From unsuccessful H2-activation with FLPs containing B(Ohfip)3 to a systematic evaluation of the Lewis acidity of 33 Lewis acids based on fluoride, chloride, hydride and methyl ion affinities

Abstract

The possibility of obtaining frustrated Lewis pairs (FLPs) suitable for H₂-activation based on the Lewis acid B(Ohfip)₃1 (Ohfip = OC(H)(CF₃)₂) was investigated. In this context, the crystal structure of 1 as well as the crystal structure of the very weak adduct 1·NCMe was determined. When reacting solutions of 1 with H₂ (1 bar) and selected phosphanes, amines, pyridines and N-heterocyclic carbenes, dihydrogen activation was never observed. Without H₂, adduct formation with 1 was observed to be an equilibrium process, regardless of the Lewis base adduct. Thus, the thermodynamics of H₂ activation of 1 in comparison with the well-known B(C₆F₅)₃ was analyzed using DFT calculations in the gas phase and different solvents (CH₂Cl₂, ortho-difluorobenzene and acetonitrile). These investigations indicated that FLP chemistry based on 1 is considerably less favored than that with B(C₆F₅)₃. This is in agreement with control NMR experiments indicating hydride transfer from [H–B(Ohfip)₃]⁻ upon reaction with B(C₆F₅)₃, giving [H–B(C₆F₅)₃]⁻ and B(Ohfip)₃ in toluene and also MeCN. Induced by these unsuccessful reactions, the Lewis acidity towards HSAB hard and soft ions was investigated for gaining a deeper insight. A unified reference system based on the trimethylsilyl compounds Me₃Si–Y (Y = F, Cl, H, Me) and their respective ions Me₃Si⁺/Y⁻ calculated at the G3 level was chosen as the anchor point. The individual ion affinities were then assessed based on subsequent isodesmic reactions calculated at a much less expensive level (RI-)BP86/SV(P). This method was validated by systematic calculations of smaller reference systems at the frozen core CCSD(T) level with correlation effects extrapolated to a full quadruple-ζ basis. Overall, 33 common and frequently used Lewis acids were ranked with respect to their FIA, CIA, HIA and MIA (fluoride/chloride/hydride/methyl ion affinity).