Differing reactivities of P≡CMe and P≡CBu^t towards a triphosphabenzene and a tetraphosphabarrelene: Synthesis of new phosphaalkyne pentamers (P₅C₅Me_nBu^t_5−n, n = 0, 1 or 2)

The reaction of excess P≡CMe with the triphosphabenzene, 1,3,5-P₃C₃Bu^t₃, yields a phosphaalkyne pentamer, P₅C₅Me₂Bu^t₃, which displays a pentaphosphaisolumibullvalene core structure. Its treatment with [W(CO)₅(THF)] gives a complex of this cage, [(W(CO)₅)₂(µ-η¹:η¹-P₅C₅Me₂Bu^t₃)], which has been structurally characterised. In contrast, the previously reported reaction of P≡CBu^t with 1,3,5-P₃C₃Bu^t₃, affords, in addition to the known tetraphosphabarrelene, 1,3,5,7-P₄C₄Bu^t₄, a new phosphaalkyne pentamer (P₅C₅Bu^t₅), which has a partially unsaturated “open cage” core. Although P≡CBu^t does not react with 1,3,5,7-P₄C₄Bu^t₄, the reaction of P≡CMe with the tetraphosphabarrelene is shown to give a mixture of products. Treatment of these with [W(CO)₅(THF)] leads to the isolation of the tungsten carbonyl complex, [(W(CO)₅)(W(CO)₄)(µ-η¹:η⁴-P₅C₅MeBu^t₄)], which has been structurally characterised. This study suggests that P≡CMe has a significantly greater reactivity towards cycloadditions than its bulkier counterpart, P≡CBu^t.