Matrix Isolation and Characterization of 1,1-Diazenes

Citation

Sylwester, Alan Paul (1986) Matrix Isolation and Characterization of 1,1-Diazenes. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/52cm-3b89. https://resolver.caltech.edu/CaltechTHESIS:10252019-122237282

Abstract

CHAPTER 1

The photochemical generation, matrix isolation and direct spectroscopic characterization of H₂NN 3 is reported. UV (VIS filtered) photolysis of carbamoyl azide 15 in a rigid glass (2-MTHF, 80°K) generates blue-violet 3. The electronic absorption spectrum of 3 reveals a structured absorption curve λ_max = 636 nm, λ_0,0 = 695 nm for the n-π* transition of 3. This transition is blue shifted in a more polar glass (2-MTHF:nPrCN, 1:1, 80°K) to λ_max = 624 nm, λ_0,0 = 681 nm. The argon matrix FT-IR spectrum of 3 shows bands at 2865.55, 2807.20, 1863.20, 1574.16, and 1003.07 cm^-1 (1:2000, Ar, 10°K). The characteristic N=N stretch for 3 at 1574.16 demonstrates the considerable double bond character in the 1,1-diazene. Incorporation of a terminal ¹⁵N label into H₂N-¹⁵N 3-¹⁵N shifts the N=N stretch to 1547.64 cm^-1. The argon matrix infrared spectra of 3, 3-¹⁵N, 3-d₂, and 3-d₂-¹⁵N are reported. Thermal decomposition of 3 (2-MTHF, 90°K) affords 2-tetrazene (λ_max = 260 nm), trans HNNH 1 (λ_max = 386 nm) and an unidentified species (λ_max = 480 nm). Subsequent thermolysis (>100 K) affords NH₃, N₂H₄, N⁺H₄N^-₃, H₂ and N₂ products. Oirect irradiation of 3 in a glass (2-MTHF = 77°K) with visible light affords H₂, N₂, and trans-HNNH 1. Photodecomposition of matrix isolated 3 (Ar, 10°K) with visible light in the presence of CO affords formaldehyde (H₂CO), trans-HNNH 1, H₂, and N₂. This represents the first direct observation of thermal and photochemical interconversion of H₂N₂ isomers.

CHAPTER 2

Preliminary studies of the low temperature matrix isolation and spectroscopic characterization of 1,1-dimethyldiazene 7 and l, 1-di-isopropyldiazene 18 are reported. The UV (VIS filtered) photolysis of carbamoyl azides 13 and 17 in a rigid medium (organic glass, 80°K or Ar matrix, 10°K) provides a new general method for the photochemical generation of reactive 1,1-diazenes. This photochemical route is considered to proceed via the photo-Curtius rearrangement of a carbamoyl azide to an aminoisocyanate followed by photodecarbonylation to a 1,1-diazene and carbon monoxide. Electronic absorption spectroscopy (2-MTHF, 80°K) reveals structured absorption curves (n-π*) λ_max = 556 nm, λ_0,0 = 643 nm for 7 and λ_max = 504 nm, λ_0,0 = 620 nm for 18. 1,1-Dimethyldiazene 7 was independently generated by UV (VIS filtered) photolysis of (Z)-3,3-dimethyl-1-phenyltriazene-1-oxide 16 to afford 7 and nitrosobenzene. Matrix isolation FT-IR spectroscopy (Ar, 10°K) reveals the characteristic N=N stretch for 7 at 1600.96 cm^-1. Incorporation of a terminal ¹⁵N label shifts this stretch to 1581.83 cm^-1. The N:N stretch for 18 at 1600.92 cm^-1 is ¹⁵N shifted to 1579.46 cm^-1. Photochemical decomposition of 7 (Ar, 10°K) yields the infrared bands of ethane and an unidentified species (U) which is subsequently photolyzed to ethane. The effects of substitution on the electronic transitions and R₂N=N stretches of 1,1-diazenes correlates with the trends of the isoelectronic carbonyl compounds. Thermolysis of 7 and 18 (2-MTHF, 90°K) yields red-orange 9 (λ_max = 464 n, ε ≃ 3000 M^-1 cm^-1) and 30 (λ_max = 474 nm, ε ≃ 3000 M^-1 cm^-1), respectively. These species are tentatively identified as the azomethinimine tautomers of the 1,1-diazenes with α-hydrogens. Irradiation of 7 and 18 at their n-π* transitions in the visible (2-MTHF, 80°K) also initially yields 9 and 30, respectively, in addition to the hydrocarbon products expected from nitrogen extrusion. Subsequent bimolecular decomposition of 9 (E_a = 8.2 ± 0.5 kcal/mol, log₁₀ A = 1.8 ± 0.6) yields tetramethyl-2-tetrazene 19. Bimolecular decomposition of 9-d₆ (E_a = 8.6 ± 0.5 kcal/mol, log₁₀ A = 1.4 ± 0.6) reveals a deuterium isotope effect k_H/k_D = 6.7 at 190°K for loss of 9. Thermal decomposition of 30 affords hydrocarbon products 32, 33, and 34 expected for nitrogen extrusion from 1,1-diazene 18. The activation parameters for unimolecular decomposition of 30 are E_a = 16.8 ± 0.5 kcal/mol, log₁₀ A = 11.8 ± 0.3.

CHAPTER 3

The low-temperature ¹⁵N NMR spectrum of the 1,1-diazene, N-(2,2,6,6-tetramethylpiperidyl)nitrene (1) is reported. The ¹⁵N double- and mono-labeled 1,1-diazenes 1a and 1b were synthesized. The nitrene and amino nitrogens of 1 have resonances in dimethyl ether at -90°C at 917.0 and 321.4 ppm, respectively, downfield from anhydrous ¹⁵NH₃, affording a chemical-shift difference of 595 ppm for the directly bonded nitrogen nuclei. The chemical shift of the ring nitrogen is consistent with an amino nitrogen whose lone pair is largely delocalized. The large downfield shift of the nitrene nitrogen is consistent with a large paramagnetic term due to a low-lying n-π* transition.

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