Gengwen Tan

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.

Isolable Diphosphorus-Centered Radical Anion and Diradical Dianion

Two salts containing diphosphorus-centered radical anion 1(•-) and diradical dianion 1(2-••) were obtained by one- and two-electron reductions of an indenofluorene-bridging diphosphaalkene (1) 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 1(2-••) has an open-shell singlet ground state. 1(•-) and 1(2-••) represent the first isolable and structurally characterized diphosphorus-centered radical anion and dianion.

A boron-centered radical: a potassium-crown ether stabilized boryl radical anion

A boron radical contact ion-pair Mes2B{4-(3,5-dimethylpyridinyl)}K(18-crown-6)(THF) (1K) has been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, UV-vis absorption spectroscopy and single crystal X-ray diffraction. The geometry, bonding and spin density distribution are shown to be affected by the NK interaction. The unpaired electron resides mainly on the boron atom and falls between those of triarylboron radical anions and neutral boron radicals. The work provides a novel boron-centered radical intermediate, connecting anionic and neutral boryl radicals.

Elusive Antimony-Centered Radical Cations: Isolation, Characterization, Crystal Structures, and Reactivity Studies

One-electron oxidation of the stibines Aryl3 Sb (1, Aryl=2,6-i Pr2 -4-OMe-C6 H2 ; 2, Aryl=2,4,6-i Pr3 -C6 H2 ) with AgSbF6 and NaBArylF4 (ArylF =3,5-(CF3 )2 C6 H3 ) afforded the first structurally characterized examples of antimony-centered radical cations 1.+ [BArylF4 ]- and 2.+ [BArylF4 ]- . Their molecular and electronic structures were investigated by single-crystal X-ray diffraction, electron paramagnetic resonance spectroscopy (EPR) and UV/Vis absorption spectroscopy, in conjunction with theoretical calculations. Moreover, their reactivity was investigated. The reaction of 2.+ [BArylF4 ]- and p-benzoquinone afforded a dinuclear antimony dication salt 32+ [BArylF4 ]2- , which was characterized by NMR spectroscopy and X-ray diffraction analysis. The formation of the dication 32+ further confirms that the isolated stibine radical cations are antimony-centered.

An Aliphatic Solvent-Soluble Lithium Salt of the Perhalogenated Weakly Coordinating Anion [Al(OC(CCl3)(CF3)2)4]−

The facile synthesis of a new highly aliphatic solvent-soluble Li(+) salt of the perhalogenated weakly coordinating anion [Al(OC(CCl3)(CF3)2)4](-) and its application in stabilizing the Ph3C(+) cation were investigated. The lithium salt Li[Al(OC(CCl3)(CF3)2)4] (4) was prepared by the treatment of 4 mol equiv of HOC(CCl3)(CF3)2 with purified LiAlH4 in n-hexane from -20 °C to room temperature. Compound 4 is highly soluble in both polar and nonpolar solvents, and it bears both CCl3 and CF3 groups, resulting in a lower symmetry around the Al center compared to that of Li[Al(OC(CF3)3)4] (1). Treatment of 4 with Ph3CCl afforded the ionic compound [Ph3C][Al(OC(CCl3)(CF3)2)4] (5) bearing the Ph3C(+) cation with concomitant elimination of LiCl, suggesting the potential application of [Al(OC(CCl3)(CF3)2)4](-) in stabilizing reactive cationic species. Compounds 4 and 5 were fully characterized by spectroscopic and structural methods.

Reversible Self-Assembling of Boryl Radical Anions to Their Diradicals with Tunable Singlet Ground States

Two novel boron-centered diradicals based on dimesitylpyridine borane (1) were synthesized by the self-assembling of the corresponding radical sodium and potassium salts, respectively. The sodium diradical was obtained by re-dissolving the crystals of the radical salt 1Na in toluene, while the potassium diradical was directly obtained by the reduction of 1 with potassium in THF. The diradicals could be converted back to their radical anions in THF solution, forming a reversible process. EPR spectroscopy and SQUID measurements, together with theoretical calculations, show that the diradicals have singlet ground states with excited triplet states. Their singlet-triplet energy gaps are tunable with metals.

Putting aniline radical cations in a bottle

Salts containing aniline radical cations have been isolated and characterized by electron paramagnetic resonance (EPR) spectroscopy, UV-Vis absorption spectroscopy and single crystal X-ray diffraction. The EPR spectra and theoretical calculations indicate the unpaired electron is delocalized on phenyl rings and nitrogen atoms. Both radical cations feature a quinoidal geometry with a partially double C–N bond, but are distinct in that the C–N bond is coplanar to the phenyl plane in one cation while deviates from the plane in the other due to steric crowding. The work provides the first unequivocal examples of stable aniline radical cations.