The Effects of Chemical and Physical Pressure on Heavy Atom Radicals

Neutral radicals have been pursued as building blocks for conductive and magnetic materials for several decades. Carbon-based materials are typically plagued by dimerization and quenching of spins, but the incorporation of heteroatoms has led to many systems that remain open-shell. Radicals based on the thiazyl subunit, which are the subject of this thesis, have evolved through many generations. While the early frameworks possessed a very high onsite Coulomb repulsion energy, U, which caused the spins to be localized so that the radicals were trapped in Mott insulating ground states, the development of resonance stabilized bisdithiazolyl radicals, and their selenazyl counterparts, has led to decreased values of U, in addition to inducing major improvements in the bandwidth, W. Variation in the ligand environment and selenium content can significantly change solid state packing and hence physical properties. This so-called effect of chemical pressure has been explored and structure-property correlations have been well established. In addition to studies involving the variation of chemical pressure, in this thesis the effect of physical pressure on these resonance stabilized heavy atom radicals is presented. In the radical building blocks of the present systems there are four possible combinations of sulfur and selenium atoms, sets of which constitute a family. The families may crystallize as an isostructural set in the solid state, but this is rare. Earlier work established that radicals in one isostructural family crystallize as undimerized π-radicals in the P4¯21m space group, the selenium variants of which all order magnetically. In this thesis, subtle molecular modification of this family is first explored. Exploration of the substituent effects with selenium fixed in the central position of the heterocycle has provided radicals that order as bulk ferromagnets in the range Tc = 9–12 K. The highest Tc ferromagnets in this group are those based on the all-selenium framework. The magnetic response of these radicals was studied under pressure, and it was discovered that with the initial application of pressure, Tc rises from 17–18 K to 21–24 K, before retreating upon further pressurization. In the 7–9 GPa range, the magnetic insulators begin to metallize, as evidenced by the loss in activation barrier to conductivity and a saturation of the resistance to a finite value at low temperature. The crystal structures in the entire pressure range have been determined and the changes in transport properties have been attributed to decreased slippage of the π-stacks with increasing pressure. Although most of the resonance stabilized bisdithiazolyl radicals and their selenium variants are undimerized in the solid state, a few derivatives exist that dimerize through unique 4-center 6-electron S•••E–E•••S σ-bonds. When E = Se, hypervalent σ-dimerization is especially prevalent. Under ambient conditions, these materials pack in crossbraced π-stacks and exhibit semiconducting behavior. Upon mild pressurization (P ≤ 5 GPa), however, conductivity increases 5–6 orders of magnitude and the activation energy decreases remarkably. Solid state structures have now been elucidated for these dimers under pressure. For two of the variants, including one (rare) E = S σ-dimer, increasing pressure results in simple contraction of the structure. This leads to enhanced orbital overlap and gradual coalescence of the valence and conduction bands, eventually leading to metallization at P > 13 GPa. This behavior is in sharp contrast to a previously reported σ-dimer, which undergoes a transition to a π-dimer at 5 GPa, the structure of which leads to abrupt closure of the HOMO-LUMO gap and, hence, the sudden onset of a weakly metallic state. As a departure from the behavior of any of the other known hypervalent σ-dimers, one bisdithiazolyl variant undergoes an abrupt S = 0 → S = ½ transition. This change can be initiated thermally, optically and with mild pressure. The thermal process, which is observed in the magnetic susceptibility measurements, is hysteretic, with T↑ = 380 K and T↓ = 375 K, giving rise to a small region of bistability. Irradiation results in the photomagnetization of the metastable S = ½ state that persists to an unprecedented relaxation temperature of 242 K. Under the influence of pressure, the same dimer-to-radical transition occurs (at room temperature) near 0.7 GPa. In all cases, the crystal structure of the metastable excited state has been determined by single crystal or powder X-ray diffraction. The novel behavior of the σ-dimer is in addition to the existence of a second polymorph of this material, which is paramagnetic and belongs to the P4¯21m space group under ambient conditions. Further exploration of the effects of chemical pressure on bisdithiazolyl radicals has led to new systems with extremely long alkyl chains. This was explored for the purpose of separating the plates to generate lower dimensional frameworks. The crystal structure of one derivative belongs to the familiar tetragonal space group P4¯21m. However, upon increasing chain length of the alkyl substituent, an isomorphous set is generated, with all three compounds crystallizing in the P21/c space group. The structures consist of pairs of radical π-stacks pinned together by strong intermolecular F•••S' bridges to create spin ladder arrays. The slipped π-stack alignment of radicals produces close non-covalent S•••S' interactions which serve as the “rungs” of a spin ladder, and the long chain alkyl substituents serve as buffers that separate the ladders from each other laterally. The “legs” of the spin ladder are generated by magnetic exchange along the π-stacks. Magnetic susceptibility measurements reveal the presence of very strong antiferromagnetic coupling in all three compounds, which have been successfully modeled as strong-leg spin ladders.1 yea

High-pressure dc magnetic measurements on a bisdiselenazolyl radical ferromagnet using a vibrating-coil SQUID magnetometer

The high-pressure magnetic properties of the iodo-substituted bisdiselenazolyl radical ferromagnet IBPSSEt have been studied by vibrating-coil SQUID magnetometry. The magnetic state at a pressure (P) of approximately 2 GPa has the highest Curie temperature (TC) of 27.5 K, and displays an ideal three-dimensional (3D) ferromagnetic interaction network. The value of TC observed by ac magnetic susceptibility measurements is consistent with that obtained from dc measurements below approximately 4 GPa. Field-cooled dc measurements at more elevated pressures reveal a slow evolution of magnetic ordering, so that atP >6 GPa the structure may be described in terms of a 1D ferromagnetic chain with predominantly antiferromagnetic lateral (interchain) interactions, in accord with the results of density functional theory calculations

Photoinduced Solid State Conversion of a Radical σ‑Dimer to a π‑Radical Pair

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 <i>S</i> = 1/2 radical state is remarkably thermally stable, persisting to 242 K before reverting to the <i>S</i> = 0 dimer

Photoinduced Solid State Conversion of a Radical σ‑Dimer to a π‑Radical Pair

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 <i>S</i> = 1/2 radical state is remarkably thermally stable, persisting to 242 K before reverting to the <i>S</i> = 0 dimer

A Bimodal Oxobenzene-bridged Bisdithiazolyl Radical Conductor

The preparation and structural characterization of the methyl-substituted oxobenzene-bridged bisdithiazolyl radical <b>3b</b> is described. Crystals of <b>3b</b> belong to the monoclinic space group <i>C</i>2/<i>c</i> and contain two distinct radical environments, <b>A</b> and <b>B</b>. There are eight <b>A</b> radicals in the unit cell, which occupy general positions and form alternating twisted π-stacks running parallel to the <i>c</i>-axis. The four <b>B</b> radicals also adopt an alternating π-stack pattern, but each molecule lies on a crystallographic 2-fold rotation axis, and the overlay of neighboring radicals is centrosymmetric. Stacks of <b>A</b> radicals are linked by close intermolecular S···O′ and S···N′ contacts into ribbon-like arrays that weave along the <i>y</i>-direction, and the <b>B</b> radical stacks are located in columnar cavities generated by the out-of-register alignment of the ribbons of <b>A</b> radicals. Variable temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically coupled system, a result in accord with DFT estimated exchange energies for intrastack radical–radical interactions. Four-probe conductivity measurements indicate a conductivity σ­(300 K) = 9.0 × 10^–4 S cm^–1, with a thermal activation energy <i>E</i>_act = 0.13 eV

Non-Innocent Base Properties of 3- and 4-Pyridyl-dithia- and Diselenadiazolyl Radicals : The Effect of N-Methylation

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.peerReviewe

A Bimodal Oxobenzene-bridged Bisdithiazolyl Radical Conductor

The preparation and structural characterization of the methyl-substituted oxobenzene-bridged bisdithiazolyl radical <b>3b</b> is described. Crystals of <b>3b</b> belong to the monoclinic space group <i>C</i>2/<i>c</i> and contain two distinct radical environments, <b>A</b> and <b>B</b>. There are eight <b>A</b> radicals in the unit cell, which occupy general positions and form alternating twisted π-stacks running parallel to the <i>c</i>-axis. The four <b>B</b> radicals also adopt an alternating π-stack pattern, but each molecule lies on a crystallographic 2-fold rotation axis, and the overlay of neighboring radicals is centrosymmetric. Stacks of <b>A</b> radicals are linked by close intermolecular S···O′ and S···N′ contacts into ribbon-like arrays that weave along the <i>y</i>-direction, and the <b>B</b> radical stacks are located in columnar cavities generated by the out-of-register alignment of the ribbons of <b>A</b> radicals. Variable temperature magnetic susceptibility measurements indicate a strongly antiferromagnetically coupled system, a result in accord with DFT estimated exchange energies for intrastack radical–radical interactions. Four-probe conductivity measurements indicate a conductivity σ­(300 K) = 9.0 × 10^–4 S cm^–1, with a thermal activation energy <i>E</i>_act = 0.13 eV

Vespa cribraria

A series of four bisdithiazolyl radicals <b>1a</b>–<b>d</b> (R₁ = Pr, Bu, Pn, Hx; R₂ = F) has been prepared and characterized by X-ray crystallography. The crystal structure of <b>1a</b> (R₁ = Pr) belongs to the tetragonal space group <i>P</i>4̅2₁<i>m</i> and consists of slipped π-stack arrays of undimerized radicals packed about 4̅ centers running along the <i>z</i>-direction, an arrangement identical to that found for <b>1</b> (R₁ = Et; R₂ = F). With increasing chain length of the R₁ substituent, an isomorphous set <b>1b</b>–<b>d</b> is generated. All three compounds crystallize in the <i>P</i>2₁/<i>c</i> 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 (R₁) serve as buffers which separate the ladders from each other laterally. Variable temperature magnetic susceptibility measurements indicate that <b>1a</b> 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 <b>1b</b>–<b>d</b>, such that the Curie–Weiss fit is no longer applicable. Analysis of the full data range (<i>T</i> = 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 <i>within</i> the ladder “legs” and “rungs”, with only very weak magnetic exchange <i>between</i> the ladders

A Pressure Induced Structural Dichotomy in Isostructural Bis-1,2,3-thiaselenazolyl Radical Dimers

The pressure dependence of the crystal and molecular structure of the bis-1,2,3-thiaselenazolyl radical dimer [<b>1b</b>]₂ has been investigated over the range 0–11 GPa by powder diffraction methods using synchrotron radiation and diamond anvil cell techniques. At ambient pressure, the dimer consists of a pair of radicals linked by a hypervalent 4-center 6-electron S---Se–Se---S σ-bond in an essentially coplanar arrangement. The dimers are packed in cross-braced slipped π-stack arrays running along the <i>x</i>-direction of the monoclinic (space group <i>P</i>2₁/<i>c</i>) unit cell. Pressurization to 11 GPa causes the unit cell dimensions <i>a</i> and <i>c</i> to undergo a slow but uniform compression, while the <i>b</i>-axis is slightly elongated. There is virtually no change in the molecular structure or in the slipped π-stack crystal architecture. This behavior is in marked contrast to that of the isostructural radical dimer [<b>1a</b>]₂, where the basal fluorine is replaced by hydrogen. Pressurization of this latter material induces a phase change near 4–5 GPa, characterized by a sharp contraction in <i>a</i> and <i>c</i>, and a correspondingly large increase in <i>b</i>. At the molecular level, the transition is associated with a buckling of the σ-bonded dimer to a more conventional π-bonded arrangement. Geometry optimized DFT band structure calculations on [<b>1b</b>]₂ replicate the observed structural changes and indicate that compression widens both the valence and conduction bands but does not induce band gap closure until >13 GPa. This result is consistent with the measured thermal activation energy for conduction <i>E</i>_act, which indicates that a metallic state requires pressures > 10 GPa