Bis(acetylacetonato)ruthenium(ii) complexes containing alkynyldiphenylphosphines. Formation and redox behaviour of [Ru(acac)2(Ph2PCCR)2] (R = H, Me, Ph) complexes and the binuclear complex cis-[{Ru(acac)2}2(µ-Ph2PCCPPh2)}2]

Abstract

Two equivalents of Ph₂PCCR (R = H, Me, Ph) react with thf solutions of cis-[Ru(acac)₂(η²-alkene)₂] (acac = acetylacetonato; alkene = C₂H₄, 1; C₈H₁₄, 2) at room temperature to yield the orange, air-stable compounds trans-[Ru(acac)₂(Ph₂PCCR)₂] (R = H, trans-3; Me = trans-4; Ph, trans-5) in isolated yields of 60–98%. In refluxing chlorobenzene, trans-4 and trans-5 are converted into the yellow, air-stable compounds cis-[Ru(acac)₂(Ph₂PCCR)₂] (R = Me, cis-4; Ph, cis-5), isolated in yields of ca. 65%. From the reaction of two equivalents of Ph₂PCCPPh₂ with a thf solution of 2 an almost insoluble orange solid is formed, which is believed to be trans-[Ru(acac)₂(µ-Ph₂PCCPPh₂)]_n (trans-6). In refluxing chlorobenzene, the latter forms the air-stable, yellow, binuclear compound cis-[{Ru(acac)₂(µ-Ph₂PCCPPh₂)}₂] (cis-6). Electrochemical studies indicate that cis-4 and cis-5 are harder to oxidise by ca. 300 mV than the corresponding trans-isomers and harder to oxidise by 80–120 mV than cis-[Ru(acac)₂L₂] (L = PPh₃, PPh₂Me). Electrochemical studies of cis-6 show two reversible Ru^II/III oxidation processes separated by 300 mV, the estimated comproportionation constant (K_c) for the equilibrium cis-6²⁺ + cis-6 ⇌ 2(cis-6⁺) being ca. 10⁵. However, UV-Vis spectra of cis-6⁺ and cis-6²⁺, generated electrochemically at −50 °C, indicate that cis-6⁺ is a Robin–Day Class II mixed-valence system. Addition of one equivalent of AgPF₆ to trans-3 and trans-4 forms the green air-stable complexes trans-3·PF₆ and trans-4·PF₆, respectively, almost quantitatively. The structures of trans-4, cis-4, trans-4·PF₆ and cis-6 have been confirmed by X-ray crystallography.