Synthesis and N-C Bond Cleavage Reactions for Cyclic Phosphazenium Dications

Phosphazenium cations were first characterized 60 years ago and yet little is known of their reactivity. The bidentate phosphonium dicationic salts [(CH)(PPhF)][B(CF)]2 and [(CH)(PPhCl)][Cl]3 were prepared. Reaction of 2 with (MeSi)NMe gave the phosphazenium dication [(CH)(PPh)(μ-NMe)][B(CF)]4, while reaction of 3 with two equivalents of MeN(SiMe) gave the related dication [(CH)(PPhNMe)][Cl]5. Compound 3 also reacted with (MeSi)NH affording the monocationic salt [(CH)(PPh)(μ-N)]Cl 6, with the liberation of HCl. Interestingly 4 reacted with [nBuN]Cl generating the related salt [(CH)(PPh)(μ-N)][B(CF)] 6' with concurrent formation of MeCl. This reactivity was extended to [(CH)(PPhF)][B(CF)]7 which afforded [(CH)(PPh)(μ-NR)][B(CF)] (R = Me 8, CHCHCH9, CHPh 10) upon reaction with (MeSi)NR. These dications were shown to react with [nBuN]Cl affording [(CHPPh)(μ-N)][B(CF)] 11 and the corresponding alkyl-chloride. The mechanism of these N-C bond cleavage reactions is considered.