Stable phosphorus radicals

[1] Radicals consisting of main group elements are often very reactive and undergo uncontrollable reactions, notably dimerization and polymerization.

al published the first electron paramagnetic resonance (EPR/ESR) spectra displaying evidence for the existence of phosphorus-containing radicals.

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. The first structurally characterised phosphorus radical [Me3SiNP(μ3-NtBu)3{μ3-Li(thf)}3X]• (X = Br, I) 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.

[8] 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 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.

[11][12] As previously mentioned, kinetic stabilisation through bulky ligands has been an effective strategy for producing persisting phosphorus radicals.

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 aP2 constant shows similar spectroscopic property to organic radicals that contain conjugated P=C doubles bond, justifying the resonance structure used for this species.

[8] 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.[4] 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 CH(SiMe3)2 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.

Theoretical calculations showed that the process of cleaving the P-P bond (endothermic), relaxation to release steric strain, and rotation about the P-C bond to yield syn,syn conformation on the monomer radical (exothermic by 67.5 kJ for each unit) is an overall exothermic process.

[1] 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.

They found that upon dissolution in THF, this cubic structure is disrupted, leaving the species to form a solvent-separated ion pair.

In 2010, the Bertrand group found that carbene-stabilised diphosphinidenes can undergo one-electron oxidation in toluene with Ph3C+B(C6F5)4− at room temperature in inert atmosphere to produce radical cations (Dipp=2,6-Diisopropylphenyl)[22].

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]−.

[26] 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]•+.

Ghadwal and co-workers were able to synthesise a diphosphene radical cation [{(NHC)C(Ph)}P]2•+ using an NHC-derived divinyldiphosphene with a high lying HOMO and a small HOMO-LUMO gap.

In 2014 the Wang group reported the synthesis of a phosphorus-centred radical anion through the reduction of a phosphaalkene using either Li in DME or K in THF yielding purple crystals.

Tan and co-workers used a charge transfer approach to synthesis the phosphorus radical anion coordinated CoII and FeII complexes.

[34] This species displays a quartet ground state showing weak antiferromagnetic interaction of the phosphorus radical with the high-spim TMII ion.

Spin Density map on phosphinyl radical found by NBO analysis.
Photolysis of three-coordinate phosphorus chloride for the synthesis of [(Me 3 Si) 2 N] 2 P • by Lappert and co-workers. [ 4 ]
Synthesis of the first stable diphosphanyl radical [Mes*MeP-PMes*] • by Grützmacher and co-workers via reduction of phosphonium salt. [ 6 ]
Synthesis of [Me 3 SiNP(μ 3 -N t Bu) 3 {μ 3 -Li(thf)} 3 X] • (X = Br, I) by Armstrong and co-workers via oxidation. [ 7 ]
Synthesis of air tolerant and air stable 1,3-diphosphayclobutenyl radical by Ito and co-workers via reduction. [ 8 ]
Resonance structures of [P{NV[N(Np)Ar] 3 } 2 ] • showing delocalisation of radical across vanadium and phosphorus nuclei. [ 10 ]
Schematic of DFT calculation results for diphosphine radical 1 in the solid state, the syn,anti- • PR 2 radical ( 1A 2 and 1A 2 ), the H optimised radical ( 1B 1 and 1B 2 ), the syn,anti - • PR 2 radical fully optimised ( 1C ), and syn,syn - • PR 2 radical in optimised geometry 2 . Energies are in kJ mol −1 . [ 13 ] Illustrating the "Jack-in-the-box" model.
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. [ 19 ] [ 20 ]
Synthesis of ( i Pr)CP] 2 radical via reduction by Rottschafer and co-workers with resonance structures. [ 21 ]
Reactivity of [P(μ-NR)] 2 (R=Hyp, Ter) radical. [ 20 ]
Solvation of lithium ions in [Me 3 SiNP(μ 3 -N t Bu) 3 {μ 3 -Li(thf)} 3 I] • in very dilute THF solutions. [ 7 ]
Synthesis of [(cAAC)2P2]•+ and [(NHC)2P2]•+ via oxidation with Ph 3 C + B(C 6 F 5 ) 4 − by Bertrand and co-workers. [ 22 ]
Synthesis of [(TMP)P(cAAC)] •+ via oxidation with Ph 3 C + (C 6 F 5 ) 4 B − by Bertrand and co-workers. [ 23 ]
Synthesis of [bis(carbene)-PN] •+ visa oxidation with h 3 C + (C 6 F 5 ) 4 B − by Bertrand and co-workers. [ 24 ]
Synthesis of Mes*P • -(C(NMe 2 ) 2 ) + via a one electron oxidation of a phosphaalkenes with [Cp 2 Fe]PF 6 by Geoffroy and co-workers. [ 25 ]
General scheme for preparation of cyclic radical cations via oxidation. [ 27 ]
Synthesis of divinyldiphosphene radical cations via oxidation with GaCl 3 by Ghadwal and co-workers. [ 30 ]
Synthesis of phosphorus-centred radical anion via reduction usgin K or Li by Wang and co-workers.
Synthesis of diphosphorus-centred radical anion and the di-radical di-anion via reduction with KC8 by Wang and co-workers. [ 32 ]
Synthesis of phosphorus radical anion coordinated with Co II and Fe II complexes by Tan and co-workers. [ 34 ]
Synthesis of phosphorus radical anion with boryl substituents by Yamashita and co-workers. [ 36 ]