Abstract:Binuclear hexacoordinate silicon chelates have been prepared and shown to have octahedral
structure by X-ray crystallography. Their ionization in CD2Cl2 solution has been studied by
29Si NMR spectroscopy. Only one Si−Cl bond in 5a−c ionizes at low temperature to form
the monosiliconium bis-chelates 11a−c. Use of a more acidic solvent, CHFCl2, facilitated
the second ionization step to the disiliconium dichloride 12c. Replacement of the chloro
ligands by better leaving groups (triflate, bromide, or iodide) cause… Show more
“…Even formation of dications has been reported in dinuclear silicon complexes 17 and 18 , where the formal positive charges are separated from each other by only three bonds. Ionization is facile in the dichloro complex 17 , with oxygen donors, but requires replacement of the chloro ligands by the more effective anions (Br, I, TfO) in 18 , with its Me 2 N ligands …”
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.
“…Even formation of dications has been reported in dinuclear silicon complexes 17 and 18 , where the formal positive charges are separated from each other by only three bonds. Ionization is facile in the dichloro complex 17 , with oxygen donors, but requires replacement of the chloro ligands by the more effective anions (Br, I, TfO) in 18 , with its Me 2 N ligands …”
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.
“…This is the common geometry for pentacoordinate bischelates with two nitrogen donor ligands 8. The N‐Si‐N angle gradually decreases along the series, while the O‐Si‐O angle increases, via a distorted TBP ( 2 a , ∼65 % TBP1→SP),6c through pure square planar (SP) geometry ( 8 ; evidenced by the N‐Si‐N≃O‐Si‐O angles),6e then through another distorted TBP with axiallike oxygen ligands ( 6 a ), and finally to the TBP2 geometry of 7 b , which is essentially an inverted TBP in which axial and equatorial groups have exchanged their positions. Throughout this process the t Bu group acts as a “pivot” for the pseudorotation.…”
Sterically hindered pentacoordinate tert‐butylsiliconium halide complexes undergo methyl halide elimination. The products have one covalent and one dative NSi bond, both in the unprecedented equatorial position. A complete pseudorotation coordinate is demonstrated with suitable crystal structures.
“…108 Similar ionisation processes are also seen in the corresponding complexes of N-isopropylidene-NЈ-(O-trimethylsilyl) hydroxides, 109 and linked dinuclear complexes. 110 The coordination chemistry of dialkylhydroxylamides has been expanded by the report of the preparation of Ge(ONMe 2 ) from germanium() chloride and the lithium salt of dimethylhydroxylamide. The complex exhibits a "4 ϩ 4" coordination geometry with short contacts to the oxygen atoms and longer contacts to the nitrogen atoms.…”
Section: Complexes With Polydentate Heterodonor Ligandsmentioning
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.