Stable phosphinous acids

Abstract

The electronic properties of organyl element compounds are strongly influenced by the electronic characteristics of the organic substituents. The bonding of two CF 3 groups to a phosphorus atom effects a drastically decreased basicity. That is the phosphorus atom is the least basic centre in the compound (CF 3) 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 (CF 3) 2POH is described. The compound (C 6F 5) 2POH, originally claimed as a phosphinous acid, is proved to exist at room temperature exclusively in the tautomeric oxide form. (C 6F 5) 2P(O)H crystallizes in the triclinic space group P 1 ¯ (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 (C 3N 3H 2) 2POH the phosphinous acid isomer to be favored by Δ E ZP = 22 kJ/mol in relation to the corresponding phosphane oxide isomer. The phosphinous acid (CF 3) 2POH (C s symmetry) is favored at the same level of theory by about Δ E ZP = 14 kJ/mol compared with the phosphane oxide structure (C s symmetry).