An N-Heterocyclic Quinodimethane: A Strong Organic Lewis Base Exhibiting Diradical Reactivity

Area of Science:

• Organic Chemistry
• Materials Science

Background:

• N-heterocyclic carbenes (NHCs) and N-heterocyclic olefins (NHOs) are important organic molecules.
• Developing new organic donors with unique electronic properties is crucial for materials science.

Purpose of the Study:

• To synthesize and characterize a new class of organic σ-donors, termed N-heterocyclic quinodimethanes (NHQs).
• To investigate the reactivity and electronic properties of NHQs, particularly their basicity and dimerization behavior.

Main Methods:

• Synthesis of the novel NHQ compound.
• Density Functional Theory (DFT) computations to understand reaction mechanisms and electronic structure.
• Competition experiments to assess relative basicity.
• Cyclic voltammetry to determine redox properties.

Main Results:

• Successful preparation of a new NHQ featuring an N-heterocyclic carbene linked to a methylidene group via a C6H4-linker.
• NHQ demonstrates significantly higher basicity compared to analogous NHCs and NHOs, driven by linker aromatization.
• Unprecedented dehydrogenative head-to-head dimerization of NHQ in solution via C-C coupling of methylidene groups.
• DFT calculations reveal an open-shell singlet pathway for dimerization, indicating diradical character.
• The resulting conjugated N-heterocyclic bis-quinodimethane is a strong organic reducing agent (E1/2 = -1.71 V vs. Fc/Fc+) with a small singlet-triplet gap (ΔES→T = 4.4 kcal mol−1).

Conclusions:

• N-heterocyclic quinodimethanes represent a novel class of organic σ-donors with tunable electronic properties.
• The unique dimerization pathway highlights the diradical nature and reactivity of NHQs.
• The strong reducing ability and small singlet-triplet gap of the dimer suggest potential applications in organic electronics and catalysis.