Unusual Coupling Reaction of C60 and Benzonitrile with Triosmium

Article pubs.acs.org/Organometallics

Unusual Coupling Reaction of C60 and Benzonitrile with Triosmium Carbonyls To Generate Fullerodiketimide Cluster Complexes Yi-Sin Lin† and Wen-Yann Yeh*,†,‡ †

Department of Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan Faculty of Medicinal and Applied Chemistry, Kaohsiung Medical University, Kaohsiung 807, Taiwan

‡

S Supporting Information *

ABSTRACT: Reaction of Os3(CO)12 (or Os3(CO)10(NCPh)2) with C60 in the benzonitrile/o-dichlorobenzene solution at reflux produces Os3(CO)5(μ-NCHPh)(μ3,η5-NCPhC60C(C6H4)N)(μ,η4-C60) (2) and Os3(CO)4(NCPh)(μ-NCHPh)(μ3,η5-NCPhC60C(C6H4)N)(μ,η4-C60) (3), bearing an unprecedented fullerodiketimide ligand from 1,4-addition of two benzonitrile molecules to the C60 cage.

Shapley and Park,10 have the metal triangle face-capped by a hexagon of the C60 core; these are structurally analogous to the face-capping benzene complexes Ru3(CO)9(μ3,η6-C6H6) and Os3(CO)9(μ3,η6-C6H6).11 In contrast, compounds 2 and 3 bear two fullerene ligands in a dumbbell fashion. It is important to note that two benzonitriles have been activated and added to C60 to generate an unprecedented fullerodiketimide species, while the second C60 is bound to the Os3 cluster in a rare μ,η2:η2 feature.12 Experiments by varying the solvents, temperatures, and reactants were carried out. Typically, treating Os3(CO)12 and C60 in refluxing benzonitrile/o-dichlorobenzene gave 1 (5%), 2 (13%), and 3 (9%), while only compound 1 was obtained (20− 26%) in refluxing chlorobenzene (133 °C) or o-dichlorobenzene (180 °C). Replacing Os3(CO)12 by the labile cluster Os3(CO)10(NCPh)2, the reaction in refluxing benzonitrile/odichlorobenzene for 45 min afforded 1 (5%), 2 (39%), and 3 (6%), whereas mainly 1 (38%) and a small amount of 2 (4%) were obtained in refluxing o-dichlorobenzene. In contrast, reactions of Os3(CO)12 (or Os3(CO)10(NCPh)2) and C60 in benzonitrile/chlorobenzene (1:20) at reflux did not produce any fullerene complexes, suggesting the benzonitrile is competing with C60 to ligate the Os3 cluster at 133 °C. The findings indicate that high temperature (180 °C) is required for C60−PhCN coupling, and the yields of 2 are greatly enhanced by using Os3(CO)10(NCPh)2 as the reactant. Heating 1 and C60 in benzonitrile/o-dichlorobenzene did not produce 2 or 3, while thermolysis of 2 in benzonitrile gave 3 via CO/PhCN substitution. Plausible reaction pathways are

One of the most fascinating aspects pertaining to organometallic chemistry concerns the activation of organic substrates at metal centers.1 Transition-metal clusters have attracted increasing attention because of their activity in reactions involving substrates which are not activated by monometallic species.2 Recently, the availability of gram quantities of the fullerene C60 has facilitated the study of the reactivity of this intriguing molecule.3,4 Subsequent work has been extensive, and many attempts have been made to coordinate fullerenes to metals5 and to incorporate metal-binding groups into their structures.6 The syntheses of such compounds offer the capability to exploit the chemical reactivity, photochemical behavior, redox properties, and novel structural features that a fullerene group provides.7 Investigation of the reactivity of fullerene-bound organometallic compounds has also become an attractive research topic.8 As part of our continuing interest in metal−fullerene chemistry,9 herein we report a novel reaction involving concomitant activation of C60 and benzonitrile molecules mediated by triosmium clusters.

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RESULTS AND DISCUSSION Os3(CO)12 (or Os3(CO)10(NCPh)2) and 2 equiv of C60 in benzonitrile/o-dichlorobenzene solution were heated to reflux for 3 h under a dinitrogen atmosphere. After purification of the products by TLC (silica gel), three cluster complexes were isolated, characterized as Os 3 (CO) 9 (μ 3 ,η 6 -C 60 ) (1), Os3(CO)5(μ-NCHPh)(μ3,η5-NCPhC60C(C6H4)N)(μ,η4-C60) (2), and Os3(CO)4(NCPh)(μ-NCHPh)(μ3,η5-NCPhC60C(C6H4)N)(μ,η4-C60) (3) (Scheme 1). The ratio of benzonitrile to o-dichlorobenzene has been varied from 1:10 to 1:30 (v/v), and 1:20 was found to give higher yields for 2 and 3. Ru3(CO)9(μ3,η6-C60) and compound 1, previously reported by © 2014 American Chemical Society

Received: November 18, 2013 Published: January 17, 2014 731

dx.doi.org/10.1021/om4011164 | Organometallics 2014, 33, 731−735

Organometallics

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Scheme 1

Scheme 2

followed by CO/PhCN exchange to produce compound 3. However, addition of C60 to B followed by PhCN activation cannot be excluded. Compound 2 forms an air-stable, greenish brown crystalline solid. The IR spectrum presents five absorption peaks from 2075 to 1946 cm−1 for terminal CO stretches. The MALDI mass spectrum displays a molecular ion peaks around m/z 2465 and fragments from the loss of CO ligands. The 1H NMR spectrum of 2 at 0 °C displays a 1H singlet at δ 9.04 (for N CH) and several 14H multiplets in the range δ 8.39−6.79 (for

portrayed in Scheme 2. Presumably, successive ligand replacements will generate Os3(CO)10(C60)(NCPh), which can lead to compound 1 or undergo C60−PhCN coupling to give the fulleroketimide complex A. Subsequent coupling of A and PhCN would produce the fullerodiketimide complex B. Alternatively, B may be obtained directly from the reaction of Os3(CO)10(NCPh)2 and C60. Next, ortho metalation of one phenyl ring of B, accompanied by activation of the third PhCN molecule, will result in the trisketimide complex C. Finally, the second C60 molecule is added to C to produce compound 2, 732

dx.doi.org/10.1021/om4011164 | Organometallics 2014, 33, 731−735

Organometallics

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Figure 1. ORTEP diagram of 2 with 30% probability ellipsoids.

Figure 2. ORTEP diagram of 3 with 30% probability ellipsoids.

each associated with three, one, and one terminal carbonyl ligands, respectively. Individual Os−CO distances range from 1.89(2) to 1.96(2) Å, C−O distances range from 1.08(2) to 1.15(2) Å, and the Os−C−O angles are in the range 169(2)− 176(2)°. It is remarkable that the carbon−nitrogen triple bonds of three benzonitrile molecules have been activated. Among them, two benzonitriles are added to one hexagon of the C60

Ph and C6H4). Crystals of 2 suitable for an X-ray diffraction study were grown from CS2/n-hexane at ambient temperature. An ORTEP diagram is given in Figure 1. The metal part consists of an open triangular cluster, in which the Os1−Os2 distance (2.887(1) Å) is significantly shorter than the other Os1−Os3 distance (2.972(1) Å), while the nonbonding Os2··· Os3 distance is 3.282(1) Å. The Os1, Os2, and Os3 atoms are 733

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core at the 1,4-position to generate a fullerodiketimide species, of which one ketimide bridges the Os1−Os2 edge with N1− Os1 = 2.06(2) Å and N1−Os2 = 2.03(2) Å and the other spans the Os2 and Os3 atoms with N2−Os2 = 2.00(2) Å and N2− Os3 = 2.04(2) Å. The CC double bond at the 2,3-positions of the same hexagon is also bonded to the Os2 atom in an η2 fashion with the distances Os2−C28 = 2.26(2) Å and Os2− C29 = 2.33(2) Å. In addition, the phenyl ring linked to the C13 atom is metalated to the Os3 atom with C15−Os3 = 2.08(2) Å. The Os1−Os3 edge is bridged by the third phenylketimide (PhHCN)13 asymmetrically with N3−Os1 = 2.13(2) Å and N3−Os3 = 2.04(2) Å, where the hydrogen atom (H20) is likely from C−H bond activation of the phenyl ring mentioned above. The bond distances N1−C6 = 1.26(2) Å, N2−C13 = 1.27(2) Å, and N3−C20 = 1.30(2) Å present a NC doublebond character for the imine moieties. Without Os−Os bonding, the Os2−N2−Os3 angle of 108.6(7)° is substantially obtuse in comparison with the Os1−N1−Os2 angle of 90.0(6)° and the Os1−N3−Os3 angle of 90.9(6)°. We note that the two benzene rings linked to the C13 and C20 atoms are parallel to each other with an interplane distance of 3.6 Å (insert), showing some π−π interactions. The three bridging ketimide ligands are located on the same side of the Os3 triangle. This leaves a space for C60 coordination at the opposite side. It is noteworthy that the second C60 molecule bridges the Os2 and Os3 atoms in an uncommon η2:η2 bonding feature,12 with the distances Os2−C87 = 2.27(2) Å, Os2−C88 = 2.27(2) Å, Os3− C89 = 2.19(2) Å, and Os3−C90 = 2.25(2) Å. Overall, the metal parts contain 50 valence electrons and require only two Os−Os bonds to satisfy the 18-electron rule, in agreement with the observation. Compound 3 forms a slightly air sensitive, greenish brown crystalline solid. The ESI mass spectrum displays the molecular ion peak around m/z 2538. The 1H NMR spectrum of 3 exhibits a 1H singlet at δ 8.78 for the NCH proton and multiplets from δ 8.09 to 6.69 for Ph and C6H4 protons (19H). Crystals of 3 suitable for an X-ray diffraction study were grown from CS2/n-hexane at ambient temperature. An ORTEP diagram is depicted in Figure 2. Overall, the structure of 3 is analogous to that of 2, except that one terminal CO ligand of 2 (bound to Os1) is replaced by a terminal PhCN ligand with the distance Os1−N1 = 2.07(1) Å and the angle Os1−N1−C5 = 174(1)°. The IR spectrum shows four absorption peaks from 2025 to 1940 cm−1 for terminal CO stretches, which are redshifted from those in 2, consistent with the stronger net donor capability of the PhCN ligand in comparison with CO. Attempts to convert the η4-C60 to η6-C60 bonding by heating 3 were not successful, presumably because of the position of the Os1 atom far away from the C35−C36 double bond. The UV−vis spectra of 2 and 3 were measured in dichloromethane (10−5 M) (Figure 3). Both compounds present absorptions around 256 and 330 nm due to π → π* transitions of fullerene and phenyl groups.14 These features are typical of a fullerene cage which has been modified only slightly by functionalization.15 The bands at ca. 445 and 570 nm may be attributed to the MLCT transition of the Os3 cluster. Compounds 2 and 3 contain redox-active fullerene cores, and their electrochemical properties were measured by cyclic voltammetry (CV) in dry, oxygen-free o-dichlorobenzene solution at 27 °C (Figure 3). Within the solvent cutoff, the pristine C60 exhibits three reversible reduction waves at E1/2 = −1080, −1480, and −1954 mV (vs Fc/Fc + couple), corresponding to the C60−/2−/3− states.16 Compound 2 exhibits

Figure 3. UV−vis absorption spectra and cyclic voltammograms for 2 and 3.

six consecutive one-electron redox waves with E1/2 values of −1055, −1243, −1490, −1849, −1997, and −2294 mV for the reduction of two fullerene moieties, while compound 3 presents redox potentials at −1264, −1504, −1654, −1905, −2075, and −2302 mV. The cathodic shifts for 3 might be attributed to the stronger net donor capability of the PhCN ligand, which results in a better π back-donation from the Os3 cluster to the fullerene cores, making them more difficult to reduce. Overall, these measured redox potentials are within the ranges reported for the fullerene derivatives in general.16,17 In summary, an unusual coupling reaction of benzonitrile and C60 with triosmium carbonyl clusters has been revealed, which involves a C−N triple bond, a C−C double bond, and C−H bond activation to generate an unprecedented fullerodiketimide species. In addition, the new products bear two fullerene ligands in an interesting dumbbell-shaped fashion, where one C60 exhibits a rare μ,η2:η2 bonding mode. This new reaction style might be applicable to other nitriles, fullerenes, and metal systems. Further studies are under way.

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EXPERIMENTAL SECTION

Reaction of Os3(CO)12 and C60 in Refluxed Benzonitrile/oDichlorobenzene Solution. An oven-dried, 100 mL two-necked Schlenk flask was equipped with a rubber serum stopper and a reflux condenser under a dinitrogen atmosphere. C60 (64 mg, 0.089 mmol) and Os3(CO)12 (40 mg, 0.044 mmol) were placed in the flask against a dinitrogen flow. Benzonitrile (1 mL) and o-dichlorobenzene (20 mL) were introduced into the flask, and the resulting solution was heated to reflux for 3 h. The volatile materials were then removed under vacuum, and the residue was purified by TLC (silica gel), with CS2/CH2Cl2 (80/1) as eluent. C60 (12 mg, 19%) was recovered from the first purple band. Isolation of the material forming the second brown band gave the known complex Os3(CO)9(μ3,η6-C60) (1; 3.4 mg, 5%). Isolation of the material forming the third greenish brown band afforded the complex Os3(CO)5(μ-NCHPh)(μ3,η5-NCPhC60C(C6H4)N)(μ,η4-C60) (2; 15 mg, 13%). Isolation of the material forming the fi fth brown band affo rd e d th e co mp l e x Os3(CO)4(NCPh)(μ-NCHPh)(μ3,η5-NCPhC60C(C6H4)N)(μ,η4-C60) (3; 10 mg, 9%). Characterization of 2. Anal. Calcd for C146H15N3O5Os3: C, 71.24; H, 0.61; N, 1.71. Found: C, 71.51; H, 0.93; N, 1.70. MS (MALDI): m/ z 2465 (M+, 192Os). IR (CS2, νCO): 2075 (vs), 2014 (s), 2010 (sh), 1978 (w), 1946 (m) cm−1. 1H NMR (CD2Cl2+CS2, 0 °C): δ 9.04 (s, 1H, NCH), 8.39 (d, JH−H = 7 Hz, 1H, Ph), 8.26 (br, 1H, Ph), 8.02 (br, 1H, Ph), 7.96 (br, 1H, Ph), 7.80 (t, JH−H = 7 Hz, 1H, C6H4), 7.74 (d, JH−H = 7 Hz, 1H, C6H4), 7.69 (br, 1H, Ph), 7.23 (d, JH−H = 7 Hz, 2H, 734

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Ph), 7.17 (t, JH−H = 7 Hz, 1H, Ph), 7.07 (m, 3H, C6H4, Ph), 6.79 (t, JH−H = 7 Hz, 1H, C6H4). Characterization of 3. HRMS (ESI): calcd for C152H20N4O4Os3 m/z 2538.0293 (190Os), found m/z 2538.0299. IR (CS2, νCO): 2025 (vs), 1974 (s), 1963 (m), 1940 (s) cm−1. 1H NMR (CDCl3 + CS2, 25 °C): δ 8.78 (s, 1H, NCH), 8.09 (d, JH−H = 7 Hz, 2H, Ph), 7.78 (m, 4H, Ph), 7.70 (t, JH−H = 7 Hz, 1H, C6H4), 7.63 (m, 3H, C6H4, Ph), 7.56 (t, JH−H = 7 Hz, 2H, Ph), 7.07 (d, JH−H = 7 Hz, 2H, Ph), 7.01 (t, JH−H = 7 Hz, 1H, Ph), 6.97 (m, 3H, C6H4, Ph), 6.69 (t, JH−H = 7 Hz, 1H, C6H4). Preparation of Os3(CO)10(NCPh)2. This reaction was carried out in a fashion similar to the preparation for Os3(CO)10(NCMe)2,18 except that benzonitrile replaced acetonitrile as the cosolvent. Os3(CO)12 (200 mg, 0.22 mmol) was placed in an oven-dried, 250 mL Schlenk flask under dinitrogen, and benzonitrile (8 mL) and dichloromethane (150 mL) were added to the flask. A solution of Me3NO (43 mg, 0.59 mmol) in benzonitrile (20 mL) was then slowly introduced into the flask by a syringe pump over 2 h at ambient temperature, and the resulting mixture was stirred for another 2 h. The solution was passed through a short silica gel column, and the filtrate was dried under vacuum. The residue was crystallized from dichloromethane/n-hexane to afford a yellow solid of Os3(CO)10(NCPh)2 (153 mg, 65%). IR (CH2Cl2, νCO): 2077 (w), 2021 (vs), 1983 (m), 1960 (w) cm−1. 1H NMR (CD2Cl2, 25 °C): δ 8.12−7.10 (m, Ph). Reaction of Os3(CO)10(NCPh)2 and C60 in Refluxed Benzonitrile/o-Dichlorobenzene Solution. A solution of C60 (27 mg, 0.038 mmol) and Os3(CO)10(NCPh)2 (20 mg, 0.019 mmol) in benzonitrile (0.5 mL) and o-dichlorobenzene (10 mL) was refluxed under dinitrogen for 45 min. The volatile materials were removed under vacuum, and the residue was purified by TLC (silica gel), with CS2/ CH2Cl2 (80:1) as eluent. Compounds 1 (1.5 mg, 5%), 2 (18.3 mg, 39%), and 3 (2.9 mg, 6%) were obtained. Reaction of Os3(CO)10(NCPh)2 and C60 in Refluxed oDichlorobenzene Solution. C60 (28 mg, 0.038 mmol) and Os3(CO)10(NCPh)2 (20 mg, 0.019 mmol) in o-dichlorobenzene (10 mL) were refluxed under dinitrogen for 45 min. The volatile materials were removed under vacuum, and the residue was purified by TLC (silica gel), with CS2/CH2Cl2 (80:1) as eluent. Compounds 1 (11.1 mg, 38%) and 2 (1.9 mg, 4%) were obtained.

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of Metal Cluster Complexes; VCH: New York, 1990. (d) Adams, R. D.; Cotton, F. A. Catalysis by Di- and Polynuclear Metal Cluster Complexes; Wiley: New York, 1998. (e) Braunstein, P.; Oro, L. A.; Raithby, P. R. Metal Clusters in Chemistry; Wiley-VCH: Weinheim, Germany, 1999. (3) Krätschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354−358. (4) (a) Taylor, R. Lecture Notes on Fullerene Chemistry; Imperial College Press: London, 1999. (b) Hirsch, A.; Brettreich, M. Fullerenes; Wiley-VCH: Weinheim, Germany, 2005; pp 231−250. (c) Murata, M.; Murata, Y.; Komatsu, K. Chem. Commun. 2008, 6083−6094. (5) (a) Balch, A. L.; Olmstead, M. M. Chem. Rev. 1998, 98, 2123− 2165. (b) Lee, K.; Song, H.; Park, J. T. Acc. Chem. Res. 2003, 36, 78− 86. (6) (a) Chen, C.-H.; Chen, C.-S.; Dai, H.-F.; Yeh, W.-Y. Dalton Trans. 2012, 41, 3030−3037. (b) Meijer, M. D.; van Klink, G. P. M.; van Koten, G. Coord. Chem. Rev. 2002, 230, 141−163. (c) Konarev, D. V.; Simonov, S. V.; Khasanov, S. S.; Lyubovskaya, R. N. Dalton Trans. 2011, 40, 9176−9179. (d) Bonuccelli, V.; Funaioli, T.; Leoni, P.; Marchetti, L.; Zacchini, S. Dalton Trans. 2013, 42, 16898−16908. (7) (a) Guldi, D. M. Chem. Soc. Rev. 2002, 31, 22−36. (b) Martín, N. Chem. Commun. 2006, 2093−2104. (c) Langa, F.; Nierengarten, J.-F. Fullerenes: Principles and Applications; RSC Publications: Cambridge, U.K., 2007. (8) (a) Park, B. K.; Miah, M. A.; Lee, G.; Cho, Y.-J.; Lee, K.; Park, S.; Choi, M.-G.; Park, J. T. Angew. Chem., Int. Ed. 2004, 43, 1712−1714. (b) Usatov, A. V.; Martynova, E. V.; Dolgushin, F. M.; Peregudov, A. S.; Antipin, M. Y.; Novikov, Y. N. Eur. J. Inorg. Chem. 2003, 29−33. (c) Yeh, W.-Y. Chem. Commun. 2011, 47, 1506−1508. (d) Yeh, W.-Y. Angew. Chem., Int. Ed. 2011, 50, 12046−12049. (9) (a) Yeh, W.-Y.; Wu, S.-H. Dalton Trans. 2013, 42, 12260−12264. (b) Chen, C.-H.; Yeh, W.-Y.; Liu, Y.-H.; Lee, G.-H. Angew. Chem., Int. Ed. 2012, 51, 13046−13049. (c) Wang, S.-J.; Yeh, W.-Y. Organometallics 2012, 31, 6491−6495. (d) Wu, Y.-Y.; Yeh, W.-Y. Organometallics 2011, 30, 4792−4795. (e) Yeh, W.-Y.; Tsai, K.-Y. Organometallics 2010, 29, 604−609. (10) (a) Hsu, H.-F.; Shapley, J. R. J. Am. Chem. Soc. 1996, 118, 9192−9193. (b) Park, J. T.; Song, H.; Cho, J.-J.; Chung, M.-K.; Lee, J.H.; Suh, I.-H. Organometallics 1998, 227−236. (11) (a) Gallop, M. A.; Gomez-Sal, M. P.; Housecroft, C. E.; Johnson, B. F. G.; Lewis, J.; Owen, S. M.; Raithby, P. R.; Wright, A. H. J. Am. Chem. Soc. 1992, 114, 2502−2509. (b) Johnson, B. F. G.; Lewis, J.; Martinelli, M.; Wright, A. H.; Braga, D.; Grepioni, F. J. Chem. Soc., Chem. Commun. 1990, 364−366. (c) Gomez-Sal, M. P.; Johnson, B. F. G.; Lewis, J.; Raithby, P. R.; Wright, A. H. J. Chem. Soc., Chem. Commun. 1985, 1682−1684. (12) Lee, G.; Cho, Y.-J.; Park, B. K.; Lee, K.; Park, J. T. J. Am. Chem. Soc. 2003, 125, 13920−13921. (13) Lausarot, P. M.; Turini, M.; Vaglio, G. A.; Valle, M.; Tiripicchio, A.; Gamellini, M. T.; Gariboldi, P. J. Organomet. Chem. 1984, 273, 239−246. (14) Leach, S.; Vervloet, M.; Desprès, A.; Bréheret, E.; Hare, J. P.; Dennis, T. J.; Kroto, H. W.; Taylor, R.; Walton, D. R. M. Chem. Phys. 1992, 160, 451−466. (15) Akasaka, T.; Ando, W.; Kobayashi, K.; Nagase, S. J. Am. Chem. Soc. 1993, 115, 1605−1606. (16) Echegoyen, L.; Echegoyen, L. E. Acc. Chem. Res. 1998, 31, 593− 601. (17) Kutner, W.; Noworyta, K.; Deviprasad, G. R.; D’Souza, F. J. Electrochem. Soc. 2000, 147, 2647−2652. (18) Braga, D.; Grepioni, F.; Parisini, E.; Johnson, B. F. G.; Martin, C. M.; Nairn, J. G. M.; Lewis, J.; Martinelli, M. J. Chem. Soc., Dalton Trans. 1993, 1891−1895.

ASSOCIATED CONTENT

S Supporting Information *

Text, a table, figures, and CIF files giving details of the reactions of C60, PhCN, and Os3(CO)12 under various conditions, structure determination details for 2 and 3, and X-ray crystal data for compounds 2·3CS2 and 3·2CS2. This material is 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.

ACKNOWLEDGMENTS We are grateful for support of this work by the National Science Council of Taiwan. We thank Mr. Ting-Shen Kuo (National Taiwan Normal University, Taipei) for X-ray diffraction analysis.

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REFERENCES

(1) (a) Jones, W. D. In Activation of Unreactive Bonds; SpringerVerlag: Heidelberg, Germany, 1999; pp 10−46. (b) Marks, T. J. Acc. Chem. Res. 1992, 25, 57−65. (c) Cordaro, J. G.; Bergman, R. G. J. Am. Chem. Soc. 2004, 126, 3432−3433. (2) (a) Cifuentes, M. P.; Humphrey, M. G. Organomet. Chem. 2011, 37, 115−132. (b) Adams, R. D.; Captain, B. Acc. Chem. Res. 2009, 42, 409−418. (c) Shriver, D. F.; Kaesz, H. D.; Adams, R. D. The Chemistry 735

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