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, RCH=NNHR'(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)N=NR'; 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.