Synthesis and microstructure of FeCo nanoalloys

M.S.Rina Tannenbau

Investigations of the origin of stereocontrol in syndiospecific Ziegler-Natta polymerizations

In order to expand our understanding of the mechanism of stereocontrol in syndiospecific α-olefin polymerization, a family of Cs-symmetric, ansa-group 3 metallocenes was targeted as polymerization catalysts. The syntheses of new ansa-yttrocene and scandocene derivatives that employ the doubly [SiMe2]- bridged ligand array (1,2-SiMe2)2{C5H-3,5-(CHMe2)2} (where R = t- butyl, tBuThp; where R = i-propyl, iPrThp) are described. The structures of tBuThpY(µ-Cl)2K(THF)2, tBuThpSc(µ-Cl)2K(Et2O)2, tBuThpYCH(SiMe3)2, Y2{µ2-(tBuThp)2}(µ2-H)2, and tBuThpSc(µ-CH3)2 have been examined by single crystal X-ray diffraction methods. Ansa-yttrocenes and scandocenes that incorporate the singly [CPh2]-bridged ligand array (CPh2)(C5H4)(C13H8)(where C5H4 = Cp, cyclopentadienyl; where C13H8 = Flu, fluourenyl) have also been prepared. Select meallocene alkyl complexes are active single component catalysts for homopolymerization of propylene and 1-pentene. The scandocene tetramethylaluminate complexes generate polymers with the highes molecular weights of the series. Under all conditions examined atactic polymer microstructures are observed, suggesting a chain-end mechanism for stereocontrol. A series of ansa-tantalocenes have been prepared as models for Ziegler-Natta polymerization catalysts. A singly bridged ansa-tantalocene trimethyl complex, Me2Si(η5-C5H4)2TaMe3, has been prepared and used for the synthesis of a tantalocene ethylene-methyl complex. Addition of propylene to this ethylene-methyl adduct results in olefin exchange to give a mixture of endo and exo propylene isomers. Doubly-silylene bridged ansa-tantalocene complexes have been prepared with the tBuThp ligand; a tantalocene trimethyl complex and a tantalocene methylidene-methyl complex have been synthesized and characterized by X-ray diffraction. Thermolysis of the methylidene-methyl complex affords the corresponding ethylene-hydride complex. Addition of either propylene or styrene to this ethylene-hydride compound results in olefin exchange. In both cases, only one product isomer is observed. Studies of olefin exchange with ansa-tantalocene olefin-hydride and olefin-methyl complexes have provided information about the important steric influences for olefin coordination in Ziegler-Natta polymerization.</p

Design of FeCo Nanoalloy Morphology via Control of Reaction Kinetics

Nanoalloys are an exciting new class of materials in the growing field of nanotechnology. Nanoalloys consist of the nanoscale co-aggregation of two or more metals with a potential to form compositionally-ordered phases or superstructures that have properties unlike those of the individual metal clusters or of bulk alloys of the constituent metals. This research seizes the opportunity that the nanoscale domain has to offer, and focuses on the synthesis of iron and cobalt nanoalloys via the simultaneous decomposition of iron cobalt organometallic precursors in a stabilizing environment, accompanied by the thorough characterization of the resulting nanoclusters. Zero-valent FeCo nanoalloys may potentially have interesting uses as magnetic materials. Since these clusters have sizes less than the size of their magnetic domain, the clusters will exhibit single domain magnetism. This magnetism may be observed by the presence of chain structures of FeCo nanoclusters due to the alignment of their single magnetic domains. In order to create a near-atomically homogeneous nanoalloy without preferential aggregation of its metal atom constituents, no clustering and phase separation should take place. In the bulk, alloys of iron and cobalt phase separate over most of the compositional range. Conversely, at the nanoscale, it may be possible to synthesize nanoalloy structures that are not normally favorable at given compositions, by the manipulation of reaction kinetics. In order to produce an atomically mixed nanoalloy, the transformation reactions of the organometallic precursors should display similar kinetic features, i.e. similar reaction rates. Therefore, the reaction kinetics of all the species in the reaction must be similar to avoid competition between them. As a result, kinetic control of the individual transformation reaction rates of each species may be used to modulate the aggregation and phase separation of the different species, and consequently control cluster morphology. This work has provided the framework for the design of synthesis methods that enable the control of the structure of FeCo nanoalloys with careful attention to precursor decomposition kinetics and the correlation between reaction kinetics and nanoalloy morphology.Ph.D.Committee Chair: Tannenbaum, Rina; Committee Member: Hamid Garmestani; Committee Member: Karl Jacob; Committee Member: Rosario Gerhard