Metalation shifts in the nuclear magnetic resonance spectra of ‘capped’ porphyrins

Abstract

The ‘capped’ porphyrins CP2, CP3, and their zinc derivatives were studied by ¹H n.m.r. spectroscopy. An examination of the changes in the chemical shifts of the capped porphyrin resonances on binding zinc in various solvents indicates that for CP2 the structure of the free-base and zinc porphyrins are similar to one another with a cavity size similar to that observed in the crystal. The spin–spin coupling constants of the aliphatic chain resonances together with the observation of only four resonances shows that dynamic processes are present for both CP2 and CP3. The metalation shifts observed for CP3 demonstrate the greater flexibility of the molecule and in particular the variable cavity size. Additionally the zinc in ZnCP3 is shown to co-ordinate DMSO with the DMSO binding from the cavity side. A consequence of the difference in behaviour of CP2 and CP3 with respect to solvents and metalation is that the interpretation of the differences in dioxygen binding of the iron(II) capped porphyrins is made complex.