Reactions of P-4 and I-2 with Ag[Al(OC(CF3)(3))(4)]: from elusive polyphosphorus cations to subvalent P3I6+ and phosphorus rich P5I2

Reactions of X-2 (X = Br, I), P-4 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 degreesC to rt the initially formed cations decomposed the Al(OR)(4)(-) anion giving, in two cases, the more stable fluoride bridged (RO)(3)Al-F-Al(OR)(3)(-) anion. When Br-2 was used as the oxidising agent the proposed intermediate phosphorus cation (P-5(+)?) reacted with the solvent CDCl3 by double insertion of a P+ unit into the C-Cl bond giving Cl2P(CDCl2)(2)[(RO)(3)Al-F-Al(OR)(3)], 1. When I-2 was used as the oxidiser the reaction led to the marginally stable P3I6[(RO)(3)Al-F-Al(OR)(3)], 2 (X-ray). By using very mild conditions throughout (-80 degreesC) the primary product of the reaction of Ag(P-4)(2)[Al(OR)(4)] and I-2 was isolated: P5I2[Al(OR)(4)], 3, containing the P5I2+ cation with a hitherto unknown C-2v-symmetric P-5 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 (P-31-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)): P-5(+) (913), P3I6+ (694), P5I2+ (792), P2I5+ (733), Ag(P-4)(2)(+) (784).