EFFECTS OF ALKYL SUBSTITUTION ON THE MULTIDENTATE ATTACHMENT OF ALKALI-METAL CATIONS BY LIGANDS IN THE GAS-PHASE - KINETICS AND THERMOCHEMISTRY OF CATION-BINDING BY ISOMERS OF DICYCLOHEXANO-18-CROWN-6

Commercially-available dicyclohexano-18-crown-6 (DC18C6) is a mixture of two easily-resolved isomers, cis-syn-cis and cis-anti-cis, differin g in whether the cyclohexyl rings are linked to the main macroring wit h both substituents on-the same side or on opposite sides, respectivel y. We have investigated the reactions of DC18C6 and its alkali metal i on complexes in a solvent-free, gas-phase environment. Both isomers ha ve greater free energies of alkali cation attachment in the gas phase than unsubstituted 18-crown-6 (18C6), with the greatest differences be tween the substituted and unsubstituted ligands (>10 kJ mol(-1)) occur ring for the smallest metal ions. This is rationalized in terms of the greater polarizability of DC18C6. Comparison with solution data indic ates there must be a greater cost in free energy for desolvating the c avity of DC18C6 than for 18C6. The efficiency of metal transfer from 1 8C6 to DC18C6 is high (greater than or equal to 20% for all alkali met al ions with either isomer) and increases with decreasing alkali ion s ize, Li+ being nearly a factor of 3 faster than Cs+ for the anti isome r. The variation in rates with cation size can be explained on the bas is of decreasing barrier height on the potential energy surface for ca tion transfer as the depths of the wells for metal binding increase. T he syn and anti isomers differ measurably in free energy of cation att achment, in kinetics of cation uptake from 18C6-alkali metal complexes , and in the rates at which 2:1 ligand-metal complexes form. The latte r for both isomers are slower than for unsubstituted 18C6, reflecting greater steric hindrance in the substituted crown.