Benzoquinone-Bridged Heterocyclic Zwitterions as Building Blocks for Molecular Semiconductors and Metals.

In pursuit of closed-shell building blocks for single-component organic semiconductors and metals, we have prepared benzoquino-bis-1,2,3-thiaselenazole QS, a heterocyclic selenium-based zwitterion with a small gap (λ = 729 nm) between its highest occupied and lowest unoccupied molecular orbitals. In the solid state, QS exists in two crystalline phases and one nanocrystalline phase. The structures of the crystalline phases (space groups R3 c and P2/ c) have been determined by high-resolution powder X-ray diffraction methods at ambient and elevated pressures (0-15 GPa), and their crystal packing patterns have been compared with that of the related all-sulfur zwitterion benzoquino-bis-1,2,3-dithiazole QT (space group Cmc2).

Magnetic Bistability in Naphtho-1,3,2-dithiazolyl: Solid State Interconversion of a Thiazyl π-Radical and Its N-N σ-Bonded Dimer.

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at T↓ = 128(2) K and T↑ = 188(2) K. The magnetic signature arises from a radical/dimer interconversion involving one of the two independent π-radicals in the P1̅ unit cell. Variable temperature X-ray crystallography has established that while all the radicals in HT-NDTA serve as paramagnetic ( S = 1/2) centers, half of the radicals in LT-NDTA form closed-shell N-N σ-bonded dimers ( S = 0) and half retain their S = 1/2 spin state.

The Power of Packing: Metallization of an Organic Semiconductor.

Benzoquino-bis-1,2,3-dithiazole 5 is a closed shell, antiaromatic 16π-electron zwitterion with a small HOMO-LUMO gap. Its crystal structure consists of planar ribbon-like molecular arrays packed into offset layers to generate a "brick-wall" motif with strong 2D interlayer electronic interactions. The spread of the valence and conduction bands, coupled with the narrow HOMO-LUMO gap, affords a small band gap semiconductor with σ = 1 × 10 S cm and E = 0.

Halide Influence on Molecular and Supramolecular Arrangements of Iron Complexes with a 3,5-Bis(2-Pyridyl)-1,2,4,6-Thiatriazine Ligand.

A series of iron centered complexes, namely, [Fe(Py2TTA)Cl2] (1), [Fe(Py2TTA)Br2] (2), and [Fe(μ-F)(Py2TTAO)F]∞ (3), were isolated via complexation of 3,5-bis(2-pyridyl)-1,2,4,6-thiatriazine (Py2TTAH) with various ferric halides (e.g., FeF3, FeCl3, and FeBr3).

Ambivalent binding between a radical-based pincer ligand and iron.

A complex exhibiting valence delocalization was prepared from 3,5-bis(2-pyridyl)-1,2,4,6-thiatriazinyl (), an inherently redox active pincer-type ligand, coordinated to iron ( ()). Complex can be prepared via two routes, either from the reaction of the neutral radical with FeCl2 or by treatment of the anionic ligand () with FeCl3, demonstrating its unique redox behaviour. Electrochemical studies, solution absorption and solid-state diffuse reflectance measurements along with X-ray crystallography were carried out to elucidate the molecular and solid-state properties.

Anion-induced Ag(I) self-assemblies with electron deficient aromatic ligands: anion-π-system interactions as a driving force for templated coordination networks.

Three novel 1D, 2D and 3D coordination polymers were successfully isolated using nitrogen based 3,6-bis(2'-pyrimidyl)-1,2,4,5-tetrazine (BPymTz) and 2,4,6-tris(2-pyrimidyl)-1,3,5-triazine (TPymT) ligands with Ag(I) ions. The formation of these supramolecular assemblies was templated through anion-π-system interactions.

Heat, pressure and light-induced interconversion of bisdithiazolyl radicals and dimers.

The heterocyclic bisdithiazolyl radical 1b (R1 = Me, R2 = F) crystallizes in two phases. The α-phase, space group P2₁/n, contains two radicals in the asymmetric unit, both of which adopt slipped π-stack structures. The β-phase, space group P2₁/c, consists of cross-braced π-stacked arrays of dimers in which the radicals are linked laterally by hypervalent 4-center 6-electron S···S-S···S σ-bonds.

Non-innocent pyridyl nitrogens: unprecedented interconversion of N-bridgehead-thiadiazolium salts and thiatriazine in the generation of thiatriazinyl.

Condensation of N-2-pyridylimidoyl-2-pyridylamidine with S2Cl2 affords fused N-bridgehead-1,2,5-thiadiazolium salts, which can be converted to 3,5-bis(2-pyridyl)-4-hydro-1,2,4,6-thiatriazine (Py2TTAH). Oxidation of Py2TTAH with iodine yields the corresponding 1,2,4,6-thiatriazinyl radical, identified by EPR spectroscopy.

From magnets to metals: the response of tetragonal bisdiselenazolyl radicals to pressure.

The bromo-substituted bisdiselenazolyl radical 4b (R(1) = Et, R(2) = Br) is isostructural with the corresponding chloro-derivative 4a (R(1) = Et, R(2) = Cl), both belonging to the tetragonal space group P(4)2(1)m and consisting of slipped π-stack arrays of undimerized radicals. Variable temperature, ambient pressure conductivity measurements indicate a similar room temperature conductivity near 10(-4) S cm(-1) for the two compounds, but 4b displays a slightly higher thermal activation energy E(act) (0.23 eV) than 4a (0.

Metallization of a hypervalent radical dimer: molecular and band perspectives.

Variable pressure and temperature conductivity measurements on the bisthiaselenazolyl radical dimer [1a](2) have established the presence of a weakly metallic state over the pressure range 5-9 GPa. To explore the origin of this metallization we have examined the crystal and molecular structure of [1a](2) as a function of pressure. At ambient pressure the dimer consists of two radicals linked by a hypervalent 4-center 6-electron S.

Heavy atom ferromagnets under pressure: structural changes and the magnetic response.

Application of physical pressure to a ferromagnetic bisdiselenazolyl radical leads to a decrease in pi-stack slippage. Initially, this leads to an increase in the ferromagnetic ordering temperature T(C), which reaches a maximum of 21 K near 1 GPa. At higher pressures, as the pi-stacks become more nearly superimposed, the value of T(C) diminishes.

Isostructural bis-1,2,3-thiaselenazolyl dimers.

Reduction of the N-methylated bis-1,2,3-thiaselenazolylium salts [2a,b,c][OTf] (with R(1) = Me and R(2) = H (2a), F (2b), Me (2c)) affords the corresponding bis-1,2,3-thiaselenazolyl radicals 2a,b,c. The radicals crystallize as centrosymmetric Se-Se sigma-bonded dimers [2a,b,c](2), in which an intramolecular Se-S bond of the radical is cleaved and replaced by an intermolecular Se-Se bond. The crystal structures of the three dimers are isomorphous, all belonging to the monoclinic space group P2(1)/c, and consist of interpenetrating, cross-braced, slipped pi-stack arrays laced together by numerous short intermolecular Se-N' and Se-S' contacts.

Structure and property correlations in heavy atom radical conductors.

The synthesis and solid-state characterization of the resonance-stabilized heterocyclic thia/selenazyl radicals 1a-4a is described. While all the radicals crystallize in undimerized slipped pi-stacked arrays, the four crystal structures do not constitute an isomorphous set; crystals of 1a and 3a belong to the orthorhombic space group P2(1)2(1)2(1), while those of 2a and 4a belong to the monoclinic space group P2(1)/n. The origin of the structural dichotomy can be traced back to the packing of the radicals in the P2(1)/n structure, which maximizes intermolecular Se-Se' contacts.

Ferromagnetic ordering in bisthiaselenazolyl radicals: variations on a tetragonal theme.

A series of five isostructural bisthiaselenazolyl radicals 2 have been prepared and characterized by X-ray crystallography. The crystal structures, all belonging to the tetragonal space group P42(1)m, consist of slipped pi-stack arrays of undimerized radicals packed about 4 centers running along the z-direction, an arrangement which gives rise to a complex lattice-wide network of close intermolecular Se---Se' contacts. Variations in R1 (Et, Pr, CH2CF3) with R2 = Cl lead to significant changes in the degree of slippage of the pi-stacks and hence the proximity of the Se---Se' interactions.

Enhanced conductivity and magnetic ordering in isostructural heavy atom radicals.

Synthetic methods have been developed to generate the complete series of resonance-stabilized heterocyclic thia/selenazyl radicals 1a-4a. X-ray crystallographic studies confirm that all four radicals are isostructural, belonging to the tetragonal space group P42(1)m. The crystal structures consist of slipped pi-stack arrays of undimerized radicals packed about 4 centers running along the z direction, an arrangement which gives rise to a complex lattice-wide network of close intermolecular E2---E2' contacts.

Spin-canting in heavy atom heterocyclic radicals.

A pair of isostructural bis-selenathiazolyl and bis-diselenazolyl radical conductors display weak (spin-canted) ferromagnetism with Tc values of 18 K and 27 K respectively.

Electronic and magnetic interactions in pi-stacked bisthiadiazinyl radicals.

The preparation of two bisthiadiazinyls (7, R1 = Me, Et; R2 = Cl, R3 = Ph), the first examples of a new class of resonance-stabilized heterocyclic thiazyl radical, are reported. Both radicals have been characterized in solution by EPR spectroscopy and cyclic voltammetry, which confirm highly delocalized spin distributions and low electrochemical cell potentials, features which augur well for the use of these materials as building blocks for neutral radical conductors. In the solid state, the radicals are undimerized, crystallizing in slipped pi-stack arrays which ensure the availability of electrons as potential charge carriers.

An alternating pi-stacked bisdithiazolyl radical conductor.

A general synthetic route to the resonance-stabilized pyrazine-bridged bisdithiazolyl framework, involving the reductive deprotection of 2,6-diaminopyrazine-bisthiocyanate and cyclization with thionyl chloride, has been developed. An N-methyl bisdithiazolyl radical, 4-methyl-4H-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyrazin-3-yl, has been prepared and characterized in solution by electron paramagnetic resonance spectroscopy and cyclic voltammetry. Its crystal structure has been determined at several temperatures.

Resonance stabilized bisdiselenazolyls as neutral radical conductors.

An efficient and versatile synthetic route to resonance stabilized bisselenathiazolyl and bisdiselenazolyl radicals 3 and 4 is described. Structural analysis of 3 and 4 confirm that lattice and pi-delocalization energies are sufficient to offset solid-state dimerization of the radicals and that the two selenium-containing radicals are isostructural with the all-sulfur based system 1. Variable temperature conductivity measurements indicate that sequential replacement of sulfur by selenium leads to a progressive increase in conductivity and reduction in thermal activation energy.

Bimodal association of a bis-1,2,3-dithiazolyl radical.

The N-ethyl pyrazine-bridged bis-1,2,3-dithiazolyl radical (R(1) = Et) associates at room temperature as a C-C bonded sigma-dimer which, on heating, converts to a laterally S-S sigma-bonded structure.

Pressure enhanced conductivity in bis-1,2,3-thiaselenazolyl dimers.

A synthetic sequence to salts of N-alkylated pyridine-bridged 1,2,3-thiaselenazolo-1,2,3-thiaselenazolylium cations [2]+ (R1 = Me, Et; R2 = H) is described. The corresponding radicals 2 (R1 = Me, Et; R2 = H) can be generated from the cations by chemical or electrochemical reduction. Crystals of the two radicals are isostructural and consist of interpenetrating pi-stacked arrays of closed-shell Se-Se sigma-bonded dimers [2]2 laced together with numerous short intermolecular Se- - -Se, Se- - -S, and Se- - -N contacts.

Bis-1,2,3-thiaselenazolyl radicals and their sigma-bonded dimers.

Resonance stabilized bis-1,2,3-thiaselenazolyl radicals associate in the solid state to afford Se-Se sigma-bonded dimers.

Resonance stabilized bis-thiadiazinyl radicals.

The resonance stabilized bis-thiadiazinyl framework holds potential as a stable and versatile building block for the design of radical-based conductors and magnetic materials.

Bistability and the phase transition in 1,3,2-dithiazolo[4,5-b]pyrazin-2-yl.

The molecular radical 1,3,2-dithiazolo[4,5-b]pyrazin-2-yl (PDTA) exhibits magnetic bistability just above room temperature, undergoing a well-defined hysteretic phase change with TC downward arrow = 297(1) K and TC upward arrow = 343(1) K. The crystal structures of the two phases of PDTA have been determined by single-crystal X-ray diffraction at 323(2) K. LT-PDTA consists of diamagnetic (S = 0) nearly superimposed pi-dimer stacks, while that of HT-PDTA comprises slipped stacks of pi-radicals (S = 1/2).