Bis(silylenyl)-substituted ferrocene-stabilized η6-arene iron(0) complexes: Synthesis, structure and catalytic application

Article abstract of DOI:10.1039/c7dt03301j

Reaction of FeX₂(thf)_n (X = Cl n = 1.5, Br n = 2) with the chelating 1,1′-bis(silylenyl)-substituted ferrocene ligand SiFcSiA (Fc = ferrocendiyl, Si = PhC(N^tBu)₂Si:) furnishes the corresponding dihalido Fe(ii) complexes [(SiFcSi)FeX₂] (X = Cl, 1 and X = Br, 2) in high yields. Reduction of the latter with an excess of KC₈ in the presence of benzene and toluene leads to the unprecedented bis(silylene) stabilized Fe⁰ complexes [(SiFcSi)Fe-η⁶(C₆H₆)] 3 and [(SiFcSi)Fe-η⁶(C₇H₈)] 4, respectively. The ⁵⁷Fe M?ssbauer spectrum of 3 at 13 K exhibits parameters (σ = 0.3676 mm s^-1; ΔE_Q = 1.334 mm s^-1) which are consistent with the presence of a pentacoordinated Fe⁰ atom in a pseudo trigonal-bipyramidal coordination environment, with two dative Si→Fe bonds and three coordination sites occupied by the η⁶-coordinated arene ligand. Results from DFT calculations, ⁵⁷Fe M?ssbauer parameters and the diamagnetic NMR spectra confirm the redox-innocent nature of these ligands and the zero oxidation state of the iron center. The catalytic ability of 3 was investigated with respect to ketone hydrogenation. In all cases, good to excellent yields to the corresponding alcohols were obtained at 50 °C and 50 bar H₂ pressure. Electron-donating as well as -withdrawing substituents were tolerated with excellent to good yields. Conversions of bulkier ketones and unactivated aliphatic ketones lead merely to moderate yields. This represents the first example of a silylene-iron metal complex which has been utilized as a highly active precatalyst in the hydrogenation of ketones. The results underline the powerful ability of chelating bis(N-heterocyclic silylene) ligands acting as strong σ-donor ligands in stabilizing a new generation of low-valent, electron-rich transition metal complexes for catalytic transformations.

Full text of DOI:10.1039/c7dt03301j

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Luecke, D. Porwal, A. Kostenko, Y. Zhou, S. Yao, M. Keck, C. Limberg and M. Oestreich, Dalton Trans.,
2017, DOI: 10.1039/C7DT03301J.
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Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Bis(silylenyl)-substituted ferrocene-stabilized ꢀ6-arene iron(0)
complexes: synthesis, structure and catalytic application†
Marcel-Philip Luecke,a Digvijay Porwal,b Arseni Kostenko,a Yu-Peng Zhou,a Shenglai Yao,a
Matthias Keck,c Christian Limberg,c Martin Oestreichb and Matthias Driess*a
Reaction of FeX2(thf)n (X = Cl n = 1.5, Br n = 2) with the chelating 1,1’-bis(silylenyl)-substituted ferrocene ligand SiFcSi A
(Fc = ferrocendiyl, Si = PhC(NtBu)2Si:) furnishes the corresponding dihalido Fe(II) complexes [(SiFcSi)FeX2] (X = Cl, 1 and
X = Br, 2) in high yields. Reduction of the latter with an excess of KC8 in the presence of benzene and toluene leads to the
unprecedented bis(silylene) stabilized Fe0 complexes [(SiFcSi)Fe-ꢀ6(C6H6)] 3 and [(SiFcSi)Fe-ꢀ6(C7H8)] 4, respectively. The
57Fe Mössbauer spectrum of 3 at 13 K exhibits parameters (σ = 0.3676 mm·s-1; ΔEQ = 1.334 mm·s-1) which are consistent
with the presence of a pentacoordinated Fe0 atom in a pseudo trigonal-bipyramidal coordination environment, with two
dative Si→Fe bonds and three coordination sites occupied by the ꢀ6-coordinated arene ligand. Results from DFT
calculations, 57Fe Mössbauer parameters and the diamagnetic NMR spectra confirm the redox-innocent nature of these
ligands and the zero oxidation state of the iron center. The catalytic ability of 3 was investigated with respect to ketone
hydrogenation. In all cases, good to excellent yields to the corresponding alcohols were obtained at 50 °C and 50 bar H2
pressure. Electron-donating as well as -withdrawing substituents were tolerated with excellent to good yields. Conversions
of bulkier ketones and unactivated aliphatic ketones lead merely to moderate yields. This represents the first example of a
silylene-iron metal complex which has been utilized as a highly active precatalyst in the hydrogenation of ketones. The
results underline the powerful ability of chelating bis(N-heterocyclic silylene) ligands acting as strong σ-donor ligands in
stabilizing a new generation of low-valent, electron-rich transition metal complexes for catalytic transformations.
Received 00th January 2017,
Accepted 00th January 2017
DOI: 10.1039/x0xx00000x
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homogeneous catalysis.[6,9] Experimental data indicated that the
σ-donor strengths of this novel bidentate ligands can be adjusted
by modification of the heterocyclic backbone (Figure 1: A, B).[10]
Introduction
After the discovery of an isolable N-heterocyclic carbene (NHC) by
Arduengo et al. in 1991,[1] Denk and West et al. isolated the first
N-heterocyclic silylenes (NHSis) a few years later.[2] The electronic
nature of divalent SiII with a singlet electronic ground state enables
NHSis to act as strong σ-donor (Si(3s)ꢀTM) and π-acceptor
(TMꢀSi(3p)) ligands in NHSi-TM complexes (TM = transition metal).
To date, a variety of NHSi-TM complexes have been isolated and
demonstrated to be active in small molecule activation reactions[3,4]
and a variety of catalytic transformations.[5,6] For the first time, the
ability of an NHSi to act as supporting ligand in a catalytically active
reaction was first demonstrated in Suzuki cross-coupling reaction
of aryl boronic acids with bromoarenes by Fürstner et al.[7] Due to
its stronger σ-donor ability compared to already broadly
established phosphines and NHCs as supporting ligands in
catalysis,[8] some of us showed that mono- and bidentate
NHSis (Chart 1) can be utilized as powerful steering ligands in
Chart 1 Examples of catalytically active mono- and bi-dentate metallylene-
transition metal complexes.
Recently, the synthesis of a new bis(silylenyl)-substituted ferrocene
ligand (SiFcSi) A (Fc = ferrocendiyl, Si = PhC(NtBu)2Si:) was reported.
This ligand is one of the strongest σ-donor ligand, as shown by
isolation of a bis(silylene)-ꢀ5-cyclopentadienyl Co(I) complex.[11] The
catalytic abilities of the Co(I) complex were probed in [2+2+2]
cycloaddition reaction of phenyl acetylene and acetonitrile.
Moreover, our group could show that a bis(silylene)-substituted
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ortho-carborane Ni complex B complex can serve as superior ligand group P-1. The molecular structures (Fig. 1) of 1 and 2 exhibit a
in the nickel-catalyzed amination of arenes.[12]
distorted-tetrahedral geometry (Σ∡: 1: 436.15 °, 2: 436.31 °), in
which the iron atom is coordinated by the bidentate chelating
bis(NHSi) ligand A and two halides. Both complexes 1 and 2 show
similar Fe-Si, Si-N and Si-C bond lengths and an average Si-Fe-Si
angle of 100.60 °. The Fe-Si distances in 1 (2.4923(4); 2.4823(4) Å)
are akin to the previously reported bond lengths in the
paramagnetic bis(silylene) pincer-type dichlorido FeII complex with
d(Fe-Si) of 2.5256(7) and 2.5110(7) Å.[15]
In the past few years, iron-based catalysis has gained considerable
attention as an alternative to established catalysts based on noble
metals due to its greater abundance, low-cost and low toxicity.[13]
The synthesis of the highly electron-rich monodentate hydrido
silylene Fe0 complex D,[14] and pincer-type iron complex C,[15]
isolated in our group, are the first examples of catalytically active
silylene-iron(0) complexes in hydrosilylation of ketones. Also,
complex C exhibit the highest electron density on the iron center
for pincer-type complexes with an additional intramolecular dative
(pyridine)NꢀFe bond. Following the isolation of the first bis(NHC)-
stabilized ꢀ6-arene Fe(0) complex E in 2013 and its application in
catalytic organic amide reduction,[16] we were highly interested in
the isolation and reactivity of the analogous bis(NHSi)-stabilized ꢀ6-
arene Fe0 complexes. Herein, we report the first ꢀ6- arene bis(NHSi)
Fe0 complexes 3 and 4 which are stabilized by the bidentate SiFcSi
chelate ligand A. Complex 3 has been successfully applied as first
example of a silylene-TM complex for the hydrogenation of
unsaturated organic substrates.
Results and Discussion
[11]
Starting from A,
we first targeted the isolation of the dihalido
FeII complexes [(SiFcSi)FeX2] 1 (X = Cl) and 2 (X = Br) as suitable Fe0
precursors. Dehalogenation of 1 and 2 wit KC8 should give access to
a
series of new SiFcSi-stabilized ꢀ6-arene Fe0 complexes.
A
stoichiometric mixture of A and FeX2(thf)n (X = Cl n = 1.5; Br n = 2)
were dissolved in benzene, resulting in an intense yellow solution
at room temperature. Monitoring the reaction progress by 1H NMR
spectroscopy (see Fig. S1 in ESI) shows the formation of
paramagnetic species with broad signals and chemical shifts in the
range of σ = 1 to 14 ppm. Crystallization from concentrated
benzene solutions resulted in orange-yellow crystals in high yields
(ca. 70 %) at room temperature, which were suitable for X-ray
diffraction analysis (Fig. 1).
Fig. 1 Molecular structures of 1 (top) and 2 (bottom) at 50 % probability
level. Hydrogen and solvent atoms are omitted for clarity. 1: Selected bond
lengths [Å]: Fe2-Cl1 2.255(6), Fe2-Cl2 2.2553(4), Si1-Fe2 2.4923(4), Si2-Fe2
2.4823(4), Si2-N3 1.8438(11), Si2-N4 1.8543(11), Si1-N2 1.8490(11), Si1-N1
1.856(12). Selected bond angle [°]: Cl1-Fe2-Cl2 119.94(2), Si1-Fe2-Si2
100.341(13), Cl2-Fe2-Si1 109.481(14), Cl1-Fe2-Si1 108.269(14), Cl2-Fe2-Si2
107.609(15), Cl1-Fe2-Si2 109.442(15). 2: Selected bond lengths [Å]: Fe2-Br1
2.402(5), Fe2-Br2 2.397(5), Si1-Fe2 2.498(8), Si2-Fe(2) 2.504(8), Si2-N3
1.850(2) , Si2-N4 1.863(2), Si1-N2 1.853(2), Si1-N1 1.859(2). Selected bond
angle [°]: Br1-Fe2-Br2 117.04(2), Si1-Fe2-Si2 100.90(3), Br2-Fe2-Si1
108.40(2), Br1-Fe2-Si1 109.82(2), Br2-Fe2-Si2 109.97, Br1-Fe2-Si2 109.48(2).
Reduction of 1 and 2 (Scheme 2) with an excess of KC8 in presence
of benzene or toluene (in THF, 1:1) gives the desired 18 electron ꢀ6-
arene SiFcSi Fe0 complexes 3 and 4 in moderate yields (≈ 60 %) after
workup. The reduction, followed by 1H NMR spectroscopy, reveals
the formation of
dehalogenation is faster (3h) compared to
a
new diamagnetic species. Using 2, the
(5h) at room
1
Scheme 1 Synthesis of the dihalido (SiFcSi) FeII complexes
1 and 2.
temperature. After removal of all volatiles in vacuum, 3 and 4 could
be extracted with diethylether and isolated as red-brown, air
When the reactions were carried out in THF, 1 and 2 can be
isolated in higher yields of 94 % (1) and 87 % (2) on multi gram
scale (up to 3 g), respectively. The complexes 1 and 2 are very air
sensitive and degrade to a black powder within a couple of minutes
in air. Complexes 1 and 2 are well soluble in benzene, toluene and
THF without decomposition. Their compositions were confirmed by
elemental analysis. Their 57Fe-Mössbauer spectra were recorded at
13 K: The isomer shift (σ) and quadrupole splitting (ΔEQ) values for
the bis-SiII-coordinated Fe center (σ [mm·s-1] = 0.6619; 0.6562 and
ΔEQ [mm·s-1] = 3.2667; 3.459 for X = Cl; Br) are in accordance with
the expected values for distorted high-spin FeII complexes in a
tetrahedral environment (S = 2) (see Fig. S2 in ESI).[15,17] This is also
in agreement with the observed paramagnetic 1H NMR spectra in
solutions. Complex 1 crystallizes in the orthorhombic space group
Pbca; the dibromido FeII complex 2 crystallizes in the triclinic space
sensitive solids which decompose above 145 °C (3, arene
=
benzene) and 143 °C (4, arene = toluene) without melting,
respectively.
Scheme 2
. Synthesis of 3 and 4 through dehalogenation of 1 and 2 in the
presence of benzene and toluene, respectively.
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The composition of the isolated complexes 3 and 4 were confirmed complexes
PAPER
like
by elemental analysis and ESI-MS spectrometry (see Fig. S4 in ESI). [(NHSi)Fe(dmpe)2] D (2.184(2) Å, dmpe = 1,2 bis(dimethylphosphany
The 1H-NMR spectra of the diamagnetic complexes in deuterated l)ethane),[14] and tridentate pincer-type (SiNSi) iron(0) complex C
benzene (C6D6) exhibit resonances in accordance with the molecule (2.1637(15); 2.1695(13) Å) with σ-donating ligands (dmpe;
structure. The resonance signal of the protons of the coordinate PMe3).[15]
benzene in 3 appears as a singlet with a chemical shift of
δ = 5.16 ppm which is shifted to lower field in comparison with
other known ꢀ6(benzene) Fe0 complexes (see Fig. S3 in ESI), bearing
Cyclic voltammetry was performed for 3 and 4 in THF showing one
reversible redox event at E1/2 = -1.56 V (vs Fc/Fc+) for 3 and at
bis(NHC) E,[16] or bis(phosphine) ligands (σ = 4.56 - 4.91 ppm).[18] E1/2 = -1.58 V (vs Fc/Fc+) for 4 (cyclic voltammogram, see Fig. S6 in
Because of their relatively low solubility of 3 and 4 in deuterated ESI). These redox waves have been assigned to a respective Fe0/Fe+1
solvents such as C6D6 and THF-d8, the respective 13C- and 29Si-NMR electron transfer process since the HOMO is metal centered as
spectra could not be recorded.
confirmed by DFT calculations (see below Fig 5). Irreversible
reduction processes occur in the cationic region, which are
overlapping with the reversible electron transfer process of the
ferrocene in the bis(NHSi) ligand A, which is assumed to result from
irreversible reduction processes of this ligand.
Fig. 3 CV of 3 (red) and 4 (black) (1mM in THF/ 0.1M TBAPF6) recorded at a
scan rate v = 100 mV·s-1 showed one reversible redox event for both
complexes.
Fig.
2 Molecular structures of 3 (top) and 4 (bottom) at the 50 %
To gain further insights into the electronic features, both 3 and 4
were additionally characterized by 57Fe Mössbauer spectroscopy
(see Fig. S5 in ESI). The 57Fe Mössbauer parameters of 3 exhibit two
doublets, shown in Fig. 4; one for the FeII center in the ferrocene
backbone (σ = 0.5301 mm·s-1; ΔEQ = 2.3012 mm·s-1) and one for the
bis(NHSi)-stabilized Fe0 site with an isomeric shift of
σ = 0.3676 mm·s-1 and quadrupole splitting of ΔEQ = 1.334 mm·s-1.
The 57Fe Mössbauer parameters for both complexes are
summarized in Table 1 together with previously reported penta-
coordinated Fe0 complexes for comparison.[15,16]
probability level. Hydrogen and solvent atoms are omitted for clarity. 3:
Symmetry transformations used to generate equivalent atoms (`): 1-x, -y,z.
Selected bond lengths [Å]: Fe2-Si1 2.1782(16), Fe2-C22 2.084(5), Fe2-C23
2.089(6), Fe2-C21 2.089(6), C21-C22 1.426(9), Si1-C16 1.899(5), Si1-N1
1.917(4), Si1-N2 1.914(4). Selected bond angle [°]:Si1-Fe2-Si1` 89.31(8), N1-
Si1-N2 68.16(7). 4: Selected bond lengths [Å]: Fe2-Si2 2.1752(8), Fe2-Si1
2.1707(8), Si1-C31 1.890(3), Si1-N1 1.913(2), Si1-N2 1.912(2), Fe2-C44
2.078(3), Fe2-C46 2.079(3), Fe2-C45 2.085(3), Fe2-C43 2.096(3), Fe2-C47
2.103(3), Fe2-C41 2.130(3).C41-C42 1.505(4). Selected bond angle [°]: C41-
C43 1.408(4). Si1-Fe2-Si1` 89.52(3), N1-Si1-N2 68.11(9).
Red rhombohedral crystals suitable for X-ray diffraction analysis
(Fig. 2) were obtained in the case of 3 in saturated benzene
solutions and for 4 in diethyl ether at room temperature. 3
crystallizes in the orthorhombic space group Fdd2 with one co-
crystallized benzene molecule in the unit cell. Compound 4 co-
crystallizes with two diethyl ether molecules in the unit cell in the
monoclinic space group C12/c1 and exhibits almost identical metric
parameters. Both η6-arene Fe complexes 3 and 4 have the same
Si-Fe-Si bond angle of virtually 90 ° (3 89.27(8) °; 4 89.52(3) °). The
SiIIꢀFe bond lengths (3, 2.1782(16) Å; 4, 2.1707(8), 2.1752(8) Å) are
shorter compared to those of 1 (2.486(6) Å) and 2 (2.499(8) Å),
because of π-back donation from the electron-rich Fe0 center to the
SiII atoms of the chelating bis(NHSi) ligand A. The Si-Fe bond lengths
are shorter compared to [(NHSi)Fe(CO)4] (2.234(2) Å) [19], but indeed
comparable to other reported silylene- or bis(silylene) stabilized Fe0
Fig. 4 57Fe Mössbauer spectrum of 3 in the solid state at 13 K. Circles are the
experimental data, lines the simulated data with parameters:
σ = 0.3604 mm·s-1; ΔEQ = 1.334 mm·s-1 for the Fe0 core.
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Table 1 57Fe-Mössbauer parameter of pentacoordinated Fe0 complexes.
Complex
σ [mm·s-1]
ΔEQ [mm·s-1]
Ref.
[SiFcSi]Fe(ꢀ6
[SiFcSi]Fe(ꢀ6
-
-
C6H6)
C7H8)
3
0.36
0.41
0.43
0.24
1.33
1.27
1.37
1.66
this work
this work
[16]
4
C6H6) E
C
[bis(NHC)]Fe(ꢀ6
-
[SiNSi]Fe(PMe3)2
[15]
The 57Fe Mössbauer parameters strongly depend on the nature of
the donors and the coordination symmetry around the iron atom.
The σ-value is directly related to the s-electron density at the iron
center, giving insights into the oxidation state of the Fe center when
compared with other pentacoordinate Fe complexes. If compared
with previously reported pentacoordinate Fe0 complexes (Table 1),
the isomeric shift of 3 and 4 (σ = 0.36 ; 0.41 mm·s-1) lies in the range
between 0.24 and 0.43 mm·s-1.[15,16] Interestingly, by changing from
toluene to benzene as stabilizing ligand, a reduced σ-value was
observed, indicating a stronger interaction between the Fe center
and the coordinated benzene versus toluene. This is in agreement
with the observed Fe-Arene distances in 3 (1.535 Å) and 4 (1.549 Å)
by structural analysis where an elongation could be observed, going
from benzene to toluene. DFT calculations (see details in ESI) show
that the exchange of toluene coordinated in 4 by benzene, forming
3, is slightly exergonic (4
+ benzene ꢀ 3 + toluene; -
ΔG = - 2.1 kcal·mol-1) which is experimentally confirmed when 4 was
exposed to benzene at room temperature. The bis(NHC) analoguos
complex E
[16]
has a notably higher σ-value (0.43 mm·s-1) which is
Fig. 5 Frontier molecular orbitals in 3 calculated at the B3LYP-D3 level with
s6-31G* basis set for Fe and 6-31G* basis set for all other atoms. The
energies in bold parentheses are the calculated energies for 4 (red = iron,
blue = nitrogen, green = silicon).
assumed to be related to the reduced σ-donor strength of CII versus
SiII.[15] The structure of 3 and 4 can be described as a penta-
coordinated Fe0 complex by comparing their quadrupole splitting
(ΔEQ) with the previously reported values for the complexes C and
E. The 57Fe Mössbauer parameter are consistent with a penta-
coordinate Fe0 atom in a pseudo-trigonal bipyramidal coordination
environment, with two coordinative Si→Fe bonds and three
coordination sites occupied by the ꢀ6-coordinated arene ring, which
is in agreement with the results of the structural analysis of both
complexes. In addition, to gain further insights into the electronical
structure of the iron core and the bonding situation, DFT
calculations were performed for 3 and 4 and additionally for the
bis(NHC) complex E. The metric parameters of the calculated
structures are in very good agreement with the experimental values
(Tab. 2).
The LUMO + 1 and LUMO of both complexes are located at the
phenyl groups of the coordinated N-heterocyclic silylene moieties in
ligand A, which corresponds to a π*-orbital. The π backdonation
from the Fe0 center into the 3p-orbital of the SiII atoms is reflected
in the HOMO. The HOMO - 1 orbital reflects the ꢁ(arene)
interaction between the coordinated arene ring (benzene/toluene)
and the bis(NHSi) stabilized Fe0 center. The HOMO-LUMO gap is
nearly identical in both complexes with 2.599 eV (3) and 2.584 eV
(4, see Fig. S17 in ESI) and smaller when compared with the
bis(NHC) complex E with a HOMO-LUMO gap of 2.710 eV.[16]
Overall, the results from DFT calculations of 3 and 4 confirm the
redox-innocent behavior of the ligands with a zero oxidation state
of the Fe atom.
Table 2 Comparison of DFT-calculated and experimental distances and
angles for 3 and 4.
Furthermore, the complex 5 was synthesized (Scheme 3) by
areneꢀCO ligand exchange under a CO atmosphere and by direct
access through reduction of the precursor complex 1 in the
presence of CO. When solutions of 3 and 4 in THF are exposed to
CO (1 atm), an immediate color change from red to yellow is
observed at room temperature. The IR spectra of the identical
products were recorded after removal of the solvent (see Fig. S14 in
ESI) resulting in three strong stretching frequencies (υCO = 1941,
1900, 1862 cm-1), which are typical for cis-L2M(CO)3 complexes with
2A` und 1A`` IR active modes. This provided further evidence of an
authentic zero oxidation state of the iron center in 3 and 4. As
expected for 5, the CO stretching frequencies are bathochromic
shifted compared to monodentate [(NHSi)Fe(CO)4] complexes due
to the bis(NHSi) coordination.[5,20] Surprisingly, when compared to a
bis(NHC) carbonyl Fe0 complex,[16] the IR stretching frequencies
(υCO = 1968, 1890, 1865 cm-1) are very similar.
complex
Fe-Si [Å]
3
4
Exp.
2.177
2.177
89.3
Calc.
2.164
2.164
89.2
Exp.
2.175
2.171
89.5
Calc.
2.171
2.172
89.7
Si-Fe-Si [°]
Fe-Arene [Å]
1.535
1.540
1.549
1.547
Both complexes show identical frontier orbitals, with very few
differences in the absolute energies of the LUMO + 1, LUMO,
HOMO and HOMO - 1 molecular orbitals as shown in Fig. 5 (see
details in chapter 4 ESI).
4 | Dalton Trans., 2017, 00, 1-3
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Scheme 3 Synthesis of 5 via a) ligand exchange reaction and b) reduction.
Table 3 Optimization of the acetophenone hydrogenation.[a]
By changing from NHC to NHSi as ligand, the υCO values of 5 are
practically identical with those of the corresponding carbonyl
complex of E. The affinity of the free bis(NHC) ligand relatively to
the bis(NHSi) ligand towards Fe0 center in 3 has been calculated
(Scheme 4), resulting in a negligible ΔG value of - 0.8 kcal mol-1,
which is a suitable explanation for the identical CO stretching
frequencies in the IR spectra.
Entry
Solvent
T [°C]
p(H2) [bar]
Conv (%)[ b]
1
2
3
4
5
6
7
THF
RT
RT
RT
RT
50
50
50
20
20
50
50
50
50
50
30
66
Toluene
THF
50
By reduction of the precursor complex 1 in presence of a CO
atmosphere, 5 can also be isolated in good yields (72 %) as an
orange solid. Complex 5 is stable under reduced pressure and shows
no decomposition in solutions after several hours. Several attempts
to crystallize 5 in benzene, THF and diethyl ether were not
successful. The recorded 1H and 13C{1H} NMR spectra in C6D6 show a
Toluene
THF
70
55
Toluene
Toluene
>99
80[c]
[a] All reactions were performed on a 0.10 mmol scale. [b] Determined by
GLC analysis with respect to starting material. [c] Conversion measured
after 10 h.
very symmetric pattern, where
a single resonance signal at
σ = 225 ppm could be assigned to the three equivalent CO ligands
through a DEPT-135 NMR experiment. The composition of 5 was
confirmed by ESI-MS spectrometry where the [M]+ peak could be
observed (calc. for [C43H54N4Si2Fe2O3]+ (m/z): 842.2428; found:
842.2430 (see Figs. S12, S13 in ESI).
This afforded clean reduction of the aldehyde to the corresponding
alcohol (c1a → c2a). Likewise, acetophenone also underwent
reduction to afford 1-phenylethan-1-ol in near quantitative yield
(c1b → c2b). Ketones with electron-donating (c1c/c1d → c2c/c2d)
as well as -withdrawing (c1e−c1g → c2e−c2g) substituents in the
para position of the aryl group were also tolerated with excellent to
moderate isolated yields. Similarly, ethyl phenyl ketone could also
be reduced affording the corresponding alcohol with 80% isolated
yield (c1h → c2h).
Scheme 4 DFT calculations of the affinity of the free bis(carbene) ligand
towards the iron(arene) center in 3.
Catalytic hydrogenation
To probe the utility of the novel Fe complex 3, we performed the
catalytic hydrogenation of various ketones and an aldehyde for
comparison (Scheme 5). Screening of the reaction parameters such
as pressure, temperature and solvent (Table 3) revealed that
2.5 mol% of 3 allow the hydrogenation of acetophenone in toluene
at 50 °C under a dihydrogen pressure of 50 bar (entry 6). Notably,
any deviation from this catalytic setup in terms of catalyst loading,
solvent or temperature led to lower conversions. To further expand
the substrate scope, we subjected benzaldehyde and several
electronically modified ketones to our optimized setup (Scheme 5).
Scheme 5 Fe(0)-ꢀ6(arene) complex 3 catalyzed reduction of various Ketones.
All reactions were performed on a 0.10 mmol scale. Isolated yields were
calculated after flash column chromatography. Conversions are determined
by GLC analysis with respect to starting material.
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DOI: 10.1039/C7DT03301J
PAPER
Dalton Transaction
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Conflict of Interest
There are no conflicts of interest to declare.
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We thank the German Academic Exchange Service (predoctoral
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Forschungsgemeinschaft (Cluster of Excellence UniCat, EXC
314/2) for financial support. M. O. is indebted to the Einstein
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Notes and References
6 | Dalton Trans., 2017, 00, 1-3
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