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 = CH2tBu; Ar = 3,5-Me2C6H3) has produced the first case of P4 activation by a metal–ligand multiple bond. Treatment of P4 with the sodium salt of the niobium phosphide complex in weakly coordinating solvents led to formation of the cyclo-P3 anion [(P3)Nb(N[Np]Ar)3]−. Treatment in tetrahydrofuran (THF) led to the formation of a cyclo-P5 anion [(Ar[Np]N)(η4-P5)Nb(N[Np]Ar)2]−, which represents a rare example of a substituted pentaphosphacyclopentadienyl ligand. The P4 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-P3 product was shown to require a 2:1 ratio of the phosphide anion to P4, while the cyclo-P5 formation requires a 1:1 ratio. The cyclo-P3 salt has been isolated in 56% yield as orange crystals of the [Na(THF)]2[(P3)Nb(N[Np]Ar)3]2 dimer or in 83% yield as an orange powder of [Na(12-crown-4)2][(P3)Nb(N[Np]Ar)3]. A solid-state X-ray diffraction experiment on the former salt revealed that each Nb–P3 unit exhibits pseudo-C3 symmetry, while 31P 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 P33– source upon treatment with AsCl3, albeit with modest yield of AsP3. The cyclo-P5 salt was isolated in 71% yield and structurally characterized from red crystals of [Na(THF)6][(Ar[Np]N)(η4-P5)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 P5 ring. The W(CO)5-capped cyclo-P3 salt was also isolated in 60% yield as [Na(THF)][(OC)5W(P3)Nb(N[Np]Ar)3] from the activation of 0.5 equiv of P4 with the sodium salt of the tungsten pentacarbonyl adduct of the niobium phosphide anion.