Oxidative Transformations of Biosourced Alcohols Catalyzed by Earth-Abundant Transition Metals

Article abstract of DOI:10.1002/cctc.201700310

The catalytic acceptorless dehydrogenative oxidation of biosourced alcohols into carboxylic acid salts was achieved using earth-abundant Fe and Mn complexes that feature aliphatic PNP pincer ligands in good to excellent yields. The Fe derivatives were characterized by using ⁵⁷Fe NMR spectroscopy. Mn pincer catalysts are catalytically more efficient than their Fe counterparts thanks to their robustness under basic conditions. Attempts to generate aldehydes from alcohols were not successful using the Fe and Mn species, but a commercially available Ru analogue achieves this transformation selectively under very mild conditions in the presence of a large excess of acetone as a hydrogen acceptor.

Full text of DOI:10.1002/cctc.201700310

Accepted Article
Title: Earth-abundant transition metals-catalyzed oxidative
transformations of bio-sourced alcohols
Authors: Duc Hanh Nguyen, Yohann Morin, Lei Zhang, Xavier Trivelli,
Frédéric Capet, Sébastien Paul, Simon Desset, Franck
Dumeignil, and Regis Gauvin
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10.1002/cctc.201700310
ChemCatChem
FULL PAPER
Earth-abundant transition metals-catalyzed oxidative
transformations of bio-sourced alcohols
Duc Hanh Nguyen,[a] Yohann Morin,[a] Lei Zhang,[a] Xavier Trivelli,[b] Frédéric Capet,[a] Sébastien Paul,[a]
Simon Desset,[a] Franck Dumeignil*[a] and Régis M. Gauvin*[a]
Abstract: Catalytic acceptorless dehydrogenative oxidation of bio-
sourced alcohols into carboxylic acid salts was achieved using earth-
abundant Fe and Mn complexes featuring aliphatic PNP pincer
ligands, in good to excellent yield. The iron derivatives were
characterized using 57Fe NMR. Mn-pincer catalysts are catalytically
more efficient than their Fe counterparts thanks to their robustness
under basic conditions. Attempts to generate aldehydes from alcohols
were not successful using Fe and Mn species, but a commercially
available Ru analogue selectively achieves this transformation under
very mild conditions, in the presence of a large excess of acetone as
hydrogen acceptor.
point of view, the dehydrogenation of primary alcohols initially
produces aldehydes that can further react with a nucleophile such
as an alcohol or an amine, to yield a hemiacetal or hemiaminal
intermediates that finally dehydrogenates to esters or amides,
respectively (Scheme 1). 5 Following the similar approach,
dehydrogenative couplings of primary alcohols in basic aqueous
media produces carboxylic acid salts in sustainable and
environmentally benign manner.6 This oxidative transformation
generates H2 as the only by-product.
Introduction
The constant dwindling of fossil resources is one of the biggest
challenges for mankind in the 21st century. It mobilizes both
academia and industry to provide solutions for improvements in
operational efficiency (“drop-in strategy”) and in product
Scheme 1. Dehydrogenative coupling reactions of primary alcohols
Several examples of “classical” organometallic complexes have
been reported for this transformation. Möller et al.7 and Madsen
et al.8 showed that Ru complexes featuring monodentate carbene
and phosphine ligands are catalytically active for a broad
substrate scope. Prechtl et al. 9 used methylated pincer-type
PMeNP ruthenium complexes while Wang et al. reported a water-
soluble binuclear Rh-terpyridine species that promotes this
reaction.10 Noteworthy, these systems do not require a hydrogen
acceptor although high catalyst loading (1.0-2.0% molar) and long
reaction times (16-24h) are usually needed to achieve satisfactory
yield.
A further category of catalysts relies on metal-ligand cooperative
systems: the ligand plays an active role in the catalytic cycle and
directly interacts with the substrate during different steps of the
catalytic cycle.11 Grützmacher et al. reported a series of Rh and
Ru catalysts that involves an amino-amido pair, however requiring
a hydrogen acceptor to proceed.12 Notably, Milstein et al. reported
a Ru-based pincer [Ru(PNN)] complex remarkably active even
under acceptor-less conditions.13 We recently demonstrated that
pincer PHNP ruthenium complexes efficiently catalyze this
reaction at low catalyst loading (0.1 mol%).14 In addition, the
related metal-catalyzed MeOH decomposition to hydrogen and
CO2 via formic acid salt intermediate was extensively studied by
different groups.15 This study was further extended with ethanol
and glycerol solutions to produce hydrogen, along with the
respective formation of acetic and lactic acids in selective
manner.16
innovations (“emerging strategy”).
1
In this perspective,
sustainable processes that explore renewable feedstocks instead
of crude oil have become industrially important alternatives.1 Of
particular interest are bio-based primary alcohols, which are
increasingly used as biofuels as well as platform molecules in the
production of higher added-value chemicals. 2 For example,
bioethanol worldwide production reached 93 billion liters (which
currently accounts for almost 90% of the global biofuel production)
while 1-butanol is considered as an advanced biofuel.3
In this context, activation of alcohols by acceptorless
dehydrogenation reactions has gained interest as an
environmentally friendly, atom-efficient catalytic process that
allows producing a wide range of products including aldehydes,
ketones, acetals, esters, amides, imines, amines, and α-
functionalised carbonyl compounds.4 From the reaction pathway
[a]
Dr. D. H. Nguyen, Dr. Y. Morin, Dr. L. Zhang, Dr. X. Trivelli, Dr. F.
Capet, Prof. S. Paul, Dr. S. Desset, Prof. F. Dumeignil, Dr. R. M.
Gauvin.
Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 -
UCCS - Unité de Catalyse et Chimie du Solide, F-59000 Lille,
France
E-mail: regis.gauvin@ensc-lille.fr, franck.dumeignil@univ-lille1.fr
Dr. X. Trivelli
[b]
Univ. Lille, CNRS, UMR 8576 - UGSF - Unité de Glycobiologie
Structurale et Fonctionnelle, F-59000 Lille, France
Supporting information for this article is given via a link at the end of
the document.
Recently there has been strong interest in the challenge of
replacing the highly active and selective well-established precious
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10.1002/cctc.201700310
ChemCatChem
FULL PAPER
metal catalysts by finding new (biocompatible) earth-abundant
metal-based counterparts.17 Indeed, use of this type of metals
would be highly beneficial in terms of sustainability and lower
catalysts cost. In the present contribution, catalytic conversion of
bio-sourced alcohols into carboxylic acid salts was accomplished
in the presence of earth-abundant Fe and Mn complexes
supported by aliphatic PNP pincer ligands. The selective
transformation of alcohols into aldehydes by using acetone as
hydrogen acceptor was also probed with these species.
HMBC pulse sequence was chosen as it provides all the coupling
constants in a single experiment (1JHFe, 2JHP, 1JPFe) in addition to
the 57Fe chemical shift. Indeed, 57Fe NMR resonance for both
stereoisomeric Fe-3a and Fe-3b are respectively determined to
be 2645 and 2743 ppm while that for the borohydride derivative
Fe-4 is significantly shifted to higher-field at 2133 ppm. In contrast,
1
1
their coupling constants values JHFe and JPFe do not vary
significantly and are of about 12 and 40 Hz, respectively. So far
only scarce examples related to one-bond 1H-57Fe scalar coupling
constant values for iron hydride complexes have been reported.21
In the course of our synthetic studies, we obtained single crystals
of Fe-1 suitable for diffraction studies. The solid state structure of
this paramagnetic, 16e- penta-coordinated iron complex Fe-1 is
displayed on Figure 2. The complex features a distorted square
pyramidal geometry highly similar to one of two solvomorphs of
reported analogous compound [FeCl2(iPr2PC2H4)2NH)],22 with the
PNP framework coordinated in meridional configuration and with
one of the bromide ligands occupying the axial position. The low
degree of distortion from square pyramidal is shown by the τ value
of 0.026.23 The Fe1-N1 bond (2.2677(15) Å) is characteristic for
iron-amino moiety. The axial bromo Br1-Fe bond length
(2.5238(3) Å) is longer with respect to that of the remaining Br2-
Fe1 bond (2.4635(3) Å). The P(1)−Fe(1)−P(2) bond angle is
151.363(18)°.
Results and Discussion
Table 1. Selected NMR data (C6D6, 300K).
11B
(ppm)
[Fe]
15N (ppm)
57Fe (ppm)
Fe-3a
Fe-3b
Fe-4
56
nd
48
47
-
2645 (1JFe-H = 12 Hz, 1JFe-P = 40 Hz)
2743 (1JFe-H = 12 Hz, 1JFe-P = 40 Hz)
2133 (1JFe-H = 10 Hz, 1JFe-P = 40 Hz)
nd
Figure 1. Transition metal complexes used in this study.
-
-32
-31
A series of known iron (Fe-1, Fe-2, Fe-3 as a mixture of two
stereoisomers Fe-3a and Fe-3b in 1:0.7 ratio, respectively, Fe-4
and Fe-5) and manganese (Mn-1, Mn-2, Mn-3) complexes were
selected for these studies (Figure 1).5k,4p,18 The synthesis of the
borohydride complex Fe-5 was improved by treating the dichloro
carbonyl [FeCl2(CO)((Cy2PC2H4)2NH)] complex with NaBH4
excess in EtOH/toluene mixture at room temperature since this
reaction was previously described as unselective in an
EtOH/CH3CN mixture.19 This revisited procedure afforded the
Fe-5
targeted complex in
a
78% yield. As complementary
spectroscopic studies, both 15N and (challenging) 57Fe NMR were
investigated (Table 1). Thanks to 1H-15N 2D HSQC NMR
sequence, 15N chemical shifts for Fe-3a, Fe-4 and Fe-5 are
respectively located at 56, 48 and 47 ppm, consistent with
nitrogen sp3 hybridization.5k Notably, we resorted to 57Fe NMR, as
valuable information may be deduced from comparison of
isotropic chemical shift of various complexes. 57Fe is a very low
sensitivity spin ½ nucleus (receptivity with respect to 13C of
4.2x10-3) with a low (2.1%) natural abundance. The very wide 57Fe
chemical shift range spreads over ca. 12 000 ppm, which makes
iron NMR an extremely powerful and direct probe of the
asymmetry of the electron distribution around the metal.20 Very
low frequency nuclei such as 57Fe can be more easily detected by
indirect experiments like HSQC/HMQC/HMBC. The 1H-57Fe-
Figure 2. ORTEP view of solid-state structure of Fe-1. Ellipsoids are given at
the 50% probability level. Selected bond distances (Å) and angles (deg): Br1-
Fe1 = 2.5238(3), Br2-Fe1 = 2.4635(3), Fe1-P1 = 2.5271(6), Fe1-P2 = 2.5227(6),
Fe1-N1 = 2.2677(15), N1-H1 = 0.84(2). Br1-Fe1-P1 = 101.177(14), Br2-Fe1-
Br1 = 108.573(11), Br2-Fe1-P1 = 95.820(14), Br2-Fe1-P2 = 98.620(15), P2-
Fe1-Br1 = 97.467(15), P2-Fe1-P1 = 151.363(18), N1-Fe1-Br1 = 98.50(4), N1-
Fe1-Br2 = 152.92(4), N1-Fe1-P1 = 78.08(4), N1-Fe1-P2 = 77.82(4).
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10.1002/cctc.201700310
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We next probed the efficiency of these readily accessible base
Under similar reaction conditions, the borohydride derivative Fe-
metal compounds in the oxidation of alcohols into carboxylic acids. 4 afforded butyric acid in 58 % yield (Table 2, Entry 3). Performing
The catalytic conditions were optimized for the acceptorless
dehydrogenative coupling (ADC) of 1-butanol and water to butyric
acid salt. The catalytic reaction was initially carried out in
presence of 2 mol% of catalyst, 2 molar equiv. of water and 1.1
molar equiv. of KOH, at 120°C with toluene as a solvent. Both
dibromo Fe-1 and dibromo carbonyl Fe-2 showed no catalytic
activity and formed black solids after 3h (Table 2, Entries 1 and
2). Fe-2 was previously shown to be catalytically inactive for the
hydrogenation of esters under basic conditions.18b This contrast
with the related ruthenium pincer complex [Ru(Cl)(µ-
Cl)((iPr2PC2H4)2NH)]2 that catalyzes this reaction under basic
conditions.14 This most probably owes to the easier formation of
catalytically active hydrido carbonyl species under reaction
conditions (i.e. in the presence of alcohol and base) in the case
of ruthenium compared to iron.24
the reaction for prolonged reaction times resulted in limited yield
improvement from 67% after 6 h to 74% after 17 h (Table 2,
Entries 4 and 5). Influence of the solvent was also studied, based
on observations made on related pincer systems.16c No catalytic
activity was observed when the reaction was carried out in polar
DMSO solvent (Table 2, Entry 7) while in 1,4-dioxane, lower
butyric acid yield of 52% was obtained with respect to toluene
(Table 2, Entry 6). This contrasts with the beneficial solvent effect
observed in the Fe-catalyzed conversion of glycerol to lactic acid
by Crabtree and Hazari.16c
Very interestingly, running the reaction in the absence of water,
allows not only achieving significantly improved yields, but also a
more efficient isolation of the products by precipitation and
extraction, as Madsen et al. very recently reported for Ru
systems.8 Thus, excellent isolated yield for butyric acid was
obtained (96%, Table 2, Entry 8). Lowering the catalyst loading
from 2 to 1 mol% leads to a lower isolated yield of 68% (Table 2,
Entry 9). As observed above, 1,4-dioxane has a harmful effect on
the catalytic performance since only 69% of butyric acid was
obtained (Table 2, Entry 10). Monohydride bromo complex Fe-3
shows the similar catalytic activity (Table 2, Entry 11) than Fe-4,
which suggests that both compounds generated similar catalytic
active species under reaction conditions, as already observed for
Ru analogues.14 The influence of substituent groups on the
phosphorus atom of the PNP ligand was also investigated.
Replacing isopropyl groups by slightly bulkier cyclohexyl ones
resulted in lower isolated yield of 74% (Table 2, Entry 12).
Furthermore, it appears that iron and manganese systems are
less tolerant towards water compared to the ruthenium ones. In
the latter case, we have demonstrated that ruthenium hydroxide
species reversibly forms under these conditions, being able to
regenerate the catalytically active amido species with water
elimination.14 It may well be that this phenomenon (trapping of the
active form of the catalyst by H2O) is not reversible in the Fe and
Mn systems, or that it irreversibly leads to deactivation species.
When discussing the compared activity of compounds Fe-1, Fe-
2, Fe-3 and Fe-4, one can rationalize these in term of active
species generation. Indeed, in the present case, it is assumed that
the entry into the catalytic cycle is the penta-coordinated
[Fe(H)(CO)((iPr2PC2H4)2N)] species bearing the PNP ligand in its
amido form. As mentioned above, the carbonyl derivative Fe-2
does not generate a hydrido carbonyl species. Thus, even if Fe-1
would form Fe-2 by alcohol decarbonylation, the reaction would
not proceed further. On the other hand, HBr elimination from Fe-
3 affords the amido species,18b,25 while BH3 release from Fe-4
generates a dihydride species [Fe(H)2(CO)((iPr2PC2H4)2NH)] and
thus leads into the catalytic cycle.26
Table 2. Oxidation of butan-1-ol into butyric acid catalysed by Fe- and Mn-PNP
complexes.
Butyric
Acid (%)b
Entry
[M]
t(h)
H2O (equiv.)
Solvent
1
Fe-1
Fe-2
Fe-4
Fe-4
Fe-4
Fe-4
Fe-4
Fe-4
Fe-4
Fe-4
Fe-3
Fe-5
Mn-2
Mn-2
Mn-3
Mn-1
Mn-3
Mn-4
3
2
2
2
2
2
2
2
0
0
0
0
0
2
0
0
0
0
0
Toluene
Toluene
Toluene
Toluene
Toluene
1,4-Dioxane
DMSO
0
2
3
0
3
3
58
67
74
52
0
4
6
5
17
6
6
7
6
8
6
Toluene
Toluene
1,4-Dioxane
Toluene
Toluene
Toluene
Toluene
Toluene
Toluene
1,4-Dioxane
Toluene
96
68
69
89
74
12
95
94
89
50
91
9 a
10
11
12
13
14
15
16
17
18
6
6
6
6
24
24
24
24
24
24
Next, manganese complexes were catalytically evaluated. This
base metal has attracted considerable attention for related
transformations. 27 In our recent report on the acceptorless
dehydrogenation coupling of alcohols into esters, the active
amido dicarbonyl species Mn-2 was shown to be thermally very
robust compared to the analogous Fe complex.5p Under
previously established standard reaction conditions (2 mol%
catalyst loading, 120 °C, 2 molar equiv. water, 1.1 molar equiv.
Reaction conditions: [M] = 0.05 mmol (2 mol%), n(butan-1-ol) = 2.5 mmol
(185.3 mg), n(H2O) = 0-2 molar equiv., n(KOH) = 2.75mmol (154 mg),
V(toluene) = 3 mL. 120°C, under argon. a: [M] = 1 mol%. Yield measured by
1HNMR. b: Yield in butyric acid, Entries 1-7: Determined by NMR, Entries 8-18:
Isolated yield.
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KOH), Mn-2 displays a low catalytic activity as only 12% of butyric
catalytically efficient since conversions up to 96% were reached
acid yield was obtained after 24 h (Table 2, Entry 13). Remarkably, after 24 h (Table 3, Entries 6 and 8). Both Fe-4 and Mn-2 were
in the absence of water, excellent isolated yield up to 95% was
achieved (Table 2, Entry 14). Using both chloro Mn-1 and
borohydride Mn-3 precatalysts produced also butyric acid with
very good isolated yield of 89 and 94%, respectively (Table 2,
Entries 15 and 16). In addition, as already observed for Fe-based
catalysts, replacement of toluene as the solvent by 1,4-dioxane
induced production of butyric acid with a lower yield of 50% (Table
x, Entry 17).
evaluated for oxidation of more reactive conjugated geraniol and
nerol substrates. Theses reactions however proceed in
unselective manner and mixtures of unidentified products were
obtained. The (S)-(-)-perillyl alcohol was converted into a mixture
of the perrilic acid and reduced dihydroperillic derivative in 44 and
50% isolated yields with respective ratio of around 1:10 in
presence of Fe-4 and Mn-2, respectively (Table 3, Entries 9 and
10).
From the mechanistic point of view, oxidation of primary alcohols
to carboxylic acid salts can proceed by an initial alcohol
dehydrogenation step to form an aldehyde intermediate. From
this point on, at least three distinct reaction pathways can take
place (Scheme 2). In Path 1, the aldehyde reacts with water or
/and OH- to afford a hydrate that is subsequently dehydrogenated
Table 3. Oxidation of bio-sourced alcohols to acid salts catalysed by Fe- and
Mn-PNP complexes.
Entry
1
Substrates
[M]
Products
Yield (%)a
and deprotonated into the carboxylic acid salt.13, 29 30 Path 2
67
(49)
,
Fe-4
consists of an ADC reaction which yields an ester that is then
saponified.14 In Path 3, the aldehyde intermediate undergoes a
Cannizzaro reaction to form a carboxylic acid and an alcohol.8
Since we did not observe any traces of esters formed, the second
pathway can be excluded under our reaction conditions.
Regarding Path 3, the lack of crotonization products in the case
of aldehyde featuring protons in the  position is a strong
indication that this pathway is not significantly operative.
99
(50)
2
3
Mn-2
Fe-4
91
(52)
99
(52)
4
Mn-2
Fe-4
Mn-2
Fe-4
Mn-2
Fe-4
Mn-2
5
50
96
(51)
6
7
51
97
(50)
8
9
(44)
(50),
(1:10 ratio)
10
Scheme 2. Reaction pathways for alcohol dehydrogenative oxidation
Reaction conditions: [M] = 2mol%, n(Substrate) = 2.5 mmol, n(KOH) = 2.75
mmol (154mg), V(toluene) = 3mL, 120°C, under argon. Yield measured by
1HNMR. Isolated yield: (In parentheses). Reaction time is 6 h with Fe-4 and 24
h with Mn-2.
Recording the 31P NMR spectrum of the reaction mixture at the
end of butanol dehydrogenative oxidation catalytic run revealed
the presence of the active Mn-2 species and a new manganese
compound of formula [Mn(OCOC3H7)(CO)2((Ph2PC2H4)2NH)]
(Mn-4) as the main organometallic species (75%), in respective
ratio of 1.4:1. Interestingly, in a closely related study devoted to
the Mn-catalyzed ADC reaction of butanol (Mn-2, 0.6 mol%,
130 °C, 72h),5p we also identified Mn-4 as the sole complex
present in the solution.
Alcohols oxidation was extended to other bio-sourced alcohols
such as fatty and terpenoic derivative alcohols (Table 3). In the
presence of Fe-4 and at 120°C, stearyl and oleyl alcohols are
respectively converted to stearic and oleic acids in 67 and 99%
yield after 6h (Table 3, Entries 1 and 3). The different reactivity
can be attributed to the lower solubility of stearyl alcohol with
respect to that of its unsaturated counterpart (oleyl alcohol). In the
presence of Mn-2 and under similar conditions (though with
longer reaction times), both stearyl and oleyl alcohols are
completely converted into the corresponding acids within 24h
(Table 3, Entries 2 and 4).28 Dihydrocitronellol and β-citronellol
are transformed into the corresponding dihydrocitronellic and β-
citronellic acids with 50% yield when using Fe-4 as catalyst during
6 h (Table 3, Entries 5 and 7). Interestingly, Mn-2 proved to be
Complex Mn-4 was isolated and fully characterized by
multinuclear NMR and IR spectroscopies and X-ray molecular
structure determination. 1H NMR displays a broad triplet centered
at 8.96 ppm (JHH = 12 Hz) that is assigned to the NH hydrogen.
Such low-field resonance signal is in line with a hydrogen bonding
established between the N-H and the carboxylato ligand. 31P{1H}
spectrum shows a singlet at 88.1 ppm. The 15N resonance is
1
located at 39.3 ppm according to H-15N 2D HSQC NMR, which
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is consistent with the sp3 hybridization for the amino nitrogen atom. As this compound was obtained from ADC of butanol, it can be
On the 13C{1H}, since 13C and 55Mn resonance frequencies are
very close (respectively 100.6 and 99.24 MHz at 9.4 T), leading
assumed that it originates from reaction of intermediate aldehyde
with adventitious water, forming an aldehyde hydrate which can
be dehydrogenated into butyric acid. This acid can in turn react
with Mn-2 to afford Mn-4. Indeed, this complex was independently
synthesized in moderate isolated yield (67%) by reaction of Mn-2
with 1.5 equiv. of butyric acid. Interestingly this shows a
decomposition pathway for this catalytic system when applied to
ADC. Although this species is no longer active for ADC reactions
under base-free conditions, it could be involved within a catalytic
cycle of the manganese-catalyzed alcohols oxidation process to
carboxylic acid salts. The efficiency of the carboxylato Mn-4
complex for the oxidation of butan-1-ol was assessed. It afforded
a 91% yield of butyric acid (Table 2, Entry 18). Consistently, after
a few minutes of reaction, the reaction mixture color turned from
yellow to dark red, which may indicate that Mn-4 generates the
(red) catalytic species Mn-2 under basic conditions.
As mentioned above, the first step of the alcohol oxidation
process consists of an alcohol dehydrogenation step to form an
aldehyde intermediate. Aldehydes are also compounds of interest,
and therefore their selective production from alcohols is most.
Attractive. Although no aldehyde traces were detected under
carboxylic acid production conditions, very low quantities of this
compound (usually accounting for less than 1.0 %) are observed
during Ru- and Mn-catalyzed ADC reactions of alcohols into
esters.5k,5p Indeed, the ADC reactions proceed throughout several
reversible steps that includes the aldehyde consumption reaction
with a second alcohol molecule to reversibly give an unstable
hemiacetal. In order to be able to produce aldehydes instead of
esters, the formation of the intermediate hemiacetal needs to be
impeded. This can be accomplished by dilution of the reaction
medium. Moreover, to avoid the aldehyde hydrogenation reaction
which is promoted by dihydride [M(H)2(CO)((R2PC2H4)2NH)] (M =
Ru, Fe) or monohydride [Mn(H)(CO)2((R2PC2H4)2NH)] species,
the concentration of these compounds has to be maintained as
low as possible. By using a sacrificial hydrogen acceptor, these
species could be therefore converted to the corresponding amido
[M(H)(CO)((R2PC2H4)2N)] (M = Ru, Fe) or Mn-2 counterpart. As a
reference point, some Ir-, Rh- and Ru- based catalytic systems
have been found to efficiently dehydrogenate alcohols to
corresponding aldehydes.32
to an efficient scalar relaxation mechanism that broadens the 13
C
signal of carbonyl ligands directly bound to the Mn center, their
chemical shifts were therefore not determined.5p The carboxylate
group however resonates as a singlet at 185.23 ppm.
In the solid-state, IR spectrum shows two intense characteristic
bands at 1915 and 1822 cm-1 that are assigned to two CO ligands
in mutually cis-position. The IR stretching C=O frequency of the
carboxylate ligand is located at 1563 cm-1. In addition, N-H
stretching band is observed at 3095 cm-1. The solid-state
molecular structure features a distorted octahedral geometry,
similar to that of [Mn(OCOH)(CO)2((Ph2PC2H4)2NH)] reported by
Boncella et al.31 The PNP tridentate ligand is coordinated in
meridional fashion. The two terminal carbonyl ligands are in
mutually cis configurations, one being trans to the amino moiety.
The residual position is occupied by the butyrato ligand, which is
arranged in syn configuration with the N-H function (Figure 3). An
intramolecular hydrogen bond is established between the
butyrate carbonyl oxygen and the N−H proton of the PNP ligand.
The hydrogen bond distance and the corresponding angle N1-
H1···O4 are 1.965(3) Å and 158.94(3)°, respectively.
Figure 3. ORTEP view of solid-state structure of Mn-4. Ellipsoids are given at
the 50% probability level. Selected bond distances (Å) and angles (deg): Mn1-
P1 = 2.2926(9), Mn1-P2 = 2.2882(9), Mn1-O3 = 2.067(2), Mn1-N1 = 2.127(2),
Mn1-C18 = 1.783(3), Mn1-C17 = 1.750(3), N1-H1 = 0.90(4), O2-C18 = 1.161(4),
O1-C17 = 1.181(4), P2-Mn1-P1 = 164.68(4), O3-Mn1-P1 = 88.58(6), O3-Mn1-
P2 = 86.25(6), O3-Mn1-N1 = 90.72(9), N1-Mn1-P1 = 82.54(7), N1-Mn1-P2 =
83.11(7), C18-Mn1-P1 = 97.16(10), C18-Mn1-P2 = 97.21(10), C18-Mn1-O3 =
89.56(11), C18-Mn1-N1 = 179.58(13), C17-Mn1-P1 = 92.79(11), C17-Mn1-P2
= 93.07(11), C17-Mn1-O3 = 177.04(11), C17-Mn1-N1 92.06(12), C17-Mn1-C18
= 87.67(14).
Figure 4. Ru complex used for dehydrogenation of butan-1-ol to butan-1-al.
The dehydrogenation of butan-1-ol to butyraldehyde was
examined using commercially available borohydride Ru-MACHO-
BH (a pre-catalyst for ADC5 and alcohol oxidation14 reactions,
Figure 4) and in the presence of acetone as both a hydrogen
acceptor and a diluent (Table 4). When the reaction was carried
out under mild conditions (60°C, 5 mL of acetone) and in the
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10.1002/cctc.201700310
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presence of 1.0 mol% of Ru-MACHO-BH, both butyraldehyde
and butyl butyrate were formed, and their respective ratio evolved
over time (Table 4, Entries 1-5). At very short reaction times, (15
minutes), butyraldehyde is the major product with a yield of 19%
while the yield of butyl butyrate is of 13% (Table 4, Entry 1). As
the reaction proceeds, the amount of butyraldehyde decreases
and the yield into butyl butyrate increases (Table 4, Entries 2-5).
This indicates that under such conditions, butyraldehyde can still
further react with butan-1-ol to give the hemiacetal that is
dehydrogenated to butyl butyrate. However, compared with neat
conditions5k (without acetone added), the final amount of
butyraldehyde is higher (6%, Table 4, Entry 5) indicating that the
addition of acetone can have a beneficial influence on the
selectivity toward aldehyde.
amounts were detected after 5 h (Table 4, Entries 12 and 13).
This may be due to their intrinsically lower activity compared to
ruthenium catalysts, as already observed in the case of ADC
reactions.5k,o,p
Conclusions
We have shown that pincer complexes of earth-abundant metals
iron and manganese readily catalyse the acceptorless
dehydrogenative oxidation of (bio-sourced) alcohols into their
corresponding carboxylic acid salts, under basic conditions.
Compared to the parent Ru catalysts, the Fe and Mn systems
needed to be used in the absence of water to reach high yields.
The manganese catalysts proved to be more efficient than their
iron counterparts, which was ascribed to their higher robustness
under the considered reaction conditions.
In addition, as this oxidative transformation proceeds via an
aldehyde intermediate, we explored the option to switch the
selectively of the systems towards this compound. Thus, the
commercially-available Ru-MACHO-BH has been found to be
able to selectively oxidize alcohols into aldehydes in presence of
a large excess of acetone as hydrogen under very mild conditions
while Mn- and Fe analogues feature only low activity.
Table 4. Dehydrogenation of butan-1-ol to butyraldehyde.
Butyl
butyrate
(%)
Acetone
(mL)
n-Butanal
(%)
Entry
[M]
t(h)
1†
2
0.25
0.5
1
19
13
10
7
13
30
50
59
71
0
3
Ru-MACHO-BH
5
4
2
Experimental Section
5
3
6
6
0.5
1
49
66
73
70
69
70
2
Experimental Details. All experiments were carried out under argon
atmosphere using a glovebox or a vacuum line using standard Schlenk
techniques unless some special conditions are pointed out. All ruthenium
complexes and tridentate ligands were stored under argon. GC–FID/MS
was carried out on Agilent 7890A (flame ionization detector and MS
detector) instruments equipped with a Zebron ZB-Bioethanol column (30
m column length x 0.25 mm internal diameter x 1.00 μm film thickness)
with helium as carrier gas. All NMR spectra were recorded on Bruker
Avance 300/400 NMR spectrometers. 1H and 13C NMR chemical shifts are
reported in ppm (δ) downfield from tetramethylsilane, 31P NMR chemical
shifts are reported in ppm (δ) and referenced to an external 85 % solution
of phosphoric acid in D2O, 15N NMR chemical shifts are reported in ppm
(δ) downfield from an external liquid ammoniac. 11B NMR chemical shifts
are reported in ppm (δ) and referenced to external solution of BF3.Et2O,
Common abbreviations used in the NMR experiments are as follows: b,
broad; s, singlet; d, doublet; t, triplet; q, quartet; sp, septet; m, multiplet; v,
virtual. Spectral assignments were made by means of routine one- (1H,
1H{31P), 31P{1H}, 13C{1H], 13C JMOD, 11B, 11B{1H}) and two-dimensional
(1H-1H COSY, 1H-1H NOESY, 1H-13C HSQC, 1H-13C HMBC, 1H-15N HSQC,
1H-15N HMBC…) NMR experiments where appropriate. IR spectra were
recorded on a Nicolet 6700 FT-IR spectrometer equipped with a Praying
Mantis mirror chamber (from Harrick Scientific) by using a DRIFT cell
equipped with KBr windows. The samples were prepared under argon in
a glove-box. Typically, 64 scans were accumulated for each spectrum
(resolution 4 cm-1). Data are reported as follows: weak (w), medium (m),
strong (s) and very strong (vs).
7
0
8
2
3
Ru-MACHO-BH
90
9
3
4
10
11
12
13
4
4
5
5
Fe-4
90
90
5
0
Mn-2
5
6
0
Catalytic conditions. [M] = 1 mol% (0.011 mmol), n(butan-1-ol) = 1;1 mmol
(81mg, 0.1 mL), applied temperature = 60 °C, under argon, Yield determined by
GC.
Noteworthy, when a very large amount of acetone (90 mL) is used,
butyraldehyde is obtained as the major product (Table 4, Entries
6-11). Butyraldehyde is initially obtained as the sole product in 49
and 66% yield after 0.5 and 1h, respectively (Table 4, Entries 6
and 7). A small amount of ester builds up only after 2 h and
marginally varies from 3 to 5% yield after 5 h (Table 4, Entries 8-
11). After 5 h, the yield into aldehyde reaches 70% while that to
ester is kept as low as 5%. In fact, under much diluted conditions
(very large excess of acetone) the chemo-selectivity can be
switched toward the production of aldehyde since aldehyde-
consuming reactions are inhibited.
57Fe NMR measurements: 57Fe chemical shifts are reported in ppm (δ) and
referenced to external (liquid) Fe(CO)5. The duration of the 57Fe 90° pulse
(39.85us) was calibrated using the 73Ge NMR signal of GeCl4 with one
drop of C6D6, since this signal is readily observed in 32 scans (receptivity
with respect to 13C 0.62) and the resonance frequency of 73Ge (13.96 MHz)
Using the same reaction conditions, both Fe-4 and Mn-2 are
significantly less active than Ru-MACHO-BH: Very low aldehyde
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10.1002/cctc.201700310
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FULL PAPER
is reasonably close to that of 57Fe (12.96 MHz).33 Very low frequency
nuclei such as 57Fe can be more easily detected by indirect experiments
like HSQC/HMQC/HMBC. The 1H-57Fe-HMBC pulse sequence was
chosen to obtain 57Fe chemical shifts and all the coupling constants in a
-OOCCH2CH2CH3), 18.85 (s, 2C, -CH3, PNP), 18.00 (s, 2C, -CH3, PNP),
14.76 (s, 1C, -CH3, -OOCCH2CH2CH3). 15N{1H} HSQC NMR (300K, C6D6,
40.565 MHz, ppm): δ 39.3 (1N). Anal. Calcd. For C22H44MnNO4P2: C
52.48; H 8.81; N 2.78. Found: C 50.70; H 8.92; N 2.78.
single experiment (1JHFe 2JHP 1JPFe). The 1H-57Fe-HMBC spectra were
, ,
recorded assuming an apparent 15 Hz coupling constant value with a 0.5
s acquisition time, a 6 s recycling delay and 256 scans.
Standard procedure for catalytic experiments: In a glovebox, the
required amount of potassium hydroxide (typically 1.1 equiv., 179 mg) and
catalyst (0.05 mmol) are weighted in a Schlenk tube containing a stir bar.
Under argon, the required amount of alcohol (2.5 mmol), water (2 molar
equiv.) and toluene (3 mL) are added. The Schlenk tube is equipped with
a reflux condenser topped with an argon bubbler. The system is heated
using an oil bath and stirred magnetically. After the desired reaction time,
water is added (5 mL) and the mixture is acidified using hydrochloric acid
and extracted with diethyl ether (5 x 10 mL). The combined organic phase
was dried over anhydrous MgSO4 and diethyl ether was removed by rotary
evaporator. The conversion was determined by 1H NMR spectroscopy
from the crude products, in comparison with spectra of pure products
reported in the literature. For isolated yields: the crude reaction suspension
was filtered and the obtained solids were washed with n-pentane (3x5mL)
and diethyl ether (2x5mL) and dissolved in water (5mL). The carboxylic
acid product was extracted as described above.
All compounds were purchased from commercial suppliers. The
substrates and solvents were degassed by argon flushing upon use.
Deuterated solvents were provided by Eurisotop and distillated prior to use.
Ruthenium
[Ru(H)(BH4)(CO)((Ph2PC2H4)2NH)] (trade name: Ru-MACHO-BH)
complexes, iron FeBr2 and FeCl2, manganese [Mn2(CO)10 and
[Ru(H)(Cl)(CO)(PPh3)3]
and
]
[MnBr(CO)5] complexes (Strem Chemicals) were stored inside an argon-
filled glove box and used as received. Ligands (iPr2PC2H4)2NH,
(Cy2PC2H4)2NH (Strem Chemicals) were stored inside an argon-filled
glove box and used as received. [FeBr2((iPr2PC2H4)2NH)] (Fe-1),18a,b
[FeBr2(CO)((iPr2PC2H4)2NH)] (Fe-2),18a,b [FeH(Br)(CO)((iPr2PC2H4)2NH)]
(Fe-3),18b
[FeH(BH4)(CO)((iPr2PC2H4)2NH)]
(Fe-4),18a,b
[Fe(Cl)2(CO)((Cy2PC2H4)2NH)],22 [Mn(Br)(CO)2((iPr2PC2H4)2NH)] (Mn-
1),18c
[Mn(CO)2((iPr2PC2H4)2N)]
(Mn-2),5p
[Mn(BH4)(CO)2((iPr2PC2H4)2NH)] (Mn-3),5p were prepared following
X-ray Structure Determination. A single crystal of each compound was
mounted under inert perfluoropolyether wax on a Mitegen MicroLoopTM
.
literature procedures.
Single-crystal X-rays measurements were performed at 100K under N2
stream from a Cryostream 700 device (OxfordCryosystems). Data were
collected using an Apex II CCD 4K Bruker diffractometer (λ = 0.71073 Å).
The structures were solved using SHELXT35 and refined by least-squares
procedures on F2 using SHELXL2014.36 All Hydrogen atoms were placed
in theoretical positions and refined riding on their parent atoms. ORTEP
drawings were generated with ORTEP-3.37 Crystallographic data have
been deposited at the Cambridge Crystallographic Data Centre as
Supplementary Publication Nos. CCDC 1532717-1532718. Copies of the
data can be obtained free of charge on application to the Director, CCDC,
12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (+44) 1223- 336-033; e-
mail, deposit@ccdc.cam.ac.uk).
Synthesis [FeH(BH4)(CO)((Cy2PC2H4)2NH)] (Fe-5). To a suspension of
[Fe(Cl)2(CO)((Cy2PC2H4)2NH)] (0.2g) in a toluene/ethanol mixture (1/2
ratio, 30 mL) was added in excess sodium borohydride (5 equiv. ) at 0°C.
The resulting reaction mixture was stirred at room temperature for 24h.
The yellow suspension thus obtained was evaporated to dryness under
reduced pressure. The product was extracted with toluene (3x15mL) and
the combined organic phases were collected and filtered through a short
celite column. The solvent was removed under reduced pressure to afford
a yellow solid which was washed with n-pentane (3x15mL) and dried under
vacuum. Yield 0.14 g (78%). NMR data are in agreement with those
reported in the literature.34
Synthesis of [Mn(OCOC3H7)(CO)2((Ph2PC2H4)2NH)] (Mn-4). To a red
solution of the manganese amido [Mn(CO)2((iPr2PC2H4)2N)] complex (Mn-
1, 154 mg) in toluene (4mL) was slowly added a solution of butyric acid
(80 mg) in toluene (2mL) at room temperature and under stirring. The
resulting reaction mixture turned immediately bright yellow. It was stirred
for 2h and evaporated under reduced pressure to afford a yellow oily-solid
which was washed with n-pentane (3x5mL) and dried under vacuum. A
single crystal suitable for X-ray determination was obtained from
crystallization from pentane. Yield (0.185g, 67%). FT-IR (KBr, cm-1): ν(NH)
= 3090 (m), ν(CO) = 1915 (s), 1821.9 (s), ν(OCO) = 1562.6 (s). 1H NMR
(300 K, C6D6, 400.33 MHz, ppm): δ 8.96 (t, 1H, NH, JHH = 12 Hz, PNP),
Acknowledgements
This work was performed in partnership with the SAS PIVERT,
within the frame of the French Institute for the Energy Transition
(Institut pour la Transition Energétique (ITE) P.I.V.E.R.T.
(www.institut-pivert.com) selected as an Investment for the Future
(“Investissements d’Avenir”). This work was supported, as part of
the Investments for the Future, by the French Government under
the reference ANR-001-01. L. Zhang thanks the China
Scholarship Council (CSC 2011008127) for providing the financial
support of his PhD. CNRS, Chevreul Institute (FR 2638),
Ministère de l’Enseignement Supérieur et de la Recherche,
Région Nord – Pas de Calais and FEDER are acknowledged for
supporting and partially funding this work.
2.84 (m, 2H, >CH2, PNP), 2.26 (t, 2H, >CH2, JHH
= 7.3 Hz, -
OOCCH2CH2CH3), 2.23-1.98 (m, 6H, 2H of CH2 and 4H of –CH, PNP),
1.84-1.69 (m, 4H, >CH2, 2H of -OOCCH2CH2CH3 and 2H of PNP), 1.62
(m, 2H, >CH2, JHH = 5.6, 14.3 Hz, PNP), 1.47 (td, -CH3, JHH = 7.3 Hz, JHP
= 7.3 Hz, PNP), 1.34 (td, -CH3, JHH = 6.9 Hz, JHP = 7.0 Hz, PNP), 1.22 (td,
-CH3, JHH = 7.1 Hz, JHP = 7.1 Hz, PNP), 1.04 (td, -CH3, JHH = 7.2 Hz, JHP
=
7.1 Hz, PNP), 0.98 (t, 3H, -CH3, JHH = 7.2 Hz, -OOCCH2CH2CH3). 31P{1H}
NMR (300K, C6D6, 162.057 MHz, ppm): δ 88.05 (s, 2P). 13C{1H} NMR
(300K, C6D6, 100.663 MHz, ppm): δ nd (CO), 185.23 (s, 1C, -OOC, -
OOCCH2CH2CH3), 52.75 (t, 2C, >CH2, JCP = 6.2 Hz, PNP), 42.29 (s, 1C,
>CH2, -OOCCH2CH2CH3), 27.88 (t, 2C, >CH2, JCP = 6.0 Hz, PNP), 26.72
(t, 2C, >CH-, JCP = 7.4 Hz, PNP), 24.60 (t, 2C, >CH-, JCP = 9.9 Hz, PNP),
20.59 (s, 2C, -CH3, PNP), 20.38 (s, 2C, -CH3, PNP), 19.96 (s, 1C, >CH2,
Keywords: iron • manganese • ruthenium • carboxylic acid •
alcohol• pincer ligands
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Entry for the Table of Contents
FULL PAPER
Acceptorless dehydrogenative
oxidation of bio-sourced alcohols
into carboxylic acid salts was
achieved using earth-abundant Fe
and Mn PNP complexes. Tuning of
reaction conditions allows selective
production of aldehydes from
alcohols using a Ru catalyst, while
Fe and Mn species proved not
effective.
Duc Hanh Nguyen, Yohann Morin, Lei
Zhang, Xavier Trivelli, Frédéric Capet,
Simon Desset, Franck Dumeignil* and
Régis M. Gauvin*
Page No. – Page No.
Earth-abundant transition metals-
catalyzed oxidative transformations
of bio-sourced alcohols.
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FULL PAPER
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