Phenolate substituent effects on ring-opening polymerization of ε-caprolactone by aluminum complexes bearing 2-(phenyl-2-olate)-6-(1- amidoalkyl)pyridine pincers

Article abstract of DOI:10.1021/om301057d

Interaction of the 2-(phenyl-2-ol)-6-ketiminopyridines 2-(4′-R ¹-C₆H₃-2′-OH)-6-{CMe=N(2″,6″- i-Pr₂C₆H₃)}C₅H₃N (R ¹ = H (L1_a-H), Bu^t (L

Full text of DOI:10.1021/om301057d

Article
pubs.acs.org/Organometallics
Phenolate Substituent Effects on Ring-Opening Polymerization of
ε‑Caprolactone by Aluminum Complexes Bearing 2‑(Phenyl-2-olate)-
6-(1-amidoalkyl)pyridine Pincers
Wafaa Alkarekshi,† Andrew P. Armitage,† Olivier Boyron,‡ Christopher J. Davies,† Matifadza Govere,†
Andrew Gregory,† Kuldip Singh,† and Gregory A. Solan*,†
†Department of Chemistry, University of Leicester, University Road, Leicester LE1 7RH, U.K.
‡Laboratoire de Chimie, Catalyse, Polymer
̀ ́ ́ ́
es et Procedes, Universite de Lyon, CNRS-UCBL, 43 Boulevard du 11 Novembre 1918,
69616 Villeurbanne, France
S
* Supporting Information
ABSTRACT: Interaction of the 2-(phenyl-2-ol)-6-ketiminopyridines 2-(4′-R1-C6H3-2′-OH)-
6-{CMeN(2″,6″-i-Pr2C6H3)}C5H3N (R1 = H (L1a-H), But (L1b-H), Cl (L1c-H), F (L1d-
H)) with AlMe3 at elevated temperature and subsequent crystallization from acetonitrile
affords the five-coordinate 2-(phenyl-2-olate)-6-(2-amidoprop-2-yl)pyridine aluminum−
methyl complexes [2-(4′-R1C6H3-2′-O)-6-{CMe2N(2″,6″-i-Pr2C6H3)}C5H3N]AlMe(NCMe)
(R1 = H (1a), But (1b), Cl (1c), F (1d)), as their acetonitrile adducts, in good yield. In each
case, complexation results in concomitant C−C bond formation via methyl migration from
aluminum to the corresponding imino unit in L1-H. On the other hand, reactions of the
aldimine-containing compounds 2-(4′-R1-C6H3-2′−OH)-6-{CHN(2″,6″-i-Pr2C6H3)}-
C5H3N (R1 = H (L1e-H), But (L1f-H), Cl (L1g-H)) afford as the major crystallized products
[2-(4′-R1C6H3-2′-O)-6-{CH(Me))N(2″,6″-i-Pr2C6H3)}C5H3N]AlMe(NCMe) (R1 = H (2a),
But (2b), Cl (2c)), in which the migrated methyl group and aluminum−methyl are disposed
mutually cis; evidence for the minor trans isomers 2a′−c′ is presented. The ring-opening
polymerization of ε-caprolactone employing 1 and 2 in the presence of PhCH2OH proceeded efficiently, producing polymers of
narrow molecular weight distribution with the catalytic activities highly dependent on the nature of the phenolate-containing 4-
R1 substituent with the F and But initiators showing the highest activities (1b ≈ 1d > 1a ≈ 1c and 2b > 2a > 2c); in general the
CMe2-containing series 1 were more active than CH(Me)-containing 2 at 30 °C. The bimetallic complex [{2-(4′-But-C6H3-2′-
O)-6-{CHMeN(2″,6″-i-Pr2C6H3)}C5H3N}AlMe(μ-OMe)AlMe2] (3), the result of adventitious oxygenation, is also reported.
The single-crystal X-ray structures are reported for L1d-H, L1f-H, 1a−d, 2a, 2b/2b′, 2c, and 3.
INTRODUCTION
■
The biodegradable and biocompatible properties of certain
types of aliphatic polyesters has led to this class of polymer
assuming considerable importance in the medical and
pharmaceutical fields.1 Ring-opening polymerization (ROP)
of cyclic esters (e.g., lactide, δ-valerolactone (δ-VL), and ε-
caprolactone (ε-CL)) has proved an effective route for
generating such materials,2 and among the most widely
employed metal-based initiators/catalysts have been aluminum
alkoxides.3−8 Indeed, numerous articles have been dedicated to
the catalytic evaluation of discrete Al−OR chelate complexes
bearing a range of different multidentate stabilizing auxiliaries
including N,N,3 N,O,4 O,O,5 O,N,N,O,6 N,N,N,O,7 and
N,N,N,N frameworks.8 On the other hand, the application of
nonsymmetric tridentate N,N,O chelates is less developed and
limited to monoanionic frameworks9 related in structure to
those employed to impart good catalytic activity on divalent
metal catalysts (e.g., Zn, Sn, Ca, Mg) for ROP.10
Figure 1. 2-(Phenyl-2-olate)-6-iminopyridine (L1) and 2-(phenyl-2-
olate)-6-(1-amidoalkyl)pyridine (L2).
applications including olefin polymerization.11,12 In particular,
we envisaged that dianionic L2 could act as a compatible ligand
framework for well-defined aluminum(III) monoalkyl/alkoxide
complexes that could be employed as initiators for ROP.
Furthermore, the amenability of the ligand synthesis to
systematic variation of the phenolate substituents (e.g., R1)
Recently, we have been interested in the development of new
families of pyridine-based N,Npy,O pincer ligands, e.g., L1 and
L2 (Figure 1), for use in a variety of metal-based catalytic
Received: November 7, 2012
Published: December 24, 2012
© 2012 American Chemical Society
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Article
a
Scheme 1 .
a
Reagents and conditions: (i) 2-Br-6-(CR2O)-C5H3N (R2 = H, Me), cat. Pd(PPh3)4, K2CO3 (aq), toluene−EtOH, heat; (ii) 2,6-i-Pr2C6H3NH2, cat.
H+, MeOH, heat; (iii) BBr3, CH2Cl2, −78 °C.
offers considerable potential for probing the role played by
electronic effects on catalytic performance. Indeed, if the
mechanism of polymerization is considered to follow a
coordination−insertion type pathway,2,13 electronic/steric
perturbations within the Lewis acidic initiator would be
expected to impact on the overall catalytic performance.
In this article, we employ a series of protonated ketimine (R2
= Me) and aldimine (R2 = H) examples of L1 (R1 = H, But, Cl,
F) as precursors to a new family of aluminum(III) methyl
complexes bearing sterically encumbered (Ar = 2,6-i-Pr2C6H3)
L2. In the presence of benzyl alcohol, these aluminum−
monomethyl complexes exhibit high efficiency for the ring-
opening polymerization of ε-CL with their catalytic activity
(propagation rate) influenced by both the nature of the
phenolate R1 group and the 1-amidoalkyl R2 group. Herein we
report a full account of the results.
view of L1d-H is depicted in Figure 2; selected bond distances
and angles are given for both L1d-H and L1f-H in Table 1. The
Figure 2. Molecular structure of L1d-H with the atom-labeling scheme
and ellipsoids at the 30% probability level. All hydrogen atoms, apart
from H1, have been omitted for clarity.
RESULTS AND DISCUSSION
■
1. Synthesis of 2-(Phenyl-2-ol)-6-iminopyridines (L1-
H). The 2-(phenyl-2-ol)-6-iminopyridines 2-(4′-R1-C6H3-2′−
OH)-6-{CR2N(2″,6″-i-Pr2C6H3)}C5H3N (R1 = H, R2 = Me
(L1a-H); R1 = R2 = H (L1e-H); R1 = But, R2 = H (L1f-H); R1 =
Cl, R2 = H (L1g-H)) could be prepared using methodologies
previously described.11a Alternatively, a two-step procedure was
developed involving Suzuki cross-coupling of the correspond-
ing 2-hydroxyphenylboronic acid with 2-bromo-6-acetylpyr-
idine followed by a condensation reaction with 2,6-
diisopropylaniline, affording 2-(4′-R1-C6H3-2′-OH)-6-{CR2
N(2″,6″-i-Pr2C6H3)}C5H3N (R1 = H, R2 = Me (L1a-H); R1 =
But, R2 = Me (L1b-H); R1 = Cl, R2 = Me (L1c-H); R1 = F, R2 =
Me, (L1d-H)) in good yield (Scheme 1). The new compounds
L1b-H, L1c-H, and L1d-H have been characterized by a
combination of 1H, 19F{1H}, and 13C{1H} NMR and IR
spectroscopy and ESI mass spectrometry (see the Experimental
Section).
structures are similar and resemble those previously reported
for L1a-H, L1e-H, and L1g-H,11a with a central pyridine ring
substituted at its 2-position by a phenyl-2-ol group and at the 6-
Table 1. Selected Bond Distances (Å) and Angles (deg) for
L1d-H and L1f-H
L1d-H
L1f-H
Bond Distances
1.359(4)
C(1)−O(1)
C(6)−C(7)
C(11)−C(12)
C(12)−N(2)
N(2)−C(13)
C(4)−F(1)
1.3591(15)
1.4764(17)
1.4668(17)
1.2571(15)
1.4230(15)
1.469(4)
1.488(5)
1.268(4)
1.429(4)
1.366(4)
C(12)−C(12′)
C(4)−C(25)
1.505(5)
1.529(2)
In addition, single-crystal X-ray diffraction studies have been
performed on L1d-H and L1f-H. Typically crystals suitable for
the structural determination could be grown by slow
evaporation of concentrated dichloromethane solutions. A
Bond Angles
122.5(3)
C(12)−N(2)−C(13)
C(11)−C(12)−N(2)
120.83(11)
121.04(12)
116.9(3)
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a
position by an imine unit (C(12)−N(2) = 1.268(4) Å (L1d-H),
1.257(2) Å (L1f-H)). The pyridine nitrogen adopts a trans
configuration with respect to the neighboring imine nitrogen
(torsion angles N(1)−C(11)−C(12)−N(2) = 174.6° (L1d-H),
167.8° (L1f-H)) while it is cis to the phenol oxygen; the latter
configuration is the result of a hydrogen-bonding interaction
between the phenol hydrogen atom and the neighboring
pyridine nitrogen (O(1)···N(1) = 2.583 Å (L1d-H), 2.621 Å
(L1f-H)). For both structures the 2,6-diisopropylphenyl rings
are inclined essentially orthogonally (C(12)−N(2)−C(13)−
C(18) = 89.3° (L1d-H), 88.4° (L1f-H)) to the plane of the
adjacent pyridylimine unit.
Scheme 2
Compounds L1-H all display peaks corresponding to the
protonated molecular ions in their ESI mass spectra, while their
IR spectra reveal characteristic ν(CN)imine bands at ca. 1642
cm−1. Further support for imine formation is provided by the
1H NMR spectra, which show signals for the ketimine protons
at ca. δ 2.2 (L1a-H−L1d-H) and aldimine proton at ca. δ 8.27
(L1e-H−L1g-H); the imino carbons are seen at ca. δ 160 in
their 13C NMR spectra. Fluorinated L1d-H displays a singlet in
its 19F{1H} NMR spectrum at δ −125.09 with a typically large
one-bond carbon−fluorine coupling constant (236 Hz)
observable in its 13C{1H} NMR spectrum.
2. Formation of Aluminum−Methyl Complexes 1 and
2. Treatment of ketimine-containing L1a-H−L1d-H with 2
equiv of trimethylaluminum in toluene at elevated temperature,
followed by workup in acetonitrile, results in complexation and
methylation of the ketimine moiety to afford the air-sensitive 2-
(phenyl-2-olate)-6-(2-amido-prop-2-yl)pyridine aluminum−
methyl complexes [2-(4′-R1-C6H3-2′-O)-,6-{CMe2N(2″,6″-i-
Pr2C6H3)}C5H3N]AlMe(NCMe) (R1 = H (1a), But (1b), Cl
(1c), F (1d)) as their acetonitrile adducts in good yield
(Scheme 2). Conversely, interaction of aldimino L1e-H−L1g-H
with AlMe3 followed by recrystallization from acetonitrile gave
as the major complex [2-(4′-R1-C6H3O)-6-{CMeHN(2″,6″-i-
Pr2C6H3)}C5H3N]AlMe(NCMe) (R1 = H (2a), But (2b), Cl
(2c)) along with trace amounts of isomeric 2a′−c′, respectively
(Scheme 2). Complexes 1 and 2 have been characterized by
NMR and IR spectroscopy and mass spectrometry and give
satisfactory microanalytical data (see the Experimental Section).
In addition, single-crystal X-ray diffraction studies were
performed on 1a−1d, 2a, 2b/2b′, and 2c.
a
Reagents and conditions: (i) 2 AlMe3, toluene, heat; (ii) MeCN,
heat; (iii) MeCN, air/water, heat.
Single crystals of 1a−d suitable for X-ray determination were
grown by slow cooling of acetonitrile solutions of each complex
that had been previously brought to reflux. A perspective view
of representative 1d is shown in Figure 3; selected bond
distances and angles for all four structures are collected in Table
2. In each structure an aluminum center is bound by a dianionic
N,N,O chelate along with a monoanionic methyl group and an
N-bound neutral acetonitrile ligand to complete a five-
coordinate geometry. On the basis of the τ parameter (where
τ = 0 implies ideal square-based pyramidal and τ = 1 ideal
trigonal bipyramidal),14 the geometries of all four complexes
display a modest tendency toward distorted trigonal bipyr-
amidal (τ = 0.55 (1a), 0.57 (1b), 0.55 (1c), 0.52 (1d)). The
presence of the gem-dimethyl group within the tridentate ligand
backbone in 1a−d confirms the successful methyl migration.
Perspective views of 2a and 2b/2b′ are shown in Figures 4
and 5; selected bond distances and angles for 2a, 2b/2b′, and
2c are given in Table 3. For 2c two “non-superimposable”
mirror images (molecules A and B) are present within the
asymmetric unit with slight variations in the relative inclinations
of the N-aryl groups apparent between each molecule. The
Figure 3. Molecular structure of 1d with atom-labeling scheme and
ellipsoids at the 30% probability level. All hydrogen atoms have been
omitted for clarity.
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Table 2. Selected Bond Distances (Å) and Angles (deg) for
1a−d
1a
1b
1c
1d
Bond Distances
Al(1)−O(1)
Al(1)−N(1)
Al(1)−N(2)
Al(1)−N(3)
Al(1)−C(1)
C(13)−C(14)
C(13)−C(27)
C(13)−N(2)
C(28)−C(29)
C(5)−R1
1.800(4) 1.8008(15)
1.995(5) 2.0032(16)
1.829(5) 1.8555(16)
2.083(6) 2.1002(18)
1.966(7) 1.975(2)
1.515(8) 1.544(3)
1.537(8) 1.543(3)
1.487(7) 1.485(2)
1.450(8) 1.452(3)
1.824(2)
2.021(3)
1.845(3)
2.088(3)
1.972(4)
1.533(4)
1.536(4)
1.486(4)
1.452(5)
1.8077(17)
2.0212(19)
1.8502(19)
2.076(2)
1.970(2)
1.541(3)
1.524(3)
1.481(2)
1.449(3)
1.535(2) (R1 = 1.754(4) (R1 = 1.367(2) (R1 =
C(30))
Cl(1))
F(1))
Bond Angles
C(1)−Al(1)−
112.7(2) 109.70(8)
100.4(2) 100.04(8)
117.8(3) 120.77(8)
112.02(13)
100.11(13)
118.80(14)
97.93(13)
113.5(3)
109.02(9)
102.47(8)
120.34(9)
95.79(9)
O(1)
C(1)−Al(1)−
N(1)
Figure 5. Molecular structure of 2b/2b′ with the atom-labeling
scheme and ellipsoids at the 30% probability level. Dotted bonds refer
to a 50:50 disorder of C(14).
C(1)−Al(1)−
N(2)
C(1)−Al(1)−
N(3)
97.2(3)
95.91(8)
Table 3. Selected Bond Distances (Å) and Angles (deg) for
2a, 2b/2b′, and 2c
N(2)−
C(13)−
C(14)
113.5(5) 113.59(15)
113.6(5) 113.10(15)
113.13(15)
2c
N(2)−
C(13)−
C(27)
113.6(3)
113.36(16)
2a
2b/2b′
molecule A
molecule B
Bond Distances
1.7944(12)
2.0059(14)
1.8529(13)
2.0810(15)
1.9741(18)
1.387(3)
Al(1)−O(1)
Al(1)−N(1)
Al(1)−N(2)
Al(1)−N(3)
Al(1)−C(1)
C(13)−C(14)
C(13)−C(14′)
C(27)−C(28)
C(27)−N(3)
1.8203(13)
2.0271(14)
1.8637(15)
2.0882(17)
1.9819(19)
1.517(3)
1.8104(17)
2.025(2)
1.863(2)
2.075(2)
1.961(3)
1.513(4)
1.8117(17)
2.019(2)
1.860(2)
2.073(2)
1.963(3)
1.502(4)
1.3508
1.449(3)
1.135(2)
1.445(2)
1.443(4)
1.132(3)
1.453(4)
1.123(3)
1.129(2)
Bond Angles (deg)
113.65(7)
C(1)−Al(1)−
106.56(8)
104.58(7)
118.32(8)
100.20(8)
155.19(6)
109.15(10)
102.65(10)
118.18(10)
98.60(10)
158.74(9)
116.3(2)
108.54(10)
102.89(10)
120.16(10)
96.80(10)
160.31(9)
116.9(2)
O(1)
C(1)−Al(1)−
100.01(7)
121.12(7)
95.11(7)
N(1)
C(1)−Al(1)−
N(2)
C(1)−Al(1)−
N(3)
N(1)−Al(1)−
164.86(6)
121.34(19)
N(3)
N(2)−C(13)− 114.87(16)
C(14)
Figure 4. Molecular structure of 2a with the atom-labeling scheme and
ellipsoids at the 30% probability level. All hydrogen atoms, apart from
H13, have been omitted for clarity.
disorder between cis (C(14)) and trans (C(14′)) occupations
of the migrated methyl group.
With regard to the N,N,O chelates in both 1 and 2, the
donor−metal distances follow the order Al(1)−N(1)pyridyl
(range 1.995(5)−2.0271(14) Å) > Al(1)−N(2)amide (range
1.829(5)−1.8637(15) Å) > Al(1)−O(1)phenolate (range
1.7944(12)−1.8203(13) Å), reflecting the anionic nature of
the Al−Ophenolate and Al−Namide interactions and consistent with
the greater oxophilicity of aluminum. When 1a (R1 = H) is
compared with 2a (R1 = H), the corresponding three metal−
ligand distances for 2a (2.0217(14), 1.8637(15), 1.8203(13) Å)
fall at the top end of the ranges, while those for 1a (1.995(5),
1.829(5), 1.800(13) Å) are at the bottom end, highlighting the
cumulative electronic or steric effect of exchanging a CMe2 for a
three structures resemble 1, with an aluminum center bound by
a tridentate dianionic N,N,O chelate along with monodentate
acetonitrile and methyl ligands to give five-coordinate geo-
metries best described as distorted square-based pyramidal (τ =
0.33 (2a), 0.43, 0.48 (2c, molecules A and B)) or distorted
trigonal bipyramidal (τ = 0.66 (2b/2b′)). This variation in
structure is likely accommodated by the flexibility of the N,N,O
chelate, which is best exemplified by the N(1)−C(8)−C(7)−
C(2) torsion angles that differ by up to 10° within this series. In
2a,c the migrated methyl groups (C(14)) are located cis to the
aluminum−methyl group, while in 2b/2b′ there is 50:50
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CHMe unit. The para substitution pattern on the phenolate
group (R1) also appears to have a slight influence on the
metal−oxygen distances for 2 (2a (1.8203(13) Å, R1 = H) > 2c
(1.8104 (17), 1.8117(17) Å, R1 = Cl) > 2b (1.7944(12) Å, R1 =
But)), although a related trend is absent for 1. The Al(1)−C(1)
distances for 1 and 2 (range 1.961(3)−1.9819(19) Å) are
typical in comparison with other five-coordinate mononuclear
aluminum monomethyl complexes.15 The Al−N(3)MeCN
distances are in general the longest of the metal−ligand
interactions (range: 2.073(2)−2.1002(18) Å),16 emphasizing
the potential lability of this monodentate ligand during the
intended polymerization process. The use of trimethylalumi-
num to facilitate the migration of a methyl group to an imino
unit of a ligand precursor has been previously reported.17 No
intermolecular interactions of note are apparent.
1
In the H NMR spectra of 1a−d (recorded in C6D6 at
ambient temperature) the peaks for the NCMe2 and Ar-o-
CHMe2 protons are all poorly resolved; attempts to improve
the resolution by recording the spectra at lower and higher
temperatures showed no appreciable differences. In contrast,
the NCHMe and Ar-o-CHMe2 protons in 2a−c are sharp with
clear multiplicities evident. Support for the solid-state structures
being maintained in solution is given by the inequivalency of
the CMeaMeb-N protons in 1a−d, which is accompanied by two
different chemical shifts for the inequivalent Ar-o-CHMe2
protons; broad resonances are evident in each case. In 2a−c
the Ar-o-CHMe2 protons are seen as four independent
doublets, highlighting the restricted rotation about the Al−
Namide bond. Unfortunately, attempts to confirm the cis
configuration of Al−Me relative to the NCHMe methyl (as
seen in the solid-state structures of 2a,c) using NOESY
spectroscopy were unsuccessful. Notably for 2b, peaks
consistent with the trans isomer 2b′ were also detected
(2b:2b′ = 90:10), supporting the mixed occupancy found in the
crystal structure for 2b (Figure 5). Closer inspection of the 1H
NMR spectra for 2a and 2c also reveals trace amounts of their
trans isomers 2a′,c′, respectively. In all cases the Al−Me
protons are seen upfield (between δ −0.05 and −0.18), with
the corresponding methyl carbon resonance occurring as
quadrupolar broadened peaks between δ −8.6 and −14.3 in
their 13C{1H} NMR spectra. The bound acetonitrile for both 1
and 2 can be seen as a singlet for the methyl group at ca. δ 0.7
Figure 6. Molecular structure of 3 with the atom-labeling scheme and
ellipsoids at the 30% probability level. All hydrogen atoms, apart from
H16, have been omitted for clarity.
Table 4. Selected Bond Distances (Å) and Angles (deg) for 3
Bond Distances
Al(1)−O(1)
Al(1)−O(2)
Al(1)−N(1)
Al(1)−N(2)
Al(1)−C(1)
1.9835(15)
1.9056(17)
2.0685(18)
1.8546(18)
1.966(2)
Al(2)−O(1)
Al(2)−O(2)
Al(2)−C(2)
Al(2)−C(3)
C(16)−C(17)
1.9056(17)
1.8239(16)
1.960(2)
1.950(3)
1.534(3)
Bond Angles (deg)
C(1)−Al(1)−O(1)
C(1)−Al(1)−O(2)
C(1)−Al(1)−N(1)
C(1)−Al(1)−N(2)
C(2)−Al(2)−C(3)
98.82(9)
109.28(8)
107.26(8)
116.54(9)
118.86(12)
C(2)−Al(2)−O(1)
C(2)−Al(2)−O(2)
Al(1)−O(1)−Al(2)
Al(1)−O(2)−Al(2)
N(2)−C(16)−C(17)
115.91(9)
109.51(10)
100.72(7)
104.35(7)
114.48(17)
the O atom of the “(MeO)AlMe2” unit in 3 has filled the site
occupied by acetonitrile found in 2b with concomitant
coordination of the phenolate oxygen atom to Al(2). This
apparent substitution has the most significant effect on the
Ophenolate−Al(1) distance with a ca. 0.189 Å elongation in
comparison to the distance in 2b. It is likely that the MeO
group arises from the inadvertent oxygenation of trimethyla-
luminum present in excess during the reaction. Indeed, related
oxygenation of aluminum−alkyls has been previously reported
as a means of forming aluminum alkoxides.9a,18 As with 2a,c the
migrated methyl group on the −CHMe− unit adopts a cis
configuration with respect to the apical methyl group on Al(1).
There are no dominant intermolecular interactions of note.
3. Ligand Effects in 1 and 2 on the Ring-Opening
Polymerization of ε-Caprolactone. Initially the gem-
dimethyl species 1a−d were screened as pre-initiators for the
ring-opening polymerization of ε-CL. Typically, 1a−d were
treated with 1.0 equiv of PhCH2OH in toluene prior to the
addition of the ε-CL (250 equiv) and the commencement of
the run at 30 °C (Scheme 3). The polymerization runs were
monitored by 1H NMR spectroscopy, and the monomer
1
in their H NMR spectra. In the FAB mass spectra of 1 and 2
(using nitrophenyl octyl ether as the matrix), fragmentation
peaks corresponding to the loss of an acetonitrile molecule and
a methyl/acetonitrile are evident in each case. The IR spectra
confirm the presence of coordinated acetonitrile with weak
bands visible between 2299 and 2319 cm−1.
Complexes 1 and 2 proved to be extremely air and moisture
sensitive, and on one occasion during the reaction of L1f-H
with AlMe3 the bimetallic complex [{2-(4′-But-C6H3-2′-O),6-
{CHMeN(2″,6″-i-Pr2C6H3)}C5H3N}AlMe(μ-OMe)AlMe2]
(3) was isolated. A single crystal of 3 was also grown during an
attempted crystallization of 2b from a concentrated acetonitrile
solution. A view of 3 is shown in Figure 6; selected bond
distances and angles are collected in Table 4. The structure of 3
consists of two aluminum centers (Al(1) and Al(2)), each
bridged by a methoxide group and by a phenolate oxygen atom
from the dianionic N,N,O ligand. The N,N,O ligand, in
addition, chelates to afford a more regular square-based
pyramidal geometry at Al(1) (τ = 0.09), which is completed
by an apical methyl group; at Al(2) two methyl groups
complete a distorted-tetrahedral geometry. It is apparent that
1
conversion was also determined by H NMR spectroscopy.
The results of the catalytic screening are collected in Table 5
(entries 1−16). Several points emerge from inspection of the
data. All systems were found to display good activity for the
polymerization of ε-CL, with p-But-containing 1b/PhCH2OH
253
dx.doi.org/10.1021/om301057d | Organometallics 2013, 32, 249−259

Organometallics
Article
a
Scheme 3 .
a
Catalytic evaluation of 1 or 2/PhCH2OH for the ROP of ε-CL.
Table 5. Ring-Opening Polymerization of ε-CL Initiated by
1/PhCH2OH Catalyst Systems
Table 6. Ring-Opening Polymerization of ε-CL Initiated by
2/PhCH2OH Catalyst Systems
a
a
pro-
pro-
initiator
time/ conversion/
M /
wd
initiator
time/ conversion/
M /
wd
b
d
c
b
d
c
entry
(R1)
min
%
Mn(GPC) Mn(calcd)
Mn
entry
(R1)
min
%
Mn(GPC) Mn(calcd)
Mn
1
1a (H)
1a (H)
1a (H)
1a (H)
1b (But)
1b (But)
1b (But)
1b (But)
1c (Cl)
1c (Cl)
1c (Cl)
1c (Cl)
1d (F)
30
60
54
17290
19970
21334
22890
27400
28668
26359
28532
17775
20452
21171
24966
27867
29726
31951
32196
15498
19203
21483
21198
26898
28328
28608
28608
14358
17395
19203
20913
27930
28608
28608
28608
1.23
1.29
1.26
1.30
1.22
1.20
1.26
1.30
1.25
1.22
1.18
1.48
1.15
1.17
1.23
1.25
1
2a (H)
2a (H)
2a (H)
2a (H)
2b (But)
2b (But)
2b (But)
2b (But)
2c (Cl)
2c (Cl)
2c (Cl)
2c (Cl)
30
75
85
88
92
96
25507
29753
29614
30206
33409
34693
34697
34414
22402
25257
27919
28361
21483
24333
25188
26328
27408
28608
28608
28608
17208
20058
22338
25758
1.25
1.20
1.28
1.24
1.23
1.24
1.27
1.32
1.20
1.25
1.20
1.28
2
67
72
2
60
3
90
3
90
4
120
30
74
4
120
30
e
5
94
5
6
60
99
6
60
100
100
100
60
7
90
100
100
50
7
90
8
120
30
8
120
30
9
9
10
11
12
13
14
15
16
60
61
10
11
12
60
70
90
67
90
78
120
30
73
120
90
e
a
98
Conditions: 2 (0.04 mmol), PhCH2OH (0.04 mmol), ε-CL (10
mmol) ([CL]/[Al] = 250), toluene, 50 °C. Estimated by H NMR
spectroscopy; Calculated from [(molecular weight of ε-CL) × [CL]/
[Al] × (conversion)] + (molecular weight of benzyl alcohol). By gel
b
1
1d (F)
60
100
100
100
c
1d (F)
90
d
1d (F)
120
a
permeation chromatography (GPC) in THF vs polystyrene standards;
Conditions: 1 (0.04 mmol), PhCH2OH (0.04 mmol), ε-CL (10
b
values corrected according to the equation Mn(PCL) = 0.56Mn(GPC
1
mmol) ([CL]/[Al] = 250), toluene, 30 °C. Estimated by H NMR
spectroscopy. Calculated from [(molecular weight of ε-CL) × [CL]/
[Al] × (conversion)] + (molecular weight of benzyl alcohol). By gel
permeation chromatography (GPC) in THF vs polystyrene standards;
e
vs polystyrene standards).19 TOF = 480 h−1.
c
d
Nevertheless, similar trends are apparent, with the But-
containing 2b displaying the highest catalytic activity and Cl-
containing 2c the lowest.
values corrected according to the equation Mn(PCL) = 0.56Mn(GPC
vs polystyrene standards).19 TOF = 490 h−1.
e
and p-F-containing 1d/PhCH2OH being the most active (with
TOFs as high as 490 h−1, 30 °C), allowing 100% conversion of
the monomer to poly(CL) after 1 h (entries 6 and 14). The p-
Cl-containing 1c/PhCH2OH and p-H-containing 1a/
PhCH2OH proved less active, reaching only 73% (entry 12)
and 74% (entry 4) after 2 h, respectively.
The molecular weight distributions (Mw/Mn) of the
polyesters are narrow, ranging from 1.15 to 1.48 with the
more active 1b,d showing maximum polydispersities of only
1.30 (1b) and 1.25 (1d), consistent with a single-site active
species. With regard to the less active 1a, an approximately
linear relationship between the number-average molecular
weight (Mn) and the conversion exists which, coupled with
the relatively low molecular weight variations, implies a
controlled polymerization. In the absence of benzyl alcohol
all four aluminum−methyl complexes showed much lower
activity and required higher temperatures (ca. 70 °C) and the
molecular weight of the polymers was much broader,
suggesting partial decomposition of the catalyst.
It is apparent from the above results that the more electron
donating the phenolate p-R1 substituent and/or the 1-amido
CMeR2N(2,6-i-Pr2C6H3) groups (R2 = Me vs H) are, the
higher the conversions. Such electronic effects are similar to
some literature reports,3d,6l,10b,20 with the activity orders herein
being 1b (But)/PhCH2OH ≈ 1d (F)/PhCH2OH > 1a (H)/
PhCH2OH ≈ 1c (Cl) /PhCH2OH > 2b (But)/PhCH2OH > 2a
(H)/PhCH2OH > 2c (Cl)/PhCH2OH. The high activity
displayed by fluorine-substituted 1d (F)/PhCH2OH was
unexpected, as aryl fluorides can display powerful negative
inductive effects. However, an aryl fluoride para substituent can
also exhibit a marked positive electron-donating mesomeric
effect (via p−π F−Ar bonding),21 and it is this property that is
viewed as responsible for the performance characteristics
exhibited by 1d/PhCH2OH. It has been proposed previously
that the overall rate of the polymerization depends on a
combination of factors, including the Lewis acidity of the metal
center and the alkoxide nucleophilicity.6l For these systems it
would appear that the fluoride (1d)- and tert-butyl-containing
systems (1b) enhance the alkoxide nucleophilicity while
maintaining sufficient Lewis acidity to allow monomer binding.
No doubt the rearrangements that ensue in the final ring
opening of the monomer further contribute to the overall
polymerization rate. Interestingly, a number of recent reports
The CH(Me)-containing species 2a−c also proved, in the
presence of benzyl alcohol, to be active catalysts for the ROP of
ε-CL (entries 1−12, Table 6). However, in this case the
polymerizations had to be conducted at 50 °C in order to
achieve conversions comparable with those of 1/PhCH2OH.
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are expressed in hertz (Hz). The electrospray ionization (ESI) and fast
atom bombardment (FAB) mass spectra were recorded using a
micromass Quattro LC mass spectrometer and a Kratos Concept
spectrometer with methanol and nitrophenyl octyl ether as the
matrices, respectively. High-resolution FAB mass spectra were
recorded on a Kratos Concept spectrometer (xenon gas, 7 kV) with
NBA as the matrix. The infrared spectra were recorded on a Perkin-
Elmer Spectrum One FT-IR spectrometer on solid samples. Melting
points (mp) were measured on a Gallenkamp melting point apparatus
(Model MFB-595) in open capillary tubes and were uncorrected.
Elemental analyses were performed on a Carlo Erba CE1108
instrument at the Department of Chemistry, London Metropolitan
University.
Size exclusion chromatography analyses were performed on an
EcoSEC semimicro GPC system from Tosoh equipped with a dual-
flow refractive index detector and a UV detector. The samples were
analyzed in THF at 30 °C using a flow rate of 1 mL min−1. All
polymers were injected at a concentration of 1 mg mL−1 in THF, after
filtration through a 0.45 μm pore size membrane. Separation was
performed with a guard column and three PL gel 5 μm MIXED-C
columns (7 μm, 300 × 7.5 mm). The average molar masses (number-
average molar mass Mn and weight-average molar mass Mw) and the
polydispersity index (PDI = Mw/Mn) were derived from the RI signal
by a calibration curve based on poly(styrene) standards. The
calibration was constructed with narrow molecular weight standards
from 580 to 3053000 g/mol. A third-degree polynomial regression was
applied. WinGPC software was used for data collection and
calculation. The Mn values of the PCLs were corrected with a factor
of 0.56 to account for the difference in hydrodynamic volumes with
polystyrene.19
The MALDI-TOF mass spectrum of PCL was obtained with an ABI
Voyager-DE STR, BioSpectrometry Workstation, Serial No. 4364,
using a nitrogen laser source (337 nm, delay time 500 ns) in reflector
mode with a positive acceleration voltage of 25 kV. The mass range
(Da) analyzed was 1000−7000 Da and 10 shots/spectrum. The matrix
was prepared as follows: a 2:1 mixture of a saturated solution of α-
cyano-4-hydroxycinnamic acid (∼10 mg/mL) (Fluka) in HPLC-
quality acetonitrile (ACN) and a 0.1% solution of trifluoroacetic acid
in ultrapure water. The sample of PCL was prepared as follows: the
sample (3.9 mg) was made to 10 mg/mL in THF and diluted 1/10,
and 1 μL was used for analysis. A 1 μL portion of a 2/1 mixture of a
saturated solution of α-cyano-4-hydroxycinnamic acid in HPLC-
quality ACN and a 0.1% solution of trifluoroacetic acid in ultrapure
water was deposited on the sample plate. After total evaporation, 1 μL
of the polymer in HPLC-quality THF was deposited. The peptide
calibration standard was used for external calibration (Bradykinin Frag
1-7, Bradykinin Frag 2-9, Angiotensin I, Glu-Fibrinopeptide B,
Adrenocorticotropic Hormone Clip 18-39, Insulin Chain B Oxidised
(Sigma)).
The reagents 2,6-diisopropylaniline and trimethylaluminum (2 M
solution in toluene) were purchased from Aldrich Chemical Co. and
used without further purification. Both benzyl alcohol and ε-
caprolactone were purchased from Aldrich and distilled prior to use.
The compounds L1e-H−L1g-H,11a 2-bromo-6-acetylpyridine,27 and
tetrakis(triphenylphosphine)palladium(0)28 were prepared according
to previously reported procedures. 2-Hydroxyphenylboronic acid, 2-
hydroxy-4-chlorophenylboronic acid, 2-hydroxy-4-chlorophenylbor-
onic acid, 2-hydroxy-4-tert-butylphenylboronic acid, and 2-hydroxy-4-
fluorophenylboronic acid were prepared by sequential lithiation of the
corresponding bromophenol in diethyl ether at −78 °C with 2 equiv of
n-BuLi, treatment with triisopropylborate, and hydrolysis with 2 M
HCl.29 All other chemicals were obtained commercially and used
without further purification.
have also highlighted the beneficial effects of introducing
fluorine into the ligand framework of an aluminum-based ROP
initiator.4a,f,6m,22,23
Attempted reaction of 1b with 1.0 equiv of PhCH2OH alone
in C6D6 at ambient temperature afforded a mixture of products,
as evidenced by a series of the peaks in the 1H NMR spectrum
in the region characteristic of the Al-OCH2Ph protons
(between δ 4.9 and 5.4).24,6n This observation would suggest
that the presence of ε-CL is vital for generating the single active
Al−benzyloxide species. Some support that the initiation could
occur through the insertion of ε-CL into an Al−OCH2Ph bond
was achieved by end group analysis (by 1H NMR spectroscopy)
of the PCL obtained using a reduced [CL]/[Al] ratio of 25:1
(with 1b/PhCH2OH as the catalyst system), revealing an
approximately 1:1 ratio between the −CH2OH chain end and
the PhCH2O ester end; nevertheless an activated-monomer
mechanism cannot be ruled out.25 Furthermore, a MALDI-
TOF mass spectrum of a PCL sample generated from 1a/
PhCH2OH showed a series of major peaks separated by a
caprolactone unit (114 g mol−1) corresponding to linear [H-
(CL)n-OBn]·Na+ cations (see Figure S21 in the Supporting
Information). In addition, minor peaks assignable to linear [H-
(CL)n-OBn]·K+, [H-(CL)n-OH]·Na+, and [H-(CL)n-OH]·K+
cations were also observable, the latter OH-capped polymers
being the likely products of hydrolysis under ionization
conditions. No evidence for cyclic isomers could be detected,
suggesting transesterification/back-biting is not significant in
the polymerization. These data also demonstrate that the
aryloxide, associated with the N,N,O ligand, acts purely as a
spectator functionality during the polymerization.
CONCLUDING REMARKS
■
A series of five-coordinate monoalkylaluminum complexes, [2-
(4′-R1C6H3-2′-O)-6-{CMe2N(2″,6″-i-Pr2C6H3)}C5H3N]AlMe-
(NCMe) (R1 = H (1a), But (1b), Cl (1c), F (1d)) and [2-(4′-
R1C6H3-2′-O)-6-{CH(Me))N(2″,6″-i-Pr2C6H3)}C5H3N]-
AlMe(NCMe) (R1 = H (2a), R1 = But (2b), R1 = Cl (2c)),
bearing 2-(phenyl-2-olate)-6-(1-amidoalkyl)pyridine pincer
ligands have been synthesized and characterized on the basis
of spectroscopic and analytical data, while their structures were
determined by single-crystal X-ray diffraction. The ROPs of ε-
CL using 1a−d, in the presence of PhCH2OH, proceeded
efficiently in a controlled fashion with the propagation rates
influenced by the nature of the phenolate R1 substituents.
Notably, the tert-butyl (1b/PhCH2OH) and fluorine containing
systems (1d/PhCH2OH) proved to be the most efficient
catalyst systems and possess good activities (with TOFs ≈ 490
h−1, 30 °C).2h The importance of electron-donating sub-
stituents on the catalytic performance was further highlighted
when the amido CMe2N(2,6-i-Pr2C6H3) arm of the pincer
ligand in 1 was replaced with a less donating CH(Me)N(2,6-i-
Pr2C6H3) unit (in 2), resulting in a less active system.
EXPERIMENTAL SECTION
■
General Procedures. All reactions, unless otherwise stated, were
carried out under an atmosphere of dry, oxygen-free nitrogen, using
standard Schlenk techniques. Solvents were distilled under nitrogen
from appropriate drying agents and degassed prior to use.26 NMR
spectra were recorded on a Bruker DPX 300 spectrometer operating at
300.03 (1H) and 75.4 MHz (13C) or a Bruker DRX400 spectrometer
at 400.13 (1H), 100.61 (13C), and 188 MHz (19F) at ambient
temperature unless otherwise stated; chemical shifts (ppm) are
referred to the residual protic solvent peaks, and coupling constants
Synthesis of L1a-H. Formation of Ketone. An oven-dried Schlenk
flask equipped with a stir bar was evacuated, back-filled with nitrogen,
and charged with 2-bromo-6-acetylpyridine (0.894 g, 4.47 mmol),
Pd(PPh3)4 (0.109 g, 0.094 mmol, 0.02 equiv), toluene (20 mL), and
an aqueous 2 M solution of potassium carbonate (4.45 mL, 8.94
mmol, 2 equiv). The solution was stirred at room temperature for 15
min before the addition of 2-hydroxyphenylboronic acid (0.800 g, 5.80
255
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Organometallics
Article
mmol) in ethanol (11 mL). After it was heated to 90 °C for 42 h, the
mixture was cooled to room temperature and 30% hydrogen peroxide
(0.4 mL) was added. The mixture was stirred at room temperature for
30 min, the product was extracted using diethyl ether (2 × 100 mL),
and the extract was washed with saturated sodium chloride solution (1
× 30 mL) and water (3 × 30 mL) to give an orange organic layer.
Following drying with magnesium sulfate, the volatiles were removed
under reduced pressure to give an orange-brown residue. The catalyst
residues were removed using a short silica gel column employing
dichloromethane/petroleum ether (80/20) as eluting solvent. After
the solvent was removed under reduced pressure, the product 2-
(C6H4-2-OH)-6-(CMeO)-C5H3N was obtained as a pale yellow
gave 2-(4-ClC6H3-2-OH)-6-(CMeO)C5H3N as a pale yellow solid
(84%, 1.496 g). Mp: 98−100 °C. 1H NMR (400 MHz, CDCl3): δ 2.70
(s, 3H, CCH3O), 6.94 (d, J = 8.8, 1H, Ar-H), 7.23 (dd, J = 8.8, 2.4,
1H, Ar-H), 7.73−7.74 (d, J = 2.4, 1H, Ar-H), 7.95−8.02 (m, 3H, Py-
H), 13.64 (s, 1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 25.1
(CMeO), 118.2 (C), 119.0, 119.4, 121.8, 123.0, 125.1, 130.8, 138.0
(CH), 149.0, 154.9, 157.1 (C), 196.0 (CMeO). IR (cm−1): 1698
(CO), 1586 (CNpyridine). ESIMS: m/z 248 [M + H]+. HRMS
(FAB): calcd for C13H11NO2Cl [M + H]+ 248.0478, found 248.0473.
Formation of Imine. Following a procedure similar to that
described for L1a-H, using 2-(4-ClC6H3-2-OH)-6-(CMeO)C5H3N
(1.38 g, 5.58 mmol), 2,6-diisopropylaniline (1.480 g, 8.37 mmol, 1.5
equiv), and methanol (10 mL) gave L1c-H as a yellow solid (1.290 g,
57%). Mp: 189−191 °C. 1H NMR (400 MHz, CDCl3): δ 1.08 (d, J =
7.0, 12H, CH(CH3)2), 2.17 (s, 3H, CCH3N), 2.62 (sept, J = 6.9,
2H, CH(CH3)2), 6.91 (d, J = 8.8, 1H, Ar-H), 7.03−7.07 (m, 1H, Ar-
H), 7.10 (s, 1H, Ar-H), 7.11 (dd, 1H, Ar-H, J 8.8, J 2.0), 7.22 (dd, J =
8.8, 2.6, 1H, Ar-H), 7.83 (d, J = 2.6, 1H, Ar-H), 7.92−7.07 (m, 2H, Py-
H), 8.28 (dd, J = 6.1, 2.6, 1H, Py-H), 14.17 (s, 1H, OH). 13C{1H}
NMR (100 MHz, CDCl3): δ 17.3 (CCH3N), 22.9, 22.2 (CH-
(CH3)2), 28.4 (CH(CH3)2), 119.8, 119.9, 112.0 (CH), 120.4 (C),
123.1, 123.0, (CH), 124.0 (C), 126.0, 131.4 (CH), 135.6 (C), 138.7
(CH), 145.9, 153.3, 155.6, 158.2 (C), 164.5 (CMeN). IR (cm−1):
1641 (CNimine), 1586 (CNpyridine). ESIMS: m/z 407 [M + H]+.
HRMS (FAB): calcd for C25H28NO2Cl [M + H]+ 407.1890, found
407.1895.
Synthesis of L1d-H. Formation of Ketone. Following a procedure
similar to that described for L1a-H, using 2-bromo-6-acetylpyridine
(1.57 g, 7.85 mmol), 2-hydroxy-4-fluorophenylboronic acid (1.50 g,
9.62 mmol), tetrakis(triphenylphosphine)palladium(0) (0.362 g, 0.314
mmol), and aqueous 2 M potassium carbonate (7.85 mL, 15.70 mmol)
gave 2-(4-FC6H3-2-OH)-6-(CMeO)C5H3N as a pale yellow solid
(85%, 1.52 g). Mp: 188−189 °C. 1H NMR (400 MHz, CDCl3): δ 2.76
(s, 3H, CMeO), 6.92−6.99 (m, 1H, Ar-H), 7.0−7.07 (m, 1H, Ar-
H), 7.45 (dd, J = 9.7, 2.9, 1H, Ar-H), 8.03−8.13 (m, 3H, Py-H), 13.37
(s, 1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 25.1 (CMe=O),
111.1 (d, JCF = 24, CH), 117.4 (d, JCF = 7, C), 118.1 (d, JCF = 24, CH),
118.6 (d, JCF = 7, CH), 119.3, 121.9, 138.0 (CH), 149.2, 154.6 (C),
154.9 (d, JCF = 236, CF), 155.2 (C), 196.2 (CMeO). 19F{1H}
(NMR (188 MHz, CDCl3): δ −124.54 (Ar-F). IR (cm−1): 1698 (C
O), 1590 (CNpyridine). ESIMS: m/z 232 [M + H]+. HRMS (FAB):
calcd for C13H11NO2F [M + H]+ 232.0774, found 232.0766.
1
solid (0.762 g, 80%). H NMR (300 MHz, CDCl3): δ 2.66 (s, 3H,
CMeO), 6.84−6.98 (m, 1H, Ar-H), 6.96 (dd, J = 8.1, 1.2, 1H, Ar-
H), 7.24−7.29 (m, 1H, Ar-H), 7.73 (dd, J = 8.1, 1.1, 1H, Ar-H), 7.88−
7.90 (m, 2H, Py-H), 8.00 (dd, J = 6.4, 2.6, 1H, Py-H), 13.42 (s, 1H,
OH). ESIMS: m/z 214 [M + H]+. These data are consistent with
those previously reported.11a
Formation of Imine. To a solution of 2-(C6H4-2-OH)-6-(CMe
O)-C5H3N (0.716 g, 3.36 mmol) in methanol (10 mL) was added 2,6-
diisopropylaniline (0.892 g, 5.04 mmol, 1.5 equiv). The solution was
stirred at reflux for 5 min before the addition of 2 drops of formic acid.
After it was refluxed for 2 days, the reaction mixture was cooled to
room temperature and the suspension filtered, washed with cold
methanol, and dried to give L1a-H as a yellow solid (1.00 g, 80%). 1H
NMR (300 MHz, CDCl3): δ 1.08 (d, J = 6.9, 12H, CH(Me)2), 2.18 (s,
3H, CMeN), 2.65 (sept, J = 6.9, 2H, CH(Me)2), 6.88 (dt, J = 8.8,
0.7, 1H, Ar-H), 6.97 (dd, J = 6.5, 0.8, 1H, Ar-H), 7.11−7.15 (m, 3H,
Ar-H), 7.28 (dt, J = 6.6, 1.0, 1H, Ar-H), 7.79 (dd, J = 6.9, 1.4, 1H, Ar-
H), 7.85−8.00 (m, 2H, Py-H), 8.23 (dd, J = 7.1, 1.2, 1H, Py-H), 14.2
(s, OH). ESIMS: m/z 373 [M + H]+. These data are consistent with
those previously reported.11a
Synthesis of L1b-H. Formation of Ketone. Following a procedure
similar to that described for L1a-H, using 2-bromo-6-acetylpyridine
(1.31 g, 6.55 mmol), 2-hydroxy-4-tert-butylphenylboronic acid (1.50 g,
7.73 mmol), tetrakis(triphenylphosphine)palladium(0) (0.302 g, 0.262
mmol), and aqueous 2 M potassium carbonate (6.55 mL, 13.10 mmol)
gave 2-(4-ButC6H3-2-OH)-6-(CMeO)C5H3N as a pale yellow solid
1
(1.146 g, 65%). Mp: 111−113 °C. H NMR (400 MHz, CDCl3): δ
1.30 (s, 9H, C(CH3)3), 2.70 (s, 3H, CCH3O), 6.95 (d, J = 8.5, 1H,
Ar-H), 7.35 (dd, J = 8.5, 2.3, 1H, Ar-H), 7.75 (d, J = 2.3, 1H, Ar-H),
7.89−7.96 (m, 2H, Py-H), 8.07 (dd, J = 7.3, 1.8, 1H, Py-H), 13.38 (s,
1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 25.1 (CMeO), 30.5
(C(CH3)3), 33.2 (C(CH3)3), 116.5 (C), 117.1, 118.6, 121.8, 121.9,
128.6, 137.7 (CH), 140.9, 149.1, 156.1, 156.7 (C), 196.6 (CMeO).
IR (cm−1): 1702 (CO), 1588 (CNpyridine). ESIMS: m/z 270 [M +
H]+. HRMS (FAB): calcd for C17H20NO [M + H]+ 270.1505, found
270.1505.
Formation of Imine. Following a procedure similar to that
described for L1a-H, using 2-(4-FC6H3-2-OH)-6-(CMeO)C5H3N
(1.31 g, 5.67 mmol), 2,6-diisopropylaniline (1.51 g, 8.51 mmol, 1.5
equiv), and methanol (10 mL) gave L1d-H as a yellow solid (0.994 g,
1
45%). Mp: 85−89 °C. H NMR (400 MHz, CDCl3): δ 1.07 (d, J =
6.9, 12H, CH(CH3)2,), 2.17 (s, 3H, NCCH3), 2.58−2.69 (sept, J =
6.9, 2H, CH(CH3)2), 6.89−6.92 (m, 1H, Ar-H), 6.95−6.99 (m, 1H,
Ar-H), 7.02−7.06 (m, 1H, Ar-H), 7.09 (s, 1H, Ar-H), 7.10 (d, J = 8.4,
1H, Ar-H), 7.45 (dd, J = 9.8, 3.0, 1H, Ar-H), 7.86 (d, J = 8.0, 1H, Py-
H), 7.92 (t, J = 7.8, 1H, Py-H), 8.27 (dd, J = 7.6, 0.8, 1H, Py-H), 13.98
(s, 1H, OH). 13C{1H} NMR (100 MHz, CDCl3): δ 16.3 (NCCH3),
21.8, 22.2, 27.4, 111.1 (d, JCF = 23.5, CH), 117.6 (d, JCF = 23.5, CH),
117.8 (d, JCF 7.5, C), 118.3 (d, JCF = 7.5, CH), 118.9 (CH), 122.1
(CH), 123.0 (CH), 134.6 (C), 137.6 (CH), 144.9 (C), 152.3 (C),
154.8 (d, JCF = 234.0, C), 154.7 (C), 154.8 (C), 156.0 (C), 163.5 (C
N). 19F{1H} NMR (188 MHz, CDCl3): δ −125.1 (Ar-F). IR (cm−1):
1645 (CNimine), 1587 (CNpyridine). ESIMS: m/z 391 [M + H]+.
HRMS (FAB): calcd for C25H28NO2F [M + H]+ 391.2186, found
391.2190.
Formation of Imine. Following a procedure similar to that
described for L1a-H, using 2-(4-ButC6H3-2-OH)-6-(CMe
O)C5H3N (0.985 g, 3.66 mmol), 2,6-diisopropylaniline (0.927 g,
5.49 mmol, 1.5 equiv), and methanol (10 mL) gave L1b-H as a yellow
solid (1.013 g, 65%). Mp: 120−122 °C. 1H NMR (400 MHz, CDCl3):
δ 1.08 (d, J = 6.7, 12H, CH(CH3)2), 1.31 (s, 9H, C(CH3)3), 2.18 (s,
3H, CCH3N), 2.65 (sept, J = 6.7, 2H, CH(CH3)2), 6.92 (d, J = 8.8,
1H, Ar-H), 7.02−7.06 (m, 1H, Ar-H), 7.10 (s, 1H, Ar-H), 7.11 (d, J =
1.6, 1H, Ar-H), 7.34 (dd, J = 8.8, 2.6, 1H, Ar-H), 7.77 (d, J = 2.4, 1H,
Ar-H), 7.92 (t, J = 7.6, 1H, Py-H), 7.98 (d, J = 7.6, 1H, Py-H), 8.22
(dd, J = 7.6, 0.8, 1H, Py-H), 13.85 (s, 1H, OH). 13C{1H} NMR (100
MHz, CDCl3): δ 17.4 (CMeN), 21.9, 22.2 (CH(CH3)2), 27.3
(CH(CH3)2), 30.5 (C(CH3)3), 33.2 (C(CH3)3), 116.9, 118.2, 120.2,
121.8, 122.0, 122.9, 128.1 (CH), 134.7, 134.8 (C), 137.3 (CH), 140.7,
145.0, 152.2, 156.1, 156.3 (C), 163.9 (CMeN). IR (cm−1): 1640
(CNimine), 1566 (CNpyridine). ESIMS: m/z 429 [M + H]+. HRMS
(FAB): calcd for C29H37NO2 [M + H]+ 429.2906, found 429.2917.
Synthesis of L1c-H. Formation of Ketone. Following a procedure
similar to that described for L1a-H, using 2-bromo-6-acetylpyridine
(1.44 g, 7.23 mmol), 2-hydroxy-4-chlorophenylboronic acid (1.50 g,
8.72 mmol), tetrakis(triphenylphosphine)palladium(0) (0.334 g, 0.289
mmol), and aqueous 2 M potassium carbonate (7.20 mL, 14.40 mmol)
Synthesis of Complexes 1a−d. Preparation of 1a. An oven-
dried Schlenk vessel equipped with a magnetic stir bar was evacuated
and back-filled with nitrogen. The vessel was charged with L1a-H
(0.220 g, 0.591 mmol) and dissolved in dry toluene (15 mL).
Trimethylaluminum (0.60 mL, 1.18 mmol, 2 equiv) was introduced
and the reaction mixture stirred and heated to 110 °C overnight. After
the mixture was cooled to room temperature, all volatiles were
removed under reduced pressure. Acetonitrile (15 mL) was
introduced, the suspension was heated until dissolution, and the
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Article
resulting solution was transferred by cannular filtration into a second
mixture stirred and heated to 110 °C overnight. After the mixture was
cooled to room temperature, all volatiles were removed under reduced
pressure. Acetonitrile (15 mL) was introduced, the suspension was
heated until dissolution, and the resulting solution was transferred by
cannular filtration into a second oven-dried Schlenk vessel. On
standing at room temperature pale yellow crystals of 2a formed (0.140
oven-dried Schlenk vessel. On standing at room temperature pale
1
yellow crystals of 1a formed (0.189 g, 68%). H NMR (300 MHz,
C6D6): δ −0.17 (s, 3H, Al-CH3), 0.67 (s, 3H, Al-NCCH3), 1.37−1.55
(m, 12H, CH(CH3)2), 1.63 (br s, 3H, NC(CH3)2), 1.88 (br s, 3H,
NC(CH3)2), 3.80 (br s, 1H, Ar-CH(CH3)2), 4.47 (br s, 1H, Ar-
CH(CH3)2), 6.93 (dd, J = 8.0, 0.8, 1H, Ar-H), 7.00 (t, J = 8.2, 1H, Ar-
H), 7.20 (d, J = 8.0, 1H, Ar-H), 7.25−7.31 (m, 4H, Ar-H), 7.49 (t, J =
8.0, 1H, Py-H), 7.58 (dd, J = 8.0, 1.2, 1H, Py-H), 7.74 (dd, J = 8.0, 1.8,
1H, Py-H). 13C{1H} NMR (100 MHz, CDCl3): δ −9.2 (Al-CH3), 1.2
(Al-NCCH3), 23.5, 24.3, 25.9, 26.2, 26.9, 27.5, 28.3, 34.9, 61.5
(NC(CH3)2), 115.3 (C), 115.9 (CH), 117.3 (CH), 118.6 (CH), 122.5
(C), 123.4 (CH), 123.6 (CH), 123.9 (CH), 127.1 (C), 127.9 (CH),
128.0 (CH), 137.7 (C), 139.1 (C), 146.6 (C), 154.1 (C), 163.6 (C),
169.1 (C). FABMS: m/z 429 [M − NCMe]+, 413 [M − NCMe −
Me]+. IR (cm−1): 2322 ν(CN)MeCN. Anal. Calcd for C29H36AlN3O: C,
74.17; H, 7.73; N, 8.95. Found: C, 74.55; H, 7.81; N, 9.01.
1
g, 52%). H NMR (400 MHz, C6D6): δ −0.06 (s, 3H, Al-CH3), 0.87
(m, 6H, Al-NCCH3/Ar-CHMeaMea′), 1.44 (d, J = 6.5, 3H, Ar-
CHMeaMea′), 1.53 (d, J = 6.6, 3H, Ar-CHMebMeb′), 1.64 (d, J = 7.0,
3H, NCHCH3), 1.86 (d, J = 6.6, 3H, Ar-CHMebMeb′), 4.09 (sept, J =
6.7, 1H, Ar-CHMe2), 4.34 (sept, J = 6.7, 1H, Ar-CHMe2), 5.26 (q, J =
6.5, 1H, NCHCH3), 6.94−7.03 (m, 2H, Ar-H/Py-H), 7.09−7.20 (m,
2H, Py-H/Ar-H), 7.25 (dd, J = 7.4,1.9, 1H, Py-H), 7.32−7.42 (m, 3H,
Ar-H/Py-H), 7.52 (dd, J = 7.7,1.8, 1H, Ar-H), 7.85 (d, J = 7.8, 1H, Ar-
H). IR (cm−1): 2327 ν(CN)MeCN. FABMS: m/z 415 [M − NCMe]+,
374 [M − NCMe − Me]+. Anal. Calcd for C28H34AlN3O: C, 73.82; H,
7.52; N, 9.22. Found: C, 74.01; H, 7.62; N, 9.15.
Preparation of 1b. The synthesis was carried out by using the same
Preparation of 2b. The synthesis was carried out by using the same
procedure as for 1a, except L1b-H (0.253 g, 0.591 mmol) was used,
forming 1b as pale yellow crystals (0.116 g, 40%). H NMR (400
procedure as for 2a, except L1f-H (0.245 g, 0.591 mmol) was used,
forming 2b as pale yellow crystals (0.121 g, 40%). H NMR (400
1
1
MHz, C6D6): −0.07 (s, 3H, Al-CH3), 0.60 (s, 3H, Al-NCCH3), 1.42−
1.62 (m, 12H, CH(CH3)2), 1.63 (s, 9H, C(CH3)3), 1.74 (br s, 3H,
NC(CH3)2), 1.99 (br s, 3H, NC(CH3)2), 3.88 (br s, 1H, Ar-
CH(CH3)2), 4.61 (br s, 1H, Ar-CH(CH3)2), 7.07 (dd, J = 7.8, 0.8, 1H,
Ar-H), 7.36 (d, J = 7.8, 1H, Ar-H), 7.39 (d, J = 7.0, 1H, Py-H), 7.42−
7.51 (m, 5H, ArH/PyH), 8.03 (s, 1H, Ar-H). 13C{1H} NMR (100
MHz, CDCl3): δ −8.6 (Al−CH3), 1.4 (Al-NCCH3), 24.0, 25.9, 26.2,
26.5, 27.3, 27.9, 28.7, 31.6, 34.3, 35.5, 60.1 (N-C(CH3)2), 115.7 (C),
118.9 (C), 121.9 (C), 123.5, 123.6, 123.7, 130.6, 139.4, 139.5 (CH),
MHz, C6D6): δ −0.05 (s, 3H, Al-CH3), 0.81 (s, 3H, Al-NCCH3), 0.83
(d, J = 7.1, 3H, Ar-CHMeaMea′), 1.44 (d, J = 6.9, 3H, Ar-CHMeaMea′),
1.50 (s, 9H, C(CH3)3), 1.52 (d, J = 6.8, 3H, Ar-CHMebMeb′), 1.54 (d,
J = 5.6, 3H, NCHCH3), 1.87 (d, J = 6.4, 3H, Ar-CHMebMeb′), 4.11
(sept, J = 6.9, 1H, Ar-CHMe2), 4.30 (sept, J = 7.1, 1H, Ar-CHMe2),
5.32 (q, J = 6.5, 1H, NCHCH3), 6.95−7.04 (m, 3H, Ar-H/Py-H), 7.11
(dd, J = 6.4, 2.3, 1H, Py-H/Ar-H), 7.30−7.41 (m, 2H, Ar-H/Py-H),
7.48 (d, J = 7.8, 1H, Ar-H), 7.62 (d, J = 2.4, 1H, Ar-H), 7.77 (d, J =
8.0, 1H, Ar-H). IR (cm−1): 2296 ν(CN)MeCN. FABMS: m/z 471 [M −
NCMe]+, 455 [M − NCMe − Me]+. Anal. Calcd for C32H42AlN3O: C,
75.11; H, 8.27; N, 8.21. Found: C, 75.32; H, 8.41; N, 8.03.
147.4, 151.2, 154.09, 161.9, 169.6 (C). IR (cm−1): 2324 ν(CN)MeCN
.
FABMS: m/z 484 [M − NCMe]+, 469 [M − NCMe − Me]+. Anal.
Calcd for C33H44AlN3O: C, 75.39; H, 8.44; N, 7.99. Found: C, 75.27;
H, 8.40; N, 7.87.
Preparation of 2c. The synthesis was carried out by using the same
procedure as for 2a, except L1g-H (0.232 g, 0.591 mmol) was used,
1
Preparation of 1c. The synthesis was carried out by using the same
forming 2c as pale yellow crystals (0.191 g, 66%). H NMR (400
procedure as for 1a, except L1c-H (0.241 g, 0.591 mmol) was used,
forming 1c as pale yellow crystals (0.260 g, 86%). H NMR (400
MHz, C6D6): δ −0.10 (s, 3H, Al-CH3), 0.85 (d, J = 7.8, 3H, Ar-
CHMeaMea′), 0.89 (s, 3H, Al-NCCH3), 1.42 (d, J = 7.8, 3H, Ar-
CHMeaMea′), 1.50 (d, J = 7.7, 3H, NCHCH3), 1.58 (d, J = 7.7, 3H, Ar-
CHMebMeb′), 1.77 (d, J = 7.7, 3H, Ar-CHMebMeb′), 4.05 (sept, J =
7.6, 1H, Ar-CHMe2), 4.29 (sept, J = 7.7, 1H, Ar-CHMe2), 5.25 (q, J =
7.6, 1H, NCHCH3), 6.98 (d, J = 7.2, 1H, Ar-H/Py-H), 7.00 (d, J = 7.3,
1H, Py-H/Ar-H), 7.09 (d, J = 6.8, 1H, Ar-H/Py-H), 7.18−7.23 (m,
2H, Ar-H), 7.28 (t, J = 6.9, 1H, Ar-H/Py-H), 7.35 (d, J = 6.7, 1H, Ar-
H), 7.65 (d, J = 1.8, 1H, Ar-H), 7.75 (d, J = 6.8, 1H, Ar-H). IR (cm−1):
2292 ν(CN)MeCN. FABMS: m/z 449 [M − NCMe]+, 433 [M −
NCMe − Me]+. Anal. Calcd for C28H33AlClN3O: C, 68.63; H, 6.79; N,
8.58. Found: C, 68.64; H, 6.62; N, 8.37.
Ring-Opening Polymerization (ROP) of ε-CL. Typical polymer-
ization procedures (Tables 5 and 6) are as follows. An oven-dried
Schlenk vessel equipped with a stirrer bar was evacuated and loaded in
the glovebox. The aluminum pro-initiator 1 or 2 (0.04 mmol) was
introduced, followed by 15 mL of a 0.0027 M solution of benzyl
alcohol (0.04 mmol, 1 equiv) in toluene. The mixture was stirred for
10 min at room temperature and then placed in an oil bath preheated
to the desired temperature. CL (1.1 mL, 10.0 mmol, 250 equiv) was
added and the mixture stirred for the designated time period. A small
aliquot (0.2 mL) of the reaction mixture was removed at selected time
intervals and treated with a few drops of methanol and the residue
analyzed by 1H NMR spectroscopy (to determine monomer
conversion) and by GPC (to determine Mn and Mw/Mn).
Crystallographic Studies. Data for L1d-H, L1g-H, 1a−d, 2a, 2b/
2b′, 2c, and 3 were collected on a Bruker APEX 2000 CCD
diffractometer. Details of data collection, refinement and crystal data
are given in Table S1 in the Supporting Information. The data were
corrected for Lorentz and polarization effects and empirical absorption
corrections applied. Structure solution by direct methods and structure
refinement based on full-matrix least squares on F2 employed
SHELXTL version 6.10.30 Hydrogen atoms were included in
calculated positions (C−H = 0.95−1.00 Å) riding on the bonded
atom with isotropic displacement parameters set to 1.5[Ueq(C)] for
methyl H atoms and 1.2[Ueq(C)] for all other H atoms. All non-H
1
MHz, C6D6): δ −0.12 (s, 3H, Al-CH3), 0.81 (s, 3H, Al-NCCH3),
1.42−1.62 (m, 12H, CH(CH3)2), 1.72 (br s, 3H, NC(CH3)2), 1.96 (br
s, 3H, NC(CH3)2), 3.84 (br s, 1H, Ar-CH(CH3)2), 4.56 (br s, 1H, Ar-
CH(CH3)2), 6.99 (d, J = 8.0, 1H, Ar-H), 7.04 (d, J = 7.6, 1H, Ar-H),
7.28 (app q, J = 7.0, 2H, Py-H/Ar-H), 7.38−7.50 (m, 4H, ArH/PyH),
7.89 (d, J = 2.9, 1H, Ar-H). 13C{1H} NMR (100 MHz, CDCl3, limited
solubility): δ −14.3 (Al-CH3), −1.39 (Al-NCCH3), 60.1 (N-
C(CH3)2), 114.4, 115.3, 117.4, 122.1, 122.6, 125.5, 131.0, 137.8. IR
(cm−1): 2322 ν(CN)MeCN. FABMS: m/z 463 [M − NCMe]+, 445 [M
− NCMe − Me]+. Anal. Calcd for C29H35AlClN3O: C, 69.10; H, 7.00;
N, 8.34. Found: C, 68.97; H, 7.05; N, 8.28.
Preparation of 1d. The synthesis was carried out by using the same
procedure as for 1a, except L1d-H (0.231 g, 0.591 mmol) was used,
1
forming 1d as orange-yellow crystals (0.210 g, 73%). H NMR (400
MHz, C6D6): δ −0.18 (s, br, 3H, Al-Me), 0.78 (s, br, 3H, NCMe),
1.30−1.51 (m, 12H, CHMe2), 1.68 (br s, 3H, NC(CH3)2), 1.90 (br s,
3H, NC(CH3)2), 3.70 (br s, 1H, CHMe2), 4.46 (br s, 1H, CHMe2),
6.91 (d, J = 8.0, 2H, Ar-H), 7.14−7.30 (m, 4H, Ar-H/Py-H), 7.33−
7.38 (m, 1H, Ar-H/Py-H), 7.39−7.44 (m, 1H, Ar-H/Py-H), 7.53 (dd,
J = 9.6, 3.0, 1H, Ar-H/Py-H). 13C{1H} NMR (100 MHz, CDCl3): δ
−9.3 (Al-CH3), 1.2 (Al-NCCH3), 23.5, 24.2, 25.9, 26.1, 26.9, 27.5,
28.3, 34.8, 61.7 (N-C(CH3)2), 112.4 (d, JCF = 23, CH), 115.4, 118.7
(C), 119.5 (d, JCF = 24, CH), 122.1 (d, JCF = 8, C), 123.4, 124.0 (CH),
124.3 (d, JCF = 8, CH), 128.0, 128.1, 128.3 (CH), 129.1 (C), 139.2
(CH), 146.5, 152.8 (C), 156.1 (d, JCF = 234, C), 159.8, 169.1 (C).
19F{1H} NMR (188 MHz, CDCl3): δ −127.9 (Ar-F). IR (cm−1): 2323
ν(CN)MeCN. FABMS: m/z 447 [M − NCMe]+, 432 [M − NCMe −
Me]+. Anal. Calcd for C29H35AlFN3O: C, 71.44; H, 7.24; N, 8.62.
Found: C, 71.21; H, 7.37; N, 8.50.
Synthesis of 2a−c. Preparation of 2a. An oven-dried Schlenk
vessel equipped with a magnetic stir bar was evacuated and back-filled
with nitrogen. The vessel was charged with L1e-H (0.212 g, 0.591
mmol) and dissolved in dry toluene (15 mL). Trimethylaluminum
(0.60 mL, 1.18 mmol, 2 equiv) was introduced and the reaction
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Organometallics
Article
atoms were refined with anisotropic displacement parameters. 2b/2b′
shows disorder at C(14) and has been split (50:50), while the tert-
butyl methyl groups are also disordered. CCDC reference numbers
905972−905981.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Table S1 and CIF files giving X-ray crystallographic data for
L1d-H, L1g-H, 1a−d, 2a, 2b/2b′, 2c, and 3 along with figures
giving representative 1H, 13C{1H}, and 19F{1H} NMR,
MALDI-ToF, and FAB mass spectra. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Tel: +44(0)116 2522096. Fax: +44 (0)116 2523789. E-mail:
gas8@le.ac.uk.
Notes
(7) Shen, M.; Zhang, W.; Nomura, K.; Sun, W.-H. Dalton Trans.
2009, 9000.
The authors declare no competing financial interest.
(8) Trofimoff, L.; Aida, T.; Inoue, S. Chem. Lett. 1987, 991.
(9) (a) Lewinski, J.; Horeglad, P.; Dranka, M.; Justyniak, I. Inorg.
Chem. 2004, 43, 5789. (b) Ma, W.-A.; Wang, Z.-X. Dalton Trans. 2011,
40, 1778. (c) Peng, K.-F.; Chen, C.-T. Dalton Trans. 2009, 9800.
(10) (a) Wheaton, C. A.; Hayes, P. G.; Ireland, B. J. Dalton Trans
2009, 4832. (b) Williams, C. K.; Breyfogle, L. E.; Choi, S. K.; Nam,
W.; Young, V. G., Jr.; Hillmyer, M. A.; Tolman, W. B. J. Am. Chem. Soc.
2003, 125, 11350. (c) Huang, Y.; Hung, W.-C.; Liao, M.-Y.; Tsai, T.-
E.; Peng, Y.-L.; Lin, C.-C. J. Polym. Sci., Part A: Polym. Chem. 2009, 47,
2318. (d) Dove, A. P.; Gibson, V. C.; Marshall, E. L.; White, A. J. P.;
Williams, D. J. Dalton Trans. 2004, 570. (e) Hung, W.-C.; Huang, Y.;
Lin, C.-C. J. Polym. Sci., Part A: Polym. Chem. 2008, 46, 6466.
(f) Zheng, Z.; Zhao, G.; Fablet, R.; Bouyahyi, M.; Thomas, C. M.;
Roisnel, T.; Casagrande, O., Jr.; Carpentier, J.-F. New J. Chem. 2008,
32, 2279. (g) Wu, J.-C.; Huang, B.-H.; Hsueh, M.-L.; Lai, S.-L.; Lin, C.-
C. Polymer 2005, 46, 9784. (h) Chen, H.-Y.; Tang, H.-Y.; Lin, C.-C.
Macromolecules 2006, 39, 3745.
(11) (a) Davies, C. J.; Gregory, A.; Griffith, P.; Perkins, T.; Singh, K.;
Solan, G. A. Tetrahedron 2008, 64, 9857. (b) Adeyi, O.; Cross, W. B.;
Godfrey, L.; Hope, E. G.; Forrest, G.; Singh, K.; Singh, A.; Solan, G.
A.; Wang, Y.-N., Dalton Trans. 2012, manuscript in preparation.
(12) (a) Giesbrecht, G. R.; Davies, C. J.; Solan, G. A. (ExxonMobil
Chemical), WO 2009082556, 2009. (b) Davies, C. J.; Solan, G. A.
(ExxonMobil Chemical), WO 2004106390, 2004.
ACKNOWLEDGMENTS
■
We thank the EPSRC (to C.J.D. and A.P.A.) and the University
of Leicester for financial assistance. We are also grateful to
Johnson-Matthey for the loan of the palladium salts.
REFERENCES
■
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