Lawesson's Reagent: Providing a New Approach to the Forgotten 6-Thioverdazyl Radical

A new method for the preparation of the underrepresented 1,5-dimethyl-6-thioverdazyl radicals has been developed employing Lawesson's reagent (LR). The synthetic route involves the direct thionation of the carbonyl group of the corresponding dialkylbishydrazone followed by cyclization to give the tetrazinanthione verdazyl precursor on a gram scale. Subsequent oxidation yields the 6-thioverdazyl radical. It was determined that thionation of substrates containing electron-withdrawing groups in the ortho- or para-positions was high yielding. In contrast, for the parent phenyl group or substrates bearing weakly electron-donating substituents, thionation efficiency was significantly reduced. This could be overcome by utilizing partial in situ cyclization, which occurs during work up, to generate the tetrazinanthione directly via a one-pot synthesis. Density functional theory suggests that the LR fragment interacts with the carbonyl prior to cycloaddition and subsequent to cycloreversion, leading to the thiocarbonyl. The electronic nature of the radical is characterized with electron paramagnetic resonance as well as the first report of 6-thioverdazyl redox properties.

Electronic structure and magnetic coupling in selenium substituted pyridine-bridged bisdithiazolyl multifunctional molecular materials

Bisdithiazolyl radicals have furnished in recent years multiple examples of molecular materials with promising conductive and magnetic properties. The electronic band structure and magnetic ordering in four different isostructural pyridine-bridged bisdithiazolyl and Selenium substituted compounds have been studied by means of hybrid DFT based methods as implemented in the CRYSTAL code. The full rationalization of the properties of these multifunctional magnetic molecular materials requires a careful description of their complex open-shell electronic structure. The results describe the systems as narrow band (0.2-0.3 eV dispersion) open-shell semiconductors with a gap of 1.15-1.40 eV between the valence and conducting bands. The bands defining the insulating gap are dominated by orbital contributions arising from the heteroatoms sitting in the outer rings. A low energy closed-shell metallic solution is found at 0.25-0.35 eV above the magnetic solutions thus suggesting a complex mechanism for electric conduction with band and hopping contributions. The observed trend of the conductivity is in line with the variation of the insulating gap but more rigorous modelling is required to take into account the details of the band structure of the systems. For all the systems the spin density is well localised on the molecular units and is independent of the magnetic solution. Thus the system can be described as an ensemble of well-defined S = 1/2 magnetic centres using a two-body Heisenberg-Dirac-van Vleck spin Hamiltonian. The lowest energy electronic solutions are in line with the observed magnetic behaviour at low temperature. The set of competing magnetic exchange interactions that emerges from using a suitable mapping to consistently describe the low energy magnetic solutions explains the variety of magnetic responses (absence of long-range magnetic order, antiferromagnetism or ferromagnetism) of the four studied compounds at low temperatures.

Combining Open-Shell Verdazyl Environment and Co(II) Spin-Crossover: Spinmerism in Cobalt Oxoverdazyl Compound

The spin states of a Co(II) oxoverdazyl compound are investigated by means of wavefunction-based calculations. Within a ca. 233 K energy window, the ground state and excited states display a structure-sensitive admixture of low-spin SM = 1/2 in a dominant high-spin SM = 3/2 Co(II) ion as read from localized molecular orbitals. This puzzling spins zoology that results from the coupling between open-shell radical ligands and a spin-crossover metal ion gives rise to this unusual scenario which extends the views in molecular magnetism. In agreement with experimental observation, the low-energy spectroscopy is very sensitive to deformations of the coordination sphere, and a growing admixture of Co(II) low-spin is evidenced from the calculations. In analogy with mesomerism that accounts for charge delocalization, entanglement combines different local spin states to generate a given total spin multiplicity, a spinmerism phenomenon.

Ferro- or antiferromagnetism? Heisenberg chains in the crystal structures of verdazyl radicals

In this study, we address the question of the origin of ferromagnetic or antiferromagnetic interactions in alkynyl-substituted 1,5-diphenyl-6-oxo verdazyl radicals. While a TMS-alkynyl derivative (3) shows antiferromagnetic ordering at low temperatures, the corresponding deprotected alkynyl verdazyl (4) shows ferromagnetic interactions. For both compounds, magnetic Heisenberg chains are characteristic, which were studied systematically by means of X-ray crystallography and quantum chemical calculations. Ferromagnetic interactions are rarely found in such radicals. Therefore, uncovering such structure-property relationships is of crucial importance in order to understand and design promising ferromagnetic networks. Using this knowledge, we were able to design and crystallize diyne derivatives showing comparable solid state characteristics and therefore antiferro- and ferromagnetic Heisenberg chain structures. We show that the understanding of such property-structure relationships is adequate for the design of organic-magnetic materials with defined cooperative effects within the class of verdazyl radicals.

Electronic Structure and Magnetic Interactions in the Radical Salt [BEDT-TTF]2[CuCl4]

The magnetic behavior and electric properties of the hybrid radical salt [BEDT-TTF]₂[CuCl₄] have been revisited through extended experimental analyses and DDCI and periodic DFT plane waves calculations. Single crystal X-ray diffraction data have been collected at different temperatures, discovering a phase transition occurring in the 250-300 K range. The calculations indicate the presence of intradimer, interdimer, and organic-inorganic π-d interactions in the crystal, a magnetic pattern much more complex than the Bleaney-Bowers model initially assigned to this material. Although this simple model was good enough to reproduce the magnetic susceptibility data, our calculations demonstrate that the actual magnetic structure is significantly more intricate, with alternating antiferromagnetic 1D chains of the organic BEDT-TTF⁺ radical, connected through weak antiferromagnetic interactions with the CuCl₄^2- ions. Combination of experiment and theory allowed us to unambiguously determine and quantify the leading magnetic interactions in the system. The density-of-states curves confirm the semiconductor nature of the system and the dominant organic contribution of the valence and conduction band edges. This general and combined approach appears to be fundamental in order to properly understand the magnetic structure of these complex materials, where experimental data can actually be fitted from a variety of models and parameters.

The magnetic fingerprint of dithiazolyl-based molecule magnets

The ferromagnetic fingerprint of dithiazolyl-based molecule materials is uncovered. Interestingly geometrical rather than electronic structure factors play the leading role.

Flexible Viologen Cyclophanes: Odd/Even Effects on Intramolecular Interactions

The ability of three bis-viologen cyclophanes to act as redox-triggered contractile switches is investigated. Odd/even effects in the formation of cyclic bis-viologens are circumvented by the use of a Zincke salt intermediate and a tetrathiafulvalene template to prepare a flexible cyclophane with hexyl linkers. Comparative spectro-electrochemical studies of this macrocycle with two other pentyl- or heptyl-linked cyclic bis-viologens show that the development of intramolecular interactions in aqueous solution depends on the length of the bridges. This dependence is confirmed by EPR and DFT studies of the magnetic coupling in the diradical dication species. The anti-ferromagnetic or ferromagnetic nature of the coupling depend, respectively, on the odd or even number of methylene groups in the spacer.

Black-box determination of temperature-dependent susceptibilities for crystalline organic radicals with complex magnetic topologies

In all but the simplest crystal structures, the identification of all relevant interactions between magnetic sites as well as the setup of magnetic model spaces, which are necessary for modeling macroscopic magnetism, are tedious and error-prone tasks. Here, we present a procedure to generate magnetic susceptibility versus temperature curves using only a crystal structure as input. The procedure, which is based on the first-principles bottom-up approach [Deumal et al., J. Phys. Chem. A, 2002, 106, 1299], is designed in a way to require as little user interference as possible. We employ quantum chemical calculations to parametrize a Heisenberg Hamiltonian, which is set up and diagonalized for different magnetic model spaces to ensure convergence of the model. We apply the procedure to several 6-oxo-verdazyl radical structures, including newly synthesized compounds, and compare the results to data we obtained from magnetic susceptibility measurements as well as published data to further benchmark our procedure. Furthermore, the different impact of certain dominating coupling constants is systematically analyzed.

Dynamical effects on the magnetic properties of dithiazolyl bistable materials

The magnetic properties of molecule-based magnets are commonly rationalized by considering only a single nuclear configuration of the system under study (usually an X-ray crystal structure). Here, by means of a computational study, we compare the results obtained using such a static approach with those obtained by explicitly accounting for thermal fluctuations, and uncover the serious limitations of the static perspective when dealing with magnetic crystals whose radicals undergo wide-amplitude motions. As a proof of concept, these limitations are illustrated for the magnetically bistable 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA) material. For its high-temperature phase at 300 K, we show that nuclear dynamics induce large fluctuations in the magnetic exchange interactions (JAB) between spins (up to 1000% of the average value). These deviations result in a ∼20% difference between the 300 K magnetic susceptibility computed by explicitly considering the nuclear dynamics and that computed using the X-ray structure, the former being in better agreement with the experimental data. The unveiled strong coupling between JAB interactions and intermolecular vibrations reveals that considering JAB as a constant value at a given temperature (as always done in molecular magnetism) leads to a flawed description of the magnetism of TTTA. Instead, the physically relevant concept in this case is the statistical distribution of JAB values. The discovery that a single X-ray structure is not adequate enough to interpret the magnetic properties of TTTA is also expected to be decisive in other organic magnets with dominant exchange interactions propagating through labile π-π networks.

Ferromagnetic intermolecular exchange interaction in ethynyl-verdazyl radical crystals

The newly synthesized p-ethynylphenyl-oxoverdazyl radical shows unusual slipped 1D chains held together by π–π interactions leading to ferromagnetic exchange interactions.