Salalen aluminium complexes and their exploitation for the ring opening polymerisation of rac-lactide

Article abstract of DOI:10.1039/c1dt11438g

In this paper we report the first use of Al(iii) salalen complexes for the ring opening polymerisation of rac-lactide. Polylactides with narrow polydispersities (PDIs range from 1.04-1.65) and moderate degrees of stereoselectivity were formed. Eight salal

Full text of DOI:10.1039/c1dt11438g

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PAPER
Salalen aluminium complexes and their exploitation for the ring opening
polymerisation of rac-lactide†
Emma L. Whitelaw, Gaynor Loraine, Mary F. Mahon and Matthew D. Jones*
Received 8th June 2011, Accepted 4th August 2011
DOI: 10.1039/c1dt11438g
In this paper we report the first use of Al(III) salalen complexes for the ring opening polymerisation of
rac-lactide. Polylactides with narrow polydispersities (PDIs range from 1.04–1.65) and moderate
degrees of stereoselectivity were formed. Eight salalen Al(III) complexes have been prepared and fully
characterised by solution-state NMR spectroscopy and, where appropriate, single crystal X-ray
diffraction. With ligand 3H2 either a monomeric or dimeric Al(III) species was formed, the dimeric
species was favoured at low concentrations. The complexes were tested for the ring opening
polymerisation of rac-lactide in toluene at 80 or 100 ◦C. Interestingly, various tacticities of polymer
were formed, which were dependent upon the nature of the group bound to the amine nitrogen centre.
are rare and notable examples are by Katsuki et al. who have
Introduction
shown that they are active for the oxidation of sulfides and the
hydrophosphonylation of aldehydes.11,12 However, due to their
facile nature of the preparation and degree of synthetic variability
the use of salalen ligands is proving more popular.13
In the last decade there has been an explosion of interest in the
field of single-site catalysts for the ring opening polymerisation
(ROP) of rac-lactide (rac-LA) to afford polylactide (PLA).1 The
polymer itself is not only sourced from sustainable materials but is
biodegradable.2 In addition the polymer properties can be tuned by
judicious choice of the catalyst, which can invoke stereoselectivity
in the resultant polymerisation. For example, stereoblock isotactic
PLA has a melting point of ca. 230 ◦C whereas poly-L-lactide has
a melting point of ca. 180 ◦C.3 There have been many impressive
advances in catalysts within the area and metals such as those
in Groups 1-4,4,5 Zn(II),6 and pertinent to this study Al(III) have
all been utilised.7–10 For example, Gibson has shown that minor
alterations to the ligand can have significant affects on the stereo-
chemistry of the resulting polymer.8 The use of Al(III) for the ROP
of rac-LA is well established.7–10 One of the earliest such reports
is that of Spassky and co-workers who used a complex based on
a chiral Schiff base ligand of R-(+)-1,1¢-binaphthyl-2,2¢-diamine.9
This was active for the production of isotactically enriched PLA.
The vast majority of Al(III) initiators for the production of PLA
are based on either salan or salen ligands,8–10 we have recently
demonstrated that Group 4 salalen complexes are active for
the polymerisation of rac-lactide under both solution and melt
conditions.5 The results herein represent the first example of the
use of salalen ligands with Al(III) for the polymerisation of rac-
LA. Catalytic reactions involving aluminium salalen complexes
Results and discussion
In this publication a series of Al(III) salalen complexes have
been prepared and tested for the ROP of rac-LA. The ligands
and complexes used in this study are shown in Scheme 1.† The
steric effects of the group on the amine, and the impact of
changing the substituents (both electronic and sterics) on the salen
phenoxide fragment have been investigated. The complexes were
Department of Chemistry, University of Bath, Claverton Down, Bath, UK,
BA2 7AY. E-mail: mj205@bath.ac.uk; Fax: +44 (0)1225 386231; Tel: +44
(0)1225 384908
† Electronic supplementary information (ESI) available: Full characterisa-
tion data, polymerisation procedure/analysis and the X-ray data. CCDC
reference numbers 829021–829025. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c1dt11438g
Scheme 1 Ligands and complexes prepared in this study.
Dalton Trans., 2011, 40, 11469–11473 | 11469
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simply prepared by the reaction of 1 equivalent of AlMe3 with 1
equivalent of the salalen ligand in toluene. Complexes based on
ligands 2H2, 3H2, 7H2 and 8H2 were characterised by single crystal
X-ray diffraction, Fig. 1.† The production of crystals of suitable
quality for diffraction studies was challenging as these complexes
were highly soluable in common organic solvents.
highly distorted trigonal pyramidal geometry with C(1)–Al(1)–
N(1) 113.68(13)◦, C(1)–Al(1)–O(1) 124.68(14)◦ and O(2)–Al(1)–
N(2) 165.45(12)◦, for Al(3)Me, Fig. 1, with the t values for these
complexes ranging from 0.56–0.80. This also suggests a bias
towards the trigonal bipyramidal geometry.14 The phenoxide trans
to the imine has the shortest Al-O bond distance and as expected
the Al-Nimine distance is considerably shorter than that of the Al-
Namine. To the best of our knowledge these are the first examples
of crystallographically characterised Al-alkyl salalen complexes.
The only previous Al(III)-salalen complex characterised in the
solid-state is the chiral Al-Cl system of Katsuki, in which an
analogue of Jacobsen’s ligand was employed.12 It should also be
noted that, to date, all Al(III) complexes of salan or salen ligands
in the Cambridge Crystallographic data base are symmetric in
their phenoxide subsituents.15 There is a necessity for novel ligand
systems, for Al(III), that can be easily prepared and derivatised.
The complexes have C1 symmetry and all adopt a b-cis
configuration in the solid-state. Upon complexation the tertiary
amine becomes chiral and due to the chelation of the ligand the
metal centre is also chiral; the S configuration at N(2) induces the
K form in the complex where the R configuration corresponds to
the D form. The complexes crystallise in centrosymmetric space
groups so both forms are present.† The 1H NMR spectra for these
complexes are in agreement with the solid-state structures being
maintained in solution, with resonances at ca. - 0.5 ppm for the
methyl group and at ca. 7.4 ppm for the imine proton, also the
CH2 groups are now diastereotopic indicating that the ligand is
“locked” in position once coordinated to Al(III).†
If the reaction was performed at a lower concentration in toluene
a second product was observed, Fig. 2. Again this is a C1 symmetric
complex, however the Al(III) centres are now tetrahedral in
geometry. In this case the salalen 3H2 has reacted with two
equivalents of AlMe3 forming Al2(3)(Me)4. Under these lower
dilution conditions both the 1 : 1 and 1 : 2 species were produced
as evidenced by the 1H NMR spectrum and it proved troublesome
to separate the different forms. However, the 1 : 1 complexes could
be isolated in high purities under more concentrated conditions.†
˚
Fig. 1 Top: Molecular structure of Al(3)Me. Selected bond lengths (A)
and angles (◦) are Al(1)–O(1) 1.757(2), Al(1)–O(2) 1.827(2), Al(1)–C(1)
1.960(3), Al(1)–N(1) 1.979(3), Al(1)–N(2) 2.266(3). O(2)–Al(1)–O(1)
90.48(11), O(2)–Al(1)–C(1) 99.65(14), O(1)–Al(1)–C(1) 124.68(14),
O(2)–Al(1)–N(1) 89.51(12), O(1)–Al(1)–N(1) 120.70(12), O(2)–Al(1)–N(2)
165.45(12). Bottom: Molecular structure of Al(2)Me. Selected bond
Fig.
2
Molecular structure of Al2(3)Me4. Selected bond lengths
◦
˚
(A) and angles ( ) are Al(1)–O(1) 1.7750(13), Al(1)–C(2) 1.952(2),
Al(1)–N(1) 1.9613(15), Al(1)–C(1) 1.9624(19), Al(2)–O(2) 1.7638(13),
Al(2)–C(3) 1.950(2), Al(2)–C(4) 1.957(2), Al(2)–N(2) 2.0577(15).
O(1)–Al(1)–C(2) 114.01(8), O(1)–Al(1)–N(1) 95.00(6), C(2)–Al(1)–N(1)
105.89(8), O(1)–Al(1)–C(1) 111.74(7), C(2)–Al(1)–C(1) 118.11(9),
N(1)–Al(1)–C(1) 109.22(8).
lengths (A) and angles (◦) are Al(1)–O(1) 1.7701(17), Al(1)–O(2)
˚
1.8193(18), Al(1)–C(1) 1.957(3), Al(1)–N(1) 1.958(2), Al(1)–N(2) 2.315(2).
O(2)–Al(1)–O(1) 93.74(8), O(2)–Al(1)–C(1) 97.56(10), O(1)–Al(1)–C(1)
126.88(11), O(2)–Al(1)–N(1) 90.12(8), O(1)–Al(1)–N(1) 116.83(9),
O(2)–Al(1)–N(2) 165.36(8).
The complexes were tested for the ROP of rac-LA in solution at
either 80 or 100 ◦C for 24 or 72 h with the addition of 1 equivalent
of benzyl alcohol (BnOH) to generate the alkoxide in situ, the
results of which are shown in Table 1.
The crystals were formed by slowly cooling a saturated solution
of the complexes in hexane to - 20 ◦C. For the crystallographically
characterised complexes the Al(III) centre was seen to be in a
11470 | Dalton Trans., 2011, 40, 11469–11473
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Table 1 Polymerisation data for the solution ROP of rac-lactide
T/◦C
Time/h
Initiator
Con.a
Mn
Mn
PDIb
Prc
b
calc
Entry
1
80
80
80
72
24
72
72
24
72
24
24
24
72
24
24
24
24
24
24
72
24
Al(1)Me
Al(2)Me
Al(2)Me
Al(2)Me
Al(2)Me
Al(3)Me
Al(3)Me
Al(4)Me
Al(4)Me
Al(5)Me
Al(5)Me
Al(6)Me
Al(6)Me
Al(7)Me
Al(7)Me
Al(8)Me
Al(8)Me
Al(8)Me
73
96
97
96
99
26
54
96
98
86
99
98
98
96
97
69
98
98
11900
11925
8100
11900
12050
7400
4800
10425
7420
6620
7700
9050
14650
8600
10600
13950
14100
13950
14350
3850
1.05
1.11
1.08
1.11
1.27
1.04
1.06
1.65
1.62
1.07
1.09
1.39
1.53
1.10
1.23
1.08
1.12
1.26
0.39
0.43
0.43
0.43
0.46
0.75
nd
0.42
0.41
0.74
0.66
0.45
0.46
0.63
0.59
0.48
0.40
0.50
2
3
4
100
100
80
100
80
100
80
100
80
100
80
100
80
80
100
5
6
7
7900
8
13950
14200
12500
14350
14200
14200
13950
14100
10050
14200
14200
9
10
11
12
13
14
15
16
17
18
7600
7950
12500
11000
Conditions [LA] : [Initiator] : [BnOH] = 100 : 1 : 1, temperature and time as given in the table.a as determined via 1H NMR, b Determined from GPC (in
THF) referenced to polystyrene. It is noted that a correction factor to account for the differences in the hydrodynamic volume of PLA and polystyrene
can be applied (typically ¥ by 0.58) this has not been applied in this case. c Calculated from the 1H homonuclear decoupled NMR (CDCl3) analysis. The
calculated molecular weights were determined by the following (144 ¥ conversion) + 108 {where 108 is the mass of the end groups (H/OCH2Ph)}.
All complexes were shown to be active for the polymerisation of
rac-LA with the addition of 1 equivalent of BnOH. Relatively
narrow polydispersity indices were observed, except with the
polymers formed with complex Al(4)Me. MALDI-ToF mass
spectrometry analysis of the polymers prepared with Al(3)Me
indicate the presence of the benzyl alcohol end group (entries 6
and 7), which would be expected from the coordination insertion
mechanism. Interestingly, the repeat unit of the polymer was seen
to be 144 mass units. This is indicative of only a small degree of
transesterification taking place. The initiators were also able to
offer a degree of stereocontrol in the polymerisations. Complexes
based on ligands 2H2, 4H2, 6H2 and 8H2 (entries 2-5, 8, 9, 12, 13
and 17) have a slight isotactic bias, whilst those based on 3H2,
5H2 and 7H2 (entries 6, 7, 10, 11, 14 and 15) afforded PLA with a
moderate heterotactic bias, especially the polymer produced from
Al(5)Me. Unlike previous work with Al(III) salan complexes where
changing the subsituents on the aromatic rings played a significant
role in the stereoselectivity,6b it would appear in this case the group
on the amine nitrogen R3 is more important. In this current work
{except for Al(1)Me} when R3 = Ph slightly isotactic PLA was
observed, but when this group was changed to Me polylactide
with an heterotactic bias was formed.
sphere of argon using standard Schlenk or glovebox techniques.
rac-LA (Aldrich) was recrystallised from toluene and sublimed
twice prior to use. All other chemicals were purchased from
Aldrich. All solvents used in the preparation of metal complexes
and polymerisation reactions were dry and obtained via SPS
1
(solvent purification system). 1H and 13C{ H} NMR spectra were
recorded on a Bruker 250, 300 or 400 MHz instrument and
referenced to residual solvent peaks. Coupling constants are given
in Hertz. Elemental analyses were performed by Mr A. K. Carver
at the Department of Chemistry, University of Bath. The ligands
were prepared according to standard literature procedures and
1
the purity confirmed via 1H/13C{ H} NMR and HR-MS prior to
use.5
Typical polymerisation procedure
For solution polymerisations a monomer:initiator ratio of 100 : 1
was used. In all cases toluene (10 ml) was added to a Schlenk
followed by the initiator and 1 eq of BnOH, the lactide (0.72
g) was added and the flask heated for the desired time at the
desired temperature. The reaction was quenched by the addition
1
of methanol (20 ml). H NMR spectroscopy (CDCl3) and GPC
(THF) were used to determine tacticity and molecular weights
(Mn and Mw) of the polymers produced; Pr (the probability of
heterotactic linkages) were determined by analysis of the methine
region of the homonuclear decoupled 1H NMR spectra.
Conclusions
In conclusion we have shown for the first time that Al-salalen
complexes are active for the ROP of rac-LA with narrow molecular
weight distributions. We have also shown that there is a correlation
between the group on the amine nitrogen and the tacticity of the
resultant polymer.
Complex preparation
A representative procedure for the preparation of Al(3)Me is
given, see supporting information for further details for all other
complexes:
3H2 (0.502 g, 1.22 mmol) was dissolved in toluene (20 cm3) to
which AlMe3 (0.6 ml of a 2.0 M solution in hexane, 1.22 mmol)
was added and stirred for 2 h. After which time the solvent was
removed in- vacuo and the product was recrystallised in hexane.
Experimental
For the preparation and characterisation of metal complexes, all
reactions and manipulations were performed under an inert atmo-
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Table 2 X-Ray crystallographic data
Compound reference
Al(2)(Me)
Al(3)(Me)
Al2(3)(Me)2
Al(7)(Me)
Al(8)(Me)
Chemical formula
Formula mass
C39H55AlN2O2
610.83
Orthorhombic
10.0400(5)
22.8750(14)
31.7450(18)
90
C27H39AlN2O2
450.58
Triclinic
C30H48Al2N2O2
522.66
Triclinic
C26H35AlCl2N2O2
505.44
Triclinic
9.708(4)
9.824(4)
C31H37AlCl2N2O2
567.51
Monoclinic
15.0450(3)
11.7530(3)
17.2050(5)
90
100.5930(10)
90
2990.41(13)
150(2)
P21/n
4
0.277
35301
5671
0.1508
0.0645
0.1334
Crystal system
˚
a/A
9.7286(7)
12.5296(10)
13.0282(10)
110.181(4)
109.510(4)
103.090(5)
1295.98(17)
150(2)
10.5560(9)
10.7180(11)
14.6300(13)
75.586(5)
72.438(5)
78.516(6)
1514.6(2)
150(2)
˚
b/A
˚
c/A
15.270(4)
82.591(15)
75.027(16)
72.308(17)
1338.3(9)
150(2)
a (◦)
b (◦)
g (◦)
90
90
3
˚
Unit cell volume/A
T/K
Space group
7290.7(7)
150(2)
Pcab
¯
¯
¯
P1
P1
P1
No. of formula units per unit cell, Z
Absorption coefficient, m/mm-1
No. of reflections measured
No. of independent reflections
Rint
Final R1 values (I > 2s(I))
Final wR(F2) values (I > 2s(I))
Final R1 values (all data)
Final wR(F2) values (all data)
Goodness of fit on F2
8
2
2
2
0.090
58588
6295
0.1130
0.0583
0.1168
0.1157
0.1393
1.041
0.103
10691
4417
0.0835
0.0615
0.1466
0.1144
0.1797
1.021
0.124
28305
6868
0.0569
0.0462
0.1065
0.0721
0.1221
1.048
0.300
15193
4732
0.0781
0.0502
0.1101
0.0973
0.1316
1.008
0.1224
0.1609
1.055
After 5 days at –20 ◦C a crop of crystals were obtained which were
filtered and dried. 1H NMR (C6D6) - 0.42 (3H, s, CH3), 1.46 (9H,
s, C(CH3)3), 1.65–1.76 (1H, m, CH2), 1.79 (9H, s, C(CH3)3), 1.80
(3H, s, CH3), 2.10–2.23 (1H, m, CH2), 2.43–2.52 (1H, m, CH2),
2.53 (3H, s, CH3), 2.59 (1H, d J = 12.0 Hz, CH2), 2.66–2.79 (1H,
m, CH2), 3.56 (1H, d J = 12.0 Hz, CH2), 6.56 (1H, t J = 7.5 Hz,
Ar-H), 6.76 (1H, dd J = 7.5 Hz, J = 1.0 Hz, Ar-H), 6.92 (1H,
d J = 2.5 Hz, Ar-H), 7.23 (1H, d J = 7.0 Hz, Ar-H), 7.35 (1H,
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˚
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The authors wish to thank the University of Bath for funding, the
EPSRC mass spectrometry service centre Swansea (MALDI-ToF
analysis) and Johnson Matthey for funding.
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