Advances in verdazyl chemistry

Applications of Verdazyl Radicals in Energy Storage, Molecular Electronics and Magnetism: Teaching Old Molecules New Tricks

Verdazyls are a fundamental class of stable organic radicals that have been traditionally overshadowed by the more synthetically accessible stable nitroxide radicals. With the advent of enhanced synthetic routes to verdazyls, particularly in recent years, these systems are now poised to realise their potential in a range of applications across emerging technologies that will be important to addressing challenges faced by modern society. This review discusses the enabling properties of a selection of verdazyl-based systems that feature promising applications in energy storage, molecular electronics and magnetic molecules. Emphasis is placed on progress in these areas over the past ~5 years.

Verdazyl-Based Radicals for High-Field Dynamic Nuclear Polarization NMR

High-field dynamic nuclear polarization nuclear magnetic resonance (DNP NMR) spectroscopy transfers polarization from unpaired electrons in polarizing agents to nuclei of interest to boost NMR sensitivity. Verdazyl biradicals are a promising choice as polarizing agents because they have been found to generate narrower electron paramagnetic resonance (EPR) signals compared to nitroxide biradicals; an advantageous characteristic for high-field DNP when operating above 400 MHz/263 GHz. The use of verdazyl radicals as DNP polarizing agents has been very limited to date, yet, recent numerical simulations have predicted that verdazyl-nitroxide hybrid biradicals could be more effective polarizing agents than nitroxide-nitroxide biradicals. Herein, the syntheses of a series of verdazyl mono- and biradicals, as well as verdazyl-nitroxide biradicals are described. These radicals were examined in high-field DNP NMR experiments (600 MHz/395 GHz), by measuring 1H signal enhancements directly and through 13C{1H} cross-polarization experiments. X-band EPR, 1H DNP field profiles, and experiments to determine the nuclear build-up times were performed for verdazyl-nitroxide biradicals VerTEMPol and VerTEKol. These hybrid biradicals provide enhancements of up to 100-fold increased signal intensities (i.e., representing >104-fold time savings), approximately four times higher than that of the nitroxide biradical TEKPol, a commonly used polarizing agent in the field.

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.

Catalytic System for Cross-Coupling of Heteroaryl Iodides with a Nitronyl Nitroxide Gold Derivative at Room Temperature

A simple and highly effective methodology for the cross-coupling of heteroaryl iodides with NN-AuPPh3 at room temperature is reported. The protocol is based on a novel catalytic system consisting of Pd2(dba)3·CHCl3 and the phosphine ligand MeCgPPh having an adamantane-like framework. The present protocol was found to be well compatible with various heteroaryl iodides, thus opening new horizons in directed synthesis of functionalized nitronyl nitroxides and high-spin molecules.

Bipolar Verdazyl Radicals for Symmetrical Batteries: Properties and Stability in All States of Charge

Redox flow batteries based on organic electrolytes are promising energy storage devices, but stable long-term cycling is often difficult to achieve. Bipolar organic charge-storage materials allow the construction of symmetrical flow batteries (i. e., with identical electrolyte composition on both sides), which is a strategy to mitigate crossover-induced degradation. One such class of bipolar compounds are verdazyl radicals, but little is known on their stability/reactivity either as the neutral radical, or in the charged states. Here, we study the chemical properties of a Kuhn-type verdazyl radical (1) and the oxidized/reduced form (1^+/- ). Chemical synthesis of the three redox-states provides spectroscopic characterization data, which are used as reference for evaluating the composition of the electrolyte solutions of an H-cell battery during/after cycling. Our data suggest that, rather than the charged states, the decomposition of the parent verdazyl radical is responsible for capacity fade. Kinetic experiments and DFT calculations provide insight in the decomposition mechanism, which is shown to occur by bimolecular disproportionation to form two closed-shell products (leuco-verdazyl 1H and triazole derivative 2).

A Theoretical Account of the Coupling between Metal- and Ligand-centred Spins

This study addresses the magnetic interaction between paramagnetic metal ions and the radical ligands taking the [Cu^II (hfac)₂ (imVDZ)] and [M^II (hfac)₂ (pyDTDA)] (imVDZ=1,5-dimethyl-3-(1-methyl-2-imidazolyl)-6-oxoverdazyl; hfac=(1,1,1,5,5,5)hexafluroacetylacetonate; pyDTDA=4-(2'-pyridyl)-1,2,3,5-dithiadiazolyl), (M=Cu, Ni, Co, Fe, Mn) compounds as reference systems. The coupling between the metal and ligand spins is quantified in terms of the exchange coupling constant (J) in the platform of density functional theory (DFT) and the wave function-based complete active space self-consistent field (CASSCF) method. Application of DFT and broken symmetry (BS) formalism results ferromagnetic coupling for all the transition metal complexes except the Mn(II) complex. This DFT-BS prediction of magnetic nature matches with the experimental finding for all the complexes other than the Fe(II)-pyDTDA complex, for which an antiferromagnetic coupling between high spin iron and the thiazyl ligand has been reported. However, evaluation of spin state energetics through the multiconfigurational wave function-based method produces the S=3/2 ground spin state for the iron-thiazyl in parity with experiment. Electronic structure analyses find the overlap between the metal- and ligand-based singly occupied molecular orbitals (SOMOs) to be one of the major reasons attributing to different extent of exchange coupling in the systems under investigation.