Double pancake bonds: pushing the limits of strong π-π stacking interactions.

Zhong-Hua Cui,Hans Lischka,Habtamu Z Beneberu,Miklos Kertesz

doi:10.1021/ja505624y

Abstract

The concept of a double-bonded pancake bonding mechanism is introduced to explain the extremely short π–π stacking contacts in dimers of dithiatriazines. While ordinary single pancake bonds occur between radicals and already display significantly shorter interatomic distances in comparison to van der Waals (vdW) contacts, the double-bonded pancake dimer is based on diradicaloid or antiaromatic molecules and exhibits even shorter and stronger intermolecular bonds that breach into the range of extremely stretched single bonds in terms of bond distances and binding energies. These properties give rise to promising possibilities in the design of new materials with high electrical conductivity and for the field of spintronics. The analysis of the double pancake bond is based on cutting edge electron correlation theory combining multireference (nondynamical) effects and dispersion (dynamical) contributions in a balanced way providing accurate interaction energies and distributions of unpaired spins. It is also shown that the present examples do not stand isolated but that similar mechanisms operate in several analogous nonradical molecular systems to form double-bonded π-stacking pancake dimers. We report on the amazing properties of a new type of stacking interaction mechanism between π conjugated molecules in the form of a “double pancake bond” which breaks the record for short intermolecular distances and provides formidable strength for some π–π stacking interactions.

Highlights

Π-Stacking in radical dimers, some of which are illustrated in Chart 1 in the form of the constituent monomers, is responsible for the formation of a very interesting class of chemical compounds which display favored packing geometries as described by the maximum multicenter overlap principle between neighboring molecules.[1]
This singly occupied molecular orbital (SOMO)−SOMO stabilization can be rationalized by the simple molecular orbital (MO) diagram shown in Chart 2a in which the bonding highest occupied molecular orbital (HOMO) is doubly occupied
Chart 2. (a) Molecular Orbital Diagram for Single Pancake-Bonded Dimers; (b) Molecular Orbital Diagram for Double Pancake-Bonded Dimers Based on a Triplet Ground State of the Monomer; (c) Molecular Orbital Diagram for Double Pancake-Bonded Dimer Based on a Singlet Diradicaloid Ground State of the Monomer with a Low HOMO−LUMO Gapa aThe formal bond order according to eq 1 is 1 for 12, 22, and 32 and 2 for 42

Summary

Introduction

Π-Stacking in radical dimers, some of which are illustrated in Chart 1 in the form of the constituent monomers, is responsible for the formation of a very interesting class of chemical compounds which display favored packing geometries as described by the maximum multicenter overlap principle between neighboring molecules.[1] This preferred orientation is primarily due to the energy lowering of the singly occupied molecular orbital (SOMO) of the radical as it overlaps with its neighbor This SOMO−SOMO stabilization can be rationalized by the simple molecular orbital (MO) diagram shown in Chart 2a in which the bonding highest occupied molecular orbital (HOMO) is doubly occupied. It will be compared to an analogous stable 7π-electrons radical, 1,2,4,6-thiatriazine,[17 3], that forms a traditional 2e/mc bonded pancake bond. 1,3,2,4,6-Dithiatriazine (4) is a neutral molecule with 8π-electrons which forms a very short pancake bonded dimer according to its crystal structure.[15]

Methods

Results

Conclusion