Dialkynyl Carbene Derivatives: Generation and Characterization of Triplet tert-Butylpentadiynylidene (t-Bu−C≡C−C̈−C≡C−H) and Dimethylpentadiynylidene (Me−C≡C−C̈−C≡C−Me)

Abstract

Triplet carbenes t-butylpentadiynylidene (t-BuC(5)H, 1a) and dimethylpentadiynylidene (MeC(5)Me, 1b) have been produced photochemically from their corresponding diazo compound precursors and studied spectroscopically in cryogenic matrices (N(2) or Ar) at 10 K. The infrared, electronic absorption, and electron paramagnetic resonance spectra of these species exhibit numerous similarities to the spectra of pentadiynylidene (HC(5)H) and methylpentadiynylidene (MeC(5)H) recorded previously. EPR spectra yield zero-field splitting parameters that are typical for triplet carbenes with axial symmetry (t-BuC(5)H, 1a: |D/hc| = 0.61 cm(-1), |E/hc| ∼ 0 cm(-1); MeC(5)Me, 1b: |D/hc| = 0.62 cm(-1), |E/hc| ∼ 0 cm(-1)). Electronic spectra are characterized by weak absorptions (T(1) ← T(0)) in the near-UV and visible region (350-430 nm) with extended vibronic progressions. The electronic transitions of several -C(5)- carbenes are compared, and an apparent dependence of the transition wavelength on the level of alkyl substitution of the carbon chain is found. Chemical trapping of triplet 1a in an O(2)-doped matrix affords carbonyl oxides derived predominantly from attack at C-3. Both t-BuC(5)H (1a) and MeC(5)Me (1b) undergo photochemical rearrangement upon UV irradiation.