Ruthenium turnover frequency

The first application of a heterocyclic carbenoid achiral ligand for hydrogenation of alkenes was reported in 2001 by Nolan and coworkers. Both ruthenium [36] and iridium [37] complexes proved to be active catalysts. Turnover frequency (TOF) values of up to 24000 b 1 (at 373 K) were measured for a ruthenium catalyst in the hydrogenation of 1-hexene. 

It is realised that both ruthenium and the substituted porphyrins are expensive catalyst components for industrial applications. Both turnover frequencies and turnover numbers are modest. Nevertheless it remains an interesting option to use dioxygen directly in epoxidation reactions. 

While ruthenium dichlorides always showed an undesirable initiation time, these novel catalysts start the reaction without delay. For compounds not containing functional groups their turnover frequencies can be several thousands per hour, but for polar molecules the total turnover may be as low as fifty, obtained in several hours. 

The ruthenium cluster [Ru2(i76-C6H6)H6]C12 is a catalyst for fumaric acid hydrogenation in aqueous solutions, with a turnover frequency of 35 h 1 at 50°C (86). 

A comparison of the theoretical treatment of the rate of synthesis of hydrocarbons with the experimental results, clearly shows that the low turnover frequencies that are measured on iron catalysts cannot be explained by a low rate constant of propagation. This result is clearly in contrast with the conclusions of Dautzenberg et al. for ruthenium. This does not signify, however, that the propagation has to be rate determining on ruthenium. 

The catalytic activity, expressed as turnover frequency (TOF), increases in the order Rhruthenium catalyst with methylphosphine instead of phenylphosphine... 

Many examples of heterogeneous ruthenium systems are available. One of the more promising seems to be ruthenium on alumina, which is an active and recyclable catalyst [155]. This system displayed a large substrate scope (see Fig. 4.62) and tolerates the presence of sulfur and nitrogen groups. Only primary aliphatic alcohols required the addition of hydroquinone. Turnover frequencies in the range of 4 h 1 (for secondary allylic alcohols) to 18 h-1 (for 2-oc-tanol) were obtained in trifluorotoluene, while in the solvent-free oxidation at 150 °C a TOF of 300 h-1 was observed for 2-octanol. 

Another class of ruthenium catalysts, which has attracted considerable interest due to their inherent stability under oxidative conditions, are the polyoxome-talates [161]. Recently, Mizuno et al. [162] reported that a mono-ruthenium-sub-stituted silicotungstate, synthesized by the reaction of the lacunary polyoxometa-late [SiWu039]8- with Ru3+ in an organic solvent, acts as an efficient heterogeneous catalyst with high turnover frequencies for the aerobic oxidation of alcohols (see Fig. 4.63). Among the solvents used 2-butyl acetate was the most... 

H4Ru4(CO),2] were shown to transform propene into butyraldehyde under a partial CO pressure of 28 bar (PcjH. 12 bar) at 100°C for 10 hours in the presence of a large excess of aqueous trimethylamine. These catalytic systems are more active for the water gas shift reaction (turnover frequency 340 hour" ) than for the production of 0x0 compounds( 12 hour" ). However, no indications about the nature of the ruthenium species produced in basic media were given. 

When the total concentration in ruthenium was decreased, the catalytic activity fell off indicating that cluster catalysis was occurring (46). Moreover, Laine has found a synergistic effect between ruthenium and iron. Indeed, whereas the turnover frequencies displayed by [Fe3(CO),2] and [Ru3(CO),2] for the hydroformylation of pent-l-ene were, respectively, 38 and 40 hour , a 1 1 mixture of [Fe3(CO),2] and [Ru3(CO),2] gave a value of 230 hour . 

Modifiers have also been used to influence the selectivity of vapor-phase partial hydrogenations of benzene. The presence of ethylene glycol increased reaction selectivity with a ruthenium black catalyst from 7% to 41% while the turnover frequency (TOF) decreased from 31 to 3. Pyridine also increased selectivity in the short term, but prolonged use poisoned the catalyst. Passivating a ruthenium black catalyst with caprolactam not only stabilized the catalyst toward deactivation but also increased reaction selectivity from 7% to 20%. 

The authors conclude that the rather low turnover frequencies of FT catalysts must be interpreted, at least in the case of ruthenium, as a low... 

These reactions have been investigated since the 1970s. Palladium, rhodium, nickel, ruthenium, and iridium complexes proved to be active catalysts, but at first with only low conversions. In the 1990s, these reactions were picked up again, first by Graf and Leitner [84] with Rh catalysts, then by Jessop and co-workers [85] and Baiker and co-workers [86] with Ru complexes. An enormous increase in activity was achieved in the synthesis of formic acid a turnover frequency (TOF) of 95 000 h 1 was obtained and for DMF a TOF of 370 000h 1. 

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