Dinuclear vs. mononuclear complexes: Accelerated, metal-dependent ring-opening polymerization of lactide

Article abstract of DOI:10.1039/c3cc47572g

Dinuclear complexes of aluminum and indium with a bis(phenoxy-imine) platform have been synthesized and used in the polymerization of lactide. Kinetic studies demonstrate that the dialuminum precursor provides a more favorable reaction pathway in terms of

Full text of DOI:10.1039/c3cc47572g

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COMMUNICATION
Dinuclear vs. mononuclear complexes: accelerated,
metal-dependent ring-opening polymerization of
lactide†
Cite this: Chem. Commun., 2013,
4
9, 11692
Received 2nd October 2013,
Accepted 20th October 2013
a
b
a
a
M. Normand, T. Roisnel, J.-F. Carpentier* and E. Kirillov*
DOI: 10.1039/c3cc47572g
www.rsc.org/chemcomm
9
Dinuclear complexes of aluminum and indium with a bis(phenoxy- of rac-LA and e-caprolactone with dialuminum systems. Although
6,8,9a,f
imine) platform have been synthesized and used in the polymerization cooperative effects have been claimed in some instances,
of lactide. Kinetic studies demonstrate that the dialuminum precursor detailed kinetic data, comparing performances of dinuclear vs.
provides a more favorable reaction pathway in terms of activation free pertinent mononuclear species, are still missing to our knowledge.
energy than that of directly related monoaluminum systems. No similar
trend is observed with the corresponding diindium–monoindium aluminum and indium complexes of Ph
systems, which is attributed to a dissimilar ROP mechanism. aldimine ligands readily promote the highly-controlled ROP of
rac-LA, yet with different kinetic regimes and, apparently, via
Multinuclear olefin polymerization catalysts have recently emerged as dissimilar ROP mechanisms: coordination/insertion for aluminum
In a previous study, we have demonstrated that mononuclear
3
Si-substituted salicyl-
1
10
a distinct class of molecular catalysts. In some cases, implicit species vs. the activated monomer mechanism for indium species.
2
cooperative behavior of two or more metal centers in active species Herein we report a dinuclear aluminum system that enables
of these systems can contribute to enhanced performance (with a 5-to-10 fold boost in activity in the ROP of rac-LA when
respect to mononuclear analogues) in terms of activity, chain-transfer compared to its monoaluminum counterparts. Interestingly,
kinetics and, also, selectivity of monomers in copolymerization no such differentiation of catalytic activity is noted between the
1
reactions. Besides, metal-catalyzed ring-opening polymerization corresponding diindium and monoindium systems.
(
ROP) of cyclic esters constitutes a highly topical research field,
As a suitable scaffold for dinuclear precursors, we selected the
0
11
especially when dealing with renewable resources such as lactide 2,2 -biphenolate motif which possesses a rigid backbone and a
3
12
(LA). ROP reactions can readily feature valuable characteristics such relatively low barrier of rotation around its aryl–aryl bond. The
as ‘‘living’’ behavior (kinitiation c kpropagation d ktermination, 100% targeted bifunctional Schiff base proligand II was obtained following
13
initiation efficiency) and even ‘‘immortal’’ behavior in the presence a literature protocol (see in ESI†). The corresponding dinuclear
3
of a co-initiator. Such effective cases, in which all side-reactions aluminum and indium tetraalkyl derivatives II–Al
2
and II–In
2
,
(
termination, formation of metal-containing dormant species, etc.) respectively, were synthesized by the same approach as that used
SiPh3
SiPh3
have been minimized, afford suitable conditions to observe possible for preparation of monometallic congeners I
–Al and I
–In,
cooperativity between active metal centers. However, only a limited namely, by direct protonolysis of trisalkyl precursors, via alkane
number of examples of dinuclear initiators based on single-frame elimination (Scheme 1). Both compounds were isolated in good
4
ligands have been explored for the ROP of cyclic esters so far: yields as yellow crystalline powders, highly soluble both in aromatic
5
6
polymerization of racemic LA (rac-LA) with dizinc, dilanthanide,
(benzene, toluene) and aliphatic (pentane, hexane) hydrocarbons as
7
dizirconium and dihafnium complexes, of b-butyrolactone with well as in polar coordinating (THF, Et O) solvents. These two
dichromium(III) complexes, and homo- and co-polymerization complexes were characterized by NMR spectroscopy in solution,
elemental analyses and X-ray diffraction studies.
2
8
1
13
The H and C NMR spectra of both II–Al
2 2
and II–In (Fig. S3–S6,
a
Institut des Sciences Chimiques de Rennes, Organometallics: Materials and
Catalysis Laboratories, UMR 6226 CNRS-Universit ´e de Rennes 1, F-35042 Rennes
Cedex, France. E-mail: evgueni.kirillov@univ-rennes1.fr,
see ESI†), recorded in C at room temperature or in toluene-d
6
D
6
8
in the temperature range of ꢀ70 to 90 1C, all showed single sets
of sharp resonances, consistent with single, time-averaged
species (no atropisomerism related to a hindered rotation
around the aryl–aryl bond).
The solid-state structures of II–Al (Fig. 1) and II–In (Fig. S11,
2 2
ESI†) feature monomeric molecules with four-coordinated metal
centers lying in a distorted tetrahedral geometry, as observed
jean-francois.carpentier@univ-rennes1.fr
b
Institut des Sciences Chimiques de Rennes, Centre de diffraction X, UMR 6226
CNRS-Universit ´e de Rennes 1, F-35042 Rennes Cedex, France
†
Electronic supplementary information (ESI) available: Experimental details,
spectroscopic and analytical data, kinetic data, and crystallographic data in CIF
for II–Al and II–In . CCDC 959436 and 959435. For ESI and crystallographic data
in CIF or other electronic format see DOI: 10.1039/c3cc47572g
2
2
1
1692 Chem. Commun., 2013, 49, 11692--11694
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0
1
0
using the rate law: ꢀd[rac-LA]/dt = kapp[LA] [Al] [iPrOH] . The
kinetic data for the ROP of rac-LA mediated by the II–Al –iPrOH
2
system indicate that the same pseudo-first-order rate law is also
operative (Fig. S12–S14, see ESI†). Yet, keeping the [rac-LA]0/
[
Al] /[iPrOH] ratio and [Al] concentration strictly constant, the
0 0 0
2
ROP LA reactions mediated by dinuclear II–Al proceed ca.
one order of magnitude faster than those with mononuclear
SiPh3
I
–Al, whichever the temperature in the range 90–120 1C
(
Table 1). Eyring analyses of the kinetic data for these two systems
a
provided the corresponding activation parameters: DH = 13.5(1)
ꢀ
1
a
ꢀ1 ꢀ1
a
kcal mol and DS = ꢀ32(5) cal mol
K
for II–Al vs. DH
=
–Al.
=
2
ꢀ
1
a
ꢀ1 ꢀ1
SiPh3
1
6.4(1) kcal mol and DS = ꢀ29(4) cal mol
K
for I
a
Hence, the free energy barrier calculated for II–Al (DG
2
2
SiPh3
10
Ph
2
298
Scheme 1 Synthesis of mononuclear (I
–M and I –Al) and dinuclear
ꢀ
1 16
SiPh3
a
3.1(1) kcal mol
)
is lower than that for I
–Al (DG298
=
(
2
II–M ) complexes.
ꢀ
1
ꢀ1
Ph
5.2(1) kcal mol ) by ca. 2 kcal mol . Complex I –Al
(Scheme 1), which represents a simplified model of the steric
environment of metal centers in the bimetallic version II–Al , was
2
also investigated in the ROP of rac-LA. Despite the more sterically
Ph
opened environment in I –Al (no ortho-substituent, no second
metal fragment), the k
value determined at 110 1C (2.5(1) ꢁ
app
ꢀ3
ꢀ1
10
s ) was again much smaller, ca. 5 fold, than that obtained
ꢀ
3
ꢀ1
for II–Al
2
(12.2(9) ꢁ 10
s ) under the same conditions. This
observation argues against the point that only steric effects can be
responsible for enhanced catalytic performances exhibited by
Fig. 1 Molecular structure of the dinuclear complex II–Al
except those of the methyl groups, are omitted for clarity; ellipsoids drawn at the
0% level).
2
(all hydrogen atoms, the dinuclear complex II–Al
. Besides these differentiated reactivities,
2
no discernible change in polymerization control (molecular weights,
distributions, and stereochemistry ) was observed for the reactions
5
17
catalyzed by bimetallic and monometallic systems, which all featured
the near perfect living character (Table 2; Fig. S18, see ESI†).
The catalyst system based on the dinuclear indium complex II–In2
featured a different kinetic regime than its dialuminum analogue, but
SiPh3
SiPh3
10
earlier for the monometallic counterparts I
–Al and I
–In.
Also, the M–O, M–N and M–C bond lengths in dinuclear complexes
II–M (M = Al, In) are in the same range as those found in I
SiPh3
10
–M.
2
The catalytic performance of the dinuclear II–M2 systems in line with that observed previously for the mononuclear indium
SiPh3
10
1
1
0
were compared with those of the mononuclear analogues system I
–In, that is ꢀd[rac-LA]/dt = k [LA] [In] [iPrOH] . Yet,
app
SiPh3
I
–M in the ROP of rac-LA, under strictly identical condi- in sharp contrast to the ROP reactions promoted by aluminum
complexes, no acceleration of polymerization rates was observed
In our initial studies, we have demonstrated that kinetics of when the dinuclear precatalyst II–In was utilized. For instance, in
–Al–iPrOH system two kinetic studies carried out at 110 1C with I
follow the Michaelis–Menten kinetic model and is described under identical conditions (Fig. S17, see ESI†), the corresponding rate
1
4
tions, upon addition of 2-propanol as a co-initiator (Table 1).
1
0
2
SiPh3
SiPh3
the ROP of rac-LA mediated by the I
–In and II–In2
1
5
ꢀ
4
ꢀ1 ꢀ1
constant values were almost identical: kapp = 7.0(5) ꢁ 10
M
s
ꢀ
4
ꢀ1 ꢀ1
Table 1 Apparent rate constants determined for the ROP of rac-lactide with and 8(1) ꢁ 10
M
s , respectively. Also, near identical kinetic
SiPh3
Ph
binary systems I
2
–Al, I –Al and II–Al –iPrOH. All reactions were performed
data, namely conversion vs. time, were obtained for these two
complexes at 80 1C (Table 2, entries 16 and 17 vs. 18 and 19).
In conclusion, we have evidenced that dinuclear aluminum
complexes are more prominent precursors for ROP of lactide than
their monometallic congeners. The significant 5- to10-fold enhance-
ment of polymerization activity observed may stem from beneficial
cooperative effects provided by the two close metal centers. In the
solid state structure of II–Al and II–In , the metal–metal distances are
2 2
8.0 and 8.5 Å, respectively. Yet, due to the relatively low barrier of
rotation around the aryl–aryl bond, these two metal centers can
approach (in principle) as close as ca. 2.8 and 3.0 Å, which could open
up e.g. dual activation mechanisms in which both metal centers
cooperate. Apparently, these cooperative effects are disabled for the
indium-based systems, although the steric changes are comparable to
those in the analogous aluminum systems. At this time, we suggest
that this change may be related to the different operative ROP
mechanisms (coordination/insertion for Al vs. activated monomers
0
with [rac-LA] = 2.0 M in toluene
b
ꢀ3 ꢀ1
Complex
Temp (1C)
[rac-LA]
0
–[Al]–[iPrOH]
0
k
app (10
s )
SiPh3
SiPh3
SiPh3
SiPh3
Ph
a
I
I
I
I
I
–Al
–Al
–Al
–Al
90
100
110
120
110
90
100
110
120
90
100
110
120
110
110
110
500 : 1 : 5
500 : 1 : 5
500 : 1 : 5
500 : 1 : 5
500 : 1 : 5
1000 : 2 : 10
1000 : 2 : 10
1000 : 2 : 10
1000 : 2 : 10
1000 : 4 : 10
1000 : 4 : 10
1000 : 4 : 10
1000 : 4 : 10
1000 : 4 : 50
1000 : 8 : 50
1000 : 20 : 50
0.34(2)
0.59(4)
1.1(1)
2.1(1)
2.5(1)
4.7(4)
8.3(6)
12.2(9)
22.2(9)
6.0(6)
–Al
II–Al
II–Al
II–Al
II–Al
II–Al
II–Al
II–Al
II–Al
II–Al
II–Al
II–Al
2
18
2
2
2
2
2
2
2
2
2
2
12
9.3(7)
18.1(1)
29.8(2)
30.2(2)
59.6(1)
113(12)
a
b
Kinetic data obtained from ref. 10. Reactions were duplicated.
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SiPh3
Ph
SiPh3
Table 2 ROP of racemic lactide promoted by catalytic systems made of aluminum or indium complexes I
–Al, I –Al, I
–In, II–Al
2
2
and II–In combined with
a
iPrOH
c
d
e
M
n,theo
M
n,NMR
M
n,SEC
a
b
ꢀ1
3
ꢀ1
3
ꢀ1
3
e
Entry [Al]
[LA]–[M]–[iPrOH]
Temp (1C) Time (h) Conv. (%) (g mol ) (ꢁ 10 ) (g mol ) (ꢁ 10 ) (g mol ) (ꢁ 10 )
w n
M /M
SiPh3
SiPh3
SiPh3
SiPh3
SiPh3
SiPh3
SiPh3
SiPh3
SiPh3
Ph
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
I
I
I
I
I
I
I
I
I
I
–Al 500 : 1 : 5
90
100
110
100
120
120
110
110
110
110
90
100
110
120
120
80
70
35
21
18
2
17
2
2
2
8
2
84
70
85
61
17
82
61
60
68
74
33
68
92
78
96
29
63
31
64
12.1
10.1
12.2
8.8
2.4
11.8
8.8
8.5
2.0
10.7
4.8
9.8
13.2
11.2
13.5
4.2
9.0
4.5
9.8
9.8
13.5
nd
2.1
11.4
8.5
5.6
1.3
11.3
5.2
10.1
14.3
nd
10.0
8.9
11.8
5.7
3.0
15.7
6.7
5.8
nd
12.1
4.8
9.7
13.9
12.9
14.3
3.7
7.4
3.2
1.06
1.05
1.09
1.11
1.06
1.33
1.11
1.08
nd
1.06
1.06
1.07
1.29
1.08
1.19
1.10
1.06
1.08
1.05
–Al 500 : 1 : 5
–Al 500 : 1 : 5
–Al 1000 : 1 : 10
–Al 500 : 1 : 5
–Al 500 : 1 : 5
–Al 500 : 3 : 5
–Al 500 : 5 : 5
–Al 500 : 5 : 25
0
1
2
3
4
5
6
7
8
9
–Al
500 : 1 : 5
1000 : 2 : 10
II–Al
II–Al
II–Al
II–Al
II–Al
I
I
2
2
2
2
2
1000 : 2 : 10
1000 : 2 : 10
1000 : 2 : 10
1000 : 2 : 10
2.25
2
1
2
1
2
1
2
13.9
3.7
9.9
4.6
8.2
SiPh3
SiPh3
–In 500 : 1 : 5
–In 500 : 1 : 5
80
80
80
II–In
II–In
2
1000 : 2 : 10
1000 : 2 : 10
2
9.2
6.6
a
b
0
Polymerization conditions unless otherwise stated: reactions performed at [rac-LA] = 2.0 M in toluene. Monomer conversion determined by
d
1
c
H NMR spectroscopy (CDCl
3
, 298 K). Calculated molecular weight calculated using Mn,theo = conv. ꢁ [rac-LA]
0
/[ROH] ꢁ MLA
.
Experimental
e
molecular weight determined by NMR from the relative intensities of the main chain and terminal resonances. Experimental molecular weight
determined by SEC vs. polystyrene standards and corrected by a factor of 0.58.
for In – a change likely related to the impossibility of the In-alkyl to
transform into In-alkoxide species upon combination with an alcohol
(c) E. L. Whitelaw, G. Loraine, M. F. Mahon and M. D. Jones, Dalton
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Dalton Trans., 2013, 42, 3860.
10
under the polymerization conditions ) rather than to the intrinsic
nature of these systems. Further investigations are focused on the
identification of the nature of cooperative effects in the dialuminum
systems and elaboration of other polynuclear precatalysts with
improved performances.
10 M. Normand, V. Dorcet, E. Kirillov and J.-F. Carpentier, Organo-
metallics, 2012, 32, 1694.
1 For an example of a related o,o -dinaphthoxy linker between two cobalt
We thank the MESR and the Universit ´e de Rennes 1 for
funding of this research (PhD grant to M.N.). We also thank
0
1
2
sites used in CO -epoxide copolymerization, see: R. M. Thomas,
Dr Vincent Dorcet for the X-ray diffraction study of II–In
2
.
P. C. B. Widger, S. M. Ahmed, R. C. Jeske, W. Hirahata, E. B. Lobkovsky
and G. W. Coates, J. Am. Chem. Soc., 2010, 132, 16520.
a
Notes and references
12 The typical range of values for barriers of interconversion (DG340) in
ꢀ
1
multisubstituted biphenyls is 14–22 kcal mol , see: G. Bott,
1
(a) M. Delferro and T. J. Marks, Chem. Rev., 2011, 111, 2450;
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2
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(
(
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2
2
010, 39, 8363; (e) P. J. Dijkstra, H. Du and J. Feijen, Polym. Chem.,
011, 2, 520.
4
5
For the use of Cl- and RO-bridged dinuclear In compounds in ROP
of cyclic esters, see: (a) C. Xu, I. Yu and P. Mehrkhodavandi, Chem. 16 Similar kinetic experiments performed using higher concentration
Commun., 2012, 48, 6806, and references cited therein; (b) I. Yu,
A. Acosta-Ramirez and P. Mehrkhodavandi, J. Am. Chem. Soc., 2012,
of II–Al
2
(1000 : 4 : 10) afforded nearly the same activation para-
a a
ꢀ1
ꢀ1
ꢀ1
meters: DH = 14.8(1) kcal mol , DS = ꢀ29(5) cal mol
K
a
ꢀ1
1
34, 12758, and references cited therein.
and DG298 = 23.2(1) kcal mol
.
(a) C. K. Williams, N. R. Brooks, M. A. Hillmeyer and W. B. Tolman, 17 The P
r
values for PLA samples, obtained with both dinuclear and
Chem. Commun., 2002, 2132; (b) P. D. Knight, A. J. P. White and
C. K. Williams, Inorg. Chem., 2008, 47, 11711.
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Trans., 2013, 42, 2870.
mononuclear precursors, were all found in the range 0.5–0.6.
18 In dizirconium and dinickel systems which exhibited pronounced
cooperative effects in a-olefin copolymerization reactions (ref. 1),
such inter-metallic distances were reported in the range 6.0–8.6 Å
and 3.1–8.6 Å, respectively. In dizinc complexes (ref. 5), such
distances are 3.1–3.3 Å. Higher ROP activities were found
for dialuminium complexes with Al–Al distances in the range
5.8–6.6 Å, while closer Al–Al distance (3.2 Å) was found to be
detrimental for activity of a dinuclear compound (ref. 9a).
6
7
T. K. Saha, V. Ramkumar and D. Chakraborty, Inorg. Chem., 2011,
5
0, 2720.
8
9
S. I. Vagin and B. Rieger, Z. Naturforsch., 2012, 67b, 614.
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1
1694 Chem. Commun., 2013, 49, 11692--11694
This journal is c The Royal Society of Chemistry 2013