Aluminum Complexes of Monopyrrolidine Ligands for the Controlled Ring-Opening Polymerization of Lactide

Article abstract of DOI:10.1021/acs.organomet.8b00161

In this paper we report the full characterization (solution-state NMR spectroscopy and solid-state structures) of a series of Al(III) half-salan complexes and their exploitation for the ring-opening polymerization of rac-lactide. Depending on the ligand e

Full text of DOI:10.1021/acs.organomet.8b00161

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Aluminum Complexes of Monopyrrolidine Ligands for the Controlled
Ring-Opening Polymerization of Lactide
James Beament, Mary F. Mahon, Antoine Buchard,* and Matthew D. Jones*
Department of Chemistry, University of Bath, Claverton Down Bath BA2 7AY, U.K.
S
* Supporting Information
ABSTRACT: In this paper we report the full characterization
(solution-state NMR spectroscopy and solid-state structures)
of a series of Al(III) half-salan complexes and their exploitation
for the ring-opening polymerization of rac-lactide. Depending
on the ligand employed and stoichiometry of the complex-
ation, structures of the form Al(X)2Me or Al(X)Me2 were
isolated. Interestingly Al(2)2Me and Al(2)Me2 produce PLA
with a strong isotactic bias (Pm up to 0.80), whereas all other
complexes produced atactic PLA. This is in contrast to recent
studies on similar salan ligand systems. PLAs with predictable
molecular weights and narrow distributions were achieved.
The results are discussed in terms of steric and electronic
properties of the ligands.
demonstrated, with a series of Al(III)-salen complexes, that the
ortho substituent on the phenyl ring of the salen can induce
distortions which can have a massive effect on the rate of
polymerization of caprolactone. In this case it is shown that
bulky substituents distort the geometry around the Al(III)
center, which is believed to be responsible for the observed
increase in polymerization rate.11
INTRODUCTION
■
Currently, there is a tremendous desire to develop new
polymeric materials that are biodegradable and are sourced
from annually renewable raw materials.1 Unquestionably one of
the most important polymers that fulfills these criteria is
polylactide (PLA). PLA is prepared from the ring-opening
polymerization (ROP) of a cyclic ester monomer, lactide (LA).
LA is typically utilized as either the enantiomerically pure L-LA
or as a racemic mixture of D and L monomers (rac-LA). When
rac-LA is utilized, atactic, heterotactic, or isotactic PLA can be
prepared, with the physical properties of the polymer being
intrinsically linked to the polymer’s microstructure. The
microstructure can be controlled by the judicious choice of
metal center and ligand. For example, there are many elegant
stereoselective polymerizations employing group 1−4 metal
centers,2 lanthanides,3 indium,4 zinc,5 and (pertinent to this
study) aluminum.6 This follows on from seminal contributions
by Feijen,6h Chisholm,7 Gibson8 and Coates2p,9 in the early
part of this century. More recently, a tremendous amount of
effort has been focused on understanding the subtle interplay
between the metal center and ligand and the consequence this
has on the stereochemistry of the resulting polymer.2o,v Subtle
changes in selectivity have been observed by Williams for
phospha-salen lanthanide complexes and by Ma for a series
aminophenolate Zn(II)/Mg(II) complexes.2o,10 However, it is
fair to say that there is a degree of serendipity in the
stereochemical outcome of the polymerization, with unpredict-
able tacticities achieved from metal−ligand combinations.
Furthermore, subtle changes to the ligand can significantly
alter the rate of polymerization.6f In this regard we have shown
that simply reducing a salalen to a salan dramatically increases
the rate of ROP of rac-LA.6f Tolman and co-workers have
Recently, we have demonstrated the importance of
bipyrrolidine-derived salan ligands for controlling the stereo-
chemical outcome for rac-LA polymerization.2u,v For example,
when a meso-bipyrrolidine salan ligand (Scheme 1) is
complexed to Zr(IV), the resulting initiator shows a high
isotactic tendency (Pm up to 0.86).2u Interestingly, when the
same ligand is complexed to Al(III), highly heterotactic PLA is
produced (Pr up to 0.87).2v Moreover, for the tBu analogue,
atactic PLA was produced in the melt (130 °C) after 48 h and it
was shown that the complex was inactive in solution.2v The
In(III) complex of the meso tBu bipyrrolidine produced
heterotactic PLA in solution (Pr up to 0.84).4b The exact
reasons for these dramatic switches in selectivity and activity are
still open to debate. Kol has also successfully shown the
importance of the these pyrrolidine rings in salalen complexes
of Al(III) for the controlled isotactic selective polymerization of
rac-LA.2s,12 As part of ongoing studies in the area of pyrrolidine
systems we have prepared a series of monopyrrolidine ligands
and their respective Al(III) complexes. These were screened for
the ROP of rac-LA to further investigate how the ligand affects
the stereochemical outcome of the polymerization. Moreover,
the ligand framework could provide a more rigid environment
Received: March 19, 2018
© XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00161
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recently reported in an attempt to gain an insight into the high
heteroselectivity of the Al−bipyrrolidine system.2v
Scheme 1. Synthesis of the Ligands and Attempted
Complexes Used in This Study and Our Previous
Bipyrrolidine Ligand for Comparison
a
While Scheme 1 shows all attempted syntheses, only the
following complexes could be isolated and characterized via
single crystal X-ray diffraction: Al(1)Me2, Al(2)Me2, Al(3)Me2,
Al(4)Me2, Al(5)Me2, Al(2)2Me, Al(3)2Me, Al(5)2Me, and
Al(6)2Me. Table 1 has the key metric data for the complexes,
and representative structures for Al(X)2Me and Al(X)Me2,
where X = 2, are shown in Figure 1. In the other cases shown in
a
Syntheses of the compounds in pale gray were attempted, but they
could not be isolated in a pure form.
Figure 1. Solid-state structures for Al(2)2Me (left) and Al(2)Me2
(right). Ellipsoids are shown at the 30% probability level, and all
hydrogen atoms and solvent of crystallization have been removed for
clarity.
around the aluminum center, which could influence the
stereochemical outcome of the ROP.
RESULTS AND DISCUSSION
■
Ligand and Complex Preparation. The ligands are easily
prepared by one of three methods (Scheme 1). Route 1
employs a reductive amination to prepare ligand 1H and 5H,
route 2 utilizes a modified Mannich reaction (2H−4H), and
route 3 proceeds via an SN2 reaction mechanism (6H).13 In our
hands these approaches were found to be the optimal synthesis
for each ligand. The ligands themselves are relatively simple,
which is a prerequisite for large-scale applications (in fact 1H is
commercially available). However, to our surprise, there is only
one crystallographically characterized metal complex of this
class of ligand: Shen and co-workers have prepared a series of
La(III) complexes, which have been screened for the
polymerization of ε-caprolactone.14 All ligands synthesized
here have been characterized via 1H and 13C{1H} NMR
spectroscopy and high-resolution mass spectrometry. In the 1H
NMR spectrum there was a sharp singlet at ca. 4 ppm for the
methylene protons, Ar−CH2−N. The ligands were reacted with
either 0.5 or 1 equiv of AlMe3 in an attempt to form the desired
complexes (Scheme 1). A further aim of this study was to
compare these ligands to the bipyrrolidine complexes we have
Scheme 1 a mixture of mono- and bis-ligated complexes was
observed in the crude mixture (regardless of stoichiometry),
which could not be purified by recrystallization. It is relatively
facile to distinguish between the two forms via 1H NMR
spectroscopy: for example, in the preparation of Al(1)Me2 it
was clear that a mixture of both forms (Al(1)Me2 and
Al(1)2Me) was observed in solution (see Figure S7 in the
Supporting Information). The mono-ligated complexes are
tetrahedral in geometry with X−Al−Y angles close to 109°
(Table 1), whereas Al(X)2Me complexes are pseudo trigonal
bipyramidal, as observed by their τ values being greater than
0.5, which are in agreement with other half salan-Al complexes
in the literature.15 As expected, the Al−N distances for the
mono-ligated system (1.9971(18)−2.040(2) Å) are signifi-
cantly shorter than those of the bis-ligated system
(2.1131(17)−2.1397(17) Å). The metric data for both systems
are in agreement with similar half-salan Al(III) reported
complexes in the CCDC.15 Moreover, on comparison of the
systems there is little difference in the Al−O (1.760(2)−
1.7918(16) Å) and Al−C distances (1.954(3)−1.987(4) Å)
a
Table 1. Selected Bond Lengths (Å) and Angles (deg) for the Complexes Characterized via Single-Crystal X-ray Diffraction
Al(1)Me2
Al(2)Me2
Al(3)Me2
Al(4)Me2
Al(5)Me2
Al(2)2Me
Al(3)2Me
Al(5)2Me
Al(6)2Me
Al(1)−O(1)
Al(1)−O(2)
Al(1)−N(1)
Al(1)−N(2)
Al(1)−C(1)
Al(1)−C(2)
O(1)−Al(1)−N(1)
N(1)−Al(1)−N(2)
O(1)−Al(1)−O(2)
C(1)−Al(1)−O(1)
τ
1.7745(10
1.7644(18)
1.760(2)
1.7769(17)
1.7606(14)
1.770(2)
1.770(2)
2.139(3)
2.132(3)
1.987(4)
1.7916(13)
1.7870(14)
2.1261(18)
2.1377(18)
1.981(2)
1.7873(17)
1.7830(15)
2.125(2)
1.7846(15)
1.7918(16)
2.1131(17)
2.1397(17)
1.983(2)
2.0300(11)
2.040(2)
2.029(2)
2.047(2)
1.9971(18)
2.125(2)
1.9639(14)
1.9579(14)
98.13(4)
1.954(3)
1.956(3)
97.43(8)
1.963(3)
1.956(3)
97.46(9)
1.964(3)
1.952(3)
97.45(8)
1.964(2)
1.956(2)
95.58(7)
1.973(2)
87.77(11)
170.41(11)
118.98(13)
118.66(15)
0.81
87.85(6)
172.06(7)
117.28(7)
121.52(9)
0.84
89.55(7)
167.88(8)
118.46(8)
119.08(9)
0.77
89.48(7)
169.29(7)
119.41(7)
120.77(10)
0.81
112.06(9)
111.22(11)
112.37(13)
111.88(10)
112.27(9)
a
The τ values have been determined via the method of Atwood et al.16
B
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Table 2. Solution and Melt Polymerization for the Range of Al(III) Complexes Prepared
d
e
f
f
g
entry
initiator
[LA]:[I]:[BnOH]
temp/°C
time/h
conversn/%
Pm
Mn
Đ
calcd Mn
a
1
Al(2)2Me
Al(2)2Me
Al(2)2Me
Al(2)2Me
Al(2)2Me
Al(3)2Me
Al(5)2Me
Al(5)2Me
Al(6)2Me
Al(6)2Me
Al(1)Me2
Al(2)Me2
Al(2)Me2
Al(2)Me2
Al(2)Me2
Al(2)Me2
Al(3)Me2
Al(4)Me2
Al(5)Me2
100:1:1
100:1:1
100:1:0
100.1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:2
100:1:0
100:1:1
1000:1:10
100:1:1
100:1:1
100:1:1
100:1:2
80
6
24
0.25
0.25
120
48
6
38
95
60
94
17
32
36
96
48
72
84
73
52
23
73
92
16
12
74
68
0.72
0.71
0.62
0.58
4210
8780
1.02
1.04
1.24
1.10
5550
13750
8750
a
2
80
b
3
130
18500
14850
b
4
130
13650
2550
c
5
25
a
6
80
0.52
0.49
0.48
0.61
0.59
0.53
0.80
0.68
24250
4650
11950
4350
12750
8250
9250
6050
1.28
1.18
1.17
1.07
1.10
1.06
1.05
1.06
4650
a
7
80
5300
a
8
80
24
6
13950
6950
a
9
80
a
10
11
12
13
14
15
16
17
18
19
20
80
24
6
10400
12154
10550
7400
a
80
a
80
6
a
80
6
a
80
6
3350
b
130
0.16
0.5
24
24
6
0.61
0.56
12550
18200
1.23
1.10
10550
13300
2400
b
130
a
80
a
80
1850
a
80
0.61
0.55
8800
5300
1.06
1.07
10700
4950
a
Al(5)Me2
80
6
a
b
c
d
e
1
Toluene solvent. Solvent free (melt conditions). CH2Cl2 solvent. Determined from analysis of the H NMR spectrum. Determined from
f
1H{1H} NMR. Determined by GPC (THF) calibrated using RI, viscometer, and light scattering detectors using the universal calibration method via
multidetection software. Theoretical molecular weight calculated from conversion ([LA]/[BnOH] × (conversn × 144.13) + 108.14) (rounded to
g
the nearest 50).
depending on the stoichiometry or ligand, both in relatively
tight ranges. The Al(X)Me2 structures appear to be maintained
in solution; this is exemplified for Al(3)Me2, where discrete and
relatively sharp resonances are observed for the −CH2−
moieties in the solution-state NMR (C6D6). Further in all cases,
one sharp 6H Al-Me resonance is observed at ca. − 0.5 ppm.
On an NMR scale Al(2)Me2 was reacted with either 1 or 2
equiv of BnOH, and the resultant spectra showed the rapid
reaction of Al-Me with BnOH, to generate the alkoxide
(Figures S16 and S17 in the Supporting Information). DOSY
investigations showed a reduction in the diffusion constant
upon reaction with BnOH (Figure S18 in the Supporting
Information), as might be expected by the removal of −Me by
−OCH2Ph, although this does not conclusively rule out the
formation of a dimeric alkoxide species. However, in the
presence of a coordinating monomer, such as lactide, a
monomeric active species is highly likely.
higher temperatures. However, in all cases at room temperature
analysis of the aromatic region of the H NMR spectrum
1
indicated the required number of resonances for this motif and
there is a single 6H resonance for the Al-Me moiety. However,
as the system cooled, the Al-Me resonance split into two
signals. This and a significantly more complex aliphatic region
at low temperature potentially indicate that multiple coordina-
tion motifs are present in solution upon cooling.
Polymerization Studies. The most pertinent comparison
in the literature to the complexes herein is with our
bipyrrolidine salan system (either meso or homochiral
versions).2u,v When it is complexed to Al(III), this bipyrrolidine
ligand (with methyl substituents in the ortho/para position)
produces either heterotactic PLA (meso chirality) or atactic
PLA (homochiral ligand). For example, with the meso-
bipyrrolidine-Al-Me complex at 100:1:1 LA:Init:BnOH in
toluene at 80 °C a conversion of 87% was achieved after 120
h (Mn = 21550, Đ = 1.05, Pr = 0.87). However, with the tBu-
substituted ligand atactic PLA was observed and it was
concluded that with this bipyrrolidine system heterotactic
PLA was due to a combination of steric bulk of the ortho
substituent and the meso chirality of the ligand. In the current
study we have removed any effect ligand chirality may have on
the stereochemical outcome of the polymerization. However,
the coordination motif is now subtly different from that in our
previously reported study.2v
The complexes were tested for the ROP of rac-LA in solution
with the addition of BnOH to generate the alkoxide in situ
(Table 2). MALDI-ToF mass spectrometry indicated the
BnO− and H− end groups as expected (entry 12); a major
series with a repeat unit of 144 g mol−1 and a minor series of 72
g mol−1 were observed due to transesterification. We suggest
that the mechanism for polymerization is the classical
coordination insertion mechanism, as both sets of complexes
react with BnOH in solution (see Figures S16 and S17 in the
For the solid-state complexes of the form Al(X)2Me the
ligands bind to the aluminum center so that the nitrogen
moieties are trans to one another, N(1)−Al(1)−N(2) being in
the range 169.29(7)−172.06(7)°. Obviously, this differs from
our bipyrrolidine system (and other tetradentate salan systems
in the CCDC) in which the tetradentate nature of the ligand
dictates them to be cis to each other.2v All metric data are in
agreement within the series regardless of the ligand’s
substituent. In solution the complexes appear to have a degree
of fluxionality, as indicated by broad resonances in the region
ca. 2−4 ppm for the pyrrolidine ring and −CH2− methylene. In
an attempt to aid resolution, a solutions of Al(3/5)2Me in
1
C6D5CD3 were investigated via variable-temperature H NMR
spectroscopy. The resulting spectra were complex with multiple
resonances being observed (see Figures S13 and S14 in the
Supporting Information). When the complexes were heated,
sharp resonances indicative of a monomeric species were
observed; this is important, as the polymerization takes place at
C
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Supporting Information).17 For the Al(X)2Me complexes there
is relatively good agreement between the observed molecular
weights and theoretical molecular weight with narrow
distributions observed in all cases. However, for Al(3)2Me
the observed molecular weight was considerably higher than
the theoretical weight; this may well be related to the increased
steric bulk of the ortho substituent hindering the formation of
the Al-alkoxide, thus reducing the concentration of the active
species in solution or poor initiation. Al(2)2Me was also trialed
in the melt (entries 3 and 4). Without the addition of BnOH
the polymer molecular weight was not predictable; however,
with the addition of BnOH there was excellent agreement and
PLA with a narrow Đ value was isolated. The polymer tacticity,
assessed by 1H{1H} NMR spectroscopy, showed that Al(2)2Me
produced PLA with a moderate isotacticity (Pm = 0.71),
whereas all other Al(X)2Me complexes afforded atactic PLA.
For the Al(X)Me2 systems again there was good agreement
between the theoretical and calculated molecular weights and
MALDI-ToF indicated the desired end groups. The high
degree of control was exemplified by a linear relationship
between conversion and molecular weight for Al(2)Me2
(Figure 2), with a gradient of ca. 144 g mol−1 indicative of
bidentate ligands to the tetradentate system. The exact reason
for the dramatic switch is not fully known but is potentially
related to change in coordination motif of the ligand around the
metal center, which in turn will alter the path of lactide
coordination to the metal center.
The rate of polymerization has been investigated, and Table
3 shows a comparison of kapp for a representative sample of
a
Table 3. kapp Values for Various Initiators
initiator
conditions (LA:Init:BnOH)
kapp/10−4 min−1
Al(2)2Me
Al(2)2Me
Al(5)2Me
Al(1)Me2
Al(2)Me2
Al(2)Me2
Al(3)Me2
Al(5)Me2
100:1:1 rac-LA
100:1:1 L-LA
6.5
10.2
5.6
100:1:1 rac-LA
100:1:1 rac-LA
100:1:1 rac-LA
100:1:2 rac-LA
100:1:1 rac-LA
100:1:1 rac-LA
19.2
18.8
19.7
1.3
13.5
a
Reaction conditions: temperature 80 °C, solvent C6D5CD3, [LA]0 =
0. 69 mol dm−3.
initiators. The mono-ligated systems appear to be faster
(Al(2)2Me vs Al(2)Me2 and Al(5)2Me vs Al(5)Me2), which
may be related to the reduce steric crowding around the Al(III)
center facilitating facile coordination of the monomer. As
expected, as the size of the ortho substituent increases (H to
Me to tBu), there is a reduction in the apparent first-order rate
constant. Again this can be explained in terms of steric
crowding around the metal center. The polymerization of L-LA
was investigated with Al(2)2Me, and in this case the
polymerization rate was faster than that with rac-LA, which is
to be expected from an isoselective initiator.
CONCLUSIONS
■
In conclusion nine half-salan Al(III) complexes have been
prepared and fully characterized in solution and in the solid
state. The ligands have been chosen to investigate the steric and
electronic effects on the polymerization. Complexes Al(2)2Me
and Al(2)Me2 produce PLA with a high isotactic bias, which is
in stark contrast to the case for previously reported complex-
es.2v The other complexes produced predominately atactic
PLA. It therefore appears that the steric requirements are the
overriding requirement for the production of isotactic PLA.
Moreover, it is interesting to compare this half-salan to the full
bipyrrolidine salan, which has a high heterotactic enchainment.
This illustrates the very subtle interplay between ligand
coordination and selectivity. Such a change between bidentate
half-salan and tetradentate salan ligands, to our knowledge, is
unprecedented.
Figure 2. Plot of measured Mn and Đ vs conversion for the
polymerization of rac-LA with Al(2)Me2 (80 °C, 100:1:1 LA:Init:B-
nOH, solvent toluene).
one chain growing per metal center, and the Đ value remained
below 1.10. Furthermore, when the amount of BnOH was
varied (Table 2, entry 12 vs entry 13), there is a concomitant
reduction in the observed molecular weight, and without
BnOH (entry 14) there is a small degree of conversion. This
system is also active under melt conditions (entries 15 and 16)
with high conversion in a relatively short time frame. It is worth
noting that at a ratio of 1000:1:10 under melt conditions a 92%
conversion was achieved in just 30 min with a predictable
molecular weight and narrow distribution. This is a significantly
shorter time frame than that for the tetradentate salan
complex.2v
For Al(2)Me2 PLA with a strong isotactic bias was observed
with Pm = 0.80. Analysis of the 1H{1H} NMR spectrum showed
a reduced sis tetrad, indicative of formation of PLA possessing a
“blocky” nature. Moreover, the sii, iis, and isi tetrads are present
in a ratio of approximately 1:1:1, which is indicative of a chain-
end mechanism in operation.2v It is interesting that, regardless
of stoichiometry, complexes with 2H have an isotactic
tendency. This is in stark contrast to the Al(III) complex of
the tetradentate version.2v It is not uncommon that minor
changes to the substituents on the phenyl group of salan/salen
or salalen ligands induce difference stereochemical out-
comes.2v,18 However, it is far less common to compare two
EXPERIMENTAL SECTION
■
The preparation and characterization of all metal complexes were
carried out under an inert argon atmosphere using standard Schlenk or
glovebox techniques. All chemicals used were purchased from Aldrich
and used as received, except for rac-LA, which was recrystallized from
dry toluene. Dry solvents used in handling metal complexes were
obtained via SPS (solvent purification system). 1H and 13C{1H} NMR
spectra were recorded on a Bruker 400 or 500 MHz instrument and
referenced to residual protio solvent peaks. CDCl3/C6D6 were dried
over CaH2 prior to use with metal complexes. Coupling constants are
given in hertz. CHN microanalysis was performed by Mr. Stephen
Boyer of London Metropolitan University. All details are given as
Supporting Information, but a representative synthesis is provided
D
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below. The general synthesis for the 1:2 metal:ligand complexes was as
follows. The ligand (1.0 mmol) was dissolved in toluene (10 mL), to
this was slowly added AlMe3 (0.5 mmol). After 1 h the solvent was
removed and the resulting product recrystallized from a hexane/
toluene mixture; typically crystals were isolated after cooling to −20
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1
°C overnight. Al(5)2Me was isolated as a yellow powder (64%). H
NMR (400 MHz, C6D6, δH, ppm, 298 K); 7.38 (2H, d, J = 2.3 Hz,
ArH), 6.52 (2H, d, J = 2.3 Hz, ArH), 3.55 (2H, m, NCH2), 3.15 (4H,
m, NCH2), 2.79 (4H, m, NCH2), 1.32 (8H, m, CH2), −0.69 (3H, s,
CH3), 13C{1H} NMR (100 MHz, C6D6, 298 K, δC, ppm): 154.7 (C−
O), 129.0 (Ar), 125.8 (Ar), 125.5 (Ar), 124.5 (Ar), 121.0 (Ar), 57.1
(NCH2), 53.1 (NCH2), 22.4 (CH2), 21.3 (CH2), −12.3 (CH3). Anal.
Calcd for C23H27N2Cl4O2Al: C, 51.90; H, 5.11; N, 5.26. Found: C,
51.65; H, 5.06; N, 5.03. For the synthesis of mono-ligated complexes, a
solution of the ligand (1.0 mmol) dissolved in toluene (5 mL) was
added slowly to a solution of AlMe3 (1.0 mmol) in toluene (5 mL).
Workup procedures followed the methods for bis-ligated complexes.
1
Al(5)Me2 was isolated as a white solid (59%). H NMR (400 MHz,
C6D6, δH, ppm); 7.38 (1H, d, J = 2.8 Hz, ArH), 6.47 (1H, d, J = 2.8
Hz, ArH), 2.89 (2H, s, CH2), 2.38 (2H, m, NCH2), 1.61 (2H, m,
NCH2) 1.11 (2H, m, CH2), 1.01 (2H, m, CH2), −0.57 (6H, s, CH3)
13C{1H} NMR (100 MHz, C6D6, δC, ppm); 155.3 (C−O), 130.5 (Ar),
127.7 (Ar), 125.5 (Ar), 123.8 (Ar), 120.9 (Ar), 58.4 (NCH2), 54.0
(NCH2), 22.4 (CH2), −11.3 (CH3). Anal. Calcd for C13H18NCl2OAl):
C, 51.66; H, 6.00; N, 4.64. Found: C, 50.85; H, 5.89; N, 5.25.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.8b00161.
Summary of NMR, GPC, and kinetic data (PDF)
Accession Codes
CCDC 1830729−1830737 contain the supplementary crystal-
lographic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Authors
*E-mail for A.B.: a.buchard@bath.ac.uk.
*E-mail for M.D.J.: mj205@bath.ac.uk.
■
ORCID
Antoine Buchard: 0000-0003-3417-5194
Matthew D. Jones: 0000-0001-5991-5617
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We wish to thank the EPSRC and University of Bath for
funding a studentship for JB. AB acknowledges Roger and Sue
Whorrod (fellowship) and the Royal Society (RG/150538;
UF/160021 fellowship. Analytical facilities were provided
through the Chemical Characterisation and Analysis Facility
(CCAF) at the University of Bath (http://www.bath.ac.uk/
ccaf.)
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