Isolation of singlet carbene derived 2-phospha-1,3-butadienes and their sequential one-electron oxidation to radical cations and dications

Abstract

A synthetic strategy for the 2-phospha-1,3-butadiene derivatives [{(IPr)C(Ph)}P(cAAC^Me)] (3a) and [{(IPr)C(Ph)}P(cAAC^Cy)] (3b) (IPr = C{(NDipp)CH}₂, Dipp = 2,6-iPr₂C₆H₃; cAAC^Me = C{(NDipp)CMe₂CH₂CMe₂}; cAAC^Cy = C{(NDipp)CMe₂CH₂C(Cy)}, Cy = cyclohexyl) containing a CC–PC framework has been established. Compounds 3a and 3b have a remarkably small HOMO–LUMO energy gap (3a: 5.09; 3b: 5.05 eV) with a very high-lying HOMO (−4.95 eV for each). Consequently, 3a and 3b readily undergo one-electron oxidation with the mild oxidizing agent GaCl₃ to afford radical cations [{(IPr)C(Ph)}P(cAAC^R)]GaCl₄ (R = Me 4a, Cy 4b) as crystalline solids. The main UV-vis absorption band for 4a and 4b is red-shifted with respect to that of 3a and 3b, which is associated with the SOMO related transitions. The EPR spectra of compounds 4a and 4b each exhibit a doublet due to coupling of the unpaired electron with the ³¹P nucleus. Further one-electron removal from the radical cations 4a and 4b is also feasible with GaCl₃, affording the dications [{(IPr)C(Ph)}P(cAAC^R)](GaCl₄)₂ (R = Me 5a, Cy 5b) as yellow crystals. The molecular structures of compounds 3–5 have been determined by X-ray diffraction and analyzed by DFT calculations.