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.