Multi-center covalency: revisiting the nature of anion–π interactions

Abstract

Exploring the nature of anion-π bonding by means of the Quantum Theory of Atoms in Molecules (QTAIM) and an energy decomposition scheme on the basis of Interacting Quantum Atoms (IQA) theory led us to conclude that these non-classical interactions benefit from "multi-center covalency" far more than from the electrostatics. Comparing anion-π systems to closely related covalent anion-σ complexes reveals that the anion-π systems benefit from an extensive degree of electron sharing between the anions and all atoms of the π-rings. Besides, decomposition of the binding energy into classical (electrostatics) and non-classical (exchange-correlation) components demonstrates that in contrast to previous reports, the anion-π complexes are local minima, if and only if the non-classical contribution to binding energy surpasses that of the electrostatics. This suggests that the stable anion-π complexes with the anions atop the π-rings might be prepared with π-systems that benefit more from the exchange-correlation term, such as extended π-systems, but not with strong electrostatic π-receptors. This conclusion is in line with the tendency of strong π-acids to form the σ-complexes with more covalent character instead of the π-complexes.

Modulating Electron Sharing in Ion-π-Receptors via Substitution and External Electric Field: A Route toward Bond Strengthening

“…The appearance of the saddle points is associated with two factors: 1) The tendency of the hard anions (mainly fluoride) to interact with the hydrogens in the periphery of our finite-sized model system, the coronene. 31 It is not expected that in a large graphene flake or graphene sheet anion-periphery interactions dominate the number of ion-π interactions; therefore, we do not discuss ion-periphery interactions in the present work. 2) Displacement of lithium cation from the ideal central position towards the carbon-carbon bonds of the internal ring in coronene.…”

“…Due to the nature of this interaction, in which a single atom interacts with the whole π-system, we employed the term multi-center covalency 30 to refer to the extensive electron sharing between anions and π-systems. 31 This phenomenon that has a non-electrostatic nature 32 has been explained in terms of charge-transfer by some authors in cases where the electrostatic potentials fail to account for the binding energy and/or bond properties. 33 Possibility of the multi-center covalency for cationic systems is investigated in this work.…”

“…50 The details of this approach are described elsewhere. 31,51 All QTAIM computations are performed by AIMAll suite of programs. 52…”

“…One may conclude that even in the absence of an external electric field, the CTESs and thus electron sharing contributes in the ion-π interactions of coronenes. 31 Failure of quadruple moment and MESPs approaches in estimating the ion-affinity of coronenes can be understood regarding the fact that these tools neglect nonelectrostatic factors between the ions and the π-systems. 33 In a nutshell, the failure of electrostatic criteria and the success of VIP and VEA suggest that electron sharing might play a significant role in bonding for the studied set of compounds.…”

The electron density of delocalized bonds (EDDB) applied for quantifying aromaticity

“…4 From the electronic-structure point of view, aromatic stabilization is usually caused by cyclic delocalization of electrons and as such it can be quantified by different delocalization indices (DI). 5,6 Indeed, the precise relation between energy and delocalization indices, originally proposed by Rafat and Popelier, 7 has been shown to be very useful for measuring the aromatic stabilization energy of aromatic molecules; 8,9 very recently the exact algebraic relationship between DI and the interatomic exchange-correlation energies has also been established. 10 Among all the ways to quantify aromaticity through the ''ground-state'' criteria (energetic, structural, and electronic), there are descriptors that have gained enormous recognition and wide acceptance: the aromatic stabilization energy (ASE), 2 harmonic oscillator model of aromaticity (HOMA), [11][12][13] and different types of the multicenter delocalization indices (MCDI).…”