Alicea A. Leitch

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).

Heavy Atom Ferromagnets under Pressure: Structural Changes and the Magnetic Response

Application of physical pressure to a ferromagnetic bisdiselenazolyl radical leads to a decrease in pi-stack slippage. Initially, this leads to an increase in the ferromagnetic ordering temperature T(C), which reaches a maximum of 21 K near 1 GPa. At higher pressures, as the pi-stacks become more nearly superimposed, the value of T(C) diminishes.

Metallization of a Hypervalent Radical Dimer: Molecular and Band Perspectives

Variable pressure and temperature conductivity measurements on the bisthiaselenazolyl radical dimer [1a](2) have established the presence of a weakly metallic state over the pressure range 5-9 GPa. To explore the origin of this metallization we have examined the crystal and molecular structure of [1a](2) as a function of pressure. At ambient pressure the dimer consists of two radicals linked by a hypervalent 4-center 6-electron S...Se-Se...S sigma-bond into an essentially coplanar arrangement. The dimers are packed in cross-braced slipped pi-stack arrays running along the x-direction of the monoclinic (space group P2(1)/c) unit cell. Pressurization to 4 GPa induces little change in the molecular structure of [1a](2) or in the slipped pi-stack crystal architecture. Near 5 GPa, however, stress on the dimer leads to buckling of the two halves of the molecule and a contraction in the metrics of the S...Se-Se...S unit. These structural changes can be understood in terms of an electronic configurational switch from a 4-center 6-electron sigma-bonded dimer to a more conventional pi-bonded arrangement. At the same time the slipped pi-stack arrays undergo a concertina-like compression, and the crystal structure experiences highly anisotropic changes in cell dimensions. DFT calculations on the molecular electronic structure of the dimer indicate a marked decrease in the HOMO-LUMO gap as the dimer buckles. Related solid-state calculations indicate a rapid closure of the valence/conduction band gap in the same pressure region and the formation of a quasi-metallic state. Metallization of [1a](2) thus arises as much from intramolecular changes, which give rise to a collapse of the HOMO-LUMO gap and near coalescence of the valence and conduction bands, as from increased intermolecular interactions, which cause widening and overlap of the band edges.

Heat, Pressure and Light-Induced Interconversion of Bisdithiazolyl Radicals and Dimers

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 β-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 β-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 β-1b to dissociate under pressure, also observed in its conductivity versus pressure profile, is in marked contrast to the behavior of the related dimer β-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 (β-1b) and 242 K (β-1a) before reverting to the dimer state. Variable-temperature optical measurements on β-1b and β-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 β-1a and β-1b is symmetry forbidden, while the photochemical transformation is symmetry allowed.

Bis-1,2,3-thiaselenazolyl radicals and their σ-bonded dimers

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

Spin-canting in heavy atom heterocyclic radicals

A pair of isostructural bis-selenathiazolyl and bis-diselenazolyl radical conductors display weak (spin-canted) ferromagnetism with Tc values of 18 K and 27 K respectively.

Halide Influence on Molecular and Supramolecular Arrangements of Iron Complexes with a 3,5-Bis(2-Pyridyl)-1,2,4,6-Thiatriazine Ligand.

A series of iron centered complexes, namely, [Fe(Py2TTA)Cl2] (1), [Fe(Py2TTA)Br2] (2), and [Fe(μ-F)(Py2TTAO)F]∞ (3), were isolated via complexation of 3,5-bis(2-pyridyl)-1,2,4,6-thiatriazine (Py2TTAH) with various ferric halides (e.g., FeF3, FeCl3, and FeBr3). Comparison of the optical and electrochemical spectroscopy, structural analysis, and magnetic studies reveal numerous similarities between the chlorido (1) and bromido (2) derivatives, which crystallize as discrete five-coordinate iron centered complexes with coordination geometries that are intermediate between trigonal bipyramidal and square base pyramid. Conversely, the fluorido derivative (3) results in a completely different structure due to oxidation of the ligand and the formation of a one-dimensional coordination polymer held together through a bridging fluoride ion. Consequently, the spectroscopic and magnetic behavior of 3 differs significantly compared with 1 and 2. Complexes 1 and 2 exhibit paramagnetic properties typical for a mononuclear S = 5/2 system with weak intermolecular antiferromagnetic interactions at low temperatures, whereas complex 3 demonstrates significant exchange couplings within the chain and weak antiferromagnetic interchain interactions, which stabilize a canted antiferromagnetic state below 4.2 K.

Elucidating the elusive crystal structure of 2,4,6-tris(2-pyrimidyl)-1,3,5-triazine

2,4,6-Tris(2-pyrimidyl)-1,3,5-triazine (TPymT) has been synthesized using two approaches: via trimerization of 2-cyanopyrimidine at 160 °C and by the reaction of 2-cyanopyrimidine with pyrimidine-2-carboximidamide. It was found that the two different synthetic pathways to TPymT yielded either a single polymorph or a mixture of two polymorphs; both of which were studied for the first time by X-ray powder diffraction (XRPD) and synchrotron X-ray powder diffraction (SXRPD). The crystal structure of the polymorph α-TPymT was determined by single crystal X-ray diffraction (SCXRD). Crystal structures of 2-cyanopyrimidine and pyrimidine-2-carboximidamide were also elucidated by SXRPD.

Optoelectronic and structural properties of a family of thiophene functionalized 1,5-dithia-2,4,6,8-tetrazocines

The dithiatetrazocine (DTTA) ring has been functionalized with thiophene (5a), 5-hexylthiophene (5b) and 5-bromothiophene (5c), and the optoelectronic and structural properties have been probed. All three derivatives possess low-lying LUMO levels near −3.5 eV and exhibit irreversible reduction and oxidation processes. Inspection of their absorption profiles reveals a small bathochromic shift in λmax with substitution of the thiophene ring. The HOMO–LUMO band gaps, determined from electrochemical, optical and computational studies, decrease across the series in the following manner: 5a > 5c > 5b. DFT calculated frontier molecular orbitals for 5a–c (R = Me for 5b) indicate delocalized electron density over the entire tricyclic framework for the HOMO, whereas the electron density for the LUMO is localized on the DTTA ring. Single crystal X-ray diffraction on 5b,c show structures consisting of molecules with planar molecular frameworks arranged in slipped π-stack arrays. Although both compounds exhibit close intermolecular interactions, the contacts observed in 5c are between π-stacks, while they are exclusively within the π-stacks for 5b.

Magnetic Bistability in Naphtho-1,3,2-dithiazolyl: Solid State Interconversion of a Thiazyl π-Radical and Its N–N σ-Bonded Dimer

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.