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

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-iPr(2)C(6)H(3); cAAC(Me) = C{(NDipp)CMe2CH2CMe2}; cAAC(Cy) = C{(NDipp)CMe2CH2C(Cy)}, Cy = cyclohexyl) containing a Cxe001;C-Pxe001;C 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 P-31 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.