Aluminium complexes containing salicylbenzothiazole ligands and their application in the ring-opening polymerisation of rac-lactide and ?-caprolactone

Article abstract of DOI:10.1039/c7dt02435e

Two series of aluminium complexes bearing salicylbenzothiazole ligands, namely four-coordinate aluminium complexes (1a-7a) and five-coordinate aluminium complexes (1b-7b), were synthesized and characterized by NMR spectroscopy, elemental analysis and X-ra

Full text of DOI:10.1039/c7dt02435e

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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a Zr-metal–organic
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DOI: 10.1039/C7DT02435E
Aluminium complexes containing salicylbenzothiazole
ligands and their application in the ring-opening
polymerisation of rac-lactide and -caprolactone
a
b
c
d
Chutikan Nakonkhet, Tanin Nanok, Worawat Wattanathana, Pitak Chuawong and
*
,a
Pimpa Hormnirun
a
Laboratory of Catalysts and Advanced Polymer Materials, Department of Chemistry and Center of
Excellence for Innovation in Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900,
Thailand
b
Department of Chemistry, Faculty of Science, Kasetsart University, Bangkok 10900, Thailand
c
Department of Materials Engineering, Faculty of Engineering, Kasetsart University, Bangkok 10900,
Thailand.
d
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science,
and Special Research Unit for Advanced Magnetic Resonance (AMR), Kasetsart University, Bangkok
10900, Thailand
*
E-mail: fscipph@ku.ac.th
†
1
Electronic supplementary information (ESI available): H NMR spectra of complexes 1a–7a
and 1b–7b, crystallographic data and structure refinement details for complexes 5a and 1b,
polymerisation data and DFT calculation data. See DOI:10.1039/x0xx00000x
ABSTRACT
Two series of aluminium complexes bearing salicylbenzothiazole ligands, four-coordinate
aluminium complexes (1a–7a) and five-coordinate aluminium complexes (1b–7b), were
synthesized and characterized by NMR spectroscopy, elemental analysis and X-ray
diffraction crystallography (for 5a and 1b). Their application for the ring-opening
polymerisation of rac-lactide and -caprolactone was studied with the aim of drawing
comparisons to closely related aluminium salicylbenzoxazole complexes previously
investigated. In the presence of benzyl alcohol, all complexes were active initiators and the
polymerisations were all well-controlled and living. Kinetic studies revealed first-order
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kinetics in the monomer. In comparison, the catalytic activity of aluminium
DOI: 10.1039/C7DT02435E
salicylbenzothiazole complexes was lower than that of aluminium salicylbenzoxazole
counterparts. Detailed DFT calculations were performed and indicated that the observed
lower catalytic activity of aluminium salicylbenzothiazole complexes agreed well with the
observed higher Gibbs free energy at the ring-opening transition state.
INTRODUCTION
Polylactide (PLA) is one of the most important synthetic polyesters due to its
1
-3
biodegradability, biocompatibility and biorenewability. In general, high molecular weight
PLA can be obtained by three different methods: (a) direct condensation polymerisation; (b)
azeotropic dehydrative condensation and (c) ring-opening polymerisation (ROP) through
4
lactide formation. Nonetheless, the ring-opening polymerisation catalyzed by discrete metal
complexes stabilized by various ligand architectures is the most effective method to control
polymer microstructure. In the case of the racemic mixture of
L
-LA and -LA or rac-LA, the
D
ROP can lead to different chain microstructures, i.e. atactic, heterotactic, stereoblock or
5
,6
stereocomplex PLA. Since the physical, mechanical and degradable properties of PLA are
correlated to the polymer chain microstructure, research into the stereocontrolled ROP of rac-
LA has thus received continuous interest over the past few decades as exemplified by large
7
-14
amount of literature in this field.
1
5-19
20,21
To access the excellent stereocontrol, various metal complexes of Al,
Ga,
have been investigated as
catalysts/initiators for the ROP of rac-LA. However, among the variety of metal complexes,
2
2-24
25,26
27-29
30-32
Zn,
Mg,
In,
and other transition metals
3
3
aluminium-based catalysts have shown significant stereocontrol in the ROP of rac-LA. In
3
4-37
particular, aluminium complexes bearing tetradentate dianionic ligands, such as salen,
3
8-43
43-47
salan,
and salalen,
displayed by far a remarkable stereocontrol of the polymer
2

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microstructure. The highly isotactic PLA or highly heterotactic PLA can be successfully
DOI: 10.1039/C7DT02435E
3
5,38,42
synthesized.
On the contrary, the exploitation of aluminium complexes containing
4
8-52
53,54
bidentate monoanionic ligands, such as phenoxy-imine,
phenoxy-amine,
5
5-58
59
60,61

-diketiminate,
amidinate, and pyrrolylaldiminate,
exhibited a lower degree of
stereocontrol due to the limited influence of steric hindrance by the lack of backbone linkers.
Furthermore, aluminium complexes with various bidentate monoanionic ligands have also
4
8-50,55,62-68
been reported as active intiators for the ROP of -caprolactone.
The effect of ligand
structure was found to have a strong influene on the catalytic activity. Recently, the activity
rating of different discrete metal based catalytic systems was reviewed and a new way of
classifying catalyst activity to enable easier comparisons between literature reports was
69
introduced by Redshaw et al. Since the nature and structure of ancillary ligands used play
an important role in the catalytic activity of a catalyst, the search for stereoselective metal
complexes of chelating bidentate ligands still remains a great challenge in this field.
Fig. 1. Structures of (a) salicylbenzoxazole (previous work) and (b) salicylbenzothiazole
ligands (this work).
Recently, we have developed aluminium complexes bearing salicylbenzoxazole
7
0
ligands (Fig.1a) as initiators for the ROP of rac-LA and -CL. The polymerisations were
well-controlled and a good isoselectivity control with the highest P value of 0.75 was
m
achieved. Recently, investigations of the effect of O/S exchange in ligand structure of various
catalyst systems on the Lewis acidity of the metal center with intriguing influence on the
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1-74
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catalytic activity were reported.
For example, aluminium complexes bearing N,S-Schiff
DOI: 10.1039/C7DT02435E
base ligands showed significantly higher polymerisation rate than aluminium complexes
73
containing N,O-Schiff base ligands in both -CL and
L
-LA polymerisations. Herein, we
report the synthesis of salicylbenzothiazole ligands which are sulfur analogs of the
7
5
salicylbenzoxazole ligands (Fig. 1b), the coordination of these ligands to the aluminium
metal, and the ROP studies of rac-LA and -CL. To the best of our knowledge, this is the
first report on the use of aluminium salicylbenzothiazole complexes for the application in the
ROP of cyclic esters. Furthermore, the mechanistic insights into the ROP processes
employing aluminium salicylbenzothiazole and aluminium salicylbenzoxazole complexes
were revealed by density functional theory (DFT) calculations.
RESULTS AND DISCUSSION
Synthesis and characterization of aluminium complexes 1a‒7a and 1b‒7b
1
7
The salicylbenzothiazole ligands (HL ‒HL ) employing in this work are outlined in Scheme
7
5
1
and synthesized according to a published procedure. The ligand structure was designed to
have a similar structure to the analogous salicylbenzoxazole ligands used for the synthesis of
7
0
aluminium complexes previously reported. The substituents at the phenoxy rings include
different combinations of electron withdrawing groups of different bulkiness (Cl and Br) and
t
1
3
the ones with different steric demands (H, Me, Bu, Ph, and CMePh ). Ligands HL ‒HL
2
were obtained in moderate to good yields (51‒77%) via condensation reactions of the
corresponding salicylaldehyde with o-aminothiophenol, followed by oxidative cyclization in
4
7
ethanol at 80 °C for 8 h. In the cases of ligands HL ‒HL with bulky phenoxy substituents,
the addition of ZrOCl 8H O as an oxidant was required for the intramolecular cyclization of
2
2
7
6
2
-((2-mercaptophenylimino)methyl)phenol derivatives. These ligands were obtained as
light yellow crystals after recrystallisation in ethanol at ‒20 °C (45‒80%).
4

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1
7
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DOI: 10.1039/C7DT02435E
The reaction of the ligand precursors HL ‒HL with one molar equivalent of
trimethylaluminium (TMA) in toluene at room temperature afforded the four-coordinate
aluminium complexes 1a‒7a in good yields (66‒94%) as shown in Scheme 2. The five-
coordinate aluminium complexes 1b‒7b were obtained by the reaction between TMA and the
corresponding ligands in the 1:2 molar ratio in toluene at 100 °C in moderate to good yield
1
13
(
45‒85%). H NMR and C NMR experiments indicated well-resolved resonances for all
proton and carbon environments. The purity of the compounds was verified by elemental
analysis.
1
7
Scheme 1. The synthetic route for proligands HL ‒HL .
Scheme 2. Synthetic routes for four-coordinate aluminium complexes (1a–7a) and
1
7
five-coordinate aluminium complexes (1b‒7b) from HL ‒HL .
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DOI: 10.1039/C7DT02435E
The H NMR spectra of complexes 1a‒7a and 1b‒7b in CDCl solution at 298 K
3
contain a single set of resonances, indicative of the existence of a highly symmetric species
on the NMR time scale (see Fig. S1–S14 in ESI). The symmetrical behavior detected by
solution NMR experiments was also observed for the analogous aluminium
7
0
salicylbenzoxazole complexes previously reported. The formation of aluminium complexes
was confirmed by the disappearance of the O–H proton signals of the free ligands and the
appearance of the new Al‒CH proton signals as a singlet in the high field region (–0.90 to
3
77-79
–
0.52 ppm).
The observation of one sharp singlet of two magnetically equivalent
aluminium methyl groups of the four-coordinate aluminium complexes 1a–7a could be
attributed to the symmetric environment around the aluminium center. In the cases of the
1
13
five-coordinate aluminium complexes 1b‒7b, the H and C NMR spectra reveal that both
salicylbenzothiazole ligands exhibit magnetically equivalent environment upon coordination
to the aluminium center.
Yellow crystals of complexes 5a (Fig. 2) and 1b (Fig. 3), suitable for single-crystal X-
ray diffraction, were grown from a mixed solution of CH Cl /hexane at 4 °C and from a
2
2
saturated toluene solution at room temperature, respectively. The crystal of complex 5a
contains two crystallographically independent molecules as shown in Fig. 2 and Fig. S15 in
ESI. Fig. 3 shows the X-ray structure of complex 1b, which contains two independent
complex molecules having fundamentally the same conformation and one toluene molecule
(see Fig. S15‒S18 and Tables S1-S6 in ESI for the crystallographic details).
Both independent complexes of 5a feature a four-coordinate aluminium center in a
distorted tetrahedral geometry (Fig. 2 and Fig. 15 in ESI) as seen in the bond angles for the
first molecule: O(1)−Al(1)−C(22) [112.78(5)°], N(1)−Al(1)−C(22) [109.08(5)°],
O(1)−Al(1)−C(23) [112.75(5)°] and O(1)−Al(1)−N(1) [93.48(4)°], and for the second
molecule:
O(2)−Al(2)−C(45)
[116.39(5)°],
N(2)−Al(2)−C(45)
[105.34(5)°],
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O(2)−Al(2)−C(46) [111.04(5)°] and O(2)−Al(2)−N(2) [93.28(4)°]. The extent of the
DOI: 10.1039/C7DT02435E
distortion of tetrahedral geometry can be numerically determined by the geometry indices: 
80
81
=
[360 – ( + )]/[360 – –2] and  = [(β − )/(360 − )] + [(180 − β)/(180 − )], where 
for both equations is 109.5°. The average  and  values are 0.91 and 0.90, indicative of the
distorted tetrahedral geometry of the aluminium centers. The aluminium center coordinated
with the phenolate oxygen and the benzothiazole nitrogen forms a six-membered N,O-
chelated ring structure. The significant decrease in bond angles involving two coordinated
atoms, the phenolate oxygen and the benzothiazole nitrogen, (O(1)−Al(1)−N(1) [93.48(4)°]
and O(2)−Al(2)−N(2) [93.28(4)°]) from the ideal tetrahedral angle enhances the rigidity of
the ligand backbone. Furthermore, all atoms regarding the six-membered chelated ring,
except aluminium, are nearly in co-planarity as the atoms lie less than 0.11 Å away from the
planes averaged from the O–C–C–C–N atoms. The dihedral angles between benzothiazole
ring and phenoxy rings of the two independent molecules are equal to 16.90° (N(1)–C(7)–
C(6)–C(1)) and 14.83° (N(2)–C(30)–C(29)–C(24)). The Al−C bond lengths (Al(1)−C(22)
[1.9615(12) Å], Al(1)−C(23) [1.9555(12) Å], Al(2)−C(45) [1.9649(12) Å], and Al(2)−C(46)
48,49,82-84
[
1.9604(12) Å] conform with the values reported in the literature.
The Al−N
distances, Al(1)−N(1) [1.9717(10) Å] and Al(2)−N(2) [1.9636(10) Å] display the
8
2-87
coordinative covalent bond character,
1.7712(8) Å] and Al(2)−O(2) [1.7743(8) Å] reveal the characteristic of -bonding.
Both the independent complexes of 1b were monomeric with a five-coordinate
while the Al−O bond lengths, Al(1)−O(1)
8
2-87
[
aluminium center in a geometry best described as distorted trigonal bipyramidal (Fig. 3 and
Fig. S17 in ESI). The amount of distortion can be calculated using the geometric criterion  =
8
8,89
( – )/60.
The averaged  value of 1b is 0.77 which is slightly higher than that of the
corresponding five-coordinate aluminium complexes bearing salicylbenzoxazole ligands
7
0
previously reported ( = 0.75). Regarding coordination geometries around the Al centers,
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the two nitrogen atoms from different molecules of the ligand are located at the axial
DOI: 10.1039/C7DT02435E
positions of the trigonal bipyramidal geometry. The diaxial angles, N(1)–Al(1)–N(2)
167.99(5)°] and N(3)–Al(2)–N(4) [168.13(5)°], are slightly distorted from linear resulting in
[
the distorted trigonal bipyramidal shape. The equatorial planes consist of two oxygen atoms
from different ligand molecules and one carbon of the methyl group. The equatorial angles at
the aluminium center fall within the narrow range from 118.47(6)° for O(2)–Al(1)–C(27) to
1
22.35(6)° for O(1)–Al(1)–C(27). The Al–N bond lengths of 2.1125(12) Å [Al(1)–N(1)],
.0964(13) Å [Al(1)–N(2)], 2.0933(13) Å [Al(2)–N(3)] and 2.0870 (13) Å [Al(2)–N(4)] are
2
8
2-87
characteristic of the coordinative covalent bond,
while the Al–O bond distances for Al(1)–
O(1) [1.7716(12) Å], Al(1)–O(2) [1.7757(12) Å], Al(2)–O(3) [1.7729(11) Å] and Al(2)–O(4)
82-87
[1.7723(11) Å] are more akin to -bond character.
The Al–C bond lengths in 1b
4
8,49,82-87
[1.9656(17) Å and 1.9743(16) Å] are typical.
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DOI: 10.1039/C7DT02435E
Fig. 2 ORTEP representation of 5a with the thermal ellipsoids drawn at 50% probability
level.
Fig. 3 ORTEP representation of 1b with the thermal ellipsoids drawn at 50% probability
level.
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DOI: 10.1039/C7DT02435E
Ring-opening polymerisation of rac-lactide
The new aluminium salicylbenzothiazole complexes 1a‒7a and 1b‒7b were tested as
initiators for the ROP of rac-LA in the presence of one equivalent of benzyl alcohol and the
results are collected in Table 1. The polymerisations were carried out in toluene at 70 °C and
the molar ratio of rac-LA to initiator was fixed at 100:1 ([LA] /[Al] = 100; [LA] = 0.83 M;
0
0
[
Al] = 8.33 mM; M (theory) = 14400). The polymerisation progress was monitored by taking
n
1
regular aliquots which were subsequently analyzed by H NMR spectroscopy to determine
the conversion. The molecular weights and molecular weight distributions (M /M ) were
w
n
determined by gel permeation chromatography (GPC) using the Mark-Houwink correction of
9
0-92
0
.58.
The experimental molecular weights of all the polymers produced with the
aluminium complexes employed in this study were closed to the theoretical values calculated
for a single PLA chain produced per metal center and the resulting polymers had unimodal
and narrow molecular weight distributions (M /M = 1.03–1.22), indicative of full activation
w
n
of the catalyst and controlled living polymerisations. The results also suggested only one
methyl group of four-coordinate aluminium complexes 1a‒7a was converted to the
benzyloxide initiating group. The polymerisations employing the four-coordinate aluminium
complexes 1a–4a and 6a proceeded to more than 90% conversion in 24 h whereas that using
complex 5a reached 86% conversion in 24 h. The lowest activity in this series was observed
for complex 7a (96% conversion in 96 h). Similar to the previously reported aluminium
70
salicylbenzoxazole complexes, the polymerisation employing five-coordinate aluminium
salicylbenzothiazole complexes was slower than that of four-coordinate aluminium
counterparts. For example, complexes 1b-3b polymerized rac-LA to ca. 90% conversion in
1
92 h and the least active initiator was complex 6b which provided 86% conversion in 480 h.
1
0

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Table 1. Polymerisation of rac-LA Using Complexes 1a‒7a and 1b‒7b in the Presence of
DOI: 10.1039/C7DT02435E
a
Benzyl Alcohol
entry complex
b
c
d
d
e
f
time
h)
conv
M (theory) M (GPC)
M /Mn
w
Pm
k
app
5
n
n
‒
1
‒1
‒
1
(
( %)
(g mol )
(10 s )
(
g mol )
1
2
3
4
5
6
7
8
9
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
24
192
24
96
14 000
14 200
1.19
1.17
1.12
1.22
1.12
1.15
1.15
1.08
1.09
1.09
1.19
1.08
1.19
1.03
0.52
0.61
0.52
0.57
0.45
4.0 ± 0.1
93
95
90
93
91
93
80
86
91
94
86
96
92
13 500
13 800
13 100
13 500
13 200
13 500
11 600
12 500
13 200
13 600
12 500
13 900
13 400
13 700
12 200
12 900
11 200
12 300
13 100
11 200
11 400
12 700
13 600
9 600
0.54 ± 0.1
3.5 ± 0.1
0.37 ± 0.1
3.2 ± 0.1
192
24
192
24
0.62 0.35 ± 0.01
0.44 2.5 ± 0.1
0.60 0.30 ± 0.01
192
24
0.62
0.61
0.52
0.57
0.66
0.60
1.9 ± 0.1
1.3 ± 0.1
2.9 ±0.1
1
0
1
2
3
4
48
1
1
1
1
24
480
96
0.13 ± 0.1
1.2 ± 0.1
13 200
12 500
192
0.22 ± 0.2
a
b
[
LA] /[Al] = 100, [Al]/[PhCH OH] = 1, [LA] = 0.83 M, [Al] = 8.33 mM, toluene, 70 ºC. As
0
2
0
1
determined via integration of the methine resonances ( H NMR) of LA and PLA (CDCl , 400 MHz).
3
c
d
Calculated by [([LA] /[Al])  144.13  conversion] + 108.14. Determined by gel permeation
0
chromatography (GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.58
e
for PLA. P is the probability of meso linkage between monomer units and was calculated from the
homonuclear decoupled H NMR spectrum of the obtained poly(rac-LA): [mmm] = P + (1 
m
1
2
m
2
2
P )P /2; [mmr] = [rmm] = (1  P )P /2; [rmr] = (1  P ) /2; [mrm] = [(1  P ) + (1  P )P ]/2.
m
m
m
m
m
m
m
m
f
[
LA] /[Al] = 50, [Al]/[PhCH OH] = 1, [LA] = 0.42 M, [Al] = 8.33 mM, toluene, 70 ºC.
0 2 0
The well-controlled living polymerisation and a single-site reaction for this catalyst
system were also highlighted by the observation of a linear correlation between M and the
n
percent conversion (Fig. 4 and Fig. S19–S31 in ESI). Furthermore, the relatively narrow
observed PDI values (<1.2) throughout the course of polymerisation suggested that the
3
4,60
polymerisation proceeded without a significant degree of transesterifications.
1
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DOI: 10.1039/C7DT02435E
8000
6000
4000
2000
0
3.0
2.5
2.0
1.5
1.0
0
20
40
60
80
100
Conversion (%)
Fig. 4 Plot of PLA M () (versus polystyrene standards) and PDI () as a function of
n
monomer conversion for a rac-LA polymerisation using 3a/PhCH OH ([LA] /[Al] = 50,
2
0
toluene, 70 ºC).
Four-coordinate aluminium complexes 1a–7a were also evaluated for rac-LA
polymerisation activity in the presence of excess benzyl alcohol to investigate their catalytic
9
3-101
performance for the immortal ROP of rac-LA.
The ratio of benzyl alcohol was increased
from 2 to 10 on a constant [LA] /[Al] ratio of 100. Excess benzyl alcohol molecules act as
0
chain-transfer agents (CTAs) which involve in a rapid and reversible exchange with the
growing alkoxide chain. As shown in Table 2, in all cases, the molecular weights of the
resulting polymers decreased with increasing amount of benzyl alcohol and the observed
molecular weights were in good agreement with the theoretical values that were proportional
to the initial [LA] /[PhCH OH] ratio. The PDI values were also relatively narrow even at a
0
2
high amount of benzyl alcohol added. These observations suggested that a fast, reversible
exchange between dormant hydroxyl-end-capped polymer chains/free benzyl alcohol and the
active growing alkoxide chain coordinated onto the aluminium center occurred significantly
1
2

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0,96
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faster than the chain propagation.
Thus, the results were consistent with controlled
DOI: 10.1039/C7DT02435E
alcohol-mediated chain transfer reactions during the ring-opening polymerisation.
Table 2. Polymerisation of rac-LA using Complexes 1a−7a in the Presence of Benzyl
a
Alcohol
b
c
d
d
entry complex [LA] :[Al]:
time
(h)
conv
(%)
M (theory)
(g mol )
M (GPC)
(g mol )
M /Mn
w
0
n
n
‒
1
‒1
[
PhCH OH]
2
1
2
3
4
5
6
7
8
9
1a
2a
3a
4a
5a
6a
7a
1a
2a
3a
4a
5a
6a
7a
1a
2a
3a
4a
5a
6a
7a
100:1:2
100:1:2
100:1:2
100:1:2
100:1:2
100:1:2
100:1:2
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:5
100:1:10
100:1:10
100:1:10
100:1:10
100:1:10
100:1:10
100:1:10
9
9
93
81
88
87
94
73
64
88
81
93
90
87
70
74
89
87
88
85
85
82
77
6 800
5 900
6 400
6400
6 500
5 400
5 900
5 800
7 200
4 600
4 000
2 000
2 000
2 800
2 200
2 400
1 600
2 000
1 100
1 000
1 100
1 100
1 000
1 000
1 000
1.20
1.12
1.14
1.10
1.09
1.09
1.05
1.19
1.09
1.11
1.12
1.10
1.12
1.07
1.17
1.10
1.17
1.14
1.14
1.15
1.11
9
9
24
9
6 900
5 400
4 700
2 600
2 400
2 800
2 700
2 600
2 100
2 200
1 400
1 400
1 400
1 300
24
6
6
1
0
1
2
3
4
5
6
7
8
9
0
1
6
1
1
1
1
1
1
1
1
1
2
2
6
9
6
24
6
6
6
6
6
1 300
1 300
1 200
b
6
24
a
[
LA] /[Al] = 100, [LA] = 0.83 M, [Al] = 8.33 mM, toluene, 70 ºC. As determined via integration of
0 0
the methine resonances ( H NMR) of LA and PLA (CDCl , 500 MHz). Calculated by ([([LA] /[Al])
144.13  conversion]/[PhCH OH]) + 108.14. Determined by gel permeation chromatography
GPC) calibrated with polystyrene standards in THF and corrected by a factor of 0.58 for PLA.
1
c
3
0
d

(
2
1
3

Dalton Transactions
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DOI: 10.1039/C7DT02435E
Kinetic studies of rac-LA polymerisation
To study the reaction kinetics of the ROP process of rac-LA initiated by complexes 1a–7a
and 1b–7b, all polymerisations were carried out in toluene at 70 °C in the presence of one
equivalent of benzyl alcohol ([LA] /[Al] = 50; [Al] = 8.33 mM; [LA] = 0.42 M). The
0
0
progress of polymerisations was monitored by taking regular aliquots of the polymerisation
1
solution, which were then analyzed by H NMR spectroscopy. Fig. 5 shows the
semilogarithmic plot of the rac-LA conversion (ln([LA] /[LA] )) versus time for the
0
t
polymerisations using 1a and 1b (see also Fig. S32–S37 in ESI for the plots of other
complexes). In all cases, the linear relationship was illustrated, indicating the apparent first-
order kinetics in the monomer. Furthermore, a straight line passing through the origin of the
plot designated that the active aluminium alkoxide species was formed instantaneously by an
in situ alcoholysis reaction utilizing benzyl alcohol. Hence, the polymerisation proceeded
x
according to the rate law −d[LA]/dt = k [LA], where k = k [Al] , in which k is the
app
app
p
p
propagation rate constant.
To determine the order in aluminium (x), kinetic experiments were conducted with
variable concentrations of the catalyst from 8.33 to 24.96 mM under identical conditions
(
[LA] ) = 0.42 M). Accordingly, Fig. 6 shows the semilogarithmic plots of rac-LA
0
conversions versus time using five different concentrations of complex 1a. The order in
aluminium was obtained from the gradient (x) of the plot of ln kapp versus ln [Al], as shown in
Fig. 7. The obtained x value of 0.98 (ca. 1.0) signified the first-order kinetics in aluminium
–
3
–1
–1
concentration. The k value of 4.27  10 s mol L was obtained from the gradient of the
p
plot between kapp versus [Al], as shown in Fig. 8. Therefore, the overall rate equation is
−
d[LA]/dt = k [LA][1a]. The stoichiometric involvement of two species, i.e. the active
p
aluminium benzyloxide and lactide, in the overall rate law indicated that the ROP of rac-LA
follows a monometallic coordination-insertion mechanism with the coordination of monomer
1
4

Page 15 of 53
Dalton Transactions
7
0
View Article Online
DOI: 10.1039/C7DT02435E
to the aluminium center, followed by the ring-opening process. The order of aluminium for
the ROP of rac-LA mediated by complex 1b was also determined using the same procedure
–4
(
see also Fig. S38–S40 in ESI). The order x of 0.99 (ca. 1.0) and the k value of 3.53  10
p
–
1
–1
s mol L were obtained, establishing the overall rate law in the form of −d[LA]/dt =
kp[LA][1b].
3.5
3.0
2.5
2.0
1.5
1.0
.
5
0.0
0
100000 200000 300000 400000 500000 600000
Time (s)
Fig. 5 Semilogarithmic plots of rac-lactide conversion versus time in toluene at 70 ºC with
complexes 1a () and 1b () ([LA] /[Al] = 50, [Al]/[PhCH OH] = 1, [LA] = 0.42 M, [Al] =
0
2
0
8
.33 mM).
3.5
3.0
2.5
2.0
1.5
1.0
.
5
0.0
0
10000
20000
30000
40000
50000
Time (s)
Fig. 6 Semilogarithmic plots of the rac-lactide conversion versus time in toluene at 70 C
with complex 1a/PhCH OH as an initiator ([LA] = 0.42 M: I, [Al] = 24.99 mM, [LA] /[Al]
2
0
0
=
=
17; II, [Al] = 20.82 mM, [LA] /[Al] = 20; III, [Al] = 16.67 mM, [LA] /[Al] = 25; IV, [Al]
0
0
12.50 mM, [LA] /[Al] = 34; V, [Al] = 8.33 mM, [LA] /[Al] = 50).
0
0
1
5

Dalton Transactions
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DOI: 10.1039/C7DT02435E
-8.5
-9.0
-9.5
-10.0
-10.5
-11.0
-5.0
-4.8
-4.6
-4.4
-4.2
-4.0
-3.8
-3.6
ln [Al]
Fig. 7 Plot of ln kapp versus ln [Al] for the polymerisation of rac-lactide with complex
1
a/PhCH OH as an initiator (toluene, 70 C, [LA] = 0.42 M).
2
0
.
.
.
.
.
.
.
.
00016
00014
00012
00010
00008
00006
00004
00002
0.00000
0.000
.005
.010
.015
.020
.025
.030
[
Al] M
Fig. 8 Plot of kapp versus [Al] for the polymerisation of rac-lactide with complex
a/PhCH OH as an initiator (toluene, 70 C, [LA] = 0.42 M).
1
2
0
According to the apparent rate constant values (k ) summarized in Table 1, each
app
four-coordinate aluminium complex was more active toward rac-LA polymerisation than its
five-coordinate congener. For example, the kapp value of four-coordinate aluminium complex
–
5
–1
1
a ((4.0 ± 0.1)  10 s ) was ca. 7 times higher than that of five-coordinate aluminium
–5 –1
complex 1b ((0.54 ± 0.1)  10 s ). The decrease in polymerisation activity was attributed to
the increase of steric shielding of the aluminium center by the presence of two ancillary
1
6

Page 17 of 53
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ligands. The data also demonstrated that the size of the alkyl phenoxy substituents had a
DOI: 10.1039/C7DT02435E
notable influence on the catalytic behavior of the complexes. For the four-coordinate
aluminium complexes 1a–7a, the catalytic activity decreased in the order o,p-H (1a) > o,p-
t
Me (2a) > o,p- Bu (5a) when the size of the alkyl phenoxy substituents at the ortho- and
t
para-positions increased from H to Me to Bu. According to our previous work, the steric
factor from the para-substituent of the closely related salicylbenzoxazole ligands has no
70
substantial impact on the catalytic performance of the complexes. Therefore, in the cases of
complexes 6a and 7a containing the bulky group at the ortho position of the phenoxy ring,
the catalytic activity reduced as the size increased from 6a (o-Ph, p-H) > 7a (o-CMePh , p-
2
Me). The observed lower activity was attributed to the steric protection at the aluminium
center by the bulky substituent, which hindered the incorporation of the monomer into the
1
02,103
growing polymer chain.
In comparison with complex 2a (o,p-Me), the introduction of
the electron withdrawing Cl atoms at the ortho- and para-positions of the phenoxy unit (3a)
resulted in a diminished catalytic activity, which can be ascribed to the different sensitivities
between the coordination and insertion steps to the Lewis acidity of the metal center. The
enhanced Lewis acidity induced by electron-withdrawing substituents may lead to preferred
coordination and activation, but it may also hinder the subsequent insertion step as a result of
1
04-107
a stronger binding of the growing alkoxide chain to the metal center.
Furthermore, the
lower activity of complexes 3a may also be attributed to the electron-donating conjugated
effect from the lone pair electron on the Cl atom, leading to the unfavorable coordination of
5
5
the lactide monomer. A decrease in catalytic activity when incorporating the halogen
5
5,108
substituent at the ancillary ligands was also observed in other catalytic systems.
Replacing the chlorine atoms with the less electronegative bromine atoms (4a) resulted in the
reduction of the catalytic activity. Therefore, the catalytic activity of the four-coordinate
1
7

Dalton Transactions
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aluminium complexes decreased in the order of 1a (o,p-H) > 2a (o,p-Me) > 3a (o,p-Cl) > 6a
DOI: 10.1039/C7DT02435E
t
(
o-Ph, p-H) > 4a (o,p-Br) > 5a (o,p- Bu) > 7a (o-CMePh , p-Me).
2
For the five-coordinate aluminium complexes 1b-7b, the steric factor of the alkyl
t
phenoxy substituent affected the catalytic activity in the order of 5b (o,p- Bu) > 1b (o,p-H) >
2
b (o,p-Me) > 7b (o-CMePh , p-Me) > 6b (o-Ph, p-H). In contrast to the four-coordinate
2
t
aluminium complexes, the five-coordinate aluminium complex 5b bearing Bu phenoxy
–5
–1
substituents exhibited the highest kapp value of (1.3 ± 0.1)  10 s whereas complex 6b
featuring the ortho-phenyl substituents at the phenoxy ring displayed the lowest kapp value of
–
5 –1
(
0.13 ± 0.01)  10 s . In addition, the halogen substituents 3b (o,p-Cl) and 4b (o,p-Br) did
not show significant influence on the catalytic activity, i.e. the kapp values of 3b and 4b were
–5 –1
comparable and also similar to those of 2b (o,p-Me) (kapp = (0.37 ± 0.1)  10 s for 2b; k
app
–
5
–1
–5 –1
=
(0.35 ± 0.01)  10 s for 3b; k = (0.30 ± 0.01)  10 s for 4b). In the series of five-
app
t
coordinate complexes, the rates of polymerisation decreased in the order 5b (o,p- Bu) > 1b
(
o,p-H) > 2b (o,p-Me)  3b (o,p-Cl)  4b (o,p-Br) > 7b (o-CMePh , p-Me) > 6b (o-Ph, p-H).
2
The kinetic data revealed that the steric and electronic influences were more
pronounced in the series of four-coordinate aluminium complexes than their homologous
five-coordinate aluminium complexes. Moreover, it was found that the aluminium
salicylbenzothiazole complexes were less active than their aluminium salicylbenzoxazole
7
0
analogs previously investigated.
Stereoselectivity of rac-LA polymerisation
The microstructure of the PLAs produced by complexes 1a–7a and 1b–7b was
determined by analyzing the signal intensities of the methine region of the homonuclear
1
109-114
1
decoupled H NMR spectra.
All homonuclear decoupled H NMR spectra are shown in
Fig. S41–S54 in ESI. The microstructure of the obtained PLA samples was changed slightly
1
8

Page 19 of 53
Dalton Transactions
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by the variation of the phenoxy substituents and the number of chelating ligands coordinated
DOI: 10.1039/C7DT02435E
to the aluminium center. In the series of four-coordinate aluminium complexes 1a–7a,
complexes 1a, 2a and 6a produced atactic PLAs with an identical P value of 0.52 whereas
m
slight heterotactic-biased PLAs were prepared by complexes 3a (P = 0.55) and 4a (P = 0.56)
r
r
containing chloro and bromo substituents at the phenoxy ring, respectively. In addition,
isotactic-enriched PLAs were obtained when increasing the bulkiness of the ortho alkyl
substituents installed on the phenoxy rings. For example, complex 5a containing tert-butyl
phenoxy substituents furnished isotactic PLA with the P value of 0.62. The most notable
m
isotacticity was observed when exchanging the ortho tert-butyl substituent with CMePh2
group, leading to an improvement of the P value to 0.66 for complex 7a.
m
In the series of five-coordinate aluminium complexes, the introduction of the bulky
phenoxy substituents did not bring further improvement of isoselectivity control. The Pm
values of PLAs produced by complexes 1b, 2b, and 5b–7b were in the range of 0.57–0.61. In
marked contrast to four-coordinate aluminium complexes containing halogen substituents (3a
and 4a), five-coordinate aluminium complexes 3b and 4b gave rise to isotactic-biased PLAs
(
P = 0.62 and 0.60, respectively). It is noted that the P values of PLAs prepared by
m
m
complexes 5a and 5b bearing tert-butyl phenoxy substituents were comparable (P = 0.62 for
m
5
a and P = 0.61 for 5b), suggesting that the increased steric congestion at the aluminium
m
center did not lead to an increase in the catalyst’s ability to select the desired stereoisomer of
the lactide. These observations were also exhibited in the related aluminium
salicylbenzoxazole system in which the four-coordinate and five-coordinate aluminium
complexes containing tert-butyl phenoxy substituents produced PLAs with the same level of
7
0
isoselectivity control. Furthermore, in the cases of complexes 7a and 7b bearing the bulky
CMePh at the ortho position of phenoxy unit, the degree of isoselectivity was diminished
2
from P = 0.66 for 7a to P = 0.60 for 7b.
m
m
1
9

Dalton Transactions
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In comparison, the level of stereocontrol established in this aluminium
DOI: 10.1039/C7DT02435E
salicylbenzothiazole system was found to be lower than that ascertained in the previously
reported aluminium salicylbenzoxazole system. For the aluminium salicylbenzoxazole
system, the isotactic-enriched PLA with the highest P value of 0.75 was prepared. However,
m
in this system, the highest P value of 0.66 was achieved for PLA produced by complex 7a.
m
To improve the degree of isoselectivity, the effect of polymerisation temperature was
investigated by changing the polymerisation temperature from 70 °C to 50 °C. Employing
complexes 5a and 7a for the rac-LA polymerisation ([LA] /[Al]/[PhCH OH] = 100/1/1, [Al]
0
2
=
8.33 mM, toluene) gave rise to isotactic PLAs, yielding a higher P value of 0.66 and 0.70,
m
respectively (5a: 96 h, 91%, M = 13 900, PDI = 1.16; 7a: 192 h, 87%, M = 11 200, PDI =
n
n
1
.07) (see Fig. S55 and S56 in ESI). Therefore, the degree of isoselectivity was increased
with the decrease of polymerisation temperature.
1
End-group analysis by H NMR spectroscopy and MALDI-TOF spectrometry
To gain more information on the mechanism entailed in the polymerisation of rac-LA, an end
1
group analysis was carried out by H NMR spectroscopy in CDCl at 298 K. The low
3
molecular weight polylactide was prepared by conducting a polymerisation experiment at a
1
low [LA] /[Al] ratio of 40. As shown in Fig. 9, H NMR analysis of this PLA sample
0
disclosed a hydroxyl end group (H = –CH(CH )OH) and a benzyl ester end group (H =
c
3
e
–
COOCH C H ) with the integration ratio of H :H = 1:5. These results indicated that the
2 6 5 c e
terminal alkoxide –OCH Ph moiety is the only initiating group involved in the
2
polymerisation process. Therefore, a coordination-insertion mechanism, involving selective
cleavage of the acyl-oxygen bond of the lactide monomer and the insertion into the alkoxide-
115,116
aluminium bond, should be operative in this system.
2
0

Page 21 of 53
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DOI: 10.1039/C7DT02435E
a
b + d
*
e
c
1
Fig. 9 H NMR spectrum of the low molecular weight PLA initiated by 1a/ PhCH OH in
2
CDCl at 298 K (* = solvent residue peak) .
3
The formation of linear polylactide capped with a benzyloxy group at one end and a
hydroxyl group at the other end was also investigated by MALDI-TOF mass pectrometry.
Fig. 10 shows the mass spectrum of polylactide prepared by 7b/ PhCH OH (Table 1, entry
2
1
4). The spectrum shows two sets of peak distributions and each set displays a (m/z)
separation of 144 Da, corresponding to the repeating unit of rac-LA. The set of high intensity
peaks is assigned to polylactide chains end-capped with the benzyloxy groups (–OCH Ph)
2
clustered with sodium ions to give adducts PhCH O–[C(O)CH(CH )OC(O)CH(CH )O] –H
2
3
3
m
+
Na (e.g. n = 80, m/z = 11663.297) whereas the set of lower intensity peaks correspond to
the polymers of the form PhCH OC(O)CH(CH )O–[C(O)CH(CH )OC(O)CH(CH )O] –H +
2
3
3
3
n
Na (e.g. n = 80, m/z = 11735.507). The results obtained from the MALDI-TOF MS are in
1
good agreement with H NMR spectroscopic analysis, supporting the ring-opening
polymerisation occurs via a coordination-insertion mechanism. In addition, there are no peak
2
1

Dalton Transactions
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envelopes with a (m/z) difference of 72 Da, which is a half unit of lactide
DOI: 10.1039/C7DT02435E
(
–OCH(CH )C(O)–), appeared in the mass spectrum. Hence, there is no evidence for
3
tranesterification side reactions during the polymerisation.
Fig. 10 MALDI-TOF mass spectrum of PLA initiated by 7b/PhCH OH in toluene at 70°C,
2
[
LA] /[Al] = 100, [Al]/[PhCH OH] = 1, [LA] = 0.83 M, [Al] = 8.33 mM (Table 1, entry 14).
0 2 0
Ring-opening polymerisation of -caprolactone
The ring-opening polymerisations of -caprolactone (-CL) using complexes 1a‒7a
and 1b‒7b were also explored to assess the control of the polymerisation under the same
conditions for those of rac-LA. The results are shown in Table 3. All complexes polymerized
-CL in controlled manner as evidenced by the experimental molecular weights closed to the
theoretical values and the narrow PDIs in the range of 1.24–1.55. As for polymerisations of
rac-LA, the four-coordinate aluminium complexes (1a‒7a) were more active than their five-
coordinate aluminium counterparts (1b‒7b). All polymerisations employing complexes
2
2

Page 23 of 53
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1
a‒7a proceeded to more than 99% within 15 min. In the series of five-coordinate aluminium
DOI: 10.1039/C7DT02435E
complexes, complexes 3b, 4b and 5b were more active toward the ROP of -CL and reached
completion within 15 min. It was also apparent that the polymerisation activity was enhanced
as the size of alkyl phenoxy substituents increased. For example, the catalytic activity
decreased in the order o-CMePh , p-Me (7b) > o-Ph, p-H (6b) > o,p-H (1b)  o,p-Me (2b).
2
Similar trend was observed in the ROP of -CL using aluminium complexes bearing
7
4
benzothiazole ligands. In addition, the catalytic activity of aluminium salicylbenzothiazole
complexes could be classified as moderate to good performance according to the activity
6
9
table for -CL polymerisation developled by Redshaw et al.
Table 3 Polymerisation of ε-caprolactone using 1a‒7a and 1b‒7b in the presence of
a
benzyl alcohol.
b
c
d
d
entry
complex
time
conv
(%)
M (theory)
(g mol )
M (GPC)
(g mol )
M /Mn
w
n
n
‒
1
‒1
(
min)
1
2
3
4
5
6
7
8
9
1a
1b
2a
2b
3a
3b
4a
4b
5a
5b
6a
6b
7a
7b
10
1080
15
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 500
11 700
11 800
11 600
11 300
8 900
1.35
1.37
1.30
1.45
1.55
1.38
1.27
1.41
1.46
1.44
1.30
1.36
1.44
1.24
1080
10
15
11 600
11 800
11 800
9 900
10
15
10
1
1
1
1
1
0
1
2
3
4
15
11 200
12 400
11 500
12 000
13 400
15
720
10
360
a
b
[
-CL] /[Al] = 100, [Al]/[BnOH] = 1, [CL] = 1.67 M, [Al] = 16.7 mM, toluene, 70 ºC. As
0
0
1
determined via integration of the methylene resonances ( H NMR) of -CL and PCL (CDCl ,
5
3
c
d
00 MHz). Calculated by [([-CL] /[Al])  114.13  conversion] + 108.14. Determined by
0
gel permeation chromatography (GPC) calibrated with polystyrene standards in THF and
corrected by a factor of 0.56 for PCL.
2
3

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DOI: 10.1039/C7DT02435E
DFT Calculations
Density functional theory (DFT) calculations have been increasingly employed in the
2
4,32,36,45,106,117-127
mechanistic study of ROP of various lactone monomers.
To acquire further
insight into the reaction mechanism and in particular to better understand the different
catalytic activities observed in aluminium salicylbenzothiazole and aluminium
salicylbenzoxazole systems, density functional theory (DFT) calculations were thus carried
out. Aluminium salicylbenzothiazole complexes 1a (four-coordinate complex) and 1b (five-
coordinate complex) and aluminium salicybenzoxazole complexes 1a′ (four-coordinate
complex) and 1b′ (five-coordinate complex) were selected as representatives. All calculations
1
28
were carried out with the GAUSSIAN 09 program package
at the M06-2X level of
1
29-131
132
theory
with the 6-311G(d,p) basis set. The validity of the methodology was verified
by comparing the experimental geometrical parameters obtained from X-ray analysis of
complexes 1b and 1b′ with their optimized geometrical data. A good agreement regarding
bond lengths and bond angles between the X-ray and the optimized structures was observed.
Fig. 11 shows the optimized structure of complexes 1a, 1a′, 1b, and 1b′ (see Tables S7 and
S8 in ESI for their selected structural parameters).
2
4

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DOI: 10.1039/C7DT02435E
Fig. 11 Optimized structures of (a) 1a, (b) 1a′, (c) 1b and (d) 1b′.
Since the active species for the initiation of the ROP is a metal alkoxide complex, we
started the reaction with the activated aluminium benzyloxide initiator. The Gibbs free energy
profiles determined for the ROP of L-LA mediated by 1a, 1a′, 1b, and 1b′ are presented in
Fig. 12. The overall geometries of the transition states and key species proposed in the
coordination-insertion mechanism are similar to those reported in literature (see Tables S9
1
17-127
and S10 in ESI for the structural parameters of key species).
The first step is the
coordination of lactide to the aluminium center via the carbonyl oxygen of the lactide through
the van der Waals complex (R). In the cases of four-coordinate aluminium initiators 1a and
1
a′, the coordination adducts were formed as indicated by the distance between the carbonyl
oxygen atoms of the lactide molecules and the aluminium center (AlOcarbonyl) of 2.224 Å and
.313 Å for 1a and 1a′, respectively, and the aluminium center is in a trigonal bipyramidal
2
2
5

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coordination environment. However, for the five-coordinate aluminium initiators 1b and 1b′,
DOI: 10.1039/C7DT02435E
the coordination adducts could not be located seemingly due to the steric congestion around
the aluminium center. The geometries of aluminium initiators 1b and 1b′and lactide
monomer in R compared with their initial geometries are very similar. The bond distances
between aluminium center and the carbonyl oxygen O(4) of the lactide are equal to 3.223 Å
for 1b and 3.143 Å for 1b′. Despite the elusive coordination adducts the benzyloxide and
lactide were brought into proximity to accomplish alkoxide insertion step. The next step is
the nucleophilic attack of the benzyloxide moiety to the carbonyl carbon of the coordinated
lactide to form the tetrahedral intermediate (INT1). At the transition state (TS1), the CO
2
bond was slightly elongated as a result of a rehybridization of thecarbonyl carbon from sp to
3
sp . The formation of INT2 by the rotation around the Oalkoxide–C bond allowed the Oacyl to
approach the aluminium center closely. Then, the polymerisation mechanism proceeded
through the second transition state (TS2) which is the ring-opening step. At TS2, the
dissociation of the LA Ocarbonyl–Al bond and the cleavage of the LA O –C bond occurred
acyl
concomitantly, along with the formation of new alkoxide bond. Further reorganization of the
growing polymer chain resulted in the formation of a five-membered metallocyclic product
(P). Overall, the free energy for the insertion of the first monomer unit is exergonic in all
cases (see Fig. S57–S60 in ESI for the schematic illustration of the optimized geometries
during the ring-opening step of
respectively).
L-LA mediated by the initiators 1a, 1a′, 1b, and 1b′,
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6

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DOI: 10.1039/C7DT02435E
Fig.12 Calculated free energy profiles for the ring-opening insertion of the first
monomer by 1a, 1a′, 1b, and 1b′.
L-LA
According to the calculated free energy profiles, the ring-opening step is rate-
determining in all cases since the Gibbs free energy barrier of the ring-opening transition
state (TS2) is higher than the insertion transition state (TS1). Therefore, we focus primarily
on the Gibbs free energies and the optimized structures at TS2 (Fig. 13). In comparison
between four-coordinate and five-coordinate aluminium complexes (1a and 1b; 1a′ and 1b′),
–1
the free energy barriers of four-coordinate complexes, 1a (GTS2 = +18.0 kcal mol ) and
–1
1
a′ (GTS2 = +15.1 kcal mol ), are lower than those of five-coordinate counterparts, 1b
–1
–1
(
GTS2= +24.3 kcal mol ) and 1b′ (G = +24.0 kcal mol ), indicating that the ring-
TS2
opening step of the four-coordinate aluminium complexes is kinetically more favorable. In
addition, when comparing the overall reaction energies for the formation of five-membered
2
7

Dalton Transactions
Page 28 of 53
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metallocyclic product (P), the four-coordinate aluminium complexes exhibit lower overall
DOI: 10.1039/C7DT02435E
reaction energies than those of the five-coordinate aluminium complexes (GProduct = –12.4
–
1
–1
–1
kcal mol for 1a; G
= –13.3 kcal mol for 1a′; G
= –6.9 kcal mol for 1b;
Product
Product
–1

GProduct = –6.6 kcal mol for 1b′), suggesting that the products P formed by 1a and 1a′ are
thermodynamically more stable than those of 1b and 1b′. The calculations were in line with
the experimentally observed activities, which indicated that the four-coordinate aluminium
complex displayed higher catalytic performance than that of the five-coordinate counterpart.
In the case of four-coordinate aluminium complexes 1a and 1a′, the ring-opening step
initiated by 1a proceeds through the higher energy transition state (TS2) compared with that
≠
–1
≠
–1
of 1a′ (G = +19.1 kcal mol for 1a; G = +16.2 kcal mol for 1a′) and the energy barrier
–
1
–1
of 1a (GTS2 = +18.0 kcal mol ) is also higher than that of 1a′ (G = +15.1 kcal mol ).
TS2
These findings are consistent with the experimental data that the initiators 1a has a lower kapp
–5
–1
–5 –1
70
value than 1a′ (kapp = (4.0 ± 0.1)  10 s for 1a; k = (6.0 ± 0.1)  10 s ) for 1a′). The
app
optimized ring-opening TS2 structures initiated by 1a and 1a′ are shown in Figure 12. The
presence of the large and less electronegative sulfur atom in the salicylbenzothiazole ligand
of 1a renders the ligand framework more distorted from planarity than for the incorporation
of an oxygen atom in the salicylbenzoxazole ligand of 1a′, as evidenced by a larger observed
dihedral angle, N(1)–C(2)–C(3)–C(4), of 20.2° for 1a and 15.6° for 1a′. The divergence from
planarity of the aromatic system in 1a reduces the degree of electron delocalization in 1a
compared with 1a′. Furthermore, the Al-N(1) bond of 1a (2.140 Å) is found to be longer than
that of 1a′ (2.105 Å). The deviation of the dihedral angle from planarity and the observed
longer Al-N(1) bond distance in 1a could be the reasons for the higher energy of TS2 for 1a.
Similar observations are also disclosed for the five-coordinate aluminium complexes
1
b and 1b′. A close examination of the TS2 geometries of 1b and 1b′ shown in Fig. 12
revealed that the Al–N(1) bond of 1b (2.065 Å) is also longer than that of 1b′ (2.011 Å) and
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the larger dihedral angle was observed in 1b (19.7° for 1b and 13.8° for 1b′). The ring-
DOI: 10.1039/C7DT02435E
–
1
–1
opening step of 1b′ (G = +22.6 kcal mol and G
= –6.6 kcal mol ) is both
TS2
Product
kinetically and thermodynamically more favorable in comparison with 1b (G = +24.3
TS2
–
1
–1
kcal mol and G
= –6.9 kcal mol ), which is in accord with the higher catalytic
Product
–5
–1
–5
activity of 1b′ compared to 1b (kapp = (0.54 ± 0.1)  10 s for 1b vs. k = (1.7 ± 0.1)  10
app
–
1
70
s for 1b′).
Fig. 13. Optimized ring-opening TS2 structures for the ring-opening polymerisation initiated
by (a) 1a, (b) 1a′, (c) 1b and (d) 1b′ (H atoms on the ligand are omitted for clarity: pink = Al;
red = O; blue = N; grey = C; yellow = S). Distances are given in Å.
2
9