Zaichao Zhang

Tuning ground states of Bis(triarylamine) Dications: from a closed-shell singlet to a diradicaloid with an excited triplet state

Three bis(triarylamine) dications were isolated by using weakly coordinating anions. Their electronic structures in the ground state were investigated by various experiments in conjunction with theoretical calculations. The ground-state electronic structures of these species were tunable by substituent effects, with two of them as closed-shell singlets and one of them as an open-shell singlet in the solid state. The excited state of the latter is thermally accessible, indicated by EPR and SQUID measurements. The work provides a new and stable diradicaloid structure motif with an excited triplet sate.

Isolation and reversible dimerization of a selenium–selenium three-electron σ-bond

Three-electron σ-bonding that was proposed by Linus Pauling in 1931 has been recognized as important in intermediates encountered in many areas. A number of three-electron bonding systems have been spectroscopically investigated in the gas phase, solution and solid matrix. However, X-ray diffraction studies have only been possible on simple noble gas dimer Xe∴Xe and cyclic framework-constrained N∴N radical cations. Here, we show that a diselena species modified with a naphthalene scaffold can undergo one-electron oxidation using a large and weakly coordinating anion, to afford a room-temperature-stable radical cation containing a Se∴Se three-electron σ-bond. When a small anion is used, a reversible dimerization with phase and marked colour changes is observed: radical cation in solution (blue) but diamagnetic dimer in the solid state (brown). These findings suggest that more examples of three-electron σ-bonds may be stabilized and isolated by using naphthalene scaffolds together with large and weakly coordinating anions.

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.

Single-Chain Magnets Based on Octacyanotungstate with the Highest Energy Barriers for Cyanide Compounds

By introducing large counter cations as the spacer, two isolated 3, 3-ladder compounds, (Ph4P)[CoII(3-Mepy)2.7(H2O)0.3WV(CN)8]·0.6H2O (1) and (Ph4As)[CoII(3-Mepy)3WV(CN)8] (2, 3-Mepyu2009=u20093-methylpyridine), were synthesized and characterized. Static and dynamic magnetic characterizations reveal that compounds 1 and 2 both behave as the single-chain magnets (SCMs) with very high energy barriers: 252(9) K for 1 and 224(7) K for 2, respectively. These two compounds display the highest relaxation barriers for cyano-bridged SCMs and are preceded only by two cobalt(II)-radical compounds among all SCMs. Meanwhile, a large coercive field of 26.2 kOe (1) and 22.6 kOe (2) were observed at 1.8u2009K.

An Isolable Diboron-Centered Diradical with a Triplet Ground State

Two new diboranes, 2,6-bis(BMes2 )mesitylene (1) and 3,3-bis(BMes2 )bimesitylene (3), were synthesized. Two-electron reduction of 1 with elemental potassium afforded the C-H activation product [(18-c-6)K(THF)2 ]2+ ⋅22- bearing a BC3 four-membered ring as colorless crystals, whereas the reduction of 3 with potassium led to the isolation of [(18-c-6)K(THF)2 ]2+ ⋅32-.. as dark blue crystals. Both reduction products were characterized by structural and spectroscopic methods. Electron paramagnetic resonance (EPR) spectroscopy and theoretical calculations revealed that the electron spin density of 32-.. mainly resides on the two boron nuclei and features a triplet ground state, which was confirmed by superconducting quantum interference device (SQUID) measurements as well as theoretical calculations. 32-.. represents the first structurally characterized boron-centered diradical with a triplet ground state. In addition, the reactivity of [(18-c-6)K(THF)2 ]2+ ⋅32-.. toward PhSeSePh and nBu3 SnH was investigated, which is consistent with its radical character.

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.

Access to Stable Metalloradical Cations with Unsupported and Isomeric Metal–Metal Hemi‐Bonds

Metalloradical species [Co2 Fv(CO)4 ](.+) (1(.+) , Fv=fulvalenediyl) and [Co2 Cp2 (CO)4 ](.+) (2(.+) , Cp=η(5) -C5 H5 ), formed by one-electron oxidations of piano-stool cobalt carbonyl complexes, can be stabilized with weakly coordinating polyfluoroaluminate anions in the solid state. They feature a supported and an unsupported (i.e. unbridged) cobalt-cobalt three-electron σu2005bond, respectively, each with a formal bond order of 0.5 (hemi-bond). When Cp is replaced by bulkier Cp* (Cp*=η(5) -C5 Me5 ), an interchange between an unsupported radical [Co2 Cp*2 (CO)4 ](.+) (anti-3(.+) ) and a supported radical [Co2 Cp*2 (μ-CO)2 (CO)2 ](.+) (trans-3(.+) ) is observed in solution, which cocrystallize and exist in the crystal phase. 2(.+) and anti-3(.+) are the first stable thus isolable examples that feature an unsupported metal-metal hemi-bond, and the coexistence of anti-3(.+) and trans-3(.+) in one crystal is unprecedented in the field of dinuclear metalloradical chemistry. The work suggests that more stable metalloradicals of metal-metal hemi-bonds may be accessible by using metal carbonyls together with large and weakly coordinating polyfluoroaluminate anions.

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.

Electrochemical Synthesis and Magnetic Properties of [Cu9W6]: The Ultimate Member of the Quindecanuclear Octacyanometallate-Based Transition-Metal Cluster?

[Cu9W6], synthesized by the electrochemical method, may be the ultimate member of the quindecanuclear octacyanometallate-based transition-metal cluster. Its single-crystal structure and magnetic properties were characterized.

Isolation and crystal structure of a dithiophene dication: controlling covalent connection and disconnection with temperature and phase

A dithienylethene (1o) undergoes a two-electron chemical oxidation to a singlet diradical as an open-isomer (1o(2+)) in solution, which cyclizes to a closed-form (1c(2+)) upon cooling. The latter crystallizes out and its structure is analyzed by single crystal X-ray diffraction. Equilibrium between 1o(2+) and 1c(2+) in solution is observed by NMR and UV spectroscopy at various temperatures and is further supported by reduction reactions with Zn powder.