Heavy ferrocene: A sandwich complex containing Si and Ge atoms

Article abstract of DOI:10.1021/ja0740162

The heavy analogue of ferrocene (η⁵-Me₅C₅)Fe[η⁵-(CPh)(CH)Si₂Ge(SiMe^tBu₂)₃] 2, incorporating three heavy group 14 elements in one of the cyclopentadienyl rings, was synthesized by the reaction of the heavy lithium cyclopentadienide [η⁵-(CPh)(CH)Si₂Ge(SiMetBu₂)₃]·[Li⁺(thf)] 1-·[Li+(thf)] with (η⁵-Me₅C₅)Fe(acac) in THF. The crystal structure analysis of 2 revealed a remarkable flattening of the C₂Si₂Ge five-membered ring, taking place upon complexation, in which the lengths of all skeletal bonds were intermediate between those of the typical single and double bonds. The one-electron oxidation of 2 occurred at more negative potentials than that of the parent (η⁵-Me5C5)2Fe: -0.53 versus -0.32 V. The fragment MO and energy decomposition analyses revealed that the bonding in 2 is predominantly electrostatic, whereas the most important covalent bonding contribution comes from the heavy Cp^- ligand to CpFe⁺ fragment π-donation. Copyright

Full text of DOI:10.1021/ja0740162

Published on Web 08/03/2007
Heavy Ferrocene: A Sandwich Complex Containing Si and Ge Atoms
Vladimir Ya. Lee,§ Risa Kato,§ Akira Sekiguchi,*,§ Andreas Krapp, and Gernot Frenking*,
Department of Chemistry, Graduate School of Pure and Applied Sciences, UniVersity of Tsukuba, Tsukuba,
Ibaraki 305-8571, Japan, and Fachbereich Chemie, Philipps-UniVersita¨t Marburg,
Hans-Meerwein-Strasse D-35032 Marburg, Germany
Received June 3, 2007; E-mail: sekiguch@chem.tsukuba.ac.jp; frenking@chemie.uni-marburg.de
Ferrocene, the archetypal sandwich iron complex (η5-H5C5)2Fe
Scheme 1
synthesized by Kealy and Pauson in 1951,1 is the first and most
famous member of the huge family of metallocenes, which
revolutionized the field of organometallic chemistry.2 Apart from
its fascinating structure and unusual bonding mode, ferrocene has
found extensive applications in the field of material science:
asymmetric catalysis, large scale olefin polymerization, and lumi-
nescent and fluorescent materials. The heteroferrocenes, incorporat-
ing skeletal atoms other than carbon in the cyclopentadienyl rings,
are most widely represented by the derivatives of group 15 elements
and 69.1 ppm (SiGe and SiCH). Such appreciable shielding can be
(E ) P, As, Sb, Bi).3 In sharp contrast, the examples of the ferrocene
attributed to two factors: a higher degree of π-delocalization in
analogues containing heavy group 14 elements are rather scarce:
the C2Si2Ge ligand of ferrocene 2 compared with that of starting
two Ru complexes with one heteroatom (η5-Me5C5)Ru[η5-Me4C4-
1-‚[Li+(thf)] and δ-backdonation from Fe to the ligand.
ESi(SiMe3)3] (E ) Si, Ge)4a,b and one Fe complex with two
The crystal structure of the heavy ferrocene 2 showed that both
heteroatoms [η5-Me4C4GeSi(SiMe3)3]2Fe.4c The most intriguing are
ligands are pentahaptocoordinated to the Fe atom, the two rings,
derivatives consisting entirely of heavy group 14 elements, of which
C5 and C2Si2Ge, are nearly coplanar, and the Fe atom is situated
persilaferrocene (η5-H5Si5)2Fe was studied computationally at HF
between them at 1.686 Å from the least-squares C2Si2Ge plane and
and MP2 levels to reveal the symmetrical D5d structure, in which
1.705 Å from the least-squares C5 plane (Figure 1).9 The heavy
the Fe-ligand binding energy is smaller than that in the parent
ferrocene 2 has a staggered conformation of the two five-membered
ferrocene: 113.6 versus 144.1 kcal/mol.5 Experimentally, however,
rings because of significant steric interaction between their sub-
the synthesis of such an attractive compound has not yet been
stituents.11 The manifestation of the cyclic π-delocalization in the
accomplished. In this paper, we report the synthesis of the first
heavy Cp ligand of 2 was clearly seen by its geometrical features.
ferrocene derivative, incorporating three heavy group 14 elements
Thus, the skeletal bond lengths in 2 are similar to those of aromatic
(two Si and one Ge) in one of the cyclopentadienyl rings, thus
1-‚[Li+(thf)]6, and markedly distinct from those of neutral 1,1,2,3-
representing a closest approach to the target ferrocenes (η5-R5E5)2-
tetrakis(di-tert-butylmethylsilyl)-4-phenyl-1,2-disila-3-germacyclo-
Fe (E ) Si-Pb), and discuss its bonding situation.
penta-2,4-diene 3,12 precursor for 1-‚[Li+(thf)]), in which SidGe
Because our initial attempts to synthesize a heavy ferrocene
and CdC double bonds are localized (below the endocyclic bond
derivative by the classical coupling of either FeCl2 or FeCl2(thf)1.44
lengths of 2 versus those of 1-‚[Li+(thf)] and 3 are shown): 2.3038-
complex with the heavy lithium cyclopentadienide 1-‚[Li+(thf)]6
(9) versus 2.3220(5) and 2.250(1) Å (Si1-Ge1), 2.2465(11) versus
were unsuccessful, we have employed a novel strategy based on
2.2403(7) and 2.364(1) Å (Si1-Si2), 1.827(3) versus 1.8269(18)
utilization of the Cp*Fe(acac) complex (Cp* ) η5-Me5C5, acac )
and 1.888(3) Å (Si2-C2), 1.419(4) versus 1.402(2) and 1.343(5)
2,4-pentanedionate) as a convenient source of the Cp*Fe fragment.7
Å (C1-C2), 1.924(3) versus 1.9303(17) and 1.972(3) Å (Ge1-
Cp*Fe(acac), generated in situ by the reaction of Fe(acac)2 with
C1). Consequently, all skeletal bonds in the heavy Cp ligand in 2
Cp*Li in THF,8 was reacted with the stoichiometric amount of
are just intermediate between the typical single and double bonds,
1-‚[Li+(thf)] in THF at -78 °C to produce the target (η5-Me5C5)-
implying effective 6π-electron delocalization. Another manifestation
Fe[η5-(CPh)(CH)Si2Ge(SiMetBu2)3] derivative 2 (Scheme 1). The
of such important delocalization is the striking flattening of the
heavy ferrocene 2 was isolated by the silica gel chromatography
heavy Cp ring upon complexation. This was clearly seen in the
in a glovebox followed by recrystallization from pentane as dark-
planarization around all skeletal atoms of heavy Cp ring of 2
red crystals in 31% yield.9 The NMR spectral data of 2 are in line
compared with those of 1-‚[Li+(thf)]6 (the sums of the bond angles
with the proposed pentahaptocoordination of the C2Si2Ge ligand
are shown): Ge1 (360.00 versus 342.89°), Si1 (355.63 versus
to Fe atom. Thus, the 1H NMR resonance of the skeletal CH proton
350.75°), Si2 (358.96 versus 357.803°), C1 (359.70 versus 359.79°),
was observed as a singlet at 4.49 ppm, in the region diagnostic of
C2 (360.00 versus 359.94°). Accordingly, the sum of the interior
Cp ligands η5-coordinated to transition metals.10 Moreover, all
bond angles of the heavy Cp ring of 2 is greater than that of 1-‚
skeletal C and Si atoms of the heavy cyclopentadienyl ligand in 2
[Li+(thf)]6: 538.0 versus 536.3°, and deviation of the skeletal atoms
were markedly shielded upon complexation, compared with those
from the C2Si2Ge least-squares plane in 2 is smaller than that in
of the starting 1-‚[Li+(thf)]6 (calculated GIAO values for 2 are given
1-‚[Li+(thf)]. The computational studies on the real compound with
in parentheses): 80.5(88.6) versus 143.2 ppm (CH), 108.1(125.1)
tBu2MeSi-substituents and on the Me3Si-, H3Si-, and H-substituted
versus 181.4 ppm (CPh), -49.3(-12.2) and 1.1(10.3) versus 54.4
models showed, that upon successive decrease in the bulkiness and
increase in the electronegativity of substituents, the heavy Cp ring
becomes progressively distorted, and the conformation of ligands
§ University of Tsukuba.
Philipps-Universita¨t Marburg.
9
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J. AM. CHEM. SOC. 2007, 129, 10340-10341
10.1021/ja0740162 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
2′ has only C1 symmetry, however we were able to identify those
orbitals, which are comparable to the σ-, π-, and δ-orbitals of
ferrocene. The fragment molecular orbital (FMO) analysis of ADF13
revealed that the most important contribution to the overall bonding
of 2′ comes from the strong π-donation from the occupied 1a and
2a orbitals of heavy Cp- ligand to the vacant e1 orbitals of the
CpFe+ unit (Figure 2). Mixing of the σ- and δ-type orbitals between
CpFe+ and heavy Cp- ligand is much less important. A previous
energy decomposition analysis (EDA) of ferrocene14 disclosed that
the covalent bonding between the CpFe+ and Cp- units comes from
63.8% π-, 14.6% σ-, and 21.6% δ-bonding,14b thus suggesting that
ferrocene and 2′ have very similar bonding situations. Since the
breakdown of the orbital interaction term in the EDA analysis of
2′ was impossible owing to the lack of symmetry, the EDA calcu-
lations considering interaction of CpFe+ and heavy Cp- ligand were
carried out by deleting the vacant orbitals of one fragment. The
results indicate that electrostatic bonding (52.4%) dominates over
covalent bonding (47.6%), and the heavy Cp- f CpFe+ donation
contributes 54.5% to the overall orbital interaction, whereas CpFe+
f Cp- backdonation contributes 45.5%. Since the former contribu-
tion can be identified as π-donation, it can be concluded that the
covalent bonding between CpFe+ and heavy Cp- in 2′ (and probably
in 2) is best described in terms of π-donation from the heavy Cp
ligand to the metal. The reactivity of heavy ferrocene 2, including
formation of charge-transfer complexes, is under current investiga-
tion.
Figure 1. ORTEP drawing of 2 (hydrogen atoms are not shown). Selected
bond lengths (Å): Ge1-Si1 ) 2.3038(9), Si1-Si2 ) 2.2465(11), Ge1-
C1 ) 1.924(3), Si2-C2 ) 1.827(3), C1-C2 ) 1.419(4), Ge1-Fe1 )
2.5313(5), Si1-Fe1 ) 2.4691(9), Si2-Fe1 ) 2.4682(8), C1-Fe1 ) 2.170-
(3), C2-Fe1 ) 2.114(3), C36-Fe1 ) 2.085(3), C37-Fe1 ) 2.093(3),
C38-Fe1 ) 2.094(3), C39-Fe1 ) 2.116(3), C40-Fe1 ) 2.102(3). Selected
bond angles (deg): C1-Ge1-Si1 ) 98.71(9), Ge1-Si1-Si2 ) 96.34(4),
Si1-Si2-C2 ) 99.49(10), C2-C1-Ge1 ) 117.8(2), Si2-C2-C1 )
125.7(2).
Acknowledgment. This work was supported by a Grant-in-Aid
for Scientific Research (Grant Nos. 17550029, 19105001, 19020012,
19022004, 19029006) from the Ministry of Education, Science,
Sports, and Culture of Japan.
Supporting Information Available: Experimental procedure,
spectral and crystallographic data for 2 including atomic positional and
thermal parameters, detailed description of theoretical calculations. This
material is available free of charge via the Internet at http://pubs.acs.org.
References
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Organomet. Chem. 2001, 637-639 (issue dedicated to 50th anniversary
of ferrocene synthesis).
Figure 2. MO correlation diagram for the interaction between the model
heavy Cp ligand [(CPh)(CH)Si2Ge(SiH3)3]- and the CpFe+ fragment.
(3) See, for example: (a) Ashe, A. J., III; Kampf, J. W.; Al-Taweel, S. M.
Organometallics 1992, 11, 1491. (b) Ashe, A. J., III; Kampf, J. W.; Pilotek,
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Al-Ahmad, S.; Pilotek, S.; Puranik, D. B.; Elschenbroich, C.; Behrendt,
A. Organometallics 1995, 14, 2689. (d) Black, S. J.; Francis, M. D.; Jones,
C. J. Chem. Soc., Dalton Trans. 1997, 2183. (e) Sava, X.; Ricard, L.;
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W. P.; Dysard, J. M.; Tilley, T. D.; Rheingold, A. L. Organometallics
2002, 21, 1734.
(5) Kudo, T.; Nagase, S. J. Mol. Struct. (Theochem) 1994, 311, 111.
(6) Lee, V. Ya.; Kato, R.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc.
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(9) For the experimental procedure, spectral and crystal data of 2, see
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Germany, 2006.
(11) Cp*2Fe itself also has a staggered conformation; see ref 10.
(12) Lee, V. Ya.; Ichinohe, M.; Sekiguchi, A. J. Am. Chem. Soc. 2000, 122,
12604.
(13) All calculations were performed using TURBOMOLE 5.8 program
package at the RI-BP86/def2-TZVPP level and the ADF2005.1 program
at the BP86/TZ2P level.
(14) (a) Rayo´n, V. M.; Frenking, G. Organometallics 2003, 22, 3304. (b) Lein,
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changes from staggered to eclipsed.13 Certainly, such a phenomenon
has both steric (flattening of the ring upon the introduction of very
bulky substituents) and electronic (hyperconjugative stabilizing
interaction: π(heavy Cp ligand)-σ*(Si-C(tBu) bonds of substit-
uents) origins.
The CV measurement of 2 displayed two irreversible oxidation
waves at Ep(1) ) -0.53 V and Ep(2) ) -0.24 V (vs Ag/Ag+, CH2-
n
Cl2, 0.1 M Bu4NClO4). The first oxidation process, apparently
corresponding to the formation of a heavy ferrocene cation-radical,
proceeded at significantly more negative potential (-0.53 V) than
the corresponding one-electron oxidation of decamethylferrocene
Cp*2Fe (-0.32 V, reversible) measured under the same conditions,
and even more negative than the oxidation of [η5-Me4C4GeSi-
(SiMe3)3]2Fe4c (-0.45 V, irreversible, vs SCE, CH2Cl2, 0.1 M
nBu4NClO4). This suggests that the heavy Cp ligand in 2 is the
more powerful electron donor to the Fe atom compared with Cp*
and even germacyclopentadienyl ligand [η5-Me4C4GeSi(SiMe3)3].
To get an insight into the bonding nature of 2, we calculated the
model compound CpFe[(CPh)(CH)Si2Ge(SiH3)3] 2′, for which the
interactions between the two fragments, heavy Cp ligand [(CPh)-
(CH)Si2Ge(SiH3)3]- and CpFe+ unit, were analyzed.13 The complex
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