Competing Modes of Hydrogen Activation in Singlet Pyridinylidenes: π-Approach <i>vs</i> 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 <i>via</i> a π-approach trajectory. Our group has recently prepared 1-iodopyridine-2-ylidene, <b>1-I</b>, introducing a new class of singlet <i>N</i>-heterocyclic carbenes featuring σ/σ* instead of the usually encountered σ/π frontier orbitals. Carbene <b>1-I</b> reacts with H₂ <i>via</i> 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.