Role of Aggregation in the Synthesis and Polymerization Activity of
SalBinap Indium Alkoxide Complexes
Dinesh C. Aluthge, Ellen X. Yan, Jun Myun Ahn, and Parisa Mehrkhodavandi*
Department of Chemistry, University of British Columbia, 2036 Main Mall, Vancouver, British Columbia, Canada
* S Supporting Information
ABSTRACT: The reaction of racemic SalBinap ligand, (±)-H
2
(ONN*O
Me
), with InCl
3
and excess NaOEt generated a mixture
of two dinuclear compounds [(μ-κ
2
-ONN*O
Me
)In(μ-OEt)]
2
(1a) and [κ
4
-ONN*O
Me
)In(μ-OEt)]
2
(1b), which were isolated
and fully characterized. Polymerization of racemic lactide with 1a and 1b was slow in refluxing THF and showed only modest
stereoselectivity. Catalyst 1b displayed better control than 1a, with the experimental molecular weights of the resulting poly(lactic
acid) in agreement with the expected values. The higher-than-expected molecular weights observed in polymers formed by 1a
were due to partial initiation of the catalyst. The reaction of (±)-H
2
(ONN*O
tBu
) with InCl
3
yielded (κ
4
-ONN*O
tBu
)InCl (2);
however, further reactivity of the compound formed a mixture of products. An attempt to prevent aggregation by reacting
(±)-H
2
(ONN*O
Me
) with InCl
3
and excess NaO
i
Pr yielded an intractable mixture, including [(μ-κ
2
-ONN*O
Me
)In]
2
(μ-Cl)(μ-
OH) (3). The thermal stabilities of compounds 1a and 1b under polymerization conditions were investigated. Examination of
the polymerization behavior of complexes 1a and 1b and the reaction equilibrium between the two illustrates the importance of
aggregation in indium salen complexes compared to their aluminum counterparts.
■
INTRODUCTION
The development of chiral metal-based catalysts for the
stereoselective polymerization of racemic lactide (rac-LA) to
generate isotactic and stereoblock poly(lactic acid) (PLA) has
attracted considerable interest over the past 20 years.
1
Spassky
et al. introduced an aluminum methoxide complex supported
by a chiral SalBinap ligand as an effective catalyst for the
stereoselective polymerization of racemic lactide (Chart 1A).
2
Development of bulkier initiators and mechanistic studies of
the system established the highly isoselective nature of this class
of compounds and provided evidence for generation of
stereoblock PLA.
3
This work was extended to chiral aluminum
complexes bearing the Jacobsen ligand (Chart 1B)
4
as well as a
number of analogous achiral aluminum complexes capable of
some degree of selective polymerization of rac-LA.
5
Thus,
aluminum alkoxide initiators supported by tetradentate Schiff-
base (salen) ligands remain the standard for site-selective and
highly isoselective catalysts capable of forming stereoblock
PLA.
1
The chiral salen motif has been used successfully by a
number of groups to generate selective catalysts with aluminum
and other trivalent metals as well as with functionalized salen
derivatives.
6
One characteristic of some salen aluminum
alkoxide initiators is their tendency to aggregate and form
multiple complexes.
7
In particular, unsubstituted SalBinap
ligands can bridge two metal centers in a κ
2
binding mode
(Chart 1C).
8
We are interested in the ring-opening polymerization (ROP)
of cyclic esters by indium amino phenolate complexes.
9
In
particular, we have been exploring the importance of
aggregation in indium catalysts and the involvement of two
metal centers in indium-catalyzed polymerization reactions.
10
Other groups have also explored indium catalyzed polymer-
ization reactions
11
and investigated the differences in reactivity
between aluminum salen complexes and the indium analogues
with the larger ionic radius.
6g,12
The chemistry of indium salen
alkoxide complexes has been explored by Atwood et al., and
different modes of aggregation in these compounds, including
dimer formation (Chart 1E), have been reported.
13
Recently, we reported that an indium-ethoxide initiator
supported by the Jacobsen salen ligand is a highly active and
isoselective catalyst for lactide polymerization (Chart 1D).
14
Herein we expand our studies to SalBinap-indium alkoxide
catalysts and report on their reactivity for the polymerization of
Received: March 21, 2014
Article
pubs.acs.org/IC
© XXXX American Chemical Society A dx.doi.org/10.1021/ic500647j | Inorg. Chem. XXXX, XXX, XXX-XXX