A 13C-NMR study of azacryptand complexes

An azacryptand has been solubilised in aqueous media containing 50% (v/v) dimethyl sulphoxide. 13C-NMR has been used to determine how the azacryptand is affected by zinc binding at pH 10. Using 13C-NMR and 13C-enriched bicarbonate we have been able to observe the formation of 4 different carbamate derivatives of the azacryptand at pH 10. The azacryptand was shown to solubilise zinc or cadmium at alkaline pHs. Two moles of zinc are bound per mole of azacryptand and this complex binds 1 mole of carbonate. By replacing the zinc with cadmium-113 we have shown that the 13C-NMR signal of the 13C-enriched carbon of the bound carbonate is split into two triplets at 2.2 °C. This shows that two cadmium complexes are formed and in each of these complexes the carbonate group is bound by two magnetically equivalent metal ions. It also demonstrates that these cadmium complexes are not in fast exchange. From temperature studies we show that in the zinc complexes both complexes are in fast exchange with each other but are in slow exchange with free bicarbonate. HOESY is used to determine the position of the carbonate carbon in the complex. The solution and crystal structures of the zinc–carbonate–azacryptand complexes are compared.