Triply Bonded Pancake π-Dimers Stabilized by Tetravalent Actinides

TL;DR

Scientists created a new type of strong molecular bond using tetravalent actinides, which could lead to new materials with unique electronic properties.

Contribution

The first experimental synthesis of six-electron triple pancake bonds stabilized by tetravalent actinides.

Findings

Triple pancake bonds were synthesized from [HAN]3– triradicals and stabilized by thorium and uranium ions.

The electronic absorption spectrum of the bonds matches computational predictions, confirming their structure.

Thin films of the π-dimers show conductivity, suggesting potential for molecular conductors.

Abstract

Aromatic π-stacking
is a weakly attractive, noncovalent interaction
often found in biological macromolecules and synthetic supramolecular
chemistry. The weak nondirectional nature of π-stacking can
present challenges in the design of materials owing to their weak,
nondirectional nature. However, when aromatic π-systems contain
an unpaired electron, stronger attraction involving face-to-face π-orbital
overlap is possible, resulting in covalent so-called “pancake”
bonds. Two-electron, multicenter single pancake bonds are well known,
whereas four-electron double pancake bonds are rare. Higher-order
pancake bonds have been predicted, but experimental systems are unknown.
Here, we show that six-electron triple pancake bonds can be synthesized
by a 3-fold reduction of hexaazatrinaphthylene (HAN) and subsequent
stacking of the [HAN]3– triradicals. Our analysis
reveals a multicenter covalent triple pancake bond consisting of a
σ-orbital and two equivalent π-orbitals. An electrostatic
stabilizing role is established for the tetravalent thorium and uranium
ions in these systems. We also show that the electronic absorption
spectrum of the triple pancake bonds closely matches computational
predictions, providing experimental verification of these unique interactions.
The discovery of conductivity in thin films of triply bonded π-dimers
presents new opportunities for the discovery of single-component molecular
conductors and other spin-based molecular materials.

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