An EPR and ab initio Study of a Phosphaalkene Radical Anion, and Comparison with other Phosphorus-Containing Radical Ions

Electron Paramagnetic Resonance (EPR) is an efficient method to obtain information about the spin densities in organic radicals containing a heteroatom and, when used in conjonction with ab initio calculations, this spectroscopy can yield a precise description of the structure of these species. In the present study, our purpose is to compare the spin delocalization on the following three paramagnetic moieties (-P=C<)', (-P=P-)" and (>P-P<) and to verify to what extent the ab initio predictions agree with the experimental hyperfine tensors. For each of these species the unpaired electron is expected to be mainly localized on both a phosphorus and an adjacent atom and the experimental parameters will consist in the P and C coupling tensors obtained from frozen solutions. Organic molecules containing a trivalent dicoordinated phosphorus atom are very reactive; they can nevertheless be isolated at room temperature when the organic group which is linked to the phosphorus atom by a single bond is very cumbersome. 2,4,6tri(tertbutyl)phenyl-phenylmethylenphosphaalkene I and bis(2,4,6-tri-tert-butylphenyl)diphosphene II are good examples of such compounds.