Phosphaalkenes with inverse electron density: Electrochemistry, electron paramagnetic resonance spectra, and density functional theory calculations of aminophosphaalkene derivatives

Cyclic voltammetry of Mes*P=C(NMe2)(2) (1) and Mes*P=C(CH3)NMe2 (2) shows that, in solution in DME, these compounds are reversibly oxidized at 395 and 553 mV, respectively. Electrochemical oxidation or reaction of 1 (or 2) with [Cp2Fe]PF6 leads to the formation of the corresponding radical cation, which was characterized by its electron paramagnetic resonance (EPR) spectra. Experimental P-31 and C-13 isotropic and anisotropic coupling constants agree with density functional theory (DFT) calculations showing that the unpaired electron is strongly localized on the phosphorus atom, in accord with the description Mes*P-.-(C(NMe2)(2))(+). Electrochemical reduction of 1 is essentially irreversible and leads to a radical species largely delocalized on the C(NMe2)(2) moiety; this neutral radical results from the protonation of the phosphorus atom and corresponds to Mes*(H)P-C-.(NMe2)(2). No paramagnetic species is obtained by reduction of 2. The presence of the amino groups, responsible for the inverted electron distribution at the P-C double bond (P--C+), confers on 1 and 2 redox properties that are in very sharp contrast with those observed for phosphaalkenes with a normal pi electron distribution (P+-C-): no detection of the radical anion but easy formation of a rather persistent radical cation. For 1, this radical cation could even be isolated as a powder, 1(.+)PF(6)(-). As shown by DFT calculations, this behavior is consistent with the decrease of the double bond character of the phosphoruscarbon bond caused by the presence of the amino groups.