High-spin products of photolysis of 1,3,5-triazido-2,4,6-trichlorobenzene in a 2-methyltetrahydrofuran solution frozen at 6 K has been studied using EPR spectroscopy in combination with quantum-chemical calculations. It has been found that the photolysis of this triazide leads to the formation of a mixture of triplet 1,3-diazido-2,4,6-trichlorophenyl-5-nitrene, quintet 1-azido-2,4,6-trichlorophenyl-3,5-dinitrene, and septet 2,4,6-trichlorophenyl-1,3,5-trinitrene. The solvent effect on the EPR spectra and the magnetic characteristics of nitrenes has been studied.

We report on W-band EPR and quantum chemical investigation of novel organic tetraradicals with negative axial zero-field splitting (ZFS) parameter D. These belong to the class of quintet 1,3,5-tribromophenylene-2,4-dinitrenes bearing different substituents in position 6 of the benzene ring (1b, N3; 1c, F; 1d, CN; 1e; Cl; 1f, Br). Analysis of the W-band EPR spectrum of dinitrene 1c reveals its large negative ZFS parameter D = -0.27 cm-1. Quantum chemical calculations show that negative D gradually grows in the row of 1c(F) < 1b(N3) < 1d(CN) < 1e(Cl) < 1f(Br) dinitrenes due to decreasing of the through-space distance between the nitrene units and neighboring bromine atoms. Shorter steric N···Br distance results in the stronger contribution of the spin-orbit coupling (SOC) to the total ZFS. The sign of D depends on the interplay of three factors: (i) the angle θ between the "easy" z-axes of the dipolar spin-spin (DSS) and spin-orbit (DSOC) interaction tensors, (ii) the ratio of DSOC/DSS values, and (iii) the rhombicity parameters ESS/DSS and ESOC/DSOC. The study demonstrates in which cases organic quintet tetraradicals may have negative ZFS owing to the presence of heavy atoms at appropriate sites nearby the nitrene units and, thus, possess the bistability property as single-molecule magnets.

The first X-band EPR spectrum containing only non-overlapping signals of septet pyridyl-2,4,6-trinitrene and triplet pyridylnitrenes is reported. This spectrum was recorded after photolysis of 2,4,6-triazidopyridine in solid argon at 5 K. The zero-field splitting (ZFS) parameters of this trinitrene as well as of intermediate triplet mononitrenes and quintet dinitrenes formed at early stages of the photolysis were determined using the combination of modern computer line-shape spectral simulations and density functional theory (DFT) calculations. It was found that septet pyridyl-2,4,6-trinitrene has the record negative parameter D-S = -0.1031 cm(-1) among all known to date septet pyridyl-2,4,6-trinitrenes and may be of interest as a model multi-qubit spin system for investigations of quantum computation processing.

Among all C-, N-, and O-centered polyradicals, high-spin nitrenes possess the largest magnetic anisotropy and are of considerable interest as multi-level molecular spin systems for exploration of organic molecular magnetism and quantum information processing. Although the first representatives of quintet and septet nitrenes were obtained almost 50 years ago, the experimental and theoretical studies of these highly reactive species became possible only recently, owing to new achievements in molecular spectroscopy and computational chemistry. Meanwhile, dozens of various quintet dinitrenes and septet trinitrenes were successfully characterized by IR, UV/Vis, and EPR spectroscopy, thus providing important information about the electronic structure, magnetic properties and reactivity of these compounds.

Experimental and theoretical studies on aromatic nitrenes bearing from three to six unpaired electrons and having quartet, quintet, sextet or septet ground spin states, published in the last 15 years are analyzed. A comparative analysis of the magnetic properties of high-spin nitrenes and all other known high-spin organic molecules is performed. Promising areas of practical application of high-spin nitrenes as molecular magnets and as qubits and qudits for quantum computations are discussed. The bibliography includes 214 references.

Di-, tri- and tetraazido-substuituted azines are of considerable interest as high-energy organic materials and precursors of carbon nitride nanomaterials as well as photoactive cross-linking agents in polymer chemistry and molecular biology, photoresists in microelectronics and as starting compounds in organic synthesis and photochemistry. This review is devoted to the recent progress in the synthesis and chemical transformations of various azines bearing several azido groups in the azine ring.

Complex multicomponent, multispin molecular system, consisting of a septet trinitrene, two quintet dinitrenes, and three triplet mononitrenes, was obtained by the photolysis of 2,4,6‐triazido‐3‐cyano‐5‐fluoropyridine in solid argon. To identify these paramagnetic products, electron paramagnetic resonance spectroscopy in combination with line‐shape spectral simulations and density functional theory calculations was used. The products of the photolysis was found to be triplet 2,4‐diazido‐3‐cyano‐5‐fluoropyridyl‐6‐nitrene ( D T = 1.000 cm −1 , E T = 0), triplet 2,4‐diazido‐3‐cyano‐5‐fluoropyridyl‐2‐nitrene ( D T = 1.043 cm −1 , E T = 0), triplet 2,6‐diazido‐3‐cyano‐5‐fluoropyridyl‐4‐nitrene ( D T = 1.128 cm −1 , E T = 0 cm −1 ), quintet 4‐azido‐3‐cyano‐5‐fluoropyridyl‐2,6‐dinitrene ( D Q = 0.211 cm −1 , E Q = 0.0532 cm −1 ), quintet 2‐azido‐3‐cyano‐5‐fluoropyridyl‐4,6‐dinitrene ( D Q = 0.208 cm −1 , E Q = 0.0386 cm −1 ), and septet 3‐cyano‐5‐fluoropyridyl‐2,4,6‐trinitrene ( D S = −0.1017 cm −1 , E S = −0.0042 cm −1 ) in a 38:4:7:22:14:4 ratio, respectively.

The septet ground state trinitrenes 1,3,5-trichloro-2,4,6-trinitrenobenzene and 1,3,5-tribromo-2,4,6-trinitrenobenzene were isolated in inert (Ar, Ne, and Xe) as well as reactive matrices (H-2, O-2, and H2O) at cryogenic temperatures. These trinitrenes were obtained in high yields by UV photolysis of the corresponding triazides and characterized by IR and UV/Vis spectroscopy. The trinitrenes, despite bearing six unpaired electrons, are remarkably unreactive towards molecular oxygen and hydrogen and are persistent in water ice up to 160 K where the water matrix starts to sublime off.

Surprisingly stable organic trinitrenes were synthesized in almost quantitative yields from their triazide precursors. In their Communication on page 12994 ff. W. Sander et al. show that the trinitrenes with six unpaired electrons and septet electronic ground states can be synthesized in inert matrices and in amorphous water at cryogenic temperatures. Under these conditions they are stable towards oxygen, hydrogen, and water, which makes these molecules promising building blocks for the design of organic magnetic materials.

A number of di-, tri-, and tetraazido-substituted azines as potential energetic dispersing agents of solid propellants for ramjet engines have been studied. The enthalpy of combustion and the enthalpy of formation of several azides (2,4,6-triazidopyrimidine, 2,4,6-triazidopyridine, 3,4,5-triazidopyridine-2,6-dicarbonitrile, and 3,4,5,6-tetraazidopyridine-2-carbonitrile) were experimentally determined. Eleven azides were compared with HMX in terms of the enthalpy of combustion in oxygen to CO2 and water (in the case of the presence of hydrogen in the component), as well as in terms of the temperature of the products of adiabatic conversion of the investigated components due to the high enthalpy of formation in the absence of an external oxidizer and the amount of gases released in this process. The enthalpy of combustion of all the investigated azides burned in air was found to be significantly higher than that of HMX, and for seven of the azides studied, the combustion temperature is significantly higher. As regards the gas release volume (24–31 mol/kg), the azides are inferior to HMX (41.9 mol/kg). Based on the combination of properties, the investigated azides can be considered as promising dispersing agents of solid propellants for ramjet engines.

Überraschend stabile organische Trinitrene wurden in nahezu quantitativen Ausbeuten aus ihren Triazidvorläufern synthetisiert. W. Sander et al. zeigen in ihrer Zuschrift auf S. 13128 ff., dass die Trinitrene mit sechs ungepaarten Elektronen und elektronischen Grundzuständen in inerten Matrizen, aber auch in amorphem Wasser bei kryogenen Temperaturen synthetisiert werden können. Unter diesen Bedingungen erwiesen sie sich als stabil gegenüber Sauerstoff, Wasserstoff und Wasser, was diese Moleküle zu vielversprechenden Bausteinen für die Entwicklung organischer magnetischer Materialien macht.

The photolysis of 4,6-diazido-N-(4,6-diazido-1,3,5-triazin-2-yl)-1,3,5-triazin-2-amine in argon matrices at 5 K has been studied, using X-band EPR spectroscopy in combination with modern line-shape spectral simulations and density functional theory computations. It was found that the photolysis of the model tetraazide occurs selectively to give only triplet 4-azido-6-nitreno-N-(4,6-diazido-1,3,5-triazin-2-yl)-1,3,5-triazin-2-amine (D =1.45 cm(-1), E = 0.0045 cm(-1)) and quintet 4,6-dinitreno-N-(4,6-diazido-1,3,5-triazin-2-yl)-1,3,5-triazin-2-amine (D = 0.276 cm(-1), E = 0.058 cm(-1)). The latter does not undergo further photolysis in the argon matrices since inappropriate spatial orientation of the second diazido-substituted triazine ring toward the UV source. The results obtained provide new data on photochemistry of aromatic tetraazides, formulating more strict requirements to the starting compounds as potential precursors of high-spin polynitrenes.

Upon electron impact, tetraazidopyridine-2-carbonitrile releases cyanogen to form tetraazidocyclobutadiene radical cation as the first representative of azido-substituted cyclo-butadiene derivatives.

2,4,6-Triazidopyrimidine-5-carbonitrile was obtained in 82% yield by the reaction of 2,4,6-trichloropyrimidine-5-carbonitrile with sodium azide in aqueous acetone and characterized with X-ray diffraction, UV, IR, 13 C and 15 N NMR spectroscopy, electron impact mass-spectrometry and quantum-chemical calculations. Compared to known 2,4,6-triazidopyrimidines, the triazide obtained has an unusual crystal structure, high melting point, high positive heat of formation and moderate sensitivity to impact and friction.

The kinetics and products of the thermal decomposition of 2,4,6-triazido-3,5-difluoropyridine in melt at temperatures of 120–160°C have been studied using pressure measurements, differential thermal analysis, and IR spectroscopy. The reaction occurs in two macroscopic steps, each described by a first-order kinetic equation. In the first, the activation energy and the common logarithm of the pre-exponential factor are 35.6 ± 1.2 kcal/mol and 15.1 ± 0.6 s–1, respectively. For the studied compound (and certain other heterocyclic azides, such as 2,4,6-triazido-1,3,5-triazine and 2,4,6-triazidopyrimidine), the pre-exponential factor is anomalously high. This is due to the fact that the 2,4,6-triazido-3,5-difluoropyridine molecule contains no hydrogen atoms. For such azides, the usual decomposition mechanism (cleavage of the azide group with subsequent attack of nitrene on a hydrogen atom of a neighboring molecule) is impossible, which favors the stability of the formed nitrene. In this case, the reaction occurs through a complex chain polymerization mechanism, which leads to the formation of specific condensed products: packs of planar polyconjugate carbon–nitrogen networks with a porphyrin-like structure.

The thermolysis of 2,4,6-triazido-1,3,5-triazine ( I ), 2,4,6-triazidopyrimidine ( II ), and 2,4,6-triazidopyridine ( III ) and its products were studied by DSC, mass spectrometry, IR spectroscopy, and electron microscopy. The thermal transformations of I gave planar nets formed by polyconjugated C–N bonds arranged into bundle aggregates. The thermolysis product of III consists of low-molecular compounds and has globular morphology. The thermolysis of II resulted in a mixture of products of both types, among which the planar nets were dominant. The relationship between the structure of the products of the thermal transformations of I , II , and III and the kinetic characteristics of these processes was discussed.

Previously unknown the steric heavy atom effect on magnetic anisotropy parameters of triplet phenyl nitrenes is reported. The heavy bromine atom effect is revealed by W-band EPR and theoretical investigations of triplet 2,4,6-tribromophenyl nitrenes bearing different substituents in positions 3 and 5 of the phenyl ring (1a, H/H; 1b, CN/CN; 1c, N3/F; 1d, N3/N3; 1e, Cl/Cl; 1f, Br/Br). The zero-field splitting parameters of nitrenes 1a ( D = 0.9930 cm-1, E = 0.0261 cm-1), 1c ( D = 1.244 cm-1, E = 0.030 cm-1), and 1d ( D = 1.369 cm-1, E = 0.093 cm-1), generated by the photolysis of the corresponding azides in frozen methylcyclohexane solution at 5 K, were determined from the W-band EPR spectra. To clarify the origin of considerable differences in the experimental D values of nitrenes 1a, 1c, and 1d, extensive DFT and CASSCF calculations of these nitrenes as well as of model nitrenes 1b, 1e, and 1f were performed. The calculations show that all nitrenes have nearly the same magnitudes of the spin-spin interactions ( DSS ∼ 1 cm-1), but drastically differ in the spin-orbit coupling parameter (from DSOC = 0.087 cm-1 for 1a to DSOC = 0.765 cm-1 for 1f). Comprehensive analysis of various computational data showed that the magnitude of DSOC of nitrenes 1a-f is the function of the N···Br distance between the nitrene nitrogen and the neighboring bromine atoms. The more bulky substituents are located in positions 3 and 5 of nitrenes 1a-1f, the smaller the N--Br distance and the larger DSOC. These features indicate that the heavy atom effect on magnetic anisotropy of triplet phenyl nitrenes originates from the through-space rather than through-bond electronic interactions between the bromine atoms and the nitrene unit.