Triplet Vinylidenes Based on (Benz)imidazole and 1,2,3-Triazole N‑Heterocycles

Triplet vinylidenes, a new class of carbon-centered diradicals containing a monosubstituted carbon atom, remain largely unexplored. A series of triplet vinylidenes based on five-membered heterocycles, featuring 2- and 4-imidazole, benzimidazole as well as 1,2,3-triazole backbones, are generated upon irradiation of stable diazoalkenes and are investigated by electron paramagnetic resonance (EPR) spectroscopy. While the calculated S/T gaps strongly vary (∼9.9-18.4 kcal/mol), the experimental zero-field splitting (ZFS) D values are positioned in a rather narrow and characteristic range of D ∼ 0.366-0.399 cm^-1. Electron nuclear double resonance (ENDOR) studies with ¹³C-labeled samples combined with quantum chemical calculations reveal a common motif of A _iso(¹³C) ≈ 50 MHz for the electronic structure of the vinylidene class. EPR decay experiments confirm that steric and electronic tuning of the heterocycle can hinder C-H activation pathways leading to the highest reported stabilities of up to 150 K. Quantum chemical studies elucidate and contrast plausible C-H insertion pathways, identifying an early triplet-to-singlet spin surface transition as the key factor that governs the stability of the vinylidenes.