Salalens and Salans Derived from 3-Aminopyrrolidine: Aluminium Complexation and Lactide Polymerisation

Article abstract of DOI:10.1002/ejic.201900417

In this paper a series of 7 salalen ligands based on an aminopyrrolidine backbone have been prepared and characterised. Several systems have been reduced to the salan ONNO type-ligand. All ligands have been complexed to Al^III with Al(1–7)Me, Al(2a)(OiPr) and Al(7a)Me being characterised by single-crystal X-ray diffraction. In general the Al^III centres are best described as being in a trigonal bipyramidal geometry. The solution and solid-state structures are discussed. All complexes have all been trialled for the production of PLA from rac-lactide, the salalen complexes had a preference for heterotactic PLA (P_r = 0.71), whereas the salan had a more isotactic bias (P_m = 0.72). In all cases PLA with low dispersities and predictable molecular weights were prepared. The activity of the two classes of ligands is compared with the salan complexes appearing to be significantly more active than the salalen systems.

Full text of DOI:10.1002/ejic.201900417

A Journal of
Accepted Article
Title: Salalens and Salans derived from 3-Aminopyrrolidine:
Aluminium Complexation and Lactide Polymerisation
Authors: Luke Britton, Daniel Ditz, James Beament, Paul Mckeown,
Helena Quilter, Kerry Riley, Mary Mahon, and Matthew David
Jones
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900417
Link to VoR: http://dx.doi.org/10.1002/ejic.201900417

European Journal of Inorganic Chemistry
10.1002/ejic.201900417
FULL PAPER
Salalens and Salans derived from 3-Aminopyrrolidine: Aluminium
Complexation and Lactide Polymerisation
Luke Britton, Daniel Ditz, James Beament, Paul McKeown,* Helena C. Quilter, Kerry Riley, Mary F.
Mahon and Matthew D. Jones*
Abstract: In this paper a series of 7 salalen ligands based on an
aminopyrrolidine backbone have been prepared and characterised.
Several systems have been reduced to the salan ONNO type-ligand.
All ligands have been complexed to Al(III) with Al(1-7)Me, Al(2a)(OiPr)
and Al(7a)Me being characterised by single-crystal X-ray diffraction.
In general the Al(III) centres are best described as being in a trigonal
bipyramidal geometry. The solution and solid-state structures are
discussed. All complexes have all been trialled for the production of
PLA from rac-lactide, the salalen complexes had a preference for
Lewis acid metal centres and ligand combinations.[1a, 1b, 4]
However, there is still an element of serendipity in the
stereochemical outcome of the polymerisation. In recent years we
have published widely on the use of salalen ligands complexed to
Al(III) and Zr(IV) for the controlled ROP of rac-LA to produce
various levels of tacticity control.[ These studies have highlighted
that subtle changes in the substituents on the ligand can have
dramatic changes in the stereochemical outcome of the
polymerisation.[ Moreover, we have also shown that the activity
of an Al(III)-salalen can be dramatically enhanced by simple
reduction of the imine moiety.[6] Kol and co-workers have
5]
5g]
heterotactic PLA (P
r
= 0.71), whereas the salan had a more isotactic
bias (P = 0.72). In all cases PLA with low dispersities and predictable
m
molecular weights were prepared. The activity of the two classes of
ligands is compared with the salan complexes appearing to be
significantly more active than the salalen systems.
published
Al(III)-OiPr
systems
based
on
a
chiral
[
7]
aminomethylpyrrolidine salalen moiety. These complexes were
highly selective for the production of PLA with a gradient isotactic
multiblock microstructure. At 80 C with a [LA]:[Al] ratio of 100,
conversions of 42 – 91 % were achieved, depending on the
m m
substituents on the ligand, whilst the P (P = the probability of
meso enchainment) values ranged from 0.23 – 0.82. Further
Introduction
examples of metal-salalen complexes that show significant
The development of biobased polymers is a key target for the 21st
Century. This is mainly due to our reliance on plastic materials for
all aspects of our everyday lives. One of the success stories in
this arena in undoubtedly the development of polylactide (PLA)
which is currently commercially produced via the controlled ring
opening polymerisation (ROP) of lactide.[1] PLA has several major
advantages, namely 1) it can be prepared from annually
renewable starch rich materials; 2) the polymer is biodegradable
under appropriate conditions; 3) PLA is biocompatible and has
found many uses as tissue engineering scaffolds, resorbable
selectivity during polymerisation include group 4 based on
phenylene-salalen ligands,[5c, 8] iron-salalen based on a variety of
values up to 0.80;[5a] further Al(III) examples
based on ethylene-salalen backbones; lanthanide examples
backbones with P
m
[9]
with ethylene-salalen complexes have shown a preference for the
production of heterotactic PLA.[10] In this paper we have prepared
a
series of salalen/salan ligands based on the racemic
aminopyrrolidine backbone and reacted the resultant ligands with
AlMe to generate a pre-catalyst for the ROP of rac-LA. A series
3
of steric and electronic effects have been studied.
[2]
sutures and stents; 4) the physical properties of the resultant
polymer can be tuned for a multitude of uses.[1c, 3] The physical
m
properties (T , and degradation rate/profile) of PLA prepared from
Results and Discussion
the racemic monomer (rac-LA) can be altered by the production
of either atactic, heterotactic or stereoblock isotactic PLA. In this
regard the choice of initiator is pivotal in determining the
stereochemical outcome of the polymerisation. There have been
many elegant examples in recent years based on a plethora of
As part of our continuing studies regarding the application of
salalen and salan complexes for the production of PLA a series
of ligands based on 3-aminopyrrolidine were synthesised in high
yield, Scheme 1. The ligands were prepared via a simple imine
condensation followed by reaction with an alkyl halide to generate
2
the tetradentate ONNO salalen (1-7H ).
Mr L. Britton, Mr D. Ditz, Dr. H.C. Quilter, Miss K. Riley, Dr. M.F.
Mahon Corresponding Author(s): Drs P. McKeown and M.D. Jones
Department of Chemistry
University of Bath
Bath BA2 7AY UK
E-mail: p.mckeown@bath.ac.uk and mj205@bath.ac.uk
Supporting information for this article is given via a link at the end of
the document. Full experimental details, NMR spectra and a
selection of GPCs, MALDI-ToF’s, kinetic data and the X-ray data in
the .cif format.
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201900417
FULL PAPER
Figure 1. Representative solid-state structure for Al(1)Me, ellipsoids are shown
at the 30% probability level and all hydrogen atoms have been removed for
clarity.
The solution-state NMR spectra (see supporting information) are
consistent with the solid-state structures being maintained in
solution. This is exemplified by a single 3H resonance at ca.  0.2
to  0.5 ppm for the Al-Me moiety, and there are discrete doublets
2
for the –CH – moieties indicating that the ligands are “locked”
once coordinated to the aluminium centre. Upon coordination to
the aluminium centre N(2) becomes chiral and with the carbon
centre in ring adjacent to the imine N(1) already being chiral
(albeit racemic). It is possible to observe two diastereoisomers in
solution (R,R/S,S or R,S/S,R enantiomeric pairs) and it is clear
that only one set of diastereoisomers is present for the isolated
Al(1-6)Me. Analysis of the solid-state structures indicates this to
be the R,R/S,S pair of enantiomers. The isolation of only one set
of diastereoisomers may explain the low isolated yields of these
complexes. However, the situation is more complicated for
Al(7)Me where in solution two sets of diastereoisomers are
observed for the isolated species.
2 2
Scheme 1. Preparation of the salalen ligands 1H -7H and reduction to salan
ligands 2a, 3a, 4a, 5a, 7aH and the respective complexes.
2
All ligands were characterised by 1H and
spectroscopy and high resolution mass spectrometry. 1-7H
a 1H resonance at ca. 8.20 – 8.30 ppm (CDCl
) for the imine
13
C{ H} NMR
have
1
2
3
moiety. These ligands were chosen as they impart differing steric
and electronic effects upon the two fragments of the salalen
system, in an attempt to discern structure-activity-relationships,
e.g. 1-3H
interrogate the sterics/electronics of the salen fragment and 1H
vs 4H and 2H vs 5H investigate the sterics of the salan moiety.
The ligands were reacted with 1 equivalent of AlMe , in toluene,
2 2
study the sterics of the salen fragment, 4-6H further
Table 1. Selected metric data for complexes Al(1-7)Me, bond lengths are
in (Å) and angles ().
2
2
2
2
Al(1)Me
1.830(2)
1.764(2)
1.985(3)
2.229(3)
1.971(4)
163.16(11)
118.23(12)
75.86(11)
0.67
Al(2)Me
1.833(2)
1.751(2)
1.982(2)
2.184(2)
1.983(3)
164.94(9)
116.09(10)
76.91(9)
0.69
Al(3)Me
1.8260(15)
1.7594(15)
1.9878(17)
2.2155(17)
1.966(2)
Al(4)Me
1.8189(13)
1.7578(12)
1.9823(15)
2.2101(15)
1.9752(18)
165.62(6)
113.65(6)
76.98(6)
Al(5)Me
1.8276(18)
1.7607(17)
1.996(2)
2.222(2)
1.971(3)
165.95(9)
112.85(9)
76.48(8)
0.69
Al(6)Me
1.834(6)
1.754(6)
1.998(6)
2.197(7)
1.954(9)
166.0(3)
115.4(3)
76.8(3)
0.70
Al(7)Me
1.804(2)
1.781(2)
1.954(3)
2.171(3)
1.962(4)
162.05(12)
118.32(15)
75.85(12)
0.69
3
Al(1)-O(1)
Al(1)-O(2)
Al(1)-N(1)
Al(1)-N(2)
Al(1)-C(1)
O(1)-Al(1)-N(2)
O(2)-Al(1)-N(1)
N(1)-Al(1)-N(2)

to generate the required complexes (Scheme 1) in low/moderate
yields (10 - 56%). For Al(2a)Me the corresponding OiPr complex
was prepared for melt studies and as a comparison to our
previous work utilising an analogous 6-membered ring salalen
system.[6] All the salalen complexes Al(1-7)Me were characterised
by multi nuclear NMR spectroscopy and in the solid-state by
single crystal X-ray diffraction, Figure 1 and Table 1 for the metric
data. The solid-state structures indicate that the aluminium centre
is best described as pseudo trigonal bipyramidal with a  value
greater than 0.5 in all cases, with similar values obtained for all
complexes. As expected the ligand coordinates in a tetradentate
fashion, with the Al-Nimine bond length being significantly shorter
164.15(7)
115.63(7)
76.66(7)
0.69
0.71
{
1.954(9) – 1.998(6)} than the Al-Namine bond length {2.229(3) –
2
.171(3)}. The Al-O and Al-C are also analogous to previously
[
7]
reported Al-salalen complexes.
It has been shown previously that reduction of the imine moiety
of the salalen to generate an ONNO salan ligand dramatically
increases the activity of the resultant aluminium complex for
the ROP of rac-LA.[ Thus, in this study a selection of salalen
6]
4
ligands were reduced with NaBH to generate the desired
salan ligands in good yield (96 – 69 %), Scheme 1. The novel
salan ligands were characterised by NMR spectroscopy,
where the loss of the salalen’s characteristic imine resonance
and the presence of a new –CH
resolution mass spectrometry. The complexes were simply
prepared in an analogous fashion to ligands 1-7H . For the
2
– group observed, and high
2
salan ligands only one Al-Me complex could be characterised
in the solid-state {Al(3a)Me} by single-crystal X-ray diffraction,
Figure 2. However, it was possible to prepare and isolate in the
2
solid-state an example of an Al-OiPr complex with ligand 2aH ,
Figure 2. For these complexes the solution-state species were
slightly more complicated with the generation of three chiral
centres upon coordination to the Al(III) centre {N(1), N(2) and
the carbon atom in the ring adjacent to N(1)}. The solution-
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201900417
FULL PAPER
state NMR spectra are more complicated, in all cases there is
one major Al-Me resonance and two smaller resonances (ca.
spectrometry, revealed that the repeat unit was 144 gmol-1 and
there was little evidence of transesterification, see supporting
information. Moreover, analysis of the data clearly indicates the
desired BnO– and H– end groups as expected from the
classical coordination insertion mechanism. The complexes
10%). DOSY spectroscopic analysis for Al(4a)Me indicated
that these have similar diffusion constants in solution and can
therefore be tentatively assigned as different stereoisomers
present in solution. In the case of our previously reported
salalen/salan system based on the six membered piperidine
ring, a subtle difference in coordination of the salan ligand was
2
with ligands 4,5,6H were also tested under melt conditions
(entries 5, 8, 11 Table 1). As expected the activity did improve
with moderate conversions being achieved in hours rather than
days. Comparison of the changes in sterics of the half-salen
[6]
noted in the solid-state. This is not the case for Al(2a)OiPr
with N(2) and O(1) adopting the “axial” positions, as observed
for the salalen systems. However, for Al(3a)Me compared to
Al(3)Me there is a stark difference in the geometry of the Al(III).
For the salan system, the Al(III) centre is best described as
square based pyramidal with a  value of 0.34 cf. 0.69 for the
salalen system. This may well be related to the greater degree
of flexibility associated with the salan ligand.
2
fragment (where R = tBu, entries 1-3) did not indicate that this
was important in enhancing the activity or selectivity, as in all
cases 10 days was necessary to obtain a respectable
2
conversion. However, when R was Me the sitution was
1
different with Al(5)Me (R = H) giving the highest conversion.
1
2
The most bulky of this mini-series, Al(4)Me (R = tBu and R =
Me), gave a pitiful conversion after 5 days and was the worst
1
performing under melt conditions. Comparing 2 vs 7 (R = H,
2
R tBu or Me) again indicated that reducing steric requirements
had a positive effect on activity. Changing the half-salen
substituents to a chloro moiety also appears to marginally
increase the activity and selectivity (entries 9-10 vs. 7-8).
Table 2. Selected polymerisation data for the ROP of rac-LA with the Al(III)
salalen complexes.
c
n
M ,Theo.
d
Mn
e
f
Pr
Entry
Init.
[LA]:[I]:[BnOH]
T °C
Time
Con./ %
Đ(GPC)
e
/
h
(GPC)
a
1
2
3
4
5
6
7
8
9
Al(1)Me
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
100:1:1
100:1:1
100:1:1
80
240
240
240
120
6.25
24
74
75
55
9
10600
10800
7900
1400
5000
3550
9050
8900
5450
10500
10350
2850
10100
12000
12100
-
1.06
1.34
1.10
-
0.53
0.42
0.42
-
a
Al(2)Me
80
a
Al(3)Me
80
a
Al(4)Me
80
b
Al(4)Me
130
80
34
24
62
61
37
72
71
19
3600
3400
5900
13600
10300
8950
20400
-
1.09
1.08
1.05
1.30
1.05
1.07
1.22
-
0.49
0.59
0.58
0.68
0.71
0.71
0.67
-
a
Al(5)Me
a
Al(5)Me
80
72
b
Al(5)Me
130
80
2.5
24
a
Al(6)Me
a
10
Al(6)Me
80
72
Figure 2. Solid-state structure of Al(2a)(OiPr) (top) and Al(3a)Me (bottom),
ellipsoids are shown at the 30% probability level. All hydrogen atoms {except
those bound to N(1)} have been removed for clarity. Selected metric data
11
12
Al(6)Me
b
130
80
1.25
240
a
Al(7)Me
{
bond lengths (Å) and angles ()}: Al(2a)(OiPr) Al(1)-O(1) 1.7943(14), Al(1)-
O(2) 1.7530(14), Al(1)-O(3) 1.7504(14), Al(1)-N(1) 2.0419(18), Al(1)-N(2)
.1185(16), O(1)-Al(1)-N(2) 167.60(7), O(2)-Al(1)-N(1) 87.46(3), N(1)-Al(1)-
N(2) 79.26(7) 0.71; Al(3a)Me Al(1)-O(1) 1.7795(14), Al(1)-O(2)
7942(15), Al(1)-C(1) 1.982(2), Al(1)-N(1) 2.2245(18), Al(1)-N(2)
.1194(17), O(1)-Al(1)-N(2) 135.60(7), O(2)-Al(1)-N(1) 155.88(7), N(1)-
[a] Toluene solvent; [b] under melt conditions; [c] Determined via 1H NMR
spectroscopy; [d] Theoretical molecular weight calculated from conversion
(rounded to the nearest 50): {[LA]:[I] × (Conversion × 144.13) / BnOH
2

=
1
2
n
equiv.} + M (BnOH). [e] Determined from GPC (in THF) referenced
against polystyrene standards. [f] P
enchainment, determined via homonuclear decoupled
r
is the probability of heterotactic
1
Al(1)-N(2) 76.03(6)  = 0.34.
H
NMR
spectroscopy.
Inspired by our recent work where we observed a dramatic
increase in selectivity and activity by reducing a salalen based
on the six membered piperidine backbone[ we also tested a
series of salan-Al systems, Table 2. The salient comparisons
are the following entries (Table 1 vs Table 2) 2 vs 1, 3 vs 5, 4
vs 6, 6 vs 9, 12 vs 11, where in all cases simply reducing the
imine fragment of the salalen dramatically increases the
activity. Due to the enhanced activity of these systems the
pseudo first order rate constants for the polymerisation of rac-
Initially the Al-salalen complexes were trialled for the ROP of
rac-LA with the addition of 1 eq. of BnOH, to generate the
alkoxide in-situ, at 80 C in toluene, Table 2. At best the
salalen-activity can be described as modest. To obtain
conversions in excess of 50% a timeframe of 3-10 days was
required. Importantly, the complexes produced PLA with
controlled molecular weight and narrow dispersities. For
Al(2)Me, PLA with a very slight isotactic bias was produced,
however for the other complexes either atactic or
6]
r
heterotactically inclined (P upto 0.71) PLA was isolated.
LA ([LA] = 0.69 M, T = 80 C, [LA]:[Al]:[BnOH] = 100:1:1, see
0
Analysis of the resultant PLA via MALDI-ToF mass
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201900417
FULL PAPER
supporting information for the plots) were determined for Al(2a,
4
salalen systems appear to indicate that the steric requirements of
the ligands are important in dictating activity. A strong
a, 5a)Me as 2.9  10 , 2.4  10 and 1.1  10-3 mins-1
-
3
-3
respectively. Due to the very modest conversions obtained with
the analogous salalen systems kinetic investigations were not
attempted. For the salan series it appears that the steric bulk
of the aryl substituent of the –NH– fragment is important (entry
enhancement in activity was observed for the complexes bearing
the ONNO salan compared to the salalen complexes. The exact
reason for this is unclear but it is postulated that the –NH- moiety
plays a crucial role in stabilizing the intermediates formed during
the polymerisation.
1
vs. 4-5) with the least sterically demanding system being the
most active. Complexes were trialed in the melt (Table 3
entries 3, 7, 8, 10) in these cases high conversion was
achieved in a significantly shorter timeframe. Moreover, the
polymerisation was controlled with narrow dispersities being
observed.
Experimental Section
The ligands were prepared via analogous procedures as previous
studies,[ full details are given in the supporting information
6]
a
In terms of selectivity only complexes based on 2-3aH
2
showed
any significant stereoselectivity with PLA possessing
moderate isotactic bias being observed. This is enhanced
comparing entry 1 (Table 2) to entry 2 (Table 1) and entry 5
a
representative example is given in the paper below. All data were collected
on a SuperNova EOS detector diffractometer using radiation CuKα (λ=
1
.54184 Å) and all data was recorded at 150(2) K. All structures were
2
solved by direct methods and refined on all F data using the SHELXL-
(
Table 2) to entry 3 (Table 1). The exact nature for this changes
2014 suite of programs. All hydrogen atoms were included in idealized
in activity and selectivity remain unclear but maybe related to
related to an –NH– interaction with the carbonyl of the in-
coming/coordinated lactide or the growing polymer chain,
stabilising the intermediates. However, it should be noted that
the salan ligands are more flexible than the salalen and this too
may affect the activity of the complexes.
positions and refined using the riding model, all refinement details are
given in the .cif file. All data was straightforward except for the following
points; Al(2)Me the structure was twinned and non-merohedral twinning of
180 about the 1 0 0 (direct space) direction was accounted for (39%).
Al(5)Me was a two component twin with the second component being
generated by a 180  rotation around 0.44 0 -0.9 reciprocal direction (57%);
Al(6)Me was twinned by virtue of a 180 ratio about the 0, 1, -1 reciprocal
direction (48%) the structure also contained disordered toluene which was
modelled with ADP restraints despite this an alert A was present in the
checkcif due to disorder in one of the toluene moieties; Al(2a)(OiPr)
contains half a molecule of hexane in the asymmetric unit. CCDC numbers
Table 3. Selected polymerisation data for the ROP of rac-LA with the
Al(III) salan complexes.
c
Mn,Theo.
d
Mn
e
Đ
(GPC)
f
Pr
Entry
Initiator
[LA]:[I]:[BnOH]
T
Time
/ h
Con./ %
e
/°C
(GPC)
a
1
2
3
4
5
6
7
8
9
Al(2a)Me
100:1:1
100:1:0
300:1:0
100:1:1
100:1:1
100:1:1
100:1:1
300:1:1
100:1:1
100:1:1
100:1:1
80
24
83
79
55
42
75
95
95
61
63
81
94
12000
11400
23900
6100
14600
24400
35000
8300
1.14
1.12
1.18
1.05
1.12
1.25
1.11
1.14
1.31
1.30
1.20
0.35
0.33
0.36
0.28
0.30
0.46
0.42
0.46
0.56
0.55
0.51
1909124-1909132 contain the necessary information.
a
Al(2a)OiPr
80
24
An example of a ligand and complexes preparation are given below.
i
b
Al(2a)OPr
130
80
2
a
Al(3a)Me
24
2H : Salicylaldehyde (1.38 ml, 12.98 mmol) was added to 3,5-di-tert-butyl-
2
2-hydroxybenzyl bromide (3.88 g, 12.98 mmol) in ethanol (50 ml) for 4
a
Al(3a)Me
80
48
10900
13800
13800
28650
9200
11600
16050
12850
35450
9700
hours. The solvent was removed in-vacuo and the product was stirred with
a
Al(4a)Me
80
16
silica (CH
2
Cl
2
), filtered and solvent removed in vacuo (orange solid, 3.29
1
g, 8.05 mmol, 62 %). H NMR (400 MHz, CDCl
3
, δ
H
, ppm); 13.03 (s, 1H),
b
Al(4a)Me
130
130
80
0.42
0.42
6
10.90 (br s, 1H), 8.30 (s, 1H), 7.29 (ddd, J = 8.3, 7.3, 1.7 Hz, 1H), 7.21 (dd,
b
J = 7.9, 1.9 Hz, 2H), 6.95 (dd, J = 8.3, 1.1 Hz, 1H), 6.88 – 6.82 (m, 2H),
Al(4a)Me
4
1
.02 (dq, J = 10.2, 4.9 Hz, 1H), 3.85 (s, 2H), 3.13 (dd, J = 10.3, 6.9 Hz,
H), 2.96 – 2.86 (m, 1H), 2.87 – 2.76 (m, 1H), 2.64 (dd, J = 10.3, 5.3 Hz,
a
Al(5a)Me
b
1H), 2.31 (dq, J = 13.3, 7.7 Hz, 1H), 1.97 (ddt, J = 12.9, 7.9, 4.7 Hz, 1H),
1
0
1
Al(5a)Me
130
80
0.33
24
11800
13500
16650
17250
1
1
.41 (s, 9H), 1.26 (s, 9H). 13C{ H} NMR (100 MHz, CDCl
3 C
, δ , ppm) 164.0
a
1
Al(7a)Me
(CN), 161.0 (Ar), 154.3 (Ar), 140.6 (Ar), 135.6 (Ar), 132.5 (Ar), 131.5 (Ar),
123.03 (Ar), 123.00 (Ar), 121.6 (Ar), 118.9 (Ar), 118.7 (Ar), 117.1 (Ar), 67.6
1
(CH
3.6 (CH
= 409.2855, found 409.2918.
2
), 60.4 (CH
2
), 59.8 (CH
2
), 52.8 (CH), 35.0 (C(CH
3
)
3
), 34.3 (C(CH
3
)
3
),
]
[
a] Toluene solvent; [b] under melt conditions; [c] Determined via H NMR
+
3
2
), 31.8 (C(CH
3
)
3
), 29.7 (C(CH
3
)
3
). Calculated m/z [C26
37 2
H N O
2
spectroscopy; [d] Theoretical molecular weight calculated from
conversion (rounded to the nearest 50): {[LA]:[I] × (Conversion × 144.13)
/
n
alkoxides} + M (end groups). [e] Determined from GPC (in THF)
r
referenced against polystyrene standards; [f] P is the probability of
2
aH
NaBH
2 hrs after which water (5 ml) was added. The solution was filtered and
the resulting solid was re-dissolved in CH Cl (30 ml) and washed water
to yield a white solid (0.75 g, 1.82 mmol,
, δ , ppm); 10.75 (br s, 2H), 7.22 (d, J =
2
: 2H
2
(1.00 g, 2.45 mmol) was dissolved in MeOH (100 ml) to which
heterotactic enchainment, determined via homonuclear decoupled 1
H
4
(4 eq, 0.37 g, 9.80 mmol) was added. The reaction was stirred for
NMR spectroscopy.
1
2
2
before being dried with MgSO
4
1
74 %). H NMR (400 MHz, CDCl
3
H
Conclusions
2.5 Hz, 1H), 7.21 – 7.12 (m, 1H), 6.96 (dd, J = 7.5, 1.7 Hz, 1H), 6.87 – 6.80
(m, 2H), 6.77 (td, J = 7.4, 1.2 Hz, 1H), 4.05 – 3.89 (m, 2H), 3.86 (d, J =
A series of salalen and salan ligands and their respective Al(III)
complexes have been prepared and characterised in solution and
in the solid-state. All complexes {apart from Al(3a)Me} show a
preference for trigonal pyramidal geometry. Comparison of the
13.4 Hz, 1H), 3.73 (d, J = 13.4 Hz, 1H), 3.42 (q, J = 6.2, 4.6 Hz, 1H), 2.91
–
=
8
2.81 (m, 1H), 2.74 (t, J = 8.5 Hz, 1H), 2.67 (t, J = 5.2 Hz, 1H), 2.57 (q, J
8.2 Hz, 1H), 2.26 (dtd, J = 13.4, 8.2, 5.2 Hz, 1H), 1.75 (dddd, J = 12.9,
.2, 6.7, 4.4 Hz, 1H), 1.42 (s, 9H), 1.28 (s, 9H). 13C{ H} NMR (100 MHz,
1
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201900417
FULL PAPER
A. M. Rodriguez, Organometallics 2015, 34, 3196-3208; f) P.
Hormnirun, E. L. Marshall, V. C. Gibson, R. I. Pugh, A. J. P. White,
Proc. Natl. Acad. Sci. U. S. A. 2006, 103, 15343-15348; g) J. W. Hu,
C. Kan, H. Y. Ma, Inorg. Chem. 2018, 57, 11240-11251; h) J. W. Hu,
C. Kan, H. B. Wang, H. Y. Ma, Macromolecules 2018, 51, 5304-
5312; i) M. D. Jones, L. Brady, P. McKeown, A. Buchard, P. M.
Schafer, L. H. Thomas, M. F. Mahon, T. J. Woodman, J. P. Lowe,
Chemical Science 2015, 6, 5034-5039; j) M. D. Jones, S. L.
Hancock, P. McKeown, P. M. Schafer, A. Buchard, L. H. Thomas,
M. F. Mahon, J. P. Lowe, Chem. Commun. 2014, 50, 15967-15970;
k) J. E. Kasperczyk, Macromolecules 1995, 28, 3937-3939; l) K. M.
Osten, P. Mehrkhodavandi, Acc. Chem. Res. 2017, 50, 2861-2869;
m) X. Pang, R. L. Duan, X. Li, C. Y. Hu, X. H. Wang, X. S. Chen,
Macromolecules 2018, 51, 906-913; n) A. Pietrangelo, S. C. Knight,
A. K. Gupta, L. J. Yao, M. A. Hillmyer, W. B. Tolman, J. Am. Chem.
Soc. 2010, 132, 11649-11657; o) A. Sauer, A. Kapelski, C. Fliedel,
S. Dagorne, M. Kol, J. Okuda, Dalton Trans. 2013, 42, 9007-9023;
p) S. Tabthong, T. Nanok, P. Sumrit, P. Kongsaeree, S. Prabpai, P.
Chuawong, P. Hormnirun, Macromolecules 2015, 48, 6846-6861; q)
H. B. Wang, Y. Yang, H. Y. Ma, Macromolecules 2014, 47, 7750-
CDCl
28.5 (Ar), 123.1 (Ar), 123.0 (Ar), 122.3 (Ar), 121.4 (Ar), 119.3 (Ar), 116.7
Ar), 59.7 (CH ), 58.9 (CH ), 56.3 (CH ), 52.4 (CH), 51.0 (CH ), 35.0
C(CH ), 34.3 (C(CH ), 31.8 (C(CH ), 29.8 (C(CH ).
3 C
, δ , ppm); 158.3 (Ar), 154.1 (Ar), 140.8 (Ar), 135.6 (Ar), 129.1 (Ar),
1
(
(
2
2
2
2
3
)
3
3
)
3
), 31.9 (CH
2
3
)
3
3 3
)
+
39 2 2
Calculated m/z [C26H N O ] = 411.3006, found 411.3041.
2
Al(2)Me: 2H (0.409 g, 1.0 mmol) was dissolved in toluene (10 ml) to which
AlMe (2 M hexane, 0.50 ml, 1.0 mmol) was added. This was stirred for 4
3
hours after which time the solvent was removed and the solid was
recrystallised from hexane/toluene mixture (pale yellow solid, 0.19 g, 0.42
1
mmol, 42%). H NMR (400 MHz, C
6
D
6 H
, δ , ppm); 7.55 (d, J = 2.5 Hz, 1H),
7
1
.19 (m, 2H), 6.89 (d, J = 2.5 Hz, 1H), 6.76 (d, J = 7.6 Hz, 1H), 6.52 (m,
H), 4.31 (d, J = 13.4 Hz, 1H), 3.00 (t, J = 9.2 Hz, 1H), 2.72 (d, J = 13.4
Hz, 1H), 2.63 (m, 1H), 2.46 (d, J = 9.4 Hz, 1H), 1.69 (s, 9H), 1.48 (s, 1H),
1
3
.44 (s, 9H), 1.34 (d, J = 9.6 Hz, 1H), 1.27 (m, 1H), 1.14 (m, 1H), -0.24 (s,
H). 13C{ H} NMR (100 MHz, C
1
6 6 C
D , δ , ppm); 168.3 (CN),166.3 (Ar), 156.3
7764; r) H. B. Wang, Y. Yang, H. Y. Ma, Inorg. Chem. 2016, 55,
(
1
(
(
7
Ar), 139.3 (Ar), 138.2 (Ar), 136.9 (Ar), 133.6 (Ar), 123.6 (Ar), 123.2 (Ar),
22.8 (Ar), 122.0 (Ar), 117.5 (Ar), 115.4 (Ar), 66.6 (CH ), 57.0 (CH ), 56.8
CH), 50.9 (CH ), 35.6 (C(CH ), 34.3 (C(CH ), 32.2 (C(CH ), 31.4
CH ), 29.9 (C(CH ). Elemental analysis (C27 ) Calcd in %: C,
2.29; H, 8.31; N, 6.24. Found: C, 72.17; H, 8.44; N, 6.19.
7356-7372; s) S. Yang, K. Nie, Y. Zhang, M. Q. Xue, Y. M. Yao, Q.
2
2
Shen, Inorg. Chem. 2014, 53, 105-115; t) Z. Y. Zhong, P. J. Dijkstra,
J. Feijen, Angew. Chem., Int. Ed. Engl. 2002, 41, 4510-4513; u) S.
Gesslbauer, H. Cheek, A. J. P. White, C. Romain, Dalton Trans.
2
3
)
3
3
)
3
3 3
)
2
3
)
3
2 2
H37AlN O
2018, 47, 10410-10414.
[
5]
a) O. J. Driscoll, C. K. C. Leung, M. F. Mahon, P. McKeown, M. D.
Jones, Eur. J. Inorg. Chem. 2018, 5129-5135; b) S. L. Hancock, M.
F. Mahon, M. D. Jones, Dalton Trans. 2013, 42, 9279-9285; c) S. M.
Kirk, G. Kociok-Kohn, M. D. Jones, Organometallics 2016, 35, 3837-
Al(2a)Me: 2aH
g, 0.49 mmol, 49%). H NMR (400 MHz, C
2
(0.411 g, 1.0 mmol), washed with hexane (white solid, 0.22
1
6 6 H
D , δ , ppm); 7.52 (s, 1H), 7.22
3
843; d) P. McKeown, J. Brown-Humes, M. G. Davidson, M. F.
Mahon, T. J. Woodman, M. D. Jones, Dalton Trans. 2017, 46, 5048-
057; e) E. L. Whitelaw, M. G. Davidson, M. D. Jones, Chem.
(m, 2H), 6.84 (s, 1H), 6.68 (m, 2H), 4.02 (d, J = 13.4 Hz, 1H), 3.89 (t, J =
1
3
0
2.5 Hz, 1H), 2.82 (d, J = 14.0 Hz, 2H), 2.64 (t, J = 11.2 Hz, 2H), 1.91 (m,
5
H), 1.59 (s, 9H), 1.42 (s, 9Hz), 1.24 (m, 1H), 1.10 (m, 1H) 1.01 (m, 1H), -
Commun. 2011, 47, 10004-10006; f) E. L. Whitelaw, M. D. Jones,
M. F. Mahon, Inorg. Chem. 2010, 49, 7176-7181; g) E. L. Whitelaw,
G. Loraine, M. F. Mahon, M. D. Jones, Dalton Trans. 2011, 40,
.34 (s, 3H). 1 C{ H} NMR (100 MHz, C
3
1
6 6 C
D , δ , ppm); 162.1 (Ar), 156.2
(
1
(
(
Ar), 139.6 (Ar), 138.4 (Ar), 130.2 (Ar), 123.8 (Ar), 123.1 (Ar), 122.6 (Ar),
21.7 (Ar), 121.3 (Ar), 120.8 (Ar), 115.5 (Ar), 58.1 (CH ), 57.0 (CH ), 53.8
CH ), 52.5 (CH), 50.4 (CH ), 35.5 (C(CH ), 34.3 (C(CH ), 32.2
C(CH ), 30.0 (C(CH ), 30.0 (CH ).
11469-11473; h) S. Dagorne, C. Fliedel, in Modern
2
2
Organoaluminum Reagents: Preparation, Structure, Reactivity and
Use, Vol. 41 (Eds.: S. Woodward, S. Dagorne), 2013, pp. 125-171.
P. McKeown, M. G. Davidson, G. Kociok-Kohn, M. D. Jones, Chem.
Commun. 2016, 52, 10431-10434.
A. Pilone, K. Press, I. Goldberg, M. Kol, M. Mazzeo, M. Lamberti, J.
Am. Chem. Soc. 2014, 136, 2940-2943.
A. Stopper, T. Rosen, V. Venditto, I. Goldberg, M. Kol, Chem. Eur.
J. 2017, 23, 11540-11548.
A. Pilone, N. De Maio, K. Press, V. Venditto, D. Pappalardo, M.
Mazzeo, C. Pellecchia, M. Kol, M. Lamberti, Dalton Trans. 2015, 44,
2
2
3
)
3
3 3
)
[
[
[
[
6]
7]
8]
9]
3
)
3
3
)
3
2
Acknowledgements
We acknowledge the University of Bath for funding and the
2
157-2165.
K. Nie, W. K. Gu, Y. M. Yao, Y. Zhang, Q. Shen, Organometallics
013, 32, 2608-2617.
[
10]
EPSRC for a postdoctoral fellowship for PM (EP/P0164051/1).
2
Keywords: lactide • catalyst • biopolymer • aluminium
[
1]
a) R. Auras, B. Harte, S. Selke, Macromol. Biosci. 2004, 4, 835-864;
b) L. T. Lim, R. Auras, M. Rubino, Prog. Polym. Sci. 2008, 33, 820-
852; c) K. M. Nampoothiri, N. R. Nair, R. P. John, Bioresour. Technol.
2010, 101, 8493-8501.
[
[
2]
3]
B. D. Ulery, L. S. Nair, C. T. Laurencin, J. Polym. Sci. Pt. B-Polym.
Phys. 2011, 49, 832-864.
a) I. Armentano, M. Dottori, E. Fortunati, S. Mattioli, J. M. Kenny,
Polym. Degrad. Stab. 2010, 95, 2126-2146; b) B. Gupta, N.
Revagade, J. Hilborn, Prog. Polym. Sci. 2007, 32, 455-482; c) O.
Martin, L. Averous, Polymer 2001, 42, 6209-6219; d) R. M. Rasal,
A. V. Janorkar, D. E. Hirt, Prog. Polym. Sci. 2010, 35, 338-356; e) J.
W. Rhim, H. M. Park, C. S. Ha, Prog. Polym. Sci. 2013, 38, 1629-
1652; f) C. M. Thomas, Chem. Soc. Rev. 2010, 39, 165-173; g) H.
Tsuji, Macromol. Biosci. 2005, 5, 569-597; h) E. T. H. Vink, K. R.
Rabago, D. A. Glassner, P. R. Gruber, Polym. Degrad. Stab. 2003,
80, 403-419.
[
4]
a) J. Beament, M. F. Mahon, A. Buchard, M. D. Jones,
Organometallics 2018, 37, 1719-1724; b) B. M. Chamberlain, M.
Cheng, D. R. Moore, T. M. Ovitt, E. B. Lobkovsky, G. W. Coates, J.
Am. Chem. Soc. 2001, 123, 3229-3238; c) H. Z. Du, X. Pang, H. Y.
Yu, X. L. Zhuang, X. S. Chen, D. M. Cui, X. H. Wang, X. B. Jing,
Macromolecules 2007, 40, 1904-1913; d) S. M. Guillaume, E.
Kirillov, Y. Sarazin, J. F. Carpentier, Chem. Eur. J. 2015, 21, 7988-
8003; e) M. Honrado, A. Otero, J. Fernandez-Baeza, L. F. Sanchez-
Barba, A. Garces, A. Lara-Sanchez, J. Martinez-Ferrer, S. Sobrino,
This article is protected by copyright. All rights reserved.

European Journal of Inorganic Chemistry
10.1002/ejic.201900417
FULL PAPER
Entry for the Table of Contents
FULL PAPER
A series of Al(III) complexes with
salan and salalen ligands have been
prepared. These have been trialled for
the controlled ROP of rac-LA. The
salan complexes have been shown to
be significantly more active for the
polymerisation.
Luke Britton, Daniel Ditz, James
Beament, Paul McKeown, Helena C.
Quilter, Kerry Riley, Mary F. Mahon and
Matthew D. Jones
Page No. – Page No.
Salalen
Salalens and Salans derived from 3-
Aminopyrrolidine: Aluminium
Complexation and Lactide
Polymerisation
Salan
This article is protected by copyright. All rights reserved.