Stable phosphorus radicals

Neutral phosphorus radicals include a large range of conformations with varying spin densities at the phosphorus. Generally, they can categorised as mono- and bi/di-radicals (also referred to as bisradicals and biradicaloids) for species containing one or two radical phosphorus centres respectively. Monoradicals In 1966, Muller et. al published the first electron paramagnetic resonance (EPR/ESR) spectra displaying evidence for the existence of phosphorus-containing radicals. Since then a variety of phosphorus monoradicals have been synthesised and isolated. Common ones include phosphinyl (R2P•), phosphonyl (R2PO•), and phosphoranyl (R4P•) radicals. In 2000, the Power group found that this species can be synthesised from the dissolution, melting or evaporation of the dimer. File:Grützmacher 2001.png|thumb|286x286px|Synthesis of the first stable diphosphanyl radical [Mes*MeP-PMes*]• by Grützmacher and co-workers via reduction of phosphonium salt. The monomer is stable below -30 ºC in the solid state for a few days. At room temperature the species decomposes in solution and in the solid state with a half life of 30 minutes at 3 × 10−2 M. File:Armstrong2004-corrected.png|center|thumb|433x433px|Synthesis of [Me3SiNP(μ3-NtBu)3{μ3-Li(thf)}3X]• (X = Br, I) by Armstrong and co-workers via oxidation. The steric bulk of the alkyl-imido groups was identified as playing a major role in the stabilising of these radicals. File:Ito2006.png|center|thumb|530x530px|Synthesis of air tolerant and air stable 1,3-diphosphayclobutenyl radical by Ito and co-workers via reduction. Sterically bulky phospholkyne (Mes*C≡P) is treated with 0.5 equiv of t-BuLi in THF to form a 1,3 diphosphaalkyl anion. This is reduced with iodine solution to form a red product. The species is a planar four-membered diphosphacyclobutane (C2P2) ring with the Mes* having torsional angles with the C2P2 plane. This is made by the synthesis of the radical precursor ClP[NV{N(Np)Ar}]3]2 followed by its one electron reduction with Ti[N(tBu)Ar]3 or potassium graphite to yield dark brown crystals in 77% yield. EPR data showed delocalisation of electron spin across the two 51V and one 31P nuclei. This was consistent with computation, supporting the reported resonance structures. This delocalisation across the vanadium atoms was identified as the source of stabilisation for this species due to the ease for transition metals to undergo one-electron chemistry. Cummins and co-workers postulated that the p-character of the system could be tuned by changing the metal centres. File:Cummins2007.png|center|thumb|618x618px|Resonance structures of [P{NV[N(Np)Ar]3}2]• showing delocalisation of radical across vanadium and phosphorus nuclei. Structure and properties (triplet), antiferromagnetically (open-shell singlet) or not interact at all (two-doublet). The bulky Ter (trimesitylphenyl) and Hyp (hypersilyl) substituents provide a large stabilising effect. This effect is more pronounced with Ter where the biradical is stable in inert atmospheres in the solid state for long periods of time at temperatures up to 224 C. Computational studies determined that the [P(μ-NTer)]2 radical shows an openshell singlet ground state biradical character. File:Villinger2011.png|center|thumb|633x633px|Synthesis of [P(μ-NR)]2 (R=Hyp, Ter) via reduction of cyclo-1,3-diphospha(III)-2,4-diazanes and subsequent CO insertion by Villinger and co-workers. The species was found to be diamagnetic with temperature-independent NMR resonances, so can be considered a non-Kekulé molecule. Structure and properties The species by Villinger can undergo reaction with phosphaalkyne forming a five-membered P2N2C heterocycle with a P-C bridge. It can also undergo halogenation and reaction with elemental sulfur. File:Villinger reactivity.png|center|thumb|522x522px|Reactivity of [P(μ-NR)]2 (R=Hyp, Ter) radical. Characterisation File:Armstrong2004 2.png|thumb|398x398px|Solvation of lithium ions in [Me3SiNP(μ3-NtBu)3{μ3-Li(thf)}3I]• in very dilute THF solutions. Phosphorus radicals are commonly characterized by EPR/ESR to elucidate the spin localisation of the radical across the radical species. Higher coupling constants are indicative of higher localisation on phosphorus nuclei. Quantum chemical calculations on these systems are also used to support this experimental data. Before the characterization by X-ray crystallography by Armstrong et al, the structure of the phosphorus centred radical [(Me3Si)2CH]2P• had been determined by electron diffraction. The diphosphanyl radical [Mes*MeP-PMes*]• had been stabilised through doping into crystals of Mes*MePPMeMes*. The radical synthesised by Armstrong et al was found to exist as a distorted PN3Li3X cube in the solid state. They found that upon dissolution in THF, this cubic structure is disrupted, leaving the species to form a solvent-separated ion pair. == Phosphorus radical cations ==

Synthesis Phosphorus radical cations are often obtained from the one-electron oxidation of diphosphinidenes and phosphalkenes. File:Bertand2010.png|thumb|382x382px|Synthesis of [(cAAC)2P2]•+ and [(NHC)2P2]•+ via oxidation with Ph3C+B(C6F5)4− by Bertrand and co-workers..  The Bertrand group reported the synthesis of [(cAAC)P2]•+ , [(NHC)P2]•+ and [(NHC)P2]++ . The EPR signal for [(cAAC)P2]•+ is a triplet of quintents, resulting form coupling to with 2 P nuclei and a small coupling with 2 N nuclei. NBO analysis showed spin delocalisation across two phosphorus atoms (0.27e each) and nitrogen atoms(0.14e each). Contrastingly, the [(NHC)P2]•+complex showed delocalisation mostly on phosphorus (0.33e and 0.44e) with little contribution of other elements. and [bis(carbene)-PN]•+ with spin density distributed over phosphorus (0.40e), central nitrogen atom (0.18e), and N atom of cAAC (0.19e). Treatment with this later cation with KC8 returns it to its neutral analogue.File:Geoffroy2003.png|center|thumb|379x379px|Synthesis of Mes*P•-(C(NMe2)2)+ via a one electron oxidation of a phosphaalkenes with [Cp2Fe]PF6 by Geoffroy and co-workers. A solution of Mes*P•-(C(NMe2)2)+ is stable in inert atmosphere in the solid state for a few weeks and a few days in solution. Hyperfine couplings on EPR show strong localisation of the spin to the phosphorus nuclei (0.75e in p orbital). In 2015, the Wang group was able to isolate the crystal structure of this species with use of the oxidant of a weakly coordinating anion Ag[Al(ORF)4]−. The electron spin density, found by EPR, resides principally on phosphorus 3p and 3s orbitals (68.2% and 2.46% respectively). This was supported by DFT calculations where 80.9% of spin density was found to be localised on phosphorus atom. Weakly coordinating anions were also used to stabilise cyclic biradical cations synthesised by Schulz and colleagues where the spin density was found to reside exclusively on the phosphorus atoms (0.46e each) in the case of [P(μ-NTer)2P]•+. In the case of [P(μ-NTer)2As]•+ the spin was found to mostly reside on the As nuclei (70.6% on As compared to 29.4% on P atom). Many other cyclic radical cations have been reported. File:Ghadwal2019.png|center|thumb|366x366px|Synthesis of divinyldiphosphene radical cations via oxidation with GaCl3 by Ghadwal and co-workers. The spin density was found to be 11-14% on each P nuclei and 17-21% on each C nuclei. Weakly coordinating anions have been used to stabilise biradical cations. == Phosphorus radical anions ==

Synthesis The most common method for accessing radical anions is through the use of reducing agents. File:Wang2016.png|center|thumb|564x564px|Synthesis of diphosphorus-centred radical anion and the di-radical di-anion via reduction with KC8 by Wang and co-workers. EPR data showed localisation of the spin on 3p (51.09%) and 3s (1.62%) orbitals of phosphorus. They later synthesised a diphosphorus-centred radial anion and the first di-radical di-anion from the reduction of the diphosphaalkene with KC8 in THF in the presence of 18-crown-6. In both cases the spin density resides principally on the phosphorus nuclei. File:Tan2017.png|center|thumb|495x495px|Synthesis of phosphorus radical anion coordinated with CoII and FeII complexes by Tan and co-workers. This species displays a quartet ground state showing weak antiferromagnetic interaction of the phosphorus radical with the high-spim TMII ion. The spin density is mostly localised on TM and phosphorus nuclei. File:Yamashitaz2020.png|center|thumb|359x359px|Synthesis of phosphorus radical anion with boryl substituents by Yamashita and co-workers. Here the diazafluorenylidene-substituted phosphaalkene is reacted with [Cp*2Ln][BPh4] (Ln = Dy, Tb, and Gd) followed by reduction with KC8 in the absence or presence of 2,2,2-cryptand yielding complexes with radical anion phosphaalkene fragments. EPR and DFT calculations indicate spin density mostly localised on the P nuclei (67.4%). == Further reading ==