Communications
firmed that the intermediate spin state prevails at the
polymerization temperature.
Keywords: a-diimines · homogeneous catalysis · iron ·
polymerization · spin state
.
In summary, we have demonstrated a correlation between
metal spin state and a simple catalytic chemical reaction, that
of polymerization. For FeIII species in a high spin state (S =
5/2), living atom transfer radical polymerization predomi-
nates, whereas with catalysts in an intermediate spin state
(S = 3/2), an organometallic pathway leads to catalytic chain
transfer. We have further shown that the metal spin state of
the iron species can be controlled by judicious choice of a-
diimine ligand substituents, highlighting the possibilities for
the rational design of catalysts on the basis of metal spin state.
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[10] Alternatively, this may be a mixture of (S = 5/2) and low-spin
species (S = 1/2).
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Experimental Section
All solvents were distilled over standard drying agents under nitrogen
and were deoxygenated before use. All reactions were performed
under an inert atmosphere. Full experimental and calculation details,
as well as characterization and polymerization data can be found in
the Supporting Information.
Ligand synthesis: To
a solution of the appropriate amine
[13] J. Bachmann, D. N. Nocera, J. Am. Chem. Soc. 2005, 127, 4730.
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Nakamura, J. Am. Chem. Soc. 2003, 125, 13028.
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M. Nakamura, Inorg. Chem. 2004, 43, 5034.
(80.0 mmol) in CH2Cl2 (30 mL) at 08C was added dropwise TiCl4
(1.0m in CH2Cl2, 12.2 mL, 12 mmol). p-Fluorobenzene (2.5 g,
10 mmol) in CH2Cl2 (10 mL) was added dropwise, and the solution
allowed to warm to room temperature and stir overnight. The
reaction was quenched with water, filtered, and extracted into
CH2Cl2. Removal of solvent left a sticky orange solid, which was
[16] K. P. Tellmann, M. J. Humphries, H. S. Rzepa, V. C. Gibson,
Organometallics 2004, 23, 5503.
recrystallized from MeOH(60 mL) to give off-white crystals of
4
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Schaefer, K. A. Peterson, J. Chem. Phys. 2004, 120, 4726.
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Phys. Chem. A 2004, 108, 5479.
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32, 3879.
[21] F. A. Cotton, R. L. Luck, K. A. Son, Inorg. Chim. Acta 1991, 179,
11.
(3.06 g, 84%) and 5 (3.65 g, 88%), respectively. Other diimine ligands
were prepared as described in the literature.[22, 23]
[R(N,N)FeCln] synthesis: To a mixture of FeCl3 or FeCl2·1.5THF
(6.00 mmol) and ligand (6.00 mmol) was added CH2Cl2 (30 mL) and
the resulting solution stirred for 24 h. Filtration of the solution,
followed by removal of solvent in vacuo gave a greasy solid. Washing
with pentane (3 15 mL) gave 1 as a yellow solid (67% yield), 2 as an
orange solid (74%), 6, 7, and 8 as purple solids (72, 77, and 60%,
respectively), and 9 as a dark-red solid (70%). [DIPP(N,N)FeCl3] (3)
was prepared by heating a solution of DIPP(N,N) and FeCl3 in THF at
reflux for 18 h, followed by extraction into diethyl ether, removal of
solvent, and washing with pentane to give a green-black solid (78%).
Alkylation experiments: [R(N,N)FeCl3] (0.500 mmol) was dis-
solved in THF (10 mL) and cooled to ꢀ788C followed by dropwise
addition of a solution of RMgCl (0.500 mmol) in THF. An aliquot
(1 mL) was removed from the reaction mixture and quenched with
distilled H2O. Subsequent aliquots were removed and quenched over
a range of temperatures. Aliquots were filtered through basic alumina
and analysed by GC or GC/MS. GC area counts were corrected
against organic components present in unreacted RMgCl.
[22] A. J. Arduengo, R. Krafczyk, R. Schumtzler, Tetrahedron 1999,
55, 14523.
[23] A. T. T. Hsieh, B. O. West, J. Organomet. Chem. 1976, 112, 285.
Polymerization experiments: Styrene (5.000 g, 48.00 mmol), 1-
phenylethyl chloride (32.0 mL, 0.240 mmol), and catalyst
(0.240 mmol) were added to an ampoule equipped with a magnetic
stirrer bar. The vessel was then sealed and heated to 1208C.
Conversion was determined by integration of signals for monomer
versus polymer backbone in the 1HNMR spectrum of crude aliquots
(0.1 mL) removed from the reaction mixture at appropriate intervals.
Upon completion of the reaction, the vessel contents were dissolved
in THF and added dropwise to acidified methanol to precipitate the
poly(styrene), which was washed and dried in vacuo. Samples were
analysed by gel permeation chromatography against styrene stand-
ards to determine the molecular weight.
Received: August 22, 2005
Revised: October 17, 2005
Published online: January 17, 2006
1244
ꢀ 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2006, 45, 1241 –1244