Silicon-Nitrogen Bonding in Hypervalent Compounds

Silicon–nitrogen interactions in hypervalent compounds embody a distinctive class of bonding in which a silicon centre exceeds the octet rule, engaging additional electron density from nitrogen donors. These systems—frequently exemplified by silatranes and related bicyclic frameworks—feature an intramolecular dative bond (N→Si) that imparts unusual structural rigidity, enhanced thermal stability and reactive versatility. The hypercoordination around silicon typically adopts a trigonal bipyramidal geometry, with the nitrogen lone pair occupying an apical site. This arrangement modulates electronic distribution, influencing reactivity in catalysis, materials science and molecular electronics. Advances in spectroscopic, crystallographic and computational techniques have clarified the interplay between bond covalency and electrostatic contributions, revealing how substituent effects, solvent interactions and external stimuli govern bond length, strength and dynamic behaviour. Emerging applications range from ion‐transport membranes to novel organosilicon scaffolds for medicinal chemistry.

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Recent electrochemical studies of pentacoordinated silatranes have explored their one–electron oxidation to short-lived cation radicals. By combining cyclic voltammetry, impedance spectroscopy and spectroelectrochemical EPR with density functional theory, investigators have shown that deprotonation is the primary decay pathway of the oxidised species. Insights into substituent effects on radical stability suggest strategies for prolonging lifetimes, with implications for redox‐active materials and electrocatalytic processes.

A comprehensive investigation into quasimetallatranes—bicyclic systems featuring Group 15 donors (N, P, As) and Group 14 acceptors (Si, Ge, Sn, Pb)—has elucidated the nature of the dative bond across a series of donor–acceptor combinations. Structural optimisation and molecular orbital analysis reveal a clear correlation between donor size and bond strength, as quantified by bond lengths and Wiberg bond indices. Conformational preferences are shown to depend critically on the balance between steric constraints and the covalent character of the N→Si interaction, informing the design of tuneable hypervalent scaffolds.

Structural and computational studies of 1-phenyl-substituted tribenzsilatranes have uncovered variations in the intramolecular N→Si bond length relative to simpler silatrane analogues. Single-crystal X-ray diffraction combined with DFT calculations indicates that increased ring rigidity leads to a ‘harder’ potential energy surface for the dative interaction, as reflected in altered bond deformation profiles. These findings provide a foundation for engineering silatrane derivatives with tailored mechanical and electronic properties for applications in supramolecular assemblies and drug delivery platforms.

Silicon-Nitrogen Bonding in Hypervalent Compounds publication trend

The graph below shows the total number of articles in silicon-nitrogen bonding in hypervalent compounds across all publications each year (not limited to Nature Index journals).

Technical terms

Hypervalency: A bonding situation in which a central atom, such as silicon, formally holds more than eight electrons in its valence shell through additional donor–acceptor interactions.

Dative bond: A covalent bond formed when both electrons of the shared pair originate from the same atom, for example, a nitrogen lone pair donating into an empty silicon orbital.

Silatrane: A tricyclic compound featuring a silicon centre coordinated by a triethanolamine framework, characterised by a stabilising intramolecular N→Si dative interaction.

References

1. Electrooxidation of Hypercoordinated Derivatives of Silicon and Reactivity of Their Electrogenerated Cation Radicals: 1-Substituted Silatranes. Molecules (2023).
2. Dative Bonding in Quasimetallatranes Containing Group 15 Donors (Y = N, P, and As) and Group 14 Acceptors (M = Si, Ge, Sn, and Pb). Inorganic Chemistry (2024).
3. Crystal Structure of New 1-Phenyl-Substituted Tribenzsilatranes. Crystals (2023).