Characterization of a Triplet Vinylidene

Abstract

Singlet vinylidenes (R₂C═C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters (D = 0.377 cm^-1 and |E|/D = 0.028) were determined, and the ¹³C hyperfine coupling tensor was obtained by ¹³C-ENDOR measurements. Most strikingly, the isotropic ¹³C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.

Figure 1

Fundamental compound classes. (A) Comparison between singlet and triplet carbenes (I/II). (B) Singlet (III) and triplet vinylidenes (IV–VI) as well as triplet nitrenes (VII). The simplified depiction of electronic structure does not imply that all relevant states are necessarily of single-reference character.

Figure 2

Irradiation of diazoalkene 1 leads to C–H insertion product 3 via ground-state triplet vinylidene 2. ¹³C labeling is shown in blue.

Figure 3

(Left) FID-detected Q-band EPR spectrum (black solid line) acquired at 6 K after the photolysis of 1 (Hg arc lamp, 1 h at 10 K) in frozen toluene solution (20 mM). The spectrum is overlaid with the best fit achieved assuming an S = 1 species (magenta dotted line). Canonical orientations of the ZFS tensor are labeled with X, Y, and Z. The superscripts refer to the M_S = −1 ↔ 0 (−) and M_S = 0 ↔ 1 (+) transitions. The intensity of the simulated half-field (M_S = −1 ↔ 1) signal at 560 mT was reduced by 86% to account for a difference in transition probabilities. The narrow signal marked with an asterisk originates from a minor fraction of photolysis byproducts (radicals and radical pairs) typically observed for divalent carbenes. (Right) CW X-band EPR spectrum acquired at 7 K after the photolysis of 1 (Xe arc lamp, 1 h at 7 K) in frozen toluene solution (6 mM), overlaid with the best fit. The radical signal is omitted for clarity. (Inset) CW EPR intensity vs inverse temperature. (Black circles) Temperature increased from 6 to 50 K. (Blue triangles) Temperature decreased from 50 to 8 K.

Figure 4

(A) ¹³C Davies ENDOR spectra of 2 acquired at canonical field positions (black) overlaid with simulations (various colors). Asterisks mark simulated ridges originating from the noncanonical orientations. (B) Simulated ¹³C ENDOR pattern derived across the EPR absorption envelope using the global fit parameters (SI). Dashed horizontal lines mark field positions where the ENDOR spectra were recorded. (C) Simulated EPR absorption spectrum.

Figure 5

(A) Optimized (B3LYP/def2-TZVP) bond lengths in angstroms and computed Mayer bond orders (in parentheses) of vinylidene 2. (B) Quasi-restricted orbitals describing the valence electronic structure of the triplet state. (C) Spin density distribution and atomic spin populations for selected atoms, resulting from the TPSSh calculations of EPR parameters.