Phosphaalkynes from Acid Chlorides via P for O(Cl) Metathesis:  A Recyclable Niobium Phosphide (P^3-) Reagent that Effects C−P Triple-Bond Formation

Reported herein is a new, metathetical P for O(Cl) exchange mediated by an anionic niobium phosphide complex that furnished phosphaalkynes (RC⋮P) from acid chlorides (RC(O)Cl) under mild conditions. The niobaziridine hydride complex, Nb(H)(^tBu(H)CNAr)(N[Np]Ar)₂ (1, Np = neopentyl, Ar = 3,5-Me₂C₆H₃), has been shown previously to react with elemental phosphorus (P₄), affording the μ-diphosphide complex, (μ₂:η²,η²-P₂)[Nb(N[Np]Ar)₃]₂, (2), which can be subsequently reduced by sodium amalgam to the anonic, terminal phosphide complex, [Na][PNb(N[Np]Ar)₃] (3). It is now shown that treatment of 3 with either pivaloyl (t-BuC(O)Cl) or 1-adamantoyl (1-AdC(O)Cl) chloride provides the thermally unstable niobacyles, (t-BuC(O)P)Nb(N[Np]Ar)₃ (4-t-Bu) and (1-AdC(O)P)Nb(N[Np]Ar)₃ (4-1-Ad), which are intermediates along the pathway to ejection of the known phosphaalkynes t-BuC⋮P (5-t-Bu) and 1-AdC⋮P(5-1-Ad). Phosphaalkyne ejection from 4-t-Bu and 4-1-Ad proceeds with formation of the niobium(V) oxo complex ONb(N[Np]Ar)₃ (6) as a stable byproduct. Preliminary kinetic measurements for fragmentation of 4-t-Bu to 5-t-Bu and 6 in C₆D₆ solution are consistent with a first-order process, yielding the thermodynamic parameters ΔH^⧧ = 24.9 ± 1.4 kcal mol^-1 and ΔS^⧧ = 2.4 ± 4.3 cal mol^-1 K^-1 over the temperature range 308−338 K. Separation of volatile 5-t-Bu from 6 after thermolysis has been readily achieved by vacuum transfer in yields of 90%. Pure 6 is recovered after vacuum transfer and can be treated with 1.0 equiv of triflic anhydride (Tf₂O, Tf = O₂SCF₃) to afford the bistriflate complex, Nb(OTf)₂(N[Np]Ar)₃ (7), in high yield. Complex 7 provides direct access to 1 upon reduction with magnesium anthracene, thus completing a cycle of element activation, small-molecule generation via metathetical P-atom transfer, and deoxygenative recycling of the final niobium(V) oxo product.