Anion-π interaction with alkenes: persistent complexes vs. irreversible reactions of anions with tetracyanoethylene

The interaction of the tetracyanoethylene (TCNE) π-acceptor with oxo- and fluoro-anions (BF₄^-, PF₆^-, ClO₄^-, NO₃^-) led to the formation of anion-π complexes in which these polyatomic anions were located over the face of alkenes, with multiple contacts being shorter than the van der Waals separations. The anion-π associations of TCNE with halides were delimited by the electron-donor strengths and nucleophilicity of the anions. Specifically, while bromides formed persistent anion-π associations with TCNE in the solid state and in solutions, only transient anion-π complexes with iodides and chlorides were observed. In the case of iodide (strong 1e reducing agent), the formation of anion-π complexes was followed by the reduction of the π-acceptor to the TCNE^-˙ anion-radical. The interaction of TCNE with Cl^- (and F^-) anions (which are better nucleophiles in the aprotic solvents) led to the formation of 1,1,2,3,3-pentacyanoprop-2-en-1-ide anions. Thermodynamics, UV-Vis spectra, and structures, as well as contributions of electrostatics, orbital interactions, and dispersion to the interaction energies in the complexes of TCNE with various anions were closely related to the characteristics of the corresponding associations with the aromatic and p-benzoquinone acceptors. This points out the general equivalence of the interactions in the anion-π complexes with different π-acceptors and the critical role of the nature of the anions in these bindings.