Organohypervalent heterocycles

This review summarizes the structural and synthetic aspects of heterocyclic molecules incorporating an atom of a hypervalent main-group element. The term "hypervalent" has been suggested for derivatives of main-group elements with more than eight valence electrons, and the concept of hypervalency is commonly used despite some criticism from theoretical chemists. The significantly higher thermal stability of hypervalent heterocycles compared to their acyclic analogs adds special features to their chemistry, particularly for bromine and iodine. Heterocyclic compounds of elements with double bonds are not categorized as hypervalent molecules owing to the zwitterionic nature of these bonds, resulting in the conventional 8-electron species. This review is focused on hypervalent heterocyclic derivatives of nonmetal main-group elements, such as boron, silicon, nitrogen, carbon, phosphorus, sulfur, selenium, bromine, chlorine, iodine(III) and iodine(V).

Synthesis and Characterization of Hypervalent Pentacoordinate Carbon Compounds Bearing a 7-6-7-Ring Skeleton

To stabilize S_N 2 transition state-like penta-coordinate carbon species, triaryl-substituted cationic carbon compounds bearing a moderately flexible 7-6-7-ring skeleton with sulfur donors were synthesized and characterized. Electronic effects of para substituents (R=Cl, F, H, CH₃ , SMe, OMe) of the two equatorial aryl groups bound to the cationic central carbon were investigated systematically along with a planar bidentate thioxanthene derivative. X-ray analysis on their solid-state structures showed that the parent (R=H), chloro-, fluoro- and methyl-derivatives were tetracoordinate carbon (sulfonium) structures, while the p-MeO and thioxanthenyl system were pentacoordinate carbocation structures. The Hammett substituent constants for the para substituents (σ_p ⁺ ) correlates well with the bonding in these compounds. The methylthio-derivative with intermediate Hammett substituent constants (p-MeS; σ_p ⁺ =-0.60) showed a tetracooridnate solid-state structure, though solution UV-Vis properties suggested the presence of a penta-coordinate structure. These findings amount to the first unambiguous solution evidence of the hypervalent apical 3c-4e interactions in pentacoordinate carbon compounds.

Bonding Analysis of Compounds with Unusual Coordination of Carbon: Proposed Symmetric Systems with Six-Coordinate Carbon

The possibility of carbon tetravalence in geometries other than tetrahedral and of carbon hypervalence has been taken seriously since the 1970s. Computational modeling and subsequent experimental validation have established the existence of molecules with carbon atoms with planar tetravalence and as many as six objects in carbon’s coordination sphere. In this work, we develop insight into the nature of bonding to carbon in these unusual environs as provided by Bader’s Atoms in Molecules (AIM) analysis of the electron density, along with the electron localization function (ELF) and the non-covalent index (NCI). We review several well-established systems (spiropentadiene dication, hexamethyl benzene dication, dimethanospiro[2.2]octaplane dication, and 1,8-dimethoxy-9-dimethoxyanthracene cation) and propose new D2d–symmetric variants of a hexacoordinated species.

The classification and representation of main group element compounds that feature three-center four-electron interactions

This article provides a means to classify and represent compounds that feature 3-center 4-electron (3c-4e) interactions in terms of the number of electrons that each atom contributes to the interaction. Specifically, Class I 3c-4e interactions are classified as those in which two atoms provide one electron each and the third atom provides a pair of electrons (i.e. LX₂), while Class II 3c-4e interactions are classified as those in which two atoms each provide a pair of electrons and the third atom contributes none (i.e. L₂Z). These classes can be subcategorized according to the nature of the central atom. Thus, Class I interactions can be categorized according to whether the central atom provides one (i.e.μ-X) or two (i.e.μ-L) electrons, while Class II interactions can be categorized according to whether the central atom provides none (i.e.μ-Z) or two (i.e.μ-L) electrons. The use of appropriate structure-bonding representations for these various interactions provides a means to determine the covalent bond classification of the element of interest.