Ability of carbenes to act as Lewis bases within a halogen bond

Abstract

A halogen bond is formed by a wide range of divalent CR₂ carbenes with ICN as the Lewis acid, and the results are examined via DFT calculations. These carbenes form much stronger halogen bonds than more common Lewis bases such as NH₃ and OH₂. The binding occurs through the carbene C lone pair and is enhanced if the central C is covalently attached to C on the R substituent, as compared to O or N. Interaction energies are as high as 30 kcal mol⁻¹, and the C⋯I bond contains a certain degree of covalent character. The two lone pairs on the C atom of the carbene, when connected through dative bonds to units such as NH₃ and OH₂, make this C especially strongly nucleophilic, enough so as to partially extract the I from the ICN unit. The carbene halogen bonds have all of the characteristics of their classic counterparts, including a high dependence upon Coulombic interaction.