Quintet Dinitrenes Research Articles

The powder X-band electron paramagnetic resonance spectroscopy of septet pyridyl-2,4,6-trinitrene.

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 DS = -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.

Molecular Magnets: The Synthesis and Characterization of High-Spin Nitrenes.

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.

EPR spectroscopy of multicomponent, multispin molecular system obtained by the photolysis of 2,4,6-triazido-3-cyano-5-fluoropyridine in solid argon.

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 (DT =1.000cm-1 , ET =0), triplet 2,4-diazido-3-cyano-5-fluoropyridyl-2-nitrene (DT =1.043cm-1 , ET =0), triplet 2,6-diazido-3-cyano-5-fluoropyridyl-4-nitrene (DT =1.128cm-1 , ET =0cm-1 ), quintet 4-azido-3-cyano-5-fluoropyridyl-2,6-dinitrene (DQ =0.211cm-1 , EQ =0.0532cm-1 ), quintet 2-azido-3-cyano-5-fluoropyridyl-4,6-dinitrene (DQ =0.208cm-1 , EQ =0.0386cm-1 ), and septet 3-cyano-5-fluoropyridyl-2,4,6-trinitrene (DS =-0.1017cm-1 , ES =-0.0042cm-1 ) in a 38:4:7:22:14:4 ratio, respectively.

W-band EPR studies of high-spin nitrenes with large spin-orbit contribution to zero-field splitting.

First W-band 94 GHz EPR spectra of randomly oriented triplet, quintet, and septet nitrenes formed during the photolysis of 1,3,5-triazido-2,4,6-tribromobenzene in cryogenic matrices are reported. In comparison with conventional X-band 9 GHz electron paramagnetic resonance (EPR) spectroscopy, W-band EPR spectroscopy allows the detection and complete spectroscopic characterization of all paramagnetic species formed at different stages of the photolysis of aromatic polyazides. This type of spectroscopy is of paramount importance for experimental determination of the sign of the zero-field splitting (ZFS) parameters of high-spin molecules with large spin-orbit contribution to the ZFS, caused by the effect of heavy atoms. The study shows that triplet 1,3-diazido-2,4,6-tribromo-5-nitrenobenzene (T1) has DT = 1.369 cm(-1), ET = 0.093 cm(-1), and g = 2.0033, quintet 1-azido-2,4,6-tribromo-3,5-dinitrenobenzene (Q1) shows DQ = - 0.306 cm(-1), EQ = 0.0137 cm(-1), and g = 2.0070, and septet 2,4,6-tribromo-1,3,5-trinitrenobenzene (S1) has DS = - 0.203 cm(-1), ES = 0, and g = 2.0073. The experimental ZFS parameters agree well with the results of density functional theory calculations at the PBE/Ahlrichs-DZ level of theory, showing that such calculations adequately describe the magnetic properties of bromine-containing high-spin nitrenes. Both experimental and theoretical data indicate that, in contrast to all known to date quintet dinitrenes, dinitrene Q1 has the negative sign of magnetic anisotropy due to the "heavy atom effect." This dinitrene along with septet trinitrene S1 possess the largest negative value of D among all known quintet and septet organic polyradicals.

High-spin intermediates of the photolysis of 2,4,6-triazido-3-chloro-5-fluoropyridine

In contrast to theoretical expectations, the photolysis of 2,4,6-triazido-3-chloro-5-fluoropyridine in argon at 5 K gives rise to EPR peaks of just two triplet mononitrenes, two quintet dinitrenes, and a septet trinitrene. EPR spectral simulations in combination with DFT calculations show that observable nitrenes can be assigned to triplet 2,4-diazido-3-chloro-5-fluoropyridyl-6-nitrene (DT = 1.026 cm−1, ET = 0), triplet 2,6-diazido-3-chloro-5-fluoropyridyl-4-nitrene (DT = 1.122 cm−1, ET = 0.0018 cm−1), quintet 4-azido-3-chloro-5-fluoropyridyl-2,6-dinitrene (DQ = 0.215 cm−1, EQ = 0.0545 cm−1), quintet 2-azido-3-chloro-5-fluoropyridyl-4,6-dinitrene (DQ = 0.209 cm−1, EQ = 0.039 cm−1) and septet 3-chloro-5-fluoropyridyl-2,4,6-trinitrene (DS = −0.1021 cm−1, ES = −0.0034 cm−1). Preferential photodissociation of the azido groups located in ortho-positions to the fluorine atom of pyridines is associated with strong π-conjugation of these groups with the pyridine ring. On photoexcitation, such azido groups are more efficiently involved in reorganization of the molecular electronic system and more easily adopt geometries of the locally excited predissociation states.

Molecular magnetic structures based on high-spin intermediates of low-temperature radiolysis of azido derivatives and possibilities of their use in undulator systems

High-spin intermediates with S = 2 generated by γ-radiolysis of triazidopyridines at 77 K were studied using the ESR method in the X range. Their thermal stability at 77–240 K with the transition to the one-electron states (S = 1/2) due to intermolecular reactions at 100–200 K was shown. The study of saturation processes elucidated an effect of a sharp nonlinear increase in the intensity of the ESR signals from quintet dinitrenes with an increase in the microwave field power higher than 10 mW. This effect can be applied as a method of identification of high-spin molecules obtained together with other paramagnetic products of low multiplicity and in the search for systems with molecular magnetism. A set of principles was developed for making compact undulators based on static molecular magnetic structures using dynamic effects upon the interaction of an electron beam with a laser pulse counter.

EPR Studies on Branched High‐Spin Arylnitrenes

The UV (λ>305 nm) photolysis of triazide 3 in 2-methyl-tetrahydrofuran glass at 7 K selectively produces triplet mononitrene 4 (g=2.003, D(T)=0.92 cm(-1), E(T)=0 cm(-1)), quintet dinitrene 6 (g=2.003, D(Q)=0.204 cm(-1), E(Q)=0.035 cm(-1)), and septet trinitrene 8 (g=2.003, D(S)=-0.0904 cm(-1), E(S) =-0.0102 cm(-1)). After 45 min of irradiation, the major products are dinitrene 6 and trinitrene 8 in a ratio of ∼1:2, respectively. These nitrenes are formed as mixtures of rotational isomers each of which has slightly different magnetic parameters D and E. The best agreement between the line-shape spectral simulations and the experimental electron paramagnetic resonance (EPR) spectrum is obtained with the line-broadening parameters Γ(E(Q))=180 MHz for dinitrene 6 and Γ(E(S))=330 MHz for trinitrene 8. According to these line-broadening parameters, the variations of the angles Θ in rotational isomers of 6 and 8 are expected to be about ±1 and ±3°, respectively. Theoretical estimations of the magnetic parameters obtained from PBE/DZ(COSMO)//UB3LYP/6-311+G(d,p) calculations overestimate the E and D values by 1 and 8 %, respectively. Despite the large distances between the nitrene units and the extended π systems, the zero field splitting (zfs) parameters D are found to be close to those in quintet dinitrenes and septet trinitrenes, where the nitrene centers are attached to the same aryl ring. The large D values of branched septet nitrenes are due to strong negative one-center spin-spin interactions in combination with weak positive two-center spin-spin interactions, as predicted by theoretical considerations.

A density functional theory study of the zero-field splitting in high-spin nitrenes

This work presents a detailed evaluation of the performance of density functional theory (DFT) for the prediction of zero-field splittings (ZFSs) in high-spin nitrenes. A number of well experimentally characterized triplet mononitrenes, quartet nitrenoradicals, quintet dinitrenes, and septet trinitrenes have been considered. Several DFT-based approaches for the prediction of ZFSs have been compared. It is shown that the unrestricted Kohn-Sham and the Pederson-Khanna approaches are the most successful for the estimation of the direct spin-spin (SS) interaction and the spin-orbit coupling (SOC) parts, respectively, to the final ZFS parameters. The most accurate theoretical predictions (within 10%) are achieved by using the PBE density functional in combination with the DZ, EPR-II, and TZV basis sets. For high-spin nitrenes constituted from light atoms, the contribution of the SOC part to ZFS parameters is quite small (7%-12%). By contrast, for chlorine-substituted septet trinitrenes, the contribution of the SOC part is small only to D value but, in the case of E value, it is as large as the SS part and has opposite sign. Due to this partial cancellation of two different contributions, SS and SOC, the resulting values of E in heavy molecules are almost two times smaller than those predicted by analysis of the widely used semiempirical one-center spin-spin interaction model. The decomposition of D(SS) into n-center (n=1-4) interactions shows that the major contribution to D(SS) results from the one-center spin-spin interactions. This fact indicates that the semiempirical SS interaction model accurately predicts the ZFS parameters for all types of high-spin nitrenes with total spin S=2 and 3, if their molecules are constructed from the first-row atoms.

Random-Orientation High-Spin Electron Spin Resonance Spectroscopy and Comprehensive Spectral Analyses of the Quintet Dicarbene and Dinitrene with meta-Topological Linkers: Origins of Peculiar Line-Broadening in Fine-Structure ESR Spectra in Organic Rigid Glasses

In high-spin chemistry, random-orientation fine-structure (FS) electron spin resonance (ESR) spectroscopy entertains advantages as the most facile and convenient method to identify high-spin systems, as frequently reported in the literature. Random-orientation ESR spectroscopy applicable to organic high-spin entities can date back to the Wasserman and co-workers' attempt on the first spin-quintet dicarbene, m-phenylenebis(phenylmethlene) (m-PBPM), in the 2-MTHF glass in 1963 and 1967, following their successful work on randomly oriented triplet-state ESR spectroscopy. The FS ESR spectrum of m-PBPM in the 2-MTHF glass, however, has never fully been analyzed due to a peculiar line-broadening appearing at many canonical peaks. Organic high-spin spectra from most quintet dinitrenes also suffer from similar phenomena. Seemingly intrinsic line-diffusing or -broadening phenomena adversely affect the reliable determination of FS parameters for organic high-spin entities in rigid glasses. In high-spin chemistry, the line-broadening has been an obstacle that masks key FS transitions in many cases. Thus, both the origin of the broadening and the comprehensive spectral analysis have been a long-standing issue. In this report, we examine the origin of the line-broadening appearing in the FS ESR spectra, illustrated by a comprehensive spectral analysis for m-PBPM in the quintet ground state and the first-documented quintet-state dinitrene, m-phenylenebis(nitrene) (m-PBN) in the 2-MTHF glass. A complete analysis of the random-orientation FS spectra from m-PBPM diluted in the benzophenone crystal has shown that the g-anisotropy of m-PBPM is not prominent. Also the higher-order FS terms such as S(i)(2)S(j)(2) group-theoretically allowed for S = 2 are not necessary in spite of the argument for a hydrocarbon-based tetraradical (S = 2) in the ground state. Our new approach to the line-broadening analysis invokes both exact analytical solutions for the resonance fields of canonical peaks and the magnetic-parameters gradient method. The D- and E-values of m-PBPM acquired by the spectral simulation in this study give different molecular structures of the quintet dicarbene in the benzophenone crystal lattice (D = +0.0703(0) cm(-1), E = +0.0212(0) cm(-1)) and in the 2-MTHF glass (D = +0.0780(0) cm(-1), E = +0.0221(0) cm(-1)). Microscopic origins of the line-broadening observed for high-spin oligocarbenes or oligonitrenes generated by photolysis in organic glasses have been proposed.

The assignment of signals in the EPR spectra of axially symmetrical quintet dinitrenes

Fine structure levels in an external magnetic field and angular dependences of resonance magnetic fields on the direction of an external magnetic field were calculated for two axially symmetrical quintet dinitrenes with the zero-field splitting parameters Dq = 0.260 cm−1, Eq = 0.000 and Dq = 0.243 cm−1, Eq = 0.003 cm−1. The EPR spectra of such dinitrenes contained lines of only three xy transitions (xy1, xy2, and xy4), two Δms = ±2 transition lines between the W−2 and W0 sublevels, and three additional lines from noncanonically oriented molecules whose magnetic axis Z made an angle of 12°–16° or 52°–54° with an external magnetic field.

The properties of quintet dinitrenes in 2,4,6-triazido-3,5-dichloropyridine crystals

The gamma radiolysis of crystalline 2,4,6-triazido-3,5-dichloropyridine (TACP) was studied by the EPR method. The molecular and crystal structure of TACP was determined by X-ray structure analysis. The prolonged radiolysis of TACP at 77 K caused the appearance of characteristic EPR signals of quintet 2,6-dinitreno-4-azido-3,5-dichloropyridine (2,6-DN) and 2,4-dinitreno-6-azido-3,5-dichloropyridine (2,4-DN). The predominant decomposition of the α-azide groups of TACP was explained by the special features of its molecular and crystal structure. The saturation of the EPR signals of quintet 2,6-DN and 2,4-DN and one-electron radicals with S = 1/2 as the microwave field power increased was studied. The quintet dinitrenes were found to be thermally unstable; they remained in TACP crystals up to 245–250 K.

Generation of quintet dinitrenes by the γ-radiolysis of crystalline 2,4,6-triazido-3,5-dicyanopyridine

High-spin nitrenes are promising organic magnetic materials for molecular electronics.1 These compounds are usually obtained by low-temperature solid-phase photolysis of aromatic azides, the aromatic ring of which is protected by appropriate substituents from undesirable intramolecular reactions with nitrenic centres.2 Unfortunately, many substituents (e.g., CF3 or CN4) in aryl nitrenes do not protect them from photochemical rearrangements and this substantially cuts down the potential of photochemical synthesis of high-spin nitrenes. Thus, for instance, our previous attempts5 to obtain high-spin nitrenes 4, 5 and 6 by the photolysis of triazide 1 were unsuccessful. In order to develop a method for generating photochemically unstable high-spin nitrenes, the γ-radiolysis of crystalline triazide 1 at 77 K was studied. Although 2,4,6-triazidopyridines are high-energy compounds and readily decompose upon the action of UV light, X-ray, temperature or pressure, triazide 1† showed unexpectedly high stability toward γ-rays. Only a very prolonged γ-irradiation of crystalline 1 in a vacuum‡ at 77 K led to the appearance of two EPR signals at 3030 and 3346 G [Figure 1(a)], which are characteristic of quintet 2,4and 2,6dinitrenopyridines, respec-tively.6 Similar EPR spectra with two characteristic signals of quintet 2,4-dinitreno-6-azido3,5-dichloropyridine at 3009 G and of 2,6-dinitreno-4-azido3,5-dichloropyridine at 3345 G have been recorded during the photolysis of 2,4,6-triazido-3,5-dichloropyridine 7 in frozen (77 K) solutions in 2-methyl-tetrahydrofuran.7 Owing to the presence of chlorine atoms in the 3and 5-positions of the pyridine ring, high-spin nitrenes were photochemically stable and available by photochemical synthesis.5 The successful generation of quintet dinitrenes 4 and 5 from triazide 1 shows that light-sensitive high-spin nitrenes can be readily prepared by low-temperature γ-radiolysis of crystalline azides. This method is insensitive to transparency of the solid phase and allows the generation of high-spin nitrenes in the whole volume of a crystalline sample but not only on its surface. Our kinetic studies of the initial stages of the γ-radiolysis of crystalline triazide 1 showed that radiation yields of quintet dinitrenes 4 and 5 are equal to 0.200 and 0.015 per 100 eV of the energy applied, respectively. Note that quintet dinitrenes 4 and 5 were also obtained in nearly the same radiation yields (0.130 and 0.017) on γ-irradiation of crystalline 1 in air‡ at 77 K. This indicates that generation of high-spin nitrenes in azide crystals by γ-radiolysis is insensitive to air. According to statistical considerations, quintet dinitrenes 4 to 5 should be formed in equal quantities. The much higher radiation yield of quintet 4 indicates that α-azido groups of triazide 1 on comparison with its γ-azido group are much more sensitive toward the action of γ-irradiation. The preferential decomposition of the α-azido groups has been observed in the γ

Effect of microwave radiation on the magnetic properties of quintet dinitrenes

Effect of microwave radiation on the magnetic properties of quintet dinitrenes

Photochemical Transformations of Quintet m-Phenylenedinitrenes

Abstract Photochemical transformations of quintet dinitrenes have been studied for the first time by FTIR spectroscopy during the photolysis of 1,3-diazidobenzene and its 2-methyl- and 2,4,6-trimethyl-substituted derivatives in solid argon at 13 K. The reactions involve two competitive processes. In the first one, the intramolecular insertion of the nitrene unit into the 1,6-aromatic bond induces ring-opening, which leads to the formation of aminoacetylenes. The second process involves the intramolecular addition of the nitrene unit to the 1,2-aromatic bond followed by ring-opening to form 4(2H-azirin-2-ylidene)-2-butenenitrile derivatives. Methyl groups in ortho-positions to the nitrene units of quintet dinitrenes efficiently protect their aromatic ring from nitrene attacks, substantially increasing the photochemical stability of such species. However, even quintet 2,4,6-trimethylphenylene-1,3-dinitrene decomposes to ring-opened products on extended irradiation. This indicates that quintet dinitrenes are much more photochemically reactive species than triplet nitrenes. Undesirable photochemical rearrangements of the latter at early stages of the photolysis of 1,3-diazidobenzenes are suppressed on introducing the methyl group in position 2 of these azides.