Characterization of a Triplet Vinylidene

Yury Kutin,Müge Kasanmascheff,Max M Hansmann,Dimitrios A Pantazis,Patrick W Antoni,Anton Savitsky,Justus Reitz

doi:10.1021/jacs.1c11062

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.

Highlights

The photochemically triggered loss of N2 should lead to vinylidene 2 in either a singlet (2S) or triplet (2T) ground state
Calculations with the benchmark-quality[33] DLPNO-CCSD(T1) coupled-cluster approach on a simplified model of 2 where ring substituent groups were replaced by hydrogens suggest a singlet−triplet gap of 13.1 kcal/mol. This result is corroborated by multireference perturbation theory NEVPT2 calculations that use a fullvalence active space of 10 electrons in 8 orbitals, which locate a closed-shell singlet state at 12.2 kcal/mol and an open-shell singlet state at 13.6 kcal/mol above the triplet ground state
(See the SI for the detailed methodology and results.) These results uniformly support the remarkable assignment of a spintriplet ground state for vinylidene 2, which is well separated from excited spin-singlet states

Summary

■ INTRODUCTION

The synthesis of paramagnetic organic compounds such as radicals or diradicals has fascinated chemists since Gomberg’s seminal discovery of the stable triphenylmethyl radical in 1900.1 In particular, high-spin ground-state species such as diradicals are typically challenging to study but are highly attractive for a series of applications due to their magnetic properties.[2−4] The isolation and characterization of the first stable divalent carbon compounds (R2C:), i.e., singlet carbenes (I) by Bertrand[5] and Arduengo[6] as well as persistent triplet carbenes (II) by Tomioka,[7] represented a breakthrough and paradigm shift for chemistry (Figure 1A).[8]. We report the photogeneration and characterization of a ground-state triplet vinylidene using electron paramagnetic resonance (EPR) spectroscopy

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES