EPR and theoretical studies of the reduction product of the fulvenephosphaallene system

Fluoren-9-ylidenemethylene-(2,4,6-tri-tert-butyl-phenyl)phosphane (2), a new type of phosphaallene with the terminal carbone incorporated in a cyclopentadienyl ring, has been synthesized and its crystal structure has been determined. The 31P and 13C (central carbon) hyperfine tensors of the reduction compound of this phosphaallene have been measured on the EPR spectra recorded after electrochemical reduction of a solution of 2 in THF. Structures of the model molecules HPCCp (where Cp is a cyclopentadienyl ring), [HPCCp]radical dot− and [HPCHCp]radical dot have been optimized by DFT and the hyperfine couplings of the paramagnetic species have been calculated by DFT and SCI methods. The comparison between the experimental and the theoretical results shows that, in solution, the radical anion [2]radical dot− is readily protonated and that the EPR spectra are due to the phosphaallylic radical.