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Issue 2, 2009
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Counter-ion modulation of long-distance π-bonding of the open-shell p-benzoquinone anions

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Abstract

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.

Graphical abstract: Counter-ion modulation of long-distance π-bonding of the open-shell p-benzoquinone anions

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Publication details

The article was received on 10 Jul 2008, accepted on 20 Oct 2008 and first published on 25 Nov 2008


Article type: Paper
DOI: 10.1039/B811816G
Citation: Phys. Chem. Chem. Phys., 2009,11, 324-332
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    Counter-ion modulation of long-distance π-bonding of the open-shell p-benzoquinone anions

    S. V. Rosokha, J. Lu, T. Y. Rosokha and J. K. Kochi, Phys. Chem. Chem. Phys., 2009, 11, 324
    DOI: 10.1039/B811816G

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