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Do cation···π interactions always need to be 1:1?

The rapid progress in structural elucidation of biomolecules and related supramolecular systems has led to an atomistic picture of the nature of the intermolecular forces that direct self-assembly. It is well accepted now that weak intermolecular forces play a major role in bestowing structural stability and functional diversity in these systems. Among the various noncovalent interactions, cation···p interactions are probably the strongest and have been shown to be crucial in many biochemical processes such as protein–ligand interactions, ion channels, receptors and steroid synthesis. 3] Cation···p interactions are conventionally considered as quadrupolar interactions between the aromatic ring and cations, though the charge-transfer component has been shown to be significant as well. Based on such a simplistic picture, one might expect that both the top and bottom surfaces of the aromatic rings should bind to cations. Such an idea has been earlier used to describe aromatic rings that bind to cation and anion simultaneously. This is not unexpected because multidecker organometallic complexes such as m-benzene(dicyclopentadienyl)divanadium have been synthesized. However, these class of complexes involve large cations like thallium(III) or Zn(II) or transition-metal ions which allow direct orbital overlap with the ligands to form bonding molecular orbitals (MOs). Therefore, these organometallic complexes formally do not qualify as cation···p complexes such as those for Li , Na , K or NH4 + , where the interactions with the aromatic rings are predominantly noncovalent. Herein, we elucidate the conditions at which both faces of the aromatic rings can be tuned to interact strongly with the M(I) ion, where M = alkali metal. A detailed search in the Cambridge structural database (CSD) for arene···M (Li , Na and K) leads to quite a large number of hits (300) (see Supporting Information for search details). Interestingly, most of them are 1:1 complexes where only one of the faces of the ring interacts with the cation. However, a few structures (7) indeed have a 2:1 stoichiometry in the cation···p complexes. For example, [(m2,h -benzene)-(hcyclopentadienyl)-(TMEDA-Li)V] (A), TMEDA = N,N,N,N tetramethylethylenediamine, (Figure 1) does show two M centers binding to the benzene ring on opposite surfaces but one of the metals is a V atom, which forms a sandwich complex with the cyclopentadienyl ligand on the other side. Also, the dipotassium complex of 1,2-diphenylbenzene dianion, K2[Ph2C6H4] (B), exhibits a 2:1 structure. But the benzene ring, being dianionic, exhibits substantial puckering distortion from planarity (~15 degrees) to avoid anti-aromatic (8pe) destabilization. Similar 2:1 cation···dianion complexes are also formed by other