HOs3(CO)10(C2H5): an alkyl complex with observable .alpha.,.beta. and reductive elimination processes

Homogeneous Hydrogenation by Defined Metal Clusters

“…No attempt was made then to identify reaction intermediates or the catalytic mechanism, although common pathways were presumed for both complexes. The same clusters [Ru 3 (CO) 12 [33][34][35][36][37][38] were evaluated by the group of Sappa and found to hydrogenate tert-butylacetylene and diphenylacetylene at 120 8C and 1 atm H 2 (TOF 200-400 h -1 ) to a mixture of cis-and trans-stilbene; complete hydrogenation to the alkane was not observed in this case, and the dihydride displayed the slowest rate. The direct participation of cluster structures in the catalytic cycle was suggested by the isolation of some intermediates that could be used as catalyst precursors with essentially the same activity.…”

“…Early investigations by Shapley [12], Basset [13], and Gladfelter [14] Basset's proposal for the silica-supported cluster [Os 3 (CO) 10 (l-H)(l-OSi)] was made on the basis of surface IR spectroscopy studies, kinetic and gas uptake measurements, and reactions of the soluble analogue [Os 3 (CO) 10 (l-H)(l-OSi-Ph)]; the supported catalyst hydrogenated ethylene at 90 8C and atmospheric pressure in a flow reactor at a TOF of 144 h -1 for extended periods of time, achieving up to 24 000 turnovers overall. Gladfelter also used kinetic measurements and IR spectroscopy to deduce the mechanism of alkene hydrogenation by anionic clusters containing isocyanate ligands [Ru 3 (l-NCO) (CO) 10 ] -; this catalyst reduced 3,3-dimethylbutene at rates of about 300 to 360 turnovers h -1 under ambient conditions.…”

“…The same group also characterized intermediates and individual reactions of the more stable, but catalytically less active, osmium analogue [Os 3 (l-NCO)(CO) 10 (1)-(3); in particular, a first-order dependence of the reaction rate on cluster concentration was taken as evidence of cluster catalysis, although the hydrogenation cycle involved a single Ru atom [16]. 12 I] -(TOF 63 h -1 ), pointed to cluster fragmentation as being responsible for the catalytic activity of these systems, which conclusion was based on a complex rate-dependence on cluster concentration and the similarity of the hydrogenation rate when a mononuclear Os complex was employed [17].…”

“…The suggestion was that the reactions proceed through cluster intermediates, although CC bond hydrogenation was accompanied by extensive isomerization and no details of reaction mechanisms were provided. Clusters derived from [H 4 Ru 4 (CO) 12 ] by substitution with chiral diphosphines exhibited reasonable activities (TOF up to 60 000 h -1 ) and moderate enantioselectivities (6 to 46% ee) in the hydrogenation of a, b-unsaturated carboxylic acids. Although the clusters were generally "recovered intact" at the end of the reactions, the participation of mononuclear species cannot be ruled out [21,22].…”

“…[Ir 4 (CO) 11 12 ] selectively reduce 1,5-cyclooctadiene to cyclooctene with high activities [average turnover number (TON) 2816], in contrast with mononuclear or metallic Ir catalysts, which quickly yielded the fully reduced product cyclooctane; this is strongly indicative of a cluster-catalyzed reaction. All the complexes lose the phosphine ligands during the course of the reactions to produce a common active species likely related to [Ir 4 (CO) 5 (C 8 H 12 ) 2 (C 8 H 10 )]; an "anchor-type" interaction between the two CC bonds of the diene to one Ir atom allows the activation and hydrogenation of only one of those bonds by the cluster.…”