Stable phosphorus radicals

Stable and persistent phosphorus radicals are phosphorus-centred radicals that are isolable and can exist for at least short periods of time. Radicals consisting of main group elements are often very reactive and undergo uncontrollable reactions, notably dimerization and polymerization. The common strategies for stabilising these phosphorus radicals usually include the delocalisation of the unpaired electron over a pi system or nearby electronegative atoms, and kinetic stabilisation with bulky ligands. Stable and persistent phosphorus radicals can be classified into three categories: neutral, cationic, and anionic radicals. Each of these classes involve various sub-classes, with neutral phosphorus radicals being the most extensively studied. Phosphorus exists as one isotope ³¹P with large hyperfine couplings relative to other spin active nuclei, making phosphorus radicals particularly attractive for spin-labelling experiments.

Neutral 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 for species containing one or two radical phosphorus centres respectively.

Monoradicals

In 1966, Muller et. al published the first electron paramagnetic resonance 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, phosphonyl, and phosphoranyl radicals.

Synthesis

Synthetic methods for obtaining neutral phosphorus mondoradicals include photolytic reduction of trivalent phosphorus chlorides, P-P homolytic cleavage, single electron oxidation of phosphines, and cleavage of P-S or P-Se bonds.
File:Lappert 1976.png|thumb|286x286px|Photolysis of three-coordinate phosphorus chloride for the synthesis of ₂P^• by Lappert and co-workers.
The first persistent two-coordinate phosphorus-centred radicals ₂P^• and ₂P^• were reported in 1976 by Lappert and co-workers. They are prepared by photolysis of the corresponding three-coordinate phosphorus chlorides in toluene in the presence of an electron-rich olifin. 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 ^• by Grützmacher and co-workers via reduction of phosphonium salt.
In 2001, Grützmacher et al. reported the first stable diphosphanyl radical ^• from the reduction of the phosphonium salt ⁺⁻ in an acetonitrile solution containing tetrakisethylene at room temperature, yielding yellow crystals. 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 x 10⁻² M.
File:Armstrong2004-corrected.png|center|thumb|433x433px|Synthesis of ^• by Armstrong and co-workers via oxidation.
The first structurally characterised phosphorus radical ^• was synthesised by Armstrong et al. in 2004 by the oxidation of the starting material with halogens bromide or iodine in a mixture of toluene and THF at 297 K. This produces blue crystals that can be characterised by X-ray crystallography. The steric bulk of the alkyl-imido groups was identified as playing a major role in the stabilising of these radicals.
In 2006, Ito et al. prepared an air tolerant and thermally stable 1,3-diphosphayclobutenyl radical. Sterically bulky phospholkyne 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 ring with the Mes* having torsional angles with the C₂P₂ plane.

Metal stabilised radicals

In 2007, Cummins et al. synthsised a phosphorus radical using nitridovanadium trisanilide metallo-ligands with similar form to Lappert, Power and co-workers' "jack-in-the-box" diphosphines. This is made by the synthesis of the radical precursor ClP₃]₂ followed by its one electron reduction with Ti₃ 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 ^• showing delocalisation of radical across vanadium and phosphorus nuclei.
Other metals stabilised radicals have been reported by Scheer et al, and Schneider et al using ligand containing tungsten and osmium respectively.

Structure and properties

As previously mentioned, kinetic stabilisation through bulky ligands has been an effective strategy for producing persisting phosphorus radicals. Delocalisation of the electron has also shown a stabilising effect on phosphorus radical species. This conversely results in more delocalised spin densities, and lower coupling constants relative to ³¹P localised electron spin. For this reason the spin localisation on the phosphorus atom varies widely for different phosphorus radical species.
Cyclic radicals like that by Ito at al have delocalisation across the rings. In this case X-ray, EPR spectroscopy, and ab initio calculations found that 80-90% of the spin was delocalised on the carbons in the C₂P₂ ring and the rest on the phosphorus atoms. Despite this, the a_P2 constant shows similar spectroscopic property to organic radicals that contain conjugated P=C doubles bond, justifying the resonance structure used for this species.
The phosphinyl radicals synthesised by Lappert and co-workers were found to be stable at room temperature for periods of over 15 days with no effect from short-term heating at 360 K. This stability was assigned to the steric bulk of the substituents and the absence of beta-hydrogen atoms. A structural study of this species conducted using X-ray crystallography, gas-phase electron diffraction, and ab initio molecular orbital calculations found that the source of this stability was not the bulkiness of the CH₂ ligands but the release of strain energy during homolytic cleavage at the P-P bond of the dimer that favoured the existence of the radical. The dimer shows a syn,anti conformation, which allows for better packing but has excessive crowding at the trimethylsilyl groups, while the radical monomer displays syn,syn conformation. Theoretical calculations showed that the process of cleaving the P-P bond, relaxation to release steric strain, and rotation about the P-C bond to yield syn,syn conformation on the monomer radical is an overall exothermic process. The stability of this species can therefore be attributed to the energy release of strain energy by the reorganisation of the ligands as the dimer converts to the radical monomer. This effect have been observed in other systems containing the CH₂ ligand and was dubbed the "Jack-in-the-box" model. Other ligand with similar flexibility, and ability to undergo conformational changes were identified as PnR₂ and ERR'₂.
In 2022, Streubel and co-workers investigated the electron density distribution across centres in metal-coordinated phosphanoxyl complexes. This study showed that tungsten-containing radical complexes have small amounts of spin density on the metal nuclei while in the case of manganese and iron, the spins are purely metal-centred.

Biradicals

Biradicals are molecules bearing two unpaired electrons. These radicals can interact ferromagnetically, antiferromagnetically or not interact at all. Biradicaloids/diradicaloids are a class of biradicals with significant radical centre interaction.

Synthesis

The first phosphorus biradical was reported in 2011 by T. Breweies and co-workers. The biradicaloid ₂ was synthesised by the reduction of cyclo-1,3-diphospha -2,4-diazanes using (Cp₂TiCl

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 ₂ radical.

Characterisation

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 ₂P^• had been determined by electron diffraction. The diphosphanyl radical ^• 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.