Differing reactivities of p[triple band]CMe and P[triple band]CBu(t) towards a triphosphabenzene and a tetraphosphabarrelene: synthesis of new phosphaalkyne pentamers (P5C5Me(n)Bu(t)(5-n) n = 0, 1 or 2)

The reaction of excess P[triple band]CMe with the triphosphabenzene, 1,3,5-P3C3Bu(t)3, yields a phosphaalkyne pentamer, P5C5Me2Bu(t)3, which displays a pentaphosphaisolumibullvalene core structure. Its treatment with [W(CO)5(THF)] gives a complex of this cage, [{W(CO)5}2(mu-eta1:eta1-P5C5Me2Bu(t)3)], which has been structurally characterised. In contrast, the previously reported reaction of P[triple band]CBu(t) with 1,3,5-P3C3Bu(t)3, affords, in addition to the known tetraphosphabarrelene, 1,3,5,7-P4C4Bu(t)4, a new phosphaalkyne pentamer (P55C5Bu(t)5), which has a partially unsaturated "open cage" core. Although P[triple band]CBu(t) does not react with 1,3,5,7-P4C4Bu(t)4, the reaction of P[triple band]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)} {W(CO)4}(mu-eta1:eta4-P5C5MeBu(t)4)], which has been structurally characterised. This study suggests that P[triple band]CMe has a significantly greater reactivity towards cycloadditions than its bulkier counterpart, P[triple band]CBu(t).