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 w ere investigated through EPR spectroscopy, cyclic voltammetry, and pulse ra diolysis. 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, dependi ng on solvent. Whilst the starting compound does not significantly absorb b eyond 300 nm, the absorption spectrum of the radical cation is characterise d by bands centred at ca. 320 and ca. 410 nm: according to EOM-CCSD calcula tions on the unsubstituted diphosphaallene the latter band should be due to a transition from the SOMO to the second LUMO, while the former might resu lt from the overlapping of three different transitions. The EPR spectra rec orded in the present study upon reduction of the title compound are far mor e complex than those recently published, and significant differences have b een observed upon chemical or electrochemical reduction. A higher spin dens ity 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 predict ed by UB3LYP calculations on the model compound diphenyl-1,3-diphosphaallen e, which also predict for both ions the existence of cis and trans geometri cal isomers, the latter being more stable. In both radical ions the unpaire d electron is mainly localised in the PCP moiety, namely in a pi-allylic ty pe 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.