe2 (Cp* = C5Me5),
Me2SiCp′2Th(C4H9)2, and Th(NEtMe)4 precatalysts have been
used to promote the dimerization of a large number of
aldehydes to give the corresponding esters.8a,c These
precatalysts incorporate alkyl or amide moieties, but mecha-
nistic studies have indicated that, prior to entering the catalytic
cycle for aldehyde dimerization, these groups are replaced by
alkoxide substituents from the incoming aldehydes.8c
that after 18 h and workup resulted in a yellow solid of
[
tBuNON]Th(OiPr)3Li·DME (1-DME). Unsurprisingly, addi-
tion of 3 equiv of LiOiPr to [tBuNON]ThCl5Li3·DME resulted
in an increase in the yield of 1-DME to 79%.
1
The H NMR spectrum of 1-DME in toluene-d8 contains
nine resonances at room temperature and is consistent with a
Cs-symmetric structure in solution; it indicates retention of the
DME molecule even after repeated washing with hexanes and
toluene. There are three resonances for the diamido ligand,
which has two inequivalent silyl methyl groups. The OiPr
substituents appear as four resonances, consistent with two
different environments for these ligands. A doublet at δ 1.26
ppm and a septet at δ 4.59 ppm that integrate respectively to
12H and 2H are consistent with two unhindered CH(CH3)2
groups. The 13C{1H} NMR spectrum of 1-DME indicates that
there are two carbon resonances at δ 28.80 and 28.99 ppm that
are coincident in the 1H NMR spectrum, obscuring the fact that
there are actually three different environments for the OiPr
groups. A second multiplet at δ 4.66 ppm that integrates to 1H
and a doublet at δ 1.41 ppm that integrates to 6H are observed
for the third inequivalent OiPr group. Finally, two resonances
are observed for the coordinated DME molecule.
A number of isolated homoleptic actinide alkoxide9 and
heteroleptic actinide alkoxide complexes have been repor-
ted.8c,9b,d,f,10 In addition, the use of cyclopentadienyl-free
actinide complexes for catalytic applications lags behind that
of the Cp*-based analogues, leaving considerable room for
additional studies and possible improvement in the design of
actinide catalyst systems. Of particular interest to us, amido-
based actinide(IV) complexes have shown increasing promise
for catalytic applications, including reports of olefin polymer-
ization and hydroamination.10o,11 As such, we sought to apply
diamido-ether actinide complexes developed in our laboratory
for use as initiators in the ROP of lactide, a monomer of
growing academic and industrial significance.
Variable-temperature 1H NMR experiments on 1-DME
provide more insight into the overlapping resonances observed
in the room-temperature spectrum. Upon cooling from 25 to
1
Thus, herein we report the synthesis of a new series of
diamido-ether actinide alkoxide heteroleptic complexes con-
taining the [(tBuNSiMe2)2O]2− ([tBuNON]2−) and
−80 °C, the H NMR spectrum of 1-DME broadens as the
overlapping resonances begin to separate, and coalescence
occurs at −40 °C. Due to the overall broadness of many
resonances at the low-temperature limit of −80 °C, full
separation of all the resonances was not observed; however, the
formerly coincident resonances for the two overlapping
CH(CH3)2 groups which integrated to 12H at δ 1.26 ppm at
room temperature have separated into two broad singlets at δ
1.21 and 1.30 ppm. To further confirm this, the multiplet at δ
4.59 ppm at room temperature has also separated into two
broad singlets at δ 4.61 and 4.69 ppm at −80 °C. As well, the
[(iPr2PhNCH2CH2)2O]2− ([iPr PhNCOCN]2−) ligand frame-
2
works. The activities of these complexes and several previously
reported diamido-ether actinide dialkyls,11c,12a,13 specifically
[
[
tBuNON]An(CH2SiMe3)2 (An = Th (6), U (7)) and
iPr PhNCOCN]An(CH2SiMe3)2 (An = Th (8), U (9)), for
2
the ROP of L-LA and rac-LA are presented. The impact on the
control of polymerization as a function of choice of actinide
metal and diamido ligand as well as of changes in solvent and
temperature is discussed.
t
overlapping resonances for the Bu-N and third CH(CH3)2
groups have also begun to separate but the separation is not as
complete as for the other CH(CH3)2 groups. In this instance, a
singlet at δ 1.58 ppm for the tBu-N resonance is observed with
a broad shoulder that spans the spectrum between δ 1.58 and
1.65 ppm.
RESULTS AND DISCUSSION
■
Synthesis of Diamido-Actinide Alkoxide Complexes.
The synthesis of new diamido actinide alkoxide complexes was
achieved through standard salt metathesis reactions of the
requisite parent diamido actinide dihalide complex with the
appropriate lithium or potassium alkoxide reagent (Scheme 1).
Thus, addition of 2 equiv of LiOiPr to [tBuNON]-
ThCl5Li3·DME ([tBuNON]2− = [(tBuNSiMe2)2O]2−; DME =
1,2-dimethoxyethane) in toluene gave a cloudy yellow solution
Despite multiple attempts to recrystallize 1-DME from DME
or toluene, no single crystals of X-ray quality were obtained;
however, recrystallization of 1-DME via slow evaporation of a
diethyl ether solution resulted in the isolation of crystals
suitable for X-ray diffraction. The structure of 1 when
recrystallized from an Et2O/toluene solution retains one Et2O
molecule in the product 1-Et2O. The structure of this adduct 1-
Et2O reveals the thorium atom in a six-coordinate distorted-
octahedral environment meridionally coordinated by the
tridentate [tBuNON]2− ligand, three isopropoxide ligands, and
a solvent-coordinated lithium cation (Figure 1). One of the
OiPr− substituents is terminally bound to the thorium, while
the other two OiPr− ligands bridge the thorium and lithium
atoms. The lithium exists in a three-coordinate environment
that is completed by the coordination of a disordered diethyl
ether molecule. The Th(1)−N(10) bond distance of 2.382(12)
Å is longer than in {[tBuNON]ThCl2}2, which has Th−N
distances of 2.29(2) and 2.291(19) Å, and longer than the Th−
N distances in (2,6-bis(2,6-diisopropylanilidomethyl)pyridine)-
ThCl2·DME (2.305(9) and 2.321(8) Å).12 These Th−N bond
lengths are also longer than in monodentate amido thorium
Scheme 1. Synthesis of Complexes 1−3
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Figure 1. Molecular structure and numbering scheme of 1-Et2O
Figure 2. Molecular structure and numbering scheme of 2 (thermal
(thermal ellipsoids shown at 30% probability). Hydrogen atoms are
ellipsoids shown at 30% probability). Hydrogen atoms are omitted,
t
omitted, and Bu-N groups are simplified for clarity. Selected bond
t
and Bu-N groups are simplified for clarity. Selected bond distances
distances (Å) and angles (deg): Th(1)−N(10), 2.382(12); Th(1)−
O(1), 2.652(11); Th(1)−O(2), 2.298(9); Th(1)−O(3), 2.153(10);
Th(1)−O(4), 2.285(11); Li(1)−O(2), 1.83(4); Li(1)−O(4), 1.87(3);
N(10)−Th(1)−N(10′), 121.3(6); N(10)−Th(1)−O(1), 61.0(3);
O(1)−Th(1)−O(2), 165.4(4); O(2)−Th(1)−O(4), 70.4(4); O(3)−
Th(1)−O(4), 158.8(4).
(Å) and angles (deg): U(1)−N(1), 2.20(2); U(1)−O(1), 2.50(3);
U(1)−O(2), 2.12(2); U(1)−O(3), 2.326(17); U(1)···U(1′),
3.895(2); N(1)−U(1)−N(1′), 126.0(13); N(1)−U(1)−O(1),
63.7(6); O(3)−U(1)−O(3′), 67.5(6); O(2)−U(1)−O(3), 158.1(7);
O(1)−U(1)−O(3′), 159.6(7).
group compares well with the U−O distance for the terminally
bound tert-butoxide ligands of the homoleptic {Li(THF)}2{U-
(OtBu)6} (2.140(8) and 2.137(9) Å).9g The bridging U(1)−
O(3) distance of 2.326(17) Å also compares well with the
bridging U−O−Li distances in {Li(THF)}2{U(OtBu)6}, which
range between 2.252(6) and 2.412(8) Å.9g This U(1)−O(3)
distance is also comparable with the bridging U−O distance of
2.29(1) Å in {U2(OiPr)10}.9a
complexes; for example, Th[N(SiMe3)2]2(NMePh)2 has Th−N
distances between 2.299(6) and 2.328(6) Å.9f It seems
reasonable that the abundance of electron density from the
three hard isopropoxide ligands reduces the impetus for
increased electron donation from the diamido ligand, resulting
in longer Th−N bond lengths in 1·Et2O. The Th(1)−O(1)
bond distance of 2.652(11) Å is also longer than in
{[tBuNON]ThCl2}2, where the Th−O distance is 2.531(17) Å
but is more comparable with the Th−O distance of 2.663(13)
Å in the “ate” complex {[(Me3 PhNSiMe2 )2O]-
ThCl3Li·THF}2.12a,13 The Th(1)−O(3) distance (to the
terminal OiPr group) of 2.153(10) Å is similar to the terminal
Th−O distances in {Th(OiPr)4}2, which range from 2.141(11)
to 2.160(11) Å, while the bridging Th(1)−O(2) and Th(1)−
O(4) distances of 2.298(9) and 2.285(11) Å, respectively, are
somewhat shorter than the bridging Th−O distances in
{Th(OiPr)4}2 (2.408(10) Å).9c
The 1H NMR spectrum of the paramagnetic 2 in benzene-d6
contains seven broad, highly shifted resonances and is
consistent with an overall C2-symmetric structure in solution.
Although not all of the resonances can be assigned on the basis
of the integrations, some observations can be made. There are
four distinct resonances that each integrate to 6H, indicating
that the silyl methyl groups are inequivalent and that there are
two environments for the OiPr substituents; this suggests that
the dimeric structure is maintained in solution. The two
resonances that integrate to 1H at δ −16.24 and 77.90 ppm can
be assigned to the two CH(CH3)2 hydrogens, and the
resonance at δ −49.55 ppm integrates to 18H and can be
assigned to the tert-butyl methyl groups of the diamido ligand.
Addition of 2 equiv of KOtBu to {[tBuNON]UCl2}2 in
toluene gave a light brown solution from which a red-brown
solid of {[tBuNON]U(OtBu)2}2 (3) was isolated in 89% yield
(Scheme 1). Crystals of 3 were grown by slow evaporation of a
pentane solution, but unfortunately they were not suitable for
Addition of 2 equiv of LiOiPr to {[tBuNON]UCl2}2 in
toluene gave a cloudy green-brown solution that after 18 h and
workup resulted in a green-brown powder of {[tBuNON]U-
(OiPr)2}2 (2) in 93% yield (Scheme 1). Crystals of 2 suitable
for X-ray diffraction were grown by slow evaporation of a
toluene solution. In the solid state, a dimeric structure is
observed in which each uranium atom exists in a distorted-
octahedral environment supported by the meridionally bound
tridentate [tBuNON]2− ligand, one terminal isopropoxide ligand
per uranium and two isopropoxide ligands that bridge the two
metal centers (Figure 2); unlike 1, this system does not retain
salt and is not an “ate” complex. The U(1)−N(1) distance of
2.20(2) Å in 2 is slightly longer than in {[tBuNON]UX2}2 (X =
Cl0.54/Br0.46),12a which has U−N distances of 2.145(16) and
2.130(18) Å. However, these U−N distances are more
comparable with the U−N distances of 2.224(8) and
2.223(8) Å in {[(iPr2PhNSiMe2)2O]UCl2}2 and the U−N
distances of 2.194(3) and 2.215(4) Å in {(Me3SiN-
(CH2CH2NSiMe3)2)UCl2}2.11c,14 The U(1)−O(1) distance of
2.50(3) Å is also similar to but slightly shorter than the U−O
distance in {[(iPr2PhNSiMe2)2O]UCl2}2 (2.567(7) Å).11c The
U(1)−O(2) distance of 2.12(2) Å to the terminally bound OiPr
1
single-crystal X-ray diffraction. The H NMR spectrum of 3 in
benzene-d6 contains four broad, highly shifted resonances
typical of paramagnetic complexes. Two of these resonances at
δ −44.57 and −42.51 ppm integrate to 6H and can be assigned
to the silyl methyl groups. The other two resonances at δ
−37.55 and −10.07 ppm both integrate to 18H and are
consistent with the tert-butyl groups of the diamido ligand and
two alkoxide ligands but could not be further assigned.
Addition of two equiv of KOtBu to [iPr PhNCOCN]-
2
ThCl2·DME in DME gave a pale yellow solution from which
a light yellow powder of [iPr PhNCOCN]Th(OtBu)2 (4) was
2
obtained in 79% yield (Scheme 2). The NMR spectra and
microanalysis data are consistent with this formulation, but no
X-ray-quality crystals could be obtained. Although no X-ray
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Scheme 2. Synthesis of Complexes 4 and 5
Chart 1. Diamido-Actinide Complexes 1−9 Used in the ROP
of Lactide
data could be obtained for 4, previous reports using the
iPr PhNCOCN]2− ligand indicate that it may coordinate in
2
[
either a facial20 or meridional fashion.11c,13 The H NMR
spectrum of 4 in toluene-d8 displays six distinct resonances
consistent with a C2v-symmetric structure in solution,
suggesting that the ligand is indeed coordinated in a meridional
1
backbone, the choice of actinide metal, and the choice of
alkoxide versus alkyl reactive group.
The results presented in Table 1 indicate that complexes 1−
8 are all active initiators for the ROP of L-LA in toluene
solution under ambient conditions, affording moderate to good
control over the polymerization parameters; only 9 proved
repeatedly inactive under the chosen experimental conditions.
In almost all cases (except complexes 4 and 5 in entries 6 and
8), nearly complete conversion of 50 equiv of monomer was
observed in less than 1 h. The distribution of the molecular
weights remained monodispersed but spanned across a
somewhat broad range, with values of Mw/Mn typically in the
range 1.1−1.6. No clear relationship emerged from the
comparison between the theoretical (calculated under the
hypothesis of growth of a single PLLA chain per metal center)
and calculated molecular weights.15 Complexes 2, 4, and 7
stood out and gave generally a good agreement between these
two values; this was accompanied by narrow molecular weight
distributions expected for a well-behaved ROP initiator (Table
1, entries 2 and 3, 6 and 7, and 10 and 11). The control proved
less efficient, however, when larger loadings of monomer were
used (500 equiv, entries 4 and 12); the decreased Mn values (as
compared to those obtained with 200 equiv) and the broader
polydispersities likely call for transesterification and/or transfer-
to-monomer side reactions under these conditions. Overall, the
influence of the identity of the alkoxo initiating group (tBuO−
vs. iPrO−) on the final outcome of the polymerization could not
be rationalized; compare, for instance, entries 2, 5, and 6 in
Table 1. Note, however, that perhaps unsurprisingly the “ate”
complex 1 generally led to fast but poorly controlled reactions
(entry 1).
fashion. Notably, the [iPr PhNCOCN]2− ligand is symmetric and
2
no hindered rotation in the room-temperature 1H NMR
spectrum is observed; one doublet integrating to 24H exists for
the aryl isopropyl groups, in addition to one septet for the
CH(CH3)2 hydrogens, two triplets for the backbone CH2
groups, and a multiplet for the aryl hydrogens. One broad
resonance that integrates to 18H is also observed for the two
tert-butoxide groups. The 13C{1H} NMR data also support this
formulation (see the Experimental Section).
Addition of 2 equiv of KOtBu to [iPr PhNCOCN]-
2
UCl3Li·2THF in toluene gave a red-brown solution that gave
after 18 h and workup a red-brown solid of [iPr PhNCOCN]-
2
U(OtBu)Cl (5) (Scheme 2); however, the product was
somewhat impure. In an effort to prepare a purer sample of
5, the reaction was repeated using 1 equiv of KOtBu. After
workup and isolation (90% yield), a sample of 5 with
microanalysis data consistent with the formulation
iPr PhNCOCN]U(OtBu)Cl was obtained. The 1H NMR
2
[
spectrum of 5 in toluene-d8 contains 10 broad, highly shifted
resonances. This is consistent with a C2v-symmetric structure in
solution. Two resonances that integrate to 12H at δ 11.50 and
12.25 ppm can be assigned to two sets of isopropyl methyl
groups. Further evidence of the ligand coordination can be
observed from two other resonances that integrate to 4H at δ
−60.16 and 56.44 ppm; both of these resonances are consistent
with the alkyl-ether backbone of the ligand, although it is not
possible to assign the two resonances to a particular methylene
in the OCH2CH2N backbone. The remaining five ligand-
related resonances all integrate to 2H and could not be further
assigned. There is also one very broad resonance that integrates
to 9H and is consistent with the presence of only one tert-
butoxide group on the metal center.
Generally, the reactivities of complexes supported by the
[
tBuNON]2− ligand are higher than those of complexes
supported by the [iPr PhNCOCN]2− ligand. Complex 2 thus
offered the best compromise between reaction rates and control
over the ROP parameters. In particular, reactions catalyzed by 2
continued to exhibit high conversions and are adequately
controlled (Mn(calcd) ≈ Mn(SEC); Mw/Mn = 1.26) even in the
presence of 200 equiv of L-LA (Table 1, entry 3); at higher
monomer loading (500 equiv), good conversions were still
observed within a reasonably short time (90 min) but, as
mentioned above, the molecular weight distributions increased
2
Lactide Polymerization. Upon developing this series of
new diamido-based actinide alkoxide complexes, we sought to
determine their ability as initiators/catalysts for the ROP of
lactide. Thus, the performance of the aforementioned
complexes 1−5 and that of the previously reported dialkyl
complexes [tBuNON]An(CH2SiMe3)2 (An = Th (6), U (7))12a
1
and [iPr PhNCOCN]An(CH2SiMe3)2 (An = Th (8), U
2
noticeably (Mw/Mn = 1.54, entry 4). The H NMR spectra of
(9))11c,13 (Chart 1) in the ROP of L- and rac-lactide were
assessed. The systematic use of different diamido ligands and
different alkoxide ligands gives the opportunity to compare
reactivities among the choice of amido-R group and ligand
the polymers produced by 1 and 2 contain resonances that are
in full agreement with the exclusive presence of both the
iPrOC(O)CH(CH3)− and −CH(CH3)OH end groups; no
indication for the presence of diamido-ether moieties at the end
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a
Table 1. Ring-Opening Polymerization of L-LA Promoted by Complexes 1−9
b
c
d
e
e
entry
compd
[L-LA]0/[An]0
time (min)
conversn (%)
Mn(calcd) (g mol−1
)
Mn(SEC) (g mol−1
)
Mw/Mn
f
1
1
2
2
2
3
4
4
5
6
7
7
7
8
9
50
50
120
30
60
90
30
30
120
30
30
30
60
90
30
95
86
91
69
47
14
100
17
86
94
89
87
79
<1
7000
6300
26300
65000
3500
1100
7300
1400
6300
6900
25800
62800
5800
9000
1.63
2
7700
1.16
1.26
1.54
1.50
1.24
1.15
3
200
500
50
28900
25700
8000
4
5
6
50
4500
7
50
6800
f
8
50
9800
1.49
9
50
29000
11100
35000
29900
14800
n/a
2.16
1.12
1.12
1.59
10
11
12
13
14
50
200
500
50
f
1.91
50
30
n/a
n/a
a
b
c
d
Conditions: 1.0 M solution of L-LA in toluene, 30 °C. The reaction time was not optimized. Isolated yield after reprecipitation. Calculated from
e
Mn(calcd) = (144.13 × ([L-LA]0/[An]0) × conversion) + Mn(ROH) (ROH = tBuOH, iPrOH), for 1 PLLA chain per metal center. Mn (g mol−1)
determined by SEC-RI against polystyrene standards and corrected by a factor of 0.58. SEC analysis indicated a bimodal distribution.
f
of the polymer chains was ever observed. This is consistent
with a coordination−insertion ROP scenario where initiation
results from acyl cleavage following nucleophilic attack of only
the isopropoxide group (and not the amido ligand
[
tBuNON]2−) on the coordinated monomer and subsequent
1
opening of the heterocycle. Similarly the H NMR spectra of
polymers produced by complexes 3−5 have resonances that are
t
consistent with BuOC(O)CH(CH3)− and −CH(CH3)OH
end groups; again, there was no evidence in the NMR spectra
to suggest that the actinide catalyst decomposed under ROP
conditions or that the diamido-ether ancillary ligand, be it
[
iPr PhNCOCN]2− or [tBuNON]2−, acts as an initiating group in
2
these ROP reactions.
We sought to investigate the reaction kinetics with these
diamido actinide catalysts via in situ monitoring of a ROP
reaction by 1H NMR spectroscopy. Ideally, 2 would be the best
candidate for these kinetic measurements because it exhibits a
well-controlled polymerization behavior and enables the ROP
of monomer loadings suited to NMR-scale reactions. However,
2 (Table 1, entries 2−4) completely converts 50−200 equiv of
L-LA in less than 1 h, and while this is desirable for an active
initiator, it makes NMR monitoring more difficult, and a slower
catalyst would be easier to follow. Furthermore, 2 is
paramagnetic, which precludes accurate NMR monitoring of
ROP reactions. Complex 1 (the available diamagnetic complex
most similar to 2 since it contains the same ancillary ligand and
Figure 3. Plot of monomer conversion vs reaction time for the ROP of
L-LA catalyzed by 4, with [L-LA]/[4] = 50/1, [L-LA]0 = 1.0 M in
toluene-d8, and 30 °C.
The bottom half of Table 1 pertains to the ROP of L-LA
catalyzed by the previously reported diamido dialkyl complexes
6−9. Unlike the related alkoxide complexes 1 and 2, the
dialkyls 6−9 portray a wider range of control over polymer-
ization. The uranium-based 7, [tBuNON]U(CH2SiMe3)2,
notably afforded the best control of all four dialkyl complexes
(entries 10−12). The thorium dialkyls 6 and 8 gave PLLAs
presenting a broad polydispersity, while the second uranium
complex, 9, did not promote any polymerization over the
allotted time period, forming instead amorphous oils that could
not be characterized. End-group analysis of a PLLA sample
produced from 7 was performed via NMR spectroscopy, in an
effort to determine which of the alkyl or diamido ligands acts as
i
reactive nucleophile, [tBuNON]2− and PrO−, respectively) was
not selected because it only afforded poorly controlled ROP
reactions. Instead, the reaction kinetics of 4 were examined,
since this compound demonstrated similar control over the
polymerization to 2 (Table 1, entries 2 and 7), is diamagnetic,
and takes somewhat longer than 2 to reach full conversion of
50 equiv of monomer, making it more compatible for NMR
monitoring. Figure 3 contains a plot of monomer conversion vs
reaction time for the ROP of L-LA catalyzed by 4 in toluene-d8
under ambient conditions. A short induction period was
observed; the reaction followed partial first-order kinetics with
respect to monomer concentration, and the value kapp = 0.408
× 10−3 s−1 was extracted from the semilogarithmic plot of
monomer conversion vs reaction time (Supporting Informa-
tion).
1
the initiating group in these ROP. The H, 13C{1H}, and 2D
(HMBC, HMQC) NMR analyses confirmed that the alkyl
group Me3SiCH2− does initiate the polymerization; no
evidence for an amide-based end group was detected. In
addition, the combined NMR data indicate that the −C(
O)CH2SiMe3 group at the polymer chain end did not remain
intact following quenching of the reaction with acidified
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1
Figure 4. H NMR spectrum (500.13 MHz, chloroform-d, 298 K) of a CH3C(O)− terminated PLLA prepared with 7 (Table 1, entry 10).
Asterisks denote 13C satellites associated with resonances d and e.
Figure 5. 13C{1H} NMR spectrum (chloroform-d, 125.78 MHz, 298 K) of a CH3C(O)− terminated PLLA prepared with 7 (Table 1, entry 10).
methanol and complete workup. Instead, the C(O)CH2−
SiMe3 bond was quantitatively hydrolyzed and a methyl ketone
group, −C(O)CH3, solely remained as this end of the
polymer chain (Figures 4 and 5),16 together with −CH(CH3)-
OH as the opposite end group. Complete, corroborative NMR
analysis is provided in the Supporting Information. To further
confirm this interpretation, the same PLLA sample was further
analyzed by MALDI-TOF mass spectrometry. The mass
spectrum shows the presence of a major population of chains
terminated by a CH3C(O)CH(CH3)− fragment, with
increments of 72 Da between consecutive peaks corresponding
to half a L-LA unit; these data were diagnostic of extended
transesterification side reactions (if transesterification would
have not occurred, only lactide repeat units would be observed
(i.e., 144 Da increments)) and provided additional evidence for
the proposed composition of the PLLA chains, namely
CH3C(O)CH(CH3)[O(C(O)CH(CH3)]nOC(O)CH-
(CH3)OH. Although a number of reports of alkyl end-capped
polylactides can be found in the literature,17 these prior
characterizations are incomplete and we therefore believe this is
the first time that ROP initiation by a −CH2SiMe3 moiety
leading to the formation of PLLA chains has been
unambiguously evidenced.
Complex 2, the most competent ROP initiator in the series
of actinide complexes 1−9, was employed for additional studies
involving rac-LA, the results of which are summarized in Table
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Organometallics
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a
Table 2. ROP of rac-LA with 2
b
c
d
e
e
entry
solvent
time (h)
conversn (%)
Mn(calcd) (g mol−1
)
Mn(SEC) (g mol−1
)
Mw/Mn
Pr
1
2
Tol
2
2
68
49
5000
3600
9600
1.24
1.25
0.61
0.73
THF
5200
a
b
c
1
Conditions: [rac-LA]0 = 1.0 M, [rac-LA]0/[2]0 = 50/1, 30 °C. The reaction time was not optimized. Conversion determined by H NMR.
d
e
Calculated from Mn(calcd) = (144.13 × ([rac-LA]0/[2]0) × conversion) + 62.10. Mn (g mol−1) determined by SEC-RI against polystyrene
standards and corrected by a factor of 0.58.
a sodium/benzophenone solution under nitrogen. Hexanes were
distilled from a sodium solution under nitrogen. Deuterated solvents
were distilled from a sodium/benzophenone solution. UCl4,18
2. The ROP reactions were relatively well controlled in toluene
with relatively low polydispersity values (Mw/Mn = 1.24), but
we found that in this case the reaction proceeded more slowly
than previously with the enantiopure L-LA. The homonuclear
ThCl4 ·2DME,1 9 (t BuNH(SiMe2 ))2 O ([t B u NON]H2 ),2 0
21
(2,6-iPr2PhNH(CH2CH2))2O ([iPr PhNCOCN]H2), {[tBuNON]-
2
1
decoupled H NMR spectrum of PLA produced with 2 in
UCl2}2,12a
[
tBuNON]ThCl5Li3·DME,11c
[
PhNCOCN]-
[
iPr2
toluene indicates a polymer with an estimated Pr value of 0.61
(entry 1), indicating a slight tendency toward heterotacticity.
This value was increased by changing the solvent to THF
(entry 2), with an estimated Pr value of 0.73, diagnostic of a
more significant heterotactic enrichment. This improvement in
stereocontrol, however, came at the expense of reaction rates,
likely attributable to competitive coordination of THF onto the
metal center. Lowering the temperature to 5 °C resulted in
poor yields after 4−12 h for no benefit in terms of
stereoselectivity, and efforts to maximize catalytic activity
were therefore not pursued; note that high conversions were
observed when the reaction time was extended to 24 or 48 h at
this temperature, but the resulting materials consisted of
intractable oils (probably as a result of excessive deleterious
transesterification side reactions over the prolonged reaction
times) which we failed to characterize adequately.
11c
tBuNON]U-
[
iPr PhNCOCN]U-
UCl3Li·2THF,13
[
iPr PhNCOCN]ThCl2·DME,
2
(CH2SiMe3)2,12a
[
tBuNON]Th(CH2SiMe3)2,11c,12a
2
(CH2SiMe3)2,13 and [iPr PhNCOCN]Th(CH2SiMe3)211c were prepared
in accordance with published literature procedures. L-lactide and rac-
lactide were purified by recrystallization once from iso-propanol and
twice more from anhydrous toluene, then dried in vacuo. All other
reagents were purchased from commercial sources and used without
further purification.
2
NMR spectra were recorded at 294 K, unless otherwise stated, on a
400 MHz Bruker Avance III spectrometer, a 500 MHz Bruker Avance
III spectrometer, or a 600 MHz Bruker Avance II spectrometer with a
1
5 mm QNP cryoprobe. H and 13C NMR shifts are reported in ppm
1
relative to residual solvent resonances: specifically for H, benzene-d6
at δ 7.15 or toluene-d8 at δMe 2.06 ppm, and for 13C, benzene-d6 at δ
128.06 or toluene-d8 at δMe 20.43 ppm. Elemental analyses (C, H, N)
were performed at Simon Fraser University by Mr. Farzad
Haftbaradaran employing a Carlo Erba EA 1110 CHN elemental
analyzer. Size exclusion chromatography (SEC) analyses of PLAs were
performed on a Polymer Laboratories PL-GPC 50 instrument
equipped with two PLgel 5 Å MIXED-C columns and a refractive
index detector. The column was eluted with THF at room temperature
at 1.0 mL min−1 and was calibrated using 11 monodisperse polystyrene
standards in the range of 580−380000 g mol−1. The molecular weights
of all PLAs were corrected by a factor of 0.58. MALDI-TOF mass
spectra of PLAs were obtained with a Bruker Daltonic MicroFlex LT
apparatus, using a nitrogen laser source (337 nm, 3 ns) in linear mode
with a positive acceleration voltage of 20 kV. Samples were prepared as
follows: 1 μL of a 2/1 mixture of a saturated solution of α-cyano-4-
hydroxycinnamic acid (Bruker Care) in HPLC-quality acetonitrile and
a 0.1% solution of trifluoroacetic acid in ultrapure water were
deposited on the sample plate. After total evaporation, 1 μL of a 5−10
mg mL−1 solution of the polymers in HPLC-quality THF was
deposited. Bruker Care Peptide Calibration Standard and Protein
Calibration Standard I were used for external calibration.
CONCLUSIONS
■
Diamido-ether actinide alkoxide complexes 1−5 were prepared
in good to high yields from the corresponding diamido actinide
halide complexes and were characterized. Complex 1 is an “ate”
complex, while compounds 2−5 are salt and solvent free.
Complexes 1−5 and the diamido actinide dialkyls 6−9 were
evaluated as initiators for the ROP of L-LA. It was determined
that complexes 1−8 were active for the polymerization of 50
equiv of monomer at room temperature. Except for complexes
4 and 5, the reaction reaches completion in less than 1 h. This
represents the first report of heteroleptic amide actinide
alkoxide complexes capable of facilitating this reaction and is
also in fact one of only a few reports of actinide-based catalysts
that have been used for this reaction. Analysis of the polymers
synthesized by 1−8 indicated a range of control over the
polymerizations where generally the uranium complexes
exhibited the best control over the polymerization parameters.
End-group analyses of the polymers showed that the diamido
ligands do not participate as initiating groups in the
polymerization; moreover, examination of PLLAs prepared by
dialkyl complexes 6−8 demonstrated initiation by the alkyl
moiety in these initiators. Complex 2 was also evaluated for the
ROP of rac-lactide. The PLAs obtained using rac-lactide
displayed a propensity toward heterotacticity, with a Pr value up
to 0.73 in THF.
[
tBuNON]Th(OiPr)3Li·Solv (1-solv). [tBuNON]ThCl2·DME (0.100
g, 0.17 mmol) was dissolved in toluene (20 mL), and 3 equiv of LiOiPr
(0.52 mL, 0.52 mmol) dissolved in toluene (20 mL) was added
dropwise via syringe at room temperature. Upon addition the reaction
became cloudy and pale yellow. Stirring was continued at room
temperature for 18 h, after which the reaction was filtered through a
Celite-padded medium-porosity glass frit, resulting in a clear, pale
yellow solution. Excess toluene was removed in vacuo to yield a beige-
yellow solid of [tBuNON]Th(OiPr)3Li·DME (1-DME; 0.106 g, 79%).
Crystals suitable for X-ray diffraction were grown by slow evaporation
of a toluene/diethyl ether solution (1-Et2O) at room temperature.
Anal. Calcd for C25H61LiN2O6Si2Th: C, 38.45; H, 7.87; N, 3.59.
1
Found: C, 38.65; H, 7.71; N, 3.57. H NMR (toluene-d8, 500 MHz,
298 K): δ 0.44 (s, 6H, Si(CH3)2), 0.50 (s, 6H, Si(CH3)2), 1.26 (d, 3J =
7.5 Hz, 12H, OCH(CH3)2), 1.41−1.50 (overlapping signals, d, 6H,
CH(CH3)2 and s, 18H, C(CH3)3), 2.81 (s, 4H, OCH2, DME), 3.04 (s,
EXPERIMENTAL SECTION
■
General Considerations. All techniques and procedures were
carried out under a nitrogen atmosphere either with an Mbraun
Labmaster 130 glovebox or using standard Schlenk and vacuum-line
techniques. All glassware was dried overnight at 160 °C prior to use.
Toluene, tetrahydrofuran (THF), and diethyl ether were distilled from
3
6H, OCH3, DME), 4.59 (br m, 2H, CH(CH3)2), 4.66 (sept, J = 7.5
Hz, 1H, CH(CH3)2). 1H NMR (toluene-d8, 400 MHz, 233 K): δ 0.50
(s, 6H, Si(CH3)2), 0.54 (s, 6H, Si(CH3)2), 1.2−1.3 (vv br s, 12H,
CH(CH3)2), 1.52−1.54 (overlapping signals, 24H, d, CH(CH3)2, and
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1
s, C(CH3)3), 2.50 (s, 4H, OCH2, DME), 2.88 (s, 6H, OCH3, DME),
4.5 (v br s, 2H, CH(CH3)2), 4.61 (br s, 1H, CH(CH3)2). H NMR
N, 3.76. H NMR (toluene-d8, 400 MHz, 298 K): δ −78.97 (2H),
−72.85 (2H), −67.99 (2H), −60.16 (4H, NCH2CH2O), 1.08 (v br,
9H, OC(CH3)3), 11.50 (12H, N−Ar−CH(CH3)2), 12.25 (12H, N−
Ar−CH(CH3)2), 45.99 (2H), 56.44 (4H, NCH2CH2O), 59.73 (2H).
Lactide Polymerization. In a typical polymerization procedure
(Table 1, entry 1), a 30 mL Schlenk flask was charged with 0.015 g of
1 (or another compound, as appropriate) and 50 equiv of L-lactide
(0.139 g). The two compounds were dissolved in 1.8 mL of toluene to
give a 1.0 M solution of lactide. The reaction mixture was placed in an
oil bath heated to 30 °C for the specified time (usually 30 min) with
magnetic stirring. After the desired time, the reaction was quenched via
the addition of a few drops of a 3% HCl/MeOH solution and the
polymer was precipitated as a white solid through the addition of
excess MeOH. The solid precipitate was collected via filtration and
dried overnight in vacuo to a constant weight.
1
(toluene-d8, 400 MHz, 193 K): δ 0.54 (s, 6H, Si(CH3)2), 0.58 (s, 6H,
Si(CH3)2), 1.21 (s, 6H, CH(CH3)2), 1.30 (s, 6H, CH(CH3)2), 1.58 (s,
ca. 18H, C(CH3)3), 1.58−1.65 (v br sh, ca. 6H, CH(CH3)2), 2.32 (s,
4H, OCH2, DME), 2.78 (s, 6H, OCH3, DME), 4.61 (br s, 1H,
CH(CH3)2), 4.69 (br s, 1H, CH(CH3)2), 4.87 (s, 1H, CH(CH3)2).
13C{1H} NMR (toluene-d8, 125 MHz, 298 K): δ 6.91 (Si(CH3)2), 6.94
(Si(CH3)2), 28.80 (CH(CH3)2), 28.99 (C(CH3)3), 35.81 (CH-
(CH3)2), 58.92 (O−CH3, DME), 66.42 (CH(CH3)2), 70.03 (O−
CH2, DME), 72.58 (CH(CH3)2).
{[tBuNON]U(OiPr)2}2 (2). {[tBuNON]UCl2}2 (0.100 g, 0.171 mmol)
was dissolved in toluene (20 mL), and 2 equiv of LiOiPr (0.34 mL,
0.343 mmol) dissolved in toluene (20 mL) was added dropwise via
syringe at room temperature. Upon addition the reaction mixture
turned golden brown. Stirring was continued at room temperature for
18 h, after which the reaction mixture was filtered through a Celite-
padded medium-porosity glass frit, resulting in a clear yellow-brown
solution. Excess toluene was removed in vacuo to yield a brown solid of
{[tBuNON]U(OiPr)2}2 (2; 0.233 g, 93%). Crystals suitable for X-ray
diffraction were grown by slow evaporation of a toluene solution at
room temperature. Anal. Calcd for C18H44N2O31Si2U: C, 34.28; H,
7.03; N, 4.44. Found: C, 33.95; H, 6.63; N, 4.24. H NMR (benzene-
d6, 400 MHz, 298 K): δ 100.18 (br, 6H), 77.90 (1H, OCH(CH3)2),
41.47 (br, 6H), −16.24 (1H, OCH(CH3)2), −24.72 (br, 6H), −31.45
(br, 6H), −49.55 (br, 18H, NC(CH3)3).
X-ray Crystallography. Crystallographic data for 1·Et2O and 2
are collected in Table 3. Both crystals were coated in Paratone oil,
Table 3. Summary of Crystallographic Data
1·Et2O
2
formula
Mw/g mol−1
cryst dimens/mm
cryst syst
space group
T/K
C50H119Li2N4O10Si4Th2
C36H88N4O6Si4U2
1261.52
0.35 × 0.15 × 0.10
orthorhombic
Cmca
1526.81
0.13 × 0.10 × 0.10
orthorhombic
Pnam
[
tBuNON]U(OtBu)2 (3). {[tBuNON]UCl2}2 (0.100 g, 0.171 mmol)
was dissolved in toluene (20 mL), and 2 equiv of KOtBu (0.038 g,
0.343 mmol) suspended in toluene (20 mL) was added slowly at room
temperature. Within 5 min of addition the reaction mixture turned
light brown. Stirring was continued for 18 h at room temperature
without any further changes, after which the reaction was filtered
through a Celite-padded medium-porosity glass frit, resulting in a clear
red-brown solution. Excess toluene was removed in vacuo to give a red-
brown solid of [tBuNON]U(OtBu)2 (3; 0.101 g, 89%). Anal. Calcd for
C45H108N4O6Si4U2: C, 38.89; H, 7.83; N, 4.03. Found: C, 39.20; H,
150
150
a/Å
26.4650(19)
11.1898(8)
12.9222(9)
90
17.5704(19)
12.6855(14)
22.676(3)
90
b/Å
c/Å
α/deg
β/deg
90
90
γ/deg
90
90
V/Å3
3826.8(5)
2
5054.2(10)
4
Z
1
7.81; N, 3.47. H NMR (benzene-d6, 400 MHz, 298 K): δ −44.57
Dc/g cm−3
μ/cm−1
1.328
1.658
(6H, Si(CH3)2), −42.51 (6H, Si(CH3)2), −37.55 (v br, 18H), −10.07
3.988
6.536
(v br, 18H).
[
a
iPr PhNCOCN]Th(OtBu)2 (4). [iPr PhNCOCN]ThCl2·DME (0.200 g,
2
2
R (I > 2.5σ(I))
0.0760
0.0983
a
0.245 mmol) was dissolved in DME, and 2 equiv of KOtBu (0.055 g,
0.490 mmol) dissolved in DME was added slowly at room
temperature. The reaction mixture immediately turned a cloudy
orange-brown. Stirring was continued for 24 h at room temperature,
after which the reaction was filtered through a Celite-padded medium-
porosity glass frit, resulting in a clear light orange solution. Excess
Rw (I > 2.5σ(I))
0.0742
0.0728
GOF
3.5262
5.4911
a
The function minimized was ∑w(|Fo| − |Fc|)2, where w−1 = [σ2(Fo) +
(nFo)2] with n = 0.01 for 1·Et2O and 0.00 for 2. R = ∑||Fo| − |Fc||/∑|
Fo|; Rw = [∑w(|Fo| − |Fc|)2/∑w|Fo|2]1/2
.
DME was removed in vacuo to give an orange solid of [iPr PhNCOCN]-
2
Th(OtBu)2 (4; 0.155 g, 79%). Anal. Calcd for C36H60N2O3Th: C,
53.99; H, 7.55; N, 3.50. Found: C, 53.17; H, 7.24; N, 3.34 (the values
are all slightly low, most likely because a small amount (less than 0.25
equiv) of KCl cannot be separated from the compound due to their
mounted on a MiTeGen Micro Mount, and transferred to the cold
stream (150 K) of the X-ray diffractometer. Crystal descriptions for
each compound are as follows: 1·Et2O was a clear, colorless
rectangular plate and 2 was a brown-green arrowhead-shaped plate.
Both crystals, while they diffracted strongly, were not of high quality
(mild twinning etc.), and despite multiple efforts to prepare good
crystals, the final R values reflect this circumstance. All data were
collected on a Bruker Smart instrument equipped with an APEX II
CCD area detector fixed at a distance of 6.0 cm from the crystal and a
Mo Kα fine-focus sealed tube (λ = 0.71073 nm) operated at 1.5 kW
(50 kV, 30 mA) and filtered with a graphite monochromator. The
temperature was regulated using an Oxford Cryosystems Cryostream;
both crystals were collected at 150 K. Data reduction and absorption
correction details can be found in the Supporting Information.
The structures were solved using direct methods (SIR 92) and
refined by least-squares procedures using CRYSTALS.22 Hydrogen
atoms on carbon atoms were included at the geometrically idealized
positions (C−H bond distance 0.95 Å) and were not refined. The
isotropic thermal parameters of the hydrogen atoms were fixed at 1.2
times that of the preceding carbon atom. The plots for the crystal
structures were generated using ORTEP-3 for windows (v. 2.00)23 and
rendered using POV-Ray (v. 3.6.1).24 Thermal ellipsoids are shown at
the 30% probability level.
1
very similar solubilities). H NMR (toluene-d8, 600 MHz, 298 K): δ
1.09 (br s, 18H, OC(CH3)3), 1.27 (d, J = 10.2 Hz, 24H, N−Ar−
3
3
3
CH(CH3)2), 3.44 (t, J = 7.8 Hz, 4H, NCH2CH2O), 3.70 (sept, J =
10.2 Hz, 4H, N−Ar−CH(CH3)2), 3.80 (t, 3J = 9.6 Hz, 4H,
NCH2CH2O), 7.09−7.13 (m, 12 H, N-Ar). 13C{1H} NMR (toluene-
d8, 150 MHz, 298 K): δ 14.32 (N−Ar−CH(CH3)2), 22.75
(OC(CH3)3), 24.53 (N−Ar-CH(CH3)2), 27.89 (OC(CH3)3), 33.27
(NCH2CH2O), 34.44 (NCH2CH2O), 124.03 (Ar−C), 124.48 (Ar−C),
142.90 (N−Ar−Cipso).
iPr PhNCOCN]U(OtBu)Cl (5). [iPr PhNCOCN]UCl3Li·THF (0.190
2
2
[
g, 0.260 mmol) was dissolved in toluene (20 mL), and 1 equiv of
KOtBu (0.029 g, 0.260 mmol) suspended in toluene (20 mL) was
added slowly at room temperature. The reaction mixture immediately
turned dark red brown. Stirring was continued for 24 h at room
temperature, after which the reaction mixture was filtered through a
Celite-padded medium-porosity glass frit, resulting in a clear red-
brown solution. Excess toluene was removed in vacuo to give a brown
solid of [iPr PhNCOCN]U(OtBu)Cl (5; 0.180 g, 90%). Anal. Calcd for
2
C32H51N2O2ClU: C, 49.96; H, 6.68; N, 3.64. Found: C, 50.10; H, 6.70;
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For 1·Et2O and 2 the coordinates and anisotropic displacement
parameters for all non-hydrogen atoms were refined, with the
exception of the terminally bound OiPr group for both crystals, one
of the bridging OiPr groups in 1·Et2O, and the ether molecule in
1·Et2O. These OiPr groups were found to be rotationally disordered in
all three cases and were treated accordingly. The ether molecule was
also found to be disordered over two positions and was treated
accordingly; the larger of the two components of the disorder is shown
in Figure 1.
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ASSOCIATED CONTENT
(6) (a) Katz, J. J.; Seaborg, G. T.; Morss, L. R. The Chemistry of the
Actinide Elements; Chapman and Hall: New York, 1986. (b) Barnea, E.;
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■
S
* Supporting Information
CIF files and figures giving complete crystallographic data for
all reported crystal structures and H and 13C NMR spectra of
1
the diamagnetic complexes, as well as representative NMR and
mass spectra of some polymers. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*D.B.L: tel, 1-778-782-4887; fax, 1-778-782-3765; e-mail,
dleznoff@sfu.ca. J.-F.C.: tel, 33-223-235-950; fax, 33-223-236-
939; e-mail, jean-francois.carpentier@univ-rennes1.fr.
■
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSERC of Canada and CNRS
of France. C.E.H. is grateful to the NSERC for a Doctoral Post-
Graduate Scholarship.
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Organometallics
Article
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