Phosphaalkenes with Inverse Electron Density:  Electrochemistry, Electron Paramagnetic Resonance Spectra, and Density Functional Theory Calculations of Aminophosphaalkene Derivatives

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Vol. 107; no. 24; pp. 4883 - 4892
• Main Authors: Rosa, Patrick, Gouverd, Cyril, Bernardinelli, Gérald, Berclaz, Théo, Geoffroy, Michel
• Format: Journal Article
• Language: English
• Published: American Chemical Society 19.06.2003
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp030023a

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 31P and 13C 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 π 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 •+PF6 - . As shown by DFT calculations, this behavior is consistent with the decrease of the double bond character of the phosphorus−carbon bond caused by the presence of the amino groups.