Synthesis and Coordination Chemistry of the Arsacyclopentadiene Ligand

Abstract

This thesis describes investigations into coordination complexes of 1H-arsacyclopentadiene (arsole) and -arsacyclopentadienyl (arsolyl) ligands, the chemistry of which has only been sporadically and non-systematically explored in the interregnum since their discovery in the late 1950's. In chapter 1, the first examples of As-ethynyl and -butadiynyl arsoles are described. These react with classic Ru(0) and Ru(II) reagents to afford the expected products of oxidative addition, hydrometallation and tautomerisation, affording the first examples of arsolylalkenyl, -acetylide and -vinylidene complexes. Both arsoles react with dicobalt octacarbonyl to provide the expected pi-alkyne adducts. Attempts to deprotonate the terminal carbon instead leads to nucleophilic substitution at the arsenic atom. In chapter 2, the first examples of cobalt complexes containing arsole ligands are described. These are thermally sensitive and mild heating furnishes pentahapto-arsolyl Co(I) species which contain both Lewis basic arsenic and cobalt centres. Accordingly, these function as novel ditopic metalloligands toward a small selection of Lewis acidic late transition metal reagents. Attempts to prepare the pentahapto-arsolyl complexes via a direct route instead produced new monohapto species. The first definitive example of a tetrahapto arsole ligand was also encountered. In chapter 3, monohapto-arsolyl complexes of Mo(II), W(II) and Fe(II) are prepared via transmetalation from the corresponding stannyl reagent, a facile process for the Group 6 metals but less-so for iron. The arsenic function is readily alkylated though the ultimate product(s) of the reaction depends on the basicity of the arsole ligand. Reactions with electron-poor alkynes leads not to cycloaddition compounds, but instead novel spiro-arsolylpropenoyl species of Mo(II). In chapter 4, bi- and trimetallic complexes of the bridging arsolyl moiety are prepared from a monohapto-arsolyl Mo(II) metalloligand, exploiting the Lewis basicity of the heteroatom to coordinate extraneous labile transition metal reagents. In one case, utilisation of the arsolyl diene function was observed which afforded the first example of a seven-electron arsole ligand which has partial arsinidene character. In another case, the vulnerability of metal-arsenic covalent bonds toward homolytic cleavage results in the first example of a biarsolyl complex. In chapter 5, the base-induced migration of the arsolyl ligand to an adjutant cyclopentadienyl ring is demonstrated, which is not only the first example of an arsenide group undergoing migration but of any entity which retains a lone pair. The corresponding Mo(II)-centred anion was isolated and characterised, and treatment with a small selection of simple electrophiles affords the neutral As-cyclopentadienyl arsole complex. Treatment with metal halide reagents gives metal-metal bonds supported by the pendant arsole donor. In one case, capture of the arsole ring was observed producing a new pentahapto species. In chapter 6, three bis(arsolyl)ferrocenes were prepared and their coordination faculty demonstrated. The most nucleophilic of these was utilised to prepare a selection of classic mono- and bidentate coordination complexes, several of which have obvious catalytic allure. No fewer than 100 new arsole compounds are described and characterised in this thesis, of which more than 80 have had their crystal structures determined. These augment the visible paucity of arsole and arsolyl transition metal complexes previously described, especially with regards to X-ray structural data. Show more