Synthesis, Structural Characterization, Ligand Displacement Reaction, and Electrochemical Property of Ruthenium Complexes Incorporating Linked Cyclopentadienyl-Carboranyl Ligands Yi Sun, Hoi-Shan Chan, Pierre H. Dixneuf, ‡ and Zuowei Xie* , Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, China, and Institut de Chimie de Rennes, UMR 6509 CNRS, Universite´ de Rennes, Organome´ talliques et Catalyse, Campus de Beaulieu, 35042 Rennes, France Received August 10, 2004 Reactions of the dilithium salts of carbon-bridged cyclopentadienyl-carboranyl ligands with 1 equiv of [RuCl 2 (COD)] x in THF afforded the organoruthenium(II) complexes [η 5 :σ-Me 2 C- (C 5 H 4 )(C 2 B 10 H 10 )]Ru(COD) (1a), [η 5 :σ-Me 2 C(C 9 H 6 )(C 2 B 10 H 10 )]Ru(COD) (2a), and [η 5 :σ-H 2 C- (C 13 H 8 )(C 2 B 10 H 10 )]Ru(COD) (3a), respectively. Treatment of 1a with bidentate tertiary phosphines in THF gave the corresponding COD displacement complexes [η 5 :σ-Me 2 C(C 5 H 4 )- (C 2 B 10 H 10 )]Ru(dppe) (1b, dppe ) 1,2-bis(diphenylphosphino)ethane), [η 5 :σ-Me 2 C(C 5 H 4 )(C 2 B 10 - H 10 )]Ru(dppm) (1c, dppm ) bis(diphenylphosphino)methane), [η 5 :σ-Me 2 C(C 5 H 4 )(C 2 B 10 H 10 )]- Ru(dppf) (1d, dppf ) 1,1′-bis(diphenylphosphino)ferrocene), and [η 5 :σ-Me 2 C(C 5 H 4 )(C 2 B 10 H 10 )]- Ru(dppc) (1e, dppc ) 1,2-(Ph 2 P) 2 -1,2-C 2 B 10 H 10 ). 1a also reacted with 2,2′-bipyridine (bipy) to offer [η 5 :σ-Me 2 C(C 5 H 4 )(C 2 B 10 H 10 )]Ru(bipy) (1f). However, 1a did not react with monodentate tertiary phosphines such as PPh 3 and PCy 3 , tertiary amines, and bidentate ligands with no π-acidity such as dimethoxyethane and tetramethylethylenediamine. These results imply that a bidentate ligand with π-acidity is critical to displace the COD in 1a. The electrochemi- cal studies showed that the electron-donating power of various ligands increases in the following order: cyclopentadienyl < indenyl < fluorenyl, and COD < dppc < dppm ≈ dppe < bipy. All of these new complexes were fully characterized by various spectroscopic techniques and elemental analyses. Their molecular structures (except for 1d) were further confirmed by single-crystal X-ray analyses. Introduction Ruthenium half-sandwich complexes of the type (η 5 - C 5 R 5 )RuXL 2 [R ) H, Me; X ) Cl, Br; L 2 ) phosphines, COD (1,5-cyclooctadiene)] are effective catalysts for C-C bond-forming reactions. 1 For example, (η 5 -C 5 H 5 )RuCl- (COD) catalyzed a variety of coupling reactions of CtC and CdC bonds for the production of functional dienes. 2 (η 5 -C 5 Me 5 )RuCl(COD) allowed the head-to-head cou- pling of alkynes to form dienes and cyclobutenes 3 or the sequential coupling of alkynes generating aromatic compounds. 4 (η 5 -C 5 Me 5 )RuCl(COD) also promoted double addition of carbene to alkynes and to enynes to generate dienes and bicyclo[3.1.0]hexane derivatives. 5 The suc- cess of these catalytic processes is attributed to the electron richness of the metal center and to the labile ligands, thus favoring oxidative coupling of the unsatur- ated molecules, and to the steric hindrance of the C 5 - Me 5 group favoring regioselective couplings. 1 By con- trast, (η 5 -C 5 R 5 )RuCl(PPh 3 ) 2 complexes underwent activa- tion of terminal alkynes into ruthenium-vinylidene key intermediates that controlled catalytic anti-Markovni- kov additions to alkynes. 6 These catalytic processes are favored by electron-releasing PR 3 ligands and illustrate how simple COD/ligand exchanges can modify the action of the catalyst. On the other hand, ruthenium half-sandwich com- plexes, in which a cyclopentadienyl ligand is tethered to a donor atom, also receive much attention. 7 The tethered donor atom in Cp-D chelating ligands can prevent rotation of the Cp ring and allow the planar chirality to be exploited in an efficient discrimination, through a strong coordination to the Ru atom, or can temporarily and reversibly coordinate to a Ru atom while stabilizing highly reactive, electronically and sterically unsaturated species, 8 to meet the require- ments of various catalytic processes. For example, a * To whom correspondence should be addressed. Fax: (852)- 26035057. Tel: (852)26096269. E-mail: zxie@cuhk.edu.hk. The Chinese University of Hong Kong. ‡ Universite´ de Rennes. (1) (a) Trost, B. M.; Toste, F. D.; Pinkerton, A. B. Chem. Rev. 2001, 101, 2067. (b) De´rien, S.; Dixneuf, P. H. J. Organomet. Chem. 2004, 689, 1382. (2) Trost, B. M. Acc. Chem. Res. 2002, 35, 695. (3) (a) Paih, J. 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Rev. 2003, 32, 130. 5864 Organometallics 2004, 23, 5864-5872 10.1021/om0493816 CCC: $27.50 © 2004 American Chemical Society Publication on Web 10/21/2004