2014 Theses Doctoral

Synthesis and Coordination Chemistry of Oxygen Rich Ligands: Bis(oxoimidazolyl)hydroborato, Tris(oxoimidazolyl)hydroborato and Tris(2-pyridonyl)methane

Al-Harbi, Ahmed Baker

In Chapter One, the sodium salt of tris (2-oxo-1-t-butylimidazolyl) hydroborate, [Toᴮᵘᵗ]Na, as an [O₃] donor ligand has been prepared. The yield for this reaction was low because there is a significant amount of side product in which the double bond of the oxoimidazole starting material is reduced. Treatment of sodium borohydride with bezannulated oxoimidazole at high temperature leads to the generation of the sodium salt of tris (2-oxo-1-R-methylbenimidazolyl) hydroborate in high yield, [Toᴿᴮᵉⁿᶻ]Na. These ligands have been prepared with different alkyl substituents, methyl, t-butyl and adamantyl, to achieve the desired steric environment. Furthermore, these benzannulated ligand have been used to synthesize a series [Toᴿᴮᵉⁿᶻ]Tl complexes, which exist as a discrete mononuclear complexes in the solid state. Finally, [Toᴿᴮᵉⁿᶻ]Tl complexes are more pyramidal than the sulfur counterpart, [Tmᴿᴮᵉⁿᶻ]Tl, but less pyramidal than those in the tris (pyrazolyl)hydroborato counterpart, [Tpᴿ𝄒ᴿ]Tl.

In Chapter Two, the properties of [Toᴿ] ligands have been evaluated versus related L₂X ligands. [Toᴿ] ligands are substantially more sterically demanding than the corresponding [Tmᴿ] sulfur donor ligand and related [O₃] donor ligands. However, electronically, the [Toᴿ] ligands exhibit weaker electron donating properties than other L₂X type ligands. Finally, the coordination chemistry of [Toᴿ] ligands with various metal compounds has been briefly investigated.

The synthesis of a new class of bidentate ligands has been detailed in Chapter Three. Namely the bis(2-oxo-1-t-butylimidazolyl)hydroborato and bis (2-oxo-1-alkylbenzimidazolyl)hydroborato, [Boᴮᵘᵗ] and [Boᴿᴮᵉⁿᶻ], have been synthesized via the reaction of MBH₄ with two equivalents of the respective 2-imidazolone. Chelation of [Boᴮᵘᵗ] and [BoᴹᵉBenz] to a metal center results in a flexible 8-membered ring that is capable of adopting a "boat-like" conformation that allows for secondary M••• H—B interactions.

Chapter Four describes the synthesis of [Boᴿᴮᵉⁿᶻ]₂Zr(CH₂Ph)₂ and [Toᴿᴮᵉⁿᶻ]Zr(CH₂Ph)₃ with different alkyl substituents. Treatment of [Toᴮᵘᵗᴮᵉⁿᶻ]Zr(CH₂Ph)₃ with ([PhNHMe₂][B{C₆F₅)₄]) in a coordinating solvent, Et₂O, generates {[Toᴮᵘᵗᴮᵉⁿᶻ]Zr(CH₂Ph)₂(OEt₂)}{B(C₆F₅)₄} which exhibit a very low activity for ethylene polymerization. However, a coordinatively unsaturated cationic zirconium alkyl complex was obtained by the treatment of ([PhNHMe₂][B{C₆F₅)₄]) with [Toᴮᵘᵗᴮᵉⁿᶻ]Zr(CH₂Ph)₃ or [Toᴬᵈᴮᵉⁿᶻ]Zr(CH₂Ph)₃ which generate [Toᴮᵘᵗᴮᵉⁿᶻ]Zr(CH₂Ph)₂[B(C₆F₅)₄ or [Toᴬᵈᴮᵉⁿᶻ]Zr(CH₂Ph)₂[B(C₆F₅)₄], respectively. Moderate activity for ethylene polymerization was obtained for t-butyl while high activity was obtained for the adamantyl derivatives.

Finally, Chapter Five describes the synthesis of new oxygen-rich ligands, namely tris (2-pyridonyl)methane, [Tpomᴿ]H. They are obtained via the reaction of 2-pyridones with CHX₃ and K₂CO₃ in the presence of [Buⁿ₄N]Br, followed by acid-catalyzed isomerization with camphorsulfonic acid. These compounds provide access to a new class of L₃X alkyl ligands that feature oxygen donors and are capable of forming metallacarbatranes, as exemplified by [Κ⁴-Tpomᴮᵘᵗ]ZnOC₆H₄Buᵗ. In addition, the [Tpomᴮᵘᵗ] ligand also allows isolation of a monovalent thallium alkyl compound, [Tpomᴮᵘᵗ]Tl, in which the Tl—C bond is long and has little covalent character.