Timothy P. Hanusa
Title and Contact Information
Ph.D., Indiana University, 1983
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Arts and Science-But Are the Chairs Silver?
Use of ligand design to control the reactivity of main-group and transition metal organometallic and coordination complexes; investigation of steric effects on the reactivity and magnetic properties of metal complexes; synthesis of precursors to materials with desirable electronic/optical properties; computational investigation of bonding and structure in inorganic systems.
Molecular chemistry of the main-group metals: Our group has been developing ligand-based approaches to manipulating the structure and reactivity of main-group metals. The synthesis of volatile and/or liquid compounds that could be useful for the preparation of thin films of metal oxides and halides by chemical vapor deposition (CVD) methods is of particular interest. We have used large, sterically demanding cyclopentadienyl ligands to turn poly- or oligomeric compounds of the heavier alkaline-metals (Ca, Sr, Ba) into monomeric species, often dramatically lowering their sublimation temperatures or melting points. Aluminum alkoxides and alkyls are being designed to deposit stochiometrically precise films of alumina on silicon wafers for microelectronics applications.
Use of sterically bulky allyl ligands in catalysis and the study of non-covalent interactions: The compact size of the allyl anion ([C3H5]-) means that its transition metal complexes are often coordinatively unsaturated and prone to facile decomposition. We have used sterically bulky substituents (e.g., SiMe3) to produce pi-allyl transition metal compounds that have no unsubstituted analogues, such as the extremely electron deficient 12- and 14-electron species [1,3-(SiMe3)2C3H3]2Cr and [1,3-(SiMe3)2C3H3]2Fe. We are synthesizing new bis- and tris(allyl) metal complexes of the transition metals, and lanthanides and examining the way that the double bonds of allyl ligands can engage in cation-pi interactions. These non-covalent interactions are important in biological systems, but commonly involve aromatic rings. We are examining non-aromatic cation-pi interactions such as that between K+ the zincate complex at right. Our investigations are helping to identify the geometric features that can affect the strength of cation-pi attractions.
Symmetryeffects on the magnetic properties of transition metal complexes: There is considerable interest in the synthesis of transition metal compounds whose magnetic properties can be influenced by external agents. In the case of spin-crossover complexes, transitions between high- and low-spin states can be induced by temperature, pressure, and light, and the effective control of such transitions could ultimately lead to applications in switching devices, magnetic storage, and photonic systems.
Metallocene-based complexes have been attractive in this regard, and variations in the metals, their oxidation states, and ring substituents have led to species displaying spin-crossover behavior, molecular ferromagnetism, and ferromagnetic/antiferromagnetic exchange. We have been studying bis(indenyl)metal complexes, which are relatives of metallocenes, but whose ligand conformations are sensitive to orbital occupancies.
Monomeric (1-RC9H6)2Cr (R = t-Bu, SiMe3) are staggered, high-spin complexes with 4 unpaired electrons. When additional bulk is added to the ligands (e.g., (1,3-R2C9H5)2Cr; R = t-Bu, SiMe3), however, rotation to a gauche (near 90)conformation is forced upon the molecule. Owing to increased metal-ligand orbital mixing, maintenance of the high-spin state is no longer possible, and the molecules adopt low-spin configurations with 2 unpaired electrons. This indicates that both steric bulk and electronic effects brought about by selective substitution of the indenyl ligand can be used to tailor the magnetic properties of the compounds, making them suitable as tunable sources of variable spin molecules. The spin state changes also suggest that there may be useful variations in the reactivity of the complexes, a possibility we are investigating.