Polynuclear osmium carbonyl

Conversely, Ti (i.e., in practice linewidth) measurements allied to (estimated) values of the quadrupolar coupling constants have been used to determine r<. and hence to study the motional behavior of various species. Thus t<. increases with the hydrocarbon chain-length in aliphatic ethers, and in acetone 0 measurements can give information not available from C, H, or H owing to rapid methyl group rotation. Cyclic ketones, cyclic ethers, and crown ethers and their complexes have also been studied, as have water reorientation in a lyotropic lamellar phase and motions of the sulfate ion in aqueous solution. Detailed studies of polynuclear osmium carbonyl dynamics have been undertaken, and allied to... 

Several other polynuclear metal carbonyls were tested as catalysts for the reduction of nitrobenzene to aniline using carbon monoxide and water as the reducing agent. Rhodium, iridium and osmium clusters were found to be very effective and they were far less susceptible to oxidative degradation than [Fe(CO)5]. 3... 

Polynuclear metal carbonyl hydrides have also been studied. The IMS spectra of [Os3(CO)io(p2-H)2] and [Os4(CO)i2H4][74] are shown in Fig. 11.21. A twofold bridging hydride has three modes symmetric and antisymmetric stretch and an out-of-plane bend. The stretches are expected to occur at about twice the frequency of the bend. Fig. 11.21a approximates to this expectation, however, there are more bands observed than expected. In particular, the observation of two bands for the out-of-plane bend at 672 and 738 cm shows that the hydrides are coupled. This is surprising since the massive osmium atoms would mechanically decouple the hydride motions. So the coupling is likely to be electronic in nature. 

Eliminations from Os(CO)4RR occur by dinuclear mechanisms only if either R or R is H. A hydride on one metal is necessary to interact with a vacant coordination site on the other in the dinuclear transition state. With Os(CO)4H2, the vacant site is created by dissociation of CO. With Os(CO)4-(H)CH the vacant site is created by a facile rate-determining isomerization which we suggest is to an acetyl hydride. The unique instability of hydridoalkyl carbonyls thus is explained. The synthesis and properties of Os(CO)4(H)C2H and various polynuclear ethyl osmium derivatives show that (3-hydrogens have no significant effect on these elimination mechanisms. Dinuclear hydridoalkyls are excellent starting points for the synthesis of more complex polynuclear alkyls. 

Reaction with carbonyl anions provides a facile route to either mixed-metal polynuclear carbonyls or to the extension of the nuclearity of the osmium series. This approach has been expanded to include a number of the higher polynuclear carbonyl species and provides a very effective method of controlled synthesis. 

The structures of anionic, triple-nuclear osmium and iron cluster catalysts supported on copolymers of styrene and divinylbenzene were analyzed by means of IR spectroscopy. Their catalytic activity during 1-hexene hydroformylation [250] and C5H5NO2 carbonylation [251] were investigated. It was found that isomerization proceeds simultaneously in the presence conventional catalysts. In the absence of moisture, a triple-nuclear osmium complex could be removed from a polymeric support after reaction. This suggests catalytic activity for this complex, particularly in the fixed state. Furthermore, a definite correlation was found to exist between polynuclearity and selectivity of heptanol formation. For iron, however, the cluster structure altered during the course of the reaction. 

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