Properties of Triplet Tetrachloro-4-pyridylnitrene Isolated in Inert Matrices

Photolysis of 4-azidotetrachloropyridine isolated in inert matrices at low temperatures leads to formation of triplet tetrachloro-4-pyridylnitrene as the sole product of the photochemical reaction. In the EPR spectrum, this nitrene gives an intense signal and is characterized by zero-field splitting parameters. The UV spectrum of nitrene in solid argon contains a series of high-intensity and medium-intensity absorption bands. The IR spectrum in solid argon agrees well with nonempirical and semiempirical calculations. On exposure to light, nitrene easily reacts with molecular carbon(II) oxide, forming the corresponding isocyanate. Irradiation with light does not result in formation of bicyclic azirine, but initiates the reverse reaction of fixation by nitrene of the molecular nitrogen contained in the solid argon, leading to the starting azide.     

Direct Observation of Carbamoylnitrenes

As the prototype Curtius rearrangement reaction, carbamoyl azide decomposes into aminoisocyanate and molecular nitrogen. However, the key intermediate carbamoylnitrene was previously undetected, even though the decomposition of carbamoyl azides has been studied frequently since its discovery in the 1890s. Upon ArF laser (λ=193 nm) photolysis, the stepwise decomposition of the two simplest carbamoyl azides H₂NC(O)N₃ and Me₂NC(O)N_3, isolated in solid noble gas matrices, occurs with the formation of the corresponding carbamoylnitrenes H₂NC(O)N and Me₂NC(O)N. Both triplet species are characterized for the first time by combining matrix‐isolation IR spectroscopy and quantum‐chemical calculations. Subsequent visible‐light irradiations cause efficient rearrangement of these nitrenes into the respective aminoisocyanates.     

Gas‐Phase Generation and Decomposition of a Sulfinylnitrene into the Iminyl Radical OSN

The dipolar oxathiazyne‐like sulfinylnitrene RS(O)N, a highly reactive α‐oxo nitrene, has been rarely investigated. Upon flash vacuum pyrolysis of sulfinyl azide CF₃S(O)N₃ at 350 °C, an elusive sulfinylnitrene CF₃S(O)N was generated in the gas phase in its singlet ground state and was characterized by matrix‐isolation IR spectroscopy. Further fragmentation of CF₃S(O)N at 600 °C produced CF₃ and a novel iminyl radical OSN, an SO₂ analogue, which were unambiguously identified by IR spectroscopy. Consistent with the experimental observations, DFT calculations clearly support a stepwise decomposition mechanism of CF₃S(O)N₃.     

Difluoro‐λ5‐Phosphinonitrile F2PN: Matrix Isolation and Photoisomerization into FPNF

Splendid isolation : Monomeric phosphazene F₂PN (¹A₁) was prepared for the first time through irradiation of F₂PN₃ in an argon matrix with an ArF excimer laser (λ=193 nm). Upon subsequent irradiation with a high‐pressure mercury arc lamp (λ=255 nm), F₂PN undergoes a 1,2‐fluorine shift to give iminophosphane cis‐FP?NF.

     

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 (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.     

EPR spectroscopy of quintet 4-amino-3,5-dichloropyridine-2,6-diyldinitrene isolated in solid argon

An EPR spectrum of solid Ar isolated quintet 4-amino-3,5-dichloropyridin-2,6-diyldinitrene that formed by the photolysis of 4-amino-2,6-diazido-3,5-dichloropyridine at 15 K was recorded. Using computer simulation based on numerical diagonalization of the quintet spin Hamiltonian matrices, it was established that this EPR spectrum corresponds to a quintet spin state with the magnetic parameters g = 2.0023, |D _q| = 0.2100 cm⁻¹, and |E _q| = 0.0560 cm⁻¹. Owing to high resolution of the experimental spectrum, the zero-field splitting parameters of the quintet intermediate were determined to an accuracy of at least 5·10⁻⁴ cm⁻¹. Calculations of the fine-structure energy levels in external magnetic field and the dependences of the EPR signal positions and intensities of the quintet dinitrene on the direction of external magnetic field were performed for the first time. This allowed unambiguous assignment of all EPR lines of quintet molecules having both in-principal-axis and off-principal-axis orientations. Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2284–2289, December, 2007.     

A comparison of the reactivity of Ni atoms toward CH4, SiH4, and SnH4: a combined matrix isolation and quantum-chemical study

Nickel atoms are shown to react spontaneously with SiH(4) and SnH(4) to give the insertion products HNiSiH(3) and HNiSnH(3). With CH(4), however, no spontaneous reaction occurs, in agreement with earlier reports; HNiCH(3) can be formed only on photolytic activation of the Ni atom. The reaction products were characterized experimentally by IR spectroscopy, including the effect of isotopic substitution (H/D), and by quantum-chemical calculations. They all have C(s) symmetry with a terminal Ni--H bond and three terminal E--H bonds (E=Si, Sn). Strikingly, the H-Ni-E bond angles are less than 90 degrees, and there is a weak interaction between the H atom bound to Ni and the E atom. The structures are compared with those of other molecules of general formula MSiH(4) that have been characterized recently in our group (M=Ti, Ni, Ga). While TiSiH(4) has three Ti-H-Si bridges, both NiSiH(4) and GaSiH(4) exhibit only terminal Ni--H and E--H bonds, but with the difference that there is no interaction between the H atom bound to Ga and the Si atom.     

Difluorophosphoryl Nitrene F2P(O)N: Matrix Isolation and Unexpected Rearrangement to F2PNO

Triplet difluorophosphoryl nitrene F₂P(O)N (X³A′′) was generated on ArF excimer laser irradiation (λ=193 nm) of F₂P(O)N₃ in solid argon matrix at 16 K, and characterized by its matrix IR, UV/Vis, and EPR spectra, in combination with DFT and CBS‐QB3 calculations. On visible light irradiation (λ>420 nm) at 16 K F₂P(O)N reacts with molecular nitrogen and some of the azide is regenerated. UV irradiation (λ=255 nm) of F₂P(O)N (X³A′′) induced a Curtius‐type rearrangement, but instead of a 1,3‐fluorine shift, nitrogen migration to give F₂PON is proposed to be the first step of the photoisomerization of F₂P(O)N into F₂PNO (difluoronitrosophosphine). Formation of novel F₂PNO was confirmed with ¹⁵N‐ and ¹⁸O‐enriched isotopomers by IR spectroscopy and DFT calculations. Theoretical calculations predict a rather long P? N bond of 1.922 Å [B3LYP/6‐311+G(3df)] and low bond‐dissociation energy of 76.3 kJ mol^?1 (CBS‐QB3) for F₂PNO.     

Direct Observation of an Imidoylnitrene: Photochemical Formation of PhC(=NMe)−N and Me−N from 1‐Methyl‐5‐phenyltetrazole

The imidoylnitrene 8 , N‐methyl‐C‐phenylimidoylnitrene, has been generated by laser photolysis of 1‐methyl‐5‐phenyltetrazole 6 at 5 K and characterized by its ESR spectrum (|D/hc|=0.9602, |E/hc|=0.0144 cm^?1). In addition, the triplet excited states of 6 and of 2‐methyl‐5‐phenyltetrazole 11 were also observed by ESR spectroscopy in the 5 K matrices ( 6 : |D/hc|=0.123 cm^?1, E/hc=0.0065 cm^?1, 11 : |D/hc|=0.126 cm^?1, |E/hc|=0.0056 cm^?1). The imidoylnitrene 8 is unstable both thermally (disappearing at 80 K) and photochemically (disappearing on continued irradiation at 266 nm). Methyl(phenyl)carbodiimide is the end product of photolysis.     

Infrared and Raman Spectroscopy of Methylcyanodiacetylene (CH3C5N)

A spectroscopic study combining IR absorption and Raman scattering is presented for methylcyanodiacetylene (CH₃C₅N). Gas‐phase, cryogenic matrix‐isolated, and pure solid‐phase substance was analyzed. Out of 16 normal vibrational modes, 14 were directly observed. The analysis of the spectra was assisted by quantum chemical calculations of vibrational frequencies, IR absorption intensities, and Raman scattering activities at density functional theory and ab initio levels. Previous assignments of gas‐phase IR absorption bands were revisited and extended.     

Contrasting behavior in azide pyrolyses: an investigation of the thermal decompositions of methyl azidoformate, ethyl azidoformate and 2-azido-N, N-dimethylacetamide by ultraviolet photoelectron spectroscopy and matrix isolation infrared spectroscopy

The thermal decompositions of methyl azidoformate (N3COOMe), ethyl azidoformate (N3COOEt) and 2-azido-N,N-dimethylacetamide (N3CH2CONMe2) have been studied by matrix isolation infrared spectroscopy and real-time ultraviolet photoelectron spectroscopy. N2 appears as an initial pyrolysis product in all systems, and the principal interest lies in the fate of the accompanying organic fragment. For methyl azidoformate, four accompanying products were observed: HNCO, H2CO, CH2NH and CO2, and these are believed to arise as a result of two competing decomposition routes of a four-membered cyclic intermediate. Ethyl azidoformate pyrolysis yields four corresponding products: HNCO, MeCHO, MeCHNH and CO2, together with the five-membered-ring compound 2-oxazolidone. In contrast, the initial pyrolysis of 2-azido-N,N-dimethyl acetamide, yields the novel imine intermediate Me2NCOCH=NH, which subsequently decomposes into dimethyl formamide (HCONMe2), CO, Me2NH and HCN. This intermediate was detected by matrix isolation IR spectroscopy, and its identity confirmed both by a molecular orbital calculation of its IR spectrum, and by the temperature dependence and distribution of products in the PES and IR studies. Mechanisms are proposed for the formation and decomposition of all the products observed in these three systems, based on the experimental evidence and the results of supporting molecular orbital calculations.     

Matrix isolation FTIR spectroscopical study of ethene secondary ozonide

A new method is used for the separation of ethene secondary ozonide (SOZ) from the other products of ethene ozonization reaction. The reaction was performed in the neat films of the reactants at 77 K. Ethene SOZ was separated from other products of the reaction by vacuum distillation at 190–210 K and analyzed by means of the matrix isolation IR absorption spectroscopy. Spectroscopic data from photolysis of the matrix isolated ozonide was used as an argument for assignment of the infrared spectral bands either to ethene SOZ or to other products of the reaction. The spectra of ethene SOZ isolated in the Ar matrix were analyzed by combining experimental results with the theoretical calculations performed at the MP2 6-311+G (3df, 3pd) level. A new assignment of some experimental fundamental bands is proposed taking in to account the Fermi resonance between CH stretch and the five membered ring vibrations. For the first time more than 30 weak infrared absorption bands were observed and assigned to various combination vibrations and overtones. By using new spectral information concerning the overtones and the combination bands it is concluded that the dissociation of unstable ethene SOZ involving breaking of any of the four CO bonds of the five membered ring of ethene SOZ has low probability. Dissociation of the ring starts from breaking of the OO bond.      

Intermediates Involved in the Oxidation of Nitrogen Monoxide: Photochemistry of the cis‐N2O2⋅O2 complex and of sym‐N2O4 in Solid Ne Matrices

Pure sym‐N₂O₄ isolated in solid Ne was obtained by passing cold neon gas over solid N₂O₄ at ?115 °C and quenching the resulting gaseous mixture at 6.3 K. Filtered UV irradiation (260–400 nm) converts sym‐N₂O₄ into trans‐ONONO₂, a weakly interacting (NO₂)₂ radical pair, and traces of the cis‐N₂O₂?O₂ complex. Besides the weakly bound ON?O₂ complex, cis‐N₂O₂?O₂ was also obtained by co‐deposition of NO and O₂ in solid Ne at 6.3 K, and both complexes were characterised by their matrix IR spectra. Concomitantly formed cis‐N₂O₂ dissociated on exposure to filtered IR irradiation (400–8000 cm^?1), and the cis‐N₂O₂?O₂ complex rearranged to sym‐N₂O₄ and trans‐ONONO₂. Experiments using ¹⁸O₂ in place of ¹⁶O₂ revealed a non‐concerted conversion of cis‐N₂O₂?O₂ into these species, and gave access to four selectively di‐¹⁸O‐substituted trans‐ONONO₂ isotopomers. No isotopic scrambling occurred. The IR spectra of sym‐N₂O₄ and of trans‐ONONO₂ in solid Ne were recorded. IR fundamentals of trans‐ONONO₂ were assigned based on experimental ^16/18O isotopic shifts and guided by DFT calculations. Previously reported contradictory measurements on cis‐ and trans‐ONONO₂ are discussed. Dinitroso peroxide, ONOONO, a proposed intermediate in the IR photoinduced rearrangement of cis‐N₂O₂?O₂ to the various N₂O₄ species, was not detected. Its absence in the photolysis products indicates a low barrier (≤10 kJ mol^?1) for its exothermic O? O bond homolysis into a (NO₂)₂ radical pair.     

The missing link: triplet fluorocarbonyl nitrene FC(O)N

The elusive triplet fluorocarbonyl nitrene, FC(O)N (X³A′′), has been generated in high yield from matrix‐isolated FC(O)N₃ by ArF excimer laser photolysis (λ=193 nm). As a side product FNCO was formed. The novel nitrene was characterized by IR, UV/Vis, EPR spectroscopy, and quantum‐chemical calculations. All six fundamental vibrations of FC(O)N at 1681.3, 1193.8, 879.8, 646.5, 588.7, and 434.8 cm^?1 (argon matrix, 16 K), their ^12/13C, ^16/18O, and ^14/15N isotopic shifts, and four electronic transitions at T₀=13 890, 25 428, 29 166, and 30 900 cm^?1 that exhibit vibrational fine structures have been detected. Under visible‐light irradiation at λ≥495 nm, FC(O)N reacted with molecular N₂ in the matrix cage at 6 K to give back FC(O)N₃, whereas near‐UV irradiation at λ≥335 nm yielded FNCO. The singlet–triplet energy gaps of different carbonyl nitrenes are discussed.     

The Vanadium(V) Oxoazides [VO(N3)3], [(bipy)VO(N3)3], and [VO(N3)5]2−

Vanadium(V) oxoazide [VO(N₃)₃] was prepared through a fluoride–azide exchange reaction between [VOF₃] and Me₃SiN₃ in acetonitrile solution. When the highly impact‐ and friction‐sensitive compound [VO(N₃)₃] was reacted with 2,2′‐bipyridine, the adduct [(bipy)VO(N₃)₃] was isolated. The reaction of [VO(N₃)₃] with [PPh₄]N₃ resulted in the formation and isolation of the salt [PPh₄]₂[VO(N₃)₅]. The adduct [(bipy)VO(N₃)₃] and the salt [PPh₄]₂₃[VO(N₃)₅] were characterized by vibrational spectroscopy and single‐crystal X‐ray structure determination, making these compounds the first structurally characterized vanadium(V) azides.     

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.