Synthesis, Characterization, and Reactions of Ruthenium(II), -(III), and -(IV) Complexes with Sterically Demanding 1,2,4-Tri-tert-butylcyclopentadienyl Ligands

Abstract

The reaction of RuCl3·3H2O with 1,3,5-C5(tBu)3H3 afforded a dimeric Ru(III) dichloride complex, [Cp‡RuCl(μ-Cl)]2 (1b; Cp‡ = η5-1,2,4-C5(tBu)3H2). The treatment of 1b with gaseous oxygen afforded the tetravalent ruthenium μ-oxo complex [Cp‡RuCl2]2(μ-O) (2b). The treatment of 2b with a primary or secondary alcohol regenerated 1b with concomitant dehydrogenative oxidation of the alcohol to afford the corresponding aldehyde or ketone, respectively. The treatment of 2b with refluxing 2-propanol or an excess of zinc powder in tetrahydrofuran efficiently afforded a novel dimeric Ru(II) monochloride with a butterfly structure, [Cp‡Ru(μ-Cl)]2 (4), with unsaturated ruthenium centers and high reactivity toward electron-donating reagents. The reaction of 1b with 4 afforded the mixed-valence Ru(III)/Ru(II) complex Cp‡Ru(μ-Cl)3RuCp‡ (7). The treatment of the appropriate precursor 1b or 2b with an excess of a hydride reagent such as LiEt3BH or LiAlH4 afforded the novel dinuclear ruthenium tetrahydride-bridged complex Cp‡Ru(μ-H)4RuCp‡ (9b). The treatment of a mixture of [Cp*Ru(μ3-Cl)]4 and 4 with LiEt3BH afforded the analogous dinuclear ruthenium tetrahydride-bridged complex bearing Cp* (Cp* = η5-C5Me5) and Cp‡ ligands, Cp*Ru(μ-H)4RuCp‡ (9c). The molecular structures of the newly synthesized complexes 1b, 2b, 4, 7, 8b, and 9b,c were determined by X-ray diffraction (XRD) studies. The reaction of 9b with ethylene exclusively afforded a μ-ethylidyne μ-ethylidene complex, Cp‡Ru(μ-CCH3)(μ-CHCH3)(μ-H)RuCp‡ (11), whereas the reaction of 9a with excess ethylene selectively afforded a dinuclear ruthenium diviny ethylene complex, Cp*Ru(μ-CHCH2)2(CH2CH2)RuCp* (10). The difference in the reactivities can be attributed to the difference in the spatial size of the reaction sites rather than their electronic properties such as electron density at the ruthenium centers. Irradiation of 11 with UV light (365 nm) effectively afforded a bis(μ-ethylidyne) complex, Cp‡Ru(μ-CCH3)2RuCp‡ (12). The structures of 11 and 12 were also determined by XRD studies.