Intramolecular Borylene Transfer Leading to the Formation of a μ3

Communication pubs.acs.org/Organometallics

Intramolecular Borylene Transfer Leading to the Formation of a μ3‑BC2 Ring on a Triruthenium Cluster Takeshi Kaneko,† Hitoshi Suwa,† Toshiro Takao,†,‡ and Hiroharu Suzuki*,† †

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: Photoirradiation of a triruthenium alkyne complex containing a μ3-borylene ligand, {Cp*Ru(μ-H)}3(μ3-η2(∥)-PhCCH)(μ3-BH) (Cp* = η5C5Me5), was examined; whereas a perpendicularly coordinated alkyne complex was obtained by the irradiation with UV light (313 nm), a trimetallic complex containing a three-membered μ3-η3-BC2 ring was preferentially formed as a result of intramolecular borylene transfer by the irradiation with visible light (436 nm).

B

Scheme 1. B−C Bond Formation across the Ru3 Plane

orylene fragments (:BR) are, similar to carbenes, hypovalent and highly reactive species that have attracted considerable attention for several decades.1 Since Braunschweig and co-workers isolated the first terminal borylene complex in 1998,2 many important aspects of the chemistry of metal− boron double bonds have been elucidated.3,4 In particular, terminal borylene complexes of group 6 metals have been demonstrated to be a facile source of borylene under mild conditions; accordingly, a variety of terminal borylene complexes of other transition metals have been obtained via borylene transfer.5 In addition, photochemical transfer of borylene to alkynes is a convenient and reliable method for preparing borirenes, which are the boron analogues of cyclopropenium ions.6 Borylene can also attach to multiple metal centers to form μ-7 and μ3-borylene8 complexes. The reactivity of the μborylene ligand is dominated by the substitution at the boron atom,9 M−B bond cleavage generating a terminal borylene complex,10 and halide abstraction from a μ-BX bond to yield a dimetalloborylene complex.11 However, little is known about its reactivity toward hydrocarbon molecules. Recently, we reported the reaction of triruthenium complex 1, which contains a μ3borylene ligand, with acetylene (Scheme 1).12 Thermolysis of μ3-borylene complex 2 results in the formation of μ3-boraruthenacyclopentenyl complex 3 as a result of B−C bond formation between the μ3-BH group and hydrocarbyl fragments on the opposite face of the Ru3 plane. Herein, we report another type of B−C bond formation across a Ru3 plane that is induced by photoirradiation, which causes intramolecular borylene transfer and leads to the formation of a threemembered μ3-BC2 ring on a triruthenium plane. As mentioned in our previous paper, μ3-η2(∥)-phenylacetylene complex 4 was prepared via the reaction of 1 with phenylacetylene.12 Since 4 shows a broad adsorption in the visible region (∼600 nm; see Figure S1 in the Supporting Information), we further investigated the irradiation of 4. When © 2013 American Chemical Society

4 was irradiated with UV light (λ = 313 nm), a perpendicularly coordinated alkyne complex 5 was formed in a 45% yield along with several unidentified paramagnetic products (eq 1).

Conversion of the coordination mode of a μ3-alkyne ligand from parallel to perpendicular in relation to the electron configuration is often observed in cluster chemistry.13 Complex 5 was fully characterized by means of 1H, 13C, and 11B NMR Received: December 12, 2012 Published: January 23, 2013 737

dx.doi.org/10.1021/om3012079 | Organometallics 2013, 32, 737−740

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spectroscopy as well as elemental analysis.14 The molecular structure of 5 was determined by an X-ray diffraction study, as shown in Figure S4 in the Supporting Information.15 In contrast to UV irradiation, visible light irradiation (λ = 436 nm) of 4 at room temperature suppressed the liberation of dihydrogen. Consequently, triruthenium complex 6, which contains a μ3-η3-BC2 ring, was obtained in a 92% yield (eq 2).16

Complex 5 was also formed in this reaction, but the yield was quite low (8%). Complex 6 was also obtained upon thermolysis of 4 at 80 °C, but 5 was preferentially obtained in this condition (5/6 ratio was 65:35). These facts suggest the salient nature of the visible light irradiation for the selective formation of 6. Complex 6 was formed by way of intramolecular borylene transfer. It is noteworthy that the mode of skeletal rearrangement varied depending on the wavelength of irradiation. Complex 6 is isostructural and isoelectronic to the cationic triruthenium complex containing a μ3-η3-C3 ring, which we reported previously.17 Although the mechanistic details were not fully elucidated, Fehlner and co-workers obtained a similar triiron complex containing a μ3-η3-BC2 ring as a minor product of the reaction of [Fe(C 5 H 5 N) 6 ][Fe 4 (CO) 13 ] with BH2Br·SMe2.18 A single crystal of 6 was obtained from a pentane solution, and the structure was determined by an X-ray diffraction study.15 Although the molecular structure clearly shows a threemembered ring composed of two carbon atoms and one boron atom on the Ru3 plane, the positions of the B(1) and C(2) atoms were not determined owing to the presence of disordered structures (Figure 1). They were solved as disordered atoms with a ratio of 60:40. The μ3-η3-BC2 ring is located nearly parallel to the Ru3 triangle in a staggered form. Although detailed discussion of the structural parameters is hampered by the disordered structure, the longest side of the triangle is 1.643(6) Å, which is considerably greater than the B−C bond length in trialkylborane (1.57 Å). The lengths of the other sides (1.577(6) and 1.587(6) Å) should be average values of the B−C and C−C bond lengths in the BC2 triangle. A density functional theory (DFT) study on 6 was performed in order to verify the structural parameters. The corresponding optimized structure is shown in Figure S6 in the Supporting Information and features long B−C bonds (1.672 and 1.664 Å) and a short C−C bond (1.500 Å). Thus, the length of 1.58 Å that was observed in the diffraction study corresponds to an average value of the B−C and C−C bonds. The B−C(Ph) bond is slightly longer than the B−C(H) bond, which is probably due to increased steric repulsion from the bulkier phenyl group. The C−C bond length of 1.500 Å in the BC2 triangle is slightly shorter than a typical C(sp3)−C(sp3) single bond (1.54 Å), but noticeably longer than the reported values for CC bonds in μ3-alkyne ligands (1.361−1.425 Å).19 The triiron analogue Fe3(CO)9{μ3-η3-C(H)C(Me)BH} reported by Fehlner and co-workers shows similar structural parameters to 6: The B−C and C−C bond lengths were

Figure 1. Molecular structure and labeling scheme of 6 with thermal ellipsoids at the 30% probability level. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) = 2.7931(4), Ru(1)−Ru(3) = 2.8272(4), Ru(2)−Ru(3) = 2.8335(4), Ru(1)−B(1) = 2.126(5), Ru(1)−C(1) = 2.110(4), Ru(2)−C(1) = 2.105(4), Ru(2)−C(2) = 2.116(4), Ru(3)− B(1) = 2.106(4), Ru(3)−C(2) = 2.099(4), B(1)−C(1) = 1.587(6), B(1)−C(2) = 1.643(6), C(1)−C(2) = 1.577(6), Ru(2)−Ru(1)− Ru(3) = 60.547(11), Ru(1)−Ru(2)−Ru(3) = 60.322(11), Ru(1)− Ru(3)−Ru(2) = 59.131(10), C(1)−B(1)−C(2) = 58.4(2), B(1)− C(1)−C(2) = 62.6(3), B(1)−C(2)−C(1) = 59.0(3).

reported to be 1.597 and 1.502 Å, respectively.18 The smaller size of the Fe3 core (Fe−Fe, av 2.58 Å) than that of the Ru3 core (Ru−Ru, av 2.82 Å) is likely responsible for the slight shortening of the B−C bond, while the length of the C−C bond is identical to that in 6. Although a σ interaction between the two p-orbitals of the ring carbon atoms was observed in the low-lying MO (MO#134), this type of interaction was not evident between the boron and carbon atoms. This implies that the B−C bond is mainly composed of a bent interaction of p-orbitals similar to a C−C bond in cyclopropane; this type of bond is known as a banana bond.20 Accordingly, natural bond orbital analysis suggests low bond orders for the B−C bonds (0.5510 and 0.5577) according to the Wiberg bond index, while that for the C−C bond corresponds to a single bond (1.0135). MO#164 shows that the p-orbitals of the ring atoms overlap with the d-orbitals (Figure 2a). In particular, the p-orbital at the boron atom interacts with the d-orbitals at the neighboring ruthenium atoms in an antibonding nature. This resembles the interaction observed in a μ-methylene complex, where an empty p-orbital of a μ-methylene ligand interacts with filled dorbitals.21 On the other hand, MO#153 shows an interaction between the pz-orbital at the boron atom and the d-orbitals of the bonding Ru−Ru bond, which can be viewed as donation from a filled pz-orbital of the boron atom to the metal fragments (Figure 2b). These MOs imply that the ring boron atom still possesses μ-borylene character. In addition, MO#164 shows antibonding character with respect to the BC2 ring, which likely causes elongation of the B−C and C−C bonds relative to those of uncoordinated borirene.6,22 In the 13C NMR spectrum of 6, two sharp signals that derive from the three-membered BC2 ring were observed at δ 142.2 (d) and 155.8 (s) ppm, even though these carbon atoms are connected to quadrupolar 11B nuclei. Braunschweig and coworkers reported that the 13C signals of the BC2 ring in a metalsubstituted borirene were not detected,23 which is attributed to 738

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Scheme 2. Plausible Mechanism for the Formation of 6

Figure 2. (a) Top view of MO#164 of 6 visualized at an electron density isovalue of 0.02 au. (b) Side view of MO#153 of 6 visualized at an electron density isovalue of 0.06 au.

methylidyne−μ3-diruthenaallyl complex that was proposed on the basis of ab initio calculations.26 According to the Wade/Mingos rule, complex 6 adopts a closo form. Similar to other polyhedral closo-borane compounds, complex 6 is robust toward thermolysis: It is stable at 80 °C for several days. However, 6 readily transforms into 5 upon irradiation with UV light (λ = 313 nm) via B−C bond cleavage (eq 3). In this reaction, two of the three hydrido ligands must be removed, probably as dihydrogen.

the severe broadening of the 13C signals due to quadrupolar coupling with 11B nuclei. Therefore, the sharp 13C signals of the BC2 ring in 6 strongly imply that the contribution of s-orbitals to the B−C bond is significantly low; this fact is quite consistent with the results of the DFT calculations, and a small JB−C value for a homoaromatic B2C2 system owing to little sorbital character in the B−C bond has been elucidated by Berndt et al.24 The 11B{1H} NMR spectrum of 6 features a broad signal (δ 59.1) that is shifted considerably upfield from that of the starting μ3-borylene complex 4 (δ 80.9). The chemical shift of 6 is comparable to that of the iron analogue reported by Fehlner and co-workers (δ 58.2).18 Information about the excited state of 4 as well as mechanistic details of the photoinduced skeletal rearrangement has not yet been clarified. However, it is likely that the μ3borylene ligand placed on the opposite face of the Ru3 plane from the alkyne ligand migrates across the Ru3 plane to form the BC2 ring. Judging from the connectivity of the product, there are two possible pathways (Scheme 2). One pathway involves B−C bond formation accompanied by reversible breaking of the Ru−Ru bond followed by further reductive B− C bond formation to form the BC2 triangle (path A). We have shown C−C bond formation accompanied by reversible Ru− Ru bond breakage during the consecutive carbyne migration on a triruthenium cluster.25 The other pathway involves reductive B−H bond formation to yield a transient μ-boryl group (path B). The μ-boryl group flips to the opposite face and forms a BC2 skeleton, which is followed by oxidative addition of a B−H bond to form 6. This corresponds to the reverse process for the skeletal rearrangement of a nido-ruthenacyclopentadiene complex into a μ3-

In conclusion, we demonstrated the photoinduced skeletal rearrangement of μ3-borylene complex 4, containing an alkyne ligand; this rearrangement results in the generation of a μ3-η3BC2 ring on a Ru3 plane. Different skeletal rearrangements leading to the formation of (⊥)-alkyne complex 5 occurred during photolysis with UV light (λ = 313 nm). It is important that the mode of transformation varies according to the wavelength. We are currently studying the detailed mechanisms of these transformations as well as further exploring of the reactivity of the μ3-borylene ligand.

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ASSOCIATED CONTENT

S Supporting Information *

Text, tables, and figures providing synthetic details for compounds 5 and 6, summary of the crystallographic data for 4, 5, and 6, and computational details for the DFT calculation 739

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of 6 as well as X-ray crystallographic files, including CIF files of 4, 5, and 6, are available free of charge via the Internet at http://pubs.acs.org.

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The signal derived from the BH group was not observed. 11B{1H} NMR (128.7 MHz, benzene-d6, 25 °C): δ 138.9 ppm. IR (KBr): ν(BH) = 2424 cm−1. (15) Crystallographic data and results of X-ray diffraction studies for 4, 5, and 6 as well as computational details are given in the Supporting Information. (16) 6: 1H NMR (400 MHz, THF-d8, 25 °C): δ −23.82 (t, JH−H = 4.4 Hz, 1H, Ru-H), −20.15 (t, JH−H = 4.4 Hz, 1H, Ru-H), −19.08 (t, JH−H = 4.4 Hz, 1H, Ru-H), 1.63 (s, 15H, C5Me5), 1.70 (s, 15H, C5Me5), 1.84 (s, 15H, C5Me5), 6.95 (tt, JH−H = 7.2, 1.7 Hz, 1H, p-Ph), 7.02 (dd, JH−H = 7.2, 7.2 Hz, 2H, m-Ph), 7.15 (d, JH−H = 7.2 Hz, 2H, oPh), 7.32 ppm (d, JH−H = 2.0 Hz, 1H, −CCH). The signal derived from the BH group was not observed. 13C{1H} NMR (100 MHz, THF-d8, 25 °C): δ 11.5 (C5Me5), 11.8 (C5Me5), 11.9 (C5Me5), 91.2 (C5Me5), 91.4 (C5Me5), 92.1 (C5Me5), 125.0 (Ph), 126.6 (Ph), 129.5 (Ph), 142.2 (−CH), 154.2 (Ph), 155.8 ppm (−CPh). 11B{1H} NMR (129 MHz, benzene-d6, 25 °C): δ 59.1 ppm. IR (KBr): ν(BH) = 2454 cm−1. (17) Takao, T.; Inagaki, A.; Murotani, E.; Imamura, T.; Suzuki, H. Organometallics 2003, 22, 1361−1363. (18) Meng, X.; Fehlner, T. P.; Rheingold, A. L. Organometallics 1990, 9, 534−536. (19) Structural data for 134 trimetallic complexes having a μ3-η2(||)alkyne ligand were obtained from Cambridge Structural Database System Version 5.33 (November 2011 + 3 updates); Allen, F. H. Acta Crystallogr. 2002, B58, 380−388. (20) Wiberg, K. B. Acc. Chem. Res. 1996, 29, 229−234. (21) Calabro, D. C.; Lichtenberger, D. L.; Herrmann, W. A. J. Am. Chem. Soc. 1981, 103, 6852−6855. (22) Eisch, J. J.; Shafii, B.; Odom, J. D.; Rheingold, A. L. J. Am. Chem. Soc. 1990, 112, 1847−1853. (23) (a) Braunschweig, H.; Fernández, I.; Frenking, G.; Radacki, K.; Seeler, F. Angew. Chem., Int. Ed. 2007, 46, 5215−5218. (b) Braunschweig, H.; Ye, Q.; Radacki, K.; Brenner, P.; Frenking, G.; De, S. Inorg. Chem. 2011, 50, 62−71. (c) Braunschweig, H.; Ye, Q.; Radacki, K.; Kupfer, T. Dalton Trans. 2011, 40, 3666−3670. (d) Braunschweig, H.; Brenner, P.; Dewhurst, R. D.; Krummenacher, I.; Pfaffinger, B.; Vargas, A. Nat. Commun. 2012, 3, 1884/1−1884/6. (24) Wrackmeyer, B.; Berndt, A. Magn. Reson. Chem. 2004, 42, 490− 495. (25) Tahara, A.; Kajigaya, M.; Moriya, M.; Takao, T.; Suzuki, H. Angew. Chem., Int. Ed. 2010, 49, 5898−5901. (26) Inagaki, A.; Musaev, D. G.; Takemori, T.; Suzuki, H.; Morokuma, K. Organometallics 2003, 22, 1718−1727.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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

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REFERENCES

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dx.doi.org/10.1021/om3012079 | Organometallics 2013, 32, 737−740