Re and Ru complexes

At present, Mo, W, Re and Ru complexes are known to catalyse alkene metathesis [7]. This unique reaction, catalysed by transition metal complexes, is impossible to achieve by other means. Later, based on studies of the reactivities of Fischer-type carbene complexes, it was discovered that carbene complexes are the intermediates in alkene metatheses. WClg reacts with EtAlCl2 to afford the diethyltungsten complex 3 by transmetallation, and subsequent elimination of a-hydrogen generates ethane and the carbene complex 4 which is the active catalyst. 

Stufkens, D. J. Aarnts, M. P. Rossenaar, B. D. Vlcek, A. A new series of Re and Ru complexes having a lowest sigma pi excited state that varies from reactive to stable and long lived. Pure Appl. Chem. 1997, 69, 831-835. 

The first indication that metal phosphine complexes were active for catalytic reactions of alkanes came from Shilov s group finding that CoH3(PPh3)3 catalyzes H/D exchange between and CH4. These systems probably involve oxidative addition (equation 1), but they were not followed up at the time because of the greater interest in the Pt system discussed above. Since 1980, a variety of Rh, Ir, Re and Ru complexes have shown activity for alkane dehydrogenation and carbonylation. The intermediate alkyl... 

ROMP is a chain growth polymerization that converts cydic olefins to a polymeric material in the presence of transition metal-based complexes such as Ti, Mo, W, Ta, Re, and Ru complexes (Scheme 54) 12,329-337 powerful polymerization method is broadly applicable for the preparation of well-defined polymers. Therefore, transformation reactions of both polycydic olefins such as norbomene (NB), norbona-diene, and dicyclopentadiene and low-strain cydic olefins induding cydopentene (CP) and cydoheptene allow extension of the range of attainable block copolymers. 

From the results described above, C02 seems to be reduced to CO in the presence of Re complexes in most cases, while RH complexes give formic acid and Ru complexes give both formic acid and CO, depending on the conditions. 

Common alcohol oxidation methods employ stoichiometric amounts of toxic and reactive oxidants like Cr03, hypervalent iodine reagents (Dess-Martin) and peracids that pose severe safety and environmental hazards in large-scale industrial reactions. Therefore, a variety of catalytic methods for the oxidation of alcohols to aldehydes, ketones or carboxylic acids have been developed employing hydrogen peroxide or alkyl hydroperoxides as stoichiometric oxygen sources in the presence of catalytic amounts of a metal catalyst. The commonly used catalysts for alcohol oxidation are different MoAV(VI), Mn(II), Cr(VI), Re(Vn), Fe(II) and Ru complexes . A selection of published known alcohol oxidations with different catalysts will be presented here. 

A large number of metal complexes have been proved active in electrochemical reduction of C02. Among these, certain Re, Ni, and Ru complexes, mostly the type [Ru(bpy)2(CO)L]"+ (bpy = bipyridyne, L = CO or H, n=l,2), have attracted much attention because of their characteristic reactivity and high efficiency.129 131 The catalytic cycles have been elucidated.132 These complexes, however, cannot be used in aqueous solution because of the competing hydrogen evolution. Re complexes, in turn, when incorporated into Nation membrane, proved to be efficient in formic acid production.133... 

Olefin metathesis is a unique reaction and is only possible by transition metal catalysis. In fact only complexes of Mo, W, Re, and Ru are known to catalyze olefin metathesis. Once it was known that metallocarbenes were the actual catalytic species, a variety of metal carbene complexes were prepared and evaluated as catalysts. Two types of catalysts have emerged as the most useful overall. The molybdenum-based catalysts developed by Schrock and ruthenium-based catalysts developed by Grubbs. 

The earlier development of these reactions were covered by a couple of re views3,306,311. Recent developments include Rh, Ir and Ru complex catalysts in place of Co2(CO)8. 

A comparison of electrochemistry of some of the free polypyridine ligands and their Ru Re and Mo complexes shows [97, 123, 134, 136, 137, 142, 230, 231] that, all other factors being constant, the potential of the first N,N-localized reduction increases in the order ... 

Pd, Pt, Rh, and Ru complexes were used as catalysts for the hydrogenation of alkenes with molecular hydrogen. In many cases, higher activity and enhanced selectivity for the desired reaction were accompanied hy successful re-use of the ionic liquid and the catalyst. Examples are reported for cyclohexadiene [10],... 

Photolysis of diethylthallium bromide in cyclohexane is a radical process involving cleavage of the thallium-carbon bond, which yields ethylcyclohexane and dicyclohexyl, as well as other products. Photoelectron transfer from benzyltributylstannanes to 10-methylacridinium ion results in cleavage of the metal-carbon bond, to give the corresponding benzyl radicals, rather than benzyl cations. Photochemical homolysis of Re- and Ru-alkyl bonds in Re(alkyl)-(CO)3(diimine) and Ru(I)(alkyl)(CO)2(diimine) complexes has been studied by Fourier transform ESR. In related manganese complexes, Mn(R)(CO)3(di-imine), elimination of CO is the predominant pathway when R = methyl, but Mn-alkyl homolysis occurs when R = benzyl. 

The metal carbene/metallacyclobutane mechanism of olefin metathesis, as outlined in Section 1.3, was first proposed by Herisson and Chauvin in 1971. By 1975 the evidence in its favour had become so compelling that the earlier pairwise mechanism had been totally discarded. From 1980 onwards well-defined carbene complexes of Ta, Mo, W, Re, and Ru were discovered which would act as initiators without the need for activation by heat, light, or cocatalyst. This in turn led to the spectroscopic detection of the propagating metal-carbene complexes in many systems, to the detection of the intermediate metallacyclobutane complexes in a few cases, and in one case to the detection of the metal-carbene-olefin complex that precedes the formation of the metallacyclobutane complex. In no individual case have all three intermediates been detected at most two have been observed, sometimes one, more often none. After 1980 metallacyclobutane complexes of Ti and Ta were found which would act as initiators at 60°C, but where the intermediate metal carbene complexes could not be detected. 

Olefin metathesis is a catalytic reaction. A wide range of transition metal compounds will catalyse the reaction, the most important being based on W, Mo, Re and Ru. A problem in the case of functionalized olefins such as unsaturated fatty acid esters is the deactivation of catalytic sites, caused by the complexation of the polar functional group to the active site. This results in turnover numbers that are much lower than those obtained for the metathesis of analogous simple olefins. Relevant catalyst systems will be discussed in Section 4. 

Hydrogenation. Gas-phase catalytic hydrogenation of succinic anhydride yields y-butyrolactone [96-48-0] (GBL), tetrahydrofiiran [109-99-9] (THF), 1,4-butanediol (BDO), or a mixture of these products, depending on the experimental conditions. Catalysts mentioned in the Hterature include copper chromites with various additives (72), copper—zinc oxides with promoters (73—75), and mthenium (76). The same products are obtained by hquid-phase hydrogenation catalysts used include Pd with various modifiers on various carriers (77—80), Ru on C (81) or Ru complexes (82,83), Rh on C (79), Cu—Co—Mn oxides (84), Co—Ni—Re oxides (85), Cu—Ti oxides (86), Ca—Mo—Ni on diatomaceous earth (87), and Mo—Ba—Re oxides (88). Chemical reduction of succinic anhydride to GBL or THF can be performed with 2-propanol in the presence of Zr02 catalyst (89,90). 

Many polypyridyl metallic complexes have been used successfully as electrocatalysts in homogeneous and heterogeneous systems and generally CO and 11 COO are the main products of reduction. Among these, the two complexes, [Re(bpy)(CO)3Cl] and [Ru(bpy)2(CO)X]" + (X = CO, Cl, H 11 2 or 1), have attracted much attention because of their characteristic... 

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