Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Catalytic Hydrogenation of Benzonitrile by Triruthenium Clusters: Consecutive Transformations of Benzonitrile on the Face of a Ru3 Plane Toshiro Takao,*,†,‡ Sachie Horikoshi,† Takashi Kawashima,† Sachio Asano,† Yuta Takahashi,† Akira Sawano,† and Hiroharu Suzuki† †
Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan ‡ JST, ACT-C, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *
ABSTRACT: Reactions of the triruthenium clusters {Cp*Ru(μ-H)}3(μ3-H)2 (1) and (Cp*Ru)3(μ3-H)(μ-H)2(μ-CO) (12) (Cp* = η5-C5Me5) with benzonitrile were investigated in relation to the selective hydrogenation of benzonitrile to benzylamine. Benzonitrile undergoes consecutive transformations into μ3-η2:η2(⊥)nitrile, μ3-η2:η2(⊥)-imidoyl, μ3-η2(∥)-imidoyl, μ3-η2-alkylideneamido, μ3-imido, and μ-amido ligands on the Ru3 plane accompanied by the uptake of dihydrogen. The reactions are analogues of nitrile hydrogenation on a metal surface. The complexes are structural models of chemisorbed species and catalyze the hydrogenation of benzonitrile. Complex 1 catalyzes benzonitrile hydrogenation without additives but exhibits only moderate selectivity toward benzylamine. Although the μ3-benzylimido complex {Cp*Ru(μ-H)}3(μ3-NCH2Ph) (4) was obtained by reaction of 1 with benzonitrile, it was readily transformed into to the μ3-imidoyl complexes (Cp*Ru)3(μ-H)2{μ3η2:η2(⊥)-PhCNH} (3) and (Cp*Ru)3(H)4{μ3-η2(∥∥)-PhCNH} (4), which are key intermediates in secondary imine formation. Two benzonitrile molecules were incorporated on the Ru3 plane under the reaction conditions, which decreases the selectivity of primary amine formation. In contrast, μ-carbonyl complex 12 suppresses the incorporation of additional benzonitrile ligands and the formation of μ3-imidoyl species due to the presence of CO. These features of 12 bring about significant improvement in the selectivity of benzonitrile hydrogenation and produce benzylamine in 92% yield.
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INTRODUCTION Hydrogenation of nitriles using heterogeneous catalysts, such as Raney-Ni and Co, is an important basis for the synthesis of primary amines.1 Nitrile hydrogenation is considered to proceed via formation of an aldimine-like intermediate on a metal surface. Because the nucleophilicity of the product, primary amine, is higher than that of the nitrile, primary amines react readily with the “surface aldimine”, leading to the formation of alkylideneamines, which are transformed into secondary amines (Scheme 1). Reaction of the aldimine with a secondary amine then affords a tertiary amine via formation of an enamine. Because these side reactions decrease the selectivity of primary amine formation, catalysis with high selectivity toward primary amines is desirable. Addition of large quantities of ammonia is commonly employed to prevent the undesirable condensation reaction.2 Protonation and acylation of primary amines, which reduce their nucleophilicity, are also sometimes used.3 However, these procedures require deprotection steps, which make the process more complicated and expensive.4 Homogeneous catalysts can selectively produce primary amines.5,6 For example, cooperative metal−ligand systems promote © XXXX American Chemical Society
selective hydrogenation of nitriles into the corresponding primary amines,6c,7 although most catalysts require the addition of significant amounts of base, typically tBuOK, to suppress byproduct formation. Catalysts exhibiting high primary amine selectivity without additives have been reported recently in both homogeneous8 and heterogeneous systems.9 These examples are of interest, because of their wide functional group tolerance and reduction of waste. Sabo-Etienne and co-workers reported the selective hydrogenation of benzonitrile via an ortho-metalated imine intermediate and observed that the N-benzylidenebenzylamine formed initially is transformed into benzylamine.8a This suggests that liberated ammonia plays a crucial role in the selective formation of benzylamine. A similar reaction pathway has been reported by Prechtl and co-workers.8e In contrast to nitrile reduction at a metal center, metal− ligand cooperation is effective in suppressing aldimine condensation. An outer-sphere mechanism of hydrogenation by Received: March 22, 2018
A
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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monometallic complexes with an η2-RCN ligand have been synthesized.19 Trinuclear complexes with a π-coordinated nitrile ligand also have been prepared. These complexes are suitable models for the parallel adsorption of nitrile on a metal surface.20 Kaesz et al. synthesized the first triply bridged π-nitrile complex, Fe3{μ3-η1(N):η2(C,N):η2(C,N)-RCN}(CO)9, by oxidation of a μ3-η2(∥)-imidoyl complex, Fe3(μ-H){μ3-η2(∥)-RCNH}(CO)9, and elucidated its interconversion via hydrogenation and dehydrogenation (Scheme 2).20a They also demonstrated
Scheme 1. Plausible Mechanistic Model of Nitrile Hydrogenation on a Metal Surface
Scheme 2. Sequential Transformation of Benzonitrile on a Triiron Cluster As Elucidated by Kaesz and Co-workers20a
[{HN(CH2PiPr2)2}Fe(H)(BH4)(CO)] has been proposed on the basis of experimental and theoretical studies.8c,d,10 Two distinctive pathways, the “M−N route” and the “M−C route”, have been proposed in heterogeneous catalysis.1a Krupka et al. suggested that the dramatic difference in selectivity among various catalysts is derived from the preferred adsorption mode of the surface-bound reaction intermediate.11 The surface nitrene in the “M−N route” is selective toward primary amines, whereas the surface aminiocarbene or the π-aldimine intermediate in the “M−C route” allows condensation with amines and leads to the formation of higher amines. Tada and co-workers reported that Ru nanoparticles supported by K-doped Al2O3 catalyze the selective hydrogenation of nitriles into the corresponding primary amines without additives and proposed that the selective formation of a nitrene-like intermediate is responsible for the high selectivity on the basis of density functional theory (DFT) calculation.9f Because the identification of surface intermediates is crucial in the mechanism evaluation, nitrile adsorption modes have been investigated extensively by near-edge X-ray fine structure (NEXAFS) spectroscopy,12 high-resolution electron energy loss spectroscopy (HREELS),13 reflection−absorption infrared spectroscopy (RAIRS),13d,14 and in situ attenuated total reflection infrared (ATR-IR) spectroscopy.15 These studies demonstrated that nitriles sometimes adsorb through π interaction with the CN bond parallel or slightly inclined to the metal surface rather than by common κ(N) coordination to a metal center. A preference for parallel adsorption of HCN on Ni(111)16 and Co(111)17 has been indicated theoretically by Oliva and co-workers. In some cases, a surface intermediate with a partially hydrogenated carbon−nitrogen bond is detected.18 These species, which are proposed to be adsorbed imidoyl (RCNH) units, are regarded as key intermediates in the “M−C route”. However, their role in the catalytic cycle has yet to be verified experimentally. Although nitriles typically are coordinated to a metal center through their nitrogen atom (κ(N) coordination), an electronrich metal center can stabilize π coordination of a nitrile. Several
the sequential transformation of the μ3-η2(∥)-imidoyl complex into μ3-η2-alkylideneamido and μ3-imido complexes, Fe3(μ-H){μ3η2(C,N)-NCRH}(CO)9 and Fe3(μ-H)2(μ3-NCH2R)(CO)9, respectively. Although elimination of primary amines was not observed, these transformations have clarified the mechanism of nitrile hydrogenation on a metal surface. Keller et al. reported that the same triiron complex can be prepared by reaction of Fe3(CO)12 with nitrile under an atmosphere of hydrogen.20c In contrast, the corresponding triruthenium and triosmium complexes were not obtained from the same procedure. Reaction of Ru3(CO)12 with nitriles under H2 afforded a μ-alkylideneamido complex, Ru3(μ-H)(μ-NCRH)(CO)10,21 whereas a mixture of μ-alkylideneamido and μ-amido complexes, Os3(μ-H)(μ-NCRH)(CO)10 and Os3(μ-H)(μ-NHCH2R)(CO)10, was obtained from Os3(CO)12.22 Cabeza and co-workers synthesized the first μ3-η2-alkylideneamido ruthenium complex by thermolysis of the μ-alkylideneamido complex formed by reaction of Ru3(CO)12 with LiNCPh2 followed by protonation.23 They showed that the μ-alkylideneamido ligand is eliminated as an amine upon hydrogenation and proposed that μ-alkylideneamido species are key intermediates in the hydrogenation of nitriles.24 We previously reported the synthesis of a triruthenium complex having a triply bridging nitrile ligand, (Cp*Ru)3(μ3-H)(μ-H)2{μ3-η2:η2(⊥)-PhCN} (2) (Cp* = η5-C5Me5).25 Unlike Kaesz’s triiron complex, the nitrile ligand in 2 is coordinated to the Ru3 plane with its nitrogen atom directed away from the Ru3 core. Søtofte and co-workers reported the same coordination mode of a nitrile ligand on a triniobium plane.26 This orientation resembles that of the alkyne ligand in {Cp*Ru(μ-H)}3{μ3-η2:η2(⊥)-RCCR′},27 and suggests that the nitrile ligand acts as a four-electron donor in a manner similar to the alkyne. It also suggests that the lone pair electrons at the nitrogen atom in 2 do not participate in coordination. Thus, intramolecular hydrido migration occurs upon thermolysis to B
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics yield a μ3-imidoyl complex, (Cp*Ru)3(μ-H)2{μ3-η2:η2(⊥)PhCNH} (3).28 The formation of μ3-η2:η2(⊥)-nitrile and μ3-η2:η2(⊥)-imidoyl complexes contrasts with the common reactivity of hydrido clusters with nitriles, which leads to formation of a bridging alkylideneamido ligand via insertion into an M−H bond.29 Fryzuk et al. synthesized μ-alkylideneamido complexes by reaction of {(dippe)Rh(μ-H)}2 (dippe = 1,2-diisopropylphosphinoethane) with nitriles and the transformation into μ-amido complexes by treatment with pressurized hydrogen.29j Because 2 and 3 adopt a 46-electron configuration, it is anticipated that they will react readily with dihydrogen. Formation of the μ3-η2(∥)-imidoyl complex (Cp*Ru)3(H)4{μ3-η2(∥)PhCNMe} was indicated upon reaction of the perpendicularly coordinated μ3-η2:η2(⊥)-imidoyl complex (Cp*Ru)(μ-H)2{μ3-η2:η2(⊥)-PhCNMe} with H2, although the μ3-η2(∥)imidoyl complex was not isolated due to facile dehydrogenation that regenerated the perpendicular form.30 Herein, we report on the consecutive transformations of a nitrile ligand on a Ru3 plane into a primary amine and the catalytic properties of the triruthenium complexes {Cp*Ru(μ-H)}3(μ3-H)2 (1) and (Cp*Ru)3(μ3-H)(μ-H)2(μ-CO) (12) toward benzonitrile hydrogenation. The reactions are suitable models for the “M−C” and “M−N” routes, respectively, of nitrile hydrogenation on a metal surface.
Scheme 3. Switchback Motion of the (⊥)-Nitrile Ligand in 2 on the Ru3 Plane
ligand moves around the Ru3 plane by altering positions of the acetylenic carbons, the isomer containing the phenyl group on the inner carbon is much more stable than the other.31 The population of the minor isomer, which has the phenyl group oriented outside the Ru3 core, is negligible, because the energy difference between the two isomers is large. This allows the kinetic parameters of the motion to be evaluated by dynamic NMR study. Dynamic NMR studies for 2 were not successful, however, because 2 equilibrates with the κ(N) isomer 5, whose μ3-nitrile ligand is coordinated with its nitrogen atom directed inside the Ru3 core as in Kaesz’s μ3-nitrile complex.20a Because of the increased population of 5, NMR spectra could not be simulated using only the rate of the switchback motion. Interaction of the inner nitrogen atom with the unique Ru atom efficiently stabilizes 5, unlike the case of the phenylacetylene complex. In the 1H NMR spectrum recorded at −50 °C, one set of hydrido signals of 2 at δ −4.01 and −24.50 ppm and a second set of hydrido signals of 5 at δ −15.08 (d, 2JH−H = 4.9 Hz) and −25.61 (t, 2JH−H = 4.9 Hz) ppm with an intensity of 2:1 were observed. The structure of 5 was not fully determined, because its Cp* and phenyl signals were not identified owing to the low population of the isomer (ca. 9%). However, spin saturation transfer (SST) between the hydrido signals of 2 and 5 was observed, which indicates that 5 equilibrates with 2. When the signal of 2 at δ −4.01 was irradiated at −10 °C, the signal at δ −25.61 disappeared and the intensity of the signal at δ −15.08 decreased by 44% (Figure S6, Supporting Information). Although no structural data are available for 5, DFT calculations provide a plausible structure containing a μ3-κ(N)-nitrile ligand (Figure S41, Supporting Information) and show that 5 is less stable than 2 by ca. 4 kJ mol−1, which agrees with the 5:2 ratio of 6:94 estimated from the 1H NMR spectrum recorded at −10 °C. Maintaining a THF solution of 2 at room temperature leads to formation of the μ3-η2-benzylideneamido complex (Cp*Ru)3(μ-H)2(μ3-η2-NCPhH) (6), which is another isomer of 2 (eq 2). The population of 6 gradually increases and stabilizes
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RESULTS AND DISCUSSION Reaction of 1 with Benzonitrile. Reaction of the triruthenium pentahydrido complex {Cp*Ru(μ-H)}3(μ3-H)2 (1) with benzonitrile at 25 °C affords a mixture of the (⊥)-nitrile complex 2 and μ3-benzylimido complex 4 (eq 1).
The time course of the reaction conducted with 10 equiv of benzonitrile in a sealed NMR tube shown in Figure S1 in the Supporting Information clearly illustrates the consecutive formation of 2 and 4. In the initial stage of the reaction, the liberation of H2 from 1 is accompanied by the formation of 2, which is detected at δ 4.46 ppm. This suggests that μ3-benzylimido complex 4 is formed from 2 by uptake of dihydrogen. In fact, (⊥)-nitrile complex 2 is obtained exclusively by reaction at reduced pressure. Treatment of 2 with 0.1 MPa of H2 at 25 °C immediately affords 4 in quantitative yield. The 1H NMR spectrum of 2 recorded at 20 °C displays two broad Cp* signals, which sharpen with decreasing temperature and resonate at δ 1.99 and 1.53 ppm with an intensity ratio of 2:1 at −50 °C. The spectral change suggests that the μ3-nitrile ligand in 2 moves among the three Ru−Ru edges in a switchback motion (Scheme 3). This motion averages the magnetic environments of the three Cp* groups. Accordingly, a time-averaged spectrum displaying a single resonance should be observed at elevated temperature. However, coalescence of the Cp* signals is not observed due to isomerization to a μ3η2:η2(⊥)-imidoyl complex, (Cp*Ru)3(μ-H)2{μ3-η2:η2(⊥)PhCNH} (3), above 40 °C (vide infra). In the switchback motion of the μ3-phenylacetylene ligand in {Cp*Ru(μ-H)}3{μ3-η2:η2(⊥)-PhCCH}, where the alkyne
after 3 h with an 2:6 ratio of 73:27 (Figure S7, Supporting Information). The 1H NMR spectrum recorded at −50 °C displays sharp Cp* signals at δ 1.90, 1.68, and 1.57 ppm. Doublet signals at δ −8.23 and −9.89 ppm (d, 2JH−H = 2.8 Hz), which broaden with increasing temperature, are observed in the hydrido region. The signal for the benzylideneamido proton, which occurs as a sharp singlet at δ 8.06 ppm, lies within the C
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics range reported for μ-alkylideneamido ligands in di- and triruthenium complexes (δ 6.5−9.1 ppm).21a,b,29l,m,32 These spectral features suggest that the μ3-benzylideneamido complex also is fluxional. However, this behavior will not be discussed further, because of the low population of 6. Fluxional behavior of the μ3-η2-benzylideneamido ligand in (Cp*Ru)3(μ-H)2(μ−η2NCPhH)(μ-CO) (13) will be discussed later. Because a single crystal of 6 suitable for X-ray diffraction (XRD) could not be obtained from the mixture, distinction between μ and μ3 coordination modes of the benzylideneamido ligand in 6 cannot be established on the basis of 1H NMR data alone. However, μ3 coordination is strongly indicated by 13C NMR. A doublet signal appears for the benzylideneamido carbon at δ 98.4 ppm (d, 1JC−H = 170 Hz). Its chemical shift differs considerably from that of μ-alkylideneamido ligands (δ 155− 179 ppm).20e,29j−m,o,33 Cabeza and co-workers noted a similar upfield shift of the alkylideneamido carbon resonance by ca. 31−37 ppm upon the μ3 coordination.23 The upfield shift is likely due to the additional η2 coordination to the Ru center in 6. The μ3 mode seems to be preferable according to the effective atomic number (EAN) rule. The number of valence electrons in 6 is 46, although η2-coordination of the CN bond is involved. Both μ and μ3 coordination modes are known for alkylideneamido ligands on a trinuclear plane. The μ3-alkylideneamido structures have been reported for Kaesz’s triiron complex and related compounds.20e,34 Although it was not confirmed in 6, μ3 coordination of a benzylideneamido ligand is unambiguously identified in 7, which is obtained by reaction of 6 with tBuNC (eq 3). Reaction of the mixture of 2 and 6 with tBuNC at 25 °C
Figure 1. Molecular structure and labeling scheme of 7 with thermal ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) 2.91365(18), Ru(1)−Ru(3) 2.87680(18), Ru(2)−Ru(3) 2.73689(17), Ru(1)−N(1) 2.0497(13), Ru(1)−C(1) 2.2290(17), Ru(2)−N(1) 1.9806(13), Ru(2)−C(8) 2.1392(17), Ru(3)−N(1) 1.9987(13), Ru(3)−C(8) 2.0186(17), N(1)−C(1) 1.351(2), N(2)−C(8) 1.227(2), N(2)−C(9) 1.478(2); Ru(2)− Ru(1)−Ru(3) 56.409(4), Ru(1)−Ru(2)−Ru(3) 61.116(4), Ru(1)− Ru(3)−Ru(2) 62.475(4), Ru(1)−N(1)−Ru(2) 92.58(5), Ru(1)− N(1)−Ru(3) 90.56(5), Ru(1)−N(1)−C(1) 78.99(9), Ru(2)− N(1)−Ru(3) 86.91(5), Ru(2)−N(1)−C(1) 146.89(11), Ru(3)− N(1)−C(1) 124.73(10), C(8)−N(2)−C(9) 134.68(16), N(1)− C(1)−C(2) 124.34(14), Ru(2)−C(8)−Ru(3) 82.28(6), Ru(2)− C(8)−N(2) 125.22(13), Ru(3)−C(8)−N(2) 152.47(14).
The μ-isocyanido ligand in 7 bridges the Ru(2) and Ru(3) atoms and adopts a bent structure (∠C(8)−N(2)−C(9) 134.68(16)°). Accordingly, the Ru(2)−Ru(3) bond (2.73689(17) Å) is noticeably shorter than the Ru(1)−Ru(2) and Ru(1)−Ru(3) bonds (2.91365(18) and 2.87680(18) Å, respectively). Because 6 reacts with tBuNC much more quickly than does 2, it can be regarded as the more reactive isomer. Thus, it is reasonable to assume that the hydrogenation of 2 occurs via its isomerization to 6. When complex 6 takes up dihydrogen, subsequent insertion of the benzylideneamido moiety into a Ru−H bond leads to the formation of the μ3-benzylimido complex 4. As shown in Scheme 2, Kaesz and co-workers showed that the triiron μ3-benzylideneamido complex Fe3(μ-H)(μ3-η2NCPhH)(CO)9 was transformed into a μ3-benzylimido complex upon treatment with H2 (1.4 MPa) for 48 h.20a Vahrenkamp and co-workers showed that the μ-benzylideneamido complex Ru3(μ-H)(μ-NCPhH)(CO)10 is hydrogenated by 0.1 MPa H2 to yield the μ3-benzylimido complex Ru3(μ-H)2(μ3-NCH2Ph)(CO)9 with elimination of CO. However, heating at 135 °C is required.21a In contrast to the preceding examples, 6 reacts smoothly with H2 to yield 4 at ambient temperature, as shown in Figure S1. The unsaturated nature of 6 may be responsible for its high reactivity toward H2. Cp* and hydrido signals with an intensity ratio of 15:1 are observed at δ 1.70 and −14.14 ppm, respectively, in the 1H NMR spectrum of 4. The methylene protons of the μ3-benzylimido group resonate as a singlet at δ 4.81 ppm. Formation of a μ3-benzylimido ligand was confirmed by XRD, as shown in Figure 2. Although three independent molecules with similar structural parameters occur in the unit cell, only one molecule is depicted. The three Ru−Ru distances are nearly equal
affords the μ-isocyanido complex 7, which is isolated in 87% yield. Complex 6 reacts much more quickly than the μ3-nitrile complex 2. Thus, the formation of 7 is accompanied by the immediate disappearance of 6 in the initial stage of the reaction, with 2 being gradually consumed via its slow isomerization of 2 to 6. The XRD results in Figure 1 clearly establish μ3 coordination of the benzylideneamido ligand in 7, where the Ru(1)−N(1) (2.0497(13) Å) and Ru(1)−C(1) (2.2290(17) Å) distances describe the η2 coordination of the CN moiety to the Ru(1) atom. The N(1)−C(1) distance (1.351(2) Å) is noticeably greater than the N(2)−C(8) distance (1.227(2) Å) of the μ-isocyanido ligand and comparable to the N−C distance in Fe3(μ3-η2-NCRH)(CO)9 (1.322−1.347 Å).34 The Ru(1)− C(1) distance also is comparable to those of the triiron complex (2.186−2.223 Å). The 13C signal derived from the μ3-η2-benzylideneamido carbon in 7 is observed at δ 106.9 (d, 1JC−H = 171 Hz), which is slightly downfield of the resonance in 6 (δ 98.4). D
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Figure 2. Molecular structure and labeling scheme of 4 with thermal ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) 2.7091(3), Ru(1)−Ru(3) 2.7003(3), Ru(2)−Ru(3) 2.7160(3), Ru(1)−N(1) 2.0000(19), Ru(2)−N(1) 2.000(2), Ru(3)−N(1) 2.016(2), N(1)−C(1) 1.457(3); Ru(2)− Ru(1)−Ru(3) 60.276(7), Ru(1)−Ru(2)−Ru(3) 59.702(7), Ru(1)− Ru(3)−Ru(2) 60.022(7), Ru(1)−N(1)−Ru(2) 85.25(8), Ru(1)− N(1)−Ru(3) 84.49(7), Ru(1)−N(1)−C(1) 120.49(14), Ru(2)− N(1)−Ru(3) 85.09(7), Ru(2)−N(1)−C(1) 133.42(16), Ru(3)− N(1)−C(1) 131.55(17), N(1)−C(1)−C(2) 117.8(2).
at 80 °C produces a mixture of μ3-imidoyl complexes 3 and 8, where μ3-η2(∥)-imidoyl complex 8 can be prepared by hydrogenation of 3 at 25 °C (eq 5). As previously reported
(Ru(1)−Ru(2) 2.7091(3) Å, Ru(1)−Ru(3) 2.7003(3) Å, and Ru(2)−Ru(3) 2.7160(3) Å), and the μ3-imido ligand lies above the center of the Ru3 triangle. Thus, the three Ru−N distances are almost equal (Ru(1)−N(1) 2.0000(19) Å, Ru(2)−N(1) 2.000(2) Å, Ru(3)−N(1) 2.016(2) Å) and are comparable to those of the isostructural μ3-phenylimido analogue (2.013(4) and 2.0243(3) Å).35 Extrusion of Benzylamine from the Ru3 Site. We previously reported that treatment of the μ3-imido complex {Cp*Ru(μ-H)}3(μ3-NH) with 0.1 MPa of H2 generates the pentahydrido complex 1 and eliminates NH3 via formation of the μ-amido intermediate (Cp*Ru)3(μ-H)4(μ-NH2).36 As shown in Scheme 2, Kaesz and co-workers also synthesized a μ3-imido complex upon hydrogenation of the triiron μ3-nitrile complex. However, the triiron μ3-benzylimido complex did not react further with dihydrogen.20a In contrast to this observation, the triruthenium complex Ru3(μ-H)2(μ3-NCPh2H)(CO)9 synthesized by Cabeza and co-workers reacts with H2 to liberate H2NCPh2H, but the triruthenium skeleton is not retained, as Ru4(μ-H)4(CO)12 forms in the reaction.24 Unlike these coordinatively saturated carbonyl clusters, the μ3-imido complex {Cp*Ru(μ-H)}3(μ3-NH) adopts a coordinatively unsaturated 46-electron configuration that presumably facilitates the uptake of dihydrogen and ammonia elimination. Isostructural 4 is expected to react readily with hydrogen to produce benzylamine. However, 4 does not react with 0.1 MPa of H2 at 80 °C. Complex 1 is obtained eventually in quantitative yield, but 0.5 MPa of H2 is required (eq 4).37 Benzylamine is formed in this reaction, but considerable N-benzylidenebenzylamine also is produced (Figure S20, Supporting Information). Unlike {Cp*Ru(μ-H)}3(μ3-NH), μ3-benzylimido complex 4 is not the true intermediate in benzylamine elimination, because the secondary imine is formed and 4 is not smoothly hydrogenated under H2. Although 6 is hydrogenated to 4 at ambient temperature, the reverse process, β-hydrogen elimination from the μ3-benzylimido ligand, is operative at an elevated temperature. Thermolysis of 4
for the analogous μ3-η2(∥)-imidoyl complex (Cp*Ru)3(μ-H)4{μ3-η2(∥)-PhCNMe},30 8 cannot be isolated due to facile elimination of H2 upon evacuation. Hydrogenation of 8 with 0.5 MPa of H2 at 80 °C results in the formation of 1. Transformation of the μ3-η2(∥)-imidoyl ligand into a μ-aminocarbene has been proposed in the “M−C route” of nitrile hydrogenation on a metal surface (Scheme 1).1a Migration of a hydride to the nitrogen atom of the μ3-η2(∥)-imidoyl ligand affords μ3-aminocarbene intermediate A, as shown in Scheme 4. Due to its unsaturated nature, A can accept dihydrogen to yield μ-aminocarbene intermediate B. B is then transformed into terminal isomer E, which is susceptible to nucleophilic attack by benzylamine, yielding N-benzylidenebenzylamine. Adams et al. have demonstrated that Os3(μ-OMe)(μ-H)(CO)10 reacts with CH2(NMe2)2 to yield a mixture of terminal- and μ3-aminocarbene complexes, Os3(μ-OMe)(μ-H){C(NMe2)H}(CO)9 and Os3(μ-OMe)(μ-H){μ3-C(NMe2)H}(CO)9, by elimination of dimethylamine.38 They also showed that the μ3-aminocarbene complex Os3(μ-OMe)(μ-H){μ3-C(NMe2)Et}(CO)9, which is obtained by reaction of Os3(μ-H)2(CO)9(MeCN) with MeC CNMe2, is converted by thermolysis to a terminal complex, Os3(μ-OMe)(μ-H){C(NMe2)H}(CO)9.39 The need for pressurized H2 in the hydrogenation of 8 is rationalized as follows. Intermediate B is stabilized under an H2 atmosphere at high pressure. Although A is unstable, the elevated H2 pressure drives the equilibrium between 8 and A to A. Subsequent hydrogenation leads to elimination of benzylamine and condensation to aldimine intermediate F. The reaction of 1 with benzylamine also was examined. The reaction proceeds at 100 °C and produces a mixture of μ3-imidoyl complexes of 3 and 8. Formation of μ3-benzylimido complex 4 is not observed in the early stages. Although it cannot be excluded that the mixture of 3 and 8 is produced from a small amount of initially formed 4, the fact suggests that benzylamine is eliminated from the μ3-η2(∥)-imidoyl complex 8 rather than the μ3-benzylimido complex 4 on the basis of the principle of E
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
intensity of the methylene proton decreases by ca. 50%, and the shape of the signal changes from a sharp singlet to a broad triplet-like shape alongside incorporation of deuterium at the CH site. The scrambling likely occurs within the amine adduct D in Scheme 4. These results imply that the benzylic C−H bond is readily broken at the Ru3 site and leads to the preferential formation of the μ3-imidoyl species. The presence of a phenyl group, which stabilizes π coordination of the CN bond, also is responsible for formation of the μ3-η2(∥) imidoyl complex 8. In other words, the electron-rich Ru3 site comprising three Cp* groups has a propensity to form imidoyl species. Catalytic Hydrogenation of Benzonitrile. Although the “M−C route” appears to be the preferred reaction mechanism, hydrogenation of 4 cleanly regenerates 1. Aminocarbene intermediate B has 46 valence electrons, and its hydrogenation is accelerated at an elevated H2 pressure, which should increase selectivity for benzylamine. Thus, we attempted the catalytic hydrogenation of benzonitrile using 1 mol % of 1 under 5 MPa of H2. When the reaction is carried out at 100 °C, almost all benzonitrile is consumed within 66 h (Table 1, entry 3). Although selectivity for benzylamine is moderate at the beginning (entry 1, 87%), it decreases to 48% in the later stages of reaction. A similar decrease in primary amine selectivity in the late stages of the reaction is often observed in nitrile hydrogenation using heterogeneous catalysts. It is widely accepted that accumulation of the primary amine promotes condensation, yielding the secondary imine.9d Reaction of 1 under 1 MPa of H2 results in low benzonitrile conversion and low selectivity (Table 1, entry 4). Although hydrogenation proceeds sluggishly below 80 °C (entry 5), the rate increases significantly above 100 °C. When the reaction is carried out at 110 °C, benzonitrile is 93% consumed after 20 h with 82% selectivity for benzylamine (Table 1, entry 6). However, the selectivity decreases to 48% at 130 °C (Table 1, entry 7). Moderate selectivity for benzylamine therefore is achieved using H2 at high pressure. Because conversion of B to C is promoted by elevated H2 pressure, the opportunity for condensation via formation of E decreases. The pathway involving hydrogenation of the μ3-benzylimido complex 4 also is possible
Scheme 4. Plausible Mechanism for Hydrogenation of the μ3-η2(∥)-Imidoyl Complex 8
microscopic reversibility. In addition, the reaction of deuterated pentahydride 1-d5 with benzylamine at 50 °C results in the scrambling of deuterium into the CH and NH sites of benzylamine (Figure S22, Supporting Information). The signal
Table 1. Catalytic Hydrogenation of Benzonitrile by Ruthenium Clusters under 5 MPa of H2
yield (selectivity)/%a,b
a
entry
cat.
time/h
temp/°C
conversn/%
BA
1 2 3c 4d 5 6 7 8 9 10 11
1 1 1 1 1 1 1 1 10e 12 15
1.5 20 20 66 20 20 20 20 20 20 20
100 100 100 100 100 80 110 130 110 120 110
6.8 56 98 98 18 3.9 93 98 24 98 34
5.9 (87) 40 (71) 27 (28) 47 (48) 12 (69) 3.2 (82) 76 (82) 47 (48) trace 92 (93) 0
b
c
BB 16 19 40
15 36 20
0.9 (28) (19) (41) 5.6 0.7 (16) (37) (83) 3.7 6.8
DB (13) 52 11 (31) (18) 15 (3.8) (20)
27
0 0.3 (53) (11) 0 0 1.2 (16) 4.1 3.3 (80)
(0.5)
(1.3) (17) (3.2)
d
Determined by GLC analysis. Based on benzonitrile. Methanol was used as solvent. Reaction carried out under 1 MPa of H2 eA mixture of 10 and unidentified complexes was prepared by reacting 3 with benzonitrile. The amount of 3 used in preparing the mixture is taken as the amount of catalyst. F
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics under this condition. However, formation of N-benzylidenebenzylamine is not completely suppressed even under 5 MPa of H2. Remarkable solvent effects have been often reported, in particular using a metal−ligand cooperation system, where a protic solvent such as 2-propanol is effective for selective primary amine formation.8c−f In our system, however, protic solvent (methanol) was not effective for the selective benzylamine formation (Table 1, entry 3). In methanol, conversion of benzonitrile is accelerated but it also causes acceleration of condensation leading to the formation of secondary amine. Consequently, benzylamine selectivity decreases in comparison to the reaction performed in THF. Reactivity of μ3-η2:η2(⊥)-Imidoyl Complex 3. We showed previously that the μ3-nitrile complex 2 begins to isomerize to the μ3-η2:η2(⊥)-imidoyl complex 3 above 60 °C (eq 6).28
Figure 3. Molecular structure and labeling scheme of 9 with thermal ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) 2.9237(2), Ru(1)−Ru(3) 2.7741(2), Ru(2)−Ru(3) 2.9650(2), Ru(1)−N(1) 2.0486(15), Ru(2)−C(1) 2.1082(17), Ru(2)−C(8) 1.9000(19), Ru(3)−N(1) 2.0934(15), Ru(3)−C(1) 2.0701(17), N(1)−C(1) 1.386(2), N(2)−C(8) 1.177(2), N(2)−C(9) 1.462(2); Ru(2)−Ru(1)−Ru(3) 62.650(5), Ru(1)−Ru(2)−Ru(3 56.204(5), Ru(1)−Ru(3)−Ru(2) 61.145(5), Ru(1)−N(1)−Ru(3) 69.65(10), Ru(1)−N(1)−C(1) 116.12(12), Ru(2)−C(1)−Ru(3) 90.40(7), Ru(2)−C(1)−N(1) 106.52(11), N(1)−C(1)−C(2) 119.52(16), Ru(2)−C(8)−N(2) 174.82(17), C(8)−N(2)−C(9) 150.3(2).
synthesis. Site exchange buries the hydrido signals in the baseline of the 1H NMR spectrum at 25 °C, but two doublets appear at δ −19.87 and −10.15 ppm at −80 °C. Two singlets at δ 157.9 and 179.1 ppm are observed in the low-field region of the 13C NMR spectrum of 9. They arise from the terminal isocyanido and the imidoyl carbons but cannot be further assigned. Complex 3 is expected to react with primary amines in a manner similar to its reaction with tBuNC. If the reaction results in the formation of secondary imines, the complex may be a promoter of side reactions. However, 3 does not react with benzylamine (100 equiv) at 110 °C. After 20 h, the majority of 3 remains unchanged (8% conversion), and 8 is the identifiable product. Although a small amount of N-benzylidenebenzylamine formation (ca. 4% based on 3) is observed, the results suggest that the reaction of 3 with benzylamine does not contribute to secondary imine formation during catalysis. Thermolysis of an equilibrated mixture of 2 and 6 in the presence of 100 equiv of benzylamine also was examined. The result does not differ from the thermolysis of 2 in the absence of amine, and only 3 is formed in 94% yield. These facts indicate that complexes 2, 3, and 6 are not involved in side reactions. Although 3 does not react with primary amines, it does with benzonitrile. Reaction of 3 with benzonitrile at 70 °C yields a mixture of several complexes, including (Cp*Ru)3(μ-H)(μ3-NCH2Ph){μ3-η2(∥)-PhCN} (10) (eq 8). Complex 10 was isolated in 26% yield by column chromatography on
Complex 3 is expected to react smoothly with two-electron donors, because it has a 46-electron configuration. Imidoyl species are regarded as key intermediates in the “M−C route”. Thus, we examined the reactivity of 3 toward two-electron donors to explore the basis of this functionality. As shown in eq 5, 3 reacts smoothly with dihydrogen at 25 °C to form the μ3-η2(∥)-imidoyl complex 8. Complex 3 also reacts rapidly with tBuNC at 25 °C to yield the isocyanido adduct 9 (eq 7). The orientation of the μ3-imidoyl ligand
changes from perpendicular to parallel relative to its adjacent Ru−Ru vector upon incorporation of tBuNC. In contrast to the μ-isocyanido complex 7, the tBuNC group in 9 coordinates to Ru in a terminal manner, as unambiguously described by XRD (Figure 3). The μ3-imidoyl group in 9 is σ-bonded to the Ru(1) and Ru(2) atoms via its N(1) and C(1) atoms, respectively, and π-bonded to the Ru(3) atom. The N(1)−C(1) distance (1.386(2) Å) is comparable to the reported range of N−C distances in triruthenium μ3-η2(∥)-imidoyl complexes (1.319− 1.381 Å)40 and to the N−C distance of μ3-η2:η2(⊥)-imidoyl complex 3 (1.391(6) Å).28 The tBuNC group is coordinated to the Ru(2) atom, where the imidoyl C(1) atom is attached. Hydrides H(1) and H(2) were successfully located at the Ru(1)−Ru(2) and Ru(1)−Ru(3) edges during the Fourier G
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics alumina. Because benzonitrile is abundant under the catalytic conditions, incorporation of second nitrile into the triruthenium core of 3 is possible. In the 1H NMR spectrum of 10 at 20 °C, two Cp* units appear as a broad signal near 1.5 ppm, while the third Cp* yields a sharp signal at δ 1.45 ppm. The broad signal resolves into two signals resonating at δ 1.30 and 1.58 ppm at −80 °C. The methylene protons of the μ3-benzylimido ligand, which are equivalent at 20 °C, separate into doublets at δ 6.21 and 6.42 ppm at −80 °C with a large geminal coupling constant (2JH−H = 17.5 Hz). These spectral changes were successfully simulated and produced estimated activation parameters of ΔH⧧ = 11.2 ± 0.2 kcal mol−1 and ΔS⧧ = −7.77 ± 0.8 cal mol−1 K−1. Details of the variable-temperature 1H NMR study of 10 are shown in Figures S26 and S27 in the Supporting Information. The spectral changes originate from a single process that involves a windshield wiper motion of the nitrile ligand in concert with hydride migration, as shown in Scheme 5. Although we could
The 13C NMR chemical shift of the imidoyl carbon of 11 (δ 179.2) is similar to that of 10. Three Cp* signals, two of which are broad, occur at δ 1.34 (br), 1.51 (s), and 1.84 (br) ppm in the 1H NMR spectrum at 25 °C. This suggests a windshield wiper motion of the μ3-η2(∥)-imidoyl ligand in 11, but the rate is much less than in 10. The molecular structure of 11 determined by XRD is shown in Figure 4. Each face of the Ru3 plane is capped by a
Scheme 5. Windshield Wiper Motion of the μ3-η2(∥)-Nitrile Ligand in 10
not determine whether the nitrile ligand pivots on carbon or the nitrogen atom, a pivoting motion on carbon by related μ3-η2(∥)-imidoyl ligands has been proposed to be lower in energy.40a,c,41 Although the position of the hydride was not determined, its presence was confirmed unambiguously by observation of a singlet resonating at δ −27.80 ppm. We tentatively locate the hydride at the Ru−Ru edge where the bridging nitrile ligand is bound, analogous to the position of the hydride in the cationic μ3-η2(∥)-imidoyl complex 11, obtained by protonation of 10. It is well-known among isoelectronic trinuclear alkyne complexes that perpendicular coordination is typical for coordinatively unsaturated 46-electron complexes, whereas parallel coordination of an alkyne ligand is typical for coordinatively saturated 48-electron complexes.42 According to the EAN rule, the number of valence electrons in 10 is 48 when the nitrile ligand acts as a four-electron donor. Therefore, the nitrile ligand in 10 is proposed to adopt the parallel mode shown in eq 8. The nitrile signal lies at δ 182.2 ppm in the 13C NMR spectrum, which is at much lower field than the carbon signal of the (⊥)-nitrile ligand in 2 (δ 97.8).25 Although μ3-η2(∥) coordination of nitrile is unprecedented in trinuclear complexes, a similar μ4-η2(∥) coordination involving additional κ(N) coordination to a metal center is known for a few clusters.20c,43 Despite the inability to establish μ3-η2(∥)coordination of PhCN by XRD, this bonding motif is suggested by the structure of the cationic μ3-η2(∥)-imidoyl complex 11. Treatment of 10 with HBF4·Et2O results in quantitative formation of 11 (eq 9). Protonation likely occurs at the nitrogen atom of the μ3-η2(∥)-nitrile ligand in a manner similar to the protonation of 2 that yields a cationic (⊥)-imidoyl complex.25
Figure 4. Molecular structure and labeling scheme of the cationic part of 11 with thermal ellipsoids at the 30% probability level. The anionic part was omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) 2.8305(4), Ru(1)−Ru(3) 2.7383(4), Ru(2)− Ru(3) 2.7529(5), Ru(1)−N(1) 2.012(3), Ru(1)−N(2) 2.089(5), Ru(1)−C(8) 2.131(5), Ru(2)−N(1) 1.995(4), Ru(2)−C(8) 2.068(4), Ru(3)−N(1) 2.034(4), Ru(3)−N(2) 2.029(5), N(1)− C(1) 1.457(6), N(2)−C(8) 1.374(7); Ru(2)−Ru(1)−Ru(3) 59.227(11), Ru(1)−Ru(2)−Ru(3) 58.718(11), Ru(1)−Ru(3)− Ru(2) 62.055(11), Ru(1)−N(1)−Ru(2) 89.90(14), Ru(1)−N(1)− Ru(3) 85.20(13), Ru(2)−N(1)−Ru(3) 86.21(15), Ru(1)−N(2)− Ru(3) 83.33(19), Ru(1)−C(8)−Ru(2) 84.76(16), N(2)−C(8)−C(9) 119.4(4).
μ3-benzylimido or μ3-η2(∥)-imidoyl group. The Ru−Ru distances are 2.7383(4), 2.7529(5), and 2.8305(4) Å. The Ru(1)−Ru(2) bond length is the greatest, likely because of steric repulsion between the phenyl group of the μ3-η2(∥)imidoyl moiety and the Cp* groups at Ru(1) and Ru(2). The phenyl group of the μ3-imido group is directed toward the midpoint of the Ru(1)−Ru(2) bond. Although the hydrogen atom of the μ3-η2(∥)-imidoyl moiety was not located, the hydride is positioned at the Ru(2)−Ru(3) edge. The μ-hydride likely moves to the Ru(1)−Ru(3) edge on the NMR time scale in concert with the windshield wiper motion of the imidoyl ligand to produce a time-averaged Cs spectrum. The imidoyl group is π-bonded to the Ru(1) atom and σ-bonded to Ru(2) and Ru(3) through the carbon and nitrogen atoms, respectively. Due to structural disorder, precise evaluation of the bond distances and angles involving the H
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
electron configuration of 12 is the same as that of 1 (44e), 12 is expected to react with nitriles in a similar manner. Furthermore, the CO ligand on 12 is not readily eliminated from the Ru3 core during the reaction. Because CO decreases electron density at the Ru3 core, its presence destabilizes the π coordination of a CN bond and suppresses the uptake of the second nitrile molecule. In fact, when the reaction is conducted using 1 mol % of 12 under 5 MPa of H2 at 120 °C, benzylamine is obtained in 92% yield (Table 1, entry 10). Although small amounts of N-benzylidenebenzylamine (3.7%) and dibenzylamine (3.3%) are produced, primary amine selectivity is improved remarkably in comparison to the reaction catalyzed by 1. The stoichiometric reaction of 12 with benzonitrile also was examined. Complex 12 reacts readily with benzonitrile at 25 °C and exclusively affords a μ3-benzylideneamido complex, (Cp*Ru)3(μ-H)2(μ3-η2-NCPhH)(μ-CO) (13) (eq 10).
imidoyl group is hampered, but the N(2)−C(8) distance of 1.374(7) Å is comparable to that found in 9. As mentioned above, μ3-η2:η2(⊥)-imidoyl complex 3 reacts with dihydrogen to regenerate the starting hydrido complex 1 via formation of the μ3-η2(∥)-imidoyl complex 8. In contrast, regeneration of 1 is not observed upon treating the mixture obtained by reacting 3 with benzonitrile. Only a complicated mixture of unidentified complexes results. Thus, formation of 10 may be related to the deactivation process. When hydrogenation of benzonitrile is carried out in the presence of the mixture obtained by reacting 3 with benzonitrile, N-benzylidenebenzylamine is produced in 20% yield with 83% selectivity (Table 1, entry 9). Although a small amount of dibenzylamine (4%) is formed in the reaction, only a trace amount of benzylamine is detected. The mechanism of secondary imine formation is not clear at present, but the results suggest that species containing two (or more) benzonitrile units promote the formation of secondary imines. Therefore, suppressing the incorporation of additional nitriles onto the Ru3 core seems to be crucial for the selective formation of primary amines. The series of transformations of benzonitrile at the Ru3 center derived from 1 is summarized in Scheme 6. Benzonitrile Scheme 6. Transformations of Benzonitrile at the Ru3 Center Derived from 1
In contrast to the reaction of 1, benzonitrile inserts into a Ru−H bond to form a μ3-η2-benzylideneamido ligand without dihydrogen elimination. Suppression of H2 elimination from the Ru3 core upon replacement of two hydrides with a CO ligand also is observed in the reaction of 12 with butadiene and ethylene.44 Complex 13 was isolated in 56% yield by recrystallization. Its molecular structure determined by XRD is shown in Figure 5. There are two independent molecules with similar structural features in the unit cell, but the structure of only one is
is hydrogenated to the μ3-imido complex 4 at the Ru3 site by formation of the π-coordinated nitrile complex 2. This process corresponds to the “M−N route”, but the Ru3 unit derived from 1 exhibits a tendency to form μ3-imidoyl complexes 3 and 8 at elevated temperature. The μ3-η2(∥)-imidoyl complex 8 apparently promotes secondary imine formation via formation of an aminocarbene intermediate, which is characteristic of the “M−C route” on a metal surface. The coordinatively unsaturated (⊥)-imidoyl complex 3 binds another nitrile molecule to yield 10. Under an atmosphere of H2, 10 decomposes to unidentified complexes, which catalyze the formation of N-benzylidenebenzylamine. These results clearly show, as noted by Krupka et al.,11 that suppression of imidoyl species is very important for the selective hydrogenation of benzonitrile. Reactivity of μ-CO Complex 12. We recently reported the synthesis of a triruthenium hydrido complex containing a μ-CO ligand, (Cp*Ru)3(μ3-H)(μ-H)2(μ-CO) (12).44 We have shown that 12 reacts with phenylacetylene in a manner similar to that for 1, yielding a μ3-η2:η2(⊥)-alkyne complex. Because the
Figure 5. Molecular structure and labeling scheme of 13 with thermal ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) 2.8444(6), Ru(1)−Ru(3) 2.8747(6), Ru(2)−Ru(3) 2.7123(6), Ru(1)−N(1) 2.062(4), Ru(1)−C(1) 2.263(6), Ru(2)−N(1) 2.003(4), Ru(2)−C(8) 1.978(6), Ru(3)− N(1) 2.011(4), Ru(3)−C(8) 2.083(5), N(1)−C(1) 1.329(7), C(8)− O(1) 1.192(6); Ru(2)−Ru(1)−Ru(3) 56.618(14), Ru(1)−Ru(2)− Ru(3) 62.253(16), Ru(1)−Ru(3)−Ru(2) 61.129(15), Ru(1)−N(1)− Ru(2) 88.81(17), Ru(1)−N(1)−Ru(3) 89.77(17), Ru(1)−N(1)− C(1) 80.6(3), Ru(2)−N(1)−Ru(3) 85.02(16), Ru(2)−N(1)−C(1) 126.9(4), Ru(3)−N(1)−C(1) 146.0(4), N(1)−C(1)−C(2) 126.0(5), Ru(2)−C(8)−Ru(3) 83.8(2), Ru(2)−C(8)−O(1) 142.0(4), Ru(3)− C(8)−O(1) 134.2(4). I
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hydrides and the alkylideneamido proton at 50 °C suggests that a μ3-imido intermediate formed by migration of a hydride to the β-carbon atom is not involved in this fluxional process. Thus, dynamic behavior via a vertically oriented μ3-alkylideneamido intermediate likely occurs in 13. Although the features of the isostructural μ3-isocyanido complex 7 are similar to those of 13, complex 7 is not fluxional on the NMR time scale. The π-acidity of CO dominates the σ-donating property of isocyanide and reduces back-donation from the Ru3 core to the π*(CN) orbital, which makes the μ3-benzylideneamido ligand in 13 fluxional. The difference in mobility of the μ3-benzylideneamido ligand between 13 and 7 is reflected in the difference in their reactivity toward H2. Despite its saturated electron configuration, 13 reacts slowly with 0.6 MPa of H2 at 25 °C to yield the μ-benzylamido complex 14 in 70% yield (eq 11). Complex 14 was purified by
depicted. The benzylideneamido ligand lies on the Ru3 plane in a μ3-η2 fashion with the N(1)C(1) moiety π-bonded to the Ru(1) atom and σ-bonded to the Ru(2) and Ru(3) atoms through the N(1) atom. The N(1)C(1) distance (1.329(7) Å) is comparable to that of the isostructural μ-isocyanido complex 7 (1.351(2) Å). The CO ligand bridges the Ru(2)−Ru(3) edge on the face of the Ru3 plane opposite to the μ3-benzylideneamido ligand. The presence of the μ-CO ligand also was confirmed by IR spectroscopy, which showed strong absorption at 1735 cm−1. The bridging hydrides were successfully located on the Ru(1)− Ru(2) and Ru(1)−Ru(3) edges. Complex 13 shows dynamic behavior arising from rotation of the μ3-benzylideneamido ligand (Scheme 7a). Two Cp* signals Scheme 7. Possible Fluxional Behaviors of the μ3-η2(∥)Benzylideneamido Ligand in 13
column chromatography and isolated in 45% yield. Prior to uptake of H2, a vacant site must be created by migration of a hydride to the alkylideneamido carbon or decoordination of the μ3-benzylideneamido ligand to form a μ-benzylideneamido intermediate. The isostructural μ-isocyanido complex 7 does not react with dihydrogen. Only 4% of 7 is converted upon treatment with 0.6 MPa of H2 at 25 °C for 50 h to afford small amounts of unidentified complexes. This result implies that the mobility of the μ3-benzylideneamido ligand on the Ru3 plane is crucial for uptake of dihydrogen. A broad signal from the amido proton is observed at δ 3.74 ppm in the 1H NMR spectrum of 14, whereas an adsorption assignable to ν(NH) is not observed in the IR spectrum. A triplet resonating at δ 93.1 ppm (1JC−H = 134 Hz) in the 13 C NMR spectrum also indicates that the CN moiety of 13 is hydrogenated. Formation of a μ-benzylamido ligand with the benzyl group oriented inside the Ru3 core was confirmed by a preliminary XRD study (Figure S42, Supporting Information).48 Formation of the μ-benzylamido ligand is accompanied by a change from μ to μ3 coordination of the CO group, which is confirmed by a strong adsorption at 1665 cm−1 in the IR spectrum. Hydrogenation of 14 to 12 occurs at 100 °C (eq 12). Complex 14 is 64% converted after 24 h, and 12 is regenerated in 47% yield. These reactions are accompanied by formation of 13 in 11% yield as a consequence of dehydrogenation despite the presence of 0.6 MPa of H2. Thermolysis of 14 at 80 °C in the absence of H2 results in the regeneration of 13 in 97% yield. This fact indicates that coordinatively saturated 13 equilibrates with unsaturated 14 via the release and uptake of H2 and that the equilibrium is shifted largely toward 13. Owing to this equilibrium, high-pressure H2 seems to be required for the hydrogenation of benzonitrile using 12. A plausible mechanism for the hydrogenation of benzonitrile by 12 is proposed on the basis of the reactions in Scheme 8. Although unsaturated μ3-benzylimido intermediate A is not observed, hydrogenation catalyzed by 12 is considered to proceed via the “M−N route”. Complex 13, unlike 6, is not
are observed at δ 1.57 (s) and 1.82 (br) ppm with an intensity ratio of 1:2 by 1H NMR at 25 °C. The broad signal at δ 1.82 ppm separates into sharp signals at δ 1.79 and 1.83 ppm at −30 °C. Sharp hydrido signals are observed at δ −21.22 and −19.47 ppm at −30 °C and become broad at higher temperatures. In contrast, the alkylideneamido proton signal (δ 6.56 ppm) is sharp from −30 to 50 °C. These spectral changes were successfully simulated to provide estimated activation parameters of ΔH⧧ = 16.5 ± 0.3 kcal mol−1 and ΔS⧧ = 4.0 ± 0.9 cal mol−1 K−1. Details of the variable-temperature 1H NMR study of 13 are shown in Figures S33 and S34 of the Supporting Information. Although the dynamics of the μ3-η2-alkylideneamido ligand have never been documented, the related μ3-alkenylidene motion in several trinuclear complexes has been discussed. We documented the pivoting motion of a μ3-vinylidene ligand in (Cp*Ru)3(μ-H)(μ−η2-CCH2){μ−η2(∥)-HCCH} to produce a time-averaged plane of Cs symmetry.45 Pivoting of the μ3-alkylideneamido ligand may explain the observed spectral changes (Scheme 7b). However, this is unlikely for 13, because the motion requires simultaneous migration of the μ-CO and μ-hydride ligands between two Ru−Ru edges. Dynamic movement of a μ3-vinylidene ligand via a vertical vinylidene intermediate has been described by Deeming and co-workers in explanation of the interconversion of two isomers of Os3(μ-H)2{μ3-η2-CC(OEt)H}(CO)9.46 A similar mechanism has been proposed for the enantiomerization of [Co3{μ3η2-CC(CHMe2)H}(CO)9]+.47 The motion produces equivalent environments for the two Cp* groups in 13 without migration of the μ-CO ligand. The lack of SST between the J
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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(1) and have identified μ3-η2:η2(⊥)-nitrile complex 2, μ3-η2benzylideneamine complex 6, μ3-η2(∥)-imidoyl complex 8, and μ3-imido complex 4 as key intermediates. The transformations of 2 to 6 and 4 correspond to the “M−N route” proposed for nitrile hydrogenation on a metal surface. However, benzylamine is not directly eliminated from 4. The true intermediate in the extrusion of benzylamine is the μ3-η2(∥)-imidoyl complex 8. This step, which follows the model of the “M−C route”, affords considerable N-benzylidenebenzylamine in the catalytic hydrogenation of benzonitrile. An aminocarbene intermediate formed from 8 seems to be responsible for secondary imine formation. The reactivity of coordinatively unsaturated μ3-η2:η2(⊥)imidoyl complex 3 also was investigated. Complex 3 reacts readily with H2 and tBuNC but not with benzylamine, although a mechanism involving nucleophilic attack of a primary amine at the imidoyl carbon has been proposed for imine formation. Complex 3 binds benzonitrile to yield the μ3-imido−μ3-η2(∥)nitrile complex 10. The mixture obtained by reacting 3 with benzonitrile catalyzes secondary imine formation. The mechanism of secondary imine formation by the mixture cannot be established at present, but it is suggested that species formed by the incorporation of two or more benzonitrile molecules, such as 10, may be responsible for secondary imine formation. In contrast to 1, the μ-carbonyl−hydrido complex (Cp*Ru)3(μH)2(μ3-H)(μ-CO) (12) reacts with one molecule of benzonitrile to catalyze highly selective hydrogenation of benzonitrile without additives. The reaction of 12 with benzonitrile affords μ3-benzylideneamido complex 13, which upon subsequent hydrogenation yields benzylamine via formation of μ-benzylamido complex 14. The most important feature of 12 is that a μ3-imidoyl species was not formed on the Ru3 core, probably due to reduced electron density within the Ru3 unit. Thus, hydrogenation of benzonitrile proceeds preferentially via the “M−N route”. Although 13 is coordinatively saturated, it does react with H2 to yield μ-amido complex 14. This reactivity is attributed to the flexible nature of the μ3-benzylideneamido ligand in 13. In contrast, the isostructural μ-isocyanido complex 7 does not react with H2. This is likely due to the π-acidity of the CO ligand on Ru3. The results show that subtle differences acting at the Ru3 core can produce marked differences in reactivity. Degradation of the triruthenium skeleton occurs at elevated temperatures and forms {Cp*Ru(μ-H)}2(μ-CO) (15). Complex 15 catalyzes secondary imine formation. Suppressing degradation of the trinuclear skeleton is crucial for increasing the selectivity of primary amine formation. We will continue to optimize reaction conditions to minimize degradation and seek a robust trinuclear skeleton that can catalyze more efficient nitrile hydrogenation.
transformed into a μ-imidoyl species. It is presumed that π-coordination of the CN bond is destabilized by the reduced electron density at the Ru3 site arising from CO ligation. Complex 13 also is obtained by reaction of 12 with benzylamine, most likely by the formation of 14. The sum of these results supports the mechanism shown in Scheme 8. Scheme 8. Plausible Mechanism for the Hydrogenation of Benzonitrile Catalyzed by 12
A small amount of {Cp*Ru(μ-H)}2(μ-CO) (15) (6%) formed during the hydrogenation of 14 is confirmed by 1H NMR signals (δ 1.79 (Cp*) and −12.88 ppm (RuH)) in C6D6.49 Complex 15 is produced by thermolysis of 13 at 110 °C under an Ar atmosphere. Complex 13 decomposes slightly upon heating at 110 °C in THF-d8 (ca. 6% conversion after 20 h), affording 15 in ca. 2% yield. Although the rate is not significant, degradation of the trinuclear skeleton occurs at elevated temperature. We also examined the catalytic activity of 15 toward hydrogenation of benzonitrile (Table 1, entry 11). Although the reactions are fairly slow, formation of N-benzylidenebenzylamine (6.8%) and dibenzylamine (27%) is observed. The result indicates that secondary imine is formed from complex 15 that is generated by the decomposition of 13.
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EXPERIMENTAL SECTION
General Procedures. All compounds were manipulated using standard Schlenk and high-vacuum-line techniques under an atmosphere of argon. Dehydrated toluene, tetrahydrofuran (THF), pentane, hexane, and methanol used in this study were purchased from Kanto Chemicals and stored under an atmosphere of argon. Diethyl ether was dried over sodium−benzophenone ketyl and distilled under an atmosphere of argon. C6D6 and THF-d8 were dried over sodium− benzophenone ketyl and stored under an atmosphere of argon. Acetone-d6 was dried over MS-4A and stored under an atmosphere of argon. Benzonitrile was dried over P2O5 and stored under an atmosphere of argon. Other reagents were used as received. Triruthenium complexes {Cp*Ru(μ-H)}3(μ3-H)2 (1), {Cp*Ru(μD)}3(μ3-D)2 (1-d5),50 and (Cp*Ru)3(μ3-H)(μ-H)2(μ-CO) (12)44
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CONCLUSION In summary, we have accomplished the consecutive hydrogenation of benzonitrile to benzylamine using {Cp*Ru(μ-H)}3(μ3-H)2 K
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
performed using the Gaussian 09 software suite.54 The molecular structures were drawn using the GaussView version 5.0 program.55 Frequency calculations at the same level of theory as geometry optimizations were performed on the optimized structure to ensure that minima exhibit only positive frequency. Information on the atom coordinates for the optimized structure is given in the Supporting Information. Reaction of {Cp*Ru(μ-H)}3(μ3-H)2 (1) with Benzonitrile in a Sealed NMR Tube. An NMR tube equipped with a Teflon valve was charged with 1 (8.9 mg, 12.5 μmol), C6D6 (0.5 mL), and cyclooctane (1.0 μL) as an internal standard. Then, 10 equiv of benzonitrile (12.8 μL, 0.124 mmol) was added to the solution at 25 °C. The 1H NMR spectra of the solution were periodically recorded, and the populations of 1, 2, and 4 were measured. Preparation of (Cp*Ru)3(μ3-H)(μ-H)2{μ3-η2:η2(⊥)-PhCN} (2). A 50 mL Schlenk tube was charged with THF (15 mL) and 1 (299 mg, 0.42 mmol). Benzonitrile (0.85 mL, 8.3 mmol) was added with vigorous stirring. Then, the pressure in the flask was reduced using the vacuum line to remove the dihydrogen produced in the reaction. The reaction mixture was stirred at 25 °C for 6 h. Dihydrogen was periodically removed from the reaction flask every 30 min during the reaction. The solution turned from reddish brown to brownish yellow. The solvent and the remaining benzonitrile were removed under reduced pressure. The residual solid was purified by the use of column chromatography on alumina (Merck, Art. No. 1097) with THF. The third reddish brown band including 2 was collected, and the solvent was removed under reduced pressure. Complex 2 was obtained as a brownish yellow solid (204 mg, 60% yield). Complex 2 equilibrated with 5 in solution, and the 2:5 ratio was estimated to be 94:6 at −10 °C; the population of 5 decreased to zero above 10 °C. Data for 2 are as follows. 1H NMR (400 MHz, acetone-d6, −50 °C): δ −24.50 (s, 1H, Ru−H), −4.01 (s, 2H, RuH), 1.53 (s, 15H, Cp*), 1.99 (s, 30H, Cp*), 5.87 (d, 3JH−H = 7.6 Hz, 2H, o-Ph), 6.73 (t, 3JH−H = 7.6 Hz, 1H, p-Ph), 6.93 ppm (t, 3JH−H = 7.6 Hz, 2H, m-Ph). 13C NMR (100 MHz, THF-d8, −40 °C): δ 11.9 (q, 1JC−H = 126 Hz, C5Me5), 12.7 (q, 1JC−H = 126 Hz, C5Me5), 84.4 (s, C5Me5), 91.2 (s, C5Me5), 97.8 (s, CN), 123.2 (d, 1JC−H = 160 Hz, Ph), 126.3 (d, 1JC−H = 158 Hz, Ph), 126.8 (d, 1JC−H = 158 Hz, Ph), 143.3 ppm (t, 2JC−H = 8 Hz, ipso-Ph). IR (ATR, cm−1): ν 2976, 2902, 1600,1592, 1478, 1448, 1373, 1284, 1248, 1156, 1070, 1025, 859. Anal. Calcd for C37H53NRu3: C, 54.53; H, 6.55; N, 1.72. Found: C, 54.18; H, 6.93; N, 1.60. 5: 1H NMR (400 MHz, acetone-d6, −50 °C): δ −25.61 (t, 2JH−H = 4.9 Hz, 1H, RuH), −15.08 ppm (d, 2JH−H = 4.9 Hz, 2H, RuH). Other signals derived from 5 were not identified due to its low population. Preparation of (Cp*Ru)3(μ-H)2{μ3-η2:η2(⊥)-PhCNH} (3). A 20 mL glass tube equipped with a Teflon valve was charged with THF (10 mL) and 2 (57.2 mg, 70.2 μmol). The reaction mixture was stirred at 100 °C for 2 h. The solution turned from brownish yellow to brown. The solvent was removed under reduced pressure. Complex 3 was obtained as a brown solid (52.3 mg, 91% yield). 1H NMR (400 MHz, acetone-d6, −20 °C): δ −8.07 (s, 2H, RuH), 1.66 (s, 15H, Cp*), 1.76 (s, 30H, Cp*), 6.12 (d, 3JH−H = 8.4 Hz, 2H, o-Ph), 6.86 (t, 3JH−H = 8.0 Hz, 1H, p-Ph), 6.99 (dd, 3JH−H = 8.4, 8.0 Hz, 2H, m-Ph), 12.61 ppm (br s, 1H, NH). 13C NMR (100 MHz, benzene-d6, 25 °C): δ 12.2 (br q, w1/2 = 18 Hz, 1JC−H = 126 Hz, C5Me5), 12.6 (br q, w1/2 = 13 Hz, 1JC−H = 126 Hz, C5Me5), 80.0 (br s, w1/2 = 9 Hz, C5Me5), 90.6 (br s, w1/2 = 10 Hz, C5Me5), 124.4 (d, 1JC−H = 161 Hz, p-Ph), 125.9 (d, 1JC−H = 158 Hz, m-Ph), 126.7 (d, 1JC−H = 157 Hz, o-Ph), 151.9 (s, ipso-Ph), 161.4 ppm (s, CN). IR (ATR, cm−1): ν 2976, 2956, 2901, 2854, 1618, 1593, 1477, 1461, 1427, 1373, 1284, 1248, 1070, 1025, 763, 702 cm−1. Anal. Calcd for C37H53NRu3: C, 54.53; H, 6.55; N, 1.72. Found: C, 54.14; H, 6.54; N, 1.77. Preparation of {Cp*Ru(μ-H)}3(μ3-NCH2Ph) (4). A 50 mL Schlenk tube was charged with THF (5 mL) and 2 (44.1 mg, 54.1 μmol). After the solution was degassed by one freeze−pump− thaw cycle, hydrogen (0.1 MPa) was added at room temperature and the mixture stirred for 24 h. The solution turned from reddish brown to dark green. The solvent was removed under reduced pressure. The residual solid was purified by column chromatography on alumina (Merck, Art. No. 1097) with a mixed solvent of hexane and THF
were prepared according to published methods. 1H and 13C NMR spectra were recorded on Varian INOVA-400 and Varian 400MR spectrometers. 1H NMR spectra were referenced to tetramethylsilane as an internal standard. 13C NMR spectra were referenced to the natural-abundance carbon signal of the solvent employed. IR spectra were recorded on a Nicolet Avatar 360 FT-IR instrument with a Ge-ATR cell and a JASCO FT/IR-4200 spectrophotometer by the diffuse reflection method using KBr. Elemental analyses were performed on a PerkinElmer 2400II CHN analyzer. GLC analyses were performed on Shimadzu GC-17A and GC-2010 instruments using a capillary column (J&W DB-1; 30 m × 0.53 mm × 1.50 μm) with helium gas as a carrier. X-ray Diffraction Studies. Single crystals of 4, 7, 9, 11, and 13 for an XRD analysis were obtained directly from the preparations described below and mounted on nylon Cryoloops with Paratone-N (Hampton Research corp.). Diffraction experiments were performed on a Rigaku R-AXIS RAPID imaging plate diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71069 Å). In all samples, cell refinement and data reduction were performed using the PROCESS-AUTO program.51 Intensity data were corrected for Lorentz− polarization effects and for numerical or empirical absorption. The structure of 4 was solved by the Patterson method, and those of 7, 9, 11, and 13 were solved by the direct method using SHELXT-2014/5 and further refined with the SHELXL-2016/6 program package.52 All non-hydrogen atoms were found by a difference Fourier synthesis and were refined anisotropically except for the disordered atoms described below. The refinement was carried out by least-squares methods based on F2 with all measured reflections. For 11, the disorder at the nitrogen atom of the μ3-imidoyl ligand, N(2), was refined in the ratio of 75:25 and the hydrogen atom attached to the N(2) atom could not be located. For 13, disorder at the Cp* ligand bonded to Ru(4) was refined in the ratio of 67:33 and refined isotropically. Hydrogen atoms attached to the Cp* ligand were not located. The metal-bound hydrogen atoms in 4, 7, 9, 11, and 13 were located in a difference Fourier map and refined isotopically. Crystal data and results of the analyses are given in Table S5 in the Supporting Information. The CIF data of 4, 7, 9, 11, and 13 are deposited with the Cambridge Crystallographic Data Center with the deposition numbers 1827738 (4), 1827742 (7), 1827741 (9), 1827739 (11), and 1827740 (13), respectively. Variable-Temperature NMR Spectra and NMR Simulations. Variable-temperature NMR studies were performed in NMR tubes equipped with a J. Young valve using a Varian INOVA-400 Fourier transform spectrometer. The NMR simulation for the 1H NMR spectrum of 8 recorded at 25 °C was performed using gNMR v5.0.6.0 (2006, Ivory Soft). Final simulated line shapes were obtained via an iterative parameter search upon the coupling constants, and the detail of the fitting procedure is shown in Figure S16 in the Supporting Information. The NMR simulations for the temperature-dependent Cp* and methylene signals of 10 and Cp* and hydrido signals of 13 were also performed via an iterative parameter search upon the exchange constants k, which was the rate for the pivoting motion of the μ3-η2(∥)-nitrile ligand in 10 and the rotation of the μ3-η2benzylideneamido ligand in 13. The details of the fitting procedure are also shown in Tables S2 and S3 and Figures S27 and S34 in the Supporting Information. The rate constants k that accurately modeled the experimental spectra at each temperature are also given in the Supporting Information (Figures S25 and S26 for 10 and Figures S32 and S33 for 13). The activation parameters ΔH⧧ and ΔS⧧ were determined from the plot of ln(k/T) versus 1/T. Estimated standard deviations (σ) in the slope and y intercept of the Eyring plot determined the error in ΔH⧧ and ΔS⧧, respectively. The standard deviation in ΔG⧧ was determined from the formula σ(ΔG⧧)2 = σ(ΔH⧧)2 + [Tσ(ΔS⧧)]2 − 2Tσ(ΔH⧧)σ(ΔS⧧). Computational Details. DFT calculations for 5, which is the possible isomer of 2, were carried out at the ωB97XD level in conjunction with the Stuttgart/Dresden ECP and associated with triple-ζ SDD basis sets for Ru.53 For C, H, and O, 6-31(d) basis sets were employed. No simplified model compounds were used for the calculations. Initial geometries for the optimization were based on the crystallographically determined structure for 2. All calculations were L
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics (5/1 in volume). The first green band including 4 was collected. A 38.6 mg amount of 4 was obtained as a dark green solid on removal of the solvent under reduced pressure (87% yield). A single crystal used for the diffraction studies was prepared from the pentane solution stored at 2 °C. 1H NMR (400 MHz, THF-d8, 25 °C): δ −14.14 (s, 3H, RuH), 1.70 (s, 45H, Cp*), 4.81 (s, 2H, NCH2), 7.08 (t, JH−H = 7.6 Hz, 1H, p-Ph), 7.16 (dd, JH−H = 7.6, 7.2 Hz, 2H, m-Ph), 7.34 ppm (d, JH−H = 7.2 Hz, 2H, o-Ph). 13C{1H} NMR (100 MHz, THF-d8, 25 °C): δ 12.5 (C5Me5), 81.7 (NCH2Ph), 91.4 (C5Me5), 126.2 (Ph), 127.5 (Ph), 128.1 (Ph), 144.3 ppm (ipso-Ph). Anal. Calcd for C37H55NRu3: C, 54.39; H, 6.79; N, 1.71. Found: C, 54.27; H, 6.47; N, 1.87. Formation of (Cp*Ru)3(μ-H)2(μ3-η2-NCPhH) (6). An NMR tube equipped with a J. Young valve was charged with 2 (4.3 mg, 5.3 μmol), THF-d8 (0.45 mL), and cyclooctane (1 μL) as an internal standard. The solution was allowed to stand at 25 °C and monitored periodically by 1H NMR spectroscopy. The distribution of 6 gradually increased and became steady after 3 h, where the ratio between 2 and 6 was estimated at 73:27. 1H NMR (400 MHz, acetone-d6, −50 °C): δ −9.89 (d, 2JH−H = 2.8 Hz, 1H, RuH), −8.23 (d, 2JH−H = 2.8 Hz, 1H, RuH), 1.57 (s, 15H, Cp*), 1.68 (s, 15H, Cp*), 1.90 (s, 15H, Cp*), 7.07 (t, 3JH−H = 7.8 Hz, 1H, p-Ph), 7.20 (t, 3JH−H = 7.8 Hz, 2H, m-Ph), 7.44 (d, 3JH−H = 7.8 Hz, 2H, o-Ph), 8.06 ppm (s, 1H, NCHPh). 13 C NMR (100 MHz, acetone-d6, 25 °C): δ 11.8 (q, 1JC−H = 127 Hz, C5Me5), 12.6 (q, 1JC−H = 126 Hz, C5Me5), 13.2 (q, 1JC−H = 126 Hz, C5Me5), 81.7 (s, C5Me5), 85.8 (s, C5Me5), 86.5 (s, C5Me5), 98.4 (d, 1JC−H = 170 Hz, N = CPhH), 125.6 (d, 1JC−H = 159 Hz, p-Ph), 127.2 (d, 1JC−H = 158 Hz, Ph), 128.2 (d, 1JC−H = 160 Hz, Ph), 146.3 ppm (s, ipso-Ph). Preparation of (Cp*Ru)3(μ-H)2(μ3-η2-NCPhH)(μ-CNtBu) (7). A 50 mL Schlenk tube was charged with toluene (5 mL) and an equilibrated mixture of 2 and 6 (58.0 mg, 0.071 mmol). An excess amount of tBuNC (65 μL, 0.58 mmol) was then added to the reaction mixture at 25 °C, and the solution was stirred for 8 h. The solution turned from brownish yellow to red. The solvent and remaining t BuNC were removed under reduced pressure. The residual solid was purified by the use of column chromatography on alumina (Merck, Art. No. 1097) with a mixed solvent of toluene and THF (5/2 in volume). The first reddish brown band including 7 was collected, and the solvent was removed under reduced pressure. Complex 7 was obtained as a reddish brown solid (55.8 mg, 62.1 μmol, 87% yield). A brown single crystal used for the diffraction study was obtained from a cold pentane solution of 7 stored at 0 °C. 1H NMR (400 MHz, THF-d8, 25 °C): δ −22.01 (s, 1H, RuH), −19.74(s, 1H, RuH), 1.40 (s, 9H, μ-CNtBu), 1.57 (s, 15H, Cp*), 1.77 (s, 15H, Cp*), 1.80 (s, 15H, Cp*), 6.62 (s, 1H, μ3-N = CPhH), 7.09 (t, JH−H = 7.4 Hz, 1H, p-Ph), 7.17 (dd, JH−H = 7.4, 7.0 Hz, 2H, p-Ph), 7.39 ppm (t, JH−H = 7.0 Hz, 2H, o-Ph). 13C NMR (100 MHz, THF-d8, −30 °C): δ 11.5 (q, 1JC−H = 127 Hz, C5Me5), 11.7 (q, 1JC−H = 127 Hz, C5Me5), 11.9 (q, 1JC−H = 127 Hz, C5Me5), 31.9 (q, 1JC−H = 126 Hz, CNCMe3), 56.0 (s, CNCMe3), 87.5 (s, C5Me5), 91.3 (s, C5Me5), 92.9 (s, C5Me5), 106.9 (d, 1JC−H = 171 Hz, μ3-η2-N = CPhH), 126.8 (dt, 1JC−H = 160 Hz, 3JC−H = 7.5 Hz, p-Ph), 127.8 (dd, 1JC−H = 160 Hz, 3JC−H = 7.7 Hz, o-Ph), 128.2 (br d, 1JC−H = 158 Hz, m-Ph), 144.0 (s, ipso-Ph), 206.4 ppm (s, μ-CNtBu). IR (KBr, cm−1): ν 3393, 2970, 2898, 2716, 1712 (ν(CN)), 1599, 1454, 1367, 1199, 1069, 1025, 876, 822, 756 cm−1. Anal. Calcd for C42H62N2Ru3: C, 56.16; H, 6.96; N, 3.12. Found: C, 56.51; H, 7.25; N, 3.27. Preparation of (Cp*Ru)3(μ-H)4{μ3-η2(∥)-PhCNH} (8). An NMR tube equipped with J. Young valve was charged with 3 (12.1 mg, 14.8 μmol), THF-d8 (0.4 mL), and 2,2,4,4-tetramethypentane (1 μL) as an internal standard. After the solution was frozen by a liquid nitrogen bath, the tube was degassed. The solution was then warmed to 25 °C, and 0.1 MPa of hydrogen was introduced. The solution immediately turned from brown to red. The 1H NMR spectrum recorded after 10 min showed the quantitative formation of 8. 1H NMR (400 MHz, THF-d8, 25 °C): δ −18.48 (ddd, 2JH−H = 5.1, 3.9, 1.3 Hz 1H, RuH), −16.76 (ddd, 2JH−H = 8.3, 5.3, 3.9 Hz, 1H, RuH), −15.03 (ddd, 2JH−H = 5.3, 5.1, 1.4 Hz, 1H, RuH), −13.67 (ddd, 2JH−H = 8.3, 1.4, 1.3 Hz, 1H, RuH), 1.65 (s, 15H, Cp*), 1.75 (s, 15H, Cp*), 1.90 (s, 15H, Cp*), 6.15 (br s, w1/2 = 8.7 Hz, 1H, PhCNH), 6.91−7.15 ppm (m, 5H, Ph).
JH−H values among the hydrido ligands were estimated on the basis of spectral simulations (Figure S16 in the Supporting Information). 13 C NMR (100 MHz, THF-d8, 25 °C): δ 11.7 (q, 1JC−H = 126 Hz, C5Me5), 12.1 (q, 1JC−H = 126 Hz, C5Me5), 12.6 (q, 1JC−H = 126 Hz, C5Me5), 86.8 (s, C5Me5), 91.0 (s, C5Me5), 93.4 (s, C5Me5), 124.3 (d, 1JC−H = 159 Hz, Ph), 126.9 (d, 1JC−H = 156 Hz, Ph), 127.4 (d, 1JC−H = 158 Hz, Ph), 153.6 (s, ipso-Ph). 169.1 ppm (s, PhCNH). Hydrogenation of {Cp*Ru(μ-H)}3(μ3-NCH2Ph) (4). Complex 4 (20.0 mg. 24.5 μmol) and THF-d8 (2 mL) were charged in a glass autoclave together with hexamethylbenzene (4.0 mg, 24.7 μmol) as an internal standard. After the reactor was degassed with a liquid nitrogen bath, the reactor was filled with 0.5 MPa of H2. The reactor was then heated at 100 °C for 21 h. After H2 gas was purged from the reactor, the solution was transferred to an NMR tube. The 1H NMR spectrum of the solution showed that all of 4 was converted into 1. (The formation of a small amount of the bis(μ3-oxo) complex (Cp*Ru)3(μ3-O)2(μ-H) was also observed in the spectrum. The bis(μ3-oxo) complex was formed by the reaction of 1 with air during the transport of the solution into the NMR tube.) Two singlet signals were observed at δ 3.77 and 3.71 ppm with a ratio of 7:3, which were assigned to the methylene signals of benzylamine and N-benzylidenebenzylamine, respectively. Formation of benzylamine and N-benzylidenebenzylamine was also confirmed by GC analysis of the solution. Reaction of 1 with Benzylamine at 100 °C. An NMR tube equipped with a Teflon valve was charged with 1 (4.2 mg, 5.9 μmol), THF-d8 (0.4 mL), and hexamethylbenzene (1.8 mg) as an internal standard. Then, benzylamine (3.0 μL, 28 μmol) was added to the solution. The tube was heated at 100 °C using an oil bath. The 1 H NMR spectrum showed that 90% of 1 was converted after 24 h and afford a mixture of 3 and 8 in a ratio of 5:4. Formation of μ3-benzylimido complex 4 was not detected. Reaction of 1-d5 with Benzylamine at 50 °C. An NMR tube equipped with a Teflon valve was charged with 1-d5 (8.6 mg, 12.0 μmol), THF-d8 (0.4 mL), and hexamethylbenzene (1.2 mg) as an internal standard. Then, benzylamine (1.2 μL, 11.0 μmol) was added to the solution. The tube was heated at 50 °C using an oil bath, and the 1H NMR spectra were periodically recorded. The 1H NMR spectrum recorded after 8 h showed that the signal intensity of the methylene proton of benzylamine decreased by ca. 50%. Preparation of (Cp*Ru)3(μ-H)2{μ3-η2(∥)-PhCNH}(CNtBu) (9). A 50 mL Schlenk tube was charged with THF (4 mL) and 3 (28.6 mg, 35.1 μmol). A 1.5 equiv amount of tBuNC (6 μL, 53 μmol) was then added to the reaction mixture at 25 °C, and the solution was stirred for 5 min. The solution immediately turned from brown to green. Removal of the solvent and remaining tBuNC under reduced pressure gave 9 as a green solid (31.2 mg, 34.7 μmol, 99% yield). A green single crystal used for the diffraction study was obtained by slow evaporation of the solvent from a pentane solution of 9 stored at 25 °C. 1H NMR (400 MHz, C6D6, 25 °C): δ 0.96 (s, 9H, CNtBu), 1.60 (s, 15H, Cp*), 1.93 (s, 15H, Cp*), 2.09 (s, 15H, Cp*), 6.34 (br s, 1H, PhCNH), 7.12 (t, 3JH−H = 7.2 Hz, 1H, p-Ph), 7.31 (dd, 3JH−H = 7.2, 7.2 Hz, 2H, m-Ph), 7.46 ppm (d, 3JH−H = 7.2 Hz, 2H, o-Ph). Hydrido signals were not observed due to severe broadening at ambient temperature. 1 H NMR (400 MHz, THF-d8, −80 °C): δ −19.87 (d, 2JH−H = 3.7 Hz, 1H, RuH), −10.15 (d, 2JH−H = 3.7 Hz, 1H, RuH), 1.05 (s, 9H, CNtBu), 1.47 (s, 15H, Cp*), 1.83 (s, 15H, Cp*), 1.92 (s, 15H, Cp*), 6.80 (br s, 1H, PhCNH), 7.0−7.7 ppm (m, 5H, Ph). 13C NMR (100 MHz, C6D6, 25 °C): δ 11.4 (q, 1JC−H = 126 Hz, C5Me5), 12.65 (q, 1JC−H = 126 Hz, C5Me5), 12.69 (q, 1JC−H = 126 Hz, C5Me5), 31.79 (q, 1JC−H = 127 Hz, CNCMe3), 55.5 (s, CNCMe3), 82.5 (s, C5Me5), 86.1 (s, C5Me5), 91.9 (s, C5Me5), 123.3 (d, 1JC−H = 157 Hz, Ph), 127.1 (d, 1JC−H = 157 Hz, Ph), 155.1 (s, ipso-Ph), 157.9 (s, CNtBu or μ3-η2(∥)-PhCNH), 179.1 ppm (s, CNtBu or μ3-η2(∥)-PhCNH). One 13 C signal derived from the phenyl group was not observed owing to obstruction by the solvent signals. IR (KBr, cm−1): ν 3442, 2972, 2900, 1999 (ν(CN)), 1598, 1453, 1369, 1205, 1038, 1025. Anal. Calcd for C42H62N2Ru3: C, 56.16; H, 6.96; N, 3.12. Found: C, 55.87; H, 6.99; N, 2.94. Preparation of (Cp*Ru)3(μ-H)(μ3-NCH2Ph){μ3-η2(∥)-PhCN} (10). A 50 mL glass tube equipped with a Teflon valve was charged M
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
109.9 (d, 1JC−H = 171 Hz, N = CPhH), 127.0 (d, 1JC−H = 160 Hz, Ph), 127.6 (d, 1JC−H = 159 Hz, Ph), 127.9 (d, 1JC−H = 160 Hz, Ph), 143.7 (s, ipso-Ph). 234.3 ppm (s, μ-CO). IR (KBr, cm−1): ν 2976, 2952, 2900, 1735 (ν(CO)), 1596, 1483, 1454, 1374, 1070, 1070, 1027, 816, 760. Anal. Calcd for C38H53NORu3: C, 54.14; H, 6.34; N, 1.66. Found: C, 54.11; H, 6.36; N, 1.83. Thermolysis of 13. An NMR tube equipped with a J. Young valve was charged with 13 (1.1 mg, 1.3 μmol), THF-d8 (0.4 mL), and 2,2,4,4-tetramethylpentane (1 μL) as an internal standard. The tube was then heated at 110 °C for 20 h. The 1H NMR spectrum of the reaction solution showed that ca. 7% of 13 was consumed. Formation of 15 (ca. 2%) was confirmed by the hydrido signal of 15 resonating at δ −13.55 ppm. Preparation of (Cp*Ru)3(μ-H)2{μ-N(CH2Ph)H)}(μ-CO) (14). A 50 mL glass autoclave was charged with THF (10 mL) and 13 (68.3 mg, 80.8 μmol). After the solution was degassed by one freeze− pump−thaw cycle, hydrogen (0.6 MPa) was introduced at 25 °C and the mixture stirred for 1 week. The solution turned from red to brownish yellow. The solvent was removed under reduced pressure. The residual solid was purified by column chromatography on alumina (Merck, Art. No. 1097) with a mixed solvent of hexane and toluene (3/2 in volume). The first green band including 14 was collected. Removal of the solvent under reduced pressure gave a 30.9 mg amount of 14 as a green solid (0.37 mmol, 45% yield). 1H NMR (400 MHz, C6D6, 25 °C): δ −10.59 (s, 2H, RuH), 1.74 (s, 30H, Cp*), 1.77 (s, 15H, Cp*), 2.67 (d, 3JH−H = 7.3 Hz, 2H, N(CH2Ph)H), 3.71 (br t, 3 JH−H = 7.3 Hz, 1H, NH), 7.07−7.23 ppm (m, 5H, Ph). 13C NMR (100 MHz, THF-d8, 25 °C): δ 10.5 (q, 1JC−H = 126 Hz, C5Me5), 12.7 (q, 1JC−H = 126 Hz, C5Me5), 79.6 (s, C5Me5), 92.9 (s, C5Me5), 93.1 (t, 1JC−H = 134 Hz, N(CH2Ph)H), 127.5 (d, 1JC−H = 160 Hz, Ph), 129.1 (d, 1JC−H = 156 Hz, Ph), 129.3 (d, 1JC−H = 159 Hz, Ph), 145.6 (s, ipso-Ph). 271.4 ppm (s, μ-CO). IR (KBr, cm−1): ν 2976, 2904, 2842, 1665 (ν(CO)), 1492, 1451, 1370, 1072, 1070, 1025, 752, 694. Anal. Calcd for C38H55NORu3: C, 54.01; H, 6.56; N, 1.66. Found: C, 54.09; H, 6.76; N, 2.03. Reaction of 12 with Benzylamine. An NMR tube equipped with a J. Young valve was charged with 12 (5.7 mg, 7.7 μmol), THF-d8 (0.4 mL), and 2,2,4,4-tetramethylpentane (1 μL) as an internal standard. After benzylamine (8 μL, 73 μmol) was added to the solution, the tube was heated at 120 °C for 70 h. The 1H NMR spectrum of the reaction solution showed that 13 was formed in 24% yield. The formation of a trace amount of 15 was also observed. Hydrogenation of (Cp*Ru)3(μ-H)2{μ-N(CH2Ph)H)}(μ-CO) (14). Complex 14 (15.2 mg. 18.0 μmol) and THF (2 mL) were charged in a glass autoclave. After the reactor was degassed with a liquid nitrogen bath, the reactor was filled with 0.6 MPa of H2. The reactor was heated at 100 °C for 24 h. The solution was then transferred to a 50 mL Schlenk tube, and the solution was removed under reduced pressure. Conversion of 14 was estimated to be 64% on the basis of the 1 H NMR spectrum of the residual solid, and the spectrum also showed that complexes 12, 13, and 15 were formed in 47, 11, and 6% yields, respectively. Catalytic Hydrogenation of Benzonitrile. A series of catalytic hydrogenations of benzonitrile were performed in a 100 mL stainlesssteel autoclave equipped with a glass cup and magnetic stirrer bar. The reaction solution (catalyst (ca. 6−10 mg), benzonitrile (100 equiv), and THF (3 mL)) was prepared in a 20 mL Schlenk tube, and the solution was transferred into the autoclave under an Ar atmosphere. The reactor was filled with H2 gas at an appropriate pressure (typically 5 MPa). The reactor was heated using an oil bath with vigorous stirring. After the appropriate reaction time (typically 20 h), the solution was analyzed by GLC. The results of catalytic reactions are given in Table 1.
with THF (7.5 mL) and 3 (92.8 mg, 0.114 mmol). After a large excess amount of benzonitrile (0.24 mL, 2.30 mmol) was added to the solution, the reaction mixture was heated at 70 °C for 45 h with vigorous stirring. The solvent and remaining benzonitrile were then removed under reduced pressure. The residual solid including 10 was dissolved in pentane (3 mL) and purified by column chromatography on alumina (Merck, Art. No. 1097). The eluent was gradually changed from pentane to toluene, THF, and methanol. The fourth green band including 10 that was eluted with THF and methanol was collected. A 26.7 mg amount of 10 was obtained as a dark green solid on removal of the solvent under reduced pressure (26% yield). Due to the difficulty in the complete removal of methanol from the residual solid, elemental analysis of 10 did not fit with the calculated values. 1H NMR (400 MHz, THF-d8, −80 °C): δ −27.80 (s, 1H, RuH), 1.30 (s, 15H, Cp*), 1.51 (s, 15H, Cp*), 1.58 (s, 15H, Cp*), 6.21 (d, 2JH−H = 17.7 Hz, 1H, NCH2Ph), 6.42 (d, 2JH−H = 17.7 Hz, 1H, NCH2Ph), 6.86 (dd, 3JH−H = 8.4, 8.0 Hz, 2H, m-Ph), 7.06−7.50 ppm (m, 10H, NCH2Ph and NCPh). 13C NMR (100 MHz, C6D6, 25 °C): δ 10.9 (q, 1JC−H = 126 Hz, C5Me5), 11.3 (q, 1JC−H = 126 Hz, C5Me5), 79.1 (t, 1JC−H = 134 Hz, NCH2Ph), 93.1 (s, C5Me5), 96.1 (s, C5Me5), 125.3 (Ph), 126.2 (Ph), 126.8 (Ph), 130.1 (br, Ph), 144.0 (s, ipso-Ph), 148.1 (s, ipso-Ph), 182.2 ppm (s, PhCN). Several 13C signals derived from two phenyl groups in 10 were not identified probably due to the obstruction by the solvent signal and broadening arising from restricted rotation of the μ3-η2(∥)-nitrile ligand. Preparation of a Tetrafluoroborate Salt of [(Cp*Ru)3(μ-H)(μ3NCH2Ph){μ3-η2(∥)-PhCNH}]+ (11). A 50 mL Schlenk tube was charged with diethyl ether (5.0 mL) and 10 (31.5 mg, 34.2 μmol). After the flask was cooled with a dry ice/methanol bath, a slight excess amount of tetrafluoroboric acid diethyl ether complex (5.5 μL, 40 μmol) was added to the solution. The solution was then gradually warmed to 25 °C and stirred for 0.5 h at ambient temperature. A dark brown precipitate immediately formed, and the supernatant was removed. The residual solid was rinsed three times with 1 mL of diethyl ether. Drying under vacuum gave 11 as a tetrafluoroborate salt (33.4 mg, 97% yield). A single crystal suitable for the diffraction study was obtained from the acetone solution of a tetraphenylborate salt of 11, which was prepared by exchanging the counteranion with tetraphenylborate, stored at 25 °C by vapor diffusion of diethyl ether. Data for 11-BF4 are as follows. 1H NMR (400 MHz, acetone-d6, 25 °C): δ −25.98 (s, 1H, RuH), 1.34 (br, 15H, Cp*), 1.51 (s, 15H, Cp*), 1.84 (br, 15H, Cp*), 6.28 (d, 2JH−H = 18.1 Hz, 1H, NCH2Ph), 6.70 (d, 2JH−H = 18.1 Hz, 1H, NCH2Ph), 7.24−7.52 (m, 10H, NCH2Ph and PhCNH), 9.08 ppm (br s w1/2 = 14 Hz, 1H, PhCNH). 13 C NMR (100 MHz, acetone-d6, 25 °C): δ 10.8 (br q, 1JC−H = 126 Hz, C5Me5), 10.9 (q, 1JC−H = 126 Hz, C5Me5), 11.1 (br q, 1JC−H = 126 Hz, C5Me5), 81.3 (t, 1JC−H = 136 Hz, NCH2Ph), 97.6 (s, C5Me5), 98.8 (s, C5Me5), 101.1 (s, C5Me5),127.8 (d, JC−H = 161 Hz, Ph), 128.0 (d, JC−H = 158 Hz, Ph), 128.66 (d, 1JC−H = 161 Hz, Ph), 128.70 (d, 1JC−H = 161 Hz, Ph), 143.2 (s, ipso-Ph), 146.2 (s, ipso-Ph), 179.2 ppm (s, PhC=NH). Elemental analysis was performed on 11-BPh4. Anal. Calcd for C68H79BN2Ru3: C, 65.95; H, 6.43; N, 2.26. Found: C, 65.95; H, 6.70; N, 2.26. Preparation of (Cp*Ru)3(μ-H)2(μ3-η2-NCPhH)(μ-CO) (13). A 50 mL Schlenk tube was charged with THF (20 mL) and 12 (263.2 mg, 0.356 mmol). An excess amount of benzonitrile (0.36 mL, 3.50 mmol) was then added to the solution at 25 °C, and the solution was stirred for 0.5 h. The solution turned from dark brown to red. The solvent and remaining benzonitrile were removed under reduced pressure. Recrystallization from a hexane solution of the residual solid stored at −30 °C gave a 167 mg amount of analytically pure 13 as a red single crystal (0.198 mmol, 56% yield). A single crystal used for the diffraction study was prepared from the THF solution of 13 stored at −30 °C. 1H NMR (400 MHz, THF-d8, −40 °C): δ −21.21 (s, 1H, RuH), −19.46 (s, 1H, RuH), 1.56 (s, 15H, Cp*), 1.80 (s, 15H, Cp*), 1.83 (s, 15H, Cp*), 6.56 (s, 1H, N = CPhH), 7.10 (t, 3JH−H = 7.6 Hz, 1H, p-Ph), 7.18 (dd, 3JH−H = 7.6, 7.2 Hz, 2H, m-Ph), 7.34 ppm (d, 3JH−H = 7.2 Hz, 2H, o-Ph). 13C NMR (100 MHz, THF-d8, 25 °C): δ 11.1 (q, 1JC−H = 127 Hz, C5Me5), 11.8 (br q, 1JC−H = 127 Hz, C5Me5), 88.1 (s, C5Me5), 93.4 (br s, C5Me5), 94.0 (br s, C5Me5),
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ASSOCIATED CONTENT
S Supporting Information *
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DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
(8) (a) Reguillo, R.; Grellier, M.; Vautravers, N.; Vendier, L.; SaboEtienne, S. J. Am. Chem. Soc. 2010, 132, 7854−7855. (b) Gunanathan, C.; Hölscher, M.; Leitner, W. Eur. J. Inorg. Chem. 2011, 2011, 3381− 3386. (c) Bornschein, C.; Werkmeister, S.; Wendt, B.; Jiao, H.; Alberico, E.; Baumann, W.; Junge, H.; Junge, K.; Beller, M. Nat. Commun. 2014, 5, 4111. (d) Neumann, J.; Bornschein, C.; Jiao, H.; Junge, K.; Beller, M. Eur. J. Org. Chem. 2015, 2015, 5944−5948. (e) Choi, J. H.; Prechtl, M. H. G. ChemCatChem 2015, 7, 1023−1028. (f) Adam, R.; Bheeter Charles, B.; Jackstell, R.; Beller, M. ChemCatChem 2016, 8, 1329−1334. (g) Adam, R.; Alberico, E.; Baumann, W.; Drexler, H. J.; Jackstell, R.; Junge, H.; Beller, M. Chem. Eur. J. 2016, 22, 4991−5002. (9) (a) Li, H.; Wu, Y.; Wan, Y.; Zhang, J.; Dai, W.; Qiao, M. Catal. Today 2004, 93−95, 493−503. (b) Li, H.; Wu, Y.; Zhang, J.; Dai, W.; Qiao, M. Appl. Catal., A 2004, 275, 199−206. (c) Chojecki, A.; Veprek-Heijman, M.; Müller, T. E.; Schärringer, P.; Veprek, S.; Lercher, J. A. J. Catal. 2007, 245, 237−248. (d) Schärringer, P.; Müller, T. E.; Lercher, J. A. J. Catal. 2008, 253, 167−179. (e) Segobia, D. J.; Trasarti, A. F.; Apesteguía, C. R. Appl. Catal., A 2012, 445−446, 69− 75. (f) Muratsugu, S.; Kityakarn, S.; Wang, F.; Ishiguro, N.; Kamachi, T.; Yoshizawa, K.; Sekizawa, O.; Uruga, T.; Tada, M. Phys. Chem. Chem. Phys. 2015, 17, 24791−24802. (10) Jiao, H.; Junge, K.; Alberico, E.; Beller, M. J. Comput. Chem. 2016, 37, 168−176. (11) Krupka, J.; Pasek, J. Curr. Org. Chem. 2012, 16, 988−1004. (12) (a) Kordesch, M. E.; Lindner, T.; Somers, J.; Stenzel, W.; Conrad, H.; Bradshaw, A. M.; Williams, G. P. Spectrochim. Acta Part A 1987, 43, 1561−1566. (b) Cassuto, A.; Tourillon, G. Vacuum 1995, 46, 583−585. (13) (a) Friend, C. M.; Muetterties, E. L.; Gland, J. J. Phys. Chem. 1981, 85, 3256−3262. (b) Sexton, B. A.; Avery, N. R. Surf. Sci. 1983, 129, 21−36. (c) Avery, N. R.; Matheson, T. W.; Sexton, B. A. Appl. Surf. Sci. 1985, 22−23, 384−391. (d) Kordesch, M. E.; Stenzel, W.; Conrad, H. Surf. Sci. 1986, 175, L687−L692. (e) Serafin, J. G.; Friend, C. M. J. Phys. Chem. 1988, 92, 6694−6700. (f) Hubbard, A. T.; Cao, E. Y.; Stern, D. A. Electrochim. Acta 1994, 39, 1007−1014. (14) Villegas, I.; Weaver, M. J. J. Am. Chem. Soc. 1996, 118, 458−466. (15) (a) Ortiz-Hernandez, I.; Owens, D. J.; Strunk, M. R.; Williams, C. T. Langmuir 2006, 22, 2629−2639. (b) Ortiz-Hernandez, I.; Williams, C. T. Langmuir 2007, 23, 3172−3178. (16) Oliva, C.; van den Berg, C.; Niemantsverdriet, J. W.; CurullaFerré, D. J. Catal. 2007, 245, 436−445. (17) Oliva, C.; van den Berg, C.; Niemantsverdriet, J. W.; CurullaFerré, D. J. Catal. 2007, 248, 38−45. (18) (a) Friend, C. M.; Serafin, J. G. J. Chem. Phys. 1988, 88, 4037− 4045. (b) Stevens, P. A.; Madix, R. J.; Friend, C. M. Surf. Sci. 1988, 205, 187−206. (19) (a) Storhoff, B. N.; Lewis, H. C., Jr Coord. Chem. Rev. 1977, 23, 1−29. (b) Michelin, R. A.; Mozzon, M.; Bertani, R. Coord. Chem. Rev. 1996, 147, 299−338. (20) (a) Andrews, M. A.; Kaesz, H. D. J. Am. Chem. Soc. 1979, 101, 7255−7259. (b) Andrews, M. A.; Knobler, C. B.; Kaesz, H. D. J. Am. Chem. Soc. 1979, 101, 7260−7264. (c) Keller, E.; Wolters, D. Chem. Ber. 1984, 117, 1572−1582. (d) Suter, P.; Vahrenkamp, H. Chem. Ber. 1995, 128, 793−797. (e) Dönnecke, D.; Halbauer, K.; Imhof, W. J. Organomet. Chem. 2004, 689, 2707−2719. (21) (a) Bernhardt, W.; Vahrenkamp, H. Angew. Chem., Int. Ed. Engl. 1984, 23, 381. (b) Lausarot, P. M.; Turini, M.; Vaglio, G. A.; Valle, M.; Tiripicchio, A.; Camellini, M. T.; Gariboldi, P. J. Organomet. Chem. 1984, 273, 239−246. (22) Lausarot, P. M.; Vaglio, G. A.; Valle, M.; Tiripicchio, A.; Tiripicchio Camellini, M.; Gariboldi, P. J. Organomet. Chem. 1985, 291, 221−229. (23) Andreu, P. L.; Cabeza, J. A.; del Río, I.; Riera, V.; Bois, C. Organometallics 1996, 15, 3004−3010. (24) Bois, C.; Cabeza, J. A.; Franco, R. J.; Riera, V. c.; Saborit, E. J. Organomet. Chem. 1998, 564, 201−207. (25) Takao, T.; Kawashima, T.; Matsubara, K.; Suzuki, H. Organometallics 2005, 24, 3371−3374.
NMR spectra of 2−11, 13, and 14, VT-NMR spectra of 2, 10, and 13, crystal data of 4, 7, 9, 11, and 13, preliminary results of an XRD study for 14, and results of DFT calculations for 2 and 5 (PDF) Atom coordinates of the optimized structures of 2 and 5 (XYZ) Accession Codes
CCDC 1827738−1827742 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.T.:
[email protected]. ORCID
Toshiro Takao: 0000-0002-5393-112X Hiroharu Suzuki: 0000-0002-9718-1375 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by JST ACT-C Grant No. JPMJCR12YA. The numerical calculations were carried out on the TSUBAME2.5 supercomputer at the Tokyo Institute of Technology, Tokyo, Japan.
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REFERENCES
(1) (a) de Bellefon, C.; Fouilloux, P. Catal. Rev.: Sci. Eng. 1994, 36, 459−506. (b) Gomez, S.; Peters Joop, A.; Maschmeyer, T. Adv. Synth. Catal. 2002, 344, 1037−1057. (2) Kukula, P.; Studer, M.; Blaser, H. U. Adv. Synth. Catal. 2004, 346, 1487−1493. (3) Xie, X.; Liotta, C. L.; Eckert, C. A. Ind. Eng. Chem. Res. 2004, 43, 7907−7911. (4) Chatterjee, M.; Sato, M.; Kawanami, H.; Yokoyama, T.; Suzuki, T.; Ishizaka, T. Adv. Synth. Catal. 2010, 352, 2394−2398. (5) (a) Werkmeister, S.; Junge, K.; Beller, M. Org. Process Res. Dev. 2014, 18, 289−302. (b) Bagal, D. B.; Bhanage, B. M. Adv. Synth. Catal. 2015, 357, 883−900. (6) (a) Yoshida, T.; Okano, T.; Otsuka, S. J. Chem. Soc., Chem. Commun. 1979, 870−871. (b) Grey, R. A.; Pez, G. P.; Wallo, A. J. Am. Chem. Soc. 1981, 103, 7536−7542. (c) Enthaler, S.; Addis, D.; Junge, K.; Erre, G.; Beller, M. Chem. - Eur. J. 2008, 14, 9491−9494. (d) Enthaler, S.; Junge, K.; Addis, D.; Erre, G.; Beller, M. ChemSusChem 2008, 1, 1006−1010. (e) Addis, D.; Enthaler, S.; Junge, K.; Wendt, B.; Beller, M. Tetrahedron Lett. 2009, 50, 3654− 3656. (f) Miao, X.; Bidange, J.; Dixneuf Pierre, H.; Fischmeister, C.; Bruneau, C.; Dubois, J. L.; Couturier, J. L. ChemCatChem 2012, 4, 1911−1916. (g) Werkmeister, S.; Bornschein, C.; Junge, K.; Beller, M. Chem. - Eur. J. 2013, 19, 4437−4440. (7) (a) Takemoto, S.; Kawamura, H.; Yamada, Y.; Okada, T.; Ono, A.; Yoshikawa, E.; Mizobe, Y.; Hidai, M. Organometallics 2002, 21, 3897−3904. (b) Li, T.; Bergner, I.; Haque, F. N.; Zimmer-De Iuliis, M.; Song, D.; Morris, R. H. Organometallics 2007, 26, 5940−5949. (c) Lu, Z.; Williams, T. J. Chem. Commun. 2014, 50, 5391−5393. (d) Mukherjee, A.; Srimani, D.; Chakraborty, S.; Ben-David, Y.; Milstein, D. J. Am. Chem. Soc. 2015, 137, 8888−8891. (e) Chakraborty, S.; Leitus, G.; Milstein, D. Chem. Commun. 2016, 52, 1812−1815. (f) Elangovan, S.; Topf, C.; Fischer, S.; Jiao, H.; Spannenberg, A.; Baumann, W.; Ludwig, R.; Junge, K.; Beller, M. J. Am. Chem. Soc. 2016, 138, 8809−8814. O
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
M.; Wolf, E.; Espitia, D. Organometallics 1997, 16, 2665−2673. (e) Gervasio, G.; Marabello, D.; Sappa, E.; Secco, A. J. Cluster Sci. 2007, 18, 67−85. (f) Cabeza, J. A.; del Río, I.; Pérez-Carreño, E.; Pruneda, V. Chem. - Eur. J. 2010, 16, 5425−5436. (41) (a) Howell, J. A. S.; Mathur, P. J. Chem. Soc., Chem. Commun. 1981, 263−265. (b) Howell, J. A. S.; Mathur, P. J. Chem. Soc., Dalton Trans. 1982, 43−50. (c) Rosenberg, E.; Kabir, S. E.; Hardcastle, K. I.; Day, M.; Wolf, E. Organometallics 1990, 9, 2214−2217. (d) Beringhelli, T.; D’Alfonso, G.; Minoja, A.; Ciani, G.; Moret, M.; Sironi, A. Organometallics 1991, 10, 3131−3138. (e) Day, M.; Espitia, D.; Hardcastle, K. I.; Kabir, S. E.; Rosenberg, E.; Gobetto, R.; Milone, L.; Osella, D. Organometallics 1991, 10, 3550−3559. (f) Rosenberg, E.; Freeman, W.; Carlos, Z.; Hardcastle, K.; Yoo, Y. J.; Milone, L.; Gobetto, R. J. Cluster Sci. 1992, 3, 439−457. (g) Casey, C. P.; Widenhoefer, R. A.; Hallenbeck, S. L.; Gavney, J. A. J. Chem. Soc., Chem. Commun. 1993, 1692−1694. (h) Day, M.; Espitia, D.; Hardcastle, K. I.; Kabir, S. E.; McPhillips, T.; Rosenberg, E.; Gobetto, R.; Milone, L.; Osella, D. Organometallics 1993, 12, 2309− 2324. (i) Casey, C. P.; Widenhoefer, R. A.; Hallenbeck, S. L.; Hayashi, R. K.; Gavney, J. A. Organometallics 1994, 13, 4720−4731. (j) Cabeza, J. A.; del Río, I.; Goite María, C.; Pérez-Carreño, E.; Pruneda, V. Chem. - Eur. J. 2009, 15, 7339−7349. (k) Takao, T.; Kawashima, T.; Kanda, H.; Okamura, R.; Suzuki, H. Organometallics 2012, 31, 4817−4831. (42) Deabate, S.; Giordano, R.; Sappa, E. J. Cluster Sci. 1997, 8, 407− 459. (43) (a) Couture, C.; Farrar, D. H. J. Chem. Soc., Dalton Trans. 1987, 2245−2252. (b) Bantel, H.; Suter, P.; Vahrenkamp, H. Organometallics 1995, 14, 4424−4426. (44) Takahashi, Y.; Nakajima, Y.; Suzuki, H.; Takao, T. Organometallics 2017, 36, 3539−3552. (45) Takao, T.; Takemori, T.; Moriya, M.; Suzuki, H. Organometallics 2002, 21, 5190−5203. (46) Boyar, E.; Deeming, A. J.; Felix, M. S. B.; Kabir, S. E.; Adatia, T.; Bhusate, R.; McPartlin, M.; Powell, H. R. J. Chem. Soc., Dalton Trans. 1989, 5−12. (47) Edidin, R. T.; Norton, J. R.; Mislow, K. Organometallics 1982, 1, 561−562. (48) The XRD study of 14 was carried out, and the formation of μbenzylamido and μ3-CO ligands were confirmed. However, owing to the presence of disordered structures regarding the positions of the μamido ligand, we could not refine the structure. The preliminary results of the XRD study are shown in Figure S42 in the Supporting Information. (49) (a) Forrow, N. J.; Knox, S. A. R. J. Chem. Soc., Chem. Commun. 1984, 679−681. (b) Kang, B. S.; Koelle, U.; Thewalt, U. Organometallics 1991, 10, 2569−2573. (50) Suzuki, H.; Kakigano, T.; Tada, K.-i.; Igarashi, M.; Matsubara, K.; Inagaki, A.; Oshima, M.; Takao, T. Bull. Chem. Soc. Jpn. 2005, 78, 67−87. (51) PROCESS-AUTO: Automatic Data Acquisition and Processing Package for Imaging Plate Diffractometer; Rigaku Corporation: Tokyo, Japan, 1998. (52) (a) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112−122. (b) Sheldrick, G. M. Acta Crystallogr., Sect. C: Struct. Chem. 2015, 71, 3−8. (53) Chai, J. D.; Head-Gordon, M. Phys. Chem. Chem. Phys. 2008, 10, 6615−6620. (54) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.;
(26) Joensen, H. A. N.; Hansson, G. K.; Kozlova, S. G.; Gushchin, A. L.; Søtofte, I.; Ooi, B.-L. Inorg. Chem. 2010, 49, 1720−1727. (27) Takao, T.; Takaya, Y.; Murotani, E.; Tenjimbayashi, R.-i.; Suzuki, H. Organometallics 2004, 23, 6094−6096. (28) Kawashima, T.; Takao, T.; Suzuki, H. Angew. Chem., Int. Ed. 2006, 45, 485−488. (29) (a) Mays, M. J.; Prest, D. W.; Raithby, P. R. J. Chem. Soc., Chem. Commun. 1980, 171−173. (b) Adams, R. D.; Katahira, D. A.; Yang, L.W. J. Organomet. Chem. 1981, 219, 85−101. (c) Adams, R. D.; Katahira, D. A.; Yang, L.-W. J. Organomet. Chem. 1981, 219, 241−249. (d) Dawoodi, Z.; Mays, M. J.; Raithby, P. R. J. Organomet. Chem. 1981, 219, 103−113. (e) Prest, D. W.; Mays, M. J.; Raithby, P. R. J. Chem. Soc., Dalton Trans. 1982, 2021−2028. (f) Churchill, M. R.; Wasserman, H. J.; Belmonte, P. A.; Schrock, R. R. Organometallics 1982, 1, 559−561. (g) Evans, W. J.; Meadows, J. H.; Hunter, W. E.; Atwood, J. L. J. Am. Chem. Soc. 1984, 106, 1291−1300. (h) Banford, J.; Mays, M. J.; Raithby, P. R. J. Chem. Soc., Dalton Trans. 1985, 1355− 1360. (i) Edwards, K. J.; Field, J. S.; Haines, R. J.; Homann, B.; Sundermeyer, J.; Woollam, S. F. J. Organomet. Chem. 1990, 386, C1− C6. (j) Fryzuk, M. D.; Piers, W. E.; Rettig, S. J. Can. J. Chem. 1992, 70, 2381−2389. (k) Garcia Alonso, F. J.; Garcia Sanz, M.; Riera, V.; Anillo Abril, A.; Tiripicchio, A.; Ugozzoli, F. Organometallics 1992, 11, 801− 808. (l) He, Z.; Neibecker, D.; Lugan, N.; Mathieu, R. Organometallics 1992, 11, 817−821. (m) Tada, K.-i.; Oishi, M.; Suzuki, H.; Tanaka, M. Organometallics 1996, 15, 2422−2424. (n) Edwards, K. J.; Field, J. S.; Haines, R. J.; Homann, B. D.; Stewart, M. W.; Sundermeyer, J.; Woollam, S. F. J. Chem. Soc., Dalton Trans. 1996, 4171−4181. (o) Schollhammer, P.; Cabon, N.; Petillon, F. Y.; Talarmin, J.; Muir, K. W. Chem. Commun. 2000, 2137−2138. (p) Cabon, N.; Petillon, F. Y.; Schollhammer, P.; Talarmin, J.; Muir, K. W. Dalton Trans. 2004, 2708−2719. (30) Kanda, H.; Kawashima, T.; Takao, T.; Suzuki, H. Organometallics 2012, 31, 1917−1926. (31) Takao, T.; Kakuta, S.; Tenjimbayashi, R.-i.; Takemori, T.; Murotani, E.; Suzuki, H. Organometallics 2004, 23, 6090−6093. (32) Michelin Lausarot, P.; Operti, L.; Vaglio, G. A.; Valle, M.; Tiripicchio, A.; Tiripicchio Camellini, M.; Gariboldi, P. Inorg. Chim. Acta 1986, 122, 103−109. (33) (a) Zippel, T.; Arndt, P.; Ohff, A.; Spannenberg, A.; Kempe, R.; Rosenthal, U. Organometallics 1998, 17, 4429−4437. (b) Lin, C.-J.; Hwang, W.-S.; Chiang, M. Y. J. Organomet. Chem. 2001, 640, 85−92. (c) Wu, C.-Y.; Chen, Y.; Jing, S.-Y.; Lee, C.-S.; Dinda, J.; Hwang, W.-S. Polyhedron 2006, 25, 3053−3065. (34) (a) Andrews, M. A.; Kaesz, H. D. J. Am. Chem. Soc. 1979, 101, 7238−7244. (b) Andrews, M. A.; Van Buskirk, G.; Knobler, C. B.; Kaesz, H. D. J. Am. Chem. Soc. 1979, 101, 7245−7254. (c) Dönnecke, D.; Wunderle, J.; Imhof, W. J. Organomet. Chem. 2004, 689, 585−594. (35) Nakajima, Y.; Suzuki, H. Organometallics 2003, 22, 959−969. (36) Nakajima, Y.; Kameo, H.; Suzuki, H. Angew. Chem., Int. Ed. 2006, 45, 950−952. (37) Nakajima, Y.; Suzuki, H. Organometallics 2005, 24, 1860−1866. In this paper, we reported the wrong description for the hydrogenation of 4. We confirmed quantitative formation of 1 by the 1H NMR analysis of the residual solid, which was obtained by the removal of the solvent. However, we overlooked the generation of N-benzylidenebenzylamine owing to its small content. We reinvestigated the reaction and analyzed the reaction solution by GC, which showed the formation of a considerable amount of N-benzylidenebenzylamine in addition to benzylamine. We correct the description of the hydrogenation of 4 as shown ineq 4. (38) Adams, R. D.; Babin, J. E. Organometallics 1987, 6, 1364−1365. (39) Adams, R. D.; Pompeo, M. P.; Tanner, J. T. Organometallics 1991, 10, 1068−1078. (40) (a) Aime, S.; Gobetto, R.; Padovan, F.; Botta, M.; Rosenberg, E.; Gellert, R. W. Organometallics 1987, 6, 2074−2078. (b) Süss-Fink, G.; Jenke, T.; Heitz, H.; Angela Pellinghelli, M.; Tiripicchio, A. J. Organomet. Chem. 1989, 379, 311−323. (c) Imhof, W. J. Chem. Soc., Dalton Trans. 1996, 1429−1436. (d) Rosenberg, E.; Milone, L.; Gobetto, R.; Osella, D.; Hardcastle, K.; Hajela, S.; Moizeau, K.; Day, P
DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D.01; Gaussian, Inc., Wallingford, CT, 2013. (55) Dennington, R.; Keith, T.; Millam, J. GaussView, version 5.0; Semichem Inc., Shawnee Mission, KS, 2009.
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DOI: 10.1021/acs.organomet.8b00165 Organometallics XXXX, XXX, XXX−XXX
