Crystallographic Insights into the Synthesis and Magnetic Properties of Oxoverdazyl Radicals Functionalized by Benzoic Acid

Direct mapping of kidney function by DCE-MRI urography using a tetrazinanone organic radical contrast agent

Chronic kidney disease (CKD) and acute kidney injury (AKI) are ongoing global health burdens. Glomerular filtration rate (GFR) is the gold standard measure of kidney function, with clinical estimates providing a global assessment of kidney health without spatial information of kidney- or region-specific dysfunction. The addition of dynamic contrast enhanced magnetic resonance imaging (DCE-MRI) to the anatomical imaging already performed would yield a 'one-stop-shop' for renal assessment in cases of suspected AKI and CKD. Towards urography by DCE-MRI, we evaluated a class of nitrogen-centered organic radicals known as verdazyls, which are extremely stable even in highly reducing environments. A glucose-modified verdazyl, glucoverdazyl, provided contrast limited to kidney and bladder, affording functional kidney evaluation in mouse models of unilateral ureteral obstruction (UUO) and folic acid-induced nephropathy (FAN). Imaging outcomes correlated with histology and hematology assessing kidney dysfunction, and glucoverdazyl clearance rates were found to be a reliable surrogate measure of GFR.

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).

Investigation by Chemical Substitution within 2p-3d-4f Clusters of the Cobalt(II) Role in the Magnetic Behavior of [vdCoLn]2 (vd = Verdazyl Radical)

1,5-Dimethyl-3-(3'-(hydroxymethyl)-2'-pyridine)-6-oxotetrazane (H₃vdpyCH₂OH) or its oxidized verdazyl form (vdpyCH₂OH) reacted with transition metal and/or lanthanide acetates to yield [(vdpyCH₂O)₂Co₂Ln₂(acO)₈] (Ln = Y(III): I_Co,Y; Gd(III): I_Co,Gd), [(vdpyCH₂O)₂M₃(acO)₄] (M = Zn(II): II_Zn; Co(II): II_Co) and [(vdpyCH₂OH)Zn(acO)₂] (III_Zn) through self-assembly implying a complex-as-ligand intermediate. Single-crystal diffraction reveals that I_MT,Ln are composed of 2p-3d-4f centrosymmetric clusters with verdazyl radicals at the two ends coordinated to the transition-metal ion in a tridentate mode and to the {Ln₂(acO)₄} lanthanide central core in a monodentate mode through its alkoxo moiety. In I_Co,Gd, the transition-metal ions adopt an irregular octahedral environment, and the {Ln₂(acO)₄} core adopts a paddlewheel motif, whereas in I_Co,Y, the transition metal is pentacoordinated, and the central core contains only two acetate bridges. Going from I_Co,Y to II_Co, the central {Y₂(acO)₄} core is replaced by an axially compressed octahedral cobalt(II) center, whereas the outer parts of the molecule remain still. The dc magnetic studies revealed that the alternate π-stacking of the verdazyl radicals in II_Zn led to the formation of alternate antiferromagnetically coupled 1D chains with J_vd-vd = -8.2(1) cm^-1 and J_vd-vd' = -7.6(1) cm^-1 (-2J convention). In I_Co,Y, a complex fitting procedure allowed us to retrieve a complete set of magnetic parameters to take into account both the magnetic anisotropy of the cobalt(II) centers and intra- and inter-molecular exchange effects. For I_Co,Y, it led to g_Co = 2.13(4), D_Co = 100(2) cm^-1, E_Co = 19.9(5) cm^-1, J_Co-vd = +26.5(4) cm^-1, and J_vd-vd = -7.95(4) cm^-1. ac magnetic susceptibility of I_Co,Y, I_Co,Gd and II_Co did not reveal any slow relaxation of the magnetization even when a dc external magnetic field up to 2000 Oe was applied.

Blatter Radicals as Bipolar Materials for Symmetrical Redox-Flow Batteries

Redox-active organic molecules are promising charge-storage materials for redox-flow batteries (RFBs), but material crossover between the posolyte and negolyte and chemical degradation are limiting factors in the performance of all-organic RFBs. We demonstrate that the bipolar electrochemistry of 1,2,4-benzotriazin-4-yl (Blatter) radicals allows the construction of batteries with symmetrical electrolyte composition. Cyclic voltammetry shows that these radicals also retain reversible bipolar electrochemistry in the presence of water. The redox potentials of derivatives with a C(3)-CF₃ substituent are the least affected by water, and moreover, these compounds show >90% capacity retention after charge/discharge cycling in a static H-cell for 7 days (ca. 100 cycles). Testing these materials in a flow regime at a 0.1 M concentration of the active material confirmed the high cycling stability under conditions relevant for RFB operation and demonstrated that polarity inversion in a symmetrical flow battery may be used to rebalance the cell. Chemical synthesis provides insight in the nature of the charged species by spectroscopy and (for the oxidized state) X-ray crystallography. The stability of these compounds in all three states of charge highlights their potential for application in symmetrical organic redox-flow batteries.

Polymorphism in a π stacked Blatter radical: structures and magnetic properties of 3-(phenyl)-1-(pyrid-2-yl)-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl

The first polymorphism example in Blatter radicals, is reported.

Verdazyls as Possible Building Blocks for Multifunctional Molecular Materials: A Case Study on 1,5-Diphenyl-3-(p-iodophenyl)-verdazyl Focusing on Magnetism, Electron Transfer and the Applicability of the Sonogashira-Hagihara Reaction

This work explores the use of Kuhn verdazyl radicals as building blocks in multifunctional molecular materials in an exemplary study, focusing on the magnetic and the electron transfer (ET) characteristics, but also addressing the question whether chemical modification by cross-coupling is possible. The ET in solution is studied spectroscopically, whereas solid state measurements afford information about the magnetic susceptibility or the conductivity of the given samples. The observed results are rationalized based on the chemical structures of the molecules, which have been obtained by X-ray crystallography. The crystallographically observed molecular structures as well as the interpretation based on the spectroscopic and physical measurements are backed up by DFT calculations. The measurements indicate that only weak, antiferromagnetic (AF) coupling is observed in Kuhn verdazyls owed to the low tendency to form face-to-face stacks, but also that steric reasons alone are not sufficient to explain this behavior. Furthermore, it is also demonstrated that ET reactions proceed rapidly in verdazyl/verdazylium redox couples and that Kuhn verdazyls are suited as donor molecules in ET reactions.

The aromaticity of verdazyl radicals and their closed-shell charged species

The aromaticity of fourteen 3-oxo-verdazyl (1–8) and Kuhn verdazyl (9–14) radicals with different substituents has been investigated computationally using the gauge-including magnetically induced current-density (GIMIC) method.