Stable Nitroxide Research Articles

Kabachnik–Fields reactions with stable nitroxide free radicals

Kabachnik–Fields reactions with stable nitroxide free radicals

TEMPOL inhibits SARS-CoV-2 replication and development of lung disease in the Syrian hamster model

Severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) has caused a worldwide outbreak, known as coronavirus disease 2019 (COVID-19). Alongside vaccines, antiviral therapeutics is an important part of the healthcare response to COVID-19. We previously reported that TEMPOL, a small molecule stable nitroxide, inactivated the RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2 by causing the oxidative degradation of its iron-sulfur cofactors. Here, we demonstrate that TEMPOL is effective in vivo in inhibiting viral replication in the Syrian hamster model. The inhibitory effect of TEMPOL on SARS-CoV-2 replication was observed in animals when the drug was administered 2 h before infection in a high-risk exposure model. These data support the potential application of TEMPOL as a highly efficacious antiviral against SARS-CoV-2 infection in humans.

Silver Nanoparticles Densely Grafted with Nitroxides as a Recyclable Green Catalyst in the Selective Oxidation of Alcohols.

The selective oxidation of alcohols, leading to appropriate aldehydes, is widely recognised as one of the most important reactions in organic synthesis. With ever-increasing environmental concerns, much attention has been directed toward developing catalytic protocols that use molecular oxygen as an oxidant. An ideal green oxidation process should employ a highly active, selective and recyclable catalyst that can work with oxygen under mild conditions. This paper presents a successful application of densely grafted silver nanostructures with stable nitroxide radicals (N-AgNPs) as an effective, easily-recovered and regenerable catalyst for the selective oxidation of alcohols. The fabricated ultra-small and narrow dispersive silver nanoparticles have been fully characterised using physicochemical methods (TEM, DLS, XPS, TGA). N-AgNPs have been successfully applied to oxidise several model alcohols: benzyl alcohol, 4-pyridinemethanol, furfuryl alcohol, 1-phenyl ethanol, n-heptanol and allyl alcohol under mild conditions using oxygen as a stoichiometric oxidant. Notably, the fabricated nitroxide grafted silver nanoparticles (N-AgNPs) were reused more than ten times in the oxidation of a series of primary alcohols to corresponding aldehydes under mild conditions with very high yields and a selectivity close to 100%.

N-Fluoroalkylpyrazolyl-substituted Nitronyl Nitroxides

The aim of the study was to prepare and investigate structure and properties of a new series of stable nitronyl nitroxides bearing N-fluoroalkylpyrazolyl substituents. According to cyclic voltammetry data, a paramagnet with a difluoromethyl substituent can reversibly undergo electrochemical oxidation and reduction. In contrast, radicals with fluorinated ethyl substituents are electrochemically oxidized reversibly, but their electrochemical reduction is quasi-reversible. Molecular and crystal structures of the nitroxides were confirmed by single-crystal X-ray diffraction analysis. The crystal structures of the radicals were studied quantitatively in terms of pairwise interaction energies and energy framework analysis. Analysis of the energies of molecular pairs revealed that in crystalline phases of all nitroxides, the binding is the strongest in dimers assembled via C–H…O interactions of nitroxide O atoms with pyrazole and/or alkyl H atoms. Other dimers are formed predominantly by C–H…π interactions, and in some cases, their binding energy is almost equal to that for the aforementioned ones. The above contacts are complemented short contacts C–H…A and others, which predetermine the final crystal packing of radicals. A consequence of their packing is the geometry of relative arrangement of nitroxide groups and, above all, distances O…O between the oxygen atoms of the paramagnetic centers. These distances were found to exceed the sum of van der Waals radii; consequently, in solid phases of the paramagnets, weak antiferromagnetic exchange interactions are predominant (|J/kB| ≤ 2.74 K, H = –2J·S1S2.

Stable nitronyl nitroxide monoradical MATMP as novel monomer of reversible addition fragmentation chain transfer (RAFT) polymerization for ultrasensitive DNA detection

In this work, it came up a hydrophilic and stable nitronyl nitroxide monoradical 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine 1-oxyl freeradical (MATMP) as new monomer of polymerization for DNA detection. The detection limit was over 1,000,000 times lower than 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) derivatives as electrochemical labels alone. Within this approach, a single biomolecule can be converted into the strong electrochemical signal, therefore lung cancer DNA can be detected at low concentration. For the first step, the HS-PNA probe was fixed on the surface of the Au electrode. After the target DNA was captured by complementary base pairing, 4-cyano-4-(phenylcarbonothioylthio) pentanoic acid (CPAD), chain transfer agent of RAFT polymerization, was bonded to target DNA as reaction via coordination bond of Zr4+. Electroactive polymers had grown by means of surface initiated thermally RAFT polymerization with MATMP as monomer. MATMP polymer significantly improves the electrochemical signal. This method can detect DNA from 0.01 fM to 10 pM, and detection limit is 1.51 aM. The sensitivity of this method is greater than that in most other reported signal amplification strategies of DNA biosensor, which indicates that it is appropriate for single nucleotide polymorphism analysis and will broaden prospects for biological molecules detection application.

A novel nitronyl nitroxide radical HPN-C6 attenuates brain damage in an acute hypobaric hypoxia mouse model through inhibition of the oxidative stress

Hypobaric hypoxia (HH) results in the increased formation of ROS and induces a wide range of deleterious effects on the brain. Nitronyl nitroxide radicals are a kind of stable organic nitroxide radicals with a good ability to scavenge ROS. Herein, a series of new nitronyl nitroxide radicals was synthesized according to the previous method. Among them, a compound with six carbon atoms alkyl chain (HPN-C6) was screened for estimating the protective effect against HH-induced brain injury in the acute HH mouse model. The results showed that HPN-C6 ameliorated oxidative stress by decreasing the activity of LDH and the content of H2O2, NO, and MDA, increasing the activities of SOD, CAT and GSH-PX in the brain tissue of HH injured mice. Moreover, HPN-C6 exhibited an anti-inflammatory effect by reducing the level of IL-1β, TNF-α, and IL-6 in the brains of mice after HH exposure through regulating the P38 MAPK signaling pathway. In addition, HPN-C6 exhibited the protection effect by inhibiting the release of mitochondrial cytochrome C, locking the activation of caspase-3, and downregulating the expression of Bax and upregulating the expression of Bcl-2. HPN-C6 pretreatment could inhibit the caspase-dependent mitochondria apoptosis pathway. These findings indicate that HPN-C6 is a potential therapeutic agent to prevent brain damage induced by HH.

SYNTHESIS, CRYSTAL STRUCTURES AND MAGNETIC PROPERTIES OF COORDINATION COMPOUNDS WITH NITRONYL NITROXIDE RADICALS AND [M(HFAC)2] (M = CUII AND MNII)

The reaction of metal hexafluoroacetylacetonato [MII(hfac)2, M = Cu, Mn] with the stable nitronyl nitroxide 2−(3−isobutyl−pyrazole)−4,4,5,5−tetramethylimidazoline−1−oxyl−3−oxide (L), resulted in one dimensional zig−zag chain systems [{Cu(hfac)2}2L2]n and [CuMn(hfac)4L2]n. The main feature inherent in the nature of [{Cu(hfac)2}2L2]n single crystals is their ability to undergo reversible structural rearrangements with temperature variation, accompanied by anomalies of magnetism. The value of cmT shown strongly antiferromagnetic at low temperature and becomes ferromagnetic when the temperature increases. And the heteronuclear complex [CuMn(hfac)4L2]n shows antiferromagnetic interactions between manganese and nitronyl nitroxide.

Cyclic Hydroxylamines as Monitors of Peroxynitrite and Superoxide-Revisited

There is a considerable need for methods that allow quantitative determination in vitro and in vivo of transient oxidative species such as peroxynitrite (ONOOH/ONOO−) and superoxide (HO2•/O2•−). Cyclic hydroxylamines, which upon oxidation yield their respective stable nitroxide radicals, have been suggested as spin probes of peroxynitrite and superoxide. The present study investigated this approach by following the kinetics of peroxynitrite decay in the absence and presence of various 5-membered and 6-membered ring hydroxylamines, and comparing the yield of their respective nitroxides using electron paramagnetic spectroscopy. The results demonstrate that hydroxylamines do not react directly with peroxynitrite, but are oxidized to their respective nitroxides by the radicals formed during peroxynitrite self-decomposition, namely •OH and •NO2. The accumulated nitroxides are far below their expected yield, had the hydroxylamines fully scavenged all these radicals, due to multiple competing reactions of the oxidized forms of the hydroxylamines with •NO2 and ONOO−. Therefore, cyclic hydroxylamines cannot be used for quantitative assay of peroxynitrite in vitro. The situation is even more complex in vivo where •OH and •NO2 are formed also via other oxidizing reactions systems. The present study also compared the yield of accumulated nitroxides under constant flux of superoxide in the presence of various cyclic hydroxylamines. It is demonstrated that certain 5-membered ring hydroxylamines, which their respective nitroxides are poor SOD-mimics, might be considered as stoichiometric monitors of superoxide in vitro at highest possible concentrations and pH.

Overhauser Dynamic Nuclear Polarization Parameters of a Nitroxide Radical in Liquid

Solution state Overhauser-effect-type dynamic nuclear polarization (ODNP) has been studied in the last years. The basis of ODNP is a polarization mechanism that presents nuclei with electron spin polarization. ODNP enhancements studies are presented 4-fluorobenzyl alcohol (4FBA) with the stable nitroxide radical as the polarizing agent for 15, 30 and 60 mM concentrations at 1.53 mT. Major ODNP parameters were obtained. The significant parameter is the coupling parameter, since it determines the maximum NMR signal enhancements and the interactions between the nuclear spin and the electron spin. The coupling parameter can vary from -0.152 to 0.001. The obtained ODNP parameters show that both scalar and dipolar interactions occur. The results show that nitroxide radical can be taken as polarizing agent for ODNP studies of 4-fluorobenzyl alcohol solvent. The effect of dissolved molecular oxygen on the enhancement factor for the sample with the smallest concentration was experimentally investigated.

Magnetic Resonance and Computational Studies on Crossover Reactions in Li-Air Batteries and Redox-Flow Batteries Using TEMPO

Redox active solution species are important in a variety of electrochemical energy storage systems. In lithium-air batteries, redox mediators are added as soluble catalysts that mitigate charge overpotentials as well as promote solution-phase reactions that improve the cycle life of a cell. Redox flow batteries are also dependent on solution redox species to carry charge and can attain high energy capacities by increasing the concentrations of these species. In both of these systems, crossover reactions, where the redox active species diffuse from one side of the cell to the other, result in capacity loss and parasitic side reactions. Here, we report a method to monitor crossover reactions using online nuclear magnetic resonance (NMR) and electron paramagnetic resonance (EPR) spectroscopy. 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) is a stable nitroxide radical used as a redox mediator in lithium-air batteries and charge species redox flow batteries. Radical concentrations can be probed and quantified during electrochemical cycling by flowing the electrolyte through the NMR and EPR spectrometers. The experimental results reveal severe crossover and self-reduction during electrochemical oxidation of TEMPO. Diffusion models performed support the in-situ data, highlighting that crossover is a major phenomenon in these systems. Figure 1

A Versatile Light‐Triggered Radical‐Releasing Surface Coating Technology

AbstractOrganic based redox active materials and coatings are increasingly being investigated as solutions for more efficient energy devices, catalysts, sensors, and biomedical technologies. Their advantage is negating the necessity for expensive and unsustainable rare earth metals seen in other redox active materials. Challenges to their wide‐spread usage remain, including versatility of coatings on a wide range of substrates, and creating smart devices that can respond to environmental stimuli. A new light responsive radical releasing redox coating has been developed, that can be rapidly and stably applied to a diverse range of substrates including silicon dioxide, plastics, and microparticles. These coatings rapidly cleave upon irradiation with UV‐A light to release stable nitroxides from the surface coating. These coatings are developed into microparticle sensors for radical oxygen species, which are able to detect the generation of free radicals in a model system for particulate matter pollution. These new responsive organic redox coatings also offer future potential to function as switchable organic electrodes, and biomedical surface coatings to prevent biofouling and reduce inflammation.

Acidic and Electrosurface Properties of Binary TiO2-SiO2 Xerogels Using EPR of pH-Sensitive Nitroxides.

The binary xerogels TiO2-SiO2 are widely used as catalysts and their carriers in organic synthesis. Characterization and adjustment of the electrostatic properties of the surface and the local acidity inside the pores, are necessary for the further development of TiO2-SiO2 xerogels applications. This research investigates acid–base equilibria in the pores, and the surface electrostatic potential (SEP) of binary TiO2-SiO2 xerogels, by the EPR of stable pH-sensitive nitroxide radicals. These radicals are small enough to penetrate directly into the pores, and to be adsorbed onto the surface of the material under study. This makes it possible to obtain valuable information on the acidic and electrosurface properties of the studied system. The highest negative surface electrical charge associated with surface electrical potential (SEP) was equal to −196 ± 6 mV. It was induced by the surface of the sample with a 7% TiO2 content. The local acidity inside the pores of this sample was found to be higher, by approximately 1.49 pH units, as compared to that in the external bulk solution.

Guest Leakage from ZIF-8 Particles under Drug Delivery Conditions: Quantitative Characterization and Guest-Induced Framework Stabilization

Metal–organic frameworks (MOFs) are promising nanomaterials for the construction of stimuli–responsive drug delivery platforms. The counterpart of the incredible sensitivity of MOFs to pathological conditions, which underpins targeted drug release, is the significant instability of nanoparticles even in normal physiological media. We report a new experimental approach for quantitative characterization of the guest release profile from MOF ZIF-8 (zeolitic imidazolate framework) suspensions under drug delivery conditions. For this sake, we used ZIF-8 with encapsulated stable nitroxides, which was exposed to physiological media, and the in situ kinetics of guest escape was monitored by electron paramagnetic resonance (EPR). The release of spin probes drastically changes the EPR spectral shapes, thus allowing quantitative characterization of leaked species. We demonstrate that the guest release from ZIF-8 particles in blood-mimicking solutions is a concentration-dependent process dictated by albumin, and nearly full guest release from nanoparticles in fetal bovine serum (FBS) occurs within a few minutes. Significant deceleration of this process and stabilization of ZIF-8 in FBS can be achieved by additional adsorption of guest inhibitors (exemplified by aromatic hydrocarbons) prior to contact with physiological media. The proposed methodology provides further perspectives for optimization of MOF-based platforms for efficient drug delivery.

In Vivo Brain Redox Imaging in Transgenic-Knockout Sickle Mice

BACKGROUND: Sickle RBCs generate excessive reactive oxygen species (ROS) due to increased HbS auto-oxidation and cell lysis. High plasma ROS lead to increased oxidative stress in the brain, where oxidative stress is linked with the etiology of a host of cerebral disorders. Noninvasive assessment of cerebral oxidative stress could be useful in understanding how ROS affects the brain in sickle cell disease. We examined a blood-brain barrier-permeable and stable nitroxide, methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-1-oxyl (MCP), which is a paramagnetic T1-shortening MRI contrast agent in its oxidized state. MCP participates in cellular redox reactions (is reduced by free radicals, ubiquinols, ascorbate, glutathione (GSH), etc.) and once reduced, it loses its ability to generate MRI contrast (becomes diamagnetic). MCP is highly lipophilic, and enters the brain rapidly after intravenous infusion. The rate of loss of contrast within the brain tissue after intravenous infusion is related to tissue ROS load. We employed MCP to evaluate the cerebral ROS levels in transgenic-knockout sickle mice.METHODS: BERK transgenic mice (n=3) and C57BL/6 (n=3, WT) were studied by MRI at 9.4 Tesla under isoflurane anesthesia. MCP was administered intravenously as a bolus (25 ul/s of an 85 mmol/l solution in dionized water) at a dose of 0.5 mmol/kg BW (one WT was IVI at 0.4 mmol/kg BW). Dynamic T1 inversion recovery (T1IR) imaging (20 seconds/T1 assessment) was performed prior to (for 6 minutes) and after (for 27 minutes) MCP infusion. T1 relaxivity kinetics were fit using 2-compartment model. Cerebral blood flow (CBF) and cerebral metabolite concentration were estimated in a different group of BERK (n=10) and control mice (n=11) (Table 2) via in vivo MRI/MRS approach (same anesthesia). In vitro assessment of the MCP T1-dependence on concentration permitted in vivo determination of MCP concentration (data not shown).RESULTS: MRI signal intensity peaked immediately after the injection of MCP (Figure 1a), due to MCP's T1 shortening effect (Figure 1b). In control mouse at a MCP dose of 0.5 mmol/kg BW, signal increased ~80% and ~100% immediately after MCP infusion in the cortex (COR) and thalamus (THAL) regions, respectively. In BERK mice, the signal peaked at lower levels than in control mice.The rate at which T1 signal recovered in BERK mice was significantly more rapid than in control mice (as shown by the steeper slope of the T1 recovery curve, indicated by the blue line in Figure 1 c, d). Signal recovery was prolonged in the BERK mice, with a slow second phase of recovery evident 200 seconds after MCP infusion (note progression of recovery after the gray dotted verticle line). The fast phase of T1 recovery in BERK cortex was double that of control mice (Table 1), and a similar trend was also observed in thalamus. A faster rate of recovery is attributed to increased MCP reduction. Peak intensity, but not rate of recovery, was affected by lower dose in the one animal (Table 1).Conslusion and Discussion: In the blood, MCP reduction begins immediately after IVI, suggesting that some MCP never reaches the brain in the oxidized (paramagnetic) state. This determines the “peak” contrast change observed, which was lower in the BERK mice with higher blood ROS. In the brain, MCP is reduced by various reductants, forming hydroxylamine and/or oxoammonium (Zhelev Z 2013 ACS Chem Neurosci), loosing its paramagnetism, so T1 returns to pre-infusion levels. Notably, the T1 recovery rate in BERK mice suggested rapid reduction of the MCP in comparison to WT mice, implying a significantly higher oxidative stress load in BERK.The MCP reduction rate is the sum of two opposing processes, i.e., the reduction of MCP to hydroxylamine and the reoxidation of hydroxylamine by ·OH to MCP, especially under iron overload (Okajo A 2009 Biol Pharm Bull). This reoxidation of hydroxylamine to MCP may explain the slow second phase predominantly observed in BERK mouse (Figure 2c). This also indicates higher free radical levels in BERK mouse brain. GSH does not appear to play a major role in MCP contrast since GSH concentration in BERK mouse brain is not different from that in control mouse as estimated using in vivo MRS (Table 2).We have demonstrated for the first time increased oxidative stress in the sickle BERK mouse brain using in vivo redox imaging. These methods can provide a means to monitor the effect of antioxidants or therapy upon brain pathology in SCD mice.. [Display omitted] DisclosuresAcharya:AIMA: Consultancy. Billett:Janssen Pharmaceutical: Research Funding.

Five‐Membered Ring Nitroxide Radical: A New Class of High‐Potential, Stable Catholytes for Neutral Aqueous Organic Redox Flow Batteries

AbstractThe utilization of redox‐active and stable cyclic nitroxide radicals (CNRs) holds a great promise in neutral aqueous organic redox flow batteries (AORFBs) for large‐scale energy storage. Herein, a new class of CNRs with five‐membered ring pyrrolidine and pyrroline motifs for AORFBs is reported. By rational molecular engineering of introducing CC double bond into the pyrrolidine‐based molecule, 3‐carbamoyl‐2,2,5,5‐tetramethylpyrroline‐1‐oxyl (CPL) with a high redox potential of 0.76 V (vs Ag/AgCl) is demonstrated, which is 160 mV higher than the common 2,2,6,6‐tetramethylpiperidine 1‐oxyl derivatives with a six‐membered ring as the core structure. Density functional theory calculations reveal that the much enhanced redox potential for CPL is largely contributed by lowered standard free energy in reduction reaction and charge population sum of NO radical head. When paired with the BTMAP‐viologen anolyte, the CPL‐based AORFB delivers constant capacity retention of up to 99.96%/cycle over 500 cycles.

Exploring Photogenerated Molecular Quartet States as Spin Qubits and Qudits.

Photogenerated molecular spin systems hold great promise for applications in quantum information science because they can be prepared in well-defined spin states at modest temperatures, they often exhibit long coherence times, and their properties can be tuned by chemical synthesis. Here, we investigate a molecular spin system composed of a 1,6,7,12-tetra(4-tert-butylphenoxy)perylene-3,4:9,10-bis(dicarboximide) (PDI) chromophore covalently linked to a stable nitroxide radical (TEMPO) by optical and electron paramagnetic resonance (EPR) techniques. Upon photoexcitation of the spin system, a quartet state is formed as confirmed by transient nutation experiments. This quartet state has spin polarization lifetimes longer than 0.1 ms and is characterized by relatively long coherence times of ∼1.8 μs even at 80 K. Rabi oscillation experiments reveal that more than 60 single-qubit logic operations can be performed with this system at 80 K. The large magnitude of the nitroxide 14N hyperfine coupling in the quartet state of PDI-TEMPO is resolved in the transient EPR spectra and leads to a further splitting of the quartet state electron spin sublevels. We discuss the properties of this photogenerated multilevel system, comprising 12 electron-nuclear spin states, in the context of its viability as a qubit for applications in quantum information science.

Application of Redox-Responsive Hydrogels Based on 2,2,6,6-Tetramethyl-1-Piperidinyloxy Methacrylate and Oligo(Ethyleneglycol) Methacrylate in Controlled Release and Catalysis.

Hydrogels have reached momentum due to their potential application in a variety of fields including their ability to deliver active molecules upon application of a specific chemical or physical stimulus and to act as easily recyclable catalysts in a green chemistry approach. In this paper, we demonstrate that the same redox-responsive hydrogels based on polymer networks containing 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO) stable nitroxide radicals and oligoethylene glycol methyl ether methacrylate (OEGMA) can be successfully used either for the electrochemically triggered release of aspirin or as catalysts for the oxidation of primary alcohols into aldehydes. For the first application, we take the opportunity of the positive charges present on the oxoammonium groups of oxidized TEMPO to encapsulate negatively charged aspirin molecules. The further electrochemical reduction of oxoammonium groups into nitroxide radicals triggers the release of aspirin molecules. For the second application, our hydrogels are swelled with benzylic alcohol and tert-butyl nitrite as co-catalyst and the temperature is raised to 50 °C to start the oxidation reaction. Interestingly enough, benzaldehyde is not miscible with our hydrogels and phase-separate on top of them allowing the easy recovery of the reaction product and the recyclability of the hydrogel catalyst.

Fluorescence Quenching Studies on the Interactions between Chosen Fluoroquinolones and Selected Stable TEMPO and PROXYL Nitroxides.

The influence of the stable 2,2,6,6-tetramethylpiperidinyl-N-oxyl (TEMPO) nitroxide and its six C4-substituted derivatives, as well as two C3-substituted analogues of 2,2,5,5-tetramethylpyrrolidynyl-N-oxyl (PROXYL) nitroxide on the chosen fluoroquinolone antibiotics (marbofloxacin, ciprofloxacin, danofloxacin, norfloxacin, enrofloxacin, levofloxacin and ofloxacin), has been examined in aqueous solutions by UV absorption as well as steady-state and time-resolved fluorescence spectroscopies. The mechanism of fluorescence quenching has been specified and proved to be purely dynamic (collisional) for all the studied systems, which was additionally confirmed by temperature dependence experiments. Moreover, the selected quenching parameters—that is, Stern–Volmer quenching constants and bimolecular quenching rate constants—have been determined and explained. The possibility of electron transfer was ruled out, and the quenching was found to be diffusion-limited, being a result of the increase in non-radiative processes. Furthermore, as the chosen nitroxides affected the fluorescence of fluoroquinolone antibiotics in different ways, an influence of the structure and the type of substituents in the molecules of both fluoroquinolones and stable radicals on the quenching efficiency has been determined and discussed. Finally, the impact of the solvent’s polarity on the values of bimolecular quenching rate constants has been explained. The significance of the project comes from many applications of nitroxides in chemistry, biology and industry.

Fluorinated Organic Paramagnetic Building Blocks for Cross-Coupling Reactions

New stable polyfluorinated nitroxide radicals for use in cross-coupling reactions, namely, N-tert-butyl-N-oxyamino-2,3,5,6-tetrafluoro-4-iodobenzene and N-tert-butyl-N-oxyamino-2,3,5,6-tetrafluoro-4-ethynylbenzene, were prepared from perfluoroiodobenzene. The reaction of the polyfluoro derivative with tert-butylamine under autoclaving conditions leading to the formation of N-tert-butyl-2,3,5,6-tetrafluoro-4-iodoaniline proved to be the key stage of the whole process. The fluorinated tert-butyl iodophenyl nitroxide was found to form in a solid state via N–O···I halogen bonds, a one-dimensional assembly of the radicals. The acceptor role of the nitroxide group in the halogen bonding changes to a donor role when the nitroxide reacts with Cu(hfac)2. In the last case, zero-dimensional assembly prevails, giving a three-spin complex with axial coordinated nitroxide groups and, as a consequence, causing ferromagnetic intramolecular exchange interactions between Cu(II) and radical spins.

Plasma polymerization of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl in a collisional, capacitively coupled radio frequency discharge.

Plasma polymerization of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) yields thin films containing stable nitroxide radicals that have properties analogous to that of nitric oxide (NO) without short lifetimes. This property gives TEMPO films a wide variety of potential applications. Typically, control of the final film chemistry is difficult and the plasma discharge conditions must be tailored to in order to maximize the retention of these nitroxide groups during the polymerization and deposition process. In this study, plasma diagnostics and surface analysis of the deposited films were carried out to determine the optimal plasma conditions for the retention of nitroxide groups. These techniques included energy-resolved mass spectrometry, heated planar probe ion current measurements, deposition rate measurements, and x-ray photoelectron spectroscopy (XPS). Results show that operating the plasma with a combination of low input powers and high pressures produces a collisional discharge in which fragmentation of the TEMPO molecule is suppressed, leading to good retention of nitroxide groups. Ion energy distribution functions and quartz crystal microbalance measurements support the soft landing theory of ion deposition on the substrate within this γ-mode, in which the flux of low energy, soft landed ions form the primary contribution to film growth. XPS analysis of deposited polymers shows 75.7% retention of N-O groups in the polymer films deposited in a 25 Pa 5 W discharge.