The Lewis Acid Induced Formation of a Stable Diradical with an Intramolecular Ion Pairing State.

A stable cross-conjugated diradical was prepared by the reaction of a donor-acceptor-donor (D-A-D) molecule with B(C6F5)3. Its geometry and electronic structure were characterized by single crystal X-ray diffraction, EPR spectroscopy, SQUID measurement, UV/vis spectroscopy, and DFT calculation. It has an open-shell singlet ground state with a thermally excited triplet state. It can be viewed as an intramolecular radical ion pair, and the formation mechanism is proposed as an intramolecular single electron transfer that occurs from the bis(triarylamine) donor fragment to the central dioxophenyl acceptor moiety, induced by the acidic boron atom. This work provides a Lewis acid induced approach to the formation of neutral and cross-conjugated diradicals.

Indeno[2,1-a]fluorene-11,12-dione Radical Anions: Synthesis, Characterization, and Properties.

The one-electron reduction of indeno[2,1-a]fluorene-11,12-dione (IF) with various alkali metals prepare the radical anion salts. The data about different structures, properties, and characterization was obtained by single-crystal X-ray diffraction, electron paramagnetic resonance (EPR) spectroscopy, superconducting quantum interference device (SQUID) measurements, and physical property measurement system (PPMS). Compound IF.- K+ (18-c-6) is regarded as a one-dimensional magnetic chain through C-H⋅⋅⋅C interaction. Theoretical calculations and magnetic results showed that [IF.- K+ (15-c-5)]2 is a dimer with an open-shell ground state. Compounds IF.- Na+ (15-c-5) and IF.- K+ (cryptand) are monoradical anion salts: IF2 .- Li+ possesses unique π-stack structure with an interplanar separation less than 3.46 Å, making it a semiconductor (δRT =1.9×10-4  S ⋅ cm-1 ). This work gives insights into multifunctional radical anions, and describes the design and development of different functional radicals.

A Magnetically Robust Triplet Ground State Sulfur-Hydrocarbon Diradical Dication.

The one- and two-electron oxidation reactions of tetrathiolated di-tert-butylpyrene compound 1 with NO[Al(ORF)4] (ORF = OC(CF3)3) resulted in radical cation salt 1•+[Al(ORF)4]- and diradical dication salt 12+••·2[Al(ORF)4]-, respectively. Both salts were isolated as stable crystals and investigated by single-crystal X-ray diffraction, EPR spectroscopy, SQUID measurements, and UV-vis spectroscopy. The electron spin density is delocalized on one single sulfur-doped ring in 1•+ and on both sulfur-doped rings in 12+••. The diradical dication 12+•• features a robust triplet ground state, representing the first example of a high-spin sulfur-hydrocarbon diradical.

Isolable Lanthanide Metal Complexes of a Phosphorus-Centered Radical.

Reaction of the diazafluorenylidene-substituted phosphaalkene 1 with [Cp*2Ln][BPh4] (Ln = Dy, Tb, and Gd), followed by subsequent reduction with KC8, afforded complexes Cp*2Dy(N,N'-1) (2), Cp*2Tb(N,N'-1) (3), and Cp*2Gd(N,N'-1) (4) in moderate yields, in which the phosphaalkene moiety is in the radical-anion state. Complexes 2 and 3 represent the first lanthanide metal complexes of a heavy main-group element-based radical. They have been characterized by single-crystal X-ray diffraction, UV/vis spectroscopy, and superconducting quantum interference device measurements. The magnetic studies reveal that the phosphorus-radical centers have a quite weak antiferromagnetic interaction with the lanthanide ions in 2-4. Furthermore, complex 2 shows slow magnetic relaxation behavior.

Stable Radical Cation and Dication of an N-Heterocyclic Carbene Stabilized Digallene: Synthesis, Characterization and Reactivity.

One- and two-electron oxidation of a digallene stabilized by an N-heterocyclic carbene afforded the first stable gallium-based radical cation and dication salts, respectively. Structural analysis and theoretical calculations reveal that the oxidation occurs at the Ga=Ga double bond, leading to removal of π electrons of the double bond and a decrease of the bond order. The spin density of the radical cation mainly locates at the two gallium centers as demonstrated by EPR spectroscopy and theoretical calculations. Moreover, the reactivity of the radical cation salt toward nBu3 SnH and cyclo-S8 was studied; a digallium-hydride cation salt containing a Ga-Ga single bond and a gallium sulfide cluster bearing an unprecedented ladder-like Ga4 S4 core structure were obtained, respectively.

Magnetic Multistability in an Anion-Radical Pimer.

Radical pimers are the simplest and most important models for studying charge-transfer processes and provide deep insight into π-stacked organic materials. Notably, radical pimer systems with magnetic bi- or multistability may have important applications in switchable materials, thermal sensors, and information-storage media. However, no such systems have been reported. Herein, we describe a new pimer consisting of neutral N-(n-propyl) benzene triimide ([BTI-3C]) and its anionic radical ([BTI-3C]-. ) that exhibits rare magnetic multistability. The crystalline pimer was readily synthesized by reduction of BTI-3C with cobaltocene (CoCp2 ). The transition occurred with a thermal hysteresis loop that was 27 K wide in the range of 170-220 K, accompanied by a smaller loop with a width of 25 K at 220-242 K. The magnetic multistability was attributed to slippage of the π-stacked BTI structures and entropy-driven conformational isomerization of the side propyl chains in the crystalline state during temperature variation.

Crystalline Diradical Dianions of Pyrene-Fused Azaacenes.

Although diradicals and azaacenes have been greatly attractive in fundamental chemistry and functional materials, the isolable diradical dianions of azaacenes are still unknown. Herein, we describe the first isolation of pyrene-fused azaacene diradical dianion salts [(18-c-6)K(THF)2 ]+ [(18-c-6)K]+ ⋅12-.. and [(18-c-6)K(THF)]2+ ⋅22-.. by reduction of the neutral pyrene-fused azaacene derivatives 1 and 2 with excess potassium graphite in THF in the presence of 18-crown-6. Their electronic structures were investigated by various experiments, in conjunction with theoretical calculations. It was found that both dianions are open-shell singlets in the ground state and their triplet states are thermally readily accessible owing to the small singlet-triplet energy gap. This work provides the first examples of crystalline diradical dianions of azaacenes with considerable diradical character.

Tunable Reduction of 2,4,6-Tri(4-pyridyl)-1,3,5-Triazine: From Radical Anion to Diradical Dianion to Radical Metal-Organic Framework.

The reduction of 2,4,6-tri(4-pyridyl)-1,3,5-triazine (TPT) with alkali metals resulted in four radical anion salts (1, 2, 4 and 5) and one diradical dianion salt (3). Single-crystal X-ray diffraction and electron paramagnetic resonance (EPR) spectroscopy reveal that 1 contains the monoradical anion TPT.- stacked in one-dimensional (1D) with K+ (18c6) and 2 can be viewed as a 1D magnetic chain of TPT.- , while 4 and 5 form radical metal-organic frameworks (RMOFs). 1D pore passages, with a diameter of 6.0 Å, containing solvent molecules were observed in 5. Variable-temperature EPR measurements show that 3 has an open-shell singlet ground state that can be excited to a triplet state, consistent with theoretical calculation. The work suggests that the direct reduction approach could lead to the formation of RMOFs.

A Main-Group Element Radical Based One-Dimensional Magnetic Chain.

The first main-group element radical based one-dimensional magnetic chain (1K)n was realized by one-electron reduction of the pyridinyl functionalized borane 1 with elemental potassium in THF in the absence of 18-crown-6 (18-c-6). The electron spin density of (1K)n mainly resides at the boron centers with a considerable contribution from central benzene and pyridine moieties. The spin centers exhibit an antiferromagnetic interaction as demonstrated by magnetic measurements and theoretical calculations. In contrast, the reduction in the presence of 18-c-6 afforded the separated radical anion salt 1K(Crown), in which the potassium cation was trapped by THF and 18-c-6 molecules. Further one-electron reduction of 1K(Crown) and (1K)n led to the diamagnetic monomer and polymer, respectively.

Tricoordinate Nontrigonal Pnictogen-Centered Radical Anions: Isolation, Characterization, and Reactivity.

The search for main-group element-based radicals is one of the main research topics in contemporary chemistry because of their fascinating chemical and physical properties. The Group 15 element-centered radicals mainly feature a V-shaped two coordinate structure, with a couple of radical cations featuring trigonal tricoordinated geometry. Now, nontrigonal compounds R3 E (E=P, As, Sb) were successfully synthesized by introducing a new rigid tris-amide ligand. The selective one-electron reduction of R3 E afforded the first stable tricoordinate pnictogen-centered radical anion salts; the pnictogen atoms retain planar T-shaped structures. EPR spectroscopy and calculations reveal that the spin density mainly resides at the p orbitals of the pnictogen atoms, which is perpendicular to the N3 E planes.

The catenation of a singlet diradical dication and modulation of diradical character by metal coordination.

A singlet bis(triarylamine) diradical dication and its zigzag 1D magnetic chain catenated by silver cations were isolated and characterized by single-crystal X-ray crystallography, EPR spectroscopy, SQUID measurements, cyclic voltammetry, and UV-Vis-NIR absorption spectroscopy in conjunction with DFT calculations. The ΔEOS-T has significantly changed due to the coordination of carbonyls (Lewis base) with Ag+ (Lewis acid), revealing that the diradical character can be modulated by Lewis acid-base coordination and interchain intermolecular coupling interactions.

S = 1/2 tetracene monoradical cation/anion: ion-based one-dimensional antiferromagnetic chains.

The monoradical cation 1˙+ and anion 1˙- based on tetracene were generated by one-electron oxidation and one-electron reduction of the bulky tetracene (1), respectively, which contain the largest π-fused skeletons reported to date. For monoradical species, 1˙+ and 1˙-, EPR spectra and DFT calculation results indicate the spin density delocalized over the whole molecules. Notably, in the solid state, 1˙+ and 1˙-, respectively, pack into (1˙+)n and (1˙--K-crown)n, characterized by single-crystal X-ray diffraction studies. The intermolecular interactions of 1˙+ and 1˙- are, respectively, through van der Waals forces and exchange couplings supported by metal ions. The monoradical cation polymer (1˙+)n was the first example based on PAHs. The EPR spectra at 90 K of 1˙+ and 1˙- all show forbidden transitions (Δms = ±2), indicating the existence of electronic coupling between the neighboring radicals, with respective 2J = -6.54 K and 2J = -0.22 K characterized by SQUID measurements.

The cis/trans conformation approach for tuning the magnetic coupling in a diradical: isolation of pure pyridine-based diradical dianions.

Two-electron reductions of 3,3'-bis(2,6-dimesitylpyridin-4-yl)-1,1'-biphenyl 1 with elemental potassium in the absence and presence of 18-c-6 afforded the diradical dianion salts [K+]2˙[trans-1]˙˙2- and [K(18-c-6)]+2˙[cis-1]˙˙2-, which exhibit trans and cis configurations, respectively. The transoid conformer could be converted to the cisoid one through reacting with 18-c-6.

A cationic sulfur-hydrocarbon triradical with an excited quartet state.

The triptycene-bridged tris(thianthrene) compound 1 was designed and synthesized. Three-electron oxidation of 1 by NO[Al(OC(CF3)3)4], followed by crystallization at two different temperatures resulted in the triradical trication salts 2a and 2b respectively, which feature different crystal packing patterns. The triradical trications in 2a and 2b both feature a doublet ground state which can be thermally populated to a quartet state, representing the first examples of cationic main-group triradicals.

Controlling the unpaired electron by electrostatic attraction in the solid state.

One-electron reduction of 2,7-tBu2-pyrene-4,5,9,10-tetraone (1) with potassium afforded two monoradicals 1K(cryp) and 1K(18c6), a radical tetramer [1K(15c5)]4 and a radical polymer (1K)2n. Using 1K(cryp) and 1K(18c6), we demonstrated large spin density modulation of an organic radical anion in the solid state by electrostatic attraction, without alternation of the molecular skeletons.

A high-spin diradical dianion and its bridged chemically switchable single-molecule magnet.

Triplet diradicals have attracted tremendous attention due to their promising application in organic spintronics, organic magnets and spin filters. However, very few examples of triplet diradicals with singlet-triplet energy gaps (ΔE ST) over 0.59 kcal mol-1 (298 K) have been reported to date. In this work, we first proved that the dianion of 2,7-di-tert-butyl-pyrene-4,5,9,10-tetraone (2,7-tBu2-PTO) was a triplet ground state diradical in the magnesium complex 1 with a singlet-triplet energy gap ΔE ST = 0.94 kcal mol-1 (473 K). This is a rare example of stable diradicals with singlet-triplet energy gaps exceeding the thermal energy at room temperature (298 K). Moreover, the iron analog 2 containing the 2,7-tBu2-PTO diradical dianion was isolated, which was the first single-molecule magnet bridged by a diradical dianion. When 2 was doubly reduced to the dianion salt 2K2, single-molecule magnetism was switched off, highlighting the importance of diradicals in single-molecule magnetism.

Persistent 2c-3e σ-bonded heteronuclear radical cations centered on S/Se and P/As atoms.

The two-center three-electron (2c-3e) bonded species are important in chemical and biological science. Reported isolable 2c-3e σ-bonded species are usually constructed in homoatomic radicals. The one-electron oxidation of main-group heteronuclear species Nap(SPh)(P(Mes)2) (1), Nap(SePh)(P(Mes)2) (2), Nap(SPh)(As(Mes)2) (3) and Nap(SePh)(As(Mes)2) (4) produced persistent radical cations 1˙+-4˙+ in solution. Large couplings of heteroatoms in EPR spectra of 1˙+-4˙+, shorter bond distances and bigger Wiberg bond orders of Ch-Pn in 1˙+-4˙+ than those in 1-4 in DFT calculations indicate large amounts of spin densities over heteroatoms and the formation of 2c-3e σ-bonds between chalcogen and pnicogen atoms. This work provides evidence of 2c-3e σ-bonds constructed between main-group heteronuclears and rare examples of radical cations involving three-electron σ-bonds between S/Se and P/As atoms.

Stable, yet "naked", azo radical anion ArNNAr- and dianion ArNNAr2- (Ar = 4-CN-2,6-iPr2-C6H2) with selective CO2 activation.

Azo radical anion 1˙- and dianion 12- have been isolated by one- and two-electron reduction of the azo compound 1 (ArNNAr, Ar = 4-CN-2,6-iPr2-C6H2) with alkali metals, respectively. The reduced species have been characterized by single-crystal X-ray analysis, EPR, UV and FT-IR spectroscopy, as well as SQUID measurements. The filling of one and two electrons in the π* orbital of the N-N double bond of 1 leads to a half-double N-N bond in 1˙- and a single N-N bond in 12-. The uncoordinated nature of these reduced species enables them to activate CO2. The exposure of 1˙- solution to CO2 led to the formation of oxalate anion C2O42-, while that of 12- solution to CO2 afforded the hydrazine dicarboxylate dianion [1-2CO2]2-, which reversibly dissociated back to 1 and CO2 upon oxidation.

Isolable cyclic radical cations of heavy main-group elements.

One-electron oxidation of 1,3-digerma-2,4-dipnictacyclobutadiene [LHGeE]2 (LH = CH[CHNDipp]2, dipp = 2,6-iPr2C6H3; 1: E = P; 2: E = As) with Ag[Al(ORF)4] (RF = C(CF3)3) afforded the stable radical cation salts 1˙+·[Al(ORF)4]- and 2˙+·[Al(ORF)4]-, respectively. The radical cation salts have been fully characterized, in conjunction with theoretical calculations. The EPR spectroscopic studies and DFT calculations reveal that the spin density mainly resides at the heavy pnictogen atoms, rather than delocalizes over the Ge2E2 ring. They represent the first structurally characterized cyclic radical species composed of both heavy group 14 and 15 elements.

A diamidinatogermylene as a Z-type ligand in a nickel(0) complex.

The reaction of the phosphine functionalized chlorogermylene 1 with Ni(COD)2 (COD = 1,5-cyclooctadiene) afforded the bis-chlorogermylene ligated nickel(0) complex 2 in high yield. The dechlorination reaction of 2 with elemental potassium serendipitously yielded the diamidinatogermylene nickel(0) complex 3. Single-crystal X-ray diffraction analysis reveals that the germanium center in 3 features a pyramidalized geometry, suggesting the germylene moiety in 3 acts as a Z-type ligand, which is further supported by theoretical calculations. Complex 3 represents the first example bearing a Z-type diaminogermylene ligand.