Metal carbonyls selected reactions

Reactions 24.18-24.23 illustrated conversions of neutral carbonyl compounds to carbonylate anions. Reduction by Na is typically carried out using Na/Hg amalgam. With Na in liquid NH3, highly reactive anions can be formed (equations 24.47-24.50). 

Hydrido ligands can be introduced by various routes including protonation (equations 24.2 and 24.52), reaction with H2 (reactions 24.53 and 24.54) and action of [BH4] (reactions 24.55 and 24.56). 

Reactions 24.57-24.59 illustrate preparations of selected metal carbonyl halides (see Section 24.9) from binary carbonyls. 

Large numbers of derivatives are formed by displacement of CO by other ligands (see equations 24.29-24.33 and discussion). Whereas substitution by tertiary phosphine ligands usually gives terminal ligands, the introduction of a secondary or primary phosphine into a carbonyl complex creates the possibility of oxidative addition of a P—H bond. Where a second metal centre is present, the formation of a bridging phosphido ligand is possible (reaction 24.60). 

We saw earlier that CO displacement can be carried out photolytically or thermally, and that activation of the starting compound (as in reaction 24.29) may be necessary. In multinuclear compounds, activation of one site can control the degree of substitution, e.g. Os3(CO)ii(NCMe) is used as an in situ intermediate during the formation of monosubstituted derivatives (equation 24.61). 

The IR spectra (see Section 23.2) of highly charged anions exhibit absorptions for the terminal CO ligands in regions usually characteristic of bridging carbonyls, e.g. 1680 and 1471 cm for [Mo(CO)4], and 1665cm for [Ir(CO)3].  

The degradation of Ni(CO)4 or Fe(CO)5 to the respective metal and CO is a means of manufacturing high-purity nickel and iron. The thermal decomposition of Ni(CO)4 is used in the Mond process to refine nickel (see eq. 21.4). Iron powder for use in magnetic cores in electronic components is produced by thermally decomposing Fe(CO)5. The Fe particles act as nucleation centres for the production of particles up to 8 pm in diameter. 

Many of the reaction types discussed in Section 24.7 are represented in the catalytic processes described in Chapter 25. Unsaturated (16-electron)metalcentresplay an important role in catalytic cycles. Selected catalysts or catalyst precursors are summarized below. 

Pd(PPh3 4 Many laboratory applications including the Heck reaction 

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The hydroformylation reaction is carried out in the Hquid phase using a metal carbonyl catalyst such as HCo(CO)4 (36), HCo(CO)2[P( -C4H2)] (37), or HRh(CO)2[P(CgH3)2]2 (38,39). The phosphine-substituted rhodium compound is the catalyst of choice for new commercial plants that can operate at 353—383 K and 0.7—2 MPa (7—20 atm) (39). The differences among the catalysts are found in their intrinsic activity, their selectivity to straight-chain product, their abiHty to isomerize the olefin feedstock and hydrogenate the product aldehyde to alcohol, and the ease with which they are separated from the reaction medium (36). 

Thus, the allyllithium, -sodium or -potassium derivatives are the ones which are most easily generated (Section D.1.3.3.3.1.1.), but they are of limited value in stereoselective carbonyl addition reactions. Usually these reagents need to be tuned" in their reactivity by metal exchange prior to application in order to achieve high selectivities. 

The induction of steric effects by the pore walls was first demonstrated with heterogeneous catalysts, prepared from metal carbonyl clusters such as Rh6(CO)16, Ru3(CO)12, or Ir4(CO)12, which were synthesized in situ after a cation exchange process under CO in the large pores of zeolites such as HY, NaY, or 13X.25,26 The zeolite-entrapped carbonyl clusters are stable towards oxidation-reduction cycles this is in sharp contrast to the behavior of the same clusters supported on non-porous inorganic oxides. At high temperatures these metal carbonyl clusters aggregate to small metal particles, whose size is restricted by the dimensions of the zeolitic framework. Moreover, for a number of reactions, the size of the pores controls the size of the products formed thus a higher selectivity to the lower hydrocarbons has been reported for the Fischer Tropsch reaction. 

Reactions carried out on the surface of inorganic oxides allow convenient high-yield and selective syntheses of various metal carbonyl complexes and clusters, starting from easily available materials (Tables 16.1-16.3). The synthetic procedures are straightforward and the recovery of products is easy. Since the use of a solid as reaction medium is not Umited in the manner in solution by boiling points and by the thermal instabiUty of some solvents, it is possible to work at atmospheric pressure even at rather high temperatures. Therefore, in many cases, yields and pressure are better and lower, respectively, than those of the traditional syntheses in solution (Tables 16.4—16.6). 

The concept that acetic acid can be prepared by carbonylation originated in use of routine acids. Carbonylation of methanol was first practiced in a high temperature and pressure process using boron trifluoride or phosphoric acid. A carbon monoxide pressure of 10,000 psi at 300 C was needed for the reaction (10). Metal salts came to replace acids as carbonylation catalysts. Carbonylation of methanol using a metal carbonyl catalyst was first discovered by Reppe and practised later by BASF. However, the process again required high pressure, 7500-10,000 psi, and the selectivity was low (11-14). 

Halcon has developed a new non-noble metal catalyst for methanol reductive carbonylation (32). It is formed under more moderate conditions (1200 psi, 120 C) and permits a selective reaction at only 1200-1800 psi of reaction pressure. Under these conditions, the catalyst s activity is comparable with noble metal catalyzed carbonylations. The conversion rate is 1.5-3.0 mol/l.hr. and acetaldehyde selectivity is 85%. In concentrated solutions, a considerable portion of product acetaldehyde (20-40%) is converted to its acetal. The acetal can be readily hydrolyzed back to acetaldehyde at 100-150 without catalyst (33). Acetal formation is actually beneficial through prevention of undesirable acetaldehyde condensation reactions. 

A very versatile preparation seems to be the elimination of organotin halides in the reaction of organostannyl-organosilylcyclopentadienes with metal carbonyl halides. This reaction is very selective and only Sn—C bonds are cleaved with formation of 7)5-cyclopentadienyl complexes (7) ... 

Finally, Basset and co-workers (88j) report that impregnation of alumina with metal carbonyl clusters leads to CO reduction through initial H2 formation from CO + OH (or adsorbed water) followed by catalyzed reaction on the surface. Above 250°C, the selectivity of the reaction toward methane formation increases greatly, but so does decomposition of the surface bound clusters. In all cases about half of the carbon monoxide was converted to C02 as would be expected for the production of H2 reducing equivalents. 

A great variety of aza macrocycle complexes have been formed by condensation reactions in the presence of a metal ion, often termed template reactions . The majority of such reactions have inline formation as the ring-closing step. Fourteen- and, to a lesser extent, sixteen-membered tetraaza macrocycles predominate, and nickel(II) and copper(II) are the most widely active metal ions. Only a selection of the more general types of reaction can be described here, and some closely related, but non metal-ion-promoted, reactions will be included for convenience. The reactions are classified according to the nature of the carbonyl and amine reactants. 

To remind ourselves that the proper objects of all catalyst research are more powerful and selective syntheses, we have R. F. Heck s chapter describing a wide range of new organic halide reaction catalyses by metal carbonyls and related catalysts. Physics may be fun but chemistry is our bread and butter, and homogeneous catalysis is an area in which we must expect to give increasing space in our Advances in Catalysis in the future. 

Catalytic Reactions. As the techniques for solid-state n.m.r. continue to improve with the simultaneous improvement in sensitivity and hence speed, there will be a growing trend to look at chemical reactions occurring on or in catalysts. There have already been a number of instances where catalytically stimulated reactions have been studied by 13C n.m.r. - the alkylation of toluene by methanol on X zeolite, for example,138 in which the influence of the cation, Na+ or Cs+, on selectivity was deduced. The adsorption binding and decomposition of various metal carbonyls on A1203 or in zeolites has been studied,139 likewise, the nature and sites of interaction of CO and C02 on X and Y zeolites.140... 

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