Homogeneous catalysis metal-catalyst bonds

Stable transition-metal complexes may act as homogenous catalysts in alkene polymerization. The mechanism of so-called Ziegler-Natta catalysis involves a cationic metallocene (typically zirconocene) alkyl complex. An alkene coordinates to the complex and then inserts into the metal alkyl bond. This leads to a new metallocei e in which the polymer is extended by two carbons, i.e. 

Today, iridium compounds find so many varied applications in contemporary homogeneous catalysis it is difficult to recall that, until the late 1970s, rhodium was one of only two metals considered likely to serve as useful catalysts, at that time typically for hydrogenation or hydroformylation. Indeed, catalyst/solvent combinations such as [IrCl(PPh3)3]/MeOH, which were modeled directly on what was previously successful for rhodium, failed for iridium. Although iridium was still considered potentially to be useful, this was only for the demonstration of stoichiometric reactions related to proposed catalytic cycles. Iridium tends to form stronger metal-ligand bonds (e.g., Cp(CO)Rh-CO, 46 kcal mol-1 Cp(CO)Ir-CO, 57 kcal mol ), and consequently compounds which act as reactive intermediates for rhodium can sometimes be isolated in the case of iridium. 

The reaction between alkenes and synthesis gas (syngas), an equimolar mixture of carbon monoxide and hydrogen, to form aldehydes was discovered in 1938 by Otto Roelen [1,2]. Originally called oxo-reaction , hydroformyla-tion is the term used today. This reflects the formal addition of formaldehyde to the olefinic double bond. Commercially, homogeneous metal complexes based on cobalt and rhodium are used as catalysts. With more than 10 million metric tons of oxo products per year, this reaction represents the most important use of homogeneous catalysis in the chemical industry. 

The breaking of carbon-to-phosphorus bonds is by itself not a useful reaction in homogeneous catalysis. It is an undesirable side-reaction that occurs in systems containing transition metals and phosphine ligands and that leads to deactivation of the catalysts. Two reaction pathways can be distinguished, oxidative addition and nucleophilic attack at the co-ordinated phosphorus atom (Figure 2.35). 

The author hopes that this chapter has convinced the readers of the value of homogeneous catalysis for the synthesis of organophosphorus compounds and for organo-heteroatom compounds in a broader sense. Hydrosilylation and hydroboration are indispensable modern synthetic reactions in this category. The H-P addition reactions herein described joins them as a third member. Although this chapter does not cover, the addition reactions of the S-P and Se-P bonds in thiophosphates [39] and selenophosphates [40] to alkynes also proceed in the presence of transition metal catalysts. In view of the wide use of phosphorus compounds, the new procedures will find practical applications. 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

Hydrogen addition to multiple bonds is catalyzed by certain complex metal salts in solution. This may be described as homogeneous catalysis and, compared to heterogeneous catalysis, is a relatively new development in the area of hydrogenation reactions. Rhodium and ruthenium salts appear to be generally useful catalysts ... 

Heterometal alkoxide precursors, for ceramics, 12, 60-61 Heterometal chalcogenides, synthesis, 12, 62 Heterometal cubanes, as metal-organic precursor, 12, 39 Heterometallic alkenes, with platinum, 8, 639 Heterometallic alkynes, with platinum, models, 8, 650 Heterometallic clusters as heterogeneous catalyst precursors, 12, 767 in homogeneous catalysis, 12, 761 with Ni—M and Ni-C cr-bonded complexes, 8, 115 Heterometallic complexes with arene chromium carbonyls, 5, 259 bridged chromium isonitriles, 5, 274 with cyclopentadienyl hydride niobium moieties, 5, 72 with ruthenium—osmium, overview, 6, 1045—1116 with tungsten carbonyls, 5, 702 Heterometallic dimers, palladium complexes, 8, 210 Heterometallic iron-containing compounds cluster compounds, 6, 331 dinuclear compounds, 6, 319 overview, 6, 319-352... 

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