Formation of an imidazoliumyl-substituted [(LC)4P4]4+ tetracation and transition metal mediated fragmentation and insertion reaction (LC = NHC)

Abstract

Tetracationic cyclo-tetraphosphane [(LC)4P4]4+ as triflate salt (3[OTf]4) (LC = 4,5-dimethyl-1,3-diisopropyl-imidazol-2-yl) is obtained in high yield from the reduction of [LCPCl2]+ (4[OTf]) with 1,4-bis(trimethylsilyl)-1,4-dihydropyrazine (6) and represents the first salt of the cationic cyclo-phosphane series with the general formula [LnPn]n+. Theoretical calculations reveal the electrophilic nature of the P atoms within the P4-ring due to the influence of the imidazoliumyl-substituents. Further reduction of 3[OTf]4 with 6 affords the unexpected formation of the notricyclane P7-type cation [(LC)3P7]3+ (9[OTf]3). Selective transition metal mediated [2 + 2]-fragmentation of 34+ is achieved when 3[OTf]4 is reacted with Fe2(CO)9, Pd(PPh3)4 and Pt(PPh3)4 leading to the formation of the dicationic diphosphene complexes [(η2-LCPPLC)Fe(CO)4]2+ (12[OTf]2) and [(η2-LCPPLC)M(PPh3)2]2+ (13[OTf]2 for M = Pd; 14[OTf]2 for M = Pt). In contrast, the reaction of 3[OTf]4 with an excess of AuCl(tht) gives rise to the formation of the five-membered ring complex [((LC)4P4)AuCl2]3+ (15[OTf]3), where the Au(i) atom reductively inserts into a P-P bond of 34+.