Elementary supramolecular chemistry in the gas phase: ligand exchange reactions of proton-bound clusters of aliphatic amides and diamides with amines

Abstract

The ligand exchange reactions of the proton-bound trimer [(DMF)₂⋯H⁺⋯H₂NCH₃] of dimethylformamide (DMF) and methylamine and of proton-bound dimers [diamide⋯H⁺⋯amine] of three aliphatic primary diamides, succinic diamide, adipinic diamide, and maleic diamide, and of three tertiary diamides N,N,N′,N′-tetramethyl succinic diamide, N,N,N′,N′-tetramethyl adipinic diamide, and N,N,N′,N′-tetramethyl bis-endo-bicyclo[2.2.1]heptane-2,5-diamide (N,N,N′,N′-tetramethyl bis-endo-BCH-2,5-diamide) were investigated by FT-ICR spectrometry. The proton-bound clusters were generated in the external CI ion source of the instrument using the appropriate amine as the CI reagent and primary ionization of the CI gas by electron ionization. The ions were transferred to the FT-ICR cell for measuring the kinetics of the exchange reaction with a reactant base which was present at a constant background pressure in the cell. The trimer [(DMF)₂⋯H⁺⋯H₂NCH₃] displays fast exchange reactions with all amines used with exception of pyridine as well as with DMF and DMSO used as polar reactants. The proton-bound trimer shows a “role-specificity” of the components: the amine ligand is exchanged specifically by a more basic amine without observation of any intermediate, while the polar reactants substitute the two DMF ligands of the cluster in two reaction steps. Like proton-bound trimers, the proton-bound dimers [diamide⋯H⁺⋯amine] contain two amide groups interacting with a proton and an amine. In the case of n-propylamine as amine ligand, exchange by a more basic amine is always efficient. However, clusters [diamide⋯H⁺⋯amine] containing trimethylamine show reduced reactivity, which is attributed to a steric hindrance of the proton transfer to the incoming base. Such steric effects can be maximized by generating proton-bound dimers of N,N,N′,N′-tetramethyl bis-endo-BCH-2,5-diamide with tertiary amines or amines containing bulky alkyl groups. Thus, the cluster [N,N,N′,N′-tetramethyl bis-endo-BCH-2,5-diamide⋯H⁺⋯di-isopropylamine] exhibits a small efficiency of only 0.35% for the exothermic exchange reaction with di-sec-butylamine. The results are used to propose a solvation model for the structure of these proton-bound clusters, in which the ammonium group of the protonated amine is solvated by two amide groups.