Osmium complexes as models for CO reduction intermediates

This thesis is concerned with the synthesis, and aspects of the chemistry, of carbon-donor complexes of osmium as organo-metallic models for CO reduction intermediates. In Chapter 1 some aspects of ligand reactivity of the carbon-donor ligands CO, CS, CSe, CTe and carbenes in transition metal complexes are reviewed. The reduction reactions of these ligands are emphasized. The preparation and structure of the osmium ??2-formaldehyde complex Os(??2-CH2O)(CO)2(PPh3)2 is described in Chapter 2. This complex has proved to be a useful synthetic precursor for stable osmium formyl, hydroxymethyl, methoxymethyl and halomethyl (-CH2X, X = Cl, Br, I) complexes and some facets of the reactivity of these ligands have been investigated. A general synthetic route to neutral osmium formyl complexes Os(CHO)X(CO)2(PPh3)2(X = halide or alkyl) has been developed. The facile preparation of a stable example of an intermediate formed during decarbonylation of a simple aldehyde by a transition metal, the osmium monohapto-acetyl-hydrido complex Os(??1-C[O]CH3)H(CO)2(PPh3)2, has been demonstrated. A preliminary study of the reactions of the osmium iodomethyl complex Os(CH2I)I(CO)2(PPh3)2 is reported in Chapter 3. The typical reaction of this species is nucleophilic substitution; in many respects reactivity is analogous to an electrophilic methylidene complex. Reaction with a variety of nucleophiles [e.g. OR-, H-, EH- (E = S, Se, Te), NH2R and PR3] has been investigated and the ligand reactivity of some of these derivatives studied. A synthetic route to ??2-CSeS and ??2-CSe2 complexes of osmium without the use of molecular CSeS or CSe2 has been developed and the isolation of the geometrical isomers of the ??2-CSeS complex Os(??2-CSeS)(CO)(CNR)(PPh3)2 (arising from ??2-C,S or ??2-C,Se coordination) has been achieved. The synthesis of a stable osmium hydrido-selenocarbonyl complex, OsHCl(CO)(CSe) (PPh3)2, has allowed the direct observation of hydride transfer from metal to CSe ligand. These latter results are discussed in Chapter 4.