Decacyclene Radical Anions Showing Strong Low‐energy Intramolecular Absorption and Magnetic Coupling of Spins in a Hexagonal Network

Spin Frustration in Organic Radicals

Spin frustration, which results from geometric frustration and a systematical inability to satisfy all antiferromagnetic (AF) interactions between unpaired spins simultaneously, is under the spotlight for its importance in physics and materials science. Spin frustration is treated as the structural basis of quantum spin liquids (QSLs). Featuring flexible chemical structures, organic radical species exhibit great potential in building spin-frustrated molecules and lattices. So far, the reported examples of spin-frustrated organic radical compounds include triradicals, tetrathiafulvalene (TTF) radicals and derivatives, [Pd(dmit)2 ] compounds (dmit=1,3-dithiol-2-thione-4,5-dithiolate), nitronyl nitroxides, fullerenes, polycyclic aromatic hydrocarbons (PAHs), and other heterocyclic compounds where the spin frustration is generated intra- or intermolecularly. In this Minireview, we provide a brief summary of the reported radical compounds that possess spin frustration. The related data, including magnetic exchange coupling parameters, spin models, frustration parameters, and crystal lattices, are summarized and discussed.

Inducing Curvature to Pyracylene upon π‐Expansion

We disclose a successive π‐expansion of pyracylene towards boat‐shaped polycyclic scaffolds. The unique structural features of the resulting compounds were revealed by X‐ray crystallographic analysis. Depending on the extent of π‐expansion the compounds display intense bathochromically shifted absorption bands in their UV/Vis spectra and are prone to several redox events as documented by cyclic voltammetry. The experimental observations are in line with the computational studies based on density functional theory, suggesting progressive narrowing of the HOMO–LUMO gap and distinct evolution of the electronic structure and aromaticity.

Macrocycle- and metal-centered reduction of metal tetraphenylporphyrins where the metal is copper(II), nickel(II) and iron(II)

The reduction of metal(II) tetraphenylporphyrins, where metal(II) is copper, nickel or iron, has been performed in toluene solution in the presence of a cryptand. Cesium anthracenide was used as a reductant. Crystalline salts {cryptand(Cs⁺)}₂{Cu^II(TPP^4-)}^2- (1) and {cryptand(Cs⁺)}{Ni^I(TPP^2-)}^-·C₆H₅CH₃ (2) have been obtained. The two-electron reduction of {Cu^II(TPP^2-)}⁰ is centered on the macrocycle allowing one to study for the first time the structure and properties of the TPP^4- tetraanions in the solid state. Tetraanions have a diamagnetic state and show essential C-C_meso bond alternation. New bands attributed to TPP^4- appear at 670, 770 and 870 nm. Unpaired S = 1/2 spin is localized on Cu^II. The one-electron reduction of {Ni^II(TPP^2-)}⁰ centered on nickel provides the formation of {Ni^I(TPP^2-)}^- with unpaired S = 1/2 spin localized on Ni^I at 100(2) K. The effective magnetic moment of 2 is 1.68μ_B at 120 K and a broad asymmetric EPR signal characteristic of Ni^I is observed for 2 and also for (Bu₃MeP⁺){Ni^I(TPP^2-)}^-·C₆H₅CH₃ (3) in the 4.2-120 K range. Since dianionic TPP^2- macrocycles are present at 100(2) K, no alternation of C-C_meso bonds is observed in 2. The excited quartet S = 3/2 state according to the calculations is positioned close to the ground S = 1/2 state. In the excited state, charge transfer from Ni^I to the macrocycle takes place resulting in the formation of Ni^II with S = 1 and TPP˙^3- with S = 1/2 in the {Ni^II(TPP˙^3-)}^- anions. Therefore, the increase in the magnetic moment of 2 above 150 K is attributed to the population of the excited quartet state with a gap of 750 K. Salt 2 is EPR silent above 150 K and manifests absorption bands characteristic of TPP˙^3- at RT. The reduction of Ni^II(TPP^2-) and Fe^II(TPP^2-) by cesium anthracenide in the presence of Bu₃MeP⁺ yields crystals of 3 and (Bu₃MeP⁺){Fe^I(TPP^2-)}^-·C₆H₅CH₃ (4) whose crystal structures and optical properties are also presented. DFT calculations have been carried out for {M^II(TPP^2-)} (M = Cu, Ni and Fe) and their anions to interpret the experimental results obtained for 1-4.

Dianionic States of Trithiadodecaazahexaphyrin Complexes with Homotrinuclear MII3O Clusters (M = Ni and Cu): Crystal Structures, Metal- Or Macrocycle-Centered Reduction, and Doublet-Quartet Transitions in the Dianions

Metal complexes of trithiadodecaazahexaphyrin (Hhp) that contain M^II₃O clusters inside a π-extended trianionic (Hhp^3-) macrocycle have been prepared. Studies of the magnetic properties of Ni^II₃O(Hhp) and Cu^II₃O(Hhp) reveal a diamagnetic and EPR-silent trianionic (Hhp^3-) macrocycle and diamagnetic Ni^II₃(O^2-) or paramagnetic Cu^II₃(O^2-) tetracations. The positive charge of M^II₃O(Hhp) is compensated by one acetate anion {M^II₃O(Hhp)}⁺(CH₃CO₂^-). The three-electron reduction of {M^II₃O(Hhp)}⁺ yields {cryptand(Cs⁺)}₂{Ni^II₂Ni^IO(Hhp^5-)}^2-·2C₇H₈ (1) and {cryptand(Cs⁺)}₂{Cu^II₃O(Hhp^•6-)}^2-·C₇H₈ (2) crystalline salts. The magnetic properties of 1 reveal the formation of Hhp^5- and the reduction of nickel(II) to the paramagnetic Ni^I ion (S = 1/2), which is accompanied by the formation of the {Ni^II₂Ni^IO(Hhp^5-)}^2- dianion. As a result, the magnetic moment of 1 is 1.68 μ_B in the 20-220 K range, and a broad EPR signal of Ni^I was observed. The Hhp^5- macrocycle has a singlet ground state, but the increase in the magnitude of the magnetic moment of 1 above 220 K is attributed to the population of the triplet excited state in Hhp^5-. The {Ni^II₂Ni^IO(Hhp^5-)}^2- dianion is transferred from the doublet excited state to the quartet excited state with an energy gap of 1420 ± 50 K. Salt 1 also shows an unusually strong low-energy NIR absorption, which was observed at 1000-2200 nm. In 2, a highly reduced Hhp^•6- radical hexaanion (S = 1/2) coexists with a Cu^II₃(O^2-) cluster (S = 1/2) in the {Cu^II₃O(Hhp^•6-)}^2- dianions. The dianions have a triplet ground state with antiferromagnetic exchange between two S = 1/2 spins with J = -6.4 cm^-1. The reduction of Hhp in both salts equalizes the initially alternated C-N bonds, supporting the increase in the Hhp macrocycle electron delocalization.