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

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

Link to VoR: http://dx.doi.org/10.1002/cctc.201901739

A Journal of

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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

<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

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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

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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+CO₂under 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 CO₂or 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 H₂pressures 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 H₂in the diazadiene ligand backbone.^[14b]Dehydrogenation of complex 1

releases two equivalents of H₂to 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)₂which 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 H₂and 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-d₈solution 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 CO₂into 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=>C_trop] 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=>C_tropbond 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

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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=>C_tropgroups 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.³⁶Complex 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=>C_tropbond [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

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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-Cl₂and [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 CO₂and H₂and 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 CO₂at 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 H₂to 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

13

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14

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

14

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15

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

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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

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>

<forr5>

<forr7>

<forr9>

3 (0.5)

75

72

68

35

23

10.1002/cctc.201901739

ChemCatChem

24

5

6

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>

Entry

1

CH₃OH

Yield [%]^[a]

86

2

3

CH₃CH₂OH

98

99

4

98

5

6

90

96

24

10.1002/cctc.201901739

ChemCatChem

25

7

8

93

87

9

58 (>99^% ee)

10

11

63

52

12

13

-

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

Hydrogenation of polar bonds catalyzed by complex 3.<W=3>

Entry

Substrate

Product

Yield [%]^[a]

1

2

3

HCOOMe

CH₃OH

80^[b]

14^[c]

50

25

10.1002/cctc.201901739

ChemCatChem

26

4

5

6

7

8

BnOH, MeOH

-

67^[d]

61^[b,e]

0^[d]

PhCOOMe

PhCOONH₂

-

0^[d]

+MeOH

+EtOH

>99^[f]

9

>99^[f]

10

11

+2 MeOH

52^[f]

>99^[f]

12

>99^[f]

[a] General reaction conditions: Substrate (1.0^^equiv), 3 (0.05^^mol%) in toluene at 80^°C,

8^^bar H₂during 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

Article Doi

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

DOI: 10.1002/cctc.201901739

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Authors:

Article abstract of DOI:10.1002/cctc.201901739

Full text of DOI:10.1002/cctc.201901739

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