Supramolecular Covalence in Bifurcated Chalcogen Bonding
Pankaj Lochan Bora and Radek Marek
Abstract
Chalcogen bond belongs to the class of supramolecular interactions that are generally classified as
non-covalent. However, recent studies have demonstrated that many of these formally noncovalent
interactions are stabilized by a significant covalent component. Hence the use of
electrostatic potential (ESP) map and maximum value of surface potential (𝑉𝑆,𝑚𝑎𝑥) to predict the
strength of these interactions is somewhat questionable. Herein we demonstrate that the
electrostatic parameters are useful for estimating the long-range electrostatic component of the
interaction but fail to characterize the short-range electrostatics that accompanies the significant
orbital interactions and electron sharing between the interacting systems. The electrostatic
potential, because of its static nature, cannot account for the polarizabilities of individual fragments
and the propensity of interacting systems to the electron sharing. It is shown for our supramolecular
systems of a general structure [MX6]2:YX2
(where M = Se or Pt, X = halogen, Y = chalcogen) –
related to the experimentally observed arrangement featuring a bifurcated chalcogen bonding –
that the characteristics of the ESPs fail to predict correct order of the binding energies in a series
of compounds. Instead, polarizabilities of individual fragments attached to the chalcogen atom and
supramolecular orbital interactions govern the trends in the binding energies. The substituent
effects on the binding energy and supramolecular electron sharing are consistently identified by
an arsenal of theoretical methods ranging from approaches based on the quantum chemical
topology (QTAIM, IQA) to analytical tools based on the localized molecular orbitals (EDANOCV,
NBO). It is demonstrated that the chalcogen bonding investigated in this work is driven
by the orbital interactions with significant electron sharing, which can be classified as the
supramolecular covalence.