Stable phosphinous acids

The electronic properties of organyl element compounds are strongly influenced by the electronic characteristics of the organic substituents. The bonding of two CF₃ groups to a phosphorus atom effects a drastically decreased basicity. That is the phosphorus atom is the least basic centre in the compound (CF₃)₂POH. This compound, synthesized in 1960 by Burg and Griffiths, is the only known example of a phosphinous acid, although there should be a general interest in this class of compounds. However, only a few investigations have been reported which may be explained by the tedious and risky synthesis. In this paper a safe one step and high yield synthesis of (CF₃)₂POH is described. The compound (C₆F₅)₂POH, originally claimed as a phosphinous acid, is proved to exist at room temperature exclusively in the tautomeric oxide form. (C₆F₅)₂P(O)H crystallizes in the triclinic space group (no. 2) with a 992.9(1) pm; b 1501.9(2) pm; c 1539.4(2) pm; α 117.48(1)°; β 100.39(1)°; γ 96.02(1)° and Z 6.Quantum chemical investigations prove the electron withdrawing effect of s-triazinyl groups (1,3,5-triazin-4-yl derivatives) to be much stronger than that of pentafluorophenyl groups. Quantum chemical calculations at the B3PW91/6-311G(3d,p) level of theory predict for the bis(s-triazinyl) derivative (C₃N₃H₂)₂POH the phosphinous acid isomer to be favored by ΔE_ZP = 22 kJ/mol in relation to the corresponding phosphane oxide isomer. The phosphinous acid (CF₃)₂POH (C_s symmetry) is favored at the same level of theory by about ΔE_ZP = 14 kJ/mol compared with the phosphane oxide structure (C_s symmetry).