Design of highly active iron-based catalysts for atom transfer radical polymerization: Tridentate salicylaldiminato ligands affording near ideal Nernstian behavior

Article abstract of DOI:10.1021/ja035458q

Iron(II) complexes bearing monoanionic tridentate salicyladiminato ligands are shown to be highly efficient catalysts for atom transfer radical polymerization (ATRP). Polymerization rates for styrene are among the highest reported for iron-mediated ATRP in nonpolar media, correlating well with E_1/2 potentials and ΔE_p values for the complexes. The rigidity of the tridentate ligands, combined with ample space around the metal center to accommodate a halogen atom, we believe to be an important factor in the efficient ATRP behavior of these systems. Copyright

Full text of DOI:10.1021/ja035458q

Published on Web 06/21/2003
Design of Highly Active Iron-Based Catalysts for Atom Transfer Radical
Polymerization: Tridentate Salicylaldiminato Ligands Affording near Ideal
Nernstian Behavior
Rachel K. O’Reilly, Vernon C. Gibson,* Andrew J. P. White, and David J. Williams
Department of Chemistry, Imperial College London, Exhibition Road, South Kensington, London, UK, SW7 2AZ
Received April 4, 2003; E-mail: v.gibson@imperial.ac.uk
Atom transfer radical polymerization (ATRP) has become
established as a powerful method for the synthesis of materials via
controlled radical polymerization.1 Following the key reports by
Matyjaszewsi2 and Sawamoto3 in the mid-1990s, of highly efficient
ATRP catalysts based on copper and ruthenium, respectively, a
number of different metal-ligand combinations have been inves-
tigated, including systems based on Pd,4 Rh,5 Ni,6 and Fe.7 There
remains, however, a need to develop new low toxicity, highly active
catalyst systems, especially for the synthesis of materials for in
vivo biomedical applications. The natural capacity of the body to
store and transport iron8 makes this metal particularly attractive
for use as an ATRP catalyst.
Figure 1. Plot of Mn vs conversion for 6 with Mw/Mn in parentheses. ([6]0:
[1-PEBr]0:[St]0 ) 1:1:100).
There are several important criteria for the successful design of
ATRP catalysts: (i) the metal must possess an accessible one-
electron redox couple, (ii) the oxidation potential should be low,
but optimal for reversible halogen atom transfer, (iii) there should
be good reversibility between the reduced and oxidized forms of
the catalystsfavored by ligands which minimize changes to the
metal coordination sphere between the reduced and oxidized states,
and (iv) the metal center must be sterically unencumbered in its
reduced form to allow a halogen atom to be accommodated. The
electrochemical parameters, E1/2 and ∆Ep, accessible via cyclic
voltammetry, provide a useful guide to evaluating (i-iii).
Figure 2. Plot of Mn vs [M]0/[I]0 for 4 with Mw/Mn in parentheses. ([4]0:
[1-PEBr]0:[St]0 ) 1:1:50, 100, 200, 300, 400).
Scheme 1. Synthesis of Complexes 1-6
We were attracted to readily accessible and derivatizable
salicyladiminato ligands which in recent years have been success-
fully exploited in a range of olefin polymerization catalyst systems.9
The starting points for our studies were the bidentate salicylaldi-
minato complexes 1-3 which were prepared by treatment of the
salicylaldimine with NaH, followed by FeCl3 (Scheme 1). These
were tested using reverse ATRP protocols10 but gave poor control
using styrene and methyl methacrylate, an observation we attribute
to the relative instability of the reduced three-coordinate Fe(II)
species. We therefore decided to investigate the tridentate deriva-
tives 4-6 where we envisaged the donor arm may afford the
reduced species additional stability as well as lower its oxidation
potential. 4-6 were isolated as dark red microcrystalline, para-
magnetic solids in good yields according to Scheme 1.
results clearly indicate that polymerizations using complexes 4-6
exhibit all the characteristics of a well-controlled and extremely
fast radical polymerization with rates comparable to those of copper-
based systems12 which are among the most active catalysts.
Encouraged by these observations, we investigated the poly-
merization of styrene in toluene solution at the lower reaction
temperature of 85 °C. Similar results were obtained as found for
the bulk polymerization except that the polymerizations were
expectedly slower (kobs(4) ) 0.06 h-1, kobs(5) ) 0.10 h-1, kobs(6)
) 0.15 h-1). However, no loss of control was observed, the
polymerizations in bulk and solvent affording polystyrene with
similar polydispersities and molecular weight control.
To obtain an understanding of the exceptional activities of these
complexes, their redox potentials (E1/2) and reversibility (∆Ep) were
probed by cyclic voltammetry. 4-6 were all found to possess
accessible redox couples with their polymerization rates showing
good correlation with the increasing reducing power of the iron
Polymerizations of styrene (100 equiv, bulk) initiated by 1-phen-
ylethyl bromide (1-PEBr) in the presence of 4-6 were monitored
at 120 °C under inert atmosphere. The semilogarithmic plot of
ln([M]o/[M]) vs time is linear in all cases with a pseudo-first-order
rate constant (kobs) of 0.39 h-1 for 4, 0.46 h-1 for 5, and 0.49 h-1
for 6, indicating that the radical concentration is constant throughout
the polymerizations. Molecular weights (Mn) increase linearly with
conversion (Figure 1) and with monomer-to-initiator ratio; a plot
of Mn vs [M]0/[I]0 using 4 is shown in Figure 2. Polydispersities
(Mw/Mn) are low, typically ca. 1.10, and decrease with monomer
1
conversion. H NMR spectra of the resultant polystyrene samples
show the presence of the halide capping group (ClCH(Ph)CH2-
≈ 4.4 ppm) which is also supported by halide microanalysis.11 These
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10.1021/ja035458q CCC: $25.00 © 2003 American Chemical Society

C O M M U N I C A T I O N S
the plane orthogonal to the ligand backbone, these factors combine
to optimize efficient halogen atom transfer. A similar correlation
of ATRP performance with ligand rigidity has recently been noted
in copper catalysts bearing tetradentate nitrogen donor ligands.14
The results of the present study suggest this may be a general effect
and an important design characteristic for ATRP catalysts.
Acknowledgment. BP chemicals Ltd is thanked for financial
support (R.K.O’R.).
Supporting Information Available: Crystallographic data (CIF)
and experimental preparations, polymerization procedures and CV data
(PDF). This material is available free of charge via the Internet at http://
pubs.acs.org.
Figure 3. The molecular structure of 4a.
References
centers [6 (E1/2 -480) > 5 (E1/2 -410) > 4 (E1/2 -280)]. Also, the
peak potentitals of all three complexes show relatively small
differences (∆Ep) (6, ∆Ep 65 < 5, ∆Ep 70 < 4, ∆Ep 75), with that
for 6, giving close to ideal Nernstian behavior (∆Ep ≈ 60 mV).
This is indicative of a small energy barrier between the reduced
and oxidized species, most likely a result of the conformational
rigidity of the attendant ligand.
With a view to obtaining a better understanding of the coordina-
tion environment around the iron centers, we attempted to grow
suitable crystals of the Fe(II) species for an X-ray structure
determination, but all attempts were unsuccessful. However, we
were able to determine the crystal structure of the (oxidized) Fe(III)
species 4a; a view is shown in Figure 3. The geometry at iron is
distorted trigonal bipyramidal with equatorial angles in the range
113.20(8)-128.8(2)°, the axial substituents, O(1) and N(10),
subtending an angle of 160.8(2)° at the metal center.
The iron atom is displaced 0.125 Å out of the equatorial plane
in the direction of O(1), a distortion similar to that observed in an
Al(Me)2 analogue where the deviation was 0.157 Å.13 The six-
membered chelate ring has a folded (asymmetric boat) conformation
with Fe and C(1) lying 0.381 and 0.040 Å out of (and on the same
side of) the {C(1),O(1),C(7),N(7)} plane, which is coplanar to
within 0.007 Å. The five-membered chelate ring has an envelope
conformation with N(10) lying 0.809 Å out of the plane of the
remaining atoms (which are coplanar to within 0.057 Å). A notable
feature of the bonding to the iron center is that the bond to the
amino nitrogen N(10) [2.250(4) Å] is substantially longer than that
to the imino nitrogen [2.071(4) Å]. This difference in bond length
was even more pronounced in the Al(Me)2 analogue and was
attributed to the weaker nature of the interaction with the axial
amino nitrogen atom. A stronger interaction was found in pyridyl-
methyl and quinolyl aluminum derivatives which most probably
accounts for their lower redox potentials here.
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An important factor in the exceptional ATRP behavior of these
complexes is likely to be the rigidity of the tridentate salicylaldi-
minato ligand, which is expected to increase from the aminoethyl
donor arm of 4 to the less flexible pyridinemethyl arm of 5, to the
highly constrained quinolyl arm of 6, and correlates well with the
trend toward Nernstian behavior for the more rigid ligand. When
combined with ample space to accommodate a halogen atom in
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