Aluminum complexes based on pyridine substituted alcohols: synthesis, structure, and catalytic application in ROP

Article abstract of DOI:10.1039/c5dt01001b

A series of substituted pyridine dialcohols (2,6-bis(hydroxyalkyl)pyridines), 1-4, was used for the synthesis of various types of aluminum complexes. Aluminum methyl derivatives, 2-4a, were obtained by the reaction of AlMe₃ with the correspondi

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DOI: 10.1039/C5DT01001B
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Cite this: DOI: 10.1039/c0xx00000x
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ARTICLE TYPE
Aluminum complexes based on pyridine substituted alcohols: synthesis,
structure, catalytic application in ROP
a
a
a
a
Marina M. Kireenko, Ekaterina A. Kuchuk, Kirill V. Zaitsev,* Viktor A. Tafeenko, Yuri F.
a
b
b
a
a
Oprunenko, Andrei V. Churakov, Elmira Kh. Lermontova, Galina S. Zaitseva and Sergey S. Karlov
5
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
DOI: 10.1039/b000000x
A series of substituted pyridine dialcohols (2,6-bis(hydroxyalkyl)pyridines), 1-4, was used for the
synthesis of various types of aluminum complexes. Aluminum methyl derivatives, 2-4a, were obtained by
reaction of AlMe3 with corresponding ligand or transmetallation reactions of germylenes. Aluminum
chloride complexes, 3-4b, were obtained by substitution of Me group under action of chlorinating agents.
Methoxy-, 2-4c, or benzyloxy-, 2d, aluminum complexes were synthesized in transalkoxylation reaction
1
0
of Me Al(OX) (X = Me, Bn) by corresponding ligand. All complexes obtained were thoroughly
2
investigated by multinuclear NMR and X-ray analysis. It is established that the structure of ligand
(number of carbon atoms) determines the nature of the complexes formed. Compounds were used as
1
5
initiators of ring-opening polymerization of L-lactide and ε-caprolactone and showed moderate activity
with controlled or immortal character.
2
9
30
Zr, Mo and some others. Yet, to the best of our knowledge,
there are neither reports concerning the use of pyridine-
containing dialcohols in ROP nor aluminum complexes based on
such ligands in the literature. According to the literature
Introduction
In recent years compounds derived from renewable resources
have been involved in industry priority processes. Creation of
materials that are readily degradable in the environment is
another important trend. Classic examples of such materials are
biodegradable polymers of cyclic esters such as polylactide (PL),
polyglycolide (PG), poly-ε-caprolactone (PCL) and their
copolymers. Synthesis of biodegradable polymers of this type is
performed via ring-opening polymerization (ROP) of cyclic
1
5
5
6
0
5
0
aluminum complexes based on other ONO-type ligands are
2
2
3
3
4
4
0
5
0
5
0
5
31-35
limited by few examples.
Such complexes based on tridentate
2
ligands have found their application in catalysis in the number of
processes. As far as pyridine-diols are concerned, the main
attention was paid to the ligands containing bulky substituents at
carbon atoms of the alkoxy group. It is obvious that the presence
of such substituents in the complex molecule can stabilize
nonpolymeric or nonoligomeric structure which is important if
the “single-site catalyst” concept is considered. Herein we report
the synthesis, structural characterization of series of novel
aluminum complexes based on 2,6-bis(hydroxyalkyl)pyridines
and their application in the ring-opening polymerization of L-
lactide and ε-caprolactone. We managed to reveal the dependence
of the structure of the aluminum complexes on the structure of
the pyridine-containing dialcohols.
3
esters. Polymerization, initiated by organic compounds or metal
complexes, allows process to be high-controlled, leading to
materials with the desired structure, stereochemistry, molecular
weight distribution. The properties of the polymer strongly
depend on the initiator that is used. There are a lot of metals
(alkali earth, Al, Zn, Ti, Zr, Sn, lanthanides) that have been
4
-7
employed in ROP as well as ligands. Aluminum complexes (in
general, phenoxide type) attract researcher’s attention due to low
cost, high control of ROP and possibility to investigate the
8
-21
polymerization process.
6
5
Results and Discussion
It is established that ligand strongly influences the catalytic
activity of the complex. Thus it is important for systematic
investigation of “structure of complex : catalytic activity”
correlation to use ligands closely related in structure because the
small changes in structure can cause crucial changes in catalytic
Ligands. In the course of this work the ligands of three types
containing 0, 1 or 2 carbon atoms between pyridine and C(OH)
parts of the ligand were used for synthesis of aluminum
2
3
36
37
complexes. Compounds 1, 2,
3
(Chart 1) have been
activity. Pyridine-containing dialcohols (ONO type ligands) are 70 synthesized previously and unsymmetrical compound 4 is a novel
of great interest because of their special steric and electronic
properties. This promising ligand platform was used to stabilize
substance (Scheme 1).
Compound 4 was prepared through the monolithiation of Me
group by the treatment with 2 equivalents of n-BuLi with
subsequent reaction with 2,2-diphenyloxirane (Scheme 1).
2
2
low-valent species such as Sn (ІІ) and Ge (ІІ) as well as for
2
3
24
24, 25
26
27, 28
synthesis of complexes of Sn (ІV), Cr, V,
Si, Ti,
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Chart 1. Ligands 1, 2, 3.
Scheme 2. Synthesis of 2a, 3a, 4a and 5.
The complexes under discussion were isolated as white
powders soluble in THF, toluene, chlorinated solvents and
insoluble in hexane and ether. Compounds 2a, 3a, 4a are
35
moisture- and air-sensitive.
1
In H NMR spectra the protons of CH -groups became
2
Scheme 1. Synthesis of ligand 4.
diastereotopic because of strong N→Al interaction. The
1
3
In addition, single crystals of ligand 4 were grown from a
corresponding phenyl and/or methyl groups (according to
C
saturated solution in toluene and the structure was investigated by 40 NMR) at each hydroxyalkyl group are diastereotopic, too.
1
3
5
X-ray analysis (Fig. 1, Table 1). It should be noted that to date
there are only three other molecular structures of pyridine
dialcohols derived from 2,6-lutidine which have been
It should be noted that the C NMR spectra of 2a, 3a, 4a
contain only one set of signals corresponding to ligand
framework (CDCl solution). This fact may be related to both the
monomer structure of aluminum complexes and possible
3
3
8-40
investigated by X-ray crystallography.
The solid state
structure clearly shows the intramolecular H-bond interactions 45 equilibrium between monomeric and dimeric structures. In the
1
0
between the pyridine N-atom and hydroxylic hydrogen. This H-
case of equilibrium it’s rapid in the NMR time scale. We
bond may be regarded as mostly electrostatic and “moderate” in
recorded the NMR spectrum of 3a in CDCl -DMSO-d6 mixture
3
4
1
terms of strength. It should be noted that 4 represents the rare
example of the molecule without chiral centers crystallizing in
Sohncke space group P1.
(dropwise addition of DMSO-d6 to the solution of 3a in CDCl3;
see Supporting Information) and two species were found in the
50 solution. It may be attributed as the complex of 3a with DMSO-
d6 formed. The nature of second species (monomeric and
dimeric) is still in question. In order to clarify the structure we
4
2-44
carried out DOSY NMR experiment
for 3a to determine the
molecular mass of 3а in DMSO-d6 solution. Two species were
detected in this spectrum (see Supplementary materials), too.
Further calculations according to the equation that relates the
molecular weight and the diffusion coefficient (see for details
Supplementary materials) gave the molecular weights for these
5
6
6
5
0
5
-
10
2
species: M= 820 Da (D= 1.74*10 m /s), M= 497.1 Da (D=
-
10
2
2.19*10 m /s) which is in good correlation with molecular
weight for dimeric 3a (774.9 Da) and monomeric complex 3a
with additional DMSO-d6 ligand (3a*DMSO-d6, 465.6 Da).
Thus, one can conclude that there is monomer-dimer equilibrium
in the solution (both DMSO and chloroform) of the methyl
derivative 3a.
Furthermore, based on this fact it may be proposed that 3a, 4a
1
2
2
3
5
0
5
0
Fig. 1 Molecular structure of 4. Displacement ellipsoids are shown at 50
have the similar behavior in solution (C
room temperature on the NMR time scale).
For structure investigation of the aluminum complexes in
70 solution one of the most useful tools is the Al NMR
spectroscopy. Due to technical reasons, it is not always possible
to register spectra of complexes, so every result is very important.
S
symmetric structure at
%
probability level. Selected interatomic distances [Å] and angles [°]:
O(1)-H(1) 0.88(5), O(2)-H(2) 0.85(5), H(2)...N(1) 1.94(5), O(2)...N(1)
.715(4), O(2)-H(2)...N(1) 151(4).
Methyl aluminum complexes
bis(hydroxyalkyl)pyridines. The methyl aluminum complexes
a, 3a, 4a were obtained by the treatment of the ligands 2-4 with
2
7
2
based
on
2,6-
2
7
2
The Al NMR (CDCl
(δ= 40 ppm) and 4a (δ= 90 ppm). According to these data it may
avoid undesirable side-reactions (such as fast reaction of AlMe3 75 be confirmed that methylaluminum complexes based on pyridine
3
) spectra were obtained for compounds 2a
o
one molar equivalent of AlMe at -30 C (Scheme 2) in order to
3
with several ligand molecules). The yields of the target
compounds are high (82 97 %). In contrast to the
abovementioned ligands, reaction of ligand in similar
containing dialcohols are dimeric or at least take part in the
equilibrium process monomer-dimer in solution (for four-
coordinate Al alkoxides δ= ~70 ppm; for five-coordinate Al
alkoxides δ= ~40 ppm; for six-coordinate Al alkoxides δ= ~0
-
1
conditions (one or two equivalents of AlMe ) afforded the
3
4
5-53
mixture of compounds from which the unusual complex 5 was 80 ppm).
It should be noted, that crystal structure (X-ray data;
isolated. All the aluminum complexes discussed herein have been
see below) of 3a was studied; this derivative formed dimer with
1
13
characterized by elemental analysis,
spectroscopy.
H
and
C
NMR
five-coordinate Al atoms due to bridging CH
is in correlation with NMR data.
CZ O-groups, what
2 2
2
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Table 1 Crystallographic and data collection parameters for 4, 3a, 4c and 5
Compound
4
3a
4c
5
Formula
C
34
H
31NO
2
C
48
2 2 4
H52Al N O •C
7
H
8
C
70
2 2 6
H64Al N O •2(C
7 8
H )
C
62
H47AlN
2
O
4
•2(CHCl
3
)
Formula weight
Crystal size /mm
Crystal system
Space group
Z
485.60
0.21×0.20×0.11
triclinic
867.01
1267.46
0.25×0.20×0.15
monoclinic
1149.73
0.13×0.10×0.08
triclinic
3
0.24×0.04×0.02
monoclinic
C2/c
P1
1
P2
1
/c
P-1
2
4
2
a /Å
b /Å
c /Å
α /°
β /°
γ /°
5.8707(13)
9.289(2)
11.820(3)
103.040(3)
92.480(3)
99.805(3)
616.5(2)
1.308
21.108(7)
14.946(5)
19.243(6)
90
130.425(4)
90
10.4452(19)
18.143(3)
18.099(3)
90
97.931(3)
90
13.2405(6)
13.2544(6)
16.8269(8)
98.855(4)
106.372(4)
95.552(4)
2768.8(2)
1.379
3
Volume /Å
calcd. /mg m
4621(3)
1.246
3397.0(11)
1.239
–
3
D
T /K
μ /mm
Total reflections
120(2)
0.080
5649
173(2)
0.112
14589
150(2)
0.101
21052
296(2)
3.396
9770
–
1
Unique data (Rint
)
2673 (0.0507)
2673/3/340
2.29 to 27.00
1.051
4166 (0.0727)
4166/0/285
2.24 to 25.25
0.968
5959 (0.0752)
5959/7/396
2.25 to 25.05
1.035
8457 (0.0808)
8457/72/684
2.79 to 62.49
1.045
Data/restraints/parameters
0
θ ( )
GoF on F
2
final R indices
R
1
= 0.0483,
R
1
= 0.0474,
R
1
= 0.0817,
1
R = 0.0901,
(
I > 2σ(I))
wR
2
= 0.0962
= 0.0759,
wR
2
= 0.0915
= 0.0972,
wR
2
= 0.2206
= 0.1266,
wR
R = 0.2187,
1
2
= 0.1927
R indices (all data)
R
1
R
1
R
1
wR
2
= 0.1068
wR
2
= 0.1028
wR
2
= 0.2494
wR
2
= 0.2522
largest diff. peak hole
0.224 / -0.286
0.260 / -0.280
0.482/ -0.375
0.581 / -0.722
3
(e/Å )
It should be noted that methylaluminum complexes of this type
5
4-56
may be obtained in high yields by transmetallation reaction
5
from corresponding germylenes. For the compound 2a this
method is preferable not only because of the higher yield of the
product (90 vs. 82 %) but also because of the higher purity of the
product (Scheme 3).
Scheme 3. Synthesis of 2a from germylene.
1
1
2
2
0
5
0
5
To date there are no aluminum complexes based on pyridine
containing alcohols investigated by X-ray analysis. Several
related structures presented on Chart 2.
At the same time it should be noted that alkyl Al complexes
based on aminobis(phenolate) ONO ligands (Chart 3) are
5
7-60
Chart 2. Molecular structures of dimeric alkyl aluminum complexes
3
2, 61
monomeric with tetrahedral aluminum atom.
under comparison of structural features of four-coordinate (Chart
) and five-coordinate Al complexes (for example, 3a, Fig. 2) it
may be concluded that increasing the coordination number results
Furthermore,
3
3
1, 32, 62
in elongation of bond lengths
(compare d(Al-C) 1.977 vs.
1.91-1.96 Å, d(Al-N) 2.185 vs. 1.98- 2.10 Å, d(Al-O) 1.759 vs.
containing ONO ligands.
1
.71- 1.75 Å).
3
0
The molecular structures of complexes 3a and 5 were
Chart 3. Molecular structures of monomeric alkyl aluminum complexes
containing ONO ligands.
investigated in the solid state by X-ray analysis (Figs. 2, 3; Table
).
1
In crystal, the molecule 3a lies on 2-fold axis. The aluminum
atom in 3a is five-coordinate, this compound in solid state is
dimeric where the almost planar Al O cycle was formed. The
35
2
2
dimerization is caused by bridging ОСМе2 group (the least
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sterically hindered near oxygen atom) from another unit. In this
compound the two methyl groups at the neighboring aluminum
atoms are situated in cis position in relation to Al O cycle and
2
2
the possible diastereomeric complex (trans-isomer) was not
formed even in solution (according to NMR spectroscopy data).
The ligands in 3a are arranged in a distorted trigonal bipyramidal
geometry (trigonal index (τ) 0.69) around the aluminum centre
5
0
5
with nitrogen and coordinating oxygen atoms in axial positions
o
(
O(2А)-Al(1)-N(1) 166.13(8) ). The equatorial positions are
1
occupied by two oxygen atoms of the 2,6-bis(alkyloxy)pyridine
ligand and the bridging alkoxide oxygen atom from the
neighboring ligand. The length of N→Al bond (2.1850(19) Å) is
expectedly longer than in related dimeric phenolic compounds
(compare with Chart 2 with imine N) because of the strong
1
intermolecular O(2A)→Al(1) interaction in 3a. The covalent
bond length Al-O(1) in 3a (based on alkyl alcohol) is somewhat
Fig. 3 Molecular structure of complex 5. Displacement ellipsoids are
shown at 30 % probability level. Hydrogen atoms (except hydroxyl H1)
are omitted for clarity. Selected interatomic distances [Å] and angles [°]:
Al(1)-O(21) 1.754(5), Al(1)-O(12) 1.920(6), Al(1)-O(11) 1.929(6), Al(1)-
O(22) 2.177(5), Al(1)-N(1) 1.983(6), Al(1)-N(2) 2.013(6); O(12)-Al(1)-
O(11) 155.5(2), O(12)-Al(1)-N(1) 79.5(2), O(11)-Al(1)-N(1) 80.7(2),
N(1)-Al(1)-N(2) 169.3(3), O(21)-Al(1)-O(22) 155.4(2).
45
50
Chemical properties of methyl aluminum complexes based on
2
,6-bis(hydroxyalkyl)pyridines. The aluminum complexes
shorter than the related one in phenolic derivatives (Chart 2).
Fig. 2 Molecular structure of complex 3a. Displacement ellipsoids are
shown at 50 % probability level. Hydrogen atoms and solvated toluene
molecule are omitted for clarity. Selected interatomic distances [Å] and
angles [°]: Al(1)-O(1) 1.7594(16), Al(1)-O(2) 1.8272(17), Al(1)-O(2А)
based on 2,6-bis(hydroxyalkyl)pyridines containing chlorine
atoms were obtained by treatment of corresponding methyl
aluminum derivatives with germanium chlorides, GeCl
55 GeCl ·dioxane (Scheme 4). It is more effective when germylene
2
and
4
2
2
3
3
4
0
5
0
5
0
compound is used in this reaction, possibly due to lower acidity.
The complexes 3b, 4b were isolated as white moisture-
sensitive powders soluble in chlorinated solvents and toluene.
The structures of compounds 3b, 4b were investigated by NMR
1
.9413(15), Al(1)-C(10) 1.977(2), Al(1)-N(1) 2.1850(19); O(1)-Al(1)-
O(2) 124.67(8), O(1)-Al(1)-C(10) 121.94(10), O(2)-Al(1)-C(10)
13.39(9).
1
Other parameters are very similar. The both chelate six- 60 spectroscopy and composition was established based on
membered Al-O-C-C-C-N rings are in half-chair conformation
elemental analysis. There are only one set of signals in spectra
with C (CH group) and N atoms as flaps.
and the dimeric structure with bridge OCPh group may be
proposed for these compounds.
2
2
5
2
The coordination number of aluminum atom in 5 is six and
aluminum atom has distorted octahedral geometry with trans-
disposition of two nitrogens of different ligand frameworks and
oxygen atoms of one ligand (mer-disposition). It should be noted
that the coordination bond Al←OH is the longest aluminum-
oxygen bond in this compound, and in solution it dissociates
1
3
(
according to C NMR spectra). The d(Al-O(2)) situated trans to
the bond mentioned above is the shortest. Besides, the bonds Al-
N in 5 are shorter than the similar in 3a, due to a greater number
of acceptor substituents in 5.
It should be noted that the hexacoordinated aluminum
complexes based on ONO ligands are very rare. To date there is
6
3
only one structure based on pyridine-2,6-dicarboxylic acid.
Scheme 4. Synthesis of aluminum complexes containing chlorine.
Alkoxy aluminum complexes based on 2,6-
65
4
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Dalton Transactions
bis(hydroxyalkyl)pyridines. Synthesis of aluminum alkoxides
stabilized by polydentate ligands is an actual synthetic problem.
So in the course of this work we investigated the approaches to
the methoxy aluminum complexes based on 2,6-
bis(hydroxyalkyl)pyridines. We performed reactions of formed in
5
0
5
situ Me Al(OMe) with the ligand 2. It was proposed that
2
substitution of methyl groups at Al atom when treating with
alcohols is more preferable. At the same time it should be noted
that numerous attempts to substitute the methyl groups in
complexes 2a, 3a, 4a under action of various alcohols (MeOH,
BnOH, HO(CH ) OCH=CH ) resulted in a mixture of compounds
1
2
4
2
including formation of free ligand. Apparently the same situation
is observed under polymerization in the presence of added co-
initiator (see below).
Scheme 7. Synthesis of methoxy aluminum complexes 2c, 3c, 4c
using Al(OMe)3.
1
It was established that methoxy aluminum complex 2c was
formed in reaction of 2 with Me Al(OMe) (Scheme 5). Reaction
The alkoxy complexes were isolated as moisture-sensitive
2
between Me Al(OBn) and 2 allowed us to obtain aluminum 45 white solids soluble in CHCl , CH Cl , THF, PhMe. The
2
3
2
2
1
complex bearing benzyloxy group at the aluminum atom (Scheme
structures of the obtained compounds were established by Н and
1
3
5
).
С NMR spectroscopy, elemental analysis, and in the case of
compounds 2c and 4c by X-ray study (Figure 4 and Figure S4,
Supporting Information). In the case of compound 2c the quality
of the crystals is not sufficient for precise X-ray experiment, but
the connectivity of atoms has clearly been determined.
In the NMR spectra of the alkoxy complexes 2c, 3c, 2d there is
only one set of signals. The protons in CH2 groups are
diastereotopic. So we may conclude that in solution these
5
5
6
0
5
0
derivatives exist as dimers with C symmetry similar to X-ray
i
structures.
Unfortunately, the DOSY spectra of methoxy and chloro
derivatives were not recorded due to bad solubility, but we
believe that these derivatives are also dimeric due to additional
bond formation between Al atom and methoxy (chloro) group of
the second monomer unit.
2
2
3
0
5
0
Scheme 5. Synthesis of aluminum complexes 2c, 2d from Me
2
Al(OAlk).
The formation of these two compounds was unambiguously
1
13
proven by elemental analysis, Н and С NMR spectroscopy and
by the parallel synthesis (for 2c).
Methoxy
aluminum
complexes
based
on
2,6-
bis(hydroxyalkyl)pyridines were obtained by exposing
corresponding methyl derivatives to dried air (oxidation)
(Scheme 6). At the same time treatment of methyl derivative 4a
with methanol does not lead to methoxy derivative but yields the
mixture of products (according to NMR) including the free ligand
4.
Scheme 6. Synthesis of methoxy complexes 2c, 4c by action of air on
methyl aluminum complexes.
The methoxy derivatives of aluminum based on 2,6-
bis(hydroxyalkyl)pyridines were synthesized using free ligand
Fig. 4 Molecular structure of complex 4c. Displacement ellipsoids are
shown at 50 % probability level. Hydrogen atoms are omitted for clarity.
Selected interatomic distances [Å] and angles [°]: Al(1)-O(1) 1.739(3),
3
5
and (MeO) Al formed in situ (Scheme 7). It should be noted that
3
in this case the transesterification reaction proceeds under more 65 Al(1)-O(2) 1.732(3), Al(1)-O(3А) 1.823(3), Al(1)-O(3) 1.915(3), Al(1)-
severe conditions than reactions mentioned above.
N(1) 2.100(3); O(1)-Al(1)-O(2) 113.24(16), O(1)-Al(1)-O(3А)
1
22.99(14), O(2)-Al(1)-O(3) 94.00(13), O(3А)-Al(1)-O(3) 75.69(13).
It should be noted that unlike the previously studied Al
5
8, 64
4
0
structures based on ONO ligands
(Chart 4) the main feature
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of the aluminum alkoxides presented in this work is
a
dimerization through the methoxy groups (not through
polydentate ligand).
5
0
Chart 5. Schematic representation of trans- and cis- isomers for Al
complex (4c) based on unsymmetrically substituted pyridine dialcohol.
Ring-opening polymerization. The aluminum complexes
obtained were tested as initiators in the L-lactide polymerization
Chart 4. The molecular structures of dimeric alkoxide aluminum
complexes based on ONO ligands investigated to date.
5
0
5
0
5
0
5
0
5
o
According to the X-ray diffraction analysis coordination
under two different conditions: in toluene solution at 80 C (for 2-
7
1
number of aluminum atoms in 2c and 4c is five, the compounds 55 4a) in the presence of BnOH as an external nucleophile and in
o
being dimeric in the solid state. Dimerization occurs due to the
formation of additional bond between the aluminum atom and the
oxygen atom of the OMe group of the second monomeric unit
bulk of the molten lactide at 100 C (for 2-4c) (Scheme 8).
Furthermore, polymerization of ε-caprolactone was studied in
bulk, too (Scheme 9). The results of the catalytic tests are
presented in Table 2.
1
1
2
2
3
3
4
4
(least sterically hindered oxygen atom), which results in the
formation of flat Al O cycle. The two Al–OMe distances within
the ring are significantly different (Al–Oeq 1.823(3) and Al–Oax
2
2
1
.915(3) Å). This is a typical feature of five-coordinate dimeric
6
5, 66
aluminum alkoxides.
The coordination polyhedron of the
central atom in 4c is distorted trigonal bipyramid (trigonal index
τ) is 0.82), with the coordinating nitrogen and oxygen atom of
6
0
Scheme 8. ROP of L-lactide promoted by complexes 2-4a and 2-
(
o
4
c.
the MeO being in axial positions (O(3)-Al(1)-N(1) 161.76(13) ).
The equatorial positions are occupied by two oxygen atoms of the
2,6-bis(dialkylalkoxy)pyridine ligand and the methoxide oxygen
atom. Coordinating Al-N bond length in 4c is 2.100(3) Å which
is shorter than that in 3a (2.1850(19) Å), due to the absence of the
donor methyl group connected with aluminum atom. It should be
noted that Al-N bond (2.100(3) Å) is somewhat longer than in
related five-coordinate Al complexes containing Py→Al
Scheme 9. Polymerization of ε-caprolactone.
All substances under investigation turned out to be moderately
active in polymerization. Practically full conversion of L-lactide
is observed within 1-5 hours when polymerization was carried
out in bulk molten lactide and 87-99% of conversion was
observed within 27 h in toluene solution. Besides, the rate of
polymerization increases with the decrease of the [cаt]/[BnOH]
ratio from 1:2 to 1:1 (full conversion for less than 7 h). This
tuning of conditions is accompanied by increase in polydispersity
3
5, 61
coordination (2.01-2.03 Å),
based on phenolate ligands.
6
7
7
5
0
5
Moreover, the Al(1)-O(1) and Al(1)-O(2) bonds formed by six-
and seven-membered rings are almost equal. In general, the Al-O
bond lengths in 4c formed by alkyl alcohols (1.72-1.75 Å) are
1
7, 31, 61, 67-69
similar to found earlier for phenolate (1.72-1.82 Å)
alkylalkoxy derivatives (1.71-1.74 Å).
or
5
8, 70
The six-membered
chelate ring Al-O(2)-C(10)-C(9)-C(8)-N is in half-chair
conformation with C(9) and N atoms as flaps and seven-
membered ring Al-O(1)-C(1)-C(2)-C(3)-C(4)-N is in twist-chair
conformation with C(3) and N atoms as flaps.
(
(
for example, entries 1 and 2, 4 and 5) and in molecular weight
entries 1 and 2, 6 and 7). It should be noted that all polymers
have narrow polydispersity indexes (M /M = 1.1-1.4) what
w
n
indicates that the process is controllable. In similar conditions the
methyl and methoxy derivatives exhibit almost similar catalytic
It should be noted that molecule 4c lies on crystallographic
inversion centre, i.e. coordinating atoms of each of the two
ligands are located in the trans-positions in relation to Al O
behavior in polymerization (compare M and PDIs).
n
2
2
Matrix-assisted laser desorption/ionization time-of-flight
cycle (trans-isomer). However, according to the NMR data there
(MALDI-TOF) mass spectrometry of PLAs at maximum
are two sets of signals in solution which do not change their
conversion was conducted in order to define structure and
o
intensity if the solution is heated (CDCl , 50 C), that indicates the
3
80 composition of polymers at low monomer/initiator ratio.
According to MALDI-TOF-MS data the sample obtained with 2a
as initiator (see Supporting Information, Fig. S5) is
presence of two diastereomeric complexes: the abovementioned
trans- and cis- isomer (Chart 5) which are stable and do not pass
into the each other or into the monomer. Numerous attempts to
crystallize the cis-isomer do not lead to the desired result.
1
monodispersed which is in agreement with H NMR spectrometry
where the polymer is Н- and BnO- end-capped (Fig. S6).
85
6
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Dalton Transactions
Table 2. Ring-opening polymerization with 2-4a, 2-4c and 2d.
d
e
f
conversion,
M
n
(theor),
M
n
(NMR),
M
n
(GPC),
h
entry initiator [M]
0
/[cаt]
0
/[BnOH]
0
t, [h]
M
w
/M
n
N
g
-1
-1
-1
[
%]
98
[g mol ]
3640
[g mol ]
4068
2572
2376
3276
2703
3276
5806
5272
[g mol ]
3100
4112
1856
2300
4320
2400
4373
4576
a
1
2a
2a
2a
3a
3a
4a
4a
4a
50/1/2
50/1/1
100/1/10
50/1/2
50/1/1
50/1/2
50/1/1
100/1/1
27
4
1.25
1.37
1.19
1.11
1.33
1.11
1.35
1.23
1.2
1.8
0.8
1.5
1.7
1.4
1.7
3.1
2
>99
>99
95
7240
3
4
5
6
7
8
20
27
5
1540
3530
>99
87
7240
27
7
3240
>99
>99
7240
4
14370
b
9
2c
2c
3c
4c
4c
50/1/-
200/1/-
200/1/-
50/1/-
1
4
2
1
2
>99
>99
>99
>99
95
7160
28550
28550
7160
6296
7116
6038
5653
5884
4400
7930
8157
4698
7070
1.18
1.22
1.48
1.38
1.34
1.6
3.6
3.5
1.5
1.9
1
1
1
1
0
1
2
3
100/1/-
13710
c
1
4
2d
300:1
0.5
>99
33960
16530
21168
3.24
1.6
a
o
b
o
c
Polymerization of L-lactide in toluene solution, 80 C, [cat]= 0.02 M. Polymerization of L-lactide in bulk, 100 C. Polymerization of ε-caprolactone in
o
d
bulk, 130 C. For solution polymerization: M
(LA)[LA] /[cat](conversion) + M (ROH) or M
spectra: M = I(CH)PLLAM (LA) + M (MeOH), M
according to the equation M = 0.58×M (GPC) for polylactide and M
crude reaction mixture. Number of polymer chains per catalyst molecule, calculated as N=M
n
(theor)= M
(theor)= M
= I(CH)PLLAM
= 0.56×M
w
(LA)[LA]
(CL)[CL]
(LA) + M
(GPC) for polycaprolactone. Obtained from H NMR spectroscopy of the
(theor)/M (exp).
o
/[BnOH](conversion) + M
/[cat](conversion) + M
(BnOH) or M = I(CH
w
(BnOH); for bulk polymerization: M
(ROH), R= Me or Bn. Calculated using H NMR
n
1
(theor)=
e
5
M
w
o
w
n
w
o
w
f
n
w
w
n
w
w
n
2
g
Ph)PCLM (CL) + M (BnOH). Calculated
w w
1
n
n
n
n
h
n
n
Thus, these results indicate the absence of side-reactions, which
is intermolecular esterification. Analogous results are obtained
with 3a as initiator. In the case of initiation with complex 4a side
process of intermolecular esterification can be observed but at
is almost similar and there is no dependence on ligand structure.
At the same time it is evident that increasing the monomer
content results in increasing the PDI that is typical for intensive
side reactions (intra- and intermolecular transesterification
1
1
2
2
3
3
0
small extent. Polymerization in bulk with 2c as initiator led to 40 reaction) in these cases.
PLAs end-capped by MeO- (Fig. S7). MALDI-TOF-MS data was
In the case of methyl aluminum complexes 2a, 3a the catalytic
1
5
0
5
0
5
also consistent with H NMR spectrometry data. There is also
behavior is almost similar, but more sterically voluminous ligand
in 4a results in some decreasing of the polymerization rate (see
entries 1, 4, 6). Increasing the monomer content results
slight flow of transesterification reactions in this case.
It should be noted that no epimerization processes are
1
72
observed. Homodecoupled H NMR analyses revealed that all 45 apparently in side processes.
PLAs are isotactic (Fig. S8).
At comparing the results obtained for polymerization in
solution and in bulk it is established that at low monomer content
The increasing of quantity of coinitiator added (alcohol) results
7
3
in immortal character of polymerization with proportional
increasing of polymer chains (compare entries 1-3) and the
molecular weight of the polymer decreases with the amount of
(50:1:1 and 50:1) the polymer characteristics are similar (up to
4
000 Da). The increasing the monomer content (to 100:1) results 50 BnOH. However, the molecular weight distribution remains
in molecular weight increasing only in the bulk polymerization.
By analyzing these data one can range initiators in accordance
with their activity: 2a>3a>4a (Fig.S9) and this row of activity is
independent on the quantity of BnOH (1 or 2 equivalents).
Polymerization with 3a and 4a is linear (first-order 55 (Table 2, lines 9, 12). On the basis of MALDI and NMR in this
dependence) (Figs. S10, S11). There is no induction
polymerization period is observed which indicates the direct
catalytic activity of the complexes. So the high control of
polymerization is observed for low monomer content. Increasing
the monomer content results in loss of control and intensive side 60 of 2,6-bis(hydroxyalkyl)pyridine as initiators,
narrow. In this case the type of polymerization is also highly
controllable.
From Table 2 it is evident that at low monomer content alkoxy
complexes 2-4c initiate one polymer chain per initiator molecule
case the polymerization performed using only OMe group
5
according to coordination-insertion mechanism of ROP.
Increasing the monomer content results in additional growth of
chains (up to 3) what may be explained by using the ligand group
7
4
and such
reactions.
For methoxy derivatives the activity for all complexes studied
polymers (containing ligand fragments) may be identified by
NMR (Figure S12, Supporting Information), but isolation of them
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Dalton Transactions
Page 8 of 12
is problematic possibly due to better solubility under isolation
conditions. For methyl complexes the situation is similar (Table
the system was carried out on polystyrene standards. Study of
PLA by mass spectrometry with ionization MALDI (Bruker
2
, lines 7, 8). Furthermore, in this case at equimolar quantity of 60 Autoflex) performed at the Chemistry Department of Moscow
coinitiator ([cat]/[BnOH]= 1/1) there are two polymer chains per
initiator (Table 2, lines 2, 5, 7). It should be noted that increasing
the quantity of added coinitiator results in decreasing the number
of polymer chains (Table 2, lines 1, 3).
State University.
Synthesis
5
0
5
0
5
of
ligands.
3-[6-(2-Hydroxy-2,2-
diphenylethyl)pyridine-2-yl]-1,1-diphenylpropan-1-ol, Py[2–
(CH CH CPh OH)-6-(CH CPh OH)] (4). Solution of n-BuLi
2
2
2
2
2
The ε-caprolactone polymerization proceeds in a rather harsh 65 (2.5 M in hexane, 11.60 ml, 29.00 mmol) was added dropwise to
conditions very quickly (full conversion over 0.5 h; only one
polymer chain per initiator molecule) but with poor controllable
character (PDI is 3). These results are suggestive of intensive
transesterification occurring during the polymerization process.
At the end it is necessary to compare the catalytic behavior of 70 dropwise at room temperature to the reaction mixture. The
various aluminum compounds. Aluminum isopropoxide has low
solution of Py[2-(CH )-6-(CH CPh OH)] (4.00 g, 13.80 mmol) in
3 2 2
1
1
2
2
THF (80 ml) at 0°С. The reaction mixture was slowly warmed to
room temperature and stirred for 4 h. Then solution of 2,2’-
diphenyloxirane (2.71 g, 13.80 mmol) in THF (10 ml) was added
mixture was stirred overnight. Then 2 N solution of NH Cl was
added, organic phase was separated and water phase was
extracted with CH Cl (3x40 ml). The combined organic phases
4
activity in polymerization of lactide (in case of rac-lactide
7
5
conversion is 60%, PDI= 2.4). It seems that aluminum
complexes based on substituted pyridine dialcohols are more
2
2
were washed with saturated solution of NaCl and dried over
active and provide polymer with better characteristics. At the 75 Na SO . All volatiles were removed under reduced pressure and
2
4
same time the phenolic ligands (related to presented on Chart 3)
provide the more controllable character of lactones
polymerization.
So it may be concluded that application of aluminum
complexes based on substituted pyridine containing dialcohols 80 6.77, 6.83-6.87, 7.06-7.12, 7.15-7.23, 7.27-7.32, 7.34-7.39, 7.44-
the residue was recrystallized from CH Cl . Compound 4 (4.70 g,
2 2
o
1
70%) was isolated as a white powder, m.p. 123 С. H NMR
o
(CDCl , 25 C): δ = 2.51-2.58 (m, 2H, CH CH CPh OH); 2.62-
3
2
2
2
2.67 (m, 2H, CH CH CPh OH); 3.69 (s, 2H, CH CPh OH); 6.72-
2 2 2 2 2
1
3
o
results in moderately active L-lactide ROP (PDI is up to 1.48)
with controlled or immortal polymerization type.
7.48 (7m, 23Н, Py and Ph) ppm. C NMR (CDCl , 25 C): δ =
3
32.23 (CH CH CPh OH); 40.93 (CH CH CPh OH); 47.00
2
2
2
2
2
2
(
CH CPh OH); 77.95, 78.55 (2CPh ); 120.90, 121.73, 126.10,
2 2 2
Experimental part
126.21, 126.47, 126.85, 127.97, 128.19, 137.18, 146.84, 147.33,
158.54, 160.58 (Py and Ph) ppm. Anal. Calcd. for C H NO
2
8
9
9
5
0
5
3
4
31
Experimental Details. All manipulations were performed under
a dry, oxygen-free argon atmosphere using standard Schlenk
techniques. Solvents were dried by standard methods and distilled
prior to use: toluene, n-hexane were refluxed under Na and
distilled; ether, THF were stored under KOH, refluxed under
Na/benzophenone and then distilled; methanol was refluxed
under Mg and then distilled off; benzyl alcohol was distilled
under vacuum. L-Lactide was recrystallized from toluene and
sublimed in vacuum, ε-caprolactone was distilled over CaH2.
Starting materials were synthesized according to the literature
(485.6155): C 84.09; H 6.43; N 2.88. Found: C 83.94, H 6.41, N
2
.95%. Crystals suitable for X-ray analysis were obtained from
o
concentrated toluene solution at –18 C.
3
3
4
4
5
5
0
5
0
5
0
5
Synthesis of complexes. Reaction of Py[2,6-(CPh OH) ] (1)
2
2
with Me Al. Synthesis of {Py[2,6-(CPh O) ]}{Py[2-(CPh O)-6-
3
2
2
2
(
CPh OH)]}Al (5). At –30°С solution of trimethylaluminum (2.0
2
M in toluene, 0.25 ml, 0.50 mmol) was added dropwise to
solution of Py(CPh OH) (1) (0.22 g, 0.50 mmol) in toluene (15
2
2
ml). Reaction mixture was slowly warmed to room temperature
and stirred overnight. Volatile materials were removed under
reduced pressure and residue was recrystallized from toluene.
Compound 5 (0.09 g, 37%) was isolated as a colorless crystals.
7
6,
77
procedures:
2,2-diphenyloxirane,
Py[2-(СH )-6-
3
3
6
23
(
(
CH CPh OH)],
Py[2,6-(CPh OH) ]
(1),
Py[2,6-
2
2
2
2
3
6
CH CPh OH) ] (2),
Py[2-(CH CMe OH)-6-(CH CPh OH)]
1
o
2
2
2
2
2
2
2
H NMR (CDCl , 25 C): δ = 6.56-6.65, 6.72-6.77, 6.79-6.85,
3
7
78
22
3
(3), GeCl ·C H O , Py(CH CPh O) Ge. n-BuLi (2.5 M
2
4
8
2
2
2
2
7
.10-7.16, 7.20-7.27, 7.30-7.36, 7.40-7.43, 7.59-7.62, 7.79-7.81
solution in hexane) (Aldrich), AlMe (2.0 M solution in toluene)
13
3
1
1
1
1
00 (9m, 23H, Py and Ph); 7.86 (br s, 1H, ОН) ppm. C NMR
(Aldrich), GeCl4 (Aldrich) were used as supplied. C D (dried
o
6
6
(CDCl , 25 C): δ = 78.17, 78.22, 78.36 (3CPh ); 122.21, 122.41,
3
2
over sodium) and CDCl (dried with CaH ) obtained from
3
2
1
1
1
22.71, 126.90, 127.04, 127.13, 127.22, 127.52, 127.68, 127.78,
27.95, 128.50, 128.80, 130.05, 131.47, 138.80, 139.09, 141.95,
43.74, 146.92, 148.38, 161.42, 164.34, 165.19 (Py and Ph) ppm.
1
13
27
Deutero GmbH. H (400.13 MHz), C (100.61 MHz) and Al
(104.26 MHz) NMR spectra were recorded on a Bruker Avance
4
00 or Agilent 400-MR spectrometers at room temperature (if
05 Anal. Calcd. for C H AlN O (911.0292): C 81.74; H 5.20; N
1
13
62 47
2
4
otherwise stated). H and C chemical shifts are reported in ppm
3
.07. Found: C 81.24, H 5.04, N 2.96%. Crystals suitable for X-
2
7
relative to Me Si as external standard; in Al NMR experiments
4
ray analysis were obtained from concentrated toluene/chloroform
3
+
Al(D O) (Al (SO ) in D O) was used as an external standard.
o
2
6
2
4 3
2
solution at –18 C.
Elemental analyses were carried out by the Microanalytical
Laboratory of the Chemistry Department of the Moscow State
University. Gel permeation chromatography (GPC) was
performed on high pressure liquid chromatograph equipped with
a column system Styrogel HR 5E, HR 4E, 300h7, 8mm and
refractometric detector. Tetrahydrofuran was used as the eluent,
flow rate 1 ml/min. The concentration of the sample solutions
was 1 mg/ml, injected sample volume of 100 μl. Calibration of
Py[2,6-(CH CPh O) ]AlMe (2a). Method 1. At –30°С solution
2
2
2
10 of trimethylaluminum (2.0 M in toluene, 1.15 ml, 2.29 mmol)
was added dropwise to solution of Py[2,6-(CH CPh OН) ] (2)
2
2
2
(1.08 g, 2.29 mmol) in toluene (20 ml). Reaction mixture was
slowly warmed to room temperature and stirred overnight.
Volatile materials were removed under reduced pressure, residue
15 washed with ether (3x5 ml) and dried in vacuum. Compound 2a
8
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Page 9 of 12
Dalton Transactions
(0.96 g, 82%) was isolated as a white powder. Method 2. At –
solution of Py(CH CPh OH) (2) (0.43 g, 0.92 mmol) was added
2 2 2
3
0°С solution of Py[2,6-(CH CPh OН) ] (2) (0.24 g, 0.50 mmol) 60 dropwise to abovementioned solution of Me Al(OBn) in toluene,
2
2
2
2
in toluene (20 ml) was added dropwise to solution of
trimethylaluminum (2.0 M in toluene, 0.25 ml, 0.50 mmol) in
toluene (20 ml). Reaction mixture was slowly warmed to room
temperature and stirred overnight. Volatile materials were
removed under reduced pressure, residue washed with ether (3x5 65 powder. H NMR (C D , 25 C): δ = 2.10 (s, 2Н, ОCH Ph); 3.02,
ml) and dried in vacuum. Compound 2a (0.15 g, 59%) was
the reaction mixture was gradually warmed to room temperature
and then heated at 90°С for 21 h. Volatile materials were
removed under reduced pressure, residue was washed with ether
(2x5 ml) and dried in vacuum to give 2d (0.35 g, 63%) as a white
5
0
5
0
5
0
5
0
5
0
5
1
o
6
6
2
2
3.37 (2d, 4H, J= 14.4 Hz, АМ system of CH ); 6.22-6.26, 6.52-
2
1
o
isolated as a white powder. H NMR (CDCl , 25 C): δ = -0.52 (s,
6.57, 6.73-6.78, 6.93-6.98, 6.99-7.03, 7.17-7.21, 7.53-7.57, 7.59-
3
2
13
o
1
1
2
2
3
3
4
4
5
5
3H, AlCH ); 3.60, 3.86 (2d, each 2H, J = 14.4 Hz, CH ); 6.93 (d,
H, J = 7.8 Hz, β-Py); 7.47 (t, 1H, J = 7.8 Hz, γ-Py); 6.96–7.00,
.27–7.33, 7.56–7.61 (3m, 20H, Ph) ppm. C NMR (CDCl3, 70 125.64, 126.65, 126.71, 127.30, 128.11, 128.51, 129.19, 129.27,
5 C): δ = -10.96 (br, AlMe); 48.60 (СН ); 78.11 (CPh ); 124.47,
7.64 (8m, 28H, Py and Ph) ppm. C NMR (C D , 25 C): δ =
6 6
3
2
3
3
2
7
2
1
47.09 (CH ); 67.08 (ОCH Ph); 78.69 (CPh ); 124.58, 125.28,
2 2 2
1
3
o
138.52, 141.99, 149.40, 152.50, 158.83 (Py and Ph) ppm. Anal.
Calcd. for C H AlNO (603.6844): C 79.58; H 5.68; N 2.32.
2
2
25.81, 125.84, 126.39, 126.40, 127.49, 128.00, 140.80, 146.96,
4
0
34
3
150.40, 157.40 (Py and Ph) ppm. Anal. Calcd. for C H AlNO
2
Found: C 79.70, H 5.93, N 2.13%.
3
4
30
(
2
511.5891): C 81.74; H 5.20; N 3.07. Found: C 81.24, H 5.04, N
.96%.
Reaction of Py(CH CPh O) Ge with Me Al, synthesis of
Reaction of Py(CH CPh OН) (2) with Al(OMe) , synthesis of
2
2
2
3
75 [2,6-Py(CH CPh O) ]AlOMe (2c). a) Synthesis of Al(OMe) : At
2 2 2 3
–30°С MeOH (0.2530 ml, 5.10 ml) was added dropwise to
solution of AlMe3 (2.0 M in toluene, 0.85 ml, 1.70 mmol) in
toluene (10 ml). Reaction mixture was slowly warmed to room
temperature and stirred overnight, the volatiles were removed
2
2
2
3
Py(CH CPh O) AlMe (2a). At –30°С solution of AlMe (2.0 M
2
2
2
3
in toluene, 0.25 ml, 0.50 mmol) was added dropwise to solution
of Py(CH CPh O) Ge (0.27 g, 0.50 mmol) in toluene (10 ml).
2
2
2
Reaction mixture was slowly warmed to room temperature and 80 under vacuum to give white powder of Al(OMe) , which was
3
stirred for 1 d. Volatile materials were removed under reduced
pressure, residue was recrystallized from toluene. Compound 2a
(0.23 g, 90%) was isolated as a white crystals. Analytic
characteristics are identical to the presented above.
used without further purification.
b) Synthesis of Py[2,6-(CH CPh O) ]AlОMe (2c): Toluene (20
2
2
2
ml) was added to abovementioned Al(OMe) , then at –30°С
3
solution of Py(CH CPh OH) (2) (0.80 g, 1.70 mmol) in toluene
2
2
2
Reaction of Py(CH CPh OН) (2) with Me Al(OMe), 85 (20 ml) was added dropwise, the reaction mixture was gradually
2
2
2
2
synthesis of Py[2,6-(CH CPh O) ]AlОMe (2c). a) Synthesis of
warmed to room temperature and then heated at 80°С for 40 h.
The volatile materials were removed under reduced pressure,
residue was washed with ether to give 2c (0.43 g, 48%) as a white
powder. Analytical data correspond to the abovementioned.
2
2
2
Me Al(OMe): At –30°С MeOH (0.0875 ml, 1.70 mmol) was
2
added to solution of trimethylaluminum (2.0 M in toluene, 0.85
ml, 1.70 mmol) in toluene (10 ml). Reaction mixture was slowly
warmed to room temperature and stirred overnight. The obtained 90 Py[2-(CH CMe O)-6-(CH CPh O)]AlMe (3a). Method 1. The
2
2
2
2
solution of Me Al(OMe) was used further without additional
procedure was analogous to that for 2a (Method 1): reaction of
2,6-Py(CH CMe OH)(CH CPh OH) (3) (0.69 g, 2.00 mmol) and
2
purification.
2
2
2
2
b) Synthesis of Py[2,6-(CH CPh O) ]AlОMe (2c): At –30°С
Me Al (2.0 М in toluene, 1.00 ml, 2.00 mmol) in toluene (30 ml)
2
2
2
3
solution of Py(CH CPh OH) (2) (0.80 g, 1.70 mmol) in toluene
gave 3a (0.75 g, 97%) as a white powder. Method 2. The
2
2
2
(20 ml) was added dropwise to abovementioned solution of 95 procedure was analogous to that for 2a (Method 2): reaction of
Me Al(OMe) in toluene, the reaction mixture was gradually
Me Al (2.0 М in toluene, 0.25 ml, 0.50 mmol) and 3 (0.17 g, 0.50
2
3
warmed to room temperature and then heated at 80°С for 20 h.
Volatile materials were removed under reduced pressure to give
mmol) in toluene (30 ml) gave 3a (0.18 g, 96%) as a white
1
o
powder. H NMR (CDCl , 25 C): δ = -0.85 (s, 3H, AlCH ); 0.66,
3 3
1
o
2
2
3
c (0.53 g, 59%) as a white powder. H NMR (CDCl , 25 C): δ =
1.37 (2s, 6H, CH ); 2.48, 3.31 (2d, 2H, J = 14.9 Hz, CH CMe );
3 2 2
3
2
2
.45 (s, 3Н, ОCH ); 3.37, 3.91 (2d, 4H, J= 14.1 Hz, АМ system 100 3.82, 3.99 (2d, 2H, J = 13.9 Hz, CH CPh ); 6.75-6.79, 6.90-6.97,
3
2
2
of CH ); 6.76-6.79, 6.84-6.88, 7.04-7.08, 7.15-7.18, 7.20-7.24,
7.10-7.19, 7.21-7.27, 7.38-7.44, 7.46-7.50, 7.63-7.67 (7m, 13H,
2
1
3
o
13
o
7
.44-7.55 (6m, 23H, Py and Ph) ppm. C NMR (CDCl , 25 C): δ
Py and Ph) ppm. C NMR (CDCl , 25 C): δ = 26.52, 30.90
3
3
= 47.77 (CH ); 49.62 (OСН ); 77.46 (CPh ); 124.56, 124.84,
(CH ); 47.05 (СН CMe ); 49.90 (CH CPh ); 73.05 (CMe ); 77.35
2
3
2
3
2
2
2
2
2
1
1
25.15, 125.22, 125.99, 127.34, 127.42, 138.88, 148.74, 151.45,
(CPh ); 122.60, 123.65, 125.41, 125.77, 126.93, 127.26, 127.27,
2
57.79 (Py and Ph) ppm. Anal. Calcd. for C H AlNO 105 127.45, 137.88, 149.16, 151.32, 158.00, 158.47 (Py and Ph) ppm.
3
4
30
3
1
3
(
2
527.5885): C 77.40; H 5.73; N 2.65. Found: C 76.94, H 5.90, N
.57%.
Reaction of Py(CH CPh OН) (2) with Me Al(OBn), synthesis
The signal of AlMe group was not found in C NMR. Anal.
Calcd. for C H AlNO (387.4503): C 74.40; H 6.76; N 3.62.
2
4
26
2
Found: C 74.03, H 6.69, N 3.84%. Crystals suitable for X-ray
2
2
2
2
of Py[2,6-(CH CPh O) ]AlОBn (2d). a) Synthesis of
analysis were obtained from concentrated toluene solution at –
2
2
2
o
Me Al(OBn): At –30°С BnOH (0.1102 ml, 1.00 mmol) in 5 ml of 110 18 C.
2
toluene was added to solution of AlMe (2.0 M in toluene, 0.5 ml,
Py[2-(CH CMe O)-6-(CH CPh O)]AlOMe (3c). The procedure
2 2 2 2
was analogous to that for 2c (reaction of Py(CH CPh OН) (2)
2 2 2
with Al(OMe) ): reaction of Me Al (2.0 М in toluene, 0.55 ml,
3 3
3
1
.00 mmol) in toluene (10 ml). Reaction mixture was slowly
warmed to room temperature and stirred overnight. The obtained
solution of Me Al(OBn) was used further without additional
1.10 mmol), MeOH (0.1336 ml, 3.30 mmol) and
2
purification.
115 Py(CH CMe OH)(CH CPh OH) (3) (0.38 g, 1.08 mmol) in
2
2
2
2
b) Synthesis of Py[2,6-(CH CPh O) ]AlОBn (2d): At –30°С
toluene (20 ml) at 90 ºС for 60 h gave 3c (0.24 g, 55%) as a white
2
2
2
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DOI: 10.1039/C5DT01001B
Dalton Transactions
Page 10 of 12
1
o
powder. H NMR (CDCl , 25 C): δ = 0.58 (s, 3H, CH ); 0.87 (s,
to solution of 2,6-Py(CH CPh O)(CH CMe O)AlMe (3a) (0.18 g,
2 2 2 2
3
3
2
3
3
H, CH ); 2.58, 2.81 (2d, 2H, J= 13.6 Hz, АМ system of CH ), 60 0.47 mmol) in toluene (10 ml). Reaction mixture was stirred at
3 2
.42, 3.89 (2d, 2H, J= 13.6 Hz, АМ system of CH ); 3.17 (s, 3Н,
ОCH ); 6.85-6.92, 6.98-7.05, 7.10-7.15, 7.21-7.29, 7.46-7.50,
7.75-7.81 (6m, 13H, Ph and Py) ppm. C NMR (CDCl , 25 C): δ
2
room temperature overnight, and then the volatile materials were
removed under reduced pressure. The residue was recrystallized
2
3
1
3
o
o
5
0
5
0
5
0
5
0
5
0
5
from toluene at –18 C to give compound 3b (0.17 g, 89%) as a
3
=
28.64, 33.19 (CH ); 49.03, 49.16 (СН ); 49.38 (OСН ); 68.77
yellowish solid. Method 2: GeCl4 (0.07 ml, 0.62 mmol) was
3
2
3
(CMe ); 77.14 (CPh ); 122.93, 123.58, 124.91, 125.39, 125.58, 65 added
slowly
to
solution
of
2,6-
2
2
1
26.30, 126.77, 127.28, 138.48, 148.16, 151.88, 157.45, 158.84
Py(CH CPh O)(CH CMe O)AlMe (3a) (0.24 g, 0.62 mmol) in
2
2
2
2
(Py and Ph) ppm. Anal. Calcd. for C H AlNO (403.4497): C
toluene (20 ml). Reaction mixture was stirred at room
temperature overnight, and then the volatile materials were
2
4
26
3
1
1
2
2
3
3
4
4
5
5
71.45; H 6.50; N 3.47. Found: C 71.23, H 6.62, N 3.24%.
Py-2-(CH CH CPh O)-6-(CH CPh O)]AlMe (4a).
[
The
removed under reduced pressure. The residue was recrystallized
2
2
2
2
2
o
procedure was analogous to that for 2a (Method 1): reaction of 70 from toluene at –18 C to give compound 3b (0.10 g, 41%) as a
Py(CH CH CPh OH)(CH CPh OH) (4) (0.24 g, 0.50 mmol) and
Me Al (2.0 М in toluene, 0.25 ml, 0.50 mmol) in toluene (20 ml)
gave 4a (0.24 g, 93%) as a beige powder. H NMR (CDCl3,
1
o
yellowish solid. H NMR (CDCl , 25 C): δ = 0.72, 1.59 (2s, 6H,
2
2
2
2
2
3
2
CH ); 2.48, 3.67 (2d, 2H, J= 15.3 Hz, CH CMe ); 3.84, 4.24 (2d,
3
3
2
2
1
2H, J = 14.3 Hz, CH CPh ); 6.83-6.87, 6.98-7.03, 7.12-7.19,
2 2
o
13
2
5 C): δ = -0.79 (s, 3H, AlCH ); 1.67-1.74, 2.83-2.88, 3.03-3.13
7.22-7.29, 7.45-7.52, 7.57-7.60 (6m, 13H, Py and Ph) ppm.
C
3
2
o
(3m, 4H, СН CH CPh ); 3.62, 3.83 (2d, 2H, J= 14.9 Hz, 75 NMR (CDCl , 25 C): δ = 26.64, 30.81 (CH ); 46.82 (СН CMe );
2
2
2
3
3
2
2
CH CPh ); 6.87-6.91, 6.94-7.03, 7.06-7.12, 7.14-7.24, 7.27-7.33,
49.18 (CH CPh ); 75.92 (CMe ); 77.56 (CPh ); 123.52, 124.64,
2 2 2 2
2
2
1
3
7
.45-7.52, 7.57-7.67 (7m, 23H, Py and Ph) ppm. C NMR
125.92, 126.30, 126.80, 126.83, 127.57, 127.68, 139.26, 148.12,
149.77, 158.06, 158.60 (Py and Ph) ppm. Anal. Calcd. for
C H AlClNO (407.8685): C 67.73; H 5.68; N 3.43. Found: C
o
(CDCl , 25 C): δ = -10.77 (AlCH ); 31.71 (СН CH CPh ); 41.66
3
3
2
2
2
(
СН CPh ); 49.45 (СН CH CPh ); 77.44, 79.35 (2CPh ); 123.08,
2 2 2 2 2 2
23 23
2
1
1
1
24.78, 125.60, 125.63, 125.87, 125.92, 126.40, 126.86, 126.94, 80 67.65, H 5.69, N 3.80%.
27.41, 127.50, 127.76, 127.98, 128.16, 141.52, 146.88, 148.57, Synthesis of 2,6-Py(CH CH CPh O)(CH CPh O)AlCl (4b).
50.23, 151.48, 158.96, 162.19 (Ph and Py) ppm. Al NMR
2
2
2
2
2
2
7
Solid GeCl ·C H O (0.08 g, 0.34 mmol) was added portionwise
2 4 8 2
to solution of 2,6-Py(CH CH CPh O)(CH CPh O)AlMe (4a)
2 2 2 2 2
o
(CDCl , 25 C): δ = 90 (ω1/2= 5000 Hz) ppm. Anal. Calcd. for
C H AlNO (525.6157): C 79.98; H 6.14; N 2.66. Found: C
3
(0.18 g, 0.34 mmol) in toluene (20 ml). Reaction mixture was
85 stirred at room temperature overnight. The volatile materials were
removed under reduced pressure to give compound 4b (0.17 g,
3
5
32
2
7
9.63, H 6.07, N 2.50%.
Py[2-(CH CH CPh O)-6-(CH CPh O)AlOMe (4c). Method 1.
The procedure was analogous to that for 2b (reaction of
Py(CH CPh OН) (2) with Al(OMe) ): reaction of Me Al (2.0 М
in toluene, 0.42 ml, 0.84 mmol), MeOH (34.0 μl, 2.55 mmol) and
Py(CH CPh OH)(CH CH CPh OH) (4) (0.40 g, 0.84 mmol) in 90 7.60-7.62, 7.66-7.68, 7.70-7.76 (8m, 23H, Py and Ph) ppm.
toluene (20 ml) at 110 ºС for 12 h gave 4c (0.26 g, 60%) as a
white powder. The compound was isolated as a mixture of two
diastereomers (1.1:1). Method 2. Under storage of solution of
complex 4a for a long time under dry air the compound 4c may
be obtained. Anal. calcd. for C H AlNO (541.6151): C 77.61; 95 148.96, 150.25, 159.93, 162.77 (Ph and Py) ppm. Anal. Calcd. for
H 5.96; N 2.59. Found: C 77.30, H 6.10, N 2.27%. Diastereomer
2
2
2
2
2
1
o
91%) as a beige solid. H NMR (CDCl , 25 C): δ = 1.72-1.81,
3
2.85-2.96, 3.14-3.22 (3m, 4H, СН CH CPh ); 3.94 (br s, 2H,
2
2
2
3
3
2
2
2
CH CPh ); 6.98-7.02, 7.15-7.28, 7.30-7.35, 7.47-7.51, 7.54-7.58,
2 2
1
3
C
2
2
2
2
2
o
NMR (CDCl3, 25 C):
δ = 31.29 (СН CH CPh ); 42.06
2 2 2
(СН CPh ); 49.28 (СН CH CPh ); 78.53, 79.14 (2CPh ); 123.70,
2
2
2
2
2
2
125.28, 125.31, 125.56, 125.91, 126.08, 126.29, 126.72, 126.77,
127.75, 127.95, 128.13, 128.21, 129.02, 142.70, 145.95, 147.36,
3
5
32
2
C H AlClNO (546.0338): C 74.79; H 5.35; N 2.57. Found: C
34 29 2
74.54, H 5.90, N 2.50%.
Typical polymerization procedure in solution. All
manipulations were performed under inert atmosphere. To the
1
o
I: H NMR (CDCl , 25 C): δ = 1.74-1.86, 2.51-2.59, 2.60-2.68,
3.25-3.34 (4m, each 1H, CH ); 3.13 (s, 3H, OMe); 3.60, 3.95 (2d,
each 1H, J= 15.3 Hz, CH Ph ); 6.54-7.66 (m, 23H, Py and Ph)
ppm. C NMR (CDCl , 55 C): δ = 31.25 (СН CH CPh ); 40.94 100 solution of initiator 2a (0.1213 g, 0.24 mmol) in toluene (12 ml)
3
2
2
2
2
1
3
o
3
2
2
2
(
(
СН CPh ); 48.25 (СН CH CPh ); 50.01 (OMe); 77.20, 78.03
2CPh ); 122.54, 124.44, 125.02, 125.20, 125.50, 125.84, 126.33,
L-lactide (1.7086 g, 11.85 mmol) was added. Then BnOH (52.0
l, 0.48 mmol) was added at stirring and the reaction mixture was
heated at 80°C for 27 h. The reaction was terminated by addition
of MeOH (1.0 ml), evaporated and purified by reprecipitation
2
2
2
2
2
2
126.86, 127.22, 127.50, 127.70, 128.25, 138.94, 147.02, 149.35,
51.93, 152.08, 152.28, 152.64, 160.80, 163.36 (Ph and Py) ppm.
1
1
o
Diastereomer II: H NMR (CDCl , 25 C): δ = 1.91-2.01, 2.69- 105 using CH Cl as solvent and methanol as a non-solvent. The
3
2
2
2
.79, 3.01-3.10, 3.39-3.47 (4m, each 1H, CH ); 3.14 (s, 3H,
polymer obtained was dried in vacuum.
2
2
OMe); 3.20, 3.69 (2d, each 1H, J= 14.9 Hz); 6.54-7.66 (m, 23H,
Py and Ph) ppm. C NMR (CDCl , 55 C): δ = 31.30
Typical polymerization procedure in bulk. All manipulations
were performed under inert atmosphere. To the initiator 2c
(0.10538 g, 0.10 mmol) L-lactide (0.7343 g, 5.09 mmol) was
13
o
3
(
(
СН CH CPh ); 40.56 (СН CPh ); 48.95 (СН CH CPh ); 50.08
2 2 2 2 2 2 2 2
OMe); 77.95, 78.13 (2CPh ); 122.46, 124.33, 124.99, 125.18, 110 added. The reaction mixture was heated at 100 °C for 4.5 h. The
2
1
1
25.28, 125.79, 126.29, 126.43, 127.14, 127.42, 127.70, 128.19,
38.79, 147.05, 149.38, 151.81, 152.00, 152.14, 152.78, 160.40,
reaction was terminated by addition of MeOH (1.0 ml),
evaporated and purified by reprecipitation using CH Cl as
2
2
163.21 (Ph and Py) ppm. Crystals suitable for X-ray analysis
were obtained from concentrated toluene solution at –18 C.
solvent and methanol as a non-solvent. The polymer obtained was
dried in vacuum.
o
Synthesis of 2,6-Py(CH CPh O)(CH CMe O)AlCl (3b). 115 Polymerization procedure for ε-CL. All manipulations were
2
2
2
2
Method 1: Solid GeCl ·C H O (0.11 g, 0.47 mmol) was added
performed under inert atmosphere. To the initiator 2d (0.0363 g,
2
4
8
2
1
0
| Journal Name, [year], [vol], 00–00
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DOI: 10.1039/C5DT01001B
Page 11 of 12
Dalton Transactions
0
.06 mmol) ε-caprolactone (2.0580 g, 18.03 mmol) was added.
† Electronic Supplementary Information (ESI) available: cif files of
compounds 4, 3a, 4b and 5; polymer characteristics; NMR spectra for
compounds obtained. See DOI: 10.1039/b000000x/
55
The reaction mixture was heated at 130 °C for 0.5 h. The reaction
was terminated by addition of MeOH (1.0 ml), evaporated and
purified by reprecipitation using CH Cl as solvent and methanol
2
2
1
.
R. H. Platel, L. M. Hodgson and C. K. Williams, Polym. Rev., 2008,
8, 11-63.
X-Ray crystallography. Crystal data and details of X-ray 60 2. M. J. Stanford and A. P. Dove, Chem. Soc. Rev., 2010, 39, 486-494.
5
0
5
0
as a non-solvent. The polymer obtained was dried in vacuum.
4
analyses are given in Table 1. Experimental datasets were
collected on a Stoe IPDS 2T (for 5) machine using Cu-Kα
radiation (λ = 1.54186 Å) and Bruker SMART APEX II (for 4, 3a
3. N. E. Kamber, W. Jeong, R. M. Waymouth, R. C. Pratt, B. G. G.
Lohmeijer and J. L. Hedrick, Chem. Rev., 2007, 107, 5813-5840.
4
.
B. J. O'Keefe, M. A. Hillmyer and W. B. Tolman, J. Chem. Soc.,
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1
1
2
and 4c) diffractometer using graphite monochromatized Mo-K 65 5. O. Dechy-Cabaret, B. Martin-Vaca and D. Bourissou, Chem. Rev.,
radiation ( = 0.71073 Å). The structures were solved by direct
2004, 104, 6147-6176.
6. J. Wu, T.-L. Yu, C.-T. Chen and C.-C. Lin, Coord. Chem. Rev., 2006,
2
methods and refined by full-matrix least-squares on F with
2
50, 602-626.
7
9
anisotropic thermal parameters for all non-hydrogen atoms
7
.
N. Ajellal, J.-F. Carpentier, C. Guillaume, S. M. Guillaume, M.
Helou, V. Poirier, Y. Sarazin and A. Trifonov, Dalton Trans., 2010,
39, 8363-8376.
except disordered solvent toluene molecule in 4c and disordered
70
75
solvent CHCl molecules in 5. All hydrogen atoms were placed in
3
8
9
.
.
P. Dubois, C. Jacobs, R. Jerome and P. Teyssie, Macromolecules,
calculated positions and refined using a riding model. High final
R-values for 5 were the result of poor quality of obtained single
crystals.
The crystallographic data for 4, 3a, 4c and 5 have been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publications under the CCDC numbers 1046664-
1
991, 24, 2266-2270.
M. Wisniewski, A. L. Borgne and N. Spassky, Macromol. Chem.
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002, 124, 5938-5939.
4
1
2
1
046667. They can be obtained free of charge from the 80 12. P. Hormnirun, E. L. Marshall, V. C. Gibson, A. J. P. White and D. J.
Cambridge
www.ccdc.cam.ac.uk/data_request/cif.
Crystallographic
Data
Centre
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Williams, J. Am. Chem. Soc., 2004, 126, 2688-2689.
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1
1
2
007, 13, 4433-4451.
2
5
Conclusions
85 15. M. Bouyahyi, E. Grunova, N. Marquet, E. Kirillov, C. M. Thomas, T.
Roisnel and J.-F. Carpentier, Organometallics, 2008, 27, 5815-5825.
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Inorg. Chem., 2008, 47, 2613-2624.
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R. Raithby and S. P. Richards, Dalton Trans., 2009, 5551-5558.
8. B. Lian, H. Ma, T. P. Spaniol and J. Okuda, Dalton Trans., 2009,
9033-9042.
19. H. Du, A. H. Velders, P. J. Dijkstra, Z. Zhong, X. Chen and J. Feijen,
Macromolecules, 2009, 42, 1058-1066.
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Organometallics, 2009, 28, 1469-1475.
21. K. V. Zaitsev, Y. A. Piskun, Y. F. Oprunenko, S. S. Karlov, G. S.
Zaitseva, I. V. Vasilenko, A. V. Churakov and S. V. Kostjuk, J.
Polym. Sci., Part A: Polym. Chem., 2014, 52, 1237-1250.
00 22. M. Huang, E. K. Lermontova, K. V. Zaitsev, A. V. Churakov, Y. F.
Oprunenko, J. A. K. Howard, S. S. Karlov and G. S. Zaitseva, J.
Organomet. Chem., 2009, 694, 3828-3832.
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Toscano, V. Santes, N. Nava and P. Sharma, J. Organomet. Chem.,
In this work we have prepared and fully characterized a number
of novel aluminum complexes based on substituted 2,6-
bis(hydroxyalkyl)pyridines with aluminum atom bearing methyl,
methoxy, benzyloxy group or chlorine. It was found that the
structure of the ligand determines the nature of the complex
formed. If there is one carbon atom in the side chain of the ligand
1
9
0
1
3
3
4
0
5
0
(ligand 1), only octahedral aluminum complex 5 was isolated. If
95
the side chain of the ligand is increased by one or two carbon
atoms (ligands 2, 3, 4) it results in dimeric in solid state
complexes with five-coordinate Al atoms. Studies of these
complexes in the ring-opening polymerization of L-lactide have
demonstrated that all complexes are active in polymerization with
controlled or immortal character. Further work is underway to
optimize polymerization conditions and employ aluminum
complexes in both polymerization of L-lactide and stereoselective
polymerization of rac-lactide.
1
1
05
2003, 672, 115-122.
4. D. A. Kurmaev, N. A. Kolosov, S. C. Gagieva, A. O. Borissova, V.
A. Tuskaev, N. M. Bravaya and B. M. Bulychev, Inorg. Chim. Acta,
2
2
2
2
013, 396, 136-143.
Acknowledgements
5. D. Esteban, D. Bañobre, Andrés Blas, T. Rodríguez-Blas, R. Bastida,
A. Macías, A. Rodríguez, D. E. Fenton, H. Adams and J. Mahía, Eur.
J. Inorg. Chem., 2000, 1445-1456.
6. E. Gómez, V. Santes, V. de la Luz and N. Farfán, J. Organomet.
Chem., 2001, 622, 54-60.
We thank Russian Fund for Basic Research (12-03-00206) for 110
financial support. This work in part was supported by President
Grant for Young Russian Scientists (MD-3634.2012.3) and by
M.V. Lomonosov Moscow State University Program of
4
5
27. K. V. Zaitsev, M. V. Bermeshev, S. S. Karlov, Y. F. Oprunenko, A.
Development.
115
V. Churakov, J. A. K. Howard and G. S. Zaitseva, Inorg. Chim. Acta,
2
007, 360, 2507-2512.
2
8. R. Fandos, B. Gallego, M. I. López-Solera, A. Otero, A. Rodríguez,
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