Anionic iron complexes with a bond between an ate-type pentacoordinated germanium and an iron atom

Article abstract of DOI:10.1021/om300915y

The first stable anionic iron(0) complexes bearing an ate-type pentacoordinated germanium(IV) ligand were synthesized. The X-ray crystallographic analysis shows trigonal-bipyramidal and piano-stool geometries of germanium and iron, respectively. The complexes have moderately electron-rich iron centers and polar Ge-Fe bonds which can be cleaved by oxidation.

Full text of DOI:10.1021/om300915y

Communication
pubs.acs.org/Organometallics
Anionic Iron Complexes with a Bond between an Ate-Type
Pentacoordinated Germanium and an Iron Atom
Naokazu Kano,*,† Naohito Yoshinari,† Yusuke Shibata,† Mariko Miyachi,† Takayuki Kawashima,*,†,∥
Masaya Enomoto,‡ Atsushi Okazawa,§ Norimichi Kojima,§ Jing-Dong Guo,¶ and Shigeru Nagase¶
†Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
‡Department of Chemistry, Faculty of Science Division I, Tokyo University of Science, Kagurazaka, Shinjuku-ku, Tokyo 162-8601,
Japan
§Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo, Komaba, Meguro-ku, Tokyo
153-8902, Japan
¶Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-Nishihiraki-cho, Sakyo-ku, Kyoto 606-8103, Japan
S
* Supporting Information
ABSTRACT: The first stable anionic iron(0) complexes bearing an ate-
type pentacoordinated germanium(IV) ligand were synthesized. The X-ray
crystallographic analysis shows trigonal-bipyramidal and piano-stool
geometries of germanium and iron, respectively. The complexes have
moderately electron-rich iron centers and polar Ge−Fe bonds which can
be cleaved by oxidation.
here have been many synthetic challenges with transition-
metal (TM) complexes bearing a ligand containing a
heavier group 14 element in an unusual coordination state this
past quarter century.1 In usual group 14 element−TM
complexes, the group 14 element is in a tetracoordinated
state (Chart 1, type I)1c and acts as a mild electron-donating
reports on neutral TM complexes bearing a pentacoordinated
or hexacoordinated group 14 element ligand,5 the TM was
frequently found to undergo σ-donation from a filled d(metal)
orbital to the vacant σ*(Si or Sn) orbital, and the group 14
element had a two-electron-accepting character (types IV and
VI).6 The σ donation was also found in the anionic iron
complex with an ate-type aluminum ligand, (Et4N)+[Ph3AlFe-
(Cp)(CO)2]−, in which the iron moiety plays a role of a Lewis
base.7
T
Chart 1
We envisaged that an ate-type pentacoordinated group 14
element ligand in an anionic complex, [R4E−MLn]− (type V),
would be sterically bulky and electron rich, both of which are
important properties for TM complexes to act as efficient
catalysts for cross-coupling reactions.8 However, such an
anionic TM complex has not been reported to date, as far as
we know, except for a few examples incorporated in a closo-
dodecaborane framework.9 Given the success in stabilizing
anionic pentacoordinated silicon10 and germanium species11 by
using a bidentate ligand derived from hexafluorocumyl
alcohol,12 a spirogermane bearing the ligand is expected to be
a good precursor for [R4Ge−MLn]− complexes. This paper
focuses on the structure and electronic properties of the anionic
iron complexes with a bond between an ate-type pentacoordi-
nated germanium(IV) and an iron(0) atom.
ligand.2 Several TM complexes with a low-coordinated group
14 element ligand have been isolated to date, and some of them
feature multiple bonds to the metal: e.g., R2EMLn and RE
MLn, where E is Si, Ge, Sn, or Pb (types II and III).1b,d,e In
these complexes, the heavier group 14 element species is used
as a ligand to mimic conventional carbon ligands such as alkyl,
alkylidene, and alkylidyne ligands. However, to create TM
complexes with novel electronic properties, it is desirable to
synthesize complexes bearing a ligand in the coordination state
which is unavailable to carbon. Heavier group 14 elements can
also adopt a high-coordinated state such as neutral and anionic
pentacoordinated structures.3 A few phosphine-bridged nickel
and palladium complexes exhibit weak intramolecular electronic
interaction between the TM and the silicon atom.4 In recent
Reaction of spirogermane 111 bearing two sets of the
bidentate ligand (−C6H3-4-CH3-2-C(CF3)2O−, denoted as Rf
hereafter) with Na+[Fe(Cp)(CO)2]− (2) in THF gave
Na+[Rf2GeFe(Cp)(CO)2]−·3THF (3a) in 58% yield (Scheme
1) (the Fe(Cp)(CO)2 fragment is denoted as Fp hereafter). A
Received: September 27, 2012
Published: November 29, 2012
© 2012 American Chemical Society
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cation-exchange reaction of 3a with benzyltrimethylammonium
chloride gave (PhCH2NMe3)+(Rf2GeFp)− (3b) in 89% yield.
are both narrower than those of the butylgermanate bearing
two sets of a similar bidentate ligand (173.8(2) and 119.2(3)°,
respectively),11 a property reflecting the steric repulsion
between the germanium ligand and the Fp moiety. Meanwhile,
the Ge1−Fe1 bond (2.4065(7) Å) in 3b shows no meaningful
elongation.
Scheme 1
In the 1H and 13C NMR spectra in THF-d8, the ortho proton
signal of the benzene ring of the Rf ligand was observed in a
low-field region (δH 8.26 ppm), as a result of the electric dipole
of the polar apical Ge−O bonds.14 The NMR data prove the
persistence of the TBP structure with dissociation of neither
Ge−O nor Ge−Fe bonds in the solution state.
Complex 3b was diamagnetic, with the SQUID measurement
showing a very low magnetic susceptibility (4.02 × 10−4 cm3
mol−1), which eliminates the monovalence or trivalence of iron.
XPS of 3b revealed the binding energy of Fe 2p3/2 of 3b as
708.2 eV (Figure 2A), and it is situated between that of bare
The crystal structures of 3a,b were determined by X-ray
crystallographic analysis (Figure 1A,B). Both of the complexes
Figure 2. Photoelectron spectra of (A) Fe 2p and (B) Ge 3d of 3b.
iron with zero valence (707.4 eV) and that of [FeCp(CO)2]2
with monovalence (708.5 eV).15 The valence of iron in 3b is
concluded to be zero in view of the result of the SQUID
measurement. In the case of Ge 3d (Figure 2B), the binding
energy of 3b (31.4 eV) is situated between that of Ph4Ge (31.0
eV) and that of GeO2 (32.5 eV), indicating tetravalence of
germanium.16
Figure 1. Thermal ellipsoid plots of complexes 3a (A) and 3b (B).
(C) CPK model view of the anion of 3b. Selected bond lengths (Å)
and angles (deg) of 3a: Ge1−Fe1, 2.404(2); Ge1−C1, 1.977(5);
Ge1−C11, 1.981(5); Ge1−O1, 2.148(4); Ge1−O2, 1.995(4); Fe−
C21, 1.752(6); Fe−C22, 1.763(6); C1−Ge1−Fe1, 117.44(15); C11−
Ge1−Fe1, 123.70(14); O1−Ge1−O2, 166.12(13); O1−Ge1−Fe1,
97.04(12); C21−Fe1−C22, 91.1(2); C21−Fe1−Ge1, 90.59(18);
C22−Fe1−Ge1, 89.57(18). Selected bond lengths (Å) and angles
(deg) of 3b: Ge1−Fe1, 2.4065(7); Ge1−C1, 1.969(4); Ge1−C11,
1.966(3); Ge1−O1, 2.043(2); Ge1−O2, 2.047(2); Fe−C21, 1.744(4);
Fe−C22, 1.748(4); C1−Ge1−Fe1, 122.96(10); C11−Ge1−Fe1,
123.31(10); O1−Ge1−O2, 169.37(10); O1−Ge1−Fe1, 97.65(7);
C21−Fe1−C22, 92.43(17); C21−Fe1−Ge1, 93.02(12); C22−Fe1−
Ge1, 85.91(12).
The s-electron density of the iron of 3b was evaluated by
57Fe Mossbauer spectroscopy at 78 K, and the spectrum
̈
showed a quadrupole doublet with an isomer shift of 0.084(2)
mm s−1 and a quadrupole splitting of 1.744(4) mm s−1 relative
to α-Fe (Figure 3A). In the case of the 57Fe Mossbauer
̈
feature a germanium−iron bond, in which germanium and iron
show trigonal-bipyramidal (TBP) and piano-stool geometries,
respectively. On the one hand, the anionic part of 3a interacts
with the sodium ion, Na1, to form an ion pair. As a result, the
Ge1−O1 bond of 3a (2.148(4) Å) is significantly longer than
the Ge1−O2 bond (1.995(4) Å), although it is within the sum
of the van der Waals radii. On the other hand, the anionic part
in 3b shows no contact with the benzyltrimethylammonium
ion. The Ge1 atom is bonded to one iron, two carbon, and two
oxygen atoms. This is the first example, as far as we know, of an
anionic iron complex bearing an ate-type pentacoordinated
germanium ligand.
The germanium ligand is bulky enough to cover one side of
the anion part of 3b (Figure 1C). This pentacoordinated
germanium ligand should be bulkier than the trimethylgermyl
group, which is known to be a bulky group with a high A value
(A = 2.07 kcal mol−1).13 The O1−Ge1−O2 angle
(169.37(10)°) and the C1−Ge1−C11 angle (113.62(14)°)
Figure 3. (A) 57Fe Mossbauer spectrum of 3b. (B) IR spectrum of 3b
in KBr.
̈
spectrum, the isomer shift decreases with increasing s-electron
density at the iron nucleus and decreasing d-electron density
responsible for the shielding effect. The isomer shift of 3b is
almost the same as those of alkyl-substituted Fp complexes
(0.076−0.105 mm s−1),17 whose small isomer shifts suggest a
large s-electron density in the iron of 3b and small d-electron
density because of the π back-donation.
The π back-donation from iron to the carbonyl ligands is
reflected in the IR spectrum. The IR spectrum of 3b showed
two carbonyl stretching bands at 1943 and 1995 cm−1 (Figure
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3B). The values are smaller than those of the cationic
[LFe(CO)2(Cp)]+ complexes, where L is a phosphine or
carbene ligand (νas, 2001−2004 cm−1; νs, 2048−2050 cm−1),18
while they are almost the same as those of Ph3GeFp (4) (νas,
1940 cm−1; νs, 1995 cm−1)19 and other alkyl derivatives (νas,
1935−1947 cm−1; νs, 1996−2005 cm−1).20 The π back-
donation from iron to the π*(CO) orbital in 3b is strong
and almost the same as that in the tetracoordinated germyl and
alkyl derivatives. A possible reason for the strong π back-
donation is the high electron density at iron, because 3b is an
anionic complex.
4C,D). In the isoelectronic neutral complexes with a hyper-
valent phosphorus ligand, π back-donation from the iron to the
antibonding orbital of the apical three-centered four-electron
bond was advocated.21 In 3b, a π-type orbital was found
between the p orbital of Ge on the equatorial plane and the d
orbital of Fe (HOMO-14, −8.00 eV) (Figure 4E). The
calculated Ge−Fe homolytic bond dissociation energy (38.9
kcal mol−1) in 3−, the anion part of 3b, is a little bit smaller
than that (42.1 kcal mol−1) of 4. This shows the slightly weak
orbital interactions between germanium and iron.
The NBO analysis of the optimized structure of 3− shows
that the σ bonding orbital forms from an sp1.93 hybrid on
germanium interacting with an sp1.24d3.56 hybrid on iron. The
polarity of the bonding σ(Ge−Fe) NBO shows 68.3%
polarization to iron. The Wiberg bond index of the Ge−Fe
bond is 0.601, which is similar to the value of 0.609 in 4.
Therefore, the germanium−iron bond of 3− is a polar bond, as
represented by Geδ+−Feδ−. The polarization level of the Ge−Fe
Chart 2
bond is almost the same as that of 4 (Ge(sp2.68)−Fe(sp1.27d3.43
)
with 67.3% polarization to iron), although the sp2 and sp3
hybridization states of germanium in 3− and 4, respectively, are
different from each other. The natural population analysis of
charge distribution of selected atoms of 1, 2−, 3−, and 4 was
carried out. In 3−, the germanium atom is positively charged
(qGe, = +1.76), being more positive than that in neutral
tetracoordinated germanium complex 4 (qGe = +1.54) and
slightly less positive than that in 1 (qGe = +1.93). All of the
atoms bonding to the germanium in 3− are negatively charged
(qFe = −1.48; qC = −0.46; qO = −0.87). The negative charge of
Despite the substituent effects similar to electron-donating
ligands on the Fp moiety, the germanium ligand in 3b may be
recognized as an electron acceptor, because the synthetic
method for 3b shows that electrons are provided from the Fp
fragment to form the Ge−Fe bond. We performed DFT
calculations on 3b at the B3PW91/6-311+G(d) level for all
atoms to obtain information on orbital interactions and the
charge distribution.
The optimized structure reproduced the crystal structure well
(Ge−Fe, 2.410 Å). The HOMO (energy level, −5.64 eV) of 3b
is mainly the σ-bonding orbital of the Ge−Fe bond (Figure
4A). In LUMO+3 (−0.74 eV), a contribution of the
corresponding σ*-nonbonding orbital is observed (Figure
4B). The HOMO-10 (−7.39 eV) and HOMO-13 (−7.58 eV)
represent σ-type and skewed π-type bonding orbitals,
respectively, in the apical O−Ge−O bond linkage (Figure
the iron atom of 3− is almost the same as that of 4 (qFe
−1.50) and slightly more negative than that in 2− (qFe
=
=
−1.19). The total anion charge (q = −1) of 3− is shared by the
Rf2Ge and Fp moieties, their charges being −0.69 and −0.31,
respectively. Supposing the formation process of 3− from 1 and
2−, the negative charges of both Cp and CO ligands of 2−
transfer to 1 and the iron atom, resulting in the negative
charges in 3− as mentioned above. The charge of the Fp moiety
of 3− is intermediate between the values of −0.26 in 4 and
−0.34 in Me3GeFp. Thus, the Fp moiety of 3− is in a
moderately electron rich state. The electronic properties are
reflected in the IR and Mossbauer spectra.
̈
Reaction of 3b with methyllithium caused nucleophilic attack
on the germanium and resulted in Ge−Fe bond fission to give
methylgermanate 5 (75%). Upon treatment with water in THF
at 50 °C for 14 days, 3b underwent hydrolysis to give
hydroxygermanate 6 together with [Fe(Cp)(CO)2]2, which
should form from the intermediate Fe(Cp)(CO)2H.22 Despite
the modest resistance to hydrolysis and thermal stability under
an inert gas atmosphere, 3b is air sensitive, suggesting its
oxidation. CV of 3b exhibited irreversible oxidation waves at
the peak potential of the anodic current of +0.31 V versus Ag+/
Ag in THF. This was much lower than that of LFe(Cp)(CO)2
(ranging from +1.335 to +1.421 V), where L is Me, SiMe3, and
GeMe3, representing the easily oxidizable characteristics of the
anionic complex.23 In fact, oxidation of 3b with silver
perchlorate gave 1 quantitatively. The oxidation corresponds
to withdrawal of an electron from the σ-bonding orbital, and
thus it leads to Ge−Fe bond fission.
In summary, we have synthesized the first anionic iron
complexes bearing an ate-type pentacoordinated germanium
ligand. This is important as the construction of a new type of
bond between a transition metal and a pentacoordinated
heavier group 14 element in the ate-type structure which
Figure 4. Selected molecular orbitals of 3b obtained by DFT
calculations (isovalue 0.03): (A) HOMO; (B) LUMO+3; (C)
HOMO-10; (D) HOMO-13; (E) HOMO-14.
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moderately electron rich iron fragment in line with Z-type
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from iron to the π*(CO) orbital was strong and almost the
same as that in the electron-donating germyl and alkyl
derivatives. The germanium ligand was found to be bulky.
Such properties will provide a new choice of ligands for TM
catalysts.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Text, tables, figures, and CIF files giving experimental and
computational details and crystallographic data. This material is
available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: kano@chem.s.u-tokyo.ac.jp (N.K.); Takayuki.
Kawashima@gakushuin.ac.jp (T.K.).
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Present Address
∥Faculty of Science, Gakushuin University, Mejiro, Toshima-ku,
Tokyo 171-8588, Japan.
(10) Kano, N.; Miyake, H.; Sasaki, K.; Kawashima, T.; Mizorogi, N.;
Nagase, S. Nat. Chem. 2010, 2, 112−116.
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Research Hub for Advanced Nano Character-
ization, The University of Tokyo, for the spectral measure-
ments. This work was supported by the Global COE program
(Chemistry Innovation), Coordination Programming, and
Specially Promoted Research from MEXT, Inamori foundation,
Noguchi foundation, Itoh science foundation, and Mitsubishi
foundation.
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