Partially Covalent Two-Electron/Multicentric Bonding between Semiquinone Radicals

Abstract

X-ray charge density was determined and analyzed for two polymorphs of the Nmethylpyridinium salt of the tetrachlorosemiquinone radical anion and its analogous closed-shell relatives, tetrachloroquinone (chloranil) and tetrachlorohydroquinone. The study, which was combined with calculations of electron delocalization, electrostatic potentials, and aromaticity, presents details of electronic structure of the semiquinoid ring. This comparative study reveals that the negative charge is delocalized over the entire semiquinone radical, and that the chlorine substituents play a crucial role in its stabilization through induction effect. In general, the semiquinoid ring has partially delocalized π-electrons and is approximately halfway between a quinoid and an aromatic ring. In the orthorhombic polymorph with stacks of equidistant radicals electron density between the rings of almost 0.05 e Å-3 and four (3,-1) saddle points between the contiguous rings were found. In the diamagnetic triclinic polymorph, comprising strongly bound radical dimers (with significant covalent character-'pancake bond'), maximum electron density between the rings exceeds 0.095 e Å-3 and multiple (3,-1) critical points are found. However, only negligible electron density is observed between the dimers. Thus, in the radical anion stacks spin coupling, along with dispersive and polarization effects, defines interplanar distance and magnetic behaviour, whereas intermolecular electrostatic potential determines the ring offset.

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).…”

“…Since neutral quinones lack delocalisation of π electrons, distribution of electron density and electrostatic potential in the ring is not uniformly distributed. [42] The electrostatic potential map of neutral chloranilic (Fig. 3) reveals an expected excess of negative charge at the carbonyl oxygen atoms, and a slight excess of electron density at the double bonds, whereas the single ones are electron-poor.…”

Towards understanding π-stacking interactions between non-aromatic rings

“…Recently, it has been documented that unusually short and strong interactions between planar radicals have a partial covalent character (Huang & Kertesz, 2007;Huang et al, 2008;Novoa et al, 2009;Tian & Kertesz, 2011;Cui et al, 2014a,b;Preuss, 2014) and this type of interaction has been termed 'pancake bonding'. In our previous analysis of the stacking interactions of the semiquinone radical, using X-ray charge-density analysis, 'pancake bonding' was described in detail (Molčanov et al, 2019).…”

“…Electrostatic potentials plotted onto electron-density isosurfaces of 0.5 e Å À3 for (a) tetrachloroquinone (Molčanov et al, 2019), (b) neutral chloranilic acid (Vuković et al, 2019) and (c) the hydrogen chloranilate monoanion (Molčanov et al, 2015). The electrostatic potentials range from À0.1 e Å À1 (red) to 1.0 e Å À1 (dark blue).…”

“…It can be regarded as stacked radical dimers with coupled spins and such structures are diamagnetic and insulating. The rings in a dimer are bent slightly towards each other; in stacked semiquinones their Cremer-Pople puckering parameter is in the range 2.0-4.3 (Molčanov et al, 2011a(Molčanov et al, , 2014a(Molčanov et al, , 2019Molčanov & Kojić-Prodić, 2017). Less common is type (ii) with long-range magnetic ordering (usually antiferromagnetic).…”