Aluminum complexes of tridentate amine biphenolate ligands containing distinct N-alkyls: Synthesis and catalytic ring-opening polymerization

Article abstract of DOI:10.1002/jccs.201200559

A series of aluminum complexes of amine biphenolate ligands that contain distinct N-alkyl substituents have been prepared and employed for ring-opening polymerization (ROP) of ε-caprolactone (s-CL). Alkane elimination of trimethylaluminum with H₂[R-ONO] ([R-ONO]^2- = [RN(CH ₂-2-O-3, 5-C₆H₂(tBu)2)2]^2-; R = tBu (1a), iPr (1b), nPr (1c)) in toluene solutions at 25 °C produces colorless crystalline [R-ONO] AlM e (2a-c). X-ray diffraction studies of these organoaluminum complexes showed them to be four-coordinate species having a distorted tetrahedral coordination core. In the presence of benzyl alcohol, complexes 2a-c are all active catalyst precursors for ROP of ε-CL. The ROP rates of s-CL are clearly a function of the steric sizes of the N-alkyl substituents, following an increasing order 2a < 2b < 2c. Interestingly, both 2b and 2c exhibit controlled ROP catalysis whereas 2a gives low molecular weight oligomers under identical conditions. In ROP catalyzed with 2b or 2c, the ε-CL disappearance rates follow pseudo-first order kinetics, with the latter being about 2.86(2) times faster than the former. Complex 2b outperforms 2c in view of maintaining controlled ROP catalysis in reactions with extremely high monomer-to-catalyst ratios. The complex constitution-reactivity relationship is discussed.

Full text of DOI:10.1002/jccs.201200559

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Aluminum Complexes of Tridentate Amine Biphenolate Ligands Containing Distinct
N-alkyls: Synthesis and Catalytic Ring-opening Polymerization
Lan-Chang Liang,* Sheng-Ta Lin and Chia-Cheng Chien
Department of Chemistry and Center for Nanoscience & Nanotechnology, National Sun Yat-sen University, Kaohsiung
80424, Taiwan
(Received: Oct. 26, 2012; Accepted: Jan. 3, 2013; Published Online: Feb. 1, 2013; DOI: 10.1002/jccs.201200559)
A series of aluminum complexes of amine biphenolate ligands that contain distinct N-alkyl substituents
have been prepared and employed for ring-opening polymerization (ROP) of e-caprolactone (e-CL).
Alkane elimination of trimethylaluminum with H2[R-ONO] ([R-ONO]2- = [RN(CH2-2-O-3,5-
C6H2(tBu)2)2]2-; R = tBu (1a), iPr (1b), nPr (1c)) in toluene solutions at 25 °C produces colorless crystal-
line [R-ONO]AlMe (2a-c). X-ray diffraction studies of these organoaluminum complexes showed them to
be four-coordinate species having a distorted tetrahedral coordination core. In the presence of benzyl alco-
hol, complexes 2a-c are all active catalyst precursors for ROP of e-CL. The ROP rates of e-CL are clearly a
function of the steric sizes of the N-alkyl substituents, following an increasing order 2a < 2b < 2c. Interest-
ingly, both 2b and 2c exhibit controlled ROP catalysis whereas 2a gives low molecular weight oligomers
under identical conditions. In ROP catalyzed with 2b or 2c, the e-CL disappearance rates follow pseudo-
first order kinetics, with the latter being about 2.86(2) times faster than the former. Complex 2b outper-
forms 2c in view of maintaining controlled ROP catalysis in reactions with extremely high monomer-
to-catalyst ratios. The complex constitution-reactivity relationship is discussed.
Keywords: Aluminum; ONO ligand; Amine biphenolate; Ring-opening polymerization.
INTRODUCTION
The search for appropriate methods to prepare bio-
ences of ortho- and para-substituents (R1 and R2, respec-
tively)45,47,59 and additional N-tethered donors46,59 on ROP
catalysis, the effect of N-alkyl substituents (R3) lacking of
extra donors is not well established. This result is some-
what surprising but may have something to do with the lack
of efficient methods to construct amine biphenolate ligands
that contain N-substituted secondary or tertiary alkyls.
Having this in mind, we have set out to explore the possibil-
ities to prepare these novel ligands. To evaluate the effects
of N-alkyl substituents on ROP catalysis, we chose to ex-
amine the activities of aluminum complexes of [RN(CH2-
2-O-3,5-C6H2(tBu)2)2]2- ([R-ONO]2-; R = tBu (1a), iPr
(1b), nPr (1c)) with e-CL. Aluminum complexes were in-
compatible and biodegradable polymeric materials1-6 con-
tinues to constitute an active area of exploratory research.
Ring-opening polymerization (ROP) of heterocyclic mole-
cules, e.g., lactides or e-caprolactone (e-CL), etc., cata-
lyzed by main-group7-23 or transition metal complexes24-31
is attractive due to their versatilities and accessibilities to
produce these biorenewable materials in a controlled man-
ner.32-35 Recent advances have shown that living ROP may
be effectively devised with judiciously tailored metal com-
plexes that contain chelating N- or O-donor ligands.36-44
For instance, metal complexes supported by chelating b-
diketiminate37,38 or phenolate-based39-42 ligands are known
as catalysts to produce polylactides or poly(e-caprolac-
tone)s (PCLs) with controlled number averaged molecular
weight (Mn) and polydispersity index (PDI) values. For
steric and/or electronic tuning, the phenolate-based ligands
may in some cases integrate extra central or peripheral do-
nor(s).45-58 Figure 1 depicts representative examples of
amine bi-phenolate ligands with and without a peripheral
donor. Notably, while several reports disclose the influ-
Fig. 1. Representative examples of amine biphenolate
ligands.
Special Issue Dedicated to Prof. Yu Wang in Honor of Her 70th Birthday
* Corresponding author. Tel: +886-7-5252000 ext. 3945; Fax: +886-7-5253908; E-mail: lcliang@mail.nsysu.edu.tw
710
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Aluminum Complexes of ONO Ligands
rated to dryness under reduced pressure. The residue was dis-
solved in Et2O (30 mL) and deionized water (20 mL) was added.
The ether solution was separated from the aqueous portion, from
which the product was extracted two more times with diethyl
ether. All ether solutions were combined and dried over MgSO4.
All volatiles were re-moved in vacuo. The residue was dissolved
in acetonitrile (5 mL). The solution was cooled to -15 °C in a
freezer to afford to product as an off-white solid. The yields were
typically 70-75%. 1H NMR (C6D6, 500 MHz) d 8.11 (s, 2, ArOH),
7.35 (d, 2, J = 1.9, ArH), 6.95 (d, 2, J = 1.8, ArH), 3.61 (s, 4,
ArCH2), 1.52 (s, 18, ArCMe3), 1.32 (s, 18, ArCMe3), 0.98 (s, 9,
NCMe3). 13C NMR (C6D6, 75 Hz) d 153.65 (C), 142.10 (C),
136.91 (C), 125.02 (C), 124.64 (CH), 123.57 (CH), 57.46
(NCMe3), 53.56 (ArCH2N), 35.48 (ArCMe3), 34.73 (ArCMe3),
32.29 (ArCMe3), 30.48 (ArCMe3), 27.05 (NCMe3). Anal. Calcd
for C34H55NO2: C, 80.09; H, 10.88; N, 2.75. Found: C, 80.23; H,
10.60; N, 2.64.
tentionally chosen on the basis of an elegant report by Chen
et al. describing controlled ROP of e-CL catalyzed by [nPr-
ONO]AlMe (2c)42 that features a primary N-alkyl. We
present herein the synthesis of H2[1a] and H2[1b] carrying
a tertiary and secondary N-bound alkyl, respectively, and
their corresponding aluminum complexes [tBu-ONO]AlMe
(2a) and [iPr-ONO]AlMe (2b), along with their ROP catal-
ysis with e-CL. Kinetic studies employing 2b and 2c were
also attempted to get insights regarding the effect of these
N-alkyls on catalysis.
EXPERIMENTAL
General considerations. Unless otherwise specified, all
experiments were performed under nitrogen using standard
Schlenk or glovebox techniques. All solvents were reagent grade
or better and purified by standard methods. Compounds 2-(bro-
momethyl)-4,6-di-tert-butylphenol,60 2-(isopropylaminometh-
yl)-4,6-di-tert-butylphenol,61 2-(tert-butylaminomethyl)-4,6-di-
tert-butylphenol,62 H2[1c],59 and 2c42 were prepared according to
literature procedures. All other chemicals were obtained from
commercial vendors and used as received. The NMR spectra were
recorded on Varian Unity or Bruker AV instruments. Chemical
shifts (d) are listed as parts per million downfield from tetra-
methylsilane. Coupling constants (J) and peak widths at half-
height (Dv1/2) are listed in hertz. 1H NMR spectra are referenced
using the residual solvent peak at 7.16 for C6D6. 13C NMR spectra
are referenced using the internal solvent peak at d 128.39 for
C6D6. The assignment of the carbon atoms for all new compounds
is based on the DEPT 13C NMR spectroscopy. 27Al NMR spectra
are referenced externally using AlCl3 in D2O at d 0. Routine cou-
pling constants are not listed. All NMR spectra were recorded at
22 °C in specified solvents unless otherwise noted. Elemental
analysis was performed on a Heraeus CHN-O Rapid analyzer.
GPC analyses were carried out at 45 °C on a JASCO instrument
equipped with two Waters Styragel HR columns in series and a
JASCO RI-2031 refractive index detector. HPLC grade THF was
supplied at a constant flow rate of 1.0 mL/min with a JASCO
PU-2080 Isocratic HPLC Pump. Molecular weights (Mn and Mw)
were determined by interpolation from calibration plots estab-
lished with polystyrene standards (Mn range: 3790-189300).
Synthesis of H2[1a]. An acetonitrile solution (10 mL) con-
taining 2-(tert-butylaminomethyl)-4,6-di-tert-butylphenol (17.2
mmol) and K2CO3 (34.4 mmol, 2 equiv) was stirred at 25 °C for
30 min. To this was sequentially added NEt3 (86 mmol, 5 equiv)
and an acetonitrile solution (10 mL) of 2-(bromomethyl)-4,6-di-
tert-butylphenol (22.4 mmol, 1.3 equiv). The reaction mixture
was stirred at 25 °C overnight and filtered. The filtrate was evapo-
Synthesis of H2[1b]. Procedures were similar to those of
H2[1a], producing the product as an off-white solid; yield 75%.
1H NMR (C6D6, 500 MHz) d 8.36 (s, 2, ArOH), 7.47 (d, 2, J = 2.5,
ArH), 6.96 (d, 2, J = 2.5, ArH), 3.43 (s, 4, ArCH2), 3.00 (sept, 1, J
= 6.5, NCHMe2), 1.62 (s, 18, ArCMe3), 1.36 (s, 18, ArCMe3), 0.75
(d, 6, J = 6.5, NCHMe2). 13C NMR (C6D6, 125 MHz) d 153.25 (C),
141.76 (C), 136.55 (C), 125.27 (CH), 123.66 (CH), 122.20 (C),
52.35 (ArCH2N), 48.40 (NCHMe2), 35.27 (ArCMe3), 34.36
(ArCMe3), 31.91 (ArCMe3), 30.06 (ArCMe3), 16.36 (NCHMe2).
Anal. Calcd for C33H53NO2: C, 79.93; H, 10.78; N, 2.83. Found:
C, 79.75; H, 10.81; N, 2.78.
Synthesis of 2a. Trimethylaluminum (0.2 mL, 2 M in tolu-
ene, 0.4 mmol) was added to a toluene solution (5 mL) of H2[1a]
(200 mg, 0.392 mmol) at 25 °C. After being stirred at 25 °C for 24
h, the reaction solution was evaporated to dryness under reduced
pressure. The residue thus obtained was washed with pentane (2
mL) to give an off-white solid; yield 144 mg (67%). Colorless
crystals suitable for single-crystal X-ray diffraction analysis were
grown from a concentrated pentane solution at -35 ºC. 1H NMR
(C6D6, 500 MHz) d 7.53 (d, 2, J = 2.3, ArH), 6.70 (d, 2, J = 2.2,
ArH), 3.53 (d, 2, J = 13.7, ArCHAHB), 3.43 (d, 2, J = 13.8,
ArCHAHB), 1.68 (s, 18, ArCMe3), 1.37 (s, 18, ArCMe3), 0.83 (s, 9,
NCMe3), -0.03 (s, 3, AlMe). 13C NMR (C6D6, 125 Hz) d 156.00
(C), 140.29 (C), 138.59 (C), 124.83 (C), 124.05 (CH), 123.68
(CH), 61.77 (ArCH2N), 55.10 (NCMe3), 35.73 (ArCMe3), 34.71
(ArCMe3), 32.46 (ArCMe3), 30.25 (ArCMe3), 26.44 (NCMe3),
-9.31 (AlMe). Anal. Calcd for C35H56NO2Al: C, 76.45; H, 10.27;
N, 2.55. Found: C, 76.33; H, 10.32; N, 2.49.
Synthesis of 2b. Procedures were similar to those of 2a,
producing the product as an off-white solid; yield 61%. Colorless
J. Chin. Chem. Soc. 2013, 60, 710-718
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crystals suitable for single-crystal X-ray diffraction analysis were
grown from a concentrated pentane solution at -35 ºC. 1H NMR
(C6D6, 500 MHz) d 7.56 (d, 2, J = 2.4, ArH), 6.70 (d, 2, J = 2.3,
ArH), 3.40 (d, 2, J = 13.5, ArCHAHB), 3.19 (d, 2, J = 13.5,
ArCHAHB), 2.66 (septet, 1, J = 6.6, NCHMe2), 1.70 (s, 18,
ArCMe3), 1.40 (s, 18, ArCMe3), 0.58 (d, 6, NCHMe2), -0.20 (s, 3,
AlMe). 13C NMR(C6D6, 125 Hz) d 156.12 (C), 140.21 (C), 138.82
(C), 125.04 (C), 124.71 (CH), 121.82 (CH), 53.75 (ArCH2N),
52.81 (NCHMe2), 35.80 (ArCMe3), 34.70 (ArCMe3), 32.43
(ArCMe3), 30.30 (ArCMe3), 16.92 (NCHMe2), -11.50 (AlMe).
27Al NMR (toluene-d8, 130 MHz) d 150 (Dv1/2 = 11186). Anal.
Calcd for C34H54NO2Al: C, 76.21; H, 10.16; N, 2.62. Found: C,
76.11; H, 10.33; N, 2.69.
Scheme I Ligand synthesis
amine, even under refluxed conditions for a prolonged pe-
riod of time (e.g., 3 days).
X-ray crystallography. Crystallographic data for all struc-
turally characterized compounds are available in Supplementary
material. Data were collected on a Bruker-Nonius Kappa CCD
diffractometer with graphite monochromated Mo-Ka radiation (l
= 0.7107 Å). Structures were solved by direct methods and re-
fined by full matrix least squares procedures against F2 using
SHELXL-97.63 All full-weight non-hydrogen atoms were refined
anisotropically. Hydrogen atoms were placed in calculated posi-
tions. The crystals of 2b were of poor quality (R1 = 0.1079) but
sufficient to establish the identity of this molecule. CCDC
889209–889214 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.
ac.uk/data_request/cif.
Compounds H2[1a] and H2[1b] were fully character-
ized by 1H and 13C NMR spectroscopy, combustion analy-
sis, and X-ray crystallography. As anticipated for pyrami-
dal amine derivatives, these molecules are Cs-symmetric in
solutions on the NMR timescale with two phenol moieties
being chemically equivalent. The benzylic methylene pro-
tons show a singlet resonance at ca. 3.5 ppm and the phenol
hydroxyl protons at ca. 8.2 ppm. Colorless crystals of
H2[1a], H2[1b] and H2[1c] suitable for X-ray diffraction
analysis were grown from a concentrated acetonitrile solu-
tion at -15 °C. The molecular structures of these amine
biphenol derivatives are depicted in Figures S1-S3 (see
supplementary material). Compounds H2[1a] and H2[1b]
are structurally similar in the solid state, containing both
intramolecular and intermolecular hydrogen bonding in-
volving hydroxyl with amine nitrogen and co-crystallized
acetonitrile nitrogen, respectively. The n-propyl substi-
tuted H2[1c], however, does not co-crystallize with aceto-
nitrile, thereby having solely intramolecular hydrogen
bonding. Neither bond distances nor angles are excep-
tional.
RESULTS AND DISCUSSION
Synthesis and characterization of ligands
Ligand H2[1c] was prepared following the known
Mannich condensation method.59 Attempts to synthesize
H2[1a] and H2[1b] under similar conditions, however, led
instead to the formation of the corresponding benzoxa-
zines64-66 on the basis of GC/MS analysis of reaction
aliquots; neither H2[1a] nor H2[1b] was detected. In con-
trast, nucleophilic substitution reactions of 2-(bromometh-
yl)-4,6-di-tert-butylphenol60 with 2-(alkylaminomethyl)-
4,6-di-tert-butylphenol61,62 in the presence of potassium
carbonate and triethylamine in acetonitrile solutions at 25
°C afforded successfully in 12 hours the desired amine
biphenol ligands in ca. 70% isolated yield (Scheme I). We
note that the participation of both potassium carbonate and
triethylamine in these reactions appears critical: no reac-
tion occurred at all in the absence of potassium carbonate;
little formation of the desired products (< 10% yield) was
found if the reactions were performed without triethyl-
Synthesis and characterization of aluminum com-
plexes
No reaction was found for Al(OiPr)3 with H2[1a],
H2[1b] or H2[1c] in C6D6 at 70 ºC for 2 days on the basis of
1H NMR studies.67 Addition of one equiv of trimethylalu-
minum to a toluene solution of H2[1a], H2[1b] or H2[1c] at
25 °C affords colorless crystalline aluminum methyl com-
plexes 2a, 2b and 2c, respectively (Scheme II). Com-
pounds 2a and 2b were fully characterized by 1H and 13
C
NMR spectroscopy, combustion analysis, and X-ray crys-
tallography. Figures 2 and 3 depict the molecular structures
of these complexes. Table 1 summarizes their selected
712
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Aluminum Complexes of ONO Ligands
appear as two doublets, contrasting sharply with what is
found for H2[1a] and H2[1b] but clearly indicative of their
diastereotopic nature as a consequence of amine coordina-
tion to aluminum. Variable temperature 1H NMR studies of
2a, 2b and 2c (in toluene-d8) revealed that the two doublet
resonances ascribed to the benzylic methylene protons do
not coalesce even at 80 ºC, a result that is suggestive of the
non-dissociative nature of the amine nitrogen donor in
these organoaluminum complexes. These complexes are all
thermally stable, remaining intact upon heating to 80 ºC for
24 hours as indicated by 1H NMR studies (30 mM in tolu-
ene-d8).
Scheme II Synthesis of aluminum complexes
bond distances and angles, in comparison with those of
2c.42 As depicted, these complexes are mononuclear, four-
coordinate species whose coordination geometry is best de-
scribed as distorted tetrahedral. Though the bond distances
and angles about aluminum are all unexceptional and com-
parable with other pseudo-tetrahedral organoaluminum
complexes ligated with phenolates or amines,42,68-72 the
Al-N bond length of 2a is slightly longer than those of 2b
and 2c, likely reflective of the steric discrepancy of these
N-alkyl substituents. The solution NMR data of 2a and 2b
are all consistent with a structure having Cs symmetry. In
ROP catalysis
The polymerization activities of 2a, 2b, and 2c with
e-CL were investigated. Table 2 summarizes the polymer-
ization results. To establish parallel comparison with litera-
ture data, we chose to examine the activities of these alumi-
num complexes with conditions similar to those reported
previously.42 Both 2b and 2c complete ROP in 20 minutes
with 100 equiv of e-CL at 50 ºC in the presence of one
equiv of benzyl alcohol, producing PCLs with extremely
narrow molecular weight distributions (entries 173 and 4,
respectively) whereas 2a reacts much slower (entry 11,
lower conversion with extended reaction time) to produce
low molecular weight oligomers, highlighting the pro-
found N-substituent effect of these complexes on ROP
rates. Elevating the polymerization temperature to 80 ºC ef-
fectively increases the catalytic efficiency of 2a but at the
expense of an increased PDI value (entry 12), suggesting
possibly the occurrence of undesired transesterification re-
actions. The n-propyl substituted 2c appears to give well-
defined PCLs at monomer-to-catalyst ratios up to 300.42
With more monomer presence, the PDIs of PCLs increase
appreciably (entry 2). In contrast, the isopropyl substituted
2b polymerizes e-CL in a much more controlled manner
under similar conditions (entry 7). The PDI of PCLs pre-
pared by 2b is significantly lower than that by 2c though
the measured Mn’s are virtually identical (entry 7 versus 2).
More significantly, extremely low PDIs could also be ob-
tained with extremely high monomer-to-catalyst ratios (en-
try 9) though a much more diluted solution is required to di-
minish inevitably increased viscosity due to the formation
of high molecular weight polymers. Catalyst 2b thus out-
performs 2c in production of well-defined polymers with a
wider range of molecular weights.
1
the H NMR spectra, the benzylic methylene resonances
Fig. 2. Molecular structures of [tBu-ONO]AlMe (2a)
with thermal ellipsoids drawn at the 35% proba-
bility level. All methyl groups in aryl tert-butyl
are omitted for clarity.
Fig. 3. Molecular structures of [iPr-ONO]AlMe (2b)
with thermal ellipsoids drawn at the 35% proba-
bility level. All methyl groups in aryl tert-butyl
are omitted for clarity.
Similar to 2c,42 complex 2b is also a well-defined
ROP catalyst for e-CL. Notably, the Mn values increase lin-
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Liang et al.
Table 1. Selected bond distances (Å) and angles (º) for [tBu-ONO]AlMe (2a), [iPr-
ONO]AlMe (2b), and [nPr-ONO]AlMe (2c)
Compound
2a
2b
2c(A) [a]
2c(B) [a]
2c(C) [a]
Al-C
Al-O
1.947(3)
1.739(2)
1.740(2)
2.022(2)
112.07(10)
98.71(10)
98.27(9)
123.69(11)
113.88(13)
109.06(13)
1.934(7)
1.739(5)
1.734(5)
1.998(6)
113.2(2)
98.1(2)
1.930(7)
1.713(4)
1.736(3)
1.985(4)
1.925(6)
1.736(3)
1.734(3)
1.988(4)
1.939(6)
1.720(4)
1.724(3)
1.991(3)
Al-N
O-Al-O
O-Al-N
114.23(19) 114.60(18) 114.63(19)
99.79(18)
99.58(16)
116.0(2)
115.3(2)
110.6(2)
99.25(18)
99.79(16)
115.2(2)
111.7(2)
114.9(2)
99.00(18)
100.15(16)
115.5(2)
115.4(2)
110.7(2)
99.1(2)
C-Al-N
C-Al-O
120.1(3)
112.4(3)
112.7(3)
[a] Data for 2c were selected from reference 42, representing three independent
molecules found in the asymmetric unit cell.
Table 2. ROP of e-CL by catalysts 2a, 2b, and 2c in the presence of benzyl alcohol initiator [a]
conv Mn (calc, kg Mn (exp, kg
corrected Mn
Entry cat.
[e-CL]/[cat]/[I] temp (ºC) time (min)
PDI[d]
(%)[b]
mol-1) [c]
mol-1) [d]
(exp, kg mol-1) [e]
1
2
3
4
5
6
7
8
2c
2c
100/1/1
400/1/1
50/1/1
100/1/1
200/1/1
300/1/1
400/1/1
1000/1/1
1000/1/1
100(100)/1/1
50
50
50
50
50
50
50
50
50
50
20
35
20
20
20
30
35
70
120
20
(20)
840
840
20
15
15
99
99
99
99
99
99
99
99
78
99
(99)
33
99
44
99
99
27
<1
11.4
45.3
5.8
16.8
58.0
9.5
9.4
32.5
5.3
1.06
1.32
1.08
1.07
1.05
1.06
1.05
1.28
1.04
1.06
(1.05)
N/A
1.28
1.10
1.08
1.08
1.15
N/A
2b
2b
2b
2b
2b
2b
2b
2b
11.4
22.7
34.0
45.3
113.1
89.1
11.4
(22.7)
3.9
11.4
5.0
5.8
5.8
3.2
13.9
23.8
37.8
58.8
88.9
82.3
13.7
(27.3)
N/A
18.9
61.0
10.1
11.2
4.5
7.8
13.3
21.2
32.9
49.8
46.1
7.7
(15.3)
N/A
10.6
34.2
5.7
9[f]
10
11
12
13
14
15
16
17
2a
2a
100/1/1
100/1/1
100/1/0
200/1/4
400/1/8
100/1/1
100/1/0
50
80
50
50
50
50
50
2b
2b
2b
6.3
2.5
N/A
AlMe3
AlMe3
20
20
<0.1
N/A
[a] Unless otherwise noted, [cat.]0 = 8.3 mM in 3 mL toluene. [b] Determined by 1H NMR analysis. [c] Calculated from {fw
of e-CL ´ ([e-CL]0/([cat.]0 [I]o)) ´ conversion} + fw of BnOH, assuming one propagating chain per aluminum atom. [d]
Measured by GPC in THF, calibrated with polystyrene standards. [e] Multiplied by a factor of 0.56. 73,74 [f] [cat.]0 = 1.7 mM
in 15 mL toluene.
early with increasing monomer consumed to catalyst ratios
while keeping consistently low PDIs (Figure 4). Signifi-
cantly, the propagating intermediates remain active even
after complete consumption of monomers; upon addition
of a second batch of monomers, the propagation resumes to
produce PCLs with controlled Mn and PDI values (entry
10).
4 and 13). Without benzyl alcohol, 2b reacts with e-CL at a
much slower rate, producing PCLs with molecular weights
much higher than the expected values, a result that is sug-
gestive of the involvement of only a portion of 2b in cataly-
sis and the reactivity discrepancy between Al-Me and Al-
O(propagating) functionalities. Once initiated, the ROP ca-
talysis takes place at a much faster rate for Al-O(propagat-
ing) than for Al-Me. In the presence of benzyl alcohol, the
catalysis was effectively initiated by the presumed forma-
The roles of benzyl alcohol were also investigated. Its
participation in catalysis is evidently demonstrated (entries
714
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JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Aluminum Complexes of ONO Ligands
of neutral aluminum complexes correlate well with their
coordination number.75 The 27Al{1H} NMR spectrum of 2b
in toluene-d8 exhibits a broad singlet resonance at 150 ppm
with the peak width at half height (Dv1/2) of 11186 Hz.
These values are within the expected region for a four-co-
ordinate organoaluminum species. Similar chemical shifts
were also found for Al2Me6 (156 ppm),76 AlEt3(thiophene)
(154 ppm),77 (iPr2-ATI)AlMe2 (154 ppm, iPr2-ATI = N,N¢-
diisopropylaminotroponiminate),78 (iPr-NP)AlMe2 (151
ppm, iPr-NP = 2,6-iPr2C6H3N(C6H4-2-PPh2)),79 and (iPr-
NO)AlEt2 (142 ppm, iPr-NO = 2,6-iPr2C6H3N(C6H4-2-
P(O)Ph2)),12 etc. In the presence of one equiv of e-CL, the
Fig. 4. Plot of Mn of PCLs versus monomer consumed
to catalyst ratios (entries 3-7 and 9 in Table 2).
The corresponding PDI values are indicated in
parentheses.
27Al chemical shift of 2b moves upfield to 129 ppm (Dv1/2
=
11157 Hz), consistent with an increase in coordination
number for a neutral organo-aluminum complex.75,80 The
1H NMR spectrum of the same solution reveals the pres-
ence of an aluminum-bound methyl group instead of ke-
tone methyl, indicating no ROP reaction occurs during
these NMR acquisitions. The upfield change of the ob-
served 27Al chemical shift of 2b upon addition of e-CL is
thus ascribable to the coordination of the heterocyclic
monomer to aluminum, presumably via carbonyl oxygen
atom. This hypothesis is consistent with the X-ray structure
of an aluminum e-CL complex supported by a biphenolate
ligand as elucidated by Lewi½ski et al.81 On the basis of the
fact that the ROP rates of 2a-c are governed by the N-sub-
stituents, we propose that the monomer coordination pro-
ceeds predominantly, though perhaps not exclusively, on
the two X-N-O (X = C for 2a-c or X = alkoxide for propa-
gating intermediates) faces of the pseudo-tetrahedral cata-
lysts. Attempts to corroborate this proposal are currently
underway with unsymmetrically substituted [ONO]2- li-
gated complexes. We note that similar justifications have
been proved for Schrock-type olefin metathesis cata-
lysts.82,83
tion of reactive [1b]AlOBn, produced from alcoholysis of
2b with HOBn. End group analysis on PCLs by 1H NMR
spectroscopy (Figure S4) clearly confirms the presence of
the benzyl oxide functionality. Addition of more than one
equiv of benzyl alcohol (entries 14 and 15) led to a decrease
in molecular weights (in comparison with entries 5 and 7,
respectively) that, however, are in good agreement with
what is anticipated for propagating intermediates to un-
dergo chain transfer reactions with benzyl alcohol at a sta-
tistical frequency corresponding to the equiv of added
benzyl alcohol. Benzyl alcohol thus also functions as an ef-
ficient chain transfer reagent. The measured Mn and PDI
values in entries 14 and 15 are well comparable to those of
entry 3; the low PDI of 1.08 in the former strongly suggests
that both initiation and chain transfer steps proceed in a
controlled fashion.
The amine biphenolate ligands also play an important
role in this ROP catalysis. Entries 16 and 17 summarize the
reactivity of trimethylaluminum with and without benzyl
alcohol, respectively. With the coordination of the amine
biphenolate ligand, 2b (entry 4) polymerizes e-CL appar-
ently more efficiently than AlMe3 (entry 16) under similar
conditions. Both 2b and 2c catalyze ROP of e-CL follow-
ing pseudo-first order kinetics (Figure S5)74 with the latter
being about 2.86(2) times faster than the former. In view of
the comparable electronic properties of isopropyl and
n-propyl, we suggest that the discrepancy in catalysis rates
is a consequence of the steric influence of these N-substitu-
ents. This result is also consistent with the much slower
ROP rates found for 2a.
CONCLUSIONS
We have demonstrated that the identity of N-alkyl
substituents in the aluminum complexes of amine bipheno-
late ligands has substantial impacts on both polymerization
rates and polymer molecular weight distributions in the cat-
alytic ROP of e-CL. Though Mannich condensation does
not afford effectively H2[1a] or H2[1b] carrying a tertiary
or secondary N-substituted alkyl, respectively, nucleo-
philic substitution of the corresponding benzyl bromide
with benzyl amines proves successful with the involvement
of appropriate bases. Controlled experiments suggest that
It has been documented that the 27Al chemical shifts
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715

Article
Liang et al.
9. Chang, Y.-N.; Liang, L.-C. Inorg. Chim. Acta. 2007, 360,
136.
the participation of both potassium carbonate and triethyl-
amine is critical under the conditions investigated. Similar
to 2c that contains an n-propyl substituent at the nitrogen
donor, aluminum methyl complexes 2a and 2b are both
four-coordinate, pseudo-tetrahedral in the solid state.
Though coordination geometry, bond distances and angles
about aluminum in these organoaluminum complexes are
similar, their ROP rates with e-CL differ apparently from
one to another as a consequence of steric influence arising
from the N-substituents. In spite of exhibiting similar con-
trolled polymerization characteristics, complex 2b appears
a superior catalyst to 2c in view of producing well-defined
PCLs with narrower molecular weight distributions in a
much wider range of molecular weights. Studies directed to
delineate the reactivity of metal complexes of these amine
biphenolate ligands involving other main group and transi-
tion metals are currently underway.
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ACKNOWLEDGEMENTS
We thank the National Science Council of Taiwan for
financial support (NSC 99-2113-M-110-003-MY3 and
99-2119-M-110-002), Mr. Ting-Shen Kuo (NTNU) for as-
sistance with X-ray crystallography, and the National Cen-
ter for High-performance Computing (NCHC) for the ac-
cess to chemical databases.
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SUPPLEMENTARY MATERIAL
X-ray crystallographic data for H2[1a], H2[1b],
H2[1c], 2a, 2b, 2c, a representative 1H NMR spectrum of
PCLs, and semilogarithmic plots of e-CL conversion with
time (employing 2b and 2c).
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