Investigating the chemistry of binuclear chromium and uranium pacman complexes

Abstract

Drawing inspiration from nature where enzymes containing multi-metallic active sites are ubiquitous, chemists have designed various ligands to bind more than one metal in precise structural arrangements. In Chapter One, a class of binucleating Schiff base pyrrole (Pacman) macrocycles which are both straightforward to synthesise and can be varied systematically to alter the metal environment and intermetallic separation are introduced, along with the state-of-the-art in this area. Previously reported complexes of these ligands with late transition metals, lanthanides and actinides are also reviewed. The results and discussion chapters of the thesis focus on the isolation and investigation of previously unexplored early transition-metal Pacman complexes and present new advances in low oxidation state uranium Pacman chemistry. In Chapter Two, binuclear chromium(II) complexes of two Schiff base macrocycles, H4LMe and H4LA are described. [Cr2(LMe)] features an ortho-phenyl spacer between the macrocycle donor compartments whereas the Cr(II) ions are separated by a larger anthracenyl spacer in [Cr2(LA)]. Both compounds have been characterised in solution and the solid state. Reactivity studies were carried out for [Cr2(LMe)]. Reactions of [Cr2(LMe)] with isocyanides and triphenylphosphine oxide were investigated leading to the isolation of the contrasting co-ordination compounds [Cr2(OPPh3)2(LMe)] and [Cr2(μ-CNR)(LMe)] (R = xylyl, tBu). Oxidation of [Cr2(LMe)] with I2 yields the Cr(III)/Cr(III) Pacman products [Cr2(μ- I)(I)(THF)(LMe)] and [Cr2(μ-I)(py)2(LMe)][I] when carried out in THF or pyridine, respectively. Cr(III) alkyl compounds are obtained by reaction of [Cr2(μ-I)(I)(THF)(LMe)] with the non-reducing alkyl transfer reagents MgBrEt and ZnEt2. When ZnEt2 in toluene is employed, one zinc cation is incorporated in the molecular cleft, whereas use of MgBrEt in THF yields the simple chromium alkyl complex [{Cr(Et)}2(endo-THF)(LMe)]. One ethyl group may be abstracted from [{Cr(Et)}2(endo-THF)(LMe)] by [CPh3][B(C6F5)4] to form a cationic alkyl complex. The activity of both the neutral and cationic alkyl species towards ethylene was investigated. Conclusions are discussed at the end of the chapter. Previously, investigation of low oxidation state uranium Pacman chemistry has been confined to the smaller macrocycle, H4LMe, and frequently resulted in the formation of insoluble polymeric materials that were intractable and challenging to analyse. In Chapter Three, metallation of the larger macrocycle, H4LA, with UI3 to generate a single soluble species is described, although this product could not be isolated or characterised in the solid state. A new synthesis of [U(BH4)3(THF)2] from UI3 and NaBH4 affords an alternative U(III) precursor to UI3. Metallation of H4LA using a sodium base and U(BH4)3(THF)2 yields the ionic product [Na(THF)4][{U(BH4)}2(μ-BH4)(THF)2(LA)] which was characterised in solution and the solid state. Reaction of this compound with KO(C6H2(tBu)3) forms the ligand substitution product [{U(OAr)}2(endo-BH4K)(THF)2(LA)] which undergoes selective reaction with excess S8 or CS2 to form [{U(OAr)}2(μ-S2)(LA)] and [{U(OAr)}2(μ-S)(LA)] respectively. It was discovered that the [U(BH4)3(THF)2] metallation strategy could be successfully extended to H4LMe to form [Li(THF)4][{U(BH4)}2(μ-BH4)(LMe)]. Protonolysis of the borohydride ligands of the complexes of the two different macrocycles was investigated using the weak acid [HNEt3][BPh4]. NMR spectroscopy indicated that both exo BH4 − groups in both complexes can be successively removed to generate neutral and cationic complexes but these were not isolated. Metallation of H4LA with UCl4 forms the ionic product [Li(THF)4][{U(Cl)}2(μ-Cl)3(LA)]. Various ligand substitution reactions were attempted but the only structurally characterised product was [{U(OtBu)(Cl)}{U(OtBu)(py)}(μ-Cl)(LA)], formed by reaction with KOtBu. Conclusions are discussed at the end of the chapter. Experimental and characterising data are provided in Chapter Four.