Ruthenium complexes with alkali metals

Although some cyclohexadienes are readily available, many can be obtained easily by Birch reduction, which involves reduction with solutions of alkali metals in liquid ammonia, a source of solvated electrons, in the presence of alcohol as a proton source.8-10 In previous years, the Bouveault-Blanc procedure, which uses sodium metal and alcohol in liquid ammonia, was frequently employed for direct reduction of aromatic esters however, it gave rise mainly to the corresponding substituted benzoic acid.11 Rabideau et al. reported a modified procedure 12 however, in our hands, this resulted in the reduction of the ester function to give benzoic acid. We have found that the Birch reduction of benzoic acid, followed by esterification, is an efficient procedure for the preparation of the corresponding 1,4-dihydro compound prior to the coordination of the arene to produce functionalized dimeric ruthenium-arene complexes.13... 

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 ... 

The results in Refs. 91 and 127 (Table 9.7) also demonstrate that some alkali salts (nitrate, carbonate, hydroxide) give rise to efficiencies similar to the alkali metals as promoters, while others (chlorides) are almost totally inactive, which is in marked contrast to alumina>supported catalysts where the addition of alkali salts has little promoting effect. The active state of the promoter on ruthenium/carbon catalysts is unlikely to be metallic, as the high vapor pressure of the alkali metals would give rise to substantial losses under synthesis conditions. The more probable state is a charge transfer complex with the graphite... 

However, while this shows that the complex is likely to be produced during catalyst reduction, it is unlikely to remain on the catalyst surface at the temperatures where ammonia synthesis catalysts operate (>623 K). The rate of desorption of the alkali is strongly dependent on the heat of adsorption which, on carbon supports or single-crystal metal surfaces, decreases markedly with coverage, as shown for nickel in Fig 9.6. Similarly, for potassium on ruthenium the heat of desorption decreases from 112 6 = 0%) to 105 kJmol" (0 = 40%) while only a 5% potassium surface coverage remains after flash desorption of Ru(OOOl) at 1000 A simple calculation, based on a desorption frequency of i = 10 s" implies that for a heat of adsorption of 270 kJ moP the residence time on the surface at 773 K would only be... 

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