Solvent effects in the metal interchange of crown ether – alkali metal cation complexes. Transition from an associative exchange in nitromethane to a dissociative exchange in acetonitrile studied by 23Na nuclear magnetic resonance

The role of the solvent in the dissociation kinetics and cation exchange mechanisms of the complex sodium–monobenzo-15-crown-5 (Na: B15C5)+ was examined by 23Na NMR. In nitromethane (NM), the cationic exchange between the complexes takes place via an associative metal interchange mechanism, 1M. In acetonitrile (AN), it takes place via a dissociative (dissociation/recombination) mechanism. In AN–NM binary mixtures, the rate constant of the dissociative exchange (k−1) increases with the AN molar fraction, XAN, from 1.1 × 10−2 s−1 for XAN = 0 to 89 × 10−2 s−1 for XAN = 0.400 (corresponding to a decrease of the free energy of activation, ΔGdi≠ from 62.1 to 51.1 kJ mol−1 respectively, at 301.5 K). The activation parameters were ΔHdi≠ = 48 kJ mol−1 and ΔSdi≠ = −20 J K−1, mol−1 for XAN = 0.200. This rate increase was related to the concentration increase in solution of the AN monosolvated complex (AN: Na: B15C5)+. In the whole range of AN mole fractions studied, the rate constant of the associative exchange, k2, was not dependent upon XAN in the error limits: k2 ≈ 9 × 104 M−1 s−1 at 301.5 K (ΔGas≠ = 45 kJ mol−1). The activation parameters were determined to be ΔHas≠ = 23 kJ mol−1 and ΔHas≠ = −75 J K−1 mol−1. These findings are in good agreement with an associative exchange controlled mainly by the conformational changes of the ligand during the concerted partial decomplexation of a sodium cation and partial complexation of a second one, while solvation of the complexed cation plays a major role in the dissociative mechanism.