Multicentric two-electron covalent bonding (pancake bonding) between semiquinone radicals determines bulk properties

Abstract

Fine details of stacking interactions in three different types of π-stacked tetrachlorosemiquinone radical anions (Cl4Q) were studied by a combination of X-ray charge density, quantum chemical computation and atoms-in-molecules (AIM) analysis: 1) stacks of pancake-bonded radical dimers in triclinic polymorph of N-MePy·Cl4Q (N-MePy = N-methylpyridinium cation), 2) stacks of trimers of partially charged semiquinones in [4-damp])2[Cl4Q]3 (4-damp = 4-dimethylamino-N-methylpyridinium) and 3) stacks of equidistant radicals in orthorhombic polymorph of N-MePy·Cl4Q. For the first time, we provide experimental evidence (based on X-ray charge density) of two-electron multicentric covalent bonding (pancake bonding) between the radicals. Typical pancake-bonded radical dimers in 1) are characterised by short interplanar distance (2.86 A) and multiple bonding (3, -1) critical points between the rings with maximum electron density exceeding 0.095 e A-3 ; in addition, a (3, +3) critical point (local minimum of electron density) was also found, indicating a cage-like electronic structure [1]. The covalent contribution to total interaction in a dimer was calculated to be -9.4 kcal mol-1. Between the dimers, interplanar separation is 3.60 A and only negligible electron density is found. In trimers there are two electrons shared between three closely interacting rings (interplanar separations are ca. 2.84 A), and (3, -1) bonding critical points are found with maximum electron density of 0.077 e A-3 ; there are also two (3, +3) local minima [2]. The calculated covalent contribution is -6.8 kcal mol-1. Maximum electron density between the rings in a stack of equidistant radicals (interplanar separation of 3.17 A) is much lower, 0.050 e A-3, and there is no local minimum of electron density [1]. However, the HOMO orbitals extend between the rings, and the calculated covalent contribution is -2.9 kcal mol-1. This compound is a 1D semiconductor [3, 4], and its semiconductivity is explained by pancake bonding extending along the stack.