Synthesis of Triruthenium Complexes Containing a Triply Bridging

Article pubs.acs.org/Organometallics

Synthesis of Triruthenium Complexes Containing a Triply Bridging Pyridyl Ligand and Its Transformations to Face-Capping Pyridine and Perpendicularly Coordinated Pyridyl Ligands Toshiro Takao, Takashi Kawashima, Hideyuki Kanda, Rei Okamura, and Hiroharu Suzuki* Department of Applied Chemistry, Graduate School of Science and Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152-8552, Japan S Supporting Information *

ABSTRACT: Unlike the reactions of carbonyl clusters with pyridine leading to the formation of μ-pyridyl complexes, the reaction of the triruthenium pentahydrido complex {Cp*Ru(μ-H)}3(μ3-H)2 (Cp* = η5-C5Me5) (1) with pyridines provided μ3-η2(//)-pyridyl complexes, (Cp*Ru)3(μ-H)4(μ3η2(//)-RC5H3N) (2a, R = H; 2b, R = 4-COOMe; 2c, R = 4COOEt; 2d, R = 4-Me; 2e, R = 5-Me), in which the molecular plane of the pyridyl group was tilted with respect to the Ru3 plane. Electron-rich metal centers of the trimetallic core enabled back-donation to the pyridyl group, which caused the additional π-coordination of the CN bond. The electron-rich metal centers of 2a−2c also promoted further transformation into face-capping pyridine complexes {Cp*Ru(μ-H)}3(μ3-η2:η2:η2-RC5H4N) (3a, R = H; 3b, R = 4-COOMe; 3c, R = 4-COOEt) upon heating. In contrast, the thermolysis of 2d did not afford a face-capping picoline complex because of the poor electronaccepting ability of the picolyl moiety. Instead, the coordinatively unsaturated μ3-picolyl complex (Cp*Ru)3(μ-H)2(μ3-η2-4-MeC5H3N) (4d) was obtained. Owing to its unsaturated nature, 4d can react with γ-picoline to yield 4,4′-dimethyl-2,2′-bipyridine. Although the reaction rate was slow, complex 1 catalyzed the dehydrogenative coupling of 4-substituted pyridines containing an electron-donating group. The protonation of 2a also afforded the coordinatively unsaturated pyridyl complex [(Cp*Ru)3(μH)2(μ3-H)(μ3-η2:η2(⊥)-C5H4N)]+ (5a), but the coordination mode of the pyridyl group in 5a was completely different from that in 4d. The pyridyl moiety in 5a was coordinated on one of the Ru−Ru bonds in a perpendicular fashion. The methylation of the face-capping pyridine complex 3a, which led to the formation of the N-methyl pyridinium complex [(Cp*Ru)3(μ-H)3 (μ3η2:η2:η2-C5H5NMe)]+ (7b) was also examined. NMR studies on 7b as well as X-ray diffraction studies suggested enhanced backdonation to the pyridinium moiety because of the localized cationic charge on the nitrogen atom.

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INTRODUCTION The chemistry of organic molecules on a metal surface has attracted considerable attention in relation to heterogeneous catalysis. The interaction of an aromatic compound with a metal surface is one of the most intensively studied subjects in this area, and various adsorption modes of arenes have thus far been elucidated by means of high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED) study, scanning probe microscopy (SPM), and so on.1 For example, Somorjai and co-workers used LEED to elucidate that benzene is adsorbed on a 3-fold site of Rh(111) in a flat manner with respect to the surface in the presence of CO.1b In contrast to benzene, pyridine can interact with a metal surface by both π electrons and the lone-pair electrons at the nitrogen atom. This produces the interesting chemistry of chemisorbed pyridine, in which the pyridine is converted from a relatively flat-lying π-bonded species to a tilted nitrogenbonded species.2 Various adsorption modes of pyridine have thus far been elucidated on several metal surfaces by means of HREELS,2a−d near-edge X-ray absorption fine structure (NEXAFS) measurements,2e,f SPM,2g,h and so on. In addition, © 2012 American Chemical Society

the chemisorption of pyridine is related to the hydrodenitrogenation process and the poisoning of catalysis. Thus, it is important to understand the interaction of pyridine with a metal surface in detail to reveal the mechanisms of these processes. Polynuclear organometallic compounds have been shown to serve as an appropriate model for the metal surface.3 The spectroscopic data of the well-defined cluster compounds is often used for the characterization of the chemisorbed species, and the structural data of these compounds provide detailed information about the chemisorbed species. More importantly, information about the reactivity of the chemisorbed species is also provided by the cluster chemistry. Lewis and co-workers succeeded in the synthesis of a trimetallic complex having a μ3η2:η2:η2-benzene ligand, which is a counterpart of the adsorbed benzene at the 3-fold site.4 They elucidated not only the structure but also its reactivities. In addition to a face-capping arene complex, various coordination modes of arenes, such as Received: May 6, 2012 Published: June 26, 2012 4817

dx.doi.org/10.1021/om300379d | Organometallics 2012, 31, 4817−4831

Organometallics

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μ3-,5 μ4-, and μ5-benzyne complexes,6 have been prepared, and their reactivities have also been investigated. While various adsorption modes of pyridine on a metal surface have been proposed, pyridine has been shown to attach to a multimetallic core in a quite limited mode. This is in contrast to the extensive chemistry of arene clusters. Since Yin and Deeming synthesized the first trimetallic μ-pyridyl complex Os3(CO)10(μ-H)(μ-C5H4N) by the reaction of Os3(CO)12 with pyridine,7 several trimetallic complexes containing a μpyridyl ligand have been prepared.8 However, whereas many μ3-benzyne complexes are known, coordination of a pyridyl group in a μ3 fashion is less well known.9 In addition, there have been no reports on a polynuclear complex containing a face-capping μ3-pyridine ligand until we reported the synthesis of {Cp*Ru(μ-H)}3(μ3-η2:η2:η2-C5H6N) (Cp* = η5-C5Me5) (2a) in a previous communication.10 We report herein novel coordination modes of pyridine on a trimetallic core, μ3-η2(//)pyridyl, μ3-η2:η2(⊥)-pyridyl, and face-capping pyridine ligands, as well as their reactivities.

Figure 1. Time course of the reaction of {Cp*Ru(μ-H)}3(μ3-H)2 (1) with pyridine, yielding μ3-η2(//)-pyridyl complex 2a and μ3-η2:η2:η2pyridine complex 3a (10 equiv of pyridine, C6D6, 80 °C).

and bond distances around the N(1) atom with those of 2c, which possesses an ethoxycarbonyl group at the 4-position (Figure 2b). The positions of the four hydrido ligands in 2a were not determined during the diffraction study, but they were located in 2c, as shown in Figure 2b. The pyridyl ligand in 2a acts as a 5e donor, and this makes complex 2a coordinatively saturated according to the EAN rule. Such a coordination mode is often observed in the trimetallic imidoyl complexes,12 but it has never been reported in a trimetallic pyridyl complex. All of the trimetallic pyridyl complexes that have ever been reported adopted a μ2coordination, which acts as a 3e donor.8 While the pyridine ring is nearly orthogonal to the trimetallic plane in the μ-pyridyl complexes, that of 2a is tilted from the normal of the Ru3 plane due to the π-coordination of the CN bond to Ru(1); the dihedral angle between the pyridine ring and the Ru3 plane is estimated at ca. 65°. While complex 2a is the first discrete μ3-η2(//)-pyridyl complex, the tilted coordination of an α-pyridyl species has been elucidated on a Rh(111) surface;2a Somorjai and coworkers proposed a tilted structure for a κ(C,N)-pyridyl species on the basis of the LEED patterns and the HREELS spectrum, in which the tilted angle was estimated to be between 40° and 60°. This fact indicates that the triruthenium core derived from 1 can faithfully reproduce the nature of a closed-packed metal surface. Unlike the traditional carbonyl clusters, complex 2 does not possess an electron-withdrawing group, such as CO. Furthermore, the trimetallic core of 2 is composed of hydrido ligands and strong electron-donating Cp* groups. Therefore, the metal centers of 2 should remain electron-rich, which enables additional back-donation to the π*(CN) of the pyridyl group. The pyridine ring is σ-bonded to Ru(2) and Ru(3) through the N(1) and the C(1) atoms, respectively (Ru(2)−N(1) = 2.077(2) Å, Ru(3)−C(1) = 2.048(3) Å). The N(1) and the C(1) atoms are also π-bonded to Ru(1) (Ru(1)−N(1) = 2.181(3) Å, Ru(1)−C(1) = 2.201(3) Å). The N(1)C(1) distance (1.413(4) Å) lies in the reported range for the NC bond distances in the μ3-η2(//)-imidoyl complexes (1.280− 1.431 Å).12 Because of this π-coordination, the π electrons in the pyridyl group should be localized; the C(1)−C(2), C(3)− C(4), and N(1)−C(5) bonds (1.427(4), 1.423(5), and 1.405(4) Å, respectively) are considerably longer than the C(2)C(3) and C(4)C(5) bonds (1.358(5) and 1.371(5) Å, respectively). The 13C signal of the imino carbon appeared at δ 168.3. This value is comparable to those of the μ-pyridyl

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RESULTS AND DISCUSSION We have shown that the reaction of triruthenium pentahydrido complex 1 with excess amounts of pyridine afforded the facecapping pyridine complex 3a.10 During the initial stage of the reaction carried out at 80 °C, the formation of intermediate 2a, which has a μ3-η2(//)-pyridyl group, was observed (eq 1).

Once the distribution of the intermediate 2a reached 60%, its concentration gradually decreased over time. At that point, the yield of face-capping pyridine complex 3a was 12%, which increased to 80% with prolonged heating. The time course of the reaction is shown in Figure 1. The μ3-pyridyl group was apparently formed via C−H bond cleavage at the α-position. The α-C−H bond cleavage of pyridine is a well-known phenomenon in cluster chemistry9 as well as in surface chemistry.2a−c,e,g,i,11 The facile C−H bond scission is thought to be caused by the cooperative interaction of the neighboring metal centers with pyridine. The nitrilesubstituted triosmium cluster Os3(CO)10(MeCN)2 has been shown to react with pyridine even at ambient temperature.9b The μ3-pyridyl complex 2a was isolated from the mixture by recrystallization, and the molecular structure was determined by X-ray diffraction. Figure 2a clearly shows that the pyridyl ligand is attached to a trimetallic core in a μ3-η2(//) fashion. The relevant bond lengths and angles are listed in Table 1 with the data of 2c. Although it is difficult to distinguish between nitrogen and carbon atoms only on the basis of the value of the cross section during the diffraction study, the position of N(1) was determined by comparing the isotropic temperature factors 4818


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Figure 2. Molecular structures of (a) 2a and (b) 2c with thermal ellipsoids at the 30% level of probability. A solvent molecule (diethyl ether) in the unit cell of 2a was omitted for clarity.

structure of 2c was determined by X-ray diffraction, and the structure is shown in Figure 2b. The results of the structural study of the face-capping pyridine complexes 3b and 3c will be mentioned later. The presence of an ethoxycarbonyl group at the 4-position enables the straightforward assignment of the N(1) atom, as shown in Figure 2b. In addition, the positions of the hydrido ligands were also determined in 2c. The H(1), H(3), and H(4) atoms are located on each Ru−Ru bond on the face opposite to the pyridyl group, and the H(2) atom bridges the Ru(2) and Ru(3) atoms on the same side as the pyridyl group. The structural parameters of 2c are almost the same as those of 2a. The μ3-η2(//)-pyridyl complex 2d was also obtained by the reaction of 1 with γ-picoline. In this case, however, a facecapping pyridine complex was not produced upon prolonged heating. This marked difference is attributable to the electrondonating nature of the methyl group on the pyridine ring because, as the formation of 3c demonstrates, the face-capping coordination of pyridine to the Ru3 plane is possible even though it contains a bulkier substituent. The face-capping coordination would receive a more significant back-donation from the trimetallic core than the μ3-η2-coordination does. The presence of an electron-donating group on a pyridine ring raises the energy level of the π* orbitals considerably, which makes the face-capping coordination of γ-picoline unfavorable. The 1H NMR spectra of 2d were recorded at various temperatures, as shown in Figure 3. While three sharp Cp* signals were observed at δ 1.88, 1.82, and 1.56 at −80 °C, the two that resonated at δ 1.88 and 1.56 became broader and flatter at −40 °C. At this temperature, the shape of the Cp* signal at δ 1.82 ppm remained unchanged. Above −20 °C, all Cp* signals became broader and coalesced into one peak resonating at δ 1.85. This spectral change is rationalized by the combination of the two dynamic processes of the μ3-pyridyl group accompanied by the hopping of a hydrido ligand between the two Ru−Ru bonds, as shown in Scheme 1. We could not determine which was the lower-energy process, the pivot motion on the carbon atom or that of the nitrogen atom, but the pivot motion of a μ3η2(//)-imidoyl ligand on carbon has often been proposed for the dynamic process in the trimetallic imidoyl complexes.15 Rosenberg and co-workers suggested that the pivot motion on the carbon atom is the lower-energy process in Os3(CO)8(L)(μ-H)(μ3-η2-CNCH2CH2CH2−) (L = CO, CNMe). Cabeza and co-workers elucidated that the pivot motion on the carbon atom is the first step for the rotation of the μ3-η2-imidoyl group

Table 1. Selected Bond Distances (Å) and Angles (deg) for 2a and 2c 2a Ru(1)−Ru(2) Ru(2)−Ru(3) Ru(1)−Ru(3) Ru(1)−N(1) Ru(1)−C(1) Ru(2)−N(1) Ru(3)−C(1) N(1)−C(1) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(4)−C(5) N(1)−C(5)

2c

2.8456(3) 2.8654(3) 2.8354(3) 2.181(3) 2.201(3) 2.077(2) 2.048(3) 1.413(4) 1.427(4) 1.358(5) 1.423(5) 1.371(5) 1.405(4) C(3)−C(6) C(6)−O(1) C(6)−O(2)

Ru(2)−Ru(1)−Ru(3) Ru(1)−Ru(2)−Ru(3) Ru(1)−Ru(3)−Ru(2) Ru(2)−N(1)−C(1) Ru(2)−N(1)−C(5) Ru(3)−C(1)−N(1) Ru(3)−C(1)−C(2)

60.581(8) 59.534(8) 59.884(8) 109.93(18) 129.8(2) 111.30(18) 129.9(2)

2.8385(5) 2.8600(5) 2.8496(5) 2.183(3) 2.173(4) 2.067(3) 2.038(4) 1.433(5) 1.420(5) 1.379(6) 1.430(6) 1.354(6) 1.386(5) 1.473(6) 1.217(5) 1.358(5) 60.370(11) 60.007(11) 59.622(12) 109.0(2) 130.1(3) 111.7(2) 130.5(3)

complexes, [Ir2(μ-ArNCH2NAr)(μ-C5H4N)2(Py)4]+ (δ 165.0, 162.4)13 and Ru3(CO)9(μ-H)(dpa) (dpa = di(2-pyridyl)amine) (δ 174.4).14 In the 1H NMR spectrum of 2a recorded at −80 °C, three sharp signals assignable to the Cp* groups were observed at δ 1.57, 1.81, and 1.87. This agreed with the molecular structure shown in Figure 2. These signals, however, became broader as the temperature increased and coalesced into one signal that resonated at δ 1.85. As will be mentioned later, this dynamic behavior arises from the motion of the μ3-η2(//)-pyridyl group moving around the Ru3 core. The four hydrido ligands resonated at δ −17.56, −17.06, −13.60, and −13.42 at −80 °C, which also became broader and flatter as the temperature increased. The reaction of 1 with methyl and ethyl isonicotinate also afforded both face-capping pyridine complexes 3b and 3c as well as μ3-η2(//)-pyridyl complexes 2b and 2c. Whereas complex 2a is decomposed on alumina, complexes 2b and 2c can be purified by column chromatography on alumina. The 4819


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Figure 3. VT-NMR spectra of μ3-η2(//)-pyridyl complex 2d in THF-d8 showing the Cp* (left) and the hydrido (right) regions. The Cp* signal appearing at δ 1.98 was derived from {Cp*Ru(μ-H)}3(μ3-H)2 (1), and the asterisked peak was derived from the residual protons of the solvent.

signals were still sharp. Thus, the signals at δ −13.68 and −13.34 were assigned to Ha and Hc. At higher temperatures, the environments of Ha, Hb, and Hc become equivalent because of the additional pivot motions of the pyridyl group. At −20 °C, the signal resonating at δ −17.48 started to broaden. At that point, the shape of the signal at δ −17.08 remained unchanged. Therefore, the signal at δ −17.48 is assignable to Hb and that observed at δ −17.08 can be assigned to Hd. Above room temperature, the signal of Hd became broader. This most likely arose from the direct site exchange between Hd and other hydrides. The reaction of 1 with β-picoline resulted in the exclusive formation of the μ3-η2(//)-picolyl complex 2e (eq 2). Although

Scheme 1. Dynamic Behavior of μ3-η2(//)-Pyridyl Complex 2

by the aid of DFT calculation.16 In the low-temperature region, the pyridyl group exhibits the first pivot motion only between the two Ru−Ru bonds, most likely a pivot motion on the carbon atom, which brings a time-averaged CS structure. Hence, the Cp* signals are seen as a set of two signals with an intensity ratio of 2:1. In the high-temperature region, the pyridyl group migrates to all of the Ru−Ru bonds. This motion requires the additional pivot motion of the pyridyl group, probably a pivot motion on the nitrogen atom. These successive pivot motions cause the environments of the Cp* groups to be equivalent. The successive pivot motions of the μ3-η2(//)-imidoyl ligand on its M−C and M−N bonds on a trimetallic plane have been clearly elucidated in (Cp*Co)3(μ-H)(μ3-η2-HCNCMe3)17 and Ru3(CO)9(μ-H)(μ3-η2-MeCNMe),18 but that of a μpyridyl group has not been reported so far. In the hydrido region, four sharp signals were observed at δ −17.48, −17.08, −13.68, and −13.34 at −80 °C. The pivot motion of the pyridyl group makes two of the four hydrido ligands, Ha and Hc in Scheme 1, to be equivalent if the pivot motion on the carbon atom is the lower-energy process. As seen in the 1H NMR spectrum at −40 °C, only the two signals appearing at the lower magnetic field coalesced, while the other

β-picoline possesses two different α-protons, C−H bond cleavage occurred only at the 6-position. This can be rationalized by the steric repulsion between the Cp* groups and the methyl group on the pyridine ring; if the C−H bond at the 2-position was broken, the μ3-picolyl group should be placed on the Ru3 plane with the methyl group directed toward the Cp*group. The fact that 1 did not react with α-picoline also shows that the position of the substituents on a pyridine ring is crucial for the μ3-η2(//)-coordination. The 1H NMR spectrum of 2e resembled that of 2d, but the characteristic 1H signal of the α-CH group, which underwent a considerable downfield shift, appears as a singlet instead of the doublet found in the 1H NMR spectrum of 2d. In addition, a face-capping β-picoline complex was not formed upon further heating of 2e. Although further thermolysis of 2d did not afford a facecapping pyridine complex, thermolysis of 2d at 160 °C resulted in the exclusive formation of the coordinatively unsaturated μ34820


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2.85 Å, the Ru3 core of 4d forms an isosceles triangle, in which the Ru(1)−Ru(2) bond (2.9142(8) Å) is the longest. As for isoelectronic μ3-alkyne complexes, it is commonly accepted that an alkyne ligand is coordinated on a trimetallic core in parallel with one of the M−M bonds in a coordinatively saturated (48-electron) complex, whereas it adopts a different coordination mode in coordinatively unsaturated (46-electron) complexes, in which an alkyne ligand is coordinated perpendicular to one of the M−M bonds.19 In a model complex, {CpRu(μ-H)}3(HCCH) (Cp = C5H5), Morokuma and co-workers estimated that the perpendicular conformation is more stable by 4.6 kcal/mol than the parallel one.20 As a consequence of the symmetric properties of the frontier orbitals of the trimetallic fragment and the alkyne π orbitals, it has been shown that the perpendicular coordination of an alkyne ligand is preferred in a cluster having a 46e configuration, while the parallel mode is stable in a 48e cluster. This classification can be applied to the isoelectronic μ3-imidoyl complexes, in which all the coordinatively saturated μ3-imidoyl complexes adopt a parallel coordination mode,15−18,21 while we showed the perpendicular coordination of a μ 3 -imidoyl ligand in (Cp*Ru)3(μ3-η2:η2(⊥)-PhCNH)(μ-H)2, which adopts a 46e configuration.22 In coordinatively saturated complex 2, the pyridyl group is attached to the Ru3 core in a parallel fashion, which is in accord with the trend of the μ3-alkyne complexes. In contrast, the structure of 4d violates the above-mentioned “rule”, meaning that the μ3-pyridyl group in 4d is not coordinated to the Ru3 core in a perpendicular fashion, despite its 46e configuration. Although it is slightly distorted, the coordination mode is viewed as a parallel mode. Because of this slight distortion, complex 4d could not be effectively stabilized. Actually, complex 4d was quite unstable toward air and moisture. This instability could be crucial for the dehydrogenative coupling of γ-picoline (vide infra). In the 1H NMR spectrum of 4d, a broad signal for the hydrido ligands was observed at −80 °C. At 25 °C, the signal became sharper and resonated at δ −11.62 as a sharp singlet. The signals for the Cp* groups were observed at δ 1.62 and 2.09 as a set of broad signals with an intensity ratio of 1:2 at −80 °C, which coalesced into a sharp signal at δ 1.79 at 25 °C. These facts indicate that the γ-picolyl group in 4d moves around the Ru3 plane in the same fashion as the μ3-η2(//)pyridyl group in 2 does, but the motion is not frozen even at −80 °C. Thus, the dynamic process in 4d seems to be significantly faster than that found in 2. In the 13C NMR spectrum, the imidoyl carbon appeared at δ 154.7, which underwent a notable upfield shift compared with that of the μ3η2(//)-pyridyl complex 2c (δ 170.4). The perpendicularly coordinated μ3-pyridyl complex 5a was obtained upon protonation of 2a. Protonation of a hydrido

pyridyl complex 4d as a consequence of the elimination of dihydrogen (eq 3). Because of this coordinatively unsaturated

nature, 4d was highly air-sensitive and thus could not be isolated in an analytically pure form. However, a red single crystal of 4d was obtained from a cold diethyl ether solution, and its molecular structure was determined by X-ray diffraction, as shown in Figure 4. The relevant structural data are listed in Table 2.

Figure 4. Molecular structure of 4d with thermal ellipsoids at the 30% level of probability.

Figure 4 clearly represents the μ3-η2-coordination of the γpicolyl group in 4d. The picolyl group bridges the Ru(2) and the Ru(3) atoms and is π-bonded to Ru(1). Unlike coordinatively saturated complex 2, the Ru(1)−C(1) bond (2.220(5) Å) is longer than the Ru(1)−N(1) bond (2.129(4) Å) by 0.09 Å. In complexes 2a and 2c, the differences are minimal: 0.02 Å in 2a and 0.01 Å in 2c. This fact shows the distorted π-coordination of the γ-picolyl group in 4d. The two hydrido ligands, H(1) and H(2), are located on the Ru(1)− Ru(2) and Ru(2)−Ru(3) edges. Unlike the Ru3 core of 2 forming an approximate equilateral triangle with sides of ca. Table 2. Selected Bond Distances (Å) and Angles (deg) for 4d Ru(1)−Ru(2) Ru(1)−N(1) Ru(3)−C(1) C(2)−C(3) N(1)−C(5) Ru(2)−Ru(1)−Ru(3) Ru(2)−N(1)−C(1) Ru(3)−C(1)−C(2)

2.9146(9) 2.129(4) 2.049(5) 1.365(8) 1.396(6) 57.69(2) 106.0(3) 131.0(4)

Ru(2)−Ru(3) Ru(1)−C(1) N(1)−C(1) C(3)−C(4)

2.7401(8) 2.220(5) 1.394(6) 1.418(9)

Ru(1)−Ru(2)−Ru(3) Ru(2)−N(1)−C(5)

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58.29(2) 123.1(3)

Ru(1)−Ru(3) Ru(2)−N(1) C(1)−C(2) C(4)−C(5) Ru(1)−Ru(3)−Ru(2) Ru(3)−C(1)−N(1)

2.7580(11) 2.081(4) 1.415(7) 1.376(8) 64.024(19) 110.9(3)


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with its nitrogen atom directed outside the Ru3 core. The direction of the pyridyl group is the same as that found in the μ3-η2:η2(⊥)-imidoyl and the cationic μ3-η2:η2(⊥)-imidoyl complexes.22 The direction of the μ3-pyridyl group was also confirmed by the marked upfield shift of the inner carbon atom in the 13C NMR spectrum (δ 136.2). The 13C signals of the inner carbon atom underwent an upfield shift by ca. 30 ppm compared with that of 2a and were comparable to that of the cationic μ3-η2:η2(⊥)-imidoyl complex 6 (δ 139.3).22a As seen in Figure 5, the μ3-η2:η2(⊥)-pyridyl group was attached to the trimetallic core with its molecular plane normal to the Ru3 plane. α-Pyridyl species normal to the metal surface have been proposed for Ni(100)2b and Pt(111)2c,f by means of HREELS, NEXAFS, and EELS; however, only the angle between the molecular plane and metal surface was shown. The coordination mode of the pyridyl group was not taken into account. The structure of 5a clearly shows the possibility of the μ3-η2:η2(⊥)-coordination of the pyridyl group on a metal surface, in addition to the common μ-coordination. While the C(1)−N(1) bond length (1.368(7) Å) is less than that of the π-coordinated C−N bonds of 2a (1.413(4) Å), it still lies in the reported range for the π-coordinated CN bonds (1.280−1.431 Å).12 The C−N bond length is quite comparable to that of the cationic μ3-η2:η2(⊥)-imidoyl complex 6 (1.351(5) Å).21 The Ru(1)−C(1) length (2.084(6) Å) implies the hypervalent nature of the inner carbon atom as seen in 6 and the perpendicularly coordinated alkyne complexes.20b Complex 5a is fluxional, similar to other μ3-η2:η2(⊥)-imidoyl and μ3-η2:η2(⊥)-alkyne complexes that shows a switchback motion.20c As a consequence, signals for the Cp* groups were observed to be equivalent at room temperature and resonated at δ 1.85 as a sharp singlet. Thermolysis of 2a at 100 °C in THF-d8 in the absence of pyridine was then monitored by NMR spectroscopy. At the initial stage of the reaction, formation of 1, 3a, and a coordinatively unsaturated μ3-pyridyl complex, (Cp*Ru)3(μ3C5H4N)(μ-H)2 (4a), was observed along with the consumption of 2a. After 5.5 h, 57% of 2a was consumed and the yields of 1, 3a, and 4a were 21, 11, and 25%, respectively. At this time, the liberation of pyridine was also observed. The rate of decrease in 2a then dropped, and the concentrations of 1 and 4a began to decrease. Complex 4a was formed by the elimination of dihydrogen from 2a, as seen in the formation of 4d. The reaction of 2a with the liberated dihydrogen resulted in the immediate regeneration of 1. Actually, pentahydrido complex 1 was exclusively obtained upon treatment of 3a with 1 atm of H2 at 60 °C with the liberation of pyridine (eq 5). Although coordinatively

complex, followed by liberation of dihydrogen, corresponds to the removal of two electrons from a metal center. Thus, it can afford a 46e species. Treatment of 2a with HBF4 in diethyl ether resulted in the immediate precipitation of the cationic μ3η2:η2(⊥)-pyridyl complex 5a (eq 4). Complex 5a was stable

enough to be isolated in an analytically pure form. The stability resembled that of the cationic μ3-η2:η2(⊥)-imidoyl complex [(Cp*Ru)3(μ-η2:η2(⊥)-PhCNH)(μ-H)2(μ3-H)]+ (6).22a This is quite a contrast with the instability of 4d that adopts the same 46e configuration. Whereas protonation of the face-capping pyridine complex 3a occurred at the nitrogen atom to yield a face-capping pyridinium complex [{Cp*Ru(μ-H)}3(μ3-η2:η2:η2-C5H5N(H))]+ (7a),10 protonation of 2a took place at a metal center, leading to the elimination of dihydrogen. Because the lone-pair electrons at the nitrogen atom do not participate in bonding interaction in the face-capping complex 3, protonation can occur at the nitrogen atom. However, the lone-pair electrons were already used for the bonding to the metal center in 2. Therefore, protonation should occur at a metal center. While the perpendicular coordination of an aromatic ring to a trimetallic core has been often proposed as an intermediate state of a dynamic process arising from the μ3-coordinated arene ligand,23 complex 5a is the first isolated example of such a species. Complex 5a can be viewed as a model of the intermediate stage of the pivot motion of the μ3-η2(//)-pyridyl on the carbon atom. An X-ray diffraction study unambiguously showed the perpendicular coordination of the μ3-pyridyl group in 5a (Figure 5). The structural data of 5a is listed in Table 3. The Ru3 core forms an isosceles triangle with sides of 2.7389(5), 2.7525(5), and 2.9873(6) Å, in which the pyridyl group is perpendicularly coordinated to the longest Ru(2)− Ru(3) bond. The pyridyl group is coordinated to the Ru3 core

unsaturated 4a was not isolated from the mixture, it was assumed that 4a would also react smoothly with dihydrogen to regenerate 2a and 1. Thus, at the initial stage, there would be a pre-equilibrium between 1, 2a, and 4a via addition and

Figure 5. Molecular structure of 5a with thermal ellipsoids at the 30% level of probability. An anionic part (BF4−) was omitted for clarity. 4822


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Table 3. Selected Bond Distances (Å) and Angles (deg) for 5a Ru(1)−Ru(2) Ru(1)−C(1) Ru(3)−N(1) C(1)−C(2) C(4)−C(5) Ru(2)−Ru(1)−Ru(3) Ru(2)−C(1)−Ru(3) Ru(1)−C(1)−C(2)

2.7389(5) 2.084(6) 2.049(5) 1.429(9) 1.345(10) 65.912(14) 74.63(17) 126.5(5)

Ru(2)−Ru(3) Ru(2)−N(1) Ru(3)−C(1) C(2)−C(3) N(1)−C(5) Ru(1)−Ru(2)−Ru(3) Ru(2)−N(1)−Ru(3)

2.9873(6) 2.119(5) 2.576(6) 1.370(11) 1.384(7) 57.262(13) 91.10(19)

Ru(1)−Ru(3) Ru(2)−C(1) N(1)−C(1) C(3)−C(4) Ru(1)−Ru(3)−Ru(2) Ru(1)−C(1)−N(1)

2.7525(5) 2.343(6) 1.368(7) 1.399(11) 56.826(13) 117.0(4)

species were converted into 3a, which indicates that the facecapping pyridine complex 3a is a thermodynamically favored product. As mentioned above, the γ-picolyl complex 2d did not afford a face-capping pyridine complex upon heating. This brought a different reactivity from those seen in 2a−2c. When the thermolysis was carried out in the presence of an excess amount of γ-picoline, dehydrogenative coupling of γ-picoline, leading to the exclusive formation of 4,4′-dimethyl-2,2′-bipyridine, was observed (Table 4). Although the reaction rate was fairly slow in comparison with that of the reaction catalyzed by diruthenium tetrahydrido complex Cp*Ru(μ-H)4RuCp*,24 the turnover number reached 80 in 120 h when the reaction was carried out using 0.2 mol % 1 at 180 °C (entry 2). It is noteworthy that only bipyridine was produced; terpyridine was not formed during the reaction. This is the crucial difference from the coupling reaction catalyzed by heterogeneous catalysts, such as Raney-Ni and Pd/C.25 This selectivity possibly arose from the shape of the triruthenium core, which suppresses the formation of a μ3-η2(//)-pyridyl complex by the reaction with 2-substituted pyridine because of the steric repulsion between the substituent and the Cp* groups. Whereas pyridine and ethyl isonicotinate were only minimally dimerized (entries 6 and 11), the dehydrogenative coupling of 4-ethylpyridine (entry 7), 4-dimethylaminopyridine (entry 8), and 4,4′-bipyridine (entry 9) proceeded in moderate yields. A solvent having strong coordination ability, in this case, 1,2-dimethoxy ethane, suppressed the reaction (entry 4). It is

elimination of pyridine and dihydrogen, as shown in Scheme 2. Complex 2a would then isomerize to the face-capping pyridine Scheme 2. Plausible Mechanism for the Transformation of μ3-η2(//)-Pyridyl Complex 2 to Face-Capping Pyridine Complex 3

complex 3a via reductive C−H bond formation to form the κ(N)-pyridine intermediate A. Upon further heating, all the

Table 4. Results of the Dehydrogenative Coupling of 4-Substituted Pyridine Catalyzed by Triruthenium Pentahydrido Complex 1a

entry 1 2 3 4 5 6 7 8 9 10 11

substrate R R R R R R R R R R R

= = = = = = = = = = =

Me Me Me Me Me H Et NMe2 OMe 4-pyridyl COOEt

temp (°C)

time (h)

solvent

substrate/catalyst

yield (%)

160 180 180 180 180 180 180 180 180 180 180

120 120 72 72 100 100 100 100 100 100 100

decane decane mesitylene DME mesitylene mesitylene mesitylene mesitylene mesitylene mesitylene mesitylene

500 500 500 500 100 100 100 100 100 100 100

3b 32b 20b 2b 43b traceb 34b 23c 8c 27c tracec

a

The reactions were carried out in a 20 mL glass tube equipped with a Teflon-seal valve. The catalyst was loaded as a decane or mesitylene solution (4 mM). A 3 mL amount of solvent was used, and biphenyl was added to the flask as an internal standard. bThe yield was determined by GLC analysis. cThe yield was determined by 1H NMR analysis. 4823


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Scheme 3. Plausible Mechanism for the Dehydrogenative Coupling of 4-Substituted Pyridines Performed on the Trimetallic Core Derived from 1

uncoordinated C−C and C−N bond lengths (1.446(5) Å) implied Kekulé distortion as an azacyclohexatriene. This is consistent with the rigid structure of 3, in which the μ3-η2:η2:η2pyridine ligand does not rotate around the Ru3 core. Consequently, the Cp* signals were observed to be inequivalent in the 1H NMR spectrum. This is quite contrasted with the μ3-η2:η2η2-benzene complexes of Ru3(CO)8(L)(C6H6), in which the benzene ligand was shown to rotate on a trimetallic core within the NMR time scale.4 In the cases of 3b and 3c, due to the presence of the substituent group at the 4-position, the position of the nitrogen atom can be unambiguously determined by X-ray diffraction. Xray diffraction studies were thus carried out for 3b and 3c using red single crystals obtained from cold diethyl ether solution. Because their molecular structures are almost the same, only the molecular structure of 3b is shown in Figure 6, and structural data are listed in Table 5 (the results of the X-ray diffraction study of 3c are shown in the Supporting Information). Figure 6 clearly shows the face-capping coordination of methyl isonicotinate to the Ru3 core. The substituent group placed on the 4-position enables the straight assignment of the nitrogen atom. The sum of the interior angles in the pyridine ring in 3b is 719.9°, which shows that the six atoms are nearly coplanar. The length of the coordinated N(1)C(1) bond is 1.395(11) Å, which is slightly shorter than the noncoordinated N(1)−C(5) bond by 0.026 Å. Differences are not apparent in the CC and C−C bond distances; the lengths of the coordinated CC bonds are 1.454(12) and 1.416(13) Å, whereas those of the noncoordinated C−C bonds are 1.428(12) and 1.462(13) Å. Back-donation to the pyridine ring causes the methoxycarbonyl group to bend away from the molecular plane. The bent-back angles are shown to be ca. 25°, which is comparable to the bent-back angle of the face-capping benzene complex, (CpCo)3{μ3-η2:η2:η2-C6H5CH(Me)Ph}(μ3-H), in which the phenethyl group is bent away from the C6 plane by 20°.28

notable that pyridine containing a dimethylamino group can be dimerized by 1, unlike the coupling reaction catalyzed by palladium on charcoal.25d Although the yields were not sufficient, these results may open a new route to developing functionalized bipyridines using a polyhydrido cluster as a catalyst precursor. Furthermore, this is a rare example of a direct C−C bond formation at the 2-position of pyridine via C−H bond activation.26 The failure of the coupling reaction of unsubstituted pyridine and ethyl isonicotinate was most likely due to the formation of thermally stable face-capping pyridine complexes 3a and 3c. Because no intermediate was observed during the reaction, the mechanistic detail is not clear at present. However, the dehydrogenative coupling of pyridines is assumed to proceed in a similar manner to the C−C bond formation between the two hydrocarbyl ligands placed on each face of the triruthenium plane,27 which we have recently demonstrated. Namely, the second pyridine molecule seems to approach from the lesshindered side of the coordinatively unsaturated μ3-pyridyl complex 4 to form intermediate I-1 (Scheme 3). The oxidative addition of the α-C−H bond forms a bis(μ-pyridyl) intermediate I-2. Reductive C−C bond formation then takes place upon breaking a Ru−Ru bond, as seen in the formation of a closo-ruthenacyclopentadiene skeleton.27a This leads to the construction of a bipyridine skeleton in I-3. Coordination of the third pyridine molecule would eliminate bipyridine from the trimetallic core and regenerate 4. The slow rate of the reaction was probably due to the sterically restricted shape of the trimetallic core, as well as to the contribution of the reverse reactions, leading to regeneration of 2 and 1 by the uptake of dihydrogen accommodated during the reaction. Face-Capping Pyridine Complexes. In our previous communication, we reported the results of a diffraction study of the face-capping pyridine complex 3a;10 although the position of the nitrogen atom was not determined because of the disordered structures around the six-membered ring, the differences between the average values of the coordinated CC and CN bond lengths (1.395(6) Å) and the 4824


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1.86. The hydrides resonated at δ −20.89, −20.67, and −19.24. These facts clearly showed the rigid structure of the pyridinium ring on the Ru3 core, as seen in the face-capping pyridine complex 3. The five 13C signals of the pyridinium ring were observed at δ 25.4, 26.8, 32.7, 48.7, and 54.6, which appeared at the notably upfield region compared with those of 3a (δ 34.5− 60.3)10 and 3c (δ 34.9−58.7). A similar upfield shift was observed in the μ3-pyridinium complex [{Cp*Ru(μ-H)}3(μ3η2:η2:η2-C5H5NH)]+ (7a).10 Since methylation and protonation occurred at the nitrogen atom, the cationic charge would be localized mainly at the nitrogen atom. This causes strong backdonation from a metal center to the pyridinium moiety, therefore, inducing a significant shielding effect. An X-ray diffraction study was carried out using a single crystal of a tetraphenylborate salt of 7b. The molecular structure is shown in Figure 7, and structural data are listed Figure 6. Molecular structure of 3b with thermal ellipsoids at the 30% level of probability.

Similar to the protonation leading to the formation of the cationic μ3-pyridinium complex 7a,10 methylation took place at the nitrogen atom. Treatment of 3a with methyl triflate in diethyl ether resulted in the immediate formation of the μ3η2:η2:η2-N-methylpyridinium complex 7b, as a purple precipitate (eq 6).

Figure 7. Molecular structure of 7b with thermal ellipsoids at the 30% level of probability. An anionic part (BPh4−) was omitted for clarity.

In the 1H NMR spectrum of 7b, three sharp signals assignable to the Cp* groups were observed at δ 1.78, 1.79, and

Table 5. Selected Bond Distances (Å) and Angles (deg) for 3b and 7b 3b Ru(1)−Ru(2) Ru(2)−Ru(3) Ru(1)−Ru(3) Ru(1)−N(1) Ru(1)−C(1) Ru(2)−C(2) Ru(2)−C(3) Ru(3)−C(4) Ru(3)−C(5) N(1)−C(1) C(1)−C(2) C(2)−C(3) C(3)−C(4) C(4)−C(5) N(1)−C(5) C(3)−C(6) C(6)−O(1) C(6)−O(2) Ru(2)−Ru(1)−Ru(3) Ru(1)−Ru(2)−Ru(3) Ru(1)−Ru(3)−Ru(2)

7b 3.0473(9) 3.0167(10) 3.0078(9) 2.134(7) 2.166(8) 2.183(8) 2.222(8) 2.158(9) 2.186(7) 1.395(11) 1.428(12) 1.454(12) 1.462(13) 1.416(13) 1.421(11) 1.465(14) 1.209(13) 1.374(12) 59.76(2) 59.47(2) 60.77(2) 4825

N(1)−C(6)

3.0709(5) 3.0186(5) 3.0168(6) 2.1219(15) 2.1314(16) 2.1602(16) 2.1604(16) 2.1635(15) 2.1150(16) 1.412(2) 1.451(2) 1.421(2) 1.449(2) 1.415(2) 1.446(2) 1.478(2)

59.444(11) 59.387(12) 61.169(10) dx.doi.org/10.1021/om300379d | Organometallics 2012, 31, 4817−4831

Organometallics

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drido complex 1 with pyridine exclusively afforded a μ3-η2(//)pyridyl complex 2, in which the pyridyl group is tilted with respect to the Ru3 plane. Such a tilted coordination mode of the pyridyl group was proposed to be formed on a metal surface, and complex 2 is the first discrete example of a tilted μ3-pyridyl species. Additional π-coordination of the pyridyl group was probably due to the electron-rich metal centers composed of the [Cp*Ru] fragments. Strong back-donation to the pyridyl group causes further isomerization into the face-capping pyridine complex. A pyridine molecule is shown to adsorb on a metal surface with its molecular plane parallel to a metal surface only in the lowtemperature region and is subsequently converted to μ-pyridyl species upon warming. In contrast to this, the face-capping pyridine complex 3 is a thermodynamically favored product. Therefore, the nature of the face-capping pyridine ligand seems to be different from that of the physisorbed pyridine on a metal surface. Somorjai showed that thermolysis of the surface pyridyl species resulted in decomposition, leading to evolution of dihydrogen and formation of CxH fragments.2a They also noted that the reaction resembles the decomposition of the adsorbed benzene, which adopted face-capping coordination. Thus, the formation of 3 implies that face-capping coordination would be also adopted in the high-temperature region on a metal surface. Protonation and methylation of the face-capping pyridine complex afforded cationic pyridinium complex 7. The diffraction and NMR data suggest that the cationic charge is localized at the nitrogen atom. As a result, interaction between the trimetallic core and the pyridinium moiety seems to be considerably strengthened. This result relates to the poisoning of the hydrodesulfurization process by pyridine. When the pyridine has an electron-donating group, the pyridine moiety did not adopt a face-capping coordination mode. Instead, coordinatively unsaturated μ3-pyridyl complex 4d was obtained upon thermolysis of 2d, accompanied by the liberation of dihydrogen, which seems to be an active species for the dehydrogenative coupling of 4-substituted pyridines. Protonation of the μ3-pyridyl complex also resulted in liberation of dihydrogen, but yielded the cationic μ3-pyridyl complex 5, whose pyridyl group is coordinated to the Ru3 plane in a perpendicular fashion. These results obtained here provide novel methods for the transformation of pyridine using a polyhydrido cluster as well as important information about the nature of an adsorbed pyridyl species in relation to hydrodenitrogenation processes and catalyst poisoning. We are currently investigating the reactivity of 4d and 5 toward other small molecules in order to reveal the reaction mechanism of the catalytic reaction.

in Table 5. The structure clearly represents the presence of a methyl group at the nitrogen atom. The N(1)−C(6) bond is bent away from the six-membered ring by 38°, which is notably larger than the bent-back angles of the methoxy group observed in 3b (25°). The average of the Ru−E bond lengths in 7b (E = C or N) is 0.03 Å less than those of 3b. These facts also suggest the enhanced back-donation to the pyridinium moiety and are quite consistent with the density functional theory (DFT) calculations for the adsorbed pyridine molecule on a MoS2 surface, which demonstrates that the pyridinium ion is adsorbed more tightly than the pyridine molecule.29 The sum of the interior angles of the six-membered ring is 719.5°, which shows that the six atoms are still coplanar. The πbonded N(1)C(1) bond (1.412(2) Å) is shorter than the uncoordinated N(1)−C(5) bond (1.446(2) Å). The average length of the π-bonded CC bond (1.42 Å) is slightly shorter than that of the uncoordinated C−C bond (1.45 Å). The N(1)−C(6) length of 1.476(2) Å is almost the same as the N− C bond length of the noncoordinated N-methylpyridinium cation (1.463−1.473 Å).30 Although complex 3a is stable upon thermolysis and stable toward air and moisture, it transformed into the μ-η2(//)pyridyl complex 2a upon irradiation. Irradiation of the THF-d8 solution of 3a in an NMR tube using a high-pressure Hg lamp for 10 h resulted in the exclusive formation of 2a in 68% yield (eq 7), which was estimated by comparing the signal intensity

of 2a to that of the internal standard (cyclooctane). The transformation also proceeded when the I line (365 nm) was irradiated by using a cutoff-filter, although the rate decreased. This transformation is quite similar to the isomerization of the triosmium complex having a face-capping benzene ligand to yield a μ3-η2(//)-benzyne complex upon irradiation, as reported by Lewis and co-workers.31 We have already reported the synthesis of the face-capping benzene complex {Cp*Ru(μH)}3(μ3-η2:η2:η2-C6H6) (8),32 which is a benzene analogue of 3a. Unlike the photoreactivity of 3a, a detectable change was not observed upon irradiation of 8. The UV−vis spectrum of 3a closely resembles that of 8; whereas complex 3a showed two adsorptions centered at 373 and 529 nm, 8 showed two peaks at 381 and 528 nm. Because of the similarity between them, it was assumed that the transitions were mainly derived from the triruthenium skeleton having Cp* groups. Thus, the difference in reactivity was probably due to the presence of a nitrogen atom in 3a, which enabled facile C−H bond activation on a trimetallic plane.

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EXPERIMENTAL SECTION

General Procedures. All experiments were carried out under an argon atmosphere. All compounds were treated with Schlenk techniques. Dehydrated toluene, pentane, methanol, THF, and diethylether used in this study were purchased from Kanto Chemicals and stored under an argon atmosphere. Mesitylene, benzene-d6, pxylene-d10, and tetrahydrofuran-d8 were dried over sodium-benzophenone ketyl and stored under an argon atmosphere. Other materials used in this research were used as purchased. {Cp*Ru(μ-H)}3(μ3-H)2 (1) was prepared according to a previously published method.33 UV− vis spectra were recorded on a Shimadzu UV-2550 spectrophotometer. 1 H and 13C NMR spectra were recorded on a Varian INOVA-400 spectrometer. 1H NMR spectra were referenced to tetramethylsilane as an internal standard. 13C NMR spectra were referenced to the naturalabundance carbon signal of the solvent employed. The numbering

■

CONCLUSION The synthesis of triruthenium complexes having an unprecedented μ3-η2(//)-pyridyl ligand and its transformations into a face-capping pyridine ligand and a perpendicularly coordinated μ3-η2:η2(⊥)-pyridyl group were discussed. Unlike the wellknown reaction of carbonyl clusters with pyridine yielding μpyridyl complexes, the reaction of the triruthenium pentahy4826


Organometallics

Article

schemes of the pyridyl and the pyridine moieties are shown in Chart 1. Elemental analysis was performed on a PerkinElmer 2400II series

mmol) was added to the solution, the reaction mixture was heated at 100 °C for 7 days. The color of the solution turned from reddishbrown to purple. Toluene and the remaining pyridine were removed under reduced pressure. The residual solid was purified by the use of column chromatography on alumina (Merck, Art. No. 1097) with tetrahydrofuran. The second purple band including 3a was collected, and 3a was obtained as a purple solid by the removal of solvent under reduced pressure (123.0 mg, 0.155 mmol, 54%). 1H NMR (400 MHz, benzene-d6, 25 °C): δ −21.50 (dd, JH−H = 3.8, 3.4 Hz, 1H, RuH), −19.68 (dd, JH−H = 4.6, 3.8 Hz, 1H, RuH), −19.65 (dd, JH−H = 4.6, 3.4 Hz, 1H, RuH), 1.66 (s, 15H, Cp*), 1.67 (s, 15H, Cp*), 1.78 (s, 15H, Cp*), 2.36 (dd, JH−H = 6.0, 4.4 Hz, 1H, CβH), 2.42 (dd, JH−H = 6.0, 4.4 Hz, 1H, CβH), 2.52 (dd, JH−H = 6.0, 6.0 Hz, 1H, CγH), 3.47 (d, JH−H = 4.4 Hz, 1H, CαH), 3.60 ppm (d, JH−H = 4.4 Hz, 1H; CαH). 13C NMR (100 MHz, benzene-d6, 25 °C): δ 10.89 (q, JC−H = 128 Hz, C5Me5), 10.93 (q, JC−H = 127 Hz, C5Me5), 11.2 (q, JC−H = 127 Hz, C5Me5), 34.5 (d, JC−H = 168 Hz, Cβ or Cγ), 35.2 (d, JC−H = 165 Hz, Cβ or Cγ), 35.7 (d, JC−H = 163 Hz, Cβ or Cγ), 58.5 (d, JC−H = 173 Hz, Cα), 60.3 (d, JC−H = 168 Hz; Cα), 89.5 (s, C5Me5), 89.6 (s, C5Me5), 90.5 (s, C5Me5) ppm. Anal. Calcd for C35H53N1Ru3: C, 53.14; H, 6.75; N, 1.77. Found: C, 53.19; H, 6.50; N, 1.73. Reaction of 1 with Methyl Isonicotinate. A 20 mL glass tube equipped with a Teflon valve was charged with {Cp*Ru(μ-H)}3(μ3H)2 (1) (64.0 mg, 0.090 mmol) and toluene (10 mL). After methyl isonicotinate (53 μL, 0.448 mmol) was added to the solution, the reaction mixture was stirred at 100 °C for 5 days. The color of the solution turned from reddish-brown to purple. The solvent and remaining methyl isonicotinate were then removed in vacuo. The residual solid was then dissolved in 5 mL of THF and purified by the use of column chromatography on alumina (Merck, Art. No. 1097) with pentane/THF = 10/1. The first black band including 2b and the second purple band including 3b were, respectively, collected. Removal of the solvents under reduced pressure afforded 2b as a dark-red solid (30.9 mg, 0.036 mmol, 41% yield) and 3b as a purple solid (20.4 mg, 0.024 mmol, 27% yield). A single crystal of 3b used for the diffraction study was prepared by the recrystallization from the cold diethyl ether solution stored at −30 °C. (Cp*Ru)3(μ-H)4(μ3-η2(//)-4-COOMe-C5H3N) (2b): 1H NMR (400 MHz, −80 °C, THF-d8): δ −17.71 (m, 1H, RuH), −16.61 (m, 1H, RuH), −13.84 (m, 1H, RuH), −13.41 (d, JH−H = 6.4 Hz, 1H, RuH), 1.58 (s, 15H, C5Me5), 1.84 (s, 15H, C5Me5), 1.90 (s, 15H, C5Me5), 3.71 (s, 3H, COOMe), 6.32 (d, JH−H = 6.4 Hz, 1H, C5H), 6.60 (d, JH−H = 6.4 Hz, 1H, C6H), 7.66 ppm (s, 1H, C3H). Anal. Calcd for C37H55NO2Ru3: C, 52.34; H, 6.53; N, 1.65. Found: C, 52.40; H, 6.57; N, 1.80. {Cp*Ru(μ-H)}3(μ3-η2:η2:η2-4-COOMe-C5H4N) (3b): 1H NMR (400 MHz, benzene-d6, 25 °C): δ −21.78 (m, 1H, RuH), −20.04 (m, 1H, RuH), −19.25 (m, 1H, RuH), 1.67 (s, 15H, Cp*), 1.69 (s, 15H, Cp*), 1.71 (s, 15H, Cp*), 2.96 (d, JH−H = 4.4 Hz, 1H, C5NH3), 3.37 (d, JH−H = 4.8 Hz, 1H, C5NH3), 3.49 ppm (s, 3H, COOMe). * Two 1H signals arising from the face-capping methyl isonicotinate moiety were obscured by the COOMe signal appearing at δ 3.49 ppm. Anal. Calcd for C37H55NO2Ru3: C, 52.34; H, 6.53; N, 1.65. Found: C, 52.56; H, 6.47; N, 1.73. Preparation of (Cp*Ru)3(μ-H)4(μ3-η2(//)-4-COOEt-C5H3N) (2c). A 20 mL glass tube equipped with a Teflon valve was charged with {Cp*Ru(μ-H)}3(μ3-H)2 (1) (156.3 mg, 0.219 mmol) and toluene (10 mL). After ethyl isonicotinate (66 μL, 0.441 mmol) was added to the solution, the reaction mixture was stirred at 100 °C for 18 h. The color of the solution turned from reddish-brown to purple. After the solvent and remaining ethyl isonicotinate were removed in vacuo, the residue was dissolved in 5 mL of THF. The residual solid was then purified by the use of column chromatography on alumina (Merck, Art. No. 1097) with pentane/THF = 10/1. The first black band including 2c was collected. Removal of the solvent under reduced pressure afforded 2c as a black crystalline solid (153.6 mg, 0.178 mmol, 81% yield). A single crystal used for the diffraction studies was prepared by the recrystallization from the cold diethylether solution of 2c stored at −30 °C. 1H NMR (400 MHz, −80 °C, THF-d8): δ −17.71 (m, 1H, RuH), −16.63 (m, 1H, RuH), −13.85 (m, 1H, RuH), −13.42 (d, JH−H

Chart 1. Numbering Schemes of the Pyridyl and the Pyridine Moieties

CHN analyzer. GLC analyses were performed on a Shimadzu GC-17A using a capillary column (J&W DB-1; 30 m × 0.53 mm × 1.50 μm) with helium gas as a carrier. UV irradiation experiments were performed using an Asahi Spectra REX-250 high power mercury light source. X-ray Diffraction Studies. Single crystals of 2a, 2c, 3b, 4d, 5a, and 7b for the X-ray analyses were obtained directly from the preparations described below and mounted on glass fibers. Diffraction experiments were performed on a Rigaku R-AXIS RAPID imaging plate diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71069 Å). Cell refinement and data reduction were performed using the PROCESS-AUTO program.34 Intensity data were corrected for Lorentz-polarization effects and numerical (2a) and empirical absorption (2c, 3b, 4d, 5a, and 7b). The structures were solved by the Patterson method using the SHELX-97 program package.35 All nonhydrogen atoms were found by the difference Fourier synthesis and were refined anisotropically, except for the disordered carbon atoms in the Cp* groups attached to the Ru(3) atom in 3b. The refinement was carried out by least-squares methods based on F2 with all measured reflections. The metal-bound hydrogen atoms of 2c, 4d, 5a, and 7b were located in the difference Fourier map and refined isotropically. The positions of the hydrogen atoms attached to the metal centers were not determined for 2a and 3b. In addition, the positions of the following hydrogen atoms attached to the carbon atoms, hydrogen atoms on the solvent molecule in 2a, hydrogen atoms on the facecapping pyridine ligand in 3b, and the hydrogen atom on C(5) in 4d, could not be refined. Crystal data and results of the analyses are listed in Table 6. Preparation of (Cp*Ru)3(μ-H)4(μ3-η2(//)-C5H4N) (2a). A 50 mL Schlenk tube was charged with {Cp*Ru(μ-H)}3(μ3-H)2 (1) (156.1 mg, 0.219 mmol) and toluene (4 mL). After pyridine (90 μL, 1.16 mmol) was added to the solution, the reaction mixture was stirred at 90 °C for 4 days. The color of the solution turned from reddish-brown to reddish-purple. After the solvent and remaining pyridine was removed in vacuo, the residue was extracted three times with 5 mL of pentane. After the combined solution was dried under reduced pressure, the residual solid was washed two times with 3 mL of MeOH and dried under vacuum. Recrystallization from the cold THF/MeOH solution stored at −20 °C afforded 2a as a purple crystal (58.8 mg, 0.0704 mmol, 34% yield). 1H NMR (400 MHz, −80 °C, THF-d8): δ −17.56 (m, 1H, RuH), −17.06 (m, 1H, RuH), −13.60 (m, 1H, RuH), −13.42 (d, JH−H = 7.2 Hz, 1H, RuH), 1.57 (s, 15H, C5Me5), 1.81 (s, 15H, C5Me5), 1.87 (s, 15H, C5Me5), 5.95 (dd, JH−H = 6.4, 6.4 Hz, 1H, C5H), 6.07 (dd, JH−H = 9.2, 6.4 Hz, 1H, C4H), 6.58 (d, JH−H = 6.4 Hz, 1H, C6H), 6.85 ppm (d, JH−H = 9.2 Hz, 1H, C3H). 13C NMR (100 MHz, −80 °C, THF-d8): δ 11.1 (q, JC−H = 125 Hz, C5Me5), 12.1 (q, JC−H = 124 Hz, C5Me5), 12.2 (q, JC−H = 124 Hz, C5Me5), 85.5 (s, C5Me5), 86.7 (s, C5Me5), 93.3 (s, C5Me5), 109.9 (d, JC−H = 160 Hz, C5H), 114.8 (d, JC−H = 159 Hz, C4H), HCCH), 143.3 (d, JC−H = 159 Hz, C3H), 149.5 (d, JC−H = 174 Hz, C6H), 168.9 ppm (s, C2). Preparation of {Cp*Ru(μ-H)}3(μ3-η2:η2:η2-C5H5N) (3a). A 20 mL glass tube equipped with a Teflon valve was charged with 1 (206.1 mg, 0.289 mmol) and toluene (10 mL). After pyridine (0.12 mL, 1.44 4827


Organometallics

Article

Table 6. Crystallographic Data for 2a, 2c, 3b, 4d, 5a, and 7b (a) crystal data empirical formula formula weight cryst description cryst color cryst size (mm) crystallizing solution cryst system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z value Dcalc (g/cm3) measurement temp (°C) μ(Mo Kα) (mm−1) (b) intensity measurements diffractometer radiation monochromator 2θ max reflns collected independent reflns reflns observed (>2σ) abs. correction type abs. transmission (c) refinement (Shelxl-97-2) R1 (I > 2σ(I)) wR2 (I > 2σ(I)) R1 (all data) wR2 (all data) data/restraints/ params GOF largest diff. peak and hole (e·Å−3)

2a

2c

C35 H53 Ru3·C4H10O 865.11 prism red 0.30 × 0.30 × 0.12 diethyl ether (−30 °C)

C38 H57 N O2 Ru3 863.06 platelet red 0.13 × 0.07 × 0.02 diethyl ether (25 °C)

monoclinic P21/n (No. 14) 11.7790(2) 18.5780(3) 16.5703(3)

3b

4d

5a

7b C60 H76 B N Ru3 1125.24 prism red 0.45 × 0.41 × 0.29 THF/Et2O (25 °C)

monoclinic C2/c (No. 15) 22.7341(11) 18.8043(8) 20.4728(8)

3543.58(11) 4 1.622 −150

C36 H53 N Ru3 803.00 platelet yellow 0.20 × 0.13 × 0.02 diethyl ether (−30 °C) triclinic P1̅ (No. 2) 10.860(4) 10.789(3) 16.040(5) 92.581(12) 97.913(14) 113.972(12) 1690.4(9) 2 1.578 −150

C35 H52 B F4 N Ru3 878.81 prism red 0.11 × 0.07 × 0.04 THF/Et2O (25 °C)

triclinic P1̅ (No. 2) 8.8245(7) 11.0624(8) 20.5986(16) 78.9596(19) 77.611(2) 72.0478(19) 1851.3(2) 2 1.548 −80

C37 H55 N O2 Ru3 849.03 needle red 0.07 × 0.04 × 0.01 diethyl ether (−30 °C) triclinic P1̅ (No. 2) 8.7652(5) 10.8209(7) 19.2871(10) 77.3390(15) 77.7690(14) 74.7230(14) 1698.54(16) 2 1.660 −150

7230.7(6) 8 1.611 −100

triclinic P1̅ (No. 2) 11.5539(17) 13.726(3) 17.284(4) 70.873(10) 86.569(10) 81.514(10) 2561.1(9) 2 1.459 −80

1.294

1.240

1.350

1.347

1.282

0.912

RAXIS-RAPID Mo Kα graphite 60° 42 782 10 586 (Rint = 0.0213)

RAXIS-RAPID Mo Kα graphite 55° 16 533 8180 (Rint = 0.0328)




9621

6376

4482

6075

7096

RAXIS-RAPID Mo Kα graphite 60° 29 674 14 691 (Rint = 0.0238) 13 409

numerical

empirical

empirical

empirical

empirical

empirical

0.7272 (min.) 0.8534 (max.)

0.6499 (min.) 1.0000 (max.)

0.2433 (min.) 1.0000 (max.)

0.6844 (min.) 1.0000 (max.)

0.7411 (min.) 1.0000 (max.)

0.8337 (min.) 1.0000 (max.)

0.0397 0.0697 0.1004 0.1021 10260/0/411

0.0374 0.0778 0.0555 0.086 8150/0/430

0.0573 0.1262 0.1255 0.1877 7724/0/391

0.0403 0.0914 0.0570 0.1082 7592/0/430

0.0530 0.1355 0.0616 0.1415 8260/0/431

0.0244 0.0597 0.0273 0.0612 14673/0/175

1.044 7.758 and −1.126

1.022 0.0695 and −1.132

1.118 1.782 and −2.250

1.055 2.184 and −1.269

1.039 5.080 and −1.190

1.043 0.946 and −0.784

102.2453(7)

= 6.4 Hz, 1H, RuH), 1.26 (t, JH−H = 7.2 Hz, 3H, −CH2CH3), 1.58 (s, 15H, C5Me5), 1.83 (s, 15H, C5Me5), 1.90 (s, 15H, C5Me5), 4.17 (m, 2H, −CH2CH3), 6.32 (d, JH−H = 6.8 Hz, 1H, C5H), 6.59 (d, JH−H = 6.8 Hz, 1H, C 6 H), 7.68 ppm (s, 1H, C 3 H). Anal. Calcd for C38H57NO2Ru3: C, 52.88; H, 6.66; N, 1.62. Found C, 53.18; H, 6.55; N, 1.91. Preparation of {Cp*Ru(μ-H)}3(μ3-η2:η2:η2-4-COOEt-C5H4N) (3c). A 20 mL glass tube equipped with a Teflon valve was charged with 1 (56.1 mg, 0.079 mmol) and toluene (5 mL). Ethyl isonicotinate (18 μL, 0.120 mmol) was added, and the reaction mixture was heated at 120 °C for 9 days. The color of the solution turned from reddishbrown to purple. Toluene was removed under reduced pressure. The residual solid was purified by the use of column chromatography on alumina (Merck, Art. No. 1097) with tetrahydrofuran. The second

124.2934(13)

purple band including 3c was collected, and 3c was obtained as a purple solid by the removal of solvent under reduced pressure (25.1 mg, 0.029 mmol, 37%). A single crystal used for the diffraction studies was prepared from the diethylether solution stored at −30 °C. 1H NMR (400 MHz, benzene-d6, 25 °C): δ −21.79 (dd, JH−H = 3.8, 3.4 Hz, 1H, RuH), −20.02 (dd, JH−H = 4.6, 3.8 Hz, 1H, RuH), −19.27 (dd, JH−H = 4.6, 3.4 Hz, 1H, RuH), 1.02 (dd, JH−H = 7.2, 7.2 Hz, 3H, CH2CH3), 1.66 (s, 15H, Cp*), 1.67 (s, 15H, Cp*), 1.78 (s, 15H, Cp*), 2.98 (dd, JH−H = 4.4, 1.6 Hz, 1H, CβH), 3.39 (d, JH−H = 4.4 Hz, 1H, CαH), 3.49 (d, JH−H = 4.4 Hz, 1H, CαH), 3.54 (dd, JH−H = 4.4, 1.6 Hz, 1H, CβH), 4.02 (dd, JH−H = 11.2, 7.2 Hz, 1H, CH2CH3), 4.12 ppm (dd, JH−H = 11.2, 7.2 Hz, 1H, CH2CH3). 13C NMR (100 MHz, benzene-d6, 25 °C): δ 10.8 (q, JC−H = 127 Hz, C5Me5), 10.9 (q, JC−H = 126 Hz, C5Me5), 11.9 (q, JC−H = 126 Hz, C5Me5), 14.6 (q, JC−H = 126 4828


Organometallics

Article

Hz, CH2CH3), 34.9 (d, JC−H = 162 Hz, Cβ), 37.7 (s, Cγ), 39.2 (d, JC−H = 161 Hz, Cβ), 55.7 (d, JC−H = 175 Hz, Cα), 58.7 (d, JC−H = 173 Hz, Cα), 59.7 (dd, JC−H = 145, 145 Hz, CH2CH3), 90.1 (s, C5Me5), 91.2 (s, C5Me5), 91.4 (s, C5Me5), 175.8 ppm (s, COOEt). Anal. Calcd for C38H57NO2Ru3: C, 52.88; H, 6.66; N, 1.62. Found: C, 53.00; H, 6.40; N, 1.94. Preparation of (Cp*Ru)3(μ-H)4(μ3-η2(//)-4-Me-C5H3N) (2d). A 20 mL glass tube equipped with a Teflon valve was charged with {Cp*Ru(μ-H)}3(μ3-H)2 (1) (131.0 mg, 0.183 mmol) and toluene (5 mL). After 4-picoline (180 μL, 1.84 mmol) was added to the solution, the reaction mixture was stirred at 120 °C for 4 days. The color of the solution turned from reddish-brown to purple. After the solvent and remaining 4-picoline were removed in vacuo, the residue was washed three times with 5 mL of MeOH. Dryness under reduced pressure afforded 2c as a brown crystalline solid (112.5 mg, 0.140 mmol, 77% yield). 1H NMR (400 MHz, −80 °C, THF-d8): δ −17.48 (m, 1H, RuH), −17.08 (m, 1H, RuH), −13.68 (m, 1H, RuH), −13.34 (d, JH−H = 7.2 Hz, 1H, RuH), 1.56 (s, 15H, C5Me5), 1.82 (s, 15H, C5Me5), 1.88 (s, 15H, C5Me5), 2.00 (d, JH−H = 0.8 Hz, 3H, −CH3), 5.87 (dd, JH−H = 6.8, 1.6 Hz, 1H, C5H), 6.57 (d, JH−H = 6.8 Hz, 1H, C6H), 6.72 ppm (m, 1H, C3H). 13C NMR (100 MHz, 25 °C, THF-d8): δ 11.8 (q, JC−H = 128 Hz, C5Me5), 20.7 (q, JC−H = 127 Hz, C4Me), 88.9 (br s, C5Me5), 113.0 (d, JC−H = 168 Hz, C5H), 124.6 (s, C4H), 141.5 (d, JC−H = 163 Hz, C3H), 149.2 (d, JC−H = 173 Hz, C6H), 170.4 ppm (s, C2). Preparation of (Cp*Ru)3(μ-H)4(μ3-η2(//)-5-Me-C5H3N) (2e). A 20 mL glass tube equipped with a Teflon valve was charged with 1 (31.2 mg, 0.044 mmol) and toluene (5 mL). After β-picoline (21 μL, 0.219 mmol) was added to the solution, the reaction mixture was heated at 120 °C for 4 days. The color of the solution turned from reddish-brown to purple. Toluene and the remaining β-picoline were removed under reduced pressure. The 1H NMR spectrum of the residual solid showed exclusive formation of 2e. The residual solid was then washed two times with 2 mL of methanol. Drying under vacuum afforded 2e as a brown solid (13.9 mg, 0.017 mmol, 40% yield). 1H NMR (400 MHz, −80 °C, THF-d8): δ −17.70 (m, 1H, RuH), −17.03 (m, 1H, RuH), −13.59 (m, 1H, RuH), −13.47 (d, JH−H = 7.2 Hz, 1H, RuH), 1.59 (s, 15H, C5Me5), 1.81 (s, 3H, NMe), 1.82 (s, 15H, C5Me5), 1.87 (s, 15H, C5Me5), 6.03 (d, JH−H = 8.8 Hz, 1H, C4H), 6.47 (s, 1H, C6H), 6.85 ppm (d, JH−H = 8.8 Hz, 1H, C3H). 13C{1H} NMR (100 MHz, −80 °C, THF-d8): δ 11.3 (C5Me5), 12.2 (C5Me5), 12.3 (C5Me5), 18.2 (C5Me), 85.4 (C5Me5), 87.0 (C5Me5), 93.2 (C5Me5), 118.0 (C4 or C5), 118.5 (C4 or C5), 143.2 (C3 or C6), 147.3 (C3 or C6), 165.5 ppm (C2). Reaction of 1 with α-Picoline. An NMR tube equipped with a JYoung valve was charged with 1 (3.0 mg, 4.2 μmol), α-picoline (2 μL, 20.3 μmol), p-xylene-d10 (0.5 mL), and ferrocene as an internal standard. The NMR tube was heated at 120 °C, and the reaction was monitored by the 1H NMR spectroscopy. After 10 days, any reaction occurred with α-picoline and 1 still remained unchanged. Reaction of (Cp*Ru)3(μ-H)4(μ3-η2(//)-4-Me-C5H3N) (2d) with H2. An NMR tube equipped with a J-Young valve was charged with 2d (6.0 mg, 7 μmol), tetrahydrofuran-d8 (0.45 mL), and cycloheptane as an internal standard. After the solution was degassed by one freeze− pump−thaw cycle, 1 atm of dihydrogen was introduced into the NMR tube at ambient temperature. The reaction mixture was heated at 60 °C. The reaction was monitored by 1H NMR spectroscopy. Exclusive formation of 1, accompanied by liberation of γ-picoline, was observed. After 216 h, all of 2d was converted into 1 quantitatively. The reaction of 2a with 1 atm of H2 also resulted in a quantitative formation of 1. Thermolysis of (Cp*Ru)3(μ-H)4(μ3-η2(//)-C5H4N) (2a) in THFd8. An NMR tube was charged with 2a (10.1 mg, 0.026 mmol), tetrahydrofuran-d8, and cycloheptane as an internal standard. The tube was sealed and heated at 100 °C. The reaction was periodically monitored by 1H NMR spectroscopy, and the distributions of the product were estimated from the intensities of the hydrido signals comparing to that of the internal standard. After 5.5 h, conversion of 2a reached 57% and complexes 1, 3a, and 4a were formed in 21, 11, and 25% yields, respectively. After 325 h, the distributions of 2a, 1, 3a, and 4a turned to 6, 5, 86, and 3%, respectively.

Thermolysis of (Cp*Ru)3(μ-H)4(μ3-η2(//)-C5H4N) (2a) in Vacuo. A 20 mL glass tube equipped with a Teflon valve was charged with 2a (21.6 mg, 0.027 mmol), toluene (3 mL), and ferrocene as an internal standard. The flask was degassed and heated at 140 °C for 4 h under vacuum. The color of the solution turned from reddish-brown to dark brown. After the solvent was removed under reduced pressure, the residual solid was dissolved in 0.5 mL of C6D6. The 1H NMR spectrum of the residual solid showed that 34% of 2a remained and complexes 4a, 3a, and 1 were formed in 59, 6, and 1% yields, respectively. Complex 4a was characterized on the basis of 1H NMR spectra of the mixture. 1H NMR (400 MHz, benzene-d6, 25 °C): δ −11.02 (s, 2H, RuH), 1.77 (s, 45H, Cp*), 6.13 (dd, JH−H = 9.2, 6.0 Hz, 1H, C3H), 6.45 (dd, JH−H = 6.4, 6.0 Hz, 1H, C4H), 6.68 (d, JH−H = 9.2 Hz, 1H, C2H), 9.58 ppm (d, JH−H = 6.4 Hz, 1H, C5H). Preparation of (Cp*Ru)3(μ-H)2(μ3-η2-4-Me-C5H3N) (4d). A 100 mL Schlenk tube was charged with {Cp*Ru(μ-H)}3(μ3-H)2 (1) (111.8 mg, 0.157 mmol) and toluene (10 mL). After 4-picoline (154 μL, 1.57 mmol) was added to the solution, the reaction mixture was stirred at 160 °C for 90 h. The color of the solution turned from reddish-brown to reddish-purple. The solvent and remaining 4picoline were removed under reduced pressure. The 1H NMR spectrum of the residual solid showed exclusive formation of 4d. Recrystallization from a cold diethylether solution stored at −30 °C afforded 4d as a purple crystal (8 mg, 0.010 mmol, 1% yield). 1H NMR (400 MHz, 25 °C, benzene-d6): δ −11.62 (s, 2H, RuH), 1.60 (s, 3H, C4CH3), 1.79 (s, 45H, Cp*), 6.38 (d, JH−H = 6.4 Hz, 1H, C5H), 6.53 (s, 1H, C3H), 9.55 ppm (d, JH−H = 6.4 Hz, 1H, C6H). 13C NMR (100 MHz, benzene-d6, 25 °C): δ 12.3 (q, JC−H = 126 Hz, C5Me5), 20.9 (q, JC−H = 125 Hz, C4CH3), 82.3 (br s, C5Me5), 114.3 (d, JC−H = 161 Hz, C5), 140.6 (d, JC−H = 160 Hz, C3), 154.7 (s, C2), 155.6 ppm (d, JC−H = 177 Hz, C6). A singlet signal derived from C4 of the μ3-picolyl group was not observed in the spectrum probably due to the obstruction by the solvent signals Preparation of [(Cp*Ru)3(μ-H)2(μ3-H)(μ3-η2:η2(⊥)-C5H4N)]+ (5a). A 50 mL Schlenk tube was charged with 2a (32.5 mg, 0.041 mmol) and diethylether (5 mL). After the tetrafluoroboric acid− dimethylether complex (5.6 μL, 0.041 mmol) was added to the solution at 25 °C with vigorous stirring, the reaction mixture was stirred for 15 min. An immediate formation of a dark red precipitate was observed. The precipitate was then separated by removing the supernatant and washed four times with 5 mL of diethylether. Dryness under reduced pressure afforded a tetrafluoroborate salt of 5a as a dark red crystalline solid (33.3 mg, 0.038 mmol, 93% yield). A single crystal used for the diffraction studies was prepared by the slow evaporation of the THF/diethylether solution of 5a stored at ambient temperature. 1 H NMR (400 MHz, acetone-d6, 25 °C): δ −11.10 (s, 3H, RuH), 1.85 (s, 30H, Cp*), 2.05 (s, 15H, Cp*), 6.54 (ddd, JH−H = 8.8, 1.2, 1.2 Hz, 1H, C3H), 6.65 (ddd, JH−H = 8.8, 6.4, 1.2 Hz, 1H, C4H), 7.24 (ddd, JH−H = 6.4, 6.4, 1.2 Hz, 1H, C5H), 10.12 (ddd, JH−H = 6.4, 1.2, 1.2 Hz, 1H, C6H) ppm. 13C NMR (100 MHz, acetone-d6, 25 °C): δ 11.5 (q, JC−H = 128 Hz, C5Me5), 12.2 (q, JC−H = 128 Hz, C4CH3), 90.8 (s, C5Me5), 98.6 (s, C5Me5), 117.3 (d, JC−H = 168 Hz, C5H4N), 126.7 (d, JC−H = 158 Hz, C5H4N), 136.2 (s, C2), 141.1 (d, JC−H = 168 Hz, C5H4N), 157.1 (d, JC−H = 191 Hz, C6) ppm. Anal. Calcd for C35H52BF4NRu3: C, 47.94; H, 5.98; N, 1.60. Found: C, 47.56; H, 6.05; N, 1.65. Preparation of [(Cp*Ru)3(μ-H)3(μ3-η2:η2:η2-C5H5NMe)]+ (7b). A 50 mL Schlenk tube was charged with 3a (49.3 mg, 0.062 mmol) and diethylether (20 mL). After methyl triflate (7.0 μL, 0.062 mmol) was added to the solution at 25 °C with vigorous stirring, the reaction mixture was stirred for 30 min. An immediate formation of purple precipitate was observed. The precipitate was then separated by removing the supernatant and washed four times with 5 mL of diethylether. Dryness under reduced pressure afforded a triflate salt of 7 as a purple solid (58.3 mg, 0.061 mmol, 98% yield). The X-ray diffraction study was carried out using a tetraphenylborate salt of 7, which was obtained by the addition of a large excess amount of NaBPh4 to a methanol solution of 7. A single crystal used for the diffraction studies was prepared by the slow evaporation of the THF/ diethylether solution of a tetraphenylborate salt of 7 stored at ambient 4829


Organometallics

Article

temperature. 1H NMR (400 MHz, acetone-d6, 25 °C): δ −20.89 (dd, JH−H = 5.6, 4.0 Hz, 1H, RuH), −20.67 (dd, JH−H = 5.6, 3.6 Hz, 1H, RuH), −19.24 (dd, JH−H = 4.0, 3.6 Hz, 1H, RuH), 1.78 (s, 15H, Cp*), 1.79 (s, 15H, Cp*), 1.86 (s, 15H, Cp*), 2.74 (dd, JH−H = 6.0, 6.0 Hz, 1H, CγH), 2.78 (dd, JH−H = 6.0, 4.8 Hz, 1H, CβH), 2.88 (s, 3H, NMe), 3.09 (d, JH−H = 5.2 Hz, 1H, CαH), 3.65 ppm (d, JH−H = 4.8 Hz, 1H, CαH). The other 1H signal derived from CβH was obscured by the Cp* signal appearing at δ 1.79. A cross-peak with the CαH (δ 5.02) and the CγH (δ 2.74) protons was found at δ 1.8 in the H−H COSY spectrum. 13C NMR (100 MHz, acetone-d6, 25 °C): δ 10.7 (q, JC−H = 127 Hz, C5Me5), 10.8 (q, JC−H = 127 Hz, C5Me5), 11.6 (q, JC−H = 127 Hz, C5Me5), 25.4 (d, JC−H = 142 Hz, Cβ), 26.8 (d, JC−H = 140 Hz, Cβ), 32.7 (d, JC−H = 166 Hz, Cγ), 48.7 (d, JC−H = 177 Hz, Cα), 52.7 (q, JC−H = 141 Hz, NMe), 54.6 (d, JC−H = 181 Hz, Cα), 93.0 (s, C5Me5), 93.8 (s, C5Me5), 98.0 ppm (s, C5Me5). Anal. Calcd for C37H56F3NO3Ru3S: C, 46.53; H, 5.91; N, 1.47. Found: C, 46.86; H, 5.81; N, 1.36. Irradiation of the Face-Capping Pyridine Complex 2a. An NMR tube equipped with a J-Young valve was charged with 3a (2.0 mg, 2.5 μmol), benzene-d6 (0.5 mL), and cyclooctane as an internal standard. The solution was exposed to UV light at 365 nm for 24 h by using a high-pressure Hg lamp equipped with a filter. The 1H NMR spectrum of the solution was then measured, and the exclusive formation of 2a in a 40% yield was observed. When the irradiation was performed without a filter, the yield of 2a increased to 68% after 10 h. Dehydrogenative Coupling of Pyridines by 1. The reactions of 1 with excess 4-substituted pyridines were performed in a glass tube equipped with a Teflon valve in appropriate reaction conditions. After the appropriate reaction time, the solution was analyzed by GLC (for pyridine, 4-picoline, and 4-ethylpyridine) and 1H NMR (for 4dimethylaminopyridine, 4-methoxypyridine, 4,4′-bipyridine, and ethyl isonicotionate) analyses. The results are listed in Table 4. A typical reaction was carried out as follows (entry 5): 3 mL of a mesitylene solution of 1 (4.0 mM, 0.012 mmol) and biphenyl (48.0 mg, 0.311 mmol) was charged in the reaction flask equipped with a Teflon valve. After a 100 equiv amount of γ-picoline (0.118 mL, 1.20 mmol) was added, the solution was heated at 180 °C for 100 h. Formation of 4,4′dimethyl-2,2′-bipyridine was analyzed by means of GLC. After the solvent was removed under reduced pressure with unreacted γpicoline, the residual solid was analyzed by means of 1H NMR spectroscopy.

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ASSOCIATED CONTENT

S Supporting Information *

Results of the X-ray diffraction study of 3c; 1H and 13C NMR spectra of 2a, 2d, 2e, and 4d; 1H NMR spectrum of 4a; and crystallographic files, including CIF files of 2a, 2c, 3b, 3c, 4d, 5a, and 7b. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research in Innovative Areas “Molecular Activation Directed toward Straightforward Synthesis” from NEXT, Japan.

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REFERENCES

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