Dimeric aluminum methyl complex bridged by 2-oxy-2-methyl-1-(phenylamino) propane: Synthesis, structure, and use in ring opening polymerization of lactide

Article abstract of DOI:10.1016/j.inoche.2012.12.025

A dimeric aluminum complex bridged by alkoxide with pendant aniline group was synthesized and characterized. X-ray analysis revealed that the two four-coordinate aluminum centers possess a distorted tetrahedral geometry with C_2h symmetry. The compound was used as an effective catalyst for the controlled ring opening polymerization of L-lactide, as shown by the linearity of the number average of the molecular weight of polylactides versus conversion, as well as narrow M_w/M_n values.

Full text of DOI:10.1016/j.inoche.2012.12.025

Inorganic Chemistry Communications 29 (2013) 157–159
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Dimeric aluminum methyl complex bridged by 2-oxy-2-methyl-1-(phenylamino)
propane: Synthesis, structure, and use in ring opening polymerization of lactide
Sungwoo Yoon a, So Han Kim a, Jungseok Heo b, Youngjo Kim a,
⁎
a
Department of Chemistry, Chungbuk National University, 52 Naesudong-ro, Heungdeok-gu, Cheongju, Chungbuk 361–763, Republic of Korea
Department of Chemistry, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305–764, Republic of Korea
b
a r t i c l e i n f o
a b s t r a c t
Article history:
A dimeric aluminum complex bridged by alkoxide with pendant aniline group was synthesized and charac-
terized. X-ray analysis revealed that the two four-coordinate aluminum centers possess a distorted tetrahe-
dral geometry with C2h symmetry. The compound was used as an effective catalyst for the controlled ring
opening polymerization of L-lactide, as shown by the linearity of the number average of the molecular weight
of polylactides versus conversion, as well as narrow Mw/Mn values.
Received 5 November 2012
Accepted 21 December 2012
Available online 8 January 2013
Keywords:
Aluminum
© 2013 Elsevier B.V. All rights reserved.
2-Methyl-1-(phenylamino)propan-2-ol
Dimeric
Polylactide
X-ray crystal structure
Polylactide (PLA) polymers are biodegradable renewable mate-
rials and many types of metal alkoxides including tin, aluminum,
zinc, magnesium, iron, lanthanide, and titanium metals have been
reported as catalysts for the ring opening polymerization (ROP) of
lactide (LA) [1]. Despite the fact that some excellent initiators have
been reported for the polymerization of LA in the literature [1], the
search for new catalysts that generate well-defined PLA polymers
are still important.
Many examples of catalytic systems for the controlled LA poly-
merization based on aluminum alkoxides have been reported in the
literature [2–5]. Examples included monomeric aluminum complexes
containing [ON], [OS] or [NN]-type bidentate ligands [2], [ONO],
[NNO], [NON], or [NNN]-type tridentate ligands [3], and [OSSO],
[ONNO] or [NNNN]-type tetradentate ligands [4]. However, relatively
few examples of dimeric aluminum complexes as catalysts for PLA
have been reported [5]. The chelating modes of dimeric aluminum
complexes are [NN]-type bidentate [5a], [NO]-type bidentate [5b,c],
[OO]-type bidentate [5d,e], and [ONO]-type tridentate [5f,g,h]. Unlike
monomeric aluminum complexes [2–4], all dimeric aluminum com-
plexes have five or six coordinate aluminum atoms in the solid
state. To our best knowledge, dimeric aluminum complexes with
four coordinate metal centers have never been used as catalysts for
ROP of LA though some dimeric and four coordinate aluminum com-
plexes have been reported [6]. Herein, we report new the synthesis
and characterization of novel 4-coordinate and dimeric aluminum
complex bridged by 2-oxy-2-methyl-1-(phenylamino)propane and
its controlled ROP behavior for L-LA.
The ligand 2-methyl-1-(phenylamino)propan-2-ol (PhNHCH2-
CMe2OH, LH) was synthesized by the reaction of isobutylene oxide
with aniline in 81.8% yield. As outlined in Scheme 1, the dimeric alu-
minum complex 1 was prepared via simple exchange reaction by
adding AlMe3 dropwise to an equimolar solution of LH in toluene in
the yield of 94.2%. Analytically pure samples were obtained as color-
less crystalline solids after the removal of volatile materials. Com-
pound 1 was not stable in air and was soluble in polar organic
solvents and in toluene, but insoluble in hexanes. Compound 1 was
characterized by 1H and 13C{1H} NMR spectroscopy, elemental analy-
sis, and single crystal X-ay diffraction analysis. (See Supplementary
Information about the synthesis and characterization data for 1).
1H and 13C{1H} NMR spectra were in accord with the suggested
structure, and all chemical shifts of the protons and carbon atoms
were in the expected range. The disappearance of the O-H signal of
LH and the appearance of the sharp singlet of Al-CH3 resonance in
the high-field region of −0.49 ppm in C6D6 are consistent with the
structure proposed in Scheme 1. The NMR data for 1 was consistent
with a centrosymmetric dimeric structure compatible with C2h sym-
metry. Complex 1 seems to possess pseudo-C2 symmetry with cou-
ples of diastereotopic –OCMe2CH2– methylene protons by well
separated and defined AB spin system doublets in solution at room
temperature. The molecular structure of 1 confirms this NMR obser-
vation. Molecular structure of 1 was determined by single crystal
X-ray diffraction analysis. Single crystals suitable for X-ray structural
determination were obtained by cooling a solution of saturated tolu-
ene at −20 °C. The complex 1 crystallized in space groups Cc. The
crystal structure for 1 is shown in Fig. 1.
In the solid state, complex 1 has a certainly C2h symmetric and di-
meric Al2O2 four-membered ring with center of symmetry. The two
⁎
Corresponding author. Tel.: +82 43 2613395; fax: +82 43 2672279.
E-mail address: ykim@chungbuk.ac.kr (Y. Kim).
1387-7003/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.inoche.2012.12.025

158
S. Yoon et al. / Inorganic Chemistry Communications 29 (2013) 157–159
Scheme 1. Synthetic routes for compounds LH and 1.
bridging O atoms in L ligand link the two AlMe2 units. The central alu-
minum atoms are four-coordinated and adopt a distorted tetrahedral
geometry. Complex 1 has a C2 symmetric axis through the centroid of
the Al1-O1-Al2-O2 plane; however, there is a slight asymmetry in the
Al2O2 ring system wherein the Al1-O1, Al1-O2, Al2-O1, and Al2-O2
distances are 1.830(9) Å, 1.915(10) Å, 1.924(9) Å, and 1.846(9) Å,
respectively. Two aluminum centers Al1 and Al2 lie in the O1-C28-C29
plane and O2-C30-C31 plane, respectively, because the sum of
three O1-Al1-C28, O1-Al1-C29, and C28-Al1-C29 angles and that
of O2-Al2-C30, O2-Al2-C31, and C30-Al2-C31 are 357.6(5)° and
357.8(5)°, respectively. However, there is no plausible existence of
the direct Al-N bond because 2.533(13) Å for Al1∙∙∙N1 and 2.495(13) Å
for Al2∙∙∙N2 is much longer than normal Al-N bond distances (less
than 2.0 Å) reported in the literatures [2–5]. The observation of N-H
proton peak for 1 in 1H NMR spectrum supports this result. Also, no di-
rect Al-Al interactions are present as indicated by the Al1∙∙∙Al2 distance
[2.918(2) Å]. Al-O and Al-C bond distances are similar to those found in
related four-coordinated aluminum complexes and are within expected
parameters.
The new compound 1 was tested as initiators for ROP of L-LA
(Table 1). Solution polymerization in toluene at 70 °C and solvent-
free polymerization at 130 °C were conducted with 1 at a [LA]/[Al]
molar ratio of 200 in the presence of 9-hydroxyfluorene, which is a
pale yellow solid and can be easily handled [7]. Well-controlled solu-
tion polymerizations of L-LA in 8 mL of toluene at a standard concen-
tration of L-LA (1 M) and using 0.5 mM concentration of Al initiator
(i.e., 1/200 loading of initiator/lactide) were systematically investi-
gated at 70 °C (Table 1, entries 1–5).
the complex in the absence of 9-hydroxyfluorene gave very little con-
version. It is reasonable to assume that the dimethylaluminum com-
pound reacts with the 9-hydroxyfluorene, giving rise to methane
and a new Al-O bond checked by in situ 1H NMR reaction between
complex 1 and 9-hydroxyfluorene. The disappearance of the Al-Me
signal in complex 1 and O-H proton in 9-hydroxyfluorene and the
downfield shifts of all protons support the formation of new complex.
The 1H NMR spectrum of the obtained PLA samples in Table 1 showed
that the polymer chains were end-capped with 9-oxyfluorene group
and a hydroxyl group. The number-average molecular weight (Mn)
determined from 1H NMR of the PLA is close to Mn calculated from
the molar ratio of monomer to 9-hydroxyfluorene. The Mn(GPC)
was found to increase linearly with monomer conversion. In addition,
the molecular weight distributions are all unimodal, ranging from
Mw/Mn =1.06–1.26, indicative of a single-site behavior. As shown in
Fig. 2, the linear relationship between Mn and conversion as well as
the narrow polydispersity indices demonstrated that compound 1
could act as an effective catalyst for the living/controlled ROP of
L-LA (Table 1, entries 1–5). Only one peak of the methine region of
PLA in the homonuclear decoupled 1H NMR spectra suggests that
there were no epimerization side reactions occurring.
As shown in entry 6 of Table 1, the bulk polymerization at 130 °C
using 1/9-hydroxyfluorene goes to completion within 60 min. In spite
of the bulk polymerization, the polymerization showed high activity
with well-controlled character (Mw/Mn =1.15). The controlled be-
havior of this polymerization was further confirmed by two consecu-
tive resumption experiments with additional L-LA (Table 1, entry 7)
and ε-caprolactone (ε-CA) (Table 1, entry 8). In this experiment
(Table 1, entry 7), an additional 400 equivalents of LA monomer
Rapid polymerization was observed and monomer conversion up
to 100% could be reached within 6 h (Table 1, entry 5). However,
Table 1
ROP data of L-LA using 1.
g
Entry Conv. (%)e Mn (NMR)e Mn (calcd)f Mn (GPC)g Mn (corr.)h Mw/Mn
1a
2a
3a
4a
5a
6b
7c
8d
26
52
77
85
3,800
7,600
3,900
7,700
1,400
7,800
800
4,500
7,600
9,100
9,700
13,500
35,800
44,900
1.26
1.10
1.07
1.06
1.09
1.15
1.12
1.18
11,300
12,400
14,900
26,600
11,300
12,400
14,600
14,600
13,100
15,700
16,700
23,200
61,700
77,400
100
100
i
i
i
–
–
–
i
i
i
–
–
–
aPolymerization condition: 40 μmol of Al complex, 80 μmol of 9-hydroxyfluorene,
8 mmol L-LA (1.15 g), [L-LA]/[Al]/[9-hydroxyfluorene]=200/1/2, toluene=8 mL, poly-
merization temperature=70 °C. bPolymerization condition: 40 μmol of Al complex,
80 μmol of 9-hydroxyfluorene, 8 mmol L-LA (1.15 g), [L-LA]/[Al]/[9-hydroxyfluorene]=
200/1/2, no solvent used, polymerization temperature=130 °C, polymerization time=
1 h. cAdditional L-LA (16 mmol, 2.31 g) was added into entry 6 after 1 h. Continuation po-
lymerization condition: [L-LA]/[Al]/[9-hydroxyfluorene]=600/1/2, no solvent used, poly-
merization temperature=130 °C, polymerization time=1 h. dε-CA (8 mmol, 0.91 g)
was added into entry 7 after 1 h. Continuation polymerization condition: [L-LA]/[ε-CA]/
[Al]/[9-hydroxyfluorene]=600/200/1/2, no solvent used, polymerization tempera-
ture=130 °C, polymerization time=1 h. eConversion (%) was obtained by 1H-NMR de-
termination. fCalculated from the molecular weight of L-LA(144) times [L-LA]0/
[9-hydroxyfluorene]0 times conversion plus the molecular weight of 9-hydroxyfluorene
(182). gObtained by GPC analysis and calibrated by polystyrene standard. hCorrected
values were obtained from GPC times 0.58. iNot determined.
Fig. 1. X-ray structure of 1 with thermal ellipsoids drawn at the 50% probability level.
Toluene and all H atoms except for N-H proton are omitted for clarity.

S. Yoon et al. / Inorganic Chemistry Communications 29 (2013) 157–159
159
033: e-mail: deposit@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.inoche.2012.12.025.
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