Study of Precatalyst Degradation Leading to the Discovery of a New Ru0 Precatalyst for Hydrogenation and Dehydrogenation

Article abstract of DOI:10.1021/acs.organomet.8b00353

The complex Ru-MACHO (1) is a widely used precatalyst for hydrogenation and dehydrogenation reactions under basic conditions. In an attempt to identify the active catalyst form, 1 was reacted with a strong base. The formation of previously unreported species was observed by NMR and mass spectrometry. This observation indicated that complex 1 quickly degraded under basic conditions when no substrate was present. X-ray crystallography enabled the identification of three complexes as products of this degradation of complex 1. These complexes suggested degradation pathways which included ligand cleavage and reassembly, along with reduction of the ruthenium atom. One of the decomposition products, the Ru⁰ complex [Ru(N(CH₂CH₂PPh₂)₃)CO] (5), was prepared independently and studied. 5 was found to be active, entirely additive-free, in the acceptorless dehydrogenation of aliphatic alcohols to esters. The hydrogenation of esters catalyzed by 5 was also demonstrated under base-free conditions with methanol as an additive. Protic substrates were shown to add reversibly to complex 5, generating Ru^II-hydrido species, thus presenting a rare example of reversible oxidative addition from Ru⁰ to Ru^II and reductive elimination from Ru^II to Ru⁰.

Full text of DOI:10.1021/acs.organomet.8b00353

Article
Cite This: Organometallics XXXX, XXX, XXX−XXX
Study of Precatalyst Degradation Leading to the Discovery of a New
0
Ru Precatalyst for Hydrogenation and Dehydrogenation
†
‡
‡
§
†,§
Aviel Anaby, Mathias Schelwies, Jonas Schwaben, Frank Rominger, A. Stephen K. Hashmi,
,†,‡
and Thomas Schaub*
†
Catalysis Research Laboratory (CaRLa) Im Neuenheimer Feld 584, D-69120 Heidelberg, Germany
BASF SE, Synthesis and Homogeneous Catalysis, Carl-Bosch-Straße 38, D-67056 Ludwigshafen, Germany
Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270, D-69120 Heidelberg, Germany
‡
§
*
S Supporting Information
ABSTRACT: The complex Ru-MACHO (1) is a widely used precatalyst for hydrogenation and dehydrogenation reactions
under basic conditions. In an attempt to identify the active catalyst form, 1 was reacted with a strong base. The formation of
previously unreported species was observed by NMR and mass spectrometry. This observation indicated that complex 1 quickly
degraded under basic conditions when no substrate was present. X-ray crystallography enabled the identification of three
complexes as products of this degradation of complex 1. These complexes suggested degradation pathways which included
ligand cleavage and reassembly, along with reduction of the ruthenium atom. One of the decomposition products, the Ru0
complex [Ru(N(CH CH PPh ) )CO] (5), was prepared independently and studied. 5 was found to be active, entirely additive-
2
2
2 3
free, in the acceptorless dehydrogenation of aliphatic alcohols to esters. The hydrogenation of esters catalyzed by 5 was also
demonstrated under base-free conditions with methanol as an additive. Protic substrates were shown to add reversibly to
II
0
II
complex 5, generating Ru −hydrido species, thus presenting a rare example of reversible oxidative addition from Ru to Ru
II
0
and reductive elimination from Ru to Ru .
i
23
INTRODUCTION
terized for [Ru(N(CH CH P Pr ) )H(CO)] and [Ru(N-
2 2 2 2
■
t
9,27
(
CH CH P Bu ) )H(CO)],
yet to the best of our knowl-
edge, such species from 1 have hitherto not been isolated.
Notwithstanding, addition products from the assumed
unsaturated species and the coordination of a substrate (e.g.,
alcohol, hydrogen, amine) have been identified,
as resting states in reaction mixtures, or isolated and used as
precatalysts. For instance, the BH adduct, commercially
named Ru-MACHO-BH, is formed by the reaction of 1 with
The commercially available Ru-MACHO complex 1 (Ru-
2
2
2 2
29
1
MACHO = [Ru(HN(CH CH PPh ) )H(Cl)CO]; Scheme
2
2
2 2
1
) has been demonstrated to be a highly robust precatalyst for
20,29
observed
Scheme 1. Dehydrochlorination of 1 to the Presumed
Unsaturated Complex 2
20
29
3
,9
4
1
NaBH . Ru-MACHO-BH is an efficient base-free homoge-
4
9
,16,30,31
neous catalyst in hydrogenation reactions.
Since 1 (or
the preactivated complex Ru-MACHO-BH) is considered to
be highly robust, it is of great interest to the chemical industry,
where high turnover numbers and turnover frequencies are
1
,2,18,30,32
required.
It is thus of high importance and scientific
numerous important transformations: hydrogenation of
2
−8
9−12
13
interest to identify the active state of the catalyst in solution.
Moreover, a well-defined homogeneous catalyst should be
available for reuse after full conversion of the substrate batch.
For this purpose, it is crucial to identify the resting state before
and after reaction and learn what form of the catalyst is carried
over for further reaction.
esters,
dehydrogenation of alcohols
and amines,
14
14−19
formic acid decomposition, carbon dioxide reduction,
and more.
1
3,20−26
In these reports, addition of base, often in
large excess, is ubiquitously required in order to convert 1 into
the active catalyst form. It is widely accepted that hydro-
II
chloride loss and formation of an unsaturated Ru −hydrido
species (depicted as 2; Scheme 1) is key to catalysis, as
3
,20,27−29
demonstrated for other analogous pincer complexes.
Such unsaturated species could be synthesized and charac-
Received: May 25, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.organomet.8b00353
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Article
In this context, an exploratory research with complex 1 was
conducted. A previously unreported decomposition of the
complex was subsequently observed and analyzed. Herein we
report the observed degradation of 1 under basic conditions,
producing several new organometallic species. Surprisingly,
0
one of these species was a Ru complex bearing a tetradentate
3
3
ligand, which was found to be active as a hydrogenation and
dehydrogenation catalyst under base-free conditions. These
findings may have an effect on future catalyst design for such
transformations.
RESULTS AND DISCUSSION
■
Figure 1. ESI-MS spectrum of the product mixture after reaction of
complex 1 with 1.5 equiv of NaHMDS (Scheme 2). For the full MS
data see the Supporting Information.
Decomposition of Ru-MACHO. In order to study the
activation of 1 with base, a procedure similar to those reported
for the isolation of related species was followed. The
commercially available complex 1 (0.04 mmol) was treated
under argon with 1.2 equiv of sodium hexamethyldisilazane
NaHMDS) in toluene-d8 in an NMR tube at −20 °C
Scheme 1). The mixture was monitored by NMR spectros-
copy, while it was shaken and warmed to room temperature.
Within 3 h, the pale cloudy solution turned dark brown. The
solution was then filtered and analyzed again by NMR (for
27
isotopic pattern at m/z 572.08 corresponded well to a
+
dechlorinated fragment of 1 ([2]H ; Scheme 2). The higher
(
(
masses suggested that dimers and clusters were present in the
mixture. Such clusters may well be formed in the ESI-MS
34
apparatus. Nevertheless, various new complexes were
obtained by crystallization from the reaction of 1 with base
and structurally characterized by X-ray crystallography. These
structures were in good agreement with the major signals
observed in ESI-MS, confirming that these species were stable
in the condensed matter (and were not merely gas-phase
fragments in the mass spectrometer).
1
details see the Supporting Information). The signals in the H
NMR spectra were very broad. Two weak hydride multiplet
signals could be observed at −9 and −11 ppm, and the hydride
and amino proton signals of 1 gradually diminished. It seemed
likely that more than one species had formed, as the observed
Ru−H signals amounted to less than 0.2 proton (with respect
One structure depicted a ruthenium dimer complex
complex 3; Figure 2), in which one of the ligand’s carbon−
(
31
1
to an internal standard). The P{ H} NMR spectrum showed
multiple new signals in the range from 30 to 70 ppm, some of
which had multiplicities. A similar experimental procedure was
t
conducted with potassium tert-butoxide (KO Bu) as base.
t
Although the reaction with KO Bu was sluggish (due to the
poor solubility of this base in toluene), the treatment with
either base led to similar observations (see the Supporting
Information).
The anticipated product from the dehydrochlorination of
complex 1 by a strong base would be complex 2 (Scheme 1),
which has been proposed in the literature as the catalytically
1
3,21,22,29
active species.
However, NMR spectroscopy could not
provide sufficient evidence for the formation of such a species.
Thus, both product mixtures, that of the reaction of complex 1
t
with KO Bu and that from the same reaction with NaHMDS,
were further investigated by ESI-MS (Scheme 2, Figure 1, and
the Supporting Information). Similar mass spectra were
observed after reaction with either base.
The strongest signal was observed at m/z 1141.15. Other
major signals were m/z 928.07, 784.16, and 572.08. The
Figure 2. Molecular structure of 3 as determined by single-crystal X-
ray structure analysis. Ellipsoids are presented at 50% probability.
Methylene and phenyl hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ru1−Ru2, 3.126(1);
Ru1−N1, 2.181(4); Ru2−N6, 2.138(4); Ru1−P3, 2.406(2); Ru2−
P3, 2.313(2); C9−C10, 1.369(8); P1−Ru1−P2, 104.51(6); P3−
Ru2−P4, 158.48(6); Ru1−P3−Ru2, 82.93(5).
Scheme 2. Reaction of Complex 1 with Strong Bases
carbon bonds was dehydrogenated and a phenyl group was
cleaved off from a phosphine atom and attached to a
ruthenium atom. This structure fits well the observed ESI-
+
MS mass at m/z 1141.15 ([3]H ; Scheme 2). Dimer formation
from unsaturated pincer intermediates has been reported by
35
Spasyuk et al. P−C bond cleavage at phosphine ligands is
known to occur (albeit not under such mild conditions) in the
coordination sphere of ruthenium and other transition
36,37
metals.
B
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A separately crystallized sample yielded another dimeric
structure, the product of N−C cleavage at the ligand, with one
ruthenium atom bearing only a monophosphine ligand:
Ph (CH ) NH (complex 4; Figure 3). To the best of our
The trigonal-bipyramidal geometry confirms an 18-electron
Ru complex.
0
The overall picture depicted by the observation of these
II
three species was quite confounding: the well-defined Ru
2
2 2
complex 1 underwent a previously unreported disproportio-
nation at the “PNP” ligand, leading to complexes bearing “NP”
0
and “NP ” type ligands. Most astounding, a Ru species formed
3
under seemingly redox neutral conditions.
Reactivity of Complex 5. Complex 1 is known to be a
highly efficient catalyst for hydrogenation and dehydrogen-
ation reactions under basic conditions. Therefore, one could
hypothesize that some of its decomposition products could
also serve as active catalysts in these reactions. While
complexes 3 and 4 retain the general structural and functional
motifs of the parent complex 1, complex 5 features a tertiary
0
amino ligand and a Ru saturated metal center. We were thus
intrigued to learn what role, if any, complex 5 may play in
catalysis. The ligand tris(2-(diphenylphosphaneyl)ethyl)amine
(
6; Scheme 3) was synthesized according to previously
Scheme 3. Synthesis of Complex 5
Figure 3. Molecular structure of 4 as determined by single-crystal X-
ray structure analysis. Ellipsoids are presented at 50% probability.
Methylene and phenyl hydrogen atoms are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ru1−Ru2, 2.734(1);
N1−Ru1, 2.138(5); N1−Ru2, 2.161(5); N2−Ru1 2.140(5); N2−
Ru2, 2.125(5); Ru1−N1−Ru2, 79.0(2); Ru1−N2−Ru2, 79.7(2);
P1−Ru1−Ru2, 120.54(4).
knowledge, this sort of reaction at the ligand (complete
3
8
published procedures. The reaction of 6 with Ru (CO)
12
(
cleavage) has not yet been reported. The depicted structure fits
3
33
+
7) was reported by Fontal; however, the product was not
well to the observed ESI-MS signal of m/z 928.07 ([4]H ;
fully characterized. Similar to the reported procedure, 6 and 7
were refluxed in toluene to yield complex 5 as a largely
insoluble orange powder (Scheme 3), which was fully
characterized (see the Supporting Information). Subsequently,
complex 5 was tested as a catalyst in the dehydrogenation of
alcohols. As shown in Table 1, complex 5 is indeed an active
catalyst for the acceptorless base-free dehydrogenative
coupling of alcohols to esters.
Scheme 2).
A third crystal structure obtained from the reaction of 1 with
t
0
KO Bu was a tripodal Ru carbonyl complex (complex 5;
Figure 4). The mass spectrum (Figure 1) showed the
corresponding mass of m/z 784.16 (fitting [5]H ; Scheme
+
2
). The crystal structure (Figure 4) exhibited C3v symmetry
with three phosphines at the equatorial positions and the
central nitrogen atom coordinated trans to the carbonyl ligand.
The dehydrogenation of aliphatic alcohols, catalyzed by 5,
was very efficient. Quantitative conversion was obtained with
1
-pentanol (8a) or 1-hexanol (8b) to the corresponding ester
(
Table 1, entries 1 and 2, respectively), with just 0.1 mol %
catalyst loading. However, benzyl alcohol (8c) was sluggish to
react (Table 1, entry 3) and required a higher catalyst loading
and longer reaction times to reach substantial conversion.
0
Since complex 5 can be readily synthesized from the Ru
carbonyl precursor 7 and the tripodal ligand 6 without the use
of base or preactivation, it represents a truly base-free
dehydrogenation catalyst which is, to the best of our
0
knowledge, the first reported well-defined homogeneous Ru
catalyst for this reaction.
The reverse reaction, hydrogenation of esters to alcohols,
was also tested using 5 as catalyst. To this end, the hexyl ester
product 9b, containing the catalyst from the dehydrogenative
coupling of 8b (Table 1, entry 2), was redissolved in toluene
and pressurized at room temperature in an autoclave to 60 bar
Figure 4. Molecular structure of 5 as determined by single-crystal X-
ray structure analysis. Ellipsoids are presented at 50% probability.
Hydrogen atoms are omitted for clarity. Selected bond lengths (Å)
and angles (deg): Ru1−N1, 2.25(1); Ru1−P1, 2.28(1); Ru1−C4,
with H (Scheme 4). Quantitative conversion back to 8b was
2
observed after 18 h of reaction at 130 °C (pressure at the
reaction temperature reached ca. 75 bar). This result attests to
the reversibility and robustness of complex 5 as a catalyst
which remains viable after reaction.
1
.79(2); N1−Ru1−C4, 180; P1−Ru1−P1a, 118.62(2); C2−N1−
C2a, 107.6(6); Ru1−N1−C2 111.3(5).
C
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Table 1. Catalytic Additive-Free Dehydrogenative Coupling
of Alcohols
themselves had an activating effect on the complex. The
successful rehydrogenation of 9b (Scheme 4) followed catalyst
activation by 8b. We also confirmed that commercial 9d
contained traces of ethanol (ca. 0.1 area % by GC-FID), which
would activate 5 toward hydrogenation. To test this
assumption, complex 5 was heated to 80 °C in a 1/10
methanol/benzene solution for 3 h. The orange slurry turned
a
II
clear yellow, and the formation of the Ru hydrido complex
1
0a (Scheme 6) was clearly observed by NMR spectroscopy.
Scheme 6. Reversible Oxidative Addition of Methanol to
Complex 5: Formation of Complex 10a
a
Reaction conditions unless specified otherwise: alcohol (4.4 mmol of
8a, 13.5 mmol of 8b, 1.4 mmol of 8c), 5, and solvent (10 mL) were
heated to reflux (oil bath temperature: 95 °C for benzene, 135 °C for
1
b
In the H NMR spectrum of complex 10a, a hydride signal
toluene) under argon for 18 h. Conversion by area percent in GC-
c
2
appears as a doublet of triplets (−6.5 ppm; J
= 83 Hz,
FID. Percent yield of product, isolated by SiO chromatography.
PH‑trans
2
d
1
2
31
1
Percent yield determined by H NMR after removal of all volatiles in
vacuo. 1 mol % of catalyst, 24 h reaction; conversion confirmed by
H NMR.
J
= 22 Hz); in the P{ H} NMR spectrum a doublet (56
PH‑cis
e
2
2
ppm, J = 15 Hz) and a triplet (45 ppm, J = 15 Hz)
PP PP
1
appear. These spectral features indicate a T-shaped phosphine
configuration, with a hydride trans to the apical phosphine and
cis to the two meridional phosphines. The methoxide which
was likely present as a counterion could not be explicitly
detected due to excess methanol in solution. N− H HMBC
NMR of 10a showed a signal at 78 ppm for the nitrogen atom,
similar to that of 5 (74 ppm) and shifted downfield by ca. 30
ppm relative to the free ligand 6. This is in line with a
Scheme 4. Additive-Free Dehydrogenative Coupling of 8b
to 9b (a) and the Reverse Reaction (b)
1
5
1
39,40
deshielded quaternary ammonium atom,
indicating that
the nitrogen atom is coordinated to the metal center.
When the solution of 10a was evaporated to dryness,
complex 5 formed again as an orange insoluble powder. Any
attempt to isolate 10a resulted in formation of complex 5.
Thus, the protonation and oxidation of 5 by the alcohol is
highly reversible. The alcohol O−H eliminates readily,
Ethyl acetate (9d) was hydrogenated neat under 60 bar of
H pressure (at room temperature) for 40 h at 130 °C
2
(
pressure at the reaction temperature reached ca. 80 bar). A
0
reducing the complex back to Ru complex 5. To the best of
catalyst loading of 0.016 mol % of 5 resulted in a 91% yield of
ethanol, corresponding to a turnover number (TON) of ca.
700 (Scheme 5a). Despite these results, when dimethyl
terephthalate (DMT, 9e) hydrogenation was attempted in
toluene, with 1 mol % of 5 under similar conditions (Scheme
b), no reaction was observed.
Since 5 showed activity toward dehydrogenation of alcohols,
completely additive free, it seemed probable that the alcohols
our knowledge, this reversible protonation, leading to a
0
II
41−43
reversible Ru /Ru system, is quite unique for ruthenium.
5
Layering a methanol/benzene solution of 10a with diethyl
ether and pentane yielded crystals suitable for X-ray
crystallography.
5
In the obtained crystal structure of 10a (Figure 5) the
methoxide counteranion could not be located due to
disordered excess methanol in the crystal. The structure
II
depicts a cationic, distorted-octahedral Ru −H species, which
Scheme 5. Additive-Free Hydrogenation of 9d (a) and
Attempted Hydrogenation of 9e (b)
confirms the structural assignment of 10a by NMR spectros-
copy. Unfortunately, obtaining analytically pure samples of 10a
failed, as removal of volatiles, even in the solid state, resulted in
methanol loss and formation of 5.
As hypothesized, complex 10a (prepared and used from a
stock solution) was found to be catalytically active in the
hydrogenation of esters. When 10a was used in the
hydrogenation of 9e, full conversion was achieved without
any addition of base (Scheme 7).
It is worth noting that hydrogenation of 9e was also
performed using 6, 7, and methanol in situ under the same
reaction conditions. Full conversion to 8e was obtained, and
the resulting solution could even be recycled again by filtration
D
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Scheme 8. Oxidative Additions of O−H (or O−D) to 5
Figure 5. Molecular structure of 10a as determined by single-crystal
X-ray structure analysis. Ellipsoids are presented at 50% probability.
+
Only the cation fragment [Ru(NP )H(CO)] could be assigned.
3
Hydrogen atoms, except the Ru hydride, are omitted for clarity.
Selected bond lengths (Å) and angles (deg): Ru1−N1, 2.231(4);
Ru1−P2, 2.361(1); Ru1−P1, 2.331(1); Ru1−C71, 1.822(6); Ru1−
H1, 1.66(4); N1−Ru1−C71, 178.0(2); P1−Ru1−P3, 150.65(5);
Ru1−N1−C2 109.1(3); C4−N1−C6, 106.7(4).
Scheme 7. Base-Free Hydrogenation of 9e Catalyzed by
Complex 10a
for further catalytic conversion of 9e (see the Supporting
Information).
II
In order to confirm that the observed Ru −H was formed by
protonation (rather than C−H activation), several experiments
Figure 6. Molecular structure of 10c as determined by single-crystal
X-ray structure analysis. Ellipsoids are presented at 50% probability.
Hydrogen atoms, except for the Ru hydride, and two molecules of
water are omitted for clarity. Selected bond lengths (Å) and angles
were carried out (Scheme 8). (i) The reaction of complex 5
with CH OD resulted in a complex with the same NMR
3
+
spectral features as the [Ru(NP )H(CO)] fragment of 10a,
3
1
albeit no hydride signal was observed in the H NMR
(deg): Ru1−N1, 2.235(2); Ru1−P1, 2.362(1); Ru1−P2, 2.362(1);
spectrum. A higher multiplicity of the 31P{ H} NMR signals
1
Ru1−C71, 1.834(3); Ru1−H1, 1.64(3), N1−Ru1−C71, 177.9(1);
P1−Ru1−P3, 104.6(1); Ru1−N1−C1, 110.6 (2); C1−N1−C3,
107.2(2).
asserted that the complex [Ru(NP )D(CO)]OMe (complex
3
1
0a-d) was formed (see spectra in the Supporting
Information). (ii) The reaction of CD OH with complex 5
3
led to the formation of complex 10a-h, with the same spectral
features as those observed with nondeuterated methanol. (iii)
When 3 equiv of phenol was added to complex 5 in benzene,
Complex 10b also showed catalytic activity toward hydro-
genation of 9e (see Supporting Information), albeit signifi-
cantly slower than that for the methanol-activated complex
10a. Complex 10c did not catalyze the hydrogenation of 9e
under the reaction conditions described above, but when
methanol was added to the reaction mixture, partial hydro-
genation of 9e was observed (see the Supporting Information).
This indicated that even benzoic acid addition to 5 is reversible
to some extent. The full catalytic scope of base-free ester
hydrogenation by complex 5 is shown in the Supporting
Information.
II
after heating to 80 °C full conversion to the new Ru −H
complex 10b was observed, which was very similar in NMR
spectral features to that of complex 10a (see the Supporting
Information). (iv) Finally, the reaction of complex 5 with 1
II
equiv of benzoic acid led to the expected cationic Ru −H
complex 10c, with a benzoate counterion, which could also be
observed by X-ray crystallography (Figure 6). The cation
+
fragment [Ru(NP )H(CO)] in the benzoate complex 10c
3
0
II
was very similar to that observed in the case of the methoxide
complex 10a by X-ray crystallography, thereby confirming the
assigned ionic nature of 10a.
Possibility of a Ru /Ru Catalytic Cycle. It is widely
accepted that hydrogenations and dehydrogenations catalyzed
by complex 1 occur through an active species of the form of
E
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Article
1
3,21,22
complex 2 (Scheme 1).
Complex 5 might follow a
[Ru(triphos)(CO) ] and [Ru(N-triphos)(CO) ] also showed
no direct reaction with H2.
2
2
43e,51
similar mechanism after activation by alcohol, perhaps through
ligand dissociation. Moreover, as shown in this work, several
organometallic species may form under the reaction con-
ditions, which are likely in dynamic equilibrium (that is,
complexes 2−5 and others may decompose and recombine
again). Nevertheless, it is also conceivable that complex 5 acts
Since the dynamic behavior of complex 5 is difficult to
monitor by spectroscopic methods (due to its poor solubility),
clear experimental evidence for its mode of action has not yet
been obtained. It may be hypothesized that decoordination of
the nitrogen atom at elevated temperatures would result in a
complex that would resemble the analogous [Ru(triphos)H-
0
II
0
in catalysis through a Ru /Ru cycle. Ru intermediates have
4
4−47
+
been cautiously suggested in related catalytic systems
and
(CO)] fragment, which was also studied computationally in
4
8−50
44
proposed in other ruthenium catalytic cycles4
but are not
the past in another context. Other plausible catalytic cycle
2,43,50
yet supported by experimental observations.
steps are currently being studied by computational methods in
0
0
Hanton et al. have reported an N-triphos-based Ru complex
N-triphos = tris(diphenylphosphinomethyl)amine) as a
catalyst in the hydrogenation of esters. In the case of the
complex [Ru(N-triphos)(CO) ] a strong oxidant was required
in order to activate the precatalyst. Nevertheless, the authors
our laboratory. Although Ru species may well be involved in
0
(
catalysis, it is of high certainty that heterogeneous Ru species
51
(as nanoparticles, or Ru black) are not present in the reaction
mixture. The fact that hydrogenation of the arene ring was not
2
55,56
observed
and the recyclability of the catalyst after reaction
0
suggested that a 14-electron Ru species was an intermediate in
attest to that fact.
the catalytic cycle, also when starting from Ru(acac) /N-
3
triphos as an additive-free hydrogenation catalyst. Complex 5
SUMMARY
■
is reminiscent of [Ru(N-triphos)(CO) ], albeit the one-
1
2
We were able to show that the widely used Ru-MACHO
complex 1) can degrade under basic conditions, which are
carbon-elongated ethylene backbone in ligand 6 (versus the
(
methylene-based N-triphos) enables nitrogen coordination to
common for most applications where it is used. While 1 is
certainly a highly active catalyst under basic conditions, this
previously unreported decomposition is of high significance in
industrial applications, which require control over the chemical
process, catalyst robustness, and catalyst recycling. This
observation is also of interest to the research community, as
it led to the discovery of a new active catalyst and may serve as
an exemplar in homogeneous catalysis research. We have
0
the metal center. This stabilizes a Ru monocarbonyl complex
which is nevertheless active toward oxidative addition.
Another analogous case is that of the reported PP3
complexes of ruthenium (PP = P(CH CH PPh ) ). Bianchini
3
2
2
2 3
52
et al. observed the dihydride complex [Ru(PP )(H) ], which
3
2
lost dihydrogen to yield the corresponding [Ru(PP )CO]
3
53
upon irradiation under a CO atmosphere. Catalytic activity
of the PP ruthenium complexes (not related to this work) was
0
3
54
shown that the tripodal Ru complex 5, found as a
limited to the noncarbonyl derivatives. Irreversible oxidative
additions to the Ru complex [Ru(triphos)(CO) ] (triphos =
decomposition product of 1, is a competent base-free catalyst
for the hydrogenation of esters and additive-free dehydrogen-
ative coupling of alcohols. Complex 5 can be obtained in a
dedicated synthesis from readily available simple precursors.
Comparable reactivity may be achieved when the synthetic
precursors for complex 5 are used in situ in catalysis, further
simplifying this new catalytic system. The form [Ru(NP )H-
CO)]OR (complex 10), obtained by O−H activation, is likely
the catalytically active species. We could clearly show a unique
0
2
1
,1,1-tris(diphenylphosphinomethyl)ethane) have also been
43e
reported by Hommeltoft et al.
On the basis of the accepted hydrogenation or dehydrogen-
20,28,29
ation mechanisms for similar ruthenium catalysts,
one
might expect a complex of the form {[Ru(NP )(H) CO]} to
3
2
3
play an active role. Thus, the precursor [Ru(PPh ) (H) CO]
3
3
2
(
(13; Scheme 9a) was heated for 16 h with the ligand 6, in
0
reversible oxidative addition of alcohols to the Ru complex 5.
Scheme 9. Formation of 5 from a Dihydride Precursor (a)
This behavior, in contrast to known ruthenium systems, can
help design a completely new class of active catalysts for other
organic oxidation and reduction reactions. Further theoretical
and experimental investigations on the detailed catalytic
mechanism when 5 is used are currently in progress.
and Reaction of 5 with H (b)
2
EXPERIMENTAL SECTION
■
General Considerations. All reagents were handled in an
MBraun glovebox using dry argon as an inert atmosphere. Organic
solvents were purchased dry from Aldrich and degassed prior to use
by bubbling argon for at least 30 min. Commercially available
chemicals (reagents, substrates, and complexes 1, 7, and 11) were
purchased from Aldrich, ABCR, or TCI and were used as received
unless otherwise stated. Ligand 6 was synthesized according to a
0
benzene-d . However, the only product observed was the Ru
6
38
complex 5, the result of dihydrogen loss (see the Supporting
Information). As seen in the case of the methoxy complex 10a,
literature procedure. Column chromatography was carried out on
Aldrich silica gel (60 Å, 70−230 mesh, 63−200 μm). NMR spectra
were recorded using a Bruker 200 instrument at CaRLa or Bruker
AVANCE III 300, Bruker AVANCE III 400, Bruker AVANCE III
II
the Ru tripodal species tends to undergo reductive
0
elimination and revert to the parent Ru complex 5. Moreover,
complex 5 showed no measurable change when it was
5
0,0 and Bruker AVANCE III 600 spectrometers at the Organisch-
1
13
Chemisches Institut der Universitat Heidelberg. H and C chemical
shifts δ are reported in ppm relative to either the residual solvent or
tetramethylsilane (TMS). C assignment was achieved via DEPT90
and DEPT135 or HSQC spectra. P chemical shifts are reported
̈
pressurized under H (see Scheme 9b and the Supporting
57
2
Information). These results, however, do not preclude the
existence of a dihydride or similar intermediate in the catalytic
cycle. According to the literature, the Ru0 complexes
13
3
1
relative to an external standard of phosphoric acid 30% in D O (0.0
2
F
DOI: 10.1021/acs.organomet.8b00353
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
ppm). 15N chemical shifts are reported downfield from liquid
ammonia. The multiplicities are reported as s = singlet, b = broad,
d = doublet, t = triplet, q = quartet, and m = multiplet. GC analyses
were performed on an Agilent Technologies 6890N gas chromatog-
raphy system equipped with an FID detector and an Agilent
Technologies DB-1 capillary column (30 m × 0.250 mm/1.0 μm;
He flow 1.0 mL/min; program initial 40 °C, ramp 15 °C/min, 300 °C
for 10 min). GC-MS analyses were performed on an Agilent
Technologies 6890N gas chromatography system coupled with an
Agilent Technologies 5975B mass spectrometer and equipped with an
Agilent Technologies HP-5MS capillary column (30 m × 0.250 mm/
Crystallization of Complexes 4 and 5. In a dry argon-filled
glovebox, complex 1 (38 mg, 0.06 mmol) was dissolved in THF in a
glass vial (2 mL). To the cream white slurry was added potassium tert-
butoxide (6.5 mg dissolved in THF, 1 mL) dropwise with stirring,
over 1 min at room temperature. The dark mixture was stirred for 2 h
at room temperature and then filtered, yielding a dark brown clear
solution. Several portions of the solution (ca. 0.6 mL each) were
layered in NMR tubes with pentane. Crystals suitable for X-ray study
were formed within 3 days at room temperature.
Preparation of Tris((2-(diphenylphosphino)ethyl)amino)-
ruthenium Monocarbonyl ([Ru(NP )CO], Complex 5). In a dry
3
0
.25 μm). All analyses (X-ray, mass spectrometry, elemental analysis)
argon-filled glovebox, a 100 mL glass Schlenk flask was charged with
Ru (CO)12 (7; 0.162 g), tris(2-(diphenylphosphaneyl)ethyl)amine
were conducted in house. X-ray crystal structures were measured on a
Bruker APEX-II-CCD or Bruker APEX-II Quazar instrument using
Mo Kα radiation. The structures were solved and refined using the
SHELXTL software package (see the Supporting Information). FD+
spectra were measured on a JEOL JMS-700 spectrometer (MStation).
ESI+ spectra were measured on a Bruker ApexQe FT-ICR mass
spectrometer.
General Procedure for the Additive-Free Acceptorless
Dehydrogenative Coupling of Alcohols (Table 1). In a dry
argon-filled glovebox, a 100 mL round-bottomed glass Schlenk flask,
with a PTFE-coated magnetic stirring bar, was charged with complex
3
(6; 0.500 g), and toluene (15 mL). The flask was equipped with a
condenser, and the system was connected under argon to a Schlenk
line. The solution was heated in an oil bath to reflux (oil temperature
135 °C) with stirring under argon for 16 h. When it was cooled to
room temperature, the product precipitated as an orange powder. The
workup procedure was performed in an argon-filled glovebox: the
supernatant solvent was removed, and the orange powder was washed
with degassed, dry benzene (20 mL). In order to ensure removal of
insoluble impurities (e.g., Ru black), the orange residue was then
introduced into a 100 mL Teflon-capped pressure vessel, with
degassed, dry benzene (25 mL). Degassed anhydrous methanol (2.5
mL) was added, and the solution was heated in the closed vessel,
under argon, to 80 °C for 3 h, until a clear yellow solution was
obtained (complex 10a). The solution was then cooled to room
temperature and filtered through a 0.2 μm PTFE filter. The filtrate
was evaporated fully under high vacuum to afford complex 5 as a
5
(0.01 mmol, 7.8 mg), alcohol (10 mmol), and toluene (10 mL).
The flask was removed from the glovebox and connected under argon
to a Schlenk line. The flask was equipped with a condenser (cooling
water at room temperature). The solution was heated in an oil bath to
reflux (oil temperature 95 °C for benzene, 135 °C for toluene) with
stirring under argon for 18 h.
1
General Procedure for the Base-Free Hydrogenation of
bright orange powder (445 mg, 74% yield). H NMR (400 MHz,
Esters (See the Supporting Information). Caution! H gas forms
benzene-d , only partial assignment due to very poor solubility): δ
2
6
highly explosive mixtures with air, and appropriate safety measures should
be taken when attempting these procedures. In an argon-filled glovebox, a
ca. 80 mL Premex stainless steel autoclave was charged with 5 (6 ×
7.53 (bm, Ph-CH2,6), 7.1−6.9 (m, Ph-CH3,4,5), 2.1 (b, NCH ),
2
1
3
1
2.1−1.9 (bm, CH P) ppm. C{ H} NMR (DEPT135, 150 MHz,
2
1
13
benzene-d , verified by H− C HSQC, only partial assignment due to
6
−
3
1
0
mmol, 4.7 mg), the specified ester (0.6 mmol), degassed,
anhydrous 1,4-dioxane (6.0 mL), and degassed anhydrous methanol
0.05 mL). Hexamethylbenzene (0.15 mmol, as internal standard)
very poor solubility): δ 131 ppm (b, Ph(C2,C6)) 127−128 ppm
(overlapping solvent, Ph(C3,C4,C5)), 58 (bm, CH P), 31 (bm,
2
3
1
1
(
NCH ) ppm. P{ H} NMR (160 MHz, benzene-d ): δ 57 (s) ppm.
2
6
1
5
1
31
was added, and the mixture was stirred vigorously at room
N− H{ P} HMBC NMR (600 MHz, benzene-d ): δ 76 ppm.
1
6
−
temperature until full dissolution. A sample (0.1 mL) was then
FTIR (KBr pellet): ν (cm ) 1851 (CO). FTIR (NaCl thin film):
̅
max
−1
taken for t analysis (GC and NMR). After it was closed, the reaction
ν̅max
(cm ) 1846 (CO). Anal. Calcd for C H NOP Ru: C, 65.98;
0
43 42 3
vessel was removed from the glovebox. The system was purged twice
with hydrogen (20 bar) and then pressurized with hydrogen (60 bar)
and placed into a preheated heating block (130 °C) over a magnetic
stirring plate. The reaction mixture was then stirred for 17 h (the
pressure reaches ca. 75 bar at the reaction temperature). The
autoclave was then placed in a cold water bath, and after it was cooled
to room temperature, it was depressurized and flushed with argon,
before being opened under a flow of argon for sampling and analysis.
Reaction of Complex 1 with Base: Formation of Complexes
H, 5.53; N, 1.79. Found: C, 65.14; H, 5.32; N, 1.77. ESI-MS (m/z):
[M + H] calcd for C H NOP Ru , 784.1596; found, 784.1624. FD-
MS (m/z): [M] calcd for C H NOP Ru , 783.15; found, 783.06.
+
+
4
3
43
3
+
+
4
3
42
3
Preparation of Tris((2-(diphenylphosphino)ethyl)amino)-
hydridocarbonylruthenium Methoxide ([Ru(NP )(H)CO]OMe,
3
Complex 10a). In a dry argon-filled glovebox, a J. Young NMR tube
was charged with complex 5 (12 mg) and benzene-d (0.7 mL).
6
Degassed anhydrous methanol (0.02 mL) was added, and the solution
was heated in the closed capped tube, under argon, to 80 °C with
intermittent shaking for 16 h (shorter reaction times are sufficient
when mixing is possible), until a clear yellow solution was obtained.
The product complex is only stable in solution with excess methanol
and was not isolated completely. Single crystals were grown by slow
diffusion of diethyl ether/pentane to a benzene/methanol solution of
2
1
−5. ESI-MS Measurements. In a dry argon-filled glovebox, complex
(23 mg, 0.038 mmol) was mixed with toluene-d (0.7 mL) in an
8
NMR tube and cooled to −38 °C in a freezer. To the cold suspension
was added potassium tert-butoxide (6.8 mg, 0.06 mmol), and the tube
was shaken and warmed to room temperature. The reaction was
monitored by NMR spectroscopy. After 18 h at room temperature,
the solution was filtered and the filtrate was evaporated under vacuum
to yield a pale brown powder. The powder was analyzed by ESI-MS
and NMR spectroscopy. The same procedure was performed using
sodium hexamethyldisilazane as base (8.4 mg).
Crystallization of Complex 3. In a dry argon-filled glovebox,
complex 1 (23 mg, 0.038 mmol) was dissolved in a glass vial in
benzene (2 mL). To the slurry was added sodium hexamethyldisi-
lazane (6.8 mg dissolved in benzene, 1 mL) dropwise with stirring,
over 1 min at room temperature. The dark mixture was stirred at
room temperature for 16 h. The volatiles were then removed under
vacuum and the brown residue was filtered in a benzene/diethyl ether
mixture (1/1). The filtrate was evaporated under vacuum and the
residue dissolved in THF (0.6 mL). Crystals suitable for X-ray study
were obtained by vapor layering (vial in vial) with pentane at room
temperature.
1
3
complex 10a. H NMR (400 MHz, benzene-d
): δ 7.89 (dt, JHH
=
6
4
3
4
12.7, JHH = 4.1 Hz, 4H, P-Ph), 7.22 (dd, JHH = 11.4, JHH = 4.2 Hz,
4H, P-Ph), 7.18−7.11 (m, 2H, P-Ph), 7.01−6.86 (m, 8H, P-Ph), 6.79
3
3
(t, JHH = 7.4 Hz, 2H, P-Ph), 6.74−6.63 (m, 6H, P-Ph), 6.60 (td, JHH
4
= 7.6, JHH = 2.0 Hz, 4H, P-Ph), 3.16−2.96 (m, 4H, N(CH
CH P-) ),
2 2
2
2.73−2.60 (m, 4H N(CH CH P−)), 2.50−2.32 (m, 4H, N-
2
2
2
2
(CH
CH
P−) ), − 6.60 (dt, JPH(trans) = 83.0, JPH(cis) = 21.7 Hz,
2
2
2
13 1
1H, Ru-H) ppm. C{ H} NMR (100 MHz, benzene-d
): δ 205.7 (td,
6
2
2
1
J
PC = 12.0, JPC = 8.8 Hz, Ru−CO), 160.0 (s, P-Ph), 138.3 (td, JPC =
3
3
27.3, J = 4.6 Hz P-Ph), 133.4 (t, J = 6.4 Hz, P-Ph), 130.6−130.4
PC
PC
3
2
(m, P-Ph), 130.0 (t, J = 5.6 Hz, P-Ph), 129.8 (d, J = 11.3 Hz, P-
PC
PC
4
3
Ph), 129.3 (s, P-Ph), 129.0 (d, J = 1.9 Hz, P-Ph), 128.6 (t, J
=
PC
PC
3
4.7 Hz, P-Ph), 128.5 (d, J = 4.6 Hz, P-Ph), 128.4−128.2 (m, P-
PC
3
3
Ph), 63.4 (t, J = 4.4 Hz, NCH CH P), 59.5 (d, J = 7.8 Hz,
PC
2
2
PC
1
N(CH CH P) ), 52.4 (s, NCH CH P), 31.5 (d, J = 23.6 Hz,
2
2
2
2
2
PC
3
1
1
N(CH CH P) ), 30.6 (vt, J = 13.4 Hz, NCH CH P) ppm. P{ H}
2
2
2
2
2
G
DOI: 10.1021/acs.organomet.8b00353
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
2
NMR (81 MHz, benzene-d ): δ 55.7 (d, J = 15 Hz, apical), 44.8 (t,
6
PP
ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications Web site. (CIF) (PDF) The Supporting
Information is available free of charge on the ACS Publications
website at DOI: 10.1021/acs.organomet.8b00353.
■
2
15
1
JPP = 15 Hz, equatorial) ppm. N− H HMBC NMR (400 MHz,
benzene-d ): δ 78 ppm. ESI-MS (m/z) prepared in 1,2-difluor-
obenzene, diluted in dry DCM: [M − OMe] calcd for
C H NOP Ru , 784.1596; found, 784.1595.
Preparation of Tris((2-(diphenylphosphino)ethyl)amino)-
hydridocarbonylruthenium Phenoxide ([Ru(NP )(H)CO]OPh,
*
S
6
+
+
43
43
3
3
Complex 10b). In a dry argon-filled glovebox, a J. Young NMR
tube was charged with complex 5 (22 mg), phenol (8 mg, 3 equiv),
Detailed analyses, chromatograms, spectra, and crystallo-
graphic data of complexes 3−5 and 10a,c (PDF)
and benzene-d (0.7 mL). The solution was heated in a closed capped
6
tube, under argon, to 80 °C with intermittent shaking for 1 h until a
clear yellow solution was obtained. Complex 10b was analyzed in
solution with excess phenol (2 equiv). H NMR (400 MHz, benzene-
Accession Codes
CCDC 1821685−1821689 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
1
3
4
d ): δ 12.92 (br, 2H, free PhOH), 7.89 (dd,, J = 12.5, J = 6.5
6
HH
HH
3
4
−
Hz, 4H, PPh ), 7.58 (dt,, J = 3.4, J = 1.7 Hz, 6H, PhO and
2
HH
HH
−
3
PhOH), 7.34−7.26 (m, 6H, PhO and PhOH), 7.13 (dd, J = 14.4,
HH
4
3
J
= 7.1 Hz, 4H, PPh ), 7.06−7.00 (m, 3H, PPh ), 6.98 (ddd, J
HH
2
2
HH
−
4
3
=
9.8, J = 3.9, J = 1.5 Hz, 7H, PPh ), 6.80−6.73 (m, 3H, PhO
3 3
HH
PH
2
and PhOH), 6.70−6.52 (m, 12H, PPh ), 3.52 (dt, J = 23.8, J
=
PH
2
HH
3
3
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.S.: thomas.schaub@basf.com.
6
.7 Hz, 2H, NCH ), 3.28 (dd, J = 33.2, J = 17.2 Hz, 2H,
■
2
HH
PH
3
3
NCH ), 2.82 (dd, J = 13.1, J = 6.5 Hz, 2H, NCH ), 2.75−2.61
2
HH
PH
2
2
(
8
m, 2H, CH P), 2.40−2.17 (m, 4H, 2(CH P)), −6.60 (dt, J
=
2
2
PH(trans)
2
13
1
2.5, J
= 21.4 Hz, 1H, Ru−H). C{ H} NMR (100 MHz,
PH(cis)
ORCID
2 2
PC
benzene-d ): δ 206.7 (dt, J = 20.7, J = 11.9 Hz, Ru-CO), 139.3
6
PC
A. Stephen K. Hashmi: 0000-0002-6720-8602
Thomas Schaub: 0000-0003-2332-0376
Notes
(
td, J = 26.9, 4.7 Hz), 134.7 (dt, J = 36.9, 2.1 Hz), 134.3 (td, J = 20.8,
.5 Hz), 134.0 (t, J = 6.4 Hz), 130.6 (t, J = 5.6 Hz), 130.6−130.5 (m,
overlapping), 130.4 (m, overlapping), 129.3 (s), 129.0 (d, J = 4.6 Hz),
1
3
1
28.8 (d, J = 9.5 Hz), 128.6 (t, J = 5.3 Hz), 63.9 (t, J = 3.7 Hz,
PC
The authors declare no competing financial interest.
3
1
NCH CH P), 60.3 (d, J = 7.2 Hz, N(CH CH P) ), 32.3 (d, J =
2
2
PC
2
2
2
PC
2
4.0 Hz, N(CH CH P) ), 31.2 (vt, J = 13.3 Hz, NCH CH P) ppm.
2 2 2 2 2
ACKNOWLEDGMENTS
CaRLa (Catalysis Research Laboratory) is cofinanced by the
31
1
2
■
P{ H} NMR (81 MHz, benzene-d ): δ 66.6 (d, J = 15 Hz,
6
PP
2
apical), 55.0 (t, J = 15 Hz, equatorial) ppm. FTIR (NaCl, thin
film): ν
difluorobenzene, diluted in dry DCM: [M − PhO] calcd for
C H NOP Ru , 784.1596; found, 784.1593.
PP
−
1
Ruprecht-Karls-Universitat
sity) and BASF SE.
̈
Heidelberg (Heidelberg Univer-
(cm ) 1934 (CO). ESI-MS (m/z) prepared in 1,2-
̅
max
+
+
43
43
3
REFERENCES
Preparation of Tris((2-(diphenylphosphino)ethyl)amino)-
■
hydridocarbonylruthenium Benzoate ([Ru(NP )(H)CO]OOCPh,
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was charged with complex 5 (15 mg), benzoic acid (2.3 mg, 1 equiv),
and benzene-d (0.7 mL). The solution was heated in the closed
capped tube, under argon, to 80 °C, with intermittent shaking, for 30
min, until a clear yellow solution was observed. Formation of complex
1
solution was cooled, and after 2 days at room temperature crystals
were obtained suitable for X-ray crystallography. Note: the
preparation can also be performed in dichloromethane, yielding a
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PhCOO and PPh ), 7.53−7.43 (m, 2H, overlapping, PhCOO), 7.33
3
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2
2
2
2
2
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(
2
05.9 (m, Ru-CO), 170.7 (s, PhCOO), 138.4 (td, J = 26.9, 4.6 Hz,
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overlapping, PPh ), 134.2 (s, overlapping, PhCOO), 134.0 (t, J = 6.2
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=
2
ipso
2
2
(
2
2
11.2 Hz, PPh ), 130.1 (d, J = 10.1 Hz, PPh ), 129.7 (s, PhCOO),
2
2
(
1
29.4 (t, J = 4.3 Hz, PPh ), 129.0−129.2 (m, PPh ), 128.2 (s), 64.7
2
2
(
t, J = 4.4 Hz, NCH CH P), 60.3 (d, J = 7.1 Hz, N(CH CH P) ),
2 2 2 2 2
(
3
2.6 (d, J = 24.0 Hz, N(CH CH P) ), 31.5 (t, J = 13.2 Hz,
2 2 2
3
1
1
NCH CH P) ppm. P{ H} NMR (80 MHz, benzene-d ): δ 57.1 (d,
2
2
6
2
2
JPP = 15 Hz, apical), 46.3 (t, J = 15 Hz, equatorial). FTIR (NaCl,
PP
−
1
thin film): νm (cm ) 1934 (CO). ESI-MS (m/z) prepared in 1,2-
difluorobenzene, diluted in dry DCM: [M − PhCOO] calcd for
C H NOP Ru , 784.1596; found, 784.1607.
̅
ax
+
+
43
43
3
H
DOI: 10.1021/acs.organomet.8b00353
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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DOI: 10.1021/acs.organomet.8b00353
Organometallics XXXX, XXX, XXX−XXX