Synthesis, structure and reactivity of B-diketiminato supported organoscandium cations
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AbstractA variety of β-diketiminato supported mononuclear dialkyl scandium compounds have been synthesized and fully characterized in both the solid and solution states. The 4-coordinate species exhibit fluxional behaviour in solution which has been quantitatively studied and is interpreted as a dynamic process which equilibrates two structures via a C2v symmetric transition state. At elevated temperatures these compounds have been determined to undergo an intramolecular metallation process with loss of RH. Polymerization experiments revealed that several of the organoscandium complexes are effective catalysts for the polymerization of ethylene. The reactivity of the neutral dialkyl organoscandium species was examined leading to the synthesis of a family of highly reactive organoscandium alkyl cations. Deactivation pathways, most notably C6F 5 transfer to the metal centre and metallation of an isopropyl group were observed. While C6F 5 transfer impeded subsequent reactivity studies, metallation proceeded sufficiently slowly so as not to hinder the development of their organometallic chemistry. Quantitative analysis of ion-pair dynamics established that intermolecular anion exchange likely occurs by way of a highly ordered transition state, most likely a low energy associative anion displacement by an incoming arene molecule. A unique class of isolable solvent separated organoscandium methyl cations was synthesized by reaction of dialkyl scandium compounds with [CPh₃][B(C₆F₅)₄]. These complexes have been found to be resistant to both C₆F₅ transfer and metallation decomposition pathways. The molecular structures show stabilization of the alkyl cations through η⁶ arene binding to the metal centre. These aromatic solvent molecules are bound sufficiently loose to permit the study of their chemistry. Quantitative kinetic experiments for arene exchange imply a partially dissociative mechanism whereby the rate-limiting step involves dissociation to lower hapticity. The mechanism for arene displacement was further investigated upon exploring the reactivity of these compounds with diphenylacetylene. In this situation it was determined that rate-limiting coordination of the alkyne to the metal centre had a slightly higher barrier than arene ring slipping. Preliminary work demonstrated that the β-diketiminato ancillary can also be used to prepare organoyttrium species with potential for further derivatization to the corresponding organoyttrium cations.
Bibliography: p. 255-266