Redox control within single-site polymerization catalysts

Article abstract of DOI:10.1021/ja061398n

The activity of a single-site titanium-based lactide polymerization initiator supported by a ferrocenyl-derivatized salen ligand is shown to be modulated by a chemical redox switch; a substantially higher rate of propagation is found for the neutral catalyst compared to its oxidized dicationic ferrocenium counterpart. Copyright

Full text of DOI:10.1021/ja061398n

Published on Web 05/19/2006
Redox Control within Single-Site Polymerization Catalysts
Charlotte K. A. Gregson, Vernon C. Gibson,* Nicholas J. Long,* Edward L. Marshall,
Phillip J. Oxford, and Andrew J. P. White
Department of Chemistry, Imperial College London, London, SW7 2AZ, U.K.
Received February 28, 2006; E-mail: v.gibson@imperial.ac.uk; n.long@imperial.ac.uk
Incorporation of a redox-active functionality within a ligand
The ethynylferrocene-substituted salicylaldimine 1 was synthesized
via the PdCl2(PPh3)2/CuI-catalyzed coupling of 3-tert-butyl-5-
iodosalicylaldehyde12 with ethynylferrocene13 (Scheme 2). Red
crystals of 1 were grown from diethyl ether/pentane solution, and
the molecular structure14 is shown in Figure 1. The ferrocenyl-
substituted salen ligand was then prepared by treatment of 1 with
ethylenediamine, and its subsequent reaction with Ti(OiPr)4 cleanly
afforded the titanium complex 2 in 88% yield. Solution NMR data
are consistent with the planar trans structure commonly found in
Ti(salen) alkoxide complexes.15
framework potentially allows the reactivity and selectivity of
complexed metal centers to be modulated through either chemical
or electrochemical switching of the redox center(s).1 A pioneering
demonstration of this concept was described by Wrighton et al.,
who showed that the hydrogenation of cyclohexene catalyzed by
the diphosphino-cobaltocene stabilized rhodium complex, Ired, is
approximately 16-fold faster than with its dicationic cobaltocenium
analogue Iox (Scheme 1).2 This observation was rationalized on the
basis of the more electron-rich Rh center of Ired being better able
to promote the oxidative addition of H2.
Scheme 1
To date, the application of redox switchable catalysts in single-
site polymerization catalyst technology has not been demonstrated,
yet it offers a number of attractive and potentially technologically
useful advantages. For example, oscillating a catalyst through
oxidized and reduced forms should allow the activity of the active
site to be modulated, by changes to the electrophilicity of the active
centers, and thereby facilitate “selectivity switching” between
different monomers during the polymerization process. Rather
unusual micro-block copolymer materials might be anticipated from
combinations of two or more monomers, where the block lengths
will be dependent upon the frequency of oscillation of the catalyst
between its reduced and oxidized forms. Here, we describe “proof
of concept” findings which show that activity can indeed be varied
by redox switching of a ferrocenyl unit contained within the ligand
backbone of a lactide polymerization catalyst. A key component
of such a catalyst is a substitutionally inert redox-active ligand.
A ligand class which has enjoyed widespread usage in polym-
erization catalysis is the bis(iminophenoxide) (salen) family, and
we therefore rationalized that a ferrocenyl-functionalized derivative
would be a suitable candidate for this studysthe chemistry of
ferrocene and its derivatives is well-established3-9 with the
electronic (redox) properties often playing an important role.10,11
Figure 1. Molecular structure of 1.
To identify a suitable chemical oxidant and reductant for the
redox switch, we determined the standard half electrode potential
1/2
Ep of 2. This was found to be 0.08 V (vs Fc/Fc+); we therefore
selected AgOTf as a suitable one-electron oxidant (Ep1/2 ) 0.43 V
vs Fc/Fc+) since the silver metal byproduct is relatively inert and
readily removed. Reaction of 2 with 2 equiv of AgOTf (in order to
oxidize both ferrocene substituents) in CH2Cl2 resulted in a color
change from orange to brown with concomitant precipitation of
Ag0; subsequent filtration and removal of solvent afforded the
dicationic complex 3 (Scheme 3). Though broadened by the
presence of the paramagnetic ferrocenium substituents, the 1H NMR
resonances of the salen component of 3 are clearly assignable.14
Decamethylferrocene was chosen as a suitable reductant for 3 not
only for its standard potential16 (E1/2 ) -0.59 V vs Fc/Fc+) but
also due to its compatibility with the system: the products of
reduction will be 2 and 2 equiv of [Cp*2Fe]+[OTf]-. The reaction
of 3 with Cp*2Fe resulted in a change from a brown back to the
pale orange/brown color of 2, a transformation confirmed by the
reappearance of the resonances attributable to the ferrocenyl
Scheme 2
1
substituents in the H NMR spectrum.14
The ring-opening polymerization of rac-lactide (LA) was initially
investigated by dissolving 0.025 mmol of both 2 and 3 in 5 mL of
toluene in separate glass ampules. The initiators may also be
conveniently prepared from the in situ reaction of (ferrocenyl salen)-
H2 with Ti(OiPr)4 followed by stringent removal of the volatile
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10.1021/ja061398n CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
the re-formation of the reduced (neutral) ligand was accompanied
by a corresponding increase in activity (Figure 3). Significantly,
neither AgOTf nor Cp*2FeOTf afforded any polymerization activity
under comparable conditions. The return in activity to approximately
the same magnitude as that observed prior to oxidation (kapp ) 4.98
× 10-6 s-1, cf. kapp ) 4.73 × 10-6 s-1 before oxidation) confirmed
the redox switch.
i
components to ensure that no residual PrOH remained. After
addition of 100 equiv of LA, the ampules were sealed and placed
in a preheated oil bath at 70 °C. Samples were then removed at
timed intervals and immediately cooled and exposed to air in order
to terminate the propagation process.
Analysis of the conversion data (Figure 2) clearly shows that
the non-oxidized ligand supports a substantially more rapid po-
lymerization (kapp[2]/kapp[3] ≈ 30). This is consistent with our recent
study of a variety of other salen-titanium initiators, which showed
that electron-withdrawing substituents within the salen framework
afford a slower rate of propagation for lactide monomer17 (contrast-
ing salen Al-based lactide initiators for which activities increase
with electron-withdrawing substituents).18 The longer reaction times
required for 3 may lead to a small amount of catalyst decomposition
as suggested by the tailing off in the conversion versus time plot
(Figure 2) after ca. 250 h. Nonetheless, in accord with Wrighton’s
Rh-catalyzed hydrogenation system,2 the redox-active ligand back-
bone is clearly shown to relay an electronic effect to the titanium
center.
Figure 3. A plot of conversion versus time for the polymerization of rac-
LA during the in situ oxidation and reduction of complex 2/3 (toluene, 70
°C, [LA]0/[Ti] ) 100).
In conclusion, these results demonstrate that remote redox-active
ligand substituents may be exploited to control the activity of single-
site polymerization initiators. Moreover, the redox switch can be
used to oscillate a catalyst between sites of differing activities. The
potential application of such redox-switchable initiators to the polym-
erization of mixed monomer pools and thereby providing access
to unusual micro-block copolymer materials is now a realistic pos-
sibility, as is the potential for electrochemical modulation of elec-
trode-attached catalysts. These, along with investigations into other
catalyst/monomer combinations, will form the focus of future work.
Acknowledgment. The authors thank the Engineering and
Physical Sciences Research Council (UK) for funding (to C.K.A.G.).
Supporting Information Available: Full experimental details,
including crystallographic and polymerization data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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Figure 2. A plot of LA consumption versus time for 2 and 3 (toluene, 70
°C, [LA]0/[Ti] ) 100).
The molecular weight distributions of the PLA samples are <1.2,
suggesting a well-controlled propagation process, mostly free from
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