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

Carbenes are promising reagents for the transition metal-free activation of molecular hydrogen. Depending on their multiplicity and electron configuration, carbenes can access different hydrogenation reaction mechanisms, with singlet carbenes usually leading to geminal hydrogenation products via a π-approach trajectory. Our group has recently prepared 1-iodopyridine-2-ylidene, 1-I, introducing a new class of singlet N-heterocyclic carbenes featuring σ/σ* instead of the usually encountered σ/π frontier orbitals. Carbene 1-I reacts with H2 via a unique sideways σ*-approach, leading to pyridinium iodide formation with N−I bond cleavage. This study investigates how nitrogen substituents with varying bond strengths (I < Br < Cl < OCF3 < OMe < NMe2 < F < Me) influence the electronic structure of the carbene and its preference for σ*- or π-approach hydrogenation. Using high-accuracy DLPNO−CCSD(T) and NEVPT2 computations, we find that I-, Br-, Cl-, and OCF3-substituted pyridinylidenes adopt a σ2σ*0 configuration and favor σ*-approach hydrogenation. In contrast, OMe-, NMe2-, and Me-substituted carbenes exhibit a σ2π0 configuration and lower barriers for the π-approach hydrogenation. Interestingly, the fluorine-substituted carbene assumes a σ2π0 electron configuration yet still preferentially undergoes σ*-approach hydrogenation.