Counter-ion modulation of long-distance π-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: rπ≈ 2.9 Å characteristic of ion-radical π-dimers. Moreover, these π-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 π-dimers (with 1 ∶ 1 cation/anion coordination), as well as to the separated dianionic [A2]2− and monoanionic [A2]−˙π-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 (rπ) between π-bonded p-benzoquinones. In addition, counter-ion bridges switch the lateral monomer offset from longitudinal in the separated π-dimers to transversal in the tethered assemblies. In dichloromethane, equilibrium constants of π-dimers formation are higher for the ion-paired salts as compared to those of the separated analogues, but the enthalpies of π-dimerization (ΔHD) are essentially independent of counter-ions. Thus, structural (rπ) and thermodynamic (ΔHD) features of the anionic p-benzoquinone π-dimers suggest a superior compensation of coulombic interactions and relatively minor role of counter-ions in the long-distance ion-radicals π-bonding.