Chemistry of ruthenium. 5. Reaction of trans-dihalobis[2-(arylazo)pyridine]ruthenium(II) with tertiary phosphines: Chemical, spectroelectrochemical, and mechanistic characterization of geometrically isomerized substitution products

Article abstract of DOI:10.1021/ic00137a040

The green trans isomer (2) of RuX₂L₂, though unreactive to pyridine bases and Ag⁺, reacts relatively slowly but smoothly with tertiary phosphines giving rise to species of types [RuXPL₂)⁺, [RuP₂L₂]²⁺, and [Ru(P-P)L₂]²⁺ in which XP, P₂, and P-P respectively occupy cis positions [X = Cl, Br, I; P = P(p-tol)₃, PPh₃, PPh₂Me, PPhMe₂; P-P = Ph₂P(CH₂)₂PPh₂, Ph₂P(CH₂)₃PPh₂; L = 2-(arylazo)pyridine]. The cations have been isolated as perchlorates. The complexes display allowed t₂(Ru) → π*(L) transitions in the visible region. A systematic shift of this MLCT band to higher energy occurs in the order X₂ < XP < P₂ ～ P-P. The ruthenium(III)-ruthenium(II) couple occurs at high potentials (>1.3 V vs. SCE). The MLCT band energies of cis-RuX₂L₂, [RuXPL₂]⁺, [RuP₂L₂]²⁺, and [Ru(P-P)L₂]²⁺ correlate linearly with ruthenium-(III)-ruthenium(II) formal potentials (E^o₂₉₈). The data point of 2 falls widely outside the line. This and other facts taken together prove that during phosphine substitution geometrical isomerization of the RuL₂ fragment occurs. The reaction of 2 (X = Cl, L = 2-(phenylazo)pyridine) with PPhMe₂ is shown to be kinetically second order with k₂ = 9.64 × 10^-4 M^-1 s^-1 (40 ± 0.2°C). The rate decreases very considerably on increasing phosphine bulk: PPhMe₂ > PPh₂Me >> PPh₃. An associative pathway involving nucleophilic trans attack or edge displacement is proposed, and the possible role of specific Ru-N(azo) bond labilization is indicated. The observed isomerization is a logical consequence of such a mechanism. Brief mention is made of certain observations and current activities. These include geometrical isomerization in nucleophilic displacement with OH^-, greater lability of the cis isomers, and the cis diaquo species [Ru(H₂O)₂L₂]²⁺ and its role as an intermediate for preparation of many interesting complexes of RuL₂.