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 [(C 6 H 4 )(PPh 2 F) 2 ][B(C 6 F 5 ) 4 ] 2 2 and [(C 6 H 4 )(PPh 2 Cl) 2 ][Cl] 2 3 were prepared. Reaction of 2 with (Me 3 Si) 2 NMe gave the phosphazenium dication [(C 6 H 4 )(PPh 2 ) 2 (μ-NMe)][B(C 6 F 5 ) 4 ] 2 4 , while reaction of 3 with two equivalents of Me 2 N(SiMe 3 ) gave the related dication [(C 6 H 4 )(PPh 2 NMe 2 ) 2 ][Cl] 2 5 . Compound 3 also reacted with (Me 3 Si) 2 NH affording the monocationic salt [(C 6 H 4 )(PPh 2 ) 2 (μ-N)]Cl 6 , with the liberation of HCl. Interestingly 4 reacted with [nBu 4 N]Cl generating the related salt [(C 6 H 4 )(PPh 2 ) 2 (μ-N)][B(C 6 F 5 ) 4 ] 6′ with concurrent formation of MeCl. This reactivity was extended to [(C 2 H 4 )(PPh 2 F) 2 ][B(C 6 F 5 ) 4 ] 2 7 which afforded [(C 2 H 4 )(PPh 2 ) 2 (-NR)][B(C 6 F 5 ) 4 ] 2 (R = Me 8 , CH 2 CHCH 2 9 , CH 2 Ph 10 ) upon reaction with (Me 3 Si) 2 NR. These dications were shown to react with [nBu 4 N]Cl affording [(CH 2 PPh 2 ) 2 (μ-N)][B(C 6 F 5 ) 4 ] 11 and the corresponding alkyl-chloride. The mechanism of these N-C bond cleavage reactions is considered. The dicationic salts, [(C 6 H 4 )(PPh 2 ) 2 (μ-NMe)][B(C 6 F 5 ) 4 ] 2 and [(C 2 H 4 )(PPh 2 ) 2 (μ-NR)][B(C 6 F 5 ) 4 ] 2 (R = Me, CH 2 CHCH 2 , CH 2 Ph) are prepared and shown to react with [nBu 4 N]Cl to generate the phosphazenium cations and the corresponding alkyl-chloride.