Precatalyst for Hydrogenation and - ACS Publications - American

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Study of Precatalyst Degradation Leading to the Discovery of a New Ru0 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 Downloaded via DURHAM UNIV on June 20, 2018 at 17:01:33 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

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(CH2CH2PPh2)3)CO] (5), was prepared independently and studied. 5 was found to be active, entirely additivefree, 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 RuII−hydrido species, thus presenting a rare example of reversible oxidative addition from Ru0 to RuII and reductive elimination from RuII to Ru0.

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terized for [Ru(N(CH2CH2PiPr2)2)H(CO)]23 and [Ru(N(CH2CH2PtBu2)2)H(CO)],9,27 yet to the best of our knowledge, such species from 1 have hitherto not been isolated.29 Notwithstanding, addition products from the assumed unsaturated species and the coordination of a substrate (e.g., alcohol, hydrogen, amine) have been identified,20,29 observed as resting states in reaction mixtures,20 or isolated and used as precatalysts.3,9 For instance, the BH4 adduct, commercially named Ru-MACHO-BH,1 is formed by the reaction of 1 with NaBH4. Ru-MACHO-BH is an efficient base-free homogeneous catalyst in hydrogenation reactions.9,16,30,31 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 required.1,2,18,30,32 It is thus of high importance and scientific 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.

INTRODUCTION The commercially available Ru-MACHO complex 1 (RuMACHO1 = [Ru(HN(CH2CH2PPh2)2)H(Cl)CO]; Scheme 1) has been demonstrated to be a highly robust precatalyst for Scheme 1. Dehydrochlorination of 1 to the Presumed Unsaturated Complex 229

numerous important transformations: hydrogenation of esters,2−8 dehydrogenation of alcohols9−12 and amines,13 formic acid decomposition,14 carbon dioxide reduction,14−19 and more.13,20−26 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 hydrochloride loss and formation of an unsaturated RuII−hydrido species (depicted as 2; Scheme 1) is key to catalysis, as demonstrated for other analogous pincer complexes.3,20,27−29 Such unsaturated species could be synthesized and charac© XXXX American Chemical Society

Received: May 25, 2018

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

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Organometallics 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, one of these species was a Ru0 complex bearing a tetradentate ligand,33 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.

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RESULTS AND DISCUSSION 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.27 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 spectroscopy, 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 details see the Supporting Information). The signals in the 1H 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 to an internal standard). The 31P{1H} NMR spectrum showed multiple new signals in the range from 30 to 70 ppm, some of which had multiplicities. A similar experimental procedure was conducted with potassium tert-butoxide (KOtBu) as base. Although the reaction with KOtBu 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 active species.13,21,22,29 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 with KOtBu 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 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.

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 apparatus.34 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). One structure depicted a ruthenium dimer complex (complex 3; Figure 2), in which one of the ligand’s carbon−

Figure 2. Molecular structure of 3 as determined by single-crystal Xray 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 ESIMS mass at m/z 1141.15 ([3]H+; Scheme 2). Dimer formation from unsaturated pincer intermediates has been reported by Spasyuk et al.35 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 metals.36,37 B


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The trigonal-bipyramidal geometry confirms an 18-electron Ru0 complex. The overall picture depicted by the observation of these three species was quite confounding: the well-defined RuII complex 1 underwent a previously unreported disproportionation at the “PNP” ligand, leading to complexes bearing “NP” and “NP3” type ligands. Most astounding, a Ru0 species formed under seemingly redox neutral conditions. Reactivity of Complex 5. Complex 1 is known to be a highly efficient catalyst for hydrogenation and dehydrogenation 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 amino ligand and a Ru0 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

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: Ph2(CH2)2NH (complex 4; Figure 3). To the best of our

Scheme 3. Synthesis of Complex 5 Figure 3. Molecular structure of 4 as determined by single-crystal Xray 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 cleavage) has not yet been reported. The depicted structure fits well to the observed ESI-MS signal of m/z 928.07 ([4]H+; Scheme 2). A third crystal structure obtained from the reaction of 1 with KOtBu was a tripodal Ru0 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.

published procedures.38 The reaction of 6 with Ru3(CO)12 (7) was reported by Fontal;33 however, the product was not 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. 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. Since complex 5 can be readily synthesized from the Ru0 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 knowledge, the first reported well-defined homogeneous Ru0 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 with H2 (Scheme 4). Quantitative conversion back to 8b was 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.

Figure 4. Molecular structure of 5 as determined by single-crystal Xray 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, 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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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 clear yellow, and the formation of the RuII hydrido complex 10a (Scheme 6) was clearly observed by NMR spectroscopy.

Table 1. Catalytic Additive-Free Dehydrogenative Coupling of Alcoholsa

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 toluene) under argon for 18 h. bConversion by area percent in GCFID. cPercent yield of product, isolated by SiO2 chromatography. d Percent yield determined by 1H NMR after removal of all volatiles in vacuo. e1 mol % of catalyst, 24 h reaction; conversion confirmed by 1 H NMR.

In the 1H NMR spectrum of complex 10a, a hydride signal appears as a doublet of triplets (−6.5 ppm; 2JPH‑trans = 83 Hz, 2 JPH‑cis = 22 Hz); in the 31P{1H} NMR spectrum a doublet (56 ppm, 2JPP = 15 Hz) and a triplet (45 ppm, 2JPP = 15 Hz) 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. 15N−1H 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 deshielded quaternary ammonium atom,39,40 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, reducing the complex back to Ru0 complex 5. To the best of our knowledge, this reversible protonation, leading to a reversible Ru0/RuII system, is quite unique for ruthenium.41−43 Layering a methanol/benzene solution of 10a with diethyl ether and pentane yielded crystals suitable for X-ray crystallography. 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 depicts a cationic, distorted-octahedral RuII−H species, which confirms the structural assignment of 10a by NMR spectroscopy. 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

Scheme 4. Additive-Free Dehydrogenative Coupling of 8b to 9b (a) and the Reverse Reaction (b)

Ethyl acetate (9d) was hydrogenated neat under 60 bar of H2 pressure (at room temperature) for 40 h at 130 °C (pressure at the reaction temperature reached ca. 80 bar). A catalyst loading of 0.016 mol % of 5 resulted in a 91% yield of ethanol, corresponding to a turnover number (TON) of ca. 5700 (Scheme 5a). Despite these results, when dimethyl terephthalate (DMT, 9e) hydrogenation was attempted in toluene, with 1 mol % of 5 under similar conditions (Scheme 5b), no reaction was observed. Since 5 showed activity toward dehydrogenation of alcohols, completely additive free, it seemed probable that the alcohols Scheme 5. Additive-Free Hydrogenation of 9d (a) and Attempted Hydrogenation of 9e (b)

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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(NP3)H(CO)]+ could be assigned. 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). In order to confirm that the observed RuII−H was formed by protonation (rather than C−H activation), several experiments were carried out (Scheme 8). (i) The reaction of complex 5 with CH3OD resulted in a complex with the same NMR spectral features as the [Ru(NP3)H(CO)]+ fragment of 10a, albeit no hydride signal was observed in the 1H NMR spectrum. A higher multiplicity of the 31P{1H} NMR signals asserted that the complex [Ru(NP3)D(CO)]OMe (complex 10a-d) was formed (see spectra in the Supporting Information). (ii) The reaction of CD3OH with complex 5 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, after heating to 80 °C full conversion to the new RuII−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 equiv of benzoic acid led to the expected cationic RuII−H complex 10c, with a benzoate counterion, which could also be observed by X-ray crystallography (Figure 6). The cation fragment [Ru(NP3)H(CO)]+ in the benzoate complex 10c 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.

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 (deg): Ru1−N1, 2.235(2); Ru1−P1, 2.362(1); Ru1−P2, 2.362(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).

Complex 10b also showed catalytic activity toward hydrogenation of 9e (see Supporting Information), albeit significantly 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 hydrogenation 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. Possibility of a Ru0/RuII 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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Organometallics complex 2 (Scheme 1).13,21,22 Complex 5 might follow a similar mechanism after activation by alcohol, perhaps through ligand dissociation. Moreover, as shown in this work, several organometallic species may form under the reaction conditions, 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 in catalysis through a Ru0/RuII cycle. Ru0 intermediates have been cautiously suggested in related catalytic systems44−47 and proposed in other ruthenium catalytic cycles48−50 but are not yet supported by experimental observations.42,43,50 Hanton et al. have reported an N-triphos-based Ru0 complex (N-triphos = tris(diphenylphosphinomethyl)amine) as a catalyst in the hydrogenation of esters.51 In the case of the complex [Ru(N-triphos)(CO)2] a strong oxidant was required in order to activate the precatalyst. Nevertheless, the authors suggested that a 14-electron Ru0 species was an intermediate in the catalytic cycle, also when starting from Ru(acac)3/Ntriphos as an additive-free hydrogenation catalyst. Complex 5 is reminiscent of [Ru(N-triphos)(CO)2], albeit the onecarbon-elongated ethylene backbone in ligand 6 (versus the methylene-based N-triphos) enables nitrogen coordination to the metal center. This stabilizes a Ru0 monocarbonyl complex which is nevertheless active toward oxidative addition. Another analogous case is that of the reported PP3 complexes of ruthenium (PP3 = P(CH2CH2PPh2)3). Bianchini et al. observed the dihydride complex [Ru(PP3)(H)2],52 which lost dihydrogen to yield the corresponding [Ru(PP3)CO] upon irradiation under a CO atmosphere.53 Catalytic activity of the PP3 ruthenium complexes (not related to this work) was limited to the noncarbonyl derivatives.54 Irreversible oxidative additions to the Ru0 complex [Ru(triphos)(CO)2] (triphos = 1,1,1-tris(diphenylphosphinomethyl)ethane) have also been reported by Hommeltoft et al.43e On the basis of the accepted hydrogenation or dehydrogenation mechanisms for similar ruthenium catalysts,20,28,29 one might expect a complex of the form {[Ru(NP3)(H)2CO]} to play an active role. Thus, the precursor [Ru(PPh3)3(H)2CO] (13; Scheme 9a) was heated for 16 h with the ligand 6, in

[Ru(triphos)(CO)2] and [Ru(N-triphos)(CO)2] also showed no direct reaction with H2.43e,51 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(CO)]+ fragment, which was also studied computationally in the past in another context.44 Other plausible catalytic cycle steps are currently being studied by computational methods in our laboratory. Although Ru0 species may well be involved in catalysis, it is of high certainty that heterogeneous Ru0 species (as nanoparticles, or Ru black) are not present in the reaction mixture. The fact that hydrogenation of the arene ring was not observed55,56 and the recyclability of the catalyst after reaction attest to that fact.

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SUMMARY We were able to show that the widely used Ru-MACHO1 (complex 1) can degrade under basic conditions, which are 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 shown that the tripodal Ru0 complex 5, found as a decomposition product of 1, is a competent base-free catalyst for the hydrogenation of esters and additive-free dehydrogenative 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(NP3)H(CO)]OR (complex 10), obtained by O−H activation, is likely the catalytically active species. We could clearly show a unique reversible oxidative addition of alcohols to the Ru0 complex 5. 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.

Scheme 9. Formation of 5 from a Dihydride Precursor (a) and Reaction of 5 with H2 (b)

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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 literature procedure.38 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 50,0 and Bruker AVANCE III 600 spectrometers at the OrganischChemisches Institut der Universität Heidelberg. 1H and 13C chemical shifts δ are reported in ppm relative to either the residual solvent57 or tetramethylsilane (TMS). 13C assignment was achieved via DEPT90 and DEPT135 or HSQC spectra. 31P chemical shifts are reported relative to an external standard of phosphoric acid 30% in D2O (0.0

benzene-d6. However, the only product observed was the Ru0 complex 5, the result of dihydrogen loss (see the Supporting Information). As seen in the case of the methoxy complex 10a, the RuII tripodal species tends to undergo reductive elimination and revert to the parent Ru0 complex 5. Moreover, complex 5 showed no measurable change when it was pressurized under H2 (see Scheme 9b and the Supporting 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 F


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Organometallics

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 tertbutoxide (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(NP3)CO], Complex 5). In a dry argon-filled glovebox, a 100 mL glass Schlenk flask was charged with Ru3(CO)12 (7; 0.162 g), tris(2-(diphenylphosphaneyl)ethyl)amine (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 bright orange powder (445 mg, 74% yield). 1H NMR (400 MHz, benzene-d6, only partial assignment due to very poor solubility): δ 7.53 (bm, Ph-CH2,6), 7.1−6.9 (m, Ph-CH3,4,5), 2.1 (b, NCH2), 2.1−1.9 (bm, CH2P) ppm. 13C{1H} NMR (DEPT135, 150 MHz, benzene-d6, verified by 1H−13C HSQC, only partial assignment due to very poor solubility): δ 131 ppm (b, Ph(C2,C6)) 127−128 ppm (overlapping solvent, Ph(C3,C4,C5)), 58 (bm, CH2P), 31 (bm, NCH2) ppm. 31P{1H} NMR (160 MHz, benzene-d6): δ 57 (s) ppm. 15 N−1H{31P} HMBC NMR (600 MHz, benzene-d6): δ 76 ppm. FTIR (KBr pellet): ν̅max (cm−1) 1851 (CO). FTIR (NaCl thin film): ν̅max (cm−1) 1846 (CO). Anal. Calcd for C43H42NOP3Ru: C, 65.98; H, 5.53; N, 1.79. Found: C, 65.14; H, 5.32; N, 1.77. ESI-MS (m/z): [M + H]+ calcd for C43H43NOP3Ru+, 784.1596; found, 784.1624. FDMS (m/z): [M]+ calcd for C43H42NOP3Ru+, 783.15; found, 783.06. Preparation of Tris((2-(diphenylphosphino)ethyl)amino)hydridocarbonylruthenium Methoxide ([Ru(NP3)(H)CO]OMe, Complex 10a). In a dry argon-filled glovebox, a J. Young NMR tube was charged with complex 5 (12 mg) and benzene-d6 (0.7 mL). 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 complex 10a. 1H NMR (400 MHz, benzene-d6): δ 7.89 (dt, 3JHH = 12.7, 4JHH = 4.1 Hz, 4H, P-Ph), 7.22 (dd, 3JHH = 11.4, 4JHH = 4.2 Hz, 4H, P-Ph), 7.18−7.11 (m, 2H, P-Ph), 7.01−6.86 (m, 8H, P-Ph), 6.79 (t, 3JHH = 7.4 Hz, 2H, P-Ph), 6.74−6.63 (m, 6H, P-Ph), 6.60 (td, 3JHH = 7.6, 4JHH = 2.0 Hz, 4H, P-Ph), 3.16−2.96 (m, 4H, N(CH2CH2P-)2), 2.73−2.60 (m, 4H N(CH2CH2P−)), 2.50−2.32 (m, 4H, N(CH2CH2P−)2), − 6.60 (dt, 2JPH(trans) = 83.0, 2JPH(cis) = 21.7 Hz, 1H, Ru-H) ppm. 13C{1H} NMR (100 MHz, benzene-d6): δ 205.7 (td, 2 JPC = 12.0, 2JPC = 8.8 Hz, Ru−CO), 160.0 (s, P-Ph), 138.3 (td, 1JPC = 27.3, 3JPC = 4.6 Hz P-Ph), 133.4 (t, 3JPC = 6.4 Hz, P-Ph), 130.6−130.4 (m, P-Ph), 130.0 (t, 3JPC = 5.6 Hz, P-Ph), 129.8 (d, 2JPC = 11.3 Hz, PPh), 129.3 (s, P-Ph), 129.0 (d, 4JPC = 1.9 Hz, P-Ph), 128.6 (t, 3JPC = 4.7 Hz, P-Ph), 128.5 (d, 3JPC = 4.6 Hz, P-Ph), 128.4−128.2 (m, PPh), 63.4 (t, 3JPC = 4.4 Hz, NCH2CH2P), 59.5 (d, 3JPC = 7.8 Hz, N(CH2CH2P)2), 52.4 (s, NCH2CH2P), 31.5 (d, 1JPC = 23.6 Hz, N(CH2CH2P)2), 30.6 (vt, J = 13.4 Hz, NCH2CH2P) ppm. 31P{1H}

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 chromatography 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/ 0.25 μm). All analyses (X-ray, mass spectrometry, elemental analysis) 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 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. General Procedure for the Base-Free Hydrogenation of Esters (See the Supporting Information). Caution! H2 gas forms 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 × 10−3 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) was added, and the mixture was stirred vigorously at room temperature until full dissolution. A sample (0.1 mL) was then taken for t0 analysis (GC and NMR). After it was closed, the reaction 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 2−5. ESI-MS Measurements. In a dry argon-filled glovebox, complex 1 (23 mg, 0.038 mmol) was mixed with toluene-d8 (0.7 mL) in an 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 hexamethyldisilazane (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. G


Organometallics

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NMR (81 MHz, benzene-d6): δ 55.7 (d, 2JPP = 15 Hz, apical), 44.8 (t, 2 JPP = 15 Hz, equatorial) ppm. 15N−1H HMBC NMR (400 MHz, benzene-d6): δ 78 ppm. ESI-MS (m/z) prepared in 1,2-difluorobenzene, diluted in dry DCM: [M − OMe] + calcd for C43H43NOP3Ru+, 784.1596; found, 784.1595. Preparation of Tris((2-(diphenylphosphino)ethyl)amino)hydridocarbonylruthenium Phenoxide ([Ru(NP3)(H)CO]OPh, 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), and benzene-d6 (0.7 mL). The solution was heated in a closed capped 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). 1H NMR (400 MHz, benzened6): δ 12.92 (br, 2H, free PhOH), 7.89 (dd,, 3JHH = 12.5, 4JHH = 6.5 Hz, 4H, PPh2), 7.58 (dt,, 3JHH = 3.4, 4JHH = 1.7 Hz, 6H, PhO− and PhOH), 7.34−7.26 (m, 6H, PhO− and PhOH), 7.13 (dd, 3JHH = 14.4, 4 JHH = 7.1 Hz, 4H, PPh2), 7.06−7.00 (m, 3H, PPh2), 6.98 (ddd, 3JHH = 9.8, 4JHH = 3.9, 3JPH = 1.5 Hz, 7H, PPh2), 6.80−6.73 (m, 3H, PhO− and PhOH), 6.70−6.52 (m, 12H, PPh2), 3.52 (dt, 3JHH = 23.8, 3JPH = 6.7 Hz, 2H, NCH2), 3.28 (dd, 3JHH = 33.2, 3JPH = 17.2 Hz, 2H, NCH2), 2.82 (dd, 3JHH = 13.1, 3JPH = 6.5 Hz, 2H, NCH2), 2.75−2.61 (m, 2H, CH2P), 2.40−2.17 (m, 4H, 2(CH2P)), −6.60 (dt, 2JPH(trans) = 82.5, 2JPH(cis) = 21.4 Hz, 1H, Ru−H). 13C{1H} NMR (100 MHz, benzene-d6): δ 206.7 (dt, 2JPC = 20.7, 2JPC = 11.9 Hz, Ru-CO), 139.3 (td, J = 26.9, 4.7 Hz), 134.7 (dt, J = 36.9, 2.1 Hz), 134.3 (td, J = 20.8, 1.5 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), 128.8 (d, J = 9.5 Hz), 128.6 (t, J = 5.3 Hz), 63.9 (t, 3JPC = 3.7 Hz, NCH2CH2P), 60.3 (d, 3JPC = 7.2 Hz, N(CH2CH2P)2), 32.3 (d, 1JPC = 24.0 Hz, N(CH2CH2P)2), 31.2 (vt, J = 13.3 Hz, NCH2CH2P) ppm. 31 1 P{ H} NMR (81 MHz, benzene-d6): δ 66.6 (d, 2JPP = 15 Hz, apical), 55.0 (t, 2JPP = 15 Hz, equatorial) ppm. FTIR (NaCl, thin film): ν̅max (cm−1) 1934 (CO). ESI-MS (m/z) prepared in 1,2difluorobenzene, diluted in dry DCM: [M − PhO]+ calcd for C43H43NOP3Ru+, 784.1596; found, 784.1593. Preparation of Tris((2-(diphenylphosphino)ethyl)amino)hydridocarbonylruthenium Benzoate ([Ru(NP3)(H)CO]OOCPh, complex 10c). In a dry argon-filled glovebox, a J. Young NMR tube was charged with complex 5 (15 mg), benzoic acid (2.3 mg, 1 equiv), and benzene-d6 (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 10c was monitored by 1H and 31P NMR spectroscopy in situ. The 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 colorless solution of 10c within 30 min at room temperature. 1H NMR (200 MHz, benzene-d6): δ 9.05 (dd, J = 8.2, 1.4 Hz, 2H, PhCOO), 7.96 (d, J = 5.8 Hz, 4H, PPh2), 7.53 (br, 1H, overlapping, PhCOO and PPh2), 7.53−7.43 (m, 2H, overlapping, PhCOO), 7.33 (d, J = 8.4 Hz, 8H, PPh2), 7.14−6.91 (m, 8H, PPh2), 6.67 (s, 10H, PPh2), 4.78 (d, J = 22.8 Hz, 2H, NCH2-), 3.95 (m, N(CH2-)2), 2.74− 2.15 (m, 4H, (-CH2P)2), −6.31 (dt, 2JPH(trans) = 82.4, 2JPH(cis) = 20.4 Hz, 1H, Ru-H). 13C{1H} NMR (150 MHz, dichloromethane-d2): δ 205.9 (m, Ru-CO), 170.7 (s, PhCOO), 138.4 (td, J = 26.9, 4.6 Hz, PPh2), 137.4 (b, CipsoCOO), 134.5 (m, overlapping, PPh2), 134.3 (m, overlapping, PPh2), 134.2 (s, overlapping, PhCOO), 134.0 (t, J = 6.2 Hz, PPh2), 131.0 (s, PhCOO), 130.7 (t, J = 5.4 Hz, PPh2), 130.3 (d, J = 11.2 Hz, PPh2), 130.1 (d, J = 10.1 Hz, PPh2), 129.7 (s, PhCOO), 129.4 (t, J = 4.3 Hz, PPh2), 129.0−129.2 (m, PPh2), 128.2 (s), 64.7 (t, J = 4.4 Hz, NCH2CH2P), 60.3 (d, J = 7.1 Hz, N(CH2CH2P)2), 32.6 (d, J = 24.0 Hz, N(CH2CH2P)2), 31.5 (t, J = 13.2 Hz, NCH2CH2P) ppm. 31P{1H} NMR (80 MHz, benzene-d6): δ 57.1 (d, 2 JPP = 15 Hz, apical), 46.3 (t, 2JPP = 15 Hz, equatorial). FTIR (NaCl, thin film): νm̅ ax (cm−1) 1934 (CO). ESI-MS (m/z) prepared in 1,2difluorobenzene, diluted in dry DCM: [M − PhCOO]+ calcd for C43H43NOP3Ru+, 784.1596; found, 784.1607.

Article

ASSOCIATED CONTENT

S 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. Detailed analyses, chromatograms, spectra, and crystallographic data of complexes 3−5 and 10a,c (PDF) Accession Codes

CCDC 1821685−1821689 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.S.: [email protected]. ORCID

A. Stephen K. Hashmi: 0000-0002-6720-8602 Thomas Schaub: 0000-0003-2332-0376 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS CaRLa (Catalysis Research Laboratory) is cofinanced by the Ruprecht-Karls-Universität Heidelberg (Heidelberg University) and BASF SE.

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REFERENCES

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