Stabilization of small reactive molecules by coordination to low valent ruthenium and osmium
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. Perhaps the most exciting discoveries of organometallic chemistry have been the isolation of complexes which contain ligands that are traditionally regarded as reactive intermediates. This thesis extends these developments by preparing new complexes which contain small reactive molecules such as phosphine(PH3), phosphido anion(PH2-), alkyl phosphinites(PH2OR), thioxophosphane(HP=S) and methylene(CH2). Stabilization in this manner allows for an examination of the structure and reactivity patterns of otherwise inaccessible species. The methylene fragment is the archetype of such traditional reactive intermediates. A neutral stable terminal methylene complex of divalent ruthenium or osmium has previously eluded preparation and in Chapter 5 the preparation, structure and reactions of the first stable examples of this class, MCl(?2-C[O]R)(=CH2)(PPh3)2 (M = Ru, Os, R = aryl), are described. The migratory insertion reactions of these species are unique and against all expectations the aryl group migrates to the carbonyl and not the methylene. In reactions which may have significance in the Fischer-Tropsch process, carbonylation of these methylene complexes affords metallaoxetene derivatives, [M(=C(R)OCH2)(CO)2(PPh3)2]+, which arise from combination of the methylene and the acyl ligands. Phosphido anions are frequently used synthons and also have an emerging coordination chemistry. Chapters 1, 2, and 3 describe the preparation and reactions of ten new examples of complexes with the simplest possible phosphido anion, PH2. The nucleophilic reactions of two of these complexes, OsCl(PH2)(CO)2(PPh3)2, and Os(?2-O2CMe)(PH2)(CO)(PPh3)2, form the core of the work described in Chapters 2 and 3 respectively. It is from these two synthetically useful compounds that complexes with a variety of unprecedented phosphorus ligands are derived. Very little is known about thioxophosphanes(RP=S) as a class, let alone of the parent species, H-P=S. Therefore, its isolation as a dihapto ligand in the osmium complex Os(?2-PHS)(CO)2(PPh3)2 permits an examination of its chemistry for the first time. As reported in Chapter 4, electrophiles such as Me+ and AuI add to the stereochemically active phosphorus lone pair. Acids, in the presence of alcohols, rupture the P-S bond and result in the formation of a thiol-alkylphosphinite complex [Os(SH)(PH2OR)(CO)2(PPh3)2]+. The most remarkable feature of this reaction is that it is reversible and addition of base to this adduct returns the original ?2-thioxophosphane complex. The alkyl esters of phosphinous acid, H2POR, and thiophosphinous acid, PH2SR, are also unknown in the free state. Chapters 2 and 3 describe the preparation of additional M(II) and M(O) examples of these species: [MH(PH2OR)(CO)2(PPh3)2]+, [OsCl(PH2SMe)(CO)2(PPh3)2]+ and M(PH2OR)(CO)2(PPh3)2. Phosphinidenes(R-P) are the phosphorus analogues of carbenes and Chapter 2 describes the preparation and structure of a ?2-phospha-alkene complex Os(P=C[O]CF3)(CO)2(PPh3)2 which can be regarded as a valence tautomer of a terminal phosphinidene complex. The phosphorus lone pair in this complex is readily protonated, alkylated and coordinated and some of these reactions, as well as certain structural characteristics, can be interpreted in terms of valence tautomerism between phospha-alkene and phosphinidene forms. In Chapter 1 the synthesis of the precursors to these complexes are described in three Parts. All of products described in Chapters 1, 2, 3 and 4 are ultimately derived from two phosphine complexes, MHCl(PH3)(CO)(PPh3)2 (M = Ru, Os). Because of this role, the organometallic chemistry of phosphine is reviewed in Part A while Part B presents the preparation and reactions of these two species, among others. The most useful derivatives of these compounds are prepared by acid cleavage of the metal-hydride bond in the presence of acetonitrile to afford an isomeric mixture of [MCl(PH3)(L')(CO)(PPh3)2]+ (L' = NCMe). A variety of ligands(L' = CO, PMe3, Cl-, MeCO2-) readily substitute for the labile acetonitrile. The forementioned terminal phosphido complexes are then prepared by deprotonation of the positively charged species (L' = CO, PMe3) with base. Deprotonation of the cationic derivative with a labile ligand(L' = NCMe) affords a phosphido bridged dimer, [OsCl(?2-PH2)(CO)(PPh3)2]2. This work demonstrates the utility of coordinated phosphine for the preparation of unusual phosphorus species and it illustrates the considerable ability of low valent osmium and ruthenium to stabilize otherwise reactive species.
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/1988