Competing Modes of Hydrogen Activation in Singlet Pyridinylidenes: π-Approach vs σ*-Approach Reaction Pathways

Area of Science:

• Organometallic Chemistry
• Catalysis
• Computational Chemistry

Background:

• Carbenes are key in transition metal-free hydrogen activation.
• Singlet carbenes typically hydrogenate via a pi-approach.
• A new class of singlet N-heterocyclic carbenes with sigma/sigma* orbitals has been introduced.

Purpose of the Study:

• To investigate the influence of nitrogen substituents on carbene electronic structure.
• To determine how substituents affect the preference for sigma*- or pi-approach hydrogenation.
• To understand the mechanism of hydrogen activation by novel N-heterocyclic carbenes.

Main Methods:

• Synthesis and characterization of 1-iodopyridine-2-ylidene and related carbenes.
• High-accuracy computational methods including DLPNO-CCSD(T) and NEVPT2.
• Analysis of electronic structure and reaction pathways for hydrogenation.

Main Results:

• Carbenes with I, Br, Cl, and OCF3 substituents favor sigma*-approach hydrogenation.
• Carbenes with OMe, NMe2, and Me substituents favor pi-approach hydrogenation.
• Fluorine-substituted carbene shows a unique preference for sigma*-approach despite a pi-configuration.

Conclusions:

• Nitrogen substituents critically tune the electronic properties and hydrogenation mechanism of singlet N-heterocyclic carbenes.
• The sigma*-approach pathway offers a novel route for transition metal-free hydrogen activation.
• Understanding these structure-reactivity relationships advances carbene chemistry and catalytic applications.