Synthesis of a Heterometallic Trinuclear Cluster Containing

Sep 5, 2012 - We have already described a dynamic behavior of the perpendicularly coordinated alkyne ligands of the Ru3 analogues called switchback ...
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Synthesis of a Heterometallic Trinuclear Cluster Containing Ruthenium and Cobalt and Its Reactivity with Internal Alkynes Masahiro Nagaoka, Toshiro Takao, 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: A heterometallic tetrahydrido cluster composed by ruthenium and cobalt, (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1) (Cp* = η5-C5Me5), was exclusively synthesized by the reaction of Cp*Ru(μ-H)4RuCp* with (Cp*CoCl)2 in the presence of LiBEt3H. X-ray diffraction and VT-NMR studies showed that 1 has a similar structure to the Ir and Rh analogues in terms of the positions of the hydrido ligands, but the size of the trinuclear site is considerably smaller than that of the iridium analogue because of the smaller covalent radius of cobalt. Like the iridium analogue, complex 1 reacted with internal alkynes to yield an equilibrated mixture of perpendicularly coordinated alkyne complexes. However, the structure of the major isomer was different from that of the iridium analogue: the alkyne ligand on the Ru2Co core was preferentially coordinated to one of the Ru−Co bonds.

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INTRODUCTION

We previously reported the synthesis of the trinuclear heterobimetallic tetrahydrido complex of rhodium and ruthenium 2 by the reaction of a 2:1 mixture of (Cp*RuCl2)28 and (Cp*RhCl2)29 with NaBH4, and Ru2Ir analogue 3 was prepared upon treatment of a diruthenium tetrahydrido complex, Cp*Ru(μ-H)4RuCp* (5),1a with an equimolar amount of Cp*IrH410 (Scheme 1). However, the reaction of a mixture of (Cp*RuCl2)2 and (Cp*CoCl)211 with a hydrido reagent, such as NaBH4 or LiBEt3H, afforded only an intractable mixture of several hydrido clusters including 4 and

We have dealt with the syntheses of polyhydrido clusters of ruthenium including bi-, tri-, tetra-, and pentanuclear clusters and have shown their high reactivities toward bond activation since 1988.1 These studies clearly demonstrated that bond activation was greatly facilitated by the cooperative interaction of the neighboring metal centers with the substrates. Simultaneously, we also developed a rational synthesis of heterometallic polyhydrido clusters composed of [Cp*Ru] (Cp* = η5-C5Me5) units and units of other transition metals containing a Cp* group, such as Mo, W,2 Re,3 Os,4 Rh, and Ir.5 These heterometallic clusters are expected to regioselectively activate the substrates because of the anisotropy of the heterometallic cluster core.6 The atomic radii of the first-row transition metals are considerably smaller than those of the second- and the third-row metals, and the energy levels of the d-orbitals of the first-row transition metals are much lower than those of the second- and third-row metals.7 These distinctive features of the first-row transition metals would give their clusters different reactivity from that of the polyhydrido clusters that we synthesized. In order to extend our knowledge of heterometallic polyhydrido cluster chemistry, we tried to introduce first-row transition metals into the cluster cores. In this article, we describe the synthesis of a trinuclear cluster containing Ru and Co, (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1), and its reactivity with internal alkynes. The differences between 1 and its congener, (Cp*Ru)2(Cp*Ir)(μ-H)3(μ3-H) (3), and the isoelectronic triruthenium pentahydrido complex, {Cp*Ru(μ-H)}3(μ3-H)2 (4), are also discussed. © 2012 American Chemical Society

RESULTS AND DISCUSSION

Scheme 1. Syntheses of Trinuclear Heterobimetallic Clusters of Ru and Rh/Ir5

Received: May 2, 2012 Published: September 5, 2012 6547

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Complex 1′ possesses three kinds of hydrido ligands, as do 2 and 3: μ3-H (H(2)), μ-H on the Ru−Co bonds (H(3) and H(4)), and μ-H on the Ru−Ru bond (H(1)). Their positions were determined during the Fourier synthesis and are shown in Figure 1. However, in the 1H NMR spectrum of 1 recorded at ambient temperature, these hydrido ligands were observed to be equivalent as a sharp singlet appearing at δ −13.48 (Figure 2). This indicates that the hydrido ligands underwent rapid site

5. Complex 1 was successfully obtained by the reaction of a 2:1 mixture of 5 and (Cp*CoCl)2 with a slight excess of LiBEt3H (eq 1).

The formation of a Ru2Co skeleton was unambiguously confirmed by an X-ray diffraction study of (Cp*Ru)2{(η5C5Me4Et)Co}(μ-H)3(μ3-H) (1′), which contains an η5C5Me4Et ligand on the Co atom in order to prevent the formation of a disordered arrangement of the three metal centers. The molecular structure of 1′ is shown in Figure 1 with the relevant bond lengths and angles.

Figure 2. VT-NMR spectra of 1 showing hydrido region (400 MHz, THF-d8/toluene-d8 = 5:1). The asterisked peak was derived from the hydrido signal of the contaminant, Cp*Ru(μ-H)4RuCp* (5).

exchange at ambient temperature. When the sample was cooled to −120 °C, the peak decoalesced into three broad signals appearing at δ −6.12, −10.90, and −30.49 with an intensity ratio of 2:1:1, which is consistent with the crystal structure. While the signal for the bridging hydrido ligands on the Ru−Co bonds was straightforwardly assigned to the largest peak found at δ −6.12, the assignment of the signals for the μ3-H and μ-H on the Ru−Ru bond was carried out by comparing the spectrum with the 1H NMR spectrum of the Ru2Rh analogue 2. The congeners 2 and 3 also showed similar dynamic behavior arising from the motion of the hydrido ligands.5b In particular, two broad signals for the hydrido ligands were observed at δ −6.88 and −14.38 with the ratio of 1:3 in the 1H NMR spectrum of 2 recorded at −90 °C. While the signal appearing at δ −14.38 was flattened at −115 °C, the signal at δ −6.88 appeared as a sharp singlet at this temperature. Because of the lack of spin−spin coupling with the 103Rh nucleus, the signal at δ −6.88 is assignable to the μ-H ligand on the Ru−Ru bond. Therefore, the signal appearing at the lower magnetic field was likely assignable to the μ-H ligand on the Ru−Ru bond in 1. The reaction of 1 with an equimolar amount of 1-phenyl-1propyne exclusively afforded an equilibrated mixture of perpendicularly coordinated alkyne complexes 6a and 7a (eq 2). As for trinuclear complexes containing an alkyne ligand, two coordination modes of the alkyne ligand have been known: parallel (μ3-η2(||)-alkyne) and perpendicular (μ3-η2:η2(⊥)alkyne) modes with respect to one of the M−M bonds.14 While the parallel coordination of an alkyne ligand is common for many carbonyl clusters adopting a coordinatively saturated 48-electron configuration, the perpendicularly coordinated

Figure 1. Molecular structure and labeling scheme of 1′ with thermal ellipsoids at the 30% probability level. Selected bond distances (Å) and angles (deg): Co(1)−Ru(1), 2.5805(8); Co(1)−Ru(2), 2.5818(9); Ru(1)−Ru(2), 2.6714(6); Ru(1)−Co(1)−Ru(2), 62.33(2); Co(1)− Ru(1)−Ru(2), 58.86(2); Co(1)−Ru(2)−Ru(1), 58.81(2).

The molecular structure of 1′ resembles that of the Ru2Ir analogue 3′ in terms of the positions of the hydrido ligands.5b However, because of the smaller covalent radius of cobalt compared to iridium, the size of the trinuclear site forming the isosceles triangle is considerably smaller than that of 3′. The Ru(1)−Co(1) and Ru(2)−Co(1) distances, 2.5805(8) and 2.5818(9) Å, are larger than the sum of the covalent radii (2.41 Å), but lie within the reported range for a Ru−Co bond (2.487−3.098 Å).12 These values are considerably smaller than the Ru−Ir distance in 3′ (av 2.695 Å).5b The Ru(1)−Ru(2) distance of 2.6714(6) Å is also smaller than the Ru−Ru distances in 3′ (2.7378(19) Å)5b and 4 (av 2.750 Å).1d The small size of the trinuclear site is responsible for the reactivity of the cluster as shown below. The structure of 1′ also resembles that of the tricobalt tetrahydrido complex {Cp*Co(μ-H)}3(μ3-H) synthesized by Theopold and Casey.13 The Ru−Co distance is larger than the Co−Co distance of the tricobalt tetrahydrido complex (av 2.476 Å) by 0.1 Å, which corresponds to the difference between the covalent radii of ruthenium (1.25 Å) and cobalt (1.16 Å). 6548

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mode is typical for the coordinatively unsaturated 46-electron complexes. Similar to other 46-electron alkyne complexes,15 complexes 6a and 7a adopt a perpendicular coordination mode. While the alkyne ligand in 6a was perpendicularly coordinated to one of the Ru−Co bonds, that of 7a was perpendicularly coordinated to the Ru−Ru bond. The ratio between the two isomers was 94:6 at −30 °C, but they could not be distinguished from each other above room temperature because of their rapid interconversion. As for trinuclear bimetallic complexes of ruthenium and cobalt containing an alkyne ligand, RuCo2 complexes RuCo2(CO)9(μ3-η2(||)-RCCR′), in which an alkyne ligand is coordinated to the RuCo2 core parallel to one of the Ru−Co bonds, have been previously reported by Braunstein’s16 and Vahrenkamp’s group,17 independently. X-ray diffraction studies clearly established that the alkyne ligand was σ-bonded to the Ru and Co atoms and π-bonded to the Co atom. In contrast to the mixture of 6a and 7a, a positional isomer where the alkyne ligand is parallel to the Co−Co vector has never been observed for RuCo2(CO)9(μ3-η2(||)-RCCR′), but was assumed on the basis of the dynamic behavior arising from the rotation of the alkyne ligand on the RuCo2 site.17b Although the reaction of the iridium analogue 3 with 1phenyl-1-propyne proceeded at ambient temperature to form a perpendicularly coordinated alkyne complex, (Cp*Ru)2(Cp*Ir)(μ3-η2:η2(⊥)-PhCCMe)(μ-H)2,18 the reaction of 1 with internal alkynes required heating above 80 °C. This was probably due to the small size of the trinuclear site of 1. The reaction of 1 with diphenylacetylene yielding 6b required an even higher temperature (120 °C) because of the bulkiness of the substrate. As in the reaction of the iridium analogue 3, two hydrido ligands of 1 were removed from the trinuclear site as dihydrogen upon the reaction with internal alkynes. This is quite different from the reaction of the isoelectronic 4 with internal alkynes, which required two molar amounts of alkyne for completion.19 The most striking feature of the reaction is that the unsymmetrical isomer 6 was formed as a major product. This is quite different from the reaction of 3 with internal alkynes, where only the symmetric isomer, in which an alkyne ligand was coordinated to the Ru−Ru bond, was obtained.18 The presence of the unsymmetrical isomer was suggested only by the VT-NMR studies for (Cp*Ru)2(Cp*Ir)(μ3-η2:η2(⊥)PhCCPh)(μ-H)2. The formation of 6a should be reasonable evidence for the presence of the unsymmetrical isomer with the Ru2Ir core. The molecular structure of 6a was confirmed by an X-ray diffraction study, which clearly showed that the alkyne moiety is perpendicularly coordinated to one of the Ru−Co bonds (Figure 3). Since there was a crystallographic mirror plane bisecting the trinuclear site, the positions of the Ru(2) and Co(1) atoms were not determined. Their locations were disordered with a ratio of 50:50. The relevant bond lengths and

Figure 3. Molecular structure and labeling scheme of 6a with thermal ellipsoids at the 30% probability level.

angles are listed in Table 1 together with those of the Ru2Ir and Ru3 analogues. The alkyne complex 6a had a similar structure to the isoelectronic triruthenium complex possessing a perpendicularly coordinated alkyne ligand.19 In this structure, the alkyne ligand is coordinated to one of the metal−metal bonds with the larger substituent located inside the trinuclear site, which minimizes the steric repulsion with the surrounding Cp* groups.20 The C(1)−C(2) distance, 1.404(10) Å, lies in the middle of the reported range for the C−C bond lengths of trinuclear (⊥)-alkyne complexes (1.300−1.465 Å)15 and is comparable to those of the Ru2Ir (1.432(8) Å)18 and the Ru3 analogues (1.410(6) Å)19 containing a 1-phenyl-1-propyne ligand. Because of the disordered structures, the precise bond lengths in the trinuclear site cannot be discussed. However, the 2.7558(6) Å of the Ru(2)−Co(1) bond, to which the alkyne ligand is perpendicularly coordinated, is significantly shorter than those of the Ru2Ir (2.9687 (8) Å)18 and Ru3 analogues (2.8502(9) Å).19 In the 1H NMR spectrum recorded at −30 °C, three signals for the Cp* groups of 6a were observed at δ 1.77, 1.78, and 1.80 (Figure 4c). Two signals for the hydrido ligands were observed at δ −20.63 and −5.98 with the same intensity (Figure 4d). These facts strongly indicate that the alkyne ligand in 6a was coordinated to one of the Ru−Co bonds in solution as well. The ortho protons of the phenyl group were observed to be inequivalent. They appeared at δ 4.99 (d, JHH = 8.0 Hz) and 5.79 (d, JHH = 7.6 Hz). This means that the rotation of the phenyl group around the C−C bond located inside the trinuclear site was seriously hindered. This is likely due to the steric repulsion arising from the Cp* groups. In addition, the inequivalence of the ortho protons also strongly suggests an unsymmetrical structure of 6a. While one of the meta protons was obscured by the signal of the residual protons of the solvent (toluene-d 8 ), they were also observed to be inequivalent: the remaining signal was observed at δ 6.88 (t, JHH = 7.6 Hz) at −30 °C. These signals coalesced into one signal with rising temperature, and the coalescence temperatures of the ortho and meta protons were estimated to be 40 and 20 °C, respectively.21 6549

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Table 1. Selected Bond Distances (Å) and angles (deg) for the (⊥)-Alkyne Complexes 6a with the Structural Data of Relevant Ru2Ir18 and Ru319 Analogues

Figure 4. VT-NMR spectra of the equilibrated mixture of 6a and 7a showing (a) phenyl, (b) PhCCMe, (c) Cp*, and (d) hydrido regions (400 MHz, toluene-d8).

The concentration of 7a was not sufficient for full characterization, and some signals derived from 7a were obscured by the signals derived from 6a. Nevertheless, the

isolated signals for the hydrido ligand, the methyl group on the alkyne ligand, and the ortho protons on the phenyl group derived from 7a were observed at δ −12.01 (s), 2.51 (s), and 6550

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Scheme 2. Switchback Motion of the Alkyne Ligand on a RuCo2 Corea

a

Hydrido ligands were omitted for simplification.

5.66 (br), respectively, with an intensity ratio of 2:3:2 at −30 °C. In particular, the hydrido ligands and ortho protons in 7a are observed to be equivalent, unlike those in 6a. These facts strongly imply that the alkyne ligand in 7a was coordinated to the Ru−Ru bond to form a symmetric structure. Instead of the symmetric structure of 7a, the hydrido signal broadened and flattened with rising temperature. This was due to the interconversion between 6a and 7a. This isomerization was confirmed by a spin-saturation transfer experiment performed at −20 °C: the intensity of the hydrido signal at δ −12.01 decreased by 70% upon irradiation at the hydrido resonance of 6a. Although the methyl group on the alkyne ligand in 6a resonated as a sharp singlet at both −30 and 80 °C, the signal became broad at 25 °C. This was also due to the equilibration between 6a and 7a within the NMR time scale. Vahrenkamp and co-workers established the fluxional behavior of the alkyne ligand in the related heterometallic alkyne complex, RuCo2(CO)9(μ3-η2(||)-HCCH), where the ethyne ligand is parallel to the Ru−Co vector; while the methine signals were observed to be inequivalent at −10 °C, they coalesced into one signal at 62 °C, at which the free energy of activation was estimated at 66 ± 2 kJ mol−1.17b This spectral change is rationalized by the successive pivot motion of the alkyne ligand over the RuCo2 core. We have already described a dynamic behavior of the perpendicularly coordinated alkyne ligands of the Ru 3 analogues called switchback motion,22 in which the alkyne ligand moves around the trinuclear site with alternating the positions of the inner and outer carbon atoms through a parallel alkyne intermediate. In the previous paper, we demonstrated the switchback motion of the diphenylacetylene ligand around a Ru2Ir core.18 When the ligand is 1-phenyl-1propyne, there are four isomers, 6a, 6a′, 7a, and 7a′, involved in the site-exchange process (Scheme 2), but the concentrations of 6a′ and 7a′, in which the phenyl group is located outside the trinuclear site, were assumed to be extremely low as a result of the instability of these isomers. It has been shown that the bulkier substituent should be located inside the trinuclear site in order to minimize the steric repulsion with the surrounding Cp* groups.20

The energy difference between 6a and 7a was estimated at ca. 1.0 kcal mol−1 at −30 °C based on the ratio of their concentrations. Although the free energy of activation for the switchback motion was obtained for the Ru3 analogue (ΔG‡298K = 17.0 ± 0.4 kcal mol−1) by the use of simulation,22 this method could not be applied to 6a because of the equilibrium with 7a. While this motion resulted in a time-averaged C3 structure for the Ru3 analogues, it resulted in a time-averaged Cs structure for the equilibrated mixture of 6a and 7a. At 80 °C, two signals assignable to the Cp* groups were observed at δ 1.76 and 1.73 with an intensity ratio of 2:1, while the signals for the hydrido ligands got buried in the baseline. In the 13C NMR spectrum of 6a, the signal for the inner acetylenic carbons connected to the phenyl group was observed at δ 75.5, while the signal for the outer carbon appeared in the lower magnetic field region characteristic of a bridging alkylidene ligand (δ 181.1). These chemical shifts were comparable to those of the Ru2Ir (Cin, δ 102.4; Cout, δ 211.4)18 and Ru3 analogues (Cin, δ 73.7; Cout, δ 181.1).19 Although an oscillation of the alkyne ligand between the two Ru−Co bonds in 6a that does not change the positions of the inner and outer carbon atoms would also produce the same spectral changes, this mechanism has been revealed to be energetically unfavorable by ab initio studies of the Ru3 analogue.20 In order to verify the motion of the alkyne ligand, VT-NMR studies were performed on a mixture of 6b and 7b, which contain a diphenylacetylene ligand.23 The 1H NMR investigation of the mixture of 6b and 7b at 70 °C produced a time-averaged spectrum, in which two Cp* signals were observed at δ 1.67 and 1.59 with an intensity ratio of 2:1. Although there should be two kinds of phenyl groups, namely, the inner and outer phenyl groups, only one set of phenyl signals was observed, at δ 6.48 (o-Ph), 6.78 (p-Ph), and 6.92 (m-Ph). This strongly indicates that the alkyne ligand underwent switchback motion around the trinuclear site. The motion of a diphenylacetylene ligand in the iridium analogue (Cp*Ru)2(Cp*Ir)(μ-η2:η2(⊥)-PhCCPh)(μ-H)2 was frozen at −30 °C, and the free energy of activation for the switchback motion was estimated to be ca. 15 kcal mol−1 at 60 6551

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°C.18 In contrast, the switchback motion of the diphenylacetylene ligand around the Ru2Co remained unfrozen even at −100 °C. The 1H NMR spectrum recorded at −100 °C had only broad signals in the phenyl group region. Thus, the free energy of activation was not estimated for the Ru2Co complex, but it is assumed that the alkyne ligand on the Ru2Co core moves considerably faster than that on the Ru2Ir core. This is likely due to the weaker M−C bond energy of the first-row metals than those of the second- and the third-row metals. While the motion of the alkyne ligand was not frozen, the signals for the hydrido ligands were clearly resolved at −30 °C: the signals derived from 6b were observed at δ −22.61 and −5.21 as broad peaks, while that of 7b was observed at δ −12.45. At this temperature, the ratio of 6b to 7b was estimated to be 78:22. This value was comparable to that of 6a/7a (94:6) observed at −30 °C. While an unsymmetrical isomer was not directly observed for the (⊥)-alkyne complexes of the Ru2Ir analogue, complex 6 was preferred rather than the symmetric isomer 7 for the Ru2Co core.

employed. Elemental analysis was performed on a Perkin-Elmer 2400II series CHN analyzer. X-ray Diffraction Studies. Single crystals of 1′ and 6a for the Xray analyses were obtained directly from the preparations described below and mounted on nylon Cryoloops with Paratone-N (Hampton Research Corp.). Diffraction experiments were performed on a Rigaku R-AXIS RAPID imaging plate diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71069 Å). Cell refinement and data reduction were performed using the PROCESS-AUTO program.24 Intensity data were corrected for Lorentz−polarization effects and empirical absorption. The structures were solved by the direct method using the SHELXS-97 program and refined by the SHELXL-97 program.25 All non-hydrogen atoms were found by the difference Fourier synthesis and were refined anisotropically except for the disordered atoms in the Cp* group bonded to Ru(1) in 6a, which were located in the ratio 50:50 and refined isotropically. For 6a, the Co(1) and Ru(2) atoms were located in the ratio 50:50 due to the presence of the mirror plane and refined anisotropically. The refinement was carried out by least-squares methods based on F2 with all measured reflections. The metal-bound hydrogen atoms in 1′ were located in the difference Fourier map and refined isotropically. Crystal data and results of the analyses are listed in Table 2. (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1). A 50 mL Schlenk tube was charged with Cp*Ru(μ-H)4RuCp* (5) (113 mg, 0.238 mmol), (Cp*CoCl)2 (82.2 mg, 0.179 mmol), and THF (8 mL). LiBEt3H (0.430 mmol in 1.5 mL of THF) was then added dropwise to the solution for 1 h, and the reaction mixture was stirred at room temperature for 24 h. After the solvent was removed under reduced pressure, the black residual solid was dissolved in 3 mL of toluene and purified by column chromatography on alumina (Merck, Art. No. 1097) at −30 °C. After the first red band including 5 was removed with toluene as an eluent, the second brown band was collected with THF as an eluent. Complex 1 was obtained as a black solid upon removal of the solvent under reduced pressure (83.3 mg, 0.124 mmol, 52%). The ethyl-tetramethyl cyclopentadiene analogue, (Cp*Ru)2(Cp**Co)(μ-H)3(μ3-H) (1′) (Cp** = η5-C5Me4Et), was prepared similarly in a 30% yield using (Cp**CoCl)2 instead of (Cp*CoCl)2. A brown single crystal used for the diffraction study was prepared by the slow evaporation of the pentane solution of 1′ stored at 25 °C. 1: 1H NMR (400 MHz, benzene-d6, 25 °C): δ −13.15 (s, 4H, μ-H), 1.75 (s, 15H, C5Me5Co), 2.02 ppm (s, 30H, C5Me5Ru); (400 MHz, THF-d8/toluene-d8 = 5:1, −120 °C): δ −30.49 (brs, 1H, μ3-H), −10.90 (brs, 1H, RuHRu), −6.12 (brs, 2H, RuHCo), 1.76 (s, 15H, C5Me5Co), 1.96 ppm (s, 30H, C5Me5Ru). 13C NMR (100 MHz, benzene-d6, 25 °C): δ 11.8 (q, JCH = 126 Hz, C5Me5Co), 13.5 (q, JCH = 126 Hz, C5Me5Ru), 84.9 (s, C5Me5Ru), 90.1 ppm (s, C5Me5Co). Anal. Calcd for C30H49Ru2Co: C, 53.72; H, 7.36. Found: C, 53.37; H, 7.70. 1′: 1H NMR (400 MHz, benzene-d6, 25 °C): δ −13.09 (s, 4H, μ-H), 0.96 (t, JHH = 7.6 Hz, 3H, CH3CH2−), 1.73 (s, 6H, C5Me4EtCo), 1.85 (s, 6H, C5Me4EtCo), 2.02 (s, 30H, C5Me5Ru), 2.43 ppm (q, JHH = 7.6 Hz, 2H, CH3CH2−). 13C NMR (100 MHz, benzene-d6, 25 °C): δ 11.7 (q, JCH = 127 Hz, C5Me4EtCo), 11.8 (q, JCH = 127 Hz, C5Me4EtCo), 13.5 (q, JCH = 126 Hz, C5Me5Ru), 14.4 (q, JCH = 126 Hz, CH3CH2−), 20.1 ppm (t, JCH = 123 Hz, CH3CH2−), 84.9 (s, C5Me5Ru), 89.9 (s, C5Me4EtCo), 90.5 (s, C5Me4EtCo), 95.3 (s, C5Me4EtCo). (Cp*Ru)2(Cp*Co)(μ-H)2(μ3-η2:η2(⊥)-PhCCMe) (6a and 7a). A glass tube equipped with a Teflon valve was charged with 1 (36.6 mg, 54.6 μmol) and toluene (4 mL). 1-Phenyl-1-propyne (17 μL, 0.136 mmol) was then added to the solution at room temperature. The reaction mixture was stirred at 80 °C for 19 h. The color of the solution turned from brown to purple. Removal of the solvent under reduced pressure gave a crude product including 6a and 7a. Complex 6a was recrystallized from the cold pentane solution stored at −30 °C as a purple crystal (26.6 mg, 33.1 μmol, 61%). The ratio between 6a and 7a was measured by the 1H NMR spectrum recorded at −30 °C as 94:6. A black single crystal of 6a used for the diffraction study was prepared by the slow evaporation of the THF solution stored at 25 °C. 1 H NMR (400 MHz, toluene-d8, 80 °C): δ 1.73 (s, 15H, C5Me5Co), 1.76 (s, 30H, C5Me5Ru), 3.08 (s, 3H, PhCCMe), 5.36 (br, 2H, o-Ph), 6.61 (t, JHH= 7.2 Hz, 1H, p-Ph), 6.88 ppm (dd, JHH = 7.6, 7.2 Hz, 2H,

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CONCLUSION A heterometallic tetrahydrido cluster of ruthenium and cobalt, (Cp*Ru)2(Cp*Co)(μ-H)3(μ3-H) (1), was synthesized by the reaction of a mixture of diruthenium tetrahydrido complex 5 and (Cp*CoCl)2 with LiBEt3H. Complex 1 has a structure quite similar to that of the Ru2Ir congener 3, but the size of the trinuclear site of 1 is considerably smaller than that of 3. Complex 1 reacted with an internal alkyne to afford a (⊥)-alkyne complex, as did the iridium analogue. These reactions proceeded in similar manners, but complex 1 required slightly more severe conditions owing to the small size of the Ru2Co core. All of the obtained (⊥)-alkyne complexes exhibited the dynamic behavior called switchback motion, which was already established for the isoelectronic triruthenium complex containing a (⊥)-alkyne ligand.22 In heterometallic clusters, the switchback motion of the alkyne ligand induces not only site exchange but also isomerization, since they are isomers with respect to the position of the alkyne ligand. Thus, they have complicated VT-NMR spectra, as shown in Figure 1 In the Ru2Co complex, the major isomer was that in which the alkyne ligand was coordinated to one of the Ru−Co bonds, while such an isomer was not directly observed for the Ru2Ir analogue. Although the factors determining the structure of a (⊥)-alkyne complex are unclear at present, it is interesting that the stability of the (⊥)-alkyne complex was affected by the composition of the trinuclear site. This relationship seems to be due to the energy level of the d-orbitals and the π*-orbital of the alkyne ligand.

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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, THF, acetone, and pentane used in this study were purchased from Kanto Chemicals 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. Other materials used in this research were used as purchased. Cp*Ru(μ-H)4RuCp* (5) and (Cp*CoCl)2 were prepared according to the previously published method in refs 1a and 11, respectively. 1H 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 natural-abundance carbon signal of the solvent 6552

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Organometallics

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H)2(μ3-η2:η2(⊥)-PhCCMe) (6a and 7a) was formed in a 50% yield (5.4 μmol) and 5.8 μmol of 1-phenyl-1-propyne was consumed. All of 1 was converted into the mixture of 6a and 7a in 4 h. (Cp*Ru)2(Cp*Co)(μ-H)2(μ3-η2:η2(⊥)-PhCCPh) (6b and 7b). A glass tube equipped with a Teflon valve was charged with 1 (117.1 mg, 0.175 mmol) and toluene (5 mL). Diphenylacetylene (96.8 mg, 0.543 mmol) was then added to the solution at room temperature. The reaction mixture was stirred at 120 °C for 40 h. The color of the solution turned from brown to dark brown. Removal of the solvent under reduced pressure gave a crude product including 6b and 7b. The residue was then extracted with 3 mL of pentane three times, and the combined solution was filtered. After the filtrate was concentrated to about 3 mL, 4 mL of acetone was added to the solution. Complex 6b was recrystallized from the cold solution stored at −30 °C as a black crystal (95.5 mg, 0.113 mmol, 64%). The ratio between 6b and 7b was measured by the 1H NMR spectrum recorded at −30 °C as 78:22. 1H NMR (400 MHz, THF-d8, 70 °C): δ 1.59 (s, 15H, C5Me5Co), 1.67 (s, 30H, C5Me5Ru), 6.48 (d, JHH = 7.6 Hz, 4H, o-Ph), 6.78 (t, JHH = 7.4 Hz, 2H, p-Ph), 6.92 ppm (dd, JHH= 7.6, 7.4 Hz, 4H, m-Ph). 13C{1H} NMR (100 MHz, THF-d8, 65 °C): 11.7 (s, C5Me5Co), 12.2 (s, C5Me5Ru), 89.9 (s, C5Me5Ru), 90.0 (s, C5Me5Co), 123.1 (s, Ph), 126.9 (s, Ph), 128.7 (s, Ph), 149.5 ppm (s, ipso-Ph). Since the 1H signals derived from the Cp* and the phenyl groups were averaged and appeared as broad peaks at −30 °C, only hydrido signals were assigned as follows. 6b: 1H NMR (400 MHz, THF-d8, −30 °C): δ −22.61 (br, 1H, μ-H), −5.21 ppm (br, 1H, μ-H). 7b: 1H NMR (400 MHz, THFd8, −30 °C): δ −12.43 ppm (br, 2H, μ-H). Averaged signals for the Cp* signals were observed at δ 1.59 (br) and 1.66 (s), while those for the phenyl groups were observed as humps of broad signals between δ 5.5 and7.5.

Table 2. Crystallographic Data for 1′ and 6a (a) Crystal Data empirical formula fw cryst description cryst color cryst size (mm) crystallizing solution cryst syst space group lattice params

V (Å3) Z value Dcalc (g/cm3) measurement temp (°C) μ(Mo Kα) (mm−1) (b) Intensity Measurements diffractometer radiation monochromator 2θmax reflns collected independ reflns reflns obsd (>2σ) abs correction type abs transmn (c) Refinement (Shelxl-97-2) R1 (I > 2σ) wR2 (I > 2σ) R1 (all data) wR2 (all data) data/restraints/params GOF largest diff peak and hole (e Å−3)

1′

6a

C31H51Ru2Co 684.79 block brown 0.07 × 0.06 × 0.04 pentane (rt) triclinic P1̅ (#2) a = 10.8082(8) Å b = 11.1196(10) Å c = 15.3516(10) Å α = 68.590(3)° β = 73.761(2)° γ = 61.346(2)° 1494.9(2) 2 1.521 −150 1.562

C39H55Ru2Co 784.90 block black 0.19 × 0.13 × 0.06 THF (rt) monoclinic C2/m (#12) a = 17.3373(6) Å b = 17.9205(6) Å c = 11.4374(5) Å

3463.9(2) 4 1.505 −150 1.359

RAXIS-RAPID Mo Kα graphite 55° 14 307 6677 (Rint = 0.0645) 4381 empirical 0.6366 (min.) 1.0000 (max.)

RAXIS-RAPID Mo Kα graphite 55° 17 359 4100 (Rint = 0.0259) 3705 empirical 0.6791 (min.) 1.0000 (max.)

0.0470 0.0865 0.0916 0.1146 6653/0/385 1.089 1.144 and −1.275

0.0346 0.0942 0.0393 0.0984 4088/0/259 0.911 0.876 and −0.989

β = 102.8920(13)°

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ASSOCIATED CONTENT

S Supporting Information *

1

H and 13C NMR spectra of 1′, 1H and 13C NMR spectra of the mixture of 6b and 7b, the preliminary results of the X-ray diffraction study for 6b, and CIF files of 1′ and 6a are available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

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

m-Ph). 6a: 1H NMR (400 MHz, toluene-d8, −30 °C): δ −20.63 (s, 1H, μ-H), −5.98 (s, 1H, μ-H), 1.77 (s, 15H, C5Me5), 1.78 (s, 15H, C5Me5), 1.80 (s, 15H, C5Me5), 3.18 (s, 3H, PhCCMe), 4.99 (d, JHH = 8.0 Hz, 1H, o-Ph), 5.79 (d, JHH = 7.6 Hz, 1H, o-Ph), 6.72 (t, JHH = 7.4 Hz, 1H, p-Ph), 6.88 (dd, JHH = 7.6, 7.6 Hz, 1H, m-Ph), 7.05 ppm (1H, m-Ph, partly obscured by the residual proton signal of the solvent). 13 C{1H} NMR (100 MHz, THF-d8, −50 °C): δ 12.1 (s, C5Me5), 12.4 (s, C5Me5), 12.7 (s, C5Me5), 25.6 (s, PhCCMe), 75.5 (s, PhCCMe), 86.2 (s, C5Me5), 89.1 (s, C5Me5), 89.2 (s, C5Me5), 121.3 (s, Ph), 123.4 (s, Ph), 124.9 (s, Ph), 126.8 (s, Ph), 126.9 (s, Ph), 148.5 (s, ipso-Ph), 181.1 ppm (s, PhCCMe). 7a: 1H NMR (400 MHz, toluene-d8, −30 °C): δ −12.01 (s, 2H, μ-H), 2.51 (s, 3H, PhCCMe), 5.66 ppm (br, 2H, o-Ph). Other signals derived from 7a were obscured by the signals derived from 6a. Anal. Calcd for C39H55Ru2Co: C, 59.68; H, 7.06. Found: C, 59.92; H, 7.45. Reaction of 1 with 1-Phenyl-1-propyne in an NMR Tube. An NMR tube equipped with a Teflon valve was charged with 1 (7.3 mg, 10.9 μmol), 1-phenyl-1-propyne (11 μL, 87.5 μmol), and benzene-d6 (0.4 mL) with 2,2,4,4-tetramethylpentane (0.2 μL) as an internal standard. The reaction tube was heated at 80 °C, and the reaction was periodically monitored by 1H NMR spectroscopy. The 1H NMR spectrum recorded after 30 min showed that (Cp*Ru)2(Cp*Co)(μ-

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(23) The preliminary result of an X-ray diffraction study of (Cp*Ru)2(Cp*Co)(μ-H)2(μ3-η2:η2(⊥)-PhCCPh) (6b and 7b) clearly showed the perpendicular coordination of a diphenylacetylene ligand on one of the metal−metal bonds. However, the positions of the individual metal centers could not be determined due to the disordered arrangement. The crystal used for the diffraction study seemed to involve both 6b and 7b, but the ratio between 6b and 7b was not determined. The preliminary result is shown in Figure S-2 in the Supporting Information. (24) PROCESS, Automatic Data Acquisition and Processing Package for a Rigaku AFC Diffractometer; Rigaku Corporation: Tokyo, Japan, 1989. (25) (a) Sheldrick, G. M. SHELX-97, Program for Crystal Structure Determination; University of Göttingen: Göttingen, Germany, 1997. (b) Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112−122.

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