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)CMe(2)CH(2)CMe(2)}; cAAC(Cy) = C{(NDipp)CMe(2)CH(2)C(Cy)}, Cy = cyclohexyl) containing a C[double bond, length as m-dash]C–P[double bond, length as m-dash]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 GaCl(3) to afford radical cations [{(IPr)C(Ph)}P(cAAC(R))]GaCl(4) (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 (31)P nucleus. Further one-electron removal from the radical cations 4a and 4b is also feasible with GaCl(3), affording the dications [{(IPr)C(Ph)}P(cAAC(R))](GaCl(4))(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.