Ruthenium complexes outer-sphere reaction, 996

Ru(CN)jNO reactions with OH , SH and SOj" resemble those of the nitroprusside ion, with attack at the coordinated nitrosyl to give analogous transients and similar second-order rate constants. Ruthenium(II) complexes of the general type Ru(N2), Nj = biden-tate hgands, are important reactants. The relative inertness of Ru(NH3) + and Ru(diimine)f+ towards substitution makes these complexes definite, although weak, outer-sphere reductants (Tables 5.4, 5.5, 5.6 and 5.1). Ruthenium(ll) complexes of the general type Ru(diimine)f +, and particularly the complex Ru(bpy)j+, have unique excited state properties. They can be used as photosensitizers in the photochemical conversion of solar energy. Scheme 8.1 ... 

Hence, the first clearcut evidence for the involvement of enol radical cations in ketone oxidation reactions was provided by Henry [109] and Littler [110,112]. From kinetic results and product studies it was concluded that in the oxidation of cyclohexanone using the outer-sphere one-electron oxidants, tris-substituted 2,2 -bipyridyl or 1,10-phenanthroline complexes of iron(III) and ruthenium(III) or sodium hexachloroiridate(IV) (IrCI), the cyclohexenol radical cation (65" ) is formed, which rapidly deprotonates to the a-carbonyl radical 66. An upper limit for the deuterium isotope effect in the oxidation step (k /kjy < 2) suggests that electron transfer from the enol to the metal complex occurs prior to the loss of the proton [109]. In the reaction with the ruthenium(III) salt, four main products were formed 2-hydroxycyclohexanone (67), cyclohexenone, cyclopen tanecarboxylic acid and 1,2-cyclohexanedione, whereas oxidation with IrCl afforded 2-chlorocyclohexanone in almost quantitative yield. Similarly, enol radical cations can be invoked in the oxidation reactions of aliphatic ketones with the substitution inert dodecatungstocobaltate(III), CoW,20 o complex [169]. Unfortunately, these results have never been linked to the general concept of inversion of stability order of enol/ketone systems (Sect. 2) and thus have never received wide attention. 

The kinetic data for a series of outer-sphere electron transfer reactions between the [Rh2(02CCH3)4(CH3CN)2] couple and nickel tetraaza macrocycles and iron and ruthenium tris(polypyridine) complexes in acetonitrile have been correlated in terms of the Marcus relationship, yielding a [Rh2] electron exchange rate constant of 3.0 1.7 x 10 M A somewhat smaller value of 5.3 1.3 x... 

The aziridination of olefins has also been studied, but fewer complexes catalyze this reaction as efficiently as iron and manganese complexes catalyze the epoxida-tion of olefins. Nevertheless, the aziridinations of olefins catalyzed by copper, ruthenium, and rhodium complexes have been reported. The source of nitrogen is usually [N-(p-toluenesulfonyl)imino]phenyliodinane (PhI=NTs) or a precursor to a related iodoarylimine. The aziridine is likely generated from these copper- and rhodium-catalyzed reactions by an outer-sphere process in which the olefin interacts with the LUMO of the complex, which is located at the nitrogen. This mechanism is more likely to be followed by these catalysts than a [2-t-2] process, followed by reductive elimination. 

More recently, Morris reported the hydrogenation of benzonitrile catalyzed by a ruthenium complex containing a P-NH-NH-P tetradentate ligand (Equation 15.121). The presence of an amine N-H and metal hydride make it likely that the reaction occurs by an outer-sphere mechanism involving two sequential simultaneous transfers of the hydride and the N-H proton, first to the nitrile and second to the imine (Scheme 15.27). This proposal was supported by DPT calculations. The hydrogenation of aryl and heteroaryl nitriles catalyzed by a combination of [Ru(COD)(2-methylallyl) J and DPPF has also been reported to occur in high yields. 

The chromium(II) reductions of a number of trans-dihalide ruthenium (III) complexes are inner sphere in nature with halide bridging groups. Reaction rates are sensitive to steric effects, whereas the corresponding reductions by vanadium(II) show little steric variation, exceed the V " ligand substitution rate, and must be considered outer sphere. 

There has been considerable recent interest in the reductions of [Fe(CN)6]. The electron exchange with A -propyl-l,4-dihydronicotinamide is catalyzed by alkali metal ions. The increase in reaction rate is attributed to the polarizability of M and the observed linear free energy relationship is discussed. An outer-sphere mechanism is postulated in the oxidation of phenothiazines. A free radical mechanism involving the alcohol anion is invoked in the reaction of 1-and 2-propanol in aqueous alkaline media, the kinetic order being unity for [Fe(CN)6], OH, and alcohol concentrations. Catalysis by metal ions has also been observed in the presence of copper(II) and ruthenium(III) complexes. In the oxidation of a-hydroxypropionic acid in alkaline media,a Cu(II)-ligand complex is formed which is oxidized slowly to a copper(III) species. Alkaline ferricyanide oxidizes butanol, the process being catalyzed by chlororuthenium complexes.The rate law is consistent with oxidation of the alcohol by the Ru(III) followed by reoxidation of the catalyst by [Fe(CN)6]. The rate law is of the form ... 

It was possible to deduce the second-order rate constants from the decay of Yb " " and Sm monitored spectrophotometrically. Variations of these rate constants with Cl concentration was also studied. Sm + is always more reactive than Yb " (as expected from the related ). It was established that the reactions take place mainly by an inner-sphere mechanism in the case of cobalt complexes and by an outer-sphere mechanism with the ruthenium complexes. The reaction with [Ru(NH3)J enabled Christensen et al. (1970) to apply the Marcus outer-sphere mechanism theory for calculating the rates of electron exchange Ln + Ln + -F e (Ln = Sm or Yb). Rates... 

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