Quantum Crystallographic Investigation of Two-Electron Multicentre Bonds in Radical Systems

Petra Stanić^a; Florian Meurer^b; Michael Bodensteiner^b; Christoph Hennig^c; Miha Virant^d; Matic Lozinšek^d; Krešimir Molčanov^d

^aRuđer Bošković Institute, ^bUniversität Regensburg; ^cThe Rossendorf Beamline (BM20), European Synchrotron Radiation Facility; ^dJožef Stefan Institute

e-mail: valentina.milasinovic@irb.hr

Two-electron multicentre bonds (2e/mc or pancake bonds) are strong intermolecular interactions that occur between planar organic radicals. [1] These bonds arise from the pairing of spins of contiguous radicals, facilitated by the delocalization of the highest occupied molecular orbital (HOMO) between both radicals. This interaction often exhibits a significant covalent character, with bond energies exceeding −15 kcal mol−1, placing 2e/mc bonds among the strongest intermolecular interactions, comparable to hydrogen and halogen bonds. Notably, the electron pair in a 2e/mc bond is delocalized over multiple centres rather than localized between two atoms. [1] The strength and extent of pancake bonding play a crucial role in defining the bulk properties of radical-based materials, [1] making this interaction of interest not only from a fundamental perspective on chemical bonding and intermolecular interactions but also from an applicative standpoint. Fine-tuning this interaction could lead to the development of novel organic magnets and (semi)conductors.

We present a systematic quantum crystallographic investigation of various 2e/mc-bonded radical systems, ranging from discrete dimers to 2D arrays. The simplest systems studied include TCNE radical anion dimers, while more complex systems feature 2D arrays of TCNQ radical anions with partial charges of −1/2. [2] Discrete (0D) pancake-bonded dimers and trimers have been well-studied and typically exhibit diamagnetic or antiferromagnetic properties of bulk samples. [1] These oligomers often stack via weaker non-bonding interactions. Extended 1D motives involve stacks of equidistant radicals, with interplanar separations shorter than 3.2 Å. These close contacts enable electron transfer between rings and promote long-range magnetic ordering, resulting in bulk materials with semiconducting and antiferromagnetic properties.[1]

Recent observations of pancake bonding in 2D arrays, such as in salts of 7,7,8,8-tetracyanoquinodimethane (TCNQ) radical anions, reveal spin interactions and electron transport in two directions, as evidenced by EPR spectroscopy. Conductivity values as high as 10−2 S cm−1 have been reported for these materials. [2]"

References:

[1] Molčanov, K & Kojić-Prodić, B. (2019). IUCrJ, 6, 156-166. Molčanov, K., Milašinović, V. & Kojić-Prodić, B. (2019). Cryst. Growth Des., 19, 5967-5980;

[2] Molčanov, K., Milašinović, V., Kojić-Prodić, B., Maltar-Strmečki, N., You, J., Šantić, A., Kanižaj, L., Stilinović, V. & Fotović, L. (2022). IUCrJ, 9, 449-467.