Whittaker, Caroline; (1994) Some oxidative addition reactions of triosmium clusters. Doctoral thesis (Ph.D.), University College London.

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Abstract

This thesis describes the chemistry of the triosmium carbonyl cluster [OS3(CO)10(MECN)2] with phosphorus ylides and with nitrogen heterocycles. The aim of the work was to synthesize triosmium clusters displaying novel modes of attachment of ligands. The reaction of [Os3(CO)10(MeCN)2] with the phosphorus ylide Ph3P=CHCHO resulted in five products, all characterised spectroscopically and by X-ray diffraction, from either thermal or photochemical routes. Initial oxidative addition of Ph3PCHCHO gave two light sensitive isomeric clusters [OS3(μ-H)( μ-Ph3PCCHO)(CO)10] and [Os3(μ-H)(μ-Ph3PCHCO)(CO)10]. The first isomer decarbonylates in daylight to give a μ3-nonacarbonyl species where the ligand is coordinated through an alkylidene bridge and a formyl group. This nonacarbonyl species further decarbonylates thermally to give an octacarbonyl species [Os3(μ-H)2(μ3-C6H4PPh2CCHO)(CO)8] by ortho-metallation at one phenyl ring of the triphenylphosphonium group. The second decacarbonyl isomer also decarbonylates in daylight to give two isomers of [Os3(μ-H)(μ3-Ph3PCHCO)(CO)9] which differ only in the stereochemistry about the carbon atom Ph3PCHCO. The X-ray structure of one isomer was determined. Cyclopentadienylidenetriphenylphosphorane (Ph3PC5H4) reacts with [Os3(CO)10(MeCN)2] to give the cluster [Os3(μ-H)( μ-Ph3PC5H3)(CO)10] which contains the C5 ring bonded through a single carbon atom to two osmium atoms. In solution the compound exists as two slowly interconverting decarbonyl isomers differing only in the substitution position of the phosphonium group. Decarbonylation gave an isomeric mixture of a noncarbonyl complex which contains the C5 ring coordinated to the three osmium atoms though two σ Os-C bonds and an η2-bond, which is a new type of cyclopentadienylidene coordination. The X-ray structures of both the deca- and nonacarbonyl species were determined. The reactions of triosium clusters with nitrogen heterocycles were examined to establish whether the chemistry always initiates at the nitrogen lone pair of electons on the ligands, whether non-hydride species may be formed and to compare the occurrence of C-H versus N-H cleavage during reaction. The initial reaction of benzotriazole, indazole, 1,2,3-triazole, pyrazole, pyrimidine and nicotine all initially gave decacarbonyl hydrido-species of the type [OS3H(ligand)(CO)10] but the compounds were resistant to thermal decarbonylation. However, the cluster [Os3H(C6H4N3)(MeCN)(CO)9] was synthesised from the corresponding decacarbonyl and shown to lose MeCN and dimerize thermally to give the new compound [Os6H2(C6H4N3)2(CO)18] which exhibits an unusual mode of bonding. The X-ray structure of the dimer was determined.

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