Use of molecular scaffolding for the stabilization of an intramolecular dative P^III-P^V 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 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.