The atomic level mechanism of white phosphorous demolition by di-iodine

Abstract

A detailed mechanism of the I₂-induced transformation of white phosphorus into PI₃ emerges from a DFT analysis. This multi-step process implies that at any stage one P–P and two I–I bonds cleavages, associated with the formation of two P–I bonds plus an in situ generated brand new I₂ molecule. Significant electron transfer between the atoms is observed at any step, but the reactions are better defined as concerted rather than redox. Along the steepest descent to the product, no significant barrier is encountered except for the very first P₄ activation, which costs +14.6 kcal mol⁻¹. At the atomic level, one first I₂ molecule, a typical mild oxidant, is first involved in a linear halogen bonding interaction (XB) with one P donor, while its terminal I atom is engaged in an additional XB adduct with a second I₂. Significant electron transfer through the combined diatomics allows the external I atom of the dangling I₃ grouping to convey electrons into the σ* level of one P–P bond with its consequent cleavage. This implies at some point the appearance of a six-membered ring, which alternatively switches its bonding and no-bonding interactions. The final transformation of the P₂I₄ diphosphine into two PI₃ phosphines is enlightening also for the specific role of the I substituents. In fact, it is proved that an organo-diphosphine analogue hardly undergoes the separation of two phosphines, as reported in the literature. This is attributable to the particularly high donor power of the carbo-substituted P atoms, which prevents the concertedness of the reaction but favors charge separation in an unreactive ion pair.