Selective conversion of alcohols in water to carboxylic acids by in situ generated ruthenium trans dihydrido carbonyl PNP complexes

Article abstract of DOI:10.1039/c4dt01634c

In this work, we present a mild method for direct conversion of primary alcohols into carboxylic acids with the use of water as an oxygen source. Applying a ruthenium dihydrogen based dehydrogenation catalyst for this cause, we investigated the effect of water on the catalytic dehydrogenation process of alcohols. Using 1 mol% of the catalyst we report up to high yields. Moreover, we isolated key intermediates which most likely play a role in the catalytic cycle. One of the intermediates was identified as a trans dihydrido carbonyl complex which is generated in situ in the catalytic process. This journal is

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ARTICLE
Selective conversion of alcohols in water to carboxylic
acids by in situ generated ruthenium trans dihydrido
carbonyl PNP complexes
Cite this: DOI: 10.1039/x0xx00000x
a
a
b
a,
Jong-Hoo Choi, Leo E. Heim, Mike Ahrens, and Martin H. G. Prechtl *
rd
Received 3 June 2014,
Accepted XXXth June 2014
DOI: 10.1039/x0xx00000x
www.rsc.org/
In this work, we present a mild method for direct conversion of primary alcohols into
carboxylic acids with the use of water as oxygen source. Applying a ruthenium dihydrogen
based dehydrogenation catalyst for this cause; we investigated the effect of water during the
catalytic dehydrogenation process of alcohols. Using 1 mol% of catalyst we report up to high
yields. Moreover, we isolated key intermediates which play most likely a role in the catalytic
cycle. One of the intermediates is identified as a trans dihydrido carbonyl complex which is
generated in situ in the catalytic process.
1
1
Introduction
with no further additives (Figure 1). Usually those pincer
typed ruthenium complexes bearing cooperative (and hemi-
Catalytic oxidation of alcohols is an essential industrial and labile) pincer-backbones are known for dehydrogenative
natural process and leads to important intermediates or products coupling of alcohols into esters and their reverse hydrogenation
such as aldehydes, ketones or carboxylic acids. Established reactions into alcohols, also for N-alkylation reactions from
1
2-15
methods usually demand strong and toxic oxidants such as alcohols and amines.
chromium or manganese oxides along with many additives.
In the presence of water, catalyst
1 is
1
-4
highly active for catalytic conversion of different alcohols into
In some cases, the use of stoichiometric oxygen supplying their corresponding carboxylic acid salts.
reactants or even the presence of pure pressurized oxygen is Other works about alcohol dehydrogenation in aqueous
5
16-18
required. In terms of the synthesis of carboxylic acids, mostly solution at low temperature, in particular methanol
and
1
9
the oxidation of aldehydes as intermediates or starting materials methanediol, also showed the possibility of acceptorless
is needed. The methods of direct oxidation of alcohols into dehydrogenation. In these certain cases the dehydrogenation
6
carboxylic acids are still behind and do not meet today’s resulted in the formation of carbon dioxide and hydrogen gas.
requirements of a clean and efficient pathway without the
necessity of aggressive and toxic oxidants and by avoiding
chemical waste products. Despite these disadvantages, only a
small number of direct alcohol conversion into carboxylic acids
7
were reported. For example Stark et al. reported a direct
oxidation method of alcohols involving tetra-n-proplyamonium
perruthenate (TPAP) in the presence of N-methylmorpholine
N-oxide (NMO) as a key additive to stabilise the aldehyde
8
hydrate intermediate. A different way was obtained by the
Grützmacher group; they reported a homogenous catalytic
transformation of alcohols to acids with high yields and under
very mild conditions applying a rhodium based catalyst with
11
Figure 1 Direct oxidation of alcohols using a bipyridine ruthenium catalyst 1.
Inspired by the latest achievements, we present a setup using a
and
RuH (CO)(Me-PNP)] 3 for catalytic dehydrogenation of
2
primary alcohols in the presence of water, respectively in
absence of any other oxidants (Figure 2). Our reactions were
9
, 10
cyclohexanone as hydrogen acceptor.
With this similar
20
ruthenium PNP pincer complexes [Ru(H )H (Me-PNP)]
2
2
2
concept they also accomplished to convert alcohols into esters
or amides. The latest method was reported by the Milstein
group in 2013 by applying a bipyridine based PNN ruthenium
[
carbonyl hydride catalyst
1 using only water as oxygen source
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conducted with aq. NaOH solution as the only additive to [Ru(H )H (Me-PNP)]
2
or [RuH (CO)(Me-PNP)]
3
was
obtain the carboxylic acid salts in up to high yields. refluxed under continuous argon flow in an open system for
Furthermore, we isolated complex intermediates and 4a-b 20 h at 120 °C. The addition of base (NaOH) is necessary to
separately and from the catalytic process (Figure 2). In our obtain the carboxylic acid salt and to shift the reaction
system, complex serves as precursor which converts in situ equilibrium towards the product. After the reaction time, the
via alcohol decarbonylation reaction to a trans dihydrido predominant single aqueous phase was treated with diethyl
complex [RuH (CO)(Me-PNP)] . Separately, complex was ether to extract the catalyst. The aqueous layer was then
used for catalytic alcohol dehydrogenation reactions in water. acidified to convert the carboxylic acid salt into its
2
2
2
3
2
3
3
2
1
2, 13, 21-25
Based on achievements in earlier reports,
investigated the decarbonylation behaviour of similar PNP extracted with ethyl acetate. Isolated yields of the carboxylic
pincer based ruthenium complex [Ru(H )H(PNP)] . Complex acids are presented in Table 1. In this catalytic oxidation of
carbonyl complex alcohols we tested a series of aliphatic alcohols along with
and converted into a trans dihydride benzyl alcohol. Best results using catalyst were obtained with
complex [RuH (CO)(H-PNP)] in hydrogen atmosphere hexanol and pentanol yielding 88 and 71% (entry 1 and 2),
Figure 2). Those complexes are important intermediates for while butanol gave a moderate yield of 63% (entry 3).
we corresponding carboxylic acid which was subsequently
5
2
5
can be transformed into
a
[
RuH(CO)(PNP)]
6
2
7
2
(
2
6, 27
different transformations reported by others.
Catalysing longer aliphatic chained alcohols (entry 4 and 5), the
isolated yields dropped down to 33%. This is probably due to a
lack of miscibility of these less polar long-chain aliphatic
alcohols with water. Benzyl alcohol and cyclohexyl methanol
gave yields between 59 and 65% (entry 6 and 7). After the
reaction and extracting the complex with diethyl ether, the
organic phase contained only traces of unreacted alcohol, but
no ester as by-product. A slight increase of the yields was
obtained with complex 3. The reaction of hexanol to hexanoic
acid gave similar yields (entry 1 and 8), for butanol and
pentanol (entry 9 and 10) an increase of around 10% were
obtained. Isolated yields for octanol and decanol (entry 11-12)
remained unchanged. The oxidation of benzyl alcohol to
benzoic acid improved from 65% yield to 85% (entry 13).
Contrary, the conversion of cyclohexyl methanol dropped to
36%. With complexes 5 and 6 the yields for the hexanol
oxidation were 53% and 61% (Table 15 and 16).
Table 1 Dehydrogenation of alcohols in the presence of water.
a
Entry
Cat.
2
2
2
2
2
2
2
Alcohol
hexanol
pentanol
butanol
octanol
decanol
benzyl alcohol
cyclohexyl
methanol
hexanol
pentanol
butanol
octanol
decanol
benzyl alcohol
cyclohexyl
methanol
hexanol
Product
hexanoic acid
valeric acid
Yield
88
71
63
42
33
65
59
1
2
3
4
5
6
7
butyric acid
caprylic acid
decanoic acid
benzoic acid
cyclohexyl
carboxylic acid
hexanoic acid
valeric acid
butyric acid
caprylic acid
decanoic acid
benzoic acid
cyclohexyl
carboxylic acid
hexanoic acid
hexanoic acid
8
9
3
3
3
3
3
3
3
92
83
73
45
32
85
36
1
1
1
1
1
0
1
2
3
4
Figure 2 Ruthenium hydride [Ru(H
complex [RuH (CO)(Me-PNP)] for alcohol oxidation, complex intermediates 4a-
, [Ru(H )H(PNP)] and carbonyl complexes [RuH(CO)(PNP)] and RuH (CO)(H-
PNP)]
2 2
)H (Me-PNP)] 2 and trans dihydrido carbonyl
2
3
1
1
5
6
5
6
53
61
b
2
5
6
2
hexanol
7.
a
reaction at 120 °C, 20 h with 1mol% cat. 2, 3, 5 or 6, 5 mmol alcohol,
5
.5 mmol NaOH.
Results and Discussion
Catalytic oxidation of alcohols
For standard catalytic procedure, a mixture of 2 ml water,
5
mmol of alcohol, 5.5 mmol of NaOH and 1.0 mol% of
2
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3
0, 32, 38
Formation of the active species and characterisation of the
isolated complex intermediates
band at lower wave numbers.
In contrary, for cis bonding
modes of metal dihydrides, the hydride trans to a pincer
backbone would have the highest wave number, followed then
by the CO band and then with lowest wave number the hydride
At the beginning of the catalysis, the trans dihydrido carbonyl
complex [RuH (CO)(Me-PNP)]
3
is formed through
2
3
8
trans to the carbonyl ligand.
decarbonylation of the primary alcohol by [Ru(H )H (Me-
2
2
PNP)]
complex
pentyl or hexyl alcohols to [Ru(H )H (Me-PNP)]
2
(Scheme 1). Separately in another experiment,
was obtained by adding 3.5 equivalents of ethyl,
in a closed
3
2
2
2
system at 80 °C for 48 h, with very good yields, which is stable
under argon atmosphere at room temperature. Furthermore, a
series of gas phase mass spectra were recorded to detect the
fragmentations of the evolved aliphatic hydrocarbons from the
decarbonylation reactions of the corresponding alcohols
(
supplementary information (SI) Fig. S1-2). The gas phase MS
Scheme 1 Decarbonylation of [Ru(H
cis/trans isomerisation reaction.
2 2 2
)H (Me-PNP)] 2 to [RuH (CO)(Me-PNP)] 3 via
analysis clearly showed the formation of methane and butane
from ethanol, respectively pentanol. Using deuterated ethanol
with these hydride catalysts, we observed the formation of
Comparing the decarbonylation reactivity of
2, similar
CD H confirming the interaction of the hydride-site with the
3
observations were made from the reaction of the analogue
substrate. Moreover, we confirmed the molar mass of complex
ruthenium complex [Ru(H )H(PNP)]
5
which synthesis was
2
2
0, 26
3
via LIFDI-MS technique (SI Fig. S3). Mechanistical
reported in earlier works (Scheme 2).
reaction of ethanol by complex
RuH(CO)(PNP)] in excellent yields. In the H-NMR
spectrum, the hydride ligand gives a triplet signal in the upfield
Decarbonylation
investigations of decarbonylation reactions with ruthenium
5
gave the carbonyl complex
2
5
1
complexes were pioneered by Kubas and Caulton. Following
[
6
2
8, 29
these observation and other indications,
Sabo-Etienne et al.
2
reported the decarbonylation reaction of alcohol by a molecular
dihydrogen ruthenium complex, whereby, similar to our
at -20.87 ppm ( J
= 16.3 Hz), which indicates the
HP
3
4
configuration of the hydride ligand cis to the pincer ligand.
2
2
1
3
system, the dihydrogen ligand is replaced by a CO ligand.
Experiments with C labeled ethanol resulted with a triplet
2
Moreover, Foxman and Ozerov reported the CO
signal at 208.8 ppm ( J = 10.5 Hz) for the CO ligand in the
CP
1
3
-1
functionalisation of a PNP typed ruthenium pincer hydride
CAPT-NMR spectrum. IR signals were found at 1872 cm for
the non labeled CO vibration along with a weaker Ru-H band
at 2052 cm which are characteristic for pincer based carbonyl
monohydride compounds. For the C labeled complex, the
CO band was detected at 1830 cm and with a Ru-H
2
3
complex obtained through decarbonylation of acetone. Based
-
1
on previous reports by Milstein et al., we accordingly assume
3
0
13
that a cis-[RuH (CO)(Me-PNP)] complex is generated in the
2
1
3
-1
first step, which undergoes then, despite the high trans
influence of the hydride ligands, a rapid cis-trans isomerisation
into the thermodynamically more stable and sterically more
-
1
vibration around 2062 cm (Fig. S13). Pressurising complex
with 1.5 bar H gas showed around 79% conversion of into
, two
triplet signals at -5.86 ppm ( J = 18.2 Hz) and -6.13 ppm
6
6
2
2
8, 30-33
favourable trans isomer
3
.
Furthermore, we observed no
trans dihydride 7, which exhibits, similar to complex 3
1
31
1
2
isomeric change in the H- and P{ H}-NMR after heating
complex at 80 °C for 10 h. [RuH (CO)(Me-PNP)] shows in
the H-NMR spectrum at 300 MHz a multiplet assigned to two
hydrides at -5.40 ppm. At higher frequencies of 600 MHz, the
multiplet resolves into two clean triplet signals at -5.43 ppm
HP
2
3
3
2
( J = 17.4 Hz). Isolation of complex 7 was not possible due
HP
1
to the rapid degeneration into 6.
2
2
(
JHP = 16.1 Hz) and -5.54 ppm ( J = 19.3 Hz). Two signals
HP
for two hydride signals next to each other in the chemical shifts
2
with a J coupling constant between 16-24 Hz is similar to
HP
other reported trans dihydride PNP pincer complexes with
3
4-37
13
aliphatic backbones by Gusev and Schneider.
In the C-
2
NMR spectrum, the CO signal was found at 210.8 ppm (t, JCP
=
13.2 Hz), which was further confirmed by decarbonylation
1
3
reactions of C labeled ethanol (SI, Fig. S16). The vCO band
was detected at 1871 cm while the comparing CO band was
found with a Δ43 at 1828 cm . Vibration of the hydrides was
found at 1642 cm for complex
-
1
13
-
1
-
1
-1
13
3
and 1640 cm for the
C
Scheme 2 Decarbonylation of [Ru(H
isomerisation and the hydrogenation of 6 to [RuH
2
)H(PNP)] 5 to [RuH(CO)(PNP)] 6 via cis/trans
(CO)(HPNP)] 7.
labeled complex (SI, Fig. S10). This is in agreement for the
case of a typical trans dihydride arrangement. The CO
stretching mode is located in the typical range with higher wave
numbers followed by the M-H vibrations as one single, sharp
2
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-
1
Catalytic cycle and the isolation of intermediates 4a and b
C=O band at 1593 cm (SI, Fig. S11-12). LIFDI-MS/MS
analysis of complex 4a showed only a fragmentation with the
mass value of 506 m/z, which can be explained by the loss of
the hexanoate under MS conditions, showing only the carbonyl
monohydride species (for more details see SI, Fig. S5-6). This
observation is in full agreement with our recent experiments
applying LIFDI-MS analysis on ruthenium pincer hydrides
Similar to the system reported by Milstein et al., we assume
that complex
3 dehydrogenates the alcohol into an aldehyde
intermediate complex. It is also possible, that the aldehyde
converts independently with water into an aldehyde hydrate
intermediate. However, due to the rapid equilibrium between
1
1
the aldehyde and the aldehyde hydrate intermediate, it seems
more plausible that the reaction with water under basic
conditions generates an aldehyde hydrate, stabilised as a
geminial diolate complex, which can be dehydrogenated into
2
0
complexes. During a soft ionisation process, a mixture of
similar fragmentations can be detected with this kind of
compound class, which can be explained by the loss of the
hydride ligands (-Δ1-2 m/z) causing a shift towards lower mass
values. In contrast to 4a, LIFDI-MS/MS analysis of 4b
revealed the molar mass of 565 m/z which is in good agreement
with the simulated isotope pattern (in red) illustrated in Figure
Compared to the simulated isotope pattern of
[RuH(CO)(OOCCH )(Me-PNP)] 565 m/z in red, the LIFDI-
MS/MS pattern is slightly shifted towards lower mass value,
which can be explained by the detection of a fragmentation of
the subspecies [Ru(CO)(OOCCH )(Me-PNP)] 564 which is
1
1
the carboxylate complex 4a (Figure 3). From there on, the
carboxylate is salted out by sodium cation. In the presence of
water, no formation of esters was observed since only unreacted
alcohol residues were found in the reaction mixture after the
appropriate reaction time. This observation confirms yet again
2
0
4.
1
1
3
that water suppresses the formation of ester. Compared to
1
1
previous studies it is unclear if the mechanism involves a
metal-ligand cooperativity during the catalytic reaction. We
achieved the conversion of alcohols to carboxylic acids using
3
generated during the ionisation process.
catalysts
2 and 3 bearing a “non-cooperative” Me-PNP-ligand.
The experimental data show that a basic position as proton
acceptor/donor is not crucial for this reaction as no H/D
exchange has been observed in the ligand backbone. The lack
of H/D exchange in the ligand backbone let us tentatively
exclude cooperative effects of the ligand. It is likely that the
acceptorless dehydrogenation and oxygen-transfer from water
solely takes place at the ruthenium core.
The attempt to isolate the complex intermediate species after
the reaction time led to the isolation of 4a which was extracted
with toluene. Separate attempts led also to the isolation of 4a
either by refluxing complex
2 in hexyl alcohol and water or by
the reaction of with hexanoic acid (Figure 3). For the latter
3
one, 4a was obtained in very good yields within minutes under
hydrogen evolution. Analogue, complex 4b was obtained by
adding acetic acid to complex
3 (Figure 3).
Figure 4. LIFDI-MS/MS analysis of [RuH(CO)(OOCCH
toluene. Isotope pattern area 558-570 (black) in comparison to the simulated
isotope pattern of [RuH(CO)(OOCCH )(Me-PNP)] 565 (red).
3
)(Me-PNP)] 565 4b in
3
A single crystal structure of 4b was obtained from crystals
grown as red prism in a mixture of benzene and heptane at
room temperature (Figure 5, selected bond distances and angles
are given in Table 2). The structure shows a distorted
octahedral coordination of the ruthenium core, where both
locations of the P-atoms of the P-Ru-P axis are twisted out-of-
plane with a P1-Ru-P2 angle of 157.24°. Same applies for the
trans-arrangement of the hydride and the carboxylate with an
angle of 169.20° (H1-Ru-O2). The x-ray pattern allowed the
localisation of the hydride H1 giving a Ru-H distance of
Figure 3 Isolation of complex 4a and 4b. Complex 4a were obtained by extraction
with toluene after the catalytic reaction with or by refluxing with hexyl
alcohol in water. Adding acetic or hexanoic acid to complex led directly to 4a
and 4b
2
2
3
.
Both complexes almost do not differ in their chemical shifts in 1.57(4) Å. Furthermore, the trans arrangement of the CO ligand
3
1
1
the P{ H} NMR showing singlets around 81.4 ppm (complex to the PNP-ligand is confirmed, which was discussed earlier in
) and 81.3 (complex 4b), while exhibiting triplet signals in this work. The N-Ru-CO angle is closer to 180° (176.16°),
4
a
2
the upfield at -17.08 ppm ( J = 20.7 Hz, complex 4a) and at consequently the H1-Ru-CO angle is near orthogonal (92.20°).
PH
2
-
17.49 ppm ( JPH = 20.4 Hz, complex 4b). IR spectra show the
-
1

CO for both complexes at 1908 cm , while exhibiting the
4
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and stored in the glove-box under argon. Non-anhydrous
solvents were dried and distilled (under vacuum or argon) prior
to use, applying standard procedures.
Analytic methods
1
13
31
1
H-, C-, P-NMR spectra were recorded at 300 MHz ( H),
1
3
31
7
5 MHz ( C) and 121 MHz ( P) on a Bruker Avance II 300
1
and on a Bruker Avance II+ 600 spectrometer at 600 MHz ( H),
1
and toluene at room temperature. H shifts were reported in
1
3
31
50 MHz ( C) and 242 MHz ( P) using deuterated benzene
1
ppm (δ ) downfield from TMS and were determined by
H
reference to the residual solvent peaks (C D : 7.16 ppm, C D :
6
6
7
8
7
.09 ppm.). Chemical shifts were reported as singlet (s),
doublet (d), triplet (t), quartet (q) and multiplet (m). Coupling
constants were reported in [Hz]. For hydrogenation
Figure 5. ORTEP diagram of the single crystal structure of complex 4b. Ellipsoids
are illustrated at 50% possibility. All hydrogen atoms not depicted here except
for H1 for clarity.
J
experiments, Young-Teflon Capped NMR tubes from Wilmad
were used. Infrared spectra (IR) were measured at room
temperature with a Bruker Alpha spectrometer equipped with a
Diamond-ATR IR unit. Data are reported as follows: absorption
a
b
Table 2. Selected bond distances and angles of complex 4b.
-
1
ṽ [cm ], weak (w), medium (m), strong (s). Mass-spectrometric
investigations of the gas composition in the gas phase were
conducted with a HPR-20 gas analysis system by Hiden
Analytical and were directly connected to the reaction set up
under argon atmosphere. The HPR-20 QIC (Hiden Analytical)
has a MS detection limit <0.09 ppm as xenon in air is
detectable. Note the MS has sensitivity down to partial
Ru1-P1
Ru1-P2
Ru1-C01
Ru1-O02
Ru1-H1
2.34(9)
2.34(6)
1.82(0)
2.21(9)
1.57(4)
2.24(7)
P1-Ru1-P2
157.24
176.16
169.30
92.20
N01-Ru1-C01
H1-Ru1-O02
H1-Ru1-C01
10
pressures of 10- torr (Note: spectrometer specific unit is torr
not MPa).
Ru1-N01
a
b
General catalytic procedure
Distances are given in Å. Angles are reported in degrees.
For standard catalytic procedure, 0.05 mmol of complex
2 or
complex were added to 5.5 mmol NaOH and 5 mmol of
3
alcohol. After the addition of 2 mL degassed water, the content
was refluxed at 120 °C for 20 h under constant argon flow in an
open system. After the reaction time, the aqueous phase was
extracted with diethyl ether to extract the catalyst and alcohol
residues. The aqueous layer was then acidified with 20% aq.
HCl and treated five times with 20 mL ethyl acetate. After the
Conclusions
In summary of this work, we presented an approach for
catalytic dehydrogenation of primary alcohols in water yielding
carboxylic acid salts using ruthenium hydride complexes.
Moreover, we confirmed that complexes
into carbonyl trans dihydrides complexes
decarbonylation reaction of alcohols. Complex intermediate 4a
which was isolated after the catalysis as well as synthesised in
different ways, is believed to be one of the complex species
taking part in the catalytic cycle.
2
and
5
convert in situ
organic layers were combined and dried 1 h over MgSO , the
4
3
and by
6
solution was filtered and the solvent was removed in reduced
pressure to obtain the isolated carboxylic acid. Yields are given
in Table 1.
,
Synthesis of [RuH
In an argon flushed Büchi glass autoclave 100 mg
(CO)(Me-PNP)] 3
2
(
6
0.198 mmol) of [Ru(H )H (Me-PNP)]
2
2
were dissolved in
is described in our
2
2
Experimental Section
mL toluene. The synthesis of complex
2
0
previous report. After the addition of 3.5 eq. (0.693 mmol) of
a primary alcohol (e.g. ethyl, pentyl, hexyl alcohol), the content
was heated at 80 °C for 48 h. After the appropriate time, the
solvent was removed in vacuo and the residue was washed
twice with pentane. The grey powder was stored at -34 °C.
Yield: 80%.
LIFDI-MS (argon collided): m/z 511.3 (2), 510.3 (19), 509.3
(
(
Reactions were generally prepared under argon atmosphere
using Schlenk techniques, flame-dried glassware and
Labmaster 200 glove-box from MBraun. High-pressure
hydrogen reactions were performed in a Büchi Tinyclave
a
(
50 mL) glass autoclave. All solvents and reagents were
purchased from Acros, Merck, Sigma-Aldrich, Fluka, Strem or
were acquired from the institute stock. Commercial anhydrous
solvents and argon-as packed reagents were used as received
55), 508.3 (33), 507.3 (100), 506.3 (73), 505.3 (74), 504.3
65), 503.3 (30), 502.3 (17), 501.3 (22).
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Dalton Trans.
1
H-NMR: (600 MHz, benzene-d ): δH [ppm] = 2.32 (m, 2H, stream of argon. The solvent was removed in vacuo and the
6
CH ), 2.11 (s, 3H, NCH ), 1.93 (m, 2H NCH ), 1.61 (m, 2H, product was washed twice with pentane. The grey powder,
2
3
2
PCH ), 1.55 – 1.52 (m, 2H, overlapped, PCH ), 1.50 (dt, 36H, yielding 81%, was stored at -34 °C.
2
2
3
2
JPH = 6.7 Hz, PC(CH ) ), -5.43 (t, 1H, J = 16.1 Hz, Ru-H), LIFDI-MS/MS: m/z 569.2 (1.9), 568.2 (14.8), 567.3 (42.6),
5.54 (t, 1H, J = 19.4 Hz, Ru-H).
CAPT-NMR: (75 MHz, benzene-d ): δ [ppm] = 210.8 ppm (t, (7.4), 560.2 (5.6).
JCP = 13.2 Hz, CO, data extracted from CO labled probe),
5.8 (t, J = 5.1 Hz, NCH ), 52.9 (NCH ), 36.4 (t, J = 8.9 OOCCH ), 2.13 (m, 4H, NCH ), 2.09 (s, 3H, NCH ), 1.63 (m,
Hz, P(C(CH ) )), 33.9 (t, J = 7.4 Hz, P(C(CH ) )), 30.3 (t, 2H PCH ), 1.49 (m, 2H, PCH ), 1.31 (t, 18H, J = 6.4 Hz,
JCP = 3.3 Hz, P(C(CH ) ), 30.1 (t, J = 2.9 Hz, P(C(CH ) )), P(C(CH ) ), 1.18 (t, 18H, J = 6.2 Hz, P(C(CH ) ),-17.49 (t,
3 3
PH
2
-
1
566.3 (18.5), 565.3 (100), 564.4 (51.9), 563.2 (27.8), 561.2
PH
3
6
C
2
13
1
H-NMR: (600 MHz, benzene-d ): δH [ppm] = 2.22 (s, 3H,
6
2
1
6
CP 2 3 CP 3 2 3
1
3
3
3
CP
3 3
2
2
PH
2
2
3
3 3
CP
3 3
3 3
PH
3 3
1
2
2
3
4.4 (t, J = 5.3 Hz, PCH2).
1H, J = 20.4 Hz, Ru-H).
PH
CP
1
1
13
P{ H}-NMR: (121 MHz, benzene-d ): δ [ppm] = 106.3 (s).
CDeptQ-NMR: (150 MHz, benzene-d ): δ [ppm] = 208.8 ppm
6 C
6
P
-
1
IR: ṽ[cm ]=2950 - 2864 (m), 1871 (s), 1640 (s), 1474 (m), (s, CO), 175.4 (s, CH COO), 65.7 (s, NCH ), 45.2 (s, NCH ),
458 (m), 1416 (w), 1383 (m), 1381 (m), 1351 (m), 1310 (w), 37.2 (t, J = 5.3 Hz, P(C(CH ) )), 36.7 (t, J = 10.2 Hz,
CP 3 3 CP
208 (w), 1171 (m), 1049 (w), 1025 (m), 930 (w), 915 (w), 881 P(C(CH ) )), 30.5 (s, P(C(CH ) ), 30.2 (s, P(C(CH ) )), 23.6 (s,
3
2
3
1
1
1
1
3
3
3 3
3 3
(
(
m), 801 (m), 739 (m), 679 (m), 644 (w), 613 (m), 566 (m), 529 PCH ).
2
1
3
1
w), 508 (w), 478 (m), 432 (m).
P{ H}-NMR: (242 MHz, benzene-d ): δ [ppm] = 81.3 (s).
6
P
-
1
IR: ṽ[cm ]=2956 - 2859 (m), 2145 - 2059 (w), 1906 (s), 1593
s), 1464 (m), 1389 (m), 1368 (m), 1354 (m), 1259 (s), 1175
In an argon flushed Schlenk flask equipped with a bubbler, (m), 1087 (s), 1021 (s), 934 (w), 907 (w), 878 (m), 800 (s), 735
0 mg (0.1 mmol) of [RuH (CO)(Me-PNP)] were dissolved (m), 680 (m), 609 (m), 569 (m), 546 (w), 529 (w), 478 (m).
Isolation of [RuH(CO)(hexanolate)(Me-PNP)] 4a
(
5
3
2
in 5 mL toluene. After the addition of 1.5 eq. (0.15 mmol)
hexanoic acid, the content was stirred for 30 min under a
Synthesis of [RuH(CO)(PNP)] 6
constant stream of argon. The solvent was removed in vacuo In an argon flushed Büchi glass autoclave 100 mg
and the product was washed twice with pentane. The grey (0.215 mmol) of [Ru(H )H(PNP)] were dissolved in 6 mL
is described in our
LIFDI-MS/MS (fragment 506): m/z 509.3 (13.1), 508.3 (33.3), previous report. After the addition of 3.5 eq. (0.753 mmol) of
07.3 (16.7), 507.2 (9.5), 506.3 (100), 505.3 (97.6), 504.3 a primary alcohol (e.g. ethyl, pentyl, hexyl alcohol), the content
5
2
powder, yielding 85%, was stored at -34 °C.
toluene. The synthesis of complex 5
2
0
5
(
47.6), 503.3 (16.7), 502.2 (16.7), 501.1 (4.8), 500.2 (9.5).
was heated at 80 °C for 48 h. After the appropriate time, the
solvent was removed in vacuo and the residue was washed
1
2
H-NMR: (600 MHz, benzene-d ): δ [ppm] = 2.51 (t, 2H, J
CH
6
H
=
7.6 Hz, OOCCH (CH ) CH ), 2.17 (s, 3H, NCH ), 2.14 (m, twice with pentane. The orange powder was stored at -34 °C.
2 2 3 3 3
4
H, NCH ), 1.70 (m, 2H, OOCCH CH (CH ) CH ), 1.55 – 1.51 Yield: 90%.
2 2 2 2 2 3
(
1
m, 8H, overlapped, 4H PCH and 4H OOC(CH ) (CH ) CH ), LIFDI-MS/MS: m/z 495.1 (1.0), 494.3 (15.9), 493.3 (46.1),
2 2 2 2 2 3
3
3
.38 (t, 18H, J = 6.5 Hz, P(C(CH ) ), 1.23 (t, 18H, J = 6.1 492.3 (14.6), 491.3 (100), 490.2 (34.7), 489.2 (39.6), 488.3
PH 3 3 PH
2
Hz, P(C(CH ) ), 0.99 (t, 3H, J = 7.3 Hz, OOC(CH ) CH ), (28.9), 487.3 (4.6), 486.2 (2.4), 485.3 (15.5).
-
1
3
3
2
CH
2 4
3
1
17.08 (t, 1H, J = 20.7 Hz, Ru-H).
CDeptQ-NMR: (150 MHz, benzene-d ): δ [ppm] = 208.5 ppm CH ), 3.14 (m, 2H, CH ), 1.88 (m, 4H, PCH ), 1.26 (dt, 36H,
s, CO), 175.8 (s, CH COO), 65.8 (s, NCH ), 45.6 (s, NCH ),
H-NMR: (300 MHz, benzene-d ): δH [ppm] = 3.49 (m, 2H,
6
PH
3
6
C
2
2
2
2
2
(
4
JPH = 14.3 Hz, PC(CH ) ), -20.87 (t, 1H, J = 16.3 Hz, Ru-
3 3 PH
3
2
3
1
0.8 (s, OOCCH (CH ) CH ), 37.5 (t, JPC = 5.1 Hz, H).
2 2 3 3
1
13
P(C(CH ) )), 36.8 (t, J = 10.3 Hz, P(C(CH ) )), 33.0 (s,
OOCCH CH (CH ) CH ), 30.6 (s, P(C(CH3)3), 30.5 (s, CO, J = 10.5 Hz, data extracted from CO labeled probe in
P(C(CH ) )), 27.2 (s, OOC(CH ) CH CH CH ), 23.7 (s, toluene-d ), 63.5 (t, J = 7.1 Hz, NCH ), 35.4 (t, J = 7.7
OOC(CH ) CH CH ),
C
-NMR: (75 MHz, benzene-d ): δ [ppm] = 208.8 ppm (t,
3
3
PC
3 3
APT
2
6
C
13
2
2
2 2
3
CP
2
1
3
3
2 2
2
2
3
8
CP
2
CP
1
23.4
(s,
PCH2),
14.2
(s, Hz, P(C(CH ) )), 33.9 (t, J = 7.4 Hz, P(C(CH ) )), 29.7 (t,
3 3 CP 3 3
2
3
2
3
2
2
OOC(CH ) CH ).
3
JCP = 3.0 Hz, P(C(CH ) ), 28.5 (t, J = 3.2 Hz, P(C(CH ) )),
3 3 CP 3 3
26.0 (t, J = 6.9 Hz, PCH2).
2
4
3
1
1
1
P{ H}-NMR: (121 MHz, benzene-d ): δ [ppm] = 81.4 (s).
6
P
CP
-
1
31
1
IR: ṽ[cm ]=2959 - 2868 (m), 2126 - 2075 (w), 1908 (s), 1595
(
(
P{ H}-NMR: (121 MHz, benzene-d ): δ [ppm] = 110.1 (s).
6 P
s), 1466 (m), 1429 (w), 1389 (m), 1369 (m), 1354 (m), 1175 IR: ṽ[cm ]=2943 - 2800 (m), 2706 (w), 2628 (w), 2068 - 2048
-
1
m), 1043 (m), 1024 (m), 958 (w), 933 (w), 907 (w), 879 (m), (m), 1869 (s),
8
5
28 (w), 807 (m), 736 (m), 680 (m), 643 (m), 609 (m), 570 (m), 1469 (m), 1454 (m), 1385 (m), 1358 (m), 1318 (w), 1262 (m),
46 (m), 531 (m). 1206 (m), 1178 (m), 1157 (w), 1106 (w), 1063 (m), 1017 (m),
9
6
67 (m), 936 (w), 806 (s), 773 (w), 729 (s), 695 (m), 674 (w),
11 (m), 579 (m), 536 (m), 471 (s).
Isolation of [RuH(CO)(OOCCH )(Me-PNP)] 4b
3
In an argon flushed Schlenk flask equipped with a bubbler,
Hydrogenation of [RuH(CO)(PNP)] 6 to [RuH (CO)(HPNP)] 7
2
3
0 mg (0.06 mmol) of [RuH (CO)(Me-PNP)]
3
were dissolved
in 5 mL toluene. After the addition of 1.5 eq. (0.09 mmol) In a Young-Teflon capped NMR tube 7 mg (0.014 mmol)
acetic acid, the content was stirred for 30 min under a constant [RuH(CO)(PNP)] were dissolved in 0.5 mL deuterated
2
6
6
| Dalton Trans., 2014, 00, 1-3
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DOI: 10.1039/C4DT01634C
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Dalton Transactions
ARTICLE
benzene. The content was pressurised with 1.5 bar H gas. After 14. C. Gunanathan, Y. Ben-David and D. Milstein, Science, 2007, 317
,
2
3
1
1
10 h, 79% conversion was detected via P{ H}-NMR. Only
790-792.
hydride signals are clearly visible.
15. J. Zhang, M. Gandelman, L. J. W. Shimon and D. Milstein, Dalton
Trans., 2007, 107-113.
1
2
H-NMR: (300 MHz, benzene-d ): δ [ppm] = -5.86 (t, 1H, J
PH
6
H
2
=
3
18.2 Hz, Ru-H), -6.13 (t, 1H, J = 17.4 Hz, Ru-H).
16. R. E. Rodriguez-Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso-
PH
1
1
P{ H}-NMR: (121 MHz, benzene-d ): δ [ppm] = 110.1 (s,
Quinones and H. Grutzmacher, Nat. Chem., 2013, 5, 342-347.
6
P
2
1%, complex
6
), 108.9 (s, 79%, complex
7
).
17. E. Alberico, P. Sponholz, C. Cordes, M. Nielsen, H. J. Drexler, W.
Baumann, H. Junge and M. Beller, Angew. Chem. Int. Ed.,
2013, 52, 14162-14166.
Acknowledgements
We acknowledge the Ministerium für Innovation, Wissenschaft 18. M. Nielsen, E. Alberico, W. Baumann, H. J. Drexler, H. Junge, S.
und Forschung NRW (MIWF-NRW) for financial support
Gladiali and M. Beller, Nature, 2013, 495, 85-89.
within the Energy Research Program for the Scientist Returnee 19. L. E. Heim, N. E. Schloerer, J.-H. Choi and M. H. G. Prechtl, Nat.
Award for M. H. G. Prechtl (NRW-Rückkehrerprogramm). For
access to LIFDI-MS analysis, we gratefully acknowledge Prof.
Commun., 2014,
5
,
Article number: 3621, doi:
3610.1038/ncomms4621.
Dr. T. Braun (Humboldt-University of Berlin, Germany). J- 20. J.-H. Choi, N. E. Schloerer, J. Berger and M. H. G. Prechtl, Dalton
H. Choi wants to thank Dr. J. Neudörfl, T. Heidemann and A.
Trans., 2014, 43, 290-299.
Krest for helpful discussion. P. Kliesen is acknowledged for 21. E. P. K. Olsen and R. Madsen, Chem.-Eur. J., 2012, 18, 16023-
technical support for the XRD measurement.
16029.
2
2. P. D. Bolton, M. Grellier, N. Vautravers, L. Vendier and S. Sabo-
Etienne, Organometallics, 2008, 27, 5088-5093.
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Department of Chemistry, University of Cologne, Greinstr.6,50939 23. R. Çelenligil-Çetin, L. A. Watson, C. Guo, B. M. Foxman and O. V.
Cologne, Germany. Fax: +49 221 470 1788; Tel: +49 221 470 1981; E-
mail: martin.prechtl@uni-koeln.de Web: www.catalysislab.de
Ozerov, Organometallics, 2005, 24, 186-189.
24. Y.-Z. Chen, W. C. Chan, C. P. Lau, H. S. Chu, H. L. Lee and G. Jia,
Organometallics, 1997, 16, 1241-1246.
b
Institute of Chemistry, Humboldt University at Berlin, Brook-Taylor-
Straße 2, D-12489 Berlin, Germany.
25. L. S. Van der Sluys, G. J. Kubas and K. G. Caulton,
Organometallics, 1991, 10, 1033-1038.
†
†
Electronic Supplementary Information (ESI) available:
DOI: 10.1039/b000000x/
For experimental section, limited spectral and crystallographic data
see ESI.
26. B. Askevold, J. T. Nieto, S. Tussupbayev, M. Diefenbach, E.
Herdtweck, M. C. Holthausen and S. Schneider, Nat. Chem.,
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DOI: 10.1039/C4DT01634C
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RSCPublishing
ABSTRACT
Selective conversion of alcohols in water to carboxylic
acids by in situ generated ruthenium trans dihydrido
carbonyl PNP complexes
Cite this: DOI: 10.1039/x0xx00000x
a
a
b
a,
Jong-Hoo Choi, Leo E. Heim, Mike Ahrens, and Martin H. G. Prechtl *
Received 2nd June 2014,
Accepted XXXth June 2014
In this work, we present a mild method for direct conversion of primary alcohols into
carboxylic acids with the use of water as oxygen source. Applying a ruthenium dihydrogen
based dehydrogenation catalyst for this cause; we investigated the effect of water during the
catalytic dehydrogenation process of alcohols. Using 1 mol% of catalyst we report up to high
yields. Moreover, we isolated key intermediates which play most likely a role in the catalytic
cycle. One of the intermediates is identified as a trans dihydrido carbonyl complex which is
generated in situ in the catalytic process.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Notes and references
a
Department of Chemistry, University of Cologne, Greinstr.6,50939
Cologne, Germany. Fax: +49 221 470 1788; Tel: +49 221 470 1981; E-
mail: martin.prechtl@uni-koeln.de Web: www.catalysislab.de
b
Institute of Chemistry, Humboldt University at Berlin, Brook-Taylor-
Straße 2, D-12489 Berlin, Germany.
†
†
Electronic Supplementary Information (ESI) available:
DOI: 10.1039/b000000x/
For experimental section, limited spectral and crystallographic data
‡
see ESI.
This journal is © The Royal Society of Chemistry 2014
Chem. Sci., 2014, 00, 1-3 | 1