Prince, Jade Olivia; (2002) Synthesis of trinuclear osmium clusters and a study of their fluxional processes. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

This thesis describes the synthesis, structure (based in IR, NMR and XRD methods) and fluxional properties (by variable-temperature NMR studies) of novel trinuclear osmium clusters, containing monophosphine and diphosphine ligands. The migration of ligands (hydride, carbonyl, etc) in transition-metal clusters can be localised (at a single atom) or delocalised (by transfer between metal atoms). A study of the energetics of such processes has thrown light on the potential of clusters to retain their stereochemistry or to be fluxional and undergo rapid enantiomerisation. The cluster [Os3(CO)12] reacts with H2S to form the more robust cluster of [Os3(μ-H)2(μ3-S)(CO)9], which in the presence of Me3NO/MeCN, and diphosphine, L, results in the formation of [Os3(μ-H)2(μ3-S)(CO)7(μ-L)]. A wide range of diphosphine ligands were utilised for this area of study; dppm, dppe, dppp, dppn and three chiral ligands, R-R-TolBINAP, R-R-BDPP and R-(+)- PROPHOS. Where appropriate 1H and 31P variable temperature NMR studies were performed and the exchange rates and mechanisms were determined using the computer program gNMR. Cluster [Os3(μ-H)2(μ3-S)(CO)9] will react, in the presence of Me3NO/MeCN, with monophosphines, L, to form clusters of the type of [Os3(μ-H)2(μ3-S)(CO)8(μ-L)]. Several monophosphine ligands were used in this study: PMe2Ph, PEt3, PCy3 and PPh3. The isomerism and fluxional features of these clusters were studied using variable temperature and 2D-EXSY 1H NMR techniques. Axial-equatorial isomerism and other processes were identified. Abstract Cluster [Os3(CO)12], in the presence of Me3NO/MeCN, reacts with monophosphines PEt2Ph, Pet3 and PCy3 to form clusters of the type [OS3(CO)11(L)]. The carbonyl migration of these clusters was studied by variable-temperature 13C NMR and the conformers present in the crystals of [Os3(CO)11(PEt2Ph)] were studied by XRD and solid-state NMR. Clusters [Os3(CO)11(PEt2Ph)] and [Os3(CO)11(PEt3)] were pyrolysed in refluxing toluene. The properties of the pyrolysis products were then investigated to determine structure and fluxionality.

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