Pancake Bonding in π‐Stacked Trimers in a Salt of Tetrachloroquinone Anion

Abstract

The crystal structure of [4-damp]) [Cl Q] (4-damp=4-dimethylamino-N-methylpyridinium, Cl Q=tetrachloroquinone) salt is built up from slipped columnar stacks of quinoid rings composed of closely bound trimers with the intra-trimer separation distance of 2.84 Å and total charge of -2 whereas the inter-trimer distance is 3.59 Å. The individual rings exhibit partial negative charges that are distributed unevenly among the three Cl Qs in the trimer. The strong interactions within a trimer (Cl Q) have a partially covalent character with two-electron/multicentered bonding, that is extended over three rings, plausibly termed as "pancake bonding". The electron pairing within this multicentre bond leads to the fact that the crystals are diamagnetic and act as insulators. The studies of the structure and nature of bonding are based on X-ray charge density analysis and density functional theory.

Malleable Electronic Structure of Chloranilic Acid and Its Species Determined by X-ray Charge Density Studies

“…Thus, the effect of chlorine substitution is similar to the one observed in tetrachlorosemiquinone radical anions. [42,43] In the coordinated species IV and V, there is no such effect and more negative charge is located at the coordinating oxygen atoms, probably due to donation of electron density to the d orbitals of the metal cations. Accordingly, electron density and the corresponding bond orders of C-O bonds are significantly lower than in the non-coordinated species (Table 2).…”

Towards understanding π-stacking interactions between non-aromatic rings

“…X-ray charge-density analysis of a trimer of tetrachlorosemiquinones (Molčanov et al, 2018a;Fig. 8) provided experimental evidence for this concept.…”

“…5(b)] (Molčanov et al, 2012(Molčanov et al, , 2016. Recently, we studied stacks of semiquinone radicals by X-ray charge-density analysis coupled with DFT calculations (Molčanov et al, 2018a, 2019) on a model system [two polymorphs of a salt of the tetrachlorosemiquinone radical anion and the N-methylpyridinium cation (N-MePyÁCl 4 Q)] and provided experimental evidence of pancake bonding (there is a good match between the experimental and calculated electron densities).…”

“…Molecular orbital calculations by p)] indicated that the HOMO extends between all three rings of the trimer; the covalent contribution to the total interaction between each pair of rings was calculated by p)] to be À6.8 kcal mol À1 (Molčanov et al, 2018a), which is somewhat less than in a pancake-bonded dimer (see above), and the bond order was estimated to be lower than 0.71 (most likely it is about 0.5).…”

Iodide···π Interactions of Perhalogenated Quinoid Rings in Co-crystals with Organic Bases

“…While preparing solid-state semiquinone systems, 17,[19][20][21][22] we serendipitously obtained a series of co-crystals of tetrachloro-and tetrabromoquinone (Cl4Q and Br4Q, respectively, Scheme 1) with iodide salts of organic cations (mostly derivatives of N-methylpyridinium, Scheme 2), in which close contacts between the quinoid ring and the iodide anion occur almost without exception.…”