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. Variable-temperature magnetic, conductivity, and near-infrared optical measurements indicate that the bulk materials behave as small band gap semiconductors with room-temperature conductivities sigma(RT) near 10(-6) S cm(-1) and thermal activation energies Ea of 0.32 eV (R1 = Me) and 0.36 eV (R1 = Et). LMTO band structure calculations on both compounds are consistent with this interpretation. The application of external pressure leads to dramatic increases in conductivity; at 4 GPa sigma(RT) reaches a value near 10(-1) S cm(-1) for R1 = Me and 10(-2) S/cm for R1 = Et. The conductivity remains activated for both compounds, but for R1 = Me the activation energy Ea is reduced to near 0.03 eV at 5 GPa, suggestive of a weakly metallic state.     

Resonance-stabilized 1,2,3-dithiazolo-1,2,3-dithiazolyls as neutral pi-radical conductors

Alkylation of the zwitterionic heterocycle 8-chloro-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridine (ClBP) with alkyl triflates affords 8-chloro-4-alkyl-4H-bis[1,2,3]dithiazolo[4,5-b:5',4'-e]pyridin-2-ium triflates [ClBPR][OTf] (R = Me, Et, Pr). Reduction of these salts with decamethylferrocene affords the corresponding ClBPR radicals as thermally stable crystalline solids. The radicals have been characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies are consistent with a low on-site Coulombic barrier U to charge transfer in the solid state. The crystal structures of ClBPR (R = Me, Et, Pr) have been determined by X-ray crystallography (at 293 K). All three structures consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S.S contacts. ClBPMe undergoes a phase transition at 93 K to a slightly modified slipped pi-stack arrangement, the structure of which has also been established crystallographically (at 25 K). Variable-temperature magnetic and conductivity measurements have been performed, and the results interpreted in light of extended Hückel band calculations. The room-temperature conductivities of ClBPR systems (sigma(RT) approximately 10(-)(5) to 10(-)(6) S cm(-)(1)), as well as the weak 1D ferromagnetism exhibited by ClBPMe, are interpreted in terms of weak intermolecular overlap along the pi-stacks. The latter is caused by slippage of the molecular plates, a feature necessitated by the steric size of the R and Cl groups on the pyridine ring.     

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. By contrast, variations in R2 (Cl, Br, Me) with R1 = Et induce very little change in either the degree of slippage or the intermolecular contacts. Variable-temperature conductivity (sigma) measurements show relatively constant values for the conductivity sigma(300 K) (10(-5)-10(-4) S cm(-1)) and thermal activation energy E(act) (0.27-0.31 eV). Variable-temperature magnetic susceptibility measurements indicate that radicals 2b and 2c (R1 = Pr, CH2CF3; R2 = Cl) behave as weakly antiferromagnetically coupled Curie-Weiss paramagnets, but in 2a, 2d and 2e (R1 = Et; R2 = Cl, Me, Br) ferromagnetic ordering is observed, with T(c) values of 12.8 (R2 = Cl), 13.6 (R2 = Me), and 14.1 K (R2 = Br). The origin of the dramatically different magnetic behavior across the series has been explored in terms of a direct through-space mechanism by means of DFT calculations on individual pairwise exchange energies. These indicate that antiferromagnetic exchange between radicals along the pi-stacks increases with pi-stack slippage.     

Isostructural bisdithiazolyl and bisthiaselenazolyl radicals: trends in bandwidth and conductivity

Reaction of N-alkylated pyridine-bridged bisdithiazolylium cations [1]+ (R1 =Me, Et; R2 =Ph) with selenium dioxide in acetic acid provides a one-step high-yield synthetic route to bisthiaselenazolylium cations [2]+ (R1 = Me, Et; R2 = Ph). The corresponding radicals 1 and 2 can be prepared by chemical or electrochemical reduction of the cations. Structural analysis of the radicals has been achieved by a combination of single-crystal and powder X-ray diffraction methods. While the two sulfur radicals 1 adopt different space groups (P3(1)21 for R1 = Me and P(-)1 for R1 = Et), the two selenium radicals 2 (space groups P3(1)21 for R1 = Me and P3(2)21 for R1 =Et) are isostructural with each other and also with 1 (R1 = Me, R2 = Ph). Variable-temperature magnetic measurements on all four compounds confirm that they are undimerized S = 1/2 systems, with varying degrees of weak intermolecular antiferromagnetic coupling. Variable-temperature electrical conductivity measurements on the two selenium radicals provide conductivities sigma(300 K) = 7.4 x 10-6 (R1 = Et) and 3.3 x 10-5 S cm-1 (R1 = Me), with activation energies, E(act), of 0.32 (R1 = Et) and 0.29 eV (R1 = Me). The differences in conductivity within the isostructural series is interpreted in terms of their relative solid-state bandwidths, as estimated from Extended Hückel band-structure calculations.     

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. However, despite these favorable electrochemical and structural properties, both materials exhibit low conductivities, with sigma(300K) < 10-7 S cm-1, a result which can be rationalized in terms of their EHT band electronic structures, which indicate that intermolecular interactions lateral to the pi-stacks are limited. The materials are thus very 1-D with low bandwidths, so that a Mott insulating state prevails. When R1 = Me, the intermolecular overlap along the pi-stacks is weak and the material is essentially paramagnetic. When R1 = Et, intermolecular pi-overlap is greater and variable-temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically coupled system, the behavior of which has been modeled in terms of a molecular-field modified 1-D Heisenberg chain of S = 1/2 centers. Broken-symmetry DFT methods have been used to estimate the magnitude of individual exchange interactions within both structures.     

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. At 295 K, the structure belongs to the space group Cmca and consists of evenly spaced radicals pi-stacked in an alternating ABABAB fashion along the x-direction. At 123 K, the space group symmetry is lowered by loss of C-centering to Pccn, so that the radicals are no longer evenly spaced along the pi-stack. At 88 K, a further lowering of space group symmetry to P21/c is observed. Extended Hückel Theory band structure calculations indicate a progressive opening of a band gap at the Fermi level in the low-temperature structures. Magnetic susceptibility measurements over the range 4-300 K reveal essentially diamagnetic behavior below 120 K. Variable-temperature single-crystal conductivity (sigma) measurements indicate that the conductivity is activated, even at room temperature, with a room-temperature value sigma RT=0.001 S cm-1 and a thermal activation energy Eact=0.19 eV. Under an applied pressure of 5 GPa, sigma RT is increased by 3 orders of magnitude, but the conductivity remains activated, with Eact being lowered to 0.11 eV at 5.5 GPa.     

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 (0.19 eV). Like 4a, radical 4b behaves as a bulk ferromagnet with an ordering temperature of T(C) = 17.5 K. The coercive field H(c) (at 2 K) of 1600 Oe for 4b is, however, significantly greater than that observed for 4a (1370 Oe). High pressure (0-15 GPa) structural studies on both compounds have shown that compression reduces the degree of slippage of the π-stacks, which gives rise to changes in the magnetic and conductive properties of the radicals. Relatively mild loadings (<2 GPa) cause an increase in T(C) for both compounds, that of 4b reaching a maximum value of 24 K; further compression to 5 GPa leads to a decrease in T(C) and loss of magnetization. Variable temperature and pressure conductivity measurements indicate a decrease in E(act) with increasing pressure, with eventual conversion of both compounds from a Mott insulating state to one displaying weakly metallic behavior in the region of 7 GPa (for 4a) and 9 GPa (for 4b).     

Syntheses, structures, physical properties, and theoretical studies of CeMxOS (M = Cu, Ag; x approximately 0.8) and CeAgOS

Black single crystals of the two nonstoichiometric cerium coinage-metal oxysulfide compounds CeCu(x)OS and CeAg(x)OS (x approximately 0.8) have been prepared by the reactions of Ce2S3 and CuO or Ag2O at 1223 or 1173 K, respectively. A black powder sample of CeAgOS has been prepared by the stoichiometric reaction of Ce2S3, CeO2, Ag2S, and Ag at 1073 K. These isostructural materials crystallize in the ZrSiCuAs structure type with two formula units in the tetragonal space group P4/nmm. Refined crystal structure results and chemical analyses provide evidence that the previously known anomalously small unit-cell volume of LnCuOS for Ln = Ce (Ln = rare-earth metal) is the result of Cu vacancies and the concomitant presence of both Ce3+ and Ce4+. Both CeCu(0.8)OS and CeAgOS are paramagnetic with mu(eff) values of 2.13(6) and 2.10(1) mu(B), respectively. CeCu(0.8)OS is a p-type semiconductor with a thermal activation energy Ea = 0.22 eV, sigma(electrical) = 9.8(1) 10(-3) S/cm at 298 K, and an optical band gap Eg < 0.73 eV. CeAgOS has conductivity sigma(conductivity) = 0.16(4) S/cm and an optical band gap Eg = 0.71 eV at 298 K. Theoretical calculations with an on-site Coulomb repulsion parameter indicate that the Ce 4f states are fully spin-polarized and are not localized in CeCuOS, CeCu(0.75)OS, or CeAgOS. Calculated band gaps for CeCu(0.75)OS and CeAgOS are 0.6 and 0.8 eV, respectively.     

Ultrafast dynamics of photochemical radical formation from

The excited-state dynamics and photochemistry of [Re(R)(CO)3(dmb)] (R=Me, Et); dmb=4,4'-dimethyl-2,2'-bipyridine) in CH2Cl2 have been studied by time-resolved visible absorption spectroscopy on a broad time scale ranging from approximately 400 fs to a few microseconds, with emphasis on the femtosecond and picosecond dynamics. It was found that the optically prepared Franck-Condon 1MLCT (singlet metal-to-ligand charge transfer) excited state of [Re(R)(CO)3(dmb)] undergoes femtosecond branching between two pathways (< or =400 fs for R=Me; approximately 800 fs for R=Et). For both methyl and ethyl complexes, evolution along one pathway leads to homolysis of the Re-R bond via a 3SBLCT (triplet sigma-bond-to-ligand charge transfer) excited state, from which [Re(S)(CO)3(dmb)]* and R* radicals are formed. The other pathway leads to an inherently unreactive 3MLCT state. For [Re(Me)(CO)3(dmb)], the 3MLCT state lies lowest in energy and decays exclusively to the ground state with a lifetime of approximately 35 ns, thereby acting as an excitation energy trap. The reactive 3SBLCT state is higher in energy. The quantum yield (0.4 at 293 K) of the radical formation is determined by the branching ratio between the two pathways. [Re(Et)(CO)3(dmb)] behaves differently: branching of the Franck-Condon state between two pathways still occurs, but the 3MLCT excited state lies above the dissociative 3SBLCT state and can decay into it. This shortens the 3MLCT lifetime to 213 ps in CH2Cl2 or 83 ps in CH3CN. Once populated, the 3SBLCT state evolves toward radical photoproducts [Re(S)(CO)3(dmb)]* and Et*. Thus, population of the 3MLCT excited state of [Re(Et)(CO)3(dmb)] provides a second, delayed pathway to homolysis. Hence, the quantum yield is unity. The photochemistry and excited-state dynamics of [Re(R)(CO)3(dmb)] (R=Me, Et) complexes are explained in terms of the relative ordering of the Franck-Condon, 3MLCT, and 3SBLCT states in the region of vertical excitation and along the Re-R reaction coordinate. A qualitative potential energy diagram is proposed.     

Straightforward route to the adamantane clusters [Sn4Q10]4- (Q = S, Se, Te) and use in the assembly of open-framework chalcogenides (Me4N)2M[Sn4Se10] (M = Mn(II), Fe(II), Co(II), Zn(II)) including the first telluride member (Me4N)2Mn[Ge4Te10

The reaction of K(2)Sn(2)Q(5) (Q = S, Se, Te) with stoichiometric amounts of alkyl-ammonium bromides R(4)NBr (R = methyl or ethyl) in ethylenediamine (en) afforded the corresponding salts (R(4)N)(4)[Sn(4)Q(10)] (Q = S, Se, Te) in high yield. Although the compound K(2)Sn(2)Te(5) is not known, this reaction is also applicable to solids with a nominal composition "K(2)Sn(2)Te(5)" which in the presence of R(4)NBr in en are quantitatively converted to the salts (R(4)N)(4)[Sn(4)Te(10)] on a multigram scale. These salts contain the molecular adamantane clusters [Sn(4)Q(10)](4-) and can serve as soluble precursors in simple metathesis reactions with transition metal salts to synthesize the large family of open-framework compounds (Me(4)N)(2)M[Sn(4)Se(10)] (M = Mn(2+), Fe(2+), Co(2+), Zn(2+)). Full structural characterization of these materials as well as their magnetic and optical properties is reported. Depending on the transition metal in (Me(4)N)(2)M[Sn(4)Se(10)], the energy band gaps of these compounds lie in the range of 1.27-2.23 eV. (Me(4)N)(2)Mn[Ge(4)Te(10)] is the first telluride analogue to be reported in this family. This material is a narrow band gap semiconductor with an optical absorption energy of 0.69 eV. Ab initio electronic band structure calculations validate the semiconductor nature of these chalcogenides and indicate a nearly direct band gap.     

Ion pair charge-transfer complexes between anionic and cationic metal-dithiolenes [M(II) = Pd,Pt

New [M(R(2)pipdt)(2)](BF(4))(2) salts [R(2)pipdt = N,N'-dialkyl-piperazine-2,3-dithione; M = Pd(II), R = Me and M = Pt(II), R = Me, Et, Pr(i)] bearing redox-active cationic dithiolene complexes have been prepared and characterized. These cations react with the redox-active [M(mnt)(2)](2-) [M = Pd(II), Pt(II); mnt = maleonitrile-2,3-dithiolate] anionic dithiolenes to form salts describable as ion pair charge-transfer complexes. X-ray crystallographic studies have shown that [M(Me(2)pipdt)(2)][M(mnt)(2)] complexes, with M = Pd(II) and Pt(II), are isomorphous. Crystal data of the Pt salt (3a): triclinic, Ponemacr; (No. 2); Z = 1; T = 293(2) K; a = 6.784(7) A, b = 8.460(6) A, c = 13.510(5) A, alpha = 100.63(2) degrees, beta = 104.04(2) degrees, gamma = 96.90(2) degrees; R1 = 0.0691 [wR2 = 0.2187 (all data)]. Structural data show that approximately square-planar [Pt(Me(2)pipdt)(2)] dications and regular square-planar [Pt(mnt)(2)] dianions form an infinite anion-cation one-dimensional stack along axis a with a Pt...Pt a/2 distance of 3.392 A and a Pt...Pt...Pt angle of 180 degrees. Anions and cations arrange themselves face-to-face so as to take on a staggered arrangement. These salts exhibit strong absorptions in the visible-near-infrared region assigned to ion pair charge-transfer transitions. A relation between the optical and thermal electron transfer in the solid state is obtained using a "Marcus-Hush model", and a solid-state electrical conductivity in agreement with expectations is observed. Vibrational spectroscopy is in agreement with the existence of charge-transfer interactions between the cationic and anionic components of the salts.     

Tuning anisotropy barriers in a family of tetrairon(III) single-molecule magnets with an S = 5 ground state

Tetrairon(III) Single-Molecule Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe4(L1)2(dpm)6] (1), [Fe4(L2)2(dpm)6] (2), [Fe4(L3)2(dpm)6].Et2O (3.Et2O), and [Fe4(OEt)3(L4)(dpm)6] (4) have been prepared by reaction of [Fe4(OMe)6(dpm)6] (5) with tripodal ligands R-C(CH2OH)3 (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, R = tBu; Hdpm = dipivaloylmethane). The iron(III) ions exhibit a centered-triangular topology and are linked by six alkoxo bridges, which propagate antiferromagnetic interactions resulting in an S = 5 ground spin state. Single crystals of 4 reproducibly contain at least two geometric isomers. From high-frequency EPR studies, the axial zero-field splitting parameter (D) is invariably negative, as found in 5 (D = -0.21 cm(-1)) and amounts to -0.445 cm(-1) in 1, -0.432 cm(-1) in 2, -0.42 cm(-1) in 3.Et2O, and -0.27 cm(-1) in 4 (dominant isomer). The anisotropy barrier Ueff determined by AC magnetic susceptibility measurements is Ueff/kB = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3.Et2O, 5.95 K in 4, and 3.5 K in 5. Both |D| and U(eff) are found to increase with increasing helical pitch of the Fe(O2Fe)3 core. The fourth-order longitudinal anisotropy parameter B4(0), which affects the shape of the anisotropy barrier, concomitantly changes from positive in 1 ("compressed parabola") to negative in 5 ("stretched parabola"). With the aid of spin Hamiltonian calculations the observed trends have been attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch.     

Multiple emissions and brilliant white luminescence from gold(I) O,O'-di(alkyl)dithiophosphate dimers

The Au(I) dimers Au(2)[S(2)P(OR)(2)](2) for R = Me, Et are found to exhibit a structure in which aurophilic interactions yield one-dimensional Au...Au chains with intermolecular contacts (3.09-3.16 A) similar to the Au...Au distances within the dimers (3.10-3.18 A). The dimers are luminescent in the solid state and become brilliantly emissive at low temperatures. At 77 K, Au(2)[S(2)P(OMe)(2)](2) shows multiple emission bands. The two higher energy bands at 415 and 456 nm are assigned to (1)MC and (3)MC on the basis of lifetime measurements (20 ns and 2.16 micros, respectively) and concentration-related effects, while the lower energy band at 560 nm is attributed to a LMCT excited state. In frozen glasses of different solvents, Au(2)[S(2)P(OMe)(2)](2) as well as the Et and n-Pr derivatives exhibit a bright luminescence of different colors and striking thermochromism of the emission.     

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. Variable temperature conductivity (sigma) measurements reveal an increase in conductivity with increasing selenium content, particularly so when selenium occupies the E2 position, with sigma(300 K) reaching a maximum (for E1 = E2 = Se) of 3.0 x 10(-4) S cm(-1). Thermal activation energies E(act) follow a similar profile, decreasing with increasing selenium content along the series 1a (0.43 eV), 3a (0.31 eV), 2a (0.27 eV), 4a (0.19 eV). Variable temperature magnetic susceptibility measurements indicate that all four radicals exhibit S = 1/2 Curie-Weiss behavior over the temperature range 20-300 K. At lower temperatures, the three selenium-based radicals display magnetic ordering. Radical 3a, with selenium positioned at the E1 site, undergoes a phase transition at 14 K to a weakly spin-canted (phi = 0.010 degrees) antiferromagnetic state. By contrast, radicals 2a and 4a, which both possess selenium in the E2 position, order ferromagnetically, with Curie temperatures of T(c) = 12.8 and 17.0 K, respectively. The coercive fields H(c) at 2 K of 2a (250 Oe) and 4a (1370 Oe) are much larger than those seen in conventional light atom organic ferromagnets. The transport properties of the entire series 1a-4a are discussed in the light of Extended Hückel Theory band structure calculations.     

The effect of the diphosphine basicity on the excited-state properties of trans-(N2)2W(R2PCH2CH2PR2)2: identification of near-degenerate, luminescent 3MLCT and 3LF terms

Absorption spectra (77 and 298 K), luminescence spectra (5-80 K), and luminescence lifetimes (5-80 K) for the title complexes have been correlated to increasing diphosphine basicity (R = 4-CF(3)-Ph < 4-H-Ph < 4-CH(3)O-Ph < Et). As a consequence, spectral peaks have been assigned to (1,3)MLCT (B(1u), W --> phosphorus) and (1,3)LF (B(2g)) terms. As the ligand basicity increases, the (3)MLCT bands observed in absorption blue-shift nearly 8000 cm(-1) and the vibrationally structured (3)LF bands observed in emission red-shift approximately 1300 cm(-1). (3)LF terms lie lowest in energy in the 4-H-Ph, 4-CH(3)O-Ph, and Et compounds, and temperature-dependent lifetime data suggest emission from each be assigned to the equilibrated, spin-orbit split levels of the (3)LF term. The (3)LF and (3)MLCT excited-state terms lie close in energy in the 4-CF(3)-Ph compound, resulting in an emission band shape that is temperature-dependent. At 77 K, the emission band is broad and structureless and is assigned to arise primarily from the (3)MLCT term. As the temperature is lowered toward 5 K, the (3)MLCT emission diminishes in intensity accompanied by the development of a vibrational structure that is characteristic of emission from the (3)LF term. These excited-state terms satisfy the requirements (different orbital origins, near-degeneracy) for separation by a Franck-Condon energy barrier, resulting in simultaneous emission from both terms between 5 and 77 K.     

Synthesis of 1,2,3,4-tetrazine 1,3-dioxides annulated with 1,2,3-triazoles and 1,2,3-triazole 1-oxides

1,2,3,4-Tetrazine 1,3-dioxides annulated with 1,2,3-triazoles and 1,2,3-triazole 1-oxides have been synthesized by the reaction of 4-amino-5-(tert-butyl-NNO-azoxy)-2-R-2H-1,2,3-triazoles (R=Me, i-Pr, t-Bu) and their 1-oxides (R=H, Me, Et, i-Pr) with the HNO₃/H₂SO₄/Ac₂O system. Their thermal stability, spectroscopic, and X-ray properties have been studied.     

A MOLECULE LIKE SODIUM

The use of neutral π-radicals as building blocks for molecular conductors holds both appeal and challenge. Such systems obviate the need for counterions, as charge transfer is not required to generate charge carriers. Essentially an array of π-radicals should function like atoms in an elemental metal, e.g., sodium, affording a half-filled energy band. Most radicals, however, tend to dimerize, and even when association can be suppressed the resulting low bandwidth W, coupled with a high on-site Coulomb repulsion U, leads to a Mott insulating state. We are pursuing the design and synthesis of stable heterocyclic thiazyl radicals, with a view to generating stable, crystalline materials with a high W/U ratio. The search for these new radicals, the molecular analogues of sodium, is the subject of this presentation.     

Inverse potassium hydride: a theoretical study

Results of an experimental study on the unusual "inverse" charge state (H(+)Na(-)) in salts where the H(+) ion is sequestered, combined with our earlier theoretical calculations on an unsequestered model compound (Me(3)N-H(+)...Na(-)), prompted us to further investigate such systems. In particular, we examined Et(3)N-H(+)...K(-) because considerations of the proton affinity of the amine and of the metal-hydride bond strength suggested that this ion-pair complex might be more stable to proton abstraction than was Me(3)N-H(+)...Na(-). In the present work, the ground-state potential energy surface of the Et(3)N-H(+)...K(-) ion pair was examined using second-order M?ller-Plesset perturbation theory and 6-311++G basis sets. We found Et(3)N-H(+)...K(-) to be metastable to dissociation with a barrier of 8 kcal mol(-1) (computed at the CCSD(T) level of theory). This barrier indeed is substantially larger than that found earlier for (Me(3)N-H(+)...Na(-)) and suggests that unsequestered inverse-charged H(+)M(-) ion-pair salts may offer a reasonable route to creating high-energy materials if a means for synthesizing them in the laboratory can be designed.     

A chiral molecular conductor: synthesis, structure, and physical properties of [ET]3[Sb2(L-tart)2].CH3CN (ET = bis(ethylendithio)tetrathiafulvalene; L-tart = (2R,3R)-(+)-tartrate)

The salt [ET](3)[Sb(2)(L-tart)(2)].CH(3)CN (1) has been obtained by electrocrystallization of the organic donor bis(ethylendithio)tetrathiafulvalene (ET or BEDT-TTF) in the presence of the chiral anionic complex [Sb(2)(L-tart)(2)](2-) (L-tart = (2R,3R)-(+)-tartrate). This salt crystallizes in the chiral space group P2(1)2(1)2(1) (a = 11.145(2) angstroms, b = 12.848(2) angstroms, c = 40.159(14) angstroms, V = 5750.4(14) angstroms(3), Z = 4) and is formed by alternating layers of the anions and of the organic radicals in a noncentrosymmetric alpha-type packing. This compound shows a room temperature electrical conductivity of approximately 1 S.cm(-1) and semiconducting behavior with an activation energy of approximately 85 meV. Analysis of the magnetic susceptibility and band structure, however, suggests that this compound should be a narrow band gap semiconductor.     

Magnetic susceptibility of alkali-tetracyanoquinodimethane salts and extended Hubbard models with bond order and charge density wave phases

The molar spin susceptibilities χ(T) of Na-tetracyanoquinodimethane (TCNQ), K-TCNQ, and Rb-TCNQ(II) are fit quantitatively to 450 K in terms of half-filled bands of three one-dimensional Hubbard models with extended interactions using exact results for finite systems. All three models have bond order wave (BOW) and charge density wave (CDW) phases with boundary V = V(c)(U) for nearest-neighbor interaction V and on-site repulsion U. At high T, all three salts have regular stacks of TCNQ(-) anion radicals. The χ(T) fits place Na and K in the CDW phase and Rb(II) in the BOW phase with V ≈ V(c). The Na and K salts have dimerized stacks at T < T(d) while Rb(II) has regular stacks at 100 K. The χ(T) analysis extends to dimerized stacks and to dimerization fluctuations in Rb(II). The three models yield consistent values of U, V, and transfer integrals t for closely related TCNQ(-) stacks. Model parameters based on χ(T) are smaller than those from optical data that in turn are considerably reduced by electronic polarization from quantum chemical calculation of U, V, and t of adjacent TCNQ(-) ions. The χ(T) analysis shows that fully relaxed states have reduced model parameters compared to optical or vibration spectra of dimerized or regular TCNQ(-) stacks.