A new anthraquinoid ligand for the iron-catalyzed hydrosilylation of carbonyl compounds at room temperature: New insights and kinetics

Article abstract of DOI:10.1039/c8dt01123k

The reaction of 1-((2-(pyridin-2-yl)ethyl)amino)anthraquinone with either Fe(HMDS)₂ or Li(HMDS)/FeCl₂ allowed the preparation of a new anthraquinoid-based iron(ii) complex active in the hydrosilylations of carbonyls. The new complex Fe(2)₂ was characterized by single-crystal X-ray diffraction, infrared spectroscopy, NMR, and high resolution mass spectrometry (electrospray ionization). Superconducting quantum interference device (SQUID) magnetometry established no spin crossover behavior with an S = 2 state at room temperature. This complex was determined to be an effective catalyst for the hydrosilylation of aldehydes and ketones, exhibiting turnover frequencies of up to 63 min^-1 with a broad functional group tolerance by just using 0.25 mol% of the catalyst at room temperature, and even under solvent-free conditions. The aldehyde hydrosilylation makes it one of the most efficient first-row transition metal catalysts for this transformation. Kinetic studies have proven first-order dependences with respect to acetophenone and Ph₂SiH₂ and a fractional order in the case of the catalyst.

Full text of DOI:10.1039/c8dt01123k

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González, I. Kuzu and I. Fernández, Dalton Trans., 2018, DOI: 10.1039/C8DT01123K.
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Efficient extraction of sulfate from water using
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A New Anthraquinoid Ligand for the Iron-catalyzed
Hydrosilylation of Carbonyl Compounds at Room Temperature:
New Insights and Kinetics
Received 00th January 20xx,
Accepted 00th January 20xx
Álvaro Raya-Barón, a Carmen P. Galdeano-Ruano, a Pascual Oña-Burgos, a Antonio Rodríguez-
Diéguez, b Robert Langer, c Rosalía López-Ruiz, a Roberto Romero-González, a Istemi Kuzu c and
Ignacio Fernández * c
DOI: 10.1039/x0xx00000x
www.rsc.org/
The reaction of 1-((2-(pyridin-2-yl)ethyl)amino)anthraquinone with either Fe(HMDS)2 or Li(HMDS)/FeCl2 allowed the
preparation of a new anthraquinoid-based iron(II) complex active in hydrosilylations of carbonyls. The new complex Fe(2)2
was characterized by single-crystal X-ray diffraction, infrared spectroscopy, NMR and high resolution mass spectrometry
(electrospray ionization). Superconducting quantum interference device (SQUID) magnetometry established no spin
crossover behavior with an S = 2 state at room temperature. This complex was determined to be an effective catalyst for
the hydrosilylation of aldehydes and ketones, exhibiting turnover frequencies of up to 63 min-1 with a broad functional
group tolerance by just using 0.25 mol % of catalyst at room temperature, and even under solvent-free conditions. The
aldehyde hydrosilylation renders it one of the most efficient first-row transition metal catalyst for this transformation.
Kinetic studies have proven first-order dependences with respect to acetophenone and Ph2SiH2 and a fractional order in
the case of the catalyst.
environmentally benign and less toxic than its relatives.9
Consequently, many iron-based catalytic systems for the
hydrosilylation of carbonyl compounds have been reported in
Introduction
The reduction of carbonyl groups into alcohols is a widely used
transformation in organic chemistry, employed in the industry
as well as in the academic field.1 This type of transformation is
frequently used as an early stage evaluation towards the
development of catalytic systems for the reduction of CO2.2
Among the different methodologies currently available,
hydrosilylation has proved to be a convenient method to
convert a carbonyl function into an alcohol via a silyl ether
intermediate,3 which can also be useful as protecting group in
synthesis.4 Since the seminal works by Piers on Frustrated
Lewis Pairs,5 non-transition-metal-catalyzed hydrosilylations
have been developed.6 However, this reaction has been
traditionally carried out using second- and third-row transition
metals as catalysts.3,7 The high cost or toxicity of these metals
have motivated the research for new catalysts based on earth-
abundant non-toxic first-row transition metals.8 In this respect,
iron meets the requisites of being inexpensive,
recent years,10,19,23 but only few of them work at room
temperature and use low catalyst loadings (≤ 1 mol %). For
instance, iron(0) NHC carbene complexes of Fe3(CO)12, provide
excellent conversions of hydrosilylated aldehydes at room
temperature with 1 mol % within a few hours.11 A more
impressive system is the one developed by Tilley and
coworkers,12 which employed an iron silylamide complex in
concentrations as low as 0.01 % to achieve similar results. In
both cases, the source of hydrogen is always primary or
secondary silanes. Chirik and coworkers investigated defined
iron dialkyl complexes of pybox ligands and bisoxazoline at 0.3
mol % of concentration in the hydrosilylation of carbonyl
compounds with excellent yields.13
( )n
N
N
O
O
Fe(HMDS)2
( )n
O
Fe
(0.5 equiv.) or
N
O
H
N
N
O
FeCl2 (0.5 equiv.)
LiHMDS (1 equiv.)
N
( )n
O
1 (n=1)
2
(n=2)
Fe(1)2 (77%)
2
Fe( )2 (75%)
Scheme 1. Synthesis of anthraquinone-based iron complexes Fe(1)2 and Fe(2)2.
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Due to its advantageous electronic structure and reversible distance of ca. 3.45 Å (centroid-centroid distance), establishing
redox activity, the anthraquinone unit has found numerous
-stacking arrangements (Figures S19).15,17
applications when incorporated in transition metal The reaction of ligand and Fe(HMDS)2 allowed us to obtain
complexes.14 Interestingly, the catalytic activity of these complex Fe(
)2 in crystalline pure form in 75% isolated yield.
π
2
2
complexes is scarce, and even more pronounce when the iron Interestingly, the iron silylamide reagent deprotonates the
metal is involved. We have already developed an aminic ligand and introduces the metal centre in a single step
anthraquinone-based iron(II) catalyst Fe(1)2 (Scheme 1) for the (Scheme 1). Alternatively, complex Fe(2)2 can be synthesized
hydrosilylation of acetophenone as a preliminary work in this by using LiHMDS as a base and FeCl2 as the iron(II) source. This
field.15 Very good conversions were obtained at room method resulted also effective but with a slightly lower
temperature in short times, although the transformation of isolated yield of 64 %.
1
acetophenone into 1-phenylethanol didn't reach complete Despite its paramagnetism, the H NMR spectrum of complex
conversion. To overcome the catalytic performance of this Fe(
2
)
in THF-d8 was partially assigned. It exhibits 11
previous iron complex, we describe herein a more flexible resonances attributable to a C2 symmetric compound with
2
ligand that significantly improves the catalytic properties of its signals in the range of
iron complex, wich represents one of the most active catalysts signal appears as
δ
a
H -43.6 to 57.3 ppm. The most up-field
rather broad resonance almost
in hydrosilylations of carbonyls described so far.
indiscernible from the noise with a half line width (W1/2) of
1955 Hz.
When the same reaction is performed with 1.0 equiv. of
Fe(HMDS)2 instead of half of it, a new set of signals is
observed. This new species shows 13 resonances consistent
with a C1 paramagnetic complex tentatively assigned to a 1:1
Results and Discussion
In the quest of alternative anthraquinone-based ligands, we
envisaged the synthesis of a more flexible chelate which would
assist the catalytic process by allowing the easy access of the
substrates to the iron center. We then synthesized N-
stoichiometry species such as Fe(2)HMDS. The resonances
with highest and lowest frequencies were located at δH 128.3
and -79.1 ppm, respectively. Interestingly, there is a signal of
prominent intensity at δH 12.6 ppm (W1/2 = 457 Hz) that was
assigned to the 18 protons associated with the N(HMDS)2
(ethylene-2-pyridine)-1-aminoanthraquinone
(ligand
2,
Scheme 1) from 1-chloroanthraquinone and 2-(2-
aminoethyl)pyridine via a simple and quick reaction in good
yields. Importantly, the method allows the recovery of most of
the unreacted 1-chloroanthraquinone for its reuse. The
procedure followed is a modification of the one described by
Barasch et al,16 where we were able to reduce the amount of
solvent and employ a minor excess of aminoethylpyridine. All
the details regarding its NMR characterization (including two-
dimensional 1H, 13C and 15N NMR) together with its X-ray
crystallographic data can be found in the supporting
information. The 1H NMR spectrum of the ligand shows a
diagnostic signal attributed to the NH resonance at δH 9.89
ppm, due to hydrogen-bonding with the C=O moiety. Among
many other 13C NMR features, we highlight the hydrogen-
1
moiety consistent with the assignment. Figure 1 shows the H
NMR spectra for both complexes. Unfortunately, all the
attempts to isolate the 1:1 complex failed, always obtaining
only complex Fe(2)2 instead.
These evidence prompted us to study the one to one
stoichiometry reaction in more detail. 1H NMR monitoring of
this reaction at ambient temperature showed that the group
of signals attributed to complex Fe(
2)
increased with
2
increasing reaction time, which also produced the progressive
diminution of the relative concentration of complex
Fe(2)HMDS.
bonded and non-bonded carbonyls with signals at δC 185.0 and
183.9 ppm, respectively. The FT-IR bands located at 3272 and
1626 cm-1 corroborates further this intra-molecular hydrogen
bonding (Figure S13). 1H,15N gHMBC experiments optimized for
1/2JNH of 83.3 ms (Figure 1), allowed the assignment of two
nitrogens at δN 84.4 (NH) and 308.6 ppm for the
aminoanthraquinone and pyridine units, respectively.
In the solid state, the X-ray crystal structure of 2, grown by
hexane diffusion in a dichloromethane solution, confirms the
chelating potential of the ligand given the intramolecular
hydrogen bond between NH and C=O (quinone) of 1.888(2) Å
(Figure S20). This interaction produced a carbonyl bond slightly
longer (C9-O1, 1.235(2) Å) than the one found for the non-
hydrogen-bonded case (C10-O2 1.222(2) Å). In the solid state,
the torsion angle between the pyridine plane and the aminic
moiety (N1-C17-C16-C15) of 178.9(13)º shows an inadequate
Figure 1. 1H NMR (500 MHz) spectra of complexes Fe(2)2 and Fe(2)HMDS in THF-d8 at
room temperature.
From a 1:8 ratio between Fe(2)2 and Fe(2)HMDS, respectively,
at the beginning of the reaction, we ended up with a 1:2 ratio
after 4 days at room temperature, that kept constant over the
following days. At this point, we decided to heat the sample up
to 60 ºC and monitor both species within time. To our surprise,
chelate conformation. As in the case of ligand
1,
anthraquinone moieties of 2 are oriented in the crystal
network in a face-to-face disposition with an inter-planar
2 | J. Name., 2012, 00, 1-3
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after 16 hours no modification of the equilibrium ratio was Complex Fe(
ARTICLE
1)
2
displays two six-membered and another two
observed what is consistent with a high activation energy five-membered metalla rings, all of them with highly planar
between these two species. Anyhow, all these findings support conformation,15 and thus with a more rigid backbone than that
the existence of a thermodynamic equilibrium between both one found in Fe(
complexes, with the former being the most stable species for accommodate any incoming substrate. From this perspective
the stoichiometry employed. Single crystals of complex Fe( complex Fe( )2 is expected to behave as a more reactive pre-
were also grown and analyzed by X-ray diffraction. The catalyst than its former analogue.
complex crystallizes with a solvent molecule as Fe( 2ꢀ0.5thf. A The FT-IR spectra (Figure S14) in THF solution showed two C=O
view of the complex is shown in Figure 2. Selected crystal data stretching bands at 1643 and 1579 cm-1, indicating the
and bond lengths and angles are given in Table S3. unsymmetrical coordination of the two quinone oxygens in
2)2, which would make it more difficult to
2)
2
2
2)
In the complex, the ethylenic unit is rearranged adopting a solution.
pseudo-gauche conformation with a torsion angle N1-C15- The cyclic voltammogram of Fe(2)2 in THF exhibits two quasi-
C16-C17 of 76.4(6)º. Another major change observed in the reversible reduction waves at -1.54 and -2.08 V versus Fc/Fc+
conformation of upon coordination of iron(II) is the loss of couple (Fc = ferrocene), which were assigned to the reduction
planarity of the anthraquinone; the three-rings system of the anthraquinone moiety of the coordinated ligand (Figure
completely planar in the free ligand, is now bent around an 3). In comparison to the uncoordinated the reduction waves
2
2
axis defined by the two carbonyl groups. The lack of are shifted to more negative potentials, providing further
coplanarity together with the elongation of 0.030Å of the evidence for the coordination of the anthraquinone group in
metal-coordinated C=O bond (Table S3), is ascribed to an solution. An irreversible oxidation wave with a peak potential
increased sp3 hybridization character of C9. The reduced angle of 0.80 V and a quasi-reversible oxidation wave at 0.91 V are
of 115.7(5)º for O1-C9-C12 in the complex compared to the attributed to the oxidation of iron(II) and one of nitrogen-
ligand together with the slight reduction on the N1-C1 distance containing donor groups.
bond, support the latter statement. These two features are
highly consistent with a partial delocalization of the negative
charge from the amide to the aromatic ring.
The geometry around the metal center can be described as a
distorted octahedron that can be viewed as a mer isomer, i.e.
the two tridentate ligands contain both trans (O1/N2 and
O3/N4) and cis (O1/N1 and O3/N3) pairs (Figure 2). A total of
four six-membered metalla rings are defined in the structure,
two of them established in the chelation of each ligand.
The metalla rings that involves the anthraquinone moiety
resembles a quite planar conformation where only the Fe
atom is out of the plane by 0.89 and 0.90 Å, respectively. The
other metalla ring, built through the more flexible ethylenic
bridge displayed a twist-boat conformation. Interestingly,
significant differences are found between Fe(2) and our
2
previously described pre-catalyst Fe(
state structures are carefully inspected.
1
)2,15 when their solid-
Figure 3. Cyclic voltammogram of complex Fe(2)2 in THF (1 mM, scan rate 100 mV/s,
Pt//0.2 M n-Bu4NPF6//Pt).
Apart from its crystallographic and potentiometric
characterization, MS spectrometry was used to verify the
structural nature of Fe(2) in more detail. The positive ESI
2
mode revealed a parent ion corresponding to [C42H30FeN4O4]+
with m/z of 710.16465, that fits the solid state structure. As
expected, the observed isotopic cluster pattern matches well
with the calculated one (Figures S15 and S16).
The magnetic properties of Fe(2) were studied by variable
2
temperature solid-state SQUID magnetometry in the
temperature range of 2-300 K (Figure S21). Data were
collected on three independently prepared, analytically pure
samples, and the individual runs were indistinguishable. The
variable-temperature magnetic properties indicate that there
is no spin crossover behavior observable in the temperature
Figure 2. X-ray diffraction structure of complex Fe(2)2. ORTEP view with ellipsoids at range from 2 to 300 K. At 300 K, χ K
MT equals 2.91 cm3 mol-1
30% probability level. Co-crystallizing solvent molecule omitted for clarity.
which established an average molar magnetic susceptibility
M) of 0.0097 cm3·mol-1, consistent with a doublet ground
(
χ
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state, 3.92 (~ 4) unpaired electrons, and therefore a high-spin considerably longer reaction times, obtaining a 91% of
Fe(II) electronic configuration. When the temperature is conversion after 14 hours (entry 11, Table 1). The reaction
lowered,
temperature,
at K, which can be attributed to very weak 94% to >99%. It is also worth mentioning that no additive
)2 in catalytic amount, was needed
for the reactions to take place.
The substrate scope was explored using (EtO)2MeSiH as the
reducing agent of choice, at 0.50 mol % of iron catalyst Fe(
χ
MT remains constant down to
∼30 K. Below this proved to be also valuable under solvent-free conditions (entry
χ
MT decreases reaching a value of 1.20 cm3 mol-1 6), even improving the conversion obtained in solution from
K
2
antiferromagnetic interactions just as other works have other than our complex Fe(
reported.18
2
Hydrosilylation of carbonyl compounds
2
)
2
We started our study of the catalytic performance of Fe(2)2 in and at room temperature as the optimized conditions set. The
carbonyl hydrosilylation by screening different hydrosilanes as catalytic outcome was excellent for the reduction of aromatic
reducing agents and varying the catalyst loading. The and aliphatic ketones as well as for sterically demanding
benchmark carbonyl was acetophenone (Table 1). From our substrates. The reaction with benzaldehyde (entry 1, Table 2)
previous experience using Fe(
1
)
as pre-catalyst,[15] we was so quick at 0.50 mol % Fe(
2
)2 load that we were not able
2
1
expected the hydrosilylation process to occur at room to record its H-NMR spectrum before it was fully converted.
temperature in THF with a catalyst need below 1 mol %. We The TOF values obtained at 95% of conversion was of 38 min-1.
first tested the most inexpensive silanes such as Et3SiH and
[a]
Table 2. Iron-catalyzed hydrosilylation of aldehydes and ketones using Fe(2)2.
PMHS, which showed undetectable (entry 2) or poor (entry 3)
hydrosilylation conversions. With Me2PhSiH as silane, the
Time
(min)
10
Conv.
(%)[c]
>99
Entry
Substrate
Cat. (mol %)
catalytic activity was also undetectable (entry 4), which shows
than even introducing an aryl group in the silane did not affect
the outcome under the conditions assayed.
1
2
0.50
0.25
30
>99 (90)
Table 1. Optimization of the reaction conditions for the hydrosilylation of
acetophenone.[a]
3
4
0.25
0.25
5
>99 (89)
Entry
1
Silane
Ph2SiH2
Cat. (mol %)
0.50
Time (h)
Conv. (%)
1
1
56
5
>99 (90)
>99 (88)
2
Et3SiH
0.50
0
3
PMHS
0.50
1
13
4
Me2PhSiH
PhSiH3
0.50
1
0
5
0.25
20
5
0.50
1
99
94 (>99)[b]
>99 (81)[c]
86
6
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
(EtO)2MeSiH
0.50
1
6
7
8
9
0.25
0.25
0.25
0.25
5
60
5
7
7
0.50
2
>99 (78)
79
8
0.25
1
9
0.25
2
92
10
11
0.05
1
37
25
>99 (77)
0.05
14
91
[a] Reaction conditions: acetophenone (0.56 mmol), silane (0.63 mmol), THF (2
mL), room temperature. Catalyst percentage relative to the carbonyl substrate.
Quenched with aqueous 1M HCl (2 mL). Conversions were determined by 1H-
NMR of the reaction crude. Control experiments using only ligand 1 as catalyst
were performed without product formation. [b] In parenthesis is shown the
conversion of the run without THF. [c] Isolated yield in parenthesis.
10
0.50
120
98 (58)
11
12
0.50
0.50
120
120
94 (79)
90 (69)
The best results among the selected reductants were achieved
with PhSiH3 and (EtO)2MeSiH (entries 5 and 6). Remarkably,
(EtO)2MeSiH yielded complete carbonyl reduction (>99% as
judged by 1H NMR spectroscopy) of acetophenone in two
hours (entry 7). The TOF value for this transformation at 68%
of conversion was 1632 h-1 (27.2 min-1), which is a significant
value in the range of the best iron-based catalysts but still far
from those provided by Stradiotto and Turculet of 393 min-1,19
or those reported by Trovitch and coworkers (>14850 h-1) with
transition metals others than iron.20
13
14
0.50
0.50
120
120
83 (41)
31
[a] Reaction conditions: ketone or aldehyde (0.56 mmol), silane (0.63 mmol), THF
(2 mL), room temperature. Catalyst percentage relative to the carbonyl substrate.
Quenched with aqueous 1M HCl (2 mL). Conversions were determined by 1H-
NMR of the reaction crude. [c] Isolated yield in parenthesis.
Lowering the loading of catalyst down to 0.25 mol % still
allowed excellent conversions of silyl ethers (entries 8-9). The
use of 0.05 mol% of catalyst resulted in the need of
To the best of our knowledge these turn-over frequencies
represent one of most rapid found for iron-catalyzed
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hydrosilylations of carbonyls at room temperature, albeit not registering a better activation step for other ketones were
always identical experimental conditions are compared. We unsuccessful due to very broad signals in the NMR spectra.
thus decided to bring the catalyst loading down to 0.25 mol % To rule out the formation of iron nanoparticles as the
for the comparative study of the whole set of aldehydes. catalytically active species, we performed one of the already
Under these conditions the conversion obtained was again reported methods for differentiating between hetero- and
quantitative but in 30 minutes of reaction time (entry 2, Table homogeneous catalysts. The experiment is called the
2). For benzaldehyde derivatives, we found that electron- “fractional poisoning experiment”,21 and uses less than one
donating substituents located at the para position decreased equivalent of a poisoning reagent such as trimethylphosphine.
the reaction times down to 5 minutes for complete In our case, we reproduced entry 7 in Table 1 but using 0.5
hydrosilylation (entries 3-4, Table 2), and that with electron- equivalents of phosphine relative to our pre-catalyst Fe(2)2.
withdrawing substituents such as fluorine, almost no effect Interestingly, the reaction was not poisoned completely and
was observed (entries 5, Table 2). In the case of nitrogen- proceeded ca. two times slower than without phosphine. In
based substrates such as picolinaldehyde and 4- fact, the observed rate mimicked the one catalyzed with only
dimethylaminobenzaldehyde, extended reaction times were 0.25 mol % of catalyst. For comparison issues we provide in
necessary to reach full conversion, with 60 minutes for the Figure S27 the kinetic profile when the loading of catalyst is of
former and 25 minutes for the latter (entries 6-9, Table 2). A 0.50 mol %.
plausible reason for this reaction retardation is the
coordination of these nitrogen-based moieties to the metal
center that could compete with substrate coordination. The
alkane functionality of ketone substrates such as 3,4-
dihydronaphthalen-1(2
H
)-one
(entry
10,
Table
2),
cyclohexanone (entry 11, Table 2) and 2-butanone (entry 12,
Table 2) produced excellent conversions but within 2 hours of
reaction times. We should consider that this decreased activity
is because, in general, ketones react slower than the
corresponding reaction of aldehydes. This could arise from an
increased steric component and the less electrophilic carbonyl
carbon atom, which was supported by the lower conversion of
83% obtained in the case of benzophenone (entry 13, Table 2)
after 2 hours of reaction. Substrates containing both carbonyl
group and C-C double bond were also assayed showing a
significant decreased on reactivity (entry 14, Table 2).
Hydrosilylation of esters, amides, cyano and alkenes were
attempted, but no reaction product was observed under
similar conditions to those used for ketones and aldehydes.
Thus, these functional groups are not expected to interfere
with the reduction of aldehydes or ketones if present in the
same molecule. Carboxylic acids, alcohols or primary amines
are not compatible with the strong basic character of complex
Figure 4. Reaction profiles of the catalytic hydrosilylation of acetophenone,
picolinaldehyde, 4-dimethylaminobenzaldehyde and benzophenone, all of them using
(EtO)2MeSiH as silane and Fe(2)2 as catalyst.
These observations support the hypothesis that the active
catalytic species is our new anthraquinonic homogeneous
complex and not any heterogenous system that could have
formed during the elapsed time such as surface atoms of
nanoparticles. Such nanoparticles have explained prolonged
induction periods in many metal-catalyzed processes which
are accepted as constituted by two steps, i.e. slow nucleation
and then fast agglomeration.[22]
Fe(2)2.
Regarding the nitro function, hydrosilylation of 4-nitro-
benzaldehyde was carried out, but very little conversion into
the corresponding alcohol was obtained, presumably due to
competing interaction of the NO2 group with the iron center in
the catalyst. No product derived from the reduction of the
nitro group was detected either.
To examine the repeatability of the transformation, we
performed eight consecutive catalytic runs of the
hydrosilylation of acetophenone on a J-Young NMR tube (entry
7, Table 1) with only one loading of catalyst (0.5 mol %). No
significant loss of catalytic activity was observed after eight
reaction cycles, where conversions higher than 99% for all the
1
The reactivity of some of these substrates was monitored by
1H-NMR spectroscopy in J-Young NMR tubes. These reaction
profiles are shown in Figure 4 and allowed us to distinguish the
maximum rates and culmination steps. We did not observe the
characteristic induction periods in the fastest catalysis but in
the case of the slowest, i.e. benzophenone, a characteristic
induction period of approximately 25 minutes was observed.
The intersection of the maximum rate tangent with the
abscissa was taken as the time required for the catalyst
activation (Figure 4).21 Further experiments focused on
successive cycles were determined by H NMR. Remarkably,
the reaction crude in all the cycles remained clean without
forming significant amounts of side products.
Kinetic study
In order to determine the rate law of the reaction, we varied
the concentration of each component of the reaction,
following a similar methodology than the one described by
Bleith and Gade.23 For the reduction of acetophenone with
(EtO)2MeSiH, we found that the initial rate of the reaction was
not appreciably affected as a function of acetophenone
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[Fe] precursor = Fe(2)2
concentration, indicating a zeroth order dependence on
ketone concentration (Figure 5a, line in triangles).
Hydride
However, when the same set of experiments were performed
using the non-coordinating Ph2SiH2 instead of (EtO)2MeSiH, the
initial rate of the reaction showed a positive linear correlation
with the acetophenone concentration, indicating first order
dependence in this case (Figure 5a, line in circles).
Regeneration
R
II [Fe]
H
R'
O
[Si]
H
O
O
Ketone
R
r.d.s
Coordination
R'
R'
R
R
When we tried to ascertain the reaction order on silane, and
varied the concentration of (EtO)2MeSiH, we found that the
initial rate of the reaction decreased as the silane
concentration increased (line in triangles, Figure 5b). However,
when the same experiments were performed using Ph2SiH2 as
the reducing agent, we found again a positive linear
correlation between the initial rate of the reaction and the
silane concentration what again corroborates a first order
dependence (Figure 5b). The intersection of the linear fit in the
latter was found to be not displaced from the origin which is
sign that there was no consumption of silane during
precatalyst activation. When we represent the initial rate of 1-
phenylethan-1-ol (acetophenone reduction product) versus
pre-catalyst concentration in the range of 0.05 to 0.5 mol %,
using either (EtO)2MeSiH or Ph2SiH2 (Figure 5c), we found in
both cases positive linear correlations with slopes of ca. 0.4 in
the Ln-Ln graphs (Figure S22-S25). At present, the exact reason
for this observed fractional rate-orders is not clear and will
require further rationalization. From these kinetic data, the
rate law for the iron-catalyzed hydrosilylation of ketones is
given by the following equation.
R'
H
H
O
[Si]
H
II[Fe]
II [Fe]
fast
R
Ketone
Reduction
Silane
Activation
H
R'
II[Fe]
O
[Si]-H
Scheme 2. Mechanistic proposal for the Fe(II)-catalyzed hydrosilylation of ketones.
In the diethoxymethylsilane case, there must be a secondary
process that participates in the activation step, inactivating
further the metal center as the concentration of silane
increases, i.e. ethoxy coordination. We actually had observed a
similar phenomenon with our previously reported complex
Fe(1
)2,15 when we carried out stoichiometric experiments in
the quest of intermediates species derived from the use of
catalyst. When 1 equivalent of (EtO)2MeSiH is added to a
solution of Fe(1)2 in THF-d8 and the reaction is monitored by
1H-NMR, the appearance of a new set of paramagnetic signals
is observed along a 24 hours period. When a second
equivalent of (EtO)2MeSiH is added to the mixture, a
progressive increase of a new set of signals together with the
complete disappearance of the resonances corresponding to
ꢀ[ꢁ]
ꢉ.ꢊ
ꢎ
ꢎ
ꢆ ꢇ
= ꢃ ∙ [ꢄꢅ 2
[
]
[
]
]
∙ ꢁℎꢈꢋꢌꢍꢈ ∙ ꢏꢅꢂꢐꢑꢅ
ꢈ
ꢀꢂ
The above findings lead us to propose a mechanistic cycle for
the iron-catalyzed hydrosilylation as depicted in Scheme 2. It
matches previous studies,15 where four main steps are
suggested: (i) coordination of the ketone (in equilibrium), (ii)
reduction of the ketone (fast); (iii) silane activation (in
equilibrium), and (iv) Fe–hydride regeneration and product
release. The first-order dependences with respect to
acetophenone and Ph2SiH2 implies that the rate-determining-
step is the latter, which involves an intermediate that
comprises both the ketone and the silane, so when deriving
the rate law, first orders result.
Fe(
1
)2 after another 24 hours is noticed (Figure S12).
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(EtO)2MeSiH molecules are involved in Fe coordination as no
irreversible reaction of the ligand was evidenced. In any case,
to prove why the initial rate of the reaction did not linearly
increase with the diethoxymethylsilane concentration, we
screened the catalytic performance of [Fe(1)2{(EtO)2MeSiH}2]
by adding acetophenone in the reaction media. To our
surprise, no hydrosilylation products nor any transformation of
the starting materials were appreciated by 1H NMR under
these conditions.
The addition of 10 equivalents of (EtO)2MeSiH together with
10 more of acetophenone did not conduct to any
transformation either, proving [Fe(1)2{(EtO)2MeSiH}2] to be
completely inactive in hydrosilylation, and therefore
corroborating that no rate dependence with this
diethoxysilane could be observable.
Interestingly, when these two equivalents are added to
complex Fe(2)2, we were not able to detect any new species
and only slow degradation of the complex into a crowded
uncharacterized mixture was observed. Even more, when
simultaneously to the diethoxymethylsilane were added
acetophenone, complete desired product formation was
immediately observed, proving the much more reactive the
complex is when ligand
2 is chelating the metal center, than
when ligand 1 is involved.
Experimental Section
General procedures of laboratory
All air-sensitive experiments were performed under an inert
atmosphere of N2 using standard Schlenk techniques or a
glovebox. Deuterated solvents were degassed and dried over
activated molecular sieves prior to use. DMSO was dried by
distillation from anhydrous CaH2 and then stored over
activated 4Å molecular sieves. 2-(2-aminoethyl)pyridine was
distilled from KOH under reduced pressure before use. 1-
Chloroanthraquinone was purchased from Aldrich and used as
received. THF and hexane were dried and degassed via elution
through a solvent column drying system.24 Fe(HMDS)2 was
synthesized by a method previously described in literature.25
LiHMDS and FeCl2 were purchased from ABCR and used as
received. All hydrosilanes employed in the catalytic runs were
purchased from Acros and used without further purification.
All ketones and aldehydes used for catalysis were purified and
dried by standard methodologies prior to use.26 Triethylamine
was distilled from CaH2 and degassed. NMR spectra were
measured in Bruker Avance III 300 and Bruker Avance III 500
spectrometers. IR spectra were recorded in a FT-IR Bruker
Alpha spectrometer. Mass spectra were acquired with and
Orbitrap mass spectrometer (Thermo Fischer Scientific).
Elemental analyses (EA) were performed on a Elementar vario
EL cube in the CHN mode. Magnetization and variable
temperature (2-300 K) magnetic susceptibility measurements
on polycrystalline samples were carried out with a Quantum
Design SQUID device operating at different magnetic fields.
Values of the magnetic susceptibility were corrected for the
underlying diamagnetic increment using tabulated Pascal
Figure 5. a) Initial rate as
a function of concentration of acetophenone using
(EtO)2MeSiH (red triangles) and Ph2SiH2 (orange circles); b) Initial rate as a function of
concentration of (EtO)2MeSiH (green triangles) and Ph2SiH2 (blue circles); c) Initial rate
as a function of concentration of pre-catalyst Fe(2)2 using Ph2SiH2 as reducing agent.
This new complex shows a similar paramagnetic behavior
exhibiting 10 signals in the range from δH -56.1 to 95.0 ppm.
High-resolution mass spectrometry (electrospray ionization)
experiments confirmed this new species to be consistent with
a complex that contains two molecules of (EtO)2MeSiH
together with the Fe(1)2 entity. The presence of a molecular
ion at m/z 950.28919 corroborates the formation of this new
complex. As expected, the observed isotope pattern matches
well with the one calculated for the exact mass of
[C50H54FeN4O8Si2] (Figure S17-18). The accurate molecular
structure and how these two silane molecules are attached to
the iron center remain unknown since all the attempts to
obtain crystals suitable for X-ray analysis were unsuccessful. To
determine that the new species arises from silane coordination
and not from the hydrosilylation of the quinonic carbonyls in
ligand
was hydrolyzed with diluted aqueous HCl. After extraction of
the organic components with ethyl acetate, ligand was
1, a sample of this new complex [Fe(1)2{(EtO)2MeSiH}2]
1
recovered unaltered, from which we concluded that the
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constants and the effect of the blank sample holders (gelatin 70.60; H, 4.64; N, 7.42. 1H NMR (500.13 MHz, THF-d8):
δ (ppm)
57.32 (W1/2 = 298 Hz, bs), 32.39 (W1/2 = 242 Hz, bs), 20.86
capsule/straw).
(W1/2 = 112 Hz, bs), 18.48 (W1/2 = 213 Hz, bs), 9.67 (W1/2 = 77
Hz, s), -17.64 (W1/2 = 157 Hz, bs), -21.20 (W1/2 = 337 Hz, bs), -
Synthesis of ligand 2
Ligand
method.16 To a pre-heated solution of 1-chloroanthraquinone (m/z) for C42H30FeN4O4 : 710.16110; found: 710.16465 [M]+. IR
(2 g, 8.1 mmol) in dry DMSO (20 mL), 2-(2-aminoethyl)pyridine (THF solution):
(cm-1) 2924w, 2854w, 1643m, 1579m,
2 was synthesized modifying a previously described 43.58 (W1/2 = 1955 Hz, bs). 5 signals not located. ESI-MS: calcd
+
υ
(2 mL, 16.7 mmol) was added and the resulting solution stirred 1518m, 1486m, 1444m, 1411m, 1347m, 1307m, 1256m,
at 150 ºC for 10 minutes. The solution was then poured into 1230m, 1160w, 1092w, 1070w, 1001w, 917w, 775w, 711w.
cold water (200 mL) and the red precipitate was filtered out
General procedures for the catalytic hydrosilylation of carbonyl
and washed with water. The pure product was obtained as a
compounds
red powder after flash chromatography on silica gel using
EtOAc/n-hexane (1:1) as eluent. Yield: 1.34 g (50%). Suitable Inside the glovebox, compound Fe(
2)2 (0.25-0.50 mol %) was
crystals for X-ray diffraction were grown by layering a dissolved in THF (2 mL) inside a small vial. The corresponding
1
with hexane. M.p. 124.8 ºC. H silane (0.63 mmol) was then added to the solution, followed by
dichloromethane solution of
NMR (500.13 MHz, CDCl3):
2
δ
(ppm) 10.89 (1H, bs, NH), 8.63 the carbonyl substrate (0.56 mmol), and the vial was sealed.
(1H, d, J = 4.2 Hz, H-21), 8.29 (1H, dd, J = 1.5 Hz, 7.5 Hz, H-8), The resulting mixture was stirred at room temperature during
8.25 (1H, dd, J = 1.5 Hz, 9 Hz, H-5), 7.78 (1H, ddd, J = 1.5 Hz, 7.2 the times indicated in Tables 2 or 3 and then the reaction was
Hz, 7.5 Hz, H-7), 7.72 (1H, ddd, J = 1.8 Hz, 7.5 Hz, 7.5 Hz, H-6), quenched by adding HCl(aq) 1M (2 mL) and stirred for 30 min.
7.67 (1H, dd, J = 1.8 Hz, 7.5 Hz, H-19), 7.62 (1H, dd, J = 1.5 Hz, to ensure the complete hydrolysis of the silyl ether product.
7.5 Hz, H-4), 7.56 (1H, dd, m, H-3), 7.25 (2H, m, H-18, H-20), The mixture was then taken to pH 7 by adding NaOH(aq) 5M
3.82 (2H, dt, J = 7.1, 5.4 Hz, H-16), 3.25 (2H, t, J = 7.1 Hz, H-15). and the organic products extracted with ethyl acetate (3x15
13C-NMR (125.76 MHz, CDCl3):
δ (ppm) 185.0 (CO, C9), 183.9 mL). The organic layers were dried using anhydrous Na2SO4,
(CO, C10), 158.8 (C17), 151.3 (C1), 149.7 (C21), 136.3 (C19), filtered and concentrated by rotatory evaporation.
135.3 (C3), 135.0 (C12), 134.7 (C14), 133.9 (C7), 133.0 (C6), Quantitative NMR was applied for conversion determination.
132.9 (C11), 126.7 (C8), 126.7 (C5), 123.6 (C19), 121.8 (C18), Isolated products were obtained after column chromatography
117.9 (C2), 115.8 (C4), 113.1 (C13), 42.8 (C16), 37.8 (C15). 15N in silica gel using ethyl acetate/hexane or dichloromethane as
NMR (500.13 MHz, CDCl3, via gHMBC):
(cm-1) 3272w, 3077w, 2933w, 1663s, 1626s, 1589s,
δ 84.4, 308.6 ppm. IR eluent.
(KBr):
υ
1568s, 1511s, 1474m, 1407m, 1366m, 1310m, 1268s, 1167m,
General procedure for NMR monitored hydrosilylation reactions
Compound Fe( )2 (0.05-0.50 mol%) was put inside a J. Young
1072m, 987m, 708s.
2
screw-capped NMR sample tube and dissolved in THF-d8 (1
Synthesis of complex Fe(2)2
mL). The corresponding silane (0.63 mmol) was then added to
A solution of Fe(HMDS)2 (34.4 mg, 0.091 mmol) in THF (12 mL) the solution, followed by the carbonyl substrate (0.56 mmol),
was added to a solution of (60 mg, 0.183 mmol) in THF (8 and the tube was closed. The sample tube was shaken to
2
mL). The resulting solution was stirred overnight at room ensure the homogeneity of the mixture and then placed inside
temperature and then all the volatiles were removed under the NMR spectrometer, where remained for the entire
reduced pressure. The dark solid was dissolved in THF (10 mL), duration of the reaction monitoring.
layered with hexane (10 mL) and kept at -20 ºC for 24 hours to
Quantitative NMR acquisition parameters
yield the pure product as dark blue crystals. Yield: 48.5 mg
(75%). X-ray diffraction suitable crystals were grown from 1H NMR determination of product’s conversion was carried out
layering a THF solution of complex Fe(
room temperature. The complex crystallizes with a solvent product. The standard acquisition parameters were one-
molecule as Fe( 2ꢀ0.5thf. The THF molecule is disordered over dimensional pulse sequence which includes a 30º flip angle
an inversion center and the positions are occupied by factor (Bruker zg30), recycle time (D1 = 30 s), time domain (TD = 64k),
0.5. Complex Fe( can alternatively be synthesized using number of scans (NS = 1), acquisition time (AQ = 2.97 s),
2)2 with n-pentane at by comparing signals arising from both carbonyl substrate and
2)
2)
2
FeCl2 as iron(II) precursor. Thus, a solution of LiHMDS (26.8 transmitter (frequency) offset (O1P = 8.0 ppm), and spectral
mg, 0.152 mmol) in THF (5 mL) was added to a solution of
2
width (SW = 22.0 ppm).
(50 mg, 0.152 mmol) in THF (12 mL). The resulting blue
solution, was poured over a suspension of FeCl2 (9.6 mg, 0.076
Single Crystal X-ray Diffraction
mmol) in THF (5 mL) with triethylamine (42 µL, 0.304 mmol) Ligand 2. X-ray data collection of suitable single crystal was
and the mixture was stirred overnight. After removing all the done at 100(2) K on a Bruker VENTURE area detector equipped
volatiles under reduced pressure, dichloromethane was added with graphite monochromated Mo-Kα radiation (λ = 0.71073
in order to dissolve Fe(2)2 and remove LiCl by filtration. Then, Å) by applying the ω-scan method. The data reduction was
dichloromethane was removed under reduced pressure and performed with the APEX227 software and corrected for
the product purified as described above. Elemental analysis: absorption using SADABS.28 Crystal structure was solved by
calcd (%) for C42H30FeN4O4: C, 70.99; H, 4.26; N, 7.88; found: C, direct methods using the SIR97 program29 and refined by full-
8 | J. Name., 2012, 00, 1-3
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matrix least-squares on F2 including all reflections using
anisotropic displacement parameters by means of the WINGX
crystallographic package.30 All hydrogen atoms were included
as fixed contributions riding on attached atoms with isotropic
thermal displacement parameters 1.2 times or 1.5 times those
of their parent atoms for the organic ligand except for
hydrogen atom pertaining to N1.
Fe(2)2ꢀ0.5thf. Data was collected at 100(2) K on an STOE IPDS
II diffractometer using Mo-Kα radiation (λ = 71.073 pm) and ω-
scan rotation. The structure was solved by direct methods and
the refinement of all non-hydrogen atoms was performed with
SHELX-97.31 All non-hydrogen atoms were refined with
anisotropic thermal parameters. Hydrogen atoms were
calculated on idealized positions using a riding model. The
structure figures were generated with ORTEP.32
1
2
a) V. B. Pukhnarevich, E. Lukevics, L. I. Kopylova, M. G.
Voronkov, Perspectives of Hydrosilylation, Institute of
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Rappoport, Y. Apeloig, Eds.; John Wiley & Sons: New York,
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Synthesis; I. Ojima, Ed.; Wiley-VCH: New York, 2000.
a) H. Koinuma, F. Kawakami, H. Kato and H. Hirai, J. Chem.
Soc., Chem. Commun., 1981, 213; b) T. Matsuo and H.
Kawaguchi, J. Am. Chem. Soc., 2006, 128, 12362; c) A. K. Roy,
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2 and
Fe( 2ꢀ0.5thf are summarized in Table S1-S4. The
2)
supplementary crystallographic data have been deposited with
the Cambridge Crystallographic Data Centre (CCDC numbers
1576113 and 1577523, respectively).
Conclusions
The synthesis and characterization of a highly active Fe(II)
catalyst for the hydrosilylation of aldehydes and ketones have
been described. Ligand deprotonation by two alternative
methods resulted in a redox innocent amino-anthraquinonic
chelate with TOFs of up to 63 min-1 using very mild conditions
such as 0.25 mol% of catalyst, room temperature, and even
under solvent-free conditions. The catalyst has also been
found to tolerate a range of functional groups where
aldehydes are the most rapidly reduced and nitrogen-based
derivatives the slowest. Kinetic studies have proven the rate-
determining step to be subject to silane concentration first
order dependence, and zeroth or first order in ketone when
(EtO)2MeSiH or Ph2SiH2 are employed, respectively.
Investigations into the enantioselectivity of these reactions are
currently undergoing in our laboratory.
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by the Junta de Andalucía (Spain)
under the project number P12-FQM-2668. A. R-B., R. L-R, and
P. O-B thank the University of Almería and MEC for Ph.D.
fellowships and a Ramón y Cajal contract (RYC-2014-16620),
respectively.
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Notes and references
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Put some iron-anthraquinoid in your chemistry. The synthesis and characterization of a highly
active Fe(II) catalyst for the hydrosilylation of aldehydes and ketones have been described.