Radical-Doped Metal–Organic Framework: Route to Nanoscale Defects and Magnetostructural Functionalities

Xerogel mesoporous materials based on ultrastable Blatter radicals for efficient sorption of nitric oxide

The reduction of hazardous nitric oxide emissions remains a significant ecological challenge. Despite the variety of possibilities, sorbents able to capture low concentrations of NO from flue gas with high selectivity are still in demand. In this work a new type of mesoporous xerogel material highly loaded with ultrastable Blatter radicals (BTR, >60 % by mass) that act as selective NO sorption sites is developed. Electron Paramagnetic Resonance (EPR) spectroscopy evidences reversible NO sorption in nanometer-scale pores of BTR-based xerogels and indicates the high NO capacity of such radical-rich sorbent. Efficient NO capture from model flue gas mixture is also evidenced in experiments with a fixed bed reactor. Such advanced properties of new materials as selectivity, strong binding with NO and an ability for mild regeneration via thermodesorption promote them for future ecological applications.

Toward N-peri-Annulated Planar Blatter Radical through aza-Pschorr and Photocyclization

Preparation of the elusive N-peri-annulated planar Blatter radicals was attempted using aza-Pschorr and photocyclization methods. In both methods, substrates containing N-Me and N-Ac groups yielded a zwitterionic heterocycle lacking the N-substituent as the main product, while in one of them a carbazole derivative representing a new heterocyclic system was also obtained. The formation of the zwitterion and the carbazole suggests the formation of the desired planar Blatter radical, which undergoes facile fragmentation through homolysis of the N-R bond. This mechanism is supported by DFT computational results, which also suggest that N-Ar derivatives should be sufficiently stable for isolation. Electronic structures of three planar Blatter radicals annulated with the O, S, and N-Ph groups are compared.

Thermally induced dimensionality changes in derivatives of a "super stable" Blatter radical

Two derivatives of a "super stable" Blatter radical (1,3-diphenyl-7-trifluoromethyl-1,4-dihydrobenzo[e][1,2,4]triazin-4-yl) with N(1)-Ar = 2-CF3C6H4 and 2-MeOC6H4 were obtained and investigated using XRD and SQUID magnetometry methods. The investigation revealed strong antiferromagnetic interactions in both radicals, which are described using the Hatfield model. For the latter radical, an abrupt and reversible change in the χ(T) plot was observed at 29 K. It was ascribed to a structural transition, consistent with a two-dimensional to one-dimensional thermally activated crossover, as supported by specific heat measurements (CvHvs. T). It is suggested that the transition is related to an order-disorder transition of the CF3 group, which is corroborated using XRD structural analysis.

Properties of Aliphatic Ligand-Based Metal-Organic Frameworks

Ligands with a purely aliphatic backbone are receiving rising attention in the chemistry of coordination polymers and metal-organic frameworks. Such unique features inherent to the aliphatic bridges as increased conformational freedom, non-polarizable core, and low light absorption provide rare and valuable properties for their derived MOFs. Applications of such compounds in stimuli-responsive materials, gas, and vapor adsorbents with high and unusual selectivity, light-emitting, and optical materials have extensively emerged in recent years. These properties, as well as other specific features of aliphatic-based metal-organic frameworks are summarized and analyzed in this short critical review. Advanced characterization techniques, which have been applied in the reported works to obtain important data on the crystal and molecular structures, dynamics, and functionalities, are also reviewed within a general discussion. In total, 132 references are included.

Deciphering the ground state of a C3-symmetrical Blatter-type triradical by CW and pulse EPR spectroscopy

3,3',3''-(Benzene-1,3,5-triyl)tris(1-phenyl-1H-benzo[e][1,2,4]triazin-4-yl) (1) is a C₃-symmetrical triradical comprised of three Blatter radical units connected at the 1, 3, 5 positions of a central trimethylenebenzene core. This triradical has an excellent air, moisture, and thermal stability. Single-crystal XRD indicates that triradical 1 adopts a propeller-like geometry with the benzotriazinyl moieties twisted by 174.1(2)° and packs in 1D chains along the c axis to form an extensive network of weak intermolecular interactions. Frozen solution continuous wave (CW) EPR spectra and variable-temperature field-sweep echo-detected (FSED) spectra revealed an intramolecular ferromagnetic exchange within the spin system, supporting a quartet S = 3/2 ground state. DFT calculations further supported these experimental findings.

"Super stable" Blatter radicals through ArLi addition: surprising chemistry of 7-(trifluoromethyl)benzo[e][1,2,4]triazine

Addition of PhLi to 7-(CF₃)benzo[e][1,2,4]triazine at -78 °C gives the "super stable" Blatter radical in high yields, while above -5 °C two additional products are formed. XRD analysis revealed the formation of a "trimer" and a benzo[f][1,2,4]triazepine via a novel mechanism. The latter is formed from the anion generated from the isolated radical, which suggests its instability in organic batteries.

Blatter Radical-Decorated Silica as a Prospective Adsorbent for Selective NO Capture from Air

Nitrogen oxides are adverse poisonous gases present in the atmosphere and having detrimental effects on the human health and environment. In this work, we propose a new type of mesoporous materials capable of capturing nitrogen monoxide (NO) from air. The designed material combines the robust Santa Barbara Amorphous-15 silica scaffold and ultrastable Blatter-type radicals acting as NO traps. Using in situ electron paramagnetic resonance spectroscopy, we demonstrate that NO capture from air is selective and reversible at practical conditions, thus making Blatter radical-decorated silica highly promising for environmental applications.

"Upper" Ring Expansion of the Planar Blatter Radical via Photocyclization: Limitations and Impact on the Electronic Structure

Photocyclization of 8-aryloxybenzo[e][1,2,4]triazines leads to the formation of π-expanded flat Blatter radicals for three phenanthryloxy and pyren-1-yloxy derivatives, whereas no photoreaction is observed for the perylen-3-yloxy precursor. Two of the new radicals are nonplanar, out of which one is unstable to isolation. The radical with the fused pyrene ring constitutes the largest thus far paramagnetic polycyclic π-system containing seven fused rings with 27 sp²-hybridized atoms and 29 π-delocalized electrons. The investigation of the reaction conditions demonstrated the higher efficiency of photoformation of the parent radical in polar solvents, which suggests a polar transition state and the S₁ photoreactive state. The effect of π expansion on the electronic structure was investigated with spectroscopic (UV-vis, electron paramagnetic resonance) and electrochemical methods augmented with density functional theory computational studies. The molecular structure of one of the radicals was determined with a single-crystal X-ray diffraction method.

The synthesis and magnetic properties of carboxylic acid-derived 1,2,4-benzotriazinyl radicals and their coordination particles

Stable magnetic coordination particles based on π-conjugated 1,2,4-benzotriazinyl radical ligands were synthesized using a sonochemical method.

Stabilization of radical active species in a MOF nanospace to exploit unique reaction pathways

We synthesized a metal-organic framework (MOF) using a ligand bearing haloalkoxy chains as a radical precursor. The radicals generated in the MOF upon photoirradiation were stable even at 250 K or under an O₂ atmosphere, despite radicals generated from the ligand decomposing at 200 K; thus, the regular arrangement of radicals effectively stabilized them. Moreover, a unique photoproduct was obtained only in the MOF, indicating that the confinement effect in the nanospace enabled a specific reaction that did not occur in the bulk state. We propose a new platform for exploring chemical reactions and materials based on reactive species.

Blatter-Radical-Grafted Mesoporous Silica as Prospective Nanoplatform for Spin Manipulation at Ambient Conditions

Quantum computing and quantum information processing (QC/QIP) crucially depend on the availability of suitable quantum bits (qubits) and methods of their manipulation. Most qubit candidates known to date are not applicable at ambient conditions. Herein, we propose radical-grafted mesoporous silica as a versatile and prospective nanoplatform for spin-based QC/QIP. Extremely stable Blatter-type organic radicals are used, whose electron spin decoherence time is profoundly long even at room temperature (up to Tm ≈2.3 μs), thus allowing efficient spin manipulation by microwave pulses. The mesoporous structure of such composites is nuclear-spin free and provides additional opportunities of embedding guest molecules into the channels. Robustness and tunability of these materials promotes them as highly promising nanoplatforms for future QC/QIP developments.

Substituent effects on the electronic structure of the flat Blatter radical: correlation analysis of experimental and computational data

Functionalized flat Blatter radicals were obtained and substituent effects on spectroscopy, electrochemistry, and stability were investigated by correlation and DFT methods.

Structural Transitions of the Metal-Organic Framework DUT-49(Cu) upon Physi- and Chemisorption Studied by in Situ Electron Paramagnetic Resonance Spectroscopy

Flexible metal-organic frameworks (MOFs) exhibit a variety of phenomena attractive for basic and applied science. DUT-49(Cu) is one of the remarkable representatives of such MOFs, where phase transitions are coupled to pressure amplification and "negative gas adsorption". In this work we report important insights into structural transitions of DUT-49(Cu) upon physi- and chemisorption of gases and volatile liquids obtained by in situ electron paramagnetic resonance (EPR) spectroscopy. In this method, phase transitions are detected via the zero-field splitting in dimeric copper(II) units. First, a new approach was validated upon physisorption of n-butane. Then, using diethyl ether, we for the first time demonstrated that chemisorption can also trigger phase transition in DUT-49(Cu). On the basis of the EPR results, the transition appears completely reversible. The developed EPR-based approach can also be extended to other flexible MOFs containing paramagnetic metal paddlewheels, where high sensitivity and spectral resolution allow in situ studies of stimuli-induced structural variability.

Mitigation of Pressure-Induced Amorphization in Metal-Organic Framework ZIF-8 upon EPR Control

Pressure-induced amorphization is one of the processes inhibiting functional properties of metal-organic frameworks (MOFs). Such amorphization often occurs when MOFs are being shaped for practical applications, as well as during certain exploitations. Typically, the porosity of MOFs, which is crucial for sorption, separation, and catalysis, suffers under external pressure. We report a new experimental approach for efficient monitoring of pressure-induced processes in MOFs that employs trace amounts of spin probes (stable nitroxide radicals) embedded in the pores of MOF and detection by electron paramagnetic resonance (EPR). EPR spectra of spin probes in MOF ZIF-8 demonstrate significant changes upon pressure-induced amorphization, whose extent can be quantitatively determined from the spectral shapes. Moreover, stabilization of ZIF-8 against amorphization via reversible adsorption of various guests was studied using this approach. Mitigation effect depends on diffusion parameters and localization of guest molecules in the cavity, and maintaining of the structure and permeability up to 80% was achieved even at 1.15 GPa applied. Therefore, the proposed methodology allows significant mitigation of MOF amorphization under external pressure and conveys further perspectives of the controlled adjustment of stabilizing agents for various MOFs and their applications.

1-(2-Methoxyphenyl)-3-phenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl: a tricky “structure-to-magnetism” correlation aided by DFT calculations

1-(2-Methoxyphenyl)-3-phenyl-1,4-dihydro-1,2,4-benzotriazin-4-yl (2) is a Blatter radical with a challenging structure-to-magnetism correlation.

Recent advances in the chemistry of benzo[e][1,2,4]triazinyl radicals

Benzo[e][1,2,4]triazinyl, or Blatter radicals, are stable free radicals with customisable magnetic, spectroscopic and electrochemical properties, and wide-ranging applications in synthesis and functional materials.