Sodium phosphaethynolate, Na(OCP), as a “P” transfer reagent for the synthesis of N-heterocyclic carbene supported P3 and PAsP radicals

Abstract

Sodium phosphaethynolate, Na(OCP), reacts as a P⁻ transfer reagent with the imidazolium salt [^DippNHC–H][Cl] [^DippNHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] to give the parent phosphinidene–carbene adduct, ^DippNHCPH, with the loss of CO. In a less atom economic reaction, the cage compound, P₇(TMS)₃ (TMS = SiMe₃) reacts likewise with the imidazolium salt to yield ^DippNHCPH thereby giving two entry points into parent phosphinidene-based chemistry. From the building block ^DippNHCPH, the carbene-supported P₃ cation [(^DippNHC)₂(μ-P₃)][Cl] was rationally synthesized using half an equivalent of PCl₃ in the presence of DABCO (1,4-diazabicyclo[2.2.2]octane). The corresponding arsenic analogue, [(^DippNHC)₂(μ-PAsP)][Cl], was synthesized in the same manner using AsCl₃. The reduction of both [(^DippNHC)₂(μ-P₃)][Cl] and [(^DippNHC)₂(μ-PAsP)][Cl] into their corresponding neutral radical species was achieved simply by reducing the compounds with an excess of magnesium. This allowed the electronic structures of the compounds to be investigated using a combination of NMR and EPR spectroscopy, X-ray crystallography, and computational studies. The findings of the investigation into (^DippNHC)₂(μ-P₃) and (^DippNHC)₂(μ-PAsP) reveal the central pnictogen atom in both cases as the main carrier of the spin density (∼60%), and that they are best described as the P₃ or PAsP analogues of the elusive allyl radical dianion. The phosphorus radical was also able to undergo a cycloaddition with an activated acetylene, followed by an electron transfer to give the ion pair [(^DippNHC)₂(μ-P₃)][P₃(C(COOMe))₂].