Dissertation Defense: Lauren Wenger, Chemistry
DISSERTATION DEFENSE
Lauren E. Wenger, Chemistry
*under the direction of Timothy Hanusa
“Mechanochemical Exploration of the p-Block”
02.20.24 | 1:45PM CST | 1220 MRBIII
As environmental concerns over organic solvents rise and chemists become more aware of the interference solvents can play in reactions, solvent-free methods have grown in popularity, including mechanochemical methods. Mechanochemical reactions are induced by the direct absorption of mechanical energy, commonly through grinding or milling, with little or no solvent. Organometallic chemistry has advanced through removing the solvent and using mechanochemical methods for synthesis. This dissertation seeks to enhance the fundamental understanding of ball milling through the preparation of bulky allyl complexes of p-block metals, specifically aluminum, germanium, arsenic, and antimony. Heteroleptic aluminum complexes of the formula [(NHC)xAlCl3‑nA′n] [NHC = N-heterocyclic carbene; A′ = [1,3-(SiMe3)2C3H3]−; x = 0−1; n = 0−3], were prepared and examined as initiators for L-lactide polymerization, a biodegradable polymer. [AlA′3], which is only accessible via ball milling, was the most active for polymerization. A homoleptic germanium tetra(allyl) ([GeA′4]) was accessed through halide metathesis of germanium halides and K[A′]. [GeA′4] is highly stable, unlike the related [SnA′4], which decomposes when exposed to air. The stability of the two systems was explored with computational and experimental studies. Tris(allyl) complexes of arsenic and antimony were prepared with salt metathesis, as two diastereomers; the ratio of diastereomers varies with mechanochemical or solution preparation. The difference in selectivity is attributed to the layered crystal lattices of the EX3 reagents, which template the stereochemistry. The effect of mechanochemical variables on isomer selectivity was explored, including the milling apparatus, milling media, liquid assisted grinding, and use of different reagents. The extent to which the anisotropic lattice is degraded during synthesis affects the diastereomeric ratio. Synthetic chemists commonly find themselves asking the question, what is the best solvent for this reaction? We seek to answer the question: what happens when there is no solvent?