Hypercoordinate Silicon Complexes Based on Hydrazide Ligands. A Remarkably Flexible Molecular System

Abstract

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.

Ylenes in the MII→SiIV (M=Si, Ge, Sn) Coordination Mode

“…[1] In addition to the thermodynamically very stable Si À F and Si À O bonds (and the relatively stable Si À N bonds), Si À C bonds are kinetically quite inert (but can be activated by Si-hypercoordination). [2] In sharp contrast, bonds between silicon and heavier donor atoms such as Br, I, and S atoms are usually significantly less stable (both thermodynamically and kinetically).…”

“…[6] Although the heavier ylenes GeCl 2 and SnCl 2 are isolable, and the portfolio of isolable silylenes did enjoy a notable growth in recent years, [7] only carbenes were reported as Group 14 ylene ligands in the coordination sphere of hypercoordinate Si atoms so far. [8] Abstract: The reaction of the methimazolyl (mt, i.e., 2-mercapto-1-methylimidazolide) substituted silane Si(mt) 4 with SnCl 2 and GeCl 2 in dioxane affords the paddlewheel-shaped complexes [ClSiA C H T U N G T R E N N U N G (m-mt) 4 MCl] (M = Sn (1) and Ge (2), respectively). These compounds represent the first crystallographically characterized hexacoordinate silicon complexes comprising a Sn or Ge atom in the Si coordination sphere.…”

Palladium‐Catalyzed CH Fluorosilylation of 2‐Phenylpyridines: Synthesis of Silafluorene Equivalents

“…[2] In silafluorene structures, p-conjugated biaryl moieties and a silicon atom are connected by two Si À C covalent bonds (Figure 1 a). [4,5] To synthesize the designed structures (Figure 1 b), we planned to use directing-group-assisted (pyridyl group), transition-metal-catalyzed direct C À H silylation. [3] This molecular design is based on the characteristics of silicon atoms bearing electron-withdrawing group(s), so that hypervalent structures can be formed as a result of the high Lewis acidity of silicon atoms.…”

Silicon(IV) Chelates of an (ONN')-Tridentate Pyrrole-2-Carbaldimine Ligand: Syntheses, Structures and UV/Vis Properties

“…Silicon compounds with a silicon coordination number of five or six [1,2] are of interest for their enhanced reactivity [3] and electronic properties [4]. Of the many ligands that are explored for their coordination behavior to silicon, our focus is on the tridentate ONNchelators and in particular on the doubly deprotonated 2-(N-2-hydroxyphenyl)pyrrolcarbaldimine (LH 2 ) [5].…”