Heterolysis of H2 Across a Classical Lewis Pair, 2,6‐Lutidine⋅BCl3: Synthesis,Characterization, and Mechanism

We report that 2,6‐lutidine?trichloroborane (Lut?BCl₃) reacts with H₂ in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut?BHCl₂. The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut?BCl₃ was calculated to react with H₂ and form the ion pair, [LutH⁺][HBCl₃^?], with a barrier of ΔH^≠=24.7 kcal mol^?1 (ΔG^≠=29.8 kcal mol^?1). Metathesis with a second molecule of Lut?BCl₃ produced Lut?BHCl₂ and [LutH⁺][BCl₄^?]. The overall reaction is exothermic by 6.0 kcal mol^?1 (Δ_rG°=?1.1). Alternate pathways were explored involving the borenium cation (LutBCl₂⁺) and the four‐membered boracycle [(CH₂{NC₅H₃Me})BCl₂]. Barriers for addition of H₂ across the Lut/LutBCl₂⁺ pair and the boracycle B?C bond are substantially higher (ΔG^≠=42.1 and 49.4 kcal mol^?1, respectively), such that these pathways are excluded. The barrier for addition of H₂ to the boracycle B?N bond is comparable (ΔH^≠=28.5 and ΔG^≠=32 kcal mol^?1). Conversion of the intermediate 2‐(BHCl₂CH₂)‐6‐Me(C₅H₃NH) to Lut?BHCl₂ may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl₃. Intramolecular protodeboronation, which could form Lut?BHCl₂ directly, is prohibited by a high barrier (ΔH^≠=52, ΔG^≠=51 kcal mol^?1).