Al(iii)-homopiperazine complexes and their exploitation for the production of polyesters

Article abstract of DOI:10.1039/c2nj40300e

In this paper we report the synthesis and characterisation of a series of Al(iii)-homopiperazine complexes. The ortho substituent has been varied from H, Me, tBu to investigate the effect this has on the solid-state structures and on the catalytic activity. Aluminium-methyl complexes involving ligands 1H ₂, 3H₂ and 5H₂ have been characterised in the solid-state and the aluminium centres are in pseudo trigonal bipyramidal geometries. The aluminium-methyl complexes were further reacted with benzyl alcohol to generate alkoxide complexes, which have been fully characterised by multinuclear NMR spectroscopy and elemental analysis. The alkoxide complexes were tested in the ring opening polymerisation of rac-lactide, δ-valerolactone and ε-caprolactone. Furthermore, triblock polyesters were also prepared with these initiators.

Full text of DOI:10.1039/c2nj40300e

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PAPER
Al(III)–homopiperazine complexes and their exploitation for the
production of polyestersw
Stuart L. Hancock, Matthew D. Jones,* Conrad J. Langridge and Mary F. Mahon
Received (in Montpellier, France) 20th April 2012, Accepted 19th June 2012
DOI: 10.1039/c2nj40300e
In this paper we report the synthesis and characterisation of a series of Al(III)–homopiperazine
complexes. The ortho substituent has been varied from H, Me, tBu to investigate the effect this
has on the solid-state structures and on the catalytic activity. Aluminium–methyl complexes
involving ligands 1H2, 3H2 and 5H2 have been characterised in the solid-state and the aluminium
centres are in pseudo trigonal bipyramidal geometries. The aluminium–methyl complexes were
further reacted with benzyl alcohol to generate alkoxide complexes, which have been fully
characterised by multinuclear NMR spectroscopy and elemental analysis. The alkoxide complexes
were tested in the ring opening polymerisation of rac-lactide, d-valerolactone and e-caprolactone.
Furthermore, triblock polyesters were also prepared with these initiators.
show activity for the polymerisation of e-caprolactone.10 Further-
Introduction
more, we have previously shown that amine bis(phenolate) ligands
based on homopiperazine ligands complexed to group 4 metals are
active initiators for the controlled ROP of rac-lactide.7b,11
In the vast majority of aluminium examples an alkoxide
initiator is generated in situ from the aluminium-alkyl and thus
the resultant polymerisation is performed in solution.3c,4c
However, this has a significant disadvantage in the fact that
the polymerisation cannot be performed under the industrially
preferred melt conditions. Therefore, in this study we have
chosen to prepare a series of aluminium–homopiperazine
alkoxide complexes for the application in ROP with a variety
of monomers for melt polymerisation studies.
Ring opening polymerisation (ROP) of lactide to produce
polylactide (PLA) has received a remarkable degree of attention
in recent years.1 This is due to the biocompatibility of the final
polymer and the fact that the starting lactide can be prepared from
sustainable sources.1b,d,2 This is truly making PLA a viable
alternative to crude oil based polymers for commodity applica-
tions. Furthermore, PLA and its copolymers are currently being
exploited for high value biomedical applications.3 Initiators for
ROP based on Al(III),4 Zn(II),5 lanthanides,6 group 4 metals7 and
group 2 metals5g,8 are prevalent in the literature. One of the most
studied monomers is rac-lactide (a 50 : 50 mixture of the D- and
L-enantiomers), this is due to the fact that different (stereoblock
isotactic, heterotactic and atactic) stereo-forms of PLA can be
prepared. The physical properties of the final polymer are directly
related to its microstructure.1b,2e However, the properties of the
polymer can also be varied by the copolymerisation of lactide with
other monomers – such as e-caprolactone or d-valerolactone.7c,9
One of the main driving forces of this approach is to produce
polymers which have desirable gas/drug permeability and
mechanical strength properties.1b
Results and discussion
Synthesis of complexes
The ligands were prepared by modified Mannich reactions and
complexes were prepared via standard literature procedures,
Scheme 1.7b,11,12 The choice of ortho substituent allows us to
probe the effect of steric influence on catalysis and solid-state
structure. All ligands were characterised via multi-nuclear
NMR spectroscopy and HR-MS.
A significant number of aluminium complexes that are
active for the ROP of rac-LA or other cyclic esters are based
on either salan or salen ligands.4c,10 One such class of salan
ligands are those utilising a piperazine or homopiperazine
backbone. For example, Fulton and Wang have shown that
bimetallic Al(III) complexes of piperazine derived phenolates
The Al-methyl complexes were prepared by addition of
1 equivalent of AlMe3 to 1 equivalent of ligand and it was
noted that products of higher purity were isolated when the
reactions were conducted at 80 1C. Complexes Al(1,3,5)Me
were characterised by single crystal diffraction studies, see Fig. 1
for Al(3)Me and Table 1 for selected bond distances and angles.
The aluminium centres are in a highly disordered trigonal
bipyramidal geometry. For complex Al(1)Me a significant degree
of disorder was observed in the homopiperazine ring moiety. For
complexes Al(3)Me and Al(5)Me the methyl group bound to the
Department of Chemistry, University of Bath, Claverton Down,
Bath BA2 7AY, UK. E-mail: mj205@bath.ac.uk;
Fax: +44 (0)1225 386231; Tel: +44 (0)1225 384908
w CCDC reference numbers 877942–877944. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/c2nj40300e
c
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Table 1 Bond lengths (A) and angles (1) for complexes Al(1,3,5)Me
Al(1)Mea
Al(3)Me
Al(5)Me
Al(1)–C(1)
Al(1)–O(1)
Al(1)–O(2)
Al(1)–N(1)
Al(1)–N(2)
O(1)–Al(1)–O(2)
N(1)–Al(1)–N(2)
O(1)–Al(1)–C(1)
N(1)–Al(1)–C(1)
1.959(5)
1.797(3)
1.767(2)
2.192(7)
2.240(6)
89.77(13)
71.9(2)
1.980(3)
1.966(7)
1.786(4)
1.796(4)
2.109(5)
2.101(5)
90.2(2)
74.48(17)
111.8(3)
102.0(2)
1.7873(19)
1.7975(18)
2.132(2)
2.108(2)
91.79(9)
73.74(8)
114.29(13)
100.09(12)
104.5(2)
96.7(3)
a
This complex exhibits disorder in the homopiperazine fragment. The
metric data given are for the –N(CH2)(CH2)N– ‘cis’ to the Me bound
to the aluminium centre.
Scheme 1 Synthesis of ligands and complexes used in this study.
between opposing ortho tBu moieties in Al(1)Me. This can be
visualised by analysis of the space filling models, Fig. 1. The
metric data for the complexes reported herein are in agreement with
Al–alkyl piperazine complexes prepared by Fulton and Wang.10
The NMR spectra of the complexes were non-trivial at
room temperature due to significant broadening and the
diastereotopic nature of the methylene groups. However, at
233 K, NMR spectra in agreement with monomeric structures
were observed in solution, with discrete doublets for the CH2
moieties. Variable temperature NMR spectra were recorded
(C6D5CD3) in an attempt to elucidate the fluctionality, see
Fig. 2 for Al(1)Me. From such experiements it is clear that
the aluminium species exhibit complex fluctional behaviour.
Noteworthily, for Al(1)Me DFT calculations indicated that
there was no discernible difference in energy between the two
isomers. Once the Al–Me complexes were prepared these were
treated with PhCH2OH to generate the alkoxide complexes
which for ligands 1H2–3H2 were isolated as white solids after
recrystallisation. Interestingly, to replace all the Al–Me groups
an excess (3 equivalents) of benzyl alcohol was required and
the reaction required heating to 80 1C for three hours. It was
noted, however, that the alkoxides prepared with ligands 4H2
and 5H2 were oily in consistency.
Fig. 1 Top: solid-state structure for Al(3)Me, the ellipsoids are
shown at the 50% probability level, the hydrogen-atoms and solvent
molecules have been removed for clarity. Middle: space fill model for
Al(3)Me. Bottom: space fill model for Al(1)Me, with the hydrogen
atoms for the space fill model being removed for clarity.
aluminium centre can be thought of as being ‘cis’ to the
–N(CH2)(CH2)N– fragment of the homopiperazine ring. How-
ever, for Al(1)Me two forms were observed in the solid-state; one
with the Me group being ‘cis’ to the –N(CH2)(CH2)N– fragment
and another with the Me ‘cis’ to the –N(CH2)(CH2)(CH2)N–
fragment. These were superimposed upon each other in a
50 : 50 ratio.
Fig. 2 Variable temperature NMR spectra Al(1)Me (only methine
region shown).
Polymerisation
Furthermore, in the case of Al(1)Me if the isomer with the
Me ‘cis’ to the –N(CH2)(CH2)N– is examined in more detail,
in comparison to Al(3,5)Me, there is a significant ‘twist’ in the
ligand. This is perhaps necessary to minimise steric clashes
Initially the Al–Me complexes were trialled for the poly-
merisation of rac-LA in a solution of toluene at 80 1C with
1 equivalent of benzyl alcohol as the co-initiator. However,
under these conditions no polymeric material was formed.
c
1892 New J. Chem., 2012, 36, 1891–1896
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Table 2 Polymerisation data
Entry Initiator Mon.
by the steric effects of the ortho substituent. All complexes
are active for the ROP of cyclic esters under solvent free
conditions.
Con. (%)a Mn
PDIb
b
1
2
3
4
5
6
7
8
Al(1)OBz rac-LA
Al(1)OBz VL
Al(1)OBz CL
87
88
97
54 400 1.31
24 800 1.77
73 100 1.54
42 400 1.66
58 200 1.40
10 100 1.12
11 400 1.15
29 500 1.63
56 400 1.40
33 000 1.54
71 900 1.63
47 800 1.76
42 100 1.42
39 600 1.50
54 100 1.42
16 800 1.08
16 000 1.25
25 800 1.48
Experimental
Al(1)OBz CL-VL-rac-LA 99, 95, 98
For the preparation and characterisation of metal complexes,
all reactions and manipulations were performed under an inert
atmosphere 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
Al(2)OBz rac-LA
Al(2)OBz VL
Al(2)OBz CL
88
31
30
Al(2)OBz CL-VL-rac-LA 88, 43, 64
9
Al(3)OBz rac-LA
Al(3)OBz VL
Al(3)OBz CL
81
94
94
10
11
12
13
14
15
16
17
18
Al(3)OBz CL-VL-rac-LA 99, 95, 22
Al(4)OBz rac-LA
Al(4)OBz VL
Al(4)OBz CL
Al(5)OBz rac-LA
Al(5)OBz VL
Al(5)OBz CL
87
90
94
56
81
83
1
obtained via SPS (solvent purification system). H and 13C{1H}
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. CHN microanalysis was performed
by Mr Stephen Boyer of London Metropolitan University.
The ligands were prepared according to standard literature
procedures and the purity confirmed via 1H/13C{1H} NMR
and HR-MS prior to use.11,12 DFT calculations utilised optimised
crystallographically determined structures with TightSCF conver-
sion. Initial calculations used the BP86 functional with RI approxi-
mation and the def2-svp basis set. The final calculations were
refined iteratively using B3LYP functional with the RIJCOSX
algorithm firstly using the def2-svp basis set and finally the
def2-TZVPP basis set. Empirical van der Waals corrections
were applied alongside a COSMO solvent effect at an infinite
dielectric constant.
a
b
Determined via 1H NMR spectroscopy. Determined via GPC at 35 1C
in THF at 1 ml minÀ1 using polystyrene as the standards. For the
homopolymerisation the monomer : initiator ratio was 300 : 1, caprolactone
= 30 min, d-valerolactone = 1 h, rac-lactide = 2 h. For the triblock the
monomer : initiator ratio was 100 : 1 for each monomer, i.e. 300 : 1 overall.
Therefore, the alkoxide complexes were prepared which could
be utilised more effectively under melt conditions (130 1C),
Table 2. All tests were run at 130 1C, regardless of the
monomer in question.
The polymerisations were very efficient for all complexes
with all monomers, with the exception of Al(2)OBz with lower
conversion for d-valerolactone (VL) and e-caprolactone (CL).
In all cases atactic PLA was produced. Noteworthy is that the
molecular weight (after similar conversions) is significantly
lower for Al(5)OBz compared to the other initiators. This is
perhaps related to the reduced steric demand of this ligand
facilitating multiple chain attachment to each metal centre.
Attempts then focused on the production of triblock polymers
with the complexes. In this case the monomers were added in a
sequential manner. It was noted that the addition of the
monomers was critical to the polymerisation success. For
example, if lactide was polymerised first followed by either
d-valerolactone or e-caprolactone then only PLA was isolated,
so a VL or CL will not insert into an Al–LA linkage. Therefore,
in the preparation of the triblock rac-LA was added last as LA
can insert into an Al–CL or Al–VL linkage. The molecular
weights of the triblocks were in agreement with the expected
values based on the GPC values for the homopolymers and the
conversion, indicating that the triblock had formed. Further-
more, the GPC traces were unimodal. The 1H NMR spectra were
complex due to overlapping of the resonances for the CL and VL
blocks. In the 13C{1H} NMR there was one resonance for each
carbonyl of the CL and VL blocks and the carbonyl region for a
rac-LA block was typical of an atactic PLA block.
Ligand preparation
All ligands were prepared using previously established protocols
except 5H2 which was prepared as follows:12 homopiperazine
anhydrous (2.20 g, 25 mmol) and formaldehyde (38% in H2O)
(5.30 ml, 75.4 mmol) were refluxed (2 h) in MeOH (40 ml). The
solution was then cooled to room temperature and 4-tert-butyl-
phenol (7.51 g, 50.0 mmol) in methanol (60 ml) was added slowly
then refluxed (16 h) and then cooled to room temperature. The
solid was filtered, washed in cold MeOH then dried under
vacuum to yield a white powder (2.31 g, 5.5 mmol, 22%).
1H NMR (CDCl3) d: 1.27 (18H, s, CH3), 1.94 (2H, quintet,
J = 6.0 Hz, CH2), 2.79 (4H, s, CH2), 2.84 (4H, t, J = 6.0 Hz,
CH2), 3.78 (4H, s, CH2), 6.76 (2H, d, J = 8.50 Hz, Ar-H), 6.95
(2H, d, J = 2.5 Hz, Ar-H), 7.19 (2H, dd, J = 8.5 Hz, J =
2.5 Hz, Ar-H), 10.47 (2H, br, O-H). 13C{1H} NMR (CDCl3) d:
26.8 (CH2), 31.6 (CH3), 34.0 (C), 53.6 (CH2), 54.7 (CH2), 62.4
(CH2), 115.5 (Ar-H), 121.0 (Ar), 125.4 (Ar-H), 125.6 (Ar-H),
141.9 (Ar), 155.4 (Ar). Calc. m/z [C27H40N2O2 + H]+
425.3168. Found 425.3233.
Complex preparation
Al(1)Me. A solution of 1H2 (0.96 g, 1.8 mmol) in toluene
(50 ml) was heated to 50 1C and 2 M AlMe3 (1 ml, 1.8 mmol)
was added slowly then stirred (30 min, 50 1C), after which the
solution was further heated and stirred (3 h, 80 1C). The crude
mixture was recrystallised from toluene : hexane (0.25 g,
0.43 mmol, 24%). 1H NMR (233 K) (C4D8O) d: À0.87
(3H, s, Al-Me), 1.26 (18H, s, CH3), 1.44 (9H, s, CH3), 1.50
(9H, s, CH3), 1.86 (1H, br, CH2), 2.33 (2H, br, CH2), 2.41
Conclusions
In conclusion a series of Al–Me and Al–OBz complexes
have been prepared and characterised. Monomeric complexes
were isolated, with the geometry of the ligand being influenced
c
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(1H, br, CH2), 2.62 (1H, br, CH2), 3.15 (2H, br, CH2), 3.23
(2H, m, CH2), 3.32 (2H, m, CH2), 3.41 (1H, br, CH2), 4.06
(1H, d, J = 11.5 Hz, CH2), 4.27 (1H, d, J = 14.0 Hz, CH2),
6.79 (1H, s, Ar-H), 6.86 (1H, s, Ar-H), 7.16 (1H, s, Ar-H), 7.20
(1H, s, Ar-H). 13C{1H} NMR (233 K) (C4D8O) d: 21.9 (CH2),
29.8 (CH3), 29.1 (CH3), 31.3 (CH3), 31.4 (CH3), 33.7 (C), 33.8
(C), 34.8 (C), 35.1 (C), 43.9 (CH2), 51.9 (CH2), 54.2 (CH2),
55.6 (CH2), 59.4 (CH2), 65.0 (CH2), 120.5 (Ar), 121.8 (Ar),
122.4 (Ar), 122.7 (Ar), 123.2 (Ar), 123.5 (Ar), 135.8 (Ar), 136.4
(Ar), 137.3 (Ar), 137.8 (Ar), 155.7 (Ar-O), 159.7 (Ar-O). Calc.
(%) for C36H57N2O2Al; C 74.96, H 9.96, N 4.86. Found (%);
C 74.96, H 9.90, N 4.81.
119.0 (Ar), 120.5 (Ar), 123.6 (Ar), 124.3 (Ar), 126.5 (Ar-H), 126.7
(Ar-H), 128.0 (Ar), 128.9 (Ar), 131.7 (Ar-H), 131.9 (Ar-H), 157.7
(Ar-O), 157.9 (Ar-O). Calc. (%) for C24H33N2O2Al; C 70.56,
H 8.14, N 6.86. Found (%); C 70.68, H 8.08, N 6.69.
Al(5)Me. The crude mixture was recrystallised from a toluene :
hexane mixture to yield a white powder (0.55 g, 0.58 mmol, 58%).
1
Please check again H NMR (233 K) (C4D8O) d: À0.53 (3H, s,
Al-Me), 1.23 (18H, s, CH3), 1.23 (2H, br, CH2), 2.30 (2H, m,
CH2), 2.94 (2H, br, CH2), 3.04 (2H, br, CH2), 3.13 (2H, br, CH2),
3.40 (2H, d, J = 13.5 Hz, CH2), 4.30 (2H, d, J = 13.0 Hz, CH2),
6.58 (2H, d, J = 8.5 Hz, Ar-H), 6.87 (2H, d, J = 2.0 Hz, Ar-H),
7.07 (2H, dd, J = 8.5 Hz, J = 2.0 Hz, Ar-H). 13C{1H} NMR
(C4D8O) d: 22.5 (CH2), 31.6 (CH3), 33.7 (C), 50.2 (CH2), 52.7
(CH2), 63.1 (CH2), 119.3 (Ar-H), 119.7 (Ar), 124.6 (Ar-H), 126.0
(Ar-H), 137.5 (Ar), 159.4 (Ar-O). Calc. (%) for C28H41N2O2Al;
C 72.38, H 8.89, N 6.03. Found (%); C 72.35, H 8.88, N 6.17.
Al(2)Me. The crude mixture was recrystallised from a
toluene : hexane mixture to yield a white powder (0.21 g,
0.4 mmol, 43%). 1H NMR (233 K) (C6D5CD3) d: À0.59
(3H, s, Al-Me), 0.54 (1H, d, J = 13.5 Hz, CH2), 0.92 (1H,
br, CH2), 1.08 (1H, br, CH2), 1.16 (2H, br, CH2), 1.30 (1H, d,
J = 7.5 Hz, CH2), 1.55 (1H, br, CH2), 1.92 (9H, s, CH3),
1.95 (9H, s, CH3), 2.27 (1H, d, J = 11.5 Hz, CH2), 2.40 (1H, d,
J = 14.5 Hz, CH2), 2.43 (1H, br, CH2), 2.43 (3H, s, 2.76), 2.48
(3H, s, 2.76), 2.57 (1H, br, CH2), 2.87 (1H, br, CH2), 3.63 (1H,
d, J = 11.5 Hz, CH2), 3.81 (1H, d, J = 14.0 Hz, CH2), 6.54
(1H, s, Ar-H), 6.60 (1H, s, Ar-H), 7.36 (1H, s, Ar-H), 7.41 (1H,
s, Ar-H). 13C{1H} NMR (233 K) (C4D8O) d: 21.3 (CH3), 21.4
(CH3), 22.9 (CH2), 30.2 (CH3), 30.9 (CH3), 35.6 (C), 35.9 (C),
44.8 (CH2), 52.7 (CH2), 55.3 (CH2), 56.8 (CH2), 60.2 (CH2),
65.6 (CH2), 122.1 (Ar), 123.3 (Ar), 123.4 (Ar), 124.8 (Ar),
127.4 (Ar), 127.5 (Ar), 128.1 (Ar), 128.5 (Ar), 138.2 (Ar), 139.4
(Ar), 156.8 (Ar-O), 161.0 (Ar-O). Calc. (%) for C30H45N2O2Al;
C 73.14, H 9.21, N 5.69. Found (%); C 73.02, H 9.25, N 5.73.
Al(1)OBz. A solution of 1H2 (0.53 g, 1.0 mmol) in toluene
(30 ml) was heated to 50 1C and 2 M AlMe3 (0.5 ml, 1.0 mmol)
was added slowly then stirred (30 min, 50 1C), after which the
solution was further heated and stirred (3 h, 80 1C). Excess
benzyl alcohol (0.31 ml, 3.0 mmol) was carefully added to the
hot solution and allowed to stir (3 h, 80 1C) the reaction was
cooled then the crude mixture was recrystallised from a toluene :
hexane mixture to yield a white powder (0.24 g, 0.36 mmol,
36%). 1H NMR (233 K) (C6D5CD3) d: 0.29 (1H, br, CH2), 0.76
(1H, br, CH2), 1.48 (4H, br, CH2), 1.51 (18H, s, CH3), 2.00
(18H, s, CH3), 2.56 (4H, m, CH2), 2.64 (2H, m, CH2), 4.32 (2H,
d, J = 13.5 Hz, CH2), 5.26 (2H, s, CH2), 6.77 (2H, s, Ar-H),
7.15 (1H, s, Ar-H), 7.21 (2H, t, J = 7.5 Hz, Ar-H), 7.36 (2H, d,
J = 7.5 Hz, Ar-H), 7.66 (2H, s, Ar-H). 13C{1H} NMR
(C6D5CD3) d: 22.1 (CH2), 30.1 (CH3), 32.1 (CH3), 34.2 (C),
36.1 (C), 50.7 (CH2), 52.3 (CH2), 63.7 (CH2), 65.9 (CH2), 119.8
(Ar), 122.8 (Ar-H), 124.0 (Ar-H), 126.1 (Ar-H), 126.7 (Ar-H),
128.1 (Ar-H), 137.9 (Ar), 139.0 (Ar), 147.0 (Ar), 157.6 (Ar-O).
Calc. (%) for C42H61N2O3Al; C 75.41, H 9.19, N 4.19. Found
(%); C 75.37, H 9.04, N 4.12.
Al(3)Me. The crude mixture was recrystallised from a
toluene : hexane mixture to yield a white powder (0.25 g,
0.5 mmol, 51%). 1H NMR (233 K) (C6D5CD3) d: À0.45
(3H, s, Al-Me), 0.18 (1H, br, CH2), 0.73 (1H, br, CH2), 1.12
(2H, d, J = 7.5 Hz, CH2), 1.39 (2H, d, J = 7.0 Hz, CH2), 1.51
(18H, s, CH3), 2.06 (1H, m, CH2), 2.45 (2H, d, J = 13.0 Hz,
CH2), 2.54 (2H, br, CH2), 2.64 (1H, br, CH2), 2.82 (6H,
s, CH3), 3.97 (2H, d, J = 13.0 Hz, CH2), 6.80 (2H, d, J =
2.0 Hz, Ar-H), 7.40 (2H, d, J = 2.0 Hz, Ar-H). 13C{1H} NMR
(C6D5CD3) d: 17.3 (CH3), 21.9 (C), 32.2 (CH3), 40.0 (CH2),
49.4 (CH2), 52.1 (CH2), 63.2 (CH2), 118.5 (Ar), 122.3 (Ar-H),
127.8 (Ar), 127.9 (Ar-H), 137.5 (Ar), 157.6 (Ar-O). Calc. (%)
for C30H45N2O2Al; C 73.14, H 9.21, N 5.69. Found (%); C
72.99, H 9.09, N 5.78.
Al(2)OBz. The reaction was recrystallised from a toluene :
hexane mixture to yield a white powder (0.20 g, 0.3 mmol,
34%). 1H NMR (233 K) (C6D5CD3) d: 0.40 (1H, d, J = 13.5 Hz,
CH2), 0.76 (1H, br, CH2), 1.20 (2H, d, J = 9.0 Hz, CH2),
1.40 (2H, br, CH2), 1.96 (18H, s, CH3), 2.43 (6H, s, CH3), 2.51
(4H, m, CH2), 2.66 (2H, m, CH2), 4.27 (2H, d, J = 13.0 Hz,
CH2), 5.25 (2H, s, CH2), 6.48 (2H, s, Ar-H), 7.14 (1H, s,
Ar-H), 7.21 (2H, t, J = 7.5 Hz, Ar-H), 7.40 (4H, m, Ar-H).
13C{1H} NMR (C6D5CD3) d: 21.0 (CH3), 22.1 (CH2), 30.9
(CH3), 35.7 (C), 50.6 (CH2), 52.2 (CH2), 63.2 (CH2), 65.9
(CH2), 120.3 (Ar), 124.3 (Ar), 126.1 (Ar-H), 126.7 (Ar-H),
126.9 (Ar-H), 127.8 (Ar-H), 128.1 (Ar-H), 139.4 (Ar), 146.7
(Ar), 157.6 (Ar-O). Calc. (%) for C36H49N2O3Al; C 73.94,
H 8.45, N 4.79. Found (%); C 73.86, H 8.51, N 4.75.
Al(4)Me. The crude mixture was recrystallised from a
toluene : hexane mixture to yield a white powder (0.45 g,
1.1 mmol, 59%). 1H NMR (298 K) (C6D5CD3) d: À0.59
(1.5H, s, Al-Me), À0.43 (1.5H, s, Al-Me), 0.50 (0.5H, d, J =
15.5 Hz, CH2), 0.89 (1H, m, CH2), 1.04 (0.5H, br, CH2), 1.22
(1H, m, CH2), 1.35 (2H, m, CH2), 1.51 (1H, m, CH2), 1.68
(1H, m, CH2), 2.28 (3H, s, CH3), 2.30 (3H, s, CH3), 2.43
(3H, s, CH3), 2.58 (2H, m, CH2), 2.62 (3H, s, CH3), 2.66 (2H,
m, CH2), 2.92 (1H, m, CH2), 3.68 (1H, d, J = 13.0 Hz, CH2),
4.04 (1H, d, J = 13.0 Hz, CH2), 6.45 (2H, d, J = 4.5 Hz,
Ar-H), 6.95 (2H, s, Ar-H). 13C{1H} NMR (C6D5CD3) d: 16.7
(CH3), 17.0 (CH3), 20.8 (CH3), 22.0 (CH2), 22.4 (CH2), 46.2 (CH2),
49.5 (CH2), 51.1 (CH2), 55.5 (CH2), 61.7 (CH2), 62.8 (CH2),
Al(3)OBz. The crude mixture was recrystallised from a
toluene : hexane mixture to yield a white powder (0.47 g,
0.8 mmol, 81%). 1H NMR (233 K) (C6D5CD3) d: 0.11
(1H, d, J = 12.0 Hz, CH2), 0.70 (1H, br, CH2), 1.10 (2H, d,
J = 8.0 Hz, CH2), 1.47 (2H, br, CH2), 1.50 (18H, s, CH3),
2.43 (4H, m, CH2), 2.71 (2H, d, J = 6.5 Hz, CH2), 2.85
c
1894 New J. Chem., 2012, 36, 1891–1896
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(6H, s, CH3), 4.31 (2H, d, J = 12.5 Hz, CH2), 5.29 (2H, s,
CH2), 6.79 (2H, s, Ar-H), 7.15 (1H, br, Ar-H), 7.24 (2H, t, J =
7.5 Hz, Ar-H), 7.40 (4H, m, Ar-H). 13C NMR (C6D5CD3) d:
17.6 (CH3), 22.3 (CH2), 32.5 (C), 32.6 (CH3), 34.5 (CH2), 50.4
(CH2), 52.9 (CH2), 63.2 (CH2), 66.7 (CH2), 119.5 (Ar), 122.7
(Ar-H), 126.5 (Ar-H), 127.2 (Ar-H), 128.1 (Ar), 128.4 (Ar-H),
128.5 (Ar-H), 138.4 (Ar), 147.3 (Ar), 157.7 (Ar-O). Calc. (%)
for C36H49N2O3Al; C 73.94, H 8.45, N 4.79. Found (%); C 73.91,
H 8.50, N 4.72.
46.8 (CH2), 50.1 (CH2), 52.4 (CH2), 55.2 (CH2), 62.6 (CH2),
66.3 (CH2), 120.9 (Ar-H), 121.4 (Ar), 125.3 (Ar-H), 126.3 (Ar-H),
127.6 (Ar-H), 127.7 (Ar-H), 128.4 (Ar-H), 139.3 (Ar), 147.3 (Ar),
159.6 (Ar-O). Calc. (%) for C34H45N2O3Al; C 73.35, H 8.15,
N 5.03. Found (%); C 73.45, H 8.14, N 4.89.
Polymerisation procedure
For solvent-free homopolymerisations the monomer : complex
ratio employed was 300 : 1 at a temperature of 130 1C, in all
cases either 1 g of rac-lactide, 1 ml of d-valerolactone, or 1 ml
of caprolactone were used. After the reaction time methanol
(20 ml) was added to quench the reaction and the resulting
solid was dissolved in dichloromethane. The solvents were
removed in vacuo and the resulting solid washed with methanol
(3 Â 50 ml) to remove any unreacted monomer. For solvent-free
triblock polymerisations the monomers were added in sequence
under an argon gas flow with the ratios 100 : 100 : 100 : 1 of
e-caprolactone : d-valerolactone : rac-lactide : complex respectively.
First e-caprolactone (0.77 ml) was stirred for 30 min, then
d-valerolactone (0.64 ml) was introduced and stirred for 1 h,
finally lactide (1 g) was added and reacted for 2 h. The
polymerisation was held at 130 1C for the duration then
quenched with methanol (20 ml) and the resulting solid was
dissolved in dichloromethane. The solvents were removed
in vacuo and the resulting solid washed with methanol (3 Â
50 ml) to remove any unreacted monomers. 1H NMR spectro-
scopy (CDCl3) and GPC (THF) were used to determine
tacticity and molecular weights (Mn and Mw) of the polymers
produced; Pr/m (the probability of heterotactic/isotactic
linkages) were determined by analysis of the methine region
of the homonuclear decoupled 1H NMR spectra. Gel Permea-
tion Chromatography (GPC) analyses were performed on a
Polymer Laboratories PL-GPC 50 integrated system using a
PLgel 5 mm MIXED-D 300 Â 7.5 mm column at 35 1C, THF
solvent (flow rate 1.0 ml minÀ1). The polydispersity index
(PDI) was determined from Mw/Mn where Mn is the number
Al(4)OBz. The crude mixture was recrystallised from a toluene :
hexane mixture to yield a tacky solid (0.22 g, 0.4 mmol, 44%).
1H NMR (233 K) (C6D5CD3) d: 0.22 (1H, d, J = 13.5 Hz, CH2),
0.75 (1H, br, CH2), 1.10 (2H, br, CH2), 1.45 (2H, br, CH2), 2.40
(6H, s, CH3), 2.45 (4H, m, CH2), 2.71 (2H, br, CH2), 2.81 (6H, s,
CH3), 4.30 (2H, d, J = 12.5 Hz, CH2), 5.25 (2H, s, CH2), 6.51
(2H, s, Ar-H), 7.15 (1H, br, Ar-H), 7.15 (2H, br, Ar-H), 7.23
(2H, m, Ar-H), 7.41 (2H, d, J = 7.5 Hz, Ar-H). 13C{1H} NMR
(C6D5CD3) d: 16.8 (CH3), 20.8 (CH3), 21.9 (C), 22.4 (C), 45.6
(CH2), 50.0 (CH2), 52.5 (CH2), 55.7 (CH2), 61.9 (CH2), 62.4
(CH2), 64.9 (CH2), 66.4 (CH2), 119.6 (Ar), 120.5 (Ar), 124.3
(Ar), 124.7 (Ar), 126.1 (Ar-H), 126.5 (Ar-H), 126.6 (Ar-H),
127.3 (Ar-H), 126.8 (Ar-H), 128.1 (Ar), 128.1 (Ar-H), 128.3
(Ar-H), 128.8 (Ar), 131.8 (Ar-H), 132.0 (Ar-H), 157.4 (Ar-O),
157.3 (Ar-O). Calc. (%) for C30H37N2O3Al; C 71.98, H 7.45,
N 5.60. Found (%); C 72.12, H 7.50, N 5.52.
Al(5)OBz. The crude mixture was recrystallised from a toluene :
hexane mixture to yield a tacky solid (0.15 g, 0.3 mmol, 27%).
1H NMR (C6D5CD3) d: 0.80 (1H, m, CH2), 1.14 (1H, m, CH2),
1.35 (18H, s, CH3), 1.50 (4H, m, CH2), 2.59 (2H, d, J = 7.5 Hz,
CH2), 3.22 (2H, d, J = 12.5 Hz, CH2), 3.36 (2H, m, CH2), 3.58
(2H, m, CH2), 5.16 (2H, s, CH2), 6.85 (2H, d, J = 2.5 Hz, Ar-H),
7.04 (1H, m, Ar-H), 7.12 (2H, m, Ar-H), 7.14 (1H, s, Ar-H),
7.14 (1H, br, Ar-H), 7.22 (1H, d, J = 2.5 Hz, Ar-H), 7.25 (1H, d,
J = 2.5 Hz, Ar-H), 7.29 (1H, m, Ar-H), 7.32 (1H, br, Ar-H).
13C{1H} NMR (C6D5CD3) d: 22.1 (CH2), 32.0 (CH3), 34.0 (C),
Table 3 Crystal structure of complexes Al(1)Me, Al(3)Me and Al(5)Me
Compound reference
Al(1)Me
36.75H58.75AlN2O2
587.59
Monoclinic
C2/c
35.6710(9)
10.4360(3)
24.9340(8)
90
Al(3)Me
Al(5)Me
Chemical formula
Formula mass
Crystal system
Space group
a/A
b/A
c/A
a/1
C
C33.50H49AlN2O2
538.73
C35H49AlN2O2
556.74
Monoclinic
C2/c
32.935(3)
7.0560(10)
31.375(4)
90
Orthorhombic
P212121
14.7810(2)
15.1010(2)
29.1190(4)
90
b/1
g/1
116.4370(10)
90.00
8311.3(4)
150(2)
8
0.075
70 559
9479
0.0411
0.1196
0.3258
0.1319
0.3350
1.090
90
90
114.289(5)
90
6645.8(14)
150(2)
8
0.092
24 788
5755
0.1342
0.1154
0.2713
0.1818
0.3088
1.077
Unit cell volume/A3
Temperature/K
6499.59(15)
150(2)
8
0.092
81 419
14 759
0.0780
0.0619
0.1606
0.0676
0.1651
1.091
0.06(14)
No. of formula units per unit cell, Z
Absorption coefficient, m/mmÀ1
No. of reflections measured
No. of independent reflections
Rint
Final R1 values (I 4 2s(I))
Final wR(F2) values (I 4 2s(I))
Final R1 values (all data)
Final wR(F2) values (all data)
Goodness of fit on F2
Flack parameter
—
—
c
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average molecular weight and Mw the weight average molecular
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Single crystal diffraction
All data were collected on a Nonius kappa CCD diffracto-
meter with MoKa radiation (l = 0.71073 A) see Table 3. T =
150(2) K throughout and all structures were solved by direct
methods and refined on F2 data using the SHELXL-97 suite
of programs.13 Hydrogen atoms, were included at idealised
positions and refined using the riding model. For Al(1)Me two
isomers were present in a 50 : 50 ratio. One tBu group was
disordered over two positions in a 50 : 50 ratio while another
tBu was disordered over three positions with equal occupancies.
The structure also had solvent accessible voids; however, the
electron density therein was very diffuse. Therefore, the
SQUEEZE programme was employed and this suggested that
the residual electron density was consistent with a diffuse hexane
moiety (recrystallisation solvent). For Al(3)Me one tBu group was
modelled over two sites in a 80 : 20 ratio a molecule of toluene was
also seen in the asymmetric unit. There is also a small (43 A3)
solvent accessible void but this was left in the final refinement. For
Al(5)Me one tBu group was modelled over two sites in a 60 : 40
ratio, while half of a molecule of toluene was observed straddling a
centre of symmetry. In addition a toluene moiety with 50%
occupancy was present for which the carbon atoms were modelled
and refined isotropically. For both Al(1)Me and Al(5)Me distance
restraints were employed in the disordered tBu moieties and
solvent molecules to aid convergence.
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This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2012