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Yttrium amidate complexes : fundamental reactivity and applications in catalysis and polymerization Stanlake, Louisa Janet Easton

Abstract

Rare-earth complexes are attractive catalyst systems due to their low cost, low toxicity and high reactivity. Modular ligand sets are ideal for complex formation since the steric and electronic properties of the resultant metal complexes can be easily varied. This thesis explores the structure and reactivity of new yttrium amidate complexes, which combine the highly reactive metal with the modular amidate ligand set. A library of tris, bis and mono(amidate) yttrium complexes have been directly synthesized from yttrium tris(trimethylsilyl)amidate and simple amide proligands. The tris(amidate) yttrium complexes are highly active initiators of ring-opening polymerization of Ɛ-caprolactone, yielding some of the largest molecular weight values for poly(Ɛ-caprolactone) reported. The initiation of this polymerization is proposed to be ligand initiated; however, a side-reaction is postulated where formation of a Ɛ-caprolactone-enolate yttrium complex results in broad polydispersity values of the resultant polymers. The bis(amidate) yttrium complexes are also excellent precatalysts for the hydroamination of aminoalkenes. Simple modification to the amidate backbone to include electron-withdrawing groups was found to significantly enhance reaction efficiency. These catalysts can mediate cyclohydroamination with both primary and secondary amine containing substrates. The mono(amidate) yttrium complexes were also investigated as novel precursors for the synthesis of the elusive terminal yttrium imido complex. Mixed anilido/amidate yttrium complexes were synthesized in high yield and a-H abstraction and deprotonation reactions were attempted in the hopes of isolating a crystalline compound. The addition of monodentate and neutral donors was required for isolation and characterization of the key reactive intermediates. This new family of yttrium complexes has proven to be very successful in preliminary catalytic studies. The ease with which the complexes can be synthesized and their steric and electronic properties make these complexes ideal for further catalytic investigations.

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