(Ethene)bis(acetylacetonato) Complexes of Divalent and Trivalent Ruthenium

Zinc dust reduction of tris(acetylacetonato)ruthenium(III), [Ru(acac)₃], in hot aqueous THF in the presence of ethene gives cis-[Ru(acac)₂(η²-C₂H₄)₂] (5), isolated as an orange solid in ca. 60% yield. A single-crystal X-ray crystallographic study shows the metal atom to be coordinated octahedrally by a pair of bidentate O-bonded acac groups and two mutually cis orthogonal ethene ligands. In the NMR spectra of 5, the ethene carbon atoms remain equivalent, while the protons are inequivalent in pairs, down to −95 °C; intermolecular exchange with ethene is slow on the NMR time scale at room temperature but becomes fast above ca. 80 °C. One of the ethene molecules of 5 is replaced by ligands at room temperature to give [Ru(acac)₂(η²-C₂H₄)(L)] [L = SbPh₃ (6), MeCN (7), NH₃ (8), and C₅H₅N (9)]. Complexes 6 and 7 are isolated as cis-isomers, 9 as a cis−trans mixture, and 8 in the form of the trans-isomer which transforms into the cis-isomer when heated or chromatographed on alumina. The structures of cis-8 and trans-9 have been confirmed by X-ray crystallography. The results indicate that ligand substitutions on 5 may occur via a square pyramidal intermediate [Ru(acac)₂(η²-C₂H₄)] in which ethene occupies the apical site. All the complexes display either a quasi-reversible or a fully reversible one-electron oxidation by cyclic voltammetry at −50 °C, with E_1/2 values in the range +0.37−0.59 V for 6−9 and +0.95 V for 5 (in 0.5 M [Bu₄N]PF₆/CH₂Cl₂ vs Ag/AgCl/MeCN). The blue oxidized species 5⁺, 6⁺, and 8⁺ can be generated by bulk electrolysis and are stable for hours at −50 °C. Their UV−vis spectra and the ESR spectra of 6⁺ and 8⁺ indicate that the oxidized species are ruthenium(III)-ethene cations. Oxidation of the known cyclooctene complex cis-[Ru(acac)₂(η²-C₈H₁₄)(SbPh₃)] (2) with AgPF₆ at −50 °C gives an isolable, deep blue ruthenium(III) salt [2]⁺[PF₆]^-, which is stable at room temperature.