Pancake Bond Orders of a Series of π‐Stacked Triangulene Radicals

A Triply Negatively Charged Nanographene Bilayer with Spin Frustration

We report on the largest open-shell graphenic bilayer and also the first example of triply negatively charged radical π-dimer. Upon three-electron reduction, bilayer nanographene fragment molecule (C₉₆ H₂₄ Ar₆ )₂ (Ar=2,6-dimethylphenyl) (1₂ ) was transformed to a triply negatively charged species 1₂ ^3.- , which has been characterized by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR) spectroscopy and magnetic properties on a superconducting quantum interference device (SQUID). 1₂ ^3.- features a 96-center-3-electron (96c/3e) pancake bond with a doublet ground state, which can be thermally excited to a quartet state. It consists of 34 π-fused rings with 96 conjugated sp² carbon atoms. Spin frustration is observed with the frustration parameter f>31.8 at low temperatures in 1₂ ^3.- , which indicates graphene upon reduction doping may behave as a quantum spin liquid.

A Redox-active Microporous Organosiloxane Containing a Stable Neutral Radical, Trioxotriangulene

A new silyl-substituted trioxotriangulene ( TOT ) neutral radical and corresponding porous organosiloxanes (POSs) were synthesized. The neutral radical exhibited a peculiarly high stability and formed a diamagnetic π-dimer characteristic to TOT neutral radicals stabilized by the strong multiple SOMO-SOMO interaction in both solution and solid states. POSs including TOT units within the organosiloxane-wall were prepared by polycondensation of the silyl groups, and formed microporous structures with ~1 nm-size diameters. Redox ability of TOT units in the POS was demonstrated by the treatment of oxidant/reductant in heterogeneous suspension condition, where the TOT units were reversibly converted between reduced and neutral radical species. Furthermore, the solid-state electrochemical measurements of the POS revealed the reversible multi-stage redox ability of TOT units involving polyanionic species within the organosiloxane-wall.

Intramolecular Magnetic Interaction of Spin-Delocalized Neutral Radicals through m-Phenylene Spacers

A new diradical having two 4,8,10-trioxotriangulene (TOT) neutral radical units linked through an m-phenylene moiety was synthesized and characterized by ESR measurements. An electrochemical study showed that the diradical undergoes two one-electron reductions to generate corresponding dianion species, suggesting the electronic interaction between two TOT units through the π-conjugated spacer. A strong intramolecular interaction between the two TOT units gives rise to the spin-projected small hyperfine couplings in comparison with those of the monomer. Furthermore, the temperature dependent ESR measurement revealed that the dimer behaves as an S=1 species in the ground state with a ferromagnetic interaction of 2 J/k_B =+7±3 K.

Pancake Bonding in π-Stacked Trimers in a Salt of Tetrachloroquinone Anion

The crystal structure of [4-damp])₂ [Cl₄ Q]₃ (4-damp=4-dimethylamino-N-methylpyridinium, Cl₄ Q=tetrachloroquinone) salt is built up from slipped columnar stacks of quinoid rings composed of closely bound trimers with the intra-trimer separation distance of 2.84 Å and total charge of -2 whereas the inter-trimer distance is 3.59 Å. The individual rings exhibit partial negative charges that are distributed unevenly among the three Cl₄ Qs in the trimer. The strong interactions within a trimer (Cl₄ Q)₃^2- have a partially covalent character with two-electron/multicentered bonding, that is extended over three rings, plausibly termed as "pancake bonding". The electron pairing within this multicentre bond leads to the fact that the crystals are diamagnetic and act as insulators. The studies of the structure and nature of bonding are based on X-ray charge density analysis and density functional theory.