Specific Features of Chlorotropism in the ENC Triad (E = PIV-PVI,C) of High-Coordination Phosphorus Chlorides and Trichloromethyl Isocyanate

The semiempirical MNDO method and its parametrized PM3 version in supermolecular approximation was used for a comparative study of the structure and alternative mechanisms of chlorotropism in the ENC triad (E = P^{I V - V I}, C) of amidinium tetrachlorophosphorate Cl₄P(NCH₃)₂CCCl₃ (I), phosphazopentachloroethane Cl₅C₂NPO₂C₆H₄, (II), trichloromethyl isocyanate Cl₃CNCO (III), and their 1:2 chloroform solvates. The absence of the thermodynamically stable intermediate as a separated ion pair in the chlorotropic transformations of structures I, III and the high enthalpy of the substrate-intermediate transformation for structure II show that the sigmatropic mechanism of chlorotropism in compounds under study is the only probable one. The activation barrier of chlorotropism in phosphorus systems I, II is much reduced. In the case of specific solvation, a weak tendency to further reduction of the activation barrier for structures I-III is observed, and the equilibrium for phosphorus systems I, II, is appreciably shifted, unlike system III, where, according to experimental data, the equilibrium is fully to the side of the carbamoyl isomer Cl₂C = NC(O)Cl.