The radical ions from bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphaallene. A multi-disciplinary approach and a reassessment

The radical ions from bis(2,4,6-tri-tert-butylphenyl)-1,3-diphosphaallene were investigated through EPR spectroscopy, cyclic voltammetry, and pulse radiolysis. Cyclic voltammetry indicated an oxidation potential bf 2.0 V vs. SCE and a reduction potential in the range -1.97 to -2.10 V vs SCE, depending on solvent. Whilst the starting compound does not significantly absorb beyond 300 nm, the absorption spectrum of the radical cation is characterised by bands centred at ca. 320 and ca. 410 nm: according to EOM-CCSD calculations on the unsubstituted diphosphaallene the latter band should be due to a transition from the SOMO to the second LUMO, while the former might result from the overlapping of three different transitions. The EPR spectra recorded in the present study upon reduction of the title compound are far more complex than those recently published, and significant differences have been observed upon chemical or electrochemical reduction. A higher spin density on the phosphorus atoms is observed in the anion than in the cation, in agreement with the different nature of the SOMO in the two species predicted by UB3LYP calculations on the model compound diphenyl-1,3-diphosphaallene, which also predict for both ions the existence of cis and trans geometrical isomers, the latter being more stable. In both radical ions the unpaired electron is mainly localised in the PCP moiety, namely in a pi-allylic type MO in the cation and in a a MO in the anion. These pictures of the SOMO are opposite to those recently published following MP2 calculations on the unsubstituted diphosphaallene.