Metal and ligand-substituent effects in the immortal polymerization of rac -lactide with Li, Na, and K phenoxo-imine complexes

Article abstract of DOI:10.1021/om501000b

A series of lithium, sodium, and potassium complexes with phenoxo-imine ligands M[(O-2-(RN=CH)C₆H₄] [R = C₆H₅; 2-^tBuC₆H₅; 2,6-ⁱPr₂C₆H₃

Full text of DOI:10.1021/om501000b

Article
pubs.acs.org/Organometallics
Metal and Ligand-Substituent Effects in the Immortal Polymerization
of rac-Lactide with Li, Na, and K Phenoxo-imine Complexes
Francisco M. García-Valle, Robert Estivill, Carlos Gallegos, Tomas
́
Cuenca, Marta E. G. Mosquera,
Vanessa Tabernero,* and Jesus Cano*
́
Departamento de Química Organ
Spain
́ ́ ́ ́
ica y Química Inorganica, Universidad de Alcala, Campus Universitario, 28871 Alcala de Henares,
*
S Supporting Information
ABSTRACT: A series of lithium, sodium, and potassium
complexes with phenoxo-imine ligands M[(O-2-(RNCH)-
t
i
C H ] [R = C H ; 2- BuC H ; 2,6- Pr C H ] and [O-2-(RN
6
4
6
5
6
5
2
6
3
t
t
i
CH)-4,6- Bu C H ] [R = C H ; 2- BuC H ; 2,6- Pr C H ] 1−
2
6
4
6
5
6
5
2
6
3
3
(a−f) have been synthesized. The molecular structures in the
solid state of some of these complexes have been determined
by X-ray diffraction. These compounds show different
nuclearities and geometries around the metal center depending on the nature and the pocket of the ligand substituents. Of
particular interest is the structure of compound 3e, being the first example of a potassium cubane complex obtained with this
kind of ligand. The structural behavior in solution has also been studied by diffusion-ordered NMR spectroscopy (DOSY),
showing a direct correlation between aggregation behavior and polymerization activity. Compounds 1−3(a−f) are extremely
active catalysts in the ring-opening polymerization (ROP) of rac-lactide, achieving conversions of 100% in less than 1 min and
heterorich-PLA that is modified by the metal atom and the ligand substituents. BnOH was used as co-initiator, and the presence
of large amounts of the alcohol produces the immortal polymerization of rac-lactide in a more controlled process. Stoichiometric
reactions involving the catalysts, BnOH, and lactide demonstrated an activated monomer mechanism for the polymerization of
rac-lactide.
5
INTRODUCTION
for the ROP of lactide owing to their nontoxic nature.
■
6
However, few examples with alkali metals such as Li, Na, and
K have been described, even though the alkali metal precursors
are cheap and easily available and their synthesis is accessible.
Morever, the new concept of immortal ring-opening polymer-
The development of petroleum-based plastics has made our
lives easier and more comfortable due to their properties and
high performance. In addition, their synthesis and processing
are both easy and inexpensive. However, their use has led to an
important environmental pollution problem. For this reason, in
recent years, the synthesis of polyesters derived from cyclic
esters such as lactide (LA) has gained increasing interest
because LA can be obtained from naturally renewable resources
such as corn, wheat, or sugar beets. Moreover, polylactide
PLA) is biodegradable and biocompatible and can be used in a
wide variety of applications including thermoplastics, films, and
fibers. As well, since PLA is biocompatible, it has found many
uses in the medical field as drug delivery systems, resorbable
7
ization (iROP), initially named as such by Inoue, allows one to
carry out the ROP with minimized amounts of a catalytic
system upon utilization of very large excess of chain transfer
agent such as BnOH. In the context of green and sustainable
chemistry, the “catalytic” iROP strategy thus appears highly
1
5h
attractive.
(
On the other hand, Schiff bases are very interesting ligand
precursors due to their straightforward preparation, high yield,
and easy purification. These molecules generate ligands that
are able to coordinate to different metals, and many catalytic
8
2
sutures, and medical implants.
9
systems are based on them. Their success in various catalytic
The most efficient method for the production of PLA is the
ring-opening polymerization (ROP) process initiated by metal-
based complexes. Different research groups have demonstrated
that by following this process well-controlled molecular weight
polymerization processes is due to the scope for suitable tuning
of the steric hindrance and the electronic properties of the
ligand precursors. As such, by changing the position and nature
of the phenyl ring substituents in the ligand it is possible to
cause variations in the catalytic properties of the complexes
3
and low polydispersity (PDI) polymers are achieved. Discrete
complexes of a wide range of metal centers such as Al, Zn, or
lanthanide have been evaluated as catalysts for the ROP of
4
d,i,5b,9
formed.
4
This work reports the preparation of a series of alkali metal
complexes bearing phenoxo-imine ligands with different
lactide. The amount of catalyst left in the resultant polymers is
usually high, which may raise concerns regarding potential
health issues associated with the toxicity of some metal-based
residues. In this sense, alkaline earth metal derivatives such as
Mg or Ca complexes have been reported recently as catalysts
Received: October 3, 2014
©
XXXX American Chemical Society
A
DOI: 10.1021/om501000b
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Article
Scheme 1. Synthesis of Phenol-imine Compounds
a
Scheme 2. Synthesis of the Alkali Metal Derivatives
a
Li: 1a−f; Na: 2a−f; K: 3a−f.
Figure 1. Molecular structure of [1b(THF)] , [1e(THF)] , and [1f(THF)] .
2
2
2
substituents to compare their activity and stereocontrol in the
effect that the bulky substituents in the aromatic rings could
play in the polymerization reaction. The condensation reaction
between the desired salicylaldehyde with a primary amine
allows us to obtain the corresponding phenol-imine com-
pounds with different steric hindrance, {La}H−{Lf}H
ROP polymerization of rac-lactide. Characterization studies by
1
H-DOSY NMR spectroscopy to explore the true nature of
these complexes in solution and by X-ray diffraction to
determine the structural disposition in the solid state are also
reported. Their catalytic activity in rac-lactide polymerization
has been analyzed, and the reaction mechanism of the
polymerization process has also been investigated, indicating
that the polymerization process depends on the presence or not
of initiator.
10
1
(
Scheme 1). The H NMR spectra of these compounds
show the characteristic low-field signal corresponding to the
OH proton in the range δ 12.50−14.00, which evidences the
acidity of this group as a consequence of intramolecular O−H···
7
N hydrogen interactions. The proton resonance of the imine
group appears in the δ 8.50−9.00 range (see the Experimental
Section).
RESULTS AND DISCUSSION
■
The reaction of the phenol-imine compounds with a
stoichiometric quantity of the appropriate metallic precursor,
[Li{N(SiMe ) }], NaH, or [K{N(SiMe ) }] has been carried
Synthesis and Spectroscopic Characterization. The
phenol-imine compounds have been tailored considering the
3
2
3 2
B
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Article
Table 1. Selected Bond Lengths and Angles for [1b(THF)] , [1e(THF)] , and [1f(THF)]
2
2
2
[
1b(THF)]2
[1e(THF)]2
[1f(THF)]2
Bond Distances (Å)
Li(1)−O(1)
Li(1)−O(1)#1
Li(1)−N(1)#1
Li(1)−O(2)
N(1)−C(1)
1.923(7)
1.915(7)
2.042(7)
1.956(7)
1.258(4)
Li(1)−O(1)
Li(1)−O(1)#1
Li(1)−N(1)
Li(1)−O(2)
N(1)−C(6)
1.874(3)
1.921(3)
2.032(3)
1.976(3)
1.285(2)
Li(1)−O(1)
Li(1)−O(1)#1
Li(1)−N(1)
Li(1)−O(2)
N(1)−C(1)
1.906(4)
1.911(4)
2.071(4)
1.961(4)
1.288(2)
Bond Angles (deg)
O(1)−Li(1)−N(1)
O(1)#1−Li(1)−O(2)
O(1)−Li(1)−O(1)#1
O(2)−Li(1)−N(1)#1
N(1)#1−Li(1)−O(1)
O(1)−Li(1)−O(2)
Li(1)#1-N(1)−C(1)
C(10)−N(1)−Li(1)#1
94.0(3)
O(1)−Li(1)−N(1)
O(1)−Li(1)−O(2)
O(1)−Li(1)−O(1)#1
N(1)−Li(1)−O(2)
N(1)−Li(1)−O(1)#1
O(2)−Li(1)−O(1)#1
Li(1)−N(1)−C(6)
Li(1)−N(1)−C(3)
93.32(14)
125.44(19)
93.88(14)
110.50(16)
129.15(18)
105.55(16)
122.33(15)
118.06(14)
O(1)−Li(1)−N(1)
O(1)−Li(1)−O(2)
O(1)−Li(1)−O(1)#1
N(1)−Li(1)−O(2)
N(1)−Li(1)- O(1)#1
O(2)−Li(1)−O(1)#1
Li(1)−N(1)−C(1)
Li(1)−N(1)−C(10)
93.89(16)
105.70(19)
100.86(16)
115.26(18)
132.3(2)
112.9(3)
98.0(3)
107.9(3)
134.4(3)
107.4(3)
119.5(3)
125.3(3)
103.93(18)
116.03(15)
128.68(15)
Figure 2. Molecular structure of [2d(THF)] and [3d(THF) ] .
2
3 2
out in dry THF. After removing the solvent, the alkali metal
cases these complexes coordinate THF in the crystallization
process.
complexes 1(a−f), 2(a−f), and 3(a−f) are isolated with high
purity and good yields (Scheme 2).
The molecular structures of the three lithium complexes are
very similar and show a dinuclear disposition where the lithium
atoms are bridged by the oxygen atom of the phenoxo ligand,
giving a Li O core (see Figure 1). The nitrogen atom from the
1
The H NMR spectra for all complexes (DMSO-d at 295 K)
6
indicate the presence of sharp signals with the expected
multiplicity, as expected for the presence of the phenoxo-imine
ligands, which confirmed that the metal derivatives have been
synthesized with high purity. Compared to the starting phenol-
imine compounds, the chemical shifts in the alkali complexes
were located at higher fields. Despite that the synthesis has
been carried out in THF, after drying all these compounds
2
2
imine group also establishes a donor interaction with the metal
center, generating a six-membered chelating ring, LiNC O. The
3
lithium atoms complete their coordination sphere with a THF
molecule, giving a distorted tetrahedral environment due to the
restrictions imposed by the chelate ring. The distances within
1
under vacuum, no solvent is observed in the H NMR spectra.
the LiNC O ring are very similar for the three complexes
3
13
11
In C NMR spectra, the most interesting signal is due to the
carbon atom bonded to the oxygen atom, which is downfield
shifted with respect to the ligand precursors (δ ca. 170.0 vs
(Table 1) and are within the range for this kind of derivative.
The LiNC O ring is nearly planar, affected by the trigonal
3
planar geometry of the nitrogen atom from the imine group.
This ring is placed almost coplanar to the phenoxo group, while
the phenyl group bonded to the nitrogen atom is in a plane
159.0 ppm), confirming the ligand coordination (see the
Experimental Section).
Solid-State Structure Determination. The molecular
structures in the solid state for three lithium, 1b, 1e, and 1f, two
sodium, 2d and 2e, and two potassium complexes, 3d and 3e,
have been determined by single-crystal X-ray crystallography.
Suitable single crystals of 1b, 1f, and 2d were obtained from a
THF/toluene solution, while complexes 1e, 2e, 3d, and 3e
were crystallized from a concentrated THF solution. In all the
nearly perpendicular to this LiNC O ring. The values of the
3
dihedral angle between the LiNC O ring and the Li O central
3
2
2
core are 42.85° for [1b(THF)] , 49.96° for [1e(THF)] , and
2
2
35.6° for [1f(THF)]2.
Sodium and potassium complexes bearing the Ld ligand,
[2d(THF)] and [3d(THF) ] , are also dinuclear like the
2
3 2
previously described lithium derivatives (Figure 2). The sodium
C
DOI: 10.1021/om501000b
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Article
a
Table 2. Selected Bond Lengths and Angles for [2d(THF)] and [3d(THF)3]2
2
[2d(THF)]2
[3d(THF)3]2
Bond Distances (Å)
Na(1)−O(1)#1
Na(1)−O(1)
Na(1)−O(2)
Na(1)−N(1)
N(1)−C(1)
2.2011(17)
2.231(2)
2.291(2)
2.4536(19)
1.284(3)
1.290(3)
K(2)−O(1)
2.675(3)
2.599(2)
2.767(3)
2.770(3)
2.769(3)
K(2)−O(1)#2
K(2)−O(10)
K(2)−O(11)
K(2)−O(12)
O(1)−C(3)
Bond Angles (deg)
O(1)#1−Na(1)−O(1)
O(1)#1−Na(1)−O(2)
O(1)−Na(1)−O(2)
O(1)−Na(1)−N(1)
C(1)−N(1)−C(22)
N(1)−C(1)−C(2)
96.39(7)
O(1)−K(2)−O(1)#2
O(1)−K(2)−O(10)
O(1)−K(2)−O(11)
O(1)−K(2)−O(12)
O(1)#2−K(2)−O(10)
O(1)#2−K(2)−O(11)
O(1)#2−K(2)−O(12)
O(10)−K(2)−O(11)
O(10)−K(2)−O(12)
O(11)−K(2)−O(12)
K(2)−O(1)−K(2)#2
C(7)−O(1)−K(2)#2
C(7)−O(1)−K(2)
84.08(7)
118.70(8)
104.61(10)
111.80(8)
114.62(18)
129.4(2)
118.06(9)
102.81(8)
125.39(12)
113.59(8)
163.56(8)
86.02(8)
O(1)−C(3)−C(2)#1
120.87(19)
76.63(10)
114.98(11)
77.82(9)
95.92(7)
158.11(19)
104.23(18)
a
Symmetry transformations used to generate equivalent atoms: for [2d(THF)] : #1 −x+1, −y+1, −z; for [3d(THF) ] #1 −x+1, −y+1, −z+1; #2
2
3 2
−
x+2, −y+2, −z.
complex 2d shows a structure very similar to the lithium
counterparts, the most remarkable differences being the
distances within the Na O core and the bond distances from
four phenoxo-imine ligands through the phenoxo moiety. The
metals saturate their coordination sphere by coordinating to the
imine nitrogen atom and one molecule of THF (Figure 3).
Examples of this kind of structure for sodium have been
2
2
the sodium atoms due to the bigger size of the metal (Table 2).
Also, because of the bigger size of sodium compared to lithium,
the metal establishes a Na···Me interactions with one methyl
11
previously described. Nevertheless [3e(THF)] constitutes
4
the first example of a potassium cubane structure bearing this
type of phenoxo-imine ligand. As in [3d(THF) ] , in
t
group from the Bu substituent in order to saturate its
3
2
11
coordination sphere (Na···C distance 3.252 Å).
[3e(THF)] an interaction between the potassium atom and
4
For the potassium derivative [3d(THF)3]2 (Figure 2)
important differences are observed with respect to the lithium
and sodium complexes. In this case the potassium atom
saturates its coordination sphere by coordinating three THF
molecules exhibiting a pentacoordinated disposition. The
phenoxo-imine ligand now acts only as a bridging ligand
through the oxygen atom, and no chelating nitrogen
coordination is observed; consequently the phenyl imine
group is located in a plane nearly perpendicular to the central
K O core and in the same plane as the phenoxo ring. The
the α-phenoxo carbon atom of the nonchelating ligand is also
observed.
Diffusion-Ordered NMR Spectroscopy (DOSY) Stud-
ies. The study of the molecular structure and aggregation
degree of the metallic complexes in solution is a very useful tool
to understand their behavior in polymerization processes under
1
these conditions. H-DOSY is a powerful method to estimate
the degree of aggregation and the molecular mass through the
13
measurement of the diffusion coefficient, D. These measure-
ments strongly depend on experimental conditions such as
viscosity changes or temperature fluctuations. Thus, to achieve
reliable molecular mass an internal reference method has been
chosen to obtain this value by their relative diffusion coefficient.
2
2
distance of the α-phenoxo carbon atom to the metal center is
2
quite short (3.35 Å) (Table 2) and could be described as an η -
C−O bond to potassium. Similar interactions have been
13b
observed in heterometallic potassium and zinc derivatives
In our study we chose as internal reference standards
N-
1
2
described by Mulvey and Hevia, where the K−C
benzylideneaniline (PhNCHPh; FW = 181.2), 1-phenyl-
naphtalene (PhN, FW = 204.7), and 1,2,3,4-tetraphenylnaph-
thalene (TPhN, FW = 432.6), which present good solubilities,
minimal overlapping of signals, and no reactivity with our
complexes.
phenoxo
distance is even shorter, 3.141 Å. This interaction could be
responsible for the perpendicular disposition of the phenoxo
ring with respect to the K O core that hampers the
2
2
coordination of the imine nitrogen to the metal. No
intermolecular Me···K interactions are present, similar to that
observed in the sodium compound.
In order to obtain a further understanding of the true nature
of these complexes in the lactide polymerization process, we
have studied complexes 1e−3e in the same conditions as in the
polymerization reactions, that is, in CD Cl and in the presence
The structures for sodium and potassium compounds with
the Le ligand, [2e(THF)] (see the Supporting Information)
4
2
2
and [3e(THF)] , have been also determined by X-ray
of benzyl alcohol together with the three reference standards in
an equimolar ratio. A correlation between log D and log FW of
the linear least-squares fit to the internal references can be set
up for each complex as shown in Table 3 (log D = A log FW +
4
diffraction methods, although the quality of the data is poor,
but the connectivity and the nuclearity of the compounds in the
solid state can be accurately determined. In both cases the
compound structure is formed by four metal atoms bridged by
13b,14
1
B).
The H-DOSY D−FW plots have a high correlation
D
DOI: 10.1021/om501000b
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Article
Figure 3. Molecular structure of [2e(THF)] (a and b) and [3e(THF)] (c and d).
4
4
Table 3. Diffusion Coefficients and Molecular Mass (m) in
CD Cl in the Presence of BnOH for 1e−3e
Chart 1. Metal Complexes Tested in the ROP of rac-Lactide
2
2
m
m*
b
%
error
a
2
entry estim. complex (g/mol)
D (m /s)
(g/mol)
6.714 × 10−
6.194 × 10−
6.095 × 10−
10
10
10
752
862
887
4.9
−4.7
−3.7
1
2
3
[1e·
790.94
(
BnOH)]2
[2e·
823.02
(BnOH)]2
[3e·
855.24
(BnOH)]2
a
b
Predicted molecular mass. Experimental molecular mass.
(
r2 > 0.99).15 By interpolating the corresponding D value in the
respective calibration curve for each compound, an approximate
14b
value of the molecular weight in solution can be obtained
see Supporting Information). In this study, all the complexes
(
show a dinuclear structure with one BnOH molecule
coordinated per metallic center. Thus, it can be considered
that in the polymerization process these structures might be
responsible for the beginning of the ROP in CH Cl .
2
2
Ring-Opening Polymerization of rac-Lactide. The
polymerization of rac-lactide with complexes 1a to 3f (Chart
1
) in the presence of benzyl alcohol as co-initiator has been
tested in CH Cl , and the results are summarized in Table 4.
2
2
E
DOI: 10.1021/om501000b
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Organometallics
Article
Preliminary polymerization experiments have been carried out
to study the influence of the metal center and the ligand
substituents on the ROP activity and stereoselectivity.
stereoselectivity has been observed. As such, some sodium
aryloxo derivatives produce a racemic enchainment in the range
0.37−0.49, while potassium bis(phenolate) complexes generate
6
a,b
atactic polymers.
Lithium and sodium bis(phenolate)
6d
complexes were tested only in the polymerization of L-lactide.
Table 4. Polymerization of rac-Lactide (Preliminary
a
In our tests, the general tendency is that sodium 2 and
potassium 3 complexes are extremely active catalysts, reaching
total conversion in less than 1 min, while lithium 1 complexes
need more than 30 min to obtain high conversions. These
differences could be ascribed to the size of the different alkali
metal atoms. The nature of the ligand substituents (Chart 1)
seems to have less influence than the metal center on the
activity, and for this parameter no clear tendencies are
observed.
Results)
c
d
entry
complex
[LA]/[cat]/[I]
t (min)
conv (%)
Pr
1
2
3
4
5
6
7
8
1a
2a
3a
1b
2b
3b
1c
2c
2c
3c
1d
2d
3d
1e
2e
2e
3e
1f
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
100:1:1
60
1
91
99
99
99
99
99
97
99
99
99
88
99
99
98
99
98
99
98
99
99
0.71
0.64
0.5
45
1
atactic
0.64
0.53
0.5
45
1
atactic
0.74
The selectivity in the polymerization of rac-lactide is also
0.56
highly influenced by the metal center, with the highest
b
9
5
0.68
5c,16
selectivity being observed for the slowest catalysts.
The
1
1
1
1
1
1
1
1
1
1
2
0
1
2
3
4
5
6
7
8
9
0
0.5
60
1
atactic
0.64
ligand substituent also plays an interesting role in this process
property. The lithium complexes 1a, 1c, and 1e give the best Pr
0.56
values, producing heterotactic-rich PLA (P = 0.71, 0.74, and
r
0.5
30
1
atactic
0.75
0
.75, entries 1, 7, and 14) at 25 °C. These complexes with no
substituents in the phenoxo ring afford more stereoregular
polymers than complexes 1b, 1d, and 1f, containing
0.50
b
5
0.68
substituents in the phenoxo ring (P = 0.64, 0.64, and 0.62,
0.5
45
1
atactic
0.62
r
entries 4, 11, and 18). As a general tendency, the complexes
with bulky substituents in the imine ring group afford better
selectivities when no substituents in the phenoxo ring are
2f
0.58
3f
0.5
atactic
a
present (P = 0.74 and 0.75, entries 7 and 14) in comparison
r
General conditions: 100:1:1 mixture of the lactide, [M], and BnOH,
b
c
1
with the complexes with both substituted rings (P = 0.64 and
CH Cl (5 mL), [LA] = 1 M, RT. T = −30 °C. Obtained from H
NMR analysis. Calculated from homonuclear decoupled H NMR
analysis.
r
2
2
d
1
0.62, entries 11 and 18).
In the polymerizations with sodium complexes, when the
temperature was decreased to −30 °C, heterorich-PLA
Few alkali complexes have been used in the polymerization
polymers are also obtained (P = 0.68 for 2c and 2e, entries
r
of rac-lactide, and in the reported studies very poor
9 and 16). The potassium complexes do not present any
a
Table 5. Immortal Polymerization of rac-Lactide Initiated by 1e and 2e with Different Amounts of BnOH
d
e
f
g
g
entry
complex
[LA]/[cat]/[I]
t (min)
conv (%)
Pr
Mn,theo
Mn,GPC
PDI
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
1e
1e
1e
1e
1e
1e
1e
1e
1e
2e
2e
2e
2e
2e
2e
2e
2e
2e
2e
2e
100:1:0
100:1:1
100:1:2
100:1:4
100:1:8
100:1:16
100:1:20
200:1:40
400:1:80
100:1:0
100:1:0
100:1:1
100:1:1
100:1:2
100:1:4
100:1:8
100:1:16
100:1:20
200:1:40
400:1:80
90
30
30
15
5
96
98
99
99
99
99
99
99
99
99
98
98
99
99
99
99
99
99
99
99
0.80
13 877
14 230
7243
3676
1892
892
11 594
8305
8342
3007
2067
1223
841
2.03
1.55
1.54
1.45
1.54
1.32
1.26
1.33
1.33
3.19
2.13
1.40
1.54
1.60
1.70
1.72
1.79
1.65
1.54
1.49
0.75
0.70
0.68
0.65
1
atactic
atactic
atactic
atactic
0.68
1
822
3
822
804
7
822
715
1
14 342
14 126
14 230
14 380
7243
3676
1892
892
14 022
16 350
7544
9489
10 710
4367
2555
1380
1517
907
b
c
5
0.68
5
0.68
1
atactic
atactic
atactic
atactic
atactic
atactic
atactic
atactic
1
0.5
0.5
0.5
0.5
2
822
822
4
822
783
a
b
c
d
General conditions: 100:1:1 mixture of the lactide, [M], and BnOH, CH Cl (5 mL), [LA] = 1 M, 25 °C. T = 0 °C. T = −30 °C. Obtained from
2
2
1
e
1
f
H NMR analysis. Calculated from homonuclear decoupled H NMR analysis. M
= {[M (lactide) × [LA]/[cat] × conv/no. equiv BnOH} +
= {[LA] × [LA]/[cat] × conv}. Determined by GPC calibrated versus polystyrene standards using a correction factor of
n,theo
w
g
M (BnOH) or M
0
n
.58.
n,theo
17
F
DOI: 10.1021/om501000b
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
selectivity under the reaction conditions used. As described, the
potassium complex 3d shows, in the solid state, a structural
disposition with the imine group of the ligand uncoordinated,
while three molecules of THF are coordinated to the potassium
atom. Therefore, the chelating coordination of the phenoxo-
imine ligand in the potassium complexes must be weaker than
in the analogous lithium or sodium derivatives. A plausible
explanation for the absence of tacticity observed for the
potassium complexes, besides their larger size, could be that in
the presence of donor molecules, such as lactide, the absence of
a chelating disposition of the phenoxo ligand prevents the
induction of stereoselectivity.
In view of these results, complexes 1e and 2e were selected
for a more detailed study (Table 5) since lithium complexes
provide the highest tacticity and sodium complexes show a very
high activity with some tacticity. These experimental polymer-
ization tests were explored with different reagent stoichiome-
tries and temperatures, and the resulting polymers were
analyzed by GPC and NMR spectroscopy. The GPC analysis
for the rac-PLA obtained in dichloromethane in the presence of
BnOH at various molar rates shows a monomodal weight
distribution ranging from 1.26 to 1.55 for the lithium complex
BnOH into the monomer, while the metal complex appears
intact at the end of the reaction. These results together with the
1
H-DOSY NMR experiments (Table 3) are in accordance with
5
a,13h,19c,20
an activated monomer mechanism.
As described
before, in the absence of initiator the mechanism seems to be
different, so we also have checked the stoichiometric reaction
under these conditions. When the reactions of a 1:1 mixture of
(1−3)e and rac-lactide were studied by NMR spectroscopy, no
terminal groups are observed in the oligomers formed. MALDI-
TOF experiments confirmed the cyclic nature as mentioned
below. Hence it can be suggested that in the absence of BnOH
the polymerization mechanism occurs by insertion of the
monomer into the M−O bond via a coordination−insertion
mechanism followed by a ring-closing termination step,
1
8c
producing cyclic polymers as reported by Kozak
and
19a
Kerton. Therefore, for our systems, the absence of BnOH
produces rac-lactide polymerization giving cyclic polymers,
while the presence of BnOH as co-initiator plays an important
role in the polymerization, giving a more controlled process
with benzyl end-groups in an immortal ROP reaction.
CONCLUSIONS
■
1e (entries 22−29) being more controlled with an excess of
A series of phenoxo-imine complexes with nontoxic alkali
metals Li, Na, and K have been synthesized and fully
characterized. In the solid state, the lithium derivatives show
a dinuclear structure, while the sodium and potassium
complexes exhibit di- or tetranuclear dispositions depending
on the ligand nature. This tetranuclear structure is not
preserved in solution in the presence of BnOH, as shown by
BnOH. In the case of the sodium derivative 2e the
polymerization is less controlled, giving polydispersities
between 1.49 and 1.79 (entries 33−40). Molecular weights,
obtained by GPC, for the resulting polymers when using a
catalyst:BnOH ratio of 1:1 are lower than those expected
assuming one growing chain per metal atom, indicating the
presence of inter- and intratransesterification reactions, which
were confirmed by MALDI-TOF experiments (separation
1
H-DOSY NMR studies that reveal a dinuclear structure for
1
e−3e under these conditions with one benzyl alcohol
−1
18
peaks of Δ (m/z) = 72 g·mol ). However, when the
molar ratio [1e−2e:BnOH] is 1:x (x = 2, 4, 8, 16, 20), the
obtained PLA polymers exhibit similar molecular weights to
those calculated, indicating that one H-[PLA]-OR polymer
chain is formed per added ROH and grows continually through
molecule coordinated per metal center. All complexes are
very active toward ROP of rac-lactide, and the activity of the
catalysts increases with the size of the metal center (K > Na >
Li), pointing to the nature of the metal center as a critical factor
in the polymerization reaction. A clear effect is exerted by the
substituents of the phenyl rings, which modifies the tacticities
of the polymer chain. The few examples of alkaline complexes
in the polymerization of rac-lactide described in the literature at
room temperature presented very poor stereoselectivity, but in
our case, heterorich-PLA are obtained with lithium complexes
19
1
the immortal ROP in a more controlled process. The H
NMR spectra of the resulting polymers show the corresponding
peaks of PLA including the terminal groups HOC(H)Me− and
−
C(O)OBn. On increasing the concentration of lactide with
respect to the metal complex in a ratio of 200:1:40 and
00:1:80 (entries 28, 29, 39, and 40), the polymerization is also
4
(
Pr = 0.75).
controlled and the activity does not decrease significantly.
As a comparison, polymerization reactions of rac-lactide with
The BnOH acts as a chain transfer agent with metal catalysts
leading to the rapid immortal ROP of cyclic esters, to produce
some of the most active ROP catalysts known to date. In the
presence of BnOH, an activated monomer mechanism is
suggested, while in the absence of co-initiator, a coordination−
insertion mechanism followed by a ring-closing termination is
1e and 2e were carried out in the absence of benzyl alcohol.
Under these conditions, the process was slower, leading to
higher heterorich-PLA (P = 0.80 for compound 1e, entry 21)
r
and greater molecular weight but with broader PDIs (from 2.03
to 3.19, entries 21, 30, and 31). No terminal polymer end-
proposed, giving heterorich-PLA (P = 0.80) with high M and
1
r
w
groups were identified by H NMR spectroscopy, indicating the
broader polydispersities. GPC and MALDI-TOF analysis show
that transesterification reactions occur when a 1:1 ratio of
catalyst to BnOH is used, but with higher amounts of BnOH,
an iROP process, a more controlled polymerization reaction
takes place.
18c,19a,b
cyclic nature of the polymers,
which was confirmed by
MALDI-TOF experiments. These results suggest that the
nature of the propagating species in the presence and absence
of BnOH is not the same.
In order to obtain insight into the polymerization
mechanism, the stoichiometric reactions involving the metal
derivatives 1e, 2e, and 3e, BnOH, and rac-lactide have been
EXPERIMENTAL SECTION
General Procedures. All manipulations were performed under an
inert atmosphere using standard Schlenk-line techniques (O < 3
■
studied in dichloromethane-d at 303 K. Initially, a mixture of
2
2
the metal derivative and BnOH (1:1) was monitored, but no
reaction was detected, and the formation of neither {Le}H nor
BnO-M (M = Li, Na, K) was observed. When the reactions
were performed adding rac-LA to generate a 1:1:1 mixture, the
products formed correspond to the ring-opening insertion of
ppm) and in an MBraun MB-20G glovebox (O < 0.6 ppm). Solvents
2
were dried by conventional procedures and freshly distilled prior to
use. Deuterated solvents were degassed by freeze−vacuum−thaw
cycles and stored in a glovebox in the presence of molecular sieves (4
Å). All reagents were purchased from Aldrich and used as received.
G
DOI: 10.1021/om501000b
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Sodium hydride was washed twice with hexane due to the commercial
reagent being an oil suspension. rac-Lactide was purified by
recrystallization from toluene twice and subsequent sublimation
under vacuum. NMR spectra were recorded with a Bruker 400
Ultrashield ( H 400 MHz, C 101 MHz) at room temperature. All
chemical shifts were determinated using the residual signal of solvents
for C H NO (365.56 g/mol): C 82.13, H 9.57, N 3.83. Found: C
25
35
82.29, H 9.71, N 4.01.
i
Synthesis of (2,6-Pr C H NCH-C H OH), {Le}H. Using the same
2
6
3
6 4
method as that for {La}H but using 2,6-diisopropylaniline (7.05 g,
1
13
0.036 mol) and 2-hydroxybenzaldehyde (4.46 g, 0.036 mol),
compound {Le}H was obtained as a yellow powder. Yield: 8.69 g,
1
and were reported versus SiMe . Assignment of signals was carried out
86%. H NMR (DMSO-d , 400 MHz, 295 K): δ 12.69 (s, 1H, OH),
4
6
1
13
1
1
13
with 1D ( H, C{ H}) and 2D ( H− C HSQC) NMR experiments.
Elemental analyses were performed with a PerkinElmer 2400 CHNS/
O analyzer Series II and were the average of a minimum of two
independent measurements. Molecular weights of polymers were
determined by gel permeation chromatography (GPC) in a Varian
HPL apparatus with a Plgel Mixed-D (30 cm × 7.5 mm × 5 μm)
column and a light scattering detector (Pl-ELS 1000) in THF at room
temperature calibrated with respect to polystyrene standards and
8.57 (s, 1H, HCN), 7.66 (d, 1H, C H ), 7.43 (m, 1H, C H ), 7.17
6
4
6
4
(m, 3H, C H ), 6.98 (m, 2H, C H ), 2.89 [m, 2H, HC(CH ) ], 1.12
6
3
6
4
3 2
13
[d, 12H, HC(CH ) ]. C NMR (DMSO-d , 101 MHz, 295 K): δ
3
2
6
167.5 (CN), 160.7 (C-OH), 146.8, 138.4, 133.9, 132.7, 125.6, 123.5,
119.6, 119.3, 117.1 (Ar-C), 28.1 [HC(CH ) ], 23.6 [HC(CH ) ].
3
2
3 2
Anal. Calcd for C H NO (281.42 g/mol): C 81.11, H 8.24, N 4.98.
19
23
Found: C 81.20, H 8.38, N 5.23.
i
t
Synthesis of (2,6-Pr C H NCH-3,5- Bu C H OH), {Lf}H. Using
2
6
3
2 6 2
17
corrected with a factor of 0.58. MALDI-TOF MAS analysis was
performed using a MALDI Agilent TOF LC/MS, and the ionization
source was Masstech AP/MALDI. The mass spectrum was recorded in
positive mode. 1,8,9-Anthracenetriol was used as matrix, and sodium
iodide was added as a cationization agent.
the same method as that for {La}H, but using 2,6-diisopropylaniline
(5.08 g, 0.026 mol) and 3,5-di-tert-butyl-2-hydroxybenzaldehyde (6.10
g, 0.026 mol), compound {Lf}H was obtained as a yellow powder.
1
Yield: 5.22 g, 53%. H NMR (DMSO-d , 400 MHz, 295 K): δ 13.57
6
(s, 1H, OH), 8.55 (s, 1H, HCN), 7.47 (s, 1H, C H ), 7.42 (s, 1H,
C H ), 7.19 (m, 3H, C H ), 2.89 [m, 2H, HC(CH ) ], 1.43 [s, 9H,
6 2 6 3 3 2
6
2
Synthesis of (C H NCHC H OH), {La}H. A solution of aniline
6
5
6
4
1
3
(
4.79 g, 0.05 mol) in ethanol (150 mL) was prepared, and to it was
C(CH )], 1.28 [s, 9H, C(CH ) ], 1.13 [d, 12H, HC(CH ) ].
C
3
3
3
3 2
added 2-hydroxybenzaldehyde (6.41 g, 0.05 mol). This mixture was
NMR (DMSO-d , 101 MHz, 295 K): δ 169.5 (CN), 158.0 (C-OH),
6
refluxed for 18−20 h under stirring. The resultant solution was
146.2, 140.8, 138.6, 136.3, 128.1, 127.9, 125.8, 123.5, 118.2 (Ar-C),
35.1, 34.4 [C(CH ) ], 31.7, 29.8 [C(CH ) ], 28.2 [HC(CH ) ], 23.7
concentrated under vacuum to just 50 mL, and MgSO was added to
4
3
3
3
3
3 2
eliminate H O. After filtration, it was concentrated under vacuum and
[HC(CH ) ]. Anal. Calcd for C H NO (393.64 g/mol): C 82.39, H
9.99, N 3.56. Found: C 82.71, H 10.09, N 3.68.
2
3 2 27 39
stored at −20 °C for a night to give a yellow powder, which was
1
characterized as compound {La}H. Yield: 9.17 g, 91%. H NMR
Synthesis of Li[(O-2-{(C H )NCH}C H )] [Li{La}] (1a). At room
6
5
6 4
(
DMSO-d , 400 MHz, 295 K): δ 13.15 (s, 1H, OH), 8.94 (s, 1H,
temperature a mixture of {La}H (1.5 g, 7.60 mmol) and Li[N{Si-
6
HCN), 7.65 (d, 1H, C H ), 7.47−7.39 (m, 5H, ArH), 7.30 (s, 1H,
(CH ) } ] (1.31 g, 7.60 mmol) in THF (50 mL) was stirred for one
night. The resultant solution was filtered and concentrated under
6
4
3 3 2
1
3
C H ), 6.95 (d, 2H, C H ). C NMR (DMSO-d , 101 MHz, 295 K):
6
5
6
5
6
δ 163.9 (CN), 160.8 (C-OH), 148.5, 138.1, 133.7, 133.1, 129.9,
reduced pressure, obtaining a white powder, which was characterized
1
1
27.4, 121.8, 119.7, 119.5, 117.1 (Ar-C). Anal. Calcd for C H NO
as compound 1a. Yield: 1.46 g, 94%. H NMR (DMSO-d , 400 MHz,
13
11
6
(
6
197.23 g/mol): C 79.16, H 5.57, N 7.10. Found: C 78.76, H 5.51, N
.99.
Synthesis of (C H NCH-3,5- Bu C H OH), {Lb}H. Using the same
295 K): δ 8.29 (s, 1H, HCN), 7.32 (m, 2H, C H ), 7.21 (m, 3H,
6
5
C H /C H ), 7.11 (m, 1H, C H ), 6.98 (m, 1H, C H ), 6.36 (d, 1H,
6 5 6 4 6 4 6 4
t
13
C H ), 6.13 (m, 1H, C H ). C NMR (DMSO-d , 101 MHz, 295 K):
6
5
2
6
2
6
5
6
5
6
method as that for {La}H but using aniline (3.01 g, 0.032 mol) and
,5-di-tert-butyl-2-hydroxybenzaldehyde (7.57 g, 0.032 mol), com-
δ 172.7 (C-O), 164.7 (CN), 153.6, 135.9, 133.4, 129.3, 124.8, 123.5,
3
122.5, 121.7, 109.3 (Ar-C). Anal. Calcd for C H NOLi (203.180 g/
13
10
1
pound {Lb}H was obtained as a yellow powder. Yield: 9.44 g, 94%. H
mol): C 76.47, H 4.89, N 6.89. Found: C 76.53, H 4.60, N 6.79.
NMR (DMSO-d , 400 MHz, 295 K): δ 13.95 (s, 1H, OH), 8.98 (s,
Synthesis of Na[(O-2-{(C H )NCH}C H )] [Na{La}] (2a). The
6
6
5
6 4
1
1
H, HCN), 7.50 (d, 1H, C H ), 7.48−7.42 (m, 4H, C H ), 7.40 (d,
procedure was as described for 1a but using {La}H (1.5 g, 7.60 mmol)
6
2
6
5
H, C H ), 7.30 (t, 1H, C H ), 1.40 [s, 9H, C(CH ) ], 1.29 [s, 9H,
and NaH (0.182 g, 7.60 mmol) in THF (50 mL). A pale yellow
6
2
6
5
3 3
13
C(CH ) ]. C NMR (DMSO-d , 101 MHz, 295 K): δ 165.6 (CN),
powder was obtained, which was characterized as compound 2a. Yield:
3
3
6
1
1
1
58.0 (C-OH), 148.2, 140.6, 136.3, 129.9, 128.1, 127.8, 127.3, 121.8,
18.7 (Ar-C), 35.1, 34.4 [C(CH ) ], 31.7, 29.7 [C(CH ) ]. Anal.
1.61 g, 96%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.77 (s, 1H,
6
HCN), 7.51 (m, 1H, C H ), 7.29 (m, 2H, C H ), 7.06 (m, 3H,
C H /C H ), 6.87 (m, 1H, C H ), 6.25 (m, 1H, C H ), 5.99 (m, 1H,
6 4 6 5 6 4 6 5
3
3
3
3
6
4
6
5
Calcd for C H NO (309.47 g/mol): C 81.50, H 8.73, N 4.53. Found:
21
27
13
C 81.17, H 8.51, N 4.44.
C H ). C NMR (DMSO-d , 101 MHz, 295 K): δ 173.5 (C-O), 161.6
6 4 6
t
Synthesis of (2- BuC H NCHC H OH), {Lc}H. Using the same
(CN), 154.9, 133.1, 129.4, 128.9, 124.2, 123.4, 123.2, 121.4, 109.2
(Ar-C). Anal. Calcd for C H NONa (219.22 g/mol): C 71.23, H
6
4
6 4
method as that for {La}H but using 2-tert-butylaniline (6.02 g, 0.040
13
10
mol) and 2-hydroxybenzaldehyde (4.93 g, 0.040 mol), compound
4.56, N 6.39. Found: C 70.88, H 4.54, N 6.13.
1
{
Lc}H was obtained as a yellow powder. Yield: 8.72 g, 87%. H NMR
DMSO-d , 400 MHz, 295 K): δ 12.53 (s, 1H, OH), 8.69 (s, 1H,
Synthesis of K[(O-2-{(C H )NCH}C H )] [K{La}] (3a). The
6
5
6 4
(
procedure was as described for 1a but using {La}H (1.5 g, 7.60
6
HCN), 7.72 (d, 1H, C H ), 7.41 (m, 2H, C H O-C H N), 7.29 (t,
mmol) and K[N{Si(CH ) } ] (1.60 g, 7.60 mmol) in THF (50 mL).
6
4
6
4
6
4
3 3 2
1
H, C H O), 7.22 (t, 1H, C H N), 7.01 (m, 3H, C H O-C H N), 1.37
A yellow powder was obtained, which was characterized as compound
6
4
6
4
6
4
6
4
13
1
[
s, 9H, C(CH ) ]. C NMR (DMSO-d , 101 MHz, 295 K): δ 162.6
3a. Yield: 1.60 g, 89%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.78
3
3
6
6
(
CN), 160.0 (C-OH), 149.6, 142.4, 133.7, 132.4, 127.8, 126.7,
(s, 1H, HCN), 7.51 (m, 1H, C H ), 7.29 (m, 2H, C H ), 7.03 (m,
6
4
6
5
1
26.44, 121.4, 120.4, 119.7, 116.9 (Ar-C), 35.4 [C(CH ) ], 30.9
3H, C H /C H ), 6.80 (m, 1H, C H ), 6.12 (m, 1H, C H ), 5.84 (m,
3
3
6 4 6 5 6 4 6 5
13
[
C(CH ) ]. Anal. Calcd for C H NO (253.34 g/mol): C 80.59, H
1H, C H ). C NMR (DMSO-d , 101 MHz, 295 K): δ 175.8 (C-O),
6 4 6
3
3
17 19
7
.49, N 5.53. Found: C 80.34, H 7.72, N 5.56.
161.2 (CN), 155.4, 133.1, 129.3, 127.5, 124.0, 123.6, 123.4, 121.3,
t
t
Synthesis of (2- BuC H NCH-3,5- Bu C H OH), {Ld}H. Using the
106.9 (Ar-C). Anal. Calcd for C H NOK (235.327 g/mol): C 66.35,
6
4
2
6
2
13 10
same method as that for {La}H but using 2-tert-butylaniline (4.12 g,
.027 mol) and 3,5-di-tert-butyl-2-hydroxybenzaldehyde (6.47 g, 0.027
H 4.28, N 5.95. Found: C 66.53, H 4.61, N 6.22.
t
0
Synthesis of Li[(O-2-{(C H )NCH}-3,5- Bu C H )] [Li{Lb}] (1b).
6
5
2 6 2
mol), compound {Ld}H was obtained as a yellow powder. Yield: 8.97
g, 89%. H NMR (DMSO-d , 400 MHz, 295 K): δ 13.59 (s, 1H, OH),
The same method as that for 1a was used but with {Lb}H (1.47 g,
4.75 mmol) and Li[N{Si(CH ) } ] (0.82 g, 4.75 mmol). Yield (1b):
1.50 g, 99%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8,23 (s, 1H,
1
6
3 3 2
1
8
.68 (s, 1H, HCN), 7.53 (s, 1H, C H ), 7.41 (s, 2H, C H ), 7.31
6
2
6
2
6
(
m, 1H, C H ), 7.24 (m, 1H, C H ), 7.08 (d, 1H, C H ), 1.43 [s, 9H,
HCN), 7,32 (m, 2H, C H ), 7.24 (m, 2H, C H ), 7.10 (m, 2H,
C H /C H ), 6.99 (s, 1H, C H ), 1.40 [s, 9H, C(CH ) ], 1.22 [s, 9H,
6 5 6 2 6 2 3 3
6
4
6
4
6
4
6
5
6
5
1
3
C(CH ) ], 1.39 [s, 9H, C(CH ) ], 1.30 [s, 9H, C(CH ) ]. C NMR
3
3
3
3
3
3
13
(
1
3
DMSO-d , 101 MHz, 295 K): δ 165.6 (CN), 157.5 (C-OH), 149.1,
C(CH ) ]. C NMR (DMSO-d , 101 MHz, 295 K): δ 170.7 (C-O),
6
3 3 6
42.4, 140.8, 136.2, 128.2, 127.9, 126.8, 126.5, 122.0, 119.1 (Ar-C),
5.3, 35.1, 34.4 [C(CH ) ], 31.7, 30.9, 29.7 [C(CH ) ]. Anal. Calcd
165.7 (CN), 154.2, 140.0, 130.0, 129.3, 127.2, 124.3, 121.7, 120.8
(Ar-C), 35.5, 33.8 [C(CH ) ], 32.1, 30.0 [C(CH ) ]. Anal. Calcd for
3
3
3
3
3
3
3
3
H
DOI: 10.1021/om501000b
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
C H NOLi (315.39 g/mol): C 79.80, H 8.31, N 4.44. Found: C
[C(CH ) ]. Anal. Calcd for C H NOLi (371.48 g/mol): C 80.83, H
21
26
3
3
25 34
7
9.82, H 8.29, N 4.38.
Synthesis of Na[(O-2-{(C H )NCH}-3,5- Bu C H )] [Na{Lb}] (2b).
9.22, N 3.77. Found: C 81.20, H 8.98, N 3.52.
t
t
t
Synthesis of Na[(O-2-{(2- BuC H )NCH}-3,5- Bu C H )] [Na{Ld}]
6
5
2
6
2
6
4
2 6 2
The same method as that for 1a was used but with {Lb}H (1.49 g,
.83 mmol) and NaH (0.108 g, 4.83 mmol). Yield (2b): 1.54 g, 96%.
(2d). The same method as that for 1a was used but with {Ld}H (0.574
4
g, 1.55 mmol) and NaH (0.037 g, 1.55 mmol). Yield (2d): 0.574 g,
1
1
9
6%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.74 (s, 1H, HC
H NMR (DMSO-d , 400 MHz, 295 K): δ 8.77 (s, 1H, HCN), 7.36
6
6
N), 7.63 (s, 1H, C H ), 7.25 (m, 1H, C H ), 7.15 (m, 1H, C H ), 7.03
(
s, 1H, C H ), 7.25 (m, 2H, C H ), 6.98 (m, 3H, C H ), 6.93 (s, 1H,
6
2
6
4
6
4
6
2
6
5
6
5
1
3
(s, 1H, C H ), 6.98 (m, 1H, C H ), 6.81 (m, 1H, C H ), 1.44 [s, 9H,
C H ), 1.31 [s, 9H, C(CH ) ], 1.18 [s, 9H, C(CH ) ]. C NMR
6 2 6 4 6 4
6
2
3
3
3 3
1
3
C(CH )], 1.38 [s, 9H, C(CH )], 1.24 [s, 9H, C(CH )]. C NMR
(
1
DMSO-d , 101 MHz, 295 K): δ 163.2 (C-O), 139.7 (CN), 129.2,
3
3
3
6
(
1
3
DMSO-d , 101 MHz, 295 K): δ 172.8 (C-O), 160.0 (CN), 154.6,
28.6, 126.6, 125.7, 123.5, 121.3, 121.0 (Ar-C), 35.3, 33.8 [C(CH ) ],
6
3
3
42.2, 139.8, 127.2, 126.2, 125.6, 123.1, 122.4, 121.3, 120.2 (Ar-C),
5.8, 35.4, 33.8 [C(CH ) ], 32.2, 30.7, 30.1 [C(CH ) ]. Anal. Calcd
3
2.1, 30.0 [C(CH ) ]. Anal. Calcd for C H NONa (331.45 g/mol):
3
3
21 26
C 76.10, H 7.91, N 4.23. Found: C 76.25, H 8.09, N 4.31.
3
3
3 3
t
for C H NONa (387.53 g/mol): C 77.48, H 8.84, N 3.61. Found: C
Synthesis of K[(O-2-{(C H )NCH}-3,5- Bu C H )] [K{Lb}] (3b).
25 34
6
5
2 6 2
7
6.77, H 8.99, N 3.64.
Synthesis of K[(O-2-{(2- BuC H )NCH}-3,5- Bu C H )] [K{Ld}]
The same method as that for 1a was used but with {Lb}H (1.51 g,
4
t
t
.89 mmol) and K[N{Si(CH ) } ] (1.03 g, 4.89 mmol). Yield (3b):
6
4
2 6 2
3
3 2
1
(3d). The same method as that for 1a was used but with {Ld}H
1.50 g, 4.10 mmol) and K[N{Si(CH ) } ] (0.862 g, 4.10 mmol).
1.60 g, 94%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.81 (s, 1H,
6
(
3
3 2
HCN), 7.39 (s, 1H, C H ), 7.27 (m, 2H, C H ), 6.99 (m, 3H,
6
2
6
5
1
Yield (3d): 1.55 g, 94%. H NMR (DMSO-d , 400 MHz, 295 K): δ
6
C H ), 6.94 (s, 1H, C H ), 1.33 [s, 9H, C(CH ) ], 1.20 [s, 9H,
6
5
6
2
3 3
13
8.64 (s, 1H, HCN), 7.55 (s, 1H, C
6 2 6 4
H ), 7.21 (m, 1H, C H ), 7.13
C(CH ) ]. C NMR (DMSO-d , 101 MHz, 295 K): δ 173.9 (C-O),
3
3
6
(
m, 1H, C H ), 6.92 (m, 2H, C H /C H ), 6.70 (m, 1H, C H ), 1.41
6
4
6
4
6
2
6
4
1
1
62.7 (CN), 156.1, 140.4, 129.2, 126.5, 126.5, 122.9, 121.3, 121.2,
1
3
[
s, 9H, C(CH )], 1.33 [s, 9H, C(CH )], 1.19 [s, 9H, C(CH )]. C
3 3 3
20.4 (Ar-C), 35.4, 33.9 [C(CH ) ], 32.3, 30.0 [C(CH ) ]. Anal.
3
3
3 3
NMR (DMSO-d , 101 MHz, 295 K): δ 173.5 (C-O), 160.1 (CN),
6
Calcd for C H NOK (347.54 g/mol): C 72.57, H 7.54, N 4.03.
Found: C 72.81, H 7.69, N 4.41.
21
26
1
55.2, 142.1, 140.2, 127.2, 126.4, 126.0, 125.5, 122.7, 122.1, 121.1,
t
119.9 (Ar-C), 35.8, 35.4, 33.7 [C(CH
) ], 32.2, 30.7, 30.0 [C(CH ) ].
3 3 3 3
Synthesis of Li[(O-2-{(2- BuC H )NCH}C H )] [Li{Lc}] (1c). The
6
4
6 4
Anal. Calcd for C H NOK (403.65 g/mol): C 74.39, H 8.49, N 3.47.
25
34
same procedure as that described for 1a was followed but with {Lc}H
Found: C 74.81, H 8.20, N 3.44.
(
1.46 g, 5.78 mmol) and Li[N{Si(CH ) } ] (1.0 g, 5.78 mmol). Yield
i
3
3 2
Synthesis of Li[(O-2-{(2,6- Pr C H )NCH}C H )] [Li{Le}] (1e). The
1
2
6
3
6 4
(1c): 1.30 g, 86%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.75 (s,
6
same procedure as that described for 1a was followed but with {Le}H
1
1
1
9
H, HCN), 7.65 (m, 1H, C H O), 7.24 (m, 1H, C H N), 7.15 (m,
6
4
6
4
(
1.50 g, 5.33 mmol) and Li[N{Si(CH ) } ] (0.919 g, 5.33 mmol).
3 3 2
H, C H O), 6.99 (m, 1H, C H N), 6.89 (m, 1H, C H O), 6.76 (m,
1
6 4 6 4 6 4
Yield (1e): 1.41 g, 92%. H NMR (DMSO-d , 400 MHz, 295 K): δ
6
H, C H O), 6.33 (m, 1H, C H N), 6.05 (m, 1H, C H N), 1.39 [s,
6
4
6
4
6
4
8
.03 (s, 1H, HCN), 7.24 (m, 1H, C H ), 7.08 (m, 2H, C H ), 7.03
13
6
4
6
3
H, C(CH )]. C NMR (DMSO-d , 101 MHz, 295 K): δ 173.9 (C-
3
6
(m, 1H, C H ), 6.97 (m, 1H, C H ), 6.35 (m, 1H, C H ), 6.09 (m,
6 4 6 4 6 3
O), 159.2 (CN), 154.3, 142.2, 132.7, 127.5, 127.3, 125.7, 124.5,
13
1
H, C H ), 3.03 [m, 2H, HC(CH ) ], 1.08 [d, 12H, HC(CH ) ]. C
6 4 3 2 3 2
1
23.7, 123.6, 120.5, 109.0 (Ar-C), 35.8 [C(CH ) ], 30.8 [C(CH ) ].
3
3
3 3
NMR (DMSO-d , 101 MHz, 295 K): δ 172.7 (C-O), 166.0 (CN),
6
Anal. Calcd for C H NOLi (259.29 g/mol): C 78.75, H 7.00, N 5.40.
Found: C 78.42, H 6.99, N 5.42.
17
18
1
2
51.4, 139.1, 133.2, 133.0, 123.6, 123.3, 122.9, 122.4, 109.0 (Ar-C),
7.4 [HC(CH ) ], 24.2 [HC(CH ) ]. Anal. Calcd for C H NOLi
t
3
3
3
3
19 22
Synthesis of Na[(O-2-{(2- BuC H )NCH}C H )] [Na{Lc}] (2c).
6
4
6 4
(
4
287.35 g/mol): C 79.42, H 7.72, N 4.87. Found: C 78.92, H 7.73, N
.88.
Synthesis of Na[(O-2-{(2,6- Pr C H )NCH}C H )] [Na{Le}] (2e).
The same procedure as that described for 1a was followed but with
{
Lc}H (1.38 g, 5.45 mmol) and NaH (0.131 g, 5.45 mmol). Yield
i
1
2
6
3
6 4
(2c): 1.49 g, 91%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.64 (s,
6
The same procedure as that described for 1a was followed but with
Le}H (1.35 g, 4.79 mmol) and NaH (0.115 g, 4.79 mmol). Yield
1
1
1
9
H, HCN), 7.58 (m, 1H, C H O), 7.23 (m, 1H, C H N), 7.14 (m,
6
4
6
4
{
(
H, C H O), 6.97 (m, 1H, C H N), 6.82 (m, 1H, C H O), 6.70 (m,
1
6 4 6 4 6 4
2e): 1.28 g, 88%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.44 (s,
6
H, C H O), 6.15 (m, 1H, C H N), 5.90 (m, 1H, C H N), 1.40 [s,
6
4
6
4
6
4
1
H, HCN), 7.59 (m, 1H, C H ), 7.03 (m, 2H, C H ), 6.93 (m, 1H,
6 4 6 4 6 4 6 3
13
6
4
6
3
H, C(CH )]. C NMR (DMSO-d , 101 MHz, 295 K): δ 175.0 (C-
3
6
C H ), 6.85 (m, 1H, C H ), 6.19 (m, 1H, C H ), 5.96 (m, 1H, C H ),
O), 158.8 (CN), 154.6, 142.2, 132.8, 127.5, 127.3, 125.6, 124.3,
13
2
.95 [m, 2H, HC(CH ) ], 1.09 [d, 12H, HC(CH ) ]. C NMR
3 2 3 2
1
23.8, 123.6, 120.3, 108.0 (Ar-C), 35.8 [C(CH ) ], 30.8 [C(CH ) ].
3
3
3
3
(DMSO-d , 101 MHz, 295 K): δ 173.3 (C-O), 162.4 (CN), 152.2,
6
Anal. Calcd for C H NONa (275.32 g/mol): C 74.16, H 6.59, N
17
18
138.1, 132.7, 127.5, 123.3, 123.0, 122.8, 108.7 (Ar-C), 27.6
5.09. Found: C 73.87, H 6.64, N 5.08.
[
HC(CH ) ], 23.8 [HC(CH ) ]. Anal. Calcd for C H NONa
3 3 3 3 19 22
t
Synthesis of K[(O-2-{(2- BuC H )NCH}C H )] [K{Lc}] (3c). The
6
4
6
4
(303.37 g/mol): C 75.23, H 7.31, N 4.62. Found: C 74.81, H 7.41,
same procedure as that described for 1a was used but with {Lc}H
N 4.46.
(
1.48 g, 5.83 mmol) and K[N{Si(CH ) } ] (1.22 g, 5.83 mmol). Yield
i
3
3
2
Synthesis of K[(O-2-{(2,6- Pr C H )NCH}C H )] [K{Le}] (3e). The
2
6
3
6 4
1
(3c): 1.55 g, 91%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.60 (s,
6
same procedure as that described for 1a was followed but with {Le}H
1
6
6
H, HCN), 7.55 (m, 1H, C H O), 7.21 (m, 2H, C H N+C H O),
.95 (m, 1H, C H N), 6.78 (m, 1H, C H O), 6.67 (m, 1H, C H O),
6 4 6 4 6 4
6
4
6
4
6
4
(1.50 g, 5.33 mmol) and K[N{Si(CH ) } ] (1.12 g, 5.33 mmol). Yield
3
3 2
1
(3e): 1.47 g, 86%. H NMR (DMSO-d , 400 MHz, 295 K): δ (s, 1H,
6
.09 (m, 1H, C H N), 5.84 (m, 1H, C H N), 1.39 [s, 9H, C(CH )].
6
4
6
4
3
HCN), 7.60 (m, 1H, C H ), 7.04 (m, 2H, C H ), 6.93 (m, 1H,
6
4
6
3
1
3
C NMR (DMSO-d , 101 MHz, 295 K): δ 172.5 (C-O), 159.0 (C
6
C H ), 6.84 (m, 1H, C H ), 6.15 (m, 1H, C H ), 5.90 (m, 1H, C H ),
6 4 6 4 6 4 6 3
1
3
N), 154.8, 142.1, 132.8, 129.4, 127.5, 127.3, 125.6, 123.9, 123.4, 120.2,
2.97 [m, 2H, HC(CH ) ], 1.09 [d, 12H, HC(CH ) ]. C NMR
3 2 3 2
107.1 (Ar-C), 35.8 [C(CH ) ], 30.7 [C(CH ) ]. Anal. Calcd for
(DMSO-d , 101 MHz, 295 K): δ 175.0 (C-O), 162.3 (CN), 152.6,
3
3
3
3
6
C H NOK (291.43 g/mol): C 70.06, H 6.23, N 4.81. Found: C
138.1, 132.7, 127.1, 123.7, 123.4, 122.7, 122.6, 107.0 (Ar-C), 27.6
[HC(CH ) ], 23.9 [HC(CH ) ]. Anal. Calcd for C H NOK (319.51
17
18
70.15, H 6.24, N 4.44.
3
2
3
2
19 22
t
t
Synthesis of Li[(O-2-{(2- BuC H )NCH}-3,5- Bu C H )] [Li{Ld}]
g/mol): C 71.43, H 6.94, N 4.38. Found: C 71.05, H 6.90, N 4.04.
6
4
2 6 2
i
t
(1d). The same method as that for 1a was used but with {Ld}H (1.47
Synthesis of Li[(O-2-{(2,6- Pr C H )NCH}-3,5- Bu C H )] [Li{Lf}]
2
6
3
2 6 2
g, 4.04 mmol) and Li[N{Si(CH ) } ] (0.696 g, 4.04 mmol). Yield
(1f). The same method as that for 1a was used but with {L6}H (1.00
3
3 2
1
(1d): 1.31 g, 87%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.74 (s,
g, 2.54 mmol) and Li[N{Si(CH ) } ] (0.438 g, 2.54 mmol). Yield
6
3 3 2
1
1
H, HCN), 7.59 (s, 1H, C H ), 7.24 (m, 1H, C H ), 7.14 (m, 1H,
(1f): 0.88 g, 87%. H NMR (DMSO-d , 400 MHz, 295 K): δ 7.81 (s,
6
2
6
4
6
C H ), 7.04 (s, 1H, C H ), 6.97 (m, 1H, C H ), 6.85 (m, 1H, C H ),
1
C(CH )]. C NMR (DMSO-d , 101 MHz, 295 K): δ 171.6 (C-O),
1H, HCN), 7.09−6.98 (m, 4H, C H -C H ), 6.91 (s, 1H, C H ),
6
4
6
2
6
4
6
4
6
2
6
3
6
2
.42 [s, 9H, C(CH )], 1.38 [s, 9H, C(CH )], 1.22 [s, 9H,
3.07 [m, 2H, HC(CH ) ], 1.43, 1.20 [s, 9H, C(CH )], 1.09 [d, 12H,
3
3
3 2 3
HC(CH ) ]. C NMR (DMSO-d , 101 MHz, 295 K): δ 170.4 (C-O),
3 2 6
13
13
3
6
1
1
60.9 (CN), 154.6, 142.1, 139.8, 128.3, 127.1, 126.2, 125.6, 123.2,
168.3 (CN), 151.6, 139.7, 139.6, 128.9, 127.0, 123.7, 123.0, 120.4
22.8, 121.7, 120.7 (Ar-C), 35.8, 35.4, 33.9 [C(CH ) ], 32.2, 30.8, 30.2
(Ar-C), 35.5, 33.7 [C(CH ) ], 32.1, 30.2 [C(CH ) ], 25.8
3
3
3
3
3 3
I
DOI: 10.1021/om501000b
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
[
HC(CH ) ], 23.7 [HC(CH ) ]. Anal. Calcd for C H NOLi (399.55
charge on application to the CCDC, 12 Union Road, Cambridge CB2
1EZ, UK (fax: (+44)1223-336-033; e-mail: deposit@ccdc.cam.ac.uk).
3
2
3
2
27 38
g/mol): C 81.17, H 9.59, N 3.51. Found: C 80.85, H 9.52, N 3.47.
i
t
Synthesis of Na[(O-2-{(2,6- Pr C H )NCH}-3,5- Bu C H )] [Na-
2
6
3
2 6 2
{Lf}] (2f). The same method as that for 1a was used but with {Lf}H
ASSOCIATED CONTENT
Supporting Information
■
(
8
1.00 g, 2.54 mmol) and NaH (0.061 g, 2.54 mmol). Yield (2f): 0.90 g,
1
*
S
5%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.46 (s, 1H, HC
6
Experimental details, X-ray crystallographic data (CIF), and
additional characterization data. This material is available free of
charge via the Internet at http://pubs.acs.org.
N), 7.49 (s, 1H, C H ), 6.97 (m, 4H, C H -C H ), 2.98 [m, 2H,
HC(CH ) ], 1.34 [s, 9H, C(CH )], 1.21 [s, 9H, C(CH )], 1.10 [d,
1
(
6
2
6
2
6
3
3
2
3
3
13
2H, HC(CH ) ]. C NMR (DMSO-d , 101 MHz, 295 K): δ 172.7
3 2 6
C-O), 163.5 (CN), 152.9, 139.9, 138.2, 126.5, 126.1, 122.6, 122.2,
1
21.4, 120.5 (Ar-C), 35.3, 33.8 [C(CH ) ], 32.3, 30.1 [C(CH ) ], 27.5
3
3
3
3
AUTHOR INFORMATION
Corresponding Authors
E-mail: vanessa.tabernero@uah.es.
*E-mail: jesus.cano@uah.es.
■
[
HC(CH ) ], 23.9, [HC(CH ) ]. Anal. Calcd for C H NONa
3 2 3 2 27 38
415.60 g/mol): C 78.03, H 9.22, N 3.37. Found: C 77.76, H 9.36,
(
*
N 3.22.
Synthesis of K[(O-2-{(2,6-Pr C H )NCH}-3,5- Bu C H )] [K{Lf}]
i
t
2
6
3
2 6 2
(3f). Using the same method as that for 1a but using {Lf}H (1.00 g,
Notes
2
1
.54 mmol) and Li[N{Si(CH ) } ] (0.534 g, 2.54 mmol). Yield (3f):
3 3 2
The authors declare no competing financial interest.
1
.01 g, 92%. H NMR (DMSO-d , 400 MHz, 295 K): δ 8.47 (s, 1H,
6
HCN), 7.51 (s, 1H, C H ), 7.02 (m, 2H, C H ), 6.96 (s, 1H, C H ),
6
2
6
3
6
2
ACKNOWLEDGMENTS
6
1
.89 (m, 1H, C H ), 3.01 [m, 2H, HC(CH ) ], 1.33 [s, 9H, C(CH )],
■
6
3
3
2
3
13
.21 [s, 9H, C(CH )], 1.10 [d, 12H, HC(CH ) ]. C NMR (DMSO-
The authors acknowledge the UAH (I3 and CCG08-UAH/
PPQ-4203) for financial support, and F.G-V. acknowledges the
UAH for a fellowship.
3
3 2
d , 101 MHz, 295 K): δ 173.2 (C-O), 163.4 (CN), 153.1, 140.0,
6
138.2, 126.0, 126.0, 122.6, 122.1, 121.3, 120.2 (Ar-C), 35.3, 33.8
[
C(CH ) ], 32.3, 30.1 [C(CH ) ], 27.5 [HC(CH ) ], 23.9 [HC-
3 3 3 3 3 2
(
8
CH ) ]. Anal. Calcd for C H NOK (431.732 g/mol): C 75.12, H
.87, N 3.24. Found: C 75.54, H 8.98, N 3.24.
Typical Procedure for the Polymerization of rac-Lactide. In
3
2
27 38
REFERENCES
■
(
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(
the glovebox, a Schlenk flask was charged with a solution of the
complex (0.05 mmol with respect to the monometallic unit) in
CH Cl (3 mL), and when required, benzyl alcohol was added in the
desired stoichiometric amount. In another Schlenk flask, a rac-lactide
solution was prepared (721 mg, 5 mmol) in CH Cl (2 mL). The
Schlenk flasks were removed from the glovebox and manipulated with
Schlenk-line techniques. The Schlenk flask was immersed in a bath at
the desired temperature. The reaction time was measured when the
complex mixture was added to the rac-lactide solution. Polymer-
izations with Li complexes were carried out in the glovebox. Small
amounts removed with a syringe were tested to determine the
conversion by 1H NMR and H NMR homonuclear decoupled
2
2
(
2
2
2
2
(3) (a) Kricheldorf, H. R.; Berl, M.; Scharnagl, N. Macromolecules
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1
experiments in CDCl . Finally the product was isolated and purified by
3
precipitation from heptane by the addition of acidified methanol. The
polymer was filtered and dried under vacuum to constant weight.
Structure Determination of Compounds [1b(THF)] , [1e(THF)] ,
2
2
[
1f(THF)] , [2d(THF)] , [3d(THF) ] , and [3e(THF)] . Details of the X-
2 2 3 2 4
ray experiment, data reduction, and final structure refinement
calculations are summarized in the Supporting Information. Suitable
single crystals of [1b(THF)] , [1e(THF)] , [1f(THF)] , [2d-
2
2
2
(
THF)] , [3d(THF) ] , and [3e(THF)] for the X-ray diffraction
2 3 2 4
study were selected. Data collection was performed at 200(2) K, with
the crystals covered with perfluorinated ether oil. The crystals were
mounted on a Bruker-Nonius Kappa CCD single-crystal diffractometer
equipped with graphite-monochromated Mo Kα radiation (λ =
0
.710 73 Å). All non-hydrogen atoms were anisotropically refined,
except in [3e(THF)] . For [3e(THF)] the quality of the data was
4
4
poor, and one coordinated THF molecule was very disordered and was
refined in two positions with fixed coordinates and left isotropic
without hydrogens. Hydrogen atoms were geometrically placed and
left riding on their parent atoms, except for the hydrogens from the
imine group that were found in the difference Fourier map and refined
in compounds [1e(THF)] , [1f(THF)] and [3d(THF) ] . For
2
2
3 2
[
1b(THF)] , [1f(THF)] , and [3e(THF)] disordered solvent
2 2 4
molecules (toluene for [1b(THF)]2 and [1f(THF)] ; THF for
2
[
3e(THF)] ) are present in the unit cell; these solvent molecules were
4
found in the difference Fourier map.
Crystallographic data (excluding structure factors) for the structures
reported in this paper have been deposited with the Cambridge
Crystallographic Data Centre as supplementary publication nos.
CCDC-1017337 [1b], CCDC-1017338 [1e], CCDC-017339 [1f],
CCDC-1017340 [2d], CCDC-1017340 [2d], CCDC-1017341 [3d],
and CCDC-1017342 [3e]. Copies of the data can be obtained free of
J
DOI: 10.1021/om501000b
Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/om501000b
Organometallics XXXX, XXX, XXX−XXX