Steric congestion in heavy pnictines alters oxidative halogenation pathways

Abstract

Dihalotriorganopnictoranes serve as important synthetic precursors in Group 15 chemistry, but the influence of the organic substituents on the synthesis and reactivity of these compounds remains underexplored for the heavy pnictogens. We recently reported the isolation of Dipp3Sb (Dipp = 2,6-diisopropylphenyl). The bulky Dipp substituents permit the isolation of monomeric tetrahedral stibine oxide Dipp3SbO, yet also allow for the formation of the trigonal bipyramidal Dipp3SbF2. We now report the isolation of a sterically crowded bismuthine, Dipp3Bi. Reaction between Dipp3Bi and XeF2 afforded Dipp3BiF2, which features a distorted trigonal bipyramidal geometry in the solid state. Reaction between Dipp3Bi and PhICl2 resulted in the generation of 1-chloro-2,6-diisopropylbenzene. Reaction between Dipp3Sb and PhICl2 resulted in the formation of Dipp3SbCl2, which features a square pyramidal geometry in the solid state. Mes3SbCl2, Mes3SbBr2, Mes3BiCl2, and Mes3BiBr2 (Mes = mesityl) were structurally characterized and all exhibit trigonal bipyramidal geometries in the solid state. Solution-phase spectroscopic measurements reflect the fluxionality of all of these molecules, but theoretical analyses in the gas phase suggest that the square pyramidal geometry of Dipp3SbCl2 is more thermodynamically favorable than the trigonal bipyramidal geometry, possibly due to the presence of intramolecular secondary interactions between the Dipp and Cl substituents. These results highlight the ability of Dipp substituents to alter reactivity pathways of heavy pnictines.