Heterolysis of H 2 Across a Classical Lewis Pair, 2,6‐Lutidine⋅BCl 3 : Synthesis, Characterization, and Mechanism

Abstract We report that 2,6‐lutidine⋅trichloroborane (Lut⋅BCl 3 ) reacts with H 2 in toluene, bromobenzene, dichloromethane, and Lut solvents producing the neutral hydride, Lut⋅BHCl 2 . The mechanism was modeled with density functional theory, and energies of stationary states were calculated at the G3(MP2)B3 level of theory. Lut⋅BCl 3 was calculated to react with H 2 and form the ion pair, [LutH + ][HBCl 3 − ], with a barrier of Δ H ≠ =24.7 kcal mol −1 (Δ G ≠ =29.8 kcal mol −1 ). Metathesis with a second molecule of Lut⋅BCl 3 produced Lut⋅BHCl 2 and [LutH + ][BCl 4 − ]. The overall reaction is exothermic by 6.0 kcal mol −1 (Δ r G °=−1.1). Alternate pathways were explored involving the borenium cation (LutBCl 2 + ) and the four‐membered boracycle [(CH 2 {NC 5 H 3 Me})BCl 2 ]. Barriers for addition of H 2 across the Lut/LutBCl 2 + pair and the boracycle BC bond are substantially higher (Δ G ≠ =42.1 and 49.4 kcal mol −1 , respectively), such that these pathways are excluded. The barrier for addition of H 2 to the boracycle BN bond is comparable (Δ H ≠ =28.5 and Δ G ≠ =32 kcal mol −1 ). Conversion of the intermediate 2‐(BHCl 2 CH 2 )‐6‐Me(C 5 H 3 NH) to Lut⋅BHCl 2 may occur by intermolecular steps involving proton/hydride transfers to Lut/BCl 3 . Intramolecular protodeboronation, which could form Lut⋅BHCl 2 directly, is prohibited by a high barrier (Δ H ≠ =52, Δ G ≠ =51 kcal mol −1 ).