White phosphorus activation at a metal-phosphorus triple bond: a new route to cyclo-triphosphorus or cyclo-pentaphosphorus complexes of niobium.

The Nb-P triple bond in P≡Nb(N[Np]Ar)(3) (Np = CH(2)(t)Bu; Ar = 3,5-Me(2)C(6)H(3)) has produced the first case of P(4) activation by a metal-ligand multiple bond. Treatment of P(4) with the sodium salt of the niobium phosphide complex in weakly coordinating solvents led to formation of the cyclo-P(3) anion (P(3))Nb(N[Np]Ar)(3). Treatment in tetrahydrofuran (THF) led to the formation of a cyclo-P(5) anion (Ar[Np]N)(η(4)-P(5))Nb(N[Np]Ar)(2), which represents a rare example of a substituted pentaphosphacyclopentadienyl ligand. The P(4) activation pathway was shown to depend on the dimer-monomer equilibrium of the niobium phosphide reagent, which, in turn, depends on the solvent used for the reaction. The pathway leading to the cyclo-P(3) product was shown to require a 2:1 ratio of the phosphide anion to P(4), while the cyclo-P(5) formation requires a 1:1 ratio. The cyclo-P(3) salt has been isolated in 56% yield as orange crystals of the Na(THF)(P(3))Nb(N[Np]Ar)(3) dimer or in 83% yield as an orange powder of [Na(12-crown-4)(2)][(P(3))Nb(N[Np]Ar)(3)]. A solid-state X-ray diffraction experiment on the former salt revealed that each Nb-P(3) unit exhibits pseudo-C(3) symmetry, while (31)P NMR spectroscopy showed a sharp signal at -223 ppm that splits into a doublet-triplet pair below -50 °C. It was demonstrated that this salt can serve as a P(3)(3-) source upon treatment with AsCl(3), albeit with modest yield of AsP(3). The cyclo-P(5) salt was isolated in 71% yield and structurally characterized from red crystals of [Na(THF)(6)][(Ar[Np]N)(η(4)-P(5))Nb(N[Np]Ar)(2)]. The anion in this salt can be interpreted as the product of trapping of an intermediate pentaphosphacycplopentadienyl structure through migration of one anilide ligand onto the P(5) ring. The W(CO)(5)-capped cyclo-P(3) salt was also isolated in 60% yield as [Na(THF)][(OC)(5)W(P(3))Nb(N[Np]Ar)(3)] from the activation of 0.5 equiv of P(4) with the sodium salt of the tungsten pentacarbonyl adduct of the niobium phosphide anion.