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We report the preparation, crystallization, and solid-state characterization of ethyl (3)- and butyl (4)-substituted spiro-biphenalenyl radicals. Both of these compounds are found to be conducting face-to-face pi-dimers in the solid state but with different room-temperature magnetic ground states. At room temperature, 4 exists as a diamagnetic pi-dimer (interplanar separation of approximately 3.1 A), whereas 3 is a paramagnetic pi-dimer (interplanar separation of approximately 3.3 A), and both compounds show phase transitions between the paramagnetic and diamagnetic forms. Electrical resistivity measurements of single crystals of 3 and 4 show that the transition from the high-temperature paramagnetic pi-dimer form to the low-temperature diamagnetic pi-dimer structure is accompanied by an increase in conductivity by about 2 orders of magnitude. This behavior is unprecedented and is very difficult to reconcile with the usual understanding of a Peierls dimerization, which inevitably leads to an insulating ground state. We tentatively assign the enhancement in the conductivity to a decrease in the on-site Coulombic correlation energy (U), as the dimers form a super-molecule with twice the amount of conjugation.

Oxidation of quinoxalineaminothiol with SCl2 or S2Cl2/Cl2 affords a series of compounds based on the quinoxaline-1,2,3-dithiazole framework QDTA. Under highly oxidizing conditions, the 1,2,3-dithiazolyl ring is opened to afford the acyclic dichlorosulfimino-sulfenyl chlorides Clx-QDTA-Cl3 (x = 0, 1, 2). Reduction of these "trichloro" compounds leads to ring closure. For x = 2, reduction using S2Cl2 affords the dithiazolylium chloride [Cl2-QDTA][Cl]. For all values of x, reduction with iodide ion (3 mol equiv) affords the corresponding dithiazolyl radical [Clx-QDTA]. The radicals can be isolated in good yield in crude form, but attempts to purify them by vacuum sublimation lead to thermal degradation. The radicals have nonetheless been fully characterized by EPR spectroscopy, and the assignments of the observed hyperfine coupling constants cross-matched with those obtained by computation at the B3LYP/6-31G** level. The structures of the trichloro compounds Clx-QDTA-Cl3 (x = 1, 2) have been confirmed by X-ray crystallography. Crystal data: Cl-QDTA-Cl3, monoclinic, space group C2/c, a = 30.561(5) Å, b = 4.9764(9) Å, c = 22.247(4) Å, β = 131.822(14)°, V = 2521.4(8) Å3, Z = 8, R(F) = 0.043, and Rw(F) [I [Formula: see text] σ (I)] = 0.049; Cl2-QDTA-Cl3, orthorhombic, space group Pnma, a = 18.627(12) Å, b = 6.848(4) Å, c = 10.926(7) Å, V = 1393.7(15) Å3, Z = 4, R(F) = 0.047, and Rw(F) [I [Formula: see text] 3σ(I)] = 0.060.Key words: thiazyl radicals, molecular conductors, EPR spectroscopy, quinoxaline, DFT calculations.

We report the preparation, crystallization, and solid-state characterization of a spiro-biphenalenyl radical. The crystal structure shows that the radical is monomeric in the solid state and withou...

Reductive coupling of 4,5-dichloro-1,2,3-dithiazolylium chloride yields trans-4,4‘-dichloro-1,1‘,2,2‘,3,3‘-tetrathiadiazafulvalene (DC-TAF), the first example of this heterofulvalene system. Ab initio molecular orbital (B3LYP/6-31G**) calculations on prototypal TAF confirm that the closed shell 1Ag state lies 22 kcal mol-1 below the 3Bu diradical triplet. Cyclic voltammetry on DC-TAF reveals two reversible oxidation waves at 0.80 and 1.25 V (in CH3CN, reference SCE). The ESR signal (g = 2.0117) of the radical cation [DC-TAF]+ (in SO2(l)) exhibits a five-line hyperfine coupling pattern with aN = 0.096 mT. DC-TAF forms a series of 1:1 radical ion salts [DC-TAF][X] by electrooxidation in the presence of tetrahedral counterions (X- = BF4-, ClO4-, FSO3-). The crystal structures of these salts are isomorphous, monoclinic space group P21/n, and consist of one-dimensional ladder-like arrays of [DC-TAF]+ radical cations bridged by S- - -S contacts ranging from 3.5 to 3.7 A. Variable-temperature conductivity and ma...

The synthesis and solid-state characterization of the heterocyclic π-radical 1,2,5-thiadiazolo[3,4-b]-1,2,3-dithiazolo[3,4-b]pyrazin-2-yl, 1,2,3-TDTA, is described. The ESR spectrum of 1,2,3-TDTA (in CH2Cl2, 293 K, g = 2.009) confirms a highly delocalized spin distribution, with observable hyperfine coupling to all five nitrogen atoms of the tricyclic molecule (aN = 0.514, 0.343, 0.109, 0.051, and 0.045 mT). While chemical and electrochemical oxidation (E1/2(ox) = 1.14 V vs SCE) of 1,2,3-TDTA requires relatively harsh conditions, reduction is extremely facile (E1/2(red) = 0.15 V vs SCE). More importantly both the observed cell potential Ecell and computed (MNDO) gas-phase enthalpy ΔHdisp for the disproportionation of this and other 1,2,3-dithiazolyls are significantly lower than those observed for their 1,3,2-isomers. Crystals of 1,2,3-TDTA are monoclinic P21/n, with a = 6.6749(16) A, b = 11.7178(14) A, c = 8.6148(14) A, β = 103.297(16)°, and Z = 4. The crystal structure consists of slipped stacks of head...

The presence of two disulfide groups in the tetrathiophenalenyl radical TTPLY leads to a highly delocalized spin distribution and the lowest cell potential ever observed for a monofunctional phenalenyl derivative. While the heteroatom substituents successfully block C-C bond formation, TTPLY nonetheless associates in the solid state to afford the hypervalent S-S-bonded dimer (TTPLY)2.

The heterocyclic π-donor benzo[2,1-c:3,4-c‘]bis(1,2,3-dithiazole) (BT) can be prepared in 60% yield by the condensation of 1,4-diaminobenzene-2,3-dithiol with sulfur monochloride. BT is also formed, in low yield, along with its 5-chloro and 5,6-dichloro derivatives, in the reaction of p-phenylenediamine with sulfur monochloride. Cyclic voltammetry on BT reveals two reversible oxidation waves at 0.61 and 1.10 V (in CH3CN, ref SCE). The ESR signal (g = 2.0175) of the radical cation BT+ [in SO2(l)] exhibits a 1:2:3:2:1 hyperfine structure with aN = 0.161 mT. BT forms radical ion salts by electro-oxidation in the presence of inorganic counterions. In addition to a 1:1 salt [BT][ClO4], the structure of which consists of a simple ionic arrangement of BT+ radical cations and ClO4- anions, BT also forms 3:2 salts, i.e., [BT]3[X]2, with X = ClO4- and FSO3-. The crystal structures of these 3:2 salts consist of triple-decker units of radical cation dimers, i.e., [BT2]2+, and neutral BT molecules. These units are pac...

Electroreduction of the 2,5-thiophene-bridged bis(1,2,3,5-diselenadiazolylium) salt [T-2,5-Se][SbF6]2 in acetonitrile, at a Pt wire and in the presence of iodine, affords a highly conductive ( sigma = 102 S cm-1 at 293 K) 1:1 charge-transfer (CT) salt [T-2,5-Se][I], the crystal structure of which has been determined by single-crystal X-ray diffraction. The crystals belong to the orthorhombic space group Fm2m, a = 3.544(2), b = 10.9808(16), c = 31.464(5) Å , V = 1224.5(7) Å 3. The structure consists of perfectly superimposed pi -stacks of molecular units bridged by columns of disordered iodines. This packing motif is similar to that of the related 1,3-benzene-bridged derivative [1,3-Se][I], but the lateral intermolecular Se···Se interactions linking adjacent pi -stacks are considerably shorter, indicative of a more isotropic electronic structure for [T-2,5-Se][I]. Magnetic susceptibility measurements on [T-2,5-Se][I] nonetheless indicate a phase transition to a diamagnetic state near 200 K, behaviour similar to that observed for [1,3-Se][I]. The electronic structures and transport properties of the two compounds are discussed in the light of extended Hückel band-structure calculations.Key words: diselenadiazolyl, diradical, charge-transfer salt, magnetic susceptibility, crystal structure.

The condensation of diaminobenzenedithiol with sulfur monochloride leads to the chloride salt of the radical cation of 3,6-dichlorobenzo(1,2-d:4,5-d')bis(1,2,3-dithiazole), dichloro-(BB-123-DTA)(Cl), which can be reduced to neutral dichloro-(BB-123-DTA) with triphenylantimony. A similar condensation with selenium tetrachloride leads, upon reduction, to the corresponding bis(1,2,3-thiaselenazole) dichloro-(BB-123-TSA). The crystal and molecular structures of both compounds have been determined by X-ray diffraction. Both compounds, which are formally antiaromatic 16﷿-systems, exhibit internal bond lengths consistent with a quinoid formulation. The radical cations of both rings have been characterized by ESR spectroscopy; for dichloro-(BB-123-DTA) + g ) 2.0114 and aN ) 0.201 mT, while for dichloro-(BB-123-TSA) + g ) 2.021 and aN ) 0.44 mT. Further oxidation of both rings affords the corresponding dications, both of which have been characterized crystallographically as their AlCl 4 - salts. The structural features of these compounds are consistent with those expected for dithiazolylium (or thiaselenazolylium) derivatives. The structure and redox chemistry of the benzo(1,2-d:4,5-d')bis(1,2,3-dithiazole) framework is discussed in the light of the results of ab initio calculations.

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

A series of four bisdithiazolyl radicals 1a-d (R(1) = Pr, Bu, Pn, Hx; R(2) = F) has been prepared and characterized by X-ray crystallography. The crystal structure of 1a (R(1) = Pr) belongs to the tetragonal space group P42(1)m and consists of slipped π-stack arrays of undimerized radicals packed about 4 centers running along the z-direction, an arrangement identical to that found for 1 (R(1) = Et; R(2) = F). With increasing chain length of the R(1) substituent, an isomorphous set 1b-d is generated. All three compounds crystallize in the P2(1)/c space group and consist of pairs of radical π-stacks locked together by strong intermolecular F···S' bridges to create spin ladder arrays. The slipped π-stack alignment of radicals produces close S···S' interactions which serve as the "rungs" of a spin ladder, and the long chain alkyl substituents (R(1)) serve as buffers which separate the ladders from each other laterally. Variable temperature magnetic susceptibility measurements indicate that 1a behaves as an antiferromagnetically coupled Curie-Weiss paramagnet, the behavior of which may be modeled as a weakly coupled AFM chain. Stronger antiferromagnetic coupling is observed in 1b-d, such that the Curie-Weiss fit is no longer applicable. Analysis of the full data range (T = 2-300 K) is consistent with the Johnston strong-leg spin ladder model. The origin of the magnetic behavior across the series has been explored with broken-symmetry Density Functional Theory (DFT) calculations of individual pairwise exchange energies. These confirm that strong antiferromagnetic interactions are present within the ladder "legs" and "rungs", with only very weak magnetic exchange between the ladders.

The preparation of the bis(1,2,3,5-dithiadiazolyl) radicals [S2N2C]−X−[CN2S2], where X is a heterocyclic bridging group (X = piperazine, thiophene, bithiophene) is described. Crystal structures of ...

Cosublimation of 2,5-furanbis(1,2,3,5-dithiadiazolyl) [F-2,5-S] and iodine affords 1:1 and 2:1 charge-transfer salts, [F-2,5-S][I] and [F-2,5-S]2[I]. Cosublimation of 2,6-pyridinebis(1,2,3,5-dithiadiazolyl) [P-2,6-S] and iodine affords uniquely the 2:1 charge-transfer salt [P-2,6-S]2[I]. Crystals of the latter belong to the space group P21/c, a = 3.434(3), b = 9.8914(9), c = 30.803(4) A, β = 91.52(3)°, V = 1045.9(9) A3, Z = 2. The structure consists of [P-2,6-S] molecules stacked along the x direction. Pairs of stacks are linked head-on through iodine atoms, which themselves form disordered stacks running parallel to x. The internal structural parameters of the two CN2S2 rings within each molecule suggest that one (linked to iodine) is formally uncharged, while the other is partially oxidized. This latter ring is heavily coordinated through S- - -N linkages to neighboring CN2S2 rings and pyridine. Magnetic susceptibility measurements on [P-2,6-S]2[I] indicate that it is diamagnetic below 220 K.

Electroreduction of 1,3- and 1,4-benzene-bridged bis(diselenadiazolium) salts [1,4-Se][SbF6]2 and [1,4-Se][SbF6]2 in acetonitrile, at a Pt wire and in the presence of iodine affords the 1:1 charge-transfer salts [1,4-Se][I] and [1,3-Se][I]. Crystals of [1,4-Se][I] belong to the monoclinic space group C2/m, with FW = 598.9, a = 10.586(2), b = 16.713(2), c = 3.5006(14) A, β = 104.26(2)°, V = 600.2(3) A3, Z = 2. Crystals of [1,3-Se][I] belong to the orthorhombic space group Ima2, with FW = 598.9, a = 28.489(7), b = 3.543(2), c = 12.283(2) A, V = 1239.8(8) A3, Z = 4. In the presence of an excess of iodine, electrocrystallization of [1,4-Se]2+ affords the mixed iodide/triiodide salt [1,4-Se][I][I3], space group C2/c, FW = 979.59, a = 12.862(3), b = 15.063(2), c = 9.028(3) A, β = 100.62(2)°, V = 1719.1(7) A3, Z = 4. The structures of the two 1:1 compounds consist of perfectly superimposed stacks of molecular units interspersed by columns of disordered iodines. Interstack contacts in both structures are limited,...

Condensation of oxamidrazone with sulfur dichloride in acetonitrile affords 4,4‘-bis(1,2,3,5-dithiadiazolium) dichloride in moderate yield. Reduction of this salt with triphenylantimony yields the diradical 4,4‘-bis(1,2,3,5-dithiadiazolyl) [S2N2C−CN2S2], which has been isolated and characterized in the solid state as its dimer [S2N2C−CN2S2]2. The diradical is disjoint, and ab initio molecular orbital methods confirm a very small energy gap (

The layered-sheet architecture of the crystal structure of the fluoro-substituted oxobenzene-bridged bisdithiazolyl radical FBBO affords a 2D π-electronic structure with a large calculated bandwidth. The material displays high electrical conductivity for a f = 1/2 system, with σ(300 K) = 2 × 10(-2) S cm(-1). While the conductivity is thermally activated at ambient pressure, with E(act) = 0.10 eV at 300 K, indicative of a Mott insulating state, E(act) is eliminated at 3 GPa, suggesting the formation of a metallic state. The onset of metallization is supported by infrared measurements, which show closure of the Mott-Hubbard gap above 3 GPa.

We discuss the effects of the spin-orbit interaction on heavy-atom organic magnets with specific reference to a series of isostructural sulfur- and selenium-based radical ferromagnets of tetragonal space group $P\overline{4}{2}_{1}m$. By using a perturbative approach, we show the spin-orbit effects lead to a pairwise anisotropic exchange interaction between neighboring radicals that provides an easy magnetic axis running parallel to the $c$-axis. Estimates of the magnitude of this magnetic anisotropy explain the significant increase in the coercive fields by virtue of selenium incorporation. Complementing this theoretical discussion are the results of ferromagnetic resonance studies, which provide an experimental verification of both the magnitude and symmetry of the spin-orbit terms. Taken as a whole, the results underscore the importance of heavy atoms and crystal symmetry in the design of molecular ferromagnets with large magnetic anisotropy and high ordering temperatures.

Reduction of 5-(methylthio)-1,4,2-dithiazolium (3, R=H, Me, Ph) salts with zinc powder yields the corresponding diazahexathioorthooxalate derivatives (RCNS 2 ) (MeS)CC(SMe) (S 2 NCR) (4, R=H, Me, Ph). Thermolysis of these materials in the presence of catalytic quantities of iodine affords the tetrathiadiazafulvalenes [RCNS 2 C=CS 2 NCR] (2, R=H, Me, Ph) in good yield. Separation of the cis and trans isomers can be achieved with relative ease for the R=Me and Ph derivatives, but co-crystallization occurs for the two isomers of the R=H compound. Tetrathiadiazafulvalenes are poorer electron donors than TTF; cyclic voltammetry nonetheless reveals two reversible oxidation processes. The ESR spectrum of the radical cation trans-2 +. (R=Ph) consists of a quintet with a N =0.008 mT, g=2.0071

For a given number of skeletal atoms, the π-electron count of an unsaturated heterocycle can be modified by the incorporation of heteroatoms from different periodic groups. Sulfur and nitrogen are particularly effective in this regard, as their high electronegativity can stabilize electron-rich π-systems. A variety of aromatic, anti-aromatic, and radical systems can be prepared from admixtures of these elements. While the properties of these ring systems are interesting in their own right, the compounds are not without relevance in the design of new materials. The strong interannular interactions that are developed between these rings in the solid state makes them ideal candidates for the fabrication of molecular conductors. The origins and development of this area of research are the subjects of this presentation.

The preparation and solid-state characterization of the 1,3,5-triazine-bridged tris(1,2,3,5-ditliiadiazolyl) complex 4,4',4''-(2,4,6-(1,3,5-triazinetriyl))tris(1,2,3,5-dithiadiazolyl) [N 3 C 3 (CN 2 S 2 ) 3 ] are described. The crystals belong to the monoclinic space group C2/c, with a=20.759(7) A, b=10.525(3) A, c=17.554(3) A, β=140.18(4) o , fw=390.5, Z=8, and V=2456.2(12) A 3 . The crystal structure consists of layers of interlocked dimers. Alternate layers are oriented in an antiparallel fashion, thereby precluding a stacked structure similar to that found in the related 1,3,5-benzene-based triradical. There are two dimerization environments

The preparation and crystal and molecular structure of the monofunctional 1,2,3,5-diselenadiazolyl radical [RCN2Se2] (R = 3,5-dicyanophenyl) are described. Crystals of [(CN)2C6H3CN2Se2] are triclinic, space group [Formula: see text] a = 7.197(2), b = 10.753(3), c = 13.345(5) Å, α = 88.00(3)°, β = 82.05(2)°, γ = 81.00(2)°, MW = 311.1, Z = 4. The structure consists of stacked arrays of radical dimers laced together by networks of short CN---Se contacts, but the steric bulk of the organic ligand leads to a pairwise aggregation of [CN2Se2] columns. These structural features are compared with those observed in related dimers, and the results interpreted in the light of extended Hückel band calculations on a series of model structures. The results provide a qualitative correlation between the size and dimensionality of the intermolecular Se---Se network and the degree of crystal orbital dispersion.

The bisdithiazolyl radical 1a is dimorphic, existing in two distinct molecular and crystal modifications. The α-phase crystallizes in the tetragonal space group P4̅2(1)m and consists of π-stacked radicals, tightly clustered about 4̅ points and running parallel to c. The β-phase belongs to the monoclinic space group P2(1)/c and, at ambient temperature and pressure, is composed of π-stacked dimers in which the radicals are linked laterally by hypervalent four-center six-electron S···S-S···S σ-bonds. Variable-temperature magnetic susceptibility χ measurements confirm that α-1a behaves as a Curie-Weiss paramagnet; the low-temperature variations in χ can be modeled in terms of a 1D Heisenberg chain of weakly coupled AFM S = (1)/(2) centers. The dimeric phase β-1a is essentially diamagnetic up to 380 K. Above this temperature there is a sharp hysteretic (T↑= 380 K, T↓ = 375 K) increase in χ and χT. Powder X-ray diffraction analysis of β-1a at 393 K has established that the phase transition corresponds to a dimer-to-radical conversion in which the hypervalent S···S-S···S σ-bond is cleaved. Variable-temperature and -pressure conductivity measurements indicate that α-1a behaves as a Mott insulator, but the ambient-temperature conductivity σ(RT) increases from near 10(-7) S cm(-1) at 0.5 GPa to near 10(-4) S cm(-1) at 5 GPa. The value of σ(RT) for β-1a (near 10(-4) S cm(-1) at 0.5 GPa) initially decreases with pressure as the phase change takes place, but beyond 1.5 GPa this trend reverses, and σ(RT) increases in a manner which parallels the behavior of α-1a. These changes in conductivity of β-1a are interpreted in terms of a pressure-induced dimer-to-radical phase change. High-pressure, ambient-temperature powder diffraction analysis of β-1a confirms such a transition between 0.65 and 0.98 GPa and establishes that the structural change involves rupture of the dimer in a manner akin to that observed at high temperature and ambient pressure. The response of the S···S-S···S σ-bond in β-1a to heat and pressure is compared to that of related dimers possessing S···Se-Se···S σ-bonds.

The 1,4-phenylene-bridged bis(1,3,2,4,6-thiaphosphatriazine) and bis(1,3,2,4,6-selenaphosphatriazine) compounds (ClEN 3 PPh 2 C)C 6 H 4 (CPPh 2 N 3 ECl) (E=S, Se) have been prepared by the reactions of 1,4-phenylenebis(phosphimidoylamidine), (Me 3 Si) 2 NPPh 2 (Me 3 SiN)CC 6 H 4 C(NSiMe 3 )PPh 2 N(SiMe 3 ) 2 , with SCl 2 and SeC 4

The title molecule, 3,7-bis(2-thienyl)-1,5,2,4,6,8-dithiatetrazocine, C 10 H 6 N 4 S 4 , is planar within 0.069 (5) A and crystallizes in the monoclinic space group P2 1 /c. There are two centrosymmetric molecules in the cell; these are stacked such that each eight-membered C 2 N 4 S 2 ring has a thienyl ring above and below it. The mean interplanar separation is 3.55 (2) A.

The methoxy-substituted bis-1,2,3-dithiazolyl radical 4a thermally decomposes to afford the zwitterionic bis-1,2,3-dithiazolopyridone 5a; EPR spectroscopic analysis indicates a rearrangement of the radical prior to loss of the methyl group.

Crystals of (Cl2C3NS)(ClC2NS2), an isothiazolyl-substituted 1,2,3-dithiazolyl radical, consist of evenly spaced, slipped π-stacks; magnetic and conductivity measurements indicate the material is a Mott insulator with σRT = 2 × 10−7 S cm−1.

The preparation and solid-state characterization of the dimers of the cyanophenyl-substituted 1,2,3,5-dithia- and 1,2,3,5-diselenadiazolyl radicals [NCC6H4CN2E2]· (E = S, Se) are described. Crystals of the 4-cyanophenyl derivatives 4 are triclinic, space group P¯1. The radicals dimerize in a cofacial fashion, with mean intradimer E- - -E spacings of 3.104/3.233 Å (E = S/Se). The dimer units form ribbonlike arrays connected by head-to-tail CN- - -E contacts. The ribbons are packed in an antiparallel fashion with no close interdimer E- - -E contacts. The 3-cyanophenyl sulfur derivative 5 crystallizes in two phases. The α-phase, along with the isomorphous selenium compound, belongs to the monoclinic space group P21/n. As in the 4-cyano derivatives, the radicals dimerize cofacially, generating ribbons connected by head-to-tail CN- - -E contacts. The ribbons, however, are layered with CN2E2 rings oriented into stacks parallel to the a axis. In addition to short intradimer E- - -E contacts (mean values = 3.13 (2)/3.27 (4) Å for E = S/Se), there are relatively short interdimer E- - -E contacts (mean values = 4.26 (5)/4.15 (5) Å (E = S/Se) along the stacking axis. The β-phase of the 3-cyanophenyl sulfur compound 5, monoclinic space group P21/n, forms discrete dimers associated in a trans antarafacial fashion, with intradimer S- - -S contacts of 3.121 (1) Å; there are no close interdimer S- - -S contacts. The 2-cyanophenyl selenium derivative 6 crystallizes in the polar space group P21. In this structure also the dimer units are laced together by short CN- - -Se contacts. The dimers form stacks parallel to the b axis, with mean intradimer and interdimer Se- - -Se contacts of 3.312/4.083 Å, respectively. Lattice constants for the six structures are: 4 (E = S), C8H4N3S2, a = 10.466 (3) Å, b = 10.342 (2) Å, c = 9.169 (2) Å, α = 104.97 (2)°, β = 112.97 (2)°, γ = 69.84 (2)°, V = 849.0 (3) Å3, Z = 4; 4 (E = Se), C8H4N3Se2, a = 10.478 (4) Å, b = 10.780 (4) Å, c = 9.432 (4) Å, α = 107.16 (3)°, β = 115.23 (2)°, γ = 68.39 (3)°, V = 883.8 (6) Å3, Z = 4; 5 (E = S, α-phase), C8H4N3S2, a = 7.295 (3) Å, b = 20.488 (2) Å, c = 11.276 (2) Å, β = 95.54 (2)°, V = 1676.0 (7) Å3, Z = 8; 5 (E = Se), C8H4N3Se2, a = 7.364 (8) Å, b = 21.085 (2) Å, c = 11.119 (4) Å, β = 94.91 (5)°, V = 1720 (7) Å3, Z = 8; 5 (E = S, β-phase), C8H4N3S2, a = 8.782 (1) Å, b = 5.638 (1) Å, c = 17.128 (4) Å, β = 102.94 (2)°, V = 826.5 (6) Å3, Z = 4; 6 (E = Se), C8H4N3Se2, a = 7.301 (1) Å, b = 11.883 (2) Å, c = 10.186 (1) Å, β = 101.50 (1)°, V = 866.1 (5) Å3, Z = 4.

The preparation and solid state characterization of the bifunctional radical furan-2,5-bis(1,2,3,5-dithiadiazolyl) 2,5-[(S2N2C)OC4H2(CN2S2)] and the related monofunctional radical 2-cyanofuran-5-(1,2,3,5-dithiadiazolyl) 2,5-[(S2N2C)OC4H2(CN)] are described. The crystal structure of 2,5-[(S2N2C)OC4H2(CN2S2)] is orthorhombic, space group Pna21, and consists of interleaved arrays of dimers, for which the mean interannular [Formula: see text] contact is 3.137 Å. The crystal structure of the monofunctional radical 2,5-[(S2N2C)OC4H2(CN)] is monoclinic, space group P21/n, and consists of a ribbon-like network of dimers (mean interannular [Formula: see text] interconnected by close head-to-tail [Formula: see text] contacts. The dimer units form stacks parallel to z, with a mean interdimer [Formula: see text] separation of 3.956 Å. The similarities and differences between these two crystal structures and those of related benzene-substituted systems are discussed. Keywords: dithiadiazolyl radicals, furan-based diradicals, cyanofuran-based radicals, radical dimers, crystal structures.

The solid-state characterization of a second or β-phase of the 1,3-phenylene-bridged diselenadiazolyl diradical 1,3-[(Se2N2C)C6H4(CN2Se2)] is reported. Crystals of this β-phase are monoclinic, space group P21/n, with a = 9.108 (6), b = 15.233 (13), c = 16.110 (5) Å, β = 103.37 (5)°, Z = 8. The crystal structure consists of chain-like arrays of discrete dimers (4 per unit cell), although one intradimer Se-Se linkage is notably longer (3.411 Å) than the other three (3.125, 3.196, 3.204 Å). The dimer units lie in chains that are linked by a complex three-dimensional array of Se-Se contacts. Variable-temperature single-crystal conductivity measurements on this phase indicate a band gap of 0.77 eV. Consistent with the conductivity measurements, extended Hückel band structure calculations suggest a relatively isotropic electronic structure.

The 1,4-phenylene-bridged bis(1,3,2,4,6-thiaphosphatriazine) and bis(1,3,2,4,6-selenaphosphatriazine) compounds (ClEN 3 PPh 2 C)C 6 H 4 (CPPh 2 N 3 ECl) (E=S, Se) have been prepared by the reactions of 1,4-phenylenebis(phosphimidoylamidine), (Me 3 Si) 2 NPPh 2 (Me 3 SiN)CC 6 H 4 C(NSiMe 3 )PPh 2 N(SiMe 3 ) 2 , with SCl 2 and SeC 4

At temperatures below 5 K the field dependence of the magnetization of the pi-stacked bis-dithiazolyl radical 1 (R(1) = Me, R(2) = H) is consistent with metamagnetic behavior.

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. There are marked differences in the transport properties of the two groups. Variable temperature conductivity measurements reveal high, but activated, conductivity for the monoclinic pair (2a/4a), with sigma(298 K)10(-3) S cm(-1). The application of physical pressure increases the conductivity of both compounds, with sigma(298 K) at 5 GPa reaching 0.5 S cm(-1) for 2a and 2 S cm(-1) for 4a. Variable-temperature magnetic susceptibility measurements indicate strong antiferromagnetic (AFM) coupling for the monoclinic pair 2a and 4a, the behavior of which has been modeled in terms of a molecular-field modified 1D Heisenberg chain of AFM coupled S = 1/2 centers. Extended Huckel theory band structure calculations and density functional theory first principles methods have been used to develop a qualitative understanding of the conductive and magnetic properties of radicals of the type 1-4 as a function of the degree and direction of slippage of the radical pi-stacks.

HeI photoelectron spectroscopy, inner-shell electron energy loss spectroscopy involving the S2p, S2s, C1s and N1s edges, and S1s synchrotron radiation photoabsorption spectroscopy have been used to probe the occupied and unoccupied valence levels of 2,1,3-benzothiadiazole, 1,3,2,4-benzodithiadiazine, and 1,3,5,2,4-benzotrithiadiazepine. The term values obtained from the S1s, S2s and N1s oscillator strength spectra are sensitive to the aromatic/anti-aromatic character in the thiazyl ring, whereas the first ionization potentials for the latter two molecules remain relatively constant. The combination of valence photoelectron and core excitation results, aided by semi-empirical (MNDO) molecular orbital calculations, provides a useful probe of the frontier molecular orbitals of these aromatic/anti-aromatic molecules.

The proton spin-lattice relaxation rate of the organic bifunctional dithiazolyl biradical 1,3-[(${\mathrm{S}}_{2}$${\mathrm{N}}_{2}$C)${\mathrm{C}}_{6}$${\mathrm{H}}_{4}$(${\mathrm{CN}}_{2}$${\mathrm{S}}_{2}$)] was measured at two NMR frequencies, 15.6 and 31.0 MHz. Data in the temperature range 350\char21{}480 K exhibit an enhancement in the rate consistent with magnetic-susceptibility results. The relaxation rate has a frequency-dependent part attributed to slow molecular motions, and a frequency-independent and strongly temperature-dependent spin motion contribution associated with free radicals. In addition to the increase in spin number, our results show an increase in the rate of spin fluctuation with temperature. A quantification of the motional rate extracted from these data does not support the speculation that these spins are the carriers that cause the increase in the electrical conductivity observed in the same temperature range.