Counter-ion modulation of long-distance pi-bonding of the open-shell p-benzoquinone anions.

X-Ray crystallography of dichlorodicyano- or tetrachloro-p-benzoquinone salts with polyether-ligated alkali metals reveals discrete supramolecular complexes: [{M(L)A}(2)]. They show pairs of co-facial p-benzoquinone anions (A(-)) arranged at close interplanar separation: r(pi) approximately 2.9 A characteristic of ion-radical pi-dimers. Moreover, these pi-bonded anions are tethered by couples of alkali-metal cations (M(+)) nested in the crown-ether (L) cavities. The structures of such assemblies are compared to the non-tethered pi-dimers (with 1:1 cation/anion coordination), as well as to the separated dianionic A(2) and monoanionic A(2)* pi-dyads. Such a comparison reveals that, contrary to electrostatic implications, counter-ion coordination (or monomer charge decrease) is accompanied by increase of the interplanar separations (r(pi)) between pi-bonded p-benzoquinones. In addition, counter-ion bridges switch the lateral monomer offset from longitudinal in the separated pi-dimers to transversal in the tethered assemblies. In dichloromethane, equilibrium constants of pi-dimers formation are higher for the ion-paired salts as compared to those of the separated analogues, but the enthalpies of pi-dimerization (DeltaH(D)) are essentially independent of counter-ions. Thus, structural (r(pi)) and thermodynamic (DeltaH(D)) features of the anionic p-benzoquinone pi-dimers suggest a superior compensation of coulombic interactions and relatively minor role of counter-ions in the long-distance ion-radicals pi-bonding.