Odd-Electron-Bonded Sulfur Radical Cations: X‑ray Structural Evidence of a Sulfur–Sulfur Three-Electron σ‑Bond

The one-electron oxidations of 1,8-chalcogen naphthalenes Nap­(SPh)₂ (<b>1</b>) and Nap­(SPh)­(SePh) (<b>2</b>) lead to the formation of persistent radical cations <b>1</b>^•+ and <b>2</b>^•+ in solution. EPR spectra, UV–vis absorptions, and DFT calculations show a three-electron σ-bond in both cations. The former cation remains stable in the solid state, while the latter dimerizes upon crystallization and returns to being radical cations upon dissolution. This work provides conclusive structural evidence of a sulfur–sulfur three-electron σ-bond (in <b>1</b>^•+) and a rare example of a persistent heteroatomic three-electron σ-bond (in <b>2</b>^•+)

A Crystalline Phosphaalkene Radical Anion

Salts containing phosphaalkene radical anions have been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, UV–vis absorption spectroscopy, and single-crystal X-ray diffraction. The radical anions feature elongated P–C bonds and an aromatization of fulvene compared to the neutral phosphaalkene. Their EPR spectra and theoretical calculations indicate the spin density of the radicals mainly resides on phosphorus atoms. This work provides the first example of a crystalline phosphaalkene radical anion

Two Stable Phosphorus-Containing Four-Membered Ring Radical Cations with Inverse Spin Density Distributions

Two phosphorus-containing four-membered ring radical cations <b>1</b>^•+ and <b>2</b>^•+ have been isolated and characterized by UV–vis absorption spectroscopy, electron paramagnetic resonance (EPR), and single-crystal X-ray diffraction. Compared with neutral molecules <b>1</b> and <b>2</b>, radical <b>1</b>^•+ has elongated P–P bonds and more pyramidalized phosphorus atoms, while shortened P–N_ring distances and larger angles around phosphorus centers are observed for <b>2</b>^•+. EPR studies indicate that for <b>1</b>^•+ spin density mainly resides on the exocyclic nitrogen atoms with very minor contribution from endocyclic phosphorus atoms, while the situation is opposite for <b>2</b>^•+. Such an inverse spin density distribution is controlled by the exocyclic substituents, which is supported by DFT calculations

Isolable Diphosphorus-Centered Radical Anion and Diradical Dianion

Two salts containing diphosphorus-centered radical anion <b>1</b>^•– and diradical dianion <b>1</b>^2–•• were obtained by one- and two-electron reductions of an indenofluorene-bridging diphosphaalkene (<b>1</b>) with K and KC₈, respectively. The salts have been characterized by electron paramagnetic resonance (EPR) spectroscopy, UV–vis absorption spectroscopy, and single-crystal X-ray diffraction analysis. EPR spectroscopy and theoretical calculations reveal that the spin density of the radicals mainly resides on the phosphorus atoms, and <b>1</b>^2–•• has an open-shell singlet ground state. <b>1</b>^•– and <b>1</b>^2–•• represent the first isolable and structurally characterized diphosphorus-centered radical anion and dianion

Two Stable Phosphorus-Containing Four-Membered Ring Radical Cations with Inverse Spin Density Distributions

Two phosphorus-containing four-membered ring radical cations <b>1</b>^•+ and <b>2</b>^•+ have been isolated and characterized by UV–vis absorption spectroscopy, electron paramagnetic resonance (EPR), and single-crystal X-ray diffraction. Compared with neutral molecules <b>1</b> and <b>2</b>, radical <b>1</b>^•+ has elongated P–P bonds and more pyramidalized phosphorus atoms, while shortened P–N_ring distances and larger angles around phosphorus centers are observed for <b>2</b>^•+. EPR studies indicate that for <b>1</b>^•+ spin density mainly resides on the exocyclic nitrogen atoms with very minor contribution from endocyclic phosphorus atoms, while the situation is opposite for <b>2</b>^•+. Such an inverse spin density distribution is controlled by the exocyclic substituents, which is supported by DFT calculations

One-Electron Oxidation of an Organic Molecule by B(C₆F₅)₃; Isolation and Structures of Stable Non-<i>para</i>-substituted Triarylamine Cation Radical and Bis(triarylamine) Dication Diradicaloid

The methylene-bridged triphenylamine <b>2</b> has been oxidized to planar radical cation <b>2</b>^<b>•+</b> by B­(C₆F₅)₃ or Ag⁺. Further reaction of <b>2</b>^<b>•+</b>[Al­(OR_F)₄]⁻ and <b>2</b> with trace amounts of silver salt resulted in dication <b>3</b>²⁺, providing a rare example of structurally characterized bis­(triarylamine) “bipolarons”. <b>3</b>²⁺ can be directly prepared by the reaction of <b>3</b> with 2 equiv of Ag⁺. X-ray structural analysis together with theoretical calculation shows that <b>3</b>²⁺ has singlet diradical character and is analogous to Chichibabin’s hydrocarbons

One-Electron Oxidation of an Organic Molecule by B(C₆F₅)₃; Isolation and Structures of Stable Non-<i>para</i>-substituted Triarylamine Cation Radical and Bis(triarylamine) Dication Diradicaloid

The methylene-bridged triphenylamine <b>2</b> has been oxidized to planar radical cation <b>2</b>^<b>•+</b> by B­(C₆F₅)₃ or Ag⁺. Further reaction of <b>2</b>^<b>•+</b>[Al­(OR_F)₄]⁻ and <b>2</b> with trace amounts of silver salt resulted in dication <b>3</b>²⁺, providing a rare example of structurally characterized bis­(triarylamine) “bipolarons”. <b>3</b>²⁺ can be directly prepared by the reaction of <b>3</b> with 2 equiv of Ag⁺. X-ray structural analysis together with theoretical calculation shows that <b>3</b>²⁺ has singlet diradical character and is analogous to Chichibabin’s hydrocarbons

Magnetic Bistability in a Discrete Organic Radical

Molecular assembly with magnetic bistability has been of considerable interest for application as electronic devices. In contrast to transition-metal complexes, magnetic bistability so far observed in organic radical crystals is mainly caused by intermolecular electron-exchange interaction. We now report that the magnetic bistability in an organic radical can also be caused by intramolecular electron-exchange interaction. The diradical salt of 1,4-di­(bisphenylamino)-2,3,5,6,-tetramethylbenzene undergoes a phase transition with a thermal hysteresis loop over the temperature range from 118 to 131 K. The phases above and below the loop correspond to two different singlet states of the diradical dication. The results provide a novel organic radical material as an unprecedented instance of an intramolecular magnetic bistability revalent to the design of functional materials

Magnetic Bistability in a Discrete Organic Radical

Molecular assembly with magnetic bistability has been of considerable interest for application as electronic devices. In contrast to transition-metal complexes, magnetic bistability so far observed in organic radical crystals is mainly caused by intermolecular electron-exchange interaction. We now report that the magnetic bistability in an organic radical can also be caused by intramolecular electron-exchange interaction. The diradical salt of 1,4-di­(bisphenylamino)-2,3,5,6,-tetramethylbenzene undergoes a phase transition with a thermal hysteresis loop over the temperature range from 118 to 131 K. The phases above and below the loop correspond to two different singlet states of the diradical dication. The results provide a novel organic radical material as an unprecedented instance of an intramolecular magnetic bistability revalent to the design of functional materials

Enhancing the Spin–Orbit Coupling in Fe₃O₄ Epitaxial Thin Films by Interface Engineering

By analyzing the in-plane angular dependence of ferromagnetic resonance linewidth, we show that the Gilbert damping constant in ultrathin Fe₃O₄ epitaxial films on GaAs substrate can be enhanced by thickness reduction and oxygen vacancies in the interface. At the same time, the uniaxial magnetic anisotropy due to the interface effect becomes significant. Using the element-specific technique of X-ray magnetic circular dichroism, we find that the orbital-to-spin moment ratio increases with decreasing film thickness, in full agreement with the increase in the Gilbert damping obtained for these ultrathin single-crystal films. Combined with the first-principle calculations, the results suggest that the bonding with Fe and Ga or As ions and the ionic distortion near the interface, as well as the FeO defects and oxygen vacancies, may increase the spin–orbit coupling in ultrathin Fe₃O₄ epitaxial films and in turn provide an enhanced damping