A Diaminopropane Diolefin Ru(0) Complex Catalyzes Hydrogenation and Dehydrogenation Reactions

Article abstract of DOI:10.1002/cctc.201901739

New ruthenium (0) complexes with a cooperative diolefin diaminopropane (DAP) or the dehydrogenated iminopropenamide ligand (IPA) were synthesized for comparison with their diaminoethane (DAE)/ diazadiene (DAD) ruthenium analogues. These DAP/IPA complexes are efficient catalysts in dehydrogenation reactions of alkaline aqueous methanol which proceeds under mild conditions (T=70 °C) and of higher alcohols, forming the corresponding carbonate and carboxylates, respectively. The scope of the reaction includes an example of a 1,2-diol as model for biomass derived alcohols. Their catalytic applications are extended to the atom-efficient dehydrogenative coupling of alcohols and amines to amides. The reaction proceeds without any additives and is applicable to the synthesis of formamides from methanol. Moreover, DAP/IPA complexes catalyze the hydrogenation of a series of esters, lactone, ketone, activated olefin, aldehyde and imine substrates. The diaminopropane Ru catalyst exhibits higher activity compared to the dehydrogenated β-ketiminate (IPA) and previously studied DAD/DAE based catalysts. We present studies on their stoichiometric reactivity with relevance to their possible catalytic mechanisms and the isolation and full characterization of key reaction intermediates.

Full text of DOI:10.1002/cctc.201901739

Accepted Article
Title: A Diaminopropane Diolefin Ru(0) Complex Catalyzes
Hydrogenation and Dehydrogenation Reactions.
Authors: Monica Trincado, Fernando Casas, Rafael Rodriguez-Lugo,
Dipshikha Baneerje, and Hansjörg Grützmacher
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To be cited as: ChemCatChem 10.1002/cctc.201901739
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DOI: 10.1002/cctc.201901739
Full Paper
Received: 12.^September.^2019
Revised: 06.Oktober 2019
A Diaminopropane Diolefin Ru(0) Complex Catalyzes Hydrogenation
and Dehydrogenation Reactions.
Fernando Casas,[a] Dr. Monica Trincado0000-0002-5912-509X,*[a] Dr. Rafael Rodriguez-
Lugo,[b] Dr. Dipshikha Baneerje,[a] Prof.^^Dr. Hansjörg Grützmacher*[a]
[a]
<orgDiv/>Department of Chemistry and Applied Biosciences
<orgName/>ETH Zürich
<city/>Zürich <postCode/>8093 (<country/>Switzerland)
E-mail: trincado@inorg.chem.ethz.ch
E-mail: hgruetzmacher@ethz.ch
[b]
<orgDiv/>Laboratorio de Bioinorgánica
<orgName/>Centro de Química
Instituto Venezolano de Investigaciones Científicas (IVIC)
<city/>Caracas <postCode/>1020^^A (<country/>Venezuela)
<pict> Supporting information for this article is available on the WWW under
<url>http://dx.doi.org/10.1002/cctc.201901739</url>
<spi> This manuscript is part of the Special Issue on New Concepts in Homogeneous
Catalysis
Homogeneous catalysis
Dehydrogenation
Hydrogenation
Ruthenium
Metal ligand cooperativity
Phosphine-free Ru catalysts: Tetradentate Ru(0) complexes with a diolefin diaminopropane
or a dehydrogenated iminopropenamide ligand can be easily interconverted by
1
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dehydrogenation or hydrogen transfer reactions. They have enabled the catalytic
dehydrogenative coupling of alcohols and water or amines to the corresponding carboxylates
and amides as well the reverse catalytic hydrogenation of carbonyl compounds.
A diaminopropane diolefin Ru(0) complex catalyzes hydrogenation and dehydrogenation
reactions from @ETH and @IVIC<?_>official
New ruthenium (0) complexes with a cooperative diolefin diaminopropane (DAP) or the
dehydrogenated iminopropenamide ligand (IPA) were synthesized for comparison with their
diaminoethane (DAE)/ diazadiene (DAD) ruthenium analogues. These DAP/IPA complexes
are efficient catalysts in dehydrogenation reactions of alkaline aqueous methanol which
proceeds under mild conditions (T=70^°C) and of higher alcohols, forming the corresponding
carbonate and carboxylates, respectively. The scope of the reaction includes an example of a
1,2-diol as model for biomass derived alcohols. Their catalytic applications are extended to
the atom-efficient dehydrogenative coupling of alcohols and amines to amides. The reaction
proceeds without any additives and is applicable to the synthesis of formamides from
methanol. Moreover, DAP/IPA complexes catalyze the hydrogenation of a series of esters,
lactone, ketone, activated olefin, aldehyde and imine substrates. The diaminopropane Ru
catalyst exhibits higher activity compared to the dehydrogenated -ketiminate (IPA) and
previously studied DAD/DAE based catalysts. We present studies on their stoichiometric
reactivity with relevance to their possible catalytic mechanisms and the isolation and full
characterization of key reaction intermediates.
Introduction
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Hydrogenation and dehydrogenation reactions are essential chemical transformations
for manufacturing fine chemicals, foods, and fuels.[1] An increased need for hydrogen storage
technologies has renewed the interest in acceptorless and reversible dehydrogenation
processes of chemical compounds. Primary or secondary alcohols can be -- under specific
catalytic conditions and by application of the principle of micro reversibility -- fully
reversibly hydrogenated and dehydrogenated which make them one of the most attractive
2
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hydrogen storage media.[2] The most promising liquid organic hydrogen carriers are based on
methanol and polyalcohols derived from biomass.[3,4]
In the last decades, alcohol dehydrogenation reactions catalyzed in homogenous phase
by catalysts containing noble metals (Ru,[5] Ir,[6] Rh,[7] or Re[8]) were intensively investigated.
More recently, complexes with earth-abundant metals (Fe,[9] Co,[10] Cu,[11] Ni,[12] and Mn[13]
joined this flock. Previously, heterogeneous catalysts were applied in the reforming of
aqueous methanol which moreover needed harsh conditions. Beller and Grützmacher
proposed independently Ru complexes which contain a cooperative aliphatic PNP or
diazadiene ligand, respectively. These complexes were able to dehydrogenate aqueous
methanol fully according to: H3COH+H2O3 H2+CO2 under atmospheric pressure and
T<100^°C.[14] There is convincing evidence that these reactions proceed step-wise with
formaldehyde and formic acid as intermediates. In related work, the direct conversion of
higher primary alcohols to carboxylates in a dehydrogenative process was successfully
accomplished by the pioneering work of Milstein et^^al., using Ru-PNN based catalysts.[15]
Recently, other pincer or N-heterocyclic carbene (NHC) complexes with Ru,[16] Ir[17] and
)
Fe[18] as metals have been investigated in the dehydrogenative coupling of alcohols and water.
Conversion of polyols containing 1,2-diols has proven to be a difficult process and very few
examples are known,[19] which mainly focus on the conversion of glycerol to lactic acid
(LA).[17,18] Milstein et^^al. also pioneered the dehydrogenative coupling of alcohols and
amines to the corresponding amides and this new “green” process was subsequently applied to
the synthesis of polymeric materials.[20] Meanwhile numerous metal complexes are able to
catalyze this dehydrogenative amidation reaction,[21] although there are still some limitations
with respect to the substrate scope. Merely four catalytic systems are able to convert methanol
and amines to formamides in the absence of harsh oxidative conditions.[22] Only one catalyst,
an aliphatic Fe-PNP[22d] complex, exhibits a TON>50 and is only active with selected
secondary amines. Progress in alcohol dehydrogenative processes seems to be coupled to the
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development of new ligand frameworks likely due to the fact that in almost all cases they play
a relevant role in substrate activation. Figure^^1<figr1> illustrates this metal ligand
cooperation. The cooperative sites can be located in the first coordination sphere (for example
metal coordinated amine/amido and hydroxy/carbonyl functions), or in a remote position like
in M-PNN and M-PNP pincer complexes, which undergo a reversible aromatization and
dearomatization transformation. Alternatively, they involve redox and chemical non-innocent
behavior as in diazadiene/diaminoethane complexes which are interconverted by
hydrogenation-dehydrogenation and intramolecular redox processes.[23]
The reverse process, the catalytic reduction of carboxylic acid derivatives has also
witnessed a rapid development. Ester hydrogenation is especially relevant in the context of
upgrading of bio-based feedstock. It is also a key step in the “green” production of methanol
by hydrogenating methyl formate (derived from CO2 or CO) to methanol.[24] These reactions,
involving molecular hydrogen as the reducing agent, can be promoted by heterogeneous and
homogeneous catalysts. Current heterogeneously catalyzed processes are operated at harsh
conditions (200--300^°C and H2 pressures of 140--300^^bar)[25] and are frequently
accompanied by side reactions and product degradation. Homogeneous catalysts can operate
at significantly lower temperatures, thereby allowing for high selectivity and are more suitable
when highly functionalized substrates need to be converted. They are also much more easily
tuned to achieve a better performance. One relevant example concerns the previously
mentioned Ru PNN complex reported by Milstein et^^al.[26] This pincer complex was
modified by placing one (CNN)[27] or two carbene groups (CNC)[28] on the ligand to give a
superior catalyst. The presence of the cooperative amine function in the first coordination
sphere of the metal center (see archetypical Noyori-type complexes) typically results in more
active ester hydrogenation catalysts when compared to these pincer-type systems.[29] We have
also demonstrated the cooperative role of an amino/amido functions in a rhodium(I) amino bis
(olefin) complex in hydrogenation and transfer-hydrogenation reactions.[30] The most active
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ester hydrogenation catalysts are obtained with metal complexes containing amino-pincer
ligands with three donor groups attached to the metal in a meridional fashion.[31] Currently,
the best catalysts for the hydrogenation of esters were reported by Gusev et al who reported a
series of Ru and Os PNN and SNS pincer complexes achieving high turnover numbers
(TON’s) under remarkably mild conditions.[32]
The development of an efficient and generally applicable homogeneous catalytic
system for reversible dehydrogenation-hydrogenation reactions of alcohols remains of great
interest. This goal may be reached by a proper design of a multifunctional cooperative ligand
which not only stabilizes the catalytic intermediates but also actively participates on substrate
binding and activation. In previous studies, the [Ru(trop2DAD)] complex 2 (trop2DAD=1,4-
bis(5H-dibenzo[a,d]cyclohepten-5-yl)-1,4-diazabuta-1,3-diene), which can be described by
two resonance structures with either a Ru(0) or a Ru(II) center, reacts with an alcohol
(methanol, ethanol or benzyl alcohol) to form a Ru(0) complex 1, which “stores” up to two
equivalents of H2 in the diazadiene ligand backbone.[14b] Dehydrogenation of complex 1
releases two equivalents of H2 to regenerate complex 2. Initial assumptions involved both
complexes as active intermediates in the catalytic cycle, but recent findings supported by DFT
calculations indicate that they may be independent catalysts, with different mechanistic
pathways, but linked by a chemical equilibrium.[33]
Results and Discussion
In order to study these phenomena of ligand non-innocence specifically in
Ru(0)/Ru(II) complexes in deeper detail, we envisioned to expand the ethylene bridge
between the two nitrogen centers in complexes 1 or 2 by an additional methylene group. The
diaminopropane ligand trop2DAP was synthesized according to the reaction sequence shown
in Scheme^^1<schr1>, which consists in a straightforward initial reductive amination of
dibenzosuberenone. 1H NMR spectroscopic analysis of the ligand in solution revealed the
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presence of three conformers. In the solid state, the amine bound trop substituents adopt an
endo/exo conformation (See Figure^^2<figr2>A). Reaction of the trop2DAP ligand with the
ruthenium precursor complex [RuCl2(PPh3)3] led to the formation of the diamine Ru(II)
complex [RuCl2(trop2DAP)] 3-(Cl)2 which can be converted to the zerovalent Ru complex
[Ru(trop2DAP)] 3 with a 16 valence electron configuartion by direct reduction with Mg
(Scheme^^1<xschr1>). Alternatively, a combination of base and superhydride can be used to
give 3 in a higher yield (87^%). The reaction must be performed at low temperature since the
coordinated ligand can be thermally dehydrogenated. In the presence of a strong base
(KHMDS or KOtBu) at 70^°C, complex 3 eliminates in solution 2.5 equivalents of hydrogen
in an open vessel flushed with Ar. In this reaction, both NH groups become deprotonated and
the propylene bridge loses three hydrogens to yield the salt K[Ru(trop2IPA)] 4, in which
formally a mono-anionic -diketiminate ligand is coordinated to a Ru(0) center
(Scheme^^2<schr2>a).[34]
In the absence of base, when 3 is heated in toluene under reflux it slowly converts
(4^^days) to the pentacoordinated ruthenium (II) complex 5. This species is best described
with mono-anionic -diketiminate and a hydride ligand resulting from a formal elimination of
2 equivalents of H2 and migration of one hydrogen atom from the ligand backbone to the
metal center (Scheme^^2<xschr2>a). 1H NMR spectra of both complexes, 4[33] and 5, exhibit
similar 1H NMR data for the ligand backbone in the region of aromatic protons. In addition,
complex 5 shows resonances at rather high frequencies for the olefinic protons at (1H)=4.09
(dd) ppm and a signal for the hydride at (1H)=<M->14.8 (s) ppm. The corresponding
spectrum of complex 3 exhibits the proton signals of the saturated backbone in the (1H)
range of 3.3--1.5^^ppm, which is close to the free ligand, and protons of the coordinated
olefins at low frequencies [(1H)=1.85 (d) and 1.71 (d) ppm]. A series of stoichiometric
reactions were performed to study the reactivity of complexes 3 and 4 towards water or an
alcohol, either under neutral or basic conditions (Scheme^^2<xschr2>). In the reaction with
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neutral water, complex 3 is converted to the octahedral hydrido hydroxo Ru(II) complex 6,
while in an alkaline solution, the reaction leads additionally to the formation of the
dehydrogenated complex 5 as minor product. Water is able to protonate complex 4 forming
the hydride species 5 in nearly quantitative yield and only complex 3 is detected as minor side
product. Addition of a strong reductant (KC8) to a THF-d8 solution of 5 reverses this reaction
and yields 4 again. While the dehydrogenation of the DAP backbone proceeds smoothly
under basic conditions or by simply heating the complex in a flask which allows the release of
gaseous products (“open” reaction system), ligand transfer hydrogenation with an alcohol as
hydrogen source is significantly more difficult. The reaction of a solution of complex 4 in
THF with benzyl alcohol or 1,2-propanediol at 50^°C and in a closed system leads to the
formation of complex 3 in low yield (12^% by NMR), in both cases. The reaction of an
excess of 1,2-propanediol and water with complex 3 in benzene at higher temperature resulted
in the formation of the hexacoordinated lactate complex 7 in more than 80^% yield. In this
complex the fully hydrogenated form of the DAP ligand remains intact (see
Figure^^2<xfigr2>D). The reaction of complex 7 with base at room temperature liberates the
lactate salt and regenerates 3 quantitatively.
The reactivity of complex 3 in the presence of methanol was also investigated under
anhydrous or aqueous conditions. All the experiments were performed at moderate
temperatures (35--50^°C). In the absence of water, the complex reacts with the O<C->H bond
of methanol forming the stable hydrido methoxy complex 9 as mayor product. The dicarbonyl
complex 8 which contains two carbonyl groups in the coordination sphere of the metal, is
formed as a minor product (10^%). The formation of the latter is likely due to the fact that in
absence of water formaldehyde is formed as first dehydrogenation product in sufficiently high
concentration, which is then decarbonylated.[35]A comparable observation was made when
trop2dad complexes like 2 were reacted with aqueous formaldehyde solutions.[36] In the
presence of water, formaldehyde will be immediately hydrated and in this case, the reaction
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proceeds further to give carbonate as fully dehydrogenated product. Indeed, we were able to
isolate single crystals of two carbonate complexes 10 and 11 formed as mayor products when
different ratios of methanol: water was used. In the presence of an excess of methanol-water
in a ratio 2^:^1, the dinuclear hydrido carbonate complex 10 is formed together with the
hydroxo complex 6 in 1^:^1 ratio. When the ratio of methanol was increased with respect to
water and complex 3, the main product isolated was the mononuclear hydrido
methylcarbonate ruthenium complex 11. This species is a particularly unstable complex and
when a solution in THF is evaporated under reduced pressure or heated at 65^°C for a short
period of time, it decomposes forming the methoxide complex 9 and CO2. To discard a
possible formation of complex 9 and insertion of adventitious CO2 into the metal-O bond, we
performed the reaction with labelled 13CH3OH and validated the incorporation of labeled
carbon in the carbonate product 11 and the subsequent release of 13CO2.
The molecular structures of compounds 3 (Scheme^^1<xschr1>), and 4,[34] 6, 7, 8, 9,
10, and 11 (Scheme^^2<xschr2>) were determined by X-ray diffraction methods using a
single crystal of each compound. Plots of the structures are shown in Figure^^2<xfigr2>. The
structure of complex 3 (Figure^^2<xfigr2>b) shows a close binding interaction between the
Ru center and the two amino and two olefin groups. The coordination sphere around the metal
center is almost planar [=11.4(1) of the planes defined by both N<C->Ru-ct, ct=centroid as
centroid of the C<C=>Ctrop] as expected for a d8-metal center with two -accepting and two
-donating binding sites each in cis- and mutually in trans-position.[37] The bonding
parameters and coordination geometry in 3 closely resemble those of the previously reported
DAE complex 1. Metal-to-ligand π-backdonation from the d-orbitals at the metal to the π∗-
orbitals of the olefins significantly elongates the C<C=>C bonds to 1.433(3) Å compared to
those in the free ligand [1.338(3) Å]. The bond lengths within the propyl-backbone are similar
to those of the 1,3-diaminopropane molecule. As previously reported,[34] the Ru species 4
shows a slightly distorted square-planar structure [=+13.2(1)°] and as such resembles other
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-diketiminato (“NacNac”) Ru complexes with a fully dehydrogenated planar backbone and
roughly symmetrical bonds. When comparing complexes 3 and 4, one can observe that 4
binds much more tightly to the nitrogen atoms with bond distances significantly shorter than
in complex 3 [2.178(2) Å and 2.192(2) Å for 3 versus 2.057(1) Å and 2.056(1) Å for 4].
However, this shorter bonding is not replicated in the Ru-ct, where complex 3 actually
features shorter distances [1.990(2) Å and 1.987(2) Å for 3, and 1.999(1) Å and 2.005(1) Å
for 4]. Complex 4 shows comparable backdonation into the olefins to 3, if measured by the
elongation of the coordinated olefins, albeit more symmetrically distributed [1.433(3) Å and
1.446(3) Å for 3, and 1.448(2) Å and 1.444(2) Å for 4]. The largest difference is present in the
bond distances located in the backbone of the trop-ligand. Here the transformation of the CH2
groups into methylidyne groups results in a significant bond shortening from 1.483(3) Å,
1.516(3) Å, 1.512(3) Å, and 1.489(3) Å along the N-(CH2)3-N backbone of 3 to 1.327(2) Å,
1.391(2) Å, 1.396(2) Å, and 1.326(2) Å along the backbone of 4. Both the distances between
backbone atoms and those between metal center and imines are remarkably shortened,
suggesting a delocalized metalloaromatic system.[38]
All complexes 7--11 preserve a hydrogenated diaminopropane ligand and with
exception of the dicarbonyl species 8, in all cases the whole tetradentate ligand remains
coordinated to the metal. Complex 8 features the longest C<C=>Ctrop bond out of all the
complexes with 1.463(6) Å. The uncoordinated double bond measures 1.344(6) Å which,
expectedly, is comparable to that of the free ligand. It also features the longest Ru<C->N
bond with 2.278(3) Å on the uncoordinated side. It is interesting to note that complexes 6
through 11 (with the exception of 8) all have Ru<C->N bonding that is shorter than that of 3.
For the Ru-ct bonding, complexes 6 to 11 all feature significantly longer bonds [2.010(6)--
2.065(7) Å] than both 3 and 4 [1.987(2)--2.005(1)Å], presumably due to the additional
substituents causing additional steric bulk around the metal center. It may also be the reason
for which complexes 6, 7, 9, 10, and 11 have smaller torsion angles around the metal center
9
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and deviations from a planar structure. Complex 6 and 9 have the smallest angles  of 0.1(1)
and 0 (due to symmetry), respectively. Complexes 7, 10, and 11 have slightly higher
distortions but not as severe as in 3 and 4. The elongation of the C<C=>Ctrop groups when
compared to those of the free ligand is apparent in all of the presented complexes. We
previously published the complex [Ru(trop2DAD)(PPh3)(CO)] and its role as an intermediate
in the catalytic dehydrogenation of aqueous formaldehyde.36 Complex 8 closely resembles the
mentioned ruthenium complex since both feature a bridged trop-type ligand that is only
partially coordinated and CO as a further ligand. Both complexes appear in a distorted
trigonal bipyramidal structure where the axial positions are occupied by an amine and a
carbonyl ligand. Complex 8 has tighter bonding to the trop double bond [2.023(3) Å versus
2.068(2) Å] and stronger backdonation from the metal, resulting in a slightly more elongated
C<C=>Ctrop bond [1.463(6) Å vs.1.445(5) Å]. The axial Ru-CO bond distance in 8 is shorter
[1.830(4) Å versus 1.857(4) Å)] as well the CO bond [1.156(5) Å versus 1.166(4) Å].
Pertinent interatomic distances and angles are given in Figure^^2<xfigr2> and Tables^^S11
and S12 in the supporting information.
The results obtained in stoichiometric experiments, encouraged testing of complexes 3
or 4 as catalysts for the dehydrogenative coupling of alcohols and water to carboxylic acids. A
comparison of the catalytic activity of complexes 1--4 is shown in Table^^1<tabr1>. In an
initial screening, the reaction was performed in the absence of an organic co-solvent.
Table^^1<xtabr1> shows that the catalytic performance between the complexes with saturated
backbone (1 and 3 - entries^^1, 3) versus those with an unsaturated backbone (2 and 4 --
entries^^2, 4) is relatively small but in favour of 1 and 3 by 16--19^%. While complexes 1, 3
and 4 were tested without additional co-solvent, complex 2 was tested using dioxane, due to
its lower solubility in an aqueous solution. We have also tested all complexes (1--4) using
toluene as co-solvent. Under these conditions, complexes 1 and 2 where practically inactive
(entries^^5 and 6) while complexes 3 and 4 show similar high activity, with a slight
10
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preference for complex 3 (entries^^7 and 8). Complexes 1 and 2 remain insoluble when
toluene is used as co-solvent. Turnover frequencies (TOF) after 1, 3 and 24^^h in water-
toluene mixtures indicate that complex 4 is a slightly better catalyst that complex 3 (see
entries^^5--8 in Table^^1<xtabr1>). After 24^^h the conversion rate drops probably because
the -ketiminate derivative shows lower stability under these reaction conditions.
The best conditions were obtained with 0.5^^mol% of the zero-valent complexes
[Ru(trop2DAP)] 3 or K[Ru(trop2IKA)] 4 in a mixture of water and toluene as solvent and at
T=120^°C (entries^^7 and 8, Table^^1<xtabr1>). We have also tested the catalytic activity of
the precursors 3-Cl2 and [RuCl2(PPh3)3] (entries^^9 and 10, Table^^1<xtabr1>) under the
optimal conditions applied with catalysts 3 and 4. These control experiments confirmed the
essential role of the tetradentate ligand (entry^^9) and the preference for a fully reduced
catalyst (entry^^10). We have also tried to identify the nature of the Ru complex after the
catalytic dehydrogenation of benzyl alcohol reaction ceded using a slightly higher catalyst
load (1^^mol%). Analysis of the reaction mixture shows that there is only one ruthenium
complex present in the organic phase (toluene). This complex was identified as benzoate
hydride complex [RuH(O2CPh)(trop2DAP)] (7-Bz) which was characterized spectroscopically
and has a structure similar to complex 7. The composition of 7 is further confirmed by
MALDI-FT-ICR (See Supporting Information). With optimized reaction conditions
established (entry^^7, Table^^1<xtabr1>), we investigated the versatility of catalyst 3 in the
DHC reaction of several alcohols. As Table^^2<tabr2> shows, benzyl alcohol (entry^^1) and
aliphatic alcohols (entries^^2 and 3) were converted with about the same efficiency. 2-Furoic
acid was isolated in rather low yield (35^%, entry^^4). The reaction with 1,2-propanediol as
substrate led to the formation of lactate in acceptable yield (55^%) but required an increased
catalyst loading of 2^% (entry^^5, Table^^2<xtabr2>). Complex 4 was also tested in the
catalytic conversion of this substrate, but disappointingly gave a very low yield of product
(entry^^6). In our previous stoichiometric studies with complex 3, facile dehydrogenation of
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aqueous methanol at T<60^°C and the formation of carbonate complexes was observed (10
and 11 in Scheme^^2<xschr2>) without adding a base. Moreover, decarboxylation of the
carbonate compound 11 under release of CO2 and H2 and formation of complex 9 occurred
quantitatively under mild conditions. These observations prompted us to test the catalytic
reforming of aqueous methanol without further additives and under mild conditions. Indeed,
complex 3 is able to catalyze the complete dehydrogenation of MeOH to 3 equivalents of H2
and CO2 at 60^°C, achieving a TON of up to 102.
For the exploration of the scope and limitation of the catalytic system in the direct
synthesis of amides from amines and alcohols, 1^^mol% of catalyst 3 was reacted in toluene
as solvent at T=120^°C (Table^^3<tabr3>). Excellent yields of amides were obtained from
the reaction of sterically unhindered alcohols and amines, including the intramolecular
amidation using 5‐aminopentanol (entries^^1--8, Table^^3<xtabr3>). In the amidation of
methanol with 1‐aminohexane, 86^% isolated yield of the corresponding formamide
(entry^^1) were obtained which is a significant improvement over previously reported
catalytic reactions with primary amines.[22] The reaction of benzyl alcohol with aliphatic
amines bearing a substituent in -position or aniline afforded lower yields (52--63^%) of the
corresponding amide (entries^^9--11, Table^^4<tabr4>). Secondary amines are not converted
(entries^^12 and 13). These results indicate that the ruthenium‐catalyzed direct amide
formation is clearly sensitive to steric hindrance. The transformation using a chiral amine
proceeds under retention of the stereo-center (entry^^9).
After these successful catalytic dehydrogenation reactions, the activity of complex 3 as
catalyst for hydrogenation reactions was studied as well (Table^^4<xtabr4>). With
0.05^^mol% of catalyst, 8^^atm of H2, and toluene at 80^°C, the cyclic ester given in
entry^^1 is cleanly converted to the corresponding dihydroxy compound. Hydrogenation of
methyl formate is rather challenging, and in THF at 65^°C only 14^% conversion (TON=280)
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was achieved (entry^^2). The conversion was improved significantly to a TON of 1’000, in
toluene at 80^°C (entry^^3). The hydrogenations of butyl butyrate and methyl benzoate
proceed with moderate yields (entries^^4 and 5). For methyl benzoate, benzaldehyde was
detected in less than 1^% and transesterification of benzyl benzoate took place with ca. 7^%
yield. Amides, either primary or secondary, are not hydrogenated under these reaction
conditions (entries^^6 and 7), which is of interest for chemoselective hydrogenations of
compounds in which both, ester and amide functions, are present. Indeed, the methyl
pyrrolidencarboxylate was cleanly and selectively hydrogenated to the corresponding alcohol,
while the amide moiety was not reduced (entry^^8). The tolerance of the presence of some
additional functional groups was investigated. Ethyl levulinate, which contains an additional
keto group in -position (entry^^9), was preferentially hydrogenated at the keto function and
under the reaction conditions 71^% ethyl 4-hydroxypentanoate and only 29^% of the fully
hydrogenated product 1,4-pentanediol/ethanol was obtained. Consequently, the aldehyde
function in hydroxymethylfurfural (HMF), a platform molecule from biomass,[39] was reduced
to 2,5-bis-(hydroxymethyl)furan (DHMF) in 52^% isolated yield (entry^^10). This process is
of interest because HMF can be obtained directly from lignocellulosic biomass and DHMF is
a valuable precursor for polymeric materials.[40] Dimethyl itaconate, which contains an ester
and an additional ,-unsaturated ester function, was hydrogenated under the uptake of three
equivalents of H2 to 2-methyl-1,4-butanediol with excellent yield (entry^^11). Finally, the
hydrogenation of an imine was successfully achieved with excellent yield (entry^^12).
Conclusions
A new d8-valence electron configured Ru(0) catalyst [Ru(tropN(CH2)3Ntrop)] (3)
which has no phosphine function is an active catalyst for the dehydrogenative coupling of
alcohols with water or primary amines to give the corresponding carboxylates or amides,
respectively. No further additives are needed. The principle of microscopic reversibility
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implies that 3 may also be an efficient hydrogenation catalyst. While this has not been
observed for amides up to 80^°C and 8^^atm of H2, other carbonyl functions such as
aldehydes, ketones, and esters are hydrogenated under neutral and mild conditions (low H2
pressure and temperature). Also C<C=>N bonds in imines are hydrogenated to the
corresponding amines. Methyl formate and HMF, both challenging substrates, are converted
although with lower efficiency.
The 1,3-diaminopropane moiety in complex 3 undergoes dehydrogenation forming a
-ketiminate ligand under thermal conditions in the presence or absence of base.[23] The
process is reversible and complex 4 can be converted to 3 by hydrogen transfer reaction with
an alcohol as hydrogen source. Although we cannot discard the direct participation of both
complexes in the same catalytic cycle, most likely, both are catalysts on different reaction
pathways as previously calculated for related complexes with a saturated DAE or unsaturated
DAD ligand (Scheme^^3<schr3/o>).[33,36,41] Here we propose that dehydrogenation of the
substrate (alcohol, hemiacetal or hemiaminal) by complex 4 occurs via a Noyori-Morris-type
mechanism to form complex 4^A, as previously proposed for the DAD complex 2.[33]
Subsequent dehydrogenation of this intermediate could occur directly or via substrate/solvent-
assisted pathways to close the cycle. A different mechanism is proposed for complex 3, which
undergoes a rearrangement to form the hydride complex 3' containing a Ru(II) center. Initial
dehydrogenation of an alcohol by complex 3’ would form the active species 3^A. This
alkoxide intermediate will start the catalytic cycle via a metal-centered pathway leaving the
ligand nitrogen (DAP) in a protonated state throughout the entire cycle. Note that the isolation
of the methoxide complex 9, carboxylate complexes 7 and 7-Bz and carbonates 10 and 11
support this assumption. But we note that both proposals are highly speculative and are
simply based on previous studies with complexes 1 and 2. Further mechanistic studies are
required to gain insight into these very complex reaction mechanisms which are currently in
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10.1002/cctc.201901739
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progress. Additionally, a detailed kinetic investigation will be accomplished and reported in
due course.”
Acknowledgements
The authors acknowledge the support from the <cgs>Schweizer Nationalfonds</cgs> (SNF,
No. <cgn>2-77199-18</cgn>) and the <cgs>Eidgenössische Technische Hochschule
Zürich</cgs>. The authors would like to thank Dr. Michael Wörle for his assistance in the
XRD analysis, and Dr. Bruno Pribanic for fruitful discussions.
Conflict of Interest
The authors declare no conflict of interest.
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Figure^^1
a) Metal ligand cooperativity in catalyzed dehydrogenation and hydrogenation
processes. b) Milstein’s PNN pincer, Beller’s PNP catalyst, and an amide Rh(I) complex. The
chemical and redox non-innocent behavior of Ru DAE / Ru DAD complexes (1 and 2) and
the new Ru DAP/Ru IPA complexes (3 and 4). Cooperative sites of the ligands are indicated
in blue.
Figure^^2
ORTEP drawings of the molecular structure of trop2DAP (a), 3 (b), 6 (c), 7 (d),
8 (e), 9 (f) 10 (g) and 11 (h). Thermal ellipsoids are drawn at a 50^% probability level.
Hydrogen atoms and solvent molecules have been omitted for clarity. ct1 and ct2 are the
centroids of the C<C=>C bonds C4<C->C5 and C19<C->C20 respectively. ct3 and ct4 are
the centroids of the C<C=>C bonds C37<C->C38 and C52<C->C53 respectively Selected
bond distances [Å] and angles [°]: a) trop2DAP: C4<C->C5 1.338(3), C19<C->C20 1.347(2),
N1<C->C31 1.471(2), C31<C->C32 1.523(2), C32<C->C33 1.517(2), C33<C->N2 1.471(2).
b) 3: C4<C->C5 1.433(3), C19<C->C20 1.446(3), Ru1<C->N1 2.178(2), Ru1<C->N2
2.192(2), Ru1<C->ct1 1.990 (2), Ru1<C->ct2 1.987(2); N2<C->Ru1<C->N1 89.3(1), N1<C-
>Ru1<C->ct1 89.2(1), N2<C->Ru1<C->ct2 89.6(1). c) 6: C4<C->C5 1.425(7), C19<C->C20
1.429(7), Ru1<C->N1 2.157(5), Ru1<C->N2 2.154(4), Ru1<C->ct1 2.043(5), Ru1<C->ct2
2.010(6), Ru1<C->O1 2.140(4), Ru<C->H1 1.47(5); N2<C->Ru1<C->N1 90.3(2), N1<C-
>Ru1<C->ct1 87.9(2), N2<C->Ru1<C->ct2 88.7(2), O1<C->Ru1<C->H1 154(2); d) 7:
C4<C->C5 1.412(3), C19<C->C20 1.417(3), Ru1<C->N1 2.148(2), Ru1<C->N2 2.158(2),
Ru1<C->ct1 2.054(2), Ru1<C->ct2 2.042(3), Ru1<C->O1 2.226(2), Ru<C->H1 1.51(2);
N2<C->Ru1<C->N1 87.5(1), N1<C->Ru1<C->ct1 89.3 (1), N2<C->Ru1<C->ct2 89.3(1),
O1-Ru1-H1 165.1(1). e) 8: C4<C->C5 1.344(6), C19<C->C20 1.463(6), Ru1–N1 2.278(3),
Ru1<C->N2 2.211(3), Ru1<C->ct2 2.023(4), Ru1-C34 1.830(4), Ru<C->C35 1.899(4), C34-
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O1 1.156(5), C35-O2 1.157(5) Ru<C->H1 1.51(2); N2<C->Ru1<C->N1 85.1(1), N2<C-
>Ru1<C->ct2 86.6(1), C34-Ru1-C35 88.5(2). f) 9: C4<C->C5 1.431(4), N1<C->C16
1.487(2), C16<C->C17 1.516(2), Ru1<C->N1 2.172(1), Ru1<C->ct1 2.042(1), Ru1<C->O1
2.192(3), Ru<C->H1 1.45(4); N1<C->Ru1<C->N1’ 90.2 (1), N1-Ru1-ct1 88.0(1), O1<C-
>Ru1<C->H1 158.0(9). g) 10: C4<C->C5 1.405(7), C19<C->C20 1.406(7), Ru1<C->N1
2.154(4), Ru1<C->N2 2.140(4), Ru1<C->ct1 2.042(4), Ru1<C->ct2 2.054(5), Ru1<C->O1
2.211(3), Ru<C->H1 1.60(4), O1<C->C34 1.252(6), C34<C->O2 1.229(7), C34<C->O3
1.368(7), O3<C->C35 1.426(7); N2<C->Ru1<C->N1 86.9(2), N1<C->Ru1<C->ct1 89.0(2),
N2<C->Ru1<C->ct2 89.8(2), O1<C->Ru1<C->H1 167(1), O1<C->C34<C->O2 128.8(5),
O2<C->C34<C->O3 115.3(5), C34<C->O3<C->C35 116.3(4). h) 11: C4<C->C5 1.41(1),
C19<C->C20 1.41(1), Ru1<C->N1 2.163(5), Ru1<C->N2 2.168(5), Ru1<C->ct1 2.064(7),
Ru1<C->ct2 2.039(7), C37<C->C38 1.39(1), C52<C->C53 1.41(1), Ru2<C->N3 2.162(5),
Ru2<C->N4 2.176(5), Ru1<C->ct3 2.065(7), Ru1<C->ct4 2.061(7), Ru1<C->O1 2.249(4),
Ru<C->H1 1.58(5), Ru2<C->H2 1.57(4), Ru2<C->O2 2.254(4), O1<C->C67 1.302(8),
C67<C->O2 1.298(8), C67<C->O3 1.265(8); N1<C->Ru1<C->N2 89.7(2), N1<C->Ru1<C-
>ct1 87.9(2), N2<C->Ru1<C->ct2 88.6(2), O1-Ru1-H1 162(3), N3<C->Ru2<C->N4 90.7(2),
N3<C->Ru2<C->ct3 87.9(2), N4<C->Ru2<C->ct4 87.3(2), O2<C->Ru2<C->H2 158(2),
O2<C->C67<C->O3 122.2(6), O3<C->C67<C->O1 121.8(6).
Scheme^^1 Synthesis of trop2DAP ligand and coordination to ruthenium.
Scheme^^2 a) Interconversion of complexes 3 and 4 and their reactivity towards water. b)
Summary of stoichiometric experiments of complex 3 with aqueous 1,2-propanediol or
methanol and under anhydrous conditions.
Scheme^^3 Proposed mechanisms of alcohol dehydrogenation catalyzed by complexes 3 or
4.
Table^^1
<forr1>
Catalyst screening for the catalyzed DHC of benzyl alcohol and water.<W=3>
Entry Catalyst
Solvent
TOF [h<M->1
]
Yield [%][a]
1^^h 3^^h
24^^h
22
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1
2
3
4
5
6
7
8
9
10
H2O
--
--
--
81
1
H2O/dioxane --
--
--
65
87
68
<5
6
2
H2O
--
--
--
3
H2O
--
--
--
4
H2O/toluene
H2O/toluene
H2O/toluene
H2O/toluene
H2O/toluene
H2O/toluene
1
0.6
4
0.2
0.5
4.6
3.7
--
1
7
2
14.5
12.6
--
10.4
16.1
--
90
86
<5
56
3
4
RuCl2(PPh3)3
3-Cl2
--
--
--
[a] NMR yields of potassium benzoate for reactions conducted on 10^^mmol scale of BnOH
(1.0^^equiv), KOH (1.1^^equiv), [Ru] (0.5^^mol%) in water (5^^mL) or mixtures with
toluene or dioxane (2^^mL), during 48^^h at 120^°C.
Table^^2
<forr2>
Catalyzed oxidation of primary alcohols to carboxylic acids[a].<W=3>
Entry
<forr3>
Product
Catalyst
[mol^%]
Isolated
yields [%]
1
2
3
4
<forr4>
<forr6>
<forr8>
<forr10>
<forr5>
<forr7>
<forr9>
<forr11>
3 (0.5)
3 (0.5)
3 (0.5)
3 (0.5)
75
72
68
35
23
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5
6
<forr12>
<forr14>
<forr13>
<forr15>
3 (2)
4 (2)
55[b]
7[b]
[a] Reaction conditions: alcohol (1.0^^equiv), water/toluene (2.5^:^1, 1.4^^M), KOH
(1.1^^equiv), reflux, 48^^h, [b] A mixture of water/toluene (4^:^1, 0.4^^M) was used. Other
products detected in this reaction include potassium acetate and polylactic acid
Table^^3
DHC of primary alcohols and amines to amides catalyzed by 3.<W=3>
<forr16>
Entry
1
<forr17>
CH3OH
<forr18>
<forr20>
<forr19>
<forr21>
Yield [%][a]
86
2
3
CH3CH2OH
<forr24>
<forr22>
<forr25>
<forr23>
<forr26>
98
99
4
<forr27>
<forr28>
<forr29>
98
5
6
<forr30>
<forr33>
<forr31>
<forr34>
<forr32>
<forr35>
90
96
24
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7
8
<forr36>
<forr39>
<forr37>
<forr42>
<forr38>
93
87
<forr40>
<forr43>
9
<forr41>
58 (>99^% ee)
10
11
<forr44>
<forr47>
<forr45>
<forr48>
<forr46>
<forr49>
63
52
12
13
<forr50>
<forr52>
<forr51>
<forr53>
-
-
0
0
[a] Isolated yields of amide products for reactions conducted on 1.0^^mmol scale of alcohol
(1.0^^equiv), amine (1.1^^equiv), [Ru] (1.0^^mol%) in toluene (2^^M), at 120^°C during
15^^h.
Table^^4
<forr54>
Hydrogenation of polar bonds catalyzed by complex 3.<W=3>
Entry
Substrate
Product
Yield [%][a]
1
2
3
<forr55>
HCOOMe
HCOOMe
<forr56>
CH3OH
CH3OH
80[b]
14[c]
50
25
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4
5
6
7
8
<forr57>
<forr58>
BnOH, MeOH
-
67[d]
61[b,e]
0[d]
PhCOOMe
PhCOONH2
<forr59>
-
0[d]
<forr60>
<forr61>
+MeOH
<forr63>+<forr64>
+EtOH
>99[f]
9
<forr62>
>99[f]
10
11
<forr65>
<forr67>
<forr66>
<forr68>
+2 MeOH
<forr70>
52[f]
>99[f]
12
<forr69>
>99[f]
[a] General reaction conditions: Substrate (1.0^^equiv), 3 (0.05^^mol%) in toluene at 80^°C,
8^^bar H2 during 18^^h. The given yields were determined by GC-FID, except for methyl
formate, which was calculated by 1H NMR, [b] 3 (1^^mol%) in THF at 65^°C, 8^^bar H2,
overnight, [c] In THF at 65^°C, 8^^bar H2, overnight, [d] 3 (1^^mol%), [e] Benzyl benzoate
was detected in ca. 7^%, [f] 3 (0.5^^mol%).
26
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