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

Abstract

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.