Synthesis and Reactivity of Gold(III) Complexes

Abstract

The interest in organogold compounds continues to grow. Gold(III) complexes are being investigated as catalysts for organic transformations as well as tested as potential anti-cancer drugs. Despite this wide-ranging interest in the properties of such complexes, the synthetic methods for preparing them are underdeveloped. Thus, Chapter 2 discusses the synthesis of cyclometalated gold(III) complexes bearing the C–N chelating ligand 2-(p-tolyl)pyridine (tpy). Monoalkylation and - arylation were possible by use of Grignard reagents, whereas alkyl and aryl lithium reagents gave the dialkylated and diarylated gold(III) complexes. By a combination of the two alkylation procedures, mixed alkyl/aryl complexes of the type AuMePh(tpy) were obtained and both isomers were available. Chapter 3 discusses the reactivity of the cyclometalated gold(III) complexes towards different gases such as carbon monoxide and oxygen. Most of the cyclometalated gold(III) complexes prepared react with acids. The monoalkylated and -arylated complexes of the type AuBrR(tpy) (R = Me, Et, CHCH₂, CCH, Ph) react with silver(I) salts to give a potential open coordination site at gold(III). Ethylene formally inserts into the Au–O bond trans to nitrogen in the chelating C–N ligand of the complex Au(OCOCF₃)₂(tpy) (62) in trifluoroacetic acid or dichloromethane, to yield Au(CH₂CH₂OCOCF₃)(OCOCF₃)(tpy) (94). In trifluoroethanol, a slightly different complex resulted due to nucleophilic attack by trifluoroethanol rather than trifluoroacetate, Au(CH₂CH₂OCH₂CF₃)(OCOCF₃)(tpy) (95). The mechanism of the insertion was investigated experimentally as well as computationally and the results are discussed in Chapter 4. The formal insertion takes place with alkenes other than ethylene, and alkynes react too. A key step in the catalytic reactions involving gold(III) is assumed to be the coordination of a C–C multiple bond to the gold centre. Various catalytic cycles involving a gold(III) π–complex have been proposed. However, gold(III) alkene, alkyne, allene, or arene complexes have until recently not been conclusively detected and characterised. Chapter 5 discusses the first, and thus far only, crystallographically characterised gold(III) alkene complex, Au(cod)Me₂BArF (133– BArF, BArF = tetrakis[3,5–bis(trifluoromethyl)phenyl]borate, cod = 1,5-cyclooctadiene).