Quinoxaline-1,2,3-dithiazolyls  Synthesis, EPR characterization, and redox chemistry

Cordes, A. W.; Mingie, James R.; Oakley, Richard T.; Reed, Robert W.; Zhang, Hongzhou

doi:10.1139/cjc-79-9-1352

Cited by 5 publications

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“…Insofar as U reaches a maximum for systems with a half-filled band ( f = 1 / 2 ), the need to control this parameter is critical. To this end, we have sought radicals with good ion energetics, that is, low gas-phase disproportionation enthalpies Δ H disp (= IP − EA) and correspondingly small solution cell potentials E cell , as these molecular observables serve as guides to the value of the more elusive U . − …”

Section: Introductionmentioning

confidence: 99%

“…Within the thiazyl radical family, the most actively studied systems, from a materials perspective, have been 1,2,3,5-dithiadiazolyls, 1 , and 1,3,2- and 1,2,3- dithiazolyls, 2 and 3 (Chart ). Attempts to suppress dimerization have met with some success, and in dithiazolyls, substituent effects can be used to modify redox properties and, hence, lower the on-site Coulomb repulsion, U . ,, Dramatic improvements in stability, structure, and redox properties can be achieved by use of resonance effects, as in the bisdithiazolyls 4 . 8b,− In the solid state, these latter radicals adopt undimerized π-stacked structures, but the loss in intermolecular overlap and hence bandwidth occasioned by slippage of the radicals along the π-stacks leads to Mott insulating ground states. Replacement of sulfur by selenium, however, leads to a marked increase in conductivity and decrease in activation energy …”

Section: Introductionmentioning

confidence: 99%

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Electronic and Magnetic Interactions in π-Stacked Bisthiadiazinyl Radicals

et al. 2007

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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.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Electronic and Magnetic Interactions in π-Stacked Bisthiadiazinyl Radicals

et al. 2007

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“…Although the a priori estimation of the magnitude of both W and U for these systems is not practical, trends in the gas-phase disproportionation enthalpy Δ H disp for a series of related radicals provide a working mirror to the trends in U . Gas-phase ionization potentials (IP) can be obtained experimentally, but reliable estimates of both IP and electron affinity (EA) values can be easily obtained by computation. , Alternatively, solution based cell potentials E cell , which are generally accessible by experiment, provide an effective mirror to solid-state characteristics . Using either criterion, the working prescription for good conductivity requires radicals with good ion energetics, i.e., low Δ H disp and E cell values.…”

Section: Introductionmentioning

confidence: 99%

Dithiazolodithiazolyl Radicals: Substituent Effects on Solid State Structures and Properties

et al. 2004

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A general synthetic route to the pyridine-bridged dithiazolodithiazolyl framework R 2 BPR 1 , involving N-alkylation of a 4-substituted 2,6-dichloropyridine, followed by amination and double Herz cyclization with S 2 Cl 2 , has been developed. The radicals R 2 BPMe (R 2 ) Me, Ph) have been prepared and characterized by EPR spectroscopy and cyclic voltammetry. Their crystal structures have been determined by X-ray crystallography. Both structures consist of undimerized slipped radical π-stacks. Lateral interactions in MeBPMe (space group P2 1 /c) generate chainlike arrays with radicals linked by inversion centers; there are no close interchain S-S contacts. By contrast, in PhBPMe (space group P3 1 21) the radical π-stacks are nested about 3 1 axes, so as to produce an extensive 2-dimensional network of intermolecular S-S interactions. Variable-temperature magnetic susceptibility measurements reveal that MeBPMe is essentially paramagnetic, whereas PhBPMe displays strong antiferromagnetic coupling. A value of J ) -149 cm -1 has been estimated by using a BonnerFisher anti-ferromagnetic chain model. Pressed pellet conductivity measurements indicate values of σ RT ≈ 10 -5 S cm -1 for both R 2 BPMe compounds (R 2 ) Me, Ph), suggesting Mott insulator ground states. The structural results and transport properties are discussed in the light of extended Hü ckel theory band structure calculations.

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“…We have therefore pursued the development of stable heterocyclic π-radicals in which U is minimized. To this end, we have sought materials with low gas-phase disproportionation enthalpies Δ H disp ( = IP − EA) and solution cell potentials E cell , as these serve as guides to trends in U . − In addition, we have tried to design radicals that do not dimerize in the solid state and yet exhibit a strong network of intermolecular interactions, so that sufficient electronic bandwidth W is generated to offset U .…”

Section: Introductionmentioning

confidence: 99%

Prototypal Dithiazolodithiazolyl Radicals: Synthesis, Structures, and Transport Properties

et al. 2003

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New synthetic routes to 1,2,3-dithiazolo-1,2,3-dithiazolylium salts, based on double Herz condensations of N-alkylated 2,6-diaminopyridinium salts with sulfur monochloride, have been developed. The two prototypal 1,2,3-dithiazolo-1,2,3-dithiazolyl radicals HBPMe and HBPEt have been prepared and characterized in solution by cyclic voltammetry and EPR spectroscopy. Measured electrochemical cell potentials and computed (B3LYP/6-31G) gas-phase disproportionation enthalpies favor a low on-site Coulombic repulsion energy U in the solid state. The crystal structures of HBPR (R = Me, Et) have been determined by X-ray crystallography (at 293 K). Both consist of slipped pi-stacks of undimerized radicals, with many close intermolecular S- - -S contacts. Magnetic, conductivity, and optical measurements have been performed and the results interpreted in light of extended Hückel band calculations. The crystalline materials are paramagnetic above 100 K, with room-temperature conductivities sigma(RT) of 10(-5)-10(-6) S cm(-1); the slightly greater conductivity of the R = Et compound can be associated with a more well developed band structure. We suggest a Mott-Hubbard insulator ground state for these materials, with an on-site Coulomb repulsion energy U of about 1.0 eV.

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Quinoxaline-1,2,3-dithiazolyls Synthesis, EPR characterization, and redox chemistry

Cited by 5 publications

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Electronic and Magnetic Interactions in π-Stacked Bisthiadiazinyl Radicals

Electronic and Magnetic Interactions in π-Stacked Bisthiadiazinyl Radicals

Dithiazolodithiazolyl Radicals: Substituent Effects on Solid State Structures and Properties

Prototypal Dithiazolodithiazolyl Radicals: Synthesis, Structures, and Transport Properties

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Quinoxaline-1,2,3-dithiazolyls  Synthesis, EPR characterization, and redox chemistry

Cited by 5 publications

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Electronic and Magnetic Interactions in π-Stacked Bisthiadiazinyl Radicals

Electronic and Magnetic Interactions in π-Stacked Bisthiadiazinyl Radicals

Dithiazolodithiazolyl Radicals: Substituent Effects on Solid State Structures and Properties

Prototypal Dithiazolodithiazolyl Radicals: Synthesis, Structures, and Transport Properties

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Quinoxaline-1,2,3-dithiazolyls Synthesis, EPR characterization, and redox chemistry