Chemoselective hydrogenation and transfer hydrogenation of aldehydes catalyzed by iron(II) PONOP pincer complexes

Article abstract of DOI:10.1021/acs.organomet.5b00105

Iron-catalyzed hydrogenation has drawn much attention, yet the scope and chemoselectivity of iron catalysis warrant further improvement. Here we report new iron pincer complexes as chemoselective hydrogenation and transfer hydrogenation catalysts. Several Fe(II) complexes supported by a 2,6-bis(phosphinito)pyridine ligand (PONOP) have been synthesized. Fe(II) hydride complexes [(ⁱPrPONOP)Fe(CO)(H)Br] (2) and [(ⁱPrPONOP)Fe(CO)(H)(CH₃CN)](OTf) (3) can activate H₂ at room temperature. Complexes 2 and 3 are hydrogenation catalysts at room temperature, and the hydrogenation is selective for aldehyde in the presence of ketone and alkene groups. 2 and 3 are also chemoselective transfer hydrogenation catalysts using sodium formate as the hydride source.

Full text of DOI:10.1021/acs.organomet.5b00105

Article
pubs.acs.org/Organometallics
Chemoselective Hydrogenation and Transfer Hydrogenation of
Aldehydes Catalyzed by Iron(II) PONOP Pincer Complexes
Simona Mazza, Rosario Scopelliti, and Xile Hu*
Laboratory of Inorganic Synthesis and Catalysis, Institute of Chemical Sciences and Engineering, Ecole Polytechnique Fed
́ ́
erale de
Lausanne (EPFL), ISCI-LSCI, BCH 3305, 1015 Lausanne, Switzerland
S
* Supporting Information
ABSTRACT: Iron-catalyzed hydrogenation has drawn much
attention, yet the scope and chemoselectivity of iron catalysis
warrant further improvement. Here we report new iron pincer
complexes as chemoselective hydrogenation and transfer
hydrogenation catalysts. Several Fe(II) complexes supported
by a 2,6-bis(phosphinito)pyridine ligand (PONOP) have been
synthesized. Fe(II) hydride complexes [(iPrPONOP)Fe(CO)-
(H)Br] (2) and [(iPrPONOP)Fe(CO)(H)(CH3CN)](OTf) (3) can activate H2 at room temperature. Complexes 2 and 3 are
hydrogenation catalysts at room temperature, and the hydrogenation is selective for aldehyde in the presence of ketone and
alkene groups. 2 and 3 are also chemoselective transfer hydrogenation catalysts using sodium formate as the hydride source.
INTRODUCTION
RESULTS AND DISCUSSION
■
■
Synthesis and Characterization of Complexes. The
Catalytic hydrogenation of organic substrates using molecular
hydrogen (H2) is one of the most commonly practiced reaction
types in organic synthesis. While numerous platinum-group-
metal complexes have been developed as active and selective
catalysts,1 there is a strong desire for iron-based catalysts.2,3
Iron catalysis4−6 is attractive not only because iron is an
abundant, inexpensive, and nontoxic metal but also because the
unique electronic structures and reactivity of iron complexes in
comparison with those of their platinum-group-metal counter-
parts may lead to novel mechanistic pathways and new
substrate scope.7−21
pincer ligand used in this study is the PONOP ligand (PONOP
= 2,6-bis(phosphinito)pyridine). Several transition-metal com-
plexes of PONOP had been reported.27−30 This ligand is
structurally similar to the PNP ligand (PNP = 2,6-bis-
(phosphinomethyl)pyridine).13,31,32 There is, however, an
important difference between the two ligands. The PNP ligand
engages in a reversible dearomatization−rearomatization
process33 to promote H2 cleavage and hydrogenation in a
bifunctional mechanism. This possibility no longer exists in the
PONOP ligand when the acidic benzylic CH2 groups are
replaced by ether O atoms.
Recently a number of iron catalysts have been developed for
hydrogenation of polar substrates, especially ketones.9−19,22−25
Despite this progress, the scope and selectivity of Fe-catalyzed
hydrogenation of carbonyl substrates remain to be improved.
For example, the selectivity between ketone and aldehyde is
challenging. Selective hydrogenation of aldehyde over ketone
normally requires stoichiometric metal hydride reagents at a
low temperature.26 Beller and co-workers recently reported the
first example of catalytic hydrogenation of aldehyde that is
chemoselective against ketone.16 However, that reaction
required an elevated temperature (120 °C) and a high H2
pressure (30 bar).
Herein, we report new iron pincer complexes that activate H2
and catalyze selective hydrogenation of aldehydes at room
temperature and under a low pressure of H2 (4 bar). The
hydrogenation is chemoselective not only to C−C double
bonds but also to the more challenging ketone moiety. The
complexes also catalyze selective transfer hydrogenation of
aldehydes using sodium formate as the hydride source.
The new Fe-PONOP complexes were prepared by following
standard procedures in Fe-pincer chemistry (Scheme 1).
Complex 1 is diamagnetic and exhibited a singlet at δ 215
ppm in the 31P{1H} NMR spectrum. The Fe-bound CO ligand
gives a strong IR band at 1961 cm−1. The crystal structure of 1
shows a distorted-octahedral geometry at the Fe center; the two
bromine ligands are trans to one another, while the CO ligand
is trans to the nitrogen of the pyridine.34
The substitution of bromide by hydride in complex 2
lowered the CO vibrational frequency to 1928 cm−1 in the IR
spectrum. In the 31P{1H} NMR spectrum of 2 one singlet was
observed at δ 239.00 ppm, while in the 1H NMR spectrum the
hydride ligand shows a characteristic triplet at δ −20.74 ppm
(2JPH = 56.6 Hz). The solid-state structure of complex 2 (Figure
1) reveals a distorted-octahedral geometry. The Fe−H bond is
1.38(3) Å. The hydride is trans to the bromide ligand, while the
CO is trans to the nitrogen of the pyridine.
Received: February 6, 2015
© XXXX American Chemical Society
A
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Scheme 1. Synthesis of New Iron Pincer Complexes 1−3
The 31P{1H} NMR spectrum of the cationic Fe-hydride
complex [(iPrPONOP)Fe(CO)(H)(CH3CN)](OTf) (3) was
nearly identical with that of 2, while the 1H NMR signal of the
hydride ligand was shifted to δ −18.53 ppm (2JPH = 53.7 Hz).
The CO IR frequency was observed at 1951 cm−1. The crystal
structure of 3 again indicates a distorted-octahedral geometry
(Figure 1). The hydride ligand is trans to the acetonitrile
molecule, and the Fe−H distance is 1.43(4) Å. The other bond
distances are comparable for complexes 2 and 3.
Hydrogen Activation. Both complexes 2 and 3 are capable
of activating H2, as confirmed by H/D exchange experiments.
When D2 (30 bar) was passed into a solution of 2 in MeOD-d4,
the formation of H2 (δ 4.58, singlet) and HD (δ 4.54 ppm,
1:1:1 triplet, JH−D = 42 Hz) was observed within minutes in the
1H NMR spectra. Meanwhile the hydride signal at δ −20.74
ppm decreased in intensity and completely disappeared after 45
min (Figure 2). This indicates that the Fe−H moiety was
converted into Fe−D during this process. The same behavior
was observed when the pressure of D2 was reduced at 1 bar,
although the hydride signal completely disappeared only after
32 h. The H/D exchange on 2 also occurred in THF but was
much slower (Table 1 and Figures S2 and S3 (Supporting
Information)). Complex 3 is stable in THF and methanol; the
CH3CN ligand is not replaced by the solvent molecule. The H/
Figure 2. Time-dependent 1H NMR spectral change of the reaction of
2 with D2 (30 bar) in MeOD-d4 at ambient temperature. The full
spectra are reported in the Supporting Information (Figure S6).
a
Table 1. Comparison of the Rates of H/D Exchange
entry
complex
solvent
time for 50% exchange (h)
0.18
1
2
3
4
5
6
2
2
2
3
3
3
MeOD-d4
THF-d8
b
>150
CD3CN
MeOD-d4
THF-d8
no H/D exchange
18
20
CD3CN
no H/D exchange
a
See Figures S2−S5 in the Supporting Information for the time-
b
dependent conversion of H/D exchange. After 150 h the conversion
was 35%. The conditions for the time-dependent H/D scrambling of
complexes 2 and 3 are reported in the Supporting Information.
D exchange reactions mediated by 3 have similar rates in
methanol and in THF. In methanol, the H/D exchange
mediated by 3 is slower than the exchange mediated by 2, but
in THF the opposite order was observed (Table 1 and Figures
S4 and S5 (Supporting Information)). In CH3CN, complex 2
Figure 1. (left) X-ray structure of 2. The thermal ellipsoids are drawn at the 30% probability level. The non-hydride hydrogen atoms are omitted for
clarity. Selected bond lengths (Å) and angles (deg): Fe1−N1 1.9839(16), Fe(1)−Br(1) 2.5156(4), Fe(1)−P(2) 2.1574(6), Fe(1)−P(1) 2.1702(5),
Fe(1)−H(1) 1.38(3), Fe(1)−C(18) 1.736(2); P(2)−Fe(1)−P(1) 160.89(2). (right) X-ray structure of the cationic complex of 3. The thermal
ellipsoids are drawn at the 30% probability level. The non-hydride hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles
(deg): Fe(1)−C(18) 1.734(5), Fe(1)−N(2) 1.972(3), Fe(1)−N(1) 1.995(3), Fe(1)−P(1) 2.1771(10),Fe(1)−P(2) 2.1815(12), Fe(1)−H(1)
1.43(4), N(2)−Fe(1)−P(2) 95.62(9); N(1)−Fe(1)−P(2) 81.94(9), P(1)−Fe(1)−P(2) 161.23(5).
B
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was slowly converted to complex 3 according to NMR, due to
the replacement of Br− by CH3CN, which is in large excess.
Neither 2 nor 3 mediated H/D exchange with D2 in CH3CN.
This suggests that H2 or D2 cannot replace the CH3CN ligand
in 3 in CH3CN.
Scheme 3. Methanol-Assisted H/D Exchange by Complex 2
The H/D exchange by complex 2 in THF might be
rationalized by the following reaction sequence (Scheme 2):
Scheme 2. H/D Exchange in THF Mediated by Complex 2
The H/D exchange rates for 3 are similar in methanol and in
THF. This suggests that the rate-determining step of H/D
exchange does not involve methanol. Instead H2 binding is rate
determining for 3, and the binding appears to have similar rates
in methanol and in THF. In contrast, for 2 H/D exchange is
significantly faster in methanol than in THF. This might be due
to a more facile replacement of Br− by H2 in methanol, thanks
to a better solvation of Br− by methanol than by THF, or a
faster H/D exchange due to the involvement of MeOH.
Catalytic Hydrogenation. The insertion reactions of iron
hydride complexes 2 and 3 with a large number of unsaturated
substrates, including ketones, aldehydes, nitrobenzene, alkynes,
and alkenes, were then tested. However, no hydride insertion
reaction took place, indicating a low hydridicity of the Fe−H
bonds in 2 and 3. However, under H2, stoichiometric
hydrogenation of benzaldehyde was observed for 2. No
reaction occurred with ketones, alkynes, or alkenes under the
same conditions. These results suggested that complex 2 might
serve as a chemoselective hydrogenation catalyst of aldehydes.
A screening of conditions indeed led to a catalytic protocol
using 2 as the catalyst. The hydrogenation could be best
achieved in MeOH using 10 mol % of 2 as catalyst, at room
temperature, under 8 bar of H2, and within 24 h. The catalysis
was slower with a lower loading of catalyst or lower pressure of
H2 (Table S4, Supporting Information). Without 2 there was
no hydrogenation. If 2 was replaced by 3, there also was no
hydrogenation. The optimized conditions were applied for the
hydrogenation of various aldehydes (Table 2).
Benzaldehyde was hydrogenated in a high yield (Table 2,
entry 1). The hydrogenation tolerates both an electron-
withdrawing Br substituent and electron-donating OMe and
Me groups on the aromatic system (88−96% yield, Table 2,
entries 2−4). Aliphatic aldehydes were hydrogenated with good
yields (Table 2, entries 6−8). The chemoselectivity of
hydrogenation is an important issue. A good test for
chemoselectivity is the hydrogenation of α,β-unsaturated
aldehydes. The Fe-PNP complex of Milstein was highly
efficient for ketone hydrogenation but was unselective for the
hydrogenation of α,β-unsaturated aldehydes due to competitive
reduction of C−C double bonds.13 So far, only two Fe catalysts
could selectively hydrogenate α,β-unsaturated aldehydes to the
corresponding allylic alcohols, but they required elevated
temperature and pressure.16,17 To our delight, the hydro-
genation reduced trans-cinnamaldehyde, yielding the corre-
the complex first binds to D2 to form the dideuterium hydride
complex 4-D, likely preceded by dissociation of Br−. Reversible
splitting of D2 in 4-D followed by H/D scrambling gives
complex 5-D. Release of HD then generates the iron deuterated
complex 2-D. Repetition of this process then also produces H2.
In principle, H2 splitting might occur via homolytic or
heterolytic H2 splitting or σ-bond metathesis.35,36 Homolytic
H2 splitting requires very electron rich Fe centers and yields
formally Fe(IV) species. The electron-poor phosphinide and
the labile CO ligands in 2 seem incompatible to such a
homolytic splitting mechanism. Heterolytic H2 splitting is
possible, as Br− might serve as a base to deprotonate
coordinated H2. Alternatively, the solvent might act as a proton
acceptor. To probe these possibilities, H/D exchange was
conducted in the presence of a base such as KOtBu and DBU
(1,8-diazabicyclo[5.4.0]undec-7-ene). Unfortunately, complex
2 decomposed upon reaction with the base. On the other hand,
σ-bond metathesis between coordinated H− and H2 ligands can
lead to the same H/D exchange. For σ-bond metathesis to
occur, isomerization of 4 so that the H− and H2 ligands are cis
to one another is required. The distinction between heterolytic
H2 splitting and σ-bond metathesis is difficult with the available
data, and it is perhaps best probed by future DFT calculations.
The mechanisms of H/D exchange by complexes 2 and 3
should be similar. The faster exchange rate by 3 is possibly due
to a substitution of CH3CN by H2 in 3 more rapid than the
substitution of Br− by H2 in 2.
Two observations indicate that the H/D exchange in MeOH
by complexes 2 and 3 is more complex. (1) In MeOD-d4 and
under H2, H/D exchange occurred in the presence of 2 or 3.
(2) No H/D exchange was observed in the absence of H2
under otherwise the same conditions. Thus, MeOH is involved
in the H/D exchange, which favors a heterolytic H2 splitting
over σ-bond metathesis for reactions in methanol. We
tentatively propose that MeOH acts as a proton acceptor in
heterolytic H2 splitting (Scheme 3). Transfer of the proton to a
H− ligand will result in a H/D exchange when MeOD is
present.
C
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the catalysis. When HCOONa was replaced by NaOtBu and
NaOiPr, HCOOH, or NH4COOH, hydrogenation was
ineffective.34 Thus, Na+ alone was not responsible for the
acceleration of hydrogenation, as reported recently in a Lewis
acid promoted Fe-catalyzed (de)hydrogenation of CO2.37 The
hydrogenation was carried out in the presence of excess Hg, but
no poisoning of the reaction was observed. This is consistent
with a homogeneous process.
Table 2. Catalytic and Chemoselective Hydrogenation of
Aldehydes
a
The optimized conditions for HCOONa-cocatalyzed reac-
tions were then applied to the hydrogenation of functionalized
aldehydes (Table 3). Interestingly, both complexes 2 and 3 can
now serve as catalysts, and they have similar efficiencies for
most substrates. Benzaldehyde was hydrogenated in over 80%
yield (Table 3, entry 1). The reaction tolerates Cl, Br, OMe,
and Me groups on the aromatic rings (Table 3, entries 2−7).
No clear steric or Hammett correlation was seen in the yields of
substituted benzaldehydes, suggesting that the yields were
influenced by side reactions. 2-Naphthaldehyde (Table 3, entry
8) and 2-thiophenecarbaldehyde were also hydrogenated in
good yields (Table 3, entries 7 and 8). Gratifyingly, selective
hydrogenation of the aldehyde group of α,β-unsaturated
aldehydes was again possible (Table 3, entries 9 and 10).
Furthermore, other potentially reducible groups such as ester
and nitro were tolerated (Table 3, entries 11 and 12). Both
primary and secondary aliphatic aldehydes could be hydro-
genated (Table 3, entries 13−16). Again, good chemoselectivity
was achieved, as either an internal or terminal C−C double
bond was tolerated (Table 3, entries 15 and 16). Significantly,
4-acetylbenzaldehyde was hydrogenated only at the aldehyde
group with a yield of 84% (Table 3, entry 17). Hydrogenation
of benzaldehyde was also conducted in the presence of
acetophenone, and only the aldehyde but not the ketone was
reduced.34 These results highlighted the chemoselectivity of
this hydrogenation method.
Transfer Hydrogenation. The promotion of catalytic
hydrogenation by HCOONa described above suggested that
complexes 2 and 3 might be transfer hydrogenation catalysts
using HCOONa as the hydride source.38−41 Transfer hydro-
genation is convenient when the handling of H2 is
undesirable.42 While a number of precious metal complexes
have been reported as selective catalysts for the transfer
hydrogenation of aldehydes,43,44 there is only one precedent for
an iron catalyst, generated in situ by reaction of Fe(BF)4·6H2O
and P(CH2CH2PPh2)3.45 Indeed, complexes 2 and 3 both
catalyzed transfer hydrogenation of aldehydes with HCOONa
in MeOH. The optimized conditions are 5 mol % of catalyst, 5
equiv of HCOONa, at 40 °C and in 6 h. Table 4 shows the
selectivity and tolerance of the catalysis.
a
Reaction conditions: substrate (0.3 mmol), catalyst (0.03 mmol, 10
b
mol %), H2 (8 bar) in MeOH (3 mL), 24 h. Yield of alcohol
determined by calibrated GC analysis; the isolated yields are given in
parentheses.
Benzaldehyde was fully converted to benzyl alcohol (Table 4,
entry 1). Halogen, methoxy, and methyl substitution at the
aromatic rings were tolerated (Table 4, entries 2−6).
Naphthalene and thiophene groups were tolerated (Table 4,
entries 7 and 8). Once again α,β-unsaturated aldehydes bearing
different functional groups were selectively reduced (Table 4,
entries 9−11). Few catalysts are capable of chemoselective
transfer hydrogenation of α,β-unsaturated aldehydes.16,43,46,47
Benzaldehydes with sensitive and reducible functional groups
such as alkene and ester were selectively reduced (Table 4,
entries 12 and 13). Aliphatic aldehydes were also reduced in
high yields (Table 4, entries 14−16).
sponding allylic alcohol as the only product in a good yield
(Table 2, entry 5). Moreover, the hydrogenation readily
tolerates alkene groups (Table 2, entries 7 and 8). Most
impressively, the hydrogenation is selective toward aldehyde
even in the presence of a ketone group. Thus, hydrogenation of
4-acetylbenzaldehyde gave exclusively 1-[4-(hydroxymethyl)-
phenyl]ethanone in high yield (Table 2, entry 9).
Interestingly, hydrogenation of o-methylbenzaldehyde was
accelerated by a catalytic amount of HCOONa. For example,
with 5 mol % of 2, under 4 bar of H2, and at room temperature,
the yield of this reaction was only 18% after 24 h. In the
presence of 10 mol % of HCOONa, the reaction was much
faster and nearly quantitative yield was obtained after 24 h.
Without H2, the yields were below 8%, indicating the
hydrogenation rather than transfer hydrogenation nature of
Mechanistic Hypothesis. The mechanism of hydro-
genation and transfer hydrogenation is unclear for the moment,
and only a hypothesis is outlined here. Because complexes 2
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a
Table 3. Catalytic and Chemoselective Hydrogenation of Aldehydes using HCOONa Cocatalyst
a
Reaction conditions: substrate (0.3 mmol), NaCOOH (0.03 mmol, 10 mol %), catalyst (0.015 mmol, 5 mol %), H2 (4 bar) in MeOH (3 mL), 24 h.
Yield of alcohol determined by calibrated GC analysis; the isolated yields are given in parentheses.
b
and 3 do not react with aldehyde, they may be considered as
NMR in the reaction of 2 with HCOONa; however, isolation
was not possible due to the decomposition of this species upon
concentration of the solution. The dihydride complex 7 is also
proposed as the active species for the transfer hydrogenation.
The establishment of a mechanism, however, requires further
experimental and computational work.
precatalysts. With 2 as an example, it might be activated by H2
to give the dihydrogen complex 4 (Scheme 4). It is proposed
that 4 undergoes heterolytic H2 splitting assisted by MeOH to
give the dihydride complex [(iPrPONOP)Fe(H)2(CO)] (7).
The proton generated in this process protonates the aldehyde
and alcohol is formed following an outer-sphere hydride
transfer from 7. The resulting monohydride complex 8 binds
H2 to regenerate 4 and complete the catalytic cycle. An
alternative mechanism involving the migratory insertion of a
hydride ligand in 7 to aldehyde is possible but will require prior
decoordination of a ligand to create an open coordination site
for aldehyde binding. Given that the Fe ion has only strong
ligands in 7, this mechanism is less likely.
CONCLUSION
■
In summary, we have synthesized and characterized several new
Fe pincer complexes based on the PONOP ligand. These
complexes not only activate H2 but also catalyze the
hydrogenation of aldehydes under mild conditions and with
high functional group tolerance. The chemoselectivity toward
aldehyde versus alkene and especially ketone groups is
remarkable and synthetically useful. The complexes also
catalyze chemoselective transfer hydrogenation of aldehydes
using HCOONa as the hydride source. The PONOP ligand is
easy to prepare and modify; therefore, catalysts with higher
For hydrogenation in the presence of HCOONa, the formate
complex [(iPrPONOP)Fe(H)(CO)(OOCH)] (9) might be
formed. β-H elimination from this complex would give the
dihydride complex 7, which then catalyzes hydrogenation. A
species consistent with the formulation of 9 was observed by
E
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a
Table 4. Catalytic and Chemoselective Transfer Hydrogenation of Aldehydes using HCOONa as Hydrogen Donor
a
Reaction conditions: substrate (0.3 mmol), NaCOOH (0.0015 mol, 5 equiv), catalyst (0.015 mmol, 5 mol %), in MeOH (3 mL), at 40 °C, 6 h.
Yield of alcohol determined by calibrated GC analysis; the isolated yields are given in parentheses.
b
efficiency (e.g., turnover number and turnover frequency) and
different chemoselectivities might be obtained by ligand tuning.
The mechanisms of H2 activation, hydrogenation, and transfer
hydrogenation will be the subject of future studies.
Scheme 4. Proposed Mechanism for Hydrogenation of
Aldehydes in Methanol by Complex 2
EXPERIMENTAL SECTION
■
Materials and Methods. All experiments were carried out under
an inert N2(g) atmosphere using standard Schlenk or glovebox
techniques. Solvents were purified using a two-column solid-state
purification system (Innovative Technology, Amesbury, MA, USA)
and transferred to the glovebox without exposure to air. Methanol
(99.8%, extra dry, over molecular sieves) was purchased from
AcroSeal. Deuterated solvents were purchased from Cambridge
Isotope Laboratories, Inc., and were degassed and stored over
activated 3 Å molecular sieves. All other reagents were purchased
from commercial sources and were degassed by standard freeze−
pump−thaw procedures prior to use. 1H and 31P spectra were
recorded at ambient temperature on a Bruker Avance 400
1
spectrometer. H NMR chemical shifts were referenced to residual
solvent as determined relative to TMS (δ0.00 ppm). GC-MS
measurements were conducted on a PerkinElmer Clarus 600 GC
equipped with a Clarus 600T MS and Agilent J&W GC column, DB-
5MS UI 25 m, 0.250 mm, 0.25 μm. IR measurements were recorded
F
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3
3
on powder samples at ambient temperature on a Varian 800 FT-IT
Scimitar Series spectrometer. Elemental analyses were performed on a
Carlo Erba EA 1110 CHN instrument. HRESI-MS measurements
were conducted at the EPFL ISIC Mass Spectrometry Service with a
Micro Mass QTOF Ultima spectrometer.
1.52 (dp, 12H, JHP = 27.0 Hz, JHH = 9.8 Hz, PCH(CH3)2), 1.28−
1.18 (m, 6H, PCH(CH3)2), 1.02 (q, 6H, J = 7.2 Hz, PCH(CH3)2),
−18.53 (s, 1H, 2JHP = 53.7 Hz, Fe-H) ppm. 31P{1H} NMR (162 MHz,
acetonitrile-d3, 20 °C): δ 237.83 (d, JHP = 49.2 Hz) ppm. 13C NMR
(101 MHz, acetonitrile-d3) δ 216.52 (t, JCP = 19.8 Hz, Fe-CO),
3
2
2
Synthesis of [(iPrPONOP)Fe(CO)Br2] (1). A 0.625 g portion of
163.50 (t, JPC = 5.7 Hz, aryl-C2,6), 144.73 (s, Ar-C4), 103.41 (s, Ar-
i
1
1
FeBr2 (0.0029 mol) and 1 g of PrPONOP (0.0029 mol) were mixed
C3,5), 32.19 (t, JPC = 7.8 Hz, PCH(CH3)2), 29.85 (t, JPC = 13.2 Hz,
2
in 50 mL of dry THF in an ACE round-bottom pressure flask in the
glovebox, and the reaction mixture was stirred for 1 h. Afterward the
flask was pressurized with 0.35 bar of CO and stirred for an additional
2 h. The solution turned immediately from dark yellow-brown to deep
blue. The solution was filtered through a PTFE filter and concentrated.
Pentane was then added to promote the precipitation of the product as
a fine blue powder, which was filtered and washed with additional
pentane (yield 85%). Single crystals suitable for X-ray analysis were
grown by diffusion of pentane in a concentrated solution of complex 1
in THF.
PCH(CH3)2), 17.47 (t, JPC = 4.04 Hz, PCH(CH3)2), 16.86 (s,
2
PCH(CH3)2), 16.61 (t, JPC = 4.04 Hz, PCH(CH3)2), 16.35 (s,
PCH(CH3)2). FT-IR: ν (cm−1) 1951 (s, νCO). ESI-MS (m/z, pos):
469.15 ([(iPrPONOP)Fe(CO)(CH3CN)]+), 428.12 ([(iPrPONOP)-
Fe(CO)]+).
General Procedure for Catalytic Hydrogenation. A 35 mL
ACE pressure tube was charged with catalyst 2 (0.03 mmol), the
aldehyde (0.3 mmol), dodecane (30 μL, 0.133 mmol), 3 mL of dry
MeOH, and 8 bar of hydrogen. The solution was stirred at ambient
temperature (20−22 °C) for 24 h. The reaction was quenched by
exposure to air and by addition of diethyl ether. The alcohol products
were identified and quantified by GC-MS with dodecane as an internal
standard. External calibration curves were made using the
commercially available products (purity >98%) or the isolated
products with dodecane as an internal standard.
General Procedure for Catalytic Hydrogenation Assisted by
HCOONa. A 35 mL ACE pressure tube was charged with catalyst 2 or
3 (0.015 mmol), 300 μL (0.03 mmol) of a freshly prepared 0.1 M
solution of HCOONa in MeOH, the aldehyde (0.3 mmol), dodecane
(30 μL, 0.133 mmol), 3 mL of dry MeOH, and 4 bar of hydrogen. The
solution was stirred at ambient temperature (20−22 °C) for 24 h. The
reaction was quenched by exposure to air and by addition of diethyl
ether. The alcohol products were identified and quantified by GC-MS
with dodecane as an internal standard. External calibration curves were
made using the commercially available products (purity >98%) or the
isolated products with dodecane as an internal standard.
General Procedure for Transfer Hydrogenation. In a vial were
placed the catalyst (0.015 mmol), the aldehyde (0.3 mmol), dodecane
(30 μL, 0.133 mmol), HCOONa (0.0015 mol, 5 equiv), and 3 mL of
dry MeOH. The solution was stirred at 40 °C for 6 h. The reaction
was quenched by exposure to air and by addition of diethyl ether. The
alcohol products were identified and quantified by GC-MS with
dodecane as an internal standard. External calibration curves were
made using the commercial available products (purity >98%) or the
isolated products with dodecane as an internal standard.
Anal. Calcd for C18H32Br2FeNO3P2: C, 36.76; H, 5.48; N, 2.38.
1
Found: C, 36.46; H, 5.35; N, 2.40. H NMR (400 MHz, CD2Cl2, 20
°C): δ 7.81 (t, 1H, 3JHH = 8.1 Hz, aryl-H4), 6.95 (d, 2H, 3JHH = 8.1 Hz,
aryl-H3,5), 3.51 (sept, 4H, 3JHH = 6.9 Hz, 3JHP = 13.4 Hz, PCH(CH3)2),
3
3
1.50 (dq, 24H, JHH = 7.5 Hz, JHP = 67.1 Hz, PCH(CH3)2) ppm.
31P{1H} NMR (162 MHz, CD2Cl2, 20 °C): δ 215.25 (s) ppm.
13C{1H} NMR (101 MHz, CD2Cl2, 20 °C): δ 165.40 (t, Fe-CO, 2JCP
=
1
6.3 Hz), 143.59 (s, aryl-C4), 104.28 (s, aryl-H3,5), 29.60 (t, JCP = 9.1
Hz, PCH(CH3)2), 17.82 (s, PCH(CH3)2), 17.39 (s, PCH(CH3)2)
ppm. FT-IR: ν (cm−1) 1961 (s, νCO). ESI-MS (m/z, pos): 508.05
(100%, [(iPrPONOP)Fe(CO)Br]+).
Synthesis of [(iPrPONOP)FeH(CO)Br] (2). A 0.5 g portion of
[(iPrPONOP)Fe(CO)Br2] (1 mmol) was dissolved in 25 mL of dry
THF, and 1.2 mL of NaHBEt3 (1 M in toluene, 1.2 mmol) was added.
The mixture turned from blue to green within a few minutes. The
reaction mixture was stirred for 3 h, filtered with a PTFE filter, and
concentrated. Addition of pentane promoted the precipitation of the
product as a fine yellow powder that was filtered and washed with
additional pentane (yield 70%). Single crystals suitable for X-ray
analysis were grown by diffusion of pentane in a concentrated solution
of complex 2 in THF.
Anal. Calcd for C18H33BrFeNO3P2: C, 42.46; H, 6.53; N, 2.75.
1
Found: C, 42.60; H, 6.52; N, 2.66. H NMR (400 MHz, CD2Cl2, 20
°C): δ 7.53 (t, 1H, 3JHH = 8.0 Hz, aryl-H4), 6.58 (d, 2H, 3JHH = 8.0 Hz,
aryl-H3,5), 3.52 (sept, 2H, 3JHH = 7.1 Hz, 3JHP = 14.4 Hz, PCH(CH3)2),
3
3
2.80 (sept, 2H, JHH = 6.6 Hz, JHP = 13.3 Hz, PCH(CH3)2, 1.51 (m,
12H, 3JHH = 6.8 Hz, 3JHP = 12.6 Hz, PCH(CH3)2), 1.19 (q, 6H, 3JHH
=
ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and CIF files giving additional
experimental and crystallographic details and characterization
data. This material is available free of charge via the Internet at
http://pubs.acs.org.
■
3
7.9 Hz PCH(CH3)2), 0.97 (m, 6H, JHH = 7.2 Hz, PCH(CH3)2)
−20.74 (t, 1H, 2JHP = 56.6 Hz, Fe-H) ppm. 31P{1H} NMR (162 MHz,
CD2Cl2, 20 °C): δ 239.00 (3JHP = 35.8 Hz) ppm. 13C{1H} NMR (101
MHz, CD2Cl2, 20 °C): δ 164.12 (t, Fe-CO, 2JCP = 6.3 Hz), 142.04 (s,
S
1
aryl-C4), 102.21 (s, aryl-C3,5), 31.88 (t, JCP = 8.2 Hz, PCH(CH3)2),
1
2
28.62 (t, JCP = 11.5 Hz, PCH(CH3)2), 18.14 (t, JCP = 4.7 Hz,
PCH(CH3)2), 17.86−17.38 (m, PCH(CH3)2), 16.46 (t, JCP = 2.2 Hz,
PCH(CH3)2) ppm. FT-IR: ν (cm−1) 1928 (s, νCO). ESI-MS (m/z,
pos): 428.08 ([(iPrPONOP)FeH(CO)]+), 400.25 ([(iPrPONOP)-
FeH]+).
AUTHOR INFORMATION
Corresponding Author
*E-mail for X.H.: xile.hu@epfl.ch.
■
Synthesis of [(iPrPONOP)Fe(H)(CO)(CH3CN)]OTf (3). A 0.150 g
portion of [(iPrPONOP)FeH(CO)Br] (0.295 mmol) was dissolved in
25 mL of dry CH3CN, and 80 μL of TMSOTf (1.5 eq, 0.442 mmol)
was added. The yellow solution was stirred for 7 h. The solvent was
then evaporated, and the resulting yellow sticky oil was dissolved in 2
mL of THF. The addition of pentane led to the precipitation of the
product as a fine yellow powder, which was filtered and washed with
additional pentane (yield 80%). Single crystals suitable for X-ray
analysis were grown by diffusion of pentane in a concentrated solution
of complex 3 in THF.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the Swiss National Science
Foundation (200020_134473/1 and 200020_152850/1) and
the COST action CM 1205 CARISMA. S.M. thanks Dr. Heron
Vrubel (EPFL) for experimental assistance.
Anal. Calcd for C21H35F3FeN2O6P2S: C, 40.72; H, 5.86; N, 4.52.
Found: C, 39.7; H, 4.72; N, 4.47. 1H NMR, 31P{1H} NMR, 13C NMR
are reported in the Supporting Information as further evidence of
purity. 1H NMR (400 MHz, acetonitrile-d3, 20 °C): δ 7.78 (t, 1H, 3JHH
= 8.1 Hz, aryl-H4), 6.76 (d, 2H, 3JHH = 8.2 Hz, aryl-H3,5), 2.92 (m, 4H,
REFERENCES
■
(1) Handbook of Homogeneous Hydrogenation; de Vries, J. G., Elsevier,
C. J., Eds.; Wiley-VCH: Weinheim, Germany, 2007.
(2) Junge, K.; Schroder, K.; Beller, M. Chem. Commun. 2011, 47,
4849−4859.
̈
3
3JHP = 18.6 Hz, JHH = 9.4 Hz, PCH(CH3)2), 1.99 (m, 3H, CH3CN),
G
DOI: 10.1021/acs.organomet.5b00105
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
(3) Morris, R. H. Chem. Soc. Rev. 2009, 38, 2282−2291.
(4) Bolm, C.; Legros, J.; Le Paih, J.; Zani, L. Chem. Rev. 2004, 104,
6217−6254.
(36) Waterman, R. Organometallics 2013, 32, 7249−7263.
(37) Bielinski, E. A.; Lagaditis, P. O.; Zhang, Y.; Mercado, B. Q.;
Wuertele, C.; Bernskoetter, W. H.; Hazari, N.; Schneider, S. J. Am.
Chem. Soc. 2014, 136, 10234−10237.
(5) Iron Catalysis in Organic Chemistry; Plietker, B., Ed.; Wiley-VCH:
Weinheim, Germany, 2008.
(38) Bar, R.; Sasson, Y.; Blum, J. J. Mol. Catal. 1984, 26, 327−332.
(39) Rajagopal, S.; Spatola, A. F. J. Org. Chem. 1995, 60, 1347−1355.
(40) Hauwert, P.; Boerleider, R.; Warsink, S.; Weigand, J. J.; Elsevier,
C. J. J. Am. Chem. Soc. 2010, 132, 16900−16910.
(6) Bauer, I.; Knolker, H.-J. Chem. Rev. 2015, DOI: 10.1021/
̈
cr500425u.
(7) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004,
(41) Wu, X.; Liu, J.; Li, X.; Zanotti-Gerosa, A.; Hancock, F.; Vinci,
D.; Ruan, J.; Xiao, J. Angew. Chem., Int. Ed. 2006, 45, 6718−6722.
(42) Brieger, G.; Nestrick, T. J. Chem. Rev. 1974, 74, 567−580.
(43) Mizugaki, T.; Kanayama, Y.; Ebitani, K.; Kaneda, K. J. Org.
Chem. 1998, 63, 2378−2381.
126, 13794−13807.
(8) Fong, H.; Moret, M. E.; Lee, Y.; Peters, J. C. Organometallics
2013, 32, 3053−3062.
(9) Casey, C. P.; Guan, H. R. J. Am. Chem. Soc. 2007, 129, 5816−
5817.
(44) Miecznikowski, J. R.; Crabtree, R. H. Organometallics 2004, 23,
629−631.
(10) Casey, C. P.; Guan, H. R. J. Am. Chem. Soc. 2009, 131, 2499−
2507.
(11) Sui-Seng, C.; Freutel, F.; Lough, A. J.; Morris, R. H. Angew.
Chem., Int. Ed. 2008, 47, 940−943.
(12) Lagaditis, P. O.; Sues, P. E.; Sonnenberg, J. F.; Wan, K. Y.;
Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 2014, 136, 1367−1380.
(13) Langer, R.; Leitus, G.; Ben-David, Y.; Milstein, D. Angew. Chem.,
Int. Ed. 2011, 50, 2120−2124.
(14) Langer, R.; Iron, M. A.; Konstantinovski, L.; Diskin-Posner, Y.;
Leitus, G.; Ben-David, Y.; Milstein, D. Chem. - Eur. J. 2012, 18, 7196−
7209.
(45) Wienhofer, G.; Westerhaus, F. A.; Junge, K.; Beller, M. J.
̈
Organomet. Chem. 2013, 744, 156−159.
(46) Selvam, P.; Sonavane, S. U.; Mohapatra, S. K.; Jayaram, R. V.
Adv. Synth. Catal. 2004, 346, 542−544.
(47) Kidwai, M.; Bansal, V.; Saxena, A.; Shankar, R.; Mozumdar, S.
Tetrahedron Lett. 2006, 47, 4161−4165.
(15) Zell, T.; Ben-David, Y.; Milstein, D. Angew. Chem., Int. Ed. 2014,
53, 4685−4689.
(16) Wienhofer, G.; Westerhaus, F. A.; Junge, K.; Ludwig, R.; Beller,
̈
M. Chem. - Eur. J. 2013, 19, 7701−7707.
(17) Fleischer, S.; Zhou, S.; Junge, K.; Beller, M. Angew. Chem., Int.
Ed. 2013, 52, 5120−5124.
(18) Zhou, S.; Fleischer, S.; Junge, K.; Beller, M. Angew. Chem., Int.
Ed. 2011, 50, 5120−5124.
(19) Chakraborty, S.; Dai, H. G.; Bhattacharya, P.; Fairweather, N.
T.; Gibson, M. S.; Krause, J. A.; Guan, H. R. J. Am. Chem. Soc. 2014,
136, 7869−7872.
(20) Li, Y. Y.; Yu, S. L.; Wu, X. F.; Xiao, J. L.; Shen, W. Y.; Dong, Z.
R.; Gao, J. X. J. Am. Chem. Soc. 2014, 136, 4031−4039.
(21) Zuo, W. W.; Lough, A. J.; Li, Y. F.; Morris, R. H. Science 2013,
342, 1080−1083.
(22) Werkmeister, S.; Junge, K.; Wendt, B.; Alberico, E.; Jiao, H.;
Baumann, W.; Junge, H.; Gallou, F.; Beller, M. Angew. Chem., Int. Ed.
2014, 53, 8722−8726.
(23) Chakraborty, S.; Brennessel, W. W.; Jones, W. D. J. Am. Chem.
Soc. 2014, 136, 8564−8567.
(24) Chakraborty, S.; Lagaditis, P. O.; Forster, M.; Bielinski, E. A.;
̈
Hazari, N.; Holthausen, M. C.; Jones, W. D.; Schneider, S. ACS Catal.
2014, 4, 3994−4003.
(25) Gorgas, N.; Stoeger, B.; Veiros, L. F.; Pittenauer, E.; Allmaier,
G.; Kirchner, K. Organometallics 2014, 33, 6905−6914.
(26) Comprehensive Organic Synthesis; Trost, B. M., Fleming, I., Eds.,
Elsevier: Amsterdam, 1991; Vol. 8.
(27) Salem, H.; Shimon, L. J. W.; Diskin-Posner, Y.; Leitus, G.; Ben-
David, Y.; Milstein, D. Organometallics 2009, 28, 4791−4806.
(28) Findlater, M.; Bernskoetter, W. H.; Brookhart, M. J. Am. Chem.
Soc. 2010, 132, 4534−4535.
(29) Kundu, S.; Brennessel, W. W.; Jones, W. D. Inorg. Chem. 2011,
50, 9443−9453.
(30) DeRieux, W.-S. W.; Wong, A.; Schrodi, Y. J. Organomet. Chem.
2014, 772, 60−67.
(31) Zhang, J.; Gandelman, M.; Herrman, D.; Leitus, G.; Shimon, L.
J. W.; Ben-David, Y.; Milstein, D. Inorg. Chim. Acta 2006, 359, 1955−
1960.
(32) Trovitch, R. J.; Lobkovsky, E.; Chirik, P. J. Inorg. Chem. 2006,
45, 7252−7260.
(33) Gunanathan, C.; Milstein, D. Acc. Chem. Res. 2011, 44, 588−
602.
(34) See the Supporting Information.
(35) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46,
2578−2592.
H
DOI: 10.1021/acs.organomet.5b00105
Organometallics XXXX, XXX, XXX−XXX