Highly electron-rich pincer-type iron complexes bearing innocent bis(metallylene)pyridine ligands: Syntheses, structures, and catalytic activity

Article abstract of DOI:10.1021/om500966t

The first neutral bis(metallylene)pyridine pincer-type [ENE] ligands (E = Si^II, Ge^II) were synthesized, and their coordination chemistry and reactivity toward iron was studied. First, the unprecedented four-coordinate complexes κ²E,E'-[ENE]FeCl₂ were isolated. Unexpectedly and in contrast to other related pyridine-based pincer-type Fe(II) complexes, the N atom of pyridine is reluctant to coordinate to the Fe(II) site due to the enhanced α-donor strength of the E atoms, which disfavors this coordination mode. Subsequent reduction of κ²Si,Si'-[SiNSi]FeCl₂ with KC₈ in the presence of PMe₃ or direct reaction of the [ENE] ligands using Fe(PMe₃)₄ produced the highly electron-rich iron(0) complexes [ENE]Fe(PMe₃)₂. The reduction of the iron center substantially changes its coordination features, as shown by the results of a single-crystal X-ray diffraction analysis of [SiNSi]Fe(PMe₃)₂. The iron center, in the latter, exhibits a pseudosquare pyramidal (PSQP) coordination environment, with a coordinative (pyridine)-Rfnet→Fe bond, and a trimethylphosphine ligand occupying the apical position. This geometry is very unusual for Fe(0) low-spin complexes, and variable-temperature ¹H and ³¹P NMR spectra of the [ENE]Fe(PMe₃)₂ complexes revealed that they represent the first examples of configurationally stable PSQP-coordinated Fe(0) complexes: even after heating at 70 °C for >7 days, no changes are observed. The substitution reaction of [ENE]Fe(PMe₃)₂ with CO resulted in the isolation of [ENE]Fe(CO)₂ and the hitherto unknown κ²E,E'-[ENE]Fe(CO)₂L (L = CO, PMe₃) complexes. All complexes were fully characterized (NMR, MS, XRD, IR, and ⁵⁷Fe M?ssbauer spectroscopy), showing the highest electron density on the iron center for pincer-type complexes reported to date. DFT calculations and ⁵⁷Fe M?ssbauer spectroscopy confirmed the innocent behavior of these ligands. Moreover, preliminary results showed that these complexes can serve as active precatalysts for the hydrosilylation of ketones.

Full text of DOI:10.1021/om500966t

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Article
pubs.acs.org/Organometallics
Highly Electron-Rich Pincer-Type Iron Complexes Bearing Innocent
Bis(metallylene)pyridine Ligands: Syntheses, Structures, and
Catalytic Activity
Daniel Gallego, Shigeyoshi Inoue, Burgert Blom, and Matthias Driess*
Department of Chemistry: Metalorganics and Inorganic Materials, Technische Universitat Berlin, Straße des 17. Juni 135, Sekr. C2,
̈
10623 Berlin, Germany
S
* Supporting Information
ABSTRACT: The first neutral bis(metallylene)pyridine pincer-type [ENE]
ligands (E = SiII, GeII) were synthesized, and their coordination chemistry
and reactivity toward iron was studied. First, the unprecedented four-
coordinate complexes κ2E,E′-[ENE]FeCl2 were isolated. Unexpectedly and
in contrast to other related pyridine-based pincer-type Fe(II) complexes, the
N atom of pyridine is reluctant to coordinate to the Fe(II) site due to the
enhanced σ-donor strength of the E atoms, which disfavors this
coordination mode. Subsequent reduction of κ2Si,Si′-[SiNSi]FeCl2 with
KC8 in the presence of PMe3 or direct reaction of the [ENE] ligands using
Fe(PMe3)4 produced the highly electron-rich iron(0) complexes [ENE]-
Fe(PMe3)2. The reduction of the iron center substantially changes its
coordination features, as shown by the results of a single-crystal X-ray
diffraction analysis of [SiNSi]Fe(PMe3)2. The iron center, in the latter, exhibits a pseudosquare pyramidal (PSQP) coordination
environment, with a coordinative (pyridine)N→Fe bond, and a trimethylphosphine ligand occupying the apical position. This
geometry is very unusual for Fe(0) low-spin complexes, and variable-temperature 1H and 31P NMR spectra of the
[ENE]Fe(PMe3)2 complexes revealed that they represent the first examples of configurationally stable PSQP-coordinated Fe(0)
complexes: even after heating at 70 °C for >7 days, no changes are observed. The substitution reaction of [ENE]Fe(PMe3)2 with
CO resulted in the isolation of [ENE]Fe(CO)2 and the hitherto unknown κ2E,E′-[ENE]Fe(CO)2L (L = CO, PMe3) complexes.
All complexes were fully characterized (NMR, MS, XRD, IR, and 57Fe Mossbauer spectroscopy), showing the highest electron
̈
density on the iron center for pincer-type complexes reported to date. DFT calculations and 57Fe Mossbauer spectroscopy
̈
confirmed the innocent behavior of these ligands. Moreover, preliminary results showed that these complexes can serve as active
precatalysts for the hydrosilylation of ketones.
bis(imino)pyridine ligand.4,5 Other research groups have
applied different pincer-type systems to stabilize other low-
valent iron complexes. Danopoulos and co-workers reported a
INTRODUCTION
■
Ligand design plays a crucial role in tuning the reactivity of
transition metal (TM) centers in organometallic chemistry.1
bis(carbene)pyridine ligand stabilizing the bis(dinitrogen)iron
[CNC]Fe(N2)2 complex II.6 It has also been shown that
pincer-stabilized iron(0) complexes are the active species in
catalytic reactions such as alkene hydrosilylation and asym-
metric hydrogenation of ketones and imines by isolating their
bis-carbonyl complexes.7,8 Recently, Milstein and co-workers
demonstrated the noninnocent behavior of bipyridine-based
PNN pincer ligands with iron biscarbonyl complexes III.9 In
addition to these seminal efforts, other ligand systems have
been applied for the stabilization of reactive iron(0) complexes.
Multidentate donor ligands have the advantage that they can
simultaneously increase the electronic density and stabilize the
coordination sphere of a TM. In particular, the pincer-type
motif [EDE], featuring a tridentate, meridional coordinating
ligand framework, offers a myriad of opportunities for
controlling the steric and electronic properties of TM
complexes.2 Generally, the side arms of a pincer ligand consist
of neutral, two-electron Lewis donor moieties (e.g., E = PR2,
NR2, or SR), which are connected through a linker group
(often CH2 or O) to a neutral or monoanionic anchoring site D
(e.g., pyridyl or phenyl group). During the past decade, several
groups have employed such ligands for stabilizing low-valent
first-row TM complexes, particularly iron(0) complexes (Figure
1). As a seminal example, employing a bis(imino)pyridine of
type [NNN], Chirik and co-workers3 stabilized a bis-
(dinitrogen)iron complex I; however the correct description
of the electronic ground state is a biradical [NNN2•−]Fe(II)-
(N2)2 rather than Fe(0), due to the noninnocence of the
Furstner and co-workers10 demonstrated the remarkable ability
̈
of iron(0)-ate complexes IV to serve as active catalysts in the
cycloisomerization of enynes. More recently, the research
groups of Peters11 and Deng12 also reported fascinating
Received: September 22, 2014
© XXXX American Chemical Society
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the electron donor properties but play a crucial role in the
catalytic behavior of their metal complexes. Inspired by these
results, we envisaged the synthesis of a neutral bis(metallylene)-
pyridine pincer-type ligand to study its coordination toward
iron as a nontoxic, inexpensive, and abundant TM, with the aim
to increase the electron density on iron(0) stabilized by a
tridentate ligand for further catalytic application (Scheme 1).
Herein, we reported the synthesis of the first neutral
bis(metallylene)pyridine pincer ligands of type ENE (E = SiII,
GeII) and their unexpected coordination mode toward iron as a
nonprecious metal center in the different oxidation states 0 and
+2. These studies revealed a novel coordination mode of Fe(II)
in the FeCl2 pincer complexes κ2E,E′-[ENE]FeCl2. In addition,
we observed a strikingly high stability in solution for the novel
iron(0) complexes of type [ENE]Fe(PMe3)2 without any
observable Berry pseudorotation within a wide range of
temperatures. These complexes represent a new family of
highly electron-rich iron(0) complexes that can moreover act as
precatalysts in the hydrosilylation of ketones.
Figure 1. Examples of isolable iron(0) complexes for activation of
strong σ-bonds and/or catalytic applications.
examples for the stabilization of highly reactive low-coordinate
iron(0) compounds (V and VI, Figure 1).
RESULTS AND DISCUSSION
In addition to their interesting structural features, iron-based
pincer-type complexes present high catalytic activity for
different reactions such as hydrogenation,8,13,14 hydrosilyla-
tion,3,15 and hydroboration.16 Recently, we reported the first
example of a highly electron-rich iron(0) complex stabilized by
a hydrido-silylene ligand (Scheme 1).17 This complex was
found to be catalytically active toward hydrosilylation of
ketones and demonstrated cooperativity between the metal
center and the hydrido-silylene ligand. We have also recently
investigated the potential application of bis(metallylene) pincer
ligands in catalytic transformations such as borylation of arenes
mediated by the [ECE]Ir complexes18 and Sonogashira cross-
coupling reactions effected by [ECE]Ni complexes19 with E =
SiII, GeII. The latter studies revealed higher σ-donor strength
compared with traditional PIII-based pincer ligands. These
studies showed that metallylenes not only dramatically enhance
■
Synthesis of the ENE (E = SiII, GeII) Pincer-Type
Ligands. Continuing our studies on the low-valent group 14
pincer ligands of the type [ECXE] (E = SiII, GeII; X = H, Br) as
monoanionic tridentate chelate ligands toward TM centers of
group 9 and 10, we envisaged the synthesis of neutral tridentate
ligands based on bis(silylenes) and bis(germylenes) as σ-donors
with a view to expand their chemistry toward the group 8 TMs.
On the basis of our synthetic approach for the synthesis of
these ligands, we planned the synthesis of the pyridine-based
pincer ligands through salt metathesis reactions of 2,6-diamine-
N,N′-diethylpyridine as the backbone. The synthesis of this
disubstituted pyridine was carried out following the reported
procedures for alkylation of amines.20−22 Acetylation of the 2,6-
diaminopyridine with concomitant reduction with lithium
aluminum hydride (LAH) produced the desired 2,6-diamino-
Scheme 1. Earlier Work on Iron-Based Silylene Complex and Nickel-Based Bis(metallylene) Pincer Complexes in Catalysis and
the Novel Iron-Based Bis(metallylene) Pincer Complexes
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N,N′-diethylpyridine. Deprotonation of the latter with 2 molar
equiv of n-BuLi in refluxing diethyl ether, followed by the
dropwise addition of 2 molar equiv of the N,N′-di-tert-
butyl(phenylamidinato)chlorosilylene or -germylene in a
toluene solution at −78 °C, afforded the desired SiNSi and
GeNGe products after warming to room temperature overnight
(Figure 2).
for iron. Reaction of the pincer ligands ENE (E = SiII, GeII)
with the in situ prepared FeCl2·(thf)1.5 adduct afforded intense
yellow solutions at room temperature (Scheme 2). Evaluation
Scheme 2. Synthesis of the κ2E,E′-[ENE]FeCl2 Complexes
(E = SiII, GeII)
1
of the reaction mixtures by H NMR showed paramagnetic
spectra with broad signals at chemical shifts within the δ = −5
to +30 ppm range. Integration of the paramagnetically shifted
signals however correlated with all the protons in the expected
structure. Extraction and crystallization in toluene afforded
yellow crystals in high yields (>70%). Their APCI HR-MS
(atmospheric pressure chemical ionization high-resolution mass
spectra) unambiguously proved the formation of the desired
[ENE]FeCl2 complexes (M•+, E = SiII: calcd 807.30915; exptl
807.30914; E = GeII: calcd 899.19765; exptl 899.19928).
Surprisingly, after growing suitable crystals for X-ray diffraction
analysis we observed that the pyridine moiety most likely does
not coordinate to the iron center, with N1−Fe interatomic
distances of 3.304 and 3.543 Å for the bis(silylene) and
bis(germylene) complexes, respectively. These distances are
larger than the sum of covalent radii for nitrogen and iron, and
the complexes are hence better described as κ2E,E′-[ENE]-
FeCl2. This result contrasts with existing Fe(II) complexes with
pyridine-based pincer ligands of type PNP, NNN, CNC, and
NNP (Figure 3),7,23a−e,24,25 in which a pentacoordinated iron
center is present. This difference might be attributed to the
enhanced σ-donor strength of the ENE ligands: first, the
stronger σ-donor properties of the metallylenes18,19 make the
iron center more electron-rich, impeding 5-fold coordination,
resulting in preferential tetrahedral coordination. Second, the
high trans effect for the metallylenes might play a role in
favoring the tetrahedral structure at the iron center.
Figure 2. Synthesis of the neutral pincer ligands ENE (E = SiII, GeII)
and the ORTEP representation of the SiNSi ligand in the solid state.
Thermal ellipsoids are drawn at the 50% probability level. Hydrogen
and solvent atoms are omitted for clarity. Selected distances [Å] and
angles [deg]: Si(1)−N(2) 1.7688(18), Si(1)−N(4) 1.8762(19),
Si(2)−N(3) 1.7894(19), Si(2)−N(6) 1.8767(19), N(1)−C(1)
1.343(3), N(1)−C(5) 1.347(3), N(2)−C(1) 1.407(3), N(3)−C(5)
1.401(3), N(4)−Si(1)−N(5) 68.83(9), N(6)−Si(2)−N(7) 68.83(8),
N(2)−Si(1)−N(4) 101.77(9), C(1)−N(2)−Si(1) 119.10(15), C(5)−
N(3)−Si(2) 120.19(15). See Supporting Information for details.
The 1H NMR spectra for SiNSi and GeNGe reveal a singlet
resonance signal for the tert-butyl, a triplet for the methyl, and a
quartet for the methylene moieties with the relative intensity of
18:3:2, respectively. Purification by extraction with n-hexane
and recrystallization leads to analytically pure SiNSi. Somewhat
noteworthy is that the GeNGe ligand exhibits a much higher
solubility in organic solvents than its silicon analogue, which
hampers crystallization. However, removal of the solvent in
vacuo produced the desired ligand in high purity (>95%). Both
1
ligands were fully characterized by H, 13C, and 29Si NMR
spectroscopy, high-resolution mass spectrometry, elemental
analysis, and single-crystal X-ray diffraction analysis for SiNSi
(see Experimental Section and Supporting Information).
Interestingly, the NMR spectra of isolated crystals of SiNSi
showed two sets of signals related to two rotational conformers
in the pendant arms. For instance, the 29Si NMR spectrum
revealed three signals: one major signal at δ = −14.9 ppm and
two other resonances at δ = −13.8 and −17.1 ppm, showing
the inequivalency of the two silylene moieties. DOSY NMR
studies permitted the unambiguous assignment of all signals to
the same chemical species.
The molecular structures depicted in Figure 4 show that the
iron center exhibits a distorted tetrahedral coordination sphere
in both complexes, being more similar to the literature
examples where bidentate ligands such as bis-carbene (Figure
3)24−26 or bis-phosphane27 are employed. Both complexes
crystallized in the monoclinic space group P21/n having a C2v
symmetry where the vertical plane crosses the iron and chloride
atoms. The Fe−Cl bond distances are within the range of the
FeCl2 complexes in a tetrahedral structure, but comparing the
κ2E,E′-[ENE]FeCl2 (E = SiII, GeII) complexes, there is an
elongation in the bond distance of about 3.5−4.6 pm with the
silylene as the σ-donor atom. Both structures are equivalent in
their coordination sphere; therefore, this elongation is mainly
due to the intrinsically higher σ-donor character of the silylene
versus the germylene subunit. This increases the electronic
density on the Fe(II) center, decreasing the interaction with the
chloride ligands, which is in accord with our earlier
observations.18,19
The single-crystal X-ray diffraction analysis for the SiNSi
ligand reveals that only one of the conformers is present in the
crystal lattice. The bond distances in the ligand backbone are
almost unchanged after the new covalent bonds between the
nitrogen and the silicon centers were formed. Both silylene
subunits are pointing out of the pyridine moiety, but its
fluxional behavior in solution, as confirmed by NMR, shows its
possibility as a potential pincer-type ligand for a metal center in
a meridional tridentate coordination.
The electronic structures of the κ2E,E′-[ENE]FeCl2
Synthesis of the [ENE]FeCl2 (E = SiII, GeII) Complexes.
The isolated ENE ligands were then employed as pincer ligands
complexes were in addition also studied by 57Fe Mossbauer
̈
spectroscopy and solution magnetic moment studies (Evans
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Figure 3. Some examples of Fe(II) complexes bearing pincer-type ligand moieties as well as a bis-N-heterocyclic carbene ligand for comparison.
Figure 4. ORTEP representation of the κ2E,E′-[ENE]FeCl2
complexes in the solid state. Thermal ellipsoids are drawn at the
50% probability level. Hydrogen and solvent atoms are omitted for
clarity. Selected distances [Å] and angles [deg]. (a) E = SiII: Fe(1)−
Si(1) 2.5256(7), Fe(1)−Si(2) 2.5110(7), Fe(1)−Cl(1) 2.2621(8),
Fe(1)−Cl(2) 2.2819(8), Si(1)−N(2) 1.761(2), Si(1)−N(4) 1.855(2),
Si(2)−N(3) 1.7608(19), Si(2)−Fe(1)−Si(1) 110.04(2), Cl(1)−
Fe(1)−Cl(2) 113.23(4), Cl(1)−Fe(1)−Si(1) 110.09(4), Cl(2)−
Fe(1)−Si(1) 107.39(3), Cl(1)−Fe(1)−Si(2) 108.82(3), Cl(2)−
Fe(1)−Si(2) 107.22(3), N(2)−Si(1)−N(4) 104.93(10), N(4)−
Si(1)−N(5) 70.22(9). (b) E = GeII: Fe(1)−Ge(1) 2.5670(4),
Fe(1)−Ge(2) 2.5509(4), Fe(1)−Cl(1) 2.2268(7), Fe(1)−Cl(2)
2.2358(7), Ge(1)−N(2) 1.8739(18), Ge(1)−N(4) 1.9795(18),
Ge(2)−N(3) 1.8661(16), Ge(2)−Fe(1)−Ge(1) 102.434(13),
Cl(1)−Fe(1)−Cl(2) 120.17(3), Cl(1)−Fe(1)−Ge(1) 108.88(3),
Cl(2)−Fe(1)−Ge(1) 107.34(2), Cl(1)−Fe(1)−Ge(2) 108.52(2),
Cl(2)−Fe(1)−Ge(2) 108.06(2), N(2)−Ge(1)−N(4) 102.00(8),
N(4)−Ge(1)−N(5) 65.95(7). See Supporting Information for details.
Figure 5. Zero-field 57Fe Mossbauer spectra of complexes κ2E,E′-
̈
[ENE]FeCl2 at 77 K. The circles are experimental data, and the lines
the simulated Lorentzian curves. (a) E = SiII: δ = 0.73(1) mm s−1, ΔEQ
= 3.06(1) mm s−1, Γfwhm = 0.34(1) mm s−1. (b) E = GeII: δ = 0.82(1)
mm s−1, ΔEQ = 2.57(1) mm s−1, Γfwhm = 0.46(1) mm s−1.
high reactivity and potential application in catalytic systems.
Due to the lack of coordination through the nitrogen on the
pyridine backbone in the Fe(II) complexes, we envisaged that
the reduction of the iron complexes under a nitrogen
atmosphere could afford homologues of the bis(dinitrogen)
iron pincer-like complexes (I and II in Figure 1) synthesized by
Chirik3 and Danopoulos.6 Recently, we reported an arene
iron(0) complex stabilized by a bis(carbene) ligand [Fe-
{(DippC:)2CH2}(η6-C7H8)] ((DippC:)2CH2 = bis(N-Dipp-imida-
zole-2-ylidene)methylene),26 and its comparison with its
heavier homologues bis(silylene) and bis(germylene) would
also be of interest, if isolable. Reduction of the Fe(II)
complexes κ2E,E′-[ENE]FeCl2 with KC8 in toluene/THF
produced intense red solutions, although after workup and
several unsuccessful attempts to crystallize a product we
obtained a red oil as crude product. The 1H NMR data suggest
a ligand reduction is the major product along with other
undefined side products but gave no hint for the formation of
the envisaged arene or N2 complex. This result prompted us to
employ an alternative coligand that would possibly better
stabilize the expected highly electron-rich iron(0) center after
method). From the 57Fe Mossbauer spectra (Figure 5) the
̈
isomer shift (δ) and quadrupole splitting (ΔEQ) values for
these complexes are in accordance with the expectations for
distorted tetrahedral high-spin Fe(II) complexes (δ = 0.73 mm
s−1, 0.82 mm s−1 and ΔEQ = 3.06 mm s−1, 2.57 mm s−1, for E =
SiII and GeII).24,25,27 This is, moreover, also in agreement with
the experimentally determined magnetic moment in solution of
μeff = 4.6 and 4.7 μB for E = SiII and GeII, respectively, which
also correspond to a high-spin S = 2 ground state.24,28
Synthesis of the [ENE]Fe(PMe3)2 (E = SiII, GeII)
Complexes. Low-valent iron complexes are extremely
attractive for the activation of small molecules due to their
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Scheme 3. Synthesis of the [ENE]Fe(PMe3)2 Pincer-like Complexes (E = SiII, GeII)
Figure 6. ORTEP representation of the [SiNSi]Fe(PMe3)2 complex in the solid state. Thermal ellipsoids are drawn at the 50% probability level. tert-
Buyl groups are drawn with wireframe, and hydrogen atoms are omitted for clarity. Selected distances [Å] and angles [deg]: Fe(1)−N(1) 2.059(3),
Fe(1)−Si(1) 2.1637(15), Fe(1)−Si(2) 2.1695(13), Fe(1)−P(1) 2.1455(11), Fe(1)−P(2) 2.1881(10), Si(1)−N(2) 1.790(4), Si(2)−N(3) 1.779(3),
Si(1)−N(4) 1.927(4), N(1)−Fe(1)−P(1) 165.48(9), N(1)−Fe(1)−Si(1) 80.63(11), N(1)−Fe(1)−Si(2) 80.83(11), N(1)−Fe(1)−P(2) 86.80(9),
P(1)−Fe(1)−Si(1) 94.19(5), P(1)−Fe(1)−Si(2) 95.38(5), P(1)−Fe(1)−P(2) 107.71(4), Si(1)−Fe(1)−P(2) 106.23(5), Si(2)−Fe(1)−P(2)
106.31(5), Si(1)−Fe(1)−Si(2) 141.28(5). See Supporting Information for details.
reduction. Hence, we anticipated the use of trimethylphosphine
as (i) a stabilizing ligand for the expected highly electron-rich
Fe(0) complex (assuming innocence of the ligand) and (ii) a
labile ligand for further reactivity studies, possibly by
elimination reaction.
Moreover, the Cs symmetry for the [SiNSi]Fe(PMe3)2 complex
is also consistent with the 31P and 29Si NMR spectra. In the
former two broad signals at δ = 7.2 and 20.8 ppm highlight the
inequivalency of both phosphorus atoms, and for the latter a
doublet of doublets (2JSi−P = 22.4 Hz, 91.9 Hz) centered at δ =
68.3 ppm showed the equivalency for both silicon atoms but
with different metric coupling constants to each phosphane.
The reduction of κ2Ge,Ge′-[GeNGe]FeCl2 by the same
synthetic strategy afforded only the side products also observed
during the reduction without PMe3 present in the reaction. As
an alternative, employing the highly reactive iron(0) precursor
Fe(PMe3)4 in a PMe3 substitution reaction with the ligand
SiNSi at 50 °C, a new species with identical spectra to the
[SiNSi]Fe(PMe3)2 complex was observed, as seen previously
using reduction with KC8. Motivated by this result, we
investigated the reaction between the ligand GeNGe and
Fe(PMe3)4 at 50 °C, which also afforded a dark red solution
after 8 h. Extraction of the crude product with pentane and
removal of the solvent, followed by three freeze−pump−thaw
cycles, afforded a foamy solid in high purity. Unfortunately,
given the high solubility of the [GeNGe]Fe(PMe3)2 complex
in aprotic organic solvents such as pentane, hexane, diethyl
ether, toluene, and THF, its crystallization was not possible.
The NMR spectra exhibit signals in close analogy to the
[SiNSi]Fe(PMe3)2 complex. However, in the 31P NMR
Reduction of κ2Si,Si′-[SiNSi]FeCl2 with KC8 in THF in the
presence of an excess of PMe3 indeed selectively afforded a new
diamagnetic species, [SiNSi]Fe(PMe3)2. The Cs symmetry of
1
the compound in solution was revealed collectively by the H,
29Si, and 31P NMR spectra, all showing the inequivalency of the
1
phosphorus atoms at the iron center (Scheme 3). The H
NMR spectrum exhibits two singlet resonances at δ = 1.09 and
1.52 ppm, two doublet signals at δ = 1.29 and 1.89 ppm, and a
triplet at δ = 1.58 ppm with a relative ratio of 18:18:9:9:6,
respectively. These signals were unambiguously assigned by 2D
NMR experiments in the following order: the nonequivalent
tert-butyl groups, the PMe3 coligands, and the methyl group
from the bound ethyl to the amine. Other aliphatic signals were
observed at δ = 3.49 and 3.70 ppm, which on first inspection
resembled sextets integrating for two protons each. These
signals reflect the diastereotopic nature of the protons on the
1
methylene units in the backbone. COSY H NMR showed a
1
correlation between both signals, and in the HSQC H−13C
NMR spectra only one correlation with the methylene carbon
was observed, confirming the assignment. The virtual sextet
arises from the value for the 2JH−H coupling (14.0 Hz), being in
this case exactly twice the value for the 3JH−H coupling (7.0 Hz).
spectrum two doublets at δ = 10.2 and 27.2 ppm (2JP−P
=
20.9 Hz) are observed instead of the broad signals for the
[SiNSi]Fe(PMe3)2 complex. This fact might be related to the
E
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Figure 7. Stacked variable-temperature (VT) 31P NMR in d8-toluene for (a) [SiNSi]Fe(PMe3)2 and (b) [GeNGe]Fe(PMe3)2.
Berry pseudorotation rate of the phosphanes on the iron center
(see VT-NMR studies below) as well as the P−Fe−P angle, as
has been previously shown in other bis-phosphane metal
complexes.29
Single crystals of [SiNSi]Fe(PMe3)2 suitable for X-ray
diffraction (XRD) analysis were obtained that revealed a Cs
symmetric molecule in the asymmetric unit. The geometric
features of [SiNSi]Fe(PMe3)2 depict the iron center in a
slightly distorted pseudo-square pyramidal (PSQP) coordina-
tion environment with τ = 0.18,30 with a trimethylphosphine
ligand occupying the apical position (Figure 6). This geometry
is unusual for Fe(0) low-spin complexes,3,31 and so a careful
study was carried out to define its electronic structure (see 57Fe
complex. This difference is related with the strong Fe→CO
π-back-bond. Comparisons to these well-defined complexes
showed that our ligand systems most likely are innocent since
bond distances are within narrow limits of actual Fe(0)
complexes rather than a ligand-centered radical.
The low symmetry of the molecule in the solid state and in
solution prompted us to explore the possibility whether the Cs
symmetry of the unusual PSQP iron complex is simply a
conformer in equilibrium with a higher (C2v) symmetric system
in a trigonal bipyramidal (TBP) conformation. For this
1
purpose, variable-temperature H and 31P NMR (VT-NMR)
experiments were carried out for both iron complexes
[ENE]Fe(PMe3)2 (E = SiII, GeII). Remarkably, the Cs
symmetry remains for the entire temperature range without
observing any Berry pseudorotation on the ligand system, even
at rather high temperatures (Figure 7). To the best of our
knowledge, these complexes therefore represent the first
examples of configurationally stable PSQP Fe(0) complexes:
even after heating at 70 °C for >7 days, no changes are
observed. The remarkable stability of the PSQP over the TBP
conformer is likely controlled by sterics: the high steric demand
on the substituents on the pincer backbone likely stabilizes the
iron center, thereby freezing out the trimethylphosphine
coligands in the molecule.
Mossbauer spectroscopy and DFT calculations below). The
̈
apical phosphorus atom features a slightly larger bond distance
to the iron center by 4.26(1) pm compared with the
phosphorus atom situated at the equatorial position, and the
Fe−P distances are comparable to the Fe(0) complexes with
silylene as coligand of type [(dmpe)2Fe(←:Si(X)L)] (L =
N,N′-di-tert-butyl(phenylamidinato), X = Cl, Me, H).17 More-
over, the Si−Fe bond distances are within the range of these
complexes (2.1634−2.200 Å) and shorter when compared with
the silylene→Fe(CO)4 complexes reported by West (:Si-
(NtBuCH)2, 2.196 Å)32 and Roesky et al. [:Si(OtBu)-
{(NtBu)2CPh}], 2.237(7) Å).33 Moreover, the bond distances
are in fact comparable to the silylene→Fe complex [Cp*Fe-
(CO)(SiMe3)SiMes2] reported by Tobita and co-workers,
where a formal double bond was described between the Si and
Fe centers,34 potentially indicating multiple-bonding character
(see the DFT calculations below). The noninnocent behavior
for the bis(imino)pyridine ligand used by Chirik and co-
workers in complex I (Figure 1) has been demonstrated, and it
is best described as an Fe(II) complex with a dianionic ligand
having a radical located on each imino subunit.4,5 This effect
was previously shown to play a role in the bond distances
between the metal center and the donor atoms.4 The N−Fe
bond distances in I can therefore be thought of as a N−Fe(II)
bonding situation changing the nature of the expected N−
Fe(0) bond. Recently, Milstein and co-workers9 reported a
series of dicarbonyl iron complexes stabilized by a bipyridine
phosphine pincer ligand (III in Figure 1), where the innocent
behavior of their ligand systems was demonstrated. For their
examples the Ncentered−Fe bond distance is shorter within a
difference of 11−13 pm than in the [SiNSi]Fe(PMe3)2
1
The diamagnetic H NMR spectrum of [SiNSi]Fe(PMe3)2
in C6D6 reveals exclusively signals in the diamagnetic spectral
window. A clear doublet resonance at δ = 5.95 ppm
corresponding to the proton at the meta position on the
pyridine moiety and a triplet of doublets (long-range coupling
6JH−P = 1.77 Hz) at δ = 7.18 ppm, which corresponds to the
proton at the para position of the pyridine backbone, are
observed. This is in close agreement with the signals observed
by Danopolous on pyridine in complex II6 but differs
substantially with the reduced bis(imino)pyridine ligand used
by Chirik in complex I3 (Figure 1), where large isotropic shifts
were observed. This again points to the ENE ligands exhibiting
innocent behavior, which was additionally confirmed by 57Fe
Mossbauer spectroscopy and DFT studies (see below).
̈
The isomer shifts and quadrupole splitting found for the
[ENE]Fe(PMe3)2 complexes in the 57Fe Mossbauer spectra at
̈
77 K (E = SiII: δ = 0.24(1) mm s−1, ΔEQ = 1.66(1) mm s−1; E =
GeII: δ = 0.36(1) mm s−1, ΔEQ = 1.87(1) mm s−1) are in the
range expected for iron(0) complexes in a pentacoordinated
environment (Figure 8). It is worth mentioning that these
F
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related to the evaluated complexes. The compounds
(dmpe)2Fe(:SnClAr) and (depe)2FeN2 bearing a TBP
structure have more differentiated ΔEQ values depending on
the type of chelate ligands and donor atoms. Moreover, the
dinitrogen complex [PDI]Fe(PEt3)N2 with a PSQP structure
shows a strong effect from the dinitrogen as coligand since its
value is lower than the [PDI]Fe(depe). Very recently, Chirik
and co-workers extended the understanding on the electronic
structure of the complex II, [CNC]Fe(N2)2 (Figure 1, Table1),
synthesized by Danopoulous et al.6 observing a redox
noninnocent behavior for this bis(carbene)pyridine ligand.
Therefore, the electronic structure for this complex is better
described as a mixed oxidation on the Fe center (+2 and 0)
with a radical located on the pyridine backbone [CNC]-
Fe0(N2)2 ↔ [CNC]FeII(N2)2.37 Table1 shows the Mossbauer
̈
values for this complex as well as its biscarbonyl [CNC]Fe-
(CO)2 complex. Comparing the ΔEQ values for these
complexes and our Fe(0) complexes, there is a large difference
but not with the well-defined biscarbonyl Fe(0) complexes III9
(Figure 1) bearing the innocent phosphane-bipyridine PNN
ligand. This points out that the electronic structures for our
Fe(0) complexes are more related with the Fe(0) center rather
than mixed Fe(II) ↔ Fe(0) oxidation states.
The δ value can be directly related to the s-electron density,
thus giving insights into the oxidation state of the Fe center.
For the examples shown in Table 1, the complexes bearing CO
as coligands show a major influence on the isomer shifts due to
the strong π-back-bonding interaction, which affects dramati-
cally the s-electron density at the iron centers, and lower value
for δ (0.02, 0.04 mm s−1). For the other complexes the δ values
are within the same range as for our complexes, showing their
similarities on the electronic structure at the Fe center. In
particular, [SiNSi]Fe(PMe3)2 exhibits the same δ value (0.24
mm s−1) as that of (depe)2Fe(N2), which is a good reference of
a pentacoordinate Fe(0) complex. The [GeNGe]Fe(PMe3)2
complex has a notably higher δ value (0.36 mm s−1), which is
related to the reduced σ-donor strength of Ge versus Si,18,19
resulting in a lower effective s-electron density at the iron
center and a concomitant increase in δ, as observed. Hence,
there appears to be some correlation of the δ values for the
[ENE]Fe(PMe3)2 (E = SiII, GeII) and other Fe(0) complexes
with comparable ligand arrangements. In conclusion, the
Figure 8. Zero-field 57Fe Mossbauer spectra of complexes [ENE]Fe-
̈
(PMe3)2 at 77 K. The circles are experimental data, and the lines the
simulated Lorentzian curves. (a) E = SiII: δ = 0.24(1) mm s−1, ΔEQ =
1.66(1) mm s−1, Γfwhm = 0.30 (1) mm s−1. (b) E = GeII: δ = 0.36(1)
mm s−1, ΔEQ = 1.87(1) mm s−1, Γfwhm = 0.35(1) mm s−1.
values are strongly dependent on the nature of the donor atoms
and in the coordination symmetry around the iron center.
Thus, a meaningful comparison can only be carried out with
those iron complexes having similar structural environment
(e.g., pincer moiety, pentacoordinated systems) and donor
atoms (i.e., σ-donors). Table 1 shows a comparison with some
reported five-coordinate iron(0) complexes with similar
features to our system. Comparing the [ENE]Fe(PMe3)2
complexes with those with PSQP structure, i.e., [PDI]Fe(N2)2
and [PDI]Fe(depe), the ΔEQ is somewhat similar to the latter
since the electronic structure and donor atoms are closely
comparison of the 57Fe Mossbauer parameters with reported
̈
values for isolated and isostructural Fe(0) and Fe(II) complexes
clearly supports that the [ENE]Fe(PMe3)2 (E = SiII, GeII)
compounds show similar features to other pentacooordinate
Fe(0) complexes, again pointing to the innocent nature of the
ENE ligands.
Table 1. Zero-Field Mossbauer Parameters for Iron(0)
Complexes Bearing Pincer-like Phosphane and Silylene
Arms
̈
compound
δ (mm s−1
)
ΔEQ (mm s−1
)
ref
To further elucidate the electronic structure of the metal
center, we conducted detailed density functional theory (DFT)
calculations at the B3LYP/6-31G(d)/LANL2DZ [Fe] level of
theory, to conclusively define the redox innocence/non-
innocence of the ligand. We focused mostly on atomic charges
from natural bond orbital (NBO) analysis and the Wiberg bond
index (WBI) to define the bonding situation within a complex.
A summary of the theoretical values is presented in Table 2.
The bonds with the Fe center are heavily polarized according to
the NBO charges on each donor atom, which is indicative of
low valence (zero valence) at the Fe center. Moreover, the
WBIs differ for each donor atom with the Fe center. The WBI
can be considered as an underestimation of bond order in polar
bonds, and the values for the Fe−Si bonds (WBI >1) therefore
indicate some multiple-bonding character.38 The molecular
[SiNSi]Fe(PMe3)2
0.24(1)
0.36(1)
0.39
1.66(1)
1.87(1)
0.53
this work
[GeNGe]Fe(PMe3)2
this work
a
[PDI]Fe(N2)2
4
b
[PDI]Fe(depe)
0.33
1.43
35
35
36
36
9
[PDI]Fe(PEt3)N2
(dmpe)2Fe(:SnClAr)
(depe)2FeN2
0.30
1.04
c
0.31
1.00
0.24
2.14
[PNN]Fe(CO)2
0.02 (P(tBu)2)
0.04 (P(Ph)2)
0.27
1.49
1.93
d
[CNC]Fe(N2)2
0.69
37
37
d
[CNC]Fe(CO)2
−0.10
0.62
a
b
Intermediate state FeII with diradical ligand PDI*. depe =
c
bis(diethylphosphino)ethane. dmpe = bis(dimethylphosphino)-
d
ethane. Mixed state FeII↔Fe0 with radical centered at the pyridine.
G
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proven by single-crystal XRD analyses (Figure 10 and
Supporting Information). For the case of E = GeII another
complex bearing one phosphine ligand, κ2Ge,Ge′-[GeNGe]Fe-
(CO)2PMe3, could also be isolated. To our knowledge this
reactivity of substitutionally labile pincer-type iron complexes
has not been observed before for related PNP, CNC, and NNN
pincer ligands, and it might again be a consequence of the
strong σ-donor capacity of our novel ligand system. Therefore,
in this case the higher electron density at the iron center is
stabilized by the extra π-acceptor ligand CO. The high trans
effect from the metallylenes, discussed above for the Fe(II)
complexes, might favor as well the formation of the TBP iron
center, having both metallylenes in a cis configuration at an
equatorial and an apical position. These complexes, moreover,
are thermally robust without observing decomposition in
solutions at 70 °C after 7 days. However, after heating for 1
day, solutions containing κ2Ge,Ge′-[GeNGe]Fe(CO)2PMe3
liberate PMe3, through intramolecular coordination of the N
atom at the pyridine backbone to iron, to give [GeNGe]Fe-
(CO)2.
Table 2. NBO Charge and Wiberg Bond Index Analyses for
[SiNSi]Fe(PMe3)2
NBO charge
−1.897
Wiberg bond index
Fe1
Si1
Si2
P1
Fe−Si
Fe−N
Fe−P
1.102
+1.617
+1.606
+1.276
+1.249
−0.458
1.115
0.456
P2
0.808
0.857
N1
orbitals (MOs) HOMO and HOMO−1 show the multiple-
bonding character for the Fe−Si bond through a π-back-
2
2
donation between the distorted dx −y and dyz from the Fe
center to the 3p orbitals from the Si(II) atoms (Figure 9). This
Table 3. Comparison of the CO Ligands in Pincer-Type
Complexes by Structural and Spectroscopic Means
IR νCO
compound
(cm−1
)
XRD C−O (Å)
ref
[SiNSi]Fe(CO)2
[GeNGe]Fe(CO)2
[CNC]Fe(CO)2
[NNN]Fe(CO)2
[PNP]Fe(CO)2
1830, 1778
1855, 1805
1928, 1865
1974, 1914
1950, 1894
1.192(7), 1.168(8)
1.189(7), 1.164(7)
1.161
this work
this work
6
1.147(2)
3, 39
39
9
1.1734(11)
[iPr-PNN]Fe(CO)2 1895, 1838
1.1580(17), 1.1615(15)
The [ENE]Fe(CO)2 complexes (E = SiII and GeII) exhibit a
C2v symmetry in solution according to the NMR spectra, which
is consistent with a TBP structure, contrary to the PSQP
structure in the precursor complexes [ENE]Fe(PMe3)2. Single-
crystal XRD analyses revealed that both [ENE]Fe(CO)2
complexes (E = SiII and GeII) possess a TBP structure in the
solid state. The Fe−Si and Fe−N bond distances are largely
invariant with the CO substitution, and the electronic structure
is comparable to the [ENE]Fe(PMe3)2 complexes calculated
by DFT (see Supporting Information). This additionally
confirms that the unusually high stability of the PSQP structure
of the [ENE]Fe(PMe3)2 complex is dominated by sterics and
not by electronics at the Fe center. A summary of the bond
distances and the IR stretching frequencies for the [ENE]Fe-
(CO)2 pincer-type complexes is presented in Table 3.
Comparing the values for the CO stretching frequencies,
lower frequencies were observed for the novel [ENE]Fe(CO)2
(E = SiII and GeII) complexes as well as elongation of the C−O
distance, indicating the higher σ-electron donor abilities of SiII
and GeII and strong Fe-CO π-back-bonding interactions. This is
consistent with a more electron-rich Fe(0) center for E = SiII
and GeII as compared to analogous complexes with E = CII, PIII,
and NIII donor atoms, again pointing out the innocent behavior
of these novel ligands.
Figure 9. Selected molecular orbitals for the [SiNSi]Fe(PMe3)2
calculated at the B3LYP/6-31G(d)/LANL2DZ [Fe] level of theory.
is in agreement with the short Fe−Si distance in the crystal
structure and comparable to our previously reported
[(dmpe)2Fe(←:Si(X)L)] (L = N,N′-di-tert-butyl-
(phenylamidinato), X = Cl, Me, H) complexes, where a related
multiple-bonding character between the Fe and Si was
observed.17 The other filled MOs, HOMO−2 and HOMO−
3, are metal centered (dxy and dxz orbitals). It is also worth
mentioning that the σ-donation from Si→Fe is observed but at
lower energy (HOMO−9), where the bonding interaction is
2
mediated by the dz orbital from the metal center (see
Supporting Information). Of importance is to note that none of
the frontier orbitals show any buildup of electron density on
the ligands and, thus, convincingly show that the ENE ligands
are innocent. This is in accord with the 57Fe Mossbauer spectra,
̈
the X-ray structural data, and the observed diamagnetic NMR
spectra (see above).
To gain further insights into the electronic nature at the
Fe(0) center in the complexes described above, we carried out
the ligand substitution reactions of the [ENE]Fe(PMe3)2
complexes with CO (Figure 10) to use it as a probe in the
IR spectrum. Stirring pentane solutions of the iron complexes
overnight under a CO atmosphere afforded replacement of the
PMe3 ligands by CO. However, the reaction consistently
produced a mixture of two major compounds: the disubstituted
[ENE]Fe(CO)2 and trisubstituted κ2E,E′-[ENE]Fe(CO)3, as
Catalytic Hydrosilylation of Ketones. During the past
decade iron catalysts have become widespread in several
chemical transformations (e.g., hydrogenation, hydrosilylation,
coupling reactions),40 having in some examples comparable
activities to their noble metal counterparts (e.g., Rh, Ir, Ru).
H
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Figure 10. Ligand substitution reaction of [ENE]Fe(PMe3)2 with CO and the ORTEP representation of the [ENE]Fe(CO)2 complexes in the solid
state. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen and solvent atoms are omitted for clarity. Selected distances [Å] and
angles [deg]: (a) E = SiII: Fe(1)−C(78) 1.701(6), Fe(1)−C(79) 1.759(7), Fe(1)−N(1) 2.089(4), Fe(1)−Si(1) 2.1579(15), Fe(1)−Si(2)
2.1664(15), Si(1)−N(2) 1.757(4), Si(1)−N(4) 1.865(5), Si(2)−N(3) 1.747(4), O(1)−C(78) 1.192(7), O(2)−C(79) 1.168(8), C(78)−Fe(1)−
C(79) 114.7(3), C(78)−Fe(1)−N(1) 131.8(3), C(79)−Fe(1)−N(1) 113.5(3), C(78)−Fe(1)−Si(1) 90.85(19), C(79)−Fe(1)−Si(1) 98.33(19),
N(1)−Fe(1)−Si(1) 81.36(12), C(78)−Fe(1)−Si(2) 91.82(19), C(79)−Fe(1)−Si(2) 99.49(19), N(1)−Fe(1)−Si(2) 81.33(11), Si(1)−Fe(1)−
Si(2) 158.95(7). (b) E = GeII: Fe(1)−C(41) 1.718(5), Fe(1)−C(40) 1.750(6), Fe(1)−N(1) 2.156(4), Fe(1)−Ge(1) 2.2086(9), Fe(1)−Ge(2)
2.2092(9), Ge(1)−N(2) 1.880(4), Ge(1)−N(4) 1.994(4), Ge(2)−N(3) 1.870(4), O(1)−C(40) 1.164(7), O(2)−C(41) 1.189(7), C(41)−Fe(1)−
C(40) 113.8(3), C(41)−Fe(1)−N(1) 126.8(2), C(40)−Fe(1)−N(1) 119.35(19), C(41)−Fe(1)−Ge(1) 97.06(17), C(40)−Fe(1)−Ge(1)
93.56(16), N(1)−Fe(1)−Ge(1) 82.06(11), C(41)−Fe(1)−Ge(2) 90.51(17), C(40)−Fe(1)−Ge(2) 96.20(16), N(1)−Fe(1)−Ge(2) 82.15(11),
Ge(1)−Fe(1)−Ge(2) 164.04(4). See Supporting Information for details.
Scheme 4. Proof of Catalytic Activity of Iron(0) Complex [SiNSi]Fe(PMe3)2 as a Precatalyst in the Hydrosilylation of
Acetophenone Derivatives
Several iron pincer-type complexes have shown to be very
robust under different catalytic conditions as well as very active
even under mild conditions.14−16 Consequently, we were
interested to probe the ability of the obtained iron(0)
complexes for the catalytic hydrosilylation of a ketone.
Previously, we reported the hydrosilylation of ketones employ-
ing the [(dmpe)2Fe(←:Si(H)L)] (L = N,N′-di-tert-butyl-
(phenylamidinato)) complex, where a cooperative behavior
was elucidated between the silicon(II) and iron(0) centers.17
This result demonstrated that a silylene can act as a cooperative
ligand in catalytic systems. However, an innocent silylene ligand
would also be of interest when a robust catalyst is required. In
preliminary experiments employing catalytic amounts (2 mol
%) of the [SiNSi]Fe(PMe3)2 complexes in the hydrosilylation
of p-methoxyacetophenone with triethoxysilane, we obtained
conversions up to 87% after 22 h at 70 °C in THF (see
Supporting Information). After the catalytic run a clear orange
solution resulted without observable decomposition of the
catalytic system. Increasing the precatalyst loading to 2.5 mol
%, a quantitative yield could be obtained for p-methoxyaceto-
phenone as well as for other ketones as substrates (Scheme 4)
in which the catalytic activity is somehow improved to our
previous system where a catalyst loading of 5 mol % was
required under the same catalytic conditions.17 Currently we
are varying the catalytic conditions and extending the substrate
scope to understand the mechanism for this chemical
transformation.
I
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FeCl2, PMe3, and (EtO)3SiH were purchase from Sigma Aldrich and
used as received. The N,N′-di-tert-butyl(phenylamidinato)-
chlorosilylene,4 1 N,N′-di-tert-butyl(phenylamidinato)-
CONCLUSIONS
■
In summary, the first bis(silylene)- and bis(germylene)-pyridine
[ENE] (E = SiII, GeII) pincer ligands were synthesized by a salt
metathesis reaction of the dilithiated 2,6-bis(ethylamino)-
pyridine and with amidinato-stabilized chlorosilylene or
chlorogermylene. The novel [ENE] ligands behave in an
innocent fashion on coordination to iron, as shown by a range
of spectroscopic, structural, and theoretical investigations, in
contrast to many other previously reported pincer-type iron
complexes, which usually show ligand-centered radical ground-
state configurations. The iron complexes κ2E,E′-[ENE]FeCl2
were synthesized by direct coordination of FeCl2, affording
tetrahedral iron(II) complexes lacking coordination through
the N atom at the pyridine backbone according to single-crystal
43
chlorogermylene,42 and Fe(PMe3)4 were prepared according to the
reported procedures.
Single-Crystal X-ray Structure Determinations. Crystals were
mounted on a glass capillary in perfluorinated oil and measured in a
cold N2 flow. The data were collected either on an Agilent
Technologies Xcalibur S Sapphire at 150 K (Mo Kα radiation, λ =
0.710 73 Å) or on an Agilent Technologies SuperNova (single source)
at 150 K (Cu Kα radiation, λ = 1.5418 Å). The structures were solved
by direct methods and refined on F2 with the SHELX-97 software
package.44 The positions of the H atoms were calculated and
considered isotropically according to a riding model.
Synthesis of 2,6-Diamine-N,N′-diethylpyridine. The synthesis
was carried out following a reported procedure for reduction of
diamides with LAH in THF under reflux.20−22 A 5.29 g (0.027 mol)
amount of the N,N′-diacetyl-2,6-diamidopyridine was dissolved in 60
mL of THF and dropwise added into a suspension of 6.56 g (0.173
mol) of LAH in 40 mL of THF at 0 °C. After stirring for 1 h at room
temperature the reaction was refluxed overnight until completion of
the reaction controlled by GC-MS. The excess of LAH was quenched
at 0 °C with a basic KOH 5% solution, obtaining two phases. The
phases were separated, and the aqueous phase was treated with diethyl
ether (3 × 30 mL). All organic fractions were collected and dried with
Na2SO4. Filtration and removal of all volatiles in vacuo afforded the
XRD, 57Fe Mossbauer, and magnetometric measurements.
̈
Either by reduction of the κ2Si,Si′-[SiNSi]FeCl2 with KC8 in
the presence of PMe3 or by a direct substitution reaction of the
iron(0) precursor Fe(PMe3)4 with the ENE pincer ligands the
unusual electron-rich [ENE]Fe(PMe3)2 complexes with Fe(0)
sites are accessible. Remarkably, these complexes exist in a
pseudo-square-pyramidal ground-state structure, which is
configurationally stable over a wide range of temperatures in
solution. A direct comparison with the dicarbonyl [ENE]Fe-
(CO)2 complexes, featuring the more expected trigonal
bipyramidal structure and (pyridine)N→Fe coordination,
showed that this stability is controlled by sterics since the
structural features and DFT calculations demonstrated that the
electronic situation on the iron center remains unchanged in
both complexes with either PMe3 or CO as coligands. Also, a
comparison with the well-established pyridine-based iron
complexes bearing pincer-like ligands with phosphines,
carbenes, or imino groups revealed that the novel [ENE]Fe-
(PMe3)2 complexes are the most electron-rich pincer-like
iron(0) complexes reported to date, due to the increased σ-
donor properties of the ENE ligands. Finally, the [SiNSi]Fe-
(PMe3)2 complex showed good catalytic activity when
employed as a precatalyst for the hydrosilylation of
acetophenones, this being the second generation of silylene-
iron-based catalysts for this type of chemical transformation.
We are currently investigating the scope of substrates and the
mechanism of the ketone hydrosilylation reaction and will
present these results in due course.
1
product in high purity as a yellow oil (3.5 g, 77% yield). H NMR
(200.13 MHz, CDCl3, 298 K): δ(ppm) = 1.22 (t, 6H, 3JH−H = 7.16 Hz,
3
3
CH3), 3.21 (q, 2H, JH−H = 7.16 Hz, CH2), 3.24 (q, 2H, JH−H = 7.16
Hz, CH2), 5.70 (d, 2H, 3JH−H = 7.91 Hz, 3,5-H py), 7.24 (t, 1H, 3JH−H
= 7.91 Hz, 4-H py). 13C{1H} NMR (50.32 MHz, CDCl3, 298 K):
δ(ppm) = 15.0 (CH3), 36.9 (CH2), 94.39 (3,5-CH py), 139.0 (4-CH
py), 158.3 (2-C py). ESI-MS (m/z): calcd for [C9H15N3 + H+]
166.13387; found 166.13379 (correct isotope pattern).
Synthesis of the Pincer-Type Ligand SiNSi. A 6.40 mL sample
of n-BuLi, 1.6 M in hexanes (10.2 mmol), was added rapidly to a 30
mL diethyl ether solution of 2,6-N,N′-diethylaminopyridine (0.847 g,
5.13 mmol) at −30 °C, forming a yellowish solution. After warming to
room temperature it was refluxed for 3 h. The resulting orange
solution was cooled to −78 °C, and a solution of N,N′-di-tert-
butyl(phenylamidinato)chlorosilylene (3.019 g, 10.2 mmol) in 30 mL
of toluene was dropwise added via cannula. The color changed with
the addition to dark red. All volatiles were removed in vacuo after
stirring overnight, with slow warming to room temperature. The
product was extracted with 60 mL of hexanes at 60 °C via cannula
filtration. The solution was concentrated to 10 mL and crystallized at
−30 °C over 3 days, affording large yellow crystals. Further filtration
and drying produced 2.90 g of the desired product (90% yield).
Suitable crystals for X-ray diffraction analysis were grown in a
EXPERIMENTAL SECTION
■
1
concentrated hexane solution at 0 °C after 2 days. H NMR (400.13
General Considerations. All experiments and manipulations were
conducted under dry, oxygen-free nitrogen using standard Schlenk
techniques or in a MBraun drybox with an atmosphere of purified
nitrogen. Solvents were dried by standard methods and freshly distilled
MHz, C6D6, 298 K): Symmetric conformer: δ(ppm) = 1.16 (s, 36H,
NC(CH3)3), 1.63 (t, 3JH−H = 6.9 Hz, 6H, NCH2-CH3), 3.77 (q, 3JH−H
= 6.9 Hz, 4H, NCH2-CH3), 6.87−7.09 (m, 10H, arom. C-H), 7.34−
7.50 (m, 3H, arom. C-H py). Asymmetric conformer: δ(ppm) = 1.14
1
prior to use. H, 13C, 31P, and 29Si NMR spectra were recorded on
3
Bruker AV 400 (1H, 400.13 MHz; 13C, 100.61 MHz; 29Si, 79.49 MHz;
31P, 161.80 MHz) or AFM 200 (1H, 200.13 MHz; 13C, 50.32 MHz;
31P, 81.01 MHz) spectrometers. The NMR signals are reported
relative to the residual solvent peaks (1H, C6D6, 7.15 ppm; C7D8, 2.09
ppm; CDCl3, 7.26 ppm; 13C, C6D6, 128.0 ppm; C7D8, 20.4 ppm;
CDCl3, 77.2 ppm) or an external standard (31P, 85% H3PO4, 0.0 ppm;
29Si, TMS, 0.0 ppm). All signals were unambiguously assigned by a
combination of 2D NMR H−H COSY, HSQC, and HMBC
correlation spectroscopy. Mass spectra were recorded using APCI or
ESI as ionization source and an LTQ Orbitrap XL as analyzer.
Elemental analyses were recorded in a Thermo FlashEA 1112 Organic
elemental analyzer. IR spectra were recorded on a PerkinElmer
Spectrum 100 FT-IR. GC-MS measurements were conducted on a
Shimadzu GC-2010 gas chromatograph (30 m Rxi-5 ms column)
linked to a Shimadzu GCMA-QP 2010 Plus mass spectrometer. The
(s, 36H, NC(CH3)3), 1.55 and 1.68 (t, JH−H = 6.9 Hz, 6H, NCH2-
3
CH3), 3.71 and 4.62 (q, JH−H = 6.9 Hz, 4H, NCH2-CH3). 13C{1H}
NMR (100.61 MHz, C6D6, 298 K): Symmetric conformer: δ(ppm) =
16.9 (NCH2-CH3), 31.6 (NC(CH3)3), 31.9 (NCH2-CH3), 52.9
(NC(CH3)3), 101.8 (3,5-Carom py), 127.6 (Carom), 128.5 (Carom),
129.3 (Carom), 129.3 (Carom), 129.4 (Carom), 130.0 (Carom), 130.5
(Carom), 130.5 (Carom), 134.7 (Carom quaternary Ph), 136.9 (4-Carom
py), 161.2 (2,6-Carom py), 161.4 (NCN). Asymmetric conformer:
δ(ppm) = 18.0 and 16.0 (NCH2-CH3), 31.4 and 31.5 (NC(CH3)3),
36.8 and 43.9 (NCH2-CH3), 53.3 (NC(CH3)3), 103.0 and 103.9 (3,5-
Carom py), 134.0 and 134.5 (Carom quaternary Ph), 136.4 (4-Carom py).
29Si{1H} NMR (79.49 MHz, C6D6, 298 K): Symmetric conformer:
δ(ppm) = −14.9. Asymmetric conformer: δ(ppm) = −13.8 and −17.1.
APCI-MS (m/z): calcd for [(C39H59N7Si2+OH)•+] 698.43924; found
698.43983 (correct isotope pattern).
J
dx.doi.org/10.1021/om500966t | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Synthesis of the Pincer-Type Ligand GeNGe. The synthesis of
the bis(germylene)pyridine pincer ligand proceeded in a similar
fashion to that for the synthesis of the pincer ligand SiNSi (see above).
After the addition of the N,N′-di-tert-butyl(phenylamidinate)-
chlorogermylene (1.788 g, 5.27 mmol) in 30 mL of toluene the
color of the reaction mixture changed to an intense yellowish solution.
All volatiles were removed in vacuo after stirring overnight, warming to
room temperature with the cold bath. The product was extracted with
60 mL of hexanes at 60 °C for 1 h via cannula filtration. Removal of
the solvent produced 1.98 g of a yellowish foamy solid with high purity
concentrated pentane solution at 0 °C after ca. 1 week. Method B: A
solution of SiNSi (160 mg, 0.235 mmol) in 10 mL of hexane was
dropwise added to a solution of Fe(PMe3)4 (85 mg, 0.24 mmol) in 10
mL of hexane at room temperature. The mixture was warmed to 50 °C
for 8 h under continuous stirring. The solvent was pumped off and the
product extracted with 40 mL of pentane. Concentration, crystal-
lization, filtration, and drying in vacuo afforded 100 mg of the desired
1
product (48% yield). H NMR (200.13 MHz, C6D6, 298 K): δ(ppm)
2
= 1.09 (s, 18H, 2 × NC(CH3)3), 1.29 (d, 9H, JH−P = 4.0 Hz,
3
PA(CH3)3), 1.52 (s, 18H, 2 × NC(CH3)3), 1.58 (t, 6H, JH−H = 7.0
1
2
according to the NMR spectra (98% yield). H NMR (400.13 MHz,
Hz, NCH′H-CH3), 1.89 (d, 9H, JH−P = 4.0 Hz, PB(CH3)3), 3.49
3
2
3
C6D6, 298 K): δ(ppm) = 1.11 (s, 36H, NC(CH3)3), 1.53 (t, JH−H
=
(virtual sextet with roof effect, 2H, JH−H = 14.0 Hz, JH−2H = 7.0 Hz,
NCH′H-CH3), 3.70 (virtual sextet with roof effect, 2H, JH−H = 14.0
Hz, 3JH−H = 7.0 Hz, NCH′H-CH3), 6.04 (d, 2H, 3JH−H = 7.7 Hz, 3−5-
H py), 6.96−7.06 (m, 6H, arom. C-H), 7.19−7.27 (m, 3H, arom. CH
and 4-H py), 7.42−7.51 (m, 2H, arom. CH). 1H NMR (400.13 MHz,
C7D8, 298 K): δ(ppm) = 1.07 (s, 18H, 2 × NC(CH3)3), 1.21 (d, 9H,
2JH−P = 4.4 Hz, PA(CH3)3), 1.51 (s, 18H, 2 × NC(CH3)3), 1.56 (t, 6H,
3
7.0 Hz, 6H, NCH2-CH3), 3.76 (q, JH−H = 7.0 Hz, 4H, NCH2-CH3),
6.34 (d, 3JH−H = 7.9 Hz, 2H, 3,5-H py), 6.88−7.11 (m, 10H, arom. C-
H Ph), 7.46 (t, JH−H = 7.9 Hz, 1H, 4-H py). 13C{1H} NMR (100.61
3
MHz, C6D6, 298 K): δ(ppm) = 16.8 (NCH2-CH3), 32.2 (NC(CH3)3),
38.8 (NCH2-CH3), 52.8 (NC(CH3)3), 95.2 (3,5-Carom py), 127.5
(Carom), 128.9 (Carom), 130.3 (Carom), 136.9 (Carom quaternary Ph),
138.7 (4-Carom py), 162.6 (2,6-Carom py), 167.3 (NCN). APCI-MS (m/
z): calcd for [C39H59Ge2N7•+] 773.32500; found 773.32666 (correct
isotope pattern). Anal. Calcd for C39H59Ge2N7: calcd, C 60.74, H 7.71,
N 12.71; found C 60.47, H 7.72, N 12.40.
2
3JH−H = 7.0 Hz, NCH′H-CH3), 1.83 (d, 9H, JH−P = 6.1 Hz,
2
PB(CH3)3), 3.46 (virtual sextet with roof effect, 2H, JH−H = 14.0 Hz,
3JH−H = 7.0 Hz, NCH′H-CH3), 3.69 (virtual sextet with roof effect,
2
3
2H, JH−H = 14.0 Hz, JH−H = 7.0 Hz, NCH′H-CH3), 5.95 (d, 2H,
3JH−H = 8.0 Hz, 3−5-H py), 7.18 (td, 1H, 3JH−H = 8.0 Hz, 6JH−P = 1.8
Hz, 4-H py), 6.99−7.08 (m, 6H, arom. CH), 7.24−7.28 (m, 2H, arom.
CH), 7.44−7.48 (m, 2H, arom. CH). 13C{1H} NMR (100.61 MHz,
C7D8, 298 K): δ(ppm) = 15.6 (NCH2-CH3), 25.9 (d, 1JC−P = 10.8 Hz,
Synthesis of [ENE]FeCl2 (E = SiII, GeII) Complexes. A
suspension of FeCl2 (1.1 equiv) was heated in 30 mL of THF at 60
°C for 2 h. After cooling to room temperature, a solution of ENE (1.0
equiv) in 20 mL of THF was dropwise added via cannula, dissolving all
suspended solid. After stirring at room temperature for 3 h, all volatiles
were removed under vacuum, forming a yellowish solid. The product
was extracted with toluene (1 × 60 mL, 1 × 30 mL) and filtered off via
cannula. Concentration in vacuo to 10 mL and crystallization at −30
°C afforded the desired product as yellow crystals.
1
3
PA(CH3)3), 33.9 (dd, JC−P = 13.07 Hz, JC−P = 5.60 Hz, P(CH3)3),
31.6 (NC(CH3)3), 33.3 (NC(CH3)3), 38.9 (NCH2−CH3), 53.3
4
(NC(CH3)3), 53.5 (NC(CH3)3), 93.3 (d, JC−P = 2.20 Hz, 3,5-Carom
5
py), 126.6 (d, JC−P = 3.26 Hz, 2,6-Carom py), 127.3 (Carom), 127.4
(Carom), 130.5 (Carom), 135.3 (Carom quaternary Ph), 165.3 (d, 3JC−P
=
Complex [SiNSi]FeCl2. At 1.0 mmol scale: 73% yield; yellow
blocks. Suitable crystals for X-ray diffraction analysis were grown from
a concentrated toluene solution at 0 °C after 3 days. 1H NMR
paramagnetic (200.13 MHz, C6D6, 298 K): δ(ppm) = −2.25 (36H),
1.24 (6H), 2.80 (2H), 5.61 (2H), 6.53 (2H), 10.29 (1H), 10.66 (2H),
3.26 Hz, 2,6-Carom py), 168.5 (NCN). 29Si{1H} NMR (79.49 MHz,
2
C7D8, 298 K): 68.3 (dd, JSi−P = 22.40 Hz, 91.87 Hz, Si:). 31P NMR
(161.97 MHz, C7D8, 298 K): 7.2 (bs, PMe3), 20.8 (bs with satellites
2JP−Si = 91.87 Hz, PMe3). 57Fe Mossbauer at 77 K (zero field): δ =
̈
0.24(1) mm s−1, ΔEQ = 1.66(1) mm s−1, Γfwhm = 0.30 (1) mm s−1.
APCI-MS (m/z): calcd for [C45H77FeN7P2Si2•+] 889.45982; found
889.45978 (correct isotope pattern).
13.32 (2H), 21.83 (4H), 24.41 (2H). Evans (C6D6, tetramethylsilylsi-
1
lane capillary, concentration 0.031 g mL−1, 200 MHz for H): μeff
=
4.65 μB. 57Fe Mossbauer at 77 K (zero field): δ = 0.73(1) mm s−1,
̈
Synthesis of [GeNGe]Fe(PMe3)2 Complex. The synthesis of this
complex was ONLY affordable by method B described above for
[SiNSi]Fe(PMe3)2 complex. Extraction with pentane and removal of
the solvent in vacuo afforded a foamy-like dark red solid in high purity.
ΔEQ = 3.06(1) mm s−1, Γfwhm = 0.34(1) mm s−1. APCI-MS (m/z):
calcd for [C39H59Cl2FeN7Si2•+] 807.30915; found 807.30914 (correct
isotope pattern). Anal. Calcd for C39H59Cl2FeN7Si2·C7H8: C 61.32, H
7.50, N 10.88. Found: C 59.61, H 7.58, N 10.99.
1
At 0.5 mmol scale: 90% yield. H NMR (400.13 MHz, C6D6, 298 K):
Complex [GeNGe]FeCl2. At 1.0 mmol scale: 83% yield; yellow
δ(ppm) = 1.08 (s, 18H, 2 × NC(CH3)3), 1.29 (d, 2JH−3P = 5.33 Hz, 9H,
PA(CH3)3), 1.47 (s, 18H, 2 × NC(CH3)3), 1.57 (t, JH−H = 7.04 Hz,
blocks. Suitable crystals for X-ray diffraction analysis were grown from
1
a concentrated toluene solution at −78 °C after 3 days. H NMR
2
6H, NCH′H-CH3), 1.76 (d, JH−P = 6.62 Hz, 9H, PB(CH3)3), 3.76
paramagnetic (200.13 MHz, C6D6, 298 K): δ(ppm) = −0.11 and 0.36
(two signals overlapping, 36H), 1.00−1.42 (5H), 3.29 (2H), 4.90
(2H), 5.17 (1H), 5.95 (2H), 8.06 (2H), 8.86 (2H), 11.93 (1H), 26.70
(4H). Evans (C6D6, tetramethylsilylsilane capillary, concentration
2
3
(“sextet” with roof effect, JH−H = 13.98 Hz, JH−H = 6.99 Hz, 2H,
2
NCH′H-CH3), 3.95 (“virtual sextet” with roof effect, JH−H = 13.98
3
3
Hz, JH−H = 6.99 Hz, 2H, NCH′H-CH3), 6.00 (d, JH−H = 7.93 Hz,
2H, 3,5-H py), 6.97−7.05 (m, 6H, arom. C-H), 7.22−7.29 (m, 3H,
0.029 g mL−1, 200 MHz for 1H): μeff = 4.71 μB. 57Fe Mossbauer at 77
̈
1
K (zero field): δ = 0.82(1) mm s−1, ΔEQ = 2.57(1) mm s−1, Γfwhm
=
]
arom. C-H and 4-H py), 7.33−7.39 (m, 2H, arom. C-H). H NMR
•+
(400.13 MHz, C7D8, 298 K): δ(ppm) = 1.06 (s, 18H, 2 × NC(CH3)3),
0.46(1) mm s−1. APCI-MS (m/z): calcd for [C39H59Cl2FeGe2N7
2
1.23 (d, JH−P = 5.33 Hz, 9H, PA(CH3)3), 1.46 (s, 18H, 2 ×
899.19765; found 899.19928 (low intensity); calcd for
[(C39H59Cl2FeGe2N7 − Cl)+] 864.22935; found 864.38190 (correct
isotope pattern). Anal. Calcd for C39H59Cl2FeGe2N7.C7H8: C 55.80, H
6.82, N 9.90. Found: C 55.06, H 7.01, N 10.20.
3
NC(CH3)3), 1.56 (t, JH−H = 6.99 Hz, 6H, NCH′H-CH3), 1.73 (d,
2JH−P = 7.04 Hz, 9H, PB(CH3)3), 3.73 (“virtual sextet” with roof effect,
2JH−H = 13.98 Hz, 3JH−H = 6.99 Hz, 2H, NCH′H-CH3), 3.93 (“virtual
2
3
sextet” with roof effect, JH−H = 13.98 Hz, JH−H = 6.99 Hz, 2H,
NCH′H-CH3), 5.93 (d, 3JH−H = 7.92 Hz, 2H, 3,5-H py), 7.16 (t, 3JH−H
= 7.92 Hz, 1H, 4-H py), 7.03−7.08 (m, 6H, arom. C-H), 7.27−7.32
(m, 2H, arom. C-H), 7.35−7.40 (m, 2H, arom. C-H). 13C NMR
(100.61 MHz, C7D8, 298 K): δ(ppm) = 16.0 (NCH2-CH3), 25.4 (d,
Synthesis of [SiNSi]Fe(PMe3)2 Complex. Method A: A strong
yellow solution of [SiNSi]FeCl2 (414 mg, 0.511 mmol) in 30 mL of
THF was dropwise added via cannula to a Schlenk flask containing
KC8 (220 mg, 1.63 mmol) at −10 °C. After 15 min stirring, a solution
of PMe3 (1.0 M in hexanes) (2.0 mL, 2.0 mmol) was added dropwise
via syringe with a change in color to red. After warming to room
temperature and 24 h stirring, the reaction mixture had a dark purple
color. The solution was filtered via cannula, and all volatiles were
removed in vacuo, producing a sticky purple solid. The product was
extracted with pentane (1 × 50 mL, 1 × 20 mL) and concentrated in
vacuo, affording 350 mg (77% yield) of a purple solid after three
freeze−thaw cycles in high purity according to the NMR spectra.
Suitable crystals for X-ray diffraction analysis were grown in a
1JC−P = 14.4 Hz, P(CH3)3), 32.9 (NC(CH3)3), 33.3 (dd, JC−P = 15.9
1
3
Hz, JC−P = 3.7 Hz, P(CH3)3), 33.4 (NC(CH3)3), 40.5 (NCH2-CH3),
53.7 (NC(CH3)3), 91.7 (d, JC−P = 1.4 Hz, 3,5-Carom py), 127.3
(Carom), 127.5 (Carom), 128.4 (d, JC−P = 3.3 Hz, 4-Carom py), 130.6
4
5
3
(Carom), 137.0 (Carom quaternary Ph), 164.1 (d, JC−P = 2.1 Hz, 2,6-
Carom py), 169.0 (NCN). 31P{1H} NMR (161.97 MHz, C7D8, 298 K):
2
2
δ(ppm) = 10.2 (d, JP−P = 20.8 Hz, PMe3), 27.2 (d, JP−P = 20.8 Hz,
PMe3). 57Fe Mossbauer at 77 K (zero field): δ = 0.36(1) mm s−1, ΔE
̈
Q
K
dx.doi.org/10.1021/om500966t | Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
= 1.87(1) mm s−1, Γfwhm = 0.35(1) mm s−1. APCI-MS (m/z): calcd for
[(C45H77FeGe2N7P2-2PMe3)•+] 827.26139; found 827.26380 (correct
isotope pattern).
ACKNOWLEDGMENTS
■
Financial support by the Cluster of Excellence UniCat
(financed by the Deutsche Forschungsgemeinschaft and
administered by the TU Berlin) is gratefully acknowledged.
We thank Dr. E. Irran for helping in the XRD structural
determinations, Dr. J. Sutter and Prof. Dr. K. Meyer (University
Synthesis of [SiNSi]Fe(CO)2 Complex. A solution of [SiNSi]-
Fe(PMe3)2 (79 mg, 0.089 mmol) in 20 mL of pentane was set up
under a CO atmosphere after 3 freeze−pump−thaw cycles. The
reaction mixture was stirred overnight, affording an orange solid in
suspension with an orange solution. The solid was filtered off by
cannula and dried in vacuo, affording a first crop of the desired product
in high purity (40 mg). Concentration, crystallization, filtration, and
drying in vacuo of the remaining solution in pentane afforded the
second crop of the product (20 mg) for a total yield of 60 mg of the
of Erlangen) for the 57Fe Mossbauer spectroscopy measure-
̈
ments, and Dr. S. Enthaler (TU Berlin) for fruitful discussions.
REFERENCES
■
1
product (85% yield). H NMR (400.13 MHz, C6D6, 298 K): δ(ppm)
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= 1.39 (s, 36H, 4 × NC(CH3)3), 1.39 (t, overlapping with singlet at
3
1.39 ppm, 6H, NCH′H-CH3), 3.52 (q, JH−H = 7.1 Hz, 4H, NCH′H-
3
CH3), 6.18 (d, JH−H = 8.0 Hz, 2H, 3,5-H py), 6.81−7.06 (m, 10H,
3
arom. C-H), 7.31 (t, JH−H = 8.0 Hz, 1H, 4-H py). 13C{1H} NMR
(100.61 MHz, C6D6, 298 K): δ(ppm) = 15.2 (NCH2-CH3), 31.1
(NC(CH3)3), 38.9 (NCH2-CH3), 54.6 (NC(CH3)3), 95.8 (3,5-Carom
py), 127.3 (Carom), 129.7 (Carom), 129.7 (Carom), 130.9 (4-Carom py),
132.4 (Carom quaternary Ph), 165.0 (2,6-Carom py), 172.3 (NCN),
222.0 (CO). 29Si{1H} NMR (79.49 MHz, C6D6, 298 K): δ(ppm) =
98.3. IR (KBr pellet): νCO(cm−1) = 1830, 1778. APCI-MS (m/z):
calcd for [C41H59FeN7O2Si2•+] 793.36127; found 793.36353.
(3) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
Synthesis of [GeNGe]Fe(CO)2 Complex. The synthesis for this
complex was carried out in a fashion similar to the synthesis of
[SiNSi]Fe(CO)2 described previously. At 0.11 mmol scale. [GeNGe]-
126, 13794−13807.
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E.; Neese, F.; Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2006, 128,
13901−13912.
1
Fe(CO)2: 39% yield. H NMR (400.13 MHz, C6D6, 298 K): δ(ppm)
3
= 1.26 (s, 36H, NC(CH3)3), 1.39 (t, JH−H = 6.8 Hz, 6H, NCH′H-
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3
3
CH3), 3.71 (q, JH−H = 6.8 Hz, 4H, NCH′H-CH3), 6.24 (d, JH−H
=
8.0 Hz, 2H, 3.5-H py), 6.86−7.05 (m, 10H, arom. C-H), 7.34 (t, 3JH−H
= 8.0 Hz, 1H, 4-H py). 13C{1H} NMR (100.61 MHz, C6D6, 298 K):
δ(ppm) = 15.5 (NCH2-CH3), 31.4 (NC(CH3)3), 40.5 (NCH2-CH3),
54.8 (NC(CH3)3), 94.9 (3,5-Carom py), 127.3 (Carom), 129.4 (Carom),
129.7 (Carom), 132.1 (4-Carom py), 133.7 (Carom quaternary Ph), 162.8
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Chem.Eur. J. 2014, 20, 4403−4413.
(2,6-Carom py), 172.0 (NCN), 220.6 (CO). IR (KBr pellet) νCO(cm−1)
•+
= 1855, 1805. ESI-MS (m/z): calcd for [C41H59FeGe2N7O2
]
883.25067; found 883.25446. [GeNGe]Fe(CO)3: 30% yield. IR
(KBr pellet) νCO(cm−1) = 1956, 1883, 1869.
Catalytic Hydrosilylation of Acetophenones. In a nitrogen-
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mmol), and (EtO)3SiH (0.15 mmol) were weighed in a Schlenk flask
containing a stir bar and dissolved in 2 mL of THF. The flask was
taken out of the drybox, stirred, and heated to 70 °C in an oil bath for
22 h. The reaction mixture was treated with an aqueous potassium
hydroxide solution (2 mL, 5%) and stirring. The reaction mixture was
stirred for 2 h at room temperature and was extracted with diethyl
ether (3 × 5.0 mL). The combined organic layers were dried with
anhydrous Na2SO4 and filtered. Anisole (internal standard) was added,
and an aliquot was taken for GC-MS analysis (30 m Rxi-5 ms column,
40−300 °C). Removal of diethyl ether and dissolution in CDCl3
permitted the control also by NMR. Furthermore, the analytical
properties of the products are in accordance with commercially
available authentic samples.
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ASSOCIATED CONTENT
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AUTHOR INFORMATION
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Corresponding Author
*E-mail: matthias.driess@tu-berlin.de.
Notes
The authors declare no competing financial interest.
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