Use of Molecular Scaffolding for the Stabilization of an Intramolecular Dative PIII‐PV System

The reaction of NapP2S4 (1; Nap = naphthalene‐1,8‐diyl) with chlorine gas gave [Nap(PCl2)(PCl4)] (2), displaying a rare σ4P‐σ6P bonding interaction. An X‐ray structure analysis confirmed the PCl5‐like, P−P bond containing phosphonium‐phosphoride structure of 2 in the crystal, which was also found in solution at low temperature. At ambient and higher temperatures, dynamic behaviour on the NMR time‐scale was observed, which was assigned to interchange of the ionic phosphonium‐phosphoride form 2 and the molecular bis(phosphorane) Nap(PCl3)2 form 3, rather than to the ionic phosphonium salt‐phosphorane form [Nap(PCl3)(PCl2)][Cl] 4. Electronic structure calculations were performed at the B3LYP/6−31G(d,p) level of theory on structures 2 and 3; structure 3 was located as a local minimum on the potential energy surface, 15 kcal·mol−1 higher in energy than structure 2. The crystal structure and calculated P−P distances are 2.34 and 2.31 Å for 2 and 3, respectively. An activation energy of 19.7 kcal·mol−1 was found for the transition state structure by coordinate driving calculations; the line‐shape analysis of variable temperature 31P{1H} NMR spectra gave an activation energy of 14.4 kcal·mol−1. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)