Bis(silylenyl)- and bis(germylenyl)-substituted ferrocenes: Synthesis, structure, and catalytic applications of bidentate silicon(II)-cobalt complexes

Article abstract of DOI:10.1002/anie.201202175

Strong bite: The two respective Si^II and Ge^II centers in ferrocene-containing bis(silylene) and bis(germylene) ligands 1 can serve as excellent donors, as shown in the formation of Co complexes 2 (see scheme, Cp=·⁵-cyclopentadienyl). Unexpectedly, only the bis(silylene)-Co complex 2 is a precatalyst for the [2+2+2] cycloaddition of phenylacetylene and MeCN, possibly because of a stronger coordination of the Ge^II donor centers to Co. Copyright

Full text of DOI:10.1002/anie.201202175

Angewandte
Chemie
DOI: 10.1002/anie.201202175
Chelate Ligands
Bis(silylenyl)- and Bis(germylenyl)-Substituted Ferrocenes: Synthesis,
Structure, and Catalytic Applications of Bidentate Silicon(II)–Cobalt
Complexes**
Wenyuan Wang, Shigeyoshi Inoue,* Stephan Enthaler,* and Matthias Driess*
Dedicated to Professor Guy Bertrand on the occasion of his 60th birthday
The chemistry of stable silylenes has received
wide interest since the first isolation of N-
heterocyclic silylenes (NHSis) by Denk and
West et al.[1] While stable silylenes have
received a lot of attention,[2] the chemistry
of bis(silylenes), compounds with two diva-
lent silicon sites in a single molecule, is much
less developed.[3] To date, bis(silylenes) have
been limited to two types of compounds,
which can be defined as follows: 1) “inter-
connected bis(silylenes)” in which the two
divalent silicon atoms are adjacent to each
other and connected by a central single bond
(I–III),[4–6] and 2) “spacer-separated bis(sily-
lenes)” in which the divalent silicon atoms
are separated by
a
spacer (A–E;
Scheme 1).[7–11] Recently, interconnected
bis(silylenes) I–III have been the subject of
active research because of their distinct
reactivity in comparison to isoelectronic
disilynes. For example, compounds I, which
bear amidinate ligands, were independently
synthesized by Roesky and co-workers,[4a]
and Jones et al.[4b] Furthermore, bis(sily-
lenes) of type II and III, which are stabilized
by an intramolecular phosphine donor and
Scheme 1. Bis(silylenes) I–III and spacer-separated bis(silylenes) A–E.
an N-heterocyclic carbene (NHC), respectively, were also
reported.[5,6] On the other hand, spacer-separated bis(sily-
lenes) are intriguing as novel bidentate s-donor ligands for
transition metals because of their unique structure and their
coordination ability. The first spacer-separated bis(silylene) A
was reported by Lappert and co-workers in 2005.[7] Bis-
(silylene) B was synthesized by the reduction of dichlorosi-
laimine NHC·Cl2Si = NR (NHC = 1,3-bis(2,6-diisopropyl-
phenyl)-imidazol-2-ylidene, R = 2,6-bis(2,4,6-triisopropyl-
phenyl)-phenyl) through initial formation of the elusive
silaisocyanide intermediate.[8] In addition, compound C was
prepared by the reaction of bis(silylene) I (R1 = tBu, R2 = Ph)
with phenylacetylene PhC ꢀ CH.[9] Very recently, we de-
scribed the synthesis of the first isolatable oxygen-bridged
bis(silylene) D and pincer-type bis(silylene) E.[10,11] This
development underlines that silylenes are no longer labora-
tory curiosities and may provide access to new silicon(II)-
based functional groups in coordination chemistry toward
transition metals. In general, complexes of silylenes and
transition metals have received much attention because they
can play a key role as intermediates in transition-metal-
catalyzed transformation of silicon compounds.[12] Very
recently, bis(silylenes) D and E have been employed as new
silicon(II)-based donor ligands to stabilize unusually elec-
tron-rich complexes of silylenes and Group 10 metals. In
order to gain access to other new bis(silylenes) as potential
bidentate s-donor ligands,[13] we set out to investigate a novel
[*] Dipl.-Chem. W. Wang, Prof. Dr. S. Inoue, Dr. S. Enthaler,
Prof. Dr. M. Driess
Department of Chemistry: Metalorganics and Inorganic Materials
Technische Universitꢀt Berlin
Straße des 17. Juni 135, Sekr. C2, 10623 Berlin (Germany)
E-mail: shigeyoshi.inoue@tu-berlin.de
matthias.driess@tu-berlin.de
[**] This work was supported by the Deutsche Forschungsgemeinschaft
(Cluster of Excellence “UniCat”, EXC 314). We thank the Alexander
von Humboldt foundation (S.I.) for support by the Sofja Kovalev-
skaja Program.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202175.
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
type of bis(silylene) with a ferrocenyl spacer, which could
result in unique properties. Herein, we present the facile
synthesis and characterization of the first bis(silylenyl)- and
bis(germylenyl)-substituted ferrocenes LSi-Fc-SiL 3 (Fc =
ferrocendiyl, L = PhC(NtBu)2) and LGe-Fc-GeL 4, respec-
tively, as well as their corresponding CpCo (Cp = h5-cyclo-
pentadienyl) complexes 5 and 6. Additionally, 5 and 6 were
probed as catalysts for [2+2+2] cycloaddition reactions of
phenylacetylene and acetonitrile to give substituted benzenes
and pyridines, respectively.
Recently, we reported the reaction of chlorosilylene LSiCl
1[14] with dilithium resorcinolate to afford the pincer-type
bis(silylene) E in high yield.[11] We anticipated that chlorosi-
lylene 1 and chlorogermylene LGeCl 2[15] could be suitable
precursors for the synthesis of the desired bismetallene-
functionalized ferrocenes 3 and 4. In fact, treatment of 1 with
1,1’-dilithioferrocene led to the formation of bis(silylene) 3 in
70% yield (Scheme 2). Similarly, the Ge analogue 4 could be
Figure 1. Molecular structure of compound 4. Thermal ellipsoids are
drawn at 50% probability level. Hydrogen atoms and solvent mole-
cules are omitted for clarity. Selected bond lengths [ꢁ] and angles [8]:
Ge1–C1 1.991(2), Ge2–C8 1.9774(19), Ge1–N1 2.0261(16), Ge1–N2
2.0247(17), Ge2–N3 2.0402(15), Ge2–N4 2.0162(16); C1-Ge1-N1
96.62(7), C1-Ge1-N2 94.45(7), N1-Ge1-N2 64.86(7), C8-Ge2-N3
94.92(7), C8-Ge2-N4 97.16(7), N3-Ge2-N4 64.59(6).
iPr2C6H3).[19] In addition, the molecular structure of bis-
(silylene) 3 could also be unambiguously determined by X-ray
diffraction. The structural features of 3 are very similar to
those of 4, however, the X-ray data are not sufficient for
discussion (Supporting Information).
A first set of experiments was undertaken to investigate
the coordination ability of 3 and 4 toward transition metals.
Inspired by the reactivity of the bis(silylenes) D and E toward
Ni0 and Pd0 [10,11] and by reports on interconnected bis(germy-
lene) iron [{LGeFe(CO)4}2][18] and spacer-separated bis(ger-
Scheme 2. Synthesis of bis(silylene)–ferrocene ligand 3 and its Ge
analogue 4.
mylene)
molybdenum
chelate
complexes
cis-
isolated in 77% yield by salt metathesis reaction of 1,1’-
dilithioferrocene with 2. Compounds 3 and 4 represent the
first examples of isolatable silylene- and germylene-substi-
tuted ferrocenes, respectively. Recently, disilene-functional-
ized ferrocene was reported by Tokitoh and co-workers.[16]
The structures of 3 and 4 were determined by multinuclear
NMR spectroscopy, elemental analysis, and X-ray crystallo-
graphic analysis (only for compound 4). Accordingly, the
13C NMR spectra of 3 and 4 each exhibit three resonances for
the ferrocendiyl moieties (d = 70.9, 72.7, and 84.6 ppm for 3;
d = 70.1, 72.3, and 92.0 ppm for 4). Furthermore, bis(silylene)
3 shows a singlet at d = 43.3 ppm in the 29Si NMR spectrum.
Single crystals of 4 suitable for an X-ray analysis were
obtained from hexane solution.[17] The crystals of 4 consist of
two conformational isomers, and one of them is shown in
Figure 1 (see also the Supporting Information). The molec-
ular structure of 4 confirms that the two N-donor-stabilized
germylene moieties are bonded to C1 and C8 of the
ferrocendiyl spacer. The Ge1–C1 and Ge2–C8 distances are
[Mo(CO)4{(CH2CMe2CH2)[NGeN(CH2tBu)-C6H4]2}],[20] we
explored the coordination behavior of potentially bidentate
ligands 3 and 4 toward CpCo complex fragments. In addition,
we wanted to test the reactivity of the resulting complexes as
precatalysts for Co-mediated [2+2+2] cycloaddition reac-
tions of phenylacetylene and acetonitrile. Suitable CpCoI
sources are accessible through treatment of cobalt(II) dibro-
mide (CoBr2) with sodium cyclopentadienide (NaCp) and
potassium graphite (KC8) in toluene or THF, resulting in the
generation of [CpCoILn] (L = toluene or THF) in solution.
Reactions of CpCo precursors with bis(silylene) 3 afforded
the desired bis(silylene) CoI complex 5 in 30% yield
(Scheme 3).[21] In the 29Si NMR spectrum, the coordination
of the bis(silylene) moieties to the Co atom leads to a drastic
downfield shift of the resonance of 5 (d = 82.0 ppm) in
À
1.991(2) and 1.9774(19) ꢀ, respectively. The Ge N bonds
(2.0261(16), 2.0247(17), 2.0162(16), and 2.0402(15) ꢀ) of 4 are
slightly shorter than those of precursor 2.[15] The GeII centers
of 4 adopt a pyramidal geometry with the sums of bond angles
of 255.93 and 256.678, respectively. The structural features of
the amidinate ligands in 4 are comparable to those reported
for related bis(germylenes), such as [{LGeN(Ph)}2][18] and
Scheme 3. Synthesis of bis(silylene)–CpCo complex 5 and its germa-
nium analogue 6.
L’GeEGeL’
(E = O,
S,
L = tBuC(NAr)2,
Ar= 2,6-
2
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
comparison to that of 3 (d = 43.3 ppm). Likewise, the
bis(germylene) CoI complex 6 could be successfully synthe-
sized from 4, and isolated in 61% yield. When the bis-
(silylene) 3 was allowed to react with two molar equivalents of
[CpCo(CO)2], the bis(silylene)–Co complex [{LSiCo-
(CO)Cp}2Fc] 7 could be obtained in 87% yield, accompanied
by elimination of CO (Scheme 4). The 29Si NMR spectrum of
7 shows a singlet resonance at d = 85.7 ppm, similar to that of
5. In the 13C NMR spectrum of 7, one characteristic signal for
Figure 3. Molecular structure of compound 6. Thermal ellipsoids are
drawn at 50% probability level. Hydrogen atoms and solvent mole-
cules are omitted for clarity. Selected bond lengths [ꢁ] and angles [8]:
Co1–Ge1 2.1967(6), Co1–Ge2 2.1979(6), Ge1–C6 1.965(4), Ge2–C11
1.961(4), Ge1–N1 2.020(3), Ge1–N2 2.031(3), Ge2–N3 2.041(3), Ge2–
N4 2.029(3); N1-Ge1-N2 64.90(11), N3-Ge2-N4 64.85(11), Ge1-Co1-
Ge2 92.79(2), C6-Ge1-Co1 132.80(10), C11-Ge2-Co1 131.40(10).
bis(2,6-diisopropylphenyl)imidazole-2-ylidene)
(2.1348(5) ꢀ),[23]
[{(NHC)Cl2Si}2Co(CO)3]+[CoCl3(thf)]À
(2.228(2) ꢀ),[24] [(LSiCl)2Co(CO)3]+[Co(CO)4]À (2.2060(6)
Scheme 4. Synthesis of bis(silylene)–Cp(CO)Co complex 7.
and 2.2017(6) ꢀ),[22] and [PNP{HSi Co(H)3(SiH2Ph)2}]
=
(PNP = {Ph2PCH2SiMe2}NÀ) (2.3990(7) ꢀ), respectively.[25]
À
the CO ligands appears at d = 207.8 ppm. Furthermore, the
IR spectrum of 7 exhibits a strong stretching band at n =
1888 cmÀ1 attributed to the carbonyl groups on the CoI atoms.
The observed CO stretching frequency of 7 is much lower
than those of [(LSiCl)Co(CO)Cp] (n = 1968 cmÀ1),[22] thus
indicating that the bis(silylene)-substituted ferrocene 3 is
a much stronger s donor than LSiCl 1.
The molecular structures of the heterobimetallic com-
plexes 5 (Figure 2) and 6 (Figure 3) were determined by
single-crystal X-ray diffraction analyses; both structures show
similar features. The silicon atoms in 5 and germanium atoms
Moreover, the Co Ge bonds of
6
(2.1967(6) ꢀ and
2.1979(6) ꢀ)
are
also
shorter
than
those
and
of
[[{(Me3Si)2N}2GeCo(CO)3]2]
(2.262(1) ꢀ)[26]
[h4-
{(tBu2MeSi)4Ge4}CoCp] (2.4616(3) ꢀ and 2.5036(3) ꢀ),[27]
À
respectively. To the best of our knowledge, the Co Ge
bonds in 6 are the shortest Co Ge bonds known to date. The
relatively short E Co bonds in 5 and 6 indicate that 3 and 4
are two of the strongest s-donor ligands in the series of
divalent silicon and germanium donors, respectively.
Numerous Co complexes have been successfully applied
as pre-catalysts in [2+2+2] cycloaddition reactions, which are
a powerful tool in organic chemistry to form arene and
heteroarene moieties.[28,29] Thus, we were interested in the
evaluation of the catalytic abilities of complexes 5 and 6 in
[2+2+2] cycloaddition reactions (Scheme 5).[28] A first set of
experiments was dedicated to the benchmark trimerization of
phenyl acetylene 8, and resulted in the quantitative formation
of isomers 9a and 9b in the presence of complex 5.
Surprisingly, similar attempts to employ 6 as precatalyst
showed no product formation, possibly because of a stronger
À
II
À
Figure 2. Molecular structure of compound 5. Thermal ellipsoids are
drawn at 50% probability level. Hydrogen atoms and solvent mole-
cules are omitted for clarity. Selected bond lengths [ꢁ] and angles [8]:
Co1–Si1 2.1252(14), Co1–Si2 2.1200(14), Si1–C6 1.894(5), Si2–C11
1.887(5), Si1–N1 1.898(4), Si1–N2 1.907(4), Si2–N3 1.904(4), Si2–N4
1.922(4); N1-Si1-N2 68.47(15), N3-Si2-N4 68.66(16), Si1-Co1-Si2
93.89(5), C6-Si1-Co1 132.54(14).
in 6 are four coordinated and show a distorted tetrahedral
À
geometry. The Co Si bond lengths of 5 (2.1252(14) and
2.1200(14) ꢀ) are similar to that of [(LSiCl)CoCp(CO)]
(2.1143(4) ꢀ).[22] Furthermore, the Co Si bonds of 5 are
shorter than those of [{(NHC)SiCl2}CoCp(CO)] (NHC = 1,3-
À
Scheme 5. Application of 5 and 6 as precatalysts in [2+2+2] cyclo-
addition reactions.
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
coordination of GeII donor centers to Co, which consequently
hampers the formation of an active site for substrate
coordination. Moreover, the formation of substituted pyri-
dines by [2+2+2] cycloaddition of phenyl acetylene and an
excess of acetonitrile was examined.[30] Again, catalytic
activity was exhibited for SiII–Co complex 5, while no product
formation could be observed with GeII–Co complex 6.
Interestingly, to some extend the application of
[CpCo(CO)2] as precatalyst gave substituted pyridines 10a
(3%) and 10b (11%) in lower yields.
In conclusion, the synthesis, characterization, and reac-
tivity of bis(silylenyl)- and bis(germylenyl)-substituted ferro-
cenes 3 and 4 have been reported. These ligands are easily
accessible by the reaction of 1,1’-dilithioferrocene with the
respective N-donor-stabilized metallene chlorides LECl (E =
Si, Ge), and represent two of the strongest bidentate MII-
based s-donor ligands, as shown by coordination of 3 and 4 to
CpCo to give the Co complexes 5 and 6, respectively. In
addition, the catalytic abilities of these ligands in Co-
mediated [2+2+2] cycloaddition reactions of phenylacetylene
and acetonitrile were probed. Unexpectedly, complex 5 is
catalytically active, while complex 6 is inactive, possibly
because of a stronger coordination of the GeII donor centers
to Co, which hampers the creation of an active site at Co for
the substrate coordination.
[8] R. S. Ghadwal, H. W. Roesky, K. Prçpper, B. Dittrich, S. Klein,
G. Frenking, Angew. Chem. 2011, 123, 5486; Angew. Chem. Int.
Ed. 2011, 50, 5374.
[9] H.-X. Yeong, H.-W. Xi, K. H. Lim, C.-W. So, Chem. Eur. J. 2010,
16, 12956.
[10] W. Wang, S. Inoue, S. Yao, M. Driess, J. Am. Chem. Soc. 2010,
132, 15890.
[11] W. Wang, S. Inoue, E. Irran, M. Driess, Angew. Chem. 2012, 124,
3751; Angew. Chem. Int. Ed. 2012, 51, 3691.
[12] For reviews on silylene complexes, see: a) T. D. Tilley in The
Chemistry of Organic Silicon Compounds (Eds.: S. Patai, Z.
Rappoport), Wiley, New York, 1989, chap. 24; b) M. S. Eisen in
The Chemistry of Organic Silicon Compounds (Eds.: Z. Rappo-
port, Y. Apeloig), Wiley, New York, 1998, chap. 35; c) H. Ogino,
Chem. Rec. 2002, 2, 291; d) M. Okazaki, H. Tobita, H. Ogino,
Dalton Trans. 2003, 494; e) R. Waterman, P. G. Hayes, T. D.
Tilley, Acc. Chem. Res. 2007, 40, 712.
[13] Bidentate (silyl)silylene transition metal complexes, see: a) H.
Tobita, H. Wada, K. Ueno, H. Ogino, Organometallics 1994, 13,
2545; b) K. Ueno, S. Ito, K. Endo, H. Tobita, S. Inomata, H.
Ogino, Organometallics 1994, 13, 3309; c) H. Wada, H. Tobita,
H. Ogino, Organometallics 1997, 16, 3870; d) H. Koshikawa, M.
Okazaki, S. Matsumoto, K. Ueno, H. Tobita, H. Ogino, Chem.
Lett. 2005, 34, 1412.
[14] C.-W. So, H. W. Roesky, J. Magull, R. B. Oswald, Angew. Chem.
2006, 118, 4052; Angew. Chem. Int. Ed. 2006, 45, 3948.
[15] S. Nagendran, S. S. Sen, H. W. Roesky, D. Koley, H. Grubmꢂller,
A. Pal, R. Herbst-Irmer, Organometallics 2008, 27, 5459.
[16] a) T. Sasamori, A. Yuasa, Y. Hosoi, Y. Furukawa, N. Tokitoh,
Organometallics 2008, 27, 3325; b) T. Sasamori, A. Yuasa, Y.
Hosoi, Y. Furukawa, N. Tokitoh, Bull. Chem. Soc. Jpn. 2009, 82,
793.
Received: March 19, 2012
Published online: && &&, &&&&
[17] CCDC 868352 (4), 868353 (4’), 868354 (5), and 868355 (6)
contain 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.
Keywords: cycloadditions · germanium · pyridines · silicon ·
transition-metal complex
.
[18] S. S. Sen, D. Kraft, D. Stern, H. W. Roesky, D. Stalke, Inorg.
Chem. 2010, 49, 5786.
[19] S.-H. Zhang, C.-W. So, Organometallics 2011, 30, 2059.
[20] A. V. Zabula, F. E. Hahn, T. Pape, A. Hepp, Organometallics
2007, 26, 1972.
[1] M. Denk, R. Lennon, R. Hayashi, R. West, A. V. Belyakov, H. P.
Verne, A. Haaland, M. Wagner, N. Metzler, J. Am. Chem. Soc.
1994, 116, 2691.
[21] Generation of the reactive CoI species, see: a) F. Baumann, E.
Dormann, Y. Ehleiter, W. Kaim, J. Krꢃcher, M. Kelemen, R.
Krammer, D. Saurenz, D. Stalke, C. Wachter, G. Wolmershꢃuser,
H. Sitzmann, J. Organomet. Chem. 1999, 587, 267; b) F. Hung-
Low, C. A. Bradley, Organometallics 2011, 30, 2636; c) F. Hung-
Low, J. P. Krogman, J. W. Tye, C. A. Bradley, Chem. Commun.
2012, 48, 368.
[2] For reviews on stable silylenes and their reactivities, see: a) M.
Asay, C. Jones, M. Driess, Chem. Rev. 2011, 111, 354; b) S. Yao,
Y. Xiong, M. Driess, Organometallics 2011, 30, 1748; c) M. Kira,
T. Iwamoto, S. Ishida, Bull. Chem. Soc. Jpn. 2007, 80, 258; d) N.
Takeda, N. Tokitoh, Synlett 2007, 2483; e) H. Ottosson, P. G.
Steel, Eur. J. Inorg. Chem. 2006, 1577; f) N. J. Hill, R. West,
J. Organomet. Chem. 2004, 689, 4165; g) M. Kira, J. Organomet.
Chem. 2004, 689, 4475; h) M. Okazaki, H. Tobita, H. Ogino,
Dalton Trans. 2003, 493; i) B. Gehrhus, M. F. Lappert,
J. Organomet. Chem. 2001, 617, 209; j) M. Haaf, T. A. Schme-
dake, R. West, Acc. Chem. Res. 2000, 33, 704.
[3] For reviews on bis(silylenes), see: S. S. Sen, S. Khan, S.
Nagendran, H. W. Roesky, Acc. Chem. Res., DOI: 10.1021/
ar2002073.
[4] a) S. S. Sen, A. Jana, H. W. Roesky, C. Schulzke, Angew. Chem.
2009, 121, 8688; Angew. Chem. Int. Ed. 2009, 48, 8536; b) C.
Jones, S. J. Bonyhady, N. Holzman, G. Frenking, A. Stasch,
Inorg. Chem. 2011, 50, 12315.
[22] A. Ramachandran, R. S. Ghadwal, H. W. Roesky, J. Hey, D.
Stalke, Chem. Asian J. 2012, 7, 528.
[23] J. Li, S. Merkel, J. Henn, K. Meindl, A. Dçrring, H. W. Roesky,
R. S. Ghadwal, D. Stalke, Inorg. Chem. 2010, 49, 775.
[24] S. Ghadwal, R. Azhakar, K. Prçpper, J. J. Holstein, B. Dittrich,
H. W. Roesky, Inorg. Chem. 2011, 50, 8502.
[25] M. Ingleson, H. Fan, M. Pink, J. Tomaszewski, K. G. Caulton,
J. Am. Chem. Soc. 2006, 128, 1804.
[26] A. Schnepf, Z. Anorg. Allg. Chem. 2006, 632, 935.
[27] V. Ya. Lee, Y. Ito, H. Yasuda, A. Sekiguchi, J. Am. Chem. Soc.
2011, 133, 5103.
[28] For reviews on [2+2+2] cycloadditions, see: a) H. Bçnnemann,
Angew. Chem. 1978, 90, 517; Angew. Chem. Int. Ed. Engl. 1978,
17, 505; b) K. P. C. Vollhardt, Angew. Chem. 1984, 96, 525;
Angew. Chem. Int. Ed. Engl. 1984, 23, 539; c) H. Bçnnemann,
Angew. Chem. 1985, 97, 264; Angew. Chem. Int. Ed. Engl. 1985,
24, 248; d) W. Hess, J. Treutwein, G. Hilt, Synthesis 2008, 3537;
e) J. Varela, C. Caꢄ, Synlett 2008, 2571; f) T. Shinata, K. J.
[5] D. Gau, R. Rodriguez, T. Kato, N. Saffon-Merceron, A.
De Cꢁzar, A. Baceiredo, Angew. Chem. 2011, 123, 1124;
Angew. Chem. Int. Ed. 2011, 50, 1092.
[6] Y. Wang, Y. Xie, P. Wei, R. B. King, H. F. Schaefer III, P. v. R.
Schleyer, G. H. Robinson, Science 2008, 321, 1069.
[7] B. Gehrhus, P. B. Hitchcock, M. F. Lappert, Z. Anorg. Allg.
Chem. 2005, 631, 1383.
4
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Tsuchikama, Org. Biomol. Chem. 2008, 6, 1317; g) C. J. Scheuer-
mann nꢅe Taylor, B. D. Ward, New J. Chem. 2008, 32, 1850; h) K.
Tanaka, Chem. Asian J. 2009, 4, 508, and references therein.
[29] For recent examples for the Co-catalyzed trimerization of phenyl
acetylene, see: a) C. C. Eichman, J. P. Bragdon, J. P. Stambuli,
Synlett 2011, 1109; b) A. Geny, N. Agenet, L. Iannazzo, M.
Malacria, C. Aubert, V. Gandon, Angew. Chem. 2009, 121, 1842;
Angew. Chem. Int. Ed. 2009, 48, 1810; c) G. Hilt, C. Hengst, W.
Hess, Eur. J. Org. Chem. 2008, 2293, and references therein.
[30] a) H. Nehl, Chem. Ber. 1994, 127, 2535; b) Y. B. Taꢆrit, Y. Diab,
B. Elleuch, M. Kerkani, M. Chihaoui, J. Chem. Soc. Chem.
Commun. 1986, 402; c) C. Brꢃndli, T. R. Ward, J. Comb. Chem.
2000, 2, 42 – 47; d) Y. Wakatsuki, H. Yamazaki, Bull. Chem. Soc.
Jpn. 1985, 58, 2715; e) C. Wang, X. Li, F. Wu, B. Wan, Angew.
Chem. 2011, 123, 7300; Angew. Chem. Int. Ed. 2011, 50, 7162.
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Chelate Ligands
W. Wang, S. Inoue,* S. Enthaler,*
M. Driess*
&&&& — &&&&
Bis(silylenyl)- and Bis(germylenyl)-
Substituted Ferrocenes: Synthesis,
Structure, and Catalytic Applications of
Bidentate Silicon(II)–Cobalt Complexes
Strong bite: The two respective SiII and
GeII centers in ferrocene-containing bis-
(silylene) and bis(germylene) ligands
1 can serve as excellent s donors, as
shown in the formation of Co complexes
2 (see scheme, Cp=h5-cyclopentadienyl).
Unexpectedly, only the bis(silylene)–Co
complex 2 is a precatalyst for the [2+2+2]
cycloaddition of phenylacetylene and
MeCN, possibly because of a stronger
coordination of the GeII donor centers
to Co.
Chelatliganden
W. Wang, S. Inoue,* S. Enthaler,*
M. Driess*
&&&& — &&&&
Bis(silylenyl)- and Bis(germylenyl)-
Substituted Ferrocenes: Synthesis,
Structure, and Catalytic Applications of
Bidentate Silicon(II)–Cobalt Complexes
Starker Biss: Die SiII- bzw. GeII-Zentren in
den Ferrocen-haltigen Bis(silylen)- und
Bis(germylen)-Liganden 1 kçnnen als
exzellente s-Donoren dienen, wie anhand
der Bildung der Co-Komplexe 2 gezeigt
wird (siehe Schema). Jedoch kann nur der
Bis(silylen)-Co-Komplex 2 als Prꢀkataly-
sator fꢂr die [2+2+2]-Cycloaddition von
Phenylacetylen und MeCN fungieren,
vermutlich aufgrund der stꢀrkeren Koor-
dination des GeII-Zentrums an Co.
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
These are not the final page numbers!

Angewandte
Chemie
Communications
Chelate Ligands
W. Wang, S. Inoue,* S. Enthaler,*
M. Driess*
&&&& — &&&&
Bis(silylenyl)- and Bis(germylenyl)-
Substituted Ferrocenes: Synthesis,
Structure, and Catalytic Applications of
Bidentate Silicon(II)–Cobalt Complexes
Strong bite: The two respective SiII and
GeII centers in ferrocene-containing bis-
(silylene) and bis(germylene) ligands
1 can serve as excellent s donors, as
shown in the formation of Co complexes
2 (see scheme, Cp=h5-cyclopentadienyl).
Unexpectedly, only the bis(silylene)–Co
complex 2 is a precatalyst for the [2+2+2]
cycloaddition of phenylacetylene and
MeCN, possibly because of a stronger
coordination of the GeII donor centers to
Co.
Angew. Chem. Int. Ed. 2012, 51, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7
These are not the final page numbers!