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

Abstract

From Na(OCP) as a “P” transfer reagent, the radicals (NHC–P–E–P–NHC) (E = P, As) were synthesized and characterized as donor–acceptor adducts by EPR spectroscopy and DFT computations.

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, ^DippNHCPH, with the loss of CO. In a less atom economic reaction, the cage compound, P ₇(TMS) ₃ (TMS = SiMe ₃) reacts likewise with the imidazolium salt to yield ^DippNHCPH thereby giving two entry points into parent phosphinidene-based chemistry. From the building block ^DippNHCPH, the carbene-supported P ₃ cation [( ^DippNHC) ₂(μ-P ₃)][Cl] was rationally synthesized using half an equivalent of PCl ₃ 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 AsCl ₃. The reduction of both [( ^DippNHC) ₂(μ-P ₃)][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-P ₃) 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 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 [( ^DippNHC) ₂(μ-P ₃)][P ₃(C(COOMe)) ₂].