Characterization of transient radicals in the reduction product of the -P=C=C=P- system: EPR and theoretical studies

The EPR spectrum obtained at room temperature after electrochemical or chemical reduction of a solution of Ar–P=C=C=P–Ar in THF exhibits hyperfine interaction (165 MHz) with two equivalent 31P nuclei. Additional couplings with two equivalent 13C are observed with Ar–P=13C=13C=P–Ar. The 31P anisotropic coupling constants could be obtained from spectra Recorded at low temperature. They indicate that the unpaired electron is mainly localized (78%) on the two phosphorus atoms. Quantum chemical calculations (DFT and ab initio SCI) were performed on the various isomers of the two radical anions: [H–P=C=C=P–H]and [H–P=CH–CH=P–H]. Although the optimized geometries of these two species are clearly different, neither of them leads to 13C/31P hyperfine tensors in conflict with the experimental results. The absence of any 1H splitting on the EPR spectrum together with the quasi-reversibility of the reduction wave make the identification of [Ar–P=C=C=P–Ar] more probable