Four-coordinate iron complexes bearing α-diimine ligands: Efficient catalysts for Atom Transfer Radical Polymerisation (ATRP)

Article abstract of DOI:10.1039/b204510a

Four-coordinate iron(II) complexes bearing α-diimine ligands with alkyl substituents are shown to be efficient catalysts for the well-controlled atom transfer radical polymerisation of styrene; catalysts containing aryldiimine ligands support competitive

Full text of DOI:10.1039/b204510a

Four-coordinate iron complexes bearing a-diimine ligands: efficient
catalysts for Atom Transfer Radical Polymerisation (ATRP)
Vernon C. Gibson,*a Rachel K. O’Reilly,a Warren Reed,b Duncan F. Wass,b Andrew J. P. Whitea and
David J. Williamsa
a Department of Chemistry, Imperial College, Exhibition Road, South Kensington, London, UK SW7 2AY.
E-mail: v.gibson@ic.ac.uk; Fax: 020 7594 5810; Tel: 020 7594 5830
b BP Chemicals, Sunbury Research Centre, Chertsey Road, Sunbury on Thames, Middlesex, UK TW16 7LN.
E-mail: WassDF@bp.com; Fax: 01932 775467; Tel: 01932 775247
Received (in Cambridge, UK) 10th May 2002, Accepted 12th July 2002
First published as an Advance Article on the web 24th July 2002
Four-coordinate iron(II) complexes bearing a-diimine li-
gands with alkyl substituents are shown to be efficient
catalysts for the well-controlled atom transfer radical
polymerisation of styrene; catalysts containing aryldiimine
ligands support competitive b-hydrogen chain transfer
processes.
One of the most significant advances in controlled polymer-
isation over the past few years has been the development of
transition metal based catalysts for atom transfer radical
polymerisation (ATRP).1 Following the key observation by
Fig. 1 The molecular structure of 2. Selected bond lengths (Å) and angles
Wang and Matyjaszewski2 that copper(
) complexes bearing
I
(°); Fe–Cl 2.231(2), Fe–N(1) 2.125(4), C(1)–N(1) 1.239(6), C(1)–C(1A)
1.495(9); Cl–Fe–N(1) 111.13(11), Cl–Fe–N(1A) 114.57(10), Cl–Fe–ClA
120.08(10), N(1)–Fe–N(1A) 78.0(2).
bipyridine ligands afford highly efficient ATRP catalysts, a
number of different metal ligand combinations have been
investigated, including systems based on Ru,3 Rh,4 Pd,5 Ni6 and
Fe.7
with respect to that of FeN2. The C(2)–H bond is rotated with
respect to its C(2A)–H counterpart, about the molecular
C(2)…C(2A) axis, by 45°. The bonding within the essentially
planar (to within 0.005 Å) five-membered chelate ring exhibits
a very distinct pattern of bond ordering with the CNN linkages
being very short at 1.239(6) Å and the C–C bond long [1.495(9)
Å] showing there to be no noticeable bond delocalisation. The
bond lengths observed here, including those to the metal centre,
do not differ significantly from those in the related diiodo(N,NA-
We have been studying the use of unsaturated nitrogen
ligands for iron-based ethylene polymerisation catalysts,8 and
became interested in iron-mediated ATRP. Iron is particularly
attractive for this purpose due to its low cost and low toxicity.
Göbelt and Matyjaszewski have reported the use of five-
coordinate bis(imino)pyridine iron complexes for the polymer-
isation of methylmethacrylate (MMA), though styrene is not
polymerised by this system.7e We considered that the activity of
the system may be raised by moving to a 4)5 coordination
manifold. Here we report a family of four-coordinate iron
ATRP catalysts based on the readily accessible and derivati-
sable a-diimine ligand frame, and the observation that alkyli-
mino substituents favour well-controlled ATRP of styrene
while arylimino substituents give rise to competitive b-
hydrogen chain transfer processes.
diisopropyl-1,4-diaza-1,3-butadiene)iron(II)
complex.9 This
pattern contrasts with the substantially delocalised bonding
6
present in the formally iron(0) species (h -toluene)(N,NA-
bis(cyclohexyl)ethylenediimine)iron(0).10 There are no inter-
molecular packing interactions of note.
The homogenous ATRP of styrene (200 equiv., bulk)
initiated with 1-phenylethyl chloride (1-PECl) and the alkyl
substituted diimine complexes, 1 and 2, was monitored at
120 °C under inert atmosphere.‡ The semilogarithmic plot of
ln([M]o/[M]t) vs. time (Fig. 2) is linear in both cases with a
pseudo-first order rate constant (kobs) of 0.27 h21 for 1 and 0.25
h21 for 2 indicating that the radical concentration is constant
throughout the polymerisation run. The molecular weight (Mn)
increases linearly with time and agrees with calculated
molecular weights, thus demonstrating good control. Poly-
dispersities (Mw/Mn) are quite low (typically ca. 1.3) and
The series of FeCl2[N,N] complexes (where [N,N]
=
[RNNCH–CHNNR] and R = tert-butyl, cyclohexyl, mesityl or
2,6-diisopropylphenyl) was readily prepared according to
Scheme 1. Stirring a solution of FeCl2 and [N,N] in diethyl ether
at room temperature followed by extraction into dichloro-
methane gave 1–4 as microcrystalline, air stable solids in
excellent yields. Crystals of 2 suitable for an X-ray structure
determination were grown from hot benzene and the structure is
shown in Fig. 1.† The molecule has crystallographic C2
symmetry about an axis passing through the metal centre and
the middle of the C(1)–C(1A) bond. The geometry at iron is
distorted tetrahedral with inter-bond angles in the range
78.0(2)–120.1(1)°, the acute angle being associated with the
bite of the N,NA-chelate ligand. There is a small torsional twist
of ca. 3° from orthogonal about the C2 axis of the FeCl2 plane
1
decrease with monomer conversion (Fig. 3). H NMR spectra
t
Scheme 1 Synthesis of complexes 1–4 where R = Bu, 1; C6H11, 2; Mes,
3 and 2,6-dipp, 4.
Fig. 2 First order kinetic plot of ln([M]o/[M]t) versus time for complex 2
([2]0 = 5.0 3 1024 mol, [PECl]0 = 5.0 3 1024 mol, [St]0 = 0.1 mol).
1850
CHEM. COMMUN., 2002, 1850–1851
This journal is © The Royal Society of Chemistry 2002

on the resultant polystyrene samples show the presence of the
halide capping group (ClCH(Ph)CH2, d 4.5) which is also
supported by halide microanalysis.§ Accordingly, the above
results clearly indicate that polymerisations with complexes 1
and 2 exhibit characteristics of a well-controlled atom transfer
radical polymerisation.
Notes and references
† Crystal data for 2: C14H24N2Cl2Fe, M = 347.1, monoclinic, I2/a (no. 15),
a = 13.028(3), b = 6.768(2), c = 20.069(5) Å, b = 101.50(1)°, V =
1734.0(8) Å3, Z = 4 (C2 symmetry), Dc = 1.330 g cm23, m(Cu-Ka) = 9.71
mm21, T = 293 K, pink/magenta dichroic platy needles; 1186 independent
measured reflections, F2 refinement, R1 = 0.052, wR2 = 0.138, 981
independent observed absorption corrected reflections [¡Fo¡ > 4s(¡Fo¡), 2q
5 120°], 87 parameters. CCDC 186199. See http://www.rsc.org/suppdata/
cc/b2/b204510a/ for crystallographic data in CIF or other electronic
format.
‡ Polymerisations were performed under dinitrogen, in a 15 cm3 glass
ampoule fitted with a Teflon stopcock. The ampoule was equipped with a
magnetic stirrer bar and the following were placed in it in the order,
monomer, initiator and catalyst in a 200+1+1 ratio. The ampoules were
transferred to a preheated oil bath, at 120 °C. After magnetic stirring for the
allotted period of time an aliquot (0.1 ml) was removed and quenched by
addition of THF (1 ml). Conversion was determined by integration of
monomer vs. polymer backbone resonances in the 1H NMR spectrum of the
crude product (in CDCl3). The polymer was purified by precipitating from
a rapidly stirred acidified (5%) methanol solution. GPC chromatograms
were recorded on a Knauer differential refractometer connected to a
Gynotek HPLC pump (model 300) and two 10 m columns (PSS) at a flow
rate of 1.00 cm3 min21 using CHCl3 as the eluent. The columns were
calibrated against polystyrene standards with molecular weights ranging
from 1560 to 128 000. Analysis was performed using Version 3.0 of the
Conventional Calibration module of the Viscotek SEC3 software pack-
age.
Fig. 3 Plot of molecular weight and PDIs (in parentheses) versus time for
complex 2.
The polymerisation of styrene using the aryl substituted
diimine complexes 3 and 4 was monitored under similar
conditions (120 °C, 200 equiv., bulk, 1-PECl initiator). In these
cases the semilogarithmic plot of ln([M]o/[M]t) vs. time was
non-linear: Mn did not increase linearly over time and did not
agree with theoretical molecular weights. Monomer conversion,
however, did increase in a semi-linear manner with time thus
suggesting that a different polymerisation mechanism may be
operating. Halide microanalyses on the resultant polymers
showed no halide content and end group analyses (by NMR)
showed vinyl end groups (d 6.05–6.65). This is consistent with
a b-hydrogen chain transfer process, most likely proceeding via
an OSET mechanism11 due to the enhanced oxidising power of
the iron complexes bearing diimine ligands with aryl sub-
stituents.
In order to gain further insight into the differing catalyst
behaviour, the redox potentials and reversibility of this series of
iron complexes were analysed by cyclic voltammetry (CV).12
The alkyl complexes 1 and 2 were found to have an accessible
and reversible one-electron redox couple, DEp(complex) 120
and 130 mV, respectively, which compare with a value of 130
mV for ferrocene. However the aryl complexes 3 and 4 were
found to possess an irreversible redox couple [DEp(complex)
210 and 270, respectively, Table 1] which accounts for their
poor behaviour in atom transfer catalysis.
§
Microanalysis for polystyrene produced using 2, Mn = 2,400; %Cl,
found (calc.): 1.38 (1.47).
CV analyses were performed in MeCN, under dinitrogen, using a Pt
counter and working electrode and a Ag/AgCl reference electrode with
[nBu4N][PF6] as an electrolyte. Benchmarked redox couple with ferroce-
ne(II)/(III) couple at E1/2 = 2150 mV and DEp = 130 mV.
1 T. E. Patten and K. Matyjaszewski, Acc. Chem. Res., 1999, 32, 895.
2 J. S. Wang and K. Matyjaszewski, J. Am. Chem. Soc., 1995, 117,
5614.
3 M. Kato, M. Kamigaito, M. Sawamoto and T. Higashimura, Macromol-
ecules, 1995, 28, 1721.
4 P. Lêcomte, I. Drapier, P. Dubois, P. Teyssié and R. Jêromé,
Macromolecules, 1998, 31, 542.
5 P. Lêcomte, I. Drapier, P. Dubois, P. Teyssié and R. Jêromé,
Macromolecules, 1997, 30, 7631.
6 (a) C. Granel, P. Dubois, R. Jêromé and P. Teyssié, Macromolecules,
1996, 29, 8576; (b) H. Uegaki, Y. Kotani, M. Kamigaito and M.
Sawamoto, Macromolecules, 1997, 30, 2249.
7 (a) T. Ando, M. Kamigaito and M. Sawamoto, Macromolecules, 1997,
30, 4507; (b) K. Matyjaszewski, M. Wei, J. Xia and N. E. Mc Dermott,
Macromolecules, 1997, 30, 8161; (c) Y. Kotani, M. Kamigaito and M.
Sawamoto, Macromolecules, 1999, 32, 6877; (d) M. Teodrescu, S. G.
Gaynor and K. Matyjaszewski, Macromolecules, 2000, 33, 2335; (e) B.
Göbelt and K. Matyjaszewski, Macromol. Chem. Phys., 2000, 201,
1619; (f) J. Louis and R. H. Grubbs, Chem. Commun., 2000, 1479; (g)
H. Takahashi, T. Ando, M. Kamigaito and M. Sawamoto, Macromole-
cules, 1999, 32, 3820; (h) Y. Kotani, M. Kamigaito and M. Sawamoto,
Macromolecules, 2000, 33, 3543.
8 (a) G. J. P. Britovsek, V. C. Gibson, B. S. Kimberly, P. J. Maddox, S.
J. Mc Tavish, G. A. Solan, A. J. P. White and D. J. Williams, Chem.
Commun., 1998, 849; (b) G. J. P. Britovsek, M. Bruce, V. C. Gibson, B.
S. Kimberley, P. J. Maddox, S. Mastroianni, S. J. McTavish, C.
Redshaw, G. A. Solan, S. Stromberg, A. J. P. White and D. J. Williams,
J. Am. Chem. Soc., 1999, 121, 8728.
Table 1 Redox potentials (E1/2) and peak separation (DEp) for complexes
1–4
Complex
E1/2/mV
DEp/mV
1
2
3
4
240
2130
280
120
130
210
270
220
The complexes described here represent a small fraction of
readily accessible a-diimine iron catalysts. Fine tuning of the
complex sterics, electronics and solubility characteristics, as
well as judicious choice of a compatible radical initiator would
be expected to yield even more active and well-controlled
polymerisation systems. Further studies are examining these
features and their effect on the polymerisation of styrene and
other monomers.
9 J. Breuer, H.-W. Fruhauf, W. J. J. Smeets and A. L. Spek, Inorg. Chim.
Acta, 1999, 291, 438.
10 P. Le Floch, F. Knoch, F. Kremer, F. Mathey, J. Scholz, W. Scholz, K.-
H. Thiele and U. Zenneck, Eur. J. Inorg. Chem., 1998, 119.
11 J. Qiu and K. Matyjaszewski, Macromolecules, 1997, 30, 5643; J. Qiu
and K. Matyjaszewski, Acta Polymerica, 1997, 48, 107; J. Qiu, K.
Matyjaszewski, L. Thouin and C. Amatore, Macromol. Chem. Phys.,
2000, 101, 1625.
BP chemicals Ltd is thanked for financial support (R. K.
O’R.). Dr Jane Boyle is thanked for NMR measurements.
12 K. Matyjaszewski, B. Göbelt, H.-J. Paik and C. P. Horwitz, Macromole-
cules, 2001, 34, 430.
CHEM. COMMUN., 2002, 1850–1851
1851