Reactions of multidentate ligands with ligated alkali cation complexes: self-exchange and “sandwich” complex formation kinetics of gas phase crown ether–alkali cation complexes

Abstract

Natural abundance isotopic labeling has been employed to study the reactions of labeled LM + complexes with L (L = triglyme, TG; 12-crown-4, C4; 15-crown-5, C5; 18-crown-6, C6; or 21-crown-7, C7; M = Li, Na, K, Rb, or Cs) in the gas phase using Fourier transform ion cyclotron resonance mass spectrometry. Reaction efficiencies for both ligand exchange and the formation of 2:1 ligand:metal “sandwich” complexes were determined. For a given ligand, self-exchange rates generally decrease with increasing metal size, while the sandwich complex formation rates show strong dependence on the relative sizes of the metal ions and ligand cavities. Acyclic TG complexes undergo self-exchange more rapidly than the analogous cyclic C4 complexes, whereas the sandwich complex formation rates are faster for the C4 complexes. Sandwich formation rates show a weak positive pressure dependence, as increased pressure leads to increased collisional stabilization of the complexes. Extrapolation of the rates to the zero pressure limits still yields significant rates, reflecting radiative stabilization. The self-exchange reactions have weak, negative pressure dependences, suggesting they are in direct competition with sandwich complex formation. Analysis of the sandwich complex formation radiative association kinetics yields estimates of binding enthalpies for attachment of the second ligand. Trends in the binding enthalpies, like the kinetics, show strong dependence on the relative sizes of the metal ions and ligand cavities. For a given metal, binding of a second TG is weaker than binding of a second C4. Binding enthalpies for the second ligand are in every case substantially less than calculated binding enthalpies for the first ligand to attach to a given metal.