Speciation of aqueous hypercoordinated organosilicate complexes / by Jiali Wen.

Abstract

Aqueous silicon chemistry plays a major and as yet unappreciated role in many fields of science. Emerging technologies such as the fabrication of biomimetic ceramics require a thorough understanding of all factors affecting aqueous silicon chemistry. In biology, silicon is known to be essential for the normal growth and development of most plants and animals, and yet almost nothing is known of the chemical mechanisms by which it is absorbed, transported and ultimately used. A careful characterization of silicon's interaction with sugars and sugar derivatives would be a significant step forward to unraveling this element's enigmatic biological role. In this study, 29Si and 13C NMR spectroscopy were employed to determine the structures and stability constants of aqueous silicon complexes formed with organic ligands containing either proximal hydroxy or vicinal ' cis'-diol functionality. Five-coordinated Si complexes formed with acyclic polyhydroxy molecules (polyols) are structurally analogous to the monomeric [(L=)2SiOH]~ species obtained with furanoidic cw-diol molecules, each with a single silicon centre bound via ester linkages to two bidentate ligands. Polyol complexes tend to be shorter lived, however, owing to ligand-ligand steric interactions. Glucoheptonic acid has a unique polyol configuration which enables it to wrap around a silicon centre with all of its hydroxyl groups directed inwards, yielding long-lived mono-ligand complexes that contain two ([L=Si(OH )3]), three ([L=Si(OH)2]) or four ([L=SiOH]') ester linkages (in addition to the usual array of labile bis-ligand complexes).