Sodium phosphaethynolate, Na(OCP), as a P transfer reagent for the synthesis of N-heterocyclic carbene supported P-3 and PAsP radicals

Sodium phosphaethynolate, Na(OCP), reacts as a P- transfer reagent with the imidazolium salt [(NHC)-N-Dipp-H]-[Cl] [(NHC)-N-Dipp = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene] to give the parent phosphinidene-carbene adduct, (NHC)-N-Dipp=PH, with the loss of CO. In a less atom economic reaction, the cage compound, P-7(TMS)(3) (TMS = SiMe3) reacts likewise with the imidazolium salt to yield (NHC)-N-Dipp=PH thereby giving two entry points into parent phosphinidene-based chemistry. From the building block (NHC)-N-Dipp=PH, the carbene-supported P-3 cation [((NHC)-N-Dipp)(2)(mu-P-3)][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, [((NHC)-N-Dipp)(2)(mu-PAsP)][Cl], was synthesized in the same manner using AsCl3. The reduction of both [((NHC)-N-Dipp)(2)(mu-P-3)][Cl] and [((NHC)-N-Dipp)(2)(m-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 ((NHC)-N-Dipp)(2()mu-P-3) and ((NHC)-N-Dipp)(2)(mu-PAsP) reveal the central pnictogen atom in both cases as the main carrier of the spin density (similar to 60%), and that they are best described as the P-3 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 [((NHC)-N-Dipp)(2)(mu-P-3)][P-3(C(COOMe))(2)].