This review describes recent advances in the development of thiazyl radicals, particularly dithiadiazolyl (RCNSSN) and dithiazolyl (RCSNSCR) as molecule-based materials. Particular emphasis is placed on their potential applications as molecular conductors, magnets and switchable materials and incorporates an overview of the structure–property relationships required in these molecular systems.

Thiazyl radicals: old materials for new molecular devices

New synthetic methods for heterocyclic 1,3,2-dithiazolyl (DTA) radicals have been developed, and trends in the molecular spin distributions and electrochemical properties of a series of DTA radicals are reported. The crystal structures of [1,2,5]thiadiazolo[3,4-f][1,3,2]benzodithiazol-2-yl (TBDTA) and [1,3,2]pyrazinodithiazol-2-yl (PDTA) have been determined. The structure of TBDTA (at 293 and 95 K) contains two molecules in the asymmetric unit, each of which generates π-stacked arrays, one consisting of antiparallel chains of centrosymmetrically associated dimers, the other comprising parallel chains of unassociated radicals. The structure of PDTA (at 293 and 95 K) is simpler, consisting of slipped stacks of π-dimers. Variable-temperature magnetic susceptibility (χP) measurements on TBDTA indicate essentially paramagnetic behavior for the unassociated radical π-stacks over the range 5−400 K. By contrast PDTA is diamagnetic at all temperatures below 300 K, but between 300 and 350 K the value of χP follows...

Bistabilities in 1,3,2-dithiazolyl radicals.

Nitroxide radicals play a central role in the recent development of organomagnetic materials or molecular-based magnetic materials. This feature article considers the recent progress in the development of molecular-based magnetic materials dealing mainly with nitroxide radical crystals and charge transfer (CT) complexes consisting of nitroxide radicals. We present a view of the progress of this rapidly growing field of materials science over the past few years, showing the results of several research groups (including ours) based on nitroxide radicals in order to introduce the readers to the present situation and to further perspectives to possible degree.

Recent progress in the development of organomagnetic materials based on neutral nitroxide radicals and charge transfer complexes derived from nitroxide radicals

The characterization of the heterocyclic radical 1,2,5-thiadiazolo[3,4-b]-1,3,2-dithiazolo[3,4-b]pyrazin-2-yl (TDP-DTA) is described. The compound is prepared by treatment of 5,6-dithiolo-1,2,5-thiadiazolo[3,4-b]pyrazine with S3N3Cl3 and purified by fractional sublimation in vacuo. The results of cyclic voltammetry and ESR analysis of TDP-DTA and related heterocyclic dithiazolyls indicate that the spin distributions and donor/acceptor properties of these radicals are extremely sensitive to the nature of the 4,5-substituents. The crystal structure of TDP-DTA has been determined at two temperatures. At 293 K, the crystals are triclinic, space group P1, with a = 4.4456(8), b = 8.407(2), c = 9.671(3) A, α = 71.34(2), β = 89.28(2), γ = 87.80(2)°, Z = 2 (for C4N5S3); at 150 K the crystals are triclinic, space group P1, with a = 7.489(7), b = 9.593(4), c = 10.759(6) A, α = 65.77(4), β = 74.10(6), γ = 74.64(6)°, Z = 2 (for C8N10S6). At 293 K the structure consists of ribbons of TDP-DTA radicals packed in a slip...

Redox, magnetic, and structural properties of 1,3,2-dithiazolyl radicals. a case study on the ternary heterocycle s3n5c4

A novel synthesis of the title compound, C2S3N3
(1) is reported. X- and K-band EPR spectra on dilute solutions of 1 indicate delocalisation of the unpaired spin density over both heterocyclic rings in agreement with DFT calculations. An XRPD study indicates that it crystallises in two morphologies with both phases formed during vacuum sublimation. The XRPD studies indicate that on cooling below 230 K, only the triclinic phase (P) becomes detectable, whereas on warming above 320 K, just the monoclinic phase (P21/c) becomes observed. The crystal structure of the monoclinic phase has been examined by variable temperature single crystal X-ray diffraction in the region 300–225 K and reveals a regular π-stacked structure. A crystal structure of the triclinic
phase is reported at 150 K and exhibits a dimeric π-stacked motif. Susceptibility measurements show that the monoclinic phase is paramagnetic whereas the triclinic phase is diamagnetic. This radical exhibits thermal hysteresis with a wide range of bistability; EPR and magnetic susceptibility measurements indicate Tc↓ = 234 K, and Tc↑ = 317 K. The magnetic behaviour of the monoclinic phase is consistent with strong antiferromagnetic exchange interactions between open shell doublet states (J = −320 K) along the π-stacking direction, although significant inter-stack interactions are required to model the data adequately. In contrast the dimeric phase is essentially diamagnetic, with the residual paramagnetism indicating a very large singlet–triplet separation (|2J| > 2000 K).
The magnetic exchange interactions in both phases are probed through a series of DFT calculations using the broken-symmetry approach. These confirm the presence of strong magnetic exchange interactions along the π-stacking direction in the high temperature phase (2J = −182 K), but with additional interstack interactions which are an order of magnitude smaller. Calculations on the triclinic phase indicate that it is best considered as a dimer with an open-shell singlet state with a very large singlet–triplet separation (2J = −2657 K). The magnitude of J for both phases from theory and experiment are in good agreement. The origin of the thermal hysteresis is attributed to the presence of two energetically similar structures which have a low energy barrier to interconversion. The thermodynamic parameters associated with the interconversion process
have been probed by DSC studies. It confirms the first order nature of the transition with Tc↓ = 232.3 K (ΔH↓ = 1.41 kJ mol−1, ΔS↓ = 6.0 J mol−1 K−1) and Tc↑ = 320.5 K (ΔH↑ = 1.86 kJ mol−1, ΔS↑ = 5.8 J mol−1 K−1).

Synthesis, crystal structures, electronic structure and magnetic behaviour of the trithiatriazapentalenyl radical, C2S3N3

Sublimation of 1,2,3,5-dithiadiazolyl in vacuo affords a triclinic phase of the dimer [HCN2S2]2. The crystals belong to the space group P¯1, a = 6.816(3), b = 13.940(2), c = 14.403(3) A, α = 116.830(14), β = 98.64(3), γ = 99.18(3)°, FW = 212.4 (for [HCN2S2]2·[N2]0.08) Z = 6. The crystal structure consists of stacked dimers, with three dimers per asymmetric unit. Pairs of asymmetric units, related by an inversion center, generate a pinwheel motif consisting of six dimers. The columnar structure associated with these pinwheels forms close-packed sets of “molecular tubes”. Cosublimation of the radical in the presence of iodine in the mole ratio (HCN2S2:I = 5:1) yields an iodine-doped hexagonal phase of composition [HCN2S2]6[I]1.1. Crystals of this material belong to the space group P61, a = b = 14.132(16), c = 3.352(5) A, FW = 128.20, Z = 6. The crystal structure consists of sixfold pinwheels in which the now evenly spaced HCN2S2 rings form a spiral about the 61 axis. The iodine atoms lie within the columnar cavity of the pinwheels in a disordered array wrapped tightly about the sixfold screw axis. The single-crystal conductivity of the doped material is 15 S cm-1 at room temperature. Raman spectroscopic and magnetic susceptibility measurements on the doped material are reported.

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Molecular Conductors from Neutral-Radical Charge-Transfer Salts: Preparation and Characterization of an Iodine-Doped Hexagonal Phase of 1,2,3,5-Dithiadiazolyl ([HCN2S2]∙)

Cosublimation of 1,3- and 1,4-benzene-bis(1,2,3,5-dithiadiazolyl) and iodine/bromine affords crystals of the mixed valence salts 1,3- and 1,4-[(S2N2C)C6H4(CN2S2)][X] (X = I, Br). The crystal structures of the two iodide salts consist of perfectly superimposed stacks of molecular units with interannular spacing along the stacks of 3.487(3) and 3.415(2) A, for the 1,3- and 1,4-derivatives. In both compounds the iodines are disordered along the stacking direction. The 1,3-derivative has a highly one-dimensional structure; there are no short intercolumnar S-S interactions. In the 1,4-derivative, however, lateral S-S contacts of 3.911 A, afford some measure of three-dimensionality. The bromide salt of the 1,4-derivative consists of ribbons of alternating 1,4-[(S2N2C)C6H4(CN2S2)]+ units and bromide ions. Within each molecule one heterocyclic ring is closed shell, i.e., a [CN2S2]+ cation, while the other is a discrete radical. The ribbons are layered in zigzag fashion that maximizes ion pairing and isolates the radical centers. The bromide salt of the 1,3-derivative also forms ribbon-like arrays, but the unit cell repeat consists of four layers of ribbons. Within these layers the [CN2S2] rings are approximately stacked. The four rings within the repeat unit along each stack consists of three rings clustered into a trimeric [CN2S2]3+ cation, while the remaining ring is a discrete [CN2S2]+ cation. Magnetic susceptibility and conductivity measurements on the two iodide salts indicate weakly metallic behavior at room temperature, with a charge density wave (CDW) driven metal-insulator phase transition occurring near 270 and 190 K for the 1,3- and 1,4-derivatives, respectively. For the 1,4-derivative, analysis of the CDW wavevector associated with the transition affords a degree of charge transfer of 1/4 of electron per radical, i.e., an overall formulation of [(S2N2C)C6H4(CN2S2)]0.5+[I]0.5-. The bromide salt of the 1,3-derivative is a closed shell insulator, while in the 1,4-bromide the isolated radical centers are antiferromagnetically coupled.

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Charge Transfer Salts of Benzene-Bridged 1,2,3,5-Dithiadiazolyl Diradicals. Preparation, Structures, and Transport Properties of 1,3- and 1,4-[(S2N2C)C6H4(CN2S2)][X] (X = I, Br)

C.D. MacKinnon

Papers

Molecular Conductors from Neutral-Radical Charge-Transfer Salts: Preparation and Characterization of an Iodine-Doped Hexagonal Phase of 1,2,3,5-Dithiadiazolyl ([HCN2S2]∙)

Charge Transfer Salts of Benzene-Bridged 1,2,3,5-Dithiadiazolyl Diradicals. Preparation, Structures, and Transport Properties of 1,3- and 1,4-[(S2N2C)C6H4(CN2S2)][X] (X = I, Br)