How do redox groups behave around a rigid molecular platform? hexa(ferrocenylethynyl)benzenes and their "electrostatic" redox chemistry

Full text of DOI:10.1002/anie.200900216

Angewandte
Chemie
DOI: 10.1002/anie.200900216
Dendrimer Models
How Do Redox Groups Behave around a Rigid Molecular Platform?
Hexa(ferrocenylethynyl)benzenes and Their “Electrostatic” Redox
Chemistry
Abdou K. Diallo, Jean-Claude Daran, Franꢀois Varret, Jaime Ruiz, and Didier Astruc*
[
1]
[2]
[10]
Ferrocenyl polymers and dendrimers are a key class of
redox materials with electronic properties that include useful
The Negishi reaction
of a fourfold excess of [ZnCl-
[
10b]
(C Fc)] (Fc = ferrocenyl) , C Br , and [Pd(PPh ) ] catalyst
2
6
6
3 4
[1]
mixed-valency-induced sensing and electrochemically based
molecular recognition. For instance, the splitting of the
ferrocenyl cyclic voltammetry wave in ferrocenyl polymers
was heated at 808C in a toluene/THF (2:1) mixture for 7 days
[Eq. (1)]. This reaction gave, in addition to [FcC Fc], a dark-
red solid that is almost insoluble in all solvents and the ether-
soluble dark-red new compound pentakis(ferrocenyl-
ethynyl)benzene (1) in 7% yield.
[
2]
4
[3]
was shown to be crucial for colorimetric sensing. When four
equivalent ferrocene redox centers are assembled around a
small metal core, the ferrocenyl cyclic voltammetry wave can
be split by using a supporting electrolyte that contains a
perfluorinated anion. Such a splitting is not seen in
ferrocenyl-terminated dendrimers, as fast single-electron
transfer to the electrode occurs because of the optimal
connection between the ferrocenyl redox centers, which
[
4]
[2c,5]
results from the flexibility of the tethers.
The situation
might be different in rigid dendrimers, however, because the
rigidly held termini cannot communicate through space,
unlike in flexible dendrimers. Although rigid polyarene
[6]
dendrimers are well-known, rigid ferrocenyl dendrimers
have not been reported to date. The electronic properties for
these compounds would solely result from intramolecular
communication and electrostatic interactions around the
ferrocenyl termini. In order to obtain an insight into such
phenomena, we have synthesized a family of hexaferroceny-
lethynylferrocenes and examined their redox and electronic
properties. This unique family of complexes is unprecedented,
presumably because of synthetic and solubility problems that
have now been overcome. Vollhardt and co-workers have
recently reported the rather extraordinary hexaferrocenyl-
benzene, including its X-ray crystal structure, which shows
[
7]
extreme bulk and distortion. Several organic-substituted
[
8]
hexaethynylbenzenes are known as well as arene-centered
stars with remote ferrocenyl termini, in which the length of
the tethers and lack of conjugation precludes electronic
[
9]
interactions.
[*] A. K. Diallo, Dr. J. Ruiz, Prof. D. Astruc
Institut des Sciences Molꢀculaires, UMR CNRS No.5255
Universitꢀ Bordeaux 1, 33405 Talence Cedex (France)
Fax : (+33)5-4000-2995
E-mail: d.astruc@ism.u-bordeaux1.fr
Homepage: http://www.u-bordeaux1.fr/lcoo/members/eastruc.htm
Dr. J.-C. Daran
Laboratoire de Chimie de Coordination,UPR CNRS No.8241
Route de Narbonne, 33000 Toulouse Cedex (France)
Prof. F. Varret
Groupe d’Etude de la Matiꢁre Condensꢀe, UMR CNRS No.8635
78000 Versailles Cedex (France)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200900216.
Angew. Chem. Int. Ed. 2009, 48, 3141 –3145
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
3141

Communications
Dehalogenation has been frequently encountered in
reactions of organometallic ethynyl reagents and hexahalo-
m/z 1742.52 (calcd m/z 1747.18, Figure S7 in the Supporting
1
13
Information), H and C NMR spectroscopy, IR and UV/Vis
spectrometry, and elemental analysis. The air stability of
solutions of 3 in various organic solvents is remarkable, even
over several weeks, whereas solutions of ferrocene in organic
solvents decompose in air during this period of time. The
crystal structure of 3 shows that, although three ferrocenyl
groups are located on each side of the benzene plane, they do
not alternate because of packing forces, including those of the
[
7,11]
genobenzenes;
this effect can possibly be attributed to
side electron transfer from the electron-rich organometallic
reagent to the bulky pentaethynylated halogenoarene deriv-
[
11b]
ative.
from the soluble residues by Soxhlet extraction with toluene,
The almost-insoluble product could be removed
1
3
and was analyzed by C NMR spectroscopy and MALDI
TOF mass spectroscopy, which confirmed the molecular
structure of hexakis(ferrocenylethynyl)benzene (2) by the
presence of the expected molecular ion peak and no other
product peaks. The Mꢀssbauer spectrum of 2 shows the
ferrocene-type frame (the clean cyclovoltammograms are
discussed below); six-electron oxidation using acetylferroce-
nium gives the corresponding hexacation that was character-
ized by elemental analysis as 2(BF ) [Eq. (2)].
[
13]
isooctane solvent molecules (Figure 1).
4
6
Figure 1. Molecular view of 3. Ellipsoids are drawn at the 30%
probability level. H atoms are omitted for clarity. (Symmetry code i:
1Àx, 1Ày, 1Àz).
Reaction of 3 with six equivalents of ferrocenium hexa-
fluorophosphate in dichloromethane yielded the dark-green
salt 3(PF ) , the ferrocenium-type structure of which was
6
6
confirmed by elemental analysis and IR and UV/Vis spec-
troscopy. The zero-field Mꢀssbauer spectra of 3 and 3(PF6)6
are compared in Figure 2 (the Mꢀssbauer doublet of 3(PF6)6
Figure 2. Zero-field Mꢂssbauer spectra of a) 3 b) 3(PF ) at 78 K.
6
6
In order to obtain a more soluble and workable analogue
[12]
of 2, we reacted 1,2,3,4,5-pentamethyl-1’-ethynyl ferrocene
with C Br in an analogous Negishi reaction under identical
collapses to a slightly broad single band, a classic feature of
[14]
ferrocenium salts ). The complex 3(PF ) was quantitatively
6
6
6 6
conditions. This reaction gave the red hexakis(1,2,3,4,5-
pentamethyl-1’-ferrocenylethynyl)benzene (3) in 27% yield.
The structure of 3 is confirmed by the molecular ion peak at
reduced back to 4 using six equivalents of decamethylferro-
cene [[FeCp* ], Eq. (1), bottom]; six equivalents of decame-
2
thylferrocenium hexafluorophosphate were also formed in
3
142
www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 3141 –3145

Angewandte
Chemie
this reaction. Thus, the six-electron transfer is fully chemically
reversible and occurs without any decomposition [Eq. (2)].
Finally, in order to introduce an intermediate situation
between the parent complex 2 and complex 3 that contains 30
methyl groups, we undertook the synthesis of a third member
of the series, which has only one methyl group on each Cp
ring. For this purpose, the synthesis of an heterodisubstituted
ethynylferrocene (Scheme 1) was required, which used the
convenient photochemical ligand exchange reaction of the
lack of CV wave splitting reported for 5 and 6 indicated that
there is no significant communication between the iron
centers of the ferrocenyl groups through the polyethynylben-
zene core in these two complexes, unlike in the complex 1,4-
(diironethynyl)benzene (7), in which the iron atoms are
[
18]
directly located on the ethynyl groups.
Thus, as expected, the CVs of 2, 3, and 4 in CH Cl also
2
2
showed a single wave when nBu NPF6 was used as the
4
supporting electrolyte. We also recorded these CVs, as well as
6
6
[15]
F
F
complex [Fe(h -C H CH )(h -toluene)]PF
followed by the
those of 5 and 6, with nBu NBAr (Ar = [3,5-C H (CF ) ])
5
4
3
6
4 4 6 3 3 2
conversion of the acetyl to an ethynyl substituent. In this case,
the Negishi reaction was as effective as that for the other two
ethynylferrocene derivatives [Eq. (1)], and afforded the
moderately soluble deep-red hexakis(1-methylferrocenyl-1’-
ethynyl)benzene complex 4 in 60% yield. These three
as the supporting electrolyte, whose interesting ion-pairing
properties and application in electrochemistry have been
[
4,19]
reported previously.
Interestingly, we find that, by using
this perfluorinated electrolyte in CH Cl , the CVs of 2 and 5
2
2
split into three well-separated waves, whereas that of 6 still
shows a single wave (Figure 3).
Scheme 1. Synthesis of the ferrocenyl precursor of 4
reactions establish the general access to this new family of
ferrocenyl-terminated rigid stars and their hexacationic
ferrocenium derivatives. The interconversion between the
neutral complexes and their oxidized form using the appro-
priate mononuclear ferrocene or ferrocenium derivative for
exergonic hexaelectron transfer reactions is virtually quanti-
tative and reversible, without any decomposition [Eq. (2)].
In the cyclovoltammetric (CV) study, we compared parent
complex 2 with both the known 1,3,5-tris(ferrocenylethynyl)-
benzene (5), which had been reported to show a single CV
[
16,17]
wave, and 1,4-bis(ferrocenylethynyl)benzene (6).
The
Figure 3. Cyclic voltammograms of a) 5 and b) 2 with NBu PF (top)
4
6
F
and NBu BAr (bottom) in CH Cl .
4
4
2
2
Electronic communication is well known to occur
between substituents around a para-substituted benzene
core, but occurs less efficiently between substituents around
[20]
a meta-substituted core.
These results therefore confirm
that electronic communication does not proceed between the
redox center (iron-based) of the ferrocenyl groups of 6
(contrary to 7), which suggests that this must also be the case
in the family of the hexakis(ferrocenylethynyl)benzenes 2, 3,
and 4. This also means that the splitting, only observed for 2
F
4
and 5 with nBu NBAr , does not result from electronic
4
communication among the redox centers, but from electro-
static effects that arise from ion pairing with the electrolyte
anion upon anodic oxidation of the neutral compounds.
It is remarkable that, although the parent complexes 2 and
2
(PF ) are almost insoluble in all solvents, the permethylated
6 6
analogues 3 and 3(PF ) are extremely soluble in pentane (3)
6
6
and CH Cl2 (3 and 3(PF ) ). This tremendous solubility
2
6 6
difference is related to the considerable change in solute–
solvent and ion-pairing energies when the bulky methyl
groups increase the distance between the organometallic
hexacation and the solvent or anions. This difference also
Angew. Chem. Int. Ed. 2009, 48, 3141 –3145
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3143

Communications
shows that the nature of the anion has an enormous influence
six waves that correspond to the six single-electron oxidation
steps (Figure 4). Thus, the influence of a single methyl group
on each ferrocenyl redox center has a dramatic effect on the
when using such ion pairs in both synthesis and electro-
chemistry.
F
4
The similarities of the CVs of 2 and 5 with nBu NBAr in
4
CH Cl (Figure 3) is also striking and indicates that the redox
2
2
process in 2 involves three two-electron waves. The appear-
ance of the para-ferrocenyl groups in the same CV wave
confirms their mutual independence. The electrostatic effect
is thus essentially the only reason for the distinction between
the three two-electron oxidation steps shown in Scheme 2,
Figure 4. CVs of 4 in CH Cl at 208C with a) nBu NPF and
2
2
4
6
F
b) nBu NBAr as the supporting electrolyte.
4
4
perturbation of the ion-pairing strength, which results in both
the CV modification and the large difference in solubility of
both the neutral compounds and hexacationic salts with the
same anions.
In conclusion, we have synthesized the first family of rigid
stars with terminal redox groups that contain both neutral
hexaferrocene and hexaferrocenium compounds, which can
be fully and reversibly interconverted. Both solubility and
cyclic voltammetry studies show that, despite the lack of
electronic communication between the redox centers, the six-
electron wave usually observed for all the family members
with the standard electrolyte nBu NPF can be split either
Scheme 2. Mechanism of the oxidation of 2 in the presence of
F
4
6
NBu BAr in CH Cl , which occurs by a cascade of three two-electron
4
4
2
2
into three two-electron waves or into six overlapping single-
electron waves by simple variation of the number of methyl
groups on the ferrocenyl groups, and of the counterion of the
supporting electrolyte. This result shows that rigidly planar
molecular platforms surrounded by redox groups undergo a
redox chemistry that is extraordinarily sensitive to ion-
pairing-dependent electrostatic effects. Such engineering of
these rigid redox dendrimer models could find applications in
materials science for the design of multielectron transfer
oxidation steps (shown in Figure 3). The ferrocenylethynyl groups are
shown as gray circles and the ferroceniumethynyl groups are shown as
black circles.
which are distinguished by the large differences between
these electrostatic effects in the ortho, meta, and para
positions of 2.
A close inspection of the CVs of 2 and 5 shows, however,
that the splitting of the three CV waves is not as well defined
for 2 as for 5, because the electrostatic (and eventually, to a
minor extent, if any, electronic) influence of each ferrocenyl
group on its neighbor in ortho position is not zero. The CVof 2
[
24]
catalysis and in biologically relevant anion sensing.
Experimental Section
F
with nBu NBAr indicates that the mixed-valent species
4
4
Negishi reaction between ethynylferrocene and hexabromobenzene:
Hexabromobenzene (42 mg, 0.076 mmol), [Pd(PPh ) ] (66 mg,
III
II
F
II
III
[
(
Fe Fe ](BAr ) (Kdisp = 172 at 208C) and [Fe Fe 4]-
2 4 4 2 2
3
4
F
4
[21]
III
II
F
BAr ) (K = 43 at 208C) , unlike [Fe Fe ](BAr ) ,
0.057 mmol, 0.75 equiv), and toluene (freshly distilled over Na,
10 mL), were successively added to a Schlenk flask. A solution of
4
disp
3
3
4 3
are rather stable (both thermodynamically and kinetically
within the electrochemical time scale), which arises from the
[
10b]
ferrocenylethynyl zinc chloride
bromide group) was then added. The mixture was heated at 808C for
24 h, an additional portion of [Pd(PPh ] (66 mg, 0.057 mmol,
.75 equiv) and ferrocenylethynyl zinc chloride (2 mmol, 4 equiv) in
in THF (2 mmol, 4 equiv per
[22]
electrostatic effect. On the other hand, the mixed-valence
III
II
6Àn
)
3 4
compounds of 2 in the series [Fe nFe
](PF ) (0 < n < 6)
6 n
0
only follow a statistical distribution, which includes a major
toluene were added by syringe under N . The mixture was kept at
2
III
II
[23]
contribution of [Fe Fe ](PF ) .
3
3
6
3
808C for six days and then cooled to room temperature. Following
Soxhlet extraction to remove soluble residues, hexakis(ferrocenyl-
ethynyl)benzene (2) was obtained as a dark-red solid (47 mg, 47%
yield), which was almost insoluble in all solvents and characterized by
using MALDI-TOF mass spectroscopy, Mꢀssbauer spectroscopy, and
cyclic voltammetry. The soluble residue was purified by column
Finally, the CVs of 3 and 4 recorded with nBu NPF show
4
6
F
a single wave, as in the CVof 2. The CVof 3 with nBu NBAr
4
4
versus decamethylferrocene is a broad envelope of over-
lapping waves between 0.3 and 0.7 V (see Figure S10 in the
Supporting Information). This observation is consistent with a
strong shielding of the electrostatic effect by the five methyl
groups around each ferrocenyl center. For 4, the envelope is
better resolved and, despite the overlap, one can distinguish
chromatography on SiO eluting with pentane to give 1,4-bis(ferro-
2
cenyl)butadiyne (75 mg), and eluting with pentane/dichloromethane
(70:30) to give pentakis(ferrocenylethynyl)benzene (1) as dark-red
solid (6 mg, 7% yield; see the Supporting Information). The same
3
144
www.angewandte.org
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2009, 48, 3141 –3145

Angewandte
Chemie
reaction conditions were used for the synthesis of 3 and 4, and the
pure compounds were separated from the reaction mixtures by
column chromatography (see the Supporting Information).
[11] a) J. Nierle, D. Barth, D. Kuck, Eur. J. Org. Chem. 2004, 867 –
872; b) D. Astruc, Organometallic Chemistry and Catalysis,
Springer, Heidelberg, 2007, p. 502.
[
12] B. Bildstein, A. Hradsky, H. Kopacka, R. Malleier, K.-H.
Ongania, J. Organomet. Chem. 1997, 540, 127 – 145.
Received: January 13, 2009
Published online: March 25, 2009
[
13] Crystal data for 8: C108H114Fe , 2C H , M = 1975.54, mono-
6
8
18
r
clinic, a = 18.7113(7), b = 12.5738(4), c = 22.4110(9) ꢃ, U =
3
5
246.1(3) ꢃ , T= 180 K, space group P2 /n (no. 14), Z = 2,
1
Keywords: dendrimers · electron transfer · ferrocene · ion pairs ·
À1
.
m(MoKa) = 0.856 mm
,
92321 reflections measured, 10708
redox chemistry
unique (Rint = 0.030) which were used in all calculations. R =
.0409 and wR2 = 0.1020 (8609 reflections with I > 2s(I)).
0
CCDC 686494 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
from The Cambridge Crystallographic Data Centre via www.
ccdc.cam.ac.uk/data_request/cif.
[
[
1] P. Nguyen, P. Gomez-Elipe, I. Manners, Chem. Rev. 1999, 99,
515 – 1548; I. Manners, Science 2001, 294, 1664 – 1666; G. R.
1
Whittell, I. Manners, Adv. Mater. 2007, 19, 3239 – 3468.
2] For reviews on ferrocenyl dendrimers and their electrochemical
properties, see: a) C. M. Casado, I. Cuadrado, M. Moran, B.
Alonso, B. Garcia, B. Gonzales, J. Losada, Coord. Chem. Rev.
[
14] R. L. Collins, J. Chem. Phys. 1965, 42, 1072 – 1080; A. Stebler, A.
Furrer, J. H. Ammeter, Inorg. Chem. 1984, 23, 3493 – 3500. The
Mꢀssbauer spectrum of 3 shows the classic ferrocene doublet
1999, 185–186, 53 – 79; b) S. H. Hwang, C. D. Shreiner, C. N.
À1
À1
with Q.S. = 2.0 mms and I.S. = 0.5 mms . The spectrum of
(PF ) collapses into a broad line (Figure 2).
Moorefield, G. N. Newkome, New J. Chem. 2007, 31, 1027 – 1038;
c) D. Astruc, C. Ornelas, J. Ruiz, Acc. Chem. Res. 2008, 41, 841 –
3
6
6
[
15] T. P. Gill, K. R. Mann, Inorg. Chem. 1983, 22, 1986; D. Catheline,
D. Astruc, J. Organomet. Chem. 1983, 248, c9 – c12; D. Catheline,
D. Astruc, J. Organomet. Chem. 1984, 272, 417 – 426; D. Cathe-
line, D. Astruc, Organometallics 1984, 3, 1094 – 1100.
856.
[
3] R. Rulkens, A.-J. Lough, I. Manners, S. R. Lovelace, C. Grant,
W. E. Geiger, J. Am. Chem. Soc. 1996, 118, 12683 – 12695; N.
Camine, U. T. Mueller-Westerhoff, W. E. Geiger, J. Organomet.
Chem. 2001, 637, 823 – 826.
[
[
[
16] P. Jutzi, B. Kleinebekel, J. Organomet. Chem. 1997, 545–546,
573 – -576.
[
[
[
4] F. Barriꢁre, N. Camine, W. E. Geiger, J. Am. Chem. Soc. 2002,
17] H. Fink, N. J. Long, A. J. Martin, G. Opromolla, A. J. P. White,
D. J. Williams, P. Zanello, Organometallics 1997, 16, 2646 – 2650.
18] a) M. Sato, Y. Hayashi, M. Katada, S. Kawata, J. Organomet.
Chem. 1994, 471, 179 – 184; b) M. Sato, Y. Hayashi, S. Kumakura,
N. Shimizu, M. Katada, S. Kawata, Organometallics 1996, 2, 721 –
124, 7262 – 7263; F. Barriꢁre, W. E. Geiger, J. Am. Chem. Soc.
2006, 128, 3980 – 3989; see also Ref. [3].
5] C. Ornelas, J. Ruiz, C. Belin, D. Astruc, J. Am. Chem. Soc. 2009,
31, 590 – 601; J. Camponovo, J. Ruiz, E. Cloutet, D. Astruc,
1
Chem. Eur. J. 2009, 15, 2990 – 3002.
7
5
28; c) F. Paul, C. Lapinte, Coord. Chem. Rev. 1998, 178, 431 –
09.
6] N. Rosenfeldt, N. Dingenouts, D. Pꢀtschke, M. Ballauff, A. J.
Berresheim, K. Mꢂllen, P. Lindner, Angew. Chem. 2004, 116,
[
19] W. E. Geiger, Organometallics 2007, 27, 5738 – 5765.
111 – 114; Angew. Chem. Int. Ed. 2004, 43, 109 – 112; C. G.
[
20] a) W. Adam, H. M. Harrer, F. Kita, W. M. Nau, J. Org. Chem.
Clark, Jr., R. J. Wenzel, E. V. Andreitchenko, W. Steffen, R.
Zenobi, K. Mꢂllen, New J. Chem. 2007, 31, 1300 – 1306.
7] Y. Yu, A. D. Bond, P. W. Leonard, U. J. Lorenz, T. V. Timofeeva,
K. P. C. Vollhard, D. W. Glenn, A. A. Yakovenko, Chem.
Commun. 2006, 24, 2572 – 2574.
1
997, 62, 1419 – 1426; b) A. H. Winter, D. E. Falvey, C. J.
Cramer, J. Am. Chem. Soc. 2004, 126, 9661 – 9668; c) B.
Le Guennic, K. Costuas, J.-F. Halet, C. Nervi, M. A. J. Paterson,
M. A. Fox, R. L. Roberts, D. Albesa-Jove, H. Puschmann,
J. A. K. Howard, P. J. Low, C. R. Chim. 2005, 8, 1883 – 1896;
d) W. T. G. Johnson, M. B. Sullivan, C. J. Cramer, Int. J. Quan-
tum Chem. 2001, 85, 492 – 508.
[
[
8] a) For the first synthesis of hexaethynylbenzene, see: J. C.
Armstrong, R. Boese, K. P. C. Vollhardt, Angew. Chem. 1986,
98, 270 – 271; Angew. Chem. Int. Ed. Engl. 1986, 25, 268 – 269;
[
[
[
21] The disproportionation constant Kdisp = exp(DE8/2RT): D. E.
Richardson, H. Taube, Inorg. Chem. 1981, 20, 1278 – 1285; D. E.
Richardson, H. Taube, J. Am. Chem. Soc. 1983, 105, 40 – 51.
b) R. Boese, J. R. Green, J. Mittendorf, K. P. C. Vollhardt,
Angew. Chem. 1992, 104, 1643 – 1645; Angew. Chem. Int. Ed.
Engl. 1992, 31, 1643 – 1645; c) J. D. Tovar, N. Jux, T. Jarrosson,
S. I. Khan, Y. Rubin, J. Org. Chem. 1997, 62, 3432 – 3433; d) J. E.
Anthony, S. I. Khan, Y. Rubin, Tetrahedron Lett. 1997, 38, 3499 –
22] Contributions to the mixed valency: DG8c = DG (entropy) +
e
DGE (electrostatic) + DGD (delocalization) + DGS (synergy):
J. E. Sutton, P. M. Sutton, H. Taube, Inorg. Chem. 1979, 18, 1017.
23] If the electrostatic and delocalization factors are zero, the
3502; e) U. H. F. Bunz, Y. Rubin, Y. Tobe, Chem. Soc. Rev. 1999,
28, 107 – 119; f) D. M. Bowles, G. J. Palmer, C. A. Landis, J. L.
[22]
distribution is only statistic (entropy) and follows a simple
binomial law, that is, for instance the half-oxidized [Fe ] product
contains 31.2% [Fe 3Fe 3], 23.4% [Fe 4Fe 2], 23.4%
Scott, J. E. Anthony, Tetrahedron 2001, 57, 3753 – 3758.
6
[
9] a) J.-L. Fillaut, J. Linares, D. Astruc, Angew. Chem. 1994, 106,
II
III
II
III
2540 – 2542; Angew. Chem. Int. Ed. Engl. 1994, 33, 2460 – 2462;
II
III
II
III
II
III
II
[
Fe 2Fe ], 9.4% [Fe Fe ], 9.4% [Fe Fe 5], 1.6% [Fe 6],
4
5
b) J. Nierle, D. Barth, D. Kuck, Eur. J. Org. Chem. 2004, 867 –
III
and 1.6% [Fe 6].
872; c) V. J. Chebny, D. Dhar, S. V. Lindeman, Org. Lett. 2006, 8,
[
24] P. D. Beer, P. A. Gale, Angew. Chem. 2001, 113, 502 – 532;
Angew. Chem. Int. Ed. 2001, 40, 486 – 516; U. Dreshler, B.
Erdogan and V. M. Rotello, Chem. Eur. J. 2004, 10, 5570 – 5579;
D. Astruc, M.-C. Daniel, J. Ruiz, Chem. Commun. 2004, 2637 –
5041 – 5044.
[
10] a) E. Negishi, F. Liu in Metal-Catalyzed Cross-Coupling Reac-
tions (Eds.: F. Diederich, P. J. Stang), Wiley-VCH, New York,
1
2
997, p. 1; b) A. Carella, G. Rapenne, J.-P. Launay, New J. Chem.
005, 29, 288 – 290.
2649.
Angew. Chem. Int. Ed. 2009, 48, 3141 –3145
ꢀ 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3145