Chemistry of ruthenium. Part 9. Electrophilic addition of a ruthenium nitrosyl complex to alkylidenearylhydrazones. Arylazo-oximates of bis(2,2′-bipyridyl)ruthenium(II)

Abstract

The electrophilic addition of [Ru(bipy)₂(NO)Cl][ClO₄]₂ to alkylidenearylhydrazones, RCHNNHR′(R = Me, Ph, or C₆H₄Me-p; R′= Ph or C₆H₄Me-p) in methanol containing sodium methoxide affords the new complex [Ru(bipy)₂(A)][ClO₄]·H₂O (2)[HA = RC(NOH)NNR′; bipy = 2,2′-bipyridyl]. An alternative synthesis of (2) is provided by the reaction of cis-[Ru(bipy)₂Cl₂] with NaA. The protonation of (2) by HClO₄ in methanol gives [Ru(bipy)₂(HA)][ClO₄]₂·H₂O (5). The proton dissociation constants (pK) of (5) and of HA in dioxan–water (1 : 1) are respectively ca. 10 and ca. 3. All complexes display a ν(NO) of the arylazo-oximate chelate ring at ca. 1 240 cm^–1 in the i.r. and one or more t₂(ruthenium)→π*(ligand) charge-transfer transition(s) in the visible region, 400–500 nm. A quasi-reversible Ru^III–Ru^II couple occurs in (2) near 0.8 V versus s.c.e. with ΔE_Pca. 75 mV (cyclic voltammetry in acetonitrile). The Ru^III species [Ru(bipy)₂(A)]²⁺ is unstable and generally decomposes with a pseudo-first-order rate constant of 0.4–0.6 s^–1(298 K). In (5) the Ru^III–Ru^II couple is at ca. 1.0 V.