Initiation metal carbonyls

The sonochemistry of solutes dissolved in organic Hquids also remains largely unexplored. The sonochemistry of metal carbonyl compounds is an exception (57). Detailed studies of these systems led to important mechanistic understandings of the nature of sonochemistry. A variety of unusual reactivity patterns have been observed during ultrasonic irradiation, including multiple ligand dissociation, novel metal cluster formation, and the initiation of homogeneous catalysis at low ambient temperature (57). 

Weitz and co-workers extended gas phase TRIR investigations to the study of coordinatively unsaturated metal carbonyl species. Metal carbonyls are ideally suited for TRIR studies owing to their very strong IR chromophores. Indeed, initial TRIR work in solution, beginning in the early 1980s, focused on the photochemistry of metal carbonyls for just this reason. Since that time, instrumental advances have significantly broadened the scope of TRIR methods and as a result the excited state structure and photoreactivity of organometallic complexes in solution have been well studied from the microsecond to picosecond time scale. ... 

In 1981, the first report on the sonochemistry of discrete organometallic complexes demonstrated the effect of ultrasound on iron carbonyls in alkane solutions (174). The transition metal carbonyls were chosen for these initial studies because their thermal and photochemical reactivities have been well characterized. The comparison among the thermal, photochemical, and sonochemical reactions of Fe(CO)5 provides an excellent example of the unique chemistry which homogeneous cavitation can... 

In addition to clusterification, ligand substitution also occurs for Fe(CO)5, and in fact for most metal carbonyls. This has proved useful as a mechanistic probe of the reactive species formed during cavitation. Sonica-tion of Fe(CO)5 in the presence of phosphines or phosphites produces Fe(CO)5 L (n = 1,2, and 3). The ratio of these products is independent of length of sonication the multiply substituted products increase with increasing initial [L] Fe(CO)4L is not sonochemically converted to Fe(CO)3L2 on the timescale of its production from Fe(CO)5. These observations are consistent with the same primary sonochemical event... 

As shown in Fig. 13, a variety of metal carbonyls upon sonication will catalyze the isomerization of 1-pentene to cis- and tram-2-pentene (186). Initial turnover rates are about 1-100 mol 1-pentene isomerized/mol of precatalyst/hour, and represent rate enhancements of 102 5 over thermal controls (174). The relative sonocatalytic and photocatalytic activities of these carbonyls are in general accord. An exception is Ru3(CO)12, which is... 

The possible mechanisms which one might invoke for the activation of these transition metal slurries include (1) creation of extremely reactive dispersions, (2) improved mass transport between solution and surface, (3) generation of surface hot-spots due to cavitational micro-jets, and (4) direct trapping with CO of reactive metallic species formed during the reduction of the metal halide. The first three mechanisms can be eliminated, since complete reduction of transition metal halides by Na with ultrasonic irradiation under Ar, followed by exposure to CO in the absence or presence of ultrasound, yielded no metal carbonyl. In the case of the reduction of WClfc, sonication under CO showed the initial formation of tungsten carbonyl halides, followed by conversion of W(C0) , and finally its further reduction to W2(CO)io Thus, the reduction process appears to be sequential reactive species formed upon partial reduction are trapped by CO. 

The generation of the initial metal-carbon bond in the catalytic cycle by reaction of methyl iodide with a metal carbonyl-containing species has been proposed as a key step in both the cobalt (2) and rhodium (4) catalyzed systems. 

These observations illustrate that there are two transformations open to metallocarboxylic acid intermediates reversible loss of OH" accompanied by oxygen exchange, and metal-hydride formation with expulsion of C02. Our entry into this area of chemistry was in 1975 when extensive studies of oxygen lability in metal carbonyl cations were initiated (10). These... 

This was the first example in which models for presumed Fischer-Tropsch intermediates have been isolated and their sequential reduction demonstrated. Neither methane nor methanol was observed from further reduction of the methyl and the hydroxymethyl complexes. The use of THF/H20 as solvent was crucial in this sytem in THF alone CpRe(C0)(N0)CH3 was the only species observed, probably because the initial formyl complex was further reduced by BH3.— When multihydridic reagents are reacted with metal carbonyl complexes, formyl species are usually not observed. The rapid hydrolysis of BH3 by aqueous THF allowed NaBH to act as a... 

Depending on the concentration of agent vapor, the effects begin to appear 1-2 minutes after initial exposure. Pulmonary edema caused by inhalation of metal carbonyls may be delayed for several hours. 

Indirect electrosynthesis of reactive formyl transition metal compounds involves an initial step of reduction of metal carbonyls to radicals followed by transfer of a hydrogen atom from trialkyltin hydrides190. Electroreduction of metal carbonyls yields products of dimerization and loss of CO from the radical anion. Electroreduction in the presence of R3SnH yields the formylmetalcarbonyls ... 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

The reactions between cyclopropenes and carbon monoxide in the presence of transition metals have been of some use in synthesis,93 and in 1978 Binger initiated a study of the reactions between metal carbonyls and cyclopropenes in order to elucidate the generality of these reactions.75 It was found that dicarbonyl 775-cyclopentadienyl(tetrahydrofuran)manganese(I) reacted with 3,3-dime thy Icy clopropene at 0°C to produce -(vinylketene) complex 81 in fair yield. The only other transition metal in Binger s study that was found to react with 3,3-dimethylcyclopropene in this manner was iron (see Section VI,B). 

Our efforts in this area of catalysis began in 1980. Our initial emphasis was on the preparation of supported phase transfer catalysts. We later became interested in the chemistry of anioni-cally activated alumina(25) and the reactivity of metal carbonyl anions prepared under these conditions. A brief description of our work in the preparation of these materials and their synthetic applications follows. 

Supported ruthenium catalysts prepared from Ru3(CO),2 have been used in CO hydrogenation because of the highly dispersed metallic phase achieved when this carbonyl-precursor is used [70,107-109]. However, under catalytic reaction conditions the loss of ruthenium from the support could take place, ft has been reported that at low temperatures it takes place through the formation of Ru(CO)s species, whereas at high temperature dodecarbonyl formation occurs [110]. Decarbonylation of the initial deposited carbonyl precursor under hydrogen could minimize this problem [107]. 

I propose to develop and apply such methods, based on ultrafast X-ray absorption spectroscopy, to study the ultrafast molecular motions of organometallics in solutions. In particular, initial studies will focus on photo-induced ligand dissociation and substitution reactions of transition metal carbonyls and related compounds in various solvent systems. 

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