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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
Goswami,Chakravarty,Chakravorty
, p. 2737 - 2742 (2008/10/08)
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 2 ～ 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 (Eo298). 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.
