Synthesis, Structural Characterization, Ligand Displacement Reaction, and Electrochemical Property of Ruthenium Complexes Incorporating Linked Cyclopentadienyl-Carboranyl Ligands

Reactions of the dilithium salts of carbon-bridged cyclopentadienyl-carboranyl ligands with
1 equiv of [RuCl2(COD)]x in THF afforded the organoruthenium(II) complexes [η5:σ-Me2C(C5H4)(C2B10H10)]Ru(COD) (1a), [η5:σ-Me2C(C9H6)(C2B10H10)]Ru(COD) (2a), and [η5:σ-H2C(C13H8)(C2B10H10)]Ru(COD) (3a), respectively. Treatment of 1a with bidentate tertiary
phosphines in THF gave the corresponding COD displacement complexes [η5:σ-Me2C(C5H4)(C2B10H10)]Ru(dppe) (1b, dppe = 1,2-bis(diphenylphosphino)ethane), [η5:σ-Me2C(C5H4)(C2B10H10)]Ru(dppm) (1c, dppm = bis(diphenylphosphino)methane), [η5:σ-Me2C(C5H4)(C2B10H10)]Ru(dppf) (1d, dppf = 1,1‘-bis(diphenylphosphino)ferrocene), and [η5:σ-Me2C(C5H4)(C2B10H10)]Ru(dppc) (1e, dppc = 1,2-(Ph2P)2-1,2-C2B10H10). 1a also reacted with 2,2‘-bipyridine (bipy)
to offer [η5:σ-Me2C(C5H4)(C2B10H10)]Ru(bipy) (1f). However, 1a did not react with monodentate
tertiary phosphines such as PPh3 and PCy3, 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 electrochemical 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.