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1.
J Am Chem Soc ; 141(45): 17989-17994, 2019 Nov 13.
Artigo em Inglês | MEDLINE | ID: mdl-31661269

RESUMO

The nitroxyl radical 1-methyl-2-azaadamantane N-oxyl (Me-AZADO) exhibits magnetic bistability arising from a radical/dimer interconversion. The transition from the rotationally disordered paramagnetic plastic crystal, Me-AZADO, to the ordered diamagnetic crystalline phase, (Me-AZADO)2, has been conclusively demonstrated by crystal structure determination from high-resolution powder diffraction data and by solid-state NMR spectroscopy. The phase change is characterized by a wide thermal hysteresis with high sensitivity to even small applied pressures. The molecular dynamics of the phase transition from the plastic crystal to the conventional crystalline phase has been tracked by solid-state (1H and 13C) NMR and EPR spectroscopies.

2.
Inorg Chem ; 58(9): 6495-6506, 2019 May 06.
Artigo em Inglês | MEDLINE | ID: mdl-31021620

RESUMO

The exceptional performance of oxobenzene-bridged bis-1,2,3-dithiazolyls 6 as single-component neutral radical conductors arises from the presence of a low-lying π-lowest unoccupied molecular orbital, which reduces the potential barrier to charge transport and increases the kinetic stabilization energy of the metallic state. As part of ongoing efforts to modify the solid-state structures and transport properties of these so-called multiorbital materials, we report the preparation and characterization of the acetoxy, methoxy, and thiomethyl derivatives 6 (R = OAc, OMe, SMe). The crystal structures are based on ribbonlike arrays of radicals laced together by S···N' and S···O' secondary bonding interactions. The steric and electronic effects of the exocyclic ligands varies, affording one-dimensional (1D) π-stacked radicals for R = OAc, 1D cofacial dimer π-stacks for R = SMe, and a pseudo two-dimensional (2D) brick-wall arrangement for R = OMe. Variable-temperature magnetic and conductivity measurements reveal strong antiferromagnetic interactions and Mott insulating behavior for the two radical-based structures (R = OAc, OMe), with lower room-temperature conductivities (σRT ≈ 1 × 10-4 and ∼1 × 10-3 S cm-1, respectively) and higher thermal activation energies ( Eact = 0.24 and 0.21 eV, respectively) than found for the ideal 2D brick-wall structure of 6 (R = F), where σRT ≈ 1 × 10-2 S cm-1 and Eact = 0.10 eV. The performance of R = OMe, OAc relative to that of R = F, is consistent with the results of density functional theory band electronic structure calculations, which indicate a lower kinetic stabilization energy of the putative metallic state arising from their reduced electronic dimensionality.

3.
Inorg Chem ; 58(5): 3550-3557, 2019 Mar 04.
Artigo em Inglês | MEDLINE | ID: mdl-30785745

RESUMO

The isostructural dimers of the 1,4-phenylene-bridged bis-1,2,3,5-dithia- and bis-1,2,3,5-diselenadiazolyl diradicals 1,4-S/Se are small band gap semiconductors. The response of their molecular and solid state electronic structures to pressure has been explored over the range 0-10 GPa. The crystal structures, which consist of cofacially aligned (pancake) π-dimers packed into herringbone arrays, experience a continuous, near-isotropic compression. While the intramolecular covalent E-E (E = S/Se) bonds remain relatively unchanged with pressurization, the intradimer E···E separations are significantly shortened. Molecular and band electronic structure calculations using density functional theory methods indicate that compression of the π-dimers leads to a widening of the gap Δ E between the highest occupied and lowest unoccupied molecular orbitals of the dimer, an effect that offsets the expected decrease in the valence-to-conduction band gap Eg occasioned by pressure-induced spreading of the valence and conduction bands. Consistent with the predicted consequences of this competition between intra- and interdimer interactions, variable temperature high pressure conductivity measurements reveal at best an order-of-magnitude increase in conductivity with pressure for the two compounds over the pressure range 0-10 GPa. While a small reduction in the thermal activation energy Eact with increasing pressure is observed, extrapolation of the rate of decrease suggests a projected onset of metallization ( Eact ≈ 0) in excess of 20 GPa.

4.
Inorg Chem ; 58(1): 419-427, 2019 Jan 07.
Artigo em Inglês | MEDLINE | ID: mdl-30570252

RESUMO

The N-methyl-4-phenyl-pyridine-bridged bisdithiazolyl radical PhBPMe is polymorphic, crystallizing from cold acetonitrile in a trigonal α-phase, space group P3121, and from hot dichloroethane in an orthorhombic ß-phase, space group Pca21. The crystal structures of both phases consist of slipped π-stacks of undimerized radicals aligned laterally into herringbone arrays. In the ß-phase, there are two independent radicals in the asymmetric unit, and the resulting π-stacks form corrugated layers interspersed by methyl and phenyl groups which block the approach of neighboring radicals. In the α-phase, the methyl/phenyl groups and the radical π-stacks separately form spirals about 31 axes, the latter giving rise to a 3D network of close radical/radical contacts. Variable temperature magnetic susceptibility measurements on the ß-phase indicate strong antiferromagnetic coupling. Weaker but predominantly antiferromagnetic interactions (θ = -20.7 K) are observed in the α-phase. A high temperature series expansion analysis of the magnetic data for the α-phase affords antiferromagnetic exchange energies for the one- and two-step radical/radical interactions about the 31 spirals ( J1 = -1.2 K, J2 = -10.9 K, respectively), with weak ferromagnetic interactions along the π-stacks ( Jπ = +1.8 K). Despite the presence of a 3D network based on the dominant J2 interactions, which affords two independent bipartite sublattices, no evidence of bulk antiferromagnetic order has been observed above T = 2 K. The magnetic results are discussed in light of exchange energies calculated using density functional theory broken symmetry methods.

5.
Inorg Chem ; 57(21): 13901-13911, 2018 Nov 05.
Artigo em Inglês | MEDLINE | ID: mdl-30351085

RESUMO

Condensation of persilylated nicotinimideamide and isonicotinimideamide with sulfur monochloride affords double salts of the 3-, 4-pyridyl-substituted 1,2,3,5-dithiadiazolylium DTDA cations of the general formula [3-, 4-pyDTDA][Cl][HCl] in which the pyridyl nitrogen serves as a noninnocent base. Reduction of these salts with triphenylantimony followed by deprotonation of the intermediate-protonated radical affords the free base radicals [3-, 4-pyDTDA], the crystal structures of which, along with those of their diselenadiazolyl analogues [3-, 4-pyDSDA], have been characterized by powder or single-crystal X-ray diffraction. The crystal structures consist of "pancake" π-dimers linked head-to-tail into ribbonlike arrays by η2-S2---N(py) intermolecular secondary bonding interactions. Methylation of the persilylated (iso)nicotinimide-amides prior to condensation with sulfur monochloride leads to N-methylated double chloride salts Me[3-, 4-pyDTDA][Cl]2, which can be converted by metathesis into the corresponding triflates Me[3-, 4-pyDTDA][OTf]2 and then reduced to the N-methylated radical triflates Me[3-, 4-pyDTDA][OTf]. The crystal structures of both the N-methylated double triflate and radical triflate salts have been determined by single-crystal X-ray diffraction. The latter consist of trans-cofacial π-dimers strongly ion-paired with triflate anions. Variable temperature magnetic susceptibility measurements on both the neutral and radical ion dimers indicate that they are diamagnetic over the temperature range 2-300 K.

6.
Inorg Chem ; 57(8): 4757-4770, 2018 Apr 16.
Artigo em Inglês | MEDLINE | ID: mdl-29620356

RESUMO

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 (λmax = 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 P21/ 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 Cmc21). Structural differences between the S- and Se-based materials are interpreted in terms of local intermolecular S/Se···N'/O' secondary bonding interactions, the strength of which varies with the nature of the chalcogen (S vs Se). While the perfectly two-dimensional "brick-wall" packing pattern associated with the Cmc21 phase of QT is not found for QS, all three phases of QS are nonetheless small band gap semiconductors, with σRT ranging from 10-5 S cm-1 for the P21/ c phase to 10-3 S cm-1 for the R3 c phase. The bandwidths of the valence and conduction bands increase with applied pressure, leading to an increase in conductivity and a decrease in thermal activation energy Eact. For the R3 c phase, band gap closure to yield an organic molecular metal with a σRT of ∼102 S cm-1 occurs at 6 GPa. Band gaps estimated from density functional theory band structure calculations on the ambient- and high-pressure crystal structures of QT and QS correlate well with those obtained experimentally.

7.
J Am Chem Soc ; 140(11): 3846-3849, 2018 03 21.
Artigo em Inglês | MEDLINE | ID: mdl-29513996

RESUMO

Crystals of the heterocyclic radical naphtho-1,3,2-dithiazolyl NDTA display magnetic bistability with a well-defined hysteretic phase transition at Tc↓ = 128(2) K and Tc↑ = 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 wide window of bistability (60 K) may be attributed to the large structural changes that accompany the phase transition.

8.
J Am Chem Soc ; 139(6): 2180-2183, 2017 02 15.
Artigo em Inglês | MEDLINE | ID: mdl-28140576

RESUMO

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 σRT = 1 × 10-3 S cm-1 and Eact = 0.14 eV for transport within the brick-wall arrays. Closure of the band gap to form an all-organic molecular metal with σRT > 101 S cm-1 can be achieved by the application of pressure to 8 GPa.

9.
J Am Chem Soc ; 139(4): 1625-1635, 2017 02 01.
Artigo em Inglês | MEDLINE | ID: mdl-28117984

RESUMO

A critical feature of the electronic structure of oxobenzene-bridged bisdithiazolyl radicals 2 is the presence of a low-lying LUMO which, in the solid state, improves charge transport by providing additional degrees of freedom for electron transfer. The magnitude of this multiorbital effect can be fine-tuned by variations in the π-electron releasing/accepting nature of the basal ligand. Here we demonstrate that incorporation of a nitro group significantly stabilizes the LUMO, and hence lowers Ueff, the effective Coulombic barrier to charge transfer. The effect is echoed, at the molecular level, in the observed trend in Ecell, the electrochemical cell potential for 2 with R = F, H and NO2. The crystal structures of the MeCN and EtCN solvates of 2 with R = NO2 have been determined. In the EtCN solvate the radicals are dimerized, but in the MeCN solvate the radicals form superimposed and evenly spaced π-stacked arrays. This highly 1D material displays Pauli-like temperature independent paramagnetic behavior, with χTIP = 6 × 10-4 emu mol-1, but its charge transport behavior, with σRT near 0.04 S cm-1 and Eact = 0.05 eV, is more consistent with a Mott insulating ground state. High pressure crystallographic measurements confirm uniform compression of the π-stacked architecture with no phase change apparent up to 8 GPa. High pressure conductivity measurements indicate that the charge gap between the Mott insulator and metallic states can be closed near 6 GPa. These results are discussed in the light of DFT band structure calculations.

10.
Chem Commun (Camb) ; 52(96): 13877-13880, 2016 Nov 24.
Artigo em Inglês | MEDLINE | ID: mdl-27841385

RESUMO

In the solid state the iodo-substituted bisdiselenazolyl radical 1c orders as a ferromagnet with TC = 10.5 K. With the application of pressure TC rises rapidly, reaching a value of 27.5 K at 2.4 GPa. The accompanying structural and magnetic changes have been examined by high resolution powder X-ray diffraction and by DFT calculations of magnetic exchange interactions.

11.
J Am Chem Soc ; 138(34): 10738-41, 2016 08 31.
Artigo em Inglês | MEDLINE | ID: mdl-27537064

RESUMO

Electro-oxidation of the quinoidal bisdithiazole BT in dichloroethane in the presence of [Bu4N][GaBr4] affords the 1:1 radical ion salt [BT][GaBr4], crystals of which belong to the trigonal space group P3. The packing pattern of the radical cations provides a rare example of an organic kagome basket structure, with S = 1/2 radical ion chains located at the triangular corners of a trihexagonal lattice. Magnetic measurements over a wide temperature range from 30 mK to 300 K suggest strongly frustrated AFM interactions on the scale of J/kb ∼ 30 K, but reveal no anomalies that would be associated with magnetic order. These observations are discussed in terms of the symmetry allowed magnetic interactions within and between the frustrated layers.

12.
J Am Chem Soc ; 137(44): 14136-48, 2015 Nov 11.
Artigo em Inglês | MEDLINE | ID: mdl-26513125

RESUMO

Pressure-induced changes in the solid-state structures and transport properties of three oxobenzene-bridged bisdithiazolyl radicals 2 (R = H, F, Ph) over the range 0-15 GPa are described. All three materials experience compression of their π-stacked architecture, be it (i) 1D ABABAB π-stack (R = Ph), (ii) quasi-1D slipped π-stack (R = H), or (iii) 2D brick-wall π-stack (R = F). While R = H undergoes two structural phase transitions, neither of R = F, Ph display any phase change. All three radicals order as spin-canted antiferromagnets, but spin-canted ordering is lost at pressures <1.5 GPa. At room temperature, their electrical conductivity increases rapidly with pressure, and the thermal activation energy for conduction Eact is eliminated at pressures ranging from ∼3 GPa for R = F to ∼12 GPa for R = Ph, heralding formation of a highly correlated (or bad) metallic state. For R = F, H the pressure-induced Mott insulator to metal conversion has been tracked by measurements of optical conductivity at ambient temperature and electrical resistivity at low temperature. For R = F compression to 6.2 GPa leads to a quasiquadratic temperature dependence of the resistivity over the range 5-300 K, consistent with formation of a 2D Fermi liquid state. DFT band structure calculations suggest that the ease of metallization of these radicals can be ascribed to their multiorbital character. Mixing and overlap of SOMO- and LUMO-based bands affords an increased kinetic energy stabilization of the metallic state relative to a single SOMO-based band system.

13.
J Am Chem Soc ; 137(11): 3720-30, 2015 Mar 25.
Artigo em Inglês | MEDLINE | ID: mdl-25767934

RESUMO

Recent developments in stable radical chemistry have afforded "heavy atom" radicals, neutral open-shell (S = 1/2) molecular species containing heavy p-block elements (S, Se), which display solid-state magnetic properties once considered exclusive to conventional metal-based magnets. These highly spin-delocalized radicals do not associate in the solid state and yet display extensive networks of close intermolecular interactions. Spin density on the heavy atoms allows for increased isotropic and spin-orbit mediated anisotropic exchange effects. Structural variations induced by chemical modification and physical pressure, coupled with ab-initio methods to estimate exchange energies, have facilitated the development of predictive structure/property relationships. These results, coupled with detailed theoretical analyses and magnetic resonance spectroscopic measurements, have provided insight into the magnetic structure of ferromagnetic and spin-canted antiferromagnetic ordered materials as well as an understanding of the importance of spin-orbit coupling contributions to magnetic hysteresis and anisotropy. Isotropic and anisotropic ferromagnetic exchange can also be enhanced indirectly by the incorporation of heavy atoms into nonspin-bearing sites, where they can contribute to multi-orbital spin-orbit coupling.

14.
J Am Chem Soc ; 137(3): 1044-7, 2015 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-25588146

RESUMO

The alternating ABABAB π-stacked architecture of the EtCN solvate of the iodo-substituted, oxobenzene-bridged bisdithiazolyl radical IBBO (space group Pnma) gives rise to strong ferromagnetic exchange along the π-stacks, and the material orders as a spin-canted antiferromagnet with T(N) = 35 K, with a spontaneous (canted) moment M(spont) = 1.4 × 10(-3) µB and a coercive field H(c) = 1060 Oe (at 2 K). The observation of spin-canting can only be understood in terms of multiorbital contributions to both isotropic and anisotropic exchange interactions, the magnitude of which are enhanced by spin-orbit effects arising from the heavy-atom iodine substituent. Pseudodipolar interactions lead to a net canted moment along the c-axis, while the sublattice magnetization is predicted to possess an easy a-axis.

15.
J Am Chem Soc ; 136(22): 8050-62, 2014 Jun 04.
Artigo em Inglês | MEDLINE | ID: mdl-24851953

RESUMO

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 ß-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. Variable-temperature magnetic susceptibility measurements on α-1b indicate Curie-Weiss behavior (with Θ = -14.9 K), while the dimer phase ß-1b is diamagnetic, showing no indication of thermal dissociation below 400 K. High-pressure crystallographic measurements indicate that the cross-braced π-stacked arrays of dimers undergo a wine-rack compression, but the dimer remains intact up to 8 GPa (at ambient temperature). The resistance of ß-1b to dissociate under pressure, also observed in its conductivity versus pressure profile, is in marked contrast to the behavior of the related dimer ß-1a (R1 = Et, R2 = F), which readily dissociates into a pair of radicals at 0.8 GPa. The different response of the two dimers to pressure has been rationalized in terms of differences in their linear compressibilities occasioned by changes in the degree of cross-bracing of the π-stacks. Dissociation of both dimers can be effected by irradiation with visible (λ = 650 nm) light; the transformation has been monitored by optical spectroscopy, magnetic susceptibility measurements, and single crystal X-ray diffraction. The photoinduced radical pairs persist up to temperatures of 150 K (ß-1b) and 242 K (ß-1a) before reverting to the dimer state. Variable-temperature optical measurements on ß-1b and ß-1a have afforded Arrhenius activation energies of 8.3 and 19.6 kcal mol(-1), respectively, for the radical-to-dimer reconversion. DFT and CAS-SCF calculations have been used to probe the ground and excited electronic state structures of the dimer and radical pair. The results support the interpretation that the ground-state interconversion of the dimer and radical forms of ß-1a and ß-1b is symmetry forbidden, while the photochemical transformation is symmetry allowed.

16.
J Am Chem Soc ; 136(3): 1070-81, 2014 Jan 22.
Artigo em Inglês | MEDLINE | ID: mdl-24400662

RESUMO

The crystal structure and charge transport properties of the prototypal oxobenzene-bridged 1,2,3-bisdithiazolyl radical conductor 3a are strongly dependent on pressure. Compression of the as-crystallized α-phase, space group Fdd2, to 3-4 GPa leads to its conversion into a second or ß-phase, in which F-centering is lost. The space group symmetry is lowered to Pbn21, and there is concomitant halving of the a and b axes. A third or γ-phase, also space group Pbn21, is generated by further compression to 8 GPa. The changes in packing that accompany both phase transitions are associated with an "ironing out" of the ruffled ribbon-like architecture of the α-phase, so that consecutive radicals along the ribbons are rendered more nearly coplanar. In the ß-phase the planar ribbons are propagated along the b-glides, while in the γ-phase they follow the n-glides. At ambient pressure 3a is a Mott insulator, displaying high but activated conductivity, with σ(300 K) = 6 × 10(-3) S cm(-1) and E(act) = 0.16 eV. With compression beyond 4 GPa, its conductivity is increased by 3 orders of magnitude, and the thermal activation energy is reduced to zero, heralding the formation of a metallic state. High pressure infrared absorption and reflectivity measurements are consistent with closure of the Mott-Hubbard gap near 4-5 GPa. The results are discussed in the light of DFT calculations on the molecular and band electronic structure of 3a. The presence of a low-lying LUMO in 3a gives rise to high electron affinity which, in turn, creates an electronically much softer radical with a low onsite Coulomb potential U. In addition, considerable crystal orbital (SOMO/LUMO) mixing occurs upon pressurization, so that a metallic state is readily achieved at relatively low applied pressure.

17.
Chem Commun (Camb) ; 50(7): 785-7, 2014 Jan 25.
Artigo em Inglês | MEDLINE | ID: mdl-24121843

RESUMO

Supramolecular CHπ interactions cause a ruffling of the otherwise coplanar ribbon-like arrays of radicals in the structure of the oxobenzene-bridged bisdithiazolyl . The material displays a conductivity σ(300 K) = 6 × 10(-3) S cm(-1) (Eact = 0.16 eV) and orders antiferromagnetically below 4 K. At applied fields above 1 kOe the material displays metamagnetic behavior.

18.
J Am Chem Soc ; 135(42): 15674-7, 2013 Oct 23.
Artigo em Inglês | MEDLINE | ID: mdl-24107231

RESUMO

Irradiation in the solid state of the hypervalent 4c-6e S···S-S···S bridged σ-dimer of a bisdithiazolyl radical leads to its photodissociation into a pair of π-radicals. The transformation has been monitored by optical spectroscopy, single crystal X-ray diffraction, and magnetic susceptibility measurements. As a result of the large electronic reorganization involved in the dimer-to-radical interconversion, the photogenerated S = 1/2 radical state is remarkably thermally stable, persisting to 242 K before reverting to the S = 0 dimer.

19.
Inorg Chem ; 52(4): 2188-98, 2013 Feb 18.
Artigo em Inglês | MEDLINE | ID: mdl-23391101

RESUMO

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.


Assuntos
Tiazóis/química , Cristalografia por Raios X , Radicais Livres/síntese química , Radicais Livres/química , Modelos Moleculares , Estrutura Molecular , Teoria Quântica , Tiazóis/síntese química
20.
Chem Commun (Camb) ; 49(16): 1603-5, 2013 Feb 25.
Artigo em Inglês | MEDLINE | ID: mdl-23338124

RESUMO

Resonance stabilized dithiazolothiadiazinyl radicals possess highly delocalized and easily tuned spin distributions; their structural features and transport properties augur well for their use in the design of magnetic and conductive materials.

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