Amidinate aluminium complexes: Synthesis, characterization and ring-opening polymerization of rac-lactide

Article abstract of DOI:10.1039/c0dt00272k

A series of aluminium alkyl complexes {PhC(NR′)(NR′′)} AlR₂ (4a-n, R′ = 2,6-ⁱPr₂C ₆H₃, 2,6-Me₂C₆H₃; R′′ = aryl groups with various ortho-, para- or meta-substituents, tert-butyl; R = methyl, ethyl) bearing non-symmetrically N-substituted benzamidinate ligands were synthesized via the reaction of trialkylaluminium and the corresponding benzamidine proligands. Complex 5 bearing symmetric amidinate ligand was also obtained for comparison purposes. The X-ray diffraction studies of complexes 4b, 4c and 5 show in each case a distorted tetrahedral geometry around the aluminium center. All the amidinate aluminium complexes were found to catalyze the ring-opening polymerization (ROP) of rac-lactide with moderate activities. The steric and electronic characteristics of the ancillary ligands have a significant influence on the polymerization performance of the corresponding aluminium complexes. The introduction of electron-withdrawing substituents at the ortho-positions of N-phenyl ring of the ligands resulted in an obvious increase in catalytic activity. Complex 4b showed the highest activity among the investigated aluminium complexes due to the high electrophilicity of the metal center induced by the ortho-chloro substituents on the phenyl ring. The existence of ortho-substituents of small steric bulkiness is also beneficial for the increase of activity of these catalysts. However, further increase of steric hindrance of the ligands by introducing bulky ortho-substituents onto the phenyl moieties resulted in a decrease of activity and an increase in the isotactic bias of the obtained polylactides. The broad molecular weight distributions (PDI = 1.13-2.02) of the polymer samples indicated that the ROP of rac-lactide initiated by these complexes was not well-controlled.

Full text of DOI:10.1039/c0dt00272k

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PAPER
www.rsc.org/dalton | Dalton Transactions
Amidinate aluminium complexes: synthesis, characterization and ring-opening
polymerization of rac-lactide†
Feng Qian, Keyin Liu and Haiyan Ma*
Received 7th April 2010, Accepted 28th May 2010
First published as an Advance Article on the web 27th July 2010
DOI: 10.1039/c0dt00272k
A series of aluminium alkyl complexes {PhC(NR¢)(NR¢¢)}AlR2 (4a–n, R¢ = 2,6-iPr2C6H3,
2,6-Me2C6H3; R¢¢ = aryl groups with various ortho-, para- or meta-substituents, tert-butyl; R = methyl,
ethyl) bearing non-symmetrically N-substituted benzamidinate ligands were synthesized via the
reaction of trialkylaluminium and the corresponding benzamidine proligands. Complex 5 bearing
symmetric amidinate ligand was also obtained for comparison purposes. The X-ray diffraction studies
of complexes 4b, 4c and 5 show in each case a distorted tetrahedral geometry around the aluminium
center. All the amidinate aluminium complexes were found to catalyze the ring-opening polymerization
(ROP) of rac-lactide with moderate activities. The steric and electronic characteristics of the ancillary
ligands have a significant influence on the polymerization performance of the corresponding aluminium
complexes. The introduction of electron-withdrawing substituents at the ortho-positions of N-phenyl
ring of the ligands resulted in an obvious increase in catalytic activity. Complex 4b showed the highest
activity among the investigated aluminium complexes due to the high electrophilicity of the metal
center induced by the ortho-chloro substituents on the phenyl ring. The existence of ortho-substituents
of small steric bulkiness is also beneficial for the increase of activity of these catalysts. However, further
increase of steric hindrance of the ligands by introducing bulky ortho-substituents onto the phenyl
moieties resulted in a decrease of activity and an increase in the isotactic bias of the obtained
polylactides. The broad molecular weight distributions (PDI = 1.13–2.02) of the polymer samples
indicated that the ROP of rac-lactide initiated by these complexes was not well-controlled.
tacticities.5–18 Aluminium complexes, especially those bearing salen
ligands and their derivatives, play an important role in the ROP
of rac-lactide among these catalysts. The rare cases of highly
isotactic polylactides prepared from rac-lactide were achieved by
Introduction
Polylactide (PLA) prepared from renewable sources is consid-
ered as an important material which exhibits unique properties,
using aluminium complexes with salen-type ligands.19–28 Apart
from bisphenolate-type ligands, ligands containing pure nitrogen
donors were also used to complex with aluminium. Bertrand
et al.7 synthesized aluminium complexes featuring tridentate di-
amidoamine ligands and investigated their catalytic performance
for lactide polymerization. Only upon being converted in situ into
alkoxides by initiating prepolymerization of propylene oxide could
these complexes initiate the ROP of lactides with low activities. Not
long ago, we reported some examples of b-diketiminate aluminium
complexes, but they could only initiate the polymerization of e-
caprolactone and showed no activity for lactide polymerization.29
Recently, considerable attention has been given to aluminium
complexes supported by various amidinate ligands for the fact
that the coordination environment of the metal center can be
easily modified by attaching substituents of various steric and
electronic properties. Jordan et al.30–32 reported the synthesis and
structural analysis of aluminium complexes [RC(NR¢)2]AlMe2
(R = Me, tBu; R¢ = iPr, Cy, Ad), while amidinate ligands
containing terphenyl group on the backbone carbon and the
corresponding dialkylaluminium complexes were synthesized by
the groups of Arnold and Clyburne.33,34 Junk et al. also reported
the synthesis of a dimethylaluminium complex featuring the
bulky N,N¢-bis(2,6-diisopropylphenyl)-4-toluamidinate ligand.35
To the best of our knowledge, most of the amidinate aluminium
complexes were explored mainly for studying the synthetic
such as biodegradability and bioassimilability, and has widely
potential applications in biomedical, pharmaceutical and agri-
cultural fields.1–4 Recently, using PLA as a new environmental-
friendly thermoplastic and an alternative material for polyolefinic
products of petroleum industry has attracted great attention of
researchers both in academy and industry. To get polylactide
with appropriate properties, different polymerization methods
based on coordination, anionic and cationic catalysts have been
explored. Among them, coordination polymerization of lactides
undoubtedly is more attractive because of its better controllability
over the polymerization process. Ring-opening polymerization
of rac-lactide initiated by discrete metal complexes of Al, Ca,
Mg, Zn, Sn, Fe and rare-earth metals is an efficient way to
produce PLAs with high molecular weights as well as various
Laboratory of Organometallic Chemistry, East China University of Science
and Technology, 130 Meilong Road, Shanghai, 200237, P. R. China.
E-mail: haiyanma@ecust.edu.cn; Fax: +86 21 64253519; Tel: +86 21
64253519
† Electronic supplementary information (ESI) available: Schematic pre-
sentation of the core structures of amidinate aluminium complexes, 1H
NMR spectra of ligand 3a, complex 4a and complex 4a with isopropanol,
1H NMR spectra of rac-LA polymerization initiated by complex 4c,
homonuclear decoupled 1H NMR spectrum of the methine region of
PLA, ESI-TOF mass spectrum of rac-lactide oligomer and summary of
crystallographic data for 4b, 4c and 5. CCDC reference numbers 772424–
772426. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c0dt00272k
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methodologies,30,31,33–45 as catalysts for olefin polymerization,46–49
or for other non-catalytic applications,50 and have not yet been
used as initiators for the ROP of cyclic esters such as lactides and
e-caprolactone. Due to the high Lewis acidity of the unsaturatedly
coordinated metal center which should facilitate the coordination
and activation of substrates, it is conceivable that amidinate
aluminium complexes may possess catalytic activity for the ROP of
lactide. At present, in most of the situations the amidinate ligands
involved all bear two identical N-moieties and only a few examples
of complexes bearing amidinate ligands with two different N-
moieties were reported.51 In further exploring the use of amidinate
ligands for the development of stereoselective initiators for the
ROP of rac-lactide, herein we report the synthesis of a range of alu-
minium complexes supported by non-symmetrically N-substituted
amidinate ligands and their catalytic behavior towards rac-lactide
polymerization, supposing that the unsymmetric coordinate sur-
rounding of the aluminium center formed by the non-symmetrical
ligands might induce some enantiomorphic site control over
the monomer coordination/insertion during the polymerization
process.
Results and discussion
Synthesis and structure of amidinate aluminium complexes
A convenient method used to synthesize amidinate aluminium
complex is to use carbodiimide as precursor to react directly with
alkylaluminium, which is however limited by the non-symmetrical
carbodiimide sources when adopted to prepare metal complexes
featuring non-symmetrical amidinate ligands where different N-
and N¢-substituents are involved. To form a potential chiral
environment around the metal center, especially in the catalytically
active state, it is considered to attach different substituents at
two nitrogen atoms. Using imidoyl chloride as an intermediate
to react with diverse aromatic or aliphatic amines affords us a way
to synthesize a variety of non-symmetrical amidine proligands
whose structure could be tuned easily from steric and electronic
aspects.51–54 As shown in Scheme 1, addition of benzoyl chloride to
a mixture of 10% NaOH and 2,6-diisopropylaniline followed by
chlorination with thionyl chloride and treatment with aryl amine
yielded the corresponding amidine compounds.52,54 Benzamidines
3a–k were reacted with AlMe3 to afford the amidinate alu-
minium complexes 4a–k which were isolated as off-white or light-
yellow crystals from hexane or toluene in moderate yields. The
monomeric structures of these amidinate aluminium complexes
were further confirmed by X-ray diffraction studies. It should be
noted that an excess amount of AlMe3 was adopted to facilitate
the conversion of the proligand in each case. The reaction of
the benzamidine proligand and AlMe3 in 1 : 1 ratio normally
resulted in low conversion even under more forcing conditions
such as refluxing in toluene, which raised problems to sequential
purification.
Scheme 1
Benzamidine compound 3l bearing N-alkyl substituents,
Scheme 2
synthesized
easily
via
the
reaction
of
N-(2,6-
diisopropylphenyl)benzylimidoyl chloride and tert-butylamine,
was also treated with excess AlMe3 to afford complex 4l as
colorless crystals in moderate yield (Scheme 2). The high
solubility of complex 4l, possibly induced by the introduction of
tert-butyl groups, made it difficult to crystallize even in n-hexane.
To minimize the steric congestion around the metal center, we
also synthesized N-(2,6-dimethylphenyl)-N¢-(2-methylphenyl)-
benzamidine (3m), N-(2,6-dimethylphenyl)-N¢-phenylbenza-
midine (3n), and the related aluminium complexes 4m–n
via similar procedures (Scheme 3). On the other hand, the
8072 | Dalton Trans., 2010, 39, 8071–8083
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Fig.
1
ORTEP diagram of the molecular structure of
[{PhC(N-2,6-iPr2C6H3)(N-2,6-Cl2C6H3)}AlMe2] (4b). Thermal ellipsoids
are drawn at the 30% probability level. Hydrogen atoms are omitted for
◦
˚
clarity. Selected bond distances (A) and angles ( ): Al–N1 1.949(4), Al–N2
1.950(4), N1–C1 1.331(5), N2–C1 1.356(5), Al–C26 1.945(5), Al–C27
1.937(6), N1–C8 1.432(5), N2–C20 1.402(5); N1–Al–N2 68.44(15),
Al–N1–C1 91.4(3), Al–N2–C1 90.6(3), Al–N1–C8 142.4(3), Al–N2–C20
142.0(3), N1–C1–N2 109.4(4), N1–C1–C2 126.1(4), N2–C1–C2 124.4(4),
C2–C1–Al 178.7(3), C26–Al–C27 120.4(3).
Scheme 3
synthesis of the proligand N,N¢-bis(2,6-diisopropylphenyl)-
benzamidine52 and the related aluminium dimethyl complex 5
provided us with the example that bears the maximum steric
congestion around the metal center among these complexes.
Though amidinate aluminium dimethyl complexes 4a–n and
5 were synthesized successfully, the reaction of these amidines
with triethylaluminium did not readily afford analytically pure
diethyl analogues except for 4a¢, [{PhC(N-2,6-iPr2C6H3)(N-2,6-
Me2C6H3)}AlEt2], due to the difficulty encountered in the purifica-
tion process.34 Even with the employment of a large excess of AlEt3
the amidine compounds could not be converted completely to the
desired aluminium complexes; in most of the cases the residual
ligands co-crystallized with the target aluminium complexes and
could not be excluded cleanly even after repeated recrystallization
from n-hexane.
are 76.44 and 75.06◦). The slight difference between bond distances
˚
˚
of N1–Al [1.949(4) A] and N2–Al [1.950(4) A] as well as N1–C1
˚
˚
[1.331(5) A] and N2–C1 [1.356(5) A] in complex 4b demonstrates
a well delocalized system. By comparison with the related N-alkyl
or -aryl substituted amidinate aluminium complexes, the non-
symmetrical substitution mode in the amidinate ligand does have
a certain influence on the molecular structure of 4b, as indicated
˚
˚
by the larger DC–N value of 0.025 A (DC–N = 0.001–0.017 A for
symmetric systems).31,32,34 Furthermore, the electronegative 2,6-
dichloro substitution shortens the bond distance of N2–C20 and
˚
gives rise to elongated Al–N bonds (1.950(4) vs. 1.912–1.940 A).
The other bond lengths and angles are however comparable to the
reported amidinate aluminium complexes.
The introduction of ortho-F substitution in complex 4c dra-
matically changes the orientation of the related aryl group, which
is approximately parallel to the metallacycle plane (N1–Al–N2–
C1) rather than perpendicular (angle between the two planes is
14.315◦, Fig. 2). A weak interaction between the fluorine atom
1
The H NMR spectra of amidine proligands obtained in this
work are generally complicated due to the presence of isomers
or possible interconversion between Z-syn and E-syn isomers that
occurs on the NMR time scale,55 which makes a precise assignment
of all the resonances difficult. The resonances become simplified in
the 1H NMR spectra of the corresponding aluminium complexes
as delocalization occurs when the amidine proligand coordinates
with the aluminium center. For instance, in the 1H NMR spectrum
of 3a, there are three singlets at 2.36, 2.13, 2.03 ppm accounting
for the aromatic methyl groups and four doublets at 1.36, 1.24,
1.02, 0.91 ppm accounting for the isopropylmethyl groups. Only
one singlet at 2.21 ppm accounting for the aromatic methyl and
two doublets at 1.17, 0.86 ppm accounting for isopropyl-methyls
are observed in the 1H NMR spectrum of complex 4a.
As depicted in Fig. 1, complex 4b is monomeric with distorted
tetrahedral geometry at the aluminium center. The Al atom and the
chelating N1–C1–N2 moiety of the amidinate ligand construct a
nearly coplanar four-membered metallacycle (the torsion angle of
N1–Al–N2–C1 = 2.0◦). The aryl rings are oriented perpendicular
to the metallacycle (angles between aryl and metallacycle planes
˚
and the aluminium center with a distance of 2.7936 A could be
observed which can be compared with the sum of the metal radius
rm(Al) and fluorine van der Waals radius rv(F) [rm(Al) + rv(F) =
˚
1.43 + 1.40 =2.83 A], and is considered to be responsible for the
unusual array of the aryl group. The interaction also forces the
whole amidinate ligand to move towards it, as indicated by the
significant difference between the two Al–N bonds (1.9701(14) vs.
˚
1.9290(14) A) in comparison with those in 4b and other amidinate
aluminium complexes.31,32,34 This deviation of amidinate ligand
also leads to smaller N1–Cl–N2 and Al–N1–C1 angles as well
as a bigger Al–N2–C1 angle. Moreover, in contrast to complex
4b, where the electron withdrawing ortho-Cl substituents result
in the elongated Al–N bonds, the weak F ◊ ◊ ◊ Al interaction in
4c shortens the corresponding Al–N2 bond and counteracts the
electron withdrawing effect of the ortho-F substituent on it.
As shown in Fig. 3, complex 5 possesses C2-symmetry with the
axis located along C2–C1 ◊ ◊ ◊ Al; the chelating nitrogen donors and
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bearing a less bulky substituted group [MeC(NC6H11)2]AlMe2
(110.4, 69◦)31 and more open than the complex bearing a
more steric bulky group [t-BuC(N-2,6-iPr2C6H3)2]AlMe2 (107.4,
68.15◦).32
Ring-opening polymerization of rac-lactide with addition of alcohol
As for most of the aluminium complexes, the initiators adopted
for lactide polymerization were generated by in situ alcoholysis
of the complexes using isopropanol or benzyl alcohol23,24,26–28,56–60
or released by the reaction of neutral proligand and aluminium
alkoxide.20,21,25,61–64 A toluene solution of amidinate aluminium
complex 4a was treated with two equiv. of isopropanol and used
directly to initiate the ring-opening polymerization of rac-lactide
at 70 ◦C. Rapid polymerization was observed and monomer
conversion up to 78% could be reached within 4 h. The 1H
NMR spectrum of the obtained polylactide sample showed that
the polymer chains were end-capped with isopropyl ester and a
hydroxyl group, respectively.
Fig.
2
ORTEP diagram of the molecular structure of
[{PhC(N-2,6-iPr2C6H3)(N-2-FC6H4)}AlMe2] (4c). Thermal ellipsoids
are drawn at the 30% probability level. Hydrogen ◦atoms are omitted for
˚
clarity. Selected bond distances (A) and angles ( ): Al–N1 1.9701(14),
In order to identify the actual active species, the reaction
of complex 4a and one equiv. of isopropanol was monitored
Al–N2 1.9290(14), N1–C1 1.3269(19), N2–C1 1.340(2), Al–C26 1.944(2),
Al–C27 1.943(2), N1–C8 1.427(2), N2–C20 1.401(2), F ◊ ◊ ◊ Al 2.7936;
N1–Al–N2 67.56(6), Al–N1–C1 91.09(10), Al–N2–C1 92.50(10),
Al–N1–C8 142.57(10), Al–N2–C20 135.85(12), N1–C1–N2 108.79(14),
N1–C1–C2 124.51(14), N2–C1–C2 126.62(14), C2–C1–Al 175.96(11),
C26–Al–C27 120.77(11).
1
by H NMR spectroscopy in CDCl3. Signals assignable to the
free proligand 3a were detected; besides, one multiple signals at
4.20 ppm and one doublet of doublet at 1.28 ppm appeared; the
singlet accounting for Al–CH3 in 4a shifted to higher field as
multiple signals ranging from -0.70 to -0.80 ppm. The integration
ratio of them appeared to be 1: 6 : 6. Clearly, the alcoholysis
reaction afforded the amine elimination product “Me2Al(OiPr)“
instead of the desired alkyl elimination one. As expected, the
further addition of a second equiv. of isopropanol afforded
“MeAl(OiPr)2” as complicated aggregates. It is indisputable that
the polymerization carried out with 4a/isopropanol system was in
fact initiated by the alkyl elimination products of “Me2Al(OiPr)”
or “MeAl(OiPr)2” or “Al(OiPr)3” depending on the [Al]/[iPrOH]
ratio. The instability of metal complex of N-containing lig-
and towards alcoholysis/enolysis was also observed by other
groups.65–67
Aluminium complexes as single component initiators
Polymerization of rac-lactide initiated by benzamidinate alu-
minium complexes 4a–n and 5 without addition of alcohol was
carefully studied. The results showed that all the complexes
displayed moderate activities for the ring-opening polymerization
of rac-lactide when used as single component initiators, and the
structure of the ancillary ligands had a significant influence on
the polymerization behavior of the corresponding aluminium
complexes.
Fig.
3 ORTEP diagram of the molecular structure of complex
[{PhC(N-2,6-iPr2C6H3)2}AlMe2] (5). Thermal ellipsoids are drawn at the
30% probability level. Hydrogen◦atoms are omitted for clarity. Selected
˚
bond distances (A) and angles ( ): Al–N1 1.9381(18), Al–C18 1.945(3),
C1–N1 1.334(2), N1–C6 1.426(2); N1–Al–N1* 68.61(9), Al–N1–C1
90.73(13), N1–C1–N1* 109.9(2), C2–C1–Al 179.999(1), N1–Al–C18
113.34(10), N1–Al–C18* 118.24(11).
Influence of ancillary ligand on catalytic activity
aluminium metal center form a consummate plane evidenced by
the torsion angle of N1–Al–N1*–C1 = 0◦. The two C–N bond
To facilitate comparing the effect of ancillary ligand on catalytic
activities, amidinate aluminium complexes obtained in this work
were divided into groups and are discussed separately.
˚
distances (1.334(2) A), lying between the C–N bond distance in
˚
˚
amidine (1.301 A) and the C–N single-bond distance (1.47 A),
suggest a symmetric delocalization in the complex. The Al–N
For
complexes
4a–f
all
possessing
one
N-2,6-
˚
bond distance of [1.938(18) A] and the bond distance of Al–
diiosopropylphenyl group, the ortho-substituents on the
other N-phenyl ring show obvious influence on the catalytic
activity of the corresponding aluminium complex. As shown
in Table 1, in contrast to most literature results, regardless of
the electronic nature of the ortho-substituents, complexes 4a–e
˚
CH3 [1.945(3) A] are comparable to the aluminium complex with
bulky ligand [{4-MeC6H4C(2,6-iPr2C6H3N)2}AlMe2].35 The N1–
C1–N1* angle of 109.9(2)◦ and the N1–Al–N1* bite angle of
68.61(9)◦ mean that the complex is more closed than the complex
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Table 1 Polymerization of rac-LA initiated by amidinate aluminium complexes with ortho-substituents on N-phenyl ringsa
c
d
e
e
f
Run
Initiator
t/h
Conv.b (%)
10-4Mc
10-4Mh
10-4Mn¢
Mw/Mn
Pm
1
2
3
4
5
6
7
8
4a (o-Me, Me)
4a¢ (o-Me, Me)
12
24
48
72
12
24
36
48
48
96
24
48
48
72
48
24
57
86
60
83
94
85
82
88
64
78
77
91
85
96
59
86
0.82
1.24
0.86
1.20
1.35
1.22
1.19
1.26
0.92
1.12
1.11
1.31
1.22
1.38
0.85
1.24
0.63
0.60
2.96
2.20
1.93
2.53
2.33
1.97
1.15
1.65
1.74
2.26
1.65
1.21
1.48
2.60
1.64
1.38
1.35
1.13
1.22
1.36
1.31
4b (o-Cl, Cl)
4c (o-F)
4d (o-Cl)
4e (o-Me)
4f (N-Ph)
0.54
0.52
0.51
0.51
9
10
11
12
13
14
15
16g
0.47
0.53
4m (o-Me)
4n (N-Ph)
5 (o-iPr, iPr)
1.47
1.46
1.20
1.38
1.77
0.51
0.65
1.45
2.24
1.60
2.03
1.34
1.63
a Conditions: [rac-LA]0/[Al]0 = 100, [rac-LA]0 = 1.0 M, in toluene, 70 ◦C. b Determined by the integration ratio of the methine protons in monomer and
polymer. c Mc = ([rac-LA]0/[Al]0) ¥ 144.13 ¥ conversion (%). d The intrinsic viscosity of polylactide was determined with an Ubbelohde viscosimeter at
◦
0.77 68
25 C in CDCl3, and the viscosity average molecular weight (Mh) was calculated from the equation: [h] (dL g-1) = 2.21 ¥ 104Mh
.
e The number average
molecular weight (Mn) and molecular weight distribution (Mw/Mn) were determined by a gel permeation chromatograph, calibrated with polystyrene
standards in THF, Mn¢ = 0.58Mn,GPC. f Pm is the probability of forming a new m-dyad, determined by homonuclear decoupled 1H NMR. g 100 ◦C.
with various ortho-substituents all exhibit superior activities
than 4f without any substituent on one of the N-phenyl groups;
and for the same substituent, methyl or chloro, the complex
with ortho-disubstituted N-aryl group exhibits superior activity
than the one with ortho-monosubstituted N-aryl group. That is,
complex 4a is more active than 4e and complex 4b is more active
than 4d (run 2 vs. 8, run 5 vs. 7). It is therefore conceived that
the introduction of substituent at ortho-positions is favorable for
the enhancement of catalytic activity and the steric effect may
dominate.
To verify the steric effect of ortho-substituents of the N-aryl
group, complexes 4m, 4n and 5 were obtained. The polymerization
results show that a similar tendency as that of 4a–f is also observed
for complexes 4m and 4n possessing the N-2,6-dimethylphenyl
group, with complex 4m being more active than 4n (Table 1).
However, the increase of steric bulkiness of the ortho-substituents
via introducing isopropyl groups is disadvantageous, lower activity
than that of 4f is observed for complex 5, and complexes 4m and
4n are even more active than their N-2,6-diisopropyl analogues 4e
and 4f, respectively.
When we reconsider complexes 4a–e, it is found that the
electronic nature of the ortho-substituents also shows a certain
influence on the activity. In general the introduction of an
electron withdrawing group leads to an enhancement of catalytic
activity. Thus complex 4b with ortho-dichloro substituents exhibits
higher activity than complex 4a with ortho-dimethyl substituents;
complex 4c with ortho-fluoro group on one of the N-phenyl groups
displays the highest activity among complexes 4c–e with ortho-
monosubstitution. Nevertheless, the number of ortho-substituents
seems more dominant than electronic effects, in comparison with
4a, complex 4c is less active.
The same fluoro substituent at different position of N-phenyl
ring brings distinct influence on the activities of corresponding
aluminium complexes 4c, 4g and 4h, which are in the order of
4c (ortho-F) > 4g (meso-F) > 4h (para-F) (Table 2). The fluorine
atom, being strongly electronegative, is generally considered to
display a marked electron-withdrawing effect, and this electron-
withdrawing strength is attenuated with the distance. On the other
hand the lone pair in the p-orbital of fluorine atom can also lead
to an electron-donating conjugation effect via p–p bonding to
its para- and ortho-positions when introduced to phenyl ring.
It is reasonable that ortho-fluoro substitution will display the
same extent of electron-donating conjugated effect as the para-
fluoro substitution but stronger electron-withdrawing effect due
to shorter distance; and the meta-fluoro substitution will only
display an electron-withdrawing effect. As a result, the latter
will often lead to highest activity, as we previously found for
b-diketiminate aluminium complexes in the polymerization of e-
caprolactone.29 In contrast to this, in this work complex 4c (ortho-
F) exhibits much higher activity than 4g (meta-F) (run 6 vs. 17
in Table 2), which implies again some special effect of ortho-
substitution. From X-ray diffraction study of complex 4c, a close
contact of the ortho-fluoro with aluminium center is observed.
Likely, this might be responsible for the extraordinary activity
in rac-lactide polymerization. A similar positive effect of ortho-
fluoro substituent on the polymerization of e-caprolactone was
also observed for aluminium complexes bearing phenoxyimine
ligands.69
In comparison with complexes 4a–e and 4m–n bearing various
ortho-substituents on one N-aryl group, complexes 4h–k with one
para-substituted N-aryl group exhibit significant lower catalytic
activities in the polymerization of rac-lactide (Table 2). On the
evidence of the catalytic activities of complexes 4f, 4j and 4k,
the obvious tendency is that an electron-donating substituent
introduced onto the para-position of the phenyl ring reduces the
activity of the aluminium complexes. Complexes 4h, 4i possessing
electron withdrawing halogen substituents, show comparable or
even lower activity than 4f in rac-lactide polymerization, possibly
suggesting a dominant electron-donating conjugated effect of the
halogen atom. Similarly, the electronic donating ability of tert-
butyl towards the active metal center in complex 4l may account
for the low activity as well.
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Table 2 Polymerization of rac-LA initiated by amidinate aluminium complexes with various N-aryl and N-alkyl groupsa
c
d
e
e
f
Run
Initiator
t/h
Conv.b (%)
10-4Mc
10-4Mh
10-4Mn¢
Mw/Mn
Pm
6
4c (o-F)
4g (m-F)
24
48
72
48
72
48
72
48
72
72
96
72
120
85
86
93
61
71
65
75
60
73
42
47
51
72
1.22
1.24
1.33
0.87
1.02
0.93
1.08
0.86
1.05
0.61
0.68
0.73
1.04
1.97
1.72
1.21
1.66
1.93
1.45
1.13
1.22
1.56
1.23
0.52
0.52
0.51
0.50
0.49
0.51
0.47
17
18
19
20
21
22
23
24
25
26
27
28
4h (p-F)
1.68
1.72
1.12
3.30
1.51
1.82
1.00
4i (p-Cl)
4j (p-iPr)
4k (p-OMe)
4l (N-tBu)
1.31
1.18
2.02
1.25
1.39
1.46
3.33
a Conditions: [rac-LA]0/[Al]0 = 100, [rac-LA]0 = 1.0 M, in toluene, 70 ◦C. b Determined by the integration ratio of the methine protons in monomer and
polymer. c Mc = ([rac-LA]0/[Al]0) ¥ 144.13 ¥ conversion (%). d The intrinsic viscosity of PLA was determined with an Ubbelohde viscosimeter at 25 ◦C
0.77 68
in CDCl3, and the viscosity average molecular weight (Mh) was calculated from the equation: [h] (dL g-1) = 2.21 ¥ 104Mh
.
e The number average
molecular weight (Mn) and molecular weight distribution (Mw/Mn) were determined by a gel permeation chromatograph, calibrated with polystyrene
standards in THF, Mn¢ = 0.58Mn,GPC. f Pm is the probability of forming a new m-dyad, determined by homonuclear decoupled 1H NMR spectroscopy.
From the above mentioned features, several unconventional
aspects could be summarized: (1) regardless of the electronic
nature, the presence of ortho-substituents of small steric bulk is
benefit for the enhancement of catalytic activity; (2) the number of
such group is also important; (3) based on these two conditions,
an electron-withdrawing nature of substituent is more favored in
comparison with electron donating one. It is therefore reasonable
to hypothesize that the ortho-substituents of small steric bulkiness
might exhibit some special effect during the polymerization
process which is beneficial for the increase of activity of these
catalysts.
(free rac-lactide) to 3.94 ppm was observed, which further shifted
to 4.13 ppm with the appearance of a new signal at 5.05 ppm
assignable to PLA after 2 h at 60 ◦C. It seemed that, under
the adopted conditions, all the monomer molecules tended to
coordinate to the aluminium center and the interaction became
even stronger with the consumption of lactide monomer. With
the progress of polymerization, the resonances of the amidinate
moiety could not be identified anymore due to significant
broadening and overlapping; the methyl group attached to the
aluminium atom displayed a group of signals in the region of
-0.20 to -0.50 ppm but with reduced integral when referring
to the sum of lactide monomer and polymer. No well-defined
active species could be characterized during the polymerization.
Based on the fact that no free amidine proligand was observed
in the 1H NMR spectra and the structure of amidinate ligand has
considerable influence on the catalytic activity, we suggest that
the amidinate segment may still bond to aluminium during the
polymerization process.
Studies concerning mechanism and active species
Being different from the polymerization carried in the presence
of alcohol, the polymerization initiated by these amidinate alu-
minium complexes alone was accompanied by the occurrence of
a yellow solution. The whole system was very sensitive, although
with strict exclusion of moisture and oxygen direct sampling from
the polymerization solution led to partial deactivation of the
remaining polymerization mixture. Thus sequential sampling at
specified interval from a single polymerization solution failed to
provide reasonable monomer conversion data and independent
polymerization runs had to be used for each data point. To clarify
whether a free radical mechanism was involved in the polymeriza-
tion, controlled polymerization runs were carried out via two ways:
(1) TEMPO serving as a free radical capture reagent was added
to the polymerization mixture in the very beginning; (2) TEMPO
was added to the polymerization mixture which had processed
for a couple of hours. Both cases showed that the addition of
TEMPO had no influence on the polymerization, the progress of
monomer conversion with time during the polymerization was the
same in each comparable run, thus excluding a possible free radical
mechanism.
In order to understand the possible initiation pathway, end-
1
group analysis of obtained polymer samples through H NMR
spectroscopy were carried out thoroughly. Resonances of the
methine proton at 4.35 ppm and hydroxyl group at 2.60 ppm
1
in the H NMR spectra of most polylactide samples could be
recognized, indicating the linear structure of obtained polymers.
The identification of the other end is not conclusive, neither
aromatic protons of amidinate moiety nor acetyl protons resulting
from Al-methyl initiation could be detected, giving no direct
support for either Al–N and Al–R initiation pathways.
Through the in situ 1H NMR polymerizations of e-CL in
the presence of diamino-aluminium complexes [N∧N]AlMe,
Chakraborty and Chen observed the initiation step involv-
ing monomer insertion into an Al–N bond.15 Very recently,
Mountford and co-workers found that the ROP of rac-lactide
by sulfonamide-supported aluminium complexes [Al(N2Ts NR)Et]
(R = OMe, py) was initiated by monomer insertion to the Al–
Et bond as evident by the existence of the EtC(O)CH(Me)O–
end group as characterized by MALDI-TOF spectra.70 The
ESI-TOF mass spectrum of the oligomer sample obtained with
[rac-LA]/[4c] = 10 at 70 ◦C however indicated the existence
Polymerization of rac-lactide initiated by complex 4c was
further monitored by H NMR spectroscopy in C6D6 at 60 ◦C
1
with [rac-LA]/[Al] = 10. Upon mixing, the resonances accounting
for complex 4c only shifted slightly, while a considerable downfield
shift of the methine proton resonance of rac-lactide from 3.79 ppm
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of both MeC(O)CH(Me)O– and {PhC(N-2,6-iPr2C6H3)(N-2-
FC6H4)}C(O)CH(Me)O– ends, thus for amidinate aluminium
alkyl complexes, both Al–N and Al–R initiation are possible, with
Al–R initiation dominant.71
to ortho-positions of the N-phenyl ring. However, further increase
the steric hindrance of the ortho-substituents results in a decrease
in catalytic activity and slight increase of isotactic bias in polymer
chain. The ROP of rac-lactide initiated by amidinate aluminium
complexes 4a–n is not well-controlled, as demonstrated by the
measured molecular weights deviating from the theoretical values.
Characterization of polylactides
Determination of the stereochemical structure of PLA is achieved
through inspection of the methine group of homonuclear decou-
pled 1H NMR spectra of the resultant polymers. The predominant
mmm tetrad peak indicates that the polymer produced by catalyst
4a is slightly isotactic bias enriched (Pm = 0.63), where [mmm] =
Experimental
General considerations and materials
All reactions and manipulations involving air-sensitive complexes
were carried out under argon atmosphere using standard Schlenk
vacuum-line and glove-box techniques. Toluene and n-hexane were
refluxed over sodium benzophenone prior to use. Chloroform-d
was dried over calcium hydride. C6D6 was refluxed over sodium
and distilled. AlMe3 (2.0 M in toluene) was purchased from
Aldrich and used as received. Neat AlEt3 (commercial product)
was dissolved in an appropriate amount of petroleum ether
to give a 0.94 M solution. N-(2,6-Diisopropylphenyl)benzamide
(1), N,N¢-bis(2,6-diisopropylphenyl)benzamidine and proligand
3a were synthesized according to the published procedures.52–72 rac-
Lactide (Aldrich) was recrystallized with dry toluene and sublimed
once under vacuum at 80 ◦C.
[Pm + (1 - Pm)2 + Pm + (1 - Pm)3]/2, [mrm] = [Pm(1 - Pm)
+ Pm(1 - Pm)2]/2, [mmr] = [rmm] = [rmr] = [Pm2(1 - Pm) +
Pm(1 - Pm)2]/2. The intensity of rmr, mmr and mrm tetrads relative
to the mmm tetrad does not change with conversion, indicating
a homogeneous distribution of isotactic sequences along the
polymer chain. Except for complexes 4a and 5, the other catalysts
only initiated polymerizations which afforded atactic polymers.
2
3
1
The results of homonuclear decoupled H NMR spectroscopy
indicate that ortho-substituents of N-phenyl rings studied in
this work show slight influence on the ability of corresponding
catalysts to control monomer insertion. For instance, changing
the isopropyl substituents in complex 5 (Pm = 0.65) to hydrogen
atoms in complex 4f results in a decrease in isotactic bias (Pm
=
Instruments and measurements
0.47). It is suggested that steric bulky groups at the ortho-positions
may block the coordination sphere of the metal center and restrict
the monomer insertion direction, leading to higher regularity
of polylactide microstructure, and low activities of aluminium
complexes in polymerization.28,29
NMR spectra were recorded on Bruker AVANCE-500 and
AVANCE-400 spectrometers with CDCl3 or C6D6 as solvent (1H:
500 MHz or 400 MHz; 13C: 125 MHz or 100 MHz). Chemical
1
shifts for H and 13C NMR spectra were referenced internally
Molecular weight information of all polylactide samples is
obtained by viscosity measurements and gel permeation chro-
matography (GPC). As shown in Tables 1 and 2, in general,
with the increase of monomer conversion, the viscosity average
molecular weights of the polymer samples increase. However, the
deviation of the molecular weights Mh and Mn¢ from theoretical
values (calculated with the assumption that each aluminium
center initiates one polymer chain) and broad molecular weight
distributions of polylactides, imply a slow initiation relative to
chain propagation and possible deactivation of the active sites
and transesterification may be inevitable.
using the residual solvent resonances and reported relative to
tetramethylsilane (TMS). Elemental analyses were performed on
an EA-1106 instrument. The intrinsic viscosity of polylactides
was measured with an Ubbelohde viscometer in chloroform at
25 ◦C. Gel permeation chromatography (GPC) analyses were
carried out on a Waters instrument (M515 pump, Optilab Rex
Injector) in THF at 25 ◦C, at a flow rate of 1 mL min-1. Calibration
standards were commercially available narrowly distributed linear
polystyrene samples that cover a broad range of molar masses.
Single crystals suitable for X-ray diffraction studies were obtained
◦
from n-hexane for 4b, 4c and from toluene for 5 at -40 C. The
crystallographic data for complexes 4b, 4c and 5 were collected on
a Bruker AXSD8 diffractometer with graphite-monochromated
Conclusion
˚
Mo-Ka (l = 0.71073 A) radiation. All data were collected at
Aluminium complexes 4a–n and 5 supported by symmetrical or
non-symmetrical amidinate ligands were synthesized via alkane
elimination reactions. The structures of these complexes were
20 ◦C using omega-scan techniques. The structures of 4b, 4c
and 5 were solved by direct methods and refined using Fourier
techniques. An absorption correction based on SADABS was
applied.73 All non-hydrogen atoms were refined by full-matrix
least-squares on F2 using the SHELXTL program package.74
Hydrogen atoms were located andrefinedbythe geometrymethod.
The cell refinement, data collection, and reduction were done
using Bruker SAINT.75 The structure solution and refinement were
performed with SHELXS-9776 and SHELXL-9777 respectively.
Molecule structures were generated using ORTEP III program.78
1
characterized by H NMR, 13C NMR and EA. The molecular
structures of complexes 4b, 4c and 5 were further confirmed by
X-ray diffraction techniques. These complexes are proved to be ef-
ficient initiators for the ring-opening polymerization of rac-lactide.
The effect of ligand substituents on polymerization is significant.
In general, an electron-donating substituent at the para-position of
the N-phenyl ring decreases the electrophilicity of the aluminium
center and is unfavorable for the coordination and insertion of
rac-lactide monomer, leading to lower catalytic activity of the
corresponding aluminium complex. The introduction of electron-
withdrawing substituents to the ortho-positions of N-phenyl ring
improves the catalytic activity. Such enhancement of catalytic
activity could also be achieved by introducing small alkyl groups
Synthesis of amidine compounds
N-(2,6-Diisopropylphenyl)-N¢-(2,6-dichlorophenyl)benzamidine
(3b). N-(2,6-Diisopropylphenyl)benzamide (6.20 g, 22.0 mmol)
was refluxed in SOCl2 (6.5 mL) for 1 h at 80 ◦C, then the reaction
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mixture was cooled to room temperature and excess SOCl2 was
evaporated off under vacuum. The residual SOCl2 could be
removed by adding toluene followed by further evaporating the
mixture. To the obtained residue a mixture of 2,6-dichloroaniline
(3.25 g, 20.0 mmol) and triethylamine (11 mL, 80 mmol) in toluene
(30 mL) was added. The reaction mixture was refluxed for 20 h
and then cooled to room temperature, washed with water, dried
over MgSO4 and concentrated. The crude product was purified by
column chromatography using silica gel (petroleum ether–ethyl
acetate = 20 : 1) and further recrystallized with ethanol to give
3b as colorless crystals (3.27 g, 35%) (Found: C, 70.11; H, 6.55;
N, 6◦.28. Calc. for C25H26Cl2N2: C, 70.59; H, 6.16; N, 6.59%); mp
(Ar-C), 128.5 (br, Ar-C), 127.8 (br, Ar-C), 126.7 (br, Ar-C), 124.1
(br, Ar-C), 123.6 (Ar-C), 122.3 (Ar-C), 28.9 [–CH(CH3)2], 28.3
[–CH(CH3)2], 23.8 [br, –CH(CH3)2].
N - (2,6 - Diisopropylphenyl) - N¢ - (2 - methylphenyl)benzamidine
(3e). The procedure was similar to that of compound 3b except
that N-(2,6-diisopropylphenyl)benzamide (6.20 g, 22.0 mmol) and
o-methylaniline (2.15 g, 20.0 mmol) was used. Crystallization in
ethanol–water afforded colorless crystals (5.5 g, 75%) (Found: C,
84.15; H, 8.24; N, 7.43. Calc. for C26H30N2: C, 84.28; H, 8.16; N,
7.56%); mp 142 ◦C; dH (500 MHz, CDCl3, 25 ◦C) 7.56 (d, 2H,
3J = 7.4 Hz, Ar-H), 7.36–7.30 (m, 4H, Ar-H), 7.20 (d, 2H, 3J =
7.4 Hz, Ar-H), 7.12–7.06 (m, 1H, Ar-H), 6.88 (t, 1H, 3J = 7.2 Hz,
Ar-H), 6.84 (t, 1H, 3J = 7.4 Hz, Ar-H), 6.45 (d, 1H, 3J = 7.4 Hz,
◦
3
116 C; dH (500 MHz, CDCl3, 25 C) 7.53 (d, 2H, J = 6.8 Hz,
Ar-H), 7.34 (m, 4H, Ar-H), 7.26 (m, 2H, Ar-H), 7.05 (d, 2H, 3J =
3
Ar-H), 5.87 (s, 1H, –NH), 3.48 [septet, 0.16H, J = 6.7 Hz, –
3
8.0 Hz, Ar-H), 6.64 (t, 1H, J = 8.0 Hz, Ar-H), 5.93 (s, 0.85H,
CH(CH3)2], 3.22 [septet, 1.84H, 3J = 6.7 Hz, –CH(CH3)2], 2.15 (s,
2.76H, –CH3), 2.11 (s, 0.24H, –CH3), 1.25 [d, 11H, 3J = 6.7 Hz,
3
–NH), 5.71 (s, 0.15H, –NH), 3.66 [septet, 1.7H, J = 6.8 Hz, –
3
CH(CH3)2], 3.34 [septet, 0.3H, J = 6.8 Hz, –CH(CH3)2], 1.41
3
–CH(CH3)2], 1.02 [d, 0.5H, J = 6.7 Hz, –CH(CH3)2], 0.94 [d,
[d, 5.1H, 3J = 6.8 Hz, –CH(CH3)2], 1.24 [d, 5.1H, 3J = 6.8 Hz, –
CH(CH3)2], 1.05 [d, 0.9H, 3J = 6.8 Hz, –CH(CH3)2], 0.93 [d, 0.9H,
3J = 6.8 Hz, –CH(CH3)2]; dC (125 MHz, CDCl3, 25 ◦C) 158.7 (C–
N), 147.1 (Ar-C), 146.0 (Ar-C), 135.4 (Ar-C), 132.4 (Ar-C), 129.9
(Ar-C), 128.5 (Ar-C), 128.4 (Ar-C), 127.8 (Ar-C), 127.7 (Ar-C),
126.9 (Ar-C), 123.5 (Ar-C), 122.4 (Ar-C), 28.5 [–CH(CH3)2], 25.0
[–CH(CH3)2], 22.9 [–CH(CH3)2].
0.5H, J = 6.7 Hz, –CH(CH3)2]; dC (125 MHz, CDCl3, 25 ◦C)
3
=
154.2 (C N), 145.2 (Ar-C), 143.4 (Ar-C), 139.2 (Ar-C), 138.8 (Ar-
C), 135.2 (Ar-C), 130.4 (Ar-C), 130.2 (Ar-C), 129.7 (Ar-C), 128.7
(Ar-C), 128.2 (Ar-C), 126.2 (Ar-C), 124.9 (Ar-C), 124.2 (Ar-C),
123.8 (Ar-C), 123.5 (Ar-C), 28.5 [–CH(CH3)2], 28.2 [–CH(CH3)2],
24.5 [–CH(CH3)2], 23.7 [–CH(CH3)2], 21.9 [–CH(CH3)2], 18.1
[–CH(CH3)2].
N-(2,6-Diisopropylphenyl)-N¢-(2-fluorophenyl)benzamidine (3c).
The procedure was similar to that of compound 3b except that
N-(2,6-diisopropylphenyl)benzamide (3.60 g, 13.0 mmol) and o-
fluoroaniline (1.1 g, 11.8 mmol) were used. Crystallization in
ethanol–water afforded colorless crystals (3.16 g, 66%) (Found:
C, 80.18; H, 7.24; N, 7.36. Calc. for C25H27FN2: C, 80.18; H, 7.27;
N, 7.48%); mp 118–119 ◦C; dH (400 MHz, CDCl3, 25 ◦C) 7.62
(br s, 1H, Ar-H), 7.50–6.86 (m, 9H, Ar-H), 6.82–6.68 (m, 2H, Ar-
H), 6.50 (br s, 0.4H, N–H), 6.13 (br s, 0.4H, N–H), 5.88 (s, 0.2H,
N-(2,6-Diisopropylphenyl)-N¢-phenylbenzamidine
(3f). The
procedure was similar to that of compound 3b except that
N-(2,6-diisopropylphenyl)benzamide (3 g, 10 mmol) and aniline
(0.84 g, 9.0 mmol) were used. Crystallization in ethanol–water
afforded colorless crystals (2.25 g, 63%) (Found: C, 83.81; H,
8.03; N 7.53. Calc. for C25H28N2: C, 84.23; H, 7.92; N, 7.86%);
mp 143–144 ◦C; dH (500 MHz, CDCl3, 25 ◦C) 7.62 (s, 2H, Ar-H),
7.38–7.34 (m, 3H, Ar-H), 7.20 (s, 2H, Ar-H), 7.12–6.93 (m, 4H,
Ar-H), 6.60 (br s, 2H, Ar-H), 6.23 (s, 1H, –NH), 3.16 [br s,
3
N–H), 3.46 [septet, 0.4H, J = 6.8 Hz, –CH(CH3)2], 3.16 [br s,
3
2H, –CH(CH3)2], 1.22 [d, 12H, J = 6.8 Hz, –CH(CH3)2]; dC
1.6H, –CH(CH3)2], 1.39 [br s, 1.2H, –CH(CH3)2], 1.41–1.06 [m,
◦
3
=
(125 MHz, CDCl3, 25 C) 153.8 (C N), 143.4 (Ar-C), 140.1
9.6H, –CH(CH3)2], 0.95 [d, 1.2H, J = 6.8 Hz, –CH(CH3)2]; dC
◦
(Ar-C), 139.1 (Ar-C), 135.1 (Ar-C), 129.7 (Ar-C), 128.9 (Ar-C),
128.7 (Ar-C), 128.7 (Ar-C), 128.3 (Ar-C), 123.8 (Ar-C), 123.57
(Ar-C), 122.5 (Ar-C), 28.2 [–CH(CH3)2], 23.9 [–CH(CH3)2], 23.5
[–CH(CH3)2].
=
(100 MHz, CDCl3, 25 C) 158.6 (C N), 153.1 (br, Ar-C), 146.7
(Ar-C), 138.9 (br, Ar-C), 135.2 (Ar-C), 134.7 (Ar-C), 133.9 (d,
1JC–F = 245 Hz, Ar-C), 129.8 (Ar-C), 129.5 (Ar-C), 128.5 (Ar-C),
128.3 (Ar-C), 128.2 (d, 2JC–F = 17.9 Hz, Ar-C), 127.6 (Ar-C), 125.0
3
(Ar-C), 123.5 (Ar-C), 122.2 (d, JC–F = 7.3 Hz, Ar-C), 115.1 (d,
N - (2,6 - Diisopropylphenyl) - N¢ - (3-fluorophenyl)benzamidine
(3g). The procedure was similar to that of compound 3b except
that N-(2,6-diisopropylphenyl)benzamide (3.9 g, 14 mmol) and
3-fluoroaniline (1.4 g, 12 mmol) were used. Crystallization in
ethanol–water afforded colorless crystals (2.7 g, 55%) (Found: C,
80.30; H, 7.13; N, 7.32. Calc. for C25H27FN2: C, 80.18; H, 7.27;
2JC–F = 20.8 Hz, Ar-C), 30.8 [–CH(CH3)2], 28.9 [–CH(CH3)2], 28.2
[–CH(CH3)2], 23.7 [br, –CH(CH3)2].
N - (2,6 - Diisopropylphenyl) - N¢ - (2 - chlorophenyl)benzamidine
(3d). The procedure was similar to that of compound 3b except
that N-(2,6-diisopropylphenyl)benzamide (6.80 g, 24.3 mmol) and
o-chloroaniline (2.4 mL, 23.0 mmol) were used. Crystallization
with toluene afforded colorless crystals (6.2 g, 65%) (Found: C,
76.87; H, 7.10; N, 7.05. Calc. for C25H27ClN2: C, 76.80; H, 6.96;
N, 7.17%); mp 140–141 ◦C; dH (500 MHz, CDCl3, 25 ◦C) 7.62
(br s, 1H, Ar-H), 7.45 (d, 3J = 6.3 Hz, 1H, Ar-H), 7.34–6.50 (m,
10H, Ar-H), 6.43 (s, 0.4H, –NH), 6.30 (s, 0.4H, –NH), 5.84 (s,
◦
◦
N, 7.48%); mp 108 C; dH (500 MHz, CDCl3, 25 C) 7.63 (br s,
1H, Ar-H), 7.43–7.31 (m, 3H, Ar-H), 7.25–7.20 (m, 2H, Ar-H),
7.13 (br s, 1H Ar-H), 7.01 (d, 1H, 3J = 6.4 Hz, Ar-H), 6.60 (b s,
1H, Ar-H), 6.47–6.24 (m, 3H, Ar-H), 5.80 (s, 1H, –NH), 3.71
[septet, 0.22H, 3J = 6.7 Hz, –CH(CH3)2], 3.40 [septet, 0.32H, 3J =
6.7 Hz, –CH(CH3)2], 3.12 [br s, 1.46H, –CH(CH3)2], 1.37 [br s,
0.96H, –CH(CH3)2], 1.27 [br s, 0.96H, –CH(CH3)2], 1.23 [d, 4.4H,
3J = 6.7 Hz, –CH(CH3)2], 1.20 [d, 4.4H, 3J = 6.7 Hz, –CH(CH3)2],
3
0.2H, –NH), 3.54 [septet, 0.4H, J = 6.5 Hz, –CH(CH3)2], 3.15
[m, 1.6H, –CH(CH3)2], 1.39 [br s, 1.2H, –CH(CH3)2], 1.41–1.04
[m, 9.6H, –CH(CH3)2], 0.94 [br s, 1.2H, 3J = 6.5 Hz, –CH(CH3)2];
1.08 [d, 0.6H, 3J = 6.7 Hz, –CH(CH3)2], 0.98 [d, 0.6H,3J = 6.7 Hz,
◦
◦
=
=
dC (125 MHz, CDCl3, 25 C) 157.5 (C N), 152.9 (br, Ar-C),
–CH(CH3)2]; dC (125 MHz, CDCl3, 25 C) 153.2 (C N), 146.5
148.7 (Ar-C), 146.9 (Ar-C), 143.3 (br, Ar-C), 138.7 (br, Ar-C),
(Ar-C), 138.9 (Ar-C), 134.7 (Ar-C), 130.1 (Ar-C), 129.8 (Ar-C),
137.3 (Ar-C), 134.9 (Ar-C), 130.0 (br, Ar-C), 129.7 (Ar-C), 129.2
129.7 (Ar-C), 129.6 (Ar-C), 128.7 , (Ar-C) 128.6 (Ar-C), 128.4
8078 | Dalton Trans., 2010, 39, 8071–8083
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(Ar-C), 128.3 (Ar-C), 128.1 (Ar-C), 124.0 (Ar-C), 123.6 (Ar-C),
123.5 (Ar-C), 118.6 (Ar-C), 117.6 (Ar-C), 28.9 [–CH(CH3)2], 28.2
[–CH(CH3)2], 23.8 [–CH(CH3)2], 23.5 [–CH(CH3)2].
N -(2,6-Diisopropylphenyl)-N¢-(4-methoxylphenyl)benzamidine
(3k). The procedure was similar to that of compound 3b except
that N-(2,6-diisopropylphenyl)benzamide (6.7 g, 24 mmol) and 4-
methoxylaniline (2.80 g, 22.6 mmol) were used. Crystallization in
toluene afforded colorless crystals (6.3 g, 68%) (Found: C, 80.59;
H, 8.05; N, 7.11. Calc. for C26H30N2O: C, 80.79; H, 7.82; N, 7.25%);
N - (2,6 - Diisopropylphenyl) - N¢ - (4 - fluorophenyl)benzamidine
(3h). The procedure was similar to that of compound 3b except
that N-(2,6-diisopropylphenyl)benzamide (6.75 g, 24.0 mmol) and
4-fluoroaniline (2.2 mL, 23 mmol) were used. Crystallization in
ethanol–water afforded colorless crystals (5.7 g, 63%) (Found: C,
80.04; H, 7.29; N, 7.25. Calc. for C25H27FN2: C, 80.18; H, 7.27; N,
7.48%); mp 110–112 ◦C; dH (500 MHz, CDCl3, 25 ◦C) 7.71 (br s,
2H, Ar-H), 7.34–7.47 (m, 3H, Ar-H), 7.21–7.25 (m, 2H, Ar-H),
7.12 (m, 1H, Ar-H), 6.78 (br s, 1.6H, Ar-H), 6.69 (m, 0.4H, Ar-H),
6.60 (br s, 1.6H, Ar-H), 6.49 (m, 0.4H, Ar-H), 6.16 (s, 0.9H, –NH),
5.75 (s, 0.1H, –NH), 3.40 [septet, 0.2H, 3J = 6.7 Hz, –CH(CH3)2],
3.16 [septet, 1.8H, 3J = 6.8 Hz, –CH(CH3)2], 1.38–1.24 [m, 10.8H,
◦
◦
3
mp 114–115 C; dH (500 MHz, CDCl3, 25 C) 7.58 (d, 2H, J =
6.8 Hz, Ar–H), 7.31–7.36 (m, 3H, Ar-H), 7.20 (d, 2H, 3J = 7.3 Hz,
Ar-H), 7.09–7.12 (m, 1H, Ar-H), 6.60 (m, 4H, Ar-H), 6.11 (s,
1H, –NH), 3.69 (s, 3H, –OCH3), 3.19 [septet, 2H, 3J = 6.8 Hz, –
CH(CH3)2], 1.23 [d, 12H, 3J = 6.8 Hz, –CH(CH3)2]; dC (100 MHz,
◦
=
CDCl3, 25 C) 156.3 (C N), 154.3 (Ar-C), 143.5 (Ar-C) 139.3
(Ar-C), 135.2 (Ar-C), 133.3 (Ar-C), 129.5 (Ar-C), 129.0 (Ar-C),
128.2 (Ar-C), 125.0 (Ar-C), 123.7 (Ar-C), 123.5 (Ar-C), 114.0
(Ar-C), 55.4 (OCH3), 28.2 [–CH(CH3)2], 24.0 [–CH(CH3)2], 23.6
[–CH(CH3)2].
3
–CH(CH3)2], 1.06 [d, J = 6.8 Hz, 0.6H, –CH(CH3)2], 0.97 [d,
3J = 6.8 Hz, 0.6H, –CH(CH3)2]; dC (125 MHz, CDCl3, 25 ◦C)
N-(2,6-Diisopropylphenyl)-N¢-(tert-butyl)benzamidine
(3l).
=
153.9 (C N), 146.5 (Ar-C), 143.3 (Ar-C), 139.2 (Ar-C), 136.3
The procedure was similar to that of compound 3b except
that N-(2,6-diisopropylphenyl)benzamide (8.0 g, 28 mmol) and
t-butylamine (2.5 g, 33 mmol) were used to react at 50 ◦C for
72 h. Crystallization in ethanol–water afforded light yellow
crystals (6.1 g, 65%) (Found: C, 82.22; H, 9.40; N, 8.31. Calc. for
C23H32N2: C, 82.09; H, 9.58; N, 8.32%) mp 88 ◦C; dH (500 MHz,
(Ar-C), 134.9 (Ar-C), 129.8 (Ar-C), 129.0 (Ar-C), 128.8 (Ar-C),
128.6 (Ar-C), 128.5 (Ar-C), 128.4 (Ar-C), 124.6 (d, 3JC–F = 6.2 Hz,
2
Ar-C), 123.9 (Ar-C), 123.6 (Ar-C), 115.5 (d, Ar-C, JC–F = 22.5
Hz), 28.9 [–CH(CH3)2], 28.2 [–CH(CH3)2], 23.9 [–CH(CH3)2], 23.5
[–CH(CH3)2].
◦
N-(2,6-Diisopropylphenyl)-N¢-(4-chlorophenyl)benzamidine (3i).
The procedure was similar to that of compound 3b except that
N-(2,6-diisopropylphenyl)benzamide (7.0 g, 25 mmol) and 4-
chloroaniline (2.9 g, 23 mmol) were used. Crystallization in toluene
afforded colorless crystals (6.3 g, 69%) (Found: C, 76.84; H, 7.11;
N, 7.10. Calc. for C25H27ClN2: C, 76.80; H, 6.96; N, 7.17%); mp
131–132 ◦C; dH (500 MHz, CDCl3, 25 ◦C) 7.71 (br s, 1H, Ar-
H), 7.35–7.40 (m, 4H, Ar-H), 6.90–7.23 (m, 5H, Ar-H), 6.47–
6.90 (m, 2H, Ar-H), 6.18 (s, 0.85H, –NH), 5.79 (s, 0.15H, –NH),
CDCl3, 25 C) 7.16 (m, 5H, Ar-H), 6.92 (br s, 2H, Ar-H), 6.85
(br s, 1H, Ar-H), 4.39 (s, 1H, –NH), 3.07 [septet, 2H, 3J =
3
6.8 Hz, –CH(CH3)2], 1.56 [s, 9H, –C(CH3)3], 1.12 [d, 6H, J =
3
6.8 Hz, –CH(CH3)2], 0.94 [d, 6H, J = 6.8 Hz, –CH(CH3)2]; dC
◦
=
(125 MHz, CDCl3, 25 C) 152.7 (C N), 146.1 (Ar-C), 138.2
(Ar-C), 136.5 (Ar-C), 128.7 (Ar-C), 127.8 (Ar-C), 127.7 (Ar-C),
122.3 (Ar-C), 121.3 (Ar-C), 51.6 [–C(CH3)3], 28.9 [–CH(CH3)2],
28.1 [–C(CH3)3], 24.4 [–CH(CH3)2], 21.9 [–CH(CH3)2].
N-(2,6-Dimethylphenyl)-N-(2-methylphenyl)benzamidine (3m).
The procedure was similar to that of compound 3b.52 N-(2,6-
Dimethylphenyl)benzamide (5.10 g, 22 mmol) and 2-methylaniline
(2.14 ml, 20 mmol) were used according to the method used before.
Distillation and crystallization the crude product afforded light
yellow crystals (1.66 g, 53%) (Found: C, 84.14; H, 7.02; N, 8.87.
3
3.39 [septet, 0.3H, J = 6.6 Hz, –CH(CH3)2], 3.13 [septet, 1.7H,
3J = 6.6 Hz, –CH(CH3)2], 1.20–1.16 [m, 10.2H, –CH(CH3)2],
3
3
1.07 [d, 0.9H, J = 6.6 Hz, –CH(CH3)2], 0.97 [d, 0.9H, J =
◦
=
6.6 Hz, –CH(CH3)2]; dC (125 MHz, CDCl3, 25 C) 153.4 (C N),
146.5 (Ar-C), 143.6 (Ar-C), 139.1 (Ar-C), 134.7 (Ar-C), 130.0
(Ar-C), 129.6 (Ar-C), 128.9 (Ar-C), 128.7 (Ar-C), 128.5 (Ar-C),
128.2 (Ar-C), 124.0 (Ar-C), 123.6 (Ar-C), 29.0 [–CH(CH3)2], 28.3
[–CH(CH3)2], 24.0 [–CH(CH3)2), 23.6 [–CH(CH3)2]. ESI-MS m/z
◦
Calc. for C22H22N2: C, 84.04; H, 7.05; N, 8.91%); mp 82 C; dH
(400 MHz, CDCl3, 25 ◦C) 7.60 (d, 1.3H, 3J = 7.2 Hz, Ar-H), 7.49
(d, 0.4H, 3J = 7.2 Hz, Ar-H), 7.37–7.29 (m 3.5H, Ar-H), 7.10 (t,
2.4H, 3J = 7.2 Hz, Ar-H), 7.03 (d, 0.4H, 3J = 7.2 Hz, Ar-H), 6.96–
6.86 (m, 3.3H, Ar-H), 6.52 (d, 0.7H, 3J = 8.0 Hz, Ar-H), 5.91 (s,
1H, –NH), 2.38 (s, 0.7H, Ar–CH3), 2.29 (s, 4.5H, Ar–CH3), 2.18
+
i
+
=
(%): 446 (M ), 264 (100, [2,6- Pr2C6H3N CC6H5] ).
N -(2,6-Diisopropylphenyl)-N¢-(4-isopropylphenyl)benzamidine
(3j). The procedure was similar to that of compound 3b except
that N-(2,6-diisopropylphenyl)benzamide (6.7 g, 24 mmol) and 4-
isopropylaniline (3.1 mL, 23 mmol) were used. Crystallization in
ethanol–water afforded colorless crystals (5.1 g, 56%) (Found: C,
84.35; H, 8.77; N, 6.84. Calc. for C28H34N2: C, 84.37; H, 8.60; N,
7.03%); mp 90–91 ◦C; dH (500 MHz, CDCl3, 25 ◦C) 7.61 (d, 2H,
3J = 6.0 Hz, Ar-H), 7.33–7.38 (m, 3H, Ar-H), 7.20 (d, 2H, 3J =
6.8 Hz, Ar-H), 7.12 (m, 1H, Ar-H), 6.91 (d, 2H, 3J = 7.0 Hz, Ar-
H), 6.52 (d, 2H, 3J = 7.0 Hz, Ar-H), 6.18 (s, 1H, –NH), 3.16 [septet,
1.4H, 3J = 6.8 Hz, –CH(CH3)2], 2.75 [septet, 0.6H, 3J = 6.8 Hz,
–CH(CH3)2], 1.23 [d, 8.4H, 3J = 6.8 Hz, –CH(CH3)2], 1.14 [d, 3.6
H, 3J = 6.1 Hz, –CH(CH3)2]; dC (125 MHz, CDCl3, 25 ◦C) 154.0
(s, 2.4H, Ar–CH3), 2.16 (s, 1.4H, Ar–CH3); dC (100 MHz, CDCl3,
◦
=
25 C) 153.8 (C N), 145.8 (Ar-C), 139.4 (Ar-C), 134.8 (Ar-C),
130.4 (Ar-C), 130.2 (Ar-C), 129.7 (Ar-C), 128.6 (Ar-C), 128.5
(Ar-C), 128.3 (Ar-C), 128.1 (Ar-C), 126.2 (Ar-C), 124.9 (Ar-C),
124.3 (Ar-C), 123.0 (Ar-C), 18.6 (Ar-CH3), 17.9 (Ar-CH3) , 17.8
(Ar-CH3).
N-(2,6-Dimethylphenyl)-N¢-phenylbenzamidine (3n). The pro-
cedure was similar to that of compound 3m.52 N-(2,6-
Dimethylphenyl)benzamide (2.5 g, 10.3 mmol) and aniline (0.96 g,
10.3 mmol) were used according to the method used before.
Crystallization in ethanol–water afforded light yellow crystals
(1.5 g, 51%) (Found: C, 84.09; H, 6.63; N, 9.13. Calc. for C21H20N2:
C, 83.96; H, 6.71; N, 9.33%); mp 84 ◦C; dH (500 MHz, CDCl3,
=
(C N), 144.2 (Ar-C), 143.5 (Ar-C), 139.2 (Ar-C), 137.9 (Ar-C),
135.3 (Ar-C), 129.6 (Ar-C), 129.0 (Ar-C), 128.9 (Ar-C), 128.3 (Ar-
C), 126.7 (Ar-C), 123.6 (Ar-C), 122.7 (Ar-C), 33.4 [–CH(CH3)2],
28.2 [–CH(CH3)2], 24.0 [–CH(CH3)2], 23.6 [–CH(CH3)2].
◦
25 C) 7.76–7.66 (m, 2H, Ar-H), 7.39–7.31 (m, 4H, Ar-H), 7.09
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3
3
4
3
(br s, 3H, Ar-H), 6.93–6.89 (m, 2H, Ar-H), 6.64 (d, 1H, J =
(tt, 1H, J = 7.2 Hz, J = 1.8 Hz, Ar-H), 7.12 (dd, 1H, J =
6.3 Hz, Ar-H), 6.18 (s, 1H, –NH), 2.23 (s, 4.3H, Ar–CH3), 2.18
(s, 0.7H, Ar–CH3), 2.10 (s, 1H, Ar–CH3). dC (125 MHz, CDCl3,
25 C) 153.7 (C N), 146.0 (Ar-C), 140.3, (Ar-C) 135.1 (Ar-C),
129.9 (Ar-C), 129.6 (Ar-C), 129.1 (Ar-C), 128.8 (Ar-C), 128.4 (Ar-
C), 128.3 (Ar-C), 123.5 (Ar-C), 123.2 (Ar-C), 122.6 (Ar-C), 18.0
(Ar-CH3).
8.6 Hz, 3J = 6.6 Hz, Ar-H), 7.07–7.00 (m, 6H, Ar-H), 6.92 (t, 1H,
3J = 8.0 Hz, Ar-H), 3.33 [septet, 2H, J = 6.8 Hz, –CH(CH3)2],
3
◦
1.18 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2], 0.86 [d, 6H, 3J = 6.8 Hz,
=
–CH(CH3)2], -0.47 [s, 6H, –Al(CH3)2]; dC (125 MHz, CDCl3,
◦
=
25 C) 173.1 (C N), 143.8 (Ar-C), 139.7 (Ar-C), 137.1 (Ar-C),
132.3 (Ar-C), 130.7 (Ar-C), 129.5 (Ar-C), 128.3 (Ar-C), 127.3 (Ar-
C), 125.9 (Ar-C), 125.4 (Ar-C), 123.5 (Ar-C), 28.0 [–CH(CH3)2],
25.6 [–CH(CH3)2], 22.9 [–CH(CH3)2], -9.9 [–Al(CH3)2]; Crystal
data for 4b: C27H31AlCl2N2, Mr = 481.42, monoclinic, space
Synthesis of amidinate aluminium complexes
[{PhC(N-2,6-iPr2C6H3)(N-2,6-Me2C6H3)}AlMe2] (4a). To a
solution of trimethyl aluminium (2.3 mL, 4.6 mmol, 2 M in
toluene) in toluene (30 mL), compound 3a (1.48 g, 3.86 mmol)
was added slowly over a period of half an hour. Instantaneous
evolution of methane and almost full dissolution of the suspension
were observed. The colorless reaction solution was kept stirring
for 24 h at 70 ◦C. After removal of all volatiles under vacuum,
˚
group C2/c, a = 24.771(4), b = 14.720(2), c = 17.004(2) A, b
◦
3
-3
˚
= 118.966(3) , V = 5424.8(13) A , Z = 8, Dc = 1.179 Mg m ,
m = 0.288 mm-1, 13973 reflections measured and 5054 reflections
unique, final R1 = 0.0638, wR2 = 0.1276 (I > 2s(I)).
[{PhC(N-2,6-iPr2C6H3)(N-2-FC6H4)}AlMe2] (4c). The proce-
dure was similar to that of complex 4a, except that proligand 3c
(1.38 g, 3.68 mmol) and a solution of trimethyl aluminium (2.7 mL,
5.4 mmol, 2 M in toluene) were used. Crystallization of the crude
product from n-hexane afforded colorless crystals (0.68 g, 44%)
(Found: C, 75.29; H, 7.58; N, 6.58. Calc. for C27H32AlFN2: C,
◦
the obtained solids were crystallized from n-hexane at -40 C to
afford colorless crystals (0.95 g, 57%) (Found: C, 79.11; H, 8.39;
N, 6◦.37. Calc. for C29H37AlN2: C, 79.05; H, 8.46; N, 6.36%); mp
◦
3
120 C; dH (500 MHz, CDCl3, 25 C) 7.15 (tt, 1H, J = 7.8 Hz,
75.32; H, 7.49; N, 6.51%); mp 121–122 ◦C; dH (400 MHz, CDCl3,
4J = 1.0 Hz, Ar-H), 7.10 (dd, 1H, J = 8.5 Hz, J = 7.0 Hz,
3
3
◦
3
4
25 C) 7.28 (tt, 1H, J = 7.2 Hz, J = 2.0 Hz, Ar-H), 7.17 (td,
Ar-H), 7.03–6.98 (m, 4H, Ar-H), 6.96 (m, 2H, Ar-H), 6.92–6.89
3
4
3
2H, J = 7.2 Hz, J = 1.6 Hz, Ar-H), 7.13–7.09 (m, 3H Ar-
H), 7.03–6.97 (m, 3H, Ar-H), 6.90–6.84 (m, 1H, Ar-H), 6.79
(m, 3H, Ar-H), 3.25 [septet, 2H, J = 6.8 Hz, –CH(CH3)2], 2.21
(s, 6H, –CH3), 1.17 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2], 0.86 [d, 6H,
3
4
3
3J = 6.8 Hz, –CH(CH3)2], -0.50 [s, 6H, –Al(CH3)2]; dC (125 MHz,
(td, 1H, J = 8.0 Hz, J = 1.2 Hz, Ar-H), 6.54 (td, 1H, J =
4
3
◦
8.0 Hz, J = 1.6 Hz, Ar-H). 3.21 [septet, 2H, J = 6.8 Hz, –
=
CDCl3, 25 C) 171.9 (C N), 143.8 (Ar-C) 141.5 (Ar-C), 137.9
3
CH(CH3)2], 1.15 [d, 6H, J = 6.8 Hz, –CH(CH3)2], 0.87 [d, 6H,
(Ar-C), 133.4 (Ar-C), 130.5 (Ar-C), 129.6 (Ar-C), 129.2 (Ar-C),
128.2 (Ar-C), 127.7 (Ar-C), 125.4 (Ar-C), 124.5 (Ar-C), 123.5
(Ar-C), 28.2 [–CH(CH3)2], 25.6 (–CH3), 22.8 [–CH(CH3)2], 19.1
[CH(CH3)2], -9.7 [–Al(CH3)2].
3J = 6.8 Hz, –CH(CH3)2], -0.51 [s, 6H, –Al(CH3)2]; dC (100 MHz,
◦
1
=
CDCl3, 25 C) 171.5 (C N), 155.7 (d, JF–C = 243.2 Hz, Ar-
3
=
=
C), 143.7 (Ar C), 137.1 (Ar C), 131.8 (d, JF–C = 10.5 Hz,
Ar-C), 130.8 (Ar-C), 129.2 (Ar-C), 129.1 (Ar-C), 128.3 (Ar-C),
125.9 (Ar-C), 123.8 (Ar-C), 123.7 (Ar-C), 123.5 (Ar-C), 123.1
(Ar-C), 115.7 (d, 2JF–C = 20.5 Hz, Ar-C), 28.2 [–CH(CH3)2], 25.7
[–CH(CH3)2], 22.9 [–CH(CH3)2], -10.9 [–Al(CH3)2]; Crystal data
[{PhC(N-2,6-iPr2C6H3)(N-2,6-Me2C6H3)}AlEt2] (4a¢). The
procedure was similar to that of complex 4a, except that
proligand 3a (1.53 g, 3.98 mmol) and a solution of triethyl
aluminium (5.12 mL, 4.8 mmol, 0.94 M in petroleum ether) were
used. Crystallization of the crude product from n-hexane for
several times afforded colorless crystals (0.76 g, 41%) (Found:
C, 78.84; H, 8.31; N, 6.27. Calc. for C31H41AlN2: C, 79.45; H,
8.82; N, 5.98%); mp 91 ◦C; dH (500 MHz, C6D6 25 ◦C) 7.03
(m, 3H, Ar-H), 6.98 (m, 2H, Ar–H), 6.86–6.83 (m, 3H, Ar-H),
¯
for 4c: C27H32AlFN2, Mr = 430.53, triclinic, space group P1, a
˚
= 9.005(7), b = 10.674(8), c = 14.0804(11) A, a = 69.3390(10),
◦
3
˚
b = 83.1210(10), g = 83.9360(10) , V = 1254.22(17) A , Z = 2,
Dc = 1.140 Mg m-3, m = 0.104 mm-1, 7464 reflections measured
and 5331 reflections unique, final R1 = 0.0511, wR2 = 0.1349 (I >
2s(I)).
3
6.62 (m, 1H, Ar–H), 6.57 (m, 2H, Ar-H), 3.54 [septet, 2H, J =
[{PhC(N-2,6-iPr2C6H3)(N-2-ClC6H4)}AlMe2]
(4d). The
6.8 Hz, –CH(CH3)2], 2.28 (s, 6H, –CH3), 1.39 [t, 6H, 3J = 8.0 Hz,
–Al(CH2CH3)2], 1.28 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2], 0.91
procedure was similar to that of complex 4a, except that proligand
3d (1.04 g, 2.66 mmol) and a solution of trimethyl aluminium
(1.7 mL, 3.4 mmol, 2 M in toluene) were used. Crystallization
of the crude product from n-hexane afforded colorless crystals
(0.54 g, 45%); (Found: C, 72.67; H, 7.21; N, 6.35. Calc. for
C27H32AlClN2: C, 72.55; H, 7.22; N, 6.27%); mp 106–107 ◦C;
3
3
[d, 6H, J = 6.8 Hz, –CH(CH3)2], 0.61 [qd, 2H, J = 8.0 Hz,
2J = 3.0 Hz, –Al(CH2CH3)2]; dC (125 MHz, CDCl3, 25 ◦C)
=
172.1 (C N), 143.5 (Ar-C), 141.6 (Ar-C), 138.0 (Ar-C), 133.1
(Ar-C), 130.5 (Ar-C), 129.6 (Ar-C), 129.2 (Ar-C), 128.2 (Ar-C),
127.7 (Ar-C), 125.4 (Ar-C), 124.4 (Ar-C), 123.4 (Ar-C), 28.2
[–CH(CH3)2], 25.5 (–CH3), 22.8 [–CH(CH3)2], 19.1 [–CH(CH3)2],
8.8 [–Al(CH2CH3]2, -0.06 [–Al(CH2CH3)2].
◦
dH (500 MHz, CDCl3, 25 C) 7.34 (m, 1H, Ar-H), 7.27 (tt, 1H,
3J = 7.5 Hz, 4J = 1.0 Hz, Ar-H), 7.15 (t, 2H, 3J = 8.0 Hz,
Ar-H), 7.11 (t, 1H, 3J = 7.9 Hz, Ar-H), 7.07 (d, 2H, 3J = 7.9 Hz,
[{PhC(N-2,6-iPr2C6H3)(N-2,6-Cl2C6H3)}AlMe2] (4b). The
procedure was similar to that of complex 4a, except that proligand
3b (0.53 g, 1.2 mmol) and a solution of trimethyl aluminium
(0.9 mL, 1.8 mmol, 2 M in toluene) were used. Crystallization
of the crude product from n-hexane afforded colorless crystals
(0.35 g, 62%) (Found: C, 67.44; H, 6.37; N, 5.85. Calc. for
C27H31AlCl2N2: C, 67.36; H, 6.49; N, 5.82%); mp 116 ◦C; dH
(400 MHz, CDCl3, 25 ◦C) 7.22 (d, 2H, 3J = 8.0 Hz, Ar-H), 7.18
Ar-H), 7.03 (d, 2H, J = 7.5 Hz, Ar-H), 6.85 (m, 2H, Ar-H),
3
3
6.45 (m, 1H, Ar-H), 3.19 [septet, 2H, J = 6.8 Hz, –CH(CH3)2],
1.17 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2], 0.87 (d, 6H, 3J =
6.8 Hz, –CH(CH3)2), -0.51 [s, 6H, –Al(CH3)2]; dC (100 MHz,
◦
=
CDCl3, 25 C) 171.5 (C N), 143.5 (Ar-C), 141.0 (Ar-C),
137.0 (Ar-C), 130.9 (Ar-C), 129.7 (Ar-C), 129.6 (Ar-C), 129.0
(Ar-C), 128.3 (Ar-C), 127.3 (Ar-C), 126.6 (Ar-C), 125.9
(Ar-C), 124.4 (Ar-C), 123.5 (Ar-C), 123.1 (Ar-C),
8080 | Dalton Trans., 2010, 39, 8071–8083
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28.1 [–CH(CH3)2], 25.6 [–CH(CH3)2], 22.9 [–CH(CH3)2],
-10.2 [–Al(CH3)2].
22 Hz, Ar-C), 109.5 (d,2JC–F = 22 Hz Ar-C), 29.2 [–CH(CH3)2],
25.7 [–CH(CH3)2], 22.8 [–CH(CH3)2], -10.8 [–Al(CH3)2].
[{PhC(N-2,6-iPr2C6H3)(N-2-MeC6H4)}AlMe2] (4e). The pro-
cedure was similar to that of complex 4a, except that proligand 3e
(1.6 g, 4.3 mmol) and a solution of trimethyl aluminium (2.6 mL,
4.6 mmol, 2 M in toluene) were used. Crystallization of the crude
product from n-hexane afforded off-white crystals of 4e (0.78 g,
42%) (Found: C, 78.39; H, 8.34; N, 6.4◦1. Calc. for C28H35AlN2: C,
78.84; H, 8.27; N, 6.57%); mp 81–82 C; dH (500 MHz, CDCl3,
25 ◦C) 7.18 (t, 1H, 3J = 7.5 Hz, Ar-H), 7.14 (d, 1H, 3J = 7.5 Hz,
Ar-H), 7.10 (t, 1H, 3J = 7.5 Hz, Ar-H), 7.06 (t, 2H, 3J = 7.8 Hz,
Ar-H), 7.02 (d, 2H, 3J = 7.5 Hz, Ar-H), 6.96 (d, 2H, 3J = 7.8 Hz,
[{PhC(N-2,6-iPr2C6H3)(N-4-FC6H4)}AlMe2] (4h). The pro-
cedure was similar to that of complex 4a, except that the proligand
3h (1.14 g, 3.05 mmol) and a solution of trimethyl aluminium
(1.9 mL, 3.8 mmol, 2 M in toluene) were used. Crystallization
of the crude product from n-hexane afforded colorless crystals
(0.43 g, 33%) (Found: C, 75.30; H, 7.41; N, 6.76. Ca◦lc. for
C27H32AlFN2: C, 75.32; H, 7.49; N, 6.51%); mp 112–114 C; dH
(500 MHz, CDCl3, 25 ◦C) 7.26 (t, 1H, 3J = 8.0 Hz, Ar–H), 7.14 (t,
2H, 3J = 8.0 Hz, Ar-H), 7.09 (t, 1H, 3J = 8.2 Hz, Ar-H), 7.02 (t,
4H, 3J = 8.4 Hz, Ar-H), 6.82 (t, 2H, 3J = 8.4 Hz, Ar-H), 6.67 (m,
2H, Ar-H), 3.22 [septet, 2H, 3J = 6.8 Hz, –CH(CH3)2], 1.16 [d, 6H,
3J = 6.8 Hz, –CH(CH3)2], 0.90 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2],
3
4
Ar–H), 6.90 (m, 2H, Ar–H), 6.55 (dd, 1H, J = 7.5 Hz, J =
1.5 Hz, Ar-H), 3.26 [septet, 2H, 3J = 6.8 Hz, –CH(CH3)2], 2.33 (s,
3H, Ar–CH3), 1.16 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2], 0.86 [d, 6H,
-0.51 [s, 6H, –Al(CH3)2]; dC (100 MHz, CDCl3, 25 ◦C) 171.4
3J = 6.8 Hz, –CH(CH3)2], -0.52 [s, 6H, –Al(CH3)2]; dC (125 MHz,
(C N), 158.9 (Ar-C, JC–F = 240 Hz), 143.8 (Ar-C), 139.8 (Ar-
1
=
◦
=
CDCl3, 25 C) 172.0 (C N), 143.7 (Ar-C), 142.8 (Ar-C), 137.6
C), 139.8 (Ar-C), 137.2 (Ar-C), 130.6 (Ar-C), 129.8 (Ar-C), 128.5
(Ar-C), 128.2 (Ar-C), 125.8 (Ar-C), 124.6 (Ar-C), 124.5 (Ar-C),
123.5 (Ar-C), 115.5 (Ar-C, 2JC–F = 20 Hz), 28.2 [–CH(CH3)2], 25.7
[–CH(CH3)2], 22.8 [–CH(CH3)2], -10.8 [–Al(CH3)2].
(Ar-C), 131.9 (Ar-C), 130.5 (Ar-C), 130.4 (Ar-C), 129.9 (Ar-C),
129.1 (Ar-C), 127.9 (Ar-C), 126.1 (Ar-C), 125.6 (Ar-C), 125.5 (Ar-
C), 123.6 (Ar-C), 123.5 (Ar-C), 28.2 [–CH(CH3)2], 25.7 (Ar-CH3),
22.8 [–CH(CH3)2], 19.2 [–CH(CH3)2], -10.3 [–Al(CH3)2].
[{PhC(N-2,6-iPr2C6H3)(N-4-ClC6H4)}AlMe2] (4i). The pro-
cedure was similar to that of complex 4a, except that the proligand
3i (1.10 g, 2.81 mmol) and a solution of trimethyl aluminium
(1.8 mL, 3.6 mmol, 2 M in toluene) were used. Crystallization
of the crude product from n-hexane afforded colorless crystals
(0.73 g, 47%) (Found: C, 72.75; H, 7.43; N, 6.22. Ca◦lc. for
C27H32AlClN2: C, 72.55; H, 7.22; N, 6.27%); mp 104–105 C; dH
(500 MHz, CDCl3, 25 ◦C) 7.27 (t, 1H, 3J = 7.4 Hz, Ar-H), 7.15 (t,
2H, 3J = 8.0 Hz, Ar-H), 7.09 (dd, 1H, 3J = 8.5 Hz, 3J = 7.0 Hz,
Ar-H), 7.07–7.03 (m, 4H, Ar-H), 7.01 (d, 2H, 3J = 7.5 Hz, Ar–H),
[{PhC(N-2,6-iPr2C6H3)(N-Ph)}AlMe2] (4f). The procedure
was similar to that of complex 4a, except that proligand 3f
(1.0 g, 2.8 mmol) and a solution of trimethyl aluminium (1.7 mL,
3.3 mmol, 2 M in toluene) were used. Crystallization of the crude
product from n-hexane afforded light yellow crystals (0.78 g, 42%)
(Found: C, 78.55; H, 8.10; N, 6.74. Calc. for C27H33AlN2: C, 78.61;
H, 8.06; N, 6.79%); mp 112 ◦C; dH (500 MHz, CDCl3, 25 ◦C) 7.24
3
(t, 1H, J = 7.5 Hz, Ar-H), 7.15–7.08 (m, 5H, Ar-H), 7.05 (d,
2H, 3J = 8.0 Hz, Ar-H), 7.00 (d, 2H, 3J = 7.5 Hz, Ar-H), 6.94 (t,
3
3
3
3
1H, J = 7.0 Hz, Ar-H), 6.70 (d, 2H, J = 8.0 Hz, Ar-H), 3.22
6.62 (d, 2H, J = 8.5 Hz, Ar-H), 3.18 [septet, 2H, J = 6.8 Hz,
3
3
[septet, 2H, J = 6.8 Hz, –CH(CH3)2], 1.14 [d, 6H, J = 6.8 Hz,
–CH(CH3)2], 1.15 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2], 0.88 [d, 6H,
–CH(CH3)2], 0.87 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2], -0.51 [s, 6H,
3J = 6.8 Hz, –CH(CH3)2], -0.53 [s, 6H, –Al(CH3)2]; dC (100 MHz,
◦
◦
=
=
–Al(CH3)2]; dC (125 MHz, CDCl3, 25 C) 171.3 (C N), 143.8 (Ar-
CDCl3, 25 C) 171.4 (C N), 143.7 (Ar-C), 142.3 (Ar-C), 137.0
C), 143.6 (Ar-C), 137.2 (Ar-C), 130.5 (Ar–C), 129.7 (Ar-C), 128.7
(Ar-C), 128.72 (Ar-C), 128.1 (Ar-C), 125.7 (Ar-C), 123.4 (Ar-C),
123.3 (Ar-C), 122.7 (Ar-C), 28.1 [–CH(CH3)2], 25.7 [–CH(CH3)2],
22.8 [–CH(CH3)2], -10.8 [–Al(CH3)2].
(Ar-C), 130.8 (Ar-C), 129.7 (Ar-C), 128.9 (Ar-C), 128.5 (Ar-C),
128.3 (Ar-C), 127.9 (Ar-C), 125.9 (Ar-C), 124.4 (Ar-C), 123.5
(Ar-C), 28.2 [–CH(CH3)2], 25.7 [–CH(CH3)2], 22.8 [–CH(CH3)2],
-10.8 [Al(CH3)2]; ESI-MS m/z (%): 446 (trace, M+), 390 (7, [M–
+
i
+
=
Al(CH3)2] ), 264 (100, [2,6- Pr2C6H3N CC6H5] ).
[{PhC(N-2,6-iPr2C6H3)(N-3-FC6H4)}AlMe2] (4g). The pro-
cedure was similar to that of complex 4a, except that the proligand
3g (1.4 g, 3.7 mmol) and a solution of trimethyl aluminium
(2.3 mL, 4.6 mmol, 2 M in toluene) were used. Crystallization
of the crude product from n-hexane afforded colorless crystals
(0.95 g, 60%) (Found: C, 75.66; H, 7.51; N, 6.45. Calc. for
C27H32AlFN2: C, 75.32; H, 7.49; N, 6.51%); mp 98 ◦C; dH
(500 MHz, CDCl3, 25 ◦C) 7.28 (tt, 1H, 3J = 7.5 Hz, 4J = 1.0 Hz
Ar-H), 7.16 (t, 2H, 3J = 8.0 Hz, Ar-H), 7.11 (dd, 1H, 3J = 8.0 Hz,
3J = 6.5 Hz, Ar-H), 7.09–7.04 (m, 3H, Ar-H), 7.02 (d, 2H, 3J =
[{PhC(N-2,6-iPr2C6H3)(N-4-iPrC6H4)}AlMe2] (4j). The pro-
cedure was similar to that of complex 4a, except that proligand 3j
(1.09 g, 2.74 mmol) and a solution of trimethyl aluminium (1.7 mL,
3.4 mmol, 2 M in toluene) were used. Crystallization of the crude
product from n-hexane afforded colorless crystals (0.66 g, 53%)
(Found: C, 78.97; H, 8.78; N, 6.14. Calc. for C30H39AlN2: C, 79.26;
H, 8.65; N, 6.16%); mp 107–108 ◦C; dH (500 MHz, CDCl3, 25 ◦C)
7.24 (t, 1H, 3J = 7.5 Hz, Ar-H), 7.13 (t, 2H, 3J = 8.0 Hz, Ar-H),
7.08 (dd, 1H, 3J = 8.5 Hz, 3J = 7.0 Hz, Ar-H), 7.05 (d, 2H, 3J =
3
4
3
3
7.0 Hz, Ar-H), 6.64 (td, 1H, J = 8.0 Hz, J = 2.5 Hz, Ar-H),
8.5 Hz, Ar-H), 7.00 (d, 2H, J = 8.0 Hz, Ar-H), 6.96 (d, 2H, J
3
4
3
6.47 (dd, 1H, J = 8.0 Hz, J = 1.5 Hz, Ar-H), 6.37 (dt, 1H,
3J = 10.5 Hz, 4J = 2.5 Hz, Ar-H), 3.18 [septet, 2H, 3J = 7.0 Hz,
–CH(CH3)2], 1.14 [d, 6H, 3J = 7.0 Hz, –CH(CH3)2], 0.87 [d, 6H,
3J = 7.0 Hz, –CH(CH3)2], -0.51 [s, 6H, –Al(CH3)2]; dC (125 MHz,
= 8.0 Hz, Ar-H), 6.62 (d, 2H, J = 8.5 Hz, Ar-H), 3.23 [septet,
3
3
2H, J = 6.8 Hz, –CH(CH3)2], 2.79 [septet, 1H, J = 6.8 Hz, –
3
CH(CH3)2], 1.18 [d, J = 6.8 Hz, 6H, –CH(CH3)2], 1.13 [d, 6H,
3J = 6.8 Hz, –CH(CH3)2], 0.87 (d, 6H, 3J = 6.8 Hz, –CH(CH3)2),
◦
CDCl3, 25 C) 171.52 (C N), 162.8 (d, JC–F = 244 Hz), 145.4 (d,
-0.51 (s, 6H, –Al(CH3)2); dC (100 MHz, CDCl3, 25 ◦C) 171.1
1
=
3JC–F = 10 Hz) 143.7 (Ar-C), 136.9 (Ar-C), 130.8 (Ar-C), 129.7
(C N), 143.9 (Ar-C), 143.2 (Ar-C), 141.1 (Ar-C), 137.4 (Ar-C),
130.4 (Ar-C), 129.6 (Ar-C), 129.0 (Ar-C), 128.0 (Ar-C), 126.6 (Ar-
C), 125.6 (Ar-C), 123.3 (Ar-C), 122.9 (Ar-C), 33.4 [–CH(CH3)2],
=
(d, 3JC–F = 10 Hz, (Ar-C)), 129.6 (Ar-C), 128.5 (Ar-C), 128.3 (Ar-
2
C), 125.9 (Ar-C), 123.5 (Ar-C), 118.9 (Ar-C), 110.1 (d, JC–F
=
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Dalton Trans., 2010, 39, 8071–8083 | 8081
©

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28.1 [–CH(CH3)2], 25.7 [–CH(CH3)2], 24.0 [–CH(CH3)2], 22.8
[–CH(CH3)2], -10.7 [–Al(CH3)2].
product from n-hexane afforded colorless crystals (0.42 g, 28%);
(Found: C, 77.32; H, 6.97; N, 7.76. Calc. for C23H25AlN2: C, 77.50;
◦
◦
H, 7.07; N, 7.86%); mp 91–92 C; dH (500 MHz, CDCl3, 25 C)
7.27 (br s, 1H, 3J = 7.0 Hz, Ar-H), 7.14 (t, 2H, 3J = 8.0 Hz, Ar-H),
7.11 (t, 2H, 3J = 8.0 Hz, Ar-H), 7.07 (d, 2H, 3J = 8.0 Hz, Ar-H),
[{PhC(N-2,6-iPr2C6H3)(N-4-MeOC6H4)}AlMe2] (4k). The
procedure was similar to that of complex 4a, except that the
proligand 3k (1.20 g, 3.11 mmol) and a solution of trimethyl
aluminium (1.9 mL, 3.8 mmol, 2 M in toluene) were used.
Crystallization from n-hexane afforded light yellow crystals
(0.79 g, 57%) (Found: C, 75.46; H, 8.19; N, 6.11. Ca◦lc. for
C28H35AlN2O: C, 75.99; H, 7.97; N, 6.33%); mp 121–123 C; dH
(500 MHz, CDCl3, 25 ◦C) 7.23 (t, 1H, 3J = 7.5 Hz, Ar-H), 7.12 (t,
2H, 3J = 8.0 Hz, Ar-H), 7.07 (dd, 1H, 3J = 8.0 Hz, 3J = 6.5 Hz ,
Ar-H), 7.03 (d, 2H, 3J = 7.5 Hz, Ar-H), 7.00 (d, 2H, 3J = 7.5 Hz,
Ar-H), 6.66 (m, 4H, Ar-H), 3.73 (s, 3H, OCH3), 3.23 [septet, 2H,
3J = 6.8 Hz, –CH(CH3)2], 1.15 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2],
3
6.95–6.90 (m, 4H, Ar-H), 6.69 (d, 2H, J = 7.5 Hz, Ar-H), 2.18
(s, 6H, Ar–CH3), -0.50 [s, 6H, Al(CH3)2]; dC (125 MHz, CDCl3,
◦
=
25 C) 171.1 (C N), 143.5 (Ar-C), 140.8 (Ar-C), 133.6 (Ar-C),
130.5 (Ar-C), 129.6 (Ar-C), 128.7 (Ar-C), 128.1 (Ar-C), 128.0
(Ar-C), 124.9 (Ar-C), 123.2 (Ar-C), 122.8 (Ar-C), 19.1 (Ar-CH3),
-10.1 [Al(CH3)2].
[{PhC(N-2,6-iPr2C6H3)2}AlMe2] (5). The procedure was
similar to that of complex 4a, except that N,N¢-bis(2,6-
diisopropylphenyl)benzamidine (1.25 g, 2.86 mmol) and trimethyl
aluminium (2.1 mL, 4.2 mmol, 2 M in toluene) were used. Crys-
tallization of the crude product with toluene afforded colorless
crystals (1.01 g, 72%); (Found: C, 79.03; H, 9.22; N 5.51. Calc.
for C33H45AlN2: C, 79.80; H, 9.13; N, 5.64%); mp 110–111 ◦C;
dH (400 MHz, CDCl3, 25 ◦C) 7.15–7.04 (m, 4H, Ar-H), 7.04
(br s, 2H, Ar-H), 7.02 (br s, 2H, Ar-H), 6.99 (t, 2H, 3J = 8.0 Hz,
0.87 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2], -0.51 [s, 6H, –Al(CH3)2];
◦
=
dC (100 MHz, CDCl3, 25 C) 171.1 (C N), 155.5 (Ar-C), 143.9
(Ar-C), 137.5 (Ar-C), 136.9 (Ar-C), 130.4 (Ar-C), 130.2 (Ar-C),
129.8 (Ar-C), 128.9 (Ar-C), 128.1 (Ar-C), 127.7 (Ar-C), 126.0
(Ar-C), 125.6 (Ar-C), 124.3 (Ar-C), 123.4 (Ar-C), 114.1 (Ar-C),
113.4 (Ar-C), 55.4 (OCH3), 28.1 [CH(CH3)2], 25.7 [CH(CH3)2],
22.9 [CH(CH3)2], -10.8 [Al(CH3)2].
3
3
Ar-H), 6.90 (d, 2H, J = 8.0 Hz, Ar-H), 3.35 [septet, 4H, J =
6.8 Hz, –CH(CH3)2], 1.19 [d, 12H, 3J = 6.8 Hz, –CH(CH3)2], 0.86
[{PhC(N-2,6-iPr2C6H3)(N-tBu)}AlMe2] (4l). The procedure
was similar to that of complex 4a, except that the proligand 3l
(1.1 g, 3.3 mmol) and a solution of trimethyl aluminium in toluene
(2 mL, 4.0 mmol, 2 M in toluene) were used. Crystallization of the
crude product from n-hexane for several times afforded colorless
crystals (0.63 g, 50%); (Found: C, 76.24; H, 9.48; N, 7.03. Calc.
for C25H37AlN2: C, 76.49; H, 9.50; N, 7.14%); mp 130–131 ◦C; dH
(500 MHz, CDCl3, 25 ◦C) 7.20–7.18 (m, 3H, Ar-H), 7.13–7.11 (m,
2H, Ar-H), 6.94 (dd, 1H, 3J = 8.5 Hz, 3J = 6.0 Hz, Ar-H), 6.88 (d,
2H, 3J = 8.0 Hz Ar-H), 3.28 [septet, 2H,3J = 6.8 Hz, –CH(CH3)2],
3
[d, 12H, J = 6.8 Hz, –CH(CH3)2], -0.50 [s, 6H, –Al(CH3)2]; dC
◦
=
(100 MHz, CDCl3, 25 C) 172 4 (C N), 143.6 (Ar-C), 138.1 (Ar-
C), 130.4 (Ar-C), 130.1 (Ar-C), 128.8 (Ar-C), 127.6 (Ar-C), 125.4
(Ar-C), 123.5 (Ar-C), 28.3 [–CH(CH3)2], 25.5 [–CH(CH3)2], 22.7
[–CH(CH3)2], -10.3 [–Al(CH3)2]; Crystal data for 5: C33H45AlN2,
Mr = 496.69, trigonal, space group P3221, a = b = 14.948(8), c =
◦
3
˚
˚
12.2099(10) A, a = b = 90, g = 120 , V = 2362.8(3) A , Z = 3,
Dc = 1.047 Mg m-3, m = 0.086 mm-1, 13607 reflections measured
and 3273 reflections unique, final R1 = 0.0550, wR2 = 0.1186 (I >
2s(I)).
3
1.16 [d, 6H, J = 6.8 Hz, –CH(CH3)2], 1.14 (s, 9H, –C(CH3)3),
1.10 [d, 6H, 3J = 6.8 Hz, –CH(CH3)2], -0.55 [s, 6H –Al(CH3)2]; dC
(125 MHz, CDCl3, 25 ◦C) 173.1 (C=N), 144.9 (Ar-C), 137.7 (Ar-
C), 132.7 (Ar-C), 129.2 (Ar-C), 127.6 (Ar-C), 127.4 (Ar-C), 125.2
(Ar-C), 122.8 (Ar-C), 52.0 [–C(CH3)3], 32.2 [–CH(CH3)2], 28.1 [–
C(CH3)3], 26.3 [–CH(CH3)2], 22.8 [–CH(CH3)2], -9.8 [Al(CH3)2].
Typical polymerization procedure
To a solution of rac-lactide (167 mg, 1.16 mmol) in toluene
(0.6 mL), a solution of aluminium amidinate complex (0.012
mmol) in toluene (0.5 mL) was added. The total volume was
1.1 mL. The mixture was then immersed into an oil bath of 70 ◦C
for polymerization. The polymerization was quenched by addition
of wet petroleum ether. After removal of the volatiles, the residue
[{PhC(N-2,6-Me2C6H3)(N-2-MeC6H4)}AlMe2]
(4m). The
procedure was similar to that of complex 4a, except that proligand
3m (1.0 g, 3.2 mmol) and a solution of trimethyl aluminium
(2.3 mL, 4.6 mmol, 2 M in toluene) were used. Crystallization
of the crude product from n-hexane afforded colorless crystals
(0.43 g, 36%) (Found: C, 76.99; H, 7.24; N, 7.48. Calc. for
C24H27AlN2: C, 77.81◦; H, 7.35; N, 7.56%); mp 90 ◦C; dH
1
was subjected to H NMR analysis. Monomer conversion was
determined by observing the methine resonance integration of
1
monomer vs. polymer in the H NMR (CDCl3, 400 MHz) spec-
trum. The purification of the polymer in each case was managed
by dissolving the crude samples in CH2Cl2 and precipitating the
polymer solution with methanol. The obtained polymers were
further dried in a vacuum oven at 60 ◦C for 24 h. The polymer
samples were subjected to viscosity measurements and in selected
cases analyzed by GPC.
3
(400 MHz, CDCl3, 25 C) 7.21 (t, 1H, J = 7.2 Hz, Ar-H), 7.14
3
3
(d, 1H, J = 6.4 Hz, Ar-H), 7.07 (t, 2H, J = 7.6 Hz, Ar-H),
3
6.98 (d, 2H, J = 7.6 Hz, Ar-H), 6.93–6.90 (m, 5H, Ar-H), 6.54
3
(d, 1H, J = 7.2 Hz, Ar-H), 2.33 (s, 3H, Ar–CH3), 2.20 (s, 6H,
Ar–CH3), -0.51 [s, 6H, –Al (CH3)2]; dC (100 MHz, CDCl3, 25 ◦C)
=
171.8 (C N), 142.7 (Ar-C), 141.1 (Ar-C), 133.5 (Ar-C), 132.2
(Ar-C), 130.5 (Ar-C), 130.4 (Ar-C), 129.8 (Ar-C), 128.9 (Ar-C),
128.1 (Ar-C), 127.9 (Ar-C), 126.1 (Ar-C), 125.4 (Ar-C), 124.7
(Ar-C), 123.7 (Ar-C), 19.1 (Ar-CH3), -9.7 [–Al(CH3)2].
Acknowledgements
This work is subsidized by the National Basic Research Program
of China (2005CB623801), National Natural Science Foundation
of China (NNSFC, 20604009 and 20774027), the Program for
New Century Excellent Talents in University (for H. Ma, NCET-
06-0413) and the Fundamental Research Funds for the Central
[{PhC(N-2,6-Me2C6H3)(N-Ph)}AlMe2] (4n). The procedure
was similar to that of complex 4a, except that proligand 3n
(1.3 g, 4.3 mmol) and a solution of trimethyl aluminium (2.6 mL,
5.2 mmol, 2 M in toluene) were used. Crystallization of the crude
8082 | Dalton Trans., 2010, 39, 8071–8083
This journal is
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©

View Article Online
Universities (WK0914042). All the financial supports are grate-
fully acknowledged. H. Ma also thanks the very kind donation of
a Braun glove-box by AvH foundation.
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