Reactivity and hemilability of ortho-phosphinoaniline complexes of rhodium and ruthenium

• Main Author: Hounjet, Lindsay
• Other Authors: Cowie, Martin (Chemistry)
• Format: Others
• Language: en
• Published: 2011
• Subjects: Hemilabile; Complexes; Rhodium; Ruthenium; Inorganic; Organometallic; Catalysis; Hybrid Ligands
• Online Access: http://hdl.handle.net/10048/1831

Molecular transition metal catalysts offer unique potential for the production of fine chemicals. Chemical processes carried out in the presence of well defined molecular catalysts often only require mild, easily accessible conditions, fewer sacrificial reagents, and can selectively produce a desired product with minimal waste. The active site of a transition metal catalyst can be varied by the use of a hybrid ligand, which employs a combination of groups with different binding affinities for the metal center. Hybrid ligands possessing both substitutionally inert and labile donors, called “hemilabile” ligands, offer an added dimension to catalysis since the weakly binding donor can be displaced from the metal center by a substrate to facilitate the chemical transformation. However this labile donor, in conjunction with an inert donor, can also offer chelate stabilization of the catalyst in the event of coordinative unsaturation at the metal center, a feature which can serve to enhance catalyst longevity. A major goal of the research reported herein is to understand the mechanisms by which hemilabile processes occur within ortho-phosphinoaniline complexes of rhodium and ruthenium and, in turn, how such features might affect catalytic characteristics. To this end, a comparison of catalytic activities of related hemi- and non-labile complexes has been carried out. The ability for two metal atoms held in close proximity to have a cooperative effect on substrate activation or catalysis has also inspired the generation of a series of binuclear compounds bridged by bis(ortho-phosphinoaniline) ligands. In addition to hemilabile and catalytic features, many unique ligand geometries and coordination modes are also observed, particularly by altering the substituents on labile amine donors. Non-labile complexes can also be prepared by deprotonation of labile amine donors to produce ortho-phosphinoanilido species, which display reactivity patterns and structural features distinct from the those of their hemilabile congeners. The amido complexes, which are effective toward ketone transfer hydrogenation and olefin silylation reactions, display interesting features, and in the first case, the possibility of a reaction mechanism unprecedented for transition metal catalysts is discussed. Evidence supporting the operation of such an unexpected mechanism could have important implications for the design and operation of new and more effective transition metal catalysts. === Chemistry