Preparation, Structural Analysis, and Reactivity Studies of Phosphenium Dications

This work expands on the recent synthesis and reactivity of the two-coordinate P(III)-containing dication [((Ph₃P)₂C)P(NⁱPr₂)]²⁺ (3a²⁺). Two additional dications of general formula [((RPh₂P)₂C)P(NR′₂)]²⁺ (3b²⁺, R = 1/2 (CH₂)₃, R′ = ⁱPr; 3c²⁺, R = Ph, R′ = Cy) have been prepared. Solution and solid-state ³¹P NMR isotropic shifts for this dication class were observed around 360 ppm. Except for [3a][SbF₆]₂, the ³¹P CP/MAS NMR spectra reveal a high degree of structural disorder around the dicationic site in the solid state. Surprisingly, the synthesis of dication 3a²⁺ was only possible in the presence of AlCl₄^-, SbF₆^-, and substituted tetraarylborates, while the use of ClO₄^-, OTf^-, BF₄^-, PF₆^-, and BPh₄^- resulted in complex product mixtures. Single-crystal X-ray analysis of dications 3a²⁺ and 3c²⁺ suggested a 4π allyl-like structure for the central CPN fragment, and this has been supported through detailed theoretical investigations. Initial reactivity studies between dications 3a²⁺/3c²⁺ and PMe₃ show an equilibrium between the free dication and the target adduct. Furthermore, 3a²⁺ has been shown to be capable of O-H bond activation using water and methanol and polymerizes THF when it is dissolved in this solvent.