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

Phosphazenium cations were first characterized 60 years ago and yet little is know of their reactivity. The bidentate phosphonium dicationic salts [(C6H4)(PPh2F)2][B(C6F5)4]2 2 and [(C6H4)(PPh2Cl)2][Cl]2 3 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, CH2CHCH2 9, 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 andthe corresponding alkyl-chloride. The mechanism of these N-C bond cleavage reactions is considered.