Reactions of P4 and I2 with Ag[Al(OC(CF3)3)4]: from elusive polyphosphorus cations to subvalent P3I6+ and phosphorus rich P5I2+

Abstract

Reactions of X₂ (X = Br, I), P₄ and Ag(CH₂Cl₂)[Al(OR)₄] [R = C(CF₃)₃] in suitable ratios to prepare naked polyphosphorus cations were carried out and led to products which suggested the presence of these elusive cations as intermediates. At temperatures above −30 °C to rt the initially formed cations decomposed the Al(OR)₄⁻ anion giving, in two cases, the more stable fluoride bridged (RO)₃Al–F–Al(OR)₃⁻ anion. When Br₂ was used as the oxidising agent the proposed intermediate phosphorus cation (P₅⁺?) reacted with the solvent CDCl₃ by double insertion of a P⁺ unit into the C–Cl bond giving Cl₂P(CDCl₂)₂[(RO)₃Al–F–Al(OR)₃], 1. When I₂ was used as the oxidiser the reaction led to the marginally stable P₃I₆[(RO)₃Al–F–Al(OR)₃], 2 (X-ray). By using very mild conditions throughout (−80 °C) the primary product of the reaction of Ag(P₄)₂[Al(OR)₄] and I₂ was isolated: P₅I₂[Al(OR)₄], 3, containing the P₅I₂⁺ cation with a hitherto unknown C_2v-symmetric P₅ cage as structural building block. P₃I₆[Al(OR)₄], 4, was directly synthesised in quantitative yield starting from P₂I₄, PI₃ and Ag(CH₂Cl₂)[Al(OR)₄] in CH₂Cl₂ solution. P₃I₆⁺ is formed through the P₂I₅⁺ stage (³¹P-NMR). P₃I₆⁺ (av.: P^2.33) is the first subvalent P–X cation (X = H, F, Cl, Br, I). P₅I₂⁺ (av.: P^0.6) is the first phosphorus rich binary P–X cation. They are the third and fourth example of a binary P–X cation after the known PX₄⁺ and P₂X₅⁺ cations. The observed reactions were fully accounted for by thermochemical Born–Haber cycles based on (RI-)MP2/TZVPP ab initio, COSMO solvation and lattice enthalpy calculations (all phases). The gaseous enthalpies of formation of several species were calculated to be (in kJ mol⁻¹): P₅⁺ (913), P₃I₆⁺ (694), P₅I₂⁺ (792), P₂I₅⁺ (733), Ag(P₄)₂⁺ (784).