Hypercoordinate silicon complexes based on hydrazide ligands. A remarkably flexible molecular system

Though only one row apart on the periodic table, silicon greatly differs from carbon in its ability to readily form five- and six-coordinate complexes, termed "hypercoordinate silicon compounds". The assorted chemistry of these compounds is varied in both structures and reactivity and has generated a flurry of innovative research endeavors in recent years. This Account summarizes the latest work done on a specific class of hypercoordinate silicon compounds, specifically those with two hydrazide-derived chelate rings. This family is especially interesting due to the ability to form multiple penta- and hexacoordinate complexes, the chemical reactivity of pentacoordinate complexes, and the observation of intermolecular ligand crossovers in hexacoordinate complexes. Pentacoordinate complexes in this family exhibit marked structural flexibility, as demonstrated by the construction of a complete hypothetical Berry-pseudorotation reaction coordinate generated from individual crystallographic molecular structures. Although hexacoordinate complexes generally crystallize as octahedra, with a decrease in the ligand donor strength the complexes can crystallize as bicapped tetrahedra. Hexacoordinate complexes bearing a halogen ligand undergo a solvent-driven equilibrium ionic dissociation, which is controlled by solvent, temperature, counterion, and chelate structure and has been directly demonstrated by conductivity measurements and temperature-dependent (29)Si NMR. Hexacoordinate silicon complexes can also undergo reversible neutral nonionic dissociation of the N-Si dative bond. Ionic pentacoordinate siliconium salts react readily via methyl halide elimination, initiated by their own counterion acting as a base. Pentacoordinate complexes can also undergo intramolecular aldol condensations of imines, which may find potential as a template for organic synthesis. In addition, these complexes are capable of performing an uncatalyzed intramolecular hydrosilylation of imine double bonds. Perhaps the most striking manifestations of flexibility are the facile and complete intermolecular ligand crossovers. Crossovers have been observed between different hexacoordinate complexes, and between complex molecules and their differently substituted precursors, and take place within minutes. Although the precise mechanisms of these transformations remain elusive, NMR and single-crystal X-ray diffraction measurements have shed light on these interesting phenomena. A profusion of crystallographic data and careful NMR experimentation has led to an improved understanding of penta- and hexacoordinate hydrazide-based silicon dichelates. The diverse chemical reactivity of these complexes demonstrates both the scope and complexity of silicon chemistry. Future exploration into the structures and chemistry of hypercoordinate silicon will continue to enhance our understanding and appreciation of this unique element.