Bis(phosphinic)diamido yttrium amide, alkoxide, and aryloxide complexes: An evaluation of lactide ring-opening polymerization initiator efficiency

Article abstract of DOI:10.1021/ic200773x

The synthesis and characterization of a series of bis(phosphinic)diamido yttrium alkoxide, amide, and aryloxide initiators are reported. The new complexes are characterized using multinuclear nuclear magnetic resonance (NMR) spectroscopy, elemental analys

Full text of DOI:10.1021/ic200773x

ARTICLE
pubs.acs.org/IC
Bis(phosphinic)diamido Yttrium Amide, Alkoxide, and Aryloxide
Complexes: An Evaluation of Lactide Ring-Opening Polymerization
Initiator Efficiency
Rachel H. Platel, Andrew J. P. White, and Charlotte K. Williams*
Department of Chemistry, Imperial College London, London SW7 2AZ, U.K.
S Supporting Information
b
ABSTRACT: The synthesis and characterization of a series of
bis(phosphinic)diamido yttrium alkoxide, amide, and aryloxide
initiators are reported. The new complexes are characterized
using multinuclear nuclear magnetic resonance (NMR) spec-
troscopy, elemental analysis, and, in some cases, X-ray crystal-
lography. The alkoxide complexes are all dimeric in both the
solid state and in solution, as are the amide complexes sub-
stituted with iso-propyl or phenyl groups on the phosphorus
atoms. On the other hand, increasing the steric hindrance of the
phosphorus substituents (tert-butyl), enables isolation of mono-
nuclear yttrium amide complexes with either 2,2-dimethylpro-
pylene or ethylene diamido ligand backbones. The complex of
2,6-di-tert-butyl-4-methylphenoxide is also mononuclear. All the new complexes are efficient initiators for rac-lactide ring-opening
polymerization. The polymerization kinetics are compared and pseudo first order rate constants, kobs, determined. The
polymerization control is also discussed, by monitoring the number-averaged molecular weight, Mn, and polydispersity index,
PDI, obtained using gel permeation chromatography (GPC). The alkoxide complexes are the most efficient initiators, showing very
high rates and good polymerization control, behavior consistent with rapid rates of initiation. The phenoxide and amide complexes
are less efficient as manifest by nonlinear regions in the kinetic plots, lower values for kobs, and reduced polymerization control. One
of the mononuclear yttrium amide complexes shows heteroselectivity in the polymerization of rac-lactide; however, this effect is
reduced on changing the initiating group to phenoxide or on changing the ancillary ligand diamido backbone group.
’ INTRODUCTION
mechanism proposes coordination of the lactide to the Lewis
acidic metal center, followed by attack/insertion of the labile
metal alkoxide/amide bond leading to acyl bond ring-opening of
the lactide and formation of a propagating metal alkoxide species.
Propagation occurs by repetition of the coordination and inser-
tion steps until all the monomer is consumed. Chain termina-
tions occur by exposure of the reaction mixture to moisture or
dilute acid which cleaves the metal alkoxide bond. Thus, the
chain end-groups are an ester/amide and a hydroxyl group,
respectively.
Various Lewis acidic metal alkoxide and amide complexes
have been shown to be initiators for lactide polymerization.2
Of these, yttrium complexes show some of the fastest rates and
even the potential for stereochemical control and polymeri-
zation control. Some of the earliest reports of yttrium initia-
tors focused on homoleptic alkoxide complexes; these were,
however, found to form clusters of the form Y5(μ-O)(OR)13
which were rather slow initiators.5,6 One strategy to overcome
the low rates was to prepare homoleptic yttrium alkoxides
Polylactide (PLA) is an aliphatic polyester comprising lactic
acid repeat units which derives from renewable resources and
which can be degraded after use to metabolites.1ꢀ3 The polymer
has suitable thermal and mechanical properties to be considered
as a replacement for some commodity polymers.1 In contrast to
such conventional polymers, PLA is degradable and can be
hydrolyzed to produce lactic acid, which can be metabolized,
ultimately yielding carbon dioxide and water. Lactic acid is
currently obtained via the fermentation of D-glucose, derived
from corn or sugar beet.1 Therefore PLA has the potential to
show a renewable life cycle, as some of the CO2 released during
its degradation is photosynthesized by the plant; however, it
should be noted that the production of the polymer does require
significant energy input. PLA is currently manufactured by the
ring-opening polymerization of lactide, a process initiated by
metal complexes (Figure 1).2
The driving force for the reaction is the relief of ring strain: for
a 1 M solution of lactide at 25 °C, ΔH = 22.1 kJ molꢀ1.4 The ring-
opening polymerization is proposed to occur via a coordinationꢀ
insertion reaction mechanism, a hypothesis supported by
polymer chain end-group analyses and theoretical analyses.2 The
Received: April 15, 2011
Published: July 13, 2011
r
2011 American Chemical Society
7718
dx.doi.org/10.1021/ic200773x Inorg. Chem. 2011, 50, 7718–7728
|

Inorganic Chemistry
ARTICLE
Figure 2. Synthesis of Complexes 3ꢀ5. Reagents and Conditions: (i) 2
ROH (R = Et, iPr), toluene, 298 K, 24 h, - 2 HNR2.
iso-propyl alcohol to the polymerization enabled improved rates
and control.21 The isolation and characterization of a series of
bis(phosphinic)diamido yttrium alkoxide complexes was targeted
to enable more detailed studies of the influence of an alkoxide
initiating group during lactide ring-opening polymerization.
We have already established, using 31P{1H} NMR spectros-
copy, that addition of iso-propyl alcohol to the [LYN(SiMe2H)2]
complexes does not protonate the ancillary ligand,21 thereby
providing a route to prepare and isolate various other yttrium
alkoxide complexes. Isolated heteroleptic yttrium alkoxide com-
plexes have not been extensively examined for lactide ROP,16,17
in contrast to the amido groups. We have previously reported the
synthesis, characterization, and polymerization activity of a series
of bis(phosphinic diamido)yttrium amido complexes, where the
phosphorus substituents have been iso-propyl or phenyl and
where the diamine groups have been propylene, ethylene, cyclo-
hexylene, or phenylene. Two of these amido complexes (1, 2)
were selected for further study (Figure 2)
The complexes were prepared by reaction between the
bis(phosphinic)diamine pro-ligands and [Y{N(SiMe2H)2}3-
(THF)2].20,21 The complexes formed dimeric structures, both
in the solid state and in solution. The complexes were specifically
chosen as they represent extremes in initiator activity, with
complex 1 showing the highest rates and complex 2 showing
the lowest.20,21 Each complex was reacted with 2 equivalents of
alcohol, in toluene/benzene solution, over 24 h at 298 K. This
enabled isolation of the alkoxide complexes 3ꢀ5, in 66ꢀ80%
yields (Figure 2).
Figure 1. Ring-opening polymerization of lactide, where ML = metal
center, for example, Y(III) with ancillary ligand, X = O-polymer for
propagation steps and X = NR2, OR for initiation, where R = alkyl, aryl.
in situ by addition of alcohol to more stabilized yttrium precursors;
this led to some very high active initiating systems.5,7 While
homoleptic yttrium alkoxides generally display excellent rates, the
requirement for in situ addition of alcohol complicated the char-
acterization of the active species.5ꢀ7 Research has focused on the
preparation of heteroleptic complexes; these have generally been of
the form LY(NR2), where L represents a sterically hindered
dianionic ancillary ligand and R is a sterically hindered alkyl or,
more commonly, a trialkylsilyl group. Notable among these are the
1,-ω-dithiaalkandiyl bridged bis(phenolate) yttrium complexes,
reported by Okuda, which showed excellent activities and high
heteroselectivity for rac-lactide or syndioselectivity for meso-lactide
polymerization.8,9 Independently, the groups of Carpentier and
Mountford, and others, have extensively investigated amine bridged
bis(phenolate) yttrium amido/alkyl/borohydride/alkoxide com-
plexes.10ꢀ12 Many of these complexes show good polymerization
rates and excellent heteroselectivity; in particular improved polym-
erization control is frequently observed on addition of exogeneous
alcohol as a chain transfer agent.13 Recently, Arnold et al. reported a
chiral alcohol which formed homochiral yttrium complexes, by a
ligand recognition process, and which showed high isoselectivity in
the polymerization of rac-lactide.14 Alternative ligand systems not
containing phenolate groups remain under-explored,13,15ꢀ17 pre-
senting an opportunity to understand the factors which influence
the behavior at the yttrium active site.
Our research group have focused on diamido ligand systems; in
particular, bis(phosphinic)diamido and bis(thiophosphinic)diamido)
yttrium amido complexes have shown excellent rates.18ꢀ22 The rate
shows a first order dependency on both monomer and initiator con-
centration; the pseudo first order rate constants, kobs, have been
among the highest reported.19 Herein, a series of bis(phosphinic)-
diamido yttrium amido and alkoxide complexes are reported, and
comparisons drawn between the catalytic activity (quantified by a
pseudo first order rate constant, kobs), the rates of initiation versus
propagation (quantified by kinetic analysis, Mn, and PDI), and the
potential for this initiator class to exert stereocontrol.
The 31P{1H} NMR spectra of 3ꢀ5 did not show any signals
for the yttrium amide precursor (1 or 2, respectively) or the free
ligand. Thus, ligand redistribution reactions with concomitant
formation of the homoleptic yttrium alkoxide complexes can be
ruled out. The 31P{1H} NMR spectrum, at 293 K, of 3 showed a
broad signal at 62.7 ppm, with a relative integral value of 25, as
well as several low intensity peaks. On cooling the sample to
183 K, the broad peak (62.7 ppm) split into four peaks: a pair at
68.5 and 59.1 ppm, with relative integrals of 2.5, and another
pair at 68.2 and 58.3 ppm, with relative integrals of 1 (Supporting
Information, Figure S1). The complex is assigned a dimeric
structure, whereby for each ligand, one oxygen atom bridges
between two yttrium centers (leading to the resonances at ∼68
ppm) and the other is coordinated to a single yttrium center
(resonances at 58ꢀ9 ppm). The appearance of four resonances
at lower temperatures suggests there are two isomeric dimeric
complexes; the isomerization could be due to the LY chelate ring
conformation, and/or changes to the yttrium coordination
geometry. The appearance of two isomers, both of which are
dimeric, was also observed for complex 1 (in a relative ratio of
13:1). In contrast to complex 1, whose spectrum is well resolved
at 293 K, complex 3 has an alkoxide coligand, and it is proposed
that the smaller size of this group increases the fluxionality of the
’ RESULTS AND DISCUSSION
Yttrium Alkoxide Complexes. Our research group have pre-
viously reported a series of bis(phosphinic)diamido yttrium bis-
(dimethylsilyl)amido complexes as initiators for the ring-opening
polymerization of rac-lactide. It was observed that addition of
7719
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728

Inorganic Chemistry
ARTICLE
complex. At lower temperatures, the fluxional processes are
sufficiently slowed enabling the phosphorus atoms attached to
the bridging oxygen atoms and the phosphorus atoms attached to
the terminally bound oxygen atoms to be distinguished. The 1H
NMR spectrum of 3 shows broadened resonances; a quartet and a
triplet, due to the methylene and methyl protons of the ethoxide
group, were shifted downfield compared to free ethanol.23 Satis-
factory elemental analyses results for compound 3 could not be
obtained despite multiple attempts; the structure is therefore
assigned on the basis of NMR spectroscopic data and the solid
state structure. The analogous iso-propyl alkoxide complex 4 gave
similar spectra; at 293 K the 31P{1H} NMR spectrum showed a
broad multiplet, and at 188 K a closely related spectrum to 3 was
obtained, with four peaks, two of relative integral 2 and two with
relative integral 1, assigned to two isomeric dimeric complexes
(Supporting Information, Figure S1).
in Supporting Information, Figure S8, and selected bond
lengths and angles for both complexes are in given in Table 1.
The complex is dimeric with a center of symmetry in the middle
of the Y2O2 ring, and the coordination geometry at the yttrium
center is distorted pentagonal bipyramidal with the ancillary
ligand occupying the basal positions and the ethoxide coligand
in the apical site. The distorted basal plane is only coplanar to
about 0.23 Å [0.23 Å] with the yttrium atom lying about 0.64 Å
[0.62 Å] out of this plane in the direction of O(30). (The initial
values refer to complex 3-A and those in square parentheses
refer to complex 3-B.) Interestingly, the ethoxide groups lie anti
to each other, which is in contrast to the yttrium amido
complexes, for example, 1, whose amido groups were arranged
syn to each other.
The use of a more rigid ancillary ligand enabled isolation of
yttrium alkoxide complexes which did not show any isomeriza-
tion on the NMR time scale at ambient temperature. Thus,
complex 5 showed a single resonance at 59.5 ppm in the 31P{1H}
NMR spectrum in CD2Cl2 at 293 K. Upon reducing the
temperature, this signal broadened, splitting to two broad signals
of equal integration at 183 K, at 64.7 and 55.8 ppm, as well as
other, sharper signals (Supporting Information, Figure S3). The
corresponding 1H NMR spectrum at 273 K showed the broad-
ening of all resonances compared to the amido complex 2 and
new signals 4.39, 1.09, and 1.05 ppm corresponding to the iso-
propyl alkoxide protons.
Crystals of complex 3 suitable for X-ray crystallographic
analysis were grown from a toluene solution. The compound
crystallized with two independent Ci-symmetric complexes
(3-A and 3-B) in the asymmetric unit, each having essentially
the same geometry; complex 3-A is shown in Figure 3, complex 3-B
Phosphorus Substituents. We have previously established
that by changing the steric hindrance of the substituents on the
phosphorus atoms it is possible to control the nuclearity of the
resulting complex.20ꢀ22 Thus, less sterically hindered substitu-
ents such as iso-propyl or phenyl resulted in formation of dimeric
yttrium complexes.20,21 In contrast, more sterically hindered tert-
butyl phosphorus substituents enabled isolation of a monomeric
yttrium amido complex, 6 (Figure 4).22 Complex 6 enabled the
heteroselective polymerization of rac-lactide (Pr: 0.85). The
reaction of 6 with 2 equiv of iso-propyl alcohol led to the
formation of a dimeric alkoxide complex, 7. Complex 7, in
common with all other dimeric complexes, showed very little
stereocontrol in the polymerization of rac-lactide. Therefore, it
was of interest to establish whether mononuclear yttrium alk-
oxide complexes could be prepared as these might be expected to
exert stereocontrol; thus 6 was reacted with more sterically
Figure 3. Molecular structure of one (3-A) of the two crystal-
lographically independent Ci-symmetric complexes present in the
crystals of 3. The labeled hetereoatoms have been drawn with 50%
probability ellipsoids.
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the Two Crystallographically Independent Ci-Symmetric Complexes
(3-A and 3-B) Present in the Crystals of 3
3-A
3-B
3-A
3-B
Y(1)ꢀO(1)
Y(1)ꢀN(7)
Y(1)ꢀO(30)
2.405(2)
2.342(3)
2.028(3)
3.9124(5)
2.436(2)
2.342(3)
2.033(2)
3.9593(6)
Y(1)ꢀN(3)
Y(1)ꢀO(9)
Y(1)ꢀO(1A)
2.372(3)
2.334(2)
2.312(2)
2.368(3)
2.344(3)
2.305(2)
Y(1) Y(1A)
3 3 3
O(1)ꢀY(1)ꢀN(3)
O(1)ꢀY(1)ꢀO(9)
O(1)ꢀY(1)ꢀO(1A)
N(3)ꢀY(1)ꢀO(9)
N(3)ꢀY(1)ꢀO(1A)
N(7)ꢀY(1)ꢀO(30)
O(9)ꢀY(1)ꢀO(30)
O(30)ꢀY(1)ꢀO(1A)
61.61(8)
132.46(8)
67.92(8)
61.03(9)
133.19(9)
66.77(9)
O(1)ꢀY(1)ꢀN(7)
O(1)ꢀY(1)ꢀO(30)
N(3)ꢀY(1)ꢀN(7)
N(3)ꢀY(1)ꢀO(30)
N(7)ꢀY(1)ꢀO(9)
N(7)ꢀY(1)ꢀO(1A)
O(9)ꢀY(1)ꢀO(1A)
Y(1)ꢀO(1)ꢀY(1A)
125.97(9)
110.46(11)
74.96(9)
127.86(10)
109.51(11)
76.03(11)
103.57(12)
62.70(10)
139.52(10)
80.48(8)
134.10(9)
129.16(8)
110.12(12)
106.65(12)
98.21(10)
134.15(10)
127.52(9)
108.08(12)
107.34(11)
98.05(10)
104.28(11)
62.99(9)
136.94(9)
78.44(8)
112.08(8)
113.23(9)
7720
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728

Inorganic Chemistry
ARTICLE
Table 2. Selected Bond Lengths (Å) and Angles (deg) for 8
Y(1)ꢀO(1)
Y(1)ꢀN(7)
Y(1)ꢀO(30)
2.4881(12) Y(1)ꢀN(3)
2.3921(15) Y(1)ꢀO(9)
2.0647(13) Y(1)ꢀO(1A)
4.0187(3)
2.3696(14)
2.3293(12)
2.3803(12)
Y(1) Y(1A)
3 3 3
O(1)ꢀY(1)ꢀN(3)
O(1)ꢀY(1)ꢀO(9)
O(1)ꢀY(1)ꢀO(1A)
N(3)ꢀY(1)ꢀO(9)
N(3)ꢀY(1)ꢀO(1A) 127.54(5)
N(7)ꢀY(1)ꢀO(30) 108.01(5)
O(9)ꢀY(1)ꢀO(30) 109.94(5)
O(30)ꢀY(1)ꢀO(1A) 101.59(5)
60.74(4)
134.04(4)
68.75(4)
O(1)ꢀY(1)ꢀN(7) 129.12(5)
O(1)ꢀY(1)ꢀO(30) 106.50(5)
N(3)ꢀY(1)ꢀN(7) 74.83(5)
N(3)ꢀY(1)ꢀO(30) 105.14(5)
N(7)ꢀY(1)ꢀO(9) 62.49(5)
N(7)ꢀY(1)ꢀO(1A) 135.96(5)
O(9)ꢀY(1)ꢀO(1A) 77.23(4)
Y(1)ꢀO(1)ꢀY(1A) 111.25(4)
130.95(5)
Figure 4. Preparation of complexes 7ꢀ9. Reagents and Conditions: (i)
ROH, toluene, 298 K, - HN(SiHMe2). (ii) 2,6-di-tert-butyl-4-methyl-
phenol, C6D6, 298 K, - HN(SiHMe2).
Table 3. Comparison of the Hydrodynamic Radii (r) of 7 and
8 Calculated Both in the Solid State and in Solution
complex
r (crystal structure) /Å
r (1H PGSE NMR) /Å
7
8
5.97
5.93
5.81
5.47
coordination geometry. The distorted basal plane is coplanar to
about 0.12 Å with the yttrium atom lying about 0.67 Å out of this
plane in the direction of O(30). The bond lengths (Table 2) are
unexceptional and are closely related to those previously found
for the yttrium amide and alkoxide complexes. It is also notable
that all the dimeric amide complexes have the amide groups on
the same face of the molecule (i.e., syn to one another) whereas
the alkoxide complexes 4, 7, and 8 have the alkoxide groups anti
to one another.
Figure 5. Molecular structure of the Ci-symmetric complex 8. The
To further investigate the nuclearity of the complexes, com-
parisons were made between the hydrodynamic radii, calculated
from the molecular structure obtained by X-ray crystallography,
and from an NMR PGSE experiment, according to the method
previously described.20,24 The hydrodynamic radii are listed in
Table 3; there is a good agreement between the values calculated
from the solid state structures and those from solution measure-
ments. This agreement, together with the 31P{1H} NMR data,
strongly suggest that the complexes maintain the dimeric struc-
tures in solution.
Since dimeric products were obtained using aliphatic alcohols,
bulkier aryl alcohols (2,4,6-trimethylphenol and 2,6-di-tert-butyl-
4-methyl-phenol) were also investigated. The reactions between
the aryl alcohols and complex 6 were slower than for aliphatic
alcohols, typically taking 24 h at 343 K to completely consume 6.
The reaction with 2,4,6-trimethyl phenol led to a mixture of
products, the major species was monomeric, as evidenced by a
doublet at 58.6 ppm in the 31P{1H}NMR spectrum; however,
low intensity multiplets, at 71.4 and 60.7 ppm and broad
resonances at 64.3 and 59.3 ppm, indicated dimeric complexes
were also present. As any catalytic studies would be likely to be
complicated by such monomerꢀdimer mixtures, this aryl alcohol
was not further investigated. The reaction with 2,6-di-tert-butyl-
4-methylphenol led exclusively to a mononuclear complex, 9.
This was established by the presence of a single doublet at 58.7
ppm in the 31P{1H} NMR spectrum and by the presence of only
one doublet signal for the tert-butyl phosphorus substituents, at
1.16 ppm, in the 1H NMR spectrum. In the 1H NMR spectrum,
labeled hetereoatoms have been drawn with 50% probability ellipsoids.
hindered alcohols including tert-butyl alcohol and 2,6-di-tert-
butyl-4-methylphenol.
The tert-butoxide complex, 8, had a dimeric structure as
evidenced by the 31P{1H} NMR spectrum which consisted of
two resonances, of equal integration, at 59.3 and 67.1 ppm, a
doublet and a triplet, respectively. The doublet arises because of
the phosphorus attached to the terminally bound oxo group
which couples with a single yttrium center. The triplet signal was
maintained even when the spectrum was collected at higher
frequency and with a greater number of data points and was
assigned to the phosphorus atom attached to the bridging oxygen
atom, which couples with two, equivalent yttrium centers.
Complex 7 showed a closely related spectrum, with a doublet
and triplet resonances. The high symmetry of these alkoxide
complexes renders the yttrium atoms equivalent; thus the
phosphorus bonded to the bridging oxygen resonates as a triplet.
The corresponding 1H NMR spectrum, of complex 8, shows four
doublets for the tert-butyl phosphorus substituents, each inte-
grating to 18 protons. The alkoxide methyl groups resonate as a
singlet at 1.14 ppm.
Crystals suitable of complex 8 suitable for X-ray crystallo-
graphic analysis were grown from a benzene solution, and the
molecular structure is illustrated in Figure 5. The structure is
similar to that previously reported for complex 7 and shows a Ci-
symmetric dimeric complex with a pentagonal pyramidal yttrium
7721
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728

Inorganic Chemistry
ARTICLE
Figure 6. Synthesis of yttrium complexes 10 and 11, where R = tert-
butyl. Reagents and Conditions: (i) [Y{N(SiHMe2)2}3(THF)2], THF,
298 K, (ii) tBuOH, 298 K.
Figure 7. Polymerization of rac-lactide by complexes 1ꢀ11. Condi-
tions: CH2Cl2 or THF, 298 K, [LA]0/[I]0 = 400 for 1ꢀ5, 7, 8, 11,
[LA]0/[I]0 = 200 for 6, 9, 10.
coordinated THF and formation of monomerꢀdimer mixtures in
noncoordinating solvents.
the signals for the ancillary ligand methylene protons are two
well-defined doublets of doublets, each with a 2JHH coupling of
3
Addition of 1 equiv of tert-butanol to either complex 10
(formed in situ in d8-THF) or to the monomerꢀdimer mixture
(in d6-benzene), yielded a single dimeric yttrium alkoxide
product, 11 (Figure 6). The 31P{1H} NMR spectrum showed
a doublet of doublets at 61.4 ppm and a doublet at 53.4 ppm.
The 1H NMR spectrum showed four doublet signals for the tert-
butyl groups on the phosphorus atoms and four multiplets for
the backbone methylene groups. In addition, a singlet at 1.49
ppm was assigned to the tert-butoxide group. The dimeric
structure of 11 highlights the dependency of nuclearity of these
yttrium complexes on the steric bulk of the ligand, the initiating
group, and the nature of the solvent.
11.4 Hz and a JHP coupling of 3.2 Hz. This coupling was also
observed in the 1H NMR spectrum of 6, but was quite different in
the 1H NMR spectra of dimeric complexes, where the methylene
group resonated as multiplets. Furthermore, the 2,6-tert-butyl
groups on the phenol were equivalent, resonating as a singlet,
integrating to 18, at 1.78 ppm, indicating that this group had free
rotation.
The tert-butyl substituents on the phosphorus atoms were
only sufficiently sterically hindered to enable isolation of mono-
meric complexes when sterically hindered coligands were
applied, for example, amido or aryl oxide. It was also of interest
to investigate the influence, if any, of the diamido backbone
group; therefore, a bis(di-tert-butyl-phosphinic)ethylene dia-
mine pro-ligand was prepared. The pro-ligand was prepared by
reacting ethylene diamine with 2 equiv of di(tert-butyl)chloro-
phosphine and base, followed by a one-pot oxidation, using
aqueous hydrogen peroxide; the product was isolated in 89%
yield. The new ligand was reacted with yttrium precursors and
alcohols, as illustrated in Figure 6.
Polymerization of rac-Lactide. All the complexes were tested
as initiators for the ring-opening polymerization of rac-lactide
(Figure 7, Table 4)
The polymerizations were carried out either in CH2Cl2
(initiators 1ꢀ6) or in THF (initiators 6ꢀ11). The polymeriza-
tions conducted in THF were somewhat slower than those in
CH2Cl2, an effect which has previously been observed and which
is ascribed to competitive binding of THF versus lactide at the
active site.12,17,19,20,22 Thus, it is not possible to directly compare
the rates of polymerization in the two solvents; however, it is still
feasible to uncover some structureꢀactivity relationships. The
polymerizations were all conducted so that the concentration of
initiating group was the same; thus dimeric complexes (with two
amido/alkoxide initiating groups) were used at [LA]0/[I]0 = 400,
while the monomeric complexes (6, 9, 10 with one amido
initiating group) were used at [LA]0/[I]0 = 200. Therefore, the
expected number averaged molecular weight, at high (>95%)
monomer conversion, is approximately 28,000 g/mol. All poly-
merizations were analyzed by taking aliquots, quenching them by
precipitation in cold hexanes, and analyzing the crude product to
determine the conversion of lactide and molecular number of the
polylactide.
Polymerization Rates. Semilogarithmic plots of ln{[LA]0/[LA]t}
versus time were constructed (Figures 8ꢀ10, where [LA]0 = 0.5 M).
The plots generally showed linear fits, with x-intercepts at the origin,
consistent with rapid initiation; the gradients of the linear fits
correspond to the pseudo first order rate constants. The kinetic data
for initiators 1 and 2 shows rather more complex behavior, with two
different regimes in the polymerization: a slower initiation regime
leading to a region where the data can be fit in a linear fashion,
corresponding to a propagation regime (Supporting Information,
Figures S3, S4).20,21 It is believed that the relatively low nucleophilicity
of a metal-amide bond and the steric hindrance of the bis-
(dimethylsilyl)amido nucleophile hinder the initiation reaction and
complicate the kinetic analysis. However, the isolation of the alkoxide
complexes, 3ꢀ5, led to a distinct improvement in the linear fits to the
The monomeric complex, 10, was prepared as the sole product
when reacting the pro-ligand and [Y{N(SiHMe2)2}3(THF)2] in
tetrahydrofuran (THF) in situ on an NMR scale. Thus, equimolar
quantities of the pro-ligand and [Y{N(SiHMe2)2}3(THF)2] were
dissolved in the minimum of THF-d8; after 16 h the 31P{1H}
NMR spectrum showed exclusive formation of 10, as evidenced by
2
a doublet at 54.3 ppm, with a JPY of 4.05 Hz, in excellent
agreement with the data obtained for 6.22 The 1H NMR spectrum
showed a doublet at 1.25 ppm for the tert-butyl groups and a
multiplet at 3.56 ppm for the methylene protons on the amine
backbone of the ligand. One molecule of bound THF was also
observed (with peaks at 3.62 and 1.78 ppm), the signals of which
were shifted downfield compared to free THF (3.58 and 1.73
ppm), although presumablythis isundergoingrapid exchangewith
free THF in the bulk solvent. Thus, the structure of complex 10
(prepared and characterized in situ) was mononuclear. On
removal of the THF and analysis of the product in d6-benzene, a
mixture of 10 and dimeric analogues were observed. The 31P-
{1H}NMR spectrum showed the major species was complex 10
(a doublet at 54.4 ppm, with a relative integration of 4 and a 2JPY
coupling constant of 3.56 Hz). However, the spectrum also
showed other low intensity signals including two doublets of
doublets of doublets at approximately 60 ppm and two doublets of
doublets at approximately 55 ppm. These multiplets, and the
associated coupling constants, are consistent with the presence of
two dimeric isomers, in a relative ratio of 2:1. It is interesting to
note the influence that the diamido group and solvent exert over
the complex nuclearity: the complex 6 was mononuclear in all
solvents, yet complex 10 was more susceptible to removal of
7722
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728

Inorganic Chemistry
ARTICLE
Table 4. Polymerization of rac-LA Using Initiators 1ꢀ11a
initiator
[LA]0/M
time/s
percentage conversionb
kobs ꢁ 10ꢀ3 c /sꢀ1
Mn /g molꢀ1 d
PDId
Prf
1
1.0
1.0
0.5
0.5
0.5
0.5
0.5e
0.5e
0.5e
1e
221
8515
150
240
390
300
480
120
59
99
99
98
99
98
98
96
94
96
91
79
99
24.3
0.66
32.3
17.6
17.3
17.8
8.36
38.8
39.5
1.93
15.6
68.6
66,409
147,900
32,550
26,820
29,060
99,590
81,390
23,180
35,670
64,700
43,900
37,200
1.66
1.66
1.34
1.12
1.15
1.30
1.38
1.77
1.31
1.23
1.26
1.19
0.62
2
3
4
5
6
0.62
0.85
0.62
0.63
0.71
0.61
0.62
6
7
8
9
1800
180
62
10
0.5e
0.5e
11
a Polymerization conditions: CH2Cl2, 298 K, [LA]0/[I]0 = 400 for 1ꢀ5, [LA]0/[I]0 = 200 for 6; THF, 298 K, [LA]0/[I]0 = 400 for 7, 8, 11, [LA]0/[I]0 =
1
200 for 9, 10. b Determined by integration of the methine region of the H NMR spectrum. c Determined from the gradients of the plots of
ln{[LA]0/[LA]t} versus time. d Determined by SEC in THF, using multiangle laser light scattering (GPC-MALLS). e Polymerizations conducted
in THF. f Determined by analysis of all the tetrad signals in the methyne region of the homonuclear decoupled 1H NMR spectrum and comparison with
the signal intensities predicted by Bernouillan statistics.25
Figure 8. Plot of ln{[LA]0/[LA]t} against time, for initiators 3ꢀ5.
Conditions: [LA]0 = 0.5 M, [LA]0/[I]0 = 400, CH2Cl2, 298 K.
Figure 9. Semilogarithmic plot for polymerizations using initiators
6ꢀ8. Conditions: [LA]0 = 0.5 M; [LA]0/[I] = 200 for 6 and
data and an increase in the value of the pseudo first order rate
constant, kobs (Figure 8). While the values cannot be directly
compared with those obtained for 1 and 2, as the latter were obtained
[LA]0/[I] = 400 for 7, 8; 298 K; THF.
at monomer concentrations of 1 M; it can be seen that even on
decreasing the monomer concentration by 0.5, the value of kobs
increases indicative of significantly improved activity. The linearity of
the fits also corresponds well with the notion that the yttrium-alkoxide
is more nucleophilic than the yttrium-amide.
For the series of initiators tested in THF, it was also noted that
yttrium-alkoxides showed higher kobs values and linear fits to the
semilogarithmic data. Figure 9 illustrates the increase in rate on
changing the initiating group from di(methylsilyl amido) (6) to
iso-propyl alkoxide (7) and tert-butyl alkoxide (8). Figure 10
shows that on substituting di(methylsilylamido) (10) group for a
tert-butyl alkoxide group (11) the values of kobs increase approxi-
mately 4-fold. Initiator 9 displays somewhat different behavior,
showing a more complex data set where a linear fit can only be
applied after an initiation period of approximately 4 min and
giving a significantly lower value of kobs (Supporting Information,
Figure S5). This is closely related behavior to that observed with
(dimethylsilyl)amido initiators and is consistent with a relatively
poor initiating behavior for the sterically hindered aryl
oxide group.
It can be seen that there is some influence of the backbone
diamido linker group on the overall rate. For example, complexes
6 and 10 differ by having 2,2-dimethylpropylene and ethylene
backbone linker groups, respectively. Complex 6 shows slower
polymerization than 10; the same effect is observed for com-
plexes 8 and 11. Thus for the series of complexes with tert-butyl
substituents on the phosphorus atoms, it appears that the
ethylene diamido ligand fragment leads to greater rates. It is
proposed that the shorter ethylene diamido linker leaves the
yttrium atom more exposed than for the complexes with a C-3
backbone diamido group, resulting in an increase in polymeri-
zation activity for the former. It should be noted that overall the
7723
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728

Inorganic Chemistry
ARTICLE
Other yttrium initiating systems have also displayed improved
control for alkoxide versus amide initiating groups, although
these are generally prepared in situ rather than isolated, as in this
case.9,11,13 It is proposed that the improved properties of the
alkoxide initiators result from both a reduction in steric hin-
drance and an increase in nucleophilicity (initiating ability)
versus amide analogues. The Mw/Mn values are also narrower
(1.12ꢀ1.39) for the alkoxide initiators, supporting the notion
that they enable rapid initiation followed by controlled polym-
erization of lactide.
Tacticity Control. The stereochemistry of rac-lactide enchain-
ment can be evaluated by comparing the integrals in the homo-
1
nuclear decoupled H NMR spectrum, at the tetrad level, with
calculated values (using Bernouillan statistics).25 Most of the
yttrium initiators display little stereocontrol, leading to PLA with
a slight heterotactic bias (∼0.6). However, we have previously
observed that complex 6 shows reasonable heteroselectivity
(0.85), when used in THF.22 This has been attributed to a chain
end control mechanism enhanced by solvent coordination at the
active site. The alkoxide analogues of complex 6, showed much
reduced heteroselectivity, even when used in coordinating
solvents. This is proposed to be due to the dinuclear nature of
these complexes which prohibits efficient stereoselecontrol. The
mononuclear aryloxide complex, 9, shows slightly enhanced
heteroselectivity (0.71) compared to dinuclear analogues,
although the value is lower than for the amido analogue 6. The
initiating group would not be expected to influence polymer
tacticity in such a manner. MALDI-ToF spectra of the polymers
produced during the early stages of polymerization (e.g., 10%
conversion) showed that significant transesterification had
occurred in both cases. These reactions reduce the level of
stereocontrol observed as they can scramble the stereochemistry
of the polymers. It is possible that the slower rate of propagation
for 9 causes the rate of transesterification to become competitive
with propagation, and transesterification becomes more signifi-
cant as a reaction pathway, causing the observed decrease in
stereocontrol, this would also be consistent with increased values
for the PDI of PLA produced using complex 9. On the other
hand, mononuclear complex 10, which has an ethylene diamido
backbone group, also shows much reduced heteroselectivity.
These findings highlight the central importance of the coordina-
tion environment in enabling chain end control. Thus, the more
open coordination geometry at complex 10, versus complex 6,
enhances the rate of polymerization but reduces the stereose-
lectivity. The open coordination site is implicated in the finding
that in noncoordinating solvents approximately half of complex
10 dimerizes, a finding not observed for complex 6.
Figure 10. Semilogarithmic plots for polymerizations using initiators
10 and 11. Conditions: [LA]0 = 1.0 M, [LA]0/[I]0 = 200 (10), 400 (11);
298 K; THF.
Figure 11. Evolution of Mn versus % conversion for polymerizations
using initiators 2ꢀ5. Conditions: [LA]0 = 1 M (2), 0.5 M (3ꢀ5),
[LA]0/[I]0 = 400 (2ꢀ5); 25 °C; CH2Cl2.
’ CONCLUSIONS
rates of polymerization of this type of yttrium initiator are high
and compete with some of the best literature systems.5
The polymerizations were all conducted so that the concen-
tration of initiating group was the same and such that the
calculated number averaged molecular weight, at high (>95%)
monomer conversions, should be approximately 28,000 g/mol.
The correlation between experimentally determined and calcu-
lated values for Mn was poor for the amido initiators, for example,
compare the values for 1ꢀ2 versus 3ꢀ5 (Figure 11) or 6 and 10
versus 7ꢀ9, 11. The poor correlation is proposed to be due to
relatively slow initiation versus propagation reactions. In con-
trast, the alkoxide initiators gave values for Mn that matched the
predicted values well.
A series of yttrium alkoxide and amide complexes, ligated by
various bis(phosphinic)diamido ancillary ligands, have been
prepared. The alkoxide complexes were prepared by alcoholysis
of the amide precursors and were fully characterized, including
using multinuclear NMR spectroscopies to confirm the solution
structures. The alkoxide complexes were all dimeric both in the
solid state and in solution, a finding supported by the multiplicity
of the peaks in the 31P{1H} NMR spectra. An aryloxide complex
was also isolated using 2,6-di-tert-butyl-4-methyl-phenol, and had
a mononuclear structure in solution. The use of bulky tert-butyl
substituents on the phosphorus atoms in the ancillary ligand
enables isolation of monomeric amide complexes with either
7724
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728

Inorganic Chemistry
ARTICLE
2,2-dimethylpropylene or ethylene diamido linkers. In contrast,
alcoholysis reactions yield dimeric alkoxide complexes regardless
of the nature of the phosphorus substituents. The new complexes
are all viable initiators for lactide ring-opening polymerization,
showing very high rates and values of kobs. The alkoxide com-
plexes all showed superior rates and improved polymerization
control compared to the amido analogues. The aryloxide com-
plex showed rather slower polymerization and reduced poly-
merization control. These findings are consistent with the sterically
hindered amido and aryl oxide initiators showing poor initiation
efficiencies. This is likely to be due to the reduced nucleophilicity
of the amido groups compared to the alkoxide counterparts and
also influenced by the steric hindrance of the amide and aryl
oxide groups. The amide complex with tert-butyl phosphorus
substituents shows good stereocontrol, yielding heterotactic
PLA, and this is proposed to be due to its monomeric structure.
Further investigations show that related monomeric amido
complex (10) shows much reduced heteroselectivity. This
underscores the importance of the active site coordination
environment in controlling the stereochemistry. The monomeric
amido complex with an ethylene diamine backbone results in a
more open coordination environment at the yttrium center, as
evidenced by the finding that in noncoordinating solvents this
complex has a tendency to dimerize.
Polymer laboratories Mixed D columns were used in series, with THF as
the eluent, at a flow rate of 1 mL minꢀ1, on a Polymer laboratories PL GPC-
50 instrument. The light scattering detector was a triple-angle detector
(Dawn 8+, Wyatt Technology), and the data were analyzed using Astra V
version 5.3.1.4. The refractive angle increment for polylactide in THF, was
taken as 0.042 mL gꢀ1 27
.
N,N0-1,2-bis(P,P0-di-tert-butylphosphinoyl)ethylenedi-
amine. To a 250 mL round bottomed flask, equipped with a stirrer bar,
was added 1,2-ethylenediamine (0.26 mL, 4 mmol), triethylamine
(2.80 mL, 20 mmol), and dichloromethane (40 mL). The flask was
cooled to 0 °C, in an ice bath, and chlorodi-tert-butylphosphine
(1.60 mL, 8.4 mmol) was added dropwise, via a syringe. The mixture
was warmed to room temperature and stirred for 20 h. The flask was
opened to air, and the mixture cooled to 0 °C, in an ice bath. Hydrogen
peroxide (2.06 mL of a 30% w/v soln., 16 mmol) was added cautiously to
the mixture. Stirring was continued at 0 °C for 10 min and then at 25 °C
for a further 30 min. Sodium thiosulphate (40 mL of a 1 M soln.,
40 mmol) was then added slowly. The aqueous and organic layers were
separated, and the aqueous layer washed with dichloromethane (2 ꢁ
10 mL). The organic layers were collected, dried (MgSO4), and the
solvents were removed under reduced pressure, to leave a colorless
residue. Recrystallization from dichloromethane and hexane gave the
title product as colorless needles (1.35 g, 89%).
M. Pt. °C: 190ꢀ193 (sublimes) 1H NMR (400 MHz, CDCl3,)
δ ppm: 3.13 (m, 4H, CH2), 2.94 (m, 2H, NH), 1.21 (d, 36 H, 3JPH = 13.6
Hz, CH3). 13C{1H} NMR (100 MHz, CDCl3,) δ ppm: 44.0 (s, CH2),
36.5 (d, 1JPC = 76.4, C(CH3)3), 26.7 (s, C(CH3)3). 31P{1H} NMR (162
MHz, CDCl3) δ ppm: 56.1 (s). m/z (FAB+): 381 [M+H]+. IR (neat)
υ cmꢀ1: 3262 (w, NꢀH), 3177 (w, NꢀH), 2970 (m, CꢀH), 2946 (m,
CꢀH), 2893 (m, CꢀH), 2865 (m, CꢀH), 1474 (m), 1388 (w), 1357
(w), 1218 (w), 1153 (s, PdO), 1086 (s, PdO), 1017 (m), 935 (w), 825
(m), 818 (s), 759 (m), 656 (s). Anal. Calcd. for C14H34N2O2P2: C,
56.82, H, 11.13, N, 7.36%. Found: C, 60.89, H, 11.21, N, 7.32%.
Di-μ-oxo-{[N,N0-1,3-bis(P,P0-di-iso-propylphosphinoyl)-
2,2-dimethylpropanediamido](ethoxy)yttrium} 3. To a solu-
tion of di-μ-oxo-{[N,N0-1,3-bis(P,P0-di-iso-propylphosphinoyl)-2,2-dimethyl-
propanediamido][bis(dimethylsilyl)amido]yttrium}, 1(0.058 g, 0.05 mmol)
in toluene (3 mL), in a Schlenk tube, was added ethanol (5.8 μL, 0.1 mmol)
via a micro syringe. The resulting colorless solution was stirred and heated at
70 °C for 24 h. The solvents were removed in vacuo to yield a colorless solid.
The title product was obtained by recrystallization from the minimum volume
of hexane at ꢀ35 °C (0.016 g, 32%).
’ EXPERIMENTAL SECTION
Materials and Methods. All reactions were conducted under a
nitrogen atmosphere, using either standard Schlenk techniques or in a
nitrogen filled glovebox, unless stated otherwise. All solvents and
reagents were obtained from commercial sources (Aldrich and Strem)
and used as received unless stated otherwise. Ethylene diamine was
stirred over CaH2, distilled, and stored under nitrogen; tert-butanol was
stirred over CaH2, distilled, degassed and stored under nitrogen. THF
and pentane were distilled from sodium and stored under nitrogen.
Dichloromethane was distilled from CaH2 and stored under nitrogen.
Benzene-d6 and THF-d8 were dried over potassium; in each case,
performing three freezeꢀthaw cycles under vacuum, refluxing them
for 1ꢀ2 days, distilling them under vacuum, and storing them under
nitrogen. Rac-lactide was donated by Purac Plc. and was recrystallized
from anhydrous ethyl acetate or toluene and sublimed at 50 °C
three times under vacuum prior to use. [Tris{N,N0-bis(dimethylsilyl)-
amido}bis(tetrahydrofurano)]yttrium, [Y{N(SiHMe2)2}3(THF)2], com-
plexes 1, 2, and 6 were prepared according to the literature procedure.20ꢀ22,26
Measurements. 1H and 13C{1H} NMR spectra were performed on a
Bruker Av-400 instrument, unless otherwise stated. 31P{1H} NMR experi-
ments were performed on a Bruker Av500 spectrometer equipped with a
z-gradient bbo/5 mm tunable probe and a BSMS GAB 10 A gradient
amplifier providing a maximum gradient output of 5.35 G/cmA. To observe
the complex PꢀY coupling patterns, 31P{1H} spectra were collected at a
frequency of 202.47 MHz using a 30 degree pulse, a spectral width of 8082
Hz (centered on 57.5 ppm), and 65536 data points with a relaxation delay of
2 s. The spectra were zero filled (0.12 Hz/point resolution) and pro-
cessed with no apodization. Spectra were processed and analyzed using
Mestrenova software. Elemental analyses were determined by Mr. Stephen
Boyer at London Metropolitan University, North Campus, Holloway Road,
London, N7. Mass spectra were performed on a Micromass Autospec
Premier machine, using LSIMS (FAB) ion sources. Melting points were
determined using a Reichert apparatus. IR spectra were acquired on a
Perkin-Elmer Spectrum 100 FT-IR spectrometer, and the data was
processed using Spectrum Express software. Polylactide molecular weights
and polydispersity indices were determined from gel permeation chroma-
tography with multiangle laser light scattering (GPC-MALLS). Two
3
1H NMR (400 MHz, C6D6) δ ppm: 4.36 (q, JHH = 6.8 Hz, 6H,
OCH2), 2.97 (m, 8H, CH2), 2.08 (m, 8H CH(CH3)2), 1.50 (t, 3JHH
=
6.8 Hz, 4H, CH3), 1.30 (m, 24H, CH(CH3)2), 1.15 (m, 24H, CH-
(CH3)2), 0.87 (m, 12H, C(CH3)2). 13C{1H} NMR (100 MHz, C6D6) δ
ppm: 63.1 (s, OCH2), 57.3 (s, CH2), 38.2 (t, C(CH3)2), 27.7 (br s,
CH(CH3)2), 25.4 (br s, CH(CH3)2), 24.6 (br s, CH(CH3)2), 24.1 (br s,
CH(CH3)2), 22.6 (s, CH(CH3)2), 17.3 (s, CH(CH3)2), 16.8 (s, CH-
(CH3)2), 15.5 (s, CH3). 31P{1H} NMR (162 MHz, C6D6) δ ppm: 56.3
(br s, 20P), 50.1 (1P), 49.3 (1P).
Crystal Data for 3. C38H86N4O6P4Y2, M = 996.81, triclinic, P1 (no. 2),
a = 10.8212(4), b = 13.0664(4), c = 20.2479(6) Å, R = 105.984(3), β =
101.401(3), γ = 101.061(3)°, V = 2604.17(17) Å3, Z = 2 [two
independent Ci symmetric molecules], Dc = 1.271 g cmꢀ3, μ(CuꢀKR) =
4.463 mmꢀ1, T = 173 K, colorless platy needles, Oxford Diffraction
Xcalibur PX Ultra diffractometer; 9922 independent measured reflections
(Rint = 0.0552), F2 refinement, R1(obs) = 0.0436, wR2(all) = 0.1241, 8151
independent observed absorption-corrected reflections [|Fo|> 4σ(|Fo|),
2θmax = 144°], 505 parameters. CCDC 812442.
Di-μ-oxo-{[N,N0-1,3-bis(P,P0-di-iso-propylphosphinoyl)-
2,2-dimethylpropanediamido](iso-propoxy)yttrium} 4. To a
solution of di-μ-oxo-{[N,N0-1,3-bis(P,P0-di-iso-propylphosphinoyl)-2,2-
dimethylpropanediamido][bis(dimethylsilyl)amido]yttrium} 1 (0.058 g,
7725
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728

Inorganic Chemistry
ARTICLE
0.05 mmol) in toluene (3 mL), in a Schlenk tube, was added iso-propanol
(7.8 μL, 0.1 mmol) via a micro syringe. The resulting colorless solution
was stirred and heated at 70 °C for 24 h. The solvents were removed,
in vacuo, to yield a colorless solid. The title product was obtained
by recrystallization from the minimum volume of hexane at ꢀ35 °C
(0.034 g, 66%).
diamido][bis(dimethylsilyl)amido] (tetrahydrofurano)}yttrium 6 (0.071 g,
0.1 mmol) in toluene (0.2 mL), was added tert-butanol (9.5 μL, 0.1 mmol)
via a microsyringe. The solution was stirred briefly, transferred to a tube,
sealed, and left to stand for 24 h, over which time colorless crystals formed.
The mixture was filtered and the solid washed with cold toluene (0.1 mL) to
give the title product as a colorless, crystalline solid (0.031 g, 52%).
1H NMR (400 MHz, C7D8) δ ppm: 4.53 (sept, 3JHH = 6.0 Hz, 2H,
OCH(CH3)2), 2.99 (dd, 2JHH = 11.0 Hz, 3JPH = 3.6 Hz, 4H, CH2), 2.89
(br s, 4H, CH2), 1.98 (br s, 8H, CH(CH3)2), 1.48 (s, 6H, CH3), 1.42 (d,
3JHH = 6.0 Hz, 12H, OCH(CH3)2), 1.36ꢀ0.98 (br m, 48H, CH-
(CH3)2), 0.87 (s, 6H, CH3). 13C{1H} NMR (100 MHz, C7D8) δ
1H NMR (400 MHz, C7D8) δ ppm: 3.16 (m, 8H, CH2), 1.53 (d, 3JHP
=
3
13.2 Hz, 18H, C(CH3)3), 1.42 (d, JHP = 10.4 Hz, 18H, C(CH3)3),
1.14 (s, 18H, OC(CH3)3), 1.40 (s, 6H, CH3), 1.24 (d, 3JHP = 12.8 Hz,
18H, C(CH3)3), 1.20 (d, 3JHP = 12.4 Hz, 18H, C(CH3)3), 0.82 (s, 6H,
CH3). 13C{1H} NMR (100 MHz, C7D82) δ ppm: 72.6 (d, 2JCY = 5.8 Hz,
OC(CH3)3), 59.5 (s, CH2), 59.0 (d, JCP = 3.1 Hz, CH2), 52.7 (s,
C(CH3)2), 40.2 (d, 1JCP = 66.6 Hz, C(CH3)3), 38.2 (d, 1JCP = 41.7 Hz,
C(CH3)3), 37.6 (d, 1JCP = 34.1 Hz, C(CH3)3), 36.8 (d, 1JCP = 60.7 Hz,
C(CH3)3), 35.1 (s, OC(CH3)3), 28.6 (s, C(CH3)3), 28.3 (s, C(CH3)3),
27.9 (s, CH3), 27.7 (s, C(CH3)3), 26.5 (s, CH3). 31P{1H} NMR (202
MHz, C7D8) δ ppm: 67.2 (dt, 1P, 2JPY = 4.24 Hz, 4JPP = 0.8 Hz), 59.2
(dd, 1P, 2JPY = 4.24 Hz, 4JPP = 0.8 Hz). Anal. Calcd. for C25H55N2O3-
P2Y: C, 51.54, H, 9.52, N, 4.81%. Found: C, 51.45, H, 9.51, N, 4.84%.
ppm: 68.1 (d, 2JCY = 5.2 Hz, OCH(CH3)2), 57.4 (s, CH2), 38.2 (t, 3JCP
=
1.6, C(CH3)2), 29.4 (OCH(CH3)2), 27.9 (s, CH3), 24.6 (br s, CH-
(CH3)2), 24.4 (s, CH3), 16.0 (s, CH(CH3)2), 15.9 (s, CH(CH3)2), 15.8
(s, CH(CH3)2), 15.7 (s, CH(CH3)2), 15.5 (s, CH(CH3)2). 31P{1H}
NMR (162 MHz, C7D8) δ ppm: 66.4 (br s, 20P), 56.9 (br s, 20P), 51.4
(1P). Anal. Calcd. for C20H45N2O3P2Y: C, 46.88, H, 8.95, N, 5.47%;
Found: C, 46.63, H, 8.88 N, 5.67%.
Di-μ-oxo-{[N,N0-1,2-bis(P,P0-di-iso-propylphosphinoyl)-
phenylenediamido](iso-propoxy)yttrium} 5. To a solution of
di-μ-oxo-{[N,N0-1,2-bis(P,P0-di-iso-propylphosphinoyl)phenylene-
diamido][bis(dimethylsilyl)amido]yttrium]} 2 (0.059 g, 0.05 mmol) in
toluene (3 mL), in a Schlenk tube, was added iso-propanol (7.8 μL, 0.1
mmol) via a micro syringe. The resulting colorless solution was stirred
and heated at 70 °C for 24 h. The solvents were removed in vacuo to
yield a colorless solid. The title product was obtained by recrystallization
from the minimum volume of hexane at ꢀ35 °C (0.041 g, 79%.).
1H NMR (400 MHz, CD2Cl2) δ ppm: 6.50 (m, 8H, ArH), 4.05 (sept,
Crystal data for 8. C50H110N4O6P4Y2 C6H6, M = 1243.23, mono-
3
clinic, P21/n (no. 14), a = 10.6303(4), b = 19.9955(7), c = 15.6431(6) Å,
β = 94.722(3)°, V = 3313.8(2) Å3, Z = 2 [Ci symmetry], Dc = 1.246
g cmꢀ3, μ(MoꢀKR) = 1.885 mmꢀ1, T = 173 K, colorless blocks, Oxford
Diffraction Xcalibur 3 diffractometer; 10912 independent measured
reflections (Rint = 0.0631), F2 refinement, R1(obs) = 0.0379, wR2(all) =
0.0949, 7645 independent observed absorption-corrected reflections
[|Fo| > 4σ(|Fo|), 2θmax = 65°], 323 parameters. CCDC 720129.
{[N,N0-1,3-bis(P,P0-di-tert-butylphosphinoyl)-2,2-dimethyl-
propanediamido](tetrahydrofurano)(2,6-di-tert-butyl-4-methyl-
phenoxy)}yttrium 9. To a solution of {[N,N0-1,3-bis(P,P0-di-tert-
butylphosphinoyl)-2,2-dimethylpropanediamido][bis(dimethylsilyl)-
amido] (tetrahydrofurano)}yttrium, 6 (0.071 g, 0.1 mmol) in toluene
(0.2 mL) was added a solution of 2,6-di-tert-butyl-4-methylphenol
(0.022 g, 0.1 mmol) in toluene. The resulting solution was stirred at
25 °C for 24 h. The solvents were removed in vacuo to leave a colorless
solid. This was washed with pentane and the colorless residue dried in
vacuo to give the title product (0.068 g, 85%).
3
3JHH = 6.0 Hz, 2H, OCH(CH3)2), 2.38 (sept, JHH = 7.2 Hz, 8H,
CH(CH3)2), 1.27 (dd, 3JHH = 7.2 Hz, 3JPH = 15.2 Hz, 24H, CH(CH3)2),
1.14 (dd, 3JHH = 7.2 Hz, 3JPH = 16.4 Hz, 24H, CH(CH3)2), 0.98 (d, 3JHH
=
6.0 Hz, 12H, OCH(CH3)2). 13C{1H} NMR (100 MHz, CD2Cl2) δ
ppm: 143.9 (d, 2JCP = 13.7 Hz, ArC), 119.1 (s, ArC), 118.8 (s, ArC), 68.4
1
(s, OCH(CH3)2), 29.1 (s, OCH(CH3)2), 25.9 (d, JCP = 75.8 Hz,
CH(CH3)2), 16.1 (s, CH(CH3)2). 31P{1H} NMR (162 MHz, CD2Cl2)
δ ppm: 59.5 (s). Anal. Calcd. for C21H39N2O3P2Y: C, 48.65, H, 7.58, N,
5.40%; Found: C, 48.62, H, 7.63, N, 5.44%.
Di-μ-oxo-{[N,N0-1,3-bis(P,P0-di-tert-butylphosphinoyl)-
2,2-dimethylpropanediamido](iso-propoxy)]yttrium} 7. To a
solution of {[N,N0-1,3-bis(P,P0-di-tert-butylphosphinoyl)-2,2-dimethylpro-
panediamido][bis(dimethylsilyl)amido];(tetrahydrofurano)}yttrium, 6 -
(0.071 g, 0.1 mmol) in toluene (0.2 mL), was added iso-propanol (7.6 μL,
0.1 mmol) via a microsyringe. The solution was stirred briefly, transferred
to a tube, sealed and left to stand for 24 h, over which time colorless
crystals formed. The mixture was filtered and the solid washed with cold
toluene (0.1 mL) to give the title product as a colorless, crystalline solid
(0.020 g, 35%).
1H NMR (400 MHz, C6D6) δ ppm: 7.21 (br s, 2H, ArH), 3.80 (m,
4H, OCH2), 3.26 (dd, 2JHH = 11.4 Hz, 3JHP = 5.6 Hz, 2H, CH2), 3.13
(dd, 2JHH = 11.4 Hz, 3JHP = 3.2 Hz, 2H, CH2), 2.34 (s, 3H, Ar CH3), 1.78
(s, 18H, Ar C(CH3)3), 1.42 (m, 4H, OCH2CH2), 1.38 (s, 3H, C-
3
(CH3)2), 1.16 (d, JHP = 13.6 Hz, 36H, C(CH3)3), 0.88 (s, 3H,
C(CH3)2). 13C{1H} NMR (100 MHz, C6D6) δ ppm: 160.8 (s, OArC),
125.9 (s, ArCC(CH3)3), 125.8 (s, ArCH), 123.5 (s, ArC(CH3)), 68.7 (s,
OCH2), 58.4 (s, CH2), 58.3 (s, CH2), 39.0 (t, 3JCP = 14.9 Hz, C(CH3)2),
38.1 (d, 1JCP = 68.3 Hz, C(CH3)3), 36.7 (d, 1JCP = 59.2 Hz, C(CH3)3),
35.7 (s, ArCC(CH3)3), 33.8 (s, ArCC(CH3)3), 32.5 (br s, ArCC-
(CH3)3), 32.3 (br s, ArCC(CH3)3), 30.5 (s, CH3), 27.8 (s, CH3), 27.7
1H NMR (400 MHz, C6D6) δ ppm: 4.58 (sept, 3JHH = 5.6 Hz, 2H,
3
OCH(CH3)2), 3.21 (m, 8H, CH2), 1.59 (d, JPH = 13.6 Hz, 18H,
(s, C(CH3)2), 27.2 (s, C(CH3)2), 25.5 (s, OCH2CH2), 21.6 (s,
C(CH3)3), 1.57 (s, 6H, CH3), 1.48 (d, 3JPH = 13.2 Hz, 18H, C(CH3)3),
2
ArCCH3). 31P{1H} NMR (162 MHz, C6D6) δ ppm: 58.7 (d, JPY
=
3
3
1.41 (d, JHH = 5.6 Hz, 6H, CH(CH3)), 1.39 (d, JHH = 5.6 Hz, 6H,
CH(CH3)), 1.31 (d, 3JPH = 12.8 Hz, 18H, C(CH3)3), 1.25 (d, 3JPH
4.54 Hz). Anal. Calcd. for C40H77N2O4P2Y: C, 59.99, H, 9.69, N, 3.50%.
Found: C, 59.88, H, 9.62, N, 3.41%.
=
12.8 Hz, 18H, C(CH3)3), 0.86 (s, 6H, CH3). 13C{1H} NMR (100 MHz,
C6D6) δ ppm: 68.2 (s, OCH(CH3)2), 59.7 (s, CH2), 58.8 (s, CH2), 48.9
(s, C(CH3)2), 40.2 (d, 1JCP = 67.6 Hz, C(CH3)3), 37.9 (d, 1JCP = 65.1
Hz, C(CH3)3), 37.8 (d, 1JCP = 56.9 Hz, C(CH3)3), 36.8 (d, 1JCP = 61.3
Hz, C(CH3)3), 29.1 (s, OCH(CH3)2), 29.0 (s, OCH(CH3)2), 28.3 (s,
CH3), 28.1 (s, C(CH3)3), 27.6 (s, C(CH3)3), 27.3 (s, C(CH3)3), 23.7 (s,
CH3). 31P{1H} NMR (162 MHz, C6D6) δ ppm: 67.0 (t, 1P, 2JPY = 4.37
Hz), 58.9 (d, 1P, 2JPY = 4.21 Hz). Anal. Calcd. for C24H53N2O3P2Y: C,
50.70, H, 9.40, N, 4.93%. Found: C, 48.70, H, 7.61, N, 5.49%.
{[N,N0-1,2-bis(P,P0-di-tert-butylphosphinoyl)ethylene-
diamido] [bis(dimethylsilyl)amido](tetrahydrofurano)} yttrium
10. To a Young’s tap NMR tube was added 1 (0.038 g, 0.100 mmol) and
[Y{N(SiHMe2)2}3(THF)2] (0.063 g, 0.100 mmol). THF-d8 (0.25mL) was
added, and the tube shaken to form a colorless solution. The tube was left at
room temperature for 24 h, or until a 31P{1H} NMR spectrum indicated the
presence of one, yttrium-complexed species.
1H NMR (400 MHz, THF-d8) δ ppm: 4.60 (m, 2H, Si-H), 3.62
(m, 4H, OCH2), 3.56 (m, 4H, CH2), 1.78 (m, 4H, OCH2CH2), 1.25
2
2
Di-μ-oxo-{[N,N0-1,3-bis(P,P0-di-tert-butylphosphinoyl)-
2,2-dimethylpropanediamido](tert-butoxy)yttrium} 8. To a solu-
tion of {[N,N0-1,3-bis(P,P0-di-tert-butylphosphinoyl)-2,2-dimethylpropane-
(d, 36H, JHP = 13.2 Hz, C(CH3)3), 0.09 (d, 12H, JHH = 3.2 Hz,
Si(CH3)2). 13C{1H} NMR (100 MHz, THF-d8) δ ppm: 49.2 (d, 2JCP
12.6 Hz, CH2), 36.9 (br m, C(CH3)3), 27.3 (br s, C(CH3)3), 3.6
=
7726
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728

Inorganic Chemistry
ARTICLE
(s, Si(CH3)2). 31P{1H} NMR (162 MHz, THF-d8) δ ppm: 54.2 (d, 2JPY
4.05 Hz).
=
(3) (a) Ragauskas, A. J.; Williams, C. K.; Davison, B. H.; Britovsek,
G.; Cairney, J.; Eckert, C. A.; Frederick, W. J.; Hallett, J. P.; Leak, D. J.;
Liotta, C. L.; Mielenz, J. R.; Murphy, R.; Templer, R.; Tschaplinski, T.
Science 2006, 311, 484. (b) Stanford, M. J.; Dove, A. P. Chem. Soc. Rev.
2010, 39, 486.
Di-μ-oxo-{[N,N0-1,2-bis(P,P0-di-iso-propylphosphinoyl)-
ethylenediamido](tert-butoxy)yttrium} 11. An NMR scale
reaction was carried out. A solution of {[N,N0-1,2-bis(P,P0-di-tert-butyl-
phosphinoyl)ethylenediamido] [bis(dimethylsilyl)amido](tetrahydro-
furano)}yttrium, 10 (0.030 mg, 0.05 mmol) in benzene-d6, in a Young’s
tap NMR tube, was prepared. To the resulting solution was added
tert-butanol (4.75 μL, 0.05 mmol) via a microsyringe. The solution was
left at room temperature for 2 h before NMR spectra were acquired.
1H NMR (400 MHz, C6D6) δ ppm: 3.67 (m, 2H, CH2), 3.55 (m, 2H,
CH2), 3.47 (m, 2H, CH2), 3.38 (m, 2H, CH2), 1.52 (d, 3JHP = 14.4 Hz,
(4) Williams, C. K. Chem. Soc. Rev. 2007, 36, 1573.
(5) McClain, S. J.; Ford, T. M.; Drysdale, N. E. Polym. Prep. 1992, 463.
(6) Simic, V.; Spassky, N.; Hubert-Pfalzgraf, L. G. Macromolecules
1997, 30, 7338.
(7) (a) Stevels, W. M.; Ankone, M. J. K.; Dijkstra, P. J.; Feijen, J.
Macromol. Chem. Phys. 1995, 196, 1153. (b) Stevels, W. M.; Ankone,
M. J. K.; Dijkstra, P. J.; Feijen, J. Macromolecules 1996, 29, 3332.
(c) Stevels, W. M.; Ankone, M. J. K.; Dijkstra, P. J.; Feijen, J. Macro-
molecules 1996, 29, 6132. (d) Simic, V.; Girardon, V.; Spassky, N.;
Hubert-Pfalzfraf, L. G.; Duda, A. Polym. Degrad. Stab. 1998, 59, 227.
(8) (a) Buffet, J.-C.; Kapelski, A.; Okuda, J. Macromolecules 2010,
43, 10201. (b) Ma, H.; Spaniol, T. P.; Okuda, J. Angew. Chem., Int. Ed.
2006, 45, 7818. (c) Ma, H.; Spaniol, T. P.; Okuda, J. Inorg. Chem. 2008,
47, 3328.
3
18H, C(CH3)3), 1.49 (s, 18H, OC(CH3)3), 1.41 (d, JHP = 14.4 Hz,
18H, C(CH3)3), 1.33 (d, 3JHP = 14.4 Hz, 18H, C(CH3)3), 1.24 (d, 3JHP
=
14.4 Hz, 18H, C(CH3)3), 31P{1H} NMR (162 MHz, C6D6) δ ppm:
61.4 (dd, unresolved, 1P), 53.4 (d, 1P, 2JPY = 3.89 Hz).
General Polymerization Protocol. To a rapidly stirred solution
of rac-lactide (144 mg, 1.0 mmol) in THF or CH2Cl2 (1.75 mL), in a
small glass vial equipped with a lid, in the glovebox, was added a solution
of the appropriate initiator in THF or CH2Cl2 (0.25 mL of a 0.02 M
solution, 0.005 mmol for dimeric initiators and 0.125 mL of a 0.02 M
solution, 0.0024 mmol for monomeric initiators), such that the total
reaction volume was 2.0 mL, with [LA]0 = 0.5 M and [6, 9, 10] =
2.5 mM, [1ꢀ5, 7, 8, 11] = 1.25 mM. The lid was replaced on the vial, and
aliquots of ∼0.1 mL were removed at known times via a syringe and
quenched in 4ꢀ5 mL of cold hexane. Upon completion of the reaction,
the quenched samples were dried in vacuo to yield colorless polymer.
The polymerization conversion was determined by normalization and
integration of the signals in the 1H NMR spectrum (CDCl3) due to the
methyne proton in polylactide (δ = 5.30ꢀ5.00 ppm) and lactide (δ =
4.92 ppm). The polymer Mn and PDI were determined by SEC, using
THF as the eluent, and triple detection was to determine the absolute
molecular weight (see Measurements section). The polymer tacticity
was determined from the homonuclear decoupled 1H NMR spectrum
and using the method described by Coudane et al. to analyze the
tetrads.25
(9) Ma, H.; Okuda, J. Macromolecules 2005, 38, 2665.
(10) (a) Ajellal, N.; Carpentier, J. F.; Guillaume, C.; Guillaume,
S. M.; Helou, M.; Poirier, V.; Sarazin, Y.; Trifonov, A. Dalton Trans.
2010, 39, 8363. (b) Grunova, E.; Kirillov, E.; Roisnel, T.; Carpentier, J. F.
Dalton Trans. 2010, 39, 6739. (c) Bonnet, F.; Cowley, A. R.; Mountford,
P. Inorg. Chem. 2005, 44, 9046. (d) Clark, L.; Cushion, M. G.; Dyer,
H. E.; Schwarz, A. D.; Duchateau, R.; Mountford, P. Chem. Commun.
2010, 46, 273. (e) Dyer, H. E.; Huijser, S.; Schwarz, A. D.; Wang, C.;
Duchateau, R.; Mountford, P. Dalton Trans. 2008, 32. (f) Liu, X. L.;
Shang, X. M.; Tang, T.; Hu, N. H.; Pei, F. K.; Cui, D. M.; Chen, X. S.;
Jing, X. B. Organometallics 2007, 26, 2747. (g) Miao, W.; Li, S. H.; Cui,
D. M.; Huang, B. T. J. Organomet. Chem. 2007, 692, 3823. (h) Nie, K.;
Gu, X. Y.; Yao, Y. M.; Zhang, Y.; Shen, Q. Dalton Trans. 2010, 39, 6832.
(i) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 1999, 121, 4072.
(j) Ovitt, T. M.; Coates, G. W. J. Am. Chem. Soc. 2002, 124, 1316.
(k) Zhang, Z. J.; Xu, X. P.; Li, W. Y.; Yao, Y. M.; Zhang, Y.; Shen, Q.; Luo,
Y. J. Inorg. Chem. 2009, 48, 5715.
(11) (a) Amgoune, A.; Thomas, C. M.; Carpentier, J. F. Pure Appl.
Chem. 2007, 79, 2013. (b) Amgoune, A.; Thomas, C. M.; Carpentier,
J. F. Macromol. Rapid Commun. 2007, 28, 693.
(12) (a) Amgoune, A.; Thomas, C. M.; Roisnel, T.; Carpentier, J. F.
Chem.—Eur. J. 2006, 12, 169. (b) Cai, C. X.; Amgoune, A.; Lehmann,
C. W.; Carpentier, J. F. Chem. Commun. 2004, 330.
(13) Ajellal, N.; Lyubov, D. M.; Sinenkov, M. A.; Fukin, G. K.;
Cherkasov, A. V.; Thomas, C. M.; Carpentier, J. F.; Trifonov, A. A.
Chem.—Eur. J. 2008, 14, 5440.
’ ASSOCIATED CONTENT
S
Supporting Information. Crystallographic data in CIF
b
format. Further details are given in Figure S1ꢀS10. This material
is available free of charge via the Internet at http://pubs.acs.org.
(14) (a) Arnold, P. L.; Buffet, J. C.; Blaudeck, R.; Sujecki, S.; Wilson,
C. Chem.—Eur. J. 2009, 15, 8241. (b) Arnold, P. L.; Buffet, J. C.;
Blaudeck, R. P.; Sujecki, S.; Blake, A. J.; Wilson, C. Angew. Chem., Int. Ed.
2008, 47, 6033.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: c.k.williams@imperial.ac.uk.
(15) (a) Alaaeddine, A.; Amgoune, A.; Thomas, C. M.; Dagorne,
S.; Bellemin-Laponnaz, S.; Carpentier, J.-F. Eur. J. Inorg. Chem.
2006, 3652. (b) Alaaeddine, A.; Thomas, C. M.; Roisnel, T.;
Carpentier, J. F. Organometallics 2009, 28, 1469. (c) Beckerle, K.;
Hultzsch, K. C.; Okuda, J. Macromol. Chem. Phys. 1999, 200, 1702.
(d) Chamberlain, B. M.; Jazdzewski, B. A.; Pink, M.; Hillmyer, M. A.;
Tolman, W. B. Macromolecules 2000, 33, 3970. (e) Chamberlain, B. M.;
Sun, Y.; Hagadorn, J. R.; Hemmesch, E. W.; Young, V. G., Jr.; Pink, M.;
Hillmyer, M. A.; Tolman, W. B. Macromolecules 1999, 32, 2400.
(f) Grunova, E.; Kirillov, E.; Roisnel, T.; Carpentier, J. F. Organometallics
2008, 27, 5691. (g) Heck, R.; Schulz, E.; Collin, J.; Carpentier, J. F.
J. Mol. Catal. A 2007, 268, 163. (h) Mazzeo, M.; Lamberti, M.; D’Auria,
I.; Milione, S.; Peters, J. C.; Pellecchia, C. J. Polym. Sci, Part A: Polym.
Chem. 2010, 48, 1374. (i) Patel, D.; Liddle, S. T.; Mungur, S. A.; Rodden,
M.; Blake, A. J.; Arnold, P. L. Chem. Commun. 2006, 1124.
(16) Aubrecht, K. B.; Chang, K.; Hillmyer, M. A.; Tolman, W. B.
J. Polym. Sci, Part A: Polym. Chem. 2001, 39, 284.
’ ACKNOWLEDGMENT
The EPSRC are acknowledged for funding this research (EP/
C544846/1 and EP/C544838/1). Rac-Lactide was donated by
Purac Plc. Mr Peter Haycock, Imperial College London is
thanked for his help with the 31P NMR experiments.
’ REFERENCES
(1) (a) Amass, W.; Amass, A.; Tighe, B. Polym. Int. 1998, 47, 89.
(b) Drumright, R. E.; Gruber, P. R.; Henton, D. E. Adv. Mater. 2000,
12, 1841.
(2) (a) O’Keefe, B. J.; Hillmyer, M. A.; Tolman, W. B. Dalton Trans.
2001, 2215. (b) Dechy-Cabaret, O.; Martin-Vaca, B.; Bourissou, D.
Chem. Rev. 2004, 104, 6147. (c) Platel, R. H.; Hodgson, L. M.; Williams,
C. K. Polym. Rev. 2008, 48, 11.
(17) Mahrova, T. V.; Fukin, G. K.; Cherkasov, A. V.; Trifonov, A. A.;
Ajellal, N.; Carpentier, J. F. Inorg. Chem. 2009, 48, 4258.
7727
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728

Inorganic Chemistry
ARTICLE
(18) Hodgson, L. M.; Platel, R. H.; White, A. J. P.; Williams, C. K.
Macromolecules 2008, 41, 8603.
(19) Hodgson, L. M.; White, A. J. P.; Williams, C. K. J. Polym. Sci.,
Part A: Polym. Chem. 2006, 44, 6646.
(20) Platel, R. H.; Hodgson, L. M.; White, A. J. P.; Williams, C. K.
Organometallics 2007, 26, 4955.
(21) Platel, R. H.; White, A. J. P.; Williams, C. K. Inorg. Chem. 2008,
47, 6840.
(22) Platel, R. H.; White, A. J. P.; Williams, C. K. Chem. Commun.
2009, 4115.
(23) Gottlieb, H. E.; Kotlyar, V.; Nudelman, A. J. Org. Chem. 1997,
62, 7512.
(24) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam, W.; Young,
V. G.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc. 2003, 125,
11350.
(25) Coudane, J.; Ustariz-Peyret, C.; Schwach, G.; Vert, M. J. Polym.
Sci., Part A: Polym. Chem. 1997, 35, 1651.
(26) Hermann, W. A.; Eppinger, J.; Spiegler, M.; Runte, O.; Anwander,
R. Organometallics 1997, 16, 1813.
(27) Dorgan, J. R.; Janzen, J.; Knauss, D. M.; Hait, S. B.; Limoges,
B. R.; Hutchinson, M. H. J. Polym. Sci., Part B: Polym. Phys. 2005, 43,
3100.
7728
dx.doi.org/10.1021/ic200773x |Inorg. Chem. 2011, 50, 7718–7728