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

Abstract

Tetracationic cyclo-tetraphosphane [(L_C)₄P₄]⁴⁺ as triflate salt (3[OTf]₄) (L_C = 4,5-dimethyl-1,3-diisopropyl-imidazol-2-yl) is obtained in high yield from the reduction of [L_CPCl₂]⁺ (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 [L_nP_n]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 [(L_C)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-L_CPPL_C)Fe(CO)₄]²⁺ (12[OTf]₂) and [(η²-L_CPPL_C)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 [((L_C)₄P₄)AuCl₂]³⁺ (15[OTf]₃), where the Au(I) atom reductively inserts into a P–P bond of 3⁴⁺.