COMMUNICATION
Over the past two decades iron catalysis has become
a powerful tool in organic synthesis.[1] Cross-coupling reac-
tions doubtless rank among the most important of these re-
actions, because they allow effective formation of carbon
scaffolds.[1f,h–q,2] Nowadays, these transformations are a stan-
dard tool for the preparation of fine chemicals and biologi-
cally active compounds on both laboratory and industrial
scales.[1d] Although cross-coupling reactions are dominated
by palladium complexes, iron complex catalysts offer an al-
ternative of increasing importance due to their easy accessi-
bility, short reaction times and broad functional group toler-
ance.[1b,g] Under established conditions alkyl or aryl
Grignard reagents are coupled with aryl chlorides, triflates
and tosylates.[1e,g,n,2a,3a]
2) Bogdanovic et al.[10] suggest a heterobimetallic inorganic
Grignard complex [FeꢀII
ACTHNUTRGEN(UNG MgCl)2]n, which would show
ꢀ
Fe Mg pairs in the EXAFS spectrum.
3) Fꢁrstner et al.[11] describe the formation of FeII organo-
ferrates [RnFe+II 2ꢀn. This type of compound would ex-
]
hibit only Fe–C contributions to the radial distribution
function.
4) Noda et al.[12] propose diaryl FeII compounds stabilized
by TMEDA, which would show similar EXAFS charac-
teristics to the organoferrates described in point 3).
5) Norrby et al.[3b] suppose a catalytically active FeI-species,
which is not further structurally specified.
Herein, these proposals will be discussed on the basis of
X-ray-spectroscopic investigations. According to the litera-
ture,[10] a maximum amount of four equivalents Grignard
compound are required to form the active iron species.
Despite the importance of iron-catalyzed cross-coupling
reactions and intensive investigations, the mechanism of this
reaction is still subject to ongoing discussion.[1m,3] Spectro-
scopic studies are very limited due to the paramagnetic char-
acter of the species formed.[3a] Hence, mechanistic findings
are mainly based on investigations of potential intermedi-
Analogously, the reaction products of iron
ACHTUNGTREN(NUNG III)-acetylaceto-
nate Fe(acac)3 and one to four equivalents phenylmagne-
AHCTUNGTRENNUNG
siumchloride PhMgCl in THF/NMP were examined to iden-
tify the species formed through activation of the pre-catalyst
ACHTUNGTRENNUNG
ates.[3a] In contrast to palladium-catalyzed cross-couplings,
for which detailed mechanistic knowledge exists,[4] this gap
prevents the directed development of improved iron cata-
lysts for cross-coupling reactions.
Fe
Figure 1 shows the energy-calibrated XANES spectra
during addition of 1–4 equivalents of PhMgCl to Fe(acac)3
(acac)3.[13]
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
X-ray absorption spectroscopy represents a method to
bridge this gap.[5] It provides element specific clarification of
local structure and oxidation state of metal centers through
EXAFS (extended X-ray absorption fine structure) and
XANES (X-ray absorption near edge structure) spectrosco-
py.[6] Employing these methods, the type and number of li-
gands as well as their distance from the catalytically active
metal center can be determined in situ.[7] Despite these ad-
vantages, which were already used for investigations of dif-
ferent cross-coupling and Grignard reactions,[5,8] to the best
of our knowledge no XAS investigations on iron-catalyzed
cross-coupling reactions have been reported to date. This is
even more surprising, because this type of investigation
would enable a comparison of the five suggested mecha-
nisms proposed to date for this type of cross-coupling:[3a]
in THF/NMP. To emphasize small changes, the first deriva-
tives[14] of the spectra are displayed as well. Two spectral re-
gions can be distinguished: The pre-edge signal (prepeak)[15]
at an energy of 7.10–7.11 keV (signal A) and the absorptions
edge at a range of 7.11–7.12 keV (signals B and C). The pre-
peak position of the pre-catalyst FeACTHNUTRGEN(UNG acac)3 is located at
7.106 keV and shifts after addition of one equivalent
PhMgCl to a smaller value of 7.104 keV. Whereas this pre-
peak is caused by a 1s!3d transition, the decreased reso-
nance energy reflects the transition from an s0d5 to an s0d6
electron configuration, which corresponds to a reduction
from FeIII to FeII. This energy does not change with addition
of further equivalents, but the prepeak intensity and the
shape and energy of the absorption edge (signals B and C)
1) Kochi et al.[9] postulate a “soluble iron species” of unspe-
cified oxidation state, which exists as an aggregate com-
ꢀ
plexed by Grignard compounds. In this case, Fe Fe pairs
characteristic of a metal cluster would be expected in the
EXAFS analysis.
[a] R. Schoch, Prof. Dr. M. Bauer
Fachbereich Chemie, TU Kaiserslautern
Erwin-Schrçdinger-Str. 54, 67663 Kaiserslautern (Germany)
Fax : (+49)631-205-4676
[b] W. Desens, Dr. T. Werner
Leibniz-Institut fꢁr Katalyse e.V. an der Universitꢂt Rostock
Albert-Einstein-Str. 29a, 18059 Rostock (Germany)
Fax : (+49)381-1281-51326
Figure 1. XANES spectra (right) and their derivatives (left) of the pre-
Supporting information for this article is available on the WWW
catalyst Fe
ACHTUNGTERN(NNUG acac)3 (1) and after addition of one to four equivalents (2–5)
PhMgCl in comparison to Fe0 (6) and under reaction conditions (7).
Chem. Eur. J. 2013, 19, 15816 – 15821
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
15817