Dehydrogenative Coupling of 4-Substituted Pyridines Catalyzed by a

Article pubs.acs.org/Organometallics

Dehydrogenative Coupling of 4‑Substituted Pyridines Catalyzed by a Trinuclear Complex of Ruthenium and Cobalt Masahiro Nagaoka,† Takashi Kawashima,† Hiroharu Suzuki,† and Toshiro Takao*,†,‡ †

Department of Applied Chemistry, Graduate School of Science and Engineering, 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: The dehydrogenative coupling of 4-substituted pyridines catalyzed by a heterometallic trinuclear complex composed of Ru and Co, (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1, Cp* = η5-C5Me5), was investigated. When the pyridine substrate contains an electrondonating group at the 4-position, complex 1 showed a high catalytic activity compared to di- and triruthenium complexes (Cp*Ru)2(μ-H)4 (4) and (Cp*Ru)3(μ-H)3(μ3-H)2 (5). The catalytic activity of 1 was also remarkably higher than the congeners of other group 9 metals, Ru2Rh (2) and Ru2Ir analogues (3). The distinctive reactivity of 1 was attributed to a paramagnetic intermediate, (Cp*Ru)2{(dmbpy)Co}(μ-H)(μ3-H)2 (12, dmbpy = 4,4′-dimethyl2,2′-bipyridine), which was formed by the reaction of 1 with 4-picoline accompanied by the dissociation of the Cp* at the Co atom. The reaction of 12 with unsubstituted pyridine resulted in the elimination of 4,4′-dimethyl-2,2′-bipyridine, indicating that the Co atom in 12 acts as a dissociation site. In contrast to the reaction of 1 with 4-picoline, the reaction of 2 and 3 with 4-picoline afforded the corresponding μ3-pyridyl complexes (Cp*Ru)2(Cp*M)(μ-H)3(μ3-η2(||)-C5H3NCH3) (15, M = Rh; 16, M = Ir). 4-(Trifluoromethyl)pyridine was not dimerized by 1; however, a similar μ3-pyridyl complex, (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-η2(||)C5H3NCF3) (13), was obtained. The stability of the μ3-pyridyl complex is probably one of the reasons for the low catalytic activity of 2 and 3 in the coupling reaction.

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INTRODUCTION 2,2′-Bipyridines are one of the most commonly used bidentate ligands in the fields of coordination, supramolecular, nanomolecular, and macromolecular chemistry, as well as analytical and photochemistry; they are also found in natural products.1 To date, 2,2′-bipyridines are prepared by two major methods. The first method involves the dehydrogenative coupling of pyridines using a heterogeneous catalyst such as Raney-Ni and Pd/C.2 Although this method is highly atom economical when unsubstituted and alkyl-substituted pyridines are used, it has been widely recognized that this method cannot be applied to the coupling of functionalized pyridines such as isonicotine and dimethylaminopyridine. Moreover, the formation of significant amounts of terpyridines as an inevitable side product sometimes becomes problematic, in particular when the conversion was increased.3 In contrast, the second method, the transition-metalcatalyzed cross-coupling reaction of 2-halopyridines with 2-metalated pyridine,1 tolerates the presence of diverse functional groups and affords functionalized bipyridines with a high selectivity. However, this method requires prefunctionalization at the 2-position of pyridines and affords an equimolar amount of salt. Therefore, efficient and step-economical methods for the synthesis of 2,2′-bipyridines are in demand. Although examples are limited, the stoichiometric formation of 2,2′-bipyridines using a transition-metal complex is known. Mashima and co-workers reported the coupling reaction of © XXXX American Chemical Society

2-arylpyridines using a diyttrium complex bearing a diamido ligand.4 Riera and Pérez reported the formation of unsymmetrical 2,2′-bipyridine ligands via the cross-coupling reaction of the two pyridine ligands on a Re complex.5 Coupling reactions between two pyridine ligands via direct C−H bond cleavage using Sc,6 Ta,7 and Ru8 complexes have also been reported. These reactions selectively afford 2,2′-bipyridine ligands; however, the extraction of the 2,2′-bipyridine ligand from the metal center was rarely investigated probably because of the strong chelation. The extraction of the generated bipyridine ligand is only reported by Mashima and co-workers upon treatment of the diyttrium complex bearing a μ-bipyridine ligand with CCl4.4 We previously reported that di- and triruthenium complexes (Cp*Ru)2(μ-H)4 (4) and (Cp*Ru)3(μ-H)3(μ3-H)2 (5) (Cp* = η5-C5Me5) catalyze the dimerization of several kinds of 4-substituted pyridines without the use of a sacrificial oxidant or hydrogen acceptor.9 In contrast to the reactions performed using heterogeneous catalysts, these reactions selectively produced 2,2′-bipyridines without the formation of terpyridine. The dehydrogenative coupling of 4-picoline proceeded rapidly in the initial few hours using diruthenium complex 4 as the catalyst precursor; the yield of 4,4′-dimethyl-2,2′-bipyridine Received: April 7, 2016

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DOI: 10.1021/acs.organomet.6b00277 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

solvents (Table S-1, Supporting Information). This result of solvent screening strongly indicates that the property of active species generated from 1 is completely different from that derived from ruthenium clusters 4 and 5. The use of a polar solvent, diglyme, significantly decreased the yield (entry 8). This is probably due to the coordination effect of the solvent, inhibiting the incorporation of 4-picoline into the cluster core. A similar effect of coordinative solvents was also observed in the reaction of 5 in DME. Although these reactions were carried out in the closed system, we confirmed that the presence of H2 was not crucial for the coupling reaction. The dimerization of 4-picoline was carried out in toluene at 180 °C for 48 h using 1 mol % of 1 under a H2 atmosphere. The yield of 4,4′-dimethyl-2,2′-bipyridine slightly decreased to 33%, while bipyridine was formed in 48% yield under an argon atmosphere. The coupling reactions of 4-picoline in heptane at 180 °C were performed using several kinds of polyhydrido clusters. The results are summarized in Table 2. This catalyst screening clearly

reached approximately 50% in 1 h. However, the yield of the bipyridine did not increase further when the reaction was continued for a longer period. The yields of other bipyridines were also limited in low to moderate amounts. Triruthenium complex 5 also catalyzes the dimerization of 4-picoline; however, the reaction rate was extremely slow compared with the reaction catalyzed by 4. In order to push the limits of the coupling reaction by the ruthenium clusters, the catalytic activity of a heterometallic cluster composed of Ru and Co, (Cp*Ru)2(Cp*Co)(μ-H)3(μ3H) (1), was investigated.10 In the first-row transition metals, the metal−ligand interactions are weaker, and the ligand-field splitting is more contracted than those of the second- and thirdrow transition metals.11 These distinctive features of the first-row transition metals would endow 1 with different reactivity from that of the polyhydrido clusters composed of second- and thirdrow transition metals. Herein, we report a remarkable improvement in the dehydrogenative coupling of pyridines catalyzed by Co-containing cluster 1. The isolation of a paramagnetic intermediate possessing a (dmbpy)Co fragment (dmbpy = 4,4′-dimethyl-2,2′-bipyridine) and the mechanistic insight are also described.

Table 2. Dehydrogenative Coupling of 4-Picoline Catalyzed by Polyhydrido Clustersa

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RESULTS AND DISCUSSION To optimize the reaction parameters, first, the dehydrogenative coupling of 4-picoline was carried out under several conditions (Table 1). The reaction carried out in heptane afforded Table 1. Dehydrogenative Coupling of 4-Picoline Catalyzed by 1a

entry 1 2 3 4 5 6 7 8

solvent heptane

decane toluene mesitylene diglyme

time [h]

temp [°C]

yieldb [%]

210 210 96 48 48 48 48 48

100 140 180 180 180 180 180 180

7 59 87 (74c) 75 60 69 48 27

entry

catalyst

yieldb [%]

1 2 3 4 5 6 7

(Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1) (Cp*Ru)2(Cp*Rh)(μ-H)3(μ3-H) (2) (Cp*Ru)2(Cp*Ir)(μ-H)3(μ3-H) (3) (Cp*Ru)2(μ-H)4 (4) (Cp*Ru)3(μ-H)3(μ3-H)2 (5) (Cp*Co)2(μ-H)3 (6) (Cp*Co)3(μ-H)3(μ3-H) (7)

87 16 trace 53 48 3 8

a The reactions were carried out using 4-picoline (43 μL, 0.44 mmol) and 22 μmol of the catalyst in 4 mL of heptane at 180 °C in a sealed reaction tube. bYield was determined by GC analysis.

shows the striking reactivity of Ru2Co complex 1 (entry 1). Although complexes 2 and 3 also contained a group 9 metal,12 they did not work efficiently, as shown in entries 2 and 3. As reported previously, the dimerization of 4-picoline using 4 and 5 also proceeded (entries 4 and 5); however, the yields were significantly lower than that obtained using 1. The results indicate that the introduction of Co into the cluster skeleton is effective for the dehydrogenative coupling of 4-picoline. We also examined the reactions using di- or tricobalt complexes 6 and 7 as the catalysts;13 however, 4,4′-dimethyl-2,2′bipyridine was obtained in very low yields (entries 6 and 7). The fact that homometallic complexes 4, 5, 6, and 7 showed a low catalytic activity indicates that the active species seems to maintain a heterometallic skeleton composed of Ru and Co during the reaction. To obtain further information about the catalysis, the coupling reactions of 4-picoline using 1, 4, and 5 were monitored. The time courses of the reactions are shown in Figure 1. The formation of 4,4′-dimethyl-2,2′-bipyridine proceeded rapidly when complex 4 was used as the catalyst, and the turnover

The reactions were carried out using 4-picoline (43 μL, 0.44 mmol) and 1 (15 mg, 22 μmol) in 4 mL of solvent in a sealed reaction tube. b Yield was determined by GC analysis. cIsolated yield. a

4,4′-dimethyl-2,2′-bipyridine above 100 °C (entry 1); the yield exceeded 87% within 96 h at 180 °C in the presence of 5 mol % of 1 (entry 3). Similar to the dimerization catalyzed by 4 and 5, the coupling reaction proceeded selectively; the corresponding terpyridine was not formed. Table 1 shows that the coupling reactions carried out using 1 were very sensitive to solvents. While the reaction performed in heptane afforded 4,4′-dimethyl2,2′-bipyridine in 75% yield within 48 h (entry 4), the yield slightly decreased when other nonpolar hydrocarbon solvents such as decane, toluene, and mesitylene were used (entries 5−7). Thus, heptane was identified as the optimal solvent based on the result of entry 4. Such a solvent effect was not observed in the dimerization of 4-picoline catalyzed by 4 and 5; the yield of 4,4′-dimethyl-2,2′-bipyridine obtained using heptane was comparable to those performed in other nonpolar hydrocarbon B


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Organometallics

Table 3. Substrate Scope and Limitation of Dehydrogenative Coupling of 4-Substituted Pyridines Catalyzed by 1a

entry

substrate

temp [°C]

yieldb [%]

1 2 3 4 5 6 7

R = Me R = tBu R = NMe2 R = OMe R=H R = COOEt R = CF3

180 180 140 140 180 140 180

87 76 58 tracec trace 0c 0c

The reactions were carried out using 4-picoline (43 μL, 0.44 mmol) and 22 μmol of the catalyst in 4 mL of heptane in a sealed reaction tube. bYield was determined by GC analysis. cYield was determined by 1 H NMR analysis. a

Figure 1. Time courses of the formation of 4,4′-dimethyl-2,2′-bipyridine in the reactions catalyzed by 1 (⧫), 4 (●), and 5 (▲). The reactions were carried out at 180 °C in a sealed reaction tube using 3 mL of the stock solution, which was prepared from a 5 mM heptane solution of the catalyst and 4-picoline (20 equiv). The yield was determined by GC analysis after the appropriate reaction time.

methoxy group that acts as a directing group, leading to orthometalation. We previously reported the synthesis of bis(μ-4-methoxypyridyl) complex 8 by the reaction of 4 with 4-methoxypyridine.9a A gentle heating of 8 at 60 °C resulted in the C−C bond formation between the two pyridyl moieties (eq 1); however, the

frequency (TOF) was 4.7 h−1 in the initial 1 h. The yield exceeded 50% within 3 h; however the yield did not increase with further increase in the reaction time. This time profile indicates that the accumulated bipyridine inhibits the reaction likely due to the recoordination to the dinuclear species. Such a product inhibition was not observed in the reaction catalyzed by 5. This can be attributed to the bulky Cp* groups surrounding the triruthenium core that effectively prevents the coordination of bipyridine. However, this steric hindrance also hinders the incorporation of pyridines into the triruthenium core, resulting in a slow reaction. Therefore, 4,4′-dimethyl-2,2′bipyridine was obtained in 56% yield even after 168 h. Complex 1 has a similar structure to 5 in terms of the trinuclear clusters bearing Cp* groups; however, the product formation rate using 1 was dramatically enhanced compared to that using 5 at the early stage of the reaction. The TOF obtained for 1 in the initial 3 h was 0.3 h−1, which was higher than that obtained for 5 (0.1 h−1). More importantly, product inhibition was not seemingly observed during the reaction. Thus, Ru2Co complex 1 provided a large amount of 4,4′-dimethyl-2,2′-bipyridine when the reaction time was sufficient. This can be attributed to the Co atom in 1, facilitating the smooth release of the product. Next, the reactions of 1 with other 4-substituted pyridines were investigated (Table 3). Complex 1 also catalyzed the dehydrogenative coupling of 4-tert-butylpyridine and 4-dimethylaminopyridine, affording the corresponding 2,2′-bipyridines (entries 2 and 3). However, the dimerization of other pyridines failed (entries 4−7). This substrate scope of the coupling reactions catalyzed by 1 is similar to that of ruthenium clusters 4 and 5. The basicity of the N atom in pyridines seems to play an important role in the catalysis, because the coupling reactions proceeded in the 4-substituted pyridines containing an electrondonating group. Although the basicity of 4-methoxypyridine (pKa = 6.47) is higher than 4-picoline (pKa = 6.00), 4-tertbutylpyridine (pKa = 5.99), and 4-dimethylaminopyridine (pKa = 6.09),14 the dimerization of 4-methoxypyridine did not proceed efficiently when 1, 4, and 5 were used. This is due to the

bipyridine was not formed. Instead, μ-κ(C,N)-2,2′-bipyrid-3-yl complex 9 was obtained as a consequence of the C−H bond cleavage at the ortho position of the methoxy group. A preferential coordination through the O atom rather than the κ(N) coordination would lead to the C−H bond cleavage. Therefore, the low yield of 4,4-dimethoxy-2,2′-bipyridine can be rationalized by the deactivation of catalyst owing to the C−H bond activation at the 3-position. The molecular structure of 9 was determined by X-ray diffraction (XRD) study as shown in Figure 2. Although several bridging pyridylpyridine ligands coordinated in a μ-κ(C,N),κ(C,N) fashion are known,15 to the best of our knowledge, this is the first example of a μ-κ(C,N),κ(N,C,C,C) coordination mode. The pyridylpyridine moiety is attached to Ru1 through N1 and C8 atoms and π-bonded to Ru2 through C1N1 and C7C8 bonds. Complex 1 was completely consumed at the end of the catalytic reaction. At that time, mononuclear species Cp*Co(dmbpy) (10) and Cp*2Ru (11) were observed in the reaction mixture (eq 2). These complexes were characterized by comparing their 1H NMR spectra with those of the authentic samples synthesized.16,17 The XRD study of 10 was performed by using a single crystal obtained from the reaction mixture at −30 °C. The molecular structure of 10 and important geometrical parameters are given in the Supporting Information. Notably, these mononuclear complexes 10 and 11 did not C


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Organometallics

Complex 12 was highly sensitive toward air and moisture. Furthermore, complex 12 gradually decomposed by contacting with Teflon. Therefore, the catalytic reaction should be carried out without a stir bar coated with Teflon. In fact, when the Teflon-coated stir bar was used, the yield of 4,4′-dimethyl-2,2′bipyridine significantly decreased.18 The observed solvent effect mentioned above was probably caused by the instability of 12. A single crystal of 12 was obtained from a cold solution of m-xylene, and the structure was determined as (Cp*Ru)2{(dmbpy)Co}(μ-H)(μ3-H)2 by an XRD study (Figure 4). The XRD structure shows that the trimetallic Ru2Co skeleton is maintained; however the Cp* group at the Co atom is eliminated. Instead, a dmbpy ligand is formed, which chelates to the Co center, orthogonal to the trimetallic plane. A similar heterometallic trinuclear complex, {(CO)5Cr}2{(bpy)Cu}(μ3-H) (bpy = 2,2′-bipyridine), has been reported by Klüfers and co-workers, in which the bpy ligand was also coordinated to the Cu atom, orthogonal to the trimetallic plane.19 The positions of the three hydrido ligands were successfully determined during the Fourier synthesis. Two of them are located above both the faces of Ru2Co plane as μ3-hydrides, and the rest bridge over the Ru−Ru edge. The trimetallic skeleton of 12 undergoes a remarkable contraction compared to that of (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1); the Ru−Ru distance of 2.5088(2) Å is significantly smaller than that of 1 (2.6714(6) Å), while the Ru−Co distances (2.5617(3) and 2.5557(3) Å) were slightly shortened compared with that of 1 (av 2.581 Å). This is likely due to the reduction in the valence electrons from 44 to 42 upon the substitution of Cp*−H with dmbpy. The C1−C7 distance of 12 (1.456(3) Å) was noticeably shortened compared with that of free 4,4′-dimethyl-2,2′bipyridine (1.493 Å).20 This shortening of the bond distance indicates the occurrence of electron transfer from the Co center to the dmbpy ligand; thus, the dmbpy moiety is considered to be a monoanionic form, (dmbpy)•−. The spin density analysis performed by a DFT calculation for 12 showed that the negative spin densities reside on the dmbpy moiety (Figure 5).21 Moreover, the positive spin densities were mainly localized at the Co center. This indicates that the oxidation state of the Co center is +2, thus adopting a d7 electron configuration. Wieghardt and co-workers reported the electronic structure of a series of bpy complexes by DFT calculations. They reported that the bpy ligand tends to undergo electron transfer and adopts a monoanionic (bpy)•− form when it binds to low-valent first-row transition metals.22 Kaim and co-workers reported the cyclic voltammograms of a series of bpy complexes of group 9 metals, [Cp*MCl(bpy)]Cl (M = Co, Rh, Ir).23 While a reversible two-electron redox wave, [Cp*M IIICl(bpy)]+ + 2e− ⇄ Cp*MI(bpy) + Cl−, was observed for the Rh and Ir complexes, two separated one-electron waves were observed for the Co analogue. The results suggest that (bpy)CoII is reasonably stable compared with (bpy)RhII and (bpy)IrII, enabling the facile electron transfer from the Co center to the bpy ligand. The singlet state of the dmbpy complex (Cp*Ru)2{(dmbpy)Co}(μ-H)(μ3-H)2 was also calculated using the same method, and the structure was optimized. However, the singlet state was shown to be less stable than the triplet state by 31.1 kcal mol−1. This is consistent with the paramagnetic nature of 12. The solution magnetic moment of 12 was measured in benzene-d6 using the Evans method24 and the μeff value of 2.5 μB at 297.7 K, which is quite consistent with the fact that 12 had a triplet state. In the 1H NMR spectrum of 12, a broad signal for the Cp* groups attached to the Ru centers, was observed at

Figure 2. Molecular structure and labeling of 9 with ellipsoids set at 30% probability. Selected distances (Å) and angles (deg): Ru1−Ru2, 2.7602(2); Ru1−N1, 2.1212(16); Ru1−C8, 2.0762(19); Ru2−N1, 2.3059(16); Ru2−C1, 2.288(2); Ru2−C7, 2.2936(19); Ru2−C8, 2.3139(19); N1−C1, 1.394(2); C1−C2, 1.428(3); C2−C3, 1.363(3); C3−C4, 1.431(3); C5−N1, 1.383(3); N2−C7, 1.388(2); C7−C8, 1.439(3); C8−C9, 1.442(3); C9−C10, 1.384(3); C10−C11, 1.411(3); C11−N2, 1.311(3); C1−C7, 1.444(3); C8−Ru1−N1, 75.98(7); Ru1− N1−C1, 119.15(13); N1−C1−C7, 112.27(16); C1−C7−C8, 115.11(16); C7−C8−Ru1, 117.21(13).

catalyze the dimerization of 4-picoline. This also indicates that the active species has a heterometallic cluster skeleton, and its fragmentation reduces the catalytic activity. Then, the reaction carried out at 160 °C was monitored by 1 H NMR spectroscopy. In the initial 3 h, the signals derived from 1 disappeared completely. Instead, a broad peak appeared at δ −27.68 ppm (Figure 3). This signal probably arises from the Cp* groups of a paramagnetic species generated in situ. While the broad signal was observed during the progress of the coupling reaction, its intensity decreased in the late stage of the reaction. This strongly suggested that the paramagnetic species is an intermediate formed during the coupling reaction. Paramagnetic complex 12 was obtained by the reaction of 1 with an excess amount of 4-picoline conducted at 120 °C (eq 3).

Because the formation of bipyridine was suppressed under these conditions, complex 12 could be isolated by removing the remaining 4-picoline under reduced pressure, followed by washing with pentane. D


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Figure 3. 1H NMR spectral changes in the dehydrogenative coupling of 4-picoline catalyzed by 1 showing Cp* and 4,4′-dimethyl-2,2′-bipyridine region (left) and the signal derived from paramagnetic complex 12 (right) (400 MHz, benzene-d6). The reactions were carried out using 0.6 mL of the stock solution in a sealed NMR tube at 160 °C. After the appropriate reaction time, the volatiles were removed under reduced pressure, and the residue was analyzed by 1H NMR spectroscopy. The stock solution was prepared by dissolving 4-picoline (75 μL, 0.77 mmol) and 1 (25.2 mg, 37.6 μmol) in heptane (5 mL). The asterisked peak was derived from the residual proton of the solvent.

Figure 4. Molecular structure and labeling of 12 with ellipsoids set at 30% probability. Selected distances (Å) and angles (deg): Ru1−Co1, 2.5617(3); Ru2−Co1, 2.5557(3); Ru1−Ru2, 2.5088(2); Co1−N1, 1.9717(18); Co1−N2, 1.9718(17); C1−N1, 1.369(3); C1−C2, 1.399(3); C2−C3, 1.383(4); C3−C4, 1.395(4); C4−C5, 1.368(4); C5−N1, 1.354(3); C7−N2, 1.370(3); C7−C8, 1.400(3); C8−C9, 1.376(3); C9−C10, 1.401(4); C10−C11, 1.369(3); C11−N2, 1.353(3); C1−C7, 1.456(3); Ru1−Co1−Ru2, 58.715(8); Co1−Ru1−Ru2, 60.524(8); Ru1−Ru2−Co1, 60.761(8).

δ −27.68 ppm, which was identical to the chemical shift observed for the reaction mixture (Figure 3). In addition to the Cp* signal, four signals with an intensity ratio of 6:2:2:2 were observed at δ −130.26, 34.57, 82.11, and 117.41 ppm, which were assignable to the signals derived from the dmbpy moiety. The signal derived

from the hydrido ligands could not be detected. This would be due to the paramagnetic nature of 12, leading to a severe broadening of the signal of hydrides directly attached to the metal center. Although other paramagnetic complexes were not isolated, formation of similar paramagnetic species was also observed in E


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Organometallics

cluster, even though the paramagnetic complex was formed in the reaction. In contrast to the reaction of 1 with unsubstituted pyridine, the reaction with 4-(trifluoromethyl)pyridine did not yield the bipyridine complex 12. Instead, a μ3-pyridyl complex, (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-η2(||)-C5H3NCF3) (13), was formed without the elimination of the Cp* group on the Co atom (eq 5). The XRD study using a single crystal obtained

Figure 5. (a) DFT-calculated spin density of 12. The blue and green areas correspond to the regions of positive and negative spin densities, respectively; (b) electronic structure of the (dmbpy)Co moiety in 12.

the reaction of 1 with 4-tert-butylpyridine, 4-dimethylaminopyridine, and pyridine. In these reactions, characteristic broad Cp* signals were observed at δ −27.93 (4-tert-butylpyridine), −29.31 (4-dimethylaminopyridine), and −27.11 ppm (pyridine), respectively. Although the unsubstituted bipyridine was not formed catalytically, this result suggested that a bpy analogue was also formed by the reaction of 1 with unsubstituted pyridine. It is noteworthy that 12 showed a comparable activity in the dimerization of 4-picoline to 1, affording 4,4′-dimethyl-2,2′bipyridine in 79% yield.25 This fact strongly indicates that 12 was the key intermediate in the dehydrogenative coupling reaction of 4-substituted pyridines. Despite the chelation to the metal center, the dmbpy ligand dissociates from the metal center relatively readily. The treatment of 12 with pyridine-d5 at 120 °C resulted in the dissociation of 4,4′-dimethyl-2,2′-bipyridine accompanied by the formation of paramagnetic species, likely (Cp*Ru)2{(bpy-d)Co}(μ-H)(μ3-H)2 judging from a broad Cp* signal (eq 4).26 When 21% of

from a cold solution of heptane unambiguously represented the μ3-η2(||)-coordination of the 4-(trifluoromethyl)pyridyl ligand to the Ru2Co plane, as shown in Figure 6. The pyridyl group is

Figure 6. Molecular structures of 13 with thermal ellipsoids at the 30% level of probability. Selected bond distances (Å) and angles (deg): Ru1− Co1, 2.8822(4); Ru2−Co1, 2.6987(4); Ru1−Ru2, 2.8063(3); Ru1−N1, 2.066(2); Co1−C1, 1.903(3); Ru2−N1, 2.218(2); Ru2−C1, 2.152(3); N1−C1, 1.406(3); C1−C2, 1.435(4); C2−C3, 1.363(4); C3−C4, 1.427(4); C4−C5, 1.355(4); C5−N1, 1.383(3); Ru1−Co1−Ru2, 60.271(10); Co1−Ru1−Ru2, 56.624(9); Co1−Ru2−Ru1, 63.106(10).

12 was converted, 4,4′-dimethyl-2,2′-bipyridine was liberated in 19% yield. This result implies that the dmbpy ligand was released from the Co center; that is, the Co atom acts as a dissociation site. Coordination of the third and fourth pyridine molecules to 12 seems to be possible due to the 42-electron configuration of 12. Thus, the dmbpy ligand in 12 seemed to be eliminated in an associative manner via incorporation of two pyridine molecules. Subsequent C−H bond scission followed by the coupling between the two pyridyl moieties led to regeneration of 12, which would make the catalytic cycle. Electron transfer from the monoanionic (dmbpy)•− to the Co center is necessary for the dissociation of the dmbpy ligand. This facile electron transfer would be the key to the elimination step, and this would be the role of the redox-active Co atom in this catalysis. In contrast, this electron transfer becomes difficult in the case of unsubstituted bipyridine because of its lower LUMO level compared to 4,4′-dimethyl-2,2′-bipyridine, making the bpy analogue (Cp*Ru)2{(bpy)Co}(μ-H)(μ3-H)2 too stable. Therefore, the unsubstituted bipyridine did not dissociate from the

bonded to the Ru1 atom via κ(N) coordination and σ-bonded to the Co atoms through the C atom next to the N atom. Moreover, the pyridyl group is π-bonded to the Ru2 atom with the CN bond, making complex 13 coordinatively saturated. The μ3-coordination of a pyridyl group on a trinuclear complex is rather rare; however, this was also observed in the isoelectronic triruthenium analogue (Cp*Ru)3(μ-H)3(μ3-η2(||)-C5H3NCH3) (14).9b Positional isomers with respect to the orientation and location of the μ3-pyridyl group may exist; however, only 13 was observed in the solution as well as in the solid state. Although the pyridyl group shows a pivot motion between the two Ru−Co edges as mentioned below, no evidence for the migration to the Ru−Ru edge or rotation leading to the κ(N) coordination to the Co atom was obtained by the VT-NMR studies. This fact implies that the formation of the isomers is thermodynamically unfavorable if they could be formed. F


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Organometallics Scheme 1. Plausible Mechanism for the Formation of 12

coordinatively saturated, a vacant site was required on the cluster for the incorporation of additional pyridine molecules. In the case of 14, the coordinatively unsaturated dihydrido complex (Cp*Ru)3(H)2(μ3-C5H3NCH3) was formed upon thermolysis, leading to the reductive elimination of dihydrogen.9b However, dihydrogen elimination did not occur in the Ru2Co complex. Instead, a vacant site for the second pyridine molecule could be generated by the transformation of the pyridyl ligand from a μ3to a μ-coordination mode. Although such transformation has never been reported, the μ-coordination mode of a pyridyl group on a multimetallic site is well known.27 While transformation into a μ-coordination mode would occur for a pyridyl group containing an electron-donating substituent, this seems to be difficult for a pyridyl ligand containing an electron-withdrawing substituent because of the susceptibleness to strong backdonation, making the π-coordination of the CN bond strong. Consequently, this strong π-coordination should impede the formation of a bipyridine complex, such as 12. According to the isolation of 13 as well as its H/D exchange reaction between deuterated pyridine, a plausible mechanism for the formation of 12 is proposed as Scheme 1. Coordination of the second pyridine molecule to μ3-pyridyl complex A or B and subsequent C−H bond scission would afford intermediate D, which corresponds to an intermediate for the H/D exchange in 13. In the reductive C−C bond forming step, formation of two pyridyl groups should be required. Formation of two μ-pyridyl groups would cause transformation of one of the Cp* ligands to a η4-cyclopentadiene or a η3-cyclopentadienyl ligand according to the EAN rule. Jones et al. demonstrated that Cp*−H eliminates from Cp*Rh(PMe3)H2 upon treatment with PMe3 via the ring slippage of the Cp* group, while an initial migration of a hydride to the Cp* ring is operative for the consecutive deuterium incorporation in the reaction with D2.28 Whereas both mechanisms are possible, formation of a η4-Cp*H ligand precludes the formation of an intermediate having a high oxidation state. In this regard, we tentatively propose a η4-Cp*H structure for intermediate D. Recently, formation of (η4Cp*H)Rh(bpy)Cl via reductive C−H bond formation between Rh−H and Cp* was reported by Miller and co-workers.29

The trinuclear skeleton of 13 is remarkably expanded compared to that of 12. The Ru1−Co1, Ru1−Ru2, and Ru2−Co1 lengths are 2.8822(4), 2.8063(3), and 2.6987(4) Å, respectively. Among these M−M bonds, the Ru1−Co1 bond is the longest due to the coordination of the μ3-pydidyl group. The C1N1 bond distance (1.406(3) Å), π-bonded to the Ru2 atom, is comparable to those of the triruthenium analogues (Cp*Ru)3(μ-H)3(μ3-η2(||)-C5H4N) (1.413(4) Å) and (Cp*Ru)3(μ-H)3(μ3-η2(||)-C5H3NCOOEt) (1.433(5) Å),9b and the π-coordination of the CN bond causes bond alternation around the pyridyl moiety. The C1−C2 (1.435(4) Å), C3−C4 (1.427(4) Å), and C5−N1 bonds (1.383(4) Å) are considerably longer than the C2−C3 (1.363(4) Å) and C4−C5 bonds (1.355(4) Å). The hydrides in 13 were successfully located above each M−M bond as a μ-hydride during the Fourier synthesis. Unlike 12, complex 13 is diamagnetic. In the 1H NMR spectrum recorded at −40 °C, three sharp Cp* signals were observed at δ 1.60, 1.64, and 1.83 ppm. As the temperature was increased, two of them appearing at δ 1.60 and 1.83 ppm broadened and coalesced into one signal. This spectral change can be attributed to the pivot motion of the pyridyl group between the Ru−Co edges, while maintaining the Co−C bond. This dynamic behavior was also observed in the isoelectronic triruthenium analogue 14. In contrast to 4-picoline, the incorporation of 4-(trifluoromethyl)pyridine into the Ru2Co core was restricted to one molecule. In spite of its coordinatively saturated nature, 13 reacted with pyridine-d5, leading to H/D exchange at the hydrido positions. Upon heating at 90 °C for 24 h, the intensity of the hydrido signals decreased to 34% without any decomposition of 13. At the same time, the intensity ratio of the residual ortho, meta, and para protons of pyridine-d5 changed from 2.4:2.0:1.0 to 2.7:2.0:1.0. These results strongly indicated that the second pyridine molecule was incorporated into the trinuclear site of 13 and a μ-pyridyl group was formed via C−D bond cleavage of pyridine-d5 at the ortho position. Although the dissociation of the Cp* at the Co atom in 13 was not observed as mentioned above, complex 13 can be regarded as a frozen intermediate for the formation of 12. Because 13 is G


Article

Organometallics Reductive elimination of Cp*−H accompanied by the reductive coupling of the two pyridyl moieties might produce 12. Such a facile elimination of Cp*−H is likely responsible for the weak metal−ligand interaction for the first-row transition metals compared to those of the second- and third-row transition metals. Although the N atom attaches to the Ru atom in 13, the bipyridine ligand chelates to the Co atom in 12. This suggests that some rearrangement occurred during the formation of 12. We previously reported the rotation of the μ3-pyridyl ligand on the Ru3 plane of 14. Like the μ3-pyridyl ligand in 13, the μ3-pyridyl ligand showed a pivot motion centered on the Ru−C bond. Moreover, another motion, namely, the pivot motion centered on the Ru−N bond, occurred at a higher temperature. A combination of these two pivot motions originated the timeaveraged spectrum, showing the coalescence of three Cp* signals for the μ3-pyridyl complex 14. Thus, the positional isomer, in which the pyridyl ligand was bonded to the Co atom through the N atom, would be formed via the rotation of the pyridyl group around the Ru2Co plane. To assess the plausibility of the mechanism shown in Scheme 1, DFT calculations on 13 and the positional isomer 13′ were carried out. The calculated structures and important geometrical parameters are given in the Supporting Information. The optimized structure of 13 exhibited reasonable agreement with the XRD data. Although the experimental evidence was insufficient, the calculations provided a plausible structure for 13′, where the N atom was attached to the Co atom. This complex was shown to be less stable than 13 by 1.1 kcal mol−1 at 25 °C, which is consistent with the fact that 13′ was not observed in the solution as well as in the solid state. One question still remains unanswered: Which metal is actually responsible for the C−H bond cleavage? The structure of 13 suggests that the Co atom seemingly acts as an activation site. However, as mentioned above, complex 13 seems to be the thermodynamic product, and the dynamic property of the μ3-pyridyl group prevents a detailed kinetic consideration. Thus, it is difficult to determine the role of the heterometallic Ru−Co skeleton in the C−H bond scission at present. Although the dehydrogenative coupling did not occur efficiently, the Ru2Rh and Ru2Ir analogues 2 and 3 also reacted with 4-picoline. Similar to the reaction of 1 with 4-(trifluoromethyl)pyridine, μ3-pyridyl complexes (Cp*Ru)2(Cp*M)(μ-H)3(μ3-η2(||)C5H3NCH3) (15, M = Rh; 16, M = Ir) were exclusively obtained (eq 6). While the reaction of 2 with 4-picoline proceeded nearly

atom through the N atom, bonded to the group 9 metal through the C atom, and π-bonded to the Ru2 atom with the CN bond. The distance of the π-coordinated C1N1 bond in 15 (1.397(4) Å) is close to that of the μ3-η2(||)-pyridyl ligand in 13 (1.406(3) Å). Although the positions of the hydrido ligands of 16 could not be determined, the similarity in the NMR data strongly indicates that 16 also has three μ-hydrides similar to 13. In the 1H NMR spectrum of 15 recorded at −80 °C, three signals derived from the Cp* groups were observed at δ 1.62, 1.81, and 1.85 ppm. In the hydrido region, three signals with the same intensity appeared at δ −22.13 (dd, JRhH = 32.4 Hz, JHH = 4.0 Hz, 1H, Rh−H−Ru), −17.13 (d, JRhH = 21.1 Hz, 1H, Rh− H−Ru), and −10.79 (d, JHH = 4.0 Hz, 1H, Ru−H−Ru) ppm, in which spin−spin coupling with the 103Rh nuclei was observed at the signals appearing at δ −22.13 and −17.13 ppm. Similar to 13, the μ3-pyridyl group of 15 showed a pivot motion between the Ru−Rh edges. Therefore, two of three Cp* signals, appearing at δ 1.62 and 1.81 ppm, broadened with increasing temperature and coalesced into one signal at −20 °C. The time-averaged signal, exhibiting two Cp*signals with an intensity ratio of 2:1 at δ 1.74 and 1.87 ppm, was obtained above room temperature. A similar spectral change was also observed for the hydrido signals; two hydrido signals stemming from Ru−H−Rh became broader and coalesced at a higher temperature. In the 13C{1H} NMR spectrum of 15 recorded at −80 °C, five signals assignable to the μ3-pyridyl ring were observed at δ 114.3, 127.2, 140.9, 151.3, and 154.8 (d, JRhC = 28 Hz) ppm. Among these signals, the spin−spin coupling with the 103Rh nuclei was observed only for the signal at δ 154.8 (d, JRhC = 28 Hz). This is consistent with the fact that the μ3-pyridyl group is σ-bonded to the Rh atom with the C atom next to the N atom as shown by the XRD analysis. The 1H and 13C NMR spectra of the Ru2Ir analogue 16 showed similar spectral features to those of 15. At −70 °C, three Cp* signals were observed at δ 1.62, 1.83, and 1.97 ppm, and the signals assignable to the hydrido ligands appeared at δ −23.98, −19.05, and −10.52 ppm. Complexes 15 and 16 did not further react with 4-picoline, and bipyridine complexes were not formed. This crucial difference would be one of the reasons for the low catalytic activity of 2 and 3 toward the coupling reaction. The Ru2Rh and Ru2Ir skeletons would stabilize the μ3-pyridyl form due to the enhanced backdonation to the π*(CN) as observed in 13.

quantitatively at 100 °C, the reaction of 3 required heating at 160 °C, and the reaction was not completed even after 5 days. The pyridyl complexes 15 and 16 were characterized by NMR and elemental analyses, and the structures were unambiguously determined by XRD analysis (Figure 7). The structural features of 15 and 16 are almost the same as those of 13, except for the expansion of the trinuclear skeleton owing to the larger size of the second- and third-row transition metals. Similar to 13, the pyridyl group is bonded to the Ru1

CONCLUSION Complex 1, composed of Ru and Co, showed a high catalytic activity toward the dehydrogenative coupling of 4-substituted pyridines compared to diruthenium complex 4 and triruthenium complex 5. In contrast to the reaction catalyzed by 4, product inhibition was not observed for the reaction catalyzed by 1. Moreover, the reaction was significantly accelerated compared to the reaction catalyzed by 5. Although Ru2Rh and Ru2Ir analogues 2 and 3 also had a heterometallic cluster skeleton, they did not work efficiently. These sharp contrasts showed that the introduction of Co into the cluster skeleton is crucial for the dehydrogenative coupling of 4-substituted pyridines. The most striking feature of the Ru2Co complex was the formation of the paramagnetic dmbpy complex 12. The corresponding complexes having (dmbpy)Rh and (dmbpy)Ir moieties were not formed by the reactions of 2 and 3 with 4-picoline. Relatively weak metal−ligand interactions for the first-row transition metals likely promoted the substitution of the Cp* ligand with the

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Organometallics

Figure 7. Molecular structures of 15 (a) and 16 (b) with thermal ellipsoids at the 30% level of probability. Selected bond distances (Å) and angles (deg) for 15: Ru1−Rh1, 2.9593(3); Ru2−Rh1, 2.8073(3); Ru1−Ru2, 2.7937(3); Ru1−N1, 2.078(2); Rh1−C1, 2.003(3); Ru2−N1, 2.213(2); Ru2−C1, 2.190(3); N1−C1, 1.397(4); C1−C2, 1.425(4); C2−C3, 1.365(4); C3−C4, 1.421(4); C4−C5, 1.354(4); C5−N1, 1.383(4); Ru1−Rh1−Ru2, 57.883(8); Rh1−Ru1−Ru2, 58.330(8); Rh1−Ru2−Ru1, 63.788(8). Selected bond distances (Å) and angles (deg) for 16: Ru1−Ir1, 2.9524(6); Ru2− Ir1, 2.8603(6); Ru1−Ru2, 2.8001(8); Ru1−N1, 2.097(6); Ir1−C1, 2.011(9); Ru2−N1, 2.179(8); Ru2−C1, 2.231(8); N1−C1, 1.375(11); C1−C2, 1.439(12); C2−C3, 1.365(15); C3−C4, 1.453(18); C4−C5, 1.366(14); C5−N1, 1.374(9); Ru1−Ir1−Ru2, 57.57(2); Ir1−Ru1−Ru2, 59.562(17); Ir1−Ru2−Ru1, 62.87(2).

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dmbpy ligand. The triplet nature of 12 was suggested by the Evans method; it was also supported by the spin density analysis performed by a DFT calculation. This indicated that the electron transfer from the Co center to the dmbpy ligand occurred. In other words, the triplet state was achieved only when the dmbpy ligand was coordinated to the redox-active Co atom. The electronic property of the Co atom is also responsible for the elimination step of the dmbpy ligand because it requires the oxidation of (dmbpy)•− to (dmbpy). The fact that the unsubstituted bipyridine analogue did not promote the catalysis clearly shows the importance of the reversible electron transfer between the Co center and the bipyridine ligand, allowing the Co center to become a dissociation site. Unlike the reaction of 1 with 4-picoline, coordinatively saturated μ3-η2(||)-pyridyl complexes 15 and 16 were obtained by the reaction of 2 and 3 with 4-picoline. A strong π-coordination of the CN bond in the μ3-pyridyl group to the Ru atom hindered the formation of a vacant site for the second pyridine molecule. Therefore, the dehydrogenative coupling of pyridine did not proceed efficiently in the case of 2 and 3. This is consistent with the fact that μ3-pyridyl complex 13 was also obtained by the reaction of 1 with 4-(trifluoromethyl)pyridine, whose CF3 group enhanced the back-donation to the π*(CN) orbital even though it was attached to the Ru2Co core. Cooperative interactions of the neighboring metal centers with the substrate facilitate the bond activation; however, the product often becomes too stable owing to the formation of multiple M−C bonds. The results obtained in this study provide some directions to utilize the multimetallic activation for catalysis, while avoiding the thermodynamic sink. The role of the Co atom in 1 in the dehydrogenative coupling of 4-substituted pyridines can be summarized as follows: (1) The Co atom enhances the elimination of the Cp* group, (2) it suppresses the π-coordination of the CN bond, and (3) it facilitates the electron transfer between the bipyridine ligands. Thus, the catalytic activity was improved using the characteristic property of the first-row transition metals, i.e., weak metal−ligand interactions and redox activity.

EXPERIMENTAL SECTION

General Procedures. All manipulations for air- and moisturesensitive compounds were carried out under an argon atmosphere using standard Schlenk techniques or in a glovebox filled with nitrogen. Dehydrated toluene, THF, diethyl ether, pentane, hexane, heptane, methanol, and acetone used in this study were purchased from Kanto Chemicals and stored under an argon atmosphere. Decane, diglyme, DME, m-xylene, and mesitylene were dried over sodium benzophenone ketyl and stored under an argon atmosphere. Benzene-d6, toluene-d8, and tetrahydrofuran-d8 were dried over sodium benzophenone ketyl and stored under an argon atmosphere. Pyridine-d5 and chloroform-d1 were dried over MS-4A and stored under an argon atmosphere. Pyridine, 4-picoline, 4-tert-butylpyridine, and 4-(trifluoromethyl)pyridine were distilled from CaH2, dried over MS-4A, and stored under an argon atmosphere. 4-Methoxypyridine and ethyl isonicotinate were dried over MS-4A and stored under an argon atmosphere. 4-Dimethylaminopyridine and hexamethylbenzene were purified by sublimation and stored in the glovebox filled with nitrogen. Other materials used in this research were used as purchased. 1H and 13C NMR spectra were recorded on Varian INOVA 400 and Varian 400-MR Fourier transform spectrometers. 1H NMR spectra were referenced to tetramethylsilane as an internal standard. 13C NMR spectra were referenced to the naturally abundant carbon signal of the employed solvent. Elemental analyses were performed on a PerkinElmer 2400II series CHN analyzer. GC spectra were obtained by means of a Shimadzu GC-2010 equipped with a TC-1 column. (Cp*Ru) 2 (Cp*Rh)(μ-H) 3 (μ 3 -H) (2), 12 (Cp*Ru) 2 (Cp*Ir)(μ-H) 3 (μ 3 -H) (3), 12 (Cp*Ru) 2 (μ-H) 4 (4), 30 (Cp*Ru) 3 (μ-H) 3 (μ 3 -H) 2 (5), 31 (Cp*Co) 2 (μ-H) 3 (6), 13 and (Cp*Co)3(μ-H)3(μ3-H) (7)13 were prepared according to the previously published method. The synthetic method of (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1) was altered from the previously published method10 as described below, in which Cp*Co(η2-CH2CH2)2 was used as a Cp*Co source. Cp*Co(η2-CH2CH2)2 was prepared according to the literature.32 X-ray Diffraction Studies. Single crystals of 9, 10, 12, 13, 15, and 16 for the X-ray analyses were obtained 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 graphitemonochromated Mo Kα radiation (λ = 0.710 69 Å). Cell refinement and data reduction were performed using the PROCESS-AUTO program.33 Intensity data were corrected for Lorentz−polarization effects and I


Article

Organometallics empirical absorption. The structures were solved by the direct method using the SHELXS-97 program and refined by the SHELXL-97 program.34 The Cp* groups attached to the Ru2 and Ir1 in 16 were refined as positional disorder, whose occupancies were 50%:50% and 55%:45%, respectively. All non-hydrogen atoms were found by the difference Fourier synthesis and were refined anisotropically, except for the disordered Cp* groups attached to Ru2 and Ir1 in 16. The refinement was carried out by least-squares methods based on F2 with all measured reflections. The metal-bound hydrogen atoms in 9, 12, 13, and 15 were located in the difference Fourier map and refined isotropically, while the positions of metal-bound hydrogen atoms in 16 were not determined. Crystal data and results of the analyses are listed in Table S-10 in the Supporting Information. Computational Details. Density functional theory (DFT) calculations on 12, 13, and 13′ were carried out at the B3PW91 level35 in conjunction with the Stuttgart/Dresden ECP36 and associated with triple-ζ SDD basis sets for the transition metals. The 6-31G(d,p) basis set was employed for the carbon, hydrogen, nitrogen, and fluorine atoms. No simplified model compounds were used for the calculations. Initial geometries for the optimization were based on crystallographically determined structures for 12 and 13. Frequency calculations at the same level of theory as geometry optimizations were performed on the optimized structure to ensure that the minimum exhibits only positive frequency. All Gibbs energies were computed in the gas phase at 298 K from the corresponding zero-point-corrected electronic energies. All calculations were carried out without symmetry constraints utilizing the Gaussian 09 program.37 The molecular structures and spin-density distribution were drawn by using the GaussView version 5.0 program.38 Information on the atom coordinates (xyz files) for all optimized structures, important geometrical parameters, and the optimized structures are collected in the Supporting Information. Preparation of (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1). A 50 mL Schlenk tube equipped with a rubber balloon filled with H2 was charged with 0.896 g of 4 (1.88 mmol), 0.479 g of Cp*Co(η2-CH2CH2)2 (1.91 mmol), and toluene (45 mL). The mixture was then cooled to −78 °C. After the Schlenk tube was evacuated, H2 was introduced to the tube. The resulting mixture was stirred at 80 °C for 12 h. Removal of the solvent under reduced pressure gave a black residual solid. The residue was extracted with toluene and filtered through a glass filter (G4) to remove insoluble material. Removal of the solvent under reduced pressure gave 1.27 g of 1 as a black solid (1.88 mmol, >99%). Data for 1 were previously published in ref 10. Dehydrogenative Coupling of 4-Picoline by 1. The dehydrogenative coupling of 4-picoline by 1 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 GC. The results are listed in Table 1. A typical reaction was carried out as follows (entry 3): A glass tube equipped with a Teflon valve was charged with 1 (14.8 mg, 22.0 μmol), 4-picoline (43 μL, 0.44 mmol), and heptane (4.2 mL). The solution was heated at 180 °C for 96 h. Biphenyl (0.12 mmol in 5.0 mL of toluene) was then added to the tube as an internal standard. Formation of 4,4′-dimethyl-2,2′-bipyridine was quantitatively analyzed by GC (87%). Dehydrogenative Coupling of 4-Picoline by 1; Isolation of 4,4′-Dimethyl-2,2′-bipyridine. A 100 mL glass tube equipped with a Teflon valve was charged with 1 (125.8 mg, 0.188 mmol), 4-picoline (365 μL, 3.75 mmol), and heptane (35 mL). The solution was heated at 180 °C for 96 h. Removal of the volatile components under reduced pressure gave a brown solid. The residue was then extracted with toluene, and the combined solution was filtered through Celite. Removal of the solvent under reduced pressure followed by washing five times with 3 mL of pentane gave 257.3 mg of 4,4′-dimethyl-2,2′-bipyridine as a white solid (1.40 mmol, 74%). 1H NMR (400 MHz, chloroform-d1, rt): δ 2.44 (s, 6H, C4Me), 7.13 (d, JHH = 5.2 Hz, 2H, C5H), 8.23 (s, 2H, C3H), 8.54 ppm (d, JHH = 5.2 Hz, 2H, C6H). 13C{1H} NMR (100 MHz, CDCl3, rt): δ 21.1, 122.0, 124.6, 148.1, 148.9, 156.0 ppm. Dehydrogenative Coupling of 4-Picoline by Several Kinds of Polyhydrido Complexes. The reactions of 1, 2, 3, 4, 5, 6, and 7 with 20 equiv of 4-picoline were performed in a glass tube equipped with a Teflon valve at 180 °C for 96 h, respectively. After the reaction, the

solution was analyzed by GC. The results are listed in Table 2. A typical reaction was carried out as follows (entry 2): A glass tube equipped with a Teflon valve was charged with 2 (15.8 mg, 22.1 μmol), 4-picoline (40 μL, 410 μmol), and heptane (4.2 mL). The solution was heated at 180 °C for 96 h. Biphenyl (24.2 μmol in 5.0 mL of toluene) was then added to the tube as an internal standard. Formation of 4,4′-dimethyl2,2′-bipyridine was quantitatively analyzed by GC (16%). Dehydrogenative Coupling of 4-Picoline by 1; Reaction Monitoring with GC. The reactions of 1 with 20 equiv of 4-picoline were performed in a glass tube equipped with a Teflon valve at 180 °C. After the appropriate reaction time, the solution was analyzed by GC. The results are shown in Figure 1 and Table S-2. A typical reaction was carried out as follows (entry 4): Complex 1 (73.9 mg, 0.110 mmol) and 4-picoline (215 μL, 2.2 mmol) were dissolved in 21 mL of heptane, the stock solution was divided into seven portions, and a 3.0 mL amount of the stock solution was charged in each glass tube equipped with a Teflon valve. The reaction mixture was heated at 180 °C for 24 h. Biphenyl (0.121 mmol in 5.0 mL of toluene) was then added to the tube as an internal standard. Formation of 4,4′-dimethyl-2,2′-bipyridine was quantitatively analyzed by GC (53%). The coupling reactions of 4-picoline by 4 and 5 were also performed as the catalytic reaction by 1. Dehydrogenative Coupling of Several Kinds of Pyridines by 1. The reactions of 1 with 20 equiv of pyridines were performed in a glass tube equipped with a Teflon valve at the appropriate temperature for 96 h. After the reaction, the solution was analyzed by GC or 1H NMR spectroscopy. The results are listed in Table 3. A typical reaction was carried out as follows (entry 2): Complex 1 (33.7 mg, 50.2 μmol) and 4-tert-butylpyridine (100 μL, 1.0 mmol) were dissolved in 15 mL of heptane, and a stock solution of the reaction mixture was formed. A 4 mL amount of the stock solution was charged in a glass tube equipped with a Teflon valve, and the solution was heated at 180 °C for 96 h. Biphenyl (0.123 mmol in 5.0 mL of toluene) was then added to the tube as an internal standard. Formation of 4,4′-di-tert-butyl-2,2′bipyridine was quantitatively analyzed by GC (76%). After the solvent and unreacted 4-tert-butylpyridine were removed under reduced pressure, the residual solid was analyzed by means of 1H NMR spectroscopy. Data for 4,4′-di-tert-butyl-2,2′-bipyridine are as follows. 1 H NMR (400 MHz, chloroform-d1, rt): δ 1.39 (s, 18H, C4tBu), 7.30 (dd, JHH = 5.2, 1.6 Hz, 2H, C5H), 8.40 (d, JHH = 1.2 Hz, 2H, C3H), 8.59 ppm (d, JHH = 5.2 Hz, 2H, C6H). Data for 4,4′-bis(dimethylamino)2,2′-bipyridine are as follows. 1H NMR (400 MHz, chloroform-d1, rt): δ 3.08 (s, 12H, C4NMe2), 6.51 (dd, JHH = 6.0, 2.4 Hz, 2H, C5H), 7.67 (d, JHH = 2.4 Hz, 2H, C3H), 8.29 ppm (d, JHH = 6.0 Hz, 2H, C6H). Data for 4,4′-bis(methoxy)-2,2′-bipyridine are as follows. 1H NMR (400 MHz, chloroform-d1, rt): δ 3.95 (s, 6H, C4OMe), 6.84 (dd, JHH = 5.6, 2.4 Hz, 2H, C5H), 7.98 (d, JHH = 2.4 Hz, 2H, C3H), 8.47 ppm (d, JHH = 5.6 Hz, 2H, C6H). Data for 2,2′-bipyridine: 1H NMR (400 MHz, chloroform-d1, rt): δ 7.32 (ddd, JHH = 7.2, 4.8, 1.2 Hz, 2H, C5H), 7.83 (ddd, JHH = 8.0, 7.2, 2.0 Hz, 2H, C4H), 8.41 (ddd, JHH = 8.0, 1.2, 1.0 Hz, 2H, C3H), 8.69 ppm (ddd, JHH = 4.8, 1.2, 1.0 Hz, 2H, C6H). Reaction of 4 with 4-Methoxypyridine; Preparation of (Cp*Ru)2(μ-H){μ-η4-C10H5N2(OMe)2} (9). A 50 mL Schlenk tube was charged with 4 (86.6 mg, 0.182 mmol) and THF (8 mL). 4-Methoxypyridine (185 μL, 1.82 mmol) was added, and the reaction mixture was stirred at 60 °C for 30 min. 4-Methoxypyridine and solvent were removed under reduced pressure to circumvent overreaction. The orange residue was dissolved in THF (5 mL), and the solution stirred at 60 °C for 24 h. The solution turned from orange to black. Removal of the solvent under reduced pressure gave a black solid. The residue was extracted with hexane and purified by column chromatography on alumina with a mixture of toluene and THF (10:1) as an eluent. Complex 9 was obtained as a black solid upon removal of the solvent under reduced pressure (53.2 mg, 0.0772 mmol, 42%). A black single crystal suitable for the XRD study was prepared by the recrystallization from the diethyl ether solution stored at −20 °C. 1 H NMR (400 MHz, benzene-d6, rt): δ −6.97 (s, 1H, μ-H), 1.71 (s, 30H, C5Me5Ru), 3.25 (s, 3H, OMe), 3.36 (s, 3H, OMe), 5.50 (d, JHH = 4.8 Hz, 1H, Ar), 6.03 (dd, JHH = 7.2, 3.2 Hz, 1H, Ar), 6.85 (d, JHH = 7.2 Hz, 1H, Ar), 7.82 (d, JHH = 3.2 Hz, 1H, Ar), 8.57 ppm (d, JHH = 4.8 Hz, 1H, Ar). 13 C{1H} NMR (100 MHz, benzene-d6, rt): δ 11.0 (C5Me5Ru), J


Article

Organometallics

(100 MHz, THF-d8, −40 °C): δ 11.2 (C5Me5), 11.6 (C5Me5), 12.6 (C5Me5), 82.7 (C5Me5), 89.7 (C5Me5), 91.1 (C5Me5), 106.1 (Ar), 117.0 (q, JCF = 31 Hz, C4), 126.1 (q, JCF = 270 Hz, −CF3), 144.6 (Ar), 150.1 (Ar), 150.7 ppm (Ar). Anal. Calcd for C36H51CoF3NRu2: C, 53.00; H, 6.30; N, 1.72. Found: C, 53.16; H, 6.48; N, 1.65. H/D Exchange between 13 with Pyridine-d5. An NMR tube equipped with a Teflon valve was charged with 13 (10.5 mg, 12.9 μmol) and pyridine-d5 (0.50 mL) with 2,2,4,4-tetramethylpentane (1.7 μL) as an internal standard. The reaction was carried out at 90 °C, and the reaction was periodically monitored by 1H NMR spectroscopy. The 1 H NMR spectrum recorded after 24 h showed that the intensity of the hydrido ligands decreased to 34% due to deuteration. At the same time, the intensity ratio of the residual ortho, meta, and para protons of pyridine-d5 changed from 2.4:2.0:1.0 to 2.7:2.0:1.0. Reaction of 2 with 4-Picoline; Preparation of (Cp*Ru)2(Cp*Rh)(μ-H)3(μ3-η2(||)-C5H3NMe) (15). A glass tube equipped with a Teflon valve and with a Teflon-coated stir bar was charged with 2 (82.5 mg, 0.115 mmol), 4-picoline (120 μL, 1.23 mmol), and toluene (5 mL). The reaction mixture was stirred at 100 °C for 48 h. The solution turned from brown to purple. Removal of the solvent under reduced pressure gave a purple residual solid. The residue was then extracted five times with 3 mL of hexane, and the combined solution was filtered through a glass filter (G4). Removal of the solvent under reduced pressure followed by washing three times with 2 mL of methanol gave 78.5 mg of 15 as a purple solid (0.0974 mmol, 85%). A purple single crystal suitable for the XRD study was prepared by recrystallization from the mixed solvent of acetone and toluene in a 2:1 volume ratio at −30 °C. 1H NMR (400 MHz, THF-d8, −80 °C): δ −22.13 (dd, JRhH = 32.4 Hz, JHH = 4.0 Hz, 1H, Rh−H−Ru), −17.13 (d, JRhH = 21.1 Hz, 1H, Rh−H−Ru), −10.79 (d, JHH = 4.0 Hz, 1H, Ru−H− Ru), 1.62 (s, 15H, C5Me5Ru), 1.81 (s, 15H, C5Me5Ru), 1.85 (s, 15H, C5Me5Rh), 2.04 (s, 3H, C4Me), 5.92 (dd, JHH = 6.4, 1.2 Hz, 1H, C5H), 6.41 (d, JHH = 6.4 Hz, 1H, C6H), 6.92 ppm (s, 1H, C3H). 1H NMR (400 MHz, THF-d8, rt): δ −19.24 (br s, 2H, Rh−H−Ru), −10.46 (s, 1H, Ru−H−Ru), 1.74 (s, 30H, C5Me5Ru), 1.87 (s, 15H, C5Me5Rh), 2.02 (br s, 3H, C4Me), 5.87 (dd, JHH = 6.4, 2.0 Hz, 1H, C5H), 6.39 (d, JHH = 6.4 Hz, 1H, C6H), 6.96 ppm (s, 1H, C3H). 13C{1H} NMR (100 MHz, THF-d8, −80 °C): δ 11.3 (C5Me5), 11.6 (C5Me5), 12.8 (C5Me5), 20.7 (C4Me), 80.7 (C5Me5Ru), 87.1 (C5Me5Ru), 96.3 (d, JRhC = 4 Hz, C5Me5Rh), 114.3 (Ar), 127.2 (Ar), 140.9 (Ar), 151.3 (Ar), 154.8 ppm (d, JRhC = 28 Hz, C2). Anal. Calcd for C36H54NRhRu2: C, 53.66; H, 6.75; N, 1.74. Found: C, 53.79; H, 6.65; N, 1.52. Reaction of 3 with 4-Picoline; Preparation of (Cp*Ru)2(Cp*Ir)(μ-H)3(μ3-η2(||)-C5H3NMe) (16). A glass tube equipped with a Teflon valve and with a Teflon-coated stir bar was charged with 3 (139.4 mg, 0.173 mmol), 4-picoline (170 μL, 1.75 mmol), and toluene (5 mL). The reaction mixture was stirred at 160 °C for 5 days. The solution turned from brown to dark red. Removal of the solvent under reduced pressure gave a dark red residual solid. The residue was extracted with hexane and purified by column chromatography on alumina with a mixture of hexane and toluene (10:1) as an eluent. Complex 16 was obtained as a dark red solid upon removal of the solvent under reduced pressure (31.7 mg, 35.4 mmol, 20%). A red single crystal suitable for the XRD study was prepared by recrystallization from the mixed solvent of acetone and toluene in a 2:1 volume ratio at −30 °C. 1H NMR (400 MHz, THF-d8, −70 °C): δ −23.98 (s, 1H, Ir−H−Ru), −19.05 (s, Hz, 1H, Ir−H−Ru), −10.52 (s, 1H, Ru−H−Ru), 1.62 (s, 15H, C5Me5Ru), 1.83 (s, 15H, C5Me5Ru), 1.97 (s, 15H, C5Me5Ir), 2.01 (s, 3H, C4Me), 5.89 (d, JHH = 6.4 Hz, 1H, C5H), 6.39 (d, JHH = 6.4 Hz, 1H, C6H), 6.72 ppm (s, 1H, C3H). 1H NMR (400 MHz, THF-d8, rt): δ −21.39 (br s, 2H, Ir−H−Ru), −10.31 (s, 1H, Ru−H−Ru), 1.74 (s, 30H, C5Me5Ru), 1.98 (s, 15H, C5Me5Ir), 2.00 (d, JHH = 1.2 Hz, 3H, C4Me), 5.86 (dd, JHH = 6.4, 2.0 Hz, 1H, C5H), 6.40 (d, JHH = 6.4 Hz, 1H, C6H), 6.75 ppm (s, 1H, C3H). 13C{1H} NMR (100 MHz, THF-d8, −70 °C): δ 10.9 (C5Me5), 11.5 (C5Me5), 12.7 (C5Me5), 20.7 (C4Me), 79.4 (C5Me5), 85.5 (C5Me5), 90.9 (C5Me5), 112.9 (Ar), 126.9 (Ar), 130.7 (Ar), 141.3 (Ar), 152.2 ppm (Ar). Anal. Calcd for C36H54NIrRu2: C, 48.30; H, 6.08; N, 1.56. Found: C, 48.63; H, 6.24; N, 1.51.

53.2 (OMe), 54.5 (3H, OMe), 84.2 (C5Me5Ru), 93.6 (Ar), 95.9 (Ar), 108.1 (Ar), 122.5 (Ar), 129.3 (Ar), 133.5 (Ar), 151.0 (Ar), 152.5 (Ar), 159.3 (Ar), 181.8 ppm (Ar). Elemental analysis did not agree with the calculated value due to the inseparable impurity. Reaction of 1 with 4-Picoline; Formation of Cp*Co(dmbpy) (10) and Cp*2Ru (11). A glass tube equipped with a Teflon valve was charged with 1 (61.6 mg, 91.8 μmol), 4-picoline (20 μL, 230 μmol), and heptane (4 mL). The reaction mixture was heated at 180 °C for 96 h. The solution turned from brown to purple. 4,4′-Dimethoxybiphenyl (45 μmol in 1.5 mL of toluene) was then added to the tube as an internal standard. Formation of 10 (40%) and 11 (25%) was quantitatively analyzed by means of 1H NMR spectroscopy. Data for 10 are as follows. 1 H NMR (400 MHz, benzened6, rt): δ 1.651 (s, 6H, Me), 1.91 (s, 15H, C5Me5Co), 6.64 (d, JHH = 6.8 Hz, 2H, C5H), 7.07 (s, 2H, C3H), 9.97 ppm (d, JHH = 6.8 Hz, 2H, C6H). Data for 11 are as follows. 1 H NMR (400 MHz, benzene-d6, rt): δ 1.649 ppm (s, 30H, C5Me5Ru). Reaction of 1 with 4-Picoline; Preparation of (Cp*Ru)2{(dmbpy)Co}(μ-H)(μ3-H)2 (12). A glass tube equipped with a Teflon valve and with a glass-coated stir bar was charged with 1 (54.1 mg, 80.6 μmol), 4-picoline (155 μL, 1.59 mmol), and toluene (8 mL). The reaction mixture was stirred at 120 °C for 21 h. The solution turned from brown to dark green. Removal of the solvent under reduced pressure gave a dark green residual solid. The residue was washed five times with 4 mL of pentane to remove unreacted 1. The residue was then extracted five times with 3 mL of toluene, and the combined solution was filtered through a glass filter (G4). Removal of the solvent under reduced pressure gave the crude product (29.6 mg, 41.2 μmol, 51%). Analytically pure 12 was obtained by recrystallization from the toluene solution stored at −30 °C (16.5 mg, 23.0 μmol, 27%). A green single crystal suitable for the XRD study was prepared by recrystallization from the m-xylene solution stored at −30 °C. 12: 1H NMR (400 MHz, benzene-d6, rt): δ −129.08 (br s, 6H, C4Me), −27.68 (br s, 30H, C5Me5Ru), 34.81 (br s, 2H, Ar), 81.47 (br s, 2H, Ar), 116.26 ppm (br s, 2H, Ar). μeff (Evans method, 298 K, benzene-d6): 2.5 μB. Elemental analysis did not agree with the calculated value due to high sensitivity toward air and moisture. Dehydrogenative Coupling of 4-Picoline by 12. A glass tube equipped with a Teflon valve was charged with 12 (13.4 mg, 18.6 μmol), 4-picoline (36 μL, 370 μmol), and heptane (3.5 mL). The reaction mixture was heated at 180 °C for 96 h. Biphenyl (123 μmol in 5.0 mL of toluene) was then added to the tube as an internal standard. Formation of 4,4′-dimethyl-2,2′-bipyridine was quantitatively analyzed by GC (79%). Thermolysis of 12 in Pyridine-d5. An NMR tube equipped with a Teflon valve was charged with 12 (2.0 mg, 2.8 μmol) and pyridine-d5 (0.50 mL). Hexamethylbenzene (0.7 mg, 4 μmol) was added as an internal standard. The tube was heated at 120 °C, and the reaction was periodically monitored by 1H NMR spectroscopy. The 1H NMR spectrum recorded after 3 h showed that 0.59 μmol of 12 was converted and 0.54 μmol of 4,4′-dimethyl-2,2′-bipyridine was formed along with the appearance of a broad signal at δ −27.45 ppm. Reaction of 1 with 4-(Trifluoromethyl)pyridine; Preparation of (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-η2(||)-C5H3NCF3) (13). A glass tube equipped with a Teflon valve was charged with 1 (72.75 mg, 0.108 mmol), 4-(trifluoromethyl)pyridine (40 μL, 0.35 mmol), and toluene (4 mL). The reaction mixture was stirred at 100 °C for 63 h. The solution turned from brown to dark green. Removal of the solvent under reduced pressure gave a dark green residual solid. The residue was then extracted five times with 2 mL of hexane, and the combined solution was filtered through a glass filter (G4). Removal of the solvent under reduced pressure followed by washing two times with 1 mL of methanol gave 71.9 mg of 13 as a dark green solid (0.0881 mmol, 82%). A green single crystal suitable for the XRD study was prepared by recrystallization from the heptane solution stored at −30 °C. 1H NMR (400 MHz, toluene-d8, −40 °C): δ −27.72 (d, JHH = 10.0 Hz, 1H, μ-H), −17.76 (s, 1H, μ-H), −11.83 (s, 1H, μ-H), 1.60 (s, 15H, C5Me5Ru), 1.64 (s, 15H, C5Me5Co), 1.83 (s, 15H, C5Me5 Ru), 5.99 (d, JHH = 6.8 Hz, 1H, Ar), 6.26 (d, JHH = 6.8 Hz, 1H, Ar), 7.67 ppm (s, 1H, C3H). 1H NMR (400 MHz, toluene-d8, 80 °C): δ 1.69 (s, 15H, C5Me5Co), 1.76 (s, 30H, C5Me5Ru), 6.09 (d, JHH = 6.0 Hz, 1H, Ar), 6.45 (d, JHH = 6.0 Hz, 1H, Ar), 7.65 ppm (br s, 1H, C3H). The signal for the hydrido ligands could not be observed due to broadening. 13C{1H} NMR K


Article

Organometallics

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(13) (a) Kersten, J. L.; Rheingold, A. L.; Theopold, K. H.; Casey, C. P.; Widenhoefer, R. A.; Hop, C. E. C. A. Angew. Chem., Int. Ed. Engl. 1992, 31, 1341−1343. (b) Casey, C. P.; Hallenbeck, S. L.; Widenhoefer, R. A. J. Am. Chem. Soc. 1995, 117, 4607−4622. (14) Lange’s Handbook of Chemistry, 15th ed.; Dean, J. A., Ed.; McGraw-Hill: New York, 1999. (15) (a) Petretto, G. L.; Rourke, J. P.; Maidich, L.; Stoccoro, S.; Cinellu, M. A.; Minghetti, G.; Clarkson, G. J.; Zucca, A. Organometallics 2012, 31, 2971−2977. (b) Zucca, A.; Petretto, G. L.; Stoccoro, S.; Cinellu, M. A.; Manassero, M.; Manassero, C.; Minghetti, G. Organometallics 2009, 28, 2150−2159. (c) Skapski, A.; Sutcliffe, V. F.; Young, G. B. J. Chem. Soc., Chem. Commun. 1985, 0, 609. (16) Lenges, C. P.; White, P. S.; Marshall, W. J.; Brookhart, M. Organometallics 2000, 19, 1247−1254. (17) Doppiu, A.; Rrivas-Nass, A.; Karch, R.; Winde, R. U.S. Pat. Appl. 20110184203A1, 2011. (18) The dimerization of 4-picoline was carried out in toluene at 180 °C for 48 h in the presence of 1 mol % of 1. Although the yield of 4,4′dimethyl-2,2′-bipyridine exceeded 48% without using a stir bar, the yield decreased to 8% when a stir bar coated with Teflon was used under the same conditions. In addition, we have tried the coupling reaction of 4picoline using 1 in the presence of 1,3-bis(trifluoromethyl)benzene. When a 52 equiv amount of 1,3-bis(trifluoromethyl)benzene was added to 1, the yield of 4,4′-dimethyl-2,2′-bipyridine was significantly decreased to 25% under optimized conditions. This strongly suggested the sensitivity of 12 to the fluorine-containing compounds. (19) Klüfers, P.; Wilhelm, U. J. Organomet. Chem. 1991, 421, 39−54. (20) Pearson, P.; Kepert, C. M.; Deacon, G. B.; Spiccia, L.; Warden, A. C.; Skelton, B. W.; White, A. H. Inorg. Chem. 2004, 43, 683−691. (21) Optimized structure and important geometrical parameters of 12 are given in the Supporting Information. (22) Scarborough, C. C.; Wieghardt, K. Inorg. Chem. 2011, 50, 9773− 9793. (23) Kiam, W.; Reinhardt, R.; Waldhör, E.; Fiedler, J. J. Organomet. Chem. 1996, 524, 195−202. (24) (a) Evans, D. F. J. Chem. Soc. 1959, 2003−2005. (b) Schubert, E. M. J. Chem. Educ. 1992, 69, 62. (c) Grant, D. H. J. Chem. Educ. 1995, 72, 39−40. (25) The yield was corrected from 84% to 79% because of the presence of dmbpy in 12. (26) In the 1H NMR spectrum recorded in pyridine-d5, the signal derived from the Cp* groups, likely derived from (Cp*Ru)2{(bpy-d) Co}(μ-H)(μ3-H)2, was observed at δ −27.45 ppm. (27) (a) Yin, C. C.; Deeming, A. J. J. Chem. Soc., Dalton Trans. 1975, 2091−2096. (b) Deeming, A. J.; Peters, R.; Hursthouse, M. B.; BackerDirks, J. D. J. J. Chem. Soc., Dalton Trans. 1982, 787−791. (c) Burgess, K.; Johnson, B. F. G.; Lewis, J. J. Organomet. Chem. 1982, 233, C55− C58. (d) Eisenstadt, A.; Giandomenico, C. M.; Fredrick, M. F.; Laine, R. M. Organometallics 1985, 4, 2033−2039. (e) Foulds, G. A.; Johnson, B. F. G.; Lewis, J. J. Organomet. Chem. 1985, 294, 123−129. (f) Bruce, M. I.; Humphrey, M. G.; Snow, M. R.; Tiekink, E. R. T.; Wallis, R. C. J. Organomet. Chem. 1986, 314, 311−322. (g) Beringhelli, T.; D’Alfonso, G.; Ciani, G.; Proserpio, D. M.; Sironi, A. Organometallics 1993, 12, 4863−4870. (h) Ellis, D.; Farrugia, L. J. J. Cluster Sci. 1996, 7, 71−83. (i) Azam, K. A.; Das, A. R.; Hursthouse, M. B.; Kabir, S. E.; Malik, K. M. A. J. Chem. Crystallogr. 1998, 28, 283−288. (28) Jones, W. D.; Kuykendall, V. L.; Selmeczy, A. D. Organometallics 1991, 10, 1577−1586. (29) Pitman, C. L.; Finster, N. L.; Miller, J. M. Chem. Commun. ASAP (DOI: 201610.1039/C6CC00575F). (30) Suzuki, H.; Omori, H.; Lee, D. H.; Yoshida, Y.; Moro-oka, Y. Organometallics 1988, 7, 2243−2245. (31) Suzuki, H.; Kakigano, T.; Tada, K.; Igarashi, M.; Matsubara, K.; Inagaki, A.; Oshima, M.; Takao, T. Bull. Chem. Soc. Jpn. 2005, 78, 67−87. (32) (a) Beevor, R. G.; Frith, S. A.; Spencer, J. L. J. Organomet. Chem. 1981, 221, C25−C27. (b) Kölle, U.; Khouzami, F.; Fuss, B. Angew. Chem., Int. Ed. Engl. 1982, 21, 131−132. (33) PROCESS, Automatic Data Acquisition and Processing Package for a Rigaku AFC.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00277. Figures and tables giving results of the dehydrogenative coupling of 4-picoline catalyzed by 1, 4, and 5, NMR spectra of 9−13, 15, and 16, results of the DFT calculations on 12, 13, and 13′, and molecular structure of 10 (PDF) Crystallographic data for 9 (CCDC 1470821), 10 (CCDC 1470822), 12 (CCDC 1470823), 13 (CCDC 1470824), 15 (CCDC 1470825), and 16 (CCDC 1470826) (CIF) Optimized Cartesian coordinates (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail (T. Takao): [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The present research was supported by a Grant-in-Aid for Scientific Research in Innovative Areas “Molecular Activation Directed toward Straightforward Synthesis” from the MEXT of Japan. M.N. also acknowledges a Sasakawa Scientific Research Grant from the Japan Science Society and the JSPS (Grant-in-Aid for JSPS fellows) for support. We are also grateful to Prof. Susumu Kawauchi (Tokyo Institute of Technology) for valuable discussions about the DFT calculations.

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REFERENCES

(1) (a) Hapke, M.; Brandt, L.; Lützen, A. Chem. Soc. Rev. 2008, 37, 2782−2797. (b) Newkome, G. R.; Patri, A. K.; Holder, E.; Schubert, U. S. Eur. J. Org. Chem. 2004, 2004, 235−254. (2) (a) Badger, G. M.; Sasse, W. H. F. J. Chem. Soc. 1956, 616−620. (b) Sasse, W. H. F. J. Chem. Soc. 1959, 0, 3046−3049. (c) Sasse, W. H. F.; Whittle, C. P. J. Chem. Soc. 1961, 0, 1347−1350. (d) Percec, V.; Barboiu, B.; Kim, H.-J. J. Am. Chem. Soc. 1998, 120, 305−316. (e) Anderson, S.; Constable, E. C.; Seddon, K. R.; Turp, J. E.; Baggott, J. E.; Pilling, M. J. J. Chem. Soc., Dalton Trans. 1985, 2247−2262. (f) Polin, J.; Schmohel, E.; Balzani, V. Synthesis 1998, 1998, 321−324. (3) (a) Rosevear, P. E.; Sasse, W. H. F. J. Heterocycl. Chem. 1971, 8, 483−485. (b) Hagelin, H.; Hedman, B.; Orabona, I.; Åkermark, T.; Åkermark, B.; Klug, C. A. J. Mol. Catal. A: Chem. 2000, 164, 137−146. (4) Shibata, Y.; Nagae, H.; Sumiya, S.; Rochat, R.; Tsurugi, H.; Mashima, K. Chem. Sci. 2015, 6, 5394−5399. (5) (a) Viguri, M. E.; Pérez, J.; Riera, L. Chem. - Eur. J. 2014, 20, 5732− 5740. (b) Viguri, M. E.; Huertos, M. A.; Pérez, J.; Riera, L.; Are, I. J. Am. Chem. Soc. 2012, 134, 20326−20329. (6) Carver, C. T.; Diaconescu, P. L. J. Am. Chem. Soc. 2008, 130, 7558− 7559. (7) Soo, H. S.; Diaconescu, P. L.; Cummins, C. C. Organometallics 2004, 23, 498−503. (8) Cockerton, B. R.; Deemig, A. J. J. Organomet. Chem. 1992, 426, C36−C39. (9) (a) Kawashima, T.; Takao, T.; Suzuki, H. J. Am. Chem. Soc. 2007, 129, 11006−11007. (b) Takao, T.; Kawashima, T.; Kanda, H.; Okamura, R.; Suzuki, H. Organometallics 2012, 31, 4817−4831. (10) Nagaoka, M.; Takao, T.; Suzuki, H. Organometallics 2012, 31, 6547−6554. (11) Mingos, D. M. P. Essential Trends in Inorganic Chemistry; Oxford University Press: New York, 1997. (12) Shima, T.; Sugimura, Y.; Suzuki, H. Organometallics 2009, 28, 871−881. L


Article

Organometallics (34) (a) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Detemination; University of Göttingen: Göttingen, Germany, 1997. (b) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, A64, 112−122. (35) (a) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B: Condens. Matter Mater. Phys. 1988, 37, 785−789. (b) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652. (c) Perdew, J. P.; Burke, K.; Wang, Y. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 16533−16539. (36) Andrae, D.; Häußermann, U.; Dolg, M.; Stoll, H.; Preuß, H. Theor. Chim. Acta 1990, 77, 123−144. (37) 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.; 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; Gaussian, Inc.: Wallingford, CT, 2009. (38) Dennington, R.; Keith, T.; Millam, J. GaussView version 5.0; Semichem Inc.: Shawnee Mission, KS, 2009.

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Dehydrogenative Coupling of 4-Substituted Pyridines Catalyzed by a

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