Radical Anions and Radical Trianions of a Phosphabenzene and Some Diphospha Derivatives of Biphenyl,p-Terphenyl andp,p′-Quaterphenyl: an ESR and ENDOR Study

Abstract

The radical anions and radical trianions of 2,4,6-triphenylphosphabenzene (1), 3,3′,5,5′-tetraphenyl-4λ3,4′λ3-diphosphabiphenyl-1,1′ (2), 3,3″,5,5″-tetraphenyl-4λ3,4″λ3-diphosphaterphenyl-1,1′:4′,1″ (3) and 3,3‴,5,5‴-tetraphenyl-4λ3,4‴λ3-diphosphaquaterphenyl-1,1′:4′,4″:1″,1‴ (4) were studied by ESR and (in part) ENDOR spectroscopy. The singly occupied orbital in the radical anions is symmetric with respect to the vertical mirror plane passing through the phosphorus atom in 1·- or two such atoms in 2·-–4·-. Accordingly, the prominent feature of these species is the large and positive 31P coupling constant (1·-,+3.31; 2·-,+2.62; 3·-,+2.02; and 4·-,+1.67 mT), which gives rise to a pronounced hyperfine anisotropy. The anions 1·-–4·- can thus be regarded as ‘P-centred radicals’ (g=2.0045±0.0003). In contrast, the trianions 1·3-–4·3- have to be classified as ‘hydrocarbon radicals’ (g=2.0027±0.0001). For 1·3-, the hyperfine data could be precisely analysed with the use of the ENDOR technique. As the nodal plane of the singly occupied orbital in this trianion passes through the phosphorus atom, the 31P coupling constant is small and negative (-0.267 mT); the largest value (-0.473 mT) belongs to the two protons in the 3,5-positions of the phosphabenzene ring. The hyperfine patterns of the radical trianions 2·3-–4·3- resemble that of 1·3- with the π-spin population being thus restricted to one phosphabenzene ring and its two phenyl substituents. Such a ‘localization’ is considered to be due to a degeneracy or near-degeneracy of the singly occupied orbital and to a perturbation caused by the association with three alkali metal counterions. © 1997 John Wiley & Sons, Ltd.