Synthesis and N–C bond cleavage reactions for cyclic phosphazenium dications

Abstract

Phosphazenium cations were first characterized 60 years ago and yet little is known of their reactivity. The bidentate phosphonium dicationic salts [(C₆H₄)(PPh₂F)₂][B(C₆F₅)₄]₂2 and [(C₆H₄)(PPh₂Cl)₂][Cl]₂3 were prepared. Reaction of 2 with (Me₃Si)₂NMe gave the phosphazenium dication [(C₆H₄)(PPh₂)₂(μ-NMe)][B(C₆F₅)₄]₂4, while reaction of 3 with two equivalents of Me₂N(SiMe₃) gave the related dication [(C₆H₄)(PPh₂NMe₂)₂][Cl]₂5. Compound 3 also reacted with (Me₃Si)₂NH affording the monocationic salt [(C₆H₄)(PPh₂)₂(μ-N)]Cl 6, with the liberation of HCl. Interestingly 4 reacted with [nBu₄N]Cl generating the related salt [(C₆H₄)(PPh₂)₂(μ-N)][B(C₆F₅)₄] 6′ with concurrent formation of MeCl. This reactivity was extended to [(C₂H₄)(PPh₂F)₂][B(C₆F₅)₄]₂7 which afforded [(C₂H₄)(PPh₂)₂(μ-NR)][B(C₆F₅)₄]₂ (R = Me 8, CH₂CHCH₂9, CH₂Ph 10) upon reaction with (Me₃Si)₂NR. These dications were shown to react with [nBu₄N]Cl affording [(CH₂PPh₂)₂(μ-N)][B(C₆F₅)₄] 11 and the corresponding alkyl-chloride. The mechanism of these N–C bond cleavage reactions is considered.