Adducts of TCNQ with Neutral Nitrogen Bases. Their Rationalization in Terms of Intermolecular Charge-Transfer (CT) or Electron Donor–Acceptor (EDA) Interactions

The structures of 8 alternate-stack adducts of TCNQ with neutral N -bases were determined by X-ray diffraction. Their packings contain 96 contacts ≤(sum vdW radii) dividable in CT or EDA classes as follows: (i) 56 σ*← n [12 inter and 5 intra X–H···:Y (X, Y = N, O, Cl); 31 inter and 8 intra C–H···:Y (Y = N,O) H-bonds]; (ii) 2 π*← n C←:N; (iii) 11 σ*←π C–H←(C,N); (iv) 21 π*←π C←(C,O); and (v) 6 σ*←σ C–H←H–C interactions. The σ*← n contacts link molecules in planar ribbons by saturating lone-pair n -donors (:Y) by a maximum of X–H or C–H acceptors. These ribbons are interlinked in alternate vertical stacks by π*←π contacts or in herringbone packings by π*← n or σ*←π ones. In spite of their alternate geometry, 4/8 crystalline adducts are black with metallic luster imputable to delocalized electronic bands, a fact successfully interpreted in terms of specific packing substructures held together by proper EDA interactions. Finally, a simple three-component model of the forces acting in neutral molecular crystals (dispersion, exchange, and CT or EDA attractions) is discussed, showing that it is theoretically well-grounded in the classical perturbation theory and suggesting that it could be profitably used in synergy with other well-established methods of crystal-packing analysis based on electron densities and electrostatic potentials.