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) (Δr G°=-1.1). Alternate pathways were explored involving the borenium cation (LutBCl2 (+) ) and the four-membered boracycle [(CH2 {NC5 H3 Me})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-(BHCl2 CH2 )-6-Me(C5 H3 NH) 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) ).