Selective Hydrogenation of Amides to Amines and Alcohols Catalyzed by Improved Iron Pincer Complexes

Article abstract of DOI:10.1021/acs.organomet.6b00251

A comparative study on the synthesis, stability, and catalytic activity of various iron pincer complexes with the general formula [(R-PN^HP)Fe(H) (CO) (BH₄)] is reported, where R denotes the substituent of the terminal PR₂-groups (R = ^tBu, Cy, ⁱPr, Ph, Et). By the example of the synthesized precatalysts, it is shown that the nature of the ligands has a surprising influence on the catalytic properties of the complexes. Bulky ligands and less electron donating ligands affect the stability of the complexes, which preferably react under the loss of CO or H₂ to deactivated products. In return, the reduced steric demand and the strong σ-donating properties of the Et-substituted precatalyst (2a) lead to an improved activity in the hydrogenation of esters to alcohols, compared to that of the previously reported ⁱPr-substituted complexes. The improved activity of complex 2a is clearly demonstrated in the direct hydrogenation of amides to alcohols and amines under mild conditions.

Full text of DOI:10.1021/acs.organomet.6b00251

Article
pubs.acs.org/Organometallics
Selective Hydrogenation of Amides to Amines and Alcohols
Catalyzed by Improved Iron Pincer Complexes
Felix Schneck,† Maik Assmann,† Markus Balmer,† Klaus Harms,† and Robert Langer*,†,‡
†Department of Chemistry, Philipps-University Marburg, Hans-Meerwein-Straße, 35032 Marburg, Germany
‡Lehn Institute of Functional Materials (LIFM), Sun Yat-Sen University (SYSU), Xingang Road West, Guangzhou 510275, PR China
S
* Supporting Information
ABSTRACT: A comparative study on the synthesis, stability,
and catalytic activity of various iron pincer complexes with the
general formula [(R-PNHP)Fe(H) (CO) (BH4)] is reported,
where R denotes the substituent of the terminal PR2-groups (R
t
i
= Bu, Cy, Pr, Ph, Et). By the example of the synthesized
precatalysts, it is shown that the nature of the ligands has a
surprising influence on the catalytic properties of the
complexes. Bulky ligands and less electron donating ligands
affect the stability of the complexes, which preferably react
under the loss of CO or H2 to deactivated products. In return,
the reduced steric demand and the strong σ-donating
properties of the Et-substituted precatalyst (2a) lead to an
improved activity in the hydrogenation of esters to alcohols,
i
compared to that of the previously reported Pr-substituted complexes. The improved activity of complex 2a is clearly
demonstrated in the direct hydrogenation of amides to alcohols and amines under mild conditions.
t
species B is isolable due to bulkier Bu-groups at the P donors.
As a consequence, CO2 and fluorinated esters can be
hydrogenated with the trans-dihydride precatalyst.31,34−36
A second example of catalyst-tuning by small variations in the
ligand manifold is the POCOP-based iron(II) catalyst C
reported by Guan et al.37,38 In this case, the catalytic activity of
complex C in the dehydrogenation of ammonia borane was
significantly increased by variation of the ancillary ligands and
the substitution pattern at the central phenyl ring. As one
ancillary phosphine ligand has to dissociate prior to substrate
binding, an electron donating methoxy group has been
introduced at the aryl backbone to accelerate the generation
of a vacant coordination site. This modification in combination
with the reduced donor properties of the ancillary phosphane
ligands led to the more active catalyst D.
Very recently, different iron(II) (pre)catalysts with an amine-
based PNHP-pincer ligand were reported to exhibit excellent
catalytic activity in different (de)hydrogenation reac-
tions.16−23,25−28 The group of Beller, for example, initially
demonstrated high catalytic activity of [(iPr-PNHP)Fe(H)
(CO)(η1-BH4)] in the dehydrogenation of methanol−water
mixtures to carbon dioxide and hydrogen.16 Shortly afterward,
the groups of Hazari, Schneider, and Jones reported about the
dehydrogenation of formic acid as well as the dehydrogenation
of primary and secondary alcohols to esters or ketones,
respectively.18,19,22 A potential hydrogen storage system using
INTRODUCTION
■
Most industrial processes based on homogeneous catalysis use
noble metals.1 As these metals are expensive, often toxic, and
not very abundant, the replacement by base metals is highly
attractive. Complexes of pincer type ligands have shown high
potential in catalytic reactions and after decades of intensive
investigation they are still subject to ongoing research. In cases
where they react in a noninnocent or cooperative way, these
complexes can enable the activation of small molecules without
changing the metal’s formal oxidation state.2−8 This makes
them an ideal ligand platform for the development of earth-
abundant metal-based catalytic systems. Especially pincer
ligands of the combination PEP (E = C, N) are popular due
to the ability to tune the electronic and steric features of the
complex by simple variation of the substituents on the
phosphorus atoms. With these ligands, significant progress
has been made by evaluating iron(II) as catalyst for
(de)hydrogenation reactions, for which the higher homologue
ruthenium(II) shows excellent activity.9−30 The pyridine-
bisphosphine ligated iron(II) system by Milstein and co-workers
is a great example of an iron(II) catalyst containing a PNPyP-
ligand, which is capable of catalyzing the hydrogenation and
dehydrogenation of various substrates under mild conditions in
high yield (Scheme 1).31−36 Aside from the great catalytic
activity, this system impressively illustrates the influence of
minor ligand variations on the complexes’ reactivity. While for
i
the Pr-substituted complex A, a dihydride species is not
isolable and does not seem to be an intermediate of the
Received: March 30, 2016
catalytic cycle during ketone hydrogenation,32,33 the dihydride
© XXXX American Chemical Society
A
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Scheme 1. Development of Iron Catalysts by Milstein and Guan Based on Rational Variation
Scheme 2. Key-Intermediates within a Possible Catalytic Cycle
1,2,3,4-tetrahydroquinaldine as liquid organic carrier and these
complexes as catalysts has been described by Jones and co-
workers.20 In parallel, the group of Beller reported the high
catalytic activity of these complexes in the hydrogenation of
nonactivated esters to alcohols.27 Shortly afterward, the groups
of Guan and Fairweather employed these complexes for the
hydrogenation of fatty acid esters in the absence of solvent.21,23
Among the known iron-based hydrogenation catalysts for
carbonyl compounds, this class of complexes with PNHP-type
ligands clearly exhibits the highest catalytic activity so far. The
catalytic cycle likely involves complexes E and F as
intermediates, which both exhibit a singlet ground state,20
thus avoiding additional barriers due to system crossing
between different spin states, which are often encountered for
elementary steps with iron complexes (Scheme 2).39,40 In
particular, the singlet ground state of the square pyramidal
complex F (τ = 0.22)41 is remarkable, and as for iron(II)
complexes with d6 electron count and square pyramidal
coordination geometry, spin states of S = 0, 1, and 2 are
possible. As homogeneous iron catalysts are generally very
sensitive to small variations of ligand and reaction parameters,
the investigation of steric and electronic ligand effects in this
system is a promising approach for catalyst improvement.
Herein, we present our results on the synthesis and stability
of hydridoborohydride precatalysts [(R-PNHP)Fe(H) (CO)-
(η1-BH4)] (R = Et, Pr, Cy, Ph, Bu). Depending on the group
R, different pathways for activation and decomposition are
viable. On the basis of these results, a catalyst with an improved
activity in the hydrogenation of esters to alcohols is reported.
The same complex exhibits catalytic activity in the
unprecedented iron-catalyzed hydrogenation of amides to
alcohols and amines.
i
t
RESULTS
■
Synthesis of Hydridoborohydride Complexes. Inspired
by the improvement of Milstein’s and Guan’s catalysts by fine-
tuning the electronic and steric properties,31−33,38 we were
interested in the influence of the substituents R at the
phosphorus donor groups in the amine-based R-PNHP ligand
(R-PNHP = (R2PCH2CH2)2NH). The synthesis of the Pr-
i
PNHP-based pincer-iron-complex 2b was first reported by
Beller and co-workers,16 adopting a synthetic procedure
B
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Scheme 3. Synthetic Route to [(R-PNHP)Fe(X)2(CO)] and [(R-PNHP)Fe(H) (CO)(η1-BH4)] Complexes (X = Cl, Br)
Scheme 4. Alternative Synthetic Route to the Desired [(R-PNHP)Fe(H) (CO)(η1-BH4)] Complexes
originally reported for the corresponding pyridine-based pincer
formation of [(Et-PNHP)2Fe](FeX4), as observed for bipyr-
32
idine-based PNN-type ligands.44,45 Apart from Bu-PNHP, all
i
t
complex A with Pr-groups. The desired complex was
obtained in good yields by complexation of FeBr2, followed
by subsequent reaction with CO. Treatment of the resulting
trans-dihalide carbonyl complex [(iPr-PNHP)Fe(Br)2(CO)]
with excess NaBH4 results in the formation of the desired
complex. Attempts to prepare derivatives of these complexes
with different substituents on the P donor groups were done by
the group of Schneider and Hazari.42 Thereby, [(Cy-PNHP)-
Fe(Cl)2] has been reported to react with CO, but the
subsequent reaction of [(Cy-PNHP)Fe(Cl)2(CO)] with
NaBH4 is described as unselective. The analogous synthesis
dihalide complexes react with carbon monoxide to give the
diamagnetic complexes 1a−d. With the dialkylphosphino-
containing ligands this reaction leads to stable trans-dibromo
complexes 1a−c, which can be isolated in good yields.21,42
Furthermore, the molecular structure of the newly synthesized
complex 1a and 1d have been confirmed by single crystal X-ray
diffraction. The reaction with the Ph-substituted ligand,
however, results in a mixture of the cis- and trans-dibromo
carbonyl complex 1d. Interestingly, the cis- and trans-complex
1d turned out to liberate carbon monoxide in the absence of
carbon monoxide atmosphere. Because of the limited stability
of complex 1d, it was not possible to prepare the corresponding
hydridoborohydride complex by the addition of NaBH4. A
similar observation has been made with complex 1c, which has
been reported to react with NaBH4 to an inseparable mixture of
products. Only the reaction of complex 1a with NaBH4 results
in the formation of the corresponding hydridoborohydride
complex 2a, as previously reported for 1b and analogous
pyridine-based pincer complexes.16,32
t
of the Bu-PNHP-based compound failed due to a lack in
reactivity of the dihalide precursor toward CO. As with bulky
PNHP-ligands, the corresponding hydridoborohydride com-
plexes were not accessible via this route, and we started to
investigate the less bulky Et-substituted ligand as well as the
more electron-poor Ph-substituted ligand (Scheme 3).
The complexation of one equivalent of iron(II) halide in
THF usually takes place with all ligands, as evident from the
formation of a colored solution and the absence of a resonance
for the uncoordinated ligand in the 31P{1H} NMR spectrum.
On the basis of these findings, we assumed the formation of
[(R-PNHP)FeX2].43 Especially, for the less bulky ligand Et-
PNHP it is important to note that the ligand still exhibits
enough steric demand to avoid 2-fold coordination and
As the synthetic route described in Scheme 3 yields the
desired hydride complex only in the case of the Et- and the iPr-
ligand, we followed a different approach that originally has been
reported for the synthesis of the trans-dihydride complex B.31
This procedure takes advantage of the selective reaction of the
C
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Table 1. Spectroscopic Data for [(R-PNHP)Fe(H) (CO) (BH4)] (2a−e)
complex
R =
νCO
̃
(cm−1
)
δH(Fe-H) (ppm)
δH(η1-BH4) (ppm)
δP(Fe-P) (ppm)
δB(η1-BH4) (ppm)
2a
Et
1892
−19.66 (2JPH = 49.0 Hz)
−19.51/−20.18 (2JPH = 50.0 Hz)
−19.59 (2JPH = 50.4 Hz)
−18.13 (2JPH = 52.3 Hz)
−20.83 (2JPH = 60.4 Hz)
−3.07
−2.84
−2.84
−3.61
−2.05
82.1
−32.9
−33.9
−31.4
−32.9
−40.6
a
2b
iPr
Cy
Ph
tBu
1892/1833
1900
99.5/100.9
91.9
2c
2d
2e
n. a.
81.7
n. a.
111.0
a
Spectroscopic data have been taken from ref 16.
Scheme 5. Solution Behavior of Complexes 2a−e, Indicating Activation and Deactivation Pathways
dicationic acetonitrile complex I (δp = 55.5 ppm, R = Cy) with
NaBH4 to give the cationic hydrido acetonitrile complex II (δp
= 80.1 ppm, R = Cy, Scheme 4). Under 1 atm of CO, one
MeCN ligand is replaced by a CO ligand, yielding the cationic
hydrido carbonyl acetonitrile complex III (δp = 89.0 ppm, R =
Cy). Continuous drying under high vacuum of this complex
results in the loss of the remaining MeCN ligand and
coordination of the BH4-counterion to give hydridoborohy-
dride complexes 2. In the present case, the intermediate
compounds I−III were not isolated, but subsequent analysis of
the reaction mixture by 31P{1H} NMR spectroscopy shows
selective reactions upon addition of the reactants.
For the extraction of the neutral complexes, 2 toluene was
used to prevent BH3 abstraction. The new complex 2c is
obtained in 31% yield, while the borane-substituted ligand 3c is
obtained as side-product.46 3c is observable by 31P{1H} NMR
spectroscopy after the addition of NaBH4, suggesting an
unselective reaction of the dicationic intermediate I with
NaBH4 that results in the desired monocationic hydrido
complex II and another unstable intermediate, which
decomposes under the formation of 3c. Applying the same
procedure with Ph-PNHP and tBu-PNHP as ligands, the desired
hydridoborohydride complexes 2d and 2e accompanied by the
phosphinoboranes 3d and 3e could be observed by NMR
spectroscopy, respectively, but the complexes are too unstable
to allow for isolation or separation. Selected values from NMR
and IR spectroscopic measurements are summarized in Table 1.
As previously reported for complex 2b,82 a mixture of two
isomers is initially formed in the case of complex 2a. The
mixture of two isomers, differing in the relative orientation of
N- and Fe-bound H atoms is slowly converted in solution to
the thermodynamic stable complex with a trans-orientation of
these two H atoms. All hydridoborohydride complexes (2a−e)
give rise to a singlet resonance in the 31P{1H} NMR spectrum,
ranging from 81.7 and 82.1 ppm for 2a and 2d, respectively, to
a low-field shifted resonance at 111.0 ppm for complex 2e with
1
the bulkiest ligand. The H NMR spectra of 2a−e exhibit
typical triplet resonances between −18.13 and −20.83 ppm for
the hydrido ligands (2JPH = 49.0−60.4 Hz), indicating a
meridional coordination of the pincer ligands with magnetically
equivalent phosphorus atoms. The broad resonances between
1
−2.05 and −3.61 with an integral of four in the H NMR
spectrum suggest the presence of an η1-coordinated BH4-
ligand.16,21,47,48 For complexes 2a−2d, the isomer with a close
proximity between the BH4-ligand and the NH-proton was
1
found to be the more stable one, as judged by H NOESY
NMR spectroscopy. For complexes 2a−d, broad resonances
between −31.4 and −33.9 ppm are observed in the 11B{1H}
D
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Scheme 6. Dehydrogenation of the Borane Adduct 3c in the Presence of 7c
NMR spectra, whereas complex 2e gives rise to a broad
resonance at −40.6 ppm, which might indicate a more weakly
bound BH4-ligand due to steric repulsion. Because of the
continuous presence of decomposition products in samples of
2d and 2e, it was not possible to unambiguously assign the CO-
stretching vibration in the acquired FT-IR spectra. Nonetheless,
complex 2d decomposes within 1 day to unidentified products
and the Ph-PNHP. As the dibromo complex 1d was labile
toward the liberation of the carbonyl ligand, such a pathway
might be viable for the hydridoborohydride complex 2d as well
and would explain the distinct instability.
Complexes 2c and 2e exhibit only limited stability in solution
too, as evident from a continuous decrease of the resonances
corresponding to hydride and the BH4 ligand and concomitant
formation of H2 in the 1H NMR spectrum. Interestingly, in the
presence of a Lewis base such as THF, which has been used as
BH3-acceptor for analogous complexes with a pyridine-based
pincer ligand,31 no BH3-transfer is detected. Such a transfer
would lead to the assumed active species E or F. However, in
the presence of the ligand borane adduct 3c/e the selective
formation of complex 7c/e can be observed (Scheme 6). These
complexes give rise to an AB spin system centered at 97.9 and
118.2 ppm as well as a broad resonance in the 31P{1H} NMR
spectrum with an almost identical chemical shift like the borane
for 2a and 2b equal wavenumbers of v = 1892 cm−1 were
̃
CO
observed for the corresponding trans-isomer in the FT-IR
spectrum, whereas complex 2c shows a slightly shifted band at
1900 cm−1. Although the C−O-vibration is usually coupled to
the Fe−H-vibration in such cases, the shifted band suggests a
weaker π-backbonding in 2c with respect to 2a and 2b.
Activation and Decomposition Pathways of 2a−e. For
the hydridoborohydride complex 2b, it has been previously
demonstrated that this complex reacts under loss of BH3 upon
heating or treatment with a Lewis base to give a mixture of the
trans-dihydride complex 4b and the penta-coordinated amido
complex 5b.21 The Et-substituted precatalyst 2a exhibits a very
similar reactivity in a solution of aprotic solvent such as
benzene, toluene, or THF (Scheme 5). The analogous trans-
dihydride complex trans-4a gives rise to a multiplet resonance
1
adducts 3c/e. The H NMR spectrum of 7c exhibits a virtual
2
triplet of doublet at −14.60 ppm (2JPH = 59.5 Hz, JHH = 4.5
Hz) for the hydrido ligand, which simplifies to doublet
resonance upon 31P-decoupling. Instead of the four proton
resonance corresponding to the coordinated BH4-ligand in
complex 2c, a broad resonance at −11.17 ppm that integrates
to one is observed in the 1H NMR spectrum of 7c. Complex 7e
give rise to a triplet of doublets at −14.93 ppm for the hydride
ligand (2JPH = 59.4 Hz, 2JHH = 4.2 Hz) and a broad one proton
resonance at −9.65 ppm for the bridging boron-bound
1
at −9.66 ppm in the H NMR spectrum for the two hydrido
ligands in trans-4a. A singlet resonance at 93.1 ppm in the
31P{1H} NMR spectrum is assigned to trans-4a. Interestingly,
minor quantities of the corresponding cis-dihydride cis-4a can
be detected when 2a is treated with a BH3-acceptor such as
PEt3, as judged by the appearance of two triplet of doublet
2
resonances at −20.00 ppm (2JPH = 51.3 Hz, JHH = 14.2 Hz)
and −8.06 ppm (2JPH = 85.2 Hz, 2JHH = 14.9 Hz). The hydride
ligand in complex 5a gives rise to a high-field shifted triplet
1
hydrogen atom in the H NMR spectrum. The resonance of
the terminal B−H-proton in 7c is observed at 3.19 ppm in the
1H NMR spectrum, whereas the resonance of the two BH3-
groups is only assignable upon 11B-decoupling, resulting in a
well resolved doublet resonance (2JPH = 14.3 Hz). Two broad
resonances at −11.8 and −42.6 ppm are observed in the
11B{1H} NMR spectrum of 7c, corresponding to the N-BH2-N-
bound boron atom and the phosphorus bound BH3-groups.
Very similar values of −11.9 and −42.3 ppm are observed in the
1
resonance at −24.40 ppm (2JPH = 50.9 Hz) in the H NMR
spectrum. The phenyl-substituted complex 2d turns out to be
much less stable in solution than its Et- and iPr-counterparts 2a
and 2b. In the 1H NMR spectrum of 2d or mixtures of 2d and
3d, usually a significant amount of H2 is detected. Because of
the limited lifetime of 2d, a mixture of 2d and 3d, was analyzed
by multinuclear NMR spectroscopy. The 31P{1H} NMR
spectrum after a couple of hours exhibits several new singlet
resonances between 75 and 100 ppm, as well as a singlet
resonance at −18.6 ppm, corresponding to the free ligand Ph-
PNHP. Only small quantities of the initial hydridoborohydride
complex 2d were detectable by 1H and 31P{1H} NMR
spectroscopy. The major species in solution at this time
exhibits a singlet resonance at 94.4 ppm in the 31P{1H} NMR
spectrum as well as a resonance with an integral of two at −8.42
ppm in the 1H NMR spectrum. A careful analysis revealed that
these signal consists of two superimposed triplet of doublet
resonances at −8.42 ppm (2JPH = 67.0 Hz, 2JHH = 10.5 Hz) and
−8.41 ppm (2JPH = 63.1 Hz, 2JHH = 9.7 Hz). Such multiplicity is
in line with a trans-dihydride complex 4d, in which the two
hydrido ligands are magnetically nonequivalent, due to a
different relative orientation to the NH-proton. Nonetheless,
t
11B{1H} NMR spectrum of complex 7e. In case of the Bu-
1
substituted ligand, an intermediate could be observed by H,
31P{1H}, and 11B{1H} NMR spectroscopy, which slowly
converts to 7e and other unidentified products. The hydrido
ligand in complex 6e gives rise to a triplet resonance at −15.85
ppm (2JPH = 63.4 Hz), whereas a broad resonance at −10.29
1
ppm in the H NMR spectrum indicates a coordinating B−H
group, similar to those in 7c/e. The appearance of a singlet
resonance at 103.7 ppm in the 31P{1H} NMR spectrum
suggests a symmetric complex with two magnetically equivalent
phosphorus atoms, while the broad resonance at −21.9 ppm in
the 11B{1H} NMR spectrum indicates a different environment
of the boron atom in 6e in comparison to those of 7c and 7e.
On the basis of the spectroscopic data, it is assumed that
E
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Figure 1. Molecular structure of 3c (left), 7c (middle), and 8c (right) in the crystal lattice with ellipsoids set to 50% probability (carbon bound
hydrogen atoms were omitted for clarity).
complex 6 is the initial product of the first H2-elimination step.
As in all hydridoborohydride complexes 2a−d, the isomer with
the N−H-proton and the BH4-ligand on the same site of the
complex is found to be the more stable one, and the
dehydrogenation step likely proceeds via the protonation of
the coordinated BH4-ligand in 2c/e, consecutive H2-elimi-
nation, and B−N-bond formation.
(Scheme 7). The new complex 9c gives rise to an AB spin
system centered at 101.3 ppm in the 31P{1H} NMR spectrum
Scheme 7. Dehydrogenative Coupling of Aniline with
Complex 2c
Finally, the molecular structure of complex 7c was confirmed
by single crystal X-ray diffraction (Figure 1). The central
iron(II) atom in 7c remains octahedrally coordinated by the
two dicyclohexylphosphino donor groups and the central
nitrogen atom as well as a hydride and a carbonyl ligand.
During the reaction with the borane-bound ligand 3c, a rare
M−N−B-H borametallacycle has been formed.49−53
The BH2-fragment likely originates from the coordinated
BH4-ligand in 2c, which subsequently forms H2 by the reaction
with an amine proton. Interestingly, the band at 1891 cm−1
corresponding to the C−O stretching vibration in the FT-IR
spectrum is shifted toward lower wavenumbers in comparison
to those of 2c, indicative of a stronger π-back bonding from the
central iron atom. If a solution of complex 7c and additional
borane adduct 2c is allowed to stand at ambient temperature
for 2 or 3 weeks, the formation of colorless crystals can be
observed. The 31P{1H} and 11B{1H} NMR spectra of these
crystals appears almost identical to those of 3c, but the single
crystal X-ray diffraction analysis of these crystals shows that
compound 3c gets slowly dehydrogenated to the dimeric B2N2-
cycle 8c in the presence of complex 7c (Figure 1). Such
dimerization reactions have been previously observed with
other ammonia boranes such as Me2NH-BH3, MePhNH-BH3,
or (CH2)4NH-BH3 in the presence of transition metal
complexes.54−58 The analysis of the supernatant solution
confirmed concomitant formation of free ligand Cy-PNHP,
indicating a BH3-group transfer in this dehydrogenation
reaction.
(JAB = 118.6 Hz, Δν = 199.7 Hz). A triplet resonance at −14.55
ppm (2JPH = 59.2 Hz), which is superimposed with the triplet
resonance at −14.60 ppm of 7c, is observed for the hydride
ligand in complex 9c, whereas the coordinating B−H group
gives rise to a broad one proton resonance at −10.49 ppm in
1
the H NMR spectrum.
For complexes 2a, 2b, and 2d, it has been shown that they
preferably release BH3, either at ambient temperature (2d) or
upon heating or treatment with a BH3-acceptor (2a/b), to give
a mixture of the trans-dihydride species 4 and the penta-
coordinated species 5.21 As the hydrido borohydride complexes
2c and 2e with more bulky ligands preferably release H2 in
favor of the BH3-release, we were investigating whether this
pathway can be suppressed in the presence of hydrogen gas. In
the case of complex 2c, a decrease in intensity of the
corresponding resonances and evolving signals for complexes
7c and 9c are still observed under 1 atm of hydrogen pressure
1
by H and 31P{1H} NMR spectroscopy. Only after approx-
imately 90% of conversion of 2c, a small amount of the trans-
dihydride complex 5c is detected by NMR spectroscopy.18 This
species gives rise to a resonance at −9.29 ppm in the 1H NMR
spectrum, which is best described as two superimposed triplet
As complex 2c undergoes a dehydrogenative coupling with
the borane protected ligand 3c, it seems likely that 2c reacts
with other primary or secondary amines as well. For this reason,
we investigated the reactivity of 2c toward aniline, which has
been previously used as a BH3-acceptor with borohydride
complexes.32 As the dehydrocoupling reaction with 3c appears
to be rather slow, the sample was heated to 50 °C and
continuously analyzed by NMR spectroscopy. With 2.2 equiv of
2
of doublet resonances (2JPH = 40.2 Hz and JHH = 8.6 Hz for
2
2
the first one and JPH = 40.8 Hz and JHH = 8.4 Hz for the
second one). A singlet resonance at 105.6 ppm in the 31P{1H}
NMR spectrum was assigned to this complex.
If a solution of complex 2e is treated with 1 atm of H2, a
decrease in intensity is still observed, but 7e is just formed to a
minor extent. Instead, two broad resonances at −6.00 and
−8.19 ppm with equal integrals exhibit increased intensities in
1
aniline, the H and 31P{1H} NMR spectra show the formation
1
of a new main product (67%) in addition to complex 7c
the H NMR spectrum (ν1/2 = 36.5 and 36.2 Hz). The former
F
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resonance appears to be a broadened triplet with 2JPH = 43.1 Hz
that simplifies to a singlet upon 31P-decoupling. This species
gives rise to a singlet resonance at 118.1 ppm in the 31P{1H}
NMR spectrum, which again is broadened with respect to the
resonances of 2e and 6e. The chemical shifts in the H NMR
spectrum allow one to propose a trans-arrangement of a
are observed at 1760 and 1824 cm−1, which is in good
agreement with values reported for [(iPr-PNHP)Fe(CO)2].42
Interestingly, the 31P{1H} NMR spectrum of 10e in benzene-d6
or toluene-d8 does not show any resonance, whereas the H
NMR spectrum provides evidence for the presence of
paramagnetic species.
1
1
hydride and a carbonyl ligand in this species (Scheme 8).
Overall, the steric demand of the ligands has a strong impact
on the stability of the synthesized precatalysts 2a−e. In this
comparative study, we were able to show that different
pathways of activation are viable for iron hydrido borohydride
complexes in solution. If the steric demand is too high, H2 is
preferably released rather than BH3, leading to deactivated
complexes. In a likewise manner, the phenyl-substituted
complex 2d in fact releases BH3 to some extent but suffers
from decomposition of the formed species that is too fast. On
the basis of the gained information about the reactivity patterns
of complexes 2a−e, the complexes with sterically less
demanding dialkylphosphino groups are expected to be the
most active (pre)catalysts.
Catalytic Hydrogenation Reactions. Within this study,
differences in stability and reactivity of the prepared hydrido
borohydride complexes 2a−e have been encountered, by
simple change of the substituents of the phosphine donor
groups. Next, we investigated the catalytic activity of sufficiently
stable precatalysts 2a, 2b, and 2c in the hydrogenation of
different substrates and compared there activity with those of
the previously reported precatalyst 2b. As complex 2d showed
no activity in the hydrogenation of acetophenone to 1-
phenylethanol, it has not been further investigated for the
hydrogenation of more inert substrates. Complex 2b exhibits
excellent activity in the hydrogenation of the nonactivated ester
methyl benzoate to benzyl alcohol and methanol (Table 2).27
Scheme 8. Reactivity of Hydrido Borohydride Complex 2e
under 1 atm of Hydrogen
Because of the limited stability of this species, a further analysis
was not possible. In addition, two high-field shifted triplet
resonances at −25.43 ppm (2JPH = 55.3 Hz) and −29.30 ppm
(2JPH = 51.8 Hz) can be located in the 1H NMR spectrum. The
strong high-field shift of these resonances and the absence of
H−H-coupling are indicative of penta-coordinated amido
complexes with a vacant coordination site trans to the hydrido
ligand. Nonetheless, all attempts to isolate these complexes
resulted in the formation of the iron(0)dicarbonyl complex
10e. Such an intermolecular carbonyl ligand transfer of
unsaturated iron hydride fragments has previously been
observed for iron pincer complexes.18,42
The molecular structure of 10e has been confirmed by single
crystal X-ray diffraction (Figure 2). On the basis of the
Table 2. Hydrogenation of Methyl Benzoate Catalyzed by
a
[(R-PNHP)Fe(H) (CO) (BH4)] (2a−c)
b
b
p(H2)
(bar)
t
conversion
(%)
yield
entry complex R =
T (°C) (h)
(%)
1
2
3
4
5
6
7
8
9
2c
2b
2b
2b
2b
2a
2a
2a
2a
2a
Cy
iPr
iPr
iPr
iPr
Et
10
10
50
50
50
10
10
10
50
50
100
100
100
80
6
6
6
6
6
6
6
6
6
6
52
91
99
87
52
68
89
32
96
>99
46
88
93
82
49
62
89
23
95
99
c
c
c
c
60
60
Figure 2. Molecular structure of 10e in the crystal lattice with
ellipsoids set to 50% probability (carbon bound hydrogen atoms were
omitted for clarity).
Et
70
Et
100
70
Et
10
Et
100
parameter τ = 0.17, the coordination geometry of the central
iron atom is best described as square pyramidal. Notably, the
Fe−C−O angle of the apical carbonyl ligand in 10e is
significantly bent (168.1°), which has been previously observed
a
Reaction conditions: methyl benzoate (0.5 mmol), precatalyst 1b−e
(0.005 mmol), and m-xylene (1.0 mmol) as internal standard and THF
(1 mL). Conversion and yield were determined by GC analysis using
m-xylene as an internal standard. Data have been taken from ref 27.
b
c
42
i
for the analogous Pr-substituted complex. The infrared
spectrum of solid complex 10e showed several strong bands in
the range of 1700 to 2100 cm−1, which is in contrast to the
assumption of a single species. A similar observation has been
made by Goldmann and co-workers for the pyridine based
complex [(tBu-PNPyP)Fe(CO)2], where this observation was
attributed to dynamic behavior in solution.59 In the present
case, different relative orientations of the NH-proton are
possible in addition to the two coordination geometries square
pyramidal and trigonal bipyramidal. The two strongest bands
Using 1 mol % catalyst loading, complex 2b showed the highest
activity at 100 °C and 50 bar of hydrogen pressure, yielding
93.0% of the corresponding alcohols (entry 3). At lower
hydrogen pressures (10 bar), the yield of benzyl alcohol and
methanol is only slightly decreased to 88.0% (entry 2), whereas
lowering the temperature results in a significant drop of activity,
ranging from 82.0% yield at 80 °C (entry 4) to 49.0% yield at
60 °C (entry 5). Complex 2c in comparison was only capable
G
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Article
a
Table 3. Hydrogenation of Amides Catalyzed by [(Et-PNHP)Fe(H) (CO) (BH4)] (2a)
a
Reaction conditions: substrate (1.00 mmol), precatalyst 2a (0.02−0.10 mmol), and m-xylene (1.00 mmol) as internal standard and THF (5 mL).
b
c
Conversion and yield were determined by GC analysis using m-xylene as an internal standard. Isolated yield.
of catalyzing the reaction with 46.5% yield at 100 °C and 10 bar
higher hydrogen pressures of 50 bar at 70 °C, complex 2a
catalyzes the hydrogenation of methyl benzoate to benzyl
alcohol in 95.0% yield (entry 9). With 50 bar hydrogen
pressure at 100 °C reaction temperature, full conversion and
formation of benzyl alcohol in more than 99% is observed with
complex 2a as catalyst. In general, complex 2a seems to be
thermally less stable than 2b at lower hydrogen pressures. As
of hydrogen pressure (entry 1). Applying the same conditions
with complex 2a gives even lower yields of 23.0% (entry 7). For
this reason, we investigated the activity of complex 2a at
different temperatures with 10 bar hydrogen pressure (entries
6−8). Under low hydrogen pressure, complex 2a exhibits a
higher activity at 70 °C than at 100 °C (entry 7). Applying
H
DOI: 10.1021/acs.organomet.6b00251
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Organometallics
Article
complex 2a and 2b are very similar in spectroscopic properties,
the reason for the higher activity in comparison to the reported
value of 2b is likely the reduced steric demand of the
diethylphosphino-groups. This seems to result in lower steric
repulsion between the catalyst and the substrate, thus lowering
reaction barriers within the catalytic cycle rather than lowering
the barriers of one of the discussed decomposition pathways.
On the basis of the catalytic activity in the hydrogenation of
methyl benzoate, we chose complex 2a as the most active
catalyst to explore new substrates and extend the scope of the
developed catalysts. Although much progress has been made in
the homogeneous hydrogenation of esters to alcohols, very few
examples for the significantly more difficult hydrogenation of
amides have been reported, and no active iron-based catalyst is
among those.60−74 In particular, the hydrogenation of amides to
alcohols and amines is desired, but with some of the reported
catalysts, C−O-cleavage and therewith the formation of
secondary amines are rather observed than C−N-cleavage.62,70
This C−O-cleavage is a result of facile hemiaminal dehydration
after the first reduction step of the amide and consecutive
hydrogenation of the formed imine. According to DFT
calculations, the C−N-cleavage is achieved by proton transfer
from the ligand to the nitrogen atom of the O-bound aminal
after the first reduction step, leading to amine-release and
formation of the aldehyde-bound complex.75
Treatment of N-phenylbenzamide with H2 (50 bar) at 70 °C
in dry THF with a catalytic amount of 2a (10 mol %) results in
the clean formation of 90% benzylalcohol and 93% aniline after
24 h, whereas the secondary amine benzylphenylamine was not
detected by GC analysis (Table 3, entry 1). If the catalyst
loading is decreased to 2 mol % benzyl alcohol, then aniline is
obtained in 45% yield. At 100 °C, benzyl alcohol and aniline are
obtained in 99% yield. With N-(3,5-bis(trifluoromethyl)-
phenyl)benzamide, complete hydrogenation to benzyl alcohol
and 3,5-bis(trifluormethyl)aniline is observed (entry 2). The
activated substrate trifluoracetanilide is completely hydro-
genated to 2,2,2-trifluorethanol and aniline with even the
lower catalyst loading of 2 mol % under otherwise identical
conditions (entry 3). Interestingly, the hydrogenation of N-
methylbenzamide proceeds only to a minor extent with
complex 2a as catalyst (entry 4), but the corresponding tertiary
amide N,N-dimethylbenzamide undergoes hydrogenation with
a modest yield of 50% (entry 5). δ-Lactams such as N-phenyl-2-
piperidone (entry 6) and the more activated N-bis-
(trifluoromethyl)phenyl-2-piperidone (entry 7) cannot be
hydrogenated with complex 2a as precatalyst. Nonetheless,
the corresponding γ-lactams, N-phenyl-2-pyrrolidone and N-
bis(trifluoromethyl)phenyl-2-pyrrolidone, are readily hydro-
genated with complex 2a as catalyst (entries 8 and 9). The
resulting amino alcohols were isolated in yields of 41% and
92%.
complexes in solution that finally allowed for the identification
of the most suitable precatalyst. Accordingly, the complexes
with the less bulky ethyl- and iso-propyl-groups showed
sufficient stability and represent the most active hydrogenation
catalysts. Furthermore, complex 2a was utilized in the
hydrogenation of amides to amines and alcohols, a reaction
only a few ruthenium complexes are reported to be capable of
catalyzing. Therewith, complex 2a represents the first example
of an iron-based catalyst for the hydrogenation of such a
challenging type of substrate.74
EXPERIMENTAL SECTION
■
Materials and Methods. All experiments were carried out under
an atmosphere of purified argon in a MBraun Labmaster glovebox or
using standard Schlenk techniques. MeCN and CH2Cl2 were dried
over CaH2, EtOH was dried over magnesium, and Et2O and THF
were dried over Na/K alloy and toluene over sodium. Benzene-d6 and
toluene-d8 were distilled from Na/K alloy. Other deuterated solvents,
such as CD2Cl2 and CDCl3 were dried over 3 or 4 Å molecular sieves.
FeCl2, FeBr2, methyl benzoate, and NaBH4 were purchased from
Sigma-Aldrich and used as received. R-PNHP (R = Et, Pr, Cy, Ph,
i
tBu)3,76−78 and amides65,79 were prepared according to literature
procedures. Complexes 1b and 2b, as well as the spectroscopic data for
4b and 5b have previously been reported.16,18,21,27 1H, 13C, 31P, and
11B NMR spectra were recorded using Bruker DRX 400, DRX 500 and
Avance 500 NMR spectrometers. 1H and 13C{1H}, and 13C-APT
(attached proton test) NMR chemical shifts are reported in ppm
downfield from tetramethylsilane. The resonance of the residual
protons in the deuterated solvent were used as internal standard for 1H
NMR spectroscopy. The solvent peak of the deuterated solvent was
used as internal standard for 13C NMR spectroscopy. Provided that the
1
complexes exhibit sufficient stability, assignments in the H and 13C
1
1
1
NMR data are based on H-COSY, H-NOESY, H,13C-HSQC, and
1H,13C-HMBC NMR spectroscopy and selective 31P- and 11B-
decoupling experiments. 31P NMR chemical shifts are reported in
ppm downfield from H3PO4 and referenced to an external 85%
solution of phosphoric acid in D2O. 11B NMR chemical shifts are
reported in ppm downfield from BF3·Et2O and referenced to an
external solution of BF3·Et2O in CDCl3. The following abbreviations
and combinations thereof are used for the description of NMR data: br
(broad), s (singlet), d (doublet), t (triplet), q (quartet), quin
(quintet), m (multiplet), and v (virtual).
FT-IR spectra were recorded by attenuated total reflection of the
solid samples on a Bruker Tensor IF37 spectrometer. The intensity of
the absorption band is indicated as vw (very weak), w (weak), m
(medium), s (strong), vs (very strong), and br (broad).
HR-ESI mass spectra were acquired with a LTQ-FT mass
spectrometer (Thermo Fisher Scientific). HR-APCI mass spectra
were acquired with a LTQ-FT mass spectrometer (Thermo Fisher
Scientific). In both cases, the resolution was set to 100.000. Elemental
analyses were performed on a Vario Micro Cube Elemental Analyzer.
X-ray Crystallography. The single crystal X-ray diffraction data
for the structural analysis has been collected using graphite-
monochromated Mo−Kα-radiation (λMoKα = 0.71073 Å) on the
imaging plate detector systems STOE IPDS2 (1a, 3c, and 7c) or on
the pixel detector system BRUKER D8 QUEST (1d, 8c, and 10e).
The structures were solved by direct methods with SHELXS-97 and
refined against F2 by full-matrix-least-squares techniques using
SHELXL-97.80 On the basis of the crystal descriptions, numerical
absorption corrections were applied.80,81 Crystallographic data for 1a,
1d, 3c, 7c, 8c, and 10e have been deposited at Cambridge
Crystallographic Data Centre (CCDC 1449037-1449042) and can
be obtained free of charge via www.ccdc.cam.ac.uk/. Details of the data
collection and the refinement can be found in the Supporting
Information.
In summary, complex 2a represents an active precatalyst for
the hydrogenation of different classes of nonactivated amides.
More activated amides can get hydrogenated with only 2 mol %
catalyst loading under even milder conditions, yielding the
corresponding amines and alcohols with excellent selectivity.
CONCLUSIONS
■
In the current study, we demonstrated that minor changes in
the ligand moiety of amine-based iron pincer complexes can
have a strong impact on complex stability and reactivity
patterns. With the synthesis of a series of possible precatalysts,
we were able to observe different reaction pathways of these
Synthesis of [(Et-PNHP)Fe(Br)2(CO)] (1a). 393 mg (1.58 mmol, 1.00
equiv) of Et-PNHP and 340 mg (1.58 mmol, 1.00 equiv) of FeBr2 were
suspended in 10 mL of THF. After stirring for 10 min, the argon
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Article
atmosphere in the Schlenk tube was replaced by 1 atm of CO, and
stirring was continued for 10 min. Twenty milliliters of n-hexane was
added, and the blue solution was stored for 16 h at −25 °C. After
removing the colorless solution, the residue was dissolved in 15 mL of
chloroform, and the solution was filtrated. After removal of the solvent
and drying under vacuum, 1a can be obtained as a blue solid. Yield:
(524 mg, 1.06 mmol, 67%). Analysis calculated for C13H29Br2FeNOP2
(M = 492.98 g/mol): C, 31.67%; H, 5.93%; N, 2.84%. Found: C,
31.78%; H, 5.90%; N, 2.79%. 1H NMR (300 MHz, CD2Cl2, 27 °C) δ:
1.32 (vsext, 12H, 3JH−H = 6.2 Hz, PCH2CH3), 2.09−2.24 (m, 10H, 2H
NCH2CH2P + 8H PCH2CH3), 2.31−2.39 (m, 2H, NCH2CH2P),
3.21−3.34 (m, 2H, NCH2CH2P), 3.53−3.58 (m, 2H, NCH2CH2P),
4.74 (br, 1H, NH) ppm. 13C{1H} NMR (75 MHz, CD2Cl2, 27 °C) δ:
stirred for 16 h. After removal of all volatiles, complex 2a was obtained
as a dark yellow solid. Despite several attempts with different
crystallization methods and different solvents, it was not possible to
grow suitable single crystals for X-ray diffraction. Yield: 266 mg (0.76
mmol, 94%). Analysis calculated for C13H35BFeNOP2 (M = 350.03 g/
mol): C 44.74%, H 9.82%, N 4.01%. Found: C 43.79%, H 9.62%, N
1
2
4.12%. H NMR (500 MHz, C6D6, 27 °C) δ: −19.8 (t, 1 H, JP−H
=
3
49.7 Hz, FeH), −3.01 (s, br, 4H, BH4), 0.98 (vquin, 6H, JH−H = 7.6
Hz, 2 x PCH2CH3), 1.49−1.40 (m, 4H, 2H NCH2CH2P + 2H
3
PCH2CH3), 1.19 (vquin, 6H, JH−H = 7.7 Hz, 2 x PCH2CH3), 1.73−
1.67 (m, 2H, NCH2CH2P), 1.56−1.50 (m, 4H, 2H NCH2CH2P + 2H
PCH2CH3), 1.98−1.89 (m, 2H, PCH2CH3), 2.23−2.17 (m, 2H,
PCH2CH3), 2.54−2.43 (m, 2H, NCH2CH2P), 3.79 (s, br, 1H, NH)
ppm. 13C{1H} NMR (75 MHz, C6D6, 27 °C) δ: 8.5 (s, PCH2CH3),
1
8.1 (s, PCH2CH3), 8.3 (s, PCH2CH3), 16.2 (vq, JP−C = 13.1 Hz,
1
2
PCH2CH3), 28.0 (vt, JP−C = 8.7 Hz, NCH2CH2P), 50.0 (vt, JP−C
=
20.3 (vt, 1JP−C = 11.1, PCH2CH3), 9.0 (s, PCH2CH3), 23.3 (vt, 1JP−C
13.9, PCH2CH3), 28.1 (vt, 1JP−C = 8.4, NCH2CH2P), 53.3 (vt, 2JP−C
=
=
4.6 Hz, NCH2CH2P). 31P{1H} NMR (122 MHz, CD2Cl2, 27 °C) δ =
64.3 (s) ppm. HR-MS (APCI+): m/z calcd 384.0304 [(Et-PNHP)Fe-
6.2 Hz, NCH2CH2P) ppm. 11B{1H} NMR (160 MHz, C6D6, 27 °C) δ:
−33.0 (s) ppm. 31P{1H} NMR (122 MHz, C6D6, 27 °C) δ: 81.3 (s)
ppm. 31P NMR (122 MHz, C6D6, 27 °C) δ: 81.4 (d, 2JP−H = 45.5 Hz)
ppm. HR-MS (ESI+, MeOH/MeCN): calcd 334.1147 [(Et-PNHP)-
Fe(H) (CO)]+; found, 334.1147; calcd, 375.1412 [(Et-PNHP)Fe(H)
̃
(Br)]+; found, 384.0318. FT-IR: ν [cm−1] = 3182 (w, NH), 2964 (m),
2935 (m), 2910 (m), 2874 (m), 2162 (w), 2036 (w), 1936 (s, CO),
1892 (m), 1461 (m), 1450 (m), 1413 (m), 1380 (m), 1305 (w), 1249
(m), 1235 (m), 1209 (m), 1174 (m), 1125 (m), 1068 (s), 1040 (s),
1029 (s), 983 (m), 972 (m), 954 (w), 868 (m), 830 (m), 787 (w), 762
(s), 736 (m), 720 (s), 693 (s), 684 (s), 629 (s), 585 (s), 568 (s), 546
(s), 510 (w), 490 (w), 415 (m).
̃
(CO) (MeCN)]+; found, 375.4107. FT-IR: ν [cm−1] = 3201 (w, NH),
2962 (m), 2934 (w), 2906 (w), 2877 (w), 2383 (w, BH), 2351 (w,
BH), 2347 (w, BH), 2300 (w, BH), 2050 (br), 1892 (s, CO), 1854
(m), 1833 (m), 1459 (m), 1412 (w), 1378 (w), 1306 (w), 1260 (s),
1207 (w), 1173 (w), 1065 (s), 1014 (s), 967 (w), 955 (m), 865 (w),
834 (m), 792 (vs), 762 (m), 731 (m), 712 (m), 689 (m), 629 (m),
602 (m), 558 (w), 495 (w), 424 (w).
Synthesis of [(Ph-PNHP)Fe(Br)2(CO)] (1d). 800 mg (1.81 mmol,
1.00 equiv) Ph-PNHP and 390 mg (1.81 mmol, 1.00 equiv) FeBr2 were
suspended in 60 mL of THF. After stirring for 1 h, the argon
atmosphere in the Schlenk tube was replaced by 1 atm of CO, and
stirring was continued for 3 h. After evaporation of the solvent, the
purple residue was suspended in 5 mL of THF and filtered over silica.
After washing with 3 × 5 mL THF, the product can be washed out of
the frit with 3 × 10 mL of DCM. After removal of the solvent and
drying under vacuum, 1d can be obtained as a red solid, containing a
mixture of trans and cis isomers. Complex 1d is moderately stable in
the solid state but reacts under loss of the CO ligand in solution,
yielding the paramagnetic dibromo complex. Yield: 120 mg (0.18
mmol, 10%). C29H30Br2FeNOP2 (M = 686.16 g/mol). Because of the
limited stability of 1d, it was not possible to obtain an elemental
Formation of [(Et-PNHP)Fe(H)2(CO)] (4a) and [(Et-PNP)Fe(H)
(CO)] (5a). When a solution of complex 2a in C6D6 was heated to
70 °C under ten atmospheres of hydrogen in a Fischer−Porter tube
and is transferred after 1 h to an NMR tube, the 1H and 31P{1H} NMR
spectra of the mixture indicate a low conversion of complex 2a.
However, in the presence of an amide virtually complete conversion
and the formation of complex 5a as the major product is observed in
addition to unidentified byproducts. A more selective reaction,
however, is observed in the presence of BH3-acceptors such as PEt3,
allowing for the detection of both the dihydride complexes 4a and the
penta-coordinated complex 5a. The 31P{1H} NMR spectrum clearly
1
analysis. H NMR (300 MHz, CDCl3, 27 °C) δ: 0.72−0.98 (m, 1H,
indicated the formation of Et3P·BH3 by a quartet at 22.05 ppm (1JPB
=
CH2), 1.93−2.16 (m, 1H, CH2), 2.31−2.55 (m, 1H, CH2), 2.63−3.15
(m, 3H, CH2), 3.29−3.51 (m, 2H, CH2), 3.53−3.71 (m, 1H, CH2),
1
58.1 Hz) after 30 min. The H NMR spectrum of the mixture gives
rise to a multiplet resonance at −9.66 ppm is, which is assigned to
trans-[(Et-PNHP)Fe(H)2(CO)] (trans-4a, δP = 93.1 ppm). A singlet
resonance at 4.47 ppm in the 1H NMR spectrum indicates the
concomitant formation of elemental hydrogen in this reaction.
Furthermore, minor quantities of the corresponding cis-isomer cis-4a
(δH = −20.00 (td, 2JPH = 51.3 Hz, 2JHH = 14.2 Hz, Fe−H), −8.06 (td,
2JPH = 85.2 Hz, 2JHH = 14.9 Hz, Fe−H) ppm; δP = 87.5 ppm) and the
2
5,06 (t, 1H, JPH = 12.9 Hz, NH), 7.32−7.51 (m, 10H, Ph-H), 7.54−
7.71 (m, 3H, Ph-H), 7.78−8.06 (m, 5H, Ph-H), 8.11−8.30 (m, 2H,
Ph-H) ppm. 31P{1H} NMR (161.9 MHz, CD2Cl2, 27 °C) δ: 70.4 (s,
trans-1d), 42.4 (s, cis-1d) ppm. Only the resonances which are
changing upon 31P-decoupling are listed in the following. 1H{31P}
NMR (400 MHz, CD2Cl2, 27 °C, o2p = 71 ppm) δ: 3.46 (d, 2H, JHH
=
3
13.4 Hz, CH2), 5.06 (s br, 1H, NH), 7.69 (d, 2H, JHH = 6.7 Hz, Ph-
3
2
H), 7.95 (d, 2H, JHH = 5.8 Hz, Ph-H) ppm. 13C-APT NMR (100.6
penta-coordinated complex 5a (δH = −24.40 (t, JPH = 50.9 Hz, Fe−
MHz, CD2Cl2, 27 °C): δ = 28.0 (d, JPC = 34.9 Hz, PCH2), 50.5 (s,
H); δP = 78.0 ppm) were detected by 1H and 31P{1H} NMR
spectroscopy.
1
NCH2), 128.0 (s, Ph-C), 128.3 (d, JPC = 4.9 Hz, Ph-C), 128.8 (s, Ph-
C), 129.2 (s, Ph-C), 130.0 (d, JPC = 19.9 Hz, Ph-C), 130.4 (s, Ph-C),
131.0 (s, Ph-C), 131.5 (s, Ph-C), 132.9 (s, Ph-C), 134.1 (s, Ph-C) ppm.
Because of the limited stability of 1d in solution and the accompanying
formation of paramagnetic compounds, quaternary carbon atoms
could not be detected. MS (ESI+): m/z (%) = 442.3 (100) (Ph-
Synthesis of [(Cy-PNHP)Fe(H)(CO)(BH4)] (2c). 150 mg (0.32 mmol,
1.00 equiv) Cy-PNHP and 108 mg (0.32 mmol, 1.00 equiv)
[Fe(H2O)6](BF4)2 are dissolved in 8 mL of MeCN to yield an
intense red solution (δp = 55.5 ppm). A solution of 183 mg (4.83
mmol, 15.0 equiv) of NaBH4 in 8 mL of ethanol was added to the red
solution, causing immediate gas evolution (δp = 80.1 ppm). The
resulting orange solution was stirred for 1 h, after which the argon
atmosphere was replaced by CO (1 atm), and the mixture was stirred
for an additional 2 h (δp = 89.0 ppm). After evaporation of all volatiles,
the residue was extracted with 3 × 10 mL n-hexane. A 31P{1H} NMR
spectrum of the reaction mixture confirms the formation of complex
2c together with approximately 50% of 3c, which can be removed by
precipitation and washing. The yellow solution was concentrated to a
volume of 5 mL and cooled to −20 °C, which causes the precipitation
of a light yellow solid. After separation of the supernatant solution, the
solid was washed again with 5 mL of n-hexane at −20 °C and dried in
vacuo to yield 60 mg (0.100 mmol, 31%) of 2c as a yellow powder. 1H
NMR (400.0 MHz, C6D6, 27 °C) δ: −19.59 (t, 2JPH = 50.4 Hz, 1H, Fe-
H), −2.84 (br, 4H, BH4), 0.60−2.34 (m, 44H, Cy-H+3xCH2), 2.56
̃
PNHP)H+; 576.2 (75) [(Ph-PNHP)Fe(Br)]+. FT-IR: ν [cm−1] = 3172
(w, NH), 3050 (w), 2934 (w), 2873 (w), 2360 (w), 2342 (w), 1940 (s,
CO), 1916 (m, CO), 1587 (w), 1573 (w), 1484 (w), 1462 (w), 1434
(m), 1409 (w), 1333 (w), 1312 (w), 1275 (w), 1247 (w), 1212 (w),
1189 (w), 1171 (w), 1161 (w), 1098 (m), 1073 (w), 1055 (m), 1027
(w), 999 (w), 963 (m), 833 (m), 783 (w), 756 (m), 744 (s), 715 (m),
695 (s), 676 (m), 667 (m), 617 (w), 595 (w), 580 (m), 570 (m), 557
(m), 536 (s), 507 (s), 498 (s), 474 (m), 444 (m), 433 (m), 414 (m).
Synthesis of [(Et-PNHP)Fe(H)(CO)(BH4)] (2a). 400 mg (0.81 mmol)
of [(Et-PNHP)FeBr2(CO)] (1a) was dissolved in 20 mL of EtOH. 153
mg (4.06 mmol, 5.10 equiv) of NaBH4 was added in one portion, and
the resulting yellow solution was stirred for 30 min. All volatiles were
removed in vacuo, and the residue was extracted with 20 mL of toluene
to give a yellow solution. The solution was filtrated over silica and
J
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(m, 4H, CH2+Cy-H), 2.86 (m, 2H, Cy-H), 2.94 (m, 2H, Cy-H), 3.97
(m, 1H, NH) ppm. 31P{1H} NMR (161.9 MHz, C6D6, 27 °C) δ: 91.9
(s) ppm. 11B{1H} NMR (128.3 MHz, C6D6, 27 °C) δ: −31.4 (br)
ppm. Only the resonances which are changing upon 31P- or 11B-
decoupling are listed in the following. 1H{31P} NMR (400 MHz,
CD2Cl2, 27 °C, o2p = 92 ppm) δ: −19.59 (s, 1H, Fe-H) ppm. 13C-
APT NMR (100.6 MHz, C6D6, 27 °C) δ: 16.7 (s, Cy-C), 20.2 (d, 2JPC
= 29.4 Hz, Cy-C), 26.9 (s, Cy-C), 27.2 (s, Cy-C), 27.3 (s, Cy-C), 27.4
(s, Cy-C), 27.9 (s, Cy-C), 28.2 (s, Cy-C), 28.4 (s, Cy-C), 29.3 (s, Cy-
C), 30.5 (s, Cy-C), 31.5 (s, Cy-C), 32.4 (d, 1JP−C = 32.4 Hz, CH), 36.8
(t, 1JP−C = 12.2 Hz, CH), 40.3 (t, 1JP−C = 8.9 Hz, CH), 45.0 (s, Cy-C),
54.4 (s, CH2), 222.4 (dt, 2JP−C = 25.6 Hz, 2JH−C = 19.2 Hz, CO) ppm.
MS (ESI+): m/z (%) = 522.7 (100) [(Cy-PNHP)Fe(H)]+; 550.5 (43)
[(Cy-PNHP)Fe(H) (CO)]+; 590.8 (52) {[(Cy-PNHP)Fe(H) (BH4)
(CO)]+Na}+. HR-MS (ESI+): m/z calcd 550.3025 [(Cy-PNHP)Fe-
62.9 Hz, 2JHH = 9.7 Hz, Fe-H) ppm. 31P{1H} NMR (162 MHz, C6D6,
27 °C) δ: 94.4 (s) ppm.
Formation of [(tBu-PNHP)Fe(H)(CO)(BH4)] (2e). 40 mg (0.11
mmol) of tBu-PNHP and 36 mg (0.11 mmol, 1.00 equiv) of
[Fe(H2O)6](BF4)2 are mixed with 6 mL of MeCN, leading to a pale
yellow solution and small amounts of a white precipitate. The mixture
is cooled to −42 °C, resulting in a pale blue solution, and a solution of
60 mg (1.56 mmol, 30.0 equiv) of NaBH4 in EtOH was added
dropwise. After 1 h of stirring at −42 °C, a greenish yellow solution
was formed. The argon atmosphere was replaced by 1 atm of carbon
monoxide, and the mixture was stirred for a further 2 h at −42 °C,
again leading to a color change and the formation of an intense orange
solution. After this period, the mixture was allowed to warm to
ambient temperature, all volatiles were removed in vacuo, and the
orange residue was extracted with n-hexane and toluene. The 31P{1H}
NMR spectrum of the extracted solutions indicates the presence of up
to four species in addition to complex 2e, including 3e, 6e, and 7e. All
attempts of purification or separation resulted in the decomposition of
2e. Furthermore, complex 2e turns out to be fairly unstable in solution
and is only detectable for a couple of hours in solutions of the
obtained mixtures. 1H NMR (400.0 MHz, C6D6, 27 °C) δ: −20.83 (t,
̃
(H) (CO)]+; found: 550.3035. FT-IR: ν [cm−1] = 3467 (br), 3199 (w,
NH), 2925(m), 2850 (m), 2361 (m, BH), 1900 (s, CO), 1821 (w),
1621 (w), 1447 (m), 1407 (w), 1343 (w), 1298 (w), 1270 (w), 1201
(w), 1176 (w), 1120 (w), 1067 (m), 1006 (w), 980 (w), 960 (w), 615
(w), 890 (w), 854 (w), 832 (w), 785 (w), 742 (w), 709 (w), 686 (w),
600 (w), 535 (w), 513 (w), 465 (w).
1
Analytical Data for 3c. H NMR (300 MHz, C6D6, 27 °C) δ:
1H, 2JPH = 60.2 Hz, Fe-H), −2.05 (br, 4H, BH4), 1.16 (d, 18H, 3JPH
=
0.90−1.07 (m br, 12H, Cy-H + BH3), 1.13−1.71 (m, 34H, 2JPH = 13.8
Hz PBH3), 1.72−1.88 (m br, 4H, Cy-H), 1.90−2.13 (m, 4H,
NCH2CH2P), 2.94−3.18 (m, 4H, NCH2CH2P), 4.83 (br s, 1H,
NH) ppm. Only the resonances which are changing upon 11B-
decoupling are listed in the following. 1H{11B} NMR (300 MHz,
9.7 Hz, C(CH3)3), 1.39 (d, 9H, JPH = 12.2 Hz, C(CH3)3), 1.41 (d,
9H, 3JPH = 11.0 Hz, C(CH3)3), 1,61 (m superimposed, 2H, CH2), 2.02
(m superimposed, 2H, CH2), 2.54 (br, 2H, CH2), 2.95 (br, 2H. CH2),
5.50 (br superimposed, 1H, N-H) ppm. 31P{1H} NMR (162 MHz,
C6D6, 27 °C) δ: 111.0 (s) ppm. 11B{1H} NMR (128 MHz, C6D6, 27
°C) δ: −40.6 (br) ppm. Only the resonances which are changing upon
31P- or 11B-decoupling are listed in the following. 1H{31P} NMR
(400.0 MHz, C6D6, 27 °C, o2p = 111.0 ppm) δ: −20.83 (s, 1H, Fe-H),
1.16 (s, 18H, C(CH3)3), 1.39 (s, 9H, C(CH3)3), 1.41 (s, 9H,
C(CH3)3) ppm.
3
2
C6D6, 27 °C) δ: 1.08 (d, 6H, JPH = 13.8 Hz, PBH3) ppm. 13C{1H}
NMR (75 MHz, C6D6, 27 °C) δ: 14.0 (d, 2JCP = 30.3 Hz), 24.1 (s, p-
2
Cy), 24.7 (vt, JCP = 2.2 Hz, o-Cy), 24.8 (s, m-Cy), 25.0 (s, m-Cy),
1
1
30.1 (d, JCP = 27.7 Hz, ipso-Cy), 30.5 (d, JCP = 27.5 Hz, ipso-Cy),
50.6 (s, NCH2CH2P) ppm. 31P{1H} NMR (122 MHz, C6D6, 27 °C) δ:
23.7 (br) ppm. 11B{1H} NMR (96 MHz, C6D6, 27 °C) δ: −43.0 (br)
ppm. MS (ESI+): m/z (%) = 480.8 (45) [(Cy-PNHP)·(BH3)]H+;
494.8 (100) [(Cy-PNHP)·(BH3)2]H+. HR-MS (ESI+): m/z calcd:
494.4391 [(Cy-PNHP)·(BH3)2]H+; found, 494.4388.
1
NMR Data for 3e. H NMR (300 Hz, C6D6, 27 °C) δ: 0.98 (d,
18H, 3JPH = 9.5 Hz C(CH3)3), 1.03 (d, 18H, 3JPH = 9.6 Hz C(CH3)3),
2.00−2.13 (m, 4H, NCH2CH2P), 3.04−3.17 (m, 4H, NCH2CH2P),
4.90 (br s, 1H, NH) ppm. Only the resonances which are changing
Formation of [(Ph-PNHP)Fe(H)(CO)(BH4)] (2d). 100 mg (0.23
mmol) of Ph-PNHP and 49 mg (0.23 mmol, 1.00 equiv) of FeBr2 are
dissolved in 8 mL of MeCN to yield a red solution (δp = 64.9 ppm). A
solution of 183 mg (4.83 mmol, 15.0 equiv) of NaBH4 in 8 mL of
ethanol is added, resulting in immediate gas evolution. After stirring
for 1 h (δp = 83.4 ppm), the argon atmosphere was replaced by 1 atm
CO, and the solution was stirred for two additional hours, leading to
an orange solution. The solvent was evaporated, and the orange
residue was extracted with 3 × 10 mL of toluene. After evaporation of
the solvent, the residue was washed with 4 × 3 mL ethanol and dried
under vacuum to yield a red solid, containing 2d and 3d according to
31P{1H} NMR spectroscopy. Because of the low stability of complex
2d, all attempts to further purify this compound resulted in
1
upon 11B-decoupling are listed in the following. H{11B} NMR (300
2
Hz, C6D6, 27 °C) δ: 1.04 (d, 6H, JP−H = 12.8 Hz, PBH3), ppm.
1
13C{1H} NMR (75 MHz, C6D6, 27 °C) δ: 15.0 (d, JC−P = 28.3 Hz,
NCH2CH2P), 27.6 (s, CCH3), 53.7 (d, 2JC−P = 2.9 Hz, CCH3), 63.3 (s,
NCH2CH2P) ppm. 31P{1H} NMR (160 MHz, C6D6, 27 °C) δ: 41.9
(br) ppm. 11B{1H} NMR (122 MHz, C6D6, 27 °C) δ: −42.5 (br)
ppm.
NMR Data for 6e. 31P{1H} NMR (162 MHz, C6D6, 27 °C) δ: 103.7
(br) ppm. 11B{1H} NMR (128 MHz, C6D6, 27 °C) δ: −21.9 (br)
1
2
ppm. H NMR (400.0 MHz, C6D6, 27 °C) δ: −15.85 (t, 1H, JPH
=
63.4 Hz, Fe-H), −10.30 (br, 1H, Fe-HB), 1.21 (superimposed,
C(CH3)3), 1.40 (superimposed, C(CH3)3), 1.61 (m, 2H, CH2), 2.08
(m, 2H, CH2), 2.40 (m, 2H, CH2), 3.12 (m, 2H, CH2) ppm. The
terminal B−H-resonance could not be assigned.
1
decomposition. H NMR (400.0 MHz, CD2Cl2, 27 °C) δ: −18.13
2
NMR Data for 7e. 31P{1H} NMR (162 MHz, C6D6, 27 °C) δ: 118.2
(AB-system, JAB = 119.0 Hz, Δν = 624.7 Hz) ppm. 11B{1H} NMR
(128 MHz, C6D6, 27 °C) δ: −11.9, −42.5 (br) ppm. 1H NMR (400.0
MHz, C6D6, 27 °C) δ: −14.93 (td, 1H, 2JPH = 59.4 Hz, 3JHH = 4.2 Hz,
Fe-H), −9.65 (br, 1H, Fe-HB), 1.26 (superimposed, C(CH3)3), 1.37
(superimposed, C(CH3)3), 1.95 (m, CH2), 2.12 (m, CH2), 2.30 (m,
CH2), 2.76 (m, CH2), 3.37 (m, CH2), 3.77 (m, CH2) ppm. Because of
the complex structure of 7e, it was not possible to assign all CH2-, tBu-,
and B−H-resonances in the obtained mixtures.
(t, 1H, JPH = 52.3 Hz, Fe−H), −3.61 (br, 4H, BH4), 2.39−2.54 (m,
2H, CH2), 2.65−2.80 (m, 2H, CH2), 2.90−3.06 (m, 2H, CH2), 3.42−
3.60 (m, 2H, CH2), 4.26 (m, 1H, NH), 7.35−7.55 (m, 12H, Ph-H),
7.68−7.85 (m, 8H, Ph-H) ppm. 31P{1H} NMR (161.9 MHz, CD2Cl2,
27 °C) δ: 81.7 (s) ppm. 11B{1H} NMR (128.3 MHz, CD2Cl2, 27 °C)
δ: −32.9 (br) ppm.
NMR Data for 3d. 1H NMR (300 MHz, C6D6, 27 °C) δ: 2.03−2.18
(m, 4H, NCH2CH2P), 2.86−3.02 (m, 4H, NCH2CH2P), 6.87−7.85
(m superimposed, 20H, phenyl-H) ppm. Only the resonances which
1
are changing upon 11B-decoupling are listed in the following. H{11B}
Synthesis of [((Cy-PNP)BH2N((CH2)2PCy2BH3)2)FeH(CO)] (7c). 106
mg (0.23 mmol) of Cy-PNHP and 77 mg (0.23 mmol, 1.00 equiv) of
[Fe(H2O)6](BF4)2 are dissolved in 8 mL of MeCN, yielding an
intense red solution. A solution of 130 mg (3.41 mmol, 15.0 equiv) of
NaBH4 in 8 mL of ethanol was added dropwise, which causes gas
evolution. The resulting orange solution was stirred for 1 h, the argon
atmosphere was replaced by CO (1 atm), and the mixture was stirred
for 2 additional hours. After evaporation of the solvent, the residue was
extracted with 3 × 10 mL n-hexane, and the yellow solution was
concentrated to a volume of 10 mL. After stirring for 1 week, the
solvent was evaporated, and the yellow residue was recrystallized from
NMR (300 MHz, C6D6, 27 °C) δ: 1.73 (d, 6H, 2JPH = 14.6 Hz, PBH3)
ppm. 31P{1H} NMR (202.5 MHz, C6D6, 27 °C) δ: 14.2 (br) ppm.
11B{1H} NMR (160.5 MHz, C6D6, 27 °C) δ: −40.5 (br) ppm.
The decomposition of complex 2d in solution was monitored by 1H
and 31P{1H} NMR spectroscopy. Two superimposed triplet of doublet
resonances in the 1H NMR spectrum were assigned to the two hydrido
ligands of the trans-dihydride complex 4d, differing in the relative
orientation of the amine proton in 4d. Assignable NMR data of 4d: 1H
NMR (400.0 MHz, C6D6, 27 °C) δ −8.55 (td superimposed, 1H, 2JPH
= 44.8 Hz, 2JHH = 10.2 Hz, Fe-H), −8.45 (td superimposed, 1H, 2JPH
=
K
DOI: 10.1021/acs.organomet.6b00251
Organometallics XXXX, XXX, XXX−XXX

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Article
diethyl ether. After drying in vacuo, the product was obtained as a
yellow powder with an approximate yield of 60% (based on Cy-
PNHP). Crystals suitable for X-ray diffraction are obtained by
Author Contributions
F.S. and M.A. contributed equally to this work.
1
Notes
recrystallizing from n-hexane at room temperature. H NMR (300.1
MHz, C6D6) δ: −14.60 (td, 1H, 2JPH = 59.9 Hz, 2JHH= 4.7 Hz, Fe-H),
−11.17 (br, 1H, ν1/2 = 53.5 Hz, BH), 1.24 (d, 1H, 2JHH = 14.3 Hz, B-
H-Fe), 0.79−2.17 (m, 98H, CH2 + Cy-H + P−BH3), 2.24 (m, 3H, Cy-
H), 2.34 (br d, 1H, Cy-H), 2.66−2.77 (m, 2H, Cy-H), 2.89−2.98 (m,
1H, CH2), 3.19 (br, 1H, BH), 3.40 (ddd, 2H, JPH = 34.1 Hz, JHH = 10.4
Hz, JHH = 8.2 Hz, CH2), 3.64 (ddd, 1H, JPH = 34.1 Hz, JHH = 11.2 Hz,
JHH = 7.8 Hz, CH2) ppm. 31P{1H} NMR (161.9 MHz, C6D6) δ: 24.3
(br, P-BH3), 97.8 (AB-system, JAB = 118.6 Hz, Δν = 963.7 Hz) ppm.
11B{1H} NMR (128 MHz, C6D6, 27 °C) δ: −11.8 (br, N-B-N), −42.6
(br, P-BH3) ppm. Only the resonances which are changing upon 11B-
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This research is supported by the DFG (LA 2830/3-1) and the
NSFC (21450110063). R.L. is grateful to Professor S. Dehnen
and Professor C. v. Hanisch for their continuous support and
̈
assistance.
1
decoupling are listed in the following. H{11B} NMR (400.0 MHz,
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ASSOCIATED CONTENT
* Supporting Information
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X-ray data (CIF)
X-ray data (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail: robert.langer@chemie.uni-marburg.de.
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DOI: 10.1021/acs.organomet.6b00251
Organometallics XXXX, XXX, XXX−XXX