[C6F5Xe]+ and [C6F 5XeNCCH3]+ salts of the weakly coordinating borate anions, [BY4]- (Y = CN, CF3, or C 6F5)

Article abstract of DOI:10.1021/ic702259c

New examples of [C₆F₅Xe]⁺ salts of the weakly coordinating [BY₄]^- (Y = CN, CF₃, or C₆F₅) anions were synthesized by metathesis of [C ₆F₅Xe][BF₄] with M^I[BY₄] (M^I = K or Cs; Y = CN, CF₃, or C₆F₅) in CH₃CN at -40°C, and were crystallized from CH ₂Cl₂ or from a CH₂Cl₂/CH ₃CN solvent mixture. The low-temperature (-173°C) X-ray crystal structures of the [C₆F₅Xe]⁺ cation and of the [C₆F₅XeNCCH₃]⁺ adduct-cation are reported for [C₆F₅Xe][B(CF₃)₄], [C₆F₅XeNCCH₃][B(CF₃)₄], [C₆F₅Xe][B(CN)₄], and [C₆F ₅XeNCCH₃][B(C₆F₅)₄]. The [C₆F₅Xe]⁺ cation, in each structure, interacts with either the anion or the solvent, with the weakest cation-anion interactions occurring for the [B(CF₃)₄]^- anion. The solid-state Raman spectra of the [C₆F₅Xe]⁺ and [C₆F₅XeNCCH₃]⁺ salts have been assigned with the aid of electronic structure calculations. Gas-phase thermodynamic calculations show that the donor-acceptor bond dissociation energy of [C₆F₅XeNCCH₃]⁺ is approximately half that of [FXeNCCH₃]⁺. Coordination of CH₃CN to [C₆F₅Xe]⁺ is correlated with changes in the partial charges on mainly Xe, the ipso-C, and N, that is, the partial charge on Xe increases and those on the ipso-C and N decrease upon coordination, typifying a transition from a 2c-2e to a 3c-4e bond.